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By Xiaobing Han, Liangzhi Kou, Xiaoli Lang, Jianbai Xia, Ning Wang, Rui Qin, Jing Lu, Jun Xu, Zhimin Liao, Xinzheng Zhang, Xudong Shan, Xuefeng Song, Jingyun Gao, Wanlin Guo,* and Dapeng Yu*
Semiconductor nanowires (NWs) are currently attracting a great deal of interest as they are expected to play an important role in the development of nanometer-scale technologies.[1] As zinc oxide (ZnO) is a typical II–VI semiconductor, ZnO NWs have aroused considerable notice because of their unique wide-bandgap semiconducting, piezoelectric, and photoelectric properties.[2] Recently, the energy-converting ability of ZnO NWs has been attracting intense attention as a result of their electronic– mechanical coupling behavior.[3] A huge number of experimental and theoretical studies have been devoted to understanding their structure, properties, and novel behavior;[4–6] the conductance of ZnO NWs has been shown to decrease with increasing strain.[7] However, how mechanical deformation can tune their electronic band structures remains elusive. Strain engineering of semiconductors has long been an important technique.[8] Strained silicon exhibits enhanced electron mobility,[9] and the optical and electronic properties of silicon NWs can be tailored through strain.[10] Tensile, bending, and torsional deformation of carbon nanotubes can significantly tune their electronic and magnetic properties.[11] Similarly, besides ZnO NWs being piezoelectric, the electronic and optical properties of ZnO NWs are also sensitive to mechanical strain,[12] but understanding the atomic-level mechanism of the intriguing multi-field coupling properties of this attractive low-dimensional material remains
[*] Prof. D. P. Yu, X. B. Han, R. Qin, J. Lu, J. Xu, Z. M. Liao, X. Z. Zhang, X. D. Shan, X. F. Song, J. Y. Gao State Key Laboratory for Mesoscopic Physics and Electron Microscopy Laboratory Department of Physics, Peking University Beijing 100871 (P. R. China) E-mail: [email protected] Prof. W. L. Guo, L. Z. Kou Institute of Nanoscience Nanjing University of Aeronautics and Astronautics Nanjing 210016 (P. R. China) E-mail: [email protected] X. L. Lang, Prof. J. B. Xia State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors, Chinese Academy of Sciences PO Box 912, Beijing 100083 (P. R. China) Prof. N. Wang Physics Department Hong Kong University of Science and Technology ClearWater Bay, Kowloon, Hong Kong (Hong Kong)
DOI: 10.1002/adma.200900956
Adv. Mater. 2009, 21, 4937–4941
COMMUNICATION
Electronic and Mechanical Coupling in Bent ZnO Nanowires
a great challenge for both theoretical and experimental researchers. In this Communication, we report how we manipulated single ZnO NWs into different shapes to create bending strain in both optical and transmission electron microscopy (TEM) environments, and that strong electronic–mechanical coupling in the NWs was found using cathodoluminescence (CL). Significant red shift and broadening of the near-edge emission in CL spectra along the bent ZnO NWs were observed. The atomic mechanism for bending deformation and strain-induced change in the electronic band structure of the NWs is revealed by lattice analysis. Theoretical calculations using both first-principles density functional theory (DFT) and effective mass envelope function theory for uniaxial stress states are used to explain the bending-induced variation in bandgaps. The ZnO NWs investigated in this work were prepared by vapor phase deposition, as described in detail in the literature.[13] The growth direction of the ZnO NWs is along the [001] c-axis. After dispersion in ethanol, ZnO NWs (100–250 nm in diameter) were transferred onto a Si substrate with a 500 nm SiO2 layer. Straight ZnO NWs of interest were bent under an optical microscope for CL investigations using a glass tip. The strong interaction between the substrate and the NWs keeps the NWs in the curved shape. The tensile and compressive strain at the outer and inner edges, respectively, of the bent NW can be estimated by its local radius of curvature r and diameter D as e ¼ D/2r. Therefore, the bending strain increases linearly with decreasing radius of curvature and increasing diameter of the NWs. The electronic bandgaps of the bent NWs were then carefully measured using CL spectroscopy (Gatan monocle 3þ) of high spatial (100 nm for ZnO at electron beam energy of 9 keV) and spectral (0.5 nm) resolution. To enhance the measuring resolution, most of the CL experiments were carried out in liquid nitrogen at ca. 81 K. Without going into details, all measurements were made by spot-scanning along the NWs with the beam focus at the center of the NWs. An optimal setting was chosen (electron beam energy 9 kV and spot size of 4) so that the spatial resolution of the CL was the highest, to obtain a high signal-to-noise ratio.[14] We focused our attention on the near-edge UV emission band between 3.0 and 3.8 eV. A typical bent ZnO NW (ca. 150 nm in diameter, and 50 mm in length) manipulated into an L-shape (with minimum radius of curvature of 3.0 mm) is shown in Figure 1a. A series of CL spectra were collected from the bent NW by scanning along its axis spot-by-spot in steps of 400–1000 nm, as indicated by colored
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Figure 1. CL measurements from a ZnO NW bent into an L-shape (a–c; diameter ca. 150 nm) at 81 K and an S-shape (d,e; diameter ca. 140 nm) at room temperature. a) Scanning electron microscopy (SEM) image of the NW; colored arrows indicate the CL measurement positions. b) CL spectra along the bent NW shown in the corresponding color for each arrowed position in (a). (Not all arrows are shown in (a).) c) FWHM of near-edge peaks of the CL spectra in (b) as a function of position along the NW. d) SEM image of the NW (left) and representation of CL spectra at different positions along the NW (right). e) Representative CL spectra revealing the red shift of the peak position along the bent NW.
Figure 2. TEM analysis of a bent ZnO NW (diameter ca. 100 nm). a) Low-magnification TEM image of the NW. b) Typical strain contrasts in the bent region within the area indicated by the red square in (a). c) Typical diffraction pattern in the bent region. d,f) High-resolution TEM images at the outer (green square in (b)) and inner (purple square in (b)) edges in the bent region, which show relatively perfect crystal structure. e,g) FFT diagrams of (d) and (f) show the lattice parameters of the deformed ZnO NW. The standard c/a ratio is ca. 1.6 for a strain-free bulk sample.
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arrows in Figure 1a. The representative CL spectrum acquired at the position of each arrow is shown in Figure 1b in the corresponding color. At liquid nitrogen temperature, the intrinsic CL peaks appear very sharp and intense. It is clear that the CL spectra collected from the straight sections have the intrinsic peak position of ca. 3.37 eV, the same as that of bulk ZnO. On entering the bent region, a red shift of the UV band from the intrinsic value occurs and increases with decreasing local radius of curvature up to a maximum of ca. 50 meV at the most bent section (with r ¼ 3.0 mm and e ¼ 2.5%). It is interesting that the bending-induced red shift is accompanied by broadening of the UV band, as shown by the variation in the full width at half maximum (FWHM) of the UV peak in Figure 1c. The same trend can be observed in the CL spectra measured at room temperature in a ZnO NW bent into an S-shape as shown in Figures 1d,e. As the radius of curvature reaches two minima along the S-shaped wire (Fig. 1d), the UV peak shifts twice from the intrinsic value and reaches two valley values, forming a W-shaped peak profile, as shown in the right-hand panel of Figure 1d. Measurements on more samples bent into different shapes presented in Figures S1 and S2 in the Supporting Information show the same trend. The reason for the broadening of FWHM in the bent region will be discussed in detail later. To reveal the atomic mechanism of the bending deformation in ZnO NWs, in situ microstructure analyses of bent NWs were conducted in a transmission electron microscope (Tecnai F30). The method we used to bend a ZnO NW on a TEM grid is similar to the method reported by Han et al.[15] That is, the ZnO NWs were dispersed on a TEM grid with a holey carbon supporting film. A selected NW of interest was bent as follows: A small hole was made in the holey carbon supporting film near the NW, and then the electron beam was focused to lacerate the carbon film above the NW. Under electron beam irradiation, the carbon film will crimp. As a result of the laceration force, the NW bends, which provides the opportunity to evaluate its atomic structure under bending, as shown in Figure 2a. The TEM image demonstrates that the ZnO NWs were grown along the [001] c-axis with a diameter of ca. 100 nm. A high-magnification TEM image such as that in Figure 2b demonstrates that the bent NW shows typical inhomogeneous strain contrast. The corresponding selected-area electron diffraction (SAED) pattern (Fig. 2c) revealed that some of the diffraction spots appear streaky
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and rotational arc-shaped, which indicates that the corresponding lattice planes suffer severe bending deformation. This is further demonstrated in high-resolution TEM images (Fig. 2d,f) taken at the outer (green square in Fig. 2b) and inner (purple square in Fig. 2b) edges of the bent area. The continuous lattice fringes are strain-contrasted, which manifests the inhomogeneous strain distribution in the bent NW. The lattice parameters c/a and a for Figures 2d and f were derived from the corresponding fast Fourier transforms (FFT) shown in Figures 2e and g, respectively. The above analysis indicates that the bent NW undergoes bending deformation, with the inner side under compressive strain and outer side under tensile strain, and the estimated strains are e ¼ (couter – cinner)/(couter þ cinner) ¼ 1.99%, respectively. We also found that such a bending deformation is elastic because it can be released with a drop of ethanol, and the bent NW recovers its straight shape, as demonstrated in Video 1 in the Supporting Information. To understand the above-demonstrated correlation between the peak shift of the UV band and the bending strain, first-principles DFT simulations have been performed on both bulk and [0001]-oriented NW ZnO under pure tensile and compressive strains to evaluate the strain-induced change in the bandgap. In the simulations, the ZnO NWs were strained axially. All the structural optimizations and energy calculations were performed based on pseudopotentials with localized atomicorbital basis sets within the Perdew–Burke–Ernzerhof general gradient approximation implemented in the code SIESTA.[16,17] The calculation results for ZnO NWs (with diameter range from 0.97 to 2.27 nm) and bulk ZnO crystal in the strain range of 5% to 5% are presented in Figure 3a, where the energy gap for the strained sample is normalized by the value of the strain-free sample. For both NW and bulk crystal, the bandgap decreases with increasing tensile strain. To obtain a direct estimate of the theoretical effect of pure bending strain on the band structure, the bandgap was also calculated as a function of the bending radius of curvature in the framework of a six-band effective-mass envelope function theory.[18] We used the same method and parameters as in a previous publication[19] except for the introduction of a strain term Hstrain in the Hamiltonian as done by Wrzesinski and Fro¨hlich.[20] To focus on strain-induced change in the bandgap, we fixed the top of the conduction band and calculated the shift in the valence band. Figure 3b shows the bandgap for a 150 nm ZnO NW as a function of radius of curvature under pure bending deformation. Significant gap reduction is obtained with the decrease of the radius of curvature. When the radius is reduced to ca. 3.6 mm, the bandgap decreases from 3.37 eV (bulk) to 3.32 eV, which is in agreement with the experiments for the L-shaped NW shown in Figure 1b. Now we will focus our attention on the conduction band. As shown in the right-hand inset of Figure 3b, the gaps between the 70 subbands (quantized states) in the conduction band minima will broaden when the NW bends into a radius of curvature of ca. 3.6 mm from a straight strain-free one. Therefore, the observed broadening of the near-edge UV peak originates from the broadening of the gaps between subbands in the conduction band under bending deformation. To confirm the theoretical correlation between the band-gap energy and the applied compressive/tensile strain, the contributions from the compressive and tensile strain parts in bent NWs
Figure 3. The variation of band structure with deformation. a) Results of DFT for c-axial strain-induced change in bandgap of ZnO NWs containing 48, 108, and 192 atoms in the cross section with diameters of 0.97, 1.62, and 2.27 nm, respectively. Inset: Sketch of the NW section. The data for the strained bulk ZnO crystal was measured in the same direction. b) Results from the effective-mass envelope-function theory for a bent ZnO NW along the c-axis (150 nm in diameter). Left inset: Sketch of bending curvature. Right inset: Bending deformation-induced broadening of gaps between the 70 subbands of conduction band valley minima.
were evaluated in radial CL measurements. Careful CL measurements were carried out by means of radial line scan across the section of a thick ZnO NW with diameter ca. 250 nm as shown in Figure 4. The line-scan step was set at ca. 25 nm across the selected bent section in Figure 4b from the inner compressive edge to the outer tensile one. The three-dimensional (3D) CL spectrum across the radial section is shown in Figure 4c, and the detailed energy distribution and peaks of the spectra are presented in Figure 4d. We should be aware that owing to scattering of the incident electron beam and the diffusion of excitons in CL measurements, the line-scan at each step cannot reflect fully the strain effect exactly at the focused point. For ZnO, the length of the exciton diffusion is ca. 110 nm,[21] which is much larger than the scanning steps. Therefore, each CL spectrum measured along the radial cross section can only be understood as a weighted average from the sampled volume. Nevertheless, the trend is clear that there is a visible red shift of the UV peak when CL collection starts from the inner compressive edge and proceeds towards the outer tensile edge. Concerns may be raised about the effect of electron beam irradiation on the in situ CL measurements. In fact, many reports[22] have investigated electron beam irradiation in ZnO. The
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ZnO nanostructures, and bending engineering can be used to design possible novel nanodevices.
Experimental Synthesis of ZnO NWs: ZnO NWs were grown by a simple physical vapor deposition method [13]. Synthesis was carried out in a conventional furnace with a horizontal alumina tube. In a typical process, a ceramic boat, loaded with a mixture of ZnO powder and graphite with a molar ratio of 1:1, was placed at the center of the furnace as the source material. Then a Si(111) wafer with a 2 nm catalyst layer of Au was placed 2 cm downstream (with respect to the gas flow) of the source. The whole system was first heated to 1050 8C in 35 min under a constant flow of 200 sccm Ar and then kept at 1050 8C for 30 min under a constant flow of 200 sccm Ar mixed with 2.6 sccm O2 under atmospheric pressure. The furnace was then cooled naturally to room temperature under Ar flow. The white product collected on the Si wafer consisted of ZnO NWs. CL Measurements: The bent ZnO NWs were acquired by glass fiber under an optical microscope. CL (using a Gatan monocle 3þ) in a scanning electron microscope (Quant 200 FEG, ESEM, FEI) Figure 4. Cross-sectional CL spectra measured from a bent ZnO NW ca. 250 nm in diameter at was used to study the electronic band structure of 81 K. a,b) SEM images of the bent ZnO NW. c) Original 3D CL spectra plot. d) Original 2D CL bent ZnO NWs. The spectra were acquired by spectra; the inset shows the normalized spectra. It reveals a red shift of the CL peak when scanned charge-coupled device (CCD) with middle wavefrom the inner to the outer part of the NW. The black dashed line shows the constant position length of 375 nm, scan range from 343 nm to across the straight part of the same NW, as shown in Figure S3 of the Supporting Information. The 408 nm. Electron beam energy of 9 kV and spot size arrows indicate the direction of CL line-scan. of 4 are the optimized settings, so that both the CL spatial resolution and the beam current are sufficient to give a good signal-to-noise ratio. defects induced by electron beam irradiation in ZnO are mainly Zn TEM Experiments: The bent ZnO NW was acquired in a transmission and O vacancies with deep donor character, and these defects have electron microscope (Tecnai F30 TEM) by the method reported in [15]. First little effect on the ultraviolet near-edge emission. Instead, they a glass fiber was used to break the carbon film on the microscope grid contribute only to the defect-related emission band. For example, randomly, and then ZnO NWs were dispersed on it. Otherwise, in TEM, the [22] Gorelkinskii and Watkins observed that electron beam irradiacarbon film would crimp under irradiation with the electron beam and bend tion at ca. 2.5 MeV produced a prominent new double-humped the ZnO NWs on it. band, centered at 750 and 900 nm, in the PL spectrum of ZnO. First-Principles Density Functional Theory: The detailed DFT methods and information about the pseudopotentials can be found in [23,24]. The Moreover, point defects or cracks are unlikely to form under the model of the NW is the same as in [25]. Single k-point and (2 2 6) bending conditions applied in our research. It should also be noted Monkhorst–Pack k-point grids were used in the structural optimizations from Figure 4 that only a red shift was observed in the bent NWs. and energy calculations, respectively. The energy convergence down to The most probable explanation is that, at an accelerating voltage of 104 hartree (1 hartree 27.2 eV 2630 kJ mol1) was guaranteed when 9 keV, the sampled CL volume is ca. 100 nm owing to electron the energy cutoff of 250 hartree was chosen in the energy calculations. All ˚. scattering, which is larger than the step (ca. 25 nm) of the radial spot atoms were allowed to relax until the forces were smaller than 0.02 eV A ˚ ˚ The obtained lattice constants of bulk ZnO (a ¼ 3.26 A , c ¼ 5.27 A , scanning of the nanowire. Therefore, each acquired CL spectrum ˚, u ¼ 0.377) are in good agreement with the experimental values (a ¼ 3.25 A represents the total average of the CL spectrum excited from the ˚ , u ¼ 0.382) [26]. The calculated bandgap is 0.834 eV at zero c ¼ 5.20 A corresponding sampled volume. Therefore, the peak shift strain, consistent with reported results [27].
measured along the radial direction of a thin nanowire represents only a trend, and is not as evident as expected. In conclusion, systematic CL spectra analyses combined with high-resolution TEM characterization of ZnO NWs bent into different shapes with bending strain range of a few percent show significant deformation-induced reduction in bandgap (red shift) and broadening of the near edge emission in the CL spectra. Theoretical calculations based on both first-principles DFT and an effective-mass envelope function theory reveal identically that tensile strain makes the main contribution to the bandgap reduction. This work should provide an atomic mechanism for the electronic and mechanical coupling behavior of deformed
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Acknowledgements This work is supported by NSFC, the national 973 program of China, MOST, and NSFC/RGC (N HKUST615/06). DPY thanks Prof. Claus Klingshirn for helpful discussion. Supporting Information is available online from Wiley InterScience or from the authors.
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Received: March 20, 2009 Published online: August 13, 2009
Adv. Mater. 2009, 21, 4937–4941
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Adv. Mater. 2009, 21, 4937–4941
[12] a) X. Wang, J. Zhou, J. Song, J. Liu, N. Xu, Z. L. Wang, Nano Lett. 2006, 6, 2768. b) W. Shan, W. Walukiewicz, J. W. Ager, III, K. M. Yu, Y. Zhang, S. S. Mao, R. Kling, C. Kirchner, A. Waag, Appl. Phys. Lett. 2005, 86, 153117. [13] Y. Zhang, H. Jia, R. Wang, C. Chen, X. Luo, D. Yu, C. Lee, Appl. Phys. Lett. 2003, 83, 4631. [14] J. Xu, L. Chen, L. Yu, H. Ling, B. S. Zhang, K. M. Lau, J. Electron. Mater. 2007, 36, 1144. [15] X. D. Han, Y. F. Zhang, K. Zheng, X. N. Zhang, Z. Zhang, Y. J. Hao, X. Y. Guo, J. Yuan, Z. L. Wang, Nano Lett. 2007, 7, 452. ´n, [16] J. M. Soler, E. Artacho, J. D. Gale, A. Garcı´a, J. Junquera, P. Ordejo D. Sa´nchez-Portal, J. Phys.: Condens. Matter 2002, 14, 2745. [17] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. [18] J. B. Xia, J. Lumin. 1996, 70, 120. [19] J. B. Xia, X. W. Zhang, Eur. Phys. J. B 2006, 49, 415. ¨hlich, Phys. Rev. B 1997, 56, 13087. [20] J. Wrzesinski, D. Fro [21] J. Yoo, G. Yi, L. S. Dang, Small 2008, 4, 467. [22] Yu. V. Gorelkinskii, G. D. Watkins, Phys. Rev. B 2004, 69, 115212. [23] C. Li, W. Guo, Y. Kong, H. Gao, Phys. Rev. B 2007, 76, 035322. [24] C. Li, W. Guo, Y. Kong, H. Gao, Appl. Phys. Lett. 2007, 90, 223102. [25] H. J. Xiang, J. Yang, J. G. Hou, Q. Zhu, Appl. Phys. Lett. 2006, 89, 223111. ¨fferlein, W. Schiessl, M. Steiner, U. Hiller, [26] H. Karzel, W. Potzel, M. Ko G. M. Kalvius, D. W. Mitchell, T. P. Das, P. Blaha, K. Schwarz, M. P. Pasternak, Phys. Rev. B 1996, 53, 11425. [27] T. R. Paudel, W. R. L. Lambrecht, Phys. Rev. B 2008, 77, 205202.
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[1] a) Y. Li, G. W. Meng, L. D. Zhang, F. Phillipp, Appl. Phys. Lett. 2000, 76, 2011. b) Y. C. Kong, D. P. Yu, B. Zhang, W. Fang, S. Q. Feng, Appl. Phys. Lett. 2001, 78, 407. c) M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 2001, 292, 1897. [2] C. Klingshirn, Phys. Status Solidi B 2007, 244, 3027. [3] a) Y. Qin, X. D. Wang, Z. L. Wang, Nature 2008, 451, 809. b) Z. L. Wang, J. H. Song, Science 2006, 312, 242. [4] C. Solbrig, E. Mollwo, Solid State Commun. 1967, 5, 625. ¨dl, F. Fuchs, J. Rurthmu ¨ller, F. Bechstedt, Appl. Phys. Lett. [5] A. Schleife, C. Ro 2007, 91, 241915. [6] C. Q. Chen, Y. Shi, Y. S. Zhang, J. Zhu, Y. J. Yan, Phys. Rev. Lett. 2006, 96, 075505. [7] X. Lin, X. B. He, T. Z. Yang, W. Guo, D. X. Shi, H. J. Gao, D. D. D. Ma, S. T. Lee, F. Liu, X. C. Xie, Appl. Phys. Lett. 2006, 89, 043103. [8] M. Ieong, B. Doris, J. Kedzierski, K. Rim, M. Yang, Science 2004, 306, 2057. [9] R. R. He, P. D. Yang, Nat. Nanotechnol. 2006, 1, 42. [10] a) D. M. Lyons, K. M. Ryan, M. A. Morris, J. D. Holmes, Nano. Lett. 2002, 2, ´. N. Mhuircheartaigh, S. M. Lipson, M. A. Morris, 811. b) G. Audoit, E W. J. Blau, J. D. Holmes, J. Mater. Chem. 2005, 15, 4809. c) A. Seike, T. Tange, Y. Sugiura, I. Tsuchida, H. Ohta, T. Watanabe, D. Kosemura, A. Ogura, I. Ohdomari, Appl. Phys. Lett. 2007, 91, 202117. d) D. Shiri, Y. Kong, A. Buin, M. P. Anantram, Appl. Phys. Lett. 2008, 93, 073114. [11] a) T. W. Tombler, C. Zhou, L. Alexseyev, J. Kong, H. Dai, L. Liu, C. S. Jayanthi, M. Tang, S. Y. Wu, Nature 2000, 405, 769. b) W. Guo, Y. Guo, Phys. Rev. Lett. 2003, 91, 115501. c) Z. Zhang, W. Guo, Appl. Phys. Lett. 2007, 90, 053114.
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By Xiaobing Han, Liangzhi Kou, Xiaoli Lang, Jianbai Xia, Ning Wang, Rui Qin, Jing Lu, Jun Xu, Zhimin Liao, Xinzheng Zhang, Xudong Shan, Xuefeng Song, Jingyun Gao, Wanlin Guo,* and Dapeng Yu*
Semiconductor nanowires (NWs) are currently attracting a great deal of interest as they are expected to play an important role in the development of nanometer-scale technologies.[1] As zinc oxide (ZnO) is a typical II–VI semiconductor, ZnO NWs have aroused considerable notice because of their unique wide-bandgap semiconducting, piezoelectric, and photoelectric properties.[2] Recently, the energy-converting ability of ZnO NWs has been attracting intense attention as a result of their electronic– mechanical coupling behavior.[3] A huge number of experimental and theoretical studies have been devoted to understanding their structure, properties, and novel behavior;[4–6] the conductance of ZnO NWs has been shown to decrease with increasing strain.[7] However, how mechanical deformation can tune their electronic band structures remains elusive. Strain engineering of semiconductors has long been an important technique.[8] Strained silicon exhibits enhanced electron mobility,[9] and the optical and electronic properties of silicon NWs can be tailored through strain.[10] Tensile, bending, and torsional deformation of carbon nanotubes can significantly tune their electronic and magnetic properties.[11] Similarly, besides ZnO NWs being piezoelectric, the electronic and optical properties of ZnO NWs are also sensitive to mechanical strain,[12] but understanding the atomic-level mechanism of the intriguing multi-field coupling properties of this attractive low-dimensional material remains
[*] Prof. D. P. Yu, X. B. Han, R. Qin, J. Lu, J. Xu, Z. M. Liao, X. Z. Zhang, X. D. Shan, X. F. Song, J. Y. Gao State Key Laboratory for Mesoscopic Physics and Electron Microscopy Laboratory Department of Physics, Peking University Beijing 100871 (P. R. China) E-mail: [email protected] Prof. W. L. Guo, L. Z. Kou Institute of Nanoscience Nanjing University of Aeronautics and Astronautics Nanjing 210016 (P. R. China) E-mail: [email protected] X. L. Lang, Prof. J. B. Xia State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors, Chinese Academy of Sciences PO Box 912, Beijing 100083 (P. R. China) Prof. N. Wang Physics Department Hong Kong University of Science and Technology ClearWater Bay, Kowloon, Hong Kong (Hong Kong)
DOI: 10.1002/adma.200900956
Adv. Mater. 2009, 21, 4937–4941
COMMUNICATION
Electronic and Mechanical Coupling in Bent ZnO Nanowires
a great challenge for both theoretical and experimental researchers. In this Communication, we report how we manipulated single ZnO NWs into different shapes to create bending strain in both optical and transmission electron microscopy (TEM) environments, and that strong electronic–mechanical coupling in the NWs was found using cathodoluminescence (CL). Significant red shift and broadening of the near-edge emission in CL spectra along the bent ZnO NWs were observed. The atomic mechanism for bending deformation and strain-induced change in the electronic band structure of the NWs is revealed by lattice analysis. Theoretical calculations using both first-principles density functional theory (DFT) and effective mass envelope function theory for uniaxial stress states are used to explain the bending-induced variation in bandgaps. The ZnO NWs investigated in this work were prepared by vapor phase deposition, as described in detail in the literature.[13] The growth direction of the ZnO NWs is along the [001] c-axis. After dispersion in ethanol, ZnO NWs (100–250 nm in diameter) were transferred onto a Si substrate with a 500 nm SiO2 layer. Straight ZnO NWs of interest were bent under an optical microscope for CL investigations using a glass tip. The strong interaction between the substrate and the NWs keeps the NWs in the curved shape. The tensile and compressive strain at the outer and inner edges, respectively, of the bent NW can be estimated by its local radius of curvature r and diameter D as e ¼ D/2r. Therefore, the bending strain increases linearly with decreasing radius of curvature and increasing diameter of the NWs. The electronic bandgaps of the bent NWs were then carefully measured using CL spectroscopy (Gatan monocle 3þ) of high spatial (100 nm for ZnO at electron beam energy of 9 keV) and spectral (0.5 nm) resolution. To enhance the measuring resolution, most of the CL experiments were carried out in liquid nitrogen at ca. 81 K. Without going into details, all measurements were made by spot-scanning along the NWs with the beam focus at the center of the NWs. An optimal setting was chosen (electron beam energy 9 kV and spot size of 4) so that the spatial resolution of the CL was the highest, to obtain a high signal-to-noise ratio.[14] We focused our attention on the near-edge UV emission band between 3.0 and 3.8 eV. A typical bent ZnO NW (ca. 150 nm in diameter, and 50 mm in length) manipulated into an L-shape (with minimum radius of curvature of 3.0 mm) is shown in Figure 1a. A series of CL spectra were collected from the bent NW by scanning along its axis spot-by-spot in steps of 400–1000 nm, as indicated by colored
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Figure 1. CL measurements from a ZnO NW bent into an L-shape (a–c; diameter ca. 150 nm) at 81 K and an S-shape (d,e; diameter ca. 140 nm) at room temperature. a) Scanning electron microscopy (SEM) image of the NW; colored arrows indicate the CL measurement positions. b) CL spectra along the bent NW shown in the corresponding color for each arrowed position in (a). (Not all arrows are shown in (a).) c) FWHM of near-edge peaks of the CL spectra in (b) as a function of position along the NW. d) SEM image of the NW (left) and representation of CL spectra at different positions along the NW (right). e) Representative CL spectra revealing the red shift of the peak position along the bent NW.
Figure 2. TEM analysis of a bent ZnO NW (diameter ca. 100 nm). a) Low-magnification TEM image of the NW. b) Typical strain contrasts in the bent region within the area indicated by the red square in (a). c) Typical diffraction pattern in the bent region. d,f) High-resolution TEM images at the outer (green square in (b)) and inner (purple square in (b)) edges in the bent region, which show relatively perfect crystal structure. e,g) FFT diagrams of (d) and (f) show the lattice parameters of the deformed ZnO NW. The standard c/a ratio is ca. 1.6 for a strain-free bulk sample.
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arrows in Figure 1a. The representative CL spectrum acquired at the position of each arrow is shown in Figure 1b in the corresponding color. At liquid nitrogen temperature, the intrinsic CL peaks appear very sharp and intense. It is clear that the CL spectra collected from the straight sections have the intrinsic peak position of ca. 3.37 eV, the same as that of bulk ZnO. On entering the bent region, a red shift of the UV band from the intrinsic value occurs and increases with decreasing local radius of curvature up to a maximum of ca. 50 meV at the most bent section (with r ¼ 3.0 mm and e ¼ 2.5%). It is interesting that the bending-induced red shift is accompanied by broadening of the UV band, as shown by the variation in the full width at half maximum (FWHM) of the UV peak in Figure 1c. The same trend can be observed in the CL spectra measured at room temperature in a ZnO NW bent into an S-shape as shown in Figures 1d,e. As the radius of curvature reaches two minima along the S-shaped wire (Fig. 1d), the UV peak shifts twice from the intrinsic value and reaches two valley values, forming a W-shaped peak profile, as shown in the right-hand panel of Figure 1d. Measurements on more samples bent into different shapes presented in Figures S1 and S2 in the Supporting Information show the same trend. The reason for the broadening of FWHM in the bent region will be discussed in detail later. To reveal the atomic mechanism of the bending deformation in ZnO NWs, in situ microstructure analyses of bent NWs were conducted in a transmission electron microscope (Tecnai F30). The method we used to bend a ZnO NW on a TEM grid is similar to the method reported by Han et al.[15] That is, the ZnO NWs were dispersed on a TEM grid with a holey carbon supporting film. A selected NW of interest was bent as follows: A small hole was made in the holey carbon supporting film near the NW, and then the electron beam was focused to lacerate the carbon film above the NW. Under electron beam irradiation, the carbon film will crimp. As a result of the laceration force, the NW bends, which provides the opportunity to evaluate its atomic structure under bending, as shown in Figure 2a. The TEM image demonstrates that the ZnO NWs were grown along the [001] c-axis with a diameter of ca. 100 nm. A high-magnification TEM image such as that in Figure 2b demonstrates that the bent NW shows typical inhomogeneous strain contrast. The corresponding selected-area electron diffraction (SAED) pattern (Fig. 2c) revealed that some of the diffraction spots appear streaky
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and rotational arc-shaped, which indicates that the corresponding lattice planes suffer severe bending deformation. This is further demonstrated in high-resolution TEM images (Fig. 2d,f) taken at the outer (green square in Fig. 2b) and inner (purple square in Fig. 2b) edges of the bent area. The continuous lattice fringes are strain-contrasted, which manifests the inhomogeneous strain distribution in the bent NW. The lattice parameters c/a and a for Figures 2d and f were derived from the corresponding fast Fourier transforms (FFT) shown in Figures 2e and g, respectively. The above analysis indicates that the bent NW undergoes bending deformation, with the inner side under compressive strain and outer side under tensile strain, and the estimated strains are e ¼ (couter – cinner)/(couter þ cinner) ¼ 1.99%, respectively. We also found that such a bending deformation is elastic because it can be released with a drop of ethanol, and the bent NW recovers its straight shape, as demonstrated in Video 1 in the Supporting Information. To understand the above-demonstrated correlation between the peak shift of the UV band and the bending strain, first-principles DFT simulations have been performed on both bulk and [0001]-oriented NW ZnO under pure tensile and compressive strains to evaluate the strain-induced change in the bandgap. In the simulations, the ZnO NWs were strained axially. All the structural optimizations and energy calculations were performed based on pseudopotentials with localized atomicorbital basis sets within the Perdew–Burke–Ernzerhof general gradient approximation implemented in the code SIESTA.[16,17] The calculation results for ZnO NWs (with diameter range from 0.97 to 2.27 nm) and bulk ZnO crystal in the strain range of 5% to 5% are presented in Figure 3a, where the energy gap for the strained sample is normalized by the value of the strain-free sample. For both NW and bulk crystal, the bandgap decreases with increasing tensile strain. To obtain a direct estimate of the theoretical effect of pure bending strain on the band structure, the bandgap was also calculated as a function of the bending radius of curvature in the framework of a six-band effective-mass envelope function theory.[18] We used the same method and parameters as in a previous publication[19] except for the introduction of a strain term Hstrain in the Hamiltonian as done by Wrzesinski and Fro¨hlich.[20] To focus on strain-induced change in the bandgap, we fixed the top of the conduction band and calculated the shift in the valence band. Figure 3b shows the bandgap for a 150 nm ZnO NW as a function of radius of curvature under pure bending deformation. Significant gap reduction is obtained with the decrease of the radius of curvature. When the radius is reduced to ca. 3.6 mm, the bandgap decreases from 3.37 eV (bulk) to 3.32 eV, which is in agreement with the experiments for the L-shaped NW shown in Figure 1b. Now we will focus our attention on the conduction band. As shown in the right-hand inset of Figure 3b, the gaps between the 70 subbands (quantized states) in the conduction band minima will broaden when the NW bends into a radius of curvature of ca. 3.6 mm from a straight strain-free one. Therefore, the observed broadening of the near-edge UV peak originates from the broadening of the gaps between subbands in the conduction band under bending deformation. To confirm the theoretical correlation between the band-gap energy and the applied compressive/tensile strain, the contributions from the compressive and tensile strain parts in bent NWs
Figure 3. The variation of band structure with deformation. a) Results of DFT for c-axial strain-induced change in bandgap of ZnO NWs containing 48, 108, and 192 atoms in the cross section with diameters of 0.97, 1.62, and 2.27 nm, respectively. Inset: Sketch of the NW section. The data for the strained bulk ZnO crystal was measured in the same direction. b) Results from the effective-mass envelope-function theory for a bent ZnO NW along the c-axis (150 nm in diameter). Left inset: Sketch of bending curvature. Right inset: Bending deformation-induced broadening of gaps between the 70 subbands of conduction band valley minima.
were evaluated in radial CL measurements. Careful CL measurements were carried out by means of radial line scan across the section of a thick ZnO NW with diameter ca. 250 nm as shown in Figure 4. The line-scan step was set at ca. 25 nm across the selected bent section in Figure 4b from the inner compressive edge to the outer tensile one. The three-dimensional (3D) CL spectrum across the radial section is shown in Figure 4c, and the detailed energy distribution and peaks of the spectra are presented in Figure 4d. We should be aware that owing to scattering of the incident electron beam and the diffusion of excitons in CL measurements, the line-scan at each step cannot reflect fully the strain effect exactly at the focused point. For ZnO, the length of the exciton diffusion is ca. 110 nm,[21] which is much larger than the scanning steps. Therefore, each CL spectrum measured along the radial cross section can only be understood as a weighted average from the sampled volume. Nevertheless, the trend is clear that there is a visible red shift of the UV peak when CL collection starts from the inner compressive edge and proceeds towards the outer tensile edge. Concerns may be raised about the effect of electron beam irradiation on the in situ CL measurements. In fact, many reports[22] have investigated electron beam irradiation in ZnO. The
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ZnO nanostructures, and bending engineering can be used to design possible novel nanodevices.
Experimental Synthesis of ZnO NWs: ZnO NWs were grown by a simple physical vapor deposition method [13]. Synthesis was carried out in a conventional furnace with a horizontal alumina tube. In a typical process, a ceramic boat, loaded with a mixture of ZnO powder and graphite with a molar ratio of 1:1, was placed at the center of the furnace as the source material. Then a Si(111) wafer with a 2 nm catalyst layer of Au was placed 2 cm downstream (with respect to the gas flow) of the source. The whole system was first heated to 1050 8C in 35 min under a constant flow of 200 sccm Ar and then kept at 1050 8C for 30 min under a constant flow of 200 sccm Ar mixed with 2.6 sccm O2 under atmospheric pressure. The furnace was then cooled naturally to room temperature under Ar flow. The white product collected on the Si wafer consisted of ZnO NWs. CL Measurements: The bent ZnO NWs were acquired by glass fiber under an optical microscope. CL (using a Gatan monocle 3þ) in a scanning electron microscope (Quant 200 FEG, ESEM, FEI) Figure 4. Cross-sectional CL spectra measured from a bent ZnO NW ca. 250 nm in diameter at was used to study the electronic band structure of 81 K. a,b) SEM images of the bent ZnO NW. c) Original 3D CL spectra plot. d) Original 2D CL bent ZnO NWs. The spectra were acquired by spectra; the inset shows the normalized spectra. It reveals a red shift of the CL peak when scanned charge-coupled device (CCD) with middle wavefrom the inner to the outer part of the NW. The black dashed line shows the constant position length of 375 nm, scan range from 343 nm to across the straight part of the same NW, as shown in Figure S3 of the Supporting Information. The 408 nm. Electron beam energy of 9 kV and spot size arrows indicate the direction of CL line-scan. of 4 are the optimized settings, so that both the CL spatial resolution and the beam current are sufficient to give a good signal-to-noise ratio. defects induced by electron beam irradiation in ZnO are mainly Zn TEM Experiments: The bent ZnO NW was acquired in a transmission and O vacancies with deep donor character, and these defects have electron microscope (Tecnai F30 TEM) by the method reported in [15]. First little effect on the ultraviolet near-edge emission. Instead, they a glass fiber was used to break the carbon film on the microscope grid contribute only to the defect-related emission band. For example, randomly, and then ZnO NWs were dispersed on it. Otherwise, in TEM, the [22] Gorelkinskii and Watkins observed that electron beam irradiacarbon film would crimp under irradiation with the electron beam and bend tion at ca. 2.5 MeV produced a prominent new double-humped the ZnO NWs on it. band, centered at 750 and 900 nm, in the PL spectrum of ZnO. First-Principles Density Functional Theory: The detailed DFT methods and information about the pseudopotentials can be found in [23,24]. The Moreover, point defects or cracks are unlikely to form under the model of the NW is the same as in [25]. Single k-point and (2 2 6) bending conditions applied in our research. It should also be noted Monkhorst–Pack k-point grids were used in the structural optimizations from Figure 4 that only a red shift was observed in the bent NWs. and energy calculations, respectively. The energy convergence down to The most probable explanation is that, at an accelerating voltage of 104 hartree (1 hartree 27.2 eV 2630 kJ mol1) was guaranteed when 9 keV, the sampled CL volume is ca. 100 nm owing to electron the energy cutoff of 250 hartree was chosen in the energy calculations. All ˚. scattering, which is larger than the step (ca. 25 nm) of the radial spot atoms were allowed to relax until the forces were smaller than 0.02 eV A ˚ ˚ The obtained lattice constants of bulk ZnO (a ¼ 3.26 A , c ¼ 5.27 A , scanning of the nanowire. Therefore, each acquired CL spectrum ˚, u ¼ 0.377) are in good agreement with the experimental values (a ¼ 3.25 A represents the total average of the CL spectrum excited from the ˚ , u ¼ 0.382) [26]. The calculated bandgap is 0.834 eV at zero c ¼ 5.20 A corresponding sampled volume. Therefore, the peak shift strain, consistent with reported results [27].
measured along the radial direction of a thin nanowire represents only a trend, and is not as evident as expected. In conclusion, systematic CL spectra analyses combined with high-resolution TEM characterization of ZnO NWs bent into different shapes with bending strain range of a few percent show significant deformation-induced reduction in bandgap (red shift) and broadening of the near edge emission in the CL spectra. Theoretical calculations based on both first-principles DFT and an effective-mass envelope function theory reveal identically that tensile strain makes the main contribution to the bandgap reduction. This work should provide an atomic mechanism for the electronic and mechanical coupling behavior of deformed
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Acknowledgements This work is supported by NSFC, the national 973 program of China, MOST, and NSFC/RGC (N HKUST615/06). DPY thanks Prof. Claus Klingshirn for helpful discussion. Supporting Information is available online from Wiley InterScience or from the authors.
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Received: March 20, 2009 Published online: August 13, 2009
Adv. Mater. 2009, 21, 4937–4941
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Adv. Mater. 2009, 21, 4937–4941
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