A New Type of Nanocrystalline Luminescent Thermometers Based on

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C: Physical Processes in Nanomaterials and Nanostructures

A New Type of Nanocrystalline Luminescent Thermometers Based on Ti /Ti and Ti /Ln (Ln =Nd , Eu , Dy ) Luminescence Intensity Ratio 3+

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Joanna Drabik, Bart#omiej Cichy, and Lukasz Marciniak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02328 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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The Journal of Physical Chemistry

A New Type of Nanocrystalline Luminescent Thermometers Based on Ti3+/Ti4+ and Ti4+/Ln3+ (Ln3+=Nd3+, Eu3+, Dy3+) Luminescence Intensity Ratio Joanna Drabik, Bartlomiej Cichy, Lukasz Marciniak* 1

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland

* corresponding author: [email protected]

Abstract The spectroscopic properties of YAG:Ti and YAG:Ti, Ln nanocrystalline powders were examined as part of the search for new functional materials devoted to luminescent thermometry. Various temperature-dependent processes occurring in the studied systems were analyzed and mechanism of absorption and emission of trivalent and tetravalent titanium ions was proposed. The first luminescent thermometer based on Ti4+/Ti3+ luminescence intensity ratio with maximum sensitivity of 0.71%C-1 is shown. It was shown that the co-doping with lanthanide ions enhances the relative sensitivity of YAG:Ti, Ln nanocrystalline luminescent thermometers due to the Ti4+→Ln3+ energy transfer. The maximal relative sensitivities in the physiological range (2.26%C-1 at 50 oC) was found for YAG:Ti, Nd3+ nanocrystals and its value increases with temperature reaching 3.70%C-1 at 200 oC.

Introduction Temperature, as one of the most important physical variables, is a key parameter for understanding the evolution of both biological and technological systems. Therefore,

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increasing the accuracy and spatial resolution of temperature measurements is a significant challenge for the scientific community. As a result, the number of research and studies on new innovative temperature measurement methods has increased significantly in recent years. Luminescent thermometry is one of the most extensively developed in a last two decades methods of temperature sensing

1,2

, which bases on dependence of luminescent response of

sensor excited by incident light on temperature. By utilizing the luminescent nanocrystals featuring fast optical response, high chemical and thermal stability, and high quantum efficiency, luminescent thermometry may reach unprecedented sub-micrometer spatial resolution of thermal mapping. Therefore nanocrystalline luminescent thermometry opens the way to new possibilities, being not explored so far, in the field of thermal sensing. Taking into consideration the type of optically active ions used, a three main groups of nanocrystalline luminescent thermometers can be distinguished, namely: (i) lanthanide ions doped, (ii) transition metal ions doped and (iii) lanthanide-transition metal ions co-doped group. In case of the first group, due to the fact that 4f orbitals are shielded from the environmental by the 5s and 5p orbitals the f state’s potential parabolas are placed concentric in the coordinate diagrams. Therefore thermal quenching of their luminescence intensity takes place via multiphonon processes. Most of the lanthanide doped luminescent thermometers take advantage from the changes of the relative emission intensities of the bands related either with emission from two Stark components of the same emitting multiplet (i.e. R1 and R2 lines of Nd3+ ions 3–8) or two different multiplets (of the same ion like in case of 2H11/2 and 4S3/2 of Er3+ 1,9–13 ions and 4F3/2 and 4F5/2 of Nd3+ 14,15 or multiplets of two different ions like in case of Nd3+, Yb3+16–22). The second category bases on optical properties of transition metal ions. For example, the comparison of the luminescence intensity of forbidden and allowed transitions, 2E→4A2 and 4T2→4A2 respectively, of Cr3+ doped phosphors

23–26

. High thermal sensitivity of such a 2

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system relies on the distortion of the parabolas of the ground and excited states, caused by the influence of the crystal field strength. Excited level is shifted in respect to the ground level in the k-space domain causing the existence of an intersection point of the parabolas, which leads to the increase of the susceptibility to thermal quenching. However, dependence of the population of these two levels is, just like in previous case, in accordance with the Boltzmann's law, which results in some fundamental limitation of the maximum sensitivity of such system. The third type of optical thermometers uses both rare earth and transition metal ions. In this approach, lanthanide emission is used as a reference, since the electronic configuration and the intersection point between ground and excited state parabolas of transition metals makes them more prone to thermal quenching of luminescence

23,27–35

. This

approach allowed one to obtain significantly higher sensitivity. In this case most of the thermometers proposed up to date base on temperature dependent intensity ratio of lanthanide ions co-doped with chromium or manganese ions. However examination of various lanthanide - transition metal pairs is a current challenge, which can lead to enhancement of sensitivity of temperature readout. Titanium is an optical active ion which was widely investigated in terms of lasing performance 36–43. Sapphire:Ti3+ was the first solid-state laser material which used Ti3+ ions as active ions. The first report on the construction of the Ti:Al2O3 laser was presented by P.F. Moulton in 1982. Due to the fact that titanium provides a broad emission range, this type of laser became very popular as a tuneable solid state laser. From the comparison reported by Bantien et al., in contrary to the Al2O3:Ti3+, the efficient laser action is not expected in YAG:Ti3+ due to the high probability of luminescence thermal quenching in this host44. This conclusion motivates us to investigate the potential of using YAG:Ti3+ nanocrystals as a sensitive luminescent thermometers. Therefore, in this paper Y3Al5O12:Ti nanocrystals of



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different sizes and different Ti concentrations were synthesized, and their temperaturedependent optical properties were investigated and discussed. Experimental The powders of YAG:Ti (Y3Al5O12:Ti) and YAG:Ti, Ln (Y3Al5O12:Ti, Ln) nanocrystals with different Ti concentrations were prepared by modified Pecchini method 45. The concentrations of Ti ions in YAG:Ti nanocrystals were fixed at x = 0.1%, 0.2%, 0.5%, 1%, 2%, and 5% mol in respect to Al3+. Due to the fact that the highest emission intensity was obtained for the YAG:0.1%Ti nanocrystals, the concentration of dopants in co-doped nanocrystals was YAG:0.1%Ti, 1%Ln3+ (Ln3+=Eu3+, Nd3+, Dy3+). The synthesis was performed using the following starting compounds: titanium(IV) nbutoxide (Ti[O(CH2)3CH3]4 of 99+% purity from Alfa Aesar), 2,4-pentanedione (CH3CH2COCOCH3 of 99% purity from Alfa Aesar), neodymium oxide (Nd2O3 of 99.95% purity from Stanford Materials Corporation), europium oxide (Eu2O3 of 99.99% purity from Stanford Materials Corporation), dysprosium oxide (Dy2O3 of 99.999% purity from Stanford Materials Corporation), yttrium oxide (Y2O3 of 99.999% purity from Stanford Materials Corporation), aluminum nitrate hydrate (Al(NO3)3·xH2O (x≈9) of 99.999% purity from Alfa Aesar), citric acid (C6H8O7 of 99.5+% purity from Alfa Aesar), polyethylene glycol (H(OCH2CH2)nOH from Alfa Aesar), and nitric acid (HNO3 of 65% purity from Avantor). At first, rare earths nitrates were produced by adding nitric acid to water solutions of oxides. Then, obtained solutions of nitrates, including aluminum nitrate, were thoroughly mixed. Afterwards, titanium(IV) n-butoxide soluted in 2, 3-pentanedione was added to the mixture. The solution containing all compounds was dried at 90 oC for one week. Obtained resin was annealed for 16 hours at 850 oC, 900 oC, 1000 oC, 1100 oC in air.


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Powder diffraction studies were carried out using the PANalytical X'Pert Pro diffractometer equipped with an Anton Paar TCU 1000 N temperature control unit using Nifiltered Cu Kα radiation (V = 40 kV, I = 30 mA). Transmission electron microscopy images were obtained using the TITAN Cubed G2 60-300 Microscope acquired from FEI company. The microscope was equipped with two spherical aberration correlators, a monochromator and a set of four EDS detectors. The samples were applied to a lacey copper mesh in a mechanical manner. The study was conducted in a classical TEM mode with parallel beam of electrons energy at 300 keV. Pictures were digitally recorded using the Gatan Ultrascan 1000XP. X-ray photoelectron spectroscopy (XPS) measurements were performed using a lamp with an aluminum anode (Al Kα emission line) with a monochromator (the lamp power was set to 300W) as the source of X-rays. The photoelectron energy spectra were collected using a hemispherical SCIENTA EW 3000 analyzer with pass energy set to 200 eV. The energy resolution was 0.6 eV, based on the FWHM of the Ag 3d 5/2 line. The pressure in the analysis chamber was kept below 5·10-10 mbar. A flood source neutralizer was used. The CasaXPS program was used to analyze the results. The scale of energy in the obtained results was calibrated to the C-C line, 285 eV. The emission spectra were measured using the 266 nm excitation line from a laser diode and measured using a Silver-Nova Super Range TEC Spectrometer form Stellarnet (1 nm spectral resolution). The temperature of the sample was controlled using a THMS 600 heating stage from Linkam (0.1 °C temperature stability and 0.1 °C set point resolution). The photoluminescence decay curves were obtained using an FLS980 fluorescence spectrometer from Edinburgh Instruments. The measurement was carried out with two detectors: R928P side window photomultiplier tube from Hamamatsu for visible range (λem=512 nm) and R5509-72 photomultiplier tube from Hamamatsu in nitrogen-flow cooled 5 ACS Paragon Plus Environment


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housing for near infrared range (λem=820 nm). Excitation line was obtained using microFlash lamp. Experimental results were also supported by the theoretical predictions. The groundstate properties of both Y3Al5O12 and Ti doped Y3Al5O12 were investigated in mixed approach involving infinite crystal and cluster-like approximations. For the first approximation numerical calculations of the ground-state properties of the Y3Al5O12:Ti nanoparticles were performed within the antiferromagnetic approximation basing on the density functional theory (DFT) and the plane wave basis. Rejecting the surface related effects the YAG:Ti nanoparticles were modelled as infinite crystal within the projector augmented wave (PAW) approach as implemented in the ABINIT software package 46. The first Brillouin zone of the 80 atom primitive cell was sampled on the 8×8×8 Monkhorst-Pack k-point grid. The Perdew-Burke-Ernzerhof (PBE) with gradient density approximation (GGA) exchangecorrelation (XC) energy functional was used in the calculations. The cut-off energy was set to 20 Ha. The valence structure of the Y, Al, O and Ti atoms was as follows 4s25s24p65p04d1, 2s22p1, 3s23p4, 3s24s13p63d3 with 11, 3, 6 and 12 valence electrons respectively. Structural optimization of the primitive cell was performed using the BroydenFletcher-Goldfrab-Shanno (BFGS) minimization algorithm. Maximal absolute force tolerance for the structural optimization was set to 5.0e-5 Ha/Bohr. Due to large size of the primate cell and large computational complexity the Ti doping was investigated within the single primitive cell allowing for occupation of both the tetrahedral and octahedral atomic sites. See the supplementary section for more details according the primitive cell and sites population by the Ti dopant (Fig. S1.). In the cluster-like approach the close proximity of the Ti dopant located in the tetrahedral and octahedral site was modelled and analysed. The molecular-like approach was solved using the GAMESS-US package

47

. The calculations were performed solving the all electron problem


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with the TZ2P polarization consistent with diffuse augmentation basis set APCseg-2. All the of the [Tix+O4]y- and [Tix+O6]y- clusters were computed using the Hartree-Fock calculations and the spin unrestricted (UHF) wave-functions. The geometry optimization threshold was set to 5·10-4 Ha/Bohr and the self-consistent field (SCF) convergence threshold was set to 10-5 Ha. More details according the numerical results may be found in the supplementary section (Fig. S2., Fig. S3. and Fig. S4.).

Results and discussion X-ray powder diffraction patterns of Y3Al5O12:Ti (YAG:Ti) nanocrystals presented in Figure 1a and Figure 1b are consistent with standard reference data of YAG (262999-ICSD). All the reflections have their corresponding representatives in the reference pattern and therefore, the obtained nanocrystals are confirmed to have an adequate phase purity. Observed monotonic narrowing of the peaks along with the annealing temperature is related to the grain size increase (Fig. S5.). The YAG crystal has a cubic phase structure and belongs to the Ia-3d space group. Calculated unit cell parameter of synthesized nanocrystals corresponds with its reference obtained for a single crystal a=1.2008 nm (Fig. S6.). Due to the large difference in yttrium and titanium ionic radii, titanium substitutes aluminum sites in this host material. The Al3+ ions in YAG structure occupy both octahedral and tetrahedral sites of mixed valency character. Appearance of the Ti3+ and the Ti4+ ions in the structure was experimentally confirmed by the XPS measurements (Fig. S7.). According to the XPS data the Ti3+ to Ti4+ ratio in the nanocrystals was estimated to be 2/3 what corresponds well to the ratio between octahedral and tetrahedral sites in the YAG structure. It is also well known that Ti3+ ions do not emit from tetrahedral sites.41,42 The comparison between ionic radii leads to the conclusion that Ti4+ may substitute both types of the Al3+ sites. Lanthanides incorporated into the host lattice substitute typically Y3+ 8-coordinate sites due to similarity of ionic radii. The 7 ACS Paragon Plus Environment


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Fig. Figure 1c and Fig. Figure 1d show a representative TEM images of nanocrystals annealed at 850oC and doped with 0.1% and 2% Ti respectively. The presented figures indicate small size distribution and aggregation of the well crystalized YAG nanoparticles (compare with Fig. S8.).

Figure 1. X-ray diffraction patterns of YAG:Ti nanocrystals annealed at 850 oC with different Ti concentrations – (a), and YAG:Ti nanocrystals doped with 0.1%Ti annealed at different temperatures – (b); representative TEM images of the nanocrystals annealed at 850 oC doped with 0.1%Ti – (c), and 2%Ti – (d).

Titanium is a transition metal of the [Ar]3d24s2 electron configuration. The structure of energy levels of Ti3+ and Ti4+ ions, characterized by 3d1 and 3d0 configurations respectively, is modulated by the crystal field of the host material determined by the neighbouring ligands. Fivefold-degenerated d-electron levels are typically split in the presence of crystal field. In the octahedral site Ti3+ energy levels split into triply 2T2g and doubly 2Eg degenerated sets of states. Due to the fact that 3d1 electron is involved in the formation of chemical bonds, the ion is also sensitive to the environmental changes and the excitation of such ion changes the spatial charge distribution. It results in the displacement of 8 ACS Paragon Plus Environment

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the parabolas on Ti3+ coordinate diagram shown in Figure 2a, which leads to a large Stokes shift of Ti3+.

Figure 2. Configurational coordinate diagram showing the energy parabolas of Ti4+ (charge transfer from oxygen ligand states) and Ti3+ ions in YAG:Ti nanocrystals – (a); emission spectra measured at -150oC of YAG:Ti nanocrystals doped with 0.1%Ti annealed at different temperatures – (b); luminescence intensity ratio of Ti4+ and Ti3+ bands for corresponding nanocrystals – (c); emission spectra measured at -150oC of YAG:Ti nanocrystals annealed at 850 oC with different Ti concentrations – (d); luminescence intensity ratio of Ti4+ to Ti3+ bands for corresponding samples – (e).

According to the experimental results obtained upon the 266 nm optical excitation, a broad emission band localized at around 820 nm may be associated with the 2Eg→2T2g electronic transitions of the Ti3+ ion (Fig. 2b and 2d). This is also well rationalized looking at the numerical calculations (Fig. 3a and 3b). The highest occupied states are formed from the t2g orbitals. The t1u ligand orbitals are typically located deep in the valence levels. The energy difference between the LUMO-HOMO orbitals ∆E for the Ti4+ was found to be 7.24 eV which is far beyond the VIS excitation line. Moreover, existence of the Ti4+ at the octahedral symmetry seems not to be energetically favoured and the Ti3+ is found as the dominative 9 ACS Paragon Plus Environment


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oxidation state (∆E=2.30 eV). Nevertheless, presence of the Ti2+ may also not be excluded from the numerical analysis. This stays in good correspondence with the DFT results obtained for the infinite crystal (Fig. S2.). Location of the Ti ions over the tetrahedral sites is more difficult to be rationalized as the 4p and 3d orbitals of the same symmetry may be mixed due to the ligand field. It is very well observed from the numerical calculations for the infinite crystals (Fig. S2.) for which the density of states DOS spectrum is dominated by hybridized Ti d-electron levels. As a result of Ti4+ electronic configuration, d-d emission from the Ti4+ ion located in the tetrahedral site is not possible. However, due to the tetrahedral field and quite high formal oxidation state of the Ti ions it is likely that charge transfer O2-→T2 transitions may be observed. From the cluster-like approach the ∆E for the Ti3+ was found to be 8.83 eV what is still far beyond the VIS band. However, much lower energies were found for the Ti2+ i.e. ∆E=2.28 eV corresponding well with the broad charge transfer band observed experimentally giving an physical premise that charge transfer at the tetrahedral Ti4+ site may be possible. It is therefore seen that the O2-→T2 transitions followed by nonradiative multiphonon relaxation lead to the occurring of a broad band centred at 512 nm (Figure 2b and Figure 2d). The Fig. 2b and Fig 2c show the dependence of the shape of the YAG:Ti emission spectrum and the Ti4+ to Ti3+ emission intensity ratio, respectively, on annealing temperature. In order to receive quantitatively information concerning the impact of grain size on changes in bands intensity, the spectra shown have been normalized to the integral intensity of the whole spectrum. An increase of annealing temperature causes grain growth, thereby reducing the amount of ions located on the surface to those located inside the nanocrystal. As it was reported for vanadium ions doped nanocrystals the ions of higher oxidation state (like V5+) prefers to occupy surface sites, while V3+ is mainly located in the core part of the nanocrystals 33

. Therefore probably similar tendency may occur in case of titanium ions. If the Ti3+ ions are


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located mainly in the core part of the nanocrystals their number increases faster for bigger grain size in respect to the Ti4+. Therefore the Ti4+ to Ti3+ emission intensity ratio decreases with annealing temperature, which can be observed in Figure 2c. From the Fig. S9 c and d it can be noticed that the size of the nanoparticle does not affect the shape of the decay profiles of neither Ti3+ nor Ti4+ luminescence, what suggest that the change of the relative emission intensity of these ions is not related with the energy transfer or nonradiative processes but rather with the growing number of Ti3+ ions for bigger nanocrystals. The Fig. 2e shows rapid growth of Ti4+/Ti3+ luminescence ratio for nanocrystals for increasing of Ti concentration. As it is clearly seen the 2Eg→2T2g emission band of Ti3+ ions is almost totally quenched above 0.5% of dopant concentration, while 2T2 → O2- charge transfer band of Ti4+ remains intense. This effect is related to the stronger luminescence concentration quenching observed for Ti3+ ions in respect to the Ti4+ ions. The interionic interactions between dopants is facilitated by the shortening of the distance between them. This observation was confirmed by analysis of the excited states kinetics of Ti3+ and Ti4+ ions (Fig. S9.). Luminescence decay curves of Ti3+ show exponential decay with τ Ti3+ = 6 ms (for 0.1% of Ti, and 850oC) while in case of Ti4+ nonexponential decay with average lifetime around = 3 ms can be observed. Unlike Ti4+, the Ti3+ lifetime shortens strongly with dopant concentration (Fig. S10.). Taking advantage from this fact, the emission intensity ratio, and thereby emission color output of YAG:Ti nanocrystals, can be modulated either by the size of the nanocrystal or dopant concentration.



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Figure 3. The electronic configurations obtained within the cluster-like approach of titanium (Ti) ion located in the tetrahedral – (a) and octahedral – (b) site including three different oxidation states i.e. 4+, 3+ and 2+. The alpha and beta spin levels were coloured respectively in red and blue. The unoccupied virtual states of alpha and beta type were coloured respectively in orange and light blue. The LUMO-HOMO energy difference ∆E for the singlet transitions were found for the (a) tetrahedral site: ∆ETi4+=13.63 eV, ∆ETi3+=8.83 eV, ∆ETi2+=2.28 eV and (b) octahedral site: ∆ETi4+=7.24 eV, ∆ETi3+=2.30 eV, ∆ETi2+=1.92 eV.

To investigate whether the YAG:Ti nanocrystals are suitable for defining a nanocrystalline luminescent thermometer based on their optical response, the measurement of emission spectra over a wide temperature range (-150oC – 300oC) was performed for all the synthesized powders. Thermal evolution of the representative emission spectra (YAG:0.1%Ti annealed at 850 oC) is presented in Fig. 4a. Both bands of Ti4+ and Ti3+ exhibit a rapid decrease of the intensity at elevating temperature, and they are almost completely quenched at about 350oC. However, the Ti4+ band intensity is stronger affected by temperature then Ti3+ band. This is related with the smaller thermal energy which is needed to overcome intersection point between ground and excited states parabolas (activation energy ∆E) for Ti4+ (∆E=0.1 eV) in respect to the Ti3+ one (∆E=0.2 eV). It is worth noting that also the Ti concentration affects the rate of the luminescence thermal quenching either for Ti3+ or Ti4+


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ions. In case of higher dopant concentration the probability of energy diffusion amongst excited states to the ‘killer centers’ increases. This additional depopulation channel increases the sensitivity of the YAG:Ti luminescence to temperature changes. It is probable that if Ti4+ ions are located mainly in the strongly defected surface part of the grain the depopulation path via energy diffusion seems to be more efficient in case of Ti4+ what is observed in Fig. 4b and Fig. 4c. Nevertheless it is worth noting that increase of dopant concentration reduces the integral emission intensity. The influence of the temperature on the LIR1 defined as the Ti4+/Ti3+ emission intensity ratio:

LIR1Ti /Ti =

I (Ti 4 + ) I (Ti 3+ )

(1)

for the powders annealed at 850 oC is presented in Figure 4d. Changes of Ti4+/Ti3+ ratio along with temperature growth are observed for all samples. Due to the fact that the luminescent responses of both Ti3+ and Ti4+ ions to temperature changes reveal different rates, their relative emission intensities can be used as a temperature sensor. One of the most important parameter which allows for a quantitative characterization of the ability of luminescent thermometer for temperature sensing is its relative sensitivity S defined as follows:

S=

1 ∆LIR(T ) ⋅100% LIR(T ) ∆T

(2)

where ∆LIR(T) stands for the changes of temperature-dependent parameter (luminescence intensity ratio - LIR(T)) corresponding to the ∆T change of temperature. In this case LIR represents relative emission intensity of Ti4+ ions to the Ti3+ ions. The sensitivity calculated from the experimental data presented in Fig. 4e shows that different concentrations of Ti doping are suitable for distinct ranges of temperatures. The highest sensitivity of 0.5-0.7 %C-1



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in a wide temperature range above 100oC was obtained for nanocrystals with the highest Ti concentration 5%. Nanocrystals doped with 0.1% of Ti ions show relatively high sensitivity (S=0.5%C-1) at temperatures below 100oC. Therefore, the usable operating range and relative sensitivity of such a thermometer may be modulated by the optimization of the dopant concentration. It is worth noting, that from the best of our knowledge, this is the first report concerning self-referenced nanocrystalline luminescent thermometer based on emission of titanium ions of a different valence states.

Figure 4. Thermal evolution of YAG:0.1%Ti annealed at 850oC representative nanocrystals emission spectra measured in -150÷500oC temperature range upon 266 m excitation wavelength – (a); normalized intensity of Ti4+ - (b), of Ti3+ band – (c), and their ratio Ti4+/Ti3+ of nanocrystals annealed at 850 oC with different Ti concentrations - (d); sensitivity calculated from curves fitted to Figure 4d – (e).

As it was recently shown transition metal ions and lanthanide ions co-doped phosphors can be used as a sensitive luminescent thermometers

27–29,31,32,48

. In this approach

luminescence of lanthanide ions, which is expected to be less temperature dependent than that of transition metal ions emission, is used as a reference signal. However the sensitivity of emission intensity of donor (transition metal ions) to the temperature changes can be further 14 ACS Paragon Plus Environment

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enhanced in the presence of acceptor (lanthanide ion) via temperature dependent donoracceptor energy transfer. When there is a energy mismatch between emitted (by donor) and absorbed energy (by acceptor) the donor acceptor energy transfer may occur with the phonon assistance according to the Miyakava-Dexter model 48. The probability of phonon-assisted energy transfer increases at higher temperature. Therefore, in order to verify how the type of lanthanide co-dopant affects the ability of the temperature readout, nanocrystals co-doped with titanium and neodymium, europium or dysprosium were synthesized and their optical response was investigated in a wide temperature range. The dependence of emission spectra of nanocrystals co-doped with 1% Nd3+, 1% Eu3+ and 1% Dy3+ on temperature are presented in Figure 5a, 5b and 5c, respectively. The characteristic narrow emission bands of the lanthanides can be observed in each case. Their emission intensities are not strongly affected by temperature rise. Therefore, they are promising candidates for luminescent reference. The luminescence intensity ratios of mentioned luminescent thermometers were defined as follows:

LIR 2 Nd /Ti =

LIR 3 Eu /Ti =

LIR 4 Dy /Ti =

I ( Nd 3+ ) ∫ I (870 − 870.5nm ) = I (Ti 4 + ) ∫ I (668.5 − 669 nm )

(3)

I ( Eu 3+ ) ∫ I (709.5 − 710 nm ) = I (Ti 4 + ) ∫ I (508 − 508.5nm )

(4)

I ( Dy 3+ ) ∫ I (583.5 − 584 nm ) = I (Ti 4 + ) ∫ I (536 − 536.5nm )

(5)

The presence of the Ln3+ co-dopants in the YAG:Ti nanocrystals significantly reduced the lifetime of the excited CT level of Ti4+ ion (see Fig. S11) which suggests the existence of efficient Ti4+→Ln3+ energy transfer. The rate of such transfer is strongly dependent on temperature according to the Miyakava-Dexter equation 48, . 15 ACS Paragon Plus Environment


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In the case of Nd3+ co-doped nanocrystals two emission band localized at 880 nm and 1064 nm associated with the 4F3/2→4I9/2 and 4F3/2→4I11/2 electronic transitions respectively, can be observed. In this case the decrease of intensity of the Ti4+ band along with the temperature growth is more rapid in respect to the singly Ti doped counterparts. The Ti4+ emission is completely quenched above 200 oC. This phenomenon suggests the efficient Ti4+→ Nd3+ energy transfer which probability enhances at higher temperature. The Ti3+ emission cannot be observed due to the overlap with the emission from lanthanide excited levels. In case of Eu3+ co-doped nanocrystals emission spectra consist of 5 emission bands at 575 nm, 590 nm, 612 nm, 650 nm and 705 nm associated with 5D0→7F0, 5D0→7F1, 5D0→7F2, 5

D0→7F3 and 5D0→7F4 electronic transitions, respectively. As it can be seen, the relative

emission intensity of Ti4+ ions in the YAG:Ti, Eu3+ nanocrystals is significantly lower in comparison to other lanthanide co-doped phosphors, what results from the fact that 266 nm excitation line overlaps with the O2-→Eu3+ charge transfer band and efficiently pump Eu3+ ions. It is noteworthy that the probability of the energy transfer from Ti4+ to Eu3+ increases with the temperature rise. Emission spectra of Dy3+ co-doped nanocrystals consist of Ti4+ emission band and narrow line at 490 nm and 580 nm which can be attributed to the 4F9/2→6H15/2 and 4

F9/2→6H13/2 electronic transition of Dy3+ ions. Due to the fact that emission bands of both of

these ions partially overlap each other the spectral range used for LIR3 definition was slightly modified (see eq. 5). The Ti4+ emission shows decrease at elevating temperature, and Dy3+ emission behaves in analogous way. In order to analyze how the thermal evolution of integral emission intensity of Ti4+ ions is affected by the presence of Ln3+ ions, the normalized emission intensity of YAG:Ti, YAG:Ti4+, Nd3+, YAG:Ti4+, Eu3+ and YAG:Ti4+, Dy3+ nanocrystals were presented in Fig. 5d. 16 ACS Paragon Plus Environment

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Reduction of the intensity of the Ti4+ band with increasing temperature has different rates for nanocrystals co-doped with different lanthanides. The faster decrease of the Ti4+ emission intensity with temperature observed for co-doped powders suggests the presence of Ti4+→Ln3+ energy transfer. The fact that thermal evolution of Ti4+ emission intensity and hence LIR reveal slower decay with temperature confirms that Ti4+→ Dy3+ energy transfer is less efficient comparing to other Ln3+ co-doped counterparts (see Fig. 5d and 5e). The Figure 5e shows lanthanide to titanium luminescence intensity ratio growth (defined according to the eq. 3-5) throughout the whole temperature range. Basing on the thermal evolution of LIR2-4 the relative sensitivity of luminescent thermometer was calculated using eq. 2 (Fig. 5f). In case of each of Ln3+ ions, S increases with temperature up to around 50-100oC. Then the upward trend breaks down and the sensitivity decreases for Eu3+ and Dy3+, but increases for Nd3+ with the growing temperature, up to 200oC, above which Ti4+ emission intensity is totally quenched. The sensitivity of Ln3+ co-doped nanocrystals is much higher in respect to the singly Ti doped counterpart (4e, black line). In a physiological temperature range (050oC), it is about 1%C-1 for Dy3+/Ti4+ nanocrystals and it reaches about 2%C-1 for Nd3+/Ti4+ and Eu3+/Ti4+ nanocrystals and its value increases up to 3.70%C-1 at 200oC for Nd3+/Ti4+ nanocrystals. In our previous work

28

, we showed that for YAG:Cr3+,Nd3+ nanocrystals we

achieved higher sensitivity of 3.48%C-1 in the physiological temperature range. However the usable temperature range was much narrower in respect to the YAG:Ti4+, Nd3+ and taking into account that sensitivity of such luminescent thermometer increases with temperature, what enables for accurate temperature readout even at 200oC, it enlarges the number of potential field of its application.



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Figure 5. Thermal evolution of YAG:Ti,Ln nanocrystals emission spectra measured in -150÷500 oC temperature range upon 266 nm excitation wavelength for nanocrystals co-doped with Nd3+ – (a), Eu3+ – (b), and Dy3+ – (c); thermal evolution of normalized intensity of Ti4+ band – (d); thermal evolution of luminescence intensity ratio of lanthanide to titanium bands - (e); sensitivity calculated from LIR presented in Fig. 5e (biological range of temperatures highlighted in blue) – (f).

Conclusions Luminescent properties of YAG doped with Ti3+ and Ti4+ ions were investigated in a wide temperature range. It was found that Ti3+ and Ti4+ ions occupy octahedral and tetrahedral sites of Al3+, respectively, in the YAG structure. The emission intensity ratio of these ions, and hence emission color output, can be modulated by altering either dopant concentration or size of the nanoparticles. For the first time self-referenced nanocrystalline luminescent thermometer based on Ti4+ to Ti3+ emission intensity ratio was presented. Its optical response was investigated in wide range of temperatures, and the mechanism of excitation and emission of trivalent and tetravalent titanium ions was proposed. The highest relative 18 ACS Paragon Plus Environment

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sensitivity of such luminescent thermometer was found to be about 0.71%C-1 at 225oC and its usable temperature can be modulated by the dopant concentration. Obtained preliminary results suggest that further optimization of host material, synthesis and doping parameters may lead to the enhancement of the relative sensitivities of described luminescent thermometers. Moreover YAG:Ti nanocrystals co-doped with Nd3+, Eu3+ and Dy3+ were examined for suitability for luminescence thermometry. The obtained results indicate that the presence of Ln3+ ions increases the luminescence thermal quenching of Ti4+ ions due to the Ti4+→Ln3+ energy transfer. Therefore obtained sensitivity were several times higher than in the case Ti4+ single doped counterparts. The maximal relative sensitivities was found for YAG:Ti,Nd3+ nanocrystals to be 2.26%C-1 at 50 oC and its value further increases with temperature reaching 3.70%C-1 at 200 oC. Due to less efficient energy transfer nanocrystalline luminescent thermometer based on YAG:Ti,Dy3+ reveals much weaker relative sensitivity of about 1%C-1 at 50 oC and increases up to 1.25%C-1 at 100 oC. Obtained results indicate that titanium luminescence strongly depends on temperature changes of its environment and, therefore, titanium is a promising candidate as a temperature sensor in luminescent thermometry.

Supporting Information The primitive cell of YAG crystal and location of Ti ion used in the performed calculations. The dispersion of local density of states of YAG crystal and YAG crystal doped with Ti ions (located in tetrahedral and in the octahedral site). Grain size evolution with increase of annealing temperature. Unit cell parameters calculated from XRD data. XPS binding energy spectrum. TEM images. Photoluminescence decay curves (emission wavelengths 512 nm and 820 nm). Decay time evaluated from luminescence decay curves.



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Acknowledgements Authors would like to acknowledge prof. George Boulon for fruitful discussion. The „High sensitive thermal imaging for biomedical and microelectronic application" project is carried out within the First Team program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund.

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