Nancy A. Burnham

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

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.

Measuring the nanomechanical properties and surface forces of materials using an atomic force microscope

An atomic force microscope (AFM) has been configured so that it measures the force between a tip mounted on a cantilever beam and a sample surface as a function of the tip–surface separation. This allows the AFM to study both the nanomechanical properties of the sample and the forces associated with the tip–surface interaction. More specifically, the AFM can measure the elastic and plastic behavior and hardness via nanoindentation, van der Waals forces, and the adhesion of thin‐film and bulk materials with unprecedented force and spatial resolution. The force resolution is currently 1 nanonewton, and the depth resolution is 0.02 nm. Additionally, the instrument itself is compact and relatively inexpensive.

Elastic modulus of ordered and disordered multiwalled carbon nanotubes

Reference LNNME-ARTICLE-1999-005doi:10.1002/(SICI)1521-4095(199902)11:2 3.0.CO;2-JView record in Web of Science Record created on 2007-04-23, modified on 2016-08-08

Comparison of calibration methods for atomic-force microscopy cantilevers

Th es cientific community needs a rapid and reliable way of accurately determining the stiffness of atomic-force microscopy cantilevers. We have compared the experimentally determined values of stiffness for ten cantilever probes using four different methods. For rectangular silicon cantilever beams of well defined geometry, the approaches all yield values within 17% of the manufacturer’s nominal stiffness. One of the methods is new, based on the acquisition and analysis of thermal distribution functions of the oscillator’s amplitude fluctuations. We evaluate this method in comparison to the three others and recommend it for its ease of use and broad applicability.

On the electrochemical etching of tips for scanning tunneling microscopy

The sharpness of tips used in scanning tunneling microscopy (STM) is one factor which affects the resolution of the STM image. In this paper, we report on a direct‐current (dc) drop‐off electrochemical etching procedure used to sharpen tips for STM. The shape of the tip is dependent on the meniscus which surrounds the wire at the air–electrolyte interface. The sharpness of the tip is related to the tensile strength of the wire and how quickly the electrochemical reaction can be stopped once the wire breaks. We have found that the cutoff time of the etch circuit has a significant effect on the radius of curvature and cone angle of the etched tip; i.e., the faster the cutoff time, the sharper the tip. We have constructed an etching circuit with a minimum cut‐off time of 500 ns which uses two fast metal–oxide semiconductor field effect transistors (MOSFET) and a high‐speed comparator. The radius of curvature of the tips can be varied from approximately 20 to greater than 300 nm by increasing the cutoff time ...

Interpretation issues in force microscopy

In this paper, we will discuss force microscopy (FM) and its potential for determining mechanical properties of thin films. We will introduce the basic principles of FM, and demonstrate how FM can be used to determine materials properties as well as image surface topography, both with nanonewton or sub‐nanonewton force resolution and sub‐nanometer position resolution. As FM is still a new field, not all of the questions concerning interpretation have been fully answered. We will elucidate four current issues that must be resolved before the full potential of FM can be realized. They are: (1) the role of water vapor and adsorbed films in imaging and force curve measurements, (2) the interpretation of force curves, (3) the influence of surface forces and loads on imaging, and (4) the nature of the imaging mechanisms.

Scanning local‐acceleration microscopy

By adapting a scanning force microscope to operate at frequencies above the highest tip–sample resonance, the sensitivity of the microscope to materials’ properties is greatly enhanced. The cantilever’s behavior in response to high‐frequency excitation from a transducer underneath the sample is fundamentally different than to its low‐frequency response. In this article, the motivations, instrumentation, theory, and first results for this technique are described.

Precision and Accuracy of Thermal Calibration of Atomic Force Microscopy Cantilevers

To have confidence in force measurements made with atomic force microscopes (AFMs), the spring constant of the AFM cantilevers should be known with good precision and accuracy, a topic not yet thoroughly treated in the literature. In this study, we compared the stiffnesses of uncoated tipless uniform rectangular silicon cantilevers among thermal, loading, and geometric calibration methods; loading was done against an artifact from the National Institute of Standards and Technology (NIST). The artifact was calibrated at NIST using forces that were traceable to the International System of units. The precision and accuracy of the thermal method were found to be 5% and 10%, respectively. Force measurements taken with different cantilevers can now be meaningfully compared.

Materials' properties measurements: Choosing the optimal scanning probe microscope configuration

Rheological models are used to represent different scanning probe microscope configurations. The solutions for their static and dynamic behavior are found and used to analyze which scanning probe microscope configuration is best for a given application. We find that modulating the sample at high frequencies results in the best microscope behavior for measuring the stiffness of rigid materials, and that by modulating the tip at low frequencies and detecting the motion of the tip itself (not its position relative to the tip holder) should be best for studying compliant materials in liquids.

Atomic force microscopy study of the role of LPS O-antigen on adhesion of E. coli

The O‐antigen is a highly variable component of the lipopolysaccharide (LPS) among Escherichia coli strains and is useful for strain identification and assessing virulence. While the O‐antigen has been chemically well characterized in terms of sugar composition, physical properties such as O‐antigen length of E. coli LPS have not been well studied, even though LPS length is important for determining binding of bacteria to biomolecules and epithelial cells. Atomic force microscopy (AFM) was used to characterize the physicochemical properties of the LPS of eight E. coli strains. Steric repulsion between the AFM tip (silicon nitride) and the E. coli cells was measured and modeled, to determine LPS lengths for three O157 and two O113 E. coli strains, and three control (K12) strains that do not express the O‐antigen. For strains with an O‐antigen, the LPS lengths ranged from 17 ± 10 to 37 ± 9 nm, and LPS length was positively correlated with the force of adhesion (Fadh). Longer lengths of LPS may have allowed for more hydrogen bonding between the O‐antigen and silanol groups of the AFM silicon nitride tip, which controlled the magnitude of Fadh. For control strains, LPS lengths ranged from 3 ± 2 to 5 ± 3 nm, and there was no relationship between LPS length and adhesion force between the bacterium and the silicon nitride tip. In the absence of the O‐antigen, we attributed Fadh to electrostatic interactions with lipids in the bacterial membrane. Copyright