Atomic structure and friction of ultrathin films of KBr on Cu(100)

The frictional properties of ultrathin films of KBr on a Cu(100) substrate have been studied on the atomic scale by means of friction force microscopy. Films as thin as two atomic layers support the sliding tip and exhibit atomic stick-slip friction similar to cleaved surfaces of bulk KBr. Steps on the film are prone to wear, while substrate steps overgrown by the film are found to be stable. The interpretation of these results is supported by a detailed analysis of the structure of the films by means of atomic-resolution noncontact force microscopy. The films are found to be compressively strained by 1.0 +/- 0.2 %. They exhibit a regular superstructure originating from the interference of lattice constants at the substrate-film interface, which also causes a weak modulation of the frictional response. Monolayer films of KBr are subject to significant wear when imaged in contact-mode force microscopy. Noncontact imaging confirms that these monolayers are rich in defects.