Mechanisms of secondary ion emission from self‐assembled monolayers and multilayers

Abstract

Alkanethiol self-assembled monolayers/multilayers (SAMs) have been applied as model organic systems with which to investigate secondary ion formation and emission processes during kiloelectronvolt ion bombardment. Self-assembled monolayer and multilayer films of 11-mercaptoundecanoic acid capped with 1-dodecanethiol were prepared on gold-coated substrates. Samples with varying number of thiolate layers were studied using static secondary ion mass spectrometry to investigate the origin of molecular secondary ions and the influence of surface chemistry and structure. The nature of the thiolate bonding affects the type and abundance of the observed ions. The intensity of atomic and cluster ions derived from the substrate decreases exponentially with increasing number of thiolate overlayers because of losses in transmission through the organic overlayers. Intact molecular and cluster ions can escape from >100Å below the surface of these structures. The variation of molecular-ion yields with multilayer thickness suggests that a significant proportion of molecular ions originate from subsurface thiolate layers. The detection of ions comprised species from the substrate or bottom of the multilayer associated with species from the top layer supports the view that chemical association at or near the surface is a viable mechanism of formation for molecular secondary ions.

Formation and electrochemical desorption of self‐assembled monolayers as studied by ToF‐SIMS

“…Alkanethiol SAMs (especially DDT) on Au have been studied using ToF‐SIMS 28–34, 37–39. Fragmentation of DDT SAMs on Au provides numerous characteristic ion fragments.…”

“…Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS)23 evolved in recent years as a fast semiquantitative tool for surface analysis24–26 which found applications in the area of thiol SAMs on Au,25, 27–39 and whose results agree well with other analytical techniques,25, 29 and as such, is suitable for the analysis of the composition of mixed SAMs.…”

An investigation of secondary ion yield enhancement using Bin2+ (n=1, 3, 5) primary ions

“…Recent experiments using layered films demonstrate that although the vast majority of secondary ions are emitted from the top ϳ2 to 3 nm of a sample, some secondary ions from deep within samples are emitted (from depths Ն 100 Å) [18]. Since we observe a few ions from the substrate in all SIMS spectra, except for the thick DL-phenylalanine samples, indicating that Bi n xϩ (n ϭ 1, 3, 5; x ϭ 1, 2) ions penetrate through the organic layer to the substrate, we estimate that the sample films are 50 to 300 Å thick.…”

An approach to differentiating between multi‐ and monolayers using MALDI‐TOF MS

“…27 -32 However, in monolayer samples both monomers and dimers are seen in the ToF-SIMS spectra. 29,30 As a result, ToF-SIMS can be used to identify species in the thin films, but establishing the height of the film may be difficult using this technique. 29 Therefore, determining monolayer vs. multilayer formation in a film is still difficult especially on oxide surfaces.…”