Carbon Nanotube Films as Sensor Material: Comparison
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Please note this is a comparison between Version 2 by Conner Chen and Version 1 by Kou Li.

The photo-thermoelectric (PTE) effect in electronic materials effectively combines photo-absorption-induced local heating and associated thermoelectric conversion for uncooled and broadband photo-detection. Formation of heterogeneous material junctions across the carbon nanotube (CNT)-film-based PTE sensors, namely photo-detection interfaces, triggers the Seebeck effect with photo-absorption-induced local heating. Typical photo-detection interfaces include a channel–electrode boundary and a junction between P-type CNTs and N-type CNTs (PN junctions). While the original CNT film channel exhibits positive Seebeck coefficient values, the material selections of the counterpart freely govern the intensity and polarity of the PTE response signals. Based on these operating mechanisms, CNT film PTE sensors demonstrate a variety of physical and chemical non-destructive inspections. 

  • carbon nanotubes
  • photo-thermoelectric effect
  • broadband photo sensing

1. Introduction

Photo-sensing or photo-imaging measurements allow non-contact acquisition of optical information for monitoring targets in a large area. The optical properties of various objects are typically variable in monitoring wavelengths or bands. This inherent nature of photo-irradiation enables hyper-spectral material identification of composite objects by aggregating optical properties at multiple wavelengths [1,2][1][2] or non-contact structural reconstructions of layered targets by analyzing the degree of transmission, reflection, and absorption against illumination [3,4,5][3][4][5]. Therefore, photo-sensing or photo-imaging measurements advantageously play a leading role in techniques for non-destructive inspections among other types of testing methods [6,7,8,9][6][7][8][9]. To handle diverse materials or objects consisting of these in non-destructive inspections, effective use of longer-wavelength light over typical X-ray, ultraviolet (UV), and visible light (Vis) bands is a potential approach. Here, longer-wavelength light includes millimeter- or terahertz-waves (MMW, THz) and infrared light (IR: far-IR (FIR), mid-IR (MIR), and near-IR (NIR)). Irradiation in these bands exhibits transparency to even non-metallic opaque objects compared to Vis, with much lower invasiveness than with X-rays. Fingerprint spectra or intrinsic absorption characteristics often exist in these bands (e.g., polymers and gases) [10[10][11],11], and such features also enrich non-destructive inspection techniques.
To perform fundamental measurements in the above electromagnetic-wave regions, careful selection of the photo-detection mechanism is indispensable. Typical MMW–IR detection methods include electronic-, photon-, and thermal-type approaches. Schottky barrier diodes, CMOS devices, and high-electron-mobility transistors are representative examples of electronic-type detectors [12,13,14][12][13][14]. Electronic-type devices function with an ultrafast photo-detection speed (order of a few picoseconds) and high sensitivities, particularly in sub-THz regions in uncooled conditions. Photon-type devices, such as quantum well and quantum dot structures, function with a fast photo-detection speed (order of several tens of picoseconds) and ultrahigh sensitivities at extremely low operating temperatures [15,16][15][16]. Bolometers and Golay cells typically serve as thermal-type devices and perform uncooled ultrabroadband photo-detection ranging from the sub-THz to IR regions [17]. Under the motivation to deal with a wider range of inspection materials and objects, employing the thermal-type photo-detection mechanism is effective because of its advantageous characteristics: ultrabroadband measurements over the electronic-type and uncooled operations over the photon-type. First, ultrabroadband photo-detection is essential for aggregating multi-spectral optical information of inspection targets. Uncooled operations facilitate compact and portable systematization without the need for bulky cooling equipment. This feature is indispensable for expanding the range of evaluable targets by relaxing their inherent location and environmental limitations. Thermal-type photo-detection includes bolometric detection and the photo-thermoelectric effect (PTE effect). Device operations under the PTE effect function without applying a voltage bias and allow photo-detection with a lower limit of thermal noise. By contrast, bolometric detection requires an external voltage bias and induces 1/f noise along with thermal noise [18]. Therefore, the device design and the associated system development of PTE effect-based photo-sensing or photo-imaging techniques potentially enrich the existing non-destructive inspection platforms.
The PTE effect generally synergizes two different energy conversion phenomena: photo-thermal and thermoelectric. The PTE effect induces thermoelectric conversion at the photo-detection interface with local heating owing to photo-absorption. This means that utilizing the PTE sensor detects external irradiation by obtaining electrical response signals. When the PTE sensor consists of a single material structure, the photo-detection interface locates at the edge of the channel to localize the photo-induced heating. Here, the PTE response is proportional to the Seebeck coefficient of the device material and photo-induced temperature gradient across the channel [19]. On the other hand, the PTE sensor consisting of multiple materials often exhibits its photo-detection interface in the bonding junction. This is because a difference between each material’s Seebeck coefficient corresponds to the effective Seebeck coefficient of the bonding junction, and proper material combinations lead to enhancing the PTE response [20]. Therefore, the PTE device design includes not only device material evaluations from the viewpoint of photonic, thermal, and electronic aspects but also material combinations and their structural management, allowing a wide range of investigation approaches. Indeed, today’s investigation field of the PTE device design is rapidly growing [21[21][22][23],22,23], and various materials exhibit their potential suitability for PTE conversion, as described later. However, development of PTE sensors satisfying sensitive photo-detection in the aforementioned MMW–NIR bands and the associated demonstrations of ultrabroadband multi-wavelength non-destructive inspections are still insufficient.

2. Sensor Material: CNT Films

As the first step in designing PTE sensors, this chapter breaks down the fundamental properties of carbon nanotube CNT(CNT) films as the device material. CNT films consist of randomly stacked CNTs and their network structures, resulting in high mechanical strength [27][24] and flexibility [28][25]. The CNT film also effectively absorbs ultrabroadband photo-irradiation at room temperature [29][26]. These features emphasize that employing a CNT film plays an essential role in developing ubiquitous non-destructive inspection devices and systems among other representative materials for PTE conversion, such as graphene [30][27], CNTs [31][28], MoS2 [32][29], SnSe [33][30], NbS3 [34][31], and PEDOT:PSS [35][32]. The CNT film collectively and advantageously satisfies the macroscopic free deformations and ultrabroadband photo-absorption among the above candidates. The CNT film further exhibits the Seebeck coefficients ranging from tens to hundreds of µV/K, which are comparable to those of PEDOT:PSS and so on. Based on these, utilizing the CNT film effectively converts external ultrabroadband photo-irradiation power to electrical signals for sensing or imaging measurements and simultaneously provides omni-directional viewing angles to 3D-structure targets. Regarding  the current-voltage characteristics of the CNT-film-based PTE sensor with and without external light irradiation to the device. The result clearly shows ohmic contacts between the CNT film channel and metal electrodes for signal readout, and this behavior allows PTE conversion under zero-bias-voltage operations, as mentioned earlier. As detailed in the following chapters, this review focuses on single-walled semiconducting-metallic-mixed CNT films. The overlapping of each energy band structure of randomly stacked CNTs is a possible factor for the ohmic contacts of the single-walled semiconducting-metallic-mixed CNT films with diverse metal electrode materials. As dispersion solutions of CNTs are widely available, suction filtration is a possible approach. Employing laser-processed thin-film masks allows selective CNT film patterning. This method freely controls the shape, size, and positions of the CNT films via laser processing on the mask. Management of the mask’s thickness and the solution’s drop amount also leads to thickness control of the CNT film up to micrometer-order. While efficient photo-absorption requires micrometer-order thickness of CNT films [36][33], the above thickness control eases the device design. Other representative CNT film patterning approaches include lithography (nanometer-order thickness) [37][34] and manual cutting (up to millimeter-order thickness) [38][35]. Although manual cutting potentially contributes to fabricating the PTE sensors for its available thickness range, the presented self-aligned suction filtration process advantageously realizes higher-yield repeatable micrometer-order spacing or patterning of the CNT film over manual handling. On the other hand, management of the solution’s drop amount controls the friction force between the mask and a membrane filter. This means that the self-aligned suction filtration of the CNT film enables both direct patterning on the membrane and embedding in the mask film. Therefore, the presented approach freely develops CNT film patterning on various thin-film materials regardless of their water absorbability.

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