Colloidal synthetic pathways of atomically-flat complex nanocrystal heterostructures

Abstract

Colloidal semiconductor nanocrystals (NCs) constitute one of the most important branches of nanoscience, with an increasingly high research interest, culminating with a Nobel Prize most recently. The nanometric size of these NCs allows for size-dependent optical properties, which provides an extra tool besides the composition to fine-tune these properties. Recent advancements in NC synthesis have been enabling important developments in the design and engineering of different shapes, compositions, and heterostructures of NCs. Accompanied by a deeper physical understanding and more sophisticated fabrication techniques, the NCs are now being integrated into many of the optoelectronic devices and are of prime importance for the next-generation optoelectronics. Despite all the progress, however, the full potential and synthesis dynamics of the NCs still need further investigation. Here, we addressed specifically four key aspects of the semiconductor NCs: shape engineering, electronic heterostructures, doping, and surface modification. In this thesis research, the synthesis dynamics, especially nucleation, growth and diffusion, were investigated in depth for different synthetic routes and conditions, and some of the important challenges were resolved. With the scarce number of proper emitters at longer wavelengths, in this thesis, a complex and thick heterostructure based on group II-VI nanoplatelets (NPLs) with relaxed quantum confinement was developed. The multi-shell design of the proposed NPLs helps overcome the unfavorable growth in the thickness direction, which, together with the cation dissolution/recrystallization and cation reorganization at high temperatures, relaxes the strain between the domains. The final NPLs, emitting in the deep-red region close to the bulk bandgap of CdSe, were used as an active layer in a light-emitting diode (LED) device and exhibited an exceptionally high external quantum efficiency (EQE) of 6.8% at electroluminescence peak wavelength of 701 nm, one of the best reported for colloids in this spectral range in the literature. Additionally, a novel heterostructure of multi-crown NPLs was designed and demonstrated, where several direct and indirect recombination pathways give rise to photoluminescence with both type-I and type-II characteristics. The design of these NPLs, especially the size of the domains, was shown to significantly impact the final optical properties that can activate/deactivate the recombination channels alongside the temperature. These multi-crown type-II NPLs exhibit an extremely high two-photon absorption cross-section with the highest value of 12.9 × 106 GM and low dark-bright exciton splitting energy critical for optoelectronic applications, including photodetectors, bioimaging and quantum devices. Next, we showed silver doping dynamics of core/shell NPLs, which previously proved challenging due to the self-purification after the shell growth. Here, the composition of the shell was shown to be an important factor in the destruction mechanism of the NPLs in the irreversible doping regime at high doping temperatures. The Ag:CdSe/CdZnS core/shell NPLs exhibit only dopant emission with superior paramagnetic properties compared to CdS-shelled NPLs thanks to better lattice preservation and higher dopant content. At last, a surface modification method was suggested and demonstrated for group I-III-VI NCs to enhance their electronic properties. Replacing the long-chain organic ligands with a S2- layer, injection of a negative charge and passivation of donor sites changed the behavior of the field-effect transistors (FETs) based on these NCs from p-type to n-type with more than a 105-fold enhancement in the carrier mobility. This method allowed fine-tuning of the optical properties of the NCs by the diffusion of the cations and shell formation. The findings of this thesis shine light on some of the important challenges in the field of semiconductor NCs while drawing a guideline for future research on the synthetic routes and optoelectronic properties. The thesis paves the way for future device integration of the developed NCs to fully realize their potential, while the demonstration of the more elaborated properties, including nonlinear absorption, paramagnetism and dark-bright exciton splitting, encourages further fundamental studies focusing on the physics of the semiconductor NCs.