Twist-controlled resonant tunnelling in graphene/boron nitride/graphene heterostructures

Probing Electronic Band Structure of Monolayer MoS2 in Gate-Controlled Resonant Tunneling Diodes

Experimental determination of band structures of monolayer transition metal dichalcogenides (TMDCs) is crucially important in the design and tailoring of the properties of TMDCs. Resonant tunneling spectroscopy (RTS) is an effective technique to probe the band structures of low-dimensional systems by measuring the density of states (DOS) and energy dispersions. Here, we report the investigation of the band structure of monolayer MoS2 (ML-MoS2) in a gate-controlled resonant tunneling diode. Three distinct resonant tunneling kinks are observed in the characteristic current-voltage curves at 0.47, 0.70, and 0.81 V, respectively, which correspond to the conduction band local minimum of ML-MoS2 at K, Q1, and Q2 points. When applying a large positive gate voltage to enhance ML-MoS2 conductivity, the three resonant kinks shift to lower bias at 0.10, 0.32, and 0.39 V, respectively, which is in excellent agreement with the theoretical calculations. Our work offers an effective and more precise way to explore the electronic band structures of TMDCs using RTS.

Mechanisms of resistive switching in two-dimensional monolayer and multilayer materials

The power and energy consumption of resistive switching devices can be lowered by reducing the dimensions of their active layers. Efforts to push this low-energy switching property to its limits have led to the investigation of active regions made with two-dimensional (2D) layered materials. Despite their small dimensions, 2D layered materials exhibit a rich variety of switching mechanisms, each involving different types of atomic structure reconfiguration. In this Review, we highlight and classify the mechanisms of resistive switching in monolayer and bulk 2D layered materials, with a subsequent focus on those occurring in a monolayer and/or localized to point defects in the crystalline sheet. We discuss the complex energetics involved in these fundamentally defect-assisted processes, including the coexistence of multiple mechanisms and the effects of the contacts used. Examining the highly localized 'atomristor'-type switching, we provide insights into atomic motions and electronic transport across the metal-2D interfaces underlying their operation. Finally, we discuss progress and our perspective on the challenges associated with the development of 2D resistive switching devices. Promising application areas and material systems are identified and suggested for further research.

Strain Engineering towards Enriched Surface Patterns in Graphene Twistronics

The remarkable electronic properties of twisted bilayer graphene (TBG) are pivotal to the realm of twistronics and are significantly regulated by surface wrinkling. In this context, strain engineering provides a novel paradigm for exploring twist-strain-electron coupling. However, prevailing studies have heavily overlooked the effects of twist angle and out-of-plane strain on the surface wrinkling of TBG. To bridge this gap, we present a pioneering strain engineering strategy that encapsulates both in-plane and out-of-plane strains to customize the surface patterns of TBG, with out-of-plane strain regulated via interlayer sp3 bonding. Starting from this method, we for the first time identify multiphase surface patterns transitioning from herringbone to hexagonal structures through extensive molecular dynamics simulations and develop an original phase diagram to intuitively illustrate pattern transitions under varying twist angles and interlayer bonding densities. To delve deeply into the mechanisms driving these transitions, we establish comprehensive scaling laws by linking pattern energies to strain, twist angle, and interlayer bonding density, thereby defining the critical conditions for phase transitions. Moreover, our results highlight that atomic reconstruction at small twist angles leads to markedly different pattern transition behaviors and geometric features. By synergistically manipulating twist and strain, our work is expected to illuminate the field of twistronics and provide valuable insights for designing novel, tailored electronic devices based on wrinkle-related TBG systems.

Anisotropic Resonant Tunneling in Twist-Stacked van der Waals Heterostructure

Resonant tunneling, with energy and momentum conservation, has been extensively studied in two-dimensional van der Waals heterostructures and has potential applications in band structure probing, multivalued logic, and oscillators. Lattice alignment is crucial in resonant tunneling transistors (RTTs) for achieving negative differential resistance (NDR) with a high peak-to-valley ratio (PVR) because twist-angle-induced momentum mismatch can break the resonant tunneling condition. Here, we report anisotropic resonant tunneling in twist-stacked ReSe2/h-BN/ReSe2 RTTs, where the PVR exhibits a strong dependence on the twist angle between the two ReSe2 layers, reaching a maximum at the twist angle of 102°. Theoretical calculations suggest that the twist angle modulates the joint density of states of the two anisotropic bands in ReSe2 layers during the tunneling process, significantly suppressing the valley current and thereby enhancing the PVR. Double NDR peaks were observed in twist-stacked RTTs, which are attributed to interband resonant tunneling. Moreover, our twist-stacked RTTs are utilized in multibit inverters and adjustable self-powered photodetectors, providing potentials for the design of high-performance RTTs and photodetectors via twist-stacked engineering.

Dielectric Integrations and Advanced Interface Engineering for 2D Field-Effect Transistors

As silicon-based transistors approach their physical limits, the challenge of further increasing chip integration intensifies. 2D semiconductors, with their atomically thin thickness, ultraflat surfaces, and van der Waals (vdW) integration capability, are seen as a key candidate for sub-1 nm nodes in the post-Moore era. However, the low dielectric integration quality, including discontinuity and substantial leakage currents due to the lack of nucleation sites during deposition, interfacial states causing serious charge scattering, uncontrolled threshold shifts, and bad uniformity from dielectric doping and damage, have become critical barriers to their real applications. This review focuses on this challenge and the possible solutions. The functions of dielectric materials in transistors and their criteria for 2D devices are first elucidated. The methods for high-quality dielectric integration with 2D channels, such as surface pretreatment, using 2D materials with native oxides, buffer layer insertion, vdW dielectric transfer, and new dielectric materials, are then reviewed. Additionally, the dielectric integration for advanced 3D integration of 2D materials is also discussed. Finally, this paper is concluded with a comparative summary and outlook, highlighting the importance of interfacial state control, dielectric integration for 2D p-type channels, and compatibility with silicon processes.

Orthogonal Control of Transport Channels in Boron-Embedded Acenes

Developing effective structural design strategies for regulating charge transport is a central focus in molecular electronics. The interplay between molecular symmetry and orbital distribution, facilitated by heteroatom substitution, presents opportunities for direct modulation in both resonant and off-resonance tunneling processes. In this study, scanning tunneling microscopy-break junction techniques and the first-principles calculations are employed to investigate the electronic properties of boron-embedded acenes. Compared to the parent acene, boron incorporation shifts the transport-dominating molecular orbital from a centrally localized distribution to a delocalized configuration across the orthogonal molecular backbone. This shift results in a 10-fold increase in conductance in the off-resonance region near zero bias and a 50-fold enhancement in conductance through near-resonant tunneling at high bias voltages. Notably, expanding the central acene fragment increases orbital asymmetry within molecular junctions, thereby compromising transport efficiency. However, applying a bias voltage gradually mitigates the symmetry-breaking effect, leading to through-backbone orbital distribution and a recovery in the near-resonant tunneling conductance. This orthogonal control of electronic transport channels provides a distinct strategy for the effective regulation of molecular conductance.

Double-sided van der Waals epitaxy of topological insulators across an atomically thin membrane

Atomically thin van der Waals (vdW) films provide a material platform for the epitaxial growth of quantum heterostructures. However, unlike the remote epitaxial growth of three-dimensional bulk crystals, the growth of two-dimensional material heterostructures across atomic layers has been limited due to the weak vdW interaction. Here we report the double-sided epitaxy of vdW layered materials through atomic membranes. We grow vdW topological insulators Sb2Te3 and Bi2Se3 by molecular-beam epitaxy on both surfaces of atomically thin graphene or hexagonal boron nitride, which serve as suspended two-dimensional vdW substrate layers. Both homo- and hetero-double-sided vdW topological insulator tunnel junctions are fabricated, with the atomically thin hexagonal boron nitride acting as a crystal-momentum-conserving tunnelling barrier with abrupt and epitaxial interfaces. By performing field-angle-dependent magneto-tunnelling spectroscopy on these devices, we reveal the energy-momentum-spin resonance of massless Dirac electrons tunnelling between helical Landau levels developed in the topological surface states at the interfaces.

Layer-Dependent Charge-State Lifetime of Single Se Vacancies in WSe_{2}

Defect engineering in two-dimensional semiconductors has been exploited to tune the optoelectronic properties and introduce new quantum states in the band gap. Chalcogen vacancies in transition metal dichalcogenides in particular have been found to strongly impact charge carrier concentration and mobility in 2D transistors as well as feature subgap emission and single-photon response. In this Letter, we investigate the layer-dependent charge-state lifetime of Se vacancies in WSe_{2}. In one monolayer WSe_{2}, we observe ultrafast charge transfer from the lowest unoccupied orbital of the top Se vacancy to the graphene substrate within (1±0.2)  ps measured via the current saturation in scanning tunneling approach curves. For Se vacancies decoupled by transition metal dichalcogenide (TMD) multilayers, we find a subexponential increase of the charge lifetime from (62±14)  ps in bilayer to a few nanoseconds in four-layer WSe_{2}, alongside a reduction of the defect state binding energy. Additionally, we attribute the continuous suppression and energy shift of the dI/dV in-gap defect state resonances at very close tip-sample distances to a current saturation effect. Our results provide a key measure of the layer-dependent charge transfer rate of chalcogen vacancies in TMDs.

High Current Gain MoS2 Bipolar Junction Transistor Based on Metal-Semiconductor Schottky Contacts

Bipolar junction transistors (BJTs) are crucial components in high-power electronic applications. However, while two-dimensional (2D) semiconductors with exceptional electrical properties have been extensively studied in field-effect transistors, their application in BJTs has received far less attention. In this study, we demonstrate high-gain MoS2 BJTs based on metal-semiconductor Schottky contacts. The emitter-base junction uses the thermal ionization properties of a Schottky diode to emit electrons, while the collector-base junction leverages the Schottky barrier to collect electrons. This design enables thermal ionization of electrons into the base region, where they are subsequently accelerated and transferred to the collector region under the influence of the collector-base junction voltage. Our MoS2 BJTs achieves a common-base current gain 0.99 and a remarkable common-emitter current gain of 1967, representing the highest performance reported for BJTs based on 2D semiconductors to date, which is comparable to traditional silicon-based BJTs.

Coulomb Drag in Graphene/h-BN/Graphene Moiré Heterostructures

We report on the observation of Coulomb drag between graphene-hexagonal boron nitride (h-BN) moiré heterostructure with a moiré wavelength of ∼14  nm and an intrinsic graphene with a lattice constant of ∼0.25  nm. By tuning carrier densities of each graphene layer independently, we find that charge carriers in moiré minibands, i.e., near satellite Dirac point (sDP), can be coupled with massless Fermions near the original Dirac point (oDP), strongly enough to generate a finite drag resistivity. At high temperature (T) and large density (n), the drag resistivities near both oDP and sDP follow a typical n^{-α} (α=1.3-1.7) and T^{2} power law dependence as expected for the momentum transfer process and it also satisfies the layer reciprocity. In contrast, at low T, the layer reciprocity is broken in both oDP-oDP and sDP-oDP coupled regions that suggest dominant energy drag. Furthermore, quantitatively, the drag resistivities near sDPs are smaller than those near oDP and they deviate from T^{2} dependence below ∼100  K. Our work demonstrates that the drag experiment can be used to investigate the coupling between the carriers in moiré minibands and those in original Dirac bands which can be extended to other moiré materials.

Spectrum of Tunneling Transport through Phonon-Coupled Defect States in a Carbon-Doped Hexagonal Boron Nitride Barrier

Defects in hexagonal boron nitride (h-BN) play important roles in tunneling transport through the h-BN barrier. Here, using carbon-doped h-BN (h-BN:C) as a tunnel barrier containing defects in a controlled manner, we investigated tunneling transport through defects in the h-BN:C/graphene heterostructures. Defect-assisted tunneling through a specific kind of carbon-related defect was observed in all measured devices, where the defect level was always located at ∼0.1 eV above the graphene's charge neutrality point. We revealed a phonon-assisted inelastic process in the defect-assisted tunneling, in which carriers tunnel through the defect with phonon emission. Furthermore, when the h-BN:C barrier was thick (12 layers, ∼4 nm), sequential tunneling through two defects became dominant, where the phonon-assisted inelastic process shows substantial effects between the two defects. This study reveals the contribution of phonons to defect-assisted tunneling transport, which is essential for the development of defect-related van der Waals (vdW) electronic techniques.

Negative Differential Resistance Device with High Peak-to-Valley Ratio Realized by Subband Resonant Tunneling of Γ-Valley Carriers in WSe2/h-BN/WSe2 Junctions

Resonant tunneling diodes (RTDs) are a core technology in III-V semiconductor devices. The realization of high-performance RTD using two-dimensional (2D) materials has been long awaited, but it has yet to be accomplished. To this end, we investigate a range of WSe2/h-BN/WSe2 RTD devices by varying the number of layers of source and drain WSe2. The highest peak-to-valley ratio (PVR) is demonstrated in the three-layer (3L) WSe2/h-BN/1-layer (1L) WSe2 structure. The observed PVR values of 63.6 at 2 K and 16.2 at 300 K are the highest among the 2D material-based RTDs reported to date. Our results indicate the two key conditions to achieve high PVR: (1) resonant tunneling should occur between the Γ-point bands of the source and drain electrodes, and (2) the Γ-point bands contributing to the resonant tunneling should be energetically separated from the other bands. Our results provide an important step to outperform III-V semiconductor RTDs with 2D material-based RTDs.

Single-crystal hBN Monolayers from Aligned Hexagonal Islands

Hexagonal boron nitride (hBN), as one of the few two-dimensional insulators, holds strategic importance for advancing post-silicon electronic devices and circuits. Achieving wafer-scale, high-quality monolayer hBN is essential for its integration into the semiconductor industry. However, the physical mechanisms behind the chemical vapor deposition (CVD) synthesis of hBN are not yet well understood. Investigating morphology engineering is critical for developing scalable synthetic techniques for the large-scale production of high-quality hBN. In this study, we explored the underlying mechanisms of the CVD growth process of hBN and found that the involvement of a small amount of oxygen effectively modulates the shape of the single-crystal hBN islands. By tuning the oxygen content in the CVD system, we synthesized well-aligned hexagonal hBN islands and achieved a continuous, high-quality single-crystal monolayer hBN film through the merging of these hexagonal islands on conventional single-crystal metal-foil substrates. Density functional theory was used to study the edges of hBN monolayers grown in an oxygen-assisted environment, providing insights into the formation mechanism. This study opens new pathways for controlling the island shape of 2D materials and establishes a foundation for the industrial-scale production of high-quality, large-area, single-crystal hBN.

Electroosmotic Sensing of Uncharged Peptides and Differentiating Their Phosphorylated States Using Nanopores

The correct characterization and identification of different kinds of proteins is crucial for the survival and development of living organisms, and proteomics research promotes the analysis and understanding of future genome functions. Nanopore technique has been proved to accurately identify individual nucleotides. However, accurate and rapid protein sequencing is difficult due to the variability of protein structures that contains more than 20 amino acids, and it remains very challenging especially for uncharged peptides as they can not be electrophoretically driven through the nanopore. Graphene nanopores have the advantages of high accuracy, sensitivity and low cost in identifying protein phosphorylation modifications. Here, by using all-atom molecular dynamics simulations, charged graphene nanopores are employed to electroosmotically capture and sense uncharged peptides. By further mimicking AFM manipulation of single molecules, it is also found that the uncharged peptides and their phosphorylated states could also be differentiated by both the ionic current and pulling force signals during their pulling processes through the nanopore with a slow and constant velocity. The results shows ability of using nanopores to detect and discriminate single amino acid and its phosphorylation, which is essential for the future low-cost and high-throughput sequencing of protein residues and their post-translational modifications.

Synthesis of Few-Layer Hexagonal Boron Nitride for Magnetic Tunnel Junction Application

Hexagonal boron nitride (hBN), a wide-gap two-dimensional (2D) insulator, is an ideal tunneling barrier for many applications because of the atomically flat surface, high crystalline quality, and high stability. Few-layer hBN with a thickness of 1-2 nm is an effective barrier for electron tunneling, but the preparation of few-layer hBN relies on mechanical exfoliation from bulk hBN crystals. Here, we report the large-area growth of few-layer hBN by chemical vapor deposition on ferromagnetic Ni-Fe thin films and its application to tunnel barriers of magnetic tunnel junction (MTJ) devices. Few-layer hBN sheets mainly consisting of two to three layers have been successfully synthesized on a Ni-Fe catalyst at a high growth temperature of 1200 °C. The MTJ devices were fabricated on as-grown hBN by using the Ni-Fe film as the bottom ferromagnetic electrode to avoid contamination and surface oxidation. We found that trilayer hBN gives a higher tunneling magnetoresistance (TMR) ratio than bilayer hBN, resulting in a high TMR ratio up to 10% at ∼10 K.

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Resonant Tunneling-Enhanced Photoresponsivity in a Twisted Graphene van der Waals Heterostructure

Leveraging its ultrahigh carrier mobility, zero-bandgap linear dispersion, and extremely short response time, graphene exhibits remarkable potential in ultrafast broad-band photodetection. Nonetheless, the inherently low responsivity of graphene photodetectors, due to the low photogenerated carrier density, significantly impedes the development of practical devices. In this study, we present an improved photoresponse within a graphene-hexagonal boron nitride-graphene vertical tunnel junction device, where the crystallographic orientation of the two graphene electrodes is aligned. Through meticulous device structure design and the adjustment of bias and gate voltages, we observe a 2 orders of magnitude increase in tunneling photocurrent, which is attributed to the momentum-conserving resonant electron tunneling. The enhanced external photoresponsivity is evident across a wide temperature and spectral range and achieves 0.7 A/W for visible light excitation. This characteristic, coupled with the device's negative differential conductance, suggests a novel avenue for highly efficient photodetection and high-frequency, logic-based optoelectronics using van der Waals heterostructures.

Non-Steady-State Symmetry Breaking Growth of Multilayered SnSe2 Nanoplates

The use of non-equilibrium growth modes with non-steady dynamics is extensively explored in bulk materials such as amorphous and polycrystalline materials. Yet, research into the non-steady-state (NSS) growth of two-dimensional (2D) materials is still in its infancy. In this study, multilayered tin selenide (SnSe2 ) nanoplates are grown by chemical vapor deposition under NSS conditions (modulating carrier gas flow and temperature). Given the facile diffusion and inherent instability of SnSe2 , it proves to be an apt candidate for nucleation and growth in NSS scenarios. This leads to the emergence of SnSe2 nanoplates with distinct features (self-growth twisting, symmetry transformation, interlayer decoupling, homojunction, and large-area 2D domain), exhibiting pronounced second harmonic generation. The authors' findings shed light on the growth dynamics of 2D materials, broadening their potential applications in various fields.

Modulating the electronic properties and band alignments of the arsenene/MoSi2N4 van der Waals heterostructure via applying strain and electric field

The two-dimensional (2D) MoSi2N4 monolayer fabricated recently has attracted extensive attention due to its exotic electronic properties and excellent stability for future applications. Using first-principles calculations, we have shown that the electronic properties of the arsenene/MoSi2N4 van der Waals (vdW) heterostructure can be effectively modulated by applying in-plane/vertical strain and vertical electric field. The arsenene/MoSi2N4 vdW heterostructure has type-II band alignment, facilitating the separation of photogenerated electron-hole pairs. The heterostructure is predicted to have an indirect bandgap of about 0.52 eV by using the PBE functional (0.87 eV by using the hybrid functional). Furthermore, under εz = 0.5 Å vertical tensile strain or -0.05 V Å-1 vertical electric field, the arsenene/MoSi2N4 heterostructure can not only experience transition from an indirect to a direct bandgap semiconductor, but also exhibit type-II to type-I band alignment transition. The calculated optical absorption properties reveal that the formation of the vdW heterostructure can effectively enhance the light absorption, and the absorption coefficient in visible and ultraviolet regions is much higher than those of the arsenene and the MoSi2N4 monolayer. Most importantly, based on charge transfer analysis, we proposed the modulation mechanism of the electronic properties of the vdW heterostructure influenced by vertical strain and electric field. Our study provides physical insights into manipulating the electronic and optoelectronic properties of MoSi2N4 based vdW heterostructures, which may be helpful for their practical applications.

Time, momentum, and energy resolved pump-probe tunneling spectroscopy of two-dimensional electron systems

Real-time probing of electrons can uncover intricate relaxation mechanisms and many-body interactions in strongly correlated materials. Here, we introduce time, momentum, and energy resolved pump-probe tunneling spectroscopy (Tr-MERTS). The method allows the injection of electrons at a particular energy and observation of their subsequent decay in energy-momentum space. Using Tr-MERTS, we visualize electronic decay processes, with lifetimes from tens of nanoseconds to tens of microseconds, in Landau levels formed in a GaAs quantum well. Although most observed features agree with simple energy-relaxation, we discovered a splitting in the nonequilibrium energy spectrum in the vicinity of a ferromagnetic state. An exact diagonalization study suggests that the splitting arises from a maximally spin-polarized state with higher energy than a conventional equilibrium skyrmion. Furthermore, we observe time-dependent relaxation of the splitting, which we attribute to single-flipped spins forming skyrmions. These results establish Tr-MERTS as a powerful tool for studying the properties of a 2DES beyond equilibrium.

Symmetry Engineering in Twisted Bilayer WTe2

The fabrication of artificial structures using a twisted van der Waals assembly has been a key technique for recent advancements in the research of two-dimensional (2D) materials. To date, various exotic phenomena have been observed thanks to the modified electron correlation or moiré structure controlled by the twist angle. However, the twisted van der Waals assembly has further potential to modulate the physical properties by controlling the symmetry. In this study, we fabricated twisted bilayer WTe2 and demonstrated that the twist angle successfully controls the spatial inversion symmetry and hence the spin splitting in the band structure. Our results reveal the further potential of a twisted van der Waals assembly, suggesting the feasibility of pursuing new physical phenomena in 2D materials based on the control of symmetry.

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.

Probing the Twist-Controlled Interlayer Coupling in Artificially Stacked Transition Metal Dichalcogenide Bilayers by Second-Harmonic Generation

Interlayer coupling plays a critical role in the electronic band structures and optoelectronic properties of van der Waals (vdW) materials and heterostructures. Here, we utilize optical second-harmonic generation (SHG) measurements to probe the twist-controlled interlayer coupling in artificially stacked WSe2/WSe2 homobilayers and WSe2/WS2 and WSe2/MoS2 heterobilayers with a postannealing procedure. In the large angle twisted WSe2/WSe2 and WSe2/WS2, the angular dependence of the SHG intensity follows the interference relations up to angles above 10°. For lower angles, the SHG is significantly suppressed. Furthermore, for the twisted WSe2/MoS2 the SHG intensity largely deviates from the coherent superposition model and shows consistent quenching for all the stacking angles. The suppressed SHG in twisted transition metal dichalcogenide (TMDC) bilayers is predominantly attributed to the interlayer coupling between the two adjacent monolayers. The evolution of the interlayer Raman mode in WSe2 demonstrates that the interlayer coupling in the twisted WSe2/WSe2 and WSe2/WS2 is highly angle-dependent. Alternatively, the interlayer coupling generally exists in the twisted WSe2/MoS2, regardless of the different angles. The interlayer coupling is further confirmed by the quenching and red-shift of the photoluminescence of WSe2 in the twisted TMDC bilayers. Combined with density functional theory calculations, we reveal that the stacking-angle-modulated interlayer coupling originates from the variation of the interlayer spacing and the binding energy in the twisted TMDC bilayers.

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Toward High-Peak-to-Valley-Ratio Graphene Resonant Tunneling Diodes

The resonant tunneling diode (RTD) is one of the very few room-temperature-operating quantum devices to date that is able to exhibit negative differential resistance. However, the reported key figure of merit, the current peak-to-valley ratio (PVR), of graphene RTDs has been up to only 3.9 at room temperature thus far. This remains very puzzling, given the atomically flat interfaces of the 2D materials. By varying the active area and perimeter of RTDs based on a graphene/hexagonal boron nitride/graphene heterostructure, we discovered that the edge doping can play a dominant role in determining the resonant tunneling, and a large area-to-perimeter ratio is necessary to obtain a high PVR. The understanding enables establishing a novel design rule and results in a PVR of 14.9, which is at least a factor of 3.8 higher than previously reported graphene RTDs. Furthermore, a theory is developed allowing extraction of the edge doping depth for the first time.

Mixing of moiré-surface and bulk states in graphite

Van der Waals assembly enables the design of electronic states in two-dimensional (2D) materials, often by superimposing a long-wavelength periodic potential on a crystal lattice using moiré superlattices1-9. This twistronics approach has resulted in numerous previously undescribed physics, including strong correlations and superconductivity in twisted bilayer graphene10-12, resonant excitons, charge ordering and Wigner crystallization in transition-metal chalcogenide moiré structures13-18 and Hofstadter's butterfly spectra and Brown-Zak quantum oscillations in graphene superlattices19-22. Moreover, twistronics has been used to modify near-surface states at the interface between van der Waals crystals23,24. Here we show that electronic states in three-dimensional (3D) crystals such as graphite can be tuned by a superlattice potential occurring at the interface with another crystal-namely, crystallographically aligned hexagonal boron nitride. This alignment results in several Lifshitz transitions and Brown-Zak oscillations arising from near-surface states, whereas, in high magnetic fields, fractal states of Hofstadter's butterfly draw deep into the bulk of graphite. Our work shows a way in which 3D spectra can be controlled using the approach of 2D twistronics.

Superior Thermoelectric Properties of Twist-Angle Superlattice Borophene Induced by Interlayer Electrons Transport

In this paper, the energy bands, interlayer interactions and thermoelectric effects of twisted bilayer borophene (TBB) synthesized on Ag (111) are studied theoretically. The results manifest the advantages of twistronics, where the high electrical conductivity and the large Seebeck coefficient are regulated to the same range, which lead to the significantly increase of figure of merit ZT than that of bilayer borophene (BB) without twist, where the BB without twist is successfully synthesized on Ag (111) film is recently experimental report [Nat. Mater. 2022, 21, 35]. For the TBB synthesized of on Ag (111) film, theoretical analysis demonstrates that TBB and Ag are relatively strongly coupled, and TBB becomes a metallic 2D material, where the top and bottom borophene layers are semiconducting and metallic, respectively. TBB exhibits excellent thermoelectric efficiency due to the charge transfer bonding between the layers, less electron localization, and the regulation of Seebeck coefficient, electrical conductivity, and ZT at the same region of chemical potential and the same temperature by twistronics. The structure-property relationship offers the possibility of applying TBB in thermoelectric devices.

A primer on twistronics: a massless Dirac fermion's journey to moiré patterns and flat bands in twisted bilayer graphene

The recent discovery of superconductivity in magic-angle twisted bilayer graphene (TBLG) has sparked a renewed interest in the strongly-correlated physics ofsp²carbons, in stark contrast to preliminary investigations which were dominated by the one-body physics of the massless Dirac fermions. We thus provide a self-contained, theoretical perspective of the journey of graphene from its single-particle physics-dominated regime to the strongly-correlated physics of the flat bands. Beginning from the origin of the Dirac points in condensed matter systems, we discuss the effect of the superlattice on the Fermi velocity and Van Hove singularities in graphene and how it leads naturally to investigations of the moiré pattern in van der Waals heterostructures exemplified by graphene-hexagonal boron-nitride and TBLG. Subsequently, we illuminate the origin of flat bands in TBLG at the magic angles by elaborating on a broad range of prominent theoretical works in a pedagogical way while linking them to available experimental support, where appropriate. We conclude by providing a list of topics in the study of the electronic properties of TBLG not covered by this review but may readily be approached with the help of this primer.

The quantum twisting microscope

The invention of scanning probe microscopy revolutionized the way electronic phenomena are visualized¹. Whereas present-day probes can access a variety of electronic properties at a single location in space², a scanning microscope that can directly probe the quantum mechanical existence of an electron at several locations would provide direct access to key quantum properties of electronic systems, so far unreachable. Here, we demonstrate a conceptually new type of scanning probe microscope-the quantum twisting microscope (QTM)-capable of performing local interference experiments at its tip. The QTM is based on a unique van der Waals tip, allowing the creation of pristine two-dimensional junctions, which provide a multitude of coherently interfering paths for an electron to tunnel into a sample. With the addition of a continuously scanned twist angle between the tip and sample, this microscope probes electrons along a line in momentum space similar to how a scanning tunnelling microscope probes electrons along a line in real space. Through a series of experiments, we demonstrate room-temperature quantum coherence at the tip, study the twist angle evolution of twisted bilayer graphene, directly image the energy bands of monolayer and twisted bilayer graphene and, finally, apply large local pressures while visualizing the gradual flattening of the low-energy band of twisted bilayer graphene. The QTM opens the way for new classes of experiments on quantum materials.

2+δ-Dimensional Materials via Atomistic Z-Welding

Pivotal to functional van der Waals stacked flexible electronic/excitonic/spintronic/thermoelectric chips is the synergy amongst constituent layers. However; the current techniques viz. sequential chemical vapor deposition, micromechanical/wet-chemical transfer are mostly limited due to diffused interfaces, and metallic remnants/bubbles at the interface. Inter-layer-coupled 2+δ-dimensional materials, as a new class of materials can be significantly suitable for out-of-plane carrier transport and hence prompt response in prospective devices. Here, the discovery of the use of exotic electric field ≈10⁶  V cm^{- 1} (at microwave hot-spot) and 2 thermomechanical conditions i.e. pressure ≈1 MPa, T ≈ 200 °C (during solvothermal reaction) to realize 2+δ-dimensional materials is reported. It is found that P_z P_z chemical bonds form between the component layers, e.g., CB and CN in G-BN, MoN and MoB in MoS₂ -BN hybrid systems as revealed by X-ray photoelectron spectroscopy. New vibrational peaks in Raman spectra (BC ≈1320 cm^-1 for the G-BN system and MoB ≈365 cm^-1 for the MoS₂ -BN system) are recorded. Tunable mid-gap formation, along with diodic behavior (knee voltage ≈0.7 V, breakdown voltage ≈1.8 V) in the reduced graphene oxide-reduced BN oxide (RGO-RBNO) hybrid system is also observed. Band-gap tuning in MoS₂ -BN system is observed. Simulations reveal stacking-dependent interfacial charge/potential drops, hinting at the feasibility of next-generation functional devices/sensors.

Emergence of Interlayer Coherence in Twist-Controlled Graphene Double Layers

We report enhanced interlayer tunneling with reduced linewidth at zero interlayer bias in a twist-controlled double monolayer graphene heterostructure in the quantum Hall regime, when the top (ν_{T}) and bottom (ν_{B}) layer filling factors are near ν_{T}=±1/2,±3/2 and ν_{B}=±1/2,±3/2, and the total filling factor ν=±1 or ±3. The zero-bias interlayer conductance peaks are stable against variations of layer filling factor, and signal the emergence of interlayer phase coherence. Our results highlight twist control as a key attribute in revealing interlayer coherence using tunneling.

Nano-engineering and nano-manufacturing in 2D materials: marvels of nanotechnology

Two-dimensional materials have attracted significant interest and investigation since the marvellous discovery of graphene. Due to their unique physical, mechanical and optical properties, van der Waals (vdW) materials possess extraordinary potential for application in future optoelectronics devices. Nano-engineering and nano-manufacturing in the atomically thin regime has further opened multifarious avenues to explore novel physical properties. Among them, moiré heterostructures, strain engineering and substrate manipulation have created numerous exotic and topological phenomena such as unconventional superconductivity, orbital magnetism, flexible nanoelectronics and highly efficient photovoltaics. This review comprehensively summarizes the three most influential techniques of nano-engineering in 2D materials. The latest development in the marvels of moiré structures in vdW materials is discussed; in addition, topological structures in layered materials and substrate engineering on the nanoscale are thoroughly scrutinized to highlight their significance in micro- and nano-devices. Finally, we conclude with remarks on challenges and possible future directions in the rapidly expanding field of nanotechnology and nanomaterial.

Synthesis of AAB-Stacked Single-Crystal Graphene/hBN/Graphene Trilayer van der Waals Heterostructures by In Situ CVD

van der Waals heterostructures based on graphene and hBN layers with different stacking modes are receiving considerable attention because of their potential application in fundamental physics. However, conventional exfoliation fabrication methods and layer-by-layer transfer techniques have various limitations. The CVD synthesis of high-quality large-area graphene and hBN multilayer heterostructures is essential for the advancement of new physics. Herein, the authors propose an in situ CVD growth strategy for synthesizing wafer-scale AAB-stacked single-crystal graphene/hBN/graphene trilayer van der Waals heterostructures. Single-crystal CuNi(111) alloys are prepared on sapphire, followed by the pre-dissolution of carbon atoms. Single-crystal monolayer hBN is synthesized on a plasma-cleaned CuNi(111) surface. Then, a single-crystal monolayer graphene is epitaxially grown onto the hBN surface to form graphene/hBN bilayer heterostructures. A controlled decrease in the growth temperature allows the carbon atoms to precipitate out of the CuNi(111) alloy to form single-crystal graphene at the interface between hBN and CuNi(111), thereby producing graphene/hBN/graphene trilayer van der Waals heterostructures. The stacking modes between as-grown 2D layers are investigated through Raman spectroscopy and transmission electron microscopy. This study provides an in situ CVD approach to directly synthesize large-scale single-crystal low-dimensional van der Waals heterostructures and facilitates their application in future 2D-material-based integrated circuits.

Resonant Tunneling between Quantized Subbands in van der Waals Double Quantum Well Structure Based on Few-Layer WSe2

We demonstrate van der Waals double quantum well (vDQW) devices based on few-layer WSe₂ quantum wells and a few-layer h-BN tunnel barrier. Due to the strong out-of-plane confinement, an exfoliated WSe₂ exhibits quantized subband states at the Γ point in its valence band. Here, we report resonant tunneling and negative differential resistance in vDQW at room temperature owing to momentum- and energy-conserved tunneling between the quantized subbands in each well. Compared to single quantum well (QW) devices with only one QW layer possessing quantized subbands, superior current peak-to-valley ratios were obtained for the DQWs. Our findings suggest a new direction for utilizing few-layer-thick transition metal dichalcogenides in subband QW devices, bridging the gap between two-dimensional materials and state-of-the-art semiconductor QW electronics.

Ab Initio Study of Graphene/hBN Van der Waals Heterostructures: Effect of Electric Field, Twist Angles and p-n Doping on the Electronic Properties

In this work, we study the structural and electronic properties of boron nitride bilayers sandwiched between graphene sheets. Different stacking, twist angles, doping, as well as an applied external gate voltage, are reported to induce important changes in the electronic band structure near the Fermi level. Small electronic lateral gaps of the order of few meV can appear near the Dirac points K. We further discuss how the bandstructures change applying a perpendicular external electric field, showing how its application lifts the degeneracy of the Dirac cones and, in the twisted case, moves their crossing points away from the Fermi energy. Then, we consider the possibility of co-doping, in an asymmetric way, the two external graphene layers. This is a situation that could be realized in heterostructures deposited on a substrate. We show that the co-doping acts as an effective external electric field, breaking the Dirac cones degeneracy. Finally, our work demonstrates how, by playing with field strength and p-n co-doping, it is possible to tune the small lateral gaps, pointing towards a possible application of C/BN sandwich structures as nano-optical terahertz devices.

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger.

Controllable Preparation of 2D Vertical van der Waals Heterostructures and Superlattices for Functional Applications

2D van der Waals heterostructures (vdWHs) and superlattices (SLs) with exotic physical properties and applications for new devices have attracted immense interest. Compared to conventionally bonded heterostructures, the dangling-bond-free surface of 2D layered materials allows for the feasible integration of various materials to produce vdWHs without the requirements of lattice matching and processing compatibility. The quality of interfaces in artificially stacked vdWHs/vdWSLs and scalability of production remain among the major challenges in the field of 2D materials. Fortunately, bottom-up methods exhibit relatively high controllability and flexibility. The growth parameters, such as the temperature, precursors, substrate, and carrier gas, can be carefully and comprehensively controlled to produce high-quality interfaces and wafer-scale products of vdWHs/vdWSLs. This review focuses on three types of bottom-up methods for the assembly of vdWHs and vdWSLs with atomically clean and electronically sharp interfaces: chemical/physical vapor deposition, metal-organic chemical vapor deposition, and ultrahigh vacuum growth. These methods can intuitively illustrate the great flexibility and controllability of bottom-up methods for the preparation of vdWHs/vdWSLs. The latest progress in vdWHs and vdWSLs, related physical phenomena, and (opto)electronic devices are summarized. Finally, the authors discuss current challenges and future perspectives in the synthesis and application of vdWHs and vdWSLs.

Intrinsic superflat bands in general twisted bilayer systems

Twisted bilayer systems with discrete magic angles, such as twisted bilayer graphene featuring moiré superlattices, provide a versatile platform for exploring novel physical properties. Here, we discover a class of superflat bands in general twisted bilayer systems beyond the low-energy physics of magic-angle twisted counterparts. By considering continuous lattice dislocation, we obtain intrinsic localized states, which are spectrally isolated at lowest and highest energies and spatially centered around the AA stacked region, governed by the macroscopic effective energy potential well. Such localized states exhibit negligible inter-cell coupling and support the formation of superflat bands in a wide and continuous parameter space, which can be mimicked using a twisted bilayer nanophotonic system. Our finding suggests that general twisted bilayer systems can realize continuously tunable superflat bands and the corresponding localized states for various photonic, phononic, and mechanical waves.

Moiré Modulation of Van Der Waals Potential in Twisted Hexagonal Boron Nitride

When a twist angle is applied between two layered materials (LMs), the registry of the layers and the associated change in their functional properties are spatially modulated, and a moiré superlattice arises. Several works explored the optical, electric, and electromechanical moiré-dependent properties of such twisted LMs but, to the best of our knowledge, no direct visualization and quantification of van der Waals (vdW) interlayer interactions has been presented, so far. Here, we use tapping mode atomic force microscopy phase-imaging to probe the spatial modulation of the vdW potential in twisted hexagonal boron nitride. We find a moiré superlattice in the phase channel only when noncontact (long-range) forces are probed, revealing the modulation of the vdW potential at the sample surface, following AB and BA stacking domains. The creation of scalable electrostatic domains, modulating the vdW potential at the interface with the environment by means of layer twisting, could be used for local adhesion engineering and surface functionalization by affecting the deposition of molecules or nanoparticles.

Revisiting Solution-Based Processing of van der Waals Layered Materials for Electronics

Following the significant discovery of van der Waals (vdW) layered materials with diverse electronic properties over more than a decade ago, the scalable production of high-quality vdW layered materials has become a critical goal to enable the transformation of fundamental studies into practical applications in electronics. To this end, solution-based processing has been proposed as a promising technique to yield vdW layered materials in large quantities. Moreover, the resulting dispersions are compatible with cost-effective device fabrication processes such as inkjet printing and roll-to-roll manufacturing. Despite these advantages, earlier works on solution-based processing methods (i.e., direct liquid-phase exfoliation or alkali-metal intercalation) have several challenges in achieving high-performance electronic devices, such as structural polydispersity in thickness and lateral size or undesired phase transformation. These challenges hinder the utilization of the solution-processed materials in the limited fields of electronics such as electrodes and conductors. In the meantime, the groundbreaking discovery of another solution-based approach, molecular intercalation-based electrochemical exfoliation, has shown significant potential for the use of vdW layered materials in scalable electronics owing to the nearly ideal structure of the exfoliated samples. The resulting materials are highly monodispersed, atomically thin, and reasonably large, enabling the preparation of electronically active thin-film networks via successful vdW interface formation. The formation of vdW interfaces is highly important for efficient plane-to-plane charge transport and mechanical stability under various deformations, which are essential to high-performance, flexible electronics. In this Perspective, we survey the latest developments in solution-based processing of vdW layered materials and their electronic applications while also describing the field's future outlook in the context of its current challenges.

Coupled Multiphysics Modelling of Sensors for Chemical, Biomedical, and Environmental Applications with Focus on Smart Materials and Low-Dimensional Nanostructures

Low-dimensional nanostructures have many advantages when used in sensors compared to the traditional bulk materials, in particular in their sensitivity and specificity. In such nanostructures, the motion of carriers can be confined from one, two, or all three spatial dimensions, leading to their unique properties. New advancements in nanosensors, based on low-dimensional nanostructures, permit their functioning at scales comparable with biological processes and natural systems, allowing their efficient functionalization with chemical and biological molecules. In this article, we provide details of such sensors, focusing on their several important classes, as well as the issues of their designs based on mathematical and computational models covering a range of scales. Such multiscale models require state-of-the-art techniques for their solutions, and we provide an overview of the associated numerical methodologies and approaches in this context. We emphasize the importance of accounting for coupling between different physical fields such as thermal, electromechanical, and magnetic, as well as of additional nonlinear and nonlocal effects which can be salient features of new applications and sensor designs. Our special attention is given to nanowires and nanotubes which are well suited for nanosensor designs and applications, being able to carry a double functionality, as transducers and the media to transmit the signal. One of the key properties of these nanostructures is an enhancement in sensitivity resulting from their high surface-to-volume ratio, which leads to their geometry-dependant properties. This dependency requires careful consideration at the modelling stage, and we provide further details on this issue. Another important class of sensors analyzed here is pertinent to sensor and actuator technologies based on smart materials. The modelling of such materials in their dynamics-enabled applications represents a significant challenge as we have to deal with strongly nonlinear coupled problems, accounting for dynamic interactions between different physical fields and microstructure evolution. Among other classes, important in novel sensor applications, we have given our special attention to heterostructures and nucleic acid based nanostructures. In terms of the application areas, we have focused on chemical and biomedical fields, as well as on green energy and environmentally-friendly technologies where the efficient designs and opportune deployments of sensors are both urgent and compelling.

All-Solution-Processed Van der Waals Heterostructures for Wafer-Scale Electronics

2D van der Waals (vdW) materials have been considered as potential building blocks for use in fundamental elements of electronic and optoelectronic devices, such as electrodes, channels, and dielectrics, because of their diverse and remarkable electrical properties. Furthermore, two or more building blocks of different electronic types can be stacked vertically to generate vdW heterostructures with desired electrical behaviors. However, such fundamental approaches cannot directly be applied practically because of issues such as precise alignment/positioning and large-quantity material production. Here, these limitations are overcome and wafer-scale vdW heterostructures are demonstrated by exploiting the lateral and vertical assembly of solution-processed 2D vdW materials. The high exfoliation yield of the molecular intercalation-assisted approach enables the production of micrometer-sized nanosheets in large quantities and its lateral assembly in a wafer-scale via vdW interactions. Subsequently, the laterally assembled vdW thin-films are vertically assembled to demonstrate various electronic device applications, such as transistors and photodetectors. Furthermore, multidimensional vdW heterostructures are demonstrated by integrating 1D carbon nanotubes as a p-type semiconductor to fabricate p-n diodes and complementary logic gates. Finally, electronic devices are fabricated via inkjet printing as a lithography-free manner based on the stable nanomaterial dispersions.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Aligned Stacking of Nanopatterned 2D Materials for High-Resolution 3D Device Fabrication

Two-dimensional materials can be combined by placing individual layers on top of each other, so that they are bound only by their van der Waals interaction. The sequence of layers can be chosen arbitrarily, enabling an essentially atomic-level control of the material and thereby a wide choice of properties along one dimension. However, simultaneous control over the structure in the in-plane directions is so far still rather limited. Here, we combine spatially controlled modifications of 2D materials, using focused electron irradiation or electron beam induced etching, with the layer-by-layer assembly of van der Waals heterostructures. The presented assembly process makes it possible to structure each layer with an arbitrary pattern prior to the assembly into the heterostructure. Moreover, it enables a stacking of the layers with accurate lateral alignment, with an accuracy of currently 10 nm, under observation in an electron microscope. Together, this enables the fabrication of almost arbitrary 3D structures with highest spatial resolution.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Potential outstanding physical properties of novel black arsenic phosphorus As0.25P0.75/As0.75P0.25 phases: a first-principles investigation

Black arsenic phosphorus As0.5P0.5 has been studied as an excellent candidate for electronic and optoelectronic applications. At the same time, the physical properties of As x P1-x alloys with other compositions were not investigated. In this work, we design seven As0.25P0.75(P-I and P-II)/As0.75P0.25(As-(I, II, III, IV and V)) phases with molecular dynamics stability. First principles calculations are used to study their electronic structures under strain as well as their carrier mobilities. By calculating Perdew-Burke-Ernzerhof (PBE) electronic bands, we reveal that these materials are direct-gap semiconductors similar to black phosphorus except for the As-IV phase. It is also found that the carrier mobility in the P-I and As-V phases can reach 104 cm2 V-1 s-1. The electronic structures of the P-I, As-IV and As-V phases under strain are studied. Finally, we design caloritronic devices based on armchair and zigzag nanoribbons. The value of the Seebeck coefficient of the armchair and zigzag devices made from the P-II phases are found to be as high as 2507 and 2005 μW K-1 at 300 K. The thermal properties of the arsenic phosphorus phases under consideration are further studied by calculating their thermoelectric figure of merit, ZT values. These values are as high as 10.88 for the armchair devices based on the As-III phase and 4.59 for the zigzag devices based on the As-V phase at room temperature, and 15 and 7.16 at 600 K, respectively. The obtained results demonstrate that the As0.25P0.75/As0.75P0.25 phases studied here can be regarded as potential candidates for thermoelectric and electronic device applications.

High performance and gate-controlled GeSe/HfS2 negative differential resistance device

A novel and astonishing p-GeSe/n-HfS2 NDR device shows a high value for the peak-to-valley current ratio in the range of 5.8.

Robust Quantum Oscillation of Dirac Fermions in a Single-Defect Resonant Transistor

The massless nature of Dirac Fermions produces large energy gaps between Landau levels (LLs), which is promising for topological devices. While the energy gap between the zeroth and first LLs reaches 36 meV in a magnetic field of 1 T in graphene, exploiting the quantum Hall effect at room temperature requires large magnetic fields (∼30 T) to overcome the energy level broadening induced by charge inhomogeneities in the device. Here, we report a way to use the robust quantum oscillations of Dirac Fermions in a single-defect resonant transistor, which is based on local tunneling through a thin (∼1.4 nm) hexagonal boron nitride (h-BN) between lattice-orientation-aligned graphene layers. A single point defect in the h-BN, selected by the orientation-tuned graphene layers, probes local LLs in its proximity, minimizing the energy broadening of the LLs by charge inhomogeneity at a moderate magnetic field and ambient conditions. Thus, the resonant tunneling between lattice-orientation-aligned graphene layers highlights the potential to spectroscopically locate the atomic defects in the h-BN, which contributes to the study on electrically tunable single photon source via defect states in h-BN.