Synthesis and characterization of Mn-doped ZnO nanorods grown in an ordered periodic honeycomb pattern using nanosphere lithography

Abstract

We report a study of the structural, optical and magnetic properties of undoped and Mn-doped ZnO nanorods grown by chemical bath deposition in a periodic honeycomb lattice formation. Mn-doping is accomplished by a diffusion process at a constant time of 8 h for different temperatures of 500 °C, 600 °C and 700 °C. Undoped and Mn-doped ZnO nanorods had a hexagonal wurtzite structure with a (0 0 2) preferred orientation. From SEM results, it was seen that Mn-doped ZnO nanorods grew vertically in the honeycomb lattice with lengths of 0.8 μm. XPS results showed that Mn³⁺ ions was successfully incorporated in the ZnO matrix by substituting for Zn²⁺ ions and that Mn-doping increased the number of oxygen vacancies in ZnO compared to undoped ZnO. This result was also supported by photoluminescence data at 10 K. Magnetic data showed that all the samples exhibited ferromagnetic character. Although the origin of undoped ZnO is related to oxygen vacancy-induced d⁰ ferromagnetism, bound magnetic polarons are responsible from the ferromagnetism of Mn-doped ZnO samples which have T_c values above the room temperature.

Introduction

In recent years, diluted magnetic semiconductors (DMS) have attracted significant interest due to their potential applications in spintronic devices. Among II–VI group semiconductors, especially, ZnO has gained lots of interest since it has a wide band gap of 3.37 eV with an excitonic binding energy of 60 meV at room temperature that make it an important material for potential optoelectronic applications [1]. ZnO is also a promising material in the investigation of DMS systems that can be achieved by doping with 3-d group elements like Mn, Co, Fe etc. and by these means the optical and magnetic properties of ZnO materials can be tuned. In particular, Mn-doped ZnO nanostructures have attracted significant interest as Mn has the highest magnetic moment and the first half of the d band is fully occupied [2]. The literature contains some reports of experimental studies on ZnO:Mn grown with diverse morphologies such as thin films, nanocrystals and nanowires and exhibiting room temperature ferromagnetism. For instance, Yang et al. produced Mn-doped ZnO thin films by the sol–gel technique on both glass and Si substrates and they found that even though undoped ZnO exhibited diamagnetic behavior, all the ZnO:Mn samples had a ferromagnetic character at room temperature. The origin of ferromagnetism was explained by the substitution of Mn²⁺ ions on Zn²⁺ sites [3]. Sain et al. synthesized ZnO:Mn nanocrystalline samples by mechanical alloying using a mixture of ZnO and MnO powders for different doping concentrations and room temperature ferromagnetism for ZnO:Mn samples was obtained. The origin of ferromagnetism was attributed to RKKY exchange interactions [4]. Furthermore, Philipose et al. grew ZnO:Mn nanowires by the vapor phase transport technique on Au-catalyzed Si substrates with Mn concentrations of 1 at%, 2 at% and 4 at%. They observed room temperature ferromagnetic character for 1 at% Mn-doping and the ferromagnetism was attributed to the interactions between Mn ions and native defects [5].

ZnO material doped with transition-metal (TM) ions has been grown in various morphologies such as nanowires, nanorods and nanotubes [6], [7], [8]. Some methods to deposit TM-doped 1-D ZnO nanostructures include RF magnetron sputtering [9], vapor phase transport [10], pulsed laser deposition [11], spray pyrolysis [12] and chemical bath deposition (CBD) [13]. Among these methods, CBD is an attractive technique and offers advantages such as simple, low cost equipment and a low growth temperature [14]. To the best of our knowledge, this is the first study investigating the structural, optical and magnetic properties of Mn-doped ZnO nanorods grown into a periodic honeycomb pattern. Additionally, the study focuses on clarifying the origin of room temperature ferromagnetism observed in both undoped and Mn-doped ZnO nanorods grown this honeycomb lattice using both photoluminescence and X-ray photoelectron spectroscopy results.