1. Introduction

Efficient luminescent materials with quantum yield higher than unit could be playing a significant role in the modern lighting and photoelectric display devices [1,2]. In theory, Dexter earlier interpreted that, for system doped with a couple of ions and a single activator ion, an absorbed vacuum ultraviolet (VUV) photon can be downconverted to two visible photons via energy transfer “sensitized luminescence” and “cascade” radiative transitions [3], respectively. The underlying physical mechanism is well known as photon cascade emission (PCE), quantum splitting (QS), quantum cutting (QC), or downconversion. Experimentally visible quantum cutting (VIS-QC) was first achieved in VUV-excited Pr³⁺-doped fluorides (YF₃ and α-NaYF₄) [4,5]. Especially the recent demonstration of VIS-QC in Gd³⁺-Eu³⁺ couple with an appreciable quantum yield exceeding 190% greatly stimulate the development of VIS-QC materials for the potential application in the ultra-efficient lighting and display devices [6,7]. In the excitation process, the feature of quantum yield greater than unit could be efficiently achieved with an intermediate level, where, through it, the absorbed energy in a couple can be stepwise transferred to activators from a sensitizer or in a single activator can be de-excited to the ground state by several “cascade” emissions [3–7]. For the potential application in the enhancement of solar cell efficiency, near-infrared (NIR) QC has been widely studied in various materials codoped by RE³⁺/Yb³⁺ (RE = Tb, Tm, Pr, Er, Nd and Ho) [8–16], where an ultraviolet-to-blue photon absorbed by RE³⁺ donor ion can be efficiently downconverted to two NIR photons re-emitted at about 1000 nm by Yb³⁺ acceptor ions. On the other hand, the NIR-QC system also has been realized in RE³⁺ (RE = Dy, Er, Ho and Tm) singly doped polycrystalline phosphors [17–20]. Particularly the three-step sequential three-photon NIR-QC of Tm³⁺ reported by Yu et al. [19] provides an interesting route to improve the photo-response of certain photoelectronic devices such as germanium solar cells. Unlike the polycrystalline phosphors, oxyfluoride glass ceramics (GCs) exhibits many advantages such as high transparency, low phonon energy, and stable physical and chemical performance [21–23]. Most importantly, due to the higher transparency for visible-to-NIR sunlight, rare earth ions doped GCs of NIR-QC is more preferable to be spectral downconverters in the front of solar cells [1,12,16]. In this paper, we fabricated a new species of fluorogermanate GC containing LaF₃:Tm³⁺ nanocrystals. Upon excitation of a blue photon around 468 nm, a sequential three-step three-photon NIR-QC of Tm³⁺ can efficiently take place with an internal quantum yield of about 160%. The underlying mechanism involved has been investigated in details.

2. Experimental

Oxyfluoride precursor glasses (PGs) were prepared by the nominal molar composition of 50GeO₂-20Al₂O₃-15LaF₃-15LiF-0.25TmF₃-xYbF₃ (x = 0, 1). The starting materials of high-purity GeO₂, LaF₃, TmF₃ and YbF₃ (99.99%), and analytical reagent grade Al₂O₃ and LiF were mixed with the specified stoichiometric ratio and further grounded completely. The batches of homogeneous mixtures (15 g) were melted in a covered corundum crucibles at 1350 °C for 1 h under air atmosphere. The melts were quenched on a preheated stainless steel plate and then annealed at 520 °C for 2 h. Here the obtained PG doped with Tm³⁺ ion and Tm³⁺/Yb³⁺ couple are donated as PG:Tm³⁺ and PG:Tm³⁺/Yb³⁺, respectively. To fabricate transparent GCs, the PG:Tm³⁺ sample was cut into several small pieces and went through the further thermal treatment at 570 °C for 4, 6 and 8 h, respectively. Accordingly, the prepared GC:Tm³⁺ samples were labeled as GC@5704h, GC@5706h and GC@5708h relative to the different thermal treatment time. Afterwards all the PG and GC samples were polished for optical measurements.

To identify the precipitated crystalline, X-ray diffractometer measurement of the crushed powders of the samples before and after heat-treatment was carried out by means of an X-ray powder diffractometer (XRD; Philips PW1830, Cu Kα). The microstructures of GC@5708h were characterized by transmission electron microscope (TEM; JEM-2010, Tokyo, Japan) combined with the selected area electron diffraction (SAED). Raman spectra of PG:Tm³⁺ and GC@5704h, GC@5706h and GC@5708h were detected by LabRAM Aramis micro Raman spectrometer (HORIBA, Jobin Yvon, France). Absorption spectra were measured on a Perkin-Elmer Lambada 900 UV/VIS/NIR spectrophotometer (Cary Model 5000, Varian Incorporation, Paloalto, CA) in the wavelength range of 250-2700 nm with the resolution of 1 nm. Steady and dynamic fluorescence spectra were recorded on a high-resolution Edinburgh FLSP920 spectrophotometer (Edinburgh Instruments, Livingston, UK) equipped with continuous wavelength 450 W xenon (Xe) lamp and microsecond Xe flash lamp as excitation sources, respectively. For FLSP920 system, a red-sensitive R928 photomultiplier tube (PMT. Hamamatsu, Japan) and a liquid nitrogen cooled R5509-72 PMT were applied for the 400-850 nm visible photon detection and the 800-1600 nm NIR photon detection, respectively. Additionally, upon pump of an 808 nm laser diode (LD) and a 980 nm LD, NIR to middle-IR (MIR) photoluminescence (PL) spectra measurements were performed on a computer-controlled Triaxial 320 spectro-flourimeter (Jobin-Yvon Inc., Paris, France) with PbSe-020 detector (operating wavelength region in 1000-4500 nm) assembled with a Standford SR510 lock-in amplifier. For comparison, all the optical measurements were operated under the identical conditions for each series of test.

3. Results and discussions

3.1 Structural characterization

Figure 1(a) shows the measured XRD patterns of the samples before and after heat-treatment. It can be seen that the XRD pattern of PG:Tm³⁺ exhibits typical amorphous with one broad diffuse humps, while, the XRD patterns of heat-treated samples exhibit several intense diffraction peaks, which can be readily indexed to the hexagonal LaF₃ phase (JCPDS Card No. 076-0510) [24]. This result indicates that LaF₃ nanocrystals have been successfully precipitated among glass matrix after heat treatment. According to the Scherrer formula, the average crystal diameters are evaluated to be about 14, 19 and 25 nm for GC@5704h, GC@5706h and GC@5708h, respectively. On the other hand, TEM micrograph in Fig. 1(b) reveals the microstructure of GC with irregular 10-20 nm LaF₃ crystallites distributing among the glass matrix. Moreover, SAED pattern in the inset of Fig. 1(b), as well as the high-resolution TEM (HRTEM) image in Fig. 1(c), further reveals the detailed crystal structure of a LaF₃ nanocrystal in the TEM micrograph.

 

Fig. 1 (a) XRD patterns of PG:Tm³⁺ and the associated GC samples, (b) typical TEM bright field image of GC@5708h, and (c) HRTEM image of LaF₃ nanocrystal. The inset of Fig. 1(b) shows the corresponding SAED pattern of GC@5708h.

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3.2 Raman spectra and UV-VIS optical absorption

Raman spectra of PG:Tm³⁺, GC@5704h, GC@5706h and GC@5708h are comparatively shown in Fig. 2.There exhibit two broad vibrational frequency bands in the region of 100-1250 cm⁻¹ with one band centered at 545 cm⁻¹ in the range of 200-700 cm⁻¹, and another one centered at 875 cm⁻¹ in the range of 700-1100 cm⁻¹. By Gaussian fitting, the Raman spectra of PG sample can be subdivided into five sub-peak with the peak centered at 367, 510, 587, 786 and 874 cm⁻¹ (Fig. 2), respectively. In actual, the low frequency band at 367 cm⁻¹ is due to the bending vibration of Ge-O-Ge bridges, and the band at 510 and 587 cm⁻¹ can be assigned to the symmetric stretching of Ge-O-Ge bonds. The high frequency band at 786 cm⁻¹ is related to the Q2 stretching vibrations, and the band at 874 cm⁻¹ is due to the Q3 stretching vibrations [25,26]. Interestingly, for the high vibrational frequency band in 700-1100 cm⁻¹, as the heat treatment time increases, the frequency peak position shifts from 875 to 844 cm⁻¹, which suggests that the maximum phonon energy of the GC sample becomes lower than the PG sample due to the precipitation of LaF₃ nanocrystals. Moreover, for the low vibrational frequency band in 200-700 cm⁻¹, there emerges one intense band located at 461 cm⁻¹ and three weak shoulders at 226, 286 and 362 cm⁻¹ after heat treatment. The frequency bands at 226, 286, 362 and 461 cm⁻¹ all can be indexed to the typical vibrational frequency bands of LaF₃ crystal lattice [27,28], which provides additional proof for the successful precipitation of LaF₃ crystallites among glass matrix after the corresponding heat-treatment regime.

 

Fig. 2 Raman spectra of PG:Tm³⁺ and associated GC samples.

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Figure 3 exhibits the absorption spectra of PG:Tm³⁺, GC@5704h, GC@5706h and GC@5708h in the wavelength region of 250-3000 nm. All the absorption spectra consist of several absorption bands centered at 356, 468, 685, 790, 1209 and 1670 nm [10], typically ascribed to the electronic transitions from the ³H₆ ground state to the ¹D₂, ¹G₄, ³F_2,3, ³H₄, ³H₅ and ³F₄ excited states of Tm³⁺, respectively. This observation confirms that Tm³⁺ ions are well distributed into the as-prepared PG and GC samples. Because of the Rayleigh scatting caused by the LaF₃ nanocrystals, the ultraviolet absorption edge of GC samples obviously shifts to longer wavelength with the prolongation of heat treatment time, which is probably induced by the growth of LaF₃ nanocrystals [29]. It is should be noticed that, following the heat-treatment at 570 °C for several hours, all the obtained GC samples still keep high transparence, as compared in the inset of Fig. 3.

 

Fig. 3 Absorption spectra of PG:Tm³⁺ and associated GC samples.

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3.3 Static-to-dynamic photoluminescence of Tm³⁺ and luminescence mechanisms

Shown in Fig. 4 are the visible-to-NIR PL spectra of GC@5708h and PG:Tm³⁺/Yb³⁺. Under excitation of 468 nm monochromatic light, not only the visible 650 and 792 nm emissions but also the NIR emissions at 1190, 1462 and 1610 nm can be intensely detected likely due to the electronic transitions of ¹G₄ → ³F₄, ³H₄ → ³H₆, ¹G₄ → ³H₄, ³H₄ → ³F₄ and ³F₄ → ³H₆ of Tm³⁺, respectively [10,19], as shown in Fig. 4(a). For a rigorous investigation into the NIR PL mechanisms, the additional PL spectra of GC@5708h are measured under excitation of 790 nm light and an 808 nm LD (Fig. 4(b)), and that of PG:Tm³⁺/Yb³⁺ are further obtained upon a 980 nm LD excitation (Fig. 4(c)). As GC@5708h excited at 790 nm, only the two emission bands around 1462 and 1610 nm can be detected because of ³H₄ → ³F₄ and ³F₄ → ³H₆ [19], respectively. Notably, for detection wavelength beyond 1600 nm, the spectral response reduces sharply for R5509-72 PMT but does inversely for the NIR-MIR PbSe-020 photoconductor. As a result, the typical ³F₄ → ³H₆ of Tm³⁺ at 1800 nm measured by R5509-72 (solid lines in Figs. 4(a-b)) performs the weak PL intensity and extreme distortion relative to that measured by PbSe-020 detector (dotted lines in Figs. 4(b-c)) [19]. Upon excitation with an 808 nm LD, besides the ³F₄ → ³H₆ at 1800 nm of GC@5708h, the NIR PL bands around 1462 nm (³H₄ → ³F₄) can be also detected by PbSe-020 photoconductor (Fig. 4(b)). Whereas, only the NIR emission peaked at 1800 nm can be recorded due to ³F₄ → ³H₆ in PG:Tm³⁺/Yb³⁺ as Tm³⁺ excited by Stokes energy transfer from Yb³⁺ pumped by a 976 nm LD [19,30].

 

Fig. 4 Visible-to-NIR PL spectra of GC@5708h under excitation of (a) 468 nm, and (b) 790 nm and 808 nm LD, respectively. Figure 4(c) presents PL spectra of PG:Tm³⁺/Yb³⁺ excited with a 976 nm LD. Solid and dotted NIR PL spectra are recorded on R5509-72 PMT and PbSe-020 detector, respectively.

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Figure 5 shows PL excitation (PLE) spectra of GC@5708h in the wavelength range of 320-850 nm. A series of PLE bands centered at 356, 468, 684 and 790 nm can be easily assigned to the electronic transitions from the ³H₆ ground state to the ¹D₂, ¹G₄, ³F_2,3 and ³H₄ excited state of Tm³⁺ [10,19], respectively. However, it can be found that, by monitoring the PL wavelength of 650 and 1190 nm, only the 356 and 468 nm PLE bands can be observed due to ³H₆ → ¹D₂ and ³H₆ → ¹G₄ of Tm³⁺, respectively, while, by monitoring the emissions at 792, 1462 and 1610 nm, respectively, additional PLE bands can be detected at 684 nm (³H₆ → ³F_2,3) and 790 nm (³H₆ → ³H₄). These observations suggest that the NIR emission at 1190 nm only can be induced from the excited ¹G₄ state and its above excited state, but the 1462 and 1610 nm emissions can originate from the ³H₄ state or it’s below state. Furthermore, decay curves are measured for the 650, 792, 1190, 1462 and 1610 nm emissions, as comparatively shown in Fig. 6. It is of great interest that the 650 and 1190 nm emissions exhibit the same decay cures with the almost identical lifetime of ~280 μs, the 792 and 1462 nm emissions also perform the similar decay curves with the average lifetime around 450 μs, while the 1610 nm emission just feature an independent decay curve with lifetime of 4.04 ms. These results clearly confirm that the 1190 nm NIR emission does originate from the same electronic level to the typical visible 650 nm PL from the ¹G₄ level of Tm³⁺, the 1462 nm NIR emission comes from the another same level to the typical visible 792 nm PL from the ³H₄ level of Tm³⁺, and the NIR 1610 nm emission is just from the ³F₄ level. Hence, it can be concluded that the simultaneous appearance of 1190, 1462 and 1610 nm NIR PL bands just originate from the “cascade” radiative transitions of Tm³⁺ with ³H₄ and ³F₄ acting as the corresponding intermediate levels [19].

 

Fig. 5 PLE spectra of GC@5708h monitored at PL wavelengths of 650, 792, 1190, 1462 and 1610 nm, respectively.

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Fig. 6 Luminescence decay curves of 650, 792, 1190, 1462 and 1610 nm of GC@5708h.

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Energy level diagram in Fig. 7 illustrates the three-step sequential three-photon NIR-QC of Tm³⁺. Under excitation of 468 nm into the ¹G₄ state of Tm³⁺, besides the radiation of ¹G₄ → ³F₄ (650 nm) and ³H₄ → ³H₆ (792 nm), the energy in excited ¹G₄ state can be decayed to the ³H₄ state by the first 1190 nm photon emitting, subsequently the populated ³H₄ state can be further relaxed to the ³F₄ state via the second 1462 nm photon radiating, and finally the excited ³F₄ state gives out the third 1800 nm photon to populate the ³H₆ ground state. Based on the energy gap law [31], the energy differences of ¹G₄ → ³F_2,3 (~6000 cm⁻¹), ³H₄ → ³H₅ (~4000 cm⁻¹) and ³F₄ → ³H₆ (~5600 cm⁻¹) are all determined to be large enough for maximum phonon energy ~461 cm⁻¹ of LaF₃ crystal lattice, enabling the radiative transitions from the excited ¹G₄, ³H₄ and ³F₄ state of Tm³⁺ ions dominate over the phonon-assisted nonradiative relaxation. In comparison, as the ³H₄ level of Tm³⁺ is directly excited at 790 nm or by an 808 nm LD (Fig. 7(b)), the sequential two-step transitions of ³H₄ → ³F₄ and ³F₄ → ³H₆ do take place efficiently with ³F₄ acting as an intermediate level, just emitting the second 1462 nm photon and the third 1800 nm photon for the aforementioned three-photon NIR-QC of Tm³⁺ ((Fig. 7(a)). Moreover, as the ³H₅ state of Tm³⁺ excited by Stokes energy transfer from the ²F_5/2 state of Yb³⁺ pumped with a 976 nm LD (Fig. 7(c)), only the third-step of ³F₄ → ³H₆ at 1800 nm occurs following the energy in ³H₅ state nonradiatively depopulated to ³F₄ of Tm³⁺.

 

Fig. 7 Energy-level diagram of Tm³⁺ interpreting the concept of three-step three-photon NIR-QC: (a) as ¹G₄ excited by 468 nm, (b) as ³H₄ excited by 790 and 808 nm, respectively, and (c) as ³H₅ excited by Stokes energy transfer from Yb³⁺: ²F_5/2 pumped by 976 nm LD.

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For the sake of exploring more luminescence properties of the three-step sequential three-photon NIR-QC of Tm³⁺, visible and NIR time resolved emission spectra of GC@5708h were further measured upon 468 nm pulsed excitation with a μF900 Xe flash lamp. As shown in Fig. 8(a), in an initial delay time around 8 μs, only the radiation of ¹G₄ → ³F₄ at 650 nm is observed clearly. Till the delay time about 10 μs, the additional ³H₄ → ³H₆ transition at ~790 nm emerges intensely, meanwhile, the PL intensity of 650 nm enhances obviously. Moreover, with the increase of delay time to 1000 μs, the PL intensity of 790 nm is shown to improve dramatically relative to that of 650 nm. These results reveal that, as delay time increases, the energy in excited ¹G₄ state could be stepwise decayed to the ³H₄ state likely by the ¹G₄ → ³H₄ transition at 1190 nm. On the other hand, as ¹G₄ excited directly, only the ¹G₄ → ³H₄ at 1190 nm first emerges at the delay time about 8 μs, and then the relatively feeble ³H₄ → ³F₄ transition at 1462 nm presents in delay time region of 10-20 μs due to the sequential energy population of ³H₄ state of Tm³⁺ (Fig. 8(b)). Furthermore, PL intensity of 1462 nm rises obviously relative to that of 1190 nm, which is resulted from the more energy population of ³H₄ state following the efficient occurrence of ¹G₄ → ³H₄ at longer delay time. These observations demonstrate that, upon excitation of 468 nm, the excited ¹G₄ state can de-excite stepwise to the ³H₄ and ³F₄ intermediate states, first emitting an 1190 nm photon and subsequently another 1462 nm photon. In principle, the populated ³F₄ level can further generate the third 1800 nm photon by the third-step of ³F₄ → ³H₆ (Fig. 4). However, due to the feeble spectral response of R5509-72 PMT at wavelength more than 1600 nm, the 1800 nm emission cannot be observed dynamically [19,20].

 

Fig. 8 Visible (a) and NIR (b) time-resolved fluorescence spectra of GC@5708h upon 468 nm pulsed light excitation with a microsecond μF900 Xe flash lamp.

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In theory, luminescence quantum yield,${\eta }_{\text{QY}}$, is defined as the ratio of the number of re-emitted photons to that of absorbed photons. On the basis of the absorption spectra (Fig. 3), Judd-Ofelt analysis was carried out to predict the radiative properties of Tm³⁺ in PG and GC samples. By applying least-squares fit method, the Judd-Ofelt intensity parameters (Ω₂, Ω₄, Ω₆) and the radiative transition probability ($A$) of Tm³⁺-doped PG and GC samples can becalculated rationally [10,24], where the corresponding Judd-Ofelt intensity parameters and the root-mean-squared (RMS) deviation δ_rms are summarized in Table 1.By considering the sequential three-step energy transfer mechanisms from the ¹G₄ state of Tm³⁺, the total ${\eta }_{\text{QY}}$ can be evaluated by the following formula [4,5,19]:

Table 1. Judd-Ofelt parameters Ω_t of PG:Tm³⁺ and the associated GC samples

View Table

4. Conclusion

In summary, efficient three-step sequential three-photon NIR QC has been demonstrated in transparent fluorogermanate GC containing LaF₃:Tm³⁺ nanocrystals. Upon excitation of a blue photon at 468 nm, three NIR photons could be obtained at 1190, 1462 and 1800 nm, respectively. In terms of steady and dynamic luminescence spectra, the underlying mechanism has been investigated theoretically and experimentally. The calculated total quantum yield of the three-step sequential three-photon NIR-QC is about 160%. Such multiphoton NIR-QC of Tm³⁺ singly doping would provide a new approach to design the high efficient NIR luminescence materials.