Hello, everyone!
I recently got a problem about band gaps of one inorganic selenide crystal. Every time I run UV-Vis-NIR diffuse reflectance spectra, the results (Fig. 1) seem that the compound has two band gaps, which is, to me, impossible. The PXRD test confirmed the sample is pure phase. Could anyone please give me any advice or help me explain this?
Thanks in advance.
Theoretically it's not possible for 2 bandgaps in a pure phase. There could other possibilities of defects in the material which is needed to be studied.
If your material is certainly pure. You can consider to take the first peak drop occurs in the graph (from right, as it gives 3.42 eV), Infact you can get an idea regarding the similar possible bandgaps of a material by literature or by computational methods.
Finally, for an accurate bandgap analysis I'd recommend to perform EIS analysis. It will give you the accurate results in terms electronic properties.
Athique Ahmed Ali Thank you so much for your answer. This problem has puzzled me for quite a long time. Your answer really helped me a lot!
The EIS analysis, as you mentioned, is accurate to determined bandgap. Could you please provide more information about it? Is it Electrochemical Impedance Spectroscopy?
Looking forward to your reply!
Hello, Mark! Do you expect (for example, from literature or calculations) that your material more likely should have a band gap of 3.4 eV instead of ~2 eV? In that case, could it be that within the 3.4 eV band gap there is a high density of shallow levels close to valence band maximum, almost forming a band by themselves?
Alternatively, I suppose that surface contamination or chemical/structural modifications may distort your reflectance spectra, and you would not be able to detect them with XRD, in case you use symmetric theta-2theta geometry, which probes very deep into the sample, so the contribution of the surface would be negligible.
For cross-correlation, I would suggest trying XPS/REELS related methods, measurement of temperature dependence of resistivity, which you could fit and try to extract an electronic band gap value, or try to clean/etch the surface to see the distortion comes from this.
Anton Khanas Hello, Anton! Thank you for your answer!
Surface contamination is what I am considering at the moment because the compounds in this system are relatively air-sensitive. They might decompose while contacting the oxygen and forming selenium, which hinders the reflectance spectra. The first-principles calculations revealed the bandgaps should be around 3.4 eV, but I was so unsure about it. I was worrying about the facticity of that value.... I tried really hard to isolate the sample from oxygen but failed as shown in the figure...
I think I will try to run XPS/REELS related methods as you recommended.
Again, thank you very much for providing so much information! I really appreciate it!
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