Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites

Nanoscale Three-Dimensional Network Structure of a Mesoporous Particle Unveiled via Adaptive Multidistance Coherent X-ray Tomography

Mesoporous nanoparticles provide rich platforms to devise functional materials by customizing the three-dimensional (3D) structures of nanopores. With the pore network as a key tuning parameter, the noninvasive and quantitative characterization of these 3D structures is crucial for the rational design of functional materials. This has prompted researchers to develop versatile nanoprobes with a high penetration power to inspect various specimens sized a few micrometers at nanoscale 3D resolutions. Here, with adaptive phase retrievals on independent data sets with different sampling frequencies, we introduce multidistance coherent X-ray tomography as a noninvasive and quantitative nanoprobe to realize high-resolution 3D imaging of micrometer-sized specimens. The 3D density distribution of an entire mesoporous silica nanoparticle was obtained at 13 nm 3D resolution for quantitative physical and morphological analyses of its 3D pore structure. The morphological features of the whole 3D pore network and pore connectivity were examined to gain insight into the potential functions of the particles. The proposed multidistance tomographic imaging scheme with quantitative structural analyses is expected to advance studies of functional materials by facilitating their structure-based rational design.

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Morphological dependent exciton dynamics and thermal transport in MoSe2 films

Thermal transport and exciton dynamics of semiconducting transition metal dichalcogenides (TMDCs) play an immense role in next-generation electronic, photonic, and thermoelectric devices. In this work, we synthesize distinct morphologies (snow-like and hexagonal) of a trilayer MoSe₂ film over the SiO₂/Si substrate via the chemical vapor deposition (CVD) method and investigated their morphological dependent exciton dynamics and thermal transport behaviour for the first time to the best of our knowledge. Firstly, we studied the role of spin-orbit and interlayer couplings both theoretically as well as experimentally via first-principles density functional theory and photoluminescence study, respectively. Further, we demonstrate morphological dependent thermal sensitive exciton response at low temperatures (93-300 K), showing more dominant defect-bound excitons (E_L) in snow-like MoSe₂ compared to hexagonal morphology. We also examined the morphological-dependent phonon confinement and thermal transport behaviour using the optothermal Raman spectroscopy technique. To provide insights into the nonlinear temperature-dependent phonon anharmonicity, a semi-quantitative model comprising volume and temperature effects was used, divulging the dominance of three-phonon (four-phonon) scattering processes for thermal transport in hexagonal (snow-like) MoSe₂. The morphological impact on thermal conductivity (k_s) of MoSe₂ has also been examined here by performing the optothermal Raman spectroscopy, showing k_s ∼ 36 ± 6 W m^-1 K^-1 for snow-like and ∼41 ± 7 W m^-1 K^-1 for hexagonal MoSe₂. Our research will contribute to the understanding of thermal transport behaviour in different morphologies of semiconducting MoSe₂, finding suitability for next-generation optoelectronic devices.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS₂ and WS₂, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS₂ with ∼200  μ m in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS₂ in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS₂ nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

Engineering Grain Boundaries in Two-Dimensional Electronic Materials

Engineering the boundary structures in 2D materials provides an unprecedented opportunity to program the physical properties of the materials with extensive tunability and realize innovative devices with advanced functionalities. However, structural engineering technology is still in its infancy, and creating artificial boundary structures with high reproducibility remains difficult. In this review, various emergent properties of 2D materials with different grain boundaries, and the current techniques to control the structures, are introduced. The remaining challenges for scalable and reproducible structure control and the outlook on the future directions of the related techniques are also discussed.

Pulsed Carrier Gas Assisted High-Quality Synthetic 3R-Phase Sword-like MoS2: A Versatile Optoelectronic Material

Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3R)-phase MoS₂ in a well-defined sword-like geometry having lengths up to 120 μm, uniform width of 2-3 μm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3R-phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS₂ swords signifies the potential of MoS₂ to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm²/V-s and I_on/I_off ratio of ∼10⁶ further confirm the electronic-grade standard of this 3R-phase MoS₂. These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.

Molten-droplet-driven growth of MoS2 flakes with controllable morphology transition for hydrogen evolution reactions

The controllable fabrication of two-dimensional transition-metal dichalcogenides (2D TMDs) and a deep understanding of the corresponding process mechanisms are of fundamental importance for their further applications. In this study, the molten-droplet-driven (MDD) growth of MoS₂ based on a Na-Mo-O molten-salt chemical vapor deposition (CVD) method is demonstrated via temperature-dependent dispersion and spreading of droplets on a surface, yielding MoS₂ flakes with morphology transition from compact triangles to fractal dendrites with the increase in temperature. By building up the dependence between the formed morphologies of grown MoS₂ flakes and the corresponding kinetics during successive growth processes, it was found that the wetting-driven force, which is governed by interface free energies (surface tension) of molten droplets, would largely determine the driven movement of the droplet, and then the formation of different morphologies. Finally, based on these MoS₂ flakes, a systematic improvement of the hydrogen evolution reaction (HER) was demonstrated in accordance with the evolution of morphologies from compact to fractal. This study presents an important advance in understanding the growth mechanisms related to the molten-salt-assisted CVD fabrication of 2D TMDs and provides a facile method for tailoring the growth and application of 2D TMDs with controllably trimmed morphologies.

Activating the Electrocatalysis of MoS2 Basal Plane for Hydrogen Evolution via Atomic Defect Configurations

Point defects of heteroatoms and vacancies can activate the inert basal plane of molybdenum sulfide (MoS₂ ) to improve its performance on catalyzing the hydrogen evolution reaction (HER). However, the synergy between heteroatoms and vacancies is still unclear. Here, a chemical vapor deposition-assisted in situ vanadium (V) doping method is used to synthesize monolayer MoS₂ with abundant and tunable vacancies and V-dopants in the lattice. Ten delicate defect configurations are prepared to provide a complex system for the relationship investigation between microstructure and catalytic performance. The combination of on-chip electrochemical tests and theoretical calculations indicates that the HER performance greatly depends on the type and amount of defect configurations. The optimal configuration is that three V atoms are aggregated and accompanied by abundant sulfur vacancies, in which, H atoms directly interact with Mo and V atoms to form the most stable metal-bridge structure. The on-chip measurements also confirm that the sample with high concentrations of this type of defect configuration exhibits the best catalytic performance, indicating the efficient synergy in the optimal configuration. The revealed effects of defect configurations are expected to inspire the design and regulation of high-efficiency 2D catalysts.

Tuning the morphology of 2D transition metal chalcogenides via oxidizing conditions

Two-dimensional transition metal chalcogenides (TMCs) are emerging as an intriguing platform to realize nascent properties in condensed matter physics, materials science and device engineering. Controllable growing of TMCs becomes increasingly important, especially for the layer number, doping, and morphology. Here, we successfully tune the morphology of MoS₂, MoSe₂, WS₂and WSe₂, from homogenous films to individual single crystalline grains only via changing the oxidizing growth conditions. The oxidization degrees are determined by the oxygen that adsorbed on substrates and the oxygen concentrations in reaction gas together. We find the homogenous films are easily formed under the reductive conditions, triangular grains prefer the weak oxidizing conditions, and medium oxidizing conditions bring in dendritic grains with higher oxygen doping and inhomogenous photoluminescence intensities from edge to interior regions shown in the dendritic grains. These growth rules under different oxidizing conditions are easily generalized to other TMCs, which also show potential for growing specific TMCs with designed oxygen doping levels.

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.

Toward High-Performance Self-Driven Photodetectors via Multistacking Van der Waals Heterostructures

The unique optoelectronic properties of layered van der Waals (vdW) heterostructures open up exciting opportunities for high-performance photodetectors. Self-driven photodetectors are desirable for reducing power consumption and minimizing the device size. Here, a semiconductor-insulator-semiconductor-type multistacking WSe₂/graphene/h-BN/MoS₂ vdW heterostructure is demonstrated to realize an enhanced self-powered photodetector with a high on-off current ratio of about 1.2 × 10⁵ and a high photoresponsivity of 3.6 A/W without applying bias, which is the highest photoresponsivity ever reported for self-powered photodetectors. Because of the difference in the Fermi level, a built-in electrical field is formed at the WSe₂/graphene junction, where the photoexcited electrons and holes can be efficiently separated and the carriers can easily tunnel through the MoS₂/h-BN junction driven by the enhanced potential. Therefore, the enhanced self-powered photodetection is attributable to highly efficient carrier tunneling through large h-BN electron barriers. By comparison, when the stacking sequence is changed to make WSe₂/MoS₂ p-n heterojunctions lay on graphene/h-BN, the self-powered photocurrent is still generated because of the type-II band alignment, which exhibits lower but still relevant values with a light on/off ratio of ∼8 × 10³ and a photoresponsivity of ∼2.39 A/W. The efficient enhancement demonstrates that multistacking heterostructures significantly elevate the performance of self-powered photodetectors, providing a feasible route to develop high-performance self-powered optoelectronic devices and extend their applications in integrated optoelectronic systems.

Wafer-Scale Production of Transition Metal Dichalcogenides and Alloy Monolayers by Nanocrystal Conversion for Large-Scale Ultrathin Flexible Electronics

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are unit-cell thick materials with tunable physical properties according to their size, morphology, and chemical composition. Their transition of lab-scale research to industrial-scale applications requires process development for the wafer-scale growth and scalable device fabrication. Herein, we report on a new type of atmospheric pressure chemical vapor deposition (APCVD) process that utilizes colloidal nanoparticles as process-scalable precursors for the wafer-scale production of TMD monolayers. Facile uniform distribution of nanoparticle precursors on the entire substrate leads to the wafer-scale uniform synthesis of TMD monolayers with the controlled size and morphology. Composition-controlled TMD alloy monolayers with tunable bandgaps can be produced by simply mixing dual nanoparticle precursor solutions in the desired ratio. We also demonstrate the fabrication of ultrathin field-effect transistors and flexible electronics with uniformly controlled performance by using TMD monolayers.

Interfacial Interactions within Amyloid Protein Corona Based on 2D MoS2 Nanosheets

The interfacial interaction within the amyloid protein corona based on MoS₂ nanomaterial is crucial, both for understanding the biological effects of MoS₂ nanomaterial and the evolution of amyloid diseases. The specific nano-bio interface phenomenon of human islet amyloid peptide (hIAPP) and MoS₂ nanosheet was investigated by using theoretical and experimental methods. The MoS₂ nanosheet enables the attraction of hIAPP monomer, dimer, and oligomer on its surface through van der Waals forces. Especially, the means of interaction between two hIAPP peptides might be changed by MoS₂ nanosheet. In addition, it is interesting to find that the hIAPP oligomer can stably interact with the MoS₂ nanosheet in one unique "standing" binding mode with an entire exposed β-sheet surface. All the interaction modes on the surface of MoS₂ nanosheet can be the essence of amyloid protein corona that may provide the venue to facilitate the fibrillation of hIAPP proteins. Further, it was verified experimentally that MoS₂ nanosheets could accelerate the fibrillation of hIAPP at a certain concentration mainly based on the newly formed nano-bio interface. In general, our results provide insight into the molecular interaction mechanism of the nano-bio interface within the amyloid protein corona, and shed light on the pathway of amyloid protein aggregation that is related to the evolution of amyloid diseases.

CVD-Grown 2D Nonlayered NiSe as a Broadband Photodetector

Two-dimensional (2D) materials have expansive application prospects in electronics and optoelectronics devices due to their unique physical and chemical properties. 2D layered materials are easy to prepare due to the layered crystal structure and the interlayer van der Waals combination. However, the 2D nonlayered materials are difficult to prepare due to the nonlayered crystal structure and the combination of interlayer isotropic chemical bonds, resulting in limited research on 2D nonlayered materials with broad characteristics. Here, a 2D nonlayered NiSe material has been synthesized by a chemical vapor deposition method. The atomic force microscopy study shows that the grown NiSe with a thin thickness. Energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy results demonstrate the uniformity and high quality of NiSe flakes. The NiSe based photodetector realizes the laser response to 830 nm and 10.6 μm and the maximum responsivity is ~6.96 A/W at room temperature. This work lays the foundation for the preparation of 2D nonlayered materials and expands the application of 2D nonlayered materials in optoelectronics fields.

Defect activated optical Raman modes in single layer MoSe2

In the last decade, transition metal dichalcogenides (TMDs) have been intensively synthesized/studied thus linking their morphological aspect to their physical properties, and consequently leading to the understanding of the possible benefits of defects in such materials. Nevertheless, for future applications, quantifying and identifying defects in TMDs is still a milestone to reach in order to better employ these materials in optoelectronic devices. Raman Spectroscopy has been successfully employed in graphene to quantify punctual or line defects. In this paper, we bombarded monolayer MoSe₂with He ions and found out the existence of three defect activated Raman bands around 250-300 cm^-1. Density functional theory calculations were employed to obtain the electronic and phonon dispersion bands, making it possible to infer that these bands arise from inter-valley Raman double resonance processes. Interestingly, the same punctual defect model, that allows one to predict the defect concentration at which graphene starts to become amorphous, also works for TMDs. Hence, this work opens the door to the macroscopic quantification of defects in TMDs, which is essential for technological applications.

Olivine Crystal Structure-Directed Twinning in Iron Germanium Sulfide (Fe2GeS4) Nanoparticles

Understanding the microstructure of complex crystal structures is critical for controlling material properties in next-generation devices. Synthetic reports of twinning in bulk and nanostructured crystals with detailed crystallographic characterization are integral for advancing systematic studies of twinning phenomena. Herein, we report a synthetic route to controllably twinned olivine nanoparticles. Microstructural characterization of Fe₂GeS₄ nanoparticles via electron microscopy (imaging, diffraction, and crystallographic analysis) demonstrates the formation of triplets of twins, or trillings. We establish synthetic control over the particle crystallinity and crystal growth. We describe the geometrical basis for twin formation, hexagonal pseudosymmetry of the orthorhombic lattice, and rank all of the reported olivine compounds according to this favorability to form twins. The work in this study highlights an area ripe for future exploration with respect to the advancement of solution-phase synthetic approaches that can control microstructure in compositionally complex, technologically relevant structures. Finally, we discuss the potential implications for olivine properties and performance in various applications.

Synthesis of High-Performance Monolayer Molybdenum Disulfide at Low Temperature

The large-area synthesis of high-quality MoS₂ plays an important role in realizing industrial applications of optoelectronics, nanoelectronics, and flexible devices. However, current techniques for chemical vapor deposition (CVD)-grown MoS₂ require a high synthetic temperature and a transfer process, which limits its utilization in device fabrications. Here, the direct synthesis of high-quality monolayer MoS₂ with the domain size up to 120 µm by metal-organic CVD (MOCVD) at a temperature of 320 °C is reported. Owing to the low-substrate temperature, the MOCVD-grown MoS₂ exhibits low impurity doping and nearly unstrained properties on the growth substrate, demonstrating enhanced electronic performance with high electron mobility of 68.3 cm² V^-1 s^-1 at room temperature. In addition, by tuning the precursor ratio, a better understanding of the MoS₂ growth process via a geometric model of the MoS₂ flake shape, is developed, which can provide further guidance for the synthesis of 2D materials.

Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe₂O₃ nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe₂O₃ nanoflakes epitaxially grown on α-Al₂O₃ substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe₂O₃ thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe₂O₃ nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

The Adatom Concentration Profile: A Paradigm for Understanding Two-Dimensional MoS2 Morphological Evolution in Chemical Vapor Deposition Growth

The two-dimensional (2D) transition metal dichalcogenide (TMD) MoS₂ possesses many intriguing electronic and optical properties. Potential technological applications have focused much attention on tuning MoS₂ properties through control of its morphologies during growth. In this paper, we present a unified spatial-temporal model for the growth of MoS₂ crystals with a full spectrum of shapes from triangles, concave triangles, three-point stars, to dendrites through the concept of the adatom concentration profile (ACP). We perform a series of chemical vapor deposition (CVD) experiments controlling adatom concentration on the substrate and growth temperature and present a method for experimentally measuring the ACP in the vicinity of growing islands. We apply a phase-field model of growth that explicitly considers similar variables (adatom concentration, adatom diffusion, and noise effects) and cross-validate the simulations and experiments through the ACP and island morphologies as a function of physically controllable variables. Our calculations reproduce the experimental observations with high fidelity. The ACP is an alternative paradigm to conceptualize the growth of crystals through time, which is expected to be instrumental in guiding the rational shape engineering of MoS₂ crystals.

Strained Epitaxy of Monolayer Transition Metal Dichalcogenides for Wrinkle Arrays

Wrinkling two-dimensional (2D) transition metal dichalcogenides (TMDCs) provides a mechanism to adjust the physical and chemical properties as per need. Traditionally, TMDCs wrinkles achieved by transferring exfoliated materials on prestretched polymer suffer from poor control and limited sample area, which significantly hinders desirable applications. Herein, we fabricate large-area monolayer TMDCs wrinkle arrays directly on the m-quartz substrate using strained epitaxy. The uniaxial thermal expansion coefficient mismatch between the substrate and TMDCs materials enables the generation of large uniaxial thermal strain. By quenching the TMDCs after growth, this uniaxial thermal strain can be quickly released as a form of wrinkle arrays along the [0001]_quartz direction. Using WS₂ as a model system, the size of as-grown wrinkles can be finely modulated within sub-100 nm by changing the quenching temperature. These WS₂ wrinkles can be locally folded and form various multilayer structures with odd layer numbers during the transfer process. Besides, the corrugated structures in WS₂ wrinkles induce significant changes to optical properties including anisotropic Raman response, enhanced photoluminescence, and second harmonic generation emissions. Furthermore, these wrinkle arrays exhibit enhanced chemical reactivity that can be selectively engineered to ribbon arrays with improved electrocatalytic performance. The developed strategy of strained epitaxy here should enable flexibility in the design of more sophisticated 2D-based structures, offering a simple but effective way toward the modulation of properties with enhanced performances.

Analyzing growth kinematics and fractal dimensions of molybdenum disulfide films

Though the positive role of alkali halides in realizing large area growth of transition metal dichalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS₂morphology for films grown under various pre-treatment conditions of the substrate with sodium chloride (NaCl). At an optimum NaCl concentration, the domain size of the monolayer increased by almost two orders of magnitude compared to alkali-free growth of MoS₂. The results show an inverse relationship between fractal dimension and areal coverage of the substrate with monolayers and multi-layers, respectively. Using the Fact-Sage software, the role of NaCl in determining the partial pressures of Mo- and S-based compounds in gaseous phase at the growth temperature is elucidated. The presence of alkali salts is shown to affect the domain size and film morphology by affecting the Mo and S partial pressures. Compared to alkali-free synthesis under the same growth conditions, MoS₂film growth assisted by NaCl results in ≈81% of the substrate covered by monolayers. Under ideal growth conditions, at an optimum NaCl concentration, nucleation was suppressed, and domains enlarged, resulting in large area growth of MoS₂monolayers. No evidence of alkali or halogen atoms were found in the composition analysis of the films. On the basis of Raman spectroscopy and photoluminescence measurements, the MoS₂films were found to be of good crystalline quality.

Oxide Inhibitor-Assisted Growth of Single-Layer Molybdenum Dichalcogenides (MoX2, X = S, Se, Te) with Controllable Molybdenum Release

Though chemical vapor deposition (CVD) methods have been widely used in the growth of two-dimensional transition-metal dichalcogenides (2D TMDCs), the controllable fabrication of 2D TMDCs is yet hard to achieve because of the great challenge of concisely controlling the release of precursors vapor, one of the most critical growth kinetic factors. To solve this important issue, here we report the utilization of oxide inhibitors covering Mo source during CVD reactions to manipulate the release of Mo vapor. In contrast to the lack of capability of conventional CVD methods, 2D molybdenum dichalcogenide (MoX₂, X = S, Se, Te) monolayers were successfully fabricated through the proposed CVD protocol with the oxide-inhibitor-assisted growth (OIAG) strategy. In this way, despite the fact that only separated MoTe₂ flakes were prepared, both MoS₂ (continuous and clean) and MoSe₂ (continuous but dotted) monolayer films at the scale of centimeter were obtained. The presented OIAG method enables a comprehensive understanding and precise control of the reaction kinetics for improved growth of 2D MoX₂.

Influence of organic promoter gradient on the MoS2 growth dynamics

Chemical vapor deposition has been demonstrated to be the most efficient, versatile and reliable technique for the synthesis of monolayers of transition metal dichalcogenides. The use of organic promoters during the growth process was a turning point in order to increase the monolayer lateral size or to obtain complete coverage of the growth substrate. In this work we clarify the influence of the promoter gradient on the growth dynamics of MoS₂. In particular, we place a sacrificial substrate covered with a promoter (a low sublimation-temperature perylene-based compound) downstream with respect to the growth substrate in order to maximize its gradient on the growth substrate through upstream diffusion. We demonstrate that the morphology and the number of layers of MoS₂ are drastically affected by the distance of the growth substrate from the promoter sacrificial substrate. The farthermost area from the promoter substrate presents micrometric MoS₂ triangular monolayers and large low hierarchy dendritic multi-layer structures. On the contrary the closest area reveals an almost continuous polycrystalline MoS₂ monolayer, with bilayer terraces, with a lateral dimension up to hundreds of micrometers.

van der Waals Epitaxial Formation of Atomic Layered α-MoO3 on MoS2 by Oxidation

The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation, and using oxidation to tune the properties of TMDs has been actively explored. In particular, because transition metal oxides (TMOs) are promising hole injection layers, a TMD-TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated because of the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional molybdenum disulfide (MoS₂) using transmission electron microscopy analysis. We find that MoS₂ starts to oxidize around 400 °C to form orthorhombic-phase molybdenum trioxide (α-MoO₃) nanosheets. The α-MoO₃ nanosheets so formed are stacked layer-by-layer on the underlying MoS₂ via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the band gap of MoS₂ is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO-TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.

Dinucleoside polyphosphates act as 5'-RNA caps in bacteria

It has been more than 50 years since the discovery of dinucleoside polyphosphates (Np_nNs) and yet their roles and mechanisms of action remain unclear. Here, we show that both methylated and non-methylated Np_nNs serve as RNA caps in Escherichia coli. Np_nNs are excellent substrates for T7 and E. coli RNA polymerases (RNAPs) and efficiently initiate transcription. We demonstrate, that the E. coli enzymes RNA 5′-pyrophosphohydrolase (RppH) and bis(5′-nucleosyl)-tetraphosphatase (ApaH) are able to remove the Np_nN-caps from RNA. ApaH is able to cleave all Np_nN-caps, while RppH is unable to cleave the methylated forms suggesting that the methylation adds an additional layer to RNA stability regulation. Our work introduces a different perspective on the chemical structure of RNA in prokaryotes and on the role of RNA caps. We bring evidence that small molecules, such as Np_nNs are incorporated into RNA and may thus influence the cellular metabolism and RNA turnover.

Nicotinamide adenine dinucleotide and coenzyme A serve as a 5′-cap of prokaryotic RNA. Here the authors report that methylated and non-methylated dinucleoside polyphosphates (Np_nNs) exist as Escherichia coli RNA caps which can be cleaved by 5′-pyrophosphohydrolase (RppH) and bis(5′-nucleosyl)-tetraphosphatase (ApaH).

Controlled growth of MoS2 via surface-energy alterations

Monolayer MoS₂ in triangular configurations with rich edges or high-quality uniform films are either catalytically active for the hydrogen evolution reaction or flexible for functional electronic and optoelectronic devices. Here, we have experimentally discovered that these two types of MoS₂ products can be selectively synthesized on graphene or sapphire substrates, which are associated with both different adsorption energy and diffusion-energy barrier for vapor precursors during growth. Our study not only provides insights into the on-surface synthesis of high-quality MoS₂ monolayers, but also can be applied to the growth of vertically-stacked and large-scale in-plane lateral MoS₂-graphene heterostructures.

Morphological evolution of atomically thin MoS2 flakes synthesized by a chemical vapor deposition strategy

The evolution of 2D MoS₂ flakes from dendritic shape to hexagonal can be realized by the reaction of S and MoO₃ powders at different growth temperatures via the chemical vapor deposition method.

UV-Visible Photodetector Based on I-type Heterostructure of ZnO-QDs/Monolayer MoS2

Monolayer MoS₂ has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic-thickness-limited adsorption and band gap-limited spectral selectivity. Here we have carried out investigations on MoS₂ monolayer-based photodetectors with and without decoration of ZnO quantum dots (ZnO-QDs) for comparison. Compared with monolayer MoS₂ photodetectors, the monolayer ZnO-QDs/MoS₂ hybrid device exhibits faster response speed (1.5 s and 1.1 s, respectively), extended broadband photoresponse range (deep UV-visible), and enhanced photoresponse in visible spectrum, such as higher responsivity over 0.084 A/W and larger detectivity of 1.05 × 10¹¹ Jones, which results from considerable injection of carries from ZnO-QDs to MoS₂ due to the formation of I-type heterostructure existing in the contact interface of them.

Mo Concentration Controls the Morphological Transitions from Dendritic to Semicompact, and to Compact Growth of Monolayer Crystalline MoS2 on Various Substrates

The domain morphology in the growth of transition-metal dichalcogenides (TMDCs) is mostly triangular but rarely dendritic. Here, we report a robust chemical vapor deposition method to fabricate atomic-thin 2H-phase MoS₂ dendrites on several single-crystalline substrates with different lattice structures, such as rutile-TiO₂(001), SrTiO₃(001), and sapphire(0001). It is found that by tuning the concentration of Mo adatoms, the morphology of MoS₂ domains on these substrates evolves from tridentate dendrites at a low Mo concentration to semicompact fractal domains at an intermediate Mo concentration, and to a compact triangular shape at a high Mo concentration. First-principles calculations reveal that the edge diffusion barrier of Mo is comparable to the attachment barrier, inhibiting fast Mo atom diffusion along the edge. Kinetics Monte Carlo simulations with varying Mo concentrations well reproduce the experimental results. Our combined experimental and theoretical analyses evidently show that the growth of MoS₂ dendritic domains at a low Mo concentration is a nonequilibrium process, which is dominated by the kinetics of Mo adatoms. Our study presents an effective route to control the morphology of TMDCs by simply tuning the transition-metal adatom concentration.

Controlled Growth of Large-Area Bilayer Tungsten Diselenides with Lateral P-N Junctions

Bilayer two-dimensional (2D) van der Waals (vdW) materials are attracting increasing attention due to their predicted high quality electronic and optical properties. Here, we demonstrate dense, selective growth of WSe₂ bilayer flakes by chemical vapor deposition with the use of a 1:10 molar mixture of sodium cholate and sodium chloride as the growth promoter to control the local diffusion of W-containing species. A large fraction of the bilayer WSe₂ flakes showed a 0 (AB) and 60° (AA') twist between the two layers, whereas Moiré 15 and 30° twist angles were also observed. Well-defined monolayer-bilayer junctions were formed in the as-grown bilayer WSe₂ flakes, and these interfaces exhibited p-n diode rectification and an ambipolar transport characteristic. This work provides an efficient method for the layer-controlled growth of 2D materials, in particular, 2D transition metal dichalcogenides, and promotes their applications in next-generation electronic and optoelectronic devices.

Shape-Engineered Synthesis of Atomically Thin 1T-SnS2 Catalyzed by Potassium Halides

Shape engineering plays a crucial role in the application of two-dimensional (2D) layered metal dichalcogenide (LMD) crystalline materials in terms of physical and chemical property modulation. However, controllable growth of 1T phase tin disulfide (SnS₂) with multifarious morphologies has rarely been reported and remains challenging. Herein, we report a direct synthesis of large-size, uniform, and atomically thin 1T-SnS₂ with multiple morphologies by adding potassium halides via a facile chemical vapor deposition process. A variety of morphologies, i.e., from hexagon, triangle, windmill, and dendritic to coralloid, corresponding to fractal dimensions from 1.01 to 1.81 are accurately controlled by growth conditions. Moreover, the Sn concentration controls the morphology change of SnS₂. The edge length of the SnS₂ dendritic flake can grow larger than 500 μm in 5 min. Potassium halides can significantly reduce the surface migration barrier of the SnS₂ cluster and enhance the SnS₂ adhesion force with substrate to facilitate efficient high in-plane growth of monolayer SnS₂ compared to sodium halides by density functional theory calculations. More branched SnS₂ with higher fractal dimension provides more active sites for enhancing hydrogen evolution reactions. Importantly, we prove that potassium halides are preferable for 1T-phase LMDs structures, while sodium halides are more suitable for 2H-phase materials. The growth mechanism proposed here provides a general approach for controllable-phase synthesis of 2D LMD crystals and related heterostructures. Shape engineering of 2D materials also provides a strategy to tune LMD properties for demanding applications.

Atomic-Scale Structural Modification of 2D Materials

2D materials have attracted much attention since the discovery of graphene in 2004. Due to their unique electrical, optical, and magnetic properties, they have potential for various applications such as electronics and optoelectronics. Owing to thermal motion and lattice growth kinetics, different atomic-scale structures (ASSs) can originate from natural or intentional regulation of 2D material atomic configurations. The transformations of ASSs can result in the variation of the charge density, electronic density of state and lattice symmetry so that the property tuning of 2D materials can be achieved and the functional devices can be constructed. Here, several kinds of ASSs of 2D materials are introduced, including grain boundaries, atomic defects, edge structures, and stacking arrangements. The design strategies of these structures are also summarized, especially for atomic defects and edge structures. Moreover, toward multifunctional integration of applications, the modulation of electrical, optical, and magnetic properties based on atomic-scale structural modification are presented. Finally, challenges and outlooks are featured in the aspects of controllable structure design and accurate property tuning for 2D materials with ASSs. This work may promote research on the atomic-scale structural modification of 2D materials toward functional applications.

Kinetics of Graphene and 2D Materials Growth

During the last 10 years, remarkable achievements on the chemical vapor deposition (CVD) growth of 2D materials have been made, but the understanding of the underlying mechanisms is still relatively limited. Here, the current progress on the understanding of the growth kinetics of 2D materials, especially for their CVD synthesis, is reviewed. In order to present a complete picture of 2D materials' growth kinetics, the following factors are discussed: i) two types of growth modes, namely attachment-limited growth and diffusion-limited growth; ii) the etching of 2D materials, which offers an additional degree of freedom for growth control; iii) a number of experimental factors in graphene CVD synthesis, such as structure of the substrate, pressure of hydrogen or oxygen, temperature, etc., which are found to have profound effects on the growth kinetics; iv) double-layer and few-layer 2D materials' growth, which has distinct features different from the growth of single-layer 2D materials; and v) the growth of polycrystalline 2D materials by the coalescence of a few single crystalline domains. Finally, the current challenges and opportunities in future 2D materials' synthesis are summarized.

Cracked eight-awn star TaS2 with fractal structures used as an efficient electrocatalyst for the hydrogen evolution reaction

A new eight-awn star TaS₂ with a specially designed fractal and cracked dendrite structure was prepared, which showed outstanding HER catalytic activities owing to its specially designed structure.

An electrochemical approach towards the controllable synthesis of highly ordered and hierarchical zinc oxide dendritic crystals composed of hexagonal nanosheets: some insights into the stacking-assembly of the hierarchical architecture

Herein, an electrochemical synthetic approach is presented to produce a highly ordered and hierarchical zinc oxide dendrite architecture composed of hexagonal nanosheets.

Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties from a potentially scalable reaction pathway

Solventless thermolysis of molecular precursors followed by liquid phase exfoliation allows access to two-dimensional IV-VI semiconductor nanomaterials hitherto unreachable by a scalable processing pathway. Firstly, the use of metal dithiocarbamate precursors to produce bulk alloys in the series Pb_1-x Sn _x S (0 ≤ x ≤ 1) by thermolysis is demonstrated. The bulk powders are characterised by powder X-ray diffraction (pXRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. It was found that there is a transition from cubic structures for the Pb-rich alloys including the end compound, PbS (0 ≤ x ≤ 0.4) to layered orthorhombic structures for Sn-rich alloys and the end compound SnS (0.5 ≤ x ≤ 1.0). A smooth elemental progression from lead-rich to tin-rich monochalcogenides across the series of materials is observed. Liquid phase exfoliation was applied to produce two dimensional (2D) nanosheets for a mixed Pb_1-x Sn _x S alloy (where x = 0.8) in 1-methyl-2-pyrrolidone (NMP) using the synthetic bulk powder as starting material. The nanosheet products were characterized by SEM, atomic force microscopy (AFM) and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). First principle calculations of Pb_1-x Sn _x S alloys show that the Sn content x modifies the size of the band gap by several 100 meV and that x changes the gap type from indirect in SnS to direct in Pb_0.2Sn_0.8S. These results are supported by UV-Vis spectroscopy of exfoliated Pb_0.2Sn_0.8S. The method employed demonstrates a new, scalable, processing pathway which can potentially be used to synthesize a range of synthetic layered structures that can be exfoliated to as-yet unaccessed 2D materials with tunable electronic properties.

Synthesis of 2D transition metal dichalcogenides by chemical vapor deposition with controlled layer number and morphology

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have stimulated the modern technology due to their unique and tunable electronic, optical, and chemical properties. Therefore, it is very important to study the control parameters for material preparation to achieve high quality thin films for modern electronics, as the performance of TMDs-based device largely depends on their layer number, grain size, orientation, and morphology. Among the synthesis methods, chemical vapor deposition (CVD) is an excellent technique, vastly used to grow controlled layer of 2D materials in recent years. In this review, we discuss the different growth routes and mechanisms to synthesize high quality large size TMDs using CVD method. We highlight the recent advances in the controlled growth of mono- and few-layer TMDs materials by varying different growth parameters. Finally, different strategies to control the grain size, boundaries, orientation, morphology and their application for various field of are also thoroughly discussed.

Recent advances in the field of transition metal dichalcogenides for biomedical applications

Nanosheets of transition metal dichalcogenide (TMDs), the graphene-like two-dimensional (2D) materials, exhibit a unique combination of properties and have attracted enormous research interest for a wide range of applications including catalysis, functional electronics, solid lubrication, photovoltaics, energy materials and most recently in biomedical applications. Their potential for use in biosensors, drug delivery, multimodal imaging, antimicrobial agents and tissue engineering is being actively studied. However, the commercial translation of exfoliated TMDs has been limited due to the low aqueous solubility, non-uniformity, lack of control over the layer thickness, and the long-term colloidal stability of the exfoliated material. There is wide interest in the synthesis and exfoliation of TMDs resulting in the reporting of increasing numbers of new methods and their biomedical applications. The unique physicochemical characteristics of the TMD nanosheets have been exploited to tether them with biological payload to achieve selective localized delivery in vivo. The large surface-to-volume ratio, good cytocompatibility, ease of surface modification, tunable bandgap, strong spin-orbit coupling, and high optical and thermal conversion efficiency of TMD nanosheets make them favorable over traditional nanomaterials for biomedical research. Moreover, the presence of abundant active edge sites on the 2D TMDs makes them suitable for catalytic activities, while the large surface area and the interspace between layers are particularly conducive to ion or small molecule intercalation, making them useful for energy storage applications with rapid redox reaction capabilities. One of the major limitations of the exfoliated TMDs has been their limited colloidal stability in aqueous media. In this review, we summarize the recent advances in the exfoliation and synthesis of single-layered TMDs, their biomedical efficacy in terms of cytotoxicity, combinatorial therapy and diagnostic imaging, as well as antimicrobial activity. We highlight the current challenges in the field and propose strategies for the future.