Optimizing physical properties of Co-doped ZnO nanoparticles: Controlling oxygen vacancy and carrier concentration

Highlights

• Dear reviewer, We submit our manuscript entitled “Optimizing physical properties of Co-doped ZnO nanoparticles: controlling oxygen vacancy and carrier concentration” to Vacuum, and there are three highlights in our manuscript as following.

• The diamagnetism of ZnO can be effectively transformed into ferromagnetism when ZnO is doped at a lower Co concentration.

• The saturation magnetization (Ms) is larger than the reported value. Moreover, there is little difference in remnant magnetization (Mr). The coercivity (Hc) is lower than that of literature, and decreases as the Co content increases.

• When Co content is 1 %, the RTFM is mainly induced by Vo defects. With the further increase of Co content, the RTFM is mainly induced by Carrier concentration, and it shows a monotonous increasing trend.

Abstract

The origination of magnetism in Co-doped ZnO-based dilute magnetic semiconductor (DMS) with room temperature ferromagnetism (RTFM) has some controversy. In this paper, Zn_1-xCo_xO nanoparticles (x = 0.00, 0.01, 0.03 and 0.05) were prepared using sol-gel method. The relationship between oxygen vacancy or carrier concentration and magnetic properties were investigated. The hexagonal wurtzite structure of all the samples was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The morphological features were investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM), and the optical properties, including energy gap, were analyzed by ultraviolet–visible spectroscopy (UV–vis). The number of oxygen vacancies (V_O) was measured by X-ray photoelectron spectroscopy (XPS), which is positively correlated with the Co content. The RTFM properties were analyzed by physical properties measurement system (PPMS). The results reveal that a higher carrier concentration is critical to promoting ferromagnetism. And, the magnetism of Co-doped ZnO may originate from V_O defects or the carrier.

Introduction

With the advancement of science and technology, diluted magnetic semiconductors (DMSs) have received increasing attention in recent years owing to their ferromagnetic and semiconducting behavior [1,2]. The advantages of DMSs include non-volatility, low power consumption, high integration density, high data processing speed, and long spin lifetime [3]. Besides, the optical, antibacterial, electronic, magneto-electronic, magneto-optical, and spintronic properties of DMSs has also raised the interest of researchers [[4], [5], [6], [7], [8], [9]].

Zinc oxide (ZnO) is an n-type semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV). It is also widely known as a sustainable, inexpensive, and eco-friendly material. ZnO-based DMS has more advantages in physical and chemical properties compared with compound semiconductors. Further, it is considered as an optimal candidate material for achieving RTFM [10,11]. Fabricating ZnO-based DMS with RTFM behavior, however, is a huge challenge in the field of material science, and the ferromagnetic origin of ZnO-based DMS is still being debated [[12], [13], [14]].

Numerous studies have reported the RTFM behavior of Co-doped ZnO and the physical origin of this magnetism. Bhardwaj et al. [15] have reported the Co-doped ZnO samples prepared via chemical solution route. They found the origin of the ferromagnetic behavior is attributed to exchange interaction between the localized spin moments of Co²⁺ that originated from oxygen vacancies. Fu et al. [16] have studied Co-doped ZnO nanoparticles by hydrothermal synthesis route and reported the RTFM originate probably from the Co³⁺ enrichment and the coexistence of Co²⁺/Co³⁺ in the particles, which may lead to a double-exchange mechanism or the super-exchange between the Co²⁺ and Co³⁺. Singh et al. [17] observed the ferromagnetic properties in their Zn_1-xCo_xO (x = 0.02, 0.03 and 0.04), oxygen and zinc related defects were found to be the main reasons for RTFM. Kumar et al. [18] have found RTFM in Zn_1-xCo_xO ($0\le \text{x}\le 0.04$) nanoparticles, and they claimed that the grain boundaries, oxygen vacancy and bound magnetic polarons (BMPs) jointly may be responsible for the RTFM. This confirms the fact that the source of the RTFM of Co-doped ZnO nanoparticles is still controversial, and it deserves confirm further.

On the other hand, with the rising of environmental awareness, the optical property of ZnO has also received increasing recognition [19]. For example, photocatalytic property allows the efficient removal of toxic substances in wastewater [20]. Besides, Poornaprakash et al. [3] demonstrated that the optical performance and photocatalytic activity of ZnO nanorods can be enhanced via Co doping.

In this study, Co-doped ZnO samples with different Co content were synthesized by sol-gel method, the structural, optical, and magnetic properties of the samples were investigated in detail.