title: Unusual kinematics-driven chemistry: cleaving C-H but not COO-H bonds with hyperthermal protons to synthesize tailor-made molecular films.
abstract: Unconventional reaction-design strategies have been developed to exploit the intriguing kinematics that occur when adsorbed organic molecules are bombarded by a beam of hyperthermal protons: kinematic energy transfer is only effective in H-->H collisions and thus only C-H bonds are cleaved. This process yields a cross-linked molecular film with its chemistry governed by the selection of appropriate precursor molecules. Unlike the conventional wet-chemistry synthesis of cross-linked polymeric films, this new route uses no chemical initiators, additives, nor catalysts, and only requires a proton beam with a kinetic energy of a few electron volts in a dry-process mode compatible with molecular-device fabrication. The reaction designs are expressed unconventionally: reaction energy is tuned by the kinetic energy of the proton beam and reactant supply is controlled precisely by the proton fluence. However, conventional considerations such as bond-strength effects on kinematic outcomes and branching-ratio statistics are also important and they can extend the reaction applicability of the kinematics concept. For example, taking advantage of the fact that COO-H bonds are stronger than C-H bonds, we show, with practical reaction conditions, synthesis results, and surface analysis using X-ray photoelectron spectroscopy and atomic force microscopy, that we can break C-H bonds without breaking COO-H and other bonds, in the production of cross-linked molecular layers with any desirable COOH concentration and with no ester nor other chemical contaminations. The new reaction-design strategies are also applicable to the synthesis of molecular layers with other functionalities such as OH, and to the synthesis of a mixture of functionalities, such as OH/COOH, with a controllable concentration ratio.
authors.affiliation: Institute of Surface Micro and Nano Materials, Xuchang University, Henan Province, Xuchang, 461000, P.R. China.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Zheng
authors.forename: Ka Wai
authors.initials: KW
authors.lastname: Wong
authors.forename: Wai Cheung
authors.initials: WC
authors.lastname: Lau
authors.forename: Raymund Wai Man
authors.initials: RW
authors.lastname: Kwok
authors.forename: Woon Ming
authors.initials: WM
authors.lastname: Lau
mesh_terms: D001696:Biomechanical Phenomena; D002245:Carbon Dioxide; D006859:Hydrogen / Q000737:chemistry*; D008567:Membranes, Artificial; D018625:Microscopy, Atomic Force; D011522:Protons
chemical_list: D008567:Membranes, Artificial; D011522:Protons; D002245:Carbon Dioxide; C041069:carboxyl radical; D006859:Hydrogen