Ion beam induced carbon clusters in polymers

Abstract

It is thought that the formation of carbon precipitates (or carbon enriched zones) during ion irradiation of polymers is responsible for the observed optical blackening as well as for the onset of electronic conductivity. The latter is explained as the result of an electronic hopping mechanism between the conducting carbon (enriched) zones. Rough estimations from electronic condition theory indicate typical cluster sizes of 10 to 50 nm diameter and surface-to-surface distances of 0.1 to 20 nm. The existence of these carbon precipitates in polymers has never been proven directly. Therefore, we used neutron scattering to verify them in 50 MeV boron implanted mylar foils. As neutron scattering is sensitive to essentially hydrogen atoms in the polymers, the carbon precipitates show up as hydrogen depleted zones. Indeed, neutron scattering data suggest the formation of small globular particles. Their dimensions (radius 6 to 7 nm) and mutual distance (around 13 nm) agree with previous predictions. The particle population seems to be quite homogeneous in size. The number density of these particles decreases with increasing deposited ion energy density. Simultaneously, larger objects with a sharp surface are observed. Their number density increases with increasing transferred ion energy, as judged from the increase of total surface area. We interpret these objects as clusters of the above described globular carbon precipitates.