Atomically Precise Lateral Modulation of a Two-Dimensional Electron Liquid in Anatase TiO₂ Thin Films

Abstract

Engineering the electronic band structure of two-dimensional electron liquids (2DELs) confined at the surface or interface of transition metal oxides is key to unlocking their full potential. Here we describe a new approach to tailoring the electronic structure of an oxide surface 2DEL demonstrating the lateral modulation of electronic states with atomic scale precision on an unprecedented length scale comparable to the Fermi wavelength. To this end, we use pulsed laser deposition to grow anatase TiO 2 films terminated by a (1 × 4) in-plane surface reconstruction. Employing photo-stimulated chemical surface doping we induce 2DELs with tunable carrier densities that are confined within a few TiO 2 layers below the surface. Subsequent in-situ angle resolved photoemission experiments demonstrate that the (1 × 4) surface reconstruction provides a periodic lateral perturbation of the electron liquid. This causes strong backfolding of the electronic bands, opening of unidirectional gaps and a saddle point singularity in the density of states near the chemical potential.Keywords: anatase titanium dioxide, angle resolved photoemission spectroscopy, two---dimensional electron liquid, lateral patterning, surface reconstruction Main textTwo dimensional electron liquids (2DELs) confined at the surface [1][2][3] 25,26 . Moreover, as shown in Figure 1c, its conduction band minimum at the Γ point has pure d xy orbital character, which strongly enhances the confinement in a band-bending potential 24,32 .Experimentally, we induce a surface band-bending potential that quantum-confines electrons along the z direction by exposing the anatase TiO 2 thin films to synchrotron UV light at a temperature < 20 K (see Figure 2a and Supplementary Information Section II for more details). The surface electron doping is caused by formation of surface oxygen vacancies through photon-induced desorption of O + ions. This process has been recognized early by Knotek and Feibelman for TiO 2 33 , and was recently used to create 2DELs on other oxides including SrTiO 3 , KTaO 3 1-3,34-36 . By controlling the irradiation time, we can tune the carrier density. As the irradiation time increases, the carrier density and corresponding Fermi wave vector increases and eventually saturates. In Figure 2b and 2c we show the photoemission intensity map of the saturated states, and the irradiation time dependent momentum distribution curves (MDC) taken at E F . Note that the photoemission intensity of the metallic states is suppressed when measured around the Γ 00 point due to the photoemission matrix effect, thus the ARPES data shown below were measured around the Γ 10 point (see Supplementary Information Section IV for more details). In Figures 2d-2f, we characterize the resulting saturated 2DEL. The first clear signature of quantum confinement is the presence of three peaks in the energy distribution curve (EDC) taken at the Γ 10 point in an energy range where the bulk band structure shows only a single state. We associate the experimentally observed p...

Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2

“…The find the electron carrier density values for the two samples n2D as-grown 3·10 13 and n2D annealed 5·10 13 cm -2 , as calculated via Luttinger's theorem for two dimensional states with spin-degenerate bands 24 . While some reports suggest that the metallic state has a 3D character 9 a model linking the metallic state to the specific anatase surface arrangement has recently been shown to provide excellent agreement with the experimental data 11 . The 2D nature of the metallic state is also supported by experiments studying both the effect of electron doping through alkaline adsorption and the influence of beam irradiation at the anatase surface 10 Figure 1e) and 1f), giving direct evidence of (partial) healing of VOs and consequent reduction of the number of free electron carriers at the surface, in agreement with previous reports 9 .…”

“…S6 and S7). As shown in Figure 3e), the potential becomes repulsive near the surface of a defect-free slab 11,29 , and this effect is further enhanced in the presence of VO1 (green line). In contrast, VO7 (red line)…”

“…Consistent results were also obtained for films grown on Nb-doped SrTiO3, indicating that the observed properties are intrinsic to the TiO2 anatase phase (see Figure S1). Details of the growth protocol and characterization results 20 Figure 1e: an outer parabola and a second (fainted) quantized sub-band related to the confinement potential at the surface 11,12 . The bottom of the two quantized states is located at ~180 meV and ~65 meV for the outer and the inner band respectively, while the Fermi momenta (kF) are at ~0.18 Å -1 and ~0.11 Å -1 .…”

The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications

“…However, the spectral intensity exhibits a periodic modulation (Figure S1, Supporting Information), resulting from the interference of photoelectrons emitted from individual crystal layers in the near surface region. This reflects the out‐of‐plane periodicity (along [111]) of the recurring In 2 O 3 layers (as seen in the schematics of the experimental geometry relative to the In 2 O 3 unit cell in Figure b) and is consistent with findings in other 2DEG systems (e.g., InN, TiO 2 , SrTiO 3 ) . Fourier transforming the wave function solutions found from the extracted quantum well potential in Figure c, we can accurately predict this photoemission intensity (Figure h), which independently validates our characterization approach.…”

Band Structure and Spin–Orbital Texture of the (111)‐KTaO3 2D Electron Gas

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metal-insulator transitions, among others. [7][8][9][10] While the various methods used to engineer such 2DEGs may differ, the physics of the resulting system is always dominated by the host crystal. This is exemplified by the observation of superconductivity, [3] magnetism, [4] strong electron lattice interactions, [5] and quasi 1D conductivity [6] in the 2DEG stabilized in (001)-oriented SrTiO 3 (STO) which is, by far, the most studied system among ABO 3 transition metal oxides.

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