R.P. Livi

Modifications in the chemical bonding and optical absorption of PPS by ion bombardment

Abstract Commercial-grade thin poly(p-phenylene sulphide) (PPS) foils, 2 μm thick, have been bombarded with 1H+ (380 keV), and with 0.61 MeV/amu 4He+, 12C2+, 16O3+, 32S3+, 79Br9+ and 127I14+ ions. The completely modified foils have been analyzed by Fourier transform infrared absorption spectroscopy (FTIR) and by ultraviolet-visible absorption spectroscopy (UV-VIS). The bond breaking and rearrangement processes were followed by using the FTIR results. The absorbance of bands related to CS, SS, CH and most of the ring carbon bonds show an exponential decrease as a function of fluence. The absorption bands most sensitive to the ion bombardment are those assigned to CS and SS bonds. The CC and CH bonds are less sensitive and certain absorption band changes indicate carbon-carbon aliphatic and or aromatic conjugation, due to their increasing intensity as a function of the ion fluence. Damage cross sections have been extracted from the plots of IR absorbance versus fluence. The values of the damage cross sections obtained scale roughly with the square of d E d x (independent of the nature of the chemical bond). The optical absorption coefficient has also been followed as a function of the ion fluence.

The fluence effect of Ar++ bombardment in PPS

Abstract The modifications induced by ion bombardment on the physical and chemical structures and on thermal, optical and electrical properties of poly(phenylene sulphide), PPS, were investigated. Thin PPS foils, 2, 6 and 125 μm thick were bombarded with Ar++ (700 keV) under initial vacuum of 10−6 torr. Changes in the chemical structure were monitored by infrared absorption spectroscopy (FTIR) and ultraviolet and visible spectroscopy (UV–VIS). Modifications in the relative atomic composition of the bombarded polymer samples were determined by elemental analysis (CHN) and Rutherford backscattering spectrometry (RBS). Processes resulting from ion implantation on the physical structure of PPS were followed by X-ray diffraction spectrometry (XRD), differential scanning calorimetry (DSC), solubility tests and electrical conductivity measurements. Thermal stability of these samples was established by thermogravimetric analysis (TGA). Ion bombardment induces electronic excitation and ionization of molecular species, which leads to crystallinity loss, chemical bonds disruptions and formation of free radicals. These reactive groups lead to the formation of cross-linking processes and absorption of atmospheric gases, like oxygen and nitrogen. Oxygen is combined with the polymer main chain, partaking in the cross-linking and in the formation of conjugated structures. Due to extensive bond conjugation the energy gap between valence and conduction bands diminishes. This process favors charge transport, leading to an increase of the macroscopic electrical conductivity. However, after bombardment, the oxygen absorption induces a continuous decrease of the conductivity, even after a period of six months. The samples irradiated with the highest fluences exhibit conductivities similar to those of semiconductors. A kinetic study of the thermal degradation of implanted samples indicates that the thermal stability, defined by the onset temperature and the activation energy of the process, decreases with increasing fluence.

Study of the thermal stability of PPS films bombarded with B+ and Ar2+ ions

Abstract The thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and solubility tests, can be advantageously used on the study of chemical effects on ion-bombarded polymeric materials. Ionic implantation affects both chemical and physical structures of these materials and at low fluences these changes are hard to detect. TGA allows a qualitative assessment of the degradation process through the analysis of residual mass thermograms and a quantitative evaluation of the bombarded samples activation parameters by means of dynamic kinetic measurements. For PPS (paraphenylene sulfide) samples irradiated with 10 12 B + /cm 2 fluence the activation energy ( E A ), calculated for low conversions (10–30%), is 290 kJ/mol, which is similar to the value of the pristine polymer. Higher fluences B + led to lower E A showed a drop of 30% in relation to the pristine. Effects on PPS, poly(paraphenylene sulfide), bombarded with B + and Ar 2+ led to a gradually increasing loss of thermal stability and higher residual mass. Multimodal thermograms revealed increasing complexity of the resulting polymeric materials as fluences become higher. Ion stopping power played a key role on the thermal behavior of the modified polymer chains. The TGA results correlated well with the FTIR results, in which C-S and S-S bonds showed to be more susceptible to the ionic bombardment and thermal degradation process than the other bonds. Progressive crosslinking in the bombarded samples is followed by solubility tests and residual mass.

Probing glass transition of PMMA thin films at the nanometer scale with single ion bombardment and scanning force microscopy

Abstract Poly(methyl methacrylate) (PMMA) thin films maintained at temperatures from −196 up to 150 °C are bombarded at grazing angles by 20 MeV Au ions at very low fluences. Immediately after the irradiation, the samples are cooled down at two distinct rates. Scanning force microscopy (SFM) images of the bombarded targets after cooling reveal nanometer-sized craters and/or raised regions (hillocks) around the point of each ion impact. The size of the craters is independent of the temperature for −196 ° C ° C , but for higher temperatures, crater dimensions increase steeply and no hillock is observed behind the crater. The temperature above which the hillocks disappear is sensitive to the cooling rate and for rapidly cooled targets it is near the tabulated value for the glass transition temperature, Tg, of the polymer. Since the variation of the morphology of ion-induced defects occurs over a narrow temperature range and depends on the cooling rate of the targets, single ion bombardment coupled to SFM may be used as an alternative method to estimate Tg and relaxation rates of nanodeformations in thin polymer films. The post-irradiation thermal annealing of the surface tracks produced at room temperature is also discussed.

Large Z1-range effect for Eu, Yb and Au ions implanted in amorphized silicon

Abstract The Z1-range oscillation amplitude is investigated as a function of energy for Eu, Yb and Au ions implanted in amorphous Si at energies from 10 to 390 keV. The obtained experimental results are not reproduced by the recent theoretical predictions of Burenkov and collaborators, showing large discrepancies for energies lower than 50 keV.

High pressure annealing of defects induced by ion implantation on graphite

Abstract The aim of this work was to investigate, by Raman spectroscopy, the effect of high pressure on the annealing of the surface disorder induced by hydrogen, carbon and nickel ion implantations on graphite samples. In the case of H implantation, the induced disorder decreased after annealing at 650°C for 7.7 GPa. For C implantation, the defect annealing started at 500°C for 7.7 GPa, while in the case of Ni implantation, it started at 850°C for the same pressure. For the three cases studied, there was practically no annealing effect for temperature treatments in vacuum up to 1100°C. The results indicated that high pressure plays an important role in the defect annealing, accelerating the graphitization process, probably due to the large volume of the defective ion implanted phase. Despite the annealing at high pressure being conducted deep inside the thermodynamic region of stability of diamond, the graphite phase was recovered, indicating that the nucleation barrier for diamond is indeed very high as compared to graphite.

Morphological changes induced in PPS by ion bombardment: amorphisation and recrystallization ability

Abstract Thin poly(paraphenylene sulfide) (PPS) films (Torelina, from Toray Co., Japan, 2 μm thick) were bombarded with energetic 1 H + (300 keV), 4 He + (380 keV), 10 B + (350 keV) and 40 Ar 2+ (700 keV) ions at fluences ranging from 10 12 to 8 × 10 14 ions/cm 2 . The beam current density was kept at 20 nA/cm 2 in order to avoid excessive heating and outgassing damage. The modified samples were analysed by differential scanning calorimetry (DSC). Both the melting onset temperature and the enthalpy of fusion, that are related to the crystalline structure, decrease with increasing ion fluence. This systematic study, made with successive heating and cooling cycles in the DSC measurements, shows that the ion bombarded polymer gradually lost its crystallinity and its ability for recrystallization, as the ion fluence increases. Over a critical fluence, which depends on the bombarding ion and energy, the amorphisation process accelerates and above a certain fluence the polymer becomes completely amorphous. The above discussed modification in the PPS morphology occurs in a fluence range where the chemical degradation was not significantly noticed by Fourier transform infrared spectroscopy (FTIR) measurements. The modification radii for the amorphisation process were extracted and the results discussed considering the energy deposition density.

Chemical damage and aging of ion bombarded PPS

Abstract Thin foils of commercial grade polyphenylene sulfide (PPS), 2 μm thick, were bombarded with energetic H+ (300 keV), He+ (350 keV), B+ (350 keV) and Ar++ (700 keV) ions at fluences ranging from 1012 to 10 15 ions/cm 2 . Fourier transform infrared spectroscopy (FTIR), Rutherford backscattering analysis (RBS) and chemical elemental analysis (CHN) were performed to evaluate the chemical changes induced by ion bombardment in the polymeric samples. It was verified that the S–S and C–S acyclic bonds were more susceptible to ion bombardment, and the aromatic ring bonds are the most resistant ones. The effective modification radii for the bond breaking and recombination processes were extracted. The aging of bombarded PPS was monitored and oxygen and nitrogen uptake increased linearly with time.

Damage and ageing effects on proton beam irradiated C60 films

Abstract In this paper, we report results on the interaction of 400 keV H + ions with thin C 60 films focusing on post-irradiation ageing effects. Infrared spectroscopy and resonant α-scattering have been used for characterization of the targets. The analysis were made at different times after bombardment during a period of several months. It has been observed that the signal from the characteristic C 60 IR bands decrease up to 50–70% after the first post-irradiation analysis of the targets. Such ageing is correlated to the appearance and evolution of the oxygen signal in the resonant α-scattering spectra, as well as to the presence of carbonyl bands in the IR spectra of aged targets. For a fixed ageing time the intensity of C 60 IR bands decrease exponentially as a function of fluence.

Bulk and surface modifications of insulating poly (paraphenylene sulphide) films by ion bombardment

Abstract Thin poly(paraphenylene sulphide) (PPS) films (2 μm thick), bombarded with He + (380 keV), B + (350 keV) and Ar 2+ (700 keV) at fluences ranging from 10 12 to 2 × 10 16 ions cm −2 , were analysed by X-ray diffractometry, UV-visible absorption spectroscopy, electrical resistance measurements and solubility tests. The polymer gradually underwent an amorphization process indicated by the decrease in the main X-ray diffraction peak area and also lost its solubility with increasing ion fluence. This behaviour revealed the formation of cross-linking and amorphous structures. A red shift of the optical absorption threshold with increasing fluence was also observed by UV-visible spectroscopy. This trend is usually attributed to the conjugation of unsaturated carbon bonds which gives rise to non-localized π electrons. From these spectral data the gap between valence band and conduction band (optical gap E g ) using a model for amorphous semiconductors can be obtained. The conjugation process increased with fluence resulting in a decrease in the optical gap. For Ar 2+ -bombarded samples the optical gap saturated at 0.9 eV for fluences around 10 15 cm −2 . The decrease in E g with increasing fluence is a direct indication that a new group of conducting structures is being formed in the polymeric material. Electrical measurements made immediately after 2 × 10 16 Ar 2+ cm −2 bombardment revealed that the electrical resistivity decreased by 18 orders of magnitude in relation to the original PPS. When the bombarded samples were exposed to air the electrical conductivity decreased. This electrical instability was assigned to the free radicals present in the polymer chain after ion bombardment. This work shows that different hydrogenated amorphous carbon films can be obtained from one common polymeric matrix by judicious choice of the projectile ion stopping mechanism.