2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum.Nevertheless,the effe...2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum.Nevertheless,the effective oscillator strengths of these transitions have bee n scarcely reported,nor is there a con sistent interpretati on of the obtained values.Here,we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect(OSE).Intriguingly,we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient.We propose that the pronounced exciton absorption line should be seen as the sum of multiple,low oscillator strength transitions,rather than a single high oscillator strength one,a feat we assign to strong exciton center-of-mass localization.Within the quantum mechanical description of excitons,this 50-fold difference between both oscillator strengths corresponds to the ratio between the cohere nee area of the exciton's center of mass and the total area,which yields a coherence area of a mere 6.1 nm2.Since we find that the coherence area in creases with reducing temperature,we conclude that thermal effects,related to lattice vibrations,contribute to exciton localization.In further support of this localization model,we show that FStark is in dependent of the n anoplatelet area,correctly predicts the radiative lifetime,and lines up for strongly confined quantum dot systems.展开更多
基金from FWO-Vlaanderen(12K8216N)Z.H.ack no wledges the Research Foundation Flanders(research projects 17006602 and G0F0920N)+1 种基金Ghent University(GOA no.01G01513)for funding.AJ.H acknowledges the ERC and NWO-TTW.S.Bisschop is acknowledged for SEM imaging of the platelet layers and K.De Nolf for help with the CdSe QD/platelet synthesis respectively.This project has received fun ding from the European Research Council(ERC)under the European Union's Horizon 2020 research and innovation program(grant agreement no.714876 PHOCONA).
文摘2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum.Nevertheless,the effective oscillator strengths of these transitions have bee n scarcely reported,nor is there a con sistent interpretati on of the obtained values.Here,we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect(OSE).Intriguingly,we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient.We propose that the pronounced exciton absorption line should be seen as the sum of multiple,low oscillator strength transitions,rather than a single high oscillator strength one,a feat we assign to strong exciton center-of-mass localization.Within the quantum mechanical description of excitons,this 50-fold difference between both oscillator strengths corresponds to the ratio between the cohere nee area of the exciton's center of mass and the total area,which yields a coherence area of a mere 6.1 nm2.Since we find that the coherence area in creases with reducing temperature,we conclude that thermal effects,related to lattice vibrations,contribute to exciton localization.In further support of this localization model,we show that FStark is in dependent of the n anoplatelet area,correctly predicts the radiative lifetime,and lines up for strongly confined quantum dot systems.
