Biaxial ZnO-ZnS Nanoribbon Heterostructures Michael W. Murphy, † P. S. Grace Kim, ‡ Xingtai Zhou, †,§ Jigang Zhou, †,| Martin Coulliard, ‡,⊥ Gianluigi A. Botton,* ,‡ and Tsun-Kong Sham* ,† Department of Chemistry, UniVersity of Western Ontario, London N6A 5B7, Canada, and Brockhouse Institute for Materials Research, McMaster UniVersity, Hamilton L8S 4M1, Canada ReceiVed: January 15, 2009; ReVised Manuscript ReceiVed: February 18, 2009 Biaxial ZnO-ZnS heterostructure nanoribbons (NRs) have been synthesized using the vapor-liquid-solid technique by the thermal evaporation of ZnS powder in the presence of a Au nanoparticles catalyst, a limited supply of oxygen and an inert carrier gas under controlled temperature and pressure. High resolution transmission electron microscopy (HRTEM) reveals that the nanoribbon is composed of two biaxially grown nanoribbons of single crystal (wurtzite) of ZnS and ZnO with widths of 10-100 and 20 -500 nm, respectively, with an Au nanoparticle tip but no clearly detectable defects. The interfacial region is smooth and narrow (∼10 nm for a wire of several 100 nm wide). It is proposed that a composite was first formed via single catalyst confined growth; that is, while ZnS grows vertically via precipitation from the saturated solution in Au, the ZnO component on the surface of the ZnS subsequently served as the substrate for laterally growth- forming ZnO nanoribbon. The implication of this technique for the synthesis of other composite nanostructures is noted. Introduction The properties of nanomaterials depend heavily on their structures, morphologies, and sizes. 1,2 Understanding the proper- ties of one dimensional (1D) nanostructures is of critical importance in comprehending the fundamental roles of dimen- sionality, surface, and quantum size effects. 3 Thus this informa- tion is vital to our ability to tailor the physical properties of 1D nanostructures for new functional nanoscale devices. In par- ticular, heterostructures can provide p-n junctions and more complex electronic structures such as bipolar transistors. 4 While the majority of work using 1D nanostructures has focused on homogeneous nanostructures, most commonly nanowires (NWs) and nanoribbons (NRs) (to study their associated optical and electronic properties) composite heterostructures have recently become of interest. Currently, a variety of heterostructured 1D nanostructures have been synthesized including Si-SiGe and GaAs-GaP superlattice NWs, biaxial TiO 2 -SnO 2 , ZnO-Ge, CdSe-Si, and ZnS-Si NRs using laser ablation, as well as (template-induced) thermal evaporation methods. 5-11 The het- erostructures used in this study are composed of ZnS and ZnO, which are both well-known semiconductors with bandgaps of 3.54 and 3.37 eV, respectively, and therefore excellent photo- luminescence materials. Several ZnS and ZnO nanostructures, including nanoneedles, NWs, NRs, and nanoflowers, synthesized by thermal evaporation techniques have been reported. 12-16 Furthermore, composite ZnO-ZnS nanostructures including ZnO-ZnS nanocables and nanocombs have been synthesized in a two-step process utilizing the sulfidation or oxidation of premade ZnS and ZnO nanostructures, respectively. 17-20 The sulfidation and oxidation processes both result in a core-shell nanostructure in which only one component is accessible to the surrounding. More recently, Shen et al. has synthesized sawlike ZnO nanobelt/ZnS NWs heterostructures through the epitaxial growth of ZnS NWs on ZnO nanobelts. 21 Wang et al. has had similar success in the epitaxial growth of ZnO nanowires on ZnS nanobelts. 22 Here, we report the synthesis and structure of biaxial ZnO-ZnS heterostructured NRs, which can be formed via single-catalyst confined anisotropic crystal growth. Experimental Method ZnO-ZnS NRs were prepared by the thermal evaporation of commercial ZnS powder (Alfa Aesar, 99.99%). ZnS was placed in an alumina boat at the center of an alumina tube inside a high temperature horizontally mounted furnace. Argon, the carrier gas, was introduced into the tube at a flow rate of 100 sccm (standard cubic centimeters per minute). A limited amount of oxygen (0.2-0.9%) was present in the carrier gas only during the reaction phase. A silicon wafer coated with gold nanoparticle catalysts was used as a substrate and placed downstream of the carrier gas. The temperature of the furnace was increased at a rate of 18 °C/min until a temperature of 1050 °C was reached and held for 2 h. The pressure was reduced to 200 Torr for the duration of the reaction. After the reaction was completed, the furnace was cooled to room temperature and a white/gray product was found covering the Si substrate. The nanostructures were characterized by X-ray diffraction (XRD, Rigaku, Co K R radiation, λ ) 0.1792 nm), scanning * To whom correspondence should be addressed. E-mail: (T.K.S.) tsham@ uwo.ca; (G.A.B.) gbotton@mcmaster.ca. † University of Western Ontario. ‡ McMaster University. § Current address: Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800 People’s Republic of China. | Current address: Canadian Light Source Inc., University of Saskatchewan, 101 Perimeter Road, Saskatoon, SK, Canada S7N 0X4. ⊥ Current address: Semiconductor Insights, 3000 Solandt Road, Kanata, ON, Canada K2K 2M8. 4755 10.1021/jp900443g CCC: $40.75  2009 American Chemical Society Published on Web 03/02/2009 2009, 113, 4755–4757