Structural evolution of in-plane hybrid graphene/hexagonal boron nitride heterostructure upon heating

In-plane hybrid graphene/hexagonal boron nitride (h-BN) heterostructure (graphene/hBN/graphene) is studied via molecular dynamics simulation. The initial configuration (6400-atom graphene/6200-atom h-BN/6400-atom graphene) is heated up from 50 K to 7500 K via Tersoff potential. To study the structural evolution, some thermal dynamics quantities are calculated such as the coordination number, the total energy per atom, the heat capacity, the angular distribution, and the distribution of rings. Some main results are calculated and presented as follows: i) The sudden increase of total energy per atom at the melting point (5500 K) exhibits the first order phase transition from the crystalline state to a liquid state of the hybrid graphene/h-BN/graphene heterostructure; ii) The heat capacity shows two peaks. The first peak (at 5500 K) represents the phase transition from the crystalline to a liquid states while the second one (at 6300 K) represents the formation of gaseous atoms of B and N in the h-BN sheet; iii) The coordination number of three decreases dramatically at temperature of 5500 K (about 10% lefts for each type of atoms) leading to the formation of the first peak in the graph of the heat capacity. The coordination number of zero for B and N in the h-BN layer increases significantly (over 55%) at 6300 K causing the formation of the second peak in the graph of the heat capacity; iv) The influence of the relative number of atoms of h-BN to graphene in the hybrid graphene/h-BN/graphene heterostructure on the structural evolution upon heating is considered as follows. The number of atoms in the graphene sheets remains constant (6400 atoms per sheet) while the one of the h-BN sheet varies in size (780, 1560, 3120, 4680, 5490, 5880, 6080, and 6200 atoms). The results show that although the phase transition is still the first order type, the phase transition temperature decreases as the size of the h-BN layer in the hybrid heterostructure increases.

Different Doping of VSe2 Monolayers as Adsorbent and Gas Sensing Material for Scavenging and Detecting SF6 Decomposed Species

The application of intrinsic and transition metals (TM)-doped VSe₂ monolayers for the detection of faulty gases in SF₆ electrical insulated equipment is investigated based on first-principles calculations. The electron density difference, density of state, and adsorption energy are analyzed to further clarify the reaction mechanism. The results show that the intrinsic VSe₂ monolayer has weak adsorption performance for SO₂ and SOF₂ molecules, but the adsorption properties of the system are significantly improved after doping TM atoms. Among them, the TM-doped VSe₂ monolayer has better sensing performance for SO₂ than for SOF₂ molecules. Furthermore, the modulating effect of biaxial strain on the gas-sensitive properties of TM-doped VSe₂ system is also analyzed. Finally, the recovery time of the gas molecules on the solid adsorbent is evaluated. The results confirm that the TM-doped VSe₂ monolayer can be used as a novel sensing material or scavenger to ensure the normal operation of SF₆ electrical insulated equipment. This will provide a prospective insight for experimenters to implement VSe₂-based sensing materials or scavengers.

Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

Structural engineering brings new electronic properties to Janus ZrSSe and HfSSe monolayers

Interfacing effects within emergent two-dimensional (2D) materials are of fundamental interest and are at the center of applications in nanoelectronics. Thus, out-of-plane and in-plane heterostructures as well as electronic heterostructures with phase boundaries and large-angle (60°) grain boundaries (GBs) of Janus ZrSSe and HfSSe are studied in this work using first-principles calculations. The out-of-plane heterostructures of T-ZrSSe and T-HfSSe illustrate quite weak interfacing interactions, thus the electronic properties are, unusually, more like the superposition of individual monolayers. The in-plane heterostructures of T-ZrSSe and T-HfSSe, interestingly, exhibit an indirect-direct band gap transition and type-II band alignment, which correspond to boosted optical properties and spatially separated excitons. For the in-plane electronic heterostructures that are constituted by T-ZrSSe and H-ZrSSe, semiconductor-metal crossover occurs due to the polar discontinuity across the T-H phase boundary, and they behave as one-dimensional metallic wires embedded in otherwise semiconducting Janus ZrSSe, creating a one-dimensional electron/hole gas. This also indicates a strategy for stabilizing the unstable and/or metastable monolayer via the phase boundary. As a result of the zero formal bulk polarization of the T-phase ZrSSe, the metallicity of 60° GBs originates mainly from the edge atoms rather than from the polar discontinuity.

First-Principle Insight into the Ru-Doped PtSe2 Monolayer for Detection of H2 and C2H2 in Transformer Oil

Using first-principles theory, this paper investigates the sensing behavior of the Ru-doped PtSe₂ (Ru-PtSe₂) monolayer for two dominant gases, namely, H₂ and C₂H₂, in the transformer oil to explore its potential as a gas sensor to evaluate the operation status of the electrical transformers. Ru-doping prefers to go through the S₁ site with the largest E _b of -3.71 eV. Chemisorption is identified in the H₂ and C₂H₂ systems with E _ad obtained as -0.83 and - 2.09 eV, respectively, indicating the stronger performance of the Ru-PtSe₂ monolayer upon C₂H₂ adsorption. Meanwhile, the obvious improvement of bandgap in the C₂H₂ system suggests the potential of Ru-PtSe₂ monolayer as a resistance-type gas sensor for C₂H₂ detection. Moreover, the applied biaxial strains ranging at 1-5% give rise to various Q _T and E _g in two systems, indicating the tunable sensing response of the Ru-PtSe₂ monolayer for gas detection with modulated strains. Our calculation proposes a novel 2D sensing material for H₂ and C₂H₂ detection, which would be beneficial to stimulate more edge-cutting research in the gas sensing field as well.

Electronic properties of Janus MXY/graphene (M = Mo, W; X ≠ Y = S, Se) van der Waals structures: a first-principles study

Based on the first-principles calculations, we studied the intrinsic dipole moment and electronic properties of Janus MXY (M = Mo, W; X ≠ Y = S, Se) monolayers, bilayers and heterostructures with graphene, and the possibility of MXY encapsulating graphene. The results show that Janus MXY monolayer has an intrinsic dipole moment and a direct band gap. However, for MXY bilayers strong interlayer coupling will cause direct to indirect band gap transition, and the existence of the dipole moment leads to a significantly large interlayer band offset, being the driving force for the formation of interlayer excitons. In MXY/graphene heterostructures, changes in the direction of intrinsic dipole moment will cause a change in Schottky barrier height and even the transition between p- and n-type Schottky contacts. Independent of the interface atomic layer of Janus MXY, on one hand, the Dirac cone still exists in graphene, proving that MXY is an ideal coating material. On the other hand, the type-II band alignment will disappear as the intrinsic dipole moment disappears, confirming that the intrinsic dipole moment plays a vital role in the formation of a large band offset. Our results provide guidance for the study of interlayer excitonic states, the experimental construction of atomically thin p-n junctions and the encapsulation of graphene.

Tunable thermal expansion coefficient of transition-metal dichalcogenide lateral heterostructures

The thermal expansion effect plays an important role in governing the thermal stability or the stable configuration of quasi-two-dimensional atomic layers, where the difference between the thermal expansion coefficient of different kinds of atomic layer in lateral heterostructure may cause strong thermal rippling of the atomic layer. We investigate the thermal expansion phenomenon in the WSe₂-MoS₂ lateral heterostructure. We find that the thermal expansion coefficient can be enhanced by more than a factor of two via varying the ratio between the WSe₂ and MoS₂ components in the heterostructure. The underlying mechanism is disclosed to be the buckling of the WSe₂ region that is induced by the misfit strain at the coherent interface between WSe₂ and MoS₂. These findings shall be helpful in handling the thermal stability of functional devices based on the transition-metal dichalcogenide lateral heterostructures and other similar quasi-two-dimensional lateral heterostructures.

Sensing Behavior of Two Dimensional Al- and P-Doped WS2 Toward NO, NO2, and SO2: an Ab Initio Study

Two-dimensional transition metal dichalcogenides (2D TMDs), such as WS2, are considered to have the potential for high-performance gas sensors. It is a pity that the interaction between gases and pristine 2D WS2 as the sensitive element is too weak so that the sensor response is difficult to detect. Herein, the sensing capabilities of Al- and P-doped WS2 to NO, NO2, and SO2 were evaluated. Especially, we considered selectivity to target gases and dopant concentration. Molecular models of the adsorption systems were constructed, and density functional theory (DFT) was used to explore the adsorption behaviors of these gases from the perspective of binding energy, band structure, and density of states (DOS). The results suggested that doping atoms could increase the adsorption strength between gas molecules and substrate. Besides, the sensitivity of P-doped WS2 to NO and NO2 was hardly affected by CO2 or H2O. The sensitivity of Al-doped WS2 to NO2 and SO2 was also hard to be affected by CO2 or H2O. For NO detection, the WS2 with 7.4% dopant concentration had better sensitive properties than that with a 3.7% dopant concentration. While for SO2, the result was just the opposite. This work provided a comprehensive reference for choosing appropriate dopants (concentration) into 2D materials for sensing noxious gases.

Toward lateral heterostructures with two-dimensional MoX2H2 (X = As, Sb)

The development of two-dimensional (2D) lateral heterostructures (LHs) with the powerful tunability of electronic properties will be of great realistic significance for next-generation device applications. Herein, we report the novel 2D MoX2 and MoX2H2 (X = As or Sb) monolayer materials with excellent stability. Using first-principles calculations, we demonstrated that 2D MoX2 layers possess the metallic characteristic while the full surface hydrogenation effect would play a role in stabilizing the 2D lattices and lead to band gap openings of 0.83 and 0.50 eV for the 2D MoAs2H2 and MoSb2H2, respectively. In addition, our results suggest that the 2D MoAs2H2 and MoSb2H2 can serve as the 'building blocks' to construct the seamless LHs exhibiting excellent thermal and dynamical stability. The obtained nL-MoAsSb LHs enable the fully tunable band gap engineering behavior with linear tendency as a function of the width of the in-plane components. The phase transition from direct to in-direct band gap was also confirmed in the LHs as the crucial value of n = 3. In view of the type-II band alignment and efficient carrier separation in nL-MoAsSb, the predicted MoX2H2 and nL-MoAsSb LHs not only highlight the promising candidates for 2D pristine materials, but also paves the way for the realization of practical integrating device applications.

Bidirectional heterostructures consisting of graphene and lateral MoS2/WS2 composites: a first-principles study

First-principles calculations have been performed to explore the structural and electronic properties of bidirectional heterostructures composed of graphene and (MoS₂) _X /(WS₂)_4-X (X = 1, 2, 3) lateral composites and compare them with those of heterobilayers formed by graphene and pristine MS₂ (M = Mo, W). The band gaps of the lateral heterostructures lie between those of pristine MoS₂ and WS₂. The weak coupling between the two layers can induce a tiny band-gap opening of graphene and formation of an n-type Schottky contact at the G-(MoS₂) _X /(WS₂)_4-X interface. Moreover, the combination ratio of MoS₂/WS₂ can control the electronic properties of G-(MoS₂) _X /(WS₂)_4-X . By applying external electric fields, the band gaps of (MoS₂) _X /(WS₂)_4-X (X = 0, 1, 2, 3, 4) monolayers undergo a direct-indirect transition, and semiconductor-metal transitions can be found in WS₂. External electric fields can also be used effectively to tune the binding energies, charge transfers, and band structures (the types of Schottky and Ohmic contacts) of G-(MoS₂) _X /(WS₂)_4-X heterostructures. These findings suggest that G-(MoS₂) _X /(WS₂)_4-X heterostructures can serve as high-performance nano-electronic devices.

Lateral heterostructures and one-dimensional interfaces in 2D transition metal dichalcogenides

The growth and exfoliation of two-dimensional (2D) materials have led to the creation of edges and novel interfacial states at the juncture between crystals with different composition or phases. These hybrid heterostructures (HSs) can be built as vertical van der Waals stacks, resulting in a 2D interface, or as stitched adjacent monolayer crystals, resulting in one-dimensional (1D) interfaces. Although most attention has been focused on vertical HSs, increasing theoretical and experimental interest in 1D interfaces is evident. In-plane interfacial states between different 2D materials inherit properties from both crystals, giving rise to robust states with unique 1D non-parabolic dispersion and strong spin-orbit effects. With such unique characteristics, these states provide an exciting platform for realizing 1D physics. Here, we review and discuss advances in 1D heterojunctions, with emphasis on theoretical approaches for describing those between semiconducting transition metal dichalcogenides MX ₂ (with M  =  Mo, W and X  =  S, Se, Te), and how the interfacial states can be characterized and utilized. We also address how the interfaces depend on edge geometries (such as zigzag and armchair) or strain, as lattice parameters differ across the interface, and how these features affect excitonic/optical response. This review is intended to serve as a resource for promoting theoretical and experimental studies in this rapidly evolving field.

Rh-doped MoSe2 as a toxic gas scavenger: a first-principles study

Using first-principles theory, we investigated the most stable configuration for the Rh dopant on a MoSe₂ monolayer, and the interaction of the Rh-doped MoSe₂ (Rh-MoSe₂) monolayer with four toxic gases (CO, NO, NO₂ and SO₂) to exploit the potential application of the Rh-MoS₂ monolayer as a gas sensor or adsorbent. Based on adsorption behavior comparison with other 2D adsorbents and desorption behavior analysis, we assume that the Rh-MoSe₂ monolayer is a desirable adsorbent for CO, NO and NO₂ storage or removal given the larger adsorption energy (E _ad) of -2.00, -2.56 and -1.88 eV, respectively, compared with other materials. In the meanwhile, the Rh-MoSe₂ monolayer is a good sensing material for SO₂ detection according to its desirable adsorption and desorption behaviors towards the target molecule. Our theoretical calculation would provide a first insight into the TM-doping effect on the structural and electronic properties of the MoSe₂ monolayer, and shed light on the application of Rh-MoSe₂ for the sensing or disposal of common toxic gases.

Visualized charge transfer processes in monolayer composition-graded WS2xSe2(1-x) lateral heterojunctions via ultrafast microscopy mapping

Two-dimensional transitional metal dichalcogenides (TMDCs) based lateral heterojunctions have emerged as appealing and intriguing materials for applications in the next generation flexible nanoelectronics. The construction of depletion region near the in-plane interface brings rich opto-electrical dynamics, which is essential for future applications. Due to the synchronous requirement of spatial and time resolution, the study of lateral heterojunction dynamics remains a challenging issue. Herein, with a home-built spatiotemporal femtosecond transient absorption (TAS) spectroscopy platform, we have investigated the ultrafast photocarrier dynamics of monolayer spatial composition-graded WS_2xSe_2(1-x) lateral heterojunctions. At the alloy interface, the charge transfer (CT) processes have been visualized and referred to occur in 1 ps time scale. The mobility difference between electrons and holes results in the space modulation of the interface and a significant broadening of rising edge on the shell region. Moreover, carrier lifetime near the interface is extraordinarily extended by over 3 times from 153 ps to 678 ps. All these results unveil its great potential in designing future low cost logic devices and ultrafast optical applications.

Straintronics in two-dimensional in-plane heterostructures of transition-metal dichalcogenides

For in-plane heterostructures between 2D transition-metal dichalcogenides (TMDs), namely, MoSe₂/MoS₂, MoS₂/MoSe₂, WSe₂/MoS₂ and MoS₂/WSe₂, intrinsic strain can be introduced resulting from lattice mismatch between two constituents, which significantly influences electronic properties (straintronics). Intrinsic strain can reduce or decrease the coupling strength between nonmetal p and metal d orbitals, and therefore modifies the splitting between bonding and antibonding states at the high-symmetry k-points. In this case, relative upward or downward shift of band edge at specific k-points leads to band gap reduction or enhancement and the indirect-direct band gap transition. Upon consideration of spin-orbit coupling (SOC) effects, energy splitting in valence bands will further shift the band edge at a specific k-point, and changes the band gap nature, such as indirect-direct band gap transition. It is of interest that intense states hybridization exists within the interline region, and therefore band alignments for in-plane heterostructures of 2D TMDs should be reconsidered, which is crucial for transport and optical features. In addition, states hybridization plays a role in the amplitude of band edge shift since individual 2D TMDs present different resistance to the strain. However, we found that intrinsic strain has no effects on the SOC-induced energy splitting in valence bands of these in-plane heterostructures, while the extent of states hybridization determines the magnitude of energy splitting. In addition, charge transfer across the interline also has effects on the band gap. In the presence of strain, the bonding strength of two 2D TMDs is reduced.

Designing lateral spintronic devices with giant tunnel magnetoresistance and perfect spin injection efficiency based on transition metal dichalcogenides

A robust spin-filtering device based on two-dimensional TMDs.

Effects of stacking method and strain on the electronic properties of the few-layer group-IVA monochalcogenide heterojunctions

Group-IV monochalcogenides (GeSe, SnSe, GeS, SnS) are a class of promising monolayer materials for nanoelectronic applications. However, the GeSe monolayer is the only direct semiconductor in the group-IV monochalcogenides, which limits their application in nanoelectronic fields. Stacking is usually a good strategy to design two-dimensional (2D) materials with novel properties. Taking these monolayer monochalcogenides as basic building blocks, various van der Waals (vdW) heterojunctions can be constructed by different stacking methods. In this study, we systematically investigated the structures, stabilities and electronic properties of thirty-six few-layer group-IV monochalcogenide heterojunctions. All the vdW heterojunctions are proved to be stable. The degree of stability of the few-layer heterojunctions is found to increase with the number of layers. The band gap values of heterojunctions are dependent not only on the components, but also on the stacking order. Five novel 2D direct semiconductors (SnSe/GeSe, GeS/SnS, SnSe/GeSe/SnSe, SnS/GeSe/SnSe and SnS/GeSe/SnSe) are obtained. It's found that biaxial strain can not only tune the values of band gap, but also change the type of the 2D materials. The band gaps of the heterojunctions monotonically increase with the increasing strain and most few-layer heterojunctions transform between direct and indirect semiconductors under biaxial strain. Five heterojunctions (SnSe/GeSe, GeS/SnS, GeSe/SnSe/SnS, SnS/GeSe/SnSe and GeSe/SnS/GeS/SnSe) are found to remain as direct semiconductors under tensile strain (0–0.1). Since the band gaps of these heterojunctions are easy to control in a suitable range, they may have potential applications in nanoelectronic fields.

We studied the effect of stacking method and biaxial strain on the electronic properties of the few-layer group-IV monochalcogenides heterojunction.

In-plane heterostructures of Sb/Bi with high carrier mobility

In-plane two-dimensional (2D) heterostructures have been attracting public attention due to their distinctive properties. However, the pristine materials that can form in-plane heterostructures are reported only for graphene, hexagonal BN, transition-metal dichalcogenides. It will be of great significance to explore more suitable 2D materials for constructing such ingenious heterostructures. Here, we demonstrate two types of novel seamless in-plane heterostructures combined by pristine Sb and Bi monolayers by means of first-principle approach based on density functional theory. Our results indicate that external strain can serve as an effective strategy for bandgap engineering, and the transition from semiconductor to metal occurs when a compressive strain of -8% is applied. In addition, the designed heterostructures possess direct band gaps with high carrier mobility (∼4000 cm² V^-1 s^-1). And the mobility of electrons and holes have huge disparity along the direction perpendicular to the interface of Sb/Bi in-plane heterostructures. It is favorable for carriers to separate spatially. Finally, we find that the band edge positions of Sb/Bi in-plane heterostructures can meet the reduction potential of hydrogen generation in photocatalysis. Our results not only offer alternative materials to construct versatile in-plane heterostructures, but also highlight the applications of 2D in-plane heterostructures in diverse nanodevices and photocatalysis.