Origin of C(1s) binding energy shifts in amorphous carbon materials

The quantitative evaluation of the carbon hybridization state by x-ray photoelectron spectroscopy (XPS) has been a surface-analysis problem for the last three decades due to the challenges associated with the unambiguous identification of the characteristic binding energy values for <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msup><a:mrow><a:mi>sp</a:mi></a:mrow><a:mn>2</a:mn></a:msup></a:math>- and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msup><b:mrow><b:mi>sp</b:mi></b:mrow><b:mn>3</b:mn></b:msup></b:math>-bonded carbon. Here, we computed the binding energy values of C(1s) core electrons on the absolute energy scale for model structures of amorphous carbon (a-C) using density functional theory (DFT). The DFT calculations show that in the case of hydrogen-free a-C, the C(1s) binding energy for <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msup><c:mrow><c:mi>sp</c:mi></c:mrow><c:mn>3</c:mn></c:msup></c:math> carbon atoms is a distribution found approximately 1 eV higher than the binding energy distribution of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msup><d:mrow><d:mi>sp</d:mi></d:mrow><d:mn>2</d:mn></d:msup></d:math>-hybridized carbons. However, the introduction of hydrogen in the a-C network reduces the distance between the characteristic signals of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msup><e:mrow><e:mi>sp</e:mi></e:mrow><e:mn>3</e:mn></e:msup></e:math>- and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msup><f:mrow><f:mi>sp</f:mi></f:mrow><f:mn>2</f:mn></f:msup></f:math>-bonded carbon due to the increased ability to screen the core hole by neighboring hydrogen atoms as compared to carbon atoms. This effect hinders the unambiguous quantification of the carbon hybridization state on the basis of C(1s) XPS data alone. This work can assist surface scientists in the use of XPS for the accurate characterization of carbon-based materials.