Optical Band Gap Estimation of ZnO Nanorods

Sáenz-Trevizo, Angélica; Amézaga-Madrid, Patricia; Pizá-Ruiz, Pedro; Antúnez-Flores, Wilber; Miki-Yoshida, Mario

doi:10.1590/1980-5373-MR-2015-0612

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Articles • Mat. Res. 19 (Suppl 1) • Dec 2016 • https://doi.org/10.1590/1980-5373-MR-2015-0612 copy

Optical Band Gap Estimation of ZnO Nanorods

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

The optical band gap energy of ZnO nanorods was estimated by different methods found in literature. These nanorods were deposited onto TiO₂ covered borosilicate glass substrates by aerosol assisted chemical vapor deposition. Characterization techniques were employed to reveal the crystalline structure, surface morphology and optical properties of the sample. The analysis proved that hexagonal cross-section ZnO nanorods in the wurtzite phase were grown onto a TiO₂ buffer thin film of anatase. Variations in the components of transmittance and reflectance were associated with the development of a rough surface. Such fluctuations were correlated with the equations employed to estimate the optical band gap. The different methods indicated band gap values of 3.17 to 3.24 eV. The energy band gap of 3.24 eV agrees to that reported in the literature for ZnO nanorods. Fluorescence spectroscopy confirmed the accuracy of the mathematical predictions since an emission at 380 nm (3.26 eV) was seen.

Keywords
ZnO nanorods; ZnO band gap; Kubelka-Munk method; rough surface

Method	Equation	Special notes
Method 1 (M1)	$α = \frac{1}{t} \ln (\frac{1}{T})$	Standard optical equation that considers the total components of T⁵5 Wang MD, Zhu DY, Liu Y, Lin Z, Zheng CX, He ZH, et al. Determination of Thickness and Optical Constants of ZnO Thin Films Prepared by Filtered Cathode Vacuum Arc Deposition. Chinese Physics Letters. 2008;25(2):743-746.^,⁷7 Sapkal RT, Shinde SS, Waghmode TR, Govindwar SP, Rajpure KY, Bhosale CH. Photo-corrosion inhibition and photoactivity enhancement with tailored zinc oxide thin films. Journal of Photochemistry and Photobiology B: Biology. 2012;110:15-21.^,⁹9 Foo KL, Hashim U, Muhammad K, Voon CH. Sol-gel synthesized zinc oxide nanorods and their structural and optical investigation for optoelectronic application. Nanoscale Research Letters. 2014;9(1):4291..
Method 2 (M2)	$α = \frac{1}{t} \ln [\frac{(1 - R)}{T}]$	Standard optical equation that considers the total components of T and R⁸8 Tabbal M, Kim T, Woolf DN, Shin B, Aziz MJ. Fabrication and sub-band-gap absorption of single-crystal Si supersaturated with Se by pulsed laser mixing. Applied Physics A. 2010;98(3):589-594..
Method 3 (M3)	$α = \frac{1 + R_{0}^{2} + T_{D}^{2}}{2 R_{0}}$	K-M theory: case of an absorbing, intensely scattering and thick material¹²12 Džimbeg-Malčić V, Barbarić-Miko čević Ž, Itrić K. Kubelka-Munk theory in describing optical properties of paper (I). Tehnički Vjesnik/Technical Gazette. 2011:18(1):117-124. 13 Kubelka P. New contributions to the optics of intensely light-scattering materials. Part I. Journal of the Optical Society of America. 1948;38(5):448-457.^-¹⁴14 Lin H, Huang CP, Li W, Ni C, Shah SI, Tseng YH. Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2-chlorophenol. Applied Catalysis B: Environmental. 2006;68(1-2):1-11..
	$R \infty = R_{0} + \frac{T_{D}^{2}}{2 b}$
	$b = \sqrt{a^{2} - 1}$
	$S = \frac{1}{t (\frac{1}{R \infty} - R \infty)} \ln [\frac{(1 - R \infty R_{0}) R \propto}{R \infty - R_{0}}] (Equation E - 1)$
	$St = \frac{1}{b} \ln \frac{2 bR \infty}{T_{D}} (Equation E - 2)$
	$K = \frac{S {(1 - R \infty)}^{2}}{2 R \infty}$
Method 4 (M4)	$R_{K - M} = R_{0} + \frac{T_{D}^{2} R_{g}}{1 - R_{g} R_{0}}$	Formulas for practical use according to K-M theory summarized by Kubelka¹³13 Kubelka P. New contributions to the optics of intensely light-scattering materials. Part I. Journal of the Optical Society of America. 1948;38(5):448-457..
	$a = 1 / 2 (R + \frac{R_{0} R_{g} - R_{K - M}}{R_{0} R_{g}})$
	$St = \frac{1}{b} (a \coth \frac{a - R_{K - M}}{b} - a \coth \frac{a - R_{g}}{b})$
	$K = S (a - 1)$

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[1] * e-mail: angelica.saenz@cimav.edu.mx