Atomic force microscopy study of photoreversible nanoscale surface relief grating patterns on side chain dendritic polyester thin films

High resolution atomic force microscopy (AFM) has been used to study the photoresponsive behavior and the photoreversible nanoscale surface relief grating patterns of side chain dendritic polyester (SCDPE) polymer thin films. These low molecular weight (<1400 g/mol) SCDPEs were synthesized by stoichiometric addition of diacylchloride azobenzene with unique first, second and third generation dendritic wedge diols, which were added to introduce additional free volume and to optimize trans–cis–trans isomerization of azobenzene moieties. The polymer samples were spin coated onto a quartz slide to obtain a homogeneous thin film with specific thickness. These samples were then irradiated with UV light (365 nm, 100 mW/cm2) through a photomask before mapping the topography of the films using AFM. The AFM images clearly showed nanoscale surface relief grating patterns for both second and third generation polymer films, whereas no detectable response was observed for the first generation polymer due to possible aggregation. The AFM results are consistent with the UV spectroscopy measurements made on the respective films. Moreover, the second and third generation films were shown to be photoreversible when exposed to visible light, thus confirming that these photoisomerized surface relief gratings can be readily patterned and erased by irradiation with non-polarized UV and visible radiation without the need for high energy lasers or separate heating elements. The photoeffectiveness of the film increased with increasing dendrimer generation and film thickness. The nano-gratings remained intact for at least 6 months after the initial exposure. These findings suggest a potential simple and cost effective system for developing reversibly generated nano-gratings for use in photonic and optical applications.