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Now showing 1 - 10 of 305
  • Publication
    Imaging and manipulation of nanometer-size liquid droplets by scanning polarization force microscopy
    (1996-03-01) Hu, Jun; Carpick, Robert W; Salmeron, Miquel; Xiao, Xu-dong
    Using atomic force microscopy in noncontact mode, we have imaged nanometer-size liquid droplets of KOH water solutions on the surfaces of highly oriented pyrolitic graphite and mica. On graphite the droplets prefer to be adsorbed on atomic step edges. Droplets on the same step tend to be evenly spaced and of similar size. The droplets can be manipulated by the atomic force microscopy tip allowing the controllable formation of droplet patterns on the surface.
  • Publication
    A variable temperature ultrahigh vacuum atomic force microscope
    (1995-11-01) Dai, Q.; Vollmer, R.; Carpick, Robert W; Ogletree, D. F; Salmeron, Miguel
    A new atomic force microscope (AFM) that operates in ultrahigh vacuum (UHV) is described. The sample is held fixed with spring clamps while the AMF cantilever and deflection sensor are scanned above it. Thus, the sample is easily coupled to a liquid nitrogen cooled thermal reservoir which allows AFM operation from ≈ 100 K to room temperature. AFM operation above room temperature is also possible. The microscope head is capable of coarse x-y positioning over millimeter distances so that AFM images can be taken virtually anywhere upon a macroscopic sample. The optical beam deflection scheme is used for detection, allowing simultaneous normal and lateral force measurements. The sample can be transferred from the AFM stage to a low energy electron diffraction/Auger electron spectrometer stage for surface analysis. Atomic lattice resolution AFM images taken in UHV are presented at 110, 296, and 430 K.
  • Publication
    Calibration of frictional forces in atomic force microscopy
    (1996-06-07) Ogletree, D. F; Carpick, Robert W; Salmeron, Miguel
    The atomic force microscope can provide information on the atomic-level frictional properties of surfaces, but reproducible quantitative measurements are difficult to obtain. Parameters that are either unknown or difficult to precisely measure include the normal and lateral cantilever force constants (particularly with microfabricated cantilevers), the tip height, the deflection sensor response, and the tip structure and composition at the tip-surface contact. We present an in situ experimental procedure to determine the response of a cantilever to lateral forces in terms of its normal force response. This procedure is quite general. It will work with any type of deflection sensor and does not require the knowledge or direct measurement of the lever dimensions or the tip height. In addition, the shape of the tip apex can be determined. We also discuss a number of specific issues related to force and friction measurements using optical lever deflection sensing. We present experimental results on the lateral force response of commercially available V-shaped cantilevers. Our results are consistent with estimates of lever mechanical properties using continuum elasticity theory.
  • Publication
    Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope
    (1996-03-01) Carpick, Robert W; Agraït, N.; Ogletree, D. F; Salmeron, Miguel
    We have studied the variation of frictional force with externally applied load for a Pt-coated atomic force microscope tip in contact with the surface of mica cleaved in ultrahigh vacuum. At low loads, the frictional force varies with load in almost exact proportion to the area of contact as predicted by the Johnson-Kendall-Roberts (JKR) theory [K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Sec. London Ser. A 324, 301 (1971)] of elastic adhesive contacts. The friction-load relation for a deliberately modified tip shape was proportional to an extended JKR model that predicts the area-load relation for nonparabolic tips, The tip shape was determined experimentally with a tip imaging technique and was consistent with the predicted friction behavior. This demonstrates that the frictional force is proportional to the area of contact between the tip and sample. Using the JKR/extended JKR model, interfacial surface energies and shear strengths can be estimated.
  • Publication
    Friction and Molecular Deformation in the Tensile Regime
    (1999-02-01) Burns, A. R; Carpick, Robert W; Michalske, T. A
    Recent molecular level studies of energy dissipation in sliding friction have suggested a contribution from adhesive forces. In order to observe this directly, we have constructed a scanning force microscope with decoupled lateral and normal force sensors to simultaneously observe the onset of both friction and attractive forces. Measurements made on self-assembling alkanethiol films with chemically different tail groups show that friction can increase with stronger adhesive intermolecular forces and from the associated tensile deformation and collective motion of the thiol chains.
  • Publication
    Lateral stiffness: A new nanomechanical measurement for the determination of shear strengths with friction force microscopy
    (1997-03-24) Carpick, Robert W; Ogletree, D. F; Salmeron, Miguel
    We present a technique to measure the lateral stiffness of the nanometer-sized contact formed between a friction force microscope tip and a sample surface. Since the lateral stiffness of an elastic contact is proportional to the contact radius, this measurement can be used to study the relationship between friction, load, and contact area. As an example, we measure the lateral stiffness of the contact between a silicon nitride tip and muscovite mica in a humid atmosphere (55% relative humidity) as a function of load. Comparison with friction measurements confirms that friction is proportional to contact area and allows determination of the shear strength.
  • Publication
    Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface
    (1998-08-01) Enachescu, M.; van den Oetelaar, R.J. A; Carpick, Robert W; Ogletree, D. F; Flipse, C. F. J; Salmeron, Miguel
    A comprehensive nanotribological study of a hydrogen-terminated diamond(111)/tungsten carbide interface has been performed using ultrahigh vacuum atomic force microscopy. Both contact conductance, which is proportional to contact area, and friction have been measured as a function of applied load. We demonstrate for the first time that the load dependence of the contact area in UHV for this extremely hard single asperity contact is described by the Derjaguin-Müller-Toporov continuum mechanics model. Furthermore, the frictional force is found to be directly proportional to the contact area.
  • Publication
    Cantilever tilt compensation for variable-load atomic force microscopy
    (2005-04-20) Canara, Rachel J; Brukman, Matthew J; Carpick, Robert W
    In atomic force microscopy (AFM), typically the cantilever's long axis forms an angle with respect to the plane of the sample's surface. This has consequences for contact mode experiments because the tip end of the cantilever, which is constrained to move along the surface, displaces longitudinally when the applied load varies. As a result, the AFM tip makes contact with a different point on the surface at each load. These different positions lie along the projection of the lever's long axis onto the surface. When not constrained by static friction, the amount of tip-displacement is, to first order, proportional to the load and is shown to be substantial for typical AFM and cantilever geometries. The predictions are confirmed experimentally to within 15% or better. Thus, care should be taken when performing load-dependent contact mode experiments, such as friction versus load, elasticity versus load, or force versus displacement measurements, particularly for heterogeneous or topographically-varying samples. We present a simple method to reliably and precisely compensate for in-plane tip displacement that depends only on the range of vertical motion used to vary the load. This compensation method should be employed in any load-varying AFM experiment that requires the tip to scan the same line or to remain at the same point at each load. ©2005 American Institute of Physics
  • Publication
    Material Anisotropy Revealed by Phase Contrast in Intermittent Contact Atomic Force Microscopy
    (2002-05-17) Marcus, Matthew S; Carpick, Robert W; Sasaki, Darryl Y; Eriksson, Mark A
    Phase contrast in intermittent-contact atomic force microscopy (AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This is surprising, because measurements of nanoscale in-plane properties typically require contact mode microscopies. Our measurements are possible because the tip oscillates not just perpendicular but also parallel to the sample surface along the long axis of the cantilever. This lateral tip displacement is virtually universal in AFM, implying that any oscillating-tip AFM technique is sensitive to in-plane material properties.
  • Publication
    Directional shear force microscopy
    (2001-01-15) Burns, A. R; Carpick, Robert W
    We describe a technique, based on shear force microscopy, that allows one to detect shear forces in a chosen direction at the nanometer scale. The lateral direction of an oscillating probe tip is determined by selecting which of the four quadrants are excited on the piezo driver. The shear forces depend directly on this lateral direction if structural anisotropies are present, as confirmed with polydiacetylene monolayers. (C) 2001 American Institute of Physics.