The inherent instability of CHNHPbX remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CHNHPbX have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CHNHPbX perovskite crystals and their effects on the stability of perovskite materials.

Bi2O2Se, a high‐mobility and air‐stable 2D material, has attracted substantial attention for application in integrated logic electronics and optoelectronics. However, achieving an overall high performance over a wide spectral range for Bi2O2Se‐based devices remains a challenge. A broadband phototransistor with high photoresponsivity (R) is reported that comprises high‐quality large‐area (≈180 µm) Bi2O2Se nanosheets synthesized via a modified chemical vapor deposition method with a face‐down configuration. The device covers the ultraviolet (UV), visible (Vis), and near‐infrared (NIR) wavelength ranges (360–1800 nm) at room temperature, exhibiting a maximum R of 108 696 A W−1 at 360 nm. Upon illumination at 405 nm, the external quantum efficiency, R, and detectivity (D*) of the device reach up to 1.5 × 107%, 50055 A W−1, and 8.2 × 1012 Jones, respectively, which is attributable to a combination of the photogating, photovoltaic, and photothermal effects. The devices reach a −3 dB bandwidth of 5.4 kHz, accounting for a fast rise time (τrise) of 32 µs. The high sensitivity, fast response time, and environmental stability achieved simultaneously in these 2D Bi2O2Se phototransistors are promising for high‐quality UV and IR imaging applications.

Based on the first-principles calculations, we systematically studied the structures and electronic properties of two-dimensional (2D) transition metal dichalcogenide (TMD) alloys with half-to-half mixing of S and Se. Using the chemical potentials of S and Se, the energetic phase diagrams for both single phases and mixed phases of TMD were constructed. A new heterolayer structure (for Sc and Ti) and alternating structure (for Cr, Mn, Fe, Zr, Mo, and W) were proposed for the first time, which were thermodynamically stable for MSSe alloys under the S-poor (relatively low chemical potential of S) and Se-rich (relatively high chemical potential of Se) conditions, and further compared with the disordered structures. Moreover, band gaps, carrier effective mass, and work functions were calculated for these stable mixed phases. Compared to the single phases of MS and MSe, MSSe alloys showed superior electronic properties including tunable band gaps and work functions. Importantly, the significantly reduced effective mass of the carriers in the MSSe alloys may induce higher carrier mobility, providing better performance of TMD materials in electronic devices.

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