Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

In situ band gap mapping of the ${\mathrm{V}}_2{\mathrm{O}}_5(001)$ crystal surface revealed a reversible metal-to-insulator transition at 350–400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl ($\mathrm{V}=\mathrm{O}$) double rows. Supported by density functional theory and Monte Carlo simulations, the results are rationalized on the basis of the anisotropic growth of vanadyl-oxygen vacancies and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures irreversible surface reduction proceeds sequentially as ${\mathrm{V}}_2{\mathrm{O}}_5(001)\to {\mathrm{V}}_6{\mathrm{O}}_{13}(001)\to {\mathrm{V}}_2{\mathrm{O}}_3(0001)$.

Figure 1

(a) Perspective and top views of ${\mathrm{V}}_2{\mathrm{O}}_5(001)$. The unit cell is indicated. The arrows indicate the vanadyl ($\mathrm{V}=\mathrm{O}$) species. (b) STM image ($15\times 15{\mathrm{nm}}^2$, $-1.1\mathrm{V}$, 0.45 nA) of the vacuum-cleaved ${\mathrm{V}}_2{\mathrm{O}}_5(001)$ surface at 300 K showing double rows structure. (c) High-resolution STM image ($4\times 2{\mathrm{nm}}^2$, $-1.1\mathrm{V}$, 0.45 nA). (d), (g) Simulated STM image of the occupied (${\mathrm{V}}_2{\mathrm{O}}_5$) and unoccupied (${\mathrm{V}}_6{\mathrm{O}}_{13}$) states based on DFT calculations. (e), (f) STM images of the ${\mathrm{V}}_2{\mathrm{O}}_5$ single crystal surface after flashing once to 800 K ($5\times 5{\mathrm{nm}}^2$, $+0.18\mathrm{V}$, 0.3 nA) and after several flashes to 800 K ($15\times 15{\mathrm{nm}}^2$, $-1.3\mathrm{V}$, 0.3 nA), respectively.

Figure 2

(a) STM image ($50\times 50{\mathrm{nm}}^2$, $-1.3\mathrm{V}$, 0.24 nA) of the ${\mathrm{V}}_2{\mathrm{O}}_5(001)$ surface at 350 K. (b) Normalized conductance spectra over areas $A$ and $B$ at 350 K, and over ${\mathrm{V}}_2{\mathrm{O}}_5(001)$ at 300 K. (c), (d) Band gap maps of the surface recorded at 350 K (c) and 400 K (d). The contours of the $B$ domains as imaged in (a) are superimposed with the band map on (c). (e), (f) Simulated gap maps at 300 K for different simulation conditions (see text). The band gap scale (in eV) is indicated.

Figure 3

Calculated density of states of the clean ${\mathrm{V}}_2{\mathrm{O}}_5(001)$ surface (blue, for online version), of the defective surface containing isolated vanadyl-oxygen vacancies (yellow), and of the missing-row structure (red). The curves are smoothed by a Gaussian level broadening of 0.2 eV.

Figure 4

(Left) 2D-periodic network used for Monte Carlo simulations. Each node is occupied by a reduced or oxidized vanadyl termination. (Middle) Geometrical characteristics of the network. (Right) Interaction parameters between a given reduced site ($X$) and other reduced sites.