Role of Microstructure on Optical Properties

                                            

 Key words- Anti reflective surface (ARS), Bioinspired, Optical properties.

I. INTRODUCTION

The Optical properties of a material can be defined as how the material interacts with light but strictly speaking it is the response of a material to the electromagnetic radiation. The major optical properties are Refraction, Polarization, Reflection, Absorption etc. 

    With the advent of the silicon age the demand for the materials which shows excellent optical properties are needed which helps in increasing the efficiency of data transfer (optical fibers), Energy production (Solar cells), etc. This paper explains the recent development in the materials microstructure leading to a humongous changes in optical properties, it also discuss how this microstructure is obtain and what are the reasons there is a high change in optical properties with major focus on reflection.  

II. FACTORS EFFECTING OPTICAL PROPERTY

A. Grain size and optical property

The light can be described as wave nature and also particle nature. With respect to the wave nature if the wavelength of the light is comparable with the size of grain boundaries and grains it has a great impact on the optical property example aluminum oxide shows different optical properties depending on its microstructure. If the aluminum oxide is manufactured into single grain microstructure then it showed as transparent material, if it is made into a structure containing grains in micrometer then it showed translucent property, if the same aluminum crystal is made with grains size in Nanometer it became opaque. 

This mainly due to the fact that the grain boundaries dimensions is comparable with the wavelength of the light due to which there is high scattering of the light which is incident on the material, so the single crystal aluminum oxide did not show any scattering of the light but with increase in the grain boundaries and grains the scattering became more predominant leading to full scattering of light when the structure is made up of small grains (Figure1).

 

Figure 1 optical property of the Aluminum oxide depending on the grains.

Even gold when it is in Nano size it loses its  shiny yellow color and gets different color like orange, black etc. Even in zinc oxide with increment on grain size the optical band gap of the material decreased. All this changes is due to the fact that wavelength of light is comparable to the dimensions of grains and grain boundary leading to change in properties. 

B. Element addition and optical property 

When Al2O3, TiO2, NiO, MgO added to the fused silica (SiO2) there is increase in the index of refraction then compared to the pure Silica this mainly due to the fact that Al3+ , Ti4+, Ni2+, and Mg2+ ions are all greater in size than the Si4+ ion (0.053, 0.061, 0.069, and 0.0.072 nm, respectively, versus 0.040 nm). This due to electronic polarization. We know that light is an electromagnetic wave so it has an varying electric field component so this electric filed which is fluctuating in its nature interacts with the electron cloud surrounding each atom in the material. This electric field shifts the electron cloud relative to the nucleus of the atom in the direction of the electric field. This is called as electronic polarization due to this phenomena there are two consequences (1) the radiation energy may be absorbed and (2) the velocity of the light may decrease when pass through the medium. Larger the ion the larger the electronic polarization so the addition of the Al3+ , Ti4+, Ni2+, and Mg2+ increased the refraction index of the materials.

Optical band energy plays a major role in optical property an high band energy signifies that the material will be opaque in nature and low band energy signifies the material will transparent so some researchers doped Copper / cobalt in zinc oxide due to the addition of copper/cobalt the grain size decreased and optical band gap also decreased leading increase in the optical property. 

C. Inspiration from Nature

The development of a microstructure with a good and enhanced optical properties from direct scratch is very difficult and also time taking with results may be futile. So some research’s thought why to bring a whole new idea rather get an idea from the nature itself. As we all know the different characteristic of species in nature are due to the thousands of years of evolution. So the microstructure in the materials is developed by getting the characteristics similar to that of the species.

D. Antireflective Microstructures (ARS)

The First inspiration is from a moth eye. In moth eye when looked through a microscope it contains a large tiny nipple like structure on its surface as shown in Figure 2.

  

Figure 2 a Moth, b Eye, c cone like structure on eye surface [3].

When a light enters a new medium (transparent) there is a sudden change in refractive index, when there is a sudden change in refractive index it causes Fresnel reflection i.e. some part of light gets transmitted some part gets reflected and the amount of reflection and transmission can be obtained by the Fresnel equation. The major reason for this is the sudden change is due to the refractive index.

      In Moths due the presence of the cone shape structure on the eye it prevents the sudden change in refractive index of the material by gradually increasing the refractive index leading to very less reflection and more transmission. So with this structure as design the scientists developed ARS on quartz substrate. The manufacturing was done by first taking the quartz as substrate then the top layers was made into the desired surface by using reactive ion exchange. In this method the etchant removes all the material which is not covered by a mask, the mask used in this process is Nano gold. So the etchant removes all the material which is not covered by the gold particle. The gold particle is in spherical shape and dimension in Nano meter and being an conductive material it attracts the electron beans towards it so in the vicinity of the Nano gold particles there is less material removal then compared to the material below the gold particle but the material away from gold particle will have greater removal rate of the material.  In this way a surface with cylinder shape can be obtained and the dimensions of the cylinder are in Nano meters.

 Figure 3 The schematic representation of the formation of hallow cone [2].

The microstructure of the surface is showed in the figure 4. From the Scanning electron micrograph it can be said that the ARS showed a quasihexagonal arrangement.

 

 

Figure 4 (a) top view at 1µm (b) top view at 200nm (c) side view but tilted at 45o angle [4].

The major advantage of this type of structure is that it can slow down the sudden change in refractive index thereby reducing the amount of reflection. To prove this the transmission and reflection at various wave length was test i.e. the quartz with ARS and quartz without ARS. Due to the ARS the transmission percentage become 98% whereas without ARS the transmission percentage was at 88% for the light with wavelength of 180-200 nm. But on overall also there is minimum 2% increase the transmission percentage. 

 

Figure 5 The variation of transmission with wavelength red line shows quartz with ARS blue line shows quartz without ARS [5].

The antireflection property is very important for the Solar cells as it effects its efficiency for producing the electricity. So scientists fabricated an ARS on quartz surface by physical vapor deposition hoping that this will increase the efficiency of the solar cells. To check whether it improved or not, the reflectivity investigation, absorption test and FDTD simulation of silicon cells where conducted and the results showed an improved ability of the solar cells in the photo electric conversion. The solar cell with ARS has 10.6% higher efficiency that one without the having any ARS in the photo electric conversion. The Solar cell with ARS also showed as excellent self-cleaning performance.

Many researchers developed ARS surface with different structures like Cylinder, Pyramid with special coatings on this Nano structure.

Some researchers made bismuth telluride (Bi2Te3) pyramid on a substrate the major reason to choose bismuth telluride is that it is naturally occurring hyperbolic structure and the substrate was silver it also showed an increase in absorption of the light due its Nano meter dimension.

Second inspiration is from butterfly wings. Butterflies also have a great antireflective surface but this ARS is limited to the black spots on their beautiful & colorful wings. The major feature to their antireflection is that it contains Nano holes and V shaped vertex (figure 6).

 

 Figure 6: Overall views, morphologies, and models of three BWs black scales (Papilio nireus, Papilio paris, and Troides helena’s forewing, respectively). A1,B1,C1) Digital photographs, A2,B2,C2) top-view SEM images, A3,B3,C3) sectional-view SEM images, and A4,B4,C4) simulation models. A–C)[4].

The light first absorbed into the V shaped vertex but some part of it may be reflected but the light reflected is directed into the Nano holes leading to more absorption even if the light escapes these two surfaces the bottom surface is flat in which some light will be absorbed and rest will be sent back to the V shaped vertex and holes leading to more absorption.

The Nano holes functions as a low refractive index layer if the wavelength of the light is larger than the dimensions of the hole if the wavelength of the light is smaller than the dimensions of the hole then they act as backscattering area.

The holes are highly disordered in the butterfly wings leading to avoid reflection with an incident or azimuth angular dependence. 

So some researchers made the same structure which is there in butter fly wings with materials silica and poly (dimethyl siloxane) (PDMS). It showed better ARS then compared to the moth like microstructure but from experiments it is found out that if the material is good absorbent then it leads to more better ARS property than the material which is not. The reason may be that the transparent materials cannot absorb the light penetrating through the first antireflection surface. Therefore, the light will arrive at subsequent multiple interfaces without attenuation and be reflected and accumulated at each surface.

III.   CONCLUSIONS

Optical property of the material is one of the most important property if the material is used in the field of communication and energy production. Designing the microstructure inspired from nature helps in developing high optical properties with 100% success ratio and also reduces the time to develop a great microstructure. 

IV. FUTURE SCOPE 

With the advent of high precision manufacturing more microstructure inspired from other species can be developed leading to enhanced optical property. 

V.    SUGGESTIONS

Microstructure inspired from peacocks can be developed as they have vibrant colors and also Camouflage materials can be developed by having microstructure with can responds to its environment.

REFERENCES

[1] Asikuzun, E., Ozturk, O., Terzioglu, R., Arda, L., & Terzioglu, C. (2020). Effect of doping on microstructure and optical properties of ternary structure of Zn1−x−yBxCyO (B=Cu, C=Co) nano thin films. Journal of Materials Science: Materials in Electronics, 31(24). https://doi.org/10.1007/s10854-020-04736-2

[2] Callister, W. D. (2010). Wiley: Materials Science and Engineering: An Introduction, 8th Edition - William D. Callister, David G. Rethwisch. In Wiley.

[3] Dou, S., Xu, H., Zhao, J., Zhang, K., Li, N., Lin, Y., Pan, L., ` Li, Y. (2020). Bioinspired Microstructured Materials for Optical and Thermal Regulation. In Advanced Materials. https://doi.org/10.1002/adma.202000697

[4] Lohmüller, T., Helgert, M., Sundermann, M., Brunner, R., & Spatz, J. P. (2008). Biomimetic interfaces for high-performance optics in the deep-UV light range. Nano Letters, 8(5). https://doi.org/10.1021/nl080330y

[5] Wang, Z., Zhang, Z. M., Quan, X., & Cheng, P. (2018). A perfect absorber design using a natural hyperbolic material for harvesting solar energy. Solar Energy, 159. https://doi.org/10.1016/j.solener.2017.11.002