Observation of highly dispersive bands in pure thin film ${\mathrm{C}}_{60}$

While long-theorized, the direct observation of multiple highly dispersive ${\mathrm{C}}_{60}$ valence bands has eluded researchers for more than two decades due to a variety of intrinsic and extrinsic factors. Here we report a realization of multiple highly dispersive (330–520 meV) valence bands in pure thin film ${\mathrm{C}}_{60}$ on a novel substrate—the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$—through the use of angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The effects of this novel substrate reducing ${\mathrm{C}}_{60}$ rotational disorder are discussed. Our results provide important considerations for past and future band structure studies as well as the increasingly popular ${\mathrm{C}}_{60}$ electronic device applications, especially those making use of heterostructures.

Figure 1

(a) LEED image of the crystalline 5 nm ${\mathrm{C}}_{60}$ film. (b) Reduced surface Brillouin zone of thin film ${\mathrm{C}}_{60}$. (c) Calculated DFT total energy for hexagon-down and pentagon-down ${\mathrm{C}}_{60}$ molecules at various distances from a single layer of Se-terminated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with van der Waals corrections included. Diagrams indicate the geometry of the bottom ${\mathrm{C}}_{60}$ face (brown) on the substrate Se surface atoms (green). (d) Energy diagram comparing the molecular energy levels for the HOMO and HOMO-1 in a single ${\mathrm{C}}_{60}$ molecule with (e) the corresponding band manifold in a thin film as shown by momentum-integrated ARPES intensity within the first Brillouin zone (solid black line), literature data [24] (dotted gray line), and the calculated density of states (DOS, calculation for single layer ${\mathrm{C}}_{60}$, solid red line). (f) ARPES data along each of the high symmetry directions indicated in (b) showing large dispersions in the HOMO and HOMO-1 band manifolds ($h\nu =45$ eV, $T=20$ K).

Figure 2

Constant energy maps of ${\mathrm{C}}_{60}$ with energies near the (a) HOMO top, (b) middle, (c) bottom, and (d) HOMO-1 top, (e) middle, and (f) bottom. The first Brillouin zone of ${\mathrm{C}}_{60}$ (${\mathrm{Bi}}_2{\mathrm{Se}}_3$) is indicated by a thick black (gray) hexagon with the high-symmetry points labeled in (a). Dashed hexagons indicate higher order Brillouin zones, while two intensity patterns are highlighted in (d) by large dotted hexagons ($h\nu =45$ eV, $T=20$ K).

Figure 3

Calculated total and atom-projected density of states for (a) bulk ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, (b) a single quintuple layer of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001) with a hexagon-down ${\mathrm{C}}_{60}$ ball at its optimal distance of 3.15 Å, (c) bulk ${\mathrm{C}}_{60}$ in the $P\overline{a}3$ structure, and (d) a single hexagonal layer of ${\mathrm{C}}_{60}$ in the (111) direction of the $P\overline{a}3$ structure. In each calculation both van der Waals corrections and the spin-orbit interaction were included. (e) Configuration of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/${\mathrm{C}}_{60}$ structure used for DOS calculations in (b). (f) Configuration of the layer ${\mathrm{C}}_{60}$ structure used for DOS calculations in (d). See text for further details.

Figure 4

(a) Curvature of the electronic band structure of ${\mathrm{C}}_{60}$ along the $\mathrm{\Gamma }\text{−}M$ (purple) and $\mathrm{\Gamma }\text{−}K$ (olive) high-symmetry directions. The calculated bands that best fit the HOMO, HOMO-1, and HOMO-2 experimental dispersions are plotted over the data as red lines. (b) Full DFT-calculated theory band structure of single layer ${\mathrm{C}}_{60}$ including the same bands plotted in (a) highlighted in red. (c) Integrated EDC (black) across ${\mathrm{\Gamma }}^{\prime }\text{−}M\text{−}\mathrm{\Gamma }\text{−}M\text{−}{\mathrm{\Gamma }}^{\prime }$ in comparison with DFT-calculated single layer ${\mathrm{C}}_{60}$ density of states (red) indicating the centroid energy of each experimental and calculated band manifold peak ($h\nu =45$ eV, $T=20$ K).