Surface-state-mediated charge-transfer dynamics in CdTe/CdSe core-shell quantum dots.

Herein, we report the synthesis of aqueous CdTe/CdSe type-II core-shell quantum dots (QDs) in which 3-mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core-shell QDs are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), steady-state absorption, and emission spectroscopy. A red shift in the steady-state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR-TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time-resolved emission studies, we observe that the average emission lifetime (τ(av)) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τ(av) =12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type-II CdTe/CdSe core-shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge-transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core-shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core-shell QDs.

Charge carrier dynamics in thiol capped CdTe quantum dots.

We report the ultrafast charge carrier relaxation dynamics of mercaptopropionic acid capped CdTe quantum dot (QD) using femtosecond transient absorption spectroscopy by exciting the particles with 400 nm laser light and monitoring the transients in the visible to near IR region. Cooling dynamics and population dynamics in different quantized states of the charge carriers were monitored by following the growth kinetics of the bleach at different excitonic positions. The cooling time second and first excitonic states were found to be 150 fs and 500 fs, respectively, which increases non-linearly with its size. Defect states of QD surface play an important role in the cooling dynamics of the charge carriers. Quenching studies have been carried out to find out cooling and trapping dynamics of the individual charge carriers. Electron and hole cooling time were measured to be 700 fs and 150 fs for the first excitonic state using quenchers. Trapping dynamics of electron and hole have been determined by monitoring transient signal at 1000 nm and by using hole and electron quencher, respectively. Electron and hole trapping times have been found to be 700 fs and 1 ps, respectively, in CdTe QD.

Interfacial electron transfer dynamics in quinizarin sensitized ZnS nanoparticles: monitoring charge transfer emission.

Water soluble cubic ZnS nanoparticles (NPs) have been synthesized at room temperature by using 3-mercaptopropionic acid (MPA) as a modifier molecule and characterized by X-ray diffraction (XRD), steady-state absorption, and emission spectroscopy. Electron transfer (ET) dynamics have been carried out in ZnS semiconductor nanoparticles and quinizarin (Qz) molecules as studied by picosecond time-resolved fluorescence spectroscopy. We have proposed that electron injection takes place from photoexcited Qz molecules into the surface states of wide band gap ZnS NPs. We have revealed that the formation of a charge transfer complex between the Qz molecule and ZnS nanoparticles facilitates electron injection into the surface states of nanoparticles. In the present investigation, we have detected charge transfer (CT) emission in the Qz-ZnS system as the injected electrons from surface states return back to the parent Qz cation radical. We have determined back ET rates by monitoring the CT emission.