An Effective Charge Neutralization Enabled by Graphene Overlayer in Ambient Pressure XPS Measurements of Insulators

Ambient pressure XPS is a powerful technique capable of performing measurements with samples kept at mbar pressure. The filled gas, also X-ray ionized, provides electrons to neutralize positive charges built up on insulating samples. However, this convenient neutralization scheme does not solve the charging problem completely. In this study, the effectiveness of how the electrons generated from conducting mechanical parts mounted in the immediate front of insulating samples neutralize positive charges on the samples during XPS measurements is tested. The mechanical parts range from fine gold mesh, holey carbon film to graphene monolayer, and measured insulating samples are polished sapphire and rough pellets of CaCO₃ powder. All these mechanical add-ons reduce the charging to different degrees at the expense of sample signal. Amazingly, the graphene monolayer is found to effect perfect charge neutralization for both smooth sapphire and rough CaCO₃ pellet, evidenced by realistic spectral profile and binding energy values. The finding that an untreated CaCO₃ pellet is often terminated with Ca(OH)(HCO₃) is consistent with earlier reports. It is speculated that a likely conformal covering with graphene on the sample with rough morphology holds the key to its superb performance in charge compensation.