general, the sections comprise previously published source material assembled and edited here to support the keynote address delivered at the 2010 Annual Meeting of the Society for Experimental Mechanics. For example, the first section, Au Break Junctions, provides a brief review of Reference [1] where we describe new NIST capabilities developed to study the mechanics and electrical properties of gold nanowires. Gold nanowires apparently neck down under tension to form a single chain of atoms, a physical constriction that leads to the coincidence of quantized force and quantized electrical impedance. It is believed that gold nanowires form in at least three different atomic arrangements. It is not known if all such structures form single-atom chains when stretched to failure and, if so, whether or not the strength of the final bond depends on the initial structure. NIST has been seeking to quantify, to the extent possible, these potential systematic variations. First principles quantum mechanics models along with a new Feedback Stabilized Break Junction (FSBJ) experimental platform have been developed and used at NIST to examine how structure and conductance evolve during elongation, with the ultimate aim to reveal their effect on force at rupture; here, we offer a brief snapshot of this exciting new work. Obviously, the interested reader is encouraged to read the source material for more detail. The intrinsic force associated with the overstretching-induced conformation change of DNA is reported to occur under a tensile force of approximately 65 pN. This force has been measured using both scanning probe microscope (SPM) and optical trapping instruments and has been proposed as both a length and force standard. The quoted force value has known dependencies on environmental factors such as pH, salinity, and temperature. NIST is working to determine how to accurately control these variables to yield reliable data, and we present a small indication of our new single-molecule expertise in the section headed DNA Overstretch. The experimental platforms necessary to investigate these disparate physical phenomena share two things in common. First, both require the accurate measurement of forces at levels at or below a few nanonewtons. Second, because the primary experiments are analogous to macroscale tensile testing, they each require a means of monitoring the elongation of a mechanical specimen with resolution of a few picometers or less. Consequently, Picometer Fiber Interferometry reviews recent developments at NIST regarding the use of fiber interferometry to measure displacements of a nanometer or less. This section provides a highly condensed introduction to the topic; for details the reader should see Reference [2]. Finally, Piconewton Force Calibration summarizes our attempts to improve the calibration of atomic force microscope (AFM) cantilever stiffness and to create known forces at or below the level of a nanonewton. It provides a succinct review of our previous research, and then communicates the key findings from Reference [3] regarding the influence of friction in cantilever-oncantilever calibration, and from References [4-7] regarding our new capabilities to apply electrostatic forces of known magnitudes directly to electrically conducting instrumented indentation and AFM probes. Au Break Junctions We have developed an experimental platform, which we refer to as a feedback-stabilized break junction (FSBJ), to create and deform stable atomic-scale contacts, and have used that platform to probe the phenomenon of quantized electrical conductance in Au nanowires (NWs) and single-atom chains (SACs) [1]. In such a system, electrical conductivity, σ, is known to be quantized in units of G0 = 2e 2/h; that is, σ = nG0 for integer n, with e the charge of the electron and h Plank’s constant [8-10]. The conductivity for the n = 1 state corresponds to a contact resistance of 12.9 kΩ. Quantized conductance has been observed many times, but experimental instabilities typically limit the time a given contact stays in a low-n state to milliseconds [11], and often the presence of quantized states must be inferred from histograms compiled from hundreds or thousands of junction breaks [12]. Because the n = 1 conduction state is believed to occur when there is only one electron conduction channel through the contact [13,14], the ability to maintain that state indefinitely would clearly demonstrate exceptional experimental stability. The NIST FSBJ described in [1] and shown in the photos of Figure 1 is proving to be just such an exceptionally stable experimental platform. By positioning an interferometer cavity directly between an Au surface and probe mount, we have significantly tightened the displacement measurement frame relative to that achieved in prior work. This has allowed us to close a servo loop around the junction separation with long-term, picometer stability, so as to remove thermal drift and low-frequency vibration artifacts and thereby reduce the need for rigorous environmental isolation that is often encountered in these types of experiments. In the near future, the NIST FSBJ will incorporate a stiff elastic force sensor (e.g., an AFM style colloidal probe) coupled with a high resolution fiber interferometer (see Figure 1), so that direct measurements can be made of bond stiffness and breaking force in SACs. 2
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