We report on a zero magnetic field transport study of a two-dimensional, variable-density, hole system in GaAs. As the density is varied we observe, for the first time in GaAsbased materials, a crossover from an insulating behavior at low-density, to a metallic-like behavior at high-density, where the metallic behavior is characterized by a large drop in the resistivity as the temperature is lowered. These results are in agreement with recent experiments on Si-based twodimensional systems by Kravchenko et al.[1] and others [2][3][4][5]. We show that, in the metallic region, the resistivity is dominated by an exponential temperature-dependence with a characteristic temperature which is proportional to the hole density, and appear to reach a constant value at lower temperatures. 71.30.+hThe study of the transport properties of twodimensional electron systems (2DES's) revealed numerous unique features associated with their reduced dimensionality. A central question is whether a metal-insulator transition (MIT) can occur in two-dimensions (2D). Using scaling arguments Abrahams et al. [6] stated that non-interacting electrons in 2D systems are localized at zero temperature (T ) for any level of disorder, and a MIT is not expected to occur at zero magnetic-field (B). This work motivated several experimental studies which investigated the T dependence of the resistivity (ρ) of low-mobility 2DES in Si metal-oxide semiconductor filedeffect transistors (MOSFET's) [7,8] and in In 2 O 3−x films [9]. The resistivity was found to increase with decreasing T , and its T -dependence changed from weak to strong as the density of the 2DES was lowered, or the disorder increased. These experiments seemed to confirm the theoretical notion that no metallic phase exists in 2D.However, several recent studies presented evidence to the contrary. In these studies 2DES's in Si MOSFET's [1,2] and Si/SiGe heterostructure [3][4][5] were used, and the resistivity was measured as a function of T for various carrier-densities. These studies demonstrated a clear crossover from metallic to insulating behavior at low T . Further, in the metallic region, the resistivity was shown to decrease with decreasing T by as much as a factor of eight, while in the insulating region the resistivity increases sharply with decreasing T . The authors of refs.[1] took these results as evidence for the existence of a MIT in 2D, and several theoretical works have tried to associate them with modified scaling [10] or raised the possibility of superconductivity [11,12]. The disagreement between these new results and earlier ones are generally attributed to the higher mobility in these samples (reaching a value as high as 75, 000 cm 2 /V ·s) and to the high effective mass of electrons in Si (m = 0.19m 0 ) which, according to the authors, combine to accentuate the effect of carrier-carrier interactions. It was therefore suggested [13] that due to the heavy mass of holes in GaAs (0.6m 0 , 0.38m 0 ) [14,15] and the superior quality of molecular beam epitaxy (MBE) growth, a ...

A novel class of micro-electrodes was fabricated by synthesizing high density carbon nanotube islands on lithographically defined, passivated titanium nitride conductors on a silicon dioxide substrate. Electrochemical characterization in phosphate buffered saline of these new electrodes reveals superb electrochemical properties marked by featureless rectangular cyclic voltammetry curves corresponding to a DC surface specific capacitance and a volume specific capacitance as high as 10 mF cm(-2) and 10 F cm(-3), respectively. These electrodes are also characterized by a slowly varying impedance magnitude over the range of 1 Hz to 20 kHz. High fidelity extracellular recordings from cultured neurons were performed and analysed to validate the effectiveness of the fabricated electrodes. The enhanced electrochemical properties of the electrodes, their flexible and simple micro-fabrication preparation procedure as well as their bio-compatibility and durability suggest that carbon nanotube electrodes are a promising platform for high resolution capacitive electrochemical applications.

Cells regulate active transport of intracellular cargo using motor proteins. Recent nanobiotechnology efforts aim to adapt motor proteins to power the movement and assembly of synthetic materials. A motor-protein-based nanoscale transport system (molecular shuttle) requires that the motion of the shuttles be guided along tracks. This study investigates the principles by which microtubules, serving as shuttle units, are guided along micrometer-scale kinesin-coated chemical and topographical tracks, where the efficiency of guidance is determined by events at the track boundary. Thus, we measure the probability of guiding as microtubules reach the track boundary of (1) a chemical edge between kinesin-coated and kinesin-free surfaces, (2) a topography-only wall coated completely with kinesin, and (3) a kinesin-free wall next to a kinesin-coated bottom surface (topography and chemistry combined). We present a guiding mechanism for each surface type that takes into account the physical properties of microtubule filaments and the surface properties (geometry, chemistry), and elucidate the contributions of surface topography and chemistry. Our experimental and theoretical results show that track edges that combine both topography and chemistry guide microtubules most frequently (approximately 90% of all events). By applying the principles of microtubule guidance by microfabricated surfaces, one may design and build motor-protein-powered devices optimized for transport.