Chemistry of Materials Highlights Colloidal Semiconductor Nanocrystals

Abstract

Colloidal semiconductor nanocrystals have been hailed as the building blocks of next-generation nanotechnology. These solution-processed nanocrystals exhibit size-tunable optical and electronic properties, making them valuable for applications ranging from high-performance quantum dot displays to advanced biomedical imaging and even in photovoltaics. Their ability to bridge the gap between molecular chemistry and solid-state physics is central to such innovation, with the 2023 Nobel Prize in Chemistry recognizing the discovery and synthesis of quantum-confined colloidal semiconductor nanocrystals. Yet, as researchers push the frontiers of their potential, challenges persist that must be addressed to unlock their full potential in commercial and scientific pursuits. This Collection of recent papers on Colloidal Semiconductor Nanocrystals from Chemistry of Materials includes an invited editorial by Raffaella Buonsanti and Brandi Cossairt (DOI: 10.1021/acs.chemmater.5c00023), where they ask: What is next? This collection is built from their insight, with papers selected that emphasize (1) Mechanistic Insight and Synthetic Design, (2) Stability and the Nanocrystal Surface, (3) New Materials and the Use of Less Toxic Elements, and (4) Translation from Fundamental Science to Application. The synthetic design of colloidal semiconductor nanocrystals relies on precise control over nucleation and growth processes to achieve nanocrystals with uniform size, shape, and composition. This achievement provides nanocrystals with precisely tuned properties. Such nanocrystals are commonly prepared by the hot-injection method, with Kenis and co-workers (DOI: 10.1021/acs.chemmater.3c02751) providing insight into this method using an automated high-throughput experimental platform to collect a large experimental data set that could be used to train models for predicting synthetic outcomes using machine learning. This method focused on the widely studied CdSe system, which has also been advanced with the synthesis of CdSe-based heterostructures. These heterostructures include CdSe-Dot/CdS-Rod/PbS-Dot nanocrystals (DOI: 10.1021/acs.chemmater.4c02553) that are dual-emissive as well as CdSe/ZnSe Core/Shell and CdSe/ZnSe/ZnS Core/Shell/Shell nanocrystals (DOI: 10.1021/acs.chemmater.3c01333), where the latter quantum dots are green-emitting with a near-unity photoluminescence quantum yield. Heterostructured nanocrystals can be achieved through seeded methods as well as through chemical transformations, such as cation exchange, which in addition to producing the dual-emissive CdS-Rod system, was used to synthesize ZnSe-Dot/CdS-Rod nanocrystals (DOI: 10.1021/acs.chemmater.2c03278) as well as wurtzite InP nanocrystals (DOI: 10.1021/acs.chemmater.3c02226) from Cu_3–xP nanocrystals. Mechanistic studies that reveal the intricacies of colloidal chemistry are central to achieving such structurally complex semiconductor nanocrystals, with advances also reported for highly luminescent In(Zn)P/ZnSe/ZnS quantum dots (DOI: 10.1021/acs.chemmater.3c01359), Ag₂Te quantum dots (DOI: 10.1021/acs.chemmater.4c00026), two-dimensional PbTe nanoplatelets (DOI: 10.1021/acs.chemmater.4c00939), Ag–In–Ga–S quantum dots (DOI: 10.1021/acs.chemmater.2c03023), and CsPbBr₃ perovskite quantum dots (DOI: 10.1021/acs.chemmater.4c00160). Such mechanistic studies also highlight the role of ligands in modulating the nucleation and growth of quantum dots, as studied recently for InP quantum dots (DOI: 10.1021/acs.chemmater.3c01309). These ligands also impact the ability to deposit high-quality shells on nanocrystals, with a new method for shelling ultrasmall PbS nanocrystals (DOI: 10.1021/acs.chemmater.3c01814) with metal-halide-perovskite-like monolayers also reported. Even with the best syntheses of colloidal semiconductor nanocrystals, their utility depends on their stability, where degradation mechanisms that include oxidation, defect generation (DOI: 10.1021/acs.chemmater.4c00602), ligand desorption, and photobleaching can compromise their optical and electronic properties. The need for stable semiconductor nanocrystals has led to studies of the chemical instability of nanocrystals, including CsPbBr₃ nanocrystals (DOI: 10.1021/acs.chemmater.4c02018) where Jiang and co-workers reported their transformation to Cs₄PbBr₆ nanocrystals being driven by the precursors used in the synthesis. Excellent perspectives by Jonathan De Roo (DOI: 10.1021/acs.chemmater.3c00638) on the surface chemistry of colloidal nanocrystals and by Emily Tsui and her students (DOI: 10.1021/acs.chemmater.3c00481) on the redox reactions at colloidal semiconductor nanocrystal surfaces provide insights into the complexity of nanocrystal surfaces (DOI: 10.1021/acs.chemmater.4c00492) and their characterization. Chemical knowledge of the surface and the thermodynamics of ligand exchange (DOI: 10.1021/acs.chemmater.2c02651) are central to effective surface passivation, whether it is with organozinc halide ligands (DOI: 10.1021/acs.chemmater.3c02461) in the case of ZnSeTe/ZnSe/ZnSeS/ZnS core/shell quantum dots or primary amines (DOI: 10.1021/acs.chemmater.4c01287) in the case of PbS nanocrystals. The development of less toxic colloidal semiconductor nanocrystals is a growing priority to mitigate environmental and health concerns associated with traditional heavy-metal-based systems. Researchers are exploring alternative compositions such as InP (DOI: 10.1021/acs.chemmater.2c02960), AgInS₂ (DOI: 10.1021/acs.chemmater.4c00263), and lead-free halide perovskites (DOI: 10.1021/acs.chemmater.3c03186). Again, synthesis plays an important role, where cation exchange of InP can provide access to coinage metal phosphide nanocrystals (DOI: 10.1021/acs.chemmater.3c03258). Also, understanding of the surface chemistry for these compositions is critically important with a comprehensive review of the InP nanocrystal system provided by Sophia Click and Sandra Rosenthal (DOI: 10.1021/acs.chemmater.2c03074). The translation of colloidal semiconductor nanocrystals from laboratory research to real-world applications hinges on addressing these challenges in synthesis, stability, and toxicity but also on their ability to integrate with existing technologies such as solar cells (DOI: 10.1021/acs.chemmater.2c03357), light-emitting diodes (DOI: 10.1021/acs.chemmater.4c00011), near-infrared devices (DOI: 10.1021/acs.chemmater.4c01619), bioimaging, and optical communication (DOI: 10.1021/acs.chemmater.3c01076). Compatibility with other device components, for example hole transport materials (DOI: 10.1021/acs.chemmater.3c00561), becomes important to such integration. At the same time, new opportunities for applications arise from considering the unique qualities of colloidal semiconductor nanocrystals. For example, coating colloidal CsPbX₃ nanocrystals with thin metal oxide coatings (DOI: 10.1021/acs.chemmater.2c03562) makes the photoexcited carriers in these nanocrystals more accessible and is anticipated to be useful in applications where carrier extraction or delocalization are important. Colloidal semiconductor nanocrystals will continue to push the boundaries of nanoscience, offering excellent control over optical and electronic properties for applications in displays, photovoltaics, and bioimaging, to name a few critical areas. Environmental and health concerns are anticipated to drive the development of heavy-metal free alternatives, which will be propelled by synthetic and physical insights. With new materials, the commercial impact of colloidal semiconductor nanocrystals will only grow, with these nanomaterials poised to shape the next generation of technologies. We hope our readers enjoy browsing this Collection, and we encourage authors to consider Chemistry of Materials as a potential venue for their high-quality research in this dynamic and exciting field. This article has not yet been cited by other publications.