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Unformatted text preview: Stability and Fluorescence Quantum Yield of CdSe–ZnS Quantum Dots—Influence of the Thickness of the ZnS Shell M ARKUS G RABOLLE , a J AN Z IEGLER , b A LEXEI M ERKULOV , c T HOMAS N ANN , b AND U TE R ESCH-G ENGER a a Federal Institute for Materials Research and Testing (BAM), Berlin, Germany b School of Chemical Sciences and Pharmacy, University of East Anglia (UEA), Norwich, United Kingdom c Laboratory for Nanosciences, Freiburg Material Research Centre (FMF), Albert Ludwig University Freiburg, Freiburg, Germany We investigated the correlation between the thickness of the ZnS shell of CdSe–ZnS quantum dots (QDs), the stability of the particles, and the fluorescence quantum yield. As a measure for stability, a new shell quality test was developed. This test is based on the reaction of the QDs with photochemically formed thiophenol radicals and communicates an imperfect ZnS shell by a rapid and complete loss of fluorescence. The quantum yield increases from less than 5% for unshelled CdSe up to 50%, with an increase in ZnS shell thickness up to 0.6–0.8 nm. At the same time, the particles become significantly more stable, as revealed by the shell test. Key words: quantum dot; nanocrystal; semiconductor; fluorescence; quantum yield; CdSe; CdSe– ZnS; shell; ZnS shell; stability; stability test Introduction Semiconductor nanocrystals, or so-called quantum dots (QDs), represent a new class of luminescent la- bels that are of increasing importance in bioanalysis and fluorescence-based imaging. 1–3 These materials show outstanding spectroscopic and (photo)chemical properties 4 , 5 and have the potential to overcome many drawbacks of organic fluorophores, which are up to now the most widely used labels in these areas. 6 , 7 QDs reveal a broad absorption spectrum, a large molar absorption coefficient that increases toward the ultraviolet (UV) region, a narrow and sym- metric emission spectrum (full width at half maxi- mum [FWHM] < 30–40 nm), and a high-fluorescence quantum yield (QY). Their onset of absorption and the spectral position of the emission band can be easily tuned by the (preparation controlled) size of the parti- cles and their material composition. 8 , 9 Because of the broad absorption spectrum, QDs can be excited far Address for correspondence: Dr. Ute Resch-Genger, Federal Institute for Materials Research and Testing, Department I.5, Richard-Willstaetter- Str. 11, 12489 Berlin, Germany. Voice: + 049-30-81041134; fax: + 049- 30-81045005. [email protected] from the emission wavelength, which renders the sep- aration from emission and excitation light straightfor- ward and results in no loss of signal. In contrast, organic fluorophores typically have relatively narrow and dis- crete absorption bands and a broad, long-wavelength tailed asymmetric fluorescence spectrum. The spectral position of the absorption and emission bands is deter- mined by the molecular structure of the fluorophore, the nature of the involved electronic transition(s), and...
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- Spring '10
- Photon, Quantum dot, QDS, zns shell, zns shell thickness