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Title: Ultrasonics and nanomechanics
Author: Skilbeck, Mark
ISNI:       0000 0004 6423 7920
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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Since its invention, atomic force microscopy (AFM) has been a valuable tool for probing sample surfaces on the nanoscale, particularly the topography and the mechanical properties. This thesis investigates a subset of techniques focussed on measuring mechanical properties, particularly those which combine AFM with ultrasound. First, a nanoindentation technique is used to measure the mechanical properties (2D elastic modulus and breaking load) of suspended 2D materials. Graphene grown by chemical vapour deposition (CVD) is tested and found to have similar mechanical properties to previously reported values for mechanically exfoliated graphene. The CVD grown graphene is then functionalised by exposure to atomic oxygen, significantly affecting the mechanical properties, making the sheets both softer and weaker, becoming comparable to the properties of graphene oxide. Comparison with the changes in atomic structure suggests that these changes in mechanical properties are likely caused by the creation of extended topological defects. Such 2D materials are of interest as ultrasonic nanoresonators, for which the resonant behaviour could potentially be investigated using AFM. Ultrasonic force microscopy (UFM), a technique where the sample is oscillated at frequencies far greater than the cantilever resonance to provide a channel with contrast due to local surface stiffness, is also investigated. By combining experiment and simulation, the influence of experimental conditions on the observed response is studied and the challenges to obtaining quantitative results (e.g. the Young’s modulus) are discussed. The combination of UFM with other contact mode AFM techniques, such as conductive AFM and friction force microscopy, is demonstrated for the first time, presenting an unusual ability to acquire multimodal information in a single pass. The combination is also shown to benefit from the superlubricity effect of UFM, using it to conductively image a delicate carbon nanotube network. Finally, the use of an AFM as a detector for ultrasonic non-destructive testing, where the interaction of ultrasonic waves with sample features is used to probe a sample, is demonstrated. Test measurements are performed on a simple aluminium plate sample with a laser micro-machined slot and compared to results gathered using traditional detectors (piezoelectric transducers and laser interferometers), showing similar signal features. The advantages and disadvantages of AFM detection are discussed, with the high spatial resolution being the primary advantage. The capability to detect in-plane surface motion using the AFM’s lateral channel is demonstrated, allowing for simultaneous and distinct measurement of two components (one in-plane component and the out-of-plane motion) of the surface, which is not easily achieved using traditional ultrasonic detection methods.
Supervisor: Not available Sponsor: University of Warwick ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics