Unraveling the Electronic Structures of Neodymium in LiLuF4 Nanocrystals for Ratiometric Temperature Sensing.

Nd3+-doped near-infrared (NIR) luminescent nanocrystals (NCs) have shown great promise in various bioapplications. A fundamental understanding of the electronic structures of Nd3+ in NCs is of vital importance for discovering novel Nd3+-activated luminescent nanoprobes and exploring their new applications. Herein, the electronic structures of Nd3+ in LiLuF4 NCs are unraveled by means of low-temperature and high-resolution optical spectroscopy. The photoactive site symmetry of Nd3+ in LiLuF4 NCs and its crystal-field (CF) transition lines in the NIR region of interest are identified. By taking advantage of the well-resolved and sharp CF transition lines of Nd3+, the application of LiLuF4:Nd3+ NCs as sensitive NIR-to-NIR luminescent nanoprobes for ratiometric detection of cryogenic temperature with a linear range of 77-275 K is demonstrated. These findings reveal the great potential of LiLuF4:Nd3+ NCs in temperature sensing and also lay a foundation for future design of efficient Nd3+-based luminescent nanoprobes.

Introduction

Trivalent neodymium (Nd3+) ion doped luminescent nanocrystals (NCs) have recently attracted considerable attention owing to their superior optical properties in the near‐infrared (NIR) spectral region.1 These Nd3+‐doped NCs are able to emit NIR light under excitation with a low‐cost 808 nm diode laser, which feature a series of advantages such as large penetration depth, minimal background interference, and little damage to the targeted samples, and thus are regarded as excellent NIR‐to‐NIR luminescent nanoprobes for various bioapplications.2 Specifically, Nd3+‐doped NCs have been frequently used as sensitive nanothermometers for physiological temperature sensing in tissues based on the temperature‐dependent energy transfer with other lanthanide (Ln3+) activators or the eletronic transitions from the thermally coupled crystal‐field (CF) energy levels of Nd3+.3 Nonetheless, the CF transition lines of Nd3+‐doped NCs are usually elusive and undistinguishable at physiological temperatures because of the line broadening and multiple sites of Nd3+ in NCs.4 As a result, the assignment of CF transition lines in previously reported Nd3+‐based nanothermometer had to rely on the reference of CF levels of bulk analogs,[[qv: 3b,5]] which could be unreliable and lead to an artificial detection result. Therefore, it is urgent to unravel the electronic structures of Nd3+ in NCs and assign its CF transition lines in the NIR of interest, which is of fundamental significance for designing novel Nd3+‐activated luminescent nanoprobes and exploring their new applications.

Lithium lutetium tetrafluoride (LiLuF4), owing to its low phonon energy and high chemical stability, is an excellent host material for Nd3+ doping to produce efficient upconverting and downshifting luminescence.6 Nd3+‐activated LiLuF4 bulk crystals have been documented as efficient solid‐state laser crystals,7 and their nanoscale counterparts have been reported as sensitive NIR‐to‐NIR luminescent nanoprobes for subtissue bioimaging.[[qv: 6d]] Moreover, the luminescence of Nd3+ in LiLuF4 lattice is characterized by sharp emission peaks even at room temperature due to the strong CF level splitting,[[qv: 6a]] which facilitates the discrimination of the CF transition lines of Nd3+ in NIR and thereby enables the assignment of CF energy levels of Nd3+ in nanoscale LiLuF4.

Herein, we report for the first time the electronic structures of Nd3+ in LiLuF4 NCs. We first use Eu3+ ion as the structural probe to unveil the local site symmetry of Ln3+ dopants in LiLuF4 NCs by means of high‐resolution photoluminescence (PL) spectroscopy, time‐resolved PL (TRPL) spectroscopy, and site‐selective PL spectroscopy at 10 K. With definite local site symmetry, we then assign the CF transition lines of Nd3+ in the NIR region and the corresponding CF levels through temperature‐dependent PL spectroscopy. Furthermore, by taking advantage of the well‐resolved CF transition lines from the thermally coupled Stark sublevels of 4F3/2 of Nd3+, we show the application of LiLuF4:Nd3+ NCs as NIR‐to‐NIR luminescent nanoprobes for ratiometric detection of cryogenic temperature with high reliability and sensitivity, thereby revealing the great potential of LiLuF4:Nd3+ NCs for temperature sensing.

Results and Discussion

The LiLuF4 crystal has a scheelite structure (space group I41/a) with Lu3+ ions surrounded by eight F− ions that form the edges of a slightly distorted dodecahedron. All Lu3+ ions occupy a single crystallographic site of S 4 symmetry (Figure a). High‐quality LiLuF4:Ln3+ (Ln = Eu and Nd) NCs were synthesized through a thermal decomposition method as we previously reported.[[qv: 6b]] The as‐synthesized NCs are hydrophobic and can be readily dispersed in a variety of nonpolar organic solvents such as cyclohexane. X‐ray diffraction (XRD) patterns (Figure 1b) show that all diffraction peaks of the NCs can be well indexed into tetragonal LiLuF4 (JCPDS No. 027‐1251) without any additional impurities. Transmission electron microscopy (TEM) images show that both LiLuF4:Nd3+ and LiLuF4:Eu3+ NCs are rhomboid with mean sizes of (28.4 ± 1.2) × (33.1 ± 1.5) and (28.0 ± 0.9) × (34.2 ± 1.0) nm, respectively (Figure 1c–e). High‐resolution TEM images (insets of Figure 1c,d) exhibit clear lattice fringes with an observed d spacing of 0.46 nm for the (101) plane of tetragonal LiLuF4, confirming pure phase and high crystallinity of the resulting NCs. Compositional analyses through energy‐dispersive X‐ray spectrum and inductively coupled plasma‐atomic emission spectroscopy reveal 1.8 mol% of Nd3+ and 4.6 mol% of Eu3+ in LiLuF4 matrix (Figure S1, Supporting Information), which are generally consistent with their nominal dopant concentrations (2 mol% of Nd3+ and 5 mol% of Eu3+).

Figure 1

a) Crystal structure of tetragonal LiLuF4 and the crystallographic site for Ln3+ dopants. b) XRD patterns of LiLuF4:2%Nd3+ and LiLuF4:5%Eu3+ NCs. The bottom lines represent the standard XRD pattern of tetragonal LiLuF4 (JCPDS No. 027‐1251). c,d) TEM images of LiLuF4:2%Nd3+ and LiLuF4:5%Eu3+ NCs. The insets show the corresponding high‐resolution TEM images. e) Size distributions of the NCs obtained by randomly calculating 200 particles in the TEM images. The blue and red bars represent the width and length of the rhomboid NCs, respectively.

Eu3+ ion is a sensitive spectroscopic probe, which can provide site symmetry information because of its nondegenerate emissive state of 5D0 and the ground state of 7F0.8 To probe the practical local site symmetry of Ln3+ dopants in LiLuF4 NCs, we measured the high‐resolution PL spectra of Eu3+ in LiLuF4 NCs. Emission and excitation spectra and PL decays were recorded at 10 K to avoid thermal broadening of spectral lines at room temperature (Figure S2, Supporting Information).9 Figure shows the high‐resolution PL spectra of LiLuF4:5%Eu3+ NCs at 10 K, which enables a detailed assignment of the CF transition lines of Eu3+. By monitoring the Eu3+ emission at 613.8 nm, a series of CF transition lines of Eu3+ from the 7F0 ground state to the excited multiplets (5DJ, 5LJ, 5GJ, 5HJ, and 5FJ) were observed (Figure 2a).10 Upon excitation to 5L6 of Eu3+ at 393.0 nm, the CF emission peaks from 5D0 and 5D1 to 7FJ (J = 0, 1, 2, 3, and 4) with full‐width at half‐maximum (FWHM) smaller than 0.5 nm were detected (Figure 2b). PL decay measurements show that both 5D0 and 5D1 display a single exponential decay with PL lifetimes of 11.3 and 2.7 ms, respectively (Figure 2c), suggesting a homogeneous CF environment around Ln3+ dopants in LiLuF4 lattice.11 The distinct PL lifetimes of 5D0 and 5D1 allow us to distinguish the emission peaks of 5D0 from those of 5D1 by means of TRPL spectroscopy. Figure 2d shows the TRPL spectra of LiLuF4:5%Eu3+ NCs at 10 K with different delay times. It was observed that the emission peaks from the short‐lived 5D1 level declined gradually with increasing the delay time and totally vanished when the delay time was longer than 6 ms, while the emission peaks from the long‐lived 5D0 level remained explicitly observed in the TRPL spectra even at a delay time of 10 ms. As a result, total numbers of 0, 2, 3, 4, and 4 CF transition lines of Eu3+ from 5D0 to 7F0, 7F1, 7F2, 7F3, and 7F4 can be discerned in LiLuF4 NCs. To check whether all these transition lines arise from the same site, site‐selective excitation spectra were measured by monitoring the three peaks of 5D0→7F2 at 610.4, 613.8, and 620.8 nm. The obtained excitation spectra were coincident (Figure S3, Supporting Information), indicating that the PL originated from Eu3+ ions occupying a single spectroscopic site, as also evidenced by the essentially identical site‐selective emission spectra upon excitation to 5L6 at 393.0, 395.8, 399.4, and 400.6 nm and the same PL lifetimes of the 5D0→7F1 emissions at 590.6 and 593.9 nm (Figure 2e and Figure S4, Supporting Information). According to the branching rules and the transition selection rules of the 32 point groups (Table S1, Supporting Information),[[qv: 8b,c]] the spectroscopic site symmetry of Eu3+ in LiLuF4 NCs was determined to be S 4, which agrees well with the crystallographic site symmetry of Lu3+ in LiLuF4. These results suggest that Ln3+ ions are prone to occupy a single spectroscopic site of S 4 symmetry in LiLuF4 NCs at low doping levels (<5 mol%) due to the close ionic radii and chemical properties of Ln3+ ions.

Figure 2

a) 10 K PL excitation spectrum of LiLuF4:5%Eu3+ NCs by monitoring the Eu3+ emission at 613.8 nm and b) their emission spectrum upon excitation at 393.0 nm. The inset in (b) enlarges the 5D0→7F4 emissions including four CF transition lines, and the asterisks represent the 5D1→7F4 emissions of Eu3+. c) PL decay curves of LiLuF4:5%Eu3+ NCs by monitoring the 5D0→7F2 and 5D1→7F1 emissions of Eu3+ at 613.8 and 582.8 nm, respectively. d) 10 K TRPL spectra of LiLuF4:5%Eu3+ NCs with different delay times under excitation at 393.0 nm. The asterisks denote the CF transition lines from the 5D1 multiplet of Eu3+. e) PL decay curves of LiLuF4:5%Eu3+ NCs by monitoring the 5D0→7F1 emissions of Eu3+ at 590.6 and 593.9 nm.

With definite local site symmetry of Ln3+ dopants, we are able to assign the CF transition lines of Nd3+ in LiLuF4 NCs by means of high‐resolution PL spectroscopy. Figure a shows the PL excitation spectrum of Nd3+ in LiLuF4 NCs at 10 K by monitoring the Nd3+ emission at 1053.2 nm, from which a series of CF transition lines of Nd3+ from the 4I9/2 ground state to the excited multiplets (4FJ, 2HJ, 4SJ, 2GJ, 4GJ, 2DJ, and 4DJ) were identified.12 Specifically, two excitation peaks at 861.4 and 865.9 nm were clearly observed (inset of Figure 3a), ascribing to the CF transitions of Nd3+ from the 4I9/2 ground state to the upper (R2) and lower (R1) Stark sublevels of 4F3/2, respectively. This implies that the CF levels of Nd3+ are doubly degenerate in LiLuF4 NCs, as expected for a Kramers ion.13 10 K PL emission spectrum (Figure 3b) shows that the NCs exhibit a set of characteristic and sharp emission peaks (FWHM < 0.9 nm) from the two Stark sublevels of 4F3/2 (R1 and R2) to those of 4I9/2, 4I11/2, and 4I13/2 of Nd3+ under xenon lamp excitation at 791.3 nm. To confirm that all these transition lines arise from a single site, we recorded PL emission spectrum of the NCs by exciting them with an 808 nm diode laser, whereby all possible spectroscopic sites of Nd3+ could be excited in view of the high power density (50 W cm−2) and relatively wide FWHM (3.2 nm) of the laser source (Figure S5, Supporting Information). It turned out that the emission pattern of the NCs under 808 nm diode laser excitation was exactly identical to that under xenon lamp excitation at 791.3 nm (Figure S6, Supporting Information), inferring that the PL originates from Nd3+ ions occupying a single spectroscopic site. From the emission spectrum, total numbers of 10, 12, and 14 CF transition lines of Nd3+ from 4F3/2 to 4I9/2, 4I11/2, and 4I13/2 were discerned, which agree well with the theoretically predicted numbers for Nd3+ in LiLuF4 with doubly degenerate CF levels.13 These results demonstrate unambiguously that Nd3+ ions occupy a single spectroscopic site of S 4 symmetry in LiLuF4 NCs, as revealed by using Eu3+ as the structural probe.

Figure 3

a) 10 K PL excitation spectrum of LiLuF4:2%Nd3+ NCs by monitoring the Nd3+ emission at 1053.2 nm and b) their emission spectrum upon excitation at 791.3 nm. The inset in (a) shows two CF transition lines from the 4I9/2 ground state to the upper and lower Stark sublevels of 4F3/2. c) Temperature‐dependent PL emission spectra (10–300 K) for the 4F3/2→4IJ (J = 9/2, 11/2, and 13/2) CF transitions of Nd3+ in LiLuF4 NCs upon 808 nm diode laser excitation at a power density of 1 W cm−2. The spectra were normalized at the maximum intensities around 880.4, 1053.1, and 1325.1 nm for the emissions from 4F3/2 to 4I9/2, 4I11/2, and 4I13/2, respectively. The dashed lines denote the CF transitions from the R1 (black) and R2 (red) Stark sublevels of 4F3/2 to those of 4IJ. d) CF energy levels of the 4F3/2 and 4IJ multiplets of Nd3+ in LiLuF4 NCs, showing all CF transitions observed in (c).

Because the energy difference between the R1 and R2 Stark sublevels of 4F3/2 is only 58 cm−1, the higher Stark sublevel (R2) is easily thermally populated from the lower one (R1). As a result, PL intensity ratio between the emissions from R2 and R1 would increase with the temperature rise, enabling discrimination of the CF transition lines of R2 from those of R1 through the temperature‐dependent PL emission spectra (Figure 3c). It was observed that the intensities of the CF emission peaks at 861.9, 872.2, 875.7, 882.3, and 904.9 nm increased significantly with the temperature rise, corresponding to the transitions from the R2 sublevel of 4F3/2 to the five CF levels (Z1, Z2, Z3, Z4, and Z5) of 4I9/2 (Figure 3d).[[qv: 12c]] By contrast, the intensities of the CF emission peaks at 866.2, 876.9, 880.4, 886.4, and 909.8 nm showed only a slight increase with the temperature rise, corresponding to the transitions from the R1 sublevel of 4F3/2. The slight increase in R1 lines is caused by the spectral overlap with R2 lines. Based on the defined CF transition lines, the CF levels of 4I9/2 can be unequivocally identified, as listed in Table . Similarly, CF transition lines from the R1 and R2 sublevels of 4F3/2 to those of 4I11/2 and 4I13/2 can be specified by virtue of the temperature‐dependent PL emission spectra, from which the CF levels of 4I11/2 and 4I13/2 were experimentally assigned (Table 1). Besides, we also found a redshift in CF transition lines of Nd3+ with the temperature rise, especially for the transitions from R2 of 4F3/2 to Z1 and Z2 of 4I11/2 (Figure 3c), as a result of enhanced electron‐phonon coupling at higher temperatures.[[qv: 4b]] Importantly, we found that the CF transition lines from the thermally coupled R1 and R2 Stark sublevels of 4F3/2 to Z1 of 4I9/2 at 862 nm (R2→Z1) and 866 nm (R1→Z1) are well resolved with little interference from other Stark components at temperatures below 300 K. This feature makes LiLuF4:Nd3+ NCs an ideal nanoprobe candidate for ratiometric luminescent detection of temperature below 300 K by using the temperature‐dependent PL intensity ratio between the R2→Z1 and R1→Z1 transitions at 862 and 866 nm (I 862/I 866), respectively.

Table 1

Experimental energy levels for the 4IJ and 4F3/2 multiplets of Nd3+ in LiLuF4 NCs

Multiplet	Energy [cm−1]	Multiplet	Energy [cm−1]	Multiplet	Energy [cm−1]	Multiplet	Energy [cm−1]
4I9/2	0	4I11/2	2003	4I13/2	3948	4F3/2	11 544
	138		2041		3978		11 602
	183		2047		3996
	265		2086		4027
	549		2241		4222
			2283		4254
					4266

To validate the applicability of LiLuF4:Nd3+ NCs for temperature sensing, we deconvoluted the 4F3/2→4I9/2 emission spectra of Nd3+ into ten Gaussian components according to the CF transitions between their Stark sublevels (Figure a), from which the PL intensity ratio between R2→Z1 and R1→Z1 (I 862/I 866) was derived. Further temperature‐correlated PL emission spectra showed that the PL intensity ratio I 862/I 866 increased gradually with increasing the temperature from 77 to 575 K (Figure S7, Supporting Information), as a result of enhanced thermal population of the R2 sublevel from the R1 sublevel of 4F3/2 at higher temperatures. Specifically, the ratio of I 862/I 866 displayed a linear dependence on temperature in the range of 77–275 K, with its value increased from 1.46 at 77 K to 3.23 at 275 K (Figure 4b). Moreover, such temperature evolution of I 862/I 866 was found to be reversible during the heating and cooling cycle between 77 and 275 K. The ratios of I 862/I 866 recorded at 77, 175, and 275 K were nearly unchanged with deviations smaller than 0.5% over a span of 20 cycles of heating and cooling processes (Figure 4c), as a merit of high photochemical stability of the NCs.14 This indicates that the PL of LiLuF4:Nd3+ NCs is fully reversible without any observable thermal hysteresis in the temperature range of 77–275 K (Figure S8, Supporting Information), which is of key importance for temperature sensing by using a luminescent nanoprobe.15 The absolute temperature sensitivity (S a) of the nanoprobe, defined as the change of response R with temperature, namely, ∂R/∂T where R is the PL intensity ratio I862/I866 and T the absolute temperature,16 was calculated to be a constant of 0.00913 K−1, which is among the highest S a values for Nd3+‐activated luminescent nanothermometer ever reported.[[qv: 3b,17]] The relative temperature sensitivity (S r), defined as the fractional rate of the change of response R with temperature, namely, (1/R)(∂R/∂T),16 was plotted in Figure 4d, from which the highest S r was determined to be 0.62% K−1 at 77 K. The highest S r obtained in LiLuF4:Nd3+ NCs is comparable to the best S r values for Nd3+‐activated luminescent nanothermometers previously reported (Table S2, Supporting Information).18 The temperature uncertainty (δT), defined as the relative error of the response (δR/R) versus the relative temperature sensitivity (S r),[[qv: 1i,19]] was calculated to be lower than 0.6 K for temperatures below 250 K (Figure S9, Supporting Information). It is worth mentioning that the spectral overlap between different Stark or CF components in the emission spectrum of Nd3+ is unavoidable at high temperatures because of the CF line broadening and shifting and the multiple sites of Nd3+ with distinct CF surroundings in NCs. Therefore, the assignment of CF transition lines in Nd3+‐doped NCs should be judiciously carried out by taking into account the interference from different CF or Stark components, which is a prerequisite for temperature sensing by using Nd3+‐doped luminescent nanoprobes. In this regard, the superior features combined with the single photoactive site symmetry, well‐resolved CF transition lines, and high photostability of LiLuF4:Nd3+ NCs, make them excellent NIR‐to‐NIR luminescent nanoprobes for ratiometric temperature sensing in practical application.

Figure 4

a) PL emission spectrum for the 4F3/2→4I9/2 transitions of Nd3+ in LiLuF4 NCs at 275 K and its Gaussian fit according to the CF transitions. b) PL intensity ratio between the R2→Z1 and R1→Z1 CF transitions at 862 and 866 nm (I 862/I 866) as a function of temperature during a heating and cooling cycle between 77 and 275 K. Each data point represents the mean (±standard deviation) of three independent measurements. c) Variation of the intensity ratio I 862/I 866 recorded at 77, 175, and 275 K measured over a span of 20 cycles of heating and cooling processes. d) The relative temperature sensitivity (S r) of LiLuF4:2%Nd3+ nanoprobes as a function of temperature. The error bars result from error propagation in the determination of S r.

Conclusions

In summary, we have systematically investigated the local site symmetry and electronic structures of Nd3+ in LiLuF4 NCs through low‐temperature and high‐resolution PL spectroscopy. By employing Eu3+ as the structural probe, a single spectroscopic site of S 4 symmetry for Ln3+ dopants was identified in LiLuF4 NCs, which is consistent with the crystallographic site symmetry of Lu3+ in LiLuF4 lattice. By means of temperature‐dependent PL spectroscopy, a total number of 36 CF transition lines of Nd3+ in LiLuF4 NCs in the NIR region were unequivocally assigned. Furthermore, by employing the sharp and well‐resolved CF transitions from the thermally coupled Stark sublevels of 4F3/2 of Nd3+, we have demonstrated the application of LiLuF4:Nd3+ NCs as sensitive NIR‐to‐NIR luminescent nanoprobes for ratiometric detection of temperature with a wide linear range of 77–275 K. The unambiguous revelation of photoactive site symmetry and electronic structures of Nd3+ in inorganic NCs is of vital importance for future design and development of Nd3+‐based NIR luminescent nanoprobes toward versatile applications such as cryogenic temperature sensing for space and energy exploration.

Experimental Section

Chemicals and Materials: Lu2O3 (99.99%), Eu2O3 (99.99%), Nd2O3 (99.99%), oleic acid (OA, 90%), oleylamine (OAm, 90%), and 1‐octadecence (ODE, 90%) were purchased from Sigma‐Aldrich (Shanghai, China). CF3COOLi·H2O (99.99%), trifluoroacetic acid (≥99.0%), ethanol (≥99.5%), acetone (≥ 99.5%), and cyclohexane (≥99.5%) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Lu(CF3COO)3, Eu(CF3COO)3, and Nd(CF3COO)3 were prepared by dissolving Lu2O3, Eu2O3, and Nd2O3, respectively, in trifluoroacetic acid at 90 °C. All the chemical reagents were used as received without further purification.

Synthesis of LiLuF: Monodisperse LiLuF4:Ln3+ NCs (Ln = Eu or Nd) were synthesized through a thermal decomposition method. In a typical synthesis of LiLuF4:2%Nd3+ NCs, 1 mmol of CF3COOLi·H2O, 0.98 mmol of Lu(CF3COO)3, and 0.02 mmol of Nd(CF3COO)3 were mixed with 6 mL of OA, 2 mL of OAm, and 2 mL of ODE in a 100 mL three‐neck round‐bottom flask. The resulting mixture was heated to 120 °C under N2 flow with constant stirring for 30 min to form a clear yellowish solution. Thereafter, the resulting solution was heated to 320 °C under N2 flow with vigorous stirring for 40 min and then cooled down to room temperature. The obtained NCs were precipitated by addition of 20 mL of acetone, collected by centrifugation, washed with ethanol several times, and finally dried in vacuum at 60 °C for 24 h.

Structural and Optical Characterization: Powder XRD patterns of the samples were collected with an X‐ray diffractometer (MiniFlex2, Rigaku) using Cu Kα1 radiation (λ = 0.154187 nm). Both the low‐ and high‐resolution TEM measurements were performed by using a TECNAI G2 F20 TEM equipped with an energy‐dispersive X‐ray spectrum. Inductively coupled plasma (ICP) analysis was conducted by using Inductively Coupled Plasma AES spectrometer (Ultima2, Jobin Yvon). PL excitation and emission spectra and PL decays were recorded on an Edinburgh Instruments FLS920 spectrometer equipped with both continuous (450 W) and pulsed xenon lamp at room temperature. For low temperature measurement, samples were mounted on a closed cycle cryostat (10–350 K, DE202, Advanced Research Systems). The emission or excitation monochromator's slits were set as small as possible to maximize the instrumental resolution, and the highest wavelength resolution is 0.05 nm. The line intensities and positions of the measured spectra were calibrated according to the correction curve of the instrument and standard mercury lamp.

Conflict of Interest

The authors declare no conflict of interest.

Supplementary

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