Multidimensional Nano-Imaging of Structure, Coupling, and Disorder in Molecular Materials

Functional properties of complex polymers and other supramolecular systems have a wide range of applications in biomedical sensing. These important functional properties originate from molecular interactions over a hierarchy of physical scales in energy and length. In condensed phases of polymers, intrachain and interchain coupling are sensitive to small variations in the kinetic and thermodynamic conditions, from the synthesis to post-processing and device fabrication. Direct access to structural details from molecular to micrometer scales is required, but multiscale heterogeneities between microscopic structure and emergent function are generally difficult to access with conventional optical and electron microscopy techniques.

In this work, Professor Raschke and coworkers combine variable temperature infrared (IR) scattering scanning near-field optical microscopy (s-SNOM) with four-dimensional scanning transmission electron microscopy (4D-STEM), and vibrational exciton modeling based on density functional theory (DFT), to link local microscopic molecular interactions to macroscopic three-dimensional order in poly(tetrafluoroethylene) (PTFE). Large spatio-spectral heterogeneities with C−F vibrational energy shifts ranging from sub-cm−1 to ≳25 cm−1 serve as a molecular indicator of the degree of local crystallinity and disorder. Spatio-spectral-structural correlations reveal a previously invisible degree of highly variable local disorder in molecular coupling as the possible missing link between nanoscale morphology and associated electronic, photonic, and other functional properties of molecular materials.

Temperature-dependent nano-FTIR s-SNOM measurements were made at 15.5, 24.5, and 35.5 °C using broadband (∼150 cm-1) excitation femtosecond IR pulses centered at ∼1200 cm-1. The symmetric and antisymmetric CF-stretching modes of PTFE occur within this spectral range. Through interferometric heterodyne detection of the tip scattered signal, demodulated at the second-harmonic of the AFM cantilever tapping frequency, complex valued nanolocalized nano-FTIR spectra were acquired at a spectral resolution of 3−6 cm-1. Hyperspectral nano-FTIR images were acquired at the edge of an ∼10-μm-sized melted PTFE bead. The hyperspectral arrays consisted of two-dimensional grids with 20 × 20 to 50 × 50 individual spectra and 50−100 nm spacing between sampling locations. The spatial variations in the peak wavenumber, bandwidth, and ratio of the antisymmetric and symmetric CF-stretching band intensities largely correlate with the topographic features of the sample. The thicker bulk areas of PTFE have sharper CF-stretching IR peaks at lower wavenumber peak positions than the thinner interface areas. Similarly, temperature-dependent hyperspectral nano-FTIR images indicate higher peak wavenumbers and linewidth broadening with increasing temperature, suggesting increased disorder.

The ability to spatially resolve low-energy structural and dynamic macromolecular landscapes demonstrated in this paper should lead to a deeper understanding of how complex molecular systems gain their macroscopic properties and functions in polymers and polymer composites, and should be readily extendable to other molecular systems, providing the critical link to associated electronic, photonic, carrier transport, and other functional properties of molecular materials.

      _{KEY TERMS:}

• s-SNOM; AFM-IR Modes, Multimodal investigation, Surface Analysis Techniques, Functional properties of polymers, Molecular disorder. Molecular coupling