Ion bombardment of Poly-Allyl-Diglycol-Carbonate (CR-39)
Introduction
Ion beam bombardment is a suitable tool to modify the physico-chemical properties of polymers. Poly-Allyl-Diglycol-Carbonate (CR-39), well-known polymer, is used in diverse fields such as neutron dosimetry, gamma and cosmic ray detection, heavy ion and nuclear physics. CR-39 is transparent in visible spectrum and almost completely opaque in infrared and ultraviolet range. It has high abrasion resistance, about half the weight of glass and index of refraction only slightly lower than that of crown glass. This makes it an advantageous material for eyeglasses and sunglasses. A wide range of colors can be achieved by dyeing the surface or the bulk of the material. The use of ion beams to modify polymer properties opened a wide area of research and utilization in various fields like industry, agriculture, ecology [1], sensorics [2], [3], microelectronics [4] and nanotechnology [5], [6], [7].
The damage processes triggered by ions can be different from those induced by classical low ionizing particles as electrons or gamma rays [8], [9]. This is due to the very high electronic stopping power of these ions. A gradual increase in absorbance [10], generation of chromophoric groups, loss of crystallinity [11], formation of carbonized inclusions along with a gradual phase transition [12], as well as creation of three-dimensional network and triple bonds [13], [14] have been found in polymers due to ion bombardment. In most cases the amount of energy deposited in the host polymer is enough for extensive breaking of the chemical bonds within the track, which makes that strong modification of the bombarded polymer material, is possible. The energy released by the ion during slowing down is deposited in the target by means of two basically different mechanisms, electronic excitation and nuclear collision, which induce quite different processes of polymer matrix rearrangement [15]. Both mechanisms act simultaneously and their additive contribution to the total energy losses of the incoming ion is characterized by stopping power (Se for the electronic mechanism and Sn for the nuclear one), which is usually expressed in eV/Å. The values of Se and Sn depend on the energy and mass of the bombarded ion as well as on the polymer composition, The process of energy deposition by incoming particles is concentrated inside the track and is accompanied by (i) radical formation, (ii) formation of carbon–carbon multiple bonds, (iii) loss of volatile molecules (iv) main chain scission, and (v) intermolecular cross-linking. The absorption of light energy by polymeric materials in the visible and ultraviolet region involves transition of electrons in n, π and σ orbitals from the ground state to a higher energy state [16]. Many molecules contain electrons that are not directly involved in bonding; these are called non-bonding or n-electrons and are mainly located in atomic orbitals of oxygen, sulphur, nitrogen and halogens. The polymers containing these molecules in the structure are expected to play an important role in UV–visible absorption. Ultraviolet-visible (UV–vis) spectroscopy has become an important tool for investigating the optical properties of polymers. It is used to estimate the value of optical band gap energy (Eg) in polymers. Carbonaceous clusters are supposed to be rich with charge carriers that enhance the electrical conductivity in ion bombarded polymers and consequently they also influence the optical properties of such materials [17]. Eventually with increasing ion fluence the surface layer can be transformed into a hydrogenated amorphous carbon [18]. Different studies reveal that carbonaceous clusters are formed along latent tracks of energetic ions in polymers [19], [20], [21]. The formation of these carbonaceous clusters can also be studied from the absorption edge of ultraviolet–visible (UV–vis) spectra of irradiated polymers [22], [23]. The possibility of organic polymers for optical applications is not limited, and developments of organic polymers have been more and more promoted.
The aim of this work is to investigate the effect of ion beam bombardment on photoluminescence and optical properties of CR-39 polymer material. The optical parameters such as band gap and Urbach's energy have been determined for the bombarded CR-39 samples.
Section snippets
Experimental details
Poly-Allyl-Diglycol-Carbonate, CR-39, sheets manufactured by Pershore, Ltd, England, are used in this study. They are of density 1.32 g/cm3 (composition C12H18O7, molecular weight 274 a.m.u.) and thickness 1.5 mm, prior to spectral analysis.
Ion bombardment of CR-39 polymer sheet was carried out in vacuum at room temperature by means of commercial Blazers MPB 202 RP ion implanter at the Institute of Electronic Material Technology (IEMT), Warsaw, Poland. Bombardment was carried out in vacuum (in the
UV–vis spectral analysis
Fig. 1, Fig. 2 show the UV–vis spectra of pristine and bombarded CR-39 polymer with different fluencies of 320 keV Ar and 130 keV He ions, respectively. From these optical spectra, it is clear that the pristine sample has a sharp decrease in absorption with increasing wavelength up to certain value, followed by a plateau region in its UV–vis spectrum. The optical spectra of CR-39 bombardment with both ions show shifting in the absorption edge towards longer wavelength with increasing ion fluence.
Conclusions
In the present study modification in optical properties of CR-39 induced by He and Ar ions bombardment has been investigated by UV–vis spectrophotometer and photoluminescence (PL) spectroscopy. Optical absorption spectrum of the pristine sample shows a sharp decrease with increasing wavelength up to a certain value, followed by a plateau region. The optical absorption increases with increasing ion fluence of He and Ar ions and this absorption shifts from UV–vis towards the visible region as the
Acknowledgment
The authors would like to thank Professor Dr. A. Turos and the staff of the Institute of Electronic Material Technology (ITME) Warsaw, Poland for providing the facilities for the ion beam implantation.
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