Large-Scale Fabrication of Suspended, Aligned, and Strained Single

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Large-Scale Fabrication of Suspended, Aligned and Strained Single-Walled Carbon Nanotube Network Jian Zhang, Siyu Liu, Jean Pierre Nshimiyimana, Jia Liu, Xiao Hu, Ya Deng, Xiannian Chi, Pei Wu, Weiguo Chu, and Lianfeng Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10755 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry

Large-scale Fabrication of Suspended, Aligned and Strained Single-Walled Carbon Nanotube Network Jian Zhang,†,‡ Siyu Liu,† Jean Pierre Nshimiyimana,†,‡ Jia Liu,†,‡ Xiao Hu,†,‡ Ya Deng,†,‡ Xiannian Chi,†,‡ Pei Wu,†,‡ Weiguo Chu,†,∗ and Lianfeng Sun†,∗

†

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, Nanofabrication

laboratory, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. ‡

University of Chinese Academy of Sciences, Beijing 100049, China.

Abstract Large-scale fabrication of suspended single-walled carbon nanotubes remain a challenge, especially at specific locations and in specific directions. In this work we demonstrate an effective, fast and large-scale technique to fabricate suspended, strained and aligned SWNT networks, which is based on a dynamic motion of silver liquid to suspend and align the SWNTs between each two prefabricated palladium patterns in high temperature. The SWNTs are aligned in eight directions: up, down, left, right, upper right, low right, upper left, low left. The simulated calculations show that the driving force leading the silver liquid motion on the substrate is around 0.66 µN. The Raman spectra of the SWNTs network were measured, and the downshift of the G+ band indicates that for the suspended SWNTs, the uniaxial strain is around 0.13%. This technique could be extended to two-dimensional material systems and open the pathway toward better optoelectronic and nanoelectromechanical systems.

Introduction ACS Paragon Plus Environment

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Single-walled carbon nanotubes (SWNTs), as one-dimensional nanomaterials, have been drawing much attention due to their excellent mechanical, electrical and chemical properties.1-5 SWNTs are the most promising materials for future industrial applications, including field effect transistors, field emission displays, high-strength fibers and sensors.6-10 Many researchers have been interested in the fabrication of suspended SWNTs, owing to their significant advantages over those adhering to a substrate, especially in high-sensitivity required electronic and sensor application.11-13 The fabrication of suspended SWNTs bridging patterned fine structures is receiving a lot of attention, owing to their ideal applications for the wiring of nanoscale devices.14-17 However, controlled fabrication of SWNTs at specific locations and in specific directions must be possible before they can be used in wiring applications.18,19 Some groups have demonstrated that the SWNTs grew at specific locations

with

controlled

orientations

and

formed

networks

to

connect

nanostructures.19-21 Cassell et al. first demonstrated the directed growth of suspended SWNTs bridging the regularly patterned silicon tower structures.20 They used contact printing techniques to selectively deposit catalyst precursor materials on top of the silicon tower arrays, and eventually, well-aligned SWNT bridges and a square of suspended tubes bridges were obtained. Oh et al. fabricated the suspended SWNTs between SiO2 pillars via a direct lithographic route using a simple mixture of catalyst precursor and conventional electron beam resist.19 Sangwan et al. presented a simple method of creating high-quality suspended SWNT-FETs using a printing/lamination process.21 Their technique does not expose the SWNTs to any chemical treatment and

ACS Paragon Plus Environment

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could be generalized to a wide variety of substrates and electrodes. However, these methods have the disadvantages that the nanotubes are slack and have poor orientation in large scale. Here we demonstrate an effective, fast and large-scale technique to fabricate suspended, strained and aligned SWNT networks, which is based on a dynamic motion of silver liquid to suspend and align the SWNTs between each two prefabricated palladium patterns in high temperature annealing treatment. At the beginning, the isolated SWNTs with random orientations are deposited onto the silver film, under which there are palladium pattern arrays. In high temperature annealing, the silver film will become silver liquid and move toward the nearby palladium patterns. During this process, the SWNTs become suspended, strained and aligned between each two silver/palladium hemispheres. The SWNTs are aligned in eight directions: up, down, left, right, upper right, low right, upper left, low left. The strained properties are investigated by simulated calculation and verified by scanning Raman spectroscopic maps.

Experimental The detailed device fabrication process is shown in Figure 1. The first step is to prepare square-shaped palladium electrode arrays on SiO2 (300 nm)/ Si substrate. The electrode pattern was made using electron-beam lithography (EBL) followed by Pd/Cr (100 nm/5 nm) evaporation and lift-off process (Figure 1a). The side length of the square-shaped pattern and the distance between each two adjacent patterns are 1.5 µm and 3 µm, respectively. Then a silver film with a thickness of 200 nm was evaporated



on the wafer surface using thermal evaporation method (Figure 1b). In this work, the SWNTs were prepared by floating catalytic chemical vapor deposition.23 The ferrocene/sulfur powder, which is used as catalyst source, is heated to 68℃and flowed into the growth zone along with the mixed gas of 1000 sccm argon and 10 sccm methane. As shown in Figure 1c, the isolated SWNTs with random orientations were deposited on the silver surface with a specific quantity controlled by the deposition time. In this work, the deposition time was 30 seconds. Afterwards, the devices were placed in a furnace for annealing treatment, where a mixed gas of 800 sccm Ar and 90 sccm H2 was introduced. The devices were heated to 960 ℃ (melting point of the silver) at a rate of 10 ℃ per minute (totally ~ 94 min), then cooled down to room temperature (totally ~ 60 min) without soaking at 960 ℃. At this stage, the SWNTs were suspended and aligned between each two silver/palladium hemispheres, as shown in the schematic in Figure 1d.

Figure 1. Fabrication process of suspended, aligned and strained SWNT networks. (a) Preparation of square-shaped pallidum electrode arrays on SiO2/Si substrate. (b) Deposition of silver film (thickness: 200 nm) onto the wafer surface. (c) Deposition of some amount of isolated SWNTs with random orientations onto the silver film. (d) Suspended, aligned and strained SWNT


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networks between each two silver/palladium hemispheres after annealing treatment.

Before the annealing process, the SWNTs adhering on the silver surface were orientated randomly. In high temperature, the silver on the silicon substrate moved toward the palladium patterns, and meanwhile, the SWNTs became suspended and aligned. The SWNTs are aligned in eight directions: up, down, left, right, upper right, low right, upper left, low left, which are indicated by the eight arrows in Figure 1d. The detailed forming mechanism will be discussed below.

Results and Discussion Under the optimal fabrication parameters (annealing temperature: 960℃, thickness of the silver film: 200 nm, side length of the square-shaped pattern: 1.5 µm, height of the Pd pattern: 100 nm, distance between each two adjacent patterns: 3 µm), the suspended and aligned SWNTs networks were fabricated on a 1.5×1.5 cm2 scale substrate. The detailed information is shown in the supporting information.



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Figure 2. Large-scale fabrication of SWNT networks. (a) Optical image of large-scale SWNT networks. (b) (c) (d) Low-, middle- and high-magnification SEM images of the SWNT networks, respectively. After the annealing process, the SWNTs with random orientations become suspended and aligned between each two silver/palladium hemispheres.

Figure 2a shows the optical image of large-scale SWNT network on a silicon substrate. Figure 2b-d show Low-, middle- and high-magnification SEM images of the SWNT networks, respectively. After the annealing process, the SWNTs with random

orientations

become

suspended

and

aligned

between

each

two

silver/palladium hemispheres. The SWNTs are aligned in eight directions, which are shown by the eight arrows.

Figure 3. Mechanisms of the suspended and strained SWNTs. (a) An isolated SWNT adhering on the surface of the silver film after deposition. (b) The break of the silver film and the motion of the silver liquid on the substrate in annealing process. (c) Suspended and strained isolated SWNT between two adjacent silver/palladium hemispheres. (d) 60°tilted high-magnification SEM image of the suspended and strained SWNTs between silver/palladium hemispheres.

We demonstrated the effective and fast fabrication of suspended and aligned SWNT networks on large-scale. To further develop this fabrication technique in


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broader application of low-dimensional materials based devices, the rational formation mechanism of the suspended SWNT networks should be discussed. The schematic illustrations of a suspended individual SWNT formation process are shown in Figure 3 a-c. These interesting behaviors of the SWNTs in annealing process mainly result from the differences of melting point and wetting behavior between the palladium patterns and silver film. There are two key points that should be considered. Firstly, at the temperature of 960℃, the silver film melts into a liquid state while the palladium patterns keep a solid state. Secondly, after the silver film changes into liquid-state particles, the particles on the silicon substrate will aggregate into the larger silver particles at the palladium patterns due to the surface tension. After the SWNTs were deposited on the silver surface, as shown in Figure 3a, the sample was placed into the tube of the furnace and the annealing treatment started. When the temperature raised above the melting point of the silver film, which was 960℃, the silver film would be unstable and turn into liquid state gradually. In order to satisfy the minimum surface energy, the silver liquid tended to form spherical particle-like shapes, which could be explained by the Plateau-Rayleigh instability.24,25 As shown in Figure 3b, in high temperature, the silver film was broken and changed into liquid-state particles. The previous work reported that the contact angles of silver liquid on SiO2 substrate and palladium surface are 123° and 36°, respectively.11 These results mean that the silver liquid has a hydrophobic-like behavior on SiO2 substrate, while on the palladium, it has a hydrophilic-like behavior. When a liquid-state silver particle is on the silicon substrate and contact with a larger particle on the palladium



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pattern, there is a surface tension force exerted on the particle due to the existence of the solid-liquid-gas interface on the particle surface. The surface tension acted along with the three-phase contact line, and of which the horizontal component would drive the particles to move. In this work, the proposed model for the surface tension is validated in a similar way. When a liquid particle with a shape of a spherical cap sits on the surface of a particle as shown in Figure 3b, the surface tension is theoretically calculated by:26

or = 2 π cos

(1)

where is the surface tension of pure silver liquid at 961 °C, which is 0.96 Nm-1.27 R is the radius of the circle of the three-phase contact line, which is estimated to be 200 nm. is the contact angle of the silver liquid on silicon substrate and the value is 123°. The Equation (1) gives F (or F’) ≈ 0.66 µN. The directions of F or F’ are toward the palladium patterns. This force drives the liquid silver particle to move toward the palladium patterns and aggregate with the larger particles on the palladium patterns. It is necessary to note that the force exerted on the SWNT is smaller than this value (0.66 uN), which depends on the diameter of the SWNT. For each palladium pattern (such as marked 0 in Figure 1d and Figure 2d), there are eight adjacent palladium patterns, which are 1 (up), 2 (down), 3 (left), 4 (right), 5 (upper left), 6 (upper right), 7 (low left) and 8 (low right). As a result, the silver surrounding the palladium pattern has eight moving directions in high temperature. Since the distance between pattern 0 to patterns 1-4 is 3 µm, which is smaller than the distance between pattern 0 to patterns 5-8 (4.2 µm), the liquid silver on the silicon


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substrate is easier to aggregate with the silver on the palladium patterns 1-4. This results in the quantity difference of SWNTs in different aligned directions. It should be noted that silver has good wetting interaction with carbon materials. As shown in the high-magnification SEM image of the SWNTs between silver/palladium hemispheres (Figure 3d), the SWNT is embedded into the silver, indicating the hydrophilic-like behavior of SWNTs to liquid silver. We usually check if the SWNTs are suspended by SEM at higher magnification and at higher angle of incidence of electron beam. As shown in Figure 3d, the SWNTs are connected to the parts of Ag electrodes that are above the substrate. This means that there exists a height difference between the SWNT and the substrate, indicating that this SWNT is suspended. Through high-magnification SEM image, we find the SWNTs are mostly in bundle form, which might be formed in the CVD growth or annealing process. In order to further characterize the properties of the suspended SWNTs, the Raman spectra were measured. The theory calculation shows that when uniaxial strain increases, the frequencies of RBM mode almost do not change, while the G band are downshifted linearly.28 Furthermore, the linear shifting rate of Raman modes under uniaxial strain is almost the same for SWNTs with different diameters.29



Figure 4. Raman spectra characterization of the SWNT networks. (a, c) Scanning Raman

spectroscopic maps plotting (a) G+ band frequency and (c) 2D band frequency of the SWNT networks. These white circles mark the positions of the patterns. (b, d) (b) G+ band spectra and (d) 2D band spectra of the SWNTs on silicon substrate (Subs-SWNTs, in black), on the silver hemispheres (Silv-SWNTs, in red), and suspended up off of the substrate (Susp-SWNTs, in blue).

Figure 4a are the scanning Raman spectroscopic maps plotting G+ band frequency. The white circles mark the positions of the patterns. In the network, the SWNTs can be divided into two parts: in part I, the SWNTs are suspended and strained; in part II, the SWNTs are on the surface of the silver and strained-free, and their typical SEM image is shown in Figure 3d. The G+ band frequency spectroscopic maps show the suspended part I has lower frequency than that in part II, which can be identified by the color difference. Figure 4b shows G+ band spectra of the SWNTs on silicon substrate (Subs-SWNTs, in black), on the silver hemispheres (Silv-SWNTs, in red), and suspended up off of the substrate (Susp-SWNTs, in blue). Compared to the frequency (1594.5 cm-1) of the SWNT on silicon substrate (in black), the frequency (1592.3 cm-1) of the SWNTs suspended up off of the substrate (in blue) is downshifted


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by 2.2 cm-1. A previous work demonstrated that there was a consistent trend of increasing downshifts in these Raman modes with increasing uniaxial strain, and the downshift rates of G+ band was 16.7 cm-1 %-1.29 Thus for our suspended SWNTs, the uniaxial strain is calculated to be about 0.13%. It should be noticed that the frequency (1596.1 cm-1) of the SWNTs on the silver hemispheres (in red) is upshifted by 1.6 cm-1, which could be the influence of the silver metal.30 Figure 4c-d are the 2D band Raman spectroscopic maps and 2D band spectra of the SWNT network, respectively. The characteristics of the 2D band frequency are similar with that of the G+ band. In Figure 4, the shifts of the G+ band frequency or 2D band frequency show directional dependence: along the lateral direction the decreasing of the band frequency is larger than that along vertical direction. The reason is attributed to that the density of SWNTs in this location is non-uniform: the number of the SWNTs along the vertical direction is more than that along the lateral direction.

Conclusions In summary, in this work we demonstrate an effective, fast and large-scale technique to fabricate suspended, strained and aligned SWNT networks, which is based on a dynamic motion of silver liquid to suspend and align the SWNTs between each two prefabricated palladium patterns in high temperature annealing treatment. The SWNTs are aligned in eight directions: up, down, left, right, upper right, low right, upper left, low left. The simulated calculations show the driving force of the silver liquid moving on the substrate is around 0.66 µN. The Raman spectra of the SWNTs network was measured, and the downshift of the G+ band indicates that for the



suspended SWNTs, the uniaxial strain is around 0.13%.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge. SEM images of the devices treated with different annealing temperatures, SEM images of the devices with different sizes of the Palladium patterns, SEM images of the devices with different height of the Palladium patterns.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (L. Sun); [email protected] (W. Chu)

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Science Foundation of China (Grant No. 51472057) and the Major Nanoprojects of Ministry of Science and Technology of China (2016YFA0200403).

REFERENCES (1) Zhang, S.; Kang, L.; Wang, X.; Tong, L.; Yang, L.; Wang, Z.; Qi, K.; Deng, S.; Li,


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Graphical TOC Entry



Figure 1. Fabrication process of suspended, aligned and strained SWNT networks. (a) Preparation of squareshaped pallidum electrode arrays on SiO2/Si substrate. (b) Deposition of silver film (thickness: 200 nm) onto the wafer surface. (c) Deposition of some amount of isolated SWNTs with random orientations onto the silver film. (d) Suspended, aligned and strained SWNT networks between each two silver/palladium hemispheres after annealing treatment. 181x124mm (300 x 300 DPI)


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Figure 2. Large-scale fabrication of SWNT networks. (a) Optical image of large-scale SWNT networks. (b) (c) (d) Low-, middle- and high-magnification SEM images of the SWNT networks, respectively. After the annealing process, the SWNTs with random orientations become suspended and aligned between each two silver/palladium hemispheres. 143x141mm (300 x 300 DPI)



Figure 3. Mechanisms of the suspended and strained SWNTs. (a) An isolated SWNT adhering on the surface of the silver film after deposition. (b) The break of the silver film and the motion of the silver liquid on the substrate in annealing process. (c) Suspended and strained isolated SWNT between two adjacent silver/palladium hemispheres. (d) 60°tilted high-magnification SEM image of the suspended and strained SWNTs between silver/palladium hemispheres. 204x121mm (300 x 300 DPI)


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Figure 4. Raman spectra characterization of the SWNT networks. (a, c) Scanning Raman spectroscopic maps plotting (a) G+ band frequency and (c) 2D band frequency of the SWNT networks. These white circles mark the positions of the patterns. (b, d) (b) G+ band spectra and (d) 2D band spectra of the SWNTs on silicon substrate (Subs-SWNTs, in black), on the silver hemispheres (Silv-SWNTs, in red), and suspended up off of the substrate (Susp-SWNTs, in blue). 186x157mm (300 x 300 DPI)


Large-Scale Fabrication of Suspended, Aligned, and Strained Single

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