Electronic Manifestation of Cation‐Vacancy‐Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO<sub>2</sub>

Zhang, Shixiong; Ogale, Satishchandra; Yu, Weiqiang; Gao, Xingyu; Liu, Tao; Ghosh, Saurabh; Das, G. P.; Wee, Andrew Thye Shen; Greene, R. L.; Venkatesan, T.

doi:10.1002/adma.200803019

Cited by 99 publications

(82 citation statements)

References 52 publications

(28 reference statements)

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“…34,35 The variation in defect formation energies with sample composition also explains the reported absence of an EPR signal for photogenerated holes in reduced TiO 2 .…”

Section: 11mentioning

confidence: 72%

Polaronic trapping of electrons and holes by native defects in anataseTiO2

2009

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We have investigated the formation of native defects in anatase TiO 2 using density functional theory ͑DFT͒ modified with on-site Coulomb terms ͑DFT+ U͒ applied to both Ti d and O p states. Oxygen vacancies and titanium interstitials are found to be deep donors that trap two and four electrons, with transition levels that explain the two features seen in deep level transient spectroscopy experiments. Titanium vacancies are deep acceptors accommodating four holes. Self-trapping of both electrons and holes is also predicted. In all cases both donor and acceptor trap states correspond to strongly localized small polarons, in agreement with experimental EPR data. Variation in defect formation energies with stoichiometry explains the poor hole-trapping of reduced TiO 2 . DOI: 10.1103/PhysRevB.80.233102 PACS number͑s͒: 71.38.Ht, 71.55.Ϫi, 82.50.Ϫm In semiconductors used in photocatalysis and photovoltaics, the primary process is the light-induced production of charge carriers.1 Photocatalysis proceeds by photoexcitation of electrons to unoccupied bands, producing free electronhole pairs. These charge carriers then typically undergo trapping, followed by diffusion to the surface where they can initiate reactions of adsorbed molecules. Electron-hole recombination is often a major competing process, and it is highly desirable to promote charge separation and inhibit subsequent charge-carrier-recombination in order to improve the quantum efficiency of photocatalytic processes.TiO 2 has been widely studied as a photocatalyst for the degradation of environmental pollutants, 1 and for water splitting. 2 Nanocrystalline anatase TiO 2 is often used, since it is more photocatalytically active than the ground-state polymorph, rutile, and the diffusion pathways of photogenerated charge carriers to the surface are shortened, resulting in increased quantum efficiencies.Following photoexcitation, trapping of charge carriers occurs on a nanosecond time scale.3 Defects are thought to play a critical role in the trapping process by acting as recombination centers. For example, n-type doping of anatase reduces the photocatalytic activity of experimental samples, 4 and reduced anatase is less efficient than stoichiometric TiO 2 in the trapping of photogenerated holes.5 Understanding the interaction of defects with charge carriers is essential for the optimization of TiO 2 samples for photocatalysis. Electronand hole traps have both been observed in EPR spectra. [5][6][7][8] These data have been interpreted as charge localization at single atoms to give small polarons: hole trapping has been associated with O − sites; O O • , and electron trapping with Ti 3+ species; Ti Ti Ј . Additional evidence for charge localization at Ti 3+ states in n-type TiO 2 comes from core-level XPS shifts, and characteristic gap-state features observable in UPS spectra, with samples annealed to produce oxygen vacancies giving identical EPR signals to those seen following uv irradiation. Deep traps 0.9 and 0.5 eV below the conduction band edge have also bee...

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“…34,35 The variation in defect formation energies with sample composition also explains the reported absence of an EPR signal for photogenerated holes in reduced TiO 2 .…”

Section: 11mentioning

confidence: 72%

Polaronic trapping of electrons and holes by native defects in anataseTiO2

2009

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“…Unless the orbital magnetization of the V Ti is extended over at least two unit cells, the direct exchange probability is very low. The fact that is FM (but see Kondo scattering [23]) is not seen in samples prepared at higher oxygen pressures, where the V Ti concentration is higher but the carrier concentration is lower, strongly argues in favour of a carrier-mediated exchange [8,42]. Figure 10 shows a schematic of the mechanism where the origin of FM is related to magnetic centres associated with the V Ti .…”

Section: (D) Origin Of Ferromagnetismmentioning

confidence: 99%

“…However, the realization of high-quality films of CaO has not yet been achieved as a result of the hygroscopic nature of CaO, which leads to rather unstable films. The first experimental observation of a local magnetic moment arising from a cationic vacancy was reported by Zhang et al [23], who observed signatures of the Kondo effect below 100 K in 5 per cent Nb-doped anatase TiO 2 thin films grown under 10 −4 Torr oxygen partial pressure by pulsed laser deposition (PLD). Using X-ray absorption spectroscopy (XAS) and Xray photoemission spectroscopy (XPS), supported by first-principles calculations, they showed that the appearance of Kondo scattering was due to the presence of localized magnetic moments associated with cationic (Ti) vacancies produced as a result of Nb incorporation.…”

Section: Introductionmentioning

confidence: 99%

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

Rusydi

Dhar

Barman

et al. 2012

Phil. Trans. R. Soc. A.

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We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti 1−x Ta x O 2 (x ∼ 0.05) thin films grown by pulsed laser deposition on LaAlO 3 substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, protoninduced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, * Authors for correspondence (phyandri@nus.edu.sg; eleds@nus.edu.sg; venky@nus.edu.sg). † These authors contributed equally to this work. electrical transport measurements, soft X-ray magnetic circular dichroism (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (V Ti ) and Ti 3+ was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the V Ti signal was strong while the Ti 3+ signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, V Ti sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements.

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“…1,2 During the past a few decades, the research efforts have been providing insights on the intriguing physics of strongly correlated electron systems such as heavy-fermion compounds and high-T c superconductors. Although the Kondo effect was first discovered in normal metals containing minute amounts of magnetic impurities, 3 recent experiments suggest its existence in several oxide-and carbon-based materials, including doped TiO 2 thin films, 4,5 SrTiO 3 , 6 and graphene. 7 These discoveries challenge the theoretical models which were originally developed to describe classic Kondo systems like Fe impurities in Au thin films.…”

Section: Introductionmentioning

confidence: 99%

Electrostatic tuning of Kondo effect in a rare-earth-doped wide-band-gap oxide

Deng

Lin

et al. 2013

Phys. Rev. B

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As a long-lived theme in solid-state physics, the Kondo effect reflects the many-body physics involving the short-range Coulomb interactions between itinerant electrons and localized spins in metallic materials. Here we show that the Kondo effect is present in ZnO, a prototypical wide-band-gap oxide, doped with a rare-earth element (Gd). The localized 4f electrons of Gd ions do not produce remanent magnetism, but interact strongly with the host electrons, giving rise to a saturating resistance upturn and negative magnetoresistance at low temperatures. Furthermore, the Kondo temperature and resistance can be electrostatically modulated using electric-double-layer gating with liquid ionic electrolyte. Our experiments provide the experimental evidence of tunable Kondo effect in ZnO, underscoring the magnetic interactions between localized and itinerant electrons and the emergent transport behaviors in such doped wide-band-gap oxides.

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Electronic Manifestation of Cation‐Vacancy‐Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO₂

Cited by 99 publications

References 52 publications

Polaronic trapping of electrons and holes by native defects in anataseTiO2

Polaronic trapping of electrons and holes by native defects in anataseTiO2

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

Electrostatic tuning of Kondo effect in a rare-earth-doped wide-band-gap oxide

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Electronic Manifestation of Cation‐Vacancy‐Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO2

Cited by 99 publications

References 52 publications

Polaronic trapping of electrons and holes by native defects in anataseTiO2

Polaronic trapping of electrons and holes by native defects in anataseTiO2

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

Electrostatic tuning of Kondo effect in a rare-earth-doped wide-band-gap oxide

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Electronic Manifestation of Cation‐Vacancy‐Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO₂

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