Epitaxial Chemical Deposition of ZnO Nanocolumns from NaOH Solutions
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Epitaxial Chemical Deposition of ZnO Nanocolumns from NaOH Solutions Renee B. Peterson1, Clark L. Fields1, Brian A. Gregg National Renewable Energy Laboratory Golden, CO 80401 1 Department of Chemistry and Biochemistry, University of Northern Colorado Greeley, CO 80639 ABSTRACT A new method of depositing epitaxial ZnO nanocolumns on sputter-coated ZnO substrates is described that utilizes supersaturated zincate species in sodium hydroxide solutions and requires no complexing agents. Uniform arrays of columns are grown reproducibly over entire substrates in 10 to 50 min. Columns are 50 to 2000 nm long and 50 to 100 nm wide. Strict substrate cleaning and/or preparations are not necessary with this method, in contrast to many other techniques. Films grow only on substrates pre-coated with ZnO, not on bare glass or ITOor SnO2-coated glass. Factors affecting the column growth are elucidated and experimental observations are correlated with crystal growth theory. INTRODUCTION In the last decade new interest has been focused on epitaxial ZnO nanocolumns having high surface area and specific crystalline orientation1, because of their unique optical, chemical, and electrical properties and that they are nontoxic, inexpensive and chemically stable.2 Our interest in these materials is for use as substrates in high surface area solar cells. We therefore required a method for forming these columnar arrays that was easily reproducible and applicable to large substrates. Due to the large surface area of the ZnO columns, the light-absorbing layers coated on them can be made thinner, leading to reduced transport limitations in the cell.3,4 These films are columnar, not particulate as are the high surface area TiO2 films,5 therefore little current should be lost due to grain boundaries.2 There are many ways to grow thin films of zinc oxide crystallites. Some common methods of deposition are metalorganic chemical vapor deposition,5 radio frequency magnetron sputtering,6 molecular beam epitaxy,7 spray pyrolysis,8 and pulsed laser deposition.9 More recently solution methods such as cathodic electrodeposition,7,10,11 thermal decomposition,1,12 and the use of complexing agents13 have been reported as low cost, low temperature methods of ZnO column production, in which several variables can be fine-tuned to produce different properties. We report here a new solution method for ZnO column deposition that employs an aqueous solution containing NaOH and Zn(NO3)2 and substrates coated with sputtered-on ZnO. The resulting quasi-epitaxial films of highly-ordered columns have the reproducibility and uniformity over large areas to be used in the development of solar cells and other devices. The parameters that control column growth are explored and correlations to crystal growth theory are proposed.
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EXPERIMENTAL DETAILS AND RESULTS The following general procedure for columnar film deposition was used. The substrate was submerged in hot aqueous solutions (covered but not sealed) of Zn(NO3)2 ▪ 6H2O (Aldrich) and NaOH (Baker). Zinc
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