| Semiconductor nanocrystals have emerged as promising building blocks in an extensive range of applications. This can be attributed to their unique size-dependent optical and electronic properties, as well as their chemical flexibility. Despite numerous studies on these materials, their enormous potential impact on industrial output faces several challenges. For instance, high-quality CdSe semiconductor nanocrystal quantum dots (NCQDs), which are the most sought after and well-studied nanocrystal system by far, require high-temperature synthesis methods (200-360ºC). As a consequence, there is still limited knowledge on its nucleation and growth mechanism due to its fast reaction kinetics. Furthermore, it makes large-scale production of these materials extremely difficult and expensive. This dissertation describes an alternative---lower-temperature (between 50-130ºC)---method for synthesizing CdSe NCQDs with structural and optical features equivalent to those obtained from conventional high temperature methods. These desired features include narrow ensemble size distribution, tunable optical properties, and efficient photoluminescence. The role of different reaction parameters on the nanocrystal nucleation-and-growth behavior, as well as on their quality, was investigated. Different optical spectroscopy tools enabled in-situ physico-chemical monitoring of the various nucleation-and-growth reaction steps. One important finding of this thesis is that the transition from CdSe magic-sized clusters to continuously-growing NCQDs was found to be the limiting factor in the formation of high-quality CdSe NCQDs. Another important insight is that growth and dissolution of nanocrystals are in a delicate tug-of-war "equilibrium" state in solution. Dissolution experiments were developed based on a simple reversible dissolution-growth reaction model. The dissolution rates of CdSe NCQDs were observed to be strongly dependent on their initial size, concentration, the type and amount of ligands present in solution, and temperature. The findings of this thesis provide new information regarding nanocrystal stability that is essential for their safe and long term use. Furthermore, the developed dissolution method can also be utilized as post-synthesis size and shape control method for CdSe NCQDs. All these methods provide greater flexibility both in nanocrystal fabrication and their incorporation into applications, which will be of great advantage for future large scale use of these unique materials. |
