NANOTRIBOLOGICAL PROPERTIES OF OCTADECYLTRICHLOROSILANE SELF-ASSEMBLED ULTRATHIN FILMS STUDIED BY ATOMIC-FORCE MICROSCOPY - CONTACT AND TAPPING MODES

Atomic force microscopy, working in contact and tapping modes, has bee n used to probe the mechanical response of octadecyltrichlorosilane, C H3-(CH2)(17)-SiCl3 (OTS), self-assembled ultrathin films grafted on si licon-treated surfaces. To assess the adhesion hysteresis and the elas ticity of the films, we have performed loading and unloading experimen ts in contact mode. The effect of compression is observed while the lo ad is being applied to the film. Once the load is fully removed, the f ilm remains compressed showing a clear deviation from perfect elastici ty, with relaxation times in the range of 0.1 s. In the tapping made, the loading/unloading experiments show again the deviation from perfec t elastic recovery of the films. Upon loading, three different regions are encountered. These can be associated with an initial disorder of the chains produced by the indenting tip; followed by a smooth compres sion and a final hardening effect due to repulsive forces between the compressed chains. The tapping mode is also used to provide strong evi dence of a ''cushioning'' effect. The tribological properties of our O TS layers seem to be influenced by molecular disorder of the alkyl cha ins.