The excited state interaction between CdSe nanocrystals and a hole acceptor, p-phenylenediamine (PPD), is probed using emission and transient absorption spectroscopies. The changes in the photophysical properties of CdSe nanocrystals arising from the interaction with PPD are compared with an aliphatic amine, n-butylamine (n-BA). At low concentrations (<0.5 mM) n-butylamine enhances the emission of CdSe quantum dots whereas PPD effectively quenches the emission at similar concentrations. The low oxidation potential of PPD (E°) 0.26 V vs NHE) enables it to act as an effective scavenger for photogenerated holes. A surface bound complexation equilibrium model has been proposed to explain the quenching phenomenon. The transient absorption measurements confirm the formation of PPD cation radical and subsequent formation of coupling product. Formation of such charged species at the surface extends the bleaching recovery over several microseconds.

One dimensional (1D) anatase Co doped TiO2 nanostructures such as nanorods, nanowires, and nanotubes were synthesized by a simple solvothermal method using CoCl2·6H2O as the cobalt source. The effect of the different solvents on the crystal structures, morphologies, and sizes of the Co doped 1D nanostructures was investigated. The doping concentration of the samples primarily depends on the solvents. The X-ray photoelectron spectroscopy studies clearly showed that the Co was incorporated into the TiO2 lattice as Co2+ and oxygen vacancies were created due to the substitution of the Ti4+ ions by Co2+ ions. Optical absorption measurements showed additional absorption bands that are due to the ligand field transitions, 4T1g(4F) → 4T1g(4P) of Co2+ and also due to the transitions from different trap states related to the oxygen vacancies. The effects of the doping concentration on the defect structures and oxygen vacancies of the 1D nanostructures were mainly investigated using steady state photoluminescence (PL) and PL decay.

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Metal halide perovskites have shown excellent properties for lighting applications, including high photoluminescence quantum yield (PLQY), compositional tunability, and narrow emission line widths. Perovskite light-emitting diodes (LEDs) have achieved external quantum efficiency (EQE) up to 20% in the green, red, and near-infrared (NIR) spectral regions. Recently, nanostructured perovskite NIR-LEDs have displayed 100 h of device stability, making this technology commercially viable and prompting greater awareness of this class of devices, as distinct from visible wavelength perovskite LEDs. Even so, the current generation of high-performance perovskite LEDs are still hampered by slow radiative recombination of charge carriers, unbalanced injection of charge carriers, and light out-coupling efficiency; therefore, more structural and morphological engineering of perovskite LEDs is needed to confine the charge carriers and collect the photons more effectively. It has been observed that 3D bulk perovskites show high performance but have poor stability and offer less control over their optical properties. In contrast, NIR-emitting perovskite nanocrystals offer precise control of their optical properties but exhibit poor optoelectronic properties due to the presence of bulky ligands. Quasi-2D perovskite systems have gained significant attention as they balance high conductivity and stability, while enabling precise color tuning of nanostructures, and the possibility to produce single crystal-LEDs. Here, we assess these and other recent advancements in NIR-emitting perovskite materials. We compare different structural frameworks and how they influence the LED performance in terms of color stability, EQE, and device stability. The practical challenges facing each of these structural classes of perovskite NIR-LED materials and the possible strategies to overcome these obstacles are thoroughly discussed.

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