Broad-Scope Thermometry Based on Dual-Color Modulation up

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Broad-Scope Thermometry Based on Dual-Color Modulation Up-Conversion Phosphor BaGdZnO : Er /Yb 5

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21

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3+

Hao Suo, Chongfeng Guo, and Ting Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11786 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Broad-scope Thermometry Based on Dual-color Modulation Up-conversion Phosphor Ba5Gd8Zn4O21: Er3+/Yb3+ Hao Suo，Chongfeng Guo*, Ting Li

National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & PhotonTechnology, Northwest University, Xi’an, 710069, China; ABSTRACT: Yb3+/Er3+ co-doped Ba5Gd8Zn4O21 up-conversion (UC) phosphors with tunable emission were synthesized using a facile sol-gel method. UC spectra are composed of green emission from 2H11/2/4S3/2 → 4I15/2 transitions and red emission from 4F9/2 → 4I15/2 transition of Er3+ ion with the excitation of 980 nm laser diodes. Modulation of emitting color from green to red could be achieved by adjusting dopant concentrations or pulse width of 980 nm laser. The mechanism of the former strategy was figured out through analyzing visible and near-infrared (NIR) down-conversion emission spectra together with the corresponding green level (4S3/2) lifetimes under excitation of 490 nm light, and the latter method was explained by the nonsteady-state up-converison process. Temperature detection range was expanded to low temperature region by utilizing red-emitting stark levels of Er3+ ion as thermally coupled levels. Thermal sensing performances based on green-emitting levels (2H11/2/4S3/2) and red emitting stark levels (4F9/2(1)/ 4F9/2(2)) of Er3+ ion were estimated and the maximum sensitivity are 0.0032 K-1 at

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490 K and 0.0029 K-1 at 200 K in our experimental range, respectively. Moreover, the effects of UC emission color from different dopant concentration and pulse width of laser on sensor sensitivity were also investigated in detail. Results imply that the present phosphor Ba5Gd8Zn4O21: Er3+, Yb3+ exhibits high and stable sensitivity in a wide temperature detection scope, which enables it as an excellent candidate for optical thermometer. 1. INTRODUCTION As one of the fundamental parameters of thermodynamics, temperature is an important index and parameter in many fields, such as scientific research, industrial manufacture and life activities. Its accurate measurement is very important and significant for practical applications, but the different thermometers with different detection temperature range and sensitivity are required in different measuring environments. Among methods to determine the temperature, optical thermometry based on fluorescence intensity ratio (FIR) technique provides high detection spatial resolution and precision, excellent sensitivity and tunable size in comparison with that of the conventional contact thermal sensing based on metal or liquid expansion. The FIR technique is realized through the varied luminescence intensities of two thermally coupled levels (TCLs) at environmental temperature, which is unaffected by spectrum losses and fluctuations of pumping intensity. This enables optical temperature sensors extremely attractive for its potential applications in biological environment, electrical power stations, oil refineries and coal mines.1-6 Especially, optical thermometers based on rare earth ions doped up-conversion (UC) phosphors have been given more attention due to their excitation in the near-infrared (NIR) region and suitable TCLs. Generally, the energy separation ∆E between the TCLs ranges from 200 to 2000 cm-1 to avoid strong overlapping of the two emission bands, and TCLs could be found in many rare earth ions, such as Er3+ (4S3/2, 2H11/2), Ho3+ (5F2, 3/3K8, 4F1/5G6), Tm3+ (3F2, 3,


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H4), Gd3+ (6P5/2, 6P7/2; 6I9/2, 6I7/2), Nd3+ (4F7/2, 4F3/2; 4F5/2, 4F3/2; 4F7/2, 4F5/2), Dy3+ (4F9/2, 4I15/2),

etc.7-12 However, two closely spaced stark sublevels of lanthanide ions whose energy gap is lower than 200 cm-1 were also considered as TCLs in recent publications, for example, Er3+ (4S3/2(1), 4S3/2(2)), Ho3+(2F5/2(1), 2F5/2(2)), Tm3+ (1G4(1), 1G4(2)) , etc.13-15 Above mentioned results imply that the larger energy separation ∆E between TCLs would lead to the higher detection temperature, and vice versa, which indicates that temperature detection range is mainly determined by the value of ∆E. Therefore, optical temperature sensor with broad-scope temperature detection could be implemented by appropriately combining two pairs of TCLs with different energy gaps.16, 17 The typical rare earth element for optical thermometric application is activator erbium ion (Er3+) with abundant ladder-like electronic energy level, and ytterbium (Yb3+) ion is usually combined used as a sensitizer to enhance UC efficiency which reduces thermal sensing deviation by taking advantages of their large absorption cross section at 980 nm and efficient energy transfer to Er3+.18 In most cases, investigations are focused on the thermally coupled levels 4S3/2 and 2H11/2 owing to its perfect luminescence property and appropriate energy gap (800 cm-1),19-22 but the red emission from 4F9/2 → 4I15/2 transition of Er3+ is hardly concerned though it situated in the “optical transparency window (650-1100 nm)” of biological tissues.23 For optical thermal sensors used in biology, red emission-based luminescence thermometers are most desirable candidates because they could offer deep penetration and low absorption by tissues.24,25 Moreover, the stark sublevels of red-emitting state 4F9/2 with small energy separation (100 cm-1) could show excellent temperature sensing property in low temperature range, whereas the green emission levels (4S3/2/2H11/2) with larger energy separation are more fit for high temperature detection. Thus, sustained high sensitivity within a broad temperature range may be achieved in a


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single host utilizing FIR technique based on both green and red emissions of Er3+, which is worthy of discussing and investigating. Sensitivity is a prominent parameter for sensors, and it is very significant to figure out what factors affect it. For the present optical temperature sensor, the effects from host materials, UC luminescence intensity and particle size are not very clear, thus it is still a challenging task to enhance the sensitivity of sensors.26-29 To reveal the influence of spectral color purity on sensor sensitivity, it is top priority to realize the tunable color purity. For Er3+-Yb3+ co-doped UC systems, the emission intensity ratio of red to green or the emitting color purity could be tuned by many methods, which provides possibilities to demonstrate the relation between color purity and sensitivity.30,31 In addition, host materials with low photon energy could achieve high UC emission efficiency by inhibiting multi-photon relaxation.32 Zincate complex oxide Ba5Gd8Zn4O21 not only has relatively low phonon energy (360 cm-1) but also exhibits stable crystal structure as well as perfect physical and thermal stability, which could be regarded as an ideal host candidate for UC luminescence thermometer.33 Here, Er3+-Yb3+ ion pairs doped Ba5Gd8Zn4O21 were synthesized by a sol-gel process. Tunable emission color was realized through adjusting dopant concentrations and excitation source pulse width, and their mechanisms were also proposed. Importantly, the thermometry behaviors of green-emitting energy levels (2H11/2, 4S3/2) at high temperature and red-emitting stark sublevels of 4

F9/2 state at low temperature of Er3+ ion were systematically investigated to broaden its

temperature detecting scope. Furthermore, a thorough inquiry about the effect of spectral color purity on sensing sensitivity was also discussed in detail. 2. EXPERIMENTAL SECTION 2.1. Sample preparation. The present Ba5Gd8Zn4O21 (BGZ) based phosphors and blank


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sample were prepared using a modified sol-gel method, and the dopant ions Yb3+ and Er3+ were expected to occupy the sites of Gd3+ ion due to similar ionic radius and identical valence. Thus the nominal formula of phosphors are written as Ba5Gd8(1-x-y)Er8xYb8yZn4O21 (x = 0.5 ~ 7% and y = 0 ~ 15%). In a typical procedure, stoichiometric amount of high purity Ln2O3 (99.99%, Shanghai Yuelong Nonferrous Metals Ltd.) were firstly dissolved in diluted HNO3 (A. R.) and surplus nitric acid was volatilized at high temperature, then suitable amount of deionized water was added with vigorous stirring to form Ln(NO3)3 (Ln = Yb, Er, Gd) solution. Subsequently, the calculated quantity of chelating agent citric acid (molar ratio of citric acid to total metal ions was 2:1) and required raw materials analytical grade BaCO3, ZnO were mixed into the above solutions under constant stirring for a few minutes to get transparent solution. Then, the obtained solution was kept in an oven at 80 ℃ for 48 h to get colorless transparent resin, and further dried at 120 ℃ for 24 h to obtain brown dried gel. Finally, the dried gels were grinded and pre-heated in furnace at 500 ℃ for 5 h and further sintered at 1200 ℃ for 3 h to get the final samples. 2.2. Measurements and characterization. The crystal structure and composition of samples were identified by powder X-ray diffraction (XRD) on a Rigaku-Dmax 3C powder diffractometer (Rigaku Corp, Tokyo, Japan) with Cu-Ka (λ = 1.54056 Å) radiation. The photoluminescence (PL) spectra and decay curves were recorded using an Edinburgh FLS920 fluorescence spectrophotometer assembled with 450 W Xe lamp as the excitation source. The upconversion emission spectra were carried out with an external power-controllable 980 nm semiconductor laser, and temperature dependent spectra were measured by an Oxford OptistatDN2 nitrogen cryogenics temperature controlling system (duration of stay at the measured temperature is 0.5 h). The pulse width modulation technique and decay curves of 4S3/2 → 4I15/2 transition were achieved by using 980 nm modulated pulsed laser which is obtained by


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coupling a 980 nm diode laser with transistor-transistor logic (TTL) pulse mode (CNI, Changchun, China) and a digital pulse generator (DG, Rigol technologies, Beijing, China). 3. RESULTS AND DISCUSSION 3.1. XRD diffraction analysis. The phase purity and structure of synthesized samples were identified by XRD, and all samples obtained in our experiments show similar results. Only the XRD patterns of blank BGZ, solely doped BGZ: 5%Er3+ and co-doped BGZ: 5%Er3+, 12%Yb3+ were displayed in Figure 1 as representatives. It is clearly observed that all diffraction peaks of samples are well coincident with those in the standard BGZ pattern (JCPDS No. 51-1686) and no any other characteristic peaks related to raw materials or impurities, suggesting that the obtained samples crystallize in the tetragonal structure and belong to I4/m space group. By comparing these patterns, a small shift of diffraction peaks from low towards the higher angles was found with the addition of lanthanide ions, and the enlarged (141) diffraction peaks were clearly presented in Figure 1b. On the basis of the obtained XRD data, the lattice constants were calculated to be a = b = 13.953 Å, c = 5.716 Å, V = 1112.826 Å3 for blank BGZ, a = b=13.898 Å, c = 5.736 Å, V = 1107.934 Å3 for BGZ: Er3+ and a = b=13.871 Å, c = 5.710 Å, V = 1098.631 Å3 for BGZ: Er3+/Yb3+, indicating that the cell volume gradually decreases with more dopant ions entering the sits of Gd3+. These results are owing to the substitute of Gd3+ (RGd = 1.00 Å) with larger radius by Yb3+ (RYb = 0.925 Å) and Er3+ (REr = 0.945 Å) with smaller radius for seven coordination number (CN = 7), which also reveal that the dopants Er3+ and Yb3+ have effectively entered into host structure and make no significant change to host lattices.


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Figure 1. (a) XRD patterns of blank, Er3+ solely doped and Er3+/Yb3+ co-doped samples as well as the standard data of BGZ; (b) enlarged (141) peak in various XRD patterns. 3.2. UC emission color modulation and mechanism by adjusting dopant contents. Under the excitation of 980 nm laser diodes, the UC spectra normalized at the strongest emission peak of BGZ: x Er3+, y Yb3+ samples, corresponding CIE coordinates, digital photos and integrated intensity of red and green emission along with their ratios were displayed in Figure 2, respectively. As shown in Figure 2a and Figure 2d, the UC spectra are composed of two green emission bands from 515 to 565 nm centered at 528 and 548 nm and a red emission band from 640 to 680 nm with two peaks at 653 and 674 nm in visible region, corresponding to the 2H11/2, 4

S3/2 → 4I15/2 and 4F9/2 → 4I15/2 intrinsic transitions of Er3+ ion, respectively. It is worth noting that

the red emissions were divided into two parts from the stark sublevels of 4F9/2, which are induced by electronic interactions and spin-orbit coupling.34 The variable Yb3+ and Er3+ concentrations make no obvious influence on the position of emission bands, but the overall UCL intensity and emission intensity ratio of red to green are greatly affected. Seen from integrated emission


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intensity histograms (Figure 2b), the overall UC intensity are greatly enhanced with introducing sensitizer Yb3+ ion owing to the efficient energy transfer from Yb3+ to Er3+. The optimal Yb3+ and Er3+ concentrations with strongest emission intensity are determined to be and 12% and 5% through fixing the concentration of activator Er3+ with variable Yb3+ and the fixed sensitizer concentration of Yb3+ with different Er3+, as shown in Figure 2b, respectively. It is clearly found that the UC intensity in both green and red region firstly increase and then sharply decrease after reaching to the maximum due to concentration quenching and the energy back transfer (EBT) Er3+ (4S3/2) + Yb3+ (2F7/2) → Er3+ (4I13/2) + Yb3+ (2F5/2).35 The variational ratio of red to green would lead to the alteration of emission color, thus the visible UC luminescence color of BGZ: x Er3+, y Yb3+ samples was finely tuned from green through yellow and finally to nearly pure red as increasing dopant content, as displayed in the digital photographs (insets of Figure 2a and Figure 2d). From the line chart (Figure 2b), the calculated red to green integrated intensity ratio enhanced steadily from 0.84 to 19.02 for fixing Er3+ contents (x = 5%) with variable Yb3+ contents and 2.69 to 12.01 for doping different Er3+ content with a constant Yb3+ (y = 10%), respectively. To visualize the modification of UC emission color, the CIE coordinates of two series of samples were calculated according to UCL spectra and listed in the Table 1, which varied from (0.411, 0.580) to (0.562, 0.435) for increasing Er3+ content (0.5 ~ 7%) and from (0.340, 0.650) to (0.608, 0.388) for enhancing Yb3+ content (0 ~ 15%). The tunable UC color from green to red region was achieved by increasing dopant content, which is clearly presented in CIE diagram (Figure 2c).


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Figure 2. The normalized UC emission spectra (λex = 980 nm) and luminescence photographs of BGZ: x Er3+, y Yb3+: (a) x = 0.5 ~ 7%, y = 10%; (d) y = 0 ~ 15%, x = 5%; (b) corresponding green and red emission peak integrated intensity along with the R/G ratio and (c) CIE chromaticity diagram. Table 1. The CIE chromaticity coordinates of BGZ: x Er3+, y Yb3+ phosphors CIE chromaticity coordinates


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Sample

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x

y

1. 10%Yb3+, 0.5%Er3+

0.411

0.580

2. 10%Yb3+, 1%Er3+

0.438

0.554

3. 10%Yb3+, 3%Er3+

0.515

0.479

4. 10%Yb3+, 5%Er3+

0.557

0.438

5. 10%Yb3+, 7%Er3+

0.562

0.435

0%Yb3+, 5%Er3+

0.340

0.650

b. 6%Yb3+, 5%Er3+

0.474

0.518

8%Yb3+, 5%Er3+

0.537

0.457

d. 12%Yb3+, 5%Er3+

0.592

0.404

e. 15%Yb3+, 5%Er3+

0.608

0.388

a.

c.

To shed light on UC process and mechanism, power-dependent UC intensities for green and red emission were investigated in detail. The mathematic relationship between UC emission intensity (I) and pump power (p) follows the equation: I ∝ Pn, where the n value denotes the required number of photons in UC process.36 As depicted in Figure 3a, the logarithmic emission intensity exhibits perfect linearity with the logarithmic power, and the calculated slopes (n) of the linear fitting for green and red emission are 2.08 and 1.45. Results reveal that the green and red emissions are assigned to two-photon absorption process, which means that one green or red photon emission need to absorb two NIR photons. According to above phenomena, the energy levels diagram of Er3+/Yb3+ co-doped system and possible UC pathways populating the emitting states were schematically illustrated in Figure 3b to completely understand the mechanism of tunable UC emission color in BGZ: Er3+/Yb3+


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phosphor. Under 980 nm laser excitation, the ground state absorption (GSA) of Yb3+ ion: 2F7/2 + a photon (980 nm) → 2F5/2 and Er3+ ion: 4I15/2 + a photon (980 nm) → 4I11/2 take place by absorbing NIR photons. Usually, the population at excited level 4I11/2 mainly occurs through an efficient energy transfer (ET) from Yb3+ to Er3+: Yb3+ (2F5/2) + Er3+ (4I15/2) → Yb3+ (2F7/2) + Er3+ (4I11/2) because of lager absorption cross section of Yb3+ than that of Er3+. For the green emission bands around 528 and 548 nm from 2H11/2, 4S3/2 → 4I15/2 radiative transitions, the meta-stable 2

H11/2 and 4S3/2 levels are populated by non-radiative (NR) transition from upper level 4F7/2. The

excited state of 4F7/2 could be populated through ET process: Yb3+ (2F5/2) + Er3+ (4I11/2) → Yb3+ (2F7/2) + Er3+ (4F7/2) or excited state absorption (ESA): Er3+ (4I11/2) + a photon (980 nm) → Er3+ (4F7/2). For the red emission band from the 4F9/2(1), 4F9/2(2) → 4I15/2 transitions, the population of stark levels 4F9/2(1) and 4F9/2(2) could be realized through three possible processes. The first is further NR from 4S3/2 level, and the second is cross relaxation (CR) process: Er3+ (4I11/2) + Er3+ (4F7/2) → Er3+ (4F9/2) + Er3+ (4F9/2). The last way is achieved by ET process: Yb3+ (2F5/2) + Er3+ (4I13/2) → Yb3+ (2F7/2) + Er3+ (4F9/2) or ESA: Er3+ (4I13/2) + a photon (980 nm) → Er3+ (4F9/2), in which the 4I13/2 level was populated through a 4I11/2 → 4I13/2 NR process. Ultimately, the stark sublevels 4F9/2(1) and 4F9/2(2) of 4F9/2 state decay radiatively to ground state 4I15/2 of Er3+ with red emission band from 640 to 680 nm with two peaks at 674 and 653 nm, respectively. Based on above analysis, it could be deduced that the increase of red to green emission intensity ratio is closely related with CR process: Er3+ (4I11/2) + Er3+ (4F7/2) → Er3+ (4F9/2) + Er3+ (4F9/2) and EBT process: Er3+ (4S3/2) + Yb3+ (2F7/2) → Er3+ (4I13/2) + Yb3+ (2F5/2) which are greatly affected by dopant concentration.37 The former process directly inhibits green-emitting and promotes red emission, and the latter process indirectly enhances the population at 4F9/2 level through subsequently ET or ESA process from intermediate 4I13/2 level.


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Figure 3. (a) Double logarithmic relationship of red and green UC emission intensities versus excitation power in BGZ: 5%Er3+, 12%Yb3+ phosphor; (b) schematic energy level diagram of Er3+/Yb3+ as well as the possible energy transfer mechanisms involved in the UC process under the excitation of 980 nm. Figure 4a and Figure 4b present the normalized temporal curves of the green emission from 4

S3/2 → 4I15/2 transition in BGZ: Er3+/Yb3+ phosphors as function of the Er3+ and Yb3+ content,

respectively. By introducing sensitizer Yb3+ ion, the shape of decay curve is distinctly different in comparison with that of the sensitizer Yb3+-free sample, which implied that Er3+-Yb3+ interaction appeared and Yb3+ modifies the UC dynamics of Er3+ in Er3+-Yb3+ co-doped samples. The effective lifetimes of green and red level could be calculated using following equation (1):38 ∞

∞

0

0

τ = ∫ tI (t ) dt / ∫ I(t ) dt

(1)

where I(t) represents the luminescene intensity at time t. The average lifetime is obtained by fitting temporal profile with a quadratic exponential function could be expressed as:39


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2

τ=

A1t1 + A2t 2 A1t1 + A2t 2

2

(2)

where the amplitude and lifetime are denoted as A and t. The green emission lifetimes of Er3+/Yb3+ co-doped samples are much larger than that of Er3+ single-doped sample, further proving the present of ET process. Insets of Figure 4 shows the dependence of decay times on dopant concentration, it is observed that the average lifetime of green and red luminescence initially increased and then decreased as raising Er3+ and Yb3+ concentration after reaching optimal value of 203.8 µs at x = 1% (fixed y = 10%) in the range of 0.5% ≤ x ≤ 7% and 164 µs at y = 6% (fixed x = 5%) in the range of 0 ≤ x ≤ 15%. This is mainly due to the effective EBT process: Er3+ (4S3/2) +Yb3+ (2F7/2) → Er3+ (4I13/2) + Yb3+ (2F5/2) at high dopant content, which greatly promotes the depopulation and thus decreases the decay time.

Figure 4. Decay curves of 4S3/2 →4I15/2 emission in BGZ: Er3+, Yb3+ phosphors with: (a) different Er3+ and 10%Yb3+, (b) variable Yb3+ contents and 5%Er3+.


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To better understand the origin of tunable UC emission color with different dopant content, the 490 nm excitation light was employed to directly populate the 4F7/2 level, and downconversion (DC) emission spectra of BGZ: Er3+/Yb3+ samples with different Er3+ content in visible and NIR region were displayed in Figure 5a and Figure 5b, respectively. As shown in Figure 5a, the red emission intensity continuously enhanced with increasing Er3+ content in comparison with that of the normalized green emission in visible UC spectra, which confirms the occurrence of cross relaxation (CR) process Er3+ (4I11/2) + Er3+ (4F7/2) → Er3+ (4F9/2) + Er3+ (4F9/2) and energy back transfer (EBT) process from Er3+ to Yb3+. It is generally accepted that the probability of CR and EBT process would be enhanced by shortening the distance between Er3+Er3+ and Er3+-Yb3+, resulting the increasing red to green emission intensity ratio. The NIR emission spectra are composed of emission peak centered at 976 nm from 2F5/2 → 2F7/2 transition of Yb3+ ion and emission band around 1550 nm from 4I13/2 → 4I15/2 of Er3+ ion. As the increase of Er3+ content, the intensities of both emissions enhance monotonously, while the green emissions in visible area exhibit opposite variation trend and keep dropping, as illustrated in line chart (Figure 5c). The corresponding calculated decay curves of 4S3/2 → 4I15/2 emission decrease gradually from about 83.03 to 17.71 µs with increasing Er3+ content from 0.5 to 7% in samples, as illuminated in Figure 5d. This can be attributed to the presence of EBT process from Er3+ to Yb3+ ion: Er3+ (4S3/2) +Yb3+ (2F7/2) → Er3+ (4I13/2) + Yb3+ (2F5/2), which populates the NIR-emiting levels 2F5/2 and decrease population at 4S3/2 level. As for varying Yb3+ content, similar results were obtained and shown in Figure S1. It should be noted that 4I11/2 → 4I15/2 transition of Er3+ ion could be observed in Er3+ single doped samples, and concentration quenching effect occurred in 976 nm NIR emission from 2F5/2 → 2F7/2 transition when Yb3+ content is over 8%. Above results


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further confirm that the dopant concentration induced tunable UC emission color is mainly caused by CR and EBT process.

Figure 5. PL spectra (λex = 490 nm) of BGZ: x Er3+, 10% Yb3+ (x = 0.5 ~ 7%) in (a) visible (normalized at green emissions) and (b) NIR region; (c) dependence of green and NIR normalized integrated intensity on Er3+ content and (d) corresponding decay curves of green emission from 4S3/2 → 4I15/2 transition.

3.3. UC color modulation and mechanism by adjusting excitation pulse width. To explore whether tunable UC luminescence in our samples could be dynamic controlled by a pulse width modulation strategy or not, the UC spectra with normalized green emission of BGZ: 5%Er3+, 15%Yb3+ phosphor were measured under the excitation of 980 nm continuous wave (C.W.) and pulse laser (100 Hz) diode, as illustrated in Figure 6. Remarkly, the red emission intensity gradually increased and the calculated value of red to green emission ratio increased from 5.78 to 19.02 with elongateing pulse durations from 300 to 1000 µs and eventually to continuous. The corresponding CIE coordinates vary from (0.411, 0.580) to (0.608, 0.388), tuning UC emission


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color from yellowish green to red region. It can be also inferred that spectrally pure green emission could be realized by irradiating with enough short pulse intervals.

Figure 6. The UC spectra (normalized at green emission) and red to green ratios of BGZ: 5%Er3+, 15%Yb3+ phosphor under excitation of a 980 nm laser with different pulse durations, inset show the corresponding CIE coordinates. The crucial mechanism that gives rise to perfect color-tunability of the prepared sample can be expressed according to a non-steady-state upconverison process. It is well known that several consecutive excitation of lower electronic states may be required to populate the upper energy levels, and the activator emissions from different emitting-levels may not occur at the same time. Additionally, it would spend milli-seconds on populating different electronic states to reach the steady states owing to abundant 4f transitions in rare earth ions.40,41 Figure 7a presents the proposed non-steady-state UC process of green and red emissions of Er3+ ion upon 980 nm pulsed laser excitation. As for Er3+ ion, two-step energy transfer and an extra non-radiative


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process are necessary for populating both green meta-stable 2H11/2 /4S3/2 levels (ET:①② and a ,○ c and NR: ○ b ). Considering that the energy gap NR:③) and red-emitting state 4F9/2 (ET: ○

between 4F7/2 and 2H11/2 (1166 cm-1) is much smaller than that of 4I11/2 and 4I13/2 levels (3609 cm1

), it is conceivable that the NR process 4F7/2 → 2H11/2 would take less time than that of 4I11/2 →

4

I13/2 transition. This indicates that the pumping rate of red-emitting 4F9/2 level is slower than that

of green emission states 2H11/2/4S3/2. When exciting at short pulse laser, the green emission would more efficiently generate than red emission due to the faster deactivation of the excitation energy by convenient NR process: 4F7/2 → 2H11/2. On the contrary, long pulse excitation could offer sufficient time for population of red emitting level 4F9/2, which facilitates the occurrence of red emission. Thus, the red to green ratio could be regulated by a pulse width modulation method, which is attributed to the different population rate between green and red emitting levels. Results provide a novel and effective method to actualize multicolor tuning in our present samples. To validate the non-steady-state UC mechanism, the time-dependent profiles of Er3+ emission in green and red region peaked at 548 nm and 674 nm in BGZ: 5%Er3+, 15%Yb3+ phosphor were displayed in Figure 7b. Under the excitation of a 980 nm laser with duration of 8 ms (100 Hz), the populating rate of Er3+ in red emission state was found to be obviously slower than that of green emission, which is consistent with above theoretic analysis. It is observed that the intensity of red emission increased gradually and then kept constant with further excitation after reaching its steady state at 5 ms, while the green emission intensity kept invariable from excitation duration at 2 ms. The plot of red to green ratio as a function of excitation duration is presented in Figure 7c, in which the ratio raised monotonously with excitation duration time and achieved the maximum value after 5 ms of excitation. Above experimental results clearly confirm the existence of non-steady-state UC process.


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Figure 7. (a) Schematic UC mechanism of green and red emission in BGZ: 5%Er3+, 15%Yb3+ phosphor upon excitation of 980 nm pulsed laser; (b) time-resolved PL investigation of Er3+ green and red emission and (c) corresponding red to green emission intensity ratio under 980 nm pulsed laser with a duration of 8 ms.

3.4. Broad-scope optical thermometry properties. According to the FIR thermal sensing theory, thermally coupled levels (TCLs) of lanthanide ions play an essential and decisive role in optical thermometric sensitivity and temperature detection range. The investigation of temperature sensing behavior of Er3+ ion is mainly focused on 2H11/2 and 4S3/2 levels, yet it is


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unclear whether closely spaced stark sublevels 4F9/2(1), 4F9/2(2) are thermal coupling or not. Thus, thermal evolution green (260-490 K) and red (200-490 K) UC emission spectra of BGZ: 5%Er3+, 12%Yb3+ were given in Figure 8 with the excitation of NIR (980 nm) light to verify its potentiality as broad-scope temperature sensors, and the laser induced thermal effect could be ignored using low enough excitation power (20 mW). It is worth noting that emission peaks have been normalized at the 528 and 653 nm to observe the relative variation of emission intensity intuitively. As displayed in Figure 8, the emission wavelengths are invariable for both green and red UC emissions as the increase of temperature, whereas the UC luminescence intensity ratios of I528 (2H11/2 → 4I15/2) to I548 (4S3/2 → 4I15/2) and I653 (4F9/2(2) → 4I15/2) to I674 (4F9/2(1) → 4I15/2) dramatically increased. With the increase of internal temperature, enhanced lattice vibrations promote the non-radiative relaxation rate between two closely spaced energy levels which keeps them quasi-thermal balanced, suggesting that both 2H11/2/4S3/2 and 4F9/2(1)/4F9/2(2) are thermally coupled. Followed by Boltzmann type distribution law, the corresponding relative population for two thermally coupled electronic states can be mathematically expressed as:42

R=

I 2 N 2 g 2σ 2ω2 ∆E ∆E = = exp(− ) = B exp(− ) I1 N1 g1σ 1ω1 KT KT (3)

where

B=

g 2σ 2ω2 g1σ 1ω1

I1 (lower level) and I2 (upper level) represent the integrated intensities of two thermally coupled levels, while N1 and N2 are the corresponding number of ions, respectively. g, σ and ω define the


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absolute temperature, degeneracy, emission cross-section and angular frequency of corresponding transitions. The Boltzmann constant and absolute temperature are symbolized by T and K, and ∆E is the energy difference between two thermally linked states.

Figure 8. Normalized temperature-dependent UC emission spectra of BGZ: 5%Er3+, 12%Yb3+ phosphor in green and red region with 980 nm excitation. To intuitively shed light on the relationship between FIR value and temperature, the plot of FIR on a monolog scale as a function of inverse absolute temperature in green (260-490 K) and red (200-490 K) region of BGZ: 5%Er3+, 12%Yb3+ sample were employed in Figure 9a, respectively. The experimental results could be perfectly fitted to straight lines whose slopes are 1144.6 for green and 143.6 for red that actually represent the value of ∆E/K. The calculated energy separation ∆E between 2H11/2 and 4S3/2 is about 790 cm-1, and approximately 100 cm-1 for that of stark levels 4F9/2(1) and 4F9/2(2). According to the variation of FIR with absolute temperature shown in Figure 9b, it is observed that the FIR increased greatly as raising temperature and the values of B for the best linear fitting curves of experimental data are found to be 6.971 and 1.662 for green and red, respectively. Considering practical applications, sensitivity (S) is a vital and prominent parameter for quantitatively characterizing the


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applicability of materials as an optical thermal sensor, which could be calculated using the following formula:43

S=

d ( FIR ) ∆E = FIR ( ) d (T ) KT 2

(4)

where all the terms have usual meaning as mentioned above. The corresponding values of sensor sensitivity as a function of temperature were calculated and plotted in Figure 9c. Impressively, the calculated S value for green-based thermometer maintained rising in experimental temperature range from 260 to 490 K and achieved the maximum about 0.0032 K-1 at 490 K. However, the sensitivity based on stark levels in red region exhibited opposite trend, which reached 0.0029 K-1 at beginning temperature (200 K) and keep dropping as further increasing temperature up to 490 K. In light of above analysis, it can be inferred that the energy separation ∆E exert essential influence on the temperature detection range of sensors. As it can be derived from Eq. (4), the value of temperature Tmax with maximum sensitivity can be determined by using the following equation:44 d ( S ) d 2 ( FIR) ∆E ∆E = 2 = FIR ( 3 − 2) =0 3 d (T ) d (T ) KTmax KTmax

(5)

Therefore:

Tmax =

∆E 2K

(6)

It is obvious that the value of Tmax is closely related with ∆E, thus a low or high valued ∆E would extend the detection range to lower or higher temperature region. Accordingly, the theoretical maximum sensitivity would reach at 568 K for green and 71 K for red-based thermometer, which


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is accordance with variation trends shown in Figure 9c. Above results reveal that dual-color thermal sensing based on two pairs of TCLs of Er3+ ion with high sensitivity within wide temperature detection range from 71 to 568 K was perfectly realized in Er3/Yb3+ co-doped single host Ba5Gd8Zn4O21, and imply that our present phosphor could be regarded as an ideal candidate for broad-scope luminescence thermometer.

Figure 9. Dependent of temperature versus FIR (a), (b) and sensitivities (c) of green and red emissions in BGZ: 5%Er3+, 12%Yb3+ phosphor.

3.5. Effects of spectral color purity on sensitivity. Optical thermal sensing behavior is related with many unknown factors, thus the optical temperature sensing capability of samples with different R/G ratio and spectral color purity were systematically demonstrated to comprehend the effect of spectral color purity on sensor sensitivity. As for green-emitting TCLs 2

H11/2/4S3/2, it is found from Figure 10a and Figure 10b that the value of B are 6.766 and 6.979

for Ba5Gd8Zn4O21: 5%Er3+ (R/G = 0.84) and Ba5Gd8Zn4O21: 5%Er3+, 15%Yb3+ (R/G = 19.02) phosphors, and the corresponding energy difference ∆E between 2H11/2 and 4S3/2 calculated by the fitting data are about 770 and 796 cm-1, respectively. As displayed in Figure 10c, within the


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margin of error, the experimental sensitivities of two samples present similar variation trend and get to optimal 0.0032 K-1 and 0.0030 K-1 at 490 K, and nearly identify with the above result 0.0032 K-1 at 490 K of Ba5Gd8Zn4O21: 5%Er3+, 12%Yb3+ (R/G = 15.81) despite their spectral color purity are distinctly different. For the red stark levels of 4F9/2 state, similar value of sensitivities was detected in three above phosphors, as depicted in Figure S2. It is worth noting that the red emission intensity in Er3+ solely doped sample is too faint to be monitored when the temperature is over 390 K due to the thermal quenching effect.45 Additionally, the highest sensitivities of green and red TCLs in Ba5Gd8Zn4O21: 5%Er3+, 15%Yb3+ sample under a pulse laser excitation with duration time 300 µs (100 Hz) with R/G of 5.78 are equal to 0.0026 K-1 at 490 K and 0.0031 K-1 at 200 K (Figure S3), respectively, which are close to that of continuous wave excitation with R/G of 19.02. Above Results adequately imply that the spectral color purity barely exert influence on optical sensing property in Er3+/Yb3+ co-doped Ba5Gd8Zn4O21 system.


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Figure 10. Temperature dependent FIR (a) (b) and sensitivitiy in green region for Ba5Gd8Zn4O21: 5%Er3+ (left) and Ba5Gd8Zn4O21: 5%Er3+, 15%Yb3+ (right) with different color purity.

4. CONCLUSIONS In summary, Ba5Gd8Zn4O21: Yb3+/Er3+ up-conversion phosphors with variable dopant concentration were prepared by a sol-gel process. Upon the excitation of 980 nm laser diodes, the UC emission color could be tuned from green through yellow and finally to nearly pure red by increasing activator Er3+ or sensitizer Yb3+ content. Possible UC mechanisms and decay curves of green emission along with 490 nm excited emission spectra in visible and NIR region were demonstrated in detail, in which the cross relaxation and energy back transfer process play vital roles for tunable emission color. Intriguingly, a novel strategy for tuning luminescence color was also achieved using a pulse laser with tunable pulse width based on non-steady-state upconverison process, and further proved by time-dependent profiles of Er3+ emissions. Through analyzing temperature sensing behavior of two pairs of thermally coupled levels (2H11/2/4S3/2 and 4

F9/2(1)/4F9/2(2)), the temperature detection range was successfully extended from high to low

temperature region and the maximum sensitivities are found to be 0.0032 K-1 at 490 K for green and 0.0029 K-1 at 200 K for red emission. And the spectral color purity has no remarkable effect on the value of sensitivity. Results suggest that Er3+ /Yb3+ co-doped Ba5Gd8Zn4O21 UC phosphor offers perfect potential application as an optical temperature sensor which exhibits relative high sensitivity and broad temperature detection range.

ASSOCIATED CONTENT Supporting Information


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This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 86-29-88302661.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 11274251, 11504295), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No.20136101110017), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (excellent), Natural Science Foundation of Shaanxi Province (No.2014JM1004), Foundation of Shaanxi Province Educational Department (15JS101, 15JK1712) and Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094).

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TOC graphic


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Broad-Scope Thermometry Based on Dual-Color Modulation up

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