Synthesis, properties and applications of 2D non-graphene materials

Defect Density and Atomic Defect Recognition in the Middle Layer of a Trilayer MoS2 Stack

Molybdenum disulfide (MoS2) is one of the most intriguing two-dimensional materials, and moreover, its single atomic defects can significantly alter the properties. These defects can be both imaged and engineered using spherical and chromatic aberration-corrected high-resolution transmission electron microscopy (CC/CS-corrected HRTEM). In a few-layer stack, several atoms are vertically aligned in one atomic column. Therefore, it is challenging to determine the positions of missing atoms and the damage cross-section, particularly in the not directly accessible middle layers. In this study, we introduce a technique for extracting subtle intensity differences in CC/CS-corrected HRTEM images. By exploiting the crystal structure of the material, our method discerns chalcogen vacancies even in the middle layer of trilayer MoS2. We found that in trilayer MoS2 the middle layer's damage cross-section is about ten times lower than that in the monolayer. Our findings could be essential for the application of few-layer MoS2 in nanodevices.

ZnO monolayer-supported single atom catalysts for efficient electrocatalytic hydrogen evolution reaction

Hydrogen is identified as one of the most promising sustainable and clean energy sources. The development of a hydrogen evolution reaction (HER) catalyst with high activity is essential to meet future needs. Considering the novel advantages of two-dimensional materials and the high catalytic activity of atomic transition metals, in this study, using density functional theory calculations, the HER on a single transition metal (10 different TM atoms) adsorbed and doped ZnO monolayer (ZnO-m) has been investigated. The Volmer-Tafel reaction mechanisms and strain engineering of the three best HER catalysts are also discussed. The results show that Pt@ZnO-m, Co-doped ZnO-m and Ir-doped ZnO-m with high stability all have a smaller absolute H adsorption free energy than Pt, and the optimal value of Pt@ZnO-m is -0.017 eV. The calculation of the reaction energy barriers shows that the Volmer-Tafel step is favorable. Co@ZnO-m and Ir@ZnO-m have high HER activity, the widest pH range, and acid-alkali resistance. Pt@ZnO-m and Co-doped ZnO-m maintain excellent HER performances in the strain range of -4% to 4%.

2D nanocomposite materials for HER electrocatalysts - a review

Hydrogen energy has the potential to be a cost-effective and strong technology for brighter development. Hydrogen fuel production by water electrolyzers has attracted attention. 2D nanocomposites with distinctive properties have been extensively explored for various applications from hydrogen evolution reactions to improving the efficiency of water electrolyzer, which is the most eco-friendly, and high-performance for hydrogen production. Recently, typical 2D nanocomposites such as Metal-Free 2D, TMDs, Mxene, LDH, organic composites, and Heterostructure have recently been thoroughly researched for use in the HER. We discuss effective ways for increasing the HER efficiency of 2D catalysts in this paper, And the unique advantages and mechanisms for specific applications are highlighted. Several essential regulating strategies for developing 2D nanocomposite-based HER electrocatalysts are included such as interface engineering, defect engineering, heteroatom doping, strain & phase engineering, and hybridizing which improve HER kinetics, the electrical conductivity, accessibility to catalytic active sites, and reaction energy barrier can be optimized. Finally, the future prospects for 2D nanocomposites in HER are discussed, as well as a thorough overview of a variety of methodologies for designing 2D nanocomposites as HER electrocatalysts with excellent catalytic performance. We expect that this review will provide a thorough overview of 2D nanocatalysts for hydrogen production.

Tuneable Schottky contact of MoSi2N4/TaS2 van der Waals heterostructure

The two-dimensional M o S i 2 N 4 monolayer is an emerging semiconductor material that offers considerable promise due to its ultra-thin profile, tuneable mechanical properties, excellent optoelectronic properties and exceptional environmental stability. The van der Waals (vdW) heterostructure formed by stacking such two-dimensional monolayers has demonstrated superior performance across various domains. In this study, a vdW heterostructure combining the two-dimensional M o S i 2 N 4 and T a S 2 monolayers is examined using first-principles density functional theory. In its ground state, this van der Waals heterostructure establishes an ohmic contact with an exceptionally low potential barrier height. By modulating the vdW heterostructure with an applied electric field of -0.1 V/Å and under vertical stress, we discovered that M o S i 2 N 4 and T a S 2 can transition from an ohmic contact to a p-type Schottky with an ultra-low Schottky barrier height (SBH). Our observations may give valuable insights for designing reconfigurable, tuneable Schottky nano-devices with enhanced electronic and optical properties based on M o S i 2 N 4 / T a S 2 .

Nanobio Interface Between Proteins and 2D Nanomaterials

Two-dimensional (2D) nanomaterials have significantly contributed to recent advances in material sciences and nanotechnology, owing to their layered structure. Despite their potential as multifunctional theranostic agents, the biomedical translation of these materials is limited due to a lack of knowledge and control over their interaction with complex biological systems. In a biological microenvironment, the high surface energy of nanomaterials leads to diverse interactions with biological moieties such as proteins, which play a crucial role in unique physiological processes. These interactions can alter the size, surface charge, shape, and interfacial composition of the nanomaterial, ultimately affecting its biological activity and identity. This review critically discusses the possible interactions between proteins and 2D nanomaterials, along with a wide spectrum of analytical techniques that can be used to study and characterize such interplay. A better understanding of these interactions would help circumvent potential risks and provide guidance toward the safer design of 2D nanomaterials as a platform technology for various biomedical applications.

Large area MoS2 thin film growth by direct sulfurization

In this study, we present the growth of monolayer MoS₂ (molybdenum disulfide) film. Mo (molybdenum) film was formed on a sapphire substrate through e-beam evaporation, and triangular MoS₂ film was grown by direct sulfurization. First, the growth of MoS₂ was observed under an optical microscope. The number of MoS₂ layers was analyzed by Raman spectrum, atomic force microscope (AFM), and photoluminescence spectroscopy (PL) measurement. Different sapphire substrate regions have different growth conditions of MoS₂. The growth of MoS₂ is optimized by controlling the amount and location of precursors, adjusting the appropriate growing temperature and time, and establishing proper ventilation. Experimental results show the successful growth of a large-area single-layer MoS₂ on a sapphire substrate through direct sulfurization under a suitable environment. The thickness of the MoS₂ film determined by AFM measurement is about 0.73 nm. The peak difference between the Raman measurement shift of 386 and 405 cm^-1 is 19.1 cm^-1, and the peak of PL measurement is about 677 nm, which is converted into energy of 1.83 eV, which is the size of the direct energy gap of the MoS₂ thin film. The results verify the distribution of the number of grown layers. Based on the observation of the optical microscope (OM) images, MoS₂ continuously grows from a single layer of discretely distributed triangular single-crystal grains into a single-layer large-area MoS₂ film. This work provides a reference for growing MoS₂ in a large area. We expect to apply this structure to various heterojunctions, sensors, solar cells, and thin-film transistors.

Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.

Enhanced Optical Response of Zinc-Doped Tin Disulfide Layered Crystals Grown with the Chemical Vapor Transport Method

Tin disulfide (SnS₂) is a promising semiconductor for use in nanoelectronics and optoelectronics. Doping plays an essential role in SnS₂ applications, because it can increase the functionality of SnS₂ by tuning its original properties. In this study, the effect of zinc (Zn) doping on the photoelectric characteristics of SnS₂ crystals was explored. The chemical vapor transport method was adopted to grow pristine and Zn-doped SnS₂ crystals. Scanning electron microscopy images indicated that the grown SnS₂ crystals were layered materials. The ratio of the normalized photocurrent of the Zn-doped specimen to that of the pristine specimen increased with an increasing illumination frequency, reaching approximately five at 10⁴ Hz. Time-resolved photocurrent measurements revealed that the Zn-doped specimen had shorter rise and fall times and a higher current amplitude than the pristine specimen. The photoresponsivity of the specimens increased with an increasing bias voltage or decreasing laser power. The Zn-doped SnS₂ crystals had 7.18 and 3.44 times higher photoresponsivity, respectively, than the pristine crystals at a bias voltage of 20 V and a laser power of 4 × 10⁻⁸ W. The experimental results of this study indicate that Zn doping markedly enhances the optical response of SnS₂ layered crystals.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Tuning photoluminescence spectra of MoS2 with liquid crystals

The photoluminescence (PL) and Raman spectra of molybdenum disulfide (MoS₂) can be tuned with liquid crystals. A nematic liquid crystal, 5CB, was aligned in a zigzag direction on an MoS₂ monolayer flake. The PL and A_1g Raman mode peaks of the MoS₂ monolayer were shifted by 46 meV and 2 cm^-1, respectively, owing to the interaction between MoS₂ and the liquid crystal. Based on Lorentzian fitting analysis, it was confirmed that the peak positions and intensity ratios of the trion PL and exciton PL varied with the phase transition of the liquid crystal. This phenomenon was possibly caused by the transfer of electrons from MoS₂ to the liquid crystal. This electron transfer varies with the temperature-dependent change in the liquid crystal phase. Therefore, the PL spectra of MoS₂ can be tuned simply by controlling the phase, without changing the type of added material.

Hydrothermal-Assisted Synthesis and Stability of Multifunctional MXene Nanobipyramids: Structural, Chemical, and Optical Evolution

Recent advancements in two-dimensional materials have brought MXene (Ti₃C₂) into attention due to its exciting properties as a very promising material for various applications. In this work, we report a novel Ti₃C₂ nanobipyramid (Ti₃C₂ NB) structure obtained through a three-step process involving exfoliation, delamination, and subsequent hydrothermal treatment. The morphological and textural properties at each step of synthesis were studied using an array of experimental techniques such as transmission electron microscopy, scanning electron microscopy, and atomic force microscopy and the chemical properties through X-ray diffraction, Raman, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis. The Ti₃C₂ NBs exhibit fluorescence with an excitation-dependent emission. Further, the effect of temperature and pH on the fluorescence was also investigated, which opens up its scope in bioanalytical applications. Ti₃C₂ NBs showed a ∼43% increase in photoluminescence intensity from pH 3 to 11 while a ∼38% increase with the temperature from 20 to 80 °C. Usually, MXenes are highly susceptible to oxidation, but the Ti₃C₂ NBs were found to be chemically and optically stable even after 30 days. Bestowed with good hydrophilicity, the material exhibited high biocompatibility on the mouse fibroblast cell line L929. Further, L929 cells also showed good cellular adhesion on a Ti₃C₂ NB-modified glass substrate. These properties pave a way for its multifunctional ability as a sensor for pH and temperature as well as bioimaging.

Graphene to Advanced MoS2: A Review of Structure, Synthesis, and Optoelectronic Device Application

In contrast to zero-dimensional (0D), one-dimensional (1D), and even their bulk equivalents, in two-dimensional (2D) layered materials, charge carriers are confined across thickness and are empowered to move across the planes. The features of 2D structures, such as quantum confinement, high absorption coefficient, high surface-to-volume ratio, and tunable bandgap, make them an encouraging contestant in various fields such as electronics, energy storage, catalysis, etc. In this review, we provide a gentle introduction to the 2D family, then a brief description of transition metal dichalcogenides (TMDCs), mainly focusing on MoS2, followed by the crystal structure and synthesis of MoS2, and finally wet chemistry methods. Later on, applications of MoS2 in dye-sensitized, organic, and perovskite solar cells are discussed. MoS2 has impressive optoelectronic properties; due to the fact of its tunable work function, it can be used as a transport layer, buffer layer, and as an absorber layer in heterojunction solar cells. A power conversion efficiency (PCE) of 8.40% as an absorber and 13.3% as carrier transfer layer have been reported for MoS2-based organic and perovskite solar cells, respectively. Moreover, MoS2 is a potential replacement for the platinum counter electrode in dye-sensitized solar cells with a PCE of 7.50%. This review also highlights the incorporation of MoS2 in silicon-based heterostructures where graphene/MoS2/n-Si-based heterojunction solar cell devices exhibit a PCE of 11.1%.

Recent advancement in biomedical applications on the surface of two-dimensional materials: from biosensing to tissue engineering

As biosensors and biomedical devices have become increasingly important to everyday diagnostics and monitoring, there are tremendous, and constant efforts towards developing and improving the reliability and versatility of such technology. As they offer high surface area-to-volume ratios and a diverse range of properties, from electronic to optical, two dimensional (2D) materials have proven to be very promising candidates for biological applications and technologies. Due to the dimensionality, 2D materials facilitate many interfacial phenomena that have shown to significantly improve the performance of biosensors, while recent advances in synthesis techniques and surface engineering methods also enable the realization of future biomedical devices. This short review aims to highlight the influence of 2D material surfaces and the properties that arise due to their 2D structure. Using recent (within the last few years) examples of biosensors and biomedical applications, we emphasize the important role of 2D materials in advancing developments and research for biosensing and healthcare.

Spin-splitting effects on the interband optical conductivity and activity of phosphorene

Being able to tune the anisotropic interband transitions in phosphorene at finite temperature offers an enormous amount of possibilities in finding new insights in the optoelectronic community. To contribute to this goal we propose a Zeeman spin-splitting field aiming at absorbing various frequencies of the incident light. Employing the tight-binding Hamiltonian to describe the carrier dynamics and the Kubo formalism to formulate the orientation-dependent interband optical conductivity (IOC) and optical activity of phosphorene we investigate the absorption and scattering mechanisms in phosphorene depending on the Zeeman field strength and optical energy parameters. The optical activity features are characterized by exploring the eccentricity and shift phase of reflected and transmitted electromagnetic waves of the incident light. Different electronic phases in the absence and presence of Zeeman field ultimate different types of interband transitions of which in all cases the IOC along the armchair direction is larger than the zigzag one. However, we observed an irregular (regular) process for IOC with the Zeeman field along the armchair (zigzag) direction, resulting in irregular (regular) absorption and scattering mechanisms. Additionally, a little to no effects for temperature-dependent IOC are provided with the Zeeman field in undoped phosphorene. Further, almost linearly and elliptically polarizations are reported for the transmitted and reflected waves, respectively, indicating that the phosphorene is almost transparent. The emergence of Zeeman spin-splitting effects in optoelectronic properties of phosphorene is pleasant to make it a great potential candidate for logic applications.

Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions

Porous single-layer molybdenum disulfide (MoS₂) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS₂. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.

Growth of 'W' doped molybdenum disulfide on graphene transferred molybdenum substrate

In the present study, a novel method has been carried out to grow tungsten (W) doped molybdenum disulfide (MoS₂) on the graphene transferred TEM grid in a chemical vapor deposition (CVD) setup. Tungsten trioxide (WO₃) has been used as a source for ‘W’ while ‘Mo’ has been derived from Mo based substrate. Different experimental parameters were used in this experiment. Higher gas flow rate decreases the size of the sample flake and on other side increases the dopant concentrations. The interaction mechanism between Mo, S, W and oxygen (O) have been explored. The influence of oxygen seems to be not avoidable completely which also imposes effective growth condition for the reaction of Mo with incoming sulfur atoms. The difference in the migration energies of Mo, WO₃, S clusters on the graphene and the higher reactivity of Mo clusters over other possibly formed atomic clusters on the graphene leads to the growth of W doped MoS₂ monolayers. Formation of MoS₂ monolayer and the nature of edge doping of ‘W’ is explained well with the crystal model using underlying nucleation principles. We believe our result provide a special route to prepare W doped MoS₂ on graphene substrate in the future.

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS₂ ) and bismuth telluride (Bi₂ Te₃ ) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high-reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS₂ and Bi₂ Te₃ is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS₂ and Bi₂ Te₃ crystals with unprecedented resolution. Real-time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single-crystalline free-standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.

Progress on Electronic and Optoelectronic Devices of 2D Layered Semiconducting Materials

2D layered semiconducting materials (2DLSMs) represent the thinnest semiconductors, holding many novel properties, such as the absence of surface dangling bonds, sizable band gaps, high flexibility, and ability of artificial assembly. With the prospect of bringing revolutionary opportunities for electronic and optoelectronic applications, 2DLSMs have prospered over the past twelve years. From materials preparation and property exploration to device applications, 2DLSMs have been extensively investigated and have achieved great progress. However, there are still great challenges for high-performance devices. In this review, we provide a brief overview on the recent breakthroughs in device optimization based on 2DLSMs, particularly focussing on three aspects: device configurations, basic properties of channel materials, and heterostructures. The effects from device configurations, i.e., electrical contacts, dielectric layers, channel length, and substrates, are discussed. After that, the affect of the basic properties of 2DLSMs on device performance is summarized, including crystal defects, crystal symmetry, doping, and thickness. Finally, we focus on heterostructures based on 2DLSMs. Through this review, we try to provide a guide to improve electronic and optoelectronic devices of 2DLSMs for achieving practical device applications in the future.

Flexible Device Applications of 2D Semiconductors

Graphene-like single- or few-layer semiconductors, such as dichalcogenides and buckled nanocrystals, possess direct and tunable bandgaps, and excellent electrical, optical, mechanical and thermal properties. This unique set of desirable properties of 2D semiconductors has triggered great interest in developing ultra-thin 2D flexible electronic devices, which ranges from realizing better material quality and simplified fabrication processes, to improving device performance and expanding the application horizon. The most explored 2D flexible devices based on transition metal dichalcogenides and black phosphorous include field-effect transistors, optoelectronics, electronic sensors and supercapacitors. By taking advantage of a large portfolio of materials and properties of 2D crystals, a new generation of low-cost, high-performance, transparent, flexible and wearable devices looks attractive and promising in advancing flexible electronic technologies.

Temperature dependence of band gap in MoSe2 grown by molecular beam epitaxy

We report on a temperature-dependent band gap property of epitaxial MoSe₂ ultrathin films. We prepare uniform MoSe₂ films epitaxially grown on graphenized SiC substrates with controlled thicknesses by molecular beam epitaxy. Spectroscopic ellipsometry measurements upon heating sample in ultra-high vacuum showed temperature-dependent optical spectra between room temperature to 850 °C. We observed a gradual energy shift of optical band gap depending on the measurement temperature for different film thicknesses. Fitting with the vibronic model of Huang and Rhys indicates that the constant thermal expansion accounts for the steady decrease of band gap. We also directly probe both optical and stoichiometric changes across the decomposition temperature, which should be useful for developing high-temperature electronic devices and fabrication process with the similar metal chalcogenide films.

Graphene/Carbon Dot Hybrid Thin Films Prepared by a Modified Langmuir-Schaefer Method

The special electronic, optical, thermal, and mechanical properties of graphene resulting from its 2D nature, as well as the ease of functionalizing it through a simple acid treatment, make graphene an ideal building block for the development of new hybrid nanostructures with well-defined dimensions and behavior. Such hybrids have great potential as active materials in applications such as gas storage, gas/liquid separation, photocatalysis, bioimaging, optoelectronics, and nanosensing. In this study, luminescent carbon dots (C-dots) were sandwiched between oxidized graphene sheets to form novel hybrid multilayer films. Our thin-film preparation approach combines self-assembly with the Langmuir-Schaefer deposition and uses graphene oxide nanosheets as template for grafting C-dots in a bidimensional array. Repeating the cycle results in a facile and low-cost layer-by-layer procedure for the formation of highly ordered hybrid multilayers, which were characterized by photoluminescence, UV-visible, X-ray photoelectron, and Raman spectroscopies, as well as X-ray diffraction and atomic force microscopy.

Exfoliation of natural van der Waals heterostructures to a single unit cell thickness

Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.

Layered materials are held together by weak van der Waals forces facilitating layer-by-layer cleavage. Here, the authors demonstrate mechanical exfoliation of a naturally occurring franckeite mineral heterostructure, possessing p-type conductivity and remarkable electrochemical properties.

Derivatization and interlaminar debonding of graphite–iron nanoparticle hybrid interfaces using Fenton chemistry

Physico-chemical phenomena endure in the nanoscale domains of organic–inorganic interfaces for exfoliation, interfacial debonding and cracking of the graphite sheets.

Penta-BxNy sheet: a density functional theory study of two-dimensional material

By using density functional theory with generalized gradient approximation, we have carried out detailed investigations of two-dimensional BxNy nanomaterials in the Cairo pentagonal tiling geometry fully composed of pentagons (penta-BxNy). Only penta-BN and BN2 planar structures are dynamically stable without imaginary modes in their phonon spectra. Their stabilities have been further evaluated by formation energy analysis, first-principles molecular dynamics simulation, and mechanical stability analysis. Penta-BN2 is superior to penta-BN in structural stability. Its stability analysis against oxidization and functional group adsorption as well as its synthesizing reaction path analysis show possibilities in fabricating penta-BN2 on experiment. Furthermore, the penta-BN2 could be transferred from metallic to semiconducting by ionizing or covalently binding an electron per dinitrogen. Also, it has been found to have superior mechanical properties, such as the negative Poisson's ratio and the comparable stiffness as that of hexagonal h-BN sheet. These studies on the stabilities, electronic properties, and mechanical properties suggest penta-BN2 as an attractive material to call for further studies on both theory and experiment.

Electrostatically tunable lateral MoTe2 p-n junction for use in high-performance optoelectronics

Because of their ultimate thickness, layered structure and high flexibility, pn junctions based on layered two-dimensional semiconductors have been attracting increasing attention recently. In this study, for the first time, we fabricated lateral pn junctions (LPNJs) based on ultrathin MoTe2 by introducing two separated electrostatic back gates, and investigated their electronic and photovoltaic performance. Pn, np, nn, and pp junctions can be easily realized by modulating the conductive channel type using gate voltages with different polarities. Strong rectification effects were observed in the pn and np junctions and the rectification ratio reached ∼5 × 10(4). Importantly, we find a unique phenomenon that the parameters for MoTe2 LPNJs experience abrupt changes during the transition from p to n or n to p. Furthermore, a high performance photovoltaic device with a filling factor of above 51% and electrical conversion efficiency (η) of around 0.5% is achieved. Our findings are of importance to comprehensively understand the electronic and optoelectronic properties of MoTe2 and may further open up novel electronic and optoelectronic device applications.

High-performance photodetectors based on bandgap engineered novel layer GaSe0.5Te0.5 nanoflakes

Layered two-dimensional (2D) gallium monochalcogenide (GaX, X = S, Se, Te) semiconductor crystals hold great promise for potential electronics and photonics application.

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

This review presents recent progress on charge transport properties, carrier scattering mechanisms, and carrier mobility engineering of two-dimensional transition metal chalcogenides.

Recent advances in transition-metal dichalcogenide based nanomaterials for water splitting

The desire for sustainable and clean energy future continues to be the concern of the scientific community. Researchers are incessantly targeting the development of scalable and abundant electro- or photo-catalysts for water splitting. Owing to their suitable band-gap and excellent stability, an enormous amount of transition-metal dichalcogenides (TMDs) with hierarchical nanostructures have been extensively explored. Herein, we present an overview of the recent research progresses in the design, characterization and applications of the TMD-based electro- or photo-catalysts for hydrogen and oxygen evolution. Emphasis is given to the layered and pyrite-phase structured TMDs encompassing semiconducting and metallic nanomaterials. Illustrative results and the future prospects are pointed out. This review will provide the readers with insight into the state-of-the-art research progresses in TMD based nanomaterials for water splitting.