An asymmetric artificial scatterer in a semiconductor microjunction is shown to dramatically affect the nonlinear transport of ballistic electrons. The chosen device geometry, defined in a GaAs-AlGaAs heterostructure, successfully guides carriers in a predetermined spatial direction, independent of the direction of the input current I. From the nonlinear current-voltage characteristic we obtain unusual symmetry relations for the four-terminal resistances with R ij,kl ͑I͒ ഠ 2R ij,kl ͑2I͒ and R ij,kl ͑B͒ ¿ R kl,ij ͑2B͒ even at zero magnetic field B. The ballistic rectifier thus realized relies on a new kind of rectification mechanism entirely different from that of an ordinary diode. [S0031-9007(98)05927-4] PACS numbers: 73.23. Ad, 73.40.Ei, 73.50.Fq The high mobilities of state-of-the art two-dimensional electron gases in semiconductor heterostructures, combined with device fabrication technologies with high spatial resolution, have made it possible to study electron transport through semiconductor devices in which characteristic feature sizes are small in comparison with the elastic mean free path between scattering events caused by residual impurities. In such microstructures, electrons do not propagate diffusively as in traditional semiconductor devices but instead ballistically, with their path largely determined by specular reflection from device boundaries. Based on ballistic electron transport, a variety of novel phenomena have been observed in such microdevices. Examples are electron focusing [1], bend resistances [2], and a quenched or negative Hall effect [3,4]. Within the framework of the Landauer-Büttiker formalism, models have been developed to explain the above linear transport phenomena [5]. However, comparatively less attention has been given to the nonlinear ballistic transport regime in which electric fields or currents become so large that they significantly affect the momentum distribution of the carriers without destroying ballistic motion by inelastic scattering processes. Only recently has it been recognized in both theoretical and experimental studies that it is rather challenging to also investigate the nonlinear ballistic transport regime [6-8]. Even though several groups have realized that the introduction of artificial asymmetries should have a significant effect on nonlinear ballistic transport [9][10][11], so far, no strong nonlinear effects induced by a broken device symmetry have been observed.Here we introduce a novel device geometry which is particularly suitable to study the effects of reduced symmetry on the nonlinear ballistic transport properties. By inserting an asymmetric scatterer into the center of a ballistic cross junction, we observe unusual nonlinear current-voltage characteristics which we show to be dominated by the symmetry properties of the scatterer. The size of the artificial scatterer is much larger than the Fermi wavelength l F of the conducting electrons and comparable to their elastic mean free path l e ͑l e ¿ l F ͒. We demonstrate a successful guidance of...

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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)

We show improvement of the optical and topographical resolution of scanning near-field optical microscopy by introducing a “tip-on-aperture” probe, a metallic tip formed on the aperture of a conventional fiber probe. The tip concentrates the light passing through the aperture. Thus the advantages of aperture and apertureless scanning near-field optical microscopy are combined. Tips are grown by electron beam deposition and then covered with metal. Fluorescent beads are imaged with a resolution down to 25 nm (full width at half maximum) in the optical signal. The near-field appears strongly localized within 5 nm in z direction, thus promising even higher resolution with sharper tips.

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.