Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals

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.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…We note that analogous concerns are expected for ultra-small 0D semiconductor materials where few atoms experience a bulk-like environment, particular given the possibility that defect states 49 involving surface reconstruction are involved in the photophysics. 99 Thus, there is an opportunity to extend this theoretical framework to capture such complex chromophores, especially where their unique properties may aid applications. In this effort, the thermodynamic quantities that we estimate for the product state resulting from photoexcitation of ultra-small PbS QDs (by characterizing its equilibrium with a molecular spin-triplet exciton with well-dened energy) may offer an intriguing opportunity for testing and calibration.…”

Ultrafast spectroscopy studies of carrier dynamics in semiconductor nanocrystals

“…Recently, Lindenberg et al utilized femtosecond electron diffraction measurements to monitor lattice distortions upon photoexcitation of CdSe nanocrystals and CdSe/CdS core/shells (Guzelturk et al, 2021).…”

“…Recently, Lindenberg et al. utilized femtosecond electron diffraction measurements to monitor lattice distortions upon photoexcitation of CdSe nanocrystals and CdSe/CdS core/shells ( Guzelturk et al., 2021 ). Changes in relative diffraction intensity over time were attributed to localized structural deformations caused by hot charge carriers distorting the nanocrystal lattice (i.e.…”

Simulations of nonradiative processes in semiconductor nanocrystals

“…For example, localized trap states at surfaces or interfaces due to atomic defects are ubiquitous in experimental studies of NCs, where they are observed to rapidly quench photoluminescence and result in significantly lower quantum yields. 86,87 An atomistic description of the NC structure allows for the introduction of site-specific defects or alloying to understand their roles in trap formation and to determine the dynamics of trapping in NC systems. 28,88 In addition to the static deformation of the crystal lattice, the effects of lattice fluctuations (i.e., phonons) play a key role in the physics of NCs and must be properly incorporated.…”