Local Structural and Optical Properties of ZnO Nanoparticles

June 1, 2017 | Autor: Sang-Wook Han | Categoria: Engineering, Technology, CHEMICAL SCIENCES, Nanoscience and nanotechnology
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Copyright © 2010 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 10, 3562–3565, 2010

Local Structural and Optical Properties of ZnO Nanoparticles Eun-Suk Jeong1 , Hyo-Jong Yu1 , Yong-Jin Kim2 , Gyu-Chul Yi3 , Yong-Dae Choi4 , and Sang-Wook Han1 ∗ 1

RESEARCH ARTICLE

Division of Science Education, Institute of Fusion Science, and Institute of Science Education, Chonbuk National University, Jeonju 561-756, Korea 2 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea 3 National CRI Center for Semiconductor Nanorods and Department of Physics, Seoul National University, Seoul 151-742, Korea 4 Department of Techno-Marketing, Mokwon University, Daejeon 302-718, Korea This study examined the local structural and optical properties of ZnO nanoparticles (NPs) with mean diameters of 4.5 and 70 nm using extended X-ray absorption fine structure (EXAFS) measurements at the Zn K edge and photoluminescence (PL) measurements. EXAFS revealed that the average bond length of atomic pairs in the NPs was shorter than that of the powder. Furthermore, a substantial amount of structural disorder existed in the NPs. From the PL measurements, we observed the direct band gap peak of 3.41 eV from the 70 nm ZnO NPs at low temperatures. This blue shift was related to the structural property by changes. Delivered Ingenta to:

Pohang Gongkwa DaehakkyoZnO, (Pohang University of ScienceQuantum & Technology) Keywords: EXAFS, Nanoparticle, Structure, Photoluminescence, Confinement. IP : 141.223.167.71 Sun, 25 Jul 2010 10:19:14

1. INTRODUCTION ZnO nanostructures have been studied extensively for possible practical applications to nanometer-scale electronics and photonics including transistors, gas sensors, light emitting diodes (LEDs), ultra-violet (UV) sensors, piezoelectric applications, biosensors, and field emission devices. Recently, researchers have paid considerable attention to ZnO nanoparticles for their quantum confinement effects. Many studies reported observations of a quantum confinement effect of ZnO nanoparticles (NPs)1–4 and nanowires5 using photoluminescence (PL) and Raman scattering measurements. The previous studies reported that the ZnO free exciton recombination peak was shifted from 3.3 eV to ∼3.4 eV at low temperatures, as the ZnO NPs reduced to a few nanometers. This blue shift was attributed to the quantum confinement effect. However, the particle size was still much larger than the Bohr radius of ZnO, which is approximately 1.5 nm. The blue shift was even observed from the ZnO nanoparticles (NPs) with a mean diameter of more than 10 nm.3 4 The blue shift could be attributed to a structural change because the energy band gap can be engineered by controlling the lattice constants of crystals.6–8 ∗

Author to whom correspondence should be addressed.

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This study compared the local structural and the optical properties of NPs. Field-emission tunneling electron microscopy (FE-TEM) is used to observe the atomic arrangement in certain areas. However, it has limited resolution and cannot detect a small amount of lattice distortion. X-ray diffraction (XRD) is a powerful tool for investigating the structures of crystalline materials but has limitations when examining nanomaterials due to a small number of scattering sources. Extended X-ray absorption fine structure (EXAFS) can reveal the bond lengths, the disorder of the bond lengths from a probe atom, the coordination numbers, and the species of the atoms.9 Therefore, EXAFS was used to quantify the local structural properties of ZnO NPs with an average particle size of 4.5 and 70 nm.

2. EXPERIMENTAL DETAILS ZnO NPs, with two different sizes as shown in Figure 1, were fabricated with a solution process.10 Zinc acetate in ethanol was boiled at approximately 80  C in air while vigorously stirring. In an ultrasonic bath, ZnO NPs were synthesized by supplying OH ions of lithium hydroxide into the zinc-ethanol solution at approximately 0  C. The 1533-4880/2010/10/3562/004

doi:10.1166/jnn.2010.2334

Jeong et al. (a)

Local Structural and Optical Properties of ZnO Nanoparticles (b)

(c) Fig. 2. XRD from the 70 nm ZnO NPs as a function of 2. Closed dots are data and solid line is a best fit with a Gaussian distribution.

3. RESULTS AND DISCUSSION The high resolution FE-TEM measurements demonstrate that the NPs have a very uniform size with a mean diameter of 4.5 nm, as shown in Figures 1(a and b). The FE-TEM analysis reveals that the NPs are a well-ordered structure with an average atomic distance of 3.2 Å, which corresponds to the bond length of the Zn–Zn or O–O pairs in wurtzite (WZ) structured ZnO crystals. The mean particle size of 70 nm was determined by using field-emission scanning electron microscopy (FE-SEM) measurements as shown in Figure 1(c). The XRD measurements for the J. Nanosci. Nanotechnol. 10, 3562–3565, 2010

Fig. 3. (a) Normalized X-ray absorption coefficients of the ZnO powder (top) and NPs with diameters of 70 nm (middle) and 4.5 nm (bottom) at Zn K edge were measured as a function of the incident X-ray energy. (b) EXAFS (k extracted from the data in (a) after removing the atomic background as a function of the photoelectron wave number,  k = 2m E − E0 / where m is the electron rest mass, E is the incident X-ray energy and E0 is the edge energy. To minimize the uncertainties, only the EXAFS data in a k-range of 2.5–10.5 Å−1 was used for further analysis.

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70 nm NPs revealed the NPs to have a WZ structure with lattice constants of a = b = 324 ± 005 Å and c = 523 ± 005 Å, as shown in Figure 2. From the full-width at half maximum (FWHM) of the XRD peaks, structural residual strains for a and c were determined to be about 0.07 Å and 0.09 Å, respectively, counting the particle size Fig. 1. FE-TEM images from (a) ZnO NPs with a mean diameter of of 70 nm.11 The XRD implied that a large amount of 4.5 nm, and (b) a single NP. (c) SEM image from the 70 nm ZnO NPs. structural disorder existed in 70 nm ZnO NPs. We did not observe any XRD peak from the 4.5 nm NPs because there NPs were dried on slide glass for the various measurewere insufficient amounts of scattering sources in the NPs. ments. The XRD measurements were performed using Figure 3(a) shows the X-ray absorption coefficients Cu K1 radiation in air. The EXAFS measurements of of the three ZnO specimens near the Zn K edge. The the commercial ZnO powder with a maximum size of X-ray absorption near edge structure (XANES) from the Delivered by Ingenta to: 25 m and the NPs at the Zn K edge (9659 eV) were three specimens was dramatically diminished as the partiPohang Gongkwa Daehakkyo (Pohang University of Science & Technology) performed with transmission and fluorescence modes by cle was reduced in size to nanometers. Since the chemical IP : 141.223.167.71 selecting incident X-ray energy with a three-quarters tuned Sun, 25 Jul 2010 10:19:14 Si(111) double monochromator at MB beamline of PNC(a) CAT at Advanced Photon Source, Argonne National Laboratory and at 3C1 beamline of Pohang Light Source. A 13-element solid-state detector was used to eliminate background noise, and the data was taken at 40 K to minimize the atomic thermal vibrations. The average diameters of the NPs are much smaller than the one absorption length of X-ray at Zn K edge, suggesting that the X-ray selfabsorption for the NPs was negligible. The PL measurements were performed with a He–Cd laser ( = 325 nm) and an f = 085 double monochromator with a liquid N2 -cooled, front-illuminated charge coupled device detec(b) tor at 5–300 K.

Local Structural and Optical Properties of ZnO Nanoparticles

Jeong et al.

of the EXAFS data fit to a distorted structure were published elsewhere.9 13 15 The best fit results are summarized in Table I. The fits of the first and second peaks in Figures 4(b and c) show that there was approximately 30% of the vacancies in both oxygen and zinc sites of the 4.5 nm NPs (b) while no vacancy was observed in neither oxygen nor zinc site of the 70 nm NPs. The vacancies could spread randomly throughout the entire NPs, and the location can also be due to a boundaries. With a simple model calculation,15 (c) the number ratio of surface atoms to total atoms is predicted to be approximately 37% and 3% for 4.5 and 70 nm NPs, respectively. This prediction and the EXAFS suggested that the vacancies of the 4.5 nm NPs were mostly located at the boundary. The bond lengths of the Zn–O pairs in the 4.5 nm NPs were nearly the same in all directions. The bond lengths of Fig. 4. Magnitude of the Fourier-transformed EXAFS from (a) ZnO Zn–Zn pairs in the NPs were shorter than that of the ZnO powder and ZnO NPs with diameters of (b) 70, and (c) 4.5 nm as a powder, particularly, in the Zn–Zn(1) pairs. The average function of distance from a Zn atom. The dotted and solid lines are the data and the best fit, respectively. For the Fourier transformation the volume of both NPs was estimated to be approximately Hanning window with the windowsill width of 1.0 Å−1 was used. 92% of the ZnO powder. The 2 value of the Zn–O pairs in the ZnO NPs was about twice as large as that of the powder. For the Zn–Zn pairs, the 2 values of the NPs valence state of the Zn atoms with the particle size was were several times larger than that of the powder, sugnot expected to be changed, the XANES change was likely gesting that there was significant structural disorder in the affected by the change of structural properties. Z–Zn pairs The local structural properties of the ZnODelivered NPs wereby Ingenta to:of the NPs. Figure 5 demonstrates temperature-dependent PL specdetermined byPohang the EXAFS measurements. The(Pohang X-ray University Gongkwa Daehakkyo of Science & Technology) tra from the 70 nm NPs. The PL spectra demonstrate absorption coefficient is contributed by the X-ray IPabsorb: 141.223.167.71 strong near-band edge emission and a weak but visible ing atoms, and the neighboring atoms, Sun, as  E = 2010 25 Jul 10:19:14 emission near 2.3 eV due to defects from the NPs. The 0 E 1 −  E , where 0 E is the X-ray absorption PL spectrum at low temperatures exhibited two distinct coefficient of an X-ray absorbing atom and  E , EXAFS, emission peaks at 3.35 and 3.41 eV. No recombination is the oscillating parts above the absorption edge due to peak was observed from the 4.5 nm NPs due to a subthe neighboring atoms around the X-ray absorbing atom. stantial amount of disorder. The direct band gap peak was After 0 is determined,  was extracted from the measured observed at 3.29 eV from ZnO powder, as shown in the data, as shown in Figure 3(b). inset of Figure 5. It should be noted that the mean diameFigure 4 demonstrated the magnitude of Fourierter of 70 nm is much larger than the ZnO Bohr radius of transformed EXAFS data. The EXAFS data were analyzed 1.5 nm. with the UWXAFS package12 and standard procedures.13 Based on EXAFS analysis, the average bond lengths of The EXAFS data were fitted to the theoretical EXAFS the atomic pairs, particularly Zn–Zn pairs in the 70 nm calculation,14 as shown in Figure 4. In the fits, the coorNPs, were shorter than those of the ZnO powder. Previous dination number, the bond lengths, and the Debye-Waller factors ( 2 , including thermal vibration and static disorPL studies of II–VI semiconductors, including ZnO-based der) of Zn–O and Zn–Zn pairs were varied. The details materials6–8 and ZnTe-based materials,16–18 reported that

RESEARCH ARTICLE

(a)

Table I. The analysis results of the EXAFS data. Coordination number (N , bond length (d and Debye-Waller factor ( 2 . For a WZ ZnO crystal, one O(1) is located just above a Zn atom in the c-axis, three O(2)s at 19 from the ab-plane, six Zn(1)s at 57 from the ab-plane, and six Zn(2)s in the ab-plane. S02 of 0.90(5) was used.15 ZnO powder

70 nm ZnO NPs 2

4.5 nm ZnO NPs

Pair

N

d (Å)



N

d (Å)



N

d (Å)

2 (Å2

Zn–O(1) Zn–O(2) Zn–Zn(1) Zn–Zn(2)

1 3 6 6

1.902(6) 1.982(5) 3.207(3) 3.245(3)

0.003(1) 0.003(1) 0.004(1) 0.004(1)

1.0(1) 3.0(2) 6.0(4) 6.0(4)

1.926(8) 1.980(8) 3.044(9) 3.203(5)

0.006(1) 0.005(1) 0.012(1) 0.005(1)

0.7(1) 2.1(2) 4.2(5) 4.2(5)

1.938(5) 1.941(5) 3.00(4) 3.23(3)

0.005(1) 0.005(1) 0.033(9) 0.026(8)

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2

2

2

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Local Structural and Optical Properties of ZnO Nanoparticles

Acknowledgment: The work at Chonbuk National University was conducted under the auspices of the Korea Research Foundation Grant (MOEHRD, KRF-2007-313C00262) and the Korea MEST through the PEFP User Program. The work at Seoul National University was supported by the National Creative Research Initiative Project (R16-2004-004-01001-0) of the Korea Science and Engineering Foundations (KOSEF).

References and Notes

Received: 5 November 2008. Accepted: 9 May 2009.

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1. A. P. Alivisatos, Science 271, 933 (1996). 2. H. K. Yadav, V. Gupta, K. Sreenivas, S. P. Singh, B. Sundarakannan, and R. S. Katiyar, Phys. Rev. Lett. 97, 85502 (2006). 3. T.-B. Hur, Y.-H. Hwang, and H.-K. Kim, Appl. Phys. Lett. 86, 193113 (2005). 4. W.-T. Hsu, K.-F. Lin, and W.-F. Hsieh, Appl. Phys. Lett. 9, 181913 Fig. 5. PL spectra of ZnO NPs with a mean diameter of 70 nm at (2007). various temperatures. The inset shows a PL spectrum from a ZnO powder 5. W. I. Park, G.-C. Yi, M. Kim, and S. J. Pennycook, Adv. Mater. at 10 K. 15, 1841 (2002). 6. W. I. Park, G.-C. Yi, and H. M. Jang, Appl. Phys. Lett. 79, 2022 (2001). the band gap of the systems was reduced or expanded by 7. S.-H. Park, K.-B. Kim, S.-Y. Seo, S.-H. Kim, and S.-W. Han, J. Elec. alloying an impurity. When these short lattice constants in Mater. 35, 1680 (2006). the NPs were compared with previous ZnMgO systems,6 7 8. F. Z. Wang, H. P. He, Z. Z. Ye, and L. P. Zhu, J. Appl. Phys. the blue shift of the 70 nm ZnO NPs can be explained 98, 84301 (2005). 9. S.-W. Han, Int. J. Nanotechnology 3, 396 (2006). by structural distortion in the NPs alone instead of by the 10. L. Vayssieres, Adv. Mater. 15, 464 (2003). quantum confinement effect. 11. S.-W. Han, S. Tripathy, P. F. Miceli, M. Covington, L. H. Greene, Delivered by Ingenta and M.to: Aprili, Jpn. J. Appl. Phys. 42, 1395 (2003). 12. E. A. Stern, Newville, B. Y. Yacoby, and D. Haskel, PhysPohang Gongkwa Daehakkyo (Pohang University ofM.Science &Ravel, Technology) 4. CONCLUSION ica B 208–209, 117 (1995). IP : 141.223.167.71 13. S.-W. Han, E. A. Stern, D. Hankel, and A. R. Moodenbaugh, Phys. The local structural and the optical propertiesSun, of ZnO 25 NPs Jul 2010 10:19:14 Rev. B 66, 94101 (2002). with the diameters of 4.5 and 70 nm were investigated 14. A. L. Ankudinov, B. Ravel, J. J. Rehr, and S. D. Conradson, Phys. using EXAFS and PL measurements. The EXAFS analyRev. B 58, 7565 (1998). sis revealed that the average volume of the ZnO NPs was 15. S.-W. Han, H.-J. Yoo, S. J. An, J. Yoo, and G.-C. Yi, Appl. Phys. Lett. 86, 21917 (2005). only 92% of ZnO powder. Furthermore, ZnO NPs had sig16. O. Maksimov and M. C. Tamargo, Appl. Phys. Lett. 79, 782 (2001). nificant amounts of structural disorder in atomic pairs, par17. C.-H. Su, S. Feth, S. Zhu, S. L. Lehoczky, and L. J. Wang, J. Appl. ticularly Zn–Zn pairs. The direct band gap peak of 70 nm Phys. 88, 5148 (2000). ZnO NPs was shifted by ∼0.12 eV toward a higher energy. 18. W. Stadler, D. M. Hofmann, H. C. Alt, T. Muschik, B. K. Meyer, This blue shift could be caused by the structural distortion E. Weigel, G. Muller-Vogt, M. Salk, E. Rupp, and K. W. Benz, Phys. Rev. B 51, 10619 (1995). of the NPs.

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