jp9b11056_si_001.pdf (3.61 MB)
AFM-IR and IR-SNOM for the Characterization of Small Molecule Organic Semiconductors
journal contribution
posted on 2020-02-25, 14:09 authored by Vaishnavi
J. Rao, Maik Matthiesen, Katelyn P. Goetz, Christian Huck, Chanyoung Yim, Rita Siris, Jie Han, Sebastian Hahn, Uwe H. F. Bunz, Andreas Dreuw, Georg S. Duesberg, Annemarie Pucci, Jana ZaumseilVibrational spectroscopies,
such as Raman and Fourier-transform
infrared spectroscopy (FT-IR), are powerful tools for the characterization
of organic semiconductor thin films and crystals in addition to X-ray
diffraction and scanning atomic force microscopy. They enable the
investigation of molecular orientation, polymorphism, doping levels,
and intra- as well as intermolecular vibrational modes albeit without
much spatial resolution. Two fundamentally different scanning probe
techniques offer two-dimensional mapping of infrared-active modes
with a spatial resolution below 100 nm: scattering-type scanning near-field
optical microscopy (IR s-SNOM) and atomic force microscopy-infrared
spectroscopy (AFM-IR). Here, we compare these two techniques with
each other and to conventional FT-IR spectroscopy measurements with
regard to their applicability to highly ordered molecular semiconductors.
For this purpose, we use organic single crystals of rubrene, perfluorobutyldicyanoperylene
carboxydiimide (PDIF-CN2), TIPS-pentacene, and TIPS-tetraazapentacene
as model systems. We find significant spectral differences depending
on the technique and polarization that are related to the anisotropy
of the crystals and the fundamentally different working principles
of the applied methods. The spatial and spectral resolution of IR
s-SNOM and AFM-IR are further tested and compared for a polycrystalline
thin film of PDIF-CN2.