Infrared correlation nanoscopy with unprecedented spectral coverage

T. Gokus
attocube Systems AG,

Keywords: nano-FTIR, s-SNOM, AFM-IR, correlation nanoscopy


Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) is scanning probe approach to optical microscopy and spectroscopy, bypassing the ubiquitous diffraction limit of light to achieve a spatial resolution below 20 nanometers. s-SNOM employs the strong confinement of light at the apex of a sharp metallic atomic force microscopy (AFM) tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip enables the extraction of the optical properties of the sample directly below the tip and yields nanoscale resolved optical images simultaneous to topography of the sample. In addition, the technology has been advanced to Fourier-Transform Infrared Spectroscopy on the nanoscale (nano-FTIR) utilizing broadband radiation from the visible spectral range to THz frequencies. Recently, the combined analysis of organic and inorganic composite surfaces by correlating near-field optical data with information obtained by other nanomechanical and -electrical scanning probe microscopy (SPM)-based measurement methodologies has gained significant interest. For example, the material-characteristic nano-FTIR spectra of a phase-separated polystyrene/low-density polyethylene (PS/LDPE) polymer blend verifies sharp material interfaces by measuring across the interface across a ca. 1µm sized LDPE island. Near-field reflection and absorption imaging at an excitation wavelength of 1600cm-1 of the ca. 50nm thin film allows to selectively highlight the distribution of PS in the blend and simultaneously map the mechanical properties like stiffness and adhesion of the different materials. Further, we demonstrate for the first time scattering near-field microscopy (s-SNOM) imaging and spectroscopy based on a fully integrated and automated commercial OPO laser source covering an ultrabroad spectral range from 1.5−18.2μm (ca. 7100 – 540 cm-1) with a narrow linewidth < 4cm-1 through the full tuning range. Extending the spectral coverage towards the far-IR spectral range, the methodology was successfully applied to selectively map the nanoscale spatial distribution of PVAC in a PS polymer matrix based on a 603cm-1 molecular absorption line to study characteristic material chemical heterogeneity and interfaces between the two phases. It is worth mentioning that sweeping the laser frequency also allows to measure spectroscopic signatures of materials and other nanostructures with unprecedented spectral coverage, enabling studies of fundamental molecular resonances and quantum states in the long wavelength IR spectral range, which until now was not possible.