Pressure-induced commensurate stacking of graphene on boron nitride

Broken-gap energy alignment in two-dimensional van der Waals heterostructures for multifunctional tunnel diodes

Two-dimensional (2D) materials are promising platforms for future nanoelectronic technologies as they provide the building blocks for atomically thin devices, including switches, amplifiers, and oscillators. When 2D materials are layered on top of each other, forming van der Waals heterostructures (vdWHs), they can provide unique properties not possessed by the individual layers. Here we consider the vdWHs HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2 as potential Esaki (or tunnel) diodes that can be incorporated into electronic devices. In this work, the strongly constrained and appropriately normed (SCAN) meta-generalised-gradient approximation (meta-GGA) functional is employed for the structural properties, whereas the Heyd-Scuseria-Ernzerhof (HSE) functional is used for the electronic properties. We establish that the band alignments in these systems form broken-band heterojunctions. We show that the electronic properties of the systems can be effectively modulated by applying lateral strain or an external electric field. Importantly, we demonstrate that the band gap of the vdWHs can be widened by up to 0.65 eV by applying an electric force field of -1 to +1 eV Å-1. This work demonstrates a set of 6 vdWHs with properties suitable for application as 2D Esaki tunnel diodes, 4 of which could be applied as multifunctional devices. These materials not only offer new device properties, but their small dimensions allow for the creation of ultrathin devices.

Janus MoSH/WSi2N4 van der Waals Heterostructure: Two-Dimensional Metal/Semiconductor Contact

Constructing heterostructures from already synthesized two-dimensional materials is of significant importance. We performed a first-principles study to investigate the electronic properties and interfacial characteristics of Janus MoSH/WSi2N4 van der Waals heterostructure (vdWH) contacts. We demonstrate that the p-type Schottky formed by MoSH/WSi2N4 and MoHS/WSi2N4 has extremely low Schottky barrier heights (SBHs). Due to its excellent charge injection efficiency, Janus MoSH may be regarded as an effective metal contact for WSi2N4 semiconductors. Furthermore, the interfacial characteristics and electronic structure of Janus MoSH/WSi2N4 vdWHs can not only reduce/eliminate SBH, but also forms the transition from p-ShC to n-ShC type and from Schottky contact (ShC) to Ohmic contact (OhC) through the layer spacing and electric field. Our results can offer a fresh method for optoelectronic applications based on metal/semiconductor Janus MoSH/WSi2N4 vdW heterostructures, which have strong potential in optoelectronic applications.

A resilient type-III broken gap Ga2O3/SiC van der Waals heterogeneous bilayer with band-to-band tunneling effect and tunable electronic property

The potential of van der Waals (vdW) heterostructure to incorporate the outstanding features of stacked materials to meet a variety of application requirements has drawn considerable attention. Due to the unique quantum tunneling mechanisms, a type-III broken-gap obtained from vdW heterostructure is a promising design strategy for tunneling field-effect transistors. Herein, a unique Ga2O3/SiC vdW bilayer heterostructure with inherent type-III broken gap band alignment has been revealed through first-principles calculation. The underlying physical mechanism to form the broken gap band alignment is thoroughly studied. Due to the overlapping band structures, a tunneling window of 0.609 eV has been created, which enables the charges to tunnel from the VBM of the SiC layer to the CBM of the Ga2O3 layer and fulfills the required condition for band-to-band tunneling. External electric field and strain can be applied to tailor the electronic behavior of the bilayer heterostructure. Positive external electric field and compressive vertical strain enlarge the tunneling window and enhance the band-to-band tunneling (BTBT) scheme while negative electric field and tensile vertical strain shorten the BTBT window. Under external electric field as well as vertical and biaxial strain, the Ga2O3/SiC vdW hetero-bilayer maintains the type-III band alignment, revealing its capability to tolerate the external electric field and strain with resilience. All these results provide a compelling platform of the Ga2O3/SiC vdW bilayer to design high performance tunneling field effect transistor.

Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Correlation-Induced Symmetry-Broken States in Large-Angle Twisted Bilayer Graphene on MoS2

Strongly correlated states commonly emerge in twisted bilayer graphene (TBG) with "magic-angle" (1.1°), where the electron-electron (e-e) interaction U becomes prominent relative to the small bandwidth W of the nearly flat band. However, the stringent requirement of this magic angle makes the sample preparation and the further application facing great challenges. Here, using scanning tunneling microscopy (STM) and spectroscopy (STS), we demonstrate that the correlation-induced symmetry-broken states can also be achieved in a 3.45° TBG, via engineering this nonmagic-angle TBG into regimes of U/W > 1. We enhance the e-e interaction through controlling the microscopic dielectric environment by using a MoS2 substrate. Simultaneously, the width of the low-energy van Hove singularity (VHS) peak is reduced by enhancing the interlayer coupling via STM tip modulation. When partially filled, the VHS peak exhibits a giant splitting into two states flanked by the Fermi level and shows a symmetry-broken LDOS distribution with a stripy charge order, which confirms the existence of strong correlation effect in our 3.45° TBG. Our result demonstrates the feasibility of the study and application of the correlation physics in TBGs with a wider range of twist angle.

Higher order gaps in the renormalized band structure of doubly aligned hBN/bilayer graphene moiré superlattice

This paper presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. Using two interfering moiré potentials, we generate a supermoiré pattern that renormalizes the electronic bands of the pristine bilayer graphene, resulting in higher order fractal gaps even at very low energies. These Bragg gaps can be mapped using a unique linear combination of periodic areas within the system. To validate our findings, we use electronic transport measurements to identify the position of these gaps as a function of the carrier density. We establish their agreement with the predicted carrier densities and corresponding quantum numbers obtained using the continuum model. Our study provides strong evidence of the quantization of the momentum-space area of quasi-Brillouin zones in a minimally incommensurate lattice. It fills important gaps in the understanding of band structure engineering of Dirac fermions with a doubly periodic superlattice spinor potential.

Improved Fluorescence and Photoelectrical Properties of CsPbBr3 by Constructing Heterojunctions under Pressure

All-inorganic cesium lead bromide quantum dots (CsPbBr3 -QD) compounds are potential candidates for optoelectronic devices, because of their excellent fluorescence luminescence and thermal stability. However, the many heterojunction interfaces and large band gap induce the low power conversion efficiency in the CsPbBr3 -QD heterojunction, limiting its practical applications. Hereby, in combination with the pressure regulation and TiO2 /CsPbBr3 -QD heterojunction, the interface interaction within the heterojunction can be enhanced and the band gap can be narrowed. The pressure-induced O─Ti─O bond softening and PbBr6 octahedron stiffening at the interface region significantly enhance the interface interactions that are favorable to the carrier transport. Compared with CsPbBr3 -QD, the atomic interaction between Pb and Br of TiO2 /CsPbBr3 -QD heterojunction can be dramatically enhanced at high pressures, leading to increased band gap narrowing rate by two times, which is useful to widen the absorption spectrum. The fluorescence intensity increases by two times. Compression increases the photocurrent and maintains it after the pressure is released, which is due to the enhanced interface interaction induced by the high pressure. The findings provide new opportunities to adjust the physical properties of perovskite heterogeneous structures, and have important applications in the field of new-generation photovoltaic devices.

Band alignment type I, II transformations in Hf2CO2/MoS2 heterostructures using biaxial strain, external electric field, and interlayer coupling: a first principal investigation

The transition to neuromorphic devices is relevant to the development of materials capable of providing electronic switching in response to external stimuli. In the present work, the Hf2CO2/MoS2 heterostructure under biaxial strain, interlayer coupling, and an electric field was investigated by first-principles calculations based on density functional theory. We have shown that the influence of lateral deformation as well as the perpendicular external electric field is more significant compared to the influence of external vertical pressure on changes in the heterojunction type of heterostructure. The lateral stretching leads to a type-I and lateral compression results in a type-II heterojunction, and an external electric field also has an effect on heterojunction type. The combination of these impacts can tune the Hf2CO2/MoS2 heterostructure. The current work suggests a compelling way to make type-I and type-II heterostructure types consisting of Hf2CO2 and MoS2 monolayers for new nanodevices in fields like photonics, electronics, optoelectronic and neuromorphic applications.

Atomic Friction Processes of Two-Dimensional Materials

In this Perspective, we present the recent advances in atomic friction measured of two-dimensional materials obtained by friction force microscopy. Starting with the atomic-scale stick-slip behavior, a beautiful highly nonequilibrium process, we discuss the main factors that contribute to determine sliding friction between single asperity and a two-dimensional sheet including chemical identity of material, thickness, external load, sliding direction, velocity/temperature, and contact size. In particular, we focus on the latest progress of the more complex friction behavior of moiré systems involving 2D layered materials. The underlying mechanisms of these frictional characteristics observed during the sliding process by theoretical and computational studies are also discussed. Finally, a discussion and outlook on the perspective of this field are provided.

First-order quantum phase transition in the hybrid metal-Mott insulator transition metal dichalcogenide 4Hb-TaS2

Coupling together distinct correlated and topologically nontrivial electronic phases of matter can potentially induce novel electronic orders and phase transitions among them. Transition metal dichalcogenide compounds serve as a bedrock for exploration of such hybrid systems. They host a variety of exotic electronic phases, and their Van der Waals nature enables to admix them, either by exfoliation and stacking or by stoichiometric growth, and thereby induce novel correlated complexes. Here, we investigate the compound 4Hb-TaS2 that interleaves the Mott-insulating state of 1T-TaS2 and the putative spin liquid it hosts together with the metallic state of 2H-TaS2 and the low-temperature superconducting phase it harbors using scanning tunneling spectroscopy. We reveal a thermodynamic phase diagram that hosts a first-order quantum phase transition between a correlated Kondo-like cluster state and a depleted flat band state. We demonstrate that this intrinsic transition can be induced by an electric field and temperature as well as by manipulation of the interlayer coupling with the probe tip, hence allowing to reversibly toggle between the Kondo-like cluster and the depleted flat band states. The phase transition is manifested by a discontinuous change of the complete electronic spectrum accompanied by hysteresis and low-frequency noise. We find that the shape of the transition line in the phase diagram is determined by the local compressibility and the entropy of the two electronic states. Our findings set such heterogeneous structures as an exciting platform for systematic investigation and manipulation of Mott-metal transitions and strongly correlated phases and quantum phase transitions therein.

Enhanced visible-light-driven photocatalytic activity in SiPGaS/arsenene-based van der Waals heterostructures

Van der Waals heterostructures (vdWHs) showcase robust and tunable light-matter interactions, establishing an intriguing realm for investigating atomic-scale photocatalytic properties. Here, we employ ab initio methods to study the photocatalytic and optical properties of semiconducting SiPGaS/arsenene-based vdWHs with a type-II band alignment. Across the heterointerfaces, there exists significant built-in electric fields and large potential drop, in turn facilitating the spatial separation of photo-generated electron-hole pairs. These vdWHs further possess high carrier mobility in the order of 102 cm2V⁻1S⁻1, which combining with appropriate band edge positions, endow the vdWHs an absorption coefficient of ∼10⁵ cm⁻1 to harvest a maximal portion of the solar spectrum for visible-light-driven photocatalytic applications. Our findings also reveal transition of the type-II band alignment in a type-III configuration via compressive strain for tunneling field-effect transistor application. Furthermore, both types of vdWHs exhibit enhanced suitability for photocatalysis under conditions with a pH of 2.

Tuning and exploiting interlayer coupling in two-dimensional van der Waals heterostructures

Two-dimensional (2D) layered materials can stack into new material systems, with van der Waals (vdW) interaction between the adjacent constituent layers. This stacking process of 2D atomic layers creates a new degree of freedom-interlayer interface between two adjacent layers-that can be independently studied and tuned from the intralayer degree of freedom. In such heterostructures (HSs), the physical properties are largely determined by the vdW interaction between the individual layers,i.e.interlayer coupling, which can be effectively tuned by a number of means. In this review, we summarize and discuss a number of such approaches, including stacking order, electric field, intercalation, and pressure, with both their experimental demonstrations and theoretical predictions. A comprehensive overview of the modulation on structural, optical, electrical, and magnetic properties by these four approaches are also presented. We conclude this review by discussing several prospective research directions in 2D HSs field, including fundamental physics study, property tuning techniques, and future applications.

Tunable Contact Types and Interfacial Electronic Properties in TaS2/MoS2 and TaS2/WSe2 Heterostructures

Following the successful experimental synthesis of single-layer metallic 1T-TaS₂ and semiconducting 2H-MoS₂, 2H-WSe₂, we perform a first-principles study to investigate the electronic and interfacial features of metal/semiconductor 1T-TaS₂/2H-MoS₂ and 1T-TaS₂/2H-WSe₂ van der Waals heterostructures (vdWHs) contact. We show that 1T-TaS₂/2H-MoS₂ and 1T-TaS₂/2H-WSe₂ form n-type Schottky contact (n-ShC type) and p-type Schottky contact (p-ShC type) with ultralow Schottky barrier height (SBH), respectively. This indicates that 1T-TaS₂ can be considered as an effective metal contact with high charge injection efficiency for 2H-MoS₂, 2H-WSe₂ semiconductors. In addition, the electronic structure and interfacial properties of 1T-TaS₂/2H-MoS₂ and 1T-TaS₂/2H-WSe₂ van der Waals heterostructures can be transformed from n-type to p-type Schottky contact through the effect of layer spacing and the electric field. At the same time, the transition from Schottky contact to Ohmic contact can also occur by relying on the electric field and different interlayer spacing. Our results may provide a new approach for photoelectric application design based on metal/semiconductor 1T-TaS₂/2H-MoS₂ and 1T-TaS₂/2H-WSe₂ van der Waals heterostructures.

Localised strain and doping of 2D materials

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS₂ (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS₂ that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Probing interlayer van der Waals strengths of two-dimensional surfaces and defects, through STM tip-induced elastic deformations

A methodology to test the interlayer bonding strength of two-dimensional (2D) surfaces and associated one (1D)- and two (2D)- dimensional surface defects using scanning tunneling microscope tip-induced deformation, is demonstrated. Surface elastic deformation characteristics of soft 2D monatomic sheets of graphene and graphite in contrast to NbSe₂indicates related association with the underlying local bonding configurations. Surface deformation of 2D graphitic moiré patterns reveal the inter-layer van der Waals strength varying across its domains. These results help in the understanding of the comparable interlayer bonding strength of 1D grain boundary as well as the grains. Anomalous phenomena related to probing 2D materials at small gap distances as a function of strain is discussed.

Nanoscale structural and electronic properties of cellulose/graphene interfaces

The development of electronic devices based on the functionalization of (nano)cellulose platforms relies upon an atomistic understanding of the structural and electronic properties of a combined system, cellulose/functional element. In this work, we present a theoretical study of the nanocellulose/graphene interfaces (nCL/G) based on first-principles calculations. We find that the binding energies of both hydrophobic/G (nCL^phob/G) and hydrophilic/G (nCL^phil/G) interfaces are primarily dictated by the van der Waals interactions, and are comparable with those of their 2D interface counterparts. We verify that the energetic preference of nCL^phob/G has been reinforced by the inclusion of an aqueous medium via an implicit solvation model. Further structural characterization was carried out using a set of simulations of the carbon K-edge X-ray absorption spectra to identify and distinguish the key absorption features of the nCL^phob/G and nCL^phil/G interfaces. The electronic structure calculations reveal that the linear energy bands of graphene lie in the band gap of the nCL sheet, while depletion/accumulation charge density regions are observed. We show that external agents, i.e., electric field and mechanical strain, allow for tunability of the Dirac cone and charge density at the interface. The control/maintenance of the Dirac cone states in nCL/G is an important feature for the development of electronic devices based on cellulosic platforms.

Interfacial Coupling and Modulation of van der Waals Heterostructures for Nanodevices

In recent years, van der Waals heterostructures (vdWHs) of two-dimensional (2D) materials have attracted extensive research interest. By stacking various 2D materials together to form vdWHs, it is interesting to see that new and fascinating properties are formed beyond single 2D materials; thus, 2D heterostructures-based nanodevices, especially for potential optoelectronic applications, were successfully constructed in the past few decades. With the dramatically increased demand for well-controlled heterostructures for nanodevices with desired performance in recent years, various interfacial modulation methods have been carried out to regulate the interfacial coupling of such heterostructures. Here, the research progress in the study of interfacial coupling of vdWHs (investigated by Photoluminescence, Raman, and Pump-probe spectroscopies as well as other techniques), the modulation of interfacial coupling by applying various external fields (including electrical, optical, mechanical fields), as well as the related applications for future electrics and optoelectronics, have been briefly reviewed. By summarizing the recent progress, discussing the recent advances, and looking forward to future trends and existing challenges, this review is aimed at providing an overall picture of the importance of interfacial modulation in vdWHs for possible strategies to optimize the device's performance.

Tunable Schottky and ohmic contacts in the Ti2NF2/α-Te van der Waals heterostructure

Two dimensional α-Te holds great promise in optoelectronic devices because of its high mobility and excellent environmental stability. In this study, the electronic structures and interfacial contact characteristics of the Ti₂NF₂/α-Te van der Waals heterostructure are investigated by means of first-principles calculations. It is found that p-type Schottky contacts with a Schottky barrier (SB) of 0.21 eV are formed at the Ti₂NF₂/α-Te interface. By applying external electric fields or controlling the interlayer coupling between the Ti₂NF₂ and α-Te monolayers, the SB height can be effectively tuned, and all the n-type Schottky, p-type Schottky, n-type ohmic and p-type ohmic contacts can be achieved. Such an extremely high tunability is further found to be closely associated with the charge transfer at the interface, as well as the interface dipole and the potential step. Our results provide an avenue for the design of future α-Te-based electronic devices with high performance.

Interlayer Coupling: An Additional Degree of Freedom in Two-Dimensional Materials

Due to their layered nature, two-dimensional nanomaterials can stack into artificial material systems, with van der Waals interaction between the adjacent constituent layers. In such heterostructures, the physical properties are largely affected by the interlayer coupling and can thus be effectively tuned by a number of means. In this Perspective, we highlight four such experimental approaches: stacking order, electric field, intercalation, and pressure, and we discuss challenges and opportunities in future studies for van der Waals heterostructures.

Dual-Scale Stick-Slip Friction on Graphene/h-BN Moiré Superlattice Structure

Using atomic force microscopy, we have shown that friction on graphene/h-BN superlattice structures may exhibit unusual moiré-scale stick slip in addition to the regular ones observed at the atomic scale. Such dual-scale slip instability will lead to unique length-scale dependent energy dissipation when the different slip mechanisms are sequentially activated. Assisted by an improved theoretical model and comparative experiments, we find that accumulation and unstable release of the in-plane strain of the graphene layer is the key mechanism underlying the moiré-scale behavior. This work highlights the distinct role of the internal state of the van der Waals interfaces in determining the rich dynamics and energy dissipation of layer-structured materials.

Recent Advances on Tuning the Interlayer Coupling and Properties in van der Waals Heterostructures

2D van der Waals (vdW) heterostructures are receiving increasing research attention due to the theoretically amazing properties and unprecedented application potential. However, the as-synthesized heterostructures are generally underperforming due to the weak interlayer coupling, which inspires the researchers to find ways to modulate the interlayer coupling and properties, realizing the tailored performance for actual applications. There have been a lot of publications regarding the controllable regulation of the structures and properties of 2D vdW heterostructures in the past few years, while a review work summarizing the current advances is not yet available, though it is significant. This paper conducts a state-of-the-art review regarding the current research progress of performance modulation of vdW heterostructures by different techniques. First, the general synthesis methods of vdW heterostructures are summarized. Then, different performance modulation techniques, that is, mechanical-based, external fields-assisted, and particle beam irradiation-based methods, are discussed and compared in detail. Some of the newly proposed concepts are described. Thereafter, applications of vdW heterostructures with tailored properties are reviewed for the application prospects of the topic around this area. Moreover, the future research challenges and prospects are discussed, aiming at triggering more research interest and device applications around this topic.

Two-Dimensional Metal/Semiconductor Contact in a Janus MoSH/MoSi2N4 van der Waals Heterostructure

Following the successful synthesis of single-layer metallic Janus MoSH and semiconducting MoSi₂N₄, we investigate the electronic and interfacial features of metal/semiconductor MoSH/MoSi₂N₄ van der Waals (vdW) contact. We find that the metal/semiconductor MoSH/MoSi₂N₄ contact forms p-type Schottky contact (p-ShC type) with small Schottky barrier (SB), suggesting that Janus MoSH can be considered as an efficient metallic contact to MoSi₂N₄ semiconductor with high charge injection efficiency. The electronic structure and interfacial features of the MoSH/MoSi₂N₄ vdW heterostructure are tunable under strain and electric fields, which give rise to the SB change and the conversion from p-ShC to n-ShC type and from ShC to Ohmic contact. These findings could provide a new pathway for the design of optoelectronic applications based on metal/semiconductor MoSH/MoSi₂N₄ vdW heterostructures.

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure and their dependence on the interlayer coupling, biaxial strain and external electric field are systematically investigated by using the first-principles...

Oscillations of the Spacing between van Hove Singularities Induced by sub-Ångstrom Fluctuations of Interlayer Spacing in Graphene Superlattices

Physical properties of two-dimensional van der Waals (vdWs) structures depend sensitively on both stacking orders and interlayer interactions. Yet, in most cases studied to date, the interlayer interaction is considered to be a "static" property of the vdWs structures. Here we demonstrate that applying a scanning tunneling microscopy (STM) tip pulse on twisted bilayer graphene (TBG) can induce sub-Ångstrom fluctuations of the interlayer separation in the TBG, which are equivalent to dynamic vertical external pressure of about 10 GPa on the TBG. The sub-Ångstrom fluctuations of the interlayer separation result in large oscillations of the energy separations between two van Hove singularities (VHSs) in the TBG. The period of the oscillations of the VHSs spacing is extremely long, about 500-1000 sec, attributing to tip-induced local stress in the atomic-thick TBG. Our result provides an efficient method to tune and measure the physical properties of the vdWs structures dynamically.

Computational discovery of PtS2/GaSe van der Waals heterostructure for solar energy applications

2D van der Waals (vdW) heterostructures as potential materials for solar energy-related applications have been brought to the forefront for researchers. Here, by employing first-principles calculations, we proposed that the PtS₂/GaSe vdW heterostructure is a distinguished candidate for photocatalytic water splitting and solar cells. It is shown that the PtS₂/GaSe heterostructure exhibits high thermal stability with an indirect band gap of 1.81 eV. We further highlighted the strain induced type-V to type-II band alignment transitions and band gap variations in PtS₂/GaSe heterostructures. More importantly, the outstanding absorption coefficients in the visible light region and high carrier mobility further guarantee the photo energy conversion efficiency of PtS₂/GaSe heterostructures. Interestingly, the natural type-V band alignments of PtS₂/GaSe heterostructures are appropriate for the redox potential of water. On the other hand, the power conversion efficiency of ZnO/(PtS₂/GaSe heterostructure)/CIGS (copper indium gallium diselenide) solar cells can achieve ∼17.4%, which can be further optimized up to ∼18.5% by increasing the CIGS thickness. Our present study paves the way for facilitating the potential application of vdW heterostructures as a promising photocatalyst for water splitting as well as the buffer layer for solar cells.

Engineering of Numerous Moiré Superlattices in Twisted Multilayer Graphene for Twistronics and Straintronics Applications

Because of their unique atomic structure, 2D materials are able to create an up-to-date paradigm in fundamental science and technology on the way to engineering the band structure and electronic properties of materials on the nanoscale. One of the simplest methods along this path is the superposition of several 2D nanomaterials while simultaneously specifying the twist angle between adjacent layers (θ), which leads to the emergence of Moiré superlattices. The key challenge in 2D nanoelectronics is to obtain a nanomaterial with numerous Moiré superlattices in addition to a high carrier mobility in a stable and easy-to-fabricate material. Here, we demonstrate the possibility of synthesizing twisted multilayer graphene (tMLG) with a number of monolayers N_L = 40-250 and predefined narrow ranges of θ = 3-8°, θ = 11-15°, and θ = 26-30°. A 2D nature of the electron transport is observed in the tMLG, and its carrier mobilities are close to those of twisted bilayer graphene (tBLG) (with θ = 30°) between h-BN layers. We demonstrate an undoubtful presence of numerous Moiré superlattices simultaneously throughout the entire tMLG thickness, while the periods of these superlattices are rather close to each other. This offers a challenge of producing a next generation of devices for nanoelectronics, twistronics, and neuromorphic computing for large data applications.

Tunable electronic properties and band alignments of InS–arsenene heterostructures via external strain and electric field

van der Waals heterostructures (vdWHs) based on two-dimensional (2D) materials have been extensively recognized as promising candidates for fabricating multi-functional novel devices.

Tunable Lattice Reconstruction, Triangular Network of Chiral One-Dimensional States, and Bandwidth of Flat Bands in Magic Angle Twisted Bilayer Graphene

The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θ_{c}. Experimentally, the θ_{c} is demonstrated to be very close to the magic angle (θ≈1.08°). Here we address the transition between reconstructed and unreconstructed structures of the TBG across the magic angle by using scanning tunneling microscopy (STM). Our experiment demonstrates that both structures are stable in the TBG around the magic angle. By using a STM tip, we show that the two structures can be changed to each other and a triangular network of chiral one-dimensional states hosted by domain boundaries can be switched on and off. Consequently, the bandwidth of the flat band, which plays a vital role in the emergent strongly correlated states in the magic angle TBG, is tuned. This provides an extra control knob to manipulate the exotic electronic states of the TBG near the magic angle.

Tunable phase transitions and high photovoltaic performance of two-dimensional In2Ge2Te6 semiconductors

Ultrathin semiconductors with great electrical and photovoltaic performance hold tremendous promise for fundamental research and applications in next-generation electronic devices. Here, we report new 2D direct-bandgap semiconductors, namely mono- and few-layer In₂Ge₂Te₆, with a range of desired properties from ab initio simulations. We suggest that 2D In₂Ge₂Te₆ samples should be highly stable and can be experimentally fabricated by mechanical exfoliation. They are predicted to exhibit extraordinary optical absorption and high photovoltaic conversion efficiency (≥31.8%), comparable to the most efficient single-junction GaAs solar cell. We reveal that, thanks to the presence of van Hove singularities in the band structure, unusual quantum-phase transitions could be induced in monolayers via electrostatic doping. Furthermore, taking bilayer In₂Ge₂Te₆ as a prototypical system, we demonstrate the application of van der Waals pressure as a promising strategy to tune the electronic and stacking property of 2D crystals. Our work creates exciting opportunities to explore various quantum phases and atomic stacking, as well as potential applications of 2D In₂Ge₂Te₆ in future nanoelectronics.

Band Engineering of Large-Twist-Angle Graphene/h-BN Moiré Superlattices with Pressure

Graphene interfacing hexagonal boron nitride (h-BN) forms lateral moiré superlattices that host a wide range of new physical effects such as the creation of secondary Dirac points and band gap opening. A delicate control of the twist angle between the two layers is required as the effects weaken or disappear at large twist angles. In this Letter, we show that these effects can be reinstated in large-angle (∼1.8°) graphene/h-BN moiré superlattices under high pressures. A graphene/h-BN moiré superlattice microdevice is fabricated directly on the diamond culet of a diamond anvil cell, where pressure up to 8.3 GPa is applied. The band gap at the primary Dirac point is opened by 40-60 meV, and fingerprints of the second Dirac band gap are also observed in the valence band. Theoretical calculations confirm the band engineering with pressure in large-angle graphene/h-BN bilayers.

Flat Bands and Mechanical Deformation Effects in the Moiré Superlattice of MoS2-WSe2 Heterobilayers

It has recently been shown that quantum-confined states can appear in epitaxially grown van der Waals material heterobilayers without a rotational misalignment (θ = 0°), associated with flat bands in the Brillouin zone of the moiré pattern formed due to the lattice mismatch of the two layers. Peaks in the local density of states and confinement in a MoS₂/WSe₂ system was qualitatively described only considering local stacking arrangements, which cause band edge energies to vary spatially. In this work, we report the presence of large in-plane strain variation across the moiré unit cell of a θ = 0° MoS₂/WSe₂ heterobilayer and show that inclusion of strain variation and out-of-plane displacement in density functional theory calculations greatly improves their agreement with the experimental data. We further explore the role of a twist angle by showing experimental data for a twisted MoS₂/WSe₂ heterobilayer structure with a twist angle of θ = 15°, which exhibits a moiré pattern but no confinement.

Flat-band superconductivity in periodically strained graphene: mean-field and Berezinskii-Kosterlitz-Thouless transition

In the search of high-temperature superconductivity one option is to focus on increasing the density of electronic states. Here we study both the normal ands-wave superconducting state properties of periodically strained graphene, which exhibits approximate flat bands with a high density of states, with the flatness tunable by the strain profile. We generalize earlier results regarding a one-dimensional harmonic strain to arbitrary periodic strain fields, and further extend the results by calculating the superfluid weight and the Berezinskii-Kosterlitz-Thouless (BKT) transition temperatureTBKTto determine the true transition point. By numerically solving the self-consistency equation, we find a strongly inhomogeneous superconducting order parameter, similarly to twisted bilayer graphene. In the flat-band regime the order parameter magnitude, critical chemical potential, critical temperature, superfluid weight, and BKT transition temperature are all approximately linear in the interaction strength, which suggests that high-temperature superconductivity might be feasible in this system. We especially show that by using realistic strain strengthsTBKTcan be made much larger than in twisted bilayer graphene, if using similar interaction strengths. We also calculate properties such as the local density of states that could serve as experimental fingerprints for the presented model.

Tunable magnetism of a single-carbon vacancy in graphene

Creating a single-carbon vacancy introduces (quasi-)localized states for both σ and π electrons in graphene. Theoretically, interactions between the localized σ electrons and quasilocalized π electrons of a single-carbon vacancy in graphene are predicted to control its magnetism. However, experimentally confirming this prediction through manipulating the interactions remains an outstanding challenge. Here we report the manipulation of magnetism in the vicinity of an individual single-carbon vacancy in graphene by using a scanning tunnelling microscopy (STM) tip. Our spin-polarized STM measurements, complemented by density functional theory calculations, indicate that the interactions between the localized σ and quasilocalized π electrons could split the π electrons into two states with opposite spins even when they are well above the Fermi level. Via the STM tip, we successfully manipulate both the magnitude and direction of magnetic moment of the π electrons with respect to that of the σ electrons. Three different magnetic states of the single-carbon vacancy, exhibiting magnetic moments of about 1.6 μ_B, 0.5 μ_B, and 0 μ_B respectively, are realized in our experiment.

Photocatalytic performance of visible active boron nitride supported ZnFe2O4 (ZnFe2O4/BN) nanocomposites for the removal of aqueous organic pollutants

ZnFe₂O₄/BN nanocomposites were synthesized using  solvothermal method. The microscopic images revealed uniform dispersion of ZnFe₂O₄ nanospheres on the surface of BN nanosheets. PL studies showed lower e⁻/h⁺ pair recombination.  ZnFe₂O₄/9.3% BN showed outstanding photocatalytic activity.

Composite super-moiré lattices in double-aligned graphene heterostructures

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.

Tunable Schottky and Ohmic contacts in graphene and tellurene van der Waals heterostructures

We systematically investigate the effects of external electric field and interlayer coupling on the electronic structures and contact characteristics of hybrid graphene and tellurene (G/Te) van der Waals heterostructures (vdWHs) based on first-principles calculations. Our results show that the G/α-Te interface is formed by an n-type Schottky contact with a negligible Schottky barrier height (SBH), while the G/β-Te interface is formed by a p-type Schottky contact with a SBH of 0.51 eV. By applying external electric fields perpendicular to the G/Te interfaces or changing the interlayer distance between the graphene and tellurene monolayers, both Schottky barriers and contact types (n-type Schottky, p-type Schottky, and Ohmic) at the G/Te interfaces can be effectively modulated. The changes in charge transfer, as well as the corresponding interface dipole and potential step with the external electric field and interlayer coupling, are revealed to account for the reason for tunable Schottky and Ohmic contacts at the G/Te interfaces. Therefore, the G/Te vdWHs show tunable Schottky and Ohmic contacts with promising applications of graphene-based field-effect transistors in future experiments.

Superlattices based on van der Waals 2D materials

Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, and electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, and 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodic perturbation, each component contributing with different characteristics. Furthermore, in a 2D material-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D material-based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D material-based superlattices and describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moiré patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications of each type of superlattices.

Computational understanding of the structural and electronic properties of the GeS–graphene contact

The combination of the revealed novel mechanical and electronic properties accelerates the applications of the GeS/G vdW heterostructure in next-generation device design.

Tunable mosaic structures in van der Waals layered materials

Intrinsic mosaic structures composed of distinctive stacking domains separated by domain walls (DWs) show the potential to regulate many outstanding properties of van der Waals layered materials. A comprehensive simulation at the atomic scale is performed to explore how the lattice/twist mismatch and the interlayer interaction influence the mosaic configuration from the incommensurate Moiré pattern to commensurate mosaic structures by adapting a complex amplitude version of the phase field crystal method. It is found that after an incommensurate-commensurate transition occurs, the topology of the mosaic structure indicated by different domain wall (DW) patterns can be drastically changed. An experimentally observed intriguing spiral domain wall (SDW) network is revealed as result of the emergent mixed dislocation driven by minimizing the elastic and interlayer energies in the presence of both lattice and twist mismatches. The transition process from a herringbone domain wall (HBDW) network to a SDW network is also simulated, elucidated by a dislocation reaction and in good agreement with the experimental observations.

Emergence of Interfacial Polarons from Electron-Phonon Coupling in Graphene/h-BN van der Waals Heterostructures

van der Waals heterostructures, vertical stacks of layered materials, offer new opportunities for novel quantum phenomena which are absent in their constituent components. Here we report the emergence of polaron quasiparticles at the interface of graphene/hexagonal boron nitride (h-BN) heterostructures. Using nanospot angle-resolved photoemission spectroscopy, we observe zone-corner replicas of h-BN valence band maxima, with energy spacing coincident with the highest phonon energy of the heterostructure, an indication of Fröhlich polaron formation due to forward-scattering electron-phonon coupling. Parabolic fitting of the h-BN bands yields an effective mass enhancement of ∼2.3, suggesting an intermediate coupling strength. Our theoretical simulations based on Migdal-Eliashberg theory corroborate the experimental results, allowing the extraction of microscopic physical parameters. Moreover, renormalization of graphene π-band is observed due to the hybridization with the h-BN band. Our work generalizes the polaron study from transition metal oxides to van der Waals heterostructures with higher material flexibility, highlighting interlayer coupling as an extra degree of freedom to explore emergent phenomena.

Computational insight into the origin of unexpected contrast in chiral markers as revealed by STM

Internal substituents can serve the double purpose of generating stereogenic centers and (potentially) being identifiable with Scanning Tunneling Microscopy (STM) in 2D self-assembled molecular layers. We investigate computationally the origin of stark contrast variations in STM images of chirally substituted self-assembled organic films. STM images of alkyl derivatives with secondary -CH₃ and -OH groups have been simulated. Density functional theory calculations reveal bias-dependent contrast reversals in the substituent regions: a lack of local density of states in the relevant energy regime results in 'dark spots' in the simulated STM images, which turn bright upon increasing the bias voltage.

Lattice-Matched Epitaxial Graphene Grown on Boron Nitride

Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band gap but requires the formation of highly strained material and has not hitherto been realized. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and coexists with a topologically modified moiré pattern with regions of strained graphene which have giant moiré periods up to ∼80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls and also the topological defects where they terminate. We relate these results to theoretical models of band gap formation in graphene/boron nitride heterostructures.

Interlayer coupling and electric field tunable electronic properties and Schottky barrier in a graphene/bilayer-GaSe van der Waals heterostructure

In this work, using density functional theory we investigated systematically the electronic properties and Schottky barrier modulation in a multilayer graphene/bilayer-GaSe heterostructure by varying the interlayer spacing and by applying an external electric field.

First-principles calculations of the electronic properties of SiC-based bilayer and trilayer heterostructures

The tunable Schottky barrier heights and optical absorption efficiency in van der Waals SiC-based heterostructures are investigated.

Evidence of Local Commensurate State with Lattice Match of Graphene on Hexagonal Boron Nitride

Transition to a commensurate state changes the local symmetry periodicity on two-dimensional van der Waals superstructures, evoking distinctive properties far beyond individual layers. We investigate the morphology of moiré superstructures of graphene on hexagonal boron nitride (hBN) with a low twist angle (≈0°) through moiré fringe analyses with dark field transmission electron microscopy. The moiré fringes exhibit local variation, suggesting that the interaction between graphene and hBN depends on the stacking configuration and that local transition to the commensurate state occurs through the reduced crystalline mismatch (that is, by lattice stretching and twisting on the graphene lattices). This moiré superstructure analysis suggests an inventive method for studying the interaction between stacked van der Waals layers and for discerning the altered electronic and optical properties of graphene on hBN superstructures with a low twist angle, even at low magnification.

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.

Physical and chemical model of ion stability and movement within the dynamic and voltage-gated STM tip–surface tunneling junction

The interaction and mobility of ions in complex systems are fundamental to processes throughout chemistry, biology, and physics. However, nanoscale characterization of ion stability and migration remains poorly understood. Here, we examine ion movements to and from physisorbed molecular receptors at solution–graphite interfaces by developing a theoretical model alongside experimental scanning tunneling microscopy (STM) results. The model includes van der Waals forces and electrostatic interactions originating from the surface, tip, and physisorbed receptors, as well as a tip–surface electric field arising from the STM bias voltage (V_b). Our model reveals how both the electric field and tip–surface distance, d_tip, can influence anion stability at the receptor binding sites on the surface or at the STM tip, as well as the size of the barrier for anion transitions between those locations. These predictions agree well with prior and new STM results from the interactions of anions with aryl-triazole receptors that order into functional monolayers on graphite. Scanning produces clear resolution at large magnitude negative surface biases (−0.8 V) while resolution degrades at small negative surface biases (−0.4 V). The loss in resolution arises from frequent tip retractions assigned to anion migration within the tip–surface tunneling region. This experimental evidence in combination with support from the model demonstrates a local voltage gating of anions with the STM tip inside physisorbed receptors. This generalized model and experimental evidence may help to provide a basis to understand the nanoscale details of related chemical transformations and their underlying thermodynamic and kinetic preferences.

Band gap opening of graphene by forming a graphene/PtSe2 van der Waals heterojunction

As a promising high performance FET, the graphene/PtSe₂ vdW heterojunction has a band gap of 0.264 eV.