Dear Dr. Amir

Have good day and nice times

ZnO is a wide-bandgap semiconductor of the II-VI semiconductor group. In literature, ZnO has a relatively large direct band gap of ~3.3 eV at room temperature. Advantages associated with a large band gap include higher breakdown voltages, ability to sustain large electric fields, lower electronic noise, and high-temperature and high-power operation. The bandgap of ZnO can further be tuned to ~3–4 eV by its alloying. 

The energy band gap Eg is strongly dependent upon the the preparative conditions, growing process and the crystal structure of ZnO. Zinc oxide crystallizes in two main forms, hexagonal wurtzite and cubic zincblende. The wurtzite structure is most stable at ambient conditions and thus most common. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral, the most characteristic geometry for Zn(II). ZnO converts to the rocksalt motif at relatively high pressures about 10 GPa.

Therefore, according to the preparative conditions, growing process and the crystal structure of ZnO

Yours,

Ahmed Saeed Hassanien

Dear Amiruddin Rafiudeen,

Adding to the colleagues above, it is observed that the band gap energy increases with the crytallite size reduction and also it increases by decreasing the nanowire diameter. Please refer to the link: http://arxiv.org/ftp/arxiv/papers/1002/1002.0427.pdf.

So, your nanowire may have gone some chemical structural changes or you may not measure precisely the energy gap. 

wish you success

Dear Ahmed Saeed Hassanien and Pavan Pagare, 

Thanks a lot for sharing your valuable comments. 

DEar Prof. Abdelhalim Zekry

Thanks a lot for your valuable suggestion and the supporting literature which you have sent. I will read the paper which you have sent and hope so I will get the answer. 

Thanks once again for your kind time and attention. 

-Amir

It is very expected that change in crystal structure and nanowire form of ZnO will have changed band gap as compared to their bulk counterpart. As we are applying confinement over two directions, we can expect the effective mass of the ZnO to get decrease which should decrease the electrical band gap but how much , this area needs an in depth exploration.

Dear Amiraddin

The energy gap of any semiconductor in its nanosize increase with the decrease of its size. There are many published articles and thesis on this subject.

The dependenc above works if all the materials parameter stay the same during the size change. Since the optical properties and other materials parameter are also a structure dependend. In this case you should be aware for your materials property

you may get some useful information on this regard from a PowerPoint on "Solid Surface and Nanoscale materials Structure" reported by M. S. Omar from university of salahaddin

In general, Tauc plot is not n proper way to get the bandgap for a crystalline material. It assumes the absorption profile following the density of states of free electrons, which is obviously incorrect because of the excitonic effect. In the free electron level theory, the absorption is zero at the bandgap (DOS = 0). However, for GaAs with merely 4 meV exciton binding energy, the absorption at the excitonic bandgap (4 meV below the free electron bandgap) is > 10^5 cm^-1 (not easy to get a very precise value). Even at room temperature, one can get an excitonic absorption peak at 1.425 eV with the 4 meV binding energy (if measured properly with good material). One might have heard the argument that no exciton at room temperature. What is means is that most exciton will be thermalized to the free electron states at room temperature. However, when doing absorption measurement, we are measuring the DOS with thermal broadening, so excitonic peak is weaker but still there. For ZnO with a much larger exciton binding energy, even at room temperature, the excitonic absorption peak should be measurable (likely somewhere in the literature). Therefore, using Tauc plot is not justified, although widely used. In particular, when the material has low crystalline quality, it could be used as an approximation. Alternatively, modulation spectroscopy can be used. Please see the attached paper where the three techniques are compared, and how excitonic absorption is affected by structural disordering.

For the ZnO nanowires, unless the diameter is reduced down to about 10 nm where quantum confinement could become significant, the bandgap should not change much for larger nanowires. The "bandgap change" from Tauc plot is likely due to structural imperfection.

Dear Prof. Yong Zhang.,

Thank you for suggesting modulation spectroscopy rather than Tauc plot calculation. From your comment, I understood that the band gap should not change for ZnO nanowires with diameter higher than 10 nm.

I will go through the attached paper.

Thank you once again for your kind time and attention

-Amir

Dear Prof. Omar M S

Thanks for your valuable suggestions.

I will go through the PPT presentation which you have mentioned.

Regards

Amir

The energy gap of nanosize increase with the decrease of its size due to increasing surface to volume ratio

This paper should be useful to explain the band gap increasing in ZnO NWs

http://aip.scitation.org/doi/abs/10.1063/1.4942653?journalCode=jap

As it well known the energy gap will increased with reduction the size and this due to increase lattice volume due unit cell (i,e increase of lattice constant), but in your structure may be is due the structure is recrystallizing again (Phase change)

Dear Amiruddin Rafiudeen I think dependent on the method when you prepared it also if you have EDX please show the ratio Zn/O it is important for you this is say you why band gap decreased