Two-dimensional transition metal dichalcogenide alloys: preparation, characterization and applications

Sub-Nanosecond Single Mode-Locking Pulse Generation in an Idler-Resonant Intracavity KTA Optical Parametric Oscillator Driven by a Dual-Loss-Modulated Q-Switched and Mode-Locked Laser with an Acousto-Optic Modulator and MoWS2

The synthesis of 2D MoWS2 nanosheets involved the liquid-phase exfoliation technique was explored in this paper. The nonlinear optical response of MoWS2 was characterized in the 1 µm wavelength range, and its suitability as a saturable absorber (SA) was confirmed. Experimental demonstrations were conducted by using MoWS2 as an SA in an idler-resonant intracavity KTA optical parametric oscillator (OPO) driven by a dual-loss-modulated Q-switched and mode-locked (QML) YVO4/Nd:YVO4 laser with an acousto-optic modulator (AOM). By appropriately tuning the pump power and the AOM repetition rate, the Q-switched envelope pulse widths for the signal and idler waves could be significantly reduced to be shorter than the cavity round-trip transit time, i.e., the interval between two neighboring mode-locking pulses. Consequently, this enabled the generation of sub-nanosecond single mode-locking pulses with a low repetition rate, high pulse energy, and remarkable stability. With a repetition rate of 1 kHz and maximal pulse energies of 318 µJ and 169 µJ, respectively, sub-nanosecond single mode-locking pulses of the signal and idler waves were generated. The theoretical model was established using coupled rate equations with a Gaussian spatial distribution approximation. The numerical simulation results for generating sub-nanosecond single mode-locking pulses for the signal and idler waves within their respective Q-switched envelopes aligned fundamentally with the experimental results, proving that MoWS2 can be a potential nanomaterial for further optoelectronic applications.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Nanostructured transition metal dichalcogenides-based colorimetric sensors: Synthesis, characterization, and emerging applications

Nanostructured transition metal dichalcogenides (TMDCs) have garnered significant attention as prospective materials for the development of highly sensitive and versatile colorimetric sensors. This work explores the synthesis, characterization, and emerging applications of TMDC-based sensors, focusing on their unique structural aspects and inherent properties. The synthesis methods involve tailored fabrication techniques, such as chemical vapor deposition and hydrothermal processes, aimed at producing well-defined nanostructures that enhance sensor performance. Characterization techniques, including microscopy, spectroscopy, and surface analysis, are employed to elucidate the structural and chemical features of the nanostructured TMDCs. These analyses provide insights into the correlation between the material's morphology and its sensing capabilities. The colorimetric sensing mechanism relies on the modulation of optical properties in response to specific analytes, enabling rapid and visual detection. The emerging applications of TMDC-based colorimetric sensors span diverse fields, including environmental monitoring, healthcare, and industrial processes. The sensors exhibit high sensitivity, selectivity, and real-time response, making them ideal candidates for detecting various target analytes. Furthermore, their integration with complementary technologies such as microfluidics, can facilitate the development of on-site and point-of-care applications. This work highlights the interdisciplinary significance of nanostructured TMDC-based colorimetric sensors and underscores their potential contributions to addressing contemporary challenges in sensing technology.

Does an intrinsic strain contribute to the effect of quantum confinement phenomenon? An alloyed transition metal dichalcogenide series, Mo(S1-xSex)2 as a case study

It is well known that the bandgap of 2D transition metal dichalcogenides (TMDs) in the quantum confinement regime increases with a decrease in the number of layers. In this work, we show the effect of lattice strain on the dependence of the gap. We have designed an ideal system in the form of common-cationic alloyed-TMDs, Mo(S1-xSex)2, for such studies. With a large difference between the ionic radii of the two chalcogens, the nanoflakes of the alloys possessed a lattice strain and have been found to yield a lower bandgap than those of both the end-members, MoS2 and MoSe2. More importantly, the dependence of the bandgap on the layer number in the nanoflakes of the alloys turned out to be steeper than in conventional binary TMDs. The experimental results imply that the lattice strain in 2D semiconductors has contributed to the effect of the quantum confinement phenomenon in addition to decreasing the bandgap, the latter being earlier predicted from a theoretical model. We have derived the electronic bandgap and the band-edge energies of the series of alloyed-TMDs in their nanoflake forms and the dependences on the number of layers from the density of states (DOS), as obtained from scanning tunneling spectroscopy (STS) recorded in a scanning tunneling microscope (STM) in an extremely localized manner.

Resonant Band Hybridization in Alloyed Transition Metal Dichalcogenide Heterobilayers

Bandstructure engineering using alloying is widely utilized for achieving optimized performance in modern semiconductor devices. While alloying has been studied in monolayer transition metal dichalcogenides, its application in van der Waals heterostructures built from atomically thin layers is largely unexplored. Here, heterobilayers made from monolayers of WSe2 (or MoSe2) and MoxW1 - xSe2 alloy are fabricated and nontrivial tuning of the resultant bandstructure is observed as a function of concentration x. This evolution is monitored by measuring the energy of photoluminescence (PL) of the interlayer exciton (IX) composed of an electron and hole residing in different monolayers. In MoxW1 - xSe2/WSe2, a strong IX energy shift of ≈100 meV is observed for x varied from 1 to 0.6. However, for x < 0.6 this shift saturates and the IX PL energy asymptotically approaches that of the indirect bandgap in bilayer WSe2. This observation is theoretically interpreted as the strong variation of the conduction band K valley for x > 0.6, with IX PL arising from the K - K transition, while for x < 0.6, the bandstructure hybridization becomes prevalent leading to the dominating momentum-indirect K - Q transition. This bandstructure hybridization is accompanied with strong modification of IX PL dynamics and nonlinear exciton properties. This work provides foundation for bandstructure engineering in van der Waals heterostructures highlighting the importance of hybridization effects and opening a way to devices with accurately tailored electronic properties.

Composition-tunable transition metal dichalcogenide nanosheets via a scalable, solution-processable method

The alloying of two-dimensional (2D) transition metal dichalcogenides (TMDs) is an established route to produce robust semiconductors with continuously tunable optoelectronic properties. However, typically reported methods for fabricating alloyed 2D TMD nanosheets are not suitable for the inexpensive, scalable production of large-area (m2) devices. Herein we describe a general method to afford large quantities of compositionally-tunable 2D TMD nanosheets using commercially available powders and liquid-phase exfoliation. Beginning with Mo(1-x)WxS2 nanosheets, we demonstrate tunable optoelectronic properties as a function of composition. We extend this method to produce Mo0.5W0.5Se2 MoSSe, WSSe, and quaternary Mo0.5W0.5SSe nanosheets. High-resolution scanning transmission electron microscopy (STEM) imaging confirms the atomic arrangement of the nanosheets, while an array of spectroscopic techniques is used to characterize the chemical and optoelectronic properties. This transversal method represents an important step towards upscaling tailored TMD nanosheets with a broad range of tunable optoelectronic properties for large-area devices.

Aerosol-Assisted Chemical Vapor Deposition of 2H-WS2 From Single-Source Tungsten Dithiolene Precursors

The tungsten carbonyl dimethyldithiolene (dmdt) complexes W(CO)4(dmdt), W(CO)2(dmdt)2, and W(dmdt)3 were evaluated as potential single-source precursors for the chemical vapor deposition of WS2. The results of TGA-MS, DIP-MS, and pyrolysis with NMR analysis were consistent with a thermal decomposition pathway in which loss of 2-butyne through a retro[3+2]cycloaddition of the dithiolene ligand generated terminal sulfido ligands. Aerosol-assisted chemical vapor deposition onto silicon substrates was performed using all three complexes, yielding 2H-WS2 thin films as characterized by Raman spectroscopy and GI-XRD. Film morphology and elemental composition of the films were determined using SEM, EDS, and XPS. Four-point probe measurements afforded a film resistivity of 8.37 Ωcm for a sample deposited from W(dmdt)3 in toluene at 600 °C.

Formulation of Hierarchical Nanowire-Structured CoNiO2 and MoS2/CoNiO2 Hybrid Composite Electrodes for Supercapacitor Applications

Hierarchical porous nanowire-like MoS2/CoNiO2 nanohybrids were synthesized via the hydrothermal process. CoNiO2 nanowires were selected due to the edge site, high surface/volume ratio, and superior electrochemical characteristics as the porous backbone for decoration of layered MoS2 nanoflakes to construct innovative structure hierarchical three-dimensional (3D) porous NWs MoS2/CoNiO2 hybrids with excellent charge accumulation and efficient ion transport capabilities. Physicochemical analyses were conducted on the developed hybrid composite, revealing conclusive evidence that the CoNiO2 nanowires have been securely anchored onto the surface of the MoS2 nanoflake array. The electrochemical results strongly proved the benefit of the hierarchical 3D porous MoS2/CoNiO2 hybrid structure for the charge storage kinetics. The synergistic characteristics arising from the MoS2/CoNiO2 composite yielded a notably high specific capacitance of 1340 F/g at a current density of 0.5 A/g. Furthermore, the material exhibited sustained cycling stability, retaining 95.6% of its initial capacitance after 10 000 long cycles. The asymmetric device comprising porous MoS2/CoNiO2//activated carbon encompassed outstanding energy density (93.02 Wh/kg at 0.85 kW/kg) and cycling stability (94.1% capacitance retention after 10 000 cycles). Additionally, the successful illumination of light-emitting diodes underscores the significant potential of the synthesized MoS2/CoNiO2 (2D/1D) hybrid for practical high-energy storage applications.

Alloying enhanced negative Poisson's ratio in two-dimensional aluminum gallium nitride (AlxGa1-xN)

The negative Poisson's ratio (NPR) effect usually endows materials with promising ductility and shear resistance, facilitating a wider range of applications. It has been generally acknowledged that alloys show strong advantages in manipulating material properties. Thus, a thought-provoking question arises: how does alloying affect the NPR? In this paper, based on first-principles calculations, we systematically study the NPR in two-dimensional (2D) GaN and AlN, and their alloy of AlxGa1-xN. It is intriguing to find that the NPR in AlxGa1-xN is significantly enhanced compared to the parent materials of GaN and AlN. The underlying mechanism mainly originates from a counter-intuitive increase of the bond angle θ. We further study the microscopic origin of the anomalies by electron orbital analysis as well as electron localization functions. It is revealed that the distribution and movement of electrons change with the applied strain, providing a fundamental view on the effect of strain on lattice parameters and the NPR. The physical origin as revealed in this study deepens the understanding of the NPR and shed light on the future design of modern nanoscale electromechanical devices with fantastic functions based on the auxetic nanomaterials and nanostructures.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Electronic Structure of the Low-Lying States of the Triatomic MoS2 Molecule: The Building Block of 2D MoS2

Molybdenum disulfide (MoS2 ) is the building component of 1D-monolayer, 2D-layered nanosheets and nanotubes having many applications in industry, and it is detected in various molecular systems observed in nature. Here, the electronic structure and the chemical bonding of sixteen low-lying states of the triatomic MoS2 molecule are investigated, while the connection of the chemical bonding of the isolated MoS2 molecule to the relevant 2D-MoS2 , is emphasized. The MoS2 molecule is studied via DFT and multireference methodologies, i. e., MRCISD(+Q)/aug-cc-pVQZ(-PP)Mo . The ground state,X ˜ ${\tilde{X}}$ 3 B1 , is bent (Mo-S=2.133 Å and ϕ(SMoS)=115.9°) with a dissociation energy to atomic products of 194.7 kcal/mol at MRCISD+Q. In the ground and in the first excited state a double bond is formed between Mo and each S atom, i. e.,a 1 2 a 1 2 b 2 2 a 2 2 ${{{\rm a}}_{1}^{2}{{\rm a}}_{1}^{2}{{\rm b}}_{2}^{2}{{\rm a}}_{2}^{2}}$ . These two states differ in which d electrons of Mo are unpaired. The Mo-S bond distances of the calculated states range from 2.108 to 2.505 Å, the SMoS angles range from 104.1 to 180.0°, and the Mo-S bonds are single or double. Potential energy curves and surfaces have been plotted for theX ˜ ${\tilde{X}}$ 3 B1 , 5 A1 and 5 B1 states. Finally, the low-lying septet states of the triatomic molecule are involved in the material as a building block, explaining the variety of its morphologies.

Progress on Two-Dimensional Transitional Metal Dichalcogenides Alloy Materials: Growth, Characterisation, and Optoelectronic Applications

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have garnered remarkable attention in electronics, optoelectronics, and hydrogen precipitation catalysis due to their exceptional physicochemical properties. Their utilisation in optoelectronic devices is especially notable for overcoming graphene's zero-band gap limitation. Moreover, TMDs offer advantages such as direct band gap transitions, high carrier mobility, and efficient switching ratios. Achieving precise adjustments to the electronic properties and band gap of 2D semiconductor materials is crucial for enhancing their capabilities. Researchers have explored the creation of 2D alloy phases through heteroatom doping, a strategy employed to fine-tune the band structure of these materials. Current research on 2D alloy materials encompasses diverse aspects like synthesis methods, catalytic reactions, energy band modulation, high-voltage phase transitions, and potential applications in electronics and optoelectronics. This paper comprehensively analyses 2D TMD alloy materials, covering their growth, preparation, optoelectronic properties, and various applications including hydrogen evolution reaction catalysis, field-effect transistors, lithium-sulphur battery catalysts, and lasers. The growth process and characterisation techniques are introduced, followed by a summary of the optoelectronic properties of these materials.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

First Principle Study on the Effect of Strain on the Electronic Structure and Carrier Mobility of the Janus MoSTe and WSTe Monolayers

Using first-principle calculations, we investigate the impact of strain on the electronic structures and effective masses of Janus WSTe and MoSTe monolayers. The calculations were performed using the QUANTUM-ESPRESSO package, employing the PBE and HSE06 functionals. Our results demonstrate that strain fundamentally changes the electronic structures of the Janus WSTe and MoSTe monolayers. We observe that deformation causes a shift in the maxima and minima of the valence and conduction bands, respectively. We find that the effective electrons and hole masses of MoSTe and WSTe can be changed by deformation. In addition, the strain's effect on carrier mobility is also investigated in this work via the deformation potential theory.

Production of Magnetic Arsenic-Phosphorus Alloy Nanoribbons with Small Band Gaps and High Hole Conductivities

Quasi-1D nanoribbons provide a unique route to diversifying the properties of their parent 2D nanomaterial, introducing lateral quantum confinement and an abundance of edge sites. Here, a new family of nanomaterials is opened with the creation of arsenic-phosphorus alloy nanoribbons (AsPNRs). By ionically etching the layered crystal black arsenic-phosphorus using lithium electride followed by dissolution in amidic solvents, solutions of AsPNRs are formed. The ribbons are typically few-layered, several micrometers long with widths tens of nanometers across, and both highly flexible and crystalline. The AsPNRs are highly electrically conducting above 130 K due to their small band gap (ca. 0.035 eV), paramagnetic in nature, and have high hole mobilities, as measured with the first generation of AsP devices, directly highlighting their properties and utility in electronic devices such as near-infrared detectors, quantum computing, and charge carrier layers in solar cells.

Modularized batch production of 12-inch transition metal dichalcogenides by local element supply

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are regarded as pivotal semiconductor candidates for next-generation devices due to their atomic-scale thickness, high carrier mobility and ultrafast charge transfer. In analog to the traditional semiconductor industry, batch production of wafer-scale TMDs is the prerequisite to proceeding with their integrated circuits evolution. However, the production capacity of TMD wafers is typically constrained to a single and small piece per batch (mainly ranging from 2 to 4 inches), due to the stringent conditions required for effective mass transport of multiple precursors during growth. Here we developed a modularized growth strategy for batch production of wafer-scale TMDs, enabling the fabrication of 2-inch wafers (15 pieces per batch) up to a record-large size 12-inch wafers (3 pieces per batch). Each module, comprising a self-sufficient local precursor supply unit for robust individual TMD wafer growth, is vertically stacked with others to form an integrated array and thus a batch growth. Comprehensive characterization techniques, including optical spectroscopy, electron microscopy, and transport measurements unambiguously illustrate the high-crystallinity and the large-area uniformity of as-prepared monolayer films. Furthermore, these modularized units demonstrate versatility by enabling the conversion of as-produced wafer-scale MoS₂ into various structures, such as Janus structures of MoSSe, alloy compounds of MoS_2(1-x)Se_2x, and in-plane heterostructures of MoS₂-MoSe₂. This methodology showcases high-quality and high-yield wafer output and potentially enables the seamless transition from lab-scale to industrial-scale 2D semiconductor complementary to silicon technology.

Laser doping of 2D material for precise energy band design

The number of excellent 2D materials is finite for nano optoelectric devices including transistors, diodes, sensors, and so forth, thus the modulation of 2D materials is important to improve the performance of the current eligible 2D materials, and even to transform unqualified 2D materials into eligible 2D materials. Here we develop a fine laser doping strategy based on highly controllable laser direct writing, and investigate its effectivity and practicability by doping multilayer molybdenum ditelluride (MoTe₂). Power-gradient laser doping and patterned laser doping, for the first time, are presented for designable and fine doping of 2D materials. The laser-induced polar transition of MoTe₂ indicates good controllability of the method for the carrier concentration distribution in MoTe₂. Multiple devices with finely tuned energy band structures are demonstrated by means of power-gradient laser doping and patterned laser doping, further illustrating the design capability of a precise energy band in 2D materials.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Recent Advances for the Synthesis and Applications of 2-Dimensional Ternary Layered Materials

Layered materials with unique structures and symmetries have attracted tremendous interest for constructing 2-dimensional (2D) structures. The weak interlayer interaction renders them to be readily isolated into various ultrathin nanosheets with exotic properties and diverse applications. In order to enrich the library of 2D materials, extensive progress has been made in the field of ternary layered materials. Consequently, many brand-new materials are derived, which greatly extend the members of 2D realm. In this review, we emphasize the recent progress made in synthesis and exploration of ternary layered materials. We first classify them in terms of stoichiometric ratio and summarize their difference in interlayer interaction, which is of great importance to produce corresponding 2D materials. The compositional and structural characteristics of resultant 2D ternary materials are then discussed so as to realize desired structures and properties. As a new family of 2D materials, we overview the layer-dependent properties and related applications in the fields of electronics, optoelectronics, and energy storage and conversion. The review finally provides a perspective for this rapidly developing field.

The growth mechanism and intriguing optical and electronic properties of few-layered HfS2

Due to electronic properties superior to group VIB (Mo and W) transition metal dichalcogenides (TMDs), group IVB (Hf and Zr) TMDs have become intriguing materials in next-generation nanoelectronics. Therefore, the growth of few-layered hafnium disulfide (HfS₂) on c-plane sapphire as well as on a SiO₂/Si substrate has been demonstrated using chemical vapour deposition (CVD). The structural properties of HfS₂ were investigated by recording X-ray diffraction patterns and Raman spectra. The XRD results reveal that the layers are well oriented along the (0001) direction and exhibit the high crystalline quality of HfS₂. The Raman spectra confirm the in-plane and out-plane vibration of Hf and S atoms. Moreover, the HfS₂ layers exhibit strong absorption in the UV to visible region. The HfS₂ layer-based photodetector shows a photoresponsivity of ∼1.6, ∼0.38, and ∼0.21 μA W^-1 corresponding to 9, 38, and 68 mW cm^-2, respectively under green light illumination and is attributed to the generation of a large number of electron-hole pairs in the active region of the device. Besides, it also exhibits the highly crystalline structure of HfS₂ at high deposition temperature. The PL spectrum shows a single peak at ∼1.8 eV and is consistent with the pristine indirect bandgap of HfS₂ (∼2 eV). Furthermore, a few layered HfS₂ back gate field-effect transistor (FET) is fabricated based on directly grown HfS₂ on SiO₂/Si, and the device exhibits p-type behaviour. Thus, the controllable and easy growth method opens the latest pathway to synthesize few layered HfS₂ on different substrates for various electronic and optoelectronic devices.

MX family: an efficient platform for topological spintronics based on Rashba and Zeeman-like spin splittings

Taking various combinations of M = (Mo, W) and X = (C, S, Se) as examples, we propose that MX (M = transition metals, X = IV,V or VI elements) family can establish an excellent platform for both conventional and topological spintronics applications based on anisotropic Rashba-like and non-magnetic Zeeman-type spin splittings with electrically tunable nature. In particular, we observe sizeable Zeeman-like and Rashba-like spin splittings with an anisotropic nature. Meanwhile, they exhibit Rashba-like and topologically robust helical edge states when grown in ferroelectric and paraelectric phases, respectively. These MX monolayers are realized to be quantum valley Hall insulators due to valley contrasting Berry curvatures. The carriers in these MX monolayers can be selectively excited from opposite valleys depending on the polarity of circularly polarized light. The amplitude of the spin splitting can be further tuned by applying external means such as strain, electric field or alloy engineering. Furthermore, considering graphene sheet over the WC monolayer as a prototype example, we show that these MX monolayers can boost the relativistic effect by coupling with the systems exhibiting extremely weak spin-orbit coupling (SOC). Depending on the surface of WC monolayer in contact with the graphene sheet, graphene over WC monolayer passes through the transformation from the semiconducting junction to the Shotcky barrier-free contact. Finally, we reveal that these MX monolayers could also be grown on the substrates such as WS2(001)and GaTe (001) with type-II band alignment, where electron and hole become layer splitted across the interface. Our analysis should be fairly applied to other systems with strong SOC and an equivalent geometrical structure to the MX monolayers.

Continuous Color-Tunable Light-Emitting Devices Based on Compositionally Graded Monolayer Transition Metal Dichalcogenide Alloys

The diverse series of transition metal dichalcogenide (TMDC) materials has been employed in various optoelectronic applications, such as photodetectors, light-emitting diodes, and lasers. Typically, the detection or emission range of optoelectronic devices is unique to the bandgap of the active material. Therefore, to improve the capability of these devices, extensive efforts have been devoted to tune the bandgap, such as gating, strain, and dielectric engineering. However, the controllability of these methods is severely limited (typically ≈0.1 eV). In contrast, alloying TMDCs is an effective approach that yields a composition-dependent bandgap and enables light emissions over a wide range. In this study, a color-tunable light-emitting device using compositionally graded TMDC alloys is fabricated. The monolayer WS₂ /WSe₂ alloy grown by chemical vapor deposition shows a spatial gradient in the light-emission energy, which varies from 2.1 to 1.7 eV. This alloy is incorporated in an electrolyte-based light-emitting device structure that can tune the recombination zone laterally. Thus, a continuous and reversible color-tunable light-emitting device is successfully fabricated by controlling the light-emitting positions. The results provide a new approach for exploring monolayer semiconductor-based broadband optical applications.

Chemical Vapor Deposition of High-Optical-Quality Large-Area Monolayer Janus Transition Metal Dichalcogenides

One-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers is reported, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these 2D semiconductor monolayers takes place upon the thermodynamic-equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe₂ single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy, transmission electron microscopy, and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton-phonon coupling and enable an exciton g-factor of -3.3.

Atomic-scale mechanisms on the stepwise growth of MoxW1-xS2 into hexagonal flakes

The systematic in situ transmission electron microscopy (TEM) analysis suggests three stepwise formation stages during the growth of Mo_xW_1-xS₂ hexagonal flakes, which are the initial assembly of precursors into vertical structures, subsequent transition into horizontal structures, and final surface relaxing and faceting into hexagonal flakes.

Transition metal dichalcogenide graded alloy monolayers by chemical vapor deposition and comparison to 2D Ising model

In this work, a chemical vapor deposition (CVD) method was developed for the synthesis of transition metal dichalcogenide alloy monolayers, with a composition gradient in the radial direction. The composition gradient was achieved by controlling the substrate cooling rate during the CVD growth. The two types of alloys, namely, WS_2(1-x)Se_2x and MoS_2(1-x)Se_2x, were found to exhibit an opposite composition gradient. This is attributed to their different cohesive energies. A two-dimensional Ising model is used to explain the growth mechanism, where two ends of the composition were modeled as a magnetically ordered phase and a paramagnetic phase. The composition as a function of substrate temperature is then represented by the thermal magnetization curve.

Bandgap Engineering in 2D Lateral Heterostructures of Transition Metal Dichalcogenides via Controlled Alloying

2D heterostructures made of transition metal dichalcogenides (TMD) have emerged as potential building blocks for new-generation 2D electronics due to their interesting physical properties at the interfaces. The bandgap, work function, and optical constants are composition dependent, and the spectrum of applications can be expanded by producing alloy-based heterostructures. Herein, the successful synthesis of monolayer and bilayer lateral heterostructures, based on ternary alloys of MoS_{2(1- x )} Se_{2 x} -WS_{2(1- x )} Se_{2 x} , is reported by modifying the ratio of the source precursors; the bandgaps of both materials in the heterostructure are continuously tuned in the entire range of chalcogen compositions. Raman and photoluminescence (PL) spatial maps show good intradomain composition homogeneity. Kelvin probe measurements in different heterostructures reveal composition-dependent band alignments, which can further be affected by unintentional electronic doping during the growth. The fabrication of sequential multijunction lateral heterostructures with three layers of thickness, composed of quaternary and ternary alloys, is also reported. These results greatly expand the available tools kit for optoelectronic applications in the 2D realm.

Machine learning-enabled band gap prediction of monolayer transition metal chalcogenide alloys

Monolayer transition metal dichalcogenide (TMD) alloys with tunable direct band gaps have promising applications in nanoelectronics and optoelectronics. The composition-dependent band gaps of ternary, quaternary and quinary monolayer TMD alloys have been systematically studied combining density functional theory and machine learning models in the present study. The excellent agreement between the DFT-calculated band gaps and the ML-predicted values for the training, validation and test datasets demonstrates the accuracy of our machine learning based on a neural network model. It is found that the band gap bowing parameter is closely related to the difference between the band gaps of the endpoint material compositions of the monolayer TMD alloy and increases with increasing band gap difference. The band gap bowing effects of monolayer TMD alloys obtained by mixing different transition metals are attributed to the conduction band minimum positions, while those of monolayer TMD alloys obtained by mixing different chalcogen atoms are dominated by the valence band maximum positions. This study shows that monolayer TMD alloys with tunable direct band gaps can provide new opportunities for band gap engineering, as well as electronic and optoelectronic applications.

Transition Metal Dichalcogenides (TMDC)-Based Nanozymes for Biosensing and Therapeutic Applications

Nanozymes, a type of nanomaterial with enzyme-like properties, are a promising alternative to natural enzymes. In particular, transition metal dichalcogenides (TMDCs, with the general formula MX2, where M represents a transition metal and X is a chalcogen element)-based nanozymes have demonstrated exceptional potential in the healthcare and diagnostic sectors. TMDCs have different enzymatic properties due to their unique nano-architecture, high surface area, and semiconducting properties with tunable band gaps. Furthermore, the compatibility of TMDCs with various chemical or physical modification strategies provide a simple and scalable way to engineer and control their enzymatic activity. Here, we discuss recent advances made with TMDC-based nanozymes for biosensing and therapeutic applications. We also discuss their synthesis strategies, various enzymatic properties, current challenges, and the outlook for future developments in this field.

Structural, Electronic and Optical Properties of four α-Se-based Heterostructures with Hyperbolic Characteristics

The physical properties and potential applications of two-dimensional (2D) materials can be effectively modulated and enriched by constructing van der Waals heterostructures (VDWHs) with two or more 2D monolayer materials....

Optoelectronic Properties of Atomically Thin MoxW(1-x)S2 Nanoflakes Probed by Spatially-Resolved Monochromated EELS

Band gap engineering of atomically thin two-dimensional (2D) materials has attracted a huge amount of interest as a key aspect to the application of these materials in nanooptoelectronics and nanophotonics. Low-loss electron energy loss spectroscopy has been employed to perform a direct measurement of the band gap in atomically thin MoxW(1-x)S2 nanoflakes. The results show a bowing effect with the alloying degree, which fits previous studies focused on excitonic transitions. Additional properties regarding the Van Hove singularities in the density of states of these materials, as well as high energy excitonic transition, have been analysed as well.

Tailoring the Interfacial Band Offset by the Molecular Dipole Orientation for a Molecular Heterojunction Selector

Understanding and designing interfacial band alignment in a molecular heterojunction provides a foundation for realizing its desirable electronic functionality. In this study, a tailored molecular heterojunction selector is implemented by controlling its interfacial band offset between the molecular self-assembled monolayer with opposite dipole orientations and the 2D semiconductor (1L -MoS2 or 1L -WSe2 ). The molecular dipole moment direction determines the direction of the band bending of the 2D semiconductors, affecting the dominant transport pathways upon voltage application. Notably, in the molecular heterostructure with 1L -WSe2 , the opposite rectification direction is observed depending on the molecular dipole moment direction, which does not hold for the case with 1L -MoS2 . In addition, the nonlinearity of the molecular heterojunction selector can be significantly affected by the molecular dipole moment direction, type of 2D semiconductor, and metal work function. According to the choice of these heterojunction constituents, the nonlinearity is widely tuned from 1.0 × 101 to 3.6 × 104 for the read voltage scheme and from 0.4 × 101 to 2.0 × 105 for the half-read voltage scheme, which can be scaled up to an ≈482 Gbit crossbar array.

Influence of Plasmon Resonances and Symmetry Effects on Second Harmonic Generation in WS2-Plasmonic Hybrid Metasurfaces

The nonlinear process of second harmonic generation (SHG) in monolayer (1L) transition metal dichalcogenides (TMD), like WS₂, strongly depends on the polarization state of the excitation light. By combination of plasmonic nanostructures with 1L-WS₂ by transferring it onto a plasmonic nanoantenna array, a hybrid metasurface is realized impacting the polarization dependency of its SHG. Here, we investigate how plasmonic dipole resonances affect the process of SHG in plasmonic-TMD hybrid metasurfaces by nonlinear spectroscopy. We show that the polarization dependency is affected by the lattice structure of plasmonic nanoantenna arrays as well as by the relative orientation between the 1L-WS₂ and the individual plasmonic nanoantennas. In addition, such hybrid metasurfaces show SHG in polarization states, where SHG is usually forbidden for either 1L-WS₂ or plasmonic nanoantennas. By comparing the SHG in these channels with the SHG generated by the hybrid metasurface components, we detect an enhancement of the SHG signal by a factor of more than 40. Meanwhile, an attenuation of the SHG signal in usually allowed polarization states is observed. Our study provides valuable insight into hybrid systems where symmetries strongly affect the SHG and enable tailored SHG in 1L-WS₂ for future applications.

Efficient photocatalytic degradation of hazardous pollutants by homemade kitchen blender novel technique via 2D-material of few-layer MXene nanosheets

To attain elevated class MXene (Ti₃C₂Tx) through a homemade kitchen blender method, high shear mechanical exfoliation is highly required for the efficient delimitations of MXene nanosheets from bulk MAX (Ti₃AlC₂). We examine large-scale industrial productions of the MXene nanosheets, where combing the predicted 2D materials using a blender is a first-time novel approach with the delaminating solvent as a dimethyl sulfoxide (DMSO). And also manually created layered MXene systems (handmade) delaminating MXene sheets (MX-H) was furthermore employed for environmental dye-degradations applications. The materials characterizations was done for both the bulk MAX, MX-H and the MX-B. Additionally, the surface morphological studies like scanning electron microscopy (SEM) were investigated for both MX-H and MX-B as-prepared samples. SEM images indicated the high shear blander technique formations highly expanded/delaminated MXene (Ti₃C₂Tx) nanosheets compared to MX-H samples. FTIR technique is employed to identify -OH, C-H, C-O stretching vibrations for both materials. Raman spectroscopy analysis of MX-H and MX-B revealed 484.80 cm^-1 Raman shift assigned to E1g phonon mode of (Ti, C, O). The ultraviolet UV visible absorption spectra explored pure and catalyst added Methylene Blue (MB) dye stock solution using annular type photoreactor with visible light source of 300 W. The comparatives of MAX, MX-H and MX-B samples was investigated as photocatalytic activity, The blender made (MX-B) sample revealed 98% of efficiency.

Effect of atomic configuration and spin-orbit coupling on thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 solid solutions

Through a combination of density functional theory calculations and cluster-expansion formalism, the effect of the configuration of the transition metal atoms and spin-orbit coupling on the thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 is investigated. Our investigation reveals that, in spite of exhibiting a weak ordering tendency of Mo and W atoms at 0 K, monolayer 2H-Mo1-xWxS2 is thermodynamically stable as a single-phase random solid solution across the entire composition range at temperatures higher than 45 K. The spin-orbit coupling effect, induced mainly by W atoms, is found to have a minimal impact on the mixing thermodynamics of Mo and W atoms in monolayer 2H-Mo1-xWxS2; however, it significantly induces change in the electronic bandgap of the monolayer solid solution. We find that the band-gap energies of ordered and disordered solid solutions of monolayer 2H-Mo1-xWxS2 do not follow Vegard's law. In addition, the degree of the SOC-induced change in band-gap energy of monolayer 2H-Mo1-xWxS2 solid solutions not only depends on the Mo and W contents, but for a given alloy composition it is also affected by the configuration of the Mo and W atoms. This poses a challenge of fine-tuning the bandgap of monolayer 2H-Mo1-xWxS2 in practice just by varying the contents of Mo and W.

Observation of Strong Valley Magnetic Response in Monolayer Transition Metal Dichalcogenide Alloys of Mo0.5W0.5Se2 and Mo0.5W0.5Se2/WS2 Heterostructures

Monolayer transition metal dichalcogenide (TMD) alloys have emerged as a unique material system for promising applications in electronics, optoelectronics, and spintronics due to their tunable electronic structures, effective masses of carriers, and valley polarization with various alloy compositions. Although spin-orbit engineering has been extensively studied in monolayer TMD alloys, the valley Zeeman effect in these alloys still remains largely unexplored. Here we demonstrate the enhanced valley magnetic response in Mo_0.5W_0.5Se₂ alloy monolayers and Mo_0.5W_0.5Se₂/WS₂ heterostructures probed by magneto-photoluminescence spectroscopy. The large g factors of negatively charged excitons (trions) of Mo_0.5W_0.5Se₂ have been extracted for both pure Mo_0.5W_0.5Se₂ monolayers and Mo_0.5W_0.5Se₂/WS₂ heterostructures, which are attributed to the significant impact of doping-induced strong many-body Coulomb interactions on trion emissions under an out-of-plane magnetic field. Moreover, compared with the monolayer Mo_0.5W_0.5Se₂, the slightly reduced valley Zeeman splitting in Mo_0.5W_0.5Se₂/WS₂ is a consequence of the weakened exchange interaction arising from p-doping in Mo_0.5W_0.5Se₂ via interlayer charge transfer between Mo_0.5W_0.5Se₂ and WS₂. Such interlayer charge transfer further evidences the formation of type-II band alignment, in agreement with the density functional theory calculations. Our findings give insights into the spin-valley and interlayer coupling effects in monolayer TMD alloys and their heterostructures, which are essential to develop valleytronic applications based on the emerging family of TMD alloys.

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

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

Promises and prospects of two-dimensional transistors

Two-dimensional (2D) semiconductors have attracted tremendous interest as atomically thin channels that could facilitate continued transistor scaling. However, despite many proof-of-concept demonstrations, the full potential of 2D transistors has yet to be determined. To this end, the fundamental merits and technological limits of 2D transistors need a critical assessment and objective projection. Here we review the promise and current status of 2D transistors, and emphasize that widely used device parameters (such as carrier mobility and contact resistance) could be frequently misestimated or misinterpreted, and may not be the most reliable performance metrics for benchmarking 2D transistors. We suggest that the saturation or on-state current density, especially in the short-channel limit, could provide a more reliable measure for assessing the potential of diverse 2D semiconductors, and should be applied for cross-checking different studies, especially when milestone performance metrics are claimed. We also summarize the key technical challenges in optimizing the channels, contacts, dielectrics and substrates and outline potential pathways to push the performance limit of 2D transistors. We conclude with an overview of the critical technical targets, the key technological obstacles to the 'lab-to-fab' transition and the potential opportunities arising from the use of these atomically thin semiconductors.

Optimal field-effect transistor operation for high-resolution biochemical measurements

Field-effect transistors (FETs) are powerful tools for sensitive measurements of numerous biomarkers (e.g., proteins, nucleic acids, and antigen) and gaseous species. Most research studies in this field focused on building discrete devices with high performance. We show that instrumentation that is commonly used in multiple areas of physics and engineering can greatly improve the performance of measurement systems that embed FET-based transducers for biological applications. We review the state-of-the-art instrumentation in the field as applied to sensing with FETs. We show how high-performance dual-gate 2D FETs that we recently developed, when operated using closed-loop proportional-integral-derivative control, can drastically improve both the sensitivity and resolution. We further show that this closed-loop control approach can be extended to commonly used single-gate silicon FETs. The generalizability of the results will allow their application to virtually any previously developed FET-based sensor. Finally, we provide insight into further optimization and performance benefits that can be extracted by using the closed-loop feedback approach for applications in biosensing.

Lattice dynamics, optical and thermal properties of quasi-two-dimensional anisotropic layered semimetal ZrTe2

In this study, an investigation was conducted on the vibrational properties exhibited by 2D layered zirconium ditelluride by employing Raman spectroscopy and confirmed by DFT calculation.

Electrode Materials for Supercapacitors: A Review of Recent Advances

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

Plasma-Treated Ultrathin Ternary FePSe3 Nanosheets as a Bifunctional Electrocatalyst for Efficient Zinc-Air Batteries

Developing novel bifunctional electrocatalysts with advanced oxygen electrocatalytic activity is pivotal for next-generation energy-storage devices. Herein, we present ultrathin oxygen-doped FePSe₃ (FePSe₃-O) nanosheets by Ar/O₂ plasma treatment, with remarkable surface atom reorganization. Such surface atom reorganization generates multiple crystalline-amorphous interfaces that benefit the kinetics of oxygen evolution reaction, achieving a low overpotential of only 261 mV at 10 mA cm^-2 with a small Tafel slope of 41.13 mV dec^-1. Density functional theory calculation indicates that oxygen doping can also modulate the electrical states at the Fermi level with a decreased band gap responsible for the enhanced electrocatalytic performance. Such unique FePSe₃-O nanosheets can be further fabricated as the air cathode in rechargeable liquid zinc-air batteries (ZABs), which deliver a high open circuit potential of 1.47 V, a small charge-discharge voltage gap of 0.80 V, and good cycling stability for more than 800 circles. As a proof of concept, the flexible solid-state ZABs assembled with FePSe₃-O nanosheets as cathode also display a favorable charge-discharge performance, durable stability, and good bendability. This work sheds new insights into the rational design of defect-rich ternary thiophosphate nanosheets by plasma treatment toward enhanced oxygen electrocatalysts in metal-air batteries.

Phase-Controlled Synthesis of Monolayer W1- x Rex S2 Alloy with Improved Photoresponse Performance

Tuning bandgap and phases in the ternary 2D transition metal dichalcogenides (TMDs) alloys has opened up unexpected opportunities to engineer optoelectronic properties and explore potential applications. In this work, a salt-assisted chemical deposition vapor (CVD) growth strategy is reported for the creation of high-quality monolayer W_{1- x} Re_x S₂ alloys to fulfill a readily phase control from 1H to DT by changing the ratio of Re and W precursors. The structures and chemical compositions of doping alloys are confirmed by combining atomic resolution scanning transmission electron microscopy-annular dark field imaging with energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy, matching well with the calculated results. The field-effect transistors (FETs) devices fabricated based on 1H-W_0.9 Re_0.1 S₂ monolayer exhibit a n-type semiconducting behavior with the mobility of 0.4 cm² V^-1 s^-1 . More importantly, the FETs show high-performance responsivity with a value of 17 µA W^-1 in air, which is superior to that of monolayer CVD-grown WS₂ . This work paves the way toward synthesizing monolayer ternary alloys with controlled phases for potential optoelectronic applications.

Catalysis Mediated by 2D Black Phosphorus Either Pristine or Decorated with Transition Metals Species

Among the novel class of mono-elemental two-dimensional (2D) materials, termed Xenes, phosphorene is emerging as a great promise for its peculiar chemical and physical properties. This review collects a selection of the recent breakthroughs that are related to the application of phosphorene in catalysis and electrocatalysis. Noteworthy, thanks to its intrinsic Lewis basic character, pristine phosphorene turned out to be more efficient and more selective than other non-metal catalysts, in chemical processes as the electroreduction of nitrogen to ammonia or the alkylation of nucleophiles with esters. Once functionalized with transition metals nanoparticles (Co, Ni, Pd, Pt, Ag, Au), its catalytic activity has been evaluated in several processes, mainly hydrogen and oxygen evolution reactions. Under visible light irradiation, it has shown a great improvement of the activity, demonstrating high potential as a photocatalyst.

Potential 2D Materials with Phase Transitions: Structure, Synthesis, and Device Applications

Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few-layers and then become a large and important subfamily of two-dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long-range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase-transition materials have vast potential for use in new-concept and functional devices. Here, potential 2D phase-transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced. The structures and experimental phase-transition properties are summarized for the bulk materials and some of the obtained monolayers. In addition, recent experimental progress on the synthesis and measurement of monolayers, such as 1T-TaS₂ , CrI₃ , and Cr₂ Ge₂ Te₆ , is reviewed.