Investigation into the surface relief grating mechanism via XPS in new azobenzene based optical material

Research into azobenzene and its compounds focuses on the molecules ability to cis-trans isomerize, this photoisomerization enables surface relief gratings (SRGs) to be formed on the azobenzene functionalized polymer films. SRG allows information to be written and then erased, an essential requirement of most modern electrical products. The interest into SRG photofabrication is thus rapidly increasing and is emerging at the forefront of photonic and nanotechnology research. The surface relief grating mechanism, however, is not fully understood, a photoplasticization process is postulated, which is thought to occur via the azo-chromophores photoisomerization, so encouraging polymer chain migration to the surface. In comparison, suggestions have shown the SRG process does not involve a change in the film surface profile but in the local refractive index. Currently, no research has yet established the surface relief grating mechanism, this paper sets out to determine the mechanism and investigate those theories already postulated by observing the thin film surfaces prior to and after SRG using X-ray photoelectron spectroscopy, (XPS). By identifying the surface components, greater insight and understanding of the SRG mechanism can be achieved. In this paper, poly(4-(N-(2-methacryloyloxyethyl)-N-ethylamino)-4'-nitroazobenzene)(90)-co-(methylmethacrylate)(10) 50:50 PMMA blend was exposed to SRGs. Using XPS the surface composition was determined prior to and after SRG. Following SRG, the spectra for O 1s, N 1s and C 1s using XPS expressed a change in the components at the surface. This is most evident in the N 1s spectra, with PMMA not containing nitrogen, the N 1s becomes the determining factor. The nitrogen absence combined with a significant increase in the carbon and oxygen peak intensity concludes the azobenzene lies not on the surface but, in fact, within the bulk after SRG. The initial light irradiation process must be a consideration. However, on X-ray analysis the sample showed the same spectra as the one prior to SRG. Copyright (C) 2002 John Wiley Sons, Ltd.