Probe Tip Radius Research Articles

Micro-surface and bulk thermal behavior of a single-grain decagonal Al–Ni–Co quasicrystal

Abstract Scanning thermal microscopy is used to examine the thermal behavior of specific surfaces of a ∼0.8 cm3 single-grain Al74Ni10Co16 decagonal quasicrystal. The response of a micro-thermal probe in contact with aperiodic and periodic surfaces in air reveals the anisotropic heat flow of the decagonal structure. Heat dissipation with the probe on the aperiodic surface is higher than when the probe is on the periodic surface. While the SThM technique is generally considered to be constrained to the surface region below the nominally 2–5 μm probe tip radius, the heat flow data obtained are clearly comprised of contributions from both the lateral surface around the probe tip and a volume normal to the surface in contact. Heat flow in the decagonal Al74Ni10Co16 quasicrystal can be modeled by an elliptical distribution of thermal diffusion. Parameters for the model used in this study were obtained by making bulk thermal diffusivity measurements using the laser flash method on specimens along the 2- and 10-fold directions. The model was applied to a surface oriented 45° to the major axes and verified from bulk measurements obtained from a sample cut along this orientation.

Contact mechanics modeling of pull-off measurements: effect of solvent, probe radius, and chemical binding probability on the detection of single-bond rupture forces by atomic force microscopy.

Pull-off forces for chemically modified atomic force microscopy tips in contact with flat substrates coated with receptor molecules are calculated using a Johnson, Kendall, and Roberts contact mechanics model. The expression for the work of adhesion is modified to account for the formation of discrete numbers of chemical bonds (nBonds) between the tip and substrate. The model predicts that the pull-off force scales as nBonds(1/2), which differs from a common assumption that the pull-off force scales linearly with nBonds. Periodic peak progressions are observed in histograms generated from hundreds of computed pull-off forces. The histogram periodicity is the signature of discrete chemical interactions between the tip and substrate and allows estimation of single-bond rupture forces. The effects of solvent, probe tip radius, and chemical binding probability on the detection of single-bond forces are examined systematically. A dimensionless parameter, the effective force resolution, is introduced that serves as a quantitative predictor for determining when periodicity in force histograms can occur. The output of model is compared to recent experimental results involving tips and substrates modified with self-assembled monolayers. An advantage of this contact mechanics approach is that it allows straightforward estimation of solvent effects on pull-off forces.

Dislocation nucleation at nano-scale mechanical contacts

The plastic deformation of metals in highly localized stress fields is critical to macroscopic phenomena such as fracture, surface wear and adhesion where crack tips and surface asperities concentrate the applied stress. Controlled-probe contact techniques can be used to measure forces and displacements on the atomic scale, thereby directly exploring elastic and plastic deformation of solids at the nanometer level. In this study we use interfacial force microscopy to quantitatively examine the nano-scale elastic and plastic response of single crystal Au(111) surfaces using controlled-probe contacts with a range of radii extending down to 500 Å. Our results show that the stress at the plastic threshold increases with decreasing probe-tip radius. We apply a model for dislocation nucleation which reproduces the scaling behavior with tip radius and discuss deficiencies in the model and plausible reasons for the underestimation in quantitative stress values.

Model database for determining dopant profiles from scanning capacitance microscope measurements

To help correlate scanning capacitance microscope measurements of silicon with uniformly doped concentrations, model capacitance curves are calculated and stored in a database that depends on the probe-tip radius of curvature, the oxide thickness, and the dopant density. The oxide thicknesses range from 5 to 20 nm, the dopant concentrations range from 1017 to 1020 cm−3, and the probe-tip radius of curvature is set to 10 nm. The cone-shaped probe is oriented normal to the sample surface, so that the finite-element method in two dimensions may be used to solve Poisson’s equation in the semiconductor region and Laplace’s equation in the oxide and ambient regions. The equations are solved within the semi-classical quasistatic approximation, where capacitance measurement depends only on the charge due to majority carriers, with inversion and charge trapping effects being ignored. Comparison with one-dimensional-related models differs as much as 200% over the given doping range. For shallow gradient profiles satisfying quasiuniformity conditions, the database is used directly to find the doping profile. Converting a 512×512 point image takes less than 2 min.

Surface defect formation on gypsum surfaces due to grazing incidence 78.2 MeV 127I ions: A comparative study employing tapping-mode scanning force microscopy and shadow-replica electron microscopy

We have performed, for the first time, comparative studies of surface tracks employing both tapping-mode scanning force microscopy (TM-SFM) and shadow-replica electron microscopy (SR-EM). The surface tracks, with dimensions of the order of 10 to 100 nm, were induced on gypsum by 78.2 MeV 127I ions impacting at surface-grazing angles of incidence. Using TM-SFM, finite probe-tip radius of curvature limited the available spatial resolution, and under some conditions, apparent deformation of the surface defects was observed, indicating that TM-SFM can be sensitive to surface properties besides topography. Rim-to-rim crater widths (perpendicular to the ion incidence azimuth) and overall lengths (parallel to the ion incidence azimuth) were measured equivalently by TM-SFM and SR-EM. However, the defect heights were somewhat overestimated by TM-SFM relative to SR-EM. Each surface track caused by an incident swift, heavy atomic ion is considered to represent the material response to an explosive vaporization process which cracks open the surface.

Scattering of evanescent light by a finite-size probe in near-field scanning optical microscopy

Scattering of the evanescent light by a finite-size SiO2 probe is calculated using a point matching method in order to improve the efficiency collecting the near-field optical signal. The scattered-light patterns can be classified into three categories. Category 1 is where the probe tip radius is very small (<λ/13). Category 3 is where the probe radius is very large (≳λ/6). Category 2 is the intermediate case (λ/13<radius<λ/6). When the radius is in category 1, the scattered light is able to be treated as a field radiated by the point dipole induced by the evanescent light at the probe tip. When the tip radius is in category 2 (λ/13<radius<λ/6), in order to obtain the angular distribution of the scattered light, the induced multipoles should be calculated, considering the probe radius and the incident angle. When the tip radius is in category 3 (radius ≳λ/6), the scattered light becomes larger in the parallel direction to a prism surface due to the forward scattering of the evanescent light.