This page is a work in progress. Please contact me if you'd like more info about this project. BACKGROUND This was my final project for my Materials Science class at Olin, which focused on electrical and magnetic properties of materials. My team (with excellent mentoring from Prof. Rebecca Christianson) identified a synthesis protocol (see [1],[6]) to create CdSe quantum dots in colloidal suspension and examined their light-emitting characteristics using confocal microscopy. ABSTRACT During UV excitation, quantum dot nanocrystals produce florescent emission spectra that can be precisely tuned by varying crystal size during synthesis. Motivated by applications in medical imaging and LED technology, we conduct colloidal synthesis of CdSe quantum dots and study the relationship between spectral properties and crystal size. We study emission maxima as reaction time increases under different quenching conditions. We compare absorption and emission peaks of various samples to investigate Stokes shift. We failed to experimentally extract CdSe nanocrystals from colloidal solutions for size calculation using XRD. Instead, we apply theoretical models and known literature values to estimate nanocrystal size. Finally, we explore possibilities for producing extremely small dots for white LEDs and very large dots for red to low infrared emission. QUANTUM DOT THEORY A quantum dot is a semiconductor which undergoes quantum confinement in all three spatial dimensions. Comparisons can be drawn from quantum dots to the “particle in a box” example from quantum mechanics. The excitons in the quantum dot are confined to a distance smaller than the Bohr exciton radius, 5.4 nm in bulk CdSe [6, 8]. Previous work on CdSe nanocrystals observed emission maxima ranging from blue (490 nm) to orange (580 nm) with estimated dot radii from 0.9 nm to 2.4 nm, which fall within the strong confinement region [1],[3]. Since the excitons are confined, only specific wavelengths corresponding to the exciton can “fit” inside the box and still satisfy the boundary conditions. Thus the available energies to the exciton are discrete. By changing the size of the quantum dot, the size of the box changes and energies that “fit” shift up and down. Smaller quantum dots have greater separation between the available energies, and thus have a higher band gap. When high enough frequency light strikes the quantum dot, an electron is excited to a higher energy state. When the electron drops back down, it loses energy equivalent to the effective band gap which is determined by the size of the dot. This lost energy is transmitted as light. Thus, quantum dots can absorb higher frequency light and reemit a lower frequency light characteristic to their size. The energy band structure of CdSe crystals as size increases from small quantum dot to large quantum dot and then to bulk semiconductor as shown below. This diagram is modified from Klimov [3].  RESULTS Samples were withdrawn from the hot reaction at semi-regular time intervals between 3 seconds and 10 minutes.  REFERENCES [1] Boatman, E., G. Lisensky, and K. Nordell. "A Safer, Easier, Faster Synthesis for CdSe Quantum Dot Nanocrystals." Journal of Chemical Education 82 (2005). [2] Bowers II, M, J. R. McBride, and S. J. Rosenthal. "White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals." J. Am. Chem. Soc. 127 (2005): 15378-5379. [3] Klimov, Victor I. "Nanocrystal Quantum Dots." Los Alamos Science 28 (2003): 214-20. Accessed 11 Dec. 2008. Available online: <http://library.lanl.gov/cgi-bin/getfile?28-32.pdf>. [4] Laheld, U. E., and G. T. Einevoli. "Excitons in CdSe quantum dots." Physical Review B 55 (1997). [5] Lisensky, G. "Preparation of Cadmium Selenide Quantum Dot Nanoparticles." Exploring the Nanoworld. 11 July 2008. University of Wisconsin, Madison. Materials Research Science and Engineering Center. Accessed 11 Dec. 2008. <http://mrsec.wisc.edu/Edetc/nanolab/CdSe/index.html>. [6] Mao, H. et al. "Photoluminescence investigation of CdSe quantum dots and the surface state effect." Physica E 27 (2005): 124-28. [7] Nirmal, M. et al. "Observation of the "Dark Exciton" in CdSe Quantum Dots." Physical Review Letters 75 (1995). [8] Norris, D. J., A. Sacra, C. B. Murray, and M. G. Bawendi. "Measurement of the Size Dependent Hole Spectrum in CdSe Quantum Dots." Physical Review Letters 72 (1994). [9] Yu, W., and X. Peng. "Formation of High-Quality CdS and Other II-VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers." Angew. Chem. Int. Ed. 41 (2002). |
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