Authors: Darwin Cordovilla Leon
Source: Deep Blue (University of Michigan) (article, 2021-01-01)
Field: Materials Science — Materials Chemistry

Exciton Transport and Strain Engineering in Atomically Thin Semiconductors The explosive energy demand by the communication and information technologies sector in the age of artificial intelligence (AI) calls for increasingly efficient electronics. One promising alternative to the inefficient electron-based information processing paradigm are the electrically neutral, Coulomb-bound electron-hole pairs known as excitons. Unlike electronics, excitonics lack the inherent capacitive delays and the associated energy losses that contribute to electronic hardware's energy inefficiency. While this particular advantage makes excitonics a promising platform for highly efficient communication and information processing hardware, conventional semiconductor materials in their bulk form are highly polarizable and screen out the Coulomb interaction that keeps excitons bound. As a result, excitonic hardware based on conventional semiconductors require cryogenic temperature operation, rendering them impractical for large-scale applications. Motivated by these challenges, the research in this dissertation broadly focuses on exploring and leveraging the properties of excitons in a new class of two-dimensional (2D) semiconductors known as transition metal dichalcogenides (TMDs). TMD monolayers offer two fundamental advantages over conventional semiconductors. First, the reduced dielectric screening and strong Coulomb interactions stemming from their 2D nature lead to strongly bound excitons with binding energies one order of magnitude higher than that in conventional semiconductors. This property makes excitons in TMD monolayers remarkably stable at room temperature. Second, the band structure of TMD monolayers is highly sensitive to mechanical deformation, enabling the manipulation of excitons via engineered energy gradients by subjecting the monolayers to non-uniform strain fields. Specifically, this dissertation centers around three main aspects. First, the physical processes responsible for the deviation of diffusive transport of excitons in TMD monolayers from the conventional Fick's law of diffusion (anomalous diffusion), are investigated. The diffusion of excitons generated in tungsten diselenide (WSe2) monolayers is studied by monitoring the time evolution of their photoluminescence (PL) intensity distribution. The two mechanisms giving rise to anomalous diffusion in the WSe2 monolayers are the multi-capture and release of mobile excitons by traps, and the relaxation of the kinetic energy of excitons created non-resonantly. Both of these mechanisms lead to the nonlinear evolution of the mean squared displacement of the exciton distribution, which is a signature of sub-diffusive transport. Second, the control of the flow of excitons via non-uniform strain in a WSe2 monolayer is demonstrated by transferring it over a substrate with nanoscale features. The strain field is measured by mapping the PL spectra across the monolayer and calculating the resonance energy shift at the strained area relative to an unstrained point. The control of the flow was verified by exciting a Gaussian distribution of excitons at different points near the strained area of the WSe2 monolayer, and monitoring the peak of the distribution over time. In all instances, the distribution's peak shifts toward the highest strain point of the monolayer, demonstrating the flow of excitons on demand. Third, the signatures of quantum coherent effects in the nonlinear absorption spectrum of a molybdenum diselenide (MoSe2) monolayer are identified via continuous-wave coherent optical spectroscopy. The nonlinear absorption spectrum of the MoSe2 monolayer reveals two unique coherent processes: excitation-induced many-body scattering, and population pulsation resonances. These effects can be leveraged to implement coherent control schemes for quantum information applications. These results represent a steppingstone in the development of hybrid quantum photonic-excitonic devices that meet the energy efficiency demands of the ongoing digital revolution fueled by the advent of AI. Exciton ; Strain (injury) ; Semiconductor ; Strain engineering ; Materials science ; Condensed matter physics ; Engineering physics ; Optoelectronics ; Physics ; Biology ; Silicon Electronic and Structural Properties of Oxides ; Semiconductor materials and devices ; Semiconductor Quantum Structures and Devices