Practical spatial resolution of electron energy loss spectroscopy in aberration corrected scanning transmission electron microscopy

The resolution of electron energy loss spectroscopy (EELS) is limited by delocalization of inelastic electron scattering rather than probe size in an aberration corrected scanning transmission electron microscope (STEM). In this study, we present an experimental quantification of EELS spatial resolution using chemically modulated 2×(LaMnO(3))/2×(SrTiO(3)) and 2×(SrVO(3))/2×(SrTiO(3)) superlattices by measuring the full width at half maxima (FWHM) of integrated Ti M(2,3), Ti L(2,3), V L(2,3), Mn L(2,3), La N(4,5), La N(2,3) La M(4,5) and Sr L(3) edges over the superlattices. The EELS signals recorded using large collection angles are peaked at atomic columns. The FWHM of the EELS profile, obtained by curve-fitting, reveals a systematic trend with the energy loss for the Ti, V, and Mn edges. However, the experimental FWHM of the Sr and La edges deviates significantly from the observed experimental tendency.     

Surface and defect structure of oxide nanowires on SrTiO3

Processing the SrTiO(3)(001) surface results in the self-assembly of reduced titanate nanowires whose widths are approximately 1 nm. We have imaged these nanowires and their defects at elevated temperatures by atomic resolution scanning tunneling microscopy. The nanowire structure is modeled with density functional theory, and defects observed in the center of the nanowire are determined to be Ti(4)O(3) vacancy clusters. The activation energy for Ti(4)O(3) vacancy cluster diffusion is explicitly measured as 4.98±0.17 eV with an exponential prefactor of μ=6.57×10(29) (s(-1).     

Identification of A- and B-site cation vacancy defects in perovskite oxide thin films

Cation vacancies on both sublattices (V(Ti), V(Sr)) have been identified in homoepitaxial pulsed laser deposited SrTiO3 films using high intensity variable energy positron annihilation lifetime spectroscopy (PALS) measurements. Film nonstoichiometry was varied by varying laser fluence. PALS showed that on increasing the fluence above the Ti/Sr～1 value, the concentration ratio [V(Sr)]/[V(Ti)] systematically increased. Reducing the fluence into the Ti-poor region below resulted in additional vacancy cluster defect formation. Vacancy concentrations greater than ～50 ppm were observed in all films.     

Electronic and magnetic reconstructions in La0.7Sr0.3MnO3/SrTiO3 heterostructures: a case of enhanced interlayer coupling controlled by the interface

We report on the magnetic coupling of La0.7Sr0.3MnO3 layers through SrTiO3 spacers in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures. Combined aberration-corrected microscopy and electron-energy-loss spectroscopy evidence charge transfer to the empty conduction band of the titanate. Ti d electrons interact via superexchange with Mn, giving rise to a Ti magnetic moment as demonstrated by x-ray magnetic circular dichroism. This induced magnetic moment in the SrTiO3 controls the bulk magnetic and transport properties of the superlattices when the titanate layer thickness is below 1 nm.     

Structure of SrTiO3 films on Si

The epitaxial deposition of oxides on silicon opens the possibility of incorporating their diverse properties into silicon-device technology. Deposition of SrTiO(3) on silicon was first reported over a decade ago, but growing the coherent, lattice-matched films that are critical for many applications has been difficult for thicknesses beyond 5 unit cells. Using a combination of density functional calculations and x-ray diffraction measurements, we determine the atomic structure of coherent SrTiO(3) films on silicon, finding that the Sr concentration at the interface varies with the film thickness. The structures with the lowest computed energies best match the x-ray diffraction. During growth, Sr diffuses from the interface to the surface of the film; the increasing difficulty of Sr diffusion with film thickness may cause the disorder seen in thicker films. The identification of this unique thickness-dependent interfacial structure opens the possibility of modifying the interface to improve the thickness and quality of metal oxide films on silicon.     

Structural basis for the conducting interface between LaAlO3 and SrTiO3

The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La(1-x)SrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti{4+}/Ti{3+} across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.     

Spontaneous 2-dimensional carrier confinement at the n-type SrTiO3/LaAlO3 interface

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3/LaAlO3 interface as a function of the sheet carrier density n(s) via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d(xy) levels within a thin (?2 nm) interface-adjacent SrTiO3 region for n(s) lower than a threshold value n(c)～10(14) cm(-2). For n(s)>n(c) a portion of charge flows into Ti 3d(xz)-d(yz) levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t(2g) states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with n(s) is provided.     

SrTiO3薄膜氧缺陷的第一性原理研究

在广义梯度近似(GGA)下,利用超软赝势对真空条件下SrTiO3超晶胞的体系能量、原子间电子云重叠布局数和电子态密度等进行了自恰计算.结果显示,对有氧缺陷的超晶胞优化后,晶格参数的几何平均值增大了2.711%,这表明在高温条件下外延生长STO薄膜时,产生了氧缺陷,并且氧空位易处于薄膜表层;另外,表层氧缺陷使表层Ti原子明显的发生偏心位移,两个Ti原子的核间距由0.3905 nm增大至0.4234 nm,b轴上的氧原子则向中心靠近了0.0108 nm、并沿c轴方向上突了0.0027 nm,使STO晶体产生自发极化,氧缺陷还使STO的电子态密度的能隙增宽了1.75 eV,达到2.48 eV,从而使STO晶体由顺电相转向铁电相.     

Interface control of emergent ferroic order in Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1)

We discovered from first principles an unusual polar state in the low n Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO(3). These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.     

Vacant-site octahedral tilings on SrTiO(3) (001), the (sqrt[13]×sqrt[13])R33.7° surface, and related structures

The structure of the SrTiO(3) (001) (sqrt[13]×sqrt[13])R33.7° surface reconstruction has been determined using transmission electron diffraction combined with direct methods and density functional theory. It has a TiO(2)-rich surface with a 2D tiling of edge or corner-sharing TiO(5)□ octahedra. Additionally, different arrangements of these octahedral units at the surface, dictated by local bond-valence sums, form 2D networks that can account for many ordered surface reconstructions as well as disordered glasslike structures consistent with the multitude of structures observed experimentally, and potentially other materials and interfaces.     

水热法制备表面修饰的钛酸锶纳米微粉

以工业原料和常用试剂ＴｉＣｌ４、Ｓｒ（）ＮＯ３）２和ＫＯＨ为基础原料，通过添加表面活性剂十二烷基苯磺酸（ＤＢＳ），并采用制备出表面包裹有ＤＢＳ的钛酸锶纳米微粉姘应用红外光谱，Ｘ射线衍射谱，透射电子显微镜，热分析等一系列手段对其微结构进行了表征。结果表明：样品为表面包裹ＤＢＳ的钛酸锶纳同粉，其形态较为规则，粒度分布较窄，单分散性较好，粒子的平均粒径为１２０ｍｍ，包裹膜的平均厚度为６ｎｍ左右，根据Ｘ光     

LaAlO3/SrTiO3界面的电子结构及光学性质的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,结合局域密度近似（LDA）研究了钙钛矿结构氧化物LaAlO3 /SrTiO3界面的电子结构及光学性质。能带结构分析表明当形成(AlO2)-/(TiO2)0界面时其禁带宽度为1.888 eV,呈现绝缘体的性质,当形成(LaO)+/(SrO)0界面时其禁带宽度为0.021 eV,呈现半导体或半金属性质。同时,对不同界面的光学性质也进行了研究,结果表明纯相的LaAlO3和SrTiO3的吸收系数、反射系数及能量损失谱强度明显高于由这两种单质形成不同界面的强度。     

First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3

Electronic and optical properties of Sr(Ti,Zr)O₃ crystals in the cubic (Pm-3m) and tetragonal (I4/mcm) phase were calculated by the first-principles calculations using the density functional theory and the local density approximation. The band structure of cubic and tetragonal phases show an indirect band gap at (R-Γ) point and at (M-Γ) point in the Brillouin zone, respectively. The linear photon-energy dependent dielectric functions and some optical properties such as the absorption coefficient, energy-loss function and reflectivity are calculated for both phases. The optical properties of tetragonal phase of Sr(Ti,Zr)O₃ were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.     

The interaction between hexahydroxytriphenylene and the rutile TiO2(110)-(1 × 1) surface at UHV conditions

The interaction between 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and the rutile TiO₂(110)–(1 × 1) surface under ultrahigh vacuum (UHV) conditions was investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The NEXAFS results showed that HHTP molecules formed a submonolayer and a monolayer that aligned along the [001]-direction with, respectively, a more or less flat downward orientation and a more upright orientation to the TiO₂ surface. The HHTP molecules that aligned along the [001]-direction were most likely grafted onto the TiO₂(110) surface by a bidentate bridge between each of the oxygen atoms of one of the catechol units within the HHTP molecule and two adjacent Ti(5f)⁴⁺ ions on the TiO₂(110) surface. The coordination is non-dissociative in the case of the submonolayer, but dissociative in the monolayer, according to the analysis of the C1s XPS, UPS, C1s NEXAFS data and complementary DFT calculations.     

Vacancy induced magnetism in SrTiO3

Vacancy-induced magnetism in perovskite SrTiO₃ is investigated by ab initio calculations and magnetic measurements. The calculations of the generalized gradient approximation (GGA), the local density approximation (LDA) and the local density approximation with on-site effect U (LDA+U) methods show that stoichiometric SrTiO₃ is nonmagnetic. The GGA calculated results indicate that Ti or O vacancy could induce magnetism rather than Sr vacancy. The LDA and LDA+U calculations show that the Ti vacancy could induce magnetism, while Sr and O vacancies couldn't. The experimental results confirm that SrTiO₃ nanocrystalline powders exhibit room-temperature ferromagnetism (FM) and the magnetic moment results from cation vacancies.     

3D atomic imaging by internal-detector electron holography

A method of internal-detector electron holography is the time-reversed version of photoelectron holography. Using an energy-dispersive x-ray detector, an electron gun, and a computer-controllable sample stage, we measured a multiple-energy hologram of the atomic arrangement around the Ti atom in SrTiO3 by recording the characteristic Ti Kα x-ray spectra for different electron beam angles and wavelengths. A real-space image was obtained by using a fitting-based reconstruction algorithm. 3D atomic images of the elements Sr, Ti, and O in SrTiO3 were clearly visualized. The present work reveals that internal-detector electron holography has great potential for reproducing 3D atomic arrangements, even for light elements.     

Electronic stabilization of the Si(111)5 × 2-Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2-Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2-Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.     

Lattice relaxation in oxide heterostructures: LaTiO3/SrTiO3 superlattices

Local density approximation + Hubbard U and many-body effective Hamiltonian calculations are used to determine the effects of lattice relaxation in LaTiO3/SrTiO3 superlattices. Large ferroelectric-like distortions of the TiO6 octahedra are found, which substantially affect the Ti d-electron density, bringing the calculated results into good agreement with experimental data. The relaxations also change the many-body physics, leading to a novel symmetry-breaking-induced ordering of the xy orbitals, which does not occur in bulk LaTiO3, or in the hypothetical unrelaxed structure.     

First-principles phonon calculations of Fe?? impurity in SrTiO?

The results of hybrid density functional theory calculations on phonons in Sr(Fe(x)Ti(1-x))O(3) solid solution within the formalism of a linear combination of atomic orbitals are presented. The phonon density of states (DOS) calculated for 6.25% Fe(4+) impurities is reported and defect-induced phonon modes are identified. Based on our calculations and group-theoretical analysis, we suggest for the first time an interpretation of experimentally observed Raman- and IR-active modes.     

A DFT + U description of oxygen vacancies at the TiO2 rutile (1 1 0) surface

Experimental observations indicate that removing bridging oxygen atoms from the TiO₂ rutile (1 1 0) surface produces a localised state approximately 0.7 eV below the conduction band. The corresponding excess electron density is thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti³⁺ sites. We consider the electronic structure and geometry of the oxygen deficient TiO₂ rutile (1 1 0) surface using both gradient-corrected density functional theory (GGA DFT) and DFT corrected for on-site Coulomb interactions (GGA + U) to allow a direct comparison of the two methods. We show that GGA fails to predict the experimentally observed electronic structure, in agreement with previous uncorrected DFT calculations on this system. Introducing the +U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (?4.0 eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (?4.2 eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values is found to differ from that predicted by previous hybrid-DFT calculations.