nano-FTIR – Nanoscale chemical mapping by local infrared fingerprint spectroscopy

An ultimate goal in modern chemistry 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. Infrared spectroscopy, on the other hand, offers high chemical sensitivity but its resolution is limited by diffraction to about half the wavelength on the order of  micrometers, thus preventing nanoscale resolved chemical mapping.

Nanoscale chemical identification and mapping of materials now becomes possible with nano-FTIR from neaspec GmbH. This technique combines the best of two worlds, the high spatial resolution of Atomic Force Microscopy and the analytical power of Fourier transform infrared (FTIR) spectroscopy. nano-FTIR allows for fast and reliable chemical identification of virtually any material at the nanometer scale – A new era in modern analytical chemistry has just begun.

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 100 nm size contamination, the determination of its chemical identity remains elusive from these images. Recording a local infrared spectrum in the center of the particle clearly reveals its chemical identity. Comparing the nano-FTIR absorption lines with standard FTIR database spectra, the contamination can be identified as a PDMS particle.

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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 min with a spectral resolution of 13 cm-1.

nano-FTIR technique from neaspec GmbH:

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Fig. 2: Working principle of nano-FTIR. A broadband mid-infrared laser source is coupled to the near-field microscope (neaSNOM) where it illuminates the metallic AFM tip. The backscattered light is analyzed with an internal Fourier Transform spectrometer, consisting of a beam splitter (BS), a reference mirror (RM) and a detector.

nano-FTIR spectra match well with standard FTIR spectra:

An important aspect of both fundamental and practical relevance is that the near-field absorption spectra of molecular fingerprints match extremely well with molecular fingerprints in conventional FTIR spectroscopy without the need of modeling (see Fig. 3). This allows for interpretation of nano-FTIR spectra by comparison with conventional FTIR spectra found in widely established databases. nano-FTIR thus allows for fast and reliable chemical identification of materials on the nanometer scale. The high sensitivity to chemical composition combined with ultra-high resolution makes nano-FTIR a unique tool for nano-analytics.

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