Study on luminescence and structural properties of vanadates phosphors

[摘要] A self-activated yellow emitting zinc vanadate (Zn2 Y20 1) was synthesized by combustionmethod. The influence of the processing parameters such as synthesis temperature anddopants concentration on the structure, morphology and luminescence properties wasinvestigated. The X-ray diffraction (XRD) analysis confirmed that the samples have atetragonal structure and no significant structural change was observed in varying both thesynthesis temperature and the dopants concentration. The estimated average grain size was 78nm for the samples synthesized at different temperatures and 77 nm for the doped samples.Scanning electron microscope (SEM) images show agglomerated hexagonal-like shapeparticles with straight edges at low temperatures and the shape of the particles changed tocylindrical-like structures at moderate temperatures but were destroyed at highertemperatures. The microstructure retained its original structure when the phosphor was dopedwith Ba, Ca and Sr. The photoluminescence (PL) of the product exhibited broad emissionbands ranging from 400 to 800 nm. The best luminescence intensity was observed for theundoped Zni Y20 1 samples and those synthesized at 600°C. Any further increase in synthesistemperature and concentration of dopants, respectively, led to a decrease in the luminescenceintensity. The broad band emission peak of Zn2 Y201 consist of two broad band'scorresponding to emission from the Em1 (3T2-1A1) and Em2(3T1 - 1A1) transitions.The Zn2 Y20 1 phosphor was prepared by a sol-gel method. The effect of annealingtemperature on the structure and photoluminescence of Zn2 Y201 was investigated. The XRDresults showed the single monoclinic phase of Zn2Y201. The crystallinity of the Zn2Y201phosphor improved while the full width at half maximum of (022) XRD peak was decreasedwith the increase in annealing temperature. SEM showed that the grains size increased withthe increase in annealing temperature, which is due to the improvement in crystallinity ofZn2 Y201. Thermal behaviour of the Zni Y20 1 phosphor was investigated byThermogravimetric analysis (TOA) and differential scanning calorimetry (DSC),respectively. TOA results showed a total weight loss of 65.3% when temperature was tisenfrom 35 to 500°C. The photoluminescence emission spectra of annealed Zn2 V201 powdersshowed a broad band emission from 400 to 800 nm. The PL intensity enhanced as theannealing temperature was increased, resulting to an improvement of the crystallinity. PLemission peaks shift from green emission towards a yellow emission.Dy doped YVQ4:0y3+ phosphors were produced by the combustion method at 600°C. Thestructure and optical properties of the powders were investigated. The XRD patterns showedthe tetragonal phase similar to the standard JCPD file (1 7-0341). SEM shows that the particlesizes were small and agglomerated, and the size increased with the o y3+ dopant concentrationand its shape changed to bulk-like particles. In PL, the emission spectra exhibited a weakband at 663 nm for the 4F 912 - 6H1 1/2 transition and a peak at 483 nm (blue) for the4F912 - 6H1 s12transition and a 574 run (yellow) peak with higher intensity for the 4F912 - 6H 1312 transition.The dependence of the properties of YV04:Dy3+ phosphor upon urea:nitrate concentrationwas investigated. The samples were synthesized by combustion method. The singletetragonal phase was observed by x-ray diffraction spectra. A highly crystalline YV04:Dy3+sample was observed when increasing the ratio of the urea to 2. The estimated crystalline sizewere found to be 20, 39, 33, 30, and 27 nm for the sample prepared with the ratio of 1, 2, 2.5,3 and 4, respectively. The formation of agglomerated particles was observed by SEM imagesand it was observed that when increasing the concentration of urea further the flake-likeparticles formed. The UV diffuse reflectance spectra of YVQ4:Dy3+ with various ratios ofurea showed the determined optical band gap ranging from 3.3 to 2.3 eV. Luminescenceproperties of YV04:0y3+showed that the phosphor emit yellow colour at 573 nm and bluecolour at 482 nm corresponding to 4 F912~6Hn12 and 4f912~6H1 s12. respectively. A very weekband at 663 nm which correspond to 4 F912~6H1 1/2 transition was also observed. It was foundthat the PL emission intensity increases with an increase in the ratio of urea and reachedmaximum at 2 then decreases when increasing the ratio of urea further.YV04:Eu thin films were well deposited by pulse laser deposition at deposition temperatureof 200, 300 and 400 °C. The oxygen pressure and deposition time were held constant. Thefilms deposited at higher temperature showed a tetragonal phase. The XRD spectra for thesample deposited at 200 °C showed a very small peak at (200) orientation. Phosphor thin filmshowed a crystalline structure when the temperature increased. SEM images indicated largerparticles at higher temperature. Atomic force microscopy (AFM) results showed the smoothsurface with small particles at lower temperature and surface roughness at higher temperaturedue to the crystallinity. The PL shows the typical emission peaks of Eu in a red region at the594 and 618 nm attributed to 5Do-7F 1 and 5Do-7F2, transitions. Also the peaks at 652 and 699nm corresponding to 5Do-7F3 and 5Do-7f4 are observed. The spectra showed an increase inintensity when deposition temperature was increased.YV04:Eu3+ thin films were prepared by pulse laser deposition (PLD). YV04:Eu3+ thin filmswere deposited at room temperature by varying the deposition time from 30, 45 to 60minutes. The XRD analysis confirmed that the samples have a tetragonal phase. Theimproved on crystallinity of the films was observed when increasing deposition time. Theestimated grain particle size increased from 52 to 69 nm as the deposition time increasedfrom 30 to 60 minutes, respectively. SEM images showed that when increasing the depositiontime, particles were agglomerated and the formation of homogeneous surface was observedfor a film deposited at 45 minutes. The rough surface with larger particles was observed forthe sample deposited at 60 minutes. PL emission spectra of YV04:Eu3+ showed the mainemission peaks which are due to the Eu3+ transition 5Dj-7fj. The strongest red emission peakat 618 nm is due to transition 5Do-7F2. The increased in deposition time showed theimprovement in intensity of the thin films.

[发布机构] University of the Free State