|Abstract:||Disorder has been a long-standing driver across all sectors of materials engineering, ranging from energy and transportation, to electronics and communications, and to biomedicine and environment. Disorder engineering has focused primarily on tuning its spatial configuration and distribution to establish desirable structure-property relations. However, while band theory and phenomenological random-walk models are available in the crystalline and amorphous limits respectively, the picture for energy and charge transport in hybrid ordered-disordered materials is still incomplete. This is especially true when structural disorder possesses long-range correlation and dynamic nature. These subjects are the concern of this work. The goal is to numerically understand how different types of disorder can modify phonon and electron transport properties.
I first establish how uncorrelated structural disorder affects vibrational energy transport in low-dimensional disordered materials. The recently synthesized amorphous graphene and glassy diamond nanothreads are studied. Modal localization analysis, molecular dynamics simulations, and a generalized analytical model together demonstrate that the thermal properties of these materials exhibit both similarities and differences from disordered 3D materials. Similar to 3D, the low-dimensional systems exhibit both propagating and diffusive vibrational modes. Different from 3D, however, diffusonic contribution to thermal transport in these low-dimensional systems is shown to be negligible, which results from the intrinsically different nature of random walks in lower dimensions. Despite the lack of diffusons, the suppression of thermal conductivity due to disorder in low-dimensional systems is shown to be mild. The mild suppression originates from the presence of low-frequency vibrational modes that maintain well-defined polarizations and help preserve the thermal conductivity in the presence of disorder. This study brings the domains of low-dimensional materials and disordered materials together, and establishes appropriate theoretical approaches to characterize the vibrational energy transport at the atomic scale when disorder is present.
The second part deals with correlated but static disorder. The model system will be a hybrid ordered/disordered nanocomposite that consists of a crystalline silicon membrane decorated by regularly patterned disordered regions. Combining molecular dynamics and the Boltzmann theory, I predict a thermoelectric figure of merit ZT ≈ 0.5 at room temperature. To facilitate the Boltzmann theory, I have derived an analytical model for electron scattering with cylindrical defective regions based on partial-wave analysis. Furthermore, I find glass-crystal duality for the vibrational transport in these hybrid systems. Lattice dynamics reveals substantial hybridization between the localized and delocalized modes, which induces avoided crossings and harmonic broadening in the dispersion. Allen/Feldman theory shows that the hybridization and avoided crossings are the dominant mechanisms of the reduction. Anharmonic scattering is also enhanced in the patterned nanocomposites, further contributing to the reduction. These findings indicate that “patterned disorder” can be a viable strategy to tailor vibrational transport, and ion beam irradiation could be a promising fabrication strategy.
In the third part, I focus on the disorder that is both correlated and dynamic. Two lead iodide perovskites, XPbI3 (X=Cs, methylammonium), are the representative materials. In these perovskites, sublattice symmetrybreaking and dynamically correlated disorder affect substantially their vibrational and thermal properties. In contrast to the conventional phononic theory, analysis of spectral energy density reveals that thermal carriers exhibit more propagonic and diffusonic characteristics. Strong anharmoncity in these perovskites, two orders higher than silicon, is observed both on the inorganic framework and surprisingly for the interactions between PbI6 framework modes and localized A-site modes. Based on first-principles calculations, I ascribe the former to long-range interactions arising from resonant bonding, while the latter to A-site rattling in CsPbI3, and polar rotor scattering instead in MAPbI3. I also observe “waterfall-like” dispersions, which I show to be an emergent phenomenon due to dynamical averaging of different dispersions that belong to energetically equivalent disordered phases. This work would be of interest to the design and functionalization of a broad family of hybrid materials, including metal-organic frameworks, molecular crystals and hierarchically organized materials.