Colloidal CsPbX3 (X = Cl, Br, and I) nanocrystals have recently emerged as preferred materials for light-emitting diodes, along with opportunities for photovoltaic applications. Such applications rely on the nature of valence and conduction band edges and optical transitions across these edges. Here we elucidate how halide compositions control both of these correlated parameters of CsPbX3 nanocrystals. Cyclic voltammetry shows that the valence band maximum (VBM) shifts significantly to higher energies by 0.80 eV, from X = Cl to Br to I, whereas the shift in the conduction band minimum (CBM) is small (0.19 eV) but systematic. Halides contribute more to the VBM, but their contribution to the CBM is also not negligible. Excitonic transition probabilities for both absorption and emission of visible light decrease probably because of the increasing dielectric constant from X = Cl to Br to I. These band edge properties will help design suitable interfaces in both devices and heterostructured nanocrystals.

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Colloidal all inorganic CsPbX (X = Cl, Br, I) nanocrystals (NCs) have emerged to be an excellent material for applications in light emission, photovoltaics, and photocatalysis. Efficient interfacial transfer of photogenerated electrons and holes are essential for a good photovoltaic and photocatalytic material. Using time-resolved terahertz spectroscopy, we have measured the kinetics of photogenerated electron and hole transfer processes in CsPbBr NCs in the presence of benzoquinone and phenothiazine molecules as electron and hole acceptors, respectively. Efficient hot electron/hole transfer with a sub-300 fs time scale is the major channel of carrier transfer thus overcomes the problem related to Auger recombination. A secondary transfer of thermalized carriers also takes place with time scales of 20-50 ps for electrons and 137-166 ps for holes. This work suggests that suitable interfaces of CsPbX NCs with electron and hole transport layers would harvest hot carriers, increasing the photovoltaic and photocatalytic efficiencies.

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 CdS x Se 1−x alloy 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).

A combination of high carrier density, high surface area, solution processability, and low cost is desired in a material for electrocatalytic applications, including H2 evolution and a counter electrode of a solar cell. Also, plasmonic-based applications in biological systems can be derived from such material. In this regard, a colloidal nanocomposite of TiN and N-doped few-layer graphene (TiN–NFG) is synthesized from molecular precursors. TiN nanocrystals (NCs) provide free electrons for electrical conductivity and plasmonics, whereas NFG is responsible for charge transport, high surface area, and colloidal stability. Colloidal TiN–NFG nanocomposites exhibit a localized surface plasmon resonance band at around 700 nm. Coatings of the nanocomposite form a counter electrode for efficient (8.9%) dye-sensitized solar cells. Furthermore, the nanocomposite acts as an efficient electrocatalyst for hydrogen evolution reaction, exhibiting an overpotential ∼161 mV at a current density of 10 mA/cm2.

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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 Ag 2 S−AgInS 2 hetero 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 AgInS 2 part, and the hole resides in the Ag 2 S 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 Ag 2 S−AgInS 2 hetero 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 AgInS 2 nanocrystals.