Luminescence enhancement of (Ca,Zn)TiO3:Pr3+ phosphor using nanosized silica powder

Article abstract of DOI:10.1063/1.2338789

The results pertaining to luminescence enhancement due to high-temperature amalgamation of nanometer-sized silica powder and the oxide mixture of calcium, zinc, titanium, and praseodymium are presented. The (Ca,Zn)TiO ₃:Pr³⁺ phosphor showed an intense and narrow band emission at ～614 nm under ambient light/ultraviolet excitation, which is associated with the typical ³H₄↑ ¹D₂ transition of Pr³⁺ The observed red emission is quite stable and long (～60 min) for a dark-adapted human eye and shows no appreciable degradation in the optical properties for prolonged exposure to surrounding/atmosphere. With the addition of nanosized silica powder, the photoluminescence was augmented nearly 1.6 times as that of unadded sample. The present results are entirely different from those reported previously for Pr³⁺ doped-(Ca,Zn)TiO₃ phosphor and silica-based persistent phosphors.

Full text of DOI:10.1063/1.2338789

Luminescence enhancement of ( Ca , Zn ) Ti O 3 : Pr 3 + phosphor using nanosized
silica powder
D. Haranath, A. F. Khan, and Harish Chander
Citation: Applied Physics Letters 89, 091903 (2006); doi: 10.1063/1.2338789
View online: http://dx.doi.org/10.1063/1.2338789
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/9?ver=pdfcov
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APPLIED PHYSICS LETTERS 89, 091903 ͑2006͒
3
+
Luminescence enhancement of „Ca,Zn…TiO :Pr phosphor using
3
nanosized silica powder
a͒
D. Haranath, A. F. Khan, and Harish Chander
National Physical Laboratory, New Delhi 110012, India
͑
Received 1 May 2006; accepted 3 July 2006; published online 28 August 2006͒
The results pertaining to luminescence enhancement due to high-temperature amalgamation of
nanometer-sized silica powder and the oxide mixture of calcium, zinc, titanium, and praseodymium
3
+
are presented. The ͑Ca,Zn͒TiO :Pr phosphor showed an intense and narrow band emission at
3
ϳ614 nm under ambient light/ultraviolet excitation, which is associated with the typical
3
1
3+
H → D transition of Pr . The observed red emission is quite stable and long ͑ϳ60 min͒ for a
4
2
dark-adapted human eye and shows no appreciable degradation in the optical properties for
prolonged exposure to surrounding/atmosphere. With the addition of nanosized silica powder, the
photoluminescence was augmented nearly 1.6 times as that of unadded sample. The present results
are entirely different from those reported previously for Pr doped-͑Ca,Zn͒TiO phosphor and
silica-based persistent phosphors. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2338789͔
3
+
3
The achievement of intense red-emitting phosphors is
one of the challenging goals in the field of display and light-
ing technologies. Considerable brightness with longer per-
and polycondensation of tetraethylorthosilicate ͑TEOS͒. The
nanosized silica powder is then mixed well with the slurry,
dried, crushed, and fired at ϳ1500 °C for 3–5 h in air,
which resulted to the desired phosphor.
1
–3
sistent time ͑afterglow͒ is of utmost importance for designing
4
5
dark-vision display devices, emergency route markings,
Figure 1͑a͒ shows the x-ray diffraction ͑XRD͒ profile
6
warning signboards, etc., and above all, possibilities of en-
ergy savings can be addressed. Recently, highly emissive
red-emitting novel phosphors prepared from a variety of per-
corresponding to orthorhombic form of CaTiO with weak
3
intensity peaks of hexagonal ZnO. It is clearly seen that the
addition of amorphous powder of nanosized silica ͑SiO2͒ gel
did not contribute for XRD. No sharp peaks were observed
in the XRD pattern, confirming the absence of Si- and
SiO2-related crystalline phases. The scanning electron micro-
scope ͑SEM͒ image of the phosphor involving stoichiometric
amount of nanosized SiO2 powder resulted in grains with
definite shapes and sizes ͓Fig. 1͑b͔͒. It is also observed that
there is a critical grain size effect at which the emission
brightness is most intense. A melting morphology associated
7
–10
voskite titanates have been reported.
In these materials
intentionally doped rare-earth ions are believed to be respon-
sible for the red photoluminescence. It has also been reported
that some of the rare-earth-doped oxysulfide phosphor exhib-
its orange-red emissions with Ͼ120 min decay of dark per-
sistence. However, persistent luminescence corresponding to
͑
Commission Internationale de I’Eclairage͒ ͑CIE͒ ideal red is
not significantly explored. Recently, we have developed an
3
+
with nano-SiO powder over the surface of the grains is also
optically efficient red-emitting CaTiO :Pr phosphor with
2
3
seen in the SEM image. Figure 2 shows the room-
the addition of stoichiometric amounts of ZnO/MgO, suit-
11
temperature photoluminescence ͑PL͒ spectra of intrinsic
able flux, and charge compensator. This phosphor exhibits
an appreciable dark persistence time of ϳ10 min under
room-light excitation. However, for any practical application
Ͼ30 min dark persistence ͑afterglow͒ is the fundamental
obligatory of lighting technologies. To circumvent the above
requisite, in this letter, we present a simple and unique meth-
odology of producing phosphors that emit light not only near
the CIE ideal red conditions but also a stable afterglow for
nearly 60 min upon ambient light excitation. This type of
luminescence augmentation has not been reported so far due
to the addition of nanosized silica powder to
3
+
SiO powder and ͑Ca,Zn͒TiO :Pr phosphor ͑in short we
2
3
call it as CZT:Pr͒ in the presence and absence of nanosized
SiO powder, respectively. Due to the band gap of CaTiO
2
3
3
+
͑
Ca,Zn͒TiO :Pr phosphor.
3
Wet mixing of the reactants, namely, calcium carbonate
͑
CaCO ͒, titanium dioxide ͑TiO ͒, zinc oxide ͑ZnO͒, and
3
2
praseodymium oxide ͑Pr O ͒, was done with de-ionized wa-
6
11
ter as solvent and a thick slurry was made. A calculated
amount of ammonium borofluoride ͑NH BF ͒ was added as
4
4
flux. Nanosized amorphous silica ͑SiO ͒ powder was pre-
2
pared separately using sol-gel method as per the procedure
1
2
suggested by Wagh et al., which is the result of hydrolysis
FIG. 1. ͑a͒ XRD pattern of ͑Ca,Zn͒TiO3 ͑CZT͒ host lattice and ͑b͒ SEM
3
+
image ͑at magnification of 1000ϫ͒ of Pr doped-CZT phosphor prepared
a͒Electronic mail: haranath@nplindia.ernet.in
with a stoichiometric addition of nanosized SiO2.
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003-6951/2006/89͑9͒/091903/3/$23.00 89, 091903-1 © 2006 American Institute of Physics
58.125.224.133 On: Tue, 25 Nov 2014 15:09:30
0
1

091903-2
Haranath, Khan, and Chander
Appl. Phys. Lett. 89, 091903 ͑2006͒
FIG. 2. Room-temperature photoluminescence ͑PL͒ spectra of
3
+
͑
Ca,Zn͒TiO :Pr phosphor ͑a͒ with and ͑b͒ without addition of nanosized
3
3
+
FIG. 3. Relative afterglow intensities of ͑Ca,Zn͒TiO :Pr phosphor ͑a͒
3
SiO2 powder. Inset shows the PL spectrum of intrinsic SiO2 powder re-
corded at 328 nm excitation.
with and ͑b͒ without addition of nanosized SiO2 powder. Inset shows the
mechanism of persistent luminescence.
host lattice and excitonic broadband in the range of
intrinsic silica powder recorded at 328 nm. It is observed that
UV excitation results in low intensity PL band ranging from
300–350 nm is observed in the excitation spectrum with a
maximum at 328 nm. The emission corresponding to this
excitation consisting of a single narrow band ͑full width at
half maximum of 45 eV at RT͒ peaking at 614 nm is ob-
served for both the samples prepared with and without nano-
4
00 to 700 nm. It has also been found that the observed PL
spectral feature is hardly dependent on the wavelength
16,18
͑
200–400 nm͒ of the UV excitation. Glinka et al.
sug-
gested that PL emission band is mainly attributed to non-
bridging oxygen hole centers and hydrogen related species
present on the surface of nanosized silica particles. Since the
silica powder has a weak absorption feature below 350 nm, it
is quite likely that the observed weak PL is attributed to the
one-photon excitation process from the midgap states asso-
ciated with the absorption, which is supposed to be created
during the sintering process.
sized SiO . The only difference is that with the addition of
2
nanosized SiO powder, the emission intensity is enhanced
2
3
+
nearly by 1.6 times. A trace doping of Pr indicates weak
absorption peaks in the range of 500–550 nm due to the
3
+
3
transition of Pr from ground state to the excited state P
J
͑
J=0,1,2͒. The characteristic emission of CZT:Pr phosphor
4
1
3+
is associated with the typical H → D transition of Pr ,
4
2
which provides an intense and narrow band emission at
14 nm. This unique red emission may offer the opportunity
to exploit novel field emission display ͑FED͒ and plasma
Figure 3 shows the room-temperature long afterglow
curves of CZT:Pr phosphors produced with and without in-
6
corporating nanosized SiO . It is interesting to note that these
2
1
3
display panel ͑PDP͒-based devices. It is also well docu-
mented that materials fired at high temperatures cannot be
defect-free. The formation of defects such as negatively
charged calcium vacancies, positively charged oxygen va-
phosphors are readily excited by ambient/or room light for
even less than 5–10 min. No appreciable photodegradation
has been observed for these samples on repeated usage/
exposure involving dark-vision applications. A distinct after-
cancies, and/or reduction of Ti4 to Ti , etc., is more likely
+
3+
19
glow mechanism proposed by Aitasalo et al., which is quite
1
4
to be observed at the surface of the CZT grains. The pres-
ence of these defects even at low concentrations can contrib-
ute to quenching of Pr3 luminescence. The role of nanosized
silica powder is identified to minimize the concentration of
the undesirable defects, by melting over the particles and
finally passivating the surface of CZT grains. This conse-
quently improved the optical performance of CZT:Pr phos-
phor in terms of augmented luminous brightness and emis-
applicable in the present system, is shown in the inset of Fig.
3. According to this, when a CZT:Pr phosphor is kept under
excitation, the energy is provided either directly to the traps
or via the conduction band and the traps are filled. The elec-
trons are thermally bleached at room temperature and con-
tribute to the electron hole recombination process. The non-
+
3
+
radiative transfer of energy results in the excitation of Pr
luminescent center and the emission results from the typical
1
5
16
3
1
sion color very close to CIE ideal red region. Glinka et al.
H → D transition. The whole process may require a close
4
2
3
+
previously reported that silica exhibits some PL bands in the
visible region under UV excitation. However, the obtained
PL spectral feature is strongly dependent on the amount and
type of residual surface OH- and H-related species on the
surface of nanosized silica powder. The concentration of the
residual OH group was rather low ͑ϳ200–400 ppm͒, which
was determined by the infrared absorption of the OH-
contact between the different defect centers and the Pr lu-
minescent center, since the conduction band is located too
high to be used for electron migration, Hence, the mecha-
nism of transfer of energy to the Pr persistent luminescent
center can be treated as that of storage phenomena observed
in wide band gap semiconductors materials and in most of
3
+
2
0
the photostimulated phosphors.
1
7
stretching bond. This indicates that most of the OH groups
at the surface of the original nanosized silica particles were
dehydroxylated during the sintering process and formed a
transparent glassy layer over the surface of the CZT:Pr phos-
In summary, we have shown that CZT:Pr phosphor pre-
pared with nanometer-sized silica particles exhibits a robust
red PL emission under ambient/room-light excitation. Al-
though the physical origin of strong red PL emission is
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phor grains. The inset of Fig. 2 shows the PL spectrum of
mainly due to the efficient energy transfer from band gap
158.125.224.133 On: Tue, 25 Nov 2014 15:09:30

091903-3
Haranath, Khan, and Chander
Appl. Phys. Lett. 89, 091903 ͑2006͒
7
8
9
0
absorption of CZT lattice to Pr3+ ions, the escalation of PL
and afterglow times due to the addition of nanosized silica
powder obtained in this work will surely open up novel prac-
tical applications in the field of dark-vision products and
display technologies.
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