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A considerable number of recent research have focused on two-dimensional (2D) black phosphorus (BP) since it was successfully prepared through mechanical exfoliation in 2014. After scaling down, BP with atomistic thickness shows fascinating semiconducting features with layer-dependent direct bandgap and high carrier mobility. The synthesis of high-quality few-layer BP thin films is critical to investigate their distinctive crystal structure, fundamental characteristics, as well as the potential applications in electronics, biomedicine, energy storage, photonics, and optoelectronics. Therefore, this review provides an overview of mono- and few-layer BP topic in the synthesis methods beyond exfoliation, including thinning treatments accompanied to exfoliation, conversion from red phosphorus to BP, and direct growth techniques. We summarize various attempts to control the BP sample's thickness and lateral dimensions during the synthesis. Furthermore, we discuss the current challenges and perspectives of large-scale growth of ultrathin BP which has been a bottleneck hindering wafer-scale device's development in this field. We hope to provide an insight into exploring some potential approaches practicable to synthesize high quality BP thin films utilized for developing high-performance nano-electronics and photonics, which may accelerate the progress of 2D BP toward real applications.

The near-infrared (NIR) emission of Er3+ ions has been extensively studied owing to their significance in optical communication applications. However, studies concerning the incorporation of lanthanide ions into the two-dimensional (2D) matrix are still in the early stages. In this work, we developed an ingenious two-step vapor-phase-transfer method to synthesize Er3+ doped MoS2 single-crystalline monolayers. The NIR emission at 1530 nm was observed from the doped MoS2 nanosheets under 980 nm diode laser excitation, corresponding to the energy transition from 4I13/2 to 4I11/2 of the Er3+ dopant. The concentration quenching effect was demonstrated with an optimal Er content of around 4 mol. %. To further understand the effect of lanthanide doping on the 2D MoS2 host matrix in terms of the growth mechanism and electronic structures, theoretical analysis was performed on Er-doped monolayer MoS2 using the density functional theory calculation. The computed band structure with the superimposed Dieke diagram was in good accordance with our experimental results. Our work offers the possibility to develop doping strategies in the 2D limit and provides an in-depth understanding of the lanthanide doping in atomically thin materials.

Ferroelectric BaTiO3 thin films were epitaxially grown on (001) GaAs substrate using SrTiO3 as a buffer layer by laser molecular beam epitaxy. The perovskite SrTiO3 buffer layer present a body centered cubic structure by formation of an interfacial layer with the [100]SrTiO3∥[110]GaAs in-plane relationship. Thereupon, a highly c-oriented BaTiO3 thin film was grown on SrTiO3/GaAs in a layer by layer mode. Enhanced electrical properties of the heteroepitaxial structure were demonstrated. The BaTiO3(150 nm)/SrTiO3/GaAs system demonstrates hysteresis loops with a remnant polarization of 2.5 μC/cm2 at 600 kV/cm and a small leakage current density of 2.9×10−7 A/cm2 at 200 kV/cm.