We report on a sequential two-step near-infrared quantum splitting in Ho3+-doped germanate glass ceramics (GC). The formation of LaF3 nanocrystals is confirmed by X-ray diffraction and transmission electron microscopy analysis. Emission of two NIR photons at 1013 and 1190 nm for one incident photon absorption within the 300-560 nm region has been demonstrated by static and dynamic photoemission and excitation spectroscopy. Using the spectroscopic parameters calculated from Judd-Ofelt theory, the quantum efficiency of Ho3+ in GC sample is estimated to be approximately 110%.
©2012 Optical Society of America
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 . The achievement of QS was firstly reported by Piper et. al.  and Sommerdijk et. al.  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 → 3HJ, 3FJ 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 (Eg) 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 Eg of c-Si. Yb3+ ion is an attractive emitter as the energy of its emission (~1000 nm) matches well with the Eg 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 .
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..
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.
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.
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 δrms is found to be 3.2 × 10−7. Furthermore, some radiative properties including radiative transition probabilities (A s−1), radiative lifetimes (τrad ms) and branching ratios (β) for different electronic transitions of Ho3+ were calculated and presented in Table 1 .
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 . Thus, it can be speculated that the 1013 and 1190 nm luminescence can be simultaneously achieved only when energy levels above 5F5 are populated.
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 → 5IJ (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.
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.
Based on J-O analysis, we can estimate the internal QE of Ho3+ (denoted as ), 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 , the crystalline environment around Ho3+ with low phonon-energy (~380 cm−1 for LaF3 crystal ) enables the two NR processes inefficient. If we ignore the NR processes from the excited 5S2,5F4 and 5I6 states, the can be calculated using the following equation :28]) are treated as one state in this article. Hence, the branching ratio and (J = 5, 6) are calculated to be 0.0019, 0.0253, and 0.0663, respectively. Finally, is estimated to be approximately 110%. It is worth noting that this value represents the upper limit of QC, and the actual QC is lower due to nonradiative losses.
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.
This work was supported by the NSFC (Grant No. 51125005, 60977060 and U0934001) and Chinese Ministry of Education (Grant no. 20100172110012).
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