V.J. Trava-Airoldi

Kinetics study of diamond electrodes at different levels of boron doping as quasi-reversible systems

Abstract Electrochemical reversibility was investigated for diamond electrodes with different boron concentrations for two redox couple. Diamond electrodes presented quasi-reversible electrochemical behavior for both electrolytes. In the range of scan rate and doping level analyzed diamond electrodes presented heterogeneous electron-transfer rate constant k 0 values of approximately 10 −4 –10 −3 cm/s for ferri/ferrocyanide and of approximately 10 −6 –10 −5 for quinone/hydroquinone. It was observed a maximum k 0 value when the electrode reaches a semi metallic behavior, associated to its higher crystalline quality and lower impurity incorporation. The acceptor concentrations were evaluated in the range of 10 18 –10 21 cm −3 from the Mott–Schottky plot curve slope. The results were compared with platinum and glassy carbon electrodes. Scanning electron microscopy (SEM) has also shown diamond crystallites decreasing with boron doping increasing. Raman spectroscopy has evidenced a drastic decrease of 1332 cm −1 peak, characteristic of boron-doped diamond. Atomic force microscopy (AFM) measurements have also demonstrated the electrode topography and its real active area for electrochemical response.

Antibacterial activity of DLC films containing TiO2 nanoparticles

Diamond-like carbon (DLC) films have been the focus of extensive research in recent years due to their potential applications as surface coatings on biomedical devices. Titanium dioxide (TiO2) in the anatase crystalline form is a strong bactericidal agent when exposed to near-UV light. In this work we investigate the bactericidal activity of DLC films containing TiO2 nanoparticles. The films were grown on 316L stainless-steel substrates from a dispersion of TiO2 in hexane using plasma-enhanced chemical vapor deposition. The composition, bonding structure, surface energy, stress, and surface roughness of these films were also evaluated. The antibacterial tests were performed against Escherichia coli (E. coli) and the results were compared to the bacterial adhesion force to the studied surfaces. The presence of TiO2 in DLC bulk was confirmed by Raman spectroscopy. As TiO2 content increased, I(D)/I(G) ratio, hydrogen content, and roughness also increased; the films became more hydrophilic, with higher surface free energy and the interfacial energy of bacteria adhesion decreased. Experimental results show that TiO2 increased DLC bactericidal activity. Pure DLC films were thermodynamically unfavorable to bacterial adhesion. However, the chemical interaction between the E. coli and the studied films increased for the films with higher TiO2 concentration. As TiO2 bactericidal activity starts its action by oxidative damage to the bacteria wall, a decrease in the interfacial energy of bacteria adhesion causes an increase in the chemical interaction between E. coli and the films, which is an additional factor for the increasing bactericidal activity. From these results, DLC with TiO2 nanoparticles can be useful for producing coatings with antibacterial properties.

Residual stresses and crystalline quality of heavily boron-doped diamond films analysed by micro-Raman spectroscopy and X-ray diffraction

Abstract X-ray diffraction analysis and micro-Raman spectroscopy measurements have been used for stress studies on HFCVD diamond films with different levels of boron doping. The boron incorporation in the film varied in the range 10 18 –10 21 boron/cm 3 . The grain size, obtained from SEM images, showed grains with 2–4-μm average size, which decreases when the doping level increases. The thickness of the films obtained by SEM cross-section view decreased from 8 to 5 μm as the doping level increased from 0 (undoped film) to 10 21 boron/cm 3 . The total residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ -values ranging from −60° to +60°, and applying the sin 2 ψ method. For the micro-Raman spectroscopy the spectral analysis performed on each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which decreases from 99 to 75% as the doping level increases. The type and magnitude of the residual stress obtained from X-ray and micro-Raman measurements agreed well only for undoped film, disagreeing when the doping level increased. We attributed this discrepancy to the domain size characteristic of each technique.

Dispersion liquid properties for efficient seeding in CVD diamond nucleation enhancement

Abstract We investigated diamond seeding as an efficient method of enhancing the nucleation density in chemical vapor deposition (CVD) diamond growth. We prepared substrates by ultrasonification for a short time in a 0.25 m diamond powder suspension and studied the effects of several dispersion liquids on seeding. We identified five key liquid properties for efficient seeding: zero or low dipole moment, low density, high vapor pressure, low surface tension and low viscosity. n-Hexane and n-pentane are the most suitable liquids, and yield seeding at the limit of the powder grit. We also observed the phenomenon of chemical consolidation of diamond particles onto silicon substrates during the initial growth stages. This consolidation ensures film adherence comparable with that obtained by other methods of nucleation enhancement.

Direct observation of chemical vapor deposited diamond films by atomic force microscopy

Diamond polycrystals deposited by the hot‐filament chemical vapor deposition method on silicon (100) substrates have been examined by atomic force microscopy (AFM) in air. Measurements of the diamond unit cell show periodic spacings between 0.34 to 0.37 nm in a very good agreement with the theoretical value of the bulk constant of natural diamond (0.356 nm). Hybridized sp3 bonds can also be observed at the (111) surface.

Efficiency study of perforated diamond electrodes for organic compounds oxidation process

Abstract The electrochemical oxidation of organic compound by bulk electrolysis has been analyzed for diamond electrodes used as anode. Boron-doped diamond thin films were grown on titanium alloy substrates (Ti6Al4V) with a geometric area of 12.2 cm 2 at 870 K and at 6.5×10 3 Pa. By using hot filament chemical vapor deposition technique and perforated and non-perforated substrates, the films were grown in both sample sides. The electrodes were characterized from their electrochemical behavior as a function of film quality, morphology and surface area, for boron concentrations of approximately 10 18 /cm 3 . The electrode performance was determined by cyclic voltammetry measurements in acid and neutral solutions and the reversibility behavior of the Fe(CN) 6 3−/4− at the Ti6Al4V/diamond electrode were also studied. High performance liquid chromatography (HPLC) and total organic carbon (TOC) techniques were used to study the oxidation products. These techniques are important to quantify the different compounds in the oxidation process steps permitting to observe the diamond electrodes efficiency. The results have shown a TOC decrease of approximately 85% and for HPLC it was observed almost complete phenol combustion to CO 2 . Scanning electron microscopy and Raman scattering spectroscopy were also used for morphology and diamond quality evaluation, respectively.

Diamond-like-carbon and molybdenum disulfide nanotribology studies using atomic force measurements

Abstract In this work, some results are presented concerning the studies of friction coefficient distribution in diamond-like-carbon (DLC) and molybdenum disulfide (MoS 2 ) thin films stored in atmosphere environment. Tribological nanoscale analysis using Lateral Force Microscopy (LFM) were performed. Experiments aimed at establishing the tribological properties of sputtered DLC and MoS 2 thin films. For friction measurements the LFM technique requires lateral movement of the Atomic Force Microscopy (AFM) tip in contact with a surface. The lateral force acts on the pyramidal tip attached to the end of the cantilever, due to friction or viscous forces, resulting in the cantilever measurable torsion and deflection related to frictions magnitude. The relationship among data from friction coefficient distribution, surface composition and crystal structure for DLC and MoS 2 are compared and are presented for the first time. Auxiliary characterization techniques as Raman scattering spectroscopy (RSS), and X-ray photoelectron spectroscopy (XPS) for both DLC and MoS 2 thin films are also presented.

Electrochemical characterization on semiconductors p-type CVD diamond electrodes

Semiconductor boron-doped CVD diamond films were prepared on Si substrates by the hot filament technique. The surface morphology analysis by SEM presented continuous and well faceted films. The samples were grown with different boron concentrations by controlling the B/C ratio in the feeding gas. Raman results showed a drastic change of diamond films for different doping levels. The characteristic line at 1332 cm-1decreases and shifts to lower energy as a function of the film resistivity. It was also observed a broad peak around 1220 cm-1 caused by the incorporation of boron in the diamond lattice. Photocurrent-voltage behaviour of undoped and boron-doped diamond electrodes was investigated in dark and UV visible irradiation. The voltammograms showed that doped electrodes illuminated with a xenon lamp exhibited currents significatively higher because of the increased conductivity. For undoped electrodes it was observed a small photocurrent for anodic and cathodic polarization in the order ofmA for the potential range of +1.0 V and -1.0 V (Ag/AgCl), for 1.0 M KCl. Mott-Schottky plots studied the interfacial processes at diamond-electrolyte junction. The flatband potential Uf b was found between 0.6 and 0.8 V (Ag/AgCl) which varies with the presence of sp2 -type carbon as an impurity. From the curve slope the acceptor concentrations were found in the range of 1018 and 1021 cm-3 .These values agree with the estimated concentration obtained by Raman measurements.

Columnar CVD diamond growth structure on irregular surface substrates

Abstract Columnar grain structure is always observed in CVD-diamond growth and is an important parameter to identify the morphology of thin and thick films. Structure defects, aspects of onset nucleation and film growth mechanisms can also be related to columnar growth. In this work we focused our attention on the columnar structure of CVD-diamond grown on irregular surfaces. We observed that there is a relationship among curvature radius of the substrate surface, the spread of the column volume and the growth rate of the diamond film. Growth rates on spherical surfaces of around 0.5 mm curvature radius have been observed to be up to three times bigger than the growth rates on planar surfaces. Also, the grain size distribution on planar and on the corner surfaces as a function of the growth rate has been studied. Characterization with scanning electron microscopy (SEM) and Raman scattering spectroscopy (RSS) has been performed.

Stress study of HFCVD boron-doped diamond films by X-ray diffraction measurements

Abstract Stress analysis on chemical vapor deposition (CVD) diamond films has demonstrated an apparent disagreement among various researchers in recent works even for similar deposition conditions. The type and the value of stress have shown a strong dependence on film thickness, which can be attributed to columnar growth and grain size and boundaries. X-Ray diffraction techniques appeared to be more suitable to study these effects and permit the evaluation of the average stress in larger sample areas when compared with micro-Raman spectroscopy, which feels a local strain inside the grains. In the case of boron-doped diamond films, boron incorporation on substitucional or interstitial sites can produce stresses according to the doping level. In order to investigate these effects, a series of diamond films were deposited on silicon (001) substrate in a hot filament (HF)-assisted CVD reactor at 800°C. The CH 4 flow is kept at 0.5 sccm for all experiments and the H 2 and B 2 O 3 /CH 3 OH/H 2 flows are controlled in order to obtain the desired B/C ratios. Stress behavior in HFCVD boron-doped diamond films has been investigated by X-ray diffraction measurements using the sin 2 ψ technique. Tensile and compressive stresses have been observed and the thermal and intrinsic components have been calculated. The diamond films were characterized by scanning electron microscopy and Raman spectroscopy.