DOI: 10.1021/cg9011636
Dual Wavelength Emissive ZnO Tetrapods: Effects of Erbium/Germanium Surface Modification
2010, Vol. 10 32–35
Xuezhen Huang and Jeffery L. Coffer* Department of Chemistry, Texas Christian University Fort Worth, Texas 76129
J. Antonio Paramo and Yuri M. Strzhemechny* Department of Physics, Texas Christian University Fort Worth, Texas 76129 Received September 22, 2009; Revised Manuscript Received November 18, 2009
ABSTRACT: Er3þ ions, both with and without a germanium (Ge) sensitizer layer, have been introduced onto the surface of ZnO tetrapod structures. Such structures, characterized by electron microscopy and X-ray diffraction, are found to emit at both the UV/ visible and near-infrared as a consequence of the presence of ZnO/Er3þ. The presence of selected absorption features in the visible luminescence spectra suggests energy transfer from ZnO to Er3þ ions in the doped materials. The introduction of Ge onto the tetrapod surface enhances the intensity of Er3þ photoluminescence significantly with a corresponding blue shift of 11 nm in the emission maxima. ZnO is known as an excellent II-VI semiconductor with a wide band gap (3.37 eV) and a large exciton binding energy (∼60 meV). The higher exposed surface area of this material in nanostructured form is providing interesting phenomena of relevance to energy conversion, spintronics, piezobased devices, and other applications.1-4 ZnO tetrapods (TPs) are one intriguing variant in this regard, with a controllable morphology based on a single crystal core.5-7 To improve the properties of electrical and optical devices, doping has been a necessary strategy. For optoelectronics in particular, erbium is an appealing candidate as dopant because of the Er intra-4f shell transition with emission at a wavelength of 1.54 μm.8-10 As a promising host material, ZnO provides erbium with an oxygen coordination sphere which is helpful to an efficient Er-based near-infrared photoluminescence (PL).11-13 Er-doped ZnO films have proven to be reliable materials for light-emitting diodes, laser diodes, and optical amplifiers operating at 1.54 μm, as well as electrode materials for carrier injection because of their high electrical conductivity and transparency.14-16 In this paper, we report the structure and properties of dual emissive wavelength Er-doped ZnO TPs based on the presence of Er3þ and ZnO. Since previous reports have noted the ability of oxidized germanium species to improve the near-IR PL intensity of Er-doped Ge nanowires (NWs)9 as well as Er-doped SnO2 nanofibers,17 Ge was deposited via vapor transport onto the surface of Er3þ/ZnO TPs to generate an analogous enhancing effect. ZnO TPs were grown by Zn oxidation in air at atmospheric pressure using a reaction temperature of 980 C without carrier gas, as reported previously,18 and then annealed at 900 C in O2 for 1 h. The incorporation of Er on the surface of TPs is achieved through the chemical vapor deposition of Er(TMHD)3. An alumina boat loaded with ZnO TPs (10-20 mg) was placed in a quartz tube reactor (of 24 mm inner diameter) heated by a ceramic furnace up to 500 C (at a rate of 30 C per min). The Er precursor (20 mg) was heated to ∼145 C (also in an alumina boat), and the vapor was carried downstream by He gas with a flow rate of 300 sccm to the furnace, with the distance between the precursor boat and the tetrapod-containing alumina boat set at approximately 6 cm. Ge deposition was fulfilled via heating Ge powder (15 mg) at 990 C in helium gas (300 sccm) for 1 h, *To whom correspondence should be addressed. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 12/01/2009
followed by cooling downstream to form a Ge coating on the surface of Er-doped or undoped ZnO TPs, with a Ge source to tetrapod container separation again of 6 cm. A SEM image (JEOL 6100) of undoped ZnO TPs (Figure 1a) shows that the legs of representative TPs have typical lengths in the range 1520 μm. In this work, two kinds of doped structures were fabricated. One has an Er-doped layer introduced between the Ge coating and the ZnO TP core (hereafter referred to as Ge/Er3þ/ ZnO TPs). The other structure has a Ge middle layer acting as a sandwich between the Er and the ZnO TP core (hereafter described as Er3þ/Ge/ ZnO TPs). The tetrapod morphology shows no remarkable change after a typical incorporation of 1-3 wt % Er (Figure 1b). In terms of composition, energy dispersive X-ray (EDX) spectra show varying amounts of the expected elements as a function of location in the tetrapod, due to the differences in ZnO thickness in the central area and legs, for example. Typically the Er or Ge concentration at the tip of a leg is about 1.5-2 times that observed near the center of the tetrapod. Under conditions of extended Ge exposure (∼25 wt % in the central area), the ZnO TPs surface remains relatively smooth at low magnification (data not shown). The two Er-doped TPs structures containing Ge have similar morphologies. The image of Ge/Er3þ/ZnO (Figure 1c) TPs and the corresponding EDX spectrum (Figure 1d) show the presence of ∼1.5% Er and ∼8% Ge in central areas after 1 h of annealing at 900 C in O2. Some slight charging is observed after such treatment. High resolution transmission electron microscopy (HREM) provides microscopic details of the surface structure (Figure 2). For the Er3þ/Ge/ ZnO TPs structure, some measurable surface roughness and associated protuberances are observed upon deposition of Er and Ge of 20-30 nm thickness (Figure 2a). The morphology increases the surface area and possibly plays a role in the enhancement of the quantum efficiency of the Er3þ PL.19,20 Analysis of higher magnification images, including measurement of lattice spacings, suggests a polycrystalline surface structure composed of a mixture of the wurtzite phase of ZnO (observed d=0.26 nm for Æ100æ and 0.28 nm for Æ002æ) along with a distinct zinc germanate phase (d = 0.71 nm, Æ110æ) (Figure 2b). Complementary X-ray diffraction measurements are consistent with this conclusion (Supporting Information), with the observed patterns dominated by the Æ100æ, Æ002æ, Æ101æ, Æ102æ, and Æ110æ reflections of ZnO; some weak, almost negligible trace features r 2009 American Chemical Society
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Figure 1. SEM images of (a) representative ZnO TPs; (b) ∼1.5 wt % Er-doped ZnO TPs; (c) Ge-coated Er/ZnO TPs; (d) EDX spectrum (after 1 h of annealing at 900 C in O2).
Figure 2. (a) TEM image of a tetrapod leg exposed to Ge vapor, followed by an Er precursor. Scale bar = 50 nm (inset: selected area electron diffraction pattern). (b) HREM image showing the lattice features associated with ZnO and Zn2GeO4. Scale bar = 5 nm.
can be seen associated with low angle reflections of Zn2GeO4 and Er2O3 if the sample is annealed above 900 C in air. Figure 3 shows normalized near-IR PL spectra at room temperature from ZnO TPs with different doping conditions. It is necessary to employ a relatively high temperature anneal (900 C) to sufficiently activate the Er3þ PL at 1540 nm luminescence associated with the (4I13/2 f 4I15/2) transition; this is likely a consequence of the relative strength of the polar Zn-O framework and the energy required to allow insertion of Er-oxo species onto the surface layers. Such treatment provides the necessary noncentrosymmetric oxygen-rich coordination environment for
optical activity. It should also be noted that this annealing also results in an enhancement in overall ZnO crystallinity, as evidenced by XRD spectra (data not shown). The PL intensity of Er-doped ZnO TPs increases by ∼10 times after Ge deposition, regardless of annealing condition. In this system, incorporation of Ge increases PL efficiency because it permits increased solubility of the rare earth ions relative to that of the zinc oxide surface (thereby reducing Er-Er interactions and associated self-quenching). For the Ge/Er3þ/ZnO structures, a preannealing step (700 C, 1 h) applied before Ge deposition is slightly helpful in increasing the Er3þ PL intensity (Figure 3).
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Figure 3. Room temperature photoluminescence spectra of ZnO TPs with doped structures: (a) Er3þ/Ge/ZnO; (b) Ge/Er3þ/ZnO with preannealing applied in O2 for 1 h before Ge deposition; (c) Ge/ Er3þ/ZnO without preannealing; (d) Er3þ/ZnO. All spectra were excited by a laser power of 200 mW, and all samples have been annealed in O2 at 900 C for 1 h.
Switching the order of addition of Ge and Er3þ has a much more significant effect on Er3þ emission intensity; Ge exposure, followed by Er3þ, yields a more intense variant, presumably because of more efficient excitation higher pump efficiency without shielding from Ge or its oxide. In any event, in all cases, a direct excitation mechanism is observed, as spikes in emission intensity are observed at excitation wavelengths of 488 and 514 nm. This is expected as a consequence of the oxygen-rich environment and subsequent complete oxidation of the germanium to an insulating matrix. It is also interesting to note that the Er3þ PL associated with ZnO TP apparently shows more Stark splitting,10,21 representing a lower symmetry of Er3þ sites in the host matrix and higher coordination numbers. A blue shift of the peak at 1540 nm was observed with a maximum value of 11 nm, especially with Ge deposition. Following the shift, the shoulder at 1526 nm diminishes while the peak at ∼1495 nm becomes stronger. We employed UV/visible PL measurements to assess the impact of erbium and germanium incorporation on the photophysical properties of ZnO TPs (Figure 4). The PL of ZnO typically includes near-band-edge (NBE) ultraviolet emission and deep-level trap features (usually broad peaks in the visible region). ZnO TPs show three low-temperature NBE emission lines at 3.367, 3.315, and 3.226 eV, and one or more peaks in the visible.22-26 In the studies reported here, intense deep defect emission appears at ∼2.4 eV in both room and low temperature PL spectra (Figure 3). One relatively weak near-band gap emission broad peak was observed at 3.163 eV in the room temperature PL spectrum. In contrast, the low temperature PL spectrum shows five peaks from 3.021 to 3.357 eV that can be assigned to surface defect-related excitation, bound exciton complexes, and phonon replicas (data not shown).10,27 The presence of the Er3þ ions demonstrates a quenching effect on the ZnO band-edge emission. Most significantly, it is noteworthy that a relatively weak adsorption structure appears at 2.383 eV (520 nm) after Er3þ deposition. The adsorption is caused by Er3þ doping and is enhanced by Ge deposition (Figure 4); the increased intensity of this feature with Ge incorporation also directly correlates with the enhanced Er3þ PL intensity observed in this type of structure in Figure 3. The fact that the absorption structure is relatively insensitive to temperature from 8 K to room temperature (data not shown) confirms the intrashell nature of this transition. Energy transfer from ZnO to Er3þ ions can probably be associated with
Huang et al.
Figure 4. Room temperature PL spectra in the UV/visible region of undoped ZnO TPs, Er3þ/ZnO TPs, and Ge/Er3þ/ZnO TPs. Inset: expanded view of the absorption features detected in the visible region at room temperature for Er3þ-doped ZnO TPs and Ge/Er3þ/ ZnO TPs.
the 2H11/2 level because of a second order cooperative upconversion interaction between Er3þ ions in the 4I11/2 state (populated from the 4I9/2 state) and ZnO.8,28,29 In conclusion, we have described the fabrication of Er doped ZnO TPs and observed both strong Er3þ PL at 1540 nm and ZnO UV/visible PL after annealing at 900 C. Considerable enhancement of Er3þ PL was made possible by Ge deposition, with the most significant enhancement observed when the erbium ions are deposited closer to the outer surface (Er3þ/Ge/ZnO). Energy transfer between ZnO and Er3þ took place with a wavelength of 520 nm verified by ZnO PL spectra, with Ge deposition again facilitating the process. The possible utility of these and related types of structures in energy conversion and photonics warrants further investigation. Acknowledgment. Financial support for this research by the Robert A. Welch Foundation (Grant P-1212 to JLC), MRCEDM (JLC), and the TCU RCAF (YMS) is gratefully acknowledged. Supporting Information Available: Selected X-ray powder diffraction spectra for ZnO tetrapods surface modified with erbium ions and germanium. This material is available free of charge via Internet at http://pubs.acs.org/.
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