Excited-state relaxations and Franck-Condon shift in Si quantum dots

TL;DR

This paper presents first-principle calculations revealing a surprisingly large Franck-Condon shift in small silicon quantum dots, with a size-dependent change in relaxation mechanisms around 1 nm.

Contribution

It provides the first quantitative analysis of excited-state relaxations and Franck-Condon shifts in silicon nanocrystals, highlighting a size-dependent transition in physical behavior.

Findings

Franck-Condon shift is ~60 meV in 2.2 nm Si nanocrystals

Relaxation mechanisms change abruptly around 1 nm in size

Size demarcates the transition from molecular to nanocrystal behavior

Abstract

Excited-state relaxations in molecules are responsible for a red shift of the absorption peak with respect to the emission peak (Franck-Condon shift). The magnitude of this shift in semiconductor quantum dots is still unknown. Here we report first-principle calculations of excited-state relaxations in small (diameter < 2.2 nm) Si nanocrystals, showing that the Franck-Condon shift is surprisingly large (~60 meV for a 2.2 nm-diameter nanocrystal). The physical mechanism of the excited-state relaxations changes abruptly around 1 nanoeter in size, providing a clear demarcation between ``molecules'' and ``nanocrystals''.