Nature Materials: Nano-FTIR – Nanoscale Infrared Spectroscopy with a Thermal Source

The absorption of infrared radiation is characteristic for the chemical composition and structure of materials. For this reason, an infrared spectrum can be considered as a material’s “fingerprint”. Infrared spectroscopy has thus become an important tool for characterizing and identifying materials and is widely applied in different sciences and technologies including materials sciences and biomedical diagnostics. However, with conventional optical instruments, such as Fourier-transform (FTIR) infrared spectrometers, the light cannot be focused to spot sizes below several micrometers. This fundamental limitation prevents infrared-spectroscopic mapping of single nanoparticles, molecules or modern semiconductor devices.

Figure: Infrared nanospectroscopy with a thermal source. The sketch shows an atomic force microscope tip probing a sample. The tip is illuminated with the broadband infrared radiation from of a thermal source and the backscattered light is analyzed with a Fourier-transform spectrometer, yielding local infrared spectra with a spatial resolution better than 100 nm. The displayed graph shows local infrared spectra of differently processed oxides in an industrial semiconductor device. Copyright F. Huth, CIC nanoGUNE.

Researchers at nanoGUNE and Neaspec have now developed an infrared spectrometer that allows for nanoscale imaging with thermal radiation. The setup – in short nano-FTIR (see Figure) – is based on a scattering-type near-field microscope (NeaSNOM, www.neaspec.com) that uses a sharp metallic tip to scan the topography of a sample surface. While scanning the surface, the tip is illuminated with the infrared light from a thermal source. Acting like an antenna, the tip converts the incident light into a nanoscale infrared spot (nanofocus) at the tip apex. By analyzing the scattered infrared light with a specially designed FTIR spectrometer, the researchers were able to record infrared spectra from ultra-small sample volumes.

In their experiments, the researchers managed to record infrared images of a semiconductor device from Infineon Technologies AG (Munich). “We achieved a spatial resolution better than 100 nm. This directly shows that thermal radiation can be focused to a spot size that is hundred times smaller than in conventional infrared spectroscopy”, says Florian Huth, who performed the experiments. The researcher demonstrated that nano-FTIR can be applied for recognizing differently processed silicon oxides or to measure the local electron density within complex industrial electronic devices. “Our technique allows for recording spectra in the near- to far-infrared spectral range. This is an essential feature for analyzing the chemical composition of unknown nanomaterials“, explains Rainer Hillenbrand, leader of the Nanooptics group at nanoGUNE.

Nano-FTIR has interesting application potential in widely different sciences and technologies, ranging from semiconductor industry to nanogeochemistry and astrophysics. “Based on vibrational fingerprint spectroscopy, it could be applied for nanoscale mapping of chemical composition and structural properties of organic and inorganic nano-systems, including organic semiconductors, solar cells, nanowires, ceramics and minerals”, adds Florian Huth.

The CIC nanoGUNE Consolider, nanoGUNE in short, is the Basque nanoscience and nanotechnology research center, inaugurated in 2009 in Donostia – San Sebastián, Spain. Neaspec GmbH has been established in 2007 as a spin-off from the Max Planck Institute of Biochemistry (Martinsried, Germany) and is the first supplier of scattering-type near-field optical microscopes. “The fruitful collaboration of research and industry has been the key for the development of nano-FTIR”, concludes Nenad Ocelic, CTO of Neaspec GmbH.

Original publication: _F. Huth, M. Schnell, J. Wittborn, N. Ocelic, R. Hillenbrand Infrared-spectroscopic nanoimaging with a thermal source Nature Materials 10, 352