- Email: [email protected]
Energy migration in YBO3:Yb3+,Tb3+ materials: Down- and upconversion luminescence studies
Accepted Manuscript 3+ 3+ Energy migration in YBO3:Yb ,Tb materials: Down- and upconversion luminescence studies Tomasz Grzyb, Konrad Kubasiewicz, Agata Szczeszak, Stefan Lis PII:
S0925-8388(16)31953-3
DOI:
10.1016/j.jallcom.2016.06.230
Reference:
JALCOM 38099
To appear in:
Journal of Alloys and Compounds
Received Date: 15 April 2016 Revised Date:
21 June 2016
Accepted Date: 22 June 2016
Please cite this article as: T. Grzyb, K. Kubasiewicz, A. Szczeszak, S. Lis, Energy migration in 3+ 3+ YBO3:Yb ,Tb materials: Down- and upconversion luminescence studies, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.230. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Energy migration in YBO3:Yb3+,Tb3+ materials: down- and upconversion luminescence studies Tomasz Grzyb*, Konrad Kubasiewicz, Agata Szczeszak, Stefan Lis Department of Rare Earths, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań
Keywords:
rare
earth
borates,
YBO3,
upconversion,
downconversion,
SC
luminescence, charge transfer
Yb3+/Tb3+,
RI PT
E-mail: [email protected]
Abstract
Down- and upconversion luminescence phenomena from YBO3 materials doped by Yb3+ and
M AN U
Tb3+ ions were observed and investigated. Materials described in the article, were synthesized by a sol-gel method. Two series of products were prepared: with changed molar concentrations of Yb3+ or Tb3+ ions in the range from 1 to 30%. This way of doping was chosen to analyse the influence of ions concentrations on luminescence and to find samples with the most intense emission. Structural and morphological properties of the obtained
TE D
materials were analysed on the basis of X-ray diffraction, infrared spectroscopy and by collecting images of samples from transmission electron microscope. The products were pure and well crystallized, showing monoclinic structure with C2/c space group. the
YBO3:Yb3+,Tb3+ materials were
investigated
by
EP
Spectroscopic properties of
measurements of excitation spectra in the ultraviolet and near infrared range; emission
AC C
spectra in the visible range; luminescence decays and dependencies of integral luminescence intensity on pumping laser energy. Based on careful studies of the obtained results, mechanisms responsible for the processes observed in these materials were proposed and explained.
1. Introduction In the recent years much attention has been paid to materials, which exhibit visible emission under near infrared (NIR) radiation [1–5]. Such phenomenon is known as upconversion (UC). In these materials, absorption of two or more low-energy photons results in emission of a single high-energy photon [5]. This process can be utilized in applications and research areas 1
ACCEPTED MANUSCRIPT as temperature, ions and molecules sensors, fingerprint identification, security markers, display technologies and solar cells design [6–15]. UC process used in biomedicine make possible to reduce photoblinking, photobleaching or autofluorescence effects [16,17]. Therefore, materials showing UC can be used in biomedical applications such as bioimaging or photodynamic therapy [2,4,17–20].
RI PT
There are several mechanisms of UC process, i.e. ground- or excited-state absorption (GSA or ESA) possible in single doped systems or energy transfer upconversion (ETU) where the Yb3+ ion play the role of sensitizer and Er3+, Tm3+ or Ho3+ act as the emitting ions [5]. However, for the Yb3+/Tb3+ dopants, the mechanism of UC differs from remaining. It is called
SC
cooperative energy transfer (CET), where at least two Yb3+ ions simultaneously transfer energy to one Tb3+ ion [21–27]. CET based upconversion occurs when the energy gap
M AN U
between the highest lying component of the ground state of Ln3+ ion and its first excited state is too large to be bridged by a single, excited Yb3+ ion. Only some host compounds are appropriate to support this relatively rare process, taking into account all investigated systems showing UC. Since the efficiency of CET is lower than that in analogue ETU-based systems, in order to improve it a high crystallinity and purity of material is demanded. This is usually connected with high temperature processing. Hence, such compounds as rare earth
TE D
borates seem to be good candidates for Tb3+ upconversion. Rare earth orthoborates, REBO3 (RE = Y, Gd, La) have been widely studied due to their suitable physicochemical properties as hosts for lanthanide ions (Ln3+), excitable in
EP
ultraviolet (UV) range: Eu3+ and Tb3+ [28–34]. Nanophosphors based on REBO3, doped by Eu3+ ions exhibit intense luminescence with high quantum yields and desired colour chromaticity
AC C
[35,36]. What is worth noting, the compound chosen by us has been investigated only once as showing upconversion, in the Yb3+ and Er3+ co-doped system [37]. Besides this example and our previous report about Yb3+ and Tb3+ doped GdBO3 material, REBO3 were not investigated as hosts for upconversion purposes [38]. However, in this report we show that UC in YBO3 may be intense and also an interesting object of investigation. 2. Experimental The YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ (1% ≤ x ≤ 30%) nanophosphors were synthesized by the sol gel method modified and optimized in order to obtain pure and single phase YBO3.[36,39] The best composition of reagents, temperature and time of annealing were found, avoiding formation of polyborates or Y2O3. 2
ACCEPTED MANUSCRIPT
2.1. Materials Rare earth oxides Y2O3, Yb2O3 and Tb4O7; (99.99% purchased from Stanford Materials, USA), nitric acid HNO3 (ultra-pure, Avantor S.A., Poland), orthoboric acid (p.a. grade, Avantor
RI PT
S.A., Poland), citric acid monohydrate (p.a. grade, CHEMPUR, Poland), ethylene glycol (p.a. grade, CHEMPUR, Poland) were used as starting materials in the synthesis. From the oxides, 1 M nitrate solutions were prepared by dissolving appropriate rare earth oxides in HNO3 diluted by distilled water (1:1).
SC
2.2. Synthesis
All of the syntheses were carried out for 6.5 mmol of the product. Firstly, the stoichiometric
M AN U
amounts of appropriate rare earth salt solutions were mixed with 100 mL of deionized water together with citric acid (CA) and ethylene glycol (EG). The used molar ratio between RE3+, CA and EG was 1:9:18. The organic compounds were used as the chelating and cross-linking reagents. Afterwards, 125% stoichiometric amount of H3BO3 was added and solutions were magnetically stirred until they became transparent. The excess of H3BO3 was used as this
TE D
compound partially evaporates from the system during an annealing process. Next, the homogenous solutions were dried in an oven at 80 °C for 48 h. Finally, the obtained gels were calcined at 900 °C in the air atmosphere for 3 h with the programmed 1.5 h increase of temperature. Annealed gels were transferred into white powders, which were additionally
EP
grounded in a mortar before analysing them. 2.3. Characterization
AC C
X-ray diffraction patterns (XRD) were collected using a Bruker AXS D8 Advance diffractometer in Debye–Scherrer geometry, with Cu Kα1 radiation (1.5406 Å) in the 2θ range from 6 – 60°. The XRD results were compared to the reference patterns from the Joint Committee on Powder Diffraction Standards (JCPDS) database. Images from the transmission electron microscope (TEM) and energy-dispersive X-ray spectra (EDX) were taken at FEI Tecnai G2 20 X-TWIN microscope, by an accelerating voltage of 200 kV. Transmittance spectra in the infrared (IR) range were registered between 400 and 4000 cm-1 on the FTIR spectrophotometer, JASCO 4200. In order to measure absorption spectra, materials were mixed with KBr and then pressed to disks. Luminescence excitation and emission spectra as well as decay curves were measured with the use of QuantaMasterTM 40 3
ACCEPTED MANUSCRIPT (Photon Technology International) spectrophotometer equipped with an Opolette 355LD UVDM (Opotek Incorporation) tunable laser (excitation source), with repetition rate of 20 Hz, and a Hamamatsu R928 or R5108 photomultipliers used as detectors. The area excited by the laser beam was 0.78 mm2. The measured luminescence decays showed a non-exponential character. Since the
calculations by applying the equation below: ∞
τ eff
∫ tI (t )dt = ∫ I (t )dt 0 ∞ 0
(1)
SC
RI PT
luminescence decay kinetics is non-exponential, effective lifetimes were used for
where τeff is the effective decay time, I(t) is the intensity at time t.[40]
To calculate the up-conversion rise times, the following equation was used: t
−
t
M AN U
−
I = [ I 0 + I1 (1 − e
τr
)]e
τd
(2)
where I0 is the initial luminescence intensity at time t = 0, I1 is the intensity added as a result of the energy transfer, and τr and τ, τd are the rise and decay times, respectively. The calculations and fit to the decay data was performed with the Origin 2016 software.
TE D
The dependence of the integral luminescence intensity on the pumping laser power was examined to analyse the mechanisms of the UC process. The relation between the UC intensity IUC and the pumping intensity IP is given by the following equation:
I UC = α (I P ) n
(3)
EP
where α is a proportionality factor, and the exponent n represents the number of photons involved in the UC process [41].
AC C
3. Results and discussion
3.1. Structural and morphological analysis Rare earth orthoborates crystallize in several structures depending on the size of RE3+ ion. These are aragonite, calcite or vaterite types. Light RE3+ ions (La – Nd) crystallize as aragonite type structures. Smaller ions (Sm – Yb and Y) are known as vaterite-type forms. Yttrium borate crystals crystallize as monoclinic, C2/c pseudovaterite-type structure determined by neutron diffraction analysis conducted by Lin et al. in 2004 [42]. Before, two other models were proposed: a disordered hexagonal P63/mmc and an ordered P63/mcm. However, studies of YBO3:Eu3+, where Eu3+ ions were used as a structural probe, indicated 4
ACCEPTED MANUSCRIPT that Eu3+ spectrum is different than should be expected from hexagonal models [43]. They proved, based on site selective spectroscopy, the monoclinic structure of YBO3. We have used this new crystal structure as reference in our studies. Two series of powders based on YBO3 doped by Yb3+ and Tb3+ ions were synthesized. The samples were obtained, once by doping with constant 10% of Yb3+ and 5% of Tb3+ ions
Tb3+ ions on the structural and spectroscopic properties.
RI PT
concentration. Such way of doping was made in order to analyse influence of both Yb3+ and
The XRD patterns and IR transition spectra shown in Figs. 1 and 2 confirmed the monoclinic crystal structure of the samples obtained, comparing to reference pattern No.
SC
01-073-7388 from the JCPDS crystallographic database. High concentrations of dopant ions were neutral for the crystal structure and any impurities appeared. It proved the great ability
M AN U
of YBO3 matrix for doping by Ln3+ ions. The intense and sharp peaks proved, that materials obtained crystallized well.
The IR spectra presented in Fig. 2, show broad and intense bands characteristic for the monoclinic YBO3 [44]. The spectra of the undoped as well as doped YBO3 consists mainly of a typical broad and intense band in the range from 866 to 1078 cm-1, that originates from the vibrations of B–O in B3O93- rings, typical for the monoclinic YBO3 structure [36,43]. Two peaks
TE D
at 866 and 917 cm-1 can be assigned to the ring stretching modes, whereas the band at 1078 cm-1 is originated from the terminal B–O stretching [45–47]. All of the IR spectra registered are very similar. The IR analysis as well as XRD patterns
EP
proved the purity of the nanomaterials obtained and lack of changes related to the dopant ions introduced into the YBO3 matrix. Appearance of other phase should introduce
AC C
absorption by the BO33- triangular groups what would be visible at around 1290 cm-1 [43]. The morphology of the YBO3 and YBO3:10%Yb3+,5%Tb3+ materials is presented in TEM images (Fig. 3). These images show the polygon- or spherical-like and agglomerated crystals. Size of the particular grain is mostly larger than 200 nm. In Fig. 3b the crystals’ planes are clearly seen, what proves that the material synthesized is well crystallized. The grains are sintered due to the high calcination temperature. Based on EDX spectrum, the presence of the Y, Yb and Tb elements was confirmed. Although, borates present interesting properties as hosts for emitting Ln3+ ions, it is difficult to obtain them as nanocrystalline materials. There is only one report about
5
ACCEPTED MANUSCRIPT nanocrystalline YBO3 prepared by a solvothermal method [48]. Other, present rather submicron sized YBO3-sized materials or with average crystal sizes close to 100 nm [49–51]. 3.2. Spectroscopic properties YBO3:Yb3+,Tb3+ samples exhibit intense visible green luminescence due to presence of emitting Tb3+ ions, which can be excited both by UV or NIR radiation. This dual-mode
RI PT
luminescence is a unique property giving materials studied possible applications as novel, advanced phosphors.
The excitation spectra in the UV region were measured by observing emission related to the 5D4→7F5 transition of Tb3+ ions (see Fig. 4). The spectra present several bands typical for
SC
the excitation of Tb3+ ions, but the most intense is connected with the 4f8→4f75d1 transition, observed at around 235 nm. For this reason the most effective excitation wavelength,
M AN U
λexc = 235 nm, was used to measure luminescence activated in the UV region (see Fig. 5). Peaks observed at the excitation spectra of Tb3+-doped materials above 300 nm (at lower energies) originate from the spin-forbidden f–f electronic transitions. Photons with higher energy (200 – 300 nm) can promote electrons from the 4f electronic levels of the Tb3+ ion to the excited 5d shell rising to two configurations: a high-spin 9D state and a 7D low-spin state [52]. The 9D state has lower energy than the 7D state according to the Hund’s rule but the F6→9D transition is spin-forbidden [52]. Therefore, the probability to excite Tb3+ into its 7D
TE D
7
state is higher and the connected transition band is usually much more intense as in the case of YBO3:Yb3+,Tb3+ samples.
EP
Interesting is the fact that increasing concentration of Yb3+ dopant ions, causes quenching of the band related to the 4f8→4f75d1 transitions. This is an effect of formation of
AC C
intermediate Tb4+-Yb2+ charge transfer state (CTS) [53,54]. Additionally, growing amount of Yb3+ ions increases absorption of light, directly by the Yb3+ ions forming their O2--Yb3+ CTS. An appropriate experiment, confirming emission of Yb3+ after excitation at 235 nm was carried out, and the results are presented below. The emission spectra presented in Fig. 5 show typical bands for the luminescence of Tb3+ ions, with the maximum at 544 nm, associated with the 5D4→7F5 transition. Increasing concentration of Tb3+ ions change the emission intensity (Fig. 5a) and 30% of Tb3+ ions in the host quenched the luminescence almost completely. The observed dependence is caused by Tb3+-Tb3+ interactions or interactions in clusters containing many of Tb3+ ions. This concentration quenching affected emission intensity especially in the highly Tb3+-doped 6
ACCEPTED MANUSCRIPT samples [55]. For this reason the optimal concentration of Tb3+, giving the highest UVexcited luminescence is 10%. By increasing concentration of Yb3+ dopant ions, the emission intensity was gradually decreasing (Fig. 5b) until 30% Yb3+, when the green emission is almost totally quenched. From the inset in Fig. 5b it is clearly seen that emission of Tb3+ ions is highly sensitive to the presence of Yb3+ co-dopants (1% of Yb3+ ions caused decrease of
RI PT
luminescence by factor two). The phenomenon responsible for these observations is CET (known also as cooperative downconversion) between the Tb3+ ion at its 5D4 exited state and two Yb3+ in their 2F7/2 ground states [56].
Synthesized materials, presented a narrow and intense peak around 972 nm observed at
SC
their excitation spectra (Fig. 6) as the result of presence of Yb3+ ions in their structure. These ions absorb light from the NIR range and undergo 7F7/2→2F5/2 transition. Relatively narrow
M AN U
spectral range of this transition band is a fingerprint of a low crystal field acting to Yb3+ ions [57]. The presence the 7F7/2→2F5/2 transition band of Yb3+ is also a confirmation of energy transfer from Yb3+ to Tb3+ ions leading to the emission at 544 nm. The intensities of excitation peaks are Tb3+ and Yb3+ concentration depended, similarly as in emission spectra presented in Fig. 8.
The spectra measured under λexc = 972 nm excitation, consist of four characteristic
TE D
emission bands of Tb3+ ions, corresponding to the 5D4→7F6,5,4,3 transitions, the same as in the spectra registered under UV radiation. Observed up-conversion was strongly depended on the concentration of Tb3+ emitter ions and Yb3+ sensitizer ions. Optimal concentration, giving
EP
the highest luminescence under NIR light was 10% Yb3+ and 15% Tb3+ ions. The relatively high concentration of Tb3+ ions is an effect of the mechanism responsible for the observed
AC C
luminescence. Cooperative energy transfer is a rather not very efficient process. What is more, in Yb3+/Tb3+ system also reverse energy transfer is possible, from Tb3+ to Yb3+ ions and the estimated optimal concentrations of these two ions are balance between processes taking place in YBO3:Yb3+,Tb3+ materials. Additionally, such processes as interactions between Tb3+ ions, multiphonon relaxation and phonon-assisted energy transfer between Tb3+ and Yb3+ have also influence on the overall luminescence properties [23,58]. The luminescence lifetimes measured for the UV or NIR excitations, presented in Figs. S1 and S2 respectively, are available in the supporting information. The calculated on their basis values are shown in Fig. 8. It is well seen, that the consequence of the increasing Tb3+ concentration as well as Yb3+ cause significant shortening of the luminescence lifetimes. 7
ACCEPTED MANUSCRIPT Luminescence lifetimes of Tb3+ ions oscillate between 0.15 – 4.08 ms, when excited by UV radiation (Fig. 8a and b). Decrease of luminescence lifetime can be explained by above mentioned concentration quenching phenomena as well as CET from Tb3+ to Yb3+ ions. This leads to the non-radiative deexcitation of Tb3+ ions. The calculated values of the upconversion luminescence lifetimes obtained under NIR
RI PT
irradiation are presented in Fig. 8c and d. The registered decays showed a short rise of the intensity after excitation. These arise from the relatively slow pumping of Tb3+-excited states being the result of the energy transfer between Yb3+ and Tb3+ ions. The values of luminescence lifetimes measured under 972 nm excitation are shorter comparing to those
SC
measured under 235 nm excitation. Also decay times presented in Fig. 8c are not such depended on the Tb3+ concentration as it is seen the case of UV-excitation. This observation
M AN U
shows that not only local environment is important on the luminescence lifetime of Tb3+ ions, but also the way how they are excited.
When concentration of Yb3+ ions grows from 1% to 30% the luminescent lifetimes are gradually shortening. But, increasing amount of the Tb3+ dopant ions had no clear influence on the luminescence values’ lifetimes at concentrations up to 20%. This arise supposition
TE D
that concentration quenching had lower effect on the upconversion of Tb3+ ions than in the case of UV-excited emission.
Emission rise times shortened with increasing concentrations of Yb3+ ions what is a direct result of decreasing statistical distance in the crystal structure between Yb3+ donor and Tb3+
EP
acceptor ions. This also explains relatively smaller changes of rise times with increasing concentration of Tb3+ ions. The small relationship between Tb3+ concentration and rise time
AC C
is observed, as the same consequence as for Yb3+ ions – increased probability of energy transfer with higher amount of energy exchanging ions in the host lattice. In order to determine the mechanism of the UC process, the dependence of the integral luminescence intensity on the pumping laser power at 972 nm was studied. The relationship describes Eq. 3 presented above. The
results
of
measurements
registered
for
two
representative
samples:
YBO3:10%Yb3+,15%Tb3+ and YBO3:1Yb3+,5%Tb3+ are presented as logarithm plots in the Fig. 9. Based on fitting with the linear function and calculated slope value of approximately 1.5 and 1.7, the number of photons involved in the UC process is 2 what should be expected on the CET upconversion. Observed in Figs. 7 and 8, concentration quenching in connection with 8
ACCEPTED MANUSCRIPT backward energy transfer processes from the Tb3+ dopant ions to Yb3+ sensitizers, led to lowered emission intensity and changed values of slopes in comparison to the theoretical mechanism. At higher energies of pumping laser, the slope of determined curves decrease what suggest the presence of “saturation effect”. The explanation of this process for materials showing upconversion as the result of ETU mechanism (usually doped by Ho3+, Er3+
RI PT
or Tm3+ ions) is a competition between linear decay, being a result of multiphonon/crossrelaxation or energy transfer to impurities and the upconversion process as the ways of the depletion of the intermediate excited states of Ln3+ ions [59,60]. However in Tb3+ ions, any of intermediate levels are possible and the CET preceded upconversion at higher energies of
SC
laser must be lowered by different reason. The emission form the 5D4 exited state of Tb3+ ions could be sensitive for thermal effects, a the result of enhanced backward, phonon
M AN U
assisted energy transfer (PET) to Yb3+ [58].
Downconversion in YBO3:Yb3+,Tb3+ materials occurs by two ways: as the conversion of UV to visible light, through to the transitions in the Tb3+ ions and as the result of energy transfer from Tb3+ to Yb3+ ions what cause conversion of UV (or visible) to NIR light. The Yb3+/Tb3+ pair of ions is known as downconverting system which allows for quantum cutting
TE D
phenomena and quantum efficiencies of NIR emission, higher than 100% [38,61]. In our materials, downconversion to NIR occurs, what is presented in Fig. 10 on the selected samples, but the resulted emission was weak. This unusual property of the synthesized samples, could be interesting for the applications in solar cells [61].
EP
Luminescence of Yb3+ ions was more intense after excitation at 235 nm, than by excitation of Tb3+ to the 5D4 excited state. Under 235 nm excitation, two competing
AC C
processes occur: Tb3+ ions can be excited into the 4f75d1 state or O2--Yb3+ CTS is formed [56]. But, what is discussed above, the Yb3+ ion can interact with Tb3+ ions forming Tb4+-Yb2+ CTS, instead of fast relaxation from O2--Yb3+ CTS to the 2F5/2 Yb3+ excited state [56]. Hence, observed decrease of Yb3+ emission (see fig. 10) with growing amount of Tb3+ in the system can be explained by the quenching electron transfer process and formation of Tb4+-Yb2+ CTS. Also luminescence lifetimes presented in Fig. 10b confirm this fact. However, the emission of single doped sample (by 10% of Yb3+ ions) under 235 nm excitation was poor, what means that Tb3+ ions are responsible for the improvement of excitation of Yb3+ ions, but most probably, at low concentrations. The lack of Yb3+ emission under 545 nm excitation from the
9
ACCEPTED MANUSCRIPT sample doped only by 10% of Yb3+ ions, confirmed that only ET between Tb3+ and Yb3+ can explain downconversion in double-doped samples. Excitation into the O2--Yb3+ CT band is much more effective than excitation through to the energy transfer from Tb3+ ions (by the excitation at 545 nm). Therefore we assume that in the studied materials quenching of Tb3+ upconversion by CET to Yb3+ ions is rather
RI PT
minimal.
The scheme of the most important processes possible in the YBO3:Yb3+,Tb3+ materials is presented in Fig. 11. The observed dual-mode luminescence, excitable by UV either as NIR light arises from down- or upconversion phenomena. After excitation into 4f75d1 level of
SC
Tb3+ ions, what can occur under radiation with wavelength of 235 nm, several processes can take place. First, is nonradiative relaxation into emitting level of Tb3+ ion, from which the 5D4
M AN U
excited state is the most important, and responsible for the green luminescence as the result of radiative relaxation into one of the 7Fj ground state components. Second process is formation of Tb4+-Yb2+ CTS state and loss of the absorbed energy by the nonradiative way. Another possibility is relaxation into 5D4 excited state of Tb3+ ions followed by CET to neighbouring Yb3+ ions, what results in the emission of NIR light. Irradiation of the YBO3:Yb3+,Tb3+ material by UV light may also cause excitation of Yb3+ ions into their O2--Yb3+
TE D
CTS. Subsequently the 2F5/2 excited state of Yb3+ ions become populated as the result of fast relaxation from the CTS state and finally relaxation to the 2F7/2 ground state of Yb3+ ions occurring with emission of the NIR light. Summarizing, downconversion from 235 nm to
EP
green and NIR light is possible by the energy migration in or between Tb3+ and Yb3+ ions. Another achievable process in YBO3:Yb3+,Tb3+ materials is upconversion, resulting in
AC C
green luminescence from Tb3+ ions. As the energy level structure of the Tb3+ ion limits the possible way of energy exchange between NIR absorbing Yb3+ and luminescent Tb3+ ions, only cooperative energy transfer is possible as lying behind observed upconversion. Only two excited Yb3+ ions, transferring simultaneously their energy can bridge the gap between the ground and excited state of the Tb3+ ion. This is in accordance with the determined number of photons responsible for upconversion (see Fig. 9). However, also absorption of another photon, when Tb3+ ions are in their exited state, is possible. Exited state absorption explains upconversion from the 5D3 and higher excited states of Tb3+ ions (visible in Fig. 12). However, ESA needs to be preceded by CET, making together a three photon process.
10
ACCEPTED MANUSCRIPT Fig. 12 shows a comparison of upconversion from different REPO4 and REBO3-based host materials, in which the optimal dopant concentrations were established by us previously [23,54,62,63]. YBO3:10%Yb3+,15%Tb3+ is one from the several studied by us materials which upconversion is very intense, much more efficient than in rare earth phosphates. However is not as intense as in GdBO3:10%Yb3+,20%Tb3+. When rare earth borates are used as hosts for
RI PT
Yb3+ and Tb3+ ions, also upconversion from higher exited states is possible. Inset in Fig. 12 shows emission bands connected with transitions from the 5G6 or 5D3 excited states of Tb3+ ions. 4. Conclusions
SC
Yttrium orthoborates doped by Yb3+ and Tb3+ were successfully synthesized by the sol-gel method at 900°C. The monoclinic crystal structure of the products with the C2/c space group
M AN U
and their phase purity were confirmed by XRD, IR and EDX analyses. TEM images showed morphology of products which were comprised of crystals with average sizes around 200 nm.
The phosphors synthesized showed green emission under excitation of UV (235 nm) or NIR (972 nm) radiation. Spectroscopic properties of the materials synthesized were analysed on the basis of excitation and emission spectra and also luminescence decays. In the
TE D
YBO3:Yb3+,Tb3+ system several processes were observed. Under UV light, transitions into the 4f75d1 excited state of Tb3+, formation of O2--Yb3+ or Tb4+-Yb2+ charge transfer states occurred. First two processes, allowed for the downconversion of UV light to green or NIR
EP
radiation. Yet, emission to visible and NIR occurred simultaneously, mechanism lying behind this observation was different. Nonradiative decay of Tb3+ ions to their 5D4 exited state
AC C
preceded emission of green photons as the result of transitions from excited 5D4 to the 7Fj ground state. NIR emission was a result of energy transfer from Tb3+ ions in the 5D4 state to Yb3+ ions and their 2F5/2→2F7/2 transition. NIR emission was additionally enhanced by transitions of Yb3+ ions from their excited states being formed by the nonradiative relaxation from the O2--Yb3+ CTS.
Another observed process was connected with irradiation of products by the light from the NIR range. Resulted upconversion to visible light occurred as the result of cooperative energy transfer from Yb3+ to Tb3+ ions. Observed transitions from the higher excited states of Tb3+ ions: 5D3 and 5G6, raised conclusion, that also excited state absorption occurred in samples analysed. Determined dependencies of integral luminescence intensity on the 11
ACCEPTED MANUSCRIPT energy of pumping laser indicated two photon process responsible for the green upconversion of Tb3+ ions. This also confirms proposed CET as the mechanism of observed upconversion. Dependence of luminescence properties on the dopants concentrations was also analysed. The most intense luminescence under UV excitation was observed for
RI PT
YBO3:10%Yb3+,10%Tb3+ sample in the series with changed Tb3+ concentration and for the YBO3:2.5%Yb3+,5%Tb3+ sample when the influence of various Yb3+ concentrations was analysed. The most effective upconversion under 972 nm excitation was observed for the YBO3:10%Yb3+,15%Tb3+ sample in the series with changed Tb3+ concertation and for the
SC
YBO3:10%Yb3+,5%Tb3+ when Yb3+ concentration effects were analysed.
Luminescence lifetimes determined from the emission decays were in the range typical
M AN U
for Tb3+ ions: 0.15 – 4.08 ms. Dependence of emission lifetimes on the Tb3+ or Yb3+ ions concentrations, indicated quenching processes, such as concentration quenching and backward energy transfers between Tb3+ or Tb3+ and Yb3+ ions.
Summarizing, the YBO3 compound is a good host for Yb3+ and Tb3+ ions allowing for the utilization of convenient sol-gel synthesis method for its production. The prepared materials showed intense luminescence under UV as well as NIR light. The presented properties of
TE D
YBO3:Yb3+,Tb3+ materials, recommend them as potentially applicable phosphors for lightening and security markers, in forensic sciences or in solar cells production.
EP
5. Acknowledgement
Financial support from the National Science Centre (grant no. DEC-2011/03/D/ST5/05701) is
AC C
gratefully acknowledged. 6. References [1] [2]
X. Liu, C.-H. Yan, J. a. Capobianco, Photon upconversion nanomaterials, Chem. Soc. Rev. 44 (2015) 1299–1301. doi:10.1039/C5CS90009C.
G. Chen, H. Qiu, P.N. Prasad, X. Chen, Upconversion nanoparticles: design, nanochemistry, and applications in theranostics., Chem. Rev. 114 (2014) 5161–214. doi:10.1021/cr400425h.
[3]
W. Feng, C. Han, F. Li, Upconversion-Nanophosphor-Based Functional Nanocomposites., Adv. Mater. (2013) 1–17. doi:10.1002/adma.201301946.
[4]
M. Haase, H. Schäfer, Upconverting nanoparticles., Angew. Chem. Int. Ed. 50 (2011) 5808–29. doi:10.1002/anie.201005159.
[5]
F. Auzel, Upconversion and anti-Stokes processes with f and d ions in solids., Chem. Rev. 104 (2004) 139–73. doi:10.1021/cr020357g. 12
ACCEPTED MANUSCRIPT K. Zheng, Z. Liu, C. Lv, W. Qin, Temperature sensor based on the UV upconversion luminescence of Gd3+ in Yb3+–Tm3+–Gd3+ codoped NaLuF4 microcrystals, J. Mater. Chem. C. 1 (2013) 5502. doi:10.1039/c3tc30763h.
[7]
S. Hao, G. Chen, C. Yang, Sensing using rare-earth-doped upconversion nanoparticles., Theranostics. 3 (2013) 331–45. doi:10.7150/thno.5305.
[8]
R. Ma, R. Shimmon, A. McDonagh, P. Maynard, C. Lennard, C. Roux, Fingermark detection on non-porous and semi-porous surfaces using YVO4:Er,Yb luminescent upconverting particles, Forensic Sci. Inter. 217 (2012) e23–6. doi:10.1016/j.forsciint.2011.10.033.
[9]
X. Li, Y. Wu, Y. Liu, X. Zou, L. Yao, F. Li, et al., Cyclometallated ruthenium complex-modified upconversion nanophosphors for selective detection of Hg2+ ions in water, Nanoscale. 6 (2014) 1020–1028. doi:10.1039/C3NR05195A.
[10]
M. Wang, M. Li, M. Yang, X.M. Zhang, A. Yu, Y. Zhu, NIR-induced highly sensitive detection of latent fingermarks by NaYF4:Yb,Er upconversion nanoparticles in a dry powder state, Nano Res. 8 (2015) 1800–1810. doi:10.1007/s12274-014-0686-6.
[11]
R. Deng, F. Qin, R. Chen, W. Huang, M. Hong, X. Liu, Temporal full-colour tuning through nonsteady-state upconversion, Nat. Nanotechnol. 10 (2015) 237–242. doi:10.1038/nnano.2014.317.
[12]
T.R. Hinklin, S.C. Rand, R.M. Laine, Transparent, Polycrystalline Upconverting Nanoceramics: Towards 3-D Displays, Adv. Mater. 20 (2008) 1270–1273. doi:10.1002/adma.200701235.
[13]
A. Rapaport, J. Milliez, M. Bass, A. Cassanho, H. Jenssen, Review of the properties of Upconversion phosphors for new emissive displays, IEEE/OSA J. Disp. Technol. 2 (2006) 68–78. doi:10.1109/JDT.2005.863781.
[14]
W.G. van Sark, J. de Wild, J.K. Rath, A. Meijerink, R.E. Schropp, Upconversion in solar cells., Nanoscale Res. Lett. 8 (2013) 81. doi:10.1186/1556-276X-8-81.
[15]
B.M. van der Ende, L. Aarts, A. Meijerink, Lanthanide ions as spectral converters for solar cells., Phys. Chem. Chem. Phys. 11 (2009) 11081–95. doi:10.1039/b913877c.
[16]
Y. Il Park, K.T. Lee, Y.D. Suh, T. Hyeon, Upconverting nanoparticles: a versatile platform for wide-field two-photon microscopy and multi-modal in vivo imaging., Chem. Soc. Rev. 44 (2014) 1302–1317. doi:10.1039/c4cs00173g.
[17]
C. Liu, Y. Hou, M. Gao, Are Rare-Earth Nanoparticles Suitable for In Vivo Applications?, Adv. Mater. 26 (2014) 6922–6932. doi:10.1002/adma.201305535.
[19]
SC
M AN U
TE D
EP
AC C
[18]
RI PT
[6]
S. Wang, J. Feng, S. Song, H. Zhang, Rare earth fluorides upconversion nanophosphors: from synthesis to applications in bioimaging, CrystEngComm. 15 (2013) 7142. doi:10.1039/c3ce40679b.
G. Mistlberger, I. Klimant, Luminescent magnetic particles: structures, syntheses, multimodal imaging, and analytical applications, Bioanal. Rev. 2 (2010) 61–101. doi:10.1007/s12566-0100017-7.
[20]
Y. Il Park, H.M. Kim, J.H. Kim, K.C. Moon, B. Yoo, K.T. Lee, et al., Theranostic Probe Based on Lanthanide-Doped Nanoparticles for Simultaneous In Vivo Dual-Modal Imaging and Photodynamic Therapy, Adv. Mater. 24 (2012) 5755–5761. doi:10.1002/adma.201202433.
[21]
F. Ostermayer, L. Van Uitert, Cooperative Energy Transfer from Yb3+ to Tb3+ in YF3, Phys. Rev. B. 1 (1970) 4208–4212. doi:10.1103/PhysRevB.1.4208.
13
ACCEPTED MANUSCRIPT L.D. Livanova, I.G. Saitkulo, A.L. Stolov, Summation process for quanta in CaF2 and SrF2 single crystals activated with Tb3+ and Yb3+ ions, Sov. Phys. Solid State. 11 (1969) 750+.
[23]
T. Grzyb, A. Gruszeczka, R.J. Wiglusz, Z. Śniadecki, B. Idzikowski, S. Lis, Multifunctionality of GdPO4:Yb3+,Tb3+ nanocrystals – luminescence and magnetic behaviour, J. Mater. Chem. 22 (2012) 22989–22997. doi:10.1039/c2jm34863b.
[24]
R. Martín-Rodríguez, R. Valiente, S. Polizzi, M. Bettinelli, A. Speghini, F. Piccinelli, Upconversion Luminescence in Nanocrystals of Gd3Ga5O12 and Y3Al5O12 Doped with Tb3+− Yb3+ and Eu3+− Yb3+, J. Phys. Chem. C. 113 (2009) 12195–12200. doi:10.1021/jp901711g.
[25]
G. Salley, R. Valiente, H. Güdel, Cooperative Yb3+-Tb3+ dimer excitations and upconversion in Cs3Tb2Br9:Yb3+, Phys. Rev. B. 67 (2003) 1–9. doi:10.1103/PhysRevB.67.134111.
[26]
T. Yamashita, Y. Ohishi, Cooperative energy transfer between and ions co-doped in borosilicate glass, J. Non-Cryst. Solids. 354 (2008) 1883–1890. doi:10.1016/j.jnoncrysol.2007.10.026.
[27]
I. Hernández, N. Pathumakanthar, P.B. Wyatt, W.P. Gillin, Cooperative infrared to visible up conversion in Tb3+, Eu3+, And Yb3+ containing polymers., Adv. Mater. 22 (2010) 5356–60. doi:10.1002/adma.201002674.
[28]
L. Yang, L. Zhou, Y. Huang, Z. Tang, Hydrothermal synthesis of GdBO3:Eu3+ nanofibres, Mater. Lett. 64 (2010) 2704–2706. doi:10.1016/j.matlet.2010.08.062.
[29]
Z. Yang, Y. Wen, N. Sun, Y. Wang, Y. Huang, Z. Gao, et al., Morphologies of GdBO3:Eu3+ onedimensional nanomaterials, J. Alloy. Compd. 489 (2010) L9–L12. doi:10.1016/j.jallcom.2009.09.073.
[30]
H. Zhu, L. Zhang, T. Zuo, X. Gu, Z. Wang, L. Zhu, et al., Sol–gel preparation and photoluminescence property of YBO3:Eu3+/Tb3+ nanocrystalline thin films, Appl. Surf. Sci. 254 (2008) 6362–6365. doi:10.1016/j.apsusc.2008.03.183.
[31]
J.C. Zhang, Y.H. Wang, X. Guo, Vuv-excited photoluminescence dependent on thermal treatment of YBO3:Tb3+ derived by hydrothermal process, J. Lumin. 122-123 (2007) 980–983. doi:10.1016/j.jlumin.2006.01.344.
[32]
T. Kim, S. Kang, Hydrothermal synthesis and photoluminescence properties of nano-crystalline GdBO3:Eu phosphor, Mater. Res. Bull. 40 (2005) 1945–1954. doi:10.1016/j.materresbull.2005.06.001.
[33]
H. Giesber, J. Ballato, G. Chumanov, J. Kolis, M. Dejneka, Spectroscopic properties of Er3+ and Eu3+ doped acentric LaBO3 and GdBO3, J. Appl. Phys. 93 (2003) 8987. doi:10.1063/1.1536724.
[35]
SC
M AN U
TE D
EP
AC C
[34]
RI PT
[22]
D. Boyer, G. Bertrand-chadeyron, R. Mahiou, C. Caperaa, J.C. Cousseins, Synthesis dependent luminescence efficiency in Eu3+ doped polycrystalline YBO3, J. Mater. Chem. 9 (1999) 211–214.
A. Szczeszak, T. Grzyb, B. Barszcz, V. Nagirnyi, A. Kotlov, S. Lis, Hydrothermal Synthesis and Structural and Spectroscopic Properties of the New Triclinic Form of GdBO3:Eu3+ Nanocrystals., Inorg. Chem. 52 (2013) 4934–4940. doi:10.1021/ic302525k.
[36]
A. Szczeszak, T. Grzyb, S. Lis, R.J. Wiglusz, Revision of structural properties of GdBO3 nanopowders doped with Eu3+ ions through spectroscopic studies., Dalt. Trans. 41 (2012) 5824–5831. doi:10.1039/c2dt12154a.
[37]
Z. Yang, D. Yan, K. Zhu, Z. Song, X. Yu, D. Zhou, et al., Color tunability of upconversion emission in YBO3:Yb,Er inverse opal, Mater. Lett. 65 (2011) 1245–1247. doi:10.1016/j.matlet.2011.01.071. 14
Q.Y. Zhang, C.H. Yang, Z.H. Jiang, X.H. Ji, Concentration-dependent near-infrared quantum cutting in GdBO3:Tb3+,Yb3+ nanophosphors, Appl. Phys. Lett. 90 (2007) 061914. doi:10.1063/1.2472195.
[39]
M. Pechini, “Method of Preparing Lead and Alkaline Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor,” US Patent No. 3330697, 1967.
[40]
J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third, Springer, Baltimore, 2006.
[41]
R. Scheps, Upconversion laser processes, Ptog. Quant. Electron. 20 (1996) 271–358. doi:10.1016/0079-6727(95)00007-0.
[42]
J. Lin, D. Sheptyakov, Y. Wang, P. Allenspach, Structures and Phase Transition of Vaterite-Type Rare Earth Orthoborates: A Neutron Diffraction Study, Chem. Mater. 16 (2004) 2418–2424. doi:10.1021/cm0499388.
[43]
G. Jia, P.A. Tanner, C.-K. Duan, J. Dexpert-Ghys, Eu3+ Spectroscopy: A Structural Probe for Yttrium Orthoborate Phosphors, J. Phys. Chem. C. 114 (2010) 2769–2775. doi:10.1021/jp910329k.
[44]
A. Szczeszak, K. Kubasiewicz, T. Grzyb, S. Lis, Spectroscopic properties of Y1−xEuxBO3 and Y1−xTbxBO3 nanopowders obtained by the sol–gel Pechini method, J. Lumin. 155 (2014) 374– 383. doi:10.1016/j.jlumin.2014.07.006.
[45]
G. Chadeyron, M. El-Ghozzi, R. Mahiou, A. Arbus, J. Cousseins, Revised structure of the orthoborate YBO3, J. Solid State Chem. 128 (1997) 261–266. doi: 10.1006/jssc.1996.7207.
[46]
G. Jia, H. You, K. Liu, Y. Zheng, N. Guo, J. Jia, et al., Highly uniform YBO3 hierarchical architectures: facile synthesis and tunable luminescence properties., Chemistry (Easton). 16 (2010) 2930–7. doi:10.1002/chem.200902834.
[47]
Z. Wei, L. Sun, C. Liao, J. Yin, X. Jiang, C. Yan, et al., Size-Dependent Chromaticity in YBO3:Eu Nanocrystals: Correlation with Microstructure and Site Symmetry, J. Phys. Chem. B. 106 (2002) 10610–10617. doi:10.1021/jp025967z.
[48]
Z. Li, J. Zeng, Y. Li, Solvothermal route to synthesize well-dispersed YBO3:Eu nanocrystals., Small. 3 (2007) 438–43. doi:10.1002/smll.200600151.
[49]
X.C. Jiang, C.H. Yan, L.D. Sun, Z.G. Wei, C.S. Liao, Hydrothermal homogeneous urea precipitation of hexagonal YBO3:Eu3+ nanocrystals with improved luminescent properties, J. Solid State Chem. 175 (2003) 245–251. doi:10.1016/S0022-4596(03)00276-7.
[50] [51]
EP
TE D
M AN U
SC
RI PT
[38]
AC C
ACCEPTED MANUSCRIPT
A. Nohara, S. Takeshita, T. Isobe, Mixed-solvent strategy for solvothermal synthesis of welldispersed YBO3:Ce3+,Tb3+ nanocrystals, RSC Adv. 4 (2014) 11219. doi:10.1039/c3ra47864e.
G. Pan, H. Song, L. Yu, Z. Liu, X. Bai, Y. Lei, et al., Structure and Photoluminescent Properties of Microstructural YBO3:Eu3+ Nanocrystals, J. Nanosci. Nanotechnol. 7 (2007) 593–601. doi:10.1166/jnn.2007.133.
[52]
L. Ning, C. Mak, P. Tanner, High-spin and low-spin f-d transitions of Tb3+ in elpasolite hosts, Phys. Rev. B. 72 (2005) 085127. doi:10.1103/PhysRevB.72.085127.
[53]
J.-L. Yuan, X.-Y. Zeng, J.-T. Zhao, Z.-J. Zhang, H.-H. Chen, X.-X. Yang, Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor, J. Phys. D Appl. Phys. 41 (2008) 105406. doi:10.1088/0022-3727/41/10/105406.
[54]
T. Grzyb, A. Gruszeczka, R.J. Wiglusz, S. Lis, The effects of down- and up-conversion on dualmode green luminescence from Yb3+- and Tb3+-doped LaPO4 nanocrystals, J. Mater. Chem. C. 1 15
ACCEPTED MANUSCRIPT (2013) 5410–5418. doi:10.1039/c3tc31100g. G. Blasse, Some considerations on rare-earth activated phosphors, J. Lumin. 1-2 (1970) 766– 777. doi:10.1016/0022-2313(70)90089-X.
[56]
X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, et al., Cooperative downconversion and nearinfrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses, Appl. Phys. B. 96 (2009) 51–55. doi:10.1007/s00340-009-3478-z.
[57]
I. Sokólska, W. Ryba-Romanowski, S. Gołab, T. Lukasiewicz, The optical properties of Yb3+ ions in LiTaO3:Nd,Yb crystals, Appl. Phys. B. 65 (1997) 495–498. doi:10.1007/s003400050302.
[58]
Y. Arai, T. Yamashidta, T. Suzuki, Y. Ohishi, Upconversion properties of Tb3+–Yb3+ codoped fluorophosphate glasses, J. Appl. Phys. 105 (2009) 83105–83111. doi:10.1063/1.3112010.
[59]
M. Pollnau, D. Gamelin, S. Lüthi, H. Güdel, M. Hehlen, Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems, Phys. Rev. B. 61 (2000) 3337– 3346. doi:10.1103/PhysRevB.61.3337.
[60]
Y. Lei, H. Song, L. Yang, L. Yu, Z. Liu, G. Pan, et al., Upconversion luminescence, intensity saturation effect, and thermal effect in Gd2O3:Er3+,Yb3+ nanowires., J. Chem. Phys. 123 (2005) 174710. doi:10.1063/1.2087487.
[61]
Y.-T. An, C. Labbé, J. Cardin, M. Morales, F. Gourbilleau, Highly Efficient Infrared Quantum Cutting in Tb3+−Yb3+ Codoped Silicon Oxynitride for Solar Cell Applications, Adv. Opt. Mater. 1 (2013) 855–862. doi:10.1002/adom.201300186.
[62]
T. Grzyb, R.J. Wiglusz, A. Gruszeczka, S. Lis, Down- and up-converting dual-mode YPO4:Yb3+,Tb3+ nanocrystals: synthesis and spectroscopic properties., Dalton Trans. 43 (2014) 17255–17264. doi:10.1039/c4dt02234c.
[63]
T. Grzyb, K. Kubasiewicz, A. Szczeszak, S. Lis, Synthesis and spectroscopic properties of Yb3+ and Tb3+ co-doped GdBO3 materials showing down- and up-conversion luminescence, Dalt. Trans. 44 (2015) 4063–4069. doi:10.1039/C4DT03667K.
AC C
EP
TE D
M AN U
SC
RI PT
[55]
16
ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of the YBO3:10%Yb3+, xTb3+ and YBO3:xYb3+, 5%Tb3+ samples (x = 1 - 30%), annealed at 900 °C. Fig. 2 IR spectra of the YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ samples (x = 1-30%),
RI PT
annealed at 900 °C. Fig. 3 TEM images of the YBO3 (a) and YBO3:10%Yb3+,5%Tb3+ (b) sample annealed at 900 °C and EDX spectrum (inset) of the YBO3:10%Yb3+,5%Tb3+ sample annealed at 900 °C.
Fig. 4 Excitation spectra of the (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples
SC
(x = 1 - 30%), annealed at 900 °C, measured by observation at λem = 544 nm.
Fig. 5 Emission spectra of the (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples (x =
M AN U
1 - 30%), annealed at 900 °C, measured under λexc = 235 nm.
Fig. 6 Excitation spectra of (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples (x = 1 30%), annealed at 900 °C, measured at λem = 544 nm.
Fig. 7 Emission of the (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples (x = 1 30%), annealed at 900 °C, measured at λexc = 972 nm.
Fig. 8 Values of the calculated luminescence lifetimes of the (a,c) YBO3:10%Yb3+,xTb3+ and (b,d)
TE D
YBO3:xYb3+,5%Tb3+ samples (x = 1 - 30%).
Fig. 9 Integral intensity of the YBO3:10%Yb3+,15%Tb3+ and YBO3:1Yb3+,5%Tb3+ samples as a function of the pumping power of the laser (λexc = 972 nm).
EP
Fig. 10 Downconversion emission (a) and luminescence decays (b) of Yb3+ ions, under excitation at 235 or 545 nm. Fig. 11
Scheme of the UC process observed in the YBO3:10%Yb3+,xTb3+ and
AC C
YBO3:xYb3+,5%Tb3+ samples. Fig. 12 Comparison of upconversion luminescence from the REPO4 and REBO3-based materials doped by optimal concentrations of ions, giving the most intense emission, measured at λexc = 972 nm (range 400-470 nm measured at slits 2 nm, 450-700nm at 0.35 nm).
17
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 1
Fig. 2
18
AC C
EP
TE D
M AN U
Fig. 3
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4
19
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 5
20
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
Fig. 6
AC C
EP
TE D
Fig. 7
Fig. 8 21
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 9
Fig. 10
22
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 11
Fig. 12
23
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
RI PT
Supporting information
Fig. S1 Luminescence lifetimes of the YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ samples
AC C
EP
(x = 1 - 30%), annealed at 900 °C, measured under λexc = 235 nm and λem = 544 nm.
24
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. S2 Luminescence lifetimes of the YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ samples (x
AC C
EP
TE D
= 1 - 30%), annealed at 900 °C, measured under λexc = 972 nm and λem = 544 nm.
25
ACCEPTED MANUSCRIPT Highlights Down- and upconverting, YBO3:Yb3+,Tb3+ materials were synthesized
•
Energy migration processes between Yb3+ and Tb3+ ions were analysed
•
Effects of dopants concentration on emission properties were observed
•
Spectroscopic properties of materials prepared were compared
AC C
EP
TE D
M AN U
SC
RI PT
•
S0925-8388(16)31953-3
DOI:
10.1016/j.jallcom.2016.06.230
Reference:
JALCOM 38099
To appear in:
Journal of Alloys and Compounds
Received Date: 15 April 2016 Revised Date:
21 June 2016
Accepted Date: 22 June 2016
Please cite this article as: T. Grzyb, K. Kubasiewicz, A. Szczeszak, S. Lis, Energy migration in 3+ 3+ YBO3:Yb ,Tb materials: Down- and upconversion luminescence studies, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.230. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Energy migration in YBO3:Yb3+,Tb3+ materials: down- and upconversion luminescence studies Tomasz Grzyb*, Konrad Kubasiewicz, Agata Szczeszak, Stefan Lis Department of Rare Earths, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań
Keywords:
rare
earth
borates,
YBO3,
upconversion,
downconversion,
SC
luminescence, charge transfer
Yb3+/Tb3+,
RI PT
E-mail: [email protected]
Abstract
Down- and upconversion luminescence phenomena from YBO3 materials doped by Yb3+ and
M AN U
Tb3+ ions were observed and investigated. Materials described in the article, were synthesized by a sol-gel method. Two series of products were prepared: with changed molar concentrations of Yb3+ or Tb3+ ions in the range from 1 to 30%. This way of doping was chosen to analyse the influence of ions concentrations on luminescence and to find samples with the most intense emission. Structural and morphological properties of the obtained
TE D
materials were analysed on the basis of X-ray diffraction, infrared spectroscopy and by collecting images of samples from transmission electron microscope. The products were pure and well crystallized, showing monoclinic structure with C2/c space group. the
YBO3:Yb3+,Tb3+ materials were
investigated
by
EP
Spectroscopic properties of
measurements of excitation spectra in the ultraviolet and near infrared range; emission
AC C
spectra in the visible range; luminescence decays and dependencies of integral luminescence intensity on pumping laser energy. Based on careful studies of the obtained results, mechanisms responsible for the processes observed in these materials were proposed and explained.
1. Introduction In the recent years much attention has been paid to materials, which exhibit visible emission under near infrared (NIR) radiation [1–5]. Such phenomenon is known as upconversion (UC). In these materials, absorption of two or more low-energy photons results in emission of a single high-energy photon [5]. This process can be utilized in applications and research areas 1
ACCEPTED MANUSCRIPT as temperature, ions and molecules sensors, fingerprint identification, security markers, display technologies and solar cells design [6–15]. UC process used in biomedicine make possible to reduce photoblinking, photobleaching or autofluorescence effects [16,17]. Therefore, materials showing UC can be used in biomedical applications such as bioimaging or photodynamic therapy [2,4,17–20].
RI PT
There are several mechanisms of UC process, i.e. ground- or excited-state absorption (GSA or ESA) possible in single doped systems or energy transfer upconversion (ETU) where the Yb3+ ion play the role of sensitizer and Er3+, Tm3+ or Ho3+ act as the emitting ions [5]. However, for the Yb3+/Tb3+ dopants, the mechanism of UC differs from remaining. It is called
SC
cooperative energy transfer (CET), where at least two Yb3+ ions simultaneously transfer energy to one Tb3+ ion [21–27]. CET based upconversion occurs when the energy gap
M AN U
between the highest lying component of the ground state of Ln3+ ion and its first excited state is too large to be bridged by a single, excited Yb3+ ion. Only some host compounds are appropriate to support this relatively rare process, taking into account all investigated systems showing UC. Since the efficiency of CET is lower than that in analogue ETU-based systems, in order to improve it a high crystallinity and purity of material is demanded. This is usually connected with high temperature processing. Hence, such compounds as rare earth
TE D
borates seem to be good candidates for Tb3+ upconversion. Rare earth orthoborates, REBO3 (RE = Y, Gd, La) have been widely studied due to their suitable physicochemical properties as hosts for lanthanide ions (Ln3+), excitable in
EP
ultraviolet (UV) range: Eu3+ and Tb3+ [28–34]. Nanophosphors based on REBO3, doped by Eu3+ ions exhibit intense luminescence with high quantum yields and desired colour chromaticity
AC C
[35,36]. What is worth noting, the compound chosen by us has been investigated only once as showing upconversion, in the Yb3+ and Er3+ co-doped system [37]. Besides this example and our previous report about Yb3+ and Tb3+ doped GdBO3 material, REBO3 were not investigated as hosts for upconversion purposes [38]. However, in this report we show that UC in YBO3 may be intense and also an interesting object of investigation. 2. Experimental The YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ (1% ≤ x ≤ 30%) nanophosphors were synthesized by the sol gel method modified and optimized in order to obtain pure and single phase YBO3.[36,39] The best composition of reagents, temperature and time of annealing were found, avoiding formation of polyborates or Y2O3. 2
ACCEPTED MANUSCRIPT
2.1. Materials Rare earth oxides Y2O3, Yb2O3 and Tb4O7; (99.99% purchased from Stanford Materials, USA), nitric acid HNO3 (ultra-pure, Avantor S.A., Poland), orthoboric acid (p.a. grade, Avantor
RI PT
S.A., Poland), citric acid monohydrate (p.a. grade, CHEMPUR, Poland), ethylene glycol (p.a. grade, CHEMPUR, Poland) were used as starting materials in the synthesis. From the oxides, 1 M nitrate solutions were prepared by dissolving appropriate rare earth oxides in HNO3 diluted by distilled water (1:1).
SC
2.2. Synthesis
All of the syntheses were carried out for 6.5 mmol of the product. Firstly, the stoichiometric
M AN U
amounts of appropriate rare earth salt solutions were mixed with 100 mL of deionized water together with citric acid (CA) and ethylene glycol (EG). The used molar ratio between RE3+, CA and EG was 1:9:18. The organic compounds were used as the chelating and cross-linking reagents. Afterwards, 125% stoichiometric amount of H3BO3 was added and solutions were magnetically stirred until they became transparent. The excess of H3BO3 was used as this
TE D
compound partially evaporates from the system during an annealing process. Next, the homogenous solutions were dried in an oven at 80 °C for 48 h. Finally, the obtained gels were calcined at 900 °C in the air atmosphere for 3 h with the programmed 1.5 h increase of temperature. Annealed gels were transferred into white powders, which were additionally
EP
grounded in a mortar before analysing them. 2.3. Characterization
AC C
X-ray diffraction patterns (XRD) were collected using a Bruker AXS D8 Advance diffractometer in Debye–Scherrer geometry, with Cu Kα1 radiation (1.5406 Å) in the 2θ range from 6 – 60°. The XRD results were compared to the reference patterns from the Joint Committee on Powder Diffraction Standards (JCPDS) database. Images from the transmission electron microscope (TEM) and energy-dispersive X-ray spectra (EDX) were taken at FEI Tecnai G2 20 X-TWIN microscope, by an accelerating voltage of 200 kV. Transmittance spectra in the infrared (IR) range were registered between 400 and 4000 cm-1 on the FTIR spectrophotometer, JASCO 4200. In order to measure absorption spectra, materials were mixed with KBr and then pressed to disks. Luminescence excitation and emission spectra as well as decay curves were measured with the use of QuantaMasterTM 40 3
ACCEPTED MANUSCRIPT (Photon Technology International) spectrophotometer equipped with an Opolette 355LD UVDM (Opotek Incorporation) tunable laser (excitation source), with repetition rate of 20 Hz, and a Hamamatsu R928 or R5108 photomultipliers used as detectors. The area excited by the laser beam was 0.78 mm2. The measured luminescence decays showed a non-exponential character. Since the
calculations by applying the equation below: ∞
τ eff
∫ tI (t )dt = ∫ I (t )dt 0 ∞ 0
(1)
SC
RI PT
luminescence decay kinetics is non-exponential, effective lifetimes were used for
where τeff is the effective decay time, I(t) is the intensity at time t.[40]
To calculate the up-conversion rise times, the following equation was used: t
−
t
M AN U
−
I = [ I 0 + I1 (1 − e
τr
)]e
τd
(2)
where I0 is the initial luminescence intensity at time t = 0, I1 is the intensity added as a result of the energy transfer, and τr and τ, τd are the rise and decay times, respectively. The calculations and fit to the decay data was performed with the Origin 2016 software.
TE D
The dependence of the integral luminescence intensity on the pumping laser power was examined to analyse the mechanisms of the UC process. The relation between the UC intensity IUC and the pumping intensity IP is given by the following equation:
I UC = α (I P ) n
(3)
EP
where α is a proportionality factor, and the exponent n represents the number of photons involved in the UC process [41].
AC C
3. Results and discussion
3.1. Structural and morphological analysis Rare earth orthoborates crystallize in several structures depending on the size of RE3+ ion. These are aragonite, calcite or vaterite types. Light RE3+ ions (La – Nd) crystallize as aragonite type structures. Smaller ions (Sm – Yb and Y) are known as vaterite-type forms. Yttrium borate crystals crystallize as monoclinic, C2/c pseudovaterite-type structure determined by neutron diffraction analysis conducted by Lin et al. in 2004 [42]. Before, two other models were proposed: a disordered hexagonal P63/mmc and an ordered P63/mcm. However, studies of YBO3:Eu3+, where Eu3+ ions were used as a structural probe, indicated 4
ACCEPTED MANUSCRIPT that Eu3+ spectrum is different than should be expected from hexagonal models [43]. They proved, based on site selective spectroscopy, the monoclinic structure of YBO3. We have used this new crystal structure as reference in our studies. Two series of powders based on YBO3 doped by Yb3+ and Tb3+ ions were synthesized. The samples were obtained, once by doping with constant 10% of Yb3+ and 5% of Tb3+ ions
Tb3+ ions on the structural and spectroscopic properties.
RI PT
concentration. Such way of doping was made in order to analyse influence of both Yb3+ and
The XRD patterns and IR transition spectra shown in Figs. 1 and 2 confirmed the monoclinic crystal structure of the samples obtained, comparing to reference pattern No.
SC
01-073-7388 from the JCPDS crystallographic database. High concentrations of dopant ions were neutral for the crystal structure and any impurities appeared. It proved the great ability
M AN U
of YBO3 matrix for doping by Ln3+ ions. The intense and sharp peaks proved, that materials obtained crystallized well.
The IR spectra presented in Fig. 2, show broad and intense bands characteristic for the monoclinic YBO3 [44]. The spectra of the undoped as well as doped YBO3 consists mainly of a typical broad and intense band in the range from 866 to 1078 cm-1, that originates from the vibrations of B–O in B3O93- rings, typical for the monoclinic YBO3 structure [36,43]. Two peaks
TE D
at 866 and 917 cm-1 can be assigned to the ring stretching modes, whereas the band at 1078 cm-1 is originated from the terminal B–O stretching [45–47]. All of the IR spectra registered are very similar. The IR analysis as well as XRD patterns
EP
proved the purity of the nanomaterials obtained and lack of changes related to the dopant ions introduced into the YBO3 matrix. Appearance of other phase should introduce
AC C
absorption by the BO33- triangular groups what would be visible at around 1290 cm-1 [43]. The morphology of the YBO3 and YBO3:10%Yb3+,5%Tb3+ materials is presented in TEM images (Fig. 3). These images show the polygon- or spherical-like and agglomerated crystals. Size of the particular grain is mostly larger than 200 nm. In Fig. 3b the crystals’ planes are clearly seen, what proves that the material synthesized is well crystallized. The grains are sintered due to the high calcination temperature. Based on EDX spectrum, the presence of the Y, Yb and Tb elements was confirmed. Although, borates present interesting properties as hosts for emitting Ln3+ ions, it is difficult to obtain them as nanocrystalline materials. There is only one report about
5
ACCEPTED MANUSCRIPT nanocrystalline YBO3 prepared by a solvothermal method [48]. Other, present rather submicron sized YBO3-sized materials or with average crystal sizes close to 100 nm [49–51]. 3.2. Spectroscopic properties YBO3:Yb3+,Tb3+ samples exhibit intense visible green luminescence due to presence of emitting Tb3+ ions, which can be excited both by UV or NIR radiation. This dual-mode
RI PT
luminescence is a unique property giving materials studied possible applications as novel, advanced phosphors.
The excitation spectra in the UV region were measured by observing emission related to the 5D4→7F5 transition of Tb3+ ions (see Fig. 4). The spectra present several bands typical for
SC
the excitation of Tb3+ ions, but the most intense is connected with the 4f8→4f75d1 transition, observed at around 235 nm. For this reason the most effective excitation wavelength,
M AN U
λexc = 235 nm, was used to measure luminescence activated in the UV region (see Fig. 5). Peaks observed at the excitation spectra of Tb3+-doped materials above 300 nm (at lower energies) originate from the spin-forbidden f–f electronic transitions. Photons with higher energy (200 – 300 nm) can promote electrons from the 4f electronic levels of the Tb3+ ion to the excited 5d shell rising to two configurations: a high-spin 9D state and a 7D low-spin state [52]. The 9D state has lower energy than the 7D state according to the Hund’s rule but the F6→9D transition is spin-forbidden [52]. Therefore, the probability to excite Tb3+ into its 7D
TE D
7
state is higher and the connected transition band is usually much more intense as in the case of YBO3:Yb3+,Tb3+ samples.
EP
Interesting is the fact that increasing concentration of Yb3+ dopant ions, causes quenching of the band related to the 4f8→4f75d1 transitions. This is an effect of formation of
AC C
intermediate Tb4+-Yb2+ charge transfer state (CTS) [53,54]. Additionally, growing amount of Yb3+ ions increases absorption of light, directly by the Yb3+ ions forming their O2--Yb3+ CTS. An appropriate experiment, confirming emission of Yb3+ after excitation at 235 nm was carried out, and the results are presented below. The emission spectra presented in Fig. 5 show typical bands for the luminescence of Tb3+ ions, with the maximum at 544 nm, associated with the 5D4→7F5 transition. Increasing concentration of Tb3+ ions change the emission intensity (Fig. 5a) and 30% of Tb3+ ions in the host quenched the luminescence almost completely. The observed dependence is caused by Tb3+-Tb3+ interactions or interactions in clusters containing many of Tb3+ ions. This concentration quenching affected emission intensity especially in the highly Tb3+-doped 6
ACCEPTED MANUSCRIPT samples [55]. For this reason the optimal concentration of Tb3+, giving the highest UVexcited luminescence is 10%. By increasing concentration of Yb3+ dopant ions, the emission intensity was gradually decreasing (Fig. 5b) until 30% Yb3+, when the green emission is almost totally quenched. From the inset in Fig. 5b it is clearly seen that emission of Tb3+ ions is highly sensitive to the presence of Yb3+ co-dopants (1% of Yb3+ ions caused decrease of
RI PT
luminescence by factor two). The phenomenon responsible for these observations is CET (known also as cooperative downconversion) between the Tb3+ ion at its 5D4 exited state and two Yb3+ in their 2F7/2 ground states [56].
Synthesized materials, presented a narrow and intense peak around 972 nm observed at
SC
their excitation spectra (Fig. 6) as the result of presence of Yb3+ ions in their structure. These ions absorb light from the NIR range and undergo 7F7/2→2F5/2 transition. Relatively narrow
M AN U
spectral range of this transition band is a fingerprint of a low crystal field acting to Yb3+ ions [57]. The presence the 7F7/2→2F5/2 transition band of Yb3+ is also a confirmation of energy transfer from Yb3+ to Tb3+ ions leading to the emission at 544 nm. The intensities of excitation peaks are Tb3+ and Yb3+ concentration depended, similarly as in emission spectra presented in Fig. 8.
The spectra measured under λexc = 972 nm excitation, consist of four characteristic
TE D
emission bands of Tb3+ ions, corresponding to the 5D4→7F6,5,4,3 transitions, the same as in the spectra registered under UV radiation. Observed up-conversion was strongly depended on the concentration of Tb3+ emitter ions and Yb3+ sensitizer ions. Optimal concentration, giving
EP
the highest luminescence under NIR light was 10% Yb3+ and 15% Tb3+ ions. The relatively high concentration of Tb3+ ions is an effect of the mechanism responsible for the observed
AC C
luminescence. Cooperative energy transfer is a rather not very efficient process. What is more, in Yb3+/Tb3+ system also reverse energy transfer is possible, from Tb3+ to Yb3+ ions and the estimated optimal concentrations of these two ions are balance between processes taking place in YBO3:Yb3+,Tb3+ materials. Additionally, such processes as interactions between Tb3+ ions, multiphonon relaxation and phonon-assisted energy transfer between Tb3+ and Yb3+ have also influence on the overall luminescence properties [23,58]. The luminescence lifetimes measured for the UV or NIR excitations, presented in Figs. S1 and S2 respectively, are available in the supporting information. The calculated on their basis values are shown in Fig. 8. It is well seen, that the consequence of the increasing Tb3+ concentration as well as Yb3+ cause significant shortening of the luminescence lifetimes. 7
ACCEPTED MANUSCRIPT Luminescence lifetimes of Tb3+ ions oscillate between 0.15 – 4.08 ms, when excited by UV radiation (Fig. 8a and b). Decrease of luminescence lifetime can be explained by above mentioned concentration quenching phenomena as well as CET from Tb3+ to Yb3+ ions. This leads to the non-radiative deexcitation of Tb3+ ions. The calculated values of the upconversion luminescence lifetimes obtained under NIR
RI PT
irradiation are presented in Fig. 8c and d. The registered decays showed a short rise of the intensity after excitation. These arise from the relatively slow pumping of Tb3+-excited states being the result of the energy transfer between Yb3+ and Tb3+ ions. The values of luminescence lifetimes measured under 972 nm excitation are shorter comparing to those
SC
measured under 235 nm excitation. Also decay times presented in Fig. 8c are not such depended on the Tb3+ concentration as it is seen the case of UV-excitation. This observation
M AN U
shows that not only local environment is important on the luminescence lifetime of Tb3+ ions, but also the way how they are excited.
When concentration of Yb3+ ions grows from 1% to 30% the luminescent lifetimes are gradually shortening. But, increasing amount of the Tb3+ dopant ions had no clear influence on the luminescence values’ lifetimes at concentrations up to 20%. This arise supposition
TE D
that concentration quenching had lower effect on the upconversion of Tb3+ ions than in the case of UV-excited emission.
Emission rise times shortened with increasing concentrations of Yb3+ ions what is a direct result of decreasing statistical distance in the crystal structure between Yb3+ donor and Tb3+
EP
acceptor ions. This also explains relatively smaller changes of rise times with increasing concentration of Tb3+ ions. The small relationship between Tb3+ concentration and rise time
AC C
is observed, as the same consequence as for Yb3+ ions – increased probability of energy transfer with higher amount of energy exchanging ions in the host lattice. In order to determine the mechanism of the UC process, the dependence of the integral luminescence intensity on the pumping laser power at 972 nm was studied. The relationship describes Eq. 3 presented above. The
results
of
measurements
registered
for
two
representative
samples:
YBO3:10%Yb3+,15%Tb3+ and YBO3:1Yb3+,5%Tb3+ are presented as logarithm plots in the Fig. 9. Based on fitting with the linear function and calculated slope value of approximately 1.5 and 1.7, the number of photons involved in the UC process is 2 what should be expected on the CET upconversion. Observed in Figs. 7 and 8, concentration quenching in connection with 8
ACCEPTED MANUSCRIPT backward energy transfer processes from the Tb3+ dopant ions to Yb3+ sensitizers, led to lowered emission intensity and changed values of slopes in comparison to the theoretical mechanism. At higher energies of pumping laser, the slope of determined curves decrease what suggest the presence of “saturation effect”. The explanation of this process for materials showing upconversion as the result of ETU mechanism (usually doped by Ho3+, Er3+
RI PT
or Tm3+ ions) is a competition between linear decay, being a result of multiphonon/crossrelaxation or energy transfer to impurities and the upconversion process as the ways of the depletion of the intermediate excited states of Ln3+ ions [59,60]. However in Tb3+ ions, any of intermediate levels are possible and the CET preceded upconversion at higher energies of
SC
laser must be lowered by different reason. The emission form the 5D4 exited state of Tb3+ ions could be sensitive for thermal effects, a the result of enhanced backward, phonon
M AN U
assisted energy transfer (PET) to Yb3+ [58].
Downconversion in YBO3:Yb3+,Tb3+ materials occurs by two ways: as the conversion of UV to visible light, through to the transitions in the Tb3+ ions and as the result of energy transfer from Tb3+ to Yb3+ ions what cause conversion of UV (or visible) to NIR light. The Yb3+/Tb3+ pair of ions is known as downconverting system which allows for quantum cutting
TE D
phenomena and quantum efficiencies of NIR emission, higher than 100% [38,61]. In our materials, downconversion to NIR occurs, what is presented in Fig. 10 on the selected samples, but the resulted emission was weak. This unusual property of the synthesized samples, could be interesting for the applications in solar cells [61].
EP
Luminescence of Yb3+ ions was more intense after excitation at 235 nm, than by excitation of Tb3+ to the 5D4 excited state. Under 235 nm excitation, two competing
AC C
processes occur: Tb3+ ions can be excited into the 4f75d1 state or O2--Yb3+ CTS is formed [56]. But, what is discussed above, the Yb3+ ion can interact with Tb3+ ions forming Tb4+-Yb2+ CTS, instead of fast relaxation from O2--Yb3+ CTS to the 2F5/2 Yb3+ excited state [56]. Hence, observed decrease of Yb3+ emission (see fig. 10) with growing amount of Tb3+ in the system can be explained by the quenching electron transfer process and formation of Tb4+-Yb2+ CTS. Also luminescence lifetimes presented in Fig. 10b confirm this fact. However, the emission of single doped sample (by 10% of Yb3+ ions) under 235 nm excitation was poor, what means that Tb3+ ions are responsible for the improvement of excitation of Yb3+ ions, but most probably, at low concentrations. The lack of Yb3+ emission under 545 nm excitation from the
9
ACCEPTED MANUSCRIPT sample doped only by 10% of Yb3+ ions, confirmed that only ET between Tb3+ and Yb3+ can explain downconversion in double-doped samples. Excitation into the O2--Yb3+ CT band is much more effective than excitation through to the energy transfer from Tb3+ ions (by the excitation at 545 nm). Therefore we assume that in the studied materials quenching of Tb3+ upconversion by CET to Yb3+ ions is rather
RI PT
minimal.
The scheme of the most important processes possible in the YBO3:Yb3+,Tb3+ materials is presented in Fig. 11. The observed dual-mode luminescence, excitable by UV either as NIR light arises from down- or upconversion phenomena. After excitation into 4f75d1 level of
SC
Tb3+ ions, what can occur under radiation with wavelength of 235 nm, several processes can take place. First, is nonradiative relaxation into emitting level of Tb3+ ion, from which the 5D4
M AN U
excited state is the most important, and responsible for the green luminescence as the result of radiative relaxation into one of the 7Fj ground state components. Second process is formation of Tb4+-Yb2+ CTS state and loss of the absorbed energy by the nonradiative way. Another possibility is relaxation into 5D4 excited state of Tb3+ ions followed by CET to neighbouring Yb3+ ions, what results in the emission of NIR light. Irradiation of the YBO3:Yb3+,Tb3+ material by UV light may also cause excitation of Yb3+ ions into their O2--Yb3+
TE D
CTS. Subsequently the 2F5/2 excited state of Yb3+ ions become populated as the result of fast relaxation from the CTS state and finally relaxation to the 2F7/2 ground state of Yb3+ ions occurring with emission of the NIR light. Summarizing, downconversion from 235 nm to
EP
green and NIR light is possible by the energy migration in or between Tb3+ and Yb3+ ions. Another achievable process in YBO3:Yb3+,Tb3+ materials is upconversion, resulting in
AC C
green luminescence from Tb3+ ions. As the energy level structure of the Tb3+ ion limits the possible way of energy exchange between NIR absorbing Yb3+ and luminescent Tb3+ ions, only cooperative energy transfer is possible as lying behind observed upconversion. Only two excited Yb3+ ions, transferring simultaneously their energy can bridge the gap between the ground and excited state of the Tb3+ ion. This is in accordance with the determined number of photons responsible for upconversion (see Fig. 9). However, also absorption of another photon, when Tb3+ ions are in their exited state, is possible. Exited state absorption explains upconversion from the 5D3 and higher excited states of Tb3+ ions (visible in Fig. 12). However, ESA needs to be preceded by CET, making together a three photon process.
10
ACCEPTED MANUSCRIPT Fig. 12 shows a comparison of upconversion from different REPO4 and REBO3-based host materials, in which the optimal dopant concentrations were established by us previously [23,54,62,63]. YBO3:10%Yb3+,15%Tb3+ is one from the several studied by us materials which upconversion is very intense, much more efficient than in rare earth phosphates. However is not as intense as in GdBO3:10%Yb3+,20%Tb3+. When rare earth borates are used as hosts for
RI PT
Yb3+ and Tb3+ ions, also upconversion from higher exited states is possible. Inset in Fig. 12 shows emission bands connected with transitions from the 5G6 or 5D3 excited states of Tb3+ ions. 4. Conclusions
SC
Yttrium orthoborates doped by Yb3+ and Tb3+ were successfully synthesized by the sol-gel method at 900°C. The monoclinic crystal structure of the products with the C2/c space group
M AN U
and their phase purity were confirmed by XRD, IR and EDX analyses. TEM images showed morphology of products which were comprised of crystals with average sizes around 200 nm.
The phosphors synthesized showed green emission under excitation of UV (235 nm) or NIR (972 nm) radiation. Spectroscopic properties of the materials synthesized were analysed on the basis of excitation and emission spectra and also luminescence decays. In the
TE D
YBO3:Yb3+,Tb3+ system several processes were observed. Under UV light, transitions into the 4f75d1 excited state of Tb3+, formation of O2--Yb3+ or Tb4+-Yb2+ charge transfer states occurred. First two processes, allowed for the downconversion of UV light to green or NIR
EP
radiation. Yet, emission to visible and NIR occurred simultaneously, mechanism lying behind this observation was different. Nonradiative decay of Tb3+ ions to their 5D4 exited state
AC C
preceded emission of green photons as the result of transitions from excited 5D4 to the 7Fj ground state. NIR emission was a result of energy transfer from Tb3+ ions in the 5D4 state to Yb3+ ions and their 2F5/2→2F7/2 transition. NIR emission was additionally enhanced by transitions of Yb3+ ions from their excited states being formed by the nonradiative relaxation from the O2--Yb3+ CTS.
Another observed process was connected with irradiation of products by the light from the NIR range. Resulted upconversion to visible light occurred as the result of cooperative energy transfer from Yb3+ to Tb3+ ions. Observed transitions from the higher excited states of Tb3+ ions: 5D3 and 5G6, raised conclusion, that also excited state absorption occurred in samples analysed. Determined dependencies of integral luminescence intensity on the 11
ACCEPTED MANUSCRIPT energy of pumping laser indicated two photon process responsible for the green upconversion of Tb3+ ions. This also confirms proposed CET as the mechanism of observed upconversion. Dependence of luminescence properties on the dopants concentrations was also analysed. The most intense luminescence under UV excitation was observed for
RI PT
YBO3:10%Yb3+,10%Tb3+ sample in the series with changed Tb3+ concentration and for the YBO3:2.5%Yb3+,5%Tb3+ sample when the influence of various Yb3+ concentrations was analysed. The most effective upconversion under 972 nm excitation was observed for the YBO3:10%Yb3+,15%Tb3+ sample in the series with changed Tb3+ concertation and for the
SC
YBO3:10%Yb3+,5%Tb3+ when Yb3+ concentration effects were analysed.
Luminescence lifetimes determined from the emission decays were in the range typical
M AN U
for Tb3+ ions: 0.15 – 4.08 ms. Dependence of emission lifetimes on the Tb3+ or Yb3+ ions concentrations, indicated quenching processes, such as concentration quenching and backward energy transfers between Tb3+ or Tb3+ and Yb3+ ions.
Summarizing, the YBO3 compound is a good host for Yb3+ and Tb3+ ions allowing for the utilization of convenient sol-gel synthesis method for its production. The prepared materials showed intense luminescence under UV as well as NIR light. The presented properties of
TE D
YBO3:Yb3+,Tb3+ materials, recommend them as potentially applicable phosphors for lightening and security markers, in forensic sciences or in solar cells production.
EP
5. Acknowledgement
Financial support from the National Science Centre (grant no. DEC-2011/03/D/ST5/05701) is
AC C
gratefully acknowledged. 6. References [1] [2]
X. Liu, C.-H. Yan, J. a. Capobianco, Photon upconversion nanomaterials, Chem. Soc. Rev. 44 (2015) 1299–1301. doi:10.1039/C5CS90009C.
G. Chen, H. Qiu, P.N. Prasad, X. Chen, Upconversion nanoparticles: design, nanochemistry, and applications in theranostics., Chem. Rev. 114 (2014) 5161–214. doi:10.1021/cr400425h.
[3]
W. Feng, C. Han, F. Li, Upconversion-Nanophosphor-Based Functional Nanocomposites., Adv. Mater. (2013) 1–17. doi:10.1002/adma.201301946.
[4]
M. Haase, H. Schäfer, Upconverting nanoparticles., Angew. Chem. Int. Ed. 50 (2011) 5808–29. doi:10.1002/anie.201005159.
[5]
F. Auzel, Upconversion and anti-Stokes processes with f and d ions in solids., Chem. Rev. 104 (2004) 139–73. doi:10.1021/cr020357g. 12
ACCEPTED MANUSCRIPT K. Zheng, Z. Liu, C. Lv, W. Qin, Temperature sensor based on the UV upconversion luminescence of Gd3+ in Yb3+–Tm3+–Gd3+ codoped NaLuF4 microcrystals, J. Mater. Chem. C. 1 (2013) 5502. doi:10.1039/c3tc30763h.
[7]
S. Hao, G. Chen, C. Yang, Sensing using rare-earth-doped upconversion nanoparticles., Theranostics. 3 (2013) 331–45. doi:10.7150/thno.5305.
[8]
R. Ma, R. Shimmon, A. McDonagh, P. Maynard, C. Lennard, C. Roux, Fingermark detection on non-porous and semi-porous surfaces using YVO4:Er,Yb luminescent upconverting particles, Forensic Sci. Inter. 217 (2012) e23–6. doi:10.1016/j.forsciint.2011.10.033.
[9]
X. Li, Y. Wu, Y. Liu, X. Zou, L. Yao, F. Li, et al., Cyclometallated ruthenium complex-modified upconversion nanophosphors for selective detection of Hg2+ ions in water, Nanoscale. 6 (2014) 1020–1028. doi:10.1039/C3NR05195A.
[10]
M. Wang, M. Li, M. Yang, X.M. Zhang, A. Yu, Y. Zhu, NIR-induced highly sensitive detection of latent fingermarks by NaYF4:Yb,Er upconversion nanoparticles in a dry powder state, Nano Res. 8 (2015) 1800–1810. doi:10.1007/s12274-014-0686-6.
[11]
R. Deng, F. Qin, R. Chen, W. Huang, M. Hong, X. Liu, Temporal full-colour tuning through nonsteady-state upconversion, Nat. Nanotechnol. 10 (2015) 237–242. doi:10.1038/nnano.2014.317.
[12]
T.R. Hinklin, S.C. Rand, R.M. Laine, Transparent, Polycrystalline Upconverting Nanoceramics: Towards 3-D Displays, Adv. Mater. 20 (2008) 1270–1273. doi:10.1002/adma.200701235.
[13]
A. Rapaport, J. Milliez, M. Bass, A. Cassanho, H. Jenssen, Review of the properties of Upconversion phosphors for new emissive displays, IEEE/OSA J. Disp. Technol. 2 (2006) 68–78. doi:10.1109/JDT.2005.863781.
[14]
W.G. van Sark, J. de Wild, J.K. Rath, A. Meijerink, R.E. Schropp, Upconversion in solar cells., Nanoscale Res. Lett. 8 (2013) 81. doi:10.1186/1556-276X-8-81.
[15]
B.M. van der Ende, L. Aarts, A. Meijerink, Lanthanide ions as spectral converters for solar cells., Phys. Chem. Chem. Phys. 11 (2009) 11081–95. doi:10.1039/b913877c.
[16]
Y. Il Park, K.T. Lee, Y.D. Suh, T. Hyeon, Upconverting nanoparticles: a versatile platform for wide-field two-photon microscopy and multi-modal in vivo imaging., Chem. Soc. Rev. 44 (2014) 1302–1317. doi:10.1039/c4cs00173g.
[17]
C. Liu, Y. Hou, M. Gao, Are Rare-Earth Nanoparticles Suitable for In Vivo Applications?, Adv. Mater. 26 (2014) 6922–6932. doi:10.1002/adma.201305535.
[19]
SC
M AN U
TE D
EP
AC C
[18]
RI PT
[6]
S. Wang, J. Feng, S. Song, H. Zhang, Rare earth fluorides upconversion nanophosphors: from synthesis to applications in bioimaging, CrystEngComm. 15 (2013) 7142. doi:10.1039/c3ce40679b.
G. Mistlberger, I. Klimant, Luminescent magnetic particles: structures, syntheses, multimodal imaging, and analytical applications, Bioanal. Rev. 2 (2010) 61–101. doi:10.1007/s12566-0100017-7.
[20]
Y. Il Park, H.M. Kim, J.H. Kim, K.C. Moon, B. Yoo, K.T. Lee, et al., Theranostic Probe Based on Lanthanide-Doped Nanoparticles for Simultaneous In Vivo Dual-Modal Imaging and Photodynamic Therapy, Adv. Mater. 24 (2012) 5755–5761. doi:10.1002/adma.201202433.
[21]
F. Ostermayer, L. Van Uitert, Cooperative Energy Transfer from Yb3+ to Tb3+ in YF3, Phys. Rev. B. 1 (1970) 4208–4212. doi:10.1103/PhysRevB.1.4208.
13
ACCEPTED MANUSCRIPT L.D. Livanova, I.G. Saitkulo, A.L. Stolov, Summation process for quanta in CaF2 and SrF2 single crystals activated with Tb3+ and Yb3+ ions, Sov. Phys. Solid State. 11 (1969) 750+.
[23]
T. Grzyb, A. Gruszeczka, R.J. Wiglusz, Z. Śniadecki, B. Idzikowski, S. Lis, Multifunctionality of GdPO4:Yb3+,Tb3+ nanocrystals – luminescence and magnetic behaviour, J. Mater. Chem. 22 (2012) 22989–22997. doi:10.1039/c2jm34863b.
[24]
R. Martín-Rodríguez, R. Valiente, S. Polizzi, M. Bettinelli, A. Speghini, F. Piccinelli, Upconversion Luminescence in Nanocrystals of Gd3Ga5O12 and Y3Al5O12 Doped with Tb3+− Yb3+ and Eu3+− Yb3+, J. Phys. Chem. C. 113 (2009) 12195–12200. doi:10.1021/jp901711g.
[25]
G. Salley, R. Valiente, H. Güdel, Cooperative Yb3+-Tb3+ dimer excitations and upconversion in Cs3Tb2Br9:Yb3+, Phys. Rev. B. 67 (2003) 1–9. doi:10.1103/PhysRevB.67.134111.
[26]
T. Yamashita, Y. Ohishi, Cooperative energy transfer between and ions co-doped in borosilicate glass, J. Non-Cryst. Solids. 354 (2008) 1883–1890. doi:10.1016/j.jnoncrysol.2007.10.026.
[27]
I. Hernández, N. Pathumakanthar, P.B. Wyatt, W.P. Gillin, Cooperative infrared to visible up conversion in Tb3+, Eu3+, And Yb3+ containing polymers., Adv. Mater. 22 (2010) 5356–60. doi:10.1002/adma.201002674.
[28]
L. Yang, L. Zhou, Y. Huang, Z. Tang, Hydrothermal synthesis of GdBO3:Eu3+ nanofibres, Mater. Lett. 64 (2010) 2704–2706. doi:10.1016/j.matlet.2010.08.062.
[29]
Z. Yang, Y. Wen, N. Sun, Y. Wang, Y. Huang, Z. Gao, et al., Morphologies of GdBO3:Eu3+ onedimensional nanomaterials, J. Alloy. Compd. 489 (2010) L9–L12. doi:10.1016/j.jallcom.2009.09.073.
[30]
H. Zhu, L. Zhang, T. Zuo, X. Gu, Z. Wang, L. Zhu, et al., Sol–gel preparation and photoluminescence property of YBO3:Eu3+/Tb3+ nanocrystalline thin films, Appl. Surf. Sci. 254 (2008) 6362–6365. doi:10.1016/j.apsusc.2008.03.183.
[31]
J.C. Zhang, Y.H. Wang, X. Guo, Vuv-excited photoluminescence dependent on thermal treatment of YBO3:Tb3+ derived by hydrothermal process, J. Lumin. 122-123 (2007) 980–983. doi:10.1016/j.jlumin.2006.01.344.
[32]
T. Kim, S. Kang, Hydrothermal synthesis and photoluminescence properties of nano-crystalline GdBO3:Eu phosphor, Mater. Res. Bull. 40 (2005) 1945–1954. doi:10.1016/j.materresbull.2005.06.001.
[33]
H. Giesber, J. Ballato, G. Chumanov, J. Kolis, M. Dejneka, Spectroscopic properties of Er3+ and Eu3+ doped acentric LaBO3 and GdBO3, J. Appl. Phys. 93 (2003) 8987. doi:10.1063/1.1536724.
[35]
SC
M AN U
TE D
EP
AC C
[34]
RI PT
[22]
D. Boyer, G. Bertrand-chadeyron, R. Mahiou, C. Caperaa, J.C. Cousseins, Synthesis dependent luminescence efficiency in Eu3+ doped polycrystalline YBO3, J. Mater. Chem. 9 (1999) 211–214.
A. Szczeszak, T. Grzyb, B. Barszcz, V. Nagirnyi, A. Kotlov, S. Lis, Hydrothermal Synthesis and Structural and Spectroscopic Properties of the New Triclinic Form of GdBO3:Eu3+ Nanocrystals., Inorg. Chem. 52 (2013) 4934–4940. doi:10.1021/ic302525k.
[36]
A. Szczeszak, T. Grzyb, S. Lis, R.J. Wiglusz, Revision of structural properties of GdBO3 nanopowders doped with Eu3+ ions through spectroscopic studies., Dalt. Trans. 41 (2012) 5824–5831. doi:10.1039/c2dt12154a.
[37]
Z. Yang, D. Yan, K. Zhu, Z. Song, X. Yu, D. Zhou, et al., Color tunability of upconversion emission in YBO3:Yb,Er inverse opal, Mater. Lett. 65 (2011) 1245–1247. doi:10.1016/j.matlet.2011.01.071. 14
Q.Y. Zhang, C.H. Yang, Z.H. Jiang, X.H. Ji, Concentration-dependent near-infrared quantum cutting in GdBO3:Tb3+,Yb3+ nanophosphors, Appl. Phys. Lett. 90 (2007) 061914. doi:10.1063/1.2472195.
[39]
M. Pechini, “Method of Preparing Lead and Alkaline Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor,” US Patent No. 3330697, 1967.
[40]
J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third, Springer, Baltimore, 2006.
[41]
R. Scheps, Upconversion laser processes, Ptog. Quant. Electron. 20 (1996) 271–358. doi:10.1016/0079-6727(95)00007-0.
[42]
J. Lin, D. Sheptyakov, Y. Wang, P. Allenspach, Structures and Phase Transition of Vaterite-Type Rare Earth Orthoborates: A Neutron Diffraction Study, Chem. Mater. 16 (2004) 2418–2424. doi:10.1021/cm0499388.
[43]
G. Jia, P.A. Tanner, C.-K. Duan, J. Dexpert-Ghys, Eu3+ Spectroscopy: A Structural Probe for Yttrium Orthoborate Phosphors, J. Phys. Chem. C. 114 (2010) 2769–2775. doi:10.1021/jp910329k.
[44]
A. Szczeszak, K. Kubasiewicz, T. Grzyb, S. Lis, Spectroscopic properties of Y1−xEuxBO3 and Y1−xTbxBO3 nanopowders obtained by the sol–gel Pechini method, J. Lumin. 155 (2014) 374– 383. doi:10.1016/j.jlumin.2014.07.006.
[45]
G. Chadeyron, M. El-Ghozzi, R. Mahiou, A. Arbus, J. Cousseins, Revised structure of the orthoborate YBO3, J. Solid State Chem. 128 (1997) 261–266. doi: 10.1006/jssc.1996.7207.
[46]
G. Jia, H. You, K. Liu, Y. Zheng, N. Guo, J. Jia, et al., Highly uniform YBO3 hierarchical architectures: facile synthesis and tunable luminescence properties., Chemistry (Easton). 16 (2010) 2930–7. doi:10.1002/chem.200902834.
[47]
Z. Wei, L. Sun, C. Liao, J. Yin, X. Jiang, C. Yan, et al., Size-Dependent Chromaticity in YBO3:Eu Nanocrystals: Correlation with Microstructure and Site Symmetry, J. Phys. Chem. B. 106 (2002) 10610–10617. doi:10.1021/jp025967z.
[48]
Z. Li, J. Zeng, Y. Li, Solvothermal route to synthesize well-dispersed YBO3:Eu nanocrystals., Small. 3 (2007) 438–43. doi:10.1002/smll.200600151.
[49]
X.C. Jiang, C.H. Yan, L.D. Sun, Z.G. Wei, C.S. Liao, Hydrothermal homogeneous urea precipitation of hexagonal YBO3:Eu3+ nanocrystals with improved luminescent properties, J. Solid State Chem. 175 (2003) 245–251. doi:10.1016/S0022-4596(03)00276-7.
[50] [51]
EP
TE D
M AN U
SC
RI PT
[38]
AC C
ACCEPTED MANUSCRIPT
A. Nohara, S. Takeshita, T. Isobe, Mixed-solvent strategy for solvothermal synthesis of welldispersed YBO3:Ce3+,Tb3+ nanocrystals, RSC Adv. 4 (2014) 11219. doi:10.1039/c3ra47864e.
G. Pan, H. Song, L. Yu, Z. Liu, X. Bai, Y. Lei, et al., Structure and Photoluminescent Properties of Microstructural YBO3:Eu3+ Nanocrystals, J. Nanosci. Nanotechnol. 7 (2007) 593–601. doi:10.1166/jnn.2007.133.
[52]
L. Ning, C. Mak, P. Tanner, High-spin and low-spin f-d transitions of Tb3+ in elpasolite hosts, Phys. Rev. B. 72 (2005) 085127. doi:10.1103/PhysRevB.72.085127.
[53]
J.-L. Yuan, X.-Y. Zeng, J.-T. Zhao, Z.-J. Zhang, H.-H. Chen, X.-X. Yang, Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor, J. Phys. D Appl. Phys. 41 (2008) 105406. doi:10.1088/0022-3727/41/10/105406.
[54]
T. Grzyb, A. Gruszeczka, R.J. Wiglusz, S. Lis, The effects of down- and up-conversion on dualmode green luminescence from Yb3+- and Tb3+-doped LaPO4 nanocrystals, J. Mater. Chem. C. 1 15
ACCEPTED MANUSCRIPT (2013) 5410–5418. doi:10.1039/c3tc31100g. G. Blasse, Some considerations on rare-earth activated phosphors, J. Lumin. 1-2 (1970) 766– 777. doi:10.1016/0022-2313(70)90089-X.
[56]
X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, et al., Cooperative downconversion and nearinfrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses, Appl. Phys. B. 96 (2009) 51–55. doi:10.1007/s00340-009-3478-z.
[57]
I. Sokólska, W. Ryba-Romanowski, S. Gołab, T. Lukasiewicz, The optical properties of Yb3+ ions in LiTaO3:Nd,Yb crystals, Appl. Phys. B. 65 (1997) 495–498. doi:10.1007/s003400050302.
[58]
Y. Arai, T. Yamashidta, T. Suzuki, Y. Ohishi, Upconversion properties of Tb3+–Yb3+ codoped fluorophosphate glasses, J. Appl. Phys. 105 (2009) 83105–83111. doi:10.1063/1.3112010.
[59]
M. Pollnau, D. Gamelin, S. Lüthi, H. Güdel, M. Hehlen, Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems, Phys. Rev. B. 61 (2000) 3337– 3346. doi:10.1103/PhysRevB.61.3337.
[60]
Y. Lei, H. Song, L. Yang, L. Yu, Z. Liu, G. Pan, et al., Upconversion luminescence, intensity saturation effect, and thermal effect in Gd2O3:Er3+,Yb3+ nanowires., J. Chem. Phys. 123 (2005) 174710. doi:10.1063/1.2087487.
[61]
Y.-T. An, C. Labbé, J. Cardin, M. Morales, F. Gourbilleau, Highly Efficient Infrared Quantum Cutting in Tb3+−Yb3+ Codoped Silicon Oxynitride for Solar Cell Applications, Adv. Opt. Mater. 1 (2013) 855–862. doi:10.1002/adom.201300186.
[62]
T. Grzyb, R.J. Wiglusz, A. Gruszeczka, S. Lis, Down- and up-converting dual-mode YPO4:Yb3+,Tb3+ nanocrystals: synthesis and spectroscopic properties., Dalton Trans. 43 (2014) 17255–17264. doi:10.1039/c4dt02234c.
[63]
T. Grzyb, K. Kubasiewicz, A. Szczeszak, S. Lis, Synthesis and spectroscopic properties of Yb3+ and Tb3+ co-doped GdBO3 materials showing down- and up-conversion luminescence, Dalt. Trans. 44 (2015) 4063–4069. doi:10.1039/C4DT03667K.
AC C
EP
TE D
M AN U
SC
RI PT
[55]
16
ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of the YBO3:10%Yb3+, xTb3+ and YBO3:xYb3+, 5%Tb3+ samples (x = 1 - 30%), annealed at 900 °C. Fig. 2 IR spectra of the YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ samples (x = 1-30%),
RI PT
annealed at 900 °C. Fig. 3 TEM images of the YBO3 (a) and YBO3:10%Yb3+,5%Tb3+ (b) sample annealed at 900 °C and EDX spectrum (inset) of the YBO3:10%Yb3+,5%Tb3+ sample annealed at 900 °C.
Fig. 4 Excitation spectra of the (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples
SC
(x = 1 - 30%), annealed at 900 °C, measured by observation at λem = 544 nm.
Fig. 5 Emission spectra of the (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples (x =
M AN U
1 - 30%), annealed at 900 °C, measured under λexc = 235 nm.
Fig. 6 Excitation spectra of (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples (x = 1 30%), annealed at 900 °C, measured at λem = 544 nm.
Fig. 7 Emission of the (a) YBO3:10%Yb3+,xTb3+ and (b) YBO3:xYb3+,5%Tb3+ samples (x = 1 30%), annealed at 900 °C, measured at λexc = 972 nm.
Fig. 8 Values of the calculated luminescence lifetimes of the (a,c) YBO3:10%Yb3+,xTb3+ and (b,d)
TE D
YBO3:xYb3+,5%Tb3+ samples (x = 1 - 30%).
Fig. 9 Integral intensity of the YBO3:10%Yb3+,15%Tb3+ and YBO3:1Yb3+,5%Tb3+ samples as a function of the pumping power of the laser (λexc = 972 nm).
EP
Fig. 10 Downconversion emission (a) and luminescence decays (b) of Yb3+ ions, under excitation at 235 or 545 nm. Fig. 11
Scheme of the UC process observed in the YBO3:10%Yb3+,xTb3+ and
AC C
YBO3:xYb3+,5%Tb3+ samples. Fig. 12 Comparison of upconversion luminescence from the REPO4 and REBO3-based materials doped by optimal concentrations of ions, giving the most intense emission, measured at λexc = 972 nm (range 400-470 nm measured at slits 2 nm, 450-700nm at 0.35 nm).
17
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 1
Fig. 2
18
AC C
EP
TE D
M AN U
Fig. 3
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4
19
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 5
20
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
Fig. 6
AC C
EP
TE D
Fig. 7
Fig. 8 21
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 9
Fig. 10
22
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 11
Fig. 12
23
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
RI PT
Supporting information
Fig. S1 Luminescence lifetimes of the YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ samples
AC C
EP
(x = 1 - 30%), annealed at 900 °C, measured under λexc = 235 nm and λem = 544 nm.
24
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. S2 Luminescence lifetimes of the YBO3:10%Yb3+,xTb3+ and YBO3:xYb3+,5%Tb3+ samples (x
AC C
EP
TE D
= 1 - 30%), annealed at 900 °C, measured under λexc = 972 nm and λem = 544 nm.
25
ACCEPTED MANUSCRIPT Highlights Down- and upconverting, YBO3:Yb3+,Tb3+ materials were synthesized
•
Energy migration processes between Yb3+ and Tb3+ ions were analysed
•
Effects of dopants concentration on emission properties were observed
•
Spectroscopic properties of materials prepared were compared
AC C
EP
TE D
M AN U
SC
RI PT
•