G. Rutins, L. Tetard
University of Central Florida,
Keywords: Plasmonics, SEIRA, nanoIR, photothermal expansion, infrared spectroscopy, AFM, fractal, surface enhanced, signal enhancement
Summary:Determining the chemical fingerprint of small, sometimes unknown, entities requires capturing their molecular fingerprint using infrared spectroscopy with high sensitivity. Nanoscale infrared (nanoIR) spectroscopy provides a way to access the information at the nanoscale, which would be undetectable with conventional Fourier Transform Infrared (FTIR) spectroscopy. NanoIR measurements are carried out by monitoring the photothermal expansion of an entity resulting from infrared absorption using an atomic force microscopy (AFM) probe. While single molecule detection has been demonstrated with this technique under optimal conditions, investigation of sub-10 nm features and their interactions with various environments remains challenging. Plasmonic structures can provide sub-wavelength regions of high field intensity in response to far-field illumination in the visible or the infrared range. Plasmonic substrates have long been exploited for Surface Enhanced Raman Spectroscopy (SERS), for which a narrow but strong enhancement band is suitable. In the mid-infrared range, structures referred to as Surface Enhanced Infrared Absorption (SEIRA) have emerged. However, boosting the signal corresponding to the molecular fingerprint of the system calls for a significant broadening of the region enhanced by SEIRA. As the resonant oscillatory plasmonic modes are highly frequency dependent, and their peak intensities are inversely proportional to frequency bandwidth, achieving broadening while maintaining good enhancement is challenging. We present a SEIRA plasmonic design that provides multi-band localized field enhancement for use with nanoIR spectroscopy. We present an overview of the effect of structure parameters on their far field response. We investigate the localized field enhancement in the near-field of the SEIRA structures with nanoscale functional imaging. In one case, the result of the field enhancement is mapped out by measuring the photothermal expansion of a thin polymer film deposited on top of the SEIRA substrate. In another example, we show that an entity-functionalized gold-coated AFM cantilever tip placed in the vicinity of the enhancement regions exhibits a stronger deflection, which is used to capture the nanoIR spectrum of the entity. This approach of using IR-absorptive functionalized AFM tips to characterize local field intensity complements and expands on the IR-absorptive polymer film approach, as it allows for characterizing a single sample structure with multiple materials while preserving the SEIRA structure over time. We investigate the effect of polarization on the spectral response. We conclude by providing a perspective of the SEIRA substrates for nanoIR spectroscopy of micro to nanoscale entities of interest in biology, life science, chemistry and beyond.