Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals

TL;DR

This study reveals how surface trapping and hot carrier dynamics cause transient lattice distortions in semiconductor nanocrystals, affecting their optoelectronic performance, through femtosecond electron diffraction and atomistic simulations.

Contribution

It uncovers the atomic-scale structural origins of nonradiative relaxations in nanocrystals driven by surface trapping and hot carrier effects.

Findings

Hot carriers induce surface lattice distortions within a few picoseconds.

Near-bandgap excitation leads to longer-timescale lattice heating (~200 ps).

Surface trapping of hot holes deteriorates optoelectronic performance.

Abstract

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in nanocrystals are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in semiconductor nanocrystals. Investigation of the excitation energy dependence 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 result in transient lattice heating that occurs on a much longer 200 ps timescale, governed 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.