I have been using the ultraviolet photoemission spectroscopy (UPS) to look at the energy levels of quantum dots. My quantum dot has a core/shell structure, where the shell is supposed to have a larger bandgap.
But how can one tell the valence bands of the core and shell respectively from the spectrum?
Also, there appears a relatively significant shoulder at the secondary-electron edge. I wonder why is that and how should I determine the cut-off?
I am really not familiar with UPS, could anyone give me a help?
P.S.
Thanks for Fabrizio Messina and V. N. Ravi Kishore V.'s suggestion in my previous question (the link below) about how to study the energy levels of quantum dots.
https://www.researchgate.net/post/How_can_I_measure_the_conduction_valence_band_edges_of_quantum_dots
Look for core level shifts to get information on reacted interfaces
The cut off at 4.3 eV will give the work function, The lower cut off might be a artefact of the lens. Do you use a sample bias? A work Funktion of 4.3eV may not be very specific if your sample is not clean, as a small surface contamination will influence the work Funktion drastically (by about 1eV in worst cases). Regarding the VB I think you do not see any structure from the core, as HeI is very surface sensitive. You may use monochromated XPS for obtaining the valence band structure with high resolution and large escape depth. You will obtain the true DOS due to free Elektrons in the final state in contrast to the JDOS in UPS where higher conduction band states with a high density of states may mix in.
@Christian Pettenkofer Thanks for your answer and suggestion. I did use a sample bias of -4.8V. I think I will try out XPS.
But do you have any experience of theoretical modelling that can simulate the QDs and obtain the band edges? This method can be much more straightforward but I wonder how accurate this can be, especially when there is an alloyed shell.
Look for core level shifts to get information on reacted interfaces
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