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Surface Photoluminescence Emission of ZnO Nanorod Arrays: Experimental and First-Principles Investigation Yongseon Kim,†,‡ Yangsoo Kim,§ and Shinhoo Kang*,† Department of Materials Science and Engineering, Seoul National UniVersity, Seoul, 151-742, Korea, SAMSUNG SDI CO., LTD, Yongin-si, Gyeonggi-do, 446-577, Korea, and Korea Basic Science Institute (Suncheon Center), Suncheon-si, Jeollanam-do, 540-742, Korea ReceiVed: July 17, 2010; ReVised Manuscript ReceiVed: September 3, 2010
The photoluminescence emission of ZnO nanorods was investigated experimentally and by first-principles calculations. Nanorod arrays of ZnO were used because it is easy to treat the surface and their morphologies simplify first-principles calculations. The nanorod array samples were coated with various oxides (TiO2, Y2O3, CeO2, and Er2O3) by sputtering. The orange-to-red emission intensity decreased dramatically on applying an oxide coating for all four coating materials, whereas the UV and green emissions were affected little by the coatings. This indicates that surface luminescent centers generate the orange-to-red emission. In this study, the electronic energy state of the surface was calculated by first-principles calculations, using the discretevariational XR molecular orbital method. The calculation results reveal that the surface energy levels of ZnO provide favorable conditions for broad orange-to-red emission. 1. Introduction Zinc oxide (ZnO) has the potential to be used in numerous applications, including in semiconductor devices, transparent conducting oxides, chemical and gas sensors, and surface acoustic wave devices.1-4 Recently, ZnO has been actively investigated for use in optical devices, as it has a wide band gap (∼3.37 eV) and a high exciton-binding energy (∼60 meV).5-7 Ultraviolet (UV) lasers and UV light emitting diodes have been fabricated from ZnO nanorods arrays and thin films.8,9 ZnO has also been investigated as a phosphor material for displays because it exhibits strong visible-light emission. Although optical applications of ZnO are being actively investigated, some aspects of the luminescence properties of ZnO remain somewhat obscure.10-26 In particular, the emission mechanism of orange-to-red light has not been fully determined. Possible causes for the emission include defects such as interstitial oxygen atoms,17-20 a pair of interstitial oxygen and zinc ions,21 a pair of interstitial zinc ion and zinc vacancy,22,23 oxygen vacancies,24 and surface imperfections.25,26 In this study, we investigated the mechanism for orange-tored emission in ZnO, both experimentally and by first-principles calculations. Arrays of ZnO nanorods grown by a solution process were used in the experiments, coated with various oxides by a sputtering process. The electronic energy states at the (010) surface were calculated by a first-principles method using the discrete-variational (DV) XR molecular orbital method. 2. Experimental Methods ZnO nanorod arrays were prepared by a solution growth technique using ZnO thin films as seed layers. The seed layers were grown on glass substrates by radio frequency magnetron sputtering at room temperature. The chamber pressure was * To whom correspondence should be addressed. Phone 82-2-880-7167. Fax 82-2-884-1578. E-mail
[email protected]. † Seoul National University. ‡ SAMSUNG SDI CO., LTD. § Korea Basic Science Institute.
maintained at about 3 × 10-2 Torr by 95% Ar/5% O2 gas flow. Commercial ZnO powder (Cerac, purity: 99.9%, 200 meshed) was used as the target material. Sputtering for 60 min produced a ∼100-nm-thick ZnO thin film. The solution for growing nanorods was prepared by dissolving zinc nitrate hexahydrate (Junsei chemicals, extra pure) and hexamethylenetetramine (Samchun chemical, 99.0%) in distilled water; both solutes had concentrations of 0.05 M in the solution. A 10 mM concentration of polyethylenimine was added to the solution to grow nanorods with high aspect ratios. Glass substrates with ZnO seed layers were immersed in the solution and the temperature of the solution was maintained at 70 °C for 20 h. The substrate was then removed from the solution, washed with distilled water, and dried. The nanorod array samples were coated with various oxides (TiO2, Y2O3, CeO2, and Er2O3) by sputtering. The respective oxide powder (Sigma-Aldrich, >99.9%) was used as the target for coating with each oxide, except that a Ti metal target (Alfa Aesar, 99.9%) was used for coating with TiO2. The shape of each sample was observed by using a field-emission scanning electron microscope (FE-SEM; JEOL, 6360F). The photoluminescence (PL) emission was excited with 325-nm-wavelength radiation from a Xe lamp and measured with a doublemonochromator system (Acton Research Corp.). The emission was amplified by a photomultiplier tube (Acton, PHV 400) prior to recording the signal. The DV-XR molecular orbital method was used for firstprinciples calculations.27 This method involves numerically solving the Schro¨dinger equation for a many-body system based on the linear combination of atomic orbitals (LCAO) and the XR potential, using the Hartree-Fock-Slater method.28 Calculations were performed with the SCAT program of the DVXR software package, using the default values of all the parameters including the R value, which is 0.978 for H, 0.733 for He, 0.781 for Li, and approaches 2/3 as the atomic number increases. A [Zn21O21] cluster model was used for bulk and surface calculations. For the bulk calculation, the cluster was placed in a uniform Madelung potential field that consisted of
10.1021/jp106646e 2010 American Chemical Society Published on Web 09/27/2010
Photoluminescence Emission of ZnO Nanorod Arrays
Figure 1. PL emission spectra of ZnO nanorods with and without an oxide coating (λex ) 325 nm). (T) and (C) indicate spectra of samples subjected to the same processes as those for TiO2 and CeO2 coatings, respectively, but with the flux of the coating material blocked.
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Figure 2. SEM images of various ZnO nanorods: (a) uncoated, (b) coated with Y2O3, (c) coated with TiO2, and (d) sample (T) in Figure 1.
point charges of +2 and -2 corresponding to the positions of zinc and oxygen ions, respectively. For the surface calculation, the center of the Madelung field was shifted so that the field terminated at a (010) surface plane. Thus, electronic states on a (010) surface plane could be calculated. Only a (010) surface was analyzed in this study, which seem to be enough because the surfaces of ZnO nanorods are mainly composed of lateral {100} planes because of their morphological character: hexagonal pillar in shape and large aspect ratio. 3. Results and Discussion Figure 1 shows PL emission spectra of ZnO nanorod arrays with and without surface coatings. The uncoated sample showed visible light emission over a very wide range of approximately 400-800 nm as well as UV emission. After coating, visible light emission in the orange-to-red wavelength range decreased significantly, whereas green emission near 500 nm remained and the UV emission showed little change. These general features were observed for all four coating materials. There are a few possible mechanisms for the reduction in the orange-tored emission: (1) sample degradation by the coating process, (2) reduction in the transmissions of the exciting and emitting light, and/or (3) the emission originating at surface centers that are passivated by the oxide coatings. The first possibility can be eliminated because the emission properties do not vary with just exposing the ZnO nanorods to the environment of the coating process. For example, some nanorod samples were subjected to the same process as the coated samples, only without being coated. The coating flux was blocked by using a shutter in the sputtering chamber. The spectra labeled (T) and (C) in Figure 1 correspond to samples subjected to the same processes as the samples coated with TiO2 and CeO2, but did not receive the flux of coating units. Their emission intensities are almost the same as those of the nontreated sample. Figure 2 shows SEM images of various samples, revealing that their morphologies are unchanged by the coating process. These results indicate that the coating process itself, including exposure to vacuum and Ar flow, is not a cause of the reduction in emission. Optical transmittances of the coating materials were measured to examine the possibility that absorption of exciting or emitting light by the coating layers may decrease PL intensity. Thin films of the coating materials were formed on glass substrates with
Figure 3. Optical transmittances of oxide thin films sputtered on glass substrates under the same conditions as those used for coating the ZnO nanorods.
Figure 4. [Zn21O21] cluster model used for calculating the electronic energy states by the DV-XR molecular orbital method.
the same sputtering times and conditions as coating of nanorods, and the transmittances were measured. The results are shown in Figure 3. For all the thin films, the transmittances near 325 and 580 nm, which are the wavelengths of exciting and emitting
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Kim et al.
TABLE 1: Energy Levels and the Contribution of Atomic Orbitals to Each Energy Level Calculated by the DV-Xr Molecular Orbital Method: (a) Bulk Calculation and (b) Surface Calculation in Which the Madelung Field Was Modified to Terminate at a (010) Surface Planea energy (eV) Zn 3s Zn 3p Zn 3d Zn 4s Zn 4p O 2s O 2p VBM (HOMO) CBM CB-1 CB-2 CB-21 VBM (HOMO) S-1 S-7 CBM CB-1 CB-21
∼0 3.860 4.545 5.224 9.806 ∼
0 0.006 1.834 3.648 4.191 7.956
(a) bulk calculation 0.000 0.000 0.666 0.000 0.000 0.063 0.000 0.000 0.004 0.000 0.000 0.005 0.000 0.000 0.009
0.008 0.839 0.825 0.788 0.024
0.021 0.017 0.033 0.064 0.907
0.004 0.055 0.040 0.032 0.008
0.303 0.083 0.099 0.111 0.052
(b) surface calculation 0.000 0.000 0.106 0.000 0.000 0.094 0.000 0.000 0.030 0.000 0.000 0.011 0.000 0.000 0.006 0.000 0.000 0.003
0.115 0.329 0.489 0.556 0.298 0.317
0.074 0.238 0.288 0.224 0.582 0.566
0.002 0.003 0.006 0.026 0.011 0.013
0.704 0.337 0.188 0.184 0.104 0.102
a VBM and CBM denote the maximum level of valence band and the minimum in conduction band, respectively, and the energy levels of surface states and those in the conduction band are numbered in ascending order from the lowest energy level (S1-S7, CB1...). Numbers underlined indicate orbitals that make significant contributions to that energy level.
Figure 5. Electronic energy levels of ZnO calculated by the DV-XR method for (a) bulk and (b) surface cases. Broken lines indicate unoccupied levels and solid lines indicate filled levels.
lights, respectively, were over 70%. This finding means that the PL emission intensity of the coated samples should be at least ∼50% (70% × 70%) of that of the noncoated sample if loss of transmittance by the coating layers is assumed to be the main factor decreasing PL intensity. Moreover, as the ZnO nanorod arrays have a much larger surface area than the glass substrate, the thickness of the coating layers on the nanorods may be thinner than that of the films on the glass substrates and the optical absorption by the coating layers on ZnO nanorods would be smaller than that by thin films on the substrates. Therefore, the loss in transmittance cannot explain the drastic decrease of PL emission shown in Figure 1. This conclusion is also supported by the relatively small change in UV emission intensity after coating. On the basis of these results, it can be concluded that the orange-to-red emission is generated by surface luminescent centers. The reduction in emission may reflect the fact that the oxide coatings passivate the luminescent centers at the surface. UV and green emissions are relatively weakly affected by the coatings, indicating that their emission mechanisms are not strongly related to the surface. As for the nature of the surface luminescent centers, adsorbed gas molecules such as hydrogen, oxygen, or water vapor may be considered. We examined PL emission of various ZnO samples: nanorod arrays grown with different conditions, commercial powders from several makers, heat treated samples under various atomospheres, dried samples, and so on. The orange-to-red emission appeared from all the samples. This indicates that the surface emission is a general feature of ZnO and generated from an intrinsic factor of ZnO surface. The emission could not be identified only for the samples heated over 400 °C with hydrogen or CO/CO2 mixed gas flow, because it merged into the green emission band that was greatly enhanced. It seems that the green emission is generated by oxygen vacancies because heating under reducing atmosphere increases the intensity dramatically. Calculation of oxygen vacancy energy levels by the DV-XR method supported this interpretation. The detailed results will be reported in the near future. To obtain more information about surface energy levels, the electronic energy states of the ZnO surface plane were calculated
Figure 6. Density of states (DOS) diagrams calculated by the DV-XR molecular orbital method. Panels a and c are total DOS of the bulk crystal and the (010) surface, respectively. Zn 3d and Zn 4p projected DOS diagrams for the bulk and for the surface are presented in panels b and d.
Photoluminescence Emission of ZnO Nanorod Arrays
Figure 7. Contour maps of electronic wave functions for the orbital of CBM (conduction band minimum) energy level (a and b) and the orbital composed mainly of the Zn 3s atomic orbital (c and d) in the ZnO cluster model: (a and c) for bulk and (b and d) for surface cases.
by first-principles calculation. Figure 4 shows the cluster model employed in this study, [Zn21O21]. The DV-XR molecular orbital method was used to calculate the electronic energy levels in this crystal cluster. Table 1 summarizes the contribution of each orbital to various energy levels of interest. The calculated energy levels are plotted in Figure 5. Figure 5a shows the energy structure of bulk ZnO with its wide band gap. The band gap energy of bulk ZnO was calculated to be 3.86 eV; this is somewhat larger than the measured value (∼3.3 eV), but it seems to lie within a reasonable error range. We expect that this discrepancy would be reduced by employing a larger cluster model. The calculated energy levels in the upper valence band
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17897 are composed mainly of Zn 3d and O 2p orbitals (Table 1a), indicating that the levels are generated by an antibonding interaction between Zn and O (see also Figure 8a,c). The lower conduction bands (CB-1 to CB-20) are composed mainly of the Zn 4s atomic orbital with minor participation by O 2p, whereas the upper conduction bands (CB-21...) are composed mainly of Zn 4p. (The energy levels in the conduction band are numbered in ascending order from the minimum of the conduction band (CBM), in the form of CB-n.) For the surface calculations (see Figure 5b), several numbers of empty levels that are narrowly separated just above the highest occupied molecular orbital (HOMO level) are formed. Such energy levels render a suitable condition for visible emission over a wide wavelength range because various electronic transitions are possible. This might explain the broad spectrum of orange-to-red emission. The calculated surface electronic states thus support the hypothesis that the broad PL emission in the orange-to-red range originates from surface centers. The composition of the HOMO level and the empty levels for the surface differed from that of the bulk: the Zn 4s orbital participates in HOMO level construction, and Zn 4p, which was related only to the higher levels in the conduction band for the bulk case (the levels over the CB-21 level), affects all the empty levels. This feature can be shown clearly in the density of states (DOS) diagrams presented in Figure 6. From the comparison between panels b and d of Figure 6, which show Zn 3d and Zn 4p projected DOS diagrams for bulk and for surface calculations, respectively, it appears that the influence range of Zn 4p is expanded to the lower energy, while that of Zn 3d changed little. At the crystal surfaces, local distortion of the potential field due to termination of the crystal structure polarizes the wave
Figure 8. Overlap population diagrams. In each figure, lines in the positive region show the amount of bonding interaction while those in the negative show antibonding. (a) Zn-O interaction of the bulk calculation, (b) Zn-Zn interaction of the bulk, (c) Zn-O interaction of the surface, and (d) Zn-Zn of the surface.
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TABLE 2: The Bond Overlap Populations (BOPs) among the Atoms in ZnO Calculated for the Bulk and the Surface by the DV-Xr Molecular Orbital Method
Zn-O Zn-Zn O-O
BOP (bulk calculation)
BOP (surface calculation)
diff (%) (BOPsurface - BOPbulk)
0.2285 0.0018 -0.0143
0.2059 0.0035 -0.0120
-9.89 +94.4 +16.1
functions, resulting in hybridization among the electronic orbitals and a large shift in their energy.28 The outer orbitals (Zn 4s and Zn 4p) would be strongly affected by the change of the field. This would produce a different trend between Zn 4s and Zn 3d orbitals. Figure 7 shows contour maps of the wave functions of the CBM and Zn 3s orbital in ZnO. The energy of the Zn 3s orbital is far below the HOMO level. Comparison of wave function shapes of the CBM level shows that contours are nonsymmetric for the case of the surface (Figure 7b), whereas the contour maps of wave functions of the bulk appear periodic (Figure 7a). This indicates that the wave functions are distorted at the surface. However, the diagrams of the Zn 3s are similar for both calculations because the inner orbital is affected little by distortion of the outer potential field. The overlap population diagrams for Zn-O and Zn-Zn are presented in Figure 8. The diagram shows bonding and antibonding interaction separately, thus providing useful information about the nature of interactions among the atoms of interest. Panels a and c of Figure 8 show Zn-O interaction for the bulk and the surface, respectively. The large bonding component and relatively smaller antibonding in the valence band indicate that there exists a strong bonding nature between Zn and O for both cases. For the Zn-Zn case (Figure 8b,d), the peaks in the valence band are very small, showing that interactions among Zn atoms play only a limited role in constructing the ZnO structure compared to those between Zn and O atoms. The bond overlap population (BOP) is defined as effective bonding interaction between atoms. It can be calculated by integrating the bonding and antibonding curves over the electron-filled energy range (under HOMO level) and finding the difference of the integration areas. The calculation results are given in Table 2, which show that Zn-O bonding is weakened on the surface, while Zn-Zn bonding interaction increases. It seems that Zn atoms have more chance to form covalent molecular orbitals because the influence ranges of Zn 4s and Zn 4p orbitals become wider on the surface as pointed out above. This can be again confirmed from the comparison of Zn-Zn interaction ranges in the bulk (Figure 8b) and the surface (Figure 8d) calculations. 4. Conclusions The visible emission of orange-to-red wavelengths from ZnO nanorod arrays was strongly reduced by applying an oxide coating. This occurred for various coating materials. The loss in transmittance due to the coatings was not very large. These results suggest that the emission is related to surface luminescent centers that are passivated by the oxide coating. The electronic energy states of the ZnO surface were calculated by firstprinciples calculations, using the DV XR molecular-orbital method. The result shows that empty levels are formed with small energy gaps just above the HOMO level. This situation is favorable for generating a broad emission spectrum of visible light, as the energy levels with narrow energy gaps can generate
Kim et al. various emission wavelengths during electronic transitions. Local distortion of the potential field at the surface affects the outer orbitals (Zn 4s and Zn 4p) and causes hybridization among the electronic orbitals, resulting in formation of the surface energy levels. This interpretation appears reasonable from the analysis of density of states, contour maps of wave functions, and bond overlap populations. Thus, based on the experimental and theoretical results, we conclude that the broad orange-tored emission in ZnO originates from luminescent centers at the surface. Acknowledgment. This work is a product of the Manpower Development Program for Energy & Resources (No. 2008EAPHMP 1300002009) supported by the Ministry of Knowledge and Economy (MKE), Korea, and was partially supported by the IT R&D program of MKE/IITA (No. 0414-20090013). We also acknowledge the use of facilities at the Research Institute of Advanced Materials, Seoul National University, and the Molecular Modeling System at the Suncheon Center of the Korea Basic Science Institute. Supporting Information Available: Photoluminescence emission spectra of various ZnO samples which include commercial powders from different makers, heat-treated samples under various atmospheres, and nanorod samples grown with different conditions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kadota, M. Jpn. J. Appl. Phys., Part 1 1997, 36, 3076. (2) Liu, J.; Huang, X.; Li, Y.; Sulieman, K. M.; Sun, F.; He, X. Scr. Mater. 2006, 55, 795. (3) Djurisˇic´, A. B.; Leung, Y. H. Small 2006, 2, 944. (4) Kim, Y.; Kang, S. Mater. Lett. 2009, 63, 1065. (5) Zamfirescu, M.; Kavokin, A.; Gil, B.; Malpuech, G.; Kaliteevski, M. Phys. ReV. B 2002, 65, 161205. (6) Comini, E.; Faglia, G.; Sbervegliery, G.; Pan, Z.; Wang, Z. Appl. Phys. Lett. 2002, 81, 1869. (7) Oztur, U.; Alivov, Y.; Liu, C.; Teke, A.; Reshchikov, M.; Dogan, S.; Avrutin, V.; Cho, S.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301. (8) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (9) Huang, H.; Fang, G.; Xiaoming, M.; Long, H.; Yuan, L.; Dong, B.; Meng, X.; Zhao, X. IEEE Electron DeVice Lett. 2009, 30, 1063. (10) Dingle, R. Phys. ReV. Lett. 1969, 23, 579. (11) Jeong, S.; Kim, B.; Lee, B. Appl. Phys. Lett. 2003, 82, 2625. (12) Usui, H.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. J. Phys. Chem. B 2005, 109, 120. (13) Vanheusden, K.; Seager, C.; Warren, W.; Tallant, D.; Voigt, J.; Gnade, B. J. Appl. Phys. 1996, 79, 7983. (14) Zhang, S.; Wei, S.; Zunger, Z. Phys. ReV. B 2001, 63, 075205. (15) Kim, Y.; Kang, S. J. Phys. Chem. B 2010, 114, 7874. (16) Chen, Y.; Jiang, J.; He, Z.; Su, Y.; Cai, D.; Chen, L. Mater. Lett. 2005, 59, 3280. (17) Pal, U.; Santiago, P. J. Phys. Chem. B 2005, 109, 15317. (18) Patra, M.; Manzoor, K.; Manoth, M.; Vadera, S.; Kumar, N. J. Lumin. 2008, 128, 267. (19) Liu, M.; Kitai, A.; Mascher, P. J. Lumin. 1992, 54, 35. (20) Wu, X.; Siu, G.; Fu, C.; Ong, H. Appl. Phys. Lett. 2001, 78, 2285. (21) Heo, Y.; Norton, D.; Pearton, S. J. Appl. Phys. 2005, 98, 073502. (22) Wei, X.; Man, B.; Xue, C.; Chen, C.; Liu, M. Jpn. J. Appl. Phys. 2006, 45, 8586. (23) Usui, H. J. Phys. Chem. C 2007, 111, 9060. (24) Ahsanulhaq, Q.; Umar, A.; Hahn, Y. Nanotechnology 2007, 18, 115603. (25) Fan, H.; Scholz, R.; Klob, F.; Zacharias, M. Appl. Phys. Lett. 2004, 85, 4142. (26) Guo, L.; Yang, S. Appl. Phys. Lett. 2000, 76, 2901. (27) Adachi, H.; Tsukada, M.; Satoko, C. J. Phys. Soc. Jpn. 1978, 45, 875. (28) Adachi, H.; Kowada, Y.; Tanaka, I.; Nakamatsu, H.; Mizuno, M. Hajimeteno Denshijoutai Kaisan; Sankyo-Shuppan: Tokyo, Japan, 1998.
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