Abstract
Searching for novel two-dimensional (2D) semiconducting materials is a challenging issue. We investigate novel 2D semiconductors ZrNCl and HfNCl which would be isolated to single layers from van der Waals layered bulk materials, i.e., ternary transition-metal nitride halides. Their isolations are unquestionably supported through an investigation of their cleavage energies as well as their thermodynamic stability based on the ab initio molecular dynamics and phonon dispersion calculations. Strain engineering is found to be available for both single-layer (1L) ZrNCl and 1L-HfNCl, where a transition from an indirect to direct band gap is attained under a tensile strain. It is also found that 1L-ZrNCl has an excellent electron mobility of about 1.2 × 103 cm2 V−1 s−1, which is significantly higher than that of 1L-MoS2. Lastly, it is indicated that these systems have good thermoelectric properties, i.e., high Seebeck coefficient and high power factor. With these findings, 1L-ZrNCl and 1L-HfNCl would be novel promising 2D materials for a wide range of optoelectronic and thermoelectric applications.
Highlights
We investigate novel 2D semiconductors ZrNCl and HfNCl which would be isolated to single layers from van der Waals layered bulk materials, i.e., ternary transition-metal nitride halides
We found that the valence band edge at the K-point (VBM at the Γ-point) is mostly from the N px/y orbitals (N pz orbitals) and conduction band minimum (CBM) at the K-point from the Zr dxy/x 2−y2 orbitals (Zr dxz/yz orbitals)
We carried out the first-principles calculations to investigate the stability, electronic structure, electric transport, and thermoelectric properties of 1L-ZrNCl and 1L-HfNCl
Summary
The first-principles calculations were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) code[20]. E1 is the DP constant defined as E1 = ΔEedge/Δδ, where ΔEedge is the energy shift of the band edge of the valence band or conduction band with respect to lattice dilation along the direction of external strain This DP method has been successfully applied to predict the mobility of 2D materials, such as graphene[29], MoS230,31, and phosphorene[11]. For the thermoelectric transport properties, we adopted the semi-classical Boltzmann transport theory as incorporated in the BoltzTraP code[32] within the constant relaxation time approximation and rigid band approximation Based on this approximation, the electrical conductivity and Seebeck coefficient tensors of a material can be written as[32]. We used a 50 × 50 × 1 k-mesh to obtain the Seebeck coefficient S, electrical conductivity σ, and power factor (PF) S2σ
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