Student Project Abstracts 2005 - Pluto - University of Washington

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EFFECTS OF SURFACE CHEMISTRY ON CADMIUM SELENIDE NANOCRYSTAL FLUORESCENCEThe similarity between the K-values of the amine and thiolpairs suggest that the alkyl chain effects are secondary to the effectsof the head group. The larger K-values for the thiols showsthat the sulfur binds better than nitrogen, which is consistent withthe Hard-Soft Acid-Base Principal (HSAB). HSAB states thatsoft acids such as cadmium preferentially bind to soft bases suchas sulfur; nitrogen is a borderline soft base and is less likely tocoordinate to the cadmium.Quenching occurs in the presence of thiol due to increasedhole traps 11 . The thiol’s sulfur atom is less electronegative than theamine’s nitrogen and therefore would donate electrons more readily.When the sulfur atom binds to the cadmium, the cadmium atombecomes more negatively charged. The increased partial negativecharge on the cadmium attracts holes in the semiconductor valenceband resulting in “hole trapping.” The trapping of holes hindersrecombination of an electron-hole pair resulting in fewer radiativedecay events and thus a lowered emission intensity.The QY enhancement occurring at amine concentrations below~0.01 M can be explained by an initial decrease in electrontraps when the nitrogen binds to cadmium sites. After all the electrontraps have been passivated, the amine will then bind creatinghole trap sites and quenches the florescence. This could suggestthat the amine preferentially binds to electron trap surface sites.The quenching by amines could result from several possibilities:(1) impurities in BA, (2) etching of CdSe NCs at high amineconcentrations, and (3) temperature changes. The molar ratio ofCdSe NCs to water is ~10 in our test solutions. However, purgingthe BA with N 2(g) did not appear to change the amount ofquenching. The quenching and blue-shifting of the absorbanceand PL peaks at high amine concentrations implies that the amineis etching the NC surface as demonstrated by Li et al. 12 AddingBA to chloroform caused the solution to heat up 10°C. This suggeststhat the heat released could be the heat of solvation of BAin chloroform solvent.To check the reversibility of the BA quenching, the BA froma test solution with 6.75 M BA concentration was pumped offand chloroform was added back. Before pumping, the CdSe NCshad a maximum absorbance peak at 563 nm and quantum yieldof 0.5%. After pumping off the BA, QY was 0.9% and the absorbancespectra appeared more broadened. However, the largeblue shift of ~22 nm in the absorbance spectra was not reversiblesuggesting that indeed the nanoparticles were etched.The restoration of the quantum yield upon addition of ODAto ODT-quenched CdSe NCs indicates that the ligand bindingmay be reversible at low concentrations. However, the initial13% QY however was not achieved with the addition of ODA.At the high ODA concentrations, a drop in QY occurs. This dropis observed at the same ODA concentration as in the earlier testsof CdSe NCs and ODA.CONCLUSIONOur studies have shown that thiols quench the fluorescenceof 4 nm diameter CdSe NCs, whereas at low concentrations (below~0.01 M) amines enhance PL and at high amine concentrationsquench PL. At low concentrations, the thiol and amine canbe reversibly exchanged. Reversibility tests indicate that the reactionwith high concentrations of amine is not completely reversibledue to surface etching. Fluorescence measurements canallow us to calculate the amount of ligand bound to the core if weassume that PL is proportional to surface coverage. In the future,H-NMR could be used to quantify surface coverage to complementfluorescence data.ACKNOWLEDGEMENTSMany thanks to my mentors, Andrea Munro and David Ginger,for their help and guidance. Thank you to the NSF Science& Technology Center for Materials and Devices for InformationTechnology Research for funding the Summer 2005 Hooked onPhotonics REU program and allowing me to enrich my summerwith research. Thanks to Loren Kruse and Rajan Paranji of theUW NMR facilities and Brenden Carlson and Kolby Allen of theDalton Lab for their help in NMR spectroscopy. Also thanks toYeechi Chen for her support and guidance.REFERENCES1. Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.;McLendon, G. “Photophysics of Quantized Colloidal SemiconductorsDramatic Luminescence Enhancement by Bindingof Simple Amines” J. Phys. Chem 1986, Vol 90, No. 23, 6074-6076.2. Landes, C.; Burda, C.; Braun, M.; El-Sayed, M. A. “Photoluminescenceof CdSe Nanoparticles in the presence of a Hole-Acceptor: n-butylamine” J. Phys. Chem. B 2001, Vol 105, No 15,2981-2986.3. Landes, C.; Braun, M.; El-Sayed, M. A.;“On the Nanoparticleto Molecular Size Transition: Fluorescence Quenching Studies”J. Phys. Chem. B 2001, Vol 105, No 43, 10554-10558.4. Hohng, S.; Ha, T. “Near-Complete Suppression of QuantumDot Blinking in Ambient Conditions” J. Am. Chem. Soc. 2004,Vol 126, No 5, 1324-1325.5. Kalyuzhny, G.; Murray, R. “Ligand Effects on Optical Propertiesof CdSe nanocrystals.” J. Phys. Chem. B 2005, Vol 109, No15, 7012-7021.6. Jeong, S.; Achermann, M.; Nanda, J.; Ivanov, S.; Klimov,V.; Hollingsworth, J. “Effect of Thiol–Thiolate Equilibrium onthe Photophysical Properties of Aqueous CdSe/ZnS NanocrystalQuantum Dots” J. Am. Chem. Soc. 2005, Vol 127, No 29, 10126-10127.92 CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005
NG7. Peng, Z.; Peng, X. “Nearly Monodisperse and Shape-ControlledCdSe Nanocrystals via Alternative Route: Nucleation andGrowth” J. Am. Chem. Soc. 2002, Vol 124, No 13, 3343-3353.8. Yu, W.; Qu, L.; Guo, W.; Peng, X. “Experimental Determinationof the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals.”Chem. Mater. 2003, Vol 15, No14, 2854-2860.9. Sachleben, J.; Wooten, E.; Emsley, L.; Pines, A.; Colvin, V.;Alivisatos, A. “NMR studies of the surface structure and dynamicsof semiconductor nanocrystals” Chem. Phys. Lett, 1992, Vol198, No 5, 431-436.10. Berrettini, M.; Braun, G.; Hu, J.; Strouse, G. “NMR Analysisof Surfaces and Interfaces in 2-nm CdSe” J. Am. Chem. Soc.2004, Vol 126, No 22, 7063-7070.11. Wuister, S.; de Mello Donega, C.; Meijerink, A. “Influenceof Thiol Capping on the Exciton Luminescence & Decay Kineticsof CdTe and CdSe Quantum Dots.” J. Phys. Chem. B 2004,Vol 108, No 45, 17393-17397.12. Li, R.; Lee, J.; Yang, B.; Horspool, D.; Aindow, M.; Papadimitrakopoulos,F. “Amine-Assisted Facetted Etching of CdSeNanocrystals.” J. Am. Chem. Soc. 2005, Vol 127, No 8, 2524-2532.CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 93
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The material is based upon work sup
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TABLE OF CONTENTSSynthesis of Dendr
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SYNTHESIS OF DENDRIMER BUILDING BLO
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throughout the work period. Five su
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12 CMDITR Review of Undergraduate R
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BARIUM TITANATE DOPED SOL-GEL FOR E
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BARIUM TITANATE DOPED SOL-GEL FOR E
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SYNTHESIS OF NORBORNENE MONOMER OF
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using different reaction conditions
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Synthesis of Nonlinear Optical-Acti
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quality of the XRD structures wasca
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Behavioral Properties of Colloidal
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Transmission electron microscopy ha
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areorient themselves with the elect
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Fabry-Perot modulators with electro
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Page 42 and 43: QUANTIZED HAMILTON DYNAMICS APPLIED
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Page 46 and 47: INVESTIGATING NEW CLADDING AND CORE
Page 48 and 49: Dr. Robert NorwoodChris DeRoseAmir
Page 50 and 51: SYNTHESIS OF TPD-BASED COMPOUNDS FO
Page 52 and 53: SYNTHESIS OF TPD-BASED COMPOUNDS FO
Page 54 and 55: OPTIMIZING HYBRID WAVEGUIDESpropaga
Page 56 and 57: At closer spaces the second undesir
Page 58 and 59: SYNTHESIS AND ANALYSIS OF THIOL-STA
Page 62 and 63: QUINOXALINE-CONTAINING POLYFLUORENE
Page 64 and 65: QUINOXALINE-CONTAINING POLYFLUORENE
Page 68 and 69: SYNTHESIS OF DENDRON-FUNCTIONALIZED
Page 72 and 73: BUILDING AN OPTICAL OXIMETER TO MEA
Page 78 and 79: TOWARD MOLECULAR RESOLUTION C-AFM W
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Page 82 and 83: SYNTHESIS AND CHARACTERIZATION OF E
Page 84 and 85: My name is Aaron Montgomery and I a
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Page 90 and 91: EFFECTS OF SURFACE CHEMISTRY ON CAD
Page 96 and 97: SYNTHESIS OF A POLYENE EO CHROMOPHO
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Page 104 and 105: CHARACTERIZATION OF THE MOLECULAR P
Page 108 and 109: OPTIMIZATION OF SEMICONDUCTOR NANOP
Page 110 and 111: OPTIMIZATION OF SEMICONDUCTOR NANOP
Page 112 and 113: CHARACTERIZATION OF THE PHOTODECOMP
Page 116 and 117: ELECTROLUMINESCENT PROPERTIES OF OR
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Page 124 and 125: HYDROGEL MATERIALS FOR TWO-PHOTON M
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Page 128 and 129: THE DESIGN OF A FLUID DELIVERY SYST
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