Carbon nanotubes in a nanocrystalline diamond film

A new process for diamond catalytic etching has been developed using metallic droplets. This process has been studied in detail and applied to the fabrication of porous membranes and composite materials made of carbon nanotubes partially embedded in a nanocrystalline diamond film.

A simple, fast and cost-effective etching technique to create oriented nanostructures such as pyramidal and cylindrical shaped nanopores in diamond was proposed. In this process, a diamond film is annealed with thin metallic layers in hydrogen atmosphere. Carbon from the diamond surface is dissolved into nanoparticles generated from the dewetted metal film, then evacuated in the form of hydrocarbons by the hydrogen flow. Consequently, the nanoparticles enter progressively the crystal volume, see figure 1. In order to understand and optimise the etching process, the role of different parameters such as type of catalyst (Ni, Co, Pt, and Au), hydrogen gas pressure, temperature and time of annealing, and microstructure of diamond (polycrystalline and nanocrystalline) were investigated. This study showed[ ] that nanopores can be formed in diamond, whatever its microstructure. The process requires a catalyst with high carbon solubility, such as nickel, in an hydrogen atmosphere at a temperature of 800 to 850 °C. The etching process is stopped for catalyst particles located deep inside the crystal volume, probably because of a limited gas exchange between the surface and the particle leading to carbon saturation. After optimisation of the process parameters, nanopores with lateral sizes in the range of 10-100 nm, and as deep as about 600 nm in diamond membranes were produced without any need for lithography process. This new etching process opens opportunities for fabricating porous diamond membranes[ ] for chemical sensing applications, for anisotropic etching of the diamond surface to reveal (111) crystallographic planes, for smooth and deep etching, and for the fabrication of diamond-carbon nanotubes composites. This last process has been studied in detail.

Fig. 2 : SEM cross-section showing carbon nanotubes grown from nickel catalyst embedded into nanocrystalline diamond.

A composite material, made of carbon nanotubes (CNTs) partially embedded in a nanocrystalline diamond film was produced[ ]. The diamond film was first decorated with palladium or nickel nanoparticles. An array of nanopores was drilled in the film in a hot filament CVD (HFCVD) reactor as described above. In this process, the metallic particles penetrate the diamond film down to a controlled depth, thus remaining at the bottom of the nanopores. The buried nanoparticles remain catalytically active and are used to grow a multiwall carbon nanotube forest using HFCVD of CH4/H2 mixtures in the same reactor without breaking the vacuum. The improvement in the adhesion and in the electrical connection between carbon the nanotubes and the boron-doped diamond substrates has been evaluated with simple sonication experiments, more sophisticated AFM measurements in the lateral force mode and finally with electrochemical analysis, including cyclic voltammetry and electrochemical impedance spectroscopy. In particular, the electrochemical measurements unambiguously showed that the composite electrode did not loose its properties after several washing steps. Obviously, thanks to the increase in the electrode apparent surface, and thanks to the diamond and carbon nanotube electrochemical properties, such electrodes can be attractive for biological applications, in particular neural interfacing or biosensing. Using this growth process, the metal nanoparticles are buried below the conductive diamond substrate, which reduces both their interaction with biological media and the dispersion of the CNTs, thus preventing toxicity issues. Any applications where CNTs must be strongly bonded to the substrate with a good electrical contact such as field emission devices could be improved thanks to this technique. The first electrochemical tests of these composite electrodes appeared promising.

Fig. 1 : SEM picture of the a 100-oriented surface of diamond etched by Ni droplets (dewetted from a 3 nm thick film) for 30 min in 60 Torr hydrogen atmosphere.

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