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Living systems employ cilia to control and to sense the flow of fluids for many purposes, such as pumping, locomotion, feeding, and tissue morphogenesis. Beyond their use in biology, functional arrays of artificial cilia have been envisaged as a potential biomimetic strategy for inducing fluid flow and mixing in lab-on-a-chip devices. Here we report on fluid transport produced by magnetically actuated arrays of biomimetic cilia whose size approaches that of their biological counterparts, a scale at which advection and diffusion compete to determine mass transport. Our biomimetic cilia recreate the beat shape of embryonic nodal cilia, simultaneously generating two sharply segregated regimes of fluid flow: Above the cilia tips their motion causes directed, long-range fluid transport, whereas below the tips we show that the cilia beat generates an enhanced diffusivity capable of producing increased mixing rates. These two distinct types of flow occur simultaneously and are separated in space by less than 5 μm, approximately 20% of the biomimetic cilium length. While this suggests that our system may have applications as a versatile microfluidics device, we also focus on the biological implications of our findings. Our statistical analysis of particle transport identifying an enhanced diffusion regime provides novel evidence for the existence of mixing in ciliated systems, and we demonstrate that the directed transport regime is Poiseuille–Couette flow, the first analytical model consistent with biological measurements of fluid flow in the embryonic node.

Nanoscale electronics seeks to decrease the critical dimension of devices in order to improve performance while reducing power consumption. Single-walled carbon nanotubes fit well with this strategy because, in addition to their molecular size, they demonstrate a number of unique electronic, mechanical and electromechanical properties. In particular, theory1,2,3,4,5,6,7,8 predicts that strain can have a large effect on the band structure of a nanotube, which, in turn, has an influence on its electron transport properties. This has been demonstrated in experiments where axial strain was applied by a scanning probe9,10,11,12. Theory also predicts that torsional strain can influence transport properties, which was observed recently in multiwalled nanotubes13. Here we present the first experimental evidence of an electromechanical effect from torsional strain in single-walled nanotubes, and also the first measurements of piezoresistive response in a self-contained nanotube-based nanoelectromechanical structure.

In many cases in experimental science, the instrument interface becomes a limiting factor in the efficacy of carrying out unusual experiments or prevents the complete understanding of the acquired data. We have developed an advanced interface for scanning probe microscopy (SPM) that allows intuitive rendering of data sets and natural instrument control, all in real time. The interface, called the nanoManipulator, combines a high-performance graphics engine for real-time data rendering with a haptic interface that places the human operator directly into the feedback loop that controls surface manipulations. Using a hand-held stylus, the operator moves the stylus laterally, directing the movement of the SPM tip across the sample. The haptic interface enables the user to “feel” the surface by forcing the stylus to move up and down in response to the surface topography. In this way the user understands the immediate location of the tip on the sample and can quickly and precisely maneuver nanometer-scale objects. We have applied this interface to studies of the mechanical properties of nanotubes and to substrate-nanotube interactions. The mechanical properties of carbon nanotubes have been demonstrated to be extraordinary. They have an elastic modulus rivaling that of the stiffest material known, diamond, while maintaining a remarkable resistance to fracture. We have used atomic-force microscopy (AFM) to manipulate the nanotubes through a series of configuration that reveal buckling behavior and high-strain resilience. Nanotubes also serve as test objects for nanometer-scale contact mechanics. We have found that nanotubes will roll under certain conditions. This has been determined through changes in the images and through the acquisition of lateral force during manipulation. The lateral force data show periodic stick-slip behavior with a periodicity matching the perimeter of the nanotube.

The consequences of quantum behavior of charge carriers in devices, while entertaining, is detrimental to the device performance. Chaos leads to unmanageable and unpredictable response in different devices to the same drive and bias inputs.

We review some experimental and theoretical results on the metal-to-insulator transition (MIT) observed at zero magnetic field (B=0) in several two-dimensional electron systems (2DES). Scaling of the conductance and magnetic field dependence of the conductance provide convincing evidence that the MIT is driven by Coulomb interactions among the carriers and is dramatically sensitive to spin polarization of the carriers., Comment: 8 pages, LaTeX, figure label changed

We present our results on the manipulation of individual viruses using an advanced interface for atomic force microscopes (AFMs). We show that the viruses can be dissected, rotated, and translated with great facility. We interpret the behavior of tobacco mosaic virus with a mechanical model that makes explicit the competition between sample-substrate lateral friction and the flexural rigidity of the manipulated object. The manipulation behavior of tobacco mosaic virus on graphite is shown to be consistent with values of lateral friction observed on similar interfaces and the flexural rigidity expected for macromolecular assemblies. The ability to manipulate individual samples broadens the scope of possible studies by providing a means for positioning samples at specific binding sites or predefined measuring devices. The mechanical model provides a framework for interpreting quantitative measurements of virus binding and mechanical properties and for understanding the constraints on the successful, nondestructive AFM manipulation of delicate samples.

This Resource Letter provides a guide to the literature on quantum interference in macroscopic systems. The main topics included are weak localization, interference‐induced fluctuations, Aharonov–Bohm effect, ballistic transport, and numerical methods. Our compilation of references is intended to help the nonspecialist who wishes to study the main topics covered. [The letter E after an item indicates elementary level or material of general interest to persons becoming informed in the field. The letter I, for intermediate level, indicates material of somewhat more specialized nature; and the letter A indicates rather specialized or advanced material.]

A brief review of quantum interference in small conductors is followed by a discussion of quantum interference in ballistic conductors. The ballistic conductors are characterized by conservation of the momentum vector and this leads to a simple model for the details of the conductance of the system. In contrast, for diffusive conductors one can only predict the statistical behavior of the interference contributions to the conductance. The response of the ballistic interferometers is governed by semiclassical physics of the electron momentum states (i.e. the modes) in the quantum conductor. Under circumstances of reduced backscattering of the modes, the interference pattern appears to imitate that from a simple optical interferometer. New results from ballistic interferometers are reviewed briefly.

We present a quartz tuning-fork-based atomic force microscopy (AFM) setup that is capable of mapping the surface contact resistance while scanning topography. The tuning-fork setup allows us to use etched Pt/Ir tips, which have higher durability and better conductivity than probes used in earlier AFM conductance measurements. The performance of the method is demonstrated with contact resistance measurements of gold lines on silicon dioxide and carbon nanotubes on graphite.

We present a fast, high yield, low cost method for the production of scanning probes with aligned carbon nanotubes protruding from the ends. The procedure is described and images of undercut films are used to demonstrate the improved probe quality for topography measurements. A magnetophoretic model of the attachment and alignment processes is discussed.

We report on the precise placement of a single carbon nanotube (CNT) onto a microlectromechanial system (MEMS) structure. Using a hybrid atomic force microscope/scanning electron microscope (AFM/SEM) system, an individual multiwalled CNT was retrieved from a cartridge by the AFM tip, translated to a MEMS device, and then placed across a gap between an actuating and a stationary structure. Progress toward a resistance versus stress/strain measurement on a CNT will be discussed, including SEM images of a MEMS structure we have designed specifically for such a measurement. © 2002 American Institute of Physics.

We report measurements of the shear modulus of each shell and the friction between the two shells of double-shell carbon nanotubes in single nanotube-based nanoelectromechanical devices operated in a transmission electron microscope. In situ nanobeam electron diffraction is applied to obtain the chiral indices of each shell of the nanotube and it allows us to establish a quantitative correlation between the atomic structure and properties of the nanotube under investigation.

Metal-Cd 0.42 Te 0.58 -metal nanowires were electrodeposited into the pores of anodized aluminum oxide (AAO) membranes, and the polarization sensitive photoconductance was analyzed for individual nanostructures. Non-linear I– V curves were observed, and the short-circuit current density, open-circuit voltage, and fill factor were determined. These nanowires exhibited a power conversion efficiency of 0.56%, which is higher than some comparable nanomaterials of greater complexity.

is preferred over sliding, and it is expected to have an equally important role in the microscopic domain. Although progress has been made in our understanding of the dynamics of sliding at the atomic level 4 ,w e have no comparable insight into rolling owing to a lack of experimental data on microscopic length scales. Here we produce controlled rolling of carbon nanotubes on graphite surfaces using an atomic force microscope. We measure the accompanying energy loss and compare this with sliding. Moreover, by repro- ducibly rolling a nanotube to expose different faces to the substrate and to an external probe, we are able to study the object over its complete surface. The microscopic aspects of tribology have been explored by means of the surface force apparatus (SFA) 5 and atomic force microscopy (AFM) 6 , which have identified the intrinsic dependence of sliding friction on contact area 7,8 and crystallographic orientation 9,10 . Further, AFM has been used to perform friction mapping of surfaces with atomic resolution 4 and has identified stick-slip motion in the sliding of nanometre-scale objects 11 . Using AFM manipulation 12,13 of multiwall carbon nanotubes (CNTs), we have observed sliding and rolling. CNTs, with a range of available sizes down to the molecular scale, serve as interesting model systems for tribological studies. In addition, nanotubes are expected to play a part in future nanometre-scale electrical-mechanical devices, and rolling fullerenes have been proposed as ideal lubricants 14-16 . Our evidence for sliding and rolling comprises sequences of topogra- phical images of manipulated nanotubes and lateral force data acquired during the manipulation to measure energy loss. Samples were prepared by solvent evaporation on mica and graphite substrates from an ethanol solution of raw carbon soot produced by the carbon arc technique 17 . The multiwall CNTs were imaged and manipulated under ambient conditions. The nanomanipulator AFM system 18,19 , designed for manipulation, comprises an advanced visual interface, teleoperation capabilities for manual control of the AFM tip and haptic (touch) presentation of the AFM data (Topometrix Discoverer). Normal force, lateral force and AFM tip trajectory are recorded simultaneously for strict correlation. The AFM tip is used to apply lateral forces at locations along the tube to produce translations and rotations. These tubes are free of pinning material and move without deformation. The lateral force values have been calibrated from measured cantilever and tip geometry, literature values for the cantilever (Si) elastic moduli and the detector response from the z-translation of the cantilever. We estimate the absolute error in this calibration to be about 30%, with the largest contribution in the uncertainty coming

We report on the characterization of nanometer-scale torsional devices based on individual single-walled carbon nanotubes as the spring elements. The axial shear moduli of the nanotubes are obtained through modeling of device reaction to various amounts of applied electrostatic force and are compared to theoretical values.

The curling of a graphitic sheet to form carbon nanotubes produces a class of materials that seem to have extraordinary electrical and mechanical properties. In particular, the high elastic modulus of the graphite sheets means that the nanotubes might be stiffer and stronger than any other known material, with beneficial consequences for their application in composite bulk materials and as individual elements of nanometre-scale devices and sensors. The mechanical properties are predicted to be sensitive to details of their structure and to the presence of defects, which means that measurements on individual nanotubes are essential to establish these properties. Here we show that multiwalled carbon nanotubes can be bent repeatedly through large angles using the tip of an atomic force microscope, without undergoing catastrophic failure. We observe a range of responses to this high-strain deformation, which together suggest that nanotubes are remarkably flexible and resilient.

We have measured the decay of local conductance in single-wall carbon nanotubes directly using conductance imaging atomic force microscopy. The decay lengths were in the range from $190\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ to well over $3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. There are strong indications that these decay lengths are the result of depletion lengths around metallic/semiconducting carbon nanotube junctions, and that they are related to defects in the tubes.

We report on the characterization of nanometer-scale resonators. Each device incorporates one multiwalled carbon nanotube (MWNT) as a torsional spring. The devices are actuated electrostatically, and their deflections, both low frequency and on resonance, are detected optically. These are some of the smallest electromechanical devices ever created and are a demonstration of practical integrated MWNT-based oscillators. The results also show surprising intershell mechanical coupling behavior in the MWNTs.

We present local conductance measurements of carbon nanotube networks with nanometer scale resolution and show that there are discrete drops in conductance that correspond to junctions of metallic nanotubes and semiconducting nanotubes. The anomalies of these networks compared to thin films are shown, and a new method of discerning between semiconducting and metallic single-wall carbon nanotubes is demonstrated.

We report on the fabrication of nanometer-scale mechanical devices incorporating multiwalled carbon nanotubes (MWNTs) as the torsional spring elements. We have employed electron beam lithography to pattern device elements directly onto individual MWNTs on a silicon dioxide substrate. The structures were suspended by etching the substrate and subsequent critical-point drying of the sample. We also briefly present characterization of the torsional properties of an individual MWNT. The techniques described are applicable to other nanometer-scale rod-like objects. © 2003 American Institute of Physics.

We demonstrate that individual carbon nanotubes (CNTs) can be visualized with fluorescence microscopy through noncovalent labeling with conventional fluorophores. Reversal of contrast in fluorescence imaging of the CNTs was observed when performing labeling procedure in a nonpolar solvent. Our results are consistent with a CNT-fluorophore affinity mediated by hydrophobic interaction. The reverse-contrast images also provide clear indication of nanotube location. © 2003 American Institute of Physics.

We report on the characterization of torsional oscillators which use multi-walled carbon nanotubes as the spring elements. Through atomic-force-microscope force-distance measurements we are able to apply torsional strains to the nanotubes and measure their torsional spring constants and effective shear moduli. We find that the effective shear moduli cover a broad range, with the largest values near the theoretically predicted value. The data also suggest that the nanotubes are stiffened by repeated flexing., 4 pages

The temperature dependence of conductivity $\sigma (T)$ of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B=0, the system displays a metal-insulator transition at a critical electron density $n_c(0)$, and $d\sigma/dT >0$ in the metallic phase. At low fields ($B\lesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) \propto B^{\beta}$ ($\beta\sim 1$), and the zero-temperature conductivity scales as $\sigma (n_s,B,T=0)/\sigma (n_s,0,0)=f(B^{\beta}/\delta_n)$ (where $\delta_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of $n_c$ at high fields., Comment: published version

The transfer of electrons from one material to another is usually described in terms of energy conservation, with no attention being paid to momentum conservation. Here we present results on the junction resistance between a carbon nanotube and a graphite substrate and show that details of momentum conservation also can change the contact resistance. By changing the angular alignment of the atomic lattices, we found that contact resistance varied by more than an order of magnitude in a controlled and reproducible fashion, indicating that momentum conservation, in addition to energy conservation, can dictate the junction resistance in graphene systems such as carbon nanotube junctions and devices.

The nanoManipulator system adds a virtual reality interface to an atomic force microscope (AFM), thus providing a tool that enables the user not only to image but also to manipulate nanometer-sized molecular structures. As the AFM tip scans the surface of these structures, the tip–sample interaction forces are monitored, which in turn provide information about the frictional, mechanical, and topological properties of the sample. Computer graphics are used to reconstruct the surface for the user, with color or contours overlaid to indicate additional data sets. Moreover, by means of a force-feedback pen, which is connected to the scanning tip via software, the user can touch the surface under investigation to feel it and to manipulate objects on it. This system has been used to investigate carbon nanotubes, fibrin, DNA, adenovirus, and tobacco mosaic virus. Nanotubes have been bent, translated, and rotated to understand their mechanical properties and to investigate friction on the molecular level. AFM lithography is being combined with the nanoManipulator to investigate the electromechanical properties of carbon nanotubes. The rupture forces of fibrin and DNA have been measured. This article discusses how some of the graphics and interface features of the nanoManipulator made these novel investigations possible. Visitors have used the system to examine chromosomes, bacterial pili fibers, and nanochain aggregates (NCAs). Investigators are invited to apply to use the system as described on the web at http://www.cs.unc.edu/Research/nano/doc/biovisit.html.

We investigate the response of multi-walled carbon nanotubes to mechanical strain applied with an Atomic Force Microscope (AFM) probe. We find that in some samples, changes in the contact resistance dominate the measured resistance change. In others, strain large enough to fracture the tube can be applied without a significant change in the contact resistance. In this case we observe that enough force is applied to break the tube without any change in resistance until the tube fails. We have also manipulated the ends of the broken tube back in contact with each other, re-establishing a finite resistance. We observe that in this broken configuration the resistance of the sample is tunable to values 15-350 kW greater than prior to breaking., Submitted to Applied Physics Letters

We have varied the disorder in a two-dimensional electron system in silicon by applying substrate bias. When the disorder becomes sufficiently low, we observe the emergence of the metallic phase, and find evidence for a metal-insulator transition (MIT): the single-parameter scaling of conductivity with temperature near a critical electron density. We obtain the scaling function $\beta$, which determines the length (or temperature) dependence of the conductance. $\beta$ is smooth and monotonic, and linear in the logarithm of the conductance near the MIT, in agreement with the scaling theory for interacting systems., Comment: 4 pages, LaTeX, physica.sty (slightly modified prabib.sty), Submitted to the 1997 Conference on Electronic Properties of Two-Dimensional Systems

We study conductance fluctuations in a two-dimensional electron gas as a function of chemical potential (or gate voltage) from the strongly insulating to the metallic regime. Power spectra of the fluctuations decay with two distinct exponents (1/v_l and 1/v_h). For conductivity $\sigma\sim 0.1 e^{2}/h$, we find a third exponent (1/v_i) in the shortest samples, and non-monotonic dependence of v_i and v_l on \sigma. We study the dependence of v_i, v_l, v_h, and the variances of corresponding fluctuations on \sigma, sample size, and temperature. The anomalies near $\sigma\simeq 0.1 e^{2}/h$ indicate that the dielectric response and screening length are critically behaved, i.e. that Coulomb correlations dominate the physics., Comment: Revised according to referee remarks

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Reports of signals travelling faster than the speed of light have captured the imagination of scientists and laymen alike. But there is an explanation for these effects that leaves Einstein's tenet intact.

not Available.

Mesure directe de la variation des fluctuations de magnetoresistance non locales en fonction de la longueur dans des fils de Sb et Au en etudiant les regions separees du parcours du courant classique par une distance L variable (3L O a 0,2LO ou L O est la longueur de coherence de phase pour les porteurs dans le metal). Analyse de la variation de la correlation du champ magnetique en fonction de la longueur

In many cases in experimental science, it is true that the instrument interface becomes a limiting factor in the efficacy of carrying out unusual experiments, or prevents the complete understanding of the acquired data. We are developing an advanced interface for Scanning Probe Microscopy (SPM) which allows intuitive rendering of datasets and natural instrument control, all in real-time. The interface, called the nanoManipulator, combines a high performance graphics engine for real-time data rendering with a haptic interface which places the human operator directly into the “feedback loop” that controls surface manipulations. In practice, the user holds a stylus in hand. By moving the stylus laterally, the user directs the movement of the SPM tip across the sample. The haptic interface enables the user to “feel” the surface by forcing the stylus to move up and down in response to the surface topography.

Winful objects to our appeal1 to pulse reshaping discussed in refs 2,3. It is these discussions, however, that show that causality is maintained in the tunnelling process and that no fundamental tenet of modern physics is called into question. Although these discussions were put forth for short pulses, they are not, in principle, limited to this case. Indeed, pulse reshaping also occurs for long pulses, even if only at the leading edge of the pulse.

A new type of cascade laser has been built that can be tuned in wavelength and gain, and can even be made to emit beams of more than one wavelength at the same time.

We have used a new technique to simultaneously measure the surface topography and surface potential of current-carrying polycrystalline Au 60 Pd 40 thin films using a scanning tunneling microscope. We find abrupt steps in the surface potential due to scattering from grain boundaries in these films.

Universal conductance fluctuations in wires and Aharonov-Bohm oscillations in loops are not symmetric about $H=0$. The observation of asymmetry in the periodic oscillations is possible when the phase-coherence length of the wave function is comparable to the separation of the voltage probes. In both cases, four-probe measurements yield resistances which depend on lead configuration. The asymmetries appear like Hall voltages, and are consistent with Onsager's relations.

Aharonov-Bohm magnetoconductance oscillations have been measured in series arrays of 1, 3, 10, and 30 submicron-diameter Ag loops. At constant temperature, the amplitude of the h/e oscillations is observed to decrease as the square root of number of loops, while the amplitude of h/2e conductance oscillations, measured in the same samples, is independent of the number of series loops. This is direct confirmation of the ensemble averaging properties of h/e oscillations in multiloop systems. The amplitude of the h/e oscillations is in good agreement with recent calculations.

The magnetoresistance of sub-0.4-\ensuremath{\mu}m-diam Au and ${\mathrm{Au}}_{60}$${\mathrm{Pd}}_{40}$ rings was measured in a perpendicular magnetic field at temperatures as low as 5 mK in search of simple, periodic resistance oscillations that would be evidence of flux quantization in normal-metal rings. However, instead of simple oscillations, a very complex structure developed in the magnetoresistance at low temperatures. Fourier analysis of all the data did not reveal convincing evidence for flux quantization in the rings. Complex structure similar to that observed in the rings was also found in the magnetoresistance of short, narrow, Au and ${\mathrm{Au}}_{60}$${\mathrm{Pd}}_{40}$ lines. This structure appears to be associated with the small size of the devices.

Random fluctuations of the electrical conductance are ubiquitous in small (typical dimension L ≲ 1 µm) metallic samples at low temperatures (typically T ≲ 1K ≃ 0.09 meV). The fluctuations result from the quantum-mechanical interference of the carrier wavefunctions. The superpositions of the wavefunctions depend randomly on the placement of impurities, on magnetic field, and on the current driven through the sample. At length scale L φ (the average distance over which the carriers retain phase information), the fluctuations always have amplitude ΔG ≃ e 2 /h, and any observations at scale larger than the coherence length yield a decreased amplitude of the fluctuations. Since the carrier wavefunctions are not classical, local objects (they extend over regions of size L φ ), the conductance contains nonlocal terms. For instance, the conductance is not zero far from the classical current paths through the sample and is not symmetric under the reversal of the magnetic field. In this article, the physics of the fluctuations is reviewed, and some of the experiments which illuminate the physics are described.

The magnetoresistance of small wires and loops is described. At low temperatures, the electrons retain quantum mechanical phase coherence for distances of a micron or so, which is the size scale over which the measurements are made. The random interference of the electron wavefunctions appears as random fluctuations in the magnetoresistance, and in loops, the Aharonov–Bohm effect causes periodic oscillations. The resistance varies randomly with the Fermi energy of the electrons, with the voltage across the sample, and with any transverse voltages applied by external gates. The nonlocal character of the wavefunction leads to resistance fluctuations in configurations where classical physics predicts null results. Throughout the discussion the physics is explained for those new to the field.

Etude des effets d'un champ electrique transversal sur les fluctuations de conductance dans une boucle de Sb. Discussion de deux mecanismes pouvant expliquer la dependance du diagramme des fluctuations avec le champ electrique. Le premier est l'effet Aharonov-Bohm electrostatique, le second est le deplacement spatial des trajectoires electroniques par le champ electrique

We report large logarithmic corrections to the conductivity of two-dimensional electrons and holes in GaSb-InAs-GaSb double heterostructures. From \ensuremath{\sim} 40 mK to 1 K, the conductivity increased with the logarithm of the temperature but with a slope as much as 30 times larger than estimated from the theories of weak localization and carrier interaction. The discrepancy apparently results from electron-hole interactions not included in the theory.

The amplitude of h/e periodic oscillations in the magnetoresistance of very small normal-metal (Au) rings, as well as the harmonic h/2e, have been studied as a function of temperature. The amplitudes depend on the temperature T roughly as ${T}^{\mathrm{\ensuremath{-}}1/2}$, as expected from the averaging of conduction channels in the absence of inelastic scattering, but may not be entirely consistent with this model. At the lowest T, the size of the fluctuations in the conductance is about \ensuremath{\Delta}G\ensuremath{\sim}${e}^{2}$/h, as predicted recently.