Multiscale dynamics in complex materials and interfaces

Habib, Adela
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Lu, T.-M. (Toh-Ming), 1943-
Wertz, Esther A.
Ullal, Chaitanya
Sundararaman, Ravishankar
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Material response to light irradiation is at the heart of a wide range of emergent technologies that are in the nano- to quantum length scale. These technologies include interferencenanolithography, optoelectronics, and spin-based quantum information processing devices. Efficient exploitation of material response is the key to the full realization of these technologies. In the context of these applications, this means investigating the charge, spin, and coupled optical dynamics with kinetic reactions in a photoresist at timescales ranging from femto- to microseconds. To this end, first-principles methods have emerged as powerful tools in simulating the dynamics with the required level of detail in disentangling multiple pro- cesses in the microscopic and macroscopic regimes. In this talk, I will present my studies of these dynamics from first-principles with three specific aims: (1) charge carrier (electron and hole) dynamics in plasmonic materials for finding key parameters affecting plasmonic hot carrier device efficiency, (2) spin carrier (electron and hole) dynamics in materials promising for spin-based technologies, and (3) coupled optical dynamics with kinetic reactions in a photoresist to explore parameters for quality nanopatterning. First, searching for new promising plasmonic materials, we find that transition metal nitrides (TMNs) and Ag-Au alloys have hot carrier properties (e.g., lifetimes and mean free paths) comparable to Au. This coupled with their optical tunability and stability as small nanoparticles in extreme conditions make them highly favorable candidates for hot- carrier applications operating in infrared to ultraviolet regimes. Second, probing the hot hole injection in Au on p-type GaN, we predict that ∼90% of hot holes can cross the interfacial barrier, suggesting hot-hole driven devices to perform similar to hot-electron driven devices for optoelectronics. A similar investigation of hot hole injection in Cu on p-GaN suggests that harnessing hot holes with p-type semiconductors is a promising strategy for plasmon-driven photodetection across the visible and ultraviolet regimes. Next, searching for materials with suitable spin properties, first, we establish a new, ac- curate and universal first-principles methodology based on Lindbladian dynamics of density matrices to calculate spin dynamics in solids with arbitrary spin mixing and crystal symmetry. Applying this method to calculate spin-phonon relaxation rates in MoS2, in addition to the excellent agreement with experimental measurements, we gain important insights about the intervalley and intravalley contributions to spin relaxation. We find that intervalley scattering is dominant for holes at low temperatures because of the large spin-orbit split in the valence bands. In contrast, for conduction electrons, it is the intravalley scattering that dominates at all temperatures. For graphene, density-matrix dynamics simulations reveal that electric fields and substrates strongly reduce spin-phonon relaxation lifetimes to the nanosecond scale as shown in experiments. We find that hBN increases the out-of-plane to in-plane lifetime ratio from 1/2 to 0.8, matching experiments, suggesting that intrinsic spin-phonon relaxation is likely the limiting factor for graphene-based spin technologies at room temperature. Lastly, we develop a coupled electromagnetic (EM) and reaction kinetics simulation method that can predict optical dynamics including absorption, diffraction, and intensity modulations coupled with the photo-activated and inhibited reaction kinetics in a material in 2 dimensions. This method, simulating a two-color interference lithography technique for a periodic pattern of lines, shows that diffraction effects are negligible (< 0.1%) for film depths up to 10 μm. Simplifying the experimental configuration from using two standing waves for both colors to a plane and a standing wave combination, we can achieve a line contrast as good as 80% at optimal exposure times. Our EM solver based on perturbation theory provides a computationally efficient method to be coupled to the kinetic reactions of any future two-color photoresist candidate for the study of optimal parameters for the nanopatterning technique.
August 2021
School of Science
Dept. of Physics, Applied Physics, and Astronomy
Rensselaer Polytechnic Institute, Troy, NY
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