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[16]
[17]
[18]
[19]
M. Li, J. Wang, L. Zhuang, S. Y. Chou, Appl. Phys. Lett. 2000, 76, 673.
D. Y. Khang H. H. Lee, Appl. Phys. Lett. 2000, 76, 870.
H.-G. Elias, Macromolecules, Vol. 1, Plenum Press, New York 1984.
M. Ohring, The Materials Science of Thin Films, Academic Press, New
York 1991.
[20] B. Crist, in Structure and Properties of Polymers (Eds: R. W. Cahn, P. Haasen, E. J. Kramer), VCH, Weinheim 1993.
[21] J. R. Sheats, B. W. Smith, Microlithography: Science and Technology, Marcel Dekker, New York 1998.
Shape-Persistent Polyphenylene DendrimersÐ
Restricted Molecular Dynamics from Advanced
Solid-State Nuclear Magnetic Resonance
Techniques**
By Michael Wind, Uwe-Martin Wiesler, Kay Saalwächter,
Klaus Müllen,* and Hans Wolfgang Spiess*
Tetrahedral polyphenylene dendrimersÐcascade molecules
with four successively branched arms made of phenyl rings
only and emanating from a central carbon coreÐare a new
class of dendritic systems that have been recently developed
in our laboratories.[1] Due to their very dense intramolecular
packing, these monodisperse polyaromatic dendrimers are of
interest with respect to the design of nanostructures with
invariant shape.[2] Besides their significantly enhanced thermal and chemical stability, their postulated rigidity as compared to aliphatic dendrimer systems provides the basis for
their potential application, e.g., as a support for catalysts, dyes,
or biological active substances in human diagnosis.[3] Whereas
both scattering and microscopy methods have successfully
been applied to the structural characterization of polyaromatic dendrimers,[4] attempts to understand their dynamic
behavior have been limited to preliminary molecular dynamic
simulations.[5,6] Advanced solid-state nuclear magnetic resonance (NMR) spectroscopy can probe the structure[7] and provides a powerful new probe for the analysis of geometry and
timescale of molecular dynamics over a wide range.[8] We
describe the synthesis and, in order to prove their shape persistence and rigidity, investigations of the dynamics of amorphous bulk dendrimers up to the third generation (G3), 1,
using recently developed high-resolution solid-state NMR
techniques, which can be applied to small amounts (typically
less than 100 mg) of as-synthesized samples. High spectral resolution is achieved by rapid magic angle spinning (MAS).
The general concept of the synthesis of the polyphenylene
dendrimers was previously presented.[1] In this work, peripherally methyl-substituted dendrimers were investigated because the methyl probes improve the NMR relaxation behav-
±
[*] Prof. K. Müllen, M. Wind, U.-M. Wiesler, Dr. K. Saalwächter,
Prof. H. W. Spiess
Max-Planck-Institut für Polymerforschung
Postfach 3148, D-55021 Mainz (Germany)
E-mail: [email protected]
[**] The authors thank Robert Graf, Steven P. Brown, and Stefan A. Reinsberg for helpful discussions. Financial support from the Deutsche Forschungsgemeinschaft (SFB 262) is gratefully acknowledged.
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Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001
ior and, consequently, the achievable signal-to-noise ratio in
the solid-state spectra as compared to their unsubstituted analogues. The methyl-substituted derivatives were obtained as
follows: sequential growth of the dendritic system is achieved
via [2+4] cycloaddition of ethynyl-substituted polyphenylenes
and tetraphenylcyclopentadienone derivatives. The starting
point for the dendrimer synthesis is the tetrahedral, tetrafunctionalized core molecule tetra-(4-ethynylphenyl)-methane 2
(Scheme 1).[9] By Diels±Alder cycloaddition of 3,4-bis-(4-triisopropylsilylethynylphenyl)-2,5-diphenyl-cyclopentadienone
in refluxing o-xylene and subsequent cleavage of the triisopropylsilyl protecting groups with ammonium fluoride and catalytic amounts of tetrabutylammonium fluoride in tetrahydrofuran (THF), the first generation of ethynyl-substituted
polyphenylene dendrimer 3 is obtained in quantitative yield.
Repetition of this cycloaddition±deprotection sequence allows the synthesis of a second generation ethynyl-substituted
dendrimer 4. Finally, the methyl-substituted polyphenylene
dendrimers 5, 6, and 1 are synthesized by another Diels±Alder
cycloaddition of 3,4-bis-(4-methylphenyl)-2,5-diphenylcyclopentadienone to 2, 3, and 4, respectively, in refluxing o-xylene.
After isolation by precipitation in methanol, 5, 6, and 1 are
obtained as off-white amorphous powders. Yields of 85 % to
95 % are achieved for the single reaction steps. For related
unsubstituted as well as functionalized systems, a more
detailed description of the synthetic concept will be published
soon.
The methyl-substituted polyphenylene dendrimers exhibit
different solubilities depending on the particular solvent. The
investigated dendrimers are insoluble in common organic solvents such as methanol or hexane. In dichloromethane, THF,
or toluene the solubility increases with the dendrimer generation: Whereas 5 is only poorly soluble (<1 g L±1), 6 and 1 show
a much higher solubility of more than 10 g L±1. Nevertheless,
all dendrimers were characterized by field desorption (FD-
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Scheme 1. Divergent synthesis of the methyl-substituted polyphenylene dendrimers Td-G1(-Me)8 5 and Td-G2(-Me)16 6.
MS) or matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF-MS) as well as by solution-state 1H and 13C NMR spectroscopy, proving their
monodispersity and purity, respectively. The thermal stability
of the dendrimers is noteworthy, as thermogravimetric analysis yielded decomposition temperatures well above 300 C.
For all dendrimers, no glass transition or any other phase
change is observed by differential scanning calorimetry
(DSC) analysis.
Molecular mechanics (MM) and molecular dynamics
(MD) simulations provide a first insight into both structural
properties and time evolution of polyphenylene dendrimers.
Ignoring, for simplicity, deviations from a fully symmetric arrangement with extended arms, the structures were modeled
and visualized with the Cerius 2 molecular modeling package, which applies an MM2(85) force field and the Conjugate
Gradient 200 algorithm for a local optimization of the conformation.[5] For the dendrimers 5, 6, and 1, maximal extensions of 2.3, 3.7, and 5.0 nm, respectively, were deduced and
were also experimentally confirmed by atomic force microscopy,[4] transmission electron microscopy, and light scattering
measurements. Further MD calculations of analogous tetrahedral dendritic systems identify an asymmetric dumbbelllike structure with an intramolecular pairing of each two
extended branches as the energetically most stable conformer.[6] In spite of intramolecular dynamics involving rearrangements of complete elongated dendritic arms as well as the
pentaphenyl benzene units, the global shape did not change
throughout the simulation, and the systems retained their
Adv. Mater. 2001, 13, No. 10, May 17
overall nano-architecture. These processes were found to involve torsion angle fluctuations with amplitudes of up to 40
occurring on a nanosecond timescale. The MD approach,
however, is restricted to an in vacuo analysis of small isolated dendrimer molecules of only the first and second generations.
Fast nanosecond torsion angle fluctuations, as implied by
the MD simulations, should lead to a directly measurable fast
dynamic averaging of the 1H±13C heteronuclear NMR dipolar
coupling tensor, thus probing the reorientation of C±H bond
directions at the aromatic rings. This information can be
obtained through extension of well-established NMR techniques aimed at measuring heteronuclear dipolar couplings
such as rotational-echo and transferred-echo double resonance (REDOR and TEDOR).[10] REPT-HDOR (recoupled
polarization-transfer heteronuclear dipolar order rotor encoding), one variant of a whole class of new NMR techniques,[11]
is a two-dimensional NMR experiment employing fast
MAS,[12] in which the site-resolved 13C chemical shift is correlated with an indirect dimension, and the dipolar coupling
information is encoded in a so-called spinning-sideband pattern. The CH dipolar coupling, as extracted from this pattern,
is a function of both the internuclear CH distance and the
dynamics on the experimental sub-microsecond timescale. For
well-defined molecular moieties with known internuclear distances e.g., CH, CH2, or CH3, the dynamical information can
be deduced directly. In Figure 1a, patterns from the aromatic
CH signals are displayed as a function of both temperature
and generation. An almost ideal sideband pattern for an iso-
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Insight into geometry and timescale of slow reorientational dynamics of polyphenylene dendrimers in the range of milliseconds to
seconds is available through 13C
two-dimensional exchange NMR,[8]
where the slow reorientation of the
13
C chemical shift tensor during a
mixing time is probed. As the main
anisotropy of the 13C chemical shift
is exhibited between the plane of
the aromatic rings and the normal to
it, this technique is very sensitive to
reorientation of aromatic rings
about the fixed links between the
phenyl groups. The very small
exchange intensity observed in static
samples was found to be restricted
to the spectral region adjacent to
the diagonal of the 2D spectra, indicating only very restricted angular
Fig. 1. a) Solid-state 13C spinning sideband patterns for the aromatic ternary CH groups (sum projections),
obtained at a spinning frequency of mR = 25 kHz and using the REPT-HDOR pulse sequence. The spectra were
displacements.[8]
recorded for different temperatures and generations. b) Corresponding simulated spectra, obtained by taking
For better quantification, a more
into account different models of phenyl ring reorientation processes on a microsecond timescale.
advanced method, CODEX (centerband-only detection of exchange),
was employed. It is the high-resolution 1D MAS analogue of
lated CH spin pair, being exclusively composed of odd-order
the conventional 2D exchange technique. The pure-exchange
sidebands, is observed in all three cases. Since the first-order
CODEX spectrum, comprised exclusively of signals from exsideband intensities are generally not reliable due to the influchanging sites, is obtained as the difference between the COence of remote protons,[11] the dipolar coupling constant was
DEX spectrum, in which segmental reorientation during a
obtained from a fit of only the higher order sidebands. The
mixing time tm leads to a reorientation angle dependent
best-fit average dipolar coupling constant (DCH) for the arodecrease in the corresponding line intensity, and the reference
matic CH group is 20.5 ± 0.6 kHz. This corresponds to an
spectrum with an infinitesimal small mixing time, during
internuclear distance of 113 ± 1 pm and matches results from
which no motion occurs. Representative, pure-exchange inaromatic CH groups in rigid, crystalline model systems very
tensities of Td-G2(-Me)16 6 as a function of mixing time towell.[11]
gether with a corresponding reference spectrum are shown in
Simulations using the best-fit dipolar coupling constant, and
Figure 2a. For long mixing times tm >> tc, the exchange buildadditionally taking fast motional averaging for different geup reaches a final intensity E¥ = fm(M ± 1)/M, which contains
ometries into account, are shown in Figure 1b. Clearly, fast
information about both the number M of equivalent orienta180 flips and full rotations of individual phenyl rings can be
ruled out, since they would lead to decreased higher order
tional sites accessible to the motional process and the fraction
sideband intensities. Motional averaging due to small-angle
fm of mobile segments. As depicted in Figure 2b, E¥ differs
significantly for the aromatic ternary (E¥ = 0.40) and quaterfluctuations, Gaussian-distributed with mean reorientation
nary carbons (E¥ = 0.20). Assuming that all mono-substituted,
angles as indicated, does not have a strong effect. However,
terminal phenyl groups take part in the motional process, the
while the fifth- to third-order sideband ratio for the ±20 case
observed plateau intensities can be explained by reorientais already considerably decreased, a corresponding spectrum
tions between two distinct molecular positions or between
(data not shown) calculated for DCH = 23.3 kHz, correspondtwo distributions of molecular positions (M = 2), since the
ing to rCH = 109 pm as a reasonable lower limit, would still be
fraction of mobile quaternary carbons (fm = 56/140 = 0.4), as
consistent with our measurement. Thus, ±20 reorientations
obtained by simple counting, is only half of that of the ternary
cannot be excluded. In summary, the dendritic systems are
ones (fm = 184/244 = 0.76). A higher number of accessible oriindeed mostly rigid on a microsecond timescale, suggesting
entational sites or a complex reorientation of extensive moshape persistence on this timescale. The observed increased
lecular segments would lead to a significantly higher asympfirst-order sideband intensities, which increase further for
totic exchange intensity.
higher temperature and higher dendrimer generation, might
As is apparent from Figure 2b, the correlation function canin part be attributed to a very small fraction of moieties
undergoing fast larger angle dynamics. This will be further
not be modeled by a single exponential. In contrast, a distrisubstantiated below.
bution of correlation times may be assumed. This poses a
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Fig. 2. a) Series of CODEX spectra of Td-G2(-Me)16 5 as a function of the mixing time tm, recorded at T = 363 K using a spinning frequency
of mR = 7.5 kHz. Top: Reference spectrum, scaled down by a factor of 4. b) Normalized pure-exchange CODEX intensities E(tm) as a function of tm for the aromatic tertiary CH and the quaternary Cquat in Td-G2(-Me)16 5 (T = 363 K). The fit curve for the tertiary carbons is a
stretched exponential exp(±(tm/tc)b) with b = 0.51, tc,KWW = 401 ms. The dotted line indicates the final CODEX exchange intensities.
problem for a stable fit of the final exchange intensity in the
long-time limit. Moreover, the plateau intensity itself is increased as a result of spin diffusion effects starting to occur at
mixing times of several seconds. Therefore, in order to obtain
a realistic estimate of the plateau, the exchange build-up was
firstly modeled with a simple biexponential function with two
correlation times tc,1 and tc,2 in the range of 10 to 1000 ms.
Then, for the thus-obtained E¥, the correlation function was
fitted with a Kohlrausch±Williams±Watts (KWW) function
exp(±(tm/tc,KWW)b) in order to obtain a mean correlation time
tc,1 < tc,KWW < tc,2 (Table 1). A mean stretch exponent b = 0.51
indicates a distribution of time constants tc over about two
orders of magnitude.[13]
of the quaternary carbon atoms Cquat is invariant under a
p-flip, which corresponds to the motional process most commonly observed for para-substituted phenylene groups.[8]
Therefore, a reorientation of 180 would not cause any Cquat
exchange intensity. The detected Cquat CODEX signal proves
that the phenyl reorientation angle must differ from 180. A
smaller reorientation angle is then rationalized realizing that
the individual CH groups of the exo-phenyls cannot pass
through the plane formed by the central ring. A graphical representation is depicted in Figure 3. A mean reorientation
angle of about 60, probably also being distributed, is in
accord with further CODEX and 2D static exchange data.
Table 1. Mean correlation times tc,KWW and b exponents of polyphenylene dendrimers Td-Gn(-Me)2(n + 2) as obtained from KWW fits to CODEX NMR
measurements at T = 363 K.
The observed distribution of correlation times may be
attributed to cooperativity or a distribution of sites with different packing densities. We believe that the steric hindrance
of a given phenyl group imposed by its neighbors, which are
bound to the same core, allows only collective reorientations
of all rings around one central core. If the correlation time distribution is wide enough to exhibit a tail of correlation times
on the timescale of the REPT experiment (10±4 s or shorter),
these would then account for the slightly increased first-order
sideband intensities observed in the REPT patterns (Fig. 1).
The observed amplification of that interference effect with
increasing temperature and dendrimer generation parallels
exactly the trend observed for the mean (slow) correlation
times. As to the geometry of the reorientation process, it
should be mentioned that the anisotropic chemical shift tensor
Adv. Mater. 2001, 13, No. 10, May 17
Fig. 3. Motional model of the localized, cooperative dynamics in polyphenylene
dendrimers, including two-site jumps of all phenyl substituents of a pentaphenyl
benzene building block. As indicated by X-ray analysis and MD simulations,
the peripheral aromatic rings are inclined by 30 with respect to an axis normal
to the face of the central benzene ring.
Analogous results obtained from Td-G1(-Me)8 5 and TdG3(-Me)32 1 exhibit essentially the same features, confirming
the notion of local dynamics, in which motion is limited to individual phenyl reorientations about fixed axes. In comparison, the average correlation times at the same temperature
decrease on going from G1 via G2 to G3 (Table 1). For tetrahedral polyphenylene dendrimers, the concept of the densest
packing,[14] implying progressively higher densities for outer
dendritic shells due to an exponential increase in the number
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of branches in contrast to a linear only increase of the diameter, is not confirmed for the investigated three generations.
Rather, the intramolecular steric hindrance, which we suppose
is the main influencing factor for the observed correlation
times, decreases upon adding the second generation, indicating an increased free volume. Investigation of G4 is currently
under way and will offer an opportunity to further study the
effect of steric crowding.
Further systematic NMR investigations on the dynamics of
polyphenylene dendrimers concerning kinetic, geometrical,
and morphological aspects as well as a comparison with
related dendritic systems with alternative core molecules and
branching structures, are in progress. In summary, our solidstate NMR characterization clearly proves the shape-persistence of the investigated polyphenylene dendrimers. The limited dynamics is generally identified as a well-localized,
restricted reorientation of single terminal phenyl substituents
around fixed axes. On the millisecond to second timescale,
angular excursions up to about ±60 occur. Fast microsecond
dynamics is further restricted to even smaller reorientations
of up to ±20. The application of technically advanced NMR
MAS recoupling techniques, allowing the study of motional
processes of the polyaromatic dendrimers thus greatly enhances the ability to understand dynamical features and provide valuable feedback to the design principles of these new
materials.
Experimental
Selected Data for 5 Td-G1(-Me)8: MS (FD, 8 kV): m/z [%]: 1954.7 (100) [M+]
(calcd. for C153H116: 1954.6); 1H NMR (300 MHz, C2D2Cl4, 303 K): d = 7.48 (s,
4H; H1); 7.21±7.13 (br, 24H; Harom); 6.96±6.62 (m, 64H; Harom); 2.13, 2.09 (s,
24H; H2).
Selected Data for 6 Td-G2(-Me)16: MS (MALDI-TOF): m/z [%]: 5131.9
[M,Na+] (calcd. for C401H292: 5133.7); 1H NMR (300 MHz, C2D2Cl4, 303 K): d
= 7.47 (s, 4H; H1); 7.38, 7.34 (s, 8H; H1¢); 7.21±6.96 (m, 64H; Harom); 6.96±6.35
(m, 172H; Harom); 2.07, 2.04, 2.03 (s, 48H; H2); 13C NMR (75 MHz, C2D2Cl4,
303 K): d = 144.5, 142.4, 142.1, 142.0, 141.1, 140.7, 140.6, 140.4, 140.3, 140.2,
139.6, 139.4, 139.3, 138.9, 138.8, 138.5, 138.2, 137.9, 137.6, 137.2, 134.8, 134.5
(Cquat); 131.9, 131.7, 131.6, 131.3, 130.2, 129.4, 128.9, 128.6, 127.7, 127.4, 126.9,
126.2, 125.6 (Ctert); 21.4 (Caliph).
Selected Data for 1 Td-G3(-Me)32: MS (MALDI-TOF): m/z [%]: 11 414.9
(100) [M,K+] (calcd. for C891H640: 11 386.1.); 1H NMR (300 MHz, C2D2Cl4,
303 K): d = 7.40±7.24 (m, 28H, H1, H1¢, H1²); 7.20±6.28 (m, 52H, Harom); 2.16±
1.92 (m, 96H; H2); 13C NMR (75 MHz, C2D2Cl4, 303 K): d = 142.4, 141.9, 140.7,
140.2, 139.4, 138.8, 137.6, 137.2, 134.8, 134.4 (Cquat); 131.7, 131.6, 130.2, 129.4,
127.7, 127.4, 126.9 (Ctert); 21.4 (Caliph).
NMR SpectroscopyÐREPT: The REPT spectra were measured on a Bruker
DRX 700 spectrometer using a narrow-bore magnet with 2.5 mm MAS equipment. Both the 1H and the 13C 90 pulses were 2 ls. The experiments were
recorded at 25 kHz MAS with a recoupling time of two rotor periods length and a
repetition time of 1 s. Acquisition of 64 slices in t1 with 512 transients each leads to
a measuring time of approximately 9 h per 2D spectrum. Details concerning the
REPT-HDOR pulse sequence can be obtained from the literature [11].
NMR SpectroscopyÐCODEX: The CODEX NMR experiments were carried
out on a Bruker DSX 300 spectrometer equipped with a commercial Bruker 4 mm
double-resonance MAS probe and operating at resonance frequencies of
300.22 MHz for 1H and 75.47 MHz for 13C. The 90 pulse lengths were 3 ls on
both channels. Proton dipolar decoupling with a B1 frequency of 83 kHz was applied during recoupling and acquisition. All spectra were recorded with cross polarization using a contact time of 2 ms and a repeating time of 2 s. The probe temperature was controlled with standard Bruker equipment. An extensive
theoretical introduction to CODEX can be found in the literature [15].
Received: November 15, 2000
Final version: January 18, 2001
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Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001
Note Added in Proof: After completion of this work we
learnt about a recent study by Haeberlen et al.,[16] who detected phenyl flips even in the crystal phase of o-terphenyl.
±
[1] F. Morgenroth, A. J. Berresheim, M. Wagner, K. Müllen, Chem. Commun.
1998, 10, 1139.
[2] V. Perec, C. Ahn, G. Ungar, D. Yeardley, M. Möller, S. Sheiko, Nature
1998, 319, 161.
[3] a) M. Zhao, R. M. Crooks, Angew. Chem. 1999, 111, 375; Angew. Chem.
Int. Ed. 1999, 38, 364. b) A. P. Alivisatos, J. Phys. Chem. 1996, 100, 13 226.
c) M. Fischer, F. Vögtle, Angew. Chem. 1999, 111, 934; Angew. Chem. Int.
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[4] H. Zhang, P. C. M. Grim, P. Foubert, T. Vosch, P. Vanoppen, U.-M. Wiesler, A. J. Berresheim, K. Müllen, F. C. De Schryver, Langmuir 2000, 16,
9009.
[5] F. Morgenroth, C. Kübel, K. Müllen, J. Mater. Chem. 1997, 7, 1207.
[6] P. Brocorens, E. Zojer, J. Cornil, Z. Shuai, G. Leising, K. Müllen, J. L.
BrØdas, Synth. Met. 1999, 100, 141.
[7] H.-M. Kao, A. D. Stefanescu, K. L. Wooley, J. Schaefer, Macromolecules
2000, 33, 6214.
[8] K. Schmidt-Rohr, H. W. Spiess, Multidimensional Solid-State NMR and
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S. Vega, J. Schaefer, J. Magn. Reson. 1992, 96, 205.
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b) K. Saalwächter, R. Graf, H. W. Spiess, J. Magn. Reson. 2001, 148, 398.
[12] a) B.-J. van Rossum, H. Förster, H. J. M. de Groot, J. Magn. Reson. 1997,
124, 516. b) A. Lesage, D. Sakellariou, L. Emsley, J. Am. Chem. Soc. 1998,
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[13] C. P. Lindsey, G. D. Patterson, J. Chem. Phys. 1980, 73, 3348.
[14] P.-G. de Gennes, H. Hervet, J. Phys. Lett. 1983, 44, L351.
[15] a) E. R. deAzevedo, W.-G. Hu, T. J. Bonagamba, K. Schmidt-Rohr,
J. Am. Chem. Soc. 1999, 121, 8411. b) E. R. deAzevedo, W.-G. Hu, T. J.
Bonagamba, K. Schmidt-Rohr, J. Chem. Phys. 2000, 112, 8988.
[16] M. Stumber, H. Zimmermann, H. Schmitt, U. Haeberlen, Mol. Phys. 2001,
in press.
Three-Dimensional Nano-objects Evolving
from a Two-Dimensional Layer Technology
By Oliver G. Schmidt,* Nicole Schmarje, Christoph Deneke,
Claudia Müller, and Neng-Yun Jin-Phillipp
Nanotubes can be formed from thin solid films once they
are freed from their substrate by a selective etching procedure.[1±3] There are two methods for making these solid-state
nanotubes,[3] involving a ªgeneralº (method I)[3] and a ªspecializedº (method II)[1±3] procedure. Both rely on the release
of thin layers from a substrate by selective etching. For the
general method I, a thin solid film of almost arbitrary composition is detached from its substrate and folded back onto its
own surface. The resultant crease forms a nanotube. The specialized method II relies on inherently built-in strain within
the thin solid film. Once the film is freed from the substrate,
the layer rolls up by itself and forms a nanotube. If etching is
performed long enough multiwall tubes are created.[1] Debonding of strained layers can also be used to fabricate freestanding helical microcoils on substrate surfaces.[1,2]
±
[*]
Dr. O. G. Schmidt, N. Schmarje, C. Deneke, C. Müller,
Dr. N.-Y. Jin-Phillipp
Max-Planck-Institut für Festkörperforschung
Heisenbergstrasse 1, D-70569 Stuttgart (Germany)
E-mail: [email protected]
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