I would like to say about the bandgap, thickness and grain size of the THIN FILM. The grain size mainly depends on the film thickness and IN ADDITION OR in other words, chemical composition of the thin film is responsible for the grain size distribution. So, if the film’s composition is not varying with the film thickness, you will get a linear relation between the grain size and the film thickness. it may increases or decreases, but the band gap never changes in this case. If the composition changed with thickness, it will directly affect the material (thin film) band gap. Lastly, The grain size of the thin film affect the average transmission of the film, due to the scattering, but not affecting the energy band gap until the composition is not varying.

On the other hand, once grain size falls below ~100 nm, quantum effects manifest themselves. Small grains work as quantum wells, and bandgap depends on the grain size.

Many examples available, look eg. fig. 5.4, p. 111, Nanophotonics and Nanofabrication. Edited by Motoichi Ohtsu, Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32121-6

The band gap of material is very sensitive parameters. Any type of irradiation effect or composition change or addition of any impurity can affect it.

yes, size affects the band gap but again it depends how large those particles are..depending on the critical size of that particular material...surface termination also affects bandgap up to some extent (if it is in nanoscale) ..

I would like to say about the bandgap, thickness and grain size of the THIN FILM. The grain size mainly depends on the film thickness and IN ADDITION OR in other words, chemical composition of the thin film is responsible for the grain size distribution. So, if the film’s composition is not varying with the film thickness, you will get a linear relation between the grain size and the film thickness. it may increases or decreases, but the band gap never changes in this case. If the composition changed with thickness, it will directly affect the material (thin film) band gap. Lastly, The grain size of the thin film affect the average transmission of the film, due to the scattering, but not affecting the energy band gap until the composition is not varying.

On the other hand, once grain size falls below ~100 nm, quantum effects manifest themselves. Small grains work as quantum wells, and bandgap depends on the grain size.

Many examples available, look eg. fig. 5.4, p. 111, Nanophotonics and Nanofabrication. Edited by Motoichi Ohtsu, Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32121-6

hello Sriramji,,, Direct experimental evidence of band-gap shrinkage or broadening due to the presence of impurities in semiconductors. By adding the impurities one can control the band-gap shrinkage or broadening. Here is one of the link for the paper regarding to that. refer it. .. PHYSICAL REVIEW B, VOLUME 11, NUMBER 6, PAGE 2251, 1975.

@Inigo Valan ...your first question was regarding particle size and grain size..first thing, if we consider the quantum confinement effect then we should always call it nanoparticle and it could have many atoms inside it (depends) ..but when we talk about grain size then it comes to macroscopic scale..though many people call in different way..

band gap could change according to the film thickness..again depending on material and thickness...see the size dependency comes when the wavefunction of that material starts to experience a geometrical barrier around it and change itself according to that..now so far I was talking about a single material ..many other things could happen if we consider a composite..

Hi

There are several factors that influence the energy band gap of semiconductors. The defects, charged impurities, disorder at the grain boundaries as well as the three dimensional quantum size effects, which could increase the energy band gap with the decrease of particle size.

The presence of defect states, charged impurities and grain boundaries led to the formation of localized states in the energy band gap.

The sub-band gap absorption can be observed clearly in the transmittance/absorption spectrum depending on the position of localized states. The localized states were formed close to the conduction band. The high concentration of impurity states causing the band structures to perturb, resulting in a prolonged tail extending into the energy gap. The width of the band tails, known as Urbach tails, was evaluated to quantify the structural disorder.

Temperature is also an another factor for affecting the band gap of the materials. Energy band gap as a function of temperature which was determined experimentally. Temperature is inversely proportional to energy band gap of the materials. the energy band gap of the any semiconducting materials tends to decrease as the temperature has increased. this behavior can be understand if an inter atomic spacing increases when the amplitude of the atomic vibrations increased due to increasing thermal energy. An increased inter atomic spacing decreases the potential seen by the electrons in the materials which in turn reduces the size of the energy band gap.

If grains in thin films are fully crystalline, the dependence of the bandgap on the grain size, in principle, no. This relationship holds for nanocrystals as a result of quantum size effect. However, if the grains is not completely crystalline (amorphous present on the surface), then such a relationship can be. I recommend you read a work that is attached.

The gap between valance band and conduction band decreases with increases of particle size.

http://prl.aps.org/abstract/PRL/v80/i17/p3803_1

Thanks

In crystals doping affects the band gap,normally it decreases..As a result of the new interactions of the guest and host materials,new additional energy levels are formed which tries to decrease the existing band gap.As the percentage of doped material increases the band gap continues to decrease.

How is the effect of the crystallinity of a material on its band gap?

Dear Shima and colleagues,

This is an interesting question. So, i will give an answer to it hopping it will be useful to research colleagues. The bandgap of a material in its crysatlline form is determined by its lattice constant and its dielectric constant. Specifically it is inversely proportional to the interatomic distance and the dielectric constant.

Then if one reduces the size of crystal to be be in form of crystallite will nanaometer size, The electrons in the crysatllites will be confined in this small volume leading to the quantization of the electron levels in the conduction band and hole levels in the valence band. This will lead to the shift of the band edges far from each others causing the increase of the energy gap as the crystallite size decreases.

Best wishes

Generally the defect core in a material doesn't affect the band gap. But it surely affects the transparency of a material.