Professor Dmitri Kilin
Department of Chemistry and Biochemistry
North Dakota State University
Fargo, ND
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, Oct. 2, 2019, at 3:00 PM
There will be a reception in the Olin Lounge at approximately 4 PM following the colloquium. All interested persons are cordially invited to attend.
ABSTRACT
Colloidal semiconductor nanostructures demonstrate favorable tuning of the optoelectronic properties facilitated by quantum confinements. The interpretation, understanding, and optimization of fabrication and characterization of nanostructures are assisted by computational modeling of excited state dynamics at the atomistic level. Dynamics of heat and light activated processes is contributed by simultaneous evolution of (I) nuclear and (II) electronic degrees of freedom. (I) The dynamics in nuclear degrees of freedom is dictated by heights of activation barriers and mechanisms to overcome such barriers, including tunneling. A recently developed time dependent excited state molecular dynamics (TDESMD), has been applied to investigate overcoming of barriers in polymerization reactions for cyclohexasilane (Si6H12) precursors for fabrication of solid silicon nanoparticles. (II) Photoinduced dynamics of electronic degrees of freedom is useful in computational characterization of semiconductor nanostructures. Two important factors provide contribution to efficiency, quantum yield (QY), and line-shape of photoluminescence (PL) signal in photoexcited colloidal nanostructures: (a) cascading process of hot carriers cooling via non-adiabatic dissipation of electronic excitation energy to lattice vibrations and (b) distribution of transition energy and oscillator strength in an ensemble, also related to exciton-to-phonon coupling, providing quantitative way to assess thermal broadening of the PL lineshape. The first principles modeling demonstrated correlation between temperature and PL lineshape of Si-quantum dots. The radiative and nonradiative relaxation and multi-exciton processes in methylammonium lead-halide MAPbI3 quantum dots are all found to be affected by quantum confinement, that positively affects PLQY. For nanostructures composed of heavy elements, such as CsPbBr3 colloidal quantum dots, the spin-orbit interaction is found to enable spin-forbidden transitions and to provide additional splitting between transitions energies of states involved in PL and affect rates and efficiencies of the PL. Nanostructures with periodicity such as nanowires and nanotubes provides specific spectral signatures, especially for materials that carry indirect gap feature in bulk form. Electronic transitions with change of electron’s momentum introduce additional pathways of nonradiative relaxation, which facilitate cooling of hot charge carriers.
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