Soft landing of polyatomic ions for selective modification of fluorinated self-assembled monolayer surfaces

Abstract

Fluorinated self-assembled monolayer (F-SAM) surfaces comprised of CF 3(CF 2) 7(CH 2) 2S- groups bound to a gold substrate were modified by deposition of mass-selected polyatomic ions at collision energies of ∼10 eV. The modified material was characterized in situ by low-energy ion bombardment and by independent high-resolution time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis. Modification of F-SAM surfaces using hyperthermal (CH 3) 2SiNCS + ( m z 116) and (CH 3) 3SiOSi(CH 3) 2 ( m z 147) projectile ion beams incorporated the intact projectile ions m z 116 and mlz 147, respectively, which were released upon subsequent 60 eV ⊎ sputtering. In addition to simple cases of soft landing of intact ions into a surface, two related soft landing channels, dissociative soft landing and reactive soft landing, are also identified. Surfaces modified by prolonged exposure to 35CICH 2(CH 3) 2SiOSi(CH 3) 2 + ( m z 181) and its isotopic variant 37CICH 2(CH 3) 2SiOSi(CH 3) 2 + ( m z 183), yielded only fragment ions derived from the projectile ions, primarily C 3H 10OSi 2 35Cl + ( m z 153) and C 3H 10OSi 2 37Cl + ( m z 155) upon ⊎ sputtering as well as in the 15 keV Ga +TOF-SIMS spectra. In these cases, facile fragmentation occurs upon initial ion impact with the surface, the fragment ion being trapped at the interface in an overall process which is described as dissociative soft landing. Consistent with this, the fragment ions C 3H 10OSi 2 35CI + ( m z 153) and C 3H 10OSi 2 37Cl + ( m z 155) generated as such in the ion source were deposited without fragmentation and subsequently released intact by 60 eV ⊎ sputtering. In the cases of some projectiles, such as protonated 2,4,6-trimethylpyridine, the sputtered ions released from the modified surface included chemically transformed products due to reaction of the projectile ion at the surface. Such reactive soft landing processes occur by ion/molecule reactions at the interface, although details of their mechanism and its timing have not been elucidated. The extent and nature of surface modification is controlled by choice of the total ion dose, the projectile ion and its energy. Total ion doses used in these experiments typically corresponded to ∼7% of a monolayer; however, inefficiencies in deposition mean that the actual surface coverages achieved are much lower. For a series of pyridine ions, correlations are evident between successful soft landing and the steric bulk of the projectile ion. It is suggested that the bulky groups facilitate energy loss by the projectile as it moves into the SAM surface and subsequently assist in its trapping there. Ex situ TOF-SIMS analysis and in situ sputtering using projectile ions with different recombination energies provide evidence that the ions deposited into the F-SAM surface are present in the matrix as isolated charged species, a result which is further supported by thermal desorption experiments. The modified surfaces are stable in both air and vacuum for several hours, although the trapped ion signal slowly decreases with time. The present method of surface modification appears to represent a new technique for producing long-lived metastable structures by allowing trapping of unusual and chemically reactive species at interfaces.