Most mineral species can take on more than one form, depending on their crystalline structure, or polymorph. While exhibiting the same chemical composition, each form will exhibit different properties. A well-known example is diamond and graphite—two different minerals composed of the same substance in different crystalline forms. The most stable form of the mineral oxide calcium carbonate is the hexagonal calcite. The orthorhombic aragonite is less abundant, and the hexagonal vaterite is the least common of the three. Two commons forms of titanium dioxide are rutile and anatase.
Raman and X-ray photoelectron are complementary spectroscopies that provide chemical information. X-ray photoelectron spectroscopy (XPS) quantifies the elemental and chemical composition of a material’s surface with extreme selectivity; the typical information depth of XPS being less than 10 nanometres. Raman spectroscopy provides information on the molecular structure of materials including crystallinity, and enables users to identify materials quickly, thanks to extensive spectral libraries.
Until recent developments in instrumentation, analysis of a material with both of these techniques required sampling on two instruments. However, the development of coincident XPS-Raman enables for straightforward and quick utilization of both techniques — opening up new exciting materials characterization opportunities.
To demonstrate, we conducted the analysis of calcium carbonate and titanium dioxide, where the determination of polymorphs serve as excellent examples of the powerful information that can be gleaned from performing XPS and Raman spectroscopy concurrently. The cleanliness, purity and stoichiometry of a sample was determined using XPS, while identification and quantification of molecular structures was determined using Raman spectroscopy.
However, as important and well-studied as mineral oxides are, there has not been until now a good characterization technique that can investigate all of the following simultaneously:
- Surface sensitivity (<10 nm)
- Elemental composition
- Chemical composition, i.e., bonding and allotrope type
- All from the same spot on a sample
Researchers have been able to investigate those properties individually by using various separate techniques:
- XPS — for elemental composition
- Raman microscopy — for chemical composition
- High-vacuum conditions — for surface study
- Marking or stage movements to get to approximately same site when moving from instrument from instrument; however, this is not ideal as the surface is exposed to non-high-vacuum environments as well as finding the exact same spot again is time consuming and very rarely accurate to the small scale of the incident spot size.