Raman spectroscopy and X-ray photoelectron spectroscopy are two of the most complementary techniques in materials analysis.
Raman spectroscopy is a light scattering technique where a fraction of the photons excited by a laser result in a spectrum that characterizes the molecular structure of the sample. Like infrared spectroscopy, each spectrum can be compared against a library of known spectra for materials identification, but unlike IR spectroscopy Raman provides information about the crystalline structure of the material, including polymorphism. Using a microscope, Raman spectroscopy is capable of measuring features as small as 1 micron.
Raman spectroscopy is extensively used for investigating structural disorder in crystalline materials based on various types of defects- namely point defects, edges, doping, stacking fault, grain boundaries, stress, and strain.
X-ray photoelectron spectroscopy uses a monochromated X-ray source to excite the sample, with a sophisticated detection system capable of reading the kinetic energy of photoelectrons in an element that have been ejected by the X-ray source. Reading this value provides information about a material’s elements, their electronic configuration and chemical state (i.e., the influence of bonded elements such as oxygen.) The value of XPS is that as the emitted photoelectrons are relatively weak, we see only the first few atomic layers of the sample, making XPS a highly useful surface analysis technique.
Taken together, Raman spectroscopy and X-ray photoelectron spectroscopy are ideal techniques for materials characterization of surfaces and small features. The two techniques are particularly useful for 2D materials such as graphene.
A problem arises when Raman and XPS analyses are performed in a serial fashion. How can one be sure that both techniques are examining the same feature? Even if the stages of each instrument can drive to the exact same X-Y position, how can an analyst be sure that something, such as oxidation, hasn’t occurred between each sampling.
The answer, of course, is run each technique coincidently. The Thermo Scientific Nexsa™ X-ray Photoelectron Surface Analysis System includes concurrent Raman spectroscopy, enabling the analyst to look at the same point (pixel) at similar spot sizes.
In a recently produced recording, we look at using coincident Raman and XPS to provide a rich understanding of 2-D materials, specifically boron nitride and molybdenum disulfide.
Here we look at the following material properties to understand
- Structural confirmation
- Surface contamination
- Chemical modification
- Electrical properties
By having both techniques on the same platform we’re able collect all this information at once without having to hunt around for the same position moving from one instrument to another. We’re able to confirm that boron nitride was created in its correct hexagonal form and identify where and what chemical changes have occurred to the molybdenum disulfide on the surface of a silicon wafer.