Photonic and phononic properties of oriented 5 nm diameter tellurium nanowires

Abstract

We study polarized optical absorption and Raman spectra (OAS and RS) of ∼5 nm diameter tellurium (Te) nanowires (NWs) confined in chrysotile asbestos nanotubes packed parallel to each other into nearly-perfect-hexagonal lattice. Te was encapsulated into the nanotubes from molt using ∼20 kbar pressure. Asbestos with Te NWs (asb-Te) shows a high OAS/RS anisotropy due to a strong NW-light interaction for the light polarized papallel to NWs (E//c) and a weak one for the light polarized perpendicular to NWs. Importantly, asb-Te RS display bands of confined acoustic phonons at ∼9 cm−1 and ∼23 cm−1, specific signatures of 5 nm diameter Te NWs. Density-functional-theory calculation shows agreement with the observed frequencies and flattening of NW acoustic phonon dispersions useful for thermo-electricity. Asb-Te optical phonon A1-mode Raman band displays frequency upshift compared to the bulk-Te band due to enhancement of the intrachain bonds on expense of interchain ones. Asb-Te 2nd-order RS confirm this. Surprisingly, E-modes display LO-phonon RS-activity in asb-Te instead of TO-phonon-activity in bulk-Te. We observe enhancement of the NW Raman signal compared to that of bulk Te, which is associated with the Mie-resonance-induced NW-light interaction enhancement for E//c, which is important for photonic and photo-electric applications.