Defining the nature of adventitious carbon and improving its merit as a charge correction reference for XPS

The chemical nature of adventitious carbon (AdC), a thin layer of carbonaceous material that deposits on the surface of most air-exposed samples and is widely used for charge correction of insulating materials in X-ray photoelectron spectroscopy (XPS), has been investigated by XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS) on a variety of air-exposed samples from various material classes. The results from this case study show that on average AdC is aliphatic in nature with ∼25% of carbon species having bonds to oxygen. D-parameter and ToF-SIMS results show that AdC is not graphitic in nature, as had been suggested in earlier studies. Using assumptions about volatile organic compounds (VOC) in air that contribute to AdC accumulation, a peak-fitting model for the C 1s XPS spectrum of AdC including beta shifted carbon peaks was developed. This model is shown to increase accuracy of the positioning of the aliphatic peak. Using this model and data from 117 samples an AdC C 1s aliphatic peak binding energy of 284.81 eV (+/-0.25 eV) was found. Average XPS and ToF-SIMS AdC spectra are presented. The average AdC XPS spectrum has been utilized to model the C 1s spectrum of complex organic compounds with AdC present.