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

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

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.