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Cooperative upconversion luminescence of Er3 + in Gd2O3 − xSx phosphor
Accepted Manuscript Cooperative upconversion luminescence of Er3+ in Gd2O3−xSx phosphor
Fei Wang, Bin Yang, Qingchun Yu, Dachun Liu, Wenhui Ma PII: DOI: Reference:
S1386-1425(17)30739-4 doi: 10.1016/j.saa.2017.09.023 SAA 15457
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received date: Revised date: Accepted date:
29 May 2017 2 September 2017 12 September 2017
Please cite this article as: Fei Wang, Bin Yang, Qingchun Yu, Dachun Liu, Wenhui Ma , Cooperative upconversion luminescence of Er3+ in Gd2O3−xSx phosphor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.09.023
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ACCEPTED MANUSCRIPT Cooperative Upconversion luminescence of Er3+ in Gd2O3-xSx phosphor Fei Wang1,2 Bin Yang*1
Qingchun Yu1 Dachun Liu1 Wenhui Ma1
1 State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization in Yunnan Province , National Engineering Laboratory for Vacuum Metallurgy, Key Laboratory for
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Nonferrous Vacuum Metallurgy of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China
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2 Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 bus 2450, 3001
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Leuven, Belgium
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Email address: [email protected]
ABSTRACT
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Gd2O3-xSx: Er crystals were prepared through high-temperature solid-state reaction method
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in vacuum, with the vacuum synthesis mechanism determined by thermal analysis. The crystal structure and upconversion luminescence properties were investigated respectively
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by XRD, TEM and spectrophotometer. Well crystallized Gd2O2S:Er phosphors were prepared
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under 1000℃ in vacuum with a certain excessive amount of sulfur content than stoichiometric. It is confirmed that with the increasing sulfur content the green emission was enhanced and red emission was weakened. The cooperative upconversion luminescence of Er3+ in non-stoichiometric Gd2O3-xSx crystals was interpreted as a result of two photon absorption and the photon avalanche process.
Keywords: upconversion luminescence; vacuum; photon avalanche process; lifetime; super lattice 1
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INTRODUCTION
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Upconversion (UC) is a process where light can be emitted with photon energies higher than those of the light generating the excitation. Cooperative luminescence is one of the
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mechanisms responsible for upconversion, which involves energy transfer processes
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between different ions[1]. In recent years lanthanide-doped upconversion nanocrystals have
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been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays
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and medical imaging[2]. Er3+ is a suitable candidates for UC processes due to their abundant energy levels and narrow emission spectral lines[3,4] and has attracted much attention as an
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activator owing to its high efficiency properties in upconversion luminescence (UCL) [5,6]. Rare earth oxysulfides, as host matrix, show several excellent properties such as chemical
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stability, low toxicity and low phonon energy (~520 cm−1 ) [7]. Several approaches have been developed to prepare fine phosphors, including
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co-precipitation method, sol–gel method, combustion method and hydrothermal method, etc.[8-11]. However, such methods have been rarely used in commercial production of fine phosphors because of long manufacturing duration, high cost, and complex operability[12]. Therefore, the solid-state reaction in vacuum was applied to prepare the rare earth oxysulfide phosphor in vacuum furnace, which is highly desirable to decrease the average particle size of the samples on the premise of energy saving. It is also well-known that impurity ions of alkali ions (K+, Na+, particularly Li+ ions), are easily coordinated in host matrix to 2
ACCEPTED MANUSCRIPT enhance the luminescence of lanthanide ions significantly[13] . In this respect, Li+ can be used as flux to promote the formation of hexagonal morphology[14], to accelerate the crystallization reaction[15], and to favor its incorporation into the crystal lattice[16]. Li+ doping at a low content significantly improves the morphology of phosphors and strongly enhanced the
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luminescent properties[17].
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Herein, the sulfur addition has played significant role in the synthesis process as well as
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the luminescence characteristics because of ready volatilization of free sulfur under vacuum condition. Therefore, Gd2O3 was determined to be present as impurity phase in the sample,
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which may play great influence of Gd2O2S:Er on crystal growth and UC luminescence
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properties. Furthermore, Gd2O3 are found in hexagonal (P32/m and Im-3m), monoclinic (C2/m) and cubic (Ia3) structures, denoted by Goldschmidt as A, B and C at room
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temperature. Gd2O3:Er has also been selected as upconversion phosphor because of its
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lower phonon energy of Gd2O3. However, most studies of the UC luminescence mainly focused on the doped cubic Gd2O3, in which rare earths are octahedrally coordinated by
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oxygen, and the structure can be viewed as a vacancy ordered variant of fluorite[18,19] with
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only a few considering on the monoclinic Gd2O3 [20-22]. With consideration of the presence of Gd2O3 in Gd2O2S:Er, which is expressed as Gd2O3-xSx:Er, the study of the photoluminescence of rare earth ions in different host lattices has been of great interest from both scientific and technological points of view[23]. The dopant site distribution, site characteristic emission properties and inter-site interactions represent key elements in understanding and optimizing the emission of lanthanide based optical materials [24]. Until recently, the study of rare earth energy-transfer systems had been viewed as simplistic
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ACCEPTED MANUSCRIPT linear and time-invariant [25]. In this paper, the microstructures of the non-stoichiometric Gd2O3-xSx crystals were investigated and Er3+ luminescence and cooperative upconversion in Gd2O3-xSx crystals were reported. The influence of impurities in the crystals was explored to clarify the luminescence mechanism. The electronic structure and optical properties were
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EXPERIMENTAL
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also calculated by first principles calculation and discussed in another paper.
The phosphor samples were synthesized by solid-state reaction method in a vacuum
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furnace system. High purity Gd2O3(99.99%) and Er2O3(99.99%) were mixed with sulfur and
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fluxes(Na2CO3 and Li3PO4) with high pressure pre-treatment (2MPa) on the thoroughly mixed raw materials, followed by heating the mixture in a covered crucible at 1000℃ for a
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sufficient time under vacuum with the vacuum degree 5~10Pa, then washing the samples
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with deionized water to remove essentially any water-soluble impurities. The solid-state reaction process was determined by METTLER TOLEDO TGA/DSC1 thermal
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analyzer in vacuum and atmosphere respectively. The phase and crystal structures were
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examined by X-ray powder diffraction (XRD, D/man-3B) with Cu K radiation (λ=0.154nm). The morphologies were imaged and analyzed using a transmission electron microscope (TEM, JEM-2100). The upconversion spectra were recorded by F-7000 FL-type spectrophotometer. Photoluminescence upconversion decay curves were determined by QuantaMaster-400 with excitation source 978nm DPSS laser with a pulsed option.
RESULTS AND DISCUSSION
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ACCEPTED MANUSCRIPT The Gd2O3-xSx phosphor samples were prepared with different sulfur content (25 wt%(a), 29 wt%(b), 32 wt%(c) and 38 wt%(d)) in raw materials at 1000℃. The XRD results as shown in Fig. 1. reveal that the well-crystallized Gd(Er)2O2S phosphors(d) are of hexagonal structure with excessive 38wt% sulfur content, which are in agreement with the standard powder peak
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positions of Gd2O2S (matching JCPDS card no. 65-3449). The sharp peaks are indications of
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good crystallization which belong to the trigonal space group P 3m1. These findings are not
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unexpected as the ionic radii for Gd3+ and Er3+ are 0.094 nm and 0.089 nm, respectively, so replacing Gd3+ with tiny amounts of Er3+ would be expected to have little or no effect on the
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crystal structure and unit cell sizes of these materials. Other XRD datasets of samples indicate
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the presence of monoclinic Gd2O3 in Gd(Er)2O2S (a and b matching JCPDS card no.43-1015 while c matching JCPDS card no.42-1465). In this Gd2O2S structure, both Gd3+ and O2- ions
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have the same site symmetry C3v, and each Gd3+ cation is coordinated to four oxygen anions
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and three sulfur anions. In addition, the Gd3+ cations experience weaker crystal fields than in Gd2O3, this means Er3+ cation dopants on the Gd3+ site will experience a weaker crystal field
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in Gd2O2S than that in Gd2O3. Therefore, the difference of the crystal field results in the slight
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difference of luminescence mechanisms. The pure and Er doped Gd2O2.5S crystals were modeled by the slab super-cell methods, which more correctly describe the physics of crystals than cluster methods. The optimized simulated crystal structure of Gd2O2.5S2.5 and Gd2O2.5S2.5: Er were shown in Fig.2 based on the crystal structure of Gd2O2S. The average crystal sizes were calculated from the Scherrer formula（Eq.3）:
K Dhkl = cos
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Dhkl means the size along the (h k l) direction and K is a constant (0.90), where θ and β are 5
ACCEPTED MANUSCRIPT the diffraction angle and full-width at half-maximum (fwhm), respectively. The wavelength (λ) of the X rays is 0.15406 nm. The crystallite sizes of the sample a, b, c and d were calculated to be about 53.70 nm, 47.47 nm, 47.19 nm and 49.09 nm. Although there’s a slight difference among the samples, the sample c with 32 wt% sulfur content shows smaller lattice in the
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structure which is ready to influence the symmetry of the host matrix. Furthermore, the
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monoclinic Gd2O3 phase should have a superior in UC efficiency due to the low symmetry
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sites in Gd2O3-xSx. and will be confirmed by spectral analysis. It may be attributed to the crystal phase transformation at high temperature and the gadolinium oxysulfide could be
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oxidized to produce new oxide phase, which is interpreted by the thermal analysis in Fig. 3.
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There is an endothermic peak at 840℃ in DSC curves which indicated that the main reaction to produce oxysulfide and exothermic peak from around 900℃ to 1020℃ which
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demonstrate the adjustment of crystal phase transformation. The primary influence of sulfur
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content on the sulfur vaporization and the reaction with Na2CO3 is clearly shown in TGA curves. It is found that there’s slow weight loss with higher sulfur content addition during the
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first stage, which means chemical reaction is the controlling step compared to diffusion
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during the reaction process. The kinetics of the solid-state reaction in vacuum will be further discussed in another paper. Fig. 4 shows the TEM image of Gd2O2S:Er phosphor (d) along with the selected area electron diffraction(SAED) pattern. It shows slightly agglomerated and well crystallized nature of the particles around 200 nm. The corresponding SAED patterns exhibit crystal diffraction patterns and are not quite consistent with those of pure Gd2O2S phase. So there is also tiny amount of impurity phase in Gd2O2S:Er crystal which can hardly be determined by
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ACCEPTED MANUSCRIPT XRD patterns. The underlying super lattice structures of Gd2O3-xSx was found in SAED pattern in Fig. 4d. The UC luminescence spectra under 980nm laser excitation were collected in Fig. 5. Emission bands observed at 410nm are assigned to (2H9/2→4I15/2) transitions, 524 nm, 528 nm,
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534 nm are assigned to (2H11/2→4I15/2) transitions, 549nm, 555nm are assigned to
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(4S3/2→4I15/2)transitions, and 658nm, 662nm, 671nm are assigned to (4F9/2→4I15/2) transitions
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respectively. The appearance of the blue, green and red emission bands can be explained on the basis of various processes such as two photon or multiphoton absorption, excited state
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absorption and energy transfer. The UC process in the Er3+ system is well understood and is
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briefly explained in the energy level diagram in several reports[12,26-29]. It is noted that with increasing sulfur content the green emission was enhanced and red emission was
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weakened. The blue emission(~410nm) was barely found in the spectra, which was enlarged
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on the left top of the figure. 5 (insert). As aforementioned, Er3+ cation on the Gd3+ site, i.e., Gd(Er)3+ will experience a weaker crystal field in Gd2O2S than in Gd2O3, resulting in the lower
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symmetry of Gd2O3-xS:Erx crystals. So the presence of Gd2O3 lead to more starks splitting
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which were found in the spectra. The Lifetimes of Gd2O3-xSx:Er3+ (c) phosphor were measured by photoluminescence upconversion decay measured at 555 nm and 671nm as were shown in Fig.6 and Fig.7. Instrument response function (IRF) was also shown in the figures. The decay analysis required a 2-exponential fitting function: F(t) = a1*exp(-t/τ1) + a2*exp(-t/ τ 2)
(4)
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ACCEPTED MANUSCRIPT For 4S3/2→4I15/2transition(555nm),τ 1= 30.3 µs, τ 2= 119 µs, a1= 6.92270E-002 (64%), a2= 3.81109E-002 (36%). For 4F9/2→4I15/2 transition(671nm), τ 1= 25.3 µs, τ 2= 198 µs, a1: -0.1236 (-99.5%), a2: 0.1242 (100%). As M.J. Weber reported in Ref. [30], there is a definite trend throughout the energy-level scheme of LaF3: Er3+ that the smaller the energy gap to the
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next-lower J multiplet, the shorter the lifetime. In general, for levels having energy gaps >
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3000 cm-1, the observed and radiative lifetimes do not differ greatly and thus the quantum
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efficiencies are near unity. For levels with energy gaps < 3000 cm-1, the discrepancies between the radiative and observed decay probabilities are larger, indicating the increased
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importance of nonradiative processes. For energy gaps <1600 cm-1, non-radiative decay is so
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dominant that fluorescence is normally not detected. So the longer decay times of red emission than the corresponding green emission are due to the larger non-radiative gap for
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red emission (18353 cm−1 vs. 15236cm−1). The analysis in Fig.7 resulted in one lifetime and
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one rise time (negative pre-exponential factor). The rise time indicates that the emitting state is not directly populated by light but from a higher energy precursor state. In this term,
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non-resonant absorption promotes an ion from ground state to metastable state 4F9/2.
2
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Absorption of a pump photon by an ion in the metastable state populates upper laser level H11/2. 2H11/2 can interact with a neighboring ground state ion through ion pair energy transfer
(ET) to produce two ions in the metastable state. UC emission results from the transition from state 2H11/2 to one of the Stark levels of the ground state[30]. Populating the metastable state from which ESA takes place maybe attributed to the efficient cross-relaxation process between ions[31] in the presence of Gd2O3. If the rate for populating the metastable state is fast compared to the metastable state lifetime, as a
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ACCEPTED MANUSCRIPT consequence, photon avalanche may take place. The red-emitting state can be populated from the upper green-emitting state 4S3/2 or ground state, which is considered to be dominant at low excitation densities by multiphonon relaxation[32] as the first step of photon avalanche[33]. The decay time is determined by the shorter lifetime of metastable
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state 4F9/2 than others involved in the generation of red emission[34]. The relative intensity
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of green and red emitting bands depends on power-density of infrared excitation, which will
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be further confirmed in Fig.8c.
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To further understand the UCL mechanisms in Gd2O3-xS:Erx samples, it is emphasized that the slope value of pump power dependence of the green and the red UCL intensities
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characterizes competition between non-radiative and radiative decays at the intermediate states. The dependences of emission intensity against pump power were measured in double
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logarithmic and plotted in a diagram, as shown in Figure 8a, 8c, 8d. Data fitting yielded
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straight lines with a slope of 3.74 and 3.52 for the green (at 549nm) and for the red emission (at 671 nm), respectively in sample a, and both 3.37 for the two emissions in sample d,
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suggesting that the upconversion is a two photon process. With respect to the sample c, the
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slop is 3.87 for the green emission. This nonlinear response of UC intensity signifies the involvement of some process other than the simple excited state absorption from 4F9/2 level. The photon avalanche process distinguishes itself by relying on an indirect method involving an intermediate level population through cross-relaxation, rather than direct ground state absorption(GSA). Usually this process takes longer time (and pump power threshold) for population building up in the lower excited level. Photon avalanche process can be explained on the basis of initial population accumulation in 4F9/2 level and formation of ion pair which 9
ACCEPTED MANUSCRIPT promote resonant energy transfer or cross-relaxation process, ultimately populating more 4
F9/2 level[35]. The similar phenomenon can be seen from the UC luminescence spectra, the
red emission of this sample with 32 wt% excessive sulfur content shows the highest intensity and no obvious splitting compared to other samples, which indicate the population
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accumulation in 4F9/2 level.
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CONCLUSION
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We have demonstrated that Gd2O3-xSx:Er phosphors exhibit highly efficient UCL emission
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under 980 nm excitation. The upconversion cooperative luminescence mechanisms of Gd2O3-xS:Er were interpreted as a result of two photon absorption and the photon avalanche
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process due to populating of metastable level (4F9/2) in the presence of monoclinic Gd2O3 crystals, which positively affected the crystalline phase, the morphology and the UC
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luminescence efficiency of the phosphor. The determination of Er3+ ions in surrounding crystal field environment by lifetime results shows that decay time of red emissions is longer
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than the corresponding green emissions due to the larger non-radiative gap for red emission. The sensitive red emission with the variations of excitation wavelength and power density
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may be very easily detected in biomedical sensing applications. Furthermore, it serves further possibility that the vacuum synthesis may also be applied to other lanthanide-doped phosphor materials. ACKNOWLEDGMENT The authors wish to express thanks to Fast Kinetics Application Laboratory of HORIBA Canada Inc. and Prof. QIU Jianbei for their assistance with measurements of upconversion luminescence properties. We gratefully acknowledge the financial support of the National 10
ACCEPTED MANUSCRIPT Natural Science Foundation of China (Youth program) under Grant No.51504120.
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--- Gd2O2S --- monoclinic Gd2O3 (B type)
(101) (100) (001)
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FIG. 1 XRD Patterns of Gd2O2S:Er Phosphor with different excessive sulfur content
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a: 25 wt%; b: 29 wt%; c:32 wt% d:excessive 38 wt%
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Gd2.5O2.5S
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Gd2.5O2.5S:Er
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FIG. 2 Optimized calculated crystal structure of Gd2.5O2.5S and Gd2.5O2.5S:Er
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Highlights
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Fei Wang, Bin Yang, Qingchun Yu, Dachun Liu, Wenhui Ma PII: DOI: Reference:
S1386-1425(17)30739-4 doi: 10.1016/j.saa.2017.09.023 SAA 15457
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received date: Revised date: Accepted date:
29 May 2017 2 September 2017 12 September 2017
Please cite this article as: Fei Wang, Bin Yang, Qingchun Yu, Dachun Liu, Wenhui Ma , Cooperative upconversion luminescence of Er3+ in Gd2O3−xSx phosphor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.09.023
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ACCEPTED MANUSCRIPT Cooperative Upconversion luminescence of Er3+ in Gd2O3-xSx phosphor Fei Wang1,2 Bin Yang*1
Qingchun Yu1 Dachun Liu1 Wenhui Ma1
1 State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization in Yunnan Province , National Engineering Laboratory for Vacuum Metallurgy, Key Laboratory for
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Nonferrous Vacuum Metallurgy of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China
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2 Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 bus 2450, 3001
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Leuven, Belgium
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Email address: [email protected]
ABSTRACT
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Gd2O3-xSx: Er crystals were prepared through high-temperature solid-state reaction method
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in vacuum, with the vacuum synthesis mechanism determined by thermal analysis. The crystal structure and upconversion luminescence properties were investigated respectively
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by XRD, TEM and spectrophotometer. Well crystallized Gd2O2S:Er phosphors were prepared
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under 1000℃ in vacuum with a certain excessive amount of sulfur content than stoichiometric. It is confirmed that with the increasing sulfur content the green emission was enhanced and red emission was weakened. The cooperative upconversion luminescence of Er3+ in non-stoichiometric Gd2O3-xSx crystals was interpreted as a result of two photon absorption and the photon avalanche process.
Keywords: upconversion luminescence; vacuum; photon avalanche process; lifetime; super lattice 1
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INTRODUCTION
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Upconversion (UC) is a process where light can be emitted with photon energies higher than those of the light generating the excitation. Cooperative luminescence is one of the
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mechanisms responsible for upconversion, which involves energy transfer processes
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between different ions[1]. In recent years lanthanide-doped upconversion nanocrystals have
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been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays
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and medical imaging[2]. Er3+ is a suitable candidates for UC processes due to their abundant energy levels and narrow emission spectral lines[3,4] and has attracted much attention as an
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activator owing to its high efficiency properties in upconversion luminescence (UCL) [5,6]. Rare earth oxysulfides, as host matrix, show several excellent properties such as chemical
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stability, low toxicity and low phonon energy (~520 cm−1 ) [7]. Several approaches have been developed to prepare fine phosphors, including
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co-precipitation method, sol–gel method, combustion method and hydrothermal method, etc.[8-11]. However, such methods have been rarely used in commercial production of fine phosphors because of long manufacturing duration, high cost, and complex operability[12]. Therefore, the solid-state reaction in vacuum was applied to prepare the rare earth oxysulfide phosphor in vacuum furnace, which is highly desirable to decrease the average particle size of the samples on the premise of energy saving. It is also well-known that impurity ions of alkali ions (K+, Na+, particularly Li+ ions), are easily coordinated in host matrix to 2
ACCEPTED MANUSCRIPT enhance the luminescence of lanthanide ions significantly[13] . In this respect, Li+ can be used as flux to promote the formation of hexagonal morphology[14], to accelerate the crystallization reaction[15], and to favor its incorporation into the crystal lattice[16]. Li+ doping at a low content significantly improves the morphology of phosphors and strongly enhanced the
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luminescent properties[17].
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Herein, the sulfur addition has played significant role in the synthesis process as well as
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the luminescence characteristics because of ready volatilization of free sulfur under vacuum condition. Therefore, Gd2O3 was determined to be present as impurity phase in the sample,
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which may play great influence of Gd2O2S:Er on crystal growth and UC luminescence
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properties. Furthermore, Gd2O3 are found in hexagonal (P32/m and Im-3m), monoclinic (C2/m) and cubic (Ia3) structures, denoted by Goldschmidt as A, B and C at room
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temperature. Gd2O3:Er has also been selected as upconversion phosphor because of its
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lower phonon energy of Gd2O3. However, most studies of the UC luminescence mainly focused on the doped cubic Gd2O3, in which rare earths are octahedrally coordinated by
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oxygen, and the structure can be viewed as a vacancy ordered variant of fluorite[18,19] with
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only a few considering on the monoclinic Gd2O3 [20-22]. With consideration of the presence of Gd2O3 in Gd2O2S:Er, which is expressed as Gd2O3-xSx:Er, the study of the photoluminescence of rare earth ions in different host lattices has been of great interest from both scientific and technological points of view[23]. The dopant site distribution, site characteristic emission properties and inter-site interactions represent key elements in understanding and optimizing the emission of lanthanide based optical materials [24]. Until recently, the study of rare earth energy-transfer systems had been viewed as simplistic
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ACCEPTED MANUSCRIPT linear and time-invariant [25]. In this paper, the microstructures of the non-stoichiometric Gd2O3-xSx crystals were investigated and Er3+ luminescence and cooperative upconversion in Gd2O3-xSx crystals were reported. The influence of impurities in the crystals was explored to clarify the luminescence mechanism. The electronic structure and optical properties were
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EXPERIMENTAL
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also calculated by first principles calculation and discussed in another paper.
The phosphor samples were synthesized by solid-state reaction method in a vacuum
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furnace system. High purity Gd2O3(99.99%) and Er2O3(99.99%) were mixed with sulfur and
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fluxes(Na2CO3 and Li3PO4) with high pressure pre-treatment (2MPa) on the thoroughly mixed raw materials, followed by heating the mixture in a covered crucible at 1000℃ for a
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sufficient time under vacuum with the vacuum degree 5~10Pa, then washing the samples
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with deionized water to remove essentially any water-soluble impurities. The solid-state reaction process was determined by METTLER TOLEDO TGA/DSC1 thermal
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analyzer in vacuum and atmosphere respectively. The phase and crystal structures were
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examined by X-ray powder diffraction (XRD, D/man-3B) with Cu K radiation (λ=0.154nm). The morphologies were imaged and analyzed using a transmission electron microscope (TEM, JEM-2100). The upconversion spectra were recorded by F-7000 FL-type spectrophotometer. Photoluminescence upconversion decay curves were determined by QuantaMaster-400 with excitation source 978nm DPSS laser with a pulsed option.
RESULTS AND DISCUSSION
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ACCEPTED MANUSCRIPT The Gd2O3-xSx phosphor samples were prepared with different sulfur content (25 wt%(a), 29 wt%(b), 32 wt%(c) and 38 wt%(d)) in raw materials at 1000℃. The XRD results as shown in Fig. 1. reveal that the well-crystallized Gd(Er)2O2S phosphors(d) are of hexagonal structure with excessive 38wt% sulfur content, which are in agreement with the standard powder peak
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positions of Gd2O2S (matching JCPDS card no. 65-3449). The sharp peaks are indications of
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good crystallization which belong to the trigonal space group P 3m1. These findings are not
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unexpected as the ionic radii for Gd3+ and Er3+ are 0.094 nm and 0.089 nm, respectively, so replacing Gd3+ with tiny amounts of Er3+ would be expected to have little or no effect on the
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crystal structure and unit cell sizes of these materials. Other XRD datasets of samples indicate
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the presence of monoclinic Gd2O3 in Gd(Er)2O2S (a and b matching JCPDS card no.43-1015 while c matching JCPDS card no.42-1465). In this Gd2O2S structure, both Gd3+ and O2- ions
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have the same site symmetry C3v, and each Gd3+ cation is coordinated to four oxygen anions
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and three sulfur anions. In addition, the Gd3+ cations experience weaker crystal fields than in Gd2O3, this means Er3+ cation dopants on the Gd3+ site will experience a weaker crystal field
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in Gd2O2S than that in Gd2O3. Therefore, the difference of the crystal field results in the slight
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difference of luminescence mechanisms. The pure and Er doped Gd2O2.5S crystals were modeled by the slab super-cell methods, which more correctly describe the physics of crystals than cluster methods. The optimized simulated crystal structure of Gd2O2.5S2.5 and Gd2O2.5S2.5: Er were shown in Fig.2 based on the crystal structure of Gd2O2S. The average crystal sizes were calculated from the Scherrer formula（Eq.3）:
K Dhkl = cos
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Dhkl means the size along the (h k l) direction and K is a constant (0.90), where θ and β are 5
ACCEPTED MANUSCRIPT the diffraction angle and full-width at half-maximum (fwhm), respectively. The wavelength (λ) of the X rays is 0.15406 nm. The crystallite sizes of the sample a, b, c and d were calculated to be about 53.70 nm, 47.47 nm, 47.19 nm and 49.09 nm. Although there’s a slight difference among the samples, the sample c with 32 wt% sulfur content shows smaller lattice in the
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structure which is ready to influence the symmetry of the host matrix. Furthermore, the
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monoclinic Gd2O3 phase should have a superior in UC efficiency due to the low symmetry
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sites in Gd2O3-xSx. and will be confirmed by spectral analysis. It may be attributed to the crystal phase transformation at high temperature and the gadolinium oxysulfide could be
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oxidized to produce new oxide phase, which is interpreted by the thermal analysis in Fig. 3.
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There is an endothermic peak at 840℃ in DSC curves which indicated that the main reaction to produce oxysulfide and exothermic peak from around 900℃ to 1020℃ which
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demonstrate the adjustment of crystal phase transformation. The primary influence of sulfur
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content on the sulfur vaporization and the reaction with Na2CO3 is clearly shown in TGA curves. It is found that there’s slow weight loss with higher sulfur content addition during the
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first stage, which means chemical reaction is the controlling step compared to diffusion
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during the reaction process. The kinetics of the solid-state reaction in vacuum will be further discussed in another paper. Fig. 4 shows the TEM image of Gd2O2S:Er phosphor (d) along with the selected area electron diffraction(SAED) pattern. It shows slightly agglomerated and well crystallized nature of the particles around 200 nm. The corresponding SAED patterns exhibit crystal diffraction patterns and are not quite consistent with those of pure Gd2O2S phase. So there is also tiny amount of impurity phase in Gd2O2S:Er crystal which can hardly be determined by
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ACCEPTED MANUSCRIPT XRD patterns. The underlying super lattice structures of Gd2O3-xSx was found in SAED pattern in Fig. 4d. The UC luminescence spectra under 980nm laser excitation were collected in Fig. 5. Emission bands observed at 410nm are assigned to (2H9/2→4I15/2) transitions, 524 nm, 528 nm,
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534 nm are assigned to (2H11/2→4I15/2) transitions, 549nm, 555nm are assigned to
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(4S3/2→4I15/2)transitions, and 658nm, 662nm, 671nm are assigned to (4F9/2→4I15/2) transitions
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respectively. The appearance of the blue, green and red emission bands can be explained on the basis of various processes such as two photon or multiphoton absorption, excited state
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absorption and energy transfer. The UC process in the Er3+ system is well understood and is
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briefly explained in the energy level diagram in several reports[12,26-29]. It is noted that with increasing sulfur content the green emission was enhanced and red emission was
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weakened. The blue emission(~410nm) was barely found in the spectra, which was enlarged
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on the left top of the figure. 5 (insert). As aforementioned, Er3+ cation on the Gd3+ site, i.e., Gd(Er)3+ will experience a weaker crystal field in Gd2O2S than in Gd2O3, resulting in the lower
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symmetry of Gd2O3-xS:Erx crystals. So the presence of Gd2O3 lead to more starks splitting
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which were found in the spectra. The Lifetimes of Gd2O3-xSx:Er3+ (c) phosphor were measured by photoluminescence upconversion decay measured at 555 nm and 671nm as were shown in Fig.6 and Fig.7. Instrument response function (IRF) was also shown in the figures. The decay analysis required a 2-exponential fitting function: F(t) = a1*exp(-t/τ1) + a2*exp(-t/ τ 2)
(4)
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ACCEPTED MANUSCRIPT For 4S3/2→4I15/2transition(555nm),τ 1= 30.3 µs, τ 2= 119 µs, a1= 6.92270E-002 (64%), a2= 3.81109E-002 (36%). For 4F9/2→4I15/2 transition(671nm), τ 1= 25.3 µs, τ 2= 198 µs, a1: -0.1236 (-99.5%), a2: 0.1242 (100%). As M.J. Weber reported in Ref. [30], there is a definite trend throughout the energy-level scheme of LaF3: Er3+ that the smaller the energy gap to the
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next-lower J multiplet, the shorter the lifetime. In general, for levels having energy gaps >
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3000 cm-1, the observed and radiative lifetimes do not differ greatly and thus the quantum
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efficiencies are near unity. For levels with energy gaps < 3000 cm-1, the discrepancies between the radiative and observed decay probabilities are larger, indicating the increased
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importance of nonradiative processes. For energy gaps <1600 cm-1, non-radiative decay is so
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dominant that fluorescence is normally not detected. So the longer decay times of red emission than the corresponding green emission are due to the larger non-radiative gap for
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red emission (18353 cm−1 vs. 15236cm−1). The analysis in Fig.7 resulted in one lifetime and
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one rise time (negative pre-exponential factor). The rise time indicates that the emitting state is not directly populated by light but from a higher energy precursor state. In this term,
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non-resonant absorption promotes an ion from ground state to metastable state 4F9/2.
2
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Absorption of a pump photon by an ion in the metastable state populates upper laser level H11/2. 2H11/2 can interact with a neighboring ground state ion through ion pair energy transfer
(ET) to produce two ions in the metastable state. UC emission results from the transition from state 2H11/2 to one of the Stark levels of the ground state[30]. Populating the metastable state from which ESA takes place maybe attributed to the efficient cross-relaxation process between ions[31] in the presence of Gd2O3. If the rate for populating the metastable state is fast compared to the metastable state lifetime, as a
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ACCEPTED MANUSCRIPT consequence, photon avalanche may take place. The red-emitting state can be populated from the upper green-emitting state 4S3/2 or ground state, which is considered to be dominant at low excitation densities by multiphonon relaxation[32] as the first step of photon avalanche[33]. The decay time is determined by the shorter lifetime of metastable
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state 4F9/2 than others involved in the generation of red emission[34]. The relative intensity
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of green and red emitting bands depends on power-density of infrared excitation, which will
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be further confirmed in Fig.8c.
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To further understand the UCL mechanisms in Gd2O3-xS:Erx samples, it is emphasized that the slope value of pump power dependence of the green and the red UCL intensities
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characterizes competition between non-radiative and radiative decays at the intermediate states. The dependences of emission intensity against pump power were measured in double
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logarithmic and plotted in a diagram, as shown in Figure 8a, 8c, 8d. Data fitting yielded
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straight lines with a slope of 3.74 and 3.52 for the green (at 549nm) and for the red emission (at 671 nm), respectively in sample a, and both 3.37 for the two emissions in sample d,
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suggesting that the upconversion is a two photon process. With respect to the sample c, the
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slop is 3.87 for the green emission. This nonlinear response of UC intensity signifies the involvement of some process other than the simple excited state absorption from 4F9/2 level. The photon avalanche process distinguishes itself by relying on an indirect method involving an intermediate level population through cross-relaxation, rather than direct ground state absorption(GSA). Usually this process takes longer time (and pump power threshold) for population building up in the lower excited level. Photon avalanche process can be explained on the basis of initial population accumulation in 4F9/2 level and formation of ion pair which 9
ACCEPTED MANUSCRIPT promote resonant energy transfer or cross-relaxation process, ultimately populating more 4
F9/2 level[35]. The similar phenomenon can be seen from the UC luminescence spectra, the
red emission of this sample with 32 wt% excessive sulfur content shows the highest intensity and no obvious splitting compared to other samples, which indicate the population
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accumulation in 4F9/2 level.
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CONCLUSION
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We have demonstrated that Gd2O3-xSx:Er phosphors exhibit highly efficient UCL emission
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under 980 nm excitation. The upconversion cooperative luminescence mechanisms of Gd2O3-xS:Er were interpreted as a result of two photon absorption and the photon avalanche
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process due to populating of metastable level (4F9/2) in the presence of monoclinic Gd2O3 crystals, which positively affected the crystalline phase, the morphology and the UC
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luminescence efficiency of the phosphor. The determination of Er3+ ions in surrounding crystal field environment by lifetime results shows that decay time of red emissions is longer
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than the corresponding green emissions due to the larger non-radiative gap for red emission. The sensitive red emission with the variations of excitation wavelength and power density
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may be very easily detected in biomedical sensing applications. Furthermore, it serves further possibility that the vacuum synthesis may also be applied to other lanthanide-doped phosphor materials. ACKNOWLEDGMENT The authors wish to express thanks to Fast Kinetics Application Laboratory of HORIBA Canada Inc. and Prof. QIU Jianbei for their assistance with measurements of upconversion luminescence properties. We gratefully acknowledge the financial support of the National 10
ACCEPTED MANUSCRIPT Natural Science Foundation of China (Youth program) under Grant No.51504120.
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--- Gd2O2S --- monoclinic Gd2O3 (B type)
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FIG. 1 XRD Patterns of Gd2O2S:Er Phosphor with different excessive sulfur content
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Gd2.5O2.5S
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Gd2.5O2.5S:Er
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FIG. 2 Optimized calculated crystal structure of Gd2.5O2.5S and Gd2.5O2.5S:Er
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DSC/(mW/mg)
85
SC
RI
PT
ACCEPTED MANUSCRIPT
NU
a
c
CE
PT E
D
MA
b
FIG. 4
AC
d
TEM image and
SAED pattern of Gd2O2S:Er phosphor
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140wt%
Intensity/(a.u.)
549 555 110wt%
50wt%
405
410
415
420
425
PT
671 662 658
430
529 534 524
Intensity/(a.u.)
Wavelength/(nm)
549
RI 674
549
534 529 524
663
570 555
400
450
500
571
550
600
NU
350
691
110wt%
658
564 528534 539 524
140wt%
673
555
SC
400
650
50wt% 700
750
MA
Wavelength/(nm)
FIG. 5 UCL spectra of Gd2O2S:Er phosphor(λex =980 nm) with different sulfur content. The blue emission band
AC
CE
PT E
D
in the range (400~430nm) was enlarged and inserted in the figure
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1,0
Intensity/(a.u.)
0,8
550nm IRF
0,6
0,4
PT
0,2
0
500
1000
1500
2000
SC
Time/(s)
RI
0,0
NU
FIG.6 Photoluminescence upconversion decay measured at 555 nm (black) and the IRF (red) with 978 nm DPSS
AC
CE
PT E
D
MA
laser excitation at 1W power
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1,0
0,6
0,4
671nm IRF
0,2
PT
Intensity/(a.u.)
0,8
0
500
1000
1500
2000
SC
Time/(s)
RI
0,0
NU
Fig. 7 Photoluminescence upconversion decay measured at 671 nm (black) and the IRF (red) with 978 nm DPSS
AC
CE
PT E
D
MA
laser excitation at 1W power
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1000
671nm 549nm
a 10
10 1000
1200
1400
1600
1800
671nm 549nm
100
c 1000
2000
1400
1600
1800
2000
RI SC
100
671nm 549nm
1000
1200
1400
1600
1800
2000
PT E
Power/mW
D
d
MA
NU
1000 Intensity/(a.u.)
1200
Power/mW
Power/(mW)
10
c
PT
100
Intensity/(a.u.)
Intensity/(a.u.)
1000
FIG.8 Upconversion intensity variations with excitation power for Gd2O2S:Er phosphor at 549nm and 671nm with
AC
CE
different sulfur content. a: 25wt%; c: 32 wt% d: 38 wt%
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1000
671nm 549nm
10
25 wt%
10
1000
1200
1400
1600
1800
671nm 549nm
100
32 wt%
PT
100
Intensity/(a.u.)
Intensity/(a.u.)
1000
1000
2000
1200
1400
1600
1800
2000
Power/mW
NU
SC
RI
Power/(mW)
MA
100
671nm 549nm
38 wt% 10 1200
1400
1600
1800
2000
PT E
1000
d
D
Intensity/(a.u.)
1000
Power/mW
CE
1,0
0,6
0,4
0,2
AC
Intensity/(a.u.)
0,8
671nm IRF
0,0
0
500
1000
1500
2000
Time/(s)
FIG. Upconversion intensity variations with excitation power for Gd2O2S:Er phosphor at 549nm and 671nm with different sulfur content and photoluminescence upconversion decay curves measured at 671 nm (black) and the
22
ACCEPTED MANUSCRIPT
IRF (red) with 978 nm DPSS laser excitation at 1W power
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D
MA
NU
SC
RI
PT
Graphical Abstract
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ACCEPTED MANUSCRIPT
671nm 549nm 1000
100
671nm 549nm
1,0
0,8
Intensity/(a.u.)
Intensity/(a.u.)
Intensity/(a.u.)
1000
100
0,6
0,4
671nm IRF
0,2
0,0
0
500
1000
1500
2000
Time/(s)
10
1000
1200
1400
1600
1800
Photoluminescence upconversion decay measured at 671 nm (black) and the IRF (red) with 978 nm DPSS laser excitation at 1W power
b 1000
2000
PT
a
10
1200
1400
1600
1800
2000
Power/mW
SC
RI
Power/(mW)
NU MA
100
671nm 549nm
c
10
1000
1200
1400
1600
1800
2000
PT E
Power/mW
D
Intensity/(a.u.)
1000
FIG. Upconversion intensity variations with excitation power for Gd2O2S:Er phosphor at 549nm and 671nm with
AC
CE
different sulfur content. a: excessive 50wt%; b: excessive 110wt% c:excessive 140wt%
Highlights
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