Luminescent properties of synthesized PbS nanoparticle phosphors
Dhlamini, Mokhotjwa Simon
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Luminescent lead sulphide (PbS) nanoparticles embedded in an amorphous silica (SiO2) matrix were synthesized at room temperature by a sol-gel process. The prepared nanocomposite materials were crushed into powders and annealed in air at 200oC. The chemical composition of the powders was analyzed with an energy dispersive x-ray spectrometer. Particle sizes, crystalline structure and morphology of the PbS nanoparticles were determined with transmission electron microscopy (TEM) and x-ray diffraction (XRD). The crystal particle sizes estimated from the XRD peaks and the TEM images were in the range of 10 to 50 nm in diameter. The SiO2:PbS powders were then irradiated with 325 nm (He-Cd) and 458 nm (Ar+) lasers for photoluminescence (PL) measurement. PL spectra were obtained for pure SiO2 as well as the encapsulated PbS nanoparticles at room temperature. Two strong broad bands, blue (450 nm) and yellow-orange (560 nm) from bulk SiO2 and PbS nanoparticles, respectively, were observed. The PL data show a blue shift from the normal emission wavelength at 3200 nm in bulk PbS to 560 700 nm in nanoparticulate PbS powders. The blue-shift of the emission wavelengths is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles. Energy transfer from ZnO nanoparticles to PbS nanoparticles was also observed. The possible mechanism for the energy transfer is reported. The powders were also subjected to prolonged 2 keV electron beam irradiation in a vacuum chamber at and different O2 pressures (5 × 10-8 2 × 10-7 Torr O2). The cathodoluminescence (CL) was measured with Ocean Optics S2000 spectrometer, and showed the emission peak to be at a wavelength of 680 nm. Changes in the CL brightness and the corresponding change in the surface chemical composition were investigated with Ocean Optics S2000, Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). The oxygen Auger peak-to-peak height decreased simultaneously with the CL intensity. XPS analysis on the degraded spot showed the development of characteristic SiO, SiOx (0<x<1) and elemental silicon peaks on the low-energy side of the SiO2 peak. The electron beam induced dissociation of SiO2 into elemental silicon and oxygen resulted in oxygen desorbing from the surface at almost the same rate as the CL intensity was decreasing. The data suggest that a non-luminescent PbSO4 was also formed on the surface. The degradation was less severe at higher oxygen pressures. All the measurements were done at room temperature. Thin luminescent films of SiO2:PbS were grown on Si(100) substrates at room temperature, 100oC, 200oC, 300oC and 400oC by the pulsed laser deposition (PLD) technique. Surface morphology and PL properties of samples were analyzed with scanning electron microscopy (SEM) and a 458 nm (Ar+) laser respectively. The PL emission wavelength of the films was red-shifted from that of the powders at 560 nm to 660 nm. The PL emission of the films was less intense than that of the powders, although the intensity of some of the films was improved marginally by post-deposition annealing in air at 400oC. The increase in the PL intensity with an increase in the deposition temperatures was observed.