Molecular dynamics study of polystyrene bond-breaking and crosslinking under C 60 and Ar n cluster bombardment Bartlomiej Czerwinski a,⇑ , Zbigniew Postawa b , Barbara J. Garrison c , Arnaud Delcorte a a Institute of Condensed Matter and Nanosciences – Bio & Soft Matter (IMCN/BSMA), Université Catholique de Louvain, 1 Croix du Sud, B-1348 Louvain-la-Neuve, Belgium b Smoluchowski Institute of Physics, Jagiellonian University, ul. Reymonta 4, 30-059 Krakow, Poland c Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA article info Article history: Received 19 July 2012 Received in revised form 31 October 2012 Accepted 1 November 2012 Available online 29 December 2012 Keywords: Molecular dynamics Cluster SIMS Molecular depth profiling Crosslinking Desorption Polystyrene abstract Molecular dynamics computer simulations are used to elucidate the bond-breaking and crosslinking pro- cesses induced by 2.5 keV C 60 and Ar n cluster bombardment in an amorphous sec-butyl-terminated poly- styrene sample. The obtained results indicate that replacement of C 60 by Ar 18 or Ar 60 projectiles leads to the decrease of the number of broken bonds and, hence, to the decrease of formation of new intra- and intermolecular (crosslinking) bonds. When the number of atoms in the Ar n cluster is increased from 60 to 250 or more, the total number of broken bonds and the total number of newly created bonds reach a zero value. Additional comparison to the case of a fullerite crystal reveals that the change of material proper- ties leads to almost 7.5-fold reduction of the efficiency of the crosslinking process. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Over the last few decades energetic ion beams have become important processing and characterization tools for a broad seg- ment of the scientific and technological manufacturing sector. Nowadays, atoms, molecules and atomic and molecular clusters are routinely used for surface analysis and treatment through the use of techniques like secondary ion mass spectrometry (SIMS) [1,2], desorption electrospray ionization (DESI) [3–5] mass spec- trometry or the recently developed desorption ionization by charge exchange (DICE) [6,7]. The main application fields are in microelec- tronics, nanotechnology and biological research. The introduction of cluster ion guns such as SF 5 ,C 60 , Au n , Bi n and Ar n resulted in a particular acceleration of the development of the SIMS technique. Their implementation allows for new proto- cols for the analysis of sensitive organic materials and, especially, biological samples [1]. The latest research reveals that large clus- ters composed of thousands of argon atoms produced by super- sonic expansion of high-pressure gas through a nozzle with an energy of a few eV per atom (originally developed for surface smoothing [8]) are able to produce relatively high yields of molec- ular ions with low (and tunable) fragmentation [9]. Unlike SF 5 and C 60 , the Ar n clusters are capable of depth profiling most types of or- ganic materials used so far for analysis [2,10]. Energetic cluster ion bombardment of organic materials initi- ates processes which lead to many chemical reactions within the bombarded samples. The main issues observed for polymers dur- ing depth profiling experiments, is thought to be the formation of crosslinks within the bombarded sample and/or the graphitiza- tion of its surface [2]. In general the crosslinking process is under- stood as the formation of new bonds between different molecules or polymeric chains which usually leads to the formation of large chunks of newly bonded material and to the significant reduction of sputtering efficiency. On the other hand, the carbonization of the surface, is the formation of a carbon layer on the surface of the bombarded material. This process is usually the consequence of the fragmentation of the organic material initiated by impinging projectiles, leading to the creation of a large number of free, highly reactive, carbon radicals which may combine to form a compact, usually amorphous, structure. With continuous bombardment, this compact structure can significantly increase its volume and, as a consequence, begin to block the emission of organic material lo- cated below [2]. In this research, classical molecular dynamics (MD) computer simulations are used as a tool for the theoretical insight into the bond-breaking and the new bond formation processes in an amor- phous sec-butyl-terminated polystyrene sample, initiated by the irradiation with C 60 and Ar n (n = 18, 60, 250, 500, 1000, 1700, 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.11.030 ⇑ Corresponding author. Tel.: +32 10478460; fax: +32 10473452. E-mail address: bartlomiej.czerwinski@uclouvain.be (B. Czerwinski). Nuclear Instruments and Methods in Physics Research B 303 (2013) 22–26 Contents lists available at SciVerse ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb