Synthesis And Growth Kinetics Of Indium Phosphide, Indium Arsenide Nanocrystals And Their Core/Shell Heterostructures

Semiconductor nanocrystals are attracting increasing interest due to their remarkable quantum confinement effect. They exhibit unique optical and electronic properties, and can be applied in biological technologies, lasers, light emitting diodes, solar cells, nanosensors, electronic devices, telecommunication, etc. While most of the research had been focused on II-VI nanocrystals, III-V nanocrystals were less developed. Nevertheless, III-V nanocrystals are very important due to their larger exciton diameters, low toxicity and unique emission wavelengths. This dissertation was to synthesize high quality indium phosphide (InP), indium arsenide (InAs) nanocrystals and their core/shell heterostructures in order to find an effective way to synthesize high quality III-V nanocrystals.InP nanocrystals were synthesized using tris(trimethylsilyl)phosphine (P(SiMe3)3) as phosphorus source in coordinating and noncoordinating solvents respectively. For the synthesis conducted in coordinating solvent, the solvent serves as both reaction medium and capping ligands, this method takes 3– 7 days for the whole synthesis, the size distribution of the resulting nanocrystals is broad, and size-selective precipitation should be adopted to obtain nanocrystals with narrower size distribution. For the synthesis conducted in noncoordinating solvent, the solvent only serves as reaction medium, and this method is quicker, because it takes only 3– 4 hours, and the quality of the as prepared nanocrystals is much higher in optical property and size distribution, so the size-selective precipitation is not needed.InAs nanocrystals were synthesized using in situ generated AsH3 as arsenic source in noncoordinating solvent for the first time. This method is economic, quick, simple and highly reproduciable. The synthesis should be controlled in 20– 30 min, or an extension of growth would lead to dissolution of the nanocrystals due to instability of the monomer formed from AsH3. In order to optimize the synthetic method, different reaction parameters were studied to explore their effects on the growth of InAs nanocrystals, involving the reaction temperature, the ratio of indium to arsenic precursors, the amount of ligand and solvent. Analogously, InP nanocrystals were synthesize using in situ generated PH3, and the results of the studies on different reaction parameters in the synthesis of InAs nanocrystals is applicable in the growth of InP nanocrystals.In order to improve the optical property, different shells with broader band gaps were epitaxially grown on core InP and InAs nanocrystals to form core/shell heterostructures, such as InP/ZnS, InAs/ZnSe, InAs/ZnS and InAs/ZnSe/ZnS nanocrystals. As a result, the photoluminescence (PL) quantum yields of the core nanocrystals were improved remarkably after the formation of the shells.The growth kinetics of InAs nanocrystals was studied in detail by monitoring the absorption spectra. The observation revealed the InAs nanocrystals would dissolve due to the instability of the monomer formed from AsH3, the monomer would decomposed to AsH3 which decomposed to As under high temperature, so the InAs nanocrystals would dissolve completely when the monomers in the solvent decomposed completely. The growth kinetics of InP nanocrystals was also studied in detail by monitoring the optical or 1H NMR spectra. In the synthesis of InP nanocrystals conducted in coordinating solvent, the growth of nanocrystals can be divided into 5 duration: accumulation of monomer, nucleation and growth, nucleation and dissolution, dissolution, stability. Similar evolution was also observed in the synthesis of InP nanocrystals using in situ PH3 as phosphorus source. Based the observation, the dissolution of InP nanocrystals was attributed to the dissolution of extremely small nuclei and growth layer.The optical stability of InP, InAs and their core/shell nanocrystals were studied systematically. Surface oxidation is the prominent factor to affect the optical stability of nanocrystals. Primary oxidation eliminates the surface dangling bonds and leads to PL enhancement while does not affect the nanocrystal size. However, further surface oxidation forms a coarse oxide layer which results in reduction in PL and nanocrystal size. Generally, due to the surface oxidation, the optical stability of semiconductor nanocrystals depends on the atmosphere where they are stored, and also depends on the methods by which the nanocrystals are synthesized. The photoexcitation of the core and core/shell nanocrystals was also studied which indicated that the photoexcitation property is another criterion to judge the amount of trapping states on the nanocrystal surface.The growth of semiconductor nanocrystals was studied theoretically through Monte Carlo simulation. Three growth modes were simulated: Ostwald ripening, monomer supplementation and dissolution, oversaturation keeps constant. Based on the detailed analysis on monomer supplementation and dissolution, the synthesis of InAs nanocrystals was improved and nanocrystals with narrower size distribution was obtained.