Engineering two-dimensional electron gases at the (001) and (101) surfaces of ${\mathrm{TiO}}_2$ anatase using light

We report the existence of metallic two-dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating ${\mathrm{TiO}}_2$ anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of $d_{xy}$ orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nanoengineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase.

Figure 1

Angle-inte-grated spectra of (a) the Ti-$3p$ peak, and (b) the in-gap state of the anatase ${\text{TiO}}_2$ (001) and (101) surfaces (upper and lower curves, respectively) measured at $h\nu =100$ eV. The black curve was measured shortly after the first light exposure, and the blue, red, and green curves at sequentially increasing times. (c) Binding energy of the ${\text{Ti}}^{4+}$ peak and concentration of oxygen vacancies (ratio between the ${\text{Ti}}^{3+}$ and total Ti-$3p$ peak area) for different durations of irradiation exposure of the (101) surface (upper panel), and binding energy and intensity of the in-gap state (lower panel). Analogous results, not shown, are obtained for the (001) surface.

Figure 2

(a) Bulk Brillouin zone of anatase, showing the relevant high-symmetry directions (green) and planes (red and blue) of the Fermi surfaces and band dispersion at the (001) surface. (b) Fermi surface map (second derivative of ARPES intensity, negative values) measured at $h\nu =47$ eV on a cleaved, bulk-insulating ${\text{TiO}}_2$ anatase (001) surface. The red lines indicate the edges of the bulk Brillouin zones in the (001) plane around ${\mathrm{\Gamma }}_{012}$. The same relative color scale indicating minimum (min) to maximum (max) ARPES intensities is used in all color figures in the rest of this work. (c) Fermi surface map (second derivative of ARPES intensity, negative values) in the $k_{\langle 001\rangle }\text{−}{k}_{\langle 010\rangle }$ or $\left(100\right)$ plane, acquired by varying the photon energy in 1 eV steps between $h{\nu }_1=38$ eV and $h{\nu }_2=85$ eV. To calculate the momentum perpendicular to the surface we set the inner potential to $V_0=13$ eV [29, 38]. The blue lines are the bulk Brillouin zones containing ${\mathrm{\Gamma }}_{102}$ and ${\mathrm{\Gamma }}_{103}$. (d) Energy-momentum map measured at $h\nu =47$ eV along the $\langle 010\rangle$ direction on the (001) surface. Dashed red lines are parabolic fits to the dispersing light bands. Red arrows show a kink attributed to electron-phonon coupling [29]. (e)–(h) Same as (a)–(d) on the anatase (101) surface. Data in panels (f) and (h) were measured at $h\nu =47$ eV. The green dashed arrow in (f) indicates the direction of measurements along $k_y$ in (g) and (h), which is slightly off $\langle 010\rangle$. The out-of-plane Fermi surface map (second derivative of ARPES intensity, negative values) in (g) was measured by varying the photon energy in 1 eV steps between $h{\nu }_1=33$ eV and $h{\nu }_2=51$ eV and between $h{\nu }_3=76$ eV and $h{\nu }_4=103$ eV. The Supplemental Material [30] includes figures for the raw spectra, Fermi surface maps, and the periodicity of the electronic structure in the (001) and (101) planes.

Figure 3

Energy-momentum maps measured at $h\nu =47$ eV along the $\langle 010\rangle$ direction at the anatase (001) surface for different UV doses. (a) After a few minutes of UV irradiation. (b) After several hours of exposure to UV light. (c) After moving the UV spot to a region in the sample not irradiated before, and illuminating again for a few minutes. The white curves correspond to the MDCs integrated over $E_F\pm 10$ meV, and are plotted using the same absolute vertical scale in the three panels.