Molecular Beam Epitaxy of Two-Dimensional Vanadium-Molybdenum Diselenide Alloys

In Situ Vanadium Modification Induced a Back Interfacial Field Passivation Effect toward Efficient Kesterite Solar Cells beyond 11% Efficiency

Realization of a high-quality back electrode interface (BEI) with suppressed recombination is crucial for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. To achieve this goal, the construction of a traditional chemical passivation effect has been widely adopted and investigated. However, there is currently a lack of reports concerning the construction of a field passivation effect (FPE) for the BEI. Herein, considering the characteristic of the negligible difference in ionic radius between Mo (0.65 Å) and V (0.64 Å) as well as the presence of one less valence electron compared to Mo, vanadium (V) was employed and in situ incorporated into the MoSe2 interfacial layer during the deposition of the Mo:V electrode and selenization process. This allowed for the establishment of a desirable in situ VI-FPE interface with p-MoSe2:V/p-CZTSSe at the BEI. The p-type characteristic in MoSe2:V is attributed to the presence of the VMo acceptor; notably, the Fermi energy level of MoSe2:V has shifted downward by 0.62 eV compared to MoSe2, thereby facilitating the formation of an optimized band alignment between MoSe2:V and the absorber. Consequently, the photovoltaic parameters of the cell-FPE have experienced a significant increase due to the enhanced carrier transportation efficiency compared to cell-ref, resulting in a remarkable improvement in efficiency from 8.28 to 11.11%.

When Machine Learning Meets 2D Materials: A Review

The availability of an ever-expanding portfolio of 2D materials with rich internal degrees of freedom (spin, excitonic, valley, sublattice, and layer pseudospin) together with the unique ability to tailor heterostructures made layer by layer in a precisely chosen stacking sequence and relative crystallographic alignments, offers an unprecedented platform for realizing materials by design. However, the breadth of multi-dimensional parameter space and massive data sets involved is emblematic of complex, resource-intensive experimentation, which not only challenges the current state of the art but also renders exhaustive sampling untenable. To this end, machine learning, a very powerful data-driven approach and subset of artificial intelligence, is a potential game-changer, enabling a cheaper - yet more efficient - alternative to traditional computational strategies. It is also a new paradigm for autonomous experimentation for accelerated discovery and machine-assisted design of functional 2D materials and heterostructures. Here, the study reviews the recent progress and challenges of such endeavors, and highlight various emerging opportunities in this frontier research area.

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.

Magnetic doping in transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) are materials with unique electronic properties due to their two-dimensional nature. Recently, there is a large and growing interest in synthesizing ferromagnetic TMDCs for applications in electronic devices and spintronics. Apart from intrinsically magnetic examples, modification via either intrinsic defects or external dopants may induce ferromagnetism in non-magnetic TMDCs and, hence expand the application of these materials. Here, we review recent experimental work on intrinsically non-magnetic TMDCs that present ferromagnetism as a consequence of either intrinsic defects or doping via self-flux approach, ion implantation or e-beam evaporation. The experimental work discussed here is organized by modification/doping mechanism. We also review current work on density functional theory calculations that predict ferromagnetism in doped systems, which also serve as preliminary data for the choice of new doped TMDCs to be explored experimentally. Implementing a controlled process to induce magnetism in two-dimensional materials is key for technological development and this topical review discusses the fundamental procedures while presenting promising materials to be investigated in order to achieve this goal.

Atomically Substitutional Engineering of Transition Metal Dichalcogenide Layers for Enhancing Tailored Properties and Superior Applications

Transition metal dichalcogenides (TMDs) are a promising class of layered materials in the post-graphene era, with extensive research attention due to their diverse alternative elements and fascinating semiconductor behavior. Binary MX2 layers with different metal and/or chalcogen elements have similar structural parameters but varied optoelectronic properties, providing opportunities for atomically substitutional engineering via partial alteration of metal or/and chalcogenide atoms to produce ternary or quaternary TMDs. The resulting multinary TMD layers still maintain structural integrity and homogeneity while achieving tunable (opto)electronic properties across a full range of composition with arbitrary ratios of introduced metal or chalcogen to original counterparts (0-100%). Atomic substitution in TMD layers offers new adjustable degrees of freedom for tailoring crystal phase, band alignment/structure, carrier density, and surface reactive activity, enabling novel and promising applications. This review comprehensively elaborates on atomically substitutional engineering in TMD layers, including theoretical foundations, synthetic strategies, tailored properties, and superior applications. The emerging type of ternary TMDs, Janus TMDs, is presented specifically to highlight their typical compounds, fabrication methods, and potential applications. Finally, opportunities and challenges for further development of multinary TMDs are envisioned to expedite the evolution of this pivotal field.

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

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

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.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Tuning the 1T'/2H phases in WxMo1-xSe2 nanosheets

Controlling materials' morphology, crystal phase and chemical composition at the atomic scale has become central in materials research. Wet chemistry approaches have great potential in directing the material crystallisation process to achieve tuneable chemical compositions as well as to target specific crystal phases. Herein, we report the compositional and crystal phase tuneability achieved in the quasi-binary W_xMo_1-xSe₂ system with chemical and crystal phase mixing down to the atomic level. A series of W_xMo_1-xSe₂ solid solutions in the form of nanoflowers with atomically thin petals were obtained via a direct colloidal reaction by systematically varying the ratios of transition metal precursors. We investigate the effect of selenium precursor on the morphology of the W_xMo_1-xSe₂ material and show how using elemental selenium can enable the formation of larger and distinct nanoflowers. While the synthesised materials are compositionally homogeneous, they exhibit crystal phase heterogeneity with the co-existing domains of the 1T' and 2H crystal phases, and with evidence of MoSe₂ in the metastable 1T' phase. We show at single atom level of resolution, that tungsten and molybdenum can be found in both the 1T' and 2H lattices. The formation of heterophase 1T'/2H W_xMo_1-xSe₂ electrocatalysts allowed for a considerable improvement in the activity for the acidic hydrogen evolution reaction (HER) compared to pristine, 1T'-dominated, WSe₂. This work can pave the way towards engineered functional nanomaterials where properties, such as electronic and catalytic, have to be controlled at the atomic scale.

WSe2-VSe2 Alloyed Nanosheets to Enhance the Catalytic Performance of Hydrogen Evolution Reaction

Tuning the electronic structures of transition metal dichalcogenides (TMD) is essential for their implementation in next-generation energy technologies. In this study, we synthesized composition-tuned WSe₂-VSe₂ (W_1-xV_xSe₂, x = 0-1) alloyed nanosheets using a colloidal reaction. Alloying the semiconducting WSe₂ with VSe₂ converts the material into a metallic one, followed by a 2H-to-1T phase transition at x = 0.7. Over a wide composition range, WSe₂ and VSe₂ are atomically immiscible and form separate ordered domains. The miscible alloy at x = 0.1 displayed enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. This trend was correlated with the d-band center via a volcano-type relationship. Spin-polarized density functional theory calculations consistently predicted the atomic immiscibility, which became more significant at the 2H-1T phase transition composition. The Gibbs free energy of H adsorption on the basal planes (Se or hole sites) and the activation barriers along the Volmer-Heyrovsky reaction pathway supported the enhanced HER performance of the alloy phase, suggesting that the dispersed V-doped structures were responsible for the best HER catalytic activity. Our study demonstrates how the atomic structure of TMD alloy nanosheets plays a crucial role in enhancing catalytic activity.

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.

Nontrivial Doping Evolution of Electronic Properties in Ising-Superconducting Alloys

Transition metal dichalcogenides offer unprecedented versatility to engineer 2D materials with tailored properties to explore novel structural and electronic phase transitions. In this work, the atomic-scale evolution of the electronic ground state of a monolayer of Nb_{1- δ} Mo_δ Se₂ across the entire alloy composition range (0 < δ < 1) is investigated using low-temperature (300 mK) scanning tunneling microscopy and spectroscopy (STM/STS). In particular, the atomic and electronic structure of this 2D alloy throughout the metal to semiconductor transition (monolayer NbSe₂ to MoSe₂ ) is studied. The measurements enable extraction of the effective doping of Mo atoms, the bandgap evolution and the band shifts, which are monotonic with δ. Furthermore, it is demonstrated that collective electronic phases (charge density wave and superconductivity) are remarkably robust against disorder and further shown that the superconducting T_C changes non-monotonically with doping. This contrasting behavior in the normal and superconducting state is explained using first-principles calculations. Mo doping is shown to decrease the density of states at the Fermi level and the magnitude of pair-breaking spin fluctuations as a function of Mo content. These results paint a detailed picture of the electronic structure evolution in 2D TMD alloys, which is of utmost relevance for future 2D materials design.

Polytypic Phase Transition of Nb1-xVxSe2 via Colloidal Synthesis and Their Catalytic Activity toward Hydrogen Evolution Reaction

Polytypes of two-dimensional transition metal dichalcogenide can extend the architecture and application of nanostructures. Herein, Nb_1-xV_xSe₂ alloy nanosheets in the full composition range (x) were synthesized by a colloidal reaction. At x = 0.1-0.3, a phase transition occurred from various hexagonal (three 2H and one 4H types) phase NbSe₂ to an atomically homogeneous 1T phase VSe₂. Density functional theory calculations also revealed a polytypic phase transition at x = 0.3, which was shifted close to 0 in the presence of Se vacancies. Furthermore, the calculations validate favorable formation of Se vacancies at the phase transition. The sample at x = 0.3 exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄. The Gibbs free energy indicates that the catalytic HER performance is correlated with the active Se vacancy sites of polytypic structures.

Electronic Properties and Carrier Dynamics at the Alloy Interfaces of WS2x Se2-2x Spiral Nanosheets

Electronic properties at the interfaces between different-composition domains of 2D-alloys are key for their optical, electronic, and optoelectronic applications. Understanding the interfacial electronic structures and carrier dynamics is essential for designing and fabricating devices that use these alloys. Here, WS_2x Se_2-2x spiral nanosheets are prepared using the physical vapor deposition method, and the nonlinear optical and electronic properties, as well as the carrier dynamics at the interfaces between the WS and WSe domains, are studied. Second-harmonic generation tests demonstrate that these nanosheets exhibit a very strong layer-dependent nonlinear optical effect. Atomic-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements reveal that S and Se atoms are non-uniformly distributed, forming WS domains, WSe domains, and defect-related areas. Atomic STM images and STS maps reveal enhanced local density of states by electron scattering at the WS/WSe interfaces, providing a detailed nanoscale interpretation of the S/Se-ratio-dependent lifetimes observed in pump-probe spectroscopy measurements. This work provides valuable interfacial characterization of 2D-alloy materials, using state-of-the-art methods with high temporal and spatial resolutions. The obtained insights are likely to be useful for prospective applications.

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano-electronics and nano-optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed-laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up-to-date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up-to-date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

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.

Phase-Transition Mo1-xVxSe2 Alloy Nanosheets with Rich V-Se Vacancies and Their Enhanced Catalytic Performance of Hydrogen Evolution Reaction

Alloys of transition-metal dichalcogenide can display distinctive phase evolution because of their two-dimensional structures. Herein, we report the colloidal synthesis of Mo_1-xV_xSe₂ alloy nanosheets with full composition tuning. Alloying led to a phase transition at x = 0.7 from the semiconducting 2H phase MoSe₂ to the metallic 1T phase VSe₂. It also produced significant V and Se vacancies, which became the richest in the 2H phase at x = 0.3-0.5. Extensive spin-polarized density functional theory calculations consistently predicted the 2H-1T phase transition at x = 0.7, in agreement with the experimental results. The vacancy formation energy also supports the formation of V and Se vacancies. Alloying in the 2H phase enhanced the electrocatalytic performance toward hydrogen evolution reaction (HER) at x = 0.3 (in 0.5 M H₂SO₄) or 0.4 (in 1 M KOH). The Gibbs free energy along the HER pathway indicates that this maximum performance is due to the highest concentration of active V and Se vacancy sites.

Two-Dimensional Magnets beyond the Monolayer Limit

Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.

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.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.