Armin Kriele

Nanocrystalline Diamond Nanoelectrode Arrays and Ensembles

In this report, the fabrication of all-nanocrystalline diamond (NCD) nanoelectrode arrays (NEAs) by e-beam lithography as well as of all-diamond nanoelectrode ensembles (NEEs) using nanosphere lithography is presented. In this way, nanostructuring techniques are combined with the excellent properties of diamond that are desirable for electrochemical sensor devices. Arrays and ensembles of recessed disk electrodes with radii ranging from 150 to 250 nm and a spacing of 10 μm have been fabricated. Electrochemical impedance spectroscopy as well as cyclic voltammetry was conducted to characterize arrays and ensembles with respect to different diffusion regimes. One outstanding advantage of diamond as an electrode material is the stability of specific surface terminations influencing the electron transfer kinetics. On changing the termination from hydrogen- to oxygen-terminated diamond electrode surface, we observe a dependence of the electron transfer rate constant on the charge of the analyte molecule. Ru(NH(3))(6)(+2/+3) shows faster electron transfer on oxygen than on hydrogen-terminated surfaces, while the anion IrCl(6)(-2/-3) exhibits faster electron transfer on hydrogen-terminated surfaces correlating with the surface dipole layer. This effect cannot be observed on macroscopic planar diamond electrodes and emphasizes the sensitivity of the all-diamond NEAs and NEEs. Thus, the NEAs and NEEs in combination with the efficiency and suitability of the selective electrochemical surface termination offer a new versatile system for electrochemical sensing.

Electrochemical hydrogen termination of boron-doped diamond

Boron-doped diamond is a promising transducer material for numerous devices which are designed for contact with electrolytes. For optimized electron transfer the surface of diamond needs to be hydrogen terminated. Up to now H-termination of diamond is done by plasma chemical vapor deposition techniques. In this paper, we show that boron-doped diamond can be H-terminated electrochemically by applying negative voltages in acidic solutions. Electrochemical H-termination generates a clean surface with virtually no carbon–oxygen bonds (x-ray photoelectron spectroscopy), a reduced electron affinity (scanning electron microscopy), a highly hydrophobic surface (water contact angle), and a fast electron exchange with Fe(CN)6−3/−4 (cyclic voltammetry).

Diamond-Modified AFM Probes: From Diamond Nanowires to Atomic Force Microscopy-Integrated Boron-Doped Diamond Electrodes

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamonds unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.

Anisotropic etching of diamond by molten Ni particles

Nanopores in insulating solid state membranes have recently attracted much interest in the field of probing, characterizing, and manipulating single linear polymers such as DNA/RNA and proteins in their native environment. Here a low cost, fast, and effective way to produce nanostructures such as pyramidal shaped nanopores and nanochannels with dimensions down to about 15 nm in diamond membranes without any need for electron-beam lithography is demonstrated. By use of a catalytic process, anisotropic etching of diamond with self-organized Ni nanoparticles in hydrogen atmosphere at 900 °C is achieved and possible etching mechanisms are discussed. It is shown that diamond planes with the crystallographic orientation of [111] are etched slowest with this method.

Tuneable optical lenses from diamond thin films

Nanocrystalline diamond (NCD) membranes of 150 nm thickness and diameters in the millimeter range grown by microwave-assisted chemical-vapor deposition were bulged to investigate their mechanical properties and their use as tuneable optical lenses. The NCD films were grown at different CH4/H2 gas mixtures to vary the sp2/sp3 ratio and thereby to tune their mechanical, optical, and surface morphology properties. By applying gas over pressure the membrane forms a lens shaped geometry. From deflection data we calculated Young’s moduli which decrease with increasing CH4/H2 ratio from 1160 GPa at 0.5% to 900 GPa at 7%. Optical lens applications show a variation in the focal point from infinity to 3.5 mm. The data indicate that NCD is a promising material for tuneable optical lenses applications.

The creation of a biomimetic interface between boron-doped diamond and immobilized proteins

Immobilization of proteins on a solid electrode is to date done by chemical cross-linking or by addition of an adjustable intermediate. In this paper we introduce a concept where a solid with variable surface properties is optimized to mediate binding of the electron-transfer protein Cytochrome c (Cyt c) by mimicking the natural binding environment. It is shown that, as a carbon-based material, boron-doped diamond can be adjusted by simple electrochemical surface treatments to the specific biochemical requirements of Cyt c. The structure and functionality of passively adsorbed Cyt c on variously terminated diamond surfaces were characterized in detail using a combination of electrochemical techniques and atomic force microscopy with single-molecule resolution. Partially oxidized diamond allowed stable immobilization of Cyt c together with high electron transfer activity, driven by a combination of electrostatic and hydrophobic interactions. This surface mimics the natural binding partner, where coarse orientation is governed by electrostatic interaction of the proteins dipole and hydrophobic interactions assist in formation of the electron transfer complex. The optimized surface mediated electron transfer kinetics around 100 times faster than those reported for other solids and even faster kinetics than on self-assembled monolayers of alkanethiols.

CVD diamond disks and shells for nuclear fusion experiments

This study presents diamond film growth, preparation, and characterization using microwave plasma-assisted chemical vapor deposition for gyrotron window materials intended for fusion plasma ignition in magnetic and inertial confinement fusion such as ITER. A surface polishing technique is utilized to polish the films to a mirror grade surface finish. A fine-grained film surface morphology is realized to avoid any deformation caused by shock wave fronts during compression.