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

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.

Review of Mn-Doped Semiconductor Nanocrystals for Time-Resolved Luminescence Biosensing/Imaging

“…Additionally, ultrafast spectroscopy, which makes use of femto- to attosecond pulses of light to study the transient absorption spectra photodynamics of various materials, has also become an emerging technique that is capable of further analyzing the host–Mn interaction . Recent studies have reported ultrafast data about the charge-transfer dynamics of Mn-doped NCs containing Cd or Pb. − The current understanding of the physics behind the energy transfer in Mn-doped multinary NCs as discussed in this paper is very limited. However, ultrafast spectroscopy may allow researchers to investigate the electronic transitions occurring in the band gap and the subsequent Mn ion electron spin-flip transition in-depth.…”

Composition effects on exciton recombination dynamics of blue-emitting alloyed Cd1−xZnxS/ZnS quantum dots

“…S8). The PA signal was not observed in CdZnS QDs without the ZnS shell, 54 and the decay lifetime of the PA signal is much longer than the biexciton transition. 64 This PA band can therefore be assigned to the excited-state absorption (EAS) of the interface-related trapping state.…”

“…S7 (ESI †). The 0.3-0.4 ps HER rate in Cd 1Àx Zn x S/ZnS is basically the same as CdZnS QDs without the ZnS shell, 54 indicating that the energy released during the electron Auger relaxation is used to excite the hole in the CdZnS core of the core/shell QDs. A positive absorption band can be seen around 470 nm before 0.2 ps.…”

Energy Transfer from Dual-Emission Manganese-Doped Quantum Dots to Graphene Oxide

“…The transient absorption spectrum of 0.4 mg/mL QD solutions with 266 nm excitation is shown in Figure d. The transient absorption spectra give mainly positive excited-state absorption signals which can be attributed to the absorption by charge carriers trapped in the defect state. − …”