Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Ion irradiation as a tool for modifying the surface and optical properties of plasma polymerised thin films
Introduction
Swift heavy ion (SHI) irradiation (energies above 1 MeV) is known to induce a number of characteristic radiation-chemical processes in polymeric materials. In common with other types of ionising radiation (e.g., gamma rays, electrons, etc.), SHIs can produce cross-linking, chain scissions, free radicals, and unsaturated bonds in hydrogenated polymers [1]. Unique to SHI irradiation however, is that as the ion traverses the polymeric medium (with penetration depths that can be tailored in the order of tens of micrometres) its large deposited energy density produces a latent track of well-defined dimensions [2], [3]. Furthermore, the energy and species of the ion employed in the irradiation process exert considerable influence on the extent of cross-linking or chain scissions within the structure of the polymer in question [4]. Many of the aforementioned radiation-chemical processes are confined to occur within the penumbra of this track [5]. The capacity for SHIs to modify physical and chemical properties of polymers has fostered a number of studies aimed at developing and enhancing commercial applications. These include lithography processes, the fabrication of nanotube templates, and nanoporous membranes [6], [7], [8].
Radio frequency (R.F.) glow discharge is a well-established dry process for the fabrication of organic thin films [9]. Typically, energy is supplied to a process gas (such as helium, argon, or atmospheric gas) leading to the formation of a non-equilibrium cold plasma within either an internal, external, or electrodeless reactor vessel [10]. Monomer units are then introduced to the plasma glow, where they are subjected to fragmentation, excitation, and ionisation by the energetic plasma. The resulting species subsequently adhere to the substrate material via either physical absorption or chemical bonding processes. The result is the formation of highly cross-linked and structurally disordered films possessing a number of advantageous physical and chemical properties, including strong substrate adhesion, pin-hole free surface topologies, and chemically functionalised surfaces. Furthermore, a large variety of deposition parameters (including vacuum pressure, precursor selection, R.F. power, reactor geometry, monomer flow rate, plasma gas, deposition time, etc.) can be varied to achieve polymer films with specific properties or combinations thereof [11]. For these reasons, R.F. glow discharge polymers have found numerous current and prospective applications as conformal coatings for inorganic electronic assemblies [12], encapsulation films for organic electronic devices [13], and coatings for various other substrate materials (exhibiting an assortment of tailored anti-fogging, corrosion resistant, and abrasion resistant properties) [14].
In this research, we focus on the application of 50 MeV I10+ SHIs to polyterpenol thin films fabricated using an R.F. glow discharge polymerisation technique. The precursor monomer for these films, terpinen-4-ol, is a non-synthetic monocyclic terpene derived from the distillation of tea tree oil. Studies undertaken by our group have demonstrated that in addition to possessing the aforementioned generic plasma polymer properties [15], [16], polyterpenol thin films demonstrate rectifying electron-blocking hole-transport behaviour [17], and the retention of terpinen-4-ol functional groups within the polymerised film imbues this polymer with antibacterial properties [18]. The primary objective of this research is to characterise the effects of iodine SHI irradiation on the surface and optical properties of polyterpenol, and, to the extent that it is possible to do so, to interpret these property changes in the context of the elementary ion energy loss mechanisms. These findings will tailor the direction of future investigations into coupling polyterpenol’s attractive material traits with SHI irradiation’s capacity to modify polymeric material properties.
Section snippets
Substrates
Plasma polymerised polyterpenol thin films were deposited on highly polished 500 μm thick 1 × 1 cm 〈1 0 0〉 photoresist coated single crystal silicon wafers (n-type Sb doped), sourced from Fondazione Bruno Kessler.
Prior to plasma deposition the wafers were rinsed in acetone to remove the photoresist coating. The wafers were then washed in a solution of Extran and distilled water, ultrasonically cleaned in a sonicator (distilled water, 50 °C, 30 min), and rinsed in isopropanol to remove inorganic
Surface properties
XRR (Cu-Kα) measurements were performed at the air–solid interface using a PANanalytical X-Pert PRO reflectometer (high tension = 45 kV, current = 40 mA). These measurements were performed as a function of incident angle (θ), observing the specularly reflected beam as a function of the momentum change perpendicular to the surface (Qz = 4πsinθ/λ).
The XRR data was modelled using the Motofit [19] reflectivity analysis software package running in the IGOR Pro environment. These data were fitted as log(R)
Surface topography – AFM
AFM imaging in Fig. 1 details the progression in surface pore density (i.e., number of pores/unit of surface area) as a function of fluence. Fig. 1A shows the surface profile of pristine polyterpenol films, and Fig. 1B shows the formation of discreet pores following irradiation to a fluence of 1 × 1010 ions/cm2. Irradiation to the fluence of 1 × 1012 ions/cm2 resulted in significant etching of and damage to the polymer film, coupled with the elimination of discernible discreet ion tracks.
The
Conclusion
In this study we have demonstrated that 50 MeV I10+ irradiation of plasma-polymerised polyterpenol thin films can be employed to modify the material’s optical properties (producing a substantial reduction in refractive index across all investigated wavelengths for specimens irradiated at 1 × 1012 ions/cm2) and surface architecture (including the incorporation of nanopores, or substantial material etching). These findings indicate that SHI irradiation of plasma-polymers may be employed to fabricate
Acknowledgements
The authors would like to thank AINSE Ltd for providing financial assistance (Award No ALNGRA14049) to enable work on ANSTO’s 10 MV ANTARES and XRR facilities to be conducted. The authors are also grateful for the assistance of Shane Askew from the JCU AAC for AFM characterisation.
D.S.G. is a recipient of an Australian Postgraduate Award (APA). K.B. acknowledges funding from JCU and ARC (DE130101550).
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