If energy gap increases on Mn doping in ZnO nanoparticles, it means some vacancies or defects are produced in host lattice.
@ Shahid Husain
ZnO is certainly a complicated system. A brief literature search revealed a well-cited paper (Solid State Comm. 135 (2005) 345–347) that had reported decrease in bulk ZnO:Mn bandgap up to 3% Mn, and then an increase in the bandgap for more Mn content. In view of your observation and the earlier, it appears that there are competing factors that control the bandgap. ZnO is a highly polar material and Mn-doping (or, at > 1% level it is more like alloying) is likely to affect sharing of electrons between Zn and O even without the participation of O vacancies.
Another more recent theoretical work (Phys. Chem. Chem. Phys., 2009, 11, 3201–3209) considered Mn-Mn interactions and clustering to show either increase or decrease in ZnMnO band gap depending on the structure of the material. They also considered OV-s, but concluded that their effect (Eg increases slightly) on ZnMnO bandgap is marginal.
Wow, that is a very good question and hard too. I allways thought and believed that dopants would not change (at least not much) the band gap. I have learned and read that what dopants do is to create electron energy levels in the gap. Now I will have to read more... ;)
Mn will often subtitute for Zn in the lattice. Mn has a smaller ionic radius which will cause a change in the lattice parameters. This can easily be confirmed by XRD study. The change in the bandgap has a kind of non-trivial dependence on the Mn concentration. For low concentrations there is a complex exchange between orbitals of Mn and the host band which will cause a decrease in the bandgap. As you increase the Mn doping the bandgap will then increase as the changing lattice parameters become more significant. In your case the bandgap is increasing. I really suggest XRD and further characterization before you attempt to draw any conclusions as to the cause of the change in bandgap. Many studies have been done on this. I suggest: http://dx.doi.org/10.1016/j.matchemphys.2005.07.045 and J. Phys. Chem. B, 2004, 108 (20), pp 6303–6310 as a starting points.
Radius of Mn(2+) is 0.08 nm and that of Zn(2+) is 0.074 nm. It means radius of Mn is larger as compared to Zn.That is why we have observed an increase in lattice parameters.But question is about band gap dependence on oxygen vacancies.
Oops I mixed up the radii! Sorry, I misunderstood the wording of your question.
Thanks Dr. Joydeep Dutta for your useful comments and suggested references.
Interesting topic.
One point needs clarification. Shahid Husain, your original question mentioned increase in the band gap of Mn-doped ZnO nanoparticles. If the increase is compared to the band gap of bulk ZnO, it may just be due to the well-known quantum size effect rather than Mn-doping or anything related to oxygen vacancies. Is it really that the band gap of Mn-doped ZnO nanoparticles is higher than undoped ZnO nanoparticles of the same size?
Prof Dutta, if I understood your comments correctly (I've not yet looked at your papers), you implied band tailing in heavily Mn-doped bulk ZnO. That would mean a decrease in the ZnO band gap rather than an increase.
Usually ZnO doped Mn forms solid solution (alloy) where Mn atoms substitute Zn atoms. These alloys present new class semiconductors named by semimagnetic semiconductors or dilute magnetic semiconductors. These materials are very promising for spintronics since they show ferromagnetism at room temperature.
The origin of high-temperature ferromagnetism in Mn–doped ZnO system, is controversial. The presence of oxygen vacancies in this material is very important. At present it was suggested that the high-temperature ferromagnetism is rather caused by an oxygen-vacancy-stabilized metastable Mn2-XZnXO3-δ phase than by carrier-induced interaction between separated Mn atoms in ZnO. This phase arises from diffusion of Zn into the Mn-oxide grains. Oxygen vacancies in ZnO:Mn induce donor states. They provide carrier-induced interaction between separated Mn atoms. Therefore, the presence these defects may be very important for appearence high-temperature ferromagnetism in ZnO:Mn.
I would also like to point out that the doping by Mn can not lead to the formation of tails of the density states in the band gap, since in this case the shallow levels are not formed.
Joydeep @ : I note that ZnMnO solid solution is formed taking into account basic physical conception. You suggest that "Solid solutions are formed only at higher temperatures". It is pure technological question. I did not indicate any temperature. Therefore I don't understand what is "higher temperature" ? You are considering the issue from a technological point of view. However, this question posed at all. So, it is important to know the physical basis. I have many years (more than forty years) experience study II-YI semiconductors containing 3d-elements. These elements have unfilled 3-d shells. Thus they usually form deep impurity levels in these semiconductors.There is a definite regularity arrangement of ground states of 3d-elements for II-YI semiconductors relative to the vacuum level. You suggest that "Mn can dislodge Zn atoms to create defect states close to the conduction band". What is the nature these states ? I can think that Mn doping can affect on the structure of intrinsic defects, which are shallow donors. Such defects can lead to the formation of tails of the density states in the band gap. However, it is necessary establish their nature and presence in ZnO:Mn. The study of defects in semiconductor materials is very complex task which demands the comprehensive physical investigations.
Dr. Kanad Mallik, We have observed an increase in bandgap on Mn doping as compared to pure ZnO in nanopowder forms. Bulk ZnO band gap is ~3.3 eV, but the bandgap of pure ZnO synthesized through sol gel process is found to be ~3.1 eV and on doping with Mn it increases.
Shahid @ : Which method you used for determining of ZnO band gap ?
What is the concentration of Mn atoms ?
We have determined bandgap using UV/Vis spectra with the help of Tauc relation.Mn concentration was 1% to 5%.
I like to point out my experience with ZnTe films doped with Gd. The doping in this case causes the lattice to contract as a result of vacancies introduced by the doping. In this case Gd is a donor and it compensates some of the acceptors associated with vacancies. The effect of compensation and reduction in lattice constant causes the band gap to remain constant. In other systems such as ZnO:Mn it is possible that the band gap will increase due to lattice contraction caused by the introduction of vacancies. See my paper in J. of Appl. Phys. Vol.103, p.23711 (2008).
@ Shahid Husain
ZnO is certainly a complicated system. A brief literature search revealed a well-cited paper (Solid State Comm. 135 (2005) 345–347) that had reported decrease in bulk ZnO:Mn bandgap up to 3% Mn, and then an increase in the bandgap for more Mn content. In view of your observation and the earlier, it appears that there are competing factors that control the bandgap. ZnO is a highly polar material and Mn-doping (or, at > 1% level it is more like alloying) is likely to affect sharing of electrons between Zn and O even without the participation of O vacancies.
Another more recent theoretical work (Phys. Chem. Chem. Phys., 2009, 11, 3201–3209) considered Mn-Mn interactions and clustering to show either increase or decrease in ZnMnO band gap depending on the structure of the material. They also considered OV-s, but concluded that their effect (Eg increases slightly) on ZnMnO bandgap is marginal.
Shahid @ : Determination of band gap with the help of Tauc relation may be very incorrect. I propose see disscussion in RG : https://www.researchgate.net/post/Can_anyone_suggest_the_most_accurate_method_for_calculating_the_band_gap_values_of_powders_and_thin_films_materials?ev=tp_feed_post_xview
I also recommend you to carry out the study the temperature dependence of the energy of exciton line by the measurement of the room temperature photoluminescence spectra.
Prof. Gnatenko, I have seen the discussion mentioned in your answer but unable to understand how energy gap can be determined using PL spectra. We have recorded PL spectra using an excitation source of 320 nm. Can you elaborate this point?
Band gap of semiconductor materials can be determined by the measurement of exciton photoluminescence spectra . An exciton is an elementary excitation of condensed matter. They are formed in semiconductors as a result of the Coulomb electrostatic interaction between electrons and holes. Thus, they present bound electron-hole pairs. Their excitation energy Eex is slightly less than the energy of the unbound electron and hole, i.e. than the band gap energy Eg of semiconductor. Thus, the band gap of semiconductor corresponds the energy position of free exciton line in PL spectrum plyus the binding energy of exciton. In the case of ZnO the binding energy equals 60 mev. (see more in attached file).
In addition : Usually the photoluminescence line at 3.276 еV corresponds to the emission of free excitons in ZnO.
Prof. Gnatenko, Please see the PL spectra as an attached file,can we find the bandgap using this spectra of Mn doped ZnO?
In the presented spectra the PL line caused by the emission of free excitons is not observed. It may be due to some inhomogeneity of the investigated films. The PL line at 386 nm (3.212 eV) may be ascribed to the emission of excitons bound to donor (may be acceptor) centers. It is necessary clarify. It is evident that for Mn concentration equals to X=0 and X=0.02 the energy of the short-wavelength line is same. Thus, the bandgaps for these sample are also same. For X=0.03 the short-wavelength shift of the PL band observe that may be caused increase of the bandgap. As regards the sample with X=0.05 it is difficult estimate the change of the PL band energy because this band is broadened.
In the future, I would advise you to also to study the low-temperature photoluminescence spectra. In this case, the photoluminescence line should be more clear, and may be a manifestation of free excitons.
Most likely that PL band at 386 nm is not the exciton band, and can be associated with transitions that include the participation of intrinsic defects. It all must be carefully analyzed and checked.
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