Mark Reitsma

Quantitative comparison of three calibration techniques for the lateral force microscope

The quantitative use of atomic force microscopes in lateral mode for friction measurements has been limited by uncertainty about reliable calibration techniques. This article describes a comparison of three methods that have been proposed for the calibration of the lateral sensitivity of atomic force microscopes: (a) one based on movement of the photodiode assembly, (b) one based on the slope of the friction-loop while the contacting surfaces are in static contact, and (c) one based on a comparison of the lateral force signal on a surface with changing slopes of known orientation. All three methods gave comparable results thereby confirming their robust nature, and also confirming the validity of atomic force microscope methods for lateral force measurement. However, (b) indicated that for the commercial instrument used here, the lateral signal sensitivity is load dependent. A simple extension to (a) revealed the nature of this dependence: a misalignment of the four-quadrant photodiode detection system wi...

Elasto-plastic and visco-elastic deformations of a polymer sphere measured using colloid probe and scanning electron microscopy

The adhesive interaction energy between a single 27 μm polystyrene sphere and a flat silica surface has been measured, as a function of applied load on the sphere, using an atomic force microscope (AFM). The pull-off force required to remove the sphere from the surface after application of a given load was found to increase as a function of the applied load. These data are indicative of a plastic or elasto-plastic deformation of the sphere. Simple analyses of these data using established elastic/plastic deformation theories indicate that, at the loads used, the system is most probably undergoing an elasto-plastic deformation. Further evidence for some plastic deformation of the sphere was obtained using scanning electron micrographs of the same sphere after an AFM experiment had been completed. Careful analysis of all these data indicated a significant time dependence of these adhesive interactions due to the viscoelastic nature of the polymer bead in question.

Atomic Force Microscope Cantilever Flexural Stiffness Calibration: Toward a Standard Traceable Method.

The evolution of the atomic force microscope into a useful tool for measuring mechanical properties of surfaces at the nanoscale has spurred the need for more precise and accurate methods for calibrating the spring constants of test cantilevers. Groups within international standards organizations such as the International Organization for Standardization and the Versailles Project on Advanced Materials and Standards (VAMAS) are conducting studies to determine which methods are best suited for these calibrations and to try to improve the reproducibility and accuracy of these measurements among different laboratories. This paper expands on a recent mini round robin within VAMAS Technical Working Area 29 to measure the spring constant of a single batch of triangular silicon nitride cantilevers sent to three international collaborators. Calibration techniques included reference cantilever, added mass, and two forms of thermal methods. Results are compared to measurements traceable to the International System of Units provided by an electrostatic force balance. A series of guidelines are also discussed for procedures that can improve the running of round robins in atomic force microscopy.

Lateral force calibration: accurate procedures for colloidal probe friction measurements in atomic force microscopy.

The colloidal probe technique for atomic force microscopy (AFM) has allowed the investigation of an extensive range of surface force phenomena, including the measurement of frictional (lateral) forces between numerous materials. The quantitative accuracy of such friction measurements is often debated, in part due to a lack of confidence in existing calibration strategies. Here we compare three in situ AFM lateral force calibration techniques using a single colloidal probe, seeking to establish a foundation for quantitative measurement by linking these techniques to accurate force references available at the National Institute of Standards and Technology. We introduce a procedure for calibrating the AFM lateral force response to known electrostatic forces applied directly to the conductive colloidal probe. In a second procedure, we apply known force directly to the colloidal probe using a precalibrated piezo-resistive reference cantilever. We found agreement between these direct methods on the order of 2% (within random uncertainty for both measurements). In a third procedure, we performed a displacement-based calibration using the piezo-resistive reference cantilever as a stiffness reference artifact. The method demonstrated agreement on the order of 7% with the direct force methods, with the difference attributed to an expected systematic uncertainty, caused by in-plane deflection in the cantilever during loading. The comparison establishes the existing limits of instrument accuracy and sets down a basis for selection criteria for materials and methods in colloidal probe friction (lateral) force measurements via atomic force microscopy.

Measurement of the adhesion of a viscoelastic sphere to a flat non-compliant substrate

Abstract The adhesion between a single polystyrene bead (radius, 27 μm) and a flat silica surface has been measured with an atomic force microscope as a function of two variables: (a) The maximum applied load and, (b) the loading time at a constant maximum applied load. Analysis of the results indicates significant plastic deformation of the bead under the action of the load forces. There is also evidence for time-dependent viscoelastic effects as a load is exerted on the bead. The contact zone of the polystyrene bead used for these experiments was examined using Scanning Electron Microscopy. The microscope images revealed a surface covered in small polymer beads with a radius of only 115 nm. In the contact zone these beads had undergone substantial and permanent deformation as a function of the applied load. Basic geometric analysis reveals that the large sphere is not contacting the flat surface under any load. The results presented here indicate the value of being able to measure adhesion using an atomic force microscope. The importance of being able to characterise the contact zone accurately is also highlighted.

Quantitative comparison of two independent lateral force calibration techniques for the atomic force microscope

Two independent lateral-force calibration methods for the atomic force microscope (AFM)--the hammerhead (HH) technique and the diamagnetic lateral force calibrator (D-LFC)--are systematically compared and found to agree to within 5 % or less, but with precision limited to about 15 %, using four different tee-shaped HH reference probes. The limitations of each method, both of which offer independent yet feasible paths toward traceable accuracy, are discussed and investigated. We find that stiff cantilevers may produce inconsistent D-LFC values through the application of excessively high normal loads. In addition, D-LFC results vary when the method is implemented using different modes of AFM feedback control, constant height and constant force modes, where the latter is more consistent with the HH method and closer to typical experimental conditions. Specifically, for the D-LFC apparatus used here, calibration in constant height mode introduced errors up to 14 %. In constant force mode using a relatively stiff cantilever, we observed an ≈ 4 % systematic error per μN of applied load for loads ≤ 1 μN. The issue of excessive load typically emerges for cantilevers whose flexural spring constant is large compared with the normal spring constant of the D-LFC setup (such that relatively small cantilever flexural displacements produce relatively large loads). Overall, the HH method carries a larger uncertainty, which is dominated by uncertainty in measurement of the flexural spring constant of the HH cantilever as well as in the effective length dimension of the cantilever probe. The D-LFC method relies on fewer parameters and thus has fewer uncertainties associated with it. We thus show that it is the preferred method of the two, as long as care is taken to perform the calibration in constant force mode with low applied loads.