Crystal structure and tribological properties of molybdenum disulfide films prepared by magnetron sputtering technology
Introduction
Applications of graphene two-dimensional nanomaterials in the fields of material science, solid-state physics, and engineering are rapidly developing and becoming a research hotspot [[1], [2], [3], [4]]. However, many defects of such nanomaterials limit their use [1,5,6]. Accordingly, other two-dimensional nanomaterials such as transition-metal disulfides [7,8], transition-metal oxides, and hexagonal boron nitride are attracting considerable research interest [[9], [10], [11]]. Among these nanomaterials, two-dimensional transition-metal disulfide has attracted considerable attention due to its excellent electrical, optical, and mechanical properties [[12], [13], [14], [15], [16]]. Molybdenum disulfide (MoS2) is a typical transition-metal disulfide. Its thickness can be reduced from large to single layer because surface dangling bond do not exist, thereby becoming a two-dimensional material [17]. MoS2 has excellent physical and chemical properties and is abundant in nature [14,18]. Moreover, it is widely used in the fields of electrochemistry [[19], [20], [21]], energy storage [[22], [23], [24]], catalysis [[25], [26], [27], [28]], lubrication, and so on [[29], [30], [31], [32]].
MoS2 follows a hexagonal-layered structure in which S and Mo are bonded by covalent bonds, whereas the covalently bonded layers of S–Mo–S are connected by weak van der Waals forces (Fig. 1). These weak van der Waals forces lead to low frictional behavior, thereby making it an effective solid lubricant [[33], [34], [35]]. MoS2 has been widely used in automotive components, sensors, optoelectronic devices, and biomedical fields [12,[36], [37], [38], [39]]. However, the MoS2 coatings prepared by different techniques have remarkable differences in friction properties. Thus, its application fields are also different. For example, Zhou et al. obtained MoS2 for fire proofing materials by chemical stripping [7,9,40]. Xue et al. used chemical deposited MoS2 films for light-emitting devices [41,42]. Pak et al. obtained magnetron sputtering for MoS2 films for detectors [43,44]. Although MoS2 films have been prepared by magnetron sputtering, their structure and properties that are formed by various sputtering conditions are remarkably different during sputtering. Thus, the deposition process of magnetron sputtering MoS2 films is worth exploring.
In this study, other conditions were constant, and a batch of MoS2 films were prepared by controlling the sputtering power. The films were characterized by structural characterization and friction properties. The composition and microstructure of the films were analyzed. The surface morphology and friction properties of the films were discussed. The best sputtering power of the magnetron sputtering MoS2 films was obtained.
Section snippets
Preparation of MoS2 films
MoS2 films were deposited onto silicon (100) substrates by magnetron sputtering using argon (Ar, purity 99.999%) as a sputtering gas. The magnetron was operated with a radio-frequency power (Prf) supply. The silicon substrates were ultrasonically cleaned in acetone and ethanol solution for 15 min, and then rinsed with deionized water. Subsequently, the substrate was dried using a heater and placed in a substrate holder for film deposition. The sputtering chamber was evacuated to 9.9 × 10−4 Pa.
Results and discussion
XRD can be used to identify the crystallization of MoS2 films. Fig. 2 (a) shows that four samples with different Prf's present two diffraction peaks in the spectrum. The search results of ICSD indicates that the diffraction peaks at 2θ is 69.4° and 69.6° correspond to the (202) crystal planes of MoS2 (JCPDS card number: 89–2905) and the (400) crystal planes of Si (JCPDS card number: 01–0791), sharp diffraction peaks indicate that all four silicon substrate surfaces have remarkable MoS2 crystals
Conclusions
MoS2 films of different Prf's were prepared and deposited onto silicon wafer substrates by magnetron sputtering. The effect of Prf on the crystal structure, chemical composition, surface morphology, and friction properties of the films were discussed. Microstructure studies show that with increased Prf, the MoS2 (202) diffraction peak gradually increases. Thus, the crystallinity of MoS2 gradually increases. Furthermore, Prf has great effect on the surface morphology of MoS2 films. When Prf is
Author contributions
Jianrong Xiao conceived and designed the experiments; Chenyang Gong performed the experiments and wrote the paper; Liwen Zhu and Meng Qi analyzed the data; all authors participated and discussed this work.
Conflicts of interests
The authors declare no conflicts of interests with regard to the publication of this paper.
Acknowledgments
The authors are grateful to the Guangxi Natural Science Foundation (Grants No. 2017GXNSFAA 198121), and the Innovation Project of Guangxi Graduate Education, China (Grants No. YCSW2019160).
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