Field-Effect Transistors Based on WS2 Nanotubes with High Current

Letter pubs.acs.org/NanoLett

Field-Effect Transistors Based on WS2 Nanotubes with High CurrentCarrying Capacity Roi Levi,† Ora Bitton,‡ Gregory Leitus,‡ Reshef Tenne,† and Ernesto Joselevich*,† †

Department of Materials and Interfaces and ‡Chemical Research Support , Weizmann Institute of Science, Rehovot, Israel S Supporting Information *

ABSTRACT: We report the first transistor based on inorganic nanotubes exhibiting mobility values of up to 50 cm2 V−1 s−1 for an individual WS2 nanotube. The current-carrying capacity of these nanotubes was surprisingly high with respect to other low-dimensional materials, with current density at least 2.4 × 108 A cm−2. These results demonstrate that inorganic nanotubes are promising building blocks for high-performance electronic applications.

KEYWORDS: Inorganic nanotubes, transistor, WS2, tungsten disulfide, conductivity

T

he discovery of carbon and subsequently inorganic nanotubes1,2 (CNTs and INTs, respectively) prompted a large interest in their structural, mechanical, and electronic properties. CNTs, in particular proved especially alluring due to the ease of their production using multiple methods3 and their ability to form either metallic or semiconducting NTs depending on their chirality and diameter. However, lack of control over the synthesized CNT properties, in particular, the mixture of metallic and semiconducting CNTs in the product,4−6 is still a daunting problem on their way to become a mainstay of the microelectronic and device industries. More recently, increased interest was pointed to other carbon-based nanostructures such as graphene.7 The zero band gap of graphene7 led researchers to study graphene nanoribbons,8−10 which do possess the needed band gap for electronic applications. However, fabricating such nanoribbons with a well-specified bandgap turned out to be very challenging, if not impossible, by the current technology.11 The recent demonstration of high field-effect mobility in single-layer MoS212 has resparked interest in inorganic nanostructures from layered compounds, in general, and single layers, in particular. Similar studies on INTs incorporated into functional devices are scarce, suggesting that this area warrants further study. WS2 nanotubes (NTs) have garnered a respectable degree of interest for various applications, in no small part due to the fact that recent developments have resulted in their semi-industrial production.13,14 Additionally, WS2 NTs are particularly appealing due to their practically defect-free structural properties resulting in excellent mechanical properties.15 The scarcity of defects in conjunction with their layered structure (Figure 2a) renders them resistant to large compressive forces16 and pressure shockwaves.17 Furthermore, preliminary studies indicate that WS2 NTs are nontoxic and biologically benign.18 INTs, in general, largely derive their electronic properties from the bulk material, thus offering a range of tailorable properties such as superconductive,19 semiconducting,20 and © 2013 American Chemical Society

Figure 1. Description of the WS2 nanotube-based field-effect transistors (NT-FETs). (a) Schematics of a two-probe WS2 NTFET showing the source, drain and back-gate electrodes. (b) Scanning electron microscope (SEM) image of panel a. (c) Schematics of a fourprobe WS2 NT-FET showing the inner pair of electrodes (sense) used to read the voltage drop in the inner WS2 NT section, while the outer pair of electrodes (force) force the voltage and read the current. (d) Atomic force microscope (AFM) image of panel c.

insulating21 behaviors. WS2 NTs, in accordance with the bulk properties, have a well-defined band gap and are hence exclusively semiconductors,22−24 albeit with indirect or direct transition depending on the chirality of the NTs.23 This attribute is, in fact, beneficial, allowing for the classical doping of the INTs with several types of dopant atoms reported Received: May 8, 2013 Revised: July 24, 2013 Published: July 30, 2013 3736

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Figure 2. Atomic structure of the WS2 nanotubes. (a) Perspective view of 2H-WS2 crystal structure.35 (The hexagonal unit cell dimensions are a = b = 3.154 Å and c = 6.18 Å.) (b) Transmission electron microscope (TEM) image of a multiwall WS2 NT showing the inner and outer diameters. (c) TEM image of the WS2 NT cap shown in panel b.

recently.25 Finally, the field of INT synthesis is experiencing a recent renaissance with the synthesis of numerous new types of NTs.26,27 Thus, INTs offer many intriguing possibilities as potential building blocks for functional devices. Despite the alluring mechanical and structural properties previously mentioned, very little work is available on the electronic properties of INTs, practically all consisting of simple two-contact I−V measurements of WS2 NTs. Previous attempts to fabricate field-effect transistors were met with limited success, having found practically insulating WS2 NT behavior,28 negligible field-effect mobility29,30 of μ = 1.3 × 10−5 to 4.1 × 10−4 cm2 V−1 s−1, and a complete lack of gate response for MoS2 NTs.31 Much higher mobilities (up to 950 cm2 V−1 s−1) have been predicted based on fitting I−V curves to theoretical models of metal−semiconductor−metal junctions32,33 but not from the actual response of WS2 NTs to field-effect gating. Thus, the most basic electronic building block based on INTs, the field-effect INT transistor, has not been demonstrated to date. Here we report the first FET based on INTs displaying a significant field-effect response. The transistor configuration is schematically described in Figure 1a,c. WS2 NTs (NanoMaterials) were dispersed from powder using previously reported procedures34 on the substrates. WS2 NT-FET devices with either end or side contacts (Figure 1a,c, respectively) were fabricated on the Si/SiO2 (oxide thickness of 200 nm) wafers. The end contacts (Figure 1b) were fabricated by standard photolithography techniques, while the side contacts (Figure 1d) were fabricated by electron-beam lithography. The lithography was followed by Au evaporation (100 nm) and lift-off. The device dimensions and external diameter of the WS2 NTs were measured by atomic force microscopy (AFM), as can be seen in Figure 1d. In total, 53 two-probe end-contact and 13 four-probe side-contact functional devices were fabricated and characterized. The external diameters of the WS2 NTs ranged 50−200 nm, and the device lengths ranged 500−3500 nm. The internal diameter of multiple WS2 NTs was measured by transmission electron microscopy (TEM) and was generally found to be around 1/2 of the external diameter (Figure 2b). The electrical measurements were performed with a Keithley SCS 4200 setup connected to a Janis Probe station, allowing for cryogenic temperatures down to 4 K and a vacuum of up to 1 × 10−5 Torr. The highly doped silicon substrate was used as the back-gate for the WS2 NT-FET channel (Figure 1a,c). To rule out gate leakage currents, we monitored the gate current, as can be seen in Figure 1a, while substrate effects on the results were ruled out by measuring devices with no WS2 NTs connecting the electrodes.

Most WS2 NT FETs exhibit n-type behavior, as shown in Figure 3a,b. A few devices were found to exhibit p-type

Figure 3. Performance of the WS2 NT FETs. Back-gate response of a typical four-probe, side-contacted WS2 NT-FET measured in a vacuum of 1 × 10−3 Torr (VG = 50 to −50 V). The device dimensions are DWS2NT,Out/LWS2NT = 75 nm/900 nm. (a) ISD−VSD curves for different values of gate voltage. (b) Gate response (ISD−VG) of the WS2 NT-FET shown in (a) at VSD = +5 V, exhibiting a steep rise with VG far from saturation.

behavior with very small mobilities (below 0.1 cm2 V−1 s−1). Using the gating response with decreasing VG in Figure 3a and 2 the expressions dI SD /dV G = μ(C/L INT )·V SD and C = (LINT2πεrε0)/ln(2h/rINT)36 it is possible to extract the fieldeffect mobility of the WS2 NTs. Correcting for the contact resistance gives a value of μ = 49 ± 4 cm2 V−1 s−1 for the NT presented in Figure 3. Whereas some hysteresis with respect to the VG sweeping direction was observed, the influence on the extracted field-effect mobility was not constant in direction or magnitude. Thus, other devices were unaffected or even exhibited an increased mobility reaching μ = 30 ± 5 cm2 V−1 s−1 for increasing VG. As no previous works on INT-FET are available it is possible to compare these values to the hall mobility of bulk WS2, μH = 100 cm2 V−1 s−1.37 Moreover, because the device does not reach saturation with respect to VG (Figure 3b), the measured field-effect mobility of the WS2 NTFETs is only a lower estimate. The four-probe measurements revealed that the Au contacts exhibited a high contact resistance, up to two orders of magnitude larger than the devices themselves. This is consistent with the variation in the measured device characteristics. Thus, the results suggest that improvement of the contacts should significantly enhance the device performance. Further investigation of the WS2 NT-FET properties revealed surprisingly high current-carrying capacity of up to 630 μA for a single WS2 NT of 135 nm diameter, as shown in Figure 4. In general, the current-carrying capacity of different 3737

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a lower estimate. Alternatively, the maximum values of the differential conductivity,40−42 σdiff = (L/A) dISD/dVSD or σdiff,max, may be used if dISD/dVSD reaches saturation with respect to VSD. The fact that no saturation of dISD/dVSD was observed with respect to VSD suggests that the value of σdiff,max is only a lower estimate as well. Thus, the ultimate currentcarrying capacity should be even higher than the values reported here. The differential conductivity values of the WS2 NT-FETs, despite being an underestimate, are nevertheless very large when compared to other materials. Using the entire cross section of the WS2 NT-FETs to calculate the differential conductivity gives up to σdiff = 3 × 103 Ω−1 cm−1. When compared with similar multilayer-WS2 devices43 with up to σdiff = 4.8 × 10−3 Ω−1 cm−1 (Figure 5), an increase of six orders of magnitude is observed here. However, in the case of layered materials, it is not immediately obvious that the entire cross section participates in the current transport. The weak van der Waals (vdW) interactions responsible for stacking the layers generally result in larger intra- versus interlayer conductivity. For WS2, the intralayer conductivity can be up to three orders of magnitude larger than the interlayer conductivity.35,44 This suggests that only a limited number of layers participate in the current transport. Thus, assuming various possible conducting cross sections results in very high differential conductivity of up to σdiff = 1.2 × 105 Ω−1 cm−1. While these values would seem surprisingly high, similar results have been recently observed for single-layer MoS2 devices39 (Figure 5). Therefore the differential conductivities of the highest current-carrying materials such as the carbon-based graphene45 and multiwall and singlewall NTs46,47 (MWCNT and SWCNT, respectively) (Figure 5) are not significantly higher than WS2 NT-FETs and inorganic materials in general.

Figure 4. WS2 NT-FET with highest current so far. The red curve traces increasing VSD and the blue curve retraces decreasing VSD. The maximum current observed for this device is ISD,max = 630 μA at VSD = 1.6 V, with device electrical breakdown at VSD = 1.8 V (VBG = 0 V). The two-probe, end-contact device is shown in Figure 1b, and the device dimensions are DWS2NT,Out/LWS2NT = 135 nm/2.55 μm.

materials and devices is compared using conductivity38 and current density,39 defined as σ = (ISDL)/(VSDA) and J = ISD/A, respectively, where A is the conducting cross section and L is the length of the device. (For the cross-section of single-wall NTs or single layer sheets, the effective thickness of the single layer is assumed to be the interlayer distance in the bulk material.) The maximum value for the conductivity is generally obtained under the condition that the current reaches saturation with respect to the source-drain voltage (VSD). Because of the extremely high currents initially observed, as in Figure 4, VSD and ISD were limited to ±5 V and 200 μA in subsequent measurements. Therefore, no current saturation with respect to VSD was observed up to the self-imposed limits in this work, and our reported current-carrying capacity is only

Figure 5. Current-carrying capacity comparison between WS2 NTs and other materials. (a) Differential conductivity (σdiff) and (b) maximum current density (Jmax) of several WS2 NT-FETs measured for two end-contacts in air (2EC Air), two end-contacts in vacuum (2EC Vac.), and four side-contacts in vacuum (4SC Vac.). The results are compared with values of devices using WS2 multilayers48 (2EC Air), MoS2 multilayers49 (2EC Air), MoS2 single layers12 (2EC Air), graphene45 (4SC in an atmosphere of Helium; 76 K) and single- and multiwall carbon nanotubes46,47 (2EC Vac. and 2SC Air, respectively). The values are extracted from the reported results of the highest current-carrying devices of each material. In the case of devices with multiple layers (MWCNT, WS2 NT, WS2 multilayers, and MoS2 multilayers), the σdiff and Jmax were calculated for several possible conducting cross sections, represented by the effective percentage of layers carrying the overall current. 3738

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intercalate between the layers of layered materials, and the presence of ambient moisture was shown to result in substantial lattice expansion along the c axis of inorganic fullerene-like WS2 nanoparticles and platelets. As moisture moieties can act as scattering centers or generate trap-states it is clear that the vacuum environment may be a significant factor in the observed WS2 NT-FET high current-carrying capacity. Regarding the WS2 NTs themselves, recent high-resolution TEM studies of WS2 NTs56 and inorganic fullerene-like nanoparticles57 reveal patches of the 1T phase, a generally unstable metallic phase of WS2.58 This suggests that the WS2 NTs could, in principle, be partially composed of a metallic phase, thus accounting for the reported results. However, the WS2 NT-FETs exhibit a decrease in conductivity with temperature (Figure 6), as typical for a semiconductor.

Comparing the results for end- versus side-contacted WS2 NT-FETs suggests that the end-contacts engage the entire WS2 NT cross section in the current transport (Figure 5), while the side-contacts connect a small number of outer layers. However, the WS2 NTs used here are frequently capped (Figure 2c) on one or both ends, and thus it is not clear whether the end contacts are connected to all of the layers.50 Recent work on the mechanical properties of the same WS2 NTs under uniaxial tension exhibited simultaneous rupture of up to three layers.51 This indicates that the cap structure may cause cross-linking of several layers, thereby creating enhanced interlayer electronic coupling. Another possibility is that because of the application of voltage the electronic coupling between the layers changes fundamentally. A recent theoretical study on bilayer transitionmetal dichalcogenides suggests that the application of a voltage may induce shrinkage of the band gap and eventually results in nanotubes with metallic characteristics.52 Considering the extreme cases, the end-contact devices conducting through the entire cross section and the side-contact devices conducting only through the outer layer, the values of the differential conductivity differ by an order of magnitude. This difference suggests that an end-contact and a side-contact to a single layer would be significantly different. This may be attributed to the dangling bonds at the layer edge forming an enhanced coupling with the contacts. Therefore, the above observations suggest some combination of effects with conduction taking place not through the entire cross section but only several layers. The surprisingly high current-carrying capacity of WS2 NTs compared with other materials as well as to previous reports on WS2 NTs may be attributed to several effects, including the device contacts, the measurement environment, and the WS2 NTs themselves: Regarding the contacts, the material used here (Au only) has exhibited Ohmic behavior in single-layer MoS2 devices.12 Density-functional tight-binding (DFTB) simulations of AuMoS2 nanowire contacts predicted a direct contact between the Au and the metallic molybdenum backbone of the MoS2 nanowire.53 The high contact resistance reported here is supported by other DFTB simulations,54 suggesting that Ti might be a better contact material. In addition, NTs are frequently dispersed from solution onto the substrate, unlike the dry dispersion used here.34 The absence of solvent residues may remove surface trap states and increase the INT−contact electronic coupling, resulting in an improved INT−contact interface. These points suggest that the contact−INT interface played a major role in the relatively low mobility and conductivity values previously reported. A first approximation of the contact−INT interface in the devices reported here may be made by considering a recent work on Schottky-barrier solar cell based on WS2 layers.55 The WS2 layers were synthesized in situ with Au contacts deposited directly on the layers, and the Schottky barrier was determined from the Mott−Schottky plot, yielding a value of ∼0.5 eV. The similarities in the device structure and fabrication suggest a similar electronic behavior and emphasize the importance of optimizing the contact fabrication. Regarding the environment, a clear difference is observed between the differential conductivity values of the WS2 NTFETs in vacuum (σdiff = 1.2 × 105 Ω−1 cm−1) and ambient (σdiff = 1.6 × 102 Ω−1 cm−1) environments (Figure 5). This is supported by I−V measurements of WS2 NTs done in ambient and vacuum.33 It is well known that various species can

Figure 6. Temperature dependence of σdiff as a function of temperature showing a decrease in σdiff with decreasing temperature, typical for semiconductor materials. The inset shows the dependence of ln(σdiff) on the reciprocal of the temperature. The slope of the extreme low end of the temperatures may be used to estimate the dopant activation energy (Ea = 0.2 meV).

Moreover, the very low temperature region can be used to extract the dopant activation energy59 according to the Arrhenius dependence ln σ ∝ −(Ea/2kb)·T−1, giving a value of 0.2 meV (inset of Figure 6). Comparing this activation energy to the thermal energy at 293 K of 25 meV suggests a very high free charge carrier concentration, which would be in line with the high current-carrying capacity. Calculating the free charge carrier concentration at room temperature using the Drude model59 and σ = enμ, where e is carrier charge, n is the free carrier concentration, and μ is the mobility, gives a value of 1019 cm−3. This value suggests that the WS2 NTs behave as a highly doped semiconductor at room temperature with the dopant level situated very closely to the conduction band. This raises the question of the dopant origin. Recent work on the chemical composition of WS2 NTs25 reveals no other elements, ruling out doping by foreign atoms. One possibility is sulfur vacancies formed during the annealing process.14 Another possibility is indicated by a study on contact−MoS 2 interfaces,60 suggesting that the contact materials might dope the WS2 NTs. This doping at the interface may also allow for enhanced band alignment61 and subsequently reduced contact resistance. Another possibility, albeit one that might be challenging to confirm experimentally, is that the inner shells might exhibit different properties due to multilayer interactions, such as 3739

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(9) Yang, X.; Dou, X.; Rouhanipour, A.; Zhi, L.; Räder, H. J.; Müllen, K. J. Am. Chem. Soc. 2008, 130, 4216−4217. (10) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Nature 2009, 458, 872−876. (11) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; Mullen, K.; Fasel, R. Nature 2010, 466, 470−473. (12) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nat. Nanotechnol. 2011, 6, 147−150. (13) Zak, A.; Sallacan-Ecker, L.; Margolin, A.; Genut, M.; Tenne, R. Nano 2009, 4, 91−98. (14) Zak, A.; Ecker, L. S.; Efrati, R.; Drangai, L.; Fleischer, N.; Tenne, R. Sens. Transducers J. 2011, 12. (15) Kaplan-Ashiri, I.; Cohen, S. R.; Gartsman, K.; Ivanovskaya, V.; Heine, T.; Seifert, G.; Wiesel, I.; Wagner, H. D.; Tenne, R. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 523−528. (16) Kalfon-Cohen, E.; Goldbart, O.; Schreiber, R.; Cohen, S. R.; Barlam, D.; Lorenz, T.; Joswig, J.-O.; Seifert, G. Appl. Phys. Lett. 2011, 98, 081908-1−081908-3. (17) Zhu, Y. Q.; Sekine, T.; Li, Y. H.; Fay, M. W.; Zhao, Y. M.; Patrick Poa, C. H.; Wang, W. X.; Roe, M. J.; Brown, P. D.; Fleischer, N.; Tenne, R. J. Am. Chem. Soc. 2005, 127, 16263−16272. (18) Goldman, E. B.; Zak, A.; Tenne, R.; Kartvelishvily, E.; Neumann, Y.; Palmon, A.; Hovav, A.-H.; Aframian, D. J. 2013, submitted. (19) Nath, M.; Kar, S.; Raychaudhuri, A. K.; Rao, C. N. R. Chem. Phys. Lett. 2003, 368, 690−695. (20) Seifert, G.; Terrones, H.; Terrones, M.; Jungnickel, G.; Frauenheim, T. Phys. Rev. Lett. 2000, 85, 146. (21) Jhi, S.-H.; Roundy, D. J.; Louie, S. G.; Cohen, M. L. Solid State Commun. 2005, 134, 397−402. (22) Frey, G. L.; Elani, S.; Homyonfer, M.; Feldman, Y.; Tenne, R. Phys. Rev. B 1998, 57, 6666. (23) Seifert, G.; Terrones, H.; Terrones, M.; Jungnickel, G.; Frauenheim, T. Solid State Commun. 2000, 114, 245−248. (24) Bar Sadan, M.; Heidelmann, M.; Houben, L.; Tenne, R. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 343−348. (25) Yadgarov, L.; Rosentsveig, R.; Leitus, G.; Albu-Yaron, A.; Moshkovich, A.; Perfilyev, V.; Vasic, R.; Frenkel, A. I.; Enyashin, A. N.; Seifert, G.; Rapoport, L.; Tenne, R. Angew. Chem., Int. Ed. 2012, 51, 1148−1151. (26) Radovsky, G.; Popovitz-Biro, R.; Staiger, M.; Gartsman, K.; Thomsen, C.; Lorenz, T.; Seifert, G.; Tenne, R. Angew. Chem., Int. Ed. 2011, 50, 12316−12320. (27) Brontvein, O.; Stroppa, D. G.; Popovitz-Biro, R.; Albu-Yaron, A.; Levy, M.; Feuerman, D.; Houben, L.; Tenne, R.; Gordon, J. M. J. Am. Chem. Soc. 2012, 134, 16379−16386. (28) Johansson, A.; Sambandamurthy, G.; Shahar, D.; Jacobson, N.; Tenne, R. Phys. Rev. Lett. 2005, 95, 116805. (29) Yang, Y.; Unalan, H. E.; Hiralal, P.; Chremmou, K.; The, A.; Alexandrou, I.; Tenne, R.; Amaratunga, G. Phototransistors Utilizing Individual WS2 Nanotubes. In NANO ’08. 8th IEEE Conference, 18−21 Aug, 2008; pp 85−87. (30) Unalan, H. E.; Yang, Y.; Zhang, Y.; Hiralal, P.; Kuo, D.; Dalal, S.; Butler, T.; Seung-Nam, C.; Jae Eun, J.; Chremmou, K.; Lentaris, G.; Wei, D.; Rosentsveig, R.; Suzuki, K.; Matsumoto, H.; Minagawa, M.; Hayashi, Y.; Chhowalla, M.; Tanioka, A.; Milne, W. I.; Tenne, R.; Amaratunga, G. IEEE Trans. Electron Devices 2008, 55, 2988−3000. (31) Remskar, M.; Mrzel, A.; Virsek, M.; Godec, M.; Krause, M.; Kolitsch, A.; Singh, A.; Seabaugh, A. Nanoscale Res. Lett. 2011, 6, 26. (32) Zhang, C.; Wang, S.; Yang, L.; Liu, Y.; Xu, T.; Ning, Z.; Zak, A.; Zhang, Z.; Tenne, R.; Chen, Q. Appl. Phys. Lett. 2012, 100, 243101. (33) Zhang, C.; Ning, Z.; Liu, Y.; Xu, T.; Guo, Y.; Zak, A.; Zhang, Z.; Wang, S.; Tenne, R.; Chen, Q. Appl. Phys. Lett. 2012, 101, 113112. (34) Tevet, O.; Goldbart, O.; Cohen, S. R.; Rosentsveig, R.; Popovitz-Biro, R.; Wagner, H. D.; Tenne, R. Nanotechnology 2010, 21, 365705. (35) Wilson, J. A.; Yoffe, A. D. Adv. Phys. 1969, 18, 193−335.

possessing a significantly different density of states. This indeed would be overlooked due to the limits of simulations62 and previous measurements probing mainly the outer shell.24,37 Thus, elucidating the nature of the dopants and the origin of the high current-carrying capacity in WS2 NTs seems to require further experimental and theoretical study beyond the scope of this Letter. In summary, we have demonstrated the first field-effect transistors based on INTs with a significant mobility. Transistors based on WS2 NTs showed a field-effect mobility up to 50 cm2 V−1 s−1, free charge carrier density of 1019 cm−3, and unexpectedly high current-carrying capacity of over 0.6 mA for an individual NT. Comparison of side- versus end-contacted WS2 NT-FETs suggests that in contrast with multiwall CNTs, where most of the current is carried by its outer wall,63−65 in WS2 NTs several layers participate in the current transport. Temperature-dependent electrical measurements of WS2 NTFETs confirm that WS2 NTs are indeed semiconducting. These results suggest that these INTs are either heavily doped or affected by a strong interlayer coupling. The significant mobility together with the high current-carrying capacity demonstrated here for individual WS2 NTs suggest that INTs may be promising components for high-power nanoelectronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

Electrical characterization of additional WS2 NT FETs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Maya Bar-Sadan and Dr. Lothar Houben for the helpful discussions. We are grateful for the support of the following agencies: the Israel Science Foundation, the Harold Perlman Foundation, the Carolito Stiftung, and the Irving and Azelle Waltcher Foundations. R.T. is the director of the Helen and Martin Kimmel Center for Nanoscale Science and the Drake Family chair in nanotechnology.

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REFERENCES

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dx.doi.org/10.1021/nl401675k | Nano Lett. 2013, 13, 3736−3741