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https://doi.org/10.1007/s11837-020-04483-z  2020 The Minerals, Metals & Materials Society

ZINC OXIDE NANOTECHNOLOGY

Electrical and Optical Properties of Well-Aligned Ho3+-Doped ZnO Nanorods as an Alternative Transparent Conducting Oxide HAKAN C ¸ OLAK

1,3

¨ SE2 and ERCAN KARAKO

1.—Faculty of Science, Department of Chemistry, C ¸ ankırı Karatekin University, 18100 C ¸ ankırı, Turkey. 2.—Vocational College, Kayseri University, 38039 Kayseri, Turkey. 3.—e-mail: [email protected]

In this study, the impacts of doping concentration on the crystal structure, morphology, electrical, and optical properties of Ho3+-doped zinc oxide (ZnO) nanorod (NR) arrays were studied. Structural and morphological characterizations showed that the Ho3+-doped ZnO NRs were crystallized in the (002) direction, and that they had a homogeneous distribution on the substrate. The crystallite sizes of the samples were between 50 nm and 65 nm. SEM analysis showed that every sample was hexagonal in shape. For the 1 and 5 mol.% Ho3+-doped ZnO NRs, the values for electrical conductivity were found to be 1.41 9 107 and 8.29 9 106 (X cm)1 at 25C and 1.70 9 105 and 1.24 9 103 (X cm)1 at 300C, respectively. The optical transmittances were between 80 and 93% for all the samples in the region from 400 to 1000 nm. The optical band gap values were determined to be between 3.180 and 3.195 eV.

INTRODUCTION Transparent conducting oxides (TCOs) are important factors in many modern devices, such as solar cells, optics, sensors, and touch screens.1 In the modern world, the necessity for all technologies has been increasing from day to day. The intensive usability of TCOs in modern instruments, the high cost of available TCO materials, and the potential prospects for future practices has focused researchers’ interests on different TCO materials.2 Metal oxides with high electrical conductivity and optical transmittance can be considered to be TCO materials. The metal oxides are also described as oxide semiconductors with high band gap energy. The optical and electrical aspects of the materials can be tailored by doping with other elements. To be accepted as a TCO, the material needs to have high electrical conductivity and optical transparency in the visible range (higher than 80%).3 Fluorine-doped SnO2 (FTO) and tin-doped In2O3 (ITO), which have excellent electrical and optical properties, are the most widely preferred TCO materials in current technologies. ZnO is an alternative TCO material with low cost, high durability, (Received August 13, 2020; accepted October 28, 2020)

non-toxicity, and abundance in nature.4 ZnO has a direct and wide band gap energy in the ultra violet region.5 To achieve the highest electrical conductivity, investigating suitable doping elements and optimal doping concentrations are important processes in improving ZnO based TCOs.1 The performance of the dopant element depends on its ionic radius and electronegativity. Regarding ZnO materials, it is thought that doping is effective when Zn2+ ions are replaced by me