UPS vs XPS. What Can We Get?

Tatyana Bendikov, Chemical Research Support, Weizmann Institute of Scince, Rehovot, Israel
Tal Toledano, Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel
Abd-Elrazek Haj-Yahia, Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel
Lee Barnea-Nehoshtan, Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel
Pabitra K. Nayak, Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel

Accurate characterization of materials is the basic requirement for their successful development and applications. In this context, the abilities of X-ray Photoelectron Spectroscopy (XPS), as a surface-sensitive technique, cannot be overstated. Using an X-ray source and analyzing core level transitions, XPS provides unique information about elemental composition, and on the chemical and electronic state of a given element in the material, with a sensitivity down to a  single atomic (near)surface layer. However, for some research goals this knowledge is not sufficient and does not provide all the information required for complete understanding of the material properties. Additional questions, like: what are the valence region transitions (close to the Fermi level) or what is the Ionization Energy of the material, become, in particular cases, crucial for the understanding of the material properties and, thus, for possible applications of those materials. Moreover, in several cases information on the interface between two materials or from the very top surface (up to 2 nm) of the sample is required, but cannot be met by XPS measurements, where the signal is collected from the top ~ 5-10 nm of the surface. To address all these issues a “softer energy” photon source is required.

Using an Ultra Violet source it is possible to obtain information from the very top surface (0.5-1.5 nm) of the sample. Ultraviolet Photoemission Spectroscopy (UPS) provides unique information on monolayers and very thin films, information that is particularly important for researchers working on optoelectronic materials. UPS allows direct measurement of the Work function, determination of Highest Occupied Molecular Orbital/Valence Band position relative to Fermi/vacuum level and yields the high resolution Valence Band spectra.

In this work we present several cases where UPS was successfully used to analyze and study the electronic properties of organic monolayers/thin films and of the resulting organic/inorganic interfaces.