Electronic properties of (Sb;Bi) 2 Te 3 colloidal heterostructured nanoplates down to the single particle level OPEN

International audienceWe investigate the potential use of colloidal nanoplates of Sb 2 Te 3 by conducting transport on single particle with in mind their potential use as 3D topological insulator material. We develop a synthetic procedure for the growth of plates with large lateral extension and probe their infrared optical and transport properties. These two properties are used as probe for the determination of the bulk carrier density and agree on a value in the 2–3 × 10 19 cm −3 range. Such value is compatible with the metallic side of the Mott criterion which is also confirmed by the weak thermal dependence of the conductance. By investigating the transport at the single particle level we demonstrate that the hole mobility in this system is around 40 cm 2 V −1 s −1. For the bulk material mixing n-type Bi 2 Te 3 with the p-type Sb 2 Te 3 has been a successful way to control the carrier density. Here we apply this approach to the case of colloidally obtained nanoplates by growing a core-shell heterostructure of Sb 2 Te 3 /Bi 2 Te 3 and demonstrates a reduction of the carrier density by a factor 2.5. Bismuth and antimony chalcogenides (tetradymite group with formula such as (Sb;Bi) 2 (Se;Te) 3) have attracted great interest in the past for their thermoelectric properties 1–5. The heavy mass of these materials leads to a large spin orbit coupling which results in an inverted band structure. Over this past decade, it is the original electronic structure of bismuth and antimony chalcogenides which has driven most of interest in these compounds. Indeed they appear as model 3D topological insulator 6–9 , with conducting surface-states and an insulating core, as long as the material can be obtained under an intrinsic form. Sb 2 Te 3 is a 0.3 eV band gap semiconductor. This material has common antisite defects 10–12 where Sb atoms replace Te atoms, which tends to result in a p-type doping 13. Controlling the bulk carrier density in topological insulator compounds is a key challenge since the Fermi level of the material needs to be close to its Dirac point for electronic transport to be dominated by topologically protected surface states. Moreover, the conductance of the material is the sum of the surface and bulk contribution. Because of the narrow band gap nature of these topolog-ical insulator materials and their deviation from stoichiometry, the bulk is generally not as insulating as desired. By reducing the bulk carrier density and the associated conductance, the weight of the surface contribution in transport is expected to increase and make the surface observation more likely to occur. The Mott criterion can be used to estimate whether the material will behave as a metal or as an insulator. Metallic behavior is expected to occur if >. a n 0 25 0 1/3 14 , where n is the carrier density and = ε ε π a h m e 0 r 2 0 2 the Bohr radius with h the Planck constant, ε 0 the vacuum permitivity, ε r the material dielectric constant, m the effective mass and e the proton charge. Due to large dielectric constant (ε > 50) of Sb 2 Te 3 , the Bohr radius is large whic