Electron–Phonon Coupling in Metal–Organic Frameworks

Abstract

Metal--organic frameworks (MOFs) are a promising class of materials that offer impressive tunability. They are thereby suitable for use as platforms for designing functional materials. However, navigating their design space is a rather Sisyphean task, with over 500,000 predicted MOFs in the literature. If we can better understand the fundamental properties of MOFs, we can develop systems for more effectively tuning MOF functionality. In particular, their high crystallinity and relative softness provide opportunities for engineering electrical and optical properties. Further, electrical conductivity in MOFs is yet limited; understanding and improving their charge transport would greatly expand MOF applications. This work seeks to advance our understanding of the MOF design space. It combines an interest in 1) applying physical theory to a traditionally chemical material platform, 2) studying transport in MOFs for practical applications, and 3) advancing the capabilities and accessibility of computational tools. In service of these goals, I implement a computational workflow for \textit{ab initio} calculations of three key determinants of MOF properties: electrons, phonons, and their coupling. In Chapter 0, I provide a gentle and patient introduction to materials science, computational materials study, and MOFs. The discussion is intended to gradually paint a rich motivation for this work, at a level accessible to the lay reader. I discuss why MOFs are an interesting material platform from both chemical and physical perspectives, as well as why electron--phonon coupling is a particularly interesting problem to address. In Chapter 1, I provide background on the theory necessary to appreciate this work. I begin with a technical review of the physics used herein to compute electron--phonon coupling. These include density functional theory, density functional perturbation theory, and Wannierization. I then provide a primer on topics in high-performance computing, which is important for understanding how I attempted to make expensive electron--phonon calculations attainable. In Chapter 2, I discuss the details of how my computational workflow is implemented. This is the heart of the work, and should be read as a ``how-to'' handbook for performing the calculations explained in Chapter 1. Whereas the preceding chapter is physics-oriented, this chapter is particularly computation-oriented. In Chapter 3, I provide sample calculations as a proof of concept for the computational workflow developed herein. In particular, I discuss results for the two-dimensional CuC7H6N4 MOF (CSD refcode DUKYAL), finding electronic features of interest and general alignment of phonon properties with the existing literature. I also note limitations in Wannierization. Taken together, this work provides a computationally practical workflow for calculating electron--phonon coupling in metal--organic frameworks, while elucidating the need for improved automation of Wannierization to achieve high-throughput scale.