InSe: a two-dimensional material with strong interlayer coupling

Realizing Ultrafast Response Speed for Self-Powered Photodetectors with a Molecular-Doped Lateral InSe Homojunction

The implementation of energy-saving policies has stimulated intensive interest in exploring self-powered optoelectronic devices. The 2D p-n homojunction exhibits effective generation and separation of carriers excited by light, realizing lower power consumption and higher performance photodetectors. Here, a self-powered photodetector with high performance is fabricated based on an F4-TCNQ localized molecular-doped lateral InSe homojunction. Compared with the intrinsic InSe photodetector, the switching light ratio (Ilight/Idark) of the p-n homojunction device can be enhanced by 2.2 × 104, and the temporal response is also dramatically improved to 24/30 μs. Benefiting from the built-in electric field, due to the formation of an InSe p-n homojunction after partial doping of F4-TCNQ on InSe, the device possesses a high responsivity (R) of 93.21 mA/W, with a specific detectivity (D*) of 1.14 × 1011 Jones. These results suggest a promising approach to get a lateral InSe p-n homojunction and reveal the potential application of the device for next generation low-consumption photodetectors.

Angle-Dependent Raman Spectra of Crystal Polymorphs of GaO: A Computational Study

Two crystal polymorphs of GaO consisting of GaO-H and GaO-T monolayers are proposed in this study. Based on the density functional theory calculations, the phonon dispersion demonstrates that both GaO-H and GaO-T monolayers could be stable. The band gaps of GaO-H and GaO-T monolayers are 1.51 and 1.43 eV, respectively. When an external electric field is applied, the band gaps of GaO monolayers are reduced dramatically, down to 0.13 eV with the field of 0.7 V/Å. Because of the decreased symmetry of C3v under an external electric field, more peaks of Raman spectra can be obtained. The angle-dependent Raman spectra ofA ' 1 1 ${{\rm{A}}{{^\prime}}_1^1 }$ andA ' 1 2 ${{\rm{A}}{{^\prime}}_1^2 }$ of GaO-H monolayer, andA 1 g 1 ${{\rm{A}}_{1{\rm{g}}}^1 }$ andA 1 g 2 ${{\rm{A}}_{1{\rm{g}}}^2 }$ of GaO-T monolayer are discussed seperately, with the incident lasers of 488 and 532 nm. Additionally, the Raman intensity distribution shows that the incident light should be parallel to the plane of the GaO monolayer to obtain more comparable Raman spectra. These investigations of the crystal polymorphs of GaO monolayers may guide the experimental investigations of GaO monolayers and potential optoelectronic applications.

Allotropic Ga2Se3/GaSe nanostructures grown by van der Waals epitaxy: narrow exciton lines and single-photon emission

The ability to emit narrow exciton lines, preferably with a clearly defined polarization, is one of the key conditions for the use of nanostructures based on III-VI monochalcogenides and other layered crystals in quantum technology to create non-classical light. Currently, the main method of their formation is exfoliation followed by strain and defect engineering. A factor limiting the use of epitaxy is the presence of different phases in the grown films. In this work, we show that control over their formation makes it possible to create structures with the desired properties. We propose Ga2Se3/GaSe nanostructures grown by van der Waals epitaxy with a high VI/III flux ratio as a source of narrow exciton lines. Actually, these nanostructures are a combination of allotropes: GaSe and Ga2Se3, consisting of the same atoms in different arrangements. The energy positions of the narrow lines are determined by the quantum confinement in Ga2Se3 inclusions of different sizes in the GaSe matrix, similar to quantum dots, and their linear polarization is due to the ordering of Ga vacancies in a certain crystalline direction in Ga2Se3. Such nanostructures exhibit single-photon emission with second-order correlation function g(2)(0) ∼ 0.10 at 10 K that makes them promising for quantum technologies.

Revealing the origin of PL evolution of InSe flake induced by laser irradiation

Two-dimensional InSe has been considered as a promising candidate for novel optoelectronic devices owing to large electron mobility and a near-infrared optical band gap. However, its widespread applications suffer from environmental instability. A lot of theoretical studies on the degradation mechanism of InSe have been reported whereas the experimental proofs are few. Meanwhile, the role of the extrinsic environment is still obscure during the degradation. As a common technique of studying the degradation mechanism of 2D materials, laser irradiation exhibits many unique advantages, such as being fast, convenient, and offering in situ compatibility. Here, we have developed a laser-treated method, which involves performing repeated measurements at the same point while monitoring the evolution of the resulting PL, to systematically study the photo-induced degradation process of InSe. Interestingly, we observe different evolution behavior of PL intensity under weak irradiation and strong irradiation. Our experimental results indicate the vacancy passivation and degrading effect simultaneously occurring in InSe under a weak laser irradiation, resulting in the PL increasing first and then decreasing during the measurement. Meanwhile we also notice that the passivation has a stronger effect on the PL than the degrading effect of weak oxidation. In contrast, under a strong laser irradiation, the InSe suffers serious destruction caused by excess heating and intense oxidation. This leads to a direct decrease of PL and corresponding oxidative products. Our work provides a reliable experimental supplement to the photo oxidation study of InSe and opens up a new avenue to regulate the PL of InSe.

Carrier and phonon transport in 2D InSe and its Janus structures

Recently, two-dimensional (2D) Indium Selenide (InSe) has been receiving much attention in the scientific community due to its reduced size, extraordinary physical properties, and potential applications in various fields. In this review, we discussed the recent research advancement in the carrier and phonon transport properties of 2D InSe and its related Janus structures. We first introduced the progress in the synthesis of 2D InSe. We summarized the recent experimental and theoretical works on the carrier mobility, thermal conductivity, and thermoelectric characteristics of 2D InSe. Based on the Boltzmann transport equation (BTE), the mechanisms underlying carrier or phonon scattering of 2D InSe were discussed in detail. Moreover, the structural and transport properties of Janus structures based on InSe were also presented, with an emphasis on the theoretical simulations. At last, we discussed the prospects for continued research of 2D InSe.

Room-Temperature Deep-UV Photoluminescence from Low-Dimensional Hexagonal Boron Nitride Prepared Using a Facile Synthesis

Identification and evaluation of defect levels in low-dimensional materials is an important aspect in quantum science. In this article, we report a facile synthesis method of low-dimensional hexagonal boron nitride (h-BN) and study light emission characteristics due to the defects. The thermal annealing procedure is optimized to obtain clean multilayered h-BN as revealed by transmission electron microscopy. UV-vis spectroscopy shows the optical energy gap of 5.28 eV, which is comparable to the reported energy gap for exfoliated, clean h-BN samples. X-ray photoelectron spectroscopy reveals the location of the valence band edge at 2 eV. The optimized synthesis route of h-BN generates two kinds of defects, which are characterized using room-temperature photoluminescence (PL) measurements. The defects emit light at 4.18 eV [deep-UV (DUV)] and 3.44 eV (UV) photons. The intensity of PL has an oscillatory dependence on the excitation energy for the defect emitting DUV light. A series of spectral lines are observed with the energy ranging between 2.56 and 3.44 eV. The average peak-to-peak energy separation is about 125 meV. The locations of the spectral lines can be modeled using Franck-Condon-type transition and associated with displaced harmonic oscillator approximation. Our facile route gives an easier approach to prepare clean h-BN, which is essential for classical two-dimensional material-based electronics and single-photon-based quantum devices.

Nearly Ideal Two-Dimensional Electron Gas Hosted by Multiple Quantized Kronig-Penney States Observed in Few-Layer InSe

A theoretical ideal two-dimensional electron gas (2DEG) was characterized by a flat density of states independent of energy. Compared with conventional two-dimensional free-electron systems in semiconductor heterojunctions and noble metal surfaces, we report here the achievement of ideal 2DEG with multiple quantized states in few-layer InSe films. The multiple quantum well states (QWSs) in few-layer InSe films are found, and the number of QWSs is strictly equal to the number of atomic layers. The multiple stair-like DOS as well as multiple bands with parabolic dispersion both characterize ideal 2DEG features in these QWSs. Density functional theory calculations and numerical simulations based on quasi-bounded square potential wells described as the Kronig-Penney model provide a consistent explanation of 2DEG in the QWSs. Our work demonstrates that 2D van der Waals materials are ideal systems for realizing 2DEG hosted by multiple quantized Kronig-Penney states, and the semiconducting nature of the material provides a better chance for construction of high-performance electronic devices utilizing these states, for example, superlattice devices with negative differential resistance.

Electronic property modulation in two-dimensional lateral superlattices of monolayer transition metal dichalcogenides

Fabricating lateral heterostructures (HSs) and superlattices (SLs) provides a unique degree of freedom for modulating the physical properties of two-dimensional (2D) materials by varying the chemical component, geometric size and interface structure in the ultra-thin atomic thickness limit. While a variety of 2D lateral HSs/SLs have been synthesized, especially for transition metal dichalcogenides (TMDs), how such structures affect quantitatively the physical properties of 2D materials has not yet been established. We herein explore electronic property modulation in 2D lateral SLs of monolayer TMDs through first-principles high-throughput calculations. The dependence of the electronic structure, bandgap, carrier effective masses, charge density overlap on chemical components, interface type, and sub-lattice size of lateral TMD-SLs are investigated. We find that by comparison with their random alloy counterparts, the lateral TMD-SLs exhibit generally type-II band alignment, a wider range of bandgap tunability, larger carrier effective masses, and stronger electron-hole charge separation tendency. The bandgap variation with a sub-lattice size shows larger bowing parameters for the SLs with heterogeneous anions, by comparison with the homogeneous anion cases. A similar behavior is observed for the SLs with an armchair-type interface, by comparison with the zigzag-type interface cases. Further analyses reveal that the underlying physical mechanism can be attributed to the synergistic interplay among the band offset of sub-lattices, quantum confinement effect, and existing internal strain.

Layer-dependent nonlinear optical properties of two-dimensional InSe and its applications in waveguide lasers

The thickness-dependent third-order nonlinear optical properties of two-dimensional β-InSe and its potential applications as a saturable absorber in pulsed laser generation are investigated. InSe sheets with different layers are prepared by the chemical vapor deposition. Using open-aperture femtosecond Z-scan technique at 1030 nm, the modulation depth and nonlinear absorption coefficient are obtained to be 36% and -1.6 × 10⁴ cm·GW^-1, respectively. The intrinsic mechanism of the layer-dependent energy band structure evolution is analyzed based on density functional theory, and the theoretical analysis is consistent with the experimental results. Based on a waveguide cavity, a Q-switched mode-locked laser at 1 µm with a repetition frequency of 8.51 GHz and a pulse duration of 28 ps is achieved by utilizing the layered InSe as a saturable absorber. This work provides an in-depth understanding of layer-dependent properties of InSe and extends its applications in laser technology for compact light devices.

Microscopic optical nonlinearities and transient carrier dynamics in indium selenide nanosheet

This work systematically investigates the third-order nonlinear optical (NLO) properties and ultrafast carrier dynamics of layered indium selenide (InSe) obtained by mechanical exfoliation (ME). The two-photon absorption (TPA) effect of layered InSe was tested using micro-Z/I-scan techniques. The results indicate that InSe flakes undergo the TPA response under the excitation of both 520 nm and 1040 nm fs pulses, and that InSe is more likely to achieve TPA saturation under visible light excitation. Furthermore, ultrafast carrier dynamics revealed that InSe flakes in the visible region undergo a transition from photoinduced absorption to photobleaching and exhibit a fast recombination time of ∼0.4-1ps, suggesting a high optical modulation speed as high as ∼1-2.5 THz.

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

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

Interlayer Quasi-Bonding Interactions in 2D Layered Materials: A Classification According to the Occupancy of Involved Energy Bands

Recent studies have revealed that the interlayer interaction in two-dimensional (2D) layered materials is not simply of van der Waals character but could coexist with quasi-bonding character. Herein, we classify the interlayer quasi-bonding interactions into two main categories (I: homo-occupancy interaction; II: hetero-occupancy interaction) according to the occupancy of the involved energy bands near the Fermi level. We then investigate the quasi-bonding-interaction-induced band structure evolution of several representative 2D materials based on density functional theory calculations. Further calculations confirm that this classification is applicable to generic 2D layered materials and provide a unified understanding of the total strength of interlayer interaction, which is a synergetic effect of the van der Waals attraction and the quasi-bonding interaction. The latter is stabilizing in main category II and destabilizing in main category I. Thus, the total interlayer interaction strength is relatively stronger in category II and weaker in category I.

Metal Halide Semiconductors beyond Lead-Based Perovskites for Promising Optoelectronic Applications

In recent decades, metal halide semiconductors represented by lead-based halide perovskites have shown broad potential in optoelectronic applications. This family of semiconductors differs from traditional tetrahedral semiconductors in crystalline structure, chemical bonding, electronic-structure features, optoelectronic properties, as well as material fabrication method. At present, difficulties arising from both intrinsic material properties (including Pb toxicity and long-term stability) and technological aspects hinder their large-scale commercialization. In this Perspective, we focus on up-and-coming lead-free metal halide semiconductors toward high-performance optoelectronic applications. We start by outlining the advantages of metal halide semiconductors and their physical and chemical underpinnings. We then review composition and structure, electronic structure, optoelectronic properties, and device applications according to classification into three material categories, i.e., three-dimensional halide perovskites, low-dimensional perovskites and perovskite-like materials, and materials beyond perovskites. We conclude with an outlook on the challenges and opportunities of metal halide semiconductors and the future development of the field.

Alternative Lone-Pair ns2 -Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications

Lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb²⁺ cation with a lone-pair 6s² electronic configuration embedded in a mixed covalent-ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors containing lone-pair ns² cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We start with the physical underpinning of how the ns² cations within the material lattice allow for superior optoelectronic properties. We then review the emerging Pb-free semiconductors containing ns² cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.

Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas

Atomically thin indium selenide (InSe) is a representative two-dimensional (2D) family that have recently attracted extensive interest for their intriguing emerging physics and potential optoelectronic applications with high-performance. Here, by utilizing molecular beam epitaxy and scanning tunneling microscopy, we report a controlled synthesis of InSe thin films down to the monolayer limit and characterization of their electronic properties at atomic scale. Highly versatile growth conditions are developed to fabricate well crystalline InSe films, with a reversible and controllable phase transformation between InSe and In₂Se₃. The band gap size of InSe films, as enhanced by quantum confinement, increases with decreasing film thickness. Near various categories of lattice imperfections, the band gap becomes significantly enlarged, resulting in a type-I band alignments for lateral heterojunctions. Such band gap enhancement, as unveiled from our first-principles calculations, is ascribed to the local compressive strain imposed by the lattice imperfections. Moreover, InSe films host highly conductive 2D electron gas, manifesting prominent quasiparticle scattering signatures. The 2D electron gas is self-formed via substrate doping of electrons, which shift the Fermi level above the confinement-quantized conduction band. Our study identifies InSe ultrathin film as an appealing system for both fundamental research and potential applications in nanoelectrics and optoelectronics.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Intercalated architecture of MA2Z4 family layered van der Waals materials with emerging topological, magnetic and superconducting properties

The search for new two-dimensional monolayers with diverse electronic properties has attracted growing interest in recent years. Here, we present an approach to construct MA2Z4 monolayers with a septuple-atomic-layer structure, that is, intercalating a MoS2-type monolayer MZ2 into an InSe-type monolayer A2Z2. We illustrate this unique strategy by means of first-principles calculations, which not only reproduce the structures of MoSi2N4 and MnBi2Te4 that were already experimentally synthesized, but also predict 72 compounds that are thermodynamically and dynamically stable. Such an intercalated architecture significantly reconstructs the band structures of the constituents MZ2 and A2Z2, leading to diverse electronic properties for MA2Z4, which can be classified according to the total number of valence electrons. The systems with 32 and 34 valence electrons are mostly semiconductors. Whereas, those with 33 valence electrons can be nonmagnetic metals or ferromagnetic semiconductors. In particular, we find that, among the predicted compounds, (Ca,Sr)Ga2Te4 are topologically nontrivial by both the standard density functional theory and hybrid functional calculations. While VSi2P4 is a ferromagnetic semiconductor and TaSi2N4 is a type-I Ising superconductor. Moreover, WSi2P4 is a direct gap semiconductor with peculiar spin-valley properties, which are robust against interlayer interactions. Our study thus provides an effective way of designing septuple-atomic-layer MA2Z4 with unusual electronic properties to draw immediate experimental interest.

Bias-controlled multi-functional transport properties of InSe/BP van der Waals heterostructures

Van der Waals (vdW) heterostructures, consisting of a variety of low-dimensional materials, have great potential use in the design of a wide range of functional devices thanks to their atomically thin body and strong electrostatic tunability. Here, we demonstrate multi-functional indium selenide (InSe)/black phosphorous (BP) heterostructures encapsulated by hexagonal boron nitride. At a positive drain bias (V_D), applied on the BP while the InSe is grounded, our heterostructures show an intermediate gate voltage (V_BG) regime where the current hardly changes, working as a ternary transistor. By contrast, at a negative V_D, the device shows strong negative differential transconductance characteristics; the peak current increases up to ~5 μA and the peak-to-valley current ratio reaches 1600 at V_D = −2 V. Four-terminal measurements were performed on each layer, allowing us to separate the contributions of contact resistances and channel resistance. Moreover, multiple devices with different device structures and contacts were investigated, providing insight into the operation principle and performance optimization. We systematically investigated the influence of contact resistances, heterojunction resistance, channel resistance, and the thickness of BP on the detailed operational characteristics at different V_D and V_BG regimes.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Atomically thin photoanode of InSe/graphene heterostructure

Achieving high-efficiency photoelectrochemical water splitting requires a better understanding of ion kinetics, e.g., diffusion, adsorption and reactions, near the photoelectrode's surface. However, with macroscopic three-dimensional electrodes, it is often difficult to disentangle the contributions of surface effects to the total photocurrent from that of various factors in the bulk. Here, we report a photoanode made from a InSe crystal monolayer that is encapsulated with monolayer graphene to ensure high stability. We choose InSe among other photoresponsive two-dimensional (2D) materials because of its unique properties of high mobility and strongly suppressing electron-hole pair recombination. Using the atomically thin electrodes, we obtained a photocurrent with a density >10 mA cm^-2 at 1.23 V versus reversible hydrogen electrode, which is several orders of magnitude greater than other 2D photoelectrodes. In addition to the outstanding characteristics of InSe, we attribute the enhanced photocurrent to the strong coupling between the hydroxide ions and photo-generated holes near the anode surface. As a result, a persistent current even after illumination ceased was also observed due to the presence of ions trapped holes with suppressed electron-hole recombination. Our results provide atomically thin materials as a platform for investigating ion kinetics at the electrode surface and shed light on developing next-generation photoelectrodes with high efficiency.

Dipole control of Rashba spin splitting in a type-II Sb/InSe van der Waals heterostructure

InSe monolayer, belonging to group III-VI chalcogenide family, has shown promising performance in the realm of spintronic. Nevertheless, the out-of-plane mirror symmetry in InSe monolayer constrains the electrons' degrees of freedom, and this will confine its spin-related applications. Herein, we construct Sb/InSe van der Waals heterostructure to extend the electronic and spintronic properties of InSe. The density functional theory is utilized to verify the tunable electronic properties and Rashba spin splitting (RSS) of Sb/InSe heterostructure. According to the obtained results, the Sb/InSe heterostructure can be considered as a direct band gap semiconductor with typical type-II band alignment, where the electrons and holes are localized in the InSe and Sb layers, respectively. The RSS is recognized at conduction band minimum around Γ point in Sb/InSe, which is induced by the spontaneous internal electric field with electric dipole moment of 0.016 e Å from Sb to InSe. The vertical strain, in-plane strain, and external electric field are employed to modulate the strength of RSS. The Rashba coefficient and dipole moment exhibit the similar variation tendency, suggesting the strength of RSS depends on the magnitude of dipole moment. The controllable RSS makes Sb/InSe heterostructure become an appropriate candidate material for spintronic devices.

Lattice Thermal Transport in Monolayer Group 13 Monochalcogenides MX (M = Ga, In; X = S, Se, Te): Interplay of Atomic Mass, Harmonicity, and Lone-Pair-Induced Anharmonicity

We perform a systematic study of the lattice dynamics and the lattice thermal conductivity, κ, of monolayer group 13 monochalcogenides MX (M = Ga, In; X = S, Se, Te) by combining an iterative solution for linearized phonon Boltzmann transport equation and density functional theory. Among the competing factors influencing κ, harmonic parameters along with the atomic masses dominate over anharmonicity. An increase in atomic mass leads to a decrease in phonon frequencies and phonon group velocities and consequently in κ. At T = 300 K, the calculated κ values are 54.9, 48.1, 44.3, 25.0, 22.3, and 17.3 W m^-1 K^-1 for GaS, InS, GaSe, InSe, GaTe, and InTe monolayers, respectively. Further analysis of anharmonic scattering rates and average scattering matrix elements evidences that the anharmonicity characterized by the third-order IFCs in GaS and InS are the largest among all monolayer group 13 monochalcogenides despite the largest κ values. This is attributed to a strong interaction between nonbonding lone-pair s electrons around the S atom and adjacent bonding electrons. In addition, the κ of these monolayers further reduces to 50% for sample sizes 300-400 nm. Our findings provide fundamental insights into thermal transport in monolayer group 13 monochalcogenides and should stimulate further experimental exploration of thermal transport in these materials for possible theromoelectric and thermal management applications.

New Polymorphs of 2D Indium Selenide with Enhanced Electronic Properties

The 2D semiconductor indium selenide (InSe) has attracted significant interest due its unique electronic band structure, high electron mobility, and wide tunability of its band gap energy achieved by varying the layer thickness. All these features make 2D InSe a potential candidate for advanced electronic and optoelectronic applications. Here, the discovery of new polymorphs of InSe with enhanced electronic properties is reported. Using a global structure search that combines artificial swarm intelligence with first-principles energetic calculations, polymorphs that consist of a centrosymmetric monolayer belonging to the point group D 3d are identified, distinct from well-known polymorphs based on the D 3h monolayers that lack inversion symmetry. The new polymorphs are thermodynamically and kinetically stable, and exhibit a wider optical spectral response and larger electron mobilities compared to the known polymorphs. Opportunities to synthesize these newly discovered polymorphs and viable routes to identify them by X-ray diffraction, Raman spectroscopy, and second harmonic generation experiments are discussed.

Ultrafast and Sensitive Self-Powered Photodetector Featuring Self-Limited Depletion Region and Fully Depleted Channel with van der Waals Contacts

Self-powered photodetectors with great potential for implanted medical diagnosis and smart communications have been severely hindered by the difficulty of simultaneously achieving high sensitivity and fast response speed. Here, we report an ultrafast and highly sensitive self-powered photodetector based on two-dimensional (2D) InSe, which is achieved by applying a device architecture design and generating ideal Schottky or ohmic contacts on 2D layered semiconductors, which are difficult to realize in the conventional semiconductors owing to their surface Fermi-level pinning. The as-fabricated InSe photodiode features a maximal lateral self-limited depletion region and a vertical fully depleted channel. It exhibits a high detectivity of 1.26 × 10¹³ Jones and an ultrafast response speed of ∼200 ns, which breaks the response speed limit of reported self-powered photodetectors based on 2D semiconductors. The high sensitivity is achieved by an ultralow dark current noise generated from the robust van der Waals (vdW) Schottky junction and a high photoresponsivity due to the formation of a maximal lateral self-limited depletion region. The ultrafast response time is dominated by the fast carrier drift driven by a strong built-in electric field in the vertical fully depleted channel. This device architecture can help us to design high-performance photodetectors utilizing vdW layered semiconductors.

An Anomalous Magneto-Optic Effect in Epitaxial Indium Selenide Layers

Single-phonon modes offer potential applications in quantum phonon optics, but the phonon density of states of most materials consist of mixed contributions from coupled phonons. Here, using theoretical calculations and magneto-Raman measurements, we report two single-phonon vibration modes originating from the breathing and opposite out-of-plane vibrations of InSe layers. These single-phonon vibrations exhibit an anticorrelated scattering rotations of the polarization axis under an applied vertical magnetic field; such an anomalous magneto-optical behavior is due to the reverse bond polarizations of two quantum atomic vibrations, which induce different symmetry for the corresponding Raman selection rules. A 180° (+90° and -90°) integrated scattering rotation angle of two single-phonon modes was achieved when the magnetic field was swept from 0 to 6 T. This work demonstrates new ways to manipulate the magneto-optic effect through phonon polarity-based symmetry control and opens avenues for exploring single-phonon-vibration-based nanomechanical oscillators and magneto-phonon-coupled physics.

Electric field control of Rashba spin splitting in 2D NIIIXVI (N = Ga, In; X = S, Se, Te) monolayer

Spin splitting of the nonmagnetic two dimensional (2D) layered N^IIIX^VI (N  =  Ga, In; X  =  S, Se, Te) monolayer is investigated based on the density functional theory. Due to the mirror symmetry, there is no Rashba spin splitting (RSS) in the freestanding NX plane. It is found that applying the external electric field perpendicular to the NX plane can result in sizable RSS around the Γ point due to the mirror symmetry breaking. The induced RSS is mainly influenced by the anions X and gradually strengthens with the increase of external electric field. The considerable RSS is observed in NTe systems. Moreover, the influence of in-plane biaxial strain on RSS is explored, and the tensile strain can enhance the RSS, especially for those bands around the Fermi level. Our theoretical investigation provides a deep insight in spin splitting behaviors in NX monolayers and agrees well with the experimental report.

Diverse electronic properties of 2D layered Se-containing materials composed of quasi-1D atomic chains

The two-dimensional (2D) atomically thin layered materials have attracted significant attention for constructing next-generation integrated electronic and optoelectronic devices. A special class of 2D materials composed of quasi one-dimensional (1D) atomic chains that show intriguing properties are less studied. Here, two Se-containing 2D layered materials α-Se and Sb₂Se₃ that have quasi-1D atomic chains are investigated via first-principles electronic structure calculations. Results shows that the electronic properties of n-monolayers (n-MLs) stacked α-Se and Sb₂Se₃ exhibit distinct layer-dependence electronic properties. The band gap of 2D α-Se remarkably decreases with increasing thickness, whereas the band gap of 2D Sb₂Se₃ show negligible change with thickness. The evolution of lattice phonon frequencies with thickness also show similar distinction. The underpinnings of the diverse electronic properties are attributed to the different electronic coupling among the layers of α-Se and Sb₂Se₃ that results in different van der Waals interactions among chains/layers. Our study demonstrates the rich diversity in the properties of 2D layered materials composed of lower-dimensional structural motifs.

ε-InSe single crystals grown by a horizontal gradient freeze method

Indium selenide (InSe) single crystals have been considered as promising candidates for future optical, electrical, and optoelectronic device applications.

Broadband Nonlinear Optical Response of InSe Nanosheets for the Pulse Generation From 1 to 2 μm

Few-layered InSe nanosheets were fabricated by the simple liquid-phase exfoliation method. The morphology and crystal structure features of InSe nanosheet sample were characterized comprehensively. The photoluminescence (PL) spectrum indicated that the liquid-phase exfoliated InSe nanosheets contained variously layered nanoflakes, where eight layers nanosheets dominate. In addition, the first-principle simulation was carried out to describe the electron density of states (DOS) and the electronic band structures. Moreover, the few-layered InSe nanosheets performed excellent nonlinear absorption properties in a broad spectral band. As an application, the stable passively Q-switched (PQS) lasers with few-layered InSe nanosheets saturable absorbers (SAs) were realized with the operating wavelengths at 1.06, 1.34, and 1.91 μm. The shortest pulse durations were 599, 520, and 210 ns, respectively. Our results confirmed that the few-layered InSe nanosheets could be an excellent candidate for pulsed lasers in wide spectral bands.

Structural engineering of bilayer PtSe2 thin films: a first-principles study

PtSe₂ is an emerging layered two-dimensional material of applied interest. Its monolayer shows promising properties for applications in electronic devices, while the bandgap of a multilayer PtSe₂ film can be tuned via changing its thickness. In this work the bilayer PtSe₂ thin films are investigated as an example of structural engineering with first-principles calculations. Various van der Waals corrections schemes are firstly discussed, and the optB86b scheme shows a better description of the semiconductor-metal transition for PtSe₂ films. Six bilayer PtSe₂ thin films in different stacking modes are constructed in order to structurally tune the electronic and transport properties. The bandgap can be effectively broadened with the structural engineering for wider potential applications. The carrier mobility, dynamical stability and Raman spectra are also calculated and discussed.

Strain Effect on Thermoelectric Performance of InSe Monolayer

Strain engineering is a practical method to tune and improve the physical characteristics and properties of two-dimensional materials, due to their large stretchability. Tensile strain dependence of electronic, phonon, and thermoelectric properties of InSe monolayer are systematically studied. We demonstrate that the lattice thermal conductivity can be effectively modulated by applying tensile strain. Tensile strain can enhance anharmonic phonon scattering, giving rise to the enhanced phonon scattering rate, reduced phonon group velocity and heat capacity, and therefore lattice thermal conductivity decreases from 25.9 to 13.1 W/mK when the strain of 6% is applied. The enhanced figure of merit indicates that tensile strain is an effective way to improve the thermoelectric performance of InSe monolayer.

Phase Identification and Strong Second Harmonic Generation in Pure ε-InSe and Its Alloys

Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), ε phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the as-prepared pure InSe, alloyed InSe_{1- x}Te _x and InSe_{1- x}S _x ( x = 0.1 and 0.2) are ε phase structures and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the noncentrosymmetric crystal structure of the ε-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multilayer pure ε-InSe and alloyed InSe_0.9Te_0.1 and InSe_{1- x}S _x ( x = 0.1 and 0.2) are around 1-2 orders of magnitude higher than that of the monolayer TMD systems and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility χ⁽²⁾ of ε-InSe is larger than that of ε-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of ε-InSe and indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.

Anisotropic ultraviolet-plasmon dispersion in black phosphorus

By means of momentum-resolved electron energy loss spectroscopy (EELS) coupled with scanning transmission electron microscopy, we have studied the dispersion relation of interband plasmonic modes in the ultraviolet in black phosphorus. We find that the dispersion of the interband plasmons is anisotropic. Experimental results are reproduced by density functional theory, by taking into account both the anisotropy of the single-particle response function, arising from the anisotropic band structure, and the damping. Moreover, our theoretical model also indicates the presence of low-energy excitations in the near-infrared that are selectively active in the armchair direction, whose existence has been experimentally validated by high-resolution EELS (HREELS) in reflection mode.

Two-Dimensional van der Waals Heterostructures Constructed via Perovskite (C4H9NH3)2XBr4 and Black Phosphorus

Heterogeneous stacking of two-dimensional (2D) perovskites with other 2D materials is a very effective strategy for designing low-cost and high-performance photovoltaic and optoelectronic devices. The structural, electronic, and optical properties of distinctive all-2D M₂XBr₄-black phosphorus (BP) [M = (C₄H₉NH₃)⁺; X = Pb²⁺, Sn²⁺, Ge²⁺] van der Waals (vdW) heterostructures have been studied by first-principle calculations. The M₂SnBr₄-BP and M₂GeBr₄-BP heterostructures show type-II band arrangement; however, the M₂PbBr₄-BP heterostructure exhibits type-I band arrangement. The energy level shift is ascribed to the difference of work function between M₂XBr₄ monolayer and BP monolayer, driving the movement of carriers spontaneously. Furthermore, the BP layers can enhance the light absorption of the total heterostructures, especially the M₂GeBr₄-BP heterostructure. These results indicate the all-2D perovskite and BP vdW heterostructures are competitive candidates for low-dimensional photovoltaic and optoelectronic applications.

Enhancement of photoluminescence and hole mobility in 1- to 5-layer InSe due to the top valence-band inversion: strain effect

Recently, two-dimensional (2D) few-layer InSe nanosheets have become one of the most interesting materials due to their excellent electron transport, wide bandgap tunability and good metal contact. However, their low photoluminescence (PL) efficiency and hole mobility seriously restrict their application in 2D InSe-based nano-devices. Here, by exerting a suitable compressive strain, a remarkable modification for the electronic structure and the optical and transport properties of 1- to 5-layer InSe has been confirmed by powerful GW-BSE calculations. Both top valence band inversion and indirect-to-direct bandgap transition are induced; the light polarization is reversed from E||c to E⊥c; and the PL intensity and hole mobility are enhanced greatly. Surprisingly, under 6% compressive strain, the light emission of monolayer InSe with E⊥c is allowed at 2.58 eV, which has never been observed previously. Meanwhile, for the 2D few-layer InSe, the PL with E⊥c polarization increases over 10 times in intensity and has a blue-shift at about 0.6-0.7 eV, and the hole mobility increases two orders of magnitude up to 103 cm2 V-1 s-1, as high as electron mobility. The strained few-layer InSe are thus a promising candidate for future 2D electronic and optoelectronic nano-devices.