J. Phys. Chem. C 2010, 114, 4331–4335
4331
The Influence of Doping on the Chemical Composition, Morphology and Electrical Properties of Si(1-x)Gex Nanowires Uri Givan, Moria Kwiat, and Fernando Patolsky* School of Chemistry, The Raymond and BeVerly Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel ReceiVed: NoVember 17, 2009; ReVised Manuscript ReceiVed: February 10, 2010
Semiconductor nanowires (NWs) have been widely investigated over the last few decades because of their unique quantum-confined physical properties which enable novel applications and their potential to play a key role in future electronics. The physical properties of binary-alloy Si(1-x)Gex NWs can be altered both by doping and by changing their chemical alloy composition. The simultaneous control of both parameters is therefore essential, although not straightforward, for the complete utilization of Si(1-x)Gex NWs. While the influence of several growth parameters on the composition of Si(1-x)Gex NW alloys was recently explored, the interplay of simultaneously controlling both dopant concentration and alloy composition has not yet been investigated. This study shows that the introduction of either boron or phosphorus as dopants, by the use of diborane (B2H6) or phosphine (PH3) during the growth of the Si(1-x)Gex NW, has a significant influence on the chemical alloy composition of the NWs produced. This leads to unexpected changes in its physical properties. Therefore, the tailoring of these properties of the NW during its growth requires a complete understanding of the interplay between the doping type and level and the chemical composition. Other aspects of the growth and properties of the Si(1-x)Gex NW, such as morphology, growth rate, growth yield, and electrical properties, were studied as well. Introduction One-dimensional semiconductor nanowires (NWs) have been the focus of growing interest over the last few decades both as promising candidates for miniaturized electronic building blocks and as functional devices for novel applications resulting from their unique quantum-confined physical properties. During this time, a variety of NW-based electronic devices such as photovoltaic cells,1 FETs,2 sensors,3 and LEDs4 have been developed, with the use of several “bottom-up” synthesis techniques. Group IV NWs (SiNWs, GeNWs) are notably widespread in nanotechnology research since they were among the first to be discovered and studied, as well as for their advantage of compatibility with conventional CMOS processes and their relatively easy and highly reproducible synthesis (via the CVD-VLS technique). In comparison with the intensively studied Si- and GeNWs produced earlier,5–13 binary-alloy Si(1-x)Gex NWs were first synthesized only a few years ago,14–19 but their unusual collection of unique properties rapidly rendered them the subject of growing interest. Traditionally, the physical properties of semiconductors such as charge-carrier density or band gap can be modulated through doping. The Si(1-x)Gex binary alloy has the advantage of enhanced tunability, achievable not only by doping but also through altering its chemical composition. The complete miscibility of silicon and germanium enables the use of practically the whole Si/Ge chemical-composition range. This has a notable influence over several of the physical properties of the binary alloy (e.g., electron and phonon mobility, band gap, and lattice parameters). This property has been widely investigated in studies of thin films and bulk binary-alloy * To whom correspondence should be addressed. E-mail: fernando@ post.tau.ac.il.
systems which have led to new applications in thermoelectricity,20 photodetectors,21 heterojunction bipolar transistors (HBT),22 optic-fiber communications,23 etc. The chemical composition of Si(1-x)Gex NW alloys can be readily modulated by the systematic control of the growth parameters. Recently, several studies investigated the influence of the ratio of the inlet precursor gases, the growth temperature, catalyst diameter, and growth pressure15,24–27 on composition, and this has led to the ability to control the composition of intrinsic Si(1-x)Gex NWs. The next step required in order to create functional devices based on Si(1-x)Gex NWs is the controlled doping of the NWs, with either phosphorus or boron, in a manner that will facilitate the simultaneous control of both alloy composition and chargecarrier density. Successful dopant introduction has been achieved by introducing either PH3 or B2H6 during the growth of the nanowire,28 or by using a PH3 plasma after the growth step.29 However, the influence of doping on the Si/Ge ratio and on the morphology of Si(1-x)Gex NWs has not yet been studied and therefore, to the best of our knowledge, the ability to simultaneously control both composition and charge-carrier density during nanowire synthesis has not yet been reported. This study examines the influence of dopant introduction on Si(1-x)Gex NW alloy composition, crystal structure, morphology, growth yield, growth rate, and electrical properties. A wide range of Si(1-x)Gex NWs, doped with phosphorus or boron, were synthesized by conventional inlet-gas doping, and then investigated. The dopant insertion had a marked influence on the composition of the Si(1-x)Gex NW alloys, resulting in Si/Ge ratios in the boron-doped Si(1-x)Gex NWs that were twice those in similarly grown intrinsic nanowires. The morphological and physical properties of the NWs were also significantly influenced by doping. These results must be thoroughly taken into account
10.1021/jp910934h 2010 American Chemical Society Published on Web 02/22/2010
4332
J. Phys. Chem. C, Vol. 114, No. 10, 2010
Givan et al.
Figure 1. Low- and high-resolution TEM images of about 20 nm diameter Si(1-x)Gex NWs grown at 350 °C, inlet precursor-gas ratio (GeH4/GeH4 + SiH4) of 0.02 and total growth pressures of 200 Torr: (a) intrinsic (undoped) Si(1-x)Gex NW; (b) 10-4 (B/Si ratio) boron-doped; (c) 5 × 10-4 (P/Si ratio) phosphorus-doped; (d) 10-4 (B/Si ratio) boron-doped; (e) 2.5 × 10-4 (P/Si ratio) phosphorus-doped; (f) 5 × 10-4 (B/Si ratio) boron-doped. Scale bars for low-resolution images are 2 (a) and 1 µm (b, c) and 6 nm for high-resolution images (d-f). Inset: FFT analysis of the HRTEM image in panel d exhibits [111] growth direction.
when tailoring the physical properties of the Si(1-x)Gex NWs during their growth. Results and Discussion The growth procedures for Si(1-x)Gex NWs have been described in detail elsewhere.27 In short, all NWs produced in this study were grown by the VLS technique in a home-built computer-controlled hot-wall CVD setup, with 20 nm gold nanoparticles (AuNPs) as the catalyst, high-purity silane (SiH4) and germane (10% GeH4 in H2 carrier) as precursor gases, and hydrogen as the carrier gas. Diborane (100 ppm in H2 carrier) or phosphine (1000 ppm in H2 carrier) served as P- and N-doping gases, respectively. Silicon substrates were cut to the required size, cleaned with acetone and isopropyl alcohol (IPA), dried in a stream of dry nitrogen, dipped in a solution of polyL-lysine, washed with deionized distilled water (DDW), and again dried in a stream of dry nitrogen. After drying, the substrates were dipped in a diluted solution of AuNPs, then washed and dried as described above. Finally, organic traces were removed by oxygen plasma. Substrates were then placed at the center of a horizontal quartz tube in a hot-wall furnace, which was then evacuated and purged in a stream of argon and hydrogen. Flow rates were determined with the use of computercontrolled calibrated mass-flow controllers (MFC), and the pressure in the growth chamber was computer-controlled via a throttle valve that limited the flow to the vacuum pump. To suppress radial growth, which might alter the chemical com-
position,15 the following growth parameters were chosen: growth temperature 350 °C; inlet-gas ratio (GeH4/(GeH4 + SiH4)) 0.02; total pressure 200 Torr. The inlet-gas flows were the following: 10%GeH4, 1 sccm (standard centimeter cube per minute at STP); SiH4, 5 sccm; H2, 100 sccm. Boron or phosphorus doping was carried out by adding a B2H6 flow of 2.5-12.5 sccm or a PH3 flow of 0.5-10 sccm during growth. Growth duration varied from 20 to 40 min, resulting in long NWs (15 to 35 µm) which enabled a detailed study of the growth rate, growth yield, and NW morphology.30 The growth process was ended by purging the sample with argon and hydrogen while it was cooling. Structural studies were carried out with the use of FEG highresolution transmission electron microscopy (FEG-HRTEM) (FEI Tecnai F20). NWs were removed from the substrate by sonication in isopropanol, followed by dispersing the solution onto lacey carbon-coated copper grids. High-resolution images and FFT analysis revealed the direction of nanowire growth and determined the lattice parameters. The compositional uniformity of nanowires was verified in this study by TEM-ESD scans employing nanometer scale e-beam spot size and the obtained results did not reveal any radial or longitudinal variation in composition. Figure 1a-f shows low-resolution, high-resolution, and FFT images of phosphorus-doped, boron-doped, and undoped Si(1-x)Gex NWs grown under similar conditions. For all dopings, the Si(1-x)Gex NWs exhibit smooth surfaces with a thin (ca. 1-3 nm) layer of amorphous silicon oxide (confirmed by EDX
Influence of Doping on Si(1-x)Gex Nanowires
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4333
Figure 2. SEM image of typical Si(1-x)Gex NWs growth substrates showing a high growth yield for (a) intrinsic, (b) boron-doped (2.5 × 10-4 B/Si ratio), (c) phosphorus-doped (2.5 × 10-4 P/Si ratio), and (d) showing a reduced growth yield for highly (2 × 10-3 P/Si ratio) phosphorus-doped. Scale bars are 20 (a) and 10 µm (b-d).
measurements). As previously reported for undoped Si(1-x)Gex NWs,5,16,18,25,31 most of the doped Si(1-x)Gex NWs examined in this study exhibit [111] growth direction, though a few bicrystalline NWs were also observed. We therefore conclude that the growth direction was not influenced by the doping type or level. The gold tip was readily observed by HRTEM (unless broken during transfer to the TEM grid), suggesting a VLS growth mechanism. TEM observations were also used to estimate the noncatalytic radial growth (tapering) by careful measurements of the nanowire diameter change along the growth direction of the nanowire (determined by the gold tip). As in the cases of previously reported phosphorus-doped silicon NWs,32,33 no observable tapering was found in phosphorusdoped Si(1-x)Gex NWs, regardless of the doping level. In contrast to this, boron-doped Si(1-x)Gex NWs exhibit notable tapering (ca. 0.5-1 nm/µm), which increases with the doping level. Previous reports of boron-doped silicon NWs showed similar results when high partial pressures of diborane were used, suggesting that a reduction in the activation energy for the deposition of silicon in the presence of diborane accounts for the observed tapering in boron-doped silicon NWs.34 In addition, the morphology and growth yield of NWs were investigated on growth substrates with the use of a fieldemission-gun high-resolution scanning electron microscope (FEG-HRSEM) (JEOL JSM-6400), or FEG environmental SEM measurements (FEG-ESEM) (Quanta 200). Variations in the growth yield were discerned upon careful examination of the SEM images of the growth substrates (Figure 2a-d). Comparison of a wide range of boron-doped Si(1-x)Gex NWs (B/Si ) 10-3-10-4) with similarly grown undoped Si(1-x)Gex NW
growth substrates did not reveal a discernible difference in growth yield. On the other hand, phosphorus doping of Si(1-x)Gex NWs seems to result in a reduction of the growth yield, which increases with increasing dopant levels until almost complete inhibition is reached. Growth rates were calculated from the length of the NW produced divided by the growth time. The effect of doping on the growth rate is shown in Figure 3. While phosphorus doping had only a minor effect on growth rate, and only at high doping concentrations, boron doping increases the growth rate by about 35%, regardless of the doping level. Similar effects of phosphorus doping on growth yield and on growth rate were observed for the growth of silicon NWs,32 in which increasing phosphorus doping reduced the growth yield but had only a minor effect on the growth rate. This phenomenon can be attributed to the high sticking efficiency of phosphorus to the silicon substrates,35 which blocks the initiation of the nucleation process, and therefore influences the growth of both silicon NWs and Si(1-x)Gex NWs. The increase of the growth rate of borondoped Si(1-x)Gex NWs is consistent with previous observations on the growth of silicon NWs36 and silicon thin films.37 These studies report an increase in growth rate with increasing B2H6/ SiH4 ratio that is ascribed to the enhancement of silane decomposition in the gas phase as a result of the interaction between silane and diborane. However, contradictory results, showing a major decrease in the growth rate of silicon NWs following an increase in the B2H6/SiH4 ratio, were reported as well.34 Some experimental limitations might account for these contradictory results: only relatively very high (1.4 × 10-2) and very low (4 × 10-6) B2H6/SiH4 ratios were tested, growth was
4334
J. Phys. Chem. C, Vol. 114, No. 10, 2010
Figure 3. The dependence of SiGe NW growth rate on doping type and level, normalized by dividing by the growth rate of an intrinsic SiGe NW, shows increased growth rate of the boron-doped SiGe NWs and a decreased rate for highly phosphorus-doped SiGe NWs. Error bars are (SD.
Figure 4. Doping type and level dependence of Si(1-x)Gex NW’s Si content extracted from energy-dispersive X-ray spectroscopy measurements (EDS). Error bars are (SD.
held at a very low total pressure, the gold-catalyst tip was consumed during growth, and results were not compared to those of undoped silicon NWs grown under the same conditions. Compositional analysis of NWs was carried out by energydispersive X-ray spectroscopy (EDS) in HRTEM experiments. EDS quantification was achieved with the use of a conventional mathematical model with computer-calculated K factors of 1 and 2.083 for the KR lines of silicon and germanium, respectively. EDS chemical-composition analysis revealed, for both the radial and longitudinal axes, and under all growth conditions, a compositional uniformity with an accuracy of about (3%. Notably, the disordered alloy is indeed expected to exhibit some diversity when high resolution and small spot sizes are used for the EDS analysis. The influence of doping on the silicon content of Si(1-x)Gex NWs is shown in Figure 4. Phosphorus insertion resulted in a slight but consistent increase of the Si/Ge ratio with doping level, in comparison with the undoped NWs grown under the same conditions. This small increase can be attributed to the reduction in the partial pressure of the precursor gases (SiH4 and GeH4) as a result of phosphine addition during the growth process. A
Givan et al. previous study of the dependence of the chemical composition of Si(1-x)Gex NWs on growth pressure27 showed a decrease in the Si/Ge ratio with increase in the total growth pressure. This latter increase is accompanied by a parallel increase in the partial pressures of the precursor gases (on condition that the inlet flow of gases remained the same). Therefore an inverse dependence of the silicon content on the partial pressure of the precursor gases has been reported and was attributed to gas-phase interactions. An additional explanation might be attributed to the change in the gases residence time in the reactor, which can affect both growth rate and chemical composition. On the other hand, boron doping doubled the Si/Ge ratio (from about 1.3 to 2.6) for all doping levels tested in this study. The significant enhancement effect of diborane on the Si/Ge ratio can be assigned to reduction of the activation energy of silane decomposition in the presence of diborane,34 which results in increasing the deposition of silicon. Germane is expected to be less strongly influenced because of its higher decomposition rate and lower activation energy, which are responsible for the lower growth temperature of germanium NWs and for the 50/1 ratio between the germanium content in the Si(1-x)Gex NWs and its content in the inlet gases. This hypothesis is consistent with the observation and argument regarding the enhancement of the Si(1-x)Gex NW growth rate by boron doping. In addition, dopant insertion during the growth of the nanowires was verified by electrical measurements. Electrical characterization was carried out with the use of back-gate fieldeffect-transistor (FET) configuration obtained by either optical or e-beam lithography.32,38 In both cases NWs were dispersed on highly doped silicon wafers coated with a 600 nm layer of thermal oxide that enables the back-gate FET configuration. For e-beam lithography, the wafer was then coated with a 200 nm layer of PMMA and four 100 nm wide contacts were defined with 1 µm separation, developed with MIBK/IPA solution (1/ 3) for 60 s, rinsed, and dried. For optical lithography, the wafer was coated with an 800 nm double layer of LOR3 adhesive and S1805 photoresist. An array of 2 µm-separated contacts was defined by exposure through a predefined mask, followed by MF319 developing, rinsing, and drying. In both cases, the NWs were then etched with 6/1 BOE (buffered oxide etchant, NH4F/HF) for 5 s, followed by e-gun evaporation of 60 nm of nickel and lift-off in acetone or PG remover. The fabrication of the FET device was concluded by annealing at 380 °C for 1-4 min in forming gas (10% H2/90% N2) employing a rapid thermal processor (RTP). Electrical-transport measurements were made with the use of a probe station (Janis, ST-500) connected to a DAQ card (National Instruments, PCI-MIO16XE) by computer-controlled rack-mounted breakout accessory (National Instruments, BNC 2090) via preamplifier (DL 1211). The electrical measurements presented in Figure 5a,b verify the success of dopant insertion. The I/V curves measured under various back-gate voltages of p-type, n-type, and nominally undoped Si(1-x)Gex NWs are characteristic of such FET devices. All NWs were grown under the above-mentioned growth conditions with in situ 1/4000 (2.5 × 10-4) doping of phosphorus or 1/8000 (1.25 × 10-4) boron for p-type and n-type accordingly. The phosphorus-doped, boron-doped, and nominally undoped Si(1-x)Gex NW I/V curves and transconductance curves shown in Figure 5a,b exhibit the expected electrical behavior as previously reported for SiNWs.39 For the intrinsic undoped nanowires, when Vg is made increasingly negative, the conductance increases (Figure 5b). This gate-dependence shows that the nominally undoped SiGeNWs behave like p-type semiconductors. I/V curves recorded for the B-doped SiGeNWs
Influence of Doping on Si(1-x)Gex Nanowires
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4335 and experimental support regarding HRTEM and EDS measurements. This work was in part financially supported by the Legacy Fund-Israel Science Foundation (ISF) and the GermanIsrael Foundation (GIF). References and Notes
Figure 5. (a) I/V curves of back-gated boron-doped SiGeNWs (doping ratio of 1.25 × 10-4 B/Si) at gate voltage values from 10 to -10 V. (b) Transconductance curves of SiGeNWs, at VSD ) 1, with (red) 2.5 × 10-4 P/Si doping ratio, (blue) 1.25 × 10-4 B/Si doping ratio, and (purple) nominally undoped SiGe NWs.
also show p-type semiconductor behavior (Figure 5a). Moreover, the resistivity of these B-doped SiGeNWs is about an order of magnitude smaller than that of the intrinsic SiGeNWs and demonstrates clearly our ability to control the conductivity of the alloy nanowires. The I/V curve recorded on the P-doped SiGeNWs nanowire shows a Vg dependence opposite of that observed for the B-doped SiGeNWs, consistent with an n-type material as expected for P-doping. Conclusion To summarize, the successful insertion of boron and phosphorus dopants into Si(1-x)Gex NWs was achieved by the coflow of SiH4 and GeH4 precursor gases mixed with either B2H6 or PH3 during the CVD-VLS growth. A remarkable influence of the doping type on the chemical composition of the Si/Ge-alloy nanowires was observed and studied. This has led to a basic understanding of the interplay between growth parameters and doping in determining the chemical composition of the SiGeNWs produced. The simultaneous control over dopant concentration and alloy chemical composition that has been achieved is necessary for the tailoring of the physical properties of the SiGeNWs during their growth. The insertion of the dopants was verified by electrical-transport measurements. The observed doping influences on growth rate, growth yield, and morphology are in good agreement with previous reports on silicon NWs doped in the same manner. Gas-phase interactions between dopant species and precursor gases are suggested to account for the modulation of the Si/Ge composition ratio and the enhancement of the growth rate. Acknowledgment. The authors thank Dr. Alex Tsukernik for E-beam lithography and Dr. Yossi Lereah for useful discussions
(1) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (2) Cui, Y.; Zhong, Z.; Wang, D.; Wang, U. W.; Lieber, C. M. Nano Lett. 2003, 149. (3) Kolmakov, A.; Zhang, Y.; Cheng, G.; Moskovits, M. AdV. Mater. 2003, 12, 997. (4) Hayden, O.; Greytak, A. B.; Bell, D. C. AdV. Mater. 2005, 17, 701. (5) Cui, Y.; Duan, X.; Hu, J.; Lieber, C. M. J. Phys. Chem. B 2000, 5213. (6) Wang, D.; Dai, H. Angew. Chem., Int. Ed. 2002, 41, 4783. (7) Wang, D.; Chang, Y.; Dai, H. J. Am. Chem. Soc. 2004, 126, 11602. (8) Cui, Y.; Lauhon, L. J.; Gudiksen, M. S.; Wang, J.; Lieber, C. M. Appl. Phys. Lett. 2001, 2214. (9) Yu, J.-Y.; Chung, S.-W.; Heath, J. R. J. Phys. Chem. B 2000, 104, 11864. (10) He, R.; Yang, P. Nat. Nanotechnol. 2006, 42. (11) Wu, Y.; Cui, Y.; Huynh, L.; Barrelet, C. J.; Bell, D. C.; Lieber, C. M. Nano Lett. 2004, 433. (12) Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Phil. Trans. R. Soc. London 2004, 1247. (13) Greytak, A. B.; Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Appl. Phys. Lett. 2004, 4176. (14) Duan, X.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (15) Lew, K.-K.; Pan, L.; Dickey, E. C.; Redwing, J. M. J. Mater. Res. 2006, 21, 2876. (16) Adu, K.; Gutierrez, H.; Chen, G.; Lew, K.-K.; Nimmatoori, P.; Zhang, X.; Dickey, E.; Redwing, J.; Eklund, P.; Lu, Q. J. Phys. Chem. 2008, 3205. (17) Kim, S.; Shuford, K. L.; Bok, H.-M.; Kim, S. K.; Park, S. Nano Lett. 2008, 800. (18) Kim, C.-J.; Yang, J.-E.; Lee, H.-S.; Jang, H. M.; Park, W.-H.; Kim, Z. H.; Maeng, S.; Jo, M.-H. Appl. Phys. Lett. 2007, 033104. (19) Yang, W. F.; Lee, S. J.; Liang, G. C.; Whang, S. J.; Kwong, D. L. Nanotechnology 2008, 225203. (20) Huxtable, S. T.; Abramson, A. R.; Tien, C.-L.; Majumdar, A.; LaBounty, C.; Fan, X.; Zeng, G.; Bowers, J. E.; Shakouri, A.; Croke, E. T. Appl. Phys. Lett. 2002, 80, 1737. (21) Bauer, M.; Schollhorn, C.; Lyutovich, K.; Kasper, E.; Jutzi, M.; Berroth, M. Mater. Sci. Eng., B 2002, 89, 77. (22) Paul, D. J. Thin Solid Films. 1997, 321, 172. (23) Washio, K. Solid-State Electron. 1999, 43, 1619. (24) Lew, K.; Pan, L.; Dickey, E. C.; Redwing, J. M. AdV. Mater. 2003, 15, 2073. (25) Zhang, X.; Lew, K.; Nimmatoori, P.; Redwing, J. M.; Dickey, E. C. Nano Lett. 2007, 7, 3241. (26) Yang, J.; Jim, C.; Kim, C.; Jo, M. Nano Lett. 2006, 6, 2679. (27) Givan, U.; Patolsky, F. Nano Lett. 2009, 9, 1775. (28) Kim, C.-J.; et al. Appl. Phys. Lett 2007, 91, 033104. (29) Yang, W. F.; Lee, S. J.; Liang, G. C.; Whang, S. J.; Kwong, D. L. Nanotechnology 2008, 19, 225203. (30) Tapering rate was measured by HRTEM in the following manner: First, the diameter vs length at a few points along the NW was measured (when gold tips were used to define the growth direction). The difference in diameters between two successive points was divided by their distance from each other to calculate the tapering per length unit; 6-8 tapering-rate measurements were taken along each NW and finally averaged. For each growth, the average tapering-rate and error bars were defined by averaging the tapering rate over 8-10 NWs. (31) Clark, T. E.; Nimmatoori, P.; Lew, K.-K.; Pan, L.; Redwing, J. M.; Dickey, E. C. Nano Lett. 2008, 8, 1246. (32) Schmid, H.; Bjork, M. T.; Knoch, J.; Karg, S.; Riel, H.; Riess, W. Nano Lett. 2009, 1, 173. (33) Zheng, G.; Lu, W.; Jin, S.; Lieber, C. M. AdV. Mater. 2004, 16, 1890. (34) Pan, L.; Lew, K.-K.; Redwing, J. M.; Dickey, E. C. J. Cryst. Growth 2005, 277, 428. (35) Jang, S.-M.; Liao, K.; Reif, R. J. Electrochem. Soc. 1995, 142, 3520. (36) Lew, K.-K.; et al. Appl. Phys. Lett. 2004, 85, 3101. (37) Lee, S.-Y.; et al. Appl. Surf. Sci. 2007, 254, 312. (38) Patolsky, F.; Zheng, G.; Lieber, C. M. Nat. Protocols 2006, 1, 1711. (39) Wang, Y.; et al. Nano Lett 2005, 5, 2139.
JP910934H
