Multiwavelength Fluorescence Intensity Ratio Nanothermometry: High

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Multiwavelength Fluorescence Intensity Ratio Nanothermometry: High Sensitivity Over a Broad Temperature Range Antonio Carlos Brandão-Silva, Maria A. Gomes, Zelia Soares Macedo, Jhon F. M. Avila, José J. Rodrigues Jr, and Marcio A. R. C. Alencar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05345 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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The Journal of Physical Chemistry

Multiwavelength Fluorescence Intensity Ratio Nanothermometry: High Sensitivity over a Broad Temperature Range

Antonio C. Brandão-Silva, Maria A. Gomes, Zélia S. Macedo, Jhon F. M. Avila, José J. Rodrigues Jr., Márcio A. R. C. Alencar*

Departamento de Física, Universidade Federal de Sergipe, Cidade Universitária Prof. José Aloísio de Campos, Rod. Marechal Rondon s/n, Jardim Rosa Elze, São Cristóvão, SE 49.100000, Brazil . KEYWORDS: nanothermometry, rare earth, yttrium oxide, fluorescence intensity ratio. .

ACS Paragon Plus Environment

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ABSTRACT

The use of multiple wavelength emissions of rare-earth doped nanomaterials was investigated for optical nanothermometry, based on a ratiometric technique. A methodology, based on the luminescence associated to different pairs of thermally coupled Starks sublevels, was proposed. This new approach enabled the measurement of broad temperature ranges, in which the absolute sensitivity was optimized. The influence of this multiwavelength fluorescence intensity ratio nanothermometry on the relative temperature sensitivity was also discussed. A proof of concept of the proposed method was done using Er3+-doped Y2O3 nanocrystals as nanoprobes. The obtained

results

suggested

that,

with

suitable

choice

of

nanophosphors,

optical

nanothermometers with high and flat absolute sensitivities in a wide range of temperature can be tailored using the proposed methodology.

1. INTRODUCTION Optical nanothermometry has evolved in recent years as a useful tool for different research areas, from nanomedicine to microelectronics.1,2 Particularly, the development of efficient nanophosphors has contributed significantly to this purpose, as they play a key role on the operation of sensitive photonic temperature sensors.2 Different luminescent methodologies, such as spectral band shape3-6 and luminescence lifetime-based nanothermometers,7 have been intensively investigated. Among them, fluorescence intensity ratio (FIR) is a very effective method to probe temperature in nanoscale using the luminescence from thermally coupled pair of energy levels of a suitable emitter, usually rare earth ions.8 However, in most of the cases, the


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obtained temperature sensor may present moderate or even relatively high sensitivities, but their optimum performances are restricted to a narrow temperature range or even to a single value of temperature. For several practical purposes, this is an undesirable feature as this may limit the possible uses for such device. FIR technique is based on the dependence of the population densities of two different excited states, which are thermally coupled with the environment temperature.8 For rare earth doped materials, the electronic states associated to the 4f levels of rare earth ion are used to measure temperature based on this method. For a myriad of hosts, the emissions from such excited states are characterized by relatively broad spectra due to the existence of multiple Stark sublevels. For very small nanocrystalline hosts, in particular, these sublevels can be separated owing to the local crystalline field to which the rare-earth ion is submitted. In these cases, sets of narrow spikes can be observed instead of a smooth emission band. Most of the proposed FIR systems are based on this collective emission of the multitude of Stark sublevels, and the energy gap that determines the sensor sensitivity is the energy difference between the center of the two excited energy levels. Many studies focused on the development of more efficient materials or strategies for nanothermometry. For instance, a significant sensitivity improvement can be achieved simply exploiting two specific Stark sublevels, instead of using the overall emissions from the entire bands.9-12 Another interesting strategy exploits pairs of thermally uncoupled levels

13,14

or from

distinct emitter ions.15 However, very few efforts have been reported on a methodology that would provide a luminescent-based temperature sensor with a broad temperature range in which the device sensitivity is high.16 - 18 In particular, Suo et al. gave an important contribution to this aim, demonstrating that it is possible to use this kind of technique within a broad range of


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temperatures, exploiting two pairs of erbium ions' Stark sublevels.16 Using this strategy, the temperature scope was broad, but the achieved sensitivity was not very high, neither as flat as desirable. Moreover, the influence of this approach on the sensor’s relative sensitivity was not analyzed. Nevertheless, the use of two sets of emission pairs is very promising and must be exploited and extended to other materials, rare earth ions and different Stark levels. The present work investigates the use of the multiwavelength luminescence of nanophosphors to produce nanothermometers with a high and flat sensitivity over a wide range of temperature. The central idea is to explore the multiple emissions due to the transitions between Stark sublevels of rare earth-inserted in nanocrystals, as well as their different behaviors due to the nanoenvironment temperature. We demonstrate that combining distinct pairs of Stark sublevels, the sensitivity of FIR-based temperature sensors can be controlled and a device with improved characteristics can be designed. The impact of this approach on the relative sensitivity was also investigated. These predictions were experimentally verified by analyzing the temperaturedependent infrared-to-green-and-red upconversion emission from erbium-doped yttrium oxide nanoparticles and the influence of the particle size on the multichannel sensor is also discussed.

2. PROPOSED METHOD The principle of FIR method can be described as follows. Consider that a luminescent material can be described by a three-level system as shown in Figure 1, in which the ground state is represented by |0, the excited state with higher energy is |2 and the lower energy excited state is |1. Typically, if the excited states are thermally coupled, the energy gap between |2 and |1, ∆E , must be smaller than 2000 cm-1. Under such condition, the ratio between the population densities of these levels can be described by a Maxwell-Boltzmann distribution. As the


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luminescence intensity is proportional to the population density, the intensity ratio of these two levels as a function of the environment temperature is given by 8

=

=

−∆ ⁄ ,

in which is the Boltzmann constant, and

(1)

!

and ! are, respectively, the frequency and

intensity of the luminescence decay from level i. For each excited state i, "! is the spontaneous emission rate and #! is the degeneracy. The experimental efficiency collection of each luminescence band is given by $! %! . For practical purposes, it is suitable to use the simplified form of this equation

= & −'/,

(2)

where α and B are empirical constants obtained directly from the experimental data. By comparison, it can be shown that these quantities are related to the pre-exponential factor and the exponent of Eq. (1). However, it has been demonstrated that although the exponential profile is still valid, the pre-exponential factor defined in Eq. (1) is a simplification, and this quantity also depends on the environment temperature.19 Hence, for different systems, the empirical Eq. (2) can be still employed, but the parameters B and α are no longer equal to the pre-exponential factor and the exponent defined by Eq. (1), respectively. The sensor performance can be evaluated by calculating its absolute ()* ) and relative sensitivity ()+ , which are given by )* =

, -.

=

1

0

,/

0

/

,

(3)

and )+ = 2 = . / -.

(4)


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The absolute sensitivity presents a maximum value, )*5*6 = 4B/ αe , achieved at 5*6 = α/2. Hence, a suitable choice of Stark sublevels may enhance the maximum absolute sensitivity and the relative sensitivity of a FIR-based sensor.

|2〉 |1〉

∆E .

I2

I1 |0〉

Figure 1. Illustration of a three-level system.

Let us assume a medium which presents three luminescent bands when excited properly. Consider that each emission band is related to transitions from three excited state bands to the ground state of a rare earth ion and that is possible to identify an internal splitting of the luminescence due to the Stark sublevels. In Figure 2(a), an illustration with the spectrum of this hypothetic system is presented. In this illustration, we identify six emission lines, two per band, which are associated to different Stark sublevels of the corresponding band. Assume also that the bands 1 and 2 are thermally coupled, as well as each pair of Stark sublevels. Due to these considerations, the luminescence of this system varies with the environment temperature.


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In principle, it is possible to use luminescence associated to any pair of thermally coupled excited state levels to measure temperature using FIR method. For this idealized system, we identify at least seven combinations of pairs of emission lines that fulfill this requirement, besides the overall spectra from bands 1 and 2. In our approach, each of those pairs is identified as a channel for temperature measurement. The energy gap associated to each pair of levels was chosen to be distinct, hence the behavior of FIR will be different for all channels, as well as their absolute sensitivities will reach maxima at different values of temperatures. This is summarized in Table 1. Now let Ik be the light intensity from the line emission k. The hypothetic FIR responses, given by each channel, as well as their corresponding absolute sensitivities are presented in Figure 2(b) and 2(c), respectively. As can be observed, the larger the energy separation between the thermally coupled levels, the higher is the value of Tmax. It was chosen that a channel’s index is related to the energy gap between the corresponding pair. The larger the index the larger is ∆E. If only two channels were employed to measure temperature as proposed in ref 16, for instance channels 3 and 6, a broad scope temperature sensor can be obtained, but with sensitivity highly dependent on the temperature. On the other hand, if more than two channels were exploited, it would be possible to design a FIR based temperature sensor with flat sensitivity in a large temperature range, as it is highlighted by the rectangle in Fig. 2(c). In summary, to employ this methodology it is necessary to: 1) characterize the FIR responses ascribed to the different thermally coupled pairs and 2) identify the best channel combinations aiming to achieve high sensitivity in a broad temperature range. However, the use of multiple channels would have an impact on the sensor’s relative sensitivity. Indeed, analyzing Eqs. (3) and (4), the differences on the maximum sensitivity and


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relative sensitivity are mutually associated to the values of the parameter ', which can vary from channel to channel. Therefore, although the multichannel approach may lead to high absolute sensitivity over a broad temperature range, this may not be the case when analyzing the relative sensitivity, as can be inferred from Figures 2(c) and 2(d).

Figure 2. (a) Spectral emission of the hypothetic system. (b) FIR curves considering seven combinations of thermally coupled Stark sublevels' pairs, each one with a distinct energy separation. Calculated (c) absolute and (d) relative sensitivities of the seven hypothetic channels. On the other hand, the energy splitting due to Stark effect is not very dramatic. Although, in principle, it is possible to identify the contribution of different excited Stark sublevels to the observed luminescence, separate them is not an easy task. Indeed, it was reported in previous works that the temperature dependence of the FIR signal can be affected by the overlap of emission lines from the two thermally coupled emitting levels and stray light from other levels or


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the excitation source. In these cases, the temperature dependence of the luminescence intensity ratio can be modeled simply by adding an offset to the Boltzmann distribution8

=

0

= < > & + < / =

5 =

0

> = @ + .

(5)

Here it is assumed that it is not possible to separate, in terms of wavelength, the individual contribution from energetically close thermally coupled levels. The measured intensity luminescence by the detector (I2), which was in principle associated only to the transition from the upper level (2), is indeed composed by the combined contribution from both levels and viceversa. The fraction of I2 that is only due to the transition from the upper level to the ground state is n2; similarly, the fraction of I1 owing to the transition from the lower level to the ground state is n1; m1 defines the fraction of the total intensity associated to the transition from level 1 which is measured by the detector for other thermalizing level.

Table 1. Summary identifying each FIR channel with their corresponding band, lines and intensity ratio of the hypothetic system.

Channel Levels’ bands 3 1 2 2 1 3 1 and 2 4 1 and 2 5 1 and 2 6 1 and 2 7

lines

FIR

5 and 6 3 and 4 1 and 2 2 and 3 2 and 4 1 and 3 1 and 4

I5/I6 I3/I4 I1/I2 I2/I3 I2/I4 I1/I3 I1/I4


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While the addition of a constant value to the FIR equation seems to be a minor modification to the original model, the impact of this overlapping effect on the sensor operation can be huge. Indeed, the absolute and relative sensitivities can now be defined as )* =

, -.

= @ / < / >,

12

B E6F< > D D . C B E6FHI

,/

0

A0

(6)

and )+ =

-.

=

C

GC

(7)

D

For the absolute sensitivity, the luminescence overlapping can cause essentially a reduction of Smax. The stronger the overlapping the smaller the value of (n2/n1) ratio and the less sensitive the sensor is. On the other hand, the absolute sensitivity dependence with temperature is not affected. Hence, although the overlap can reduce the overall absolute sensitivity, a careful choice of sublevels may still lead to temperature sensor with optimized performance over a broad temperature range. The modifications on the relative sensitivity must be analyzed carefully. From Eq. (7), )+ can present a maximum point that can be obtained solving a transcendental equation. It can be also noticed that at elevated temperature values, Eq. (7) approaches to Eq. (3) and the luminescence overlap effect is negligible to this quantity. However, at low temperatures, the relative sensitivity might present very low values due to the emission superposition. Therefore, care must be taken whenever one decide to exploit thermally coupled levels that present significant luminescence overlap to the development of FIR-based temperature sensors.


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It must be stressed that in this methodology the number of available channels, the FIR and Sa and Sr are strongly dependent on the physical and chemical properties of the host and the rareearth ions. For instance, size,19,20 shape

21

and crystalline phase

22

are just a few examples of

variables that can modify the sensor performance significantly. However, a suitable choice of nanophosphor and FIR channels based on Stark sublevels would certainly enhance the temperature operation range and the sensitivity’s flatness of the developed multiwavelength nanosensor.

3. EXPERIMENTAL This methodology was experimentally verified using Y2O3:Er nanocrystals as the temperature probes. The samples were prepared by means of a modified PVA-assisted sol-gel route. The synthesis procedure involves the dissolution of Y(NO3)3 (Alfa Aesar, 4N) and ErCl3.6H2O (Sigma-Aldrich, 4N) in distilled water according to the formula (Y1.96Nd0.02)O3 in a concentration of 0.2 g/mL. A PVA solution (10% w/v) was added to the metal solution keeping magnetic stirring for 30 min. After that, the viscous gel was dried and calcined at 600 and 1000 °C for 5 h in order to obtain the yttrium oxide crystals with different particle sizes, which will be referred hereafter as YOEr600 and YOEr1000, respectively. Simultaneous differential thermal analysis (DTA) and thermogravimetry (TG) were performed in the dried materials on a SDT 2960/TA Instruments equipment, using a heating rate of 10 °Cmin-1, in a flow of synthetic air, from room temperature up to 1000 °C. XRD patterns were obtained in a Rigaku RINT 2000/PC diffractometer using Co Kα radiation, with beam voltage and current of 40 kV/40 mA, in a 2θ range of 10 to 80°, step width of 0.02° and counting time of 10 s. Transmission electron microscopy (TEM) images were acquired on a JEOL JEM-1400 plus


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equipment operating at 120 kV, equipped with a CCD camera (Gatan) and the powder suspension was deposited onto a holey carbon film-coated 400 mesh cooper grid. For the temperature-dependent luminescence experiment, the produced powders were conformed as roughly small discs with 1 mm thickness. The discs were kept under controlled temperature while excited under a CW diode laser with 379 mW, emitting at 800 nm. The light beam was focused on the disk surface at a 60° incident angle. The infrared to green and red upconverted emission was analyzed by a compact CCD spectrometer and the sensor characteristics, using different FIR channels, were obtained. All measurements were performed isothermally, under the same excitation and fluorescence collection conditions, in the temperature interval of 20 to 230 ºC.

4. RESULTS AND DISCUSSION The polyvinyl alcohol (PVA), used in the synthesis of samples, is a synthetic polymer that contains one hydroxyl group per monomer. When mixed to the solution of precursor nitrates, the negatively-charged OH– groups of PVA are capable of solvate the metal ions, forming metallic hydroxide nuclei. Figure 3(a) presents the thermal profile of this material during the synthesis, investigated via DTA/TG curves of the dried powder. Two distinct regions can be noticed. The first one corresponds to the thermal events occurring from 200 to 600 °C, characterized by two endothermic peaks and a mass loss of 53% approximately. These events are possible related to decomposition reactions of yttrium hydroxide, according to equation 2.Y(OH)3 Y2O3 + 3H2O,


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and residual PVA.23 From 600 to 1000 °C, corresponding to the second region, no further thermal event was observed. Based on thermal analysis, fully crystallized samples could be obtained from 600 ºC. This fact was confirmed by the XRD patterns of the samples treated at 600 and 1000 ºC, showed in Figure 3(b). As can be observed, all the diffraction peaks were indexed to the cubic yttrium oxide, in accordance with the ICDD card No. 41-1105. No diffraction peak from erbium oxide was detected, confirming the complete incorporation of Er3+ ions in Y2O3 matrix.

Figure 3. (a) DTA/TG curves of dried xerogel. (b) XRD patterns of samples calcined at 600 and 1000 °C for 5 h. Figure 4 presents the TEM images of YOEr600 (Fig. 4(a)) and YOEr1000 (Fig. 4(b)) samples. In the images, it can be noticed the formation of roughly spherical particles with increasing size as the temperature of calcination was raised. Based on a set of images from different regions (not shown), the average particle size could be estimated as 21 and 86 nm, respectively, for YOEr600 and YOEr1000.


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Figure 4. TEM images of YOEr600 (a) and YOEr1000 (b) samples.

Figure 5 presents the luminescence of the YOEr1000 and YOEr600 nanocrystals measured at 296 K, as well as the distinct FIR curves of the investigated channels. Three typical emission bands associated to the transitions from the 2H11/2, 4S3/2 and 4F9/2 excited states to 4I15/2 ground state of Er3+ are observed. The luminescence mechanisms exhibited by these two samples were discussed in detail in a previous work. 24 As highlighted in this figure, two emission peaks were selected per band and the temperature-dependent relative emission of pairs of lines were associated to a FIR sensor channel. The overall behavior of the bands 1 and 2, which are thermally coupled, was also associated to another channel. Table 2 presents the analyzed emission wavelengths, as well as their corresponding bands, luminescence intensity ratios and channels' label. In this case, Ik corresponds to the light intensity of the line k and Aj is the integrated intensity of the band j. It should be noticed that the YOEr600 luminescence signal was weaker. Indeed, owing to the low signal of the red emission, the FIR analysis related to the channel 2 could not be evaluated for the sample with smaller crystallites. On the other hand, all the other YOEr600 channels displayed similar FIR behavior of YOEr1000 sample.


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Figure 5. Emission spectra of (a) YOEr1000 and (b) YOEr600 samples. (c) FIR curves for YOEr1000 sample of the seven channels associated to the thermally coupled Stark sublevels (CH1-7) and the one calculated considering the complete emissions from 2H11/2 and 4S3/2 levels (CH0). (d) FIR curves for YOEr600 sample of the six channels associated to the thermally coupled Stark sublevels and the one calculated considering the complete emissions from 2H11/2 and 4S3/2 levels (CH0). Channel 2 could not be investigated for this sample due to its low signal level.


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Table 2. Summary identifying each FIR channel with their corresponding band, lines, emission wavelength and intensity ratio.

Channel 0 1 2 3 4 5 6 7

Levels’ bands 2

H11/2 and 4S3/2 4

S3/2 F9/2 2 H11/2 2 H11/2 and 4S3/2 2 H11/2 and 4S3/2 2 H11/2 and 4S3/2 2 H11/2 and 4S3/2 4

Lines Whole bands 3 and 4 5 and 6 1 and 2 2 and 3 2 and 4 1 and 3 1 and 4

Emission wavelength (nm)

FIR

530 and 555

A1/A2

553 and 563 654 and 660 522 and 538 538 and 553 538 and 563 522 and 553 522 and 563

I3/I4 I5/I6 I1/I2 I2/I3 I2/I4 I1/I3 I1/I4

As stressed in section 2, a careful analysis of the Stark sublevels’ emission lines must be performed to identify accurately the contribution of different thermally coupled levels to the signals exploited on the FIR temperature sensing. If no significant overlap occurs among the emission lines of a chosen channel, the corresponding data must be analyzed using equations (2), (3) and (4). On the other hand, if the channel’s lines present significative superposition, equations (5), (6) and (7) must be employed. The Stark splitting of Er-doped Y2O3 have been investigated previously25. Of particular interest for our work are the energy separation among the Stark sublevels of the 4I15/2, 4F9/2, 4S3/2 and 2H11/2 bands. The combined transitions between the sublevels of the above mentioned


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excited states to the 4I15/2 ground state produce the rich spectrum that here is exploited to measure temperature. A detailed description of this analysis is presented in the supplementary material of this work. Based on that analysis, it was possible to identify that overlapping between the emission lines of the thermally coupled Stark sublevels occurred in CH1, CH2 and CH3. Indeed, for CH1, line 3 is formed by a combination of 4S3/2(1) and 4S3/2(2) emissions to ground state sublevels. Similarly, for CH 2, line 5 is a combination of emission lines from 4F9/2(5), 4F9/2(4) and 4F9/2(3), while line 6 is composed by emissions from 4F9/2(4), 4F9/2(3), 4F9/2(2) and 4F9/2(1) sublevels. In CH3, on the other hand, an interesting behavior can be observed. Due to the splitting, the six Stark’s sublevels can be divided in two groups, higher energetic 2H11/2(6), 2H11/2(5) and 2H11/2(4), and lower energetic 2

H11/2(3), 2H11/2(2) and 2H11/2(1) sets of sublevels. By analyzing the luminescence wavelength from

these groups, it is possible to identify that, even in this case, emission lines 1 and 2 must present contributions from both groups. On the other hand, the emission bands from distinct excited states do not present significant overlap. Indeed, from the results presented in the supplementary materials, it can be observed for the closest emission lines belonging to 2H11/2 and 4S3/2, in terms of wavelength, are separated by ≈ 6 nm. Comparing the emission lines from 4S3/2 and 4F9/2, the separation of the closest lines is ≈ 83 nm. Consequently, no overlap between lines from distinct bands was considered in the following analysis. As expected, different emission lines present distinct energy gaps as well as FIR slopes, indicating that the sensitivities are not equal among channels. Using Eq. (1), the FIR sensor parameters for CH0, CH4, CH5, CH6 and CH7 channels were obtained, while Eq. (5) was used


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to characterize the results from CH1, CH2 and CH3. A summary of the results is presented in table 3 and 4. As it can be observed, the larger the values of α, the steeper is the channel FIR curve in logarithm scale. As expected, Y2O3 particles presented slightly distinct sensor parameters.24 Moreover, the CH0 curve, associated to the overall bands response presents moderate slope and FIR absolute values, which indicates that there should be channels that present higher absolute and relative sensitivities than CH0 along the whole temperature operation range. Nevertheless, the performance of all channels in comparison with the CH0 must be deeper analyzed.

Table 3. FIR parameters for investigated channels of YOEr1000 sample.

Channel 0 1 2 3 4 5 6 7

α (K) 1087±11 400±100 600±300 720±90 926±9 1082±12 1344±16 1501±19

β = ln B 2.72±0.03 2.76±0.02 2.55±0.03 3.50±0.04 3.26±0.05

D 0.61±0.03 1.7±0.5 2.6±0.3 -

E 0.34±0.05 0.6±0.2 0.27±0.05 -

Tmax (K) 543.5 200 300 360 463 541 672 750.5

Smax(x10-4 K-1) 76 ± 2 8.3 ± 0.3 15 ± 3 20 ± 3 92 ± 2 64 ± 2 133 ± 6 94 ± 5

Table 4. FIR parameters for investigated channels of YOEr600 sample.

Channel 0 1 3 4 5 6 7

α (K) 1089±10 600 ± 200 500 ± 100 928 ± 10 1103 ± 9 1309±12 1485±12

β = ln B 2.67±0.27 2.71 ± 0.02 2.55 ± 0.02 3.27 ± 0.03 3.11 ± 0.03

D 0.8±0.1 1.9±0.2 -

E 0.38±0.04 0.1±0.1 -

Tmax (K) 544.5 300 250 464 551.5 654.5 742.5

Smax (x10-4 K-1) 71 ± 1 7±3 21 ± 5 88 ± 2 63 ± 1 109 ± 3 82 ± 2


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Using the results from Table 3 in Eqs. (3) and (4) the absolute and relative sensitivities of CH0, CH4, CH5, CH6 and CH7 were calculated, for temperatures below 1273 K, that is, below the calcination temperature of YOEr1000. The CH1, CH2 and CH3 sensitivities were obtained employing the results from Table 3 in Eqs. (6) and (7). In Figure 6, the behavior of calculated Sa and Sr for all YOEr1000 channels are presented. It must be emphasized that absolute sensitivity of each channel possesses a characteristic behavior with very distinct values of Tmax and Smax. The original proposal of FIR temperature sensing exploits the complete emission from 2H11/2 and 4S3/2 sublevels. This methodology is represented by the response of CH0 in our study. As can be observed, for YOEr1000 sample, the value of Smax is quite high, 76 x 10-4 K-1 and the values of Sr are also high in comparisons with other systems.24 However, other channels present better performance. As expected, the channels associated to low energy gaps (CH1, CH2 and CH3) present better performance (higher absolute sensitivity) at low temperatures (< 130 K). On the other hand, high energy gaps channels have greater absolute sensitivities for temperatures above 300 K. Another important feature is that channels 4, 6 and 7 have the highest values of absolute sensitivities, even higher than CH0. It should be also noticed that the larger the value of the constant α, the higher the relative sensitivity of the corresponding channel is. For this quantity, there is no crossover among the channels curves. Moreover, although this quantity is approximately the same for all channels measured at higher temperatures, its value can be very different comparing to distinct channels at temperatures below 500 K. From this complete picture, it is possible to implement sensors with distinct optimized characteristic, choosing the suitable combination of channels for that purpose.


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Figure 6. Calculated (a) absolute and (b) relative sensitivities of the YOEr1000 FIR channels. Considering the performance of CH6 alone, it presents the highest values of Smax and Sr. Although this channel exhibits the highest absolute sensitivity between 380 and 1273 K, it presents an inferior performance for temperature values smaller than 380 K. Moreover, despite of the fact that the relative sensitivity increases as the temperature decreases, its variation is quite high using only CH6 as a temperature sensor. In this case, Sr varies continuously from 13.44 % K-1 to 0.083 %K-1 within 380-1273 K. Confining the analysis to absolute sensitivity, Ref 16 proposed that the sensor performance could be improved exploiting CH1 and CH6. However, owing to the differences among the nanocrystalline probes and the experimental setups, the optimized operation temperature subintervals used in the present work is not the same employed by Suo et al. Moreover, as CH1 is affected by luminescence superposition among the exploited thermally coupled Stark levels, we have restricted the temperature operation range between 120 K and 672 K, in such a way that


20

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CH1 is used between 120 K and 180 K, while CH6 is employed in the range of 180 K to 672 K. This would give a wider operation range and higher values of Sa, but with wider variation amplitude, with maximum (Smax) of 129.4 x 10-4 K-1 and minimum (Smin) equal to 6.0 x 10-4 K-1, in comparison to Ref 16. However, in our proposal, this optimized operation range can be enlarged using these two channels up to 1153 K. Here, we do not limit the operation interval to the Tmax of the channels. Instead, fixing the minimum value of Smin to 6.0 x 10-4 K-1, the lower temperature interval limit would be 120 K, while the higher operation temperature is limited to the calcination temperature, of 1273 K. It must be emphasized that the channels do not need to be chosen in pairs. Keeping that in mind, different combinations can be exploited aiming different responses. For instance, consider the following channels: CH5 (from 250 K to 350 K), CH7 (from 350 K to 500 K) and CH4 (from 500 K to 1270 K). This multichannel combination would provide an operation range of 1020 K, with Smax of 91 x 10-4 K-1 and Smin = 29.2 x 10-4 K-1. This result would keep the same minimum value for absolute sensitivity as achieved in Ref. 16, but a much larger temperature scope and higher values of Sa within this interval. If narrower absolute sensitivity variation was desired, different choices of channels could be exploited, optimizing this quantity in distinct temperature scopes. A summary of different combinations exploited in this work is presented in Table 5. The same analysis was performed using YOEr600 samples. As can be observed on Table 4, this material presents less intense luminescence and slightly smaller values of Smax than YOEr1000 under the same experimental conditions. Moreover, its use as a temperature sensor has a different operation temperature upper limit, chosen to be 870 K, as this calcination


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temperature of the material was 873 K. Those differences imply that the multichannel results using YOEr600 presents, in principle, smaller values of absolute and relative sensitivities, as well as small temperature operation range if elevated temperature measurements were needed. On the other hand, due to its smaller dimension, these nanocrystals are more suitable for temperature measurements of highly localized sites and, at some channels configuration (see Table 5), the variations on Sa and Sr can be relatively small.

Table 5. Characteristics of some analyzed multichannel configurations. Temperature range (K)

Smax

Smin

Srmax

Srmin

(x10-4 K-1)

(x10-4 K-1)

(% K-1)

(% K-1)

1 and 6

120 to 672

129.4

6.0

4.1

0.16

1 and 6

120 to 1270

129.4

26.8

4.1

0.08

4, 5 and 7

250 to 1270

91.0

29.2

1.73

0.06

YOEr1000 1, 4 and 6

120 to 1270

129.4

6.0

5.48

0.08

2 and 3

160 to 980

19.5

9.3

0.40

0.06

4 and 6

280 to 1270

129.4

68.3

1.27

0.08

4 and 7

330 to 1270

110.9

81.2

0.85

0.09

1 and 6

200 to 490

32.1

17.1

1.46

0.20

1 and 6

120 to 870

109.0

2.8

4.53

0.05

3 and 5

120 to 870

63.0

10.2

4.90

0.14

3 and 6

120 to 870

109.0

10.2

2.70

0.17

3, 4 and 6

120 to 870

109.0

10.2

4.12

0.17

4 and 6

250 to 870

109.0

54.5

1.48

0.17

4 and 7

340 to 870

88.0

78.7

0.80

0.19

Sample

Channels’ combination

Ref. 16

YOEr600


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It is important to notice that this multichannel approach have a significant impact on the relative sensitivity of the sensor. The change from one channel to another produces abrupt changes on this sensor quantity at the switching channels’ temperature. To illustrate this fact, it is presented in Figure 7 the absolute and relative sensitivities of three multichannel combinations: Ref. 16 and YOEr1000 sample’s CH2-CH3 and CH4-CH6. In all cases, the ratio between the maximum and minimum value of Sa was approximately equal to 2. As can be observed, the appropriate choice of nanoprobe and lines (channels) may lead to improved sensors performance: higher values of Sa within even broader scopes. Additionally, the use of multiple channels clearly induces jumps on the relative sensitivity. However, the magnitude of this abrupt change on the Sr depends on the difference between the values of the sensor parameter α of the employed channels. The smaller this difference is, the less abrupt is this variation.

Figure 7. Calculated (a) absolute and (b) relative sensitivities of the YOEr1000 multichannel FIR sensors, using the CH2-CH3 and CH4-CH6 channels’ combinations and the results using the data of Ref. 16.


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Owing to our technical limitations, we could only analyze experimentally temperature nanosensors operating in the range between 295 and 495 K. Within this region, it was possible to combine only CH4 and CH6 to increase the sensor performance. Figure 8 presents the experimental absolute and relative sensitivities of the channels CH4, CH6 and CH0 within the temperature range between 295 and 497 K employed in this work. Let us use the Smax of CH0, 77 x10-4 K-1, as a sensitivity inferior limit parameter, and seek for the configuration that would allow the highest sensitivities with the widest temperature range. As can be observed, using only CH6, the temperature sensor operates with absolute and relativity sensitivities variations smaller than 0.004 K-1 and 0.5 %K-1, respectively, within a scope of 142 K. On the other hand, CH4 alone display a relatively flat absolute sensitivity behavior (∆Sa = 0.0015 K-1) and 0.6 %K-1 relative sensitivity variation, but inferior values of absolute sensitivity above 355 K and smaller relative sensitivity in comparison with CH6.

Figure 8. Experimental (a) absolute and (b) relative sensitivities of the YOEr1000 FIR channels CH0, CH4 and CH6.


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Using a multichannel approach, the sensor performance can be optimized. Mixing CH4 and CH6 responses, the sensor absolute sensitivity was maximized through the whole investigated temperature range. The device would operate using CH4 from 310 to 395 K and exploiting CH6 from 395 to 495 K. Moreover, in this case, the same absolute and relative sensitivity variation of CH6 was achieved, but the operation temperature scope was increased by 47 K, which is a much better performance in comparison with the conventional band-shape approach. The same procedure was performed with YOEr600 sample. In Figure 9, the experimental results for absolute and relative sensitivities are presented. It can be observed that the combination of CH4 and CH6, in this case, increases in up to 45% the scope range operating above the 0.007 K-1 in comparison with response of CH6 alone. In this case, the sensor would operate using CH4 from 300 to 400 K and exploiting CH6 from 400 to 483 K. The overall absolute and relativity sensitivities variations were 0.0027 K-1 and 0.46 %K-1, which are smaller than the observed for sample with larger nanocrystals. This indicates that the choice of the nanoparticles' size is also a key factor to be considered in the design of multichannel FIR sensors.

Figure 9. Experimental (a) absolute and (b) relative sensitivities of the YOEr600 FIR channels CH0, CH4 and CH6.


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A comparison between the results obtained using this multichannel approach to previous work reported in literature focusing on a single pair of emission lines was also performed. In terms of absolute sensitivity, the values of Smax obtained in our work, 133 x 10-4 K-1 and 109 x 10-4 K-1 are relatively high in comparison with previous works exploiting Er3+ emissions,24, 26-28 but slightly smaller than the result reported by Pandey et al using Er3+–Yb3+-codoped SrWO4.29 Although this was a very interesting result, if exploiting more than on pair of lines, they would probably be able to reduce even more the Sa variation in their system. On the other hand, focusing on the Sr values, which are the best comparison parameter among sensors based on distinct detection systems, both Er-doped Y2O3 nanoprobes investigated in our work present a better behavior, as the values of α obtained here are larger than the reported value in Ref. 29. The same analysis can be extended to the result of Ref. 28, in which by using a multichannel approach, Lu et al would be also able to optimize their sensing capability.

5. CONCLUSION In summary, we have proposed a new temperature sensor methodology based on multichannel fluorescence intensity ratio. Using Er3+-doped Y2O3 nanocrystals as test probe, we have shown that, exploiting more than a pair of emission lines associated to different Stark sublevels it is possible to design sensors with very broad temperature scope, optimizing their absolute sensitivity within different temperature ranges. Moreover, a suitable choice of channels combinations may provide devices with small sensitivity variation within a relatively wide temperature operation range. It was also observed that the use of a multichannel affects the sensor’s relative sensitivity, inducing abrupt jumps in its value at the switching temperatures. The effect of luminescence overlap among Stark sublevels was also analyzed. It was verified


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that, although it is possible to exploit these sublevels in the multichannel approach, care must be taken, as the absolute and relative sensitivities are reduced due to the overlap. Additionally, we confirmed that the variation of particles size also affects the multichannel optimization. Our results indicate that improved temperature sensors, with high absolute and relative sensitivities but small variation throughout a very wide temperature range can be designed applying this methodology with a suitable choice of nanoprobe species.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Marcio Alencar: 0000-0003-1436-9901 Jose Joatan Rodrigues Jr.: 0000-0001-6473-9339 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements


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The authors acknowledge CAPES, CNPq, FINEP and FAPITEC/SE for the financial support. This work was performed in the framework of the National Institute of Photonics (INCT de Fotonica), Grant Number 465763/2014-6, MCTI/CNPq/FACEPE. . REFERENCES (1) Childs, P. R. N. In Thermometry at the Nanoscale: Techniques and Selected Applications; Carlos, L. D.; Palacio, F., Eds.; Royal Society of Chemistry: Cambridge, 2016; pp 3-15.

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Multiwavelength Fluorescence Intensity Ratio Nanothermometry: High

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