《科学》：南京大学制备出基于氧化物钙钛矿体系的新颖二维材料

<正>南京大学聂越峰教授课题组采用分子束外延技术对非层状结构的氧化物钙钛矿材料进行单原子层精度的生长与转移,结合王鹏教授课题组的透射电子显微镜的结构分析,成功制备出基于氧化物钙钛矿体系的新颖二维材料。由于氧化物钙钛矿体系具有优异的电子特性,该成果开启了一扇通往具有丰富强关联二维量子现象的大门。     

钛酸锶光催化材料研究进展

近年来,钙钛矿型化合物以其特有的结构在光催化领域成为国内外研究的热点。钛酸锶(SrTiO3)是一种典型的钙钛矿型复合氧化物,具有稳定性高、无毒性,载流子迁移率高,光催化活性高等特点,作为半导体有低阻的电子运输结构和对小分子高效的氧化还原能力,是一种极具开发潜力和应用前景的光催化材料。主要介绍了SrTiO3材料的结构、制备方法与光催化机理,并对近年来SrTiO3在光催化领域的改性研究进行综述。最后在现有研究成果的基础上,对SrTiO3光催化材料的未来发展动向进行简要分析,有助于研究者取得进一步突破。     

钙钛矿锰氧化物低维纳米结构研究进展

钙钛矿结构锰氧化物由于同时存在电荷、自旋、轨道、晶格等多种自由度, 它们之间很强的相互作用和相互竞争导致了一系列新颖的物理现象, 如庞磁电阻效应、巨磁熵效应、绝缘体-金属转变、电子相分离、电荷/轨道有序等现象, 使其成为凝聚态物理学研究的热点。随着微电子器件日趋集成化和微型化, 其特征尺寸越来越小, 目前基于钙钛矿结构锰氧化物微电子器件的特征尺寸已经进入纳米尺度。在纳米尺度钙钛矿结构锰氧化物具有显著的尺寸效应, 表现出与薄膜及块材不同的电、磁输运特性, 在新一代微电子器件领域具有重要的应用价值。近年来人们在钙钛矿锰氧化物低维纳米结构制备、电磁输运特性测量、微结构表征及理论模拟方面, 都取得了较大的研究进展, 本文对此进行了评述。首先, 概述了钙钛矿锰氧化物低维纳米结构的微结构研究进展; 介绍了钙钛矿锰氧化物低维纳米结构的电子相分离及电荷有序现象; 评述了其电磁输运特性的纳米尺度表征; 讨论了钙钛矿锰氧化物低维纳米结构在自旋电子学、磁随机存储器和传感器方面的应用进展。最后指出了未来钙钛矿锰氧化物低维纳米结构研究需要重点解决的一些问题。     

SrTiO3材料中氧空位的密度泛函理论

计算了SrTiO3-δ（δ=0，δ=0．125）体系电子结构，分析了氧空位对SrTiO3晶体的价键结构、能带、态密度、分波态密度、差分电荷密度的影响。所有计算都是基于密度泛函理论（DFT）框架下的第一性原理平面波超软赝势方法。计算结果表明：当氧空位浓度δ=0．125时，空位在母体化合物SrTiO3中引入了大量的传导电子，费米能级进入导带，体系显示金属型导电性。由于空位掺杂，导带底附近的态密度发生了畸变，刚性能带模型不再适合描述SrTiO2.875体系。同时，在导带底附近距离费米能级0．3eV处引入了空位能级，这和实验测得的SrTiO3材料内中性氧空位的电离能约为0．4eV相符。此外，Mulliken布局分析、分波态密度和差分电荷密度分析表明，该空位能级主要由与其最近邻的两个Ti原子的3d电子态贡献，并且由该空位引入的导电电子大部分都局域在空位最近邻的两个Ti原子周围。最后，计算了三种典型平衡条件下SrTiO3晶体内中性氧空位的形成能。     

调控La2/3Sr1/3MnO3基异质结中磁各向异性变化的进展(英文)

钙钛矿锰酸盐(La_2/3Sr_1/3MnO₃)基异质结由于存在强自旋轨道耦合作用、高温铁磁性、强金属导电性以及丰富的氧八面体变化等特点,近年来在氧化物自旋电子学、拓扑电子态以及磁存储功能材料等应用方面备受关注.本综述主要针对锰酸盐异质结中磁各向异性变化,及其在氧化物自旋电子学器件中可能的应用进行了及时系统的总结.首先,本文简要介绍了锰酸盐异质结中的热点研究内容以及本文的出发点;其次,介绍了利用应变和衬底取向对锰酸盐异质结磁各向异性的调控;再次,重点介绍了界面耦合效应、氧八面体旋转、对称性破缺等新型界面工程手段对磁各向异性的调控;最后,介绍了电场调控磁各向异性的可逆变化方法,并总结了一些新型调控磁各向异性的手段.本综述将有效推动锰氧化物中磁各向异性的进一步研究,并且为锰氧化物在高效磁存储器件中的应用指明方向.     

新型SiAsP二维Janus材料电子结构和光吸收性质的第一性原理研究

基于密度泛函理论的第一性原理计算,通过对最新报道出的二维材料单层SiP₂进行原子替换,预测出单层SiAs₂,α-SiAsP,β-SiAsP 3种新型二维材料,并对3种材料的晶体结构、稳定性、电子能带结构和光学性质进行分析验证。通过静电势计算验证出3种材料在与平面垂直的方向具有非对称性,是Janus材料;声子谱计算结构表明3种材料具有良好的结构稳定性;电子结构计算结果表明上述材料为间接带隙半导体,带隙大小分别为2.21,2.43,1.76 eV;吸收光谱计算结果表明3种材料可以有效地吸收可见光和紫外光,其中β-SiAsP甚至可以吸收近红外光。因此,预测出单层SiAs2,α-SiAsP和β-SiAsP二维Janus材料在光学和电子领域具有一定的应用前景。     

中国应用表面物理国家重点实验室

实验室密切注视国际学术界发展的新动向,适时拓展研究方向。开展了计算凝聚态物理、表面物理与工程、半导体光电子物理、表面与超薄膜磁性、光子晶体等5个方面的研究工作。主要研究内容包括：表面界面与电子态声子态理论,微结构和介观体系理论,化合物半导体表面的结构与电子态,半导体异质界面,用STM／AFM研究表面结构,硅基光电子材料与低维量子结构,     

四面体掺杂尖晶石结构Co1-xRexCr2O4(Re=Li、Na、K、Rb)的电子结构和光学性质

采用基于密度泛函理论(DFT)的平面波超软赝势方法, 计算了CoCr₂O₄及Li、Na、K和Rb四面体掺杂CoCr₂O₄的基态结构、电子结构和光学性质。计算结果表明: 一价离子四面体掺杂都导致晶格有微小的畸变, 使体系的稳定性降低, Rb掺杂的体系最稳定; 电子态密度的计算结果表明: 掺杂体系的导带主要有Co-3d和Cr-3d轨道电子构成, 掺杂离子改变了CoCr₂O₄导带的电子结构, 主要引起了导带Co-3d态密度峰的下移, 随着掺杂浓度的增大, 费米能级进入价带更深; 光学性质计算表明: 掺杂体系的吸收光谱发生红移, 并在低能区有很强的吸收, 表明掺杂能极大地提高CoCr₂O₄对可见光的吸收和光催化效率。     

Zn1-xFexO精细结构的第一性原理研究

计算了Fe掺杂ZnO体系几何结构和电子结构,分析了掺杂对ZnO晶体的电子结构、磁矩、电子态密度、磁学性质的影响.所有计算都是基于密度泛函理论(DFT)框架下的第一原理平面波赝势方法.计算结果表明:Fe3 并不能占据准确的Zn2 位置,而是沿c轴方向位移了一段距离Δz,这是引起Zn1-xFexO铁磁性的主要原因,理论计算结果与试验结果吻合.     

有机电子传输材料在反式钙钛矿太阳能电池中的研究现状

近年来,反式结构的钙钛矿太阳能电池凭借制备工艺简单、可低温成膜、迟滞效应低、适合与传统太阳能电池结合制备叠层器件等优点,受到了人们广泛的关注,经过几年的发展,反式钙钛矿太阳能电池的光电转化效率已从3.9%提升到25.37%。其中电子传输层作为钙钛矿太阳能电池的重要组成部分,在提取和运输载流子、阻挡空穴、调节界面能级结构和抑制电荷复合等方面起着关键性的作用。一些有机材料(富勒烯及其衍生物、苝二酰亚胺、萘二酰亚胺等)凭借容易合成和纯化、能级可调、电子迁移率高、溶解性好、化学/热稳定性良好等优势,已经广泛应用于反式钙钛矿太阳能电池。本文主要介绍了不同有机电子传输材料在反式钙钛矿太阳能电池中的研究现状,还介绍了电子传输层掺杂和界面修饰两种提升器件性能的改性手段,旨在为开发全新的有机电子传输材料提供基础性的理论指导。     

Switchable induced polarization in LaAlO3/SrTiO3 heterostructures

Demonstration of a tunable conductivity of the LaAlO(3)/SrTiO(3) interfaces drew significant attention to the development of oxide electronic structures where electronic confinement can be reduced to the nanometer range. While the mechanisms for the conductivity modulation are quite different and include metal-insulator phase transition and surface charge writing, generally it is implied that this effect is a result of electrical modification of the LaAlO(3) surface (either due to electrochemical dissociation of surface adsorbates or free charge deposition) leading to the change in the two-dimensional electron gas (2DEG) density at the LaAlO(3)/SrTiO(3) (LAO/STO) interface. In this paper, using piezoresponse force microscopy we demonstrate a switchable electromechanical response of the LAO overlayer, which we attribute to the motion of oxygen vacancies through the LAO layer thickness. These electrically induced reversible changes in bulk stoichiometry of the LAO layer are a signature of a possible additional mechanism for nanoscale oxide 2DEG control on LAO/STO interfaces.     

Collective Control of Potential-Constrained Oxygen Vacancies in Oxide Heterostructures for Gradual Resistive Switching

Filamentary resistive switching in oxides is one of the key strategies for developing next-generation non-volatile memory devices. However, despite numerous advantages, their practical applications in neuromorphic computing are still limited due to non-uniform and indeterministic switching behavior. Given the inherent stochasticity of point defect migration, the pursuit of reliable switching likely demands an innovative approach. Herein, a collective control of oxygen vacancies is introduced in LaAlO₃/SrTiO₃ (LAO/STO) heterostructures to achieve reliable and gradual resistive switching. By exploiting an electrostatic potential constraint in ultrathin LAO/STO heterostructures, the formation of conducting filaments is suppressed, but instead precisely control the concentration of oxygen vacancies. Since the conductance of the LAO/STO device is governed by the ensemble concentration of oxygen vacancies, not their individual probabilistic migrations, the resistive switching is more uniform and deterministic compared to conventional filamentary devices. It provides direct evidence for the collective control of oxygen vacancies by spectral noise analysis and modeling by Monte-Carlo simulation. As a proof of concept, the significantly-improved analog switching performance of the filament-free LAO/STO devices is demonstrated, revealing potential for neuromorphic applications. The results establish an approach to store information by point defect concentration, akin to biological ionic channels, for enhancing switching characteristics of oxide materials.     

Electronic effects in manganite/insulator interfaces: interfacial enhancement of the insulating tunneling barriers

The transport properties across perovskite oxides heterointerfaces are analyzed. Epitaxial La(2/3)Ca(1/3)MnO3/SrTiO3 (LCMO/STO) heterostructures with different STO insulating-barrier thicknesses are systematically investigated and their behavior compared with LCMO/metal junctions. Atomic force microscopy (AFM) measurements in current-sensing mode show typical features associated with tunneling conduction. Careful analysis of the I-V curves across LCMO/STO heterointerfaces, using the Simmons model in the intermediate voltage range, clearly shows the existence of an interface-induced enhancement of the tunneling barrier of about 1.6 nm on the LCMO side. These results confirm recent theoretical studies predicting electronic phase segregation and the formation of an orbital-ordered insulating phase at the manganite-insulator interface that is a result of the reduction in the number of charge carriers at the interface.     

Fabrication of KTa0.65Nb0.35O3 film by pulsed laser deposition on glass substrate with various buffer layers

KTa_0.65Nb_0.35O₃ (KTN) thin films were deposited on amorphous glass substrates using a range of single buffer layers such as indium tin oxide (ITO), zinc oxide (ZnO), 3 at% Al-doped ZnO (AZO), and 3 at% Ga-doped ZnO (GZO), as well as a variety of multi-buffer layers such as SrTiO₃ (STO)/ITO, STO/ZnO, STO/AZO, and STO/GZO using a pulsed laser deposition system. All films showed a polycrystalline perovskite phase with the exception of all single buffer layers and STO/ITO multi-buffer layers. The STO buffer layer is important for crystallizing KTN films due to the similar lattice constant and same crystal structure. The optical transmittance of all films exhibited a transmittance ?90% in the wavelength range.     

Extreme Reconfigurable Nanoelectronics at the CaZrO3/SrTiO3 Interface

Complex oxide heterostructures have fascinating emergent properties that originate from the properties of the bulk constituents as well as from dimensional confinement. The conductive behavior of the polar/nonpolar LaAlO₃/SrTiO₃ interface can be reversibly switched using conductive atomic force microscopy (c‐AFM) lithography, enabling a wide range of devices and physics to be explored. Here, extreme nanoscale control over the CaZrO₃/SrTiO₃ (CZO/STO) interface, which is formed from two materials that are both nonpolar, is reported. Nanowires with measured widths as narrow as 1.2 nm are realized at the CZO/STO interface at room temperature by c‐AFM lithography. These ultrathin nanostructures have spatial dimensions at room temperature that are comparable to single‐walled carbon nanotubes, and hold great promise for alternative oxide‐based nanoelectronics, as well as offer new opportunities to investigate the electronic structure of the complex oxide interfaces. The cryogenic properties of devices constructed from quasi‐1D channels, tunnel barriers, and planar gates exhibit gate‐tunable superconductivity, quantum oscillations, electron pairing outside of the superconducting regime, and quasi‐ballistic transport. This newly demonstrated ability to control the metal–insulator transition at nonpolar oxide interface greatly expands the class of materials whose behavior can be patterned and reconfigured at extreme nanoscale dimensions.     

Diluted Oxide Interfaces with Tunable Ground States

The metallic interface between two oxide insulators, such as LaAlO₃/SrTiO₃ (LAO/STO), provides new opportunities for electronics and spintronics. However, due to the presence of multiple orbital populations, tailoring the interfacial properties such as the ground state and metal‐insulator transitions remains challenging. Here, an unforeseen tunability of the phase diagram of LAO/STO is reported by alloying LAO with a ferromagnetic LaMnO₃ insulator without forming lattice disorder and at the same time without changing the polarity of the system. By increasing the Mn‐doping level, x, of LaAl_1?xMn_xO₃/STO (0 ≤ x ≤ 1), the interface undergoes a Lifshitz transition at x = 0.225 across a critical carrier density of n_c = 2.8 × 10¹³ cm^?2, where a peak T_SC ≈255 mK of superconducting transition temperature is observed. Moreover, the LaAl_1?xMn_xO₃ turns ferromagnetic at x ≥ 0.25. Remarkably, at x = 0.3, where the metallic interface is populated by only d_xy electrons and just before it becomes insulating, a same device with both signatures of superconductivity and clear anomalous Hall effect (7.6 × 10¹² cm^?2 < n_s ≤ 1.1 × 10¹³ cm^?2) is achieved reproducibly. This provides a unique and effective way to tailor oxide interfaces for designing on‐demand electronic and spintronic devices.     

Creation and Control of Two-Dimensional Electron Gas Using Al-Based Amorphous Oxides/SrTiO(3) Heterostructures Grown by Atomic Layer Deposition

The formation of a two-dimensional electron gas (2-DEG) using SrTiO(3) (STO)-based heterostructures provides promising opportunities in oxide electronics. We realized the formation of 2-DEG using several amorphous layers grown by the atomic layer deposition (ALD) technique at 300 °C which is a process compatible with mass production and thereby can provide the realization of potential applications. We found that the amorphous LaAlO(3) (LAO) layer grown by the ALD process can generate 2-DEG (～1 × 10(13)/cm(2)) with an electron mobility of 4-5 cm(2)/V·s. A much higher electron mobility was observed at lower temperatures. More remarkably, amorphous YAlO(3) (YAO) and Al(2)O(3) layers, which are not polar-perovskite-structured oxides, can create 2-DEG as well. 2-DEG was created by means of the important role of trimethylaluminum, Me(3)Al, as a reducing agent for STO during LAO and YAO ALD as well as the Al(2)O(3) ALD process at 300 °C. The deposited oxide layer also plays an essential role as a catalyst that enables Me(3)Al to reduce the STO. The electrons were localized very near to the STO surface, and the source of carriers was explained based on the oxygen vacancies generated in the STO substrate.     

High Quality YBCO Film Growth on SrTiO_3-Buffered LaAlO_3 Substrate by Full Solution Method

A full solution method has been developed as a low cost process of YBa2Cu3OT-x （YBCO） coated conductor fabrication. In this study, highly biaxially textured SrTiO3 （STO） buffer layers were fabricated on LaAlO3 （LAO） single crystal substrates by sol-gel method using metal alkoxides as the staring precursor materials. High quality YBCO superconducting film was then fabricated on STO-buffered LAO substrate by trifluoroacetic metalorganic deposition （TFA-MOD） method. For the YBCO superconducting film, only （001） diffraction peaks can be detected by XRD （X-ray diffraction） analysis with no other phases detectable. Especially, Inplane texture of YBCO film is improved compared to that of STO buffer layer from phi scans analysis, which indicates the self-epitaxy phenomenon explained by considering interracial energy. STO and YBCO films both show c-axis oriented grains growth and have uniform surface microstructure. A critical transition temperature, TC （R=0） of 89.5 K and a critical current density of 2 mA/cm2 （77 K, self-field） were obtained for a 0.2μm thick YBCO film on STO-buffered LAO substrate. No reaction between YBCO and STO was detected by XRD analysis. This full solution process may provide a promising low cost fabrication route for YBCO coated conductors on metal tape.     

Controlling the Magnetic Properties of LaMnO3/SrTiO3 Heterostructures by Stoichiometry and Electronic Reconstruction: Atomic‐Scale Evidence

Interface‐driven magnetic effects and phenomena associated with spin–orbit coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital, and lattice degrees of freedom is the key to boosting device performance. In LaMnO₃/SrTiO₃ (LMO/STO) polar–nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The interfacial electronic structure plays a critical role in the understanding of the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above, yet such a study is missing. Here, an atomic scale understanding of LMO/STO ambipolar ferromagnetism is offered by quantifying the interface charge distribution and performing first‐principles density functional theory (DFT) calculations across this abrupt magnetic transition. It is found that the electronic reconstruction is confined within the first 3 ucs of LMO from the interface, and more importantly, it is robust against oxygen nonstoichiometry. When restoring stoichiometry, an enhanced ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm (5 uc) is achieved, making LMO readily applicable as barriers in spin filters.     

Optical Study of the Strain Effects on Pr0.6Ca0.4MnO3 Thin Films

We report a systematic Raman and infrared (IR) study of Pr_0.6Ca_0.4MnO₃ (PCMO) thin films grown on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates (PCMO/STO and PCMO/LAO, respectively). The measurements were performed for a temperature range 78 K<T<295 K. The A_g(2) tilting Raman mode has shown substrate dependence, implying an effect of the lattice mismatch on the tilting angle, as the MnO₆ octahedra are tilted to optimize the structure. The Raman and the IR modes are affected by the different kinds of strain induced by different substrates. In the mid-IR (MIR) region, there are broad bands implying that FM and AFM clusters are formed and competing even at RT.