Instead of pure inorganic crystalline phosphor, polycrystalline porous phosphor has been synthesized. The luminescence efficiency of the later one is three times higher than the previous one. What could be the reason for it?
(one interesting thing in that is 'the same in XRD patterns')
@anuj soni: Porosity is confirmed by nitrogen isotherm as well as with TEM.
In my case, the luminescence intensity is increasing for the porous sample (poly crystalline) compared to the crystalline sample. If we consider that 'there is decrease in luminescence efficiency for porous sample compared to non-porous, as you said', then what will be the role of porosity against luminescence efficiency apart from the crystallinity...?
It's common to see luminescent enhancement in porous materials, you can look into the literature on porous silicon. There has been lots of research on porous silicon after it was noted that the photoluminescence was strongly enhanced.
Here's my speculative explanation: in INDIRECT band gap semiconductors, a phonon is needed in addition to the photon to promote an electron from the valence band-edge to the conduction band-edge. This is because both the energy AND momentum of the band edges are different in indirect band gap semiconductors, so while the photon provides most of the energy, the phonon provides most of the momentum. In a perfectly crystalline material, the phonon modes are severely constrained - only very specific values of momentum are possible.
But in a porous (and therefore highly defective) crystal, the phonon modes are relatively unconstrained. At the surfaces of the pores, there is a much broader set of possible values for phonon momentum. Thus, there is a much richer set of phonons that the photon can interact with to promote an electron from the valence band-edge to the conduction band-edge.
So that's my speculation - porous materials have surfaces with less-constrained phonon momenta, while monocrystalline materials have highly constrained values for the momentum of the phonons. (This is all based off my own intuitive sense of semiconductor physics, not based off literature, so if anyone has references to support or refute this explanation I'd be happy to hear.)
A more mundane explanation for enhanced luminescence from a porous semiconductor is just that there is more scattering. With more scattering, you might be redirecting the photons towards the direction you are measuring them from.
How did you measure luminescence efficiency? If it's not absolute PLQY then it is probably mistake. But if efficiency determined correctly, the results are very interesting.
@Ruslan Gadirov:
Internal quantum efficiency is found to be 25% for crystalline phosphor while the porous phosphor showed 75%....
I don't think that there is any correlation between porosity and polycrystallinity...
I just got a polycrystalline pattern in SAED while TEM and N2 isotherm confirm its porosity & high surface area.
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