C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy

Abstract

The C 1s signal from ubiquitous carbon contamination on samples forming during air exposure, so called adventitious carbon (AdC) layers, is the most common binding energy (BE) reference in X‐ray photoelectron spectroscopy studies. We demonstrate here, by using a series of transition‐metal nitride films with different AdC coverage, that the BE of the C 1s peak EnormalBF varies by as much as 1.44 eV. This is a factor of 10 more than the typical resolvable difference between two chemical states of the same element, which makes BE referencing against the C 1s peak highly unreliable. Surprisingly, we find that C 1s shifts correlate to changes in sample work function ϕnormalSnormalA , such that the sum EBnormalF+ϕSA is constant at 289.50±0.15 eV, irrespective of materials system and air exposure time, indicating vacuum level alignment. This discovery allows for significantly better accuracy of chemical state determination than offered by the conventional methods. Our findings are not specific to nitrides and likely apply to all systems in which charge transfer at the AdC/substrate interface is negligible.

Compromising Science by Ignorant Instrument Calibration—Need to Revisit Half a Century of Published XPS Data

“…[22] Hence, the magnitude of C1sshifts caused by the substrate is larger than typical chemical shifts,w hich definitely prevents any meaningful bonding assignments.S etting the C1 sp eak at arbitrary chosen BE value from the range suggested by ISO standard, 284.6-285.2 eV,i sn ot justified and may lead to unphysical results,like anon-zero density of states above the Fermi level. [23] But, why does the C1speak shift so much from substrate to substrate?Interestingly,inthe same set of experiments that revealed the magnitude of these shifts,t he close correlation between the measured BE of the C1speak from adventitious carbon E F B and the sample work function 0 SA ,w as demonstrated. [22] As depicted in Figure 3, within the measurement accuracy the sum E F B þ 0 SA is constant.…”

Compromising Science by Ignorant Instrument Calibration—Need to Revisit Half a Century of Published XPS Data

“…Hence, the magnitude of C 1s shifts caused by the substrate is larger than typical chemical shifts, which definitely prevents any meaningful bonding assignments. Setting the C 1s peak at arbitrary chosen BE value from the range suggested by ISO standard, 284.6–285.2 eV, is not justified and may lead to unphysical results, like a non‐zero density of states above the Fermi level …”

Highly Stable and Efficient Lignin‐PEDOT/PSS Composites for Removal of Toxic Metals

“…The binding energy scale is calibrated according to the procedure described in ref. [35]. Spectra deconvolution and quantification were performed using CasaXPS software package.…”