The effect of annealing on structural defects and d0 ferromagnetism in SnO2 nanoparticles prepared by solution combustion method is investigated. The as-synthesized SnO2 nanoparticles were annealed at 400–800 °C for 2 h, in ambient conditions. The crystallinity, size, and morphology of the samples were studied using x-ray diffraction and transmission electron microscopy studies. The annealing results in grain growth due to coarsening as well as reduction in oxygen vacancies as confirmed by Raman spectroscopy, photoluminescence spectroscopy, and x-ray photoelectron spectroscopy. All the as synthesized and annealed samples exhibit room temperature ferromagnetism (RTFM) with distinct hysteresis loops and the saturation magnetization as high as ∼0.02 emu/g in as-synthesized samples. However, the saturation magnetization is drastically reduced with increasing annealing temperature. Further the presence of singly charged oxygen vacancies (Vo− signal at g-value 1.99) is confirmed by electron paramagnetic resonance studies, which also diminish with increasing annealing temperature. The observed diminishing RTFM and simultaneous evidences of diminishing O vacancies clearly indicate that RTFM is driven by defects in oxide lattice and confirms primary role of oxygen vacancies in inducing ferromagnetic ordering in metal oxide semiconductors. The study also provides improved fundamental understanding regarding the ambiguity in the origin of intrinsic RTFM in semiconducting metal oxides and projects their technological application in the field of spintronics.
Unusual optical bandgap narrowing is observed in undoped SnO2 nanoparticles synthesized by the solution combustion method. The estimated crystallite size is nearly 7 nm. Though the quantum confinement effect predicts a larger optical bandgap for materials with small crystallite size than the bulk, the optical bandgap in the as synthesized materials is found to be 2.9 eV compared to the reported value of 3.6 eV for bulk SnO2 particles. The yellow-green photoluminescence emissions and the observed narrowing of the bandgap can be attributed to the deep donor levels of oxygen vacancies, owing to the high exothermicity of the combustion reaction and the faster cooling rates involved in the process.
The nanoscale interface between multi-component (Ag–Au–Cu–Pd–Pt) alloy nanoparticles on MoS2 sheets increase its work function making an ohmic contact into Schottky with gold electrodes. This drastically enhances response towards hydrogen gas.
The effect of Radio Frequency (RF) power on the properties of magnetron sputtered Al doped ZnO thin films and the related sensor properties are investigated. A series of 2 wt% Al doped ZnO; Zn0.98Al0.02O (AZO) thin films prepared with magnetron sputtering at different RF powers, are examined. The structural results reveal a good adhesive nature of thin films with quartz substrates as well as increasing thickness of the films with raising RF power. Besides, the increasing RF power is found to improve the crystallinity and grains growth as confirmed by X-ray diffraction. On the other hand, the optical transmittance is significantly influenced by the RF power, where the transparency values achieved are higher than 82% for all the AZO thin films and the estimated optical band gap energy is found to decrease with RF power due to an increase in the crystallite size as well as the film thickness. In addition, the defect induced luminescence at low temperature (77 K) and room temperature (300 K) was studied through photoluminescence spectroscopy, it is found that the defect density of electronic states of Al 3+ ion found to increase with increase of RF power due to the increase in the thickness of the film and the crystallite size. The gas sensing behavior of AZO films was studied for NO2 at 350 °C. The AZO film shows good response towards NO2 gas and also a good relationship between the response and the NO2 concentration, which is modeled using an empirical formula. The sensing mechanism of NO2 is discussed.
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