High-Pressure and Temperature Upconversion Luminescence

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Functional Inorganic Materials and Devices

Multifunctional Optical Sensors for Nanomanometry & Nanothermometry: High-Pressure and Temperature Upconversion Luminescence of Lanthanide Doped Phosphates - LaPO4/YPO4:Yb3+-Tm3+ Marcin Runowski, Andrii Shyichuk, Artur Tymi#ski, Tomasz Grzyb, Víctor Lavín, and Stefan Lis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02853 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Multifunctional Optical Sensors for Nanomanometry & Nanothermometry: High-Pressure and Temperature Upconversion Luminescence of Lanthanide Doped Phosphates - LaPO4/YPO4:Yb3+-Tm3+ Marcin Runowski,1,* Andrii Shyichuk,2 Artur Tymiński,1 Tomasz Grzyb1, Víctor Lavín,3 and Stefan Lis1 1

Adam Mickiewicz University, Faculty of Chemistry, Umultowska 89b, 61-614 Poznań,

Poland 2

Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland

3

Departamento de Física, MALTA Consolider Team, and IUdEA, Universidad de La Laguna,

Apdo. 456, E-38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain

KEYWORDS: Ln3+ doped luminescent nanoparticles; Upconversion optical pressure and temperature gauge; Energy transfer; Lifetimes; Nanomaterials compression in DAC; Rare earth ions

ABSTRACT Upconversion luminescence of nano-sized Yb3+ and Tm3+ co-doped rare earth phosphates, i.e. LaPO4 and YPO4, has been investigated under high pressure (up to ~25 GPa) and temperature (293-773 K) conditions. The pressure-dependent luminescence properties of the nanocrystals, i.e. energy red-shift of the band centroids, changes of the band ratios, shortening of upconversion lifetimes, etc., make the studied nanomaterials suitable for optical pressure sensing in nanomanometry. Furthermore, thanks to the large energy difference (~1800 cm-1), the thermalized states of Tm3+ ions are spectrally well separated, providing high temperatureresolution, required in optical nanothermometry. The temperature of the system containing

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such active nanomaterials can be determined on the basis of the thermally-induced changes of the Tm3+ band ratio (3F2,3→3H6/3H4→3H6), observed in the emission spectra. The advantage of such upconverting optical sensors is the use of NIR light, highly penetrable for many materials. The investigated nanomanometers/nanothermometers have been successfully applied, as a proof-of-concept of a novel bimodal optical gauge, for the determination of the temperature of the heated system (473 K), which was simultaneously compressed under high pressure (1.5 and 5 GPa).

INTRODUCTION Multifunctional nanomaterials doped with trivalent lanthanide (Ln3+) ions are extensively studied due to their appealing optical, magnetic and structural features.1–9 Thanks to the small size of their particles (≤100 nm), such materials exhibit favorable morphological and mechanical properties, e.g. formation of colloidal systems and possibility of their manipulation in the nano-sized regions. Furthermore, their relatively low-cytotoxicity, intense multicolor luminescence of Ln3+ dopant ions (e.g. Eu3+, Tb3+, Er3+ and Tm3+), facile synthesis, thermal- and photo-stability make them excellent candidates for bio-application purposes, the use in forensics, as new light sources, multimodal tracers, optical gauges, etc.2-5,10–15 The favorable spectroscopic properties of Ln3+ ions originate from their unique electronic structure. Multicolor photoluminescence of Ln3+-doped materials excited with UV or NIR (upconversion) light comes from intrinsic 4f-4f transitions within Ln3+ ions, which are forbidden by Laporte selection rules, resulting in the long radiative lifetimes (µs-ms range).16– 18

Whereas, the crystal-field effect results in a configuration mixing and relaxes the selection

rules, which makes the transitions partially allowed and observable. Due to the shielding of 4f electrons by 5s and 5p ones, 4f-4f transitions reveal narrow absorption and emission bands.19–


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22

Moreover, luminescence properties of Ln3+-doped materials can be affected by pressure and

temperature.15,23–26 Among various Ln3+-doped luminescent nanomaterials, up-converting rare earth orthophosphates such as LaPO4 and YPO4 doped with Yb3+-Tm3+ ions reveal superior physicochemical properties.27,28 They are resistant to photodegradation, oxidation and high temperature.29 Moreover, such materials are insoluble in water (forming stable aqueous colloids), and their synthesis is easy, reproducible and cheap.27 However, their most favorable feature can be an effective upconversion luminescence (violet-blue), excited with NIR light (≈ 975 nm), which is highly penetrable for many media. In such systems, Yb3+ ions act as efficient sensitizers, due to their high absorption cross-section in the mentioned NIR range, transferring the excitation energy to the emitting Tm3+ ions.27,29 Due to the fact, that Tm3+ emission bands originate from two, three and four-photon transitions, and the presence of thermalized levels, the optical response of such materials is very sensitive to pressure and temperature alternations, which is important for optical sensor applications. By applying pressure one can vary the volume of the material. Thus a continuous change of the interatomic distances can be performed, and the effect of crystal structure on the f-electron states can be observed directly. The compression of Ln3+-based materials under high-pressure conditions is usually performed in a diamond anvil cell (DAC), and leads to the spectral shift of their absorption/emission bands (blue/red-shift), changes in band intensity ratios, bands broadening, longer/shorter luminescence lifetimes, etc.15,23,30–32 These changes can be used for pressure calibrations purposes (nanomanometry). As some Ln3+-doped materials are sensitive to temperature alterations, their luminescence properties may change with temperature significantly. Due to the thermalization processes, some energy levels, such as the 3F2 and 3F3 multiplets, involved in the 3F2,3→3H6 transitions in Tm3+ ions, are thermally populated, resulting in the presence of new bands emerging in the emission spectra.33,34 The



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ratios of the thermally populated/depopulated bands are also used for temperature determination of the system (nanothermometry).25 Here we report results of the extensive spectroscopic studies of the luminescent nanomaterials, i.e. Yb3+-Tm3+ co-doped rare earth phosphates, LaPO4 and YPO4, performed as a function of pressure (P) and/or temperature (T). When these variables change, materials exhibit variations in their luminescence properties, which can be used for the in-situ pressure and temperature calibration of the system, as novel bimodal high-pressure (HP) and hightemperature (HT) optical sensors in the nanoscale (nanomanometers and nanothermometers). The research novelty is focused on the pressure-modulated upconversion luminescence of the materials compressed in a DAC. It is worth noting that, to the best of our knowledge, this is the first time when the temperature (473 K) of a system compressed under HP conditions (≈1.5 and ≈5 GPa) has been determined using upconverting NPs.

EXPERIMENTAL PART The synthesis of the LaPO4 and YPO4 nanomaterials co-doped with 20 mol% of Yb3+ and 0.5 mol% of Tm3+ was performed via a simple precipitation process: a mixture of metal nitrates was precipitated with ammonium phosphate, and the product was calcined at 1273 K.27 Due to technical reasons and a relatively low intensity of the signal collected from the samples in DAC (sample size < 150 µm), all emission spectra at HP conditions were measured with a cooled R928P photomultiplier (PMT) from Hamamatsu coupled to a 750 mm focal length monochromator, Jobin-Yvon Spex 750M, and a tunable CW Ti:Sapphire laser system, Spectra Physics 3900-S pumped by a 15W 532 nm Spectra Physics Millenia, fixed at 975 nm as excitation source (1 W; spot size ≈0.5 mm2). Spectra at HT and ambient pressure were recorded using a FC-CW SSDP 975 nm laser (0.5 W; spot size ≈1 mm2) and a cooled Andor Newton CCD camera, (having a much higher sensitivity than the PMT in the 700-900 nm


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spectral range, where the thermalized bands of interest were located) with Andor Shamrock 328i spectrometer. The spectra used for temperature determination were additionally corrected for the instrumental response. Luminescence decay curves were collected using a 200 MHz LeCroy WS424 oscilloscope, coupled to the PMT, and a tunable EKSPLA/NT342/3/UVE 10 ns pulsed laser, optical parametric oscillator (OPO), with repetition rate of 10 Hz as excitation source. Transmission electron microscopy (TEM) measurements were performed using a FEI Tecnai G2 20 X-TWIN microscope, operating at 200 kV. Powder XRD patterns were recorded with a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å). The Raman spectra were recorded in a backscattering geometry using a Renishaw InVia confocal micro-Raman system with a power-controlled 100 mW 532 nm laser diode. The laser beam was focused using an Olympus x20 SLMPlan N long working distance objective. Technical details concerning measurements, sample preparation for DAC, and synthesis are presented in Supporting Information (SI).

RESULTS AND DISCUSSION Properties at ambient conditions Powder X-ray diffraction patterns of the LaPO4 and YPO4 NPs doped with Yb3+ and Tm3+ fit well with the reference patterns from the Inorganic Crystal Structure Database (ICSD): monoclinic structure for LaPO4 with 2 ⁄ space group (card no. 79747), and tetragonal for YPO4 with 4⁄amd space group (card no 79754), respectively (see Figure 1a). Reflexes broadening is related to the nanocrystallinity of the materials. Raman spectra of the materials, shown in Figure 1b, agree well with the literature data reported for the pure LaPO4 and YPO4.35,36 Absorption spectra of the products (Figure 1c) reveal typical narrow bands of Tm3+ ions located in the visible range, and a very broad and intense band in the NIR region, characteristic of Yb3+ ions. The average crystals size determined from TEM images for the



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LaPO4:Yb3+-Tm3+ NPs (Figure 1d; top) ranges from 70 to 150 nm, whereas for the smaller YPO4:Yb3+-Tm3+ NPs (Figure 1d; bottom) it ranges from 20 to 50 nm, respectively. The determined interplanar distances for both nanomaterials are very similar to the ones from ICSD for their bulk counterparts, i.e. ≈0.50 nm (110) for LaPO4 and ≈0.35 nm (200) for YPO4, respectively.

Figure 1. (a) Experimental powder XRD and ICSD reference patterns, (b) Raman spectra, (c) absorption spectra and (d) TEM images of the LaPO4: 20% Yb3+-0.5% Tm3+ and YPO4: 20% Yb3+0.5% Tm3+ samples.


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Properties at HP conditions Monoclinic, monazite-type LaPO4 (bulk modulus, B0=144(2) GPa) transforms to its HP phase above ≈24 GPa,37 whereas tetragonal, zircon-type YPO4 (B0=186(5) GPa) undergoes a phase transition around 14 GPa to a monoclinic, monazite-type phase (B0 = 260(29) GPa), and above ≈30 GPa to a HP tetragonal, scheelite-type phase.36 Raman spectra at HP conditions of the synthesized compounds were recorded (Figures S1 and S2), to study changes in the energy of the phonon modes, caused by compression of the system. All of the Raman peaks reversibly shift toward higher energy (see Figure 2), due to the shortening of the average distances between the ions (shorter bonds). The observed shifts are nearly linear up to ≈20 and 15 GPa, for LaPO4:Yb3+-Tm3+ and YPO4:Yb3+-Tm3+, respectively, where the mentioned phase transitions of LaPO4 and YPO4 occur. Noteworthy, the dominant high-energy phonon modes (responsible for multiphonon relaxation of luminescence) situated for both materials initially around 1000 cm-1, increase their energies with pressure at a different rate, i.e. +2.7 cm-1/GPa for LaPO4:Yb3+-Tm3+ and +4.3 cm-1/GPa for YPO4:Yb3+-Tm3+.



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Figure 2. Pressure dependences of the phonon modes for LaPO4:Yb3+-Tm3+ and YPO4:Yb3+-Tm3+; filled spheres represent compression data, and empty triangles decompression data.

HP upconversion luminescence The nanomaterials studied exhibit bright violet-blue upconversion (non-linear anti-Stokes) emission under 975 nm NIR laser irradiation, in resonance with the absorbance maximum of the 2F7/2→2F5/2 transition of Yb3+ ions. In these systems Yb3+ ions act as sensitizers, transferring the absorbed excitation energy to the emitting Tm3+ ions (activators), pumping their excited states via energy transfer upconversion (ETU) mechanisms.38–40 Upconversion emission of the Yb3+/Tm3+ ions pair is very sensitive to the compression of the system. The higher is the number of photons engaged in the transition, the more elementary energy transfer processes participate in its pumping. Energy transfer rates are also highly sensitive to the changes of interatomic distances.15 In the Yb3+/Tm3+ system, several radiative transitions require multi-photon absorption processes, i.e. 1D2→3F4 at ≈450 nm (four-photons); 1G4→3H6 at ≈480 nm and 1G4→3F4 at ≈650 nm (three-photons); 3F2,3→3H6 at ≈700 nm and 3H4→3H6 at ≈800 nm (two-photons).27 The systematic changes of the band ratios of these transitions and their energy, i.e. spectral position (red-shift), can be used for pressure sensing in nano-sized areas. The compression of the materials under HP conditions reveals a significant change of the shape of their upconversion emission spectra (Figures 3a and 3b). It can be observed that the relative intensity of the four-photon 1D2→3F4 band at ≈450 nm decreases the most, whereas intensity of the two-photon 3H4→3H6 band at ≈800 nm increases with pressure for both nanomaterials. As the three-photon 1G4→3H6 band at ≈480 nm shows high intensity at ambient 3

conditions,

which

decreases

with

pressure,

we

have

compared

the

H4→3H6/1G4→3H6 band ratios (Figure 3c). The band ratio reversibly increases about 3 times

for LaPO4:Yb3+-Tm3+ and about 20 times for YPO4:Yb3+-Tm3+ with increasing pressure. The


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significantly higher change for the second material is due to the sudden, non-linear and huge increase of the band ratio around 14 GPa, related to the phase transition of yttrium phosphate, which can be clearly observed in the emission spectra. The total luminescence intensity of both materials decreases with pressure, as observed in the non-normalized emission spectra (Figure 3d). This phenomenon is related to the increased probability of non-radiative energy transfer cross-relaxation (shorter interionic distances) and multiphonon (higher phonon energy and larger electron-phonon coupling) relaxation processes in the compressed materials. The relative decrease of the luminescence intensity is much higher for YPO4:Yb3+-Tm3+, probably due to the stronger phonon-assisted quenching in this host. This is because, as it was mentioned before, the energy of the highest phonon modes increase with pressure much faster for that material. Another plausible explanation is that pressure-induced phase transition results in a mixed-phase material, which means more defects capable of quenching and more vibrational modes that participate in non-radiative relaxation processes. Spectra recorded during decompression cycles are presented in the Supporting Information (Figures S3-S6). It is worth noting that the small hysteresis observed in the emission intensity, and further in the luminescence lifetimes during the compression-decompression cycle, is normally related to inelastic structural deformations, e.g. formation of crystal defects, permanently quenching some of the active centers (Yb3+-Tm3+).23



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Figure 3. (a, b) Normalized and (c) non-normalized upconversion emission spectra, and (d) 3

H4→3H6/1G4→3H6 band ratio of the LaPO4:Yb3+-Tm3+ and YPO4:Yb3+-Tm3+ at HP; λex= 975 nm.

All of the peak centroids (Figure 4a,b) reversibly red-shift with increasing pressure, due to the shorter interionic Tm-O distances and stronger interactions between the ions, which results in a smaller energy difference between Tm3+ ground and excited states.30 In general, the energies of the 4f ground configuration and the barycenters of the multiplets decrease with pressure, since the coulomb and spin-orbit interactions between f-electrons decrease, whereas the crystal-field strength increases with pressure, thus increasing the splitting of each 10 ACS Paragon Plus Environment

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multiplet. The shift is larger for the bands with lower number of photons engaged in the transition, i.e. lower energy ones observed at longer wavelengths (+0.03 nm/GPa for 1

D2→3F4; +0.1 nm/GPa for 1G4→3H6 and 1G4→3F4; +0.25 (in LaPO4) and +0.8 nm/GPa (in

YPO4) for 3H4→3H6. The shift is nearly linear for the LaPO4:Yb3+-Tm3+ up to ≈23 GPa, and for the YPO4:Yb3+-Tm3+ up to ≈13 GPa. The apparent nonlinear bias/slope above these values is related to the above mentioned phase transitions of the materials, occurring at slightly lower pressure values than the reported ones. This can be related to the high sensitivity of Tm3+ luminescence to structure and site symmetry alterations, as well as nanocrystallinity of the products, since phase transitions can take place in NPs at slightly different pressure values in comparison to their bulk analogues.41,42 The width of the bands, i.e. full width at half maximum (FWHM) reversibly increases with pressure (see Figures 4c and 4d), due to the increasing strains/distortions of the materials, formation of crystal defects and deviations from hydrostaticity observed at higher pressure values. The sudden bias/slope, i.e. huge increase of FWHM is observed in the mentioned phase transition pressure range, it is related to the formation of the second emitting Tm3+ site in the nanomaterials studied. Due to the phase transition of YPO4:Yb3+-Tm3+ around 14 GPa, the changes in Tm3+ peak positions and their broadening are more pronounced in this material, than in LaPO4:Yb3+-Tm3+. It can be taken also into account that there is always a hysteretic behavior of the structure when compressing to high pressures and releasing the pressure back to ambient conditions. The material never recovers exactly the initial structure. This can be even more dramatic when a phase transition occurs during compression.



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Figure 4. (a, b) Spectral positions of the emission bands and (c, d) FWHM for the LaPO4:Yb3+-Tm3+ and YPO4:Yb3+-Tm3+ as a function of pressure.

Pressure calibration via band ratio and luminescence red-shift The alternative pressure calibration curves, based on the upconversion luminescence of LaPO4:Yb3+-Tm3+ were determined (see Figures S7 and S8). This compound reveals reversible

and more linear optical response in a broader pressure range, in comparison to the YPO4:Yb3+-Tm3+ one, which is crucial for sensor applications. The 3H4→3H6/1G4→3H6 band ratios were correlated with pressure according to the 2nd order polynomial, with R2 = 0.964 (Figure S7). Whereas, a linear fit was applied to the pressure-induced red-shift of the most 12 ACS Paragon Plus Environment

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intense 1G4→3H6 emission band, with R2 = 0.962 (Figure S8). The accuracy of the applied methods was estimated to be about ±1 GPa. Such upconverting optical nanosensors may be used for estimation of the pressure values of some non-transparent systems, especially in the nano-sized regions. HP lifetimes Compression of the materials studied influences greatly the energy transfer rates between Yb3+ and Tm3+ ions, manifested in the altered upconverted luminescence decay profiles and lifetimes, shown for LaPO4:Yb3+-Tm3+ in Figure 5 and YPO4:Yb3+-Tm3+ in Figure 6. Upconversion lifetimes (τ) of the compressed materials were determined via fitting the recorded decay profiles to the exponential functions:

I = A0 (1 – exp(–t/τrise))n (exp(–t/τ1-decay) + A3 exp(–t/τ2-decay))

(1)

that can be simplified, if A3 = 0, to: I = A0 (1 – exp(–t/τrise))n exp(–t/τdecay)

(2)

where I is the emission intensity at time t; A0 is the amplitude; τ1 is the rise time; and τ2 and τ3 are the luminescence lifetimes. These are the simplified functions; the functions that were actually used in fitting also comprised vertical and horizontal offsets. The detailed lists of the determined lifetime values and fitting parameters used are shown in Tables S1-S8 in the SI. The description of the fitting routine, as well as some comments on functions (1) and (2), are presented in the SI. The reason to select these particular functions was two-fold. On the one hand, they result in almost perfect fit, with high determination coefficients, R2, and fitting residual (i.e. fitted function minus original function) that contained only noise. With many other functions, the residual obviously contained some signal, exhibiting a noisy curve shape. To the best of our concerns, the residual of a properly made fit must contain a (noisy) horizontal line at y = 0. On the other hand, such curves correspond well to what can be the actual physics of the observed rise and decay profiles. Such functions present mutually 13 ACS Paragon Plus Environment


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dependent rise and decay processes. They correspond to a simplified dynamical model, in which a certain emitting level is populated by energy transfer process from another level undergoing an exponential decay. The emitting level thus undergoes exponential rise is represented by lifetime τ1. The very same level is emitting, i.e. undergoes a decay. Classical exponential decay assumes constant initial population, I0, and goes as I = I0 exp(–t/τdecay). In this case, the initial population undergoes exponential rise, I = I0 (1 – exp(–t/τrise)). Functions (1) and (2) represent exponential decays of exponentially rising level, i.e. (1 – exp(–t/τrise)) times exp(–t/τdecay), a rise-times-decay kinetics. The increase of pressure causes a reversible (with some hysteretic effect) and nearly monotonic decrease of the luminescence rise and decay times (≈5-fold above 20 GPa), for all transitions of both materials, up to the phase transition pressure range. In the case of the LaPO4:Yb3+-Tm3+, at around 23 GPa the upconversion lifetime starts to increase dramatically (longer lifetime of the HP phase). For YPO4:Yb3+-Tm3+, around 14 GPa, the average lifetime values drastically decrease and the second lifetime component (τ2-decay) has appeared (clearly seen as a “bend” of the decay profiles observed at HP values in Figure 6; for the clarity, the values of τ2-decay are given only in SI, Tables S5-S8). As observed, the upconversion lifetimes are very sensitive to pressure-induced phase transitions, manifesting an evident non-linear behavior, so they can be potentially used as an optical sensor for phase transitions. As all of the ions get closer to each other in the compressed materials (shorter interionic distances), the Yb3+→Tm3+ ET rates are enhanced, manifested in a decrease of the rise times. Similarly, within Tm3+ ions the radiative and non-radiative relaxation increase, leading to the shortening of the luminescence decay times.15 Besides the cross-relaxation processes, the multiphonon quenching is also enhanced with pressure and contributes to the shortening of lifetime values.15 This is because of the previously mentioned increase of the energy of the phonon modes in the compressed phosphate matrices.


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Figure 5. (left) Luminescence decay curves and (right) determined upconversion lifetimes, i.e. rise and decay times for LaPO4:Yb3+-Tm3+ at HP; λex= 975 nm, λem= 451, 480, 648 and 790 nm for 1

D2→3F4, 1G4→3H6, 1G4→3F4, 3H4→3H6 transitions, respectively. 15 ACS Paragon Plus Environment


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Figure 6. (left) Luminescence decay curves and (right) determined upconversion lifetimes, i.e. rise and decay times (right) for YPO4:Yb3+-Tm3+ at HP; λex= 975 nm, λem= 451, 480, 648 and 796 nm for 1

D2→3F4, 1G4→3H6, 1G4→3F4, 3H4→3H6 transitions, respectively. 16 ACS Paragon Plus Environment

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HT upconversion luminescence Thanks to the presence of thermalized states within Tm3+ ions, i.e. 3F2,3 (3H4∆ 3F2,3), the →

Tm3+-doped nanomaterials might also act as optical temperature sensors.25 Comparing the 3

H4→3H6/3F2,3→3H6 band ratio in the emission spectra, the temperature of the system can be

determined.33,34 Since both these bands correspond to two-photon transitions, and the 3

F2,3→3H6 one is populated from the 3H4 level by thermalization, the pressure effect on the

3

H4→3H6/3F2,3→3H6 band ratio is negligible. Hence, it is possible to determine the

temperature of the systems compressed under HP conditions independently of the pressure. The impact of temperature on the nanomaterials luminescence is presented in Figure 7. The increase of temperature of the system results in a significant decrease of all emission bands (Figure 7a, b), except the one at ≈700 nm, related to the 3F2,3→3H6 transition, whose intensity measured at ambient conditions was initially low. Due to the thermalization process (3H4∆ 3F2,3) the intensity of this band has significantly increased with temperature for the →

samples studied. Thermalization of states can effectively occur for the energy levels separated from 200 to 2000 cm-1.43 As the temperature-populated 3F2,3 level (level 2) is situated about 1843 (LaPO4:Yb3+-Tm3+) and 1778 cm-1 (YPO4:Yb3+-Tm3+) above the 3H4 level (level 1), the thermalization process, favored at higher temperature, takes place according to the Boltzmann distribution: ≡

∆

=

(3)

where: FIR is fluorescence intensity ratio; I2 is intensity of 3F2,3→3H6 transition; I1 is intensity of 3H4→3H6 transition; ∆E is energy difference between the centroids of these two transitions (E2 – E1); kB is Boltzmann constant; T is absolute temperature; and B is a constant that depends on degeneracies of states, total spontaneous emission rates, transitions branching ratio relatively to the ground state and angular frequencies of the transitions).



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The FIR of these two transitions is known as a thermometric parameter, as it is directly connected with temperature of the luminescent material and conforms to Boltzmann distribution.24,44 The curves in Fig. 7c,d are least-squares fits of the Eq. (3) to the experimental plots of FIR as a functions of temperature (both B and ∆E varied during fit). The calculated ∆E values used for fitting were smaller than the ones derived from the emission spectra, i.e. 1790.5 and 1395.2 cm-1 for LaPO4:Yb3+-Tm3+ (B = 3.8) and YPO4:Yb3+-Tm3+ (B = 1.5), respectively. Figure 7c, d shows the evolution of the thermometric parameter for the luminophores studied, in the temperature range from 293-773 K. The FIR value was initially (at ambient conditions) close to zero, and then increased up to ≈0.140 (LaPO4:Yb3+-Tm3+) and 0.115 (YPO4:Yb3+-Tm3+) at the highest temperature. Another commonly used luminescence thermometer property is its relative sensitivity, Sr.25 Sr shows (in %) how is the peak ratio change (per 1 K change of absolute temperature) in respect to initial value of FIR. Sr is defined as: !

= 100% × &

1 ' ∆) ( = 100% × ' *+ ,

(4)

The insets in Figure 7c, d present the relative sensitivity, Sr (%K-1) of the luminescent materials, applying the experimental values of ∆E. This parameter is generally used to compare the performance of different thermometers.25 The relatively high Sr value, comparing to other luminescence thermometers, is due to the large energy separation between the thermalized states. It is worth noting that another benefit of the large energy difference in the case of the Tm3+-based thermometers, is a good spectral separation of the thermalized bands,33 in contrast to the other thermometers based on Er3+ or Nd3+ ions, suffering from overlapping bands generating difficulties in the precise temperature determination.43,45,46 The increasing temperature also influences the spectral positions of the peak centroids, as blueshift is observed for the all recorded transitions (Figure 7e, f), implying a slight thermally-


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induced increase of the energy difference between Tm3+ ground and excited states. The thermal shift of an emission line, as well as the simultaneous thermally-induced increase of its FWHM, comes from two contributions: the static contribution ∆Est(T), due to the changes in the geometry of site occupied by the lanthanide ion in the crystal caused by the lattice thermal expansion, and the vibrational contribution ∆Evib(T), due to the electron–phonon interactions. However, the detailed analysis is very complex, especially for the highly overlapped emission peaks we´ve got for the Tm3+ emission bands, and since many parameters are needed its study is a difficult task and would require further studies.



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Figure 7. Emission spectra (CCD) (a, b); FIR - 3F2,3→3H6/3H4→3H6 ratio and relative sensitivity insets (c, d); spectral position of the emission bands (e, f) for LaPO4:Yb3+-Tm3+ (a, c, e) and YPO4:Yb3+Tm3+ (b, d, f) at HT; λex= 975 nm.

HP and HT upconversion luminescence For the next experiment performed under simultaneous HP and HT conditions, LaPO4:Yb3+-Tm3+ was selected as a potential temperature and pressure sensor. This material exhibits monotonic and more linear response to the pressure-induced changes of the luminescence properties, e.g. energy red-shift of the bands and their ratio, as well as higher thermal sensitivity than YPO4:Yb3+-Tm3+. Initially the pressure value in DAC was adjusted to ≈1.5 GPa (Pressure I) and temperature 293 K. The increase in temperature caused a slight variation of the pressure values in the DAC (determined by ruby fluorescence shift), as the metal elements of the chamber, pressure transmitting medium and the sample expand differently as a function of temperature. Moreover, the precision of pressure determination with ruby deteriorates with temperature, as its emission intensity decreases, and the bands reveal a significant thermal broadening effect (see Figure S11 in SI). When the temperature reached 473 K, the system was left to cool down. At the next step, the pressure was increased to ≈5.0 GPa (Pressure II), and the temperature was again elevated to 473 K. The upconversion luminescence of the LaPO4:Yb3+Tm3+ sample in DAC at HP and HT (Figure 8) changes considerably, i.e. shape of the spectra (band ratios) varies and emission intensity decreases as a function of both factors. Due to the complex influence of both pressure and temperature on the luminescence properties of the sample, we were unable to precisely determine the pressure of the system using LaPO4:Yb3+Tm3+ luminescence (shift of the peak centroids, change of the band ratios, etc.) as a pressure sensor working at the elevated temperature. However, we have successfully determined the 21 ACS Paragon Plus Environment


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approximated values of the local temperature of the sample in DAC at HP, using the calibration curve from Figure 7c and the band ratio of the thermalized levels, extracted from the measured spectra. For both pressure values (I and II, i.e. ≈1 and 5 GPa) and the temperature of 473 ±1 K (determined with a thermocouple), the calculated temperature value using a luminescence-based LaPO4:Yb/Tm nanothermometer was ≈468 ±4 K (FIR ≈0.0155), which is very close to the value measured with a thermocouple. This was possible, as both thermalized levels are related to two-photon transitions (3F2,3→3H6 and 3H4→3H6 bands), and the pressure affects them similarly. It can be better seen comparing the normalized spectra (Figure S12); namely, the band ratio (FIR) in the spectra recorded at different pressure values and at the same temperature is nearly the same. Figure S13 shows the spectra corrected for the instrumental response, which were used for temperature determination. It is worth noting, that the temperature calibration experiments (Figure 7) have been performed using a different detection systems, excitation sources, pressure values, sample form/preparation and surrounding (DAC) of the specimen, in comparison to the measurements performed under HP conditions. The obtained results confirm the realapplication potential of the LaPO4:Yb3+-Tm3+ nanomaterial as a sensor of temperature (nanothermometer) at both ambient and HP conditions, and the possibility of its application by others using various configurations/setups for the luminescence-based temperature determination. Moreover, after appropriate calibration, the nanomaterials studied can be potentially used as sensors of pressure (nanomanometer) at constant temperature, thanks to the pressure-induced shifts/changes of their luminescence properties.


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Figure 8. Emission spectra (PMT) for LaPO4:Yb3+-Tm3+ at HP and HT; λex= 975 nm.

CONCLUSIONS The measurements of upconversion emission of LaPO4:Yb3+-Tm3+ and YPO4:Yb3+-Tm3+ nanocrystals under HP and HT conditions revealed their strong luminescence dependence on the state functions studied. Increase of the pressure and/or temperature of the system results in a shift of the spectral position of the bands, changes of the band ratios, shortening of luminescence rise and decay times, decrease of emission intensity and significant broadening of the emission bands for the Ln3+-doped nanoluminophores. These phenomena are related to the shorter interionic distances in the compressed materials, formation of crystal defects, increased probability of cross-relaxation, multiphonon quenching and thermalization processes. The observed changes of luminescence were correlated with the pressure and temperature values of the system, resulting in the determined calibration curves. We report for the first time the materials that can be used as novel bimodal optical sensors, i.e. nanomanometers and nanothermometers. As the thermalized levels of Tm3+ ions (two-photon 3

F2,3→3H6 and 3H4→3H6 transitions) are similarly affected by the HP, their band ratio has

been successfully used for the temperature determination of the system compressed under HP



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conditions. Due to the ease of temperature calibration, the materials studied have a real potential application as novel, remote (non-contact) optical sensors of local temperature, especially in nano- and micro-sized regions subjected to HP.

ASSOCIATED CONTENT Supporting Information Raman spectra; emission spectra and luminescence decay curves (decompression); pressure calibration curves; luminescence lifetimes and fitting parameters; ruby emission spectra at HP and HT. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author M.R.: Phone: +48618291778 E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Polish National Science Centre, grant No. 2016/23/D/ST4/00296, Polish Ministry of Science and Higher Education, grant No. IP2014 014573, Spanish MINECO (MAT2016-75586-C4-4-P), the Agencia Canaria de Investigación, Innovación y Sociedad de la Información ACIISI (ProID2017010078) and by EU-FEDER funds.

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High-Pressure and Temperature Upconversion Luminescence

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