Nondiamond Carbon Impurities Research Articles

Morphological and Microstructural Stability of Boron‐Doped Diamond Thin Film Electrodes in an Acidic Chloride Medium at High Anodic Current Densities

Boron‐doped diamond thin films have been examined before and after high‐current‐density electrolysis to investigate the morphological and microstructural stability of this new electrode material. The diamond thin films were used to generate chlorine from a solution of 1.0 M + 2.0 M NaCl at current densities of 0.05 and 0.5 A/cm2 for times up to 20 h. Comparative studies were made using common “graphitic” electrodes including highly oriented pyrolytic graphite, glassy carbon, and Grafoil®. The electrodes were characterized using four‐point probe resistivity measurements, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, x‐ray photoelectron spectroscopy, and cyclic voltammetry. In all cases, no severe morphological or microstructural damage (i.e., corrosion) was observed on films exposed to the highest current density. There were surface compositional changes in the forms of oxygenation and nondiamond carbon impurity etching that produced an increase in the reaction overpotential. Specifically, the overpotential was supposed to result from a combination of decreased surface conductivity due to the formation of carbon‐oxygen functional groups and loss of kinetically active redox sites due to the oxidative etching of nondiamond carbon impurities.

The Formation and Electrochemical Activity of Microporous Diamond Thin Film Electrodes in Concentrated KOH

The corrosion resistance and electrochemical activity of boron‐doped, polycrystalline diamond thin films were evaluated before and after potentiodynamic cycling between −0.7 and 0.7 V vs. Ag/AgCl in 15 weight percent KOH. The maximum anodic current densities at 0.7 V ranged from 0.3 to 6.0 mA/cm2. Two types of corrosion processes were observed. In one case, low‐quality films grown using a 2.9% C/H ratio developed circular pits (100 to 300 nm diam) located almost exclusively at the intercrystalline grain boundaries, creating a microporous film. For the most part, the diamond microcrystallites were morphologically unaffected by the polarization, but in a few areas, extreme pitting and smoothing of the microcrystallites occurred. In a second case, low‐quality films grown using a 1% C/H ratio experienced corrosion that advanced laterally across the diamond microcrystallite surfaces commencing at facet corners and step edges. High‐quality films grown using a 1.4% C/H ratio exhibited no signs of corrosion or morphological damage. A corrosion mechanism is proposed whereby the nondiamond carbon impurities exposed at the surface are preferentially etched away by anodic polarization. The electrochemical activity of the films, before and after cycling, was probed using cyclic and differential pulse voltammetry with . The morphological and surface chemical changes produced during the microporous film formation led to an increase in the oxidation and reduction peak currents by up to a factor of eight. The heterogeneous electron‐transfer rate constant, , decreased slightly by a factor of four after formation of the microporous film. It is supposed that the electron‐transfer reactions are not occurring exclusively through the nondiamond carbon impurity sites but rather that these impurities serve primarily an electronic role by supplying charge carriers to the film, thereby reducing resistivity.

UV Raman Microspectroscopy: Spectral and Spatial Selectivity with Sensitivity and Simplicity

The high sensitivity, selectivity, spatial resolution, and ease of operation of UV Raman microspectr oscopy is demonstrated with the use of a new highly efficient UV Raman microspectrometer with excitation at 244 nm. Single spectrograph dispersion combined with special new filters for the rejection of Rayleigh scattering improves the throughput efficiency by a factor of approximately 4 in comparison to a triple-stage spectrograph. The instrument has a spatial resolution of approximately 3 μm × 9 μm in the lateral (X–Y) plane, and 10 μm or less in the axial (Z) plane. UV resonance Raman spectra of nucleic acids are selectively excited from spatially resolved areas of a single paramecium by using low continuous-wave (cw) excitation powers and short accumulation times to minimize sample damage. High signal-to-noise Raman spectra are excited from spatially resolved areas of chemical-vapor-deposited (CVD) diamond films. We demonstrate, for the first time, the ability to probe the spatial distribution of the nondiamond carbon impurities in CVD diamond films. The amorphous carbon band at ∼ 1553 cm−1 is resolved from the normally broad ∼ 1600-cm−1 nondiamond carbon band.

Hydrogen in chemical vapour deposited diamond films

Thin films of diamond were deposited at 20, 40 and 70 Torr by MPCVD and at 40, 100 and 140 Torr by HFCVD while keeping the substrate temperature constant. The films were characterised using micro-Raman spectroscopy, X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Micro-Raman spectra show a sharp peak centered near the natural diamond line (≈ 1332 cm −1) accompanied by a non-diamond band at ≈ 1450 − 1550 cm −1. Intense peaks corresponding to (111), (400) and (311) reflections are observed in XRD. Diamond crystallites with sharp facets are seen in SEM. Plasma diagnostics in the MPCVD system were also performed at various conditions in order to estimate the H atom concentration in the gas phase. Elastic recoil detection analysis was used to measure the H concentration. It is observed that the H concentration shows contrasting trends in the films grown by MPCVD and HFCVD at various growth pressures. H concentration is found to be 1.0, 0.8 and 0.7at % in the films grown at 20, 40 and 70 Torr, respectively, in MPCVD apparatus. Conversely, the H concentration in the films grown by HFCVD increases systematically with growth pressure (0.5, 1.1 and 1.4 at % at 40, 100 and 140 Torr, respectively). Non-diamond carbon impurities correlate well with H content. Results of micro-Raman spectroscopy, SEM and plasma diagnostic are discussed in the framework of a growth model. Results of H content in the diamond films obtained by various techniques are compared.

Cyclic Voltammetric Studies of Charge Transfer Reactions at Highly Boron-Doped Polycrystalline Diamond Thin-Film Electrodes

Cyclic voltammetry and ac impedance analysis were used to measure the background current response and capacitance of interfaces formed at as grown (untreated) boron-doped polycrystalline diamond thin-film electrodes in contact with aqueous electrolytes. The diamond films (∼1 cm 2 , 15 μm thick ; carrier concentration, ∼10 17 cm -3 ) were grown on conducting Si substrates by plasma-enhanced chemical vapor deposition. Cyclic voltammetry was also used to determine the charge transfer reactions of several redox analytes at as grown and chemically wet etched diamond thin-film electrodes and to study the effect of surface pretreatment, including Fe(CN) 6 3-/4- , IrCl 6 2-/3- , Ru(NH 3 ) 6 3+/2+ , dopamine, 4-methylcatechol, MV 2+/+/0 , and ferrocene. The electrochemical response exhibited by the films is explained using two models : (i) traditional electron transfer at a p-type semiconductor-electrolyte interface and (ii) electron transfer at a composite electrode composed of nondiamond carbon impurities contained within a diamond matrix such that k° nondiamond >> k° diamond.

Deposition of diamond films on sapphire: studies of interfacial properties and patterning techniques

Polycrystalline diamond films were grown on single crystal sapphire substrates using hot filament chemical vapour deposition (CVD). Problems with poor adhesion, stress and film cracking became severe for deposited areas greater than about (100 μm) 2. Scanning electron microscopy analysis showed the films to be failing both at the interface and in the diamond layer itself. Transmission electron microscopy cross-sections of the interface showed that the interface was clean and free from non-diamond carbon impurities. Spallation problems in the diamond film could be reduced by introducing a barrier layer of epitaxial silicon grown on the sapphire prior to the diamond CVD step. Patterned silicon-on-sapphire wafers were then used as substrates for CVD of diamond in order to define features of linewidth more than 10 μm in the diamond films. Two methods were used: selective nucleation and lift off.