High-resolution lateral differentiation using a macroscopic probe: XPS of organic monolayers on composite Au-SiO2 surfaces

X-ray photoelectron spectroscopy (XPS), an essentially macroscopic probe, i s used to analyze mesoscopic systems at a lateral resolution given by the s ubstrate structure. The method is based on controlled differential charging of multi-component surfaces, using a simple, commonly available XPS functi on, the electron flood gun. This new approach is applied here to a novel co mposite surface comprising SiO2 clusters on a {111} gold substrate, onto wh ich different molecules are self-assembled to form a mixed organic monolaye r. The method allows direct correlation of adsorbed molecules with surface sites, by analyzing XPS line shifts, which reflect local potential variatio ns resulting from differential surface conductivity. This provides a powerf ul tool for resolving complex ultrathin films on heterogeneous substrates, on a length scale much smaller than the probe size.