Superlattices in charge Band occupation and electron transport in 2D PbSe and CdSe superlattices

Abstract

Two-dimensional (2D) lead-chalcogenide and cadmium−chalcogenide NC superlattices, one monolayer in thickness, present outstanding opto-electronic properties due to their unique band structure. The honeycomb superlattice, with its similarity to graphene, theoretically results in a semiconductor with Dirac-type valence and conduction bands with massless holes and electrons. Here, we investigate the band occupation and electron transport in 2D PbSe and CdSe superlattices, a NC monolayer sheet of (truncated) cubic NCs that are epitaxially connected only via their {100} facets and thus electronically coupled in the lateral directions. The absorptivity of a monolayer superlattice is about 1.6 ± 0.1%, independent of the material (PbSe or CdSe) and geometry (square or honeycomb). One additional layer of superlattice adds another 1.6 % to the absorptivity. This value of the absorptivity appears to be universal in 2D systems including III-V quantum wells and graphene. From an electronic viewpoint, the 2D superlattices are intrinsic semiconductors. To populate the conduction bands with electrons in a controlled way, chemical doping or external gating is required. It is shown, in this thesis, that the PbSe and CdSe superlattices with just a monolayer thickness can be incorporated in a transistor setup with an electrolyte gate. The electron mobility at room temperature is between 2 and 18 cm2V-1s-1, no Dirac carriers’ behavior has been observed. If honeycomb transistor-type devices can be cooled down to cryogenic temperatures, eventually, this research would enable us to display the Dirac character of the carriers.