Vol. 36, No. 12
Journal of Semiconductors
December 2015
Preparation, electronic structure, and photoluminescent properties of Eu2C
activated BaSi2 O5 powder phosphors for solid-state lighting
Cao Donghua(曹东华), Wang Hui(王惠)Ž , Wei Hongjun(魏红军), and Yang Weiqiang(杨伟强)
Jingao Solar Holdings Co. , Ltd, Ningjin 055550, China
Abstract: The green-emitting phosphor BaSi2 O5 :Eu2C was synthesized by the conventional solid state reaction.
Using the CASTEP code, BaSi2 O5 is calculated to be an intermediate band gap semiconductor with an indirect
energy gap of about 3. 2 eV. As expected, the calculated optical band gap of BaSi2 O5 is lower compared to the
experimentally determined values. Eu2C -activated BaSi2 O5 phosphor can be excited efficiently over a broad spectral range between 200 and 400 nm, and has an emission peak at 500 nm with a full width at half maximum of
95 nm. The study of concentration-dependent emission intensity shows the optimal concentration of the Eu2C is
0.05 mol, and that concentration quenching occurs when the Eu2C content is beyond the critical value. The external
quantum efficiency of the optimized BaSi2 O5 :Eu2C is 96. 1%, 70. 2% and 62. 1% under excitation at 315, 350 and
365 nm, respectively. The superior optical properties of the sample show the potential as an ultraviolet converting
green-emitting phosphor for white light emitting diodes.
Key words: light emitting diode; the green-emitting phosphor BaSi2 O5 ; Eu2C
DOI: 10.1088/1674-4926/36/12/123008
PACC: 4270G
1. Introduction
White light-emitting diodes (LEDs), the next-generation
of solid-state lighting, have received increasing interest in recent years for their promising properties such as energy efficiency, long lifetime, compactness, environmentally friendly
and designable featuresŒ1 7 . These devices with dramatic
global energy-saving capability have the potential to entirely
replace both incandescent and fluorescent lamps. The light revolution is sweeping all over the world and is quietly coming in
and improving our everyday lifeŒ8 . In view of convenience and
economy, the most important way to realize white emission is
to combine a blue light emission InGaN chip with a yellow
Y3 Al5 O12 :Ce3C (YAG:Ce3C / phosphor. However, this type
of white light has poor color rendering caused by the color deficiency in the red region of the phosphor. Although many efforts have been devoted to improving the performance of the
method of “blue chip C yellow phosphor”, including modifying the host lattice, and exploring the new luminescent system, progress has not been obviousŒ6; 9 14 . The combination
of a UV LED with red, green, and blue (RGB) phosphors is
the alternative way to achieve white light. Compared with the
above “blue C yellow”, the “UV C RGB phosphors” would
exhibit high color rending properties and low color point variation against the forward-bias currents. With the development
of efficient LEDs that emit light in the UV range, most research
interest has been devoted to the development of RGB phosphorsŒ5; 15 18 . In the existing green-emitting phosphor for the
above application, the rare-earth ion activated sulfide-based
and (oxy)nitride-based compounds have been widely developed, however, the related synthesis processes are harsh. In
addition, the sulfide-based phosphors suffer a problem in the
aspect of low chemical stabilities, which causes the strong temperature dependence of chromaticity and degradation of the lu-
minous efficiency of the white LEDs. Therefore, it is necessary
to develop alternative green phosphors, which have properties
such as superior luminescence phenomenon, easy fabrication
and high chemical stabilities.
Silicates, as the most important class of compounds, have
been widely used as the matrix of inorganic phosphors due
to their merits, namely facial synthesis, high chemical stability, good thermal quenching and ideal excitation and emission wavelengthsŒ13; 19; 20 . As a member of the silicate host,
BaSi2 O5 has attracted attention and related work which aims
to value the potential, as the suitable host lattice for inorganic phosphor has been processedŒ21 24 . In previous reports,
Eu2C activated BaSi2 O5 phosphors have been synthesized by
the pulse laser deposition and melting method, the related luminescence properties have been studied. However, attention
should be paid to the fact that the results obtained in the existing reports are inconsistent with each other. The differences
in the report may have resulted from the poorer crystallinity
of the samples. The solid state reaction is the most important
method in the technology, and the samples obtained by this
method have the better crystallinity due to the high synthetic
temperature. Taking these aspects into consideration, we believe that it is necessary to process the work related to the
solid state reaction in view of the theory and technology applications. Furthermore, the first-principles electronic structure calculations have been widely used in the study of inorganic phosphor, which aims to gain the value information
about the host lattice in terms of the band structure and density of statesŒ14; 25 28 . In the thesis, an explorative research is
presented on the electronic structure and luminescent properties of the Eu2C -activated BaSi2 O5 phosphor in the calculation
and experiment aspect, the obtained information reveals that
the BaSi2 O5 :Eu2C has the potential as an ultraviolet converting green-emitting phosphor for white light emitting diodes.
† Corresponding author. Email: 15613925832@163. com
Received 26 March 2015, revised manuscript received 23 June 2015
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© 2015 Chinese Institute of Electronics
J. Semicond. 2015, 36(12)
Cao Donghua et al.
2. Experimental section
Table 1. Crystal lattice constants for BaSi2 O5 crystal.
BaSi2 O5
Lattice parameters
a (Å)
b (Å)
c (Å)
Calc.
4.893
8.483
13.569
Expt.
4.629
7.688
13.523
2.1. Density functional theory (DFT) calculations
All calculations were performed in the density functional
theory (DFT) framework using the CASTEP (Cambridge Serial Total Energy Package) moduleŒ29 of Materials Studio 4. 0.
The exchange-correlation effects were treated within the generalized gradient approximation (GGA) with the Perdew-BurkeErnzerhof functionalŒ30 . Two steps were necessary to calculate the electronic band structure of BaSi2 O5 . The first step
was to optimize the crystal structure using the crystallographic
data reported in Reference [31]. The second step was to calculate the band structure and density of states of BaSi2 O5 for the
optimized structure. Lattice parameter and atomic coordinates
were fixed at the values obtained by the crystal structure optimization process in the first step. For the two steps, the basic
parameters were chosen as follows in setting up the CASTEP
run: the kinetic energy cutoff D 340 eV, k-point spacing D
0.05 Å 1 , sets of k points D 5 3 2, self-consistent field
tolerance thresholds D 1.0 10 5 eV/atom, and space representation D reciprocal. The reliability of the calculation was
demonstrated by the result of the convergence test.
Figure 1. Band structure of host lattice BaSi2 O5 .
2.2. Synthesis
Undoped and Eu2C -doped BaSi2 O5 powders were prepared by a solid state reaction approach using BaCO3 [analytic reagent (A.R.)], SiO2 [analytic reagent (A.R.)] and Eu2 O3
(99.99%) as the starting materials. The stoichiometric amounts
of raw materials were weighed out and thoroughly mixed by
grinding in an agate mortar, and subsequently the mixture was
prefixed at 873 K for 2 h. After slowly cooling down to room
temperature, the prefixed samples were thoroughly reground
and then calcined at 1473 K for 3 h in the CO reducing atmosphere.
2.3. Characterizations
X-ray powder diffraction measurements were performed
on a D8 focus diffract meter (Bruker) at a scanning rate of
0.2ı /min in the 2 range from 10ı to 80ı , with graphitemonochromatized Cu K˛ radiation ( D 0.15405 nm) at 40 kV
and 40 mA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were
recorded with a Hitachi F-4500 spectrophotometer equipped
with a 150 W xenon lamp as the excitation source. Photoluminescence quantum yield (QY) was measured by absolute PL
quantum yield measurement system C9920-02. All the measurements mentioned above were performed at room temperature.
3. Results and discussion
3.1. Electronic structure calculations
The convergence test of the geometry optimization and energy calculation showed well, demonstrating our basic parameters were suitable. The lattice parameters obtained by the
calculations are in accordance with the experiment results,
which are shown in Table 1.
Figure 2. Diffuse reflection spectrum of the host lattice BaSi2 O5 . The
inset shows the absorption spectrum as calculated by the KubelkaMunk function.
Figure 1 shows the band structure of BaSi2 O5 . It is seen
that BaSi2 O5 shows an indirect optical band gap. The gap between the lowest energy of the conduction band and highest energy of the valance band is about 3.2 eV, which is lower than
the experimental value obtained from the absorption spectra
(about 3.7 eV, seen in Figure 2). Such an underestimation of
the calculated band gaps is related to well-known DFT limitations, namely not taking into account the discontinuity in
the exchange-correlation potentialŒ32 , which is a common feature of all DFT calculations. Composition of the calculated
energy bands can be resolved with the help of a partial density of states (PDOS) and total density of states (DOS) diagrams. Figure 3 describes the total and partial density of states
of BaSi2 O5 . These diagrams allow us to conclude that the conduction in BaSi2 O5 is about 3 eV wide and is formed by the
Ba 5d, 6s states with a minor contribution coming from the Si
3s, 3p states. The valence band is about 12 eV wide and consists of three sub-bands, clearly seen in the band structure as
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J. Semicond. 2015, 36(12)
Cao Donghua et al.
Figure 3. (Color online) Total and partial density of states of
Figure 5. Excitation and emission spectra for BaSi2 O5 :0.05Eu2C .
3.3. Luminescence properties of BaSi2 O5 : Eu2C
Figure 4. XRD profile for BaSi2 O5 :0.05Eu2C phosphor.
well: the upper one (between 9:3 and 0.8 eV) is mixed by the
Ba 6s states, which are hybridized with Si 3s, 3p states and O
2p states. The lower one is narrow (between –11.3 eV) and is
composed by the Ba 5p, 6s states and Si 3s, 3p states.
3.2. Phase formation and structural characters
Figure 4 presents the XRD patterns of the sample
BaSi2 O5 :0.05Eu2C . All the diffraction peaks of the samples
can be basically indexed to the standard data of BaSi2 O5
(JCPDS card no. 72–0171). No other phase is detected, indicating that the obtained sample is single phase and the activator ions have been successfully incorporated in the host lattices by replacing the Ba2C due to their similar ionic radiiŒ33 .
BaSi2 O5 occurs in a low and a high temperature form. In the
low-temperature phase, also the crystal phase obtained in the
work, BaSi2 O5 crystallizes in the orthorhombic space group
Pcmn with unit cell dimensions of a D 4.639 Å, b D 7.688 Å,
c D 13.523 Å. V D 482.29 Å3 , and Z D 4.33. There is one
barium site which is coordinated by nine oxygen ions. When
the Eu2C substitutes the Ba2C site in the host, interesting luminescent properties of the corresponding activators are expected.
The PLE and PL spectra of BaSi2 O5 :0.05Eu2C are shown
in Figure 5. Two broad absorption bands can be seen from the
PLE spectrum, which covers the wavelength range of 200–
275 nm and 275–400 nm and is attributed to the absorption
of the host lattice and 4f–5d transition of the Eu2C ions, respectively. It is worth noting that the absorption band can
match well with the radiative light from GaN-based LEDs,
indicating the potential application. Under the 365 nm excitation, the sample shows a broad emission peaking at about
500 nm. The emission band is due to the Eu2C ions 4f–5d
transition and the single luminescent center can be concluded
by the high symmetry profile of the emission band, which is
consistent with the fact that there is one Ba2C ion crystallographic site in the crystal structure. Compared with other
silicate-based phosphors in the previous publications, such as
Li2 CaSiO4 :Eu2C , BaCa2 Si3 O9 :Eu2C and Sr2 MgSi2 O7 :Eu2C ,
the BaSi2 O5 :Eu2C emits at a longer wavelength, which may
have originated from the special nature of the BaSi2 O5 host
latticeŒ35 37 . In the BaSi2 O5 structure, the Ba2C ions site in a
parallel row along the c axis and experience negative charges
from the nearest O2 anion and positive charges of the neighboring Ba2C ; as a consequence, the crystal field orients the d
orbital in the chain direction preferentially. The preferred orientation lowers the energy of the d orbital and will result in the
photoluminescence emission of Eu2C at a longer wavelength
when Eu2C ions substitute the Ba2C crystallographic sitesŒ20 .
Taking the general illumination into consideration, the FWHM
(full width at half maximum) is an important parameter because it is related with the color rendering. The FWHM of the
BaSi2 O5 :0.05Eu2C is 95 nm, a superior value for the WLED
application. The above optical properties are consistent with
those of Reference [24] but inconsistent with the related results
in References [22, 23], as well as the crystallinity of the obtained samples. Therefore, the above phenomena can indicate
that the possible reason for the inconsistence of the previous
results is the difference in crystallinity.
Figure 6 displays the emission intensity of BaSi2 O5 :
xEu2C phosphors as a function of the Eu2C concentration (x/.
The preparation conditions and the test conditions of all the
samples are the same. With increasing concentration of Eu2C ,
the emission intensity increases up to x D 0.05, a further in-
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J. Semicond. 2015, 36(12)
Cao Donghua et al.
Figure 6. (Color online) Emission spectra of BaSi2 O5 :Eu2C at different Eu2C concentrations.
Figure 7. Plot of lg(xEu2C / versus lg(I /xEu2C / in the BaSi2 O5 :
xEu2C samples (ex D 365 nm).
crease of the Eu2C content leads to a decrease of the emission intensity, which is due to the concentration quenching.
Concentration quenching is mainly caused by nonradiative energy transfer among Eu2C ions, and the possibility of which
increase as the concentration of Eu2C increases. The critical
energy transfer distance (Rc ) between Eu2C ions is approximately equal to twice the radius of a sphere with the volume
of the unit cell, which can be calculated by the following equationŒ38 .
1=3
3V
RC 2
;
(1)
4xc Z
where V is the volume of the unit cell, xc is the critical concentration of the activator ion, and Z is the number of formula units per unit cell. For the BaSi2 O5 matrix, Z D 4, V
D 482.29 Å3 and the critical concentration of Eu2C is found to
be 0.05. Therefore, Rc of Eu2C is determined to be 16.64 Å.
Nonradiative energy transfer among Eu2C ions usually occurs as a result of an exchange interaction, radiation reabsorption or a multipole–multipole interactionŒ39 . The exchange interaction is responsible for the energy transfer for forbidden
transitions and a typical critical distance is then about 5 Å, and
can be excluded in the case of allowed-transition and the calculated Rc of Eu2C ions, respectively. The mechanism of radiation reabsorption comes into effect only when there is broad
overlap of the fluorescent spectra. In view of the emission
and excitation spectra of BaSi2 O5 : Eu2C phosphor, the radiation reabsorption mechanism cannot be responsible for nonradiative energy transfer between the Eu2C ions. Therefore, the
multipole–multipole interaction dominated the concentration
quenching mechanism of Eu2C emission, and a detailed mechanism can be proposed as follows.
The emission intensity (I / per activator ion is given by the
equationŒ40; 41
k
I
D
;
(2)
x
1 C ˇ.x/=3
where x is the activator concentration; D 6, 8, or 10 is for
dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole
interaction, respectively; while k and ˇ are constants for the
same excitation condition for a given host lattice. Figure 7
illustrates the plot of lg(xEu2C / versus lg(I /xEu2C / in the
BaSi2 O5 :xEu2C samples (ex D 365 nm) is linear and the
Figure 8. (Color online) CIE chromaticity diagram for the
composition-optimal BaSi2 O5 :0.05Eu2C phosphor.
slope is 0:86: The value of is found to be approximately
6, indicating that the concentration quenching mechanism of
Eu2C emission was dominated by the dipole–dipole interaction.
In the practical application, the LED phosphor should have
high quantum efficiency under the UV or blue light excitation.
Higher quantum efficiency means less energy loss during the
luminescence process. Upon excitation at the wavelengths of
315, 350 and 365 nm, the absolute quantum efficiencies of
the composition-optimized phosphor BaSi2 O5 :0.05Eu2C are
96.1%, 70.2%, and 62.1%, respectively. Considering the work
was very primary, and did not study the effect of technological
parameters, such as the synthesis temperature and the flux, the
value of quantum efficiency could be enhanced by process optimization. From the corresponding PL spectra upon 365 nm
excitation, the commission International de I’Eclairage chro-
123008-4
J. Semicond. 2015, 36(12)
Cao Donghua et al.
2C
maticity coordination of the phosphor BaSi2 O5 :0.05Eu has
been calculated to be (0.247, 0.455), which is depicted in the
CIE 1931 chromaticity diagram in Figure 8. The above characteristic indices of the sample show that the optimized BaSi2 O5 :
0. 05Eu2C phosphor has the superior luminescent properties to
meet the general requirements of the LED phosphor, namely
the suitable PLE and PL spectra, the wide FWHM value, and
the high quantum efficiency, thus the as-prepared sample has
a potential to be a green-emitting phosphor for the UV excited
solid state lighting.
4. Conclusion
In summary, a green-emitting phosphor BaSi2 O5 :Eu2C
was synthesized by the conventional solid state reaction. The
electronic structure of the host matrix was analyzed using the
CASTEP module of Materials Studio 4.0. The calculation results show that BaSi2 O5 is an intermediate band gap semiconductor with an indirect energy gap of about 3.2 eV. For the
Eu2C -activated samples, the related optical properties were investigated in detail. BaSi2 O5 :Eu2C phosphor can be excited by
a broad excitation band between 200 and 400 nm and gives an
emission band centered at 500 nm. The optimal concentration
of the activator in the host lattice is determined to be 0.05 mol.
The potential of the composition-optimized phosphor as the
LED phosphor have been confirmed by the superior optical
properties of BaSi2 O5 :0.05Eu2C in terms of the PLE and PL
spectra, the FWHM value, and the quantum efficiency.
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