Piezoresistive,optical and electrical properties of diamond like carbon and carbon nitride films

In the present study diamond like carbon (DLC) and carbon nitride (a-CN_x:H) films were deposited by closed drift ion source from the acetylene and nitrogen gas mixture. The piezoresistive, electrical and optical properties of ion beam synthesized DLC films were investigated. Piezoresistive properties of the diamond like carbon and carbon nitride films were evaluated by four point bending test. The piezoresistors were fabricated on crystalline alumina substrates using Al-based interdigitated finger type electrodes. Effects of the nitrogen concentration on the piezoresistive gauge factor were investigated. The dependence of the resistance of the metal/a-CN_x:H/metal structures on temperature has been studied. Current–voltage (I–V) and capacitance–voltage characteristics were measured for a-CN_x:H/Si heterostructures. The main current transport mechanisms were analyzed. Optical parameters of the synthesized films such as optical bandgap and B parameter (slope of the linear part of the Tauc plot) were investigated to study possible correlation with the piezoresistive properties.     

Preparation of diamond like carbon thin films above room temperature and their properties

Diamond like carbon (DLC) thin films were deposited on p-type silicon (p-Si), quartz and ITO substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at different substrate temperatures (RT ∼ 300 °C). Argon (Ar: 200 sccm) was used as carrier gas while acetylene (C₂H₂: 20 sccm) and nitrogen (N: 5 sccm) were used as plasma source. Analytical methods such as X-ray photoelectron spectroscopy (XPS), FT-IR and UV–visible spectroscopy were employed to investigate the structural and optical properties of the DLC thin films respectively. FT-IR spectra show the structural modification of the DLC thin films with substrate temperatures showing the distinct peak around 3350 cm^− 1 wave number; which may corresponds to the sp² C–H bond. Tauc optical gap and film thickness both decreased with increasing substrate temperature. The peaks of XPS core level C 1 s spectra of the DLC thin films shifted towards lower binding energy with substrate temperature. We also got the small photoconductivity action of the film deposited at 300 °C on ITO substrate.     

Electrochemical characterization of diamond like carbon thin films

Diamond-like carbon (DLC) thin films were deposited from pure graphite target by DC magnetron sputtering method. Experimental parameters, i.e., substrate temperature and negative bias voltage, have been changed to finely tune the chemical bonding property (sp²/sp³) of the as-deposited DLC films. The as-deposited DLC films were characterized as anode materials for Li–ion batteries and special attentions were paid to the effects of sp²/sp³ ratio on the electrochemical properties of the DLC films. The results indicated that a high fraction of sp² bonding in the DLC films is preferred for high lithium storage capacity, flat and low charge voltage plateau, and long cycling retention.     

Microstructure and mechanical properties of CeO2 doped diamond-like carbon films

A kind of rare earth oxide, CeO₂, was doped into the diamond-like carbon (DLC) films with thickness of 180–200 nm, using unbalanced magnetron sputtering. All the adhesion strength of CeO₂ doped DLC films is increased, while the residual compressive stress is obviously decreased compared to pure DLC film. Specially, the residual compressive stress of the deposited films are reduced by 90%, when the CeO₂ content is in the range of 5–7%, from a value of about 4.1 GPa to 0.5 GPa. When the CeO₂ content is increased to 10%, the deposited films possess the highest adhesion strength of 85 mN, 37% higher than that of pure DLC film. The nanohardness and elastic modulus exist a transition point at 8% of CeO₂ content within the DLC film. Before this value, nanohardness and elastic modulus of CeO₂ doped DLC films are lower than those of pure DLC film, and after this value, they are higher or adjacent to those of pure DLC film. Auger electron spectroscopy shows a more widened interface of 6% CeO₂ doped DLC film compared to pure DLC film. The enhancement of adhesion strength is mainly attributed to the widening of the film-substrate interface, as well as the decrease of residual compressive stress.     

Synthetic diamond electrodes: The effect of surface microroughnesson the electrochemical properties of CVD diamond thin films on titanium

The electrochemical behavior of B-doped diamond films on Ti substrates subjected to different pretreatment procedures (annealing, sand-blasting, and etching in HCl) is evaluated as a function of surface microroughness. Generally, the differential capacitance follows the true surface area of the electrodes. The width of the potential window also increases, but slightly, with the roughness. The electrode reversibility in the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox system increases with increasing surface roughness. The apparent increase in the reversibility of the reaction may be also explained by the decrease in the true current density. Although the variations in the electrochemical parameters are not strongly pronounced, the tendencies observed can be used to optimise the electrode properties.     

The effect of annealing on mechanical and tribological properties of diamond-like carbon multilayer films

The diamond-like carbon (DLC) multilayer films have been deposited by plasma CVD deposition onSi wafer substrate. The deposited films have then been post-annealed in vacuum at 250 °C for 2 h. Changes in internal stress, hardness, critical load, friction coefficient and wear have been investigated toassess the influence of annealing on mechanical and tribological properties of DLC multilayer films. At the same time, DLC single layerfilms are also deposited and annealed in the same method for a comparison.The results show that there is 28–33% decrease in internal stress and 10–13% decrease in hardness of theDLC single layer films after the anneal treatment. However, for the DLC multilayer films, there is 41–43% decreasein internal stress and less than 2% decrease in hardness. In addition, the annealed DLC multilayer filmhas the same friction and wear properties as that un-annealed film. This result indicates that the anneal treatment isan effective method for the DLC multilayer films to reduce the internal stress and to increase the critical load.The by-effect of the annealing, decrease of hardness and wear resistance of the multilayer film, can be restrictedby the multilayer structure.     

The stress reduction effect by interlayer deposition or film thickness for diamond like carbon on rough surface

The effects of thermal stress resulting from thermal cooling for diamond like carbon (DLC) films on a rough surface are investigated by a finite element analysis. Two different schemes, interlayer deposition (metallic interlayer, multi-interlayer, grade interlayer) and film thickness increase are proposed to relieve the stress. The results show that the film thickness and coefficient of thermal expansion (CTE) are critical in stress reduction for a single interlayer. The metallic interlayer is effective in stress reduction only if the CTE of the interlayer is between that of the substrate and the film. Grade interlayer can further reduce the stress if the plasticity is graded in the interlayer. The increase of film thickness can also reduce the thermal stress, even though it is not as effective as the interlayer approach.     

Surfactant functionalization of carbon nanotubes (CNTs) for layer-by-layer assembling of CNT multi-layer films and fabrication of gold nanoparticle/CNT nanohybrid

This paper describes a new strategy through noncovalent functionalization of multi-walled carbon nanotube (MWNTs) with supramolecular surfactant for layer-by-layer (LbL) assembling MWNT multi-layer film onto indium tin oxide (ITO)-coated glass plate and for attaching gold nanoparticles (GNPs) onto the MWNTs to fabricate GNP/MWNT nanohybrid. Surfactant (i.e., sodium dodecyl sulfate, SDS) can interact with the MWNTs through hydrophobic interaction between the hydrophobic chain of SDS and the sidewall of the MWNTs. Such an interaction essentially leads to noncovalent adsorption of SDS onto the MWNTs, resulting in an enhanced solubilization of the MWNTs in distilled water and providing some negative charges on the tube surface. Both properties make it possible to assemble MWNT multi-layer films onto the ITO plate through an alternative adsorption of oppositely charged SDS-functionalized MWNTs and polyelectrolyte [i.e., poly(diallyldimethylammonium chloride), PDDA] as revealed by scanning electron microscopy (SEM), ultraviolet-visible-near-infrared spectroscopy (UV-vis-NIR), quartz crystal microbalance (QCM), and cyclic voltammetry (CV). The same properties of the SDS-functionalized MWNTs are demonstrated to be useful for mediating the attachment of GNPs onto the tube surfaces to form GNP/MWNT nanohybrid as verified with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemistry.     

氧离子注人对金刚石薄膜微结构和力学性能的影响

采用扫描电镜（SEM）、Raman光谱和纳米压痕法研究了氧离子注入对低硼掺杂金刚石薄膜微结构和力学性能的影响。结果表明,薄膜中注入较高剂量的氧离子并退火后,晶粒尺寸减小。氧离子注入导致薄膜中金刚石含量减小;1000℃退火后,薄膜中金刚石含量增加为99．8％。氧离子注入后,薄膜中的内应力由拉应力转变为压应力;退火后,薄膜内应力再转变为柱应力。氧离子注入后的金刚石薄膜的硬度较注入前的薄膜硬度有所降低,但其硬度仍然大于40GPa并具有良好的弹性恢复率。薄膜的力学性能与薄膜中的金刚石含量、晶粒尺寸和应力值有直接关系。     

Effects of surface modification on rheological and mechanical properties of CNT/epoxy composites

Despite superior properties of carbon nanotubes (CNTs), physical properties of the CNT/epoxy composites are not improved significantly because interfacial bonding between the CNTs and the polymer matrix is weak. CNTs were treated by an acidic solution to remove impurities and modified subsequently by amine treatment or plasma oxidation to improve interfacial bonding and dispersion of nanotubes in the epoxy matrix. The functional groups on the surface of treated CNTs were investigated by X-ray photoelectron spectroscopy. The surface modified CNTs were embedded in the epoxy resin by ultra-sonication and the cured nanotube containing composites were characterized by field emission scanning electron microscopy. Rheological properties of nanotube containing epoxy resin and mechanical properties of the modified CNT/epoxy composites were improved because the modification of CNTs improved dispersion and interaction between the CNT and the epoxy resin.     

Effect of surface treatments on the electronic properties of ultra-nanocrystalline diamond films

We present the soft x-ray spectroscopic study of the ultra-nanocrystalline diamond (UNCD) films with different surface treatments. The samples were prepared by means of microwave plasma enhanced chemical vapor deposition (MPECVD) and the different surface treatments are applied to alter their field emission properties. The electronic properties were subsequently examined by the soft x-ray absorption and x-ray emission spectroscopy at carbon 1s threshold. From the experimental results, there is no significant variation in electronic structure of oxygen- and hydrogen-plasma treated UNCD films. On the other hand, the biased treated UNCD film shows more remarkable change on the sp² and sp³ states. The formation of sp² bonding and the reduction of sp³ bonding are the consequence of the improved electron field emission properties.     

The influence of carbon fibre surface treatment on the mechanical properties of carbon/carbon composites

The influence of carbon fibre type and carbon fibre surface treatment on the mechanical properties of phenolic-based, unidirectionally reinforced carbon/carbon composites has been investigated.It was found that only the reinforcement of carbon/carbon composites with untreated type I fibres results in best mechanical properties. Surprisingly in that case also an optimised surface treatment of the fibres improve the yield of fibre strength. The experimental results have shown that the applicability of the rule of mixtures for the precalculation of the strength of carbon/carbon composites is limited and that the fracture behaviour is controlled by the amount of adhesion between carbon fibres and carbon matrix.     

Characteristic of silver doped DLC films on surface properties and protein adsorption

Ag-incorporated diamond-like carbon (DLC) films were prepared on Si substrate using a hybrid deposition system composed of an end-Hall-type hydrocarbon ion gun and a silver DC magnetron sputter source. Ag was selected due to their potential values of biomaterial. The concentration of Ag in the films was varied from 0.1 to 9.7 at.% by controlling the fraction of Ar in the reaction gas mixture with benzene. In order to understand the influence of incorporated Ag on wettability, the surface energy and the protein adsorption as an indirect haemo-compatibility were measured. The surface energy of the Ag-incorporated DLC film decreased gradually with the increase of the Ag concentration. The haemo-compatibility was examined by the adsorption ratio of albumin/fibrinogen as an indirect method and improved with the increase of Ag concentration. The surface and biological behaviors of the films will be discussed in terms of the atomic bond characteristic and microstructure induced by Ag incorporation. Our results demonstrate that the Ag-incorporated DLC films are potentially useful as biomedical devices having good haemo-compatibility and hydrophobic characteristics.     

Annealing effects on the mechanical properties of near-frictionless carbon thin films

In this work, the near-frictionless carbon (NFC) thin films developed at Argonne National Laboratory were annealed at 100 °C, 150 °C, 200 °C, 400 °C, and 600 °C in nitrogen atmosphere. The changes of the NFC mechanical properties were measured with both static and dynamic nanoindentation methods. It was found that the Young's modulus and hardness decreased with increasing annealing temperatures. Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the film's structural change before nanoindentation testing. Raman characterization indicated that the G peak shifted upwards as the annealing temperature was increased above 150 °C, which indicated decreasing sp³ content. The intensity of the D peak was shown to increase with annealing temperature indicating that the NFC film became more graphite-like. AFM analysis showed an increase of sp² clustering with annealing temperature, which resulted in an increase in surface roughness. SEM characterization indicated that as the films were annealed large cracks and numerous pinholes were generated. The characterization results were in good agreement with the measured mechanical properties.     

The effect of surface treatment on oxidation of oxalic acid at nanocrystalline diamond films

The surface investigation of undoped and boron doped nanocrystalline diamond (NCD/BDND) films associated to their electrochemical behavior of oxalic acid after four pre-treatments was studied. The films were produced using Hot Filament CVD technique on Si substrate with a gas mixture of CH₄/H₂/Ar. Surface pre-treatments were carried out to analyze the surface chemical changes induced by hydrogen and oxygen plasma and as well as cathodic and anodic treatments performed in 0.1 mol L^? 1 HClO₄. The films wetting analyzed by contact angle presented a strong dependence of their surface before and after each treatment was also confirmed by the electrochemical response from cyclic voltammograms. Independent of the surface pre-treatments, all the electrodes exhibited response for oxalic acid oxidation, but the electrode submitted to hydrogen plasma presented the lowest starting oxidation potential and the highest current density. Nonetheless, the BDND electrode presented higher oxidation current than that for NCD electrodes, after all pre-treatments studied. The use of square wave voltammetry with BDND electrode treated by hydrogen plasma for the analytical determination of oxalic acid is described. The detection limits of 0.75 μmol was obtained from the linear relationship between the peak currents of voltammograms as a function of the oxalic acid concentrations.     

Defect studies and photoelectrical properties of phosphorus doped amorphous carbon films

This paper reports on the successful deposition of n-type phosphorus doped carbon (n-C:P) thin films and fabrication of n-C:P/p-Si cells by pulsed laser deposition (PLD) using graphite target at room temperature. The cell performances have been tested in the dark for the current–voltage (I–V) rectifying curve and I–V working curve under illumination when exposed to AM 1.5 SUN illumination condition (100 mW/cm², 25 °C). The cells fabricated using 7% of phosphorus by weight percentages in the graphite target (Pwt%) show the highest energy conversion efficiency, η=1.14% and fill factor, FF=41%. The quantum efficiency of the cells is observed to improve with Pwt%. The dependence of Pwt% on the optical and physical properties of the deposited films and the photovoltaic characteristic of the n-C:P/p-Si heterojunction cells are discussed.     

Electro- and magneto-transport properties of amorphous carbon films doped with iron

Electro- and magneto-transport properties of amorphous carbon films doped with iron element have been systematically studied. The electro-transport mechanism of the films is dominated by thermal activation at T > 200 K, Mott-type variable range hopping (VRH) at 200 K > T > 60 K and Efros-Shklovskii type (ES-) VRH at T < 60 K. An anomalous giant positive magnetoresistance (MR) 6.40% is found at the ES-VRH range, which is attributed to the spin blockage effect. At high temperatures, an anomalous Hall effect (AHE) is also found with a large AHE coefficiency 49.6 μΩcm/T. Electron energy loss spectroscopy (EELS) reveals that iron atoms chemically bond with carbon matrix. These iron carbides exist as amorphous nanoparticles with a diameter of 6-12 nm, which is regarded as the origin of the MR and AHE. Besides, the films are p-type conductive at high temperature, which might be related with the iron doping. These properties make iron doped amorphous carbon films applicable in carbon-based solar cells, magnetic sensors or some other multifunctional devices.     

Effect of non-diamond carbon content on the spatial distributions of emission sites of the diamond films

The influence of non-diamond carbon in diamond films on the spatial distributions of emission sites was investigated, and consequently the field emission mechanism was discussed. It was confirmed that when increasing the positive substrate bias or the CH₄/H₂ concentration ratio, the non-diamond carbon content in the resulting films was markedly increased and the surface morphologies of the films lost their unique facet shape. An increase of non-diamond carbon content improved significantly the field emission properties and caused a dramatic increase of emission sites. It was observed from the images of the films containing a small amount of non-diamond carbon that electron emission occurs predominantly at the sample edges and the reason for this phenomenon could be attributed to the surface electron emission. With increasing non-diamond carbon, the emission occurs at the center as well as at the edge of the sample. From this observation, it is proposed that the emission from central region was caused by a preferential transport through the conducting pathways such as grain boundaries.     

The relationship between structure and mechanical properties of hydrogenated amorphous carbon films

There is increasing interest in the relations between Raman fit parameters and the mechanical properties of diamond-like carbon films. The present work describes these relations in hydrogenated diamond-like carbon films (a-C:H) deposited by an ion beam source operated at varied discharge voltages, i.e. kinetic carbon species energies. A number of highly distinct relations between Raman fit parameters and mechanical properties are identified for the a-C:H films investigated. For example the nanohardness (H) and reduced elastic modulus (E) increase almost linearly with an increase in full width at half maximum of the G-band (FWHM (G)). The film elasticity, expressed as H³/E² increases with increasing FWHM (G). In addition, H and E increase linearly with decreasing intensity ratio of the D-band and the G-band (I_D/I_G). H and E also increase with the G-band dispersion (Disp. (G)), i.e. the rate of change of the G-band position vs. excitation energy. Hydrogen contents in all films are approximately equal and range from 21.2 to 23.5 at.% over the entire set of investigated samples.