Synthesis and multicolor upconversion of Tm3+/Er3+/Yb3+ doped Na (Y1.5Na0.5) F6 single-crystal nanorods

Article abstract of DOI:10.1016/j.jallcom.2009.12.133

Er³⁺/Tm³⁺/Yb³⁺ tri-doped Na (Y_1.5Na_0.5) F₆ single-crystal nanorods were synthesized by a facile hydrothermal treatment method. The tri-doped sample was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM). The sample showed a pure hexagonal phase structure with space group P6₃/m and nanorod morphology. Under 980 nm excitation, intense red, green and blue upconversion emissions were observed. The red and green upconversion are consistent with ⁴F_9/2 → ⁴I_15/2 and ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺, respectively, while the blue upconversion originates from ¹G₄ → ³H₆ transition of Tm³⁺. Moreover, the Er³⁺/Tm³⁺/Yb³⁺ tri-doped Na (Y_1.5Na_0.5) F₆ nanorods exhibit strong red and green emission which is ascribed to the sensitization of Tm³⁺ to Er³⁺. The energy transfer upconversion mechanisms for the fluorescent intensity are also discussed.

Full text of DOI:10.1016/j.jallcom.2009.12.133

Journal of Alloys and Compounds 493 (2010) 476–480
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
3
+
3+
3+
Synthesis and multicolor upconversion of Tm /Er /Yb doped Na (Y Na ) F
6
1
.5
0.5
single-crystal nanorods
∗
Songjun Zeng, Guozhong Ren, Qibin Yang
Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China
a r t i c l e i n f o
a b s t r a c t
Er3+/Tm3+/Yb3+ tri-doped Na (Y1.5Na0.5) F single-crystal nanorods were synthesized by a facile hydrother-
mal treatment method. The tri-doped sample was characterized by X-ray diffraction (XRD), transmission
electron microscopy (TEM). The sample showed a pure hexagonal phase structure with space group P63/m
and nanorod morphology. Under 980 nm excitation, intense red, green and blue upconversion emissions
Article history:
Received 30 September 2009
Received in revised form
6
1
8 December 2009
Accepted 19 December 2009
Available online 4 January 2010
4
4
2
4
4
were observed. The red and green upconversion are consistent with F9/2 → I15/2 and H11/2, S3/2 → I15/2
3
+
1
3
3+
transition of Er , respectively, while the blue upconversion originates from G4 → H6 transition of Tm
.
3
+
3+
3+
Moreover, the Er /Tm /Yb tri-doped Na (Y1.5Na0.5) F6 nanorods exhibit strong red and green emission
Keywords:
Nanorods
Upconversion mechanisms
Hydrothermal method
Na (Y1.5Na0.5) F6
3
+
3+
which is ascribed to the sensitization of Tm to Er . The energy transfer upconversion mechanisms for
the fluorescent intensity are also discussed.
© 2009 Elsevier B.V. All rights reserved.
1
. Introduction
In recent years, one dimensional (1D) nanoscale materials, such
crystals doped with the lanthanide ions [19,20] and investigate the
influence of crystal phase on the luminescence properties [21,22].
Recently, Ln3+ tri-doped NaYF4 nanoparticles [23] and Ln doped
3+
as nanowires, nanotubes, nanobelts and nanorods, have attracted
much interest due to their potential applications in nanoscale opto-
electronics [1,2], electronics [3,4], lasers [1,5] and biological labels
NaYF /polymer composite fibers prepared by electrospinning [24]
4
have been reported. However, there were rare reports on highly
uniform rare earth ions doped hexagonal phase NaYF4 with 1D
nanostructure such as nanorod shape.
[
6], etc. Many unique and fascinating properties for this kind of
materials such as high luminescence efficiency and a lowered lasing
threshold have already been reported in literatures [7,8]. Among
all the unique properties, the upconversion (UC) properties have
aroused great interest due to their potential for use in color displays
In the present work, the highly uniform Na (Y1.5Na0.5) F single-
6
crystal nanorods doped with Er3+/Tm /Yb were synthesized via
a simple hydrothermal treatment. The as-prepared nanorods emit
strong multicolor NIR-to-visible UC fluorescence via excitation by
diode laser (LD) with wavelength of 980 nm. And the corresponding
UC mechanism was investigated in detail.
3+
3+
[
9], solar cells [10], biological detection [11,12], and so on.
Among all of the commonly used UC host materials, including
oxyfluorides [13,14], oxysulfides [15], fluorides [16] and others,
NaYF4 is one of the most efficient UC host materials by means of
doping with lanthanide ions owing to the low phonon energy of
the crystal lattice [17]. The crystal structure of NaYF4 exhibits two
polymorphic forms, namely, cubic and hexagonal phases, depend-
ing on the synthesis conditions and methods. It has been reported
that hexagonal phase NaYF4 is a much better host lattice than
the cubic phase NaYF4 for the upconversion luminescence [18].
Hereby, it is essential to synthesize pure hexagonal phase NaYF4
crystal with uniform size and high quality structure under sim-
ple processes without any other phases. Several reports have been
devoted to synthesize hexagonal phase NaYF4 nano- and micro-
2.
Experimental details
All the rare-earth oxides were 99.99% purity. Rare earth nitrate RE(NO3)3 (RE = Y,
−
1
Tm, Er, Yb) solution with 0.1 mol L was formed by dissolving RE2O3 (99.99%) in
HNO3 and excess nitric acid was removed by evaporation under elevated tempera-
ture. Other chemicals were of analytical grade and used as received without further
purification.
The NaYF4 nanorods were synthesized using the method described in Ref. [25]
with some modifications. First, 1.2 g NaOH, 2 mL distilled water, 9 mL ethanol, and
2
0 mL oleic acid were mixed together under agitation to form a homogeneous
solution. Next, 1.77 mL Y(NO)3 (0.5 M), 0.2 mL Tm(NO)3 (0.05 M), 0.1 mL Er(NO)3
0.05 M), 0.2 mL Yb(NO)3 (0.5 M), and 8 mL NaF (1.0 M) aqueous solution were added
(
to the solution under vigorous stirring. The resulting mixture was vigorously stirred
for another 30 min. The resulting solution was then transferred into a 50 mL stainless
Teflon-lined autoclave, sealed, and heated at 190 C for 24 h. After that the system
◦
was then allowed to cool down to room temperature naturally and the products
were deposited at the bottom of the vessel. The obtained nanorods were washed
with ethanol and water to remove oleic acid and other remnants. Finally dried in air
∗ Corresponding author. Tel.: +86 0732 8292113.
E-mail addresses: yqb@xtu.edu.cn, qibinyang2009@yahoo.com (Q. Yang).
0
925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.12.133

S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
477
Fig. 1. XRD pattern of the as-prepared Na (Y1.5Na0.5) F6 sample.
◦
at 60 C for 6 h. In addition, the final products can be well-dispersed in a nonpolar
solvent such as cyclohexane and aggregated by adding polar solvent such as ethanol.
The phase composition and phase purity of the obtained nanorods were exam-
ined by XRD on a D/max-␥A System X-ray diffractometer at 40 kV and 40 mA with
Cu K␣ radiation. TEM and the high-resolution transmission electron microscopy
(
HRTEM) images were obtained using JEM-2100 microscope operated at the acceler-
ating voltage of 200 kV. The samples were dispersed in nonpolar cyclohexane solvent
and were then subjected to ultrasonication to achieve a well-dispersed suspension.
One drop of the suspension was placed on copper grid covered with hollow carbon
films for TEM assays. The upconversion emission spectra at room temperature were
detected by a spectrophotometer (R500) under the excitation of 980 nm LD. All the
above measurements were carried out at room temperature.
3
. Results and discussion
Fig. 1 shows a typical XRD pattern of the as-prepared Na
(
Y1.5Na0.5) F sample. All of the strong and sharp diffraction peaks in
6
Fig. 1 can be readily indexed to hexagonal phase of ␤-Na (Y1.5Na0.5
)
F6 (space group: P6 /m), which are consistent with the litera-
3
ture data (JCPDS 16-0334). No other impurity peak was detected
which revealed that the pure hexagonal phase of Na (Y1.5Na0.5) F6
had been fabricated. As can be seen from XRD pattern, the high
crystallinity can be obtained at the low hydrothermal treatment
◦
temperature (190 C). High crystallinity is important for phosphors,
because it generally means less traps and stronger luminescence
[
26].
A typical TEM image in Fig. 2a shows that the obtained ␤-Na
3
+
3+
3+
(
Y1.5Na0.5) F : Tm /Er /Yb from the hydrothermal condition
6
exhibits rod-like morphology with high quality and uniform size of
the 1500 nm in length and 90 nm in diameter, as estimated from the
TEM images. The aspect ratio is about 16. Fig. 2b shows a single as-
synthesized Na (Y1.5Na0.5) F6 nanorod. The HRTEM image (Fig. 2c)
taken from the single nanorod presented in Fig. 2b shows clearly
resolved lattice fringes. The interplanar distance d = 0.257 nm and
0.295 nm match that of the (2 0 0) and (1 1 0) lattice planes of hexag-
onal phase Na (Y1.5Na0.5) F6 respectively. In addition, the growth
direction of the as-prepared nanorods along the [1 1 0] direction can
be also confirmed by using HRTEM pattern. Its fast Fourier trans-
form (FFT) of the HRTEM image shown in the inset of Fig. 2c also
reveals single-crystalline nature of as-synthesized nanorods and
can also be readily indexed as hexagonal phase Na (Y1.5Na0.5) F6,
which is in good agreement with the XRD results presented in Fig. 1.
Under 980 nm excitation, the strong multicolor fluorescence
Fig. 2. TEM and HRTEM images of the as-prepared Na (Y1.5Na0.5) F6 nanorods: (a)
overview image, (b) single nanorod image, and (c) the typical HRTEM image for
single nanorod. The inset of Fig. 2c is corresponding Fourier transform of HRTEM
image.
can be observed in the Tm3+/Er /Yb doped Na (Y1.5Na0.5) F6
3+
3+
single-crystal nanorods. The upconversion luminescence spectra
of Na (Y1.5Na0.5) F6 nanorods doped with Tm3+/Yb , Er /Yb
3+
3+
3+

4
78
S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
Fig. 4. Upconversioin spectra of x% Tm +, 0.5% Er3+, 10% Yb3+ co-doped Na (Y1.5Na0.5)
F6 nanorods with different Tm concentrations under 980 nm excitation.
3
Fig. 3. Room temperature upconversion spectra of Na (Y1.5Na0.5) F6 single-crystal
3
+
3+
3+
nanorods under 980 nm excitation (a) doped with 0.5 mol%Tm , 10 mol%Yb ions,
3
+
3+
3+
(
0
b) doped with 0.5 mol%Er , 10 mol%Yb ions, and (c) doped with 0.5 mol%Tm ,
.5 mol%Er3 and 10 mol%Yb ions.
+
3+
To understand the mechanism, the dependence of the blue,
3
+
3+
green and red upconversion emissions of 0.5% Er , 0.5% Tm , and
and Tm3+/Er3+/Yb3+ under 980 nm excitation are shown in Fig. 3.
3+
10% Yb ions co-doped Na (Y1.5Na0.5) F6 nanorods on excitation
3+
3+
As can be observed, in Tm /Yb double doped Na (Y1.5Na0.5) F6
nanorods (Fig. 3a) two visible emissions 476 nm (blue), 650 nm
and a weak peaks at 690 nm (red) were observed which can be
power was analyzed. Fig. 6 shows the log–log plot of the UC emis-
sion intensity as a function of the pump power. As shown in Fig. 6,
all the green and red emission bands centered at 520, 545, 660 and
690 nm exhibit a quadratic relationship (slope = 1.96, 1.78, 1.73 and
1.91 for 520, 545, 660 and 690 nm, respectively), which reveals that
two photons steps are involved in these upconversion emissions.
While, a slope of 2.59 for the blue emission indicates that three
phonons are necessary for the blue upconversion emission centered
at 476 nm.
ascribed to G → H , G → F and 3F → H transitions. And
1
3
1
3
3
4
6
4
4
3
6
in Er3+/Yb co-doped Na (Y Na0.5) F6 nanorods (Fig. 3b), there
3+
1
.5
were three well-known emission bands centered at 520, 545 and
2
4
6
60 nm which are associated with the transition from H11/2, S3/2
4
4
3+
and F9/2 level to ground state ( I15/2) of Er . As compared with
Fig. 3a and b, in Fig. 3c the blue emission near 476 nm comes from
1
3
3+
Fig. 7 shows simplified energy level diagrams of the Er3+, Tm3+
the G → H transition of Tm ions, the sharp peaks in the green
,
4
6
and Yb3+ ions as well as proposed upconversion mechanisms to
region around 520 and 545 nm are assigned to the transition emis-
sions 2H11/2 → I
4
and 4S3/2 → I
4
of Er ions, respectively,
while the peak in the red region of 660 nm corresponding to the
3+
3+
3+
3+
produce the multicolor radiation. In Er /Tm /Yb doped sys-
tem, excitation of a single level F5/2 of Yb3+ is only considered
15/2
15/2
2
4
4
3+
3+
3+
F9/2 → I
of Er ions and the weak red emission band centered
because Yb ions have larger absorption cross-section and Tm
15/2
3
3
3+
at 690 nm is ascribed to F → H transition of Tm ions. It is noted
3
6
that the emission band at 650 nm in Fig. 3a arising from the tran-
sition 1G → H of Tm ions coincides with that of Er ions in
3
3+
3+
4
4
3+
Fig. 3b. However, the contribution of Tm ions to the red emission
band in Fig. 3c can be neglected due to the intensity of 650 nm band
in Fig. 3a is much smaller than that of the blue band, which is also
expected in Fig. 3c.
As shown in Fig. 3, the introduction of Tm3+ ions into the Na
(
Y
.5Na0.5) F6 nanorods significantly influences the emissions of
1
3
+
Er ions. The emission intensity of red and green is obviously
3+
enhanced. Furthermore, the blue upconversion of Tm decreases
in some extent when Er3 , Tm and Yb ions are tri-doped.
+
3+
3+
3+
3+
3+
Fig. 4 shows the upconversion spectrum of Tm , Er , Yb co-
doped Na (Y1.5Na0.5) F6 nanorods with different concentrations of
Tm ion under 980 nm excitation. The blue, red and green emission
intensity increases remarkably with Tm doping content increas-
ing from 0.2 to 0.5 mol%. However, it falls abruptly when Tm3
content reaches 0.7 mol%, indicating the occurrence of the con-
centration quenching. The upconversion spectrum of as-prepared
Na (Y1.5Na0.5) F6 nanorods with varying Yb3+ ion concentrations
excited by 980 nm is presented in Fig. 5. With the increase of
3+
3+
+
3+
Yb concentration, the whole upconversion emission intensity
3+
increases gradually. However, when the Yb doping concentration
increased up to 15%, the upconversion emission intensity decreased
due to the concentration quenching.
Fig. 5. Upconversioin spectra of 0.5% Tm3+, 0.5% Er3+, x% Yb3+ co-doped Na (Y1.5Na0.5)
3
+
F6 nanorods with different Yb concentrations under 980 nm excitation.

S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
479
above. For the Er3+, Tm3+and Yb3+ doped samples, Tm3+ behaved
as a sensitizer. The intensification of red luminescence is possibly
3+
due to the efficient energy transfer processes between Tm and
Er . (1) 4I1
3
+
(Er ) + F (Tm ) → F
3+
3
3+
4
(Er ) + H (Tm3+); (2)
3+
3
1/2
3
4
9/2
3
6
4
3+
3+
4
(Er ) + F (Tm3+). The red emis-
3+
F7/2 (Er ) + H (Tm ) → F
6
9/2
4
3
+
3+
sion is weak in the Er /Yb doped sample, and it is significantly
enhanced when Tm3+ is introduced to the system, indicating that
the energy transfer processes play a dominant role in populating
4
3+
3+
the F9/2 excite state of Er [27]. And Tm behaves as the sen-
sitizer to Er for the green upconversion luminescence through
3+
3
3+
4
3+
4
the energy transfer process [28]: H4 (Tm ) + I
(Er ) → I
15/2
9/2
3
+
3
3+
3+
(
Er ) + H (Tm ). On the other hand, the sensitization of Tm to
6
3+
3
3
Er depopulates H and F energy level and weakens the 476 nm
4
4
emission. Blue luminescence can be quenched at a relative lower
3
+
3+
3+
3+
concentration of Tm ions in Er , Yb and Tm tri-doped glasses
3+
3+
1
than in Yb /Tm co-doped sample by the following process: G4
3+
3
3+
3
3+
3
3+
(
Tm ) + H (Tm ) → H (Tm ) + H5 (Tm ). Therefore, the blue
6
4
3+
emission of the tri-doped sample decreases compared with Tm
and Yb3+ ions co-doped sample. Based on the analyses above,
3+
3+
by properly adjusting the concentrations of the Tm , Er and
Yb ions, the intensity of the blue, green and red upconversion
emissions can be changed in the Na (Y1.5Na0.5) F6 nanorods.
3+
FIg. 6. Dependence of upconversion emission intensity at 496, 520, 545, 660, 690 nm
3+
3+
3+
on excitation power under 980 nm in 0.5% Tm , 0.5% Er , 10% Yb co-doped Na
Y1.5Na0.5) F6 nanorods.
(
4. Conclusions
ions have no corresponding energy level. It is well know that the
In summary, Tm3+/Er3+/Yb3+ doped Na (Y1.5Na0.5) F6 single-
3+
3+
3+
Yb can efficiently sensitize Er and Tm ions. Under 980 nm
crystal nanorods were synthesized via a simple hydrothermal
method. TEM and XRD results show that these nanorods present
uniform morphology and high quality. Under 980 nm excitation,
intense green, red and blue upconversion was obtained. In addi-
tion, the upconversion mechanism was fully discussed in detail. For
excitation, the Yb ions are excited from the 2F7/2 level to the
3
+
2
3+
F
level, then transfer their energies to the nearby Tm and
5
/2
3
+
3+
4
4
3+
Er ions. Thus Er ions are raised from I15/2 to I11/2, and Tm
3
3
3+
ions are raised from
H
6
to H5 excited states. The Tm ions in
level will non-radiative transmit to F4
3
H5 level and Er in 4I1
3+
3
1/2
3+
3+
3+
the Tm /Er /Yb /tri-doped Na (Y1.5Na0.5) F6 nanorods, the blue,
green, and red upconversion emission can be attributed to the tran-
4
3+
3
and I13/2 levels respectively. Secondly, the Tm in F4 level are
promoted to F2 level and Er ions in 4I11/2/ I
3
3+
4
to 4F / F
4
13/2
7/2
9/2
1
3
3+
2
4
4
, and 4F9/2 → 4I
sitions G → H of Tm
,
H11/2/ S → I
4
6
3/2
15/2
15/2
levels by the same energy transfer. The Tm ions in 3F2 level
3+
3+
of Er ions, respectively. In addition, the efficient energy trans-
fer processes between Er and Tm occurred. Tm behaves as
the sensitizer to Er for red and green luminescence of Er and
Er acts as quenching center for the blue upconversion lumines-
and Er ions in F7/2 level relax non-radiatively to 3H4 level and
3
+
4
3+
3+
3+
2
4
H11/2/ S levels respectively. The third energy transfer processes,
3+
3+
3/2
the Tm in 3H4 level is raised to 1G4 level. Finally, the blue emis-
3
+
3+
sion at 476 nm is generated by transition of Tm ions 1G → H
3+
3
4
6
3+
cence of Tm ions. This hexagonal phase Na (Y1.5Na0.5) F nanorods
6
transition; the green emission at 520 and 545 nm are assigned to
with multicolor upconversion emission might promise further
fundamental research and be found application in nanoscale opto-
electronics and solid-state lasers.
3
+
2
4
4
4
the Er ions H11/2 → I
and S3/2 → I
transitions; the red
(660 nm) the Tm
15/2
15/2
emission are due to the Er ions 4F9/2 → I
3
+
4
3+
15/2
ions 3F → H transitions (690 nm). However, some other mech-
3
3
6
anism must exist, since the intensity of the red, green and blue
emissions were changed in our triply doped sample as mentioned
Acknowledgments
This work is supported by the National Natural Scientific Foun-
dation of China (No. 10874144) and the Scientific Foundation of
Education Department of Hunan Province (No. 08C885).
References
[
[
[
[
1] X.F. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature 421 (2003) 241.
2] X.D. Wang, C.J. Summers, Z.L. Wang, Nano Lett. 4 (2004) 423.
3] J. Cao, Q. Wang, H. Dai, Nat. Mater. 4 (2005) 745.
4] A. Javey, H. Kim, M. Brink, Q. Wang, A. Ural, J. Guo, P. Melntyre, P. Mceuen, M.
Lundstrom, H. Day, J. Nat. Mater. 1 (2002) 106.
[
5] J.C. Johnson, H.J. Choi, K.P. Knutsen, R.D. Schaller, P.D. Yang, R.J. Saykally, Nat.
Mater. 1 (2002) 106.
[
[
6] J.H. Zeng, J. Sun, Z.H. Li, R.X. Yan, Y.D. Li, Adv. Mater. 17 (2005) 2119.
7] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D.
Yang, Science 292 (2001) 1897.
[
[
8] L.D. Hicks, M.S. Dresselhaus, Phys. Rev. B 47 (1995) 16631.
9] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185.
[
[
10] J.F. Liu, Q.H. Yao, Y.D. Li, Appl. Phys. Lett. 88 (2006) 17319.
11] G.S. Yi, H.C. Lu, S.Y. Zhao, G. Yue, W.J. Yang, D.P. Chen, L.H. Guo, Nano Lett. 4
(
2004) 2191.
[
[
12] F. Auzel, Chem. Rev. 104 (2004) 139.
13] L.J. Lin, G.Z. Ren, M.P. Chen, Y. Liu, Q.B. Yang, Opt. Mater. 31 (2009) 1439.
Fig. 7. Simplified energy level diagram of Yb3+, Tm3+ and Er3+, as well the possible
mechanism for the upconversion luminescence.
[14] L.J. Lin, G.Z. Ren, M.P. Chen, Y. Liu, J. Alloys Compd. 486 (2009) 261.
[15] X.X. Luo, W.H. Cao, J. Alloys Compd. 460 (2008) 529.

4
80
S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
[
[
[
[
[
[
16] Y.Q. Qu, Y. Yu, X.G. Kong, Y.J. Sun, Q.H. Zeng, H. Zhang, Mater. Lett. 63 (2009)
285.
17] J.F. Suyver, A. Aebischer, S. Garcia-Revilla, P. Gerner, H.U. Gudel, Phys. Rev. B 71
2005) 125123.
18] K.W. Kra¨mer, D. Biner, G. Frei, H.U. Gu¨del, M.P. Hehlen, S.R. Lu¨thi, Chem.
Mater. 16 (2004) 1244.
19] M. Wang, C.C. Mi, J.L. Liu, X.L. Wu, Y.X. Zhang, W. Hou, F. Li, S.K. Xu, J. Alloys
Compd. 485 (2009) L24.
20] G.F. Wang, W.P. Qin, J.S. Zhang, L.L. Wang, G.D. Wei, P.F. Zhu, R. Kim, J. Alloys
Compd. 475 (2009) 452.
[22] G. Pushpal, P. Amitava, J. Phys. Chem. C 112 (2008) 19283.
[23] B.A. Dong, H.W. Song, H.Q. Yu, H. Zhang, R.F. Qin, X. Bai, G.H. Pan, S.Z. Lu, F.
Wang, L.B. Fan, Q.L. Dai, J. Phys. Chem. C 112 (2008) 1435.
[24] B.A. Dong, H.W. Song, R.F. Qin, X. Bai, S.Z. Lu, H. Zhang, F. Wang, L.B. Fan, J. Phys.
Chem. C 8 (2008) 3921.
1
(
[25] L.Y. Wang, Y.D. Li, Chem. Mater. 19 (2007) 727.
[26] X.C. Jiang, L.D. Sun, W. Feng, C.H. Yan, Cryst. Growth. Des 42 (2004) 517.
[27] G.Y. Chen, Y. Liu, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Appl. Phys. Lett. 91
(2007) 133103.
[28] M. Liao, L. Hu, Y. Fang, J. Zhang, H. Sun, S. Xu, L. Zhang, Spectrochim. Acta Part
A 68 (2007) 531.
21] G. Pushpal, P. Amitava, J. Phys. Chem. C 112 (2008) 3223.