Student Project Abstracts 2005 - Pluto - University of Washington

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