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Effect of growth time on the optical constants of ZnO nanowires by hydrothermal synthesis for application in optoelectronic devices

E. MUCHUWENI1,2,* , T. S. SATHIARAJ1, H. NYAKOTYO1,2, C. SHONHIWA2, M. T. MAGAMA2

Affiliation

  1. Department of Physics and Astronomy, Botswana International University of Science and Technology (BIUST), P. Bag 16, Palapye, Botswana
  2. Department of Physics and Engineering, Bindura University of Science Education (BUSE), P. Bag 1020, Bindura, Zimbabwe

Abstract

Recently, an assortment of zinc oxide (ZnO) nanostructures, such as nanowires, nanorods, nanobelts, nanoribbons and nanoflowers, have attracted significant research attention in the fabrication of optoelectronic devices, especially as transparent conductive electrodes and electron transport layers (ETLs) due to their high optical transmittance in the visible region and excellent electrical conductivity. Among these nanostructures, ZnO nanowires have been prepared using hydrothermal synthesis, and their optical constants, such as refractive index (n) and extinction coefficient (k), were determined as a function of growth time. In the visible region, n and k showed an increasing trend with growth time, which was associated with the decrease in optical transmittance owing to the growth of longer nanowires with relatively high reflectance and high absorbance. Despite the decrease in transmittance with increasing growth time, there was an increase in electron concentration (Nc), favourable for transparent electrode and ETL applications. This was ascribed to an increase in the mean crystallite sizes, which produces less grain boundaries; hence, few charge carrier traps and scattering centres. The effect of growth time on the optical band gap (Eg), Urbach energy (Eu), effective single oscillator energy (Eo), dispersion energy (Ed), zero frequency dielectric constant (ε0), zero frequency refractive index (n0) and high frequency dielectric constant (ε∞) was also investigated. Raman spectroscopy also revealed the presence of E2 (high) and A1 (LO) modes, whose intensity increased with growth time, demonstrating an improvement in crystallinity and an increase in structural defects, such as oxygen vacancies, which act as donors that increase Nc, in consistency with optical and electrical measurements. Thus, optical constants conceivably provide more insight into the optoelectronic properties of materials, and tuning of the hydrothermal parameters, e.g., growth time, plays a significant role in tailoring the optoelectron.

Keywords

ZnO nanowires, Optical constants, Hydrothermal growth time, Raman spectroscopy, Optoelectronic devices.

Submitted at: Jan. 23, 2021
Accepted at: Aug. 10, 2022

Citation

E. MUCHUWENI, T. S. SATHIARAJ, H. NYAKOTYO, C. SHONHIWA, M. T. MAGAMA, Effect of growth time on the optical constants of ZnO nanowires by hydrothermal synthesis for application in optoelectronic devices, Journal of Optoelectronics and Advanced Materials Vol. 24, Iss. 7-8, pp. 380-384 (2022)