Nontraditional techniques

1996 ◽  
Author(s):  
Wolfgang Schmickler
Keyword(s):  
Fermi Level ◽  
Electrode Surface ◽  
Electrochemical Techniques ◽  
Tunneling Current ◽  
Current Mode ◽  
Constant Current ◽  
Scanning Tunneling ◽  
Electronic Density ◽  
Direction Parallel ◽  
Metal Tip

The traditional electrochemical techniques are based on the measurement of current and potential, and, in the case of liquid electrodes, of the surface tension. While such measurements can be very precise, they give no direct information on the microscopic structure of the electrochemical interface. In this chapter we treat several methods which can provide such information. None of them is endemic to electrochemistry; they are mostly skillful adaptations of techniques developed in other branches of physics and chemistry. The scanning tunneling microscope (STM) is an excellent device to obtain topographic images of an electrode surface . The principal part of this apparatus is a metal tip with a very fine point, which can be moved in all three directions of space with the aid of piezoelectric crystals. All but the very end of the tip is insulated from the solution in order to avoid tip currents due to unwanted electrochemical reactions. The tip is brought very close, up to a few Ångstroms, to the electrode surface. When a potential bias ΔV, usually of the order of a few hundred millivolts, is applied between the electrode and the tip, the electrons can tunnel through the thin intervening layer of solution, and a tunneling current is observed. The situation is illustrated in Fig. 15.2: A potential energy barrier exists between the tip and the substrate. Application of a bias potential shifts the two Fermi levels of the tip and of the substrate. Electrons can tunnel from the metal with the higher Fermi level through the barrier to empty states on the other metal. Roughly speaking, electrons with energies between the two Fermi levels can be transferred. A detailed calculation shows that the current is proportional to the electronic density of states at the Fermi level of the substrate. The tip is moved slowly in the yz direction parallel to the metal surface, and simultaneously the distance x from the electrode is adjusted in such a way that the tunneling current is constant (constant-current mode).

Experiments with a scanning tunneling microscope (STM) mounted in a scanning electron microscope (SEM)

1986 ◽  
Vol 44 ◽  
pp. 636-639
Author(s):  
Oliver C. Wells ◽  
Mark E. Welland
Keyword(s):  
Electron Microscope ◽  
Scanning Electron Microscope ◽  
Mechanical Stability ◽  
Tunneling Current ◽  
Feedback System ◽  
Scanning Tunneling ◽  
Surface Profiling ◽  
Close Proximity ◽  
Metal Tip ◽  
Scanning Electron

Scanning tunneling microscopes (STM) exist in two versions. In both of these, a pointed metal tip is scanned in close proximity to the specimen surface by means of three piezos. The distance of the tip from the sample is controlled by a feedback system to give a constant tunneling current between the tip and the sample. In the low-end STM, the system has a mechanical stability and a noise level to give a vertical resolution of between 0.1 nm and 1.0 nm. The atomic resolution STM can show individual atoms on the surface of the specimen.A low-end STM has been put into the specimen chamber of a scanning electron microscope (SEM). The first objective was to investigate technological problems such as surface profiling. The second objective was for exploratory studies. This second objective has already been achieved by showing that the STM can be used to study trapping sites in SiO2.

Conductance Imaging of the Depletion Region of Biased Silicon PN Junction Device

2001 ◽  
Vol 669 ◽  
Author(s):  
Jeong Young Park ◽  
R. J. Phaneuf ◽  
E. D. Williams
Keyword(s):  
Tunneling Current ◽  
Current Mode ◽  
Depletion Region ◽  
Constant Current ◽  
Band Bending ◽  
Pn Junction ◽  
Junction Device ◽  
Modulation Signal ◽  
Passivated Silicon ◽  
Lock In

ABSTRACTSimultaneous conductance imaging and constant current mode STM imaging have been used to delineate Si pn junction arrays over a range of reverse bias conditions. Conductance has been obtained by adding a modulation signal to voltages applied in the p and n regions of a model device, and by measuring the modulation signal of the tunneling current with a lock-in amplifier. Both constant current and conductance imaging ofthe electrically different regions (n, p, and depletion zone) show a pronounced dependence on applied pn junction bias. The conductance contrast is mainly due to electrically different behaviors of metal-gap-semiconductor junction which are determined by the tip-induced band bending of the oxide-passivated silicon surface.

Scanning Tunneling Microscopy Imaging and Spectroscopy of a Single Viologen Molecule

2008 ◽  
Vol 8 (9) ◽  
pp. 4621-4625
Author(s):  
Nam-Suk Lee ◽  
Chang-Heon Yang ◽  
Won-Suk Choi ◽  
Young-Soo Kwon
Keyword(s):  
Self Assembly ◽  
Current Mode ◽  
Scanning Tunneling Spectroscopy ◽  
Constant Current ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Microscopy Imaging ◽  
Current Voltage ◽  
Imaging And Spectroscopy ◽  
Stm Images

A low-temperature ultrahigh-vacuum scanning tunneling microscope (UHV-STM) was used to image viologen (N-methyl-N′-di (8-mercaptooctyl)-4,4′-bipyridinium; HSC8VC8SH) molecules and to perform local spectroscopic measurements on these molecules. Self-assembly of viologen molecules was conducted on Au (111), which had been thermally deposited onto freshly cleaved, heated mica. Here, we demonstrate a novel SAM matrix appropriate for the isolation of viologen molecules composed of octanethiol (C8) in which HSC8VC8SH was inserted at defects in the molecular lattice. The isolated single molecules of viologen inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. STM images at 298 K showed protrusions with a topographic height of about 2.71 nm (HSC8VC8SH) with viologen molecules that self-assembled on the substrate. The current–voltage (I–V) characteristics were measured while the electrical properties of the formed monolayer were scanned using scanning tunneling spectroscopy (STS). We found the high peak current-like rectification at +1.14 V (HSC8VC8SH). The rectification ratios, RR = J (at +2.5 V)/J (at −2.5 V), are in the range of 4.47.

Fast scanning tunneling microscope in constant current mode

1989 ◽  
Vol 333 (4-5) ◽  
pp. 340-342 ◽  
Author(s):  
S. Bräuer ◽  
B. Krämer ◽  
H. Pagnia ◽  
N. Sotnik ◽  
K. -H. Vetter ◽  
...  
Keyword(s):  
Scanning Tunneling Microscope ◽  
Current Mode ◽  
Constant Current ◽  
Scanning Tunneling ◽  
Tunneling Microscope ◽  
Constant Current Mode

Design of Decouple on MEMS Tunneling Gyroscope

2007 ◽  
Author(s):  
Wenwang Li
Keyword(s):  
Relative Motion ◽  
Tunneling Current ◽  
Scanning Tunneling ◽  
Quantum Mechanical ◽  
Sense Mode ◽  
Tunneling Microscopy ◽  
Drive Mode ◽  
Comb Drive ◽  
Quantum Mechanical Tunneling ◽  
Metal Tip

The proposed microelectromechanical (MEMS) tunneling gyroscope uses a general principle of operation that is commonly used for scanning tunneling microscopy (STM). In STM a bias voltage is applied between a sharp metal tip and a conducting sample. When the tip and sample are brought to within a few Angstroms (Å) of each other, a tunneling current can flow due to quantum mechanical tunneling effects. Because the tunneling current is exponentially dependent on the separation between the tip and the sample, the distance between the tip the sample can be measured to within 10−3 (Å). There are two motions in two directions in tunneling gyroscope: drive mode and sense mode. For decoupling drive mode and sense mode, two schemes are studied forward: (1) doubly decoupled comb drive plate oscillation gyroscope: There is only longitudinal relative motion, no landscape orientation relative motion between two electrodes.(2) Cantilever tunnel gyroscope: By using of ladders cantilever, relative motion of the silicon tip in the drive direction was reducing.

scholarly journals Deconvolution of the density of states of tip and sample through constant-current tunneling spectroscopy

2011 ◽  
Vol 2 ◽  
pp. 607-617 ◽  
Author(s):  
Holger Pfeifer ◽  
Berndt Koslowski ◽  
Paul Ziemann
Keyword(s):  
Barrier Height ◽  
Density Of States ◽  
Probability Function ◽  
Current Mode ◽  
Transmission Probability ◽  
Wkb Approximation ◽  
Constant Current ◽  
Scanning Tunneling ◽  
Energy Position ◽  
Sample Separation

We introduce a scheme to obtain the deconvolved density of states (DOS) of the tip and sample, from scanning tunneling spectra determined in the constant-current mode (z–V spectroscopy). The scheme is based on the validity of the Wentzel–Kramers–Brillouin (WKB) approximation and the trapezoidal approximation of the electron potential within the tunneling barrier. In a numerical treatment of z–V spectroscopy, we first analyze how the position and amplitude of characteristic DOS features change depending on parameters such as the energy position, width, barrier height, and the tip–sample separation. Then it is shown that the deconvolution scheme is capable of recovering the original DOS of tip and sample with an accuracy of better than 97% within the one-dimensional WKB approximation. Application of the deconvolution scheme to experimental data obtained on Nb(110) reveals a convergent behavior, providing separately the DOS of both sample and tip. In detail, however, there are systematic quantitative deviations between the DOS results based on z–V data and those based on I–V data. This points to an inconsistency between the assumed and the actual transmission probability function. Indeed, the experimentally determined differential barrier height still clearly deviates from that derived from the deconvolved DOS. Thus, the present progress in developing a reliable deconvolution scheme shifts the focus towards how to access the actual transmission probability function.

“Stm” Scanning Tunneling Microscopy

1987 ◽  
Vol 45 ◽  
pp. 196-197
Author(s):  
J. A. Kubby
Keyword(s):  
Scanning Tunneling Microscopy ◽  
Tunneling Current ◽  
Sample Surface ◽  
Real Space ◽  
Three Dimensions ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Scanned Image ◽  
Metal Tip ◽  
First Time

Scanning Tunneling Microscopy is a recently developed technique within the area of Scanned Image Microscopy that is based on tunneling between two conducting electrodes. This method offers, for the first time, the possibility of direct, real space determination of surface atomic and electronic structure in three dimensions on an atomic length scale, including nonperiodic structures.In this technique a sharp metal tip, mounted on a piezoelectric tripod that forms an orthogonal coordinate system, is brought to within a few Angstroms of the sample surface without “touching” the region to be scanned. A tunneling current I, on the order of 0.1 to 1 nA, is established by applying a bias between the tip and sample. The tunneling current is given to first order by;

TUNNELING SPECTROSCOPY OF NANOMETER-SIZE CLUSTERS DEPOSITED ON GRAPHITE

1996 ◽  
Vol 03 (01) ◽  
pp. 979-982 ◽  
Author(s):  
A. WAWRO ◽  
A. KASUYA ◽  
R. CZAJKA ◽  
Y. NISHINA
Keyword(s):  
Scanning Tunneling Microscope ◽  
Electronic Structures ◽  
Energy Gap ◽  
Current Mode ◽  
Constant Current ◽  
Vacuum System ◽  
Graphite Substrate ◽  
Scanning Tunneling ◽  
Nanometer Size ◽  
Au Clusters

The electronic structures of Sb, Ni, and Au clusters of nanometer size are discussed. Clusters were deposited on a graphite substrate and analyzed with a scanning tunneling microscope in an ultrahigh-vacuum system. Scanning at constant-current mode at different bias voltages was used as a method of spectroscopic measurements. Disappearing of the clusters images at certain range of voltage biases is attributed to occurrence of the energy gap in electronic structure of clusters, suggesting their nonmetallic behavior.

Scanning tunneling microscopy of planar biomembranes

1989 ◽  
Vol 47 ◽  
pp. 14-15
Author(s):  
K. A. Fisher ◽  
S. Whitfield ◽  
R. E. Thomson ◽  
K. Yanagimoto ◽  
M. Gustafsson ◽  
...  
Keyword(s):  
Scanning Tunneling Microscope ◽  
Surface Deformation ◽  
Pyrolytic Graphite ◽  
Current Mode ◽  
Constant Current ◽  
Scanning Tunneling ◽  
Biological Macromolecules ◽  
Tunneling Microscopy ◽  
Aspect Ratios ◽  
Planar Membranes

The scanning tunneling microscope (STM) is capable of imaging conductive surfaces at atomic resolution. When STMs are used to image biological samples, however, STM resolution is limited to nanometer levels whether samples are hydrated, air-dried, or metal-coated. Lateral resolution is poor due to the nature of biological macromolecules (large Image aspect ratios) as well as to STM tip effects (shape, multiple tips, and tip/sample Interactions). If samples are adsorbed to highly-oriented pyrolytic graphite (HOPG) surfaces and scanned in the topographic (constant current) mode, vertical resolution is also uncertain due to contamination-mediated surface deformation artifacts. Nevertheless, because the STM is capable of detecting sub-Ångstrom displacements in z (e.g. to 0.02 Å in UHV), we have examined the feasibility of using the STM to determine the thickness of planar membranes attached to glass and mica surfaces. Planar membrane monolayers also uniquely provide the opportunity to correlate biochemical and TEM information with STM topographic images.

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