Enhanced chemical vapour deposition of monolayer MoS2 films via a clean promoter

Technical Summary: Enhanced Chemical Vapour Deposition of Monolayer MoS2 Films via a Clean Promoter

Problem Statement
Two-dimensional (2D) transition metal dichalcogenides (TMDCs), specifically molybdenum disulfide (MoS2), hold significant promise for next-generation electronic and optoelectronic devices due to their unique electrical and mechanical properties. However, their integration into advanced technologies is hindered by challenges in the controllable synthesis of high-quality, large-area monolayer films. While Chemical Vapour Deposition (CVD) is a primary method for scalable synthesis, growing MoS2 directly on amorphous SiO2/Si substrates (to avoid transfer-induced contamination and strain) is difficult due to the lack of epitaxial templating, leading to random nucleation and inhomogeneity. Existing strategies using seeding promoters, such as alkali metal halides (e.g., NaCl, KI) or organic compounds (e.g., PTAS, F16CuPc), often introduce residual metal ions or non-volatile species that contaminate the film, alter intrinsic properties, or fail to provide precise control over nucleation sites and film thickness.

Methodology
The authors developed a contamination-free CVD growth method for monolayer MoS2 on amorphous SiO2 substrates using a novel "clean" nano-promoter.

• Promoter Material: The promoter consists of conventional photoresist S1813 (Shipley), a mixture of cresol novolak resin and a photoactive compound (PAC). This was diluted in isopropyl alcohol and spin-coated onto half of the substrate, creating a controlled interface between a promoter-decorated region and a pristine region.
• Growth Process: The CVD process utilized MoO3 powder as the molybdenum source and sulphur powder as the sulphur source. The substrate was positioned downstream from the MoO3 source. The sulphur temperature was independently controlled to vary the reactant concentration and S/Mo ratio.
• Experimental Design: To isolate the promoter's effect, the study compared growth on the promoter-treated half versus the pristine half of the same substrate under identical gas flow and thermal conditions. The sulphur temperature was systematically varied (180°C to 300°C) to investigate the transition from homogeneous to heterogeneous nucleation and to optimize film quality.
• Characterization: The synthesized films were analyzed using optical microscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Raman spectroscopy, Photoluminescence (PL), X-ray Photoelectron Spectroscopy (XPS), and High-Resolution Transmission Electron Microscopy (HRTEM). Thermogravimetric analysis (TGA) was also employed to understand the thermal behavior of the reactants.

Key Results

• Promoter Effect: At an optimal sulphur temperature of 250°C, the promoter region exhibited significantly enhanced growth compared to the pristine region. The promoter region showed a continuous dark olive-green film, whereas the pristine region remained largely clean with sparse particles.
• Quantitative Improvements: Statistical analysis revealed that the promoter region achieved:
  • A monolayer ratio of 88.9% (approximately six times higher than the pristine region).
  • A flake coverage of 15.0% (four times higher than the pristine region).
  • An average flake size of 14.4 μm for the main population (more than three times larger than the pristine region's average of 3.9 μm).
  • A population density of large flakes (≥10 μm) of 30.2%, compared to 0.5% in the pristine region.
• Film Quality: Raman spectroscopy confirmed the monolayer nature of the MoS2 in the promoter region, with an E12g/A1g peak spacing (∆ω) of 20.8 cm⁻¹ and a strong PL A exciton peak at 1.845 eV, matching the characteristics of mechanically exfoliated samples. HRTEM and Fourier transform analysis confirmed a single-crystal hexagonal 2H phase with a disorder-free lattice.
• Cleanliness: XPS analysis of C 1s and O 1s core levels showed no evidence of promoter-induced contamination (e.g., no C-Mo or C-S bonds). The observed carbon signals were attributed to adventitious carbon typical of air-exposed samples, confirming the growth process itself was clean.
• Temperature Dependence: The study identified a critical dependence on sulphur temperature.
  • Low Temperature (<220°C): Insufficient sulphur concentration led to small, defective flakes even in the promoter region.
  • Optimal Temperature (250°C): Balanced reactant supply facilitated heterogeneous nucleation, resulting in large, high-quality monolayer triangular flakes with minimal sulphur vacancies (lowest LA peak intensity in Raman and lowest MoS2-x phase in XPS).
  • High Temperature (>270°C): Excessive sulphur led to supersaturated reactant concentrations, causing a shift to homogeneous nucleation. This resulted in a high density of particles and multilayer films, diminishing the promoter's effectiveness and reducing the monolayer ratio.

Significance and Claims
The paper claims to establish a robust pathway for the practical implementation of 2D MoS2 in next-generation electronic devices by addressing the dual challenges of scalability and contamination. The primary significance lies in the demonstration of a "clean" growth promoter (S1813) that avoids the introduction of detrimental metal ions or non-volatile residues common in previous methods. By systematically tuning the sulphur temperature, the authors successfully controlled the nucleation mechanism, transitioning from random homogeneous nucleation to controlled heterogeneous nucleation. This approach enables the site-specific, scalable synthesis of high-quality, large-area monolayer MoS2 films directly on amorphous SiO2 substrates, thereby eliminating the need for transfer steps that often degrade film quality. The findings offer a novel strategy for improving the controllability and performance of 2D electronic devices by reducing material variability and enhancing film uniformity.