I have seen many people are explaining the color change of metal nanoparticles (silver, gold nanoparticles) through SPR (surface plasmon resonance). And some are explaining with the fact that when size reduce band gap increase and thus change in color is observed according to band gap of that material (they do not consider SPR).
For example, if silver nanoparticles color is changing from yellow to reddish, then the size is increasing & band gap is decreasing. Can we relate this with the SPR?
You can relate the absorption coefficient, which is used via Tauc plot to find the band gap energy, to the size and morphology of plasmonic nanoparticles.
This paper may be useful:
https://cyberleninka.ru/article/n/optical-analysis-of-ag-nps-containing-methyl-ammonium-lead-tri-iodide-thin-films
That is strange. As far as I know, the color change is due to size, shape, material, surrounding environment and areal density of the particles. With reduction in size, the absorption cross section increase and there will be an increase in interparticle distance due to which coupling between nanoparticles blue shifts in energy scale. Each metal has a bandgap and plasmon resonance is below this energy. The reflectance will reduce when interband transitions start.
Surface plasmon resonance is observed in a dispersion of metal nanoparticles. Metal does not have a band gap. Your question relates to the absorption and photoluminescence of semiconductor nanoparticles. With a decrease in their size, the band gap increases and a “blue shift” is observed upon absorption and photoluminescence.
Dear Yuri Mirgorod, as per the wikipedia first line "Quantum dots (QDs) are tiny semiconductor particles" which means if we are talking about any strong quantum confined material then it must be a semiconductor only. Which further related to an idea that when we are cutting any material to very small nanosize, tends to break the continuous band structure of metals and thus creates band gap for that material. This should be the reason people always say that when size of the nanomaterial reduces then the band gap is increased (even in the case of metals), which finally leads to the change in the color of that material.
Dheeraj Kumar
Try to find the link where the dependence of plasmon absorption on the band gap of metal nanoparticles was investigated. Always investigate the dependence of plasmon absorption on the size of the nanoparticles (or surface).
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