Addressing the exciton fine structure in colloidal nanocrystals: the case of CdSe nanoplatelets

TL;DR

This study investigates the exciton fine structure in colloidal CdSe nanoplatelets using multiple optical methods, revealing how the bright-dark splitting varies with thickness and aligning experimental results with theoretical predictions.

Contribution

It provides a comprehensive comparison of optical techniques for measuring exciton splitting and demonstrates the size dependence in colloidal CdSe nanoplatelets.

Findings

Bright-dark exciton splitting ranges from 3.2 to 6.0 meV.

Splitting is inversely proportional to nanoplatelet thickness.

Experimental results agree with theoretical size dependence.

Abstract

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton slitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques.