Hi all,
Does anyone have any experience of using the Lascovich method for calculating the hybridization ratio of DLCs by X-ray induced Auger?
This involves differentiating the KLL Auger signature and using the distance (i.e. the D-parameter) between the prominent maximum and minimum of the derivative spectrum to calculate the relative proportions of sp2 and sp3 bonded carbon.
This is done by taking reference spectra in the same BE region for graphite (100% sp2-bonded, where the D-parameter is ~21eV) and for diamond (100% sp3, D-parameter ~ 13eV). A linear relationship is assumed to exist between D parameter width and sp3 content.
I've used the method described in the training video on the NEXUS website, using CasaXPS, first adding a Shirley background and then replacing the spectrum with a synthetic envelope, fitting the curve using 4-5 peaks to approximate the data and ensure that the derivative spectrum is smooth.
Now, I am able to get sensible results for diamond and graphite, but my DLC samples are throwing up odd results. I've also analysed these by visible Raman spectroscopy and have arrived at sp3 contents of 40-50%, which seems sensible in light of the mechanical properties of the films. However, using the XAES method of Lascovich, I am getting figures of roughly double that, i.e. 80-90% sp3, which seems too high.
I wondered if anyone had any ideas as to why this may be? The spectra don't look unusual, but my films are multilayers and the lower layers contain quantities of silicon and/or germanium, and the films are also hydrogenated, so not pure carbon. (H content is probably around 30% at. in the top layer).
My thoughts are either that it's possible that the linear relationship between D-parameter and sp3 content is maybe not valid for hydrogenated (a-C:H) DLCs, or that I have made some mistake in the fitting or processing.
Any ideas or pointers would be much appreciated!
High,
I don't know about lascovich method howaevr not all carbon films are DLC, some are polymers; You should first measure the microhardness and look for Cn-Hm bonding in IR spectroscopy. If the films are soft and C-H bands are very high then you are dealing with polymers which are very different from DLC films.
Thanks for your input!
There are C-H bonds present - Raman analysis of these samples showed a strong photoluminescence background, which is indicative of bound H within the sample matrix. The films are definitely a-C:H type DLCs though, rather than PLCH, as the optical and mechanical properties are consistent with this class of amorphous carbon.
We have used this XAES method too, and found similar figures to yours (see link) we compared to XPS C1s peak deconvolution and had figures that better match the usual Raman analysis - we had a few films in this sample set, but observed the same trend with both methods, putting the absolute difference down to the O and H content of the films/surfaces.
Article Substrate and material transfer effects on the surface chemi...
Very interesting! Thanks for the link.
There does seem to be an additive effect in XAES with H-containing films.
You must be very careful in obtaining hybridisation data from surface analysis techniques, and then comparing these to bulk techniques. There are bound to be many issues with the assignment as well as the impact of hydrogen on the technique used. One of the key routes that can be used and is easily available is EELS and do have a look at the following analysis in the PRB paper below.
http://epubs.surrey.ac.uk/2492/1/electron-energy-loss_SILVA_00.pdf
But, a full comparison of techniques and potential issues using different techniques to obtain hybridisation data is covered inteh relevant chapters of this book:
http://books.google.co.uk/books?id=OemOXv486j0C&printsec=frontcover&dq=properties+of+amorphous+carbon&hl=en&sa=X&ei=PG52UYb4N8KLOYTEgUA&ved=0CDkQ6AEwAA
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