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

Abstract

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 Infra-Red (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 which 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.

Mercury Chalcogenide Quantum Dots: Material Perspective for Device Integration

“…(g) High resolution TEM in the dark field mode of a 10 ML HgSe/CdSe/HgSe heterostructure. Parts f and g are adapted with permission from ref 166 Copyright (2018) American Chemical Society.…”

Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals

“…The HgTe NPLs obtained by cation exchange show narrow absorption and PL in the near infrared (IR) range (around 900 nm). The cation exchange is nevertheless self-limited, meaning that only alloys 22,23 have been obtained in the case of quantum dots and only surface exchange over the first two monolayers is achieved for NPLs 24 . In practice attempts to conduct the process on NPLs thicker than 3 monolayers have led to CdX-HgX core-shell heterostructures.…”

“…In practice attempts to conduct the process on NPLs thicker than 3 monolayers have led to CdX-HgX core-shell heterostructures. 24 The exchange is limited to the two external planes of the NPLs. These 2D heterostructures based on HgX were not able to reach the SWIR range.…”

Seeded Growth of Mesoscale Quantum-Confined Semiconductor Nanoplatelets

“…3 Monolayer CdTe Seed Nanoplatelets (slow injection). Adapted from Izquierdo et al, 4,5 260 mg of cadmium propionate, 160 μL of oleic acid, and 20 mL of ODE were degassed under vacuum at 90 °C for 1 hr. Under Ar flow, the reaction was heated to 210 °C and 0.200 mL of 1M TOP-Te mixed with 3.75 mL of octadecene was injected with a syringe pump at a rate of 5 mL/hr.…”