School of Chemistry - Theses
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ItemNanoparticle assembly using an atomic force microscope( 2017)Friction is caused by the energy dissipation between two interfaces in relative motion. Almost 100 million terajoules of energy is lost to overcome friction every year. It is almost one fifth of the energy produced globally. Therefore, understanding friction behaviour is one of the most fundamental topics of scientific research. At the macro-scopic size scale, three empirical laws of friction were discovered almost 400 years ago. Although these three laws of friction work well in the macroscale world, the understanding of friction on the microscale and nanoscale is still very challenging and remains controversial. Atomic force microscope (AFM) is a powerful tool to quantify nanoscale forces. In our thesis, we demonstrate the normal and lateral force calibration. Beyond these, as the optical lever sensitivity calibration of the AFM is becoming a crucial parameter in force measurements, we calculate the ratio of the dynamic and static sensitivities for several common AFM cantilevers, whose shapes vary considerably, and experimentally verify these results. The dynamic-to-static optical lever sensitivity ratio is found to range from 1.09 to 1.41 for the cantilevers studied – in stark contrast to the constant value of 1.09 used widely in current calibration studies. This analysis shows that accuracy of the thermal noise method for the static spring constant is strongly dependent on cantilever geometry – neglecting these dynamic-to-static factors can introduce errors exceeding 100%. We also discuss a simple experimental approach to non-invasively and simultaneously determine the dynamic and static spring constants and optical lever sensitivities of cantilevers of arbitrary shape, which is applicable to all AFM platforms that have the thermal noise method for spring constant calibration. With good force calibration in both the normal and lateral direction, this Ph.D project focuses on the investigation of friction of nanospheres of different sizes. Chemically synthesised gold nanospheres with various diameters were manipulated on Si/SiO2 and gold substrates. Using atomic force microscopy in contact mode and dynamic mode, the static friction and sliding friction of gold nanospheres on the Si/SiO2 substrates were measured and analysed. The friction experiment data in contact mode manipulation indicated that the static friction of gold nanospheres did not reveal size dependence. However, the static friction is higher related to the surface chemistry between the particle ligand and the substrate. Also, we have observed that the relative humidity also plays an important role in the quantify of static friction coefficient of gold nanospheres on a substrate. In contact mode AFM manipulation, the sliding friction of nanospheres revealed size dependence. However, with plotting the logarithm of sliding friction as a function of the logarithm of nanoparticles’ diameter, the slope of linear fitting did not correspond to any contact area models. We believe that this discrepancy is caused by the large deviation of sliding friction in contact mode manipulation. The dynamic mode nanoparticle manipulation was also performed in this Ph.D project. Different sizes of gold nanospheres were manipulation on a gold substrate. Our experiment data indicates the sliding friction is clearly dependent on the size of nanospheres. From plotting the logarithm of sliding friction as a function of the logarithm of nanoparticles’ diameter, the slope of linear fitting suggests that the experimental data is in accord with the DMT contact area model. A new fabrication method, which combines electron beam lithography and atomic force microscopy manipulation, is demonstrated to build a series of symmetric or asymmetric 3D gold nanostructures with nanoscale interparticle separation. The topography of these structures is provided by scanning electron microscopy, and the plasmon modes of these structures were elucidated by the polarised dark field microscopy. The spectra of the symmetric structures exhibit little polarisation dependence, which are in good accord with the COMSOL modelling. More interestingly, the symmetric 3D pentamer exhibits a Fano-like resonance and can provide a drastically enhanced localized electric field in the interstices of 3D structures. For the asymmetric 3D pentamer structures, where one disk is situated 5 nm away from the center of the bottom tetramer, a Fano-like dip is only revealed at a particular polarisation angle. When the top particle is manipulated onto a single disk of the bottom tetramer, there is no Fano-like resonance.