Chemistry Reference
In-Depth Information
Optical Anisotropy of Thin and Ultrathin
Porphyrin Layers
Claudio Goletti
Abstract The use of porphyrins as the sensing elements in technical applications
(e.g. electronic nose and electronic tongue) has anticipated the real comprehension
of the mechanisms determining at the microscopic level the interaction between the
analyte molecules and the layer itself. A huge bulk of phenomenological data has
been accumulated, showing the high efficiency of these organic materials versus
specific gaseous particles. It has been also reported that the sensitivity to gas is
higher for highly disordered layers, probably due to the enhanced surface area of the
resulting assembly. A better comprehension of the microscopic behavior would
definitely allow a deeper exploitation of the properties of the layer in view of the
most efficient electronic devices . Consequently, recently there has been an effort to
gain a higher level of accuracy in characterizing the electronic properties and the
morphology of the thin (sometimes ultrathin) layers deposited.
In this chapter, I will present and discuss the most significant results obtained by
studying the optical anisotropy of thin and ultrathin porphyrin films. In particular, I
will mostly limit to data obtained by using a powerful optical technique (originally
developed in traditional surface science), namely, reflectance anisotropy spectros-
copy (RAS). I will show how the measured spectral features in RAS spectra have
their origin in the structure of the layer and can be connected to the interaction
existing between molecules in the layer. I will briefly describe the technique, giving
some details about the experimental setup and explaining how experimental data
can be interpreted. A review of results will follow, from the very early applications
on Langmuir-Blodgett and Langmuir-Schaefer films to the more recent study of
ultrathin layers evaporated in ultrahigh vacuum.
Keywords Analyte discrimination
Langmuir-Blodgett
layers
Langmuir-
Schaefer layers
Optical anisotropy of Langmuir-Blodgett
layers
Optical
properties of thin layers
Optical sensors
Porphyrin layers
Reflectance anisotropy
spectroscopy (RAS)
