The so-called Buckminster fullerene (C60) has a spherical shape and assembles into a cubic structure at all temperatures. At room temperature, the fullerenes can spin around their axes and hence, the molecules are randomly oriented. At lower temperature, this spinning motion is frozen and all the C60 molecules are orientationally ordered in a certain direction. The transition to ordered structure with cooling is typically observed as first order structural transition from face-centered-cubic to simple cubic structure below 260 K. While thick layers of fullerenes on metal and semiconductor substrates have been studied previously, the C60 structural transition in single layer and its impact on substrate surface electronic properties are still unexplored.
In this work, Pandeya et al. studied the growth of single layer long-range crystalline order of a single layer fullerene film on a novel substrate. Since the expected effect of C60 on the substrate is rather small because of the van der Waals interaction, a topological insulator (TI), Bi4Te3, with spin-polarized electronic states located at the surface was chosen as substrate. The sample was grown at Forschungszentrum Jülich (Germany) by molecular beam epitaxy and capped with a protective layer so that it could be safely transported to Elettra synchrotron. The surface character of the topological insulator electronic states made it possible to study the interaction with adsorbed fullerenes.
To probe the electronic structure of both topological insulator surface and the C60 thin film, high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements were carried out at the BaDElPh beamline of Elettra, taking advantage of high brightness, high energy resolution, photon energy tunability, and most importantly polarization tunability of the photon source. The study was conducted at two different temperatures: room temperature, at which the fullerenes are spinning, and 30 K, at which the spinning motion is frozen out. Careful analysis of the ARPES data (see Figure 1) enabled the research team to identify a significant electron transfer to the TI surface state from C60 layer at room temperature without affecting substrate surface and thin film electronic properties. Interestingly, at low temperature where C60 molecules are frozen, a negligible charge transfer to TI surface was observed, indicating that both the substrate and thin films preserve the pristine electronic properties.
Read more on Elettra website