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Synthesis and characterization of boron nanotubes and other related boron nanomaterials by dual pulsed laser ablation.

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Developments in nanotechnology in the last 30 years have provided the increasing usage of nanostructured particles in many applications. Boron nanomaterials (BNMs) are a good example of such nanoparticles. The uniqueness of boron nanomaterials set it apart from other similar nanomaterials. It has a wide field of promising applications ranging from quantum computing to materials science and medicine. Quasi-one-dimensional nanomaterials like nanowires, nanotubes, bamboo-like – nanotubes, nanosheets, nanoribbons, and nanorods are of great interest in fundamental research as well as their potential uses in many technological applications, such as field emitters, electronics, pharmaceutical, and novel circuitry elements, sensor devices and after appropriate functionalization in biomedical due to their applications. In this dissertation, the laser furnace technique was utilized to study the synthesis of boron nanomaterials. In the study, we achieved the synthesis of single-walled boron nanotubes and among other nanostructured materials of boron such as bamboo like – nanotubes, nanowires, and nanorods. These kinds of nanomaterials were synthesized through the use of the dual pulsed laser ablation technique. Experiments were conducted in a tube furnace. The boron composite target was ablated by sequential 1064 and 532 nm laser pulses at a furnace temperature of 1000 °C or 1100 °C in argon/nitrogen gas using different pressure and flow rates. The investigation involved varying the parameters such as gas pressure, gas flow rate, and furnace temperature. We observed the effect of varying these parameters on the synthesis of boron nanomaterials. To verify these effects, the as – prepared boron nanomaterials were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy (RS), ultraviolet-visible (UV- VIS), photoluminescence (PL) and vibration sample magnetometer (VSM). It was found that the variation of the argon/nitrogen pressure and flow rate influence the quantity, quality and type of the as-prepared boron nanomaterials, since these parameters affected the plasma dynamics. The low flow rate and low pressure reduced the cross section for a collision between the plasma constituents, in particular, between the boron atoms and the metal catalysts which affected the probability of nucleation and growth of boron nanomaterials.The temperature was found to be the critical process parameter in the nucleation and the growth of boron nanomaterials. It was found that as the synthesis temperature increased to 1100 °C, there is also an increase in the nucleation and the growth of boron nanomaterials. It was discovered that an argon/nitrogen pressure of 400 Torr, the flow rate of 200 sccm and temperature of 1100 °C produced more boron nanomaterials at a rate of 100 mg/h with the highest quality of single-walled boron nanotubes (SWBNTs) with diameters ranging from 0.4 to 2.0 nm and 2.0 microns in length. The density gradient ultracentrifugation was carried out on boron as – prepared nanomaterials which resulted in the separation into different diameters of single-walled boron nanotubes. It was discovered that SWBNTs with smallest diameters of about 1.43 nm were found at the lower density, while the SWBNTs with larger diameters of 2.05 nm were obtained at higher density. These diameters were obtained by HRTEM analyses and the length up to one micrometer was also observed. The lattice fringes of 0.34 nm were found by HRTEM imaging in bamboo like-nanotubes, nanowires and nanorods. The lattices spacing from the fringes is consistent with the recent theoretical calculations of bulk boron of 0.35 nm. SEM analyses revealed the tubular and spherical structures of SWBNTs synthesized. XRD results showed that α – boron and β – boron were the solid phases formed in the products. It was observed that the crystallinity and size of the materials increased with an increase in furnace temperature. Raman analyses showed certain peaks below 500 cm-1, which is attributed to tubular structures similar to the peaks observed in a single-walled carbon nanotube. Raman peaks attributed to α – boron cluster was also observed at 788, 878 and 965 cm-1, confirming that SWBNTs formed from most stable sheets of α – boron. The statistical analyses of width distributions of the synthesized SWBNTs revealed variations in their diameters obtained according to each sample layer. The strong exciton absorption peak SWBNTs around 279 nm was revealed by UV-VIS results, while the luminescence of SWBNTs was found around 332 nm with photoluminescence analyses. Ferromagnetic properties of SWBNTs materials at room temperature were determined through VSM measurements. Finally, a simple model of vapour liquid solid (VLS) mechanism process has been developed to describe the formation of the SWBNTs.


Doctoral Degree. University of KwaZulu-Natal, Durban.