Tunneling Probe Research Articles

Energy Redistribution Between Quasiparticles in Mesoscopic Silver Wires

We have measured with a tunnel probe the energy distributionfunction of quasiparticles in silver diffusive wires connectedto two large pads (“reservoirs”), between which a biasvoltage was applied. From the dependence in energy and biasvoltage of the distribution function we have inferred theenergy exchange rate between quasiparticles. In contrast withpreviously obtained results on copper and gold wires, thesedata on silver wires can be well interpreted with the theoryof diffusive conductors either solely, or associated withanother mechanism, possibly the coupling to two-level systems.

Novel physical sensors using evanescent microwave probes

Local probes, such as electron and photon tunneling, atomic force, and capacitance probes, are excellent sensing means for displacement and other related sensors. Here we introduce applications of a new local probe using evanescent microwave probe (EMP) in displacement sensing with a very high vertical spatial resolution (0.01 μm at 1 GHz), very high bandwidth (100 MHz), and stability. The EMP has been used in the characterization and mapping of the microwave properties of a variety of materials in the past and its application in gas sensing and thermography was recently explored and reported. The interesting feature of the EMP is that its characteristics can be easily altered for a specific sensing application by changing its geometry and frequency of operation.

Scanning tunneling and atomic force microscopy probes of self-assembled, physisorbed monolayers: peeking at the peaks.

The imaging and control of self-assembled, physisorbed monolayers have been the subject of numerous scanning tunneling microscopy and atomic force microscopy investigations. The successful interpretation of the structures observed in scanning probe images of molecules self-assembled at liquid-solid and gas-solid interfaces has benefited greatly from recent experimental and theoretical work. These studies are converging on a general tunneling mechanism that accounts for the images of weakly bound, insulating adsorbates. Experiments in which the dynamical behavior of these monolayers has been monitored as a function of time both statically and after the introduction of an external perturbation are described, and novel studies of the selective control of monolayer structure that make use of internal and external electric fields, photons, and solvent coadsorption are reviewed.

Local Electronic Structure in Ordered Aggregates of Carbon Nanotubes: Scanning Tunneling Microscopy/scanning Tunneling Spectroscopy Study

The recent application of tunneling probes in electronic structure studies of carbon nanotubes has proven both powerful and challenging. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), local electronic properties in ordered aggregates of carbon nanotubes (multiwalled nanotubes and ropes of single walled nanotubes) have been probed. In this report, we present evidence for interlayer (concentric tube) interactions in multiwalled tubes and tube-tube interactions in singlewalled nanotube ropes. The spatially resolved, local electronic structure, as determined by the local density of electronic states, is shown to clearly reflect tube-tube interactions in both of these aggregate forms.

Energy Distribution Function of Quasiparticles in Mesoscopic Wires

We have measured with a tunnel probe the energy distribution function of Landau quasiparticles in metallic diffusive wires connected to two reservoir electrodes, with an applied bias voltage. The distribution function in the middle of a 1.5-\ensuremath{\mu}m-long wire resembles the half sum of the Fermi distributions of the reservoirs. The distribution functions in 5-\ensuremath{\mu}m-long wires are more rounded, due to interactions between quasiparticles during the longer diffusion time across the wire. From the scaling of the data with the bias voltage, we find that the scattering rate between two quasiparticles varies as ${\ensuremath{\varepsilon}}^{\ensuremath{-}2}$, where $\ensuremath{\varepsilon}$ is the energy transferred.

Erratum to: Energy distribution of electrons in an out-of-equilibrium metallic wire

We have measured with a tunnel probe the energy distribution function of quasiparticles in the middle of a 1.5 μm-long copper wire placed between two reservoirs at different potentials. We find a distribution function close to half the sum of the Fermi distributions in the reservoirs. The deviation from this behavior is not yet understood.

Optical probes of proton tunneling in molecular solids

Low-temperature optical spectroscopy of guest-host molecular crystal systems provides a very sensitive tool to study proton tunneling processes. Examples include the light-induced creation and evolution of “proton defects”, translational tunneling along hydrogen bonds as well as rotational tunneling of methyl groups. Absorption spectra at 2 K of γ-picoline single crystals evolve over time scales of days due to the slow spin conversion of the methyl groups which leads to an ordering of the crystal.

Energy distribution of electrons in an out-of-equilibrium metallic wire

We have measured with a tunnel probe the energy distribution function of quasiparticles in the middle of a 1:5 µm-long copper wire placed between two reservoirs at different potentials. We find a distribution function close to half the sum of the Fermi distributions in the reservoirs. The deviation from this behavior is not yet understood.

X-ray calibrated tunneling system utilizing a dimensionally stable nanometer positioner

To determine to what extent tunneling probes can be used for metrology or as position-sensitive transducers on the atomic scale, accurate characterization of tunneling current as a function of distance must be ascertained. This report describes a system for making such characterizations and observing their repeatability in air. Also, it is often necessary to position two objects to within nanometers in order to perform precision measurements. Included in the narrative, a dimensionally stable screw-wedge-lever system is described for manually moving a tunneling probe to within 15 nm of a conducting surface. The conducting surface is translated via a magnetically actuated single crystal flexure system previously calibrated through direct x-ray interferometry over a 20-nm range. This limited calibration range dictated dimensional positioning below 15 nm. Also, tunneling current measurements demanded dimensional stability below one nanometer. The complete system is demonstrated to hold the relative probe-surface separation to within 1 nm over a 15-minute duration. Also, the vibrational resonance of the structure is determined using the power spectrum analysis during electron tunneling. Resulting data using platinum, palladium, and silver tips in conjunction with a carbon surface show that current-displacement values vary from run to run with a positional difference of up to 1 nm for a given current. These variations suggest limitations for using tunneling probes in air to perform angstrom metrology.

Design and implementation of a combined scanning tunneling and near-field optical microscope

The design and the implementation of an integrated form of far-field optical, near-field optical and scanning tunneling microscope are presented. Gold coated optical fiber points are used as both tunneling and near-field optical probe, allowing a simultaneous recording of scanning tunneling and near-field optical images of objects. Photon tunneling images are also observed with the same set-up under total internal reflection condition. Finally, we present near-field local spectroscopy using the same system, applied to a high quality Zn 0.8Cd 0.2Se/ZnSe quantum well sample.

Analog ramp generator for a fast scanning tunneling microscope

We present an analog ramp generator for use in fast scanning tunneling or scanning probe microscopes. The analog design achieves low noise with a relatively simple circuit layout. The circuit requires minimal computer overhead because the X and Y ramps are generated using just two clocks and one digital control line. The resulting X and Y ramps are automatically set equal in magnitude, yielding images which remain square despite changes in scan size, number of lines per image, and tip velocity. We have used the circuit at X ramp frequencies above 10 kHz.

Simultaneous scanning tunneling microscope and collection mode scanning near-field optical microscope using gold coated optical fiber probes

Simultaneous scanning tunneling microscope (STM) and collection mode scanning near-field optical microscope (SNOM) images using tunneling regulation are presented. The images were obtained using a gold coated single optical fiber as both a tunneling and near-field optical probe. The method counts on the reliable STM distance regulation and the short separation between tip and sample (<1 nm). Using optimized fiber probes, both STM and SNOM images show good resolution and image contrast. We describe the STM/SNOM system and present the first images obtained in the collection mode with calibration samples of gratings of period down to 200 nm.

Aharonov-Bohm-type effect in graphene tubules: A Landauer approach

In this paper we show that (1) for graphene tubules an axial magnetic field can induce a semimetal-semiconductor transition periodically at zero bias. This is an Aharonov-Bohm-type effect and is generally true for thin cylindrical conductors; (2) the Landauer approach can be used to estimate the conductance of individual tubules measured by scanning tunneling probes as a function of applied bias, temperature, tubule geometric factors, and magnetic field. Numerical calculations are presented showing that the conductance modulation should be observable even at room temperature.

Tunneling probe manipulation of individual rhodium nanoparticles supported on titania

Active metals finely dispersed on oxide supports are useful in a broad class of applications. We have used scanning tunneling microscopy (STM) to characterize and to manipulate 30-50-A rhodium particles grown on TiO 2 (001) rutile. Tunneling barrier height values of ≃5 eV (close to the mean tipsample work function) indicated a good vacuum tunneling gap. The tunneling probe was used to manipulate individual nanoparticles via intimate tipsample point contacts. The results are consistent with cohesive/adhesive tip-particle interactions. This study indicates that supported catalyst and sensor research can be done on a particle-by-particle basis

The use of theodolite intersections to calibrate a wind tunnel probe

Abstract The 30x60 feet wind tunnel at the NASA Langley Research Center is a major facility capable of testing full size aircraft. Air pressure measurements recorded during aerodynamic tests of aircraft or models are made using a probe attached to a three axis carriage. Positioning of the carriage during tests is made using manual observations of survey tapes and an electronic counter. Observations with three theodolites were used to calibrate the location of the carriage relative to an orthogonal coordinate system with a vertical axis. Calibrations were carried out with both the air flow off and the air flow on to assess the influence of induced vibrations on the measurements and the displacement of the carriage under wind load. A variety of coordinate transformations were used to analyse the data, demonstrate the subjectivity of the manual observations and provide a mathematical correction for the observed data.

Electron-barrier interaction in a vacuum tunneling probe

A model for dealing with energy and momentum exchanges between ballistic electrons and the vacuum barrier in a tunneling probe used as an electromechanical transducer is studied and its physical significance in devices of size comparable to the mean free path of the tunneling electrons is discussed.

Nanometer scale gold particle identification by the scanning tunneling optical microscope

Pyrolytic graphite surfaces coated with gold particles of nanometer scale size have been studied to demonstrate the possibility of obtaining additional chemical surface data by observing photon emission from the STM tunnel contact. The design of a new tunneling probe for an STM is described.

Exponential relationships among multiple hydrogen isotope effects as probes of hydrogen tunneling

For hydrogen transfer reactions with moderate degrees of tunneling, the semiclassical relationship of deuterium, tritium to protium, tritium isotope effects is predicted to break down, such that ( k D k T ) 3.26 < k H k T . By contrast, kinetically complex reactions produce ( k D k T ) 3.26 > k H k T . We now show that these limiting cases may not pertain under the conditions of (i) measured secondary kinetic isotope effects which fall below their equilibrium limits and (ii) extensive hydrogen tunneling. Although condition (i) could lead to a false signature of tunneling, ambiguities of interpretation can be avoided by the simultaneous measurement of exponential relationships for primary and secondary isotope effects. With the exception of reactions where the H-transfer process is known to be fully rate determining, condition (ii) may lead to an underestimate of quantum effects as judged by exponential relationships.

Talking point. The scanning tunneling probe as a feynman ‘sewing’ machine?

Research News/Talking Point: The prospect of using the scanning tunneling microscope (STM) to ‘stitch’ together molecules of, for example, thiophene or pyrrole one by one to form macromolecular chains, or in the production of organic superconductors from Langmuir‐Blodgett arrays containing metal ions is discussed.

The scanning tunneling microscope as a high-gain, low-noise displacement sensor

We consider the capabilities of the tunneling probe of the scanning tunneling microscope as a displacement sensor as distinguished from its better established application to surface imaging. Electromechanical transducers that operate on this principle can achieve very large gain and a noise temperature equal to the minimum required by quantum mechanics for any linear amplifier. We present a two-port network representation of the tunneling transducer, including noise, that allows us to discuss the differences between the tunneling transducer and more conventional electromechanical transducers and to draw analogies between a tunneling transducer and a transistor. We present a simple equivalent circuit for the tunneling transducer including two uncorrelated noise generators, the tunneling current shot noise and the fluctuating force that the tunneling probe exerts on a test mass. In practice the fluctuating ‘‘back action’’ force spectral density is exceedingly small. We give an example of a system in which a tunneling transducer is used to monitor the motion of a very small mechanical harmonic oscillator. A transducer gain of approximately 108 should be achieved in this system that makes negligible the noise contribution of conventional following electronics. The contribution to the noise of the tunneling transducer itself should be near the quantum limit and the most significant remaining source of noise is the mechanical oscillator’s Brownian motion. The tunneling transducer represents a new approach for measuring mechanical displacements and may profoundly influence transducer technology in applications from gravity wave detectors down to measurements on single molecules.