Ultrafast Quenching of Excitons in the ZnxCd1-xS/ZnS Quantum Dots Doped with Mn2+ through Charge Transfer Intermediates Results in Manganese Luminescence.

Dmitry Cherepanov,Fedor Gostev,Yuriy Kabachii,Andrei Kostrov,Sergei Kochev,Ivan Shelaev,Victor Nadtochenko,Mikhail Motyakin

doi:10.3390/nano11113007

Abstract

For the first time, a specific time-delayed peak was registered in the femtosecond transient absorption (TA) spectra of ZnxCd1−xS/ZnS (x~0.5) alloy quantum dots (QDs) doped with Mn2+, which was interpreted as the electrochromic Stark shift of the band-edge exciton. The time-delayed rise and decay kinetics of the Stark peak in the manganese-doped QDs significantly distinguish it from the kinetics of the Stark peak caused by exciton–exciton interaction in the undoped QDs. The Stark shift in the Mn2+-doped QDs developed at a 1 ps time delay in contrast to the instantaneous appearance of the Stark shift in the undoped QDs. Simultaneously with the development of the Stark peak in the Mn2+-doped QDs, stimulated emission corresponding to 4T1-6A1 Mn2+ transition was detected in the subpicosecond time domain. The time-delayed Stark peak in the Mn2+-doped QDs, associated with the development of an electric field in QDs, indicates the appearance of charge transfer intermediates in the process of exciton quenching by manganese ions, leading to the ultrafast Mn2+ excitation. The usually considered mechanism of the nonradiative energy transfer from an exciton to Mn2+ does not imply the development of an electric field in a QD. Femtosecond TA data were analyzed using a combination of empirical and computational methods. A kinetic scheme of charge transfer processes is proposed to explain the excitation of Mn2+. The kinetic scheme includes the reduction of Mn2+ by a 1Se electron and the subsequent oxidation of Mn1+ with a hole, leading to the formation of an excited state of manganese.

Highlights

We report for the first time that the transient absorption (TA) spectra of the manganese-doped quantum dots (QDs) revealed a time-delayed absorption peak appearing at the red side of the edge exciton
The femtosecond transient absorption (TA) spectra of quantum dots (QDs) of the manganese-doped Mn2+ :Zn~0.5 Cd~0.5 S/ZnS alloy reveals a specific feature that manifests itself as an absorption peak that appears with a time delay
The delayed development and decay kinetics of this Stark peak in manganese-doped QDs significantly distinguish it from the known Stark peak associated with electrochromic shift caused by exciton–exciton interactions in undoped QDs at the initial time delays

Summary

Introduction

Introduction published maps and institutional affilDoped quantum dots (QDs) are promising materials for different applications in photovoltaics, spintronics, bioimaging, photonics, and other modern technological applications [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. Doping II-VI semiconductor QDs with Mn2+ ions makes it possible to obtain brightly luminescent nanomaterials, which are used in bioimaging, photocatalysis, and other applications [5,11,20,21,22]. The Mn2+ ion effectively quenches the exciton, and the Mn2+ 4 T1 -6 A1 luminescence quantum yield can reach ~45–65%, which means a high probability of energy transfer from the exciton to Mn2+ [24,25,26]. The mechanism of energy transfer from the exciton to the d5 levels of the Mn2+ ion remains controversial [10,11,12,15,19,21,23,27,28].

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