Chemical Vapor Deposition of Silica Micro- and Nanoribbons Using Step-Edge Localized Water Michael P. Zach, † John T. Newberg, Luiza Sierra, John C. Hemminger, and Reginald M. Penner* Department of Chemistry, UniVersity of California, IrVine, California 92697-2025 ReceiVed: February 21, 2003 Amorphous silica (SiO 2 ) ribbons were prepared by hydrolyzing SiCl 4 at highly oriented pyrolytic graphite (HOPG) surfaces on which water had condensed. Exposure to humid air (>30% relative humidity (RH)) at room temperature caused water to condense selectively at hydrophilic step edges on an otherwise hydrophobic graphite surface. Upon exposure of this surface to SiCl 4 vapor, a reaction with condensed water at step edges caused the formation of SiO 2 according to the reaction SiCl 4 + 2H 2 O f SiO 2 + 4HCl. The shape and size of the SiO 2 nanostructures varied with the RH: Below 20% RH, nanoparticles of SiO 2 , aligned at step edges, were observed. For RH of 35-50%, continuous nano- and microribbons of SiO 2 were obtained. For higher RH, micron-scale silca particles and ribbons were both observed. At a RH of 35%, silica ribbons as small as 80 nm (width) x 20-40 nm (height) up to 500 μm in length could be prepared. Repetitive dosing of the HOPG surface in the humid environment with SiCl 4 produced nano- and microribbons with a height and width that increased in proportion to square root of the SiCl 4 dose. I. Introduction Micron-scale fibers composed of silicon dioxide and other dielectrics are technologically important as conduits for light. 1,2 New methods for preparing dielectric nanofibers have recently emerged. Wang, Gole, and co-workers 3-6 have described a technique for preparing “nanobelts” composed of a variety of metal oxides including SiO 2 , SnO 2 , ZnO, In 2 O 3 , and others. These single crystalline structures are prepared by thermal evaporation and vapor transport of the metal oxide from powders at ≈1000-1400 °C. Wang et al. 7 have also demonstrated that amorphous silica nanowires can be obtained by vapor-liquid- solid (VLS) synthesis at 1150 °C using gallium catalyst droplets. Wu and co-workers 8 prepared amorphous SiO 2 nanowires by a carbothermal reduction reaction between silicon dioxide and activated carbons at 1350 °C. Leiber and co-workers 9,10 have employed laser ablation to control the diameters of crystalline silicon nanowires prepared by VLS at 440 °C. In this paper we demonstrate that graphite step edges can be used to template the formation of SiO 2 nanoribbons at room temperature using the strategy shown schematically in Figure 1. The basal plane surface of a highly oriented pyrolytic graphite (HOPG) crystal is traversed by a high density (10 3 -10 4 cm -2 ) of step edge defects that are approximately linear and up to a millimeter in length. These step edges can be used to template the growth by electrodeposition of metal 11 or conductive metal oxide (e.g., MoO 2 ) 12,13 nanowires, but insulators such as SiO 2 cannot be electrodeposited. Upon equilibration of a graphite surface with humid air, water condenses preferentially at hydrophilic defects on the surfacesespecially step edges. SiO 2 nano- and microribbons were obtained by exposing this “wet” HOPG surface to the SiCl 4 vapor: Previously, reaction 1 has been used to prepare silica films using atomic layer epitaxy (ALE) and atomic layer processing (ALP). In experiments by George et al., 14-16 silicon surfaces were exposed alternately to water and SiCl 4 under conditions where both reagents underwent surface-limited reactions. Ultrathin (<15 nm) silica layers of the desired thickness were thereby built up in 0.8-1.0 nm steps. Implementation of ALE involving reaction 1 has required either elevated temperatures (>600 K 16 ) or the use of catalysts (NH 3 , 17,18 pyridine 19 ). We demonstrate here that high-purity SiO 2 ribbons can be obtained using reaction 1 at room temperature without catalysts. * Corresponding author. E-mail: rmpenner@uci.edu. † Current address: Department of Earth and Planetary Sciences, Uni- versity of California, Berkeley, California 94720-4767. Figure 1. Method for forming SiO2 micro- and nanoribbons on HOPG step edges at room temperature. SiCl 4 + 2H 2 O f SiO 2 + 4HCl (1) 10.1021/jp034452k CCC: $25.00 © xxxx American Chemical Society PAGE EST: 4.4 Published on Web 00/00/0000