Green Synthesis of Fe-Doped ZnO Nanoparticles obtained
using Lemongrass Extract (Cymbopogon citratus)

Diluted magnetic semiconductors (DMSs) are solid solutions in which a fraction of the cations in the crystalline lattice is replaced by magnetic impurities, such as transition metals. However, most of these materials exhibit low Curie temperatures, which limits their technological applications [0]. Fe-doped ZnO also exhibits magnetic properties; however, the origin of this magnetism remains controversial, as different studies report both paramagnetic and ferromagnetic behaviors under varying experimental conditions. The origin of ferromagnetism in ZnO, doped or not, remains an open question, being influenced by the synthesis method and structural conditions of the samples, such as grain size, dopant concentration, morphology, and crystalline structure [1]. In this work, ZnO nanoparticles (nps) doped with 3%, 5% and 10% Fe were produced through an environmentally friendly route, employing lemongrass extract as a chelating agent, via the sol-gel method [2], [3]. Morphological analysis by TEM revealed that Fe doping increased the average particle size from 9 nm to 12 nm. UV-Vis spectroscopy results indicated that the presence of Fe induces the formation of sub-band gap states in ZnO, reflected by the arising of Urbach levels. Raman spectroscopy demonstrated that the Fe incorporation introduced defects into the crystalline structure of the material. XRD analysis confirmed that Fe was successfully incorporated into the ZnO wurtzite structure without the formation of secondary phases. Magnetization measurements at room temperature revealed a paramagnetic behavior for the doped samples. Additionally, Mössbauer spectroscopy, performed at room temperature using a Co-57 source, confirmed this paramagnetic doublet character of Fe3+ sites, showing spectrum consistent with only quadrupole electric interactions.

Impact of the sputtering pressure on the structural, morphological, optical,
electrical and gas sensor properties of ZnO films grown by DC magnetron sputtering

Nanostructured oxide materials are currently used as biosensors in technological applications. These materials are of great interest due to their potential for use in medical diagnostics, as they enable rapid, non-invasive, and low-cost evaluations. In this work, zinc oxide (ZnO) films were produced by the sputtering deposition method, varying the working pressure, in order to obtain materials with specific features which can be used as gas sensors. Surface analysis using SEM revealed the formation of microrods and two sizes of grains, one larger than 1 micrometer and the other smaller than 500 nm. X-ray diffraction analysis revealed the formation of ZnO's wurtzite phase with preferential orientation along the [002] direction. It was determined that the film thickness decreases as the working pressure is increased. XRD data analysis also indicates that the mean crystallite size and lattices constants decreases as the working pressure is increased. UV-Vis spectroscopy results revealed that the band gap energy of the films does not show a clear tendency with the working pressure. However, Urbach Energy increases from 0.1 to 0.3 eV, suggesting the formation of more defects as the film thickness become thinner. The temperature dependence of the resistivity of the films in the range from 300 to 573K shows a typical semiconducting behavior and indicates the presence of two activation energies. Photocurrent measurements for the films showed a good sensitivity to UV light. Data analysis indicates the presence of trap energies at 0.58 and 0.63 eV, suggesting fast and slow response, respectively. Tests of sensor response using methane gas were carried out, which showed that the ZnO films can be used as a gas sensor material.