Real-time X-ray Scattering Studies of Oxide Epitaxial Growth

该项目由材料研究部的电子和光子材料(EPM)和陶瓷(CER)项目联合资助。非技术描述：X射线是研究材料的一种非常强大的工具：由于其波长短，它们可以使用衍射测量原子尺寸的结构，并且由于它们不被强烈吸收，它们可以到达材料内部。同步加速器是科学家可用的最明亮的X射线源。在这个项目中，实时X射线散射被用来研究晶体金属氧化物薄膜在专门为安装在同步加速器源上而定制的薄膜沉积室中的生长。一部来自生长表面的X射线散射图案随时间变化的电影被用来了解晶层从汽相组装时的原子结构，这在寻求创造具有改进或新颖性能的新材料方面产生了重要的见解。有用特性的例子包括通过晶格中正离子和负离子的相对移动来存储电荷的能力(铁电效应)，或者在变形时产生电压的能力(压电效应)。这些效应以及其他几种更奇特的效应可能会通过人工生产的薄膜(例如，由两种不同材料交替形成的层，每层只有几个原子厚)来改变。由于其新颖的性质，金属氧化物在包括电子存储器、探测器、致动器和能量收集器在内的许多应用中具有重要的意义。该项目在一个高度跨学科的领域提供研究生培训，并向本科生介绍X射线科学和薄膜沉积技术。技术细节：该项目利用布鲁克海文国家实验室(NSLS II)的新世界级同步加速器设备，对脉冲激光沉积薄膜生长过程中由氧化物材料组成的表面、界面和多层膜的演变进行现场时间分辨X射线散射研究。钙钛矿氧化物表现出各种复杂的结构扭曲和相关的功能性质，这也可以通过非平衡结构的生长来连续调节。然而，目前通过外延薄膜和多层膜的生长来实现合成结构需要耗时的试验和错误的方法，这极大地限制了新材料和器件的进展速度。原位同步加速器X射线散射提供了在几秒钟内生成相互作用的空间地图的能力。这些图在一次快照中提供了关于面内和面外晶格参数、极区结构、表面粗糙度和表面终止的实时信息，从而提供了关于生长过程变量和所产生的原子结构之间关系的关键反馈。该项目还侧重于与碰撞诱导的能量过程在促进平面界面形成中的作用以及如何控制这种过程以生产改进材料有关的基本问题。正在使用原位X射线散射来测试特定的原子尺度的机制，以跟踪氧化物薄膜在沉积过程中的表面动力学。该项目还为研究生提供了一个令人兴奋的机会，使他们能够通过先进的方法获得薄膜生长和材料表征方面的专业知识，并为本科生参与一个涉及大学与国家实验室合作的跨学科项目提供了一个令人兴奋的机会。

This project is jointly funded by the Electronic and Photonic Materials (EPM) and Ceramic (CER) Programs in the Division of Materials Research. NON-TECHNICAL DESCRIPTION: X-rays are an enormously powerful tool for the study of materials: they can measure structures down to atomic dimensions using diffraction because of their short wavelength, and they can reach inside materials because they are not strongly absorbed. Synchrotrons are the brightest sources of X-rays available to scientists. In this project, real-time X-ray scattering is used to study the growth of crystalline metal oxide thin films in a special film deposition chamber customized for installation at a synchrotron source. A movie of the X-ray scattering pattern from the growing surface as a function of time is used to learn about the atomic structure of the crystalline layers as they assemble from the vapor phase, which yields important insights in the quest to create new materials with improved or novel properties. Examples of useful properties include the ability to store an electrical charge via a relative shift of positive and negative ions in the crystal lattice (ferroelectric effect), or to produce an electrical voltage when deformed (piezoelectric effect). These effects, and several other more exotic effects can potentially be modified through growth of artificially-produced thin films (e.g., with alternating layers of two different materials, each only a few atoms thick). Due to their novel properties, metal oxides are significant in a number of applications, including electronic memories, detectors, actuators and energy harvesters. This project provides graduate student training in a highly interdisciplinary area, and it introduces undergraduate students to X-ray science and thin film deposition technology.TECHNICAL DETAILS: This project utilizes a new world-class synchrotron facility at Brookhaven National Laboratory (NSLS II) to carry out in situ time-resolved X-ray scattering studies of the evolution of surfaces, interfaces, and multilayers composed of oxide materials during thin film growth by pulsed laser deposition. Perovskite oxides exhibit an extraordinary variety of complex structural distortions and associated functional properties, which may also be continuously tunable through growth of non-equilibrium structures. However, realizing synthetic structures through the growth of epitaxial thin films and multilayers currently involves a time-consuming trial and error approach, which greatly limits the pace of progress towards new materials and devices. In situ synchrotron X-ray scattering provides the capability to generate reciprocal space maps in a matter of seconds. These maps provides real time information on the evolution of both in and out of plane lattice parameters, polar domain structure, surface roughness and surface termination, in a single snapshot, thus providing critical feedback on the relationship between growth process variables and the resulting atomic structure. This project also focuses on fundamental issues related to the role of impact-induced energetic processes in promoting the formation of planar interfaces, and how such processes can be controlled to produce improved materials. Specific atomic-scale mechanisms are being tested using in situ X-ray scattering to follow the dynamics of surfaces of oxide thin films during deposition. This project also provides an exciting possibility for graduate students to gain expertise in both thin film growth and characterization of materials by advanced methods, and for undergraduate students to participate in an interdisciplinary project involving university-national laboratory collaborations.