| AVS 58th Annual International Symposium and Exhibition | |
| Nanometer-scale Science and Technology Division | Tuesday Sessions |
| Session NS-TuP |
| Session: | Nanometer-scale Science and Technology Division Poster Session |
| Presenter: | Ponnusamy Nachimuthu, Pacific Northwest National Laboratory |
| Authors: | P. Nachimuthu, Pacific Northwest National Laboratory A.S. Karakoti, Pacific Northwest National Laboratory S.P. Sanghavi, Pacific Northwest National Laboratory P. Yang, Pacific Northwest National Laboratory V. Shutthanandan, Pacific Northwest National Laboratory L.J. Terminello, Pacific Northwest National Laboratory S. Thevuthasan, Pacific Northwest National Laboratory |
| Correspondent: | Click to Email |
The CdSe quantum dots have been studied for various applications ranging from biomedical imaging and sensing to hybrid solar cells. When the size of the nanocrystals reduces below the exciton Bohr’s radius, the energy levels become discrete (quantized). Consequently, the band gap of quantum dots increases with decreasing the size of the dots. This is inherently reflected in the electronic structures of these quantum dots. In addition, these CdSe quantum dots are known to exhibit a phase transition from a stable hexagonal phase in larger dots to a metastable cubic phase at smaller dot sizes during their synthesis. As the size of quantum dot decreases, the number of surface atoms and the energy associated with the surface increase leading to creation of vacancies, which results in the non-stoichiometric CdSe. However, the mechanism is not clear at atomic level. Therefore, we took a systematic approach to study the size-induced structural and electronic properties of CdSe quantum dots in toluene and drop-casted on Si by various in-situ and ex-situ imaging, spectroscopy and diffraction techniques to obtain the correlation between the quantum confinement and the corresponding stoichiometry, crystalline phases and the effect of surface ligands. The CdSe quantum dots capped with trioctylphosphine oxide (TOPO) or hexadecylamine (HDA) in toluene exhibit predominantly wurtzite crystal structure, which undergoes a phase transformation to zinc blende crystal structure following drop casting on Si and this phase transition increases with decreasing the size of the CdSe quantum dots. A systematic increase in the core level binding energies of Cd 3d and Se 3d, the band gap and the Cd/Se ratio is found as the size of the quantum dots decreases from 6.6nm to 2.1nm. This is attributed to the quantum confinement of CdSe crystallites by the capping ligands in toluene which increases with decreasing the size of the quantum dots thereby increasing the Se vacancies. However, drop-casting of CdSe quantum dots on Si alters the arrangement of capping ligands on the quantum dots which facilitates significant phase transformation. To gain further insights in understanding these transitions we are seeking first principles investigation on model CdSe particles using density functional theory (DFT). The relative stability between the two phases as a function of particle sizes will be reported.