Van der Waals Epitaxial Growth of Two-Dimensional Single-Crystalline GaSe Domains on Graphene

Moiré-Pattern Modulated Electronic Structures of GaSe/HOPG Heterostructure

Conventional two-dimensional electron gas (2DEG) typically occurs at the interface of semiconductor heterostructures and noble metal surfaces, but it is scarcely observed in individual 2D semiconductors. In this study, few-layer gallium selenide (GaSe) grown on highly ordered pyrolytic graphite (HOPG) is demonstrated using scanning tunneling microscopy and spectroscopy (STM/STS), revealing that the coexistence of quantum well states (QWS) and 2DEG. The QWS are located in the valence bands and exhibit a peak feature, with the number of quantum wells being equal to the number of atomic layers. Meanwhile, the 2DEG is located in the conduction bands and exhibits a standing-wave feature. Additionally, monolayer GaSe/HOPG heterostructures with different stacking angles (0°, 33°, 8°) form distinct moiré patterns that arise from lattice mismatch and angular rotation between adjacent atomic layers in 2D materials, which effectively modulate the electron effective mass, charge redistribution, and band gap of GaSe. Overall, this work reveals a paradigm of band engineering based on layer numbers and moiré patterns that can modulate the electronic properties of 2D materials.

Emergence of ferroelectricity in a nonferroelectric monolayer

Ferroelectricity in ultrathin two-dimensional (2D) materials has attracted broad interest due to potential applications in nonvolatile memory, nanoelectronics and optoelectronics. However, ferroelectricity is barely explored in materials with native centro or mirror symmetry, especially in the 2D limit. Here, we report the first experimental realization of room-temperature ferroelectricity in van der Waals layered GaSe down to monolayer with mirror symmetric structures, which exhibits strong intercorrelated out-of-plane and in-plane electric polarization. The origin of ferroelectricity in GaSe comes from intralayer sliding of the Se atomic sublayers, which breaks the local structural mirror symmetry and forms dipole moment alignment. Ferroelectric switching is demonstrated in nano devices fabricated with GaSe nanoflakes, which exhibit exotic nonvolatile memory behavior with a high channel current on/off ratio. Our work reveals that intralayer sliding is a new approach to generate ferroelectricity within mirror symmetric monolayer, and offers great opportunity for novel nonvolatile memory devices and optoelectronics applications.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Janus Ga2SeTe/In2SSe heterostructures: tunable electronic, optical, and photocatalytic properties

Vertically stacking two-dimensional materials into van der Waals (vdW) heterostructures (HS) is deemed to be an effective strategy to tailor their physical properties and enrich their applications in modern nanoelectronics. Here, we study the geometry, electronic, and optical properties of Janus Ga₂SeTe/In₂SSe heterostructures by using first-principles calculations. We consider four models of Ga₂SeTe/In₂SSe heterostructures with an alternative chalcogen atom layer sequence and five potential stacking configurations, and find that the most energy favorable stacking pattern is AB stacking for each model. The heterostructures form type II alignment with a direct band gap. Moreover, the band gap values are highly dependent on the magnitude of the electric dipole, which is related to the sublayer intrinsic dipole direction and interface charge transfer. Additionally, the optical absorption of the heterostructures is intensified in the visible and ultraviolet regime. Furthermore, we predict two heterostructures with the band edge straddling the water redox potential level. These findings can help in understanding the tailored properties of the heterostructures based on Janus two-dimensional materials, and guide experiments in designing novel optoelectronic devices.

Understanding Heterogeneities in Quantum Materials

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entanglement, and topological effects in the host system. Therefore, a critical need is to understand the roles of heterogeneities in order to endow materials with new quantum functions for energy and quantum information science applications. In this article, several representative examples are reviewed on the recent progress in connecting the heterogeneities to the quantum behaviors of real materials. Specifically, three intertwined topic areas are assessed: i) Reveal the structural, electronic, magnetic, vibrational, and optical degrees of freedom of heterogeneities. ii) Understand the effect of heterogeneities on the behaviors of quantum states in host material systems. iii) Control heterogeneities for new quantum functions. This progress is achieved by establishing the atomistic-level structure-property relationships associated with heterogeneities in quantum materials. The understanding of the interactions between electronic, magnetic, photonic, and vibrational states of heterogeneities enables the design of new quantum materials, including topological matter and quantum light emitters based on heterogenous 2D materials.

Understanding Substrate-Guided Assembly in van der Waals Epitaxy by in Situ Laser Crystallization within a Transmission Electron Microscope

Understanding the bottom-up synthesis of atomically thin two-dimensional (2D) crystals and heterostructures is important for the development of new processing strategies to assemble 2D heterostructures with desired functional properties. Here, we utilize in situ laser-heating within a transmission electron microscope (TEM) to understand the stages of crystallization and coalescence of amorphous precursors deposited by pulsed laser deposition (PLD) as they are guided by 2D crystalline substrates into van der Waals (vdW) epitaxial heterostructures. Amorphous clusters of tungsten selenide were deposited by PLD at room temperature onto graphene or MoSe₂ monolayer crystals that were suspended on TEM grids. The precursors were then stepwise evolved into 2D heterostructures with pulsed laser heating treatments within the TEM. The lattice-matching provided by the MoSe₂ substrate is shown to guide the formation of large-domain, heteroepitaxial vdW WSe₂/MoSe₂ bilayers both during the crystallization process via direct templating and after crystallization by assisting the coalescence of nanosized domains through nonclassical particle attachment processes including domain rotation and grain boundary migration. The favorable energetics for domain rotation induced by lattice matching with the substrate were understood from first-principles calculations. These in situ TEM studies of pulsed laser-driven nonequilibrium crystallization phenomena represent a transformational tool for the rapid exploration of synthesis and processing pathways that may occur on extremely different length and time scales and lend insight into the growth of 2D crystals by PLD and laser crystallization.

Tunable Schottky barrier in InTe/graphene van der Waals heterostructure

The structures and electronic properties of InTe/graphene van der Waals heterostructures are systematically investigated using the first-principles calculations. The electronic properties of InTe monolayer and graphene are well preserved respectively and the bandgap energy of graphene is opened to 36.5 meV in the InTe/graphene heterostructure. An n-type Schottky contact is formed in InTe/graphene heterostructure at the equilibrium state. There is a transformation between n-type and p-type Schottky contact when the interlayer distance is smaller than 3.56 Å or the applied electric field is larger than -0.06 V Å^-1. In addition, the Schottky contact converts to Ohmic contact when the applied vertical electric field is larger than 0.11 V Å^-1 or smaller than -0.13 V Å^-1.

Controlling Defects in Continuous 2D GaS Films for High-Performance Wavelength-Tunable UV-Discriminating Photodetectors

A chemical vapor deposition method is developed for thickness-controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS_0.87 and stoichiometric GaS. The unique degradation mechanism of GaS_0.87 with X-ray photoelectron spectroscopy and annular dark-field scanning transmission electron microscopy is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo-induced oxidation at defects. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis-to-UV to UV-discriminating. The stoichiometric GaS is suitable for large-scale, UV-sensitive, high-performance photodetector arrays for information encoding under large vis-light noise, with short response time (<66 ms), excellent UV photoresponsivity (4.7 A W^-1 for trilayer GaS), and 26-times increase of signal-to-noise ratio compared with small-bandgap 2D semiconductors. By comprehensive characterizations from atomic-scale structures to large-scale device performances in 2D semiconductors, the study provides insights into the role of defects, the importance of neglected material-quality control, and how to enhance device performance, and both layer-controlled defective GaS_0.87 and stoichiometric GaS prove to be promising platforms for study of novel phenomena and new applications.

Screw-Dislocation-Driven Growth Mode in Two Dimensional GaSe on GaAs(001) Substrates Grown by Molecular Beam Epitaxy

Regardless of the dissimilarity in the crystal symmetry, the two-dimensional GaSe materials grown on GaAs(001) substrates by molecular beam epitaxy reveal a screw-dislocation-driven growth mechanism. The spiral-pyramidal structure of GaSe multi-layers was typically observed with the majority in ε-phase. Comprehensive investigations on temperature-dependent photoluminescence, Raman scattering, and X-ray diffraction indicated that the structure has been suffered an amount of strain, resulted from the screw-dislocation-driven growth mechanism as well as the stacking disorders between monolayer at the boundaries of the GaSe nanoflakes. In addition, Raman spectra under various wavelength laser excitations explored that the common ε-phase of 2D GaSe grown directly on GaAs can be transformed into the β-phase by introducing a Se-pretreatment period at the initial growth process. This work provides an understanding of molecular beam epitaxy growth of 2D materials on three-dimensional substrates and paves the way to realize future electronic and optoelectronic heterogeneous integrated technology as well as second harmonic generation applications.

Charge Separation in Epitaxial SnS/MoS2 Vertical Heterojunctions Grown by Low-Temperature Pulsed MOCVD

The weak van der Waals bonding between monolayers in layered materials enables fabrication of heterostructures without the constraints of conventional heteroepitaxy. Although many novel heterostructures have been created by mechanical exfoliation and stacking, the direct growth of 2D chalcogenide heterostructures creates new opportunities for large-scale integration. This paper describes the epitaxial growth of layered, p-type tin sulfide (SnS) on n-type molybdenum disulfide (MoS₂) by pulsed metal-organic chemical vapor deposition at 180 °C. The influence of precursor pulse and purge times on film morphology establishes growth conditions that favor layer-by-layer growth of SnS, which is critical for materials with layer-dependent electronic properties. Kelvin probe force microscopy measurements determine a built-in potential as high as 0.95 eV, and under illumination a surface photovoltage is generated, consistent with the expected Type-II band alignment for a multilayer SnS/MoS₂ heterostructure. The bottom-up growth of a nonisostructural heterojunction comprising 2D semiconductors expands the combinations of materials available for scalable production of ultrathin devices with field-tunable responses.

Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors

Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A-1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

Dipole controlled Schottky barrier in the blue-phosphorene-phase of GeSe based van der Waals heterostructures

Controlling the interface structure is of utmost importance to regulating the nanoscale Schottky barrier height (SBH). Herein, by using first-principles calculations, the electronic properties of the graphene (G) based blue-phosphorene-phase of GeSe van der Waals (vdW) heterostructures, including M/G and X/G interfaces (M = Ge; X = Se), are systematically investigated. When the layer spacing exceeds the vdW gap, n-type Schottky contacts are formed for both MX/G and XM/G heterojunctions. With the layer spacing decreasing to equilibrium distances, due to different charge transfer across the interface, MX/G and XM/G heterojunctions display n- and p-type Schottky contacts, respectively. Further decreasing the layer distance makes both heterojunctions transit into p-type ones. The layer-spacing-dependent SBHs can be rationalized by the increased charge transfer across the interface and the resulting interfacial dipole enhancement. Enlightened by the finding of dipole-controlled SBHs, using MX as building blocks, two different stacking patterns, i.e., nMX-MX-G and nXM-XM-G (n = 1 and 2), are designed to further modulate the SBH. Interestingly, due to the presence of the intrinsic dipole of MX, it is found that the magnitude and orientation of the interfacial dipole can be artificially engineered. With n increasing from 0 to 2, nMX-MX-G with an X/G interface changes from the n-type Schottky contact to Ohmic contact. The Fermi level meets the conduction band and G shows a p-type doping feature finally. Likewise, transition from p-type Schottky contact to Ohmic contact is observed for the nXM-XM-G with M/G interface, accompanied by the Fermi level touching the valence band and the feature of n-type doping for G. The role of nMX stacking seems like the role of applying an external electric field (E-field): applying positive E-field is equivalent to the increase of dipole moment while negative E-field corresponds to the offset of dipole moment. In brief, the SBHs of GeSe/G contact are found to be tunable which originates from the intrinsic dipole of MX. The predictable SBHs for these kinds of charming built-in dipole systems are expected to be highly desirable in electronic devices.

Direct transmission electron microscopy observation of the oriented edge-attachment processes between single-layer graphene flakes

Oriented attachment works on super-lubric surfaces, which could be helpful for 2-dimensional material deposition.

Mirror twin grain boundaries in molybdenum dichalcogenides

Mirror twin grain boundaries (MTBs) exist at the interface between two grains of 60° rotated hexagonal transition metal dichalcogenides (TMDC). These grain boundaries form a regular atomic structure that extends in one dimension and thus may be described as a one-dimensional (1D) lattice embedded in the 2D TMDC. In this review, the different atomic structures and compositions of these MTBs are discussed. The obvious formation of MTBs is by coalescence of two twinned grains. In addition, however, in MoSe₂ and MoTe₂ a different formation mechanism has been revealed for the formation of Mo-rich MTBs. It has been shown that excess Mo can be incorporated into the TMDC lattices. These excess Mo atoms can then reorganize into closed, triangular MTB-loops that can grow in size by adding more Mo atoms to them. This mechanism allows the formation of dense MTB networks in MoSe₂ and MoTe₂. Such MTB networks have been observed in samples grown by molecular beam epitaxy (MBE) and consequently their presence needs to be considered in understanding the properties of MBE grown MoSe₂ and MoTe₂. Density functional theory as well as photoemission spectroscopy of MTB networks have shown that MTBs exhibit dispersing 1D-bands that intersect the Fermi-level, thus suggesting that these are 1D electron systems. Consequently, experimental data have been interpreted to reveal a charge density wave (or Peierls) instability, as well as a Tomonaga-Luttinger liquid behavior for electrons confined in 1D. We discuss these observations and the controversies that remain in the interpretation of some data. The metallic properties of the MTBs and their formation in dense networks also sparked the potential use of such crystal modifications for making metallic contacts to MoTe₂ or MoSe₂. Moreover, these crystal modifications may also boost the catalytic properties of these materials.

Realization of vertical metal semiconductor heterostructures via solution phase epitaxy

The creation of crystal phase heterostructures of transition metal chalcogenides, e.g., the 1T/2H heterostructures, has led to the formation of metal/semiconductor junctions with low potential barriers. Very differently, post-transition metal chalcogenides are semiconductors regardless of their phases. Herein, we report, based on experimental and simulation results, that alloying between 1T-SnS₂ and 1T-WS₂ induces a charge redistribution in Sn and W to realize metallic Sn_0.5W_0.5S₂ nanosheets. These nanosheets are epitaxially deposited on surfaces of semiconducting SnS₂ nanoplates to form vertical heterostructures. The ohmic-like contact formed at the Sn_0.5W_0.5S₂/SnS₂ heterointerface affords rapid transport of charge carriers, and allows for the fabrication of fast photodetectors. Such facile charge transfer, combined with a high surface affinity for acetone molecules, further enables their use as highly selective 100 ppb level acetone sensors. Our work suggests that combining compositional and structural control in solution-phase epitaxy holds promises for solution-processible thin-film optoelectronics and sensors.

Recent Advances in Synthesis and Applications of 2D Junctions

Recent progress in the methods of integration of 2D materials is reviewed. Integrated 2D circuits are one of the most promising candidates for advanced electronics and flexible devices. Specifically, methods such as mechanical transfer, chemical vapor deposition growth, high temperature conversion, phase engineering, surface doping, electrostatic doping, and so on to fabricate 2D heterostructures are discussed in detail. Applications of these integrated 2D heterostructures in p-n junctions, ohmic contact, high-performance transistors, and phototransistors are also highlighted. Finally, challenges and opportunities of methods to integrate 2D materials are proposed.

A broadband, self-biased photodiode based on antimony telluride (Sb2Te3) nanocrystals/silicon heterostructures

Low bulk band gaps and conductive surface electronic states of tetradymite topological insulators (TTI) make them potential candidates for next generation ultra-broadband photodevices. Here, we demonstrate a broadband and self-biased photodiode based on a Sb2Te3-Si heterostructure. A low-cost thermal evaporation technique was employed to fabricate the photodiode. The self-biased nature of the photodiode was due to the built-in potential at the Sb2Te3-Si interface. Upon characterizing the Sb2Te3 nanocrystalline film via AFM, SEM, EDX, and XPS it was found that the film exhibited p-type behavior due to antimony vacancies or antisites. The fabricated photodiode showed an excellent rectification ratio of 3388 with n-Si confirming a robust Schottky barrier at the interface and a well-defined photocurrent upon illumination. Due to the p-type behavior of the Sb2Te3 nanocrystalline film, a rectification ratio of only 0.38 was observed with p-Si. The barrier at the interface also increases the carrier lifetimes, thereby eliminating one of the biggest drawbacks of ultrafast carrier recombination times in TTI as a photodetection material. Moreover, the photodiode exhibited excellent Ion/Ioff of three orders of magnitude under the self-biased conditions, and photocurrents ranging from 520 nm to 980 nm wavelengths were observed.

Semiconducting edges and flake-shape evolution of monolayer GaSe: role of edge reconstructions

Group-III metal monochalcogenides have emerged as a new class of two-dimensional (2D) semiconductor materials. For the integration of 2D materials for various potential device applications, there is an inevitable need to reduce their dimensionality into specific sized nanostructures with edges. Owing to the properties of finite-sized 2D nanostructures strongly related to the edge configurations, the precise understanding of the edge geometric structures at an atomic level is of particular importance. By means of first-principles calculations, the geometric structures and electronic properties of stable zigzag and armchair edges in a prototype example GaSe monolayer have been identified. Our results demonstrate that both Ga- and Se-terminated zigzag edges prefer to the (3 × 1) reconstructions, and the armchair edges with the perfect flat configuration are energetically favorable. It is unexpectedly found that both zigzag and armchair GaSe nanoribbons with reconstructed edges are semiconductors, which is different from previous recognition where the zigzag edges are metallic. Moreover, the edge-dependent flake shape in GaSe has been plotted using the Wulff construction theory, and the shape evolution with chemical potentials can be applied to explain broad experimental observations on the morphologies of GaSe flakes. Importantly, similar reconstructions and electronic properties also appeared at InSe edges, suggesting that the reconstruction induced semiconducting edges are a fundamental phenomenon for 2D group-III metal monochalcogenides.

Novel structured transition metal dichalcogenide nanosheets

Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their unique properties and great potential in a wide range of applications. Great efforts have been devoted to the preparation of novel-structured TMD nanosheets by engineering their intrinsic structures at the atomic scale. Until now, a lot of new-structured TMD nanosheets, such as vacancy-containing TMDs, heteroatom-doped TMDs, TMD alloys, 1T'/1T phase and in-plane TMD crystal-phase heterostructures, TMD heterostructures and Janus TMD nanosheets, have been prepared. These materials exhibit unique properties and hold great promise in various applications, including electronics/optoelectronics, thermoelectrics, catalysis, energy storage and conversion and biomedicine. This review focuses on the most recent important discoveries in the preparation, characterization and application of these new-structured ultrathin 2D layered TMDs.

Abnormal band bowing effects in phase instability crossover region of GaSe1-xTe x nanomaterials

Akin to the enormous number of discoveries made through traditional semiconductor alloys, alloying selected 2D semiconductors enables engineering of their electronic structure for a wide range of new applications. 2D alloys have been demonstrated when two components crystallized in the same phase, and their bandgaps displayed predictable monotonic variation. By stabilizing previously unobserved compositions and phases of GaSe_1−xTe_x at nanoscales on GaAs(111), we demonstrate abnormal band bowing effects and phase instability region when components crystallize in different phases. Advanced microscopy and spectroscopy measurements show as tellurium is alloyed into GaSe, nanostructures undergo hexagonal to monoclinic and isotropic to anisotropic transition. There exists an instability region (0.56 < x < 0.67) where both phases compete and coexist, and two different bandgap values can be found at the same composition leading to anomalous band bowing effects. Results highlight unique alloying effects, not existing in single-phase alloys, and phase engineering routes for potential applications in photonic and electronics.

Alloys of two-dimensional materials normally occur when two components crystallize in the same phase. Here, the authors observe an anomalous phase instability, accompanied by a band bowing effect, in GaSe_1-xTe_x alloys on GaAs(111).

Synthesis, structure and applications of graphene-based 2D heterostructures

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

P-GaSe/N-MoS2 Vertical Heterostructures Synthesized by van der Waals Epitaxy for Photoresponse Modulation

The important role of p-n junction in modulation of the optoelectronic properties of semiconductors is widely cognized. In this work, for the first time the synthesis of p-GaSe/n-MoS₂ heterostructures via van der Waals expitaxial growth is reported, although a considerable lattice mismatching of ≈18% exists. According to the simulation, a significant type II p-n junction barrier located at the interface is expected to be formed, which can modulate optoelectronic properties of MoS₂ effectively. It is intriguing to reveal that the presence of GaSe can result in obvious Raman and photoluminescence (PL) shift of MoS₂ compared to that of pristine one, more interestingly, for PL peak shift, the effect of GaSe-induced tensile strain on MoS₂ has overcome the p-doping effect of GaSe, evidencing the strong interlayer coupling between GaSe and MoS₂ . As a result, the photoresponse rate of heterostructures is improved by almost three orders of magnitude compared with that of pristine MoS₂ .

Curvature-dependent flexible light emission from layered gallium selenide crystals

Flexible optoelectronics devices play an important role for technological applications of 2D materials because of their bendable, flexible and extended two-dimensional surfaces. In this work, light emission properties of layered gallium selenide (GaSe) crystals with different curvatures have been investigated using bending photoluminescence (BPL) experiments in the curvature range between R⁻¹ = 0.00 m⁻¹ (flat condition) and R⁻¹ = 30.28 m⁻¹. A bendable and rotated sample holder was designed to control the curvature (strain) of the layered sample under upward bending uniformly. The curvature-dependent BPL results clearly show that both bandgaps and BPL intensities of the GaSe are curvature dependent with respect to the bending-radius change. The main emission peak (bandgap) is 2.005 eV for flat GaSe, and is 1.986 eV for the bending GaSe with a curvature of 30.28 m⁻¹ (the maximum bending conditions in this experiment). An obvious redshift (i.e. energy reduction) for the GaSe BPL peak was detected owing to the c-plane lattice expansion by upward bending. The intensities of the corresponding BPL peaks also show an increase with increasing curvature. The correlations between BPL peak intensity, shiny area and bond-angle widening of the bent GaSe under laser excitation have been discussed. The lattice constant versus emission energies of the bending GaSe was also analyzed. An estimated lattice constant vs. bandgap relation was present for further application of the layered GaSe in bendable flexible light-emission devices.

Curvature-dependent luminescence enhancement and bandgap shift of 2D layered GaSe under upward bending have been clearly analyzed and demonstrated.

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.

Stability, electronic and phononic properties of β and 1T structures of SiTe x (x = 1, 2) and their vertical heterostructures

By performing first-principles calculations, we predict a novel, stable single layer phase of silicon ditelluride, 1T-[Formula: see text], and its possible vertical heterostructures with single layer β-SiTe. Structural optimization and phonon calculations reveal that 1T-[Formula: see text] structure has a dynamically stable ground state. Further analysis of the vibrational spectrum at the [Formula: see text] point shows that single layer 1T-[Formula: see text] has characteristic phonon modes at 80, 149, 191 and 294 [Formula: see text]. Electronic-band structure demonstrates that 1T-[Formula: see text] phase exhibits a nonmagnetic metallic ground state with a negligible intrinsic spin-orbit splitting. Moreover, it is shown that similar structural parameters of 1T-[Formula: see text] and existing β-SiTe phases allows construction of 1T-β heterostructures with a negligible lattice mismatch. In this regard, it is found that two energetically favorable stacking orders, namely AA and [Formula: see text]B, have distinctive shear and layer breathing phonon modes. It is important to note that the combination of semiconducting β-SiTe and metallic 1T-[Formula: see text] building blocks forms ultra-thin Schottky barriers that can be used in nanoscale optoelectronic device technologies.

Recent progress in van der Waals heterojunctions

Following the development of many novel two-dimensional (2D) materials, investigations of van der Waals heterojunctions (vdWHs) have attracted significant attention due to their excellent properties such as smooth heterointerface, highly gate-tunable bandgap, and ultrafast carrier transport. Benefits from the atom-scale thickness, physical and chemical properties and ease of manipulation of the heterojunctions formulated by weak vdW forces were demonstrated to indicate their outstanding potential in electronic and optoelectronic applications, including photodetection and energy harvesting, and the possibility of integrating them with the existing semiconductor technology for the next-generation electronic and sensing devices. In this review, we summarized the recent developments of vdWHs and emphasized their applications. Basically, we introduced the physical properties and some newly discovered phenomena in vdWHs. Then, we emphatically presented four classical vdWHs and some novel heterostructures formed by vdW forces. Based on their unique physical properties and structures, we highlighted the applications of vdWHs including in photodiodes, phototransistors, tunneling devices, and memory devices. Finally, we provided a conclusion on the recent advances in vdWHs and outlined our perspectives. We aim for this review to serve as a solid foundation in this field and to pave the way for future research on vdW-based materials and their heterostructures.

Booming Development of Group IV-VI Semiconductors: Fresh Blood of 2D Family

As an important component of 2D layered materials (2DLMs), the 2D group IV metal chalcogenides (GIVMCs) have drawn much attention recently due to their earth-abundant, low-cost, and environmentally friendly characteristics, thus catering well to the sustainable electronics and optoelectronics applications. In this instructive review, the booming research advancements of 2D GIVMCs in the last few years have been presented. First, the unique crystal and electronic structures are introduced, suggesting novel physical properties. Then the various methods adopted for synthesis of 2D GIVMCs are summarized such as mechanical exfoliation, solvothermal method, and vapor deposition. Furthermore, the review focuses on the applications in field effect transistors and photodetectors based on 2D GIVMCs, and extends to flexible devices. Additionally, the 2D GIVMCs based ternary alloys and heterostructures have also been presented, as well as the applications in electronics and optoelectronics. Finally, the conclusion and outlook have also been presented in the end of the review.

van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties

Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems that are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by molecular beam epitaxy on graphene. Reflection high-energy electron diffraction images exhibited sharp streaky features indicative of a high-quality GaSe layer produced via a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW heterointerface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting to an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of the few layers of GaSe are located at the Γ point at a binding energy of about -0.73 eV relative to the Fermi level (p-type doping). From the ARPES measurements, a hole effective mass defined along the ΓM direction and equal to about m*/m₀ = -1.1 was determined. By coupling the ARPES data with high-resolution X-ray photoemission spectroscopy measurements, the Schottky interface barrier height was estimated to be 1.2 eV. These findings allow a deeper understanding of the interlayer interactions and the electronic structure of the GaSe/graphene vdW heterostructure.

Synthesis, properties and applications of 2D layered MIIIXVI (M = Ga, In; X = S, Se, Te) materials

Group III-VI compounds M^IIIX^VI (M = Ga, In; X = S, Se, Te) are one class of important 2D layered materials and are currently attracting increasing interest due to their unique electronic and optoelectronic properties and their great potential applications in various other fields. Similar to 2D layered transition metal dichalcogenides (TMDs), M^IIIX^VI also have the significant merits of ultrathin thickness, ultrahigh surface-to-volume ratio, and high compatibility with flexible devices. More impressively, in contrast with TMDCs, M^IIIX^VI demonstrate many superior properties, such as direct band gap electronic structure, high carrier mobility, rare p-type electronic behaviors, high charge density, and so on. These unique characteristics cause high-performance device applications in electronics, optoelectronics, and optics. In this review, we aim to provide a summary of the state-of-the-art of research activities in 2D layered M^IIIX^VI materials. The scope of the review covers the synthesis and properties of 2D layered M^IIIX^VI materials and their van der Waals heterostructures. We especially focus on the applications in electronics and optoelectronics. Moreover, the review concludes with some perspectives on future developments in this field.

Interlayer Coupling in Twisted WSe2/WS2 Bilayer Heterostructures Revealed by Optical Spectroscopy

van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe2/WS2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS2 and WSe2 grown by chemical vapor deposition. After annealing the WSe2/WS2 bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm(-1) that is sensitive to the number of WSe2 layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe2/WS2 bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed broadening of the WSe2 A exciton absorption band in the bilayers suggests fast charge separation between the layers, which was supported by direct femtosecond pump-probe spectroscopy. Density functional calculations of the band structures of the bilayers at different twist angles and interlayer distances found robust type II heterojunctions at all twist angles, and predicted variations in band gap for particular atomistic arrangements. Although interlayer excitons were indicated using femtosecond pump-probe spectroscopy, photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition is very weak. However, the interlayer coupling for the WSe2/WS2 bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics.

Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures

Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EG(FH)) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EG(FH) displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EG(PH) forms a Schottky barrier junction.

Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy

Two-dimensional (2D) heterostructures hold the promise for future atomically thin electronics and optoelectronics because of their diverse functionalities. Although heterostructures consisting of different 2D materials with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) epitaxy, constructing heterostructures from layered semiconductors with large lattice misfits remains challenging. We report the growth of 2D GaSe/MoSe2 heterostructures with a large lattice misfit using two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientation between the two layers, forming a periodic superlattice. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe2 monolayer domains in lateral GaSe/MoSe2 heterostructures, GaSe monolayers are found to overgrow MoSe2 during CVD, forming a stripe of vertically stacked vdW heterostructures at the crystal interface. Such vertically stacked vdW GaSe/MoSe2 heterostructures are shown to form p-n junctions with effective transport and separation of photogenerated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These GaSe/MoSe2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells.

Low-Temperature Growth of Two-Dimensional Layered Chalcogenide Crystals on Liquid

The growth of high-quality two-dimensional (2D) layered chalcogenide crystals is highly important for practical applications in future electronics, optoelectronics, and photonics. Current route for the synthesis of 2D chalcogenide crystals by vapor deposition method mainly involves an energy intensive high-temperature growth process on solid substrates, often suffering from inhomogeneous nucleation density and grain size distribution. Here, we first demonstrate a facile vapor-phase synthesis of large-area high-quality 2D layered chalcogenide crystals on liquid metal surface with relatively low surface energy at a growth temperature as low as ∼100 °C. Uniform and large-domain-sized 2D crystals of GaSe and GaxIn1-xSe were grown on liquid metal surface even supported on a polyimide film. As-grown 2D GaSe crystals have been fabricated to flexible photodetectors, showing high photoresponse and excellent flexibility. Our strategy of energy-sustainable low-temperature growth on liquid metal surface may open a route to the synthesis of high-quality 2D crystals of Ga-, In-, Bi-, Hg-, Pb-, or Sn-based chalcogenides and halides.

Interlayer coupling of a direct van der Waals epitaxial MoS2/graphene heterostructure

A triangular MoS₂ monolayer directly synthesized on CVD-G on Pt reveals a strong correlation between each other.