Motivated by a recent experiment that reported the successful synthesis of hexagonal (h) AlN [Tsipas et al., Appl. Phys. Lett. 103, 251605 (2013)], we investigate structural, electronic, and vibrational properties of bulk, bilayer, and monolayer structures of h-AlN by using first-principles calculations. We show that the hexagonal phase of the bulk h-AlN is a stable direct-band-gap semiconductor. The calculated phonon spectrum displays a rigid-layer shear mode at 274 cm −1 and an E g mode at 703 cm −1 , which are observable by Raman measurements. In addition, single-layer h-AlN is an indirect-band-gap semiconductor with a nonmagnetic ground state. For the bilayer structure, AA -type stacking is found to be the most favorable one, and interlayer interaction is strong. While N -layered h-AlN is an indirect-band-gap semiconductor for N = 1 − 9, we predict that thicker structures (N 10) have a direct band gap at the point. The number-of-layer-dependent band-gap transitions in h-AlN is interesting in that it is significantly different from the indirect-to-direct crossover obtained in the transition-metal dichalcogenides.
The electronic properties, carrier mobility, and strain response of TiS 3 nanoribbons (TiS 3 NRs) are investigated by first-principles calculations. We found that the electronic properties of TiS 3 NRs strongly depend on the edge type (a or b). All a-TiS 3 NRs are metallic with a magnetic ground state, while b-TiS 3 NRs are direct band gap semiconductors. Interestingly, the size of the band gap and the band edge position are almost independent of the ribbon width. This feature promises a constant band gap in a b-TiS 3 NR with rough edges, where the ribbon width differs in different regions. The maximum carrier mobility of b-TiS 3 NRs is calculated by using the deformation potential theory combined with the effective mass approximation and is found to be of the order 10 3 cm 2 V −1 s −1 . The hole mobility of the b-TiS 3 NRs is one order of magnitude lower, but it is enhanced compared to the monolayer case due to the reduction in hole effective mass. The band gap and the band edge position of b-TiS 3 NRs are quite sensitive to applied strain. In addition we investigate the termination of ribbon edges by hydrogen atoms. Upon edge passivation, the metallic and magnetic features of a-TiS 3 NRs remain unchanged, while the band gap of b-TiS 3 NRs is increased significantly. The robust metallic and ferromagnetic nature of a-TiS 3 NRs is an essential feature for spintronic device applications. The direct, width-independent, and strain-tunable band gap, as well as the high carrier mobility, of b-TiS 3 NRs is of potential importance in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors.
Theoretical and experimental studies present that metal halogens in MX3 forms can show very interesting electronic and magnetic properties in their bulk and monolayer phases. Many MX3 materials have layered structures in their bulk phases, while RuBr3 and RuI3 have one-dimensional chains in plane. In this paper, we show that these metal halogens can also form two-dimensional layered structures in the bulk phase similar to other metal halogens, and cleavage energy values confirm that the monolayers of RuX3 can be possible to be synthesised. We also find that monolayers of RuX3 prefer ferromagnetic spin orientation in the plane for Ru atoms. Their ferromagnetic ground state, however, changes to antiferromagnetic zigzag state after U is included. Calculations using PBE+U with SOC predict indirect band gap of 0.70 eV and 0.32 eV for the optimized structure of RuBr3 and RuI3, respectively. Calculation based on the Monte Carlo simulations reveal interesting magnetic properties of RuBr3, such as large Curie temperature against RuI3, both in bulk and monolayer cases. Moreover, as a result of varying exchange couplings between neighboring magnetic moments, magnetic properties of RuBr3 and RuI3 can undergo drastic changes from bulk to monolayer. We hope our findings can be useful to attempt to fabricate the bulk and monolayer of RuBr3 and RuI3.
We have studied the structural stability of monolayer and bilayer arsenene (As) in the buckled (b) and washboard (w) phases with diffusion quantum Monte Carlo (DMC) and density functional theory (DFT) calculations. DMC yields cohesive energies of 2.826(2) eV/atom for monolayer b-As and 2.792(3) eV/atom for w-As. In the case of bilayer As, DMC and DFT predict that AA-stacking is the more stable form of b-As, while AB is the most stable form of w-As. The DMC layer-layer binding energies for b-As-AA and w-As-AB are 30(1) and 53(1) meV/atom, respectively. The interlayer separations were estimated with DMC at 3.521(1) Å for b-As-AA and 3.145(1) Å for w-As-AB. A comparison of DMC and DFT results shows that the van der Waals density functional method yields energetic properties of arsenene close to DMC, while the DFT + D3 method closely reproduced the geometric properties from DMC. The electronic properties of monolayer and bilayer arsenene were explored with various DFT methods. The bandgap values vary significantly with the DFT method, but the results are generally qualitatively consistent. We expect the present work to be useful for future experiments attempting to prepare multilayer arsenene and for further development of DFT methods for weakly bonded systems.
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