  A systematic experimental and theoretical study about substrate induced
molecular conformation in rubrene thin films by varying film thickness from
sub-monolayer to multilayer, which currently attracts substantial attention
with regard to its application in organic electronics, is performed. The clean
polycrystalline Au and Ag were used as noble-metals, whereas, H passivated and
SiO2 terminated Si (100) were used as dielectric substrates. Angle dependent
near edge x-ray absorption fine structure spectroscopy (NEXAFS) was employed to
understand the molecular conformation whereas atomic force microscopy (AFM) was
used to investigate the surface morphologies of the films. X-ray absorption
spectra (XAS) of rubrene molecules with flat and twisted conformations were
calculated using density functional theory (DFT). All the observed NEXAFS
spectra of rubrene thin films at various thicknesses and onto different
substrates were explained in terms of different combination of the spectral
intensity from the twisted and the flat molecules. In contrast to general
findings, comparatively rough polycrystalline Ag surface is found to support
growth of better quality rubrene films. The results have important implications
for the understanding of the substrate induced molecular conformations in
rubrene thin films with its thickness and are beneficial for the improvement of
the device performance.
