A novel NIR long phosphorescent phosphor:SrSnO₃:Bi²⁺

Abstract

A novel phosphor with near-infrared (NIR) long persistent luminescence, SrSnO₃:Bi²⁺ was successfully synthesized by traditional solid-state reaction. The phosphor shows deep red-NIR persistent luminescence with a broad emission band from 700 to 900 nm peaking at 808 nm, as well as a persistence time of >30 min. This phosphor has broken the domination of conventional Cr³⁺, Mn²⁺ and Mn⁴⁺-activated long persistent phosphors and provides a new perspective on the NIR phosphorescence of Bi-doped long phosphorescent phosphors. Meanwhile, the characteristic operational waveband offer the possibility of potentially being applied in vivo bio-imaging of this new NIR phosphorescent material.

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1. Introduction

Long persistent phosphorescence, also called long-lasting afterglow, is a phenomenon whereby luminescence can last for a long time after ceasing the excitation. The delay in emission is caused by the presence of defect levels ionized under external field (including irradiation, electron beam, temperature and mechanical strain). Such trap site levels have strong abilities to capture, store and release charge carriers (electrons or holes) in the forbidden gaps.^1–3 The works on long phosphorescent phosphors (LPPs) on the early stage were in pursuit of applications in decorations, displays and dials of sulphide phosphors. After discovering a bright green-emitting phosphor SrAl₂O₄:Eu²⁺, Dy³⁺ in 1996 and a remarkable Cr³⁺ doped Zn₃Ga₂Ge₂O₁₀ near-infrared (NIR) phosphor in 2012,^2,3 numbers of optoelectronic applications of LPPs have been burgeoned. Especially for the persistent phosphors in the region of the biologically transparent window (650–1100 nm).⁴ They are anticipated to open the possibility of advanced optical imaging with high resolution and weak light disturbance for realistic analysis of the structural and functional processes in cells, tissues and other complex systems, owing to the distinctive features of low autofluorescence, high signal-to-noise ratio, and deep tissue penetration.⁵

Over the past few years, substantial achievements have been obtained in the research and development of NIR LPPs. The researches mainly focused on transition metals (Cr³⁺, Mn²⁺, Mn⁴⁺)-activated NIR LPPs, which have a broad emission band. Some of the most famous were Zn₃Ga₂Ge₂O₁₀:Cr³⁺, Zn₃Ga₂SnO₈:Cr³⁺, ZnGa₂O₄:Cr³⁺, Zn₂SnO₄:Cr³⁺, MAlO₃:Mn⁴⁺, Ge⁴⁺ (M = La, Gd), Ca_0.2Zn_0.9Mg_0.9Si₂O₆:Eu²⁺, Dy³⁺, Mn²⁺.^3,6–10 However, we must deal with the fact that the number of known phosphorescent activators is relatively limited. Besides these main activators, scarcely any other transition metal ion has been found to exhibit NIR persistent luminescence, even though a wide variety of activation ions can be used as NIR luminescent centres.^11–16

To address this issue, we expect to develop a new long persistent phosphor with a wondrous activation center. In the context of materials selection, this system successfully avoids the existing ubiquitous reliance on Cr³⁺, Mn⁴⁺, and Mn²⁺ as activation center. Bi²⁺ represents the intermediate species of Bi, which typically absorbs in the UV-visible region, and emits from orange to red spectral range. Photoluminescent properties of Bi²⁺ have been widely investigated in recent years. For example, spectroscopic properties of orange phosphor SrB₄O₇:Bi²⁺ have been reported by Peng et al. in 2010.¹⁷ Three distinct photoluminescent excitation peaks and featured photoluminescent peaks were observed at 245 nm (²P_1/2 → ²S_1/2), 478 nm (²P_1/2 → ²P_3/2(2)), 578 nm (²P_1/2 → ²P_3/2(2)), and 588 nm (²P_3/2(1) → 1²P_1/2), respectively. Similar results were also observed in Bi²⁺-doped SrB₆O₁₀. Even more remarkable, optical properties of Bi²⁺ ion strongly depend on ligand field strength of the employed host material. In addition to the yellow luminescence, red luminescence at 627 nm was also investigated in BaSO₄:Bi²⁺ phosphor.¹⁸

SrSnO₃ is selected as the host candidate to take advantages of the ingenious crystalline structure. Alkaline earth borates, sulfates, phosphates and borophosphates have already considered as the most suitable host materials for Bi²⁺. Because usual sites for incorporating Bi²⁺ ions are Ca²⁺, Sr²⁺ and Ba²⁺ ions vacancies, based on the predominance of tetrahedral anion groups. Both compounds are highly efficient for stabilizing the Bi²⁺-ion over its divalent form.^19,20 Therefore, during the last decade, alkaline earth orthostannates (MSnO₃, where M = Ca, Sr and Ba) can be considered as the host matrixes for new phosphors because of their stable crystalline structure and high physics-chemical stabilities.

In this paper, we report a novel NIR persistent phosphor SrSnO₃:Bi²⁺ as a promising material for vivo imaging application due to its emission at 808 nm and the wide emission band ranging from 700 to 900 nm. It may largely extend the operation waveband in biological tissue window. Here, we show the capabilities of SrSnO₃:5% Bi²⁺ as an example for investigation.

2. Experimental

Preparation

The phosphor sample of SrSnO₃:5% Bi²⁺ was synthesized by conventional high temperature solid state reaction method. Typically, stoichiometric amounts of starting materials Bi₂O₃ (99.99%), SnO₂ (99.99%) and SrCO₃ (analytical reagent) were weighted and mixed thoroughly in an agate mortar. The process comprised a two-step thermal treatment. Initial calcination at 800 °C for 2 h in air, after the prefired powders were thoroughly ground again, secondary calcination at 1200 °C for 4 h in air. After that, the samples were furnace-cooled to room temperature and ground again into powder for subsequent use.

Characterization

The crystalline structure of the powder SrSnO₃:5% Bi was analyzed by X-ray diffractometer (XRD) (Cu Kα) and the data was collected between 10° and 90°. The data for Rietveld analysis were collected between 10° and 100°, the step scan mode was used with a setup of 40 kV × 40 mA, 0.01° per step and 0.2 s per step. Rietveld refinement of XRD data was performed by using the TOPAS-Academic program. The morphology and particle size of phosphors were observed by Nova Nano SEM 430 Scanning Electron Microscopy (SEM). X-ray photoelectron spectra (XPS, K-Alpha, Thermo Scientific, UK) were measured by the physical electronics 5600 multi-technique system. Achromatic 200W Al Kα X-rays were chosen as the excitation source. The resolution was 0.05 eV. Photoluminescence excitation (PLE), photoluminescence (PL), long-lasting phosphorescence (LLP) spectra and decay curve were measured with a high resolution spectrofluorometer (Edinburgh Instruments FLS920) equipped with a 500 W Xe lamp as excitation source. All PL results were corrected over the lamp intensity with the PMT spectral response. Thermo-luminescence glow curves were measured after irradiation with the UV lamp for 5 min using a FJ-427A TL meter (Beijing Nuclear Instrument Factory) by heating the irradiated samples from room temperature to 650 K with a fixed heating rate of 2 °C s⁻¹. All the measurements of optical properties were carried out at room temperature. Long persistent luminescence imaging, were performed with a modified imaging system including a Germany pco dicam pro camera as the signal collector. All images were taken in a dark room and analysed with home-made software.

3. Results and discussion

Phase of the samples

All the main diffraction peaks are indexed to those of the orthorhombic SrSnO₃ crystal (Fig. 1, JCPDS no. 70-4389). Obviously, the higher doping amount of bismuth do not change the crystalline phase of SrSnO₃, and the pattern is in perfect match with the standard powder peak positions of SrSnO₃, indicating the successful incorporation of Bi²⁺ into SrSnO₃ host. Rietveld refinement results of the XRD patterns of SrSnO₃:5% Bi are summarized in Tables 2 and 3, and the corresponding refinement profiles are presented in Fig. 2. The lattice of SrSnO₃:5% Bi has a cubic symmetry with space group of Pm3̄m.

Fig. 1 XRD pattern of SrSnO₃:5% Bi sample, which is in agreement with JCPDS 70-4389.

Table 1 Positions of some optical transitions of Bi²⁺ in several host lattices

Materials	Excitation peak (nm)			Emission peak (nm)	Ref.
	^2P1/2 → ^2S1/2	^2P1/2 → ^2P3/2(2)	^2P1/2 → ^2P3/2(1)	^2P3/2(1) → ^2P1/2
SrB4O7:Bi^2+	245	478	578	660 nm (orange)	17
SrBPO5:Bi^2+	234	432	619	688 nm (red)	25
Sr2P2O7:Bi^2+	261	428	628	667 nm (red)	26
SrB6O10:Bi^2+	286	380	560	660 nm (red)	27
BaSO4:Bi^2+	260	452	592	627 nm (red)	28

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Table 2 Rietveld refinements of the XRD data of SrSnO₃:5% Bi

Compounds	SrSnO3
Space group	Pm3̄m
a (Å)	4.0339(1)
Cell volume (Å^3)	65.641(6)
Rwp	13.70

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Table 3 Crystallography data for SrSnO₃:5% Bi determined by Rietveld refinements

Compound	SrSnO3
Atoms	wyck	x	y	z
Sr1/Bi	1	0.5000(0)	0.5000(0)	0.5000(0)
Sn1	1	0.0000(0)	0.0000(0)	0.0000(0)
O1	3	0.5000(0)	0.0000(0)	0.0000(0)

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Fig. 2 The experimental (blue solid line) and calculated (red dotted line) XRD patterns as well as their difference profile (gray line) for the Rietveld structure analysis of SrSnO₃:5% Bi sample.

Fig. 3 is the SEM micrograph of SrSnO₃:5% Bi powders. It can be clearly seen that the sample has irregular morphology with round edge, and the grain sizes varied from 0.3 to 1 μm. To further investigate the divalent bismuth in SrSnO₃ material in the air condition, representative X-ray photoelectron spectroscopy (XPS) at Bi 4f position of the SrSnO₃:5% Bi particles is shown in Fig. 4. It is distinctly seen that two peaks around 164 and 159 eV present for the samples sintered in air. The shapes combined with binding energies of the Bi 4f signals agree well with the signals of 4f_5/2 and 4f_7/2, respectively. It is worth to mention that XPS observed two peaks at 163.66 eV (4f_5/2) and 158.25 eV (4f_7/2), which were related to the lower-valence bismuth ions, in addition to the peak at 164.56 eV and 159.12 eV for the dominant trivalent Bi³⁺.^21–24 Thus, the XPS spectra reveal the coexistence of Bi²⁺ and Bi³⁺ in air condition.

Fig. 3 SEM images of SrSnO₃:5% Bi phosphors.

Fig. 4 XPS analysis, showing the presence of Bi²⁺ in the SrSnO₃:5% Bi phosphor.

Optical properties

Photoluminescent property. Emission and excitation spectra during fluorescence should be analyzed primarily, in order to identify which activator is taking part in the luminescent process. Fig. 5a shows the normalized excitation and emission spectra of the SrSnO₃:5% Bi²⁺ sample at room temperature. The sample exhibits a broadening NIR emission band ranging from 700 nm to 900 nm peaking at 808 nm under the excitation of 298 nm light. The excitation spectrum monitored at 808 nm covers a very broad spectral region (from 250 to 750 nm) and consists of three main excitation bands at 298, 528 and 700 nm.

Fig. 5 (a) Normalized excitation and emission spectra of the SrSnO₃:5% Bi phosphor at room temperature. The emission spectrum is acquired under 298 nm light excitation and the excitation spectrum is obtained by monitoring at 808 nm emission. (b) Simplified energy level diagram of SrSnO₃:Bi phosphor.

It is known that bismuth exhibits various optoelectronic properties due to the native electron configuration. Bi²⁺ ion shows the broad absorption band in UV-vis region and emission band from orange, red, to NIR spectral range.²⁵ The normal electronic configuration of Bi²⁺ is [Xe]4f¹⁴5d¹⁰6s²6p¹. In addition to the ground state ²P_1/2, the first excited state ²P_3/2 also is derived from 6s²6p¹ state, which is usually split into two sublevels, ²P_3/2(1) and ²P_3/2(2). The excited level ²S_1/2 derives from 6s²7s¹. Normally, ²P_1/2 → ²P_3/2 transition is parity forbidden in contrast to the inter transition ²P_1/2 → ²S_1/2. In fact, after being incorporated into one crystal lattice, the strong lattice vibrations and electron-phonon coupling will promote the admixture of ²P_1/2, ²S_1/2, and ²P_3/2 wave functions of Bi²⁺, leading to the permissible transition of ²P_1/2 → ²P_3/2. Therefore, in a solid matrix, three absorption transitions (²P_1/2 → ²P_3/2(1), ²P_1/2 → ²P_3/2(2), and ²P_1/2 → ²S_1/2, ordered by increasing transition energy) and one emission transition (²P_3/2(1) → ²P_1/2) should be visible.¹⁹ The positions of some optical transitions of Bi²⁺ in different host lattices are listed in Table 1.

The first one was a clear confirmation of Blasse's postulation of the existence of an excitation peak below 312.5 nm.²⁷ Latter three peaks were consistent with Blasse's observations. Similar excitation bands were observed at 260 nm (²P_1/2 → ²S_1/2), 452 nm (²P_1/2 → ²P_3/2(2)) and 592 nm (²P_1/2 → ²P_3/2(1)) respectively. All the bands were consistent with Hamstra's observations.²⁸

Therefore, three excitation peaks should be assigned to inner transitions of Bi²⁺ (²P_1/2 → ²S_1/2), (²P_1/2 → ²P_3/2(2)) and (²P_1/2 → ²P_3/2(1)), while the distinct emission band corresponds to the transition (²P_3/2(1) → ²P_1/2). Fig. 2b shows the simplified energy level diagram of SrSnO₃:Bi²⁺ phosphor.

Long persistent luminescence properties. Not only the steady-state emission spectrum during excitation, but also the afterglow emission spectrum needs to be investigated. The long persistent phosphorescence spectrum of SrSnO₃:5% Bi²⁺ phosphor exhibits a similar spectral profile as the PL spectrum, i.e. a broad emission band from 700 to 900 nm peaking at 808 nm, indicating that Bi²⁺ is taking part in the persistent luminescence (Fig. 6a). Fig. 6b shows the phosphorescent decay curve of SrSnO₃:5% Bi²⁺ sample monitored at 808 nm after an irradiation by a xenon lamp for 10 min. The data were recorded as a function of persistent luminescence intensity (I) versus time (t); the recording lasts for 800 s.

Fig. 6 (a) LLP spectrum of SrSnO₃:5% Bi after 500 W Xe lamp illumination for 10 min. (b) LLP decay curve of SrSnO₃:5% Bi phosphor monitored at 808 nm emission after illumination for 10 min. (c) Afterglow intensity I_{10 s} monitored at 808 nm as a function of the excitation wavelengths over the 250–750 nm spectral range.

The afterglow attenuates exponentially. It contains a rapid decay process and a slow decay process. The decay curve is similar to that of typical long afterglow materials and it can be evaluated by fitting double exponential equation which reflects the trend of the decay. The equation is as follows:²⁹

where I represents the phosphorescent intensity; I₁ and I₂ are two constants; t represents the time; τ₁ and τ₂ are decay constants which decide the decay rate for the rapid and the slow exponentially decay components. The fitting results of the parameters of τ₁ and τ₂ and the fitting curves are shown in Fig. 6b.

We also studied the relationship between afterglow intensity and excitation wavelengths. Fig. 6c shows the afterglow intensity I_{10 s} (afterglow intensity recorded 10 s after the stoppage of irradiation were used as reference points) of SrSnO₃:5% Bi²⁺ powders monitored at 808 nm as a function of excitation wavelengths, indicating that persistent luminescence can be effectively excited by the light from 250 to 430 nm.

The long-lasting NIR afterglow of the SrSnO₃:5% Bi phosphor was also visually evaluated using a supersensitive camera in a dark room. Fig. 7a–d shows the changes of NIR emission brightness with a decay time up to 20 min for phosphor after irradiation at ultraviolet lamp for 10 min. The images were taken in a dark room using a Germany pco dicam pro camera for a decay period lasting at least 20 min. It is shown that phosphor can be effectively activated by an ultraviolet lamp and result in persistent NIR emission.

Fig. 7 Long persistent luminescence imagings of sample SrSnO₃:5% Bi. (a) 1, (b) 5, (c) 10 and (d) 20 min post-irradiation phosphorescence images. The samples were pre-excited by ultraviolet lamp for 10 min.

Defect investigation

It is well known that trapping centers play an essential and crucial role for photo-energy storage in afterglow phosphors.^30,31 Thermoluminescence (TL) spectrum is very useful for evaluating the trap depth, which is an important factor affecting emission of afterglow. In order to investigate trap states of SrSnO₃:Bi²⁺, TL measurement was carried out. Fig. 8 shows normalized TL curve for sample SrSnO₃:Bi²⁺ measured 30 s after irradiation ceased. The TL curve consists of two broad bands with maxima at 335 and 583 K, corresponding to the shallow and deep traps, respectively. At room temperature, shallow traps are easily emptied, whereas deep traps are difficult to be emptied; and parts of captured electrons remain stored there. If a trap is too deep, the captured electrons cannot escape, and no persistent phosphorescence is detected. Engineering a suitable trap depth is essential for achieving room temperature persistent phosphorescence. The depth of traps is estimated using the following equation:³²where E is the average depth of traps, T_m is the peak temperature, w is the FWHM of the peak, and k is the Boltzmann constant. Therefore, the calculated results of trap depths are 0.623 eV and 1.310 eV, respectively.

Fig. 8 Thermo-luminescence curve of the sample SrSnO₃:5% Bi measured at 10 min after the stoppage of irradiation.

4. Conclusions

To the best of our knowledge, such long persistent phosphors with Bi²⁺ as the phosphorescent center have never been reported. There is also still no any study on the afterglow properties of Bi²⁺ doped SrSnO₃ host. Bi₂O₃ was just proved to be an effective codopant to enhance red persistent luminescence, for instance, in ZnGa₂O₄:Cr, Bi spinel or Ca₃Ga₂Ge₃O₁₂:Cr, Bi.^33,34

But in this paper, we successfully developed a novel NIR persistent phosphor SrSnO₃:Bi²⁺, which exhibits NIR persistent luminescence ranging from 700 nm to 900 nm. The phosphor has broken the domination of the conventional Cr, Mn-activated long persistent phosphors and provides a new perspective on the afterglow of Bi-doped phosphorescent phosphors. This new NIR persistent material has potential applications in vivo bio-imaging. Further research is underway on the mechanism analysis, nanocrystallization and functionalization of LPPs, which are expected to open up new possibilities for visualizing structural and functional processes in cells, tissues and other complex systems.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (Grant No. 51132004, 51072754, 51472091), Guangdong Natural Science Foundation (Grant No. S2011030001349, 2014A030310444), National Basic Research Program of China (Grant No. 2011CB808100), China Postdoctoral Science Foundation (Grant No. 2015M570707) and Fundamental Research Funds for the Central Universities (Grant No. 2013ZM0001, 2015ZM089). This work was also supported by the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics.).

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