A Novel Technique to Evaluate the Physical Structure with X-Ray Photoelectron Spectroscopy for 2D Transition Metal Dichalcogenide Alloy MoS2(1-x)Te2x
Authors: Hibino Yusuke , Nakamura Kodai , Yamazaki Kota , Hashimoto Yusuke , Wakabayashi Hitoshi , Ogura Atsushi 
Published: May 2020

Recently, the research on layered material is progressing rapidly. Transition metal dichalcogenides (TMDs) has a finite bandgap which is driving its high expectations for various applications in electronics and optoelectronics and so on. Alloying of TMD has been drawing attention since the alloying allows the tuning of the bandgap. Among the TMD alloys, MoS2(1-x)Te2x
is expected to show the widest range of tunable bandgap. However, the alloy of MoS2 and MoTe2 is expected to be thermally unstable [1-2]. That is, there is a chance that MoS2 and MoTe2 would segregate rather than distribute evenly throughout the film [3]. Thus it is crucial to confirm whether a uniform alloy is fabricated or the film underwent phase segregation. There are several methods to confirm if there is a phase separation, such as XRD, STEM, etc. While methods like STEM can determine whether there is phase separation in a definitive manner, it is a destructive method and sample preparation may be troublesome. XRD is another powerful tool to determine the physical structure, however, the peak deconvolution may yield an ambiguous results. On the other hand, it is reported that when a uniform alloy is formed, the probability of S or Te atoms occupying the closest neighbor to Mo atoms can be described by binomial theorem [4-6]. The difference in how many S or Te atoms are bonding with Mo should shift the chemical states of the Mo atoms. X-ray photoelectron spectroscopy (XPS) is a powerful tool to investigate the chemical states of the elements in the film. Therefore a novel technique to determine whether the film has undergone phase separation or not with the use of XPS was investigated.

In this study, various MoS2(1-x)Te2x
samples are evaluated with XPS to evaluate its alloy formation. Peak fitting of XPS spectra were performed with 3 double and 1 single voigt peaks (2 doublet for Mo and S/Te bonds where Mo-S bond binding energy is higher than then Mo-Te bonds, 1 doublet corresponding to Mo-O, and 1 single for S 2s). The peak position difference showed change according to Te concentration. However, the trend seen with sulfurization samples and tellurization samples are different. More specifically, for sulfurization samples, or S-rich samples, the peak position difference for the two peaks corresponding to the alloy becomes larger as the Te concentration x becomes larger. This may be explained with the fact that at x = 0, there is only Mo-S bond, hence 2-peak fitting would result in ideally 0 peak energy difference. As x becomes larger, there are Mo-Te bonds which give rise to second (or more) XPS peaks. As x becomes even larger, there are more lower energy peaks corresponding to different Te bond numbers around Mo which cause the lower energy peak to shift away from the higher energy peak. On the other hand, for the Te-rich samples, or tellurized samples, the peak position difference does not show clear trend, rather show a steady or constant trend even when the Te concentration x changes. This may be attributed to the fact that although the amount of Te is changing, it is not changing the Mo bonding state in the same manner as the sulfurized samples discussed above. The two peak difference staying somewhat constant may indicate that Mo bonding state is only in two forms: Mo-S and Mo-Te, a state where phase separation is occurring. This shows that XPS may be used to evaluate not just the chemical states but also the physical structure of the alloy.

This work was partly supported by JST CREST Number JPMJCR16F4, Japan. This work was also partly supported by JSPS KAKENHI Grant Number 18J22879.

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