Sign of the Gap Temperature Dependence in CsPb(Br,Cl)3 Nanocrystals Determined by Cs-Rattler Mediated Electron-Phonon Coupling

TL;DR

This study reveals that the sign reversal in the temperature dependence of the bandgap in CsPb(Br,Cl)3 nanocrystals is caused by a unique electron-phonon coupling mechanism activated in Cl-rich compositions, affecting their optoelectronic properties.

Contribution

It identifies the origin of the sign change in gap temperature dependence as a specific electron-phonon interaction linked to vibrational modes in Cl-rich nanocrystals.

Findings

Electron-phonon interaction causes the sign reversal in gap temperature dependence.

Activation of anomalous electron-phonon coupling occurs in Cl-rich CsPb(Br,Cl)3 NCs.

The effect is linked to vibrational modes involving octahedral tilting and Cs rattling.

Abstract

So far, the striking sign reversal in the near-ambient slope of the gap temperature dependence of colloidal CsPbCl3 perovskite nanocrystals (NCs) compared to its Br counterpart, remains unresolved. Pure bromide NCs exhibit a linear gap increase with increasing temperature, to which thermal expansion and electron-phonon interaction equally contribute. In contrast, the temperature slope for the chlorine compound gap is outspoken negative. By combining temperature and pressure-dependent photoluminescence on a series of CsPb(Br1-xClx)3 NCs, we unravel the origin of such inversion. Responsible is solely the electron-phonon interaction, undergoing a sudden change in sign and magnitude due to activation of an anomalous electron-phonon coupling mechanism linked to vibrational modes characterized by synchronous octahedral tilting and Cs rattling. This takes place in the shrunken orthorhombic NC lattice for Cl concentrations exceeding ca. 40%. We have thus clarified a puzzling result directly impacting the optoelectronic properties of lead halide perovskite NCs.