Chemical imaging with scanning near-field infrared microscopy and spectroscopy

The development of a scanning near-field microscope that utilizes infrared absorption as the optical contrast mechanism is described. This instrument couples the nanoscale spatial resolution of a scanning probe microscope with the chemical specificity of vibrational spectroscopy. This combination allows the in situ mapping of chemical functional groups with subwavelength spatial resolution. Key elements of this infrared microscope include: a broadly tunable infrared light source producing ultrafast pulses with a FWHM bandwidth of 150 cm-1, an infrared focal plane array-based spectrometer which allows parallel detection of the entire pulse bandwidth with 8 cm-1 resolution, and a single mode fluoride glass fiber probe which supports transmission from 2200 to 4500 cm-1. A novel chemical etching protocol for the fabrication of near-field aperture probes is described. Infrared transmission images of a micropatterned thin gold film are presented that demonstrate spatial resolution of (lambda) /8 at 2900 cm-1, in the absence of artifacts due to topography induced contrast. Images of thin film polymer blends and nanocomposites acquired in the C-H stretching region are used to benchmark the chemical imaging capabilities of this microscope, focusing particularly on the absorption sensitivity of the spectrometer.