Widely-applicable gold substrate for the study of ultrathin overlayers

Ultrathin films on gold substrates have been the subject of enormous scientific and technological interest. Comprehensive study of such systems requires concomitant application of a variety of complementary characterization techniques. The reliability of the result is frequently hampered by the fact that different characterization methods impose different requirements on the Au substrate, resulting in the need to use different types of Au substrates for different measurements, possibly influencing the overlayer structure. This results in an average, rather than exact, structure determination. Here, we show that 15-nm-thick Au films evaporated at 0.5 Angstrom/sec on silanized glass and annealed are semi-transparent, electrically conducting, and morphologically well-defined, showing a smooth, {111} textured surface. Such Au films provide a high-quality, widely applicable and relatively inexpensive platform for ultrathin overlayers, enabling characterization by a wide spectrum of experimental methods, applied to the same substrate. The exceptional qualities and analytical capabilities of such substrates are demonstrated with several different systems: (i) Cu underpotential deposition (upd); (ii) alkanethiol self-assembly; (iii) formation of Au nanoparticle layers; (iv) binding of the chromophore protoporphyrin IX (PPIX) to a monolayer of 11-mercaptoundecanoic acid (MUA). In the latter case it is shown that the use of Cu2+ ions for binding between the carboxylate groups of PPIX and MUA promotes better organization of the porphyrin layer.