Characterization of pulsed laser deposited MoS 2 by transmission electron microscopy
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V. J. Dyhouse Research Institute, University of Dayton, Dayton, Ohio 45469-0168 (Received 10 August 1992; accepted 7 July 1993)
Molybdenum disulfide is a technologically important solid phase lubricant for vacuum and aerospace applications. Pulsed laser deposition of M0S2 is a novel method for producing fully dense, stoichiometric thin films and is a promising technique for controlling the crystallographic orientation of the films. Transmission electron microscopy (TEM) of self-supporting thin films and cross-sectional TEM samples was used to study the crystallography and microstructure of pulsed laser deposited films of M0S2. Films deposited at room temperature were found to be amorphous. Films deposited at 300 "C were nanocrystalline and had the basal planes oriented predominately parallel to the substrate within the first 12-15 nm of the substrate with an abrupt upturn into a perpendicular (edge) orientation farther from the substrate. Spherically shaped particles incorporated in the films from the PLD process were found to be single crystalline, randomly oriented, and less than about 0.1 fim in diameter. A few of these particles, observed in cross section, had flattened bottoms, indicating that they were molten when they arrived at the surface of the growing film. Analytical electron microscopy (AEM) was used to study the chemistry of the films. The x-ray microanalysis results showed that the films have the stoichiometry of cleaved single crystal MoS 2 standards.
I. INTRODUCTION Molybdenum disulfide is presently the metal dichalcogenide solid lubricant of choice for precision satellite mechanisms. The hexagonal crystal structure of M0S2 provides the possibility of an ultralow friction surface because of its highly anisotropic lamellar morphology with a van der Waals bonded basal plane oriented parallel to the surface. Unfortunately, current thin film deposition technology is able to only approach the friction qualities inherent in basal plane oriented single crystal MoS 2 . The most commonly used method of depositing MoS 2 is rf sputtering, a technology that is well developed.1"3 The properties of rf sputtered M0S2 films have been investigated extensively, and researchers have yet to achieve the optimal stoichiometry, structure, and orientation. There is a need for improved film deposition processes to advance the state-of-the-art in producing adherent, dense, nonporous, stoichiometric, MoS 2 thin films. Pulsed laser deposition (PLD) is an emerging thin film deposition method that has recently been applied to tribological materials with great success. 4 " 6 PLD has several inherent advantages over conventional deposition techniques, including excellent film adhesion, replication of target chemistry, and low film deposition temperature. J. Mater. Res., Vol. 8, No. 11, Nov 1993
For example, excimer PLD has been used to grow dense, nonporous, stoichiometric MoS 2 films, which were long lived and exhibited low friction (cf = 0.02).6 The utility of the PLD process was demonstrated where film deposition temperatur
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