Covalent chemical modification of self- assembled fluorocarbon monolayers by low- energy CH₂Br₂^+· ions: a combined ion/surface scattering and X-ray photoelectron spectroscopic investigation

Abstract

Specific covalent chemical modification at the outermost atomic layers of fluorinated self-assembled monolayers (F-SAMs) on gold is achieved by bombardment with low-energy polyatomic ions (<100 eV). The projectile ion CH₂Br₂^+· (m/z 172), mass and energy selected using a hybrid ion/surface scattering mass spectrometer and scattered from the F-SAM surface, CF₃(CF₂)₇(CH₂)₂-S-Au, undergoes ion/surface reactions evident from the nature of the scattered ions, CH₂F⁺ (m/z 33), CHBrF⁺ (m/z 111), and CF₂Br⁺ (m/z 129). The chemical transformation of the reactive F-SAM surface was independently monitored by in situ chemical sputtering with the projectile Xe^+·. Representative species sputtered from the modified surface include CF₂Br⁺, an indicator of terminal CF₃ to CF₂Br conversion. X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of organic bromine at the surface; Br (³P_3/2) and Br (³P_½) peaks were present at binding energies of 182 and 190 eV, respectively. XPS analysis also revealed increased surface modification at higher collision energies in these reactive ion bombardment experiments, as exemplified by the increased hydrocarbon/fluorocarbon peak ratio in the C(1s) region and incorporation of oxygen in the surface seen in the observation of an O(1s) peak.