Applied Surface Science 258 (2011) 169–175 Contents lists available at ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc KrF laser ablation of a polyethersulfone film: Effect of pulse duration on structure formation Hedieh Pazokian a,b,∗ , Alexandros Selimis c,1 , Emmanuel Stratakis c , Mahmoud Mollabashi a , Jalal Barzin d , Saeid Jelvani b a Department of Physics, Iran University of Science and Technology, Tehran, 1684613114, Iran b Laser and Optics Research School, Tehran, 1439951113, Iran c Institute of Electronic Structure and Laser, Foundation for Research and Technology- Hellas, Heraklion, 71110 Crete, Greece d Biomaterial Department, Iran Polymer and Petrochemical Institute, Tehran, 1497713115, Iran a r t i c l e i n f o Article history: Received 6 April 2011 Received in revised form 22 June 2011 Accepted 7 August 2011 Available online 11 August 2011 Keywords: Polyethersulfone KrF laser Structure formation Pulse duration a b s t r a c t Polyethersulfone (PES) films were processed with KrF laser irradiation of different pulse durations (). Scanning electron microscopy (SEM) and Raman spectroscopy were employed for the examination of the morphology and chemical composition of the irradiated surfaces, respectively. During ablation with 500 fs and 5 ps pulses, localized deformations (beads), micro-ripple and conical structures were observed on the surface depending on the irradiation fluence (F) and the number of pulses (N). In addition, the number density of the structures is affected by the irradiation parameters (, F, N). Furthermore, at longer pulse durations ( = 30 ns), conical structures appear at lower laser fluence values, which are converted into columnar structures upon irradiation at higher fluences. The Raman spectra collected from the top of the structures following irradiation at different pulse durations revealed graphitization of the ns laser treated areas, in contrast to those processed with ultra-short laser pulses. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Pulsed laser ablation of materials leading to various micro/nano structures has been the domain of extensive research investiga- tions [1–12]. Following UV laser ablation, a variety of structures (e.g. cones and micro-ripples) can be developed on polymers. Great efforts have been dedicated to both the elucidation of the mecha- nisms leading to the structure formation and the clarification of the irradiation parameters of influence on the laser induced struc- tures. Several models have been proposed in order to interpret the formation of these structures on various polymers. However, the similarity of the structures following irradiation of polymeric materials owning to different thermal and mechanical properties suggests that a combination of several mechanisms is probably required to account for structure formation over such a wide range of irradiation conditions [1]. The contribution of each mechanism to the structure formation depends on the irradiation conditions such as the laser wavelength, polarization, medium atmosphere, as well as on the material parameters. ∗ Corresponding author at: Iran University of Science and Technology, Depart- ment of Physics, Farjam Street, Tehran 1684613114, Iran. Tel.: +98 21 82062563. E-mail addresses: h pazokian@iust.ac.ir (H. Pazokian), selimis@iesl.forth.gr (A. Selimis). 1 Tel.: +30 2810 391323. Briefly, the most commonly suggested mechanisms leading to structure formation on different polymer materials comprise of the following: (i) Thermal processes (heat generation and diffusion) at the laser irradiated polymer surface layer during and following the laser pulse combined with local thermal expansion lead to the prop- agation of bulk and surface capillary/thermoelastic waves and, in the case of sufficiently thin samples, to a simultaneous front and back surface motion [2,3]. (ii) The nonlinear relation between the etching rate and the inci- dent laser fluence which dominates as a result of two- photon stepwise absorption by chromophores [4]. (iii) The more efficient etching of amorphous regions compared to crystalline ones (e.g., on PET surfaces) [5]. (iv) The interference between incident and surface scattered parts of the UV laser beam [6] and spatial melting and crystallization [7]. (v) Devitrification of the film surface at temperatures above the characteristic T g of the polymers leads to LIPSS formation in amorphous spin-coated polymer films [8]. (vi) The laser beam shielding by the re-deposited ablation debris [9,10] and possible impurities [11,13]. Conical structures induced by laser irradiation of polymer surfaces have been investigated by numerous research groups 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.025