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Metal halide perovskites (MHPs) have emerged as highly promising candidates for the next generation of photonics and optoelectronic devices, owing to their prominent optical and excitonic properties, as well as the convenience of fabrication. Particularly, ultrathin two-dimensional (2D) MHPs, which are generally prepared by exfoliation, solution growth, and chemical vapor deposition method, have attracted dramatically increasing attentions owing to their combined features of ultrathin 2D morphology and superior performance of MHPs. Despite the growing interest in ultrathin 2D MHPs, there is currently a lack of a comprehensive and systematic overview of the distinct advantages offered by each growth method for producing these materials. This review critically assesses the preliminary studies on the materials design and preparation of ultrathin MHPs. Furthermore, it explores heterostructures based on ultrathin MHPs and offers insights into the challenges and opportunities that lie ahead for this enticing class of 2D materials. [ABSTRACT FROM AUTHOR]

The liquid‐air interface offers a platform for the in‐plane growth of free‐standing materials. However, it is rarely used for inorganic perovskites and ultrathin non‐layered perovskites. Herein the liquid‐air interfacial synthesis of inorganic perovskite nanosheets (Cs3Bi2I9, Cs3Sb2I9) is achieved simply by drop‐casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid‐air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid‐air interfacial growth of ultrathin perovskites provides multi‐degree‐of‐freedom for constructing perovskite‐based heterostructures and devices at atomic scale. [ABSTRACT FROM AUTHOR]

We investigate the manifestations of band structure engineering in few-layer PbI2-based heterostructures by probing their tunable optical properties. First, we have successfully prepared atomically thin flakes from PbI2 solution by two distinct approaches. A drop-casting of PbI2 solution onto various substrates followed by a simple heating process yields abundant flakes with different thickness and regular shape. Mechanical exfoliation of PbI2 bulk crystals, obtained from a low-temperature recrystallization process of PbI2 solution, also gives ultrathin PbI2 flakes of high quality. Moreover, these PbI2 flakes are employed to construct various van de Waals heterostructures. A significant enhancement of photoluminescence in MoSe2 interfaced with PbI2 was observed at different laser excitation intensity, due to the forming of type-I band alignment. Type-I band alignment can also be investigated in MoS2/PbI2 heterostructure, while type-II band alignment is built-in WSe2/PbI2 heterostructure. These results demonstrate that the strong interfacial coupling between PbI2 and other two-dimensional semiconductors can modulate their band alignment, and as a result, the exciton properties noticeably, which provides new insights of building a designer heterostructure device at the atomic level. [ABSTRACT FROM AUTHOR]

Hexagonal silicon (Si-IV) has recently attracted considerable research interest due to its extraordinary optical properties and possibility to convert into a direct semiconductor under strain with great potential for applications. However, the mechanical properties of Si-IV, which are critical for its applications, have not been experimentally studied yet. In this study, combining nanoindentation and in situ high-pressure synchrotron X-ray diffraction, we thoroughly investigated the mechanical properties of Si-IV. The results suggest that the elastic moduli and hardness of Si-IV are close to those of the common diamond cubic silicon. The similar mechanical properties of Si-IV and diamond cubic silicon are beneficial for integrating Si-IV into conventional Si-based devices using similar industrial processing and for its photovoltaic and optoelectronic applications. [Display omitted] [ABSTRACT FROM AUTHOR]