Epitaxial Electrodeposition of Copper(I) Oxide on Single-Crystal Copper Julie K. Barton, Alexey A. Vertegel, Eric W. Bohannan, and Jay A. Switzer* Department of Chemistry and Graduate Center for Materials Research, University of MissourisRolla, Rolla, Missouri 65409-1170 Received September 5, 2000. Revised Manuscript Received December 6, 2000 Epitaxial thin films of copper(I) oxide (Pn3m, a ) 0.427 nm) were electrodeposited onto [110]-, [111]-, and [100]-oriented single-crystal copper (Fm3m, a ) 0.3615 nm) by reduction of copper(II) lactate in solution. Cu 2 O films grown on Cu(110) and Cu(111) exhibited both an out-of- and in-plane orientation following that of the substrate as measured by 2θ and azimuthal X-ray scans, up to a thickness of 0.8 μm. X-ray diffraction studies showed that Cu 2 O films deposited onto Cu(100) grow initially with a near-[111] orientation up to a critical thickness, beyond which film growth is primarily in the [100] direction. The films were found to be both in- and out-of-plane oriented throughout, as measured by azimuthal X-ray scans. In situ 2θ X-ray measurements showed a critical thickness for growth in the [100] direction of about 360 nm. As determined from scanning electron microscopy images, the Cu 2 O films deposited onto Cu(100) grew with triangular facets consistent with the [111] orientation prior to the critical thickness, and then as pyramidal islands over the initial triangular layers above this thickness. A proposed interface model of Cu 2 O(111) over Cu(100) yields a low mismatch and a high number of atomic contact points per unit area, offering a possible explanation for the initial [111]-oriented deposition. Introduction Electrodeposition is a simple and inexpensive tech- nique for preparing epitaxial films. We have recently electrodeposited epitaxial thin films of δ-Bi 2 O 3 , Cu 2 O, PbS, Tl 2 O 3 , Fe 3 O 4 , and a Cu 2 O/PbS heterojunction onto single-crystal Au. 1-7 However, little work has been done with epitaxial electrodeposition onto substrates other than Au. In the present work, it is shown that this technique may be extended to the deposition of epitaxial thin films of Cu 2 O onto single-crystal Cu. Cu 2 O is a p-type semiconductor that has been elec- trodeposited previously on a variety of substrates. 8-11 In previous studies of the electrodeposition of Cu 2 O films, we have demonstrated that the out-of-plane orientation of a film deposited on a polycrystalline substrate is dependent on the solution pH. 12 In addition, scanning electron microscopy (SEM) images revealed that these films grow with a faceted microstructure corresponding to the out-of-plane orientation of the film, yielding four-sided pyramids or triangular structures for [100]- and [111]-oriented films, respectively. 12 Lee et al. deposited Cu 2 O onto Pt from a weakly acidic solution and reported a film with a [111] out-of-plane orienta- tion. 13 In both experiments, geometric facets grew with no particular orientation within the plane of the film. In other studies, we have shown that the deposition current for the Cu 2 O is limited by a Schottky-like barrier that forms between Cu 2 O and the deposition solution. 12 We have also shown that under certain galvanostatic conditions the working electrode potential spontaneously oscillates, 14,15 forming a Cu/Cu 2 O-layered nanostructure that shows a negative differential resis- tance (NDR) feature during perpendicular transport measurements. 16 The 1/L 2 dependence of the NDR feature on the Cu 2 O layer thickness suggests quantum confinement of carriers in the nanoscale Cu 2 O. 17 Al- though these layered nanostructues have, at present, only been deposited on polycrystalline substrates, it * E-mail: jswitzer@umr.edu. (1) Switzer, J. A.; Shumsky, M. G.; Bohannan, E. W. Science 1999, 284, 293. (2) Bohannan, E. W.; Jaynes, C. C.; Shumsky, M. G.; Barton, J. K.; Switzer, J. A. Solid State Ionics 2000, 131, 97. (3) Vertegel, A. A.; Shumsky, M. G.; Switzer, J. A. Angew. Chem., Int. Ed. 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Langmuir 1999, 15, 813. (15) Switzer, J. A.; Hung, C.-J.; Huang, L.-Y.; Miller, F. S.; Zhou, Y.; Raub, E. R.; Shumsky, M. G.; Bohannan, E. W. J. Mater. Res. 1998, 13, 909. (16) Switzer, J. A.; Hung, C.-J.; Huang, L.-Y.; Switzer, E. R.; Kammler, D. R.; Golden, T. D.; Bohannan, E. W. J. Am. Chem. Soc. 1998, 120, 3530. (17) Switzer, J. A.; Maune, B. M.; Raub, E. R.; Bohannan, E. W. J. Phys. Chem. B 1999, 103, 395. 952 Chem. Mater. 2001, 13, 952-959 10.1021/cm000707k CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001