# Highlights - Dynamics of Photoexcited Nanomaterials

Principal Investigator: Andrei Kryjevski (Physics, North Dakota State University)

Dr. Kryjevski's group focuses on theoretical description of electronic properties of nanomaterials. The goal is to develop a comprehensive high-precision computational technique for modeling the dynamics of a photoexcited nanoparticle starting from photon absorption and until recombination into ground state. They study properties of photoexcited semiconductor nanomaterials composed of quantum dots (QDs), nanowires (NWs), nanofilms, carbon nanotubes (CNTs), etc.

The study requires a comprehensive description of electrons, photons, and atomic vibrations (phonons), all of which are interacting quantum mechanical particles. The group, therefore, employs powerful methods of quantum field theory, which have been mostly used in theoretical nuclear and particle physics and combine them with the results of density-functional theory (DFT) simulations using advanced computational capabilities.

There are two major projects:

(i) Description of electronic and optical properties of QD arrays and of NWs and nanofilms as a function of the composition, morphology, doping, and structural disorder. The group also studies CNTs and CNT-QD and CNT-NW composites. This helps to determine the features that control systems’ properties, such as multiple exciton generation (MEG) and electron energy dissipation. Maximizing MEG while minimizing heat losses is a major pathway to increase nanomaterial-based solar cell efficiency [1–5].

(ii) Development of a novel first-principles high-precision Monte Carlo (MC) approach for the electronic structure modeling of semiconductor nanoparticles. Here, they apply stochastic importance sampling to the path integral representation of electron dynamics, using DFT output as a basis. Preliminary results suggest that the infamous Fermion sign problem can be overcome in this case. The idea is similar to that of the lattice gauge theory approach, which has been successful for quantum chromodynamics–the theory of strong interactions. In this method, properties of a whole spectrum (electrons, holes, trions, excitons, etc.) are obtained from the same MC simulation.

Both research projects require the use of high-performance computing capabilities provided by CCAST.

**References**

[1] A. Kryjevski, D. Mihaylov, and D. Kilin, "Dynamics of charge transfer and multiple exciton generation in the doped silicon quantum dot–carbon nanotube system: Density functional theory-based computation,"

*J. Phys. Chem. Lett.***9**, 5759 (2018).[2] A. Kryjevski, D. Mihaylov, S. Kilina, and D. Kilin, "Multiple exciton generation in chiral carbon nanotubes: Density functional theory based computation,"

*J. Chem. Phys.***147**, 154106 (2017).[3] A. Kryjevski, D. Mihaylov, B. Gifford, and D. Kilin, "Singlet fission in chiral carbon nanotubes: Density functional theory based computation,"

*J. Chem. Phys.***147**, 034106 (2017).[4] S. Brown, J. Miller, R. Anthony, U. Kortshagen, A. Kryjevski, E. Hobbie, "Abrupt size partitioning of multimodal photoluminescence relaxation in monodisperse silicon nanocrystals,"

*ACS Nano***11**, 1597 (2017).[5] D. Vogel, A. Kryjevski, T. Inerbaev, D. Kilin, "Photoinduced single- and multiple-electron dynamics processes enhanced by quantum confinement in lead halide perovskite quantum dots,"

*J. Phys. Chem. Lett.***8**, 3032 (2017).