Black Phosphorus Properties

The structure of black phosphorus results in pronounced anisotropic properties such as carrier mobility, thermal conductivity, and mechanical strength. The exploration of black phosphorus and phosphorene opens new avenues in the development of advanced materials with tailored electronic, optical, and mechanical properties for future technologies.

Properties of Phosphorene

Like other van der Waals 2D materials, such as MoS₂, the optical, electronic, and mechanical properties of phosphorene differ from that of the bulk state due to a combination of factors. These include:

High surface-to-volume ratio
Out-of-plane charge carrier confinement
No interlayer interactions
Increased Coulomb interaction between charge carriers (reduced dielectric screening)

A hole mobility up to ~1000 cm²V^-1s^-1 has been measured in few-layer phosphorene in a FET structure (Peimyoo et al., 2015). The anisotropy of carrier mobility travelling along the AC and ZZ directions has also been demonstrated (Xia et al., 2014).

Specifically, after exfoliation of black phosphorus crystals or powder, black phosphorus typically has the following properties:

Orthorhombic C puckered honeycomb structure
0.3 eV ~ 1.5 eV thickness-dependent bandgap
High charge carrier mobility ~1000 cm²V^-1s^-1
Thermal conductivity of 86 (34) Wm^-1K^-1 in ZZ (AC) direction (few-layer)
On/off ratio of 10⁵

Black phosphorus powder can be combined with liquid-phase exfoliation or further chemical modification to control the physical and electronic properties of the resultant semiconductor material. Liquid-phase exfoliation is often used to create black phosphorus quantum dots. Black phosphorus quantum dots have an average size of 4.9 nm and a thickness of 1.9 nm.

Band Gap of Phosphorene

Black phosphorene has a thickness-dependent direct band gap, ranging from 1.88 eV (for a single monolayer) to 0.3 eV (for the bulk material).

Black phosphorus monolayer and bulk band structure — Electronic band structure of bulk (left) and monolayer (right) black phosphorus. Note that the band gap of the monolayer has been underestimated by the DFT (density functional theory) calculation.

A comparison between the band gaps of several 2D materials is shown below, along with those of conventional bulk semiconductors. The wide tunability of the phosphorene bandgap by changing the sample thickness makes it attractive for a number of applications.

2D and conventional bulk semiconductor bandgap chart — Bandgap energies of 2D materials and conventional bulk semiconductors

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The Allotropes of Phosphorus

Depending on environmental conditions and how it is processed, elemental phosphorus can take several forms: white, red, black, and violet phosphorus.

References

Peimyoo et al. (2015). Thermal conductivity determination of suspended mono- and bilayer WS2 by Raman spectroscopy. Nano Research, 8(4). doi: 10.1007/s12274-014-0602-0
Xia et al. (2014). Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics. Nature Communications, 5. doi: 10.1038/ncomms5458