A research group has observed the complex orbital character and phase effects in a C₆₀ (or carbon buckyball) molecular solid thin film. As reported recently in ACS Nano by Claudia Ojeda-Aristizabal of California State University at Long Beach, Alessandra Lanzara and Alex Zettl of the University of California—Berkeley, and J.D. Denlinger of Lawrence Berkeley National Laboratory, these results could drive further research into molecular solids and other novel quantum materials.

“Molecular solids are a useful platform for achieving new and exciting quantum states,” says Yue Cao of Argonne National Laboratory, who was not involved in this study. While normal solids are held together by covalent, ionic, or metallic bonds, molecular solids are a class of materials that are held together by weaker intermolecular forces, such as van der Waals forces or hydrogen bonding. Several interesting properties can arise in molecular solids, such as strongly correlated behavior, because these weaker forces dominate.

“Bulk or thin film C₆₀ is a molecular solid; the crystal or the thin film is supposed to be ruled by what happens in a single [buckyball] molecule,” says Ojeda-Aristizabal. As a molecular solid, the bond length is smaller between atoms within the same C₆₀ molecule than between atoms belonging to neighboring C₆₀ molecules. However, recent experiments by the same collaborative team demonstrated that the van der Waals forces between neighboring C₆₀ molecules are important in the formation of the thin film’s electronic band structure, with dispersive energy bands that indicate long-range interactions between separate molecules. If these interactions within the molecular solid are to be understood or modeled using density functional theory (DFT), the contributions to these dispersive bands from different molecular orbitals must be measured.

To study these interactions, Ojeda-Aristizabal and colleagues performed polarization-dependent angle-resolved photoemission spectroscopy (ARPES) on 5-nm-thick films of C₆₀ on a Bi₂Se₃ substrate. ARPES is an experimental technique that simultaneously measures the energy and momentum of photoemission from a sample to measure its electronic band structure. Here, the researchers used different polarizations of light to create the photoelectrons, which allowed them to analyze the full “electronic wave function of the C₆₀ molecular solid, especially the relative phase between different orbitals and also the occupancy of the electrons on each orbital,” Cao says. “Experimentally, ARPES is the best method [for doing this].”

To analyze the band structure of the C­₆₀ molecular solid, the research team took many steps to eliminate disorder in the thin film. Because of the Bi₂Se­_3­ substrate, “the C₆₀ molecules prefer to have a hexagonal face down [as opposed to a pentagonal face], and that imposed an orientational constraint on the orientation of the molecules,” Ojeda-Aristizabal says. Experiments were carried out at 20 K to eliminate movement of the C₆₀ molecules within the thin film.

The different electronic bands, the highest occupied molecular orbital (HOMO) and the two lower energy bands just below it (HOMO–1 and HOMO–2, respectively), showed striking differences when different polarizations of light were used. According to Ojeda-Aristizabal, “These are dispersive bands that have a more complex orbital character.” Her team attributed differences observed in the electronic bands under different polarizations to contributions from orbitals with different angular momentum.  

The researchers hope that this work excites further interest into C₆₀. “C₆₀, even though it hasn’t received as much attention in past years as it did in the ‘90s, can play an important and exciting role in the new emerging field of van der Waals [2D] structures,” Ojeda-Aristizabal says.

Read the abstract in ACS Nano.