Electron Beam Irradiation Stiffens Zinc Tin Oxide Nanowires

LETTER pubs.acs.org/NanoLett

Electron Beam Irradiation Stiffens Zinc Tin Oxide Nanowires Jianfeng Zang,† Lihong Bao,† Richard A. Webb,‡ and Xiaodong Li*,† † ‡

Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, United States Department of Physics and USC NanoCenter, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States

bS Supporting Information ABSTRACT: We report a remarkable phenomenon that electron beam irradiation (EBI) significantly enhances the Young’s modulus of zinc tin oxide (ZTO) nanowires (NWs), up to a 40% increase compared with the pristine NWs. In situ uniaxial buckling tests on individual NWs were conducted using a nanomanipulator inside a scanning electron microscope. We propose that EBI results in substantial atomic bond contraction in ZTO NWs, accounting for the observed mechanically stiffening. This argument is supported by our experimental results that EBI also reduces the electrical conductivity of ZTO NWs. KEYWORDS: Electron beam irradiation, Young’s modulus, zinc tin oxide, nanowires, buckling, nanomanipulator

N

anowires (NWs) have attracted intensive studies in recent years as critical structural/functional building blocks for nanophotonic, nanoelectronic, and nanoelectromechanical devices.1 NWs exhibit remarkable optical, electronic, magnetic, and mechanical properties that are different from their bulk counterparts. The reliability and/or functionality of these nanodevices are heavily dependent on the mechanical properties of individual NWs. The strategies that have been developed to measure the mechanics of individual NWs include observing thermal or electric field-induced vibration of NWs in a transmission or scanning electron microscope (TEM/SEM),2
4 performing lateral bending on suspended NWs by atomic force microscopy (AFM),5,6 indenting NWs on substrate,7 and conducting uniaxial tensile testing or buckling on NWs inside a TEM/SEM with an electromechanical device or a nanomanipulator.8
13 The reported experimental results of Young’s moduli of carbon nanotubes, silicon NWs, and zinc oxide NWs exhibit a significant scattering.4,12,14,15 The discrepancy of the Young’s moduli of the same type of NWs may come from several aspects summarized as: (1) size, structure, defect type, and density in NWs;4,12 and (2) nanomechanics testing strategies and parameters such as time, temperature, and strain rate.14,15 We have noticed that many of the nanomechanical tests rely on electron microscopes. The e-beam provides an unique range of properties and characteristics, making SEM/TEM powerful tools in many applications, such as e-beam lithography, e-beam induced deposition (EBID),16 e-beam assisted engineering of nanomaterials,17 and e-beam assisted welding, milling, cutting, and processing.18,19 Integrating a mechanical testing device into an electron microscope chamber allows researchers to in situ observe deformation and/or fracture of individual NWs while simultaneously measuring the applied load and recording images. However, mild e-beam also produces undesirable side effect, such as damage to the materials of interest. Hence, a key question is r 2011 American Chemical Society

raised but not answered: Does e-beam irradiation (EBI) on NWs alter their physical properties, e.g., mechanical properties? The pioneering studies on the effect of ion irradiation on the mechanical properties may provide us a clue.20,21 However, the effect of EBI on the mechanical properties of individual NWs has not yet been investigated. In this Letter, we report a remarkable nanotechnology for the first time that EBI induces stiffening in zinc tin oxide (ZTO) NWs at a selected location. The test was conducted inside a SEM using a homemade nanomanipulator as an actuator and an AFM cantilever as a load sensor. The Young’s modulus of the individual NWs was determined by buckling instability upon the application of a uniaxial compressive load. We found that the Young’s modulus increases up to 40% upon EBI. The effect of EBI on the electrical conductivity of NWs has also been investigated to understand the mechanism involved the EBIinduced stiffening in NWs. ZTO NWs are known to have excellent electrical conductivity and low visible absorption, demonstrating a wide range of potential applications, such as humidity sensors, flat panel displays, and photovoltaic devices.22,23 The synthesis and characterization of ZTO NWs were described in detail previously24 and in Supporting Information (Figure S1). A homemade nanomanipulator was employed to perform mechanical uniaxial buckling tests on individual ZTO NWs inside a FEI Quanta 200 environmental SEM. The nanomanipulator consists of two independent operating stages (x, y) and (z, θ). Each axial is driven by a picomotor and allows nanometer resolution linear motion (x, y, z) or 360° rotational motion (θ).10,13 A rigid tungsten tip mounted on the (x, y) stage was employed to pick up a protruding ZTO Received: August 8, 2011 Revised: September 20, 2011 Published: October 03, 2011 4885

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Figure 1. Schematic showing the priciple of the uniaxial buckling of an individual NW. The uniaxial buckling test of the individual NW before (a) and after (b) EBI.

Figure 2. Series of SEM images of an individual ZTO NW in a process of buckling (a
g).

NW from a silicon wafer and to clamp the NW with localized EBID of carbonaceous materials inside the SEM chamber.16 After the NW was clamped on the tungsten tip, the carbonaceous source materials were removed from the SEM chamber to eliminate the effect of EBID induced polymerization on the NW during the EBI in the following tests. The diameter of the ZTO NWs was measured before the carbon deposition, thus ruling out the effect of a possible thin layer of carbon deposition on the NWs during the imaging process. A compliant AFM cantilever with a spring constant of 0.1 K m
1 mounted on the (z, θ) stage was served as a load sensor. The load (P) applied on the NWs is calculated by the deflection of the cantilever (d) multiplied with its spring constant (k). Before the test, we moved the NW toward the AFM probe and adjusted the four axis to make sure: (1) the NW and the AFM cantilever were in the same plane, which was perpendicular to the e-beam direction; (2) the NW was perpendicular to the AFM cantilever; and (3) the free end of the NW touched the AFM cantilever at the point where its tip was located. The schematic setup of the uniaxial buckling test is presented in Figure 1. Figure 2 shows a series of SEM images taken during the buckling test for a NW with diameter of 248 nm before EBI with a reduced window. The Figure 2a is the image prior to loading. As the tungsten tip gradually moved toward the compliant AFM probe, the cantilever started to deflect, indicating a gradually increased compressive load applied on the NW (Figure 2b). At the same time, the NW was kept straight (Figure 2b). As the NW continued to move toward the cantilever with an increased deflection, the NW started to bend but remained in neutral equilibrium, as presented in Figure 2c. Once the applied axial compressive load reached the critical load (Pcr), the NW immediately lost its stable equilibrium and buckling instability

Figure 3. Effect of EBI on the buckling instabilities of the individual ZTO NW. (a) Plots of length of the NW versus applied load extracted from the buckling tests before and after 30, 60, 90, and 120 min EBI. (b) Elastic modulus of NW calculated from panel (a) was plotted as a function of the duration of EBI. (c) Plots of length of the NW versus applied load extracted two consecutive buckling tests after 30 min EBI.

occurred, as depicted in Figure 2d. After the critical point, the deflection of the cantilever increased slowly (from Figure 2d and e) and then kept unchanged (Figure 2e
g) with continued movement of the NW toward the cantilever. The whole buckling process recorded in a movie is presented in the Supporting Information (SV1). The buckling instability was released by slowly moving the NW away from the cantilever until it no longer touched the cantilever. The e-beam scanning was applied to irradiate the NW with reduced window mode for 30 min, as shown in Figure 1b. The current density was estimated to be 5  10
2 A cm
2 in reduced window mode during EBI. The buckling test was repeated after the EBI. We repeated the 30 min EBI and buckling test several times for the same NW. The corresponding uniaxial buckling processes recorded in movies are presented in the Supporting Information (SV2
SV6). The imaging process for each buckling instability test usually lasted 10 min with much larger image window; the exposure area is approximately100 times larger than that at EBI (reduced window) mode. The total exposure dose during each buckling test in this work is only ∼1/ 300 of that at EBI mode. Therefore, the effect of EBI during the test was well controlled to minimize its influence. The typical shape of the NW under buckling instability is presented in Figure 2d, suggesting the buckling process fits well 4886

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Figure 4. Effect of EBI on the conductance of individual ZTO NWs. (a) Representative individual ZTO NW wired out by two Ti/Au electrodes. EBI area on the NW is highlighted by the box in white lines. (b) I
V curves of an individual NW before and immediately after 30 min EBI (0 min) and its resultant time-dependent relaxation (15, 38, and 1474 min). (c) I
V curves of another individual ZTO NW before and immediately after 60 min EBI (0 min) and its resultant time dependent relaxation (25, 238, and 1620 min). (d) The normalized conductance retention of the two NWs as a function of relaxation time after 30 and 60 min EBI, respectively.

with the Euler’s uniaxial buckling model of fixed-pinned ends. One end of the NW was fixed on the tungsten tip. The other end was free and pressed on the cantilever probe, which was treated as a pinned end. Therefore, elastic modulus (E) of the individual NW follows the equation:25 E¼

Pcr ðkLÞ2 π2 I

ð1Þ

where I is the moment of inertia and k is 0.7 when the model fits the condition of fixed-pinned ends. Figure 3a shows the length versus applied load of the NW in buckling tests. Surprisingly, a higher load was needed to reach instability for the same NW after 30 min EBI, indicating a stiffening effect occurred in the NW. The pronounced stiffening effect continued as the irradiation duration increased to 60 min and reached saturation after 90 and 120 min irradiation. The corresponding Young’s moduli of the NW were calculated according to eq 1 and presented as a function of irradiation duration in Figure 3b. The increase of Young’s modulus of the individual NW of 15.6, 34.5, 36.1, and 40.1% was observed corresponding to 30, 60, 90, and 120 min of irradiation. In order to confirm the assumption that the observed stiffening in the NW is not a result of hysteresis effect from the previous buckling test, we conducted two consecutive buckling tests after 30 min irradiation (movies in Supporting Information SV2 and SV3). The extracted plots of the NW length versus applied load were depicted in Figure 3c. The two plots look similar. According to eq 1, the calculated Young’s modulus in the second buckling test shows a slight decline, a 4.8% decrease. Such a difference of the Young’s modulus between two consecutive buckling tests might be introduced by several possibilities, such as the measurement errors inherent in this test strategy, the hysteresis effect that previous buckling deformation of the NW might degrade a little

bit of its mechanical properties, and/or other factors. Clearly, the observed slightly decline in Young’s modulus of the NW in the two consecutive buckling tests helps us to rule out the possibility that the stiffening was attributed to the hysteresis effect. To understand the mechanism of the EBI-induced stiffening in the ZTO NWs, we further investigated the electrical conductivity of individual NWs as a function of EBI duration. Two Ti/Au electrodes were built on the individual NWs by e-beam lithography,26 as shown in Figure 4a. We found that the conductivity decreased 13.9% immediately after 30 min EBI in the first NW (Figure 4b). We continued to measure I
V properties of the NW as a function of relaxation time. Interestingly, we found that the conductivity continued to decrease gradually with the relaxation time without further EBI. We examined the EBI effect on the second NW, as the irradiation duration increased to 60 min. The resultant conductivity of the NW exhibited an even bigger drop, 35.6%, as shown in Figure 4c. The conductivity showed the similar trend in the relaxation period without further EBI. The conductivity of each NW was normalized to its corresponding pristine conductivity and was plotted as a function of relaxation time, as shown in Figure 4d. The normalized conductivity continued to decrease until 60 and 45% of its pristine conductivity reached after 30 and 60 min EBI (Figure 4d). This observation reveals two facts: (1) a relaxation process was occurred in the electronic structure of the individual NWs after EBI and (2) the impact of EBI on the NWs was permanent (electronic structure and mechanical properties in our experiments). E-beam irradiating materials may bring damage in two principal forms depending on the energy level of the incoming e-beam: (1) radiolysis, e-beam with mild-energy level (low voltage), only generates inelastic scattering (mainly ionization) that breaks the 4887

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chemical bonds and (2) knock-on damage, e-beam with sufficient high-energy, produces direct displacement of atoms from the crystal lattice that creates point defects.27
30 The primary mechanism on EBI of ZTO NWs at low voltage (30 KV here in SEM) is e-beam-induced local modification of bonding structure in ZTO NWs, resulting in local bond contraction. Point defects, if generated at low concentration without obvious materials degrading and crystal structure damage, also caused lower coordination numbers of the surrounding atoms. According to Goldschmidt31 and Paulling,32 the atom with lower coordinated number holds a reduced radius, resulting in shortened bonds with strengthened or stiffened bonds.33 Regardless of the complexity of the e-beam
NW interaction, as a result of EBI, bond contraction occurred in the NW. The higher bond contraction effect resulted in a higher single bond energy (Ei), Ei = c
m i Eb and a higher single atomic cohesion (Ebi), Ebi = zibc
m i Eb, which have been summarized as bond-order-length-strength (BOLS) correlation by Sun.33 The i and b denote an atom in the ith atomic layer and in the bulk. The bond contraction coefficient, ci, varies with effective atomic coordination number (zi). The index m, discriminates bond nature alternation, for alloys or compounds, m = 4.34 According to the BOLS treatment, the relative change of the Young’s modulus can be normalized to be a dimensionless form when the temperature is far below the melting point: ΔEi
ðm þ 3Þ ¼ ci
1 E

ð2Þ

Therefore, the enhanced bond contraction effect resulted in an enhanced Young’s modulus in the NWs that experienced EBI. The enhanced bond contraction effect by the EBI also induces pronounced surface potential barrier.33 The potential barrier trapped carriers in the NWs, leading to a decreased electrical conductivity. Such a modification on the electronic structure of the NWs is consistent with the experimental results observed in Figure 4. In summary, we have demonstrated for the first time that EBI on ZTO NWs increased the Young’s modulus, as high as up to 40%. We proposed that EBI caused substantial bond contraction in the NWs, accounting for the observed mechanical stiffening. This argument is supported by the EBI-induced degradation of electrical conductivity in ZTO NWs. Our results not only elucidate the discrepancy among the existing nanomechanics results but also provide a new aspect to examine in situ electron microscopy nanomechanics, electron/ion milling processing, and nanodevice fabrication. In addition, our findings offer a novel nanotechnology to in situ modifying of physical properties of individual NWs or nanodevices at selected locations.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details about the preparation of ZTO nanowires, SEM/TEM/HRTEM/SAED characterization of ZTO, in situ uniaxial buckling tests for individual ZTO NWs. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the U.S. Army Research Office under agreement/grant W911NF-07-1-0320 and the National Science Foundation (CMMI-0653651, CMMI-0824728, and CMMI0968843). ’ REFERENCES (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. Adv. Mater. 2003, 15 (5), 353–389. (2) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381 (6584), 678–680. (3) Poncharal, P.; Wang, Z. L.; Ugarte, D.; de Heer, W. A. Science 1999, 283 (5407), 1513–1516. (4) Chen, C. Q.; Shi, Y.; Zhang, Y. S.; Zhu, J.; Yan, Y. J. Phys. Rev. Lett. 2006, 96 (7), 075505. (5) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277 (5334), 1971–1975. (6) Wu, B.; Heidelberg, A.; Boland, J. J. Nat. Mater. 2005, 4 (7), 525–529. (7) Li, X. D.; Gao, H. S.; Murphy, C. J.; Caswell, K. K. Nano Lett. 2003, 3 (11), 1495–1498. (8) Yu, M. F.; Lourie, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Science 2000, 287 (5453), 637–640. (9) Yu, M. F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. Rev. Lett. 2000, 84 (24), 5552–5555. (10) Lin, C. H.; Ni, H.; Wang, X. N.; Chang, M.; Chao, Y. J.; Deka, J. R.; Li, X. D. Small 2010, 6 (8), 927–931. (11) Zhu, Y.; Espinosa, H. D. Proc. Natl. Acad. Sci. U.S.A. 2005, 102 (41), 14503–14508. (12) Zhu, Y.; Xu, F.; Qin, Q. Q.; Fung, W. Y.; Lu, W. Nano Lett. 2009, 9 (11), 3934–3939. (13) Wu, L. Y.; Zang, J. F.; Lee, L. A.; Niu, Z. W.; Horvatha, G. C.; Braxtona, V.; Wibowo, A. C.; Bruckman, M. A.; Ghoshroy, S.; zur Loye, H. C.; Li, X. D.; Wang, Q. J. Mater. Chem. 2011, 21 (24), 8550–8557. (14) Ruoff, R. S. Proc. Natl. Acad. Sci. U.S.A. 2006, 103 (18), 6779–6780. (15) Buongiorno Nardelli, M.; Yakobson, B. I.; Bernholc, J. Phys. Rev. B 1998, 57 (8), R4277–R4280. (16) Ding, W.; Dikin, D. A.; Chen, X.; Piner, R. D.; Ruoff, R. S.; Zussman, E.; Wang, X.; Li, X. J. Appl. Phys. 2005, 98 (1), 014905. (17) Zheng, K.; Wang, C. C.; Cheng, Y. Q.; Yue, Y. H.; Han, X. D.; Zhang, Z.; Shan, Z. W.; Mao, S. X.; Ye, M. M.; Yin, Y. D.; Ma, E. Nat. Commun. 2010, 1, 8. (18) Xu, S. Y.; Tian, M. L.; Wang, J. G.; Xu, H.; Redwing, J. M.; Chan, M. H. W. Small 2005, 1 (12), 1221–1229. (19) Utke, I.; Hoffmann, P.; Melngailis, J. J. Vac. Sci. Technol., B 2008, 26 (4), 1197–1276. (20) Shim, S.; Bei, H.; Miller, M. K.; Pharr, G. M.; George, E. P. Acta Mater. 2009, 57 (2), 503–510. (21) Nagar, R.; Teki, R.; Koratkar, N.; Sathe, V. G.; Kanjilal, D.; Mehta, B. R.; Singh, J. P. J. Appl. Phys. 2010, 108 (6), 063519. (22) Wang, L. S.; Zhang, X. Z.; Liao, X.; Yang, W. G. Nanotechnology 2005, 16 (12), 2928–2931. (23) Zhang, Y. J.; Wang, J. J.; Zhu, H. F.; Li, H.; Jiang, L.; Shu, C. Y.; Hu, W. P.; Wang, C. R. J. Mater. Chem. 2010, 20 (44), 9858–9860. (24) Bao, L. H.; Zang, J. F.; Li, X. D. Nano Lett. 2011, 11 (3), 1215–1220. (25) Timoshenko, S. P.; Gere, J. M. Theory of Elastic Stability; McGraw-Hill: New York, 1961. (26) Zang, J. F.; Xu, Z.-H.; Webb, R. A.; Li, X. D. Nano Lett. 2011, 11 (1), 241–244. (27) Williams, D. B.; Carter, C. B. Transmission electron microscopy: a textbook for materials science; Springer: New York, 1996. (28) Kis, A.; Csanyi, G.; Salvetat, J. P.; Lee, T. N.; Couteau, E.; Kulik, A. J.; Benoit, W.; Brugger, J.; Forro, L. Nat. Mater. 2004, 3 (3), 153–157. 4888

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(29) Duchamp, M.; Meunier, R.; Smajda, R.; Mionic, M.; Magrez, A.; Seo, J. W.; Forro, L.; Song, B.; Tomanek, D. J. Appl. Phys. 2010, 108 (8), 084314. (30) Lu, B.-R.; Lanniel, M.; Alexandar, M.; Liu, R.; Chen, Y.; Huq, E. J. Vac. Sci. Technol., B 2011, 29 (3), 030604. (31) Goldschmidt, V. M. Ber. Dtsch. Chem. Ges. 1927, 60, 1263– 1296. (32) Pauling, L. J. Am. Chem. Soc. 1947, 69 (3), 542–553. (33) Sun, C. Q. Prog. Solid State Chem. 2007, 35 (1), 1–159. (34) Sun, C. Q.; Li, S.; Li, C. M. J. Phys. Chem. B 2005, 109 (1), 415–423.

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