Modeling HgTe Complex Optical Index from Bulk to Nanocrystal Layers

Quantum confinement opens a large energy gap Eg in HgTe nanocrystals (NCs), which is a non-trivial semi-metal (Egbulk < 0) in the bulk crystal limit. In this context, we present a theoretical study of the evolution of the dielectric and optical properties of HgTe from bulk material to isolated NCs up to compact films of NCs. In the latter case, we compare the calculated complex dielectric constant with the one measured by broadband ellipsometry on synthesized colloidal NC film. The theory-experiment agreement is excellent, demonstrating the interest of simulations based on tight-binding calculations combined with a Bruggeman-type effective medium model to predict the optical properties of HgTe NC layers. Our calculations also reveal that the complex dielectric constant εNC(ω) of isolated NCs differs considerably from that of bulk HgTe and depends strongly on their size, especially for the real part. This variation results only from surface effects for energies ℏω≪Eg. This behavior is identical to that of NCs from semiconductors such as Si, InAs, or CdTe. On the other hand, we reveal the existence of an important variation of εNC(ω) of another nature; when going from a normal semiconductor (Eg > 0) to a semi-metal (Egbulk < 0), the material undergoes a transition of topological nature that we predict for a NC diameter of ∼26 nm.