Aluminium

Aluminium metal exhibits significant plasmon structure in its XPS spectra, and as such may be tricky should the correct energy window not be selected.

Section from the survey spectrum for metallic Al, showing bulk and surface plasmon structure as well as the Al(2p) and Al(2s) core-level peaks

Experimental Advice

When analysing aluminium, it is often sensible to collect an extended region to the higher binding energy side, in order to capture any plasmons and enable accurate background modelling.

Aluminium metal plasmons, identified in reference 1.

Whilst it is commonplace to fit the sharp metallic doublet as a pair of peaks, and the alumina overlayer as a single voight-type convolution (due to the small doublet separation – 0.44 eV – relative to the broader oxide peaks for Al 2p), it is nonetheless good practice to fit every species as a doublet – particularly when deconvoluting multiple species.

Al native oxide fit, using a metal lineshape of LA(1,2.44,69), and an oxide lineshape of GL(30)

Whilst this no doubt gives a good approximation for native oxides, and determining overlayer thicknesses – decoupling of chemical species surely benefits from a thorough treatment of the spin-orbit doublet in order to minimise errors in fitting shoulders and broadenings – such as in the below example studying Al-grafted overlayers onto a silica interface from Parlett et al.

Al 2p fitting for a deconvoluted multiple state composition – reproduced with permission from author.

1. Alexander, M. R., et al. “Quantification of oxide film thickness at the surface of aluminium using XPS.” Surface and Interface Analysis: An International Journal devoted to the development and application of techniques for the analysis of surfaces, interfaces and thin films 34.1 (2002): 485-489. Read it online here.
2. Van den Brand, J., et al. “Acid− base characterization of aluminum oxide surfaces with XPS.” The Journal of Physical Chemistry B 108.19 (2004): 6017-6024. Read it online here.
3. Parlett, Christopher MA, et al. “Alumina-grafted SBA-15 as a high performance support for Pd-catalysed cinnamyl alcohol selective oxidation.” Catalysis Today 229 (2014): 46-55. Read it online here.