Ultrafast Charge Carrier Relaxation and Charge Transfer Dynamics of CdTe/CdS Core−Shell Quantum Dots as Studied by Femtosecond Transient Absorption Spectroscopy

Abstract

We are reporting ultrafast charge carrier and charge transfer dynamics of the CdTe quantum dot (QD) and type II CdTe/CdS core-shell QD materials with different shell (CdS) thicknesses. Herein, we have synthesized CdTe and CdTe/CdS core-shell quantum dots by using 3-mercaptopropionic acid as a capping agent. Steady state absorption and emission studies confirmed successful synthesis of CdTe QD and CdTe/CdS core-shell QD materials. Time-resolved emission studies indicate a longer emission lifetime of the CdTe/CdS core-shell as compared to CdTe QD materials, where in both cases only CdTe gets excited. We have carried out femtosecond transient absorption studies of these QD and core-shell materials by exciting them with 400 nm laser light and monitoring the transients in the visible to near-IR region to study charge carrier and charge transfer dynamics in the ultrafast time scale. On laser excitation, electron-hole pairs are generated which are confirmed by induced absorption signal for the charge carriers in the visible and near-IR region and an immediate bleach at excitonic position for both QD and QD core-shell. The carrier relaxation was found to be slower and the carrier lifetime was found to be longer in the QD core-shell as compared to the QD indicating charge transfer from core to shell. Carrier quenching studies have been carried out for both CdTe and CdTe/CdS by using benzoquinone (BQ, electron quencher) and Pyridine (Py, hole quencher) to assign the different relaxation processes. Details about the relaxation of hot carriers and the quenching effect on the relaxation dynamic of the charge carriers have been discussed for both QD and core-shell nanostructures.

Photophysical studies of CdTe quantum dots in the presence of a zinc cationic porphyrin

“…S9. This is presumably due to contributions from 25 more than one species. It was found that the decay phenomena were similar over the energy range 5-100 nJ (data recorded at 100 nJ excitation are also supplied in the see ESI Fig.…”

“…1 The II-VI metal chalcogenides such 20 as ZnS, CdS, CdSe, CdTe, PbSe, and PbTe have been extensively studied and their variable direct band gap and resultant tuneable photoluminescence has led to numerous applications including photodynamic therapy, imaging and sensing applications. [2][3][4][5][6][7][8][9][10][11] Of particular interest is the use of quantum dots as acceptor/donor 25 components for the conversion of light energy into electricity in a photovoltaic device. [12][13][14][15][16] A significant obstacle to the development of high-performance photovoltaic cells is the effective separation of photogenerated electron-hole pairs and the transfer of the electrons to the electrode.…”

“…Study of the system using steady-state absorption and emission spectroscopy reveals a high affinity of the porphyrin for the QD surface which is accompanied by highly efficient quenching of the QD luminescence. The possible formation of 25 small QD aggregates due to ZnTMPyP4 cross linking is considered by small angle X-ray scattering (SAXS). Finally, pump-probe transient absorption (TA) spectroscopy is used to profile the ultrafast processes that occur between the ZnTMPyP4 and CdTe QDs.…”

Synthesis of Multifunctional Mn‐Doped CdTe/ZnS Core/Shell Quantum Dots

“…In general, two strategies, ice bath method 28 and the H 2 Te gas route, 29 were used to prepare fresh Te precursor. However, the ice bath method is time-consuming, usually taking over 6 h, and the H 2 Te gas route is laborious.…”

Transient absorption studies of CdSe nanocluster passivated with phenyldithiocarbamate ligands.

“…To better understand the underlying dynamics and interpret the observed ultrafast kinetics, control TA experiments with electron and hole quenchers [44,236] were also performed. Benzoquinone (BQ) was added into the solution to quench electrons in photoexcited CdSe SCNCs.…”