Raman Spectroscopy Measurements of Interface Effects in C 60 /Copper-Oxide/Copper
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Raman Spectroscopy Measurements of Interface Effects in C60/Copper-Oxide/Copper Y. Li, J. H. Rhee, D. Singh, and S. C. Sharma* Department of Physics, University of Texas at Arlington, Arlington, TX 76019, USA *
Author to whom correspondence should be addressed, [email protected]
ABSTRACT We have investigated interface effects in C60/Cu-oxide/Cu structures. C60 thin films were grown under high vacuum by thermal evaporation and the well-known “pentagonal pinch mode” of C60, Ag(2) band was examined by Raman spectroscopy. We observe Raman-active bands centered at 1420, 1448, 1455, and 1465 cm-1 in C60/Cu-oxide/Cu samples with a thin (~40 nm) C60 overlayer. The 1420, 1448, and 1455 cm-1 Raman bands are not observed in the spectra of pristine C60 powder, copper-oxide substrate, and C60/Cu-oxide/Cu samples with thick (~1.3 µm) C60 overlayer. We associate these new Raman-active bands with C60/copper-oxide interface. INTRODUCTION The nature of the C60-substrate interface, and in particular, possible charge transfer between certain substrates and C60 overlayers continues to be of significant interest and scientific activity. It is generally believed that the charge transfer in such a microstructure takes place via transfer of electrons from metal to the surface layers of C60 at the interface. The charge transfer issue has been studied recently for C60 thin films deposited on metallic substrates by using several techniques, e. g., photoemission spectroscopy, electron energy loss spectroscopy, Raman spectroscopy, etc [1-4]. In one of the early works, Chase et al [5] observed substantial shifts in the Ag(2) pentagonal pinch mode of C60 due to charge transfer to the C60 cluster. Tsuei et al [6] conducted photoemission and photo-absorption study on C60 adsorbed on Cu(111) surfaces and concluded that charge transfer takes place from the substrate to LUMO-derived states of C60 molecular orbitals and consequently the overlayer becomes metallic. Herein we examine interface effects for C60 overlayers deposited on copper oxide, a technologically important material for photothermal and photoconductive applications.
EXPERIMENTAL DETAILS C60 powder of 99.9% purity and containing 1-10 µ m particles was obtained from MER [7]. The C60/copper-oxide/copper structures were deposited by using a high vacuum (10-8 Torr)
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thermal evaporation system discussed by Govinthasamy et al elsewhere in this volume [8]. Our deposition system consists of a high current (50A) power supply, quartz crystal thin film thickness monitor, and temperature control and monitor equipments. For thermal evaporation of thin films, an Al2O3 crucible containing C60 powder was heated to 600K and chemically etched polycrystalline copper substrates (99.999% elemental purity) were maintained at ≈ 350K under high vacuum. Two different C60/Cu-oxide/Cu samples were deposited, a “thin sample” with about 40 nm thick C60 film, and a “thick sample” with a C60 film thickness of about 1.3 µm. The interface effects in thin films, as well as, the pressure effects in pristi
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