All data of UV-Vis and fluorescence were collected at Dickinson College
Quantum dots (QDs) are nanoscale semiconductor particles with unique optical and electronic properties due to their small size. In Band Theory, the number of orbitals is proportional to the number of atoms that are bound to each other. Bulk materials have countless combined atoms, so their band is very closely spaced, meaning that the energy gap between bands is negligible. For nanomaterials, every particle has only several hundreds to thousands atoms, making their energy gap much wider. This is the reason why nanoparticles can display different colors compared to their bulk counterparts. The CdSe semiconductor particles synthesized in this project were only a few nanometers in diameter, and the fluorescence they display at this size does not occur in bulk materials. We can use the quantum size effects to consider the nanocrystals as a model system for a particle in a box.
Selenium was dissolved in 1-octadecene, with trioctylphosphine being added as ligand. Se solution was then combined with reaction solution containing proportionate amounts of CdO, oleic acid, and octadecene that was just heated to 225 degree Celcius. Ater that, Pasteur pipet was used to rapidly remove approximately 1 mL samples at frequent intervals. 9 CdSe quantum dots samples were sucessfually collected.
UV-Vis and fluorescence measurements were performed on all samples to estimate the quantum dots' diameter value. The excited wavelength for fluorescence is 360 nm.
**Data of the first sample were not included in the analysis for its experimental error.
Quantum dots are treated as a particle in a box so its energy is calculated based on this model. For UV-Vis, Brus equation (Quantum confinement effect) is used as the absorbance is directly related to QDs band gap energy, which is obtained by adding the band gap energy of CdSe bulk materials and particle-in-a-box energies of electron and hole. In Brus equation, Column interaction is also considered so effective masses of electron and hole are used and binding exciton energy is taken into account. For Fluorescence, emission energy is estimated by adding the ground state energy of CdSe with particle-in-a-box energies of free electron and hole. Fluorescence model is much more simplified and open compared to UV-Vis model.
Relevant parameters that affect the synthesis process, such as reaction time, instantaneous temperature, and its rising time, are studied to generate models to predict UV absorbed wavelength.