Showing papers by "Jari Koskinen published in 2016"

Correlation between sp3-to-sp2 Ratio and Surface Oxygen Functionalities in Tetrahedral Amorphous Carbon (ta-C) Thin Film Electrodes and Implications of Their Electrochemical Properties

Abstract: Detailed information about the physicochemical properties of a given surface is important in order to understand and predict the performance of materials in electrochemical applications. Here we present a detailed X-ray absorption spectroscopy study of two different tetrahedral amorphous-carbon (ta-C) thin films and their subsequent electrochemical characterization. The results show marked differences in ta-C surface and bulk properties, namely differences in the amount of surface functional groups and their sp3/sp2 ratios, respectively. In particular, the variation in the oxygen content of the surface leads to significantly different behavior in electrochemical measurements, such as a 10-fold increase in sensitivity for dopamine and stronger response to ascorbic acid. Results of the surface properties were further analyzed by simulations carried out within the framework of density functional theory (DFT) as well as by utilizing Raman spectroscopy.

What Does Nitric Acid Really Do to Carbon Nanofibers

Abstract: Understanding the chemical nature of the surface of carbon nanofibers (CNF) is critical in assessing their fundamental properties and tailoring them for the right application. To gain such knowledge, we present here a detailed X-ray adsorption spectroscopy (XAS) study accompanied by high resolution transmission electron microscopy (TEM) micrographs of two morphologically different CNF pairs (tetrahedral amorphous carbon (ta-C) grown “open structured” fibers and traditional bamboo-like “closed structured” fibers), where the surface chemical properties and structural features of the fibers are investigated in depth and the effects of nitric acid treatment on the fibers are revealed. The morphology of the fiber and/or the original seed- and adhesion layers markedly affect the response of the fibers to the acid treatment. Results also show that the nitric acid treatment increases the observed sp2 intensity and modifies the two types of fibers to become more-alike both structurally and with respect to their ox...

Effect of tetrahedral amorphous carbon coating on the resistivity and wear of single-walled carbon nanotube network

Abstract: Single walled carbon nanotube networks (SWCNTNs) were coated by tetrahedral amorphous carbon (ta-C) to improve the mechanical wear properties of the composite film. The ta-C deposition was performed by using pulsed filtered cathodic vacuum arc method resulting in the generation of C+ ions in the energy range of 40–60 eV which coalesce to form a ta-C film. The primary disadvantage of this process is a significant increase in the electrical resistance of the SWCNTN post coating. The increase in the SWCNTN resistance is attributed primarily to the intrinsic stress of the ta-C coating which affects the inter-bundle junction resistance between the SWCNTN bundles. E-beam evaporated carbon was deposited on the SWCNTNs prior to the ta-C deposition in order to protect the SWCNTN from the intrinsic stress of the ta-C film. The causes of changes in electrical resistance and the effect of evaporated carbon thickness on the changes in electrical resistance and mechanical wear properties have been studied.

Nanodiamond embedded ta-C composite film by pulsed filtered vacuum arc deposition from a single target

Abstract: Detonation Nanodiamonds (DNDs) are known to have sp3 core, sp2 shell, small size (few nm) and are gaining importance as multi-functional nanoparticles. Diverse methods have been used to form composites, containing detonation nanodiamonds (DNDs) embedded in conductive and dielectric matrices for various applications. Here we show a method, wherein DND-ta-C composite film, consisting of DNDs embedded in ta-C matrix have been co-deposited from the same cathode by pulsed filtered cathodic vacuum arc method. Transmission Electron Microscope analysis of these films revel the presence of DNDs embedded in the matrix of amorphous carbon. Raman spectroscopy indicates that the presence of DNDs does not adversely affect the sp3 content of DND-ta-C composite film compared to ta-C film of same thickness. Nanoindentation and nanowear tests indicate that DND-ta-C composite films possess improved mechanical properties in comparison to ta-C films of similar thickness.

Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films

Abstract: Thin carbon films were grown at room temperature on (0 0 1) n -Si substrate using dual cathode filtered vacuum arc deposition system. Graphite was used as a source of carbon atoms and separate metallic electrode was simultaneously utilized to introduce Ni or Cu atoms. Films were irradiated by 100 MeV Ag 7+ ions to fluences in the range 1 × 10 10 –3 × 10 11 cm −2 . Rutherford backscattering spectroscopy, Raman scattering, scanning electron microscopy and atomic force microscopy in conductive mode were used to investigate film properties and structure change under irradiation. Some conductive channels having metallic conductivity type were found in the films. Number of such channels is less than number of impinged ions. Presence of Ni and Cu atoms increases conductivity of those conductive channels. Fluence dependence of all properties studied suggests different mechanisms of swift heavy ion irradiation-induced transformation of carbon matrix due to different chemical effect of nickel and copper atoms.

The influence of PVD coatings on the wear performance of steel dental curettes

Abstract: The wear properties of steel dental curettes coated with wear resistant Diamond Like Carbon (DLC) and metal nitride coatings deposited by Physical Vapor Deposition (PVD) were studied by using a customized mechanical device simulating motion and load of dental curettes against human teeth. The wear surfaces were studied by optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Nanoindentation. Rockwell indentation testing was used to evaluate the coating adhesion. With the optimized combination of substrate steel and wear resistant coating reduction of 80% in the wear of instruments could be achieved.