Richard Sonnenfeld

Atomic-resolution microscopy in water.

The scanning tunneling microscope is revolutionizing the study of surfaces. In ultra-high vacuum it is capable not only of imaging individual atoms but also of determining energy states on an atom-by-atom basis. It is now possible to operate this instrument in water. Aqueous optical microscopy is confined to a lateral resolution limit of about 2000 angstroms, and aqueous x-ray microscopy has yielded a lateral resolution of 75 angstroms. With a scanning tunneling microscope, an image of a graphite surface immersed in deionized water was obtained with features less than 3 angstroms apart clearly resolved. Further, an image measured in saline solution demonstrated that the instrument can be operated under conditions useful for many biological samples.

Tunneling microscopy, lithography, and surface diffusion on an easily prepared, atomically flat gold surface

We show that a gold surface with atomically flat terraces as large as (150 nm)2 can be easily prepared in air by melting a gold wire with an oxyacetylene torch. Features with characteristic dimensions as low as 10 nm can be written and observed on these terraces with a scanning tunneling microscope. The features are appreciably distorted by diffusion within an hour.

Tunneling microscopy in an electrochemical cell: Images of Ag plating

A scanning tunneling microscope (STM) operating in an electrochemical cell was used to observe Ag films that had been electrodeposited on a graphite substrate. Before Ag deposition and after stripping the deposited Ag, the atomic features of the graphite substrate were observed. The observed STM images are consistent with an island film growth mechanism. This initial study demonstrates that STM studies of electrochemical processes occurring at electrodes in solution are now possible.

Tunneling microscope for operation in air or fluids

A tunneling microscope that is a hybrid between IBM Zurich designs and squeezable tunnel junctions has been operated in air, oil, and liquid nitrogen. Key design goals were (1) maximum rigidity and (2) minimum thermal drift. Images of individual atoms in a close packed layer have been obtained under liquid nitrogen.

Semiconductor topography in aqueous environments: Tunneling microscopy of chemomechanically polished (001) GaAs

Scanning tunneling microscopy (STM) of (001) GaAs samples immersed in aqueous solutions has been used to assess the effectiveness of a standard bromine‐methanol chemomechanical polish to produce flat surfaces over length scales from 5 to 1000 nm. The STM images reveal irregular 100‐nm features coexisting with large areas of average roughness of the order of a few nanometers. The precision, stability, and reproducibility of these images suggest that immersion STM could be used to study surface chemical processes in real time.

Squeezable electron tunnelling junction

A mechanically adjustable tunnelling junction includes two electrodes defining a gap supported on substrates. Spacers maintain the electrodes in spaced apart relation. At least one of substrates is mechanically deformable, whereby the application of an external force to the substrates decreases the gap to the range where tunnelling will occur.

Contactless tunneling to semiconductors

We present current versus voltage characteristics for two metal‐insulator‐semiconductor systems: lead‐insulator‐mercury cadmium telluride and silver‐insulator‐silicon. A previously developed new technique allows us to suspend the metal <20 A above the semiconductor so that the insulator is simply the space between them. The Si studies at room temperature show features suggestive of electron tunneling between the metal and semiconductor, while the HgCdTe studies in liquid helium conclusively show the Pb superconducting energy gap, demonstrating that contactless tunneling studies of semiconductor surfaces are possible.

Scanning tunneling microscopy and atomic force microscopy of the liquid–solid interface

The liquid–solid interface is important not only for science, but also for technology. Scanning tunnel microscopes (STM’s) and atomic force microscopes (AFM’s) can image and even manipulate solids covered with liquids. An image of a line 75 nm long and 5 nm wide drawn with an STM on a liquid‐covered Au(111) surface demonstrates the potential for manipulating surfaces. Images of a Pt film demonstrate the ability of STM’s to find new features by zooming from large‐area scans down to the atomic scale. Finally, an AFM image of a liquid‐covered graphite surface demonstrates atomic resolution.

On the relationship between continuing current and positive leader growth

It has long been speculated that the source of continuing current (CC) for a negative cloud-to-ground flash is provided by the growth of its positive leader into negative charge regions. In this study, data from the Langmuir Electric Field Array (LEFA) and Lightning Mapping Array (LMA) are used to investigate these speculations. LEFA and LMA data provide a way to estimate the occurrence and duration of CC and channel growth throughout a flash, respectively. By connecting LMA VHF sources onto contiguous channels, the growth of the positive leader associated with each return stroke is inferred. A linear correlation between positive-channel growth and CC duration is found, providing evidence that the positive leader grows with a constant velocity, but no obvious correlation of this velocity with CC occurrence is found. Each return stroke is then sorted by its channel growth rate and further identified by its CC type. This analysis also provides no identifiable correlation linking the positive-channel growth rate to CC occurrence or duration. Finally, the positive-channel growth rate for the whole flash is calculated in 10 ms windows so that any trends occurring before, during, or after the CC can be observed. This analysis too shows no correlation, which implies that positive-channel growth is not the primary mechanism that determines CC occurrence and duration.

Electric Field Reversal in Sprite Electric Field Signature

AbstractIn measurements of the electric field associated with the current of a sprite 450 km from ground-based field sensors, it was observed that the sign of the electric field was positive when positive charge was lowered from the ionosphere. A recent model for the electric field associated with the sprite current also predicts positive field changes at 450 km from the sprite. A well-known analysis of a vertical dipole in a thundercloud shows that the electric field on the ground reverses its sign at an easily computed distance from the dipole. A similar simplified electrostatic analysis of a sprite predicts a field reversal distance around 130 km. A more accurate electrodynamic analysis based on Maxwell’s equations indicates that the field reversal distance should be between 70 and 80 km.