Study on luminescence and structural properties of vanadates phosphors
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Date
2016-01
Authors
Foka, Kewele Emily
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
A self-activated yellow emitting zinc vanadate (Zn2 Y20 1) was synthesized by combustion
method. The influence of the processing parameters such as synthesis temperature and
dopants concentration on the structure, morphology and luminescence properties was
investigated. The X-ray diffraction (XRD) analysis confirmed that the samples have a
tetragonal structure and no significant structural change was observed in varying both the
synthesis temperature and the dopants concentration. The estimated average grain size was 78
nm for the samples synthesized at different temperatures and 77 nm for the doped samples.
Scanning electron microscope (SEM) images show agglomerated hexagonal-like shape
particles with straight edges at low temperatures and the shape of the particles changed to
cylindrical-like structures at moderate temperatures but were destroyed at higher
temperatures. The microstructure retained its original structure when the phosphor was doped
with Ba, Ca and Sr. The photoluminescence (PL) of the product exhibited broad emission
bands ranging from 400 to 800 nm. The best luminescence intensity was observed for the
undoped Zni Y20 1 samples and those synthesized at 600°C. Any further increase in synthesis
temperature and concentration of dopants, respectively, led to a decrease in the luminescence
intensity. The broad band emission peak of Zn2 Y201 consist of two broad band's
corresponding 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 annealing
temperature on the structure and photoluminescence of Zn2 Y201 was investigated. The XRD
results showed the single monoclinic phase of Zn2Y201. The crystallinity of the Zn2Y201
phosphor improved while the full width at half maximum of (022) XRD peak was decreased
with the increase in annealing temperature. SEM showed that the grains size increased with
the increase in annealing temperature, which is due to the improvement in crystallinity of
Zn2 Y201. Thermal behaviour of the Zni Y20 1 phosphor was investigated by
Thermogravimetric analysis (TOA) and differential scanning calorimetry (DSC),
respectively. TOA results showed a total weight loss of 65.3% when temperature was tisen
from 35 to 500°C. The photoluminescence emission spectra of annealed Zn2 V201 powders
showed a broad band emission from 400 to 800 nm. The PL intensity enhanced as the
annealing temperature was increased, resulting to an improvement of the crystallinity. PL
emission peaks shift from green emission towards a yellow emission.
Dy doped YVQ4:0y3+ phosphors were produced by the combustion method at 600°C. The
structure and optical properties of the powders were investigated. The XRD patterns showed
the tetragonal phase similar to the standard JCPD file (1 7-0341). SEM shows that the particle
sizes were small and agglomerated, and the size increased with the o y3+ dopant concentration
and its shape changed to bulk-like particles. In PL, the emission spectra exhibited a weak
band at 663 nm for the 4F 912 - 6H1 1/2 transition and a peak at 483 nm (blue) for the4F912 - 6H1 s12
transition 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 concentration
was investigated. The samples were synthesized by combustion method. The single
tetragonal 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 size
were 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 images
and it was observed that when increasing the concentration of urea further the flake-like
particles formed. The UV diffuse reflectance spectra of YVQ4:Dy3+ with various ratios of
urea showed the determined optical band gap ranging from 3.3 to 2.3 eV. Luminescence
properties of YV04:0y3+showed that the phosphor emit yellow colour at 573 nm and blue
colour at 482 nm corresponding to 4 F912~6Hn12 and 4f912~6H1 s12. respectively. A very week
band at 663 nm which correspond to 4 F912~6H1 1/2 transition was also observed. It was found
that the PL emission intensity increases with an increase in the ratio of urea and reached
maximum at 2 then decreases when increasing the ratio of urea further.
YV04:Eu thin films were well deposited by pulse laser deposition at deposition temperature
of 200, 300 and 400 °C. The oxygen pressure and deposition time were held constant. The
films deposited at higher temperature showed a tetragonal phase. The XRD spectra for the
sample deposited at 200 °C showed a very small peak at (200) orientation. Phosphor thin film
showed a crystalline structure when the temperature increased. SEM images indicated larger
particles at higher temperature. Atomic force microscopy (AFM) results showed the smooth
surface with small particles at lower temperature and surface roughness at higher temperature
due to the crystallinity. The PL shows the typical emission peaks of Eu in a red region at the
594 and 618 nm attributed to 5Do-7F 1 and 5Do-7F2, transitions. Also the peaks at 652 and 699
nm corresponding to 5Do-7F3 and 5Do-7f4 are observed. The spectra showed an increase in
intensity when deposition temperature was increased.
YV04:Eu3+ thin films were prepared by pulse laser deposition (PLD). YV04:Eu3+ thin films
were deposited at room temperature by varying the deposition time from 30, 45 to 60
minutes. The XRD analysis confirmed that the samples have a tetragonal phase. The
improved on crystallinity of the films was observed when increasing deposition time. The
estimated grain particle size increased from 52 to 69 nm as the deposition time increased
from 30 to 60 minutes, respectively. SEM images showed that when increasing the deposition
time, particles were agglomerated and the formation of homogeneous surface was observed
for a film deposited at 45 minutes. The rough surface with larger particles was observed for
the sample deposited at 60 minutes. PL emission spectra of YV04:Eu3+ showed the main
emission peaks which are due to the Eu3+ transition 5Dj-7fj. The strongest red emission peak
at 618 nm is due to transition 5Do-7F2. The increased in deposition time showed the
improvement in intensity of the thin films.
Description
Keywords
Sol-gel, Dopants, Thin films, Vanadates, Deposition, Transitions, Combustion, Crystallinity, Luminescence, Thermogravimetric, Thesis (Ph.D. (Physics))--University of the Free State, 2016