J. Phys. Chem. B 1998, 102, 3959-3966
3959
Chemical Modification of Polystyrene Surfaces by Low-Energy Polyatomic Ion Beams Earl T. Ada, Oleg Kornienko, and Luke Hanley* Department of Chemistry, m/c 111, UniVersity of Illinois at Chicago, Chicago, Illinois 60607-7061 ReceiVed: December 30, 1997; In Final Form: March 18, 1998
The chemical modification of polystyrene surfaces by low-energy (10-100 eV) SF5+, C3F5+, and SO3+ ions was studied by X-ray photoelectron spectroscopy and two-laser ion trap mass spectrometry. The mechanism of fluorination was found to be dissimilar for SF5+ and C3F5+ ions in this energy range at fluences of 10141016 ions/cm2. SF5+ was found to induce fluorination of the polymer surface by grafting reactive F atoms upon dissociation at impact. SFn fragments were not found to be grafted or implanted into the polymer. Sulfur was detected on the polymer surface only at incident energies above 50 eV and was found to be sulfidic in nature. In contrast, C3F5+ ions induced grafting of both reactive F atoms and molecular CmFn fragments from the dissociation of the incident projectile. Larger proportions of highly fluorinated sites and thicker fluorocarbon layers were found for C3F5+ at all energies and fluences. A variety of aliphatic and aromatic fluorine bonding environments were detected on both SF5+ and C3F5+ modified polystyrene surfaces.
I. Introduction Interest in low-energy ion beam-surface interactions is driven by two general applications: (1) the development of analytical techniques such as low-energy ion scattering spectroscopy, secondary ion mass spectrometry, and surface-induced dissociation mass spectrometry1-7 and (2) materials processing.1,8,9 Ion beam processing of materials involves the modification of surfaces by direct exposure to energetic ions. Ion beams have been used to graft specific functional groups to polymer surfaces for biomaterials, adhesion, and printing applications.10-12 Ion beams have also been used to synthesize or deposit thin films on metal and semiconductor surfaces with chemical and physical properties different from the bulk substrate.1,14 Finally, ion beams have been used to etch features on surfaces for pattern generation.9 Another important motivation for ion beam-surface experiments is to model plasma-surface interactions. Plasma-based processing of materials has gained widespread technological prominence due to its several advantages over other methods of surface treatment. However, the extreme complexity of the plasma environment makes it difficult to gain a detailed understanding of the fundamental plasma-surface interaction, especially for plasmas of organic molecules.14 Therefore, several studies have used ion beams of specific mass, kinetic energy, and fluence incident on well-defined surfaces to model the energetic species in plasmas that lead to fluorination,13,15,16 nitridation,17,18 hydroxylation, amination,11 and reactive ion etching.12,16,19 These and other model systems revealed some of the mechanistic details in the reaction processes, particularly the importance of energetic ion bombardment of the surface during plasma treatment.20 For example, specific chemical functional groups can be grafted onto the surface of polystyrene by bombardment with OH+ and NH+ ions at specific kinetic energies (
