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Recent years have witnessed the rapidly growing interests in the rediscovered black phosphorus (BP), an elemental group‐V layered material with very high carrier mobility among all semiconducting layered materials. As a layered semiconductor, the bandgap of intrinsic BP varies from ≈0.3 to 2 eV depending on the thickness. This bandgap value can be tuned to below 50 meV by a moderate external electric field. Adsorption doping and external pressure can also effectively modify its bandgap. The largely tunable bandgap of BP makes it a promising material for infrared optics. Moreover, its unique puckered structure leads to the anisotropic in‐plane properties, making it ideal for the exploration of exotic physical phenomena and the realization of novel devices. Here, the fundamental optical properties are reviewed and latest developments on BP photonic and optoelectronic devices are discussed.

Black phosphorus (BP) has recently attracted significant attention due to its exceptional physical properties. Currently, high‐quality few‐layer and thin‐film BP are produced primarily by mechanical exfoliation, limiting their potential in future applications. Here, the synthesis of highly crystalline thin‐film BP on 5 mm sapphire substrates by conversion from red to black phosphorus at 700 °C and 1.5 GPa is demonstrated. The synthesized ≈50 nm thick BP thin films are polycrystalline with a crystal domain size ranging from 40 to 70 µm long, as indicated by Raman mapping and infrared extinction spectroscopy. At room temperature, field‐effect mobility of the synthesized BP thin film is found to be around 160 cm²V⁻¹s⁻¹along armchair direction and reaches up to about 200 cm²V⁻¹s⁻¹at around 90 K. Moreover, red phosphorus (RP) covered by exfoliated hexagonal boron nitride (hBN) before conversion shows atomically sharp hBN/BP interface and perfectly layered BP after the conversion. This demonstration represents a critical step toward the future realization of large scale, high‐quality BP devices and circuits.

For the electrochemical hydrogen evolution reaction (HER), the electrical properties of catalysts can play an important role in influencing the overall catalytic activity. This is particularly important for semiconducting HER catalysts such as MoS₂, which has been extensively studied over the last decade. Herein, on‐chip microreactors on two model catalysts, semiconducting MoS₂and semimetallic WTe₂, are employed to extract the effects of individual factors and study their relations with the HER catalytic activity. It is shown that electron injection at the catalyst/current collector interface and intralayer and interlayer charge transport within the catalyst can be more important than thermodynamic energy considerations. For WTe₂, the site‐dependent activities and the relations of the pure thermodynamics to the overall activity are measured and established, as the microreactors allow precise measurements of the type and area of the catalytic sites. The approach presents opportunities to study electrochemical reactions systematically to help establish rational design principles for future electrocatalysts.