Abstract: Ultrathin film of perfluorodecanoic acid expected to be excellent lubricant for micromachines was prepared successfully on single crystal silicon substrate.The film was characterized by means of Xray photoelectron spectroscopy (XPS) and contactangle meter.The chemical reaction involved in the preparation of the ultrathin film was discussed as well.After being immersed in a dilute aqueous solution of polyethyleneimine (PEI) for 15 minutes and rinsed with distilled water,the silicon substrate was coated with a thin film of PEI,which was then put into a dilute solution (1×10－3 mol•L－1) of perfluorodecanoic acid in hexadecane.Subsequently the steady perfluorodecanoic acid ultrathin film was developed on PEI coating in the presence of a covalent amide bond between carboxylic group and the primary or secondary amine groups of PEI.This process was accompanied by the contact angle changes of water droplet on the Si surface (see Table 1).Moreover,the reaction between perfluorodecanoic acid and PEI was significantly influenced by N,N′dicyclohexylcarbodiimide (DCCD).The contact angle on the ultrathin film of perfluorodecanoic acid is only 66.3° in the absence of DCCD in the reacting solution; it rises to 89.4° in the presence of DCCD.This indicates that the reaction between perfluorodecanoic acid and PEI was accelerated by DCCD,and the quality of perfluorodecanoic acid ultrathin film thus improved.XPS analysis of the ultrathin film indicates that the derivatization of PEI with perfluorodecanoic acid was accompanied by several changes.First,a large and highly symmetrical F 1s peak appeared at 688.3 eV (C－F).Secondly,a new peak of N 1s appeared at 400.7 eV (chemical shift 1.4 eV),which was attributed to the N atom attached to the carbonyl group (O=C－N).Thirdly,three new peaks of C 1s appeared at 286.1 eV (chemical shift 1.5 eV),288.1 eV (chemical shift 3.5 eV),and 291.0 eV (chemical shift 5.4 eV),respectively.These C 1s peaks were attributed to the C atom attached to the O=C－N group (O=C－N－C),the carboxyl C atom (O=C－N),and the C atom in －CF3 group (C－F),respectively.Therefore it can be concluded that perfluorodecanoic acid has been chemically adsorbed onto the surface of PEI and perfluorodecanoic acid ultrathin film prepared successfully.