IMPROVEMENT OF HEIGHT UNIFORMITY OF ZnO NANOWIRE ARRAYS BY USING ELECTROPOLISHING METHOD Nano-Scale Measurement & AnaLysis Lab. Mee Na Park 1,*, Ee-Le Shim 1, Joonho Bae 3, C. J. Kang 1,2, and Young Jin Choi 1,2** 1 Department of Physics, MyongJi University, Yongin, , Korea 2 Dept. of Nanoscience & engineering, MyongJi University, Yongin, , Korea 3 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA , USA Motivation & Purpose Synthesis of ZnO nanowire Experimental Summary & Future work Results An aqueous solution of phosphoric acid is used as the electrolyte to planarize the top surfaces of ZnO nanowire arrays. Applied voltage ranged from 7 to 15 V in phosphoric acid (pH 5.0). To control and improve the uniformity of heights of ZnO nanowire arrays Electropolishing method ◎ Applications of ZnO nanostructures - solar cells, chemical sensors, field emitters and nanogenerators. ◎ In order to be successfully incorporated into nanodeives, the nanowires have to be grown with a high degree of control in uniformity. Principle of Electropolishing Ir electrolyte stirrer Anode cathode N.W cation anion DC Power supply V A ∆ up ∆ down Diffusion layer Additives Before E.P After E.P Bulk concentration additive species Electropolishing(EP) has been conventionally used to remove a thin layer of material on the surface of a metal part. The basic mechanism of EP is based on the differential rates of metal removal in an aqueous solution of acid. Areas of metal with high current densities are rapidly dissolved, resulting in the smoothing of metal surfaces. EP is the electrolytic removal of metal in a highly ionic solution by means of an electrical potential and current. Before E.P After E.P The height variation of nanowires was reduced after EP had been performed. In order to optimize EP conditions for the planarization of nanowire arrays, the statistical analysis of the roughness change is in progress. * Main experimental parameters - pH value of the acid - applied voltage - current - reaction time Apparent planarization of protruded ZnO nanowires Roughness measurement. Zinc solution : Zinc nitrate hexahydrate + Hexamethylenetetramine + Deionized water. Concentration of solution : 7.5 mM. Temperature : 70~90 ℃. Synthesis time : 6h Hot plate Zinc solution Hydrothermal Method ※ To directly observe the morphology change when nanowires were electropolished, patterned ZnO nanowire arrays were fabricated on Si substrate, and the same regions of nanowire arrays were compared before and after the EP process. Reference [1] M.Al-Ajlouni and A.Al-Hamdan, J. Applied Sci. 8(10), pp (2008). [2] Shih-Chieh Chang, Jia-Min Shieh, Electrochemical and Solid-State Letters. 6(5), G72-G74 (2003). [3] Sheng Xu, Yaguang Wei, Zhong Lin Wang*, J. Am. Chem. Soc. 130,pp.14958–14959(2008). [4] M. Law, L. E. Greene, P. D. Yang, Nature Materials 4, pp. 455, (2005). ※ SEM image of patterned ZnO nanowire arrays (CH 2 ) 6 N H 2 O ↔ 4 NH HCHO, (1) NH 3 + H 2 O ↔ NH 3 ·H 2 O, (2) NH 3 ·H 2 O ↔ NH OH −, (3) Zn OH − ↔ Zn(OH) 2, (4) Zn(OH) 2 → ZnO + H 2 O. (5) Same position Before E.P After E.P (b) Line profile E.P condition ; 15 V, 1min, pH 5.0 (a) As-grown ZnO nanowire arrays (b) After EP 7V, 1h, pH 5.0 (a) Typical AFM image of as-grown ZnO nanowire arrays planarization