Overlayer growth and molecular structures of ${\mathrm{C}}_{84}$ and other large fullerenes: A scanning-tunneling-microscopy study

Thin films of ${\mathrm{C}}_{84}$, ${\mathrm{C}}_{82}$, ${\mathrm{C}}_{78}$, ${\mathrm{C}}_{76}$, and ${\mathrm{C}}_{70}$ grown on cleaved GaAs(110) were studied with scanning tunneling microscopy. Well-ordered two-dimensional arrays were formed at submonolayer molecular coverage for each fullerene. High-resolution imaging of ${\mathrm{C}}_{84}$ revealed intramolecular features that we attribute to atomic features on the fullerene cage. Variations in the symmetry of these structures reflect differences in the molecular orientation. Similar intramolecular features were observed for ${\mathrm{C}}_{82}$ but none were imaged for ${\mathrm{C}}_{78}$, ${\mathrm{C}}_{76}$, and ${\mathrm{C}}_{70}$, implying that substrate interaction is less restrictive and the fullerenes rotate at 300 K.