The unique properties of colloidal semiconductor nanocrystals, or quantum dots, have attracted enormous interest in a wide range of applications, including energy, lighting, and biomedical fields. ...However, widespread implementation is hampered by the difficulty of developing large-scale and inexpensive synthesis routes, mainly due to our limited knowledge of formation reaction parameters. We report here a simple yet powerful method to experimentally determine critically important reaction parameters such as rate constants, activation barriers, equilibrium constants and reaction enthalpies. This method was applied to wurtzite cadmium selenide nanocrystals, yielding activation energies for growth and dissolution of 14 ± 6 kJ mol–1 and 27 ± 8 kJ mol–1, respectively, and a reaction enthalpy for nanocrystal growth of −15 ± 7 kJ mol–1. Moreover, the Gibbs free energy for growth was found to be negative at low temperatures, whereas dissolution becomes the spontaneous process above 150 °C.
The dissolution behavior of colloidal CdSe nanocrystal quantum dots is investigated. A series of experiments is conducted to study the stability of nanocrystals based on a simple reversible ...dissolution-growth reaction model. Immersing purified CdSe quantum dots in a fresh solvent at elevated temperatures resulted in their dissolution with rates determined by their concentration, size, type and amount of ligands present in solution, and temperature. In general, highest dissolution rates were observed for diluted quantum dot solutions in the presence of phosphonic acids and carboxylic acids ligands, at temperatures below 200 °C. Importantly, the dissolution process is reversible and regrowth of quantum dots occurs at temperatures above 220 °C. It is concluded that the most stable state of nanocrystal quantum dots in solution is an equilibrium between formed crystals and free molecular precursors in solution.
A method for fabricating colloidal CdSe nanocrystals at low reaction temperatures was developed. The transition from CdSe clusters to continuously-growing nanocrystals was found to be crucial in the ...formation of high-quality quantum dots with narrow size distribution and efficient, tunable optical properties.
The reaction parameters of low-temperature synthesis of colloidal nanocrystals were investigated. It was found the structural and optical properties of CdSe nanocrystal quantum dots synthesized at ...130 degree C are particularly sensitive to alkyl-chain lengths of ligands, the cadmium source, and ligand to precursor ratios. Under optimized synthesis conditions CdSe quantum dots with narrow ensemble size distribution and excellent photoluminescence quantum efficiencies of up to 40 percent were obtained.
A method for fabricating colloidal CdSe nanocrystals at low reaction temperatures was developed. The transition from CdSe clusters to continuously-growing nanocrystals was found to be crucial in the ...formation of high-quality quantum dots with narrow size distribution and efficient, tunable optical properties.
A low-temperature (between 50 and 130 °C) organometallic method was developed for the synthesis of wurtzite CdSe nanocrystal quantum dots with narrow size distribution, tunable optical properties, and efficient luminescence.
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.
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