Observation of room temperature photoluminescence in proton irradiated SrTiO3 single crystal

We have irradiated SrTiO₃ single crystal with 3 MeV-proton (H⁺) beam and found that blue -, green - and infrared - frequency photoluminescence (PL) are induced simultaneously at room temperature. TEM and EELS analyses show that an oxygen-deficient amorphous layer is formed at the crystal surface by the proton irradiation. Possible origin of the PL-effect is discussed.     

Unconventional photoluminescence effect of SrTiO3 thin film under UV-illumination

We find that PL intensity I(t) of SrTiO₃ thin film measured under UHV condition increases with UV-laser illumination over long time scale of ∼ 2 h. The intensity increase takes place at lower sample temperature as well, 200, 100 K, and 20 K. When O₂ and N₂ gas are introduced into the sample chamber the PL intensity decreases with the UV-illumination time, opposite to the UHV-case. We consider a quantitative thermal energy flow model of the laser-power and heat absorption by the sample, but find that temperature change of the sample is not large enough to account for the time dependent I(t). We propose photo-catalysis effect on STO surface as possible scenario of the PL intensity change.     

Blue and infrared cathodoluminescence induced by carbon-irradiation in SrTiO3 single crystal

We show that blue-CL can be induced in SrTiO₃ single crystal at room temperature by irradiating it with 60 KeV carbon ion (C⁻) beam. An infrared CL-emission is induced simultaneously. Transmission electron microscopy (TEM) measurement exhibits an 200 nm thick surface layer formed by the C⁻ irradiation. We show that the emitting region can be patterned into any desired shape by fabricating the STO surface using Pt-based micro-lithography technique.     

Large enhancement of photoluminescence in SrTiO3:Pr powders by fluorhydric acid treatment

We have investigated the effect of fluorhydric acid (HF) treatment on the photoluminescence (PL) properties of SrTiO₃:Pr³⁺ powders prepared by sol-gel method. The red emission intensity increased significantly up to 18 times when the powders were subjected into a water-diluted 5% HF solution for 10 min. The origin of the PL enhancement was ascribed to the increase of oxygen vacancies in HF-treated SrTiO₃:Pr³⁺ powders. This study also manifested that HF etching is more effective to improve the red PL intensity than vacuum-annealing for the sol-gel made SrTiO₃:Pr³⁺ powders.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO_3（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3（STO）（110） substrates.Inclined ZnO and cobalt-doped ZnO（ZnCoO） thin films are grown on unetched STO（110） substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO（110） surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E（2G）→ 4A2（4F） transition of Co 2＋ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2＋ ions at the lattice positions of the Zn 2＋ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.     

Effect of polar distortion on electronic structures of (001) LaGaO3/SrTiO3 interface

First-principles calculations of electronic structures of (001) epitaxial LaGaO₃/SrTiO₃ heterostructures were performed in the framework of density functional theory. The effects of atomic relaxation on electronic characteristics of both n-type (LaO)⁺/(TiO₂)⁰ and p-type (GaO₂)⁻/(SrO)⁰ interfaces are investigated. It is found that the n-type interface remains metallic, whereas the p-type interface becomes insulating after atomic relaxation. Polar distortion in the LaGaO₃ layers associated with the atomic relaxation strongly screens the intrinsic electric field induced by periodically stacking (LaO)⁺ and (GaO₂)⁻ charged atomic layers on SrTiO₃ with charge neutral (001) atomic layers. This relieves the trend to a polar catastrophe and reduces the carrier charge density on the interface.     

First-principle study of strain-driven phase transition in incipient ferroelectric SrTiO3

First-principle calculations were carried out to investigate the epitaxial strain dependence of the structural stability and related properties of SrTiO₃. The P4mm-to-P4/mmm-to-Amm2 phase transitions in SrTiO₃ with the epitaxial strain changing from compressive to expansive have been identified by analysis of the soft-mode eigendisplacement. The zero-temperature critical strains for the P4mm-to-P4/mmm and P4/mmm-to-Amm2 phase transitions are −0.72% and 0.83%, respectively, which is consistent with the previous predictions based on the phenomenological thermodynamic model and the parameterized total-energy model. The influences of the epitaxial strain on the dielectric tensor and spontaneous polarization along the phase transition path are discussed in terms of charge transfer. Further investigations on the vibration modes show unusual softening behaviors of the A₁ and B₂ modes in the Amm2 phase, which may be associated with the soft phonon modes away from Γ point and possibly result in other phase transitions unreported by any previous studies.     

Y掺杂SrTiO3晶体材料的电子结构计算

采用基于第一性原理的密度泛函理论平面波超软赝势法, 研究了Y掺杂SrTiO₃体系的空间结构和电子结构性质, 得到了优化后体系的结构参数, 掺杂形成能, 能带结构和电子态密度. 对比掺杂浓度为0125, 025, 033时,Sr_1-xY_xTiO₃和SrTi_1-xY_xO₃的掺杂形成能,发现Y替代Sr能形成更稳定的结构. 对Sr_1-xY_xTiO₃（x=0, 0125, 025, 033） 的结构进行了优化,结果表明Y替代Sr后, 随着掺杂浓度增大, 体系的晶格常数逐渐减小, 稳定性逐渐增强. 对不同掺杂浓度的Sr_1-xY_xTiO₃能带结构的计算结果表明：纯净的SrTiO₃是绝缘体, 价带顶在R点, 导带底在Γ点, 费米能级处于价带顶; 掺杂Y后, 费米能级进入到导带底中, 体系呈金属性;掺杂浓度越大,费米能级进入导带的位置越深,禁带宽度也近似变宽. 关键词： 3')" href="#">SrTiO₃ 电子结构 掺杂 VASP     

Preparation and luminescence properties of SrTiO3:Pr,Al phosphor from the glycolate method

Spherical SrTiO₃:Pr³⁺,Al³⁺ phosphor with high crystallinity and uniform particle size distribution was formed from the glycolate precursor. The glycolate precursor was obtained by heating the mixed solution of metallic nitrates and titanium oxychloride in ethylene glycol up to 200 °C. The thermal decomposition of the glycolate precursor proceeded through three major stages, i.e., (i) evolution of glycols (∼200 °C), (ii) decomposition of glycolate precursor, and (iii) decomposition of strontium carbonate and crystallization of SrTiO₃:Pr³⁺,Al³⁺ phosphor.SrTiO₃:Pr³⁺,Al³⁺ phosphor exhibited a strong red emission, peaking at about 617 nm. SrTiO₃:Pr³⁺,Al³⁺ phosphor obtained from the glycolate complex has higher luminescent properties than the conventional solid state reaction and the Pechini method in terms of photoluminescence (PL) and cathodoluminescence (CL). High crystallinity, low residual carbon content and small grain size with uniform shape would enhance the luminescence intensity of phosphor by the glycolate method due to high surface area per unit volume and low organic content compared with the Pechini method. Also, Al³⁺ ion is more effective than Ga³⁺ ion to enhance PL intensity of SrTiO₃:Pr³⁺,Al³⁺ phosphor because of smaller Al³⁺ ion radius. Therefore, the glycolate method has been demonstrated to be a convenient and unique process for the production of muticomponent oxide with smaller grain size and higher crystallinity compared with the conventional mixed oxide reaction and the polymer precursor method.     

Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO₃ (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.     

Combinatorial discovery of anomalous substrate effect on the photochemical properties of transition metal-doped epitaxial SrTiO3 heterostructures

Epitaxial Sr(V_xCr_yTi_1−x−y)O₃ (0≤x+y≤0.05) ternary composition spreads were grown on two different single crystal substrates, LaAlO₃ and Nb-doped SrTiO₃, by use of combinatorial laser molecular beam epitaxy with a specially patterned slide masking plate. The photocatalytic activity on the composition spreads was evaluated by the photo-reduction of Ag⁺ in an AgNO₃ aqueous solution to deposit Ag metal on the spreads. The V-doping effect was found to depend greatly on the substrate: the photodeposition of Ag was much enhanced in the composition region of SrV_0.05Ti_0.95O₃ only on the Nb-doped SrTiO₃, but not on the LaAlO₃ and non-doped SrTiO₃.     

Structural, magnetic and electric properties of HoMnO3 films on SrTiO3(001)

HoMnO₃ films were grown on pure and Nb-doped SrTiO₃ (001) substrates by pulsed laser deposition. The films grew epitaxially with the c-axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn₃O₄ precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c-axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.     

Atomic and electronic structures of Mg-doped perfect SrTiO3 and crystals containing oxygen vacancies from first principles

The electronic properties of Mg-doped perfect SrTiO₃ and crystals containing oxygen vacancies systems are investigated by first principles calculation. Dopant formation energy results show that the Mg atoms preferentially enter the Sr site in SrTiO₃. Substitution of Ti by Mg brings some acceptor levels, which introduces the p-type conductivity of SrTiO₃. Creation of oxygen vacancies in SrTiO₃ introduces donor levels, which can contribute to the n-type conductivity. In SrTiO₃ containing oxygen vacancies system, a self-compensation effect will occur when Ti is substituted by Mg, and the system undergoes n-type to p-type transition.     

Morphology and growth modes of metal-oxides deposited on SrTiO3

Classical molecular dynamics simulations are used to examine the growth of SrO and TiO₂ thin films on SrTiO₃ (STO). In particular, the simulations consider the deposition of SrO and TiO₂ molecules at incident energies of 0.1, 0.5, and 1.0 eV/atom onto the (0 0 1) surface of STO. The role of surface termination layer (SrO vs. TiO₂) is analyzed. In the case of SrO deposition, smooth, ordered films are produced for all incident energies considered and for both surface terminations. By contrast, in the case of TiO₂ deposition, three-dimensional islands are formed under all conditions. These predictions are in good agreement with experimental data. Importantly, the simulations explain why these differing morphologies are produced for SrO and TiO₂ deposition.     

Study on Mn-doped SrTiO3 with first principle calculation

The defect formation energies and electronic structures of Mn doped strontium titanate have been studied using CRYSTAL-09 code. The defect formation energies for Mn_Sr and Mn_Ti, under different chemical potential conditions, have been obtained to determine the way Mn prefers to occupy in the Mn doped SrTiO₃ crystal. From the electronic structures of Mn doped SrTiO₃, it is shown that Mn_Sr cannot change the band gap of SrTiO₃. However, Mn_Ti can effectively reduce the band gap of SrTiO₃ and improve the photocatalysis.     

Improved photoluminescence and afterglow in CaTiO3:Pr with addition of nanosized SiO2

Enhancement of the ¹D₂-³H₄ red emission in CaTiO₃:Pr³⁺ with addition of nanosized SiO₂ fabricated by a solid state reaction method is reported. The dynamical processes for the improvement of red emission were systematically investigated using photoluminescence (PL) and PL excitation spectra, and diffused reflectance spectra as well as time decay patterns of PL and persistent afterglow. Higher efficiency of energy transfer from CaTiO₃ host to the activator Pr³⁺ due to the improvement of crystallinity by SiO₂ addition was discussed in comparison with that of the SiO₂ free sample. The enhancement of persistent afterglow after the cessation of excitation in SiO₂ added CaTiO₃:Pr³⁺ was also analyzed by theoretically fitted results.     

Spin-Hamiltonian parameters and local rotational order parameters for Gd ion in the tetragonal phase of SrTiO3 crystal

Using a diagonalization method, the spin-Hamiltonian parameters (g factors g_||, g_⊥ and zero-field splittings b₂⁰, b₄⁰, b₂⁴, b₆⁰, b₆⁴) for Gd³⁺ ion at the 12-fold coordinated Sr²⁺ site in the tetragonal phase of SrTiO₃ at T≈4.2 and 77 K are calculated in a unified way. The calculated results are in reasonable agreement with the experimental values. The local rotational order parameters ?_l of Gd³⁺ center at the two temperatures are also obtained from the calculations. It is found that the local rotational order parameters ?_l of Gd³⁺ impurity center at both temperatures are different from the corresponding parameters ?_h in the host SrTiO₃ crystal and the changes of spin-Hamiltonian parameters with temperature are caused by the change of local rotational order parameter?_l with temperature. The results are discussed.     

Characteristics of photoluminescence, thermoluminescence and thermal degradation in Eu-doped BaMgAl10O17 and SrMgAl10O17

The relation between photoluminescence and thermoluminescence from Eu-doped BAM (BaMgAl₁₀O₁₇) and SAM (SrMgAl₁₀O₁₇) are investigated. The emission peak of SAM:Eu shifts from 463 to 489 nm whereas that of BAM:Eu only shifts 3 nm at 450 nm as temperature decreased from 300 to 50 K under 146 nm excitation. This can be explained by the fact that there are Beevers–Ross (BR) and mid-oxygen (mO) sites for Eu ions in SAM. The emission peak around 463 nm from SAM:Eu is ascribed to Eu ions in the mO site, while the peak around 489 nm is ascribed to ones in the BR site in SAM host. From the result of thermal degradation of SAM:Eu, it is confirmed that the Eu ions located at mO site are easy to degrade compared with those located at BR site. The thermal degradation of BAM:Eu phosphor becomes large with the increase in Eu concentration. We suggest that the thermal degradation of BAM:Eu phosphor is due to the tendency of Eu ions to occupy the mO site.     

Characterization of the Landau expansion equation for SrTiO3 under pressure

This work focuses on the characterization of the expansion of Landau's main equation for Pm3?m structures, which are transformed with symmetry subgroups, associated with M+3 and R+4 special points of the Brillouin zone, under hydrostatic pressure. This solution has been adapted to include the general influence of hydrostatic pressure and the evolution of the order parameter. We present a simple scheme in order to search systematically the ground state of all perovskite structures derivable from octahedral rotations under high pressure. We have applied this adaptation to SrTiO₃ perovskite. Results of the characterization of Landau′s equation for the cubic-tetragonal phase transition under pressure are discussed here.     

钛酸锶纳米颗粒界面层特性的光谱学研究

利用拉曼光谱和紫外—可见光漫反射吸收光谱研究了颗粒度为10—80 nm的钛酸锶纳米颗粒的界面层特性.研究发现,当样品颗粒度从80 nm减小至10 nm时,一阶极化TO₂模的拉曼强度有显著增加,表明在样品边界层中存在由增强表面缺陷偶极子造成的微极化区.相反,一阶非极化TO₃模的拉曼强度随颗粒度的减小而降低.同时,发现TO₂和TO₃模的频率随着颗粒度的减小而发生软化,表明Ti—O 键的键长随颗粒度减小而增加,与XRD观察到 关键词： 纳米钛酸锶颗粒 微极化区 晶格扩张 表面缺陷态