Resonant soft X-ray scattering reveals hierarchical structure in a multi-component vapor-deposited glass
arxiv(2024)
Abstract
Multi-phase vapor-deposited glasses are an important class of materials for
organic electronics, particularly organic photovoltaics and thermoelectrics.
These blends are frequently regarded as molecular alloys and there have been
few studies of their structure at nanometer scales. Here we show that a
co-deposited system of TPD and Disperse Orange 37, two small molecule
glass-formers, separates into compositionally distinct phases with a domain
size and spacing that depends on substrate temperature during deposition.
Domains rich in one of the two components become larger and more pure at higher
deposition temperatures. We use resonant soft X-ray scattering (RSoXS)
complemented with Atomic Force Microscopy (AFM) and photo-induced force
microscopy (PiFM) to measure the phase separation, topography, and purity of
the deposited films. A forward-simulation approach to RSoXS analysis, the
National Institute of Standards and Technology (NIST) RSoXS Simulation Suite
(NRSS), is used with models developed from AFM images to evaluate the energy
dependence of scattering across multiple length scales and interpret the RSoXS
with respect to structure within the films. We find that the RSoXS is sensitive
to a hidden length scale of phase separation that is not apparent from the AFM
characterization alone. We demonstrate that vacuum scattering, which is often
ignored in RSoXS analysis, contributes significantly to the features and energy
dependence of the RSoXS pattern, and then illustrate how to properly account
for vacuum scattering to analyze films with significant roughness. We then use
this analysis framework to understand structure development mechanisms that
occur during vapor deposition of a TPD-DO37 co-deposited glass with results
that outline paths to tune morphology in multi-component materials.
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