Christoph E. Nebel

Size-dependent reactivity of diamond nanoparticles

Photonic active diamond nanoparticles attract increasing attention from a wide community for applications in drug delivery and monitoring experiments as they do not bleach or blink over extended periods of time. To be utilized, the size of these diamond nanoparticles needs to be around 4 nm. Cluster formation is therefore the major problem. In this paper we introduce a new technique to modify the surface of particles with hydrogen, which prevents cluster formation in buffer solution and which is a perfect starting condition for chemical surface modifications. By annealing aggregated nanodiamond powder in hydrogen gas, the large (>100 nm) aggregates are broken down into their core ( approximately 4 nm) particles. Dispersion of these particles into water via high power ultrasound and high speed centrifugation, results in a monodisperse nanodiamond colloid, with exceptional long time stability in a wide range of pH, and with high positive zeta potential (>60 mV). The large change in zeta potential resulting from this gas treatment demonstrates that nanodiamond particle surfaces are able to react with molecular hydrogen at relatively low temperatures, a phenomenon not witnessed with larger (20 nm) diamond particles or bulk diamond surfaces.

Periodic light coupler gratings in amorphous thin film solar cells

Efficient light trapping structures for amorphous hydrogenated silicon (a-Si:H) solar cells have been realized using periodically structured aluminum doped zinc oxide (ZnO:Al) with periods between 390 and 980 nm as a transparent front contact. Atomic force microscopy, optical reflection, and diffraction efficiency measurements were applied to characterize solar cells deposited on such gratings. A simple formula for the threshold wavelength of total internal reflection is derived. Periodic light coupler gratings reduce the reflectance to a value below 10% in the wavelength range of 400–800 nm which is comparable to cells with an optimized statistical texture. Diffraction efficiency measurements and theoretical considerations indicate that a combination of transmission and reflection gratings contribute to the observed reduction of the reflectance.

GaN-based heterostructures for sensor applications

The potential of AlGaN/GaN heterostructures for novel sensor devices is discussed. The two-dimensional electron gas which is formed at the AlGaN/GaN interface due to the difference in the spontaneous polarisation of the two adjacent III-nitride layers is shown to respond very sensitively to changes in the electrostatic boundary conditions caused by the adsorption of ions, wetting by polar liquids, exposure to gases, or the piezoelectric polarisation due to mechanical strain. Possibilities to use III-nitride heterostructures in future biosensors or integrated sensor devices are also mentioned.

Vertically Aligned Nanowires from Boron-Doped Diamond

Vertically aligned diamond nanowires with controlled geometrical properties like length and distance between wires were fabricated by use of nanodiamond particles as a hard mask and by use of reactive ion etching. The surface structure, electronic properties, and electrochemical functionalization of diamond nanowires were characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) as well as electrochemical techniques. AFM and STM experiments show that diamond nanowire etched for 10 s have wire-typed structures with 3-10 nm in length and with typically 11 nm spacing in between. The electrode active area of diamond nanowires is enhanced by a factor of 2. The functionalization of nanowire tips with nitrophenyl molecules is characterized by STM on clean and on nitrophenyl molecule-modified diamond nanowires. Tip-modified diamond nanowires are promising with respect to biosensor applications where controlled biomolecule bonding is required to improve chemical stability and sensing significantly.

Electronic properties of CVD diamond

The electronic properties of chemical vapour deposited (CVD) diamond are reviewed based on data measured by transient and spectrally resolved photoconductivity experiments, photo-thermal deflection spectroscopy (PDS) and electron paramagnetic resonance (EPR) where substitutional nitrogen (P1-centre) and carbon defects (H1-centre) are detected. The results show that nominally undoped high quality polycrystalline CVD diamond is a n-type semiconductor due to the presence of substitutional nitrogen. The sub-band-gap optical absorption is governed by amorphous graphite present at grain boundaries. Spectrally resolved photoconductivity experiments measured in the same regime are partially dominated by diamond bulk properties which are comparable to single crystalline Ib and IIa diamond and partially by grain boundaries. Mobilities and drift length of carriers are discussed and compared to properties of single crystalline diamond.

Physics and applications of CVD diamond

Marshall Stoneham: Thinking about diamond Olivier Williams, Milos Nesladek: Growth and properties of nanocrystalline diamond Films Tokuyuki Teraji: Chemical Vapor Deposition of Homoepitaxial Diamond Films Yutaka Anado, Atsuhito Sawabe: Heteroepitaxy of diamond C. E. Nebel, B. Rezek, D. Shin, H. Watanabe: Surface electronic properties of H-terminated diamond in contact with electrolytes Shin, B. Rezek, C.E. Nebel: Photo- and electrochemical bonding of DNA to single crystalline CVD diamond Vincent Mortet, Ken Haenen, Oliver Williams: Diamond: Acoustic wave filters and sensors applications Jonathan Goos: Defects and dopants in diamond Satoshi Koizumi, Mariko Suzuki, Julien Pernot: n-Type doping of diamond: growth, electrical transport and devices Jelezko, J. Wrachtrup: Single defect centers in diamond Hideyo Okushi, Hideyuki Watanabe, Satoshi Yamasaki, Shokichi Kanno: Emission properties from dense exciton gases in diamond Heinz Pernegger: High Mobility Diamonds and Particle Detectors Etienne Bustarret: Superconducting diamond

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).

Nanocrystalline 3C-SiC Electrode for Biosensing Applications

Silicon carbide has been proved as a candidate for power and high-frequency devices. In this paper, we show the application of nanocrystalline 3C-SiC as an electrochemical electrode and its electrochemical functionalization for biosensing applications. SiC electrodes show a wider potential window and lower background current than glassy carbon electrodes. The surface can be electrochemically functionalized with diazonium salts, as confirmed by electrochemical techniques and X-ray photoelectron spectroscopy. The nitrophenyl film is used as linker layer to bond DNA molecule to SiC. These results show that 3C-SiC can be an interesting transducer material for applications in electro- and bioelectrochemical applications.

Diamond-integrated optomechanical circuits

Diamond integrated photonic devices are promising candidates for applications in nanophotonics and optomechanics. Here I present active modulation of diamond-based devices by exploiting mechanical degrees of freedom in free-standing electro-optomechanical resonators.