Scanning-tunneling-microscopy and spectroscopy studies ofC70thin films on gold substrates

Thin films of high-purity ${\mathrm{C}}_{70}$ were deposited on polycrystalline gold surfaces and studied by scanning tunneling microscropy (STM) in ultrahigh vacuum. Topographic images reveal initial-stage growth patterns ranging from close packing with a twofold symmetry to random stacking. ${\mathrm{C}}_{70}$ molecules appear oval-shaped, in agreement with the proposed ${\mathit{D}}_{5\mathit{h}}$ structure of ${\mathrm{C}}_{70}$. Static orientational disorder is observed in both close-packed and random-stacked regions. The adsorbed ${\mathrm{C}}_{70}$ molecules show the highest STM contrast at bias voltages well below the HOMO-LUMO gap voltage. These and scanning-tunneling-spectroscopy results are discussed and compared with those obtained previously on similarly prepared ${\mathrm{C}}_{60}$ thin films.