Tuning the Optical Absorption of Anatase Thin Films Across the Visible-To-Near-Infrared Spectral Region

Abstract

The electronic properties of anatase titanium dioxide (TiO 2 ) thin films epitaxially grown on LaAlO 3 substrates are investigated by synchrotron-x-ray spectroscopy [x-ray absorption spectroscopy (XAS), xray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES)] and infrared spectroscopy. The Ti 3+ fraction in TiO 2−x is varied either by changing the oxygen pressure during deposition or by postgrowth annealing in ultrahigh vacuum (UHV). Structural investigation of the TiO 2 thin films provides evidence of highly uniform crystallographic order in both as-grown and in situ UHVannealed samples. The increased amount of Ti 3+ as a consequence of UHV annealing is calibrated by in situ XPS and XAS analysis. The as-grown TiO 2 samples, with a low Ti 3+ concentration, show distinct electronic properties with respect to the annealed films, namely, absorption in the midinfrared (MIR) region correlated with polaron formation, and another peak in the visible range at 1.6 eV correlated with the presence of localized defect states (DSs). With the increasing level of Ti 3+ induced by the postannealing process, the MIR peak disappears, while the DS peak is redshifted to the near-infrared region at about 1.0 eV. These results indicate the possibility of tailoring the optical absorption of anatase TiO 2 films from the visible to the near-infrared region.

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

“…), implying that in this last case the anatase crystal structure is not stabilized. As a matter of fact, in agreement with our previous investigation by in situ STM and ex situ HR-TEM and low-angle X-ray reflectivity, [11,27] the TiO 2 surface is characterized by a very low surface roughness, consistently with a pure 2D growth mode, and the 0.75 u.c. sample corresponds to a film fully covered by 0.75 u.c.…”

“…Films were grown in situ by pulsed laser deposition (PLD) and subsequently transferred across the continuous ultra-high vacuum manifold to the synchrotron radiation spectrometers of the NFFA-APE beamlines at the Elettra facility in Trieste. [11,23,27] Angular-resolved photo-emission spectroscopy (ARPES), X-ray absorption spectroscopy (XAS) and X-ray photo-emission spectroscopy (XPS) allow to detect the development of 2DEG along with that of the spectroscopic fingerprint of anatase. We observe that as soon as a single-unit-cell of anatase TiO 2 is formed, the 2DEG is also observed.…”

Spectroscopic Evidence of a Dimensionality-Induced Metal-to-Insulator Transition in the Ruddlesden–Popper Lan+1NinO3n+1 Series

“…In this case, XAS features are determined by a 1–2-nm-thick region of materials close to the surface and then duplicated within the superlattices, therefore again making the measured XAS features representative of the bulk of the structure and, therefore, comparable with those investigated by the infrared spectroscopy reported in the following paragraph. 12 , 13 …”

Unveiling Oxygen Vacancy Superstructures in Reduced Anatase Thin Films

“…6,9 Creating such in-gap states by V O is a way to expand the photocatalytic activity of the material into the visible range of light. 10,11 Furthermore, V O play a major role in memristive devices, where highly conductive nanofilaments are attributed to formation and disruption of Magneĺi phases, 12 which are also claimed to occur nanoscopically in anatase powders after thermal treatment above 800 °C13 and to be at the origin of the storage of alkali metals in TiO 2 -based battery materials. 14,15 Understanding the low energy properties of anatase is key to guide the material engineering for optimal functionalities and the intrinsic self-doping due to the oxygen vacancy concentration and site distribution is one central aspect to be clarified.…”

“…Anatase, however, shows superior photocatalytic activity as compared to rutile and brookite and is thus the preferred configuration for applications. , Even though stoichiometric anatase is known to be a wide band gap semiconductor with an indirect optical band gap of 3.2 eV, its electronic properties are largely determined by the presence of excess electrons, which can be induced by intrinsic defects, dopands, or photoexcitation. − It is known that oxygen vacancies (V O ) are inherently present in anatase with a typical concentration of 10 17 cm –3 , which act as donors in the n-type semiconductor . V O induce localized electronic states within the band gap, correlated to the formation of Ti 3+ ions. , Creating such in-gap states by V O is a way to expand the photocatalytic activity of the material into the visible range of light. , Furthermore, V O play a major role in memristive devices, where highly conductive nanofilaments are attributed to formation and disruption of Magnéli phases, which are also claimed to occur nanoscopically in anatase powders after thermal treatment above 800 °C and to be at the origin of the storage of alkali metals in TiO 2 -based battery materials. , Understanding the low energy properties of anatase is key to guide the material engineering for optimal functionalities and the intrinsic self-doping due to the oxygen vacancy concentration and site distribution is one central aspect to be clarified.…”