Introduction

Since the discovery of carbon nanotubes (CNTs)1,2,3,4,5,6,7, their hybrids with transition metals have been extensively developed to achieve synergetic performances of these materials in various advanced fields such as flexible/stretchable electronics8,9, sensors10, energy storage devices11, and catalysts12. CNTs can be hybridized with transition metal components using diverse processes, including electroplating, physical vapor deposition, chemical vapor deposition, hydrothermal processes, and dispersion13,14,15,16,17,18. Among them, the solution-based wet process is expected to be widely used in the future because dispersion-based wet processes (e.g., printing, coating, and wet spinning) are extensively employed in large-area and scalable production of electronic devices.

To date, many attempts have been made to use CNTs (multi-walled and single-walled CNTs (SWCNTs)) for ink- or paste-type dispersions, which can be applied to several existing printing processes19,20,21,22,23,24,25,26. However, nano-hybridized SWCNTs/transition metal dispersions have not been achieved because SWCNTs predominantly exhibit substantial bundled structures that lead to their unfavorable dispersions in media due to the strong van der Waals (vdW) force2,4. Thus, in addition to colloidal interaction, nanoscale dispersibilities of SWCNTs are a prerequisite for the nanoscale hybridization of SWCNTs with transition metals in various media.

Generally, to overcome the inferior dispersibilities of SWCNTs, their dispersions are obtained through three processes: (i) hexagonal carbon wall oxidation for functionalizing SWCNTs; (ii) introduction of additional reagents, such as organic solvents27, ionic salts28, polymers29, alkali metals30, and superacids31, as additives into SWCNTs; (iii) application of a harsh mechanical mixing method such as ultrasonication. Although all these methods can successfully disperse SWCNTs, many of the methods induce defects in the carbon lattices of SWCNTs that deteriorate the electrical properties of SWCNTs. Furthermore, some methods disperse SWCNTs only at low concentrations, thereby hindering the attainment of SWCNT dispersions with niche rheological properties to realize wet processes. Additionally, ensuring a safe process conduction under mild conditions is difficult, and several steps are required during the process to achieve neat SWCNTs.

Recently, several effective SWCNT dispersion methods that do not severely degenerate the carbon lattices have been developed32,33,34. The small organic molecules diethyltoluenediamine and dimethyldicyane have been employed to disperse SWCNTs with bundle sizes narrower than those obtained using the conventional dimethylformamide solvent33. Moreover, to avoid hazardous and harsh conditions or destructive processes during SWCNT dispersion, researchers have used p-toluenesulfonic acid and methanesulfonic acid to disperse SWCNTs34. In these studies, researchers have examined the relationships between charge transfer on SWCNTs and interactions with the dispersion medium using organic/acid-based liquid media. Charge transfer on SWCNTs can render the SWCNT dispersions stable, resulting in counterbalanced electrostatic interactions that provide concentrated dispersions of SWCNTs, which can attain suitable rheological properties when subjected to printing or coating.

Nevertheless, transition metal hybridization of the dispersions of SWCNTs is challenging with inherently implicit difficulties because compatible intermolecular interactions cannot be easily achieved for a stable colloidal system with extensive intrinsic properties (e.g. surface energy, density, melting point, and reactivity) of constituent heterogeneous materials. Therefore, SWCNT dispersion systems that comprise transition metal components and do not involve harsh, destructive, or additive-free methods have not been established thus far. Furthermore, obtaining highly concentrated dispersions of SWCNTs hybridized with transition metal components and performing wet processing at room temperature using these dispersions are challenging and thus hamper the development of SWCNT dispersion systems with diverse wet processibilities. Thus, a cutting-edge solution for the simultaneous hybridization of transition metal components and dispersion of SWCNTs at the molecular level is required to address existing challenges.

Herein, we report a SWCNT colloidal suspension system that uses a metal-complex-based medium with high dispersion stability and wet processibility at room temperature to produce CNTs/transition metal hybrid nanostructured electrodes. As metal complexes can accept or donate electrons at a molecular level, they are promising candidates to disperse SWCNTs via charge transfer in solutions35. The Cu-complex selected for hybridization in this study can disperse SWCNTs in a solution by simple mixing without the destruction of SWCNTs. Thus, the resulting mixture demonstrates viscoelastic behavior and a liquid crystalline phase, which are observed in concentrated dispersions originating from SWCNT self-assembly (SA). We discovered a solution consisting of a Cu-complex that acted as an n-type dopant for SWCNTs, playing a crucial role in the dispersion mechanism, and proposed plausible counterbalanced interactions in a gel network system. We anticipate that the proposed approach can be expanded to other transition metals and can serve as one of the promising methods for forming hybrid colloidal suspensions based on transition metal complexes and CNTs.

Moreover, we provide a facile deposition method for direct fabrication of flexible electrodes on polymeric substrates, including polyimide (PI) films and polyester fabrics, and cylindrical glass surfaces by screen printing, direct writing extrusion printing, blade coating, and dip coating using dispersions. After reduction under the ambient thermal condition of 200 °C, the nanostructured electrodes based on SWCNTs/Cu nanoparticle (SWCNTs/CuNP) exhibit multifunctional electrical performances. These electrodes demonstrate electrothermal properties (heating up to 110 °C in 10 s at an applied voltage of 10 V) and electromagnetic interference (EMI) shielding performances (over 60 dB at 10 GHz) with moderate flexibilities. Thus, CNTs/metal hybrid materials can exhibit multifunctionalities and can be extensively employed in other versatile applications.

Results

Outline of this study

Schematic of the entire fabrication of the multifunctional electrodes using the dispersion of SWCNTs hybridized with a Cu-complex solution (SWCNTs/Cu-complex) is shown in Fig. 1. As Cu is one of the widely used metals for electrodes, herein, a Cu formate-based complex was chosen as the metal component among the complexes of many transition metals. Cu-complex was coordinated with 2-ethyl-1-hexylamine (EH-NH2) followed by mixing with SWCNTs in an amine compound solution through a simple stirring step (Fig. 1a, b). The simple stirring step afforded the stable colloidal suspensions because the Cu-complex solution acted as a doping reagent, resulting in the de-bundling of SWCNTs. Furthermore, as EH-NH2 exists as a liquid at temperatures between its melting point (−76 °C) and boiling point (169 °C), it allowed suitable dispersion processibility at room temperature. SWCNTs/Cu-complex samples with different solid contents were printed or coated, followed by single-step reduction, which was thermally triggered at low temperatures and produced Cu nanoparticles (CuNPs) (Fig. 1a). The wet-processed electrodes formed the SWCNTs/CuNP hybrid nanostructured networks demonstrating multifunctional electrical performances (Fig. 1c).

Fig. 1: Schematics of the dispersion and fabrication of multifunctional electrodes.
figure 1

a Thermal reduction reaction of the EH-NH2-coordinated Cu-complex. b SWCNT dispersion with a Cu-complex solution and process design to fabricate the SWCNTs/CuNP nanostructured electrode. c Flexible electrodes on a polymeric substrate and Joule heating and EMI shielding performances of the electrodes.

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Viscoelastic behavior and liquid crystalline phase of SWCNTs/Cu-complex

We produced dispersions of SWCNTs/Cu-complex with up to 2 wt% SWCNTs (the highest concentration) under stirring and subjected them to a wet process using various printing/coating methods. With an increase in the SWCNT concentration from 0.1 to 2 wt%, viscosities of the dispersions increase (Fig. 2a). All dispersions exhibit pseudoplastic behaviors, wherein the viscosity decreases with an increase in shear rate. Shear thinning behavior is attributed to the aligned flow of the dispersion with high aspect ratio of SWCNTs. Dispersion with the lowest SWCNT concentration (0.1 wt%) demonstrates a shear rate (1 s−1) viscosity of 2.78 Pa·s, whereas that with a SWCNT concentration of 2 wt% exhibits a shear rate (1 s−1) viscosity of 122 Pa·s. Cu-complex solution behaves as a Newtonian fluid demonstrating a shear rate (1 s−1) viscosity of 0.114 Pa·s. Shear rate viscosities of the SWCNTs/Cu-complex samples are in the range of the typical shear rate viscosities needed for printing. Viscoelastic behaviors of the dispersions were confirmed by measuring their storage modulus (G′) and loss modulus (G″) values at shear stress. In the lower shear stress region, before the crossover point (G′ = G″), G′ representing the stored deformation energy is dominant; in contrast, in the higher shear stress region, G″ is dominant (Fig. 2b). As G″ is defined as the deformation energy lost during flow, all dispersions exhibiting predominant G′ values ensure printing/coating processabilities.

Fig. 2: Viscoelastic properties of the Cu-complex solution and dispersions of SWCNTs/Cu-complex with different SWCNT concentrations.
figure 2

a Shear thinning (pseudoplastic) behaviors of the dispersions and Newtonian fluid behavior of the Cu-complex solution with respect to the shear rate. b Storage modulus (G′) and loss modulus (G″) values of the dispersions versus shear stress. c Ratios of modulus (G′/G″) in a relatively low angular frequency region. d Photograph of the Cu-complex solution and dispersions with different SWCNT concentrations of 0.1, 0.5, 1, and 2 wt%. e Description of rheology with an image of the paste (the dispersion with a SWCNT concentration of 1 wt%). All rheological behaviors were analyzed at room temperature.

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These rheological results indicate that viscosities and shear rate viscosities of dispersions can be controlled by varying the concentration of SWCNTs and adopted for use in appropriate printing/coating methods that consider G′/G″ (Fig. 2c)36. All the SWCNTs/Cu-complex samples are in the regions of viscoelastic soft solid and gel at relatively low frequencies, whereas SWCNTs/Cu-complex with a relatively low SWCNT concentration of 0.1 wt% demonstrates viscoelastic liquid-like behavior at frequencies higher than 1 Hz. The observed viscoelastic behaviors are elucidated hereafter (Fig. 2d, e). SWCNTs/Cu-complex with a SWCNT concentration of 0.1 wt% exhibits behaviors ranging from a viscoelastic liquid to viscoelastic soft solid, slowly flowing down the glass wall. Dispersions with higher SWCNT concentrations maintain their shapes as viscoelastic soft solids and gels. Moreover, the stabilities of the dispersions are retained without severe separation of liquid and solid phases even after storage for 2 months, implying stable colloidal suspension states (Supplementary Fig. S1). Dispersion stabilities of SWCNTs in the presence of Cu-complex are verified by optical microscopy and zeta potential measurements (Supplementary Fig. S2)37,38. All dispersions demonstrate shear thinning behaviors with the presumption of parallelly aligned morphologies arising from the SWCNT SA when they flow due to driving forces (e.g., gravity). If the driving forces are not applied in a certain direction, the dispersions remain in the equilibrium states as G′ is dominant.

These viscoelastic properties are consistent with the liquid crystalline natures of the dispersions obtained by the de-bundling of SWCNTs. Polarized optical microscopy (POM) images show shear-aligned SWCNTs with Cu-complex solution (Figs. 3a–c and S3), confirming the SWCNT SA. Upon inducing a circularly directed shear force, the dispersion exhibits a continuous nematic liquid crystalline morphology, which is verified at the cross-polarized angles of 10° (Fig. 3a), 0° (Fig. 3b), and −10° (Fig. 3c). The nematic liquid crystalline phase reveals the division of large bundles of SWCNTs into smaller bundles, which are surrounded by Cu-complex in the colloidal system (Fig. 3d), leading to the SA morphology. Thus, SWCNTs intercalated by Cu-complex can be aligned in the length direction under shear force, demonstrating shear thinning behaviors. Morphology of the SWCNT bundle was examined by high-resolution transmission electron microscopy (HR-TEM) after washing out the Cu-complex solution from the dispersions (Figs. 3d–f and S4), showing nanoscale bundle size.

Fig. 3: Descriptions of the dispersion state of SWCNTs.
figure 3

POM images of the dispersion with a SWCNT concentration of 1 wt% acquired at the cross-polarized angles of (a) 10, (b) 0, and (c) −10° (Inset scale bars: 50 µm). Schematics depicting (d) the de-bundling of SWCNTs in the Cu-complex solution and (e) left over SWCNTs after being washed out of the medium, in accordance with the (f) TEM image. g Main energy components affecting the colloidal system and counterbalancing the vdW forces of SWCNTs.

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Therefore, based on all these results, we speculate that the viscoelastic behaviors and liquid crystalline phases of the dispersions generate from both the attractive vdW force between SWCNTs and repulsive force based on charge transfer (Fig. 3g). This phenomenon may be attributed to the intermolecular charge transfer on SWCNTs owing to the Cu-complex solution doping. Once the SWCNT bundle faces the Cu-complex and residual ligand component after mixing, the doped charge (in this case, electron) is initially localized on the outer surface of the bundle. The doped charge can be instantly delocalized in the circumferential directions of SWCNTs by charge diffusion. Thus, neighboring SWCNTs repulse each other and can be appropriately dispersed in the medium even by a weak shear force when the Cu-complex and residual ligand component surround SWNCTs.

Charge transfer characteristics of sp2 carbon nanomaterials doped with the Cu-complex solution

We conducted Raman spectroscopy measurements to verify and specifically characterize the Cu-complex-induced doping effect on the SWCNTs and their dispersions (Fig. 4a–c). The G band originating from the sp2-hybridized carbons of SWCNTs exhibits two prominent peaks (Fig. 4b). The G+ peak at approximately 1580 cm−1 arises from the C–C vibrations parallel to the nanotube axis, whereas the G− peak at approximately 1560 cm−1 originates from the vibrations in the perpendicular direction39. In the case of SWCNTs/Cu-complex, where SWCNTs are surrounded with the Cu-complex solution, perpendicular vibration is reduced owing to the coverage of SWCNTs by the Cu-complex solution, leading to a weaker G− peak with a slight downshift. Furthermore, the full-width-half-maximum (FWHM) of the G+ peak is considerably lower for SWCNTs/Cu-complex when compared with that for pristine SWCNTs. These results are in good agreement with those reported in previous studies, where FWHM of the isolated nanotubes was smaller than that of the bundled nanotubes40. Doping of SWCNTs was confirmed by the position and intensity of G+ and G′ peaks, respectively. G+ peak of SWCNTs/Cu-complex shifted to lower wavenumbers, and the intensity ratio of the G+ and G′ peaks was significantly lower relative to that for pristine SWCNTs. This is a general characteristic of n-doped nanotubes41,42. Phonon softening by charge transfer from the Cu-complex solution to SWCNTs results in downshift and intensity reduction. In addition to the G and G′ band, intensity of the radial breathing mode (RBM) is also decreased (Supplementary Fig. S5) due to modification of the band gap in the desity of state of the nanotubes upon doping33,43,44,. Work function measurements using ultraviolet photoelectron spectroscopy (UPS) (Supplementary Fig. S6) provided additional evidence for charge transfer. The measured work function of the pristine SWCNTs was 4.43 eV, which decreased to 4.35 eV for SWCNTs/Cu-complex. The calculated shift of 0.08 eV originated from the Fermi level upshift of the SWCNTs due to the n-type doping-induced charge transfer. To further verify the function of the Cu-complex solution as an n-type dopant for sp2 carbons, the Raman spectra of pristine graphene and graphene doped with the Cu-complex solution (Gr/Cu-complex) were obtained (Fig. 4a). Similar to the case of SWCNTs, the intensity ratio of G and G′ peaks for the graphene sample decreased from 2.2 to 1.6 after Cu-complex solution doping. This result is in accordance with the previously reported result that the G′ intensity of doped graphene is significantly lower than that of undoped graphene45. Moreover, the positions of both G and G′ peaks shifted to lower wavenumbers46, confirming that the Cu-complex solution is an effective n-type dopant for sp2 carbon nanomaterials.

Fig. 4: Raman spectra and electrical characteristics for evaluating the doping effect of the Cu-complex solution on the sp2 carbons.
figure 4

a Full Raman spectra and Raman spectra of the (b) G and (c) G′ band regions. d Transfer characteristics of doped and undoped graphene FETs and (e) corresponding electron mobilities relative to electron concentration.

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Furthermore, we explored the n-doping effect induced by the Cu-complex solution using doped and undoped graphene field-effect transistors (FETs). Transfer curves (drain–source current (Ids) versus gate–source voltage (Vg)) were acquired over a Vg range of –40–40 V at a constant drain–source voltage (Vds) of 0.01 V. The characteristic V-shaped ambipolar behavior representing both electron and hole conductions is observed for both devices (Fig. 4d). Charge–neutral point of the pristine graphene, namely, the Dirac voltage (VDirac), was evaluated as 6 V, demonstrating p-type transport behavior, which was attributed to hole doping of graphene by the SiO2 substrate47. Upon coating the graphene FET with Cu-complex, VDirac considerably shifted from 6 to –7 V, implying electron doping of graphene by the Cu-complex solution. These results are consistent with those achieved by Raman spectroscopy. The Fermi level was initially located under the valence band and then upshifted toward the conduction band for n-type graphene (Inset of Fig. 4d). Hole and electron mobilities were calculated from the transfer curves as follows:

$$\mu =\frac{1}{Cg}\frac{L}{W}\left(\frac{d{I}_{{ds}}}{d{V}_{g}}\right)\frac{1}{{V}_{{ds}}}$$
(1)

where Cg denotes the gate capacitance, L represents the channel length, and W is the channel width48. Hole mobility (\(\mu\)h) decreased from 1366 cm2 V–1 s–1 (pristine graphene) to 713 cm2 V–1 s–1 (Gr/Cu-complex). However, the Cu-complex solution doping in graphene only negligibly reduced the electron mobility (\(\mu\)e). Instead, the electron mobility of the Gr/Cu-complex sample was slightly higher than that of the pristine graphene at relatively low electron concentrations (Fig. 4e). This result was in reasonable agreement with the previously reported result for intermolecular n-type doping of graphene with a charged layer49. Moreover, the electron mobilities of the doped and undoped graphene were significantly different at high electron concentrations. When the dielectric materials were located on the top surface of the graphene FET, the electron mobility was considerably high because charged impurity-induced long-range Coulomb scattering was reduced by the dielectric screening effect50,51. The reported dielectric constant of the Cu formate-based compound, which is the same ligand used in this study, is as high as 3852. Thus, the high electron mobility of the Gr/Cu-complex FET is ascribed to the reduction in the interaction of electrons with charged materials, suggesting that the sp2 carbons can be effectively doped with the Cu-complex solution (n-type dopant).

Fabrication of nanostructured SWCNTs/CuNP electrodes

Consequently, the dispersions produced herein could be printed or coated by screen printing, direct writing extrusion printing, blade coating, and dip coating (Figs. 5a and S7) because of molecular interactions, which were verified by rheological and electrophysical properties of these dispersions. Long-term environmental stability of the casted film was also confirmed (Supplementary Fig. S8). After being deposited on glass or polymeric substrates, SWCNTs/Cu-complex was thermally treated for the thermal reduction of Cu-complex to CuNP that afforded the nanostructured hybrid electrodes SWCNTs/CuNP (Fig. 5b). In this study, Cu-complex acted as an n-type dopant for dispersing SWCNTs and also formed CuNP, with a high electrical performance, after reduction. X-ray diffraction (XRD) patterns of pristine SWCNTs, Cu-complex, SWCNTs/Cu-complex, and nanostructured SWCNTs/CuNP obtained after reduction are shown in Fig. 5c. XRD pattern of pristine SWCNTs exhibits a distinct graphitic peak (Supplementary Fig. S9) at 26.5°53; in contrast, the liquid-state Cu-complex is amorphous, and its XRD pattern demonstrates no specific peak. Furthermore, SWCNTs/Cu-complex exhibits an amorphous state, and its XRD pattern does not demonstrate the graphitic peak of SWCNTs. XRD pattern of nanostructured SWCNTs/CuNP exhibits diffraction peaks at 43.28, 50.40, and 74.81° corresponding to the (111), (200), and (220) planes of the Cu cubic lattice with small peaks of SWCNTs. XRD profiles reveal excellent dispersion of SWCNTs in the Cu-complex solution without substantial SWCNT rebundling and a definite production of CuNP. Morphologies of nanostructured SWCNTs/CuNP with different SWCNT contents of 1.2, 5.6, 10.5, and 19.0 wt% (5.3, 22.0, 36.0, and 53.0 vol%, respectively) were analyzed (Fig. 5d). All SWCNTs/CuNP samples demonstrate nanoscale SWCNT networks. The higher the SWCNT content, the more dense the SWCNT network, as observed with a high SWCNT content:Cu content ratio of approximately 1:1 vol.%. Notably, the SWCNT content can be regulated to 53 vol.% in the nanoscale network of SWCNTs/CuNP for lightweight and flexible electrode applications.

Fig. 5: Fabrication of the nanostructured electrodes.
figure 5

a Printing/coating methods adopted in this study: Direct writing extrusion printing, screen printing, blade coating, and dip coating using the dispersions of SWCNTs/Cu-complex on a polymeric or glass substrate. b Thermal reduction in a quartz tube furnace after the deposition of the dispersions. c XRD patterns of the raw materials, dispersions, and nano-hybridized film. d SEM images of the nanostructured SWCNTs/CuNP electrodes (SWCNT contents of 1.2, 5.6, 10.5, and 19.0 wt%, and inset scale bars: 200 nm).

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To evaluate the electrical performances of the fabricated electrodes, light-emitting diode (LED) lighting tests were performed by connecting SWCNTs/CuNP at a certain applied voltage using a direct current (DC) power supply. The electrode was fabricated on a polyester fabric with a specific area of 5 × 5 cm2 (Fig. 6a) by screen printing, which is a promising method for large-scale and cost-effective patterned electrode manufacturing. The low- and high-magnification scanning electron microscopy (SEM) images displayed in Fig. 6b show well-cast SWCNTs/CuNP nanostructures. The electrode exhibited efficient lighting performance (Fig. 6c), indicating that the fabricated nanostructured SWCNTs/CuNP electrically connected even under severe bending or folding conditions. This electrical conducting behavior was confirmed for a curved electrode on a cylindrical glass substrate constructed by direct writing extrusion printing (Supplementary Fig. S10).

Fig. 6: Screen-printed electrode on polyester.
figure 6

a Image of the screen-printed electrode on polyester. b SEM images of the electrode (Inset scale bars: 500, 100, 50, and 2 µm). c Flexibility and LED lighting tests of the electrode.

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Joule heating performances

Electrothermal properties of the SWCNTs/CuNP electrode were investigated using SWCNTs/CuNP film heaters constructed on PI substrates. With a stepwise increase in voltage from 2 to 12 V with a step size of 2 V, the nanostructured SWCNTs/CuNP film (Supplementary Fig. S11) demonstrated a fast temperature increase, implying that the electrode temperature can be easily adjusted by controlling the voltage (Fig. 7a). The electrode temperature reached over 110 °C in 10 s and remained in its equilibrium state (Fig. 7b and Supplementary Fig. S12). Moreover, bending cycle test of the electrode was conducted to verify the flexibility of the electrode at a bending radius of 5 R. The SWCNTs/CuNP film retained electrical properties and Joule heating performance after 2000 bending cycles (Fig. 7c, d). Infrared (IR) camara image of the electrode after 2000 bending cycles is shown in Fig. 7e. Uniform temperature distribution noticed over the entire electrode surface indicated a negligible change in electrode resistance, revealing effective flexibility of the SWCNT/CuNP electrode. Adhesion test result indicates that the hybrid electrode shows a good mechanical property with an adhesion level of almost 5B (Supplementary Fig. S13).

Fig. 7: Joule heating performances of the electrodes separately fabricated on a PI film and glass vial.
figure 7

a Stepwise increment of the temperature profile of the electrode fabricated on a PI film with an increase in the applied voltage from 2 to 12 V. b Temperature profile for over 9000 s at an applied voltage of 10 V. c Current measurement of the electrodes along with bending cycles. d Comparison of the temperature profiles before and after bending. e IR camera image of the electrode in the bending state. f Photographs and IR camera images of the electrodes prepared on the vial by dip coating. Temperature gradient through voltage-on time at an applied voltage of 15 V.

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Additionally, water in the glass vial was boiled using the SWCNTs/CuNP-based heater directly fabricated outside the vial by dip coating (Fig. 8f). IR camera images depict the temperature gradient along with the operating time, and water was boiled with a specific heat of 1 cal·g−1·K−1 in 300 s. Joule heating performances were analyzed at an applied voltage of 15 V, implying that the SWCNTs/CuNP-based heater was useful even when low-power sources, such as a portable battery and supercapacitor, were used.

Fig. 8: EMI shielding properties of the SWCNTs/CuNP thin films with different thicknesses.
figure 8

a EMI SEs versus frequency. b EMI SE and SSE, and c SSE/t with respect to film thickness. d Comparison of the SSE/t performance versus film thickness of the SWCNTs/CuNP nanostructured film with those of previously reported materials. e Description of the EMI shielding performance of the material reported in this study (inset scale bar of the SEM image: 200 nm). f EMI SEs before and after the bending test (2000 cycles at a bending radius of 20 R).

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EMI shielding performance

SWCNTs/CuNP electrode can be efficiently applied as an EMI-shielding film due to its hybrid network structure comprising the combination of zero-dimensional CuNP and one-dimensional SWCNTs, which exhibit high electrical conductivities. We measured the EMI shielding effectivenesses (SEs) of nanostructured SWCNTs/CuNP films in the frequency range of 1.5–10 GHz including the partial L-, full S-, C-, and partial X-bands divided by a dashed line. This range belongs to most of the fifth-generation (5 G) wireless communication frequency region54,55 and sixth-generation (6 G) for the prime candidate region of next-generation communication. EMI SEs of the SWCNTs/CuNP films with various thicknesses are shown in Fig. 8a. As expected, EMI SE enhanced with an increase in the film thickness. The results measured with the experimental EMI SE equation which is defined as:

$${{{{{\rm{EMI\; SE}}}}}}\, \left({{{{{\rm{dB}}}}}}\right)=10 \, {{{{\mathrm{log}}}}}\, ({{{{{{\rm{P}}}}}}}_{{{{{{\rm{i}}}}}}}/{{{{{{\rm{P}}}}}}}_{{{{{{\rm{t}}}}}}})$$
(2)

where Pi and Pt represent the incoming and transmitted powers, respectively (Fig. 8a).

To achieve a light and thin film with a high EMI shielding performance, we employed the most concentrated SWCNTs/Cu-complex (SWCNT concentration of 2 wt% in this study). The corresponding film comprised 19.1 wt% SWCNTs (53 vol%) and 80.9 wt% CuNP (47 vol%), resulting in a measured film density of 2.968–3.719 g·cm−3. The EMI SE values ranged from 29.5 dB (over 99% shielding) at a film thickness of 3 µm to 71.7 dB at a film thickness of 32 µm (over 99.99999% shielding), and these values were comparable to those of other materials (Table S1). As EMI SE definitely depends on the thicknesses of the materials, specifically in the cases of nanostructured materials, specific EMI SE (SSE) and thickness averaged specific EMI SE (SSE/t) were evaluated for more rational comparison. EMI SE and SSE increased with an increase in film thickness (Fig. 8b). Nevertheless, SSE/t of the thinnest film (3 µm) is the highest (26,440.8 dB·cm2·g−1) (Fig. 8c). Note that considering its SSE/t, SWCNTs/CuNP manufactured by printing/coating by a simple fabrication method is a competitive material to several previously reported materials (Fig. 8d and Table S1). High SSE/t of the abovementioned film is attributed to multiple reflections from the hybrid network structure of SWCNTs/CuNP (Fig. 8e). Furthermore, the SWCNTs/CuNP film demonstrated almost same EMI SE after the bending cycle test (2000 cycles at a bending radius of 20 R) (Fig. 8f). Consequently, the nanostructured SWCNTs/CuNP film exhibits potential for application in flexible and wearable EMI shielding electrodes.

Discussion

In this study, we developed concentrated SWCNT dispersions and facilely fabricated SWCNTs/CuNP electrodes by dispersing SWNCTs in a Cu-complex solution. The SWCNT dispersions surrounded by Cu-complex via charge transfer demonstrated SA morphologies with pseudoplastic flow behavior. Such behavior with liquid crystalline natures allows printing or coating of these dispersions for printed electronics. Accordingly, the corresponding nanostructured electrode exhibited multifunctional performances: electrothermal properties, EMI shielding, and adequate flexibility. Fast temperature rising rate and stable Joule heating performances with an increase in temperature to over 110 °C at an applied voltage of less than 15 V were observed. Moreover, a lightweight (density = 3.719 g·cm−3) thin film (thickness: 3 µm) with an EMI SSE/t of 26440.8 dB·cm2·g−1 was realized. In conclusion, the proposed metal-complex-hybridized SWCNT dispersions at the molecular level can successfully undergo wet processing. Moreover, we believe that they are a breakthrough toward versatile electrode utilization in wearable electronics and 5 G/6 G communication applications as well as in semiconductor, energy storage, and catalyst research fields, where CNT-transition metal hybrid materials are in huge demand.

Methods

Materials and preparation of SWCNTs/Cu-complex dispersions

SWCNTs (lengths: >5 µm, diameters: <2 nm, and purities >93%) were purchased from OCSiAl. Copper (II) formate tetrahydrate and 2-ethyl-1-hexylamine were bought from Merck and Thermo Fisher Scientific, respectively, and used without further purification. To synthesize Cu-complex, 2-ethyl-1-hexylamine (4 equivalent) was added to the copper (II) formate tetrahydrate (1 equivalent) powder followed by stirring using a previously reported synthetic method with some modifications56. After the powder was completely dissolved, 0.1, 0.5, 1, and 2 wt% SWCNTs were separately placed in the Cu-complex solution followed by stirring with a magnetic stirring bar and an overhead stirrer to achieve SWCNT dispersions.

Characterization of SWCNTs and SWCNTs/Cu-complex dispersions

Viscoelastic rheological properties were evaluated by a rheometer (MCR 102, Anton Paar) using RheoCompass. Dispersion states of SWNCTs were examined by OM (ECLIPSE LV100, Nikon) using polarizer accessories and TEM (Titan G2 60-300, FEI company). Zeta potential measurements were performed using Litesizer 500 (Anton Paar). To obtain the Raman spectra, Raman spectroscopy was conducted using LabRAM HR Evolution (HORIBA) with the following equipment specifications: a 532 nm laser, 1800 grooves (lines)/mm grating, and 0.6 cm−1 spectral resolution. A home-built Raman spectroscopy system with a 514.5 nm Ar ion laser and power of 0.5 mW was complementarily used. Raman signals were spectrally resolved using an optical spectrometer featuring a 2400 grooves/mm grating (Horiba iHR550) and acquired using a liquid N2-cooled charge-coupled device (CCD) detector (Horiba Symphony II CCD).

Device fabrication and electrical characterization

Monolayer graphene flake was exfoliated on a SiO2/Si substrate. A graphene FET was constructed by conventional e-beam lithography (JSM-IT200, JEOL). Cr/Au (5 nm/50 nm) electrode was deposited using a thermal evaporator. Electrical characterization was performed using Keithley 4200A-SCS (Tektronix, USA) in a vacuum probe station (~10−6 Torr). For work function calculations, UPS spectra were obtained using a UPS system (Nexsa XPS system, Thermo Fisher Scientific, UK) with a 21.2 eV He I UV source. XRD data were acquired using X’Pert PRO MPD (PANalytical).

Fabrication and characterization of the printed SWCNTs/CuNP electrodes

A flexible SWCNTs/CuNP electrode (with a SWCNTs/Cu-complex dispersion concentration of 1 wt%) on polyester was screen-printed. The process was conducted 10 times. Direct writing extrusion printing was performed using a 12 mL syringe with a needle on the glass substrates. For the heater electrode, blade coating and dip coating were adopted on a PI film and vial, respectively. Dip coating was implemented by putting the vial in the dispersion to coat the outer surface of the vial, and then, the vial was taken out. Thin-film electrodes for EMI shielding were also blade-coated on the PI film. All blade coatings were conducted using thickness-adjustable applicator. Printed/coated materials achieved using the dispersions were placed in a quartz furnace. The furnace was heated to 150 °C with a ramping rate of 3.75 °C·min−1 under an Ar atmosphere. DC power supply (OPS-303, ODA TECHNOLOGIES) was used to perform LED lighting tests and evaluate the Joule heating performances of the fabricated SWCNTs/CuNP electrodes. Temperature profiles were measured using an IR camera (RSE30, FLUKE) with a software (Smartview IR) produced by FLUKE. IR images were also acquired by the IR camera. Bending tests were conducted using a source meter (2636B, Keithley) equipped with current measurement bending equipment. EMI characterization was performed using a network analyzer (ZVA 40, Rohde & Schwarz) with the sample holding zig EM-2108. Structural morphologies were determined using SEM (S-4800, Hitachi).