Influence of Polyethyleneimine and Ammonium on the Growth of ZnO

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Influence of Polyethyleneimine and Ammonium on the Growth of ZnO Nanowires by Hydrothermal Method Liang-Yih Chen,*,† Yu-Tung Yin,† Ching-Hsiang Chen,‡ and Jau-Wern Chiou§ †

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei, 106, Taiwan ‡ Protrustech Corporation Limited, 3F.-1, No. 293, Section 3, Dongmen Rd. East District, Tainan, 701, Taiwan § Department of Applied Physics, National University of Kaoshiung, Kaoshiung, 811, Taiwan

bS Supporting Information ABSTRACT: In this study, ZnO nanowire arrays were prepared using a hydrothermal method. During growth, polyethyleneimine (PEI) and ammonia were added to adjust the structure and optical properties of the ZnO nanowires. Emission analysis revealed visible photoluminescence emissions from ZnO nanowires produced under various growth conditions. To correlate the relationship between visible emissions and structural defects in ZnO nanowires, we employed X-ray absorption spectroscopy (XAS) to characterize the coordination number and bond length of the ZnO nanowires. On the basis of analytical results, we determined that the red emission is attributed to interstitial zinc defects (Zni) and the yellow emission is attributed to interstitial oxygen defects (Oi).

’ INTRODUCTION Zinc oxide (ZnO) nanomaterials have demonstrated tremendous potential in a wide variety of practical applications, such as photonic crystals, optical waveguides, surface acoustic wave devices, variators, transparent conductive oxides, chemical and gas sensors, UV emitters, and solar cells.1 ZnO features wide band gap (∼3.37 eV at room temperature1a), excellent chemical and thermal stability, and the specific electronic and optoelectronic properties of II VI semiconductors with high exciton binding energy.1a Various chemical, electrochemical, and physical deposition techniques have been employed in the synthesis of ZnO nanomaterials, yielding ZnO nanowires and nanowires with an oriented structure and aspect ratio of 50 250.2 The optical properties of ZnO have been studied extensively using techniques such as electron paramagnetic resonance,3 positron annihilation spectroscopy,4 and photoluminescence (PL).5 Theoretical calculations investigating defects in ZnO have also been reported.6 Nonetheless, despite the numerous studies of ZnO, a number of unresolved issues remain. These issues include contradictory explanations dealing with the nature of shallow donors in ZnO as well as the origin of the visible emission. Most ZnO nanowires grown under high temperature gas-phase methods show green visible emissions depending on the growth temperature and availability of oxygen.7 It has been reported that the green emission is the result of electron recombination with holes trapped in singly ionized oxygen vacancies (V+O).5a,8 Recently, Vayssiers et al. reported the controlled fabrication and direct growth of highly oriented ZnO nanowires onto r 2011 American Chemical Society

various substrates using a hydrothermal method.2a However, only a few papers have discussed the optical properties of ZnO nanowires via this method. Greece et al. reported that the room temperature spectra of as-grown ZnO nanowires show a weak band-edge emission at 378 nm and a very strong and broad yellow-orange emission at 605 nm.9 Li et al. used electron paramagnetic resonance (EPR) and photoluminescence (PL) to investigate ZnO nanowires prepared using hydrothermal methods.3 The strong g ≈ 1.96 EPR signal was present only in the sample exhibiting yellow emission. Vanheusden et al. observed a correlation between the intensities of g ≈ 1.96 EPR signal and the green emission, proposing that the green emission originated from a transition between single charged oxygen vacancies and photoexcited holes.8a Clearly, the assignment of g ≈ 1.96 to singly ionized oxygen vacancies is controversial. In this work, well-aligned ZnO nanowires were grown on indium-doped tin oxide (ITO) transparent conductive glass substrates using a hydrothermal method. For ZnO nanowires synthesis, Yang et al. proposed that polyethyleneimine (PEI) molecules could assist in the growth of high aspect ratio ZnO nanowire arrays.10 Yong et al. replaced HMTA with ammonium (NH3) as a base on which to grow ZnO nanowires.11 In this study, we were interested in the influence of PEI and NH3 on the growth of ZnO nanowires by hydrothermal methods. Microphotoluminescence Received: June 15, 2011 Revised: September 19, 2011 Published: September 19, 2011 20913

dx.doi.org/10.1021/jp2056199 | J. Phys. Chem. C 2011, 115, 20913–20919

The Journal of Physical Chemistry C

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(μ-PL) and micro-Raman spectra (μ-Raman) were used to analyze optical properties and structural information, respectively. To identify the relationship between optical properties and structural defects, X-ray absorption spectroscopy (XAS) was employed to analyze variations in coordination number and bond length of ZnO nanowires synthesized under various growth conditions.

’ EXPERIMENTAL SECTION I. Detailed Growth Procedure of ZnO Nanowires. ZnO nanowires were grown on indium tin oxide (ITO) glass substrates with ZnO seed layer at 90 °C. For seed layer preparation, ITO glass substrate was wetted with a droplet of 0.005 M zinc acetate dihydrate (Zn(CH3COO)2 3 2H2O, 99%, SHOWA) in ethanol, rinsed with clean ethanol after 30 s, and then blown dry with a stream of argon (Ar). This coating step was repeated several times. After coating, the substrate was heated to 350 °C in air for 30 min to yield the seed layer on the substrate for the growth of ZnO nanowire arrays. In sample S1 without extra reagent, an equal molar aqueous solution of Zn(OAc)2 3 2H2O and hexamethylenetetramine ((CH2)6N4, HMTA, ACROS) at a concentration of 25 mM was placed in a regular oven for 24 Hr. For sample S2, 5 mM of polyethyleneimine (PEI, branched, molecular weight 800, Sigma-Aldrich) was added into the reactor, and NH3 was added to adjust the pH value of solution around 10.8 for sample S3. Subsequently, the samples are thoroughly washed with DI water to remove any residual salt or amino complex and were allowed to dry in air at room temperature. II. X-ray Diffraction Analysis. To characterize ZnO nanowires, X-ray diffraction (XRD) experiments were performed on a Rigaku ATX-E diffraction spectrometer with a Cu Kα line at 1.5406 Å. III. Raman Scattering and Photoluminescence Measurements. A 32 nm diode laser excited μ-Raman scattering spectroscope (Uni-RAM system) and the μ-PL system including two lasers with 325 nm He Cd laser and 248 nm quasi-cw Ne Cu laser (1 20 Hz) were integrated by Protrustech Corp. Ltd. Andor iDus CCD with 1024 128 pixels was used to take the μ-Raman and μ-PL signals. IV. X-ray Absorption Spectroscopy Measurements. The X-ray absorption spectra (XAS) were recorded at the beamline BL17C at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The electron storage ring of NSRRC was operated at 1.5 GeV. A double Si(111) crystal monochromator was employed for the energy selection with a resolution ΔE/E better than 1 10 4 at the Zn K-edge (9659 eV). All of the experiments for the film of ZnO nanowires were conducted in a combination of reflection and transmittance cells made with aluminum as used in the edge measurements. All of the spectra were recorded at room temperature in fluorescence mode. Higher harmonics were eliminated by detuning the double crystal Si(111) monochromator. Three gas-filled ionization chambers were used in series to measure the intensities of the incident beam (I0), the beam reflected by the sample and filtered by Cu foil (If), the beam transmitted by the sample (It), and the beam subsequently transmitted by the reference foil (Ir). The third ion chamber was used in conjunction with the reference sample, which was a Zn foil for Zn K-edge measurements. The control of parameters for extended X-ray absorption fine spectroscopy (EXAFS) measurements, data collection modes, and calculation of errors was all done as per the guidelines set by the International XAFS Society Standards and Criteria Committee.12

Figure 1. FE-SEM tile-view and cross-sectional images of (a) S1, (b) S2, and (c) S3 of ZnO nanowires prepared by hydrothermal method. The scale bars of tilt-view and cross-sectional images are 1 and 2 μm, respectively.

V. X-ray Absorption Spectroscopy Data Analysis. The EXAFS data reduction was conducted by utilizing the standard procedures. The EXAFS function, χ, based on fluorescence mode (If/I0) was obtained by subtracting the postedge background from the overall absorption and then normalized with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space, where “k” is the photoelectron wave vector. The χ(k) data were multiplied by k3 for the Zn K-edge to compensate the damping of EXAFS oscillations in the high kregion. Subsequently, χ(k) data in the k-space ranging from 3.48 to 11.86 Å 1 for the Zn K-edge were Fourier transformed (FT) to r-space to separate the EXAFS contributions from the different coordination shells. A nonlinear least-squares algorithm was applied to the curve fitting of an EXAFS with phase correlation in the r-space between 1.5 and 4.1 Å for the Zn K-edge depending on the bond to be fitted. The effective scattering amplitude [f(k)] and phase shift [δ(k)] for the Zn O and Zn Zn were generated by using FEFF7 code based on the ZnO structure with space group P63mc. The lattice parameters a = 3.249 Å and c = 5.206 Å were used in the FEFF7 calculation. All of the computer programs were implemented in the UWXAFS 3.0 package13 with the backscattering amplitude and the phase shift for the specific atom pairs being theoretically calculated by using the FEFF7 code.14 From these analyses, structural parameters like coordination numbers (N) and bond distance (R) have been calculated. The amplitude reduction factor (S20) values, which account for energy loss due to multiple excitations, for Zn were obtained by analyzing the Zn foil reference sample and by fixing the coordination number in the FEFFIT input file. The S20 value was found to be 0.89.

’ RESULTS AND DISCUSSION Figure 1 shows representative scanning electron microscopic (SEM) images of the structure of ZnO nanowires fabricated under three growth conditions. Three kinds of ZnO nanowires were prepared using Zn(OAc)2 3 2H2O and HMTA without additional reagent (S1, Figure 1a), by adding PEI (S2, Figure 1b), and by adding PEI and NH3 (S3, Figure 1c). According to SEM analysis, ZnO nanowires were vertical aligned on the ITO substrate with a high aspect ratio. In comparison, the diameter and length of S2 decreased with the addition of PEI. With the addition of NH3, the diameter of the ZnO nanowires 20914

dx.doi.org/10.1021/jp2056199 |J. Phys. Chem. C 2011, 115, 20913–20919


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Figure 2. (a) X-ray diffraction patterns of ZnO nanowires S1 S3 on ITO glass substrates; and (b) enlarged (0002) diffraction peaks. The dashed line represents the (0002) diffraction peak position of standard crystalline ZnO powder (JCPDS no. 79-0205).

Figure 3. (a) Raman spectra of S1 ∼S3 and (b) E1(LO) modes of S1 S3. The black dash line represents the E1(LO) mode of bulk ZnO.

was reduced even further. (The distribution of diameter and aspect ratio for the various conditions were provided in Supporting Information.) Figure 2 reveals that the XRD patterns of ZnO nanowires under different growth conditions can be indexed to a ZnO (JCPDF card, no. 79-0205) wurtzite structure, and no impurity phase was observed. As shown in Figure 2a, this study found highly oriented ZnO nanowires along the c-axis grown on ZnO seeds coated in ITO substrates. We also enlarged the (0002) diffraction peaks of S1 S3, as shown in Figure 2b. The positions of the peaks in these samples shifted to lower angles as compared to that of the (0002) standard XRD pattern, which is indicated by a dashed line in Figure 2b. The plane distance along the c-axis of ZnO nanowire arrays, grown using the hydrothermal method, is notably greater than that of the bulk ZnO under all growth conditions. Micro-Raman (μ-Raman) and microphotoluminescence (μ-PL) were used to investigate the influence of PEI and NH3 on the structural and optical properties of ZnO nanowires. The

vibrational properties of ZnO nanowires, analyzed by Raman scattering, are shown in Figure 3. In Figure 3a, no significant differences in the E1(TO), A1(TO), and E2(H) modes frequencies were observed among S1 S3. Unlike TO phonon modes, the peak at 580 cm 1 corresponded to the E1(LO) mode of oxygen defects. Among S1 S3, the intensities of LO phonon mode increased following S1 S3. To understand the structure of defects of S1 S3, peak fitting techniques were applied to the LO phonon mode. Two peaks, 588 582 cm 1 and 540 cm 1 belonging to E1(LO) and second-order Raman spectrum arising from zone boundary (M point) phonons 2-LA(M), are shown in Figure 3. With respect to the E1(LO) mode of the bulk ZnO with 591 cm 1,15 a red shift of the frequencies among S1 S3 was tentatively assigned to the existence of point defects within the films;16 see Figure 3b. Figure 4a and b shows the room-temperature μ-PL spectra of S1 S3 with two types of excitation, continuous 325 nm excitation using an He Cd laser, and quasi-cw 248 nm excitation from an Ne Cu laser. For the results shown in Figure 4a, the PL spectra 20915



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Figure 4. Micro-PL spectra of S1 S3 under the excitation of (a) quasi-cw Ne Cu laser with 248 nm and (b) continuous He Cd laser with 325 nm.

Figure 5. (a) XANES of S1 S3, Zn foil, and ZnO powder at Zn K-edge; (b) the oxidation state of S1 S3, as compared to Zn foil and ZnO powder; and (c) the intensity of the white line, positioned between 9665 and 9675 eV in the XANES, has been identified via charge-transfer effects.

were obtained under a quasi-cw 248 nm laser. For quasi-cw 248 nm excitation, a direction band-edge transition dominates to recombine the electron hole pairs, which are abundantly photogenerated

under nonequilibrium conditions.17 In this study, S1 and S2 exhibited strong UV emissions with a relatively narrow spectral line width center of 383 nm. For S2, PEI can hinder the lateral growth of 20916



Figure 6. Fourier transformed at k3-weighted EXAFS Zn K-edge spectrum of S1 S3, Zn foil, and ZnO powder.

ZnO nanowires and reduce the diameter of ZnO nanowires, as shown in Figure 1b. Although the diameter of ZnO nanowires was reduced to create high surface states by adding PEI, it also passivated the surface states to enhance UV emissions simultaneously. Thus, different from S2, NH3 created additional defects, and UV emissions are low at S3. According to a previous study, emissions caused by recombination centers, such as lattice imperfections, have high transition probability under continuous excitation.17,18 Therefore, to study the defect emissions, continuous 325 nm excitation was used, as shown in Figure 4b. This study observed a small peak in the ultraviolet (UV) region and a broad emission in the visible region. Visible emission bands are typically attributed to deep level defects within the ZnO crystals. In this study, S1 possessed the dominant emission in the red emission region (approximately 650 nm), whereas S2 and S3 demonstrated another emission in the orangeyellow emission region (590 620 nm). Various defects were induced by the addition of PEI and NH3 molecules during the growth stage. In a previous study, Cross et al. analyzed as-grown nanowire samples under room temperature PL using 325 nm laser excitation. This study observed a weak band-edge emission centered on 381 nm and a strong and broad red emission of 640 nm. The results indicate that excess oxygen is responsible for this emission and contributes to the presence of oxygen interstitials (Oi).19 Gomi et al. prepared ZnO nanoparticles at low temperatures and studied their lattice imperfections using PL properties.17 The ZnO nanoparticles exhibited a broad emission of approximately 652 nm, strongly suggesting that interstitial Zn (Zni) is responsible for the lattice disorder along the c-axis and contributes to the observed red emission. Greene et al. prepared ZnO nanorods using the hydrothermal method. The PL spectra of as-grown ZnO nanorods were measured at room temperature using an unfocused 325 nm He Cd laser as the excitation source.9 Their results revealed a major peak centered on the yellow-orange

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emission (605 nm) and a shoulder at 730 nm. They proposed that the yellow-orange emission was caused by interstitial oxygen ions (Oi). Similar results were also reported in other studies.3,5a,20 Recently, detailed studies of native point defects in ZnO have been conducted using first-principle calculations.6,21 Erhart and Albe used the density functional calculation with the climbing image-nudged elastic band method to determine the migration barriers of Zn vacancy, interstitial, and interstitialcy jumps.21a Their study revealed that zinc interstitials are the most mobile defects contributing to annealing at temperatures as low as 90 130 K. Janotti and Van de Walle also performed a native point defect formation of ZnO based on density function theory and pseudopotentials.21c Their investigation revealed that zinc interstitials are shallow donors that have high formation energies in n-type ZnO, and zinc vacancies are deep acceptors and may act as compensating centers in n-type ZnO. They also reported that zinc interstitials are rapid diffusers in n-type ZnO. For oxygen interstitials, oxygen atoms can occupy the octahedral or tetrahedral interstitial sites, but interstitials occupying the octahedral site are electrically active.21c Oxygen at the octahedral site introduces states in the lower part of the band gap that can accept two electrons. In this study, we attempted to identify the correlation between structural defects and the mechanism underlying the visible emissions of hydrothermally produced ZnO nanowires. X-ray absorption spectroscopy (XAS) was employed to analyze variations in coordination number and the bond length of ZnO nanowires prepared under various growth conditions. In XAS, X-ray absorption near edge spectroscopy (XANES) representing to a large extent a fingerprint for the oxidation state and site symmetry of the element from which the absorption spectrum was measured. This high degree of sensitivity to the coordination environment of the absorber atom is due to perturbations in the wave function of the ejected photoelectron, due to multiple scattering events from the surrounding atoms.22 The Zn K-edge spectra of ZnO nanowires (S1 S3) are compared to the reference compounds Zn foil and commercial ZnO powder in Figure 5a. The postedge energy of S1 S3 was between that of the Zn foil and the commercial ZnO powder, indicating that the oxidation state of the zinc species in those samples was between Zn(0) and Zn(II). As seen in Figure 5b, the oxidation state of those samples followed S3 > S2 > S1. It is believed that the local structure of the zinc species develops defects during growth. The intensity of the white line, positioned between 9665 and 9675 eV in the XANES, has been identified with charge-transfer effects. The white line at the Zn K-edge is due to the electron transition from 1s to 4p (actually 4sp due to hybridization), which is the lowest unoccupied energy level.23 However, special caution has to be paid to this direct assignment, which is related to the intensity of the white line and the charge transfer varying the number of empty 4sp states of Zn in the absorption process.24 This is because both the spectral shape and the intensity of the white line features are intimately linked to the local structure of the absorbing atoms.25 Figure 5c shows that the intensity of the white line of S1 is much lower than that of S2, S3, and the standard of commercial ZnO powder, indicating a lower number of empty 4sp states in the Zn of S1. The number of empty 4sp states was in the order of S3 > S2 > S1. This implies that both the reagents, PEI and NH3, played a role in the generation of empty 4sp states of the local Zn species. Fourier transformed k3-weighted EXAFS spectra at the Zn K-edge of the S1 S3 are shown in Figure 6, and the relative parameters are summarized in Table 1. Figure 6 illustrates a peak 20917



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Table 1. Structural Parameters Derived from the EXAFS Results at Zn K-Edge of S1 S3a states S1 S3 S3 ZnO powder a

N

R (Å)

Zn O

2.93 (0.08)

1.974 (0.008)

Zn Zn

9.62 (0.11)

3.250 (0.007)

shell

σ2 (Å2) 10

ΔE0 (eV)

r-factor

4.9 (1.3)

0.6 (0.3)

0.0058

3.28

1.646

6.5 (0.4)

0.8 (0.5) 0.0054

3.12

1.646

0.0047

3.02

1.646

0.0092

3.00

1.640

3

Zn O

2.87 (0.08)

1.974 (0.008)

4.9 (1.3)

0.3 (0.3)

Zn Zn

8.94 (0.07)

3.250 (0.007)

12.0 (0.4)

1.0 (0.2)

Zn O

2.83 (0.06)

1.971 (0.003)

5.3 (0.4)

0.2 (0.3)

Zn Zn

8.54 (0.09)

3.245 (0.010)

12.2 (1.2)

1.1 (0.4)

Zn O

4.00 (0.12)

1.965 (0.010)

5.1 (1.6)

1.5 (0.5)

Zn Zn

11.98 (0.09)

3.223 (0.012)

8.2 (0.3)

3.6 (1.0)

NZn

Zn/NZn O

RZn

Zn/RZn O

N: coordination number. R (Å): bonding distance. σ2 (Å2): Debye Waller factor. ΔE0 (eV): inner potential shift.

between 1.5 and 2.5 Å corresponding to the Zn O bond and a peak between 2.6 and 4.1 Å corresponding to Zn Zn bond as conformed by FEFF7. Bond length analysis of RZn O and RZn Zn of the S1 without additional reagents gave 1.974 and 3.250 Å, which is larger than that of the well-crystallized structure of ZnO powder. When PEI was added to the reaction, the bond lengths of RZn O and RZn Zn of the S2 were the same as for the S1. When NH3 was also added, the bond lengths of RZn O and RZn Zn of S3 were reduced to 1.971 and 3.245 Å, which is still larger than the crystallized structure of ZnO powder. The results are consistent with that of the XRD analysis, which indicates an expanded plane distance along the c-axis, as shown in Figure 2b. According to the theoretical calculation by Janotti et al. based on density functional theory and pseudopotentials, the zinc interstitial is stable at the octahedral site and induces significant local lattice relaxation. The Zn atom is slightly displaced along the c-direction toward the plane of its three oxygen neighbors, strongly repelling its three zinc neighbors.21c Interestingly, the ratio of RZn Zn/RZn O in S1 S3 maintained the same value of 1.646. These results indicate that adding PEI does not affect the stacking mechanism during growth. However, the addition of NH3 during growth shrinks the length of bonds, but maintains the stacking mechanism. As shown in Figure 1c, the size of the basal plane of ZnO nanowires reduced along the growth direction. Additionally, the Fourier transfer (FT) magnitude of Zn O bonds in those samples was similar; however, the FT magnitude of Zn Zn bond was different, S1 > S2 > S3. If the FT magnitude of the Zn Zn bond is higher, the number of the Zn(II) surrounding the core zinc species is higher, closely correlated with the number of the holes, according to the intensity of the white line in Zn K-edge XANES. For a well-crystallized structure of ZnO with space group P63mc, the coordination numbers of NZn O and NZn Zn were 4.00 and 12.00. In S1, the values of NZn O and NZn Zn were lower than those of commercial ZnO powder. Because the sample was reduced to nanomaterial, the coordinate numbers of NZn O and NZn Zn also decreased due to an increase in the surface area. For S1, with a well-crystallized structure of ZnO stacking, the ratio of the coordination number, NZn Zn/NZn O, should also be 3, which is the same as that of commercial ZnO powder. However, the ratio of NZn Zn/NZn O was 3.28, greater than 3, which implies that Zn(II) surrounding the core zinc species was higher with the oxidation state of the core zinc species lower than double valence. When PEI and NH3 were added during the growth of ZnO nanowire arrays, the coordination number of the NZn O is maintained at approximately 2.9 and the coordination number of the NZn Zn changed remarkably. Thus, the ratio of the coordination number NZn Zn/ NZn O decreased to 3.12 and 3.02 for S2 and S3, respectively.

From previous reports, we know that PEI is a nonpolar polymer with a large quantity of side amino-groups ( NH2), which can be protonated and positively charged over a wide range of pH values (3 11).26 The pH value of the growth solution in the present work was approximately 9.5, falling within the range of the proponated. Note that ZnO nanowires should be negatively charged in the facets because the potential of zero charge point (ZCP) of ZnO is 7.2.27 Therefore, positively charged PEI molecules should be adsorbed on the facets of ZnO nanowires to reduce the activity of Zn(II) resulting from electrostatic affinity. Furthermore, when NH3 was added to the reactor, the negative charge on the facets of ZnO nanowires would increase. Both growth mechanisms may result in a higher number of O atoms incorporated into the structures of ZnO nanowires. By correlating the PL spectra with XAS analysis, we deduce that the mechanism underlying the emission of red emission region of S1 without added reagent was due to a high number of Zn atoms in the structure (i.e., interstitial Zn, Zni) and the yellow emission region of the S2 and S3 was caused by a high number of O atoms in the construction of the ZnO nanowires (i.e., interstitial O, Oi). We believe that the methodology of EXAFS fitting is applicable to evaluating the mechanisms associated with emissions in the visible band from ZnO nanowires prepared by hydrothermal methods.

’ CONCLUSIONS We have conducted a new methodology to characterize the mechanism of visible emission bands of ZnO nanowires by using X-ray absorption spectroscopy. All three ZnO nanowires had orange-red emission near 640 nm. However, ZnO nanowires grown without any additive had another dominant emission in the near-infrared emission region, and ZnO nanowires grown by adding PEI and NH3 demonstrated another emission in the yellow emission region. According to coordinate number analysis of NZn Zn/NZn O, we deduce that ZnO nanowires grown by normal hydrothermal growth condition own a high number of Zn atoms in the structure (i.e., interstitial Zn, Zni). When PEI and NH3 were added into the reactor, high numbers of O atoms were in the construction of the ZnO nanowires (i.e., interstitial O, Oi). This is the first demonstration of the visible emission mechanism of ZnO nanowires by structural analysis directly. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figure and table. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: +886-2-27376615. Fax: +886-2-27376644. E-mail: sampras@ mail.ntust.edu.tw.

’ ACKNOWLEDGMENT Financial support from the National Science Council, under the NSC-99-2221-E-011-123 contract, the National Taiwan University of Science and Technology, and the National Synchrotron Radiation Research Center (NSRRC) is gratefully acknowledged. ’ REFERENCES (1) (a) Choy, J.-H.; Jang, E.-S.; Won, J.-H.; Chung, J.-H.; Jang, D.-J.; Kim, Y.-W. Adv. Mater. 2003, 15, 1911. (b) Frenzel, H.; Lajn, A.; Von Wenckstern, H.; Lorenz, M.; Schein, F.; Zhang, Z.; Grundmann, M. Adv. Mater. 2010, 22, 5332–5349. (c) Hong, Y. J.; Jung, H. S.; Yoo, J.; Kim, Y.-J.; Lee, C.-H.; Kim, M.; Yi, G.-C. Adv. Mater. 2009, 21, 222–226. (2) (a) Vayssieres, L. Adv. Mater. 2003, 15, 464–466. (b) Vayssieres, L.; Keis, K.; Lindquist, S.-E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3350–3352. (c) Konenkamp, R.; Boedecker, K.; Lux-Steiner, M. C.; Poschenrieder, M.; Zenia, F.; Levy-Clement, C.; Wagner, S. Appl. Phys. Lett. 2000, 77, 2575–2577. (d) Yu, H.; Zhang, Z.; Han, M.; Hao, X.; Zhu, F. J. Am. Chem. Soc. 2005, 127, 2378–2379. (e) Wong, M. H.; Berenov, A.; Qi, X.; Kappers, M. J.; Barber, Z. H.; Illy, B.; Lockman, Z.; Ryan, M. P.; MacManus-Driscoll, J. L. Nanotechnology 2003, 14, 968–973. (f) Fan, H. J.; Lotnyk, A.; Scholz, R.; Yang, Y.; Kim, D. S.; Pippel, E.; Senz, S.; Hesse, D.; Zacharias, M. J. Phys. Chem. C 2008, 112, 6770–6774. (g) Song, T.; Choung, J. W.; Park, J.-G.; Park, W. I.; Rogers, J. A.; Paik, U. Adv. Mater. 2008, 20, 4464–4469. (h) Chen, L.-Y.; Wu, S.-H.; Yin, Y.-T. J. Phys. Chem. C 2009, 113, 21572–21576. (3) Li, D.; Leung, Y. H.; Djuriic, A. B.; Liu, Z. T.; Xie, M. H.; Shi, S. L.; Xu, S. J.; Chan, W. K. Appl. Phys. Lett. 2004, 85, 1601–1603. (4) Uedono, A.; Koida, T.; Tsukazaki, A.; Kawasaki, M.; Chen, Z. Q.; Chichibu, S. F.; Koinuma, H. J. Appl. Phys. 2003, 93, 2481–2485. (5) (a) Tam, K. H.; Cheung, C. K.; Leung, Y. H.; Djurisic, A. B.; Ling, C. C.; Beling, C. D.; Fung, S.; Kwok, W. M.; Chan, W. K.; Phillips, D. L.; Ding, L.; Ge, W. K. J. Phys. Chem. B 2006, 110, 20865–20871. (b) Ahmad, M.; Zhu, J. J. Mater. Chem. 2011, 21, 599–614. (6) Kohan, A. F.; Ceder, G.; Morgan, D.; Van de Walle, C. G. Phys. Rev. B 2000, 61, 15019. (7) Liu, M.; Kitai, H.; Mascher, P. J. Lumin. 1992, 54, 35. (8) (a) Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Appl. Phys. Lett. 1996, 68, 403–405. (b) Djuriic, A. B.; Leung, Y. H. Small 2006, 2, 944–961. (9) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem., Int. Ed. 2003, 42, 3031–3034. (10) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455–459. (11) Tak, Y.; Yong, K. J. Phys. Chem. B 2005, 109, 19263–19269. (12) (a) http://ixs.iit.edu/subcommittee_reports/sc/sc00report. pdf. (b) http://ixs.iit.edu/subcommittee_reports/sc/errorep.pdf. (13) Stern, E. A.; Newville, M.; Ravel, B.; Yacoby, Y.; Haskel, D. Phys. B 1995, 208 209, 117. (14) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. Rev. B 1995, 52, 2995. (15) Ashkenov, N.; Mbenkum, B. N.; Bundesmann, C.; Riede, V.; Lorenz, M.; Spemann, D.; Kaidashev, E. M.; Kasic, A.; Schubert, M.; Grundmann, M.; Wagner, G.; Neumann, H.; Darakchieva, V.; Arwin, H.; Monemar, B. J. Appl. Phys. 2003, 93, 126–133. (16) Xing, Y. J.; Xi, Z. H.; Xue, Z. Q.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; Zhang, S. L.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 1689–1691.

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(17) Gomi, M.; Oohira, N.; Ozaki, K.; Koyano, M. Jpn. J. Appl. Phys., Part 1 2003, 42, 481–485. (18) Djuriic, A. B.; Leung, Y. H.; Tam, K. H.; Ding, L.; Ge, W. K.; Chen, H. Y.; Gwo, S. Appl. Phys. Lett. 2006, 88, 10317. (19) Cross, R. B. M.; De Souza, M. M.; Sankara Narayanan, E. M. Nanotechnology 2005, 16, 2188–2192. (20) (a) Sekiguchi, T.; Miyashita, S.; Obara, K.; Shishido, T.; Sakagami, N. J. Cryst. Growth 2000, 214, 72–76. (b) Liu, X.; Wu, X.; Cao, H.; Chang, R. P. H. J. Appl. Phys. 2004, 95, 3141. (c) Ong, H. C.; Du, G. T. J. Cryst. Growth 2004, 265, 471–475. (d) Heo, Y. W.; Norton, D. P.; Pearton, S. J. J. Appl. Phys. 2005, 98, 073502. (e) Pal, U.; Santiago, P. J. Phys. Chem. B 2005, 109, 15317–15321. (f) Romero-Gomez, P.; Toudert, J.; Sanchez-Valencia, J. R.; Borras, A.; Barranco, A.; GonzalezElipe, A. n. R. J. Phys. Chem. C 2010, 114, 20932–20940. (21) (a) Erhart, P.; Albe, K. Appl. Phys. Lett. 2006, 88, 201918. (b) Oba, F.; Nishitani, S. R.; Isotani, S.; Adachi, H.; Tanaka, I. J. Appl. Phys. 2001, 90, 824–824. (c) Janotii, A.; Walle, C. G. V. d. J. Cryst. Growth 2006, 287, 58. (d) Kim, Y.; Kim, Y.; Kang, S. J. Phys. Chem. C 2010, 114, 17894–17898. (e) Moreira, N. H.; Aradi, B. l.; Rosa, A. L. d.; Frauenheim, T. J. Phys. Chem. C 2010, 114, 18860–18865. (f) Angelis, F. D.; Armelao, L. Phys. Chem. Chem. Phys. 2010, 13, 467. (g) Tuomisto, F.; Saarinen, K.; Look, D. C.; Farlow, G. C. Phys. Rev. B 2005, 72, 085206. (h) Zhang, S. B.; Wei, S. H.; Zunger, A. Phys. Rev. B 2001, 63, 075205. (i) Huang, G.-Y.; Wang, C.-Y.; Wang, J.-T. Solid State Commun. 2009, 149, 199. (22) Waychunas, G. A.; Fuller, C. C.; Davis, J. A.; Rehr, J. J. Geochim. Cosmochim. Acta 2003, 67, 1031–1043. (23) (a) Yoshida, H.; Shimizu, T.; Murata, C.; Hattori, T. J. Catal. 2003, 220, 226–232. (b) Kelly, R. A.; Andrews, J. C.; DeWitt, J. G. Microchem J. 2002, 71, 231–245. (c) Larcheri, S.; Armellini, C.; Rocca, F.; Kuzmin, A.; Kalendarev, R.; Dalba, G.; Graziola, R.; Purans, J.; Pailharey, D.; Jandard, F. Superlattices Microstruct. 2006, 39, 267–274. (24) Chaboy, J.; Boada, R.; Piquer, C.; Laguna-Marco, M. A.; García-Hernandez, M.; Carmona, N.; Llopis, J.; Ruíz-Gonzalez, M. L.; Gonzalez-Calbet, J.; Fernandez, J. F.; García, M. A. Phys. Rev. B 2010, 82, 064411. (25) Zhang, P.; Sham, T. K. Phys. Rev. Lett. 2003, 90, 245502. (26) Oztekin, N.; Alemdar, A.; Gungor, N.; Erim, F. B. Mater. Lett. 2002, 55, 73–76. (27) Degen, A.; Kosec, M. J. Eur. Ceram. Soc. 2000, 20, 667.

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