Organic

NANO LETTERS

Broad Spectral Response Using Carbon Nanotube/Organic Semiconductor/C60 Photodetectors

2009 Vol. 9, No. 9 3354-3358

Michael S. Arnold,†,‡,§ Jeramy D. Zimmerman,†,§ Christopher K. Renshaw,† Xin Xu,†,| Richard R. Lunt,†,⊥ Christine M. Austin,† and Stephen R. Forrest*,† Departments of Physics, Electrical Engineering and Computer Science, and Materials Science and Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Departments of Electrical Engineering and Chemical Engineering, Princeton UniVersity, Princeton, New Jersey 08544 Received May 24, 2009; Revised Manuscript Received July 21, 2009

ABSTRACT We demonstrate that photogenerated excitons in semiconducting carbon nanotubes (CNTs) can be efficiently dissociated by forming a planar heterojunction between CNTs wrapped in semiconducting polymers and the electon acceptor, C60. Illumination of the CNTs at their nearinfrared optical band gap results in the generation of a short-circuit photocurrent with peak external and internal quantum efficiencies of 2.3% and 44%, respectively. Using soft CNT-hybrid materials systems combining semiconducting small molecules and polymers, we have fabricated broad-band photodetectors with a specific detectivity >1010 cm Hz1/2 W1- from λ ) 400 to 1450 nm and a response time of τ ) 7.2 ( 0.2 ns.

Carbon nanotubes have been the focus of intense investigation for nearly two decades due to their high tensile strength and resiliency and their potential for use in a wide range of photonic and electronic applications.1 For example, the electron mobility in semiconducting carbon nanotubes exceeds 105 cm2 V-1 s-1, opening up possibilities for a new class of high performance electronic devices.2 Furthermore, the spectral response of heterogeneous populations of carbon nanotubes can be tailored to extend from the visible to well into the infrared by exploiting the polydispersity in their diameters.1,3 Yet, to this time, the demonstration of practical photodetector and photovoltaic devices that take advantage of the broad-band absorption of CNTs have been limited. Indeed, most photodetectors reported thus far have been based on single or a few semiconducting nanotubes used in transistors or unipolar conducting structures.1,4 In this work, we demonstrate a device based on bulk films of CNTs that functions as an efficient detector with spectral bandwidth extending from the blue to the NIR. Previous efforts for extending the sensitivity of organic semiconductor photodetectors beyond λ)1000 nm have been * Corresponding author. E-mail: [email protected]. † University of Michigan. ‡ Current address: Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706. § These authors contributed equally to this work. | Department of Electrical Engineering, Princeton University. ⊥ Department of Chemical Engineering, Princeton University. 10.1021/nl901637u CCC: $40.75 Published on Web 07/28/2009

 2009 American Chemical Society

problematic;5-7 therefore, hybrid devices have been sought to cover this wavelength band while preserving the potential advantages of organic thin films, such as low cost and ability to fabricate devices on plastic or flexible foil substrates. To date, the most successful broad-band hybrid devices have consisted of polymers blended with inorganic nanocrystals composed of small band gap semiconductors such as PbS or PbSe.8,9 However, these blends exhibit only weak NIR photoresponse, for example, EQE ) 8 × 10-4% at λ ) 975 nm,10 to EQE ) 0.03% at 1240 nm,8 (although significantly better results have been achieved using PbS inorganic nanocrystals, alone.11) Furthermore, there have been few successful attempts to utilize bulk CNT films in practical photodetectors due to the strong binding of electron-hole pairs, or excitons, in the nanotubes.1,12 Thin films of percolating nanotube networks have exhibited photoconductivity.13,14 However, the photoconductivity has been attributed to thermal (i.e., bolometric) or other slow phenomena distinct from the photovoltaic effect, which is driven by the efficient and fast separation of photogenerated electrons and holes even without an externally applied bias. Additionally, CNTs have been explored as charge transport adducts to improve charge collection in solar cells, and it has been hypothesized that CNTs act as electron acceptors in semiconducting polymer blends.15-20 However, thus far, polymer/CNT blends have exhibited either zero or limited NIR sensitivity, despite the inclusion of the near-infrared absorbing CNTs.

Figure 1. Carbon nanotube/organic heterojunctions. (A) Schematic of device architectures without and with SnPc. (B) Film morphology. Top: scanning electron micrograph of doctor-bladed thin films of 1:1 MDMO-PPV:carbon nanotubes, by weight (scale ) 500 nm). Bottom: scanning electron micrograph of bare carbon nanotubes prepared by vacuum filtration (scale ) 500 nm). The presence of the MDMO-PPV results in more uniform surface morphology than do the bare CNT films (with a root-mean-square roughness of 4 nm compared with 20 nm for the bare films).

To fully exploit the potential of CNTs as broad-band absorbers in photodetectors, the CNTs must be isolated from each other, and paired with semiconductor layers that have the appropriate energetics for efficiently dissociating CNT excitons. Here, we demonstrate bulk CNT photodetectors with a wide spectral response and a short response time. We find that photogenerated excitons in CNTs are efficiently dissociated at interfaces formed between CNTs and thin films of C60. We also find that CNT excitons can be efficiently dissociated in simultaneous heterojunctions7 containing both the acceptor C60 and the donor SnPc, enabling further broad enhancement of the spectral response. Devices in Figure 1A are fabricated by combining both solution-processed polymer and vacuum-thermal-evaporated small molecular weight constituents. First, unsorted HiPCO carbon nanotubes varying in diameter from 0.7 to 1.1 nm are dispersed in chlorobenzene by wrapping21 with a semiconducting polymer, either poly[2-methoxy-5-(3′,7′dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) or poly[3-hexylthiophene] (P3HT). Polymer wrapping is done by ultrasonicating CNTs along with polymer, centrifuging, and decanting (see Supporting Information).21 The presence of isolated CNTs in solution is confirmed by strong photoluminescence, and films are subsequently applied by doctor blading the solution onto an indium tin oxide (ITO) substrate. This is followed by vacuum deposition of the small molecular weight material, C60, and in some cases SnPc (see Supporting Information). The primary role of the polymer wrapping is to impart solubility to the CNTs. The polymer also provides Nano Lett., Vol. 9, No. 9, 2009

isolation between the carbon nanotubes, thus minimizing the direct electronic coupling between the optically active semiconducting nanotubes and the metallic nanotubes present in the mixture (at approximately 30 wt %) that can quench the photogenerated excitons, or short the layer. Finally, the polymer ensures uniform film morphology in contrast to films consisting only of CNTs, as shown in the micrographs of Figure 1B. The polymer/CNTs and bare CNT films had a root-mean-square roughnesses of 4 and 20 nm, respectively, as measured by atomic force microscopy. The current-voltage characteristics and spectrally resolved photoresponse for a 1:1 MDMO-PPV:CNT/C60 heterojunction detector are depicted in Figure 2. The diodes have dark current rectification ratios >104 at (1 V, which is notable given that the polymer/CNT layer consists of a high density of metallic tubes whose presence would be expected to result in large shunt currents. The absence of such parasitic effects from the metal tubes suggests that they are, indeed, electrically and energetically isolated from the semiconducting tubes by the wrapped polymer. The continuous C60 layer also prevents metallic CNTs from directly bridging the anode to the cathode and shorting out the rectifying junction. A fit to the forward bias current-voltage characteristics (solid line) follows the ideal diode equation22 with an ideality factor of 2.0 and a specific series resistance of 0.99 Ω cm2; this low ideality factor suggests that carrier recombination is the dominant source of dark current, which is again evidence that the high density of metallic tubes are isolated by their polymer wrappers. The NIR responsivities of the diodes at 0 and -0.7 V are compared in Figure 2B. Due to the diametric heterogeneity of the nanotubes, photoactive response is observed over a wide range from both E11 (λ ≈ 900-1450 nm) and E22 (λ ≈ 550-900 nm) absorption features,3 with the peak polymer response at λ ) 500 nm. The observation of response from individual chiralities is again strong evidence that the majority of the CNTs are individually dispersed. The indices (n, m) of the nanotube responsible for each absorption feature are labeled in the figure, as are the absorption regions of the polymer and small molecule constituents of the device. The very broad spectral coverage results from the diametric polydispersity of the CNTs, acting as multiple individual parallel detectors which collectively cover the spectrum from 550 to 1600 nm. The detector responsivity at λ ) 1155 nm at a bias of 0 and -0.7 V was 12 and 23 mA/W, respectively, corresponding to a peak EQE ) 2.3%. At λ ) 1300 nm, the detector responsivity was 11 and 21 mA/W (EQE ) 2.0%), respectively, whereas the response of devices lacking CNTs at these wavelengths was not measurable (15 nm did not result in an increased EQE, suggesting an exciton diffusion length in the polymer-nanotube film of less than this distance. At a MDMO-PPV/CNT layer thickness of 14 nm, the IQE of the CNTs in the NIR (Figure 2C) was >25% between λ ) 1000 and λ ) 1400. The large IQE indicates highly efficient dissociation of photogenerated excitons in CNTs, and the subsequent collection of the resulting electron-hole pairs. CNT-based devices with substantially 3355

Figure 2. Nanotube/C60 heterojunction device characteristics. Device structure is as follows: ITO, 1:1 MDMO:PPV/CNTs (14 nm), C60 (100 nm), BCP (10 nm), Ag (100 nm). (A) Current density vs voltage characteristics. Solid line is a fit of the experimental data (circles) to the ideal diode equation, using an ideality factor of 2.0 and a specific series resistance of 0.99 Ω cm2. (B) Photoresponsivity at zero bias (dashed line) and -0.7 V (solid line). The responsivity is divided into three regions: λ < 550, λ ) 550-900, λ > 900 nm corresponding to absorption by the MDMO-PPV and C60, the E22 optical transitions of semiconducting CNTs, and the E11 optical transitions of semiconducting CNTs, respectively. The E22 and E11 transitions of the most common semiconducting HiPCO CNTs are labeled according to their (n, m) indices. (C) Internal quantum efficiency, IQE, (solid line) at 0.7 V and the fraction of incident light absorbed by CNTs (dashed line). Across the near-infrared, the IQE > 25% indicating the efficient dissociation and collection of photogenerated electron-hole pairs. (D) Temporal response of the detector at 0 V and λ ) 847 nm yields a RC-limited rise time of 1.84 ( 0.56 ns, indicating the upper limit for the time scale for exciton diffusion to the CNT/C60 interface, and a fall time of 7.16 ( 0.18 ns, which is due to the carrier extraction time.

higher EQE are therefore expected for structures where the total absorption in the thin films can be increased, such as has been demonstrated by using light trapping or broad spectral bandwidth resonant cavities.23,24 There are several possible loss mechanisms that may result in IQE < 100%. First, the polymer wrapping on the surface of the CNTs may act as a tunnel barrier that decreases the rate of electron transfer from the CNTs to the C60 molecules, thus resulting in exciton recombination prior to dissociation and electron transfer. Similarly, excess polymer embedded in the film might also act as an electron barrier. Additionally, electron back-transfer from the C60 to the CNTs, due to the small barrier across the interface can be followed by recombination. Finally, metallic CNTs or defects in semiconducting CNTs can result in the recombination of photogenerated electron-hole pairs. Furthermore, the detector responsivity at λ ) 450 nm at a bias of 0 and -0.7 V was 41 and 64 mA/W, respectively, corresponding to a peak EQE ) 18%, which is comparable to that for planar organic single heterojunction diodes. The high visible responsivity of the CNT-based diodes indicates that here the metallic CNTs are not efficient recombination sites. Temporal response of a 0.3 mm diameter device with the structure in Figure 2, was measured at 0 V and λ ) 847 nm using a laser pulse width of ∼1 ns (see Supporting Information), with the result in Figure 2D. The rise time, limited by the RC time constant of the diode and thus setting an upper limit for the time scale of exciton transport to the CNT/C60 interface, is τR ) 1.84 ( 0.56 ns, with a fall time of τF ) 3356

7.16 ( 0.18 ns due to charge drift to the contacts. The fall time corresponds to a 3 dB bandwidth of ∼31 MHz, making this detector suitable for a wide range of high-speed imaging applications. The responsivity in the E22 range (λ ≈ 550-900 nm) is considerably weaker than in the E11 range (λ ≈ 900-1450 nm), which is partially due to reduced absorptivity of the E22 peaks (Figure 3A). To increase the responsivity between λ ) 600 and 950 nm, the C60 was codeposited in a 1:3 (by volume, 100 Å total thickness) mixture with the NIR absorbing material, SnPc. Previously, it has been shown that SnPc can extend the spectral response across this wavelength region through a combination of monomer and aggregate absorption, with rapid and efficient exciton dissociation at the SnPc/C60 interface.25 The dark current of 1 µA/cm2 (with a corresponding rectification ratio of 105 at (1 V) of the P3HT-based device in Figure 3B is approximately 2 orders of magnitude lower than for the MDMO-PPV based detector shown in Figure 2. The decrease in dark current may be due to the approximately 50% reduction in CNT concentration in the former structure or due to differences in the orbital energies of MDMO-PPV and P3HT. A fit to the data gives a relatively high specific series resistance 2.45 Ω cm2 and an ideality factor of 1.34, which indicates that the device performs as a nearly ideal Shockley diode, minimizing the effects of charge conduction by metallic tubes. On the basis of these dark current characteristics, the specific detectivity (D*) of a CNT:P3HT/ C60:SnPc at 0 V was calculated by assuming the diode noise is primarily thermal in origin (see Supporting Information). Nano Lett., Vol. 9, No. 9, 2009

Figure 3. Ultra-broad-band photodetector response. Device structure is as follows: ITO, 3.2:1 P3HT/CNTs (45 nm), 1:3 SnPc:C60 (10 nm), C60 (100 nm), BCP (10 nm), Ag (100 nm). (A) Optical absorptivity of constituent materials (orange dot-dashed curve, thin film of C60; red dashed curve, SnPc; green dotted curve, MDMO-PPV; purple double-dot-dashed curve, P3HT; blue solid curve, MDMO-PPV wrapped CNTs). The combination of the small molecule organic and polymer semiconductors with the CNTs provides broad spectral sensitivity from λ ) 400 to 1450 nm. (B) Current density vs voltage characteristics. The solid line is a fit of the experimental data (circles) to the ideal diode equation, using an ideality factor of 1.34 and a specific series resistance of 2.54 Ω cm2. (C), Device specific detectivity (see Supporting Information). Broad-band sensitivity is achieved using the parallel junctions of P3HT/C60, CNT/C60, and SnPc/C60. Responsivity due to the P3HT and C60, SnPc, and CNTs is labeled by the symbols “#”; “%”, and “*”, respectively.

The photodetector exhibits D* > 1010 cm Hz1/2W-1 from λ < 400 nm to λ ) 1450 nm, as shown in Figure 3C. The response in the ranges of λ < 600 nm, λ ) 600-950 nm, and λ > 950 nm is predominantly due to the presence of C60 and P3HT, the SnPc, and the CNTs, respectively. The responsivity significantly decreases at λ > 1450 nm due to of a lack of absorption corresponding to the high end of the CNT diameter range at ∼1.1 nm. To our knowledge, there are no reports of organic photodetectors with D* extending beyond λ ) 1000 nm, and with appreciable responsivity at λ > 1200 nm.26 In comparison with inorganic devices, the CNT-based detector has a similar D* in the NIR to that of cryogenically cooled InSb, although D* for InGaAs photodetectors have a peak room temperature detectivity of 1.35 × 1013 cm Hz1/2 W-1 at λ ) 1500 nm, primarily resulting from a higher EQE.27 The mechanism for the efficient separation of photogenerated electron-hole pairs in the CNTs was studied using adeviceinwhicha40nmthicklayerofboron-subphthalocyanine (SubPc) was inserted between the layer of polymer wrapped CNTs and C60 as a means to physically separate the two layers, as shown in Figure 4. Here, the lowest unoccupied molecular orbital (LUMO) energy of SubPc is similar to that of the electron affinity (EA) of the semiconducting nanotube. While NIR responsivity was observed when the CNTs were in contact with C60 (Figures 2 and 4A), no such response was observed for the structures with SubPc (Figure 4B). The lack of NIR responsivity suggests a negligible exciton dissociation efficiency at the CNT/SubPc and CNT/MDMOPPV interfaces and that the CNT/C60 heterojunction is primarily responsible for exciton dissociation and charge generation in the NIR spectral region. Nano Lett., Vol. 9, No. 9, 2009

Figure 4. Model for exciton dissociation at the polymer wrapped CNT/C60 interface. (A) Energy levels for a CNT/C60 heterojunction. In devices in which the polymer wrapped CNTs form an interface with C60, efficient dissociation of CNT excitons is observed. A 0.2 eV offset in the electron affinity/lowest unoccupied molecular orbital (EA/LUMO) of the CNTs and C60, respectively, is expected to result in efficient exciton dissociation. The thin unlabeled layer at the heterojunction between the CNTs and the C60 depicts the possible electron barrier due to the polymer wrapper. (B) In contrast, when the polymer wrapped CNT/C60 interface is interrupted by the insertion of a SubPc buffer layer, CNT exciton dissociation is not observed due to the high ionization energies of MDMO-PPV and SubPc relative to the CNTs.

An offset energy of ∼0.2 eV exists between the EA of a (9, 4) nanotube that is characteristic of the CNT population in our films, and the LUMO of C60 (see Supporting Information), as shown in Figure 4. For comparison, the binding energy of an exciton in a (9, 4) nanotube in a highly polarizable medium is also ∼0.2 eV.12 Therefore, the energy offset is sufficient to result in exciton dissociation and charge transfer, which is consistent with the observed photoresponse. 3357

Previous studies of CNT/polymer composites have suggested that semiconducting CNTs and donor polymers such as P3HT or MDMO-PPV form a type-II (staggered) heterojunction in which CNTs are the acceptor.15-17,19,20 In a typeII heterojunction, responsivity is expected from both the donor and acceptor materials. However, while strong visible responsivity is reported from the polymer/CNT composites in these studies, zero or limited NIR responsivity arising from semiconducting CNTs is demonstrated,15-17,19,20 which is inconsistent with a polymer/CNT type-II heterojunction picture. Our experiments also do not support the polymer/CNT type-II heterojunction interpretation. The lack of NIR responsivity in devices with the SubPc separation layer (Figure 4B) indicate that the interactions between the MDMO-PPV and the CNTs are insufficient to result in CNT exciton dissociation. In further support of the type-I heterojunction picture, Lebedkin et al. and Nish et al. have reported on the observation of intense E11 photoluminescence from polymerwrapped semiconducting CNTs in response to optical excitation at the polymer absorption band.28,29 We have demonstrated that excitons generated on CNTs using NIR illumination can be dissociated at interfaces with C60. Using this advance, we fabricated photodetectors with a specific detectivity >1010 cm Hz1/2 W-1 over 1100 nm of spectral bandwidth by employing a film of solution-processed polymer-wrapped carbon nanotubes along with the vacuumdeposited molecules, SnPc and C60. We have demonstrated photodetectors with a peak EQE ) 2.3% at λ ) 1155 nm corresponding to an IQE ) 44% that indicates efficient dissociation of photogenerated excitons in the CNTs and the subsequent collection of free electrons and holes. Devices with EQE approaching IQE can be realized by implementing light-trapping strategies, or alternatively by blending the CNTs with acceptors in a bulk-heterojunction-like geometry to increase the optical density. It is expected that the extension of the spectral responsivity to near λ ) 2000 nm can be achieved by employing larger diameter carbon nanotubes with their smaller optical band gaps. The combination of carbon nanotubes, organic polymers, and smallmolecular-weight organic semiconductors represents a new family of semiconductor devices useful in a wide range of optoelectronic applications. In particular, semiconducting carbon nanotubes, with their broad spectral absorbance, chemical stability in ambient environments, and excellent charge transport characteristics have potential for use in solution-processable, high-efficiency photodetectors and photovoltaic cells. Acknowledgment. We gratefully thank the Department of Defense (the Air Force Office of Scientific Research and the Defense Advanced Research Projects Agency HARDI Program), Global Photonic Energy Corp., and Universal

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Display Corp. for their support of this work. Additionally, the authors thank S. Kena-Cohen and N. C. Giebink for useful discussions. Supporting Information Available: Experimental details on solution processing, device fabrication, device characterization, calculation of energy levels, and calculation of specific detectivity. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Avouris, P.; Freitag, M.; Perebeinos, V. Nat. Photonics 2008, 2 (6), 341–350. (2) Durkop, T.; Kim, B. M.; Fuhrer, M. S. J. Phys.-Condes. Matter 2004, 16 (18), R553–R580. (3) Weisman, R. B.; Bachilo, S. M. Nano Lett. 2003, 3 (9), 1235–1238. (4) Lee, J. U. Appl. Phys. Lett. 2005, 87, (7),art. #073101. (5) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; De Boer, B. Polym. ReV. 2008, 48 (3), 531–582. (6) Perzon, E.; Zhang, F. L.; Andersson, M.; Mammo, W.; Inganas, O.; Andersson, M. R. AdV. Mater. 2007, 19 (20), 3308. (7) Yang, F.; Lunt, R. R.; Forrest, S. R. Appl. Phys. Lett. 2008, 92, (5),. (8) Jiang, X. M.; Schaller, R. D.; Lee, S. B.; Pietryga, J. M.; Klimov, V. I.; Zakhidov, A. A. J. Mater. Res. 2007, 22 (8), 2204–2210. (9) McDonald, S. A.; Konstantatos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4 (2), 138– U14. (10) McDonald, S. A.; Konstantatos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4 (2), 138–142. (11) Clifford, J. P.; Konstantatos, G.; Johnston, K. W.; Hoogland, S.; Levina, L.; Sargent, E. H. Nat. Nanotechnol. 2009, 4 (1), 40–44. (12) Capaz, R. B.; Spataru, C. D.; Ismail-Beigi, S.; Louie, S. G. Phys. Status Solidi B-Basic Solid State Phys. 2007, 244 (11), 4016–4020. (13) Itkis, M. E.; Borondics, F.; Yu, A. P.; Haddon, R. C. Science 2006, 312 (5772), 413–416. (14) Pradhan, B.; Setyowati, K.; Liu, H. Y.; Waldeck, D. H.; Chen, J. Nano Lett. 2008, 8 (4), 1142–1146. (15) Kazaoui, S.; Minami, N.; Nalini, B.; Kim, Y.; Hara, K. J. Appl. Phys. 2005, 98, (8),art. #084314. (16) Kymakis, E.; Amaratunga, G. A. J. ReV. AdV. Mater. Sci. 2005, 10 (4), 300–305. (17) Sgobba, V.; Rahman, G. M. A.; Guldi, D. M.; Jux, N.; Campidelli, S.; Prato, M. AdV. Mater. 2006, 18 (17), 2264–2269. (18) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110 (50), 25477–25484. (19) Kanai, Y.; Grossman, J. C. Nano Lett. 2008, 8 (3), 908–912. (20) Landi, B. J.; Raffaelle, R. P.; Castro, S. L.; Bailey, S. G. Prog. PhotoVoltaics 2005, 13 (2), 165–172. (21) Nish, A.; Hwang, J. Y.; Doig, J.; Nicholas, R. J. Nat. Nanotechnol. 2007, 2 (10), 640–646. (22) Sze, S. M., Physics of Semiconductor DeVices 2nd ed.; WileyInterscience Publication: New York, 1981. (23) Agrawal, M.; Sun, Y.; Forrest, S. R.; Peumans, P. Appl. Phys. Lett. 2007, 90, (24),art. #241112. (24) Peumans, P.; Bulovic, V.; Forrest, S. R. Appl. Phys. Lett. 2000, 76 (19), 2650–2652. (25) Yang, F.; Lunt, R. R.; Forrest, S. R. Appl. Phys. Lett. 2008, 92, (5),art. #053310. (26) Perzon, E.; Zhang, F. L.; Andersson, M.; Mammo, W.; Inganas, O.; Andersson, M. R. AdV. Mater. 2007, 19 (20), 3308–3311. (27) Rogalski, A. Prog. Quantum Electron. 2003, 27 (2-3), 59–210. (28) Lebedkin, S.; Hennrich, F.; Kiowski, O.; Kappes, M. M. Phys. ReV. B 2008, 77, (16),art. #165429. (29) Nish, A.; Hwang, J. Y.; Doig, J.; Nicholas, R. J. Nanotechnology 2008, 19, (9),art. #095603 .

NL901637U

Nano Lett., Vol. 9, No. 9, 2009