**6. Heavy ion interaction with gold nanoparticles**

Energetic heavy ions, which will be defined as ions heavier than a helium atom, can result from recoil following a nuclear reaction, decay, or fission events, though sources relevant to gold nanoparticle applications will often likely come from particle accelerators. Initiation of nuclear reactions with heavy ion irradiation is improbable, and energy is typically deposited via ionization and Coulombic interaction similar to their less massive counterparts. The primary distinction is that, due to their size, heavy ions are capable of transferring much more energy to the gold atoms they interact with, resulting in large displacement damage cascades (see **Figures 1** and **2**). Cascade clustering, in which several point defects are formed as a result of a single interaction event and coalesce into larger and less mobile defect structures, results in a much more disordered microstructure and production of a smaller fraction of freely migrating defects. Complex interactions between these point- and multi-defect structures can significantly affect their stability and mobility [49]. As alluded to in the introduction, the effects of limited dimensions in the formation of surface cascades and sputtering in gold have been well known for over two decades [50]. At these limited dimensions, the effect of viscous flow during ballistic interactions is thought to have a significant role [50, 51]. These and many other size effects resulting from heavy ion irradiation in gold nanoparticles will be explored in greater detail in the following subsections.

## **6.1 Radiation stability of freestanding gold nanoparticles**

In general, the response of nanostructured materials to radiation damage is still poorly understood [29]. Despite the limited understanding in the general field, freestanding gold nanoparticles (usually drop casted onto carbon or silicon nitride TEM

grids) have been used as the model system for testing and validation of TEM with in-situ ion irradiation capabilities [52–54]. Expanding on the known enhanced sputtering rates observed in gold thin foils exposed to a range of noble gas ions [55], it was later shown by Ilinov et al*.* that gold nanorods irradiated with 80 keV Xe demonstrated sputtering rates with three orders of magnitude higher than predicted by classical sputtering simulations [27]. These results have been verified multiple times in various facilities around the world. A detailed qualitative example can be seen in **Figure 5** [56]. By performing in-situ TEM experiments, individual nanoparticles, as well as the individual number of ion strikes on that particle, can be tracked for the duration of the experiment. This surprising set of results means that classical models and expectations no longer hold true when predicting the radiation response of gold nanoparticles.

These experimental results contradict the classic Monte Carlo-based simulations of sputtering effects [57]. A more catastrophic image of radiation damage in various sizes and morphologies of gold nanoparticles is predicted by molecular dynamic simulations of heavy ion irradiation [20, 26, 27, 56, 58]. An example of these types of simulations comparing the expected sputtering from a flat surface versus significantly increased sputtering from a nanoparticle can be seen in **Figure 6** [58]. The recent work has correlated the in-situ TEM observation of nanorod evolution and sputtering with molecular dynamic simulations that provide greater insight into the role of local crystal orientation on the effects of individual ion strikes relative to the nanoparticle orientation [26, 56]. The sputtering of the nanoparticles has also been tied to sintering of clustered gold nanoparticles, which is not surprising due to the work imparted into such a small volume. This is best seen when the particles are examined via electron tomography, as can be seen in **Figure 7** [28]. Despite the higher number of studies into the radiation stability of gold nanoparticles, few studies explore the effects of gold particle size and morphology or the myriad of irradiation environmental variables. One of the few systematic studies looked at the response of gold nanoparticles during self-ion

#### **Figure 5.**

*Changes to Au nanowire due to irradiation with 80 keV Xe ions. (a) Segmentation due to "necking" and breaking of nanowire at grain boundaries following irradiation to a fluence of 2.1 × 1014 cm<sup>−</sup><sup>2</sup> , (b) nanorod at starting point for volume measurements—white arrow indicates projected direction of the ion beam which was incident at 60° to the specimen plane, (c) nanorod following irradiation to (additional) fluence of 1.6 × 1013 cm<sup>−</sup><sup>2</sup> (*≈*227 impacts on nanorod), (d) nanorod following irradiation to (additional) fluence of 5.5 × 1013 cm<sup>−</sup><sup>2</sup> (*≈*316 additional impacts on nanorod). All are bright-field TEM images and (e) plot of atom loss versus ion impacts for Au nanorod shown in panels (b)–(d) [56]. Source: https://journals.aps.org/prl/abstract/10.1103/ PhysRevLett.111.065504. American Physical Society. Reprinted figure with permission from G. Greaves et al., Physical Review Letters, Vol. 111, pp. 065504-1, 2013. Copyright 2013 by the American Physical Society .*

**61**

**Figure 6.**

*Evolution of Gold Nanoparticles in Radiation Environments*

irradiation. This study explored particles with average diameters of 5, 20, and 60 nm and altered the gold ion energy between 46 keV, 2.8 MeV, and 10 MeV [59]. It is very clear from the results presented in **Figure 8** that the response and stability of nominally spherical gold nanoparticles are heavily dependent on the order of magnitude changes in particle diameter, as the diameter approaches that of the cascade volume. Another heavy ion irradiation effect that has been noted at an even higher energy regime (956 MeV Pb) is that the nanoparticles can be ejected or desorb from the surface as a result of ion irradiation [60]. The combination of the

*20nm sized Au nanoparticles," pp. 91-97, Copyright 2008, with permission from Elsevier.*

*Perspective view of 16 keV Au impact on (a) plane surface and on (b) nanoparticle at time = 20 ps after impact. Color denotes the local temperature [58]. Source: https://www.sciencedirect.com/science/article/pii/ S1387380608000195. Elsevier B.V. Reprinted from International Journal of Mass Spectrometry, Vol. 272, S. Zimmermann and H.M. Urbassek, "Sputtering of nanoparticles: Molecular dynamics study of Au impact on* 

*DOI: http://dx.doi.org/10.5772/intechopen.80366*

*Evolution of Gold Nanoparticles in Radiation Environments DOI: http://dx.doi.org/10.5772/intechopen.80366*

*Gold Nanoparticles - Reaching New Heights*

grids) have been used as the model system for testing and validation of TEM with in-situ ion irradiation capabilities [52–54]. Expanding on the known enhanced sputtering rates observed in gold thin foils exposed to a range of noble gas ions [55], it was later shown by Ilinov et al*.* that gold nanorods irradiated with 80 keV Xe demonstrated sputtering rates with three orders of magnitude higher than predicted by classical sputtering simulations [27]. These results have been verified multiple times in various facilities around the world. A detailed qualitative example can be seen in **Figure 5** [56]. By performing in-situ TEM experiments, individual nanoparticles, as well as the individual number of ion strikes on that particle, can be tracked for the duration of the experiment. This surprising set of results means that classical models and expectations no longer hold true when predicting the radiation response of gold nanoparticles.

These experimental results contradict the classic Monte Carlo-based simulations of sputtering effects [57]. A more catastrophic image of radiation damage in various sizes and morphologies of gold nanoparticles is predicted by molecular dynamic simulations of heavy ion irradiation [20, 26, 27, 56, 58]. An example of these types of simulations comparing the expected sputtering from a flat surface versus significantly increased sputtering from a nanoparticle can be seen in

**Figure 6** [58]. The recent work has correlated the in-situ TEM observation of nanorod evolution and sputtering with molecular dynamic simulations that provide greater insight into the role of local crystal orientation on the effects of individual ion strikes relative to the nanoparticle orientation [26, 56]. The sputtering of the nanoparticles has also been tied to sintering of clustered gold nanoparticles, which is not surprising due to the work imparted into such a small volume. This is best seen when the particles are examined via electron tomography, as can be seen in **Figure 7** [28]. Despite the higher number of studies into the radiation stability of gold nanoparticles, few studies explore the effects of gold particle size and morphology or the myriad of irradiation environmental variables. One of the few systematic studies looked at the response of gold nanoparticles during self-ion

*Changes to Au nanowire due to irradiation with 80 keV Xe ions. (a) Segmentation due to "necking" and* 

*starting point for volume measurements—white arrow indicates projected direction of the ion beam which was incident at 60° to the specimen plane, (c) nanorod following irradiation to (additional) fluence of 1.6 × 1013*

 *(*≈*227 impacts on nanorod), (d) nanorod following irradiation to (additional) fluence of 5.5 × 1013 cm<sup>−</sup><sup>2</sup> (*≈*316 additional impacts on nanorod). All are bright-field TEM images and (e) plot of atom loss versus ion impacts for Au nanorod shown in panels (b)–(d) [56]. Source: https://journals.aps.org/prl/abstract/10.1103/ PhysRevLett.111.065504. American Physical Society. Reprinted figure with permission from G. Greaves et al., Physical Review Letters, Vol. 111, pp. 065504-1, 2013. Copyright 2013 by the American Physical Society .*

*, (b) nanorod at* 

*breaking of nanowire at grain boundaries following irradiation to a fluence of 2.1 × 1014 cm<sup>−</sup><sup>2</sup>*

**60**

**Figure 5.**

*cm<sup>−</sup><sup>2</sup>*

#### **Figure 6.**

*Perspective view of 16 keV Au impact on (a) plane surface and on (b) nanoparticle at time = 20 ps after impact. Color denotes the local temperature [58]. Source: https://www.sciencedirect.com/science/article/pii/ S1387380608000195. Elsevier B.V. Reprinted from International Journal of Mass Spectrometry, Vol. 272, S. Zimmermann and H.M. Urbassek, "Sputtering of nanoparticles: Molecular dynamics study of Au impact on 20nm sized Au nanoparticles," pp. 91-97, Copyright 2008, with permission from Elsevier.*

irradiation. This study explored particles with average diameters of 5, 20, and 60 nm and altered the gold ion energy between 46 keV, 2.8 MeV, and 10 MeV [59]. It is very clear from the results presented in **Figure 8** that the response and stability of nominally spherical gold nanoparticles are heavily dependent on the order of magnitude changes in particle diameter, as the diameter approaches that of the cascade volume. Another heavy ion irradiation effect that has been noted at an even higher energy regime (956 MeV Pb) is that the nanoparticles can be ejected or desorb from the surface as a result of ion irradiation [60]. The combination of the

#### **Figure 7.**

*Source electron micrographs and discrete 4D electron tomograms of Au nanoparticles irradiated with 3 MeV Cu3+. (a, c, and e) Example micrographs from tilt series at increasing fluences up to ~1015 cm<sup>−</sup><sup>2</sup> . (b, d, f) Corresponding 3-D tomogram reconstructions, rotated to a different angle from the source micrograph. Source: http://pubs.rsc.org/en/content/articlelanding/2014/cc/c3cc49479a#!divAbstract. The Royal Society of Chemistry. Reproduced from [28] with permission of The Royal Society of Chemistry.*

increased sputtering, sintering, ballistic destruction, and possible ejection demonstrates that the response of gold nanoparticles to heavy ion irradiation is far from that expected by classical theories and models based on bulk sample geometries. As such, significant further investigation is needed before any commercialization in displacement damage environments is considered.
