Direct Patterning of Carbon Nanotube via Stamp Contact Printing Process for Stretchable and Sensitive Sensing Devices
A dry transfer method for the mass production of transparent conductive carbon nanotube (CNT) films inspired by typography has been proposed.
The strain sensors based on the CNT films have high stretchability and repeatability (gauge factor up to 9960 at 85% strain).
These ultrathin strain sensors can detect human motion, sound, and pulse, suggesting promising application prospects in wearable devices.
KeywordsCarbon nanotube Strain sensor Dry transfer Stamp contact printing process
Flexible electronic devices have a wide range of needs and attracted tremendous attention in wearable electronics, soft robots and implantable medical devices [1, 2, 3, 4, 5]. Among the various functions of flexible electronic devices, strain sensing is the most fundamental and indispensable one [6, 7, 8, 9]. These applications require the strain sensor to be ultrathin, transparent, integrative and easy to fabricate. Besides, the strain sensor also needs to be flexible and conformable for electronic skins or wearable electronics [10, 11, 12]. Generally, the performance of the strain sensor including sensitivity, stability, and respond speed depends not only on the stretchable substrate but also on the conductive network which transforms the strain from deformation into an electrical signal [13, 14, 15]. Despite some sensors with remarkable performance have been fabricated by fancy structural design [16, 17], or special materials modified , high-efficiency, low-cost and environmentally friendly manufacturing strategy for embedding conductive materials into polymer substrate are still one of the technological difficulties in fabricating strain sensors [19, 20, 21, 22, 23].
Recently, some methods have been developed for constructing a conductive network on stretchable polymer substrate. For instance, conductive nanomaterials were first dispersed in organic solutions and deposited on a polymer substrate by spin-coating [24, 25, 26], dip-coating [27, 28], or printing technology [21, 29, 30, 31]. Although many sensors fabricated by these technologies possess high sensitivity and larger stretchability, the dispersion process of conductive nanomaterials will destroy their structure and lead to an enormous decrease in conductivity. In addition, most of the organic solutions can damage the structure of the polymer substrate, which will lower the stretchability of the devices. Therefore, dry transfer, which can reduce the structural failure of conductive materials, is a feasible strategy for strain sensor fabrication. Recently, some new dry transfer methods have been explored for the fabrication of wearable devices. For example, Qiao et al.  demonstrated graphene epidermal artwork sensors based on laser scribed graphene. Gilshteyn et al.  developed a one-step technique to transfer CNT films on to hydrogel surface. Liao et al.  fabricated graphite-based strain sensors by pencil drew on printing paper. However, the above fabricating technologies are low-efficiency, high-cost, and complicated. A high-efficiency, eco-friendly, and low-cost method for fabricating strain sensors with excellent performance has not been explored, especially for the sensors with high sensitivity and stretchability.
Here, we proposed a simple, low-cost, environmentally friendly stamp contact printing method for the mass production of transparent conductive carbon nanotube (CNT) film. Stamp, similar to typography technology, has been widely used in official documents for nearly 3000 years. During the stamping process, the seal with well-designed pattern adsorbs and transfers liquid ink onto the target substrate. Afterward, the pattern will be printed on the substrate. Herein, inspired by stamp, a versatile stamp contact printing technology to prepare transparent CNT film on polymer substrate was developed. In this stamping method, a porous CNT block was used as both the seal and the solid ink. After the stamping process, the surface layer of CNT will be separated from the seal and transferred onto the Ecoflex surface with the help of the van der Waals’ interaction. The patterns on the CNT seal engraved by the laser can be transferred onto the target substrate and form a patterned CNT film. Further, we can fabricate strain sensors merely by connecting electrodes on both ends of the as-prepared CNT film. The strain sensor based on this CNT film shows not only high sensitivity and stretchability (gauge factor (GF) of 9959.8, at strain 85%), but also high repeatability (> 5000 cycles). Besides, even after stretching, bending and twisting for 1000 cycles, the resistance of the strain sensors had a tiny change, which shows the excellent recoverability of the CNT percolation network. To excavate the potential application of our sensors, pulse detection, motion monitoring, and voice recognition are demonstrated.
2.1 Fabrication of CNT Film and Strain Sensor
The CNT seals were synthesized by chemical vapor deposition (CVD) method using ferrocene and 1,2-dichlorobenzene as the catalyst precursor and carbon source, as reported in our previous work . The Ecoflex substrates were fabricated by mixing the A and B components of Ecoflex 00-30 (Smooth-On) rubbers in a volume ratio of 1:1 and coating on a glass substrate. After curing at room temperature for 10 h, the Ecoflex thin film, with a thickness of 300 μm, can be separated from the glass substrate. The CNT seal was fixed on the linear motor, which can move along z-axis and stamp the CNT seal on the Ecoflex substrate to form the CNT film. After the dry transfer process, UV/O3 treatment was used to enhance the interaction between CNT and the substrate. To fabricate the CNT strain sensor, two silver wires were connected on both ends of the CNT conductive film by silver conductive gel.
2.2 Device Characterization
The morphology of the CNT block and transferred CNT film were characterized by Hitachi S-4800 field emission scanning electron microscope. A homemade system was used to measure the electromechanical performance of the strain sensor. This system consists of a digital multimeter (Keithley 2400) and a commercial linear mechanical motor (Zolix, TSA 300). Optical transmittances of CNT film were characterized by Ocean Optic Spectrometers (Maya 2000 Pro) together with a balanced deuterium halogen light source (Ocean Optics DH-2000-BAL).
3 Result and Discussion
As discussed above, a successful stamp depends on both the seal and target substrate. The microstructure of the CNT seal has been characterized by scanning electron microscopy (SEM), as shown in Fig. 1d. It indicates that the CNT seal consists of CNT which were self-assembled into the anisotropic, porous and interconnected framework. This characteristic ensures that the transferred CNT film on the Ecoflex substrate has a homogeneous network structure. As shown in Fig. 1e, the transferred CNT film formed a uniformity network structure and embedded on the surface of the Ecoflex. Like the stamp, we can also engrave different words or patterns on the CNT seal by laser ablation to fabricate CNT films with any expected patterns. As a demonstration, a CNT word “S” was directly stamped on the Ecoflex substrate by an engraved CNT seal (Fig. 1f). The strong attachment of the sample to fingerprint shows the excellent conformability of our fabricated film, which is very favorable for the strain sensor.
The transferred CNT films have strong interaction with the substrate. Even under ultrasonic washing, most of the CNT still stick on the surface of the Ecoflex substrate. The resistance of the sample only increases by less than 50% after 40 s of ultrasonic washing, as shown in Fig. 2c. This can be further confirmed during the second transfer. By using a fresh Ecoflex to dry transfer the as-prepared CNT/Ecoflex thin film, there was a tiny change in the resistance of the as-prepared CNT film (Fig. S4b). Van der Waals’ force and hydrogen bonding forces are the main interaction between the transferred CNT and substrate, which can be enhanced by UV/O3 treatment. As the UV/O3 treatment time increases, the resistance of the CNT film decreases, indicating that the connection between the CNT is better (Fig. S4a). UV/O3 treatment strengthens both the CNT–CNT interaction and CNT-Ecoflex interaction. Because of the strong interaction between CNT film and Ecoflex substrate, the electrical properties of the CNT films are stable. After the CNT film was stretched (ε = 50%), bent (θ = 180°) and twisted (θ = 90°) for 1000 cycles, its sheet resistance has tiny change (Fig. 2d–f). This indicates that the CNT conductive network is stable with little displacement and fracture.
Furthermore, the thickness of the transferred CNT plays a very important role in the sensitivity of the strain sensor. Thinner CNT film possesses better flexibility and smaller sheet resistance change, compared to the thicker one under the same applied strain. Due to the thickness of CNT can be adjusted by controlling the transfer pressure or transfer times (Fig. S7), the sensitivity of the strain sensor can be improved by increasing the transfer pressure or repeating the dry transfer process. For example, under the same applied strain (80%), the relative resistance changes of strain sensor with higher transfer pressure (1000 kPa) are about 530 times larger than the one with lower transfer pressure (1 kPa). Besides, by repeating the stamping transfer process for four times, the sensitivity of the strain sensor can improve about 3 times.
The CNT-based strain sensor also possesses high sensitivity and large stretchability. As shown in Fig. 3b, the GF of the strain sensors is 9960 at 85% applied strain. The GF increases with the applied strain. This is mainly due to the more junctions of the CNT connection will be broken with the increase in strain (Fig. S5), resulting in the dramatic increase in resistance. The maximum tensile strain exceeds 200% (Fig. S7b). Besides, the CNT-based strain sensor exhibited high stability and excellent recoverability in 5000 stretching/releasing cycles (as shown in Fig. 3c). The dynamic response of the strain sensor is shown in Figs. 3d and S8 under different maximum applied strain. The repeatability was excellent during 10 stretching/releasing cycles under both small and large strain. It demonstrates that the relative resistance monotonically increases with the increasing applied strain (Fig. S9). Figure 3e shows a small drift in the stretching/releasing cycles. In the first few cycles, some unrecoverable damages will be caused in the CNT film. This small drift only occurred when we first extend the maximum working strain, which will be eliminated after a few cycles. As shown in Fig. S10, during 10 stretching/releasing cycles at 70% strain, there was no drift between each cycle and the hysteresis between loading and unloading was negligibly small. Besides, the conductivity of CNT can be fully recovered after releasing from strain up to 70%. Figure 3f illustrates the GF versus the maximum working strain of strain sensors using different materials such as CNT [9, 44, 45], nanowires [46, 47], graphene [48, 49], and other conductive materials [27, 50, 51, 52]. Some of these strain sensors possess high sensitivity while others can withstand the large strain. However, few sensors can work under 50% strain with GF larger than 100. By controlling the thickness of CNT film, our strain sensors can work under 85% strain with GF up to 9960, and under 200% strain with GF up to 274.
In this paper, we have developed a simple and environmentally friendly method via stamp contact printing process for the fabrication of stretchable and sensitive patternable carbon nanotube sensing devices. This dry transfer technique provides new insight and has a promising future in the mass production of the strain sensor. The strain sensor possesses impressively high sensitivity (GF up to 9960) in a wide range of applied strain. Besides, the strain sensors can detect tiny signals such as pulse, breath, and voice as well as larger deformation such as finger motion and robot movement. It suggested that this typography-inspired dry transfer method can be extended to other conductive materials and other substrates.
This work was financially supported by National Natural Science Foundation of China (Grant No. 51772335), Guangdong Youth Top-notch Talent Support Program (No. 2015TQ01C201), and the Fundamental Research Funds for the Central Universities.
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