Wenqi Xiong

n- and p-type dopants in the InSe monolayer via substitutional doping

On the basis of density functional theory, we investigate the characteristics of n- and p-type dopants in the InSe monolayer by group V and VII atoms substituting Se atom. The results show that group V and VII atoms substituting Se atom have significant influences on the electronic structures of the InSe monolayer, and all the considered doping cases can be easier to be realized under In-rich experiment conditions. For group V atom-doped InSe monolayer, the magnetic ground states are obtained and its transition levels are too large to provide effective p-type carrier. In contrast, for group VII atom, I atom substituting Se atom has lower formation energy and shallowest transition level, which indicates that I substituting Se atom can induce effective n-type doping in the InSe nanosheets.

A type-II GeSe/SnS heterobilayer with a suitable direct gap, superior optical absorption and broad spectrum for photovoltaic applications

Van der Waals (vdW) heterobilayers are emerging as unique structures for next-generation electronic and optoelectronic devices. In this work, we predict that the GeSe/SnS heterobilayer has a direct band structure with a gap value of about 1.519 eV and typical type-II band alignment. Moreover, it possesses the characteristics of superior optical absorption (∼105) and a broad absorption spectrum from the visible light to the near ultraviolet region. In addition, the GeSe/SnS heterobilayer also exhibits obviously anisotropic electronic transport and optical properties with larger current and stronger optical absorption along the zigzag direction. Meanwhile, interlayer coupling and applying an external electric field are identified to be effective methods to modify its electronic and optical properties. Thus, these predicted results indicate that the GeSe/SnS heterobilayer will have promising applications in photovoltaic devices.

Gate-tunable diode-like current rectification and ambipolar transport in multilayer van der Waals ReSe2/WS2 p–n heterojunctions

Vertically stacked van der Waals (vdW) heterojunctions of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention due to their fascinating properties. In this work, we report two important gate-tunable phenomena in new artificial vdW p-n heterojunctions created by vertically stacking p-type multilayer ReSe2 and n-type multilayer WS2: (1) well-defined strong gate-tunable diode-like current rectification across the p-n interface is observed, and the tunability of the electronic processes is attributed to the tunneling-assisted interlayer recombination induced by majority carriers across the vdW interface; (2) the distinct ambipolar behavior under gate voltage modulation both at forward and reverse bias voltages is found in the vdW ReSe2/WS2 heterojunction transistors and a corresponding transport model is proposed for the tunable polarity behaviors. The findings may provide some new opportunities for building nanoscale electronic and optoelectronic devices.

Modulation of the band structures and optical properties of holey C2N nanosheets by alloying with group IV and V elements

The band structures and optical characteristics of group IV (Si, Ge) and V (P, As) element-alloyed C2N monolayers are investigated by means of first-principles methods. The results indicate that C2N1−xPx and C2N1−xAsx alloys are easier to fabricate than C2−xSixN and C2−xGexN alloys. Moreover, it is feasible to construct mixed C2N1−xPx and C2N1−xAsx alloys with tunable composition and band gap. When the doping concentration increases, the band gap shows a decreasing tendency, and the absorption edges exhibit a red shift in these alloys. These obtained results predicate that C2N1−xPx and C2N1−xAsx alloys may be promising candidates for optoelectronic applications.

Tuning electronic structures of the stanene monolayer via defects and transition-metal-embedding: spin–orbit coupling

The electronic structures and magnetism of defect- and transition metal (TM)-embedded stanene monolayers are investigated by using first-principles methods. Single vacancy (SV) and double vacancy (DV) cannot induce magnetism, while embedding a TM can effectively tune the magnetic moments of the stanene monolayer. Moreover, the results show that all 3d TM-embedded stanene monolayers are stable. The TM-embedded SV is easier to form than DV. For TM-embedded SV systems, the Ti-embedded case presents half-metallic properties. However, for TM-embedded DV systems, the Ti-embedded system is a magnetic semiconductor and spin-orbit coupling (SOC) effects remarkably increase its band gap. Interestingly, the SOC interaction induces electronic phase transition from the semiconductor to the half-metal (metal) for Ni (Zn)-embedded DV systems. These results provide a promising route to design stanene-based spintronics devices.

Electric field-tunable electronic structures of 2D alkaline-earth metal hydroxide–graphene heterostructures

Based on first-principles calculations, we study the electronic structures of 2D alkaline-earth metal hydroxide X(OH)2/graphene (X = Ca, Mg) heterostructures. The results show that the characteristics of the band gap size of X(OH)2 and Dirac cone of graphene are preserved well, and p-type Schottky barriers with a small Schottky barrier height (SBH) are formed in the hetero-multilayers. Moreover, double Dirac cones are also found in the X(OH)2/bilayer graphene (X(OH)2/BLG) cases. Interestingly, negative electric fields can easily induce the transition from p-type Schottky to Ohmic contact, while the p-type to n-type Schottky transition can be realized by positive electric fields. In addition, the electric field-modulations of the Schottky barrier are more sensitive in the X(OH)2/BLG systems. These studies may open the possibility of using X(OH)2/graphene as building blocks in the fabrication of Schottky devices.

Band engineering of the MoS2/stanene heterostructure: strain and electrostatic gating

In a fast developing field, it has been found that van der Waals heterostructures can overcome the weakness of single two-dimensional layered materials and extend their electronic and optoelectronic applications. Through first-principles methods, the studied MoS2/stanene heterostructure preserves high-speed carrier characteristics and opens the direct band gap. Simultaneously, the band alignment shows that the electrons transfer from stanene to MoS2, which forms an internal electric field. As an effective strategy, the out-of-plane strain remarkably changes the band gaps of the heterostructure and enhances its carrier concentration. In addition, the combined effects of the internal and external electric fields can further open the band gaps and induce a direct-to-indirect gap transition in the heterostructure. More interestingly, when the external electric field is equal to the reverse internal one, the heterostructure regains a Dirac cone. Our results show that the MoS2/stanene heterostructure has potential applications in high-speed optoelectronic devices.

Type‐I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects

Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type-II band alignment and interlayer exciton. However, the studies of type-I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX2 (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer-dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type-I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX2 homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type-I homojunction based 2D semiconductor materials.

Magnetism induced by 3d transition metal atom doping in InSe monolayer

Based on density functional theory, we study the electronic structures and magnetism of 3d transition metal (TM)-doped two-dimensional (2D) InSe monolayer by means of first-principles methods. The results show that all the doping cases can be easily realized under Se-rich experimental environments. For the Sc- and Cu-doped InSe monolayers, the nonmagnetic semiconducting properties can be retained. The Ti-, Cr- and Ni-doped InSe monolayers possess the half-metal behavior. Moreover, the diluted magnetic semiconductor characteristics can be found in the V-, Mn-, Co-, Fe- and Zn-doped cases. Interestingly, the Ti-, V-, Cr- and Fe-doped 2D InSe systems exhibit ferromagnetic ground states, while antiferromagnetic ground states occur in the Mn-, Co- and Ni-doped InSe monolayers.

Type-I Ca(OH)2/α-MoTe2 vdW heterostructure for ultraviolet optoelectronic device applications: electric field effects

The van der Waals (vdW) heterostructure is attracting intensive attention as a unique building block in the construction of future nano-devices. In this work, through first-principle calculations, we systemically predict the stability, electronic structures, band alignment, interface charge transfer and optical absorption of a Ca(OH)2/α-MoTe2 vdW heterostructure, considering stacking patterns and electric field (E-field) effects. The studies show that the heterostructure possesses the outstanding characteristics of direct band structures, type-I alignment, high ultraviolet absorption strength (∼105 cm−1) and broadband spectrum. Moreover, the external E-fields can tune the band alignment transformation from type I to type II. These results indicate that the Ca(OH)2/α-MoTe2 vdW heterostructure will be a promising candidate for ultraviolet optoelectronic device applications.