ADVANCED
MATERIALS
Nanochemistry :
Synthesis in Diminishing Dimensions**
By Geoffrey A. Ozin*
1. Introduction to Nanophysics and Nanochemistry
Nanochemistry, as opposed to nanophysics, is an emerging subdiscipline of solid-state chemistry that emphasizes the
synthesis rather than the engineering aspects of preparing
little pieces of matter with nanometer sizes in one, two or
three dimensions. Currently there is considerable interest in
nanoscale objects, since they exhibit novel materials properties, largely as a consequence of their finite small size. The
nanochemist can be considered to work towards this goal
from the atom “up”, whereas the nanophysicist tends to
operate from the bulk “down”. Building and organizing
nanoscale objects under mild and controlled conditions “one
atom at a time” instead of “manipulating” the bulk, should
in principle provide a reproducible method of producing
materials that are perfect in size and shape down to the
atoms. A cartoon illustration of this comparison is shown in
Figure 1. These little objects can be made of organic, inorganic and/or organometallic components. Their structureproperty relationships are designed to yield new materials
with novel electronic, optical, magnetic, transport, photo-
chemical, electrochemical, catalytic and mechanical behavior. Areas of application that can be foreseen to benefit from
the small size and organization of nanoscale objects include
quantum electronics, nonlinear optics, photonics, chemoselective sensing, and information storage and processing.
Nanophysics fabrication methods have improved remarkably over the last few years. For example, using a combination of sophisticated metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) planar
engineering (deposition) and scanned optical, X-ray, ion and
electron beam lateral engineering (nanolithographic) techniques, one can routinely produce submicron-scale objects,
with essentially any desired architecture!’] A state-of-the-art
quantum dot array with individual component boxes of size
300 & 50 A is displayed in Figure 2. In order to go beyond
Fig. 2. State-of-the-art reactive
ion-beam etching of an organized
array of 300 8, diameter quantum
dots 111.
+
Fig. 1 . Nanochemistry and nanophysics compared
[*] Prof. G. A. Ozin
[**I
61 2
Advanced Zeolite Materials Science Group
Lash Miller Chemistry Department, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 1A1 (Canada)
The generous financial assistance of the Natural Sciencesand Engineering
Research Council of Canada is gratefully appreciated.
‘cj V C N Ver/u~s~esell.~rhufft
mhH.
this limit one must apply the recent amazing breakthroughs
in the manipulation and imaging of atomic scale objects
utilizing the nanotips of scanning probe microscopes.[21This
approach can be considered to represent the ultimate limit in
nanoengineering physics and miniaturization. It is hard to
imagine anything much more impressive than the writing of
the IBM logo with individual Xe atoms adsorbed on a cold
nickel (110) surface,[3]the transfer of single atoms of Si, Ge,
S and Se from site to site on Si, Ge, MoS, and WSe, surf a c e ~ , ‘the
~ ] layer-by-layer nanometer scale etching of twodimensional SnSe,, TiSe, and NbSe, substrate^,'^] the deposition of nanoscale metal features from an organometallic
gas,[61the local pinning and movement of organic molecules
on a surface,”] the machining of atomic- to nanometer-size
holes and channels in various substrates,[81the building and
placement of nanoscale clusters on different surfaces,[g]and
the remarkable construction of an electronic (bistable)
switch from a single Xe atom,”’] all of which are illustrated
in Figure 3.
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ADVANCED
MATERIALS
G. A . OzinlNanochemistry
WRITING I B M LOGO MACHINING ATOMIC
WITH Xe ATOMS ON P TO NM SIZE HOLES
ON A SURFACE
BUILDING AND PLACE LAYER -BY - LAYER
MENT OF NM SCALE NM SCALE ETCHING
CLUSTERS ON A
OF 2-D SnSe,, TtSe,.
SURFACE
NbSe2
MOVEMENT AND LOCAI METAL DE POSl T I ON
PINNING OF AN ORGA FROM AN ORGANONIC MOLECULE ON A
METALLIC GAS
SURFACE
\IM SIZE LINES ON
A SURFACE
RANSFERENCE OF
;INGLE ATOMS OF SL,
e,S,Se FROM SITE'0-SITE ON SURFA.ES OF St.Ge.MoS2,
ISe, RESPECTIVELY
CONSTRUCTION OF
AN ELECTRONIC
BISTABLE) SWITCH
FROM A SINGLE
Xe ATOM
Fig. 3. Manipulation and imaging of atomic-scale objects usmg the nanotip of
a scanning probe microscope.
The ability to engineer nanometer-sized objects on a surface has potential for important applications, primarily in
the construction of electronic devices with atomic dimensions. This is the size regime in which quantum transport and
confinement effects in conducting, semiconducting o r insulating elements can be observed, controlled and exploited.
But this will not be so easy in practice. Local atomic-scale
surface modification requires the reproducible and rapid
movement of a sharp stylus or surface over a wide range of
distances, and the ability to return to within 1 nm of the same
position. It is clear that serious obstacles need to be over-
come before practical atomic-scale logic and memory devices
can be realized.
Chemists, on the other hand, pride themselves on being
able to synthesize perfect objects having nanometer dimensions. To be able to make nanostructures that are useful in
electronic, optical and information processing systems,
chemists also have to dream LIPsynthetic methods that have
the ability to position these tiny objects in appropriately
connected organized arrays. The chemical alternatives for
meeting this challenge and building such nanoscale devices
from scratch involve patterning and templating methods. In
the former nanolithography is used to spatially define chemically active foundation sites, usually on planar substrates,
upon which subsequent site-specific chemical synthesis allows the growth of nanoscale objects." '1 The latter exploits
the perfectly periodic, single-size-and-shape channel, layer
and cavity spaces of crystalline nanoporous host structures
for performing host-guest inclusion chemistry." Both approaches benefit from the principle of synthesis and self-organization in pre-existing regions of a planar substrate or a
porous solid, both with restricted dimensions on the nanoscale.
With respect to the template-based preparation, organization and stabilization of nanoscale objects, the nanochemist
generally dreams up chemical syntheses inside the void
spaces of nanoporous host materials. The strategy involves
the judicious selection of the host material and Suitable precursors to the desired guest(s). There now exists a huge range
of hosts.['31 They can be of an inorganic, organic or
organometallic compositional type, with one-dimensional
(I-D) tunnel, 2-D layer and 3-D framework structures.
Hosts may be of the insulating, semiconducting, metallic or
superconducting type, or may attain these properties following inclusion of the chosen guest.['41 On surveying known
host structures, one finds that channel, interlamellar and
cavity dimensions vary widely in size, separation and perfection, spanning the size range from barely being able to accommodate the smallest ionic or molecular guests all the way
to channel dimensions of about 5-10000 A, interlamellar
spaces of 3-50 8, and cavity diameters of 6-1000O A. Representative examples of 1 -D, 2-D and 3-D host structures are
listed in Tables 1-3.
Geoffrey A . Ozin was born in London in fY43. He completedhis undergraduate ~ . o r in
k chcrnistr~y
at King's College, London University, and obtained his P1i.D. in inorganic cheniistrj crt Oriel
College, Oxford University in 1967. He was an ICI Post Doctorul Fellow ut Southumpton
University fbom 1967 to I969 before,joining the Chemistrjs Facultj. of'tlic' Univrrsity of' Toronto
us an Assistant Professor, where he is now a Full Professor. His currem reserirch is exclusively
in the area of solid-state chemistry with a thrust towards advanced zeolite nmteriuls science. The
emphasis of' his work is on the synthesis and characterization qf'novel nanoporous materids,for
the development of systems with value in quantum electronics and nonlinear optics, high density
erasable optical data storage and chenioselective sensing.
Adv. Maler. 1992. 4, No. 10
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able 1. 1-0 tunnel hosts
G. A . OzinlNanochemistry
Table 3.
3-D framework hosts
Host
faphite tubes
lucleopore membranes
lrea channels
‘hosphazene tunnels
>organic tunnels
!eolites, molecular sieves
#oft chemistry
iligned polymer channels
ipid bilayer vesicles
Host
Pillared clay galleries
Carbon sieves
Porous glasses
Polymer cavities
Block-copolymer nanodomains
2eolites.molecular sieves
2eotypes.non-oxide frameworks
Micelles. capped materials
This article focuses mainly on host-guest inclusion chemstry aimed at the synthesis of new hosts and the organizaion and assembly of a range of guest precursors on the
nternal surfaces of the nanometer-dimension channel, layer
md cavity spaces of a variety of hosts. This kind of hostcuest inclusion chemistry can be considered to be a form of
opotaxy, to accentuate the resemblance to expitaxy, a simiar type of chemistry performed on a two-dimensional surNanochemistry of this kind is able to yield novel
rable 2.
2-D layered hosts
Y
Host
Clays, cationic, anionic
Graphite,boron nitride,boron graphites,nitrogen graphite5
Oxide layers
Halide layers
Chalcogenide l a y e r s
Organic,inorganic LB films
Self-assembled mono- and rnultilayers
Miscellaneous inorganic layers
Protein cages
Inorganic insertion materials
Buckyball endohedral complexes and insertion compounds
composite materials with properties of potential value in the
fabrication of nanoscale electronic, optical, optoelectronic,
photonic and magnetic devices.
When contemplating how to make “well-defined” hostguest inclusion systems, it is pivotal to really appreciate the
effects of the topology and reactivity of the internal surface
of the host on the occluded guest. These factors will control,
for instance, details of the chemistry between the host and
the guest, as well as the location, population and distribution
of the guest inside the host lattice. In this paper, hosts surveyed will be mainly chosen from inorganic and organic
Langmuir-Blodgett f i l m and tunnel, layer and open-framework materials, a representative list of which is given in
Tables 1-3. Within this context it is useful to think about the
comparison between the nanophysics fabrication philosophy
and the nanochemistry synthesis method of, for example, a
GaAs quantum dot array, as illustrated in Figure 4. This
scheme brings out the interrelationship between the planar
deposition and lateral engineering methodology of the nanophysicist and the host-guest inclusion techniques of the nanochemist. The former involves the organization of the atomic
components of GaAs from Me,Ga/AsH, gaseous precursors, using a “one-step” MOCVD procedure followed by
scanning-beam nanolithographic procedures, to form an array of GaAs quantum dots on a planar substrate. The latter
requires a “two-step” MOCVD encapsulation-anchoringreaction sequence of events to create an assembly of GaAs
quantum dots (clusters) inside a nanoporous host lattice.
G. A . OzinlNanochemistry
a ) Organic and Inorganic Channel Hosts
0
0
MOCVD PRECURSOR
DEPOSITION
PRISTINE HOST
J.
0
Me3Ga
GaAs
Si
GaAs QUANTUM LAYER
0
0
0
0
0
ENCAPSULATED AND
ANCHORED GUEST Me3Go
PRECURSOR
bH3
NANOLITHOGRAPHIC
FABRICATION CF GuAs
QUANTUM WIRE ARRAY
b ) Organic and Inorganic Luyer Hosts
0
0
0
NANOLITHOGRAPH IC
FABRICATION OF GaAs
QUANTUM DOT ARRAY
ENCAPSULATED AND
ANCHORED Go As
QUANTUM DOT ARRAY
Fig. 4. Nanophysics fabrication (left) compared to nanochemistry synthesis
(right) of a GaAs quantum dot array.
0
0
0
0
The properties of these materials depend on the degree of
quantum, spatial and dielectric confinement. The ability to
create lattice-matched quantum dot arrays of precise size
and shape benefits enormously from a detailed knowledge of
the structural and dynamical aspects of the surface chemistry
of the precursors used, for example, in MOCVD routes to
such nanostructures. Reasoning of a related type can be
applied to the structural and reactivity properties of the internal surfaces of nanoporous hosts towards different kinds
of guests. Some of these guests are listed in Table 4.
The following list provides an overview of the host-guest
nanocomposites topics that are briefly surveyed in this article.
in
1-D.2-D and 3-D host
0
0
0
0
0
0
0
Atoms
Cations (simple. complex)
0
Anions (simple. complex)
Molecules (organic.inorganic1
0
Radicals (organic.inorganic)
Coordination complexes
Orgonometallic compounds
0
Clusters (insulator,semiconductor,rnetal components1
0
Oligomers a n d polymers ~insuloting.conducting)
0
Adv Muter 1992, 4, N o 10
Organization of nonlinear optically active chromophoric
molecules in Langmuir-Blodgett (LB) and self-assembled
monolayer (SAM) hosts.
Synthesis of quantum-confined semiconductor layers in
LB hosts.
Preparation of sensitized layered oxide semiconductor
particles.
Organic macromolecules between the sheets of layered
inorganic hosts.
Polymer-ceramic nanocomposites.
Molecular recognition in layered metal phosphonates.
Superlattice reagents.
c ) Organic and Inorganic Open-Framework Hosts
0
Table L. Range of guests included
structures
Synthesis of oriented polymer fibrils in nucleopore membranes.
Electric-field-aligned molecular sieve crystals for optical
second-harmonic generation.
Self-organized electron transport chains and photosynthetic biomimetics in zeolite hosts.
Nanoscale semiconductor particles in surfactant vesicle
hosts.
Helical graphitic carbon nanotubes.
Preparation of quantum wires in zeolites.
Zeolite encapsulated selenium chains and rings.
Extra-large-pore and ultralarge-pore zeolite molecularsieve-type materials.
Nanometer-sized semiconductor clusters.
Synthesis of quantum-confined, capped, soluble semiconductor clusters in micellar hosts.
Assembly of semiconductor clusters within zeolite hosts.
Stepwise synthesis of 11-VI metal chalcogenide clusters
inside zeolite Y supercages using MOCVD-type precursors : the phenylate, phenylthiolate, zeolate cluster capping analogy.
Intrazeolite phototopotaxy : redox interconvertible tungsten oxide superlattices.
Sodalite superlattices: from molecules to clusters to expanded insulators, semiconductors and metals.
Visible photoluminescence and electroluminescence from
quantum-confined silicon.
Cloverite: exploring the 30 A supercage of a novel gallophosphate molecular sieve for advanced materials science
applications.
Synthesis and biosynthesis of inorganic nanophase materials inside polypeptide cages.
Synthesis of ordered arrays of semiconductor clusters of
predictable size and shape within the nanodomains of
block-copolymers.
Template synthesized nanoporous organic polymer hosts.
Nanochemistry with fullerenes: a bright future.
Nanoelectrochemistry.
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ADVANCED
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G. A . OzinlNanochemistry
2. Organic and Inorganic Channel Hosts
channel hosts, as well as the assembly of nanophase materials inside channel hosts.
2.1. The Classics
The crystallization of certain organic and inorganic molecular species, exemplified by urea and cyclophosphazenes, creates long channels with diameters of around 5.5 and 4.55.0 A, respectively. Materials of this type can act as hosts for
a wide variety of guests, the latter usually being packed
lengthwise along the channel axis. Motion of included guests
tends to be constrained to rotation about the channel direction. Interchannel interactions between guests are usually
insignificant compared to intrachannel ones.112a1
A number of conducting organic polymers and low-dimensional redox-active inorganic chain compounds have
framework structures with vacant lattice channels.['61 Representative examples of this class of tunnel hosts include polyacetylene, polyphenylene, NbSe, and Mo,Se,. They usually
exhibit electronic conductivity, together with an appropriate
band structure for the uptake of additional electrons and a
system of interconnected vacant tunnel lattice sites to accommodate charge-balancing cations. Electrochemical and/or
chemical techniques can be used to inject electrons/cations
topotactically into these materials, the critical diameter of an
acceptable guest cation being determined by the geometry of
the channel system. The primary chemical properties of these
materials that can be controlled by electron/cation transfer
processes are changes in composition, stoichiometry, redox
state, and structure. These processes are often reversible.
Physical properties that can be controlled by these reversible
topotactic electronic/cation injection reactions concern electronic (transport, metal/semiconductor and metal/superconductor transitions, charge-density waves, magnetic ordering
phenomena), optical (linear and nonlinear behavior), ionic
conductivity and mechanical behavior. These qualities provide
a wide range of applications for these materials, including
reversible electrodes, energy storage in high energy density
secondary batteries, passive display systems, smart mirrors
and windows, ion-sensitive chemical sensors and microelectronic/microelectrochernical devices.
Many 2-D layer and 3-D open-framework host lattic
listed in Tables 2 and 3 can also participate in similar electrc;,,,
cation intercalation and injection processes. Layered host systems and their intercalated guests can display a number of
structural oddities, including staging, stacking, order/disorder transitions and incommensurate phases. Similar effects
have been observed for I-D host lattices. As the focus of this
paper is concerned with nanochemistry inside different dimensionality organic and inorganic host compounds, we will
not pursue the insertion/intercalation reactions of metal
cation (and proton) guests any further. The interested reader
is referred to numerous excellent recent reviews and books
that exhaustively cover this tremendously interesting area of
host-guest inclusion chemistry.['2' 1 6 , ''I Let us now take a
brief look at the synthesis of some nanometer-dimension
w
2.2. Template Synthesis of Polymer Fibrils, Cylinders
and Wires in Nucleopore Membranes
The synthesis of nanoscale organic polymer fibers, cylinders and wires has recently created a tremendous amount of
excitement in the chemistry, materials science and physics
communities.[", 191 Such nanoscale sculptures are intrinsically scientifically interesting systems with a myriad of potential applications. In the case of polypyrrole and poly(3methylthiophene), the synthesis method utilizes a nanoporous
nucleopore membrane as a template during the synthesis. A
simple scheme for achieving this goal is shown in Figure 5a.[18,1 9 ] The nucleopore polycarbonate membrane filter contains linear cylindrical pores of equivalent pore diameter covering the range 300 to 10000 A. A solution of
monomer is separated from a ferric salt polymerization
agent by the nucleopore membrane arranged as sketched in
the top part of Figure 5 . Nascent polymer chains adsorb on
1-1
/-F/
,Glass
tube
4cm
Polymerization reagent
solution
Fig. 5. Top) Equipmcnt used to synthesize polyheterocyclic nanoscale fibers,
cylinders and wires inside a nucleopore template membrane filter [18, 191. Bottom) Scanning electron micrograph of chemically synthesized polypyrrolc
nanotubules [18. 191.
the pore walls, yielding a thin polymer coating which thickens with time to eventually yield a solid wire. The templatemediated polymerization can also be performed electrochemically by mounting the nucleopore membrane onto the surface
ADVANCED
MATERIALS
G. A. OzinlNanochemistry
of a Pt disk electrode.[18,191 Polyheterocyclic fibers, cylinders and wires are generated within the pore structure of the
template membrane by controlling the conditions of the
chemical or electrochemical synthesis. The most significant
advantage of this method over earlier synthetic routes is its
ability to produce nanoscale objects with monodisperse diameters and lengths. De-encapsulation of these objects can
be accomplished by dissolving the membrane in CH,CI,,
followed by filtration collection of the fibers, cylinders or
wires. An electron micrograph of some beautiful polypyrrole
nanotubules 500 nm in diameter and 8 pm in length is shown
in the lower part of Figure 5.[''. 19] Interestingly, the polyheterocyclic nanotubules and nanowires show redox reactions typical of the parent polymer. Furthermore, it has been
found that template synthesis dramatically enhances their
electronic conductivity with respect to the bulk form of the
analogous polymer. This increases with decreasing size of the
cylinders and wires. A combination of dc and optical measurements of conductivity, X-ray diffraction and polarized
infrared absorption spectroscopy showed that the polymer
chains in the narrowest template-synthesized fibrils are preferentially oriented parallel to the cylinder axes of these
fibrils.['8. 19] Whether the observed enhanced conductivity in
these fibrils results from a highly oriented collection of polymer chains running down the axis of the nucleopore channels
or a surface layer of polymer chains adsorbed to the walls of
the pore has yet to be established. Whatever the outcome, the
template-mediated polymerization of pyrrole and thiophene
monomers down a nucleopore channel nicely illustrates the
nanochemistry approach to tiny electronically conducting
polymer objects built of aligned polymer chains.["%'91
la1
lC)
L-Fig. 6. Large AIPO,-5 crystals before (a) and after (b) their alignment by n
o f an electric field of strength E = 3 kVkm. Aligned crystals are fixed by
1
glass. (c) The relationship between the crystal habitus and the unitnoda
unidirectional channels is shown schematically [20].
(-NO,) vectors arranged to reinforce one another, w
placed in the oscillating field of a light wave are abk
produce light at the doubled frequency.['l] Stucky and
workers["' first demonstrated that the acentric AlPC
2.3. Electric Field Alignment of Large Molecular
Sieve Crystals
Recently Car0 and co-workers["' have demonstrated that
large zeolite and molecular sieve needle-shaped crystals
(AIPO,-5, H-SAPO-5, Silicalite, ZSM-5) with dimensions of
around 200 pm x 50 pm x 50 pm can be aligned by kV/cm
electric fields and fixed in the aligned state by a thin film of
water glass or an epoxy resin. Figure 6 shows an assembly of
as-synthesized randomly oriented and electric-field-aligned
AIPO,-5 crystals. From an advanced materials applications
standpoint it is important to realize that the 12 T-atom ring,
7.5 A unidimensional channel system of A1P04-5 (Fig. 6)
runs parallel to the c-axis of the hexagonal crystals. Therefore aligned AlPO,-5 crystals not only allow the orientation
of a variety of guests within the channel spaces to be controlled, but also the organization of the molecular sieve host
itself. This provides opportunities for lining-up large ensembles of hyperpolarizable molecules in acentric and aligned
AIP0,-5 single crystals, an elegant approach for tuning the
composite material for optical second harmonic generation
(SHG). Recall that easily polarized molecules of p-nitroaniline, having their electron donor (-NH,)-acceptor
Adv Mater 1992, 4, No 10
1
I
-103
-50
1.0-
Q
I
I
0
50
1
[degrees]
Fig. 7 . A) Picosecond Nd:YAG laser, where 'p denotes the angle betwee
length axis of the AIPO,-5 crystal and the polarization plane of the inc
laser beam. F, filter; P, polarizer; M, monochromator; PM, photomultil
BCI, gated boxcar integrator; KDP, potassium dihydrogen phosphate cr:
(JJ and 2 0 denote the frequencies ofthe incident and emitted light, respecti
B) SHG intensity for p-nitroaniline in a large A1P04-5 crystal a5 a functi<
the angle 'p between the c-axis of the A1P04-5 crystal and the polarization
of the incident light [20].
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molecular sieve absorbs and forces the alignment of p-nitroaniline guest molecules within its 7.5 8, diameter channels. The pores are maximally filled at a loading of 13%
p-nitroaniline by weight and the SHG signal is at its highest
for randomly oriented crystal samples. For individual crystals of A1P04-5 containing p-nitroaniline, Caro and coworkers[201have shown that the SHG effects are strongly
direction dependent. Maximum SHG is obtained when the
electric field vector of linearly polarized incident laser light
oscillates in the direction of the molecular dipole moments of
the p-nitroaniline guest, Figure 7. Consequently the maximum SHG effect is obtained with optimally loaded and
aligned assemblies of AIPO,-S/p-nitroaniline crystals. This is
a beautiful example of a nanochemistry approach for assembling and organizing dipolar molecules for applications in
ectro-optical and optical devices.
G. A . OzinlNanochemistry
constructed photodiodes that resemble the donor-acceptoracceptor triad in the photosynthetic reaction center of purple
bacteria, in which extremely efficient and irreversible lightinduced charge separation occurs. The system takes molecules B-C and D shown in Figure 9. The former is a photo-
2.4. Self-organization of Electron Transport Chains
in Zeolite Hosts
Mallouk and c o - w o r k e r ~have
[ ~ ~ exploited
~
the size-exclusion and ion-exchange properties of zeolites to cause as
many as four electroactive species to fall into line a t a zeolite/
aqueous solution interface. The zeolite can thus act as a
template for the self-organization of electron transport chains,
which can function as biomimetic photosynthetic systems,
current rectifiers and photodiodes. Let us briefly explore
Mallouk's methodology using the generic four-subunit example of an anion A, which is charge excluded from the
anionic zeolite pore structure, a11 interfacial cation B-C, on
account of the size exclusion of subunit B, and intrazeolite
cation D. A cationic polymer bound to anion A holds the
self-assembled A-B-D triad at an electrode, Figure 8.
+
~~+-zeoiite
+ B-c"+
+ D"+Na+
Water
D
Fig. 9. Photochemically active donor-acceptor diad B C and acceptor D [23].
chemically active donor-acceptor diad, constrained to the
surface cation sites of the zeolite crystals. Internal electron
transfer occurs from the Ru(bpy): center to the diquaternary 2,2'-bipyridine moiety when excited by visible light.
Because D is a better electron acceptor than the acceptor end
of C, it captures the electron before reverse electron transfer
can occur within B to regenerate the ground state. Uphill
electron transfer from D to the acceptor C, followed by electron-hole recombination within B, slows down the back
electron transfer process, thereby conferring upon this selfassembled nanoscale triad the quality of a photosynthetic
biomimetic.
@
Z e o l i t e L or Y
2.5. Nanoscale Semiconductor Particles
in Surfactant Vesicles
Fig. 8. Self-asembly of a four subunit redox chain at the zeolite/water interface [23].
The triad Fe(CN)~e-Os(bpy)~@-(trimethylamino)methylferrocene@,(Fc@),where bpy is bipyridine, acts a5 a current
rectifier because the iutrazeolite Fc@/"couple is unable to
communicate directly with the electrode. Electrons can only
pass from Fc@/Oto the electrode as the O~(bpy):@'~@
couple
is more oxidizing than FcO''. Current rectification in the
opposite sense has been realized with an Fe(CN);@Ru(bpy):%obalticeniuni
triad, (CcO), since the
Ru(bpy):@'@ couple is a better reducing agent than the
CcQ" couple. By incorporating suitable light absorbing
molecules into the chain, Mallouk and c o - ~ o r k e r s [ ' ~have
]
61 8
Nanometer-sized semiconductor clusters have electronic,
optical and chemical properties that depend on the cluster
size, a phenomenon often referred to as the quantum size or
confinement effect. Blue shifts in the exciton energy and
enhanced oscillator strengths of the exciton as the cluster size
decreases are diagnostics of such quantum confinement effects in semiconductor nanoscale particles. Several review
papers summarize our current understanding of their sizeHere we briefly examine the use of
dependent
surfactant vesicles as a tubular host for nanoscale 11-VI
semiconductor particles. For example, Fendler and co-worke r ~ ~showed
' ~ ] that dihexadecyl phosphate (DHP) ultrasonically dispersed in water, under appropriate conditions of pH
and temperature, spontaneously forms negatively charged
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ADVANCED
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G. A . OzinlNanochemistry
vesicles. ZnS, ZnSe, CdS and CdSe particles were generated
in situ from M2@-coatedDHP vesicles (where M = Zn, Cd)
by exposure to gaseous H,S or H,Se. The vesicles provide
nucleation sites, control the growth of the particles, act as
stabilizers against aggregation, and enable compartmentalization of reactants and products. Sulfide and selenide particle formation was monitored by absorption spectroscopy.
The observation of a red-shifting absorption edge with H,S
or H,Se loading indicated the formation of larger particles.
Band-gap irradiation of the surfactant-vesicle-supported
semiconductor particles led to charge separation and to electron transfer to acceptor methyl viologen in the presence of
glucose, a sacrificial electron donor. Photocurrent measure-
Fig. 10. Photocurrents from the illumination of DHP-vesicle-incorporated
ZnSe particles In the presence of glucose and methylviologen [25].
ments and absorption spectroscopy verify the electron transfer scheme illustrated in Figure 10 and laid out below:
Electron-hole photoproduction
ZnSe -% ZnSe(eo h@)
+
Electron transfer to methylviologen MV2@
ZnSe + MV'@
ZnSe(e@)+ MV2@
Reoxidation of MV'@ at collector electrode surface
MV'Q --t MV=@+ e0
Depletion of holes by electron transfer from glucose
t ZnSe + RCHO
ZnSe(h@)+ RCH,OH
~
2.6. Helical Graphitic Carbon Nanotubes
IijimaL261recently reported on the amazing finding that
capped helical nanotubules of graphitic carbon, called "buckytubes", ranging from about 30 to 300 8, in diameter can be
produced under conditions similar to those used for fullerene
synthesis. This discovery suggests that engineering of carbon
structures should be possible on scales considerably greater
than those relevant to C,, and the fullerenes. Let us place
this breakthrough in context.
Since Kratschmer and ~ o - w o r k e r s ' [ ~discovery
~I
of the
arc-discharge evaporation method for synthesizing preparative scale amounts of C,, and other fullerenes, reasearch on
these materials has been progressing at a furious pace. The
Adv. Mater. 1992, 4, No. 10
(3
history and chemical and physical properties of fullerenes
have been described in several exceptionally interesting recent reviews,[28'and the intense activity on new methods of
preparation, their fundamental properties, reactivity patterns and theoretical aspects can be appreciated from numerous recent reports.[291Many aspects of fullerene chemistry
are of great significance. These include derivatives of C,,
with hydrogen, fluorine, phenyl, benzyl and both externally
and encapsulated metal atoms. The different modes of bonding of C,, as a ligand are being discovered from the crystal
structures of recently reported transition metal complexes,
consisting of the metal, C,, and ancillary ligands. Of great
interest are stable superconducting phases of C,, doped with
metals. These can be viewed as insertion compounds (see
Sec. 5 ) where the metal center occupies octahedral and tetrahedral interstitial sites in the fcc lattice of C6". Charge transfer occurs from the metal center to a vacant conduction band
formed through interaction of the C,, and metal valence
orbitals in the close packed lattice. The first doped phase
identified was K,C,, with a superconducting transition of
18 K. Optimization of the critical temperature
in these
superconducting phases has been achieved by manipulating
the composition and the type of the metal dopants. The
highest T, of 45 K thus far observed has been found in Rb/Tl
co-doped C,, .[301
In 1980, using equipment of the type used for fullerene
synthesis, Iijimar311observed electron-diffraction and microscopy evidence for the formation of spherical graphitic
shell structures of diameter 50-200 A deposited around the
negative end of the electrode used for the arc discharge. The
existence and form of these spherical particles have played
an important role in our understanding of the mode of formation of fullerenes and soot particles.[321Very recently,
Iijima[261noticed that capped helical nanotubes of graphitic
carbon coexist with these spherical graphitic shell structures.
This new type of finite carbon structure comprises seamless
coaxial cylinders of graphitic sheets ranging in number from
2 to 50, Figure 11. Electron diffraction and microscopy data
showed that on each tube the carbon-atom hexagons are
arranged in a helical fashion about the tube axis, the helical
pitch varying from needle to needle and from tube to tube
within a single needle. The formation of the graphitic tube
structure can be appreciated from examination of a cut and
unrolled tube, illustrated in Figure 11. This structure may
turn out to be a model for giant fullerenes. The smallest
diameter innermost tube observed was 22 8, and corresponds roughly to a ring of 30 carbon hexagons. In this case,
two neighboring hexagons on the ring meet at an angle of
about 6", indicating more six-ring strain than in planar
graphite, but less than in C,, ,where the bending angle is 42".
It is fascinating to realize that the ABAB hexagonal stacking
sequence found in graphite cannot be retained for coaxial
tubes in the observed size range. Outer to inner adjacent
cylinders are separated by 3.4 A, implying a reduction of
8-9 hexagons on the circumference of a tube on going from
one tube to the one inside it. This hexagon mismatch phe-
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G. A . OzinlNanochemistry
nomenon is known in the turbostratic stacking forms of
graphite.
Since Iijima’s[261discovery of collections of concentrically
arranged graphitic-like tubes with fullerene-like caps,
Ebbesen and A J a ~ a n [have
~ ~ ]unraveled an extremely simple
way to synthesize multigram quantities of these hollow carbon tubules. Their method employs an arc discharge between two different-diameter graphite rods, separated by a
1 mm space in a 500 Torr pressure of He. The plasma in this
gap gradually consumes the smaller rod. Nanotubes of 20200 8, diameter and 1 pm length form in this way. Interestingly, these ndnotubes are predicted to have size-dependent
electronic properties, where diameter and helicity dictate either semiconductive or metallic behavior. High thermal and
mechanical stabilities are also predicted for carbon nanotubes, and therefore they are a possible competitor for carbon fibers and carbon fiber composite materials.[33]
Just as fullerenes can act as novel hosts for encapsulated
guests in the so-called endohedral complexes, Iijima’s helical
nanotubes of graphitic carbon are anticipated to host a wide
range of organic, inorganic and organometallic guests. Furthermore, the recently reported “exohedral” transition metal
complexes of C,, imply that analogous systems are also likely to exist for the cylindrical form of graphitic carbon.
2.7. Encapsulation of Conjugated Polymers in Zeolites
[loo]
a
[;lo]
h
0 9
Fig. 1 I , Top) Electron micrographs ofnanotuhulcs of graphitic carbon. Parallel dark lines correspoiid to the (002) lattice images orgraph~te.A cross section
ofeach tubule is illustrated. a) Tube consisting of five graphitic sheets, diameter
6.7 n m . b) Two-sheel tube. diamcter 5 5 nm. cJ Seven-sheet tube, diameter
6.5 n m . which has ihe smallest hollow diameter (2.2 nm). Bottom) a) Schematic
diagram sho\\iiig ii helical arrangemcnt of a graphitic carbon tubule, which is
uiirolled for the purposes ofexplanalion. The tuhc axis I S indicaicd by the heavy
line and the hcxagons labclcd A. B and A’, 8’ are supcriinposed to form the
tube. b) The row of shaded hexagons forms a helix on thc tube. The number of
hexagons doe5 iioi reprcseni a i-cal tube size. but the orientation is correct [26].
Bein and E n ~ e l [have
~ ~ I essentially single-handedly established the groundwork required to encapsulate conjugated
polymers, with the potential to be conductive, within the
confines of zeolite hosts. One of the driving forces of their
work was to enhance the understanding of the conduction
mechanism of conjugated polymers, by isolating and decoupling these low-dimensional materials in structurally welldefined hosts, like zeolites. In the context of nanochemistry,
isolated conducting chains could provide the molecule-sized
interconnections between electronic components in nanometer-dimension circuitry of the future. In an even broader
sense, the intrazeolite polymer work of Bein and Enzel is part
of an on-going multiprong approach to the synthesis of encapsulated polymers in a variety of 1-D, 2-D and 3-D host
materials-see Secs. 3 and 4. The goal here is usually to
enhance the carrier mobilities or optical nonlinearities
through molecular organization of conjugated polymer
chains. Already, this subject has been touched upon with
reference to polypyrrole and polythiophene in the channels
of nucleopore membranes. Later on in this paper, two-diinensional and three-dimensional versions of these will be
examined. In what follows, highlights of the work of Bein
and Enzel will be reviewed.
Polyaniline, polypyrrole, polythiophene, polyacrylonitrile
and some substituted derivatives thereof were synthesized
inside the channels of mordenite and the cavities of zeolite Y.
Internal oxidants, such as extraframework Cu2@,Fe3@were
used to polymerize pyrrole and thiophene monomers, while
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
/\n/u
CN CN CN
200-300°C
WLYACRYLDNITRILE
LADDER STRUCTURE
LADDER STRUCTURE
&,&A&
GRAPH ITE-LIKE
STRUCTURE
Bein and Enzel explored the non-charged polyacrylonitrile
system in mordenite and zeolite Y Gas phase chromatography (GPC) showed molecular weights of about 10000 and
1000 for the recovered polymer (dilute HF dissolution) from
these zeolites, respectively. Pyrolysis of the intrazeolite polyacrylonitrile in N, at 650-700 "C produces a black product
devoid of nitrile and hydrogen, but with N/C atomic ratios
in the range 0.18-0.20. Steric considerations favor ladder
rather than sheet-like structures for the intrazeolite compared to the bulk graphitized polymer, Figure 12. The zeolite
drastically changes the pyrolysis properties of polyacrylonitrile compared to the bulk form. Recovered, pyrolyzed
polyacrylonitrile shows DC conductivity of about
S/cm.
Fig. 12. Pictorial representation ofpyrolyzed polyacrylonitrile in the bulk form
compared to that in mordenite and zeolite Y [34].
2.8. Zeolite-Encapsulated Selenium Chains and Rings
external oxidants like (NH,),S,O, successfully polymerized
aniline and acrylonitrile. Both the reaction medium and the
zeolite type influence the observed polymerization rates,
products and chain lengths. For instance, water adversely
screens extraframework oxidizing cations from the monomers, necessitating non-aqueous solvents for successful
intrazeolite polymerizations of pyrrole and thiophene;
pyrrole polymerizes more slowly in mordenite than in zeolite Y, and steric effects prevent the polymerization of poly(2-ethylaniline) in mordenite but not in zeolite Y ; the average chain length of polyacrylonitrile is shorter in mordenite
and zeolite Y than the average size of the crystal host. The IR
and UV-visible spectroscopic properties of intrazeolite polypyrrole, polythiophene and polyaniline are diagnostic of oxidized chains with bipolaronic carriers in the first two and
polarons in the latter. Conductivity measurements, however,
show significant differences between the transport properties
of the bulk and intrazeolite forms of these polymers. It seems
that charge carrier trapping by host-lattice cation or defect
centers seriously impedes carrier mobility for the encapsulated form of these polymers. To circumvent this phenomenon,
The trigonal and monoclinic forms of bulk selenium contain helical Se, chains and crown-shaped Se, rings, respectively. Trigonal selenium is both semiconductive and photoconductive. Therefore the inclusion of selenium into various
zeolite structure types is intriguing from the point of view of
the host dependence of the structure of the encapsulated
guest, the nature of host-guest interactions, and the possibility of gaining access to spatially, quantum and dielectrically
confined selenium having altered intermolecular chain-chain
and ring-ring interactions compared to the bulk forms. Experimentally, the encapsulation of selenium into various
cation exchanged forms of FAU, LTA, MOR and L zeolite
structure types was achieved from the vapor phase using the
dehydrated host in the temperature range 250-350 "C. Numerous groupsr3']have studied the products of the selenium
impregnation process by an array of diffraction, microscopy
and spectroscopy techniques. The general consensus for the
structure of selenium in the 7.5 8, channel system of mordenite is that of isolated spiral chains with bond lengths, bond
angles and helical pitch not very different to that found in the
bulk trigonal form. The absorption edge of Sen-MOR is blue
shifted compared to bulk trigonal selenium and is interpret-
(c)
white
black
t
lOOB+
Fig. 13. HRTEM images of Sen-MORtaken on a 1 MV electron microscope. a) Low, b) high magnification image, and c) shape of a mordenite crystal and the nature
of its Si/Al compositional variation along [OIO]. Dark bands correspond to aluminous regions [37].
Adv. Muter. 1992, 4 , No. 10
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ed in terms of the absence of interchain interactions. Absorption bands around 2.5 eV and 3.5 eV are assigned to selenium 4p lone-pair and selenium-selenium a-electron transitions into the vacant u*-orbital system. Interestingly, the
quantity of Se that can be incorporated into FAU-type zeolites depends strongly on the Si/Al ratio, being greatest for
the most aluminous hosts.[361The absorption spectra suggested that the structure of the imbibed selenium transforms
from Se, rings, to Senchains, to Se, double chains on passing
from high to low Si/Al ratios. These results, when combined
with the saturation Se loading values, indicate that the encapsulation process is mainly controlled by Se atom interactions with extraframework cationic centers. In this context,
it is worth recalling the reported[351black and white contrast
observed in the high-resolution transmission electron microscopy (HRTEM) images of Sen-MORthat was attributed
to the modulation of the Se content along [OIO], Figure 13.
At that time, the origin of this effect was not understood.
With the recent observation that the Si/Al ratio appears to
control the saturation loading of Se in FAU-type zeolites,
TerasakiI3’1 has re-interpreted the contrast modulation phenomenon in Se,-MOR in terms of compositional fluctuations of Si/A1 along [loll, and that the dark regions in the
HRTEM images probably correspond to higher Se concentrations in Al-rich areas of MOR. In fact, it was proposed
that Se atoms can be used as probes of compositional variation of A1 across zeolite crystals by monitoring the contrast
in the HRTEM images.[371
2.9. Extra-Large-Pore and Ultralarge-Pore Zeolite
and Molecular-Sieve-Type Materials
After roughly four decades of zeolite and molecular sieve
synthesis, it appeared that the experimentally accessible
range of pore sizes had an intrinsic upper stability limit of the
12 T-atom ring, with a diameter of about 7-8 .&r381 (T refers
to apex-linked tetrahedral TO, building units, where T is
most commonly Si, Al, P.) However, a dramatic turn of
events occurred in 1988 with the discovery by Davis and
co-workers[391of an extra-large-pore molecular sieve based
on aluminophosphate, denoted VPI-5, having an essentially
circular 18 T-atom ring unidimensional channel structure,
with a diameter of about 12-13 A. This breakthrough shattered the “psychological” 12 T-atom ring barrier, and had
the effect of revitalizing synthetic efforts aimed towards extra-large-pore and even ultralarge-pore materials. Not long
after this amazing breakthrough, a method was discovered
in 1990 for the controlled phase transformation of VPI-5 to
a related yet smaller pore 14 T-atom ring material, denoted
anALPO-8.[401In 1991, Estermann and co-worker~[~’]
nounced another spectacular discovery with the synthesis
and structure determination of an extra-large-pore gallophosphate molecular sieve called cloverite (see Sec. 4.5 for
more details). This novel material contains a 20 T-atom
622
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VC‘H VerlagcgeseN$chuftmhH, W-6940 Wernheu
G. A . Ozin/Nanochemistry
cloverleaf-shaped entrance window leading into an impressive 29-30 8, diameter supercage. The overall structure of
the cubic unit cell is based on two non-intersecting, threedimensional, 8 T-atom and 20 T-atom channel systems built
of LTA/RPA cages and cloverite supercages, respectively.
An intriguing feature of cloverite is the existence of
P(OH)OGa(OH) terminal hydroxyl groups completely covering the inside surface of the supercage and protruding into
the cloverleaf-shaped pore openings. Zeolite and molecular
sieve frameworks normally contain TO, (T = Si, AI, P) tetrahedral building units connected by T-0-T bonds, with
only a low level of terminal T(0H) defect groups, whereas
cloverite contains large numbers of terminal T(0H) groups
as an integral part of the structure and is referred to as
having an “interrupted framework”. The thermal stability
and chemical reactivity of cloverite is under intense investigation, as the properties of the interrupted framework will
probably determine the use of the material for catalytic, adsorption and advanced materials applications (see Sec. 4.5).
Shortly after Estermann and co-workers discovered
[ ~ ~ ~ the synthesis of
cloverite, Xu and ~ o - w o r k e r sdisclosed
JDF-20, an aluminophosphate molecular sieve with an elliptically shaped 20 T-atom ring unidimensional channel structure. Curiously, JDF-20 also contains terminal hydroxyl
groups lining the inside surface of the 20 T-atom channel and
protruding into the pore opening, just like cloverite. This
raises the interesting question as to whether interrupted
frameworks are an intrinsic characteristic of such extralarge-pore molecular sieves, so far represented by cloverite
and JDF-20, and are possibly a kind of extended “giant
defect”.
A common theme pervading the synthesis of the majority
of known zeolites and molecular sieves is the use of quaternary alkyl ammonium and/or amine organic additives
under hydro- or organo- or aminothermal reaction condit i o n ~ .431[ ~It~ is~ generally agreed that these template moieties serve one or more structure-directing, space-filling and
charge-balancing functions in the self-assembly of, for example, silicate, aluminate and phosphate basic building
units. One scientifically appealing way of thinking about the
template-mediated nucleation and growth of zeolite and
molecular sieve materials is in terms of density fluctuations
of transient local order, involving the cationic, anionic and
neutral constituents of a synthesis mixture under hydrothermal reaction conditions. Equipoteiitial periodic minimal energy surfaces (EPMESs) having zeolite and molecular-sievetype topologies have recently been proposed to be associated
with these fleeting structure domains in the liquid and/or gel
phases.[441Tessellating on the loci of these EPMESs, the
basic building units are imagined to undergo a spatially constrained condensation-polymerization to form the seed that
is responsible for the nucleation of a particular zeolite or
molecular sieve structure type. The surfaces of the crystal
nuclei themselves bear their own characteristic EPMESs,
which are viewed as being responsible for the continued
growth and crystallization of the product material.
1992
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With this in mind, it is interesting to note that some zeolites and molecular sieves synthesized to date, based on monomeric, oligomeric and even polymeric quaternary alkylammonium and/or amine templates, appear to display
framework topologies that bear a resemblance to the shape
of the template. Thus spherically shaped templates may lead
to cavity-type structures and rod-shaped ones may result in
channel-type structures. However, the pore sizes, channel
and cavity spaces have been generally restricted roughly to
the molecular diameter of individual spherically shaped templates or the width of the rod-shaped ones, resulting in the
current pore size maximum of the 20 T-atom rings found in
cloverite and JDF-20, mentioned above.
A leap into the ultralarge pore size range of 15-100 8, has
very recently been disclosed in the patent and open literat ~ r e , [ with
~ ~ ] the creative use of surfactant liquid crystal
templates. These consist of hexagonal or cubic close packed
aggregates of cylindrically shaped micelles, the latter being
composed of surfactant molecules containing hydrophobic
alkane chains, hydrophilic alkylammonium cationic head
groups and proximal charge-balancing hydroxyl and/or
halide anions. It is believed that under the hydrothermal
conditions of a zeolite or molecular-sieve-type synthesis, silicate, aluminate and/or phosphate building units undergo a
spatially constrained condensation-polymerization reaction
on a cylindrically shaped EPMES associated with the close
packed arrays of cylindrically shaped micelles. The latter
effectively serve as “massive templates” for the creation of
ultralarge cylindrically shaped pore, zeolite and molecularsieve-like structures. By judiciously selecting the alkyl chain
1
A
Micellar Rod
exagonal
Micellar
Array
1
Silicate
Silicate
1
Calcination
Fig. 14. Proposed mechanistic pathway for the formation of ul1ralarge-pore
zeolite and molecular-sieve-type materials [45].
lengths of the surfactant, the solution chemistry, and the use
of auxiliary organic molecule fillers, the effective diameter of
the cylindrical micellar aggregates can be adjusted to yield
20A
1ooA
40A
Fig. 15. Representative TEM images of 20A, 40A. 60 A, and I00 A ultralarge-pore zeolite and molecular-sieve-type materials. The latter two are prepared with
mesitylene as the organic molecule filler [45].
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C. A . OzinlNanochemistry
ultralarge-pore zeolite-like structures with diameters in the
range of 15- I00 A.A possible mechanism for the formation
of this family of mesoporous materials is illustrated in Figure 14. Calcination procedures appear to successfully remove the surfactant liquid crystal templates from these ultralarge pore materials without any noticeable evidence of
structure collapse up to about 800 “C to 1000 “C. Powder
X-ray diffraction, HRTEM lattice imaging, small angle neutron scattering and gas adsorption techniques have all been
effectively used in the preliminary characterization of these
materials before and after removal of the micellar tem40 A,
plate.r451Some representative T E M images of 20
60 A. and 100 8, pore size materials are shown in Figure 15,
where the latter two are prepared with mesitylene as the
auxiliary organic filler. They appear to behave as quasi-crystalline solids, with amorphous-like, roughly 8-10 A thick
walls and a hexagonal or cubic close packed array of essentially single-size pores in the 1 5 - 100 A range. Details of the
precise shape of the pores (e.g. hexagonal, cylindrical) are
under intense i n ~ e s t i g a t i o n . ‘ ~ ~ ]
From the standpoint of a range of basic scientific issues
and technological applications, this discovery of the ultralarge-pore silicates, aluminosilicates and aluminophosphates
must be considered a landmark in the history of the synthesis
of zeolite and molecular-sieve-type materials. Presumably
one can tune the pore sizes and even dimensionality further
by the appropriate choice of “secondary” chemistry involving the aluminum sites and/or the surface hydroxyl groups
on the inside walls of the cylindrical channels, following
removal of the micellar template. These ultralarge-pore materials have tremendous potential for very large molecule
size-and-shape-selective catalysis, gas adsorption and separation, as well as advanced materials for future nanoscale
device applications.
The 6-13 A pore size barrier of known zeolite and molecular-sieve-type materials has been dramatically broken by
the discovery of the ultralarge-pore materials. The 6-100 8,
range of window and channel spaces now available enormously expands the kinds of host-guest inclusion chemistry
accessible to the nanochemist. This provides an unprecedented opportunity to make interesting and significant contributions to the world of small, perfect and organized nanomaterials for various kinds of nanoscale device applications.
The future of the newly emerging field of nanochemistry,
based on these kinds of ultralarge-pore materials, looks ext raordi n aril y bright.
A,
3. Organic and Inorganic Layer Hosts
This section briefly explores recent developments involving the synthesis of nanoscale objects of dimensionality two.
Such materials are usually anisotropic and display novel
chemical/physical properties that arise from both the nature
of their constituents and how they are organized into 2-D
arrays. Rational design of such supramolecular assemblies
624
(
often requires that the synthesis be carried out within 2-D
regions intentionally built into Langmuir-Blodgett films or
self-assembled multilayers (SAMs), or existing as an intrinsic
part of the overall architecture of inorganic layer host materials. Nanolayer constituents of recently synthesized materials include aligned hyperpolarizable molecules and polymers, magnetically active cations, quantum-confined
semiconductors, conducting polymers and polymer electrolytes. Composite layered materials of this type often exhibit novel properties making them attractive potential candidates in, for example, integrated electronics and optics,
information processing and storage, solar energy conversion, electrochromic devices and electrode materials in advanced batteries.
3.1. Nonlinear Optically Active Self-Assembled
Multilayers
The Langmuir-Blodgett (LB) monolayer transfer method
provides a versatile technique for generating controlled-architecture multilayer films on surfaces. In essence, film formation involves the coherent transfer of a compressed amphiphilic monolayer at an air/water interface onto a chosen
substrate. Precise control over the assembly and growth of
large numbers of superimposed layers can be achieved by
repeatedly dipping the substrate through the air/water interface. Details of this technique and application examples have
been documented in many reviews and books.[461In the next
section, this approach is used for the assembly of quantnmconfined semiconductor layers. Here we present some recent
work aimed at improving the stability of rather delicate LB
films through the preparation of organic and inorganic
multilayers using sequential adsorption-reaction strategies.
Marks and c o - w ~ r k e r s [ ~
have
’ ~ employed an elegant organic
superlattice approach to thin-film nonlinear optical (NLO)
materials. Their strategy involved the synthesis of covalently
linked chromophoric self-assembled NLO multilayers according to the scheme laid out in Figure 16. The key features
of this study include:
0 The stilbazole chromophore precursor.
0 The layer-building quaternization step.
The anchoring of a high 8-center.
0 The incorporation of soft polymeric layers transverse to
the stacking direction.
0 The control and measurement of layer growth and thickness by UV-visible absorption, X-ray photoelectron spectroscopy (XPS), ellipsometry and contact angle measurements.
The results of this study for optical S H G showed considerable improvements over earlier attempts involving, for instance, poled polymers and acentric LB films. The multilayer
films of Marks and c o - w o r k e r ~ [ ~
adhered
’~
strongly to glass,
were insoluble in common organic solvents and could only
be removed by diamond polishing. The S H G data revealed
no in-plane anisotropy and showed films with uniaxial sym-
VClf V ~ r l a ~ ~ ~ c \ c l l \ mhH
~ h u f t W-6Y40 Weinherm, lYY2
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$ 3 50+ 2510
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ADVANCED
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G. A . OzinlNanochemistry
Mallouk and c o - w o r k e r ~ ' [approach
~~]
to circumventing
the fragility problem of LB films has been to exploit the
anticipated greater stability of inorganic multilayer films attached to Si and Au substrates. The essence of this strategy
is laid out in Figure 17. Here a phosphonic acid monolayer
is anchored to a Si or Au surface, followed by sequential
adsorption of Zr4@and 1,lO-decanediylbisphosphonicacid.
This produces an ordered multilayer film with a structure
analogous to that of a LB multilayer film, Figure 18. Ellip-
OH OH
HO HO OH OH
1) ? i,)
j
layer
Ch
fp
c-----nonpolar
tail/
polar head group
layer
f
TP P P P P - l
___f
b
f 1t 1i
LPPPPPJ
/surface-active
Fig. 16. Chroinophoric self-assembled multilayers. Organic superlattices as
thin-film NLO materials. a) Benzene, 25"C, b) retlux in n-PrOH, c)
CI,SiOSiCI,OSiC1, in tetrahydrofuran (THF). d) polyvinylalcohol in dimethyl
sulfoxide (DMSO) [47]. (PVA: polyvinyl alcohol; Ch: chroinophoric layer; Cp:
capping layer).
metry having no azimuthal dependence of chromophore
molecular orientation. The actual xi:? measured for a single
22 A layer exceeded the best value for poled polymers and
compared well with high quality acentric LB films. This is
consistent with the anticipated high chromophore number
density having a high degree of acentric alignment. With
respect to temporal stability, a five-layer structure was found
to have less than a 10% decline in SHG in 30 days. The
observation of a square root dependence of the SHG intensity with up to five chromophoric layers provides a compelling
NLO diagnostic for the existence of a high degree of structural regularity and non-centrosymmetric chromophore ordering in the addition of sequential layers. This is an impressive demonstration of rationally designed nanochemistry in
two dimensions.
F L : F > r i
,03pWPO:-
--
0 3 p w p g - (2)
-
(3)- (2),etc?
group
m
m
Metal Phosphonate
Film
Langmuir- Blodgett
Film
Fig. 18. Structural analogy between surface-adsorbed metal phosphonate and
LB multilayer films [48].
sometry shows stepwise growth of multilayers with precisely
controlled thickness from the spontaneous self-assembly of
soluble Zr4@ and a,w-phosphonic acid components. Even
thin films are good insulators, as demonstrated by the complete blocking of electron transfer by a 24 8, layer synthesized on Au from solution redox couples such as
Fe(CN)iG'4Q. The impressive compositional flexibility of
the zirconium phosphonate layers (side chains, main chains,
metal center and metal oxidation state) confers great potential on the system for tailoring NLO chromophoric assemblies, building thin-film molecular-sieve-type electrodes, and
achieving chiral molecular recognition, light-induced vectorial electron transfer and substrate-specific heterogeneous
catalysis.
multilayer film
Fig. 17. Inorganic multilayer films on silicon and gold surfaces [48].
Adv. Muier. 1992, 4, No. 10
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Katz and c o - ~ o r k e r s adopted
~ ~ ~ ] this approach to construct accntric inorganic multilayers. Their scheme for the
monolayer deposition sequence used to form multilayers
with polar order is shown in Figure 19. The building block
used was 1 1 -hydroxyundecyl phosphonic acid. Gold-surface-bound 11-hydroxyundecanethiol was employed to initiate the process. Ellipsometry confirmed that the monolayer
deposition sequence gave stepwise multilayer growth. XPS
experiments provided support for the phosphate/phosphonate linkages shown in Figure 19. Katz and c o - ~ o r k e r s [ ~ ' ]
(A)
Step1
I
Step3
Step2
I
I
,
(B)
%
INJ
HO
Qz
Dye monolayer
fh
DhOsDhonate
interlayer
< I
Dye monolayer
N,
%
w
phosphonate
inter layer
,-
I-
7////AS'PZ'//////
Fig. 19. A ) Self-assembly of organic multilaycrs with polar order using rirconium phosphate bonding between layers. B) Idealized schematic of dye multilayer
film [49].
used this self-assembly procedure for preparing multiple polar dye monolayers on silicon and glass substrates, joined by
zirconium phosphate-phosphonate interlayers. An idealized
schematic of the structure of their specific dye multilayer film
is shown in Figure 19. Polar order was convincingly demon626
,(:I
strated in their multilayers from the thickness, absorbance
and SHG as a function of the number of monolayers in their
samples. Maintenance of the degree of orientation of the dye
chromophore in each layer, considerable second-order optical nonlinearity and excellent thermal stability are significant accomplishments of this study. The future of this kind
of nanochemistry in 2-D looks exceedingly bright.
3.2. Size-Quantized 11-VI Semiconductors in LB Films
Numerous groups[501have recently reported on the synthesis of quantum confined TI-VI semiconductors, probably
with 2-D features, in the hydrophilic interlayer of LB films of
fatty acids. One of these groups has shown that, in the case
of CdS formed therein, the material could be grown by repetition of the process without destroying the proposed layered structure. Manipulation of the layer dimension and
composition of size-quantized IT-VI semiconductors of this
type allows one to fine-tune the optical bandgap, the redox
levels of the conduction and valence bands, and the linear
and nonlinear optical properties of the material.
This methodology can be illustrated with the work of
Moriguchi and c o - w o r k e r ~ [using
~ ~ ' LB films of cadmium
stearate. The proposed scheme for the production and
growth of CdS in the hydrophilic interlayers of a LB matrix
is laid out in Figure 20. Monolayers of cadmium stearate
were deposited on well-cleaned CaF, or borosilicate substrates using a standard Langmuir rough. Substrates carrying 15-20 monolayers permitted diagnostic IR, UV-visible
spectroscopy and X-ray diffraction measurements to be
recorded at each stage of the film-forming procedure. Sulfidation of the cadmium stearate film on exposure to H,S
converts the (RCO,),Cd groups absorbing at 1548 cm-' to
the 2RC0,H acidic function appearing at 1702 cm-', Figure 20. The concurrent formation of quantum confined CdS
is indicated by the growth of the UV absorption having its
onset at 370 nm, blue shifted with respect to that of bulk CdS
at 520 nm, Figure 20. Maintenance of the integrity of the LB
films is seen from the invariance of the vCH frequencies and
intensities of the stearic acid or cadmium stearate layers,
Figure 20. Growth of the CdS in these composite LB films
without destroying the layered structure is achieved as follows. Re-immersion of the composite acid form of the film in
aqueous CdZ@creates the salt form of the composite multilayer film of CdS and cadmium stearate, Figure 20. The
1702 cm-' IR band is totally replaced by the 1548 cm-'
band, while the original CdS UV absorption remains unchanged, Figure 20. Sulfidation-intercalation transforms
the 1548 cm-' TR band back to the 1702 cm-' one, concomitant with a growth of the intensity and a red shift of the
UV absorption onset, Figure 20.
This process can be repeated many times, the trend being
continued growth of size-quantized CdS at the hydrophilic
interlayers of the LB film, without destroying the layered
structure. X-ray diffraction analysis shows that the 50 A
VCH Y ~ ~ l u ~ . s ~ ~ ~ . s r lmhH,
l s t / l a fW-6940
t
Weinhein*,1992
09.ZS-964s/92/i0lU-U626 $3.50+ .2S/fJ
Ad". Maier. 1992,4, No. 10
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
I
charge separation, vectorial electron transport and multielectron redox reactions. Compartmentalized assemblies like
these form the basis of nanoscale electronic devices, photosynthetic biomimetics and chemical sensors. More recently
Mallouk and ~ o - w o r k e r shave
~ ~ ~described
~
a structured
photocatalyst that converts H,O/I@ into HJIF. This system
is a rationally designed supramolecular assembly of an internally platinized layered metal oxide semiconductor of the
type K,,,H,,,Nb,O,,, H,Ti,O, and HTiNbO,, sensitized
by external surface-confined RuL:@, (L = 4,4'-dicarboxy2,2'-bipyridene), Figure 21. The nanochemistry required to
Fig. 21. Photocatalyst with a layered oxide semiconductor structure [53].
300
400
k/n rn
500
2
J/cm-'
Fig. 20. I) Proposed scheme for the production and growth of CdS in the
hydrophilic interlayers of an LB matrix. A) Sulphidation by exposure to H,S
gas. B) Intercalation of Cd2@Ions by immersion in aqueous CdCI,. 11) IR
spectra of cadmium stearate LB film and composite films of CdS and LB
matrix: a) cadmium stearate LB film + b) sulfidation +c) intercalation + d)
sulfidation + e) intercalation -t f) sulfidation. H I ) UV-vis spectra of cadmium
stearate LB film and composite films of CdS and stearic acid multilayers: a 4
correspond to a. b. d , and f i n 11, respectively [50].
basal-plane spacing of the original LB film of cadmium
stearate increased steadily with the growth of CdS, reaching
50.9 A and 52.3 8, after the first and second sulfidationintercalation cycles, respectively. It has yet to be established
whether the CdS exists in the form of continuous two-dimensional nanosheets or planes of nanoparticles. In this context
it is worth noting that by using an "inert" ion-dilution techniquerSo1involving mixed cadmium/calcium stearate multilayers one can begin to manipulate the form and degree of
size-quantization of the interlayer CdS nanoguests. This
methodology has been applied to Cd2@/Ca2@
ion-exchanged
Nafion, in order to produce size-quantized, polymer-encapsulated CdS
3.3. Layered Oxide Semiconductors
assemble this kind of structured photocatalyst is described in
the original
Laser flash photolysis and transient
diffuse reflectance techniques defined the photophysical/
photochemical steps involved in this system. In the absence
of interlamellar Pt, clusters one observes the reactions given
in Equation 1, where CB stands for conduction band.
The I;@/e& charge-separated
state decays slowly
(> 500 ps) whereas the Io oxidation step is fast (< 100 ns).
On placing Pt, clusters between the slabs of a layered metal
oxide (Figure 21), H, and I," are produced, where the rate of
H, evolution declines to zero concurrent with a build-up of
I," in solution. Fresh KI restores the HJI," cycle, showing
that the photocatalyst is still active. This implies that the H,
forming reaction, Equation 2, is intercepted by the back reaction with oxidized donors, Equations 3 and 4, before the
e& reaches the Pt, cluster site. Future work to improve the
system involves promotion of the 1;'/eFB charge-separated
state by band bending at the semiconductor/solution interface by doping, as well as preventing I;@/IF reduction by
blocking the semiconductor through suitable tailoring of the
steric congestion at the surface.[s31
In a creative series of papers, Mallouk and co-workersrs2]
have effectively demonstrated how zeolite, clay and layered
metal phosphonate solid-state templates can spontaneously
organize multicomponent assemblies of electroactive and
photoactive moieties capable of efficient light absorption,
Adv. Muter. 1592, 4, No. 10
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ADVANCED
MATERIALS
G. A . OzinlNanochemistry
3.4. Organic Macromolecules in Layered Inorganic Hosts
The idea of being able to control the polymerization of an
organic monomer in the interlamellar 2-D spaces of a layered
inorganic host is an intriguing idea that has been the focus of
considerable activity during the last five years. Nanoscale
sheets of organic polymers sandwiched between thin slabs of
inorganic components are expected to display unique properties. For example, the topochemical photopolymerization
of diacetylenes within the interlayer spacing of layer perovskite halide salts and layered metal phosphate salts1s4]
produces ultrathin sheets of polydiacetylenes with a fully
conjugated backbone. Materials of this type are expected to
have large non-resonant third-order nonlinear optical susceptibilities of interest in optical computing applications.
The intercalative oxidative polymerization of pyrrole, thiophene and aniline between the sheets of FeOCl and V,O,
yields layered materials containing monolayers of conductive polymers inserted in the interlamellar spaces of an electroactive host. Such composite systems are expected to be of
interest as new materials for battery electrodes, electrochromic displays, mirrors and windows, and molecular
electronic components. The intercalation of poly(ethy1ene
oxide) and Li0 cations in the layer spaces of Vz051ss1and
silicates like montmorillonite[s61yield inorganic hosts bearing nanoscale sheets of polymer-inorganic salt complexes
that are expected to display interesting 2-D ionic mobility
properties. Solid polyelectrolytes of this type are likely to
find new electrochemical applications in solid-state batteries.
In what follows we briefly examine highlights of each of the
above systems in turn.
Mallouk and Cao1s4b1
explored the UV and X-ray induced
polymerization of diacetylene groups in layered salts of 3,5octadiyne bisphosphate (ODBP) organized in the manner
envisaged in Figure 22. The solid-state polymerization is rationalized to proceed as radical stepwise 1,4-addition to the
conjugated triple bonds as illustrated in Figure 22. The observed trend in the degree of polymerization for the diand trivalent metal salts studied was Zn, Mg, Mn >
.
.
Metal Ion. .Layer
,
1
(A)
La, Y, Sm > Ca, Cd, as monitored by UV-visible diffuse reflectance spectroscopy. The established correlation between
conjugation length of polydiacetylene and its electronic absorption onset was used to estimate the chain length of the
interlamellar polymer in this study. The observed behavior
was rationalized in terms of a structural polymerization
model requiring packing of the interlamellar diacetylene
groups with optimum contact between the C4 atom of one
diacetylene rod and the C1' of the neighboring rod, as illustrated in Figure 22.
FeOCl possesses a 2-D polymeric structure in which
FeOCl layers are separated by van der Waals gaps between
chlorine atoms, Figure 23. The coordination geometry around
Fig. 23. The layered structure of FeOCl viewed perpendicular to the h-axis [57].
the Fe"' center is distorted octahedral, composed of two axial
trans-chloride and four equatorial oxygen ligands. Kanatddis
and c o - ~ o r k e r s [ ~showed
']
that slow intercalative oxidative
polymerization of CH,CN/aniline can be achieved into
single crystals of FeOCl to produce (polyaniline),,,,FeOC1.
The resulting crystal quality, although inferior to that of the
starting FeOCI, was sufficiently high for preliminary X-ray
diffraction (XRD) experiments. The structure that emerged
from this XRD study involves the emeraldine form of the
polymer, stacked side-by-side and running along the diagonal [I011 direction, with the NH-backbone groups lying
a a a a
Po:-
I
,
POy
.:Metal
I o n Layer
Po;-
, :,
N
Po:,
1
Fig. 22. A) Possible structural arrangement of ODBP molecules in
M"(ODBP),., H,O (M = Mg, Mn, and Zn). B) Illustration of the topochemical principle for diacctylene polymerization [51].
'
628
7
A
Fig. 24. Proposed hydrogen bonding of polyaniline to the chlorines of the
FeOCl lattlce [57].
VCH ~,rlus\srrellr(li(Ifrff
nihH, W-6940 Weinherm, f992
0935-9648/92/1010-0628$ 3 5 0 i 2510
Adv Muter 1992, 4, No 10
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
a
above every other row of C1 atoms and an interchain distance of around 5 A, Figures 24 and 25. The layer-to-layer
orientation of the polyaniline chains could not be established
with the available XRD data.
-a'=2a
-
Fig. 25. Proposed orientation of interlamellar polyaniline with respect to the
FeOCI lattice a s projected in the (u,e)-plane [57]-
I
I
Fig. 26. A) Two views of the structure o f a V,O, layer in orthorhombic V,O,.
oxygen: open circles; vanadium: solid circles. B) Representation of V,O, lihril
and approxiinate dimensions [%I.
c o - w o r k e r ~ ~ ~ ~organization
~
From the recent work of Kanatzidis and
one learns that V,O, . nH,O xerogels are porous solids with
a layered structure capable of a diverse intercalation chemistry. The lamellar structure is built of corrugated rigid ribbons like those found in orthorhombic V,O, shown in Figure 26. The local coordination of a significant fraction of the
V5@centers in the xerogel layers is that of a square pyramid
each with an axial V=O and having about one half of these
axially coordinated to water as V-OH,. Roughly one equivalent of water is loosely held by hydrogen-bonding in the
interlamellar space. A schematic of the shape, approximate
dimensions and basic overall organization of the ribbons in
V,O, . nH,O xerogel is shown in Figure 26. These xerogels
can be easily fabricated into films and coatings, and they can
act as layered inorganic host lattices by accepting neutral or
charged guest species via cation exchange, acid-base, coordination and redox reactions. V,O, . nH,O itself is electroactive. Chemical and electrochemical redox intercalation in
these materials forms bronzes that can exhibit semiconducting or metallic properties. The insertion of several conducting polymers and alkali metal cations has been reported. An
especially interesting recent example from Kanatzidis and
c o - w o r k e r ~ [focuses
~~]
attention on a new class of V,O, .
nH,O intercalation compounds containing the insulating
polyethylene oxide, (PEO)/V,O, . nH,O. Because of the great
interest in PEO/aIkali metal salts as solid electrolytes and
e l e c t r o c h r ~ m i c s ,s91
~ ~Lie
~ * intercalation into (PEO)/V,O, .
nH,O is definitely worthy of investigation. Mixing aqueous
solutions of PEO (MW = 10') with aqueous gels of V,O, .
Adv. Muter. 1992, 4. No. 10
nH,O (interlayer spacing 11.55 A), followed by slow evaporation of H,O, yields a layered hydrated PEO/V,O, red-colored composite whose interlayer spacing has increased to
13.2 A. The net intralayer distance associated with this spacing is 4.5 A, consistent with a straight-chain rather than
coiled conformation of PEO in V,O, . nH,O as shown in
Figure 27. These PEO/V,O, . nH,O compounds can be
made to swell in water, and they can be cast into flexible thin
films. They are also capable of Lie redox intercalation using
LiI/CH,CN according to the reaction stoichiornetry of
Equation 5 . The resulting dark-blue bronze-like compounds
are significantly more conductive than the pristine counterparts. The interlayer spacing remains essentially unchanged
VCH Vrrlugsgesellschuft mbH, W-6940 Weinhrim, 1992
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ADVANCED
MATERIALS
G. A . OzinlNanochemistry
A.
at around 13 The properties of these new materials are
under investigation.
xLiI
+ PEO/V,O,
nH,O
--f
Li,PEO/V,O,
x
nH,O +? I,
-
(5)
Similar PEO intercalation experiments have been conducted with homoionic montmorillonite layer silicates.[56.'1
Interestingly, the interlayer distance of about 8
deduced
from the XRD pattern of the PEO/Li@-MONT@compound
is more consistent with interlamellar helical rather than
straight PEO chains as illustrated in the top part of Figure 28. Temperature-dependent AC impedance measure-
A,
or solvents, crystallinity and stoichiometry. Charge transport in the intercalated polymer-salt composites, on the other
hand, is limited to the cations, as the negatively charged
silicate layers effectively act as "infinite mass" immobile anions. This in principle eliminates the undesirable properties
of anion transport, concentration gradients and unwanted
electrode reactions for possible solid-state battery applications of PEO/Li@-MONT'. The enhanced ionic conductivity found in PEO/Li@-MONTOcompared to LiQ-MONTe is
envisaged to arise from PEO tunnels in the interlamellar
region, coupled possibly with a pillaring effect of the PEO,
Figure 28, top.
3.5. Polymer-Ceramic Nanocomposites
intercalated
PEO-sol1 compl
/
/
0
interlayer c a t i o n
I
+
Fig 28. A) Structural representation of PEO intercalated in a homoionic
montmorillonitc layer silicate [56, 591. B) Composite structures obtained using
layered ceramics. a ) Single polymer layers iiitercalated in the ceramic galleries.
b) Composites obrained by dispersion of delaminated ceramic Payers in a continuous polymcr matrix 159 b].
ments showed that the conductivity perpendicular to the
(a,b)-plane of PEO/Li@-MONT@maximized at 575 K, which
is a value about an order of magnitude higher than Li@MONTO. Above this temperature, the conductivity decreased, due to the expulsion of PEO from between the layers, and gradually converged to the value of collapsed
LiO-MONTO. Recall that in conventional polymer-salt
complexes it has been demonstrated that both cation and
anion moieties contribute, to different extents, to the electrical cond~ctivity."~"~
The anion mobility depends on a number offactors, including the type of cation, presence of water
The intercalation of polymers between the sheets of layered
ceramics provides access to novel polymer-ceramic nanocomposites. They exhibit unique physical and mechanical
properties attributed to the synergism between the individual
components. These are not necessarily the same as those
found on dispersion of single ceramic (delaminated) layers in
a continuous polymer matrix. The self-assembling nature of
the components yields two-dimensional arrays with nanometer dimensions in a single processing step with a high
degree of organization.
A nice case in point concerns the recent work of Giannelis
and c ~ - w o r k e r s [using
~ ~ ~polymer-mica-type
]
layered silicate
composites. The silicate component is described by the unit
cell formula M,,,Mg,Si,~,Al,O,,(OH),,
where 0 < x < 2.
Their structure consists of two-dimensional layers formed by
sandwiching two apex-sharing SiO, tetrahedral sheets to an
edge sharing MgO, octahedral sheet, with isomorphous substitution of Si'" by Al"' in the silicate layers. The negative
charge on the layers is balanced by ion-exchangeable cations
that lie in the galleries between the layers of the structure.
Aniline, for example, diffuses rapidly into the silicate galleries of Cu2@ion-exchanged mica-type layered silicates
(MTSs) to form intercalated polyaniline (PANI) by an oxidative-polymerization reaction, lower parts of Figure 28.
Doping with HCI vapor protonates the intercalated PANI to
form a metallic polaron lattice with phase segregation between metallic and insulating islands. Four-probe electrical
conductivity measurements show oIl = 0.05 S/cm and
ol =
S/cm with electrical anisotropy o , , / o ,% 5 x 10'.
Variable temperature conductivity measurements are consistent with tunneling of charge carriers between interlayer
metallic islands in the doped PANI. The lower conductivity
and blue-shifted optical absorption spectra of the PANIhybrid compared to bulk PANI are thought to originate in
the localization of charge carriers and molecular confinement, respectively, of the interlayer PANT. Here the extensive electronic coupling between chains which normally
occurs in bulk PANI is minimized in the PANI-hybrid.
Fracture toughness, dynamic modulus, differential scanning calorimetry and thermally stimulated current analy-
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
sis[s9b1of the PANI-hybrid show that its properties are modified with respect to the individual components. The PANIhybrid is found to be more oriented than the pristine host,
thus requiring a large energy dissipation during fracture due
to the increased surface area of the former. The PANI-hybrid
also shows no glass transition, whereas bulk PANI has T, =
220 “C. Thus the polymer stiffness and thermal expansion
coefficient have been modified in the PANI-hybrid compared to pristine PANI. Here intercalation and confinement
of PANI reduces segmental motion and cooperative effects
within and between polymer chains.
The alternative synthetic strategy[59b1
of homogeneously
dispersing single ceramic layers (10 A thick) of delaminated
MTSs in a continuous polymer matrix causes the host layers
to tend to orient parallel to each other through mainly dipoledipole interactions, Figure 28, bottom. Thus, as little as
2 wt.% of MTSs dispersed in polyimide causes a 60% decrease in the permeability of water, reduces the thermal expansion coefficient by 25 YOand enhances the in-plane storage modulus, while maintaining the dielectric characteristics
of the bulk polymer. This combination of new properties
makes the composites very attractive candidates for dielectric layers in electronic packaging applications, as well as for
lightweight composites. Increased ordering of the composite
compared to the pristine host and induced ordering of the
polymer chains up to 100 A from the polymer/ceramic interface are both believed to contribute to the enhanced properties of these kinds of hybrid-polymer materials. This new
field of polymer-ceramic nanocomposites can look forward
to a bright future.
0
a-Zr(O;POH),.H,O
1
I
1
I
Zr(O 3 PR)?
;
Q
;
I
0 0
Ca(H03PR12
Fig. 29. Idealized structure of a-zirconium phosphate, a-Zr(HOPO,), . H,O,
showing the pseudohexagonal unit cell, and schematic drawings of related
metal phosphonate structures Zr(O,PR), and Ca(HO,PR), [60. 611.
expansion of the layers, whereas branched alcohols at the
a-position, such as ’BuOH and ‘BuOH, are sterically excluded. In the layered divalent metal phosphonates M(0,PR).
M ( 0 3 P C 6 Hj )
3.6. Molecular Recognition in Layered Metal
Phosphonates
Earlier, we mentioned Cao’s and Mallouk’s elegant
nanochemical investigations of the topochemical polymerization of diacetylenes in the interlamellar spaces of layered
metal phosphonate salts.[541More recently, Mallouk and
co-workers[60,6 1 1 have cleverly exploited the attributes of
layered metal phosphonates (Fig. 29) for supramolecular assembly by organizing the organic groups in such a way as to
facilitate an intralayer molecular recognition event, such as
the binding of a particular analyte. Their method relies on
the formation of a “shape-selective pocket” for either coordination of a ligand or non-covalent binding of a guest
molecule in the interlamellar gap of layered metal phosphonates.
The approach involves synthesis around a “template”
moiety. Johnson and co-workers[621first recognized this
phenomenon in the vanadyl phosphonates VO(0,PR).
H,O. R O H , where the R O H alcohol occupies one of six
vanadium coordination sites and can be removed topochemically (i.e., with retention of the remaining structure) to yield
VO(0,PR). H,O. Primary alcohols bind at the open coordination sites of this “deintercalated host” material, causing
Adv. Mnter. 1992, 4, No. 10
colloidal Zr-modified
zinc phosphonate
surface-modified QCM
QCM
Fig. 30. Top) NH, intercalation into M(O,PC,H,). M = Zn, Co, prepared by
topochemical dehydration ofM(O,PC,H,) H,O. Bottom) Derivatizdtion procedurc for quartz crystal microbalances (QCMs). OTS: Octadecyltrichlorosilane.
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H,O, M = Mg, Mn, Co, Zn, the water molecule occupying
one of six metal coordination sites can be thermally removed
topochemically (Fig. 30). As found in their aqueous coordination chemistry, the magnesium compounds greatly prefer
oxygen-containing ligaiids over amines, whereas the reverse
is true for the zinc ones. When R = CH,, M = Zn, only
amines lacking x-branching are reactive; however, when
R = C,H,, M = Zn, greater steric congestion at the zinc
binding site restricts intercalation to only NH,. Based on
these reactions Mallouk and co-workersr601have developed
a shape-selective sensor for NH, and RNH,. This involves
the selective anchoring of Zr4@capped Zn(0,PR) H,O particles (reaction with aqueous ZrOCI,) to the gold electrode of
a piezoelectric quartz crystal microbalance (Fig. 30) via a
thiol-phosphonate anchoring agent (the quartz surface first
being functionalized by an octadecyltrichlorosilane selective
blocking agent). Activation of the sensor requires a 100°C
thermal dehydration, while NH, or RNH, deiiitercalation
occurs at 165 ”C. No evidence for degradation of the device
is observed over several cycles.
Selective binding of a single enantiomer from a racemic
solution has also recently been achieved by Mallouk and
co-workers[6’1using a chiral layered metal phosphonate receptor/analyte matched set. An enantiomerically pure amino
acid derivative is ion-exchanged for protons in x-Zr(HOPO,), .H,O (Fig. 31). This expands the layers from 7.6
to 19 A. This material intercalates the analyte, a chiral naph-
’””
I
o
n
Solid
Solution
a
50
60
70
80
.
90
* I
100
anaiytei
receptor
= 411
110
Concentration, mM
Fig. 31. TopJ Schcmatic structure ot’tliccationic .‘Pirkle-phase” rcccptor intercalated i n t o ?-zirconium phosphate, and sti-uctureof thc i-aceniic iinalyte. Bottom) Solution and solid phaae enantiomeric excesses (EEs) plotted against
linalyte concentration. The dashed h i e represents the maximum theoretical
solution EE, 33% for molar composition 411 analyte!receptor 160. 611.
thylamine derivative (Fig. 31) through a three-point (two
hydrogen bonds and one n-stacking interaction Pirkle collection) recognition mechanism, to form a 30 8, phase. This
intercalation occurs above a certain analyte threshold concentration, at which point the selectivity for the correct enantiomer is excellent, Fig. 31. Moreover, the separation capacity of the material exceeds that of conventional chiral
stationary phases by about an order of magnitude.
This beautiful kind of 2-D nanochemistry within the interlamellar spaces of layered salts of phosphoric and phosphonic acids has a promising future. These layered hosts are
amenable to systematic structural design by modulation of
both their ionic frameworks and organic constituents. Clearly the pioneering work with these materials as ion-exchangers and catalysts[631can be elegantly extended to include a
much wider range of reactivities and nanoscale device applications. This derives from their versatility, 1.e. structural tunability, chemical and thermal stability, and morphological
similarity to well-studied and useful phases such as LB films
(see Sec. 3.2).
3.7. Superlattice Reagents
Multilayer structures in which atomic, molecular, polymer
or extended network planes are stacked coherently are
known as superlattices. Periodic structures of this kind can
be composed of equal or unequal thickness layers. As mentioned earlier, the techniques for assembling such artificially
layered superlattices are of the chemical and physical variety.
The more chemical approaches to the synthesis of layered
organic and inorganic superlattices include Langmuir-Blodgett, self-assembly, electrochemical and photoelectrochemical film-forming methods. The more physical methods of
achieving this goal are usually of the vacuum deposition
type. These include molecular layer deposition (MLD),
chemical vapor deposition (CVD), molecular beam epitaxy
(MBE), organic molecular beam epitaxy (OMBE) and metal
organic chemical vapor deposition (MOCVD).
In the particular case of nanomodulated semiconductor
atomic layered materials, quantum confinement of carriers is
observed when each layer thickness is in the range of electron, hole o r exciton length scales. This leads to thickness-,
composition- and doping-dependent quantum optical, electronic or optoelectronic effects and a multitude of device
options. For nanoscale metal superlattices, interest stems
from, for example, their enhanced mechanical and magnetic
properties.
From the perspective of nanocheinistry using atomic layer
superlattice reagents (Fig. 32a), the thickness, composition
and ordering sequence of the layers can be uniquely tailored
to provide low-temperature access to metastable amorphous
intermediate and crystalline
It turns out that
there exist well-defined length scales for elemental layers that
separate the evolution of the chemistry of a “bulk” diffusion
couple from that of a “thin film” diffusion couple.[641This
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
Ultrathin-film modulated
composites
Homogeneous
amorphous alloy
Temperature (“C)
\L
Crystalline compound
$4
ibl
-
0.20
9
E
0.15
g
0.10
v
;;i 0.05
E
2
O.CQ+
i
, $$ ”
A
\ c
D
0
0
I
I
I
100
200
ux)
I
m
I
I
I
I
1
20
40
60
SO
500
600
Fig. 32. a) Reaction mechanism for ultrathin-film composities showing the
formation o f a homogeneous, amorphous alloy as the key reaction step. Differential scanning calorimetry (DSC) and XRD traces of a layered Mo/Se composite for a repeat unit of b) 74 A and c) 26 A. The points A-D indicated on the
DSC therrnograms show the temperatures at which the respective X R D diffraction patterns were obtained [64].
Temperature (“C)
70x10’
so
l0
I
Angle (213) in degrees
\
l A
B
5
20
40
60
8C
Two-Theta
involves the concept of competing time scales for diffusion
versus nucleation. The time scale for heterogeneous nucleation of a crystalline compound occurring at interfaces in a
inultilayer composite should be independent of the size of a
repeat unit. By contrast, if a sample diffuses rapidly enough
(diffusion time proportional to the square of the diffusion
distance, Fick’s law), the sample will become homogeneous
before heterogeneous nucleation can occur.r64]
This interesting nanosynthesis idea is nicely illustrated in
the Mo/Se system built of varying layer thicknesses which
crystallizes to MoSe, having a simple 2-D layered struct ~ r e . [The
~ ~ progress
]
of these chemical reactions in ultrathin
multilayer composites is most conveniently monitored by
grazing angle X-ray diffraction and differential scanning
calorimetry (DSC). These in situ methods permit one to distinguish the process of diffusion and phase nucleation in the
layers. For samples whose modulation length is 38 8, or
Adv. Mater. 1992. 4 , No. 10
(2
larger, the films evolve upon heating as bulk diffusion couples, initially giving an amorphous layer and then crystallizing MoSe, at the interface between individual elemental layers, Figure 32 b. For samples of 27 8, or smaller, the films
evolve to a homogeneous, amorphous alloy before crystallizing MoSe,, Figure 32c. This implies a critical layer thickness, below which it is possible to form a homogeneous,
amorphous alloy. It is significant that there is a difference in
nucleation temperature of several hundred degrees between
the chemistry in these two length scales, reflecting the importance of interfaces in aiding nucleation.
This research convincingly demonstrates the synthetic advantages of using diffusion distances in nanolayers to control the intermediates and products of solid-state reactions.
The use of nanodimension superlattice reactants has great
potential for directly nucleating desired binary and ternary
compounds from amorphous precursors, as well as controlling the nucleation and synthesis of single-crystal thin
films from homogeneous amorphous alloy intermediates.
4. Organic and Inorganic Open-Framework Hosts
4.1. Nanometer-Sized Semiconductor Clusters
Nanometer-dimension clusters of uniform size and shape
built up of the atomic components of bulk semiconductors
V C H VerlugsgrseIlschu/i mhH. W-6940 Weinhrrrn, 1992
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ADVANCED
MATERIALS
have evoked great interest in the solid-state chemistry and
physics communities, because of their unique electronic and
optical properties as a consequence of quantum, spatial and
dielectric confinement effects involving excitons, electrons
and holes. Size effects of this type confer upon these quantized semiconductor materials qualities that make them of
considerable interest in, for example, (photo)electrochemistry, quantum electronics and nonlinear optics. One of the
impediments, however, to the practical utilization of these
kinds of materials centers around the synthetic challenge of
fabricating organized arrays and narrow size distributions of
the clusters. In the ideal case of a perfect semiconductor
“cluster” crystal, deleterious inhomogeneous size and shape
contributions to their electronic and optical properties are
reduced to a minimum, thereby permitting optimum evaluation and exploitation of quantum confinement effects. Hostguest inclusion chemistry provides an attractive method for
achieving the goal of monodispersion for these nanoscale
semiconductor cluster materials. The selection of hosts has
encompassed organic, inorganic, polymeric and biological
materials. Here chemical protection and stabilization, together with spatial restrictions imposed by the host, allow
the synthetic nanochemist to nucleate, grow and arrest the
“crystallization” of nanometer semiconductor clusters in the
desirable quantum size regime. In the following sections we
will take a cursory look at some recent semiconductor
nanochemistry conducted in micellar, protein, zeolite and
feldspathoid hosts.
G. A . OzinlNanochemistry
clusters could also be formed by growing, for example, controlled-thickness ZnS layers upon CdSe particles, and vice
versa, in inverse micelle media. Steigerwald’s type of
nanochemistry1”‘1 is a useful way to prepare soluble, purifiable and processable nanometer-sized semiconductor and
mixed semiconductor clusters. They are considered to represent a new class of large molecules.
Ph
S‘
Ph)’\
Ph
,-d/
I
Ph
‘Ph
z
S
0
b
s
Ph
2
Fig. 33. The analogy between a) phenylate. b) phenylthiolate. and c ) zeolate
capping (stabilization) of nanoscale CdS particles.
In a similar vein, Herron and ~ o - w o r k e r s ~ ~used
~ . ~ an
’]
aqueous coordination chemistry approach involving Zn2 ,
Cd2@and PhSH reagents to grow fixed nuclearity phenylthiolate-capped TI-VI clusters, as illustrated in Figure 33.
These can be further expanded by a fascinating kind of cluster “fusion” chemistry illustrated in Equation 6.
@
4.2. Quantum-Confined, Capped,
Soluble Semiconductor Clusters in Micellar Hosts
The utilization of chemical capping procedures for producing narrow dispersions of zinc and cadmium sulfide and
selenide clusters, using phenylate (Phe) and phenylthiolate
(PhSO) ligands, as we explain later, turns out to have an
interesting analogy to zeolite cavities in their ability to encapsulate and stabilize uniform arrays of these same clusters.[6sl
Steigerwald and Brus1661have elegantly demonstrated how
the 11-VI semiconductor particles formed in the nanometersized water pools of inverse micelle media by the method of
Fendler‘25b1can be considered to be “living polymers” capable of further growth, surface functionalization, solubilization, and chemical passivation. They discovered that in the
CdSe system, for example, the surface-attached surfactant
ligand is labile and can be transiently displaced by sequential
additions of SeZe and [email protected] grow on the encapsulated CdSe seed to make a larger CdSe crystallite. By treating
a Cd2@-richsurface with PhSeSiMe,, it proved possible to
isolate phenylate-capped (CdSe)-Ph semiconductor crystallites, Figure 33. These powders were soluble in organic solvents. Variation of the organic capping group had little effect
on the optical properties of the particles but a profound
effect on their solubilities. Concentric layered semiconductor
634
+ 5SZe
2Cd2,S,,(SPh)8,? + 2S2’
2Cd,,S4(SPh):p
+ Cd,,S,,(SPh)i:e
+ 10SPh’
+Cd,,S,,(SPh)~~
+ 5 Cd” + 16SPh’
(6)
Here the clusters expand from about 7 8, to 10 A to 13 8,
in diameter. The important point in both Steigerwald’s[66]
and Herron’srZ4.671 methods is that a semiconductor cluster
growth process is size-controlled using Ph’ and PhSO capping/protecting/stabilizing/solubilizingligands.
4.3. Assembly of Semiconductor Clusters
Within Zeolite Hosts
Zeolite hosts allow the ordered inclusion of particles with
dimensions in the range 6-100 8, and having a very uniform
size distribution.r681Zeolites of different architecture permit
fine control over the spatial arrangement of and interactions
between these quantum-size particles. The zeolite framework
provides a stabilizing medium, constraining the cluster growth
to nanometer dimensions. Modification of framework charge
.r! VCH Ver/y~.sge.rclI.~~liu/t
m h l f , W-6Y40 Weinhcbn. 1992
0935-Y648/Y2/10lO-U634 B 3.50f .25/0
Adv. M u m . 1992. I , No. 10
ADVANCED
MATERIALS
G . A . OziizlNanochemistry
density and intracavity electric field gradients, by manipulation of the host composition and extraframework cations,
enables one to fine-tune the electronic and optical properties
of the encapsulated clusters (dielectric confinement effect).
Naonochemistry techniques for synthesizing zeolite encaged semiconductor clusters are summarized below, with a
selected example for each case. SOD is sodalite.
-
Aqueous ion-exchange (Y is zeolite Y): [h91
Cd2@
Na,,Y
Cd,,Y 2 (CdS),,H,,Y
Melt ion-exchange;[”I
8 Na, 2 Cl-SOD A% 8 Ag, 2 CI-SOD
Vapor phase impregnation (A is zeolite A): [”I
Na,,A 3 (PbI,),Na,,A
Intrazeolite MOCVD:
Me Cd
H,,Y 2
(MeCd),,H,Y
*H
(Cd,Se,),H,,Y
Let us illustrate the methodology with reference to the
MOCVD-type synthesis of 11-VI metal chalcogenide clusters
inside zeolite Y super cage^,[^^' the phototopotactic synthesis
of tungsten oxide supralattices in zeolite YL7’]and sodalite
supra~attice~.[~~]
4.3.1. Stepwise Synthesis of It- VI Metal Chalcogenide
Clusters Inside Zeolite Y Supercages
One of the objectives of this study was the preparation of
quantum size effect (QSE) 11-VI metal chalcogenide clusters
in a zeolite Y
In devising a scheme to achieve this
goal, there are stringent specifications that must be met in
the final product. By working in the QSE regime, one alters
the electronic architecture of the bulk material from that of
a band description to one involving discrete electronic states
in the cluster. Transition energies involving these discrete
levels may be fine-tuned by adjusting the size of the cluster,
where blue shifts in the optical spectra are observed as the
cluster size is reduced. The importance of size in tuning the
electronic properties of the cluster emphasizes the need to
control the cluster nuclearity during synthesis, in order to
minimize deleterious inhomogeneous broadening effects of
the cluster electronic states. Just as guest-host interfacial
effects can influence the cluster geometry, electronic and optical properties, variations in the cluster shape can also contribute to unwanted broadening effects of cluster electronic
levels. Another adjustable parameter is cluster composition,
the control of which provides an extra dimension in tailoring
the electronic properties of the cluster. A mild, controlled
and versatile synthetic method that can quantitatively deliver a wide range of precursors and dopants to a specific locaAdv. Mulrr. 1992, 4 , No. 10
tion in the host zeolite would be a tremendous asset in the
continuing quest for improved techniques for fabricating
uniform arrays of size-, shape- and composition-tunable
semiconductor nanoclusters. In this context Ozin et al.[”]
have recently achieved this goal by using a stepwise
MOCVD-type synthesis of 11-VI and IV-VI QSE nanoclusters in a zeolite Y host. This method yields a novel class of
intrazeolite 11-VI and IV-VI nanoclusters, inaccessible by
the traditional ion-exchange routes.[691The following is a
synopsis of this new development for the case of the 11-VI
nanoclusters.
The in situ mid-IR probe provides a quantitative and convenient means by which to monitor the sequence of intrazeolite events involving (CH,),M precursors, zeolite Y
Brmsted acid sites and H,S and H,Se reactants. For the
chosen example presented in some detail
the reaction stoichiometry and cluster nuclearity that is consistent
with the elemental analysis results, the number of reacted
and regenerated Brmsted acid sites and the EXAFS structure analysis for a H,,Na,,Y host follows the reaction
scheme of Equation 7.
H,,Na,,Y + 44(CH3),M + (CH,M),,Na, , Y + 44CH,
(CH,M),,Na,,Y + 29.84HZX-+
(7)
(M5 5X3.73)8HL5.64NalL y + 44CH4
The EXAFS (extended X-ray absorption fine structure)
analysis of the 11-VI clusters formed in the H,,Na,,Y host,
together with support from other spectroscopy, microscopy
and elemental analysis methods, was found to be most consistent with an anchored “ideal” M,X:@ cluster housed in
every a-cage (Figure 34). Bond lengths and coordination
Fig. 34. Proposed cubane structure of the cadmium sulfide cluster anchored
with two site I1 Cd’O cationr within the 1-cage of zeolilc Y.Hatched circles
represent cadmium calions. open cii-cles repfesent sulfide anions [65].
numbers determined for cadmium sulfide encapsulated in
zeolite Y are: R,,, = 2.24 A, Ncdo = 1.4, R,,, = 2.52 A,
NCds= 2.2. The cluster, coexisting protons and sodium
cations serve to compensate the charge on the zeolite Y oxide
framework. The M-0 coordination numbers indicate that
0 VCH Verlagsjicsrlkcl~uftmhH, W-6940 Weinheirn, 1952
093S-9648/52/lOlO-O63S$3.50+.ZS/O
635
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
the cluster is tethered to the oxide framework, probably at
a-cage site 11. The M or X coordination numbers are more
consistent with a M,(M,X4)40 “capped” cubane geometry
rather than an M,XZ@ adamantane-type structure for the
imbibed cluster. The absence of any obvious M and X second coordination shells in the EXAFS data is consistent with
a model of “isolated” M,(M,X4)40 clusters in every cc-cage.
This does not exclude through space or framework electronic
coupling between clusters.
As the anchoring of zinc or cadmium metal centers
occurs via (CH,)M@ species (Rietveld P X R D structure refinement[6si)and is controlled by the reaction of (CH,),M
precursor with the Brmsted acid sites, the maximum attainable loading of metal chalcogenide guests into the zeolite Y
host through the MOCVD-type route ofthe present study is
twice that which can be achieved using the traditional M z o
ion-exchange route with the same zeolite Y host.[691In addition, the MOCVD method is designed to homogeneously
locate the anchored (CH,)MQ guest and the cluster product
in the x-cage, rather than in the /I-cage as in the case for the
ion-exchange approach. The intrazeolite MOCVD method
therefore allows one to extend the accessible range of 11-VI
cluster nuclearities beyond the limit of four available to the
ion-exchange method.[6yi
11-VI bulk semiconductors adopt the cubic zinc blende or
hexagonal wurtzite structures in which both M and X are
tetrahedrally coordinated. Cubane-like 11-VI clusters are
rare, and those that do exist are usually only found with
sterically demanding ligands, exemplified by [(‘Bu)GaS],
74i In these
and [N(CH,),][Cd,(SC,H,‘Pr,),] . C,H,,
cases, further aggregation is inhibited by the surrounding
ligand sheath and the structure adapts to one that satisfies
normal coordination-number and valency requirements.
The genesis of the cubane-type M,XZ@ cluster in zeolite Y
requires the Brmsted acid sites, the anionic oxide framework, the 11-VI precursors and the size-and-shape constraints of the supercage. The anionic nature of the framework prohibits M:X ratios lower than unity. Competition of
the anionic framework and the chalcogenide anions for the
available metal cations results in the cluster stoichiometries
that have M:X ratios greater than unity. Charge balance is
provided by regenerated Brmsted acid sites and residual
extraframework sodium cations. I t is interesting that all four
systems appear to behave similarly, in that the elemental
analysis, 1R and EXAFS data point to the existence of
M,(M4X4)4e capped clusters for M = Zn, Cd; X = S, Se.
With the above as minimal background material, the relationship of the intrazeolite MOCVD work of Ozin et
to the inverse micelle work of Steigerwald[66i and cluster
fusion work of Herron[2”,h7ican be appreciated. In the
latter two cases, a TILVT metal chalcogenide cluster growth
process is size controlled using Phe and PhSe capping/protecting/stabilizing ligands, Figure 33. In the former case,
MeJn, Me,Cd, H,S, and H,Se MOCVD-type precursors
are used to produce uniform arrays of single-size-and-shape
11-VI “zeolate” capped clusters, Figure 33. The interesting
.[733
connection between phenylate, phenylthiolate and zeolate
capped 11-VI clusters now becomes clear.[24.6 5 - 6 7 1
4.3.2. Intrazeolite Phototopotaxy:
Redox Interconvertible Tungsten Oxide Supralattices
The volatile hexacarbonyl of tungsten has been used as a
precursor in the synthesis of highly organized assemblies of
molecular-dimension tungsten oxides encapsulated exclusively within the cc-cages of zeolite Yc7’]Following sublimation of the precursor into the host, it is next converted in an
0, atmosphere to the W6@oxide by photo-oxidation, and
may be subsequently thermally reduced in vacuum to yield
W 0 , - clusters (where 0 2 x 5 1) and then reversibly reoxidized by heating in 0, at 300- 400 “C. The electronic properties of these oxide clusters can be easily manipulated as a
result of their facile redox interconvertibility, and there is the
further possibility of fine-tuning their electronic environments by choosing which charge balancing cation is present.
A maximum of two hexacarbonyl precursor molecules can
be anchored in each cc-cage of the zeolite. Following photooxidation, half of the a-cage void volume in the host is freed
so that subsequent precursor impregnations/photo-oxidations can be carried out. Thus, the stepwise loading proceeds
according to the set of Equations 8.
16 W(CO),-Na,,Y
8 W(CO),, 16W0,-Na,,Y
4 W(CO),, 24 WO,-Na,,Y
1 W(CO),, 31 WO,-Na,,Y
+
4
+
~
16WO,-Na,,Y
24 W0,-Na,,Y
28 WO,-Na,,Y
(8)
32 W0,-Na,,Y
Altogether, the series of materials is denoted by the general unit cell formula n[WO,- x]-M56Y, where 0 < n 432,
0 5 x 5 1 and M = H, Li, Na, K, Rb, Cs.
Structural and electronic details of the various tungsten
oxides have been elucidated through the use of HRTEM.
gravimetry, EXAFS, FTIR (Fourier transform infrared),
29Si MAS-NMR (magic angle spinning nuclear magnetic
resonance), 27A1/23NaDOR-NMR (double rotation
NMR), XPS, UV-vis and EPR (electron paramagnetic resonance) spectroscopies. In all of these materials, the W-containing moieties are strictly confined within the internal void
space of the zeolite host, and their presence has been shown
to result in perturbation of neither the host lattice crystallinity nor its itegrity, and only very slight changes in the unit cell
For the various tungsten oxide materials, structural characterization has revealed that well-defined monomeric, dimeric and tetrameric molecular tungsten oxides n[WO,- JNa,,Y can be produced, Figure 35. In the case where x = 0,
Nao-cation-anchored W,O, dimers were observed for values of n in the range 16 to 32. In the first-stage reduction
products (where x = I/,), the structure was a Na@-cationanchored W,O, dimer for n = 16, but a Na@-cation-
ADVANCED
MATERIALS
G. A. OzinlNanochemistry
anchored W40,, tetramer when n = 32. The second-stage
reduction products, corresponding to x = 1, were all of a
monomeric, framework-oxygen-anchored structure type
over the loading range of n = 16 to 32. Measurements made
by XPS have clearly demonstrated that the oxidation states
W6@,WSO,and W4@(representing x = 0, x = '/z, and x = 1,
respectively) can be assigned to the W centers in n[WO,- ,INa,,Y In the special case of the W,O, dimer, XPS results
indicated that both tungsten centers were in the W5@/W5@
oxidation state rather than members of a mixed-valence
W4Q/W60moiety. Band assignments in the UV-visible spectra also showed agreement with this conclusion.
400T
- 'I20 2
$
(SIP)
4
and displays; pH-sensitive microelectrochemical transistors;
chemical sensors, electrochemical cells and selective hydrocarbon oxidation catalysts.
4.3.3. Sodalite Supevlattices
Sodalite is an ancient material with great potential for
advanced applications. In the context of solid-state chemistry and condensed matter physics, this Federovian framework material is considered to provide a unique opportunity
for studying metal-non-metal transitions, quantum, spatial
and dielectric confinement effects, quantum electronic and
nonlinear optical phenomena in expanded insulators, semiconductors and metals. One of the major goals of some of
our recent research has been to assemble novel nanostructures by encapsulating clusters consisting of the components
of insulators, semiconductors and metals inside the framework of aluminosilicate ~odalite.['~]
This nanoporous host
acts as a stabilizing dielectric matrix capable of organizing
single-size-and-shape clusters in perfectly periodic arrays.
Such materials, and the related zeolite analogues, have an
interesting variety of potential applications such as nanoporous molecular electronic materials, molecular wires, chemical sensors, zeolite electrodes, nonlinear optical materials
and high density data storage materials. Advanced materials
of this type have been recently reviewed.[681
In our research, the sodalite framework was used as a host
material to confine clusters composed of the components of
insulators, semiconductors and metals. Sodalites are aluminosilicates of the type M,X,(SiAIO,),, where M is a cation and
X an anion. The structure of sodalites is best described by
considering the archetype, sodium chlorosodalite (NaCISOD). Figure 36 shows a single sodalite cage. It consists of
0"C
Fig. 35. Structures of various zeolite-encapsulated tungsten oxides and their
thermal vacuum reduction products, n{W0,3- x}-Na,,Y. n =16, 32, I = 0, 0.5,
1 .O [72].
Great interest in these materials revolves around the concept of hand-gup engineering, that is, being able to fine-tune
the electronic properties of these zeolite-encapsulated oxide
semiconductor materials. One means of achieving this is to
alter local electrostatic fields through the substitution of different cations in the surrounding a-cage.['*] This capability
is particularly attractive when one considers that materials
like bulk WO,- ,are well-known oxide semiconductors having solid-state applications such as rechargeable solid-state
batteries; electrochromic devices, smart mirrors, windows
Adv. Muter. 1992, 4, No. 10
(A)
Sodalite Cage
(0)
Sodolite
Fig. 36. A) Sodalite cage showing ii single cuboctahedron. a central iiiiion, and
four cations in the six-ring sites. Each corner represents il TO, unit (T = Si or
Al). B) Sodalite framework, emphasizing the close packing of cages [70, 751.
cj VCH V e r l a ~ s ~ r s e l b c hmbH,
f ~ f t W-6Y4O Wrinheim, i Y Y 2
OY35-964S/Y2/iOlO-O637 X 3.50+.25,'0
637
ADVANCED
MATERIALS
twelve A102 and twelve SiO, tetrahedral units linked together by oxygen bridges in an alternating pattern to form a
truncated octahedron with eight single six-ring openings and
six single four-rings. Typically, the cage has a diameter of
6.6 A, and the diameters of the hcxagonal and tetragonal
ring-openings are quoted as 2.2-2.6 8, and 1.5-1.6A, respectively. Sodalite cages, or P-cages, are fundamental building units of many zeolites.
Because of the valence difference between aluminum and
silicon, the sodalite lattice possesses a negative charge equal
to the number of aluminum atoms. This charge is balanced
by exchangeable cations at C, sites near the six-rings. Another cation and an anion at the center of the cage are often
present as well. In order to maintain a charge balance, divalent anions require the presence ofdivalent cations or a complementary cage lacking an anion. Different kinds of both
cations and anions may be mixed throughout the lattice.
Thus, a vast range of guests can be incorporated during the
synthesis of the sodalites or produced by thermal, photochemical and other reaction processes. These include materials that are insulators, semiconductors, photoconductors
and metals in their bulk form. The unit cell dimensions,
charge balance requirements and cage-filling can be tuned by
incorporating the appropriate ions during the sodalite synthesis and cation exchange processes.
The sodalites must be synthesized in their sodium forms
according to the recipes summarized in Figure 37. Depending on the choice and relative amounts of X and Y one can
generate sodalite compositions described as class A, B, and
C, as depicted in Figure 34. Except for differences in the
degree of hydration, only single crystallographic phases were
found in as-synthesized class A, B, and C sodium sodalites.
Combined results from powder XRD, Rietveld refinement,
23Na DOR/MAS-NMR, Far-IR and mid-IR indicate that
the anions (or small domains of anions) are distributed
statistically throughout the sodalite lattice, forming a homogeneous solid solution.
In class A materials, all cages are filled with M,X clusters.
Class C sodalites are closely related, except that they can
contain two different anions, that is, mixed M,X and M,Y
semiconductor component clusters. Class B sodalites contain isolated M,X clusters “diluted” in cages with M, “spectator triangles” or M,OH clusters. The sodalite framework
itself is a wide band-gap insulator. Materials trapped inside
the sodalite cages were found to be isolated and to exhibit
molecular behavior at low loading levels, and also to communicate through the sodalite walls at higher concentrations, forming an expanded cluster lattice or quantum supralattice after a percolation threshold had been reached. For a
sodalite containing sodium with a very low bromide concentration, the material contains isolated ionic Na,Br3@ units
with sodium bromide distances shorter than in the bulk salt.
As the NaBr concentration is increased one finds greater
coupling between the Na4Br3@units until a certain percolation threshold is reached and a mini band structure is formed.
At this point the product obviously still does not have the
G. A . Ozin/Nanochemistry
rock-salt structure of bulk sodium bromide, but instead one
can propose the formation of an expanded insulator within
a sodalite framework. A similar scheme is found for trapped
semiconductor species such as the I-VII silver halides. By
tuning the silver loading in a series of sodium silver chloro-,
bromo- and iodosodalites it is possible to span the range
found from an isolated silver halide molecule to an expanded
supralattice of what are normally I-VTI semiconductors with
different band gaps. Silver ions in sodalites containing an
internal electron source (such as OH’, C,Oio) can be reduced within the sodalite cages to form silver atoms or
trapped silver clusters. Interaction between silver clusters
results in the formation of an expanded metal, where intercage silver-silver distances are significantly longer than in
the regular metal, but electronic interaction is still possible.
SYNTHESIS
+ S102 + H20
CLASS A
NaWOD
.
.
CLASS c
Na&-pYp-SOD
CUSS B
Nad(100HpSOD
-
AQUEOUS AgN03 SOLUTION (25 100°C)
AgNOdNaNO3 MELT (32OOC)
STOlCHlOMETRlC
Fig. 37. Synthesis of class A, B, and C sodalite supralattices [70. 751. In the
formular represents an “empty” (anion-free) sodalite cage.
Clearly, all possible device applications for sodalites require much further study. However, the unique structural
properties of the sodalite host and the variety of guests that
can be included in controlled ways certainly make this ancient
material an ideal “model system” for probing the physicochemical properties of assemblies of single-size-and-shape
encapsulated clusters built up of the components of bulk
insulators, semiconductors and metals (expanded materials),
as well as a promising candidate for advanced materials in
the 21st century.
G. A . Osin/Nanochemistry
4.4. Visible Photoluminescence and Electroluminescence
from Quantum-Confined Silicon
Silicon is the archetype semiconductor. It is the benchmark standard upon which the modern microelectronics and
computer industries are founded. Yet silicon-based technologies are constantly challenged by other semiconductors that
can compete in size, speed and efficiency of products and
processes. Gallium arsenide 111-V-type semiconductor materials represent the major opposing technology. The key
issue here is not so much the physical size of electronic components on a chip or the mobility of carriers therein, but
rather the fact that GaAs can emit light, whereas Si cannot.
This difference originates in the direct band gap of GaAs
compared to the indirect band gap of Si. Electron-hole recombination in GaAs-based semiconductors produces “useful” photons whereas Si yields only “useless” heat in the
form of photons. Moreover, compositional, size and dimensional tuning of GaAs materials (bulk and quantum confined) allows one to tailor the band properties and lightemitting characteristics over a wide range of technologically
desirable frequencies. As a result, GaAs-based semiconductors are currently the industry’s choice for many real and
perceived electronic, optoelectronic and optical device applications. Compared to Si, however, the fabrication chemistry,
physics and engineering of GaAs materials is complex, fraught
with safety hazards and costly. Breakthroughs involving new
forms of Si with direct band gap light-emitting qualities are
eagerly awaited because optical devices made of silicon
could integrate far more easily with standard silicon-based
electronic circuitry. One obvious way to achieve this most
desirable goal is to synthesize quantum-confined nanostructures of silicon that fulfil these requirements.
In this context, nanochemists have very recently learned
how to synthesize different types of nanoscale Si-based objects that have the intriguing ability to emit visible light when
optically and/or electrically excited.[761All of these new materials appear to be composed of filaments and/or clusters of
Si and the light emission is believed to originate from quantum size effects of the type discussed above. The different
strategies adopted so far to produce these nanoscale forins of
Si involve electrochemical and chemical dissolution of Si
wafers, ultrasonic dissolution of Si wafers in different solvents, and microwave and pulsed exciiner laser decomposition of gaseous ~ilanes.[’~]
The size-dependent luminescence
behavior of these materials generally conforms to that expected for quantum-confined silicon. When the characteristic dimensions of these Si nanoscale objects drops below that
of the size of the free exciton in Si (Bohr radius about 50 A),
quantum size effects are observed. In particular, UV-excited
visible luminescence, well above that of the band gap of bulk
crystalline Si in the infrared, is found to blue-shift with decreasing size of the Si nanostructure. This is consistent with
the property of a Si quantum wire and quantum dot, wherein
respectively 2-D and 3-D quantum confinement of carriers
has appreciably widened the band gap of silicon. Let us
A d v Muter. 1992, 4, No. 10
0 VCH
briefly examine the anodic dissolution of Si wafers a little
more closely, as this strategy provides a beautiful example of
a kind of nanochemical patterning for fabricating Si nanostructures small enough to display quantum confinement
effects on the band structure. A Euclidean idealistic rather
than a more realistic fractal view of the process of partial
electrochemical dissolution of a Si wafer (usually p-type) in
aqueous H F to create porous Si is shown in Figure 38. Ideal-
T
Fig. 38. Idealized Euclidean view of the changes in porosity of a bulk Si wafer
achieved by cylindrical pore enlargement through chemical or electrochemical
dissolution.
ly, exceedingly small initially formed pores of less than 20 8,
in width can be grown in size, in a form depending on the
substrate resistivity and the anodizing conditions. At low
porosities one has some array of noninteracting cylindrical
pores running perpendicular to the surface. In practice, variations are to be expected in pore size, shape and separation
with perhaps pore branching. Increasing the porosity further
eventually causes merging of adjacent pores and the creation
of arrays of isolated Si columns. The jury is actually still out
as to whether quantum wires or dots are indeed responsible
for the light emission from these Si nanostructures. Rather
than emanating from quantum size effects, the light could
possibly originate from oxygen, hydrogen and/or fluorine
surface impurities on the Si formed in the aforementioned
Very recently a fascinating alternative mterpretation was forwarded for the visible luminescence of porous
silicon which does not invoke quantum confinement in crystalline silicon, but rather involves luminescent siloxene
Si,O,H, derivatives formed on the surface of silicon during
the chemical/electrochemical etching process.[’”
Incidentally, the process of transforming a bulk semiconductor into a nanoporous one, as conceptualized for silicon
in Figure 38, is actually a rather insightful way of beginning
to think about the interrelationship between endosemiconductors and exosemiconductors of the type discussed later.
In essence, one can be thought of as a type of “inverse or
inside-out quantum superlattice” of the other. Before adjacent pores merge (lower porosityj, one has a crystalline open
framework solid built of the atomic components of a bulk
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G. A . OzinlNanochemistry
semiconductor. After they merge (higher porosity), one has
a cluster crystal built of the atomic components of a bulk
semiconductor. Therefore the endosemiconductor and
exosemiconductor can be visualized as quantum wires and/
or quantum dots with different electronic coupling strengths
(see Sec. 7).
4.5. Cloverite
The recent report detailing the structure of a synthetic
gallophosphate molecular sieve called cloverite, with a 20
T-atom cloverleaf-shaped entrance window into a 30 8, diameter supercage, has created great excitement in the chemistry
The generous dimensions of the cubic
network of 30 8, cavities of cloverite provides a giant step
above the spatial limit imposed by the diamond network of
13 A diameter supercages found in the previously largest
known pore system of zeolite Y. Cloverite thus greatly enhances the opportunities for innovative host-guest nanochemistry aimed at advanced materials applications.
As outlined in this paper, considerable effort has been expended within our laboratory to prepare a novel series of
materials called endosemiconductors, which are anticipated
to have some exciting, device oriented, optical and electronic
properties. These properties depend upon the degree of spatial, quantum and dielectric confinement conferred upon the
clusters by the host lattice. To date, the principal host employed has been zeolite Y. The extreme degree of spatial confinement imparted by the 13 8, supercages of this host has
resulted in the formation of cluster guests whose dimensions
fall below those which can be readily dealt with using the
tenets of theoretical solid-state semiconductor physics, size
quantization and the effective mass approximation. As a
result of this, most of the contemporary evaluations of the
potential of endosemiconductors as new materials in, for
example, quantum electronics and nonlinear optics point to
the small cavity size as bcing the major limiting factor. Utilization of a molecular sieve such as cloverite as a 3-D host
with a cubic network of 30 b; supercages completely transforms the situation and could therefore be extremely beneficial to research in this area.
The purpose of our early research on cloverite"*] was therefore to assess the physicochemical properties of the material
pertinent to its utilization for host-guest inclusion chemistry
aimed at advanced materials science applications. A multiprong analytical approach was employed to explore the fate
of the 30A supercage of cloverite subjected to a range of
critical post-treatment conditions.r781
The following compositional recipe has been used to synthesize cloverite under hydrothermal conditions (150 "C,
22 h) employing quinuclidine as a template and fluoride as a
mineralizer :
Ga,0,:P20, :6Q:0.75H F : 7 0 H 2 0
640
(I i.
Fig. 39. CHEM-X molecular graphics reprcscntations o f cloverite. Top) Thc
cloverite 29-30 A supercage. Bottom) A (100)projection ofthe ki1?3C cubic unit
cell of cloverite clearly depicting the "clover shaped" 20 T-atom window into
the supercavity, the 8 T-atom windows into the LTA cages and the 6 T-atom
windows into the RPA cages [78].
The cubic unit cell of cloverite as reported by Estermann
and co-workers["'] is
A CHEM-X representation['*' of a section of the unit cell
as well as a view of the supercage of cloverite is displayed in
Figure 39. It is interesting to note that cloverite can be entirely
constructed from layers of interlinked GaPO, double fourrings each containing a centrally located fluoride. One can
imagine that the cloverite unit cell assembles from a spatially
controlled condensation-polymerization of 192((Ga,P40, 2 (OH),F)'QH - ) ion-pairs. Half of these double four-rings
end up in the structure with terminal Ga(OH)OP(OH)
groups, where 96 of these serve to line the entire surface of
the central cloverite supercage (denoted an interrupted
framework). Structurally the system is best viewed as a 3-D
cubic network of two distinct non-intersecting channel systems. The smaller of these channel systems is accessed through
8 T-atom rings and is lined with 8 T-atom ring interconnected LTA and RPA cages. Entry to the large channel system
occurs via 20 T-atom rings and is lined with interlinked
VCH ~~rlug.sjii~.sells~/lufi
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G. A . Ozin,"anochemistry
cloverite
Xe gas can penetrate the larger but not the
smaller channel system of as-synthesized cloverite (variable
temperature lz9XeNMR).[781
Removal of H,O occurs in two distinct stages at 90 "C and
110 "C [thermogravimetric analysis (TGA)/DSC/MS (mass
spectroscopy)] and is reversible, with little effect on the
cloverite framework [in situ powder XRD (PXRD)].[781This
differential H,O loss probably corresponds to different binding states of extraframework H,O in the cloverite and LTA/
RPA channel systems respectively. The supercage of cloverite under these conditions can be considered to be uniquely
"bifunctional" with respect to the coexistence of Brarnsted
acid P(0H) and Brarnsted base Ga(0H) framework sites (in
situ mid-IR probe of reactions with anhydrous NH, and
HBr, respectively).[781Xe is still only able to access the
supercages of dehydrated cloverite (' 29XeNMR). Strong
hydrogen-bonding probably exists between adjacent P(0H)
and Ga(0H) groups in the supercage (mid-IR). These can be
exchanged with D, at 300°C and D,O at room temperat~re.[~~]
The next thermal event occurs around 350 "C and involves
the loss of about one half of the quinuclidine, simultaneous
with the evolution of some small organics plus H, and HF/
H,O [mid-IR, NMR, TGA/DSC/MS, dilatometry]. Hydrogen and small organic molecules are produced by the pyrolysis of some of the quinuclidine template. The HF/H,O
originates from those double four-rings that are part of the
interrupted framework in the cloverite cage, and is believed
to form by the type of chemistry laid out in Figure 40.r7*]
Fig. 40. Proposed template removal chemistry of cloverite (300-600 "C) involving the double four-rings of the interrupted gallium phosphate framework
1781.
This step has very little effect on the crystallinity of cloverite
at the unit cell level, but a little short-range disorder is induced due to the random production of dehydroxylated-dehydrofluorinated and/or partially fragmented double fourrings in the cloverite cage according to the idea shown in
Figure 40 (in situ PXRD). Even at this stage, Xe gas can still
only enter the supercages of c l o ~ e r i t e . [ ~ ~ ~
The next thermal event occurs around 450°C and also
involves the loss of mainly H, plus small organics and HF/
Adv. Mnfer. 1992, 4, No. 10
0 VCH
H,O probably from continuation of a process similar to the
one that occurs at 350 "C. This step is accompanied by very
slight deterioration in crystallinity of the sample and further
slight loss of short-range order at the unit cell level (in situ
PXRD). Around 550°C another thermal event occurs with
the evolution of large amounts of H, and some more small
organics most likely emanating from pyrolysis of the remaining strongly bound quinuclidine, probably residing in the
LTA/RPA channel system.[781
One can appreciate from the above model how the interrupted framework might be considered to be directly responsible for the creation of "dehydroxylated-dehydrofluorinated and/or partially fragmented" double four-rings, causing a
small loss of short-range order and crystallinity over the
range of 100-550 "C, which leads eventually to catastrophic
breakdown of the cloverite lattice at around 700 "C with loss
of additional H,O (in situ PXRD, TGA/DSC/MS).'781At
this stage the collapsed material is amorphous but can be
induced at 850 "C and 1000 "C to recrystallize into the dense
phase tridymite and cristobalite forms of GaPO, respectively
(in situ PXRD).[7B1Experiments are continuing, especially
those aimed at synthesizing and characterizing 30 A quantum-confined semiconductor nanoclusters located in the
supercage of cloverite.
4.6. Synthesis and Biosynthesis of Inorganic Nanophase
Materials Within Polypeptide Cages
Chemists involved in the controlled synthesis of inorganic
nanophase materials have recently realized that they have
much to learn from mother nature in the realm of biominera l i ~ a t i o n . [Here
~ ~ ] organic supramolecular assemblies, e.g.
intracellular membrane vesicles, organic polymeric matrices,
control the nucleation and growth of many familiar biogenic
inorganic materials, such as those found in bones, teeth,
shells and iron oxide storage proteins. The inorganic
nanophase in these materials often displays an impressive
degree of organization and regularity. Cellular forces can
shape these nanoparticles into patterned architectures that
transcend the supramolecular level. Ideas and concepts of
bioinineralization have recently been adapted by nanochemists, using, for example, the protein ferritin as a reaction
cage for the synthesis of nanometer-size inorganic particle~.['~]
The quaternary structure of ferritin derives from
the self-assembly of 24 polypeptide subunits arranged into a
hollow sphere of 80-90 A internal diameter. The 50 8, iron
oxide cluster that exists in the native protein cavity can be
removed by reductive dissolution to give intact empty
protein cages called apoferritin. This extraction process occurs through hydrophilic and hydrophobic channels that
penetrate the protein shell. Nanoscale particles can be reassembled inside the protein cage by introducing reagents
through the same channel system. Thus, iron sulfide cores
were easily synthesized by in situ chemical reaction of native
iron oxide cores with H,S gas, Figure 41. Discrete particles
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....
( A ) :.
I'
HzS/ Naps
(C)
(B)
Fig. 41. Schematic representation of the use of a protein, ferritin, in the synthesis of nanophase materials: A) Fe sulfide formation by in situ reaction of native
iron oxide cores. B) Mn oxide reconstitution by redox-driven reactions within
apofcrritin. C) Uranyl oxyhydroxidc deposition by ion-binding and hydrolytic
polymerization [79].
having an average size of 78 A and a Fe/S ratio close to unity
were produced in this way. In vitro reconstitution of empty
protein cages with a known amount of iron oxide [Fe(ir)
oxidation/condensation] allows one to control the particle
size of the encapsulated iron sulfide product. This approach
allowed the synthesis of 70 8, manganese oxide cores (Mn2@
precursor) and 60 A uranyl oxyhydroxide cores (UO:@ precursor).[7y1In all cases studied, the encapsulated material
was amorphous by electron diffraction.
certain yeasts, cultured in the presIn this same
ence of Cd2@,generate sulfide. Chelating peptide coatings,
with cysteinyl thiolates as capping ligands, control the nucleation and growth of peptide-capped intracellular CdS
nanocrystallites of diameter 20 1- 3 A corresponding to a
(CdS),,-cdp cluster. The SjCd ratio of these particles is
about 0.7, the Cd-S bond length is 3--6 Yoshorter than in the
bulk, and the structure of the cluster tends towards a rocksalt rather than the zinc-blende structure of the bulk. Interestingly, larger extracellular CdS particles of about 29 5 8,
are also formed, some of which show a zinc-blende structure.
These biogenic, short-peptide-capped CdS nanoparticles appear to have narrower size dispersions than synthetic ones
capped with organic groups or polymeric ligands.[801
bers contained in the domains of phase-separated diblock
copolymers. The latter are known to self-organize in films to
yield lamellar, cylindrical or spherical domains, the spacing
and size (100-2000 A) of which can be manipulated in a
predictable manner. This is achieved by adjusting the length
of each block, the total molecular weight, and by blending
with a homopolymer. Schrock and co-workers[811concept
involves the ring-opening metathesis polymerization
(ROMP) of diblock copolymers having metal complexes selectively attached to one block, with nanophase domain separation in solvent-cast films, followed by reaction of the
anchored metal coinplexes to form encapsulated semiconductor nanoclusters still localized in the parental nanoreactor. This creative form of semiconductor nanochemistry is
sketched in Figure 42. F o r instance, norbornenes were employed that could bind Sn'", Sn", Pb", Zn" and Cd" complexes in a dative fashion through N, 0, P, or S donor groups,
with complexation constants optimized for localizing the
precursor-to-product nanochemistry within the block-copolymer domains. An example of a ROMP to a block-copolymer of an oxygen ligand functionalized norbornene with
methyltetracyclodecene (MTD) is shown in Figure 42. Metal
4ZQbPh
CMe,Ph
H p S , 115°C.
12h, 1.3alm.
*
4.7. Synthesis of Semiconductor Clusters
Within the Nanodomains of Block-Copolymers
Within the context of nanochemistry, Schrock and coworkers["] have very recently discovered how to spatially
constrain the nucleation and growth of organized assemblies
of semiconductor clusters to the nanoscale reaction cham642
:O
'
CMeZPh
[5 - OI,,
MTD],
/ ( ZnS )x
Fig. 42. Synthesis strategy for producing zinc sultidc nanoclusters encapsulatblock-copolymer [XI].
ed in a (5-O)80(MTD)22(l
complexes that have been bound to this system are Ph,Zn
and [3.5-C6H,(CF,),],Cd as precursors to encapsulated ZnS
and CdS nanoclusters, respectively. A solvent-cast film of
the Ph,Zn-containing block-copolymer, before and after
reaction with H,S at 115°C for 12 h at 1.3 atm, shows that
the lamellar nanodomaiii morphology is maintained, as re-
VC'II Yi.rlrigsjie.sr/l.s~hrrflmhM. W-6Y40 Weinheim, 1992
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G. A . OzinlNanochemistry
Fig. 43. (5-O),,(MTD),,o~XOZnPh2after treatment with H,S (115'C. 12 h,
1.3 atm): a) TEM bright field image; b) sulrur X-ray fluorescence map; c) zinc
X-ray fluorescence map [XI].
der carefully chosen conditions of pH, temperature and crystallization time.[38,431 It is believed that co-added alkali
metal cations, quaternary alkylammonium cations and
amines function as structure-directing, space-filling and
charge-balancing agents, in what can be envisaged to be a
spatially constrained condensation-polymerization reaction
to the final product.[43, Interestingly, similar methods
have recently been under active development in the field of
molecular recognition, involving the template-mediated synthesis of substrate-selective polymers. Wulff and co-worke r ~ [ ' devised
~]
a novel approach to synthesizing "imprinted"
polymers that consists of covalent linking of polymerizable
groups around a template molecule and subsequent crosslinking polymerization of the resulting assembly. Just as in
zeolite synthesis, the template molecules sculpt the architecture of the rigid porous polymer product. Dhal and
Arnold[84]reported a novel variation of this template polymerization technique using polymerizable bis(i1nidazo1e)Cu" complexes (protein analogues) as templates. Their template mediated synthesis is laid out in Figure 44. Polymerization is initiated in the presence of a cross-linking agent. Removal of the bisimidazole templates with aqueous methanol
leaves the Cu" ions in the porous templated polymer. More
than 95 % of these Cu" ions can be removed using methalnoic triazacyclononane followed by aqueous EDTA. Quantitative reloading of the Cu" ions back into the polymer can be
achieved with aqueous CuCI,. Substrate recognition and selective binding of these templated copper-complexing copolymers were demonstrated by saturation rebinding and
competitive rebinding experiments with their templates and
close structural analogues.[s41These open-framework organ-
D
vealed by STEM-EDX (scanning transmission electron microscopy with energy dispersive X-ray fluorescence analysis)
with both Zn and S X-ray fluorescence mapping, Figure 43.
Lamellar versus spherical domain structure is controlled by
the fraction of M T D in the block-copolymer. This type of
nanochemistry imparts impressive control over the size,
shape and distribution of the imbibed semiconductor aggregates in the block-copolymer film. It is simple, broad in
scope, limits cluster growth to a predictable size, and yields
materials that are straightforwardly processable into
convenient forms. This type of nanochemistry is anticipated
to have an exciting future with numerous possible real world
applications.
macroporous
ternplated polymer
Fig. 44. Template-mediated synthesis or metnl-complexing organic polymer$
~841.
4.8. Template-Synthesized Nanoporous
Organic Polymer Hosts
Zeolites, all-silica and aluminophosphate molecular sieves
are inorganic open-framework materials that are usually
synthesized under hydrothermal conditions from aluminosilicate, silicate and aluminophosphate gels, respectively, unAdis.
M u m . 1992, 4, No. 10
ic polymer hosts appear to be functioning like zeolites. Further work is needed to decide whether orientation of the
anchoring sites or shape-selectivity govern the binding selectivities of these new materials. They should have a promising
future as hosts for nanophase materials.
VCH Veri[rgsgrsellJ_~~lufi
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G. A . Ozin/Nanochemistry
5. Nanochemistry with Fullerenes: A Bright Future
Isolated hollow spherical, ellipsoidal and cylindrical shells
of interconnected, conjugated hexagonal and pentagonal
rings of naked carbon atoms found respectively in buckminsterfullerene C,, , fullerenes and buckytubes (mentioned earher), have a natural and exciting appeal to nanochemists
intrigued by the idea of encapsulating and decorating guests
in their tiny void spaces (endohedral class) and on their outside surfaces (exohedral class), respectively. Most of the
chemistry so far involves C,, . The endohedral class to date
is represented by metal atoms in pairs (La, Y) or singly.[s51
However, the range of exohedral materials is much more
extensive and includes single and multiple attachment of
hydrogen, fluorine, chlorine, bromine, methyl, trimethylsilyl, methoxy, phenyl and benzyl groups as well as osmium,
platinum, silver, iridium, chromium, iron and nickel complexes.[26- 2 9 . 3 1 - 33.861 Ch emistry has been discovered that
can expand the C,, icosahedral shell to C,, by one carbon
atom at a time.[871I t also appears that C,, molecules can be
strung together, like pearls on a necklace, through buckyball
polymerization reactions.'881In the solid state, C,, spheres
pack neatly together into beautiful close packed lattices (Figure 45) in which the orientation of neighboring C,, mole-
C
t
+b
Fig. 45. Illustration ofthe crystal structure ofC,,,. The ABC packing ofthe fcc
unit cell IS shown in thc top vicw. where the central iiiolecule b can be seen to
be surrounded by its 12 neighbors. The bottom view shows 21 projection of the
fcc unit cell of C , , , along Lhe [I001 axis 1891.
cules is arranged to maximize intermolecular interactions
between surface carbon atoms in adjacent
This lattice architecture of C,, icosahedra is reminiscent of,
but distinct in detail to the kind of open framework structures found in zeolites and molecular sieves!go1 (Amazingly,
C,, has actually been introduced inside activated zeolite X
where it was shown by EPR to be trapped at C:0.'911) In
essence, C,, crystallizes so as to create a lattice bearing endohedral and exohedral nanometer-size void spaces, in which
one can conduct host-guest inclusion chemistry. Some nice
examples of this have already been documented and have
important implications in the areas of superconductivity,
ferromagnetism and nonlinear optics. In this context, interand intramolecular interactions between carbon 2pz functions in solid C,, can be considered in terms of Block wavefbnctions and electronic bands. Delocalization of electronic
charge within and between the extended orbitals of C,,
icosahedra is of central importance in understanding many
of the physical properties of C,, in the condensed phase.
Examples of this include the alkali-metal-type, loading- and
temperature-dependent transformations of M,C,, (0 I
x5
6) between the insulating, semiconducting, metallic and superconducting
921 The delocalization of n-electronic charge over the entire network of coupled C,, molecules
is vital to the understanding of the optical nonlinearities of
thin films of C,, in its pure, functionalized and doped
The chemical, electrochemical and photochemical
reduction (n-doping) of C,, and other f ~ l l e r e n e s [prompt~~l
ed an exploration of the use of strong organic reducing
agents. Unexpectedly the addition of tetrakis(dimethy1amino)-ethylene (TDAE) to C,, afforded a metallic organic
molecular ferromagnet C,,TDAE,.,, with a Curie temperature of T, = 16.1 K.IY4]This is a higher temperature than any
reported for other molecular ferromagnets strictly based on
first row elements of the periodic table. The temperature
dependence of the magnetization below T, is consistent with
that expected for a very soft three-dimensional ferromagnet
with a remanence magnetization of zero.
That C,, forms a zoo of fascinating endohedral and exohedral compounds, shows amazing reversible multielectron
redox character involving up to six electrons. can have its
shell expanded, contracted and built into polymers, can allow its surface to be multiply decorated with metal-ligand
complexes, can act as a veritable sponge for free radicals, and
is capable of exhibiting nonlinear optical, metallic, superconductivity and ferromagnetic behavior are very strong arguments in favor of ensuring a fascinating nanochemistry future for buckyballs, fullerenes and buckytubes.
6. Nanoelectrochemistry
As mentioned in Sec. 1, the scanning tunneling microscope (STM) is gaining in importance for modifying and
imaging nanometer-scale regions of conductive surfaces,
the ultimate goal being the fabrication of nanoscale
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MATERIALS
G. A . OzinlNanochemistry
devicesL2 l o ] An exquisite recent development that makes
use of this nanoengineering technology concerns the assembly and operation of a nanometer-scale electrocheinical
cell.[9s1The cell, having a largest total dimension of 700 A,
was constructed on a highly oriented pyrolytic graphite crystal by electrochemically depositing (STM voltage pulse), in a
sequential fashion, Ag (0.5 mM AgF) and Cu (0.5 m M
CuSO,) pillars (radius, R 150-200 A; height, H 2050 A) closely positioned with respect to each other, Figure46
-
-
A,B. When immersed in a CuSO, plating solution, the copper pillars spontaneously decrease and the silver pillars concurrently increase in size, Figure 46C.
The thermodynamic driving force for thesc changes appears to originate from work function differences between
the Cu and Ag nanostructures. Two possible origins of the
effect are under-potential and contact-potential deposition
phenomena, the former being illustrated in Figure 46D. The
direction and magnitude of the observed changes in the
nanocell have been quantified to yield an overall current
efficiency of about 75 YO,which corresponds to the transfer
of about 70000-80000 Cu atoms (volume changes of pillars). This translates into the total passage of about 12fC
equivalents of charge, with about 4-5 monolayers of Cu
deposited on the Ag pillars (smooth, hemispherical approximation). This in turn corresponds to an average current
passed, over the 46min period of operation of the nanometer-scale galvanic cell, of approximately 2--5 aA (aA =
l o - ” A). Overall, the behavior of the nanocell mimics that
of the analogous macroscopic cell.
Nanoelectrochemistry of the type described above opens
new vistas for atomic level probing of electrochemical reactions at surfaces and the corrosion of metal surfaces.
Nanoscale electrochemical devices are also a possibility for
the future.
7. Future Directions in Molecular-Sieve
and Zeolite Science
n
Fig. 46. STM images of a copper-silver nanometer-scale cell on graphite (A)
immediately following pulsed electrochemical deposition of two silver structures from aqueous 0.5 mM AgF (B) after exchange of 0.5 mM CuSO, for
0.5 mM AgF and pulsed electrochemical deposition of two additional copper
structures ( C )following 46 min of spontaneous reaction in 0.5 mM CuSO,. (D)
Schematic diagram illustrating thc discharge of the cell by the proposed underpotential discharge mechanism [95].
Adv. Muter. 1992, 4, N o . 10
(0VC‘H
In this section some entirely speculative suggestions and
ideas are briefly presented for a potpourri of potential
nanoscale devices based on molecular-sieve and zeolite materials. They are formulated on sound solid-state chemistry
and physics principles and are intended only to intellectually
challenge and hopefully stimulate research activity amongst
nanoscientists interested in investigating and expanding
upon these possibilities.
As mentioned earlier in this paper, zeolite and molecularsieve hosts allow the ordered inclusion of semiconductor
nanoclusters with dimensions in the range 6-100 A and having a very uniform size d i s t r i b u t i ~ n . [ ~ ~ .Weakly
~”~~’~~~]
coupled quantized objects built of the atomic components of
bulk semiconductors and housed inside a nanoporous “oxide” host lattice can be considered to be a quantum supralattice, the electronic and optical properties of which can be
considered in terms of a miniband diagram of the type illustrated in Figure 47.[65‘1 With the recent discovery of template-based hydrothermal synthetic routes to “non-oxide”
zeotype framework materials,[9b]it is now possible to begin
thinking about the construction of crystalline nanoporous
solids built up from the atomic components of bulk semiconductors. Such materials can be considered to be nanoporous
quantum superlattices composed of more strongly coupled
quantized objects. Their electronic and optical properties
can be envisaged in terms of the type of miniband diagram
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/--
/
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MINIBAND DIAGRAM
I
ENDOSEMICONDUCTOR
-
MINIBAND DIAGRAM
EXOSEMICONDUCTOR
sketched in Figure 47. These two exciting classes of solidstate materials can be thought of as “reconstituted” bulk
semiconductors, the electronic and optical properties of
which have yet to be defined in detail. The first class of
material can be viewed as a semiconductor cluster crystal or
an expanded semiconductor, which we refer to as an endosemiconductor. The second class of material can be regarded as a crystalline nanoporous semiconductor, referred
to as an exosemiconductor. All of these ideas are illustrated
in Figure 47.
Now that we have at our disposal wide band-gap insulating zeolites and molecular sieves, as well as endosemiconductors and exosemiconductors, let us proceed to fanciful speculation as to how they might be used in the future. In this
context, twelve nanoscale device ideas based on the aforementioned materials with some amusing yet appropriate
names are illustrated in Figure 48.
First, the insulating, nanoporous characteristics of zeolites
and molecular sieves could be put to effective use as the
dielectric spacer layer in a field effect transistor having a
porous metal gate electrode. The gate threshold potential to
switch the device on will probably depend inversely on the
capacitance of the zeolite thin film with and without the
adsorbate. Such a device could be designed to be molecule
discriminating and could form the basis of a chemoselective
sensor, which we call a ZEOFET (Fig. 48).
Endosemiconductors are envisioned to combine the properties of quantum, spatial and dielectric confinement of bulk
semiconductors. The physics literature teaches one to anticipate enhanced quantum electronic and excitonic optical
nonlinearities for endosemiconductors whose guest clusters
have dimensions comparable to electron, hole and exciton
length scales of the bulk semiconductor and dimensionalities
of zero and one, known as quantum dots and wires respectively. Therefore, endosemiconductor crystals and single
646
Fig. 47. Illustration of the “reconstitution”
of a bulk seiniconductor into an endosemiconductor and an exosemiconductor [65c].
crystal thin films could possibly be designed and fabricated
to function as resonance tunneling multiple dot or wire transistors (ZEORESTUN), multiple quantum dot or wire laser
arrays (ZEOLASER) and nonlinear optical switches, called
transphasors (ZEOPHASOR). These concepts are also
sketched in Figure 48.
Molecule discriminating transistor behavior (ZEOTRANS)
and light emitting diode/laser action (ZEOLED) can be imagined for pnp- and np-junction devices where one of the components is an exosemiconductor. This makes the development of chemoselective devices a possible scenario. Here one
attempts to design the system to display chemoselective electronic switching or chemoselective electroluminescence, possibly originating from changes in the mini-valence-band/
mini-conduction-band structure, induced by specific molecule
adsorption into the channel or cavity spaces of the doped
exosemiconductor junction (Fig. 48).
A real-world zeolite-based device idea involves the elegant
ZEODOUBLER of Car0 and co-workers.[201Here polar
alignment of anisotropic hyperpolarizable PNA guest molecules running down the channels of large single crystals of
ALPO-5 and ZSM-5 serves to efficiently double the frequency of incident ps-Nd3@:YAG laser light, Figure 48.
Another example of a zeolite-based device idea recently
brought to realization is the inventive ZEOSAW of Bein and
co-worker~,[~’~
Figure 48. The attachment of a chemically
modified, zeolite-silica composite thin film to the silica part
of a surface acoustic wave device (or zeolite crystals anchored
to the gold electrode of a quartz crystal microbalance via a
self-assembled monolayer of a thiol-alkoxysilane coupling
agent) allows the selective detection of molecules at the
nanogram level .c9 ’1
As described earlier, exosemiconductors are envisaged to
behave as nanoporous quantum superlattices. If this turns
out to be the case, they could possibly exhibit large third-
< VCH Vrrlqrgc >ells<hafrmhH W-6940 Wernherm fYY2
0935-9648/92/iOlO-O646S 3 SO+ 2510
Ad\ Marer 1992, 4 No 10
ADVANCED
MATERIALS
G. A . OzinlNanochemistry
SODAWR ITE
~~
1
7
ZEOWIRE
ZEOFET
GATE
f
SODALITE ERASABLE
OPTICAL DATA
STORAGE MEDIA
n-CHANNEL CHEMO
-SELECTIVE F I E L D
EFFECT TRANSISTOF
ISOLATED
INTRAZEOLITE
POLYMER CHAINS
Z EO RESTUN
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M O D RESONANT
T U N N E L I NG
TRANSISTOR
order excitonic nonlinearities. In this event, one can imagine
devices based on the concept of the ZEOCHI-3, in which the
intensity of transmitted light of an incident laser could be
molecule tunable, and therefore they could form the basis of
a chemoselective nonlinear optical switch, Figure 48.
The idea of the "zeolite nose" was actually first documented by Ozin and co-workers through the ZEOLIGHT, a
proof-of-concept chemoselective luminescence intensity and
lifetime sensor, illustrated in Figure 48. This concept is based
on the selective adsorption of guest molecules in luminescent
Eu3@zeolites and molecular sieves.r98JMolecule-discriminating intensity and lifetime quenching of the Eu3@emission
formed the basis of the transducer action in this system.
The scheme of storing information at extremely high densities in a crystalline nanoporous material was first illustrated in the SODAWRITE concept of Ozin et al.[99"1Figure 48.
In this work, the use of silver-containing sodalites as novel
media for reversible optical data storage was proposed. A
system containing oxalate or hydroxide as an intracavity
reducing agent for Ag@cations could be reversibly marked
with a laser beam over many cycles. The proposed mechanism for reversible color changes involves electron transfer
between two types of tetrahedral silver clusters occluded in
the sodalite framework.[99]
A first step towards the experimental realization of the
idea of molecular wire interconnects for possible use i n future nanometer-dimension electronic circuitry can be appreciated from the ZEOWIRE studies of Enzel and be it^,[^^]
illustrated in Figure 48.
r-
Z EOSAW
ZEOCH 1-3
8. A Nanoscale Future
ZEOLITE
SINGLE
WLECULE SENSITIVE
ZEOLITE MASS SENSOl
ZEOLASER
ZEOPHASOR
I
I
It
ir-
MCCECULE TUNABLE
NLO SEMlCDNDUCTOl
ZEoL'GHT
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M Q D LASER
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TRANSPHASOR
CHEMOSELECTIVE
LUMINESCENCE INTENSIT
We are entering an era of solid-state chemistry and physics
in which there will be increasing demands for structured
nanophase materials with stringent requirements of size,
shape and dimensionality, as well as the type and concentration of dopants, defects and impurities. In such a world of
tiny objects, processes and devices, the undisputed production workhorse of the nanophysicist over the next decade or
so will be sophisticated forms of planar deposition and lateral engineering techniques. The practical limit of these methods appears to be around 100 A. Beyond this size, the nanotips of scanning probe microscopes will continue to be developed towards achieving the ultimate in miniaturization,
namely atomic and molecular scale devices. This promising
technology could show some practical utility in the 21st century provided that the huge challenge of rapidly and reproducibly moving matter at the atomic level can be surmounted. Meanwhile the elegant patterning and templating
methods of the chemist for producing spatially controlled
nanophase materials are likely to receive increasing attention
in the exciting nanoscale world of the future.
Fig. 48. Potpourri of speculative nanoscale device ideas based on zeolites and
molecular sieves.
Adv Mater. 1992, 4. No. 10
6
VCH Verfugsgesellschaft mbH, W-6940 Wemhecm, 1992
Received: March 9, 1992
Final version: August 12, 1992
093s-964~192jlOf0-0647$3.500+ 2510
647
ADVANCED
MATERIALS
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VerlugsgeseIl~chuJt
mhH, W-6940 Weinherm, 1992
0935-9648/92/10l0-0648 $3.50+ .25/0
Ad". Muter 1992, 4, No. 10
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Correction: In the September issue (p. 564) one of the author's names for the Communication shown below was
printed incorrectly. The names should read as shown here:
Synthesis and Characterization of New Annelated
Terheterocycles**
By Peter Bauerle,* Giinther Gotz, Peter Emele,
and Helmut Port
Verlugsgesellschaft mhH. W-6940 Weinheim, 1992
0935-9648~92~1010-0649
$3.50+ ,2510
649