Discharge Plasma Oxidation Research Articles

A Combined Experimental and Theoretical Investigation of Atomic-Scale Defects Produced on Graphite Surfaces by Dielectric Barrier Discharge Plasma Treatment

The characteristics and nature of atomic-scale defects produced on graphite surfaces by dielectric barrier discharge (DBD) plasma oxidation have been investigated, both experimentally and theoretically. Two main types of defect visualized by scanning tunneling microscopy (STM) were studied: protrusions ∼1-5 nm in diameter and smooth circular depressions 5-7 nm wide, the latter constituting a novel type of defect on carbon surfaces that was only very recently reported for the first time. STM and atomic force microscopy (AFM) experiments indicated that both the protrusions and the depressions are not associated to topographical features on the graphite surface and that their observation by STM should be related to electronic effects. The thermal behavior of the protrusions, which could only be removed at a temperature of ∼900 °C, as well as their reactivity toward molecular oxygen, allowed their identification as multiatomic vacancies. In comparison, the depressions displayed a higher thermal stability (they could be eliminated only at ∼1200 °C) and a lower reactivity toward oxidation. Density functional theory (DFT) calculations suggested that the depressions are associated with two-dimensional clusters of interstitial oxygen formed by the agglomeration of migrating oxygen atoms. Such clusters induce a lowering in the local density of electronic states on the graphite surface and are therefore detected as a depression by STM. Taken as a whole, the findings reported here provide a consistent picture of the basic mechanism underlying the modification of graphitic surfaces by this type of plasma, which is driven by physical processes (i.e., ion bombardment).

Formation of ultrathin nanocomposite SiO 2:nc-Au structures by Pulsed Laser Deposition

A method for the formation of Au nanocrystal (nc) arrays embedded in an ultrathin SiO 2 layer in one vacuum cycle is proposed. The method is based on the co-deposition in vacuum of ∼1 nm thick uniform Si–Au amorphous layer at a specific composition ratio by Pulsed Laser Deposition on the pre-oxidized Si(1 0 0) substrate, followed by its oxidation in the glow discharge oxygen plasma at room temperature, resulting in the precipitation of Au ncs at the bottom interface and/or at the surface of the forming SiO 2 layer. The capping SiO 2 layer is formed by the glow discharge plasma oxidation of further deposited ultrathin Si layer. Au ncs 2–5 nm in size and with the separation of ∼3–20 nm from each other segregate during the oxidation of Au–Si mixture as evidenced by transmission electron microscopy (TEM). The evolution of Au and Si chemical state upon each step of the SiO 2:nc-Au nanocomposite structure formation is monitored in situ by X-ray photoelectron spectroscopy (XPS). The metrology of nanocomposite SiO 2:nc-Au structures describing the space distribution of Au ncs as a function of Au/Si ratio is presented.

Commercially Pure Titanium and Ti6Al4V: XPS Comparison Between Different Commercial Ti Dental Implants and Foils Prepared by Various Oxidation Procedures

Commercially pure (c.p., 99.6%) titanium and the Ti6Al4V alloy are widely used in biomedical applications. The surface (oxide) properties of these materials play a key role for their function in biomaterial applications, and it is therefore important to analyze their surface composition. XPS survey and high resolution spectra (Ti 2p, O 1s, and C 1s) are presented from three commercially available dental implants, all manufactured by machining but with different post treatments (grit blasting, cleaning, sterilizing, and packaging). Comparison is made with a TiO2 single crystal (rutile (110)) reference sample, and with Ti and Ti6Al4V foils prepared by the following treatments commonly used for titanium biomaterials: ASTM F86/B600 passivation, electropolishing in perchloric acid based electrolyte, anodization in sulfuric acid and phosphoric acid, and glow discharge plasma oxidation. Survey spectra from the implants are qualitatively similar but quantitatively different, with Ti, O, and C as the main elements and trace amounts of preparation-specific impurities. The high resolution spectra show differences in oxide composition (varying deviations from pure TiO2 stoichiometry), thickness, and carbon overlayer composition. Spectra from the differently prepared Ti and Ti6Al4V foils showed preparation-specific differences mainly with respect to oxide composition, oxide thickness, and trace impurities. For the Ti6Al4V the different preparations lead to various amounts of detected Al. A general difference between the differently prepared Ti and Ti6Al4V foils was that Al and V were always detected on the latter, but only sporadically on the Ti. These results clearly show that the composition of titanium surfaces varies considerably depending on the surface preparation used. Important implications are: (i) surface preparation and characteristics need to be considered when results from biological studies on Ti surfaces are interpreted and compared, (ii) Ti and Ti6Al4V form oxides with different composition, and (iii) different commercial Ti implants cannot a priori be assumed to have similar surface composition.