Formation of Plasmonic Polarons in Highly Electron-Doped Anatase TiO₂

Abstract

The existence of various quasiparticles of polarons because of electron−boson couplings plays important roles in determining electron transport in titanium dioxide (TiO 2 ), which affects a wealth of physical properties from catalysis to interfacial superconductivity. In addition to the well-defined Froḧlich polarons whose electrons are dressed by the phonon clouds, it has been theoretically predicted that electrons can also couple to their own plasmonic oscillations, namely, the plasmonic polarons.Here we experimentally demonstrate the formation of plasmonic polarons in highly doped anatase TiO 2 using angle-resolved photoemission spectroscopy. Our results show that the energy separation of plasmon-loss satellites follows a dependence on √n, where n is the electron density, manifesting the characteristic of plasmonic polarons. The spectral functions enable to quantitatively evaluate the strengths of electron−plasmon and electron− phonon couplings, respectively, providing an effective approach for characterizing the interplays among different bosonic modes in the complicate many-body interactions.

Evidence of a 2D Electron Gas in a Single‐Unit‐Cell of Anatase TiO2 (001)

“…Recently, experimental and theoretical reports suggest that the delocalized oxygen vacancies in the subsurface region are responsible for the formation of the 2DEG, [23] supporting a 2D character of the dispersive features. On the other hand, Ma et al [24] propose a coexistence of both 2D and 3D characters, similar to what has been observed in other oxides. [25,26] In this framework, the only way to provide conclusive evidence regarding the nature and the dimensionality of the 2DEG is to check the evolution of the 2DEG versus material thickness in carefully controlled thin films.…”

“…On the other hand, Ma et al. [ 24 ] propose a coexistence of both 2D and 3D characters, similar to what has been observed in other oxides. [ 25,26 ] In this framework, the only way to provide conclusive evidence regarding the nature and the dimensionality of the 2DEG is to check the evolution of the 2DEG versus material thickness in carefully controlled thin films.…”

“…It is worth noting that, even in the ultra-thin regime, the electron density is within the high carrier's concentration range reported in literature. [8,9,24] When the film thickness increases, the 2DEG occupancy saturates, as seen by the minor changes between 3 and 5 u.c. and also between 5 and 20 u.c.…”

Hydrogen-Bond Network Promotes Water Splitting on the TiO2 Surface

“…In addition to the GS, a sharp contribution appears just below the E F (black arrow in Figure a) with a parabolic dispersion in the 2D E b ( k || ) maps (Figure b). This feature can be assigned to the metallic state (MS) formed by electron doping at the anatase-TiO 2 (001) surface. , The produced H defects from water splitting provide excess electrons to form a confined electron gas at the surface region. Considering the 2D Luttinger volume, we can estimate the electron density of n 2D ≈ 4.1 × 10 13 cm –2 within the wavevector k F = (0.16 ± 0.02) Å –1 , via n 2D = k F 2 /2π, at sample 3 with 60 min UV irradiation.…”

Near-Surface Composition, Structure, and Energetics of TiO2 Thin Films: Characterization of Stress-Induced Defect States in Oxides Prepared via Chemical Vapor Deposition versus Solution Deposition from Sol–Gel Precursors

“…1,19,20,39,40,42,51−57 Figure 1 provides a schematic view of a range of defect states that have been postulated for the surface of TiO 2 ; in this case we show the (001) termination of the anatase unit cell, which guides our understanding of interactions with perovskite precursors. 17,19,20,35,39,41,42,[51][52][53]55 Both 5-and 6-coordinate (Ti 4+ 6c and Ti 4+ 5c ) are present in such surfaces along with an undercoordinated electron-rich Lewis acidic defect (Ti 3+ 4c ) associated with oxygen vacancies (V O prominent in the studies discussed below) and an interstitial form (Ti i ). 16,24,40,41,58−62 These electron-rich states are important in n-doping the oxide film, providing adequate electrical conductivity for its use in efficient device platforms with low series resistance and have even been shown recently to provide for two-dimensional metallic and "plasmonic" states when the defect concentrations are sufficiently high in the near-surface region.…”