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.

We report a simple and reproducible method to fabricate switchable Ag(2)S devices. The alpha-Ag(2)S thin films are produced by a sulfurization process after silver deposition on an Si substrate. Structure and composition of the Ag(2)S are characterized using XRD and RBS. Our samples show semiconductor behaviour at low bias voltages, whereas they exhibit reproducible bipolar resistance switching at higher bias voltages. The transition between both types of behaviour is observed by hysteresis in the I-V curves, indicating decomposition of the Ag(2)S, increasing the Ag(+) ion mobility. The as-fabricated Ag(2)S samples are a good candidate for future solid state memory devices, as they show reproducible memory resistive properties and they are fabricated by an accessible and reliable method.

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.

Environmentally friendly Cu2−x S compounds exist in many different mixed phases in nature, while their nanoscale counterparts can be pure phase with interesting localized surface plasmon resonance properties. Because of the complexity of composition and phase, controllable synthesis of Cu2−x S nanocrystals becomes an important scientific issue in colloidal chemistry. In this work, a hot‐injection method is developed to synthesize Cu2−x S nanocrystals by injecting a sulfur precursor into a copper precursor using oleylamine and octadecene as solvents. By varying the reaction parameters (temperature, volume ratio of oleylamine/octadecene, molar ratio of Cu/S in the precursors), hexagonal CuS, monoclinic Cu1.75S, and rhombohedral Cu1.8S, nanocrystals can be selectively synthesized, providing a platform to illustrate the mechanism of crystal phase control. The crystal phase control of Cu2−x S nanocrystals is oleylamine‐determined by controlling the molar ratio of Cu/S in the reaction precursors as well as the ratio of Cu2−x S clusters/Cu+ in the subsequent reaction. More importantly, temperature plays an important role in varying the molar ratio of Cu/S and Cu2−x S clusters/Cu+ in the reaction system, which significantly influences the crystal phase of the resulting Cu2−x S nanocrystals. The understanding into crystal control provides a guideline to realize reproducible phase‐selective synthesis and obtain well‐defined high‐quality materials with precise control.

Owing to their high electrical conductivity, tunable plasmonic absorption spectra, low cost, and abundance in nature, Cu2-x S nanocrystals are of great interest as functional materials for photovoltaic and photothermal applications. With the aim of developing low-cost high-efficiency quantum-dot-sensitized solar cells, solution-processed Cu2-x S nanocrystal films are synthesized and their phase transformations upon thermal treatment are investigated. A combination of experimental results and theoretical analysis illustrates the thermodynamic evolution of the crystal structures and the composition caused by the thermal-annealing process. The use of Cu2-x S nanocrystal films as counter electrodes in quantum-dot-sensitized solar cells is also explored. The devices have an optimized power-conversion efficiency of 5.81 % for tetragonal Cu2 S nanocrystal films that are derived from annealed Cu1.8 S nanocrystal films.