Variable Index of Refraction Ultrathin Films Formed from Self-Assembled Zirconium Phosphonate Multilayers: Characterization by Surface Plasmon Resonance Measurements and Polarization/Modulation FT-IR Spectroscopy

Abstract

Ultrathin (submicrometer) organic films of specified index of refraction are constructed by the sequential deposition of self-assembled zirconium phosphonate (ZP) molecular monolayers that contain two different bisphosphonate ions, 1,10-decanediylbis(phosphonate) (DBP) and 4,4'-azobis[(p-phenylene)methylene]bis(phosphonate) (AZO). By varying the ratio of DBP to AZO monolayers in the ZP multilayer film, the index of refraction can be controlled. A combination of surface plasmon resonance (SPR) measurements and polarization/modulation Fourier transform infrared reflection-absorption spectroscopy (PM/Fr-IRRAS) is used to examine the structure of the ZP multilayers on vapor-deposited gold substrates, and ellipsometric measurements are utilized to determine the index of refraction of the ZP multilayers on transparent substrates. ZP films consisting of 100% DBP and 100% AZO molecules are found to have indices of refraction of 1.51 and 1.64, respectively, at an optical wavelength of 632.8 nm. In both the 100% DBP and the 100% AZO multilayers, an average monolayer thickness of 16 ± 0.5 A is determined from the SPR measurements. The spectroscopic and ellipsometric data indicate that the index of refraction of mixed ZP multilayer films (i.e., films with both DBP and AZO self-assembled monolayers) can be varied systematically between 1.51 and 1.64, but that the average thickness of the self-assembled monolayers is greater in the mixed films than in either of the 100% DBP or 100% AZO multilayers.