Modification of topological phenomena at hybrid Bi₂Se₃/organic interfaces

Background

Topological insulators such as Bi₂Se₃ have interesting transport properties, including undamped transport via topologically-protected surface electronic states¹. Manipulation of surface plasmons has been previously demonstrated using magnetic dopants, but these cannot be altered after growth. In contrast, the use of organic molecular overlayers such as C₆₀ could be advantageous because it is possible to ‘gate’ the C₆₀ interaction through electrical biasing and thereby develop tunable devices². Here, we investigate plasmonic interactions at the interface of an as-deposited thin film sample of Bi₂Se₃/C₆₀ using electron energy loss spectroscopy (EELS). We show changes in the characteristic plasmonic behaviour of the Bi₂Se₃ surface in the presence of C₆₀, providing greater understanding of the topological insulator-organic interface.

Methods

Bi₂Se₃ thin films were grown on a c-plane sapphire substrate by molecular beam epitaxy (MBE) with immediate subsequent deposition of C₆₀ overlayers to avoid interfacial contamination. Cross-sections were extracted from the thin films by focused ion beam techniques and thinned to an electron transparent thickness of 35 nm for imaging. STEM and EELS were carried out on a mono-chromated instrument at 60 kV (the Nion UltraSTEMTM 100MC 'HERMES' instrument at the UK SuperSTEM facility).

Momentum-resolved EELS was used to map the plasmon dispersion with a convergence and collection semi-angles of 2 mrad and 1.1 mrad respectively. Spectra were collected containing only electrons scattered with specific momentum transfer from the lattice at momenta 0.0±0.3, 0.7±0.3, 1.0±0.3 and 1.4±0.3 1/Ȧ along the ΓM direction of the first Brilliouin zone of Bi₂Se₃ to map the dispersion.

Results

EELS data and plasmon dispersions were obtained across two interfaces Al₂O₃/Bi₂Se₃ and Bi₂Se₃/C₆₀ and are presented in figure 1. Panel is a HAADF STEM image of the stack, showing excellent film quality that includes clear crystallinity in the C₆₀ layers. EELS spectra of bulk Bi₂Se₃ from the centre of the thin film (annotated blue in figure 1a) revealed two volume plasmons, at 7.2 and 17.3 eV, as well as two Bi core-loss edges between 25-28 eV (not shown). The dispersion of the 17.3 eV volume plasmon is shown in figure 1d and follows a parabolic trendline as expected from literature implying the classical nature of this plasmon. A surface plasmon from Bi₂Se₃ was observed at 5 eV, localised to the Al₂O₃/Bi₂Se₃ interface, as shown in figure 1b. Its plasmon dispersion was observed to follow either a linear or root trend, plotted in figure 1e, which is similar to that predicted for π-electrons in graphene³ and suggests the presence of a strongly confining interfacial potential. Comparison with simulations suggests that the nature of this surface plasmon could be a result of Bi₂Se₃ πelectrons confined in 2D to the surface³. The carrier density obtained from a fit of the surface plasmon dispersion concurs with the predicted number of carriers arising from Bi₂Se₃ πelectrons by DFT simulations.

At the other side of the thin film, additional features could be isolated as originating from interaction with C₆₀ due to their absence at the Al₂O₃/Bi₂Se₃ interface. Across the Bi₂Se₃/C₆₀ interface, the surface plasmon energy was shifted higher in energy to 5.9 eV, shown in figure 1c. This interface contained features from bulk Bi₂Se₃ and C₆₀ along with the additional surface plasmon. In bulk C₆₀, three interband transitions were observed, at 3.6, 4.7 and 5.8 eV, consistent with literature⁴; these exhibited very little dispersion with increasing momentum. At the Bi₂Se₃/C₆₀ interface, some contribution of these non-dispersive interband transitions remained present. In momentum-resolved EELS spectra of this interface, dispersion of the observed surface plasmon was more difficult to map due to the additional spectral features present, however, the spectra could be decomposed into a linear combination of distinct contributions, revealing the presence of a dispersing feature, localised at the interface and that we identify as the interfacial plasmon.

Unusual plasmon dispersion of the Bi₂Se₃ surface plasmon was observed by momentum-resolved EELS similar to dispersion of 2D π-electrons in graphene. Upon fitting, an agreement in carrier concentration suggests the surface plasmon origin could be from the 2D confinement of Bi₂Se₃ π-electrons to the surface. Through the introduction of organic molecules such as C₆₀, the surface plasmon of Bi₂Se₃ was altered. Further work will characterise the interface between Bi₂Se₃ and other organic molecules such as H₂Pc.