When I've done PL Spectroscopy of my Perovskite film, I've found a secondary peak with lower intensity at a longer wavelength. What are the reasons behind this?
Secondary peaks are very common in some of the materials, actually in our case (anode material for Li-ion batteries) , every time we used to get secondary peaks for all our samples with very high intensity but we neglect it and concentrate on one particular peak where it should match to excitation and emission peak values one another.
Dear Mahfuzul
If you can show your spectrum, that would help us answer your question. Each peak has its own explanations. Some may come from stray light when excite your sample.
If it is at double of excitation wavelength it is a derivative peak and not of your compound.
All other peaks have logical origin and can not be ignored for good research.
Check the spectrum.
To summarize/comment the previous comments:
- It is not safe to simply disregard bands unless you know where they are coming from and therefore have good reason to do so.
- Additional peaks may come from second order spectrum, light scattering, Raman effect, or artefacts in the samples (e.g. impurities, even at very dilute concentrations).
- If you use a grating in your spectometer to disperse light, you will see higher orders of the spectrum. Usually only second order, i.e. another spectrum, with lower intensity at twice the wavelength: if you have genuine bands at 350 nm, 400 nm and 515 nm, then the second order spectrum will occur at 700, 800, and 1030 nm. You may cut it out by inserting a low-pass filter but be cautious not also cutting out some of genuine bands: in the above example, if you have a genuine band at 780 nm, then it would be difficult to cut out the second order features at 700 and 800 nm.
- Rayleigh scattering is easy to determine since it occurs at the excitation wavelength (1st order; never scan this range while measuring the emission spectrum otherwise you may damage the detector)) and at twice that wavelength (2nd order); for 350 nm excitation, you will see it at 350 nm and 700 nm.
- Raman bands are essentially seen when measuring solutions and usually correspond to the most intense vibration of the solvent. For water for instance, this translates to approx. 3400 cm-1. Therefore when exciting at 350 nm, you will see a Raman band at 397 nm (1st order) and a smaller one at 792 nm (2nd order). These are so-called Stokes bands, but you may also see anti-Stokes bands at shorter wavelength; in this example, at 313 nm, weaker than the Stokes one) In the case of solid samples and if you are using high power laser excitation, the Raman spectrum of the sample may also be generated.
- Impurities...well anything can happen...
Long wavelength emission of perovskite films can be related to trap states. But all depends on your composition, and if you have PbI2 left in the film it can also show up in emission. If your film consists of several different domains with different composition it will also show different emission bands.
Some representative emission spectra can be found in these open access papers:
Article Minimizing Defect States in Lead Halide Perovskite Solar Cel...
Article Perovskite Thin Film Materials Stabilized and Enhanced by Zi...
Article Air-stable and oriented mixed lead halide perovskite (FA/MA)...
https://doi.org/10.26434/chemrxiv.5484073.v2
It is good to show the PL spectra measured. Every PL peak can be assigned to a electron transition. First, be sure that it is a real peak, not a peak form the light source.
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