Browsing by Author "Hoffmann, P. M."
Now showing 1 - 8 of 8
Results Per Page
Sort Options
Item Open Access Direct measurement of interatomic force gradients using an ultra-low-amplitude atomic force microscope(The Royal Society Publishing, 2001) Hoffmann, P. M.; Oral, A.; Grimble, R. A.; Özer, H. Ö.; Jeffery, S.; Pethica, J. B.Interatomic force gradients between a W tip and a 7 × 7 reconstructed Si(111) surface were measured using an off-resonance, ultra-low-amplitude atomic force microscope (AFM) technique. The amplitudes used were less than 1 Å (peak-to-peak), which allowed direct measurement of the interaction force gradients as a function of separation. The force gradient curves are shown to consist of an attractive van der Waals part and short-range attractive and repulsive interactions. The van der Waals background can be subtracted, leaving a short-range interaction with an energy parameter of 1.9-3.4 eV and an interaction length-scale of 0.54-1.26 Å, characteristic of a single atomic bond. This correlates well with our observation of single-atom resolved force gradient images. In general, the interaction is reversible up to the zero intercept of the force gradient (inflection point of the energy). Beyond this point hysteresis tends to be observed and the onset of inelastic deformation can be clearly discerned. An analysis of the atomic scale contact gives reasonable values for the interfacial energy, yield strength, and the energy per atom needed to initiate plastic deformation.Item Open Access Direct measurement of molecular stiffness and damping in confined water layers(The American Physical Society, 2004) Jeffery, S.; Hoffmann, P. M.; Pethica, J. B.; Ramanujan, C.; Özer, H. Ö.; Oral, A.We present direct and linear measurements of the normal stiffness and damping of a confined, few molecule thick water layer. The measurements were obtained by use of a small amplitude (0.36 Å), off-resonance atomic force microscopy technique. We measured stiffness and damping oscillations revealing up to seven molecular layers separated by 2.526 ± 0.482 Å. Relaxation times could also be calculated and were found to indicate a significant slow-down of the dynamics of the system as the confining separation was reduced. We found that the dynamics of the system is determined not only by the interfacial pressure, but more significantly by solvation effects which depend on the exact separation of tip and surface. The dynamic forces reflect the layering of the water molecules close to the mica surface and are enhanced when the tip-surface spacing is equivalent to an integer multiple of the size of the water molecules. We were able to model these results by starting from the simple assumption that the relaxation time depends linearly on the film stiffness.Item Open Access Energy dissipation in atomic force microscopy and atomic loss processes(American Physical Society, 2001) Hoffmann, P. M.; Jeffery, S.; Pethica, J. B.; Özer, H. Ö.; Oral, A.Atomic scale dissipation is of great interest in nanomechanics and atomic manipulation. We present dissipation measurements with a linearized, ultrasmall amplitude atomic force microscope which is capable of measuring dissipation at chosen, fixed separations. We show that the dynamic dissipation in the noncontact regime is of the order of a few 10–100 meV per cycle. This dissipation is likely due to the motion of a bistable atomic defect in the tip-surface region. In the contact regime we observe dc hysteresis associated with nanoscale plasticity. We find the hysteretic energy loss to be 1 order of magnitude higher for a silicon surface than for copper.Item Open Access A highly sensitive atomic force microscope for linear measurements of molecular forces in liquids(American Institute of Physics, 2005) Patil, S.; Matei, G.; Dong, H.; Hoffmann, P. M.; Karaköse, M.; Oral, A.We describe a highly improved atomic force microscope for quantitative nanomechanical measurements in liquids. The main feature of this microscope is a modified fiber interferometer mounted on a five axis inertial slider which provides a deflection sensitivity that is significantly better than conventional laser deflection based systems. The measured low noise floor of 572.0 fmHz provides excellent cantilever amplitude resolution. This allows us to operate the instrument far below resonance at extremely small cantilever amplitudes of less than 1 Å. Thus linear measurements of nanomechanical properties of liquid systems can be performed. In particular, we present measurements of solvation forces in confined octamethylcyclotetrasiloxane and water with amplitudes smaller than the size of the respective molecules. In general, the development of the instrument is important in the context of quantitative nanomechanical measurements in liquid environments.Item Open Access Linear measurements of nanomechanical phenomena using small-amplitude AFM(Materials Research Society, 2004) Hoffmann, P. M.; Patil, S.; Matei, G.; Tanülkü, A.; Grimble, R.; Özer, Ö.; Jeffery, S.; Oral, Ahmet; Pethica, J.Dynamic Atomic Force Microscopy (AFM) is typically performed at amplitudes that are quite large compared to the measured interaction range. This complicates the data interpretation as measurements become highly non-linear. A new dynamic AFM technique in which ultra-small amplitudes are used (as low as 0.15 Angstrom) is able to linearize measurements of nanomechanical phenomena in ultra-high vacuum (UHV) and in liquids. Using this new technique we have measured single atom bonding, atomic-scale dissipation and molecular ordering in liquid layers, including water.Item Open Access Low-amplitude, force gradient imaging of Cu(100) surface using tunnel current feedback(Institute of Physics Publishing, 2004) Özer, H. Ö.; Norris, A.; Oral, A.; Hoffmann, P. M.; Pethica, J. B.The large corrugation amplitudes in scanning tunnelling microscope (STM) images of metal surfaces have been commonly attributed to the action of forces between the tip and the sample. We have investigated the Cu(100) surface using a high-resolution non-contact atomic force microscope/scanning tunnelling microscope (nc-AFM/STM) in UHV. Force gradient and STM topography images were acquired simultaneously using constant tunnelling current feedback. Force gradient images showed atomic resolution whereas STM scans exhibited almost no contrast, corresponding to a flat tip trajectory during scans. The corrugation height in force gradient images was found to increase as the set tunnelling current was increased. Force gradient and tunnel current were directly measured as a function of separation, to determine the operating conditions during imaging. The STM operation regime is found to lie between the minimum of the stiffness curve and the start of repulsive force.Item Open Access Nanomechanics using an ultra-small amplitude AFM(Cambridge University Press, 2001) Hoffmann, P. M.; Jeffery, S.; Oral, Ahmet; Grimble, R. A.; Özer, H. Özgür; Pethica, J. B.A new type of AFM is presented which allows for direct measurements of nanomechanical properties in ultra-high vacuum and liquid environments. The AFM is also capable to atomic-scale imaging of force gradients. This is achieved by vibrating a stiff lever at very small amplitudes of less than 1 Å (peak-to-peak) at a sub-resonance amplitude. This linearizes the measurement and makes the interpretation of the data straight-forward. At the atomic scale, interaction force gradients are measured which are consistent with the observation of single atomic bonds. Also, atomic scale damping is observed which rapidly rises with the tip-sample separation. A mechanism is proposed to explain this damping in terms of atomic relaxation in the tip. We also present recent results in water where we were able to measure the mechanical response due to the molecular ordering of water close to an atomically flat surface.Item Open Access Solid or liquid? Solidification of a nanoconfined liquid under nonequilibrium conditions(American Chemical Society, 2006) Patil, S.; Matei, G.; Oral, A.; Hoffmann, P. M.There has been a long-standing debate about the physical state and possible phase transformations of confined liquids. In this report, we show that a model-confined liquid can behave both as a Newtonian liquid with very little change in its dynamics and as a pseudosolid, depending solely on the rate of approach of the confining surfaces. Thus, the confined liquid does not exhibit any confinement-induced solidification in thermodynamic equilibrium. Instead, solidification is induced kinetically when the two confining surfaces are approached with a minimum critical rate. This critical rate is surprisingly slow (on the order of 6 Å/s), explaining the frequent observation of confinement-induced solidification.