Synthesis of Germanium Nanowires Mingyuan Ge, ² J. F. Liu,* ,² Haiping Wu, ² Changwa Yao, ² Yuewu Zeng, ²,‡ Z. D. Fu, § S. L. Zhang, § and J. Z. Jiang* ,² International Center for New-Structured Materials (ICNSM) and Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, Analysis and Testing Centre, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and Department of Physics, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: February 13, 2007; In Final Form: April 20, 2007 We report here a novel and simple method, without using any seeds or catalysts, to synthesize single-crystal germanium nanowires with tunable 10-115 nm diameters and of 10-20 μm length by the decomposition of a precursor [(CH 3 (CH 2 ) 7 CHdCH(CH 2 ) 7 CH 2 NH 2 ) 4 Ge] 4+ ‚(Cl - ) 4 (TOG) at 360 °C in trioctylamine. The diameter of Ge nanowires can be controlled by varying the concentration of TOG in solution. Introduction One-dimensional nanostructured semiconductors, such as nanowires, nanorods, nanotubes, and nanobelts, are receiving increasing attention for their unique size effects in electrical and optical properties and also for their possible use in modern electronics. 1-5 Germanium is an interesting group IV semicon- ductor known for its high carrier mobility and has been considered for application in high-speed electronics. It also displays unique optical properties at the nanoscale and holds promise for application in photonics. 6,7 Many synthesis methods for preparation of Ge nanowires have been reported, e.g., laser ablation, 8 vapor transport, 9 low-temperature chemical vapor deposition (CVD), 10 and supercritical fluid-liquid-solid syn- thesis. 11,12 A vapor-liquid-solid (VLS) growth mechanism is often applied. The VLS process can be divided into two main steps: (1) the formation of a small liquid droplet, and (2) the alloying, nucleation, and growth of the nanowires. The catalysts used in preparing Ge nanowires include Au, 11-16 Bi, 17 and Fe. 18 Recently, a few synthesis methods have been reported to grow Ge nanowires without using any catalyst. The growth mechanisms of these methods include self-catalyzed Ge, 19 high boiling solvent catalysis, 20 and electrochemical etching. 21 In this work, we synthesized cubic-Ge (C-Ge) nanowires with a simple thermal decomposition of [(CH 3 (CH 2 ) 7 CHd CH(CH 2 ) 7 CH 2 NH 2 ) 4 Ge] 4+ ‚(Cl - ) 4 (TOG) without any help from catalysts and proposed a new mechanism for wire growth that was different from normal VLS. In the growth process of Ge nanowires, phase transformation was observed: amorphous Ge particles first transfer into liquid Ge nanodroplets and then crystallize into Ge nanowires. The diameter of Ge nanowires can be tuned between 10 and 115 nm by varying the concentra- tion of TOG in solution. Experimental Section Synthesis of TOG. The synthesis of [(CH 3 (CH 2 ) 7 CHd CH(CH 2 ) 7 CH 2 NH 2 ) 4 Ge] 4+ ‚(Cl - ) 4 (TOG) was as follows. 22 At room temperature, 0.47 mL (4 mmol) of GeCl 4 (99.99%) was dropped into 5 mL (16 mmol) of oleylamine (97%). After the first droplet was added, the reaction processed rapidly, vigor- ously releasing heat. The whole reaction took about 30 min. To avoid the hydrolysis of GeCl 4 in air, the reaction was protected in a glovebox under argon atmosphere. The product was a pale yellow colloid. More study is needed to well characterize the structure and properties of the precursor TOG. Synthesis of Ge Nanowires with Different Diameter Sizes. Ge nanowires were synthesized based on TOG decomposition at 360 °C. First, 10 mL of tri-n-octylamine (TOA) was heated in a three-neck, round-bottom flask to 200 °C in argon atmosphere. To collect the products, a piece of single-crystal silicon wafer (0.5 cm × 0.5 cm) was placed at the bottom of the flask. In another flask, colloid with a given mass was added to 5 mL of TOA. The mass of the colloid affects the diameter of the synthesized Ge nanowires. To dissolve the colloid, the * Corresponding authors. Phone: +86 571 8795 2107. Fax: +86 571 8795 2107. E-mail: jiangjz@zju.edu.cn (J.Z.J.); nanoljf@zju.edu.cn (J.F.L.). ² Department of Materials Science and Engineering, Zhejiang University. ‡ Analysis and Testing Centre, Zhejiang University. § Peking University. Figure 1. Characterization of C-Ge nanowires by thermal decomposi- tion of 1.8 × 10 -4 mol of TOG at 360 °C for 4 h. (a) SEM image of Ge nanowires. (b) HRTEM image of one C-Ge nanowire; the inset is an FFT image corresponding to the HRTEM image. (c) HRTEM image for one C-Ge nanowire with 25 nm diameter and an about 4 nm layer of GeO 2 on the surface. (d) HRTEM image for one end of one 20 nm C-Ge nanowire with [110] growth direction. 11157 J. Phys. Chem. C 2007, 111, 11157-11160 10.1021/jp0712553 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007