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Chem. Mater. 2005, 17, 4858-4864
Phase Changes in Ge Nanoparticles Hsiang Wei Chiu, Christopher N. Chervin, and Susan M. Kauzlarich* Department of Chemistry, UniVersity of California DaVis, One Shields AVenue, DaVis, California 95616 ReceiVed March 29, 2005. ReVised Manuscript ReceiVed May 20, 2005
Butyl-capped crystalline germanium (Ge) nanoparticles were synthesized at room temperature in dimethoxyethane by reduction of GeCl4 with Na(naphthalide) and subsequent reaction with butyl Grignard. The nanoparticles were isolated in hexane and characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX), elemental analysis, and X-ray powder diffraction (XRD). The product from this room-temperature reaction was heated under vacuum at temperatures of 200-600 °C at 50 °C intervals. The product obtained from the 300 °C treatment was soluble in hexane, while the products from temperatures greater than 300 °C were not. SAED was consistent with crystalline Ge from the initial synthesis at room temperature and amorphous Ge for the product heated under vacuum to 300 °C. X-ray powder diffraction of the 300 °C product shows the transition from amorphous to crystalline nanoparticles occurring between 550 and 600 °C. TEM shows that the nanoparticles remain dispersed and nonaggregated up to 600 °C. Differential scanning calorimetry (DSC) shows a crystallization exotherm at 561 °C and a melting endotherm at 925 °C for nanoparticles with average diameter of 8 nm.
Introduction In the history of solar cells and satellite arrays, germanium has been an important component in the preparation of solar cells.1-3 Gallium arsenide (GaAs) cells have been available since the 1960s and have been much improved to make highefficiency photovoltaics.4 It has been found that highefficiency photovoltaics can be made by using germanium as a substrate for GaAs cell.5,6 Germanium is lattice-matched to GaAs and it is less expensive than GaAs. Ge is also stronger and more robust than GaAs; hence, the cells can be made on thinner substrates. Thinner substrates translate into a lighter, high-efficiency cell using less GaAs than ones without Ge. A current challenge is the production of largearea, low-cost, flexible GaAs photovoltaic cells. With the rise of nanotechnology, it is envisioned that a lightweight, flexible photovoltaic may be constructed from nanoparticles.7,8 Quantum dots offer the option of one material with different band gap energies. An alternate use of nanoparticles (NPs) is as a soluble form of precursor that could be deposited as a thin film using ink-jet or laser printing technologies.9 This would also require that the film could be made into a single-crystal substrate and that the NPs be (1) McConnell, R. D. Future Generation PhotoVoltaic Technologies; American Institute of Physics: Woodbury, NY, 1997; Vol. 404. (2) Marti, A.; Luque, A. Next Generation PhotoVoltaics; Institute of Physics Publishing: Bristol, UK, 2004. (3) Bailey, S. G.; Flood, D. J. Prog. PhotoVoltaics 1998, 6, 1-14. (4) Hardingham, C.; Wood, S. P. GEC ReV. 1998, 13, 163-171. (5) Blondeel, A.; Clauws, P.; Depuydt, B. Mater. Sci. Semicond. Process. 2001, 4, 301-303. (6) Mauk, M. G.; Balliet, J. R.; Feyock, B. W. J. Cryst. Growth 2003, 250, 50-56. (7) Bukowski, T. J.; Simmons, J. H. Crit. ReV. Solid State Mater. Sci. 2002, 27, 119-142. (8) Valeev, R. G.; Deev, A. N.; Ruts, Y. V. Surf. Interface Anal. 2004, 36, 955-958. (9) Huang, D.; Liao, F.; Molesa, S.; Redinger, D.; Subramanian, V. J. Electrochem. Soc. 2003, 150, G412-G417.
compatible with printing technology. This idea has been demonstrated for metal NPs and holds promise for other types of NPs.9 The use of NPs may enable low-cost fabrication of lightweight photovoltaic cells while keeping their high-energy conversion capabilities. There have been several recent reports of the production of Ge NPs prepared from the reduction of GeCl4 under moderate or ambient conditions.10-13 These NPs are initially terminated with either -Cl or -H and then terminated with an organic ligand. The particles produced show a fairly narrow size distribution and photoluminescence. Similar results have also been obtained for Ge NPs produced from supercritical reactions14,15 and there has been some speculation concerning the origin of the photoluminescence.15 This group has investigated several routes to the production of group IV NPs.16-19 There is an early report that the reduction route leads to amorphous group IV NPs.20 In an interest to (10) Wilcoxon, J. P.; Provencio, P. P.; Samara, G. A. Phys. ReV. B: Condens. Matter 2001, 64, 035417-035419. (11) Hope-Weeks, L. J. Chem. Commun. 2003, 2980-2981. (12) Fok, E.; Shih, M.; Meldrum, A.; Veinot, G. C. Chem. Commun. 2004, 386-387. (13) Gerung, H.; Bunge, S. D.; Boyle, T. J.; Brinker, C. J.; Han, S. M. Chem. Commun. 2005, 1914-1916. (14) Lu, X.; Ziegler, K. J.; Ghezelbash, A.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2004, 4, 969-974. (15) Gerion, D.; Zaitseva, N.; Saw, C.; Casula, M. F.; Fakra, S.; Van Buuren, T.; Galli, G. Nano Lett. 2004, 4, 597-602. (16) Taylor, B. R.; Kauzlarich, S. M.; Lee, H. W. H.; Delgado, G. R. Chem. Mater. 1998, 10, 22-24. (17) Taylor, B. R.; Kauzlarich, S. M.; Delgado, G. R.; Lee, H. W. H. Chem. Mater. 1999, 11, 2493-2500. (18) Taylor, B. R.; Fox, G. A.; Hope-Weeks, L. J.; Maxwell, R. S.; Kauzlarich, S. M.; Lee, H. W. H. Mater. Sci. Eng. B 2002, 96, 9093. (19) Tanke, R. S.; Kauzlarich, S. M.; Patten, T. E.; Pettigrew, K. A.; Murphy, D. L.; Thompson, M. E.; Lee, H. W. H. Chem. Mater. 2003, 15, 1682-1689. (20) Kornowski, A.; Giersig, M.; Vogel, R.; Chemseddine, A.; Weller, H. AdV. Mater. 1993, 5, 634-636.
10.1021/cm050674e CCC: $30.25 © 2005 American Chemical Society Published on Web 08/17/2005
Phase Changes in Ge Nanoparticles
gain an understanding of the product obtained from the ambient reduction route, butyl-terminated Ge NPs have been produced by the reduction of GeCl4 with Na naphthalenide. These NPs were further reacted with a Grignard reagent to produce alkyl-terminated NPs. This product shows small polydispersity (∼10%) and electron diffraction consistent with crystalline Ge. Elemental analysis is consistent with the product being a mixture of Ge NPs and hydrocarbon. Heating this product under vacuum to various temperatures shows that this material undergoes a transition to amorphous (or paracrystalline21) NPs by 300 °C and can be converted back to crystalline NPs by 561 °C. In situ X-ray powder diffraction, along with TEM, SAED, and calorimetry, studies will be presented. Experimental Section Materials. Ethylene glycol dimethyl ether (glyme) (Acros, 99+%) was dried and distilled twice from a Na-K alloy under argon. The sodium-potassium (Na-K) alloy was freshly prepared from a mixture of sodium (Aldrich, 99%) and potassium pieces (Aldrich, 98%). Naphthalene (C10H8) (Fisher, refined, 99.98%), germanium tetrachloride, (GeCl4) (Acros, 99.99%), and butylmagnesium chloride (Aldrich, 2 M) were used without further purification. HPLC grade water (EM Science) and HPLC grade hexane (Aldrich) were used as received. Manipulations of these chemicals were handled either in a N2-filled glovebox or on a Schlenk line using standard anaerobic and anhydrous techniques. Sodium Naphthalenide (Na(naphth)) Synthesis. Na metal (0.5190 g,0.02258 mol) was added to a Schlenk flask in a drybox and transferred to a Schlenk line. Then 2.894 g (0.02258 mol) of naphthalene was added under Ar. Approximately 70 mL of freshly distilled and degassed glyme was added to the solids and the mixture was stirred overnight. Upon dissolution of the Na metal, the solution changed from colorless to a dark green color. Germanium Nanoparticle Synthesis. A 70 mL solution of Na(naphth) was added rapidly via cannula to 0.70 mL (0.00602 mol) of GeCl4 in 300 mL of glyme in a Schlenk flask at room temperature. The solution immediately changed from a clear to a black suspension upon the addition of the Na(naphth) mixture. After 10 min of stirring, the suspension was allowed to settle. Once settled, there was a dark black solid at the bottom of the flask and a dark yellow solution on the top. The orange solution was vacuumdried to remove the naphthalene. Freshly distilled and degassed glyme (250 mL) was then added to the flask, followed by 3.01 mL (0.00602 mol) of n-BuMgCl. The mixture was left to stir for 12 h. The mixture was pumped down to dryness, the NPs were extracted with hexane, and the extract was rinsed with acidified water. After the removal of hexane in vacuo, with mild heating, ca. 500 mg of viscous orange oil was obtained. The elemental analysis of the orange oil, as-prepared Ge NPs were Ge(C4H9)2.3. Calculated: Ge, 35.60; C, 54.17; H, 10.23. Found: Ge, 31.56; C, 56.13; H, 9.1; Cl, 0.67; Na, 0.57; Mg, 0.03. Heat Treatment. The as-prepared Ge NPs were loaded into an alumina boat which was then placed into a quartz tube. The tube was attached to a high-vacuum apparatus and was inserted into a temperature-controlled furnace. The sample was brought up to a targeted temperature under vacuum conditions with a heating scheme of 1 °C/min to 90 °C, dwell for 3 h, 1 °C/min to the desired temperature, then dwell for 12 h. The final temperatures were 200, 250, 280, 300, 320, 360, 400, 420, and 450 °C. For temperatures (21) Treacy, M. M. J.; Gibson, J. M.; Keblinski, P. J. J. Non-Cryst. Solids 1998, 231, 99-110.
Chem. Mater., Vol. 17, No. 19, 2005 4859 above 300 °C the products were black solids. An orange solid was produced at 300 °C and below 300 °C orange oil remained. All the black solids were insoluble in any common nonpolar and polar solvents. The 300 °C products along with the products obtained at temperatures lower than 300 °C were soluble in hexane. The elemental analysis of the orange solid after heating to 300 °C, Ge(C4H9)0.75. Calculated: Ge, 55.98; C, 27.77; H, 5.24. Found: Ge, 58.04; C, 29.23; H, 5.4; Cl,
