Sachin Rawalekar

Charge carrier cascade in Type II CdSe–CdTe graded core–shell interface

We report the synthesis, structural characterisation and charge separation behaviour in an interface graded Type II CdSe–CdTe core–shell nanostructure. The gradation was accomplished by the use of two layers consisting of an alloyed composition with increasing Te composition between the CdSe core and finally a CdTe shell grown by the successive ionic layer adsorption and reaction (SILAR) method. The effect of gradation was analysed by steady state UV-Vis absorption, photoluminescence (PL) spectroscopy, time-resolved luminescence and femtosecond transient absorption spectroscopy and was found to be immensely beneficial in improving the quantum yield as compared to an ungraded core–shell. Improvement in charge separation was further ascertained by temperature dependent luminescence studies and time correlated single photon counting studies. Better charge separation behaviour accompanied by a more robust PL yield is indicative of better surface passivation and band alignment for charge carrier funnelling. The reduction in stress was further verified by Raman studies where the Raman peak position was used as an index for stress in the film.

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.

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.