chapter  2
72 Pages

Chapter 2

Abstract ................................................................................... 8 2.1 Introduction .................................................................... 9 2.2 Crater and Hole Formation ......................................... 11

2.2.1

Ex Situ

Studies of Crater Formation .............. 11 2.2.2

In Situ

Studies of Crater Formation ............... 14 2.2.2.1 Gold ....................................................... 14 2.2.2.2 Silver .................................................... 21 2.2.2.3 Lead ...................................................... 21 2.2.2.4 Indium .................................................. 22

2.2.3 Crater Annihilation ........................................... 23 2.2.4

In Situ

Studies of Hole Formation .................. 26 2.2.5 Craters and Holes — Discussion ..................... 33

2.2.4.1 Crater and Hole Annihilation — Discussion ............................................ 39

2.3 Nanocluster Emission .................................................. 41 2.3.1 Craters and Nanoparticles ............................... 41 2.3.2 Nanoparticle Collection..................................... 42 2.3.3 Radiation Effects on Nanoparticles ................. 45 2.3.4 Nanoparticle Ejection Rates ............................. 47 2.3.5 Relationship of Nanoparticle Ejection to

Cratering and Cascade Events.......................... 49 2.3.6 Nanoparticle Ejection Mechanisms ................. 50 2.3.7 Ejected Nanoparticle Size Distribution ........... 52 2.3.8 Shock Wave Model ............................................. 53 2.3.9 Relationship of Nanoparticle Ejection to

Sputtering .......................................................... 55 2.3.10 Synthesis ............................................................ 56 2.3.11 Summary of Nanoparticle Experiments .......... 57

2.4. MD Molecular Dynamics Simulations of Crater Production ..................................................................... 58 2.4.1 Monte Carlo Simulations versus Molecular

Dynamics ............................................................ 58 2.4.2 Channeling Effects ............................................ 59 2.4.3 MD Simulation Method .................................... 60 2.4.4 Formation of Ordinary Craters ........................ 62

2.4.4.1 Surface Damage Mechanisms ............. 62 2.4.4.2 Basic Crater Formation Mechanism .. 63

2.4.5 Formation of Exotic Crater Structures ............ 68 2.4.6 Analysis Based on MD ...................................... 71 2.4.7 Observations of Nanocluster Ejection ............. 73

2.5 Conclusions ................................................................... 74 Acknowledgments ................................................................. 75 References ............................................................................ 76

ABSTRACT

The combination of

in situ

electron microscopy with molecular dynamics (MD) simulations gives important insights into the processes occurring during ion-beam engineering of thin films. This chapter compares and contrasts experimental observations and MD simulations of individual heavy-ion impacts on metal films. These impacts result in the formation of craters

and other surface features on metals and the ejection of nanoparticles. Images in the manuscript and video sequences on the accompanying CD-ROM illustrate the processes. The simulations of ion impacts match the experiment and give remarkable insight into the processes that give rise to the observed surface structures. Liquid flow and micro-explosions have been unequivocally identified in the MD work and provide an atomic-level understanding of the processes giving rise to cratering. An incomplete understanding exists of the emission of nanoclusters by ion impacts where the experimental size distribution of the emitted particles exhibits a powerlaw relationship, suggesting that this could be a shock-wave phenomenon. Although this is not, as yet, supported by the MD work, further simulations giving rise to improved statistics on nanocluster emission should enable a better comparison between experiment and simulation and thus serve to test this interpretation.