Abstract
Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.
Highlights
Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance
Earlier works focused on identifying optical signatures associated with such nonradiative processes in NCs20,21, more recent works have begun to point to the fundamental role of dynamic structural fluctuations interrelated with nonradiative relaxation in NCs22–29
Neutron scattering measurements corroborated by molecular dynamics (MD) simulations[22] and correlative transmission electron microscopy (TEM) studies[27] have indicated that NC surfaces are mechanically soft, and may accelerate the nonradiative relaxation process
Summary
Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Earlier works focused on identifying optical signatures associated with such nonradiative processes in NCs20,21, more recent works have begun to point to the fundamental role of dynamic structural fluctuations interrelated with nonradiative relaxation in NCs22–29 In this context, neutron scattering measurements corroborated by molecular dynamics (MD) simulations[22] and correlative transmission electron microscopy (TEM) studies[27] have indicated that NC surfaces are mechanically soft, and may accelerate the nonradiative relaxation process. We unveil that localized disordering is induced in addition to transient heating when multiexcitons are generated predominantly in the shell by photons with energies much larger than the bandgap of the shell These localized structural deformations arise from localization of hot holes at NC surfaces forming surface small polarons (Fig. 1a). Kinetic models considering these nonradiative relaxations capture the experimentally measured dynamics well
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