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The focus of the first phase of the project was to demonstrate the feasibility of implementing infrared vibrational spectroscopy with nanometer spatial resolution in scattering-scanning near-field microscopy (sSNOM) as a prerequisite for spectroscopic access with chemical sensitivity to the characteristic nanodomains in block-copolymers and polymer nano-composites.

The approach of this work is based on the use of sharp scanning probe tips (such as an AFM probe) as an optical antenna for local field enhancement and confinement. The improved spatial resolution is realized by using a nanoscale metallic probe tip of an atomic force microscope to enhance and then scatter radiation interacting with the sample. The combination of the field confinement, along with sensing scattered radiation in the extreme near field allows this technique to surpass conventional diffraction limits on resolution. Using external illumination, the nanometer radius of the apex of the pointed metal tips near the sample, and the resulting mutual optical tip-sample polarization in the optical near-field region, provide the nanometer scale imaging contrast. The imaging contrast is based on the following mechanism: the incident light induces an optical polarization in the tip apex. With the tip in close proximity to the sample at a distance comparable to its apex radius the evanescent components of the optical field of the driven apex dipole need to match the boundary conditions of the field equations when penetrating the sample surface. This gives rise to an optical tip-sample coupling and the resulting field distribution of the combined tip-sample system sensitively depends on the optical properties of the sample material. The latter can be expressed as the dielectric function or the index of refraction with their respective optical resonances due to vibrational (in the IR spectral region) and electronic (in the UV/Vis) transitions.

Mid-infrared spectroscopy, by probing the fundamental vibrational modes, allows for identification of molecular functional groups. sSNOM thus provides a spatially resolved surface map of the chemical properties simultaneously with the AFM surface topography. Few group have yet demonstrated IR spectroscopy using sSNOM due to limited tuning range of lasers sources used and associated challenges in signal detection. In this project, we have successfully demonstrated the general applicability of spectroscopic IR-sSNOM achieving sub-30 nm spatial resolution for IR spectroscopic imaging. This is an improvement of 500x in spatial resolution over conventional far-field IR microscopy.

For that purpose we have developed a new experimental setup based on a modified atomic force microscope (AFM, Veeco) with sample scanning and dynamics force control of the cantilever tips. A sample heater allows for controlling the sample temperature between room temperature and 180 degree Celsius. Tip illumination and signal detection is performed using a Cassegrain objective implemented in side-on configuration with polarization control of incident and tip-scattered radiation, and interferometric signal detection.

A new laser system has been set up and broad-band mid-IR laser pulses are generated using a Ti:S oscillator pumped optical parametric oscillator (OPO). Mid-IR laser pulses over the IR range of interest from 700 to 5000 cm-1 (14 to 2 micron) are obtained by difference frequency mixing (DFG) between signal and idler beam. This provided quasi-continuous IR pulses at 80 MHz repetition rate, 200 fs pulse duration, and 1-10 mW average power. Due to the repetition rate exceeding the cantilever oscillation frequency by three orders of magnitude, no special synchronization is necessary.

With the spectral width of the IR pulses with FWHM of 30 cm-1 using interferometric homodyne detection and Fourier transform, spectroscopic information of individual scan positions at specific vibrational resonances can be obtained. The interferogram (with free-induction decay) of the near-field signal from an individual nanodomain of a block-copolymer surface is obtained by splitting the incident laser beam at a beamsplitter, with part of it incident on the tip and the other part passing through the interferometer. The tip-scattered light is then recombined with the reference light from the interferometer and directed to the MCT detector. Varying the delay of the interferometer while lock-in detecting the MCT signal provides the interferogram of the near-field signal. This approach is analogous to the signal detection in conventional FT-IR spectroscopy. The resulting Fourier transform of the interferogram allowed for the identification domains in PS-b-P2VP and with two characteristic vibrational modes within the laser bandwidth, already sufficient for the identification of one kind of the domains as composed of polystyrene. We have successfully imaged different block-copolymer systems including PS-b-P2VP, PS-b-PMMA, as well as polymer nanocomposites such as MDMO-PPV-PCBM. The results demonstrate the first systematic approach to spectroscopic IR-s-SNOM imaging in a robust way.