Superhydrophobic Silicon Surfaces with Low Light Reflectivity
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Superhydrophobic Silicon Surfaces with Low Light Reflectivity Yonghao Xiu1,2, Yan Liu2, Dennis W Hess1 and C. P. Wong2* 1 School of Chemical and Biomolecular Engineering, 311 Ferst Drive, Atlanta, GA 30332, U.S.A. 2 School of Materials Science and Engineering, 771 Ferst Drive, Atlanta, GA 30332, U.S.A. *[email protected] ABSTRACT Creation of superhydrophobic self-cleaning surfaces is an important objective for a variety of applications. Indeed, numerous routes to generate superhydrophobic surfaces have been proposed. In this paper, a facile way of forming superhydrophobic surfaces is reported that uses Au assisted HF/H2O2 etching of silicon wafers. The Au layer was deposited onto a silicon wafer via e-beam evaporation. By controlling the evaporation and etching times, the surface roughness can be manipulated and superhydrophobic surfaces with reduced light reflection can be generated. Contact angles were measured with a CCD camera equipped goniometer; these values determined the water repellency. Light reflection on the as prepared black surfaces was measured to assess the efficiency for low cost solar cell applications. This approach offers a new way both to theoretically study the surface roughness effect and to investigate engineering applications of self-cleaning surfaces in solar cells, MEMS, anti-bacteria coating, and microfluidic devices. INTRODUCTION Over the last decade, much attention has been directed to semiconductor materials with surface micro/nano structures, primarily due to potential applications in opto-electronics, chemical and biochemical sensing and the possibility of creating material properties not readily obtained in the corresponding crystalline materials[1]. The surface micro/nano structures of semiconductors can be produced by anodic etching, where the semiconductor is biased positively in a conductive electrolyte to facilitate oxidation and subsequent removal of surface atoms. The magnitude of the applied voltage and current and the electrolyte composition are used to control etch rate and, thus, morphology and properties. This approach is hampered by the need for a conducting substrate that is stable under the electrochemical etching conditions and the need to explore the relatively large parameter space to identify the optimum etching conditions. To circumvent these problems, an efficient chemical etching method to produce Si surface structures that requires no external bias was developed recently[2, 3]. In this process, a discontinuous layer of Pt or Au (20 - 200 Å thick) is deposited on the silicon surface before immersing it in HF/H2O2. Reduction of the oxidant (H2O2) injects holes into the valence band of the semiconductor material, which then participate in oxidative etching of the semiconductor surface. Superhydrophobic self-cleaning surfaces are of interest for a variety of applications such as self-cleaning, water repellency and anticorrosion[4, 5] properties. Numerous approaches to generate superhydrophobic surfaces have been proposed[6-11]. In these studies,
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