Journal of Luminescence 161 (2015) 389–394 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin Structure and photoluminescence characterization of Tb3 ỵ -doped LaPO4 nanorods prepared via the microwave-assisted method Duong Thi Lien, Duong Thi Mai Huong, Le Van Vu, Nguyen Ngoc Long n Centre for Materials Science, Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam art ic l e i nf o a b s t r a c t Article history: Received 29 August 2014 Received in revised form January 2015 Accepted 25 January 2015 Available online February 2015 LaPO4 nanopowders doped with 1, 2… 10, 11, 15 and 20 mol% Tb3 ỵ have been prepared by the microwaveassisted method Transmission electron microscope (TEM) images indicated that the nanopowders are composed 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 angle side with increasing Tb3 ỵ dopant concentration Raman scattering measurement found out that some of scattering lines were shifted and broadened to higher wavenumber side with increasing Tb3 þ concentration It is discovered that the photoluminescence (PL) of Tb3 ỵ ions results from the radiative intra-congurational ff transitions that occur from the 5D4 exited state to the 7FJ (Jẳ0,16) ground states; the photoluminescence excitation (PLE) of Tb3 ỵ ions takes place from the 7F6 ground state to the 5DJ (J¼0,1–4), 5L10, 5GJ (J¼2–4), 5HJ and 5IJ exited states It was observed that the photoluminescence intensity reached a maximum in the samples doped with 15 mol% Tb3 ỵ Double-exponential decay of the 5D4-7F5 emission was observed with lifetimes of $ ns and 6.35 ms & 2015 Elsevier B.V All rights reserved Keywords: Lanthanum orthophosphate Nanorods Microwave-assisted method Raman scattering Photoluminescence 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 presentday 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 nanoscale [4,5] Recently, lanthanide orthophosphate (LaPO4) has been reported to act as an excellent host for cerium, terbium, and europium 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-earthdoped LaPO4 is one of the best candidates for biomedical applications such as fluorescence 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 various fields 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: Ce3 þ , Eu3 þ , Tb3 þ nanorods with hexagonal structure prepared by the microwave method Hence, we tried to use this method for synthesis of LaPO4:Tb3 ỵ nanorods In this paper we report the preparation of LaPO4 nanorods doped with terbium (Tb3 ỵ ) ions by microwave-assisted technique Our studies are focused on the Tb3 ỵ concentration effect on XRD, Raman scattering spectra, PL properties of the LaPO4:Tb3 ỵ nanorods Experimental n Corresponding author E-mail addresses: longnn@vnu.edu.vn, ngocnglong@gmail.com (N Ngoc Long) http://dx.doi.org/10.1016/j.jlumin.2015.01.050 0022-2313/& 2015 Elsevier B.V All rights reserved La1 À xTbxPO4 nanopowders with x¼ 0, 0.01… 0.11, 0.15 and 0.20 have been prepared from the chemicals such as La2O3 powders, 390 D Thi Lien et al / Journal of Luminescence 161 (2015) 389–394 bulk Tb, HNO3 acid, and NH4H2PO4 by 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 to form La(NO3)3 transparent solution A stoichiometric amount of Tb was dissolved in diluted HNO3 acid (30%) under vigorous stirring for 15 to produce Tb(NO3)3 aqueous solution An appropriate amount of NH4H2PO4 was dissolved in double distilled water under constant stirring for 15 to prepare NH4H2PO4 solution Stoichiometric amounts of La(NO3)3 and Tb(NO3)3 aqueous solutions were mixed, and then appropriate amounts of NH4H2PO4 solution 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 ml flask 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 The obtained precipitate was centrifuged, washed with distilled water and absolute alcohol to remove chemicals possibly remaining in the final products, and was dried at 75 1C for 12 h in air Crystal structure of the powders was analyzed by X-ray diffraction (XRD) using an X-ray diffractometer SIEMENS D5005, Bruker, Germany with Cu-Kα1 (λ ¼1.54056 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 spectrometer (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 Results and discussion Typical XRD patterns of LaPO4 nanopowders doped with 0, 7, 15 and 20 mol% Tb3 ỵ concentrations prepared with an irradiation power of 450 W for 30 are shown in Fig 1a All the peaks in the XRD patterns clearly indicate that the LaPO4 undoped and doped with Tb3 ỵ samples possess hexagonal crystal structure No other diffraction peaks are detected except for the LaPO4 related peaks The lattice constants determined from the XRD patterns are a¼b¼ 7.088 Å, c¼ 6.489 Å and c/a¼0.9155 which are in agreement with the standard values a¼ b¼ 7.042 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 Fig (a) XRD patterns of LaPO4 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 Tb3 ỵ concentration 0:9 cos where β is the full width at half maximum (FWHM) in radians of the diffraction peaks, θ is Bragg's diffraction angle and λ ¼1.54056 Å The average sizes of the LaPO4 nanocrystallites were estimated to be 10 nm It is noted from Fig 1a that with increasing Tb3 ỵ concentration in the XRD patterns was as well observed an amorphous phase The XRD analysis results for the samples prepared with different microwave 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 2θ angle side with increasing Tb3 ỵ concentration (see Fig 1b), which exhibited a shrink of host lattice The reason for this shrink is because the radius of Tb3 ỵ dopant ion Fig EDS spectra of LaPO4 nanopowders doped with mol% and 20 mol% Tb3 ỵ concentrations with coordination number of (1.095 ) is smaller than that of La3 ỵ ion (1.216 Å) [28] Typical EDS spectra of the LaPO4 nanopowders doped with and 20 mol% Tb3 ỵ are shown in Fig The amount of Tb obtained by EDS analysis in and 20 mol% doped LaPO4 samples was 1.43 and 3.26 at%, D Thi Lien et al / Journal of Luminescence 161 (2015) 389–394 respectively 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 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 LaPO4 nanorods 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 cm À Fig depicts room-temperature Raman spectra of Tb3 þ -doped LaPO4 nanopowders with hexagonal crystal structure, which are in good agreement with the results reported recently by Ref [32] for Tb3 ỵ -doped LaPO4 nanowires prepared by hydrothermal technique at low temperature Fig (a) Low magnified and (b) high magnified TEM images of LaPO4 nanopowders prepared by the microwave-assisted method 391 Fig Room-temperature Raman scattering spectra of Tb doped LaPO4 nanopowders prepared by the microwave-assisted method Various Tb3 ỵ concentrations are shown in the gure As can be seen from the figure, the Raman spectrum of hexagonal LaPO4 nanopowders exhibits five scattering line groups First group consists of 226 cm À line in the range of 100–300 cm À 1; second group: 377, 413 and 466 cm À lines in the range of 375–500 cm À 1; third group: 544, 573 and 624 cm À lines in the range of 525–625 cm À 1; fourth group: 974 cm À line in the range of 950–980 cm À 1; and fifth group: 1029, 1049 and 1084 cm À lines in the range of 990–1100 cm À The observed Raman bands were assigned to the lattice vibrations and typical vibrational bands of the (PO4)3À tetrahedron [33–35] Indeed, a free (PO4)3À ion has the four normal vibrational modes: O–P–O E-bending ν2(E), O–P–O F2-bending ν4(F2), P–O symmetric stretching ν1(A1) and P–O asymmetric stretching ν3(F2) The Raman scattering lines of hexagonal LaPO4 nanorods and their assignment are listed in Table compared with infrared (IR) absorption lines From Table it is found that the ν2(E) mode appears only in Raman spectra, meanwhile the lattice vibrational, ν4(F2), ν1(A1) and ν3(F2) modes appear both in Raman and in IR spectra To investigate inuence of Tb3 ỵ dopant, the Raman spectra of the LaPO4 nanorods undoped and Tb3 ỵ -doped with different concentrations were measured The results are presented in two regions: from 100 cm À to 700 cm À (Fig 5a) and from 925 cm À to 1100 cm À (Fig 5b) It can be found that some scattering lines (226, 573, 624, 974 and 1084 cm À 1) are clearly shifted and broadened to higher wavenumber side Of which a considerable change is observed for the 974 cm À line Table lists the position and FWHM of the 974 cm À line assigned to the P–O symmetric stretching modes ν1(A1) As can be seen from Table 2, for the undoped LaPO4 samples the Raman peak of ν1(A1) modes is at 974.6 cm À with FWHM of 5.5 cm and when increasing Tb3 ỵ dopant concentration up to 20 mol%, the corresponding Raman peak of ν1(A1) modes is at 979.1 cm À with FWHM of 12.9 cm À 1, i.e is shifted by 5.5 cm À and broadened by 7.4 cm À towards the higher frequency Phenomenon that the Raman scattering lines are shifted and broadened to higher wavenumber when doping 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 structure stability of LaPO4 [29,30] and TbPO4 [31] by Raman spectroscopy, 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 Tb3 ỵ -doped LaPO4 nanorods, the radius of Tb3 þ ion is smaller 392 D Thi Lien et al / Journal of Luminescence 161 (2015) 389–394 Table Experimental Raman scattering and infrared (IR) absorption frequencies (cm À 1) at room temperature of the hexagonal LaPO4 and assignment Groups, assignment [33–35] Raman (this work) Raman [32] IR [35] IR [36] IR [37] 178 202 228 253 Not seen Not seen Not seen Not seen Not seen 540 570 593 615 949 542 570 615 971 (Raman) Not fully resolved Lattice vibrations 100–300 226 228 ν2(E) 375–500 377 413 466 544 573 376 414 466 545 575 624 974 625 977 ν4(F2) 525–625 ν1(A1) 950–980 ν3(F2) 990–1100 1029 1049 1084 1039 1085 991 1020 1043 1070 1090 IR [38] IR [39] IR [40] IR [41] Not seen Not seen Not seen Not seen Not seen Not seen Not seen 542 569 541 570 542 542 566 538 567 617 966 (Shoulder) Not fully resolved 613 956 619 Not seen 615 Not seen 615 961 1052 1050 1012 1044 215 Not fully resolved Table The P–O symmetric stretching modes ν1(A1) in Raman spectra of the LaPO4 nanorods doped with different Tb3 ỵ concentrations LaPO4 Undoped mol% Tb3 ỵ 11 mol % Tb3 ỵ 15 mol% Tb3 ỵ 20 mol% Tb3 ỵ Position (cm 1) FWHM (cm 1) 974.6 5.5 975.6 8.0 976.7 10.7 977.7 11.7 979.1 12.9 Fig PLE and PL spectra measured at room temperature of LaPO4 nanopowders doped with 15 mol% Tb3 ỵ Fig Room-temperature Raman spectra in the wavenumber region (a) from 100 cm À to 700 cm À and (b) from 925 cm À to 1100 cm À of the LaPO4 nanopowders undoped and doped with 5, 11, 15 and 20 mol% Tb3 ỵ than that of La3 ỵ ion Hence replacement of the La3 þ ions 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 ỵ -dopant concentration Fig represents the room-temperature PLE spectrum monitored at 543 nm and the PL spectrum under excitation wavelength of 368 nm of Tb3 ỵ -doped LaPO4 nanorods with 15 mol% As seen below, the lines in the two spectra are interpreted as the optical intra-congurational ff transitions in the Tb3 ỵ ions The roomtemperature PL spectra under excitation wavelength of 368 nm of LaPO4 nanorods doped with various concentrations of Tb3 ỵ are illustrated in Fig It can be seen from the inset of Fig that the PL intensity achieved its maximal value for the samples doped with 15 mol% Tb3 ỵ , in good agreement with the reported value of 16 mol% Tb3 ỵ for Tb-doped LaPO4 lms [18] When increasing Tb3 ỵ concentration higher than 15% the PL intensity decreased This is a conventional concentration quenching effect Room-temperature PL spectrum under 368 nm excitation wavelength of LaPO4 nanorods doped with 15 mol% of Tb3 ỵ is illustrated D Thi Lien et al / Journal of Luminescence 161 (2015) 389–394 Fig Room-temperature PL spectra with 368 nm excitation wavelength of LaPO4 nanopowders doped with various concentrations of Tb3 þ 393 Fig Typical PLE spectrum monitored at 543 nm emission line of LaPO4 nanopowder doped with 15 mol% of Tb3 ỵ Fig Room-temperature PL spectrum under 368 nm excitation wavelength of LaPO4 nanopowders doped with 15 mol% of Tb3 ỵ in Fig 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 the 7F6, 7F5, 7F4 and 7F3 ground states, respectively Some groups of very weak emission lines at 645, 667 and 681 nm are assigned to 5D4-7F2, 7F1 and 7F0 transitions, respectively (see the inset of Fig 8) It is noted that in the case of our LaPO4:Tb3 ỵ nanorods the PL lines of Tb3 ỵ 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% Tb3 þ nanorods is depicted in Fig The groups of excitation lines located around 283, 303, 317, 340, 350, 368, 377 and 486 nm are attributed to the absorption transitions from the 7F6 ground state to the 5IJ, 5HJ, 5D0,1, GJ, 5D2, 5L10, 5D3 and 5D4 excited states, respectively It should be noted that LaPO4 has bandgap energy of around eV and we used xenon lamp as an excitation source, therefore, the Tb3 ỵ 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 ỵ 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 Fig 10 Luminescence decay curves at 550 nm of Tb3 þ ions doped in LaPO4 nanorods under 337 nm laser excitation with (a) MΩ load resistor and (b) 50 Ω load resistor Decay curves of emission at 550 nm (5D4-7F5) of Tb3 ỵ ion were measured with M and 50 Ω load resistors under 337 nm laser excitation Typical decay curves in semi-logarithmic scale are depicted in Fig 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 wellknown that Tb3 ỵ ff transitions are spin and parity forbidden and therefore the observed Tb3 ỵ luminescence decay is very long (in the 394 D Thi Lien et al / Journal of Luminescence 161 (2015) 389–394 millisecond scale) However, from Fig 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 $ ns (‘fast’ component) and 6.35 ms (‘slow’ one) Similar non-exponential decay was ready detected for the (5D4-7F5) Tb3 ỵ 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) Tb3 ỵ 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 ỵ emission: (i) the direct energy transfer from the excited state of Tb3 ỵ 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 luminescence centers in nanostructures: ‘surface’- and ‘core’-related centers The centers located close to surface are responsible for ‘fast’ component, 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 Tb3 ỵ ions to the surface states or the decay of ‘surface’-related Tb3 þ ions Another reason is assumed to be the energy transfer between 5D3 level and 5D4 one of Tb3 ỵ ions Conclusion LaPO4:Tb3 ỵ nanorods doped with different Tb3 þ concentrations from 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 the first time we revealed that the replacement of the La3 ỵ ions with the Tb3 ỵ ions causes a shrinking host lattice, that leads to the shift and broadening of some vibrational modes towards higher wavenumber with increasing Tb3 þ dopant concentration The PL and PLE spectra of Tb3 þ 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 Tb3 þ ions to the surface states or the decay of surface-related Tb3 ỵ ions Another reason is assumed to be the energy transfer between 5D3 level and D4 one of Tb3 ỵ ions References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] 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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 Tb3 ỵ ions to the surface states or the decay of. .. in the range of 990–1100 cm À The observed Raman bands were assigned to the lattice vibrations and typical vibrational bands of the (PO4)3À tetrahedron [33–35] Indeed, a free (PO4)3À ion has the. .. 7F1 and 7F0 transitions, respectively (see the inset of Fig 8) It is noted that in the case of our LaPO4: Tb3 ỵ nanorods the PL lines of Tb3 ỵ ions are poorly resolved, that is the same as the