Coupled HgSe Colloidal Quantum Wells through a Tunable Barrier: A Strategy To Uncouple Optical and Transport Band Gap

Among semiconductor nanocrystals (NCs), 2D nanoplatelets (NPLs) are a special class of nanomaterials with well controlled optical features. So far, most of the efforts have been focused on wide band gap materials such as cadmium chalcogenide semiconductors. However, optical absorption can be pushed toward the infrared (IR) range using narrow band gap materials such as mercury chalcogenides. Here we demonstrate the feasibility of a core/shell structure made of a CdSe core with two HgSe external wells. We demonstrate that the optical spectrum of the heterostructure is set by the HgSe wells and this, despite the quasi type II band alignment, makes the band edge energy independent of the inner core thickness. On the other hand, these core/shell NPLs behave, from a transport point of view, as a wide band gap material. We demonstrate that the introduction of a wide band gap CdSe core makes the material less conductive and with a larger photoresponse. Hence, the heterostructure presents an effective electric band gap wider than the optical band gap. This strategy will be of utmost interest to design infrared effective colloidal materials for which the reduction of the carrier density and the associated dark current is a critical property.