Nanoscale Origins of Thermal Transport Phenomena for Hybrid Layered Perovskites

Abstract

An exciting and fundamentally powerful modern methodology for materials development is the process by which artificial solids are rationally built piece-by-piece from nanoscale “building blocks”. Among the library of nanomaterials currently at the forefront of this pursuit, two-dimensional layered lead halide perovskites (2D LHPs), are of particular interest. These materials, solid crystals composed of alternating layers of atomically thin organic and inorganic subphases, possess novel optical and electronic properties that make them particularly suited for use in devices including solar cells, LEDs, flexible electronics, and even lasers. While significant early strides have been made in investigating charge carrier transport through dynamic models and sophisticated experiments, comparatively little attention has been given to understanding the manner in which the design of these nanostructured solids impacts their macroscopic thermal properties via thermal carrier (phonon) transport. This knowledge, however, is critical to addressing the thermal management constraints necessary to the design of reliable and stable devices. To this end, this dissertation seeks to elucidate the thermal stability and fundamental pathways for heat transport within 2D LHP artificial solids. I first present an experimental investigation into the thermal and structural stability of these 2D LHPs near room temperature using differential scanning calorimetry and x-ray diffraction. This analysis reveals near-room temperature melting transitions isolated to the organic component of the nanomaterials. The existence of such an isolated phase transition indicates thematerials behave thermophysically as composites, a hypothesis that is supported by the effective use of a lever rule in estimation of the heat capacity of the materials. I discuss the theoretical foundation and experimental construction of a frequency domain thermoreflectance technique to effectively measure the cross-plane thermal conductivity of 2D LHPs. This technique is then utilized to perform the first measurement of the thermal conductivity of 2D LHPs. This experimental study reveals that even in terms of their thermal transport pathways, 2D LHPs can be treated as composite materials. Specifically, lead bromide 2D LHPs exhibit structure-property relationships characteristic of ballistic phonon transport within isolated subphases and diffuse scattering at the organic-inorganic interfaces between layers. Finally, I report the first measurements of the vibrational spectrum for 2D LHPs via low frequency Raman spectroscopy. This probe identifies the persistence of bulk-like phonons even in the atomically thin 2D LHPs, in addition to identifying coherent acoustic phonons in lead iodide 2D LHPs potentially capable of carrying thermal energy across the organic-inorganic interfaces without scattering. Each of the observations made throughout this dissertation suggest the thermophysical representation of 2D LHPs as composite materials is a useful framework for understanding their thermal transport properties. That so many material properties can be effectively predicted simply from the bulk properties of the component phases is surprising given both the long-range order of the artificial solids and the sub-nanometer length scale of the individual component layers, and underlines the potential for intelligent engineering of the thermal properties of 2D LHPs without deleterious influences on the sterling optoelectronic properties.