Near-Infrared Quantum Cutting Long Persistent Luminescence

By combining the unique features of the quantum cutting luminescence and long persistent luminescence, we design a new concept called “near-infrared quantum cutting long persistent luminescence (NQPL)”, which makes it possible for us to obtain highly efficient (>100%) near-infrared long persistent luminescence in theory. Guided by the NQPL concept, we fabricate the first NQPL phosphor Ca2Ga2GeO7:Pr3+,Yb3+. It reveals that both the two-step energy transfer of model (I) and the one-step energy transfer of model (IV) occur in 3P0 levels of Pr3+. Although the actual efficiency is not sufficient for the practical application at this primitive stage, this discovery and the associated materials are still expected to have important implications for several fields such as crystalline Si solar cells and bio-medical imaging.

Scientific RepoRts | 6:24884 | DOI: 10.1038/srep24884 photons can be created via a quantum cutting channel (two-step ET). The net effect of the NQPL process is that the theoretical quantum efficiency of LPL may reach 200% in maximum, a very interesting phenomenon that has not been reported previously.

Results and Discussion
X-ray diffraction analyses show that the incorporation of the 0.1 mol% Pr 3+ and 0.06-2 mol% Yb 3+ ions into the Ca 2 Ga 2 GeO 7 host does not induce obvious impurity, but the some impurity peaks arise when the content of Yb 3+ is more than 2 mol% (see Fig. S1 in the supplemental material). Figure 2 shows the LPL spectra of the Pr 3+ (a-b), Yb 3+ (c-d) single doped and Pr 3+ -Yb 3+ codoped (e-f) samples recorded after ultraviolet (254 nm) lamp irradiation for 10 min and again after a delay of 60 s. The LPL emissions due to 3 P 0 and 1 D 2 levels of Pr 3+ in Ca 2 Ga 2 GeO 7 :Pr 3+ sample can be clearly observed. The occurrence of 3 P 0 emissions of Pr 3+ is very significant for the QCL of the Pr 3+ -Yb 3+ pairs, and it should be associated with the low phonon energy of the Ca 2 Ga 2 GeO 7 crystal, which partly prevents the multiphonon relaxation from 3 P 0 to 1 D 2 . On the contrary, the NIR LPL of Yb 3+ in the Ca 2 Ga 2 GeO 7 :Yb 3+ sample is not recorded. Therefore, it can be clearly concluded that the characteristic NIR LPL (977 nm) of Yb 3+ in the Ca 2 Ga 2 GeO 7 :Pr 3+ ,Yb 3+ sample which can last for more than 100 h must originate from the ET of Pr 3+ → Yb 3+ . As shown in Fig. 2(e), the LPL band in range 560-660 nm due to 1 D 2 → 3 H 4 , 3 P 0 → 3 H 6 and 3 P 0 → 3 F 2 transitions clearly reduces in intensity after codoping Yb 3+ , and it further suggests that the ET of Pr 3+ → Yb 3+ mainly originates from 3 P 0 and 1 D 2 levels of Pr 3+ . LPL in rare-earth ion doped crystals and glasses is a complex process and might be dependent on a combination of host-dopant defect state energy exchange. Thus, the samples were prepared in different atmospheric conditions including oxygen (1 atm), air and without air to gain the information of defect shown in Fig. 2(f) 25,26 . The intensity of LPL decreases with the partial pressure of oxygen increases. It is obvious that the LPL derives from the defect of vacancy oxygen, and the higher concentration of vacancy oxygen, the more traps in the bandgap which strengthen the intensity of LPL.
By comparing the relative absorption strengths of the 3 H 4 → 3 P J , 3 H 4 → 1 I 6 , and 3 H 4 → 1 D 2 transitions with the corresponding relative photon fluxes in the excitation spectrum, the occurrence of the possible quantum cutting effect i.e., model (I) can be determined 2 . In Fig. 4, the normalized excitation (black line) and the diffuse reflectance (red line) spectra are shown for the Ca 2 Ga 2 GeO 7 :Pr 3+ ,Yb 3+ sample. The excitation spectrum is monitored by Yb 3+ emission (977 nm). It can be seen that the area ratio (R E ) of the 3 P J band to the 1 D 2 band in the excitation spectrum is 1.90, while that (R A ) of the absorption spectrum is 1.15. When we assume that the quantum efficiencies to Yb 3+ from 3 P J and that from 1 D 2 (100%) are equivalent, the emission intensity of Yb 3+ ions by excited 3 P J levels should be also 1.15 times as strong as that of the 1 D 2 level. In fact, it is found in Fig. 4 that the excitation intensity by 3 P J is 1.90 times greater than that by 1 D 2 , and this is direct evidence of quantum cutting as indicated in the model (I) of Fig. 3(a) 2 . However, if the model (I) is the only channel, the ratio of R E /R A should be 2 in theory. The roughly estimated value of 1.65 indicates that both the two-step ET of model (I) and the one-step ET of model (IV) occur in the Pr 3+ -Yb 3+ codoped samples. As mentioned earlier, both the PL and LPL occurs through the direct recombination of the conduction electrons with the emission centers, and the only difference is that the electrons in the conduction band originate from direct excitation in PL or from traps in LPL. Both the processes are achieved through conduction band, and the electrons would finally reach the 3 P 0 level of Pr 3+ via relaxation. Therefore, the electrons in the 3 P 0 level face the same choice in PL and LPL processes (also evidenced by the highly similar spectra profiles). At this stage, the occurrence of the QCL and NQPL at 3 P 0 levels of Pr 3+ can be demonstrated. Note that the actual quantum efficiency should be lower than the theoretical value of 165% due to the quenching effect, which reduces the Yb 3+ emission. An estimate of the overall ET efficiency, which is the fraction Scientific RepoRts | 6:24884 | DOI: 10.1038/srep24884 of 3 P 0 excited states that relax through ET rather than radiative decay, can be obtained from the integrals under the normalized fluorescent decay curves, as outlined in refe. 27. From the fluorescent decay curves in Fig. S2, it is determined that the roughly estimated ET efficiency from 3 P 0 level including the one-step and the two-step processes is only 17.7% for the optimal Ca 2 Ga 2 GeO 7 :Pr 3+ ,Yb 3+ sample, and thus the actual quantum efficiency should be less than 117.7%. The low ET efficiency may be due to the low quenching concentration of Yb 3+ in this host. As previously mentioned, when the codoping content of Yb 3+ is more than 2 mol%, some impurities clearly arise and thus badly quench the NIR emission of Yb 3+ (Fig. S1). However, although the efficiency is not sufficient for the practical applications at this primitive stage, this study is of significance both in the theoretical research on NQPL and in the future developmental practices of the crystalline Si solar cells and the biomedical imaging.  Additionally, the LPL duration time is also significant for the applications in the c-Si solar cells and biomedical imaging, and thus it is necessary to measure the LPL time of this material. Generally speaking, the duration time of visible LPL could be evaluated by the 0.32 mcd/m 2 , a value commonly used by the safety signage industry (about 100 times the sensitivity of the dark-adapted eye) 28 . However, NIR LPL is less efficiently sensed by the human eye. Instead, radiance is more appropriate than luminance for the evaluation of NIR LPL 29 . According to previous practices 19,30 , the NIR LPL around 977 nm of this materials could be recorded for more than 100 hours after irradiated for 15 min as shown in Fig. 5, although after such time from the end of the irradiation, the signal-to-noise ratio was strongly reduced making the Yb 3+ emission barely detectable. The inset of Fig. 5 also gives the LPL spectrum acquired at different decay time. It is reasonable that the detectability of the NIR LPL at a given time strongly depends on the experimental conditions. Accordingly, Fig. 6 exhibits a schematic representation of the NQPL mechanism. The trap levels continuously distribute over a wide range of energies and localize near the Pr 3+ sites. Under ultraviolet light excitation, the electrons can be promoted to the conduction band (process ① ). The electrons are subsequently captured by the traps below conduction band (process ② ). The captured electrons are gradually released from the traps and are backtracked to the excited 3 P 0 level of Pr 3+ via the conduction band (process ③ ). Finally, the energy is transferred  from Pr 3+ to Yb 3+ via the one-step (model IV) and the two-step (model I) ET processes, and gives the NIR LPL of Yb 3+ (④ ).
In summary, A new NQPL concept by combining the unique QCL and LPL processes is proposed for the first time. According to this idea, we designed the first NQPL phosphor Ca 2 Ga 2 GeO 7 :Pr 3+ ,Yb 3+ by incorporating acceptor Yb 3+ ions into the LPL phosphor Ca 2 Ga 2 GeO 7 :Pr 3+ . It reveals that a two-step ET process from Pr 3+ ( 3 P 0 → 1 G 4 ) → Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Pr 3+ ( 1 G 4 → 3 H 4 ) → Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) occur in this phosphor, demonstrating the occurrence of the QCL and NQPL in 3 P 0 levels of Pr 3+ . Even though the actual QC efficiency still need to be improved, this interesting discovery enables the Ca 2 Ga 2 GeO 7 :Pr 3+ ,Yb 3+ phosphor to find potential applications in many important areas, particularly in c-Si solar cells and biomedical imaging that requires highly efficient, less environmental limitation, super-long and near-infrared LPL.

Methods
Synthesis. All phosphors were fabricated by a simple solid-state method. Stoichiometric amounts of CaCO 3 (A.R.), Ga 2 O 3 (A.R.), GeO 2 (A.R.), Pr 6 O 11 (4N) and Yb 2 O 3 (4N) were used as starting materials. The ingredients were ground homogeneously in an agate mortar with anhydrous alcohol. Then the mixtures were sintered at 1573 K for 2 h in air (or oxygen (1 atm) and without air ). After cooled down to room temperature, the final products were obtained.
Characterization. The X-ray diffraction patterns were obtained on a Rigaku D/max-2400 powder diffractometer by using Cu Kα radiation at 40 kV and 60 mA. The luminescence decay curves were measured by a FLS-920T fluorescence spectrophotometer with a nF900 microsecond flashlamp as the light source. The photoluminescence and the long persistent luminescence spectra were recorded by FLS-920 fluorescence spectrophotometer (Edinburgh Instruments). The absorption spectra were recorded by a PerkinElmer Lambda 950 spectrometer in the region of 400-700 nm, while BaSO 4 was used as a reference.