Reversible Tuning of Individual Carbon Nanotube Mechanical

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Reversible tuning of individual carbon nanotube mechanical properties via defect engineering Bin Zhang, Longze Zhao, Yong Cheng, Dmitri Golberg, and Ming-Sheng Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02287 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 26, 2016

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Reversible tuning of individual carbon nanotube mechanical properties via defect engineering Bin Zhang 1, Longze Zhao 1, Yong Cheng 1, Dmitri Golberg 2, Ming-Sheng Wang 1* 1

Department of Materials Science and Engineering, College of Materials, and Pen-Tung Sah

Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian 361005, China 2

International Centre for Materials Nanoarchitectonics (MANA), National Institute for Materials

Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 3050044, Japan. *Address correspondence to: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The structural defects that inevitably exist in real-world carbon nanotubes (CNTs) are generally considered undesirable since they break the structural perfection and may result in drastically degraded CNT properties. On the other hand, the deliberate defect introduction can provide a possibility to tailor the tube mechanical properties. Herein, we present a fully controllable technique to handle defects by using in situ transmission electron microscopy (TEM). Young’s modulus, quality factor of the resonation and tensile strength of CNTs, can be controllably, reversibly and repeatedly tuned. Parallel high resolution visualizing of structural defects suggests that the property tuning cycles are primarily attributed to the reversible conversion of defects at the atomic scale: the defects are created in the form of vacancies and interstitials under electron irradiation, and they vanish through the recombination via current-induced annealing. For applications, such as reversible frequency-tuned CNT resonators, this defect-engineering technique is demonstrated to be uniquely precise; the frequency may be tuned with 0.1%/min accuracy, improved by 1 order of magnitude compared with the existing approaches. We believe these results will be highly valuable in a variety of property-tunable CNT-based composites and devices.

KEYWORDS In-situ Transmission Electron Microscopy (TEM), Carbon nanotube (CNT) resonator, Defect engineering, Property-structural relationship, Mechanical properties tuning, Resonant frequency


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Sp2 carbon nanomaterials, constructed of superstrong covalent C-C bonds within the perfect honeycomb lattice planes, possess amazing mechanical properties. For carbon nanotubes (CNTs) as an example, theoretical calculations have predicted that defect-free structures have the Young’s modulus and tensile strengths values of ~1 TPa and >100 GPa, respectively. 1, 2 Such extremely high values, however, are hardly achievable in most real tests due to the inevitable existence of defects in CNTs. 3-5 Both theory and experiments have proven that the mechanical properties of sp2 carbon structures are highly sensitive to the structural defects that lead to bond reconstruction and lattice disorder, and therefore to the significant mechanical properties degradation. It has been found, for instance, that a single vacancy or step-like atomic defect in CNTs can cause a drastic drop in fracture strength, by 20-50%.

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Thus, defect existence is

generally undesirable for CNTs and graphene, especially in reinforcing applications. On the other hand, the defects in sp2 carbon nanomaterials are not always playing a negative role in regard to mechanical applications. A controlled way of defect handling, i.e. defect introduction and their elimination can be smartly utilized for carbon nanostructures property engineering. 9 Such reversible defect control is highly desirable in a variety of applications where the reversible tuning of device mechanical properties is required. For example, CNT resonators, emerged as important nano-electromechanical devices, need a tunable resonant frequency.10-19 Tuning of such nanomechanical resonators has previously been achieved by altering the tube length or its tension in doubly-clamped resonators,12,13 or by changing the position of the attached/encapsulated metal particles along the CNT cantilever in singly-clamped resonators. 14,15

The defect engineering route proposed by us in this work, would create another efficient and

smart way of resonant frequency tuning.


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Among all known defect introduction approaches, electron beam (or other energetic particles) irradiation has been demonstrated to be an effective means to modify or engineer carbon nanostructures.20-23 However, the generation of defects due to electron irradiation-induced atom displacements, i.e. knock-on collisions, usually leads to the irreversible changes of the microstructures and their properties. A reversible process, in which the defects are introduced during irradiation and are removed later, is necessary for reversibly-tunable devices. Thus, in this work, we present a technique which allows for delicate and fully controlled defect handling and reversible tailoring of the individual CNT mechanical properties. This technique produces defects under electron irradiation, and then eliminates them through Joule heating. All the experiments were carried out inside high-resolution transmission electron microscopes (HRTEMs) equipped with a scanning tunneling microscopy (STM) specimen holder and a conducting atomic force microscope (AFM) unit. The mechanical properties of thus processed tubes were investigated by linking the Young’s modulus, quality factor and tensile strength with the structural defects directly visualized under parallel tube lattice imaging. We also demonstrate reversible frequency tuning of CNT resonators by employing this defectengineering technique. This can provide large frequency tuning ranges exceeding 40%, under excellent fine tuning, as precise as 0.1%/min or even better. Defect engineering and electromechanical resonance experiments were performed inside a JEOL-2100 200 keV TEM by using a STM holder from “PicoFemto” company.

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The multi-

walled CNTs (MWNTs) were synthesized by an arc-discharge method. As shown in Figure 1, a single CNT protruding from the gold electrode was first picked up by a piezo-driven tungsten probe, and then clamped onto the W tip via amorphous carbon deposition by focusing the electron beam within the area of CNT/W contact (see Fig.1b). The electron beam was then


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immediately weakened in order to minimize its effect while monitoring the manipulations with the CNT. The W tip was retracted to a certain position, where the mechanical resonance of the singly clamped CNT was then initiated via applying an AC electric field across the CNT and Au electrode (see Fig.1a).

Figure 1. (a) A selected carbon nanotube clamped on a W probe; (b) The enlarged image of the CNT/W contact covered by amorphous carbon; (c) The fundamental harmonic resonance (ν1=7.88 MHz), and (d) the second harmonic resonance (ν2=47.4 MHz).

Figures 1c and 1d show the selected CNT at its fundamental and second harmonic resonance, respectively. This was achieved by adjusting the frequency of the applied potential. The Euler-Bernoulli equation for a cantilevered beam was employed to analyze the mechanical properties. The resonance frequency v can be expressed as: 4

ν

(1)

where D is the cantilever diameter, L is its length, Eb is the Young’s modulus, ρ is the mass density (for MWNT, ρ=2.2 g/cm3), and βj is a constant for the jth harmonic: β1 =1.875 and β2


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=4.694. The tube diameters D and length L were directly measured under HRTEM imaging. As a basic property parameter for a material, Young’s modulus is closely related to its microstructures, and can be determined from the measurements of ν, D, and L.

Figure 2. Defect engineering of a CNT resonator. (a) The time evolution of the nanotube’s resonant frequency and Young’s modulus under electron irradiation as well as their responses to current annealing processes. The CNT is brought into contact with counter Au electrode for pulse current annealing processes (b), and the recorded I-V curves show the reduced resistance after six consecutive voltage sweeps (c); (d-g) HRTEM images revealing the structural evolution of the area near CNT root (marked with a red square in (b)) during defect-engineering processes, corresponding to the points d-g in (a), respectively.


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Since the resonant frequency is proportional to the square root of the Young’s modulus (as seen in Eq. 1), it is possible to tune the resonant frequency by changing the CNT microstructure, and accordingly, its Young’s modulus. We accomplished this via defect introduction by irradiating the CNT with an electron beam of relatively high current density. As shown in Figure 2a, we observed a continuous drift of the resonant frequency when the electron beam with a current density of 0.98 A/cm2 had been controlled to cover the entire nanotube area. The resonant frequency changes were recorded at the time intervals of 1 min. After exposing the CNT to the e-beam for 40 min, the resonant frequency had been shifting linearly with the irradiation time, from 7.64 MHz (point d) to 6.49 MHz (point e), so a decrease of 15%. According to the Eq. 1, the corresponding Young’s modulus was reduced by 28%, from 150 GPa to 108 GPa, as plotted with red circles in Figure 2a. Such reductions in both resonant frequency and Young’s modulus indicate a structural change of thus-irradiated CNT, which can be clearly confirmed under HRTEM imaging (Fig. 2 d-g). The CNT with originally perfect shells (Fig. 2d) became heavily defective and even amorphized (Fig. 2e). A closer examination shows that the irradiation defects in the present MWNTs are highlighted by breaking, tilting and bending of the basal planes, similar to the observations in the early literature.20, 21 Such morphologies have been attributed to the formation of vacancies and interstitials and their agglomerates under irradiation. 21 These defects at the atomic scale result in the disordering and weakening of carbon bonds, and lead to a large reduction in the strain energy associated with the Young’s modulus parallel to the basal planes. This phenomenon should account for decreasing of the Young’s modulus and resonant frequency of a CNT resonator. It needs to be pointed out the frequency decrease derived from other mechanisms, such as the mass loss of carbon by sputtering, can basically be ruled out, because the loss of mass, instead, should


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cause the resonant frequency to increase. 19 Actually, no visible shrinkage of the MWNT due to electron irradiation was found (see Fig. 2d-g), suggesting the knock-on collisions only lead to defect formation within the MWNT, while the sputtering effect can be neglected. To achieve reversible tuning of the CNT frequency, another process is needed for the tube to recover from its defective state. Thus, the irradiated CNT was brought into contact with the counter electrode (Figure 2b). Consecutive fast (within 1s) voltage sweeps of 0-2 V were then applied to the bridged CNT (to avoid the breakdown of the CNT, the current was limited to less than 10µA). The obtained current-voltage (I-V) curves in Figure 2c show that the resistance of the nanotube was successively reduced, from initial 101 kΩ to final 66 kΩ. The improvement of the electrical conduction implies a healing/recrystallization process taking place within the defective tube shells at each voltage sweep. Six sweeps later, no visible changes in the I-V curves could be found, indicating that the defects should have been annihilated to the maximum extent possible at such pulse current level. The regarded structural transformation can be verified by the corresponding HRTEM imaging (Figure 2f) as well as through checking the resonant frequency. As expected, the resonant frequency of the annealed nanotube greatly increased to 7.51 MHz (point f), almost totally regaining the starting value of 7.64 MHz. The above established defect annihilation can be understood in terms of recombination of vacancies and interstitials at elevated temperatures. Since the amorphized structures within the irradiated CNT have been largely crystallized, the temperature on the tube is estimated to be at least 1000°C. 25, 26 Due to the low migration energies, adatoms possess enough mobility for the annealing of adatom-vacancies pairs. Interstitial agglomerates, at such high temperature, may shrink by emitting interstitial atoms, followed by the atoms migration along the tube shell and recombination with vacancies. 21 In addition, since the applied current density reaches the values


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as high as 106-108 A/cm2, the electromigration effects may also contribute to the interstitials/vacancies diffusion and their recombination. To investigate the repeatability of the above processing cycle, the annealed nanotube was once again exposed to the electron beam with the same current density. The resonant frequency of the CNT was found to decrease again from 7.51 MHz (point f) to 6.65 MHz (point g). The following current annealing also significantly improved the resonant frequency as previously seen (not shown here). Such reversible tuning cycles can be multiply repeated, while a special care must be taken to avoid the accident damage to CNTs caused by improper manipulation. In addition, the frequency tuning range larger than 40% can be routinely obtained, if the prolonged exposure time or an intense electron beam is employed for irradiation (see Supporting information, Fig. S2). Electron beam intensity has a profound impact on the frequency shift, which can be employed as an effective way for a precise control of the resonant frequency, as well as the Young’s modulus. To target the resonant frequency as precisely as possible, we have also studied the relationship between the frequency shift rate and e-beam intensity used for irradiation, as shown in Figure 3a. In this case, the CNT had a length of 1920 nm, and an external diameter of 15 nm. The starting fundamental frequency was 5.69 MHz, and the Young’s modulus was 210 GPa. Electron irradiations with different current densities of 1.52 A/cm2, 0.88 A/cm2 and 0.33 A/cm2 were applied to this CNT. Accordingly, we observed the resonance frequency decreasing shift rates of 20.0 kHz/min, 14.7 kHz/min, 8 kHz/min, corresponding to the control precision of 0.35 %/min, 0.26 %/min, and 0.14 %/min, respectively. Apparently, the frequency shift rate shows a positive correlation with the electron beam intensity. By choosing a lower e-beam intensity, we were able to achieve a fine control of the resonant frequency with a precision better


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than 0.1%/min, corresponding to a Young’s modulus tuning precision of

Reversible Tuning of Individual Carbon Nanotube Mechanical

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