Direct structural and spectroscopic investigation of ultrathin films of tetragonal CuO: Six-fold coordinated copper

¹ MESA + Institute for Nanotechnology, University of Twente - P.O. Box 217, 7500AE, Enschede, The Netherlands
² EMAT, University of Antwerp - Groenenborgerlaan 171, B-2020 Antwerp, Belgium
³ Department of Chemical Engineering and Materials Science, University of California-Davis - Davis, CA 95616, USA
⁴ Materials Science and Technology Division, Oak Ridge National Laboratory - Oak Ridge, TN 37831, USA
⁵ Advanced Light Source, Lawrence Berkeley National Laboratory - 1 Cyclotron Rd, MS6R2100, Berkeley, CA 94720, USA

^(b) g.koster@utwente.nl (corresponding author)

Received: 17 November 2013
Accepted: 22 December 2013

Abstract

Unlike other 3d transition metal monoxides (MnO, FeO, CoO, and NiO), CuO is found in a low-symmetry distorted monoclinic structure rather than the rocksalt structure. We report here of the growth of ultrathin CuO films on SrTiO₃ substrates; scanning transmission electron microscopy was used to show the stabilization of a tetragonal rocksalt structure with an elongated c-axis such that and the Cu-O-Cu bond angle , pointing to metastable six-fold coordinated Cu. X-ray absorption spectroscopy demonstrates that the hole at the Cu site for the CuO is localized in orbital unlike the well-studied monoclinic CuO phase. The experimental confirmation of the tetragonal structure of CuO opens up new avenues to explore electronic and magnetic properties of six-fold coordinated Cu.