Surface-diffusion-limited growth of atomically thin WS2 crystals from core-shell nuclei

Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations

Hexagonal boron nitride (h-BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS2 and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS2 crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS2. Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS2 crystals.

Confined VLS Growth of Single-Layer 2D Tungsten Nitrides

Two-dimensional (2D) metal nitrides have garnered significant interest due to their potential applications in future electronics and quantum systems. However, the synthesis of such materials with sufficient uniformity and at relevant scales remains an unaddressed challenge. This study demonstrates the potential of confined growth to control and enhance the morphology of 2D metal nitrides. By restricting the reaction volume of vapor-liquid-solid reactions, an enhanced precursor concentration was achieved that reduces the nucleation density, resulting in larger grain sizes and suppression of multilayer growth. Detailed characterization reveals the importance of balancing the energetic and kinetic aspects of tungsten nitride formation toward this ability. The introduction of a promoter enabled the realization of large-scale, single-layer tungsten nitride with a uniform and high interfacial quality. Finally, our advance in morphology control was applied to the production of edge-enriched 2D tungsten nitrides with significantly enhanced hydrogen evolution ability, as indicated by an unprecedented Tafel slope of 55 mV/dec.

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.

The Hidden Flower in WS2 Flakes: A Combined Nanomechanical and Tip-Enhanced Raman Exploration

We report on tungsten disulfide (WS₂) flakes grown by chemical vapor deposition (CVD), which exhibit a flower-like surface structure above the primary few-layer flake with a triangular shape. The fine structure is only revealed in the mechanical, chemical, and electronic properties of the flake but not in the topography. The origin of this structure is the peculiar one-step growth during the CVD process that permits to control the sulfur concentration at any time. A high concentration of S at the onset of the deposition process leads to a rapid growth of the flake, resulting in tungsten vacancies. Reducing the sulfur concentration toward the end of the growth slows down the reaction and leads to sulfur vacancies. These microscale domains were studied by confocal- and tip-enhanced Raman spectroscopy revealing their chemical composition with high spatial resolution. A strong quenching of the photoluminescence in the tungsten-vacancy domains is observed. Atomic force microscope measurements, performed in intermittent contact mode, force modulation mode (including lateral force mode), and PeakForce quantitative nanomechanics mode, show that the mechanical properties of these domains differ. Within the tungsten-vacancy domains, the adhesion force is reduced, while the friction force increased. Kelvin probe force microscopy measurements show that the electronic properties of the flakes are modulated by these domains. The combined nanomechanical and nanospectroscopy measurements provide detailed insights on the inhomogeneous surface properties of the single WS₂ flake, further highlighting how its multidomain properties can be finely tuned using CVD.

Bandgap modulation in the two-dimensional core-shell-structured monolayers of WS2

Tungsten disulfide (WS2) has tunable bandgaps, which are required for diverse optoelectronic device applications. Here, we report the bandgap modulation in WS2 monolayers with two-dimensional core-shell structures formed by unique growth mode in chemical vapor deposition (CVD). The core-shell structures in our CVD-grown WS2 monolayers exhibit contrasts in optical images, Raman, and photoluminescence spectroscopy. The strain and doping effects in the WS2, introduced by two different growth processes, generate PL peaks at 1.83 eV (at the core domain) and 1.98 eV (at the shell domain), which is distinct from conventional WS2 with a primary PL peak at 2.02 eV. Our density functional theory (DFT) calculations explain the modulation of the optical bandgap in our core-shell-structured WS2 monolayers by the strain, accompanying a direct-to-indirect bandgap transition. Thus, the core-shell-structured WS2 monolayers provide a practical method to fabricate lateral heterostructures with different optical bandgaps, which are required for optoelectronic applications.

Millimeter-Scale Continuous Film of MoS2 Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor

While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS₂ have limited controllability due to their lack of self-controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS₂ film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N₂ atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS₂ grows next to the coating. At 800 °C, the synthesized MoS₂ showed a uniform single-layer MoS₂ film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS₂ lateral growth. The synthesized MoS₂ with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS₂ showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS₂-based electronic and optoelectronic devices such as transistors and photodetectors.

Recent Advances in WS2 and Its Based Heterostructures for Water-Splitting Applications

The energy from fossil fuels has been recognized as a main factor of global warming and environmental pollution. Therefore, there is an urgent need to replace fossil fuels with clean, cost-effective, long-lasting, and environmentally friendly fuel to solve the future energy crisis of the world. Therefore, the development of clean, sustainable, and renewable energy sources is a prime concern. In this regard, solar energy-driven hydrogen production is considered as an overriding opening for renewable and green energy by virtue of its high energy efficiency, high energy density, and non-toxicity along with zero emissions. Water splitting is a promising technology for producing hydrogen, which represents a potentially and environmentally clean fuel. Water splitting is a widely known process for hydrogen production using different techniques and materials. Among different techniques of water splitting, electrocatalytic and photocatalytic water splitting using semiconductor materials have been considered as the most scalable and cost-effective approaches for the commercial production of sustainable hydrogen. In order to achieve a high yield of hydrogen from these processes, obtaining a suitable, efficient, and stable catalyst is a significant factor. Among the different types of semiconductor catalysts, tungsten disulfide (WS2) has been widely utilized as a catalytic active material for the water-splitting process, owing to its layered 2D structure and its interesting chemical, physical, and structural properties. However, WS2 suffers from some disadvantages that limit its performance in catalytic water splitting. Among the various techniques and strategies that have been constructed to overcome the limitations of WS2 is heterostructure construction. In this process, WS2 is coupled with another semiconducting material in order to facilitate the charge transfer and prevent the charge recombination, which will enhance the catalytic performance. This review aims to summarize the recent studies and findings on WS2 and its heterostructures as a catalyst in the electrocatalytic and photocatalytic water-splitting processes.

Plasma driven nano-morphological changes and photovoltaic performance in dye sensitized 2D-layered dual oxy-sulfide phase WS2 films

The present work examined dye sensitized 2D layered tungsten disulfide (WS₂) as a photo-anode in solar cells with no use of nanocrystalline metal oxides as electron acceptors such as titanium dioxide. It is observed that coating WS₂ directly onto a fluorine doped tin oxide (FTO) transparent conductor, annealed at 530 °C, resulted in a mixed oxy-sulfide dual phase as confirmed by transmission electron microscopy and X-ray diffraction studies. Further studies on the surface morphology of the dual phase WS₂-WO₃ film showed a random distribution of platelets which further shaped into precisely regulated hexagonal platelets upon plasma treatment. High resolution transmission electron microscopic studies elucidated two different phases, WS₂ and WO₃, with d-spacing values of 0.26 nm and 0.37 nm, respectively. A well-defined grain boundary was also observed which separated the oxy-sulfide phase in the sample. The dual WS₂-WO₃ phase films showed optical absorption in the wavelength range of 350 nm-800 nm with a systematic increase in plasma exposure duration. Photovoltaic devices fabricated using the WS₂-WO₃ mixed phase photo-anodes resulted in 0.61% efficiency (η) which further was observed to be sensitive to the plasma exposure as it was observed that the 20 minute plasma treated sample increased the η value to 0.67%. Plasma treatment on the dual-phase samples orients and modifies the shapes of the platelets with a significant change in the surface which eventually influences the charge transport in resulting photovoltaic devices and thus the variation with respect to exposure duration.