Bombardment of HOPG with multiple charged ions

Abstract

Electron cyclotron resonance ion sources and electron string ion sources play an important role in the production of multi-charged ion beams. We have used these devices to irradiate highly oriented pyrolytic graphite with highly charged ions Kr27+ at a fluence of 4.2 x 109 cm-2 and Ti5+ at a fluence of 1.6 x 1011 cm-2. The two instruments were used to irradiate highly oriented pyrolytic graphite samples at Veksler and Baldin Laboratory of High Energies (VBLHE, JINR, Dubna) with the Krion-2 electron string ion source and electron cyclotron resonance ion sources at the Flerov Laboratory of Nuclear Reactions (FLNR, JINR, Dubna) irradiation facility. We have designed the sample holder for electron string ion source and electron cyclotron resonance ion source. The sample holder was manufactured at University of Pretoria at the Department of Physics in South Africa. For this study I also assisted in the assembling of the vacuum chamber in Dubna, Russia and tested whether it had any vacuum leaks. The basic principles and equations for the production HCI were reviewed together the design and operating principles of the electron string ion source and electron cyclotron resonance ion source. In a review of ion-solid interactions, special attention was given to the interaction of HCI with solid surfaces and the possible mechanisms operating during this type of interaction. The interaction of highly charged ions with solid surface depends on potential energy rather than kinetic energy. Because in all other ion-solid interactions kinetic energy transfer to the substrate dominates, the study of HCI interactions where the potential energy of the ions are much larger than their kinetic energy becomes more interesting. Highly charged ions have a high amount of potential energy due to the ionization process. This high power deposition creates nano-structures on the surface such as craters and nano-hillocks on the nanometre scale and also provide a high amount of secondary ions and electrons. The surface damage (or modifications) of highly oriented pyrolytic graphite after irradiated with Kr27+ and Ti5+ ions was investigated using the scanning tunnelling microscope in NanoScan Company, Moscow State University. The potential energy and kinetic energy of Kr27+ ions were 15.5 keV and 2.7 keV, respectively, while for the Ti5+ ions they were 0.185 keV and 50 keV. For Kr27+ ion bombarded sample the STM measurements showed protrusion (nano-dots) on the highly oriented pyrolytic graphite (HOPG) surface. For Ti5+ ions bombardment we could not identify with certainty any topographical changes on the highly oriented pyrolytic graphite (HOPG) surface. The STM results in this study were compared to similar studies by other groups. Other groups used different highly charge ions (HCI) with various charge states (and hence potential energy of highly charge ions) and different incident ion kinetic energies as compare to ours. Compared to our results and previous results, we found that there was a correlation between the average protrusion diameter and the potential energy of the bombarding highly charge ion and an agreement between our Kr27+ ion bombardment results and the published results.