Topological insulators can be used as plasmonics materials

LOH Kian Ping (Group Leader, Chemistry)

March 11, 2016

11 Mar 2016 Scientists in NUS discovered visible surface plasmon modes in Bi_²Te_³ topological insulator nanoplates and obtained real space imaging of the plasmon modes for the first time.Bi_²Te_³ is a three dimensional (3D) topological insulator (TI). Such material mostly behaves as insulators but has metallic surfaces. There are frequent discoveries of new electronic properties in TI due to its unique surface states. However, experimental research on the plasmonic properties of TI is only beginning and this is usually focused on the Terahertz to Infrared regions and there has been no report of visible plasmons on TI.

Plasmonics is a key branch of nanophotonics due to its extraordinary photon confinement at the subwavelength scale. While plasmonics promises numerous applications in sensing, energy harvesting, and information transformation, most plasmon enhancement effects rely on the use of noble metals, which show rapid plasmon decay at visible wavelengths. This has stimulated the search for new plasmonics building blocks which offer tunability and design flexibility beyond noble metals.

Prof LOH Kian Ping and his research fellow Dr ZHAO Meng from the Department of Chemistry in NUS, along with Dr Michel BOSMAN from the Institute of Materials Research and Engineering (IMRE) under the Agency for Science, Technology and Research (A*STAR) studied the plasmonic property of individual Bi_²Te_³ nanoplates using transmission electron microscopy (TEM)-based electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy. Taking advantage of the high spatial precision and energy resolution of TEM-EELS and TEM-CL, the spatial distribution of plasmon modes supported on a single nanoplate was fully characterized. The study revealed that a hexagonal Bi_²Te_³ nanoplate can support multiple surface plasmon modes in the entire visible range from 1.6 to 3.1 eV (400-800 nm). Real space imaging of the plasmon modes in TI was obtained for the first time. Different modes were observed in the centre and edge of the nanoplate, and a dark breathing plasmon mode excited in the centre was first discovered in non-noble metal plasmonic materials. Density functional theory calculation shows that all the observed plasmon modes are related to the strong spin-orbit coupling induced surface states of Bi_²Te_³. The anisotropic crystal shape of solution synthesised topological insulators affects surface polarisation and gives rise to tunable, shape-dependent plasmon modes, which can be further used to enhance plasmonic effects in photovoltaics.

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Surface plasmon modes of a hexagonal Bi_²Te_³ nanoplate. Left: EELS of the nanoplate when the electron beam is positioned at the middle of the edge (blue line) and centre (red line) of the nanoplate. Right: Experimental EELS maps the nanoplate at different energies corresponding to the three peaks in the left spectra [Image credit: Zhao Meng].

Reference

Zhao M, Bosman M, Danesh M, Zeng M, Song P, Darma Y, Rusydi A, Lin H, Qiu CW, Loh KP. “Visible Surface Plasmon Modes in Single Bi_²Te_³ Nanoplate.” Nano Letters (2015) 15 8331.