XPS – UPS – ISS: Surface Spectroscopic Techniques for Chemical Analysis
Covalent Metrology previously announced the completed installation of its new Thermo Scientific Nexsa XPS system, which incorporates 3 of the most useful surface chemical techniques available: X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and ion-scattering spectroscopy (ISS). This webinar recording will deepen your knowledge on those 3 techniques: what they do best, how they can work together, and what each can do for you!
Surface characterization expert, Dr. Roland Barbosa, will introduce key theoretical and operational concepts for these tools before exploring the practicalities of their applications.
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This Webinar Will Answer:
- What is XPS? UPS? ISS? How do they work?
- What should I consider when applying each technique?
- How can they be used together?
- What are the kinds of applications they work for?
- How can I ensure the best data with each technique?
The Most Surface-Specific Chemical Analysis
Signal photo-electrons and scattered-ion fragments escape only the outermost few nanometers of a surface. This allows you not only to measure true surface chemistry, but also to detect contaminants and chemical features most relevant to the surface properties of your material: its roughness, adhesion, bonding potential, grain structure, wettability, diffusion characteristics, zeta potential, and many more.
Learn how to use these 3 techniques to probe the chemical properties of your surface, and how correlative analysis between them can clarify greater insights into your sample.
Q&A Session
Is the intensity peak Lorentzian, Gaussian, or a mix. If Gaussian, or partially so, what is the phenomena responsible for the distribution in binding energies?
It’s a combination. The contribution of the spectrometer is Gaussian in shape while the natural core level lineshape is Lorentzian. The atomic-level should also be considered, i.e., with metallic species, the ejected photoelectrons may interact with the conduction electrons, loosing very small portions of energy which account for the peaks being asymmetric.
What is the collection angle of the electron lens for the Nexsa? Is it the same as K-alpha?
60°. Yes.
What is the spot size of UPS and ISS?
Few hundred microns to a few mm.
For the ISS are you limited to the Ar from the ion gun or can other inert gasses be used?
Other inert gases can be used like Ne, Ar and Kr. Alkali ion beams may also be used.
How can I post-process data obtained from XPS, outside the standard XPS software?
I use Origin and Excel outside of the XPS software (CASA, Avantage, Multipak, etc).
For ISS, what would be the error quantitatively determining the composition of a film composed of heavy atoms (e.g. atomic mass >Si)? What is the penetration depth?
The mass resolution and/or sensitivity is insufficient for heavier elements when using He+ ions. Additionally, large background from tails are expected for samples with heavier elements resulting in the poorer detection limits. The penetration depth depends on the ion beam energy but usually less than 10 nm with ISS. That is why you go to higher mass for better sensitivity.
Is surface roughness a problem for ISS?
It could be. The roughness on the surface can sometimes contribute to some of the surface atoms that cannot be seen by the analyzer due to shadowing effects.
Can depth profiling be done to remove the top layer and also see how composition changes with depth?
Yes. This is the point of depth profiling; determining the composition as a function of depth.
Do powder samples need special preparations before analysis?
Powders are usually mounted by pressing it on carbon tape or indium foil. Some make a pellet out of the powder to be able to analyze in XPS
What is the uncertainty in the atom% conc of Li?
In general, the uncertainty in atomic % is ~ +/- 10%. Weaker signals have much higher uncertainties.
For the li ion battery expt, why did the Mn conc is higher in the cycled compared to the pristine sample? Where is the extra Mn component coming from? from the bulk?
The Mn content on the cycled sample is only slightly higher than that of the pristine sample and it is within the % error.
For the Li example, do you expect sample nonhomogeneity to affect the composition measurement? What was the analysis area size?
Yes. The spot size varies from tens of microns to a few hundred microns or even mm size depending on the XPS instrument.
Can ISS measure Hydrogen?
Hydrogen has been detected through ISS using a 1 keV 4He+ ion beam and narrow scattering angles from 0° to 15° (F. Shoji et. al. Surf. Sci. Lett., 220, 1989, L719-L725). The Hydrogen Forward Scattering Spectrometry is (HFSS) usually used for the detection and quantification of H in films. This is similar to ISS technique but instead of the reflected ion beam, the H atoms that are sputtered away are analyzed using a solid state detector.
How does charging of the sample affect the XPS signal and how do you mitigate it?
As photoelectrons are emitted from the surface, a positive charge will build up on the sample resulting in a shift to higher binding energies of the observed peaks in the spectrum. The resulting spectra may also be broadened significantly. The most common solution to this problem is the use of low energy electron flood gun to overcompensate for the positive charge.
How well does UPS work for thin films?
The information depth for UPS is about 2-3 nm. The usual problem encountered in thin film analysis using UPS is surface contamination. UPS is very surface sensitive that the mere presence of adventitious C due to atmosphere exposure may affect UPS measurements (i.e. valence band spectra, work function measurements). Argon cluster sputtering is usually done prior to the analysis to remove any contaminants without changing the chemistry of the surface.
I know that an XPS survey scan can contain Auger peaks. Do you ever use this for analysis?
Yes. The Auger peaks can sometimes reveal differences in binding energies between oxidation states of some metals and its corresponding XPS cannot (i.e. Cu0 and Cu+1 peaks overlaps in Cu 2p while bigger chemical shifts is observed for these two states in Cu LMM).
During depth profiling one of the problems with older tools was that the bottom of the crater became rougher and rougher which affected the data. How does roughness affect XPS data?
For the depth profile, the sputtering process can cause roughness to occur and this degrades the depth resolution as well as the accuracy of the quantification. This effect is usually mitigated by rotating the sample (azimuthal rotation) during the sputtering part of a profiling cycle.
I assume if one does an XPS measurements on a porous structure that you get data from both the surface of the material and potentially from inside the pores. Is this correct and how would you verify it.
It really depends on the size, geometry and porosity of the material, but it is possible to get the XPS information both from the surface and inside the pores. To confirm, changing the take-off angle to be more surface sensitive will change the relative composition and adding TEM measurements to get a general idea on the differences between the surface and inside the pores.