Colloidal heterostructures of semiconductor quantum wells : synthesis, characterization and applications

Colloidal semiconductor quantum wells, also known as nanoplatelets (NPLs), have recently emerged as a new class of colloidal semiconductor nanocrystals enabling fascinating excitonic properties. With their quasi two-dimensional structure resembling epitaxially-grown quantum wells, these atomically- at nanoplatelets exhibit narrow emission linewidths, giant linear and nonlinear absorption cross-sections, and ultrafast uorescence lifetimes when compared to other classes of semiconductor nanocrystals. These appealing features have led to achievement of low lasing thresholds and high color purity by using simple heterostructures of these NPLs. To further exploit the benefits of these solutionprocessed NPLs and develop next-generation colloidal optoelectronic devices, novel heterostructures of NPLs with superior excitonic properties are in high demand. In this thesis, to address these needs, we proposed and demonstrated novel heterostructured NPLs. This thesis includes the rational design and systematic synthesis and characterization of these hetero-NPLs. To overcome the lower photoluminescence quantum yield (PL-QY) and stability issues of core/shell NPLs, we successfully synthesized CdSe/CdS/CdS core/crown/shell NPLs resembling platelet-in-a-box. With this advanced architecture, we accomplished substantially enhanced PL-QY and absorption crosssection as well as stability, allowing for the achievement of low-threshold optical gain. However, due to the pure vertical confinement observed in these NPLs, these exciting excitonic features of NPLs suffered from the limited spectral tunability. By developing homogenously alloyed CdSexS1 NPLs together with their alloyed core/crown and alloyed core/shell heterostructures, we succeeded in obtaining highly tunable excitonic features and further extending tunability of the optical gain from these NPLs. In addition to the NPLs having Type-I electronic structure, we demonstrated the highly uniform growth of CdSe/CdTe core/crown NPLs having Type-II electronic structure exhibiting unique excitonic properties Additionally, to realize the evolution of Type-II electronic structure, we synthesized CdSe/CdSe1-xTex core/crown NPLs by precisely tailoring the composition of the crown region. Without changing their vertical thicknesses, we achieved again highly tunable excitonic features and near-unity PL-QY from these hetero- NPLs. Based on the proposed architectures of these heteronanoplatelets, we believe the findings of this thesis provide important guidelines and inspiration for the synthesis of highly efficient and stable heterostructured NPLs to construct high-performance colloidal optoelectronic devices, possibly challenging their conventional epitaxially-grown counterparts.