Nanometer Scale Electronic Reconstruction at the Interface between ${\mathrm{LaVO}}_3$ and ${\mathrm{LaVO}}_4$

Electrons at interfaces, driven to minimize their free energy, are distributed differently than in bulk. This can be dramatic at interfaces involving heterovalent compounds. Here we profile an abrupt interface between V $3d^2$ ${\mathrm{LaVO}}_3$ and V $3d^0$ ${\mathrm{LaVO}}_4$ using electron energy loss spectroscopy. Although no bulk phase of ${\mathrm{LaVO}}_x$ with a V $3d^1$ configuration exists, we find a nanometer-wide region of V $3d^1$ at the ${\mathrm{LaVO}}_3/{\mathrm{LaVO}}_4$ interface, rather than a mixture of V $3d^0$ and V $3d^2$. The two-dimensional sheet of $3d^1$ electrons is a prototypical electronic reconstruction at an interface between competing ground states.

Figure 1

V-$L_{2,3}$ and O-$K$ electron energy loss spectra taken from bulk ${\mathrm{LaVO}}_3$, ${\mathrm{SrVO}}_3$, and ${\mathrm{LaVO}}_4$. The dashed lines indicate the $L_3$ peak position, which shifts to higher energies as the vanadium valence increases. The shaded region of the spectra is due to V-$L_{2,3}$ excitations only, whereas at higher energies the EELS signal is composed of V and O-$K$ edge contributions. The measurements were performed on the FEI Tecnai F20-ST equipped with a GIF 865-ER.

Figure 2

(a) Cross-sectional ADF image of a ${\mathrm{LaVO}}_x$ film grown at $T_g=800°\mathrm{C}$ and $P_{\mathrm{O}2}={10}^{-4}\mathrm{Torr}$ showing mixed phase growth of 3D ${\mathrm{LaVO}}_4$ islands and flat ${\mathrm{LaVO}}_3$ on a ${\mathrm{SrTiO}}_3$ substrate. (b) $\mathrm{V}\mathrm{\text{−}}{L}_{2,3}/\mathrm{O}\mathrm{\text{−}}K$ electron energy loss spectra taken from an island and from the smooth film, respectively. (c) AFM image of the same film showing both flat regions of ${\mathrm{LaVO}}_3$ and 3D islands of ${\mathrm{LaVO}}_4$.

Figure 3

(a) Low magnification ADF image of a ${\mathrm{LaVO}}_3$ film grown at $T_g=800°\mathrm{C}$ and $P_{\mathrm{O}2}={10}^{-4}\mathrm{Torr}$, where ${\mathrm{LaVO}}_4$ 3D islands are marked with arrows. (b) Higher magnification ADF image of a ${\mathrm{LaVO}}_3$ film grown at $T_g=800°\mathrm{C}$ and $P_{\mathrm{O}2}={10}^{-5}\mathrm{Torr}$ showing a bright band at the interface between the ${\mathrm{LaVO}}_4$ and the ${\mathrm{SrTiO}}_3$ substrate. (c) The crystal structure of the monoclinic ${\mathrm{LaVO}}_4$ recorded along the [010] crystallographic direction for the area marked by the box in (b). (d) The sharp interface between the ${\mathrm{LaVO}}_4$ and the bright band.

Figure 4

(a) V-$L_{2,3}$ electron energy loss spectra recorded across the ${\mathrm{LaVO}}_4/{\mathrm{LaVO}}_3$ interface (dotted lines). The solid lines show the fits to the experimental spectra using reference spectra shown in (b). (c) ADF image of the interface where the electron beam position is denoted by the white dotted line. The measurements (a),(c) were performed on the VG HB-501 100 kV dedicated STEM equipped with a parallel electron energy loss spectrometer.

Figure 5

(a) The fractions of ${\mathrm{V}}^{3+}$, ${\mathrm{V}}^{4+}$, and ${\mathrm{V}}^{5+}$ across the interface obtained by a three-component fit to the experimental data shown in Fig. 4a using the reference spectra in Fig. 4b. These fractions were used to color code the fits to the experimental data in Fig. 4a. The root-mean square residuals of the fit are shown below. (b) A two-component fit of the data using ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{5+}$ reference spectra results in large residuals at the interface. (c) Total vanadium valence across the ${\mathrm{LaVO}}_4/{\mathrm{LaVO}}_3$ interface as determined from (a).