Ultrathin Polymer Film Formation by Collision-Induced Cross-Linking of Adsorbed Organic Molecules with Hyperthermal Protons

Abstract

A new synthetic approach for the formation of ultrathin polymer films with customizable properties was developed. In this approach, the kinematic nature of proton collisions with simple organic molecules condensed on a substrate is exploited to break C-H bonds preferentially. The subsequent recombination of carbon radicals gives a cross-linked polymer thin film, and the selectivity of C-H cleavage preserves the chemical functionalities of the precursor molecules. The nature and validity of the method are exemplified with theoretical results from ab initio molecular dynamics calculations and experimental evidence from a variety of characterization techniques. Its applicability is demonstrated by the synthesis of ultrathin polymer films with precursor molecules such as dotriacontane, docosanoic acid, poly(acrylic acid) oligomer, and polyisoprene. The approach is fundamentally different from conventional chemical synthesis as it involves an unusual mix of physical and chemical processes including charge exchange, projectile penetration, kinematics, collision-induced dissociation, inelastic energy transfer, chain transfer, and chain cross-linking.

Using a low-energy proton beam to cross-link polymer films for the protection of inorganic substrates

“…c and d), we demonstrated again that the surface dotriacontane molecules were fixed by cross‐linking after H + ion treatment. This resulted in the indissolubility of the treated film, which is consistent with the results for the silicon …”

“…The proton‐collision‐induced reactions were performed in a mass‐separated low‐energy ion beam system, with protons at ion energies of 10 eV and with an energy spread of less than 0.6 eV incidents on the sample under ultrahigh vacuum (H + , H 2 + , H 3 + , room temperature, 5 × 10 −8 Torr during ion beam bombardment of the sample). We selected dotriacontane as the precursor molecule, because it is large enough that it does not desorb under vacuum at room temperature but small enough that its solubility in benzene allows uniform deposition on convenient inorganic substrates by spin‐casting …”

Polyatomicity and kinetic energy effects on surface polymerization by ion-assisted deposition

“…While atomic Ar + ions behave only as catalysts of polymerization during SPIAD, C 4 H 4 S + polyatomic ions and their fragments additionally behave as reagents which covalently bind to 3T. Protons or atomic hydrogen produced mostly from fragmentation of the incident T + ion also induce polymerization, as has been recently observed through direct deposition of 10 eV protons on organic films [23]. Polyatomic and atomic ions lead to significantly different film chemistry and optical properties, which also vary for 100 versus 200 eV ion kinetic energies.…”

“…10 eV protons have recently been shown to induce polymerization of various adsorbed organic species by C H bond cleavage followed by recombination of the resultant carbon radicals to form crosslinked polymeric thin films [23]. Hyperthermal protons or atomic hydrogen also play a role in SPIAD.…”

Study of the Hyperthermal Proton Bombardment Effects on Self-Assembled Monolayers of Dodecanethiol on Au(111)

“…Some of the cross-linked molecules may have multiple C12 base units, and the probability of this will increase with the degree of cross-linking which can be raised by either increasing the fluence or energy of the incident projectiles. 24 Although the partial conversion of the impact energy to a transient thermal energy at the impact site is likely a driving force of surface diffusion, raising the substrate temperature by the bombardment is below the detection limit by direct temperature measurements with a thermocouple and with an infrared pyrometer. In a relevant set of experiments with the same bombardment conditions, we bombard a layer of physiosorbed CH 3 (CH 2 ) 30 CH 3 on a silicon wafer and observe no desorption.…”