Shape-Selective Dependence of Room Temperature Ferromagnetism Induced by Hierarchical ZnO Nanostructures

Abstract

We report on the room temperature ferromagnetism of various highly crystalline zinc oxide (ZnO) nanostructures, such as hexagonally shaped nanorods, nanocups, nanosamoosas, nanoplatelets, and hierarchical nano "flower-like" structures. These materials were synthesized in a shape-selective manner using simple microwave assisted hydrothermal synthesis. Thermogravimetric analyses demonstrated the as-synthesized ZnO nanostructures to be stable and of high purity. Structural analyses showed that the ZnO nanostructures are polycrystalline and wurtzite in structure, without any secondary phases. Combination of electron paramagnetic resonance, photoluminescence, and X-ray photoelectron spectroscopy studies revealed that the zinc vacancies (VZn) and singly ionized oxygen vacancies (VO(+)) located mainly on the ZnO surface are the primary defects in ZnO structures. A direct link between ferromagnetism and the relative occupancy of the VZn and VO(+) was established, suggesting that both VZn and VO(+) on the ZnO surface plays a vital role in modulating ferromagnetic behavior. An intense structure- and shape-dependent ferromagnetic signal with an effective g-value of >2.0 and a sextet hyperfine structure was shown. Moreover, a novel low field microwave absorption signal was observed and found to increase with an increase in microwave power and temperature.

The modification of structural and optical properties of nano- and submicron ZnO powders by variation of solvothermal syntheses conditions

“…The origin of the green PL emission, as the most commonly observed defect emission in ZnO nanostructures, is quite controversial. In the recent studies (Ghosh et al 2013;Motaung et al 2014;Panigrahy et al 2010), this emission is associated with singly ionized oxygen vacancies V O ? and/or zinc vacancies V Zn , located mainly on the ZnO surface.…”

Characteristics of the mechanical milling on the room temperature ferromagnetism and sensing properties of TiO2 nanoparticles

“…The observed RTFM is thought to be due to higher V O on the TiO 2 NPs surface (see PL results) and structure defect induced by milling process which result in coalescence of ferromagnetic domains [43] such as reduced crystallites size (see Table 1). Previous studies [9,[44][45][46] showed that larger crystals are detrimental to magnetic properties. Moreover, the origin of ferromagnetism for TiO 2 NPs diluted magnetic semiconductors can be explained by various carrier mediated mechanisms that include double-exchange, super-exchange or Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interactions [43].…”

“…The detected carbon is associated to the carbon tape used during the measurements and that adsorbed on the surface during the exposure of the sample to the ambient atmosphere. The binding energy for the C1s peak at 284.6 eV was used as the reference for calibration [9]. Fig.…”

“…Recent reports ruled out the possibility of RTFM being due to extrinsic effects associated to 3d cations demonstrating that the 3d electronic shells of the cations in these diluted magnetic oxides (DMOs) do not carry any measurable ferromagnetic moment using innovative characterization tools such as X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD). Scientists also reported RTFM in undoped oxides such as TiO 2 , HfO 2 , ZnO, and In 2 O 3 , RTFM related to surface defects and/or interface defects [6][7][8][9] which maybe during the preparation process. Previous results calculated that the FM observed in undoped TiO 2 largely originates from the d-orbitals of low-charge-state Ti cations converted from Ti 4+ cations, which are induced by the surface oxygen vacancies [10,11].…”

Multiferroic Properties of Zn1−x Mg x O Nanoparticles

“…Figure 3 shows the ESR spectra of undoped and Mgdoped ZnO nanoparticles. A broad resonance signal has been observed for undoped ZnO nanoparticles which can be attributed to long-range ferromagnetic ordering between the electrons trapped in oxygen vacancies [17]. Generally, ZnO loose oxygen resulting in formation of oxygen vacancies, owing to its low formation energy (3.8 eV).…”