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Nanoscale heterojunctions with type-II band alignment can efficiently separate a photogenerated electron–hole pair, and therefore find applications in solar cells and photocatalysis. Here, we prepare a nanojunction in the form of Ag2S–AgInS2hetero dimer nanocrystal that does not contain toxic Cd and Pb. A combination of photophysics, cyclic voltammetry, and quantum dot-sensitized solar cell properties shows that the junction/interface has a type-I band alignment, but still electron–hole separation takes place with efficacy across the interface because of defect states. The electron gets localized in a defect state within the AgInS2part, and the hole resides in the Ag2S part of the hetero dimer nanocrystal. This type-II-like defect-mediated electron–hole separation, irrespective of the nature interfacial band alignment, is an interesting phenomenon, and can be utilized to tune optoelectronic properties of heterostructured nanocrystals. For example, very long (13 μS) photoluminescence lifetime has been observed for Ag2S–AgInS2hetero dimer nanocrystals because of this defect-mediated spatial separation of electron and hole wave functions, which in turn improve the solar cell efficiency by more than 3 times as compared to that of AgInS2nanocrystals.

Cyclic voltammetry has been used to investigate the interaction between reduced graphene oxide (r-GO) and CdTe quantum dots (Q-CdTe). For that, the composite of Q-CdTe with r-GO (r-GO-CdTe) was prepared by carrying out the reduction of graphene oxide and the synthesis of Q-CdTe simultaneously, in a single bath. r-GO-CdTe was characterized by UV–visible, steady state fluorescence, time-resolved fluorescence, X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM). Cyclic voltammetry was employed to determine the quasi-particle gap and band edge parameters of Q-CdTe and r-GO-CdTe. The blue shifts in the quasi-particle gap of r-GO-CdTe have been attributed to the strong interaction of graphene with CdTe. These interactions were further verified by time-resolved fluorescence and Raman spectroscopy which suggested strong electronic coupling between Q-dots and graphene.

The band gap bowing effect in oleic acid-stabilized CdSxSe1–xalloy quantum dots (Q-dots) with varying composition has been studied experimentally by means of cyclic voltammetry and theoretically using density functional theory based calculations. Distinct cathodic and anodic peaks observed in the cyclic voltammograms of diffusing quantum dots alloy are attributed to the respective conduction and valence band edges. The quasi-particle gap values determined from voltammetric measurements are compared with interband transition peaks in UV–vis and PL spectra. Electronic structure for alloy Q-dots is determined computationally with projector augmented wave method for a particular size of dots. The band gap bowing is observed predominantly in the conduction band states. The bowing parameter determined experimentally (0.45 eV) has been found to be in good agreement with the one estimated from DFT (0.43 eV).