Bridging Two Worlds: Colloidal versus Epitaxial Quantum Dots

Abstract

An overview is given of two distinct classes of semiconductor quantum dots, epitaxial and colloidal structures that have been studied intensely for more than 30 years by now, however, without large interconnection between the two involved research communities. The largely parallel and independent evolution of the two structure classes may be partly related to the origin of colloidal systems from chemistry, while epitaxial quantum dots have been addressed mostly by the physics community. These independent evolutions are somewhat surprising because the interest in optics-related applications is shared by both communities. Here, a short summary of the development of the two structure classes, the present status of activities, and some perspectives for future developments are presented.

Prospects of Coupled Organic–Inorganic Nanostructures for Charge and Energy Transfer Applications

“…[20] This, in conjunction with the electron-hole exchange interaction and crystal field effects, leads to a fine-splitting of the excitonic states with a different total angular momentum (J) of a few 10 meV (in II-VI materials) or even less (in III-V QDs). [21] Since this is smaller than k b T at not very low temperatures, QDs act as spin mixers with fast thermalization of spins. [9] In many QDs, the excitonic state with the lowest energy is characterized by a total angular momentum of J =AE 2 and referred to as "dark", manifesting in long lifetimes and low quantum yield at low temperature, when thermalization of spins is inhibited.…”

“…[52] In epitaxially grown QDs, this is achieved by embedding the QDs in another inorganic material with matching lattice constant. [21] In colloidal quantum dots, this passivation scheme is adopted in so-called core-shell structures where the inorganic quantum dot core is passivated by a thin shell of another inorganic material with matching lattice constant. Depending on the band edge alignment between core and shell, these core-shell QDs are termed "type I" (a wide band-gap shell encloses the narrow band gap core) or "type II" (core and shell are of comparable band gap in a staggered alignment).…”

“…applications as quantum objects, where a single charge carrier in an isolated quantum object may serve as a qbit -the fundamental unit of information in a quantum computer -if its spin can be controlled and preserved. [21] Furthermore, triplet harvesting and fast triplet transfer is often the ratelimiting step in multiexciton generation by singlet fission (see section 4.1), which bears the potential to exceed the Shockley-Queisser efficiency limit for solar cells. [128] Material combinations tested in this regard so far are summarized in Table 1.…”

“…Another factor with deleterious consequences for the spin purity are dangling bonds and other surface defects, which can invoke spin flipping due to the interaction with the unpaired electrons in dangling bonds and are, thus, to be avoided. [21] The most common strategies involve the design of surface molecules with the right electronic structure to remove trap states from the band gap, the growth of an epitaxial shell onto the NC core or filling the gaps within NC solids by an inorganic, macroscopic matrix, all of which have been outlined in section 1.1. Finally, it should be emphasized that the stability of triplets is hugely affected by the presence of oxygen, such that efficient triplet harvesting will benefit strongly from operation under inert conditions, for instance by encapsulation.…”

Advances in Physics of Semiconductors

“…The special issue includes two Review articles. One of them discusses the properties of semiconductor quantum dots by comparing the nano‐objects coming from two different worlds, namely colloidal and epitaxially‐grown quantum dots (M. Bayer). The other describes the optical properties of a novel type of nanomaterials – transition metal dichalcogenides nanotubes, which are able to emit bright photoluminescence and can act as efficient optical resonators (T. V. Shubina et al …”