NANO LETTERS
Evidence for Metal-Semiconductor Transitions in Twisted and Collapsed Double-Walled Carbon Nanotubes by Scanning Tunneling Microscopy
2008 Vol. 8, No. 10 3350-3356
Cristina E. Giusca, Yann Tison,† and S. Ravi P. Silva‡ Nano-Electronics Centre, AdVanced Technology Institute, UniVersity of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom Received June 20, 2008; Revised Manuscript Received August 1, 2008
ABSTRACT The atomic and electronic structure of a twisted and collapsed double-walled carbon nanotube was characterized using scanning tunneling microscopy and spectroscopy. It was found that the deformation opens an electronic band gap in an otherwise metallic nanotube, which has major ramifications on the use of carbon nanotubes for electronic applications. Fundamentally, the importance of the intershell interaction in this double-walled carbon nanotube points to the potential of a reversible metal-semiconductor junction, which can have device applications, as well as a caution in the design of semiconductor components based on carbon nanotubes. Lattice registry effects between the two neighboring walls evidenced by atomically resolved images confirm earlier first principle calculations indicating that the helicity influences the collapsed structure and show excellent agreement with the predicted twisted-collapse mode.
Carbon nanotubes (CNTs) may undergo a variety of mechanical deformations from their otherwise perfect cylindrical shape owing to their long-tubular and high-aspect geometric ratio, under diverse circumstances. These can be correlated with changes in their electronic properties predicted to be sensitively dependent on the atomic structure. Distortions such as buckling, bending, and twisting, as well as collapse, have been evidenced experimentally and have been subject to extensive theoretical treatment in order to determine the effect they exert on their electronic properties. Bending nanotubes was found to decrease their transmission function, which is directly related to the conductance of the nanotube as described by the Landauer-Bu¨ttiker formalism.1,2 As a result, the deformation leads to anomalous electrical transport properties due to significant σ-π hybridization resulting from the increased curvature. This aspect was further confirmed by scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) studies of bent carbon nanotube junctions which showed the presence of localized states and superstructures on the nanotube due to electron scattering and interference at the defect site in the junction surroundings.3-7 Other reports on the electronic conductance of sharp nanotube bends provide evidence for the existence of semiconductor-metal and metal-metal ‡ To whom correspondence should be addressed. E-mail: s.silva@ surrey.ac.uk. † Present address: Technical University of Denmark, Department of Physics, Lyngby, 2800, Denmark.
10.1021/nl801782k CCC: $40.75 Published on Web 09/11/2008
2008 American Chemical Society
heterojunctions, possibly due to pentagon-heptagon defect pairs present in the hexagonal carbon network.8-10 Twisting is found to strongly affect the electronic structure of carbon nanotubes as revealed by theoretical studies11 and extended-Hu¨ckel calculations.2 Metallic armchair nanotubes in particular develop a band gap which scales linearly with the twisting angle up to a critical value at which the nanotube collapses into a flat ribbon structure.2 A twist distortion of several degrees evidenced by an anomalous angle orientation between the zigzag and armchair direction was reported by STM measurements for an armchair single walled carbon nanotube, although with no information on the electronic response to the deformation.12 A different type of deformation, observed experimentally by transmission electron microscopy (TEM),13-15 atomic force microscopy (AFM),16,17 and more recently STM,18 is the collapsed state configuration, which appears in some cases along with the twist deformation. On the basis of the experimental observations, the collapsed configuration was theoretically explained to occur when balancing the energy due to the nanotube curvature with the energy due to the intersheet attraction between opposing layers of carbon nanotubes.13,14 The interlayer attraction, mainly van der Waals interactions, can also consist of other potentially less important contributions, such as correlation-borne and kinetic contributions.14 Numerous studies have been dedicated to analyzing the structure and energetics of the collapsed
configuration employing classical molecular dynamics19 and atomic-scale finite-element simulation methods.20,21 In the case of single-walled carbon nanotubes (SWNTs), these studies reveal different regimes of stability for the collapsed deformation depending on the diameter of the nanotube with collapse being favored in tubes with larger diameters. It was also found that the interlayer lattice registry could lead to various structural morphologies such as straight ribbon, a warping or twisted ribbon depending on the degree of commensurability between the opposing layers, and thus on the helicity of the nanotube. The electronic properties of the collapsed carbon nanotubes have also been investigated using tight-binding molecular dynamics methods22-25 and demonstrate metal-semiconductor and semiconductor-metal transitions in response to the collapse perturbation, depending on the helicity of the tube and the degree of deformation. However, most of these calculations are restricted to the simplest case of monoshell carbon nanotubes due to the complexity of the collapsed nanotube systems containing more than one constituent layer for which electronic band structure calculations are almost lacking in the literature, whereas most experiments involving collapsed tubes deal with multiwalled samples. Although without establishing a correlation with the local atomic coordination, due to the nature of the AFM technique used, such experiments have proved that the collapse deformations can significantly alter the electronic behavior of carbon nanotubes.26,27 In contrast to earlier experiments, we present here for the first time the electronic behavior of a twisted and collapsed double-walled carbon nanotube (DWNT) directly probed at the atomic level by STM, which will elucidate the detailed relationship between the local electronic structure and the mechanical deformations. The data is ideal to formulate and fine-tune realistic atomistic modeling and calculations that combine first principle structure with density functional theory. The use of the data to both future electronic devices, and NEMS structures could be invaluable. The potential of using CNT devices for sensing with unprecedented sensitivity due to the atomic hybridization influences could open a route to building molecular bottom up structures. The effect of conformational strain on the chemical reactivity of singlewalled carbon nanotubes has been shown by simulations to significantly enhance reactivity locally by controlled deformations. For example, the chemisorption of hydrogen atoms is predicted to be enhanced by as much as 1.6 eV at regions of high conformational deformation, such as kinks in bent nanotubes and ridges resulting from torsional strain, suggesting that local reactivity will be significantly enhanced.28,29 Experimental Section. The double-walled carbon nanotubes used in this work have been purchased from Carbon Nanotechnologies Incorporated and have been acid treated prior to the experiment (reflux in HNO3, 2 mol·L-1, 150 °C for 24 h followed by a reflux in HCl, 2 mol·L-1, 150 °C for 45 min) and oxidized in air (300 °C, 1 h) in order to remove most of the catalyst and amorphous carbon impurities. The suitability of the nanotube material for STM experiments was checked by TEM which established that the sample Nano Lett., Vol. 8, No. 10, 2008
contained mostly DWNTs (>95%), occasionally showing the presence of SWNT as well as some tubes containing three or four layers with the results reported elsewhere.30 For STM experiments, a suspension of carbon nanotubes was prepared by sonication in 1,2- dicholoroethane for approximately 15 min and then drop cast on Au substrates prepared as described in ref 31. The sample was left in the fast entry lock of the system prior to introducing it into the STM chamber and pumped overnight to allow for the specimen to outgas. The instrument used for the STM studies is a commercial Omicron VT Multiscan STM combining STM with scanning electron microscopy (SEM). Prior to STM experiments, scanner calibration was carried out on highly oriented pyrolitic graphite (HOPG), which also helped with assessing the quality of the tip by rendering the atomic resolution. STM tips were electrochemically etched W wire prepared immediately before introducing them into the UHV chamber, and their cleanliness was checked against HOPG. I-V curves recorded periodically during experiment on clean regions of the Au substrate displaying metallic behavior also provide confirmation of a suitably clean STM tip. All measurements were carried out at room temperature in constant current mode. Typical scanning conditions used with this sample were 0.1 nA for the tunneling current and a positive sample bias of 0.1 V. The STS measurements were performed by interrupting the scanning and the feedback at preset locations and recording the tunneling current variations during the sample bias ramp (usually between -2 and +2 V). STM images were processed using WSxM software available for download at http://www.nanotec.es.32 Results and Discussion.Typical large-scale STM images on this sample show the presence of bundles of two-three to tens of nanotubes, as well as larger bundles. Individual DWNTs are more rarely observed, owing probably to the purification process that, by removing some of the amorphous carbon, allows the tubes to effectively stick together in bundles. Figure 1 illustrates such an example, showing also the presence of an individual nanotube at the periphery of a bundle (indicated by the black arrow at the bottom of the image). This individual tube has a shape that presents a few clearly different features along its length, as revealed by additional subsequent higher resolution STM images in Figure 1b-f. Starting at the point where the tube extends out of the bundle (Figure 1b) and recorded consecutively along the tube length, up to end of the nanotube (Figure 1f), the total length extending out of the bundle was estimated to ∼40 nm. In order to assist with identification of the observed distinct features described above, Figure 1g presents the schematic model of a twisted carbon nanotube, also flattened into a ribbonlike structure, highlighting regions corresponding to similar features in the STM images obtained experimentally. The schematic model shows the top view of the twistedcollapsed carbon nanotube and was generated using the Nanotube Modeler program (http://www.jcrystal.com). Figure 1b,c reveals that this section of the nanotube is collapsed, as suggested by the two pronounced round bulbs at both sides along the nanotube and evidenced also by the flat ribbonlike shape, better illustrated in Figure 1c. Further3351
Figure 1. STM image showing bundles of DWNTs on the Au substrate (a). Sequence of atomically resolved STM images of a twisted and collapsed nanotube recorded along the nanotube indicated by the arrow in panel a, as it extends out of the bundle and all the way to its free end (b-f). Symbols (*) indicate locations where I(V) curves were recorded. (g) Schematic model of a collapsed and twisted carbon nanotube, highlighting regions similar to the ones observed in the STM images recorded by experiment.
more, the shape of the cross sectional profiles (not shown) taken across the nanotube implies a collapsed configuration. Figure 1c shows in addition to the collapse, a twist of the nanotube, indicated by the arrow on the figure, with the section of the nanotube following the twist still maintaining the collapse, as evidenced in Figure 1d. Further to the right, this image appears to indicate buckling of the nanotube due to a severe twist in this region, possibly causing the nanotube to coil up resulting in a drastic morphological change at this point of deformation. This observation would be supported by the change in the orientation of the nanotube with respect to the substrate, as illustrated by Figure 1e, which also indicates a slight “arched” shape of the nanotube. As shown by molecular dynamics simulations,33 a nanotube under torsional load above a critical angle can result in a ribbonlike flattened shape which loses the straight axis and buckles sideways. Furthermore, the behavior in Figure 1e seems to 3352
be similar to cupping instabilities previously observed in TEM experiments, which cause bulbs to bulge asymmetrically on the sides of the flat region and concentrate the bulb on one side of the tube, owing to a symmetry-breaking deformation of the twisted ribbon.14 The analysis of the image in Figure 1d is complicated by the presence of some extraneous material present adjacent to and possibly extending over on top of the nanotube, which makes drawing a definite conclusion regarding this section difficult due to the speculative nature of the problem. The extra features observed in Figure 1d and shown at a slightly higher magnification in Figure 1e appear to resemble the asymmetrical shape of the opened ends of the purified DWNTs, as evidenced by additional STM images30 and TEM which most of the time showed C fragments at the end of the asymmetric tips of the nanotubes.30 Nano Lett., Vol. 8, No. 10, 2008
In Figure 1f, the remaining section that follows all the way to the free end shows that this segment is partially flattened, and it follows the irregularities existent on the substrate surface. It is not yet clear what exactly determined the twist and the collapse formation for the nanotube presented here, was it the twist deformation induced by the collapse, or was it the collapse which induced the twist. One possible reason for the presence of these deformations could be due to the sample preparation methodology. The Au substrate is placed for drop-casting on a hotplate maintained at ∼70 °C to allow for quick evaporation of the solvent and consequently to prevent tube aggregation after solution sonication. The quick evaporation, occasionally through “bubbling”, of the solvent could be responsible for twist deformations induced to nanotubes due to fluctuation dynamics present in the liquid environment. As shown by earlier studies, the elastic deformations induced by the van der Waals interaction of the nanotube with the substrate can be substantial, so this interaction might be strong enough to maintain a twist, given the large binding energy state and the high forces involved.16 The fact that the nanotube investigated in this study is part of a bundle might also be an explanation for the twist, as it was shown that tubes having different helicities in a bundle arrange themselves respecting the atomic lattice commensurability which could lead to twisted tubes within the bundle. It is worth stating that while one end of these tubes is fixed within the bundle, the other end is free as revealed by the STM images. An interesting observation is that the free end did not allow for the untwisting of these tubes owing possibly to a strong van der Waals interaction with the substrate, responsible for effectively locking the twist, which could presumably induce the collapse of the nanotube. The existence of twists in collapsed freestanding MWNTs has been also observed before in TEM experiments and a model based on the analysis of energetics was proposed,15 supporting the hypothesis that twists in MWCNT ribbons are stabilized through more favorable atomic registry (commensurability) between neighboring layers of the innermost shell in the twisted state. This is because of the 0.012 eV/ atom energy difference in the interlayer binding energy of the AA and the AB stacking configurations, that exist for two graphitic layers spaced 0.34 nm.34 Imperfect registry would produce internal interlayer stress that can contribute to the mechanical deformation, generating or maintaining a twist. In accordance to this model, the appearance of the twist in the already collapsed DWNT could be related to the achievement of an energetically favorable structure in consideration with the effects that the inner layer would impact. Depending on various configurations of tube pairs, lattice registry effects may be established between the opposing walls of the inner tube and between the inner and the outer tube. Computational methods however show that in the majority of cases of DWNTs, the structure of the collapsed DWNTs is determined by the inner tube, while the outer tube acts only as a constraint to the deformation.21,35 For this purpose, Nano Lett., Vol. 8, No. 10, 2008
the atomic scale finite-element method20 was used, which describes both the atomistic interactions between the carbon atoms belonging to the same nanotube, using a recent manybody interatomic potential for carbon of Brenner et al.,36 as well as the van der Waals interaction between carbon atoms belonging to different walls modeled with a Lennard-Jones potential for carbon. It was established by these studies that for DWNTs the collapse and stability is governed by the radius of the inner wall and that the helicities of the two constituent tubes has a strong influence on the collapsed structure. A stability map designed for the collapse deformation of DWNTs, showed the critical radius of the inner wall determined for the fully collapsed DWNTs to be stable is 2.48 nm, whereas for a radius less than 2 nm, the circular cross section becomes stable.21,35 The radius of the DWNT presented in our study can be determined based on the fact that the perimeter of the nanotube should remain constant in both the collapsed and the uncollapsed state, as described in a previous study on collapsed SWNTs18 and using 20 averaged cross-sectional profiles measured across the nanotube presented in Figure 1b. The DWNT diameter so determined is 2.7 ( 0.1 nm, in its uncollapsed state, which gives an inner wall of ∼ 2 ( 0.1 nm, by considering an average interlayer spacing of 0.34 nm. This places the currently studied nanotube at the boundary to the collapsed metastable region on the stability map of collapsed DWNTs, where both collapsed and circular forms are energetically possible. As shown by Xiao et al.,21 although for DWNTs the collapse is governed by the radius of the inner tube, the collapse mode will be strongly influenced by the helicity of the nanotube, more precisely by the difference in the wrapping angle of the inner and outer tube and thus the lattice registry between the inner and outer tube on one side, and also by the lattice registry between the two collapsed walls of the inner tube on the other side. This will consequently lead to flat-, warped- and twisted-collapsed nanotubes. Generally, the helicity determination from STM measurements in the case of carbon nanotubes is complicated by the finite size of the tip and the circular shape of the tube. The geometry of the tip apex results in convolution effects producing an apparent broadening of the nanotube, whereas the circular shape of the tube, combined with the tendency of the tunneling current to follow the shortest path induce distortions in the STM image of the nanotube lattice making it to appear stretched in the direction perpendicular to the tube.37-39 For the present case, the wrapping angle determination is complicated due to the multiple distortions inducing deviations of the C hexagon rows from the true wrapping angle. Based on the section of the nanotube presented in Figure 1d, which ensures a sufficiently long and flat region for which the effects associated with the curvature of the tube and other distortions do not affect the correct angle, we estimate the value of the wrapping angle to be 4 ( 1°. From this value and the measured diameter, within the uncertainties calculated for the diameter and wrapping angle, either a (33, 3) 3353
Figure 2. (a,b) STS spectra recorded on the collapsed nanotube, showing a difference in conductivity between the flat ribbon at the center and the various other locations on the tube.
or (34, 2) wrapping indices can be assigned to the outer tube of the present DWNT. As shown in a separate STM study of DWNTs,30,40 the helicity of the inner tube can be determined on the basis of the van Hove singularities present in the experimental tunneling spectra. However, given the presence of so many deformations which can drastically impact on the electronic band structure, this method will not render a reliable estimation here. We have used the study of Guo et al.41 to infer the helicity of the inner wall. Guo et al.41 studied the energetically favored configurations of two neighboring tubes, based on a helicity dependent potential specially parametrized for layered carbon structures by Kolmogorov and Crespi42 and found that for a given set of wrapping indices (n, m) of the inner tube with a radius less than 3 nm, the optimum outer tube is described by the wrapping indices (n + 9, m). According to ref 41, the most likely pairs for the inner tube can either be (24, 3) and (25, 2), leading to two possible sets of DWNTs: (24, 3)@(33, 3) and (25, 2)@(34, 2), consistent with metal-metal and semiconductorsemiconductor inner-outer combinations, respectively. The helicity of the inner wall will help relate our present observations to earlier studies postulating various modes of collapse in DWNTs, based on the lattice registry/alignment angle between the opposing sides of the inner wall (selfregistry) and between the inner and the outer wall (cross registry).21 An atomic-scale finite element method21 predicts that with no lattice registry between any of these layers, a flat collapsed structure is obtained, whereas lattice registry between the opposing sides of the walls of the inner nanotube leads to a warped collapsed structure. The same study21 finds that a twisted collapsed configuration is consistent with lattice registry effects both between the inner and outer layer, as well as between the two layers of the inner tube. By considering the difference between the inner and outer wrapping angle for the two possible DWNTs, i.e. (24, 3)@(33, 3) and (25, 2)@(34, 2), we obtain a value of 1.5° for the first pair and 1° for the latter pair. These values are in excellent agreement with the computational predictions of Xiao et al.21 for the twisted and collapsed case and are consistent with rotations of the inner tube or rotations between the inner and outer tube in order to achieve a better commensurability between the layers. Further evidence for the twist and collapse-related lattice registry effects is 3354
portrayed in the complex constructive and destructive electronic interference effects presented in Figure 1e displaying nonconventional patterns. The observed pattern resembles previous STM observations near defects, on graphite or on a carbon nanotube bend junction.5 Electronic Properties. In order to examine the effect of these structural distortions on the electronic properties, tunneling spectra have been recorded on various distinct locations, indicated by the “*” symbols on the tube displayed in Figure 1b-f. The normalized differential conductance, V/I(dI/dV), which has been shown to provide a good measure of the local density of states (LDOS),43 was computed using standard methods and the resulting spectra are presented in Figure 2. The spectra were collected on the central part of the flat ribbon and on the bulbs at each side, at the twist and the defect region, and close to the tube end. The LDOS all generally present very consistent behavior for curves corresponding to similar locations on the tube wall, showing the reproducibility of the measurements. However, very different characteristics are exhibited between the data recorded on the distinct locations indicated above. The data recorded near the end of the nanotube (Figure 1f), the region which should be the least affected by the collapse and should thus exhibit the closest response to the electronic properties of the undeformed tube, shows a positive and finite DOS at the Fermi energy, also found for the LDOS recorded on the section of the tube in the closest proximity to its end (Figure 1e). A positive, finite DOS is consistent with metallic behavior and consistent with the expectation derived from the wrapping indices determined in the previous section. One set of wrapping indices we obtain corresponds to a metallic nanotube nested inside another metallic nanotube, rendering a metallic DWNT system. The other set of wrapping indices indicates a semiconducting innersemiconducting outer DWNT, which would also be expected to be metallic. This is based on recent STM measurements of DWNT30,40 and earlier in situ transport measurements in a TEM, coupled with selected area electron diffraction,44 showing a metallic character for semiconducting-semiconducting DWNTs, as well as NMR spectroscopy experiments which have evidenced the highly uniform metallic character of DWNT samples as a result of the interlayer interaction.45 In addition to experimental studies, computational studies Nano Lett., Vol. 8, No. 10, 2008
also show major changes in the electronic structure due to the interlayer interaction in DWNTs. For example, firstprinciple calculations show evidence that the electronic energy bands of semiconducting nanotubes for a curvatureinduced downward shift of the π*- and π-states of the inner tubes, causing overlap with those of the outer tube and consequent metallization of the DWNT system.46 Significant contrasting behavior is observed in the data recorded on the flat part of the collapsed tube, exhibiting zero DOS at the Fermi energy between ∼ -0.2 and +0.2 V, corresponding to well-defined valence and conduction band edges, respectively, typical of a semiconductor with a band gap. The observed semiconducting behavior is consistently shown by the defect site presented in Figure 1d, and on the twist region (Figure 1c). The twist region displays a slightly increased DOS at the Fermi energy but still retains a similar configuration of the gap and the valence and conduction band edges. Unlike the flat central part, a finite DOS is found in the tunneling spectra recorded on the curved bulbs at the edges of the collapsed nanotube (Figure 1b,c), which shows remarkable resemblance to the spectra measured on the regions in Figure 1, panels e and f near the end of the nanotube, correlated to the electronic behavior of the undeformed nanotube. To summarize, our spectroscopy data show semiconducting behavior on the flat part of the ribbon and the twist, and metallic behavior on the curved bulbs at the edges and on the end section of the nanotube. The interpretation of the tunneling spectra is not an easy task for such a geometrically complex system, where the effects induced by the twist, by the collapse and also the fact that the nanotube comprises of two constituent layers have all to be taken into account in order to examine the electronic behavior of this nanotube. The appearance of the gap is a direct consequence of the strong modifications of the electronic structure induced by the twist and collapse of the tube. Qualitatively, the observed changes in the electronic properties can be understood as arising from shifts of the Fermi point away from the Brillouin zone vertices when strain is involved, perturbing the low-energy states and causing shifting, merging, or splitting of the van Hove singularities.47 As the Fermi point kF is driven by the collapse and by the twist deformation to move away from the K-point of the Brillouin zone, between different allowed onedimensional electronic states, the first van Hove singularities positions change accordingly, moving further apart from each other with respect to the undeformed case, giving rise to the band gap we record. These experimental results are consistent with earlier predictions according to which twisting a nanotube will strongly perturb their electronic band structure11 and that twisting above a certain critical angle leads to a collapsed structure, similar to a twisted ribbon, which is also a semiconductor.2 Although an electronic band gap could also open on the curved bulbs at the edges due to finite curvature-induced shift of the Fermi wave vector,3,48 the finite DOS we observe could Nano Lett., Vol. 8, No. 10, 2008
be an indication of more complex interaction due to the presence of the inner tube coupled with effects due to increased curvature for sufficiently small diameters predicted to cause mixing of the bonding π/σ and antibonding π*/σ* orbitals49 which can dramatically change the band structure. The interlayer interaction between the inner and outer layer, and the contribution of the inner layer to the global electronic structure of the DWNT, cannot be neglected as shown by recent STM studies 30,40 and by several previous theoretical calculations46 revealing strong perturbations of the band structure due to the coupling between the two layers. Furthermore, the band structure of the inner layer seems to be more sensitive to the radial squashing than that of the outer layer, as shown in the case of a semiconductorsemiconductor DWNT, for which the inner tube undergoes a semiconductor to metal transition upon radial collapse, whereas the outer tube remains semiconducting.50 The contribution of the inner layer of the twisted and collapsed DWNT is thus expected to bring more changes to the overall band structure, due to the twist and collapsed deformation combined with strong interlayer interaction, likely to be triggered by the inner layer trying to achieve a better commensurability with the outer layer in an attempt to minimize the total energy of the system. We believe this case deserves further attention in detailed electronic band structure calculations in order to fully elucidate the driving force for the large deformations of the nanotubes and the impact these deformations have on their electronic structure. Due to the impact that these deformations have on the electronic structure, the susceptibility of the nanotubes to collapse presents relevance for their future integration into electronic and micro/nanoelectro mechanical devices. Conclusions. Scanning tunneling microscopy and spectroscopy have been used to characterize the atomic and the electronic structure of a twisted and collapsed DWNT. Atomically resolved images allow for the helicity of the outer constituent tube to be determined, which enables the most energetically favored inner tube helicity to be inferred based on a helicity-dependent potential specially parametrized for layered carbon structures. The lattice registry effects observed confirm earlier predictions that the helicity influences the mode of collapse, showing excellent agreement with the predicted twisted-collapsed mode. Tunneling spectroscopy measurements made simultaneously on the atomically resolved nanotube exhibit metallic behavior that changes to semiconducting behavior in response to the twist and collapse deformation. While changes in the conductivity near the Fermi energy are expected to hinder the electronic transport properties of these systems, the metal to semiconductor transition could find practical applications for novel nanoscale applications, such as nanoswitches or NEMS devices. Acknowledgment. The authors acknowledge the financial support received via the EPSRC Portfolio Partnership award which enabled this study. 3355
References (1) Rochefort, A.; Salahub, D. R.; Avouris, P. Chem. Phys. Lett. 1998, 297 (1-2), 45–50. (2) Rochefort, A.; Avouris, P.; Lesage, F.; Salahub, D. R. Phys. ReV. B 1999, 60 (19), 13824–13830. (3) Odom, T. W.; Huang, J. L.; Kim, P.; Lieber, C. M. J. Phys. Chem. B 2000, 104 (13), 2794–2809. (4) Avouris, Ph.; Martel, R.; Ikeda, H.; Hersam, M.; Shea, H. R.; Rochefort, A. Science and Applications of Nanotubes; Tomanek D., Enbody, R. J., Eds.; Kluwer Academic/Plenum Publishers: New York, 2000. (5) Tapaszto, L.; Nemes-Incze, P.; Osvath, Z.; Darabont, A.; Lambin, P.; Biro, L. P. Phys. ReV. B 2006, 74, 235422-1–235422-6. (6) Ouyang, M.; Huang, J.-L.; Lieber, C. M. Phys. ReV. Lett. 2002, 88, 066804-1–066804-4. (7) Ouyang, M.; Huang, J.-L.; Cheung, C. L.; Lieber, C. M. Acc. Chem. Res. 2002, 35, 1018–1025. (8) Venema, L. C.; Janssen, J. W.; Buitelaar, M. R.; Wildoer, J. W. G.; Lemay, S. G.; Kouwenhoven, L. P.; Dekker, C. Phys. ReV. B 2000, 62 (8), 5238–5244. (9) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Nature 1999, 402 (6759), 273–276. (10) Ouyang, M.; Huang, J.-L.; Cheung, C. L.; Lieber, C. M. Science 2001, 291, 97–100. (11) Kane, C. L.; Mele, E. J. Phys. ReV. Lett. 1997, 78 (10), 1932–1935. (12) Clauss, W.; Bergeron, D. J.; Johnson, A. T. Phys. ReV. B 1998, 58 (8), R4266-R4269. (13) Chopra, N. G.; Benedict, L. X.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Nature 1995, 377 (6545), 135–138. (14) Benedict, L. X.; Chopra, N. G.; Cohen, M. L.; Zettl, A.; Louie, S. G.; Crespi, V. H. Chem. Phys. Lett. 1998, 286 (5-6), 490–496. (15) Yu, M. F.; Dyer, M. J.; Chen, J.; Qian, D.; Liu, W. K.; Ruoff, R. S. Phys. ReV. B 2001, 64, 241403-1–241403-4. (16) Hertel, T.; Walkup, R. E.; Avouris, P. Phys. ReV. B 1998, 58 (20), 13870–13873. (17) Yu, M. F.; Kowalewski, T.; Ruoff, R. S. Phys. ReV. Lett. 2001, 86 (1), 87–90. (18) Giusca, C. E.; Tison, Y.; Silva, S. R. P. Phys. ReV. B 2007, 76, 0354291–035429-6. (19) Gao, G. H.; Cagin, T.; Goddard, W. A. Nanotechnology 1998, 9 (3), 184–191. (20) Liu, B.; Yu, M. F.; Huang, Y. G. Phys. ReV. B 2004, 70, 161402-1– 161402-4. (21) Xiao, J.; Liu, B.; Huang, Y.; Zuo, J.; Hwang, K. C.; Yu, M. F. Nanotechnology 2007, 18, 395703-1–395703-7. (22) Park, C. J.; Kim, Y. H.; Chang, K. J. Phys. ReV. B 1999, 60 (15), 10656–10659. (23) Lu, J. Q.; Wu, J.; Duan, W. H.; Liu, F.; Zhu, B. F.; Gu, B. L. Phys. ReV. Lett. 2003, 90, 156601-1–156601-4. (24) Lammert, P. E.; Zhang, P. H.; Crespi, V. H. Phys. ReV. Lett. 2000, 84 (11), 2453–2456.
3356
(25) Mehrez, H.; Svizhenko, A.; Anantram, M. P.; Elstner, M.; Frauenheim, T. Phys. ReV. B 2005, 71, 155421-1–155421-7. (26) Tombler, T. W.; Zhou, C. W.; Alexseyev, L.; Kong, J.; Dai, H. J.; Lei, L.; Jayanthi, C. S.; Tang, M. J.; Wu, S. Y. Nature 2000, 405 (6788), 769–772. (27) Gomez-Navarro, C.; Saenz, J. J.; Gomez-Herrero, J. Phys. ReV. Lett. 2006, 96, 076803-1–076803-4. (28) Srivastava, D.; Brenner, D. W.; Schall, J. D.; Ausman, K. D.; Yu, M. F.; Ruoff, R. S. J. Phys. Chem. B 1999, 103 (21), 4330–4337. (29) Park, S.; Srivastava, D.; Cho, K. Nano Lett. 2003, 3 (9), 1273–1277. (30) Giusca, C. E.; Tison, Y.; Stolojan, V.; Borowiak-Palen, E.; Silva, S. R. P. Nano Lett. 2007, 7 (5), 1232–1239. (31) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291 (1-2), 39–46. (32) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78, 0137051–013705-8. (33) Yakobson, B. I.; Brabec, C. J.; Bernholc, J. Phys. ReV. Lett. 1996, 76 (14), 2511–2514. (34) Qian, D.; Wagner, G. J.; Liu, W. K.; Yu, M. F.; Ruoff, R. S. Appl. Mech. ReV. 2002, 55 (6), 495–533. (35) Xiao, J. L.; Liu, B.; Huang, Y. G.; Hwang, K. Z.; Yu, M. F. Trans. Nonferrous Met. Soc. China 2006, 16, S776–S779. (36) Brenner, D. W.; Shenderova, L. A.; Harrison, J. A.; Stuart, S. J.; Ni, B.; Sinnott, S. B. J. Phys.: Condens. Matter 2002, 14, 783–802. (37) Lambin, P.; Mark, G. I.; Meunier, V.; Biro, L. P. Int. J. Quantum Chem. 2003, 95 (4-5), 493–503. (38) Venema, L. C.; Meunier, V.; Lambin, P.; Dekker, C. Phys. ReV. B 2000, 61 (4), 2991–2996. (39) Meunier, V.; Lambin, P. Phys. ReV. Lett. 1998, 81 (25), 5588–5591. (40) Tison, Y.; Giusca, C. E.; Stolojan, V.; Hayashi, Y.; Silva, S. R. P. AdV. Mater. 2008, 20 (1), 189–194. (41) Guo, W.; Guo, Y. J. Am. Chem. Soc. 2007, 129 (10), 2730–2731. (42) Kolmogorov, A. N.; Crespi, V. H.; Schleier-Smith, M. H.; Ellenbogen, J. C. Phys. ReV. Lett. 2004, 92, 085503-1–085503-4. (43) Stroscio, J. A.; Feenstra, R. M.; Fein, A. P. Phys. ReV. Lett. 1986, 57 (20), 2579–2582. (44) Kociak, M.; Suenaga, K.; Hirahara, K.; Saito, Y.; Nakahira, T.; Iijima, S. Phys. ReV. Lett. 2002, 89, 155501-1–155501-4. (45) Zo´lyomi, V.; Rusznya´k, A.; Ku¨rti, J.; et al. Phys. Status Solidi B 2006, 243, 3476–3479. (46) Song, W.; Ni, M.; Lu, J.; et al. Chem. Phys. Lett. 2005, 414 (4-6), 429–433. (47) Yang, L.; Han, J. Phys. ReV. Lett. 2000, 85 (1), 154–157. (48) Ouyang, M.; Huang, J. L.; Cheung, C. L.; Lieber, C. M. Science 2001, 292, 702–705. (49) Blase, X.; Benedict, L. X.; Shirley, E. L.; Louie, S. G. Phys. ReV. Lett. 1994, 72, 1878–1881. (50) Yang, X.; Dong, J. Phys. Lett. A 2004, 330 (3-4), 238–244.
NL801782K
Nano Lett., Vol. 8, No. 10, 2008
