Nanoparticles and nanostructured materials are a hot topic in materials research and can be found already in several technical products. Raman microscopy and tip-enhanced Raman spectroscopy (TERS) provide spatially resolved chemical analysis, which allows the identification of chemical compounds and crystal structures. This information helps to improve production methods in materials research and is needed in quality control of technical products.

Experimental Setup

The setup used for the investigation of inorganic samples is an upright microscope, which combines AFM/STM, confocal microscopy, and Raman microspectroscopy. Different AFM/STM and optical measurements can be performed simultaneously on the same part of the sample. The upright configuration allows the investigation of both, transparent and opaque samples.

Fig. 1: Combined AFM/STM-confocal microscopy-Raman spectroscopy instrument for transparent and opaque samples (NTEGRA spectra, upright system, NT-MDT). Left: front view showing the main parts of the system. Right: back view with choice of detectors.

Raman Microscopy on Solar Cell Materials

The analysis of thin-film solar cell absorbers (collaboration with the Helmholtz Centre Berlin for Materials and Energy) has demonstrated the potential of Raman microscopy in the investigation of new materials. The efficiency of such solar cells is greatly dependent on the composition as well as on crystal structure and purity of the compounds involved. This information can be obtained by Raman microscopy.

Fig. 2: Raman microscopy map of a cross-sectional sample of a thin film solar cell showing the distribution of different crystal structures of the absorber material CuInS 2 (CIS) as well as the contaminant carbon and the segregate copper sulfide. Chalcopyrite-type (CH) CIS is an efficient absorber material, whereas the presence of CuAu I-type (CA) CIS, segregates and contaminants is known to reduce the efficiency of the solar cell.

Raman spectroscopy allows the identification/determination of:

- Chemical species (e.g. CuInS2, CuxSy)
- Stoichiometries (e.g. CuGaSe2 , CuGa3Se5)
- Crystal structures (e.g. chalcopyrite-type CIS, CuAu I-type CIS)
- Modifications (e.g. graphite, diamond, amorphous/nanocrystalline carbon)
- Chemical identity of segregates and contaminants (e.g. CuxSy, carbon)

By Raman mapping experiments the lateral distribution of chemical compounds and crystal structures can be determined with a resolution of approx. 400 nm . The combination with AFM allows imaging of the same part of the sample with nanometer resolution . TERS with this setup for chemical analysis of inorganic materials with high spatial resolution is under development.

Applications

Inorganic samples under investigation are for example:

-Thin-film solar cell materials (e.g. CuInS2, CuGaxSey)
-Titanium dioxide nanoparticles (rutile, anatase, brookite)
-Diamond nanoparticles

Contact:

Dr. Thomas Schmid, schmid@org.chem.ethz.ch
Johannes Stadler, stadler@org.chem.ethz.ch
Prof. Dr. Renato Zenobi, zenobi@org.chem.ethz.ch

Literature: