Analyzing Growth Kinematics and Fractal Dimensions of Molybdenum Disulfide Films

TL;DR

This study systematically analyzes how NaCl pre-treatment influences MoS2 film growth, revealing that optimal NaCl concentration significantly enhances monolayer size, coverage, and film quality by modulating gaseous phase chemistry.

Contribution

It provides a detailed understanding of NaCl's role in MoS2 growth, including the relationship between fractal dimension and coverage, and demonstrates improved film properties without doping.

Findings

NaCl increases monolayer domain size by nearly 100 times.

Optimal NaCl concentration suppresses nucleation and enlarges domains.

NaCl-assisted growth yields high-quality, undoped MoS2 monolayers.

Abstract

Through the positive role of alkali halides in realizing large area growth of transition metal-di-chalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS$_2$ morphology 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 MoS$_2$. 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, MoS$_2$ film growth assisted by NaCl results in $\approx$ 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 MoS$_2$ monolayers. The monolayers were found to be free of unintentional doping with alkali metal and halogen atoms and exhibit high crystallinity and excellent opto-electronic quality.