8 research outputs found
Orientation of Phenylphosphonic Acid Self-Assembled Monolayers on a Transparent Conductive Oxide: A Combined NEXAFS, PM-IRRAS, and DFT Study
Self-assembled monolayers (SAMs) of dipolar phosphonic
acids can
tailor the interface between organic semiconductors and transparent
conductive oxides. When used in optoelectronic devices such as organic
light emitting diodes and solar cells, these SAMs can increase current
density and photovoltaic performance. The molecular ordering and conformation
adopted by the SAMs determine properties such as work function and
wettability at these critical interfaces. We combine angle-dependent
near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and
polarization modulation infrared reflection absorption spectroscopy
(PM-IRRAS) to determine the molecular orientations of a model phenylphosphonic
acid on indium zinc oxide, and correlate the resulting values with
density functional theory (DFT). We find that the SAMs are surprisingly
well-oriented, with the phenyl ring adopting a well-defined tilt angle
of 12â16° from the surface normal. We find quantitative
agreement between the two experimental techniques and density functional
theory calculations. These results not only provide a detailed picture
of the molecular structure of a technologically important class of
SAMs, but also resolve a long-standing ambiguity regarding the vibrational-mode
assignments for phosphonic acids on oxide surfaces, thus improving
the utility of PM-IRRAS for future studies