Modification of Soft Inorganic Thin Films through the use of van der Waals Epitaxial Strain

人类社会对更好的信息传播手段的一贯追求和对它的有效利用？美国的能源供应，需要在基础技术材料基础设施上不断创新。变革性技术依赖于其科学基础的突破，例如材料和加工。最近，一些软无机固体在能量转换和信息处理方面显示出潜在的应用潜力，并有望对物质的奇异状态的物理学产生新的见解。理论研究表明，这些材料的可逆机械变形可能导致其物理性质的逐渐或突然变化。然而，这些软质材料，特别是重卤化物和硫族化合物，由于其独特的材料结构和原子排列，很难利用传统的化学方法来诱导变形。这项工作通过探索通过范德华外延拉伸软但重的卤化物和硫系材料的可能性来解决这个问题。该研究将以应变薄膜的形式引入一套新的有用的光电、电光、逻辑和存储材料。研究人员还将通过建立“学习图形”计划来追求教育目标，该计划将通过信息的图形表示来培训工程本科学生，特别是来自代表性不足的群体的学生。外延电子和光学材料中的应变已被用于产生在非应变状态下无法实现的理想材料性能。虽然在传统半导体中有很大的效果，但在范德华键合化合物中引入应变是很困难的。由元素周期表中较深的元素形成的范德华化合物，被称为重卤化物和硫族化物，可能为这些材料的应变修饰提供了新的机会。这项资助旨在通过范德瓦尔斯外延在外延层内产生显著弹性应变的条件下，将较重的元素合成为薄膜的“软”和“重”材料。重点将放在两种模型材料上：范德华固体PbI2和非范德华固体CH3NH3PbCl3。其他几种材料包括CdTe、CdS、SbI3和Sb2S3也将进行研究。通过x射线衍射、拉曼光谱、透射电子显微镜和角分辨光致发光光谱对材料的应变大小、应变松弛机制和应变诱导的物理性质进行表征。第一性原理计算将用于阐明实验观察。在建立卤化物和硫族化合物的范德华形核/生长模型后，对这些外延材料的合成进行修改，将通过范德华外延工艺实现弹性应变工程。外延应变的大小和各向异性以及对材料性能的影响将用于发展对外延“重”范德华材料的潜在变化和行为的机械理解。

Human society's consistent pursuit of better means of information communication and effective use of it?s energy supplies, requires continual innovation in the underlying technological materials infrastructure. Transformative technologies rely on breakthroughs in their scientific foundations, such as materials and processing. Recently, a number of soft inorganic solids have shown potential for applications in energy conversion and information manipulation, and promising new insight into the physics of exotic states of matter. Theoretical works suggest that reversible mechanical deformations in these materials could lead to either gradual or abrupt shifts of their physical properties. However, these soft materials, particularly, heavy halides and chalcogenides, experience difficulty in utilizing conventional chemical approaches to induce for deformation purpose due to their unique materials structures and atomic arrangement. This work addresses this problem by exploring the possibility of stretching soft but heavy halide and chalcogenide materials through van der Waals epitaxy. The research will introduce a new set of useful optoelectronic, electro-optic, logic and memory materials in the form of strained thin films. Researchers will also pursue educational goals by establishing a 'Graphics for Learning' program, which will train engineering undergraduate students, especially from underrepresented groups, through the graphical presentation of information. Strain within epitaxial electronic and optical materials has been used to produce desirable materials properties unachievable in the unstrained state. While used to great effect in conventional semiconductors, the introduction of strain into the class of van der Waals bonded compounds has been difficult. The van der Waals compounds formed from elements deeper in the periodic table, termed heavy halides and chalcogenides, may offer new opportunity to strain-modify these materials. This grant looks to synthesize 'soft' and 'heavy' materials as thin films out of the heavier elements via van der Waals epitaxy onto useful substrates under conditions that lead to significant elastic strains within the epitaxial layers. The focus will be on two model materials: the van der Waals solid PbI2 and non-van der Waals solid CH3NH3PbCl3. Several other materials including CdTe, CdS, SbI3 and Sb2S3 will also be studied. The strain magnitude, strain relaxation mechanisms and strain-induced physical properties will be characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and angle-resolved photoluminescence spectroscopy. First-principle calculations will be employed to elucidate the experimental observations. After establishing a van der Waals nucleation/growth model for halides and chalcogenides, modifications to the synthesis of these epitaxial materials will be directed at achieving elastic strain engineering via the van der Waals epitaxial process. The magnitude and anisotropy of the epitaxial strain and the effect on materials properties will be used to develop a mechanistic understanding of the underlying changes in and behavior of the epitaxial 'heavy' van der Waals materials.