RYU ET AL . VOL. 7 ’ NO. 3 ’ 2090–2098 ’ 2013 www.acsnano.org 2090 February 17, 2013 C 2013 American Chemical Society On Demand Shape-Selective Integration of Individual Vertical Germanium Nanowires on a Si(111) Substrate via Laser-Localized Heating Sang-gil Ryu, † Eunpa Kim, † Jae-hyuck Yoo, † David J. Hwang, § Bin Xiang, †,‡ Oscar D. Dubon, ‡ Andrew M. Minor, ‡ and Costas P. Grigoropoulos †, * † Department of Mechanical Engineering and ‡ Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States, and § Department of Mechanical Engineering, State University of New York at Stony Brook, New York 11794, United States T he size, shape, surface roughness, and facets of nanowires (NWs) regulate the electrical 1,2 and thermal 3À5 trans- port, as well as the optical properties. 6,7 In particular, light interactions with semicon- ductor materials can be engineered for absorp- tion and scattering enhancement and reflec- tance reduction, depending on the NW shape. 8,9 Hence, NWs have been used in the context of photovoltaic, 7,10 optoelec- tronic, 11À13 and sensing applications. 14À16 Despite interesting characteristics of NWs, it has not been straightforward to embed multiple properties and functionalities in a single NW-based device. The composition can be varied along the axial direction by controlling the lateral growth rate by a gas onÀoff process. 17 However, the broad inter- facial length induced by the vaporÀliquidÀ solid (VLS) growth mechanism hinders sharp property di fferentiation. On-demand discrete assembly and integration of individual NWs with di fferent properties in a planar layout would require elaborate optical/optoelectronic tweezing. 18,19 On the other hand, out-of-plane and vertically oriented architectures are more desirable for the direct synthesis and discrete integration of functionalized NWs with enhanced device performance. 20À23 Vertical silicon nanowire (SiNW) arrays or single Si nanopillars having different diam- eters that were fabricated by top-down etching combined with electron beam litho- graphy (EBL) could induce changes of reflected color or enhance the Raman scattering. 24,25 Bottom-up processes, including VLS assisted by EBL, could also yield NWs of di fferent size, 26 although lack control of the NW geometric features, including tapering and facets. In chemical vapor deposition (CVD) pro- cesses, a focused laser beam has been uti- lized to achieve site-selective growth of semi- conductor materials since the respective kinetics follow an Arrhenius behavior. 27,28 Specifically, spatially confined and tempo- rally modulated continuous wave (CW) laser irradiation can impose a short heating and cooling time scale and hence sufficient pre- cision for controlling the localized growth time and temperature. 29 By taking advantage * Address correspondence to cgrigoro@me.berkeley.edu. Received for review September 22, 2012 and accepted February 16, 2013. Published online 10.1021/nn400186c ABSTRACT Semiconductor nanowire (NW) synthesis methods by blanket furnace heating produce structures of uniform size and shape. This study overcomes this constraint by applying laser- localized synthesis on catalytic nanodots defined by electron beam lithography in order to accomplish site- and shape-selective direct integration of vertically oriented germanium nanowires (GeNWs) on a single Si(111) substrate. Since the laser-induced local temperature field drives the growth process, each NW could be synthesized with distinctly different geometric features. The NW shape was dialed on demand, ranging from cylindrical to hexagonal/irregular hexagonal pyramid. Finite difference time domain analysis supported the tunability of the light absorption and scattering spectra via controlling the GeNW shape. KEYWORDS: germanium nanowires . vertical orientation . site- and shape-selective integration . laser-assisted growth . facet . taper ARTICLE