We approach this problem purely from the sample side and disregard the specific apparatus. Non the less of course an integrating sphere with reabsorbtion correction will be used.
Lets say we want to determine a quantum yield (QY) of a solid, crystalline substance. What i gathered from literature is that particle size matters when You are measuring a solid sample, this is the case also for me:
Example: Crystals formed via vapor diffusion (big orderly crystals suitable for X-Ray) gave QY of ~ 5%, when i ground this material in a pestle a powder like substance was obtained and it gave QY of ~ 19%, on the other hand if the substance was directly precipitated from saturated solution using a solvent in which the material does not dissolve (DCM -> pentane was added) a similar powder like substance was obtained, in this case the QY ~ 22%
All of these measurements were done with the same substance (same batch) at the same purity etc. several X-Ray structures (from different solvent systems) revealed that solvent does not participate in crystallization pattern and the pattern is always the same.
Which of these measurements should i choose?
Are there any other aspects we should take in consideration?
Thanks in advance,
Kaspars
Dear Kaspars,
when you ground your crystal I suppose you change your sample a lot - then it's not a surprise that QY is changed as well.
Dear Kaspars,
i suppose, that QY in a single crystal might be smaller mainly due to these two reasons:
1) There occurs less reabsorbtion in the grounded or precipitated sample (smaller crystallite size). For that You should compare fluorescence/luminescence spectra of these samples;
2) There might be less deffects (non-emissive species) in a small crystallite than in a large single crystal, considering diffusion length of excitons in your system is typical. For that you should try to compare fluorescence/luminescence lifetimes of your samples;
Dear Kaspars,
This is an interesting point. It is clear that the emission intensity and therefore the QY depends on crystal size and morphology. This is the reason because it is very dangerous compare absolute intensities from different samples even if the same setup was used.
My suggestion is that all measurements are valid, but you should mention the conditions you performed them. When light interact with matter you have different processes depending on the excitation wavelength and the intrinsic properties of the material: absorption, reflection, refraction, scattering, emission, etc. The scattering will change a lot depending on the size of your samples. My point is that it is the main reason of the different values you obtained. It depends on size and excitation wavelength and, of course, on the refractive index of the media surrounding your sample powders.
I hope the explanation is clear enough.
Chears,
Rafa
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