Light emission spectra of the monolayer-island of C₆₀ molecules on Au(1 1 1) induced by scanning tunneling microscope

We have measured the light emission (LE) spectra of the ordered C₆₀ monolayer-island on the Au(1 1 1) surface and the bare Au surface, induced by the scanning tunneling microscope (STM). The optical measurements were performed at 78 K using electrochemically etched W-tips at a constant tunneling current of 2.0 nA for a sample bias voltage with respect to the tip of 2.3 V. For the bare Au surface, we observed an asymmetric peak at ∼2.01 eV, which is the typical STM-LE spectrum for the Au sample-W tip system. For the C₆₀ monolayer-island, we observed a broad peak centered at ∼1.7 eV, which consists of the red-shifted emission from the underlying Au substrate (∼1.93 eV) and a new emission peak around 1.65 eV. The energy range of the new emission is consistent with that of the photoluminescence of the isolated molecules and a 100 nm-thick film of C₆₀. Thus we conclude that the new emission peak is the fluorescence from the C₆₀ molecules in the monolayer-island.