Thickness, Surface Morphology, and Optical Properties of Porphyrin Multilayer Thin Films Assembled on Si(100) Using Copper(I)-Catalyzed Azide−Alkyne Cycloaddition

We report the structure, optical properties and surface morphology of Si(100) supported molecular multilayers resulting from a layer-by-layer (LbL) fabrication method utilizing copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC), also known as “click” chemistry. Molecular based multilayer films comprised of 5,10,15,20-tetra(4-ethynylphenyl)porphyrinzinc(II) (1) and either 1,3,5-tris(azidomethyl)benzene (2) or 4,4′-diazido-2,2′-stilbenedisulfonic acid disodium salt (3) as a linker layer, displayed linear growth properties up to 19 bilayers. With a high degree of linearity, specular X-ray reflectivity (XRR) measurements yield an average thickness of 1.87 nm/bilayer for multilayers of 1 and 2 and 2.41 nm/bilayer for multilayers of 1 and 3. Surface roughnesses as determined by XRR data fitting were found to increase with the number of layers and generally were around 12% of the film thickness. Tapping mode AFM measurements confirm the continuous nature of the thin films with roughness values slightly larger than those determined from XRR. Spectroscopic ellipsometry measurements utilizing a Cauchy model mirror the XRR data for multilayer growth but with a slightly higher thickness per bilayer. Modeling of the ellipsometric data over the full visible region using an oscillator model produces an absorption profile closely resembling that of a multilayer grown on silica glass. Comparing intramolecular distances from DFT modeling with experimental film thicknesses, the average molecular growth angles were estimated between 40° and 70° with respect to the substrate surface depending on the bonding configuration.