Novel thermal and electron-beam approaches for the fabrication of boron-rich nanowires
Ignacio Guillermo Gonzalez Martinez
PhD thesis (Dissertation), TU Dresden, July 2016
Pursuing the development and implementation of novel synthesis techniques to produce nanostructures with an interesting set of properties is a goal that advances the frontiers of nanotechnology. Also of fundamental importance is to revisit well-established synthesis techniques employing a new set of materials as precursors, substrates and catalysts. Fundamental breakthroughs in the field of nanotechnology can be achieved by developing new synthesis procedures as well as by adapting known procedures to new materials. This thesis focuses on both kinds of experiments.
A variant of chemical vapor deposition (CVD) has been used to produce Al5BO9 nanowires out of sapphire wafers without the need of a catalyst material. The novelty of the work relies on the formation mechanism of the Al5BO9 nanowires. Essentially, the process can be described as a large-scale topological transformation taking place on the substrate’s surface as its chemical composition changes due to the arrival of precursor molecules. Dense mats of Al5BO9 nanowires cover large areas of the substrate that were previously relatively flat. The process is enhanced by a high temperature and the presence of pre-existing superficial defects (cracks, terraces, etc.) on the substrates. Al5BO9 nanowires as well as B/BOX nanowires and BOX nanotubes were also produced via a novel in-situ electron beam-induced synthesis technique. The process was carried out at room temperature and inside a transmission electron microscope. Au nanoparticles were used as catalyst for the case of B/BOX nanowires and BOX nanotubes, while the Al5BO9 nanowires were synthesized without the need of a catalyst material. The formation and growth of the nanostructures is solely driven by the electron beam. The growth mechanism of the B/BOX nanowires and BOX nanotubes relies on interplay between electrostatic charging of the precursor material (to produce and transport feedstock material) and electron stimulated desorption of oxygen which is able to activate the catalytic properties of the Au nanoparticles. For the case Al5BO9 nanowires a nucleation process based on massive atomic rearrangement in the precursor is instigated by the e-beam, afterwards, the length of some of the nanowires can be extended by a mechanism analogous to that of the growth of the B/BOX nanowires.