Silver fractal dendrites for highly sensitive and transparent polymer thermistors

Jongyoun Kim , Donghwa Lee , Kyutae Park , Hyeonjin Goh and Youngu Lee *
Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333, Techno Jungang Daero, Hyeonpung-Eup, Dalseong-Gun, Daegu, 42988, Republic of Korea. E-mail: youngulee@dgist.ac.kr

Received 17th May 2019 , Accepted 20th June 2019

First published on 21st June 2019


Effective temperature measurement using non-invasive sensors finds applications in virtually every field of human life. Recently, significant efforts have been made toward developing polymer positive temperature coefficient (PTC) thermistors because they have advantages including flexibility, conformability, and biocompatibility. However, most polymer PTC thermistors still have issues such as low sensitivity, low optical transparency, and poor operational durability because of low electrical conductivity and inefficient hopping transport of conventional conductive filler. Here, a highly sensitive and transparent polymer thermistor composed of silver fractal dendrites (AgFDs) and a polyacrylate (PA) matrix has been successfully demonstrated. A AgFDs–PA composite film exhibits a superior PTC effect (about 104 Ω °C−1) around 35 °C because of the high electrical conductivity of the AgFDs and the quantum tunneling effect among them. A thermistor based on the AgFDs–PA composite shows excellent sensitivity, PTC intensity (∼107), and sensing resolution through dramatic resistance changes from thousands to billions of ohms in the human body temperature range (34–37 °C). Moreover, it exhibits excellent optical transparency (82.14%), mechanical flexibility, and operational durability. An electrical impedance spectroscopy analysis shows that the distance between the AgFDs increases with temperature, which implies that the quantum tunneling effect amplified by the branches of the AgFDs has a significant influence on the changes in resistance. This characteristic makes the thermistor immediately suitable for monitoring body temperature. We anticipate that the new thermistor based on the AgFDs–PA composite can be a key component of various sensing applications.


Introduction

Temperature is an important physical quantity for measuring the state of a material. Temperature sensors, which measure the degree of heat energy in a body, are used for different applications.1–12 They are found in different types of electronic devices and electrical appliances. Recently, temperature monitoring in real-time has found use in a variety of applications, such as electronic skin,1–5 medical diagnosis,6 personal health monitoring,7–10 and food processing.11,12 The most widely used temperature sensors are thermocouples, which consist of a pair of junctions formed from two dissimilar metals. Thermocouple-based temperature sensors enable rapid response, mechanical robustness, and non-invasive temperature measurement.13–15 However, it is difficult to apply thermocouples to flexible and biocompatible temperature sensors because they are made from inflexible materials and involve complex fabrication processes.15 Moreover, since they are essentially opaque, it is impossible to observe the visible changes in the target system with temperature. Therefore, it is necessary to fabricate a new type of temperature sensor with functional features, such as softness, flexibility, optical transparency, biocompatibility, and low fabrication cost.

Recently, thermistors have received significant attention as flexible and biocompatible temperature sensors because they are inexpensive, easy to use, readily available, and adaptable temperature sensors.16,17 They are made from semiconducting materials whose resistivities are sensitive to temperature. The resistance of a thermistor changes with varying temperatures in such a way that a change in temperature can be detected by the corresponding change in resistance. There are two types of thermistors: negative temperature coefficient (NTC)18–23 and positive temperature coefficient (PTC)24–34 thermistors. The resistance of NTC thermistors decreases as their temperature rises and resistance increases with temperature for PTC thermistors. Recently, significant efforts have been made toward developing polymer PTC thermistors because they have several advantages including high sensitivity, flexibility, conformability, biocompatibility, and biodegradability.33,35–38 Polymer PTC thermistors consist of semi-crystalline polymer matrices with conductive fillers. So, conductive fillers used as polymer PTC thermistors include graphite,25,26 carbon nanotube (CNT),27,28 reduced graphene oxide,29 graphene,30,31 and nickel microparticles.32–34 The temperature dependence of polymer PTC thermistors is mainly due to the hopping transport mechanism between the conductive fillers embedded in the polymer matrix. However, most polymer PTC thermistors still have issues such as low sensitivity, low reproducibility, and poor operational durability because conventional conductive fillers have low electrical conductivity and inefficient hopping transport. Thus, it is very difficult to use polymer PTC thermistors for practical applications. Moreover, they are generally opaque because a large amount of conductive fillers with low electrical conductivity is embedded in the polymer matrix to achieve sufficient conductivity in the initial state. Until now, polymer PTC thermistors with high optical transparency and sensitivity have not been reported.26,28,34 Therefore, it is very important to develop novel conductive fillers for polymer PTC thermistors with excellent sensitivity, optical transparency, mechanical flexibility, and operational durability.

In this work, we report a highly sensitive, flexible, and transparent polymer PTC thermistor consisting of silver fractal dendrites (AgFDs) and polyacrylate (PA) matrix. The AgFD is synthesized by controlling the concentration ratio and injection rate of silver nitrate (AgNO3) and hydroxylamine (NH2OH) aqueous solutions. We fabricated a AgFDs–PA composite film using a simple melt-mixing process for the AgFDs and molten PA. It exhibits a superior PTC effect (about 104 Ω °C−1) around 35 °C because of the high electrical conductivity of the AgFDs and the quantum tunneling effect among them. A flexible and transparent thermistor was fabricated with the AgFDs–PA composite and patterned indium tin oxide (ITO) electrodes. It shows excellent sensitivity, sensing resolution, and fast sensing response through dramatic resistance changes from thousands to billions of ohms in the human body temperature range. Moreover, it exhibits excellent optical transparency (82.14% at 550 nm), mechanical flexibility, and operational durability. An electrical impedance spectroscopy (EIS) analysis shows that the distance between the AgFDs increases with temperature, which implies that the quantum tunneling effect amplified by the branches of the AgFDs has a significant influence on the changes in resistance. In addition, we successfully demonstrated that the flexible and transparent thermistor based on AgFDs–PA composite can detect human body temperature changes.

Results and discussion

Fig. 1a illustrates the fabrication process for the AgFDs–PA composite film. The AgFDs were synthesized using silver nitrate (AgNO3) and hydroxylamine (NH2OH). The NH2OH acts as a reductant as well as a surface coupling agent. The size and shape of the AgFDs were carefully controlled by monitoring the concentrations and injection rate of AgNO3 and NH2OH solutions. When the ratio of NH2OH to AgNO3 was lower than 4, Ag was deposited only on the specific surface due to the difference in binding energy of each plane of the Ag seed. Therefore, Ag nanoparticles with primary branches were mainly synthesized. However, the reduction rate of Ag ions exceeded the capping rate, resulting in the growth of new branches on main branches. When aqueous solutions of AgNO3 and NH2OH were injected at a ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4 at 5 mL min−1, the fractal branches of AgFDs grew to second generation. The average diameter of the AgFDs was 3–5 μm and the average length of secondary fractal branches of the AgFDs was 0.4–0.8 μm (Fig. S1, ESI). This result indicates that the size and shape of the AgFDs can be determined by tuning the rate of silver ion reduction at a specific surface of the silver seed. Next, the PA was prepared by dissolving octadecyl acrylate (OA, 54 mol%), butyl acrylate (BA, 46 mol%), and 2,2-dimethoxy-2-phenylacetophenone (DMPA, 1 wt%) in tetrahydrofuran. The mixture was then exposed to UV light at 365 nm for 24 h. Finally, a AgFDs–PA composite was prepared by a simple melt-mixing process of AgFDs and molten PA at 60 °C. The AgFDs–PA composite was sufficiently cooled at room temperature and crushed to form a powder. The powder was then hot-pressed to obtain a thin AgFDs–PA composite film whose thickness was 25 μm. The AgFDs–PA composite film was characterized by optical microscopy (OM) and field-emission scanning electron microscopy (SEM). As shown in Fig. 1a and S3, the AgFDs were well dispersed and fully covered with PA, indicating that their unique multi-dimensional fractal branches are well maintained during the fabrication process. It is expected that the formation of a conductive network due to the spreading of nanoscale fractal branches in three dimensions has high potential for resistive type sensors.39,40 To measure the electrical characteristics of the AgFDs–PA composite film, a computer controlled custom-made system with digital source meter and temperature controller was designed for current and voltage (IV) measurement (Fig. S4, ESI). The temperature controller consists of a heater and a Peltier device and could control the temperature in units of 0.1 °C through a proportional-integral-derivative (PID) control.
image file: c9nr04233d-f1.tif
Fig. 1 (a) Schematic illustration of fabrication of the AgFDs–PA composite films. (b) Initial resistivity of the AgFDs–PA and AgMPs–PA composites with various loading concentrations of fillers in the PA matrix. (c) Changes in resistivity of AgFDs–PA composite film as a function of temperature. (d) DSC melt endotherms of the AgFDs–PA composites under different loading concentrations of the AgFDs.

The resistivity of the AgFDs–PA composite can easily be tuned by controlling the loading concentration of the AgFDs in the PA matrix. Fig. 1b illustrates the resistivity of the AgFDs–PA composites with various loading concentrations of AgFDs in the PA matrix. It shows a significantly high resistivity (0.12 MΩ cm) for AgFDs–PA composite with a low loading concentration of AgFDs (0.4%) because the loading concentration of AgFDs was not high enough to achieve percolation threshold and induce quantum tunneling effect. For AgFDs–PA composites with high AgFDs concentration (up to 0.6%), it exhibited excellent resistivity (as low as 103 Ω cm) at 25 °C, indicating that it is sufficient to satisfy the loading concentration of AgFDs required to achieve percolation threshold. Moreover, the highly elongated AgFDs tips can enhance the quantum tunneling effect among the AgFDs, leading to an increase in electrical conductance through the PA matrix.

To investigate the cause of the low percolation threshold in the AgFDs–PA composite film, we synthesized spherical silver microparticles (AgMPs) with diameters similar to that of the AgFDs. We also prepared a AgMPs–PA composite film with a AgMPs loading concentration of 0.4% showed an extremely high resistivity (77 MΩ cm) at 25 °C. Even when the loading concentration of AgMPs was 5 wt%, the AgMPs–PA composite film showed a resistivity >104 Ω cm. These results clearly confirm that the conductive pathway in the PA matrix was more effectively generated by the unique shape of the AgFDs. It is well known that highly elongated tips at the surface of metallic materials can increase electrical conductance through the insulating polymer, leading to the amplification of electric field or tunneling probability among metallic materials.

Fig. 1c and S7 illustrates changes in the resistivity of the AgFDs–PA composite film as a function of temperature. The resistivity of the AgFDs–PA composite film was 103 Ω cm below 30 °C. However, as the temperature of the AgFDs–PA composite film increased from 30 °C to 37 °C, its resistivity increased dramatically from 103 to 1010 Ω cm. In addition, there was no changes in resistivity even if the temperature rose above 37 °C. These results prove that the AgFDs–PA composite film exhibits superior PTC effect from 34 °C to 37 °C, making it an ideal thermistor.

To investigate the effect of the loading concentration of the AgFDs on the melting point of the AgFDs–PA composite, the thermal characteristics of the AgFDs–PA composite were analyzed by differential scanning calorimetry (DSC). As shown in Fig. 1d, the peak temperatures of the AgFDs–PA composites were constant at 33.7 °C even though the loading concentration of the AgFDs was varied from 2.5% to 50%. This result suggests that the AgFDs could be used as a conductive filler in a thermistor because it does not affect the thermal characteristics of the PA matrix. In addition, the sensitive temperature of the AgFDs–PA composite ranges from 30 °C to 37 °C, indicating that it can be used for body temperature monitoring.

Fig. 2a shows the structure of a flexible and transparent thermistor based on the AgFDs–PA composite. A thin AgFDs–PA composite film was placed between two polyimide (PI) films and pressed at 60 °C. The loading concentration of AgFDs in the thermistor was 2.5 wt% to ensure stable operation. After cooling, the top PI film was peeled off and the exposed surface attached to a ITO/polyethylene terephthalate (PET) film. An ITO electrode with a zigzag pattern was used as a lateral directed conductive pathway to ensure rapid response and high optical transmittance of the thermistor. The spacing between the zigzag patterns was maintained at 30 μm to increase the surface area of the inter-electrode space. Finally, the bottom PI film was peeled off to obtain a thermistor based on the AgFDs–PA composite. The thickness of the AgFDs–PA composite layer was about 20 μm.


image file: c9nr04233d-f2.tif
Fig. 2 (a) Schematic illustration of a structure of a AgFDs–PA-based transparent and flexible thermistor. (b) Changes in output resistance of the thermistor as a function of temperature. (c) Output resistance of the thermistor from 34 °C to 36.5 °C. The resistance was measured for 30 s. (d) Optical transmittance of the thermistor with 2.5 wt% AgFDs. Inset: A photograph of a AgFDs–PA-based transparent and flexible thermistor.

Fig. 2b shows the output resistance versus temperature characteristics of the thermistor based on the AgFDs–PA composite. It shows a temperature sensing range from 30 °C to 37 °C, which is in a good agreement with the thermal behavior of the AgFDs–PA composite. In particular, it was observed that a dramatic resistance change (about 104 Ω °C−1) occurs around 35 °C, which is the temperature of the human body. PTC intensity, defined as the peak resistivity divided by the initial resistivity (at room temperature), is one of the important indicators to evaluate the performance of PTC thermistors. It is noteworthy that the thermistor based on the AgFDs–PA composite shows superior PTC intensity (∼107) compared to previously reported polymer thermistors (Table S1, ESI).25,27–31,34 Moreover, it exhibits slight fluctuations in output resistance at 0.5 °C resolutions from 34 °C to 36.5 °C, as shown in Fig. 2c.

The thermistor also showed a fast response in the rapid temperature change (ΔT = 20 °C) in real-time for a single heating and cooling cycle (Fig. S10, ESI). The optical transparency of the thermistor is 82.14% at 550 nm, as shown in Fig. 2d, indicating that it possesses superior optical transparency compared with conventional conductive filler-PTC polymer matrix composites. To the best of our knowledge, this is the highest reported optical transparency among thermistors. The superior PTC effect and excellent optical transparency of thermistors based on the AgFDs–PA composite can be attributed to their multi-dimensional spike structure and low loading concentration. When the temperature of the PA matrix is below 34 °C, the percolation thresholds in the PA matrix could effectively be achieved by the multi-dimensional spike structure of the AgFDs even when the loading concentration of the AgFDs is as low as 2.5%. The highly elongated tips of the AgFDs can enhance the quantum tunneling effect among the AgFDs, leading to an increase in electrical conductance through the PA matrix. When the PA matrix heats up, it expands, forcing the AgFDs apart. Thus, the quantum tunneling effect among the AgFDs disappears abruptly, leading to a dramatic increase in resistance.

To evaluate the mechanical flexibility of the thermistor based on the AgFDs–PA composite, we designed a custom-made device bending system (Fig. S11, ESI) composed of the bending machine, with computer controlled motor and IV measurement system with digital source meter. It is well known that it is challenging to maintain a stable measurement capability in an environment where bending is inevitable because the strain generated by bending tends to cause significant problems in the structure or sensing mechanism of the thermistor. In particular, the thickness limitation of thermistors using conductive fillers is one of the major factors that reduce mechanical flexibility. However, the designed thermistor based on AgFDs–PA composite exhibits superior sensing performance even at a thickness of 20 μm, showing that it overcomes the thickness limitation. Fig. 3a illustrates the change in resistance of the designed thermistor with temperature at various bending radii. At several steps from low temperature (25 °C) to high temperature (45 °C), it showed the same output resistance irrespective of the bending radius, indicating that it can be used to measure the temperature without difficulty even in a dynamic environment. Fig. 3b shows changes in the real-time initial resistance ratio during mechanical flexibility test. The initial resistance was maintained at a relatively low rate of change of up to 3% even at 45% bending condition. This result verifies that the initial change in resistance due to bending is not large, and it is well restored when returning to the flat state. In addition, it indicates excellent operational durability, as shown in Fig. 3c. These results confirm that the thermistor possesses excellent mechanical flexibility and operational durability. The mechanical robustness of the thermistor suggests that it can be applied to targets that are easily deformed by external environments such as E-skin. Fig. 3d shows that the thermistor has superior stability because the characteristics of the resistance change with temperature are clearly maintained even after 500 h.


image file: c9nr04233d-f3.tif
Fig. 3 (a) Changes in resistance as a function of temperature for different bending radii. (b) Changes in real-time initial resistance ratios during mechanical flexibility test. In each test, the device was bent by 5%, 25%, and 45% of its total length. (c) Changes in resistance with temperature after repeatedly bending the thermistor 100 times. Each bending cycle was performed to 45% of the total length of the thermistor. (d) Changes in resistance as a function of temperature after 500 h.

To elucidate the outstanding temperature sensing capability of the thermistor based on the AgFDs–PA composite, an electrical impedance spectroscopy (EIS) analysis was performed under several temperature conditions. EIS measurements were carried out in a frequency range of 10 to 1 MHz using a sinusoidal signal with an amplitude of 100 mV at zero bias. Fig. 4a and b, respectively, show the Nyquist and Bode plots under different temperature conditions. The Nyquist plot of the thermistor is in the form of one semicircle only, suggesting that it can be described as a simple resistor–capacitor (RC) circuit.41 Furthermore, the increase in the relative change in the real part of impedance with increasing temperature corresponds to the increase in the complex impedance (|Z|). The resistance of the thermistor has a significant effect on the |Z| in the low-frequency region of the Bode plot. Thus, the change in impedance mainly depends on the change in resistance. In addition, the increase in the transition frequency, which is defined as the frequency at 10% drop in |Z|, implies an increase in the distance between the conductive connections in the polymer–conductor system, proving that the distance between the AgFDs increases with temperature.41,42 Therefore, the quantum tunneling effect amplified by the branches of the AgFDs has a significant influence on the increase in resistance.


image file: c9nr04233d-f4.tif
Fig. 4 (a) Nyquist plot (Zvs. Z′′) and (b) Bode plot (|Z| vs. frequency) of the thermistor at different temperatures.

Since skin temperature can be an indicator of the surrounding environment, biological activities, or changes in metabolism, various methods have been attempted for monitoring the skin temperature. Our flexible and transparent thermistor changes temperature directly into electrical signals, so it can be integrated into medical electronic devices as well as monitoring body temperature. Fig. 5a–c show thermographic images of the changes in hand temperature measured by an IR camera when blood flow is controlled through the cuff. It has been reported that measuring the temperature changes in the fingertip through blood flow control can be used in determining clinical illnesses, such as coronary artery disease.43 The thumb temperature of the occluded right hand decreased to 33.3 °C due to the obstruction of the blood flow for 5 min while the thumb temperature of the non-occluded left hand was maintained at 35.0 °C. After the blood flow regulation was released, the thumb temperature of the non-occluded hand rose to 36.5 °C, and the thumb temperature of the occluded hand rebounded to the thumb temperature of the opposite hand and gradually converged to the initial state (Fig. S12, ESI). As shown in Fig. 5d, the AgFDs–PA based thermistor exhibited a change in resistance similar to that of finger temperature when attached to the thumb of the occluded hand. In addition, the change in color of the occluded fingers could be observed simultaneously with the measurement due to the excellent optical transparency of our thermistor. In addition, we measured the change in real-time resistance of the thermistor attached to the human wrist while raising the temperature artificially (Fig. S13 and S14, ESI). In the relaxation state in which the wrist was not bent, the resistance of the thermistor increased from about 103 Ω to 1011 Ω with heating, and recovered to 105 Ω when the heating stopped. It also showed a similar change in resistance under tensile and compressive conditions. Consequently, we proved that our thermistors can be used as clinical thermometer tools and medical devices without problems in any part of the flexible body.


image file: c9nr04233d-f5.tif
Fig. 5 Thermographic images of human hand in (a) initial, (b) after occlusion, and (c) after reperfusion state. (d) Output resistance of the thermistor attached to the thumb of the occluded hand and its temperature.

Conclusions

In conclusion, we successfully fabricated a highly sensitive, flexible, and transparent polymer PTC thermistor composed of silver fractal dendrites (AgFDs) and a polyacrylate (PA) matrix. AgFDs with high electrical conductivity were synthesized by controlling the concentrations and injection rate of AgNO3 and NH2OH solutions. A AgFDs–PA composite film was fabricated using a simple melt-mixing process of AgFDs and molten PA. It exhibited a superior PTC effect from 34 °C to 37 °C because of the high electrical conductivity of AgFDs and their quantum tunneling effect. A flexible and transparent thermistor was successfully fabricated with the AgFDs–PA composite and zigzag-patterned ITO electrodes. It showed excellent sensitivity, PTC intensity (∼107), and sensing resolution through dramatic resistance changes from thousands to billions of ohms in the human body temperature range. Moreover, it exhibited excellent optical transparency (82.14% at 550 nm), mechanical flexibility, and operational durability. We also conducted an electrical impedance spectroscopy analysis to further illustrate the outstanding temperature sensing capability of the thermistor based on the AgFDs–PA composite. The analysis showed that the distance between the AgFDs increases with temperature, which implies that the quantum tunneling effect amplified by the branches of the AgFDs has a significant influence on the changes in resistance. In addition, we successfully demonstrated that the flexible and transparent thermistor based on the AgFDs–PA composite can detect temperature changes in the human body. We anticipate that the thermistor based on the AgFDs–PA composite will be as a key component in a variety of sensing applications.

Experimental

Synthesis of AgFDs

AgFDs were synthesized by injecting silver nitrate (AgNO3, 0.06 M, Daejung Chemicals & Metals Co., Korea) as a silver ion source and hydroxylamine (NH2OH, 0.24 M, Alfa Aesar) aqueous solution as reducing and capping agent into an Erlenmeyer flask, respectively. Peristaltic pump was used to adjust the feed rate of each reactant to 5 mL min−1. During the synthesis, the solution was mixed for about 10 min using an orbital shaker. The resulting precipitate was washed several times with deionized water through filtration and dried under vacuum for 3 h to remove the remaining solvent and ligands.

Synthesis of PA

PA was prepared by dissolving octadecyl acrylate (OA, 54 mol%) and butyl acrylate (BA, 46 mol%) in additional 100 wt% of tetrahydrofuran (THF). 1 wt% of 2,2-dimethoxy-2-phenylacetophenone (DMPA) was also added as a photo-initiator. All chemicals were purchased from Sigma Aldrich. The solution was exposed to UV light at 365 nm for 24 h while being covered with a glass slide to block the oxygen inflow. After the solution was changed to a white solid, the residual THF was evaporated under vacuum for 12 h. The dried polymer was ground for mixing with AgFDs.

Preparation of AgFDs–PA composite film

AgFDs were immersed in molten PA at various concentrations at 60 °C and stirred for about 12 h. The AgFDs–PA composite was sufficiently cooled at room temperature and pulverized into powder. To make the AgFDs–PA composite film, the composite powder was loaded between 25 μm-thick PI films and pressed at 60 °C for 3 min. The PI/AgFDs–PA/PI film was obtained and each PI cover can be removed for the next process.

Fabrication of AgFDs–PA based thermistor

Micro-patterned ITO/PET electrodes (Rs = ∼200 Ω sq−1, Hansung Ind Co., Ltd) were fabricated by photolithography. A photoresistor (AZ Electronic Materials Co., USA) was spin-coated onto the ITO/PET substrate at 3500 rpm. It was annealed at 110 °C for soft bake and exposed to 365 nm UV light under the shadow mask. Then, it was annealed at 110 °C again for hard bake and developed using a AZ-300 developer. After unexposed photoresistor was removed, the ITO was etched using a LCE-12 etchant. A PI cover of PI/AgFDs–PA/PI film was peeled off to attach AgFDs–PA composite film to the fabricated electrode. Then, exposed surface of the film was attached to the electrode at 60 °C for 1 min. After sufficiently cooling, the remaining PI film was removed to obtain an AgFDs–PA based thermistor.

Device characterization

The images of AgFDs and AgFDs–PA composite nanostructure were observed using a high-resolution field emission scanning electron microscope (SU-8020, Hitachi). The images of AgFDs–PA composite film, ITO micro-patterns and fabricated thermistor were observed using an optical microscope (Nikon, Eclipse LV100). DSC was carried out using a DISCOVERY (TA Instruments) in the temperature range of −10 °C to 80 °C with a heating rate of 10 °C min−1. The optical transmittance was measured using a UV-Vis-NIR spectrophotometer (CARY 5000 spectrophotometer, Agilent). The electric characterization of AgFDs–PA composite film and the thermistor were performed using a digital source meter (2636A, Keithley) with a 1 V DC bias. The mechanical flexibility test was performed using a custom-made bending system (SNM) with a speed of 1 mm min−1. The EIS were investigated by grade potentiostat system (VSP, Bio Logic). The temperature was measured with an IR camera (FLIR C3, FLIR Systems, Inc.).

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research was supported by Basic Science Research Program thorough the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2018R1A2B2001926). This research was also supported by the DGIST R&D program of the Ministry of Science and ICT (19-BD-0404).

Notes and references

  1. G. Y. Bae, J. T. Han, G. Lee, S. Lee, S. W. Kim, S. Park, J. Kwon, S. Jung and K. Cho, Adv. Mater., 2018, 30, 1803388 CrossRef PubMed .
  2. Z. Wang, W. Gao, Q. Zhang, K. Zheng, J. Xu, W. Xu, E. Shang, J. Jiang, J. Zhang and Y. Liu, ACS Appl. Mater. Interfaces, 2019, 11, 1344–1352 CrossRef CAS PubMed .
  3. S. Zhao and R. Zhu, Adv. Mater., 2017, 29, 1606151 CrossRef PubMed .
  4. Q. Gui, Y. He, N. Gao, X. Tao and Y. Wang, Adv. Funct. Mater., 2017, 27, 1702050 CrossRef .
  5. H. Joh, S.-W. Lee, M. Seong, W. S. Lee and S. J. Oh, Small, 2017, 13, 1700247 CrossRef PubMed .
  6. D. Son, J. Lee, S. Qiao, R. Ghaffari, J. Kim, J. E. Lee, C. Song, S. J. Kim, D. J. Lee, S. W. Jun, S. Yang, M. Park, J. Shin, K. Do, M. Lee, K. Kang, C. S. Hwang, N. Lu, T. Hyeon and D.-H. Kim, Nat. Nanotechnol., 2014, 9, 397–404 CrossRef CAS PubMed .
  7. X. Wang, Z. Liu and T. Zhang, Small, 2017, 13, 1602790 CrossRef PubMed .
  8. W. A. D. M. Jayathilaka, K. Qi, Y. Qin, A. Chinnappan, W. Serrano, C. Baskar, H. Wang, J. He, S. Cui, S. W. Thomas and S. Ramakrishna, Adv. Mater., 2019, 31, 1805921 CrossRef PubMed .
  9. T. Q. Trung, H. S. Le, T. M. L. Dang, S. Ju, S. Y. Park and N.-E. Lee, Adv. Healthcare Mater., 2018, 7, 1800074 CrossRef PubMed .
  10. Y. Yamamoto, D. Yamamoto, M. Takada, H. Naito, T. Arie, S. Akita and K. Takei, Adv. Healthcare Mater., 2017, 6, 1700495 CrossRef PubMed .
  11. M. K. Law, A. Bermak and H. C. Luong, IEEE J. Solid-State Circuits, 2010, 45, 1246–1255 Search PubMed .
  12. F. Bibi, C. Guillaume, N. Gontard and B. Sorli, Trends Food Sci. Technol., 2017, 62, 91–103 CrossRef CAS .
  13. K. Kim, W. Jeong, W. Lee and P. Reddy, ACS Nano, 2012, 6, 4248–4257 CrossRef CAS PubMed .
  14. G. S. Shekhawat, S. Ramachandran, H. J. Sharahi, S. Sarkar, K. Hujsak, Y. Li, K. Hagglund, S. Kim, G. Aden, A. Chand and V. P. Dravid, ACS Nano, 2018, 12, 1760–1767 Search PubMed .
  15. D. Liu, P. Shi, W. Ren, Y. Liu, G. Niu, M. Liu, N. Zhang, B. Tian, W. Jing, Z. Jiang and Z.-G. Ye, J. Mater. Chem. C, 2018, 6, 3206–3211 RSC .
  16. T. Q. Trung, S. Ramasundaram, B.-U. Hwang and N.-E. Lee, Adv. Mater., 2016, 28, 502–509 CrossRef CAS PubMed .
  17. Q. Li, L.-N. Zhang, X.-M. Tao and X. Ding, Adv. Healthcare Mater., 2017, 6, 1601371 CrossRef PubMed .
  18. D. Katerinopoulou, P. Zalar, J. Sweelssen, G. Kiriakidis, C. Rentrop, P. Groen, G. H. Gelinck, J. Brand and E. C. P. Smits, Adv. Electron. Mater., 2019, 5, 1800605 CrossRef .
  19. C. Yan, J. Wang and P. S. Lee, ACS Nano, 2015, 9, 2130–2137 CrossRef CAS PubMed .
  20. J. H. Oh, S. Y. Hong, H. Park, S. W. Jin, Y. R. Jeong, S. Y. Oh, J. Yun, H. Lee, J. W. Kim and J. S. Ha, ACS Appl. Mater. Interfaces, 2018, 10, 7263–7270 CrossRef CAS PubMed .
  21. F.-F. Chen, Y.-J. Zhu, F. Chen, L.-Y. Dong, R.-L. Yang and Z.-C. Xiong, ACS Nano, 2018, 12, 3159–3171 CrossRef CAS PubMed .
  22. N. Neella, V. Gaddam, M. M. Nayak, N. S. Dinesh and K. Rajanna, Sens. Actuators, A, 2017, 268, 173–182 CrossRef CAS .
  23. J. Wu, S. Han, T. Yang, Z. Li, Z. Wu, X. Gui, K. Tao, J. Miao, L. K. Norford, C. Liu and F. Huo, ACS Appl. Mater. Interfaces, 2018, 10, 19097–19105 CrossRef CAS PubMed .
  24. S. K. Ameri, R. Ho, H. Jang, L. Tao, Y. Wang, L. Wang, D. M. Schnyer, D. Akinwande and N. Lu, ACS Nano, 2017, 11, 7634–7641 CrossRef PubMed .
  25. P. Zhang and B. Wang, J. Appl. Polym. Sci., 2018, 135, 46453 CrossRef .
  26. T. Yokota, Y. Inoue, Y. Terakawa, J. Reeder, M. Kaltenbrunner, T. Ware, K. Yang, K. Mabuchi, T. Murakawa, M. Sekino, W. Voit, T. Sekitani and T. Someya, Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 14533–14538 CrossRef CAS PubMed .
  27. G. Li, C. Hu, W. Zhai, S. Zhao, G. Zheng, K. Dai, C. Liu and C. Shen, Mater. Lett., 2016, 182, 314–317 CrossRef CAS .
  28. Y. Zeng, G. Lu, H. Wang, J. Du, Z. Ying and C. Liu, Sci. Rep., 2014, 4, 6684 CrossRef PubMed .
  29. L. He and S. C. Tjong, RSC Adv., 2015, 5, 15070–15076 RSC .
  30. H. Pang, Y.-C. Zhang, T. Chen, B.-Q. Zeng and Z.-M. Li, Appl. Phys. Lett., 2010, 96, 251907 CrossRef .
  31. Y. Wang, J. Yang, S. Zhou, W. Zhang and R. Chuan, J. Mater. Sci.: Mater. Electron., 2018, 29, 91–96 CrossRef CAS .
  32. S. Stassi and G. Canavese, J. Polym. Sci., Part B: Polym. Phys., 2012, 50, 984–992 CrossRef CAS .
  33. Z. Chen, R. Pfattner and Z. Bao, Adv. Electron. Mater., 2017, 3, 1600397 CrossRef .
  34. J. Jeon, H.-B.-R. Lee and Z. Bao, Adv. Mater., 2013, 25, 850–855 CrossRef CAS PubMed .
  35. T. H. Lee and J. Y. Jho, Macromol. Res., 2018, 26, 659–664 CrossRef CAS .
  36. Q. Fang, T. Chen, Q. Zhong and J. Wang, Macromol. Res., 2017, 25, 206–213 CrossRef CAS .
  37. S. Phetrong, C. Sansuk, P. Tangboriboonrat and P. Paoprasert, Macromol. Res., 2017, 25, 799–805 CrossRef CAS .
  38. E. Tarabukina, E. Seyednov, A. Filippov, M. Constantin, V. Harabagiu and G. Fundueanu, Macromol. Res., 2017, 25, 680–688 CrossRef CAS .
  39. D. Lee, H. Lee, Y. Jeong, Y. Ahn, G. Nam and Y. Lee, Adv. Mater., 2016, 28, 9364–9369 CrossRef CAS PubMed .
  40. D. Lee, J. Kim, H. Kim, H. Heo, K. Park and Y. Lee, Nanoscale, 2018, 10, 18812–18820 RSC .
  41. C. S. S. Sangeeth, A. Wan and C. A. Nijhuis, J. Am. Chem. Soc., 2014, 136, 11134–11144 CrossRef CAS PubMed .
  42. B. E. Kilbride, J. N. Coleman, J. Fraysse, P. Fournet, M. Cadek, A. Drury, S. Hutzler, S. Roth and W. J. Blau, J. Appl. Phys., 2002, 92, 4024–4030 CrossRef CAS .
  43. N. Ahmadi, V. Nabavi, V. Nuguri, F. Hajsadeghi, F. Flores, M. Akhtar, S. Kleis, H. Hecht, M. Naghavi and M. Budoff, Int. J. Cardiovasc. Imaging, 2009, 25, 725–738 CrossRef PubMed .

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr04233d

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