Electronic Theses and Dissertations (PhDs)

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Browsing Electronic Theses and Dissertations (PhDs) by Author "Derry, Trevor"

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    Modification of boron nitride nanostructures induced by medium energy ion irradiation
    (University of the Witwatersrand, Johannesburg, 2023-08) Lisema, Lehlohonolo Innocent; Madhuku, Morgan; Derry, Trevor
    This research focused on using Chemical Vapour Deposition (CVD) to synthesize boron nitride nanostructures, particularly nanotubes, and selectively introducing defects into them through ion implantation. Boron ion implantations were carried out at ambient temperature at 150 keV energy and fluences 1x1014 and 5x1014 ions/cm2. The synthesized samples were analyzed using scanning electron microscopy (SEM), Raman spectroscopy, and Grazing incidence X-ray diffraction (GIXRD). Ion implantation was found to introduce defects into the surface of the samples, resulting in increased stress levels and a higher local density that favoured more crystallized nanostructures. SEM images showed clear evidence of BN nanostructures and boron nitride nanotubes (BNNTs), with the latter appearing as long, thin structures with diameters ranging from ⁓30-90nm. After ion implantation, the Raman spectra of samples implanted with ion fluence 5×1014 ions/cm2 at 1000oC, show an amorphous h-BN peak, and a narrower, intense E2g vibrational mode of h-BN is observed around 1366 cm-1 for samples synthesized at 1100oC and 1200oC. Raman analysis did not show any E2g mode of vibration of h-BN for all samples at implanted with ion fluence 1×1014 ions/cm2. The samples synthesized at 900 ºC had no active 1366 cm-1 Raman peak present. Grazing incidence X-ray diffraction (GIXRD) spectra revealed a prominent peak between 54 and 56 ° 2θ, corresponding to the (004) h-BN reflection, which was used to determine the average a and c lattice parameters 0.249 ± 0.0002 nm and 0.662 ± 0.001 nm, respectively, yielding an interplanar distance of 0.166 ± 0.0001 nm representing the stacking direction of the BN layers. The majority of the samples had broad peaks, indicative of a nanocrystalline material. The only exception was the sample grown at 1200 °C, which was found to have a Scherrer crystallite size >100 nm. In contrast, the rest of the samples had an average size of 3.5 ± 0.3nm. The average crystalline domain size values confirmed that after ion implantation, the phonon lifetime would be longer due to a large domain size, indicating that the BN nanostructures were more crystallized. The fluence of 5x1014 ions/cm2 showed to be the optimal growth condition for BNNTs. Overall, BNNTs and BN nanostructures were effectively synthesized at 900°C, 1000°C, 1100°C, and 1200°C CVD temperatures, and insights into the influence of ion implantation on the composition as well as properties of BN nanostructures are presented. The most noteworthy finding of the experiment was the substantial increase in the size of the Raman derived crystallite domains in the 1100°C and 1200°C samples following ion implantation with boron ions at a fluence of 5x1014 ions/cm2.