Non-diamond Impurities Research Articles

Electrochemical characterization of porous boron-doped diamond prepared using SiO2 fiber template

Porous boron-doped diamond (BDD) is fabricated by consecutive plasma enhanced chemical vapor deposition on a 3D porous SiO2 fiber template deposited by spin coating (SC). The fabricated highly doped and mechanically stable porous BDD layers are characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The roughness factor of the prepared porous BDD, depending on the number of porous layers, was determined from cyclic voltammetry (RFCV) and from krypton adsorption isotherms – BET, (RFBET). Differences in determination of the roughness factor using these two methods are discussed. Electrochemical measurements (cyclic voltammetry and galvanostatic charge/discharge cycling) of porous BDD are performed in aqueous electrolyte solutions with different composition and pH values. The highest electric double-layer capacitance of ca. 2 mF·cm−2, related to the projected geometric (2D) surface area, is obtained for the thickest (26 μm) porous BDD electrode measured in 0.5 M H2SO4 electrolyte solution. The capacitance of the same porous BDD normalized to the total physical surface area (3D) determined by BET is 45 μF·cm−2. The electrocatalytic activity of porous BDD electrodes is studied using a hexaammineruthenium(III/II) redox probe, and the electrochemical cycle stability is determined by galvanostatic charge/discharge. The charge retention of the thickest porous BDD samples after removal of non-diamond impurities by an oxidative treatment is ca. 77% after 3000 cycles.

Influence of growth conditions on microstructure and defects in diamond coatings grown by microwave plasma enhanced CVD

Diamond coatings were grown on SiO2/Si substrate under various process conditions by microwave plasma chemical vapour deposition (MPCVD) using CH4/H2 gas mixture. In this paper, we present a microstructural study to elucidate on the growth mechanism and evolution of defects, viz., strain, dislocations, stacking faults, twins and non-diamond impurities in diamond coatings grown under different process conditions. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the diamond coatings. It has been shown that our new approach of prolonged substrate pre-treatment under hydrogen plasma yielded a new growth sequence that the SiO2 layer on the Si substrate was first reduced to yield Si layer of ∼150 nm thickness before diamond was allowed to grow under CH4–H2 plasma, created subsequently. It has also been shown that Si and O as impurity from the substrate hinders the initial diamond growth to yield non-diamond phases. It is being suggested that the crystal defects like twins, stacking faults, dislocations in the diamond grains and dislocations in the intermediate Si layer are generated due to the development of non-uniform stresses during diamond growth at high temperature.

Boron-doped Diamond Electrodes: Electrochemical, Atomic Force Microscopy and Raman Study towards Corrosion-modifications at Nanoscale

Comparative studies of boron-doped diamonds electrodes (polycrystalline, single-crystalline, H-/O-terminated, and with different sp3/sp2 ratios) indicate morphological modifications of diamond which are initiated by corrosion at nanoscale. In-situ electrochemical AFM imaging evidences that the textural changes start at non-diamond carbonaceous impurity sites treated at high positive potentials (>2.2V vs. Ag/AgCl). The primary perturbations subsequently develop into sub-micron-sized craters. Raman spectroscopy shows that the primary erosion site is graphite-like (sp2-carbon), which is preferentially removed by anodic oxidation. Other non-diamond impurity, viz. tetrahedral amorphous carbon (t-aC), is less sensitive to oxidative decomposition. The diamond-related Raman features, including the B-doping-assigned modes, are intact during reversible electrochemical charging/discharging, which is a salient difference from all usual sp2-carbons. The electrochemical oxidation partly transforms a hydrogen-terminated diamond surface to O-terminated one, but the electrocatalytic activity of plasmatically O-terminated diamond is not achieved for a model redox couple, Fe3+/2+. Electrochemical impedance spectra were fitted to six different equivalent circuits. The determination of acceptor concentrations is feasible even for highly-doped diamond electrodes.

Grid and substrate bias effects on mechanical properties of diamond films prepared by HFCVD

Diamond films were prepared at different grid bias and substrate bias in hot filament chemical vapor deposition (HFCVD) system. The Raman and SEM results show that grain size decreases and non-diamond impurities increase for applying grid and substrate bias currents. The defects and impurities in the film increase with the decrease of grain size, which causes the decrease of hardness and elastic modulus of diamond films. The fracture toughness of film increases because of the grain size effects by applying bias. The grid bias and substrate bias make the friction coefficient smaller because of the smaller grain size and lubricating effect of graphite in the film. But the excessively high substrate bias current will lead to the dramatic decrease of mechanical properties of CVD diamond as a lot of non-diamond impurities appear in the film.

Using active carbon pellet as substrate for deposition diamond by MPCVD

In this letter, we proposed the use of active carbon pellet as substrates for the deposition of diamond films. A series of depositions were done on the different substrates prepared in different ways and compositions. The morphologies of the as-deposition films were analyzed by scanning electron microscopy. Raman spectroscopy was used to investigate the crystal structure and non-diamond impurities in the films deposited on various substrates. The ball-shaped diamond was evident, and the homogeneous and dense diamond films could be obtained by microwave plasma assisted CVD using acetone as C source diluted in hydrogen on the active carbon substrates cleaned in a solvent ultrasonic bath.

Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp 3 and sp 2 network and the quantitative analysis were done by Fourier transform infrared spectroscopy.

Electrochemical Polishing of Boron-Doped Diamond in Organic Media

The electropolishing of a B-doped diamond (BDD) film deposited by CVD was studied in HOAc-H2SO4 media. The electropolished surface was characterized by SEM, XPS, and electrochem. measurements. In 1 M H2SO4 no polishing of the surface occurred even after prolonged time; on the contrary, in electrolyte contg. 3 M HOAc in 1 M H2SO4 the morphol. changed rapidly. The surface became smoother and the roughness decreased as the typical results of a polishing process. All the nondiamond impurities were also removed by the electropolishing treatment in HOAc media. [on SciFinder (R)]

Interesting trends in direct current electrical conductivity of chemical vapor deposited diamond sheets

Self-supported diamond sheets of the thickness ranging from 15 to 30 μm were prepared using hot filament chemical vapor deposition technique. The controlled variation of the deposition parameters resulted in the sheets with varying amount of nondiamond impurities. Routine characterization of the sheets was carried out using scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, Fourier transform infrared spectroscopy, and Positron annihilation spectroscopy techniques. Detailed measurements of room temperature electrical conductivity (σ300), current–voltage (I–V) characteristics, and annealing studies on the sheets deposited with various structural disorder have yielded useful information about the electrical conduction in this interesting material. σ300 and I–V characteristic measurements were done in sandwiched configuration taking care off the surface effects. The diamond sheets deposited at low deposition pressure (Pd<60 Torr) contain negligible nondiamond impurities and show σ300≅10−6–10−7 S.cm−1. The I–V characteristics in these sheets show space charge limited conduction behavior with I∝Vn and n>1, in high voltage range. In contrast the sheets deposited at higher pressure (60 Torr and higher), containing high concentration of nondiamond impurities, show a sharp reduction in the values of σ300. Interestingly, the conduction in these sheets is ohmic with n values nearly equal to unity. Similarly the sheets deposited with nitrogen also show a sharp reduction in σ300. Annealing of all types of diamond sheets results in a decrease in σ300 values by several orders of magnitude. In the sheets deposited at low Pd, the n values increase sharply with annealing. On the other hand the values of n in the sheets deposited at higher pressure remain constant with annealing. The above results are explained in terms of hydrogen abstraction from the traps and compensation of donor–acceptor pairs.

DC electrical conductivity of chemical vapour deposited diamond sheets: a correlation with hydrogen content and paramagnetic defects

Diamond sheets were grown on p-type Si(100) substrates varying the deposition pressure from 20 to 140 Torr at 1163 K by a hot filament chemical vapour deposition (HFCVD) process. A mixture of 0.8% methane in balance hydrogen was used as a precursor gas. Diamond sheets deposited at low pressures are translucent with very low concentration of hydrogen and non-diamond impurities. The amount of non-diamond impurities and the hydrogen content increase in the sheets significantly with the growth pressure. A systematic variation in the value of the room temperature DC electrical conductivity (σ300) of diamond sheets was observed. Highly pure sheets show very high values of σ300. It is suggested that the paramagnetic defects intrinsic to CVD diamond might be one of the controlling parameters of σ300 of the sheets.