Synchrotron-based high-resolution photoelectron spectroscopy was applied to study the modification of the alkanethiol (AT) self-assembled monolayers on gold and silver substrates by nitrogen-oxygen downstream microwave plasma. Because of the low density and energy of the ionizing particles, the long-lived nitrogen and oxygen radicals provided the major impact of plasma treatment. The treatment resulted in massive damage and disordering of the initially well-ordered and chemically homogeneous AT films. The most pronounced processes are the complete (AT/Au) or partial (AT/Ag) oxidation of the pristine thiolate species, partial desorption of hydrogen and carbon-containing fragments with subsequent cross-linking within the residual hydrocarbon layer, and partial oxidation of this layer, and appearance of the nitrogen-containing entities. The plasma-treatment-induced changes in the alkyl matrix and at the S-substrate interface are only partly correlated. The rate and extent of the oxidation processes at this interface are noticeably larger for C18/Au than for C18/Ag, which suggests a stronger S-metal bond in the latter system. The results demonstrate that a smallest oxygen contamination should be avoided if one wants to perform a soft modification of thin organic layers or definite molecular entities attached to these layers through the exposure to plasma.
Angle-dependent x-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy measurements were performed to differentiate local electronic structures at the tips and sidewalls of highly aligned ZnO nanorods. The overall intensity of the O K-edge XANES spectra is greatly enhanced for small photon incident angles. In contrast, the overall intensity of the Zn K-edge XANES is much less sensitive to the photon incident angle. Both valence-band photoemission and O K-edge XANES spectra show substantial enhancement of O 2p derived states near the valence band maximum and conduction band minimum, respectively. The spatially resolved Zn 3d core level spectra from tip and sidewall regions show the lack of chemical shift. All the results consistently suggest that the tip surfaces of the highly aligned ZnO nanorods are terminated by O ions and the nanorods are oriented in the [0001̄] direction.
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.
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