Two-dimensional Layered Semiconductors Research Articles

Synergistic SERS enhancement of NiCo-LDHs microurchins and silver nanoparticles for ultra-sensitive and reusable detection of thiabendazole

Two-dimensional layered semiconductor materials as a distinctive class of materials are comprehensively explored for widespread applications due to narrow bandgap, controllable morphology, and tunable metal cation composition. Herein, we constructed a sensing platform of surface enhanced Raman spectroscopy (SERS) by combination of nickel‑cobalt layered double hydroxide (NiCo-LDH) microurchins and plasmonic silver nanoparticles (Ag NPs) for fungicide detection of thiabendazole (TBZ). The NiCo-LDHs/Ag-NPs microcomposites consist of NiCo-LDHs microurchins having a large number of nanoneedles deposited with photoreduced Ag NPs. The SERS platform with NiCo-LDHs/Ag-NPs shows an excellent SERS performance for TBZ detection, including an ultra-low detection limit of 1.49 × 10−11 M, a sublime enhancement factor of 1.71 × 109, high uniformity, good reproducibility, and long-term storage stability. The ultrahigh SERS activity of NiCo-LDH/Ag-NPs can be attributed to strong electromagnetic enhancement in the nanoscale gaps between Ag NPs, massive charge transfer through large-area NiCo-LDH/Ag-NPs interfaces, and the synergistic action of electromagnetic and charge transfer mechanisms. Besides, the unique morphology of NiCo-LDHs/Ag-NPs microcomposite provides a broad surface area for adsorption of TBZ molecules for further Raman signal enhancement. The practicability of the proposed SERS platform is confirmed by detecting TBZ in the real samples of apple juice and river water. The exceptional self-cleaning capability of the NiCo-LDHs/Ag-NPs microcomposite with an retention rate of 81.97 % even after the fifth degradation cycle underscores its impressive sustainable reusability and cost-effectiveness. The findings in this work lay the foundation for the development of high-performance SERS platforms to ensure food safety and environmental protection.

Dissolution of g-C3N4 Using Zinc Chloride Molten Salt Hydrates for Nanobelt Fabrication and Photocatalytic H2O2 Production.

Graphitic-carbon nitride (g-C3N4), a metal-free two-dimensional layered semiconductor material, holds great potential for energy conversion, environmental remediation, and sensing. However, the limited solubility of g-C3N4 in conventional solvents hinders its widespread application. Improving the dissolution of g-C3N4 in the liquid phase is highly desired but challenging. Herein, we report an innovative approach to dissolve g-C3N4 using ZnCl2 molten salt hydrates. The solubility of g-C3N4 in the solution reaches up to 200 mg mL-1. Density functional theory (DFT) results suggest that ZnCl+H2O is the key species that leads to charge redistribution on g-C3N4 surface and promotes the dissolution of carbon nitride in the solution. Furthermore, through dilution, the dissolved carbon nitride can be effectively recovered while maintaining its intrinsic chemical structure. The resultant regenerated C3N4 (r-C3N4) exhibits nanobelt morphology and demonstrates a substantially improved photocatalytic activity in H2O2 production. The rate of H2O2 production over the r-C3N4 reaches 20,228 μmol g-1 h-1, which is 6.2 times higher than that of pristine g-C3N4. This green and efficient dissolution route of g-C3N4 offers an effective approach for its diverse applications.

Research progress on the application of two-dimensional layered semiconductors in optoelectronic devices

In recent years, traditional 2D layered materials such as graphene have been widely used in the photoelectric area. As the emerging research and development of 2D layered materials, there are more and more newly developed 2D layered semiconductors being applied to this field, such as MoS2, phosphorene, and other nanomaterials. 2D layered MoS2 can be utilized as sensitive elements to obtain high-performance photodetectors with high responsivity, high current response rate, and short recovery time. Moreover, because phosphorene has adjustable bandgap width and strong light absorption, it has more potential in aspects of other semiconductors, and application in photoelectric devices. This article introduced the structures of some common 2D layered semiconductors, discussed their applications in photoelectric fields, and pointed out their limitations. For example, transition metal dichalcogenide (TMDC) materials generally have low carrier mobility, and black phosphorus is prone to adsorb water molecules from the air, leading to changes in its physical and chemical properties. Finally, improvement methods of applications and prospects for future development were provided.

Inheritance of the exciton geometric structure from Bloch electrons in two-dimensional layered semiconductors

Inheritance of the exciton geometric structure from Bloch electrons in two-dimensional layered semiconductors

Interlayer-coupling-engineerable flat bands in twisted MoSi2N4bilayers.

The two-dimensional layered semiconductor MoSi2N4, which has several advantages including high strength, excellent stability, high hole mobility, and high thermal conductivity, was recently successfully synthesized using chemical vapor deposition. Based on first-principles calculations, we investigate the effects of the twist angle and interlayer distance variation on the electronic properties of twisted bilayer MoSi2N4. The flat bands are absent for twisted bilayer MoSi2N4when the twist angleθis reduced to 3.89°. Taking twisted bilayer MoSi2N4withθof 5.09° as an example, we find that flat bands emerge as the interlayer distance decreases. As the interlayer distance can be effectively modulated by hydrostatic pressure, we propose hydrostatic pressure as a knob for tailoring the flat bands in twisted bilayer MoSi2N4. Our findings provide theoretical support for extending the applications of MoSi2N4in strong correlation physics and superconductivity.

Nanoscale Probing of Electrical Memory Effects in van der Waals Layered PdSe2.

Tunable electronic materials that can be switched between different impedance states are fundamental to the hardware elements for neuromorphic computing architectures. This "brain-like" computing paradigm uses highly paralleled and colocated data processing, leading to greatly improved energy efficiency and performance compared to traditional architectures in which data have to be frequently transferred between processor and memory. In this work, we use scanning microwave impedance microscopy for nanoscale electrical and electronic characterization of two-dimensional layered semiconductor PdSe2 to probe neuromorphic properties. The local resolution of tens of nanometers reveals significant differences in electronic behavior between and within PdSe2 nanosheets (NSs). In particular, we detected both n-type and p-type behaviors, although previous reports only point to ambipolar n-type dominating characteristics. Nanoscale capacitance-voltage curves and subsequent calculation of characteristic maps revealed a hysteretic behavior originating from the creation and erasure of Se vacancies as well as the switching of defect charge states. In addition, stacks consisting of two NSs show enhanced resistive and capacitive switching, which is attributed to trapped charge carriers at the interfaces between the stacked NSs. Stacking n- and p-type NSs results in a combined behavior that allows one to tune electrical characteristics. As local inhomogeneities of electrical and electronic behavior can have a significant impact on the overall device performance, the demonstrated nanoscale characterization and analysis will be applicable to a wide range of semiconducting materials.

Van der Waals Epitaxy, Superlubricity, and Polarization of the 2D Ferroelectric SnS.

Tin monosulfide (SnS) is a two-dimensional layered semiconductor that exhibits in-plane ferroelectric order at very small thicknesses and is of interest in highly scaled devices. Here we report the epitaxial growth of SnS on hexagonal boron nitride (hBN) using a pulsed metal-organic chemical vapor deposition process. Lattice matching is observed between the SnS(100) and hBN{11̅0} planes, with no evidence of strain. Atomic force microscopy reveals superlubricity along the commensurate direction of the SnS/hBN interface, and first-principles calculations suggest that friction is controlled by the edges of the SnS islands, rather than interface interactions. Differential phase contrast imaging detects remnant polarization in SnS islands with domains that are not dictated by step-edges in the SnS. The growth of ferroelectric SnS on high quality hBN substrates is a promising step toward electrically switchable ferroelectric semiconducting devices.

Pseudocapacitive charge storage behavior of cation-doped WS2 nanosheets electrode

Nanostructured two-dimensional layered semiconductors MX2 (M = Mo, W, and X = S, Se, Te) are potential candidates for electrochemical energy conversion and storage applications. Recently, different research strategies, including phase and heterostructure engineering, surface modification, and chemical doping have been employed with the aim to obtain high energy density MX2 supercapacitor electrodes. Here, we report the electrochemical supercapacitor performance of cation, Co, Ir, and Ru, doped WS2 nanosheets electrode in 1 M Na2SO4 and 2 M H2SO4 aqueous electrolyte. The structure of WS2 nanosheets is stabilized in the 2H hexagonal phase and the interlayer separation expanded after cation doping. The cation-doped WS2 nanosheet electrodes exhibit pseudocapacitive behavior with improved performance in both electrolytes. Among all, the best electrode, Ru doped WS2 nanosheets shows excellent energy storage capacity and attains a specific capacity of 438 Fg−1 in 2 M H2SO4 electrolyte. The analysis reveals that cation doping is one possible research strategy to improve the supercapacitor performance of WS2 nanosheets.

Large Size Few-Layer Ambipolar MoS<sub>2</sub> Metal-Oxide-Semiconductor Field Effect Transistors by Nitrogen Plasma Doping

Molybdenum disulfide (MoS2), a typical two-dimensional layered semiconductor material, is widely studied due its excellent electronic properties in atomic scale. In this study, we achieved the growth of large size few-layer MoS2 films by using molybdenum boat with vertical shield at the end to carry MoO3 precursor in the chemical vapor deposition (CVD) system. The optical microscopy reveals the morphology and lateral size of as-grown films. The Raman spectrum testified that the synthesized films are few-layer MoS2 with defects. Metal-oxide-semiconductor field effect transistors (MOSFETs) based on CVD-grown MoS2 are fabricated, presenting n-type transportation with ION/IOFF ratio about 103. The transportation behaviour of MoS2 MOSFETs is changed from n-type to ambipolar by introducing nitrogen plasma into MoS2 films. The electron or hole transportation in MoS2 is controlled by gate-source voltage. The ambipolar MOSFETs show ION/IOFF ratio about 103.

Optothermal Raman Spectroscopy of Black Phosphorus on a Gold Substrate.

With black phosphorus being a promising two-dimensional layered semiconductor for application to electronics and optoelectronics, an issue remains as to how heat diffusion is managed when black phosphorus is interfaced with metals, namely in a typical device heterojunction. We use Raman spectroscopy to investigate how the laser-induced heat affects the phonon modes at the interface by comparing the experimental data with a finite element simulation based on a localized heat diffusion. The best convergence is found taking into account an effective interface thermal conductance, thus indicating that heat dissipation at the Au-supported black phosphorus nanosheets is limited by interface effect.

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.

Improving thermoelectric performance of GeSe compound by crystal structure engineering

In the thermoelectric field, GeSe is a two-dimensional layered semiconductor with a large band gap, intrinsically low carrier concentration and poor thermoelectric figure of merit <i>ZT</i>. In this work, a series of GeSe<sub>1–<i>x</i></sub>Te<sub><i>x</i></sub> (<i>x</i> = 0, 0.05, 0.15, 0.25, 0.35, 0.45) polycrystalline samples is prepared by melting and quenching combined with spark plasma activation sintering process. The influences of Te content on the phase structure and thermoelectric transport properties of GeSe are systematically studied. The results indicate that with the increase of Te content, the crystal structure of GeSe gradually changes from orthorhombic to rhombohedral structure. This reduces the band gap of the material, and simultaneously increases the carrier concentration and mobility. Meanwhile, the energy band degeneracy of the compound increases significantly because of enhanced crystal symmetry in this process, thereby considerably improving the effective mass of carriers. Altogether, the power factor of the rhombohedral GeSe is increased by about 2 to 3 orders of magnitude compared with that of the orthorhombic phase GeSe. In addition, the rhombohedral phase GeSe has abundant cationic vacancy defects and softened phonons arising from its ferroelectric feature, leading the lattice thermal conductivity to be 60% lower than orthorhombic one. The GeSe<sub>0.55</sub>Te<sub>0.45</sub> sample achieves a peak <i>ZT</i> of 0.75 at 573 K, which is 19 times that of pristine GeSe. Crystal structure engineering could be considered as an effective way of improving the thermoelectric performance of GeSe compounds.

Solution-processed and high-performance ionic-paper gated field-effect transistors from two-dimensional layered semiconductor nanosheets with high thermal resolution

The remarkable properties of layered semiconductor nanosheets (LSNs), such as scalable production, bandgap tunability and mechanical flexibility, have promoted them as promising building blocks for nanoelectronics and bioelectronics.

Phonon Coupling and Dielectric Response in 2D Semiconductor

The impact of phonon and their surrounding environment on exciton and its complexes were investigated in monolayer WSe2 semiconductor. Phonon up-conversion has been studied in past for conventional III–V semiconductors, but its role in two-dimensional layered transition metal dichalcogenides has rarely been explored. We investigated the photoluminescence up-conversion mechanism in WSe2 monolayer and found that a lower energy photon gain energy upto 64[Formula: see text]meV to be up-converted to emission photon at room temperature. Moreover, the phonon-exciton coupling mechanism has also been investigated and the role of dielectric screening has been explored to get complete insight of coulomb’s interaction in these electron-hole pairs. Investigations of charge carrier’s lifetime reveal that boron nitride encapsulated monolayer has shorter recombination time as low as 41 ps as compared to a bare monolayer on SiO2 substrate. These results are very promising for realizing spintronics-based application from two-dimensional layered semiconductors.

Preparation of highly efficient Ni-doped layered perovskite grafted graphene oxide NixSr1-xTiO3-RGO heterojunction photocatalyst with enhanced visible-light photocatalytic activity for MB degradation

Two-dimensional layered semiconductor photocatalysts have attracted much attention due to their excellent performance. Layered photocatalyst, however, generally has low visible-light utilization and is easy to self-polymerize. In this study, layered photocatalyst NixSr1-xTiO3 was prepared by doping Nickelin perovskite, and then loaded on RGO with a larger specific surface area. Heterojunction photocatalyst with high performance was successfully prepared. The results show that doping Ni will introduce an occupied energy level between the band gaps, effectively reducing the forbidden bandwidth, and the wavelength of 600 nm still has a high optical response. Much elevated photocatalytic degradation was investigated after further loading NixSr1-xTiO3 onto RGO. As a carrier, RGO can improve the self-polymerization of the catalyst and form heterojunction with NixSr1-xTiO3. The transfer of photogenerated carriers is promoted relying on the potential difference between the two sides of the heterojunction interface and the electron transport channel of RGO. We also noticed that the unique amphoteric molecular properties of RGO can improve the dispersion of NixSr1-xTiO3-RGO in dye wastewater. To sum up, the synergistic effect of reducing band gaps, inhibiting heterojunction structure and improving dispersion are the reasons for more efficient activity.

Theoretical investigation of lattice dynamics, infrared reflectivity, polarized Raman spectra and nature of interlayer coupling in two-dimensional layered gallium sulfide

Gallium sulfide (GaS) is a highly promising two-dimensional layered semiconductor owing to its remarkable thickness dependent electronic and physical properties. In this article, we perform a comprehensive ab initio study of lattice dynamics, mode symmetry assignments, polarized Raman and infrared (IR) reflectivity spectra of GaS system. Polarized Raman spectra are obtained for different light polarization set-ups of incoming and scattered light. The frequencies of all allowed vibrational modes at the zone-centre are calculated and symmetry labels are assigned. Furthermore, the variation of frequencies & intensities of Raman/IR active modes of ultrathin GaS films (few layers) as function of film thickness is studied. In addition, we also explore the nature of weak interlayer coupling in GaS. The weak forces between the GaS layers are usually assumed to be due to interlayer van der Waals (vdW) interaction. However, this assumption has not been reasonably explained in reported experimental studies. Our study strongly suggests that weak interlayer interactions in GaS may be primarily electrostatic (Coulomb) in nature and therefore the contribution of vdW interactions to layer–layer coupling and lattice dynamics may be significantly lower than that of electrostatic interaction. The suggested nature of interlayer coupling in GaS and related III–VI semiconductors may have important implications in determination of their various physical properties.

Effect of annealing on structural and optical properties of SnS2 thin films grown by thermal evaporation and post sulphur annealing technique

Recently, two-dimensional layered semiconductors caught attention due to their tuneable thickness dependent properties and its optoelectronic device applications. This paper reports the effect of annealing temperature and annealing duration on structural, compositional and optical properties of tin disulphide thin films (SnS2) fabricated using a bi- stage process. The process involves thermal evaporation of metallic tin precursor films and post sulphur annealing process in a non-vacuum furnace. The annealing temperatures ranged from 300 °C to 500 °C while annealing time varied from 0.5 to 3 hours. The annealing at 300°C improved the crystallinity of SnS2 thin films. Raman analysis showed a prominent peak at 207 cm−1, while at higher temperatures nanocrystalline SnS2 phase is observed. Optical bandgap got reduced as annealing temperature increased and at 300°C, the bandgap was 2.7 eV. The above obtained properties of SnS2 opens a pathway for using these films for potential applications.

Preparation and photodetection performance oftwo-dimensional In<sub>2/3</sub>PSe<sub>3</sub> nanosheets

Two-dimensional layered semiconductors have been the research focus of electronics and optoelectronics owing to their atomic thickness, light-matter interaction, and large-scale integration. Metal phosphorus trichalcogenides (MPTs), possessing the structure of [P2S6]4– or [P2Se6]4– surrounded with metal cations, have emerged as newly rising stars and drawn widespread interests of researchers due to their novel electrical and optical properties. Compared with common transition metal dichalcogenides, two-dimensional (2D) MPTs have larger optical bandgaps (from 1.3 to 3.5 eV), higher light absorption efficiency and richer bandgap structures due to the increased degree of freedom from the elements and atomic ratio, which highlight its great advantages in the field of optoelectronics. Among MPTs, In2/3PSe3 owns special structure, in which two-thirds of the octahedral holes are occupied by the indium ions and the other one-third are vacant. The unique structure endows In2/3PSe3 with intrinsic broken-symmetry, showing potential in non-linear optics. It was reported that bulk In2/3PSe3 is a direct-bandgap semiconductor (2.0 eV), in which the electron transition process from the conduction band to valence band is direct without phonon assisting, indicating great potential in application of optoelectronic devices. However, there is no research reported on the optoelectronic property of In2/3PSe3. According to the promising properties mentioned above, it is extremely urgent to characterize the optical and optoelectronic properties of 2D In2/3PSe3 nanosheets. In this work, high-quality In2/3PSe3 single crystals are synthesized by chemical vapor transport method and for the first time we achieve 2D In2/3PSe3 nanosheets on SiO2/Si substrate by mechanical exfoliation. X-ray diffractometer (XRD) and transmission electron microscope (TEM) are performed on In2/3PSe3 nanosheets, which demonstrate their high quality and single-crystalline nature. Considering that the non-centrosymmetric space group R 3, we further explored the SHG properties of In2/3PSe3 nanosheets. The intrinsic SHG response further verifies the broken-symmetry nature of In2/3PSe3. In addition, photoluminescence spectra of In2/3PSe3 nanosheets have revealed the exciton-like transition mechanism and longer PL lifetime than previously reported 2D materials, which may own to the high crystalline and low defects of as-synthesized crystals. To investigate the optoelectronic behavior of 2D In2/3PSe3 nanosheets, two-electrodes photodetectors based on In2/3PSe3 nanosheets are fabricated. Impressively, the In2/3PSe3 nanosheets-based photodetector exhibits an ultra-low dark current (25 fA), excellent detectivity (6.28 ´ 1011 Jones), high on/off ratio (4 ´ 104), and ultra-fast response rate (14 μs/24 μs) under 365 nm illumination, which are superior to other typical 2D ultraviolet photodetectors. The main photocurrent generation mechanism of the photodetector is photoconductive effect. The photogenerated free electrons and holes are separated rapidly to the opposite electrodes without trapping in defect states, leading to the ultra-fast response. These excellent optoelectronic behaviors of In2/3PSe3 nanosheet suggest its appealing prospect in future application of photodetection.

Gate-controlled ambipolar transport in b-AsP crystals and their VIS-NIF photodetection.

As a new two-dimensional elemental layered semiconductor, black phosphorus (b-P) has received tremendous attention due to its excellent physical and chemical properties and has potential applications in the fields of catalysis, energy, and micro/nano-optoelectronic devices. However, studies have found that b-P is very unstable and will decompose within a few minutes under humid air conditions. Element doping is an effective method for adjusting the physical and chemical properties of crystals. Theoretical and experimental studies have confirmed that the stability of b-P crystals is significantly improved after arsenic doping, and the crystals also exhibit excellent photoresponse and electrical transport performances. In this work, we investigate the physical properties of a component of black arsenic phosphorus crystals (b-As0.084P0.916) and the potential applications in field effect transistors (FETs) and broadband photodetectors. An obvious ambipolar behavior is observed in the transfer characteristics of b-As0.084P0.916 based FETs, with drain current modulation on the order of 105 and the highest charge-carrier mobility of up to 147 cm2 V-1 s-1. The physisorption of atmospheric species on the surface of the FETs is the main factor for the formation of Schottky contacts between the Au electrodes and the b-As0.084P0.916 crystal. Temperature-dependent electrical characteristics show that the Fermi level shifts from the valence band to the middle level between the conduction band and valence band as the temperature decreases. In addition, the FETs also exhibit excellent photoresponse properties from the visible to near-infrared region (450-2200 nm), with a responsivity of 37 A W-1, a specific detectivity of 7.18 × 1010 Jones, and a fast response speed (τrise ≈ 0.04 s and τdecay ≈ 0.14 s). These results suggest that b-As0.084P0.916 crystals are a promising candidate for future electronic and optoelectronic devices.

Two-dimensional transistors with reconfigurable polarities for secure circuits

Security is a critical aspect in modern circuit design, but research into hardware security at the device level is rare as it requires modification of existing technology nodes. With the increasing challenges facing the semiconductor industry, interest in out-of-the-box security solutions has grown, even if this implies introducing novel materials such as two-dimensional layered semiconductors. Here, we show that high-performance, low-voltage, two-dimensional black phosphorus field-effect transistors (FETs) that have reconfigurable polarities are suitable for hardware security applications. The transistors can be dynamically switched between p-FET and n-FET operation through electrostatic gating and can achieve on–off ratios of 105 and subthreshold swings of 72 mV dec−1 at room temperature. Using the transistors, we create inverters that exhibit gains of 33.3 and are fully functional at a supply voltage of 0.2 V. We also create a security primitive circuit with polymorphic NAND/NOR obfuscation functionality with sub-1-V operation voltages, and the robustness of the polymorphic gate against power supply variations is tested using Monte Carlo simulations. Transistors that use two-dimensional black phosphorus as the active material can dynamically switch between p-type and n-type operation, and can be used to create security primitive circuits with polymorphic NAND/NOR obfuscation functionality.