Channeling effects in gold nanoclusters under He ion irradiation: insights from molecular dynamics simulations

Gregor Hlawacek,Arkady V Krasheninnikov,Silvan Kretschmer,Mahdi Ghorbani-Asl,Sadegh Ghaderzadeh

doi:10.1088/1361-6528/ab4847

Gregor Hlawacek, Arkady V Krasheninnikov + Show 3 more

Open Access

https://doi.org/10.1088/1361-6528/ab4847

Copy DOI

Abstract

The interpretation of helium ion microscopy (HIM) images of crystalline metal clusters requires microscopic understanding of the effects of He ion irradiation on the system, including energy deposition and associated heating, as well as channeling patterns. While channeling in bulk metals has been studied at length, there is no quantitative data for small clusters. We carry out molecular dynamics simulations to investigate the behavior of gold nanoparticles with diameters of 5–15 nm under 30 keV He ion irradiation. We show that impacts of the ions can give rise to substantial heating of the clusters through deposition of energy into electronic degrees of freedom, but it does not affect channeling, as clusters cool down between consecutive impact of the ions under typical imaging conditions. At the same time, high temperatures and small cluster sizes should give rise to fast annealing of defects so that the system remains crystalline. Our results show that ion-channeling occurs not only in the principal low-index, but also in the intermediate directions. The strengths of different channels are specified, and their correlations with sputtering-yield and damage production is discussed, along with size-dependence of these properties. The effects of planar defects, such as stacking faults on channeling were also investigated. Finally, we discuss the implications of our results for the analysis of HIM images of metal clusters.

Highlights

Due to a small system size with respect to ion ranges in the corresponding bulk material, only a part of ion energy can be adsorbed by the target, so that the cross section for defect production decreases with increasing ion energy
While analytic solutions exist for high energy channeling into bulk samples [53] they cannot be applied without further changes to low energy channeling into nano-objects
It is appropriate at this point to draw attention to the fact that our calculations ignore the effects of the anisotropy of the electron density on the electronic stopping

Summary

Introduction

Properties related to size-quantization effects [1, 2] and high surface-to-volume ratio. This provides vast opportunities for tuning their optical and electronic properties for a wide range of applications, including optoelectronics and catalysis, through only changing the system size [3, 4]. Ion irradiation and implantation, which are routinely used nowadays in the semiconductor industry, work well in this case, as system size is comparable to or less than the typical ion range of ions in the Nanotechnology 31 (2020) 035302 commercially available accelerators, so that the whole system can be treated by the beam. The evolution of the target structure under ion irradiation can be different due to high forward and sideward sputtering rates, reduction or absence of collisional cascades and longer lifetimes of electronic excitations in the target

Methods

Results

Conclusion