X-ray diffraction XRD analysis showed that the prepared nanorods exhibit hexagonal crystal structure and the diffraction peaks were shifted towards the higher 2θangle side with increasin
Trang 1Structure and photoluminescence characterization of Tb 3 þ -doped
Centre for Materials Science, Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
a r t i c l e i n f o
Article history:
Received 29 August 2014
Received in revised form
7 January 2015
Accepted 25 January 2015
Available online 3 February 2015
Keywords:
Lanthanum orthophosphate
Nanorods
Microwave-assisted method
Raman scattering
Photoluminescence
a b s t r a c t LaPO4nanopowders doped with 1, 2… 10, 11, 15 and 20 mol% Tb3þhave been prepared by the microwave-assisted method Transmission electron microscope (TEM) images indicated that the nanopowders are comp-osed of nanorods X-ray diffraction (XRD) analysis showed that the prepared nanorods exhibit hexagonal crystal structure and the diffraction peaks were shifted towards the higher 2θangle side with increasing Tb3þ dopant concentration Raman scattering measurement found out that some of scattering lines were shifted and broadened to higher wavenumber side with increasing Tb3þconcentration It is discovered that the photoluminescence (PL) of Tb3þions results from the radiative intra-configurational f–f transitions that occur from the5D4exited state to the7FJ(J¼0,1–6) ground states; the photoluminescence excitation (PLE) of Tb3þ
ions takes place from the7F6ground state to the5DJ(J¼0,1–4),5L10,5GJ(J¼2–4),5HJand5IJexited states It was observed that the photoluminescence intensity reached a maximum in the samples doped with 15 mol% Tb3þ Double-exponential decay of the5D4-7F5emission was observed with lifetimes of5 ns and 6.35 ms
& 2015 Elsevier B.V All rights reserved
1 Introduction
Rare earth phosphates have become a topic of growing interest
during the past few years due to their potential applications in
optical materials, phosphors, lasers, light-emitting diodes (LED),
sensors, displays and luminescent lamps[1–3] One of the
present-day actual tasks is the fabrication of relatively cheap rare earth
phosphate nanopowders because their luminescence efficiency is
expected to increase when the size of particles decreases to
nano-scale[4,5] Recently, lanthanide orthophosphate (LaPO4) has been
reported to act as an excellent host for cerium, terbium, and
euro-pium ions, to synthesize phosphors emitting a variety of colors for
development of photoluminescent materials with applications in
optoelectronic devices, solid-state lasers, white LEDs, displays, and
phosphors[6–13] Very recently, it has been found that
rare-earth-doped LaPO4is one of the best candidates for biomedical
applica-tions such asfluorescence resonance energy transfer (FRET) assays,
biolabelling, optical imaging or phototherapy[14–16] For synthesis
of the rare-earth-doped LaPO4 various methods including sol–gel
[16–18], hydrothermal synthesis [13,16,19,20], co-precipitation
[21–23], polyol-mediated synthesis [8] and microwave-assisted
technique[9–12]have been developed The LaPO4 nanostructures
with various morphologies such as nanowires[19,23], nanorods
[12,17,24,25], thin films [18], nanoparticles [8–11,13,14,16,21,22]
and microspheres[20]were obtained
Microwave irradiation is used as a heating method, which is generally quite fast, simple and efficient in energy In the last two decades microwave-assisted technique has been developed and is widely used in variousfields of chemistry[26] The effect of heating
is created by the interaction of the dipole moment of the molecules with the high frequency electromagnetic radiation (2.45 GHz) The application of microwave irradiation to materials science has shown very rapid growth due to its unique reaction effects such as rapid volumetric heating and the consequent dramatic increase in reaction rates, etc By using the microwave-assisted technique, authors of the earlier works [9–11] have synthesized LaPO4 nanoparticles with monoclinic crystal structure Only Ma et al.[12]reported on LaPO4:
Ce3þ, Eu3þ, Tb3 þ nanorods with hexagonal structure prepared by the microwave method Hence, we tried to use this method for synthesis of LaPO4:Tb3 þnanorods
In this paper we report the preparation of LaPO4nanorods doped with terbium (Tb3 þ) ions by microwave-assisted technique Our studies are focused on the Tb3þconcentration effect on XRD, Raman scattering spectra, PL properties of the LaPO4:Tb3þnanorods
2 Experimental
La1xTbxPO4nanopowders with x¼0, 0.01… 0.11, 0.15 and 0.20 have been prepared from the chemicals such as La2O3 powders,
Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/jlumin
Journal of Luminescence
http://dx.doi.org/10.1016/j.jlumin.2015.01.050
0022-2313/& 2015 Elsevier B.V All rights reserved.
n Corresponding author.
E-mail addresses: longnn@vnu.edu.vn , ngocnglong@gmail.com (N Ngoc Long).
Trang 2bulk Tb, HNO3acid, and NH4H2PO4by using microwave irradiation.
In a typical synthesis, a stoichiometric amount of La2O3 was
dissolved in diluted HNO3 acid (30%) under vigorous magnetic
stirring for 15 min to form La(NO3)3 transparent solution A
stoi-chiometric amount of Tb was dissolved in diluted HNO3acid (30%)
under vigorous stirring for 15 min to produce Tb(NO3)3 aqueous
solution An appropriate amount of NH4H2PO4 was dissolved in
double distilled water under constant stirring for 15 min to prepare
NH4H2PO4 solution Stoichiometric amounts of La(NO3)3 and
Tb(NO3)3 aqueous solutions were mixed, and then appropriate
amounts of NH4H2PO4solution were added into the mixed nitrate
solution under stirring for 30 min, obtaining an opalescent solution
The resulting solution was then transferred into a 100 mlflask The
flask containing the above solution was put in a microwave oven
(SANYO, EM-D9553) with irradiation powers of 300, 450 and 750 W
for 20, 30 and 40 min The obtained precipitate was centrifuged,
washed with distilled water and absolute alcohol to remove
chemi-cals possibly remaining in thefinal products, and was dried at 75 1C
for 12 h in air
Crystal structure of the powders was analyzed by X-ray
diffrac-tion (XRD) using an X-ray diffractometer SIEMENS D5005, Bruker,
Germany with Cu-Kα1 (λ¼1.54056 Å) irradiation The surface
morphology of the samples was observed by using a JEOL JEM
1010 transmission electron microscope (TEM) The composition of
the samples was determined by an energy-dispersive X-ray
spectro-meter (EDS) OXFORD ISIS 300 attached to the JSM 5410 LV, JEOL,
Japan scanning electron microscope (SEM) Raman measurements
were carried out by using LabRam HR800, Horiba spectrometer
with 632.8 nm excitation The PL and the PLE spectra measured
at room temperature were carried out on a spectrofluorometer
Fluorolog FL 3-22 Jobin-Yvon-Spex, USA with a 450 W xenon lamp
as an excitation source
3 Results and discussion
Typical XRD patterns of LaPO4nanopowders doped with 0, 7, 15
and 20 mol% Tb3 þ concentrations prepared with an irradiation
power of 450 W for 30 min are shown inFig 1a All the peaks in the
XRD patterns clearly indicate that the LaPO4 undoped and doped
with Tb3þ samples possess hexagonal crystal structure No other
diffraction peaks are detected except for the LaPO4related peaks
The lattice constants determined from the XRD patterns are
a¼b¼7.088 Å, c¼6.489 Å and c/a¼0.9155 which are in agreement
with the standard values a¼b¼7.042 Å, c¼6.445 Å and c/a¼0.9158
(JCPDS card no 04-0635) The average sizes of the crystallites were
estimated by Debye–Scherrer's formula[27]:
D¼ 0:9λ
β cosθ
whereβis the full width at half maximum (FWHM) in radians of
the diffraction peaks, θ is Bragg's diffraction angle and
λ¼1.54056 Å The average sizes of the LaPO4nanocrystallites were
estimated to be 10 nm
It is noted fromFig 1a that with increasing Tb3þconcentration in
the XRD patterns was as well observed an amorphous phase The
XRD analysis results for the samples prepared with different
micro-wave irradiation powers showed that XRD patterns of the samples
synthesized with a low (300 W) irradiation power exhibited a little
amorphous phase; the samples synthesized with a higher (450 and
750 W) irradiation power possessed better crystallinity Learning
XRD patterns in more detail, we revealed that the diffraction peaks
were shifted towards the higher 2θangle side with increasing Tb3þ
concentration (seeFig 1b), which exhibited a shrink of host lattice
The reason for this shrink is because the radius of Tb3þ dopant ion
with coordination number of 9 (1.095 Å) is smaller than that of La3 þ
ion (1.216 Å)[28] Typical EDS spectra of the LaPO4nanopowders doped with 9 and
20 mol% Tb3þare shown inFig 2 The amount of Tb obtained by EDS analysis in 9 and 20 mol% doped LaPO samples was 1.43 and 3.26 at%,
Fig 1 (a) XRD patterns of LaPO 4 powders doped with different Tb concentrations prepared with a microwave irradiation power of 450 W for 30 min, and (b) the shift
of diffraction peaks to the higher 2θ angle side with increasing Tb 3þ concentration.
Fig 2 EDS spectra of LaPO 4 nanopowders doped with 9 mol% and 20 mol% Tb3þ concentrations.
Trang 3respectively It can be seen that the amounts of Tb obtained by EDS
analysis is much less than those added during synthesis The carbon
(C) and aluminium (Al) weak peaks observed in the EDS spectra
originated from the carbon tape used to glue the powders in the EDS
measurement
The TEM images of LaPO4 nanopowders depicted in Fig 3
indicate that the powders are composed of the nanorods which
are around 10–15 nm in diameter and the length ranging from
300 to 800 nm Most of earlier works [9–11] indicated that the
products obtained by the microwave-assisted method were LaPO4
nanoparticles with monoclinic crystal structure The only work of
Lin Ma et al.[12]informed that by using the microwave method
they received LaPO4nanorods with hexagonal structure
It is well known that Raman scattering spectroscopy is becoming a
powerful technique for the characterization of materials For example,
Raman spectroscopy has been applied to LaPO4[29,30] and TbPO4
[31]phosphors for studying pressure effect on their structure stability
Our Raman measurements were performed at room temperature
in the wavenumber range from 100 to 1200 cm1 Fig 4 depicts
room-temperature Raman spectra of Tb3þ-doped LaPO4
nanopow-ders with hexagonal crystal structure, which are in good agreement
with the results reported recently by Ref.[32]for Tb3þ-doped LaPO4
nanowires prepared by hydrothermal technique at low temperature
As can be seen from thefigure, the Raman spectrum of hexagonal LaPO4 nanopowders exhibits five scattering line groups First group consists of 226 cm1line in the range of 100–300 cm1; second group: 377, 413 and 466 cm1 lines in the range of
375–500 cm1; third group: 544, 573 and 624 cm1lines in the range of 525–625 cm1; fourth group: 974 cm1line in the range
of 950–980 cm1; and fifth group: 1029, 1049 and 1084 cm1
lines in the range of 990–1100 cm1 The observed Raman bands were assigned to the lattice vibrations and typical vibrational bands of the (PO4)3 tetrahedron [33–35] Indeed, a free (PO4)3 ion has the four normal vibrational modes:
O–P–O E-bending ν2(E), O–P–O F2-bending ν4(F2), P–O symmetric stretchingν1(A1) and P–O asymmetric stretchingν3(F2) The Raman scattering lines of hexagonal LaPO4nanorods and their assignment are listed in Table 1compared with infrared (IR) absorption lines FromTable 1it is found that theν2(E) mode appears only in Raman spectra, meanwhile the lattice vibrational, ν4(F2),ν1(A1) andν3(F2) modes appear both in Raman and in IR spectra
To investigate influence of Tb3þ dopant, the Raman spectra of the LaPO4 nanorods undoped and Tb3 þ-doped with different concentrations were measured The results are presented in two regions: from 100 cm1to 700 cm1(Fig 5a) and from 925 cm1
to 1100 cm1(Fig 5b)
It can be found that some scattering lines (226, 573, 624, 974 and
1084 cm1) are clearly shifted and broadened to higher wavenumber side Of which a considerable change is observed for the 974 cm1 line Table 2 lists the position and FWHM of the 974 cm1 line assigned to the P–O symmetric stretching modesν1(A1) As can be seen fromTable 2, for the undoped LaPO4samples the Raman peak of
ν1(A1) modes is at 974.6 cm1with FWHM of 5.5 cm1and when increasing Tb3þ dopant concentration up to 20 mol%, the corre-sponding Raman peak ofν1(A1) modes is at 979.1 cm1with FWHM
of 12.9 cm1, i.e is shifted by 5.5 cm1and broadened by 7.4 cm1 towards the higher frequency
Phenomenon that the Raman scattering lines are shifted and broadened to higher wavenumber when doping 6 H-SiC crystal with N-, Al-, B-, V-impurities was investigated in detail by Refs
[42,43] and was attributed to the coupling interaction between phonons and carrier plasmon Studying pressure effect on struc-ture stability of LaPO4[29,30]and TbPO4[31]by Raman spectro-scopy, the authors of Refs [29,31]had revealed that all Raman modes were shifted and broadened to higher wavenumber with increasing pressure According to Lacomba-Perales et al.[30], the pressure induced a decrease in lattice parameter and bond length, resulting in the shift and broadening of Raman modes In our case
of Tb3þ-doped LaPO nanorods, the radius of Tb3þ ion is smaller
Fig 3 (a) Low magnified and (b) high magnified TEM images of LaPO 4
nanopow-ders prepared by the microwave-assisted method.
Fig 4 Room-temperature Raman scattering spectra of Tb doped LaPO 4 nanopow-ders prepared by the microwave-assisted method Various Tb3þconcentrations are shown in the figure.
Trang 4than that of La3þion Hence replacement of the La3þions with the
Tb3 þ ions causes a shrinking host lattice (as observed from XRD
measurements), i.e causes a decreasing lattice parameter and
bond length; and as a result of this, the vibrational modes are
shifted and broadened to higher wavenumber with increasing
Tb3 þ-dopant concentration
Fig 6 represents the room-temperature PLE spectrum
mon-itored at 543 nm and the PL spectrum under excitation wavelength
of 368 nm of Tb3þ-doped LaPO nanorods with 15 mol% As seen
below, the lines in the two spectra are interpreted as the optical intra-configurational f–f transitions in the Tb3þ ions The room-temperature PL spectra under excitation wavelength of 368 nm of LaPO4 nanorods doped with various concentrations of Tb3þ are illustrated inFig 7 It can be seen from the inset ofFig 7that the
PL intensity achieved its maximal value for the samples doped with 15 mol% Tb3 þ, in good agreement with the reported value of
16 mol% Tb3 þ for Tb-doped LaPO4 films [18] When increasing
Tb3þ concentration higher than 15% the PL intensity decreased This is a conventional concentration quenching effect
Room-temperature PL spectrum under 368 nm excitation wave-length of LaPO nanorods doped with 15 mol% of Tb3 þis illustrated
Table 1
Experimental Raman scattering and infrared (IR) absorption frequencies (cm1) at room temperature of the hexagonal LaPO 4 and assignment.
Groups, assignment [33–35] Raman (this work) Raman [32] IR [35] IR [36] IR [37] IR [38] IR [39] IR [40] IR [41]
202
253
593
ν 3 (F 2 ) 991 Not fully resolved Not fully resolved Not fully resolved
Fig 5 Room-temperature Raman spectra in the wavenumber region (a) from
100 cm1 to 700 cm1 and (b) from 925 cm1 to 1100 cm1 of the LaPO 4
nanopowders undoped and doped with 5, 11, 15 and 20 mol% Tb 3 þ
Table 2 The P–O symmetric stretching modes ν 1 (A 1 ) in Raman spectra of the LaPO 4
nanorods doped with different Tb 3 þ concentrations.
LaPO 4 Undoped 5 mol%
Tb 3þ
11 mol
% Tb 3þ
15 mol%
Tb 3þ
20 mol%
Tb 3þ
Position (cm1) 974.6 975.6 976.7 977.7 979.1
Fig 6 PLE and PL spectra measured at room temperature of LaPO 4 nanopowders doped with 15 mol% Tb 3 þ
Trang 5inFig 8 The groups of emission lines located at 489, 543, 585 and
620 nm are assigned to the emission transitions from the 5D4
excited state to the7F6,7F5,7F4and7F3ground states, respectively
Some groups of very weak emission lines at 645, 667 and
681 nm are assigned to5D4-7F2,7F1and7F0transitions,
respec-tively (see the inset ofFig 8) It is noted that in the case of our
LaPO4:Tb3þ nanorods the PL lines of Tb3þ ions are poorly
resolved, that is the same as the result of Pankratov et al.[44,45]
for LaPO4:Ce,Tb nanopowder The reason of this was suggested to
be a strong perturbation of the crystal field due to the small
nanoparticles size[44,45]
Our experimental results (not shown here) indicated that all the
emission line groups have the same excitation spectra, which prove
that all these lines possess the same origin Typical PLE spectrum
monitored at 543 nm emission line of LaPO4-15% Tb3þnanorods is
depicted inFig 9 The groups of excitation lines located around
283, 303, 317, 340, 350, 368, 377 and 486 nm are attributed to the
absorption transitions from the7F6ground state to the5IJ,5HJ,5D0,1,
5
GJ,5D2,5L10,5D3and5D4excited states, respectively It should be
noted that LaPO4has bandgap energy of around 8 eV and we used
xenon lamp as an excitation source, therefore, the Tb3þ ions could
be directly excited only by f–f transitions in the range of 2.5–4.4 eV,
as presented above In the work[45]authors excited LaPO4:Ce,Tb
nanopowder by synchrotron radiation with energy of 3.7–40 eV
They revealed that the PL of Tb3 þ ions in nanosized LaPO4:Ce,Tb
can be excited by excitonic transitions (including self-trapped and/
or bound excitons) in the range of 6.5–8.5 eV
Decay curves of emission at 550 nm (5D4-7F5) of Tb3þ ion were measured with 1 MΩand 50Ωload resistors under 337 nm laser excitation Typical decay curves in semi-logarithmic scale are depicted inFig 10
One can note that the decay curve in Fig 10a almost obeys a mono-exponential law with a long lifetime of 6.35 ms It is well-known that Tb3þ f–f transitions are spin and parity forbidden and therefore the observed Tb3 þluminescence decay is very long (in the
Fig 7 Room-temperature PL spectra with 368 nm excitation wavelength of LaPO 4
nanopowders doped with various concentrations of Tb3þ.
Fig 8 Room-temperature PL spectrum under 368 nm excitation wavelength of
LaPO 4 nanopowders doped with 15 mol% of Tb 3 þ
Fig 9 Typical PLE spectrum monitored at 543 nm emission line of LaPO 4 nanopowder doped with 15 mol% of Tb3þ.
Fig 10 Luminescence decay curves at 550 nm of Tb3þ ions doped in LaPO 4
nanorods under 337 nm laser excitation with (a) 1 MΩ load resistor and (b) 50 Ω load resistor.
Trang 6millisecond scale) However, fromFig 10b it is obvious that in the
initial stage, 550 nm emission decays very fast with a lifetime of
5 ns Hence the overall decay curve of 550 nm emission can be
considered as the sum of two exponents with lifetimes of 5 ns
(‘fast’ component) and 6.35 ms (‘slow’ one) Similar non-exponential
decay was ready detected for the (5D4-7F5) Tb3 þ emission in
nanosized CePO4:Tb [46,47]; LaPO4:Tb,Pr [48] and LaPO4:Ce,Tb
[7,8,11,18,49,50] It should be noted that the single-exponential
decay was observed for the (5D4-7F5) Tb3þ emission in nanosized
LaPO4:Tb[18,19]and LaPO4:Ce,Tb[13,50]
There are several reasons that can lead to the deviation from the
single-exponential decay of Tb3 þ emission: (i) the direct energy
transfer from the excited state of Tb3þ to the co-doped rare earth
impurities that may play a role as quenching centers[46–48] (ii) The
non-exponential decay may be attributed to the surface states, which
may act as quenching centers[47,51] (iii) The non-exponential decay
may be also assigned to two non-equivalent positions of
lumines-cence centers in nanostructures:‘surface’- and ‘core’-related centers
The centers located close to surface are responsible for‘fast’
compo-nent, while those located in the core are accountable for ‘slow’
component [11,51,52] For the case of our LaPO4:Tb nanorods, the
‘fast’ component in the initial part of the decay curve may be related
to the energy transfer from the Tb3þions to the surface states or the
decay of‘surface’-related Tb3 þions Another reason is assumed to be
the energy transfer between5D3level and5D4one of Tb3þ ions
4 Conclusion
LaPO4:Tb3þ nanorods doped with different Tb3 þconcentrations
from 1 to 20 mol% have been successfully synthesized by the
microwave-assisted method With microwave irradiation powers
higher than 300 W, the samples displayed a good crystallinity For
thefirst time we revealed that the replacement of the La3 þions with
the Tb3þ ions causes a shrinking host lattice, that leads to the shift
and broadening of some vibrational modes towards higher
wave-number with increasing Tb3 þdopant concentration The PL and PLE
spectra of Tb3þ ions result from the optical intra-configurational
f–f transitions The ‘fast’ component in the initial part of the decay
curve may be related to the energy transfer from the Tb3þ ions to
the surface states or the decay of‘surface’-related Tb3 þions Another
reason is assumed to be the energy transfer between5D3level and
5D4one of Tb3þ ions
Acknowledgments
The authors wish to express their gratitude to W D de Marcillac,
P Bénalloul, L Coolen, C Schwob, A Maître at Sorbonne University,
UPMC Univ Paris 06, UMR 7588-CMRS, NanoSciences Institute of
Paris (INSP), Paris F-75005, France for measurements of the samples'
luminescence decay and helpful discussions The authors would like
to thank Vietnam National University forfinancially supporting this
research through Project no QGTD 13 04 Authors also thank the
VNU project“Strengthening research and training capacity in fields
of Nano Science and Technology, and Applications in Medical,
Pharmaceutical, Food, Biology, Environmental protection and climate
change adaptation in the direction of sustainable development” for
creating favorable conditions for equipment to complete this work
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