AFM-IR and IR-SNOM for the Characterization of Small Molecule Organic Semiconductors

Abstract

Vibrational spectroscopies, such as Raman and Fouriertransform 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 infraredactive 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-CN 2 ), TIPS-pentacene, and TIPStetraazapentacene 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-CN 2 .

Probing amyloid fibril secondary structures by infrared nanospectroscopy: experimental and theoretical considerations

“…For the first time, a complete study of three different amyloid fibrils (Aβ, α-syn and PrP) was performed using polarized resolved AFM-IR in different illumination configurations and using two types of AFM probes. Several other polarizedresolved AFM-IR studies, 41,[47][48][49][50][51][52][53][54][55] were performed using either the bottom-up or top-down illumination configuration, but none of them has discussed in detail the effect of these different illumination and tip's gold-coating.…”

Recent Progress on the Characterization of Cellulose Nanomaterials by Nanoscale Infrared Spectroscopy

“…Thus, the detected signal can be converted into the vibrational spectrum of the sample. The IR s-SNOM technique, by contrast, determines the amount of scattered light from the specimen [ 35 , 36 ]. The light scattered was influenced by the comprehensive optical properties of the AFM probe, specimen, and underlying substrate.…”

“…A pulsed tunable IR source with an AFM is also an essential accessory for IR s-SNOM ( Figure 1 b,d). On the other hand, IR s-SNOM collects the scattered near-field of light from a metallic AFM tip with the sample underneath [ 35 ]. The wavelength or wavenumber-dependent optical and chemical properties can be obtained via the collected light with amplitude and phase properties [ 36 , 37 ].…”

“…Advances of AFM-IR and IR s-SNOM technology make it possible to perform IR spectroscopic mapping and chemical analysis at the nanoscale with spatial resolution beyond the Abbe diffraction limit for the characterization of nanomaterials and nanostructures. AFM-IR is now recognized as one of the most important novel techniques for analyzing various systems, such as polymer blends, organic fibers, composites, multilayer thin films in addition to biological samples [ 33 , 34 , 44 ], while IR s-SNOM has demonstrated great popularity in studying monolayered and inorganic samples as well as soft materials [ 35 , 38 , 45 ].…”

Infrared Scattering-Type Scanning Near-Field Optical Microscopy in Water

“…33,34 Although simple and versatile, AFM IR provides spectral information transduced via its thermal expansion which might affect the IR contrast for anisotropic samples. 35 Here, we show an alternative illumination and detection scheme which permits IR sSNOM experiments on biological samples, here biomembranes, immersed in bulk aqueous solution. We further prove the imaging capabilities of this geometry and outline strategies for imaging wide fields of view as those typically achieved with atomic force microscopes.…”