Multifunctional Near-Infrared Luminescence Performance of Nd³⁺ Doped SrSnO₃ Phosphor

Abstract

The phosphors with persistent luminescence in the NIR (near-infrared) region and the NIR-to-NIR Stokes luminescence properties have received considerable attention owing to their inclusive application prospects in the in vivo imaging field. In this paper, Nd³⁺ doped SrSnO₃ phosphors with remarkable NIR emission performance were prepared using a high temperature solid state reaction method; the phase structure, morphology, and luminescence properties were discussed systematically. The SrSnO₃ host exhibits broadband NIR emission (800–1300 nm) with absorptions in the near ultraviolet region. Nd³⁺ ions emerge excellent NIR-to-NIR Stokes luminescence under 808 nm laser excitation, with maximum emission at around ~1068 nm. The concentration-dependent luminescence properties, temperature dependent emission, and the luminescence decay curves of Nd³⁺ in the SrSnO₃ host were also studied. The Nd³⁺ doped SrSnO₃ phosphors exhibit exceptional thermal stability; the integrated emission intensity can retain approximately 66% at 423 K compared to room temperature. Most importantly, NIR persistent luminescence also can be observed for the SrSnO₃:Nd³⁺ samples, which is in the first and second biological windows. A possible mechanism was proposed for the persistent NIR luminescence of Nd³⁺ based on the thermo-luminescence spectra. Consequently, the exciting results indicate that multifunctional NIR luminescence has been successfully realized in the SrSnO₃:Nd³⁺ phosphors.

1. Introduction

Recently, the fluorescence-based optical imaging technique has attracted considerable attention, owing to the incomparable merits such as portability, non-invasiveness, and time effectiveness. Therefore, this technique has been regarded as a crucial means for cancer research, clinical translation, and medical practice [1,2,3]. Up to now, a variety of representative fluorescent materials have been reported, which may have potential for in vivo bio-imaging [4,5,6,7,8]. Generally, NIR light in the 650–950 nm (the first biological window) and 1000–1700 nm (the second biological window) wavelength range would be more beneficial to high-resolution imaging, attributed to the small light loss and being less damaging to organism, especially for NIR light in the second biological window [9,10]. As a consequence, the investigation on phosphors that can emit NIR light in biological windows is particularly significant.

Interest in Nd³⁺ activated luminescence materials has been widespread in recent years. On one hand, Nd³⁺ can effectively improve the pumping efficiency of 808 nm laser owing to its appropriate absorption around 800 nm [11]. More importantly, 808 nm possesses a significant advantage of avoiding the overheating effect in biological systems in comparison with 980 nm [12]. On the other hand, the optimal emission wavelength of Nd³⁺ is usually located in the 1050–1100 nm wavelength range, which perfectly matches with the second biological window. Hence, Nd³⁺ has been considered as an ideal candidate for in vivo optical imaging thanks to the characteristic of NIR excitation (such as 808 nm) and NIR emission (the so called NIR-to-NIR Stokes luminescence) [13,14,15]. Some typical materials have been reported, such as Nd³⁺-sensitized yolk-shell GdOF@SiO₂ [16], CaF₂:Nd³⁺ [17], ZnSnO₃:Nd³⁺ [18], NaSrVO₄:Nd³⁺ [19], YVO₄:Nd³⁺ [20], and the co-doped phosphors La₂CaZrO₆:Cr³⁺,Nd³⁺ [21], Ca₃In₂Ge₃O₁₂:Cr³⁺,Nd³⁺ [22], CaZnOS:Nd³⁺,Mn²⁺ [23], BaY₂O₄:Yb³⁺,Nd³⁺ [24], NaYF₄:Nd³⁺,Yb³⁺,Ho³⁺ [25], and NaGdF₄:Yb³⁺,Tm³⁺@NaGdF₄:Nd³⁺ [26].

Persistent luminescence is a process with luminescence that lasts for minutes to hours after the stoppage of irradiation [27]. In particular, persistent luminescence with an emission wavelength in the biological windows is extremely attractive in bio-labels, which possess the priority such as excitation-free and noise-free imaging conditions [28,29,30]. To take full advantage of both persistent luminescence and NIR luminescence for biomedical applications, it is essential to employ a suitable persistent luminescence material with NIR emission in biological windows.

Considering the wide application prospect, NIR emission and persistent luminescence materials motivate our research interests. In this work, a perovskite structure compound (SrSnO₃) was chosen as a host lattice. The NIR-to-NIR Stokes luminescence and persistent NIR luminescence properties of the SrSnO₃:Nd³⁺ phosphors were reported, which is in good accordance with the biological windows. The persistent luminescence mechanism was also proposed according to the thermo-luminescence curves. Herein, the realization of multifunctional luminescence may be beneficial for further investigation on NIR phosphors.

2. Experimental Section

2.1. Sample Preparation

In this research, the undoped SrSnO₃ compound and Nd³⁺ doped SrSnO₃ phosphors were prepared via a convenient high temperature solid state reaction method. The raw starting reactants are analytical reagents SnO₂ (A. R.), SrCO₃ (A. R.), and high purity Nd₂O₃ (99.99%). In the synthetic procedure, the stoichiometric amounts of raw materials were first weighed accurately, then the mixed raw materials were ground thoroughly in an agate mortar for approximate 30 min. Subsequently, the mixtures were transferred into a muffle furnace and preheated at 800 °C for 2 h in air ambience. The preheated samples were then thoroughly reground and sintered at 1200 °C for 4 h in air. At last, the target products were achieved after cooling down naturally to room temperature.

2.2. Characterization

The phase structure and phase purity were verified by the X-ray diffraction (XRD) method, which was conducted on a BRUKER D8 ADVANCE type powder X-ray diffractometer (Bruker, Germany). The structure refinement was performed using the Topas program. The diffuse reflection spectra were measured on a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies, USA). The luminescence excitation and luminescence emission spectra at room temperature, persistent luminescence emission spectra, and thermo-luminescence (TL) curves were collected using a Fluorolog-3 spectrofluorometer (HORIBA, USA); a 450 W xenon lamp and an 808 nm semiconductor laser were employed as excitation sources, respectively. For TL measurement, the heating rate was 2 K/s. The temperature dependent emission spectra and the luminescence decay curves were collected using a FLS 1000 spectrometer (Edinburgh Instruments, Livingston, UK), and the excitation source was a 60 W μF flash lamp for the decay curve measurement. The SEM images (scanning electron microscopy) and EDS (energy-dispersive spectroscopy) were conducted on a Hitachi SU5000 field emission scanning electron microscopy (Hitachi, Tokyo, Japan).

3. Results

3.1. Phase Structure and Morphology

As reported, the SrSnO₃ host lattice belongs to the perovskite structure class; however, it is somewhat distorted from cubic symmetry by an octahedral tilting distortion, which crystallizes in an orthorhombic P_bnm space group [31]. In this host, there is only one crystallographic site for Sr²⁺ and Sn⁴⁺, respectively. Sn⁴⁺ ions are coordinated by six oxygen atoms, whereas Sr²⁺ ions are twelve-fold coordinated by oxygen atoms [32,33]. In the present case, considering the ionic radius similarity of Nd³⁺, Sr²⁺, and Sn⁴⁺, Nd³⁺ ions may prefer to enter Sr²⁺ site in the host.

The Rietveld refinement of the SrSnO₃ host lattice was performed according to the X-ray diffraction patterns; the refinement results are shown in Figure 1a. The obtained reliability factors are R_wp = 8.47%, R_p = 6.16%, and R_b = 1.62%, which imply a good fitting quality for the phase structure. There is no obvious impurity phase for the prepared sample. The refined cell parameters are a = 5.7065(4) Å, b = 5.7027(3) Å, c = 8.0699(7) Å, and V = 262.62(3) ų, respectively. Figure 1b depicts the XRD patterns of Nd³⁺ doped SrSnO₃ samples (Sr_1−xNd_xSnO₃); the observed diffraction patterns agree well with each other, and all the diffraction peaks are in good consistency with the standard card 77-1798 [SrSnO₃], indicating that all the as-prepared Sr_1−xNd_xSnO₃ samples are nearly a pure phase in the 0–0.03 doping concentration range of Nd³⁺. However, an impurity phase with diffraction at 29.20° could be observed for the x = 0.05 sample, as marked in Figure 1b. Therefore, only the luminescence spectra for the samples with a doping concentration smaller than 0.03 (x ≤ 0.03) are presented and discussed in this work.

The as-prepared SrSnO₃ host compound exhibits irregular morphologies with a particle size of ~1–2 μm, and a certain degree of aggregations can be observed for the sample particles; the SEM images are shown in Figure 2a. In addition, the target elements Sr, Sn, and O can be detected in the elemental mapping images in Figure 2b (the selected area was marked by a red rectangle), and all the observed elements uniformly distributed within the phosphor particle.

3.2. Diffuse Reflection and Host Luminescence

Figure 3a displays the diffuse reflection spectra of the SrSnO₃ and Sr_0.99Nd_0.01SnO₃ samples. Obviously, the two typical samples both exhibit strong broadband absorption in the 250–350 nm range, which is principally due to the host-related absorption from the SrSnO₃ host. Additionally, several additional weak absorption peaks (such as 582 nm, 802 nm) also appear for the Sr_0.99Nd_0.01SnO₃ sample, which are assigned to the 4f-4f electron transition absorptions of Nd³⁺ in the host. Herein, the diffuse reflection spectrum of the SrSnO₃ host lattice can be transformed according to Kumar’s method [34], as shown in the inset of Figure 3a, which is helpful to estimate the band gap between valence band and conduction band. According to reference [31], the SrSnO₃ compound possesses a direct band gap, hence the band gap is estimated to be 4.12 eV [inset of Figure 3a], which is in good accordance with the reported value [35].

Upon 312 nm ultraviolet light excitation, intense broadband NIR emission ranging from 800 nm to 1300 nm with a maximum at around 970 nm can be observed for the SrSnO₃ host lattice in Figure 3b, which matches with the biological windows. The FWHM (full width at half maximum) of this broadband NIR emission is approximately 140 nm. Herein, the broadband NIR emission is ascribed to the Sn²⁺ 5s→VB (valance band) transition emission from the SrSnO₃ host, which was also observed for BaSnO₃ in the reference [36]. Detecting the emission wavelength at 970 nm, the corresponding excitation spectrum was obtained, as displayed in Figure 3b. The excitation spectrum contains a broad absorption band peaking at 312 nm and a shoulder band at 287 nm, owing to the host-related absorption, which is in good accordance with the diffuse reflection spectra in Figure 3a. Hence, the SrSnO₃ host can be effectively excited by ultraviolet light and generate intense broadband NIR light emission.

3.3. Luminescence Performance of Nd³⁺ in SrSnO₃

Figure 4a depicts the luminescence emission and excitation spectra of the Sr_0.99Nd_0.01SnO₃ sample at room temperature. Under the excitation of 582 nm light, a series of sharp emission lines are observed in the 800–1500 nm wavelength range, which can be assigned to the electron transitions from the ⁴F_3/2 state to the ground states ⁴I_J of Nd³⁺. Specifically, the emission bands in the 850–1000 nm, 1000–1200 nm, and 1300–1400 nm are attributed to the ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2 transition emissions, respectively. The most intense emission band is located at ~1068 nm (in the second biological window). Upon 312 nm ultraviolet light excitation, the NIR emissions from the SrSnO₃ host lattice and Nd³⁺ ions can be observed simultaneously, demonstrating that the designed phosphors can realize multiple luminescence under different light excitations. Monitoring the emission wavelength at 1068 nm, the corresponding excitation spectrum emerges a series of peaks in the 400–800 nm wavelength range, attributing to the electron transitions from the ⁴I_9/2 ground state to the excited states of Nd³⁺. The assignments for the excitation peaks are as follows [15,37,38,39]: ⁴I_9/2→²P_1/2 (400–440 nm), ⁴I_9/2→²G_9/2,²K_15/2 (440–485 nm), ⁴I_9/2→⁴G_9/2,⁴G_7/2 (485–545 nm), ⁴I_9/2→⁴G_5/2,²G_7/2 (545–610 nm), ⁴I_9/2→⁴F_9/2 (660–690 nm), ⁴I_9/2→⁴F_7/2,⁴S_3/2 (710–770 nm), and ⁴I_9/2→⁴F_5/2,²H_9/2 (770–840 nm). Furthermore, the excitation band at 312 nm shows a somewhat redshift in comparison with the undoped SrSnO₃, and another shoulder band at around 350 nm also appeared, which mainly relates to the absorption of the host lattice and Nd³⁺ ions, respectively.

Emission spectra of the Sr_1−xNd_xSnO₃ samples upon 312 nm light excitation are illustrated in Figure 4b. Obviously, the relative emission intensity from the host lattice gradually declines with increasing Nd³⁺ doping concentration, and the emission intensity of Nd³⁺ increases. We did not observe the concentration quenching of Nd³⁺ under 312 nm excitation. The decrease in emission intensity from the host lattice may relate to the energy transfer from the SrSnO₃ host lattice to Nd³⁺ ions. Under 312 nm excitation, the excitation energy was mainly absorbed by the host lattice; the sensitization of Nd³⁺ via the host lattice subsequently occurred, resulting in the NIR emission of Nd³⁺. Energy transfer efficiency from the SrSnO₃ host lattice to Nd³⁺ can be obtained using the formula [40]:

$$\eta =1-{\displaystyle \frac{{\mathrm{I}}_s}{{\mathrm{I}}_{s0}}}$$

(1)

where I_s and I_s0 stand for the luminescence intensities of the host lattice with and without Nd³⁺ ions. Consequently, the energy transfer efficiencies are 17.46%, 38.12%, 50.79%, and 79.45% for x = 0.002, 0.005, 0.01, and 0.03, respectively. We should also notice that the emission of Nd³⁺ is inconspicuous under 312 nm (or 287 nm) light excitation when the doping concentration (x value) is lower than 0.01. Therefore, the Sr_0.99Nd_0.01SnO₃ sample was chosen as a typical sample to discuss the persistent NIR luminescence in the following section.

Since NIR-to-NIR Stokes luminescence is available for in vivo probe applications, the luminescence of Nd³⁺ under 808 nm laser excitation was also investigated, as displayed in Figure 4c. Upon 808 nm laser excitation, the emission spectra exhibit similar peaks (850–1000 nm, 1000–1200 nm, and 1300–1400 nm) with that under 582 nm light excitation in Figure 4a, but the resolution is much higher due to the smaller slit size during the measurement procedure. In this case, the excitation wavelength and emission wavelength both locate perfectly in the “tissue-transparent window”, and accordingly promises high-quality imaging applications. The integrated emission intensity tends to increase with the doping concentration of Nd³⁺ ions, as shown in the inset of Figure 4c, which confirms that there is no concentration quenching effect in the investigated concentration range.

Upon 582 nm light excitation and detecting the emission wavelength at 1068 nm, the luminescence decay curves of Nd³⁺ in the Sr_1−xNd_xSnO₃ samples were achieved; the curves are displayed in Figure 4d. All the luminescence decay curves satisfy the first-order exponential behavior and can be well fitted using Equation (2) [41]:

I_t = I₀ + Aexp(−t/τ)

(2)

where I_t and I₀ are the intensities at time t and t = 0, respectively. τ is the luminescence lifetime. Based on Equation (2), the decay constants are estimated to be 284 μs, 275 μs, and 265 μs for x = 0.005, 0.01, 0.03, respectively. In general, the luminescence lifetime τ is regulated by both the radiative transition (W_R) and the non-radiative transition (W_NR) processes, which can be expressed by Equation (3) [42]:

$$\tau ={\displaystyle \frac{1}{w_R+w_{NR}}}$$

(3)

The luminescence lifetime shows little decrease with increasing Nd³⁺ concentration, owing to the somewhat enhanced non-radiative energy transfer in the phosphors.

Temperature-dependent emission spectra of the Sr_0.99Nd_0.01SnO₃ sample under 582 nm excitation are depicted in Figure 5a; all the emission spectra are similar with each other in the 298–523 K temperature range, but the emission intensity declines progressively with rising temperature. Figure 5b demonstrates the integrated emission intensity depending on temperature; the plotted curve has been normalized by the integrated intensity at 298 K. The integrated emission intensity tends to decrease at high temperatures; it is exciting that the emission intensity can still keep about 66% at 423 K compared with that at 298 K, suggesting a good thermal stability of the as-prepared NIR emitting phosphors. To further evaluate the thermal properties, the activation energy (ΔE) for thermal quenching can be determined through the Arrhenius function [43]:

$$I_T={\displaystyle \frac{I_0}{1+\mathrm{A}\mathrm{\ast }\exp (-∆E/k\mathrm{T})}}$$

(4)

where I_T and I₀ are the luminescence emission intensity of Nd³⁺ at temperature T and the initial emission intensity, respectively. k is the Boltzmann constant. Accordingly, Equation (4) can also be rewritten as the following [44]:

$$\ln \left({\displaystyle \frac{I_0}{I_T}}-1\right)=\mathrm{l}\mathrm{n}\mathrm{A}-{\displaystyle \frac{∆E}{kT}}$$

(5)

Therefore, the ΔE value was obtained by plotting the relationship between ln[(I₀/I) − 1] and 1/(kT), as shown in Figure 5c. The slope of the fitted straight line is −0.2562, demonstrating the ΔE value is equal to 0.2562 eV.

In addition, the luminescence decay curves of the Sr_0.99Nd_0.01SnO₃ sample in the 298–523 K temperature range are illustrated in Figure 5d. The decay process became faster at a high temperature (especially when T > 423 K), verifying the increase in non-radiative transitions at high temperatures. The luminescence decay curves exhibit somewhat deviations from the first-order exponential; therefore, a second-order exponential decay mode can be used, as described by Equation (6) [45]:

$${\mathrm{I}}_t={\mathrm{I}}_0+A_1{e}^{{\displaystyle \frac{-t}{{\tau }_1}}}+A_2{e}^{{\displaystyle \frac{-t}{{\tau }_2}}}$$

(6)

where I_t represents the luminescence intensity at time t, A₁ and A₂ are constants of fitting parameters, and τ₁ and τ₂ refer to the luminescence lifetimes for the non-exponential decay curve, respectively. Consequently, the average luminescence decay time (τ_ave) can be calculated using the following Formula (7) [45]:

$${\tau }_{ave}={\displaystyle \frac{A_1{\tau }_1^2+A_2{\tau }_2^2}{A_1{\tau }_1+A_2{\tau }_2}}$$

(7)

The fitting results are displayed in

Table 1. Average luminescence lifetime (τ_ave) of Nd³⁺ at different temperatures for the Sr_0.99Nd_0.01SnO₃ sample.

Temperature (K)	A1	τ1 (μs)	A2	τ2 (μs)	τave (μs)
298	0.9213	149.45	0.9400	338.48	287.70
323	1.0831	180.20	0.6981	370.19	288.56
373	1.0685	181.87	0.6966	374.19	292.02
423	0.9174	150.37	0.9467	344.34	286.66
473	1.1418	128.31	0.8640	330.13	261.65
523	1.9923	84.20	0.7547	324.05	226.46

; the luminescence decay curves effectively support the occurrence of thermal quenching at a high temperature.

3.4. Persistent NIR Luminescence

In addition to the NIR luminescence and NIR-to-NIR Stokes luminescence properties, the persistent NIR emission and the possible mechanism were demonstrated as well for Nd³⁺ ions in the SrSnO₃ host. Figure 6a illustrates the persistent luminescence emission spectra of the SrSnO₃ and Sr_0.99Nd_0.01SnO₃ samples. For the undoped SrSnO₃ host lattice, broadband emission peaking at 970 nm can be detected after irradiating by 287 nm ultraviolet light for 5 min, which is similar to the emission spectra in Figure 3. Correspondingly, a shoulder band at around 1068 nm appeared after the excitation of 287 nm ultraviolet light for the Sr_0.99Nd_0.01SnO₃ sample, demonstrating the existence of persistent luminescence for Nd³⁺ ions in the host. Therefore, the persistent NIR luminescence in the first and second biological windows have been successfully achieved. The persistent luminescence decay curves monitored at 970 nm and 1068 nm for the Sr_0.99Nd_0.01SnO₃ sample were investigated, respectively; the results are displayed in Figure 6b. Before measurements, the phosphor was first pre-irradiated by 287 nm ultraviolet light for 5 min, and then the persistent luminescence intensity was recorded with increasing time. The persistent luminescence can be maintained for more than 120 s for both the emission from the SrSnO₃ host and Nd³⁺ ions. Thus, the Nd³⁺ doped SrSnO₃ may be a multifunctional phosphor with persistent NIR luminescence and NIR-to-NIR Stokes luminescence in the tissue-transparent window.

To reveal the persistent luminescence mechanism in depth, TL curves were collected for the Sr_0.99Nd_0.01SnO₃ sample after exciting by 287 nm ultraviolet light for 5 min; the results are exhibited in Figure 6c. A broad band peaking at around 378 K was observed in the TL curves, which is concerned with the depth of defect traps. The TL curves are very similar as shown by monitoring the emissions at 970 nm and 1068 nm; no additional peaks were detected for the luminescence of Nd³⁺ (1068 nm emission). As a consequence, we can deduce that the traps that contributed to the persistent luminescence of Nd³⁺ are most probably related to the intrinsic defects in the host; both the persistent luminescence from SrSnO₃ host lattice and Nd³⁺ ions are in connection with the similar defects. Some possible defects in SrSnO₃ have been discussed in detail [46,47], which could support the persistent luminescence in this research. The characteristic trap depth (ET) can be simply calculated via the relationship ET = T/500 eV [48], where T is the temperature (K). Herein, the obtained ET value is 0.756 eV. Based on the above results, we can propose a possible persistent luminescence mechanism of Nd³⁺ in Figure 6d. Under ultraviolet light excitation, the excitation energy was absorbed by the host lattice through electron transitions from the valance band to conduction band; then, the absorbed energy could transfer to Nd³⁺ ions. At the same time, part of the excitation energy will also be absorbed directly by Nd³⁺ ions (marked as process ①). Subsequently, the excited electrons can be captured by defect traps via the conduction band, marked as process ②. After stopping excitation, the captured electrons would escape from the defect traps with the assistance of thermal activation energy, and gradually recombine with the host lattice and the ionized Nd³⁺ through the conduction band (process ③). Finally, persistent NIR luminescence from Nd³⁺ was observed, denoted as process ④. Though we report the NIR-to-NIR Stokes luminescence and persistent NIR luminescence of Nd³⁺ in this research, this investigation is still in its infancy, and further efforts are needed to explore in the future.

4. Conclusions

In summary, we successfully designed and fabricated a multifunctional NIR emitting phosphor SrSnO₃:Nd³⁺ through a high temperature solid reaction method. The SrSnO₃ host lattice shows broadband NIR emission in the 800–1300 nm wavelength range with a FWHM of 140 nm, owing to the Sn²⁺ 5s→VB (valance band) electron transition emission. Nd³⁺ doped SrSnO₃ phosphors exhibit strong NIR-to-NIR Stokes luminescence with optimal emission at ~1068 nm upon 808 nm laser excitation. The as-prepared NIR phosphors exhibit attractive thermal stability, retaining approximately 66% at 423 K compared with room temperature. Notably, the prepared phosphors also reveal persistent NIR luminescence in the first and second biological windows. The possible mechanism for persistent luminescence was verified by TL curves, which may relate to the intrinsic defects in the host. The findings in this research may be beneficial for the further investigation of NIR phosphors.