Flexible terahertz imaging systems with single-walled carbon nanotube films

Journal Pre-proof Flexible terahertz imaging systems with single-walled carbon nanotube films Daichi Suzuki, Yukio Kawano PII:

S0008-6223(20)30136-6

DOI:

https://doi.org/10.1016/j.carbon.2020.01.113

Reference:

CARBON 15048

To appear in:

Carbon

Received Date: 2 October 2019 Revised Date:

23 January 2020

Accepted Date: 31 January 2020

Please cite this article as: D. Suzuki, Y. Kawano, Flexible terahertz imaging systems with single-walled carbon nanotube films, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.01.113. 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. © 2020 Published by Elsevier Ltd.

CRediT author statement Daichi Suzuki: Device fabrication, Simulation, Measurement, Writing- Original draft preparation Yukio Kawano: Supervision, Conceptualization, Writing- Reviewing and Editing

Flexible terahertz imaging systems with single-walled carbon nanotube films

Daichi Suzuki1 and Yukio Kawano2∗

ABSTRACT Sensing systems that can ensure product reliability in a broad range of environments are urgently required for broader application of the internet of things. Imaging technologies based on the terahertz (THz) frequency are considered a promising solution to the challenge of inspecting industrial products in a nondestructive manner. However, current THz imaging systems require bulky and complicated components that hamper their practical application. Therefore, we herein present flexible THz imaging systems based on single-walled carbon nanotube (CNT) films that leverage the material’s advantages of mechanical strength, broad THz absorption, high thermoelectric power, and flexibility. This work investigates and optimizes the physical parameters that govern the detection sensitivity of the proposed CNT THz detector, and then tests a flexible THz imaging system based on the optimization. These imaging systems can be used in a wide range of industrial sensing applications, including inline pharmaceutical quality screening, multi-view imaging, and portable THz imaging for nondestructive quality testing of industrial products, with no bulky measurement components.

Keywords: Carbon nanotube, Terahertz imaging, Nondestructive inspection, Flexible device

CONTENTS 1. Introduction .................................................................................................................................................. 2 2. Design principles for flexible THz detectors with CNT films ..................................................................... 3 1

RIKEN Center for Emergent Matter Science, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan 2 ∗

Corresponding author. Tel:81-3-5734-3811. E-mail: [email protected] (Yukio Kawano).

2.1 THz detection mechanism ...................................................................................................................... 4 2.2 Preparation of CNT films ....................................................................................................................... 6 2.3 THz absorption of CNT films ................................................................................................................ 6 2.4 Chemical doping for p–n junction .......................................................................................................... 8 2.5 Thermoelectric device design ................................................................................................................. 9 3. THz imaging ............................................................................................................................................... 11 3.1 THz spectroscopic imaging .................................................................................................................. 11 3.2 Flexible THz imaging with CNT THz detectors .................................................................................. 13 Outlook and conclusion .................................................................................................................................. 14 Competing financial interests ......................................................................................................................... 15 Acknowledgements ........................................................................................................................................ 15 References ...................................................................................................................................................... 15

1. Introduction

A wide range of research and development targets are being vigorously pursued in anticipation of the broader adoption of the internet of things (IoT), in which machines will automatically communicate with each other and robotics will handle manufacturing processes [1–3]. Ensuring the safety and security of these innovative products is essential to their implementation, and these objectives require the development of effective inspection technologies. Technologies based on optical science have historically played a significant role in inspection applications for quality assurance, including in modern functions such as X-ray transmission measurements [4–6], membrane assessments by near-infrared spectroscopy [7–11], and inspections for contamination by image

processing using visible light [12,13]. In particular, sensing technologies based on terahertz (THz) electromagnetic waves have gained increasing attention in recent years. THz bands have long been unexplored because their working speeds were too slow for electronics and their energies were insufficient for photonics. However, recently developed THz sensing technologies such as solid-state light sources [14– 22], detectors [23–36], and spectroscopy [37–46] have shown nondestructive inspection capabilities in applications such as diagnosing infrastructure degradation in concrete and power lines, identifying drug components, and inspecting for electrolyte leakages from electronic devices [47–51]. Although these developments in THz sensing have attracted significant attention, these technologies continue to face challenges to implementation. An IoT society, where diverse products coexist in various places, will require an inspection technology that operates with a high degree of freedom to accommodate any environment. However, existing THz measurement systems based on compound semiconductors and nonlinear crystals require large optical systems and cooling pools maintained at extremely low temperatures, and these systems cannot be easily transported. Therefore, to develop a more convenient measurement system—one that is not limited by the measurement environment and can be applied immediately in various places and products—we developed a flexible and wearable THz detector based on single-walled carbon nanotube (CNT) films. In this paper, we describe how high-sensitivity THz detection can be achieved with CNT films in THz imaging systems with no bulky components. These flexible THz detectors are shown to offer significant potential as a next-generation quality control technology that can be broadly applied in an IoT society.

2. Design principles for flexible THz detectors with CNT films

One of the keys to building a versatile THz device is enabling flexibility. In contrast to conventional solid-state rigid semiconductors, flexible electronics are desirable for emerging applications, including in

transistors [52–54], thermoelectric generators [55–57], and electro-luminescence [58,59]. Bendable organic thin films such as poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) are generally employed because of their high flexibility and high conductivity. Flexible THz detectors will also require a high conversion efficiency from THz irradiation to electric signals. In the THz detector presented here, we utilized CNT film as the flexible material. Since the invention of the CNT by Iijima in 1991 [60], many studies have been conducted on CNT theory [61–63], synthesis [64–68], and applications in electronics, photonics, energy, and biology [69–76]. In relation to conventional semiconductor materials, CNT films consisting of networked CNT bundles have shown the following advantageous properties: i) mechanical strength with a high degree of flexibility; ii) high absorption spectra over the entire THz frequency range; and iii) high thermoelectric power, with a Seebeck coefficient exceeding 100 µV/K [77,78]. Leveraging these features, we developed a flexible and wearable THz detector that can sense broadband THz irradiation from 0.14 to 39 THz at room temperature. In this section, we describe the mechanism and experimental results of high-sensitivity THz detection with CNT films.

2.1 THz detection mechanism One of the notable features of THz waves is that because the THz frequency region lies between microwaves and light, THz waves can be detected as a wave (electronics), a plasma wave (plasmonics), heat (thermoelectronics), or photons (photonics) [23–36]. Each detection mechanism has both advantages and disadvantages in terms of operation temperature, detectable frequency range, sensitivity, and detection speed (Table 1). Therefore, detectors must be selected with respect to their applications. Because of the diversity of sensing environments in an IoT society, THz imaging applications for this purpose will require broadband THz detection at room temperature. Therefore, we selected the photothermoelectric effect as the THz detection mechanism [79,80]. Figure 1 depicts the mechanism of the photothermoelectric effect. When the THz wave is irradiated onto the detector, the CNT film absorbs the THz illumination and becomes hotter. Along this temperature

gradient, carriers diffuse and a THz-detected voltage signal V is generated, as expressed by: =

( )

,

(1)

where S is the Seebeck coefficient and T is the temperature of the material. This equation is well-known as the Seebeck effect. Figure 1b shows a typical THz response based on the photothermoelectric effect and a response map of the CNT THz detectors, in which the voltage/current signals were generated by THz irradiation. Because the photothermoelectric signal is proportional to the total amount of absorbed heat, as displayed in Fig. 1a, the materials’ THz absorption spectra can influence their detectable frequency ranges. Therefore, the high absorption ratio of the CNT film over the entire THz frequency range was expected to enable the proposed THz detector to operate under broadband THz irradiation. As shown in Fig. 1b, the detection mechanism of the photothermoelectric effect enables broadband THz detection without any cryogenic system. An additional advantage of the photothermoelectric effect is that because the detector can work under zero bias voltage, as shown in Fig. 1b, the system avoids the effects of shot noise and 1/f noise. Indeed, the experimentally obtained noise voltage spectra of the detector (Fig. 1c) show that the noise voltage was reduced to 2 nV Hz-1/2, in good agreement with the theoretical thermal noise voltage VThernal of 2.75 nV Hz-1/2 given by: =

4

∆ ,

(2)

where kB is the Boltzmann constant, T is the temperature, R is the resistance, and ∆f is the frequency bandwidth [81]. This close match indicated that the proposed detector would offer low-noise THz detection capability. To evaluate detection performance in terms of both sensitivity and noise, the noise equivalent power (NEP) index is generally used, given by: NEP =

!"# %! & '

( )#"&"%"&*

=

+,- ./ 0 5 23 65 1( )24

× 89::

;&

,

(3)

where PEffect is the effective power of the THz irradiation. Equation (3) indicates that reduced resistance R and increases in the THz absorption ratio α, Seebeck coefficient S, and THz-induced thermal gradient ∆T enhance the detection performance of the CNT film.

2.2 Preparation of CNT films Three types of CNTs, with different average diameters of 1.0, 1.4, and 1.7 nm, were synthesized by enhanced direct-injection pyrolytic synthesis, and each CNT was dispersed with a nonionic surfactant by a modified free solution electrophoresis process. Each CNT was mixed into a deuterium oxide (D2O) solution of 1 wt.% polyoxyethylene (100) stearyl ether, and was then sonicated by a horn-type ultrasonic homogenizer. After precipitating bundles and impurities through an ultracentrifugation process, an electric field was vertically applied to a monodispersed CNT solution to obtain electrophoretically separated metallic and semiconducting CNT solutions [66]. We then formed the CNT film through a filtration process, depicted in Fig. 2. The CNT solution was deposited on a cellulose membrane filter and was vacuumed until fully filtered (typically for several hours). The film was then dried, after which it was picked up using tweezers. The film’s structural parameters, such as its scale, thickness, and density, were controlled by adjusting the concentration of the CNT solution.

2.3 THz absorption of CNT films Two processes convert THz irradiation to heat: i) carrier excitation via the absorption of incoming THz photons and ii) conversion of photon energy to phonon energy (heat) via carrier relaxation. Based on the law of conservation of energy, we assume that the photon energy was totally converted to heat, and therefore we focus on the carrier excitation. In contrast to the visible frequency region, the photon energy of a THz wave (~4 meV at 1 THz) is much lower than the band gap energy; the absorption mechanism of CNT films is thus irrelevant to the interband transition and can be modeled by considering both the Drude-like free electron response and the quasi-1D plasmon resonance [82], and hence the carrier density and the Fermi-level position

of a CNT film should be closely related to the film’s THz absorption ratio. We therefore investigated the relationship between the THz absorption spectra and the carrier densities of the CNT films [83]. The tuning of electrical properties of a single CNT by the field effect technique is a well-known approach reported in many studies. However, Fermi level tuning of a microscale-thickness CNT film remains difficult because the gating does not work inside the film’s network structure [84]. In this study, we employed the electric double layer (EDL) technique to tune the Fermi level of the whole CNT film continuously. Figure 3a shows a schematic of an EDL-based CNT transistor. Two CNT films, one for the channel and the other for the gate, were placed on a glass substrate and connected via an ionic liquid of N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium bis(trifluoromethane sulfonyl)imide (DEME-TFSI). In contrast to a conventional field effect transistor, which uses electric fields directed through electric insulators, ions in the EDL-based transistor move along with the applied electric field, and the cations selectively cling to CNTs, resulting in the Fermi level tuning. The ions migrate to the inside of the networked materials, and thus we can tune the Fermi level of the whole area of a CNT film, as shown in Figs. 3b and 3c. We measured the THz absorption spectra of the CNT films as a function of Fermi level by using THz time-domain spectroscopy (THz-TDS) as illustrated in Fig. 4a, employing a Cherenkov-type THz wave emitter (0.5–4.5 THz) with a nonlinear optical crystal (LiNbO3) waveguide and a photoconductive detector mounted on a hyper-hemispherical silicon lens. The transmission measurements were conducted under the following conditions: room temperature, dry air, averaging 1028 measurements per film, with a time resolution of 2 fs, a frequency resolution of 3.8 GHz, a scan range of 262 ps, a throughput of 16 ms/scan, and a frequency accuracy of 10 GHz (at 1.41 THz). Figure 4b displays the relatively flat THz absorption over a broad THz frequency region, and this absorption strongly depended on the gate voltage. The change in the absorption under 2-THz irradiation (Fig. 4c) revealed that the THz coupling efficiency varied by a factor of 16 (6.1% at the minimum and 98.2% at the maximum). These results can be explained by the energy band diagram of the CNT film shown in Fig. 4d. When the Fermi level lies in the band gap, most of the irradiated THz waves are transmitted through the CNT film because of the lower photon energy of the THz waves

relative to the band gap energy. In contrast, when the Fermi level enters the higher or lower band and consequently carriers are generated, the THz waves are absorbed well by the free carriers in the CNT films. The measured THz absorption spectra were in good agreement consistent with this explanation, and therefore the results indicated that carrier doping in CNT films contributes significantly to THz wave coupling, and thus Fermi-level control is crucial to optimizing the detector performance.

2.4 Chemical doping for p–n junction In order to enhance thermoelectric responses, p–n junctions are generally used in thermoelectric materials owing to the large difference in Seebeck coefficients associated with this structure. In this study, we utilized the interface at the p-n junctions in the CNT films as the THz detection area [83]. For the CNT THz detector with the p-n junction illustrated in Fig. 5, Equation (1) can be simplified as follows: =

( )

=(

<

−

)×∆ ,

(4)

where SP (SN) is the Seebeck coefficient of the p-type (n-type) CNT film, and ∆T is the thermal gradient induced by THz irradiation. Equation (4) indicates that the photothermoelectric voltage can be enhanced by the p-n junction because the Seebeck coefficient of the p-type (n-type) is generally positive (negative). To change the electrical properties of a CNT film from p-type to n-type, the Fermi level can be tuned via the EDL technique as shown in Fig. 3. However, in THz detection based on the photothermoelectric effect, the presence of the ionic liquid inevitably increases the total heat capacity of the device and hampers the temperature rise induced by THz irradiation, which in turn deteriorates and slows down the THz response. For this reason, as an alternative to the EDL technique, we utilized chemical doping to form the p-n junction inside the CNT films. Motivated by high demands for energy harvesting, several types of doping reagents for CNT films have been reported [85,86], and the doping solution of a simple salt (sodium hydroxide: NaOH) with 15-crown-5-ether has been shown to enable efficient n-type doping with long-term stability under exposure in air [87]. Therefore, in this study, we investigated the capability of the crown ether as the n-type doping solution for the CNT THz detector.

Figure 6a schematizes the doping mechanism of the crown ether solution. When a simple salt of NaOH and 15-crown-5-ether are mixed in pure water, the cation Na+ is tightly bound inside the ring structure of the crown ether; consequently, the counter anion OH- chemically reacts with the CNT and injects electrons into the CNT film, which varies the Fermi level of the CNT film. Figure 6b shows the Seebeck coefficient as a function of the concentration of NaOH and crown ether solution. These results showed that the pristine p-type CNT films (Seebeck coefficient of 44.4 µV/K) were sequentially shifted into n-type (Seebeck coefficient of -41.1 µV/K) with increasing concentrations of NaOH. We then fabricated the p-n junction inside the CNT film by dropping the doping solution on half of the CNT film, and measured the THz response profile by scanning THz irradiation (Fig. 6c). The observed profile showed that a strong positive signal was generated when the THz wave was irradiated at the p-n junction (X = 0 mm) and weak negative signals were generated at the metal-CNT interface (X = ±5 mm), indicating that the THz detection sensitivity was improved by a factor of 10 by the large difference in Seebeck coefficients of the p-n junction. In addition, we found that the doped CNT films were very stable in air, were never eroded after drying, and did not deteriorate the THz detection performance based on NEP values over 25 days (Fig. 6c and 6d). These cumulative results indicated that the NaOH and crown ether solution can be considered a promising doping method for photothermoelectric detectors, including flexible THz imaging technologies.

2.5 Thermoelectric device design We then investigated the macroscopic thermoelectric design of a device containing the CNT film to enhance the detection sensitivity and speed through the thermal conduction analysis [27]. According to Equation (4), one of the key factors that governs the photothermoelectric detection performance is the thermal gradient ∆T induced by THz irradiation. The temperature distribution in the device can be calculated by solving the heat equation given by:

>?

@

@A

=

B

CD

CED

+

CD

C*D

+

CD

CGD

H

+I,

(5)

where ρ is the density, C is the specific heat capacity, k is the thermal conductivity, and Q is the total amount of heat. Based on the model shown in Fig. 7a, we derived temperature distributions for various device shapes by performing simulations with the following conditions: the thermal conductivity of the CNT film was 10 W m-1K-1 in the XY-plane and 0.1 W m-1K-1 on the Z axis [87]; the density of the CNT film was 1 g cm-3; the specific heat capacity of the CNT film was 0.7 J g-1 K-1 [88]; the heat transfer coefficient of air was 10 W m-2K-1, and the temperature was maintained at a constant 300 K. We also assumed that the power of an incident THz wave was totally absorbed by the CNT film and was converted into heat, as indicated by the experimental results for the THz absorption spectrum (Figs. 4). We thus directly added the heat onto the CNT film. Figures 7b and 7c summarize the simulated temperature gradients and experimental THz responses as a function of CNT film width and thickness. The simulation results revealed that miniaturizing the film’s width and thickness led to a larger thermal gradient. These behaviors originate from both the reduction in the total heat capacity and the thermal localization due to the high thermal resistance of the CNT film. The experimental results of the THz response were consistent with the simulation results and with Equation (4), showing that the THz detection sensitivity increased with the larger thermal gradient generated by the device miniaturization. The performance enhancement due to miniaturization was limited to 150 µm owing to the diffraction limit of half the wavelength of a 1-THz wave, and the minimum film thickness required to retain sufficient mechanical strength for bendable imaging applications was approximately 1 µm. We also found that the THz detection speed increased with thicker CNT films (Fig. 7d). The simulation and experimental results showed similar tendencies: higher-speed THz detection with thicker CNT films. We achieved the fastest response of 6 ms with a 1 µm thickness (Fig. 7e). In terms of the detection sensitivity, Figure 8 depicts THz responses with variation in air pressure [79].

With decreasing air pressure, which is related to the thermal conductivity of air, the THz response was improved as a result of increasing the maximum temperature inside the CNT films. As shown in Fig. 8, we enhanced the THz response by a factor of 2.5 by suppressing the thermal diffusion into the surrounding air. However, this higher detection sensitivity requires a vacuum package and thus sacrifices the flexibility of the proposed CNT THz detector. By incorporating the above techniques, we fabricated a thermally optimized CNT THz detector with the best NEP value of 17 pW Hz-1/2 [27], which is competitive with current state-of-the-art room-temperature thermal detectors.

3. THz imaging

In general, imaging measurements are based either on the diffraction and penetration of electromagnetic waves or on the resonance of molecular vibrations, energy bands, or similar phenomena. These characteristics are governed by the frequency of the electromagnetic wave. Because the THz frequency region is between the microwave and visible regions, THz waves have both a high penetration ratio and high spatial resolution. In addition, because inter-molecular vibration spectra are in the THz frequency region, THz spectroscopy is well-suited to clarify the details of polymers and water. Because of these unique abilities, THz imaging techniques are expected to be powerful tools for nondestructive inspection in a variety of fields, including security, organic/inorganic materials characterizations, pharmaceutical quality control, agriculture, and medical and biological inspections [47–51]. The following subsections review useful examples of THz imaging applications and flexible THz imaging systems based on our flexible CNT THz detectors.

3.1 THz spectroscopic imaging Molecules vibrate at their natural frequencies because of their particular stretching, bending, rotating, slippage, and coordination dynamics. These frequencies are referred as to the fingerprint region, and by

investigating their fingerprint spectra, we can identify unknown compounds. Infrared spectroscopy is utilized to clarify high-frequency vibration modes associated with stretching (typically 4000–400 cm-1) [7–11]. In contrast, the THz fingerprint region corresponds to low-frequency vibration modes (typically less than 400 cm-1) associated with rotational structure and coordinated movements between macromolecules, and hence THz imaging can be used to probe higher order structures of polymers. We measured THz absorbance spectra of several types of medicines (scopolamine butylbromide, lactomin, rebamipide, famotidine, metoclopramide, and a mixture of famotidine and metoclopramide), by using the proposed THz-TDS as illustrated in Fig. 4a [89,90]. Figure 9a presents the different absorption peaks that appeared, indicating the capacity of the THz-TDS to identify components from these spectra nondestructively. In contrast to the visible light image (left of Fig. 9b), the THz images (right of Fig. 9b) enabled clear differentiation among the components from the different absorption peaks (0.96 THz for the coating agent, 2.28 THz for famotidine, and 1.36 and 2.55 THz for metoclopramide). The results also visualized contaminations from a metal and a polymer (Scotch tape), in which the THz signals appeared in all frequency range for the metal and above 4 THz for the Scotch tape. These material identification results demonstrate one of the powerful applications of THz imaging. As THz imaging application, we performed crack inspections of acrylic plates based on the diffraction of THz waves. In contrast to visible and infrared irradiation, most THz irradiation penetrates the acrylic owing to the material’s low THz absorption ratio; however, when the acrylic plate was defective, THz waves were diffracted at the edge of the defect, and consequently, the THz-detected signal was attenuated at the defect area, causing high THz absorption in that area. These features allow us to visualize cracks and contaminations on or inside the polymer films (Fig. 10), although the spatial resolution depends on the wavelength of the incident wave (300 µm at 1 THz) because of the diffraction limit. These results demonstrate another powerful THz imaging application in nondestructive quality inspections of industrial products.

3.2 Flexible THz imaging with CNT THz detectors In spite of their considerable potential in the above applications, the bulky components required by current THz imaging systems significantly restrict the shapes and locations of measured samples. In order to realize THz imaging as a core inspection technology in the IoT, flexible THz imaging systems must be developed. The proposed CNT THz detectors described in the previous section was demonstrated to offer the situational flexibility required for IoT applications. Figure 11 displays the THz imaging results of samples concealed behind an opaque object [79,83]. THz waves from several types of THz emitters (an impact-ionization avalanche transit-time diode, a frequency multiplier, and a THz laser pumped by a CO2 laser) were irradiated onto the samples, and the THz transmission images were acquired by scanning the CNT THz detector relative to the samples investigated. Metals concealed behind opaque objects (a piece of paper, an envelope, and a germanium plate) were visualized with 0.14, 1.0, and 29 THz irradiation, and objects (a pillar and chewing gum) inside a plastic box were also imaged under 1.4-THz irradiation. The spatial resolution depends on the wavelength λ, and therefore the image acquired at 29 THz (λ = 10.2 µm) is clearer than those acquired at 0.14 THz (λ = 2.1 mm), 1.0 THz (λ = 300µm), and 1.4 THz (λ = 214 µm). To demonstrate real-time monitoring, we further constructed an inline inspection system for medicines. Figure 12 depicts the measurement system and the THz response of the inline medical impurity screening. By comparing the normalized THz-detected signal, corresponding to the THz transmittance of the medicine, with the threshold signals, we could identify metal impurities and distinguish the medicines (A, B, and C), even when their visible colors were the same, in a nondestructive manner. Although we here employed only a single THz emitter with an oscillation frequency of 1 THz, we anticipate that the inspection capability could be improved by combining several THz frequency sources according to the frequency of the absorption peaks based on their fingerprint spectra as shown in Fig. 9a. A strong advantage of our CNT THz detector over conventional THz detectors is that the CNT THz detector can be easily bent and thus be fitted around objects of many shapes. Figure 13a demonstrates the

THz multi-view imaging of a syringe with a flexible THz scanner that integrated 23 CNT detector elements into a single array [79]. By wrapping our THz scanner around a syringe, the omnidirectional THz image was acquired without any bulky systems or mechanical rotations, and thus a break on the back side of the cylindrical syringe was successfully identified. We also fabricated a wearable THz sensor, the portability of which allows nondestructive quality inspections of products even when the products were still in use in places such as manufacturing plants that can be difficult to access (Fig. 13b) [83]. These results indicate that the proposed flexible THz imaging system has significant potential in future nondestructive sensing technologies in the IoT society.

Outlook and conclusion

In this work, we presented a CNT-based THz imaging system that eliminated bulky and complicated components and expanded the range of measurement objects. We first fabricated flexible THz detectors with CNT films and improved the detection performance by optimizing the Fermi level and thermoelectric design. We then demonstrated the practical applicability of this flexible THz imaging system in several types of industrial sensing applications such as inline medical impurity screening, multi-view imaging, and portable THz imaging for nondestructive quality testing of industrial products. Recent technological innovations have enabled various manufactured goods to penetrate our daily lives, which has led to a more comfortable and convenient society. However, inspection technologies that secure the safety of such products are essential for continued technological advances in this field. In the future, a more precise inspection system is needed to implement IoT technology. The flexible THz inspection device proposed here can easily inspect an object of any shape at any given location without a large-scale measurement system, and can thus be applied in situations that were previously difficult to address, such as quality inspections in congested environments and immediate inspections performed by visiting medical services. We hope that the achievements made by this study will be further developed into an inspection

system that contributes to the approaching IoT society.

Competing financial interests Y.K. and D.S. have filed Japanese patent applications related to this work.

Acknowledgements We thank the National Institute of Advanced Industrial Science and Technology (AIST) and ZEON Corporation for providing the CNT solution. This work was supported in part by the Mirai Program and Center of Innovation Program from the Japan Science and Technology Agency, the Toray Science Foundation, and JSPS KAKENHI Grant Numbers JP17H02730, JP18H03766, JP19K22099, JP19H02199, JP19K15002, and JP19H04539 from the Japan Society for the Promotion of Science, the Special Postdoctoral Researcher Program of RIKEN, the Hattori Hokokai Foundation, the Murata Science Foundation, the Yazaki Memorial Foundation for Science and Technology, and Support for Tokyo Tech Advanced Researchers (STAR).

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Fig. 1. (a) Schematic of the detection mechanism and the equivalent circuit of the photothermoelectric effect: along the temperature gradient induced by THz irradiation, carriers diffuse and a THz-detected voltage signal is generated. (b) Typical THz response of the photothermoelectric effect and response map of CNT THz detectors. (c) Noise voltage spectrum of a THz detector. The experimental value of 2 nV Hz-1/2 reached the theoretical value of the thermal noise limit of 2.75 nV Hz−1/2 (dashed line) calculated by Eq. 2. The peak value observed at 50 Hz originated from the power source of the measurement system, not from the THz detector. Reprinted with permission from [27,79].

Fig. 2. Schematic of the CNT filtration process. A monodispersed CNT solution was deposited on a cellulose membrane filter and was vacuumed until the CNT solution was fully filtered (typically over several hours). Reprinted with permission from [83].

Fig. 3. (a) Schematic and photograph of the EDL transistor with an ionic liquid (DEMETFSI). As a result of the EDL formation by ions, the Fermi level can be effectively tuned using CNT films with thicknesses of several tens of micrometers. (b) Resistance of 30-µm-thick CNT film versus gate voltage. (c) Noise voltage spectra at VG = 0.7 and −1.5 V, where the dashed lines indicate the theoretical values for the thermal noise. Reprinted with permission from [83].

Fig. 4. (a) Measurement system of the THz-TDS. (b) THz absorption as a function of the gate voltage for an irradiation frequency of 2 THz. (c) Variation of the absorption under 2-THz irradiation. (d) Schematic of the energy band diagram of the CNT film. Reprinted with permission from [83].

Fig. 5. Schematic of the photothermoelectric effect of the CNT THz detector under THz irradiation at the p-n junction interface.

Fig. 6. (a) CNT THz detector with the p-n junction formed by a chemical doping solution of NaOH and 15-crown-5-ether. (b) Measurement results for the Seebeck coefficient of a CNT film as a function of the concentration of the dopant (NaOH). (c, d) Stability check for the position of the p-n junction (c) and the NEP of the detector (d). Reprinted with permission from [83].

Fig. 7. (a) Simulation model and results of the steady-state thermal analysis. (b, c) Calculated temperature gradients and experimental THz responses as a function of CNT film width (b) and thickness (c). (d) Detection speed of THz detector versus CNT film thickness. (e) Transient response with a CNT film thickness of 1 µm. The solid line (black) and the dashed line (red) indicate the experimental results and the fitting curve calculated by V/VMax = (1- exp(-t/τ)), respectively. Reprinted with permission from [27].

Fig. 8. Measurement system and the THz response under vacuum. Reprinted with permission from [79].

Fig. 9. (a) THz absorption spectra of scopolamine butylbromide, lactomin, rebamipide, famotidine, metoclopramide, and a mixture of famotidine and metoclopramide. The arrows indicate the absorption peaks in the fingerprint spectra. (b) Nondestructive inspection of medical drugs. In contrast to the visible light image (left), the THz images (right) enable us to clearly distinguish the components (medicines and contaminations) based on the differences in their absorption spectra. Reprinted with permission from [89].

Fig. 10. Nondestructive inspections of cracks and contaminations (hair) on and inside the polymer films. Reprinted with permission from [89].

Fig. 11. THz imaging of samples concealed behind opaque objects with irradiation at different frequencies: (a) 0.14 THz, (b) 1.0 THz, (c) 1.4 THz, and (d) 29 THz. Reprinted with permission from [79,83].

Fig. 12. Measurement system and the THz response in inline medical quality screening using the CNT THz detector. The results of the normalized THz-detected signal (the THz transmittance of the medicine) enables us to identify the metal impurities and the medicines (A, B, and C) by comparing the detected signal with the threshold signals.

Fig. 13. Flexible THz imaging. (a) Photographic image of a flexible THz scanner and multi-view nondestructive inspection of a syringe. (b) Wearable THz sensing device and breakage detection of a pipe. Reprinted with permission from [79,83].

Table 1. Performance of THz detectors.

Detector type

Operation temperature (K)

Detectable frequency range

NEP (W/√Hz)

Detection speed

Refs.

Golay cell

300

0.02–20 THz

10 × 10-9

25 ms

Commercial

Pyroelectric

300

0.1–0.14 THz

2 × 10-9

2.3 ms

[23]

300

1.1–100 THz

0.21 × 10-12

N.A.

[24]

1.6

0.06–1 THz

3.6 × 10-15

3 ms

Commercial

TES

4.2

0.65 THz

50 × 10-15

N.A.

[25]

MEMS Bolometer

300

N.A. (Broadband)

20 × 10-12

55 µs

[26]

Thermocouple

300

0.14–39 THz

17 × 10-12

13 ms

[27]

SBD

300

0.823 THz

36.2 × 10-12

1 µs

[28]

FMB diode

300

0.3–1 THz

3.0 × 10-12

N.A.

[29]

Backward diode

300

0.1 THz

0.18 × 10-12

N.A.

[30]

CMOS

300

0.823 THz

36.2 × 10-12

1 µs

[31]

Bolometer

50 × 10

-12

8 µs

[32]

38 ps

[33]

HBT

300

0.65–1 THz

HEMT

300

0.28 THz

0.5 × 10-12

Plasmonics

300

0.2 THz

0.48 × 10-12

N.A.

[34]

Quantum well

4.2

3–7 THz

0.4 × 10-12

160 ps

[35]

*TES: Transition edge sensor, MEMS: Microelectromechanical systems, SBD: Schottky barrier diode, FMB: Fermi-level managed barrier, CMOS: Complementary metal–oxide–semiconductor, HBT: Heterojunction bipolar transistor, HEMT: High electron mobility transistor.

List of corrections i) Text - Please see “Supporting Information For Review.docx” ii) Table - Table 1 1. We have added the list for performance of other THz detectors.

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Y.K. and D.S. have filed patent applications related to this work.