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Two-dimensional transition metal dichalcogenide heterostructures are potential candidates for next-generation semiconductor materials due to their extremely thin atomic structure and better optoelectronic properties. As a common method for growing heterostructures, chemical vapor deposition (CVD) can synthesize both lateral and vertical heterostructures. However, the controlled synthesis of heterostructures still faces challenges due to fewer studies about the evolution of heterostructure morphology under different conditions. In this study, heterostructures with sharp interfaces are fabricated via the vapor–liquid–solid (VLS) growth-based CVD technique using mixed sodium molybdate and sodium tungstate solutions. By optimizing a series of growth parameters, the modulation of MoxW1−xS2/MoS2 heterostructure morphology is achieved. In addition, lateral and vertical MoxW1−xS2/MoS2 heterostructures are synthesized by changing the molybdenum–tungsten molar ratio. This study provides a new strategy for synthesizing heterostructures with different growth ways. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDCs) films are potential two‐dimensional materials for next‐generation optoelectronic and electronic devices. Chemical Vapor Deposition, Atomic Layer Deposition, and Metal‐Organic Chemical Vapor Deposition are usually used for the synthesis of the MoS2 films by taking Mo(CO)6 as precursors. However, the mechanism of adsorption and reaction of Mo(CO)6 on the substrate at the molecular level are hardly to know only by the experiments. In this study, the detailed reactions of Mo(CO)6 on α‐Al2O3(0001) are investigated by density functional theory to demonstrate the growth mechanism of MoS2. It is found that, Mo(CO)6 could form a stable chemical bond with the Al‐1 site on the surface, which has large adsorption energy and charge transfer. The decomposition of Mo(CO)6 into Mo(CO)3 exists in a clear order, with the top carbonyl composing firstly, follow by the two parallel carbonyls. In order to determine the stable model of Mo(CO)3, adsorption energy and DOS are analyzed. The Ead is −5.65 eV and the 4d5 s orbitals of Mo atom overlap with the 2 s2p orbitals of O atom, indicating the new chemical bond is forming in 3O‐Al‐2 model. Finally, the LST/QST (Linear Synchronous Transit/Quadratic Synchronous Transit) method is used to search for transition states of Mo(CO)3 sulfuration reactions with CH3S2CH3, H2S, and S2 respectively. This paper provides a theoretical basis for the experimental preparation of MoS2 and provides research ideas for the general synthesis of TMDCs. [ABSTRACT FROM AUTHOR]

High quality and high quantity few-layer graphene was successfully prepared using a new impinging jet method. Natural graphite flakes were first agitated in N-methyl pyrrolidone (NMP) with the assistance of supercritical CO2, then the half-exfoliated graphite was further stripped using the shear stress derived from the impinging jets. After the energy conversion and stress analysis of the graphite particles during the whole exfoliation process, it was revealed that the size of the target mesh, the distance between the nozzle and the target, the decompression rate, and the size of the raw materials had a significant influence on the exfoliation process. Additionally, a microscopic view of the exfoliation and dispersion mechanism of graphene in the CO2–NMP system was investigated using molecular dynamics simulation, and CO2 was found to be beneficial for the penetration of NMP into the graphite sheets. Finally, the concentration and quality characteristics of the prepared graphene were characterized using ultraviolet–visible spectroscopy, transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The maximum concentration was as high as 0.689 mg ml−1, the thickness of 68% of the product was less than 2.5 nm, and the lateral dimension was from 0.5 to 3.0 μm. These results indicate that this impinging jet method is promising for large-scale industrial production. [ABSTRACT FROM AUTHOR]

Sulfuration reactions dominate the synthesis of transition-metal dichalcogenides via chemical vapor deposition. A neglected critical issue is the evolution of crystal domain morphology and growth models caused by boundary layer development. In this study, we propose two growth models within a laminar flow field to investigate the kinetic mechanism of uniformly grown MoS 2 . We used supercritical fluid pre-deposition to obtain a well-distributed and low-crystallinity Mo precursor on the surface of a substrate to avoid non-stoichiometric supply in sulfuration. The development of the boundary layer was suppressed through mainstream force by adjusting the substrate slope angle. For growth within the underdeveloped laminar boundary layer, monolayer MoS 2 with a size of 50 μm uniformly distributed on the full substrate with R = 85% (relative change in boundary layer thickness). Moreover, the growth constrained by surface chemical reactions tended to promote spatially uniform growth. However, within the fully developed laminar flow, the crystal domains preferentially grew vertically, which was attributed to the excessive crystal growth rate ( g ). Our results provide new insights into the controllable preparation of two-dimensional materials.

In this study, a novel pressure-induced supercritical phase nucleation method is proposed to synthesize monolayer MoS 2 films, which is promoter free and can avoid contamination of films derived from these heterogeneous promoters in most of the existing techniques. The low-crystallinity and size-controlled MoO 2 (acac) 2 particles are recrystallized on the substrate via the pressure-sensitive solvent capacity of supercritical CO 2 and these particles are used as growth sites. The size of single-crystal MoS 2 on the substrate is found to be dependent on the wetting area of the pyrolyzed precursor droplets (MoO 2 ) on the surface, and the formation of continuous films with high coverage is mainly controlled by the coalescence of MoO 2 droplets. It is enhanced by the increase of the nucleation site density, which can be adjusted by the supersaturation of the supercritical fluid solution. Our findings pave a new way for the controllable growth of MoS 2 and other two-dimensional materials and provide sufficient and valuable evidence for vapor-liquid-solid growth.

High quality and high quantity few-layer graphene was successfully prepared using a new impinging jet method. Natural graphite flakes were first agitated in N-methyl pyrrolidone (NMP) with the assistance of supercritical CO 2 , then the half-exfoliated graphite was further stripped using the shear stress derived from the impinging jets. After the energy conversion and stress analysis of the graphite particles during the whole exfoliation process, it was revealed that the size of the target mesh, the distance between the nozzle and the target, the decompression rate, and the size of the raw materials had a significant influence on the exfoliation process. Additionally, a microscopic view of the exfoliation and dispersion mechanism of graphene in the CO 2 -NMP system was investigated using molecular dynamics simulation, and CO 2 was found to be beneficial for the penetration of NMP into the graphite sheets. Finally, the concentration and quality characteristics of the prepared graphene were characterized using ultraviolet-visible spectroscopy, transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The maximum concentration was as high as 0.689 mg ml -1 , the thickness of 68% of the product was less than 2.5 nm, and the lateral dimension was from 0.5 to 3.0 μm. These results indicate that this impinging jet method is promising for large-scale industrial production.

Distributions of CH₄ and N₂O concentrations in Weihe River in Xinxiang City were monitored in spring of 2015, and their influencing factors were discussed. The result showed that CH₄ and N₂O were super-saturated in surface water of Weihe River. The variation ranges of two gases' saturations in the surface water of Weihe River were 147.59-2667.85 (CH₄) and 4.06-188.25 (N₂O). In the urban area, significant correlation existed between N₂O and NH₄⁺-N concentrations (P < 0.01), but in the new district, dissolved N₂O concentration showed sharp increase because of the water input from the urban sewage plants, illustrating that the controlling mechanism on N₂O production varied as pollutant characteristics changed. Stepwise regression analysis showed that CH₄ concentrations could be explained by NH₄⁺-N concentrations and water temperature, and CH₄ concentrations in the surface water of Weihe River was significantly correlated with NH₄⁺-N concentrations (R² = 0.70, P < 0.01), suggesting that NH₄⁺-N was the key factor in regulating the production and assumption of CH₄oxidation in Weihe River in spring. Besides, this study showed that when there was less NO₃⁻-N but more NH₄⁺-N in river water, CH₄and N₂O concentrations would be positively correlated, indicating that different nitrogen sources would impact the coupling mechanism of CH₄and N₂O productions.