Controlled surface charging as a depth-profiling probe for mesoscopic layers

Probing the structure of material layers just a few nanometres thick requir es analytical techniques with high depth sensitivity. X-ray photoelectron s pectroscopy(1) (XPS) provides one such method, but obtaining vertically res olved structural information from the raw data is not straightforward. Ther e are several XPS depth-profiling methods, including ion etching(2), angle- resolved XPS (ref. 2) and Tougaard's approach(3), but all suffer various li mitations(2-5). Here we report a simple, non-destructive XPS depth-profilin g method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfa ces; the former contain 'marker' monolayers that have been inserted at pred etermined depths. A controllable potential gradient is established vertical ly through the sample by charging the surface of the dielectric overlayer w ith an electron flood gun. The local potential is probed by measuring XPS l ine shifts, which correlate directly with the vertical position of atoms. W e term the method 'controlled surface charging', and expect it to be genera lly applicable to a large variety of mesoscopic heterostructures.