Abstract
The ability to characterise material properties at sub-micron length scales is imperative for understanding phenomena such as corrosion and fatigue. Atomic force microscopy (AFM) is one tool to achieve this; it creates topographic maps of materials with nanome-tre spatial resolution. The high-speed AFM (HS-AFM), developed at the University of Bristol, captures these topographic images at frame-rate speeds, orders of magnitudes faster than conventional AFMs. However, alongside the topography of a sample, it has long been thought that the AFM is capable of the characterisation of non-topographic properties. In this thesis, we develop the means for stiffness measurements and combine it with the high–speed AFM to create a valuable tool for material characterisation at sub-micron length scales.The quantification of non-topographic measurements is complicated by uncertainties in the AFM probe, the influence of hydrodynamic forces, as well as lateral forces, on the cantilever and the scan speed of the HS-AFM. We achieve several important results by investigating each of these influencing factors in turn, such as our new calibration method to measure the effective stiffness of a probe and our custom-built FEM solver that calculates the influence of hydrodynamic effects on a probe as it is brought towards a sample. Both results are used in our development of a method for quantified stiffness
measurements.
Stiffness measurements of a material can be achieved with AFM by utilising a shift in the resonant frequency of the AFM probe as it comes into contact with a sample. We describe the theory that underpins this technique, as well as showing a new method to estimate two critical parameters of the system, the tip height and offset. We then give experimental evidence that our findings can be used to quantify the Young’s moduli of stiff materials, specifically for steel and gold samples, and highlight the existence of sources of error intrinsic to the method. Finally, we implement our findings on the HS-AFM to calculate the Young’s moduli of graphene flakes on a silicon substrate. This demonstrates the sensitivity of our work, which is capable of video-rate mapping of the elastic properties of materials that are sub-micron in height. The quantification of stiffness at these length scales is expected to enable important results in material science applications, such as to predict life of system components, and to be of value to the nuclear industry, as well as the wider material science community.
| Date of Award | 23 Jan 2019 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Martin E Homer (Supervisor) & Oliver Payton (Supervisor) |