Electrochemical synthesis and STM-STS studies of thin diamond-like films on the surface of oxidized aluminum

Thin carbon films on the surface of aluminum containing native oxide were prepared at room temperature by electrochemical deposition from a solution of lithium acetylenide in dimethylsulfoxide. The structure of the coatings obtained was studied by scanning tunnel microscopy and spectroscopy. The mechanical characteristics of carbon coatings were found to depend strongly on the main parameters determining deposition conditions. The presence of a considerable amount of sp ³ carbon (diamond-like phases) in the films was substantiated.     

The nature of the adsorption bond between graphite islands and iridium surface

To reveal the nature of adsorption bonds between two-dimensional graphite islands and iridium (111) and (100) faces, a study has been made of the adsorption of potassium and cesium atoms on the surface of these systems, using thermal desorption and Auger electron spectroscopy, as well as surface ionization and thermionic emission techniques. The graphite islands are shown to be weakly bound to the iridium substrate by Van der Waals forces. The unsaturated valence bonds at the periphery of the graphite islands are “lowered down” on to the metal. The recess between the graphite layer and the metal is filled by adsorbing particles through defects in the graphite layer. The atoms can penetrate into the recess in two ways: at T > 1000 K directly from the flux incident on the surface, and at T < 1000 K also by migration from the graphite island surface. The adsorption capacity of this state is ～ (2?3) × 10¹⁴cm^-2. Thermal destruction of the islands at T > 1900 K liberates the potassium and cesium atoms from under the graphite islands. Our study suggests that the reason for the “raised” position of the islands lies in the valence bonds of the graphite layer being saturated, the valence bonds of the metal and its crystallographic orientation being less significant. Therefore one may expect the graphite layer to be raised also above other metals as well. The filling by cesium of the recess between the graphite layer and iridium and of the adsorption phase on the graphite surface, does not change the general “graphitic” shape of the carbon Auger peak. This cesium results, however, in a pronounced splitting of the negative spike on the carbon peak (which provides information on its location relative to the graphite layer) indicating the appearance in the valence band of graphite near the Fermi level of two narrow (～ 2?3 eV) regions with an enhanced density of states originating from the presence of the alkali metal.     

Diamondlike nanocomposite a-C:H:Cr Coatings: Structure,mechanical, and tribological properties

The results from a comprehensive investigation of the structure, phase and chemical composition, microhardness, and nanomechanical and tribological properties of chromium-doped coatings of hydrogenised amorphous carbon a-C:H:Cr are presented. The coatings are deposited via reactive magnetron sputtering in an Ar + C₂H₂ + N₂ gas mixture at various volume concentrations of nitrogen and acetylene. It is found that the carbon in the coatings is formed as disordered mixtures of domains with tetrahedral (sp ³) and hexagonal (sp ²) carbon coordinations. In addition, the doping metal in the coating consists of nanosized inclusions of metallic chromium and its carbide and nitride phases. Additional nitrogen doping resulting in the formation of chromium nitride is shown to improve the micromechanical and tribological properties of the obtained coatings.     

Formation mechanisms of nanocomposite layers based on multiwalled carbon nanotubes and non-stoichiometric tin oxide

Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnO _x ) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume, fixed by SnO _x islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands, which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite layers to (NO₂)⁻ molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs.     

Effects of electron drag of gold in pores of anodic aluminum oxide: Reversible resistive switching in a chain of point contacts

The current–voltage characteristics of Au/AAO(Au)/probe structures based on anodic aluminum oxide with pores incompletely filled with gold have been studied. It has been found that an electric field initiates the mass transfer of a rear Au electrode and the subsequent growth of the metal in unfilled parts of pores of the oxide matrix in the form of chains of gold islands. It has been established that this transfer, which appears at a positive potential of the probe, is due primarily to the effect of electron drag of the metal (Au). Estimates have been obtained for the effective radius of Au islands (2 nm), the width of a gap between islands (0.5 nm), the height of potential barriers (100 meV), and the characteristic resistance of tunnel junctions (30 kΩ ~ h/e²), which is typical of point quantum contacts. The structures demonstrate reversible resistive switching between low- (~1 MΩ) and high-resistance (>100 GΩ) states.     

Deposition of silicon–carbon coatings from the plasma of a non-self-sustained arc discharge with a heated cathode

Amorphous hydrogenated carbon doped with silicon oxide (a-C:H:Si:O), which is referred to as silicon–carbon coatings in this work, consists of thin amorphous films, which are used as commercial solid lubricants due to their higher stability under extreme environmental conditions as compared to amorphous hydrogenated carbon. The deposition of silicon–carbon coatings from the plasma of a non-self-sustained arc discharge with a heated cathode is considered. Silicon–carbon coatings are deposited using polyphenul methylsiloxane as a precursor at a flow rate of 0.05 mL/min in an argon atmosphere at a pressure of 0.1 Pa. A high-frequency power supply is used to apply a high-frequency bias voltage to a substrate during deposition. After deposition, the mechanical properties of the coatings are studied. The maximum hardness of the coating is 20 GPa at a minimum friction coefficient of 0.16 and a wear rate of 1.3 × 10^–5 mm³ N^–1 m^–1. Energy dispersive analysis shows that the coatings contain a significant content of carbon and oxygen (about 80 and 15%, respectively) and a low content of silicon (about 5%).     

Emissivity of powders prepared from nanoporous carbon

Powders prepared from nanoporous carbon are promising for creating cold emitters, which are essential to the development of reliable next-generation monitors. The results of an experimental study of the temperature and time dependences of the emission current from nanoporous carbon coatings are reported. It is shown that the stable emission may last at least 20 h under continuous operation if the emission current density does not exceed 0.6 mA/cm² at room temperature and an accelerating field strength of 800–1200 V/mm. The highest values of the unstable-in-time current density vary from 2.5 to 3.2 mA/cm².     

Features of degradation and recovery of the optical properties of coatings based on ZnO powder modified with nanoparticles after irradiation

The effect of electron irradiation with energy of 30?keV and fluence up to 7?×?10¹⁶?cm^?2 on diffuse reflection spectra in situ of coatings based on ZnO powders unmodified and modified with zirconium dioxide and aluminum oxide nanopowders was investigated. The higher radiation stability of coatings based on modified pigments in comparison to unmodified pigments has been established. A significant recovery of the reflection spectra of irradiated coatings after exposure to residual vacuum and air was shown.     

Microwave plasma deposition of diamond like carbon coatings

The promising applications of the microwave plasmas have been appearing in the fields of chemical processes and semiconductor manufacturing. Applications include surface deposition of all types including diamond/diamond like carbon (DLC) coatings, etching of semiconductors, promotion of organic reactions, etching of polymers to improve bonding of the other materials etc. With a 2.45 GHz. 700 W, microwave induced plasma chemical vapor deposition (CVD) system set up in our laboratory we have deposited diamond like carbon coatings. The microwave plasma generation was effected using a wave guide single mode applicator. We have deposited DLC coatings on the substrates like stainless steel, Cu-Be, Cu and Si. The deposited coatings have been characterized by FTIR, Raman spectroscopy and ellipsometric techniques. The results show that we have achieved depositing ∼95% sp³ bonded carbon in the films. The films are unform with golden yellow color. The films are found to be excellent insulators. The ellipsometric measurements of optical constant on silicon substrates indicate that the films are transparent above 900 nm.     

The anomalous behaviour of the ESID O+ yield from a Ni(110) surface during oxygen adsorption

The behaviour of the ESID O⁺ yield is studied as a function of the exposure of a Ni(110) surface to oxygen. Measurements performed during the adsorption process reveal a contribution to the ESID O⁺ yield, which is ascribed to a precursor state. If there is a time lapse between exposure and the measurement, the precursor state is not observed. For this situation the O⁺ ions, generated by electron impact, are shown to originate from NiO islands. The disappearance of the precursor state is explained as a transition into a chemisorption state and a depletion by ESD. The behaviour of the O⁺ yield from the NiO islands during adsorption is assumed to be due to the formation of Ni₂O₃ on top of the NiO islands.     

The 2-D growth of gold on single-layer graphene/Ru(0001): Enhancement of CO adsorption

The growth and morphology of two-dimensional (2-D) gold islands on a single-layer graphene supported on Ru(0001) have been studied by scanning tunneling microscopy (STM). Our findings show that gold exhibits 2-D structures up to a gold dosage of 0.75 equivalent monolayers, and that these 2-D gold islands are thermally stable at room temperature. Parallel polarization modulation infrared reflection absorption spectroscopic (PM-IRAS) and high resolution electron energy loss spectroscopic (HREELS) studies indicate that carbon monoxide (CO) adsorbs on these 2-D gold islands at 85 K, showing a characteristic CO stretching feature at 2095 cm^? 1 for a saturation coverage of CO. The red shift of the CO stretching frequency compared to that on charge neutral gold is consistent with electron transfer from graphene to gold, i.e., an electron-rich gold overlayer. Preliminary data obtained by dosing molecular oxygen onto this CO pre-covered surface suggest that the 2-D gold islands catalyze the oxidation of CO.     

Nanoscale coatings for control of interfacial bonds and nanotube growth

This paper describes the usefulness of nanoscale coatings in improving some engineering materials having porous and uneven surfaces (microcellular foam, nanofibers, nanotubes, etc.). It is shown that 3-5 nm coatings deposited in microwave plasma can influence crucial properties for a wide variety of applications. Two coatings resulting in opposite chemistries have been studied, an oxide layer that increases surface reactivity_, and a similar fluorocarbon layer that makes it inert. In-depth atomic level microscopic and spectroscopic investigations of nucleation and growth of these layers on various substrates have been reported earlier. The effectiveness of such coatings in modifying bond strength, wettability and catalytic activity of various porous and uneven carbon surfaces have been shown here. The following influences of nanoscale functional coatings have been elaborated upon: (a) modification of carbon-polymer interfaces (b) controlled metallization of carbon (c) influence of nano-coatings on catalytic activity, for formation of carbon nanotubes on larger structures.     

Electrolytical production of Ni + Mo + Si composite coatings with enhanced content of Si

Ni + Mo + Si composite coatings were prepared by co-deposition of nickel with molybdenum and silicon powders from a nickel solution in which Mo and Si particles were suspended by stirring. The layers have been deposited on a carbon steel substrate (St3S) under galvanostatic conditions. The content of Si in deposited layers was about 2-5 wt.% depending on deposition current density and the value of electric charge. For comparison Ni + Mo composite coatings were obtained under analogous current conditions. Composite coatings of enhanced Si content (15 wt.%) were deposited from an electrolyte in which 40 g/dm³ of Si covered with electroless plated nickel was dispersed. Deposition current density was equal 0.1 A/cm² and the value of electric charge Q = 500 C/cm². The thickness of the coatings was about 100-300 μm depending on their kind, electric charge and the deposition current density. Surface and cross-section morphology were investigated by scanning electron microscope (SEM). All deposited coatings are characterized by great, developed surface area. No internal stresses causing their cracking were observed. Chemical composition of the layers was determined by X-ray fluorescence spectroscopy (XRF) method and quantitative X-ray analysis (QXRD). It was stated, that the content of molybdenum and silicon in Ni + Mo + Si coatings depends on deposition current density and the amount of the powder in bath. The results of structural investigation of the obtained layers by the X-ray diffraction (XRD) method show, that they consist in crystalline Mo or Mo and Si phases built into Ni matrix. Moreover, Ni + Mo + Si composite coatings were modified by thermal treatment. It has been found that the thermal treatment of Ni + Mo + Si composite coatings caused that the new phases (NiSi, Mo₂Ni₃Si and Ni₆Mo₆C_1.06) were obtained.     

Wettability and antibacterial activity of modified diamond-like carbon films

Diamond-like carbon (DLC) films can be used in a numerous industrial applications, including biomedical coatings with bactericidal properties. It has been demonstrated that DLC surface can be modified with oxygen plasma treatment. The purpose of this paper is to study the wettability and bactericidal activity of oxygen plasma-treated DLC films produced by plasma enhanced chemical vapor deposition technique. The sp³/sp² ratio increased after the treatment due to the increase in the generation of the unstable carbon bonds caused by the energetic ions, especially O-H group. The treated DLC surface becomes superhydrophilic and rougher, although the roughness values are still lower. DLC antibacterial activity did not increased with plasma treatment. Therefore, oxygen plasma treatment can be used to make superhydrophilic DLC but not to increase its bactericidal properties.     

Pulsed laser deposition of hard coatings in atmospheric air

A new laser plasma technique for non-vacuum deposition of thin films has been proposed and experimentally realized. It is based on the fact that the plasma plume, which occurs under ablation of a target in air by high-intensity short laser pulses, can penetrate through a dense gas environment without significant cooling at the distance of about 1 mm. The technique has been applied to deposit diamond-like carbon (DLC) coatings on stainless steel substrates using four different values of pulse duration: 10 ns, 300 ps, 5 ps and 130 fs. Optimization of different experimental parameters including distance between the target and the substrate, laser intensity and gases (He, Ar, N₂, compressed air) blown in the deposition zone, has been performed. The deposition rate in the experiments was estimated as 2–5×10^-4 nm/(cm²pulse) for the pulse energy of 1–4 mJ. The deposited amorphous carbon films with thickness of several hundred nanometers have shown high average nanohardness (10–25 GPa depending on the irradiation conditions) and good adhesion to substrates (60 MPa). According to X-ray electron spectroscopy analysis the films consist of both sp²- and sp³-bonded carbon and contain 3–7% of free oxygen in bulk. The mechanisms of DLC non-vacuum laser deposition are discussed. To demonstrate the large potential of this technique, the first results on deposition of titanium nitride using ablation of titanium in air with nitrogen jet assistance are also presented. PACS 52.38.Mf; 81.15.Fg; 81.05.Uw     

Structural features of carbon materials synthesized by different methods

This paper presents the results of investigations of three types of carbon structures synthesized by different methods, such as arc discharge plasma enhanced chemical vapor deposition of carbon in a magnetic field, chemical dehydrohalogenation of the poly(vinyl chloride)/poly(vinylidene chloride) precursor, and pulsed plasma ion assisted deposition. It has been found that the samples prepared by different methods have a common feature, i.e., the presence of three-dimensional clusters based on sp²- or sp³-bonds surrounded by quasi-one-dimensional carbon chains. It has been shown that the structure of carbon materials changes depending on the synthesis conditions.     

Effect of pulsed ion irradiation on the electronic structure of multi-walled carbon nanotubes

The effect of pulsed ion irradiation and vacuum annealing on the ratio of sp ²- and sp ³-hybridized orbitals of carbon atoms in the layers of oriented multi-walled carbon nanotubes has been studied by analyzing the photoemission spectra of the C1s core level and the valence band of carbon, which were obtained using the equipment of the BESSY II Russian-German beamline of synchrotron radiation and a Riber analytical system. It has been shown that the ion irradiation leads to a significant decrease in the fraction of atoms with the sp ³ hybridization of electrons. On the contrary, the annealing reduces the fraction of the sp ³-component in the spectra of carbon. Typical features of the valence band of multi-walled carbon nanotubes in the annealed and irradiated states have been established.     

Investigation of the Structure and Functional Properties of Diamond-Like Coatings Obtained by Physical Vapor Deposition

Diamond-like coatings with a total thickness of ~0.6 μm are obtained by physical vapor deposition with plasma separation and a pulsed carbon arc source with a cooled cathode and laser arc ignition; the substrates are titanium alloy (VT4), stainless steel (12Cr18N10T), and copper (M1). Scanning electron microscopy and profilometry are used to study the coatings surface and structure. The composition of the coatings and the fraction of sp³ bonds are studied using Raman spectroscopy. A wide peak in the 1580 cm^-1 region is observed characteristic of diamond-like coatings. The coatings have a dense, nonporous structure. The tribological properties of the coatings are evaluated by the ball-on-disk method using a friction pair with WC and technical diamond. The strength characteristics are determined using linear scratch testing and nanoindentation measurements. The strength characteristics of the coatings vary and depend on the substrate materials. The friction coefficient of a diamond-like coating on VT4 alloy is ~0.1 in a friction pair with WC and ~0.01 with technical diamond.     

Comparison of two different carbon nanotube-based surfaces with respect to potassium ferricyanide electrochemistry

This paper describes the electrochemical investigation of two multi-walled carbon nanotube-based electrodes using potassium ferricyanide as a benchmark redox system. Carbon nanotubes were fabricated by chemical vapor deposition on silicon wafer with camphor and ferrocene as precursors. Vertically-aligned as well as islands of horizontally-randomly-oriented carbon nanotubes were obtained by varying the growth parameters. Cyclic voltammetry was the employed method for this electrochemical study. Vertical nanotubes showed a slightly higher kinetic. Regarding the sensing parameters we found a sensitivity for vertical nanotubes almost equal to the sensitivity obtained with horizontally/randomly oriented nanotubes (71.5 ± 0.3 μA/(mM cm²) and 62.8 ± 0.3 μA/(mM cm²), respectively). In addition, values of detection limit are of the same order of magnitude. Although tip contribution to electron emission has been shown to be greatly larger than the lateral contribution on single carbon nanotubes per unit area, the new findings reported in this paper demonstrate that the global effects of nanotube surface on potassium ferricyanide electrochemistry are comparable for these two types of nanostructured surfaces.