Evidence of Optical Band Gap in Few-layered Topological Insulator Bismuth Selenide

When: Wednesday, March 13, 2013 at 12:00 pm
Where: DA 114
Speaker: Anthony Vargas
Organization: Northeastern University
Sponsor: Introduction to Physics Research

In recent times, Bi2Se3 has received a lot of attention as a model Topological Insulator material. The strong spin-orbit coupling in Bi2Se3 is known to result in topologically protected co-existance of gapless metallic surface states and semiconducting bulk states. An important question of considerable fundamental interest is how the electronic properties of this material modifies under nanoscale confinements. In this work, we present optical absorption studies of single-crystal Bi2Se3 nano-flakes. Samples of high-quality single-crystalline Bi2Se3 were fabricated on Si and quartz substrates using a catalyst-free chemical vapor deposition technique. Uniformly flat, hexagonal or triangular nanoflakes of Bi2Se3 could be fabricated with growth-condition-dependent thicknesses and lateral sizes. Experiments were also performed on separately prepared samples mechanically exfoliated from bulk Bi2Se3. In both cases, photon energy-dependent optical absorbance (1.1 eV < E_{photon} <6.5 eV) were measured for samples with different thickness-ranges, from bulk all the way down to a few quintuple layers (QLs). Bulk samples show strong optical absorbance at the lowest photon energies, in good agreement with our theoretical calculations. As the layer-thickness reduces, we find a dramatic change in their optical absorbance – with an optical gap of 2-3 eV appearing in the thinnest samples (~few QLs). The results will be discussed in the framework of the evolution of its electronic structure and optical absorbance as the layer-thickness approaches a single QL limit.