A novel fullerene structure on a topological insulator surface

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.

Figure 1: Angle-resolved photoemission spectroscopy of (a) topologically protected surface state and (b) C₆₀ molecular bands measured using 22 eV photon energy at 30 K. (c) Momentum distribution curves at the Fermi level from the ARPES spectra measured on pristine TI (300 K) and C₆₀ deposited on TI surfaces (300 K and 30 K). (d) Energy distribution curves obtained at 0.0 Å^-1 and 0.21 Å^-1 from ARPES spectra measured on C₆₀ thin film. Two intense peaks at ~2.0 eV and ~ 3.3 eV are from C₆₀.

The ARPES experiments were complemented by temperature dependent Raman and photoluminescence spectroscopies, carried out at the Institute of Solid State Electronics (TU Wien, Austria), enabling the detection of the structural transition of the single layer C₆₀ on TI surface. The experiments were substantiated by density functional theory, carried out at Forschungszentrum Jülich in Germany, which unveiled the origin of the charge transfer to the TI surface state at room temperature and at low temperatures.

Continuing in this research direction, the authors aim to realize an interface between single layer magnetic fullerenes with topological insulator and even superconducting alkali metal doped fullerene films (e.g. Rb₃C₆₀) on topological insulator substrate.

This research was conducted by the following research team:

Ram Prakash Pandeya^1,2, Konstantin P. Shchukin^1,2, Yannic Falke², Gregor Mussler³, Abdur Rehman Jalil³, Nicolae Atodiresei⁴, Eddwi H. Hasdeo^5,6, Alexander Fedorov^2,7, Boris V. Senkovskiy², Daniel Jansen², Giovanni Di Santo⁸, Luca Petaccia⁸, Alexander Grüneis^1,2
1 Institut für Festkörperelektronik, Technische Universität Wien, Vienna, Austria
² II. Physikalisches Institut, Universität zu Köln, Köln, Germany
³ Peter Grünberg Institut (PGI-9), Forschungszentrum Jülich, Jülich, Germany
⁴ Peter Grünberg Institut (PGI-1), Forschungszentrum Jülich, Jülich, Germany
⁵ Department of Physics and Materials Science, Universite’ du Luxembourg, Luxembourg, Luxembourg
⁶ Research Center for Quantum Physics, National Research and Innovation Agency, South Tangerang, Indonesia
⁷ Leibniz Institute for Solid State and Materials Research,  Dresden, Germany
⁸ Elettra - Sincrotrone Trieste, Trieste, Italy

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