Dear Christian and Joachim

Thank you for your comments and sorry for my vague question. My question is concerned with SnO2 films deposited on sapphire substrates by PLD. I have been observed a systematic band gap blueshift for undoped SnO2 films when films’ crystallite size increase from ~15 nm to ~30 nm.

Very interesting observation I would like to investigate myself if I had the opportunity. I am not too sure but I suspect that you have hydrated SnO2 nanoparticles. What we know from our first principle that ligands, such as water, when they chelate to a metal center, such as tin in this case, would split it's d orbitals and so increase the band gap between combined molecular orbitals. The larger the size of the SnO2 nanoparticles the larger the surface area and the more water molecules can adsorb onto the surface. The more water molecules adsorbed onto the surface the greater the extent of d orbital splitting and the greater the degree of blue shift. One way to find out if you run an elementary analysis of your sample to see if there are certain amount of hydrogen element present apart from tin and oxide. If you also subject the sample to FTIR or IR analysis and compare that with free form of water to see if there are any shift in (O-H) bond had taken place.

I hope this helps and I wish you the best of luck,

Keep in tough.

Your work is very interesting. I guess that your samples with small size should have higher concentration of oxygen vacancies than ones with large size. Generally, high concentration of oxygen vacancies in oxides can induce the band gap narrowing. The attached paper can be helpful.

Dear Baibiao,

Thank you for your answer. However, I can't open your file (I dont know why). Could you please send me the paper to my email: [email protected].

Thank you

António

Dear all

Thank you for your help.

António

In my opinion, this situation may be in the case the presence of nanoparticles which include different crystalline modifications, which have different values ​​of the band gap. An example of such nanoparticles can be TiO2. This material may have several different crystal modifications bandgap value, in particular, anatase (Eg=3.4 eV) and rutile (Eg=3.0 eV). If percentage these phase is changed with the nanoparticle size, it is possible wait increase of TiO2 band gap with increase of nanoparticle size.

Many materials have an oscillating gap due to the change in shape and size such as carbon in its diamond nano struture. Although the change of the gap due to shape can be understood easily, the oscillation due to size is more difficult to understand. This oscillation simulates a damping osillation as the nanocrystal grow up in size.

How is it possible that the band gap energy increases by enhancing the doping level but the particle size is the same or increasing ?I used PbTe and doped PbTe.thaks