Erbium-Doped Mixed Oxide Nanowires of Zinc and Germanium: Role

Mar 26, 2012 - For the gray areas, the diminution of emission intensity with increasing wavelength in the 470−514 nm range is a clear signature of a...
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Erbium-Doped Mixed Oxide Nanowires of Zinc and Germanium: Role of Host Structure on Near Infrared Emission Xuezhen Huang and Jeffery L. Coffer* Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129, United States ABSTRACT: This work describes the fabrication of Er-doped oxide nanowires containing both zinc and germanium through selected sol−gel reactions in concert with electrospinning and thermal removal of polymer templates. These nanowires exhibit strong Er3+ near-IR photoluminescence at 1.54 μm after annealing; it is found that the mechanism of this emission can be mediated through the selection of appropriate composition and annealing temperature. Relatively high Er concentrations (up to 3 at. %, excluding oxygen) can be incorporated into these structures, with retention of Er3+ emission due presumably to an absence of erbium self-quenching and associated nonradiative pathways.

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INTRODUCTION Er-doped nanostructures have been extensively studied due to optical intra-4f transitions at 1.54 μm (4I13/2→4I15/2), an important telecommunication wavelength since standard silica-based optical fibers have their maximum transparency at this wavelength.1 The identity and associated properties of the oxide host play a significant role in the observed properties of the erbium impurity phase.2 Among possible host materials for Er3+ centers, germanium-containing glasses are attractive because of their low transmission loss in the infrared region and outstanding mechanical strength, high thermal stability, as well as higher refractive index.3 For the case of nanofibers/ nanowires, earlier work has established the superiority of germanium oxide over silicon (oxide) in terms of Er3+ near-IR emission intensity (for a given rare earth concentration);4 a similar trend is observed for the case of the corresponding elemental semiconductors (Ge and Si) as well.5−7 Given the thermal and photophysical stability of zinc oxide (ZnO), recent investigations have also probed the impact of a zinc oxide passivating shell layer over germanium core motifs doped with Er3+ and grown by vapor−liquid−solid methods.8 In such structures, thermal annealing results in diffusion-limited intermixing of elements present and can transform the discrete core/shell architecture into a Zn2GeO4 phase, with the Er3+ playing a role in mediating such a transformation.8 In this work, we take advantage of sol−gel condensation reactions, in conjunction with electrospinning and thermal removal of a polymer template, to produce polycrystalline nanowires containing oxides of both germanium and zinc, along with optically active Er3+ impurity centers. Relative to vapor transport methods, sol−gel reactions ideally permit more sensitive control of uniform composition via kinetic control of alkoxide hydrolysis and condensation,4,9 especially in mixed metal systems such as these containing Zn and Ge, thereby allowing for more detailed investigations of photophysical © 2012 American Chemical Society

dependence on composition and structure. In addition, we evaluate the role of thermal annealing of the nanowire platform on the resultant structure and associated near-IR photophysics.

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EXPERIMENTAL METHODS

Instrumentation. Structural characterization of nanowire products was principally achieved by the use of scanning electron microscopy (SEM, JEOL-6100) with an energy-dispersive X-ray spectroscopy (EDX) system, high-resolution transmission electron microscopy (HRTEM, JEOL-2100), and X-ray diffraction (XRD, Philips 3100). Near-infrared wavelength photoluminescence (PL) was measured using an Applied Detector Corp. liquid-N2-cooled Ge detector in conjunction with a Stanford Research Systems chopper/lock-in amplifier and an Acton Research Corp. 0.25 m monochromator. Excitation was provided by a Coherent Ar+ laser in the range 476−514 nm. Raman spectra were recorded by using a Raman microimaging system containing an exciting argon ion laser operating at 514 nm and an Olympus BH-2 microscope with a 100× objective for the collection of scattered light. Fabrication of Er-Doped ZnO/GeO2 Nanowires. In a typical preparation with a target Zn/Ge ratio of 1:3, 0.04 g of poly(ethylene oxide) (PEO, mol wt = 900 000, Aldrich) was dissolved into 2 mL of CHCl3 (99.5%, Pharmco) to form solution I, and then 0.03 g Er(III) ipropoxide (99.9%, Strem), 0.6 mL tetraethoxygermane (TEOG, Gelest), and 0.2 g zinc acetate hydrate (Gelest) were dissolved into a mixture of 3.0 mL of MeOH (99.99%, Pharmco) and 1 mL of acetic acid (Mallinckrodt AR) to form solution II. Solutions I and II were mixed together by vortex shaking; this mixture was then loaded into a glass syringe equipped with a 21 gauge stainless steel needle for electrospinning. A 20 kV accelerating voltage (dc) was applied to the solution through the metal needle, 15 cm away from a drum covered by a piece of aluminum foil for fiber collection. Upon completion of electrospinning, as-fabricated nanowires were exposed to air for a 24 h period to ensure complete hydrolysis. Such mixtures typically provide Received: January 6, 2012 Revised: March 22, 2012 Published: March 26, 2012 2362

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relative atomic ratios of 2:25:73 Er:Zn:Ge (at %, neglecting oxygen) in the as-fabricated nanowires. Nanowire composition can be adjusted by varying the ratio of Er, Zn, and Ge precursors in the initial formulation process. The polymer template component is subsequently removed by high temperature annealing (O2 for 2−8 h at either 500, 650, or 700 °C) to produce the desired mixed oxide nanowires containing erbium.

composition of the treated nanowires remains essentially constant before and after annealing (Figure 1d). Interestingly, the above second stage annealing at temperatures of 600 °C and above typically creates nanowires that exhibit a porous morphology (Figure 2a). Such annealed nanowires usually possess pore sizes ranging from 20 to 80 nm due to the removal of the polymer template (Figure 2b). Structural Influence of ZnO Content and Associated Er3+ Photoluminescence. The introduction of a zinccontaining precursor in the reaction mixture manifests itself in the form of a zinc germanate phase in the final reaction products, as evidenced by both X-ray powder diffraction (Figure 3) as well as high resolution lattice imaging in the TEM

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RESULTS AND DISCUSSION Nanowire Fabrication and Morphology. A typical SEM image and associated EDX spectrum of an unannealed Ercontaining zinc/germanium oxide nanowire film (Zn:Ge ratio 1:3.4) still containing the PEO template is shown in Figure 1a.

Figure 1. SEM images of typical Er-doped GeO2/ZnO nanowires. (a) Before annealing. After annealing in air for 4 h at (b) 600 °C and (c) 700 °C. All scale bars are 5 μm. (d) EDX spectrum of Er-doped GeO2/ZnO nanowires before annealing. The relative atomic ratios present in a typical nanowire sample (neglecting O): 3:22:75 Er:Zn:Ge.

Figure 3. XRD spectra of Er-doped ZnO/GeO2 nanowires annealed at 700 °C in air for 4 h. Relative composition in these samples (based on O exclusion) is (a) Er:Zn:Ge = 1:6:26; (b) Er:Zn:Ge = 1:34:65; (c) Er:Ge = 1:99 (no Zn).

The average nanowire diameter of ∼400 nm does not measurably change after annealing at 600 °C in air for 4 h (Figure 1b). Increasing the annealing temperature to 700 °C results in an increase in fiber width (average diameter ∼540 nm) along with additional fusion of fibers to form more of a network structure (Figure 1c). EDX spectra indicate that the

(Figure 4). While only GeO2 related reflections are observed for the case of Er-doped oxide nanowires containing no zinc, for a representative sample containing a Ge/Zn atomic ratio of ∼2:1, strong reflections at 30.66° and 33.21° associated with Zn2GeO4 are readily detected. Further information is provided by HREM imaging (Figure 4b), where the lattice spacing

Figure 2. TEM images of Er-doped GeO2/ZnO porous nanowires (annealed at 650 °C for 4 h in air) at two different magnifications. 2363

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Figure 4. HRTEM images of typical Er-doped GeO2/ZnO nanowires. (a) Before annealing (inset, an amorphous SAED image). After annealing in air for 4 h at 650 °C with lattice planes observed for (b) zinc germinate (Zn2GeO4) and (c) ZnO. (Insets are FFT images). Scale bars are 1 μm, 5 nm, and 5 nm in images a, b, and c, respectively.

associated with the ⟨110⟩ plane of zinc germanate (∼0.713 nm) is readily observed. The presence of excess zinc in the system results in the formation of separate ZnO domains (hexagonal phase), as noted by d-spacings of 0.269 nm associated with the ⟨0002⟩ index (Figure 4c).10,11 Spectroscopically, the impact of increasing Zn2+ concentration on the 1540 nm Er3+ PL is presented in Figure 5 for a

Er3+ Concentration. It is found that relatively significant amounts of emissive erbium can be accommodated into the nanowire framework (with relative atomic ratios of Er:Zn:Ge = 3:18:79, excluding O). Structurally, such large concentrations of Er3+ impurity centers direct an overall lack of detectable crystallinity in the product, as exemplified in the X-ray diffraction spectrum of sample labeled a (Figure 3). Extremely weak 1540 nm emission is detected for Er3+ concentrations less than 0.5 at. %, but increases sensitively with increasing Er3+ up to values of 3 at. % and beyond (excluding O) (Figure 6). Presumably, it is the disordered

Figure 5. Er3+ near-IR PL spectra from Er-doped ZnO/GeO2 nanowires annealed at 500 °C with ∼2% Er. Spectra were measured at λex = 488 nm at a power of 200 mW. The spectral intensity of trace B is reduced by a factor of 13 for purposes of visualization. Figure 6. Er3+ near-IR PL spectra from Er-doped GeO2/ZnO nanowires annealed at 700 °C with varying Er concentrations. The Ge concentration is fixed at 78% (at.).

series of nanowire films containing Er at a concentration of 2 at. %. The intensity of Er3+ PL is enhanced with increasing zinc concentration, reaching a maximum value when the ratio of zinc:germanium reaches a value of 1:1. Beyond this value, the Er3+ near-IR PL intensity diminishes, as presumably the ZnO host does not permit facile insertion of the Er3+ impurity centers into polar Zn−O bonds.8 Previous studies on Er3+surface enriched ZnO nanowires and ZnO tetrapod structures are consistent with this observation.12,13 For example, the introduction of a surface Ge layer, subsequently transformed to a Zn2GeO4 phase by annealing, onto Er-doped ZnO tetrapods produces superior near-IR emission relative to similarly annealed Er-doped ZnO tetrapod structures; this is presumably a function of more facile Er3+ insertion into the germanium-containing oxide framework.13 3+

Zn2GeO4/ZnO framework and the initial formation of Er−O− Ge or Zn−O linkages (under solution conditions) that permit accommodation of optically active Er3+ at such relatively high levels. This retention of Er3+ emission is due presumably to reduced contributions from erbium self-quenching and associated nonradiative pathways. Precedent exists for wide bandgap oxide hosts (such as Al2O3) where widely spread Stark levels are in place and erbium concentrations at 1% have been observed.14 The significantly broader near-IR PL linewidths for the Er3+ emission emanating from GeO2 containing hosts (from this work as well as previously reported systems4,7,8) are consistent with this observation. Disorder in local 2364

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Er3+coordination (away from Oh symmetry) is also known to be a plus for the associated near-IR emission intensity.15,16 Effect of Thermal Annealing. As anticipated, annealing temperature strongly influences final product structure and associated photophysics. Removal of the PEO polymer component is typically achieved by annealing at 500 °C; at such a temperature no crystallinity can be discerned, as evidenced by a lack of features in the selected area electron diffraction pattern as well the associated X-ray data. Upon annealing at the elevated temperature of 650 °C, the Zn2GeO4 reflections do emerge in the electron diffraction pattern along with the X-ray powder spectra. Visually, there are regions of gray and white in the nanowire film annealed at 500 °C. Such gray regions are consistent with graphitic carbon domains,17 as broad scattering near 1100 cm−1 is observed in the corresponding micro-Raman spectra (Figure 7). While the presence of white and gray regions do not reflect

Earlier studies have shown that incorporation of carbon into Ercontaining Si thin films improves near-IR PL18 by influencing the location of the Er impurity centers.19 More recent investigations suggest that defects in C-doped oxide phases are likely sensitizers for responsible for energy transfer in such structures.20 In the white areas, the characteristic spikes in the intensity at 488 and 514 nm are a consequence of direct excitation into the 4 F7/2 and 2H5/2 ligand field states (followed by nonradiative decay to the 4I13/2 and accompanying 1540 nm emission). Increasing the annealing temperature to 600 °C results in a uniform whitish appearance and direct Er3+excitation throughout the sample (data not shown).

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CONCLUSIONS

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AUTHOR INFORMATION

Use of sol−gel chemistry in conjunction with electrospinning and removal of polymer template permits facile fabrication of polycrystalline mixed oxide nanowires of zinc and germanium incorporating erbium impurity centers emissive in the nearinfrared. Carbon residue present in the nanowire matrix resulting from incomplete polymer degradation after annealing at low temperature (500 °C) results in a carrier-mediated excitation process which enhances the intensity of Er3+ PL. Increasing the composition of ZnO in the system also contributes to stronger Er3+ PL until a maximum value is reached at an atomic ratio of 1:1 Ge/Zn. Relatively high Er concentrations in these mixed oxide nanowires likely inhibits crystallinity in the final nanowire product, an attribute helpful to reducing clustering of the Er3+ ions and associated selfquenching in the corresponding near-IR PL.

Corresponding Author

Figure 7. Raman spectra of Er-doped GeO2/ZnO nanowires after annealing at: (a) 500 °C, gray areas; (b) 500 °C, white areas; (c) 650 °C, white areas.

*E-mail: j.coff[email protected]. Notes

The authors declare no competing financial interest.

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significant differences in Er3+ PL intensity (Figure 8a), a clear effect on the excitation mechanism is observed. For the gray areas, the diminution of emission intensity with increasing wavelength in the 470−514 nm range is a clear signature of a carrier-mediated pathway (Figure 8b). It is the presence of carbon domains in the nanowire film, and an associated lower bandgap, that presumably drives this change in PL mechanism.

ACKNOWLEDGMENTS Financial support by the Robert A. Welch Foundation (Grant P-1212) is gratefully acknowledged. The authors also wish to sincerely thank Professor Waldek Zerda of TCU Physics for his assistance with the Raman measurements.

Figure 8. (a) PL spectra of Er-doped GeO2/ZnO nanowires (relative atomic ratios of Er:Zn:Ge =3:54:43) annealed at 500 °C. (b) Corresponding PLE spectra for Er-doped GeO2/ZnO nanowires in (1) white and (2) gray areas. 2365

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