Densification of sol–gel silica thin films induced by hard X-rays generated by synchrotron radiation

Abstract

In this article the effects induced by exposure of sol-gel thin films to hard X-rays have been studied. Thin films of silica and hybrid organic-inorganic silica have been prepared via dip-coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X-ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si-C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X-rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.

Hard X-rays for processing hybrid organic–inorganic thick films

“…The interplay between these three functional groups follows a complex mechanism as it involves simultaneous and delayed interactions triggered by X-rays. As shown in our previous work (Innocenzi et al, 2011), the residual water and the hydroxyl groups, which are present in the as-deposited films, produce upon X-ray exposure radical species that induce different chemical reactions. Radicals, in fact, promote the polycondensation of the siloxane oligomers and, at the same time, allow the degradation and the removal of the organic part of the hybrid matrix.…”

“…A sharp, less intense band at 3600 cm À1 is attributed to the presence of isolated and twin silanols groups (Innocenzi, 2003). The trend shown by these bands is quite unexpected because they increase in intensity with the dose whilst an opposite response has been observed previously in silica and hybrid films with a low content of methyl groups (Innocenzi et al, 2011). On the other hand, the main silica band at 1073 cm À1 [ as (Si-O-Si)] increases in intensity with the dose, which is an indication of the proceeding silica backbone condensation at higher X-ray exposure values.…”

“…One of the most applied lithographic techniques for sol-gel films is UV-patterning which is a simple and very effective method. Another possibility is the exposure to soft (Brusatin et al, 2008) and hard X-rays (Innocenzi et al, 2011) which has shown to be a powerful lithographic tool to pattern thick films (Falcaro et al, 2008(Falcaro et al, , 2009. These techniques apparently cannot be competitive with UV lithography in terms of costs (Ma et al, 2001;Lasagni et al, 2005), because they may require a synchrotron radiation source.…”

Hybrid materials with an increased resistance to hard X-rays using fullerenes as radical sponges

“…These results clearly show the radical scavenging effect of fullerenes at low X-ray doses. Under these conditions, in fact, the C 60 molecules interact with the surrounding matter by absorbing the radical hydroxy species which are responsible for the most part of the densification and damaging of the materials (Innocenzi et al, 2011) (Fig. 4).…”

Tuning the phase transition of ZnO thin films through lithography: an integrated bottom-up and top-down processing

“…Hard X-ray exposure, in fact, is responsible for the formation of radicals, mainly due to the presence of residual water or -OH groups in the as-deposited films. The radicals trigger the polycondensation of the inorganic matrix and degrade the organic groups or moieties that are present in the films after deposition (Pinna et al, 2012;Innocenzi et al, 2011). Recently, exposure to hard X-rays has been exploited for the fabrication of patterns and the formation of nanoparticles in the film matrix.…”