Excitonic Transport in Van der Waals Solids: Insights from Experiment and Predictive Calculations

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Nontechnical description: When a semiconductor absorbs a photon, an electron is excited to a higher energy state. The negatively charged excited electron and the positive charge left behind (hole) attract each other and form a bound pair known as an exciton. These excited pairs are capable of transporting energy in materials and play an important role in natural processes such as photosynthesis as well as organic devices such as photovoltaics and light-emitting diodes. Understanding the transport of excitons adds a new dimensionality to not only bolster the performance of the current excitonic devices but also introduce a platform for next-generation optoexcitonic devices. The current work studies the energy transport and its relation to the separation between the bound electron and hole pair that form the exciton. Above and beyond addressing fundamental and technological challenges, this research at the frontiers of optoelectronic and quantum materials provides an ideal venue for truly interdisciplinary education at all levels. In addition to PhD training, the results of the research findings are being incorporated into the curriculum of graduate and undergraduate courses as well as used for training of undergraduate students. The knowledge gained from this research has a potential to push forward the frontiers of research, innovations and educating the next generation of scientists and engineers for a better future. Technical description: Exciton transport in two-dimensional semiconductors has recently received significant attention due to the prospects of achieving room-temperature-stable excitonic devices. These van der Waals (vdW) semiconductors support stable room temperature excitons due to reduced dielectric screening that results in high binding energies and small Bohr radii. However, since these excitons remain delocalized over a few lattice spacings, qualitative as well as quantitative understanding of the transport behavior has been difficult. This is reflected from the fact that classifying the excitons as Wannier-Mott or Frenkel excitons is not trivial in these materials. Understanding the transport is crucial for the development of the material system as a device platform as it determines the architecture of the devices. To meet this challenge, the research team undertake a joint experimental and computational research effort using diffusion imaging microscope, ultrafast pump-probe and nonlinear optical techniques, photoluminescence spectroscopy and first-principles calculations based on density functional theory to conduct a systematic study of excitonic energy transport properties that can provide insight on the excitonic states. The team investigates excitonic energy transport in varying Lead Iodide layers that are sandwiched between hexagonal Boron Nitride. This material system enables two independent knobs (i) thickness of Lead Iodide (ii) thickness of Boron Nitride, to control the dielectric screening of the excitons and hence the amount of delocalization. Such atomic level control over the excitonic states as well as dielectric screening provides an opportunity to study the relationship between localized and delocalized excitonic energy transport in the same material system. In doing so, it could potentially enable control over band-like or hopping-like energy-transport mechanisms. Control over transport behavior is potentially transformative as it will change the rules on how charge and/or excited states are exploited for various devices such as photovoltaics, light generation, transistors, etc. Through this work, the research team attempts to gain control over the energy flow between and within nanoscale system that will enhance progress in quantum-information science, energy harvesting, metrology, and light sources.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.