Description

Colloidal semiconductor nanocrystals (or quantum dots) have evolved during the last few decades from fundamental theoretical concepts to real commercial products (one recent example is a line of Samsung QLED TVs in which quantum dots are employed as color converters), owing to intensive efforts by a plethora of research groups worldwide. These nanomaterials benefit on the one hand from their unique size-dependent optoelectronic properties, based on quantum confinement. On the other hand, their solution-based synthesis is a remarkably simple process that can be implemented in nearly any chemistry lab. Both these factors greatly promote investigation of semiconductor nanocrystals, making this field truly interdisciplinary. Among the main players involved are chemists, physicists, biologists, material researchers, and engineers.

Despite its simplicity, the colloidal synthesis of nanoparticles in its current state allows us to prepare nanocrystals from a wide variety of semiconductors, mainly of the II-VI, IV-VI, III-V, I-III-VI, I-II-IV-VI and I-IV-VII families (corresponding examples are CdSe, PbS, InP, CuInS₂, Cu₂ZnSnS₄, CsPbBr₃). Furthermore, manipulating the conditions of synthesis makes it possible to tune the size of these particles, their shape, and their crystal structure. Moreover, one can combine two or more semiconductors with different designs—such as alloys and heterostructures (core/shell, Janus-type, segmented particles)—within one nano-object. The surface chemistry of these nanocrystals can also be adjusted by means of post-synthetic functionalization and ligand exchange. The first is very important for the biomedical application of nanoparticles; the latter is of paramount importance for their integration into optoelectronic devices. Thus, complex chemistry and physics undergird these apparently simple materials. Being soluble objects, these nanomaterials can be processed via common solution-based techniques, such as drop-casting, spin- and spray-coating, and inkjet-printing. Most of their optoelectronic applications, e.g. in light-emitting diodes, solar cells, photodetectors, and field-effect transistors, involve nanocrystals in the form of solid thin films.

Another important aspect of research on semiconductor nanomaterials is found in computational studies that seek to explain their unique properties observed in experiments, as well as to predict novel properties that can be achieved through the manipulation of size, composition, and structure.

Among current trends in the field of semiconductor nanoparticles, one can define 2D nanomaterials with two typical representatives: CdSe nanoplatelets and MoS₂ nanosheets, as well as perovskite-type nanocrystals, made of alkylammonium/cesium lead halides. Accounting for the fact that many semiconductor compounds contain toxic elements, particularly metals such as Cd, Pb and Hg, researchers are seeking more environment-friendly alternatives. Restrictions imposed on the emission of toxic metals into environment have stimulated investigation into semiconductor nanocrystals based on InP, CuInS₂ and Cu₂ZnSnS₄. Now, as nanoparticles march toward commercialization, they assume an ever greater importance in large-scale synthesis methods developed by chemists in close collaboration with engineers, as processes of mass- and heat-management of reaction mixtures become very significant in large volumes or in continuous production schemes.

Keywords