Wide dynamic range 2-D nanoindentation: Friction and partial slip at contacts

A new nanomechanical testing system is described. It provides the same force controlled displacement sensing capability as nanoindentation, but now with two completely separated orthogonal axes. Load modulation enables direct determination of contact area and stiffness, both lateral and vertical, along with energy losses from the phase shifts. Two features in particular, wide dynamic ranges of several orders of magnitude of stiffness and a very high degree of mechanical separation (low crosstalk) between the axes, distinguish the technique from AFM. AFM is one of the few techniques to date to investigate tribological single asperity contacts but its mechanical limitations make it difficult to discern the underlying mechanisms. With this new technique, the evolution of a contact under 2-D stresses from deformation-free atomistic scale to initial plasticity along with the associated changes in geometry, can be monitored. Results will be presented showing that unlike in elastic contacts, Mindlin partial slip does not occur immediately under lateral stress in plastically deformed contacts. The evolution of contact area in the initial stages of sliding in the presence of plastic flow will be described, and resembles the predictions of classical Tabor and Johnson models. It will be shown that energy dissipation measured from phase shift of a modulating signal is largely due to interfacial friction rather than volumetric deformation. Prospects for further studies using both shear and normal loading will be discussed

Densification of polymer glass film under combined high pressure and shear flow revealed via scanning X-ray microscopy

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Direct measurement of molecular stiffness and damping in confined water layers

We present {\em direct} and {\em 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 $\textrm{\AA}$), off-resonance Atomic Force Microscopy (AFM) technique. We measured stiffness and damping oscillations revealing up to 7 layers separated by 2.56 $\pm$ 0.20 $\textrm{\AA}$. 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. Thus ` solidification\rq seems to not be merely a result of pressure and confinement, but depends strongly on how commensurate the confining cavity is with the molecule size. We were able to model the results by starting from the simple assumption that the relaxation time depends linearly on the film stiffness.Comment: 7 pages, 6 figures, will be submitted to PR

Quantitative STM imaging of metal surfaces

Many deductions made about STM images are based upon the model of Tersoff and Hamann, in which images are given in principal by a combination of surface atomic positions and local charge density. There is a now a need for a fuller understanding of this technique in order to explain experimental evidence which indicates that the tip and sample can interact strongly during normal imaging. In order to investigate the fundamental STM imaging process, a method for deducing the tunnel barrier height has been developed which is based on corrugation height measurements of constant current topographs. From experiments on clean Cu(100), values of the tunnel barrier height have been shown to be somewhat below the workfunction (~ 1-2.5eV) but are in good agreement with other reports of atomically resolved barrier height data. At large values of the tunnel conductance (~ 1μS), a fall-off (based upon extrapolation of large separation data) in the corrugation heights is observed with increasing conductance. This effect is quantitatively explained using a Molecular Dynamics simulation of the tip approaching the sample. The simulation gives a good estimate of both the absolute tip-sample separation and site-dependent tip-surface forces. Distributions of corrugation heights indicate that variations in both tip geometry and chemistry are likely to occur in practice and strongly influence the phenomena described above. Similarly, it is found that increased local tunnel barrier heights are measured when the Cu(100) surface is modified with small numbers of single halogen atoms. This data has been used to estimate the contributions to the increase in local barrier height of both adsorbate induced dipoles and geometric topography. Values for the charge transfer between the surface and adsorbate have been established. The process of tip-induced adsorbate manipulation has also been demonstrated at room temperature.</p

Quantitative electrostatic force measurement in AFM

We describe a method for measuring forces in the atomic force microscope (AFM), in which a small amplitude oscillation(similar to 1 Angstrom(p-p)) is applied to a stiff(similar to 40 N/m) cantilever below its first resonant frequency, and the force gradient is measured directly as a function of separation. We have used this instrument to measure electrostatic forces by applying an ac voltage between the tip and the sample, and observed a variation in contact potential difference with separation. We also show how the benefits of this instrument may be exploited to make meaningful capacitance measurements, especially at small tip-surface separations, and demonstrate the potential of this technique for quantitative dopant profiling in semiconductors

Silicon surfaces

The fundamental atomic and electronic behaviour of clean silicon surfaces has been studied within a simple tight-binding picture of bonding in solids. Of the various contributions to the surface binding energy, the lowering in the promotion energy (i.e. rehybridization) which accompanies localized Jahn-Teller distortions has been identified as a major electronic driving force underlying the stability of silicon surfaces.The structure of Si(113) has been experimentally determined by the technique of scanning tunnelling microscopy (STM). Despite its high index, the Si(113) surface is found to be highly stable. STM images of both empty and filled states provide strong evidence for a particular structural model with a 3x2 unit cell. The STM results are explained in terms of a general rehybridization principle, suggested by the earlier theoretical study, which accounts for the low surface energy as well as the observed spatial distribution of empty and filled states. In addition, the STM images reveal a high density of domain boundaries which introduce energy states that pin the Fermi level and explain earlier reports of a 3x1 reconstruction for this surface.Voltage-dependent STM image simulations for the Si(113)3x2 surface have been carried out using a simple tight-binding description of surface electronic structure. Quantitative agreement with experiment is obtained confirming the qualitative rehybridization arguments used previously. The local barrier for tunnelling electrons is shown to have an important effect on the interpretation of STM images.The high stability of clean Si(l 13) is shown by STM to be disrupted by adsorption of submonolayer amounts of atomic hydrogen which saturates dangling bonds. Mass transport of silicon occurs and structural models are proposed for the resultant mixed 2x2 and 2x3 surface.</p