Tunable bandgap of black phosphorus by arsenic substitution toward high-performance photodetector

Multilayer black phosphorus (BP) has been widely used in many infrared applications due to its narrow bandgap below 3.7-µm wavelength range. Approaches that reduce the bandgap to less than 0.33 eV would extend the cutoff wavelength and are of paramount importance. Moreover, the poor air stability of BP severely limits its practical applications. However, none of the current methods ensure effective bandgap tuning while enhancing air stability of BP for long-term device operation. This paper solved both problems by substituting BP with congener arsenic (As) atoms. We achieved millimeter-sized As-substituted BP (b-As_xP_1−x) by optimizing the chemical vapor transport parameters, with As concentrations (x) varying as 0, 0.3, 0.4, 0.5, and 0.6. According to the scanning tunneling microscopy results, arsenic atoms are resolved to be randomly embedded in the phosphorus host lattice, while maintaining a well-ordered lattice arrangement. Therefore, the electrical bandgap of pristine BP is narrowed to ∼0.16 ± 0.02 eV for the b-As_0.6P_0.4 sample, accompanied by the accumulated p-doping and red-shifting BP Raman feature. Furthermore, the device based on b-As_0.6P_0.4 exhibits no signs of oxidation under ambient conditions (temperature ∼20°C; humidity ∼33%) for 48 h and shows a photoresponsivity of up to ∼882 mA W⁻¹, exceeding the values of ∼314 mA W⁻¹ for pristine BP devices due to the strong doping without obvious lattice distortion. Our findings indicate that arsenic-substituted BP has potential application in developing ambient-stable photodetectors and optical modulators. [Figure not available: see fulltext.].