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Bi-doped CsI crystals exhibited near-infrared ultra-broadband photoluminescence around 1216 nm and 1560 nm, depending on the bismuth doping levels, which were ascribed to Bi+ and Bi2+ centers, respectively. The crystal chemistry of the Bi3+ to Bi+ reduction and Bi2+ dimer formation in CsI lattice were investigated. Thermal treatments including annealing and quenching were carried out to study the thermal behaviors of the two emission bands. The evolution of absorption and emission spectra of Bi:CsI crystals indicating the Bi-aggregation and valence conversions under thermal activation. The process of Bi aggregation was observed to be a second-order reaction with activation energy of 0.33 eV. Bi2+ was identified as the origin of the 1560 nm emission band with ESR spectra. A simple lattice structure diagram was developed to illustrate the physical processes in Bi:CsI crystals induced by thermal activation.
©2012 Optical Society of America
1. Introduction
The last quarter of the twentieth century and the beginning decade of the twenty-first witnessed spectacular discoveries in the chemistry of the heavier main-group elements, which have fundamentally different electronic properties from their lighter congeners [1]. Among them, bismuth is one of the most thoroughly investigated elements concerning cationic cluster chemistry, which has indeed been coined ‘the wonder metal’ because of its profound propensity to form such clusters [2]. Since 2001, Bi-doped glasses and fibers have been found to have ultrabroad NIR luminescence properties in the wavelength region from 1000 to 1700 nm with a FWHM over 200 nm, which are promising for advanced optical communication systems, medicine, and astrophysics [3–12]. Laser operation has been demonstrated in Bi-doped fibers at room temperature from 1150 nm to 1500 nm wavelength [13,14]. A novel application (such as vivo photoluminescence bioimaging) of bismuth activated materials has also been demonstrated [15].
However, the identification of NIR emitting active centers of bismuth is relatively difficult in Bi-doped glasses or fibers, due to the disordered structure. In previous publications, the NIR emission was controversially attributed to electronic transition of Bi5+ [3,4], Bi2+ [5], Bi+ [6–8], color centers [9], Bi clusters [11], Bi2- and Bi22- dimers [16] or Bi atom in glass [17]. A polycation of subvalent bismuth, Bi53+, was also suggested as the origin of NIR emission centers [18–20]. It would be much easier to understand the nature of the Bi-related active centers in crystal hosts than in glasses or fibers due to rigid and ordered crystal lattices. Several works have reported the NIR luminescence properties of Bi-doped single crystals, including RbPb2Cl5, BaF2, and α-BaB2O4 (BBO) single crystals [21–26]. Recently, the CsI crystal was selected as a host to study the nature of NIR luminescence of bismuth and to resolve the discrepancies. The simple composition and structure of CsI crystal are beneficial to understand the existing forms of bismuth [27].
In this work, further and systematic studies were carried out on Bi:CsI crystals. Bi+ and Bi2+ were discovered to be the origins of NIR luminescence.
2. Experiment
The Bi-doped CsI crystals were grown by the temperature gradient technique (TGT). The starting materials of CsI and BiCl3 mixture were dried at 473 K under continuous evacuation before being loaded into the sealed molybdenum crucible. BiI3 was not used in this work due to its heat decomposition. The Bi:CsI crystals with doping concentration of 0.02 at%, 0.2 at%, and 1.0 at%, respectively, were grown in high vacuum atmosphere with a freezing rate of 3-5 K/h.
Absorption spectra were recorded at room temperature with a Jasco V-570 spectrophotometer. The NIR luminescence was recorded with an InGaAs detector in 1000–1700nm wavelength range, pumped with 800 nm laser diode (LD). Emission decay curves were measured with a Tektronix TDS3052 storage digital oscilloscope. The measurements were performed at room temperature and all the emission spectra were corrected for the setup characteristic. Electronic spin resonance (ESR) spectra were measured by a Bruker EMX-10/12 spectrometer with a microwave frequency of 9.38 GHz.
3. Results and discussion
Absorption and emission spectra of the as-grown Bi:CsI crystals are presented in Fig. 1 , together with the corresponding excitation spectra. Two NIR broadband luminescence bands were observed in the as-grown Bi:CsI crystals, peaking at 1216 nm and 1560 nm respectively. The decay curves of the emissions at 1216 nm and 1560 nm of the as-grown 0.2 at% Bi:CsI crystal are shown in Fig. 2 , under excitation of 800 nm LD. The two curves both fit well to a single exponential. The emission lifetimes at 1216 nm and 1560 nm were determined to be 132 μs and 213 μs, respectively. As reported previously, for Bi-doped BaF2 and α-BaB2O4 crystals, the additional treatment, such as irradiation, heat annealing, etc, is necessary to produce NIR luminescent centers. On the contrary, as-grown Bi:CsI crystals have strong NIR photoluminescence. Since the univalent state of Cs ions in CsI lattice will reduce the dopant ions to low valence states Bi+ ions should be considered as the possible origin of NIR luminescent centers. Tl and Pb, the neighbors of Bi in the periodic table of elements, have been extensively studied as the NIR active lasing ions in the 1980s [28,29]. The research results should be helpful to analyze the state of Bi-related centers. It is well known that isoelectronic ions show analogous spectroscopic properties. For example, Bi2+, Pb+ and Tl0 with the same electron configuration of 6s26p1 have emission lifetimes of several microseconds [28–30]. The photoluminescence properties of Bi:CsI are similar to Pb0 in alkaline-earth fluorides, with emission wavelength of 1.6 μm and radiative lifetime of 3.3 ms at room temperature [3]. As the analogue of Pb0, with the same electron configuration of 6s26p2, Bi+ or Bi+-related centers should be primarily considered as the origin of the NIR luminescence.
Fig. 1 (Left) Absorption spectra of Bi-doped CsI crystals: (a) 0.2 at% Bi, (b) 0.02 at% Bi; corresponding excitation spectra for 0.2 at% Bi: (c) ex = 1560 nm, (d) ex = 1216 nm. (Right) Emission spectra: (e) 1.0 at% Bi, (f) 0.2 at% Bi, (g) 0.02 at%. All data were recorded at room temperature.
Fig. 2 Decay curves of the emissions at 1216 nm and 1560 nm of the as-grown 0.2 at% Bi:CsI crystal under excitation of 800 nm LD.
In the lightly doped Bi:CsI crystal with doping concentration of 0.02 at%, only one emission band at 1216 nm was observed in the NIR region with FWHM of 174 nm. With doping concentration increased to 0.2 at%, an additional band at 1560 nm with FWHM of 140 nm appeared. When the doping concentration further increased to 1.0 at%, the emission intensity of the 1560 nm band sharply enhanced. On the contrary, the intensity of the 1216 nm band remained almost unchanged when the Bi doping concentration was increased from 0.02 at% to 1.0 at%. Obviously, the two emission bands originated from different centers. The excitation spectra in the visible region for the two emission bands are also presented in Fig. 1. An excitation band at 358 nm for the 1216 nm emission can be observed, while there are four bands for the 1560 nm emission at 358 nm, 473 nm, 556 nm, and 710 nm, respectively.
As shown in Fig. 1, there are two apparent absorption bands at 272 nm and 298 nm in the absorption spectrum of 0.02 at% Bi:CsI crystal, due to Bi3+ ions as identified in [31,32]. Five absorption bands at 360 nm, 430 nm, 483 nm, 564 nm, and 710 nm can be clearly observed in the spectrum of 0.2 at% Bi:CsI crystal. The absorption band at 360 nm can be assigned to Bi3+ aggregations, as identified in Bi-doped NaCl and KCl crystals [31,32]. Both excitation spectra for the emission at 1216 nm and 1560 nm have the same excitation band around 360 nm, indicating the existence of energy transferring from Bi3+ clusters to the NIR luminescent centers.
Thermal treatment was carried out to identify the two different NIR luminescent centers. NIR emission spectra of the annealed and quenched 0.2 at% Bi:CsI crystals at the temperature of 573K in an inert Ar atmosphere are shown in Fig. 3 . Heat annealing enhanced the emission intensity of both the 1216 nm and 1560 nm bands, but the enhancement factor of the latter was larger than the former. On the other hand, heat quenching enhanced the emission intensity of the 1216 nm band but depressed the 1560 nm band. For cationic impurities or dopants in dielectric crystals, heat quenching is an effective way to dissociate ion pairs or aggregates. Moreover, the 1216 nm emission band can be observed in the lightly doped Bi:CsI crystal, while 1560 nm band appeared with higher doping concentrations, as shown in Fig. 1. So, the 1560 nm emission band can be attributed to Bi+ aggregations and the other to single Bi+ ions. In fact, several papers have assigned the NIR luminescent centers to subvalent bismuth species, including Bi+ and cluster ions [18–20,33]. That the two emission bands of single Bi+ ions and Bi+ aggregates were both enhanced after heat annealing suggests Bi-valence converting from + 3 to + 1.
To further evidence the presence of Bi+ dimers, ESR spectra of the as-grown and heat-annealed Bi:CsI crystals were recorded at 77 K and are displayed in Fig. 4 . No signal can be distinguished in the as-grown and 573 K-annealed samples. The signal emerged in the 773 K-annealed sample with a g-value of 2.0086, close to the g value of the free ion [34]. As discussed above, the concentration of Bi+ dimers increased more than one order of magnitude after heat annealing at 773 K, which should be considered to be involved with the ESR signal. Therefore, Bi2+, i.e., an electron trapped by a Bi+-Bi+ dimer can be attributed to the ESR signal.
Fig. 4 ESR spectra of the as-grown, 573 K- and 773 K-annealed 0.2 at% Bi:CsI crystals, measured at 77 K.
Absorption spectra of 0.2 at% Bi:CsI crystals annealed at 573 K, 673 K, and 773 K in Ar atmosphere are shown in Fig. 5 , together with the as-grown crystal for a comparison. The Bi:CsI crystal turned into pale gradually with increasing the annealing temperature and became colorless at 773 K. The absorption band of Bi3+ aggregations at 360 nm and the other three bands at 430 nm, 483 nm, and 564 nm all decreased after heat annealing. On the contrary, the absorption band at 710 nm was gradually enhanced, synchronously growing with the emission band at 1560 nm. The former is the counterpart of the excitation band at 710 nm for the latter. Then, the 710 nm absorption band should be ascribed to Bi+ aggregations.
Fig. 5 Left: Room-temperature absorption spectra of the as-grown and Ar-annealed 0.2 at% Bi:CsI crystals (annealing at 573 K, 673 K, and 773 K). Upper right: Absorption coefficients at 710 nm of Ar-annealed Bi:CsI crystals with annealing temperature from 473 K to 773 K. Bottom right: The logarithm of absorption coefficient at 710 nm as a function of the reciprocal annealing temperature.
The inset in Fig. 5 shows the absorption coefficient at 710 nm as a function of annealing temperature. With annealing temperature rising from 473 K to 773 K, the absorption coefficient increased from 0.029 to 0.41 cm−1, an increase of more than one order of magnitude. The concentration of bismuth in the Bi:CsI crystal was measured to be 0.05 wt% by ICP-AES analysis, corresponding to an atom concentration of 6.5 × 1018 cm−3. Using the Bi concentration determined by ICP-AES in Bi:CsI crystal as that of Bi+ aggregations after Ar-annealed at 773 K, the absorption cross section of Bi+ aggregations can be calculated to be 6.3 × 10−20 cm2, which is undoubtedly underestimated.
The stimulated emission cross section (σem) can be estimated by use of the Füchtbauer-Ladenburg equation:
where λ0 = 1560 nm is the peak wavelength, n = 1.75 is the refractive index of CsI, c is the velocity of light, τem = 213 μs is the emission lifetime, and Δνem = 577 cm−1 is the FWHM of the emission band. The calculated value of σem of Bi:CsI crystal is 8.4 × 10−20 cm2, one order of magnitude higher than those of Bi-doped glasses as previously reported [13,33]. The important parameter σem × τem responsible for laser oscillation is found to be 17.9 × 10−24 cm2s in the annealed Bi:CsI crystal, one order of magnitude higher than that of Ti:sapphire (1.58 × 10−24 cm2s) [35]. Therefore, Bi-doped CsI crystal is promising for the broadly-tunable and ultra-fast laser applications in the eye-safe 1.5 μm region.The activation energy (Ea) of the formation of Bi+ aggregations in Bi:CsI crystal can be determined using the Arrhenius equation:
where k is the rate constant of the formation of Bi+, A is the pre-exponential factor, kB is Boltzmann constant, and T is the annealing temperature. The rate constant k is the quotient of the difference of Bi+ aggregations concentration (C) divided by the annealing time (t), while C is the quotient of the difference of the absorption coefficient (α) divided by the absorption cross section (σ) at 710 nm. Therefore, the Arrhenius equation can be expressed as:where A′ is the product of σ, t, and A. Since both σ and t are constant, the A′ is the new pre-exponential factor. Taking the logarithm of the above equation yields:The inset in Fig. 5 shows the experimental values of lnα as a function of the reciprocal annealing temperature. Then, the activation energy Ea = 0.33 ± 0.02 eV of the formation of Bi+ can be derived from the slope of the fitted straight line. The linear relationship between lnα and 1/T, corresponding to a second-order reaction, indicates the Bi+ aggregates are dimers.
The physical mechanism of Bi-aggregation and valence conversion under heat annealing can be illustrated by the model shown in Fig. 6 . The electrons trapped in the color centers VCs′ are released by thermal activation and then captured by Bi3+ ions. As a result, Bi3+ is reduced to Bi+ after capturing two electrons. With increasing Bi+ levels, the concentration of nearest-neighbor Bi+-Bi+ dimers rises rapidly. These dimers can trap electrons and then Bi2+ is formed. The physical processes can be expressed as:
4. Conclusion
In conclusion, two NIR ultra-broadband luminescence bands at 1216 nm and 1560 nm were observed in Bi-doped CsI crystals with different doping levels. According to their optical properties, thermal behaviors, and ESR spectra, Bi+ and Bi2+ dimers are identified to be the origins of the two emission bands at 1216 nm and 1560 nm, respectively. A simple lattice structure diagram was developed to illustrate the chemistry and physics processes of the Bi3+ to Bi+ reduction and Bi2+ dimer formation in Bi:CsI crystals. The activation energy of the formation of Bi2+ dimers was further calculated to be equal to 0.33eV. The identification of NIR luminescent centers of bismuth in this work will be helpful to design and develop novel and highly efficient laser materials activated by bismuth and other main-group metal ions.
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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