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The PbS Quantum Dots (QDs)-doped silica optical fiber is fabricated using atomic layer deposition (ALD) technique in combination with modified chemical vapor deposition (MCVD) technology. PbS materials are introduced into the fiber core and then formed to QDs in the optical fiber materials during the preparation process. Its structure features and optical properties are investigated. The element distribution and stoichiometry of the core materials are revealed by μ-X-ray absorption near edge structure (μ-XANES), μ-X-ray fluorescent (μ-XRF) and energy dispersive spectrometer (EDS) analysis. The experiment results indicate that PbS QDs are distributed at the region between core and cladding layers with the concentration about 0.11 mol%. High resolution transmission electron microscopy (HRTEM) further reveals the dispersion of PbS QDs is uniform and its nanocrystalline size is about 2-6 nm. This is basically in agreement with the evolution results with effective-mass method. Additionally, PbS QDs-doped optical fiber pumped with 980 nm exhibits photoluminescence property in the 1050-1350 nm range. The special doping fibers will show application potential in optical fiber amplifiers, fiber lasers and optical sensing.
© 2015 Optical Society of America
1. Introduction
Lead sulfide (PbS) QDs has received much attention in lasers [1], sensor [2], optical communication [3–5], and so on, because of their unique electrical and optical properties. PbS has small band gap and Bohr radii about 0.4 eV and 18 nm, respectively. The photoluminescence (PL) spectra of PbS QDs with different particle sizes, being quantum confinement effect, nearly cover a broad band from 800 to 1600 nm, which matches well with the whole optical communication window [3–9]. It has been reported that PbS QDs with PL bands at 1330 and 1550 nm were prepared in silicate glasses through post annealing [8].
A variety of methods are used to manufacture PbS semiconductor materials on glass, such as melt-quenching method [9], successive ionic layer adsorption and reaction technique [10], chemical bath deposition [11], colloidal method in combination with solution-derived approach [12], and so on. However, the deposition methods present some disadvantages: bigger particle size, poor homogeneity, easier introduction of impurities, and lower step coverage. These lead to limitation of its application in the preparation of doped silica optical fiber. ALD, previous stage called atomic layer epitaxy (ALE) [13], is a chemical vapor deposition technique based on the sequential use of self-terminating gas-solid reactions. The outstanding advantages of ALD including: good uniformity and adhesion, excellent step coverage, high dispersability, and controlling of the doping concentration, which meets the requirements of doped silica optical fiber. Recently, there are also reports about the fabrication of rare earth doped optical fibers using ALD technique [14,15]. However, there are no relevant investigations about nanometer semiconductor-induced into the silica optical fiber using ALD technique.
In this paper, PbS QDs-doped silica optical fiber is prepared using ALD combining with conventional MCVD technique. In the core of the QDs-doped silica fibers, its structure features are investigated by μ-XANES, μ-XRF and EDS analysis. And the dispersion of PbS QDs and its nanocrystalline size are further analyzed by HRTEM. In addition, we also study the optical properties of the PbS QDs-doped optical fibers.
2. Fabrication of optical fiber
The fabrication process of PbS-doped optical fiber consists of four steps: Firstly, the porous soot layer was deposited inside silica substrate tube using MCVD process; Secondly, PbS and Al2O3 doped layers were deposited on the surface of the porous soot layer using ALD technique; Thirdly, Ge-doped SiO2 materials were deposited as core layers by MCVD, and then the doped fiber preform was formed by collapsing process; Finally, the preform was drawn into optical fiber with typical dimensions of single mode fiber (SMF).
The PbS and Al2O3 nano-materials deposition were carried out with an ALD system (TFS-200, Beneq Inc., Finland). PbS was deposited using Pb(tmhd)2 (Bis (2,2,6,6-tetramethyl-3,5-heptanedionato) lead(II)) (Shanghai HongRui New-Materials Technology Co., Ltd) as Pb precursor and H2S as S precursor, respectively. Here, Pb(tmhd)2 was used as precursor because of its higher growth rates, lower evaporating temperature, and good stability in air [16, 17]. For Al2O3, Al(CH3)3 and O3 are used as Al and O precursor, respectively. Here, the introduction of Al2O3 is to promote the compatibilities between PbS material and silica substrate materials, and also can adjust the refractive index distribution in the fiber core [18, 19].
3. Results and discussion
3.1 μ-XANES and μ-XRF measurement of PbS-doped fiber preform
To analyze and verify the trace elements in the optical fiber, we use synchrotron radiation analysis method (Shanghai Synchrotron Radiation Facility (SSRF), China). The experiment samples are come from the fiber preform, its diameters of cladding and core are about 9 and 1 mm, respectively, as shown in Fig. 1.The optical fiber preform is cut into slices with thickness 3 mm, and double sides are polished. We have investigated local structure states, coordination environment of Pb ion, and element distribution in the fiber core layer. μ-XANES and μ-XRF measurements were carried out with beamline BL15U1 (3.5 GeV/210 mA). It is an undulator beamline equipped with Si(111) double crystal monochromator. The electron beam energy is ranging from 5 to 20 keV and spot size is less than 3 μm. Measurements were made at 80 K at Ge KLIII edge (13.1035 keV) and Pb LIII edge (13.055 keV).
Fig. 1 LIII XANES spectra of Pb-related materials (Reference data of PbS and PbO materials reported in [20, 21]), (inner) PbS-doped optical fiber preform and its cross-section.
μ-XANES spectrum is measured in a transmission mode with X-ray absorption data collected at 200 energy points ranging from 13010 to 13110 eV. The Pb LIII μ-XANES spectrum for the test sample is shown in Fig. 1(c line). There are two peaks at about 13040 and 13054 eV, which is around the Pb LIII-edge absorption peak at 13050 eV. The result confirms the presence of Pb ion [20,21]. In order to further study, the Pb LIII μ-XANES spectrum fitting analyses are performed by total area method [22], as shown in Fig. 1(c0-c2). One peak at 13041.6 eV is similar to that of the pure PbS material. Another at 13030–13080 eV is similar shape to that of PbO materials at 13054.5 eV. These are shown using a and b lines in Fig. 1. In addition, there is a broad band, as shown c2 line in Fig. 1. It reveals that the local structure has lower symmetry. These results suggest that the main structure states of Pb ion in optical fiber are Pb-S and Pb-O, such as -Si-O-Pb-S-, -Si-Pb-S-, and so on. Comparing with those of standard reference materials, the curve shape of Pb LIII-edge absorption peak of the sample is a bit different to PbS and PbO reference. This can be attributed to the influence of silica substrate material and other dopant.
The μ-XRF is measured to analyze the distribution of Pb and Ge ions in optical fiber preform, as shown in Fig. 2.Inset shows cross-section of the fiber preform. The sample is scanned along the red line from top to bottom. So, x axis is the scanning distance, and the point at 4.5 mm is the centre of the fiber preform. We can see from Fig. 2, Ge ions are uniformly distributed in the fiber core, while Pb ions are mainly located at the region between core and cladding layers, which is due to the role of deposition technology. Additionally, the intensity of Ge element optical spectrum is about twenty times higher than that of Pb element. Therefore, we can predict that the doping concentration of Pb ions in the fiber core would be twenty times lower than that of Ge element. It is basically in agreement with the results of EDS in the following study.
Fig. 2 μ-XRF spectra of Pb and Ge elements in the fiber core, (inner) cross-section of PbS-doped optical fiber preform.
3.2 HRTEM-EDS analysis of PbS-doped optical fiber
The optical fiber preform, the same preform in Fig. 1, is drawn into optical fiber, and its diameters of cladding and core are about 127 and 8.8 μm, respectively, as shown in Fig. 3.Refractive index difference (RID) between the core and cladding of the fiber sample is analyzed by optical fiber index analyzer (S14, Photon Kinetics Inc., USA). Its RID is about 0.75% which is larger than that of SMF. It may result from the role of PbS doping in the fiber core.
To further analyze the characteristics of PbS QDs-doping in the fiber core, the structure features and compositions of the doping materials are examined by HRTEM (JEM-2010F, Japan) combining with EDS (OXFORD, England). Focused Ion beam (FIB) microdissection technology (600i, FEI Hongkong Co., LTD, Czekh) is used for the measurement of HRTEM. The procedures of FIB machining are depicted in Fig. 4.First, metal spraying on cross-section of the fiber is carried out, and then the fiber core section with 2 × 10 μm size is pre-coated with Pt, as shown in Fig. 4(a); Next, microdissection is performed. The sample size with a typical dimension is 10 × 2 × 3 μm (L × W × H), and then it is picked up by Omniprobe AutoProbe 200.2, as shown in Fig. 4(b-c). At last, the sample with thickness lower 100 nm is obtained using ion beam thinning method for HRTEM analysis, as shown in Fig. 4(d).
Fig. 4 Procedure of FIB machining: (a) metal spraying on cross-section, (inner) the pre-coated platinum on the core of fiber; (b) slicing process; (c) sample separates from the fiber; (d) ion beam thinning process.
The test sample with 100 nm thickness, prepared according to the FIB microdissection method above, is analyzed using HRTEM. We can see from low-resolution TEM (LRTEM) in Fig. 5(a) that some nanoparticles are uniformly dispersed in cross-section of the fiber core, and particle size is about 2-6 nm. Combining with EDS analysis, we find that there exist PbS-doped materials, as listed in Table 1.We further studied and find that the PbS-doped materials exist as spherical particles with crystalline structure. In addition, selected-area electron diffraction (SAED) patterns (inset of Fig. 5(b)) also prove the lattice phenomenon of PbS nanoparticles. The lattice spacing, which can be calculated from the diffraction rings, is 2.064 and 1.79 Å, respectively. It corresponding to the lattice spacing of lattice planes (2 2 0), (3 1 1) for cubic PbS, which is also reported in [23]. According to the discussion above, we think that PbS materials are successfully doped in the fiber core, and PbS QDs are formed, due to the phase change process in the fabrication process.
Fig. 5 TEM images of optical fiber core with PbS-doped materials: (a) LRTEM; (b) HRTEM lattice image, (inner) SAED.
Table 1. The content of different elements in the fiber core
The composition of the core materials is analyzed by EDS. The results show the concentration of Pb, S, and Al elements are 0.11, 0.12, 0.63 mol%, respectively. The molar ratio of Pb to S is also 1:1. And the concentration of Ge element is about 20 times more than that of Pb element, which is consistent with the μ-XRF analysis result. Another, there exist a little Au element. This mainly results from the sample contaminated using FIB treatment process.
3.3 Optical properties
The PbS QDs display unique optical properties, such as spectral tunability by altering the QD diameter [3–9]. Accordingly, the absorption and PL characteristics of PbS QDs-doped optical fiber would be also affected by the structure of QDs, which we need to further investigate. Absorption spectrum is measured using cut-back technique with a broadband optical spectrum analyzer (OSA, Yokogawa AQ-6315A) in the 550-1500 nm wavelength region, and resolution is 0.2 nm. The optical fiber lengths are 10 m. We can see from Fig. 6(a) there are two absorption peaks at 628 and 930 nm bands. According to literatures [24–26], they are attributed to the typical absorption peaks of PbS materials, and its absorption coefficients are 0.3 and 0.8 dB/m, respectively. Other peak at 1380 nm band is attributed to -OH group and the background loss of the fiber sample is much lower, less than 0.2 dB/m.
Fig. 6 Optical properties of PbS-QDs doped silica optical fiber: (a) absorption spectrum; (b) PL spectrum.
The PL spectrum of PbS-doped silica optical fiber is measured with pumping wavelength of 980 nm. And the PL spectrum covers in the 1050-1350 nm range, as shown in Fig. 6(b). According to literature [6,7] reported, the size of PbS QDs in silica optical fiber is about 4 nm, which is basically agreement with our analysis result above. We can speculate the broadband emission may be caused by quantum dimension effect from PbS dopant materials.
According to absorption spectrum, the effective band gap energies and average sizes of PbS QDs are estimated using the following relation [27,28]:
Here, α is the absorption coefficient, ħ is Planck constant, ħω is photo energy, m is the reduced mass, and m = 0.085 me, where me is the effective mass of electron. R and Eg(R) are the radius and effective bandgap energy of PbS QDs, while Eg (0.41 eV) is the direct band gap of bulk PbS. The effective bandgap energy is obtained by fitting the absorption spectrum with Eq. (1), and the size of PbS QDs can be estimated by Eq. (2). The average size and effective band gap, according absorption wavelength at 930 nm and 628 nm, are about 3.8 nm-1.34 eV and 2.5 nm-1.97 eV, respectively. These results are basically in agreement with HRTEM analysis above and literature reported [25,26]. From the preceding discussion, it is clear that the optical properties of the optical fiber mainly depend on PbS QDs in the fiber core. Therefore, through optimizing fabrication technology, we would get high quality and size-controlled PbS QDs in the fiber core, which is of great interest in optical communication and other fiber optic devices.
4. Conclusion
We fabricated PbS QDs-doped silica optical fiber by using ALD combining with MCVD technique. PbS QDs are successfully introduced into the optical fiber core. With μ-XANES, μ-XRF and EDS analysis methods, we confirmed element distribution and stoichiometry in fiber core. The concentration of PbS QDs-doped materials is about 0.11 mol%, and distributed at the region between core and cladding layers. HRTEM further revealed the dispersion of PbS QDs is uniform and its nanocrystalline size is about 2-6 nm. And then there are two broad absorption bands at 628 and 930 nm. Which may mainly result from the quantum dimension effect of PbS QDs, and this is also basically in agreement with theory analysis. Additionally, PbS QDs-doped optical fiber pumped with 980 nm exhibited good photoluminescence property in the 1050-1350 nm range. Next step we would fabricate the optical fibers with the optical properties of broad bands and high gain by improving the preparation process. The specialty silica optical fibers would have application potential in optical fiber amplifiers, optical fiber lasers, and optical sensing.
Acknowledgment
We acknowledge Shanghai Synchrotron Radiation Facility (SSRF) and Instrumental Analysis and Research Center of Shanghai University for our experiments. This work is supported by National Program on Key Basic Research Project (973 Program, Grant No: 2012CB723405); Natural Science Foundation of China (Grant Nos: 61177088, 61275051, 61275090, 61227012, 61475096); Shanghai Natural Science Foundation (12ZR1411200); Open Project of State Key Laboratory (SKLD11KZ03). The authors are also thankful for the support through the International Science Linkages project (CG130013) by the Department of Industry, Innovation, Science and Research, Australia, and for two LIEF grants (LE0883038 and LE100100098) from the Australian Research Council to fund the national fiber facility at the University of New South Wales.
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