Upconversion luminescence of Sm²⁺ ions

Abstract

Herein, we report the phenomenon of upconversion luminescence from Sm²⁺ ions, which demonstrates that changeable valence lanthanides can serve as ions for optical frequency transformation. Upon excitation with a 980 nm diode laser, the doped Sm²⁺ ions in the hybrid material BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺ emit red upconversion fluorescence peaks at 631 nm, 644 nm, 665 nm, 689 nm, 704 nm, and 729 nm from the ⁵D_0,1 → ⁷F_0,1,2 transitions. By transient dynamic analysis, we attributed the excitation of Sm²⁺ ions to the cooperation energy transfer process, in which two excited Yb³⁺ ions simultaneously transfer their energy to one Sm²⁺ ion.

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Introduction

Upconversion luminescence (UCL) refers to a nonlinear optical process in which the sequential absorption of two or more photons leads to the emission of light at a shorter wavelength than the excitation wavelength (anti-Stokes type emission). Since the 1960s, optical frequency upconversion (UC) has been developed as a luminescence technology. The most important evolution of UC focuses on visible (VIS) UCL including red, green, and blue.^1–6 Owing to their excellent spectroscopic properties, lanthanide (Ln)-doped UCL materials have gained significant attention in the fields of 3D displays, light-emitting devices, biomarkers, bioassays, solid lasers, light-emitting diodes (LEDs), NIR photocatalysis, and temperature sensors.^7–16 In general, trivalent lanthanide ions, such as Tm³⁺, Er³⁺, Tb³⁺, and Ho³⁺, usually act as activators in UC materials, and Yb³⁺ ions with a large absorption cross-section in the NIR region^17,18 act as popular sensitizers to increase optical absorption.^19,20 In the past few decades, researchers used trivalent lanthanides with stable valence states in various UCL materials, and to date, almost no reports have been found on UCL from a divalent lanthanide ion or a changeable valence lanthanide, except for the study reporting UCL in Tm²⁺ materials in 2006 by Hans U. Güdel et al.²¹ However, changeable valence lanthanides may produce more abundant wavelengths beyond those from trivalent lanthanides.

As a popular changeable valence lanthanide ion, Sm²⁺ has been heavily studied in the past few decades due to its red downconversion (DC) luminescence as well as its ability in optical storage.^22–25 Sm²⁺ has a similar electronic structure to that of Eu³⁺ and emits mainly bright red fluorescence in a downconversion way. Since the persistent spectral hole burning (PSHB) based on Sm²⁺ ions was reported, Sm²⁺-doped fluoride halide mixed crystals have been exhibiting fascinating potential application in ultrahigh density optical data storage.^26–28 However, Sm²⁺ ion has never been considered as an UCL ion working in the regime of NIR excitation because it cannot be excited by a NIR photon or sensitized by an excited Yb³⁺ ion. In principle, two-photon absorption can possibly lead to the UCL of Sm²⁺ ions. However, to date, this has not been achieved because of the low efficiency (η = ∼10⁻¹³) of two-photon absorption.²⁹ Compared with two-photon absorption, the cooperative energy transfer (CET) from a Yb³⁺-dimer to another lanthanide ion is more efficient (η = ∼10⁻⁶).³⁰ CET is the physical process in which two identical excited ions simultaneously exhibit transitions and transfer energy to another ion. Cooperative transition, which is closely associated with clustered lanthanide ions in solid hosts, involves cooperative luminescence (CL), cooperative absorption (CA), and CET. Clusters of lanthanide ions, such as Yb³⁺-dimers, can easily form in the alkaline-earth fluoride crystal AF₂ (A = Ca, Sr, and Ba), depending on the type of lanthanide ion and its doping concentrations.^31,32 In ref. 30, the Yb³⁺ emission is a featureless band peaking at 497 nm, whereas a much more complex band with at least five individual peaks centered at around 520 nm was observed in the materials mentioned in this study. Calcium fluoride consists of a simple cubic lattice of fluorine ions in which every other body center position is occupied by a Ca²⁺ ion. Yb³⁺ ions occupy the Ca²⁺ sites when they are introduced into the CaF₂ lattice. Cooperative sensitization based on Yb³⁺-dimers can excite Sm²⁺ ions by bridging the energy gap between the ground state and excited state of Sm²⁺ ions, yet it has not been performed to date.

The main reason for this should be that it is a real challenge to prepare a material simultaneously containing Yb³⁺ and Sm²⁺ ions using the conventional solid state reduction reaction. Samarium is a lanthanide that usually takes on a trivalent state, and an intense reduction reaction is needed to change it from the trivalent state to the divalent state. However, trivalent ytterbium has a similar reduction-oxidation potential as that of trivalent samarium; thus, it will also be reduced to the bivalent state under the same conditions for the reduction of trivalent samarium ions. Therefore, a reasonable synthetic route should be considered for the coexistence of Sm²⁺ ions and Yb³⁺ ions in a material.

Herein, we synthesized a hybrid material containing the Yb³⁺-dimer and Sm²⁺ ions via a stepwise synthetic reaction and report the first experimental observation of UC emissions from Sm²⁺ ions upon 980 nm excitation.

Experimental

Yb³⁺-doped CaF₂ and Sm²⁺-doped BaFCl_0.5Br_0.5 were prepared via high temperature solid state reactions in argon and reducing atmospheres, respectively. Stoichiometric amounts of the raw materials CaF₂ and YbF₃ (1 mol%) were thoroughly mixed by grinding and heated to 1200 °C for 2 h in an Ar atmosphere. Sm³⁺-doped BaFCl_0.5Br_0.5 was prepared in the same way but in a H₂ reducing atmosphere. A suitable melting temperature is the key to ensure a close enough distance between Yb³⁺ and Sm²⁺ ions and achieve efficient energy transfer from the Yb³⁺-dimers to Sm²⁺ ions. To make this a reality, the prepared powders BaFCl_0.5Br_0.5:1%Sm²⁺ and CaF₂:1%Yb³⁺ were mixed in the mass ratio of 1 : 1 and annealed at 900 °C in an Ar atmosphere for 10 min to obtain the final hybrid material BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺.

Characterization

A Rigaku RU-200b X-ray powder diffractometer (XRD) was used to analyze the crystal structures using nickel-filtered Cu-Kα radiation (λ = 0.15405 nm) in the range of 10° ≤ 2θ ≤ 70°. The accelerating voltage was 40 kV, and the emission current was 200 mA. Absorption spectra in the UV to NIR region were obtained using a Shimadzu UV3600 spectrophotometer in the range of 200–1100 nm. Excitation and emission spectra were obtained using a Hitachi F-4500 spectrometer equipped with a Hamamatsu R928 photomultiplier (PMT) and a 2 W 980 nm continuous wave diode laser as the excitation source. Luminescence spectra were obtained using a one-meter monochromator (SPEX 1000M; HORIBA Jobin Yvon Inc., Edison, NJ, USA) equipped with an 1800 line mm⁻¹ grating. The excitation light source was a power adjustable continuous wave laser diode (978 nm, 10 W; BWT Beijing Ltd, Beijing, China). Spectral measurements at low temperature were performed using a helium-cycled cryostat (ARS-2HW; Advanced Research Systems, Macungie, PA, USA). A digital oscilloscope (DPO4104B, bandwidth 1 GHz, sampling rate 5 GSs⁻¹; Tektronix, Shanghai, China), a power-adjustable continuous wave laser diode (CW978 nm, 10 W), and a chopper were used to obtain the decay curves.

Results and discussion

The composition and phase purity of the products were first examined by XRD. It is well known that CaF₂ has a cubic crystal structure and its lattice parameter values are a = 5.463 nm, b = 5.463 nm, and c = 5.463 nm. The blue line in Fig. 1a shows the XRD patterns of the as-prepared CaF₂:1%Yb³⁺. All the patterns can be indexed to the pure cubic phase of CaF₂ (JCPDS 4-864) and no impurity peaks were observed. There is no standard card corresponding to the sample BaFCl_0.5Br_0.5:1%Sm²⁺ since it is a fluoride halide-mixed crystal (Fig. 1a, red line). According to the XRD patterns of the products, neither impurity peaks nor a second phase can be detected at the current mixing level. This clearly implies that the doping of Yb³⁺ and Sm²⁺ ions does not cause any significant change in the host structure.

Fig. 1 (a) XRD patterns of BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺ (hybrid), BaFCl_0.5Br_0.5:1%Sm²⁺ and CaF₂:1%Yb³⁺. (b) Absorption spectra of the obtained samples. (c) Emission spectrum of BaFCl_0.5Br_0.5:Sm²⁺ upon 408 nm excitation, and UC spectrum of Sm²⁺ in the hybrid material under 978 nm excitation. (d) Photoluminescence excitation spectra of hybrid and BaFCl_0.5Br_0.5:1%Sm²⁺ by monitoring the emission of Sm²⁺ ions at 689 nm.

The absorption spectra of obtained powder samples, with the characteristic bands of Sm²⁺ ions and Yb³⁺ ions in the UV-vis and the near-infrared (NIR) regions, respectively, reveal the existence of both Sm²⁺ and Yb³⁺ ions, as shown in Fig. 1b. The absorption band at around 950 nm is attributed to the electronic transition from the ²F_7/2 ground state to the ²F_5/2 excited states of Yb³⁺ ions. For the samples doped with Sm²⁺ ions, an obvious broad absorption band with four peaks (black and blue curves) appears in the UV-vis region, which are assigned to the 4f5d bands of Sm²⁺ ions. The emission spectrum of BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺ was obtained under 408 nm excitation, which is shown as the black line in Fig. 1c. The emissions consist of several sharp peaks in the region from 600 to 750 nm, which are assigned to the ⁵D₀ → ⁷F_j (j = 0, 1, and 2) and ⁵D₁ → ⁷F_j (j = 0, 1, and 2) transitions of Sm²⁺ ions. It is well-known that the 689 nm emission from the ⁵D₀ → ⁷F₀ transition of Sm²⁺ ions is efficient and well isolated. To understand the origin of the emissions, the excitation spectra of the 4f5d bands were obtained by monitoring the transition of ⁵D₀ → ⁷F₀ (689 nm), as shown in Fig. 1d. It can be seen that there are four main peaks in the 4f5d band. The half diffraction peak of the emission ⁵D₀ → ⁷F₀ is located at about 345 nm. The UC emissions of BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺ was obtained under 978 nm excitation, which is shown as the red line in Fig. 1c. The emission curve is almost the same as that in the DC spectrum of BaFCl_0.5Br_0.5:1%Sm²⁺ without Yb³⁺ ions, indicating that two different ways of excitation can induce the emissions of Sm²⁺ ions. Therefore, it is reasonable to attribute the UC emission peaks at 631 nm, 644 nm, 665 nm, 689 nm, 704 nm, and 729 nm to the ⁵D_i (i = 0, 1) → ⁷F_j (j = 0, 1, 2) transitions of the Sm²⁺ ion.

Under the pumping of an NIR laser, a pair of Yb³⁺ ions (Yb³⁺-dimer) were both exited from the ²F_7/2 to ²F_5/2 level, and then depopulated from the excited state ²F_5/2 simultaneously to emit a visible photon, which is called cooperative luminescence. Fig. 2a shows the photoluminescence spectrum of the mixture, hybrid, and CaF₂:1%Yb³⁺ under 980 nm excitation. A broad band ranging from 470 to 570 nm is observed, which originates from the cooperative luminescence of the Yb³⁺-dimers. It was found that this broad band is composed of a few sharp emission peaks connected with each other. It is well-known that the ²F_5/2 to ²F_7/2 levels of Yb³⁺ ions split into three and four Stark components in a tetragonal crystal field, respectively.³³ Therefore, a random combination of two Stark components in the cooperative luminescence of a Yb³⁺-dimer forms emissions located at different energies but close to each other.³⁴ These approaching emission peaks are connected to form a broad band.

Fig. 2 (a) Emission spectra (450–750 nm) of the hybrid, CaF₂:1% Yb³⁺ and mixture materials upon 978 nm excitation at room temperature. (b) Schematic for the energy level diagram of Yb³⁺ and Sm²⁺ ions and possible UC population and emission processes.

As shown in Fig. 2a and 1d, there is obvious spectral overlap between the emission of the Yb³⁺-dimers and the excitation of Sm²⁺ ions. Radiation reabsorption may happen when the excitation and emission spectra overlap. However, there is no UCL in the physical mixture of BaFCl_0.5Br_0.5:1%Sm²⁺ and CaF₂:1%Yb³⁺, which indicates that the mechanism of radiation reabsorption does not play a main role in the process of UCL of Sm²⁺ ions. In contrast, the emissions of Yb³⁺-dimers are still very strong in the hybrid material, and Yb³⁺-dimers have a partial radiative transition accompanied with CL, thus demonstrating that only some of the energy of the Yb³⁺-dimers transfers to Sm²⁺ ions and realizes the UCL of Sm²⁺ ions. Fig. 2b schematically describes the possible UC processes in the energy level diagrams of the cooperation energy transfer from Yb³⁺ to Sm²⁺ ions. The large energy gap between the ground state and the excited state in Sm²⁺ ions requires the cooperation sensitization of Yb³⁺-dimers because there is no intermediate level in Sm²⁺ that can be resonant with a 980 nm photon or an excited Yb³⁺ ion. Under the NIR laser excitation, an Yb³⁺-dimer simultaneously absorbs two NIR photons and then transfers their energy to a Sm²⁺ ion through a cooperative sensitization process to promote it to the state of ⁵D₃. The lower energy levels ⁵D_0,1 can be populated through a series of non-radiative relaxations from the higher ⁵D₃ state.

To further confirm the CET from the Yb³⁺-dimers to Sm²⁺ ions, the excited state dynamics was investigated under 978 nm excitation by monitoring the emission of Yb³⁺-dimers at 500 nm, as shown in Fig. 3a. The decay time of the Yb³⁺-dimers decreases in the hybrid material, indicating that CET occurred between the Yb³⁺-dimers and Sm²⁺ ions. It can be seen that these decay curves have no rising edge, which means that the CL of the Yb³⁺-dimers originates from the direct excitation of 978 nm and no energy transfer process exists. However, the UCL of Sm²⁺ ions shows a rising edge, as shown in Fig. 3b, which indicates that the energy that stimulates Sm²⁺ ions originates from the energy transfer of the Yb³⁺-dimers. The decay times were measured at around 1.13 ms and 1.86 ms for the ⁵D₁ → ⁷F_0,1,2 and ⁵D₀ → ⁷F_0,1,2 transitions of Sm²⁺ ions, respectively. The decay parts of these curves can be fitted well into a single-exponential function as I = I₀ exp(−t/τ) (I₀ is the initial emission intensity at t = 0, and τ is the lifetime), demonstrating the populating processes as discussed above.

Fig. 3 (a) Decay curves of cooperative luminescence from Yb³⁺-dimers by monitoring the 500 nm emission of BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺, CaF₂:1%Yb³⁺ and the mixture under 978 nm excitation. (b) UC photoluminescence decay curves of the ⁵D_i → ⁷F_j transitions of Sm²⁺ ions at room temperature.

To fully analyse the nature of the UC processes, the dependence of the integral luminescence intensities on the pumping laser power (at 978 nm) was examined. For an unsaturated UC process, the integrated UCL intensity I_f is proportional to Pⁿ, where, P denotes the pumping power, and the exponent n represents the number of photons involved in populating the upper emitting state in the UC process. Fig. 4 shows the typical double-logarithmic plots of UCL intensities versus the pump power densities. The values of the photon number n are listed in Table 1. It can be seen from the table that these transitions are of two-photon UC processes. This result corresponds to the analysis of the UC population and emission processes in the schematic energy level diagram of the Sm²⁺ ion and Yb³⁺-dimer.

Fig. 4 (a) Excitation power dependence of the UCL of ⁵D₁→ ⁷F_j (j = 0, 1, 2) in Sm²⁺ ions. (b) Excitation power dependence of UCL of ⁵D₀ → ⁷F_j (j = 0, 1, 2) in Sm²⁺ ions.

Table 1 The photon number n with different transitions

Emitting level	^5D1 → ^7F0	^5D1 → ^7F1	^5D1 → ^7F2	^5D0 → ^7F0	^5D0 → ^7F1	^5D0 → ^7F2
Number of photons	1.59	1.64	1.68	2.11	2.04	1.93

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Conclusion

In summary, the hybrid phosphor BaFCl_0.5Br_0.5:1%Sm²⁺–CaF₂:1%Yb³⁺ was prepared via a stepwise high temperature reaction to achieve a material containing both Yb³⁺ and Sm²⁺ ions. The UCL of the ⁵D_i (i = 0, 1) → ⁷F_j (j = 0, 1, 2) transitions from Sm²⁺ was observed under NIR excitation. The analysis results indicate that the population of Sm²⁺ excited states originates from the cooperative energy transfer of Yb³⁺-dimers. Based on our results, it is actually possible to achieve the upconversion luminescence of changeable valence lanthanides under NIR excitation, which moreover, opens up a new field for the upconversion of rare earth ions.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (Grants 11474132 and 11274139).

Notes and references

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