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Applying strain on semiconductors is a powerful method to modulate its electronic structures and optical properties. In this study, the behavior of liquid-nitrogen exciton emissions and the longitudinal optical phonon–exciton interactions of tensile strained [0001]-orientated ZnO whiskers were investigated using in situ cathodoluminescence spectroscopy. It has been found that, under the axial tensile strain, various exciton emissions shift to the long wavelength and their shifts have a linear relationship with the applied strain. This linear relationship and reversible shift suggest that the strain plays a dominating role in manipulating light emissions of axially strained ZnO whiskers.
© 2014 Optical Society of America
1. Introduction
Nanomaterials can survive with ultra-large elastic strains with high strength [1–3]. It has been revealed that metals [4], semiconductors [5,6], and metallic glass [7] possess ultra-large elasticity by up to 7.2% in the form of low-dimensional nanostructures, such as nanowires [8]. With the ultra-large elastic strain, the lattice constants and the band-gap of the nanomaterials can be tuned largely, from which new physical properties and in turn novel applications can thus be expected [9]. The rapid growth of strain-engineering based on nanomaterials has been witnessed. For examples, for GaAs nanowires, over 290 meV of the light emission can be tuned by a 3.5% tensile strain [10]. By applying tunable tensile strain, the energy offsets between the Г and L valleys of energy band of Ge was reduced and its light emission was largely improved by enhancing the radiative recombination [11]. As a direct wide band-gap semiconductor, ZnO has been paid extensive attention in recent decades. ZnO has a relatively large exciton binding energy (60 meV) that leads to an effective near band edge (NBE) excitonic emission of ultraviolet light region even above room temperature, so that ZnO nanostructures have been considered as a promising candidate for the application in waveguide, laser photonic and optoelectronic devices [12–14]. It has been demonstrated that ZnO nanostructures possess ultra-large elastic strains and unusual strain-induced luminescence properties [15–20]. Nevertheless, there were only limited studies related to luminescence properties of one-dimensional ZnO nanomaterials under in situ axial tensile strain because of experiment difficulties. In particular, the luminescence behavior of strained one-dimensional ZnO nanostructures is unclear, which deserves an urgent attention if such an exceptional material system can be employed in practical applications, particular when these nanostructures are under deformation [21].
In this paper, we investigate the behaviour of exciton emissions of strained [0001]-orientated ZnO whiskers at liquid nitrogen temperature using the spatially resolved cathodoluminescence (CL) under the in situ axial tensile strain. Through careful correlation of measured CL signals with simultaneous strained ZnO whiskers, the exciton emissions of ZnO whiskers were observed to have a linear red-shift with the applied axial tensile strain. The behaviour of the near band edge (NBE) emissions of our ZnO whiskers, including the free exciton (FX), the bound exciton (D0X), the first order (FX-1LO) and the second order (FX-2LO) longitudinal-optical (LO) phonon–exciton interaction, are explored.
2. Experimental details
The [0001]-orientated ZnO whiskers were prepared by powder thermal evaporation without catalysts. The obtained ZnO whiskers were deposited on the Si substrate. A tungsten tip with paste was used to pick up the whiskers from the Si substrate, and locate them to the Si cantilever(s) and the manipulating tip, as shown in Fig. 1(a) - A schematic illustration of the experiment setup. The epoxy glue and conductive Ag epoxy were used to fix the two ends of a ZnO whisker on the Si cantilever and the manipulating tip under an optical microscope. A mechanical-CL correlated experimental setup combined with liquid nitrogen-cooled stage was established in a high-resolution environmental scanning electron microscope (SEM, FEI Quanta 600F) for low-temperature deformed optical characterization [22]. All CL spectra were recorded at T = 80 K from the obtained [0001]-orientated ZnO whiskers, and were measured using a spectrometer (Gatan Mono 3 + ) with an accelerating voltage of 10 kV.
Fig. 1 A mechanical-CL combined experiment setup and experiment results of a strain-free ZnO whisker under the liquid nitrogen (a)The schematic illustration of the experiment setup. (b) SEM image of a ZnO whisker in the experiment setup, the inset showing the diameter of ZnO whisker. (c) A cross section of a fractured ZnO whisker showing the hexagonal cross-section. (d) CL spectrum of the NBE peaks of a strain-free ZnO whisker.
During the experiment, each individual ZnO whisker was axially tensile-elongated until fracture, and their corresponding exciton spectra were recorded by a CL spectrometer for different tensile strains. The manipulating tip, mounted on a nano-manipulator (KleindiekTM) in the SEM, was served to drive the ZnO whiskers elongation; the ZnO whisker can then be elastically strained. The circulative loading-unloading states can be accurately controlled by the fine steps of the nano-manipulator. The minimal moving step of the linear axis of the nano-manipulator reaches 5 nm. The loading force was recorded by deflection of the Si cantilever, so that the axial tensile strains can be calculated as [22, 23]
where is the Young’s modulus in [0001] direction (for bulk ZnO, E3 = 140 GPa, cij is the elastic constants [24]), σc is the applied axial stress on the whisker, is the displacement of the Si cantilever,is a force constant of the Si cantilever, S is the area of the whisker’s cross-section. According to Eq. (1), the strain experienced by the ZnO whisker can be determined by the change of .3. Results and discussions
Figure 1(b) is a SEM image and shows the typical morphology of a ZnO whisker. The inset is an enlarged SEM image, from which the whisker’s diameter can be measured as 1.26 μm. Figure 1(c) is a magnified SEM image and shows a hexagonal shaped cross-section of a ZnO whisker, suggesting that the measured diameter in the inset of Fig. 1(b) is the edge-to-edge diameter. In Fig. 1(a), the electron beam excited light emission was firstly collected by the elliptical mirror, dispersed by the monochromator, and then collected by the charge-coupled device. Using the electron backscatter diffraction measurement (not shown here), the axial direction of ZnO whiskers can be confirmed to be along the [0001] direction. Figure 1(d) shows a CL spectrum taken from a strain-free ZnO whisker, in which the peak located at 3.384 eV belongs to the free exciton (FX) transition [18]. Due to the existence of intrinsic defects and impurities, the free excitons are bounded by the defects, a weak FX peak is expected. On the other hand, the exciton peak caused by neutral donor like complex (D0X), located at 3.357eV with a full-width at half maximum (FWHM) of ~16 meV has the strongest intensity. In addition, the first, second and third order longitudinal optical (LO) phonon replicas of the free excitons can be found at 3.312 eV (FX-1LO), 3.238 eV (FX-2LO) and 3.166 eV (FX-3LO), respectively [19].
The ZnO whisker shown in Fig. 1(b) was under tensile strains and its corresponding CL signals were simultaneously recorded. Figure 2 is a set of CL spectra of the ZnO whisker with different strains until fracture, and shows the variations of exciton emission spectra. As revealed, the D0X emission peak of the strain-free ZnO whisker is located at 3.357 eV. When the axial strain was applied up to 2.06%, this D0X peak was shifted from 3.357 eV to 3.276 eV. Simultaneously, the FX-1LO peak was shifted from 3.312 eV to 3.233 eV, and the FX-2LO shifted from 3.238 eV to 3.159 eV. In addition, the FWHM of these peaks are nearly unchanged under different strains. During the loading and unloading cycles, these peaks in the CL spectra are fully reversible, suggesting that these changes of the band structures for our ZnO whiskers can be reversibly tuned by the elastic strain. Therefore, the corresponding light emission energies, radiative recombination and carrier mobility can then be reversibly tuned using the applied strain.
Fig. 2 The excition emission spectra of a 1.26 μm thick ZnO whisker under different tensile strains.
To clarify the relationship between the FX, D0X, FX-1LO, FX-2LO emissions and the applied axial tensile strain, these emissions are plotted as a function of the applied tensile strain. Figure 3 shows such plots, from which the red shift in NBE emissions induced by the axial tensile strain exhibits a linear relationship with the elongation strain for the ZnO whisker. These measured values fit well with the relationship between the energy band gap (Es) of strained whiskers and the applied axial tensile strain (εc) [20]
where E0 is the energy band gap of the strain-free ZnO, is the band edge shifted due to the axial tensile strain, and is the deformation potential. In Eq. (2), the E0 can be measured from the strain-free whiskers, εc can be estimated from Eq. (1), and the Es can be determined from the CL spectra, so that ac can be calculated from Eq. (2). In fact, from the mathematic point of view, ac represents the slope of the linear plot. From Fig. 3, the fact that all plots are almost parallel to each other suggests that all ac should have a similar value. Our detailed calculation shows acFX = −3.96 eV, acD0X = −3.90 eV, acFX-1LO = −3.84 eV and acFX-2LO = −3.82 eV. The uniform strain changes the energy of FX phonon replicas, which can be expressed as [25, 26]The measured deformation potentials is similar (), indicating that a homogeneous strain significantly change the energy of exciton emission, rather than the phonon-exciton interaction energy. We believe that the variation of LO phonon energy is not affected by the homogeneous strain [26, 27].For ZnO whiskers with diameters of 1-5 μm, our extensive experiments indicate that acavg = - (3.86 ± 0.10) eV, similar to that for strained bulk ZnO [28–31]. The linear relationship between variation of the light emissions and axial tensile strains indicates that the strain effect plays a dominating role in tuning exciton emissions of tensile strained ZnO whiskers. In addition, the deformation potential in our axial tensile test is larger than that of Dietrich’s results (−2.04 eV) of bending ZnO microwires, which can be attributed to different types of deformations i.e., a strain gradient along radial bending ZnO microwire and a homogeneous strain in axial tensile ZnO whisker.
To gain the spatially resolved CL mapping along radial direction and its variation as a function of the tensile strain, line-scan CL spectra along the cross section (perpendicular to the c-axis) of a ZnO whisker under axial tensile strains were collected. Figure 4(a) is a schematic setting of the electron beam line-scan along the cross section of the ZnO whisker. In this arrangement, the electron beam was gradually moved to irradiate the three top-facets along the cross section [refer to the marked arrow in Fig. 4(a)]. Figure 4(b) is spatial-resolved CL spectra of a 1.60 μm thick ZnO whisker under strains of 0% (unstrained case) and 1.48%, respectively. For each of spatial-resolved CL spectra, the x-axis presents the energy dispersive spectrum and y-axis is the location of the electron beam in the cross-section, so that the line scans of the CL spectra were along the y-axis, in which the two horizontal dashed lines divide three facets of the ZnO whisker with the hexagonal cross-section. The fact that all recorded emissions are vertically aligned indicates that the line scan is precisely along the cross-section of the whisker. It is of interest to note that the CL intensities from the two-side facets are relatively lower than that of the central facet, possibly due to the synergetic effect of (a) the smaller contribution from electrons which are excited within smaller volume at the edge of hexagonal cross-section and (b) the difference in detecting the generated photons at azimuthal angles, which originate from the three facets of the hexagonal shape [32, 33]. When the axial tensile strain gradually increases to 1.48%, these fusiform peaks homogeneously shift toward the lower energies, indicating the homogeneous strain reversibly tuned the light emission along cross section of ZnO whisker.
Fig. 4 A linescan of the excition emission spectra from the entire cross section of ZnO whisker (a) A schematic setting for the electron beam line-scan CL spectrum measurement. (b) CL line-scan spectrum maps of a 1.60 μm thick ZnO whisker under strain-free and axial tensile strain of 1.48%, respectively.
4. Summary
In conclusion, the behavior of exciton emissions of axial tensile strained ZnO whiskers was investigated by in situ CL spectroscopy at liquid nitrogen temperature. It has been found that the exciton emission shifts toward the long wavelength under the tensile strain, and the shift of the exciton emission varies linearly with the applied axial tensile strain. In this study, the deformation potential of bound exciton of ZnO whiskers under the axial tensile strain has been determined as - (3.86 ± 0.10) eV, similar to the bulk ZnO. The features of constant deformation potential of our [0001]-orientated ZnO whiskers, i.e. the linear relationship between the change of the photon emissions and their experienced axial tensile strain, indicates that the strain effect plays a key role in manipulating exciton emissions of our strained ZnO whiskers. These results provide direct evidence of how electronic structure of ZnO whiskers can be altered by the external tensile strain, which is critical for ultimate design of ZnO nanostructure based nanoelectronic and optoelectronic devices, particularly when these devices are under deformation.
Acknowledgments
This study was supported by the Chinese National Outstanding Young Investigator Grant (10825419), the Key Project Program of the Chinese Natural Science Foundation (50831001), the National 973 Program of China (2009CB623700), the Beijing High-level Talents Grants (PHR 20100503), the Beijing PXM201101420409000053, the Chinese Natural Science Foundation (11004004), the Beijing Municipal Natural Science Foundation (1112004), Doctoral Fund of Innovation of BJUT (YB201313), the Beijing 211 Project, and the Australian Research Council.
References and links
1. E. W. Wong, P. E. Sheehan, and C. M. Lieber, “Nanobeam mechanics: elasticity, strength, and toughness of nanorods and nanotubes,” Science 277(5334), 1971–1975 (1997). [CrossRef]
2. X. D. Han, Y. F. Zhang, K. Zheng, X. N. Zhang, Z. Zhang, Y. J. Hao, X. Y. Guo, J. Yuan, and Z. L. Wang, “Low-temperature in situ large strain plasticity of ceramic SiC nanowires and its atomic-scale mechanism,” Nano Lett. 7(2), 452–457 (2007). [CrossRef] [PubMed]
3. Q. Deng, Y. Cheng, Y. Yue, L. Zhang, Z. Zhang, X. Han, and E. Ma, “Uniform tensile elongation in framed submicron metallic glass specimen in the limit of suppressed shear banding,” Acta Mater. 59(17), 6511–6518 (2011). [CrossRef]
4. Y. Yue, P. Liu, Z. Zhang, X. Han, and E. Ma, “Approaching the theoretical elastic strain limit in copper nanowires,” Nano Lett. 11(8), 3151–3155 (2011). [CrossRef] [PubMed]
5. K. Zheng, X. Han, L. H. Wang, Y. F. Zhang, Y. H. Yue, Y. Qin, X. N. Zhang, and Z. Zhang, “Atomic Mechanisms Governing the Elastic Limit and the Incipient Plasticity of Bending Si Nanowires,” Nano Lett. 9(6), 2471–2476 (2009). [CrossRef] [PubMed]
6. R. Shao, K. Zheng, Y. Zhang, Y. Li, Z. Zhang, and X. Han, “Piezoresistance behaviors of ultra-strained SiC nanowires,” Appl. Phys. Lett. 101(23), 233109 (2012). [CrossRef]
7. Q. Jiang, P. Liu, Y. Ma, Q. Cao, X. Wang, D. Zhang, X. Han, Z. Zhang, and J. Jiang, “Super elastic strain limit in metallic glass films,” Sci. Rep. 2, 852 (2012).
8. B. Chen, Q. Gao, Y. Wang, X. Liao, Y.-W. Mai, H. H. Tan, J. Zou, S. P. Ringer, and C. Jagadish, “Anelastic Behavior in GaAs Semiconductor Nanowires,” Nano Lett. 13(7), 3169–3172 (2013). [CrossRef] [PubMed]
9. Y. F. Hu, Y. F. Gao, S. Singamaneni, V. V. Tsukruk, and Z. L. Wang, “Converse Piezoelectric Effect Induced Transverse Deflection of a Free-Standing ZnO Microbelt,” Nano Lett. 9(7), 2661–2665 (2009). [CrossRef] [PubMed]
10. G. Signorello, S. Karg, M. T. Björk, B. Gotsmann, and H. Riel, “Tuning the Light Emission from GaAs Nanowires over 290 meV with Uniaxial Strain,” Nano Lett. 13, 917–924 (2012). [PubMed]
11. J. R. Jain, A. Hryciw, T. M. Baer, D. A. Miller, M. L. Brongersma, and R. T. Howe, “A micromachining-based technology for enhancing germanium light emission via tensile strain,” Nat. Photonics 6(6), 398–405 (2012). [CrossRef]
12. M. Willander, O. Nur, Q. X. Zhao, L. L. Yang, M. Lorenz, B. Q. Cao, J. Zúñiga Pérez, C. Czekalla, G. Zimmermann, M. Grundmann, A. Bakin, A. Behrends, M. Al-Suleiman, A. El-Shaer, A. Che Mofor, B. Postels, A. Waag, N. Boukos, A. Travlos, H. S. Kwack, J. Guinard, and D. Le Si Dang, “Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers,” Nanotechnology 20(33), 332001 (2009). [CrossRef] [PubMed]
13. A. Little, A. Hoffman, and N. M. Haegel, “Optical attenuation coefficient in individual ZnO nanowires,” Opt. Express 21(5), 6321–6326 (2013). [CrossRef] [PubMed]
14. M. Ding, D. Zhao, B. Yao, S. e, Z. Guo, L. Zhang, and D. Shen, “The ultraviolet laser from individual ZnO microwire with quadrate cross section,” Opt. Express 20(13), 13657–13662 (2012). [CrossRef] [PubMed]
15. F. Fang, D. Zhao, B. Li, Z. Zhang, D. Shen, and X. Wang, “Bending-induced enhancement of longitudinal optical phonon scattering in ZnO nanowires,” J. Phys. Chem. C 114(29), 12477–12480 (2010). [CrossRef]
16. H. Xue, N. Pan, M. Li, Y. Wu, X. Wang, and J. G. Hou, “Probing the strain effect on near band edge emission of a curved ZnO nanowire via spatially resolved cathodoluminescence,” Nanotechnology 21(21), 215701 (2010). [CrossRef] [PubMed]
17. B. Yan, R. Chen, W. W. Zhou, J. X. Zhang, H. D. Sun, H. Gong, and T. Yu, “Localized suppression of longitudinal-optical-phonon-exciton coupling in bent ZnO nanowires,” Nanotechnology 21(44), 445706 (2010). [CrossRef] [PubMed]
18. Z.-M. Liao, H.-C. Wu, Q. Fu, X. Fu, X. Zhu, J. Xu, I. V. Shvets, Z. Zhang, W. Guo, Y. Leprince-Wang, Q. Zhao, X. Wu, and D.-P. Yu, “Strain induced exciton fine-structure splitting and shift in bent ZnO microwires,” Sci. Rep. 2,452 (2012).
19. C. P. Dietrich, M. Lange, F. J. Klupfel, H. von Wenckstern, R. Schmidt-Grund, and M. Grundmann, “Strain distribution in bent ZnO microwires,” Appl. Phys. Lett. 98(3), 031105 (2011). [CrossRef]
20. X. B. Han, L. Z. Kou, X. L. Lang, J. B. Xia, N. Wang, R. Qin, J. Lu, J. Xu, Z. M. Liao, X. Z. Zhang, X. D. Shan, X. F. Song, J. Y. Gao, W. L. Guo, and D. P. Yu, “Electronic and Mechanical Coupling in Bent ZnO Nanowires,” Adv. Mater. 21(48), 4937–4941 (2009). [CrossRef]
21. W. Yang, Y. Ma, Y. Wang, C. Meng, X. Wu, Y. Ye, L. Dai, L. Tong, X. Liu, and Q. Yang, “Bending effects on lasing action of semiconductor nanowires,” Opt. Express 21(2), 2024–2031 (2013). [CrossRef] [PubMed]
22. B. Wei, K. Zheng, Y. Ji, Y. F. Zhang, Z. Zhang, and X. D. Han, “Size-Dependent Bandgap Modulation of ZnO Nanowires by Tensile Strain,” Nano Lett. 12(9), 4595–4599 (2012). [CrossRef] [PubMed]
23. M. R. He and J. Zhu, “Defect-dominated diameter dependence of fracture strength in single-crystalline ZnO nanowires: In situ experiments,” Phys. Rev. B 83(16), 161302 (2011). [CrossRef]
24. C. Q. Chen, Y. Shi, Y. S. Zhang, J. Zhu, and Y. J. Yan, “Size dependence of Young’s modulus in ZnO nanowires,” Phys. Rev. Lett. 96(7), 075505 (2006). [CrossRef] [PubMed]
25. R. Mendelsberg, M. Allen, S. Durbin, and R. Reeves, “Photoluminescence and the exciton-phonon coupling in hydrothermally grown ZnO,” Phys. Rev. B 83(20), 205202 (2011). [CrossRef]
26. S. Xu, W. Guo, S. Du, M. M. Loy, and N. Wang, “Piezotronic Effects on the Optical Properties of ZnO Nanowires,” Nano Lett. 12(11), 5802–5807 (2012). [CrossRef] [PubMed]
27. X.-W. Fu, Z.-M. Liao, R. Liu, J. Xu, and D. Yu, “Size-Dependent Correlations between Strain and Phonon Frequency in Individual ZnO Nanowires,” ACS Nano 7(10), 8891–8898 (2013). [CrossRef] [PubMed]
28. A. Mang, K. Reimann, and S. Rübenacke, “Band gaps, crystal-field splitting, spin-orbit coupling, and exciton binding energies in ZnO under hydrostatic pressure,” Solid State Commun. 94(4), 251–254 (1995). [CrossRef]
29. J. Wrzesinski and D. Fröhlich, “Two-photon and three-photon spectroscopy of ZnO under uniaxial stress,” Phys. Rev. B 56(20), 13087–13093 (1997). [CrossRef]
30. J. Rowe, M. Cardona, and F. Pollak, “Valence band symmetry and deformation potentials of ZnO,” Solid State Commun. 6(4), 239–242 (1968). [CrossRef]
31. A. Segura, J. Sans, F. Manjon, A. Munoz, and M. Herrera-Cabrera, “Optical properties and electronic structure of rock-salt ZnO under pressure,” Appl. Phys. Lett. 83(2), 278–280 (2003). [CrossRef]
32. T. Onuma, T. Yamaguchi, and T. Honda, “Electron‐beam incident‐angle‐resolved cathodoluminescence studies on bulk ZnO crystals,” Phys. Status Solidi 10(5c), 869–872 (2013). [CrossRef]
33. H. Z. Xue, N. Pan, R. G. Zeng, M. Li, X. Sun, Z. J. Ding, X. P. Wang, and J. G. Hou, “Probing the Surface Effect on Deep-Level Emissions of an Individual ZnO Nanowire via Spatially Resolved Cathodoluminescence,” J. Phys. Chem. C 113(29), 12715–12718 (2009). [CrossRef]
