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As seen previously, a change of bending direction of the whole or a part of the nanowire can sometimes happen due to the motion of the nanowire during bending. Indeed, when the nanowire moves during IIB it may happen that the ion-beam facing side of the nanowire changes due to its motion [122]. However, whilst such geometric considerations (i.e. the ion

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beam hitting one side or the other) typically influence the bending direction of nanowires in all proposed mechanisms, nanowires have also been observed to either bend away from or towards the ion beam depending on factors unrelated to a change of the irradiated side [97], [114]. A mechanism based on an uneven distribution of point defects proposed by Borschel et al. has the advantage of explaining both the bending towards the ion beam and the bending away from it.

In their work, Borschel et al. irradiated single-crystal gallium arsenide nanowires or single- crystal zinc oxide nanowires [97], [114]. They observed that the bending direction was dependent on the diameter of the nanowires, the energy of the ion beam and the ion species. In their experiments, they irradiated the nanostructures under conditions that would either promote radiation damage in the whole cross section of the nanowire (deep implantation) or within the first half of it (shallow implantation) [97], [114]. Furthermore, in [97] by choosing medium energy conditions (i.e. such that the range of the ions is somewhere in the middle of the nanowire at the beginning of the irradiation) they observed that the bending direction of the nanowires changed depending on the way in which the implantation varied during the nanowire motion. In their studies, the authors wanted to visualize the distribution of point defects in the irradiated structures which they did using MC calculations [97], [114]. SRIM is routinely used to simulate the interaction of ions with flat target materials. However, due to its inability to reproduce the circular cross section of nanowires, the authors used either a TRIM based program “3dTRIM” or their in-house code “Iradina” [97], [114]. The two aspects that were monitored using the simulation were the distribution within the nanowires of the implanted ions and of the induced point defects.

The authors reported that when shallow implantation was performed such that the range of the ions was less than half of the nanowire cross section, the nanowire bent away from the ion

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beam [97], [114]. For instance in [114] 60 nm diameter zinc oxide nanowires were observed to bend away from the ion beam when irradiated with a 20 keV argon ion beam [114].

As explained above, during a nuclear collision when an atom is displaced from its lattice site it can leave behind a vacancy and travel within the material until it comes to rest at some distance. Therefore, as stated by Borschel et al. [97], [114] one might expect that overall more self-interstitials will be located deeper (and more vacancies shallower) within the target material. As annihilations of pairs of self-interstitials and vacancies are more likely to occur when the two point defects are located close to each other, the self-interstitials and vacancies formed within the same simulation voxel were considered as having recombined [97], [114]. As can be seen in figure 2.27, this method showed that an excess of vacancies is expected very close to the surface and an excess of self-interstitials a few nanometres deeper [114]. To determine the final distribution of these point defects, the authors also considered sputtering effects and concluded that a thin surface layer which was rich in vacancies was removed during sputtering thus leading to a net excess of self-interstitials in the irradiated part of the nanowire [114]. The self-interstitials would therefore induce a volume expansion of the upper part (i.e. the ion-beam-facing side) of the nanowire while the lower part (i.e. the opposite side) would remain unaffected. According to the authors, such differences between the two sides of the nanowire would induce opposite stresses and therefore a bending moment leading the nanowire to bend away from the ion beam [97], [114].

On the other hand, as also reported in the same work, for the irradiation of 90 nm zinc oxide nanowires by a 100 keV argon ion beam, the authors observed that deep implantation resulted in the nanowires bending towards the ion beam [114]. As explain above, the nature of nuclear collisions during ion irradiation is likely to generate an uneven distribution of vacancies and self-interstitials. However, as shown in figure 2.27, in the case of the deep implantation conditions the unequal distribution results in an excess of vacancies in the upper part of the

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nanowire and an excess of self-interstitials in the lower part. In this case, according to the authors the bending moment will be due to a volume reduction in the upper part richer in vacancies and a volume expansion in the lower part which is richer in self-interstitials and thus, according to the authors, will induce the bending of the nanowire towards the ion beam [114].

Figure 2.27: Iradina MC simulation results and schematic depicting shallow and deep

implantation in the top and the bottom row, respectively. In the left column (a), (d), implanted ions are represented in a purple colour gradient. In the middle column (b), (e), excess of interstitials are represented in purple whilst excess of vacancies are shown in red. In the right column (c), (f), the direction of the ion beam is represented by the dark blue arrow and the part of the nanowire affected by the ions is coloured in light blue. From [114].

Furthermore, the same authors dismissed the influence of the implanted ions in the volume change of the nanowires as their effect must be marginal compared to that of the point defects as the number of vacancies and self-interstitials generated during irradiation is much higher (orders of magnitude higher) than the number of implanted ions. For this purpose, the authors showed that the number of point defects generated during the experiments performed on zinc oxide is about 5×103 per incoming ion [114].

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Choosing an energy such that the range of the incoming ions was about half of the nanowire diameter, the authors irradiated gallium arsenide nanowires with xenon ions [97]. By imaging the nanowires ex situ in a scanning electron microscope (SEM) at successive irradiation steps, they observed that the bending direction of the nanowires changed during the experiment. When the irradiation commenced the nanowire initially started to bend away from the ion beam, then during the bending the range was increased as the relative angle of irradiation changed (due to the bending) and the nanowire then started to bend towards the ion beam. In terms of the mechanism described earlier, this bending behaviour was explained by invoking the change of range due to the angle of incidence which affects the distribution of point defects thus, switching the irradiation conditions from shallow implantation to deep implantation conditions, leading to an alternate bending direction [97].