Flexible Thermoelectric Generator Based on Dip‐Coated SWCNT/PEDOT:PSS Composite Films for High Power Generation

We investigated the origin of n-type thermoelectric properties in single-wall carbon nanotube (SWCNT) films with anionic surfactants via experimental analyses and first-principles calculations. Several types of anionic surfactants were employed to fabricate SWCNT films via drop-casting, followed by heat treatment at various temperatures. In particular, SWCNT films with sodium dodecylbenzene sulfonate (SDBS) surfactant heated to 350 °C exhibited a longer retention period, wherein the n-type Seebeck coefficient lasted for a maximum of 35 days. In x-ray photoelectron spectroscopy, SWCNT films with SDBS surfactant exhibited a larger amount of sodium than oxygen on the SWCNT surface. The electronic band structure and density of states of SWCNTs with oxygen atoms, oxygen molecules, water molecules, sulfur atoms, and sodium atoms were analyzed using first-principles calculations. The calculations showed that sodium atoms and oxygen molecules moved the Fermi level closer to the conduction and valence bands, respectively. The water molecules, oxygen, and sulfur atoms did not affect the Fermi level. Therefore, SWCNT films exhibited n-type thermoelectric properties when the interaction between the sodium atoms and the SWCNTs was larger than that between the oxygen molecules and the SWCNTs.

Single-walled carbon nanotube (CNT) film is a transparent, conductive, and flexible material that exhibits uniform physical and electronic properties1. Several promising optoelectronic and photovoltaic device applications of these films have recently been demonstrated2. However, in previous works, the properties of the junction between the CNT film and the semiconductor substrate (typically Si) have not been properly characterized2. Here, we analyze the interface and transport properties of the junction between the CNT film and Si substrates by fabricating metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) structures, where the CNT film acts as the metal and Si is the semiconductor. Our results help to better understand the electrical properties of the CNT film-Si contacts and to improve the design of optoelectronic and photovoltaic devices which use CNT films as transparent conductive electrodes. Device fabrication begins by preparing CNT films using a vacuum filtration approach1,3 (Fig. 1a) and opening windows in SiO 2 layers on 1015–1016 cm−3 doped n- and p-type Si substrates (Figs. 1c and 1d). For MIS structures, a thin oxide layer is then thermally grown on the exposed Si areas (Fig. 1d2). Next, the CNT films are deposited over both MS and MIS samples (Figs. 1d) and then patterned by O 2 plasma etching3 to form individual devices (Figs. 1e). Finally, metallic rings are deposited on the films for electrical probing (Figs. 1e). For comparison, control samples in which CNT film is replaced with a Ti/Au layer (10/90 nm) have also been fabricated and characterized. An optical image of a CNT film-Si MS structure is shown in Fig. 1b.

Development of hydrogel microstructures on single-walled carbon nanotube films

Ultrafast wafer-scale assembly of uniform and highly dense semiconducting carbon nanotube films for optoelectronics

Carbon nanotubes are promising candidates for thermoelectric power generation because of their one-dimensionality mediated high Seebeck coefficient, high electrical conductivity with added advantages of flexibility, light weight, and scalability. We report the temperature-dependent thermoelectric properties of single-walled carbon nanotube (SWCNTs) films. The SWCNTs films exhibit p-type metallic conduction with high Seebeck coefficient (∼69.5 μVK−1) and moderate electrical conductivity (∼76 Scm−1). The films exhibit low thermal conductivity (∼0.1 Wm−1 K−1) due to phonon scattering at the interjunction region. The synergetic combination of thermoelectric properties resulted in a high figure-of-merit of ∼0.11 at 305 K. A flexible thermoelectric generator based on SWCNTs films mounted on a curved hot surface exhibited an output of 17 mV and 54 μA under a small temperature gradient of 10 K. The present work provides possible avenues for developing wearable SWCNTs based thermoelectric power generation modules for harvesting body heat.

Solvent-optimized monolithic SWCNT-based thermoelectric generator for efficient electricity harvesting from body heat and sunlight

In this study, we analyzed the morphological changes and molecular structure changes on the surface of single-walled carbon nanotube (SWCNT) films during oxygen plasma (O2) etching of SWCNT surfaces formed by the spray method and analyzed their potential use as electrochemical electrodes. For this purpose, a SWCNT film was formed on the surface of a glass substrate using a self-made spray device using SWCNT powder prepared with DCB as a solvent, and SEM, AFM, and XPS analyses were performed as the SWCNT film was O2 plasma etched. SEM images and AFM measurements showed that the SWCNT film started etching after about 30 s under 50 W of O2 plasma irradiation and was completely etched after about 300 s. XPS analysis showed that as the O2 plasma etching of the SWCNT film progressed, the sp2 bonds representing the basic components of graphite decreased, the sp3 bonds representing defects increased, and the C-O, C=O, and COO peaks increased simultaneously. This result indicates that the SWCNT film was etched by the O2 plasma along with the oxygen species. In addition, electrochemical methods were used to verify the damage potential of the remaining SWCNTs after O2 plasma etching, including cyclic voltammetry, Randles plots, and EIS measurements. This resulted in a reversible response based on perfect diffusion control in the cyclic voltammetry, and an ideal linear curve in the Randles plot of the peak current versus square root scan rate curve. EIS measurements also confirmed that the charge transfer resistance of the remaining SWCNTs after O2 plasma etching is almost the same as before etching. These results indicate that the remaining SWCNTs after O2 plasma etching do not lose their unique electrochemical properties and can be utilized as electrodes for biosensors and electrochemical sensors. Our experimental results also indicate that the ionic conductivity enhancement by O2 plasma can be achieved additionally.

Dye-fixed TiO2 films and doubly layered Fe2O3 films were prepared and used in solar cells. For the both cells a single walled carbon nanotube (SWNT) film, prepared by simply pasting over ITO glass, was used as a counter electrode. A stable current (I)-Voltage (V) curve was found under UV-visible light irradiation using KI+I2 in water as an electrolyte. A comparison of I–V curves between Pt and SWNT films has been made. Under UV visible light irradiation almost similar results were found for both Pt and SWNT films. In the case of Fe2O3, the open circuit voltage was found to be low (about 90 mV) but the short circuit current was found to be comparatively high (about 0.75 mA cm/t-2) whereas those for the Eosin Y (EY)-fixed TiO2 film with the SWNT film as a counter electrode, were found to be 350 mV and 0.27 mA cm/t-2, respectively.

Fluctuation-induced tunneling dominated electrical transport in multi-layered single-walled carbon nanotube films

Polarization influence on the photovoltaic current raised due to the photon-drag effect in the single-walled carbon nanotube (SWNT) films and nanostructured silver-palladium (Ag/Pd) resistive films is examined at the wavelengths of 532 and 1064 nm of nanosecond laser pulses. The SWNT films were synthesized by the aerosol chemical vapor deposition technique. Ag/Pd films, consisting of AgPd alloy and palladium oxide (PdO), were prepared by burning a special paste on a ceramic substrate. The films obtained were characterized by Raman spectroscopy. It is shown that the Ag/Pd films Raman spectra consist of PdO peak that moves from 650 cm-1 to 628 cm-1 as the excitation He-Ne laser power increases. The photocurrent was measured at the oblique incidence of the laser beam on the film in the direction perpendicular to the plane of incidence. It is found that the transverse photocurrent in the SWNT films at circular polarization is absent and does not depend on the direction of the electric field vector rotation (the sign of the circular polarization) of the incident irradiation. The photocurrent in the Ag/Pd films at circular polarized irradiation is significant and depends on the circular polarization sign. The results obtained demonstrate the potential applications of the Ag/Pd resistive films as a sensor of the circular polarization sign of the incident light pulse in a wide wavelength range.

Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) are one-dimensional materials with high thermal conductivity and similar crystal structures. Additionally, BNNTs feature higher thermal stability in air than CNTs. In this work, a single-walled carbon nanotube (SWCNT) film was used as a template to synthesize a BNNT coating by the chemical vapor deposition (CVD) method to form a coaxial heterostructure. Then, a contact-free steady-state infrared (IR) method was adopted to measure the in-plane sheet thermal conductance of the as-synthesized film. The heterostructured SWCNT-BNNT film demonstrates an enhanced sheet thermal conductance compared with the bare SWCNT film. The increase in sheet thermal conductance shows a reverse relationship with SWCNT film transparency. An enhancement of over 80% (from ∼3.6 to ∼6.4 μW·K-1·sq-1) is attained when the BNNT coating is applied to an SWCNT film with a transparency of 87%. This increase is achieved by BNNTs serving as an additional thermal conducting path. The relationship between the thermal conductance increase and transparency of the SWCNT film is studied by a structured modeling of the SWCNT film. We also discuss the effect of annealing on the thermal conductance of SWCNTs before BNNT growth. Along with the preservation of high electrical conductance, the enhanced thermal conductance of the heterostructured SWCNT-BNNT films makes them a promising building block for thermal and optoelectronic applications.

We demonstrated a simple transfer method enables to fabricate the single-walled carbon nanotube (SWCNT) film on the plastic substrate. SWCNT network was separated from the initial substrate and transferred onto another substrate using the nitric acid. We also found that electrical conductivity of transferred film was improved. The sheet resistance of SWCNT films was changed from a 150∼300 Ω/sq to a 80∼150 Ω/sq after transferring the SWCNT film in the range of 70∼80% transmittance. This is contributed to the densification of the SWCNT network. During the transfer process, the nitric acid fills the voids between the tubes and form homogeneous interfaces among the tubes, liquid and air. Pressure differences at the interface exert bending moments to the tubes and induce deformation. By the way, deformation leads to reduce the distance between the tubes and it is stuck together due to the van der Waals forces. We suggest that this transfer method provides electrically improved SWCNT films as well as easy, contrallable and inexpensive way to fabricate. This work can open up new possibility for the flexible electronics.

Carbyne is a carbon allotrope whose structure is a one-dimensional chain of sp-hybridized carbon atoms. Carbyne’s mechanical and electrical properties, as predicted by theoretical studies, have attracted great interest because they would lead to many promising applications. Thus, much effort has been devoted to the synthesis of carbyne. Long linear atomic carbon chains encapsulated in carbon nanotubes have recently been produced by high-temperature heat treatment of double-wall carbon nanotubes (DWCNTs). Here, we present an alternative approach to produce long linear carbon chains: field electron emission accompanied by electrical discharge from single-wall carbon nanotube (SWCNT) films. Raman spectroscopy and transmission electron microscopy were performed on SWCNT films after the electrical discharge during field electron emission. The results showed that a large number of long linear carbon chains were formed within the SWCNTs and DWCNTs. For DWCNTs with an inner diameter of 0.7 nm, the atomic carbon chains lay directly along the central tube axis. However, for SWCNTs with an inner diameter of 1.0 nm, the encapsulated carbon chains were bent in some places and positioned close to the nanotube wall, away from the central tube axis.

This report presents n-type single-walled carbon nanotubes (SWCNT) films with ultra-long air stability using a cationic surfactant and demonstrates that the n-type Seebeck coefficient can be maintained for more than two years, which is the highest stability reported thus far to the best of our knowledge. Furthermore, the SWCNT films exhibit an extremely low thermal conductivity of 0.62 ± 0.08 W/(m·K) in the in-plane direction, which is very useful for thin-film TEGs. We fabricated all-carbon-nanotube TEGs, which use p-type SWCNT films and the n-type SWCNT films developed, and their air-stability was investigated. The TEGs did not degrade for 160 days and exhibited an output voltage of 24 mV, with a maximum power of 0.4 µW at a temperature difference of 60 K. These results open a pathway to enable the widespread use of carbon nanotube TEGs as power sources in IoT sensors.

Transparent conducting single-walled carbon nanotube (SWCNT) films were fabricated using the spin coating technique. UV-ozone treated and poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) coated glass substrates together with SWCNT dispersed in 1,2-dichlorobenzene were used to promote the adhesion of SWCNT at room temperature. The produced film had a sheet resistance of 430 Ω/□ for 80% optical transparency at 550 nm. The spin coated SWCNT film after a post fabricated treatment in a mixer of isopropyl alcohol and nitric acid solution had a sheet resistance as low as 120 Ω/□ for 80% optical transparency at 500 nm. Besides reduction in sheet resistance, we obtained stable and strongly adherent SWCNT films on substrate that could serve as an alternative to transparent conducting oxides in display and optoelectronic applications.