Fabrication of Transparent and Conductive SWCNT/SiO2 Composite Thin-Film by Photo-Irradiation of Molecular Precursor Films

A single-walled carbon nanotube (SWCNT)-silica composite thin film on a quartz glass was formed by ultraviolet irradiation (20–40 °C) onto a spin-coated precursor film. With 7.4 mass% SWCNTs, the electrical resistivity reached 7.7 × 10−3 Ω·cm after UV-irradiation. The transmittance was >80% at 178–2600 nm, and 79%–73% at 220–352 nm. Heat treatment increased the transparency and pencil hardness, without affecting the low electrical resistivity. Raman spectroscopy and microscopic analyses revealed the excellent film morphology with good SWCNT dispersal. The low refractive index (1.49) and haze value (<1.5%) are invaluable for transparent windows for novel optoelectronic devices.


Introduction
Transparent conductive oxide (TCO) films are important for various optoelectronic devices, such as transparent electrodes for light-emitting diodes and solar cells. Transparent thin films of indium tin oxide (ITO) and fluorine tin oxide (FTO) on glass substrates are the most widely used for these purposes [1]. These common TCO thin films provide an electrical resistivity of 10 −3 Ω·cm and a high transmittance of over 80% in the visible-light region. However, these films absorb deep ultraviolet (DUV) and vacuum ultraviolet (VUV) light because of their bandgap energies. Therefore, a new type of transparent conductive film that is adaptable over a wide spectral range, spanning VUV to near-infrared (IR) light, would spur the development of novel optoelectronic devices. The refractive index of ITO films is approximately 1.9 in the visible-light region [2], which reduces the transmittance of the electrode owing to reflections at the interface between the ITO and the glass substrate, for which the typical value is~1.5. Thus, a transparent conductive film with a low refractive index is required.
Carbon nanotubes (CNTs) are conductive materials with excellent strength, electron conductivity, and flexibility [3]. CNTs are generally used to fabricate composite materials comprising rubber, polymers, and so on [4,5]. The CNT networks in the composites are intertwined, and electrically conductive paths are formed by contact between CNTs [6], where the conductivity depends on the diameter of the CNTs and the low resistance associated with the networks of CNTs with small diameters [7]. Therefore, SWCNTs with

Preparation of SWCNT/SiO 2 Composite Precursor Solution
An SiO 2 precursor solution (S Silica ) was prepared according to our previous study [13]. A mixed solution of TEOS (1.3 g) and oxalic acid (1.1 g) dissolved in ethanol (10 g) was refluxed for 1 h (Si 4+ concentration of 0.5 mmol g −1 ).
Ethanol was added to dilute a solution (S CNT ) of the ethanol dispersion of SWCNTs (eDIPS-INK) to adjust the concentration to 0.1 mass%. S Silica and S CNT at room temperature were mixed to prepare seven different precursor solutions (C/Si molar ratios of 0.05-0.5) for fabricating the composite thin film. Each mixed solution was mechanically stirred for 1 h. The obtained solutions are denoted as Sx, where "x" indicates an alphabetical order from "a" to "g" that corresponds to the C/Si molar ratios of 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, respectively (see Table 1).

Fabrication of SWCNT-SiO 2 Composite Thin Films by UV-Light Irradiation, and Post-Annealing Treatment
Each Sx (300 µL) solution was dropped onto a quartz glass substrate and was subsequently spin-coated in double-step mode (1st: 500 rpm for 5 s and 2nd: 2000 rpm for 30 s). The precursor films (Px) were obtained after preheating the spin-coated films in a drying oven at 70 • C for 10 min. A germicidal lamp was used to irradiate each Px set with a UV light (4 mW cm −2 at 254 nm) for 64 h in a clean bench. A digital thermocouple and hygrometer were respectively used to monitor the surface temperature of Px and the relative humidity of the clean bench during UV irradiation. The UV-irradiated films on the quartz glass substrate are denoted as Fx, corresponding to the applied Sx in the spin-coating process. The obtained thin film Ff with a C/Si ratio of 0.4 was heat treated at 500 • C for 1 h in an electric furnace to yield Ff-HT.
Additionally, the abovementioned procedure was used to obtain a CNT thin film (F CNT ), following spin coating of the S CNT and preheating. The heat-treated F CNT -HT film was also obtained following heat treatment of F CNT at 500 • C for 1 h in an identical furnace. A UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan) in double-beam mode with air as a reference was used to measure the transmittance spectra of Ff and Ff-HT in the 220-2600 nm range. A far-ultraviolet monochromator in single-beam mode (based on KV-200, Bunkohkeiki Co., Ltd., Tokyo, Japan), which was purged with nitrogen gas, was used for measurement in the wavelength range of 130-300 nm; the purging gas was used as a reference.

Electrical and Optical Characterization of Thin Films
A MARY-102 ellipsometer was used to measure the refractive indices of Ff and Ff-HT (Five Lab, Kanagawa, Japan) at a wavelength of 632.8 nm and an incident angle of 70.07 • . The refractive index was measured at 14 points on the films, and SiO 2 (refractive index of 1.52) was used as the reference.
A haze meter COH7700 (Nippon Denshoku Industries Co., Ltd., Tokyo, Japan) was used to measure the haze of Ff and Ff-HT. The haze was calculated as the average value from five points on the films, excluding the highest and lowest values.

Structural Characterization of Thin Films
A Raman microspectrometer (LaBRAM HR800, Horiba Ltd., Kyoto, Japan) with a charge-coupled device detector in back-scattering geometry was used to measure the Raman spectra of five films: Pf, Ff, Ff-HT, F CNT , and F CNT -HT. A Nd-YAG laser (532 nm) with a spot diameter of 1 µm was used as the excitation source at an intensity of 13 mW. The exposure time was changed to determine the optimum conditions for peak detection. The spectra of Pf, Ff, and Ff-HT in the range of 250-2100 cm −1 were obtained with an exposure time of 60 s. The spectra of F CNT and F CNT -HT were obtained with an exposure time of 30 s. The spectra of Pf, Ff, and Ff-HT in the range of 100-250 cm −1 were obtained with an exposure time of 360 s, while the spectra of F CNT and F CNT -HT in the identical range were obtained with an exposure time of 180 s.
A Raman peak of the Si substrate at 520.64 cm −1 was used for wavelength calibration. A nonlinear least-squares method, which involved a Lorentz function, was used to deconvolute the Raman peaks in the range of 1200-1700 cm −1 . OriginPro2018b (OriginLab Corporation, Northampton, MA, USA) was used for calculations, and the peak fitting converged with a χ 2 tolerance value of 1 × 10 −9 .

Surface Morphology, Film Thickness, and Pencil Hardness of Thin Films
Field-emission scanning electron microscopy (FE-SEM) with a JSM-6701F microscope (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 5 kV was used to determine the surface morphologies of Pf, Ff, and F CNT . The average thickness of the SWCNT bundles in Pf and Ff was calculated from five randomly selected bundles.
Atomic force microscopy (AFM) equipment (OLS4500, OLYMPUS, Tokyo, Japan) was used to obtain three-dimensional (3D) AFM images of Ff, Ff-HT, F CNT , and F CNT -HT by scanning a 2 × 2 µm 2 area, and the surface roughness was estimated.
Scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) in JEM-2100F (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 200 kV were used to analyze the cross-sectional morphology and elemental distribution of Ff and Ff-HT. The film surface was covered with a carbon film before the center of the sample was cut with a focused ion beam (FIB).
A stylus profilometer (DEKTAK-3, Sloan, Santa Barbara, CA, USA) was used to measure the film thicknesses of Pf, Fx, and Ff-HT. A portion of each precursor film was removed during sample preparation, using ethanol to expose the substrate. The differences at each level between the substrate and the resultant film were measured at five positions for each sample. The film thickness was calculated as the average value, excluding the highest and lowest values. In the case of Pf, the measurement was performed two days after the film formed.
A pencil scratch test (the Japanese Industrial Standard (JIS) K5400) was used to evaluate the pencil hardness of Ff and Ff-HT. The test involved an MJ-PHT pencil hardness meter (Sato Shouji Inc., Kanagawa, Japan) with a 0.75 kg load. Pencils (UNI, Mitsubishi Pencil Co., Ltd., Tokyo, Japan) with various hardness values were standardized in the hardening order from 6B to 9H and were used to scratch the film.

Results and Discussion
Although an aqueous dispersion of MWCNTs was used in our previous study [13], the precursor solutions (Sx) for the SWCNT-silica composite thin film can be prepared by mixing the Si 4+ complex of oxalic acid in ethanol and an ethanolic dispersion of SWCNTs. The coatability of Sx on the quartz glass substrate was excellent and was independent of the SWCNT concentration. The surface temperature of each precursor film during exposure to UV was 20-40 • C, and the humidity in the clean bench was 20-50%. The various properties of the composite film Ff with a C/Si ratio of 0.4 are examined in detail in the following sections because this thin film exhibited the lowest electrical resistivity. Table 1 lists the electrical resistivities of the composite thin films, Fa-Fg (thickness: 60-120 nm, Supplementary Materials Figure S1). The volume concentration of SWCNTs was calculated on the assumption that the SWCNTs and SiO 2 were close packed; therefore, respective densities of 1.3 and 2.2 g cm −3 were used [14,15]. Generally, the electrical resistivity of the composite decreased drastically near the percolation threshold, resulting in electrical resistivity at 3.3 vol.%, and reached the lowest value at 12 vol.% (Table 1 and Supplementary Materials Figure S2). The electrical resistivity of Ff on the order of 10 −3 Ω·cm is comparable to that of the ordinary ITO and FTO glass substrates. This low resistivity is associated with the formation of a conductive network unique to CNTs [10,16]. Contrastingly, the lowest resistivity of the SWCNT-silica film was 6.6 Ω·cm; this film was formed by the sol-gel method, in which the maximum CNT concentration was 4 mass% [17]. This concentration corresponds to films Fb and Fc, which had a proportionally low resistivity. In the case of the present MPM, the resistivity of Ff was reduced by as much as three orders of magnitude because the CNT concentration could be further increased. Importantly, the Si 4+ complex did not interfere with the dispersion of the SWCNTs at high concentrations. Figure 1 shows the transmittance spectra of Ff, Ff-HT, and the quartz glass substrate in the range of 130-2600 nm. The transmittance of Ff was higher than 80% from 178 nm in the VUV region to the near IR region, and higher than 73% in the range of 220-352 nm. The corresponding value of Ff-HT was 1-3% higher than that of Ff. Because the quartz glass solely absorbed VUV light, it had the shortest wavelength absorption, and the transmittance consequently decreased because of the substrate thickness. The weak peak at 273 nm (4.5 eV) is due to the π-plasmon absorption of the CNTs, related to the polarization dependence and the optical properties of graphite [18]. The refractive indices of Ff and Ff-HT were 1.49 and 1.48, respectively, and were comparable to 1.52 for SiO2. The haze values of Ff and Ff-HT were 1.5% and 1.0%, respectively, which indicate the flatness of the film surface (vide supra). Notably, current thin films with a low refractive index and haze are much more transparent than ordinary ITO and FTO. The refractive indices of Ff and Ff-HT were 1.49 and 1.48, respectively, and were comparable to 1.52 for SiO 2 . The haze values of Ff and Ff-HT were 1.5% and 1.0%, respectively, which indicate the flatness of the film surface (vide supra). Notably, current thin films with a low refractive index and haze are much more transparent than ordinary ITO and FTO. Figure 2 shows the Raman spectra of Ff, Ff-HT, Pf, F CNT , and F CNT -HT. The three peaks at 1340, 1570, and 1590 cm −1 in all films can be assigned to the D, G − , and G + bands of CNT [19,20]. These three peaks were deconvoluted to evaluate the structural changes of the SWCNTs (Supplementary Materials Figure S3).  Ff-HT (-), and quartz glass substrate (-). Spectra were measured in two separate wavelength regions by using different spectrometers and conditions: first region shorter than 300 nm by using KV-200 under N2 gas flow and second longer than 300 nm by using UV-3600 in air. The vertical dash-dotted auxiliary line indicates the wavelength at which the spectrometer was changed.

Structural Characterization of Ff, Ff-HT, Pf, F CNT , and F CNT -HT
The refractive indices of Ff and Ff-HT were 1.49 and 1.48, respectively, and were comparable to 1.52 for SiO2. The haze values of Ff and Ff-HT were 1.5% and 1.0%, respectively, which indicate the flatness of the film surface (vide supra). Notably, current thin films with a low refractive index and haze are much more transparent than ordinary ITO and FTO. Figure 2 shows the Raman spectra of Ff, Ff-HT, Pf, FCNT, and FCNT-HT. The three peaks at 1340, 1570, and 1590 cm −1 in all films can be assigned to the D, G − , and G + bands of CNT [19,20]. These three peaks were deconvoluted to evaluate the structural changes of the SWCNTs (Supplementary Materials Figure S3).  The (G − + G + )/D ratios of F CNT and Pf were 52 and 42, respectively, suggesting that upon mixing the SWCNTs with the Si 4+ complex of oxalic acid in ethanol, the number of defect sites in graphite preferentially increased. The (G − + G + )/D ratio for Ff is 7.1 and 1/6 of that for Pf, indicating a significant increase in the number of defect sites upon UV irradiation. Thus, UV irradiation of Pf induced structural changes of the SWCNTs. Despite these significant structural changes, this method is still advantageous for forming highly conductive composites at room temperature.

Structural Characterization of Ff, Ff-HT, Pf, FCNT, and FCNT-HT
The (G − + G + )/D ratios of F CNT -HT and Ff-HT were respectively 26 and 15, which are respectively half and twice the ratios before heat treatment. Therefore, the number of defect sites in the SWCNTs alone increased than for those embedded in the silica matrix, suggesting that the aforementioned increase in the number of defect sites after UV irradiation might be temporary and that the original graphite ones recovered after heat treatment. Notably, the SiO 2 matrix plays an important role in preventing degradation of the graphite sites of the SWCNTs during heat treatment.

Changes in the Film Thickness and Hardness for Pf after UV Irradiation and for Ff after Heat Treatment
The film thicknesses of Pf, Ff, and Ff-HT were 110, 60, and 70 nm, respectively. Because the as-coated precursor film was too soft, the thickness measurement for Pf was performed after keeping the film in the dark for 48 h at 25 • C; nevertheless, the pencil hardness of Pf was less than 6B. Comparatively, the pencil hardness of Ff and Ff-HT was 4H and 9H, respectively. Therefore, UV irradiation of Pf at room temperature and heat treatment of Ff were both useful for forming robust composite thin films due to the stepwise removal of organic residues in the original precursor film and the UV-irradiated composite thin film, respectively. The film thickness of Ff-HT may increase when oxygen atoms are embedded into the Si-O dangling bonds, as observed for an MWCNT composite that we previously prepared [13]. Figure 3 shows the FE-SEM images of Ff, Pf, and F CNT . The SWCNT bundles in Ff and Pf (Figure 3a,b) are randomly arrayed and the intertwined networks of SWCNT bundles have average diameters of~16 nm. Comparatively, F CNT , which was solidified with its dispersant, has many pinholes on its surface (Figure 3c).

Changes in the Film Thickness and Hardness for Pf after UV Irradiation and for Ff after Heat Treatment
The film thicknesses of Pf, Ff, and Ff-HT were 110, 60, and 70 nm, respectively. Because the as-coated precursor film was too soft, the thickness measurement for Pf was performed after keeping the film in the dark for 48 h at 25 °C; nevertheless, the pencil hardness of Pf was less than 6B. Comparatively, the pencil hardness of Ff and Ff-HT was 4H and 9H, respectively. Therefore, UV irradiation of Pf at room temperature and heat treatment of Ff were both useful for forming robust composite thin films due to the stepwise removal of organic residues in the original precursor film and the UV-irradiated composite thin film, respectively. The film thickness of Ff-HT may increase when oxygen atoms are embedded into the Si-O dangling bonds, as observed for an MWCNT composite that we previously prepared [13]. Figure 3 shows the FE-SEM images of Ff, Pf, and FCNT. The SWCNT bundles in Ff and Pf (Figure 3a,b) are randomly arrayed and the intertwined networks of SWCNT bundles have average diameters of ~16 nm. Comparatively, FCNT, which was solidified with its dispersant, has many pinholes on its surface (Figure 3c).  One bundle contains up to 250 molecules, assuming that one SWCNT molecule is 1 nm in diameter and that adjacent molecules connect directly to form a bundle with a diameter of 16 nm. These bundles with high aspect ratios are arranged horizontally toward the surface and are intertwined. Therefore, it is acceptable that the SWCNT molecules that were dispersed in finer units cannot be observed with FE-SEM. As the Si 4+ complex photodegraded, the molecular precursor enhanced the contact between the SWCNTs during film formation, resulting in densification of the SiO 2 matrix and a low electrical resistivity. In fact, the film thickness of Ff decreased to almost half that of the precursor film, Pf. Comparing the distribution of the SWCNT bundles in Ff and Pf, it is assumed that densification of the matrix occurred during UV irradiation without aggregation of the SWCNT bundles. From the 3D-AFM images, the surface roughness of Ff and Ff-HT was 3 nm, which was smaller than 7 and 9 nm for F CNT and F CNT -HT, respectively (Supplementary Materials Figure S4). Figure 4 shows the cross-sectional STEM images and corresponding EDS elemental maps of Ff and Ff-HT. The interface and surfaces were smooth in both cases; there were no cracks or pinholes in the entire film. Importantly, there was little aggregation of the SWCNTs, resulting sufficient dispersion of the carbon atoms.

Morphology of Surface and Cross-Section of Thin Films
which was smaller than 7 and 9 nm for FCNT and FCNT-HT, respectively (Supplementary Materials Figure S4). Figure 4 shows the cross-sectional STEM images and corresponding EDS elemental maps of Ff and Ff-HT. The interface and surfaces were smooth in both cases; there were no cracks or pinholes in the entire film. Importantly, there was little aggregation of the SWCNTs, resulting sufficient dispersion of the carbon atoms.

Conclusions
SWCNT/SiO2 composite precursor solution was prepared by mixing a novel SiO2 precursor solution with an ethanol dispersion of SWCNTs. As a result, the precursor film involves the well dispersed SWCNTs in the matrix that consisted of the dried Si complex having oxalic acid as its ligand. By the weak UV-irradiation, the oxalic acid contained as a ligand could be effectively decomposed at room temperature and carbon atoms, originally belonging to oxalic acid, were removed from the film as carbon oxides, producing SiO2 matrix. The electrical resistivity of the composite thin film with an SWCNT concentration of 12 vol.% (7.4 mass%) was of the order of 10 -3 Ω cm, indicating high transparency over a wide wavelength range spanning the VUV to near IR regions. This resulted from the facile preparation of the precursor solutions via the molecular precursor method, which involved Si 4+ complex and unprecedentedly high SWCNT content. The current thin film is much more conductive than the MWCNT-SiO2 composite thin film. Heat treatment in air hardened the composite thin film without damaging the electrical resistivity and transmittance. The apparent percolation threshold of the SWCNT-SiO2 composite thin

Conclusions
SWCNT/SiO 2 composite precursor solution was prepared by mixing a novel SiO 2 precursor solution with an ethanol dispersion of SWCNTs. As a result, the precursor film involves the well dispersed SWCNTs in the matrix that consisted of the dried Si complex having oxalic acid as its ligand. By the weak UV-irradiation, the oxalic acid contained as a ligand could be effectively decomposed at room temperature and carbon atoms, originally belonging to oxalic acid, were removed from the film as carbon oxides, producing SiO 2 matrix. The electrical resistivity of the composite thin film with an SWCNT concentration of 12 vol.% (7.4 mass%) was of the order of 10 −3 Ω·cm, indicating high transparency over a wide wavelength range spanning the VUV to near IR regions. This resulted from the facile preparation of the precursor solutions via the molecular precursor method, which involved Si 4+ complex and unprecedentedly high SWCNT content. The current thin film is much more conductive than the MWCNT-SiO 2 composite thin film. Heat treatment in air hardened the composite thin film without damaging the electrical resistivity and transmittance. The apparent percolation threshold of the SWCNT-SiO 2 composite thin film was found at an SWCNT volumetric fraction of 3.3 vol%, and this value may be relevant to the formation of SWCNT bundles. This thin film not only has high transmittance in the visible light region, but also has a refractive index equivalent to that of quartz glass. Therefore, the thin film is promising as an alternative material for ITO because it reduces the transmittance of electrodes, associated with light reflection. The composite thin film is sufficiently transparent in the VUV and DUV regions, which will contribute to developing new optoelectronic devices and improving the performance of various devices, including LEDs and solar cells.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano11123404/s1, Figure S1: Relationship between the film thickness of the SWCNT/SiO 2 composite thin films (Fa-Fg) and their volumetric fraction of SWCNT in each film. A stylus profilometer was used to measure the film thicknesses of Fx. Figure S2: Relationship between the electrical resistivity of the SWCNT/SiO 2 composite thin films (Fb-Fg) and their volumetric fraction of SWCNT in each film. The electrical resistivity of Fb-Fg at 25 • C was determined using four-probe method and film thickness. The plot of Fa was removed from the figure because the value is >10 3 Ω·cm. Figure S3: Deconvoluted Raman peaks of the obtained thin films, (a) Ff, (b) Ff-HT, (c) Pf, (d), F CNT , and (e) F CNT -HT, along with each original spectrum in the range of 1200-1700 cm −1 . The thick line indicates the observed spectra, while three lines, (---), ( ••• ), and (-) indicate the theoretically fitted curves assignable the D, G − , and G + bands, respectively. All the calculated curves completely overlapped the corresponding observed curves. Figure S4