Clément Hébert

Formation of oriented nanostructures in diamond using metallic nanoparticles

A simple, fast and cost-effective etching technique to create oriented nanostructures such as pyramidal and cylindrical shaped nanopores in diamond membranes by self-assembled metallic nanoparticles is proposed. In this process, a diamond film is annealed with thin metallic layers in a hydrogen atmosphere. Carbon from the diamond surface is dissolved into nanoparticles generated from the metal film, then evacuated in the form of hydrocarbons and, consequently, the nanoparticles enter the crystal volume. In order to understand and optimize the etching process, the role of different parameters such as type of catalyst (Ni, Co, Pt, and Au), hydrogen gas, temperature and time of annealing, and microstructure of diamond (polycrystalline and nanocrystalline) were investigated. With this technique, 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 a lithography process, which opens the opportunities for fabricating porous diamond membranes for chemical sensing applications.

Biocompatibility of nanostructured boron doped diamond for the attachment and proliferation of human neural stem cells.

OBJECTIVE We quantitatively investigate the biocompatibility of chemical vapour deposited (CVD) nanocrystalline diamond (NCD) after the inclusion of boron, with and without nanostructuring. The nanostructuring method involves a novel approach of growing NCD over carbon nanotubes (CNTs) that act as a 3D scaffold. This nanostructuring of BNCD leads to a material with increased capacitance, and this along with wide electrochemical window makes BNCD an ideal material for neural interface applications, and thus it is essential that their biocompatibility is investigated. APPROACH Biocompatibility was assessed by observing the interaction of human neural stem cells (hNSCs) with a variety of NCD substrates including un-doped ones, and NCD doped with boron, which are both planar, and nanostructured. hNSCs were chosen due to their sensitivity, and various methods including cell population and confluency were used to quantify biocompatibility. MAIN RESULTS Boron inclusion into NCD film was shown to have no observable effect on hNSC attachment, proliferation and viability. Furthermore, the biocompatibility of nanostructured boron-doped NCD is increased upon nanostructuring, potentially due to the increased surface area. SIGNIFICANCE Diamond is an attractive material for supporting the attachment and development of cells as it can show exceptional biocompatibility. When boron is used as a dopant within diamond it becomes a p-type semiconductor, and at high concentrations the diamond becomes quasi-metallic, offering the prospect of a direct electrical device-cell interfacing system.

Graphene in the Design and Engineering of Next-Generation Neural Interfaces

Neural interfaces are becoming a powerful toolkit for clinical interventions requiring stimulation and/or recording of the electrical activity of the nervous system. Active implantable devices offer a promising approach for the treatment of various diseases affecting the central or peripheral nervous systems by electrically stimulating different neuronal structures. All currently used neural interface devices are designed to perform a single function: either record activity or electrically stimulate tissue. Because of their electrical and electrochemical performance and their suitability for integration into flexible devices, graphene-based materials constitute a versatile platform that could help address many of the current challenges in neural interface design. Here, how graphene and other 2D materials possess an array of properties that can enable enhanced functional capabilities for neural interfaces is illustrated. It is emphasized that the technological challenges are similar for all alternative types of materials used in the engineering of neural interface devices, each offering a unique set of advantages and limitations. Graphene and 2D materials can indeed play a commanding role in the efforts toward wider clinical adoption of bioelectronics and electroceuticals.

Microfabrication, characterization and in vivo MRI compatibility of diamond microelectrodes array for neural interfacing

Neural interfacing still requires highly stable and biocompatible materials, in particular for in vivo applications. Indeed, most of the currently used materials are degraded and/or encapsulated by the proximal tissue leading to a loss of efficiency. Here, we considered boron doped diamond microelectrodes to address this issue and we evaluated the performances of a diamond microelectrode array. We described the microfabrication process of the device and discuss its functionalities. We characterized its electrochemical performances by cyclic voltammetry and impedance spectroscopy in saline buffer and observed the typical diamond electrode electrochemical properties, wide potential window and low background current, allowing efficient electrochemical detection. The charge storage capacitance and the modulus of the electrochemical impedance were found to remain in the same range as platinum electrodes used for standard commercial devices. Finally we observed a reduced Magnetic Resonance Imaging artifact when the device was implanted on a rat cortex, suggesting that boron doped-diamond is a very promising electrode material allowing functional imaging.

Interfacing neurons on carbon nanotubes covered with diamond

A recently discovered material, carbon nanotubes covered with diamond (DCNTs) was tested for its suitability in bioelectronics applications. Diamond shows advantages for bioelectronics applications (wide electro chemical window and bioinertness). This study investigates the effect of electrode surface shape (flat or three dimensional) on cell growth and behavior. For comparison, flat nanocrystalline diamond substrates were used. Primary embryonic neurons were grown on top of the structures and neither incorporated the structures nor did they grow in between the single structures. The interface was closely examined using focused ion beam (FIB) and scanning electron microscopy. Of special interest was the interface between cell and substrate. 5% to 25% of the cell membrane adhered to the substrate, which fits the theoretical estimated value. While investigating the conformity of the neurons, it could be observed that the cell membrane attaches to different heights of the tips of the 3D structure. However, the aspect ratio of the structures had no effect on the cell viability. These results let us assume that not more than 25% of cell attachment is needed for the survival of a functional neuronal cell.

Monitoring the evolution of boron doped porous diamond electrode on flexible retinal implant by OCT and in vivo impedance spectroscopy.

Nanocrystalline Boron doped Diamond proved to be a very attractive material for neural interfacing, especially with the retina, where reduce glia growth is observed with respect to other materials, thus facilitating neuro-stimulation over long terms. In the present study, we integrated diamond microelectrodes on a polyimide substrate and investigated their performances for the development of neural prosthesis. A full description of the microfabrication of the implants is provided and their functionalities are assessed using cyclic voltammetry and electrochemical impedance spectroscopy. A porous structure of the electrode surface was thus revealed and showed promising properties for neural recording or stimulation. Using the flexible implant, we showed that is possible to follow in vivo the evolution of the electric contact between the diamond electrodes and the retina over 4months by using electrochemical impedance spectroscopy. The position of the implant was also monitored by optical coherence tomography to corroborate the information given by the impedance measurements. The results suggest that diamond microelectrodes are very good candidates for retinal prosthesis.

Photoemission properties of nanocrystalline diamond thin films on silicon

The authors have built up a dedicated ultrahigh vacuum setup to measure ultraviolet (266 nm photons) photoemission properties of nanocrystalline diamond thin films obtained by chemical vapor deposition on silicon substrates. The authors validated their setup by measuring polycrystalline copper quantum efficiency of ∼10−6, which is in good agreement with literature. The authors also measured quantum efficiency of bare silicon (highly p and n doped) and demonstrate strong influence of doping type. The authors then measured quantum efficiency of silicon samples coated with submicron (50 and 100 nm thick) nanocrystalline diamond layers. This coating reveals to have major influence on the photoemission properties when deposited on highly n-doped silicon samples. The authors obtain quantum yield as high as 1.60 × 10−5. The relatively high quantum efficiency of such structure associated with its high stability in air and easy processing make it a good candidate as fast electron source for electron gun based syst...

CVD nanodiamond thin films as high yield photocathodes driven by UV laser pulses

We present here an UV (266nm) photoemission setup dedicated to measure properties of conductive materials under DC extraction field as photocathodes. We have successfully tested copper, as reference material, and silicon samples. It allowed us testing photoemission properties of thin CVD nanodiamond films on silicon substrates. We demonstrate a strong influence on silicon doping type on the photoemission yield, pointing out a clear influence of the nanodiamond-silicon interface in the photoemission process. Furthermore, the nanodiamond-silicon structure exhibit one order of magnitude higher photoemission current compared to copper test samples.