A high performance graphene/few-layer InSe photo-detector

Tunable Negative and Positive Photoconductance in Van Der Waals Heterostructure for Image Preprocessing

The processing of visual information occurs mainly in the retina, and the retinal preprocessing function greatly improves the transmission quality and efficiency of visual information. The artificial retina system provides a promising path to efficient image processing. Here, graphene/InSe/h-BN heterogeneous structure is proposed, which exhibits negative and positive photoconductance (NPC and PPC) effects by altering the strength of a single wavelength laser. Moreover, a modified theoretical model is presented based on the power-dependent photoconductivity effect of laser:I ph = - mP α 1 + nP α 2 ${\rm I}_{\rm ph}\,=\,-{\rm mP}^{\alpha _{1}} + {\rm nP}^{\alpha _{2}}$ , which can reveal the internal physical mechanism of negative/positive photoconductance effects. The present 2D structure design allows the field effect transistor (FET) to exhibit excellent photoelectric performance (RNPC = 1.1× 104 AW-1, RPPC = 13 AW-1) and performance stability. Especially, the retinal pretreatment process is successfully simulated based on the negative and positive photoconductive effects. Moreover, the pulse signal input improves the device responsivity by 167%, and the transmission quality and efficiency of the visual signal can also be enhanced. This work provides a new design idea and direction for the construction of artificial vision, and lay a foundation for the integration of the next generation of optoelectronic devices.

Contemporary innovations in two-dimensional transition metal dichalcogenide-based P-N junctions for optoelectronics

Two-dimensional transition metal dichalcogenides (2D-TMDCs) with various physical characteristics have attracted significant interest from the scientific and industrial worlds in the years following Moore's law. The p-n junction is one of the earliest electrical components to be utilized in electronics and optoelectronics, and modern research on 2D materials has renewed interest in it. In this regard, device preparation and application have evolved substantially in this decade. 2D TMDCs provide unprecedented flexibility in the construction of innovative p-n junction device designs, which is not achievable with traditional bulk semiconductors. It has been investigated using 2D TMDCs for various junctions, including homojunctions, heterojunctions, P-I-N junctions, and broken gap junctions. To achieve high-performance p-n junctions, several issues still need to be resolved, such as developing 2D TMDCs of superior quality, raising the rectification ratio and quantum efficiency, and successfully separating the photogenerated electron-hole pairs, among other things. This review comprehensively details the various 2D-based p-n junction geometries investigated with an emphasis on 2D junctions. We investigated the 2D p-n junctions utilized in current rectifiers and photodetectors. To make a comparison of various devices easier, important optoelectronic and electronic features are presented. We thoroughly assessed the review's prospects and challenges for this emerging field of study. This study will serve as a roadmap for more real-world photodetection technology applications.

Effect of 10 MeV electron irradiation on the electrical properties of bulk α-In2Se3 crystals

In this work, the effect of 10 MeV electron irradiation on the structure and electrical properties of bulk α-In2Se3 crystals is studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray microanalysis, atomic-force microscopy, and Raman spectroscopy methods. Droplets of 200-500 nm in size were detected on the bulk α-In2Se3 crystal surface. The droplets can be formations with the γ-In2Se3 crystalline phase. The current-voltage characteristics measured by conductive atomic-force microscopy are different on and outside the droplets after electron irradiation. On the droplets, slightly better conductive properties were detected after irradiation with the electron fluence of 1015 cm-2. It is found that local resistance increases significantly for both on and outside the droplets after irradiation with the electron fluence of 1017 cm-2. Our study shows that electron irradiation can contribute to the disappearance of ferroelectric domains in the bulk α-In2Se3 crystals. Also, the distribution of surface potentials measured by Kelvin probe force microscopy becomes more uniform after electron irradiation. The results obtained in the work allow us to calculate the operating time of the device containing α-In2Se3 under conditions of long-term electron irradiation with high-energy electrons. The study shows that α-In2Se3 is a very promising material for applications in the aerospace and nuclear industries.

Visible and infrared photodiode based on γ-InSe/Ge van der Waals heterojunction for polarized detection and imaging

Broadband photodetectors are a category of optoelectronic devices that have important applications in modern communication information. γ-InSe is a newly developed two-dimensional (2D) layered semiconductor with an air-stable and low-symmetry crystal structure that is suitable for polarization-sensitive photodetection. Herein, we report a P-N photodiode based on 3D Ge/2D γ-InSe van der Waals heterojunction (vdWH). A built-in electric field is introduced at the p-Ge/n-InSe interface to suppress the dark current and accelerate the separation of photogenerated carriers. Moreover, the heterojunction belongs to the accumulation mode with a well-designed type-II band arrangement, which is suitable for the fast separation of photogenerated carriers. Driven by these advantages, the device exhibits excellent photovoltaic performance within the detection range of 400 to 1600 nm and shows a double photocurrent peak at around 405 and 1550 nm. In particular, the responsivity (R) is up to 9.78 A W^-1 and the specific detectivity (D*) reaches 5.38 × 10¹¹ Jones with a fast response speed of 46/32 μs under a 1550 nm laser. Under blackbody radiation, the room temperature R and D* in the mid-wavelength infrared region are 0.203 A W^-1 and 5.6 × 10⁸ Jones, respectively. Moreover, polarization-sensitive light detection from 405-1550 nm was achieved, with the dichroism ratios of 1.44, 3.01, 1.71, 1.41 and 1.34 at 405, 635, 808, 1310 and 1550 nm, respectively. In addition, high-resolution single-pixel imaging capability is demonstrated at visible and near-infrared wavelengths. This work reveals the great potential of the γ-InSe/Ge photodiode for high-performance, broadband, air-stable and polarization-sensitive photodetection.

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.

Interfacial Charge Transfer and Ultrafast Photonics Application of 2D Graphene/InSe Heterostructure

Interface interactions in 2D vertically stacked heterostructures play an important role in optoelectronic applications, and photodetectors based on graphene/InSe heterostructures show promising performance nowadays. However, nonlinear optical property studies based on the graphene/InSe heterostructure are insufficient. Here, we fabricated a graphene/InSe heterostructure by mechanical exfoliation and investigated the optically induced charge transfer between graphene/InSe heterostructures by taking photoluminescence and pump-probe measurements. The large built-in electric field at the interface was confirmed by Kelvin probe force microscopy. Furthermore, due to the efficient interfacial carrier transfer driven by the built-in electric potential (~286 meV) and broadband nonlinear absorption, the application of the graphene/InSe heterostructure in a mode-locked laser was realized. Our work not only provides a deeper understanding of the dipole orientation-related interface interactions on the photoexcited charge transfer of graphene/InSe heterostructures, but also enriches the saturable absorber family for ultrafast photonics application.

Strong In-Plane Optical and Electrical Anisotropies of Multilayered γ-InSe for High-Responsivity Polarization-Sensitive Photodetectors

Recently, identifying promising new two-dimensional (2D) materials with low-symmetry structures has aroused great interest for developing monolithic polarization-sensitive photodetectors with small volume. Here, after comprehensive research of the in-plane anisotropic structure and electronic and optoelectronic properties of layered γ-InSe, a superior responsivity polarization-sensitive photodetector based on multilayer γ-InSe is constructed by a facile method. Notably, the conductance and carrier mobility of the device along the armchair direction are 11.8 and 2.35 times larger than those along the zigzag direction, respectively. Benefitting from the high efficiency of light absorption and excellent carrier mobility (221 cm² V^-1 s^-1) of our multilayered γ-InSe along the armchair direction, the device exhibits a superior responsivity of 127 A/W and an external quantum efficiency (EQE) of 10⁴%. Especially, the highest responsivity along the armchair direction of our γ-InSe polarization-sensitive photodetectors can reach as high as 78.5 A/W under polarized light. This value is much higher than those of other devices even under unpolarized light. This work not only provides an insight into the in-plane anisotropic properties of 2D layered γ-InSe but also proposes a stable and environmentally friendly candidate for anisotropic optoelectronic applications.

Layer Number-Dependent Raman Spectra of γ-InSe

The two-dimensional layered semiconductor InSe, with its high carrier mobility, chemical stability, and strong charge transfer ability, plays a crucial role in optoelectronic devices. The number of InSe layers (L) has an important influence on its band structure and optoelectronic properties. Herein we present systematic investigations on few-layer (1L-7L) γ-InSe by optical contrast and Raman spectroscopy. We propose three quantified formulas to quickly identify the layer number using optical contrast, the frequency difference of two A₁ modes, and ultralow-frequency Raman spectroscopy, respectively. Moreover, angle-resolved polarization Raman spectra show that γ-InSe is isotropic in the a-b plane. Furthermore, using Raman mapping, we find that the relative strength of the low-frequency interlayer shear modes is particularly sensitive to the interaction between the sample and the substrate.

Atomic insights into the influence of Bi doping on the optical properties of two-dimensional van der Waals layered InSe

As a narrow-gap semiconductor, III-VI two-dimensional (2D) van der Waals layered indium selenide (InSe) has attracted a lot of attention due to excellent physical properties. For potential optoelectronic applications, the tunability of the optical property is challenging, e.g., the modulation of optical bandgap commonly by element doping. However, the deep understanding of the influence of element doping on the microstructure and the optical properties lacks of systematic investigation. In this work, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we investigate the influence of Bi doping on controlling of the microstructure and optical properties of InSe single crystal in detail. The results show that Bi doping can introduce additional stacking faults in InSe single crystal, and more importantly, the atomic spacing and lattice constant of Bi-doped InSe are changed a lot as compared to that of the undoped one. Further optical characterizations including photoluminescence and transmission spectra reveal that Bi-doping can broaden the transmission wavelength range of InSe and make its optical bandgap blue-shift, which can also be physically interpreted from the doping-induced structure change. Our work expands new ideas for the optical property modulation of 2D thin-layer materials and brings new possibilities for the development of thin-layer InSe optical devices.

Out-of-plane and in-plane ferroelectricity of atom-thick two-dimensional InSe

Two-dimensional (2D) ferroelectric materials are promising substitutes of three-dimensional perovskite based ferroelectric ceramic materials. Yet most studies have been focused on the construction of non-centrosymmetric 2D van der Waals materials and only a few are constructed experimentally. Herein, we experimentally demonstrate the co-existence of voltage-tunable out-of-plane (OOP) and in-plane (IP) ferroelectricity in few-layer InSe prepared by a solution-processable method and fabricate ferroelectric semiconductor channel transistors. The reversible polarization can initiate instant switch of resistance with high ON/OFF ratios and a comparable subthreshold swing of 160 mV/dec under gate modulation. The origins of such unique OOP and IP ferroelectricity of the centrosymmetric structure are theoretically analyzed.

Electronic Structures of Twisted Bilayer InSe/InSe and Heterobilayer Graphene/InSe

Building vertical van der Waals heterojunctions between two-dimensional layered materials has become a promising strategy for modulating the properties of two-dimensional materials. Herein, we investigate the electronic structures of non-twisted/twisted bilayer InSe/InSe and heterobilayer graphene/InSe (Gr/InSe) by employing density functional theory calculations. For twisted bilayer InSe/InSes, their interlayer distances and band gaps are almost identical but a bit larger than those of the AB-stacking one due to the spontaneous polarization. Differently, the band gaps of twisted Gr/InSe are found to vary with the rotation angles. Our results provide an effective way to tune the electronic properties of two-dimensional materials.

Visible to near-infrared photodetector with novel optoelectronic performance based on graphene/S-doped InSe heterostructure on h-BN substrate

van der Waals heterostructures of two-dimensional (2D) materials have attracted considerable attention due to their flexibility in the design of new functional devices. Despite numerous studies on graphene/2D semiconductor heterostructures, their optoelectronic applications are significantly hindered because of several disadvantages, such as large band gaps and chemical instability. In this work, we demonstrate the fabrication of graphene/S-doped InSe heterostructure photodetectors with excellent photoresponse performance, and this is attributed to the moderate band gap and band gap engineering by element doping of InSe as well as the high carrier mobility of graphene. In particular, the graphene/InSe0.9S0.1 device achieves an ultrahigh photoresponsivity of ∼4.9 × 106 A W-1 at 700 nm and an EQE of 8.7 × 108%, and it exhibits broadband photodetection (visible to near-infrared). More importantly, by virtue of the interaction between n-type graphene arising from the influence of h-BN as a dielectric layer and S-doped InSe with a high work-function, our devices always exhibited positive photocurrent when the polarity of the gate voltage is adjusted, and is different from that the previously reported graphene/2D semiconductor photodetectors. This work not only provides a promising platform for highly efficient broadband photodetectors but also sheds light on tuning the optoelectronic performance through band gap engineering and designing novel heterostructures-based various 2D materials.

Reduction of the ambient effect in multilayer InSe transistors and a strategy toward stable 2D-based optoelectronic applications

Indium selenide (InSe) photodetection devices attract significant research interest. However, InSe is unstable and degrades rapidly in ambient conditions, thus it is still a challenge to fabricate stable optoelectronic devices. In this work, multilayer InSe FETs are fabricated, and their photoresponse properties are investigated. Both positive and negative photoconductivities are observed for the first time in the same InSe FET in a wide spectral range from 450 nm to 660 nm, which can be tuned through changing either the gate bias or the source-drain bias. A physical mechanism is proposed to explain the dual-photoresponse phenomenon in our devices. Based on the proposed physical mechanism, as a proof of concept, a facile and simple approach is used to eliminate the negative photoconductivity of the InSe FET. Our results will offer valuable strategies for stable multilayer InSe optoelectronic device design, and a practical scheme for improving the performance of other transition metal dichalcogenide devices as well.

Anharmonic lattice dynamics and thermal transport of monolayer InSe under equibiaxial tensile strains

Two-dimensional (2D) InSe, which exhibits high electron mobility and a wide band gap has emerged as a promising material for photoelectric and thermoelectric applications. The inadequate understanding of the lattice thermal conductivity (κ), however, hampers the advancement of 2D InSe. Herein, by taking into account anharmonicity up to the fourth order and introducing the equibiaxial tensile strain (ϵ), we have performed an in-depth study on the lattice dynamics of 2D InSe. Interestingly, theκexhibits a non-monotonic behaviour as a function of equibiaxial tensile strain, which can be attributed to the changes in acoustic phonon lifetimes. At the Γ point, a blue-shift of the lowest optical mode and a red-shift of the uppermost optical mode are reported for the first time. More strikingly, the blue-shift can be largely suppressed by equibiaxial tensile strain. Further study indicates that the unique transition of the potential energy surface is responsible for the disappearance of the blue-shift. Our work may enlighten the future research on phonon engineering and management of the lattice thermal conductivity of 2D InSe.

Ultrafast dynamics of hot carriers in a quasi-two-dimensional electron gas on InSe

Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas-Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high-κ dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Tunable Schottky barrier in InTe/graphene van der Waals heterostructure

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

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.

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.

Photo-excited carrier relaxation dynamics in two-dimensional InSe flakes

Carrier relaxation dynamics of InSe flakes is investigated by using time-resolved pump-probe reflectivity measurement. The photocarriers associated with the P _xy orbital band-edge transition at 2.40 eV, which is coupled to the in-plane polarized light, is observed to possess a lifetime of ∼19 ps at room temperature and ∼99 ps at 10 K. The temperature and power dependent carrier lifetime suggests that Shockley-Read-Hall process is the dominant nonradiative recombination mechanism responsible for the carrier relaxation. In addition, the electron scattering with a 14.5 meV optical phonon plays an active role in the carrier relaxation with increasing temperatures. A broad absorption around 1.65-1.90 eV is observed. The photocarriers associated with this broad transition show a long lifetime of ∼200 ps that is nearly independent of temperature and photon energy. This is indicative of bound carriers by defects. Our experimental results provide essential information for the characteristics of carrier dynamics and defects in InSe flakes. The experimental findings are fundamentally important for further development of microelectronics and optoelectronics based on InSe.

Recent advances in two-dimensional-material-based sensing technology toward health and environmental monitoring applications

Monitoring harmful and toxic chemicals, gases, microorganisms, and radiation has been a challenge to the scientific community for the betterment of human health and environment. Two-dimensional (2D)-material-based sensors are highly efficient and compatible with modern fabrication technology, which yield data that can be proficiently used for health and environmental monitoring. Graphene and its oxides, black phosphorus (BP), transition metal dichalcogenides (TMDCs), metal oxides, and other 2D nanomaterials have demonstrated properties that have been alluring for the manufacture of highly sensitive sensors due to their unique material properties arising from their inherent structures. This review summarizes the properties of 2D nanomaterials that can provide a platform to develop high-performance sensors. In this review, we have also discussed the advances made in the field of infrared photodetectors and electrochemical sensors and how the structural properties of 2D nanomaterials affect sensitivity and performance. Further, this review highlights 2D-nanomaterial-based electrochemical sensors that can be used to check for contaminations from heavy metals, organic/inorganic compounds, poisonous gases, pesticides, bacteria, antibiotics, etc., in water or air, which are severe risks to human wellbeing as well as the environment. Moreover, the limitations, future prospects, and challenges for the development of sensors based on 2D materials are also discussed for future advancements.

Photodetector based on heterostructure of two-dimensional WSe2/In2Se3

Heterojunctions formed by two-dimensional (2D) layered semiconducting materials have been studied extensively in the past few years. These van der Waals (vdW) structures have shown great potential for future electronic and optoelectronic devices. However, the optoelectronic performance of these devices is limited by the indirect band gap of multilayer materials and low light absorption of single layer materials. Here, we fabricate photodetectors based on heterojunctions composed of n-type multilayer α-indium selenide (In₂Se₃) and p-type tungsten diselenide (WSe₂) for the first time. The direct band gap of multilayer α-In₂Se₃ and type-II band alignment of the WSe₂/In₂Se₃ heterojunction enable high optoelectronic performance of the devices at room temperature in the air. Without light illumination, the dark current is effectively suppressed to 10^-13 A under -1 V bias and a high rectification ratio of 7.37 × 10³ is observed. Upon laser illumination with a wavelength of 650 nm, the typical heterojunction device exhibits a photocurrent on/off ratio exceeding 1.24 × 10⁵, a maximum photo responsivity of 26 mA W^-1 and a short photoresponse time of 2.22 ms. Moreover, the heterojunction photodetectors show obvious light response in the wavelength range from 650 nm to 900 nm. The present 2D vdW heterojunctions composed of direct band gap multilayer materials show great potential for future optoelectronic devices.

Direct Observation of Band Gap Renormalization in Layered Indium Selenide

Manipulation of intrinsic electronic structures by electron or hole doping in a controlled manner in van der Waals layered materials is the key to control their electrical and optical properties. Two-dimensional indium selenide (InSe) semiconductor has attracted attention due to its direct band gap and ultrahigh mobility as a promising material for optoelectronic devices. In this work, we manipulate the electronic structure of InSe by in situ surface electron doping and obtain a significant band gap renormalization of ∼120 meV directly observed by high-resolution angle resolved photoemission spectroscopy. This moderate doping level (carrier concentration of 8.1 × 10¹² cm^-2) can be achieved by electrical gating in field effect transistors, demonstrating the potential to design of broad spectral response devices.

Robust Piezo-Phototronic Effect in Multilayer γ-InSe for High-Performance Self-Powered Flexible Photodetectors

The piezo-phototronic effect has been promising as an effective means to improve the performance of two-dimensional (2D) semiconductor based optoelectronic devices. However, the current reported monolayer 2D semiconductors are not regarded as suitable for actual flexible piezotronic photodetectors due to their insufficient optical absorption and mechanical durability, although they possess strong piezoelectricity. In this work, we demonstrate that, unlike 2H-phase transition-metal dichalcogenides, γ-phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers and has a strong second-harmonic generation effect. Moreover, driven by the piezo-phototronic effect, a flexible self-powered photodetector based on multilayer γ-InSe, which can work without any energy supply, is proposed. The device exhibited ultrahigh photon responsivity of 824 mA/W under light illuminations of 400 nm (0.368 mW/cm²). Moreover, the responsivity and response speed of this photodetector were enhanced further by as much as 696% and 1010%, respectively, when a 0.62% uniaxial tensile strain was applied. Our devices exhibit high reliability and stability during a 6 month test time. These significant findings offer a promising pathway to construct high-performance flexible piezo-phototronic photodetectors based on multilayer 2D semiconductors.

Formation and Healing of Defects in Atomically Thin GaSe and InSe

Two dimensional III-VI metal monochalcogenide materials, such as GaSe and InSe, are attracting considerable attention due to their promising electronic and optoelectronic properties. Here, an investigation of point and extended atomic defects formed in mono-, bi-, and few-layer GaSe and InSe crystals is presented. Using state-of-the-art scanning transmission electron microscopy, it is observed that these materials can form both metal and selenium vacancies under the action of the electron beam. Selenium vacancies are observed to be healable: recovering the perfect lattice structure in the presence of selenium or enabling incorporation of dopant atoms in the presence of impurities. Under prolonged imaging, multiple point defects are observed to coalesce to form extended defect structures, with GaSe generally developing trigonal defects and InSe primarily forming line defects. These insights into atomic behavior could be harnessed to synthesize and tune the properties of 2D post-transition-metal monochalcogenide materials for optoelectronic applications.

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.

Two Dimensional β-InSe with Layer-Dependent Properties: Band Alignment, Work Function and Optical Properties

Density functional theory calculations of the layer (L)-dependent electronic band structure, work function and optical properties of β-InSe have been reported. Owing to the quantum size effects (QSEs) in β-InSe, the band structures exhibit direct-to-indirect transitions from bulk β-InSe to few-layer β-InSe. The work functions decrease monotonically from 5.22 eV (1 L) to 5.0 eV (6 L) and then remain constant at 4.99 eV for 7 L and 8 L and drop down to 4.77 eV (bulk β-InSe). For optical properties, the imaginary part of the dielectric function has a strong dependence on the thickness variation. Layer control in two-dimensional layered materials provides an effective strategy to modulate the layer-dependent properties which have potential applications in the next-generation high performance electronic and optoelectronic devices.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

p-MoS2/n-InSe van der Waals heterojunctions and their applications in all-2D optoelectronic devices

A library of 2D semiconductors are prepared providing a new platform for developing high-performance optoelectronic devices. All-2D optoelectronic devices based on type-II p-MoS₂/n-InSe vdWs heterojunctions operate at the near-IR wavelength range.

Synthesis of Large-Area InSe Monolayers by Chemical Vapor Deposition

Recently, 2D materials of indium selenide (InSe) layers have attracted much attention from the scientific community due to their high mobility transport and fascinating physical properties. To date, reports on the synthesis of high-quality and scalable InSe atomic films are limited. Here, a synthesis of InSe atomic layers by vapor phase selenization of In₂ O₃ in a chemical vapor deposition (CVD) system, resulting in large-area monolayer flakes or thin films, is reported. The atomic films are continuous and uniform over a large area of 1 × 1 cm² , comprising of primarily InSe monolayers. Spectroscopic and microscopic measurements reveal the highly crystalline nature of the synthesized InSe monolayers. The ion-gel-gated field-effect transistors based on CVD InSe monolayers exhibit n-type channel behaviors, where the field effect electron mobility values can be up to ≈30 cm² V^-1 s^-1 along with an on/off current ratio, of >10⁴ at room temperature. In addition, the graphene can serve as a protection layer to prevent the oxidation between InSe and the ambient environment. Meanwhile, the synthesized InSe films can be transferred to arbitrary substrates, enabling the possibility of reassembly of various 2D materials into vertically stacked heterostructures, prompting research efforts to probe its characteristics and applications.

Effect of illumination and Se vacancies on fast oxidation of ultrathin gallium selenide

Gallium selenide (GaSe) has recently emerged as a unique platform due to its exciting properties, namely, large and fast photo-response, high carrier mobility and non-linear optical properties. However, exposure for a few days causes the fast oxidation of ultrathin GaSe under ambient conditions and the oxidation mechanism remains unclear. By means of density functional theory calculations and ab initio molecular dynamics simulations, we comprehensively investigated the possible sources that cause oxidation of ultrathin GaSe. Our results show that illumination and Se vacancies induce the fast oxidation of GaSe. Under illumination, photo-excited electrons from the surface of GaSe are effectively transferred to oxygen molecules and thus, superoxide anions (O2-) are generated that react with GaSe. Moreover, Se vacancies directly react with O2. In both the cases, the Ga-Se bonds are continually replaced by Ga-O bonds, which eventually leads to complete degradation of GaSe, accompanied with the formation of the oxidation products Ga2O3 and elemental Se. The comprehensive degradation mechanism unveiled herein lays an important foundation for the development of suitable protecting strategies in GaSe-based devices.

Phase-Engineered Type-II Multimetal-Selenide Heterostructures toward Low-Power Consumption, Flexible, Transparent, and Wide-Spectrum Photoresponse Photodetectors

Phase-engineered type-II metal-selenide heterostructures are demonstrated by directly selenizing indium-tin oxide to form multimetal selenides in a single step. The utilization of a plasma system to assist the selenization facilitates a low-temperature process, which results in large-area films with high uniformity. Compared to single-metal-selenide-based photodetectors, the multimetal-selenide photodetectors exhibit obviously improved performance, which can be attributed to the Schottky contact at the interface for tuning the carrier transport, as well as the type-II heterostructure that is beneficial for the separation of the electron-hole pairs. The multimetal-selenide photodetectors exhibit a response to light over a broad spectrum from UV to visible light with a high responsivity of 0.8 A W^-1 and an on/off current ratio of up to 10² . Interestingly, all-transparent photodetectors are successfully produced in this work. Moreover, the possibility of fabricating devices on flexible substrates is also demonstrated with sustainable performance, high strain tolerance, and high durability during bending tests.

Effects of graphene/BN encapsulation, surface functionalization and molecular adsorption on the electronic properties of layered InSe: a first-principles study

A proper adoption of the n- or p-type dopants allows for the modulation of the work function, the Fermi level pinning, the band bending, and the photo-adsorbing efficiency near the InSe surface/interface.

Band offset and an ultra-fast response UV-VIS photodetector in γ-In2Se3/p-Si heterojunction heterostructures

High-quality γ-In₂Se₃ thin films and a γ-In₂Se₃/p-Si heterojunction were prepared using pulse laser deposition (PLD).

2D library beyond graphene and transition metal dichalcogenides: a focus on photodetection

Two-dimensional materials beyond graphene and TMDs can be promising candidates for wide-spectra photodetection.

High White Light Photosensitivity of SnSe Nanoplate-Graphene Nanocomposites

The multi-functional nanomaterial constructed with more than one type of materials has gained a great attention due to its promising application. Here, a high white light photodetector prototype established with two-dimensional material (2D) and 2D nanocomposites has been fabricated. The 2D-2D nanocomposites were synthesized with SnSe nanoplate and graphene. The device shows a linear I-V characterization behavior in the dark and the resistance dramatically decreases under the white light. Furthermore, the photosensitivity of the device is as large as 1110% with a rapid response time, which is much higher than pristine SnSe nanostructure reported. The results shown here may provide a valuable guidance to design and fabricate the photodetector based on the 2D-2D nanocomposites even beyond the SnSe nanoplate-graphene nanocomposites.

The Advent of Indium Selenide: Synthesis, Electronic Properties, Ambient Stability and Applications

Among the various two-dimensional semiconductors, indium selenide has recently triggered the interest of scientific community, due to its band gap matching the visible region of the electromagnetic spectrum, with subsequent potential applications in optoelectronics and especially in photodetection. In this feature article, we discuss the main issues in the synthesis, the ambient stability and the application capabilities of this novel class of two-dimensional semiconductors, by evidencing open challenges and pitfalls. In particular, we evidence how the growth of single crystals with reduced amount of Se vacancies is crucial in the road map for the exploitation of indium selenide in technology through ambient-stable nanodevices with outstanding values of both mobility of charge carriers and ON/OFF ratio. The surface chemical reactivity of the InSe surface, as well as applications in the fields of broadband photodetection, flexible electronics and solar energy conversion are also discussed.

Oxidation Mechanism and Protection Strategy of Ultrathin Indium Selenide: Insight from Theory

Ultrathin indium selenide (InSe), as a newly emerging two-dimensional material with high carrier mobility and a broad absorption spectrum, has been the focus of current research. However, the long-term environmental instability of atomically thin InSe seriously limits its practical applications. To develop an effective strategy to protect InSe, it is crucial to reveal the degradation mechanism at the atomic level. By employing density functional theory and ab initio molecular dynamics simulations, we provide an in-depth understanding of the oxidation mechanism of InSe. The defect-free InSe presents excellent stability against oxidation. Nevertheless, the Se vacancies on the surface can react with water and oxygen in air directly and activate the neighboring In-Se bonds on the basal plane for further oxidation, leading to complete degradation of InSe into oxidation products of In₂O₃ and elemental Se. Furthermore, we propose an efficient strategy to repair the Se vacancies by thiol chemistry. In this way, the repaired surface can resist oxidation from oxygen and retain the original high electron mobility of pristine InSe simultaneously.

Wafer-Scale Synthesis of High-Quality Semiconducting Two-Dimensional Layered InSe with Broadband Photoresponse

Large-scale synthesis of two-dimensional (2D) materials is one of the significant issues for fabricating layered materials into practical devices. As one of the typical III-VI semiconductors, InSe has attracted much attention due to its outstanding electrical transport property, attractive quantum physics characteristics, and dramatic photoresponse when it is reduced to atomic scale. However, scalable synthesis of single phase 2D InSe has not yet been achieved so far, greatly hindering further fundamental studies and device applications. Here, we demonstrate the direct growth of wafer-scale layered InSe nanosheets by pulsed laser deposition (PLD). The obtained InSe layers exhibit good uniformity, high crystallinity with macro texture feature, and stoichiometric growth by in situ precise control. The characterization of optical properties indicates that PLD grown InSe nanosheets have a wide range tunable band gap (1.26-2.20 eV) among the large-scale 2D crystals. The device demonstration of field-effect transistor shows the n-type channel feature with high mobility of 10 cm² V^-1 s^-1. Upon illumination, InSe-based phototransistors show a broad photoresponse to the wavelengths from ultraviolet to near-infrared. The maximum photoresponsivity attains 27 A/W, plus a response time of 0.5 s for the rise and 1.7 s for the decay, demonstrating the strong and fast photodetection ability. Our findings suggest that the PLD grown InSe would be a promising choice for future device applications in the 2D limit.

Extremely high-performance visible light photodetector in the Sb2SeTe2 nanoflake

The photocurrent was performed in the Sb₂SeTe₂ topological insulator at a wavelength of 532 nm. It exhibits extremely high performance that the responsivity and the photoconductive gain reach 2293 AW⁻¹ and 5344 at 1 V. This high photoresponse is orders of magnitude higher than most reported values in topological insulators and two-dimensional transitional metal dichalcogenides. This finding suggests that the Sb₂SeTe₂ nanoflake has great potential for future optoelectronic device applications.

Highly responsive photoconductance in a Sb2SeTe2 topological insulator nanosheet at room temperature

SEM picture of the Sb₂SeTe₂ nanosheet. The top-right figure shows the linear current–voltage curve indicating the ohmic contact between the Pt electrodes and Sb₂SeTe₂ nanosheet.

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

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

Visible to short wavelength infrared In2Se3-nanoflake photodetector gated by a ferroelectric polymer

Photodetectors based on two-dimensional (2D) transition-metal dichalcogenides have been studied extensively in recent years. However, the detective spectral ranges, dark current and response time are still unsatisfactory, even under high gate and source-drain bias. In this work, the photodetectors of In2Se3 have been fabricated on a ferroelectric field effect transistor structure. Based on this structure, high performance photodetectors have been achieved with a broad photoresponse spectrum (visible to 1550 nm) and quick response (200 μs). Most importantly, with the intrinsic huge electric field derived from the polarization of ferroelectric polymer (P(VDF-TrFE)) gating, a low dark current of the photodetector can be achieved without additional gate bias. These studies present a crucial step for further practical applications for 2D semiconductors.

Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.

The influence of chemical reactivity of surface defects on ambient-stable InSe-based nanodevices

We demonstrate that, in contrast to most two-dimensional materials, ultrathin flakes of InSe are stable under ambient conditions. Despite their ambient stability, InSe-based nanodevices show an environmental p-type doping, suppressed by capping InSe with hexagonal boron nitride. By means of transport experiments, density functional theory and vibrational spectroscopy, we attribute the p-type doping assumed by uncapped InSe under an ambient atmosphere to the decomposition of water at Se vacancies. We have estimated the site-dependent adsorption energy of O2, N2, H2O, CO and CO2 on InSe. A stable adsorption is found only for the case of H2O, with a charge transfer of only 0.01 electrons per water molecule.