J. Phys. Chem. C 2007, 111, 18663-18671
18663
Modification of 1H,1H,2H,2H-Perfluorooctyltrichlorosilane Self-Assembled Monolayers by Atomic Hydrogen Justin M. Gorham,† Adam K. Stover,†,‡ and D. Howard Fairbrother*,†,‡ Department of Chemistry and Department of Materials Science and Engineering, The Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed: July 2, 2007; In Final Form: October 1, 2007
The effect of atomic hydrogen (AH) on the chemical composition and interfacial properties of a semifluorinated self-assembled monolayer (SAM) formed on Si ((CF3)(CF2)5(CH2)2-Si) has been studied by using a combination of X-ray photoelectron spectroscopy (XPS) and contact angle measurements. XPS results indicate that AH reacts with the SAM in a kinetically controlled process where the rate-determining step involves H atom abstraction from C-H bonds to form carbon-centered radicals tethered to the Si substrate. Subsequent reactions of these carbon-centered radicals with AH lead to desorption of species that contain the fluorocarbon (CF3(CF2)5) portion of the adsorbate chain. This proposed mechanism accounts for a number of experimental observations, including the fact that the ratio of CF3 to CF2 groups within the film remains constant during AH exposure and that the loss of fluorine exhibits first-order kinetics with a rate constant proportional to the AH flux. The rate constant for the loss of CF3 + CF2 groups is estimated to be 1.3 × 10-18 AH-1 cm2. Desorption of fluorocarbons from the adsorbate layer is followed by AH-mediated chemical erosion of residual hydrocarbon species. Contact angle measurements conducted on the 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS) monolayer as a function of AH exposure revealed a correlation between wettability and the CF3 (and CF2) group coverage. The ability of AH to control the chemical composition of the monolayer as a route to create patterned surfaces that exhibit different interfacial properties is also demonstrated.
I. Introduction Fluorine-containing self-assembled monolayers (SAMs) and thin films have been a subject of intense scientific interest due to their desirable properties, including chemical and thermal stability and low surface energy.1-3 Indeed, fluorocarbon thin film coatings are used to regulate chemical reactivity and control interfacial properties such as lubrication and adhesion.4 In microelectromechanical systems (MEMS), well-ordered fluorinated SAMs formed from organosilanes can create protective coatings with low friction.5 The high electron affinity of fluorine, as well as the extremely hydrophobic nature of the surface,4,6,7 has also made CF3-terminated SAMs attractive candidates for the generation of patterned resists in lithographic applications8 and as resists in the atomic layer deposition of high κ-dielectrics.9 In many applications of fluorine-containing thin films and SAMs, the ability to control the film’s surface chemistry to tailor interfacial properties would be advantageous. Due to the strength of the C-F bond (≈480 kJ mol-1) the use of conventional wet chemical routes to modify fluorocarbon surfaces is difficult. Reactive gas-phase species such as electrons and ions can initiate C-F bond cleavage and alter the chemical composition and surface properties of fluoropolymers and fluorinated interfaces.10-13 However, one intrinsic drawback of these low-pressure surface modification processes (e.g., plasma treatments) is the complexity of the reactive environment and the range of chemical reactions that are initiated within the film during the modification process.14,15 Consequently, control of the resultant film’s * Address correspondence to this author. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Materials Science and Engineering.
chemical and physical properties is difficult to achieve with traditional vacuum-based treatment strategies. Compared to electrons or ions, radicals are more selective in their reactions with fluorocarbons. For example, both C-F and C-H bonds are cleaved by low-energy electrons or ions.10-13 In contrast, C-F bonds are inert toward atomic oxygen at incident energies
