Monolayer MoS 2 , processing flexibility and remarkable physical properties derived from its direct bandgap feature, has been endowed to be one of the potential materials for practical applications such as integrated circuits and logic devices. Recently, a facile CVD method using alkali metal compounds as promoters attracted a lot of attention. Here, we systematically investigated the mechanism of alkali metal compoundpromoted growth of monolayer MoS 2 by CVD and proposed a eutectic intermediate model. In the presence of alkali metal compounds, large monolayer MoS 2 was obtained, regardless of the anions. However, nonalkali metal compounds did not promote the growth of monolayer MoS 2 . We proposed that the formation of eutectic intermediates, containing alkali metal molybdates and molybdenum oxides, played a crucial role in promoting the growth of monolayer MoS 2 . It is because the low melting point of eutectic intermediates could facilitate their mobility, favoring less nuclei and lateral growth. The proposal of the eutectic intermediates model could not only contribute to growing ultralarge monolayer MoS 2 and other 2D materials but also inspire new ideas about growing 2D materials based on low melting point and high mobility of eutectic intermediates.
Monolayer MoS2 has emerged as one of the most promising candidate materials for future semiconductor devices because of its fascinating physical properties and optoelectronic performance. Recently, the utilization of alkali metal compounds as promoters in CVD growth has been demonstrated to be a facile strategy for growing monolayer MoS2 and other 2D TMDs with large domain sizes. In this work, we systematically investigated the residues derived from alkali metal compounds and the spontaneous n-doping effect on monolayer MoS2 in alkali metal compound-promoted CVD growth. When using NaOH and other alkali metal compounds as promoters, it is found that the Raman peak of the A1g mode red shifted with a broadening width and the PL intensity of the A peak decreased with a red shift, which was attributed to the spontaneous n-doping effect during growth. Moreover, the growth using varying amounts of NaOH promoter suggests that the n-doping level could be controlled by the amount of promoter. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) showed the existence of cation-derived residues in the form of a Na–O cluster physiosorbed on top of monolayer MoS2, which was also confirmed by the transfer experiment. The NaOH treatment experiment and density functional theory (DFT) calculations demonstrate that sodium hydroxide clusters, which could be converted from a combination of Na–O clusters and water vapor, could produce an n-doping effect on monolayer MoS2. This study provides a facile route to controllably grow monolayer 2D materials with a desired doping level without further treatment.
Optical microscopy is believed to be an efficient method for identifying layer number of two-dimensional 2D materials. However, since illuminants, cameras and their parameters are different from lab to lab, it is impossible to identify layer numbers just by comparing a given optical image with standard or calculated images under standard conditions. Here we reported an image reconstruction method, converting raw optical images acquired by arbitrary illuminants and cameras into reconstructed images at specified illuminant and specified camera. After image reconstruction, the color differences of each layer number roughly equaled those calculated under specified condition. By comparing the color differences in reconstructed image with those calculated under specified condition, the layer numbers of 2D materials in our lab and published papers, including MoS2, WS2 and WSe2, were ambiguously identified. This study makes optical _____________________________ a) Author to whom correspondence should be addressed. Electronic mail: gaobo@hit.edu.cn microscopy a precise method for identifying layer numbers of 2D materials on known substrate.
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