We report the electronic structures of Ta-doped anatase TiO2 thin films grown by pulsed laser deposition (PLD) with varying magnetization using a combination of first-principles calculations and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The roles of Ta doping and Ti vacancies are clarified, and the observed room-temperature ferromagnetism is attributed to the localized magnetic moments at Ti vacancy sites ferromagnetically ordered by electron charge carriers. O K-edge spectra exhibit significant polarization dependence which is discussed and supported by first-principles calculations in relation to both the crystal symmetry and the formation of defects. In particular, anomalous spectral-weight transfer across the entire O K edge for the ferromagnetic thin film is associated exclusively with the occurrence of Ti vacancies and strong correlation effects, which result in the enhancement of the direct interaction between oxygen sites and of the anisotropy of the eg-pσ hybridizations in the out-of-plane component. Our results show that O K-edge NEXAFS spectra can provide reliable experimental probes capable of revealing cationic defects that are intimately related to the ferromagnetism in transition metal oxides. © 2013 American Physical Society.

Sankar Dhar

Physics

(SoNS) School of Natural Sciences

Qi D.-C.

Barman A.R.

Debbichi L.

Santoso I.

Asmara T.C.

Omer H.

Yang K.

Krüger P.

Wee A.T.S.

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Shiv Nadar University

Physical Review B - Condensed Matter and Materials Physics

Advancement of the memristor-based artificial synapse (AS) is urgently needed for rapid progress in neuromorphic devices. The precise structural and chemical engineering of metal oxide layers by metal dopants (Ni) is presented as an innovative way to set off a decent performance of the AS. An ON/OFF ratio of 103as well as data retention and endurance capabilities of 104s and 103cycles, respectively, are achieved. With these properties, the symmetric alteration in conductance states, short-term plasticity (STP) and long-term plasticity (LTP) are realized within the same device, and compared with the reported values to establish its excellent cognitive behavioural ability. Our combined experimental and the DFT-based first-principles calculation results reveal that the rational designing of AS using metal-cations (Ni) can promote an ultra-low-power of ∼2.55 fJ per pulse (lower than human brain ∼10 fJ per pulse) for STP, promising for next-generation smart memory devices. Here, Ni endorses strong electronic localization, which in turn familiarizes trap states within the forbidden energy gap and improves short-term memory loss. Further, it modifies the local electrostatic barriers to stimulate modulatory action (as commonly observed in the mammalian brain) for LTP. Overall, this work provides a novel pathway to overcome the technological bottleneck. © The Royal Society of Chemistry 2021.

Journal of Materials Chemistry C

Metal-induced progressive alteration of conducting states in memristors for implementing an efficient analog memory: a DFT-supported experimental approach

Titanium dioxide (TiO2) has rich physical properties with potential implications for both fundamental physics and new applications. To date, the main focus of applied research is to tune its optical properties, which is usually done via doping and/or nanoengineering. However, understanding the role of d electrons in materials and possible functionalization of d-electron properties are still major challenges. Herewith, within a combination of an innovative experimental technique, high-energy optical conductivity, and state-of-the-art ab initio electronic structure calculations, we report an emerging, novel resonant exciton in the deep ultraviolet region of the optical response. The resonant exciton evolves upon low-concentration Ta substitution in anatase TiO2 films. It is surprisingly robust and related to strong electron-electron and electron-hole interactions. The d- and f-orbital localization, due to Ta substitution, plays an unexpected role, activating strong electronic correlations and dominating the optical response under photoexcitation. Our results shed light on a new optical phenomenon in anatase TiO2 films and on the possibility of tuning electronic properties by Ta substitution. © 2016 American Physical Society.

Physical Review B

Emerging giant resonant exciton induced by Ta substitution in anatase TiO2: A tunable correlation effect

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences

Cationic-vacancy-induced room-temperature ferromagnetism in transparent, conducting anatase Ti 1− x Ta x O 2 ( x ∼0.05) thin films

AIP Advances

Interplay between carrier and cationic defect concentration in ferromagnetism of anatase Ti1-xTaxO2 thin films

Screened hybrid density functional study on Nb- and Ta-doped TiO2

Theory of high-energy optical conductivity and the role of oxygens in manganites

Nature Materials

A ten-year perspective on dilute magnetic semiconductors and oxides

Cationic vacancies and anomalous spectral-weight transfer in Ti 1-xTaxO2 thin films studied via polarization-dependent near-edge x-ray absorption fine structure spectroscopy