Because of the rich polymorphs and lower diffusion energy barriers of copper chalcogenide systems, the phase transformation of colloidal Cu2–x S (0 ≤ x ≤ 1) nanocrystals is critical for understanding their fundamental properties and designing convenient synthetic routes. In this work, high quality digenite Cu1.8S nanocrystals with rhombohedral structure were synthesized at gram-scale. The as-prepared colloidal nanocrystals undergo an in situ phase transformation from rhombohedral Cu1.8S nanocrystals to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. The observed transformation was explored by a combination of structural and spectroscopic analyses, including powder X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. A possible mechanism is proposed and thoroughly discussed. We further determined the evolution of plasmonic absorption spectra during the transformation. The Cu1.8S nanocrystals and CuS clusters exhibit composition-dependent local surface plasmon resonance absorption (LSPR) in the near-infrared region, which are in good agreement with calculated extinction spectra based on Mie-Drude model. Combined experimental and theoretical analyses demonstrated that both the phase induced dielectric constant change and the composition induced carrier concentration variation account for the spectroscopic evolution.

Room-temperature-operated continuous-wave lasers have been intensively pursed in the field of on-chip photonics. The realization of a continuous-wave laser strongly relies on the development of gain materials. To date, there is still a huge gap between the current gain materials and commercial requirements. In this work, we demonstrate continuous-wave lasers at room temperature using rationally designed in situ fabricated perovskite quantum dots in polyacrylonitrile films on a distributed feedback cavity. The achieved threshold values are 15, 24, and 58 W/cm 2 for green, red, and blue lasers, respectively, which are one order lower than the reported values for the conventional CdSe quantum dot-based continuous-wave laser. Except for the high photoluminescence quantum yields, smooth surface, and high thermal conductivity of the resulting films, the key success of an ultralow laser threshold can be explained by the interaction of polyacrylonitrile and perovskite induced "charge spatial separation" effects. This progress opens up a door to achieve on-chip continuous-wave lasers for photonic applications.

We report a combined experimental and theoretical study of the synthesis of CH NH PbBr nanoplatelets through self-organization. Shape transformation from spherical nanodots to square or rectangular nanoplatelets can be achieved by keeping the preformed colloidal nanocrystals at a high concentration (3.5 mg mL ) for 3 days, or combining the synthesis of nanodots with self-organization. The average thickness of the resulting CH NH PbBr nanoplatelets is similar to the size of the original nanoparticles, and we also noticed several nanoplatelets with circular or square holes, suggesting that the shape transformation experienced a self-organization process through dipole-dipole interactions along with a realignment of dipolar vectors. Additionally, the CH NH PbBr nanoplatelets exhibit excellent polarized emissions for stretched CH NH PbBr nanoplatelets embedded in a polymer composite film, showing advantageous photoluminescence properties for display backlights.

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Colloidal Cu 2−x S nanocrystals are potential abundant, low-cost, and environment-friendly candidates for photovoltaic and photothermal applications. The fabrication of high-quality nanocrystal films through a solution process is a key step toward the exploration of their applications. In this work, we fabricated high-quality Cu 1.8 S nanocrystal films, characterized their phase transformation under thermal annealing treatments, and investigated the evolution of the corresponding optical and electrical properties. It was demonstrated that the Cu 1.8 S nanocrystal films undergo a phase transformation from metastable rhombohedral phase to stable tetragonal phase (Cu 2 S) after annealing at a temperature higher than 240 °C, which is much lower than that of the bulk materials (544 °C). Along with the transformation, both optical and conductivity properties exhibit well-defined evolution from nonstoichiometric semiconductor to stoichiometric semiconductor, which can be interpreted through a combined electronic structure analysis and theoretical modeling. The correlations between the crystal structure, composition, optical and electrical properties enable us to gain further insights into the structure−property relationship in Cu 2−x S nanocrystals. More importantly, a highly conductive Cu 2−x S nanocrystal film with electrical conductivity up to 6.7 S/cm was obtained, implying the potential to be used as conductive electrodes. We further integrated the annealed Cu 2−x S nanocrystal films into a photovoltaic device by adopting a FTO/TiO 2 /Cu 2−x S:CdS/MoO 3 /Au structure, and a preliminary power conversion efficiency of 0.24% was achieved.