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Disruption of the Chemical Environment and Electronic Structure in p-Type Cu<sub>2</sub>O Films by Alkaline Doping
In this work we present an experimental and theoretical
study of
Cu<sub>2</sub>O films doped with alkaline ions (Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, and Cs<sup>+</sup>) prepared by Cu anodization.
By X-ray photoelectron spectroscopy we determined dopant incorporation
as high as 1% for Na<sup>+</sup>. Three oxygen species were found:
O<sup>2ā</sup> ions in the bulk cuprite structure, adsorbed
OH<sup>ā</sup> and oxygen
in hydroxylated dopant sites. The main effects of the alkaline doping
on the optical properties were a reduction in the direct band gap
and an approach of the acceptor level edge to the maximum of the valence
band. Electrochemical tunneling microscopy experiments confirmed that
the valence band maximum energy position is almost invariant. Additional
electrochemical impedance, photoelectrochemical activity, and current
sensing atomic force microscopy measurements showed an increase of
the carrier density and electrical conductivity and a reduction in
the photocurrent response with the dopant ion size. Urbach tail parameter
analysis suggested additional interaction between copper vacancy derived
states and dopant states. From first-principles calculations with
the B3LYP hybrid functional on models for the alkaline-doped Cu<sub>2</sub>O systems we determined that the main effect of the alkaline
substitution of copper atoms consists of polarizing the O states,
which causes a reduction in the insulating gap and splitting of the
density of states just below the Fermi level. The nature of the oxygenādopant
interaction was also calculated: there is a net attractive interaction
for LiāO, a slightly repulsive interaction for NaāO,
and a net repulsive interaction for KāO and CsāO. The
repulsive interactions between K<sup>+</sup> or Cs<sup>+</sup> and
O cause an accumulation of the dopant at the surface of the crystallites,
whereas for Na<sup>+</sup> and Li<sup>+</sup> the doping ions are
more uniformly distributed in the film bulk. It was found that the
surface accumulation of K<sup>+</sup> and Cs<sup>+</sup> hinders vacancy
diffusion and therefore blocks film growth, leading to a reduction
of roughness and thickness as the ion size increases