Modification of Aliphatic Monomolecular Films by Free Radical Dominant Plasma:  The Effect of the Alkyl Chain Length and the Substrate

Abstract

High-resolution photoelectron spectroscopy was applied to study the modification of alkanethiolate (AT) self-assembled monolayers (SAMs) on gold and silver substrates by nitrogen-oxygen downstream microwave plasma. The dominating plasma-induced processes are the oxidation of the alkyl matrix and the thiolate headgroups and the desorption of the alkylsulfonate species. The rates and extent of these processes and the mechanism of film modification depend on the substrate and the length of the alkyl chain. The films on Ag were found to be much more resistant to degradation by the reactive plasma, which is related to stronger thiolate-metal and sulfonate-metal bonds as compared to those on Au and to a partial polymerization of the alkyl matrix in AT/Ag at the initial stages of the plasma treatment. On a given substrate, the length of the aliphatic chain noticeably affects the rates and extent of the oxidation and desorption processes. The major effect stems from a two-step character of the thiolate-sulfonate transformation process, which requires both the penetration of the reactive oxygen species to the headgroupsubstrate interface and the oxidation event itself. Whereas for short-chain SAMs the oxidation step is rate-determining, the process becomes diffusion-limited for the long-chain films.

Perspective on the Development of Microplasma Technology: The Role of Oxygen in Downstream Microwave Plasma

“…The energy resolution was 0.25-0.3 eV. The binding energy (BE) scale was referenced to the Au 4f 7/2 peak of DDT/Au at 84.00 eV [7,15].…”

“…The S 2p spectrum of this film exhibited a single S 2p doublet with a BE of ≈ 162.0 eV (S 2p 3/2 ). The observed doublet is commonly related to the thiolate species bonded to gold [7,18,18]. HRXPS spectra of pristine (bottom curves) and argon-plasma-processed DDT/Au.…”

“…Presumably, only a small fraction of electrons and ionized species survives during the transport to the afterglow region. However, there is still a noticeable portion of long-living free radicals [6][7][8], which can become the dominant reactive species in the afterglow region.…”

“…In our previous studies, we have shown that low-density (∼ 10 6 particles/cm 3 ) downstream plasma can be successfully used to modify ultra-thin organic films, such as, e.g., aliphatic and aromatic thiol-derived selfassembled monolayers (SAMs) [1,2,7,8]. There are several major processes, which occur both simultaneously and subsequently during the exposure of SAMs to the plasma.…”

“…All these processes result in chemical modification and partial or even complete desorption of the monomolecular film, which can be, e.g., used for its patterning. [1,2,[7][8][9][10]. Particular techniques based on weakly-ionized micro-plasma [11,12] and/or micro-plasma jet at atmospheric pressure [13] are also well suited for advanced treatment of sensitive objects.…”