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Near-infrared (NIR) emitting active centers can exist abundant in Bi:CsI crystal. In addition, Bi:CsI have the simplest crystal structure, body-centered cubic (BCC). In this paper, annealing and quenching treatments were carried out in detail to identify the nature of NIR emitting active centers in Bi:CsI crystals. The changes of absorption and emission spectra with increasing the thermal treatment temperature indicated that the two NIR emission bands at 1210 nm and 1580 nm were related to Bi+ and Bi2+, respectively. Besides, the assignments of absorption bands and the thermal behaviors of Bi3+, Bi2+, Bi+ and Bi2+ were discussed as well.
©2013 Optical Society of America
1. Introduction
Since the 1960s, the active ions of laser materials have been always limited in transition metal and rare earth ions. Till 2001 when Fujimoto first found the near-infrared (NIR) broadband photoluminescence (PL) in bismuth doped silica glasses [1], main group metal (MGM) ions especially Bi ions, as the third type of active center, began to catch attention of many researchers. Showing the property of broadband NIR-PL which covers the telecommunication and biological optical windows, Bi doped materials have vast potential applications in ultrafast lasers [2], fiber optical communications [3], bioimaging [4], laser facial [5], and so on. However, although Bi can be doped abundantly in some laser materials, the luminous efficiency can hardly be enhanced [1–23]. The reason for that is because Bi has the valences of + 1, + 2, + 3, + 4 and + 5, and can easily form varied clusters, but not all of them are responsibility for the broadband NIR-PL. So identification of the NIR emitting active centers is rather essential.
Now, more and more researchers believe that the NIR emitting active centers are Bi+ [5,6], and subvalent cluster of Bi ions, such as Bi53+ and Bi85+ [7–12]. However, these centers, which were just stabilized in Lewis acidic melts or molecular crystals [13], are still controversial. We think that the nature of NIR emitting would be more understandable in crystals because of the regular close-packed structure.
Up to now, crystals reported to have NIR emitting properties included Bi:BaB2O4 crystal [14], Bi:RPC crystal [15], Bi:BaF2 crystal [16], Bi:CsI crystal [17,18], Bi:CdWO4 crystal [19], and so on. Only the Bi:CsI crystal shows the bright NIR-PL and the strong absorption, which means NIR emitting active centers can exist abundant in Bi:CsI crystal. In addition, CsI has the simplest crystal structure, body centered cubic (BCC). Although it has been reported that Bi+ and Bi2+ were the NIR active centers in Bi:CsI crystal [17,18], the influence of thermal treatment on the active centers has not been researched in detail. Considering the thermal treatments could change the species and distribution of impurities in crystals [24,25], we did a series of annealing and quenching treatments on the Bi:CsI crystal, to further demonstrate the active centers and find ways to improve the PL properties.
2. Experiment
The 0.2at% Bi:CsI crystal was grown by the vertical Bridgman method. High purity CsI and BiI3 powders were chosen as the starting materials, which were accurately weighed, well mixed and then dried at 523K for three hours in argon atmosphere before sealed into vacuum quartz crucible. Chips with the same size (10x10x3 mm3) were cut from the middle parts of the crystal for thermal treatments and spectroscopy investigations.
Thermal treatments were carried out in a HIRH VS 100-600/16/3G precision atmosphere annealing furnace. Argon was chosen as the protective atmosphere. In the annealing treatments, after three hours kept at each target temperature, the samples were slowly cooled down at the same rate of 20K/h. In the quenching experiment, the crystals were kept three hours at target temperatures and then taken out of the furnace directly for shock chilling in the air. Absorption spectra were recorded by a Jasco V-570 spectrophotometer. A ZOLIX SBP300 spectrophotometer with an InGaAs detector was used to measure the luminescence spectra. The excitation spectra measurements were taken by a Jobin-Yvon spectrophotometer with a Xe lamp as the exciting source. A Tektronic TDS 3052 storage digital oscilloscope was used to record the fluorescence decay curves. A VSTA/AX inductively coupled plasma (ICP) atomic emission spectrometer was used to detect the actual concentration of bismuth.
3. Data analysis
The actual concentration of bismuth is about 0.008 at% measured by ICP technique, rather lower than the initial concentration 0.2 at%. When BiI3 was doped into CsI crystal, a Bi3+ will replace a Cs+ and two VCs’ color centers are needed to compensate the charge balance [17,18], which increased the formation energy of the impurity. In addition, the radius of Bi3+ is smaller than that of Cs+, and BiI3 has a high vapor pressure at the growth temperature. These all would hinder the doping concentration.
Two strong NIR-PL bands were detected at 1210 nm and 1580 nm upon 808 nm excitation, shown in Fig. 1 , with FWHMs of 174 nm and 184 nm and lifetimes of 0.13 ms and 0.21 ms, respectively, even though the excitation source was Xe-lamp. This indicates that the NIR active centers are abundant in CsI crystal. And in CsI crystal, bismuth existed in the form of Bi3+, Bi2+, Bi+ and small clusters like dimers (Bi20, Bi22+)and Bi2+ [17,18]. Large clusters like Bi53+ and Bi85+ [7–12] could not exist because of the regular close-packed structure. Two independent NIR-PL bands should correspond to two active centers. Bi3+ and Bi2+ have been shown to produce no NIR-PL [22]. So the active centers could have some relationship to Bi+ and the small clusters.
Absorption and excitation spectra of the 0.2 at% Bi:CsI crystal at room temperature were also measured, shown in Fig. 1. In absorption spectra, absorption bands are located at 372 nm, 492 nm, 568 nm and 711 nm. In the excitation spectra, the curve for 1210 nm emission has two excitation bands at 350 nm and 711 nm, while the curve for 1580 nm emission has four bands at 350 nm, 465 nm, 550 nm and 711 nm, shown in Fig. 1. The bands of excitation are generally consistent with the bands of absorption.
For analyzing the nature of absorption bands, annealing treatments were carried out on the 0.2 at% Bi:CsI crystal. Figure 2 shows the absorption spectra of the as-grown and annealed 0.2 at% Bi:CsI crystal respectively at 523K, 673K and 823K. The top-left graph shows the absorption spectra for the whole measurement range from 300 nm to 800 nm, while the other three graphs display the enlarged top-left graph at three important bands, for convenient observation. As the annealing temperature rose, the absorption coefficient at 372 nm decreased gradually, the absorption coefficients at 492 nm and 568 nm first increased and then decreased, and the absorption coefficient at 711 nm increased markedly after 673K.
Fig. 2 Top-left graph shows absorption spectra of the as-grown and annealed 0.2 at% Bi:CsI crystals at 523K, 673K and 823K, respectively. The other three graphs show the same absorption spectra in top-left graph enlarged at three important absorption bands.
The possible thermal behaviors of Bi3+, Bi2+, Bi+ and small clusters in CsI crystal were inferred, revealed in Fig. 3 . VCs’ easily loses its electron [17,18], and monovalence is the most stable in the lattice of Cs+. So, as the temperature rises gradually, Bi3+ would first take electrons from VCs’ to become Bi2+, then Bi2+ would take electrons to become Bi+, and next Bi+ ions would take electrons to become clusters. So, as the temperature rises, the concentration of Bi3+ ions would decrease continuously, the concentration of Bi2+ and Bi+ ions would first increased and then decrease, and the concentration of small clusters would increase continuously (see Fig. 3). It is reported that the characteristic absorption of Bi3+ is between 300 nm and 400 nm, due to the electron transition from ground state 1D2 to excited state 3P2 [22], and the two characteristic absorption bands of Bi2+ were at 470 nm and 590 nm due to the electron transitions from ground state 2P1/2 to two excited states 2P3/2(1) and 2P3/2(2) split by the crystal field effect [6]. In addition, comparing the changes of absorption coefficients (Fig. 2) with the concentration changes of Bi3+, Bi2+, Bi+ and small clusters (Fig. 3), we assigned the absorption band at 372 nm to Bi3+, the absorption bands at 492 nm and 568 nm to Bi2+, and the absorption band at 711 nm to a small cluster.
Fig. 3 The schematic of ion conversion and inferred concentration change of Bi3+, Bi2+, Bi+ and small clusters, with the increase of temperature.
Figure 4 is the luminescence spectra of the as-grown and annealed 0.2 at% Bi:CsI crystals at 523K, 673K and 823K, respectively, under excitation of 808 nm. FWHMs, lifetimes and peak positions are constant with an increase of the annealing temperature. The inset graph shows the changes of peak intensities respectively at 1210 nm and 1580 nm, as a function the annealing temperature. The curve of 1210 nm rises successively at 523K and 673K, and then decreases at 823K, while the curve of 1580 nm first decreases at 523K, and then rises up sharply at 673K and 823K. It has been mentioned that the active centers could have some relationship to Bi+ and the small clusters. So, comparing the curve changes (inset graph in Fig. 4) with the concentration changes of Bi+ and small clusters (Fig. 3), the NIR active centers at 1210 nm should be attributed to Bi+, and the NIR active centers at 1580 nm should be attributed to small clusters.
Fig. 4 Emission spectra of the as-grown and annealed 0.2 at% Bi:CsI crystals at 523K, 673K and 823K, respectively, under excitation of 808 nm. The inset graph shows the change of peak heights at 1210 nm and 1580 nm, with increase of annealing temperature.
Through quenching experiment, high temperature would separate the clusters in Bi:CsI crystal, and then the shock chilling process would retain the separated ions [24,25]. This will help us research the nature of clusters. So quenching experiments were carried out on the 0.2 at% Bi:CsI crystals. Before being quenched at different temperatures, all the samples had been annealed at 773K to simplify the species of defects, removing most of the Bi3+ and Bi2+ and leaving Bi+ and small clusters. Unlike previous annealing treatments carried out at 523K, 673K and 823K, these quenching treatments were done at 473K, 573k, 673K and 773K, for observing the changes in detail.
Absorption and emission spectra are shown in Fig. 5 . In the top-left absorption spectra, after quenched at 473K, 573K, 673K and 773K, concentrations of Bi2+ (at 492 nm and 568 nm) and Bi3+ (at 372 nm) have no obvious variation, which means both Bi2+ and Bi3+ are not part of the small clusters. A new strong absorption band appeared at 324 nm after quenching at 773K. Considering that the peak intensity of Bi+ at 1210 nm was enhanced sharply at 773K (lower-left graph), this new absorption band was attributed to Bi+. The concentration increase of Bi+ at high temperature indicates that Bi+ could be a component of some small clusters. In the top-right graph, the absorption band at 711 nm slowly disappeared when the quenching temperature increased from 473K to 573K, and then slowly appeared again when the quenching temperature continued increasing from 573K to 773K. This corresponds to the curve change of 1580 nm and verifies that the 711 nm absorption band and 1510 nm emitting band came from the same center. After quenching at low temperature near 573K, the emitting peak intensity of the 1580 nm band weakened which means temperature nears 573K destroyed the NIR active cluster. Although we cannot explain that mechanism, we judge that the NIR active cluster is stable at high temperature.
Fig. 5 Absorption and emission spectra of 0.2 at% Bi:CsI crystal annealed at 773K and the same annealed crystals again quenched at 473K, 573K, 673K and 773K, respectively, and the changes of peak intensities at 1210 nm and 1580 nm with increase of quenching temperature. Top two graphs are the same absorption spectra enlarged at different bands.
Here we have had some clues about the NIR active cluster. First, the NIR active cluster is not large for the close-packed crystal structure. Second, its valence should be low, because neither Bi2+ nor Bi3+ is involved. Third, the cluster would be stable at high temperature, and the monovalence is most stable. Considering all of the above, we assign the cluster to Bi2+.
4. Conclusion
Investigation of NIR-PL properties of Bi:CsI crystal and experiment data analysis allow us to conclude that Bi+ and Bi2+ are responsible for NIR-PLs at 1210 nm and at 1580 nm, respectively. Through annealing experiment data, the absorption bands at 324 nm, 372 nm, 492 nm, 568 nm and 711 nm are assigned to Bi+, Bi3+, Bi2+, Bi2+ and Bi2+, respectively. Through quenching treatments, we know that Bi2+ is stable at high temperature, but it will be destroyed if the crystal is held near 573K. All in all, Bi:CsI is a promising NIR laser crystal. The following effort should be devoted to finding ways to obtain the high concentrations of Bi+ and Bi2+ in crystals.
Acknowledgments
This work was supported by the National Natural Science Foundation of China under the numbers of 61078053, 51002175, and 60938001.
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