Toward the perfect membrane material for environmental x-ray photoelectron spectroscopy

Abstract

We outline our achievements to develop electron transparent, leak tight membranes required for environmental photoelectron spectroscopy (PES). We discuss the mechanical constraints limiting the achievable membrane size and review the development of growth protocols for the chemical vapor deposition (CVD) of single crystalline graphene on highly (111) textured Cu foils serving as membrane material. During CVD growth Cu tends to develop a mesoscopic staircase morphology consisting of alternating inclined surface planes, irrespective of whether or not the covering graphene film or the substrate is single crystalline. This morphology remains imprinted even when converting the film into freestanding graphene which affects its mechanical properties. Determining the number of carbon layers in freestanding graphene, we show that membranes reported to suspend over distances larger than 20 µm most likely consist of few layer graphene. The Raman band signature often used to confirm monolayer- rather relates to graphene with turbostratic stacking. The vertical corrugation of freestanding graphene was shown to be almost absent for tri- and four-layer thick graphene but substantial for bilayer- and especially for monolayer graphene. The corrugation is reduced when mechanically straining the freestanding graphene through thermal expansion of the supporting frame, especially flattening membrane areas with imprinted staircase morphology. The electron signal attenuation through supported and freestanding graphene was determined as a function of the electron kinetic energy verifying that large area graphene based electron windows will have sufficient electron transparency required for environmental PES. Meanwhile, we managed to cover 100 µm sized single holes by few layer graphene up to a coverage fraction better than 99,9998 % as deduced when applying 10 mbar air on one side of the sealing membrane without detecting any measurable pressure increase on its ultra high vacuum side. The reported achievements will pave the way towards the development of laboratory based environmental PES.