Journal
NANOTECHNOLOGY
Volume 26, Issue 25, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0957-4484/26/25/255703
Keywords
quantum dot; photoluminescence; pH; reversibility; quantum-confined Stark effect
Funding
- KU Leuven Start grant
- KU Leuven BOF [GOA/14/007]
- European Research Council under the European Union's Seventh Framework Programme (FP7)/ ERC Grant [291593 FLUOROCODE]
- Flemish government [METH/08/04 CASAS]
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The photoluminescence (PL) of colloidal quantum dots (QDs) is known to be sensitive to the solution pH. In this work we investigate the role played by the organic coating in determining the pH-dependent PL. We compare two types of CdSe/ZnS QDs equipped with different organic coatings, namely dihydrolipoic acid (DHLA)-capped QDs and phospholipid micelle-encapsulated QDs. Both QD types have their PL intensity quenched at acidic pH values, but they differ in terms of the reversibility of the quenching process. For DHLA-capped QDs, the quenching is nearly irreversible, with a small reversible component visible only on short time scales. For phospholipid micelle-encapsulated QDs the quenching is notably almost fully reversible. We suggest that the surface passivation by the organic ligands is reversible for the micelle-encapsulated QDs. Additionally, both coatings display pH-dependent spectral shifts. These shifts can be explained by a combination of irreversible processes, such as photo-oxidation and acid etching, and reversible charging of the QD surface, leading to the quantum-confined Stark effect (QCSE), the extent of each effect being coating-dependent. At high ionic strengths, the aggregation of QDs also leads to a spectral (red) shift, which is attributable to the QCSE and/or electronic energy transfer.
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