The Impact of Cr3+ Doping on Temperature Sensitivity Modulation in Cr3+ Doped and Cr3+, Nd3+ Co-doped Y3Al5O12, Y3Al2Ga3O12, and Y3Ga5O12 Nanothermometers

Introduction

In response to the requirements imposed by technology, micro/nanoelectronics or photonics as well as by biomedical applications, new approaches to the luminescent nanothermometers (LNTs) have to be proposed to secure fast and accurate temperature sensing with submicrometer spatial resolution, and highly sensitive temperature readout (Brites et al., 2012; Jaque and Vetrone, 2012; Chen et al., 2016; del Rosal et al., 2016a,b; Dramićanin, 2016; Marciniak et al., 2016b; Suo et al., 2017; Wang et al., 2017; Gao et al., 2018; Liao et al., 2018; Liu et al., 2018; Malysa et al., 2018; Runowski et al., 2018; Zhong et al., 2018). One of the most promising one, relies on exploiting transition metal (TM) ions, whose highly temperature dependent emission is referred to emission of barely temperature dependent lanthanides ions (Marciniak et al., 2017a; Drabik et al., 2018; Elzbieciak et al., 2018; Kniec and Marciniak, 2018; Marciniak and Trejgis, 2018; Trejgis and Marciniak, 2018). Materials which could be applied as real time temperature sensors in biomedicine, must also accomplish some other important requirements like sufficient sensitivity to temperature changes, high stability, low cytotoxicity (Brites et al., 2012; Jaque and Vetrone, 2012; Benayas et al., 2015) and operation in spectral range of optical transparency windows of biological tissues (Anderson and Parrish, 1981; Jaque and Jacinto, 2016). Because the transition metal ions based luminescent nanothermometers meet abovementioned flagship demands, they can be considered as distinctively attractive research area. However, in order to be applicable, the thorough understanding of the correlation between structure of the host material and the thermal quenching of luminescence has to be studied. In our previous work (Elzbieciak et al., 2018), we have shown that by appropriate adjustment of the stoichiometry of the host matrix, relative sensitivity of Cr³⁺ ions-based luminescent thermometer can be intentionally modulated. The presented tuning occurred as a result of modification of the metal-to-oxygen ionic distance, which modified the strength of the crystal field (CF) (Struve and Huber, 2000; Xu et al., 2017). Therefore, taking advantage from the fact that CF influences the position of ⁴T₂ parabola, the activation energy, responsible for determination of thermal stability of luminescence, can be deliberately reduced. Efficient thermal quenching of the luminescence intensity is beneficial for sensitive luminescent thermometry. As it was already presented (Marciniak and Bednarkiewicz, 2016; Marciniak et al., 2016a, 2017b; Azkargorta et al., 2017), significant changes of the CF strength may also result from rising the concentration of active or passive dopants. Recently, Deren et al. (2012) showed that the increase of the Cr³⁺ concentration in Y₃Ga₅O₁₂:Cr³⁺ causes the diminishment of ²E → ⁴A₂ narrow emission band and enhancement of ⁴T₂→⁴A₂ band's intensity. Lowering the CF strength by growing number of Cr³⁺ ions, which is related with the elongation of the M-O distance, should relevantly enhance the relative sensitivity to temperature changes of such Cr³⁺ based luminescent thermometer. These observations motivated us to verify this hypothesis through comprehensive investigations of the impact of Cr³⁺ concentration (0.01–50%) on the temperature sensing capability in Y₃Al₅O₁₂, Y₃Al₂Ga₃O₁₂ and Y₃Ga₅O₁₂ nanocrystals.

Experimental

Nanopowders of (0.1; 0.5; 2; 5; 10; 20; 50%) Cr³⁺ and (0.1; 0.5; 2; 5; 10; 20; 50%) Cr³⁺,1% Nd³⁺ doped Y₃Al₅O₁₂, Y₃Ga₅O₁₂ and (0.06; 0.3; 1.2; 3; 6; 9; 12; 30%) Cr³⁺, (0.06; 0.3; 1.2; 3; 6; 9; 12; 30%) Cr³⁺, 1%Nd³⁺ doped Y₃Al₂Ga₃O₁₂ garnets were synthesized by the modified Pechini method (Pechini, 1967). Whole synthesis procedure was analogous as described in our earlier work (Elzbieciak et al., 2018). In brief, in order to obtain metal nitrates, calculated amount of yttrium oxide and neodymium oxide were dissolved in deionized water with addition of ultrapure nitric acid. After triple recrystallization process, yttrium or yttrium and neodymium nitrates were mixed together with aluminum, gallium and chromium nitrates. Afterwards, aqueous solution of citric acid and PEG was added to the mixture, than it was stirred for 3 h. In order to form a resin the obtained solution was heated for 1 week at 90°C.

All the synthesized materials were annealed for 16 h at 850°C. The following starting materials were used for synthesis: yttrium oxide (Y₂O₃ with 99.995% purity from Stanford Materials Corporation), neodymium oxide (Nd₂O₃ with 99.998% purity from Stanford Materials Corporation), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O Puratronic 99.999% purity from Alfa Aesar), gallium(III) nitrate nonahydrate (Ga(NO₃)₃·9H₂O Puratronic 99.999% purity from Alfa Aesar), chromium nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 99.99% purity from Alfa Aesar), citric acid (C₆H₈O₇ with 99.5+% purity from Alfa Aesar) and poly(ethylene glycol) (PEG C₂H₆O₂ BioUltra 200 from Sigma).

All of the obtained materials were examined by XRD (X-ray diffraction) measurements carried out on PANalitycal X'Pert diffractometer, equipped with an Anton Paar TCU 1000 N temperature control unit, using Ni-filtered Cu-K_α radiation (V = 40 kV, I = 30 mA). Transmission electron microscope (TEM) images were taken using FEI TECNAI G2 X-TWIN microscope equipped with EDS detector. Powders were dispersed in methanol solution in ultrasounds and applied for lacey type copper lattices. The studies were performed in conventional TEM microscope with 300 keV parallel beam electron energy. Images were digitally recorded using the Gatan Ultrascan 1000XP.

Excitation spectra were measured using FLS980 Fluorescence Spectrometer form Edinburgh Instruments. Temperature dependent emission spectra were measured using 450 nm excitation line from laser diode and recorded using a Silver-Nova Super Range TEC Spectrometer from Stellarnet of 1 nm spectral resolution. Temperature during the measurement was controlled using the THMS 600 heating stage from Linkam (0.1°C stability and 0.1°C set point resolution).

Results and Discussion

The yttrium aluminum/gallium garnets, crystallize in cubic structure with Ia3-d space group. As it is known, structures of these materials (A₃B₂C₃O₁₂) provide three types of cation sites, namely dodecahedral Y³⁺ site and also octahedral (B) and tetrahedral (C) Al³⁺/Ga³⁺ sites which, because of similarity in ionic radii and the same coordination number, could be occupied by lanthanide Ln³⁺ (A) and transition metal ions TM³⁺ (B, C), respectively. Representative XRD patterns of Y₃Al₂Ga₃O₁₂: Cr³⁺ nanocrystals with different chromium concentration are presented in Figure 1a. All the reflection peaks correspond to the reference patterns confirming phase purity of the synthesized materials even for high dopant concentration (see also Supporting Information, Figures S1–S5). It is worth noting that in case of YAG (Figures S2, S3) and YGG (Figures S4, S5) with doping above 20% of Cr³⁺ ions, additional peaks occur in the XRD pattern. The cell parameter, as it can be seen in Figure 1b, is strongly affected by the Cr³⁺ concentration. In the case of YAG:Cr³⁺, parameter a increases from 11.99 Å for 0.1%Cr³⁺ to 12.16 Å for 50%Cr³⁺. On the other hand for YGG:Cr³⁺ an opposite tendency can be found –the a parameter decreases from 12.09 Å for 0.1%Cr³⁺ to 12.05 Å for 50%Cr³⁺. This is due to differences in ionic radius between host ions Al³⁺ (67.5 pm) and Ga³⁺(76 pm) ions being substituted by a larger dopant Cr³⁺ ions (75.5 pm). When atom with shorter ionic radius is substituted by atom with longer one, as in the case of Y₃Al₅O₁₂, the volume of the unit cell increases, due to the local expansion of the structure. This was also confirmed by the increase of the microstrains in the YAG structure (calculated using Rietveld refinement, Figure S7). On the other hand, in the case of substitution of larger host ion with a smaller dopant ion, the parameter a and microstrains decrease likewise in the Y₃Ga₅O₁₂. The reduction of the cell parameter for increasing Cr³⁺ concentration observed in YAGG, is therefore a direct confirmation that Cr³⁺ ions substitute octahedral sites of Ga³⁺ ions. The average grain size of the nanocrystals, calculated using Rietveld refinement technique (around 60 nm), was in agreement with the nanoparticle size distribution determined from TEM images-70 ± 10 nm (Figures 1c,d, Figure S6). TEM images revealed good crystallization and some agglomeration of the obtained powders.

FIGURE 1

Figure 1. Structural and morphological characterization of synthesized materials: XRD spectra for Y₃Al₂Ga₃O₁₂ (a) with different concentrations of Cr³⁺ ions, dependence of unit cell parameter (a) of YGG, YAG, and YAGG on Cr³⁺ concentration (b); representative TEM images for Y₃Al₂Ga₃O₁₂:0.06%Cr³⁺,1%Nd³⁺ (c) and Y₃Al₂Ga₃O₁₂:30%Cr³⁺,1%Nd³⁺ (d) nanocrystals.

The simplified configurational coordinates diagram of Cr³⁺ is presented in Figure 2A. The luminescence of Cr³⁺ ions occurs through radiative depopulation of ²E and/or ⁴T₂ states to the ⁴A₂ ground state. Due to the fact that strength of the crystal field determines the emission of Cr³⁺ ions, sharp emission line corresponding to the ²E → ⁴A₂ transition and broadband emission corresponding to the ⁴T₂→⁴A₂ transition can be observed in the emission spectra of Cr³⁺ ion in strong and weak crystal field, respectively. At higher temperatures, when the thermal energy is sufficient to reach the intersection point between the ²E parabola and ⁴T₂ or ⁴A₂ parabolas, the process of nonradiative, multiphonon relaxation results in lowering of their emission intensity. Analysis of the emission spectra of Y₃Al₂Ga₃O₁₂:Cr³⁺ for different dopant concentration obtained upon 450 nm excitation, clearly indicates that at higher Cr³⁺ concentration the broad ⁴T₂→⁴A₂ emission band localized at around 870 nm increase its intensity in respect to the ²E → ⁴A₂ band at 692 nm (Figure 2B). Obviously, the total emission intensity decreases at higher dopant concentration due to the concentration quenching of luminescence. Nevertheless, it can be distinctly seen that ⁴T₂→⁴A₂ dominates in the spectra for 30% of Cr³⁺ ions. Similar observation can be done in the emission spectra of Y₃Al₅O₁₂:Cr³⁺ and Y₃Ga₅O₁₂:Cr³⁺ nanocrystals presented in Figures S8, S9, respectively. However, in these cases similar trends can be observed for lower Cr³⁺ concentrations and to a lesser extent also in Y₃Al₂Ga₃O₁₂. Two main consequences of the lowering of ⁴T₂ state parabola can be found. First, at low dopant concentration the gradual reduction of its energy facilitates thermal depopulation of ²E state–lowered ΔE₂ energy (the consequence of the intersection point between ²E and ⁴T₂ parabolas). Secondly, at higher dopant concentration, when the energy ⁴T₂ state becomes lower than ²E one, the broadband emission appears. The characteristic red-shift of the Cr³⁺ absorption bands localized around 400 nm and 575 nm, which can be attributed to the ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ electronic transitions, respectively, is a result of lowering of the CF strength (Figures S12–S14). In order to quantify the observed change of the crystal field strength in the examined nanocrystals, the Dq/B parameter was calculated for each of the samples as follows (Casalboni et al., 1994):

$\begin{array}{cc}Dq=\frac{E\left({}^4{A}_2{\to }^4{T}_2\right)}{10}& (1)\end{array}$

$\begin{array}{cc}\begin{array}{r}\frac{Dq}{B}=\frac{15(x-8)}{(x^2-10x)}\end{array}& (2)\end{array}$

where x could be defined as (Casalboni et al., 1994):

$\begin{array}{cc}x=\frac{E\left({}^4{A}_2{\to }^4{T}_1\right)-E\left({}^4{A}_2{\to }^4{T}_2\right)}{Dq}& (3)\end{array}$

As it can be observed in Figure 2C the Dq/B gradually decreases with Cr³⁺ concentration from 2.79 to 2.44 for Y₃Al₅O₁₂; from 2.88 to 2.55 for Y₃Ga₅O₁₂ and from 3.42 to 2.76 for Y₃Al₂Ga₃O₁₂ nanocrystals. In order to obtain self-referenced luminescent thermometer based of the Cr³⁺ emission intensity the examined nanocrystals with different Cr³⁺ concentration were co-doped with 1% Nd³⁺ ions. Emission intensity of Nd³⁺ is expected to be significantly less temperature dependent in respect to the chromium emission. Moreover, the Nd³⁺ ions emit in the first (band around 880 nm attributed to the ⁴F_3/2→⁴I_9/2 electronic transition) and the second (bands around 1064 nm and 1350 nm attributed to the ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 electronic transitions, respectively) optical window of the biological tissues making it well suited for biological applications.

FIGURE 2

Figure 2. Configurational coordinates diagram of Cr³⁺ ions (A); comparison of normalized emission spectra for Y₃Al₂Ga₃O₁₂: xCr³⁺ (B) and for Y₃Al₂Ga₃O₁₂: xCr³⁺, 1%Nd³⁺ nanocrystals (D); dependence of Dq/B parameter (C) and Cr³⁺/Nd³⁺ ratio at T = −150°C (E) on Cr³⁺ concentration.

Following excitation spectra, a 450 nm excitation line was chosen, which provides the condition of direct excitation of each individual optically active ion-Cr³⁺ (⁴A₂→⁴T₁) and independently Nd³⁺ (⁴I_9/2→²G_5/2). As it was recently showed, this is an important condition to enhance the relative sensitivity of this kind of LTs (Marciniak et al., 2017a). In case of Y₃Al₂Ga₃O₁₂: Cr³⁺, Nd³⁺ (Figure 2D) nanocrystals the presence of Nd³⁺ ion significantly quenched the Cr³⁺ emission intensity due to the Cr³⁺ → Nd³⁺ energy transfer. Therefore, the ²E → ⁴A₂ emission band can be barely seen for Y₃Al₂Ga₃O₁₂: 30%Cr³⁺, 1%Nd³⁺ while in the case of Y₃Al₅O₁₂ and Y₃Ga₅O₁₂ above 10% of Cr³⁺, no chromium emission was detected (Figures S10, S11). Moreover, the presence of broad Cr³⁺ absorption bands in the excitation spectra when monitoring Nd³⁺ emission (⁴F_3/2→⁴I_9/2 emission band), is an additional confirmation of the interionic energy transfer which takes place between dopants (Figures S12–S14). The change of contribution of the emission of particular optically active ions in the total emission intensity related with Cr³⁺ concentration is presented in Figure 2E. Initially, at low Cr³⁺ amount the ²E → ⁴A₂ emission band dominates in the spectra. However, around 6% of Cr³⁺ its emission intensity equalize with Nd³⁺ amount. Above this value, the chromium emission intensity rapidly decreases. Therefore, this energy transfer strongly limits the usable Cr³⁺ concentration which can be applied for luminescent thermometry.

To understand the role of Cr³⁺ on the luminescence thermal quenching in the Y₃Al₅O₁₂ and Y₃Ga₅O₁₂ and Y₃Al₂Ga₃O₁₂ nanocrystals, their emission spectra were measured in a wide temperature range. The representative thermal evolution spectrum of Y₃Al₂Ga₃O₁₂:1.2%Cr³⁺ nanocrystals is presented in Figure 3A. It is clearly seen that sharp R-line of the ²E → ⁴A₂ emission band is rapidly quenched at around 50°C in contrary to the ⁴T₂→⁴A₂ emission. Therefore, due to this difference in the rates of thermal quenching of these particular emission bands, their luminescence intensity ratio was chosen as a temperature sensor LIR₁:

$\begin{array}{cc}LI{R}_1=\frac{C{r}^{3+}{(}^4{T}_2{\to }^4{A}_2)}{C{r}^{3+}{(}^{2}E{\to }^4{A}_2)}=\frac{{\displaystyle \int I(850-855)nm}}{{\displaystyle \int I(730-735)nm}}& (4)\end{array}$

At low Cr³⁺ concentration the LIR₁ decreases with temperature due to the fact that ²E state population feeds the ⁴T₂ state at higher thermal energy. However, for higher Cr³⁺ (above 6%) different tendency can be found. Initially the LIR₁ increases up to temperatures around 100°C above which saturation of its value can be found. This effect occurs because ⁴T₂→⁴A₂ band's intensity starts to play an important role in the total emission intensity. Due to the strong electron-phonon coupling, this emission band is expected to be efficiently reduced by the temperature. Therefore, its much higher rate of thermal quenching in respect to the ²E → ⁴A₂ results in the enhancement of LIR₁ value. To quantitative describe the observed changes, relative sensitivity (S) of LIR₁-based luminescent thermometer was calculated according to the following formula:

$\begin{array}{cc}\begin{array}{r}S(T)=\frac{1}{LIR}\frac{\Delta LIR}{\Delta T}100\%\end{array}& (5)\end{array}$

Independently from the Cr³⁺ concentration, the relative intensities reach maximal value at temperatures below 100°C. Above this value, low changes of LIR₁ are manifested as a minor value of S (Figure 3C). It is clearly seen that, according to our expectation, S significantly increases proportionally to Cr³⁺ content. The S_max at 9°C increases from 0.2%/°C for Y₃Al₂Ga₃O₁₂: 0.06%Cr³⁺ to 2.2%/°C for Y₃Al₂Ga₃O₁₂: 30%Cr³⁺ nanocrystals. Analogous tendency was found for the Y₃Al₅O₁₂:Cr³⁺ and Y₃Ga₅O_12:Cr³⁺ where for 10% Cr³⁺, S_max equals to 2.7%/°C at −105°C and 2%/°C at −78°C respectively (Figures S15, S16). The observed enhancement of the rate of the thermal quenching is obviously caused by the lowering of the CF strength. The optically active ions in the heavily Cr³⁺-doped nanocrystals are located in the lower CF sites (Figure 3D) which reduce the activation energy and facilitate luminescence thermal quenching. Therefore, the highest S were found for high Cr³⁺ concentration (low Dq/B values). However, above 130°C this correlation is suppressed due to the fact that above this temperature no ²E → ⁴A₂ emission was observed. It is worth noting that the temperature range of high S overlaps with physiological temperature range (10–50°C) what indicates the importance of these LTs for biomedical applications.

FIGURE 3

Figure 3. Comparison of thermal evolution of emission spectra for Y₃Al₂Ga₃O₁₂:1.2%Cr³⁺ (A); evolution of LIR₁ (B) with the corresponding relative sensitivities (S₁) (C) in the function of temperature; sensitivity map for different temperatures depending on Dq/B (D).

Due to the fact that emission intensity of both ²E → ⁴A₂ and ⁴T₂→⁴A₂ bands decreases at higher temperatures, relative sensitivity of luminescent thermometer based on their intensity ratio is reduced. Therefore, Nd³⁺ co-dopant, whose emission intensity is expected to be less temperature dependent, were used as a luminescent reference. The luminescent properties of Nd³⁺, Cr³⁺ co-doped nanocrystals were investigated in the analogous temperature range as in the case of singly Cr³⁺ doped counterparts. Representative thermal evolution of Y₃Al₂Ga₃O₁₂: 1.2%Cr³⁺, 1%Nd³⁺ nanocrystals is presented in Figure 4a. According to the expectations the intensity of bands at 880 and 1,060 nm attributed to ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I_11/2 electronic transition of Nd³⁺ ions, respectively, is almost independent on the temperature, while the ²E → ⁴A₂ emission intensity is strongly thermally quenched. It is worth noting that at temperatures above 50°C, additional Nd³⁺ bands appears which can be attributed to the ⁴F_5/2, ⁴S_3/2→⁴I_9/2 electronic transition. This band occurs at higher temperature due to the fact that population of ⁴F_5/2, ⁴S_3/2 states increases in respect to the ⁴F_3/2 with temperatures according to Boltzmann population. Taking advantage from these changes of the emission spectra with temperature, two types of luminescent thermometers based on the Nd³⁺/Cr³⁺ luminescence intensity ratio have been defined as follows:

$\begin{array}{cc}LI{R}_2=\frac{N{d}^{3+}{(}^4{F}_{3/2}{\to }^4{I}_{9/2})}{C{r}^{3+}{(}^4{T}_2{\to }^4{A}_2)}=\frac{{\displaystyle \int I(869-869.5)nm}}{{\displaystyle \int I(705-705.5)nm}}& (6)\end{array}$

$\begin{array}{cc}LI{R}_3=\frac{N{d}^{3+}{(}^4{F}_{5/2}{\to }^4{I}_{9/2})}{C{r}^{3+}{(}^4{T}_2{\to }^4{A}_2)}=\frac{{\displaystyle \int I(810-810.5)nm}}{{\displaystyle \int I(710-710.5)nm}}& (7)\end{array}$

The thermal dependence of LIR₂ and LIR₃ for different concentration of Cr³⁺ ions are presented in Figures 4b,d, respectively. Obviously, due to the Cr³⁺ → Nd³⁺ energy transfer the usable concentration range of Cr³⁺ is strongly limited (to 12% Cr³⁺). Independently from dopant concentration, both LIR₂ and LIR₃ increase at higher temperature due to the thermal quenching of ⁴T₂→⁴A₂ band of Cr³⁺ (Figures 4b,d). However, more rapid changes can be found for LIR₃ what is obviously related with the increase of the ⁴F_5/2, ⁴S_3/2→⁴I_9/2 emission band of Nd³⁺. Both LIR₂ and LIR₃ are strongly modulated by the Cr³⁺ concentration. In agreement with the results obtained for singly Cr³⁺ doped systems, significant enhancement of LIR's thermal changes can be found. At heavily doped phosphors the activation energies of ⁴T₂ level is meaningfully diminished facilitating its nonradiative depopulation. Small activation energy is beneficial for enhancement of the relative sensitivity of luminescent thermometers. Therefore, these effects substantially affect the relative sensitivity of both S₂ and S₃ (Figures 4c,e). The highest S₂ at 150°C increases from 0.17%/°C for 0.06% of Cr³⁺ to 1.17%/°C for 12% of Cr³⁺ ions. On the other hand the S₃ at −48°C increases from 0.95%/°C for 0.06% of Cr³⁺ to 2.16%/°C for 12% of Cr³⁺ ions. Comparing these results with the Figure 3, evident profitable effect of the Nd³⁺ ions use as a luminescent reference can be found. The relative sensitivity for 0.06% Cr³⁺ increases from S₁ = 0.14%/°C to S₂ = 0.17%/°C and S₃ = 0.95%/°C, while for 12% Cr³⁺ from S₁ = 1.37%/°C to S₂ = 1.17%/°C and S₃ = 2.16%/°C. Analogous beneficial effect of high Cr³⁺ concentration can be found for Y₃Ga₅O_12:Cr³⁺, Nd³⁺ as well as Y₃Al₅O_12:Cr³⁺, Nd³⁺ (Figures S10, S11). It is also worth noting that the LIR₂ reveals high sensitivity at high temperature range (above 100°C) in contrast to LIR₃, which unveils good performance for non-contact temperature sensing at low temperatures (below 0°C). Therefore, by simultaneous employment of both of these thermometers the temperature range where high accuracy temperature readout can be achieved, is widened.

FIGURE 4

Figure 4. Comparison of thermal evolution of emission spectra for Y₃Al₂Ga₃O₁₂:1.2%Cr³⁺, 1%Nd³⁺ (a); thermal evolution of LIR₂ (b); and LIR₃ (d) and the corresponding relative sensitivities S₂ (c) and S₃ (e) for different Cr³⁺ concentration for Y₃Al₂Ga₃O₁₂:1.2%Cr³⁺, 1%Nd³⁺ nanocrystals.

The highest recorded sensitivity (S = 2.64%/°C) was found for YAG nanocrystals at −100°C for 10% of Cr³⁺ (Figure S15B) due to the fact of the lowest CF strength for this host material. Nevertheless, CF strength was tuned in the widest range for Y₃Al₂Ga₃O₁₂: Cr³⁺, Nd³⁺ via the Cr³⁺ doping the enhancement of S₃ was the strongest in this case. The most important finding presented in this paper is that highly sensitive luminescent thermometers can be intentionally designed by the modification of the CF strength through elongation of Cr³⁺-O²⁻ distance and enlargement of the Cr³⁺ concentration.

Conclusions

In this work, we proposed a new strategy to modulate the relative thermal sensitivity of Cr³⁺ doped nanophosphors. We considered three types of garnet matrices doped with different Cr³⁺ concentrations, with or without Nd³⁺ co-dopant. It was shown that the increase of the Cr³⁺ concentration causes elongation of the average Cr³⁺-O²⁻ distance leading to the reduction of the crystal field strength in Y₃Al₂Ga₃O₁₂, Y₃Ga₅O₁₂ and Y₃Al₅O₁₂ nanocrystals. A gradual increase of the broad emission band attributed to the ⁴T₂→⁴A₂ spin allowed transition of Cr³⁺ is observed. Moreover, the reduction of the ⁴T₂ state energy facilitates thermal quenching of ²E state. Therefore, the relative sensitivity of ⁴T₂→⁴A₂ to ²E → ⁴A₂ emission intensity increases from S = 0.2%/°C for 0.06%Cr³⁺ to S = 2.2%/°C for 30%Cr³⁺ at 9°C in Y₃Al₂Ga₃O₁₂ nanocrystals, from S = 0.027%/°C for 0.5%Cr³⁺ to S = 2.7% for 10%Cr³⁺ at −105°C in Y₃Al₅O₁₂ nanocrystals, and from S = 0.14%/°C for 0.5%Cr³⁺ to S = 2% for 10%Cr³⁺ at −78°C in Y₃Ga₅O₁₂ nanocrystals. In the case of Nd³⁺ co-doped nanocrystals, due to the Cr³⁺ → Nd³⁺ energy transfer, the usable concentration range of Cr³⁺ dopants is strongly reduced and no evidence of Cr³⁺ emission was found above 10% of Cr³⁺. However, by taking advantage from this energy transfer and the fact that ⁴F_5/2, ⁴S_3/2→⁴I_9/2 emission intensity increases proportionally to the temperature according to the Boltzmann distribution, the relative sensitivities of luminescent thermometers defined as ⁴F_5/2, ⁴S_3/2→⁴I_9/2 to ⁴T₂→⁴A₂ luminescence intensity ratio enhances from 1.3%/°C at −50°C without Nd³⁺ dopant to 2.2%/°C at −50°C for nanocrystals doped with Nd³⁺ ions Y₃Al₂Ga₃O₁₂: 10%Cr³⁺, Nd³⁺). The presented results confirm that the relative sensitivity of luminescent thermometers can be effectively modulated by the dopant concentration. Moreover, it was proved that the presence of Nd³⁺ dopant contributes to faster quenching of Cr³⁺ luminescence, what is favorable for highly sensitive non-contact luminescent thermometers. Our studies may be considered as a next step toward intentional designing of nanocrystalline luminescent thermometers with fully controllable thermooptical response.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The High sensitive thermal imaging for biomedical and microelectronic application project is carried out within the First Team programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00424/full#supplementary-material

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