Bismuth Nanocrystal-Seeded III-V Semiconductor Nanowire Synthesis

and the reaction flask was allowed to cool to room temperature, the crude nanowire dispersions ..... Summer Heat Transfer Conf. 2003, HT2003-47263, 1r...
0 downloads 0 Views 974KB Size
Download PDF
CRYSTAL GROWTH & DESIGN

Bismuth Nanocrystal-Seeded III-V Semiconductor Nanowire Synthesis Dayne D. Fanfair and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712-1062 Received June 8, 2005;

2005 VOL. 5, NO. 5 1971-1976

Revised Manuscript Received July 19, 2005

ABSTRACT: Bismuth (Bi) nanocrystals are used for solution-liquid-solid (SLS) synthesis of crystalline InAs, GaP, GaAs, and InP nanowires at temperatures between 300 and 340 °C in trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO), and trioctylamine (TOA). Bi nanocrystals are observed at the nanowire tips, confirming their role as crystallization seeds. The nanowires are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Introduction Nanowires are intriguing materials: they provide experimental models to study size- and shape-dependent optical,1-10 electronic,11-15 thermoelectric,16-18 and mechanical properties,19,20 self-assembly,21-24 and might be integrated like “building blocks” into future nanoelectronic circuits.11-14,25-30 Chemically synthesized nanowires can be dispersed in solvents and processed at low temperatures for deposition on plastic substrates or mixing with polymers and other molecular materials for hybrid organic/inorganic structures.31,32 Nanowires of Group III-V compounds, such as GaAs and InAs, are of particular interest because of their suitability for optoelectronic devices such as light-emitting diodes, lasers, and optical detectors.33 A wide range of synthetic approaches have been developed for high aspect ratio nanowires, including vapor-liquid-solid (VLS),34 laser catalytic growth (LCG),35 oxide-assisted growth (OAG),36 supercritical fluid-liquid-solid (SFLS),1,37 solution-liquid-solid (SLS) growth,38 colloidal oriented attachment,30,39 and various templating strategies.40-42 “VLS”-type growth has been very successful for a wide range of nanowire compositions with controlled diameter by using nanometer diameter metal particles as seeds in either vapor-43-45 or solution-1,2,4,46,47 phase syntheses. As compared to gasphase methods, solution-phase methods can be scaled up to obtain much larger reaction quantities through homogeneous-phase continuous processing. The primary thermodynamic requirement for VLS nanowire synthesis is that the growth temperature exceed the metal/semiconductor eutectic.34 Au:Si has a eutectic at ∼360 °C, and Au forms a eutectic with III-V semiconductors in the temperature range of 550-650 °C. To achieve these relatively extreme temperatures in solution, solvents can be pressurized above their critical points and sterically stabilized Au nanocrystals have been used to produce crystalline Si,1,47,48 Ge,46,49 GaAs,50 and GaP51 nanowires in hexane at 350-500 °C.37 VLS-growth can be induced in conventional sol* Corresponding author. Phone: (512) 471-5633. Fax: (512) 4717060. E-mail: [email protected].

vents at much lower temperatures using low melting metals, such as In, Ga, and Bi. In 1995, Buhro observed (self-catalyzed) VLS growth of InP, InAs, and GaAs fibers and nanowhiskers in conventional solvents (i.e., SLS growth) induced by the low melting metals, In and In/Ga alloy.38 The challenge with low-temperature SLS growth was initially the difficulty in synthesizing small diameter nanocrystal seeds of low melting metals, which has been recently overcome,52,53 and, in the past few years, Buhro2,4,54 and more recently Nozik55 and Kuno56 demonstrated very good success with SLS using narrow diameter low melting In4,54,55 and Bi2,56 nanocrystals to seed high-quality GaAs,54 InP,4,55 and CdSe2,56 nanowires. Here, we demonstrate the general use of Bi nanocrystals to seed crystalline III-V semiconductor nanowires of GaAs, GaP, InAs, and InP in conventional solvents by SLS growth. Experimental Section Chemicals. All manipulations were done using Schlenk line techniques under nitrogen. Tris(trimethylsilyl)arsine ((SiMe3)3As) was prepared according to literature methods.32 Indium(III) chloride (InCl3), gallium(III) acetylacetonate (Ga(acac)3), gallium(III) chloride (GaCl3), tris(trimethylsilyl)phosphine ((SiMe3)3P), and bismuth(III) 2-ethylhexanoate were used as received from STREM. Ethylenediamine, dioctyl ether, sodium borohydride (NaBH4), tri-n-octylphosphine (TOP), myristic acid, butanol, trioctylamine (TOA), and trioctylphosphine oxide (TOPO) were used as received from Sigma-Aldrich. All other solvents were used as received from Fisher Scientific without further purification. Bismuth Nanocrystal Synthesis. Bi nanocrystals were prepared by arrested precipitation in dioctyl ether and TOP. 0.1 mL of Bi(III) 2-ethylhexanoate (Bi[OOCCH(C2H5)C4H9]3) and 0.15 mL of TOP were added to 11 mL of dioctyl ether (C16H34O) and stirred for 15 min. Separately, the reducing agent (30 mg of NaBH4) was dissolved in 3.4 mL of ethylenediamine by heating to 45 °C and stirring for 15 min. The reducing agent solution was cooled to room temperature and injected into the Bi precursor solution under vigorous stirring. The reaction mixture progressed from a milky appearance to a black color as the Bi precursor reduces to Bi metal nanocrystals. After 30 min, the crude nanocrystal dispersion was removed from the Schlenk line. The nanocrystals were precipitated with ∼5 mL of ethanol and centrifuged at 8000 rpm for 5 min. The supernatant was discarded. The dried nanocrystals were stored under nitrogen until needed, as the Bi

10.1021/cg0502587 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005

1972

Crystal Growth & Design, Vol. 5, No. 5, 2005

nanocrystals were found to be very sensitive to oxidation and air exposure. Nanowire Synthetic Procedure. Reagent solutions (detailed below) were mixed in a nitrogen filled glovebox and injected into a hot (320-340 °C) coordinating solvent under nitrogen on a Schlenk line. Injection of the precursor solution decreased the solvent temperature by ∼40 °C. Once the temperature increased back to the initial injection temperature, nanowire growth was allowed to proceed for 5 min before taking the reaction flask off the heating mantle and allowing it to cool to room temperature. InAs Nanowires in TOP/TOPO. A mixture of 61.4 mg of InCl3, 14.6 µL of (SiMe3)3As, 5.8 mg of Bi nanocrystals, 300 µL of toluene, 24 µL of oleic acid, and 850 µL of TOP was injected into 1 g of hot (330 °C) TOPO. The reaction yield was ∼10%. InAs Nanowires in TOA. A mixture of 18 mg of InCl3, 8.6 µL of (SiMe)3As, 1.7 mg of Bi nanocrystals, 300 µL of toluene, 24 µL of oleic acid, and 850 µL of TOA was injected into a hot (340 °C) solution of 5.6 mg of myristic acid in 2.5 mL of TOA. The yield was ∼17%. GaP Nanowires. A mixture of 60.4 µL of (SiMe3)3P, 1.7 mg of Bi nanocrystals, 300 µL of toluene, 24 µL of oleic acid, and 850 µL of TOP was injected into a hot (320 °C) solution of 76 mg of Ga(acac)3 in 1 g of TOPO. The yield was ∼20%. GaAs Nanowires. A mixture of 8.6 µL of (SiMe3)3As, 1.7 mg of Bi nanocrystals, 300 µL of toluene, 24 µL of oleic acid, and 850 µL of TOA was injected into a hot (340 °C) solution containing 14.3 mg of GaCl3 and 5.6 mg of myristic acid dissolved in 2.5 mL of TOA. The yield was ∼40%. InP Nanowires. A mixture of 18 mg of InCl3, 23.6 µL of (SiMe3)3P, 1.7 mg of Bi nanocrystals, 300 µL of toluene, 24 µL of oleic acid, and 850 µL of TOP was injected into a hot (340 °C) solution of 2 g of TOPO and 5.6 mg of myristic acid. The yield was ∼57%. Nanowire Purification. Once the reaction was complete and the reaction flask was allowed to cool to room temperature, the crude nanowire dispersions were diluted with 10 mL of toluene and centrifuged at 8000 rpm for 10 min. The supernatant was discarded. The nanowires were redispersed in CHCl3 and precipitated again with butanol or ethanol and recentrifuged to obtain a purified nanowire product in the precipitate. Nanowires were stored dry under nitrogen prior to characterization. Materials Characterization. Purified nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). SEM samples were prepared by drop casting chloroform-dispersed nanowires onto glassy carbon substrates. SEM was performed on a LEO 1530 field emission gun SEM, operating at 3 kV accelerating voltage, and digital SEM images were acquired using an Inlens detector and LEO 32 software system. For TEM, nanowires were drop cast from chloroform onto 200-mesh lacey carboncoated Cu grids (Electron Microscopy Sciences). TEM and EDS were performed on a JEOL 2010F field emission gun electron microscope operating at 200 kV accelerating voltage. Energydispersive X-ray spectra were obtained with an attached Oxford INCA spectrometer. Digital TEM images were acquired with a Gatan multipole scanning CCD camera. XRD data were acquired from ∼0.5 mg of nanowires on quartz slides using a Bruker-Nonius D8 Advance θ-2θ powder diffractometer with Cu KR radiation (λ ) 1.5418 Å) and collecting with a scintillation detector for 6-12 h with an incremental angle of 0.02° at a scan rate of 12°/min.

Results and Discussion Bi Nanocrystals. Bi is a low melting point (semi)metal that forms a liquid eutectic with GaAs, InAs, GaP, and InP near its melting temperature (∼270 °C), making it a suitable metal seed for SLS nanowire growth in conventional solvents. Key to SLS growth is the

Fanfair and Korgel

availability of high-quality organic monolayer-coated Bi nanocrystals. However, nanocrystals of low melting metals, such as In and Bi, are difficult to synthesize. One approach to obtaining these nanocrystals has employed Au nanocrystals as a platform for core/shell Au/In52 or Au/Bi2,53 nanocrystals with good dispersion stability to serve as growth seeds. Because the presence of Au could potentially affect the nanowire synthesis, we sought to use pure Bi nanocrystals to synthesize the III-V nanowires. Several different syntheses were attempted with widely varying results, and most plausible recipes did not provide Bi nanocrystals with sufficient steric stabilization and size control. As one example, a hightemperature reduction of bismuth acetate, in the presence of oleic acid and TOP, at 175-200 °C in dioctyl ether using either NaBH4, superhydride (LiBEt3H), or 1,2-hexadecanediol gave Bi nanocrystals that were too large, with a very broad size distribution and an average diameter greater than 25 nm. In an alternative biphasic route, bismuth nitrate pentahydrate was transferred from an aqueous solution using tetraoctylammonium bromide as a phase transfer catalyst for an interfacial reduction using an aqueous NaBH4 solution. It was impossible to avoid Bi oxidation using this approach, even when the reaction was done on a Schlenk line under nitrogen with rigorous attempts to exclude oxygen from the system. Many different capping ligands, including dodecylamine, oleylamine, dodecanethiol, butanethiol, TOP, and oleic acid, were explored. None of these would stabilize Bi metal nanocrystals using the two-phase reduction, and surface oxidation could not be avoided. The highest quality Bi nanocrystals were made by a room-temperature reduction of bismuth(III) 2-ethylhexanoate (Bi[OOCCH(C2H5)C4H9]3) in dioctyl ether in the presence of TOP. TOP was found to provide reasonable steric stabilization, yielding Bi nanocrystals with ∼20 nm diameter. Figure 1 shows TEM images and XRD data of the Bi nanocrystals used to synthesize the nanowires. The average nanocrystal diameter is ∼20 nm. The size distribution is relatively broad; however, there are very few particles much larger than 20 nm in diameter. The particles are spherical and crystalline and composed of Bi metal. The XRD pattern (Figure 1) corresponds to rhombohedral Bi (JCPDS Card 44-1246). The peak broadening in the XRD data is consistent with nanocrystals with an average diameter of 20 nm based on the Scherrer equation. InAs and GaP Nanowires. Figure 2 shows SEM and TEM images of InAs and GaP nanowires synthesized in TOP/TOPO with Bi nanocrystals. InAs nanowires were formed through the dehalosilylation reaction between InCl3 and (SiMe3)3As, and GaP nanowires were synthesized by reacting Ga(acac)3 with (SiMe3)3P. Nanowires did not form without Bi nanocrystals. Both InAs and GaP nanowires were obtained with relatively high yield and nanowires ranged between 1 and 10 µm long. The best nanowire reaction yields were observed with the addition of oleic acid and myristic acid. TOP is generally regarded as a relatively weak-binding capping ligand, and the carboxylated ligands most likely provide additional Bi particle steric stabilization under the nanowire growth conditions.

Bismuth III-V Semiconductor Nanowire Synthesis

Crystal Growth & Design, Vol. 5, No. 5, 2005 1973

Figure 1. Organic monolayer-coated Bi nanocrystals used for SLS III-V nanowire synthesis. (A) TEM image of a field of Bi nanocrystals. (B) XRD from Bi nanocrystals, matching rhombohedral Bi (JCPDS Card 44-1246). Peak broadening and the Scherrer equation give an average particle diameter of 20 nm. (Inset) High-resolution TEM image of a Bi nanocrystal showing the internal crystallinity.

Figure 2. (A-C) InAs and (D-F) GaP nanowires: (A,D) SEM and (B,C,E,F) TEM images. The FFTs in the insets in (B) and (E) index to sphalerite InAs and GaP, respectively, and indicate that the nanowires in (B) and (E) are imaged down the [011] zone axes. The nanowires exhibit a 〈111〉 growth direction. Both InAs and GaP nanowires were produced in TOP/TOPO in the presence of Bi nanocrystals.

XRD shows that the nanowires are crystalline and exhibit cubic (sphalerite) crystal structure (Figure 3). Both InAs and GaP grow predominantly in the 〈111〉 direction, and Figure 2 shows examples of 〈111〉-oriented InAs and GaP nanowires. The lattice spacings in Figure 2 match the (111) d spacings of InAs (3.498 Å) and GaP (3.14 Å), and the angles between the [1h 11 h ] and [200] directions in the InAs (Figure 2B) and GaP (Figure 2E) nanowires are 55° and 56.3°, respectively, which are characteristic of sphalerite (cubic) InAs and GaP. InP Nanowires. InP nanowires were synthesized in TOP/TOPO in the presence of Bi nanocrystals by the dehalosilylation reaction between InCl3 and (SiMe3)3P. The nanowire yield was again relatively high, and XRD shows that the InP nanowires are crystalline with sphalerite (cubic) structure (Figure 3). Figure 4A shows

SEM and TEM images of an InP nanowire sample. The InP nanowires are crystalline with the 〈111〉 growth direction. However, the InP nanowires are not as straight as the InAs and GaP nanowires, and TEM images of the nanowire bends (Figure 4B-D) reveal stacking faults as the possible source of bending. In comparison, SFLS-grown GaAs50 and GaP51 nanowires exhibit planar (111) twinning faults cross-sectioning the nanowires, unlike these stacking faults that appear at random orientations with respect to the growth direction. Although a very careful crystallographic analysis of the faults has not been carried out, they are most likely (111) stacking faults because the (111) planes are the slip planes in a sphalerite crystal.57 Under poor growth conditions, such as starved In and P supply, stacking faults can be introduced into the nanowires to

1974

Crystal Growth & Design, Vol. 5, No. 5, 2005

Fanfair and Korgel

Figure 3. XRD from (A) InAs, (B) GaP, (C) GaAs, and (D) InP nanowires. Bi peaks are labeled with a “*”. The peak labeled in (A) with “9” is coesite (monoclinic SiO2), the XRD substrate. InAs and GaAs nanowires were produced in TOA. InP and GaP nanowires were produced in TOP/TOPO.

Figure 4. (A) SEM and (B,C,D) TEM images of InP nanowires produced in TOP/TOPO in the presence of Bi nanocrystals. The inset in (A) displays an InP nanowire with the sphalerite crystal structure and 〈111〉 oriented growth. The (111) d spacing (3.388 Å) is labeled on the inset. (B) TEM image of an InP nanowire with stacking faults indicated by the ovals. (C) and (D) show magnified images of the stacking faults indicated in (B).

give rise to a tortuous morphology, as has been observed in Si nanowires.47 GaAs Nanowires and the Effect of Solvent and Precursor Reactivity. In contrast to GaP, GaAs

nanowires could not be synthesized with high yield in TOP/TOPO. Figure 5A shows SEM images of GaAs nanowires produced in TOP/TOPO from GaCl3 and (SiMe3)3As in the presence of Bi nanocrystals; the reaction generates a low yield of poor quality nanowires. To improve the yield in TOP/TOPO, alternative Ga precursors were tested, including (tBu)3Ga and Ga(acac)3. Although the nanowire yield was slightly better with GaCl3 and Ga(acac)3 than with (tBu)3Ga, the yield was relatively poor in each case. Trioctylamine, in place of TOP/TOPO as the solvent, was found to give much better results, with high-quality GaAs nanowires in high yield from the dehalosilylation reaction with GaCl3 and (SiMe3)3As. The phosphoruscontaining TOP/TOPO mixture quenches GaAs nanowire formation. TOP/TOPO forms a relatively strong complex with Ga, and complex formation competes with the dehalosilylation reaction. Figure 5 compares GaAs nanowires produced in TOP/TOPO and TOA, showing the much better result in TOA. The GaAs nanowires produced in TOA from GaCl3 and (SiMe3)3As were crystalline with sphalerite crystal structure, as shown by the XRD pattern in Figure 3. The GaAs nanowires grow in the 〈111〉 direction. InAs nanowires could also be produced in higher yield in TOA than TOP/TOPO, although TOP does not quench the InAs formation reaction nearly as much as the GaAs reaction. Figure 6 compares SEM images of InAs nanowires synthesized in TOA and TOP/TOPO. TOP influences the reaction significantly, but the In-TOP complex is not as chemically stable as the Ga-TOP complex and InAs nanowires form to a significant extent. This is consistent with observations from nanocrystal synthesis: InAs nanocrystals can be grown by arrested precipitation in TOP/TOPO mixtures, but GaAs nanocrystals cannot.58-60 Bi Nanocrystals at the Tips of the Wires. The XRD data in Figure 3 for the GaP, GaAs, InP, and InAs nanowires confirmed their sphalerite structure (InAs (JCPDS Card 15-0869), GaP (JCPDS Card 12-0191), GaAs (JCPDS Card 32-0389), and InP (JCPDS Card 320452)). In all of the nanowire XRD data, diffraction peaks also appear from Bi metal. The diffraction peaks are slightly narrower than those obtained from XRD of the starting Bi nanocrystals, indicating that Bi particle

Bismuth III-V Semiconductor Nanowire Synthesis

Crystal Growth & Design, Vol. 5, No. 5, 2005 1975

Figure 5. (A) SEM image of GaAs nanowires produced using GaCl3 in TOP/TOPO. (B) SEM image of GaAs nanowires produced in TOA. The inset shows a TEM image of a GaAs nanowire with the sphalerite crystal structure and 〈111〉 oriented growth. The (111) d spacing (3.263 Å) is labeled on the inset. Both syntheses were done in the presence of Bi nanocrystals.

Figure 6. InAs nanowires synthesized using InCl3 and (SiMe3)3As in (A) TOP/TOPO and (B) TOA.

Figure 7. (A,C,E,G) EDS data obtained from nanocrystals at the tip of (A) InAs, (C) GaP, (E) InP, and (G) GaAs nanowires as compared to EDS from the (B) InAs, (D) GaP, (F) InP, and (H) GaAs nanowires. InAs, InP, and GaP nanowires were produced in TOP/TOPO. GaAs nanowires were produced in TOA.

aggregation during the nanowire synthesis does occur to some extent. However, the peaks are still relatively broad, consistent with an average domain size in the 20-40 nm range. This size range is consistent with TEM images that reveal 20-40 nm diameter Bi particles residing at the tips of the nanowires, as shown in Figure 7. EDS line scans (Figure 7) confirmed that the

particles are Bi, and are free of Ga and In metal, and that the nanowires are free of Bi. Conclusions Bi nanocrystals seed SLS growth of crystalline high aspect ratio GaP, GaAs, InP, and InAs nanowires in

1976

Crystal Growth & Design, Vol. 5, No. 5, 2005

conventional solvents such as TOPO/TOP and TOA at 320-340 °C, which is above the Bi:semiconductor eutectic. Good dispersion stability of Bi nanocrystals with reasonably tight size control is needed to produce high-quality nanowires, and the precursor degradation kinetics are very important, determining the yield and quality of the wires. III-As nanowire synthesis works much better in solvents free of P, which tend to form relatively strong metal-phosphorus complexes. Solution-phase metal nanocrystal seeded nanowire synthesis provides one scalable, rational, and general approach for obtaining high aspect ratio crystalline semiconductor nanowires. Because the nanowires are produced in solution as dispersions, these synthetic routes are naturally compatible with low-temperature processing with polymers, small organic molecules, and flexible substrates, providing a useful alternative to higher temperature vapor-phase methods. Acknowledgment. We thank Robert Wiacek, Jennifer Moore, and Alan Cowley for synthesis of (SiMe3)3As and (tBu)3Ga precursors. We thank J. P. Zhou for TEM assistance. We acknowledge the National Science Foundation, SPRING (Strategic Partnership in Research in Nanotechnology in collaboration with the Air Force Research Laboratory), the Welch Foundation, and the AMRC in collaboration with International SEMATECH for financial support for this work. References (1) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471-1473. (2) Yu, H.; Li, J.; Loomis, R. A.; Gibbons, P. C.; Wang, L.-W.; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168-16169. (3) Ahrenkiel, S. P.; Micic, O. I.; Miedaner, A.; Curtis, C. J.; Nedeljkovic, J. M.; Nozik, A. J. Nano Lett. 2003, 3, 833837. (4) Yu, H.; Li, J.; Loomis, R. A.; Wang, L.-W.; Buhro, W. E. Nat. Mater. 2003, 2, 517-520. (5) Kan, S.; Mokari, T.; Rothenberg, E.; Banin, U. Nat. Mater. 2003, 2, 155-158. (6) Shabaev, A.; Efros, A. L. Nano Lett. 2004, 4, 1821-1825. (7) Li, J. B.; Wang, L. W. Chem. Mater. 2004, 16, 4012-4015. (8) Steiner, D.; Katz, D.; Millo, O.; Aharoni, A.; Kan, S.; Mokari, T.; Banin, U. Nano Lett. 2004, 4, 1073-1077. (9) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897-1899. (10) Shieh, F.; Saunders, A. E.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 8538-8542. (11) Cui, Y.; Lieber, C. M. Science 2001, 291, 630-633. (12) Wang, D. W.; Chang, Y. L.; Wang, Q.; Cao, J.; Farmer, D. B.; Gordon, R. G.; Dai, H. J. J. Am. Chem. Soc. 2004, 126, 11602-11611. (13) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466-15472. (14) Hanrath, T.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 5518-5524. (15) Schricker, A. D.; Sigman, M. B.; Korgel, B. A. Nanotechnology 2005, 16, S508-S513. (16) Lin, Y. M.; Sun, X. Z.; Dresselhaus, M. S. Phys. Rev. B 2000, 62, 4610-4623. (17) Lin, Y. M.; Dresselhaus, M. S. Phys. Rev. B 2003, 68, 075304. (18) Yu, C.; Jang, W.; Hanrath, T.; Kim, D.; Yao, Z.; Korgel, B.; Lhi, L.; Wang, Z. L.; Li, D.; Majumdar, A. Proc. 2003 ASME Summer Heat Transfer Conf. 2003, HT2003-47263, 1-6. (19) Wang, Z. L.; Dai, Z. R.; Gao, R. P.; Gole, J. L. J. Electron Microsc. 2002, 51, S79-S85. (20) Jin, Z. Q.; Ding, Y.; Wang, Z. L. J. Appl. Phys. 2005, 97, 074309. (21) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360-4361.

Fanfair and Korgel (22) Jana, N. R.; Gearheart, L. A.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909-2912. (23) Jana, N. R. Angew. Chem., Int. Ed. 2004, 43, 1536-1540. (24) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; Fo¨rster, S.; Weller, H. J. Am. Chem. Soc. 2004, 126, 12984-12988. (25) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149-152. (26) Duan, X.; Niu, C.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.; Goldman, J. L. Nature 2003, 425, 274-278. (27) Wang, D. W.; Wang, Q.; Javey, A.; Tu, R.; Dai, H. J.; Kim, H.; McIntyre, P. C.; Krishnamohan, T.; Saraswat, K. C. Appl. Phys. Lett. 2003, 83, 2432-2434. (28) Goldberger, J.; Sirbuly, D. J.; Law, M.; Yang, P. J. Phys. Chem. B 2005, 109, 9-14. (29) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659-663. (30) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140-7147. (31) McAlpine, M. C.; Friedman, R. S.; Jin, S.; Lin, K. H.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 1531-1535. (32) McAlpine, M. C.; Friedman, R. S.; Lieber, C. M. Nano Lett. 2003, 3, 443-445. (33) Kalt, H. Optical Properties of III-V Semiconductors: The Influence of Multi-Valley Band Structures; Springer: New York, 1995. (34) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 8990. (35) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208-211. (36) Zhang, R.-Q.; Lifshitz, Y.; Lee, S.-T. Adv. Mater. 2003, 15, 635-640. (37) For a recent review, see: Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 9574-9587. (38) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791-4. (39) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237240. (40) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120-2123. (41) Ragan, R.; Chen, Y.; Ohlberg, D. A. A.; Medeiros-Ribeiro, G.; Williams, R. S. J. Cryst. Growth 2003, 251, 657-661. (42) Yun, M. H.; Myung, N. V.; Vasquez, R. P.; Lee, C. S.; Menke, E.; Penner, R. M. Nano Lett. 2004, 4, 419-422. (43) Gudiksen, M. S.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 8801-8802. (44) Cui, Y.; Lauhon, L. J.; Gudiksen, M. S.; Wang, J. F.; Lieber, C. M. Appl. Phys. Lett. 2001, 78, 2214-2216. (45) Wu, Y. Y.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 31653166. (46) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 1424-1429. (47) Lu, X.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2003, 3, 93-99. (48) Lee, D. C.; Hanrath, T.; Korgel, B. A. Angew. Chem., Int. Ed. 2005, 44, 3573-3577. (49) Hanrath, T.; Korgel, B. A. Small 2005, 1, 717-721. (50) Davidson, F. M., III; Schricker, A. D.; Wiacek, R. J.; Korgel, B. A. Adv. Mater. 2004, 16, 646-649. (51) Davidson, F. M., III; Wiacek, R.; Korgel, B. A. Chem. Mater. 2005, 17, 230-233. (52) Yu, H.; Gibbons, P. C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem. Soc. 2001, 123, 9198-9199. (53) Grebinski, J. W.; Richter, K. L.; Zhang, J.; Kosel, T. H.; Kuno, M. J. Phys. Chem. B 2004, 108, 9745-9751. (54) Yu, H.; Buhro, W. E. Adv. Mater. 2003, 15, 416-419. (55) Nedeljkovic, J. M.; Micic, O. I.; Ahrenkiel, S. P.; Miedaner, A.; Nozik, A. J. J. Am. Chem. Soc. 2004, 126, 2632-2639. (56) Grebinski, J. W.; Hull, K. L.; Zhang, J.; Kosel, T. H.; Kuno, M. Chem. Mater. 2004, 16, 5260-5272. (57) Takeuchi, S.; Suzuki, K. Phys. Status Solidi A 1999, 171, 99-103. (58) Guzelian, A. A.; Banin, U.; Kadavanich, A. V.; Peng, X.; Alivisatos, A. P. Appl. Phys. Lett. 1996, 69, 1432-1434. (59) Stowell, C.; Wiacek, R. J.; Saunders, A.; Korgel, B. A. Nano Lett. 2003, 3, 1441-1447. (60) In our laboratory, we have never been able to synthesize GaAs nanocrystals in TOP/TOPO.

CG0502587