Near-infrared emitting quantum dots: Recent progress on their synthesis and characterization

Abstract Near-infrared emitting lead-based quantum dots (QD) have gained considerable attention in the last decade due to their potential applications in for instance solar cells, bioimaging, telecommunications, and quantum computing. These QDs can be tuned to emit from 750 to 3700 nm which makes them very viable for the aforementioned applications. Although the synthesis was developed over the last decade, which had led to fairly uniform and narrow size distribution of lead-based quantum dots, the stability of these QDs under atmospheric conditions has rather been a big challenge. The stability of these QDs is of course of paramount importance for various applications. A passivation shell is normally grown over the quantum dots to form a core/shell structure which in turn improves the stability. There are semiconductors which potentially could grow over PbSe or PbS QDs by the conventional epitaxial method, such as EuS, CdS, and CdSe. However, the conventional epitaxial method of growing a larger band gap as the passivating shell has not yet yielded the desired core/shell structure for PbSe or PbS QDs. Furthermore, a few groups have worked on improving the stability of the lead-based QDs. The first part of the review focuses on the synthesis of the lead-based QDs and what modifications have been applied to improve the quality of the nanocrystals. The second part of the review will emphasize the core/shell synthesis of PbSe/CdSe and PbS/CdS and how their structure has been validated. The third part of the review will focus on the surface modification of these QDs with different methods and what challenges still lie ahead. The fourth part will discuss the optical properties of the core and core/shell QDs. The last part of the review will briefly deal with NIR QDs other than lead-based ones. Finally, we will conclude what challenges lie ahead in the field of lead-based QDs and their application in the field of biology.