Surface/interface electronic structure in $C60$ anchored aminothiolate self-assembled monolayer: An approach to molecular electronics

Electronic structure in self-assembled monolayers (SAMs) of $C60$ anchored 11-amino–1-undecane thiol $(C60-11-AUT)$ on Au(111) was studied by means of ultraviolet photoelectron spectroscopy and hybrid density functional theory calculations. Valence band features of the molecular conformation revealed the interface electronic structure to be dominated by σ(S–Au), localized at the thiolate anchor to Au. Formation of a localized covalent bond as a result of hybridization between $N Pz$ orbital of $–NH2$ group of the thiolate SAM and the π level of $C60$ resulted in a symmetry change from $Ih$ in $C60$ to C1 in $C60-11-AUT$ SAM. Appearance of low, but finite amplitude surface electronic states of bonded $C60,$ much beyond the Fermi level, ruled out $Au–C60$ end group contact. The band gap $Eg$ of the SAM, determined to be 2.7 eV, was drastically reduced from the insulating alkanethiol SAMs (∼8.0 eV) and fell intermediate between the $C60$ ground state (N electrons, 1.6 eV) and $C60$ solid (N±1 electrons, 3.7 eV).