New highly efficient green
phosphor for LEDs
Mihail Nazarov, Boris Tsukerblat & Do Young Noh
To develop a highly
energy-efficient white light sources, one needs to design highly efficient
yellow, green and red phosphors, which are able to absorb excitation energy of
blue LEDs and generate emissions. In this paper, results on production of a new
green multiphase thiogallate phosphor {SrGa2S4
+ MgGa2O4}:Eu2+
with improved luminescence
properties have been presented. The detailed XRD analysis, Raman spectroscopy
and PL study have been carried out in order to enlarge the understanding of the
radiative processes in {SrGa2S4
+ MgGa2O4}:Eu2+, based on two different
crystallographic phases {SrGa2S4}
and {MgGa2O4},
prepared by solid state reaction at 900-1000°C during 4 h
with a carbon reduction atmosphere. A
model of 4f-5d optical transitions with due account for the
electron-vibrational interaction is discussed. An approximate simple expression for the shape function of the broad
vibronic bands with allowance for the terms corresponding to the emission
(absorption) from both ground and first excited vibrational levels is given.
Within this approximation we analyze the room temperature experimental data on the 4f-5d transitions in thiogallates doped with Eu2+ ions.
The vibronic coupling parameters, Stokes shift and nephelauxetic
effect for
systems under study are estimated. The theoretical results are in a good
agreement with the experimental data.
POSL studies on indigenously developed
α-Al2O3:C
M S Kulkarni, D R Mishra, K P Muthe, S K Gupta
& D N Sharma
The pulsed optically stimulated luminescence
(POSL) technique is known for its good signal to noise ratio, fast, and
multiple readout features in radiation dosimetry in general and personnel
monitoring in particular. Apart from this the POSL method also provides the
estimation of the luminescence lifetime t which cannot be measured in CW-, LM OSL techniques. The POSL signal
is recorded on the indigenously developed α-Al2O3:C
using 532 nm diode pumped solid state laser and high intensity blue (470 nm)
light emitting diode cluster for different pulse widths (5–20 μs) and
different repetition rates. The results are also compared with commercially
available single crystal α-Al2O3: C. The
luminescence life time is estimated to be ≈ 35 ms is observed in both the
samples. The POSL studies show the developed sample do not have long decay tail
in time profile of POSL curve indicating suitability of the developed material
for POSL routine dosimetry and qualitative difference of defect formation in
developed sample as compared to those formed in the commercially available
sample .
Keywords: Optically stimulated luminescence, POSL, a-Al2O3:C
Delayed
optically stimulated luminescence of Al2O3:Si,Ti phosphor
P S Page, B C Bhatt, N S Rawat, D R Mishra & M S Kulkarni
In some Al2O3 samples,
the OSL is seen to decay much more slowly that we would expect due to
re-trapping of the released charge in shallow traps. The preliminary studies
indicate that Al2O3:Si,Ti has some component of delayed
optically stimulated luminescence (DOSL). Therefore, DOSL in this system has
been investigated in detail and relative contribution of DOSL to the CW-OSL
signal has been determined. Therefore, OSL readout has been performed at
elevated temperatures (in the range 27-100ºC) in order to eliminate the
contribution of DOSL in the CW-OSL signal. The dependence of DOSL signal on
readout temperature can be explained in terms of Arrhenius equation of
thermally released charges from shallow traps. The total contribution of the
DOSL to the CW-OSL intensity is estimated to be about 1.4%. Although the
contribution of DOSL to the CW-OSL integral signal is much less, it is
advisable that OSL measurements be performed at 75ºC to avoid any contribution
of DOSL signal to CW-OSL signal. The DOSL decay shows the second order
exponential fit. The activation energy calculated for the trap responsible for
DOSL is 0.5 ±0.04 eV.
Electroluminescence
of CdS nanoparticles-polyvinyl carbazole composites
Pooja Chouksey, B P Chandra
& M Ramrakhiani
In the present study thin films of CdS
nanoparticle-polyvinyl carbazole (PVK) composite have been prepared using
chemical method. The absorption spectra and electroluminescence of the films
doped with different concentrations of CdS nanoparticle have been measured. The
absorption of pure PVK film starts at 290 nm wavelength in which a peak appears
at 270 nm, indicating that the optical energy gap of PVK film is 4.26 eV. The
absorption onset of CdS-PVK thin films is obtained at 300 nm, which gives the band
gap of CdS nanoparticles as 4.13 eV. It is observed that the current varies
linearly, whereas EL intensity varies non-linearly with increasing voltage. At
a particular frequency, the emission starts at a particular threshold voltage
and then it increases rapidly with increasing voltage. At a particular
frequency and voltage, the EL intensity of the composite increases with
concentration of CdS nanoparticles in PVK polymer and attains saturation at 5%
CdS concentration. It is shown that the EL of CdS-PVK composites can be
explained on the basis of electron acceleration-collision mechanism.
Swift
heavy ion induced thermoluminescence studies in
polycrystalline aluminum oxide
K R Nagabhushana, B N Lakshminarasappa,
D Revannasiddaiah, D Haranath & Fouran Singh
When energetic swift heavy ions interact
with matter, inelastic collision (leading to electronic energy loss-Se) and elastic collision
(leading to nuclear energy loss -Sn)
take place. In the present study, the effect of energetic ion species on
thermoluminescence (TL) of polycrystalline aluminum oxide (PAO) is reported.
PAO pellets of 6 mm
diameter are irradiated with energetic Au9+, Ni7+ and Si7+
ions for the fluence of 1×1013 ions cm-2. A single well resolved prominent TL glow with
peak at 538 K is observed in Si7+ irradiated samples. However, in
Ni7+ and Au9+ irradiated samples a prominent TL glow with
peak at 610 K along with a shoulder at 513 K is observed. On the other hand,
when PAO samples are irradiated with g-rays two
well separated TL glows with peaks at 483 K and 638 K are observed. A prominent
PL emission with peak at 430 nm besides a weak emission with peak at 480 nm and
a shoulder at 525 nm are observed in 120 MeV Au9+ion irradiated
samples when excited with 320 nm. These PL peaks are attributed to F, F2+
and F22+-centers respectively. However, in Si7+
irradiated samples a single PL emission peak at 430 nm is observed and it is
attributed to F-centers.
Vol.
16, June 2009, pp. 165-168
Synthesis and thermoluminescence studies of Sr4Al14O25:Eu2+
phosphor
Suchinder Sharma, Shreyas Pitale, M M Malik, R N Dubey, M S Qureshi, S K Vasisth
& P Pathakb
Trapping
parameter, the activation energy (E),
the order of kinetics (b) and the
frequency factor (s), are determined
for the solution combustion synthesized Sr4Al14O25:
Eu2+ phosphor using 60Co irradiation source. The glow
curve is a double peak structure with prominent peak at 487 K with shoulder at
423 K. Non-shifting Tm
property is utilized for the trap parameter determination. Glow curve
deconvolution (GCD) is carried out on recorded glow curves after gamma
radiations. The trap depths calculated ranges between 0.5-1.36 eV. The
linearity characteristic of the present phosphor and the fading experiments
suggests the aptness of present phosphor towards dosimetric applications.
Thermoluminescence
dosimetric study of X-ray irradiated BAM:Eu
K V R Murthy, A S Sai Prasad,
B Subba Rao & Louis Rey
Thermoluminescence
of 0.5Gy X-ray irradiated BAM:Eu has been studied. The irradiated phosphor has
been studied for its TL properties after one week of irradiation and after 100
weeks of storage. The TL of BAM recoded after one week of irradiation and after
100 weeks of storage. Before storage for 100 weeks the TL of 0.5Gy X-ray
irradiated phosphor displays a broad TL with a hump around 180oC
followed by a peak at 273°C. After storage for 100 weeks the TL
pattern entirely changed, i.e., the mixed peak emerged as two well resolved and
high intensity peaks at 215 and 300oC. Normally after storage to 100
weeks, the peak at 190oC reduces its intensity or disappears in most
cases, instead of that the peak appears at 215°C is a well resolved and the intensity
is almost to the intensity of 300oC peak. Since the TL phenomenon
observed is interesting. The two well resolved, isolated, high intensity peaks
observed after 100 weeks of storage approves this BAM:Eu can be used as TL
dosimetric material. The photoluminescence (PL) of BAM:Eu excited with 254 nm
given an emission peak at 450 nm.
Keywords:
Thermoluminescence (TL), Photoluminescence
(PL)
Vol.
16, June 2009, pp. 172-174
Energy response study of thermoluminescent dosimeters to
synchrotron
radiation in the energy range 10-35 keV
A K Bakshi, B S Dhabekar, S Chatterjee,
V J Joshi & R K Kher
LiF:Mg,Ti,
LiF:Mg,Cu,P and CaSO4:Dy TLDs are worldwide used in the field of
medical dosimetry, personnel monitoring and environmental monitoring. In view
of the wide spread applications of synchrotron radiation for material research
and diagnostic radiology in particular medical imaging, the present study of
the energy response of some popular TLDs was undertaken. For the energy
response study the most useful energy range 10-35 keV of synchrotron radiation
was chosen and the present experimental work was carried out at SYRMEP beam
line of Elettra Synchrotron Accelerator facility, Bassoviza, Trieste, Italy.
The LiF based TLDs in the form of chips and disc, and CaSO4:Dy in
the form of disc of thicknesses 0.8 mm and 0.4 mm made up of mixture of CaSO4:Dy
phosphor and Teflon in the weight ratio 1:3 were used. The energy of the
synchrotron radiation was calibrated using a Si (111) double crystal
monochromator and several thin foils
starting from Zn with Kedge
9.659 keV to KI compound with Kedge
33.169 keV. The energy response of the TLDs were normalized with respect its
response to 60Co and corresponding relative response was evaluated.
It was found that for CaSO4:Dy discs the relative response is about
12 at 25 keV which is maximum and subsequently falls down on either side of
energy for both 0.4 mm and 0.8 mm thick discs. The relative response at 10 keV
was found to be about 4.5 and 6 for 0.8 mm and 0.4 mm thick discs of CaSO4:Dy
respectively. Maximum relative response of LiF:Mg,Cu,P chip of thickness 0.4 mm
was found to be about 1.26 at 25 keV and it also exhibited lower response on
either side of energy. LiF:Mg,Ti chips and discs show maximum response at the
30 keV which were 1.33 and 1.39, respectively. At 10 keV which was the lowest
energy used in the present study, the relative response was always found to be
less than 1 for LiF based TLDs. The relative response of the TLDs evaluated in
the present experiment was compared with the published value in the literature
and also confirmed through theoretical calculation using ratio of mass
absorption coefficient of these materials with respect to air.
Keywords: Synchrotron radiation; energy
response; TLDs;
Preparation of Eu3+ doped Y2O3
and core-shell Y2O3:Eu-Y2O3
nanoparticles: Photoluminescence study
Dorendrajit Singh & L Robindro Singh
Nanocrystalline of Y2O3
doped with Eu3+ at different concentrations and also core-shell of Y2O3
is prepared by wet chemical method using urea hydrolysis at low temperatures.
XRD, TEM and FTIR are used to characterize the prepared nanoparticles. The
sizes determined by XRD are well in agreement with TEM results. As-prepared
samples show luminescence peak
of Eu3+
transition 5D0 à 7F2
at 615 nm but annealed samples at 610 nm. Core-shell study of the nanoparticles
show reduction in the luminescence intensity compared to the core particles.
Luminescence studies during combustion synthesis of a long afterglow
phosphor Sr4Al14O25:Eu2+, Dy3+
N Suriyamurthy & B S Panigrahi
The long glow persistent
phosphor Sr4Al14O25:Eu2+, Dy3+
has been synthesized through combustion route. SrAl4O7
and SrAl2O4
are identified as the intermediate products formed at different temperatures
during the synthesis of Sr4Al14O25:Eu2+,Dy3+.
The photoluminescence and afterglow emission of the intermediate products have
been systematically studied. The photoluminescence and afterglow emission
profiles of the intermediate products varied with the temperature of synthesis.
For a sample with similar aluminate phases (at a particular synthesis
temperature) the λAGL and λPL are different.
However, for Sr4Al14O25:Eu2+,Dy3+
annealed at 1200-1300ºC, the AGL and PL emissions are
similar.
Eu3+ luminescence in Na3SO4F
and NaMgSO4F halosulphate phosphor for mercury-free lamps
I M Nagpure, S J Dhoble, S V Godbole, M K
Bhide & R B Pode
Polycrystalline Na3SO4F:Eu
and NaMgSO4F:Eu phosphors prepared by a wet chemical method has been
studied for its photoluminescence (PL) characteristics. PL emission spectra of Na3SO4F and NaMgSO4F,
phosphors under 393 nm excitation (mercury-free
excitation) shows very strong prominet peak at 591 nm due to 5Do
à7F1 transition and 613 nm
corresponding to 5Do à7F2
transition in the red region of the spectrum. The PL emission spectra of Eu3+ ion in the
Na3SO4F:Eu and NaMgSO4F:Eu phosphors are
applicable for mercury free lamp industries.
Role of SiO2
in Zn2SiO4:Mn2+ phosphor used
in optoelectronic materials
M C Parmar，W D Zhuang, K V R Murthy, X W Huang, Y S Hu & V Natarajan
Zn2-xMnxSiO4
is a classical green phosphor, and it has been used in many applications, such
as CCFL, CRT, PDP etc. In this paper, Zn2SiO4:Mn2+
green phosphor is fabricated with the solid-state reaction method.
Photoluminescence and crystalline properties are examined as functions of the
firing temperatures. Zn2-xMnxSiO4 is ground
and fired at 1200oC for 4 h in air, but intensity of emission is
weak. Therefore, the conditions of firing are changed and Zn2-xMnxSiO4
are ground and fired at 1000oC for 2 h in air and again ground and
fired at 1200 oC for 4 h in air respectively. When varies SiO2
value from 0.9 to 1.8 however the ZnO is 1.92 as well as MnCO3 is
0.08 are constant, The SiO2 concentration (1.1) show good PL
spectra. The PL peak observed at 524 nm in these phosphors suggests their
potential as green emitting phosphors under 254 nm excitation.
Keywords: Phosphors, X-ray method,
Photoluminescence, Zn2SiO4-Mn2+
Synthesis
and optical properties of CdTe nanocrystals with
improved optical properties
Kiran Jain, Vibha Srivastava & Abhilasha Chouksey
CdTe nanocrystals are prepared by
hydrothermal route using the reaction between Cd2+ and KHTe in the
presence of thioglycolic acid as the stabilizing agent. Hydrothermal synthesis
under optimum conditions resulted in a rapid growth. CdTe nanocrystals with
high PL intensity and narrow PL spectra are obtained in less time. The growth
mechanism of CdTe nanocrystals is investigated. The growth rate in the initial
stage of synthesis is higher in hydrothermal synthesis resulting in an
increased growth in diffusion controlled focused region.
Keywords: CdTe, Nanostructures, Hydrothermal,
Luminescence
Spectroscopic
characteristics of Sm3+ doped alkaline earth potassium titanium
phosphate glasses
D V R Moorthy, M Jayasimhadri, Kiwan Jang, J Suresh Kumar, A
Mohan Babu & L Rama Moorthy
The paper reports on the preparation and
characterization of Sm3+ doped alkaline earth potassium titanium
phosphate (RKTP) glasses with molar composition of 24(NaPO3)6
+ 30KH2PO4 + 25TiO2 + 20RCl2 + 1Sm2O3
(R=Mg, Ca & Sr). Optical absorption spectra in the UV-VIS-NIR region reveal
twelve absorption bands and are assigned to different transitions arising from
the 6H5/2 ground state. The energy level structure of Sm3+
(4f5) has been analyzed using the free-ion Hamiltonian model. The
root mean square deviations between the experimental and calculated energies of
Sm3+ doped MgKTP, CaKTP and SrKTP glasses are 44, 35 and 56 cm-1
respectively and are comparable with the earlier reports. By measuring the
areas under the absorption peaks, the experimental oscillator strengths are
calculated and the phenomenological JO-intensity parameters (Ω2,
Ω4 and Ω6) have been determined with the help
of least-squares fitting approach. These parameters exhibit a similar trend as
Ω2>Ω6>Ω4 in all the three
glasses. The evaluated JO parameters are then used to calculate the excited 4G5/2
level radiative properties such as radiative transition probabilities (AR), branching ratios (βR)
and lifetimes (τR). The emission spectra recorded by exciting
with 476 nm shows four peaks at 563, 599, 649 and 714 nm and are assigned to 4G5/2
→ 6H5/2, 6H7/2, 6H9/2,
6H11/2 respectively. The experimentally observed
decay profiles of the 4F5/2 level of Sm3+ are
found to be single exponential yielding the lifetimes (τmes)
2330, 1750 and 2320 μs in MgKTP, CaKTP and SrKTP glasses respectively,
which are comparable with the calculated lifetimes (τR)
obtained from the Judd-Ofelt theory.
Effect of
pre-thermal treatment on the OSL response of Al2O3:Si,Ti
B C Bhatt, P S Page, N S Rawat, D R Mishra
& M S Kulkarni
We have recently
reported optically stimulated luminescence (OSL) characteristics of the Al2O3:Si,Ti
phosphor along with some thermoluminescence (TL) and photoluminescence (PL)
studies. The TL glow curve of this phosphor shows peaks around 158, 191, 276
and 355ºC for a heating rate of 4 Ks-1. In order to investigate the contribution of
individual TL traps to OSL signal, CW-OSL was studied as a function of
pre-thermal treatment in the temperature range 27-450ºC. The integrated CW-OSL
intensity of the phosphor remains constant up to 100ºC; beyond this temperature
the integrated intensity goes on decreasing with increase in temperature. After
a pre-thermal treatment at 225ºC the CW-OSL intensity falls by 73% of its
initial value. Hence, it can be concluded that the TL traps up to 225ºC are
responsible for the major contribution to the CW-OSL signal in this phosphor.
The OSL indicates that all the TL traps, up to 400ºC, give measurable
contribution to the OSL signal. The OSL signal becomes insignificant when the
exposed phosphor is annealed beyond 400ºC. It decreases nearly to zero for the
sample annealed at 400ºC as compared to the OSL intensity of the sample without
any pre-thermal treatment. Optical bleaching results indicate that all the TL
peaks decrease continuously as a function of bleaching time.
