J. Phys. Chem. C 2007, 111, 17265-17267
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Strong Surface Effect on Cathodoluminescence of an Individual Tapered ZnO Nanorod Nan Pan, Xiaoping Wang,* Ming Li, Fanqing Li, and J. G. Hou Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: August 21, 2007; In Final Form: September 19, 2007
Spatial-resolved cathodoluminescence spectra are collected at spots with different diameters on individual single-crystalline tapered ZnO nanorods to study the size-dependent surface effect. A significant increase in the intensity ratio of the deep level to the near band edge emission is observed with ever-increasing nanorod surface-aspect ratio. This ratio shows notable superlinear behavior especially on the larger surface-aspectratio side, which can be well understood with consideration of both surface effect and self-absorption. Further discussion reveals that surface recombination dominates the deep-level emission. Our results indicate that the surface effect is crucial to modulation of the physical properties of nanostructures even before their sizes close to quantum size regime and the self-absorption effect must be taken into account for quasi-quantitative analysis at the nanoscale level, which may find its practical applications in the design of functional nanodevices.
Introduction ZnO 1D nanostructures possess a brilliant future for versatile applications because of their remarkably peculiar properties. They have been studied intensively for years and have been regarded as one of the most promising materials in nanoscale electronic,1-3 optoelectronic,4-7 sensing,8,9 energy transfer,10-12 piezoelectric,13-15 and field-emitting16 devices. However, the understandings on how the physical properties are affected by shrinking diameter are still quite limited despite their importance of putting the 1D ZnO nanodevice in practice.17-19 A surfacerelated anomalous blueshift in the near band edge (NBE) emission of ZnO nanorods with size shrinkage has been observed by both photoluminescence (PL) and cathodoluminescence (CL).18,19 The intensity ratio of the deep level (DL) to NBE emission, which is very useful in exploring the origin of DL emission and the distribution of recombination centers, was also found to be size related in ZnO nanorods by PL. Shalish et al.17 found that the ratio increases with decreasing size, whereas the result from Chang et al.19 contradicts with them; both studies used nanowire bulks for PL measurement. Considering that nanowires of different diameters or different batches underwent different synthesis conditions, this will probably bring confusion to the true surface effect. As far as we know, no systematical experiment has been performed on individual ZnO nanorods to investigate the surface effect on both DL and NBE recombination, and no effort has been stressed on revealing the recombination paths and their distribution responsible for DL emission by studying the surface effect. Here we present a CL study on tapered ZnO nanorods whose diameters shrink continuously from several hundred nanometers to ∼20 nm. Although the diameter in this region is still too large to meet the quantum confinement regime, it is small enough to show a dramatically enhanced surface effect. CL spectra are collected at spots with different diameters on individual tapered nanorods so that variance in samples (of different batches) and the averaged effect in assembled nanorods * To whom correspondence should be addressed. E-mail: xpwang@ ustc.edu.cn. Tel: +86-551-3607090. Fax: +86-551-3606266.
Figure 1. (a) FESEM image of an as-dispersed tapered nanorod. (b) HRTEM image of a tapered nanorod; the corresponding FFT pattern is shown in the inset. (c and d) CL images of the same nanorod in a respectively recorded at 380 and 500 nm.
can be effectively eliminated. Our results demonstrate that surface recombination dominates the nanorod DL emission, and the surface effect brings strong modulation on nanorod luminescence, which will be entirely dominant when the nanorod diameter shrinks to a critical value. These results are vital for device design based on nanostructures with ever decreasing size. Experimental Section Single-crystalline tapered ZnO nanorods are synthesized by a vapor-phase transport and condensation method on the basis of carbon-thermal reduction reaction. Details of the preparation were described elsewhere.20 For the CL study, as-grown nanorods are mechanically scratched off the growth substrate and transferred onto a cleaned silicon wafer. Afterward, clean and dry nitrogen gas flow is used to blow the nanorod-covered silicon wafer for several seconds to avoid nanorods from aggregation and entanglement. The dispersion quality is checked by field emission scanning electron microscopy (FESEM), and only those well-dispersed nanorods are selected for subsequential study. CL experiments are performed on a Gatan Mono-CL3 system attached to a FEI SIRION-200 FESEM, with an
10.1021/jp076708i CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2007
17266 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Pan et al.
Figure 2. (a) FESEM image of a tapered nanorod with four spots marked by white arrows. (b and c) CL spectra from the four spots using arbitrary intensity and intensity normalized to NBE emission peak, respectively. (d) Variations of IDL (hollow circle), INBE (hollow square), and IDL/INBE (solid triangle) with decreasing diameter.
accelerating voltage at 10 kV and a constant beam current at ∼1 nA to avoid damage to nanorods. All CL images and spectra are collected at room temperature under the same conditions to ensure convictive comparison. Results and Discussion The morphology of as-transferred nanorod is characterized by FESEM, and the crystal structure is examined by highresolution transmission electron microscopy (HRTEM). A typical nanorod is shown in Figure 1a. As seen, the length of the nanorod exceeds 5 µm and the diameter shrinks smoothly and gradually from ∼190 nm to ∼25 nm, which facilitates us to study the surface effect in a single nanorod. A hexagonal wurtzite lattice is clearly shown in Figure 1b with no observable defects or strains, indicating high crystallization quality throughout the nanorod. The corresponding fast Fourier transform (FFT) pattern in the inset of Figure 1b indicates that the nanorod is grown along the axis. Figure 1c and d shows CL images of the same nanorod in Figure 1a respectively recorded at 380 and 500 nm, suggesting that neither NBE nor DL emission is negligible. To study the surface effect, CL spectra are collected at spots with different diameters on a series of individual nanorods. CL spectra on a representative nanorod are shown in Figure 2.21 As seen in Figure 2a, four spots are marked by white arrows and numbered with 1-4 in descending diameter order. CL spectra collected from these spots are shown in Figure 2b in which the emissions at ∼380 nm and ∼500 nm are generally ascribed to NBE and DL recombination, respectively. Although both the integral intensity of NBE (INBE) and that of DL (IDL) emission decrease with shrinking diameter, INBE decreases notably faster. This feature can be clearly observed in the normalized CL spectra to the NBE emission peak in Figure 2c. To further reveal the difference between these two peaks, variations of INBE, IDL, and their ratio IDL/INBE with diameter are plotted together in Figure 2d. As seen, while the diameter shrinks from ∼100 nm to ∼30 nm, IDL/INBE increases by an order of magnitude, indicating that DL emission becomes more dominant at spots with smaller diameters. Though the exact origin of DL emission is still not clear, it is often related to recombination from bulk or surface defects.17,19,22 Supposing that the recombination centers responsible for DL emission distribute uniformly in a nanorod, IDL/INBE will be independent of diameter, which is obviously opposite to our observations. Consequently, we believe that the origin of DL emission is surface-related and the monotonic diameter-dependent change in IDL/INBE is due to surface modulation because a nanorod with a smaller diameter holds a larger surface-aspect ratio (R); thus, the surface effect enhances.17 Most recently, Zhou et al.22 have revealed that DL emission in ZnO nanostructures exhibits a strong dependence on the surface crystal plane. DL emission from the polar surfaces
Figure 3. IDL/INBE as a function of reciprocal of nanorod diameter: experimental data (hollow square) and fitting curves based on the model proposed in ref 17 (dot line), model ignoring the self-absorption effect (dash line), and our model (solid line). The inset is the scheme of surface-recombination-layer approximation.
(101h1) and (0001) are much stronger than that from the nonpolar (101h0). This is because nonpolar surfaces are electrically neutral and have relatively low surface energy so that very low defect could present on these surfaces, while the polar surfaces hold surplus charges that must be eliminated by segregation, relaxation, reconstruction, or absorption to form stable surfaces so that plenty of surface defects are introduced. In our experiments, because the nanorod is smoothly tapered and grown along its axis, the side surface of the nanorod must be terminated by a polar crystal plane with high indices. Similarly, notable DL emission is observed from this polar surface in our experiments. This is consistent with ref 22 and can be also attributed to the defects introduced onto the nanorod surface during polar surface stabilization (surplus charges elimination), such as surface oxygen vacancies. We also note a discrepancy in a recent report by Chang et al.23 where IDL/INBE was found to decrease with the diminishing nanoneedle diameter. Because their samples were prepared by a hydrothermal downward process and ours by vapor-phase epitaxial growth, this diversifying trend can perhaps be attributed to the difference in the two synthesis methods. Also, the discrepancy may partially be related to the averaged effect and variance in samples in their measurements, which will certainly affect the CL intensity. Quasi-quantitative analysis of surface modulation on nanorod luminescence has been executed by surface-recombination-layer approximation.17 Under simplification, a linear relation between IDL/INBE and reciprocal of diameter d-1 (proportional to R) has been deduced. Our IDL/INBE data collected at a series of representative nanorods are denoted by hollow squares in Figure 3. In contrast with the previous result,17 the data show a notable deviation from a linear relation to superlinear with d-1especially when d-1 becomes larger. This deviation is possibly caused by the self-absorption effect of the nanorod, which is wavelengthrelated and size-dependent.24 To further understand our observations, we proposed an improved model with consideration of both the surface effect and self-absorption by which IDL/INBE can be described as the following expression25
Strong Surface Effect on Cathodoluminescence
(
)
1 t 0.2 + 1IDL d d t × 4ws ) 0.643 × × + wb INBE 1 2t 2 d 0.05 + 1d d
(
)
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17267
(1)
where d is the nanorod diameter, t is the depth that defines the effective volume beneath nanorod surface that contributes to surface recombination entirely, and ws and wb (wb + ws ) 1) are the weight of surface and body recombination contributive to DL emission, respectively. The scheme of this model is given in the inset in Figure 3. As seen in eq 1, the expression comprises three items. The first item, a constant of 0.643, denotes a collection efficiency difference at 380 and 500 nm, which can be determined by linear fitting the experimental data within a proper ranges of d-1 (typically 0.005-0.02 nm-1 where the curve shows satisfying linear behavior).17 The second item is a correction account for self-absorption in the nanorod.24,25 This is based on the fact that ZnO has a large absorption coefficient of ∼2 × 105 cm-1 at ∼380 nm while a smaller one of ∼5 × 104 cm-1 at ∼500 nm,26 different self-absorption losses will lead to different light extraction efficiencies at the two wavelengths. The last item in the bracket describes the surface effect. A series of variational t and ws are chosen to fit the experimental data.27 The optimized fitting curve (t ) 9.9 nm and ws ) 0.836) is denoted by solid line in Figure 3; for comparison, the fitting results based on a model ignoring the self-absorption effect and the model proposed in ref 17 (on the assumption that t , d) are also shown. As seen, our model can fit the experimental data much better than other two models, indicating that self-absorption must be taken into account at the nanoscale level and the presupposition of t , d is inapplicable in smaller diameter regions. The optimized fitting parameters in our model suggest that surface recombination exclusively prevails within a thin volume of ∼10 nm depth beneath the nanorod surface;28 the optimized ws of ∼0.84 indicates the contribution of surface recombination to DL emission is predominant while body recombination takes just minor role. The results also imply that the surface recombination layer tends to run through the whole nanorod when the diameter of the nanorod keeps shrinking and is comparable with ∼10 nm; in this case, surface recombination will entirely dominate the luminescence. Therefore, this strong surface modulation must be considered and controlled carefully in the design and fabrication of nanosized ZnO devices. In conclusion, the surface effect on the luminescence property is comparatively studied by spatial-resolved CL spectra collected at spots on individual tapered ZnO nanorods. Strong surface modulation is found to prevail with ever-decreasing nanorod diameter, behaving as remarkable superlinear increase in intensity ratio IDL/INBE with increasing nanorod surface-aspect ratio. This behavior can be well described by an improved model with consideration of both the surface effect and self-absorption. Further discussion demonstrates that DL emission is dominated by surface instead of body recombination, and the luminescence will be entirely dominated by the surface effect when the nanorod diameter shrinks to a critical value. These results imply that surface-state modulation is extremely important in nanomaterials even far beyond the quantum size regime and must
be tailored and controlled properly for nanoscale device engineering. Acknowledgment. We thank Mr. Jinghang Chen for his assistance with optimizing the fitting parameters by computer programming. This work is supported by the National Natural Science Foundation of China under Grant Nos. 60676030, 90406009, and 50121202. X. P. Wang also acknowledges the support from CAS and MOST (2006CB922002) of China. References and Notes (1) Park, W. I.; Kim, J. S.; Yi, G. C.; Lee, H. J. AdV. Mater. 2005, 17, 1393. (2) Ju, S. H.; Lee, K.; Janes, D. B.; Yoon, M. H.; Facchetti, A.; Marks, T. J. Nano. Lett. 2005, 5, 2281. (3) Keem, K.; Jeong, D. Y.; Kim, S.; Lee, M. S.; Yeo, I. S.; Chung, U. I.; Moon, J. T. Nano. Lett. 2006, 6, 1454. (4) Kind, H.; Yan, H. Q.; Messer, B.; Law, M.; Yang, P. D. AdV. Mater. 2002, 14, 158. (5) Ko¨nenkamp, R.; Word, R. C.; Godinez, M. Nano. Lett. 2005, 5, 2005. (6) Bao, J. M.; Zimmler, M. A.; Capasso, F.; Wang, X. W.; Ren, Z. F. Nano. Lett. 2006, 6, 1719. (7) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. Nano. Lett. 2007, 7, 1003. (8) Fan, Z. Y.; Lu, J. G. Appl. Phys. Lett. 2005, 86, 123510. (9) Wei, A.; Sun, X. W.; Wang, J. X.; Lei, Y.; Cai, X. P.; Li, C. M.; Dong, Z. L.; Huang, W. Appl. Phys. Lett. 2006, 89, 123902. (10) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. (London) 2005, 4, 455. (11) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (12) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano. Lett. 2007, 7, 1793. (13) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (14) Buchine, B. A.; Hughes, W. L.; Degertekin, F. L.; Wang, Z. L. Nano. Lett. 2006, 6, 1155. (15) Wang, Z. L. AdV. Mater. 2007, 19, 889. (16) Zhao, Q.; Zhang, H. Z.; Zhu, Y. W.; Feng, S. Q.; Sun, X. C.; Xu, J.; Yu, D. P. Appl. Phys. Lett. 2005, 86, 203115. (17) Shalish, I.; Temkin, H.; Narayanamurti, V. Phys. ReV. B 2004, 69, 245401. (18) Chen, C. W.; Chen, K. H.; Shen, C. H.; Ganguly, A.; Chen, L. C.; Wu, J. J.; Wen, H. I.; Pong, W. F. Appl. Phys. Lett. 2006, 88, 241905. (19) Chang, P. C.; Chien, C. J.; Stichtenoth, D.; Ronning, C.; Lu, J. G. Appl. Phys. Lett. 2007, 90, 113101. (20) Geng, C. Y.; Jiang, Y.; Yao, Y.; Meng, X. M.; Zapien, J. A.; Lee, C. S.; Lifshitz, Y.; Lee, S. T. AdV. Funct. Mater. 2004, 14, 589. (21) The CL experiment is performed on a series of dispersed individual tapered ZnO nanorods; the spectra and the trends of their changes with diameter are fairly consistent. (22) Zhou, X.; Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.; Xu, T.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. C 2007, 111, 12091. (23) Chang, Y. C.; Chen, L. J. J. Phys. Chem. C 2007, 111, 1268. (24) Yacobi, B. G.; Holt, D. B. Cathodoluminescence Microscopy of Inorganic Solids; Plenum: New York, 1990, and references therein. A correction factor account for sample self-absorption can be expressed as fA ) ICL(obs)/ICL(gen) ) (1 + RL)-1, where R is the material absorption coefficient at certain wavelength and L is related to the diameter of the 1D nanostructure. (25) This expression is derived from IDL/INBE ) ηDL/ηNBE × fA(DL)/fA(NBE) × (wsVsurface + wbVbody)/Vbody, where η denotes the total collection efficiency at a certain wavelength, fA ≈ (1 + Rd)-1, Vsurface ) π[(d/2)2-(d/2 - t)2]l and Vbody ) π(d/2 - t)2l. (26) Landolt-Bo¨rnstein, New Series, Group III, Vol. 41, Pt. B; SpringerVerlag: Berlin, 1999. (27) By programming, a series of t and ws values are generated to fit the experimental data. The fitting quality of each pair of t and ws values is n [(IDL/INBE)i(fit) - (IDL/ evaluated by an error function defined as σ ) ∑i)1 2 INBE)i(exp)] /(n - 1), minimum of σ is obtained if t ) 9.9 nm and ws ) 0.836. (28) This depth strongly relies on the density and distribution of surface states, which could vary notably in samples undergoing different fabrication/ treatment processes.
