J. P. Duraud

Electron trapping in amorphous SiO2 studied by charge buildup under electron bombardment

Electron bombardment of thick pure SiO2 induces the buildup of a negative charge which can be observed through a ‘‘mirror’’ effect in a conventional Auger scanning microscope. A mechanism for the creation of this charge is proposed in terms of trapping of an electron in defects due to the irradiating beam. The influence of temperature is studied on amorphous and monocrystalline SiO2. The temperature dependence of the existence of high negative charge shows around 270 °C an anomalous effect which depends on the irradiation time. The role of electronic excitation to produce defects in silica is discussed.

Electronic structure of an AlN film produced by ion implantation, studied by electron spectroscopy

N+ ion implantation in a pure Al (111) monocrystal triggers a crystalline and stoichiometric thin AlN film. A complete description of the electronic states of the film is obtained by combining different spectroscopies carried out in situ. The density of electronic states in the valence band is given by x ray and UV photoemission spectroscopy; excitation of a core level (Al2p) by electrons provides information on the density of unoccupied states in the conduction band. Low‐electron energy‐loss spectroscopy allows one to study transitions between occupied and unoccupied states, as well as localized levels in the band gap, due to the presence of structural defects.

Modifications of polyvinylidene fluoride (PVDF) under high energy heavy ion, X-ray and electron irradiation studied by X-ray photoelectron spectroscopy*

Polyvinylidene fluoride (PVDF) samples have been irradiated with energetic heavy ions. Their structural and electronic modifications were studied by X-ray photoelectron spectroscopy (XPS) using a monochromatised X-ray source. A non-monochromatised X-ray source was used to irradiate the sample and to study the modifications induced by both X-rays and electrons. In addition a foil of polymer which was subjected to energetic heavy ions irradiation in the GANIL accelerator was also studied and compared with the samples irradiated with electron and X-ray. Changes in the shape, intensity and energy position of the C 1s line enable us to suggest a two step mechanism for the polymer evolution after irradiation: i) creation of allenes produced by dehydrofluorination of the polymer backbone; ii) disappearance of the double bonds by addition of species (atoms or free readicals) generated under the beam.

Modifications induced in polyvinylidene fluoride by energetic ions

Abstract Modifications induced by energetic ions have been investigated in the regime of electronic stopping power by X-ray photoemission spectroscopy (XPS), infrared absorption spectroscopy (IRAS) and differential scanning calorimetry (DSC). Stacks of thin foils of polyvinylidene fluoride (PVDF) have been exposed to ion beams with energies ranging from 1 to 50 MeV/amu and with different atomic number Z ( 16 O , 84 Kr , 129 Xe ) . XPS measurements provide information about the surface modifications of irradiated PVDF: (1) oxygen ions bombardment leads to the creation of cumulene compounds (CCC)n following hydrogen fluoride desorption, the intensity of which increases with the irradiation dose δE, the fluence φt of the ion and the energy loss ΔE in the polymer; (2) for krypton or xenon irradiation, no changes can be detected on the surface composition. Differential scanning calorimetry (DSC) performed on a pristine sample and on PVDF irradiated with xenon ions shows a decrease of the melting point TFand of the enthalpy ΔHF with the irradiation dose δE. This behaviour is characteristic of a loss of crystallinity due to the internal reorganization of the polymer (creation of polyallenic compounds, chain scission, decrease of the molecular weight). Infrared transmission spectra (IRAS) and attenuated total reflexion spectroscopy (ATR) show that only surface modifications are induced by oxygen ions. For energetic heavy ions (Kr, Xe), the bulk of the polymer is loaded with unsaturated compounds like allenes (2038, 1720, 1610 and 1560 cm−1), and is depleted in fluorine atoms. On the other hand, the PVDF surface studied in ATR mode exibits a much smaller evolution. There results are consistent with XPS data. All these experimental data can be explained by gas formation and desorption, migration in the bulk of reactive species like free radicals and internal recombinations.

(011̄0) α-quartz surface: a LEED, XANES and ELS study☆

Abstract We studied the structure of the (0110) α-quartz surface after different treatments (900°C vacuum or air heating and ion bombardment). The long-range order was investigated by low-energy electron diffraction (LEED), the mean-range order by X-ray absorption spectroscopy at the oxygen K edge (XANES) and the electronic structure of the surface band gap by low-energy electron loss spectroscopy (ELS). The (1 × 1) structure is obtained only after a slight chemical etching in HF. A 900°C air-annealed sample exhibits a (3 × 1) or (1 × 3) LEED pattern associated with reconstructed surface regions, which structure is either related to the bulk α → β transition phase occurring at 573°C, or to a new SiO2 phase present only at the surface (tridymite). Finally, a 900°C heating in vacuum provides a glassy surface, also containing oxygen vacancies. These three structures lead to different surface electronic properties: the reconstructed surface presents a larger surface band gap than the (1 × 1) (9.5 instead of 8.6 eV). The disordered surface shows an 8.8 eV wide surface band gap, containing two electronic levels associated with surface oxygen vacancies.

Electronic and structural modification of polyvinylidene fluoride under high energy oxygen ion irradiation

Stacks of polyvinylidene fluoride (PVDF) foils were exposed to an oxygen beam of 800 MeV. Electronic and structural modifications induced by the radiations were studied by X-ray photoelectron spectroscopy (XPS) using a monochromatized X-ray Al K α source, and by electron spin resonance (ESR) experiments. It was observed that i) the creation of allenes compounds produced by the dehydrofluorination of PVDF; ii) the presence of primary free radicals and peroxide radicals. The amount of the created defects is related to the electronic ion stopping power d E /d x and to the deposited dose.

Radiation induced charges IN SiO2

Abstract The influence of different treatments (radiation, temperature) is studied on SiO2 which has been negatively and positively charged. Electrons, X-rays, and UV photons are able to neutralize a positive and/or a negative charge through interaction with the localized levels in the band gap of the insulator. The temperature dependence of the existence of high negative charge shows at 300°C an anomalous effect.

Surface processes occurring on PVDF under energetic ion irradiation

Abstract Modifications induced in polyvinylidene fluoride by energetic ions (E ⩾ 1 MeV/amu) have been reported in previous papers [1–3]. It was shown by using methods such as X-ray photoemission spectroscopy (XPS), infrared absorption spectroscopy (IRAS), electron spin resonance (RPE) or differential scanning calorimetry (DSC) that light and very heavy ions lead to opposite modifications of the polymer. Superficial unsaturations characterized as cumulene compounds, and HF gas desorption were produced under oxygen ions. On the other hand, with heavy ions, the polymer surface appears not to be damaged and a rebuilding process was suggested, which involves a migration of reactives species from the bulk to the surface. In this paper, we report desorption experiments and XPS measurements which support a readdition mechanism of HF molecules on cumulene compounds. Gas desorption measurements carried out with a quadrupole mass spectrometer, during the polymer exposure to the ions, lead to a two-step desorption mechanism: migration of HF and F2, followed by desorption of these species from the surface. XPS spectra of PVDF samples irradiated at a high fluence with a focussed krypton ion beam show new structures which correspond to the readdition of reactive species (HF∗, H∗, F∗), on unsaturations produced during the first step of the modification. A PVDF sample of 50 μm thickness previously irradiated with oxygen ions which led to superficial allenic structures was irradiated again with krypton ions. The XPS measurements of the C1s and F1s photoemission lines, the shape of which is similar to that obtained for a pristine PVDF sample, clearly show that a readdition mechanism occurs on the unsaturations previously produced under energetic light ions.

Radiation effects in SiO2 under electronic excitation

Abstract Excitation of electrons in pure amorphous and crystalline bulk SiO2 may induce transient or stable modifications of their geometrical and electronic structure. Such situation is found either under electrons, X-rays, γ-rays, UV laser irradiation or along the path of very energetic ions (>62; MeV/amu). In turn, the bulk characteristics of the material such as loss tangent, Youngs modulus can be changed. A review is given of this type of radiation effects. It includes surface studies, and bulk defect production. It has been adduced that defects production mechanism involves the self trapping of excitons. This process will be discussed in the frame of new results related to high local densities of electronic excitation.

Aging of fluoropolymers irradiated by X-Rays, low energy electrons and energetic heavy ions

Abstract Polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) samples have been irradiated with X-rays, electrons and energetic heavy ions. Changes of electronic structure and chemical bonding have been studied by X-ray photoelectron spectroscopy (XPS). Under electron bombardment, a surface modification of PTFE is first observed (C =. C bond formation). Then bulk reticulation and new bond creation appears The variation of the C15 line with detection angle shows that the graphitization like process is restricted to the surface atomic layer. After the electron bombardment, a slow evolution of the polymer is observed as a function of time (aging process). PVDF appears to be more sensitive to ionizing radiation than PTFE. Indeed, modifications are observed under the X-ray probe. Rational mechanisms are proposed to explain the observed changes.