Probing the structure of material layers just a few nanometres thick requires analytical techniques with high depth sensitivity. X-ray photoelectron spectroscopy (XPS) provides one such method, but obtaining vertically resolved structural information from the raw data is not straightforward. There are several XPS depth-profiling methods, including ion etching, angle-resolved XPS (ref. 2) and Tougaard's approach, but all suffer various limitations. Here we report a simple, non-destructive XPS depth-profiling method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfaces; the former contain 'marker' monolayers that have been inserted at predetermined depths. A controllable potential gradient is established vertically through the sample by charging the surface of the dielectric overlayer with an electron flood gun. The local potential is probed by measuring XPS line shifts, which correlate directly with the vertical position of atoms. We term the method 'controlled surface charging' and expect it to be generally applicable to a large variety of mesoscopic heterostructures.
Metal-organic coordination is an attractive means for constructing supramolecular systems, providing versatility, simple synthesis, and a defined geometry. The convenience of changing "building blocks" during multilayer assembly is exploited for the fabrication of novel ion-coordinated hybrid multilayers on gold. Two bifunctional linkers are used, a tetrahydroxamate and an organic diphosphonate, while the connection between layers is accomplished through Zr(IV) coordination, to form a well-defined hybrid multilayer. The two ion binders are compatible with respect to multilayer assembly, allowing the change of linkers during construction while maintaining the film structural integrity and organization. The different chemical reactivity of the binders enables rational structural manipulation of the multilayer, by selective dissolution of the acid-sensitive hydroxamate layers while keeping the acid-resistant phosphonates (and underlying hydroxamates) intact. The process demonstrates the multilayer structural quality, where two diphosphonate monolayers are capable of effectively blocking proton penetration to underlying hydroxamate layers. This allows nanometer-scale reshaping of the molecular film according to a scheme introduced during its construction.
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