Local Structural Properties of ZnO Nanoparticles, Nanorods and Powder Studied by Extended X-ray Absorption Fine Structure

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1 Journal of the Korean Physical Society, Vol. 53, No. 1, July 2008, pp Local Structural Properties of ZnO Nanoparticles, Nanorods and Powder Studied by Extended X-ray Absorption Fine Structure E.-S. Jeong, H.-J. Yu and S.-W. Han Institute of Fusion Science, Institute of Science Education and Division of Science Education, Chonbuk National University, Jeonju Sung Jin An, Jinkyoung Yoo, Y.-J. Kim and Gyu-Chul Yi National CRI Center for Semiconductor Nanorods and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang (Received 10 September 2007) We investigated the local structural properties of ZnO nanoparticles (NPs), nanorods and powder by using extended X-ray absorption ne structure (EXAFS). The average diameter and size of the ZnO NPs and nanorods were about 4.5 nm and 40 nm, respectively. The EXAFS analysis revealed that the bonding lengths of atoms in the ZnO nanorods were slightly elongated along the c-axis while they were shrunken in the ab-plane, as compared with those of the ZnO powder. We did not observe a vacancy or an extra disorder for the ZnO nanorods with an average length of 0.5 m. However, there were about 30 % vacancies in the oxygen and the zinc site of the ZnO NPs and the Zn-Zn bonding length had a substantial amount of disorders. ZnO NPs had a distorted-wurtzite structure with the same bonding length of four Zn-O pairs. The vacancy, the disorders and the distortion should be counted if the changes in the properties of the ZnO NPs are to be understood. PACS numbers: Ht, Dz, Bc, Gh Keywords: ZnO, Nanoparticle, Nanorod, Structure, EXAFS, MOCVD I. INTRODUCTION ZnO (wurtzite, p63m) is known to be a wide band gap semiconductor with a band gap energy of 3.3 ev at room temperature (RT). It is suitable for RT device applications because of its RT exciton binding energy of 60 mev [1]. The band gap can be engineered by alloying Mg or Cd with ZnO [2,3]. Previous studies also reported its potential applications to spintronics by adding a small amount of a transition metal [4]. Because of these merits, ZnO has been widely investigated for practical applications to electronic and photonic devices, including ultraviolet (UV) lasers, vacuum uorescent or eld emission displays, high power and high frequency devices, eldeect transistors [5] and light-emitting diodes [6,7]. Recently, ZnO nanostructures have attracted considerable attention for their potential applications to nanometerscale electronics and photonics, as well as academic research. As the particle size becomes of nanometer scale, the nanoparticle (NP) properties are changed resulting from the size eect and structural changes [8]. The blue shift of the NPs is generally understood in term of the quantum connement eect [9{11]. However, the shan@chonbuk.ac.kr; Fax: blue shift can also be caused by structural changes, such as short atomic bonding lengths [12, 13]. For a NP, we expect a strong tensile stress to exist among the surface atoms of the NP resulting from their dangling bonds. The surface tension can cause a shortened bonding length. X-ray diraction (XRD) is a canonical technique for structural analysis of crystals. However, XRD is not very useful for NPs because a NP does not have many X-ray scattering sources and NPs mostly have a large amount of structural disorder and displacement. Furthermore, the XRD averages over the atomic distances. Therefore, the XRD cannot measure the exact structural properties of NPs. We employed extended X-ray absorption ne structure (EXAFS) to investigate the local structural properties of ZnO NPs and nanorods. We, then, compared the results with those for a ZnO powder. EXAFS can describe the bonding lengths, bonding length disorders and atomic species of neighboring atoms around a probe atom. When the EXAFS data are taken with the incident X-ray electric eld parallel to a certain crystalline direction, the structural properties in that direction can be independently determined. For the nanorod study, the EXAFS data were taken with the incident X- ray electric eld parallel and perpendicular to the length direction of the nanorods. We could independently de

2 -462- Journal of the Korean Physical Society, Vol. 53, No. 1, July 2008 Fig. 1. (a) FE-TEM image of ZnO NPs and (b) FE-SEM image of ZnO nanorods. termine the structural properties along the c-axis and in the ab-plane of the nanorods. II. EXPERIMENTAL DETAILS ZnO NPs were synthesized by using a solution method [14]. Zinc acetate in ethanol was boiled at 80 C under an ambient atmosphere while vigorously stirring. In an ultrasonic bath, ZnO NPs were synthesized by supplying OH ions of lithium hydroxide into the zinc-ethanol solution at 0 C. The NPs had a fairly uniform size and an average diameter of 4.5 nm, as shown in Figure 1(a). The NPs were dried on slide glass for the EXAFS measurements. Vertically well-aligned ZnO nanorods were fabricated on Al 2 O 3 (0001) substrates at 500 C by using catalyst-free metal-organic vapor-phase epitaxy [15, 16]. The average diameter of the nanorods was 40 nm and the mean length along the c-axis was about 0.5 m, as shown in Figure 1(b). XRD and high-resolution transmission electron microscope measurements revealed that the nanorods were well-ordered single crystals with a wurtzite (WZ) structure and that they were vertically well-aligned [16]. A commercial ZnO powder with a purity of % was used as the counterpart for this EX- AFS study. EXAFS measurements at the Zn K edge (9659 ev) were performed on the ZnO NPs and nanorods with a uorescence mode at 20 K and on the ZnO powder with a transmission mode at 30 K. The incident X-ray energy was selected with a three-quarter tuned Si(111) double monochromator. For the orientation-dependent EXAFS measurements, the electric eld direction of the incident X-ray was aligned perpendicular (^? ^c) and parallel (^ k ^c) to the ZnO nanorod length direction. III. RESULTS AND DISCUSSION Figure 2(a) demonstrates the X-ray absorption near edge structure (XANES) near the Zn K edge from ZnO Fig. 2. (a) Normalized X-ray absorption spectra from ZnO powder (1 st ), ZnO nanorods with the incident X-ray electric eld direction perpendicular, ^? ^c (2 nd ) and parallel, ^ k ^c (3 rd ), to the nanorod-length direction and ZnO nanoparticles near the Zn K edge. (b) After the atomic background was removed, the EXAFS (k) was obtained from the data in (a) as a function of the photoelectron wave number, k = p me(e E 0)=~, where m e is the electron rest mass, E the incident X-ray energy and E 0 the absorption edge energy. powder, nanorods and NPs. The XANES is mainly determined by the chemical properties of the probe atom and the energy bands [17]. The XANES (2 nd and 3 rd ) from the ZnO nanorods are considerably dierent from the XANES of ZnO powder (1 st ) because the ZnO had a wurtzite structure and its energy bands were orientation dependent. The XANES (4 th ) of the ZnO NPs is also dissimilar to that of the ZnO powder. When the size of ZnO becomes nanometer scale, the local band structures around Zn are expected to be dierent from those of the bulk ZnO because of the size eect, structural distortion and disorder. The local structural properties around the probe atom can be obtained by looking at the oscillations (EXAFS) above the X-ray absorption edge of the probe atom [17]. After the atomic background absorption was determined, the EXAFS () was extracted from the measured data, as shown in Figure 2(b). The EXAFS data in Figure 2(b) demonstrate the local structural dierences between the ZnO powder, nanorods and NPs. For a quantitative comparison of the local structural properties of the three systems, the data were Fourier transformed to r-space as shown in Figures 3. To minimize errors, we used the data of Figure 2(b) in the k-ranges of , and for the powder, nanorods and NPs, respectively, for further analysis. In Figure 3(a), the rst and second peaks

3 Local Structural Properties of ZnO Nanoparticles, Nanorods { E.-S. Jeong et al Table 1. Fit results of the EXAFS data taken from ZnO powder, nanorods and NPs at low temperatures. N is the coordination number, d is the distance between atoms and 2 is the Debye-Waller factor. For the model calculation, lattice constants of a = b = A, c = A and oxygen z = were used. Zn-O(1) Zn-O(2) Zn-Zn(1) Zn-Zn(2) specimen N d (A) 2 (A 2 ) N d (A) 2 (A 2 ) N d (A) 2 (A 2 ) N d (A) 2 (A 2 ) model powder (4) 0.003(1) (4) 0.003(1) (3) 0.004(1) (3) 0.004(1) nanorods 1.0(1) 1.946(5) 0.005(1) 3.0(2) 1.966(6) 0.005(1) 6.0(1) 3.203(3) 0.006(1) 6.0(1) 3.255(2) 0.004(1) NPs 0.7(1) 1.940(5) 0.005(1) 2.1(3) 1.940(5) 0.005(1) 4.0(7) 3.10 (5) 0.030(5) 4.0(7) 3.30(5) 0.030(5) Fig. 3. Magnitude of the Fourier-transformed EXAFS from (a) ZnO powder, ZnO nanorods for (b) ^? ^c and (c) ^ k ^c and (d) ZnO nanoparticles as functions of the distance from a Zn atom. Data in the range of r = A were tted. For the Fourier transformation, a Hanning window with windowsill widths of 1.0 A 1 was used. correspond to 4 oxygen and 12 zinc atoms located around a zinc atom. For the data analysis, we used the UWX- AFS package [18] and standard analysis procedures [17, 19]. It should be noted that the peaks shifted by about 0.4 A on the r-axis from their true bond lengths because of the phase shift of the back-scattered photoelectrons by the neighboring atoms. To obtain quantitative structural properties, we tted the data to the EXAFS theoretical calculation [20]. We started the t of the powder data with a fully occupied WZ structure, including single- and multi-scattering paths. In the t, we varied the bonding length and the Debye-Waller factor ( 2, including thermal vibration and static disorder) of each atomic shell. The t results are summarized in Table 1. For the EXAFS data analysis of the ZnO nanorods, the two sets of the data were simultaneously tted with the same variables, as shown in Figures 3(b) and (c). The rst peaks in Figures 3(b) and (c) correspond to three O(2) atoms located near the Zn ab-plane and one O(1) atom located along the c-axis, respectively. The second peak in Figure 3(b) correspond to 6 Zn(2) atoms located in the Zn ab-plane while the second peak in Figure 3(c) is contributed by 6 Zn(2) atom and 6 Zn(1) located at 55 from the Zn ab-plane. Therefore, the structural properties of the Zn-O(1) and the Zn-O(2) pairs were independently determined by the ts to the two sets of data. The details of the orientationdependent EXAFS data analysis have been described elsewhere [17,19,21]. The t results summarized in Table 1 indicate that the distance of the Zn-O(1) pair located along the c-axis is slightly elongated while that of the Zn-O(2) pairs in the ab-plane is shrunken. These results agree well with a previous study of ZnO nanorods at room temperature [21]. The 2 values of all atomic pairs were also similar to those of the bulk counterpart, which implied that extra structural disorder and distortion were negligible in the ZnO nanorods. For the NP data t shown in Figure 3(d), we used a distorted WZ model because with the WZ structure determined from the ZnO powder, we could not obtain a satisfactory t. The ts for the NP data indicated that the bonding lengths of the Zn-O pairs were the same. This structure is close to a zinc-blende (ZB) structure. The bonding length of the Zn-O(1), located along the c-axis, was slightly elongated while the length of the Zn-O(2) pairs, located near the ab-plane, was shrunken, compared with the bulk counterpart. The bonding length was about 0.05 A shorter than previously reported [22, 23]. The oxygen site had about 30 % vacancy. The 2 value of the Zn-O pairs was similar to that of the ZnO bulk counterpart. This small 2 value implied a tight bonding between the Zn and O atoms. The t revealed that about 8 Zn atoms were placed at zinc site, implying that approximately 30 % vacancy existed at the zinc site. The bond length of about 4 Zn-Zn pairs shrank while that of the other 4 pairs expanded. This contrasted with previous studies that reported a bonding-length expansion of all atomic pairs [22,23]. Based on the bonding lengths of the Zn-O and the Zn-Zn pairs, we concluded that the ZnO NPs had a distorted WZ structure. The

4 -464- Journal of the Korean Physical Society, Vol. 53, No. 1, July 2008 Debye-Waller factor, 2, at low temperatures is mainly contributed by the zero-point motion of atomic pairs and the static disorders of the atomic bonding length because at a low temperature, the thermal vibrations of atomic pairs mostly disappear. The large 2 value of the Zn- Zn pairs implies that a substantial amount of structural disorders existed in the Zn-Zn pairs of the ZnO NPs, compared with ZnO bulk and nanorods. The 30 % oxygen vacancy in the oxygen site might be correlated with the Zn vacancy in the Zn site. There were at least two possibilities for the location of the vacancies. The vacancies can randomly spread over the NPs and can be due to the boundary. For spherical particles, the number ratio of surface atoms to total atoms can be predicted with a simple model of (R 3 (R D) 3 )/R 3, where R is the radius of a NP and D is the distance from the boundary. When assuming R = 2.25 nm and D = 0.32 nm (Zn-Zn or O-O distance), about 37 % of the atoms were estimated to be placed in the boundary [21]. If zinc atoms are the terminating atoms, 37 % of the Zn atoms are located in the boundary and have less than 2 oxygen neighbors. This means that the oxygen vacancy of the ZnO NPs is approximately 25 %. If the Zn atoms at the boundary have only 3 zinc neighbors, the vacancy of the zinc site is estimated to be about 28 %. This model calculation agrees well with the EXAFS analysis, suggesting zinc termination in the NPs. The structural changes, including vacancies, disorders, distortion and Zn termination of the ZnO NPs, should be counted to understand the physical properties including photoluminescence (PL) and Raman scatterings, of the NPs. ZnO NPs with a diameter of 4.5 nm have a spherical shape and include about 44,000 atoms of zinc and oxygen. The ZnO nanorods have a diameter and a length of about 40 nm and 500 nm, respectively. Strictly speaking, the NPs are not dots and the nanorods are not onedimensional (1D). However, they are still dots and 1D when compared with their bulk counterparts. For a nanodot, a large portion of atoms are located at its surface and have imperfect bonding. The imperfect bonding, creating a large surface tension, likely causes short atomic bonding lengths in all directions. For the ZnO nanorods, the c-axis is parallel to the nanorod's length. Therefore, we expect the bonding lengths in the ab-plane to be shrunken due to the surface tension. The EXAFS results provided clear evidence for a surface tension existing on the atomic surface. Furthermore, the bonding length distortion should aect the band gap energy of the nanostructures. In general, the quantum connement effect is considered by observation of a blue shift in the photoluminescence (PL) spectra [9{11]. The blue shift can also be caused by structural changes, such as short lattice constants [12, 13]. The EXAFS results demonstrated that the atomic bonding length was shorter for a smaller particle size. Therefore, the blue shift observed in NPs could originate from the quantum connement eect and the structural distortion. IV. CONCLUSION The structural properties of ZnO NPs, nanorods and bulk were investigated by using EXAFS at low temperatures. From the EXAFS measurements, we found that there were substantial amounts of structural distortion and disorder existing in the NPs, compared with the ZnO bulk counterpart. The Zn-O pairs in all directions had the same bonding length while Zn-Zn(2) pairs in the abplane had bonding lengths dierent from the other Zn-Zn pairs(1) located at 55 degrees from the ab-plane. This result implies that the ZnO has a distorted WZ structure, an intermediate structure between the WZ and the ZB structures. ZnO likely has a ZB structure at the beginning of crystal formation and nally becomes a WZ structure when it is perfectly crystallized. There were about 30 % vacancies in the oxygen and the zinc sites, suggesting terminating zinc atoms at the boundary of the ZnO NPs. From the analysis of the ZnO nanorod EXAFS data, we also observed bonding length changes, shrunken in the ab-pane and elongated along the c-axis. These results strongly suggested the presence of atomic surface tension. The structural changes in the nanostructures should be counted if their physical properties, including PL and Raman spectra, are to be understood. The blue shift in a PL spectrum from NPs may have originated from the short bonding lengths and from the quantum connement eect. ACKNOWLEDGMENTS The work at Chonbuk National University was conducted under the auspices of the Korea Basic Research Program (KOSEF-R ), the Korea Research Foundation Grant (MOEHRD, KRF C00262) and the Korea Ministry of Science and Technology (MOST) through the PEFP User Program as a part of the 21C Frontier R&D Program. 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