INFLUÊNCIA DA TEMPERATURA E DA PRESSÃO DE TRATAMENTO TÉRMICO NA FOTOLUMINESCÊNCIA DO ZnO

Abstract

Zinc oxide (ZnO) is a material with great technological interest, showing a direct gap 3,37 eV and high electron-hole bond energy (excitons) of 60 meV at room temperature, what makes it applicable in a variety of areas, especially in optoelectronic dispositives. In this work samples of ZnO were prepared through the polymeric precursors method (Pechini Method) and then they were submitted through a pressure assisted thermal treatment (TTAP) and another posterior thermal treatment. The study evaluated influence of temperature and pressure, varying those parameters through factorial planning during the treatments, on the photoluminescence properties of ZnO. The Pechini method shown to be efficient for obtaining the ZnO powders. The structure was determined through x-ray diffraction analysis (DRX) and by refining through the Rietveld method, it was possible to assure that the samples had nanometric size cristalites and good structural stability. With images obtained through scanning electron microscopy field emission (MEV-FEG) it was possible to define that the particles shown an almost spherical and homogeneous morphology. The optical properties of the samples were measured and compared through the photoluminescence spectroscopy (PL) and UVvisible spectroscopy. The ZnO shown two main peaks, one in the ultraviolet (UV) region and the other one in the visible region of the electromagnetic spectra. In the UV region the parameters that had bigger influence on the photoluminescence intensities were pressure and the interaction between pressure and temperature and between pressure and posterior thermal treatment. In the visible region, the isolated pressure had bigger influence. The variations of the band gap energy (Egap) values confirmed effects of the treatments in relation to the sample without treatment. The chemical composition of the materials was evaluated through Raman spectroscopy, Fourier Transform Infrared (FTIR) and by x-ray excited photoelectrons (XPS). Raman spectra shown that residual tensions (traction and compression) caused by the increase pressure and temperature grant an increase in the ZnO crystallinity and favor bigger photoluminescence intensities. By FT-IR it was possible to observe that due to the high superficial area of the nanoparticles, a bigger desorption of hydroxyl (-OH) occur on material’s surface. The XPS results confirmed the effects caused by the superficial layer of ZnO, where defects related to oxygen and –OH groups on the surface fundamental and rule the changes in the photoluminescence intensities.