Scanning tunneling microscopy study of alanes/Al(111): Contrasting neat and molecularly-terminated W tips

Scanning tunneling microscopy has proven to be a very powerful method for r eal space imaging of molecules on surfaces. Here we demonstrate how the mor phological and chemical structure of the tunneling tip alters the apparent topography of STM images. When imaged with conventional W tips, isolated al uminum hydride (alane) oligomers appear as 2.8 Angstrom protrusions on Al(1 11), When imaged with alane-terminated tips, STM image contrast is both enh anced and inverted, with alane oligomers appearing as depressions. We attri bute this image reversal to the electronic levels of tip-adsorbed alanes, w hose distance from the surface Fermi levels reduces the coherence in the tu nneling junction. The absence of a bias dependence in these images suggests predominant tunneling through tip-alane LUMO states.