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YVO4:Yb3+:Er3+ nanoparticles and their mixture with Au nanoparticles have been fabricated by a simple chemical method. With the excitation of a 975-nm laser, both samples had strong upconversion emissions at 2.18, 2.25, and 2.36 eV, which correspond to the transitions 4F9/2 → 4I15/2, 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 of Er3+ ions, while the emission at 2.36 eV has a large overlap with the resonant absorption energy, 2.34 eV, of the Au nanoparticles. With the coupling of the Au nanoparticles, the upconversion emission at 2.36 eV presents a strong quenching effect which is due to the energy transfer from the Er3+ ions to the Au nanoparticles. This depends on the energy difference between the emission of the Er3+ ions and the absorption of the Au nanoparticles. The upconversion power dependence shows that the slope factor of the power dependence is about 3 for a high pump power, which means that the upconversion process is a three-photon process for both samples. [ABSTRACT FROM AUTHOR]

With the excitation of a 978 nm laser, the upconversion (UC) emissions of the YVO4 : Er3+ , Yb3+ , Cr3+ nanoparticles include three emission bands located at 524, 552 and 658 nm, respectively. The UC emission characterization confirms that the UC emission of the YVO4 : Er3+ , Yb3+ nanoparticles can be modulated by the doping of the Cr3+ ions, the increase of the incident laser power and the change of the laser pulse-width. With the doping of the Cr3+ ions, the two green emissions decrease because of the energy transfer from the Er3+ states H211/2 and S3/24 to the Cr3+ ions. With the increase of the incident laser power, the two green emissions increase due to the decrease of the transfer rate of the energy transfer path to the state F9/24 of the Er3+ ions. With the increase of the incident laser pulse-width from 0.2 to 6 ms the intensity ratio of the emission at 524 nm to the emission at 658 nm increases from 0.063 to 0.19, which is due to the number accumulation at the state H211/2 due to the small transfer rate from the Er3+ state H211/2 to the Cr3+ ions. The experimental results confirm that the concentration of the Cr3+ ions has less effect on the average life-time of the UC emissions at 524, 552, and 658 nm. [ABSTRACT FROM AUTHOR]

Cadmium sulfide (CdS) quantum dots (QDs) prepared by a convenient chemical method have been characterized using absorption, fluorescence, and photoluminescence excitation techniques. The photoluminescence excitation studies show that there is an electron transfer from the surface adsorbate (thiourea) to CdS QDs in aqueous solution. The excitation band with peak maximum at 5.8 eV is assigned to the electronic transitions in the chemisorbed thiourea, whereas the excitation band between 3.45 and 3.7 eV corresponds to the band-to-band transition within the nanocrystalline CdS host. The absorption spectroscopy of the CdS QD solutions shows a strong absorption peak which is generated from thiourea. The band-edge fluorescence of the CdS QDs has also been investigated. It is shown that the fluorescence property of the CdS QDs can be enhanced by adding cadmium chloride (CdCl2) solution. [ABSTRACT FROM AUTHOR]

The Er3+, Yb3+ and Mn2+ co-doped perovskite NaMgF3 submicron particles have been successfully fabricated and the related upconversion (UC) emission was investigated in this paper. The UC emission spectra confirm that the emission color of the Er3+ ions can be modulated by the doping of the Mn2+ ions, the incident laser power and the laser pulse-width. The UC emission results demonstrate that the key factor to modulate the UC emission color of the Er3+ ions is the Mn2+ ions in the submicron particles, in which the NaMgF3 submicron particles have the largest relative green emission bands with the 3% Mn2+ concentration. The experimental results also confirm that the UC intensities ratio of the green to the red emission will increase with the increase of the incident laser power and the decrease of the laser pulse-width. All the UC emission results imply that there are two energy transfer paths from the Mn2+ state 4T1 to the Er3+ ions, in which the transfer rate to the state 4S3/2 is far larger than the transfer rate to the state 2H11/2 and the energy transfer mechanism may related with the special structure of the Mn2+ ions and the phonon-assisted processes. [ABSTRACT FROM AUTHOR]