Since 2014, black phosphorous can be split into layers 10 to 20 atoms thick. Black phosphorous has an inherent bandgap, something that graphene lacks. The bandgap can be tuned from 0.3 to 2.0 electron volts. No other 2D material has this range. It bridges the bandgaps of graphene (0 eV) and of transition-metal dichalcogenides such as molybdenum disulfide, which range from 1.0 to 2.5 eV.
Black phosphorus’s bandgap range means it can absorb and emit light with wavelengths of 0.6 to 4.0 micrometers—covering the visible to infrared. That spectrum could be key to its use in sensors and in optical communications.
However, black phosphorous is tough to make. High temperature and pressure is needed to make it. It degrades in a few hours when exposed to air.
Background on doping to tune bandgap
Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices.
Devices made from Black Phosphorous
Black phosphorus (P), a layered material that can be isolated down to individual 2D crystalline sheets, exhibits highly anisotropic mechanical properties due to its corrugated crystal structure in each atomic layer, which are intriguing for two-dimensional (2D) nanomechanical devices. Here we lay the framework for describing the mechanical resonant responses in free-standing black P structures, by using a combination of analytical modeling and numerical simulation. We find that thicker devices (over 100 nm) operating in the elastic plate regime exhibit pronounced signatures of mechanical anisotropy, and can lead to new multimode resonant characteristics in terms of mode sequences, shapes, and orientational preferences that are unavailable in nanomechanical resonators made of isotropic materials. In addition, through investigating devices with different geometries, we identify the resonant responses’ dependence on crystal orientation in asymmetric devices, and evaluate the effects from the degree of anisotropy. The results suggest a pathway towards harnessing the mechanical anisotropy in black P for building novel 2D nanomechanical devices and resonant transducers with engineerable multimode functions.