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2007, 111, 16088-16091 Published on Web 10/11/2007
Strongly Emissive Erbium-Doped Tin Oxide Nanofibers Derived from Sol Gel/ Electrospinning Methods Ji Wu and Jeffery L. Coffer* Department of Chemistry, Texas Christian UniVersity, Fort Worth, Texas 76129 ReceiVed: August 7, 2007; In Final Form: September 24, 2007
One-dimensional nanofibers of tin oxide doped with erbium can be readily prepared by a combination of sol gel condensation processing and electrospinning. Such structures are strongly emissive in the near-infrared at 1540 nm whose intensity is directly proportional to erbium concentration. A combination of electron microscopies, selected area electron diffraction, and X-ray diffraction reveal a polycrystalline structure for a given one-dimensional nanofiber whereby the erbium influences nanocrystal feature size.
Facile routes to the fabrication of free-standing erbium-doped one-dimensional group IV oxide nanofibers are of significant interest because of their potential value in generating complex nanoscale photonic devices such as waveguides and amplifiers.1-3 In addition, these structures also provide an alternative, welldefined experimental model analogous to oxidized Er-doped Ge and SiGe nanowires (NWs) such that the mechanism of erbium luminescence in these semiconducting materials could be understood in greater detail.4-6 In terms of bottom-up assembly methods, electrospinning has been demonstrated to be an efficient technique to produce polymer nanofibers with a high yield,7 and it is known that sol-gel processes can be employed to fabricate Er-doped group IV oxide film waveguides and amplifiers.8 Therefore, a combination of electrospinning methods with sol gel-type condensation reactions is one possible route to obtain erbium-doped group IV oxide nanofibers. In this letter, we describe a straightforward, general route to crystalline Erdoped nanofibers with controllable erbium concentrations and tunable widths. Although a variety of structures can be made with this approach (Er2O3;9 MO2, M ) Si,9 Ge,9 and Sn), our specific focus here involves SnO2 fibers characterized by highresolution TEM, selected area electron diffraction (SAED), X-ray diffraction (XRD), and energy dispersive X-ray (EDX) linescan analysis. It is found that nanofibers strongly emissive at 1540 nm and containing up to 4% erbium can be obtained by this method. Since this fabrication method requires condensed phase reactions whereby reactants can be precisely weighted, the concentrations of erbium ions in the host materials can be easily controlled by adjusting the ratios of the erbium precursor to these group IV oxide sol-gel precursors. Unlike vapor transport methods, all Er3+ ions are ideally left in the oxide matrix after the reaction without any loss of material. In addition, erbium ions can be homogeneously dispersed in the host materials * To whom correspondence should be addressed. E-mail: j.coffer@ tcu.edu.
10.1021/jp076338y CCC: $37.00
without serious erbium clustering, provided that the concentrations of Er3+ ions are below its solubility limit in a given host material. In terms of potential host materials for erbium ions, SnO2 possesses several appropriate properties as an optical waveguide. First, it is highly optically transparent (80-90% transparency in the visible light region).10 Second, Er3+ ions could substitute for interstitial Sn4+ ions in the rutile SnO2 lattice so that the direct Er-Er coupling interaction would be greatly suppressed.11 Third, SnO2 is a wide band gap n-type semiconductor (Eg ) 3.6 eV) due to the presence of donor sites such as oxygen vacancies and interstitial Sn4+ ions, which implies that electroluminescence could be obtained from an Er-doped SnO2 sample via electrical pumping.10 In a typical procedure, tin oxide nanofibers containing 4.0 atomic percent erbium can be prepared by first dissolving polyvinylpyrrolidone (PVP, 0.30 g, Mw ) 1.3 M, Aldrich) in 2.5 mL of absolute ethanol and then mixed well with 0.030 g of Er(III) i-propoxide and 0.90 g of Sn(IV) t-butoxide (Gelest, Inc.) in a mixture of 1.0 mL of absolute ethanol and 1.0 mL of acetic acid by vortex shaking. The mixture was then loaded into a glass syringe equipped with a 21 gauge stainless steel needle. The needle was connected to a high voltage direct current (DC) supply using a metal clamp. A piece of aluminum foil was used to wrap the surface of a drum that serves as a grounding electrode to collect these nanofibers. A 25 kV accelerating voltage and a 15 cm working distance between the needle and the drum were used for the electrospinning process. The as-formed nanofibers were exposed to air for about a week in order to completely hydrolyze the alkoxide precursors. Asfabricated nanofibers were annealed in air at 500 °C or higher temperatures for 3 h to remove the PVP templates completely.12 Undoped SnO2 nanofibers were prepared in an analogous manner, except that no erbium precursor was added. A SEM image of as-prepared Er-doped SnO2/PVP fibers is shown in Figure 1a. After annealing, these nanofibers experience a 28% reduction in diameter due to loss of the PVP and associated organic components. Figure 2b shows the TEM image © 2007 American Chemical Society
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J. Phys. Chem. C, Vol. 111, No. 44, 2007 16089
Figure 1. (a) SEM image of 4% (atomic percent) Er-doped SnO2 nanofibers that were prepared using a 0.2 g PVP/1.2 mL ethanol solution; (b) TEM image of a 4% Er-doped SnO2 nanofiber with a diameter of 590 nm (annealed at 800 °C for 3 h in air; scale bar ) 200 nm); (c) EDX spectrum of a 4% Er-doped SnO2 nanofiber sample confirming the presence of Er and Sn; (d) EDX linescan of an Er-doped SnO2 nanofiber that was annealed at 900 °C for 3 h in air; inset: High angle annular dark field (HAADF) image of this wire.
(JEM-3010) of an Er-doped SnO2 nanofiber annealed at 900 °C for 3 h in air. This fiber has a diameter of ∼590 nm and is composed of numerous nanoparticles. An EDX spectrum of 4 at. % Er-doped SnO2 fibers (Figure 1c) confirms the presence of Sn and Er, where the Sn LR and Er MR peaks lie at 3.5 and 1.4 KeV, respectively. Information concerning spatial distribution of erbium ions can be obtained from EDX linescans; importantly, measurements across a ∼60 nm Er-doped SnO2 nanofiber annealed at 900 °C in air for 3 h (Figure 1d) suggest that erbium ions are uniformly distributed among the SnO2 host matrix without significant clustering of Er2O3. Selected-area electron diffraction (SAED) and HRTEM were used to further investigate the structure and composition of Erdoped SnO2 nanofibers. Figure 2a shows the SAED pattern obtained from a single fiber shown in the inset. This SAED pattern is consistent with a polycrystalline structure. The inner 6 diffraction rings in turn are from (110), (101), (200), (211), and (301) crystal planes of tetragonal SnO2, which are assigned according to their calculated d-spacing values. Analysis of complementary X-ray powder diffraction of this type of sample confirms the presence of this tetragonal phase. Figure 2b shows the HRTEM image of an Er-doped SnO2 fiber annealed at 900 °C for 3 h in air. The lattice image shows that the nanocrystals have a d-spacing value of 3.389 Å in the circled areas, which is a 1.3% deviation from the standard value (3.347 Å for (110) crystal plane of tetragonal SnO2, JCPDS-ICDD PDF No. 411445). The HRTEM image also shows that a typical size of SnO2 nanocrystals is approximately 10 nm. XRD and TEM imaging were also used to investigate the effect of erbium ions on the size of SnO2 nanocrystals in the fibers for a given preparative condition. For undoped SnO2 and Er-doped SnO2 nanofibers that were annealed at 1000 °C in air
for 3 h, nanocrystal diameter was indirectly assessed using the Scherrer formula
D (HKL) ) K λ/(β cos θ)
(1)
where D is the average nanocrystal diameter, HKL is the Miller indices of the associated crystal planes, K is the shape factor (0.90), λ is the wavelength of the X-ray (1.54178 Å), β is the fwhm of the peak at the diffraction angle, and θ is the diffraction angle. The (110) diffraction peak was used to calculate the average diameters of SnO2 nanocrystals. Such a result shows that the average size of SnO2 nanocrystals is about 30.0 nm in pure SnO2 fibers that were annealed at 1000 °C, whereas the value is only ∼13.3 nm for 4% Er-doped SnO2 fibers annealed at the same temperature. This suggests that the presence of erbium ions can greatly increase the number of nucleation sites for the growth of SnO2 nanocrystals resulting in the formation of smaller nanocrystals. It is also possible that erbium ions on the surface of these SnO2 nanocrystals can serve as a diffusion barrier so that the growth of larger particles is suppressed. This indirect analysis is confirmed by direct TEM imaging of these samples (Figure 3), where a typical size of SnO2 nanocrystals in undoped nanofibers is ∼36 nm, which is close to the value (30 nm) calculated using the Scherrer formula above. The typical size of nanocrystals in Er-doped SnO2 nanofibers is clearly much smaller than that of undoped SnO2 nanofibers. With regard to control of nanofiber diameter, it is found that variation of PVP template solution concentration is a sensitive parameter to alter the diameter of Er-doped SnO2 nanofibers. For example, a comparison of 4% (at.) Er-doped SnO2 nanofibers that were prepared using a 0.30 g PVP/1.2 mL ethanol solution with a 0.20 g PVP/1.2 mL ethanol solution, the
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Figure 3. TEM images of (a) 4% Er-doped SnO2 nanofibers and (b) pure SnO2 nanofibers. All fibers were annealed at 1000 °C in air for 3 h.
Figure 2. (a) SAED pattern obtained from a single Er-doped SnO2 fiber shown in the inset (Scale bar ) 200 nm). (b) HRTEM images of SnO2 nanocrystals in an Er-doped SnO2 nanofiber showing that the nanocrystals have a d-spacing value of 3.347 Å (scale bar ) 5 nm). Note: All Er-doped SnO2 nanofibers have been annealed at 900 °C in air for 3 h.
nanofibers prepared using a 0.3 g PVP/1.2 mL ethanol solution are almost two times thicker on average than those prepared using a 0.2 g PVP/1.2 mL ethanol solution. This phenomenon is retained upon annealing and removal of the organic template (annealed at 800 °C in air for 3 h), as the doped SnO2 fibers prepared from the more dilute PVP solution are approximately 203 nm ((58 nm) in width, contrasted with the 486 nm average thickness of Er-doped SnO2 nanofibers from the more concentrated PVP viscous solution. The concentration of polymer solution (and accompanying viscosity) is one of the most important factors that affect the diameters of electrospun nanofibers.12 Such results are consistent with earlier studies reporting that the average diameter of TiO2/PVP nanofibers can be reduced from 100 to 50 nm while the PVP polymer concentration was decreased from 0.04 to 0.02 g/mL.12 Figure 4a shows the normalized 1540 nm PL intensities of a 4% Er-doped SnO2 nanofiber sample annealed at various temperatures. When excited by the 488 nm line of an Ar+ laser,
the sample does not emit strong luminescence at 1540 nm until it is annealed at 900 °C. The PL intensity of the sample annealed at 900 °C is almost 10 times stronger than that of the sample annealed at 800 °C. A higher temperature annealing is critical to enhancing the luminescence because it can help remove most residual hydroxy groups introduced during the sol-gel process. It was reported that excited Er3+ ions can de-excite through a resonant energy transfer to O-H vibrations resulting in the quenching of the erbium PL.13 It is also possible that erbium ions can substitute for interstitial Sn4+ ions at such a high annealing temperature so that the direct Er-Er interaction could be suppressed resulting in a significant enhancement of PL intensity.11 However, PL intensity of the fibers annealed at 1000 °C decreases by ∼5% (relative to the fibers annealed at 900 °C). This is likely due to the precipitation of erbium ions as XRD analysis of an Er-doped SnO2 nanofiber sample annealed at 1000 °C in air for 3 h reveals the appearance of two new peaks at 29.58° and 58.64°, which are from the (222) and (622) crystal planes of bcc (body centered cubic) Er2O3 (ICDD PDF # 08-0050). This suggests that erbium ions are starting to precipitate at a 1000 °C annealing temperature. Figure 4b shows the PL spectra of Er-doped SnO2 nanofibers with concentrations ranging from 1 to 4 atomic percent. It was found that the PL intensity of the fibers with a 4% Er concentration is almost 5 times stronger than the fibers with a 2% Er concentration. Significantly, no concentration-related quenching effect is observed, which suggests that Er3+ ions have a relatively higher solubility in SnO2. This observation is also consistent with our EDX linescan analysis (shown in Figure 1d) indicating that there is no significant erbium clustering in a 4% Er-doped SnO2 nanofiber sample. This value is in contrast to previously reported Er3+ concentrations in silica hosts prepared by flame hydrolysis (0.5 wt. %) or mixed tin oxide/ silica films (1.25 wt. %).14 Incorporation of such large Er3+ concentrations is a useful parameter in achieving optimal excitation efficiencies in these rare earth-containing optical materials.
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J. Phys. Chem. C, Vol. 111, No. 44, 2007 16091 dimensional nanostructures of the transparent conductor SnO2 containing erbium ions in relatively high concentrations. This process is very adaptable to oxides of other group IV elements of technological relevance as well.9 Further work in erbiumdoped nanofibers of these oxides, as well as aligned structures, is in progress. Acknowledgment. The authors gratefully acknowledge the Robert A. Welch Foundation and the National Science Foundation for their support of this research. The expertise of Dr. Alan Nichols of the Electron Microscopy Facility of the Research Resources Center of the University of Illinois-Chicago is also greatly appreciated. References and Notes (1) Polman, A.; van Veggel, F. C. J. M. J. Opt. Soc. Am. B 2004, 21, 871. (2) Kik, P. G.; Polman, A. J. Appl. Phys. 2002, 91, 534. (3) Daldosso, N.; Navarro-Urrios, D.; Melchiorri, M.; Pavesi, L.; Gourbilleau, F.; Carrada, M.; Rizk, R.; Garcı´a, C.; Pellegrino, P.; Garrido, B.; Cognolato, L. Appl. Phys. Lett. 2005, 86, 261103. (4) Wang, Z.; Coffer, J. L. Nano Lett. 2002, 2, 1530. (5) Wang, Z.; Coffer, J. L. J. Phys. Chem. B 2004, 108, 2497. (6) Wu, J.; Coffer, J. L.; Punchaipetch, P.; Wallace, R. M. AdV. Mater. 2004, 16, 1444. (7) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151. (8) Selvarajan, A.; Srinivas, T. IEEE J. Quantum Electron. 2001, 37, 1117. (9) Wu, J.; Coffer, J. Chem. Mater. submitted for publication. (10) Ray, S. C.; Karanjai, M. K.; Dasgupta, D. Surf. Coat. Technol. 1998, 102, 73. (11) Morais, E. A.; Scalvi, L. V.; Ribeiro, S. J.; Geraldo, V. Phys. Status Solidi A 2005, 202, 301.
Figure 4. (a) Normalized 1540 nm PL intensities of a 4% Er-doped SnO2 nanofiber sample annealed at various temperatures. (b) PL spectra of Er-doped SnO2 nanofiber samples with erbium concentrations ranging from 1 to 4 at. %. Note: All fibers were annealed at 900 °C in air for 3 h.
(13) Righini, G. C.; Pelli, S.; Ferrari, M.; Armellini, C.; Zampedri, L.; Tosello, C.; Ronchin, S.; Rolli, R.; Moser, E.; Montagna, M.; Chiasera, A.; Ribeiro, S. J. Opt. Quantum Electron. 2002, 34, 1151.
In summary, we have successfully developed a straightforward process for the fabrication of strongly emissive one-
(14) Hattori, K.; Hittagawa, T.; Oguma, M.; Okazaki, H.; Ohmori, Y. J. Appl. Phys. 1996, 80, 5301. Laliotis, A.; Yeatman, E. M.; Ahmad, M. M.; Huang, W. IEEE J. Quantum Electron. 2004, 40, 805.
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