Hello everyone. I have conducted PL measurements on CsPbBr3 nanocrystals at 10 K. I observed a significant power-dependent blueshift (~50 meV) and spectral broadening as the laser power was raised from 1 W/cm^2 to 30 W/cm^2. I estimated that the range of induced carrier densities is around 10^14 to 10^15 cm^-3.
Given the typical temperature-dependent shift of the PL energy in this material, a 50 meV blue shift could be explained by an increase in temperature of around 150 K. However, I do not have any feeling for how strong the laser-induced heating should be in this material, under the given conditions.
As such, I was wondering if there was a simple way to confirm or dismiss the laser-heating argument in this situation.
Any insight or suggestions of relevant publications on this topic would be highly appreciated.
Hello Claudio,
My careful answer (I don't know the material) is the following :
I would expect a redshift if T increases due to the gap reduction.
A blueshift is typically in the case of band filling. The excitation is almost very high. This produces excess carriers in valence and conduction band. Consequently, the recombination energy increases.
Best Regard
R. Mitdank
Dear Rüdiger,
Thank you for your answer. Indeed if this were a typical semiconductor like GaAs, I would expect a heating induced red shift. However, lead halide perovskites like CsPbBr3 exhibit a blue shift with increasing temperature. To my understanding, this is because the orbitals contributing to the valence and conduction band edges are reversed, as compared to typical semiconductors.
As such, there is no simple way to distinguish between band filling and laser heating.
However, band filling is unlikely at my excitation density, because the exciton binding energy is around 40 meV, meaning that there are no free carriers available, even up to room temperature. To observe band filling i would need to have significant exciton screening, which would require around 10^21 cm^-3, by my estimate.
Similarly, I believe I may be able to exclude laser heating as well, since the blue shift stops above a certain laser power (i did not mention this initially, to keep the discussion as simple as possible, but after a while I realised that this might just be my answer). So if the material were indeed around 150K, then i would still expect some further shift and broadening as I increase the laser power further. Since this does not happen, I can exclude laser heating as the reason for my observations.
I am wondering if the blue shift may be due to a filling of shallow trap states, as I expect the material to have a trap concentration somewhere between 10^14 and 10^17 cm^-3. Then, excitation from 10^14 to around 10^15 cm^-3 should be enough to observe some trap filling. However, I haven't found many examples of this in the literature so I cannot say if this always results in a blue shift and broadening, as I observed. Are you, by chance, familiar with shallow trap filling?
Kind regards,
Claudiu
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