Single-layer Janus black arsenic-phosphorus (b-AsP): Optical dichroism, anisotropic vibrational, thermal, and elastic properties

By using density functional theory (DFT) calculations, we predict a puckered, dynamically stable Janus single-layer black arsenic-phosphorus (b-AsP), which is composed of two different atomic sublayers, arsenic and phosphorus atoms. The calculated phonon spectrum reveals that Janus single-layer b-AsP is dynamically stable with either pure or coupled optical phonon branches arising from As and P atoms. The calculated Raman spectrum indicates that due to the relatively strong P-P bonds, As atoms have no contribution to the high-frequency optical vibrations. In addition, the orientation-dependent isovolume heat capacity reveals anisotropic contributions of LA and TA phonon branches to the low-temperature thermal properties. Unlike pristine single layers of b-As and b-P, Janus single-layer b-AsP exhibits additional out-of-plane asymmetry which leads to important consequences for its electronic, optical, and elastic properties. In contrast to single-layer b-As, Janus single-layer b-AsP is found to possess a direct band gap dominated by the P atoms. Moreover, real and imaginary parts of the dynamical dielectric function, including excitonic effects, reveal the highly anisotropic optical feature of the Janus single-layer. A tight-binding (TB) model is also presented for Janus single-layer b-AsP, and it is shown that, with up to seven nearest hoppings, the TB model reproduces well the DFT band structure in the low-energy region around the band gap. This TB model can be used in combination with the Green's function approach to study, e.g., quantum transport in finite systems based on Janus single-layer b-AsP. Furthermore, the linear-elastic properties of Janus single-layer b-AsP are investigated, and the orientation-dependent in-plane stiffness and Poisson ratio are calculated. It is found that the Janus single layer exhibits strong in-plane anisotropy in its Poisson ratio much larger than that of single-layer b-P. This Janus single layer is relevant for promising applications in optical dichroism and anisotropic nanoelasticity.

Figure 1

Optimized single-layer Janus b-AsP crystal: (a) top and side views with the corresponding bond lengths and Brillouin zone, (b) phonon band dispersion and the corresponding Raman activity, and (c) the vibrational motion of the individual atoms of the phonon modes indicated in (b).

Figure 2

(a) Isovolume heat capacity of single-layer b-P, b-As, and b-AsP. The quadraticlike behaviors of the curves at low temperature are shown in the inset. (b) The directional contributions of the different phonon branches to the heat capacity.

Figure 3

The electronic band dispersions for single layers of (a) b-As, (b) b-P, and (c) Janus b-AsP. Here, red and green curves represent the contributions of As and P atoms to the electronic bands. The Fermi level is set to zero.

Figure 4

For single layers of b-As, b-P, and b-AsP, the imaginary-part-dependent dielectric function along the AC (solid) and ZZ (dashed) directions.

Figure 5

(a) The vectors show the different bonds defined in our TB model. The blue and red vectors denote the interlayer bonds between P and As atoms, and the yellow vector indicates the intralayer bond. (b) The red curves are the TB band structures which are compared to that of the ab initio HSE06 results. (c) Three-dimensional plot of the valence and conductance bands of the TB model.

Figure 6

(a) Schematic representation of the orientation angle $\theta$. (b) The angle-dependent in-plane stiffness and (c) that of Poisson ratio.