Achieving ballistic energy flow in materials at room temperature is a long-standing goal that could unlock lossless energy harvesting and wave-based information technologies. I will describe two avenues to achieve ballistic transport by harnessing strong interactions between coherent and incoherent excitations in 2D materials. The first is to leverage strong interactions between photons and semiconductor excitons, yielding part-light part-matter particles known as polaritons. The second is to leverage strong interactions between electrons and delocalized phonons, forming coherent polarons. In both cases, we image the propagation of these particles using unique ultrafast microscopies on femtosecond and few-nanometer scales.
We spatiotemporally resolve exciton transport in two-dimensional transition metal dichalcogenides (TMD) in which we incorporate local dielectric inhomogeneities known as nanobubbles. We observe highly efficient exciton funneling in these dielectric inhomogeneities, a process entirely driven by momentum-indirect (dark) excitons whose energies are much more sensitive to dielectric perturbations than bright excitons.
The ability of energy carriers to move within and between atoms and molecules underlies virtually all material function. Understanding and controlling energy flow requires observing it on ultrasmall and ultrafast spatiotemporal scales, where energetic and structural roadblocks dictate the fate of energy carriers. I will describe a new optical ultrafast microscope based on stroboscopic elastic scattering that allows direct visualization of energy carrier transport in 3D with few-nm spatial precision and picosecond temporal resolution. I will demonstrate the wide applicability of the method for watching all forms of energy carriers – free charges, excitons, phonons and ions – move in materials ranging from silicon to conjugated polymers via 2D transition metal dichalcogenides and metal halide perovskites. Beyond quantifying carrier mobilities, our approach directly correlates material resistivities to local morphology, shedding light on how disorder affects transport pathways in 3D.
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