A scanning near-field microscopy that allows the fast acquisition of mid-infrared absorption spectra is described. The microscope couples the nanoscale spatial resolution of a scanning probe microscope with the chemical specificity of vibrational spectroscopy. Key design elements of the microscope include a tunable broadband infrared light source; an infrared focal plane array-based spectrometer which allows parallel detection of the entire pulse bandwidth (200 cm-1); and a single mode, fluoride glass, near-field probe fabricated with a novel chemical etching protocol. Infrared transmission images of a micropatterned thin gold film are presented that demonstrate spatial resolution conservatively estimated to be λ7.5 at 3.4 υm, in the absence of optical artifacts due to topography. Constant height mode images of a polymer nanocomposite demonstrate instrumental sensitivity to fractional transmission changes of 1 x 10-3. Near-field absorption spectra (λ = 3.4 υm) of a 2 υm thick polystyrene film are presented which demonstrate the instrumental sensitivity required for high spatial resolution, near-field absorption imaging. These spectra are acquired in 2 s and indicate a film thickness detection limit for polystyrene of 200 nm. Measurements exploring the coupling between the infrared absorption magnitude and changes in tip-sample separation suggest that near-field absorption imaging is relatively insensitive to topographic artifacts.
Citation: Journal of Applied Physics
Issue: No. 8
Pub Type: Journals
chemical imaging, chemical microscopy, infrared microscopy, infrared spectroscopy, near-field microscopy, NSOM, ultrafast lasers, vibrational spectroscopy