Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots

TL;DR

This study uses ultrafast supercontinuum transient absorption spectroscopy to investigate carrier multiplication and biexcitonic effects in excited states of PbS quantum dots, revealing detailed excitonic dynamics and multiexciton generation efficiencies.

Contribution

It provides the first detailed ultrafast spectroscopic analysis of multiexciton dynamics and carrier multiplication in PbS quantum dots over a broad spectral range.

Findings

Observation of first and second order bleaches in excitonic transitions.

Evidence of carrier multiplication at energies 2-4 times the band gap.

Biexciton peak is red-shifted, indicating positive binding energy.

Abstract

We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient spectra show the presence of first order bleach of absorption for the 1S_h-1S_e transition and second order bleach along with photoinduced absorption band for 1P_h-1P_e transition. We also report evidence of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed photon) from analysis of the first and second order bleaches, in the limit of low absorbed photon numbers (<N_{abs}>~ 10^-2), at pump energies from two to four times the semiconductor band gap. The multiexciton generation efficiency is discussed both in terms of a broadband global fit and the ratio between early- to long-time transient absorption signals.. Analysis of population dynamics shows that the bleach peak due to the biexciton population is red-shifted respect the single exciton one, indicating a positive binding energy.