Recent Approaches for Bridging the Pressure Gap in Photoelectron Microspectroscopy

Ambient-pressure photoelectron spectroscopy (APPES) and microscopy are at the frontier of modern chemical analysis at liquid–gas, solid–liquid and solid–gas interfaces, bridging science and engineering of functional materials. Complementing the current state-of-the art of the instruments using differentially pumped analyzers, we survey in this short review several alternative APPES approaches, developed recently in the scanning photoelectron microscope (SPEM) at the Elettra laboratory. The reported set-ups allow for performing dynamic near-ambient pressure experiments without introducing additional differential pumping stages. They include implementation of pulsed-gas injection in the vicinity of samples or placing the sample inside reaction cells with very small apertures. The major part of the review is dedicated to construction and performance of novel environmental cells, where ultrathin electron-transparent but molecularly impermeable membranes are used to isolate the gas or liquid ambient from the electron detector operated in ultra-high vacuum (UHV). We demonstrate that two-dimensional materials, such as graphene and derivatives, are mechanically robust to withstand atmospheric—UHV pressure differences and are sufficiently transparent for the photoelectrons emitted from samples immersed in liquid or gaseous media. Representative results illustrate the performance of reported APPES approaches using tunable synchrotron X-rays, combined with the sub-micrometer lateral resolution of SPEM. They demonstrate the unique opportunities for addressing the chemical composition and electronic structure of surfaces and interfaces under realistic operation conditions with unprecedented lateral and spectral resolution.