Ion irradiation of germanium foils and germanium nanowires

In the work presented in this thesis, ion irradiations were carried in situ within a transmission electron microscope (TEM) allowing the consequences of radiation damage on germanium to be investigated. In general, the study of radiation damage on semiconductors is of utmost importance as the use of ion beams during the processing of semiconductors is now a standard technique. Furthermore, as germanium materials in general, and germanium nanowires especially, are currently being considered as replacements for bulk silicon in future microelectronic devices, this thesis will address the effects of radiation damage on both germanium thin foils and germanium nanowires. Concerning, the use of ion beam on nanowires, a remarkable, but yet to be fully explained consequence of radiation damage, is investigated in this work: the ion induced bending (IIB) effect. In the literature, it has been reported that during ion irradiation of nanostructures they may bend towards or away from the ion beam. However, the mechanisms invoked to explain IIB are various and still debated. Following 30 and 70 keV xenon ion irradiation experiments, it is shown in this thesis that out of the proposed mechanisms only those based on dynamical rearrangement of the damage can explain the bending of the irradiated germanium nanowires towards the ion beam. In contrast, it is demonstrated that the mechanisms based on the accumulation of point defects or on the presence of an amorphous phase cannot explain the bending of the germanium nanowires irradiated in the current work. In a set of experiments where germanium nanowires were irradiated by 30 keV xenon ions at 400°C, bending was observed even though the accumulation of point defects and the collapse of the crystalline phase into an amorphous one is prevented by the relatively elevated temperature (i.e. 400°C). Similarly, in another set of experiments performed at room temperature it is shown via Monte Carlo calculations that there is a discrepancy between the distribution of the damage within the nanowires and that which would be required in order for the mechanisms based on damage accumulation to operate. Furthermore, the work in this thesis also solves several issues regarding the use of IIB as a potential technique in industrial processing of nanowires. Whilst IIB can be considered as an unwanted side effects occurring during the ion beam doping of nanostructures, it also represents a powerful nanomanipulation technique. However, to make full use of IIB as such a technique, the bending direction must be controllable. For this reason, using an in-house MATLAB code combined with Monte Carlo calculations it was determined that the damage depth normalised with respect to the diameter of the nanowire could be used to forecast the bending direction. Lastly, as germanium nanowires became amorphous or partially amorphous during IIB, annealing experiments were performed. However, it is shown in this work that the shape modification obtained via ion beam irradiation can be unstable during recrystallisation. Consequently, irradiating the germanium nanowires at elevated temperature (e.g. 400°C in this work) is proposed as a novel way to use the IIB effect as it allows the nanowires to maintain their single-crystalline character during the nanomanipulation. As stated above, ion beams are routinely used to process semiconductors. However, unless the irradiation is performed at elevated temperature, the damage accumulation may induce amorphisation. To investigate the currently debated mechanisms behind amorphisation and the effect of the ion mass on the amorphization rate, germanium foils were irradiated in situ within the TEM using either 300 keV xenon, 200 keV krypton, 100 keV argon, 80 keV neon or 70 keV helium ions. By monitoring the diffraction patterns during the in situ ion irradiations and modelling the collision cascades, it was shown that amorphisation must occur gradually via a heterogeneous damage accumulation mechanism where each ion induces an amorphous region at the core of the cascade surrounded by a highly defective crystalline shell. It was also revealed that the threshold displacement per atom (dpa) for amorphisation was not always lower for heavier ions as may be expected. This feature of the threshold dpa trend was shown to depend on the spatial distribution of the point defects in a collision cascade. Furthermore, the correlation between the experiments and the modelling of 1000 collision cascades induced by helium ions has shown that, in the case of helium, the amorphisation mechanism can be understood only when taking into account the stochastic nature of collision cascades. Indeed, it is revealed that the average collision cascade induced by helium ions could not induce amorphisation. On the other hand, it is shown that it is only occasional collision cascades involving a relatively larger number of defects which are responsible for the amorphisation process

Effects of temperature on the ion-induced bending of germanium and silicon nanowires

Nanowires can be manipulated using an ion beam via a phenomenon known as ion-induced bending (IIB). While the mechanisms behind IIB are still the subject of debate, accumulation of point defects or amorphisation are often cited as possible driving mechanisms. Previous results in the literature on IIB of Ge and Si nanowires have shown that after irradiation the aligned nanowires are fully amorphous. Experiments were recently reported in which crystalline seeds were preserved in otherwise-amorphous ion-beam-bent Si nanowires which then facilitated solid-phase epitaxial growth (SPEG) during subsequent annealing. However, the ion-induced alignment of the nanowires was lost during the SPEG. In this work, in situ ion irradiations in a transmission electron microscope at 400°C and 500°C were performed on Ge and Si nanowires, respectively, to supress amorphisation and the build-up of point defects. Both the Ge and Si nanowires were found to bend during irradiation thus drawing into question the role of mechanisms based on damage accumulation under such conditions. These experiments demonstrate for the first time a simple way of realigning single-crystal Ge and Si nanowires via IIB whilst preserving their crystal structure

Anomalous nucleation of crystals within amorphous germanium nanowires during thermal annealing

In this work, germanium nanowires rendered fully amorphous via xenon ion irradiation have been annealed within a transmission electron microscope to induce crystallization. During annealing crystallites appeared in some nanowires whilst others remained fully amorphous. Remarkably, even when nucleation occurred, large sections of the nanowires remained amorphous even though the few crystallites embedded in the amorphous phase were formed at a minimum of 200 °C above the temperature for epitaxial growth and 100 °C above the temperature for random nucleation and growth in bulk germanium. Furthermore, the presence of crystallites was observed to depend on the diameter of the nanowire. Indeed, the formation of crystallites occurred at a higher annealing temperature in thin nanowires compared with thicker ones. Additionally, nanowires with a diameter above 55 nm were made entirely crystalline when the annealing was performed at the temperature normally required for crystallization in germanium (i.e. 500 °C). It is proposed that oxygen atoms hinder both the formation and the growth of crystallites. Furthermore, as crystallites must reach a minimum size to survive and grow within the amorphous nanowires, the instability of crystallites may also play a limited role for the thinnest nanowires

The effect of cobalt on morphology, structure, and ORR activity of electrospun carbon fibre mats in aqueous alkaline environments

An innovative approach for the design of air electrodes for metal–air batteries are free-standing scaffolds made of electrospun polyacrylonitrile fibres. In this study, cobalt-decorated fibres are prepared, and the influence of carbonisation temperature on the resulting particle decoration, as well as on fibre structure and morphology is discussed. Scanning electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, and inductively coupled plasma optical emission spectrometry are used for characterisation. The modified fibre system is compared to a benchmark system without cobalt additives. Cobalt is known to catalyse the formation of graphite in carbonaceous materials at elevated temperatures. As a result of cobalt migration in the material the resulting overall morphology is that of turbostratic carbon. Nitrogen removal and nitrogen-type distribution are enhanced by the cobalt additives. At lower carbonisation temperatures cobalt is distributed over the surface of the fibres, whereas at high carbonisation temperatures it forms particles with diameters up to 300 nm. Free-standing, current-collector-free electrodes assembled from carbonised cobalt-decorated fibre mats display promising performance for the oxygen reduction reaction in aqueous alkaline media. High current densities at an overpotential of 100 mV and low overpotentials at current densities of 333 μA·cm−2 were found for all electrodes made from cobalt-decorated fibre mats carbonised at temperatures between 800 and 1000 °C

UNDERSTANDING GAS ADSORPTION OF PAN-BASED CARBON NANOFIBERS

Polyacrylonitrile-based carbon nanofibers (PAN-based CNFs) have great potential to be used for carbon dioxide (CO2) capture due to their excellent CO2 adsorption properties. The porous structure of PAN-based CNFs originates from their turbostratic structure, which is composed of numerous disordered stacks of graphitic layers. During the carbonization process, the internal structure is arranged toward the ordered graphitic structure, which significantly influences the gas adsorption properties of PAN-based CNFs. However, the relation between structural transformation and CO2 capture is still not clear enough to tune the PAN-based CNFs. In this paper, we show that, with increasing carbonization temperature, the arrangement of the PAN-based CNF’s structure along the stack and lateral directions takes place independently: gradually aligning and merging along the stack direction and enlarging along the lateral direction. Further, we correlate the structural arrangement and the CO2 adsorption properties of the PAN-based CNFs to propose a comprehensive structural mechanism. This mechanism provides the knowledge to understand and tailor the gas adsorption properties of PAN-based CNFs