1. Introduction

Research on luminescent materials with quantum efficiency (QE) larger than unity was initially aroused aiming at developing high-performance mercury-free lamps and plasma display panels, where an absorbed vacuum ultraviolet (VUV) photon is expected to be converted into more than one visible photon [1–3]. The underlying mechanism is referred to as quantum splitting (QS), quantum cutting (QC) or photon cascade emission (PCE) in the visible region, which was predicted theoretically by Dexter in 1957 [4]. The achievement of QS was firstly reported by Piper et. al. [5] and Sommerdijk et. al. [6] in Pr³⁺-doped YF₃ and α-NaYF₄, where the ¹S₀ level of the 4f² configuration is located below the lowest 4f5d level. After the absorption of an incident VUV photon, Pr³⁺ is initially excited to 4f5d levels followed by non-radiative relaxations (NR) down to ¹S₀ level. Two visible photons are emitted as the excited Pr³⁺ ion decays via two-step sequential transitions: firstly ¹S₀ → ¹I₆ for 407 nm emission, and secondly ³P₀ → ³H_J, ³F_J for 610 nm emission. The QE for visible light was determined to be 140 ± 15%.

To avoid the ultraviolet (UV) and infrared losses encountered with a single ion, Wegh et. al. [7,8] proposed an alternative route known as “downconversion (DC)” to visible QS, which is based on energy transfer (ET) between lanthanide (Ln) ions. The Gd³⁺-Eu³⁺ couple is a well-known example for such mechanism, exhibiting QE as high as 190%. Recently much attention has been shifted to the near infrared (NIR) QS to explore its potential application in crystalline Si (c-Si) solar cell, which only performs an energy conversion efficiency of 15% lower than Shockley–Queisser limit (29%) due to the energy mismatch between the solar spectrum and the bandgap (E_g) of c-Si [9–11]. To boost the solar cell efficiency, DC materials seem applicable via spectrum modification, i.e., DC of one high-energy incident photon into two or more photons with energy above the E_g of c-Si. Yb³⁺ ion is an attractive emitter as the energy of its emission (~1000 nm) matches well with the E_g of c-Si. Thus, there has been a continuous interest in the NIR QS through DC in Ln³⁺-Yb³⁺ (Ln = Tb, Tm, Pr, Er, Nd and Ho) couples to convert one UV-visible photon to two NIR photons around 1000 nm [12–20]. However, in the most of the cases, the NIR emission might only originate from Yb³⁺ via the first-order ET from Ln³⁺ rather than QS. Besides, NIR-QS on a single Ln³⁺ ion has seldom been reported till now, and the main mechanisms for the NIR QS are still not well understood [21].

Recently, transparent oxyfluoride glass ceramics (GC) have been receiving significant attention as promising hosts for upconversion (UC) and DC luminescence of Ln³⁺ ions. As composite materials containing fluoride nanocrystals within glass matrix, they not only exhibit excellent properties of oxide glass such as easy fabrication technique, high chemical and mechanical stability but also provide low phonon energy environment for Ln³⁺ ions incorporated in nanocrystals and, therefore facilitate efficient luminescence. In this paper, we report on the NIR-QS effect in Ho³⁺-doped germanate GC containing LaF₃ nanocrystals. The luminescence measurements present that one absorbed UV or visible photon within the range of 300-560 nm can be converted into two NIR photons at 1013 and 1190 nm via sequential two-step radiative transitions. NIR-QS presents an intriguing path for the design of ultra-efficient optical converters, e.g. for application in low-bandgap solar cells thermo-photovoltaic energy conversion, sensing etc..

2. Experimental

Glass with nominal molar composition of 49.9GeO₂–22 Al₂O₃–13LaF₃–15LiF–0.1HoF₃ was prepared by melting mixtures of raw materials in covered corundum crucibles at 1350 °C for 1 h in air. The melt was then quenched on a preheated stainless steel plate followed by annealing at 450 °C for 2 h. The obtained glass was heat-treated at 560 °C for 8 h to fabricate transparent GC sample. Afterwards the sample was polished for optical measurements. The crystalline phase was identified by X-ray powder diffractometer (X’ Pert PROX, Cu Kα). The microstructures of the sample were characterized by a transmission electron microscope (TEM, JEM-2010) assembled with the selected area electron diffraction (SAED). Optical absorption spectrum was measured on a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer with the resolution of 1 nm. Excitation and emission measurements with the resolution of 1 nm were performed using fluorescence spectrometer (Edinburgh FLS920-combined with Time Resolved and Steady State Spectrophotometer equipped with thermo-electronic cooled R928 visible photomultiplier tube and R5509-72 NIR photomultiplier tube in a liquid nitrogen housing). All the measurements were carried out at room temperature.

3. Results and discussions

3.1 Structural characterization

The XRD patterns of Ho³⁺-doped precursor glass (PG) and GC are presented in a comparative way (see Fig. 1(a) ). It is noted that the PG is structurally amorphous with only two diffuse humps detected in the curve, whereas it exhibits several characteristic diffraction peaks readily indexed to the hexagonal LaF₃ phase (JCPDS Card No. 01-076-1500) for the sample heat treated at 560 °C for 8h. It is clear that LaF₃ crystals have been crystallized from the glass matrix. The microstructures of GC sample was further investigated by TEM, as shown in Fig. 1(b), which reveals the composite structure of GC with 12-20 nm sized LaF₃ nanocrystals distributing among glass matrix. Figure 1(c) illustrates the detailed lattice structure of LaF₃ nanocrystals.

 

Fig. 1 (a) XRD patterns of the precursor glass (PG) and GC heat-treated at 560 °C for 8 h. (b) TEM bright field image and corresponding selected area electron diffraction pattern (inset) of the GC. (c) High-resolution TEM image of GC.

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3.2 UV-VIS optical absorption and J-O analysis

The absorption spectrum of Ho³⁺-doped GC in the 300-2200 nm spectral range is presented in Fig. 2 . The detected absorption bands can be readily ascribed to the transitions from the ground state ⁵I₈ to the excited states as labeled in the figure. Besides, each band shows great similarity in shape and position to the counterpart in silicate GC. The corresponding energy level diagram of Ho³⁺ along with emission transitions is illustrated in the inset of Fig. 2.

 

Fig. 2 Absorption spectrum of 0.1 mol% Ho³⁺ single-doped GC sample. The inset shows the simplified energy level diagram of Ho³⁺ along with the mechanisms of sequential two-step two-photon NIR-QS.

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Based on the absorption spectrum, Judd-Ofelt (J-O) analysis [22,23] was carried out to predict the radiative properties of Ho³⁺ in GC. The three J-O parameters (Ω₂, Ω₄ and Ω₆) are calculated to be 4.21 × 10⁻²⁰, 2.59 × 10⁻²⁰ and 1.81 × 10⁻²⁰ cm², respectively, by using least-squares fit method. The root-mean-squared (RMS) deviation δ_rms is found to be 3.2 × 10⁻⁷. Furthermore, some radiative properties including radiative transition probabilities (A s⁻¹), radiative lifetimes (τ_rad ms) and branching ratios (β) for different electronic transitions of Ho³⁺ were calculated and presented in Table 1 .

Table 1. Radiative Properties of Ho³⁺ in GC

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3.3 Static photoluminescence of Ho³⁺

Figure 3 presents the excitation and emission spectra of Ho³⁺-doped GC. Excitation at 450 nm yields the emission spectrum (see Fig. 3(b)), which exhibits intense emission bands in the visible to NIR region, assigned to the transitions of ⁵S₂,⁵F₄ → ⁵I₈ (540 nm), ⁵F₅ → ⁵I₈ (650 m), ⁵S₂, ⁵F₄ → ⁵I₇ (750 nm), ⁵F₅ → ⁵I₇ (978 nm), ⁵S₂, ⁵F₄ → ⁵I₆ (1013 nm) and ⁵I₆ → ⁵I₈ (1190 nm), respectively. Meanwhile, the excitation spectra have been measured by monitoring emissions at 540, 1013, and 1190 nm, respectively. As shown in Fig. 3(a), all the excitation spectra exhibit similar bands in the 300-500 nm wavelength range, which correspond to the electronic transitions of Ho³⁺ from the ground state ⁵I₈ to the labeled excited states, respectively. Among these bands, the most efficient excitation wavelength is located at 450 nm, due to the ⁵I₈ → ⁵G₆ + ⁵F₁ transition. In the range of 500-670 nm, only one excitation band originated from the ⁵I₈ → ⁵S₂ + ⁵F₄ (536 nm) transition has been observed in the excitation spectrum for 1013 nm emission, whereas it exhibits additional band at 638 nm (⁵I₈ → ⁵F₅) for the 1190 nm emission. As a matter of fact, the effective pumping wavelength range for the ⁵I₆ → ⁵I₈ emission can be further extended down to 910 nm because of the ⁵I₈ → ⁵I₅ transition [24]. Thus, it can be speculated that the 1013 and 1190 nm luminescence can be simultaneously achieved only when energy levels above ⁵F₅ are populated.

 

Fig. 3 (a) Excitation spectra of the sample monitored at 540, 1013, and 1190 nm, respectively, and (b) visible-to-NIR emission spectrum under 450 nm excitation.

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To further explore the luminescence mechanisms of Ho³⁺, NIR emission spectra excited by 536, 638, and 880 nm lights have been recorded as shown in Fig. 4 . It is noted that the emission spectrum of Ho³⁺ excited at 536 nm is composed of emission bands at around 978, 1013, and 1190 nm corresponding to the ⁵F₅ → ⁵I₇, ⁵S₂, ⁵F₄ → ⁵I₆, and ⁵I₆ → ⁵I₈ transitions, respectively. On the other hand, only 978 (⁵F₅ → ⁵I₇) and 1190 nm (⁵I₆ → ⁵I₈) emission bands can be observed in the spectrum under 638 nm excitation, as presented in Fig. 4(b). Besides, excitation Ho³⁺: ⁵I₅ state with 880 nm light yieds the only NIR emission at around 1190 nm, corresponding to the ⁵I₆ → ⁵I₈ transition. These observations are consistent with the excitation spectra shown in Fig. 3(a). It should be mentioned that the emission bands for GC sample are not only remarkably intensified but also characteristic of well-resolved Stark-splitting peaks as compared with those of PG (see s-Fig. 1). This could be related to the change of the ligand field of Ho³⁺ resulted from partition of some Ho³⁺ into LaF₃ nanocrystals. Based on the spectroscopic measurements, the involved mechanisms for Ho³⁺ luminescence have been illustrated with the help of the simplified energy-level diagram depicted in the inset of Fig. 2. For 880 nm excitation, the ⁵I₆ state populated by NR decays to the ground state, giving rise to an 1190 nm photon. The radiative transition of ⁵I₆ → ⁵I₈ also occurs in the case of 638 nm excitation, where the ⁵I₆ state is populated via NR from the excited ⁵F₅ state. Besides, the electrons in the excited ⁵F₅ level could radiatively relax to the ⁵I₇ level, yielding the emission at 978 nm. Upon excitation at 450 nm, Ho³⁺ is initially excited from the ground state to ⁵G₆ + ⁵F₁ state, followed by NR process to the ⁵S₂, ⁵F₄ levels. Thus, the NIR emission spectrum obtained is like the one excited by 536 nm light. Obviously, similar luminescence spectra can be expected when excitation bands in the range of 300-560 nm are selected as pumping wavelength. Then, ions in the populated ⁵S₂, ⁵F₄ levels could radiatively relax to the lower levels, yielding not only visible emissions at 540 (⁵S₂, ⁵F₄ → ⁵I₈) and 750 nm (⁵S₂, ⁵F₄ → ⁵I₇) but also two NIR photons via two-step sequential transitions, i.e., Step 1: ⁵S₂, ⁵F₄ → ⁵I₆ (1013 nm) and Step 2: ⁵I₆ → ⁵I₈ (1190 nm). Alternatively, the excited ⁵S₂, ⁵F₄ states can also be deactivated by NR process to ⁵F₅, from which the radiative transitions of ⁵F₅ → ⁵I_{J (J = 8, 7)} occur, resulting in 650 and 978 nm emissions. All these observations lead to the conclusion that two NIR photons can be emitted from each Ho³⁺ ion excited to the ⁵S₂ + ⁵F₄ levels or higher ones, i.e., emission of two sequential NIR photons at 1013 and 1190 nm per absorbed UV or visible photon within the range of 300-560 nm.

 

Fig. 4 NIR emission spectra of the sample under excitation of (a) 536 nm, (b) 638 nm and (c) 880 nm, respectively.

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3.4 Dynamic NIR photoluminescence and luminescence mechanisms

To verify the above luminescence mechanisms, time resolved luminescence spectra upon excitation into the ⁵S₂, ⁵F₄ levels at 536 nm were recorded, as shown in Fig. 5 . At early stage for delay time less than 110 µs, only emission bands at 978 and 1013 nm appear on the spectra, corresponding to ⁵F₅ → ⁵I₇ and ⁵S₂, ⁵F₄ → ⁵I₆ transitions. It is interesting to note that the emission band due to ⁵I₆ → ⁵I₈ transition emerges when the delay time increases up to 110 µs. Moreover, the intensity of 1190 nm luminescence is found to increase with increasing the delay time at the expense of 978 and 1013 nm band intensity. These results demonstrate that the NIR photons at 1013 and 1190 nm are generated by two sequential steps. The excited Ho³⁺ ions in the ⁵S₂, ⁵F₄ levels initially relax to ⁵I₆ state, giving rise to the first photon at 1013 nm (Step 1). Also, the presence of 978 nm emission reveals that part of the excited electrons decay nonradiatively to ⁵F₅ state. As the delay time increases up to 110 µs, the second photon at 1190 nm can be emitted through the radiative transition from the densely populated ⁵I₆ level to ⁵I₈ level (Step 2). In short, one absorbed visible photon (536 nm) can be split into two NIR photons at 1013 and 1190 nm by the two-step sequential radiative transitions.

 

Fig. 5 Time resolved luminescence spectra (λ_ex = 536 nm) of the sample with the time delays106, 108, 110, 120 and 140 μs in the wavelength range of 900-1270 nm.

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Based on J-O analysis, we can estimate the internal QE of Ho³⁺ (denoted as ${\eta }_{H{o}^{3+}}$), which refers to the ratio of the number of photons re-emitted over the number of photons absorbed. It is worthwhile to mention that the energy gaps of (⁵S₂,⁵F₄)-⁵F₅ and ⁵I₆-⁵I₇ are about 2900 and 3500 cm⁻¹, respectively. According to the energy-gap law [25], the crystalline environment around Ho³⁺ with low phonon-energy (~380 cm⁻¹ for LaF₃ crystal [26]) enables the two NR processes inefficient. If we ignore the NR processes from the excited ⁵S₂,⁵F₄ and ⁵I₆ states, the ${\eta }_{H{o}^{3+}}$ can be calculated using the following equation [27]:

4. Conclusion

In summary, Ho³⁺-doped germanate GC containing LaF₃ nanocrystals has been fabricated by melting-quenching and subsequent heating. XRD and TEM analyses indicate that 12-20 nm sized LaF₃ nanocrystals are formed among glass matrix. From the absorption spectrum, radiative properties such as radiative transition probabilities, lifetimes and branching ratios have been determined by using the J-O parameters derived according to J-O theory. A comparison of excitation and emission spectra of Ho³⁺-doped GC reveals the generation of two NIR photons for one UV or visible photon absorption within the 300-560 nm region. A sequential two-step two-photon NIR-QS of Ho³⁺ has been demonstrated by examining the time resolved luminescence spectra. Using the spectroscopic parameters calculated from J-O theory, the QE of Ho³⁺ is estimated to be approximately 110%. It is believed that this NIR QS mechanism would offer a promising approach to designing the advanced NIR-QS materials with QE greater than unity.