Black phosphorus is a layered semiconductor that is intensely researched in view of applications in optoelectronics. In this Letter, we investigate a multi-layer black phosphorus photo-detector that is capable of acquiring high-contrast (V>0.9) images both in the visible ({\lambda}_{VIS}=532nm) as well as in the infrared ({\lambda}_{IR}=1550nm) spectral regime. In a first step, by using photocurrent microscopy, we map the active area of the device and we characterize responsivity and gain. In a second step, by deploying the black phosphorus device as a point-like detector in a confocal microsope setup, we acquire diffraction-limited optical images with sub-micron resolution. The results demonstrate the usefulness of black phosphorus as an optoelectronic material for hyperspectral imaging applications.

A black phosphorus photo-detector for multispectral, high-resolution imaging

There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.

http://www.thenanoresearch.com/upload/justPDF/0446.pdf

Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization

Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ∼20 and ∼40 W m(-1) K(-1) for black phosphorus films thicker than 15 nm, respectively, and decrease to ∼10 and ∼20 W m(-1) K(-1) as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ∼2 for thick black phosphorus films and drops to ∼1.5 for the thinnest 9.5-nm-thick film. Theoretical modelling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long mean-free-path acoustic phonons.

/pdf/anisotropic-in-plane-thermal-conductivity-observed-in-few-42m03htnug.pdf

Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus.

Black phosphorus is a narrow-gap semiconductor newly attracting attention because of recent success in growing single crystals at a high temperature under a high pressure. This review covers recent research work on the preparation, the crystal structure, the band structure, the electrical and optical properties, and the superconductivity of black phosphorus.

Semiconducting black phosphorus

There have been continuous efforts to seek for novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and the following Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast combined with atomic force microscope (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows monotonic discrete increase with the decrease of phosphorene thickness down to monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor efficiently. This work for monolayer phosphorene fabrication and thickness determination will facilitates the research of phosphorene.

The lattice vibration of black phosphorus was investigated under hydrostatic high pressure by polarized Raman scattering. On applying pressure the crystal structure changes to the rhombohedral A7-structure from the layered orthorhombic structure at 52 kbar. At atmospheric pressure four phonon modes are observed at 362 cm -1 with A g -symmetry, 467 cm -1 A g , 194 cm -1 B 1 g and 439 cm -1 B 2 g . The Gruneisen parameters are 0.74 for the 362 cm -1 A g -mode, 0.10 for the 467 cm -1 A g -mode and 0.22 for the 439 cm -1 B 2 g -mode. The energies of the latter two modes change little with pressure, since in these modes the atoms displace to deform the rigid zigzag chain along the a -axis. In the A7-structure the A 1 g -mode was observed at 466 cm -1 and the E g -mode at 392 cm -1 . The comparison with the phonon modes in other group V materials was made.

Toru Ueda

Papers

Pressure Dependence of the Lattice Vibration in the Orthorhombic and Rhombohedral Structures of Black Phosphorus