The quantum efficiency (η) or the photoluminescence quantum yield (QY) is a key parameter for the characterisation of luminescent samples.
The absolute QY is the ratio of the number of photons that the materials emits over the number of photons that the material absorbs. Measurement of the absolute QY is now favoured over measuring the relative QY, which requires using a fluorescent standard of known QY.
Measurement of the absolute QY also is a simpler method, requiring only a fluorescence spectrometer equipped with an integrating sphere, than measurement of the QY using thermal lens/beam deflection and photoacoustic methods.
Methods and Materials
The excitation and emission wavelengths in the ultraviolet to the near infrared region were measured for solid and liquid samples which included thin films and LED phosphor powders doped with lanthanides. In the NIR range, lead sulfide (PbS) semiconductor nanocrystal quantum dots (QD-NIR-1V, Ocean Optics) in toluene were transferred into 10mm path-length quartz cuvettes.
The excitation and emission spectra were measured using a FS5-NIR fluorescence spectrometer with the assistance of Hamamatsu’s single photon R2658P PMT detector and a SC-30 integrating sphere module which expanded the detection range of emission to 1010 nm.
In the case of liquid samples, measurement was made over the sample’s emission Esam and scattering Lsam, followed by measuring the solvent, also known as blank or reference, Lref and Eref and the following equation was used the determine the absolute quantum yield (QY):
In the case of thin film and solid powder samples three measurements were carried out to determine the quantum yield. These include:
- Measurement of sample emission under direct excitation, Esamdir
- Measurement of sample scattering under indirect excitation, Esamind
- Measurement of scattering without the sample, Lref
The following equation was then used:
QY calculation was carried out in the instrument's operating software, Fluoracle.
Results and Discussion
The scattering and emission spectra of Ce3+ -doped phosphor and PbS quantum dots are shown in Figures 1 and 2, respectively.
Figure 1. Scattering and emission spectra of LaPO4:Ce3+ phosphor. The measurement conditions were: Δλexc=10nm, Δλem=1nm, step=0.5nm, dwell=0.2s.
Figure 2. Scattering and emission spectra of PbS quantum dots. Conditions were: Δλexc=5nm, Δλem=2nm, step=1nm, dwell=0.5s.
This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.
For more information on this source, please visit Edinburgh Instruments.