Anomalously enhanced Raman scattering from C60 on Ag surfaces

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C₆₀-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Deciphering the metal-C60 interface in optoelectronic devices: evidence for C60 reduction by vapor deposited Al

The formation of interfacial midgap states due to the reduction of buckminsterfullerene (C60) to amorphous carbon upon subsequent vapor deposition of Al is confirmed using Raman spectroscopy and X-ray, ultraviolet, and inverse photoemission spectroscopies. We demonstrate that vapor deposition of Al results in n-type doping of C60 due to an electron transfer from Al to the LUMO of C60, resulting in the formation of midgap states near the C60 Fermi level. Raman spectroscopy in ultrahigh vacuum clearly identifies the presence of the C60 anion radical (C60(•-)) as well as amorphous carbon created by further degradation of C60(•-). In contrast, the interface formed by vapor deposition of Ag shows only a slight Ag/C60 interfacial charge displacement with no evidence for complete metal-to-C60 electron transfer to form the anion radical or its further degradation products. These results confirm previous speculations of metal-induced chemical damage of C60 films after Al deposition, which is widely suspected of decreasing charge collection efficiency in OPVs, and provide key insight into charge collection at metal/organic interfaces in such devices.

Synthesis and surface self-assembly of [3]rotaxane-porphyrin conjugates: toward the development of a supramolecular surface tweezer for C60

Surface immobilization of pristine C60 by supramolecular interactions is an attractive way to introduce C60 on surfaces since the pi-electron network and the electronic properties of C60 remain intact. Several hosts have been developed for surface complexation of C60. With few exceptions, the hosts reported to date are "electronically inert", limiting the potential applications of pristine C60-based devices. In this study, we present the synthesis and self-assembly of a potential tweezer-like host for C60 having a light-harvesting moiety and an electron-donating unit. More precisely, an azide-containing [3]rotaxane scaffold having ferrocene moieties as blocking group and thioctic acid as anchoring group for a gold surface has been synthesized. This [3]rotaxane has been self-assembled on gold in its protonated (NH2+) (1p) and neutral (NH) (1n) forms and characterized using electrochemistry, XPS, and contact angle measurements. The SAMs were functionalized with free-base and zinc porphyrin using copper-catalyzed 1,3-dipolar cycloaddition in optimized conditions. In combination with C60, this new host is expected to form a triad that could potentially be used as active building block in the preparation of nanostructured electrodes for photoelectrochemical application.