Sharp contrast in the electrical and optical properties of vanadium Wadsley (VmO2m+1,m>1) epitaxial films selectively stabilized on (111)-oriented Y-stabilized ZrO2

Abstract

Four oxidation states $({\mathrm{V}}^{2+},{\mathrm{V}}^{3+},{\mathrm{V}}^{4+},\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{5+})$ in vanadium oxides and the conversion between them have attracted attention for application to batteries and electronics. Compared to single-valence counterparts, however, there have been few reports on the fundamental properties of mixed-valence vanadium oxide films, as their complexity and closeness in thermodynamic phase diagrams hinder the formation of pure phases in film. Here, using an epitaxial growth technique with precise control of oxygen partial pressure (20--100 mTorr) on (111)-oriented Y-stabilized $\mathrm{Zr}{\mathrm{O}}_{2}$, we selectively stabilize pure phases of $\mathrm{V}{\mathrm{O}}_{2}(\mathrm{B})$ $(m=\ensuremath{\infty}), {\mathrm{V}}_{6}{\mathrm{O}}_{13}$ $(m=6)$, and ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ $(m=2)$, so-called Wadsley phases $({\mathrm{V}}_{m}{\mathrm{O}}_{2m+1},mg1)$ in which ${\mathrm{V}}^{4+}$ and/or ${\mathrm{V}}^{5+}$ can coexist. Fractional increase of ${\mathrm{V}}^{4+}$ changes the electrical ground state, insulating $\mathrm{V}{\mathrm{O}}_{2}(\mathrm{B})$ and ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$, metallic ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ transition into insulators below 150 K. While $\mathrm{V}{\mathrm{O}}_{2}(\mathrm{B})$ and ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ exhibit strong spectral weights at low photon energy in the room-temperature extinction coefficients, the band-edge absorption shifts toward higher photon energy for smaller $m$, opening an indirect band gap of 2.6 eV in ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$. The sharp contrast of electrical and optical properties between vanadium Wadsley phases highlights the importance of precisely controlling the oxidation state of vanadium.