An ultimate goal in modern chemistry and materials science is the non-invasive chemical mapping of materials with nanometer scale resolution. A variety of high-resolution imaging techniques exist (e.g. electron microscopy or scanning probe microscopy), however, their chemical sensitivity cannot meet the demands of modern chemical nano-analytics. Optical spectroscopy, on the other hand, offers high chemical sensitivity but its resolution is limited by diffraction to about half the wavelength, thus preventing nanoscale resolved chemical mapping.
Nanoscale chemical identification and mapping of materials now becomes possible with nano-FTIR, an optical technique that combines scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. By illuminating the metalized tip of an atomic force microscope (AFM) with a broadband infrared laser, and analyzing the backscattered light with a specially designed Fourier Transform spectrometer, the researchers could demonstrate local infrared spectroscopy with a spatial resolution of less than 20 nm. "Nano-FTIR thus allows for fast and reliable chemical identification of virtually any infrared-active material on the nanometer scale", says Florian Huth, who performed the experiments.
An important aspect of enormous practical relevance is that the nano-FTIR spectra match extremely well with conventional FTIR spectra, while the spatial resolution is increased by more than a factor of 300 compared to conventional infrared spectroscopy. "The high sensitivity to chemical composition combined with ultra-high resolution makes nano-FTIR a unique tool for research, development and quality control in polymer chemistry, biomedicine and pharmaceutical industry" concludes Rainer Hillenbrand, leader of the Nanooptics group at nanoGUNE.
Fig. 1: Chemical identification of nanoscale sample contaminations with nano-FTIR. In the topography image (left), a small sample contaminant (B) can be found next to a thin film of PMMA (A) on a Si substrate (dark region). In the mechanical phase image (middle) the contrast already indicates that the particle consists of a different material than the film and the substrate. Comparing the nano-FTIR absorption spectra at the positions A and B (right panel) with standard IR databases reveals the chemical identity of the film and the particle. Each spectrum was taken in 7 minutes with a spectral resolution of 13 cm-1.
For example, nano-FTIR can be applied for the chemical identification of nanoscale sample contaminations. Fig. 1 shows AFM images of a PMMA film on a Si surface. While the AFM phase contrast indicates the presence of a 100 nm size contamination, the determination of its chemical identity remains elusive from these images. Using nano-FTIR to record a local infrared spectrum in the center of the particle and comparing it with standard FTIR database spectra, the contamination can be identified as a PDMS particle.