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Date of Award
2016
Publisher
North Dakota State University
Document Type
Dissertation
Degree Awarded
Doctor of Philosophy (PhD)
Department
Chemistry and Biochemistry
College
Science and Mathematics
Faculty Advisor
Kilina, Svetlana V.
Description
Quantum dots (QDs) have drawn a special attention since recent past due to their properties such as broad range absorption ability, size tunable narrow emission, high extinction coefficients, and charge carriers ability. Nonetheless, imbalanced surface passivation/defects leads to the appearance of surface trap states inside the band gap, affecting both radiative and non-radiative dynamics. Experimentally, it is difficult to explore the effect of surface states as they are optically inactive. However, computations provide valuable insights to these characteristics. We performed calculations using density functional theory (DFT) and time-dependent DFT (TDDFT) to provide our insights to such effects. Firstly, we performed DFT studies to understand the effect of QD- ligand interactions on their photophysical properties. Our studies on thiols passivated CdSe QDs showed that passivation of their surface by equilibrium concentration of neutral thiols and negatively charged thiolates is essential to achieve photoluminescence (PL) enhancement. Additionally, we investigated the effect of surface defects on photophysical properties of silicon QDs. Our results showed that defects introduce mid gap states inside band-gap. Absorption spectra showed the appearance of dark/semi-dark states at the first energy band, proving that the surface states quench PL efficiency in QDs. Secondly, we studied the effect of QD-QD interactions on their optoelectronic properties. In collaboration with the experimental group from Prof. Hobbies’ lab, we studied interactions between defective and non-defective QDs. Calculated Forster Resonance Energy Transfer rates suggest that all the trap states in a defective QD would be filled by the excited electrons from the non-defective QD and thus emission happens from the highest bright energy state. We proposed this as the reason for the experimental observation of the increased on-time blinking and overall enhancement of PL in these QDs. Furthermore, in collaboration with experimentalists from Los-Alamos National Lab, we have provided our insights into the chemical engineering of self-assembling of PbSe QDs into (100) directed 2D nanoplates. Our surface energy calculations on the oriented attachments revealed that 2D nanoplates grown in (100) are more feasible than 3D quantum dots. Overall, our calculations not only supported the experimental findings, but also provided solutions to questions raised by experimentalists.
Recommended Citation
Dandu, Naveen Kumar, "Effect of Surface Ligands and Surface Defects on the Electronic and Optical Properties of Quantum Dots" (2016). Chemistry and Biochemistry. 51.
https://digitalcommons.ndsu.edu/chem-biochem/51
Rights
NDSU Policy 190.6.2
Rights Link
https://www.ndsu.edu/fileadmin/policy/190.pdf
Author ORCID Identifier
0000-0001-7122-8537
Handle Identifier
https://hdl.handle.net/10365/25913