Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.

https://science.sciencemag.org/content/sci/248/4954/454.full.pdf

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

A scanning tunnelling microscope can image surfaces down to the atomic scale. Its very high resolution is caused by its local probing mechanism: a tunnel current which flows through the outermost atom of a scanning tip to the sample. Its invention, about ten years ago, was based on an ingenious blend of quantum physics, mechanical design and electronic control. Since then, it has proved to be a very versatile instrument. Its applications range from the characterization of surface roughness to atomic reconstructions, and from doped semiconductors to cell membranes. It can operate in ultra high vacuum, in air, in reactive gases, in corrosive solutions or at cryogenic temperatures. This paper reviews the evolution of scanning tunnelling microscopy in the first decade after its invention. Attention is focused on the basis of STM: tunnelling theory, mechanical design and modes of operation. Representative examples of applications in various fields of research are discussed.

http://idwebhost-202-73.ethz.ch/praktika/analytisch/stm/Rep%20Prog%20phys%2055,%20p1165.pdf

Scanning tunnelling microscopy

We have investigated methods for cleaning dc-etched polycrystalline tungsten tips for scanning tunnelling microscopy (STM). The cleaning methods include Ar-ion sputtering, heating, chemical treatments and Ne-ion self-sputtering. We correlate transmission electron microscopy images of the tip, field-emission data from the tip and images of a clean Cu(111) surface to find an optimum procedure for STM imaging. Clean and sharp tips are made by sputtering, combined with careful heating by electron bombardment. We found that optimum sputtering was obtained either by use of a 4 keV Ar-ion gun for a few seconds or by self-sputtering with Ne ions for a few seconds or until decapitation occurs.

/pdf/preparation-and-characterization-of-electrochemically-etched-109kkzznti.pdf

Preparation and characterization of electrochemically etched W tips for STM

Atomistic mechanisms of adhesive contact formation and interfacial processes

This review article describes progress made in scanning force microscopy of polymers during the last 5 years including fundamental principles of SFM and recent developments in instrumentation relevant to polymer systems. It focuses on the analytical capabilities of SFM techniques in areas of research where they give the most unique and valuable information not accessible by other methods. These include (i) quantitative characterisation of material properties and structure manipulation on the nanometer scale, and (ii) visualisation and probing of single macromolecules.

Imaging of Polymers Using Scanning Force Microscopy: From Superstructures to Individual Molecules

We have been successful in obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current–voltage (I–V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. Our observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.

/pdf/imaging-graphite-in-air-by-scanning-tunneling-microscopy-4vl1istwkj.pdf

Imaging graphite in air by scanning tunneling microscopy: Role of the tip

We report a new tip for scanning tunneling microscopy and a tip repair procedure that allows one to reproducibly obtain atomic images of highly oriented pyrolytic graphite with previously inoperable tips. The tips are shown to be relatively oxide‐free and highly resistant to oxidation. The tips are fabricated with graphite by two distinct methods.

/pdf/oxide-free-tip-for-scanning-tunneling-microscopy-22q9li8ofs.pdf

W. J. Kaiser

Papers

Imaging graphite in air by scanning tunneling microscopy: Role of the tip

‘‘Oxide‐free’’ tip for scanning tunneling microscopy