J. Phys. Chem. C 2007, 111, 7335-7338
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Intense Infrared Luminescence in Transparent Glass-Ceramics Containing β-Ga2O3:Ni2+ Nanocrystals Shifeng Zhou,† Gaofeng Feng,† Botao Wu,‡ Nan Jiang,§ Shiqing Xu,† and Jianrong Qiu*,† State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, China, Shanghai Institute of Optics & Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China, and Department of Physics, Arizona State UniVersity, Tempe, Arizona 85287-1504 ReceiVed: December 6, 2006; In Final Form: March 14, 2007
Transparent glass-ceramics containing β-Ga2O3:Ni2+ nanocrystals were synthesized and characterized by X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. Intense broad-band luminescence centering at 1200 nm was observed when the sample was excited by a diode laser at 980 nm. The room-temperature fluorescent lifetime was 665 µs, which is longer than the Ni2+-doped ZnAl2O4 and LiGa5O8 glass-ceramics and is also comparable to the Ni2+-doped LiGa5O8 single crystal. The intense infrared luminescence with long fluorescent lifetime may be ascribed to the high crystal field hold by Ni2+ and the moderate lattice phonon energy of β-Ga2O3. The excellent optical properties of this novel material indicate that it might be a promising candidate for broad-band amplifiers and room-temperature tunable lasers.
1. Introduction 3d ions such as Ni2+ and Cr4+ are important activators since they show broad-band luminescence in the near-infrared region when incorporated into crystal hosts, and they have wide applications in tunable lasers and broad-band optical amplifiers.1,2 However, the difficulty in elaboration of crystals limits their applications. In addition, 3d ions only exhibit weak or even no luminescence in amorphous hosts, owing to the strong nonradiative relaxation. Glass-ceramics (GCs) combine the particular optical properties of activators in the crystal hosts with the elaboration and manipulation advantages of glasses.3-5 As we know, 3d ions are very sensitive to their local ligand fields, and usually, tetrahedral (Cr4+) and octahedral (Ni2+) environments are often optically active for transition-metal (TM) ions.6-9 From this point of view, the well-studied transparent oxyfluoride GCs,3 which often provide an 8-fold coordinated environment, are probably not suitable hosts for 3d ion luminescence. Therefore, it is absolutely necessary to search for favorable GCs hosts for TM ions as tunable laser and broadband amplifier materials especially those that show excellent properties at room temperature. Wide band gap semiconductors are good host materials for activators since the thermal-quenching effects are inversely proportional to the band gap of the hosts.10,11 β-Ga2O3 is a typically wide band gap semiconductor with band gap of 4.7∼5 eV, and it has lower phonon energy compared to other oxide crystals such as Zn2SiO4, Al2O3, and ZnAl2O4. Therefore, Ga2O3 could also be a good candidate for hosting 3d ions luminescence. On the other hand, it was reported that Ga2O3 nanocrystals can precipitate in glass.12 In this study, the Ni2+-doped β-Ga2O3 nanocrystals embedded in the transparent silicate GC were synthesized and characterized. Its optical properties were also investigated. The intense emission centering at 1200 nm as well * To whom correspondence should be addressed. Tel: +86-57188925079. Fax: +86-571-88925079. E-mail:
[email protected]. † Zhejiang University. ‡ Chinese Academy of Sciences. § Arizona State University.
as long room-temperature decay time indicates that the Ni2+doped β-Ga2O3 GC might be acceptable materials as roomtemperature tunable lasers and broad-band optical amplifiers. 2. Experimental Section Na2O-Ga2O3-SiO2 (NGS) glasses with composition of 66.5 SiO2-19.5Ga2O3-6.5Al2O3-7.5Na2O-0.15NiO were prepared by a conventional melt-quenching method. A 50 g reagent grade stoichiometric mixture of Na2CO3, Ga2O3, Al2O3, SiO2, and NiO was melted in a corundum crucible at 1580 °C for 2 h and then was casted into a slab on a heated iron plate. The transparent NGS GC was obtained just by annealing the glasses at 900 °C for 2 h. The samples were cut and polished with the thickness of 2 mm. The crystalline phase in the NGS GC was identified by X-ray diffraction (XRD) using Cu/KR1 radiation. Raman scattering was measured using a laser Raman spectrophotometer (INVIA PLUS). Transmission electron microscopy (TEM) was performed using a JEOL 2010F (scanning) transmission electron microscope operating in TEM mode. Absorption spectra were recorded by a double-beam spectrophotometer (JASCO FP6500). The infrared luminescent spectra were obtained using a ZOLIX SBP300 spectrophotometer with InGaAs as detector in 1000-1800 nm wavelength region, pumped by a diode laser at 980 nm. The fluorescence decay curves of the samples were recorded by a Tektronix TDS3052 storage digital oscilloscope. 3. Results and Discussion Figure 1a shows XRD patterns of as-made NGS glass and NGS GC. The broad peak can be attributed to the amorphous phase in the as-made NGS glass, while the superimposed sharp peaks in the NGS GC can be indexed by the diffraction peaks of monoclinic β-Ga2O3 structure (JCPDS [43-1012]). Figure 1b compares the Raman scattering spectra of the as-made NGS glass with that of the NGS GC. Three strong peaks at 201, 653, and 767 cm-1 and several weak peaks at 273, 335, 357, 384, and 573 cm-1 are observed in the NGS GC, but they are absent
10.1021/jp068370i CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007
7336 J. Phys. Chem. C, Vol. 111, No. 20, 2007
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Figure 3. Photographs of the as-made NGS glass (left) and NGS GC (right).
Figure 1. (a) XRD patterns of as-made NGS glass and NGS GC; (b) FT-Raman spectra of as-made NGS glass and NGS GC. Figure 4. Absorption spectra of Ni2+ in the as-made NGS glass and NGS GC.
Figure 2. (a) TEM bright-field images of NGS GC. The inset is the corresponding selected-area electron diffraction (SAED) pattern; (b) HRTEM image of one particle; (c) EELS spectra of the Ga L23 edge for individual particle and interspace region between the particles.
in the as-made NGS glass. The Raman active modes of β-Ga2O3 can be classified into three groups: GaO4 tetrahedron in high-frequency region (∼770-500 cm-1), Ga2O6 octahedron (∼480-310 cm-1) in mid-frequency region, and tetrahedronoctahedron chain in low-frequency region ( ∆2 > ∆3. For this reason, the longer luminescent decay time of Ni2+-doped NGS GC compared to ZnAl2O4 GC and LiGa5O8 GC can be partially ascribed to the larger activation energy. Furthermore, the nonradiative transition needs to overcome the potential barrier (i.e., ∆), and the probability of process depends on the number and energy of bridging phonons. As mentioned above, the Raman modes of Ga2O6 octahedral where Ni2+ ions incorporate are in the mid-frequency region (∼480-310 cm-1), and tetrahedral-octahedral chains are below 200 cm-1. The cutoff phonon energy that local environment Ni2+ ions occupy is relatively low in the NGS GC. In this simple model, one would expect that the nonradiative decay process becomes harder in NGS GC than in ZnAl2O4 GC and LiGa5O8 GC, and it results in the long luminescent decay lifetime. The integrated room-
7338 J. Phys. Chem. C, Vol. 111, No. 20, 2007 temperature luminescence intensity I is give by22 I ) I0(1 τ/τnr) where I0 is the beam intensity incident on the sample. So, the high luminescence intensity is expected when the nonradiative lifetime τnr is long. It is consistent with the result of the intense infrared luminescence in Ni2+-doped NGS GC at room temperature (Figure 6). As for the case of emission peak position, Ni2+-doped NGS GC (1200 nm) also shows distinct difference in comparison with other hosts Ni2+-doped ZnAl2O4 GC (1400 nm) and LiGa5O8 GC and single crystal (1300 nm). The prominent difference of emission peak positions can be easily understood from the difference of the crystal fields as shown in Figure 5 and Figure 7. The increase of the crystal field results in the augment of the transition energy from 3T2(F) to 3A2(F). That is to say, tunable luminescence can be acquired by changing the crystal field of Ni2+ ions. Here, intense broad-band emission in the high energy of near-infrared region at room temperature was acquired in transparent glass-ceramics containing β-Ga2O3:Ni2+ nanocrystals, which have potential applications in tunable lasers and broad-band optical amplifiers. 4. Conclusion In summary, transparent GCs containing β-Ga2O3:Ni2+ nanocrystals were prepared and characterized by XRD measurement, Raman spectrum, HRTEM, and EELS. Intense broadband luminescence centered at 1200 nm has been observed excited by lasers with different wavelength. The high efficiency along with long lifetime of luminescence at room temperature demonstrates that β-Ga2O3 is a good host for Ni2+ ions and potentially for other TM elements. Acknowledgment. This work was financially supported by National Natural Science Foundation of China (Grant No. 50672087) and National Basic Research Program of China (2006CB806000b). The work at Arizona State University (N.J.) was supported by NSF DMR0603993.
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