Near-Infrared Light-Triggered Visible Upconversion Emissions in Er3

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Materials and Interfaces

Near-infrared light-triggered visible upconversion emissions in Er3+/Yb3+-codoped Y2Mo4O15 microparticles for simultaneous noncontact optical thermometry and solid-state lighting Peng Du, and Jae Su Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02938 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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Industrial & Engineering Chemistry Research

Near-infrared light-triggered visible upconversion emissions in Er3+/Yb3+codoped Y2Mo4O15 microparticles for simultaneous noncontact optical thermometry and solid-state lighting Peng Du and Jae Su Yu* Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea.

Abstract The Er3+/Yb3+-codoped Y2Mo4O15 microparticles, which possessed admirable optical thermometric and upconversion (UC) emission behaviors, were synthesized through the traditional sintering technology of sol-gel route. Under near-infrared (NIR) light excitation, the obtained compounds showed the featured green and red UC emissions of Er3+ ions. The most intense luminescent intensity was detected when the Yb3+ ion content was 40 mol%. Through studying the temperature-dependent UC emission intensity ratio of the monitored green emissions, the optical thermometric properties of the resultant micropaticles were investigated. The maximum sensor sensitivity of the studied microparticles with optimum doping concentration was approximately 0.00846 K-1. Moreover, the achieved maximum sensor sensitivity was independent on the dopant content that was identical with the theoretical analysis through taking the advantage of the Judd-Ofelt theory. Furthermore, a significant increment in the compounds’ temperature was achieved when the excitation pump power verified in the range of 320-1040 mW. Ultimately, a green-emitting light-emitting diode device was developed with the aid of synthesized microparticles and a 940 nm NIR chip so as to identify their applicability for solid-state lighting. These aforementioned characteristics make the Er3+/Yb3+-codoped Y2Mo4O15 microparticles not only suitable for non-contacting optical thermometry but also for solid-state lighting.

Keywords: Rare-earth, Thermometry, Upconversion, FIR technique, Solid-state lighting

*Corresponding authors: E-mail addresses: [email protected] (J. S. Yu) 1 ACS Paragon Plus Environment

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1. Introduction The interest in the lanthanide ions doped upconversion (UC) inorganic luminescent materials, where the lanthanide ions act as activator and sensitizer to convert low frequency photon to high frequency photon through multiphoton process, has been gained because of their wide range applications in fluorescent bioprobe, indoor illumination, photocatalysis, water detection, optical thermometry, latent fingerprint detection, solar cells and so on.1-8 By comparison with other trivalent rare-earth ions, the trivalent Er3+ ions were extensively studied as green-emitting activators for upconverting materials owing to its sharp green emission (2H11/2 → 4I15/2 and 4S3/2 → 4I15/2) bands.9-12 Nevertheless, for the lack of intense absorption in the region of near-infrared (NIR), the single Er3+-doped UC materials usually suffer from low luminescent efficiency. For the purpose of making up this kind of disadvantage, the Yb3+ ions, which can efficiently deliver the obtained energy to the Er3+ ions, were introduced due to its merits of impressive absorption cross section near 980 nm, resulting in the enhanced UC luminescent efficiency in the Er3+/Yb3+codoped upconverting materials.13-15 It was reported that the enhanced UC emission properties were realized in the Na0.5Bi2.5Nb2O9:Er3+ phosphors after adding the Yb3+ ions and the resultant compounds exhibited potential applications in optical data storage.16 Furthermore, Wang et al. also revealed that the Bi3Ti1.5W0.5O9:Er3+/Yb3+ ferroelectric ceramics did not only have better temperature sensing performance but also exhibited superior UC emission behaviors than those of the Bi3Ti1.5W0.5O9:Er3+ compounds.17 Obviously, the introduction of Yb3+ ions into the Er3+ ions based systems is an efficient route to simultaneously improve their UC emission efficiency and extend their vivid applications. Recently, there is an increasing interest in accurate temperature detection since it plays an irreplaceable role in disparate fields ranging from scientific research and commercial manufacture to daily life. However, the traditional contact temperature meters, which mainly make use of the principle of either liquid or metal expansion, possess several intrinsic drawbacks, such as it is impossible for them to detect the temperature of fluctuation of microcircuit and monitor the temperature distribution of living cells which restrict their further applications in the future life.18,19 Nowadays, the non-contact optical thermometry, which shows honorable characteristic of real-time detection with high spatial resolution and fast response, by means of studying the fluorescence intensity ratio (FIR) of two emissions from the thermally coupled levels at various temperatures was proposed to replace these conventional contact 2 ACS Paragon Plus Environment

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thermometers.20-22 Until now, the researchers pointed out that many trivalent lanthanide ions (Tm3+, Nd3+, Er3+, Eu3+, Ho3+ etc.) exhibited a couple of thermally coupled levels in which their energy band gaps can not over range of 200-2000 cm-1.23-28 Among these rare-earth ions doped luminescent thermometers, the Er3+/Yb3+-codoped systems were widely studied because of its good optical thermometric properties and feasibility of use. Unfortunately, the maximum sensitivities of recently developed Er3+/Yb3+-codoped luminescent temperature meters were still not high enough for practical application.29,30 Thus, to enhance the optical thermometric properties and allow them for practical application, some practicable routes should be proposed. As disclose in previous reports, it is widely accepted that maximum sensor sensitivities of the trivalent rare-earth ions were tremendously influenced by the host materials.31,32 Therefore, searching a proper host material will be an available approach to tune the temperature sensing ability of the trivalent rare-earth ions. Considering the competition between the efficient radiative transition (i.e., UC emission) process and the nonradiative transition process, a luminescent host matrix, which shows small phonon energy, would be a prior candidate. In comparison, molybdates with superior inherent luminescent properties and high thermal stability are regarded as good host matrixes for the trivalent rare-earth ions.33,34 Specially, the interest in Y2Mo4O15 with monoclinic structure, as a member of molybdates, is increasing. Luo et al. stated that the Ho3+/Yb3+-codoped Y2Mo4O15 compounds possessed strong UC emission properties.35 Meanwhile, it was revealed that the Eu3+-doped Y2Mo4O15 phosphors were suitable for indoor illumination.36 In spite of these interesting research results, to out knowledge, the optical thermometric along with the UC emission performance of Yb3+ ion concentration-dependent Er3+/Yb3+-codoped Y2Mo4O15 microparticles have not been investigated yet. In this work, with the aid of the mature preparation technology of sol-gel method, series of Er3+/Yb3+-codoped Y2Mo4O15 microparticles were synthesized so as to analyze their phase composition, microstructure as well as the UC emission performance. Through the FIR technology, we also systematically investigated the involved temperature sensing capacity of the as-prepared microparticles. Furthermore, in order to theoretically interpret the doping concentration-independent sensor sensitivity, the Judd-Ofelt theory was employed. Ultimately, by integrating the synthesized microparticles and an NIR chip, we successfully prepared a green-emitting light-emitting diode (LED) device so as to state their potential application for solid-state lighting. 3 ACS Paragon Plus Environment


2. Experimental 2.1 Sol-gel preparation of Er3+/Yb3+-codoped Y2Mo4O15 microparticles The Y1.96-2xMo4O15:0.04Er3+/2xYb3+ (Y2Mo4O15:Er3+/2xYb3+; where the x value was 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7) novel compounds were successfully calcinated by a convention synthetic route of the critic acid assisted sol-gel method. The powders including Y(NO3)3·6H2O (99.8%), Er(NO3)3·5H2O (99.9%), Yb(NO3)3·5H2O (99.9%), (NH4)6Mo7O24·4H2O (99%) and critic acid (C6H8O7; 99.5%) were bought from Sigma-Aldrich Co and employed as the raw materials. In general process, 1 mmol (NH4)6Mo7O24·4H2O, 0.04 mmol Er(NO3)3·5H2O, 2x mmol Yb(NO3)3·5H2O and (1.96-2x) mmol Y(NO3)3·6H2O were weighted by using the electronic balance and evenly dissolved into the de-ionized water (200 ml) under continuous agitation. Afterwards, the citric acid with the content of 12 mmol, which played the role of the chelating agent, was weighted and transferred to the above mixture. Then, the beaker was sealed. After that, the achieved mixture was heated to 80 °C and maintained for 30 min. To obtain the wet-gel, we shifted the lid and the obtained solution was continually heated at 80 °C until all the water was evaporated. Subsequently, the wet-gel was transferred to an oven in which the temperature was set at 120 °C for 12 h to prepare the gray xerogel. At last, these gray xerogels were filled into the alumina crucibles and calcinated at 700 °C for 12 h in the furnace so as to prepare the Y2Mo4O15:Er3+/2xYb3+ microparticles. 2.2 Sample characterization The phase component, morphology and elemental composition of the Y2Mo4O15:Er3+/2xYb3+ microparticels were characterized detailedly via a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα (λ = 1.5406 Å) radiation and a field-emission scanning electron microscope (FE-SEM) (LEO SUPRA 55, Carl Zeiss), respectively. Under the excitation of a 980 nm pump power tunable laser diode, the UC emission spectra of the Y2Mo4O15:Er3+/2xYb3+ microparticels were checked through the Scinco FluroMate FS-2 spectrofluorometer. The temperature surrounding the prepared compounds, which ranged from 298 to 486 K, was adjusted with the aid of a temperature controller (NOVA ST540). The multi-channel spectroradiometer (OL 770) along with an integrating sphere was adopted to monitor the electroluminescence (EL) spectrum of the packaged green-emitting LED device. 3. Results and discussion 3.1 Crystal structure and microstructure performance 4 ACS Paragon Plus Environment

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For the sake of examining the phase compositions and crystal structure, the X-ray diffraction (XRD) spectra of the Y2Mo4O15:Er3+/2xYb3+ microparticles with various Yb3+ ion contents were tested. Fig. S1 illustrates the XRD patterns of Y2Mo4O15:Er3+/2xYb3+ microparticles prepared at 700 °C for 12 h. As demonstrated in Fig. S1, all the compounds exhibited similar results and the observed diffraction peaks of the microparticles were assigned to those of the Y2Mo4O15 (JCPDS#53-0358),

implying

the

successful

synthesis

of

new

solid

solution

of

Y2Mo4O15:Er3+/2xYb3+ and the dopants were entirely entered into the Y2Mo4O15 host lattices. By comparing the electrovalence and ionic radii of the involved positive ions, one knows that the dopants were expected to substitute the Y3+ ions to form the Y2Mo4O15:Er3+/2xYb3+ compounds. As we know, for the formation of a new compound, the radius percentage difference (Dr) between the replaced ions and dopant must be small enough (≤ 30%). Through utilizing the following expression, the Dr value can be esmitated:28,37

Dr =

R1 ( CN ) − R2 ( CN ) R1 ( CN )

× 100% ,

(1)

where R1(CN) is related to the ionic radius of the replaced ions, whereas the R2(CN) shows the ionic radius of the dopants. Taking into account of the involved coordinate number (CN) of the cations (CN = 7), the valid ionic radii for Y3+, Yb3+ and Er3+ ions were 0.96, 0.925 and 0.945 Å, respectively. Thus, the values of Dr for the Y3+/Er3+ and Y3+/Yb3+ ions were revealed to be approximately 1.5 and 3.6%, respectively. Clearly, these calculated Dr values were far below 30%, further demonstrating that the Er3+ and Yb3+ ions were able to diffuse into the Y2Mo4O15 host lattices by replacing the Y3+ ions which coincided well the result achieved from the XRD curves. Utilizing the General Structure Analysis System (GSAS) software, the typical Rietveld refinement of the Er3+/Yb3+-codoped Y2Mo4O15 microparticles with the composition of 40 mol% of Yb3+ ions was done to deeper understand the phase structure of the resultant microparticles. The Rietveld refinement result shown in Fig. 1(a) confirmed that the measured and calculated XRD patterns matched well with each other, indicating that the resultant products were made up of single pure monoclinic phase with space group of P121/c(14). Furthermore, as listed in Table S1, the values of weighted profile R-factor (Rwp), expected R-factor (Rp) and χ2 were 5.94%, 4.65% and

2.52,

respectively.

Meanwhile,

the

calculated

lattice

parameters

of

the

Y2Mo4O15:Er3+/0.8Yb3+ microparticles were slightly smaller than those of the undoped Y2Mo4O15 host lattice, as listed in Table S1. The volumetric constriction with the introduction of 5 ACS Paragon Plus Environment


dopants (Er3+ and Yb3+) also revealed that the dopants can take up the Y3+ ions’ sites in the Y2Mo4O15 host lattices. The partial structural overview of the Y2Mo4O15 is shown in Fig. 1(b). As disclosed, the Y3+ ions formed the YO7 polyhedron, whereas the Mo6+ ions were surrounded by four O2- ions and formed the MoO4 tetrahedron. The representative FE-SEM images of Y2Mo4O15:Er3+/2xYb3+ microparticles with the composition of 20, 40, 60 and 70 mol% of Yb3+ ions are shown in Fig. 2(a)-2(d), respectively. From these images, it is evident that the synthesized compounds were made up of closely packed microparticles and the particle size ranged from 3 to 7 µm. Generally, the closely packed particles are beneficial to the decrement of light scattering, resulting in high luminescent efficiency.38 As displayed in Fig. 2(a)-2(d), with elevating the Yb3+ ion concentration, the microstructure as well as the particle size changed little, implying that the morphology performance of the prepared products was independent of the doping concentration. From Fig. 2(e)-2(j), it is obvious that the chemical elements presented in the Er3+/Yb3+-codoped Y2Mo4O15 microparticles were distributed equally over all the particles. To further investigate the elemental compositions in the studied samples, the typical energy-dispersive X-ray (EDX) spectrum of Y2Mo4O15:Er3+/0.8Yb3+ microparticles was detected. It can be seen that the EDX spectrum mainly consisted of Y, Mo, O, Er and Yb without detecting extra peaks from other elements (see Fig. 2(k)), further proving the successful synthesis of Y2Mo4O15:Er3+/2xYb3+ microparticles. 3.2 Upconversion emission behaviors of Y2Mo4O15:Er3+/2xYb3+ microparticles To examine the effect of dopant content on the luminescent behaviors of the final products, the ambient temperature UC emission spectra of Y2Mo4O15:Er3+/2xYb3+ microparticles with diverse Yb3+ ion contents were recorded in the visible region with 980 nm excitation and the corresponding monitored results are described in Fig. 3(a). As presented, under NIR light (980 nm) irradiation with a laser pump power of 320 mW, three emission bands at approximately 528, 550 and 664 nm, which are ascribed to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions,10,19 respectively, were detected in all the compounds. Although the UC emission profiles were invariable, the UC emission intensities of both green and red emissions were impacted by the Yb3+ ion content (see Fig. 3(a)). It can be seen from Fig. S2 that the UC emission intensity was greatly improved after introducing the Yb3+ ions due to the strong absorption of Yb3+ ions at 980 nm as well as the efficient energy transfer (ET) from Yb3+ to Er3+ ions. Significantly, when x = 0.4, the Y2Mo4O15:Er3+/2xYb3+ microparticles exhibited the 6 ACS Paragon Plus Environment

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strongest UC emission intensity. After that, the UC emission intensity quenched when the dopant concentration was over 40 mol%. Because the luminescent intensity of the green UC emission was almost 15 times stronger than that of the red UC emission, the studied samples excited by 980 nm light emitted glaring green emission (see inset of Fig. 3(b)). The representative Commission International De I’Eclairage (CIE) coordinate diagram of Y2Mo4O15:Er3+/0.8Yb3+ microparticles is demonstrated in Fig. 3(b). The achieved CIE value was (0.273,0.706), which was situated in the green region, further indicating that the synthesized samples can emit visible green emissions (see Fig. 3(b)). 3.3 Upconversion emission mechanism of Y2Mo4O15:Er3+/2xYb3+ microparticles For the sake of interpreting the above NIR-induced-visible UC emission mechanism in Er3+/Yb3+-codoped Y2Mo4O15 microparticles, the simplified energy level diagram of Er3+ and Yb3+ ions was modeled and displayed in Fig. 3(c). Here, considering the high concentration of Yb3+ ions and its impressive absorption ability in NIR region, the ET from Yb3+ to Er3+ ions was the main process to contribute the observed green and red UC emissions. First of all, the energy from the incident NIR photons was successfully observed by the Yb3+ ions with 980 nm light excitation, leading to the electron transition of 2F7/2 → 7F5/2. Considering the admirable energy level overlap between these two different dopants, the Yb3+ ion would transfer the energy to the adjacent Er3+ ion to populate the 4I11/2 level via the efficient ET process of (Yb3+ (2F5/2) + Er3+ (4I15/2) → Yb3+ (2F7/2) + Er3+ (4I11/2)), as described in Fig. 3(b). Subsequently, the second ET between the Yb3+ and Er3+ ions, Yb3+ (2F5/2) + Er3+ (4I11/2) → Yb3+ (2F7/2) + Er3+ (4F7/2), took place and the majority of the populated electrons in the 4I11/2 level would be jumped to the higher level of 4F7/2. However, due to the nonsteady state of 4F7/2 level, electrons can be nonradiatively (NR) relaxed to the excited states of 2H11/2 and 4S3/2. After that, two intense green emissions arising from the 2H11/2 → 4I15/2 (528 nm) and 4S3/2 → 4I15/2 (550 nm) transitions were monitored in the studied compounds due to the radiative transition processes. Besides, the electrons situated in the 4I11/2 level have two different transition routes, namely, they can either be transferred to the higher excited level of 4F7/2 or nonradiatively decayed to the lower excited level of 4I13/2. Note that, the electrons populated in the 4I13/2 level was able to be pumped to the higher excited level of 4F9/2, which is helpful for the generation of red emission, through the ET from Yb3+ to Er3+ ions. Apart from this process, the 4S3/2 level can also decay to the 4F9/2 level. Ultimately, the red

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emission (4F9/2 → 4I15/2) was monitored in the Y2Mo4O15:Er3+/2xYb3+ microparticles (see Fig. 3(c)). For the sake of verifying the above proposed UC emission mechanism, the representative UC emission behaviors of Y2Mo4O15:Er3+/0.8Yb3+ microparticles as a function of excitation pump power were investigated and the corresponding obtained results are demonstrated in Fig. 3(d). Evidently, through tuning the excitation pump power in the range of 320-1040 mW, the luminescent profiles did not verify, whereas UC emission intensities of both green and red emissions increased continuously. As is known, the UC emission intensity (IUC) and the excitation pump power (P) have the following relation, as defined:39-41 IUC = P n or ln ( IUC ) = n ln P .

(2)

Here, n is associated with the incident photon number that is needed to generate the higher excited level. From the plots of ln(IUC) versus lnP, as illustrated in Fig. 3(e) and 3(f), it is significant that the n values for the radiation transitions of 2H11/2 → 4I15/2 (528 nm), 4S3/2 → 4I15/2 (550 nm) and 4F9/2 → 4I15/2 (664 nm) were fitted to be 1.69, 1.45 and 1.49, respectively, implying that the obtained green and red UC emissions in the Y2Mo4O15:Er3+/2xYb3+ microparticles all pertained to a two-photon process, which is in good agreement with above discussion. 3.4 Optical thermometric performance of Y2Mo4O15:Er3+/2xYb3+ microparticles The energy separation between the 2H11/2 and 4S3/2 levels, which was evaluated from the recorded UC emission spectrum (see Fig. 3(a)), was around 757.5 cm-1. Consequently, when the temperature surrounding the compounds was elevated, electrons can be populated from the lower 4

S3/2 level to the upper 2H11/2 level and achieved the quasi-thermal equilibrium state because of

the relatively small energy gap, leading to the inconsistent response of the green emissions to the temperature. As a result, the studied samples may be suitable for non-invasion temperature monitoring by utilizing the FIR route. Upon 980 nm light irradiation with a fixed pump power of 320 mW, the typical green UC emission spectra of the YMo4O15:Er3+/0.8Yb3+ microparticles at diverse temperatures were measured to study its temperature sensing ability. As disclosed in Fig. 4(a), it is significant that the positions of the emission bands were insensitive to the temperature, whereas the relative luminescent emission intensities between the 2H11/2 → 4I15/2 (528 nm) and 4

S3/2 → 4I15/2 (550 nm) transitions monotonously increased when the surrounding temperature

was elevated in the range of 298-486 K. The temperature related FIR value of the green 8 ACS Paragon Plus Environment

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emissions, which was computed from the detected UC emission spectra, in the range of 298-486 K is illustrated in Fig. 4(b). A gradual increment was observed in the temperature-dependent FIR values and its maximal value reached up to approximately 1.89 when the surrounding temperature was 486 K. From Boltzmann distribution theory, the FIR value between the 2H11/2 and 4S3/2 levels of Er3+ ions can be defined as:42,43

FIR =

IU I H  ∆E  = = A exp  − , IL IS  kT 

(3)

In present formula, IU, IL, IH and IS show the integral fluorescence emission intensities originating from the radiative transitions of the upper excited level, lower excited level, 2H11/2 and 4S3/2 thermally coupled levels to the ground state, respectively, A denotes the coefficient, k presents the Boltzmann constant, ∆E is associated with the energy gap between the involved thermally couples levels (i.e., 2H11/2 and 4S3/2) and T is the temperature. Clearly, the calculated FIR values demonstrated in Fig. 4(b) were well fitted to above expression, meanwhile, the values of A and ∆E/k were calculated to be approximately 16.37 and 1042.02, respectively. Herein, the energy gap was found to be 719 cm-1 approaching to the value obtained from the UC emission spectrum. As analyzed above, the electron distribution between the excitation levels of 2H11/2 and 4S3/2 was very sensitive to the surrounding temperature. For the sake of deeply comprehending the electron redistribution phenomenon between the involved thermally coupled levels at elevated temperature, we systematically investigated the population redistribution ability (PRA) at different temperatures, as defined below:19,40 PRA =

IU IH A = = . IU + I L I H + I L A + exp ( ∆E kT )

(4)

All the parameters, which were presented in this expression, exhibited the same meaning and the same values as described in Eq. (3). Moreover, the temperature-dependent PRA value of the 2

H11/2 and 4S3/2 thermally coupled levels in the range of 298-486 K, which was estimated from

Eq. (4), is shown in Fig. 4(c). As presented, these obtained PRA values sharply relied on the temperature. Specially, the PRA value was only about 0.33 at 298 K, while it sharply increased to 0.65 at 483 K. These observed results verify that the Y2Mo4O15:Er3+/2xYb3+ microparticles possess towards applications in noncontact optical thermometry.



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For practical applications, the sensor sensitivity can be applied to quantitatively evaluate the temperature sensing capability of the optical thermometers. It is known to all that the sensor sensitivity is related to the ratio of FIR depending on the temperature, as given below:7,13 S=

dFIR ∆E ∆E  ∆E  = FIR × 2 = × A exp  − . 2 dT kT kT  kT 

(5)

With the help of Eq. (5) as well as the fitting results obtained from Eq.(3), we calculated the temperature-dependent sensor sensitivity of the Y2Mo4O15:Er3+/0.8Yb3+ microparticles. Obviously, the estimated sensor sensitivity shown in Fig. 4(d) exhibited a monotonously upward tendency in the temperature range of our interest and its maximal value reached up to 0.00846 K1

when T = 486 K. To compare the optical thermometric properties of present work with previous

reported Er3+ ions-based sensing materials, the relevant results are displayed in Table 1. Significantly, the maximum sensor sensitivity of present work was comparable with those of other developed optical sensing materials, such as molybdates, fluorides and oxdes, suggesting that the Y2Mo4O15:Er3+/2xYb3+ microparticles may have towards application in noninvasion temperature measurement by means of the FIR technology. It has been widely demonstrated that the sensor sensitivity of rare-earth ions based sensing materials can affected by some factors, including host matrix, particle size, doping concentration and excitation pump power.44-46 To investigate the impact of dopant content on the temperature sensing ability of the final products, the green UC emission spectra of Y2Mo4O15:Er3+/2xYb3+ with various doping compositions of 20 and 60 mol% at various temperatures were measured. As revealed

in

the

normalized

temperature-dependent

green

UC

emission

spectra

of

Y2Mo4O15:Er3+/0.4Yb3+ and Y2Mo4O15:Er3+/1.2Yb3+ microparticles, which are shown in Fig. 5(a) and 5(b), respectively, the green UC emission intensity originating from the 2H11/2 → 4I15/2 (528 nm) transition gradually increased compared to the

4

S3/2 →

4

I15/2 (550 nm) transition.

Furthermore, by fitting the temperature-dependent FIR values achieved from the monitored UC emission spectra with Eq. (3), the ∆E/k values of the Y2Mo4O15:Er3+/0.4Yb3+ and Y2Mo4O15:Er3+/1.2Yb3+ microparticles were demonstrated to be 1060.15 and 1007.74 (the value of coefficient A was 16.65 and 15.27, respectively), respectively, as described in Fig. 5(c) and 5(d). Additionally, as demonstrated in Fig. 5(e) and 5(f), all the calculated sensor sensitivities were

alive

to

the

temperature

and

the

maximal

sensor

sensitivities

for

the

Y2Mo4O15:Er3+/0.4Yb3+ and Y2Mo4O15:Er3+/1.2Yb3+ microparticles were determined to be 10 ACS Paragon Plus Environment

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around 0.00843 and 0.00819 K-1 when T = 483 K, respectively, which was nearly the same as that of the Y2Mo4O15:Er3+/0.8Yb3+. Clearly, although the Yb3+ ion content was inconsistent, these studied samples exhibited similar optical thermometric behaviors, implying that the temperature sensing ability of the Y2Mo4O15:Er3+/2xYb3+ microparticles was independent of the dopant concentration within the error range. Similar phenomenon was also found in trivalent rare-earth ions doped other inorganic compounds, such as BiOCl:Er3+, Ba5Gd8Zn4O21:Tm3+/Yb3+ and Na0.5Gd0.5MoO4:Er3+/Yb3+.12,47,48 3.5 Judd-Ofelt analysis To get deeper insight into the doping concentration-independent sensor sensitivity in Y2Mo4O15:Er3+/2xYb3+ microparticles, a theoretical discussion was done with the help of JuddOfelt theory. According to the reports of Judd and Ofelt, the transition rate of the J → J´ radiative transition can be expressed as:49,50 AJ → J '

2 64π 4 e 2 v 3 ( n + 2 ) = 3h ( 2 J + 1) 9n

2

∑ λ

Ωλ ψ J U λ ψ ' J '

2

,

(6)

= 2,4,6

In aforementioned equation, the parameters, such as e stands for the elementary charge, h represents the Planck constant (h = 6.626 × 10-34 J·s), v corresponds to the wavenumber of the radiative transition of J → J´, n is associated with the host refractive index, Ωλ denotes the ଶ

optical transition parameter and ห‫ܬ߰ۦ‬ฮܷ ఒ ฮ߰ᇱ ‫ܬ‬ᇱ ห is related to the reduced matrix element for the radiative transition of J → J´. As a consequence, the FIR value of the green emissions arising from the radiative transitions of 2H11/2 → 4I15/2 (528 nm) and 4S3/2 → 4I15/2 (550 nm) of Er3+ ions can be written as a function of intensity parameters, as defined below:12,46

FIR =

IH 0.7158 × Ω2 + 0.4138 × Ω4 ∝ . IS 0.2225 × Ω6

(7)

Herein, the sensor sensitivity, which was defined in Eq. (5), can be roughly rewritten as:

S=

dFIR  0.7158 × Ω2 + 0.4138 × Ω4 ∝ 0.2225 × Ω6 dT 

 ∆E .   kT

(8)

Obviously, for the Er3+ ions-based optical sensing materials, the sensor sensitivity is determined by the optical transition parameters of Ωλ. Among these optical transition parameters, the Ω2 value is greatly affected by the local crystal field, whereas the resultant intensity parameters, namely, Ω4 and Ω6, are only dependent on the host material and have little influence on the 11 ACS Paragon Plus Environment


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sensor sensitivity. Generally, the products, which possess small particles, low spatial symmetry and poor crystallinity, exhibit higher Ω2.46,51 In present work, as shown in Fig. 2, all the Y2Mo4O15:Er3+/2xYb3+ microparticles possessed the same microstructure. Besides, the XRD patterns shown in Fig. S1 revealed that all the Y2Mo4O15:Er3+/2xYb3+ microparticles exhibited pure monoclinic structure and the diffraction peak intensities varied little when the Yb3+ ion content was boosted from 10 to 70 mol%, implying that Y2Mo4O15:Er3+/2xYb3+ microparticles had similar crystallinity and spatial symmetry. Based on these characteristics, it is reasonable to consider that the Ω2 values in the Y2Mo4O15:Er3+/2xYb3+ microparticles were hardly changed by adjusting the dopant concentration. As a result, the sensor sensitivity of the prepared Er3+/Yb3+codoped Y2Mo4O15 microparticles was found to be insensitive to the doping concentration. 3.6 Internal heating properties of Er3+/Yb3+-codoped Y2Mo4O15 microparticles Since the activator can not convert all the incident photons into useful emissions, part of the captured energy could be transferred into heat through the inevitable route of the involved NR transition process. As a consequence, the temperature of the studied samples could be improved. Upon NIR light (980 nm) irradiation with diverse excitation pump powers, the representative UC emission spectra of Y2Mo4O15:Er3+/0.8Yb3+ microparticles were measured so as to clarify the existence of the light-source induced thermal effect in the resultant compounds. As illustrated in the normalized green emission spectra at various excitation pump powers (see inset of Fig. 6), the green emissions at 528 and 550 nm, which corresponded to the 2H11/2 → 4I15/2 and 4S3/2 → 4

I15/2 transitions, respectively, exhibited various responses to the laser pump power, resulting in

inconsistent FIR values under different pump powers. In particular, when we adjusted the laser pump power in the range of 320-1040 mW, it is found that the FIR value presented in Fig. 6 was gradually enhanced from 0.48 to 0.65. Furthermore, on the basis of the above FIR technology, it is evident that the samples’ temperature under a certain FIR value could be computed by utilizing the following expression:   ∆E 1 . T =   × ln ln A FIR k − ( )  

(9)

In this formula, the A and ∆E/k value were known and their values were 16.37 and 1042.02, respectively. As a result, the temperature of the synthesized compounds at different FIR values or excitation pump powers can be achieved from Eq. (9). As presented in Fig. 6, when the laser pump power was 320 mW, the temperature of the Y2Mo4O15:Er3+/0.8Yb3+ microparticles was 12 ACS Paragon Plus Environment

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296 K, while it rapidly increased to 323 K when we improved the excitation pump power to 1040 mW, suggesting that the Y2Mo4O15:Er3+/2xYb3+ microparticles possessed the ability to transfer the laser pump power into heat. 3.7 EL properties of developed LED device As disclosed above, the Er3+/Yb3+-codoped Y2Mo4O15 microparticles could emit brightly visible green emissions with high color purity under NIR light excitation, which allows its potential applicability in NIR chip-based indoor illumination. For the purpose of confirming this deduction,

we

successfully

prepared

a

LED

device

by integrating

the

prepared

Y2Mo4O15:Er3+/0.8Yb3+ microparticles and a NIR (940 nm) chip. It is evident that the EL emission spectrum, which was detected under an injection current of 300 mA, contained three dominated emission bands at about 528 nm (2H11/2 → 4I15/2), 550 nm (4S3/2 → 4I15/2) and 664 nm (4F9/2 → 4I15/2) which were assigned to the intra-4f transitions of Er3+ ions, as illustrated in Fig. 7(a). Fig. 7 (b) illustrates the developed LED device. Under an injection current of 300 mA, dazzling visible green light was observed in the packaged LED device (see Fig. 7(c)). This observed result further clarifies that the Y2Mo4O15:Er3+/2xYb3+ compounds are potential greenemitting microparticles towards feasibility in solid-state lighting by employing the NIR chip as the excitation lighting source. 4. Conclusion In summary, through the citrate acid-assisted sol-gel technique with the sintering temperature of 700 °C, the Er3+/Yb3+-codoped Y2Mo4O15 microparticles were prepared. Upon 980 nm light irradiation, the bright visible UC emissions were observed in the entire compounds and these involved UC emission mechanisms all belonged to the traditional two-photon process. The UC emission intensity was very sensitive to the Yb3+ ion content and the optimal doping content was 40 mol%. The temperature related FIR values of the green emissions from the transitions of 2

H11/2 → 4I15/2 (528 nm) and 4S3/2 → 4I15/2 (550 nm) in the range of 298-486 K were measured so

as to examine the temperature sensing properties of the achieved microparticles. The maximum sensor sensitivity of the synthesized products with optimal doping concentration reached up to 0.00846 K-1 when T = 486 K. Besides, the maximum sensor sensitivities of the Y2Mo4O15:Er3+/2xYb3+ compounds were revealed to be insensitive to the dopant concentration which was further proved by the theoretical discussion based on the Judd-Ofelt theory. Additionally, the prepared compounds also had the capacity to transfer the incident light source 13 ACS Paragon Plus Environment


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into heat and the microparticles’ temperature was changed from 296 to 323 K when the pump power of the incident light was verified in the range of 320-1040 mW. Ultimately, through coating the synthesized Y2Mo4O15:Er3+/0.8Yb3+ microparticles onto a NIR chip, a LED device was fabricated and it can emit visible green light when the injection current was 300 mA. These aforementioned results implied that the Y2Mo4O15:Er3+/2xYb3+ microparticles were promising candidates for simultaneous noncontact optical thermometry and NIR chip-based indoor illumination. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998)

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(18) Wang, X.; Liu, Q.; Bu, Y.; Liu, C.; Liu, T.; Yan, X. Optical temperature sensing of rareearth ion doped phosphors, RSC Adv. 2015, 5, 86219-86236. (19) Du, P.; Luo, L.; Yu, J. S. Facile synthesis of Er3+/Yb3+-codoped NaYF4 nanoparticles: a promising multifunctional upconverting luminescent material for versatile applications, RSC Adv. 2016, 6, 94539-94546. (20) Suo, H.; Zhao, X.; Zhang, Z.; Guo, C. 808 nm Light-Triggered Thermometer–Heater Upconverting Platform Based on Nd3+-Sensitized Yolk-Shell GdOF@SiO2, ACS Appl. Mater. Interfaces, 2017, 9, 43438-43448. (21) Suo, H.; Guo, C.; Li, T. Broad-Scope Thermometry Based on Dual-Color Modulation upConversion Phosphor Ba5Gd8Zn4O21:Er3+/Yb3+, J. Phys. Chem. C 2016, 120, 2914-2924. (22) Marciniak, L.; Bednarkiewicz, A.; Strek, W. The impact of nanocrystals size on luminescent properties and thermometry capabilities of Cr, Nd doped nanophosphors, Sens. Actuators. B 2017, 238, 381-386. (23) Min, Q.; Bian, W.; Qi, Y.; Lu, W.; Yu, X.; Xu, X.; Zhou, D.; Qiu, J. Temperature sensing based on the up-conversion emission of Tm3+ in a single KLuF4 microcrystal, J. Alloys. Compd. 2017, 728, 1037-1042. (24) Zhang, Z.; Suo, H.; Zhao, X.; Sun, D.; Fan, L.; Guo, C. NIR-to-NIR Deep Penetrating Nanoplatforms Y2O3:Nd3+/Yb3+@SiO2@Cu2S toward Highly Efficient Photothermal Ablation, ACS Appl. Mater. Interfaces, 2018, 10, 14570-14576. (25) Suo, H.; Zhao, X.; Zhang, Z.; Shi, R.; Wu, Y.; Xiang, J.; Guo, C. Local symmetric distortion boosted photon up-conversion and thermometric sensitivity in lanthanum oxide nanospheres, Nanoscale, 2018, 10, 9245-9251 (26) Zhao, L.; Cai, J.; Hu, F.; Li, X.; Cao, Z.; Wei, X.; Chen, Y.; Yin, M.; Duan, C. Optical thermometry based on thermal population of low-lying levels of Eu3+ in Ca2.94Eu0.04Sc2Si3O12, RSC Adv. 2017, 7, 7198-7202. (27) Chai, X.; Li, J.; Wang, X.; Li, Y.; Yao, X. Upconversion luminescence and temperaturesensing properties of Ho3+/Yb3+-codoped ZnWO4 phosphors based on fluorescence intensity ratios, RSC Adv. 2017, 7, 40046-40052. (28) Du, P.; Huang, X.; Yu, J. S. Yb3+-Concentration dependent upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped Gd2MoO6 nanocrystals prepared by a facile citric-assisted sol-gel method, Inorg. Chem. Front. 2017, 4, 1987-1995. 16 ACS Paragon Plus Environment

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(51) Hao, H.; Lu, Z.; Lu, H.; Ao, G.; Song, Y.; Wang, Y.; Zhang, X. Yb3+ concentration on emission color, thermal sensing and optical heater behavior of Er3+ doped Y6O5F8 phosphor, Ceram. Int. 2017, 43, 10948-10954. (52) Sinha, S.; Mahata, M. K.; Kumar, K.; Tiwari, S. P.; Rai, V. K. Dualistic temperature sensing in Er3+/Yb3+ doped CaMoO4 upconversion phosphor, Spectrochim. Acta. A 2014, 173, 369-375. (53) Mukhopadhyay, L.; Rai, V. K.; Bokolia, R.; Sreenivas, K. 980 nm excited Er3+/Yb3+/Li+/Ba2+: NaZnPO4 upconverting phosphors in optical thermometry, J. Lumin. 2017, 187, 368-377. (54) Mondal, M.; Rai, V. K.; Srivastava, C.; Influence of silica surface coating on optical properties of Er3+-Yb3+:YMoO4 upconverting nanoparticles, Chem. Eng. J. 2017, 327, 838-848.



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Table 1 Optical thermometric performance of Er3+ ions-based luminescent thermometers Compounds

Temperature range

Sensitivity

∆E/k

λex

Reference

YF3:Er3+/Yb3+

260-490 K

0.0027 K-1

997.40

980 nm

5

YNbO4:Er3+/Yb3+

298-673 K

0.0072 K-1

759.60

980 nm

8

La2MoO6:Er3+

303-463 K

0.0097 K-1

1105.00

379 nm

10

260-490 K

0.0032 K

-1

1144.60

980 nm

21

Gd2O3:Er3+/Yb3+

298-723 K

0.0084 K-1

1175.34

980 nm

46

Na0.5Gd0.5MoO4:Er3+/Yb3+

298-778 K

0.00856 K-1

1195.11

980 nm

48

CaMoO4:Er3+/Yb3+

300-760 K

0.00721 K-1

1072.00

980 nm

52

NaZnPO4:Er3+/Yb3+/Li+

300-603 K

0.00646 K-1

1102.38

980 nm

53

1274.46

980 nm

54

1042.02

980 nm

This work

3+

Ba5Gd8Zn4O21:Er /Yb

3+

YMoO4:Er /Yb

3+

Y2Mo4O15:Er3+/Yb3+

3+

-1

301-368 K

0.024 K

298-483 K

0.00846 K-1


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Figure 1 (a) Rietveld XRD refinement for the Y2Mo4O15:Er3+/0.8Yb3+ microparticles utilizing the GSAS software. (b) Schematic diagram of the unit cell crystal structure of Y2Mo4O15.



Figure 2 FE-SEM images of Y2Mo4O15:Er3+/2xYb3+ microparticles (a) x = 0.2, (b) x = 0.4, (c) x = 0.6 and (d) x = 0.7. (e)-(j) Elemental mapping and (k) EDX spectrum of Y2Mo4O15:Er3+/0.8Yb3+ microparticles.


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Figure 3 (a) Room-temperature UC emission spectra of Y2Mo4O15:Er3+/2xYb3+ (x = 0.12, 0.2, 0.3,

0.4,

0.5, 3+

0.6

and

0.7)

microparticles.

(b)

CIE

3+

chromaticity 3+

diagram

of

3+

Y2Mo4O15:Er /0.8Yb microparticles. (c) Energy transfer UC mechanism in Er /Yb -codoped Y2Mo4O15 microparticles. (d) UC emission spectra of Y2Mo4O15:Er3+/0.8Yb3+ microparticles with various pump powers. Dependence of pump power on the (e) green and (f) red emission intensities.



Figure 4 (a) UC emission spectra of Y2Mo4O15:Er3+/0.8Yb3+ microparticles in the temperature range of 298-486 K. (b) FIR value related to the temperature. (c) Temperature-dependent PRA values. (d) Sensor sensitivity as a function of temperature.


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Figure 5 Dependence of temperature on the optical thermometric properties for (a), (c), (e) Y2Mo4O15:Er3+/0.4Yb3+ and (b), (d), (f) Y2Mo4O15:Er3+/1.2Yb3+ microparticles.



Figure 6 Dependence of FIR and product temperature on the excitation pump power for the Y2Mo4O15:Er3+/0.8Yb3+ microparticles. Inset displays the normalized UC emission spectra as a function of excitation pump power.


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Figure 7 (a) EL emission spectrum of the developed LED device at 300 mA forward bias current. (b) Developed LED device. (c) Luminescent image of the fabricated LED device under a forward bias current of 300 mA.



Table of Contents

Er3+/Yb3+-codoped Y4Mo4O15 microparticles with splendid UC emission performance for optical temperature sensors and indoor illumination.


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Figure 1 (a) Rietveld XRD refinement for the Y2Mo4O15:Er3+/0.8Yb3+ microparticles utilizing the GSAS software. (b) Schematic diagram of the unit cell crystal structure of Y2Mo4O15. 110x55mm (300 x 300 DPI)

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116x66mm (300 x 300 DPI)


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Figure 3 (a) Room-temperature UC emission spectra of Y2Mo4O15:Er3+/2xYb3+ (x = 0.12, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7) microparticles. (b) CIE chromaticity diagram of Y2Mo4O15:Er3+/0.8Yb3+ microparticles. (c) Energy transfer UC mechanism in Er3+/Yb3+-codoped Y2Mo4O15 microparticles. (d) UC emission spectra of Y2Mo4O15:Er3+/0.8Yb3+ microparticles with various pump powers. Dependence of pump power on the (e) green and (f) red emission intensities. 99x66mm (300 x 300 DPI)



Figure 4 (a) UC emission spectra of Y2Mo4O15:Er3+/0.8Yb3+ microparticles in the temperature range of 298-486 K. (b) FIR value related to the temperature. (c) Temperature-dependent PRA values. (d) Sensor sensitivity as a function of temperature. 108x81mm (300 x 300 DPI)


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Figure 4 (a) UC emission spectra of Y2Mo4O15:Er3+/0.8Yb3+ microparticles in the temperature range of 298-486 K. (b) FIR value related to the temperature. (c) Temperature-dependent PRA values. (d) Sensor sensitivity as a function of temperature. 121x61mm (300 x 300 DPI)



Figure 6 Dependence of FIR and product temperature on the excitation pump power for the Y2Mo4O15:Er3+/0.8Yb3+ microparticles. Inset displays the normalized UC emission spectra as a function of excitation pump power. 81x53mm (300 x 300 DPI)


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Figure 7(a) EL emission spectrum of the developed LED device at 300 mA forward bias current. (b) Developed LED device. (c) Luminescent image of the fabricated LED device under a forward bias current of 300 mA. 85x47mm (300 x 300 DPI)


Near-Infrared Light-Triggered Visible Upconversion Emissions in Er3

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