R. C. Jaklevic

Scanning tunneling microscopy study of metals: Spectroscopy and topography

Abstract We will describe experiments on metal surfaces using the scanning tunneling microscope. Our scanning tunneling microscope was designed to operate in ultra-high vacuum together with sample cleaning and characterization. The major component for vibration damping is an external isolation table. The tip positioning mechanism is a quartz lever arm combined with coarse and fine adjusting screws. The STM and feedback control system are designed to obtain both topographic and spectroscopic information. Topographic images of Au(111) have been obtained which show very smooth planes containing only single atomic steps. Other surfaces show monatomic step structure in the form of an ordered array whose period corresponds to a previously studied reconstruction on Au(111). Further, on the same surface we find steeper sloped regions with multiple steps of equal height with wide facets. Spectroscopic data for Au and Pd show the presence of surface and bulk electronic states with good signal to noise. The energy values of these states are compared directly with the results of other experimental methods.

Electron tunneling spectroscopy—External doping with organic molecules

We report the observation of inelastic electron tunneling spectra of molecules introduced into completely fabricated tunnel junctions. By using an external doping method several kinds of molecules have been introduced into Al–Al oxide–Pb diodes yielding strong spectra characteristic of the particular molecules. This technique is a new way of detecting and studying molecules interacting with surfaces. The high sensitivity and greater simplicity of this method increases possible applications of tunneling spectroscopy.

Scanning tunneling microscope tip structures

Studies of electrochemically etched tungsten scanning tunneling microscope tips, using scanning electron microscopy, show that (i) the tips are often convolved or bent if the mass of the tungsten wire submerged in the etchant is large (an effect ascribed to surface plastic flow), (ii) bent tips nevertheless often produce good quality scanning tunneling microscopy images of Au films in air, but (iii) tips, once crashed clumsily into the Au films, no longer produce images.

Direct observation of an ordered step surface reconstruction on Au(111) by scanning tunneling microscopy

Abstract Experiments with a scanning tunneling microscope (STM) are reported which include the first surface topographic images of Au(111), cleaned and annealed in ultrahigh vacuum. The STM system used for this work includes in situ sample manipulation for sample cleaning, annealing and characterization. Topographs with very large atomically flat (111) regions are obtained and no corrugation on the scale of about 0.1 A is observed. Some single atomic steps are observed. Surprisingly, ordered arrays of single steps are observed over a large region. These represent a type of surface reconstruction whose repeat period agrees with values derived from previous LEED, TEM and ion scattering measurements. These results represent the first critical information about the surface topography of the Au(111) surface.

Nano-machining of gold and semiconductor surfaces

Using a scanning tunnelling microscope tip formed by cutting a platinum wire, we have modified the surfaces of gold and Hg1‐xCdxTe on a nanometre scale by mechanical contact between the tip and the surface. By using the same tip to form images, we have been able to gain ‘before’ and ‘after’ pictures of surfaces that have been selectively ‘sanded’, controllably ‘chiselled’, and ‘swept’.

Observation of electron standing waves in Mg by tunneling

Abstract Electron standing wave states in Mg films are observed by electron tunneling measurements. An electron group velocity is determined by this experiment.

Observation of electromagnetic resonances in crystals of K2Pt (CN)4 Br0.3.3H2O at microwave frequencies

Abstract Electromagnetic dielectric resonances in the range 8–11 GHz have been observed in millimeter sized crystals of K 2 Pt(CN) 4 Br 0.3 .3H 2 O. This observation results directly from the existence of large dielectric constants; values for the longitudinal and transverse dielectric constants at 4°K are estimated to be ϵ ‖ >∼ 3000 and ϵ ⊥ ≅4.

Ultrathin Au films studied with the scanning tunneling microscope: Topography of insulating surfaces

The scanning tunneling microscope (STM) can produce high‐resolution topographic images of surfaces which, normally, are electrically conducting. We have made STM images of a variety of insulating surfaces which are made conducting by a smooth ultrathin (>20 A thickness) Au overlay film. Topographic features of the underlying substrate as small as 10 A are resolved. Surprisingly, no features of the overlay Au film are seen. These films are useful for obtaining topographic images of a wide variety of surfaces and for STM measurements where a stable conducting surface is required for support of molecules or other microscopic structures.

Anisotropic dielectric constant of TTF-TCNQ observed by dielectric resonance

Abstract Independent measurements at low temperatures of the anisotropic dielectric constant (∈ 1 ) of TTF-TCNQ are reported. The measurements utilize dielectric resonance techniques to determine ∈ B 1 and ∈ a 1 on the same crystal. The results confirm the unusually large value for ∈ b 1 (4.2 K) > 10 3 and the anisotropic behavior ∈ b 1 (4.2 K) > 10 2 found earlier in cavity perturbation studies of TTF-TCNQ.

Strain modulation electron tunneling: Observation of energy states in metals

Electron tunneling experiments have been carried out on metal-metal oxide-metal diodes. Observed structure in the current versus voltage curves is attributed to band states of the metal electrodes. A strain modulation technique is employed that provides a further means of identification of band effects and provides a way to eliminate other strain independent tunneling phenomena. In particular, the L2 and L1 band edges of Au have been studied as well as standing wave structure in Pb and Mg. Band edge effects in Sn, Ag, and Mg have also been studied. This technique provides a new way to study energy levels in metals.