Determination of absolute quantum yields of luminescing nanomaterials over a broad spectral range: from the integrating sphere theory to the correct methodology

Abstract

Determination of fluorescence or photo- and electro-luminescence quantum yield (QY) is of increasing importance for standardization of novel light-emitting materials like semiconductor quantum dots. The most straightforward spectroscopic QY measurement technique is based on the exploitation of an integrating sphere (IS). We propose a compact set-up and procedure for reliable measurements of QY over a broad excitation spectral range which uses excitation by light-emitting diodes. Starting from the general IS theory we show that (i) a variable excitation pattern has no fatal influence on the QY determination and (ii) a simple two-configuration experiment is an adequate basis of the correct experimental procedure. Then we give guidelines for the QY experiment and calculations as well as a few practical examples and error analysis. Our comprehensive description of absolute luminescence QY techniques based on the IS theory has a general applicability not restricted to the apparatus described in this paper.

Keywords:

• luminescence
• quantum yield
• efficiency
• radiometry
• nanocrystals

Acknowledgements

The author thanks Prof. I. Pelant (Institute of Physics, Czech Academy of Sciences, Prague) for critical reading of the manuscript and Prof. M. Zacharias’ group from IMTEK, University of Freiburg, Germany, for providing excellent nanocrystalline silicon multilayer samples. He also expresses his appreciation to former and current students, A. Fucikova, A. Raichlova and M. Greben for using the PL QY set-up and so enabling to uncover some problems of the technique. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007−2013) under grant agreement no. 245977 (project NASCEnT).

Notes

1. The direct coupling of rays from either an excitation source or an emitting sample into an output channel without several reflections from IS walls must be avoided. Otherwise, the randomizing (integrating) role of an IS will be violated and strong signal artefacts generated. The narrow acceptance cone of an output channel itself can possibly avoid the direct signal coupling without using baffles.

2. Note, photons can be wavelength-shifted also by non-elastic scattering (mainly Raman scattering). We suppose that it can be either neglected or corrected by using the RS. Non-linear optical effects are excluded as very low excitation power densities are used.

3. In the case of a sample placed on an IS wall, incident photons passing through a sample are reflected back and pass for the second time (if not absorbed). Neglecting reflections on sample interfaces as well as losses during the back-reflection from an IS wall, the double passage is equivalent to the single passage through a sample with the double thickness. For convenience, the absorbance A used from now on will have meaning of the absorption probability during a double passage.