Electronic Structure and Optical Spectrum of Thick HgTe Colloidal Nanoplatelets

The electronic structure and optical transitions of HgTe nanoplatelets (NPLs) are calculated by means of an 8-band k·p Hamiltonian. We show that, for NPL thickness between the limits of strong confinement (∼1 nm) and that of band gap collapse (∼6 nm), the photophysics is largely governed by the Γ6 – Γ8 band coupling. This leads to a nontrivial size dependence of the energy spectrum and charge distribution. A prominent effect is the formation of hybrid states in the conduction band, which evolve gradually from volume to surface localization as the thickness increases. This property enables controlled switching from direct to indirect exciton behavior without the need of using type-II heterostructures, which suppresses interband recombination. By contrast, intraband transition rates are enhanced. The small band gap of thick HgTe NPLs, together with the large binding energy of excitons, suggests that the excitonic insulator phase may be within reach.