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A new strategy to prepare carbon nanotube thin film by the combination of top-down and bottom-up approaches
Journal Pre-proof A New Strategy to Prepare Carbon Nanotube Thin film by the Combination of Topdown and Bottom-up Approaches
Yun Wang, Dong Lu, Fei Wang, Dongxing Zhang, Jing Zhong, Binghao Liang, Xuchun Gui, Li Sun PII:
S0008-6223(20)30109-3
DOI:
https://doi.org/10.1016/j.carbon.2020.01.090
Reference:
CARBON 15025
To appear in:
Carbon
Received Date:
14 November 2019
Accepted Date:
24 January 2020
Please cite this article as: Yun Wang, Dong Lu, Fei Wang, Dongxing Zhang, Jing Zhong, Binghao Liang, Xuchun Gui, Li Sun, A New Strategy to Prepare Carbon Nanotube Thin film by the Combination of Top-down and Bottom-up Approaches, Carbon (2020), https://doi.org/10.1016/j. carbon.2020.01.090
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A New Strategy to Prepare Carbon Nanotube Thin film by the Combination of Top-down and Bottom-up Approaches a,b Yun
Wang, cDong Lu, bFei Wang, a,* Dongxing Zhang, c,* Jing Zhong, dBinghao Liang, d,* Xuchun Gui, b,* Li Sun
a School
of Material Science and Engineering, Harbin Institute of Technology, Harbin 150090,
China. b Department c Key
of Mechanical Engineering, University of Houston, Houston, TX 77204, USA
Lab of Structure Dynamic Behavior and Control (Harbin Institute of Technology), Ministry
of Education, Harbin 150090, Heilongjiang, China d
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and
Engineering, Sun Yat-sen University, Guangzhou 510275, China.
Abstract: Bottom-up and top-down are two different strategies to prepare nano-materials and their assemblies, each with their own pro-and-cons. Here, we demonstrate a combined approach in preparing large area uniform carbon nanotube (CNT) thin films within minutes, several orders of magnitude faster than conventional methods. This strategy takes full advantage of the unique micro-structure of CNT sponges, synthesized by a bottom-up process and composed of physically entangled CNTs with uniform size and spatial distribution. Such a 3D network structure allows us to firstly transfer certain amount of structured CNTs from the sponge onto a Scotch tape. Through a second-step stamping process, which is essentially a top-down process, transparent and conductive (TCF) CNT films with controlled micro-structures are produced on viscous substrates. Bonding strength between the stamped CNT network and elastomer substrate can be adjusted to optimize the physical properties including the transparency and sheet resistance of the CNT thin films. Upon stretching, the device exhibits high piezoresistive responsiveness; combining high sensitivity, low hysteresis and large working strain range. The proposed methodology had been extended to fabricate micro-electrodes on patterned elastomers with same effectiveness. These experimental results highlight the high efficiency, low-cost and versatility of this approach in preparing TCF electrodes with different sizes and shapes.
Email: [email protected]; [email protected]; [email protected]; [email protected].
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1. Introduction Science and engineering communities are actively searching for flexible, transparent and conductive films in recent years [1-5]. Through decades of research-led advancement, ceramic based transparent and conductive films(TCFs) have already found extensive applications in solar cells, lasers, optical communication components, and solid-state lighting [6-10] devices. However, brittleness and cost-effectiveness severely limit their adoptions in recyclable, wearable and adaptable applications, in which devices availability, stretchability and environmental friendliness becomes important. Flexibility/stretchable ability in TCFs can often be realized by integrating various conductive nano-materials, including graphene, carbon nanotube (CNT), or metallic nanomaterials, with soft substrates [11-15]. Among these active materials, CNT has the advantages of light weight, mechanical robustness, chemical stability and tunable electrical/thermal conductivity. CNTs can be grown by various CVD processes, which are usually not compatible with polymeric substrates, thus the formation of CNT TCFs often utilizes coating approaches to obtain a conductive network structure on substrate after CVD growth of CNTs. Indeed, a number of wet coating strategies have been explored since 2004, including drop-drying from solvent, airbrushing, electrophoretic deposition, Langmuir-Blodgett deposition, vacuum-assisted filtration and centrifugal casting [1113]. For instance, Wu prepared a TCF of pure single-walled carbon nanotubes by filtration, achieving a sheet resistance of 30 /sq and a transmittance of 70% [3]. Pei et al reported the synthesis of uniform CNT coatings on substrates by electrophoretic deposition, with a resistance and transmittance of 220 /sq and 81%, respectively [11]. Such wet coating processes often require the preparation of highly uniform colloidal dispersion using toxic solvent and/or surfactants, thus create negative environmental impacts and also cause damages to CNT . Other than wet coating approaches, direct transfer of CVD grown CNT onto substrates has also been explored. In 2005, Zhang prepared TCFs by direct drawing CNTs from a forest structure and realized a resistance and transmittance of 750 /sq and 70%, respectively [12]. Through drawing a super-aligned CNT forest, Feng further demonstrated a sheet resistance of 208 /sq with a transmittance of 90% can be achieved [13]. However, the adoption of such CVD strategy is oftern 2
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limited by the size of CVD tools, thus not be suitable for the synthesis of large area TCFs. Obviously, CNT TCFs performances will firstly be influenced by the structure, size and quality of CNTs, which heavily rely on CVD synthesis [6-8]; yet factors including CNT layer micro-structure, inter-CNT interaction and CNT-substrate interaction are even more important. Retooling and optimizing CVD synthesis parameters are not very effective for these approaches, and new strategy is needed to improve efficiency, reproducibility and cost-effectiveness. From the perspective of CNT TCF preparation methodology, both the wet spin and CVD process represent bottom-up approaches. Normally, the bottom-up strategy is considered as having higher accuracy, yet with relatively lower efficiency as compared to top-down methods. Therefore, the combination of the two strategies can help to overcome their intrinsic limitations and utilize their unique benefits in preparing stretchable CNT TCFs. In this study, for the first time, we report on the synthesis of CNT TCFs through a two-step dry transfer-stamping process using CNT sponge as the starting material. The CNT sponge was CVD grown and have a porous aerogel structure [14] in which the CNTs are randomly distributed and entangled in 3D space. The microstructure of the CNT sponge, including the CNT size, density, tortuosity, entanglement degrees, can be tuned during the CVD growth process (bottom-up process). We then peel off a thin layer of CNT sponge with a modified structure by dry transfer process. Although the CNT sponge used here is also synthesized by CVD, its thickness will allow large number of transfer operations, which are conducted in ambient environment. Recently, we demonstrated the fabrication of CNT films on rubber substrates by direct stampingas well as the potential applications of such a film[16]. However, the quality of the CNT structure needs to be significantly improved. In this work, we report on the development of a new two-step transfer process, using a tape as medium to reconstruct the distribution of the peeled-off CNTs from CNT sponge, which greatly improves the uniformity of the CNT thin film as compared to the previous work. Furthermore, we studied related physical mechanisms and demonstrated potential applications of such CNT layers as stretchable electrodes for flexible devices. This two-step transfer-stamping process can yield nanometer thick CNT network electrodes on various polymeric substrates. By combining the bottom-up sponge growth and top-down dry 3
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transfer, the high throughput, low cost and reproducible fabrication of stretchable TCFs can be realized. Adjusting substrate viscosity and stamping repeats, transparency and sheet conductance of the TCFs can be manipulated d. In addition, adhesion between the TCFs and substrate can be tuned to improve bending fatigue life. Furthermore, we demonstrate the fabrication of micro-sized CNT electrodes using this transfer method, on polymeric patterns printed by micro-plotting. As a proof of concept, piezoresistive sensors directly utilizing such TCFs were evaluated in terms of stability and repeatability.
2. Experimental 2.1 Materials CVD grown CNT sponge, as having been reported in ref. 14, was used in this study [14]. The sponge growth process include: to first dissolve ferrocene powders (as catalyst) in 1,2dichlorobenzene (carbon source) to form a ~0.06 g ml−1 solution; the solution is then injected into a 2-inch diameter resistively quartz tube furnace use a syring pump. A tyical flow rate is 0.13 ml min−1 and the reaction temperature is 860 °C. A mixture of Ar and H2 is used as carrier gas, with respective flowrate of 2000 ml min−1 and 300 ml min−1. CNT sponge was collected from a quartz sheet place in the reaction zone. The stretchable substrates used in this study were polydimethylsiloxane (PDMS) layers prepared from Dow Corning (SYLGARD 184) with four mixing ratios of 10:1, 15:1, 20:1 to 30:1 of pre-polymer (base) and cross-linker (curing agent), corresponding samples are named as PDMS10, PDMS15, PDMS20 and PDMS30. Polymeric patterns were printed by Sono Micro-plotter II using PDMS10. Micro-plotter pipette has an opening diameter of 100μm. 2.2 Two-step CNT TCFs synthesis from CNT sponge A two-step transfer method has been developed. First a strip of Scotch tape was applied to the CNTs sponge with a pressure using a doctor blade with a constant speed at 1mm/s, the blade is adjusted to be perpendicular to the CNT sponge surface. No visible deformation is induced to the sponge, ensuring that only the CNTs from the top layers of the sponge were adhered to the Scotch tape. After ~1min, the Scotch tape was then peeled off from the sponge along the 4
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longitudinal direction of the strip as shown in Figure 1(h), at a peeling rate of ~1 mm/s. Peeling rate and angle can affect the quality of transferred CNT film, and slower rate was found to be more beneficial in reducing the CNT film roughness formed on the tape. The second stamping process helped to transfer the CNTs from Scotch tape to flexible substrate. For the preparation of CNT TCFs with random CNT distribution, the stripe of Scotch tape with adhered CNTs was gently laminated onto a PDMS substrate in a parallel fashion as shown in Figure 1. In contrast, for the CNT TCFs consists of aligned CNTs, the Scotch tape was slide downwards. The transfer process can be carried out multiple times, and the corresponding sample is named as PDMS303, for example, which refers to film formed on PDMS30 substrate with 3 times of stamping from the Scotch tape. Finally, the PDMS can be delaminated from the Scotch tape by peeling off the tape. This stamping process can be repeated in increasing the transferred CNT layer thickness on substrate. 2.3 Characterization The microstructure of CNT TCFs was investigated by a scanning electron microscope (SEM; Nova Nano SEM 463). Electrical conductivity and transparency measurements were conducted with a standard four-probe arrangement using an electrochemical workstation (Cordova, TN, USA) and a laser reflection interferometry, respectively. The flexibility of the CNT TCFs was tested by repeatedly bending the film with a home-made two-point bending device, with a sample radius of curvature set at ~10 mm. The piezoresistive properties were characterized in the tensile mode using a DMA Q800 (TA Instruments).
3. Results and discussion: Direct transfer of CNTs from CNT sponge to substrate was found to result in highly non-uniform transferred layers with poor thickness and uniformity control, thus greatly hinders their applications [16]. This might be related to the characteristic length scale (SLS) at dozens of micrometers of the average distance of CNT segment between adjacent physical CNT cross-points, which endow CNT sponge with relative high strength, elasticity and conductivity [14]. When a CNT sponge is directly pressed onto a PDMS substrate, only a few loose CNTs can be transferred 5
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with high degree of non-uniformity at micro-meter scale. Namely, the uniformity of CNT film should be controlled by the SLS. To improve thin CNT film uniformity, it is needed to de-entangle transferred CNTs from sponge micro-structure, which could significantly increase SLS. Therefore, we consider the first transfer to a Scotch tape to be a critical step in controlling final TCFs quality in such a two-step method. Figure 1 (a, b) illustrates the two-step transfer-stamping process for the preparation of CNT TCFs. When sponge came into contact with the tape surface, first adhered CNTs would act as anchors during the afterward peeling process. As shown in Figure 1(h), significant amount of CNTs pullout and re-arrangement occurs during this first transfer process, depending on applied pressure and how to peel off the tape from the sponge. As shown in Figure 1 (c, d) those transferred CNTs on Scotch tape form a more fluffy and uniform surface structure, compared to the CNTs at sponge surface; and more interestingly, the CNTs on Scotch tape exhibit improved degree of alignment (Figure 1(c)), indicating the successful de-entanglement of the original physical crosslinks, which is critical to the quality of TCF. In other words, transferred CNT layer on Scotch tape (Figure 1 (c, d)), serving as an intermediate buffer layer, possesses an improved uniformity as compared to the sponge surface (Figure 1(e)). Figure 1(c) also shows that the CNTs on Scotch tape have a denser bottom layer that strongly binds to the adhesive tape, and a loose top layer with the CNTs partially aligned. It is the top layers of the CNTs will then be transferred to the stretchable substrate to form TCFs, which will result in randomly distributed and transparent CNTs film. Figures 1 (f, g) show the morphology of CNTs transferred onto a PDMS 10 substrate. It can be seen that that CNTs are more or less randomly distributed, after first transfer . One of most important advantages of the two step transfer approach is that the properties of the transferred CNT TCFs can be easily controlled by both the viscosity of the PDMS substrate and the number of repeating transfer. As shown in Figures 1 (j, m-q), as the number of transfer increases, the thickness of the TCF increase at the cost of decreasing in transparency. A continuous CNT network already forms covering the entire PDMS substrate surface after onetime transfer (Figure 1(m)), resulting in a combination of high transparency (~95%) and conductivity. Such a conductivity at transmittance rate of 95% is higher than most of the 6
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reported results, which were obtained by complex procedures such as vacuum assisted filtration, spraying or electro-templating [11-13]. Such a performance indicates the formation of high percentage of conductive physical contacts in the current CNT network , with few isolated CNTs that reducesthe transparency. This characteristic essentially inherits the unique microstructure of the CNT sponges, highlighting the advantage of this method. Figure 1(i) demonstrates the uniformity of macroscopic transparency of a sample after five transfer repeats, and the inset SEM image in Figure 1(i) confirms the microscopically uniform CNT distribution. In comparison, for a sample with fewer transfer repeats, the SEM images also show decreasing CNT distribution uniformity Figure 1(f, g). Fuigure 3(a) also shows that both of the transparency (80%) and conductivity (~8 K/sq) saturate when then transfer repeated 5 times, with this specific material system and transfer procedure. This phenomenon suggests that the formation of TCF on PDMS substrate in our approach is mainly controlled by the Van der Waal’s interaction between the CNT layer and PDMS substrate, rather than that between CNT layers from consecutive transfer processes. Considering that the Van der Waal’s interaction range is generally limited to dozens of nanometers, which is smaller than the diameter of CNT itself, the areas on PDMS substrate where there are already CNTs exist from the previous transfer, could screen the Van der Waal’s interaction from PDMS substrate. In such a scenario, it is only the blank and exposed areas on PDMS substrate can adhere CNTs from the following transfer process. Note that, this is very different from the general understanding of CNT interaction, for which the attraction force between CNTs is belived to be relative strong and results in CNT bundling and aggregation. We believe that it is because of the loose CNT structure with certain degree of alignment on the Scotch tape obtained from the first transfer step. Specifically, when the aligned CNTs on Scotch tape are brought to contact with PDMS substrate partially covered by CNTs, CNTs will either directly contact with exposed PDMS substrate or CNTs that already existed from previous transfer. For the later case, CNTs will re-distribute to the nearby bare PDMS substrate and bending and buckling can occur due to external compressive force and gradual increase of the Van der Waal’s attraction from the PDMS substrate. Therefore uniformity of the CNT layer can be improved .
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Transparency and conductivity of CNT based TCF can be adjusted through changing PDMS viscosity. Figure 3(b) shows the effect of substrate viscosity where samples with one-time stamp are compared. For the substrate with highest viscosity (PDMS30), the sheet resistance was lowered to 29 K/sq, at the expense of transmittance (73%). However, the morphology of CNT TCF with high conductivity is very different from that obtained from multiple transfer processes. As shown in Figure 1(l), when PDMS with higher viscosity is used (PDMS30), the transferred CNT thin film forms micro-wrinkles with an average wavelength of ~2 μm. This is very important for the realization of stable piezoresistive properties without noticeable hysteresis as will be elaborated below. Upon the peeling off the Scotch tape from the PDMS substrate, the surface layer of PDMS modified with adhered CNT TCF will be stretched accordingly because of the high PMDS stickiness, which could re-orientate the CNTs to certain more stable directions as have been reported by many groups. When the Scotch tape eventually separated from the PDMS substrate, the elastic PDMS springs back to its original shape, generating significant compressive stress on the CNT TCF which results in the formation of wrinkles (Figure 1(l)). We emphasize here that the current method is mainly taking the advantage of the existed porous and percolated CNT structure transferred from the sponge to Scotch tape. This is very different from the vacuum assisted filtration method, in which a percolated and conductive network is gradually formed, and thus is time-consuming and highly sensitive to the dispersion quality of CNT. Therefore, the dry transfer formation of TCF reported here can be considered as a top-down approach, while the CNT growth of sponge is based on a bottom up strategy. In other words, we essentially combine the unique advantages of bottom-up and top-down. Conventional optical techniques like spectroscopic ellipsometry, laser reflection interferometry is widely used for determining thin film thickness or growth rate [15, 17]. Here laser absorption measurements on CNT TCFs were performed to reveal thickness variation and uniformity. A diode laser with 473nm wavelength was loosely focused on sample (Figure 2(a)). A photodiode was used to collect transmitted light intensity, which was sensitive to film thickness and film light absorption. Since the size of laser spot is ~50 microns, the accuracy of this test for the uniformity of TCF is also the average over a range of tens of microns. A 3-axis tranlation stage enabled us to 8
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get optical information over a large sample area. As we can see from Figure 2(b) and Figures 1 (m-q), as transfer repeat times increase, less transmitted light measured, indicating more light absorption in CNT TCFs due to increased effective film thickness. The low standard deviation of scanned intensity data got from individual sample mean relatively constant transmitted light intensity values in different transfer repeat times thin films. From uniformity point view, in Figure 2(d), as transfer time increase, uniformity increase with smaller standard deviation data value, which means better thin film quality. Further, considering different curing agent ratio, higher curing agent brings higher viscosity PDMS, leading to better uniformity during each transfer process. Figure1 (c, e) show thatthe film transparency decreases when less curing agent is added in the PDMS, which means the formed CNTs films have a increasing thickness, which is confirmed by the . corresponding SEM images of Figure 1(j lower) and Figure 1(l). To further analyze underlining physics of the CNT networks formed by our stamping process, we adopted selective models to investigate the thin film conductivity and transparency relationship to the CNT microstructure. For two dimensional randomly distributed stick model [18], the critical density can be estimated by 𝑙√𝑁𝑐 = 4.236 Here, 𝑙 is the length of CNT and 𝑁𝑐 is the value of the critical density, above which a percolated network is formed. Assuming an average CNT length of ~20 μm (as we reported in [14]), we calculated a theoretical critical density of 1.43 CNT per 100 μm2, which is way below the density of the CNTs on PDMS10 substrate as can be seen in Figure 1f. This suggests that CNTS with much lower density or higher porosity can be used to further increase the transparency while maintaining the electrical conductivity. The transparency T of a thin metallic film in air can be modeled by Equation 2, assuming that the thickness of the film is much smaller than the wavelength [19]. Here, dc is the DC conductivity, ac is the optical conductivity, and d is the film thickness. 𝑇=
1
(
2 1 + 𝑎𝑐 𝑑 𝑐
= 2
) ( 9
1 2 𝑎𝑐 1+ 𝑐𝑅𝑑𝑐𝑑𝑐
)
2
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Using the film thickness on the order of 50-100 nm and optical wavelength around 550 nm, one can estimate the value of
𝑎𝑐 . 𝑑𝑐
We find that 𝑎𝑐=2.5𝑑𝑐 provides the best fit to this model, in
general agreement with reported by other groups [20-21]. As a proof of concept, electromechanical properties of the CNT TCFs were studied under different deformation modes. As can be seen in Figure 4(a), the CNT TCF on PDMS30 exhibits high electrical robustness upon mechanical deformation after 10000 cycles of repeated bending (bending radius of ~10 mm), while for the TCFs on PDMS10 and PDMS20, significant increment of resistance emerges after ~2000 cycles. This is due to the different CNT morphologies on PDMS with different viscosity. As been explained, the CNT TCF on PDMS30 substrate, which is very sticky, can be reorientated and form wrinkles during the second transfer step. Such features could help the sensor to accommodate external strain without destroying the CNT conductive network [22-24]. Again, it should be emphasized that the wrinkle forms naturally during our dry transfer process, rather than through substrate pre-stretching followed by transferring as reported before [23]. Therefore, substrate with highest viscosity (PMDS30) could be used for the preparation of highly stable bendable conductor, whereas low viscosity substrate for strain sensors. We further tested the piezoresistive performances of the CNT TCF on PDMS15 under tensile strain. As shown in Figure 4(b), the change of resistance with strain exhibits a three-stage variation. In the first stage (0-5% of strain), the resistance increases linearly with a gauge factor as high as ~50, making our device one of the most sensitive sensors reported so far [25-26]. In the second stage (5% to 27% of strain), the resistance increases nonlinearly with certain fluctuations and a gauge factor of ~8. This probably resulted from the Poisson's ratio effects, in which the compression along the transverse direction could increase the backbone density of CNT conductive network, which can partially compensate the resistance increase resulted from the extension along longitudinal direction. When the strain exceeds 27%, corresponding to third stage, the sensor exhibits linear characteristic again with a gauge factor of 37. Such re-entry into the linear range suggests the complete of the structuring of the CNT network maybe caused by the Poisson's ratio effect. The loading and unloading cycles of the sample exhibit stable responses with no-significant hysteresis (symmetric loading and unload branches). As the amplitude of the strain oscillation increases, 10
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the change of resistance also increases accordingly. Note that hysteresis is mainly due to the reorientation of CNTs upon the stretching, indicating a high interfacial bonding strength between CNTs and PDMS, which has been discussed by Jin [27]. Such a high and stable CNT-substrate interfacial bonding is probably resulted from our unique transfer process. In other words, the fluffy and loosely connected CNTs structure on the tape allow CNTs to adjust themselves for the position and orientation to eventually achieve minimum Gibbs energy for the system, which helps to reduce hysteresis. A combination of high sensitivity, low hysteresis, and wide working range (~ 40% of strain) as been demonstrated here had been rarely reported before because these characteristics generally go against each other for a network structure. For instance, the high piezoresistive sensitivity often relies on the creation of delicate conductive network structures, while large measurement range requires the conductive network to be able to be robust. In the meanwhile, low hysteresis requires high “elasticity” of a conductive network. In this study, CNTs in direct contact with PDMS have strong bonding with the substrate and result in low hysteresis. In addition, the strain sensors tested in the frequency range from 0.1 Hz to 10 Hz all show consistent and stable piezoresistive responses. The symmetry of the loading and unloading curves increases with the frequency, which is a typical characteristic of polymer substrate. To reduce CNT/polymer structure viscoelastic effect, we are in the process of developing a bilayer strain sensor structure where a thick layer of elastic polymer is coupled with the sensing layer. And to improve this behavior without losing the advantage of using substrate viscosity, we propose that a double layered polymer substrate consists of a thick polymer layer with low viscosity and a thin polymer layer with targeted viscosity can be employed, which will be studied systematically in future. To demonstrate the practicality and versatility of this dry transfer method, this technology was combined with micro-plotting to produce micro-CNT TCF electrodes. In this demonstration, viscous PDMS was used as ink for micro-plotting to create sinusoidal patterns on glass substrates, CNTs were then selectively transferred from Scotch tape (with CNTs from sponge) onto the PDMS patterns. Transferred CNTs on PDMS patterns also show uniform surface distribution (Figure 5(a)) at miron sizes . Further study on improving microelectrode performances is underway. 11
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The CNT sponge in this study can be repeatedly used, thus the transfer synthesis proposed here is efficient and cost effective. CNT sponge properties including CNT size, density, porosity, alignment can be adjusted to tune the performance of TCF. This method can be easily scaled up. Without the need of using any chemicals and intermediate processing steps, it is an environmental-friendly nano-manufacturing technique. Furthermore, the orientation control of CNTs in the transferred CNT TCFs is currently under study, as shown Figure 5 (b, c). The alignment of CNTs can be realized by a rubbing process before transferring on to the final substrate.
4. Conclusion: In this study, a new strategy to prepare CNT TCF with tunable transparency, conductivity and piezoelectric properties have been demonstrated. This method takes the advantages of the porous and self-standing properties of CNT Sponge, which allows facile transfer of CNTs first to a Scotch tape. Transferred CNTs on tape exhibits smoother, more uniform and weaker inter-tube interactions which allows further controlled secondary transfer to viscous substrates through stamping. This two-step transfer method is low cost, high through put and easy to scale up fabrication of CNT TCFs. This method is able to produce transparent and stretchable electrodes of different sizes on different substrate contours. This transfer technique creates a new path for preparation of more delicate electronics, and also open new angle to combine bottom-up and top-down strategy to fabricate electronics.
Acknowledgements: Financial support from China Scholarship Council (CSC) of P. R. China (grants to Yun W.), and University of Houston are gratefully acknowledged.
Reference:
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[24] S. H. Chae, W. J. Yu, J. J. Bae, D. L. Duong, D. Perello, H. Yun, et al. Transferred wrinkled Al2O3 for highly stretchable and transparent graphene–carbon nanotube transistors. Nat. Mater, 12 (2013), pp. 403-409. [25] N, Hu, Y. Karube, M. Arai, T. Watanable, C. Yan, Y. Li, et al. Investigation on sensitivity of a polymer/carbon nanotube composite strain sensor. Carbon, 49(3) (2010), pp. 680687. [26] O. Kanuoun, C. Muller, A. Benchirout, A. Sanli, T. N, Dinh, A. Ai-Hamry, et al. Flexible Carbon Nanotube Films for High Performance Strain Sensors. Sensors, 14(6) (2014), pp. 10042-10071. [27] L. H. Jin, A. Chortos, F. Lian, E. Pop, C. Linder, Z. Bao, W. Cai. Microstructural origin of resistance–strain hysteresis in carbon nanotube thin film conductors. PNAS, 115(9) (2018), pp. 1986-1991.
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Credit Author Statement D. Z, J. Z, X. G and L. S. designed the research project and supervised the experiment. Y. W, D. L, F. W and B. L. carried out experiments and analysed data. All authors discussed, revised and approved the manuscript.
Journal Pre-proof Declaration of Interest Statement The authors declared that they have no conflicts of interest to this work. They do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Email: [email protected]; [email protected]; [email protected]; [email protected].
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Figure 1. Process of the two-step dry transfer and optical/SEM pictures of CNT TCFs samples. (a, b) Illustrations of the two-step dry transfer of CNT TCFs from CNT sponge to a Scotch tape and from the Scotch tape to a flexible substrate. (c) SEM image of the sideview of the CNT layer on scotch tape. (d) Top view SEM image of the CNT layer on Scotch tape. (e) SEM image of the bulk CNT sponge used for transfer. (f, g) Top view and cross-section view of CNT layer of a PDMS10-1 (PDMS10 substrate and transferred once) sample. Scale bars in c, d, e, f, g are 10μm. (h) Photo of the peeling process controlled by a robot arm. (i) Photograph of the demonstration of a PDMS10-5 (PDMS10 substrate with 5 times repeat of transfer) sample transparency film in front of a school badge and building (The transparency of the TCF is around 80%), inset shows the SEM image surface morphology of transferred CNT layer (scale bar is 10μm). (j) (Upper) Photos of CNT TCF samples with different transfer repeats 1 to 5 on PDMS10 substrates, (Lower) Photos of PDMS, PDMS10-1, PDMS15-1, PDMS20-1 and PDMS30-1 samples. (m, n, o, p, q) SEM images of cross section of CNT TCFs with 1, 2, 3, 4, 5 transfer times on PDMS 10, scale bar is 5μm. (k, l) SEM images of PDMS15-1 and PDMS30-1 samples, scale bar is 2μm.
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Figure 2. Optical CNT TCF uniformity characterizations. The motion of sample is controlled by a 3-axis stage and the path of it is shown following the red arrow directions in (a). (b) Measured transmitting laser intensity for CNT TCFs with various transfer times on PDMS15. (c) Measured transmitting laser intensity of CNT TCFs on PDMS substrates with different substrates viscosity (curing agent ratio) with just one transfer. (d, e) Averaged transparency and corresponding data standard deviation (SD) for samples shown in c and d.
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Figure 3. Transparency and conductivity dependence the of CNT TCF on number of transfer (a) and substrates viscosity (curing agent ratio) (b).
Figure 4. (a) Stability of sheet resistance (Rs/R0) of different CNT TCFs measured in bending mode as a function of number of repeated bending cycles. The radius of curvature of bending is 10 mm. The insets are photos of the bending device with a testing sample. (b) Loadingunloading behavior measured at a rate of 1 mm min -1. Inset cycling signal image presents the piezoresistive performance of the PDMS15-1 at 40% strain. Frequency responses of the sample piezoresistive behavior measured at 0.1 Hz (c), 1 Hz (d), 10 Hz (e) for two strain amplitudes.
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Figure 5. (a) Optical photos of PDMS10 patterns plotted on a glass substrate, scale bar is 400μm. (c, d) SEM photographs show well alignment of CNTs configuration can be achieved by a rubbing process, scale bars are 10μm and 2μm, respectively.
Yun Wang, Dong Lu, Fei Wang, Dongxing Zhang, Jing Zhong, Binghao Liang, Xuchun Gui, Li Sun PII:
S0008-6223(20)30109-3
DOI:
https://doi.org/10.1016/j.carbon.2020.01.090
Reference:
CARBON 15025
To appear in:
Carbon
Received Date:
14 November 2019
Accepted Date:
24 January 2020
Please cite this article as: Yun Wang, Dong Lu, Fei Wang, Dongxing Zhang, Jing Zhong, Binghao Liang, Xuchun Gui, Li Sun, A New Strategy to Prepare Carbon Nanotube Thin film by the Combination of Top-down and Bottom-up Approaches, Carbon (2020), https://doi.org/10.1016/j. carbon.2020.01.090
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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A New Strategy to Prepare Carbon Nanotube Thin film by the Combination of Top-down and Bottom-up Approaches a,b Yun
Wang, cDong Lu, bFei Wang, a,* Dongxing Zhang, c,* Jing Zhong, dBinghao Liang, d,* Xuchun Gui, b,* Li Sun
a School
of Material Science and Engineering, Harbin Institute of Technology, Harbin 150090,
China. b Department c Key
of Mechanical Engineering, University of Houston, Houston, TX 77204, USA
Lab of Structure Dynamic Behavior and Control (Harbin Institute of Technology), Ministry
of Education, Harbin 150090, Heilongjiang, China d
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and
Engineering, Sun Yat-sen University, Guangzhou 510275, China.
Abstract: Bottom-up and top-down are two different strategies to prepare nano-materials and their assemblies, each with their own pro-and-cons. Here, we demonstrate a combined approach in preparing large area uniform carbon nanotube (CNT) thin films within minutes, several orders of magnitude faster than conventional methods. This strategy takes full advantage of the unique micro-structure of CNT sponges, synthesized by a bottom-up process and composed of physically entangled CNTs with uniform size and spatial distribution. Such a 3D network structure allows us to firstly transfer certain amount of structured CNTs from the sponge onto a Scotch tape. Through a second-step stamping process, which is essentially a top-down process, transparent and conductive (TCF) CNT films with controlled micro-structures are produced on viscous substrates. Bonding strength between the stamped CNT network and elastomer substrate can be adjusted to optimize the physical properties including the transparency and sheet resistance of the CNT thin films. Upon stretching, the device exhibits high piezoresistive responsiveness; combining high sensitivity, low hysteresis and large working strain range. The proposed methodology had been extended to fabricate micro-electrodes on patterned elastomers with same effectiveness. These experimental results highlight the high efficiency, low-cost and versatility of this approach in preparing TCF electrodes with different sizes and shapes.
Email: [email protected]; [email protected]; [email protected]; [email protected].
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1. Introduction Science and engineering communities are actively searching for flexible, transparent and conductive films in recent years [1-5]. Through decades of research-led advancement, ceramic based transparent and conductive films(TCFs) have already found extensive applications in solar cells, lasers, optical communication components, and solid-state lighting [6-10] devices. However, brittleness and cost-effectiveness severely limit their adoptions in recyclable, wearable and adaptable applications, in which devices availability, stretchability and environmental friendliness becomes important. Flexibility/stretchable ability in TCFs can often be realized by integrating various conductive nano-materials, including graphene, carbon nanotube (CNT), or metallic nanomaterials, with soft substrates [11-15]. Among these active materials, CNT has the advantages of light weight, mechanical robustness, chemical stability and tunable electrical/thermal conductivity. CNTs can be grown by various CVD processes, which are usually not compatible with polymeric substrates, thus the formation of CNT TCFs often utilizes coating approaches to obtain a conductive network structure on substrate after CVD growth of CNTs. Indeed, a number of wet coating strategies have been explored since 2004, including drop-drying from solvent, airbrushing, electrophoretic deposition, Langmuir-Blodgett deposition, vacuum-assisted filtration and centrifugal casting [1113]. For instance, Wu prepared a TCF of pure single-walled carbon nanotubes by filtration, achieving a sheet resistance of 30 /sq and a transmittance of 70% [3]. Pei et al reported the synthesis of uniform CNT coatings on substrates by electrophoretic deposition, with a resistance and transmittance of 220 /sq and 81%, respectively [11]. Such wet coating processes often require the preparation of highly uniform colloidal dispersion using toxic solvent and/or surfactants, thus create negative environmental impacts and also cause damages to CNT . Other than wet coating approaches, direct transfer of CVD grown CNT onto substrates has also been explored. In 2005, Zhang prepared TCFs by direct drawing CNTs from a forest structure and realized a resistance and transmittance of 750 /sq and 70%, respectively [12]. Through drawing a super-aligned CNT forest, Feng further demonstrated a sheet resistance of 208 /sq with a transmittance of 90% can be achieved [13]. However, the adoption of such CVD strategy is oftern 2
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limited by the size of CVD tools, thus not be suitable for the synthesis of large area TCFs. Obviously, CNT TCFs performances will firstly be influenced by the structure, size and quality of CNTs, which heavily rely on CVD synthesis [6-8]; yet factors including CNT layer micro-structure, inter-CNT interaction and CNT-substrate interaction are even more important. Retooling and optimizing CVD synthesis parameters are not very effective for these approaches, and new strategy is needed to improve efficiency, reproducibility and cost-effectiveness. From the perspective of CNT TCF preparation methodology, both the wet spin and CVD process represent bottom-up approaches. Normally, the bottom-up strategy is considered as having higher accuracy, yet with relatively lower efficiency as compared to top-down methods. Therefore, the combination of the two strategies can help to overcome their intrinsic limitations and utilize their unique benefits in preparing stretchable CNT TCFs. In this study, for the first time, we report on the synthesis of CNT TCFs through a two-step dry transfer-stamping process using CNT sponge as the starting material. The CNT sponge was CVD grown and have a porous aerogel structure [14] in which the CNTs are randomly distributed and entangled in 3D space. The microstructure of the CNT sponge, including the CNT size, density, tortuosity, entanglement degrees, can be tuned during the CVD growth process (bottom-up process). We then peel off a thin layer of CNT sponge with a modified structure by dry transfer process. Although the CNT sponge used here is also synthesized by CVD, its thickness will allow large number of transfer operations, which are conducted in ambient environment. Recently, we demonstrated the fabrication of CNT films on rubber substrates by direct stampingas well as the potential applications of such a film[16]. However, the quality of the CNT structure needs to be significantly improved. In this work, we report on the development of a new two-step transfer process, using a tape as medium to reconstruct the distribution of the peeled-off CNTs from CNT sponge, which greatly improves the uniformity of the CNT thin film as compared to the previous work. Furthermore, we studied related physical mechanisms and demonstrated potential applications of such CNT layers as stretchable electrodes for flexible devices. This two-step transfer-stamping process can yield nanometer thick CNT network electrodes on various polymeric substrates. By combining the bottom-up sponge growth and top-down dry 3
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transfer, the high throughput, low cost and reproducible fabrication of stretchable TCFs can be realized. Adjusting substrate viscosity and stamping repeats, transparency and sheet conductance of the TCFs can be manipulated d. In addition, adhesion between the TCFs and substrate can be tuned to improve bending fatigue life. Furthermore, we demonstrate the fabrication of micro-sized CNT electrodes using this transfer method, on polymeric patterns printed by micro-plotting. As a proof of concept, piezoresistive sensors directly utilizing such TCFs were evaluated in terms of stability and repeatability.
2. Experimental 2.1 Materials CVD grown CNT sponge, as having been reported in ref. 14, was used in this study [14]. The sponge growth process include: to first dissolve ferrocene powders (as catalyst) in 1,2dichlorobenzene (carbon source) to form a ~0.06 g ml−1 solution; the solution is then injected into a 2-inch diameter resistively quartz tube furnace use a syring pump. A tyical flow rate is 0.13 ml min−1 and the reaction temperature is 860 °C. A mixture of Ar and H2 is used as carrier gas, with respective flowrate of 2000 ml min−1 and 300 ml min−1. CNT sponge was collected from a quartz sheet place in the reaction zone. The stretchable substrates used in this study were polydimethylsiloxane (PDMS) layers prepared from Dow Corning (SYLGARD 184) with four mixing ratios of 10:1, 15:1, 20:1 to 30:1 of pre-polymer (base) and cross-linker (curing agent), corresponding samples are named as PDMS10, PDMS15, PDMS20 and PDMS30. Polymeric patterns were printed by Sono Micro-plotter II using PDMS10. Micro-plotter pipette has an opening diameter of 100μm. 2.2 Two-step CNT TCFs synthesis from CNT sponge A two-step transfer method has been developed. First a strip of Scotch tape was applied to the CNTs sponge with a pressure using a doctor blade with a constant speed at 1mm/s, the blade is adjusted to be perpendicular to the CNT sponge surface. No visible deformation is induced to the sponge, ensuring that only the CNTs from the top layers of the sponge were adhered to the Scotch tape. After ~1min, the Scotch tape was then peeled off from the sponge along the 4
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longitudinal direction of the strip as shown in Figure 1(h), at a peeling rate of ~1 mm/s. Peeling rate and angle can affect the quality of transferred CNT film, and slower rate was found to be more beneficial in reducing the CNT film roughness formed on the tape. The second stamping process helped to transfer the CNTs from Scotch tape to flexible substrate. For the preparation of CNT TCFs with random CNT distribution, the stripe of Scotch tape with adhered CNTs was gently laminated onto a PDMS substrate in a parallel fashion as shown in Figure 1. In contrast, for the CNT TCFs consists of aligned CNTs, the Scotch tape was slide downwards. The transfer process can be carried out multiple times, and the corresponding sample is named as PDMS303, for example, which refers to film formed on PDMS30 substrate with 3 times of stamping from the Scotch tape. Finally, the PDMS can be delaminated from the Scotch tape by peeling off the tape. This stamping process can be repeated in increasing the transferred CNT layer thickness on substrate. 2.3 Characterization The microstructure of CNT TCFs was investigated by a scanning electron microscope (SEM; Nova Nano SEM 463). Electrical conductivity and transparency measurements were conducted with a standard four-probe arrangement using an electrochemical workstation (Cordova, TN, USA) and a laser reflection interferometry, respectively. The flexibility of the CNT TCFs was tested by repeatedly bending the film with a home-made two-point bending device, with a sample radius of curvature set at ~10 mm. The piezoresistive properties were characterized in the tensile mode using a DMA Q800 (TA Instruments).
3. Results and discussion: Direct transfer of CNTs from CNT sponge to substrate was found to result in highly non-uniform transferred layers with poor thickness and uniformity control, thus greatly hinders their applications [16]. This might be related to the characteristic length scale (SLS) at dozens of micrometers of the average distance of CNT segment between adjacent physical CNT cross-points, which endow CNT sponge with relative high strength, elasticity and conductivity [14]. When a CNT sponge is directly pressed onto a PDMS substrate, only a few loose CNTs can be transferred 5
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with high degree of non-uniformity at micro-meter scale. Namely, the uniformity of CNT film should be controlled by the SLS. To improve thin CNT film uniformity, it is needed to de-entangle transferred CNTs from sponge micro-structure, which could significantly increase SLS. Therefore, we consider the first transfer to a Scotch tape to be a critical step in controlling final TCFs quality in such a two-step method. Figure 1 (a, b) illustrates the two-step transfer-stamping process for the preparation of CNT TCFs. When sponge came into contact with the tape surface, first adhered CNTs would act as anchors during the afterward peeling process. As shown in Figure 1(h), significant amount of CNTs pullout and re-arrangement occurs during this first transfer process, depending on applied pressure and how to peel off the tape from the sponge. As shown in Figure 1 (c, d) those transferred CNTs on Scotch tape form a more fluffy and uniform surface structure, compared to the CNTs at sponge surface; and more interestingly, the CNTs on Scotch tape exhibit improved degree of alignment (Figure 1(c)), indicating the successful de-entanglement of the original physical crosslinks, which is critical to the quality of TCF. In other words, transferred CNT layer on Scotch tape (Figure 1 (c, d)), serving as an intermediate buffer layer, possesses an improved uniformity as compared to the sponge surface (Figure 1(e)). Figure 1(c) also shows that the CNTs on Scotch tape have a denser bottom layer that strongly binds to the adhesive tape, and a loose top layer with the CNTs partially aligned. It is the top layers of the CNTs will then be transferred to the stretchable substrate to form TCFs, which will result in randomly distributed and transparent CNTs film. Figures 1 (f, g) show the morphology of CNTs transferred onto a PDMS 10 substrate. It can be seen that that CNTs are more or less randomly distributed, after first transfer . One of most important advantages of the two step transfer approach is that the properties of the transferred CNT TCFs can be easily controlled by both the viscosity of the PDMS substrate and the number of repeating transfer. As shown in Figures 1 (j, m-q), as the number of transfer increases, the thickness of the TCF increase at the cost of decreasing in transparency. A continuous CNT network already forms covering the entire PDMS substrate surface after onetime transfer (Figure 1(m)), resulting in a combination of high transparency (~95%) and conductivity. Such a conductivity at transmittance rate of 95% is higher than most of the 6
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reported results, which were obtained by complex procedures such as vacuum assisted filtration, spraying or electro-templating [11-13]. Such a performance indicates the formation of high percentage of conductive physical contacts in the current CNT network , with few isolated CNTs that reducesthe transparency. This characteristic essentially inherits the unique microstructure of the CNT sponges, highlighting the advantage of this method. Figure 1(i) demonstrates the uniformity of macroscopic transparency of a sample after five transfer repeats, and the inset SEM image in Figure 1(i) confirms the microscopically uniform CNT distribution. In comparison, for a sample with fewer transfer repeats, the SEM images also show decreasing CNT distribution uniformity Figure 1(f, g). Fuigure 3(a) also shows that both of the transparency (80%) and conductivity (~8 K/sq) saturate when then transfer repeated 5 times, with this specific material system and transfer procedure. This phenomenon suggests that the formation of TCF on PDMS substrate in our approach is mainly controlled by the Van der Waal’s interaction between the CNT layer and PDMS substrate, rather than that between CNT layers from consecutive transfer processes. Considering that the Van der Waal’s interaction range is generally limited to dozens of nanometers, which is smaller than the diameter of CNT itself, the areas on PDMS substrate where there are already CNTs exist from the previous transfer, could screen the Van der Waal’s interaction from PDMS substrate. In such a scenario, it is only the blank and exposed areas on PDMS substrate can adhere CNTs from the following transfer process. Note that, this is very different from the general understanding of CNT interaction, for which the attraction force between CNTs is belived to be relative strong and results in CNT bundling and aggregation. We believe that it is because of the loose CNT structure with certain degree of alignment on the Scotch tape obtained from the first transfer step. Specifically, when the aligned CNTs on Scotch tape are brought to contact with PDMS substrate partially covered by CNTs, CNTs will either directly contact with exposed PDMS substrate or CNTs that already existed from previous transfer. For the later case, CNTs will re-distribute to the nearby bare PDMS substrate and bending and buckling can occur due to external compressive force and gradual increase of the Van der Waal’s attraction from the PDMS substrate. Therefore uniformity of the CNT layer can be improved .
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Transparency and conductivity of CNT based TCF can be adjusted through changing PDMS viscosity. Figure 3(b) shows the effect of substrate viscosity where samples with one-time stamp are compared. For the substrate with highest viscosity (PDMS30), the sheet resistance was lowered to 29 K/sq, at the expense of transmittance (73%). However, the morphology of CNT TCF with high conductivity is very different from that obtained from multiple transfer processes. As shown in Figure 1(l), when PDMS with higher viscosity is used (PDMS30), the transferred CNT thin film forms micro-wrinkles with an average wavelength of ~2 μm. This is very important for the realization of stable piezoresistive properties without noticeable hysteresis as will be elaborated below. Upon the peeling off the Scotch tape from the PDMS substrate, the surface layer of PDMS modified with adhered CNT TCF will be stretched accordingly because of the high PMDS stickiness, which could re-orientate the CNTs to certain more stable directions as have been reported by many groups. When the Scotch tape eventually separated from the PDMS substrate, the elastic PDMS springs back to its original shape, generating significant compressive stress on the CNT TCF which results in the formation of wrinkles (Figure 1(l)). We emphasize here that the current method is mainly taking the advantage of the existed porous and percolated CNT structure transferred from the sponge to Scotch tape. This is very different from the vacuum assisted filtration method, in which a percolated and conductive network is gradually formed, and thus is time-consuming and highly sensitive to the dispersion quality of CNT. Therefore, the dry transfer formation of TCF reported here can be considered as a top-down approach, while the CNT growth of sponge is based on a bottom up strategy. In other words, we essentially combine the unique advantages of bottom-up and top-down. Conventional optical techniques like spectroscopic ellipsometry, laser reflection interferometry is widely used for determining thin film thickness or growth rate [15, 17]. Here laser absorption measurements on CNT TCFs were performed to reveal thickness variation and uniformity. A diode laser with 473nm wavelength was loosely focused on sample (Figure 2(a)). A photodiode was used to collect transmitted light intensity, which was sensitive to film thickness and film light absorption. Since the size of laser spot is ~50 microns, the accuracy of this test for the uniformity of TCF is also the average over a range of tens of microns. A 3-axis tranlation stage enabled us to 8
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get optical information over a large sample area. As we can see from Figure 2(b) and Figures 1 (m-q), as transfer repeat times increase, less transmitted light measured, indicating more light absorption in CNT TCFs due to increased effective film thickness. The low standard deviation of scanned intensity data got from individual sample mean relatively constant transmitted light intensity values in different transfer repeat times thin films. From uniformity point view, in Figure 2(d), as transfer time increase, uniformity increase with smaller standard deviation data value, which means better thin film quality. Further, considering different curing agent ratio, higher curing agent brings higher viscosity PDMS, leading to better uniformity during each transfer process. Figure1 (c, e) show thatthe film transparency decreases when less curing agent is added in the PDMS, which means the formed CNTs films have a increasing thickness, which is confirmed by the . corresponding SEM images of Figure 1(j lower) and Figure 1(l). To further analyze underlining physics of the CNT networks formed by our stamping process, we adopted selective models to investigate the thin film conductivity and transparency relationship to the CNT microstructure. For two dimensional randomly distributed stick model [18], the critical density can be estimated by 𝑙√𝑁𝑐 = 4.236 Here, 𝑙 is the length of CNT and 𝑁𝑐 is the value of the critical density, above which a percolated network is formed. Assuming an average CNT length of ~20 μm (as we reported in [14]), we calculated a theoretical critical density of 1.43 CNT per 100 μm2, which is way below the density of the CNTs on PDMS10 substrate as can be seen in Figure 1f. This suggests that CNTS with much lower density or higher porosity can be used to further increase the transparency while maintaining the electrical conductivity. The transparency T of a thin metallic film in air can be modeled by Equation 2, assuming that the thickness of the film is much smaller than the wavelength [19]. Here, dc is the DC conductivity, ac is the optical conductivity, and d is the film thickness. 𝑇=
1
(
2 1 + 𝑎𝑐 𝑑 𝑐
= 2
) ( 9
1 2 𝑎𝑐 1+ 𝑐𝑅𝑑𝑐𝑑𝑐
)
2
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Using the film thickness on the order of 50-100 nm and optical wavelength around 550 nm, one can estimate the value of
𝑎𝑐 . 𝑑𝑐
We find that 𝑎𝑐=2.5𝑑𝑐 provides the best fit to this model, in
general agreement with reported by other groups [20-21]. As a proof of concept, electromechanical properties of the CNT TCFs were studied under different deformation modes. As can be seen in Figure 4(a), the CNT TCF on PDMS30 exhibits high electrical robustness upon mechanical deformation after 10000 cycles of repeated bending (bending radius of ~10 mm), while for the TCFs on PDMS10 and PDMS20, significant increment of resistance emerges after ~2000 cycles. This is due to the different CNT morphologies on PDMS with different viscosity. As been explained, the CNT TCF on PDMS30 substrate, which is very sticky, can be reorientated and form wrinkles during the second transfer step. Such features could help the sensor to accommodate external strain without destroying the CNT conductive network [22-24]. Again, it should be emphasized that the wrinkle forms naturally during our dry transfer process, rather than through substrate pre-stretching followed by transferring as reported before [23]. Therefore, substrate with highest viscosity (PMDS30) could be used for the preparation of highly stable bendable conductor, whereas low viscosity substrate for strain sensors. We further tested the piezoresistive performances of the CNT TCF on PDMS15 under tensile strain. As shown in Figure 4(b), the change of resistance with strain exhibits a three-stage variation. In the first stage (0-5% of strain), the resistance increases linearly with a gauge factor as high as ~50, making our device one of the most sensitive sensors reported so far [25-26]. In the second stage (5% to 27% of strain), the resistance increases nonlinearly with certain fluctuations and a gauge factor of ~8. This probably resulted from the Poisson's ratio effects, in which the compression along the transverse direction could increase the backbone density of CNT conductive network, which can partially compensate the resistance increase resulted from the extension along longitudinal direction. When the strain exceeds 27%, corresponding to third stage, the sensor exhibits linear characteristic again with a gauge factor of 37. Such re-entry into the linear range suggests the complete of the structuring of the CNT network maybe caused by the Poisson's ratio effect. The loading and unloading cycles of the sample exhibit stable responses with no-significant hysteresis (symmetric loading and unload branches). As the amplitude of the strain oscillation increases, 10
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the change of resistance also increases accordingly. Note that hysteresis is mainly due to the reorientation of CNTs upon the stretching, indicating a high interfacial bonding strength between CNTs and PDMS, which has been discussed by Jin [27]. Such a high and stable CNT-substrate interfacial bonding is probably resulted from our unique transfer process. In other words, the fluffy and loosely connected CNTs structure on the tape allow CNTs to adjust themselves for the position and orientation to eventually achieve minimum Gibbs energy for the system, which helps to reduce hysteresis. A combination of high sensitivity, low hysteresis, and wide working range (~ 40% of strain) as been demonstrated here had been rarely reported before because these characteristics generally go against each other for a network structure. For instance, the high piezoresistive sensitivity often relies on the creation of delicate conductive network structures, while large measurement range requires the conductive network to be able to be robust. In the meanwhile, low hysteresis requires high “elasticity” of a conductive network. In this study, CNTs in direct contact with PDMS have strong bonding with the substrate and result in low hysteresis. In addition, the strain sensors tested in the frequency range from 0.1 Hz to 10 Hz all show consistent and stable piezoresistive responses. The symmetry of the loading and unloading curves increases with the frequency, which is a typical characteristic of polymer substrate. To reduce CNT/polymer structure viscoelastic effect, we are in the process of developing a bilayer strain sensor structure where a thick layer of elastic polymer is coupled with the sensing layer. And to improve this behavior without losing the advantage of using substrate viscosity, we propose that a double layered polymer substrate consists of a thick polymer layer with low viscosity and a thin polymer layer with targeted viscosity can be employed, which will be studied systematically in future. To demonstrate the practicality and versatility of this dry transfer method, this technology was combined with micro-plotting to produce micro-CNT TCF electrodes. In this demonstration, viscous PDMS was used as ink for micro-plotting to create sinusoidal patterns on glass substrates, CNTs were then selectively transferred from Scotch tape (with CNTs from sponge) onto the PDMS patterns. Transferred CNTs on PDMS patterns also show uniform surface distribution (Figure 5(a)) at miron sizes . Further study on improving microelectrode performances is underway. 11
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The CNT sponge in this study can be repeatedly used, thus the transfer synthesis proposed here is efficient and cost effective. CNT sponge properties including CNT size, density, porosity, alignment can be adjusted to tune the performance of TCF. This method can be easily scaled up. Without the need of using any chemicals and intermediate processing steps, it is an environmental-friendly nano-manufacturing technique. Furthermore, the orientation control of CNTs in the transferred CNT TCFs is currently under study, as shown Figure 5 (b, c). The alignment of CNTs can be realized by a rubbing process before transferring on to the final substrate.
4. Conclusion: In this study, a new strategy to prepare CNT TCF with tunable transparency, conductivity and piezoelectric properties have been demonstrated. This method takes the advantages of the porous and self-standing properties of CNT Sponge, which allows facile transfer of CNTs first to a Scotch tape. Transferred CNTs on tape exhibits smoother, more uniform and weaker inter-tube interactions which allows further controlled secondary transfer to viscous substrates through stamping. This two-step transfer method is low cost, high through put and easy to scale up fabrication of CNT TCFs. This method is able to produce transparent and stretchable electrodes of different sizes on different substrate contours. This transfer technique creates a new path for preparation of more delicate electronics, and also open new angle to combine bottom-up and top-down strategy to fabricate electronics.
Acknowledgements: Financial support from China Scholarship Council (CSC) of P. R. China (grants to Yun W.), and University of Houston are gratefully acknowledged.
Reference:
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Credit Author Statement D. Z, J. Z, X. G and L. S. designed the research project and supervised the experiment. Y. W, D. L, F. W and B. L. carried out experiments and analysed data. All authors discussed, revised and approved the manuscript.
Journal Pre-proof Declaration of Interest Statement The authors declared that they have no conflicts of interest to this work. They do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Email: [email protected]; [email protected]; [email protected]; [email protected].
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Figure 1. Process of the two-step dry transfer and optical/SEM pictures of CNT TCFs samples. (a, b) Illustrations of the two-step dry transfer of CNT TCFs from CNT sponge to a Scotch tape and from the Scotch tape to a flexible substrate. (c) SEM image of the sideview of the CNT layer on scotch tape. (d) Top view SEM image of the CNT layer on Scotch tape. (e) SEM image of the bulk CNT sponge used for transfer. (f, g) Top view and cross-section view of CNT layer of a PDMS10-1 (PDMS10 substrate and transferred once) sample. Scale bars in c, d, e, f, g are 10μm. (h) Photo of the peeling process controlled by a robot arm. (i) Photograph of the demonstration of a PDMS10-5 (PDMS10 substrate with 5 times repeat of transfer) sample transparency film in front of a school badge and building (The transparency of the TCF is around 80%), inset shows the SEM image surface morphology of transferred CNT layer (scale bar is 10μm). (j) (Upper) Photos of CNT TCF samples with different transfer repeats 1 to 5 on PDMS10 substrates, (Lower) Photos of PDMS, PDMS10-1, PDMS15-1, PDMS20-1 and PDMS30-1 samples. (m, n, o, p, q) SEM images of cross section of CNT TCFs with 1, 2, 3, 4, 5 transfer times on PDMS 10, scale bar is 5μm. (k, l) SEM images of PDMS15-1 and PDMS30-1 samples, scale bar is 2μm.
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Figure 2. Optical CNT TCF uniformity characterizations. The motion of sample is controlled by a 3-axis stage and the path of it is shown following the red arrow directions in (a). (b) Measured transmitting laser intensity for CNT TCFs with various transfer times on PDMS15. (c) Measured transmitting laser intensity of CNT TCFs on PDMS substrates with different substrates viscosity (curing agent ratio) with just one transfer. (d, e) Averaged transparency and corresponding data standard deviation (SD) for samples shown in c and d.
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Figure 3. Transparency and conductivity dependence the of CNT TCF on number of transfer (a) and substrates viscosity (curing agent ratio) (b).
Figure 4. (a) Stability of sheet resistance (Rs/R0) of different CNT TCFs measured in bending mode as a function of number of repeated bending cycles. The radius of curvature of bending is 10 mm. The insets are photos of the bending device with a testing sample. (b) Loadingunloading behavior measured at a rate of 1 mm min -1. Inset cycling signal image presents the piezoresistive performance of the PDMS15-1 at 40% strain. Frequency responses of the sample piezoresistive behavior measured at 0.1 Hz (c), 1 Hz (d), 10 Hz (e) for two strain amplitudes.
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Figure 5. (a) Optical photos of PDMS10 patterns plotted on a glass substrate, scale bar is 400μm. (c, d) SEM photographs show well alignment of CNTs configuration can be achieved by a rubbing process, scale bars are 10μm and 2μm, respectively.