M. M. Dovek

Nanometer‐scale hole formation on graphite using a scanning tunneling microscope

We present a reproducible technique for forming holes on a graphite surface with a scanning tunneling microscope. The holes have an average diameter of 40 A (20 A minimum) with an average resolvable spacing of 60 A. Holes are produced by applying a short voltage pulse (3–8 V, 10–100 μs) across the tunneling gap which removes one or more layers of graphite in a small region directly below the tip. Arrays of hundreds of holes have been formed with yields as high as 99.6%. The writing process has a higher success rate in air or in the presence of water vapor. This suggests that the physical mechanism is a chemical process.

Imaging and modification of polymers by scanning tunneling and atomic force microscopy

Direct imaging of ultrathin organic films on solid surfaces is important for a variety of reasons; in particular, the use of such films as ultrathin resists for nanometer scale fabrication and information recording requires that we understand their microstrucure. We have used the Langmuir–Blodgett technique to prepare monolayer and submonolayer films of poly(octadecylacrylate) (PODA) and poly(methylmethacrylate) (PMMA) on graphite substrates. Atomic scale images obtained with the scanning tunneling microscope (STM) and the atomic force microscope of the PODA films showed a variety of structures, including isolated narrow fibrils, parallel groups of fibrils, and an ordered structure consistent with the side chain crystallization expected with that material. The fibrils observed are interpreted as individual polymer chains or small bundles of parallel chains. Images of the PMMA samples show no ordered regions. By applying voltage pulses on the STM tip, we were able to locally modify and apparently cut throu...

Characterization of gold surfaces for use as substrates in scanning tunneling microscopy studies

We have used scanning tunneling microscopy to characterize the surface of epitaxial gold on mica in air. We find that these surfaces are simple to prepare, are relatively inert to exposure to air or to water, and have atomically flat terraces extending for up to several hundred angstroms. The observed topography is consistent with the Au(111) surface. It is possible to produce bumps on the surface of less than 100 A in size in a controlled manner by pulsing the tip voltage while scanning. Self‐diffusion of gold is observed in the decay of written features and well as in the movement of existing terrace edges. In some cases, a periodicity in both the geometry of terrace edges and the spatial variation of surface diffusion rates suggest the presence of the 22×1 surface reconstruction.

Preparation of STM tips for in‐situ characterization of electrode surfaces

The total current between the tip and the sample in a scanning tunnelling microscopy study of a solid/liquid interface can be dominated by Faradaic charge transfer currents. In such a situation, feedback control of the tunnelling gap, and imaging, is precluded. In this contribution we describe the preparation of glass and polymer coated STM tips that possess < 100 Å2 of exposed metal. These tips effectively discriminate against Faradaic current and enable STM imaging in the presence of reversible electroactive solution species at appreciable tip/sample biases.

Au(111) autoepitaxy studied by scanning tunneling microscopy

Abstract Using a scanning tunneling microscope in ultra-high vacuum, we have studied several stages of autoepitaxy on Au(111) from submonolayer up to twenty monolayer coverage at room temperature. The substrate, Au(111) epitaxially grown on mica, exhibits several hundred →ngstrom wide atomatically flat terraces separated by monoatomic steps. At submonolayer coverages, the gold nucleates into single layer clusters arranged preferentially in rows along 〈11 2 〉 directions. As the metal coverage increases, cluster coalescence by growth is observed. Cluster size distributions and spatial correlation functions have been extracted from the STM data. Higher layers start forming before the lower ones are completely filled. The number of incomplete layers increases with deposition rate and total thickness of the film. Room temperature diffusion smooths the terrace structure over a period of several hours. This process is observed to accelerate with a moderate anneal.

Characterization and local modification of atomically flat gold surfaces by STM

In order to characterize the surface of epitaxial gold on mica in air we have used a scanning tunnelling microscope (STM) to image and to modify this surface. It was possible to create controlled features by applying voltage pulses on the tip while scanning. The voltage threshold for writing (about 3 V, 100 ns pulses) was dependent of the tip condition. The lowest pulses were associated with sub‐50 Å feature size. We observed that at ambient temperature the written features disappeared in a time scale of half an hour for the smallest (<30 Å) to a few hours for the bigger features (∼500 Å). We have used the same surface as a substrate for organic imaging. We also present images of a polymer deposited on gold.

Design of a scanning tunneling microscope for electrochemical applications

A design for a scanning tunneling microscope that is well suited for electrochemical investigations is presented. The construction of the microscope ensures that only the tunneling tip and the sample participate in electrochemical reactions. The design also allows rapid replacement of the tip or sample, and enables facile introduction of auxiliary electrodes for use in electrochemical experiments. The microscope utilizes stepper motor driven approach mechanics in order to achieve fully remote operation and to allow reproducible coarse control of tip/sample spacings for electrochemical experiments. Highly ordered pyrolytic graphite images at atomic resolution in air and aqueous solutions can be obtained with this microscope.

Low‐temperature ultrahigh‐vacuum scanning tunneling microscope

We describe a multipurpose scanning tunneling microscope designed to operate in ultrahigh vacuum as well as in air, over a range extending from room temperature to liquid‐nitrogen temperatures. It is a single‐tube scanner design with a differential flexing approach mechanism mounted on a vibration isolation stack. The instrument features a novel in situ tip and sample exchange mechanism for extended operation under vacuum. A unique characteristic is that the vacuum chamber and all components with the exception of the gas‐cooled sample holder are at room temperature. We present preliminary data taken with this instrument, demonstrating atomic resolution constant current, constant height, and multiple‐bias imaging, gap‐modulated current‐voltage spectroscopy or simultaneous topography, and work function measurements, as well as lithography on the surfaces of graphite, Au(111) on mica, and GaAs(110).

Observation and manipulation of polymers by scanning tunnelling and atomic force microscopy

The properties of monolayer films of organic materials are important for a variety of technologies. We have employed STM and AFM to study Langmuir‐Blodgett films of a variety of polymers on substrates of graphite, MoS2, and Au(111) on mica. The polymers were poly(octadecyl acrylate) (PODA), atactic and syndiotactic poly(methyl methacrylate) (PMMA) and poly(2‐methyl‐1‐pentene sulphone) (PMPS). One striking feature was the degree of order observed; a second was the morphological difference between films of submonolayer thickness (long, thin fibrils) and those of at least monolayer thickness (lumpy structures arranged in domains). By pulsing the STM bias voltage to values in excess of 4V, we were able to bring about local modification of the polymer morphology.

Atomic resolution imaging of electrode surfaces in solutions containing reversible redox species

Procedures are described for insulating metal scanning tunneling microscope (STM) tips with either glass or polymer coatings. In solutions containing 0.10 M of a reversible redox couple, Fe(CN)−3/−46, the faradaic limiting current to polymer coated tips was 200–500 pA and that for glass coated tips was <10 pA. For polymer insulated tips, steady‐state currents of 10–100 pA were observed at tip‐sample displacements less than 0.3 μm. The suppression of faradaic current achieved by these coating procedures enabled the collection of the first atomic resolution STM images of highly ordered pyrolytic graphite electrodes in contact with redox‐active electrolytes. Preliminary data for the in situ electrochemical characterization of these tips are also discussed.