While glasses prepared by different routes have different properties (and therefore must have different structures), it is often difficult to directly measure these structural differences. Below we discuss methods for determining the density of vapor-deposited glasses. In addition, vapor-deposited glasses can be anisotropic and we have ways of characterizing this as well.
Ellipsometry is a polarized light optical technique that lets us measure the thickness of a film, its index of refraction, and any light absorption properties it might have. By using a broadband light source we are able to determine the thickness and the optical properties for the near-UV, visible and near-IR parts of the spectrum. These spectral ranges are particular important for applications related to organic semiconductors.
The most important tunable parameter in our sample preparation is the temperature of the substrate during deposition. Unfortunately, this is also a tedious variable to search systematically. With the development of the temperature gradient stage, we are able to measure properties of essentially all the interesting glasses of a particular material in a single experiment. We do this by controlling the temperature of the two ends of a silicon wafer and calculating the temperature of all the places in between. The density of vapor-deposited glasses of indomethacin is determined by measuring the thickness of the as-deposited film. We then transform the sample to the equilibrium supercooled liquid and compare the thickness of our non-equilibrium material to the thickness of an equilibrium state which has a known density.
Based upon density, many of our glasses appear to be in equilibrium – by any other means, it would take thousands of years to produce materials this dense. Many of our vapor-deposited films are birefringent, i.e., the index of refraction for light polarized in the plane of the sample is different than for light polarized normal to the substrate. This indicates that the molecules in these samples are oriented anisotropically. This orientation could have a big impact on the electrical conductivity of semiconductor materials.
We have recently started wide angle x-ray scattering experiments (WAXS) on samples prepared with the temperature gradient stage. More information on this will be added shortly!