Controlled vapor phase growth of single crystalline, two-dimensional GaSe crystals with high photoresponse

Tunable non-volatile memories based on 2D InSe/h-BN/GaSe heterostructures towards potential multifunctionality

Floating-gate memories based on two-dimensional van der Waal (2D vdW) heterostructures play an important role in the development of next-generation information technology. The diversity of 2D vdW materials and their heterostructures provides flexibility in the design of novel storage architectures. However, 2D InSe/h-BN/GaSe heterostructures are rarely reported in the field of tunable non-volatile memories, probably due to the quality limitation of materials and complex interfaces from stackings. Herein, a floating-gate 2D InSe/h-BN/GaSe memory with high performance and atmosphere stability is demonstrated. It exhibits both a large ON/OFF current ratio of ∼105 and a good extinction ratio of ∼103, with an estimated maximum storage capacity of 5.1 × 1012 cm-2. Moreover, the storage performance can be regulated by optimizing the thickness of the insulating h-BN layer. Different device configurations have been explored to validate the working mechanism. Furthermore, a simulation of biological synaptic behavior is achieved on the same prototype device. The enhanced non-volatile characteristics enable the exploration of the integrated 2D memory and potential multifunctionality.

Tunable Electronic Properties of Two-Dimensional GaSe1-xTex Alloys

In this work, we performed a theoretical study on the electronic properties of monolayer GaSe1-xTex alloys using the first-principles calculations. The substitution of Se by Te results in the modification of a geometric structure, charge redistribution, and bandgap variation. These remarkable effects originate from the complex orbital hybridizations. We demonstrate that the energy bands, the spatial charge density, and the projected density of states (PDOS) of this alloy are strongly dependent on the substituted Te concentration.

Salt-Assisted Selective Growth of H-phase Monolayer VSe2 with Apparent Hole Transport Behavior

Vanadium diselenide (VSe₂) exhibits versatile electronic and magnetic properties in the trigonal prismatic (H-) and octahedral (T-) phases. Compared to the metallic T-phase, the H-phase with a tunable semiconductor property is predicted to be a ferrovalley material with spontaneous valley polarization. Herein we report an epitaxial growth of the monolayer 2D VSe₂ on a mica substrate via the chemical vapor deposition (CVD) method by introducing salt in the precursor. Our first-principles calculations suggest that the monolayer H-phase VSe₂ with a large lateral size is thermodynamically favorable. The honeycomb-like structure and the broken symmetry are directly observed by spherical aberration-corrected scanning transmission electron microscopy (STEM) and confirmed by giant second harmonic generation (SHG) intensity. The p-type transport behavior is further evidenced by the temperature-dependent resistance and field-effect device study. The present work introduces a new phase-stable 2D transition metal dichalcogenide, opening the prospect of novel electronic and spintronics device design.

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.

Suspended-core fiber with embedded GaSe nanosheets for second harmonic generation

We report an all-fiber scheme for the second harmonic generation (SHG) by embedding gallium selenide (GaSe) nanosheets into a suspended-core fiber (SCF). Based on modes analysis and theoretical calculations, the phase-matching modes from multiple optional modes in the SHG process and the optimal SCF length are determined by calculating the effective refractive index and balancing the SHG growth and transmission loss. Due to the long-distance interaction between pumped fundamental mode and GaSe nanosheets around the suspended core, an SHG signal is observed under a milliwatt-level pump light, and exhibits a quadratic growth with the increased pump power. The SHG process is also realized in a broad wavelength range by varying the pump in the range of 1420∼1700 nm. The SCF with the large air cladding and suspended core as an excellent platform can therefore be employed to integrate low-dimensional nonlinear materials, which holds great promise for the applications of all-fiber structures in new light source generating, signal processing and fiber sensing.

Enhanced photoresponse of PVP:GaSe nanocomposite thin film based photodetectors

Two-dimensional materials have become the focus of attention of researchers in recent years. The demand for two-dimensional materials is increasing day by day, especially with the inadequacy of graphene in optical applications. In this context, the optical and electrical characteristics of the PVP:GaSe thin film nanocomposites were investigated. The surface morphologies of the samples were characterized by SEM, the thin film thicknesses and refractive index parameters were measured by the Ellipsometer method, the structural characteristics were obtained by XRD, and Raman and PL spectroscopy was used to determine the optical characteristics. Critical parameters of Au/PVP:GaSe/n-Si photodetector were calculated under various illumination intensities. It is observed that photodetector with PVP:%5GaSe thin film has the best performance results. According to the experimental results, its responsivity, external quantum efficiency, and detectivity values are 0.485 A W^-1, %86, and 1.14 × 10⁷cm Hz^1/2W^-1respectively.

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.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

A pathway toward high-throughput quantum Monte Carlo simulations for alloys: A case study of two-dimensional (2D) GaSxSe1-x

The study of alloys using computational methods has been a difficult task due to the usually unknown stoichiometry and local atomic ordering of the different structures experimentally. In order to combat this, first-principles methods have been coupled with statistical methods such as the cluster expansion formalism in order to construct the energy hull diagram, which helps to determine if an alloyed structure can exist in nature. Traditionally, density functional theory (DFT) has been used in such workflows. In this paper, we propose to use chemically accurate many-body variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) methods to construct the energy hull diagram of an alloy system due to the fact that such methods have a weaker dependence on the starting wavefunction and density functional, scale similarly to DFT with the number of electrons, and have had demonstrated success for a variety of materials. To carry out these simulations in a high-throughput manner, we propose a method called Jastrow sharing, which involves recycling the optimized Jastrow parameters between alloys with different stoichiometries. We show that this eliminates the need for extra VMC Jastrow optimization calculations and results in significant computational cost savings (on average 1/4 savings of total computational time). Since it is a novel post-transition metal chalcogenide alloy series that has been synthesized in its few-layer form, we used monolayer GaS_xSe_1-x as a case study for our workflow. By extensively testing our Jastrow sharing procedure for monolayer GaS_xSe_1-x and quantifying the cost savings, we demonstrate how a pathway toward chemically accurate high-throughput simulations of alloys can be achieved using many-body VMC and DMC methods.

Liquid-Metal-Assisted Growth of Vertical GaSe/MoS2 p-n Heterojunctions for Sensitive Self-Driven Photodetectors

van der Waals (vdW) vertical p-n junctions based on two-dimensional (2D) materials have shown great potential in flexible, self-driven, high-efficiency electronic and optoelectronic applications. However, due to the complex nucleation dynamics, the controllable synthesis of vertical heterostructures remains a daunting challenge. Here, we report the controlled growth of vertical GaSe/MoS₂ p-n heterojunctions via a liquid gallium (Ga)-assisted chemical vapor deposition method. The growth mechanism can be interpreted by theoretical calculations based on the Burton-Cabrera-Frank theory. By analyzing the diffusion barriers and the Ehrlich-Schwoebel barriers of adatoms, we found that the growth modes between vertical and lateral can be precisely switched by means of adjusting the amount of Ga. Based on the achieved high-quality vertical GaSe/MoS₂ p-n heterojunctions, photosensing devices are further designed and systematically investigated. Upon light illumination, prominent photovoltaic effects with large open-circuit voltage (0.61 V) and broadband detection capability from 375 to 633 nm are observed, which can further be employed for self-powered photodetection with high responsivity (900 mA/W) and fast response speed (5 ms). The developed liquid-metal-assisted strategy provides an effective method for controllable synthesis of vdW heterostructures and will give impetus to their applications in high-performance optoelectronic device.

Optoelectronics and Nanophotonics of Vapor-Liquid-Solid Grown GaSe van der Waals Nanoribbons

2D/layered semiconductors are of interest for fundamental studies and for applications in optoelectronics and photonics. Work to date focused on extended crystals, produced by exfoliation or growth and investigated by diffraction-limited spectroscopy. Processes such as vapor-liquid-solid (VLS) growth carry potential for mass-producing nanostructured van der Waals semiconductors with exceptionally high crystal quality and optoelectronic/photonic properties at least on par with those of extended flakes. Here, we demonstrate the synthesis, structure, morphology, and optoelectronics/photonics of GaSe van der Waals nanoribbons obtained by Au- and Ag-catalyzed VLS growth. Although all GaSe ribbons are high-quality basal-plane oriented single crystals, those grown at lower temperatures stand out with their remarkably uniform morphology and low edge roughness. Photoluminescence spectroscopy shows intense, narrow light emission at the GaSe bandgap energy. Nanophotonic experiments demonstrate traveling waveguide modes at visible/near-infrared energies and illustrate approaches for locally exciting and probing such photonic modes by cathodoluminescence in transmission electron microscopy.

Controllable Thin-Film Approaches for Doping and Alloying Transition Metal Dichalcogenides Monolayers

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit exciting properties and versatile material chemistry that are promising for device miniaturization, energy, quantum information science, and optoelectronics. Their outstanding structural stability permits the introduction of various foreign dopants that can modulate their optical and electronic properties and induce phase transitions, thereby adding new functionalities such as magnetism, ferroelectricity, and quantum states. To accelerate their technological readiness, it is essential to develop controllable synthesis and processing techniques to precisely engineer the compositions and phases of 2D TMDs. While most reviews emphasize properties and applications of doped TMDs, here, recent progress on thin-film synthesis and processing techniques that show excellent controllability for substitutional doping of 2D TMDs are reported. These techniques are categorized into bottom-up methods that grow doped samples on substrates directly and top-down methods that use energetic sources to implant dopants into existing 2D crystals. The doped and alloyed variants from Group VI TMDs will be at the center of technical discussions, as they are expected to play essential roles in next-generation optoelectronic applications. Theoretical backgrounds based on first principles calculations will precede the technical discussions to help the reader understand each element's likelihood of substitutional doping and the expected impact on the material properties.

High thermoelectric performance in two dimensional chalcogenides systems: GaSe and GaTe

Among the group-III chalcogenides, the two-dimensional (2D) GaSe and GaTe materials have been synthesized, but recent theoretical studies have raised controversial results regarding their thermoelectric (TE) properties. Hereby, systematically investigated the temperature and carrier concentration dependent TE properties of 2D GaSe and GaTe. We found that the GaSe had an indirect band gap of 2.94 eV while the GaTe had an indirect band gap of 1.88 eV. Both materials had almost the same Seebeck coefficients, but the p-type GaTe had the longest carrier relaxation time. We obtained the largest electrical conductivity over the thermal conductivity ratio in p-type GaTe compared with all other systems. This results in a very high p-type ZT of 0.91. Moreover, this high ZT performance is only changed by approximately 7% in a wide range of temperatures (300-700 K) and carrier concentration (10¹¹-10¹³ hole cm^-2). Compared with previously reported results, we find that it is necessary to consider the carrier relaxation time and spin-orbit coupling effect for determining reliable TE property. Overall, we propose that the p-type GaTe have outstanding TE property, and it can be utilized for potential TE device applications.

Substrate-induced strain in 2D layered GaSe materials grown by molecular beam epitaxy

Two-dimensional (2D) layered GaSe films were grown on GaAs (001), GaN/Sapphire, and Mica substrates by molecular beam epitaxy (MBE). The in situ reflective high-energy electron diffraction monitoring reveals randomly in-plane orientations of nucleated GaSe layers grown on hexagonal GaN/Sapphire and Mica substrates, whereas single-orientation GaSe domain is predominant in the GaSe/GaAs (001) sample. Strong red-shifts in the frequency of in-plane [Formula: see text] vibration modes and bound exciton emissions observed from Raman scattering and photoluminescence spectra in all samples are attributed to the unintentionally biaxial in-plane tensile strains, induced by the dissimilarity of symmetrical surface structure between the 2D-GaSe layers and the substrates during the epitaxial growth. The results in this study provide an important understanding of the MBE-growth process of 2D-GaSe on 2D/3D hybrid-heterostructures and pave the way in strain engineering and optical manipulation of 2D layered GaSe materials for novel optoelectronic integrated technologies.

Two dimensional ruthenium carbide: structural and electronic features

The design and realization of novel 2D materials and their functionalities have been a focus of research inspired by the successful synthesis of graphene and many other 2D materials. In this study, in view of first principles calculations, we predict a novel 2D material ruthenium carbide (RuC) in graphene-like honeycomb hexagonal lattice with planar geometry. Phonon dispersion spectra display a dynamically stable structure. Comprehensive molecular dynamics calculations confirm the stability of the structure up to high temperatures as ≈1000 K. The system is a narrow gap semiconductor with a band gap of 53 meV (345 meV) due to GGA-PBE (HSE) calculations. Band gap exhibits significant changes by applied strain. Elastic and optical properties of the system are examined in monolayer form. RuC/RuC bilayer, RuC/graphene and RuC/h-BN heterostructures are also investigated. By calculating the phonon dispersion it is verified that RuC bilayer is the most stable in AA type-stacking configuration where Ru and C atoms of both layers have identical lateral coordinates. The effects of atomic substitutions on electronic band structures, acting as p-type and n-type doping, are revealed. A novel 3D RuCLi structure is also predicted to be stable and the isolation of its monolayer forms are discussed. Ruthenium carbide, as a 2D material which is dynamically and thermally stable, holds promise for applications in nanoelectronics.

Asymmetric electrode incorporated 2D GeSe for self-biased and efficient photodetection

2D layered germanium selenide (GeSe) with p-type conductivity is incorporated with asymmetric contact electrode of chromium/Gold (Cr/Au) and Palladium/Gold (Pd/Au) to design a self-biased, high speed and an efficient photodetector. The photoresponse under photovoltaic effect is investigated for the wavelengths of light (i.e. ~220, ~530 and ~850 nm). The device exhibited promising figures of merit required for efficient photodetection, specifically the Schottky barrier diode is highly sensitive to NIR light irradiation at zero voltage with good reproducibility, which is promising for the emergency application of fire detection and night vision. The high responsivity, detectivity, normalized photocurrent to dark current ratio (NPDR), noise equivalent power (NEP) and response time for illumination of light (~850 nm) are calculated to be 280 mA/W, 4.1 × 10⁹ Jones, 3 × 10⁷ W⁻¹, 9.1 × 10⁻¹² WHz^−1/2 and 69 ms respectively. The obtained results suggested that p-GeSe is a novel candidate for SBD optoelectronics-based technologies.

Polytypism in few-layer gallium selenide

Gallium selenide (GaSe) is one of the layered group-III metal monochalcogenides, which has an indirect bandgap in the monolayer and a direct bandgap in bulk unlike other conventional transition metal dichalcogenides (TMDs) such as MoX₂ and WX₂ (X = S and Se). Four polytypes of bulk GaSe, designated as β-, ε-, γ-, and δ-GaSe, have been reported. Since different polytypes result in different optical and electrical properties even with the same thickness, identifying the polytype is essential in utilizing this material for various optoelectronic applications. We performed polarized Raman measurements on GaSe and found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even with the same thickness due to different stacking sequences of the polytypes. By comparing the ultra-low-frequency Raman spectra with the theoretical calculations and high-resolution electron microscopy measurements, we established the correlation between the ultra-low-frequency Raman spectra and the stacking sequences of trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.

Two-Dimensional BAs/InTe: A Promising Tandem Solar Cell with High Power Conversion Efficiency

Tandem solar cells (SCs) connecting two subcells with different absorption bands have the potential to reach the commercialized photovoltaic standard. However, the performance improvement of tandem architectures is still a challenge, primarily owing to the mismatch of band gaps in two subcells. Here, we demonstrate a two-dimensional (2D) BAs/InTe-based tandem SC, which could achieve solar-to-electric conversion efficiency higher than 30%. First, the narrow band gap of hexagonal single-layer BX (X = P and As) and wide band gap of single-layer YZ (Y = Ga and In, Z = S, Se, and Te) are found to have high thermodynamic stability based on density functional theory calculations. Next, considering narrow and wide band gaps at the HSE06 functional, single-layer BX/YZ-based tandem SCs are built to effectively capture a broad-band solar spectrum by combining such two subcells. Since the band gap of single-layer BAs matches well with that of the InTe monolayer, the power conversion efficiency of BAs/InTe-based tandem SC can reach as high as 30.2%. Moreover, it is important to note that the used materials, including few-layer GaZ and InSe, have been experimentally prepared, which strongly supports the high feasibility of the designed 2D tandem SCs in this work. Our constructed 2D-material-based devices can be competitive in realizing commercialized high-performance tandem SCs.

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.

Effective Hexagonal Boron Nitride Passivation of Few-Layered InSe and GaSe to Enhance Their Electronic and Optical Properties

Indium selenide (InSe) and gallium selenide (GaSe), members of the III-VI chalcogenide family, are emerging two-dimensional (2D) semiconductors with appealing electronic properties. However, their devices are still lagging behind because of their sensitivity to air and device fabrication processes which induce structural damage and hamper their intrinsic properties. Thus, in order to obtain high-performance and stable devices, effective passivation of these air-sensitive materials is strongly required. Here, we demonstrate a hexagonal boron nitride (hBN)-based encapsulation technique, where 2D layers of InSe and GaSe are covered entirely between two layers of hBN. To fabricate devices out of fully encapsulated 2D layers, we employ the lithography-free via-contacting scheme. We find that hBN acts as an excellent encapsulant and a near-ideal substrate for InSe and GaSe by passivating them from the environment and isolating them from the charge disorder at the SiO₂ surface. As a result, the encapsulated InSe devices are of high quality and ambient-stable for a long time and show an improved two-terminal mobility of 30-120 cm² V^-1 s^-1 as compared to mere ∼1 cm² V^-1 s^-1 for unencapsulated devices. On employing this technique to GaSe, we obtain a strong and reproducible photoresponse. In contrast to previous studies, where either good performance or long-term stability was achieved, we demonstrate a combination of both in our devices. This work thus provides a systematic study of fully encapsulated devices based on InSe and GaSe, which has not been reported until now. We believe that this technique can open ways for fundamental studies as well as toward the integration of these materials in technological applications.

Thermoelectric Performance of Two-Dimensional AlX (X = S, Se, Te): A First-Principles-Based Transport Study

By using the first-principles calculations in combination with the Boltzmann transport theory, we systematically study the thermoelectric properties of AlX (X = S, Se, Te) monolayers as indirect gap semiconductors. The unique electronic density of states, which consists of a rather sharp peak at the valence band maxima and an almost constant band at the conduction band minima, makes AlX (X = S, Se, Te) monolayers excellent thermoelectric materials. The optimized power factors at room temperature are 22.59, 62.59, and 6.79 mW m-1 K-2 under reasonable electronic concentration for AlS, AlSe, and AlTe monolayers, respectively. The figure of merit (zT) increases with temperature and the optimized zT values of 0.52, 0.59, and 0.26 at room temperature are achieved under moderate electronic concentration for AlS, AlSe, and AlTe monolayers, respectively, indicating that two-dimensional layered AlX (X = S, Se, Te) semiconductors, especially AlSe, can be potential candidate matrices for high-performance thermoelectric nanocomposites.

Shear-force exfoliation of indium and gallium chalcogenides for selective gas sensing applications

Layered chalcogenides AIIIBVI of gallium and indium form a group of semiconducting nanomaterials with huge potential in electronic, sensor and energy storage applications. However, the preparation method predetermines the usage of the prepared nanomaterial. In this paper, we investigated shear-force milling exfoliation in a surfactant free water/ethanol mixture on indium and gallium chalcogenides and their utilization in the gas sensing of volatile organic compounds (VOCs). The exfoliation of bulk materials in a surfactant-free environment helped to avoid any surface contamination and allowed the preparation of materials without non-covalently bonded large organic molecules. Furthermore, the gas-sensing properties were evaluated by electrical impedance spectroscopy on VOCs. Our results showed high sensitivity and selectivity towards methanol. This suggests that shear-force milling is an effective method for the exfoliation of indium and gallium chalcogenides which can find application in the selective gas sensing of VOCs.

Predicted high thermoelectric performance in a two-dimensional indium telluride monolayer and its dependence on strain

Group IIIA–VIA monolayers are predicted to exhibit high thermoelectric performance, owing to their low thermal conductance and unique band structures.

Transformation of 2D group-III selenides to ultra-thin nitrides: enabling epitaxy on amorphous substrates

The experimental realization of two-dimensional (2D) gallium nitride (GaN) has enabled the exploration of 2D nitride materials beyond boron nitride. Here we demonstrate one possible pathway to realizing ultra-thin nitride layers through a two-step process involving the synthesis of naturally layered, group-III chalcogenides (GIIIC) and subsequent annealing in ammonia (ammonolysis) that leads to an atomic-exchange of the chalcogen and nitrogen species in the 2D-GIIICs. The effect of nitridation differs for gallium and indium selenide, where gallium selenide undergoes structural changes and eventual formation of ultra-thin GaN, while indium selenide layers are primarily etched rather than transformed by nitridation. Further investigation of the resulting GaN films indicates that ultra-thin GaN layers grown on silicon dioxide act as effective 'seed layers' for the growth of 3D GaN on amorphous substrates.

Recent Advances in Growth of Novel 2D Materials: Beyond Graphene and Transition Metal Dichalcogenides

Since the discovery of graphene just over a decade ago, 2D materials have been a central focus of materials research and engineering because of their unique properties and potential of revealing intriguing new phenomena. In the past few years, transition metal dichalcogenides (TMDs) have also attracted considerable attention because of the intrinsically opened bandgap. The exceptional properties and potential applications of graphene and TMDs have inspired explosive efforts to discover novel 2D materials. Here, emerging novel 2D materials are summarized and recent progress in the preparation, characterization, and application of 2D materials is highlighted. The experimental realization methods for these materials are emphasized, while the large-area growth and controlled patterning for industrial productions are discussed. Finally, the remaining challenges and potential applications of 2D materials are outlined.

Photoresponse of Graphene-Gated Graphene-GaSe Heterojunction Devices

Because of their extraordinary physical properties, low-dimensional materials including graphene and gallium selenide (GaSe) are promising for future electronic and optoelectronic applications, particularly in transparent-flexible photodetectors. Currently, the photodetectors working at the near-infrared spectral range are highly indispensable in optical communications. However, the current photodetector architectures are typically complex, and it is normally difficult to control the architecture parameters. Here, we report graphene-GaSe heterojunction-based field-effect transistors with broadband photodetection from 730-1550 nm. Chemical-vapor-deposited graphene was employed as transparent gate and contact electrodes with tunable resistance, which enables effective photocurrent generation in the heterojunctions. The photoresponsivity was shown from 10 to 0.05 mA/W in the near-infrared region under the gate control. To understand behavior of the transistor, we analyzed the results via simulation performed using a model for the gate-tunable graphene-semiconductor heterojunction where possible Fermi level pinning effect is considered.

Comparative study of structural and electronic properties of GaSe and InSe polytypes

Equilibrium crystal structures, electron band dispersions, and bandgap values of layered GaSe and InSe semiconductors, each being represented by four polytypes, are studied via first-principles calculations within the density functional theory. A number of practical algorithms to take into account dispersion interactions are tested, from empirical Grimme corrections to many-body dispersion schemes. Due to the utmost technical accuracy achieved in the calculations, nearly degenerate energy-volume curves of different polytypes are resolved, and the conclusions concerning the relative stability of competing polytypes drawn. The predictions are done as for how the equilibrium between different polytypes will be shifted under the effect of hydrostatic pressure. The band structures are inspected under the angle of identifying features specific for different polytypes and with respect to modifications of the band dispersions brought about by the use of modified Becke-Johnson (mBJ) scheme for the exchange-correlation potential. As another way to improve the predictions of bandgaps values, hybrid functional calculations according to the HSE06 scheme are performed for the band structures, and the relation with the mBJ results are discussed. Both methods nicely agree with the experimental results and with state-of-the-art GW calculations. Some discrepancies are identified in cases of close competition between the direct and indirect gap (e.g., in GaSe); moreover, the accurate placement of bands revealing relatively localized states is slightly different according to mBJ and HSE06 schemes.

Photoelectric Detectors Based on Inorganic p-Type Semiconductor Materials

Photoelectric detectors are the central part of modern photodetection systems with numerous commercial and scientific applications. p-Type semiconductor materials play important roles in optoelectronic devices. Photodetectors based on p-type semiconductor materials have attracted a great deal of attention in recent years because of their unique properties. Here, a comprehensive summary of the recent progress mainly on photodetectors based on inorganic p-type semiconductor materials is presented. Various structures, including photoconductors, phototransistors, homojunctions, heterojunctions, p-i-n junctions, and metal-semiconductor junctions of photodetectors based on inorganic p-type semiconductor materials, are discussed and summarized. Perspectives and an outlook, highlighting the promising future directions of this research field, are also given.

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.

Multifunctional Binary Monolayers Ge xP y: Tunable Band Gap, Ferromagnetism, and Photocatalyst for Water Splitting

The most stable structures of two-dimensional Ge _xP _y and Ge _xAs _y monolayers with different stoichiometries (e.g., GeP, GeP₂, and GeP₃) are explored systematically through the combination of the particle-swarm optimization technique and density functional theory optimization. For GeP₃, we show that the newly predicted most stable C2/ m structure is 0.16 eV/atom lower in energy than the state-of-the-art P3̅m1 structure reported previously ( Nano Lett. 2017, 17, 1833). The computed electronic band structures suggest that all the stable and metastable monolayers of Ge _xP _y are semiconductors with highly tunable band gaps under the biaxial strain, allowing strain engineering of their band gaps within nearly the whole visible-light range. More interestingly, the hole doping can convert the C2/ m GeP₃ monolayer from nonmagnetic to ferromagnetic because of its unique valence band structure. For the GeP₂ monolayer, the predicted most stable Pmc2₁ structure is a (quasi) direct-gap semiconductor that possesses a high electron mobility of ∼800 cm² V^-1 s^-1 along the k _a direction, which is much higher than that of MoS₂ (∼200 cm² V^-1 s^-1). More importantly, the Pmc2₁ GeP₂ monolayer not only can serve as an n-type channel material in field-effect transistors but also can be an effective catalyst for splitting water.

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).

High-Performance All 2D-Layered Tin Disulfide: Graphene Photodetecting Transistors with Thickness-Controlled Interface Dynamics

Tin disulfide crystals with layered two-dimensional (2D) sheets are grown by chemical vapor deposition using a novel precursor approach and integrated into all 2D transistors with graphene (Gr) electrodes. The Gr:SnS₂:Gr transistors exhibit excellent photodetector response with high detectivity and photoresponsivity. We show that the response of the all 2D photodetectors depends upon charge trapping at the interface and the Schottky barrier modulation. The thickness-dependent SnS₂ measurements in devices reveal a transition from the interface-dominated response for thin crystals to bulklike response for the thicker SnS₂ crystals, showing the sensitivity of devices fabricated using layered materials on the number of layers. These results show that SnS₂ has photosensing performance when combined with Gr electrodes that is comparable to other 2D transition metal dichalcogenides of MoS₂ and WS₂.

Band Structure of the IV-VI Black Phosphorus Analog and Thermoelectric SnSe

The success of black phosphorus in fast electronic and photonic devices is hindered by its rapid degradation in the presence of oxygen. Orthorhombic tin selenide is a representative of group IV-VI binary compounds that are robust and isoelectronic and share the same structure with black phosphorus. We measure the band structure of SnSe and find highly anisotropic valence bands that form several valleys having fast dispersion within the layers and negligible dispersion across. This is exactly the band structure desired for efficient thermoelectric generation where SnSe has shown great promise.

Orbital Symmetry and the Optical Response of Single-Layer MX Monochalcogenides

We show that the absorption spectra of single-layer GaSe and GaTe in the hexagonal phase feature exciton peaks with distinct polarization selectivity. We investigate these distinct features from first-principles calculations using the GW-BSE formalism. We show that, because of the symmetry of the bands under in-plane mirror symmetry, the bound exciton states selectively couple to either in-plane or out-of-plane polarization of the light. In particular, for a p-polarized light absorption experiment, the absorption peaks of the hydrogenic s-like excitons emerge at large angle of incidence, while the overall absorbance reduces over the rest of the spectrum.

High-Quality GaSe Single Crystal Grown by the Bridgman Method

A high-quality GaSe single crystal was grown by the Bridgman method. The X-ray rocking curve for the studied GaSe sample is symmetric and the Full Width at Half Maximum (FWHM) is only 46 arcs, which is the smallest value ever reported for GaSe crystals. The IR-transmittance is about 66% in the range from 500 to 4000 cm⁻¹. The photoluminescence spectrum at 9.2 K shows a symmetric and sharp excition peak in 2.1046 eV. The results indicate that the as-grown GaSe crystal is of high crystalline quality. The as-grown ε-GaSe crystal has a p-type conductance with the resistivity of 10³ Ω/cm, and the Hall mobility is ~25 cm² V⁻¹ s⁻¹. Few-layer GaSe crystals were prepared through mechanical exfoliation from this high-quality crystal sample. Few-layer GaSe-based photodetectors were fabricated, which exhibit an on/off ratio of 10⁴, a field-effect differential mobility of 0.4 cm² V⁻¹ s⁻¹, and have a fast response time less than 60 ms under light illumination.

Tuning Schottky barriers for monolayer GaSe FETs by exploiting a weak Fermi level pinning effect

Monolayer gallium selenide (GaSe), an emerging two-dimensional semiconductor, holds great promise for electronics and optoelectronics.

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.

Hybrid functional calculations of electronic and thermoelectric properties of GaS, GaSe, and GaTe monolayers

We have investigated the structural, electronic and thermoelectric properties of GaS, GaSe and GaTe monolayers based on the first-principles approach by using density functional theory and the semi-classical Boltzmann transport equation.

Ab initio phonon thermal transport in monolayer InSe, GaSe, GaS, and alloys

We compare vibrational properties and phonon thermal conductivities (κ) of monolayer InSe, GaSe, and GaS systems using density functional theory and Peierls-Boltzmann transport methods. In going from InSe to GaSe to GaS, system mass decreases giving both increasing acoustic phonon velocities and decreasing scattering of these heat-carrying modes with optic phonons, ultimately giving [Formula: see text]. This behavior is demonstrated by correlating the scattering phase space limited by fundamental conservation conditions with mode scattering rates and phonon dispersions for each material. We also show that, unlike flat monolayer systems such as graphene, in InSe, GaSe and GaS thermal transport is governed by in-plane vibrations. Alloying of InSe, GaSe, and GaS systems provides an effective method for modulating their κ through intrinsic vibrational modifications and phonon scattering from mass disorder giving reductions ∼2-3.5 times. This disorder also suppresses phonon mean free paths in the alloy systems compared to those in their crystalline counterparts. This work provides fundamental insights of lattice thermal transport from basic vibrational properties for an interesting set of two-dimensional materials.

High-quality monolayer superconductor NbSe2 grown by chemical vapour deposition

The discovery of monolayer superconductors bears consequences for both fundamental physics and device applications. Currently, the growth of superconducting monolayers can only occur under ultrahigh vacuum and on specific lattice-matched or dangling bond-free substrates, to minimize environment- and substrate-induced disorders/defects. Such severe growth requirements limit the exploration of novel two-dimensional superconductivity and related nanodevices. Here we demonstrate the experimental realization of superconductivity in a chemical vapour deposition grown monolayer material—NbSe₂. Atomic-resolution scanning transmission electron microscope imaging reveals the atomic structure of the intrinsic point defects and grain boundaries in monolayer NbSe₂, and confirms the low defect concentration in our high-quality film, which is the key to two-dimensional superconductivity. By using monolayer chemical vapour deposited graphene as a protective capping layer, thickness-dependent superconducting properties are observed in as-grown NbSe₂ with a transition temperature increasing from 1.0 K in monolayer to 4.56 K in 10-layer.

Two-dimensional superconductors will likely have applications not only in devices, but also in the study of fundamental physics. Here, Wang et al. demonstrate the CVD growth of superconducting NbSe2 on a variety of substrates, making these novel materials increasingly accessible.

Quantum size confinement in gallium selenide nanosheets: band gap tunability versus stability limitation

Gallium selenide is one of the most promising candidates to extend the window of band gap values provided by existing two-dimensional semiconductors deep into the visible potentially reaching the ultraviolet. However, the tunability of its band gap by means of quantum confinement effects is still unknown, probably due to poor nanosheet stability. Here, we demonstrate that the optical band gap band of GaSe nanosheets can be tuned by ∼120 meV from bulk to 8 nm thick. The luminescent response of very thin nanosheets (<8 nm) is strongly quenched due to early oxidation. Oxidation favors the emergence of sharp material nanospikes at the surface attributable to strain relaxation. Simultaneously, incorporated oxygen progressively replaces selenium giving rise to Ga₂O_3, with a residual presence of Ga₂Se₃ that tends to desorb. These results are relevant for the development and design of visible/ultraviolet electronics and optoelectronics with tunable functionalities based on atomically thin GaSe.

Thermoelectric and phonon transport properties of two-dimensional IV-VI compounds

We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures, Seebeck coefficients, electrical conductivities, lattice thermal conductivities, and figures of merit of these two-dimensional materials, which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT = 2.63, 2.46), SnS (ZT = 1.75, 1.88), GeSe (ZT = 1.99, 1.73), and GeS (ZT = 1.85, 1.29) at 700 K along armchair and zigzag directions, respectively. Phonon dispersion calculations confirm the dynamical stability of these compounds. The calculated lattice thermal conductivities are low while the electrical conductivities and Seebeck coefficients are high. Thus, the properties of the monolayers show high potential toward thermoelectric applications.