Experimental Evidences of Negative Thermal Expansion in a

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C: Physical Processes in Nanomaterials and Nanostructures

Experimental Evidences of Negative Thermal Expansion in a Composite Nanocable of Single-Walled Carbon Nanotubes and Amorphous Carbon along Axial Direction Xiannian Chi, Lei Wang, Jian Zhang, Jean Pierre Nshimiyimana, Xiao Hu, Pei Wu, Siyu Liu, Jia Liu, Weiguo Chu, Qian Liu, and Lianfeng Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07372 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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The Journal of Physical Chemistry

Experimental Evidences of Negative Thermal Expansion in a Composite Nanocable of Single-Walled Carbon Nanotubes and Amorphous Carbon along Axial Direction Xiannian Chi,a,b Lei Wang,a,b Jian Zhang,a,b Jean Pierre Nshimiyimana,a,b Xiao Hu,a,b Pei Wu,a,b Siyu Liu, a Jia Liu,a,b Weiguo Chu,*a Qian Liu,*a Lianfeng Sun*a (a) CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China (b) University of Chinese Academy of Sciences, Beijing 100049, P. R. China *Corresponding author: [email protected]；[email protected]; [email protected]

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ABSTRACT: When a composite nanocable of single-walled carbon nanotubes (SWNTs) and amorphous carbon is fixed at one end and the other end is free, the free end is observed to be bent within a scanning electron microscope (SEM) and transmission electron microscope (TEM). The bending of the free end of the nanocable is found to be toward the reverse direction of incident electrons. And, the bending of the composite nanocable depends on the energy and exposure time of the incident electron beam. The mechanism of these observations is attributed to inhomogeneous temperature rise by absorption of incident electrons and a negative thermal expansion (NTE) of the composite nanocable along axial direction. The NTE property demonstrates the potential for using electron beam irradiation to investigate and manipulate SWNTs in nanodevices and making a zero expansion composite material.

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INTRODUCTION Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. Abnormal thermal expansion can be found in some materials, which include zero and negative thermal expansion materials, respectively. Negative thermal expansion (NTE) is an unusual physicochemical process in which some materials contract upon heating.1-2 NTE materials which undergo this unusual process have a range of potential engineering, photonic, electronic and structural applications.3 Since the discovery of single-walled carbon nanotubes (SWNTs), their unique one-dimensional, curved tube structure and outstanding electrical,4 mechanical, and chemical characters have attracted much attention. Fabrication of varieties of nanostructures based on SWNTs (cantilevers, pillars, springs, etc.) is needed for nano/micro-electromechanical systems (NEMS/MEMS), actuators, nanosensors and structures for biological applications.5-8 Among these studies, a great deal of effort has been put into the processing of SWNTs through energetic electron beam (e-beam) irradiation for their potential applications.7-8 Such as e-beam irradiation can induce shrinkage, modifying, cutting, or jointing of SWNTs.9-14 However, due to the nonequilibrium and nonlinear nature during the e-beam irradiation, the mechanisms underlying the experimental observations of SWNTs have been less understood.15 There are some theoretical calculations,16-20 which predict the existence of NTE behavior in graphene and carbon nanotubes. However, there has been a lack of direct experimental data to verify these theoretical simulations. 19 3



In this work, composite cables of SWNTs and amorphous carbon are irradiated under SEM and TEM. The cables are found to be bent toward the reverse direction of the incident electron beam. This bending is attributed to negative thermal expansion (NTE) of the composite cable due to inhomogeneous temperature rise by absorption of incident electrons. These results suggest the electron beam in SEM and TEM, which show the advantage of high special resolution, offers a high potential for engineering nanotubes at nanoscale.

EXPERIMENTAL SECTION The SWNT films were grown by floating catalytic chemical vapor deposition as reported previously.21 Ferrocene/sulfur powder is used as the catalyst and heated to 6575 °C at 1st furnace, which flows into the reaction zone in a mixture of 1000 sccm argon and 10 sccm methane at grown zone (2nd furnace) of SWNTs at 1100 °C. The time of the synthesis process depends on the thickness of SWNTs film, which takes about 1-2 hours in this work. The RBMs (radial breath modes) of Raman spectra of SWNTs are the most characteristic modes of SWNTs (Figure S1) and are used to determine the tube diameters. The diameter of individual SWNTs is in the range of 0.9-2.0 nm. These individual SWNTs aggregate into the form of bundles which are curled and randomly oriented. The bundles’ diameter is in the range of 30-50 nm and their length is about 10-20 µm (Figure S2). The SWNTs are purified by immersing SWNTs into hydrochloric acid (concentration is 1:10) for 24 h to remove the iron catalyst. 4


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Figure 1. Schematic diagram of the laser ablation method. (1) Fabrication of trenches on the substrate of SiO2/Si. (2) A SWNTs film covering the trenches and the substrate. (3) Laser direct patterning of SWNT above the trench. (4) At the edge of the trench, a composite nanocable can be obtained, which is fixed at one end while the other end is suspended and free to move. The composite nanocables of SWNTs and amorphous carbon were fabricated by a laser ablation method as shown in Figure 1. First, a purified SWNTs film was transferred onto a silicon wafer with trenches (width: 1-2 µm, depth: 200 nm). Then, the SWNTs film was condensed by dropping ethanol onto the film and dried in the air. Finally, laser direct patterning with power of 20 mW was carried out at the area above the trenches. At the edge of the trench, composite nanocables can be obtained as shown in Figure 1. For TEM characterization, copper grids with ordinary lacey support film are used and the size is 200 mesh. A SWNTs film was spread out on the copper grid. 5



Some ethanol was dropped onto the film to condense the SWNTs and the film was dried in air. Laser patterning with the power of 20 mW was carried out on SWNTs (Figure 1 (3)). At the edge of the laser pattern, composite nanocables can be found (Figure1 (4)). The SEM experiments were carried out with chamber pressure about 10−3 Pa. The ebeam irradiation and the in situ observations were carried out in an SEM operated at 10 kV or 15 kV and a TEM (FEI Tecnai F20) operated at 200 kV, respectively. Representative consecutive SEM or TEM images of the composite cables were taken during electron beam irradiation.

RESULTS AND DISCUSSION A high resolution TEM image of the SWNTs bundles induced by the laser ablation method is shown in Figure S3. It can be seen that SWNTs are in the middle of bundle, which are surrounded by amorphous carbon. This indicates that SWNTs bundles have become composite nanocables after laser etching. The amorphous carbon is formed during the etching process by laser in air and its thickness is in the range of 8-30 nm, which cannot be controlled and varies from one cable to another cable. The formation of the composite nanocable includes a chemical reaction of SWNTs with oxygen at high temperature. Thermo-gravimetric (TG) analysis of SWNTs indicates that oxidation temperature of SWNTs and amorphous carbon is ~638 °C and 411 °C in air, respectively.22 This means that when the temperature is higher than 638 °C, the SWNTs can be etched. Because air has low thermal conductivity, when the laser power is 6


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appropriate, the SWNTs on the substrate are etched little. But for SWNTs near the trenches, the temperature is high enough to etch most SWNTs and just several SWNTs bundle are left because the heat can be transferred to nearby SWNTs and substrate. The outer SWNTs becomes amorphous carbon and inner SWNTs remain because lack of oxygen. Thus, composite nanocables can be obtained at the edge of the trench with one end fixed and the other freely suspended. After laser direct patterning of SWNT above the trenches on the SiO2/Si substrate, composite nanocables can be obtained at the edge of the trenches. These nanocables are fixed at one end and the other ends are suspended and free to move as schematically shown in Figure 2a. When the sample is irradiated with electron beam, the SWNTs bundles are bended towards the reverse direction of incident electrons (Figure 2b).

Figure 2. (a) A diagram showing the composite cable (one end is fixed and the other end is free) before e-beam irradiation at room temperature. No temperature gradient and thermal stresses exist. (b) The composite nanocable is bended towards the reverse direction of incident electrons due to the e-beam irradiation. This observation is 7



attributed to inhomogeneous temperature rise by absorption of incident electrons negative thermal expansion (NTE) of the composite nanocable. In order to have a better understanding of the mechanisms for these observations, the samples were first studied with SEM, which is carried at room temperature with voltage of 10 kV. Figure 3 presents a sequence of observation of SEM images snapped to illustrate representative bending of a typical composite nanocable. Figure 3a shows the initial position of the nanocable before irradiation, which is straight and protruded from the substrate. One end of the nanocable is fixed and the other end is suspended and free to move, which is highlighted by a white arrow in Figure 3a. Figure 3b–3i show the subsequent e-beam irradiation-induced bending of the nanocable with the increasing irradiation time. The irradiation time is 0 s (a), 90 s (b), 210 s (c), (d) 350 s; (e) 490 s; (f) 710 s; (g) 1200 s; (h) 1500 s; (i) 1900 s, respectively. It is clearly observed that the nanocable is bended toward the reverse direction of the incident electrons, which is novel and interesting. When the irradiation time is larger than 1200 seconds (Figure 3g-3i), no obvious the bending of the nanocable is found. It should be mentioned that the white lines crossing the images are the edges of the trench on the substrate.

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Figure 3. (a−i) Representative in situ consecutive SEM images showing the bending of a typical nanocable. The typical nanocable is highlighted by a white arrow. The experiment was carried out under the accelerating voltage of 10 kV with varying irradiation time: ((a) 0 s; (b) 90 s; (c)210 s; (d) 350 s; (e) 490 s; (f) 710 s; (g) 1200 s; (h) 1500 s; (i) 1900 s. The e-beam direction is perpendicular to the surface of the X ”. The short nanocable is marked with a red SiO2/silicon substrate and marked with “○

square in (a) and (i), suggesting the dependence of bending on length of composite cables. The variables that affect the bending process of composite nanocables include accelerating voltage, beam current, exposure time and the diameter, length of nanocables. In Figure 4, the bending of another composite nanocable is shown under a higher voltage of 15 kV in SEM. The irradiation time is 0s (a), 90 s (b), 210 s(c), 300 s 9



(d), 420 s (e), 540 s (f), 760 s (g), 980 s (h) and 1300 s (i), respectively. Here, laser direct etching is carried out on a SWNTs film and composite nanocables can be found at wrinkles of SWNTs film, which act like the trenches on a SiO2/Si substrate. A typical nanocable is highlighted by a white arrow in Figure 4a. Figure 4b–4i show the subsequent e-beam irradiation-induced bending of the composite nanocable with increasing irradiation time. It can be seen that the nanocable also bends toward the reverse direction of the incident e-beam, indicating that this experimental phenomenon can be repeated. The diameter and length of the cable also have effect on the bending process, which is shown in Figure S4. It can be seen that all the bending of nanocables with different size is toward the reverse direction of the incident electrons. It should be mentioned that if SWNTs are replaced by MWCT, no one end-fixed cables are found (Figure S5), suggesting different conditions between MWCNT and SWCNT films by laser etching.

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Figure 4. (a−i) Top view of SEM images showing the bending of a composite nanocable of single-walled carbon nanotubes and amorphous carbon. It is highlighted by a white arrow. This was carried out under the accelerating voltage of 15 kV with varying irradiation time : (a) 0 s; (b) 90 s; (c)210 s; (d) 300 s; (e) 420 s; (f) 540 s; (g) 760 s; (h) 980 s; (i) 1300 s, respectively. The e-beam direction is perpendicular to the X ”. The short nanocable is surface of the SiO2/silicon substrate and marked with “○

marked with a red square in (a) and (i), suggesting the dependence of bending on length of composite cables. To study the effect of voltage on the bending process, the composite cables are also investigated with TEM under a voltage of 200 KV. Due to the much higher voltage, the bending of the cable usually happens so quickly that it is difficult to take images, which represent the position of the cable without irradiation. A typical example of bending of 11



a composite cable is shown in Figure 5. The original straight composite bends in less than one second (Figure 5a). And the bending process of the cable becomes much slower (Figure 5b-5f), where the irradiation time is 45 s (b), 10 2s (c), 145 s (d), 208 s (e), 328 s (f), respectively. In Figure 5, the red lines represent the projective distance between the tip end of the nanocable (marked with x) and the bending part (marked with y), respectively. The length between “x” and “y” is about 275 nm (a), 317 nm (b), 353 nm (c), 371 nm (d), 391 nm (e), and 430 nm (f), respectively.

Figure 5. (a−f) Consecutive TEM images showing the bending of an original straight composite cable of single-walled carbon nanotubes and amorphous carbon during electron beam irradiation. This is carried out with an accelerating voltage of 200 kV with varying irradiation time: (a) < 1 s; (b) 45 s; (c) 102 s; (d) 145 s; (e) 208 s; (f) 328 s. The red lines represent the projective length between the tip end of the composite cable (marked with “x”) and the bending part (marked with “y”), respectively. Scale 12


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bar: 100 nm. It is obvious that the bending of the cable toward the reverse direction of the incident electrons cannot be explained by collision mechanisms.13-14,

23-27

Based on

irradiation-induced temperature rise and temperature gradient along the composite cable, the novel bending of the composite cable can be properly explained with negative thermal expansion (NTE) of the cable. It should be noted that thermal expansion is properly represented as a second order tensor quantity for anisotropic materials. In this work, the composite cable is not isotropic and the CTEs (coefficient thermal expansion) of the cable can be represented as the components in axial and radial directions. From the observations of the bending of the composite cable towards the incident electrons, it is shown that the CTE along axial direction of the composite cable is negative. It should be mentioned that even for SWNTs alone, there are reports about negative thermal expansion along axial and radial directions. 20, 28 We hope to point out that loss of carbon atoms of SWNTs can be usually observed under TEM as reported previously.26 After electron beam irradiation for 110 seconds, obvious shrinkage is observed but no bending of SWNTs is found.26 Meanwhile, the loss of carbon atoms and depositions of amorphous carbon bas been reported by Fujita et al.29 The individual SWNT becomes shorter and thicker after being irradiated with electron doses of 1.0 C/cm2 but no bending of SWNTs towards the reverse direction of incident electrons is observed.29 As for loss of carbon atoms, it is usually observed in previous reports.26 However, no bending of SWNTs towards the 13



reverse direction of incident electron is observed, 29 suggesting the loss of carbon will not necessarily result in the bending of SWNTs. In this work, the nanocable becomes bended toward the reverse direction of e-beam without obvious shrinkage. This may be due to the nanocables are very thick, and the loss of carbon atoms cannot be observed. Thus, the unexpected bending is attributed to the NTE of the nanocable. Therefore, negative thermal expansion is the only reasonable cause for the unexpected bending of the composite cable. It should be mentioned that the observed phenomena in this experiment are stable. If the samples are kept at room temperature, the morphologies of the composite nanocables are almost the same after several months. And, the bending shown in Figure 3, Figure 4 and Figure 5 are not reversible. This means that the nanocable does not recover its original shape when the irradiation is turned off. This irreversibility of the bending of the cable is attributed to the loss of carbon atoms and depositions of amorphous carbon. It should be mentioned that two bending junctions should be observed if negative thermal expansion of the composite nanocables exists. Meanwhile, the observation of the bending of the cable also depends on other factors, such as loss of carbon atoms, depositions of amorphous carbon and length of the cable. For example, in Figure 3 and Figure 4, the bending of short cables marked with squares has not clearly observed. This may explain why a single bending junction of the composite cable is observed in this work. It should be mentioned that there are two kinds of technique to study the NTE of 14


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materials: direct and indirect measurements. For macroscale materials, expansion and compression behaviors can be observed when temperature increases or decreases in order to study NTE or normal thermal properties. However, for nanomaterials, when it is difficult to measure the local temperature and its gradient, indirect evidences are usually used. For example, to study the NTE in graphene16, the conclusions are obtained through Raman spectra. In this work, it is still a great challenge to measure the temperature and its gradient along a nanocable when the experiments are carried out in SEM and TEM. Thus, indirect evidences are provided to draw the conclusions of NTE in composite cable of SWNTs.

CONCLUSION In summary, the bending of composite nanocables of single-walled carbon nanotubes and amorphous carbon has been observed to be towards the reverse direction of the incident electrons during the electron beam irradiation in SEM and TEM. Continuous bending of the composite nanocable was observed and monitored as a function of the energy of incident electrons and exposure time. The mechanism of these observations is attributed to inhomogeneous temperature rise by absorption of incident electrons and a negative thermal expansion (NTE) of the composite nanocable along axial direction. The SEM and TEM based in situ observations demonstrate the potential of using electron beam irradiation to manipulate composite SWNTs cables. The NTE property demonstrates the potential for using electron beam irradiation to investigate and manipulate SWNTs in nanodevices and making a zero expansion composite material. 15



Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The radial breath modes in SWNTs, a typical SEM image of a SWNTs film, High resolution TEM image of a composite nanocable, SEM image of the nanocables with different size bended along the reverse direction of the incident electrons, SEM images of the MWCNT treated by laser etching. Acknowledgement X.C. and L.W. contributed equally to this work. This work was supported by the Major Nanoprojects of Ministry of Science and Technology of China (Grant No. 2018YFA0208403, 2016YFA0200403), National Natural Science Foundation of China (Grant No. 51472057, 11874129) and Baotou Rare Earth Research and Development Centre, Chinese Academy of Sciences (GZR 2018001).

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Experimental Evidences of Negative Thermal Expansion in a

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