Letter pubs.acs.org/NanoLett
Size-Dependent Bandgap Modulation of ZnO Nanowires by Tensile Strain Bin Wei,† Kun Zheng,*,† Yuan Ji,*,† Yuefei Zhang,† Ze Zhang,‡ and Xiaodong Han*,† †
Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, China Department of Materials Science, Zhejiang University, Hangzhou 310058, China
‡
ABSTRACT: We quantified the size-dependent energy bandgap modulation of ZnO nanowires under tensile strain by an in situ measurement system combining a uniaxial tensile setup with a cathodoluminescence spectroscope. The maximal strain and corresponding bandgap variation increased by decreasing the size of the nanowires. The adjustable bandgap for the 100 nm nanowire caused by a strain of 7.3% reached approximately 110 meV, which is nearly double the value of 59 meV for the 760 nm nanowire with a strain of 1.7%. A two-step linear feature involving bandgap reduction caused by straining and a corresponding critical strain was identified in ZnO nanowires with diameters less than 300 nm. The critical strain moved toward the high strain level with shrunken nanowires. The distinct size effect of strained nanowires on the bandgap variation reveals a competition between core-dominated and surface-dominated bandgap modulations. These results could facilitate potential applications involving nanowire-based optoelectronic devices and band-strain engineering. KEYWORDS: Bandgap modulation, size effect, uniaxial tension, cathodoluminescence, ZnO nanowire
E
Therefore, we focused on measuring the bandgap variations caused by strain modulation of ZnO NWs with different diameters via an in situ measurement system combining a uniaxial tensile setup with a high-resolution cathodoluminescence spectroscope in an environmental scanning electron microscope (ESEM). The [0001]-orientated ZnO NWs were fabricated by physical vapor deposition. The morphology and crystal structures were analyzed by a high-resolution transmission electron microscope (JEOL 2010F) and a thermal field emission ESEM (FEI Quanta 600F). The diameters of NWs ranged between 100 and 1000 nm, and the uniaxial tensile strain was along the [0001] direction. For in situ tension, the two ends of the NW were individually fixed to the tip and the cantilever by either electron beam−induced deposition (EBID) for thinner NWs or glue bonding for thicker ones. The markers on the nanowire surface, formed by the EBID or adhesion, were used to determine the elongation lengths of strained NWs. The adhesion reduced the intensity but did not change the peak position of the CL spectra. The tension-luminescence measurement system comprised the uniaxial tensile setup and the CL spectroscope (Gatan mono 3 plus). The applied loads for the individual nanowire were manipulated by the piezo-manipulator (Kleindiek). The conditions of CL spectra collection at room temperature included an accelerating voltage of 15 kV, a beam
arly research found that the piezoresistive effect exists in semiconductors and that the strain effect is a useful method to modulate the physical properties of semiconductors.1 Recently, strain engineering of semiconducting nanostructures has attracted attention because the higher elastic limit induces a larger tunable range for the bandgap in comparison to what is possible with bulk materials.2−7 ZnO, a wide-bandgap semiconductor, has significant advantages and many potential applications.8 The mechanical-optical coupling property of ZnO nanowires (NWs) has become a promising field for piezoelectric-optical and photon-mechanical devices.8−10 Many strain-luminescence measurement techniques for nanowires have been carried out by cathodoluminescence (CL),11−14 photoluminescence,15−18 Rayleigh scattering spectroscopy,19 and Raman spectroscopy.20 However, most previous experiments were based on bent NWs under a mixture of tensile and compressive strain states.9−16 The values of the bending strain of ZnO NWs provided by previous works were less than 3%, which was far below the elastic strain limit.21−23 Furthermore, there is a lack of reports in the literature describing uniaxial tension−induced bandgap variations because of experimental difficulties. Moreover, theoretical calculations on the bandgap shifts of strained NWs, including uniaxial compressive and tensile states, have been widely investigated.24,25 The size-dependent bandgap shifts with the strain can be revealed by simulation.25 However, there are few reports containing experimental confirmation of simulation predictions or systematic investigations of this phenomenon. © 2012 American Chemical Society
Received: May 21, 2012 Revised: July 21, 2012 Published: August 13, 2012 4595
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current of 10−8 to 10−9 A and a spectral shift of less than 0.5 nm with a UV-NIR blazed grating of 0.1 to 0.4 nm. Figure 1a shows an in situ tension-luminescence measurement system built in the ESEM. The uniaxial tensile setup
individual NW and the variations in the length of the NW under loading, respectively (eq 1) σ=
4Kδ , πD 2
ε=
(Ls − L0) L0
(1)
where D is the diameter of the NWs (the cross-section is considered to be round), δ is the displacement of the cantilever from the reference lever, K is the force constant of the silicon cantilever, and L0 and Ls are lengths of strain-free and strained NWs. The values of δ, L0, and Ls are recorded by the secondary electron images. To ensure measurement reliability, the experiments that produced the σ-ε curves and the matched CL spectra were repeated at least twice. Figure 1b shows the σ−ε curves of ZnO NWs with diameters of 100, 260, and 760 nm. The inset shows the secondary electron images of an individual nanowire that is well fixed between the tip and the cantilever. The linear σ−ε relationship of the nanowire indicates that an elastic strain feature existed. Table 1 shows the in situ measurement data, including the stresses, strains, CL spectra, and strain- and stress-band coefficients for size-changed NWs. The fracture strength increased with decreasing diameters. For the 100 nm NW, the maximal stress and strain reached 13.2 GPa and 7.8% (without matched CL spectra data). In contrast, these values decreased to 2.4 GPa and 1.7% for the 760 nm NWs. The corresponding Young’s modulus is 168 GPa for the 100 nm NW, whereas it is 141 GPa for the 760 nm NW, a value approaching the bulk value.22,26 The largely elastic strain of 7.8% for the 100 nm ZnO NWs agreed with the values reported for various NWs, that is, 7.3,22 7.1,27 and ∼7.2%28 for ZnO NWs, Si NWs, and Cu NWs, respectively. Moreover, crystal defects, impurities, and the surface adsorption may also influence the mechanical properties of the nanowire.29,30 To assess the impact of the uniaxial strain on the energy band of ZnO NWs with various diameters, we measured a series of near-band-edge (NBE) emissions of CL spectra in the loadunload process. Figure 2a shows the variations in the NBE peaks of a 100 nm NW with an increased strain of 7.3%. The secondary electron images shown at the top of the figure recorded the length changes of the nanowire that were caused by tension. Clearly, NBE peaks move toward the lower energies as the result of strain. The bandgaps of 3.28 eV (dashed line) and 3.17 eV (solid line), respectively indicating strain-free and 7.3% strain, resulted in a large bandgap modulation of ∼110 meV. Figure 2b shows the variations in the NBE peaks of a 260 nm NW in a load−unload cycle from 0 to 3.5% strain, which resulted in a bandgap variation of ∼84 meV. For the 760 nm NW, the bandgap variation decreased to ∼59 meV from a 1.7% strain, shown in Table 1 (spectral data are not shown here).
Figure 1. (a) Schematic description of an in situ tension-luminescence measurement system including a uniaxial tensile setup and a cathodoluminescence spectroscope in the ESEM, (b) The stress− strain curves of ZnO nanowires of 100, 260, and 760 nm in diameter. The inset is a secondary electron image of an individual NW fixed between the tip and the cantilever.
includes a silicon cantilever, a reference lever, and a tungsten tip based on a piezo-manipulator. The manipulator provides a tunable force to control the loading−unloading process for individual NWs, resulting in uniaxial elongation-relaxation with a step accuracy between 5 and 20 nm. The stress (σ) and the strain (ε) are calculated from the force (Kδ) applied to the
Table 1. Measurement Data of ZnO NWs with Diameters of 100, 260, and 760 nm, Including Stresses, Strains, CL Spectra, Stress-Band, and Strain-Band Coefficients diameter of NWs, D maximal stress, σf (GPa) maximal strain, εf (%) band gap shift, ΔE (meV) IDef/INBE of CL spectra (strain-free state) strain-band coefficient A=ΔE/Δε (eV) stress-band coefficient B=ΔE/Δσ (meV/GPa)
low-strain stage, A1 high-strain stage, A2 low-stress stage, B1 high-stress stage, B2 4596
100 nm
260 nm
760 nm
12.2 (13.2) 7.3 (7.8) 110 0.01 0.6 (3.2%) 3.0 (5.5 GPa)
5.3 3.5 84 0.26 0.9 (1.3%) 5.6 (2 GPa)
2.4 1.7 59 2.9 2.0 (0.2%) 15 (0.3 GPa)
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phenomenon later in detail. Furthermore, the strain-luminescence coupling property can be expressed by a strain-band coefficient, A = ΔE/Δε, or a stress-band coefficient, B = ΔE/ Δσ. The total energy band shift (ΔE) can be expressed by eq 2 ΔE = A1[ε1 − εc(D)] + A 2 [ε2 − εc(D)]
(2)
where A1 and A2 are strain-band coefficients in low- and highstress stages and ε1 and ε2 are tensile strains in low- and highstress stages. Figure 3b indicates that in the low-stress stage, A1 rapidly decreases and approaches a small constant with a decreased diameter (D), resulting in a smaller bandgap shift. In contrast, in the high-stress stage, A2 decreases linearly, resulting in a larger bandgap shift. Moreover, the measured value of A2 for the 760 nm NW is close to that obtained from calculations.24,25 Third, a linear relationship is found to exist between the critical strain (εc) and the inverse of the diameter (1/D), shown in Figure 3c. From a linear fit, a critical diameter (Dc), Dc ≥ 950 nm, can be deduced. Beyond Dc, the nanowires exhibit bulklike properties. Fourth, size-related bandgap shifts in response to different strain levels can be further demonstrated by the relationship between the strain-band coefficient and the strain, shown in Figure 3d and Table 1. The dashed line on the top indicates an extrapolated strain-band coefficient for the 950 nm NW (A = 3.6 eV and B = 26.5 meV/GPa), which is close to the bulk value. The bandgap of the 100 nm NW has a very small response (A1 = 0.6 eV and B1 = 3.0 meV/GPa) in the large strain range from zero to 3.2%, while that of the 760 nm NW has a large response (A1 = 3.5 eV and B1 = 25 meV/GPa) in the strain range of 0.2−1.7%. Furthermore, the values of A1 are only approximately one-third of the values of A2 for nanowires smaller than 260 nm, indicating a large difference in the response to low and high strain levels. The versatile modulating features of the ZnO NWs can be expected to be used in opticalmechanical devices and in strain-band engineering. The size-dependent bandgap shifts caused by the tensile strain could contribute to both the surface-shrinking strain along the radial direction and the tensile strain along the c-axis direction,33−35 as illustrated in Figure 4. Under the axial strainfree state, the shrinking strain due to the surface reconstruction shortens the Zn−O bond parallel to the radial direction in the shell of nanowires.33 Therefore, the bandgap increases as the diameter decreases (Eg2(0) > Eg1(0)) due to the increased surface-volume ratios. Under a state of tension, a competition exists between the shrinking strain on the surface and the uniaxial tensile strain in the core. The energy (Ec) of the conduction band minimum (CBM) state concentrated in the core is sensitive to the axial bond length variation (axial strain), while the energy (Ev) of the valence band maximum (VBM) state localized on the surface is sensitive to the radial bond length variation (surface in-plane strain).36,37 As a result of the tension, the increase in axial Zn−O bond lengths results in proportional shifts downward of the CBM state (ΔEC‑L = ΔEC−H). Simultaneously, Zn−O bond lengths parallel to the radial direction continuously shorten, resulting in VBM shifts upward, but with different bandgap reduction rates. Considering the size effect, the VBM shift of the thinner NWs is smaller than that of the thicker ones (ΔEV2‑L < ΔEV1‑L, ΔEV2−H < ΔEV1−H) due to the enhanced surface-shrinking strain. Considering the uniaxial tensile state, the shrunken shell is less sensitive to the axial tensile in the low-strain stage, resulting in a smaller VBM shift than that in the high-strain stage, which
Figure 2. Series of the NBE emission during the loading−unloading process. (a) The shifts of the NBE peak for the 100 nm NW by tension, inset is a SE image; bar = 500 nm. The SE image shows the adhered markers on the NW surface. (b) The NBE peak shift for the 260 nm NW in a loading−unloading circle. The dashed and solid lines indicate peak positions of unstrained and strained states, respectively.
Moreover, the experiments that produced the CL spectra and σ−ε curves were repeated, and the data exhibit a clear trend, suggesting that the in situ experimental system had relatively high levels of reliability and precision. The size-dependent bandgap shifts caused by the tensile strain are further explained by Figure 3 and Table 1. First, under the strain-free state, the bandgap visibly increased from 3.230 to 3.284 eV as the diameter of the NWs decreased from 760 to 100 nm. The corresponding increase in the bandgap is 54 meV (Figure 3a). The phenomenon could be attributed to the surface shrinking strain of the NWs, which increased as the diameter decreased.31,32 Second, the bandgap reduction rate caused by straining is size-dependent (Figure 3a). It is interesting to note that photon energy-strain curves exhibit a two-step linear (dual-monotonic) feature for diameters less than 300 nm. The turning point on each curve, called the critical strain (εc), move toward higher strain levels with decreasing diameters. When the strain is less than the εc, the surface-dominated band modulation plays a distinct role, and beyond this point the core-dominated band modulation makes a visible contribution. We will discuss the 4597
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Figure 3. The energy band shifts by the tensile strain for ZnO NWs with different diameters. (a) The photon energy versus strain curves. (b) The stress-band coefficient as a function of the inverse of the diameter (1/D). (c) The critical strain (εc) versus the 1/D. (d) The strain-band coefficient as a function of the strain.
band modulations for strained ZnO NWs brings out the versatile responses of the band to strain, that is, “Smaller is more versatile”. The crystal defects of ZnO NWs can be evaluated by the CL spectra from ultraviolet to green bands, including NBE and defect emission (Figure 5). Under a strain-free state, the
Figure 4. The illustration of size-dependent bandgap shifts under strain-free and strained states. Eg1(0) and Eg2(0) are the bandgap of thicker and thinner NWs under strain-free state, Eg1(ε) and Eg2(ε) are the bandgap of thicker and thinner NWs by straining, ΔEC‑L and ΔEC−H are the conduction band variation in low and high stress levels, ΔEV1‑L and ΔEV1−H are the valence band variation of thicker NWs in low and high stress levels, ΔEV2‑L and ΔEV2−H are the valence band variation of thinner NWs in low- and high-stress levels.
is responsible for the smaller bandgap shifts (ΔEV1‑L < ΔEV1−H, ΔEV2‑L < ΔEV2−H) and gentle slopes (Figure 3a,b). As a result, the total bandgap shift of the thinner NWs is smaller than that of the thicker ones (Eg2(ε) > Eg1(ε)). The surface-dominated band modulation plays a dominant role until the tensile strain reaches the critical value (εc), beyond which the nanowire exhibits corelike features and a steep slope (Figure 3a). Note that the surface-shrinking state weakens but does not fade away even in the higher strain stage, as shown in Figure 3a−d and Table 1. The phenomena could be supported by the localized reinforcement of the surfaceshrinking state under the c-axis tension.35,36 The underlying physical mechanisms need to be further explored because the surface-shrinking strain is not a unique effect for NWs. The competition between surface-dominated and core-dominated
Figure 5. CL spectra from UV to green bands for strain-free NWs with different diameters. The inset shows the ratio of defect-to-NBE peak intensity (IDef/INBE) as a function of the tensile stress for the NWs of 260 and 450 nm in diameter.
thinner NWs exhibit weak defect peaks, implying that thinner NWs are almost defect-free and have a high quality. The thicker NWs exhibit a distinct enhancement in defect peaks caused by intrinsic defects or oxygen vacancies. The ratios of the defectto-NBE peak intensity (Idef/INBE) increase nearly 30 times with increasing diameters from 100 to 760 nm (Table 1). Under the larger tension states, the ratios IDef/INBE of the 450 and 260 nm NWs increase by nearly two times in comparison to those of 4598
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unstrained NWs (inset in Figure 5). The results indicate that thinner NWs have fewer original and grown defects. The results support the idea that “Smaller is stronger”. The size-related responses of the band-to-tension cannot be well predicted by previous theoretical calculations. For instance, according to first-principles calculations, the bandgap of thin NWs exhibits a linear or near-linear relationship with uniaxial tension.24,25 The calculation results agree with our measured data for the 760 nm NWs but not with the data for the thinner NWs with diameters less than 760 nm. It could be assumed that in addition to the surface shrinking strain localized strain distributions in the shell and core and defect levels could also influence the bandgap shift. In this case, the in situ tensileluminescence experiments have provided a considerable amount of valuable information. In summary, we studied the size-dependent bandgap variation caused by the tensile strain for ZnO nanowires with diameters ranging between 100 and 760 nm. The in situ measurement system combining a uniaxial tensile setup and a cathodoluminescence spectroscope provides good repeatability and high precision. The 100 nm NW has a high strength of 13.2 GPa and a maximal strain of 7.8% due to fewer original and grown defects. The corresponding bandgap modulation reaches approximately 110 meV, which is nearly two times the value for the 760 nm NW with 1.7% strain. The two-step linear feature of the bandgap reduction caused by straining was observed when the nanowires were less than 300 nm in diameter. A critical strain (εc) exists that shifts toward the high-strain level for shrunken NWs. The competition between core-dominated and surface-dominated bandgap modulation involves a mixture of shrinking strain on the surface and axial tension in the core. The competition brings out the versatile responses of the band to strain for the nanowires. The large bandgap modulation of thinner NWs caused by the large tensile strain could be used in photon-mechanical devices and energy-strain engineering.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (K.Z.)
[email protected]; (Y.J.)
[email protected]. cn; (X.H.)
[email protected]. Tel: +86-10-67392251. Fax: +86-10-67392251. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Outstanding Young Investigator Grant of China (10825419), the Key Project of CNatural Science Foundation (50831001), the National 973 Program of China (2009CB623700), the Beijing High-level Talents (PHR20100503), the Beijing PXM201101420409000053, the National Natural Science Foundation (11004004), the Beijing Municipal Natural Science Foundation (1112004), and the Beijing 211 Project.
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REFERENCES
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