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Final Presentation Jamie Golden CHEM 496 04/30/10
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Final Presentation Jamie Golden CHEM 496 04/30/10

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  1. Photophysical Properties of CdSe/ZnS Quantum Dots Embedded in Polymer Films and Solubilized in Toluene Final Presentation Jamie Golden CHEM 496 04/30/10

  2. Introduction to Quantum Dots • QDs are semiconductor particles; size = 1-99nm • Photophysical properties (absorption & emission spectra) controlled by size and shape • Ex = Atkin’s P. Chem Book p.(307) • Use in applications such as LEDs, flat screen monitors, and solar cells. • Must be suspended in solid (polymer) matrix for applications to be materialized • Interaction between QDs and polymer matrix is of interest to investigate

  3. Selection of polymers to be used • PMMA polymer of choice • Optically transparent • Water resistant • Chemically stable • Amorphous thermoplastic • Convenient rheological properties • High strength-weight ratio

  4. Previous Studies • Previous studies involving CdSe/ZnS quantum dots involved the use of a thermal lens to measure the quantum yield of QDs in PMMA suspended in three different solvents • toluene, tetrahydrofuran, and chloroform. • Pilla et al found that the quantum yield of the QDs exhibiting fluorescence ranged from 0.60-0.85 at room temperature when suspended in organic solvents.4 • were not able to explain the quenching mechanisms involved of the quantum yield in function of increasing concentration because it was not completely understood. • Proposed behavior could be due to the formation of a cluster or due to particle agglomerations.4 • In another study involving the same QDs and polymer film (PMMA) by Tamborra et al, optical and physical properties of nanocomposties were investigated. • It was evident from fluorescence microscopy images that there is a presence of larger aggregates in CdSe/ZnS in PMMA than for CdS.5 4 Pilla V, Alves LP, Munin E, Pacheco MTT. Radiative quantum efficiency of CdSe/ZnS quantum dots suspended in different solvents. Opt. Comm 2007; 280: 225-229. 5 Tamborra M, Striccoli M, Curri ML, Agostiano A. Hybrid Nanocomposites Based on Luminescent Colloidal Nanocrystals in Poly(methyl methracrylate): Spectroscopical and Morphological Studies. J Nanoscience and Nanotechnology 2008; 8: 628-634.

  5. Experimental • QDs from Evident Technology; no further modification • Temperature resolved laser photolysis setup using 3ns laser pulse • Polymer film placed in quartz dewar to cool to 77 K using liquid nitrogen • Fluoresceien used as a standard in order to calculate quantum yield of QDs in toluene • Used simple formula: QY1/QY2 = I1/I2

  6. Emission Spectra • Emission spectra of QDs solubilized in toluene and embedded in PMMA at RT and 77 K • No significant change in toluene and PMMA film • Indicates no change in QD size in film process • Slight change may be due to close proximity

  7. Luminescence Quantum Yield of Quantum Dots Suspended in Toluene Solution • Fluorescein used as a standard QY = 0.92 • Absorption and Emission Spectra of Fluorescein and QDs in toluene • excitation wavelength chosen to ensure identical spatial distribution of excited molecules in cell • Relative QY ratio = areas under emission spectra ratio • QY1/QY2 = (Area)1/(Area)2 • QY of QDs in toluene = 0.52 λx

  8. Quantum Yield of QDs in PMMA Film • Laser photolysis used in order to avoid technical difficulties to determine QY of QDs in PMMA film • Compared areas under emission decay curves of QDs in toluene and embedded in polymer film • QY1/QY2 = (Area)1/(Area)2 • QY of QDs in PMMA = 0.25 which is only 46% of that measured in toluene Fig. Emission decays of QDs in toluene and embedded in PMMA at RT

  9. Emission Decays of QDs in PMMA Film at RT and 77 KInsert: Laser Pulse Profile • Normalized RT quantum yield of QDs in PMMA film. From this value, can get QY at any temp. • Like to explore temp dependence on QY because electron transfer quenching is expected to slow on lowering the temp Fig showing 2 normalized emission decays at RT and 77 K of QDs in PPMA film. Insert : laser pulse profile and QD luminescence

  10. Laser Pulse Profile and QDs Luminescence • Insert is laser pulse profile and QDs luminescence • Wanted to determine if laser pulse is short enough and detection system fast enough to accurately monitor QD’s luminescence decays • Decay part of laser pulse profile was compared to actual decay of QD luminescence • Found decay part of QD luminescence had a half-lifetime ~ 7.5ns • Found decay part of laser pulse profile had half-lifetime ~ 1ns • This proves that the laser photolysis apparatus is fast enough to accurately measure the decays

  11. Quantum Yield of QDs in PMMA Film • QY at 77K is still less than that of toluene at RT • Why is that?

  12. PMMA and PP Structures PP PMMA

  13. Temperature Dependence of QY • Temperature resolved laser photolysis technique • Relative QY of QDs in PMMA film and relative QY of QDs in PP film as a function of temperature • Found as temperature decreases, the QY increases continuously

  14. Summary of PMMA & PP Study • Found that both PMMA and PP matrices reduce the QY of QDs to the same extent • Conclusions about decrease in QY (from QDs in toluene • Neither energy or electron transfer observed • PP is an inert polymer matrix where energy or electron transfer cannot happen • Decided to see QDs go through liquid to solid phase (Freeze toluene) • Compare using another polymer to further investigate interaction; using polyestyrene (PS)

  15. Introduction to PS • Amorphous polystyrene • Similar chemical composition as toluene • Soluble in toluene • Interest to study • Effects of a liquid to solid phase transition (QDs suspended in toluene) Polystyrene: Toluene:

  16. Quantum Yield of QDs in PS Film • Fluorescein used as a standard QY = 0.92 • Experimental parameters kept same • Relative QY ratio = areas under emission spectra ratio • QY of QDs in toluene = 0.52 • Laser photolysis used in order to avoid technical difficulties to determine QY of QDs in PS • Ratio of Area under emission decay curves = Ratio of QY (QDs in PS and QDs in toluene) • QY of QDs in PS film equals 0.27

  17. Emission Decays of QDs in Toluene and Embedded in PS at RT Insert: Excitation Wavelength

  18. Results/Discussion • To further gain insight in this difference, QY’s of QDs in PS/Toluene solution measured as PS concentration increased • Found: • No change in emission decays for QDs in toluene upon adding 10% PS addition to solution • Continuous decrease of QY as the weight by weight percent of PS increased

  19. QY as a Function of PS Concentration

  20. Results/Discussion • Study liquid to solid phase transitions using emission decays of QDs in PS at 77 K and 298.1 K • Comparing QY’s for QDs in Toluene and QDs in PS film as a function of temperature • QY increases continuously as the temperature decreases for QDs in PS film • DSC analysis did not show phase transition • Different behavior observed for QDs in Toluene: • QY constant, then decreases abruptly as temperature increases at approximately 250 K • QY increases as temperature decreases at ~ 200 K • Note melting point of toluene is -93C (180 K)

  21. Temperature Dependence of QY for QDs in Toluene and QDs in PMMA Melting Point: 180 K

  22. Conclusions • Quantum yield of quantum dots in polymer films is lower than that of quantum dots in toluene (by about half) • Faster emission decay for QDs in polymer film at RT than 77 K • Study liquid to solid phase transitions QDs in polymer film from RT to 77 K • QY of QDs in Polymer film increases as temperature decreases • QY of QDs in Toluene decreases as temperature increases and around melting point, QY begins to increase as temperature decreases • DSC analysis did not show phase transition