Observation of Electrical Conductivity During XPS Analysis of Organic Oils

Abstract

Under the conditions of x-ray photoelectron spectra (XPS) analysis organic liquids appear to behave as electrical conductors. Here we report x-ray photoelectron spectra for pentaphenylether and pentaphenyltrimethyltrisiloxane oils at room temperature. These materials are commercially available as diffusion pump fluids (Santovac 5 and DC705, respectively). Their room temperature vapor pressures are ∼2 × 10−8 Pa and they can be studied by XPS without hazard to the spectrometer vacuum system. Both materials were investigated as relatively thick liquid films (∼10 μm, as determined by weighing) on a piece of silicon wafer. A solid organic film of this thickness would be expected to be an XPS insulator. The oil films on silicon required no external charge compensation and gave high resolution C 1s spectra with the first component of the envelope very close to 285 eV binding energy. Measured binding energies closely followed a bias voltage applied to the silicon wafer. Oil films thicker than 10 μm behaved as XPS insulators. Insulating behavior was also observed on cooling the samples by passing liquid nitrogen through the spectrometer manipulator during analysis. The explanation of these phenomena is not yet clear but the apparent electrical conductivity allows XPS study of low vapor pressure organic liquids in a straightforward way, without charging effects.