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Investigations into the structural and down-shifting and up-conversion luminescence
properties of Ba2Na1−3x Er x Nb5O15 (0 ≤ x ≤ 0.06) nanocrystalline phosphor synthesized
via sol-gel route
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2015 Mater. Res. Express 2 105015
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Mater. Res. Express 2 (2015) 105015
doi:10.1088/2053-1591/2/10/105015
PAPER
RECEIVED
15 July 2015
REVISED
23 September 2015
ACCEPTED FOR PUBLICATION
1 October 2015
Investigations into the structural and down-shifting and upconversion luminescence properties of Ba2Na1−3xErxNb5O15
(0„x„0.06) nanocrystalline phosphor synthesized via sol-gel
route
PUBLISHED
27 October 2015
Swarup Kundu1,4, Rajasekhar Bhimireddi1,4, Kavita Mishra2, S B Rai3 and K B R Varma1
1
2
3
4
Materials Research Centre, Indian Institute of Science, Bangalore-560 012, India
Department of Physics, University of Lucknow, Lucknow-226007, India
Department of Physics, Banaras Hindu University, Varanasi-221005, India
These authors contributed equally to this work
E-mail: [email protected]
Keywords: nanocrystals, photoluminescence, up-conversion emission
Abstract
The present work deals with the structural and efficient down-shifting (DS) and up-conversion (UC)
luminescence properties of erbium ion (Er3+) doped nanocrystalline barium sodium niobate (Ba2Na1
−3xErxNb5O15, where x=0, 0.02, 0.04 and 0.06) powders synthesized via novel citrate-based sol-gel
route. The monophasic nature of the title compound was confirmed via x-ray powder diffraction
followed by FT-IR studies. High-resolution transmission electron microscopy (HRTEM) facilitated
the establishment of the nanocrystalline phase and the morphology of the crystallites. The Kubelka–
Munk function, based on diffused reflectance studies and carried out on nano-sized crystallites, was
employed to obtain the optical band-gap. The synthesized nanophosphor showed efficient DS/PLphotoluminescence and UC luminescence properties, which have not yet been reported so far in this
material. The material emits intense DS green emission on excitation with 378 nm radiation.
Interestingly, the material gives intense UC emission in the visible region dominated by green
emission and relatively weak red emission on 976 nm excitation (NIR laser excitation). Such a dualmode emitting nanophosphor could be very useful in display devices and for many other applications.
Introduction
Functional materials at nanoscale are of prime importance as the science and technology associated with them
are intriguing. Therefore, researchers around the globe have been directing their efforts towards the synthesis of
materials in nanodimensions using a variety of routes and studying their physical properties from the viewpoint
of exploiting them in devices that include optoelectronic, photonic, displays, etc. Therefore, nanotechnology is a
fast-growing area of interest for researchers, involving the fabrication and use of nano-sized materials and
devices. Nanocrystallites doped with rare earth (RE) ions have been of technological interest as these affect their
optical [1–4], electrical properties [5–7] etc. The luminescence properties of these materials are of industrial
importance [8–11]. The crystallite size-dependent physical properties, especially the optical property, are of
paramount importance as these facilitate miniaturization of devices which include digital displays, light emitting
diodes etc.
Trivalent rare earth (RE) ions have been extensively used as luminescent centres because of their unique
optical features, e.g. narrow emission bandwidth, long lifetime of excited states, good chemical durability and
large emission cross-section encompassing a wide range of spectra in the UV–Vis–NIR regions. The energy
levels in some of these are suitable for direct pumping by UV/Vis or NIR diode lasers [12–15]. The dense energy
levels in these permit one to observe the dual-mode emissions, i.e. DS-downshifting and UC-up-conversion
luminescence properties [16]. DS is defined as a normal photoluminescence process which involves
© 2015 IOP Publishing Ltd
Mater. Res. Express 2 (2015) 105015
S Kundu et al
transformation of one absorbed high-energy photon into a lower-energy photon, whereas UC is a non-linear
process in which low energy photons are used to generate high energy photons, usually using a visible/infrared
excitation source to get a visible photon [17, 18]. However, selection of appropriate host material to visualize
efficient luminescence is a crucial step. Recently, up-conversion of light in Er3+ doped nanocrystalline BaTiO3
with conversion efficiency higher than that of TiO2 was reported by Patra et al [19].
Another interesting class of materials is the tungsten bronze family of compounds. Barium sodium niobate,
Ba2NaNb5O15 (BNN), belongs to the tungsten bronze family associated with the general formula
(A1)4(A2)2B10O30, where A1 is fifteen coordinated, A2 is twelve coordinated and B is six coordinated atoms. BNN
belongs to the orthorhombic crystal structure (space group Pba2) at room temperature. It is an interesting
material from the research point of view as it finds applications in electro-optic [20], non-linear optic [21, 22],
acousto-optic [23, 24], and piezoelectric [25] based devices etc. However, to date this material has not been fully
exploited for industrial applications, despite the fact that it possesses promising physical properties. This might
be due to the problems that one faces in obtaining crack-free single crystals [22]. In addition, intense second
harmonic generation in several RE ion–doped BNN single crystals has been reported by Yoshikawa et al [26].
Based on spectroscopic studies, it was reported that the rare earth ions (Nd3+ and Yb3+) could occupy Na+ or
Ba2+ sites but not Nb5+ sites in BNN crystals [27]. Based on ionic considerations, it was argued that these ions
primarily occupy the 12-fold coordinated Na+ sites with a remote possibility of occupying 9-fold coordinated
Ba2+ sites. Indeed, several single crystals corresponding to the formula Ba2Na1–3xRExNb5O15 were grown by the
Czochralski technique at 1700 K and these were reported to be monophasic and free from macroscopic defects,
(up to x=0.06). The physical quality of the grown crystals was found to be not adequate for device applications
and the growth process became complex for the compositions corresponding to x<0.06 [27, 28].
Recently, our group successfully synthesized BNN nano-crystallites embedded in glass matrices and nanopolycrystalline materials by a novel sol-gel synthesis technique and demonstrated the potentialities of these for
photoluminescence (PL) applications under ultraviolet excitation [29, 30]. The authors have also made a
successful attempt to visualize the effect of the RE ion (Er3+) doping on the PL characteristics of BNN
nanocrystals. To the best of our knowledge, no literature exists on erbium-doped BNN polycrystalline samples
and their physical properties probably because of the experimental limitations associated with the synthesis
technique in obtaining chemically homogeneous polycrystalline powders. Therefore, an attempt was made to
synthesize fine powders of BNN via the soft-chemistry route as this route guarantees chemical homogeneity,
high purity, and uniform grain size due to molecular level mixing, besides providing an opportunity to study
their physical properties as a function of crystallite size etc. In this article, we report the details concerning the
synthesis of nanocrystalline powders of Er3+ doped BNN by citrate-assisted sol-gel method. These powders were
subjected to structural, optical and luminescence studies at room temperature; the details are reported in the
following sections. The synthesized nanophosphor shows efficient dual-mode luminescence properties (DS and
UC) under UV and NIR excitation, which has not been reported so far in this material. Such a dual-mode
emitting nanophosphor could be very useful in display devices and for many other applications.
Experimental section
Material synthesis
The precursors employed to synthesize Ba2Na1–3xRExNb5O15 (henceforth denoted as BNN-Erx) nanocrystalline
powders were ammonium niobate (V) oxalate pentahydrate (99%, H. C. Starck GmbH), barium nitrate (99%,
Sigma-Aldrich), sodium acetate (99%, Fisher Scientific Co.), erbium (III) oxide (99.9%, Sigma-Aldrich),
anhydrous citric acid (99.5%, SDFCL), hydrogen peroxide (H2O2, 30% w/v, SDFCL), ammonia solution
(SDFCL) and nitric acid. Erbium doped and undoped BNN nanocrystalline powders were synthesized using the
citrate-assisted sol-gel method. The process began with the synthesis of the peroxocitro-niobium precursor
solution using a method similar to that reported by earlier researchers [31]. For this, an appropriate amount of
citric acid was dissolved in 30 ml of H2O2 on constant stirring. After 30 min, the appropriate amount of
ammonium niobate (V) oxalate pentahydrate was added to the above solution and again subjected to vigorous
stirring for about an hour at 343 K, which yielded a clear yellow colour solution. The pH of the solution was
adjusted to 6.5 to 7 by adding drop by drop aqueous ammonia. Appropriate amounts of barium nitrate, sodium
acetate and citric acid were dissolved in 25 ml of deionized water in a separate beaker and added to the above
solution slowly. The desired amounts of erbium oxide were dissolved in concentrated nitric acid separately and
added to the above solution under continuous stirring. In order to maintain the pH of the solution in the range
of 6.5–7.0, ammonia solution was added to it. The temperature was raised to 403 K at which point water started
evaporating and a viscous gel was formed. The temperature was further raised to 573 K, and at this stage the gel
started burning, leaving behind a residue. This residue was subjected to different stages of heat-treatment for
further analysis to identify the different phases that evolve at various stages of heating.
2
Mater. Res. Express 2 (2015) 105015
S Kundu et al
Figure 1. Thermo-gravimetric traces obtained (at the heating rate of 10 K min−1) for the BNN-Erx (x=0.02, 0.04 and 0.06) precursor
in N2 ambience.
Material characterization
To identify the BNN-Erx nanocrystalline phase formation, x-ray powder diffraction studies were carried out on
the samples heat treated at different temperatures using a PANalytical x-ray diffractometer in the 2θ range of
10–80° with CuKα (1.5408 Å) radiation. Crystallite size distribution and its morphology in the synthesized
powder were studied using scanning electron microscope (FEI Inspect F50). The recorded SEM images were
analyzed using the Sigma Scan Pro 4.0 software package developed by Jandel Scientific to find out the statistical
size distribution of the crystallites. For image analysis, the well-separated crystallites were marked individually
with a pixel-calibrated scale bar and then a statistical average was calculated. Transmission electron microscopy
(TEM) was used to confirm the crystallinity and structure. The TEM images were recorded with the aid of a JEOL
JEM-2100F at an accelerating voltage of 200 kV.
The FT-IR spectra were recorded using a Perkin–Elmer (model: Spectrum 1000) spectrometer in the range
of 400–4000 cm−1. For this, the powdered sample was mixed with KBr and pelletized. Thermo-gravimetric (TG)
studies were carried out on the as-prepared powders in the temperature range of 300–1020 K using a
commercial TA-instrument, a TG-DTA thermal analyzer, at a heating rate of 10 K min−1. The diffuse
reflectance spectra were recorded for powders using a UV–Vis Perkin–Elmer Lambda 750 instrument. The
Kubelka–Munk function was used to calculate the band gap. Photoluminescence excitation (PLE) and emission
(PL) measurements were performed using a Fluorolog-3 spectrofluorometer (model: FL3−11, Horiba Jobin
Yvon) equipped with 450 W xenon flash lamp, whereas the decay curve measurement was performed using a
pulsed xenon lamp (25 W) attached to the same spectrofluorometer. The up-conversion emission
measurements were carried out by exciting the samples with a 976 nm radiation from a diode laser (continuous
mode, model-III 980, Chengchun New Industries Optoelectronics Tech. Co. Ltd). For decay time measurement
in the case of UC, data were acquired using a 976 nm laser light, a monochromator and an oscilloscope
(analogue digitalscope, HM1507) and software (SP107).
Results and discussion
Thermal behaviour
The thermo-gravimetric traces obtained for the as-synthesized precursor powder of BNN-Erx (x=0.02–0.06) is
shown in figure 1. About 57% weight loss was observed during the temperature sweep up to 1223 K for BNNErx=0.02; subsequently the weight loss became insignificant. The weight loss from room temperature to 500 K is
attributed to the desorption of water and the subsequent weight loss is assigned to the pyrolysis of citric acid and
complex oxalates. This pyrolysis behaviour is similar to that observed in the case of undoped barium sodium
niobate (BNN) via the sol-gel method [29].
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Mater. Res. Express 2 (2015) 105015
S Kundu et al
Figure 2. Fourier transformed infrared (FT-IR) spectra recorded at room temperature for the precursor and other samples heattreated at various temperatures.
FT-IR analysis
The BNN-Erx phase formation was confirmed by analyzing the Fourier transform infrared (FT-IR) spectra
recorded at room temperature. Figures 2(a)–(f) depict the FT-IR spectra of the precursor powder and the other
samples heat-treated at different temperatures. The broad band (3450 cm−1) observed in the spectra recorded
for the precursor powder of BNN-Erx=0.02 corresponds to the stretching vibration of water and the other
hydroxyl groups. The band at 1636 cm−1 is due to the angular deformation of the -OH group present in water
molecules. The band at 2920 cm−1 represents aliphatic C–H vibrations present in complex precursor. The broad
band at 1630 cm−1 is assigned to C=O vibrations of carboxylates. The broad bands observed at 3160 cm−1 and
3028 cm−1 are ascribed to ν3(N–H) and ν1(N–H) vibrations respectively [31]. The sharp band observed around
2340 cm−1 is due to the asymmetric stretching mode of carbon dioxide present/trapped in the samples. With
thermal treatment, the high energy band at 3450 cm−1 shifts and the intensity diminishes with dehydroxylation. The FT-IR spectra (figure 2(d)) corresponding to the BNN-Erx=0.02 precursor heat-treated at
1023 K shows only a single broad peak in the 450–830 cm−1 range and centred around 620 cm−1 which is due to
the metal–oxygen (Nb–O) vibrations. The absence of all other absorption peaks indicates that at this
temperature there is no residue of hydroxyl and organic groups left confirming the complete decomposition of
the precursor. The bands observed at 900, 655 and 570 cm−1 are assigned to Nb=O in highly distorted NbO6
octahedra, symmetric stretching of the niobia polyhedra and ν(Nb–O) present in slightly distorted NbO6
octahedra respectively [32]. Figures 2(e) and (f) represent the FT-IR spectra of the BNN-Er x=0.04 and
x=0.06 samples respectively and these evidently confirm the total decomposition of the complex metal–
organic precursors.
X-ray powder diffraction studies
The phase purity and crystal structure of all the samples were identified by x-ray powder diffraction analyses.
Figure 3 depicts the x-ray powder diffraction patterns of Ba2Na1–3xErxNb5O15 (x=0.02, 0.04, 0.06) phosphors
heat-treated at different temperatures. All the diffraction peaks encountered in these patterns were indexed to
the polycrystalline BNN phase as reported in the ICSD PDF no. 01-086-0739. The x-ray pattern obtained for the
precursor powder is amorphous in nature. As the heat-treatment temperature is increased the diffraction peaks
corresponding to the BNN phase evolved. However, the samples subjected to lower calcination temperatures
showed low intensity impurity peaks. However, these peaks disappeared completely on increasing the
calcination temperature suggesting the incorporation of Er3+ in the BNN lattice.
Transmission electron microscopy
A TEM micrograph and selected area electron diffraction (SAED) pattern were recorded to analyze the
crystallinity and crystallite morphology of the compound (x=0.02) heat-treated at 1023 K/2h. The bright field
image (figure 4(a)) clearly shows the presence of agglomeration of the nanocrystalline powder associated with
4
Mater. Res. Express 2 (2015) 105015
S Kundu et al
Figure 3. X-ray powder diffraction patterns recorded at room temperature for BNN-Erx samples.
Figure 4. (a) Bright field low magnification TEM image for the sample BNN-Erx=0.02, (b) HRTEM micrograph of the same sample, the
left inset depicts the SAED pattern and the right inset represents the magnified lattice fringes.
rod-shaped morphology. The lattice fringes in the high-resolution transmission electron microscopy (HRTEM)
image (figure 4(b)) revealed good crystallinity of the sample and also the spot SAED pattern (left inset of
figure 4(b)) endorsed the polycrystalline nature of the sample. A zoomed-in portion of the HRTEM
micrograph is shown in the inset of figure 4(b), confirming the lattice spacing to be 3.9 Å, close to the c-lattice
parameter of the BNN unit cell. These indeed corroborate the XRD data obtained for the samples in the present
investigations.
Scanning electron microscopic studies
To visualize the crystallite and its morphology, SEM image analysis was done on the calcined samples. The
secondary electron images along with the crystallite size distribution are shown in figure 5. The representative
morphology of BNN-Erx=0.02 calcined at 1023 K/2h (figure 5(a)) shows a typical elongated microstructure
having average length and width of 75 nm and 39 nm respectively. As the calcination temperature is increased
the dimensions of the crystallites are increased. There were agglomerated regions in the samples. Analysis of the
crystallite size (length and width) is shown in the inset of each micrograph and the statistical mean and standard
deviations are listed in table 1.
5
Mater. Res. Express 2 (2015) 105015
S Kundu et al
Figure 5. Secondary electron SEM images of the BNN-Erx samples heat-treated at different temperatures (1023 K/2h and 1073 K/2h).
(a)–(b) x=0.02, (c)–(d) x=0.04 and (e)–(f) x=0.06.
Table 1. Average particle size and optical band gap of the BNN-Erx samples obtained from different
calcined temperatures.
Particle size by SEM (nm)
Sample code
x=0.02
x=0.04
x=0.06
Temperature (K/2h)
1023
1073
1023
1073
1023
1073
Length
Width
82±23
230±64
158±46
171±41
107±39
115±34
40±13
93±26
77±31
109±24
51±20
66±23
Optical band gap (eV)
3.18
3.16
3.21
3.2
3.26
3.25
Optical properties
UV–Vis–NIR measurement
To estimate the optical band gap of the synthesized samples, diffuse reflectance spectra were recorded in the
250–1100 nm wavelength range. The spectra recorded for the BNN-Erx=0.02–0.06 samples calcined at 1073 K/2h
are shown in figure 6(a). A sharp cut-off around 342 nm was observed for all the samples. Some dips were found
in the reflection spectra in different wavelength locations which characterize the absorption bands of Er3+ ions
present in the sample [33]. The band located at 377 nm is caused by the transition from 4I15/2 to 4G11/2. Several
other bands observed are assigned appropriately, and these are given in table 2. From the reflection data the
Kubelka–Munk function has been calculated and is depicted in figure 6(b). The Kubelka–Munk [34] function,
6
Mater. Res. Express 2 (2015) 105015
S Kundu et al
Figure 6. (a) Optical diffused reflection spectra of BNN-Erx samples and (b) Kubelka–Munk function plot for different samples.
Table 2. UV–Vis–NIR bands and the associated transitions of Er3+ present
in the BNN-Erx samples (very similar positions for all the compositions).
Band position
(nm) (±1 nm)
Energy (eV)
(±0.01 eV)
408
488
526
543
653
Ground
state
Excited
state
2
3.04
2.55
2.36
1.92
1.90
H9/2
F7/2
2
H11/2
4
S3/2
4
F9/2
4
4
I15/2
which relates the diffuse reflectance (R∞) emanating from an infinitely thick sample, absorption coefficient (K)
and the scattering coefficient (S), is given by
2
( )
F R¥
(1 - R )
=
¥
2R¥
=
K
.
S
(1)
Figure 6(b) depicts the plot between [F(R∞)(hν)]1/2 and photon energy (hν) from which the optical band gap
has been calculated by a straight-line fit to the linear region of the plot. The band gaps of the BNN-Erx=0.02, 0.04,
0.06(1073 K/2h) samples are 3.16, 3.2 and 3.25 eV respectively and are listed in table 1. The band gaps obtained
for the samples heat-treated at different temperatures are also included in table 1.
PL/DS measurement
Figure 7 shows the PLE and PL data of the BNN-Erx nanocrystalline phosphors using a xenon lamp. Figure 7(a)
shows the excitation spectrum (300–500 nm) of BNN-Erx monitored at 542 nm (λem=542 nm corresponding
to the 4S3/2→4I15/2 transition). The spectrum shows peaks at 378 nm, 398 nm; 408 nm; 450 nm and 486 nm
which correspond to transition from ground state 4I15/2 to 4G11/2; 2H9/2; 4F3/2 and 4F7/2 levels, respectively. The
most intense peak at 378 nm is consistent with the absorption spectra peaking at 377 nm. Therefore, 378 nm
radiation was selected to monitor the PL emission of the BNN-Erx nanocrystalline phosphor. Figure 7(b) shows
the PL emission of BNN-Erx sample in the 500–700 nm range on excitation with 378 nm radiation. The peaks
are observed at 467 nm, 491 nm (inset, figure 7(b)), 522 nm, 542 nm and 660–695 nm with sharp peaks centred
at 680 nm and 692 nm corresponding to the 2F3/2→4I15/2; 2F7/2→4I15/2; 2H11/2→4I15/2; 4S3/2→4I15/2;
and 4F9/2→4I15/2 transitions (see the partial energy diagram in figure 8) [33]. In this case, the 4G11/2 is directly
excited by 378 nm, thereby resulting in population accumulation in 2H11/2, 4S3/2 and 4G9/2 levels through nonradiative relaxation of ions from the 4G11/2 level and thus different emissions of the Er3+ ion are observed in
this case.
Though they appear similar, the PL emission measurements with 378 nm excitation for four different
concentrations of the Er3+ ion (x=0.01, 0.02, 0.04 and 0.06) show one interesting feature. The samples with
first three concentrations show a similar emission pattern, i.e. a dominant green emission with relatively weak
red emission. But, at higher concentration of Er3+ (x=0.06) ion, the red emission is increased while there is a
decrease in intensity of the green emission. The ratio of intensities of red to the green emission (Ired/Igreen) is
7
Mater. Res. Express 2 (2015) 105015
S Kundu et al
Figure 7. (a) Room temperature photoluminescence excitation (PLE) of BNN-Er3+ nanocrystalline phosphor monitored at
λem=542 nm corresponding to the 4S3/2→4I15/2 transition, (b) photoluminescence (PL) emission of BNN-Er3+ nanocrystalline
phosphor with λexc=378 nm with the inset showing the enlarged portion of blue emission in the range 450–500 nm, (c) CIE
chromaticity diagram showing the dominant green emission of BNN-Er3+
x nanocrystalline phosphor, (d) decay curve measurement
of λem=542 nm with λexc=378 nm.
Figure 8. Schematic partial energy level diagram of Er3+-ion showing the mechanisms involved in DS and UC. Upward arrows
indicate the excitation whereas downward arrows indicate the emission. Dotted arrows show the non-radiation relaxation. GSA and
ESA stand for ground state absorption and excited state absorption involved in the UC process.
8
Mater. Res. Express 2 (2015) 105015
S Kundu et al
0.34, 0.54, 0.46 and 0.87 for x=0.01, 0.02, 0.04 and 0.06, respectively. Thus, the green and red emissions have
almost equal intensity at higher concentrations. The enhancement in the intensity of the red emission is due to
the enhancement in population of Er3+ ions in 4F9/2 level as compared to the 2H11/2/4S3/2 levels at higher
concentrations of the Er3+ ion. When the concentration of Er3+ ions is increased in the host, the ions would be
in close proximity and the cross-relaxation (CR) mechanism between the same or different Er3+ ions plays a key
role [35, 36]. In the present case, the following CR mechanism is proposed to take place (for example, in the case
of three Er3+ ions, Er1, Er2 and Er3):
4
F7/2 (Er1) 4 I11/2 (Er1) «4 I15/2 (Er2) 4 I11/2 (Er2)
4
F7/2 (Er2) 4 F9/2 (Er2) «4 I11/2 (Er1) 4 F9/2 (Er1)
4
F7/2 (Er3) 4 F9/2 (Er3) «4 I11/2 (Er2) 4 F9/2 (Er2) .
In this way, most of the ions are accumulated in 4F9/2 level and the red emission of the Er3+ ion is, therefore,
enhanced.
To further verify mathematically the obtained emissions, the Commission Internationale de l’Eclairage
(CIE) chromaticity coordinates have been calculated for the four samples under study. The CIE chromaticity
coordinates are based on the spectral luminous efficiency function for photopic vision [37]. The calculated
colour coordinates are (0.40, 0.57), (0.28, 0.69), (0.29, 0.69) and (0.30, 0.66) for x=0.01, 0.02, 0.04 and 0.06,
respectively and are shown in figure 7(c). The colour coordinates are that in the greenish region show the purity
of the colour. Thus, the studies confirmed the synthesized BNN-Erx nanocrystalline phosphors to be good green
emitting phosphors.
The decay curve measurement for green emission at 542 nm (4S3/2→4I15/2 transition) has also been carried
out to have an idea about the lifetime. The decay curve follows the following relation:
I (t ) = I0 exp ( - t /t )
(2)
where I(t) is the emission intensity of a particular transition at time t, I0 is the emission intensity at time t=0,
and τ is the lifetime of the emitting level. Figure 7(d) shows the room temperature decay curve for 4S3/2→4I15/2
transition at 542 nm on excitation with 378 nm. The decay curve fitting is found to be mono-exponential and the
obtained lifetime of 4S3/2 level is ∼114 μs.
UC measurement
RE ion-doped materials are very promising since they exhibit peculiar optical characteristics such as UC, DS,
QC, etc. In general, a material emits photons of higher wavelength than that of the incident photons. This
process is termed as normal photoluminescence or DS. Contrary to DS, UC is an anti-Stokes type of emission in
which the emitted photons are of lower wavelength than that of the incident photons. Actually, RE ions have a
number of levels with lifetimes of the order of μs to ms, which allow the UC process to take place easily even with
a small laser power. UC takes place in many different ways. Two important channels for UC are excited state
absorption (ESA) and energy transfer up-conversion (ETU). ESA is a single ion process whereas ETU involves
two similar/different excited ions, in which one acts as donor/sensitizer and the other one acts as an acceptor.
The authors have tried to monitor the UC emission in the present work using 976 nm radiation from a diode
laser. Figure 9(a) shows the room temperature UC emission spectra for BNN samples with different
concentrations of Er3+ heat-treated at 1073 K/2h on excitation with a 976 nm diode laser. The spectra show
strong green emission with relatively weak blue and red emissions for all the concentrations of the Er3+ ion.
Figure 9(b) shows the dopant concentration–dependent up-conversion emission intensity of the Er3+ ion and it
is found that the overall UC emission intensity decreases at higher concentrations of dopant ion. The optimized
UC is observed for the composition pertaining to x∼0.02.
The prominent peaks of Er3+ ion are observed at 486 nm, 528, 542 nm and 650–690 nm corresponding to
4
the F7/2→4I15/2, 2H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions. The observed UC emission peaks arise
due to the following mechanism: when the material is exposed to 976 nm radiation (∼10 000 cm−1), the ions
absorb the energy from the incident photon and promote to the excited 4I11/2 level (since it lies at
∼10 000 cm−1) through ground state absorption (GSA) as shown in figure 8. Since the 4I11/2 level is metastable,
the ions present in this level can absorb another 976 nm photon and promote to the 2H11/2 level through ESA
[35, 36]. Since the separation between 2H11/2 and 4S3/2 level is very small (∼667 cm−1), a part of excited ions in
2
H11/2 level is accumulated in 4S3/2 level. Thus, an intense green emission is observed corresponding to the 2H11/
4
4
3+
ions is another pathway to
2, S3/2→ I15/2 transition. Also, energy transfer (ET) between two or more Er
3+
populate the emitting levels of Er ions. In terms of blue emission, this blue peak is very poor and is rarely
observed in very few hosts. The possible mechanism for this emission is understood to be that a small population
of ions in 2H11/2 and 4S3/2 level is promoted to 2G7/2 and after non-radiative relaxation between the closely
spaced energy levels the ions are finally accumulated in the 4F7/2 level and thus a very weak blue emission
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S Kundu et al
Figure 9. (a) Room temperature UC emission spectra of BNN-Er3+
x nanocrystalline phosphors on excitation with 976 nm radiation
from a diode laser. Inset to the figure shows the enlarged portion of the blue emission (475–500 nm) of Er3+ ion. (b) Integrated
intensity of dominant green emission with respect to different concentrations of Er3+. (c) CIE chromaticity diagram showing the
dominant green emission of BNN-Er3+
x nanocrystalline phosphor. (d) Decay curve measurement of λem=542 nm with
λexc=976 nm.
corresponding to 4F7/2→4I15/2 transition is observed in this case. A part of the population in the 4I11/2 level is
accumulated in the 4I13/2 level through non-radiative transition. The ions in the 4I13/2 level are excited to the
4
F9/2 level by absorption of the incident 976 nm photon. The ions are relaxed to the ground level 4I15/2 emitting
red emission (4F9/2→4I11/2 transition). All these mechanisms are illustrated in figure 8.
Again, the CIE chromaticity colour-coordinates (x, y) for the UC emission dominated by green colour of
BNN-Erx nanocrystalline phosphor have been calculated and found to be ∼(0.32, 0.66), (0.30, 0.58), (0.32, 0.55)
and (0.35, 0.47) for x=0.01, 0.02, 0.04 and 0.06, respectively. The CIE chromaticity colour coordinates are
shown in the chromaticity diagram in figure 9(c); they are lying in the greenish region, demonstrating the purity
of the colour. Further, the decay curve measurement of the dominant green UC emission (corresponding to (the
4
S3/2→4I15/2 transition of the Er3+ ion) has also been monitored using the same excitation wavelength (see
figure 9(d)). The decay curve was fitted according to equation (1) and the corresponding lifetime was found to be
∼326 μs. The increase in the lifetime in the case of UC as compared to the DS measurement clearly shows the
process of energy transfer (ET) between the neighbouring Er3+ ions along with the excited state absorption
(ESA) process. Similar results for lifetime enhancement have been reported by Vetrone et al [38 and references
therein] and Boyer et al [39 and references therein].
Further, the UC efficiency for the optimized sample has been calculated by taking the integrated intensity
ratio of the UC emission in the visible region to that of the incident NIR radiation and is found to be ∼3%, which
is good enough to support the candidature of the present material. Therefore, the BNN-Erx nanocrystalline
phosphors may be considered as potential candidates for up-conversion.
Conclusions
Dual mode luminescence emitting Er3+-ion doped BNN nanophosphors were developed by citrate-assisted solgel route and their structural and photoluminescence characteristics were studied. The structure and phase
purity of the samples were confirmed by x-ray diffraction, TEM and FT-IR studies. The synthesized materials
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S Kundu et al
emit intense DS green and red emission with 378 nm radiation from normal PL measurement. Interestingly, the
material gives intense UC emission in the visible region dominated by green emission and relatively weak red
emission on 976 nm (NIR) excitation. Also, the UC efficiency for the present material is quite good at ∼3%.
Thus, the present BNN-Erx nanocrystalline phosphor shows efficient dual-modal luminescence properties (DS/
PL and UC) which has not been reported so far in this material. The promising optical properties of erbiumdoped nanocrystalline BNN could be employed in display and related device applications.
Acknowledgments
Two authors, Dr Rajasekhar Bhimireddi and Dr Kavita Mishra, acknowledge the University Grants
Commission, Govt. of India for the financial support provided through the Dr D. S. Kothari Post-Doctoral
Fellowship.
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