Structural and mechanical properties of (Co/Mg) co-doped nano ZnO

Abstract

The (Co/Mg) co-doped ZnO systems (ZnMgCoO) were synthesized as polycrystalline nanoparticles using the sol–gel technique. The effects of Co/Mg co-addition to the structural and mechanical properties of ZnO nanoparticles were investigated. Microstructural properties, such as phases, crystal structures and grain sizes are characterized by using X-Ray Diffraction (XRD) analysis and Scanning Electron Microscope (SEM). The microhardness values of the samples were evaluated by the Vickers tester. In the samples of the Co>Mg ratio, Indentation Size Effect (ISE) behaviors were observed. On the other hand, in the samples of the Mg>Co ratio, Reverse Indentation Size Effect (RISE) was observed. These were analyzed in detail utilizing the models for materials hardness analysis found in literature.

Introduction

Wurzite structured ZnO exhibits a direct bandgap of 3.37 eV, high electrochemical stability, absence of toxicity, good mechanical strength, and a large exciton binding energy of 60 meV at room temperature. These properties and other possible properties of ZnO have led to constitute an extensive interest in many areas of the technological applications such as optical and magnetic memory devices, heat protecting windows, solar cells and sensors [1], [2], [3], [4], [5].

The Dilute Magnetic Semiconductors (DMS) are formed by doping ZnO with small amounts of magnetic Transition-Metals (TM) such as Cr, Ni, Mn, Co, Fe and V. In recent years, these have been extensively studied to explain the origin of magnetism in the DMSs by means of determining the correlation between magnetic properties and the TM doping concentrations in spintronic applications. Among these doping transitions metals, the Co doped ZnO may be the most promising DMS for the room temperature ferromagnetism according to the experimental reports and theoretical arguments in the literature [6], [7], [8], [9].

A main interesting feature of ZnO applications is that it could be tuned like a bandgap which can be controlled via divolent substitution on the cation site. Many doping elements have been applied to modulate the bandgap of ZnO. While some elements, such as Cd, decrease the bandgap, substituting Mg on Zn site increases the bandgap. The properties and applications of ZnO are affected by the bandgap modulation and strongly depend on the preparation method and condition. Sol–gel is low cost, simple and easy method to control the composition of nano particle and thin films. The structural, electrical, magnetic and mechanical properties of Mg, Co, Ni doped ZnO and Mg/Al, Mg/Cr, Mg/Co, MgNi, Cu/Co Co-doped ZnO nano particles via the sol–gel method are available in the references [10], [11], [12], [13], [14], [15], [16], [17], [18].

The determination of mechanical properties of ZnO-based semiconductors is a key factor in choosing the suitable applications such as sensors, solar cell, electrical varistors, transparent high-power electronics, window materials for display etc. The Vickers hardness test is one of the well-known and easy method to estimate the mechanical properties [15], [16]. In industry, the hardness tests are frequently used to determine both the production quality of the materials and specifically the control of the effectiveness of the heat treatment based on the simple nondestructive test. Microhardness value of a solid depends on the applied test-load. This phenomenon is known as Indentation Size Effect (ISE), and it decreases with increased applied load. On the other hand, Reverse Indentation Size Effect (RISE) means microhardness value increases with increased applied load. Therefore, ISE or RISE behaviors of materials show where, when, and to what extent the materials can be used. In the practical applications of DMS materials, in the future, the key point will be to switch from RISE to ISE behavior. It is possible to adjust Mg ratio by the user according to the need in the practical applications. In the present study, only the microhardness values of DMS materials are tuned by using Mg ratio.

The researchers in this study are interested in investigating characterization and sol–gel parameters by varying Co/Mg co-doped ratio of the ZnMgCoO nano particles using the sol–gel technique, by doping ratio effects on the mechanical, structural and microstructure properties of nano particles, and by finding critical doping ratio of Co/Mg when switched from RISE to ISE behavior.