Indian Journal of Pure & Applied Physics
Vol. 51, March 2013, pp. 149-155
Review Article
Three strategies for fabrication of I-VII semiconductor nano(particles)structures
C S Sunandana & D Rajesh
School of Physics, University of Hyderabad, Hyderabad 500 046, India
E-mail:[email protected]
Received 12 June 2012; revised 3 December 2012; accepted 16 January 2013
Three methods of I-VII nanoparticle/nanostructure synthesis/fabrication as applied to AgI and AgI-CuI solid
solutions (1) hot solution controlled precipitation, (2) mechanochemical reaction for metastable nanopowders such as
Ȗ-AgI and and (3) controlled iodization of Ag, Ag-Cu and Cu nanostructured thin films using a specially fabricated
hourglass iodinator jig, are briefly outlined. They have applications in sensor and optoelectronic technologies. Focused
though on I-VII semiconductors, they are equally applicable to chalcogenides and chalcohalides of transition and
post-transition metals. These materials are used to investigate such exotic physics as stress-induced confinement of excitons
in thin films and plasmon-exciton ‘transitions’. The efficacy of brief iodization for a modification of nanostructured
Ag surfaces for photonic applications has recently been demonstrated. III-V and II-VI–type nanostructures could as well be
fabricated and studied for I-VII semiconductors and their chalcogenide combinations I-VII-VI-an area that beckons the
explorer and the exploiter.
Keywords: Nanoparticles, Semiconductor, Fabrication, Sensor, Optoelectronics
1 Introduction
Nanoscale synthesis of materials-in fluid media or
unconsolidated dry powder or as low-dimensional
thin film structures on substrates or in tubes (such as
carbon nanotubes) are all endeavors in metastable
materials science based on one single problem of third
order (or higher) complexity namely precipitation and
crystallization-involving shape, size and grain density
as parameters to be ascertained and controlled. Thus,
it is a contemporary challenge whose success can
allow the nano-industry to move on. By way of
conceptualization and a working mandate, it is vital to
briefly consider tetrahedrality-a unique concept in
nanoscience and nanotechnology. The tetrahedron is
one of the perfect geometric shapes naturally
occurring in the macroscopic world of matter.
Tetrahedrality of the chemical bond arising from the
microscopic reality of the carbon and carbon-like
atoms is at once a dynamic attribute symbolizing
material evolution itself-nanomaterial evolution
through a specific 3D growth process in the current
context. Tetrahedrality assures 3D connectivity in
general but flexible tetrahedrality promises much
more in the fascinating world of dimensions lower
than three. Bond tetrahedrality and hybridization put
together enable us to understand, explore and predict
many things in the realm of non-equilibrium world of
nanomaterials, such as phase stability, phase
transitions and nanocrystallization. Quantum reality
and thermodynamic necessity together make
tetrahedrality a geometrical design template cutting
across the organic and inorganic worlds of rational
(nano)materials design and development. A dramatic
manifestation of the flexible tetrahedral bond
visualized as a ‘glassy-chemical bond’ occurs in the
sol-gel synthesis of a silicate glass based on tetraethyl orthosilicate as discussed earlier1. The Ag/Cu-I
bond is an unusual tetrahedral 70% ionic 30%
covalent bond and the covalency arising from metal
d-halogen p-hybridization that makes the I-VII’s AgI
and CuCl, CuBr and CuI superionic conductors as
well as large gap (~2.8 eV) semiconductors-a kind of
Janus-faced compounds.
It is not easy to obtain an array of say uniformlysized metal nanoparticles such as silver on a
'workhorse' kind of substrate such as Si as
demonstrated by a recent study2. Even trivial
precipitation reaction in water medium such as given
in Eqs (1) and (2):
AgNO3 + KIĺAgI(ppt) + KNO3
…(1)
2CuSO4 + 4 KI ĺ 2K2SO4 + 2 CuI + I2
…(2)
does not yield monodisperse AgI nanoparticles unless
nontrivial
strategies
such
as
specific
precursors/media3 and " boiling solution of KI added
dropwise" are employed4. Chemical doping,
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INDIAN J PURE & APPL PHYS, VOL 51, MARCH 2013
mechanochemical synthesis, and novel techniques
such as oxidative surface modification could be
employed to create applicable nanoparticles and
structures.
AgI4 and CuI4 tetrahedra (and also their chalcogen
counterparts) (Fig. 1) with marginal (~30%)
covalency are perhaps the only two unique building
blocks that remind you of benzene in organic
chemistry. Deep physics-not fully explored and much
nanotechnology not yet fully exploited could be based
on these inorganic counterparts of benzene! Water
insolubility, mechanical softness and the nascent
nature of the oxidizable (iodization and sulphidation
to be specific) noble metal surface are the key
characteristics that enable nano-scale synthesis. This
paper focuses on three case studies. We note in
passing that although we are dealing with I-VII
semiconductors in present paper. These or allied
concepts are at work in the synthesis of say CdS or
CuSbS2 which are of course involved in solar energy
conversion or Cu2Se which is the potential new
thermoelectric material.
2 Precipitation Synthesis of I-VII Nanoparticles
and Solid Solutions
The basis of this synthesis is the extreme water
insolubility of silver iodide: the saturated solution at
298 K is 2.4 × 1O−7 normal or 0.056 mg/L). Its
solubility in ammonia is much less than that of the
chloride or the bromide; the tendency to complex
formation is not strong enough to overcome the effect
of the extremely small solubility product. However,
silver iodide is moderately soluble in sodium
thiosulphate and in concentrated hydrochloric acid.
Copper iodide does marginally better: 22×10−7
normal5 or 0.42 mg/L.
Fig. 1 — Solid-state structure and the tetrahedral bonding with 3D
connectivity that relate metal-halide and metal-chalcogenide
materials such as AgI, CuI, Ag2S and Cu2S. The size and charge
of the respective ions, the d-electron count of the cations, and the
type of A−X, A−A, and X−X bonding interactions are involved in
nanosynthesis [after J D Martin et al.,Chem Mater, 10 (1998)
2699.]
Solid solutions of AgI and CuI with zincblende or
sphalerite structure may be prepared so as to change
the property of the material in a single component.
This is possible because of the extended mutual solid
solubility range of AgI and CuI despite their unequal
ionic radii (rAg=1.26 Å; rCu=0.96 Å; rAg/rCu=1.31).
This large size mismatch in mobile cation size points
to the possibility of a mixed mobile ion (Ag+ and Cu+)
effect controlled ionic conductivity in the AgI-CuI
system. Furthermore, the fact that CuI is an electronic
conductor in the temperature range of investigation
brings into play factors such as p–d hybridization to
the conduction process-essentially including subtle
changes in ion mobility and concentration. Non-trivial
modification of the cation sublattice in the
zincblende/bcc phases and the possibility of realizing
interfacial polytypes are expected to decrease the
defect concentration ‘n’ and hence the conductivity
‘ı’ of the high temperature Į-phase thereby increasing
the ‘Tc’ of AgI. Thus, the nanoparticle synthesis is
connected with fundamental issues such as metalhalogen bonding, defects, polymorphism and
applicable physical properties such as ionic
conductivity.
The solubility difference between CuI and AgI has
been utilized in the synthesis of solid solutions using
the simple conversion process in an aqueous
suspension to synthesize Ag1−xCuxI (x=0.05, 0.1, 0.15,
0.2 and 0.25) compositions. These samples were
prepared by the gradual addition of the required molar
aqueous solutions of the nitrates of silver and copper
(Analar grade, LOBA, India) under continuous
stirring and precipitating them with a 2 to 5% excess
solution of potassium iodide (LOBA, India) just
below the normal boiling temperature. After several
decantations with double distilled water to remove the
excess nitrates present if any, the samples were
dehydrated in an oven at 60 °C for several hours.
Fig. 2 shows typical X-ray powder diffractograms
(Cu-KĮ radiation) evidencing formation of a
zincblende phase of the Ag-rich Ag1−xCuxI solid
solutions (x=0.05, 0.10, 0.15, 0.20 and 0.25).
Dilatometry and high-temperature X-ray diffraction
experiments on these samples have enabled a
successful search for a zero-thermal expansion
material in this system6 and a detailed phase diagram
study7, respectively.
Liu et al8. have recently described a simple and
efficient method for preparing homogeneous nanoAg1−xCuxI (x = 0-0.5) solid solutions with wurtzite
structure. The single-phase formation is attributed to
SUNANDANA & RAJESH: FABRICATION OF I-VII SEMICONDUCTOR NANO(PARTICLES)STRUCTURES
151
chemical reactions such as potassium nitrate and citric
acid. Wet powders were obtained which were
dehydrated in oven at 80°C to obtain powders ready
to use for measurement or application.
Fig. 2 — Typical X-ray powder diffraction patterns for the Agrich Ag1−xCuxI solid solutions (x=0.05, 0.10, 0.15, 0.20 and 0.25).
The presence of the major sphalerite Ȗ-AgI structure is evidenced
by the dominant (111), (220) and (311) Bragg peaks. The ‘circle’
marked weak intensity peaks which correspond to the random
distribution of ‘impurity’ wurtzite ȕ-AgI phase arising from the
stacking disorder in the sample4
the active role of citric acid participating in the
chemical reactions leading to the controlled synthesis
of these compositions. It is interesting and worthwhile
to examine systematically the specific role of citric
acid which also happens to be an important fuel in
combustion synthesis of nanomaterials.
Silver nitrate (AgNO3) and copper nitrate
(Cu(NO3)2) and potassium iodide (KI) and citric acid
(C6H8O7) of analytical reagent grade were dissolved
in deionized water by agitation, respectively.
Corresponding to series of the final nominal
composition of the Ag1−xCuxI (x = 0-0.50) solid
solutions powders, the aqueous solutions with
different contents of AgNO3 and Cu(NO3)2 were
mixed together to obtain precursor solutions with the
total metal concentration of 0.1 M. C6H8O7 as the
chelating agent was added to the precursor solutions
according to cation concentration. Under the
condition of continuous stirring and its normal boiling
temperature, 2–5% excess aqueous solutions of KI
with the concentration of 0.1M were added drop wise
to the solutions with different contents of AgNO3 and
Cu(NO3)2 and C6H8O7. The desired solid solutions
powders were obtained. Then, the powders in the
aqueous solution were filtered and washed repeatedly
with de-ionized water to remove the byproducts of the
3 Mechanochemical Synthesis
Mechanochemistry — An essentially ‘dry’ processing
route-exploits the effects of non-hydrostatic
mechanical stress and plastic strain on the chemical
processes inducing (a) change of the energy and
entropy (b) change of the structure and chemical
composition of molecules, crystals and other
aggregates of matter.
Mechanochemical effects directly stem from the
directional character of mechanical stresses. Purely
mechanical forces could selectively break and re-form
covalent bonds in individual molecules, thus
promoting or depressing the chemical reactivity of
covalent systems. Although in many of the solids,
mechanical forces cannot be selectively applied to
individual chemical species located at the sites of the
crystalline lattice, localized mechanical stresses can
still affect a relatively large volume due to the
interaction forces operating across the lattice.
Therefore, atomic, ionic, or molecular solids could all
exhibit a cooperative response to mechanical
deformation, mediated by the nucleation and
migration of point, line, and planar lattice defects9.
Due to the different intensity of interactions, the
elementary units forming the crystal, atomic, ionic,
and molecular solids must be expected in general to
exhibit a different response to mechanical
deformation. For example, the absence of direct
chemical bonds between individual molecules, as well
as the weakness of intermolecular forces, permit the
plastic deformation of molecular crystals at low
mechanical stresses. Molecules can be displaced from
their original position and the crystalline lattice has
been considerably disordered, without any noticeable
chemical behaviour due to the activation of
intramolecular bonds. Conversely, the chemical
behaviour of metallic phases is intimately connected
with their plastic deformation. In the regions of solid
in which mechanical stresses are more intense,
unusual mass transport processes take place, which
induce in turn the intimate mixing of atomic species.
Coordination shells are severely affected, finally
resulting in new crystalline or polymorphic or
amorphous chemical systems.
Ionic crystals can exhibit in principle a broader
spectrum of behaviour. Cations and anions strongly
interact with each other through very intense
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INDIAN J PURE & APPL PHYS, VOL 51, MARCH 2013
electrostatic forces, resulting in high cohesive
energies. At the same time, the intensity of
electrostatic forces also results in the brittle character
of ionic crystals. Even relatively small deformations
can produce a brittle fracture as a consequence of the
intense repulsive forces arising when ions of the same
charge face with each other. On this basis, it can be
expected that ionic crystals could be significantly
reduced in size during the mechanical processing,
which has been experimentally observed-this is the
basis for mechanochemical reactive synthesis of nano
superionic conductors10.
Although aforementioned behaviour is common to
all of the ionic crystals, more variable responses to
deformation can be obtained when either cations, or
anions, or both, exhibit relatively complicated
structures, strongly suggesting the extension of the
mechanochemical reactive synthesis to transition
metal and post-transition metal chalcogenides
[for Cu2−xS see Ref. (12)] and chalco-halides. The
intense electrostatic forces arising from deformation
processes can significantly destabilize the molecular
ions. Deformation can give rise to intense
intramolecular stresses originating from electrostatic
repulsion and attraction which can potentially activate
the rupture of covalent chemical bonds and then
induce a strongly localized chemical reaction. This
mechanism could also produce metastable paramagnetic
centers such as Ag-based hole and electron centres
probed by EPR in mechanochemically synthesized
zincblende AgI nanoparticles13.
A remarkable example of mechanochemical
synthesis is the realization at ambient temperature of
the complete set of solid solutions in the system9 AgICuI. Not only does this method yield a ready-to use
applicable nanopowder to applications such as
electrochemical sensors and solid electrolytes, it has
provided an opportunity to investigate phase diagram,
phase stability and phase transitions10-12. Significantly,
it has illustrated how Cu as acted as an agent to
reduce the particle size of the solid solutions
[Fig. 3(a-c)].
Compositions corresponding to AgI, Ag-rich
(Ag1−xCuxI where x =0.05; i.e. 5%, 0.10, 0.15, 0.25),
intermediate Ag1-xCuxI (x= 0.50); Cu-rich (Cu1-yAgyI,
y = 0.05; 0.10, 0.15, 0.25) portion of the AgI–CuI
system and CuI were synthesized by mechanical
grinding in a 6-inch diameter agate mortar and pestle
for 5 h at room temperature in an unilluminated room
by using appropriate quantities (in wt%) of copper
(LOBA, India), silver (Special materials project,
Fig. 3(a) — Gradual increase of Bragg angle (2θ) accompanied by
a broadening of the (111) hkl plane of the zincblende structure of
AgI-CuI solid solution stabilized by ambient temperature
mechanochemistry upon progressive Cu addition to Ag. (a) AgI;
(b) Ag0.95Cu0.05I; (c) Ag0.90Cu0.10I; (d) Ag0.85Cu0.15I; (e)
Ag0.75Cu0.25I; (f) Ag0.50Cu0.50I; (g) Cu0.75Ag0.25I; (h) Cu0.85Ag0.15I;
(i) Cu0.90Ag0.10I; (j) Cu0.95Ag0.05I; (k) CuI; and (l) commercial
powder of CuI. Note also the effect of Cu addition on the average
size of the nanoparticle reflected through FWHM a very sensitive
indicator indeed10.
Fig. 3(b) — Plot of lattice parameter (a) versus Cu composition in
the AgI–CuI system of solid solutions showing smooth linear
decrease of ‘a’ from AgI to CuI obeying Vegard’s law10
Hyderabad, India) and I (Rasayana Laboratory, India).
When attrition was carried out, the charge (Ag or Cu
or I or Ag + Cu or Cu + Ag or Ag + Cu + I or Cu +
Ag + I) present at the bottom of the mortar is sheared
SUNANDANA & RAJESH: FABRICATION OF I-VII SEMICONDUCTOR NANO(PARTICLES)STRUCTURES
Fig. 3(c) — Crystallite size changes in AgI–CuI solid solutions
made by mechanochemical reaction. Smallest crystallite size
occurs for Cu0.75Ag0.25I but unagglomerated crystallites10 of 17 nm
are seen in Ag0.50Cu0.50I.
by the pestle at the start of the spiral type motion
executed by it. This motion causes the charges to
spread over the inner surface of the mortar, say 1/3 of
the inner surface area (294.5 cm3). In about a minute
of attrition, the charge undergoes ‘reactive mixing’ as
the pestle circles about 60 times in the mortar—a
motion that is fairly uniform and periodic. The
periodic motion of the pestle causes Ag/Cu grains to
get compressed and the compressed surface is
exposed to iodine molecules on the breaking-up and
sublimating iodine flakes. This is but a simple-minded
description of a process that deserves not only a
detailed fundamental investigation but also a
computer simulation such as molecular dynamics.
Note that the exceptional stability of the iodine
molecule and the easy vibrational excitability and the
fact that room temperature is already higher than the
triple point if iodine all combine to produce
nanoparticles of these relatively soft solids AgI and
CuI and their solid solutions possessing zincblende
structure.
4 Partial/Complete Iodization of Ag and Ag-Cu
Nanostructured Films
Silver nanostructured films fabricated by thermal
evaporation or RF sputtering among a host of other
techniques are perhaps the most investigated
structures among metal nanostructures. It was realized
during our earliest investigations14that these Ag (and
153
Fig.4 — Hourglass-type Iodinator. A simple glass-and stainless
steel jig used for iodinating metal thin films to generate
nanostructures such as AgI both for physical investigations and
applications. [after Ref.21]
later Ag-Cu and Ag-Sb ‘alloy’ films15,16 offer a very
versatile and fascinating platform to fabricate at
ambient
temperature
and
pressure
I-VII
nanostructures that could serve to investigate
electronic structure development as well as to develop
optoelectronic/photonic devices. While partial
iodization
creates
metal-semiconductor
nanocomposites
complete
iodization
creates
applicable nano semiconductors. This technique could
be extended to the fabrication of nanoparticle arrays,
nanowires, I-VII versions of quantum wells, ternary
superionic conductors based on Ag and Cu as well as
to investigate novel superionic superlattices-an area
waiting to be explored.
How is this iodization implemented? The Ag or
Ag-based alloy/multilayer film is fabricated in the
usual way either by vacuum thermal evaporation onto
suitable substrates (glass, quartz, PVA among others)
and characterized by diffraction and microscopy.
Subsequently they are mounted on top of a closed
(airtight but with provision of gas passage/evacuation
if required) hourglass iodinator (Fig. 4). Suitably
weighed Iodine charge (flaky crystals) is placed at the
bottom of the jig. The process begins right away and
can easily be timed. Interruptions help ex-situ
monitoring of partially iodized Ag films by XRD and
microscopy. How does the process occur? We have
looked at the kinetics of iodization and modeled it17as
an interface-controlled reaction somewhat like the
oxidation of silicon. Plasmon-exciton ‘transitions’ in
RF sputtered ultrathin films (<20 nm average
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INDIAN J PURE & APPL PHYS, VOL 51, MARCH 2013
Fig. 5 — Atomic Force Miicroscope (AFM) pictures of sputtered Cu film (left) and Cu film iodized using the jig of Fig. 5 for 60 min
(right) showing CuI formation subsequently confirmed by X-ray diffraction [after Ref. 20]
thickness) which involves the initial formation of
surface plasmons (note that the films are
rough/disordered) and their gradual conversion to the
so-called Z1,2 and Z3 excitons18,10 of zincblende AgI
have been investigated. In a recent work20, plasmonexciton transitions in iodized RF sputtered Cu thin
films have been investigated (see Fig. 5 for AFM
picture).
Our recent studies on Ag films21 have shown that
short-term (< 10 min) iodination22 can be an easy but
effective method for creating Ag nanoparticle
assemblies of controllable median particle size for
applications such as fabrication of SERS-active
substrates. This method is perhaps superior in that it
does not involve messy solutions and there is no need
to generate22 wurtzite AgI.
5 Conclusions
In this brief overview, we have outlined three
methods of nanoparticle/nanostructure synthesis/
fabrication as applied to AgI and AgI-CuI solid
solutions: (1) hot solution controlled precipitation,
(2) mechanochemical reaction for metastable
nanoparticle synthesis (zincblende AgI and AgI-CuI
solid solutions) and (3) controlled iodization of Ag
and Ag-Cu and Cu nanostructured thin films. Theses
I-VII semiconductors/superionics have applications to
sensor and optoelectronic technologies. Though we
have focused on I-VII semiconductors, they are
equally applicable to chalcogenides and chalcohalides
of transition and post-transition metals. Although not
discussed here, we have used these materials to
investigate such exotic physics as stress-induced
confinement of excitons in thin films and plasmonexciton ‘transitions’. We have recently demonstrated
the efficacy of brief iodization for a modification of
nanostructured Ag surfaces for photonic applications.
III-V and II-VI–type nanostructures could as well be
fabricated and studied for I-VII semiconductors and
their chalcogenide combinations I-VII-VI-an area that
beckons the explorer and the exploiter.
Acknowledgement
We thank Mr J R G Patnaik, Drs P Senthil Kumar,
D Bharathi Mohan and Dr Gnanavel for their
enthusiastic participation in this I-VII nanostructures
research.
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