Enhanced Infrared Absorption of C 60 on Thin Evaporated Pd Island Films

Abstract

Infrared transmission spectra of C 60 multilayers on thin Pd films deposited onto surface-oxidized Si(100) and hydrogen-terminated Si(111) substrates are reported. In both cases, the spectra in the 1500-1100 cm -1 region exhibited bands at 1444, 1429, and 1182 cm -1 due, respectively, to the A g (2), T 1u (4), and T 1u (3) modes. The appearance of the A g (2) mode, which is originally infrared inactive (Raman active), reveals electron transfer from the metal to chemisorbed C 60 . Indeed, increasing the thickness of C 60 , the A g (2) mode intensity saturated more rapidly than the T 1u (4) and T 1u (3) modes. The originally infrared active T 1u (4) and T 1u (3) modes were enhanced in intensity depending upon the Pd thickness. Actually, while both substrates gave nearly the same magnitude of enhancement, the optimum Pd thickness was smaller on the hydrogen-terminated surface than on the surface-oxidized surface. On the other hand, the A g (2) mode was less intense on the hydrogen-terminated surface than on the oxidized surface, suggestive of a shortage of chemisorbed C 60 and thus pointing out the importance of the metal film morphology. Indeed, Pd films deposited on the two substrates gave rise to quite different AFM images. We also show that, regardless of the substrate, the A g (2) mode is an order of magnitude smaller than for Ag deposition, though no remarkable intensity differences were observed with respect to the T 1u (4) and T 1u (3) modes.