Xiaolin Cai

Tunable electronic properties of arsenene/GaS van der Waals heterostructures

Finding novel atomically thin heterostructures and understanding their electronic properties is critical for developing better nanoscale electronic and optoelectronic devices. In this work, we investigate the structural and electronic properties of arsenene/GaS van der Waals (vdW) heterostructures using first-principles calculations. Our results suggest that this heterostructure has an intrinsic type-II band alignment and an indirect band gap. Comparing the calculated band edge positions to the redox potentials of water, we identify that the arsenene/GaS vdW heterostructure is a promising photocatalyst for water splitting. Moreover, we also find that intriguing indirect–direct and semiconductor–metal transitions can be induced by strain. In particular, under certain strain, degenerate valleys of conduction band bottoms will be created, which suggests potential applications in valleytronics.

Electronic structures and enhanced photocatalytic properties of blue phosphorene/BSe van der Waals heterostructures

Constructing van der Waals heterostructures can enhance two-dimensional (2D) materials with desired properties and greatly extend the applications of the original materials. On the basis of density functional theory calculations, we verify that a blue phosphorene (BlueP)/BSe inter-layer heterostructure possesses an indirect gap and intrinsic type-II band alignment. In particular, this heterostructure is found to be a potential photocatalyst for water splitting under different pH conditions and exhibits enhanced optical properties in the visible and ultraviolet light zones. Besides, we confirm that the band gap, band edge position, and optical absorption of the BlueP/BSe heterostructure can be tailored by biaxial strain. And the tensile strain increases the optical absorption significantly over the entire energy range of visible light, which can increase the efficiency of solar energy conversion. Furthermore, we determine that adjusting the number of sublayers is another effective method to modulate the band gaps and band alignments of heterostructures. Our studies provide a promising route to design new BlueP-based vdW heterostructures and explore their potential applications in electronic and optoelectronic devices.

Arsenene/Ca(OH)2 van der Waals heterostructure: strain tunable electronic and photocatalytic properties

Vertical stacking of two-dimensional materials has recently emerged as an exciting method for the design of novel electronic and optoelectronic devices. In this work, we investigate the structural, electronic, and potential photocatalytic properties of arsenene/Ca(OH)2 van der Waals (vdW) heterostructures using first-principles calculations. It is found that all of the heterostructures are semiconductors with indirect band gaps and present similar electronic properties, almost irrespective of the stacking arrangement. However, among these heterostructures, the β-stacking heterostructure is found to be the most stable and its band gap and band edge position can be tuned by biaxial strain. In particular, comparing the band edge positions with the redox potentials of water shows that the strained β-stacking arsenene/Ca(OH)2 vdW heterostructure is a potential photocatalyst for water splitting. Meanwhile, this heterostructure exhibits significantly improved photocatalytic properties under visible-light irradiation by the calculated optical absorption spectra. Our findings provide a detailed understanding of the physical properties of arsenene/Ca(OH)2 vdW heterostructures and a new way to improve the design of photocatalysts for water splitting.

Strain induced quantum spin Hall insulator in monolayer β-BiSb from first-principles study

Topological insulator (TI) is a peculiar phase of matter exhibiting excellent quantum transport properties with potential applications in lower-power-consuming electronic devices. Searching for inversion-asymmetric quantum spin Hall (QSH) insulators persists as an effect for realizing new topological phenomena. Using first-principles density functional theory calculations, we investigate the geometry, dynamic stability, and electronic structures of monolayer β-BiSb. We find that it presents QSH state under biaxial tensile strain of 14%. The nontrivial topological situation in the strained system is confirmed by the identified band inversion, Z2 topological invariant (Z2 = 1), and an explicit presence of the topological edge states. Owning to the asymmetric structure, remarkable Rashba spin splitting is produced in both the valence and conduction bands of the strained system. These results provide an intriguing platform for applications of monolayer β-BiSb in future alternative quantum Hall spintronic devices.

Molecular dynamics simulation studies on the plastic behaviors of an iron nanowire under torsion

The plastic deformation mechanism of iron (Fe) nanowires under torsion is studied using the molecular dynamics (MD) method by applying an external driving force at a constant torsion speed. We find that the deformation behavior depends on the orientation of the wire. The dislocations in 〈100〉 and 〈111〉 oriented nanowires propagate through the nanowires under torsion, whereas those in 〈110〉 oriented nanowires divide the wire into two parts. The situation that there is a low angle twist grain boundary (GB) in the nanowires is also under consideration. The results reveal that the dislocations are concentrated on the GB in the initial state, presenting different patterns of dislocation network. The networks change depending on the twist direction. They shrink with increase in twist angle but expand with the decreasing twist angle, presenting an asymmetric phenomenon. Our findings can help us more thoroughly understand the plastic deformation mechanism of Fe nanowires under torsion.

Strain-Tunable Electronic Properties and Band Alignments in GaTe/C 2 N Heterostructure: a First-Principles Calculation

Recently, GaTe and C2N monolayers have been successfully synthesized and show fascinating electronic and optical properties. Such hybrid of GaTe with C2N may induce new novel physical properties. In this work, we perform ab initio simulations on the structural, electronic, and optical properties of the GaTe/C2N van der Waals (vdW) heterostructure. Our calculations show that the GaTe/C2N vdW heterostructure is an indirect-gap semiconductor with type-II band alignment, facilitating an effective separation of photogenerated carriers. Intriguingly, it also presents enhanced visible-UV light absorption compared to its components and can be tailored to be a good photocatalyst for water splitting at certain pH by applying vertical strains. Further, we explore specifically the adsorption and decomposition of water molecules on the surface of C2N layer in the heterostructure and the subsequent formation of hydrogen, which reveals the mechanism of photocatalytic hydrogen production on the 2D GaTe/C2N heterostructure. Moreover, it is found that in-plane biaxial strains can induce indirect-direct-indirect, semiconductor-metal, and type II to type I or type III transitions. These interesting results make the GaTe/C2N vdW heterostructure a promising candidate for applications in next generation of multifunctional optoelectronic devices.

Stable GaSe-Like Phosphorus Carbide Monolayer with Tunable Electronic and Optical Properties from Ab Initio Calculations

On the basis of density functional theory (DFT) calculations, we propose a stable two-dimensional (2D) monolayer phosphorus carbide (PC) with a GaSe-like structure, which has intriguing electronic and optical properties. Our calculated results show that this 2D monolayer structure is more stable than the other allotropes predicted by Tománek et al. [Nano Lett., 2016, 16, 3247–3252]. More importantly, this structure exhibits superb optical absorption, which can be mainly attributed to its direct band gap of 2.65 eV. The band edge alignments indicate that the 2D PC monolayer structure can be a promising candidate for photocatalytic water splitting. Furthermore, we found that strain is an effective method used to tune the electronic structures varying from direct to indirect band-gap semiconductor or even to metal. In addition, the introduction of one carbon vacancy in such a 2D PC structure can induce a magnetic moment of 1.22 µB. Our findings add a new member to the 2D material family and provide a promising candidate for optoelectronic devices in the future.

Thickness-dependent nanofriction of a rare gas monolayer sliding on Pb(111) ultrathin films

The friction can be affected dramatically by quantum size effects (QSEs) and edge effects at nanoscale. The modulations of QSEs on nanofriction of a rare gas (RG) monolayer sliding on Pb(111) ultrathin films were investigated by using the first-principles approach within density functional theory (DFT) with van der Waals (vdW) interaction correction. Our findings revealed that there exist even-odd oscillations in the friction with the thickness of Pb(111) substrate and the friction can be tuned up to 30% by the different thicknesses of Pb(111) films. Moreover, such modulation is more obvious for the RG adatoms with larger radius. The underlying physics is that the oscillations of the electronic density of states at Fermi level induce different interactions and energy barriers between RG and Pb(111) films with different thicknesses. Overall, we here propose an approach to tune friction and a way to identify the electronic contribution to friction via the different thicknesses of substrates at nanoscale.