High-Mobility Ambipolar Magnetotransport in Topological Insulator Bi2Se3 Nanoribbons

Nanoribbons of topological insulators (TIs) have been suggested for a variety of applications exploiting the properties of the topologically protected surface Dirac states. In these proposals it is crucial to achieve a high tunability of the Fermi energy, through the Dirac point while preserving a high mobility of the involved carriers. Tunable transport in TI nanoribbons has been achieved by chemical doping of the materials so to reduce the bulk carriers' concentration, however at the expense of the mobility of the surface Dirac electrons, which is substantially reduced. Here we study bare ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ nanoribbons transferred on a variety of oxide substrates and demonstrate that the use of a large relative permittivity ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ substrate enables the Fermi energy to be tuned through the Dirac point and an ambipolar field effect to be obtained. Through magnetotransport and Hall conductance measurements, performed on single ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ nanoribbons, we demonstrate that electron and hole carriers are exclusively high-mobility Dirac electrons, without any bulk contribution. The use of ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ allows therefore an easy field effect gating in TI nanostructures providing an ideal platform to take advantage of the properties of topological surface states.