Enhanced Crystallinity and Electrical Properties of Reduced Graphene Oxide through Homoepitaxial Growth

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Kanda, T. Yamashita, S. Kurosu, F. Sakamoto, T. Hanajiri, Y. Nishina, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8773050/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 15 You are reading this latest preprint version Abstract Graphene oxide (GO), which can be synthesized inexpensively and in large quantities, is regarded as a promising starting material for electronic device applications due to its ability to recover electrical conductivity through reduction. However, oxygen-containing functional groups and structural defects introduced during the oxidation and reduction process significantly impair the electrical performance of reduced graphene oxide (rGO), posing a major challenge for practical implementation. In this study, we demonstrate that high-temperature thermal reduction in the presence of a carbonaceous gas not only facilitates the repair of vacancies in rGO thin films but also induces the homoepitaxial growth of two-dimensional graphene islands, guided by the underlying rGO template. By precisely controlling the growth driving force of the carbonaceous gas, epitaxial graphene islands were successfully formed, resulting in a significant improvement in electrical performance, with Hall mobilities reaching up to 365 cm²/V·s. These results suggest that the homoepitaxial growth of graphene islands plays a crucial role in enhancing both the crystallinity and electrical properties of rGO films. Physical sciences/Energy science and technology Physical sciences/Materials science Physical sciences/Nanoscience and technology Physical sciences/Physics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Graphene oxide (GO) is a scalable two-dimensional material that can be synthesized at low cost and with high yield 1 , 2 . Owing to its potential applications in a wide range of electronic devices such as sensors, transparent conductive films and supercapacitors, GO has attracted significant attention in recent years for both its fundamental properties and practical utility 3 – 7 . GO is typically produced by oxidizing graphite, a process that increases the interlayer spacing and weakens the van der Waals forces between the layers 2 . The most common method to chemically exfoliate into single-layer GO is the modified Hummers method 2 . The resulting GO can be deposited onto various device substrates to form large-area GO thin films. However, GO exhibits insulating behavior due to the presence of numerous oxygen-containing functional groups and structural defects. Therefore, a reduction process is essential to restore its electrical conductivity for use in electronic devices. Several reduction methods have been explored for GO, including chemical reduction 8 – 11 with agents such as hydrazine, photoreduction 12 – 14 under light irradiation, and thermal reduction conducted in inert atmospheres 15 , 16 such as argon. Among these approaches, thermal reduction has proven particularly effective in removing oxygen-containing functional groups—such as hydroxyl, carboxyl, and carbonyl moieties—thereby yielding reduced graphene oxide (rGO) with improved electrical conductivity 13 , 16 – 18 . Nevertheless, it has been reported that structural defects introduced during the oxidation process—such as vacancies, 5- and 7-membered rings, and dislocations—are not repaired by reduction treatments and remain embedded within the graphene lattice 19 , 20 . These structural defects disrupt the intrinsic hexagonal lattice, thereby degrading the electrical transport properties of the material. Plasma and high-temperature thermal reduction of GO have been extensively investigated under carbonaceous atmospheres such as methane 21 – 23 and ethanol 24 – 27 . Under these conditions, carbonaceous gases decompose into reactive carbon precursors through plasma irradiation or high-temperature thermal reduction, which react with dangling bonds at vacancies and thereby promote partial repair of structural defects 21 – 27 . Alcohol-based carbon sources such as ethanol differ from hydrocarbon gases in that they generate hydroxyl radicals during thermal decomposition. These hydroxyl radicals are known to suppress the formation of amorphous carbon in carbon aggregates by an etching effect 28 . Indeed, high-temperature thermal reduction using ethanol has been reported to dramatically enhance the crystallinity of rGO with increasing reduction temperature 24 – 26 , suggesting its effectiveness not only in removing oxygen-containing functional groups but also in repairing vacancies. However, under excessive conditions, ethanol decomposition proceeds beyond the optimal range, leading not only to carbon-precursor-mediated repair of vacancies but also to the deposition of carbon aggregates on the rGO surface 27 , 29 . Thus, thermal reduction in an ethanol atmosphere intrinsically encompasses a paradox—simultaneously enabling repair of vacancies that improves crystallinity while inducing amorphous carbon deposition as a side reaction—thereby making process control critically important. Nevertheless, the influence of surface morphological evolution on the crystallization behavior of rGO during high-temperature thermal reduction in an ethanol atmosphere remains insufficiently understood. In this study, we systematically varied the reduction temperature and ethanol supply to investigate the relationship between crystallinity and surface morphology in rGO thin films. Our results reveal that the formation of highly crystalline rGO film is primarily driven by a homoepitaxial growth mechanism, which plays a central role in enhancing the film’s crystallinity. Results High crystallinity of rGO by thermal reduction in an ethanol atmosphere Figure 1 (a) shows typical Raman spectra of rGO reduced at various temperatures in an ethanol atmosphere with a total pressure of 1.33 kPa (ethanol partial pressure: 1.33 Pa). The spectra exhibit peaks in the D band (~ 1350 cm⁻¹), G band (~ 1600 cm⁻¹), and 2D band (~ 2700 cm⁻¹) regions. Since the D peak originates from structural defects of graphene, the intensity ratio I(D)/I(G), calculated from the maximum intensities of the D and G peaks in the Raman spectrum, serves as an indicator of the crystallinity of graphene-based materials 24 , 26 , 29 , 30 . At 980°C, the intensity ratio I(D)/I(G) is 0.99, while at 1400 ℃, it decreases to 0.31, indicating improved crystallinity of rGO as a result of high-temperature thermal reduction in an ethanol atmosphere. Figure 1 (b) shows the reduction temperature on the horizontal axis and both the intensity ratio I(D)/I(G) and the full width at half maximum (FWHM) of the D peak ( \(\:{{\Gamma\:}}_{D}\) ) as the vertical axis. \(\:{{\Gamma\:}}_{D}\) is determined by fitting the D and G peaks with Lorentzian functions and extracting the full width at half maximum of the D peak (Supplementary Fig. S1 ). \(\:{{\Gamma\:}}_{D}\) narrows as the sp² carbon network expands, which reflects improved structural ordering 26 , 31 . Based on the variations in the intensity ratio I(D)/I(G) and \(\:{{\Gamma\:}}_{D}\) with respect to the reduction temperature, as shown in Fig. 1 (b), the temperature ranges were classified into distinct stages. The temperature range where both an increase in the intensity ratio I(D)/I(G) and a narrowing of \(\:{{\Gamma\:}}_{D}\) are observed is defined as Stage (I). The temperature range in which \(\:{{\Gamma\:}}_{D}\) remains constant is defined as Stage (II). The temperature range where the intensity ratio I(D)/I(G) begins to decrease sharply and \(\:{{\Gamma\:}}_{D}\) narrows again is defined as Stage (III). The changes in the intensity ratio I(D)/I(G) and \(\:{{\Gamma\:}}_{D}\) with increasing reduction temperature within the temperature ranges corresponding to Stage (I) and Stage (II) are consistent with previously reported trends 26 . Stage (I) is primarily associated with desorption of oxygen-containing functional groups from GO 26 . This desorption reveals the underlying sp ² carbon network, thereby enhancing structural order and crystallinity (Supplementary Fig. S2). Since the D peak originates from sp ² carbon network in proximity to structural defects, the exposure of the sp ² carbon network contributes to an increase in D peak intensity 32 . Consequently, the intensity ratio I(D)/I(G) increases and \(\:{{\Gamma\:}}_{D}\) gradually narrows as the reduction temperature rises. Stage (II) is characterized by the repair of vacancies in rGO through the adsorption of carbon precursors onto dangling bonds 24 – 27 , 29 . This repair process facilitates lateral expansion of the sp ² carbon network and results in a lower defect density compared to Stage (I). In this stage, the crystallinity of rGO, as assessed by the intensity ratio I(D)/I(G), increases only marginally, and the change is sufficiently small that \(\:{{\Gamma\:}}_{D}\) remains nearly constant (Fig. 1 (b)). At higher reduction temperatures corresponding to Stage (III), a pronounced decrease in the intensity ratio I(D)/I(G) and further narrowing of \(\:{{\Gamma\:}}_{D}\) are observed. The significant crystallization occurring in Stage (III) is clearly distinct from the trends observed in Stages (I) and (II), suggesting the involvement of a previously unreported crystallization mechanism (Supplementary Fig. S3). Estimation of Defect Structures The reduction of GO at the temperatures corresponding to Stage (III) in an ethanol atmosphere was found to markedly enhance the crystallinity of rGO. To clarify the crystallization mechanism operative in Stage (III), the evolution of defect structures associated with this pronounced crystallization behavior was analyzed. Raman spectra can be deconvoluted into multiple peaks (Supplementary Fig. S1 ). Structural defects in graphene typically include sp ³-defects, vacancies, and grain boundaries. Since the D' peak is highly sensitive to the type of defect, the intensity ratio I(D)/I(D') between the D and D' peaks can be used to infer the nature of the defects 26 , 30 , 33 , 34 . The intensity ratio I(D)/I(D') corresponds to structural defects of sp ³-defects (~ 13), vacancies (~ 7), and grain boundaries (~ 3.5), respectively 34 . Figure 2 shows the intensity ratio I(D)/I(D') and I(D)/I(G) (same as Fig. 1 (b)) in the temperature range from Stage (II) to Stage (III). The intensity ratio I(D)/I(D') is observed to transition linearly from Stage (II) to Stage (III). Fitting results for each of the peaks yielded I(D)/I(D') ≈ 4.4 at 1200 ℃ and I(D)/I(D') ≈ 3.3 at 1400 ℃ (Supplementary Fig. S1 ). Therefore, the structural defects of rGO are suggested to change from a coexistence of grain boundaries and minor vacancies to predominantly grain boundaries as the reduction temperature increases from Stage (II) to Stage (III). This is consistent with previously reported models of rGO crystallization, in which the repair of vacancies progresses through the adsorption of carbon precursors onto dangling bonds within rGO 21 – 27 . In contrast, a pronounced decrease in the intensity ratio I(D)/I(G) is observed at Stage (III). This suggests that the dramatic crystallization observed at Stage (III) involves a mechanism distinct from defect repair in rGO. Change in Surface Morphology of rGO Thin Films as a Function of Ethanol Partial Pressure Analysis of defect structures based on Raman spectroscopy indicates that mechanisms beyond the simple repair of vacancies contribute to the pronounced crystallization observed in Stage (III). Previous studies have reported that high-temperature thermal reduction in an ethanol atmosphere leads to the formation of carbon aggregates on the surface of rGO 27 , 29 . To clarify the crystallization mechanism operative in Stage (III), we investigated the influence of these carbon aggregates on the surface morphology and crystallinity of rGO. Figures 3 (a)-(c) show the atomic force microscopy (AFM) phase images of rGO surfaces thermally reduced at 1340°C under varying ethanol partial pressures. At a low partial pressure of 0.27 Pa (Fig. 3 (a)), small amorphous carbon aggregates originating from carbon precursors are observed. At an intermediate partial pressure of 0.67 Pa (Fig. 3 (b)), two‑dimensional island structures emerge. At a higher partial pressure of 2.67 Pa (Fig. 3 (c)), the islands further increase in lateral size and exhibit multilayered structures with hexagonal edges. These edge morphologies reflect crystallographic orientations associated with slower growth kinetics; among graphene edge structures, the zigzag edges exhibit the slowest growth rate 35 . Consequently, the graphene islands preferentially adopt hexagonal shapes. Figure 3 (d) summarizes the variation in the intensity ratio I(D)/I(G) and the grain size of rGO as a function of ethanol partial pressure during thermal reduction at 1340°C in an ethanol atmosphere. The grain size \(\:{L}_{a}\) is evaluated using Eq. (1) 36 . $$\:\begin{array}{c}{L}_{a}\left[nm\right]=\frac{560}{{{E}_{l}}^{4}}{\left(\frac{I\left(D\right)}{I\left(G\right)}\right)}^{-1}\#\left(1\right)\end{array}$$ As shown in Fig. 3 (d), the crystallinity of rGO increases with increasing ethanol partial pressure. Because the surface morphology of the rGO is dramatically changed by the introduction of a carbonaceous gas, this enhancement is attributed to the formation of graphene islands on the rGO surface during thermal reduction in an ethanol atmosphere. However, when the ethanol partial pressure exceeds 1.33 Pa, the improvement in crystallinity gradually saturates, and asymptotically approaches a constant value. Evaluation of Surface Morphology of rGO Thin Films as a Function of Reduction Temperature Figures 4 (a)–(c) show the AFM phase images observed on the rGO surface after thermal reduction at 1100°C (Stage (I)), 1250°C (Stage (II)), and 1400°C (Stage (III)), in an ethanol atmosphere with a partial pressure of 1.33 Pa. At Stage (I), no ethanol-derived carbon aggregates are observed on the rGO surface. In contrast, at Stage (II), minute carbon aggregates are uniformly formed across the surface. At the higher temperature of Stage (III), multilayered islands with hexagonal edges appear and are widely distributed over the rGO surface. Figures 4 (d)–(f) show the height profiles along the blue lines in Figs. 4 (a)–(c) obtained from AFM height images. The value of RMS (Root Mean Square) surface roughness at Stage (I) is 0.174 nm, which is comparable to that of rGO reduced in an argon atmosphere. At Stage (II), the value of RMS surface roughness slightly increases to 0.249 nm, likely due to the formation of small carbon aggregates. In contrast, Stage (III) exhibits a significant increase in RMS surface roughness to 0.989 nm. This increase is attributed to the multilayered islands, featuring pyramid-like three-dimensional structures with heights of several nanometers. As highlighted by the green arrows in Fig. 4 (c), part of the uppermost island layer appears to be bonded to the underlying layer. The detailed formation mechanism of these multilayered islands is discussed later. In the temperature range corresponding to Stage (III), where a significant enhancement in crystallinity is observed, three-dimensional structures are observed to form on the rGO surface as shown in Fig. 4 (c). Figure 5 shows the AFM height images and corresponding height profiles of multilayered islands on the rGO surface thermally reduced at 1400°C (Stage III)). The multilayered islands can be classified into two types: layer-by-layer structures (Fig. 5 (a)) and spiral structures (Figs. 5 (b) and 5(c)). The multilayered islands shown in Fig. 5 (a) are attributed to the homoepitaxial growth of graphene, proceeding via layer-by-layer growth in which each monolayer is sequentially stacked uniformly on the rGO surface, forming distinct step edges. As shown in the height profile in Fig. 5 (d), steps with monolayer height are clearly observed. These observations indicate that the crystal growth process yields a highly crystalline structure with excellent lattice matching and no discernible structural defects such as dislocations. By contrast, the island with the spiral structure shown in Fig. 5 (b) is similar to the graphene spirals reported in previous studies 37 , 38 . This structure is thought to result from screw dislocations formed at edges or defect sites on the rGO surface acting as nucleation sites, inducing spiral growth. The spiral structure exhibits a geometric feature where each layer is continuously connected along the out-of-plane direction. The morphology shown in Fig. 5 (c) exhibits a characteristic feature in which a portion of the uppermost layer is continuously connected to the underlying layer, similar to the multilayered island indicated by the green arrow in Fig. 4 (c). This morphology is attributed to spiral growth induced by a dislocation originating from a Frank–Read source. Such a morphology is consistent with geometric features that have been experimentally and theoretically reported in other material systems 39 , 40 . The dislocation structure generated from a Frank–Read source consists of paired right-handed and left-handed screw dislocations 38 . As these two screw dislocations grow in mutually outward rotational directions, multilayered islands, as observed in Fig. 5 (c), are formed. Accordingly, this structure is classified as a spiral structure. The height profiles shown in Figs. 5 (e) and 5(f) reveal that the step heights of the spiral-structured multilayered islands exceed the interlayer spacing of 0.335 nm typically observed in AB-stacked graphene. This observation suggests the formation of turbostratic stacking, characterized by a defect-free in-plane orientation and weak interlayer interactions along the c-axis, arising from spiral growth. Furthermore, the area indicated by the purple arrow in Fig. 5 (b) shows where adjacent islands are connected, appearing to exhibit smoothly merged boundaries between island edges. These observations indicate that the lateral expansion of graphene islands occurs not only through layer-by-layer growth but also via the coalescence of independently grown islands originating from multiple nucleation sites. This process facilitates the formation of a highly crystalline thin film with minimal structural defects. Figure 6 (a) shows the relationship between the grain size of the rGO thin film, evaluated from Raman spectra, and the Hall mobility measured using the Van der Pauw method. The grain size is controlled by varying the reduction temperature under constant gas-phase conditions. As the grain size increases, indicating improved crystallinity, the Hall mobility increases linearly and reaches a maximum of 365 cm²/V·s at room temperature. This mobility represents one of the highest values reported so far for rGO-based materials 15 . Figure 6 (b) shows the variation in sheet resistance as a function of the number of layers in the rGO thin film. The number of rGO layers is controlled by varying the reduction time, while maintaining the GO thin film at 1–3 layers and keeping gas-phase conditions constant. The layer number is evaluated using the intensity ratio I(G)/I(sub.) between the G band peak and the substrate-derived peak from fused quartz at ~ 460 cm⁻¹ in the Raman spectra 41 . The solid red line represents the theoretical sheet resistance calculated based on a parallel resistance model, as illustrated on the right side of the graph, using the sheet resistance of rGO thin films without graphene islands as a reference. For films with five or more layers, the measured sheet resistance deviates from the theoretical curve and exhibits lower values. This reduction in resistance is attributed to the stacking of graphene islands that possess higher crystallinity than the original rGO thin film as the template. Discussion Figure 7 shows a schematic illustration of the crystallization mechanisms of rGO thin films across different temperature ranges. In Stage (I), the thermal energy supplied during the reduction process promotes the desorption of oxygen-containing functional groups, leading to progressive crystallization. However, due to the relatively low temperature, the driving force for graphene growth remains insufficient, and the generation of carbon precursors is negligible. In Stage (II), vacancies are repaired as carbon precursors adsorb onto the dangling bonds present in rGO. Meanwhile, it has also been reported that carbon precursors can adsorb and aggregate on the rGO surface, leading to the formation of carbon aggregates 27 , 29 . These adsorbed carbon aggregates lack sufficient activation energy to undergo structural reorganization into a stable sp ² carbon network, resulting instead in the formation of an amorphous-like structure containing sp ³-defects. In Stage (III), the high-temperature conditions provide sufficient activation energy for structural reorganization, allowing graphene islands composed of sp² carbon networks to crystallize on the rGO surface, originating from carbon precursors. Two types of multilayer graphene islands are observed in this stage: layer-by-layer structures and spiral structures. Both types exhibit well-defined step features corresponding to the thickness of monolayer graphene. At the reduction temperature corresponding to Stage (III), the structural analysis using Raman spectra and AFM images indicates that the rGO thin film exhibits enhanced crystallinity, attributed to the crystal growth of graphene islands on the rGO template. This observation implies that the graphene islands formed atop the rGO template possess higher crystallinity than the template itself. Indeed, the 002 diffraction peak obtained from X-ray diffraction (XRD) measurements shows that the graphene islands exhibit a sharper peak than the rGO template, confirming their superior crystallinity (Fig. S4). This result, indicating that the graphene islands exhibit higher crystallinity than the rGO template, is in good agreement with the observed dependence of sheet resistance on the number of layers. The tendency for the upper epitaxially grown layers to possess higher crystallinity than the underlying template is considered a characteristic feature of van der Waals epitaxial growth in layered materials. This enhanced crystallinity of the grown layers is attributed to the weak π‑bonding interlayer interactions along the c-axis direction of graphene. The 2D band in the Raman spectrum of multilayer graphene is known to be highly sensitive to interlayer interactions 42 , 43 . In this study, analysis of the 2D peak reveals a turbostratic stacking ratio of approximately 63.4% (Fig. S5). This ratio suggests that, during the crystal growth of graphene islands, interlayer interactions with the rGO template are weak. As a result, crystallization proceeds relatively independently of the crystallinity of the underlying template and is strongly influenced by the surrounding gas-phase conditions. Figure 8 illustrates the crystallization processes of graphene islands via two distinct crystal growth modes: layer-by-layer growth and spiral growth. These differences in behavior are attributed to variations in the crystal structure of the rGO template, which serves as a nucleation site for the carbon precursor. In the absence of structural defects such as dislocations (Fig. 8 (a)), nucleation of graphene is followed by epitaxial growth, resulting in the formation of islands with a layer-by-layer structure. In contrast, when a screw dislocation is present in the rGO template (Fig. 8 (b)), the carbon precursors are adsorbed at the dislocation, which serves as a preferential nucleation site, leading to the development of the spiral structure. Furthermore, when the template contains a dislocation originating from a Frank–Read source (Fig. 8 (c)), a geometric feature characteristic of the Frank–Read mechanism is observed in the uppermost layer of the resulting graphene islands. These islands also exhibit continuously connected spiral structures, analogous to those formed via spiral growth, indicating that the sp ² carbon network is preserved throughout the vertical growth process. Importantly, regardless of the crystal growth mode, in-plane crystallization proceeds through the homoepitaxial growth mechanism, enabling the formation of an extended sp ² carbon network with minimal defects. These findings indicate that, although the crystal growth modes are governed by the structure of the underlying template, highly crystalline graphene islands can be grown on rGO templates. Summary In this study, we systematically investigated the crystallization mechanisms of rGO thin films during thermal reduction in an ethanol atmosphere, with particular emphasis on the effects of reduction temperature and ethanol partial pressure on the structural, morphological, and electrical properties of rGO. Comprehensive characterization using Raman spectroscopy, AFM, XRD, and electrical transport measurements revealed a multi‑stage evolution of crystallinity and surface morphology. Notably, we identified a previously unreported crystallization mechanism dominated by the homoepitaxial growth of graphene islands on the rGO surface. These findings provide fundamental insights into the structural transformation of GO through thermal reduction in an ethanol atmosphere and suggest a viable strategy for the scalable and controllable fabrication of high-crystallinity rGO thin films with superior electronic properties. Methods Preparation of GO thin films GO was synthesized following previously reported methods 2 . GO thin films were fabricated using an electrostatic self-assembly approach 26 ,45 . A fused silica substrate (Shin-Etsu Chemical) was first immersed for 1 h in a solution of 3-aminopropyltrimethoxysilane (APTMS, purity > 96.0%, Tokyo Chemical Industry) and ethanol mixed at a volume ratio of 1:9, thereby forming an amino-terminated self-assembled monolayer (SAM) on the substrate surface. After annealing the SAM-modified substrate at 120°C for 30 min on a hot plate, the substrate was immersed overnight in a 0.003 wt% aqueous GO dispersion, resulting in the formation of GO thin films composed of one to three layers. Thermal reduction of GO thin films The reduction of GO was carried out using a custom-built cold-wall infrared heating furnace. Unlike conventional hot-wall furnaces, the cold-wall configuration enables localized heating of only the substrate, thereby allowing reduction at elevated temperatures. This design also suppresses undesired heating of the surrounding chamber, effectively inhibiting secondary gas-phase association reactions of ethanol decomposition products and preventing the deposition of carbon aggregates on the rGO surface. Argon, an inert gas, was employed as the carrier gas, while ethanol served as the carbonaceous. The flow rates of all gases were precisely controlled using mass flow controllers. The temperature ramp rate was set to 100°C/min up to 1000°C, and 50°C/min up to 1400°C. Upon reaching the target temperature, the total chamber pressure was maintained at 1.33 kPa. During the 60-minute reduction process, the Ar flow rate was kept constant at 500 sccm, while the ethanol flow rate was varied between 0 and 3 sccm depending on the experimental conditions. Characterization of rGO Thin Films To investigate the surface morphology of the rGO thin film, atomic force microscopy (AFM; Jupiter, Oxford Instruments) was performed in AC mode (tapping mode). The crystallinity of the rGO thin film was evaluated by measuring Raman spectra using a micro-Raman spectrometer (LabRAM HR-800, HORIBA Jobin Yvon). Raman measurements were carried out at room temperature with a 100× objective lens and a laser excitation wavelength of 514.5 nm. The chemical bonding states of the rGO films were analyzed using X-ray photoelectron spectroscopy (XPS; PHI Quante, Ulvac PHI) with a monochromatic Al Kα X-ray source (1486.6 eV). In addition, θ-2θ scans were conducted using an X-ray diffractometer (XRD; SmartLab, Rigaku) to assess the stacking structure of the rGO thin films. Measurements were performed over a 2θ range of 5°-60° with a step size of 0.02°, using a monochromatic Cu Kα radiation source (λ = 0.154 nm). Transistor fabrication and Electrical transport measurements The carrier mobility and sheet resistance of the rGO thin films were determined by Hall-effect measurements using the van der Pauw configuration with a Bio-Rad HL5500PC system. Device fabrication was carried out using photolithography (Mask-less Exposure System, DL-1000/NC2P, NanoSystemSolutions) followed by electron beam lithography (ELS-G125, Elionix). Declarations Competing interests The authors declare no competing interests. Funding Declaration This work was supported by Grants-in-Aid for Scientific Research (C) (No. 22K04865) from the Japan Society for the Promotion of Science (JSPS) and partly by the Inoue Enryo Memorial Grant, Toyo University. Author Contribution S. Kanda and Dr. R. Negishi led the study and wrote the manuscript. T. Yamashita and Dr. R. Negishi performed the measurements and analysis of the electrical transport properties of the rGO samples. S. Kanda carried out the Raman spectroscopy, AFM, XPS, and XRD measurements and analysis. Dr. S. Kurosu assisted with Raman spectroscopy measurements and contributed to discussions of the results. F. Sakamoto assisted with XPS measurements and analyses and contributed to discussions of the results. S. Kanda, Dr. R. Negishi, and T. Hanajiri discussed and interpreted the results. Dr. Y. Nishina synthesized the GO samples. All authors reviewed and approved the final manuscript. Acknowledgement This work was supported by Grants-in-Aid for Scientific Research (C) (No. 22K04865) from the Japan Society for the Promotion of Science (JSPS) and partly by the Inoue Enryo Memorial Grant, Toyo University. This work was carried out using the advanced facilities of Bio-nano Electronics Research Centre. We gratefully acknowledge the technical assistance in photolithography provided by Dr. T. Yamaguchi and Dr. K. Ishibashi at the Nano Science Building, RIKEN. 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Express . 9 , 025103. https://doi.org/10.7567/Apex.9.025103 (2016). De Silva, K. K. H., Huang, H. H., Suzuki, S., Badam, R. & Yoshimura, M. Ethanol-assisted restoration of graphitic structure with simultaneous thermal reduction of graphene oxide. Jpn J. Appl. Phys. 57 , 08NB03. https://doi.org/10.7567/Jjap.57.08nb03 (2018). Negishi, R., Akabori, M., Ito, T., Watanabe, Y. & Kobayashi, Y. Band-like transport in highly crystalline graphene films from defective graphene oxides. Sci. Rep. 6 , 28936. https://doi.org/10.1038/srep28936 (2016). De Silva, K. K. H., Shibata, K., Viswanath, P., Huang, H. H. & Yoshimura, M. High-quality monolayer reduced graphene oxide films via combined chemical reduction and ethanol-assisted defect restoration. Adv. Mater. Interfaces . 9 , 2200503. https://doi.org/10.1002/admi.202200503 (2022). Maruyama, S., Kojima, R., Miyauchi, Y., Chiashi, S. & Kohno, M. Low-temperature synthesis of high-purity single-walled carbon nanotubes from alcohol. Chem. Phys. Lett. 360 , 229–234. https://doi.org/10.1016/S0009-2614(02)00838-2 (2002). Su, C. Y. et al. Highly efficient restoration of graphitic structure in graphene oxide using alcohol vapors. Acs Nano . 4 , 5285–5292. https://doi.org/10.1021/nn101691m (2010). Negishi, R., Nakagiri, T., Akabori, M. & Kobayashi, Y. Promotion of the structural repair of graphene oxide thin films by thermal annealing in water-ethanol vapor. Thin Solid Films . 775 , 139841. https://doi.org/10.1016/j.tsf.2023.139841 (2023). Cancado, L. G. et al. Quantifying defects in graphene via Raman spectroscopy at different excitation energies. Nano Lett. 11 , 3190–3196. https://doi.org/10.1021/nl201432g (2011). Lucchese, M. M. et al. Quantifying ion-induced defects and Raman relaxation length in graphene. Carbon 48 , 1592–1597. https://doi.org/10.1016/j.carbon.2009.12.057 (2010). Eckmann, A., Felten, A., Verzhbitskiy, I., Davey, R. & Casiraghi, C. Raman study on defective graphene: effect of the excitation energy, type, and amount of defects. Phys. Rev. B . 88 , 035426. https://doi.org/10.1103/PhysRevB.88.035426 (2013). Eckmann, A. et al. Probing the nature of defects in graphene by Raman spectroscopy. Nano Lett. 12 , 3925–3930. https://doi.org/10.1021/nl300901a (2012). Ma, T. et al. Edge-controlled growth and kinetics of single-crystal graphene domains by chemical vapor deposition. P Natl. Acad. Sci. USA . 110 , 20386–20391. https://doi.org/10.1073/pnas.1312802110 (2013). Cançado, L. G. et al. General equation for the determination of the crystallite size La of nanographite by Raman spectroscopy. Appl. Phys. Lett. 88 , 163106. https://doi.org/10.1063/1.2196057 (2006). Tay, R. Y. et al. Concentric and spiral few-layer graphene: growth driven by interfacial nucleation vs screw dislocation. Chem. Mater. 30 , 6858–6866. https://doi.org/10.1021/acs.chemmater.8b03024 (2018). Wang, Z. J. et al. Conversion of chirality to twisting via sequential one-dimensional and two-dimensional growth of graphene spirals. Nat. Mater. 23 , 331–338. https://doi.org/10.1038/s41563-023-01632-y (2024). Long, C. et al. Frank-Read mechanism in nematic liquid crystals. Phys. Rev. X . 14 , 011044. https://doi.org/10.1103/PhysRevX.14.011044 (2024). Hudson, T., Rindler, F. & Rydell, J. A quantitative model for the Frank-Read dislocation source based on pinned mean curvature flow. P Roy Soc. a-Math Phy . 481 , 20240740. https://doi.org/10.1098/rspa.2024.0740 (2025). Wei, C. P. et al. Turbostratic multilayer graphene synthesis on CVD graphene template toward improving electrical performance. Jpn. J. Appl. Phys. 58, SIIB04 (2019). https://doi.org/10.7567/1347-4065/ab0c7b Cançado, L. G. et al. Measuring the degree of stacking order in graphite by Raman spectroscopy. Carbon 46 , 272–275. https://doi.org/10.1016/j.carbon.2007.11.015 (2008). Malard, L. M., Pimenta, M. A., Dresselhaus, G. & Dresselhaus, M. S. Raman spectroscopy in graphene. Phys. Rep. 473 , 51–87. https://doi.org/10.1016/j.physrep.2009.02.003 (2009). Gómez-Navarro, C. et al. Electronic transport properties of individual chemically reduced graphene oxide sheets. Nano Lett. 7 , 3499–3503. https://doi.org/10.1021/nl901209z (2007). Additional Declarations No competing interests reported. Supplementary Files supplementaryManuscriptKandaSatoshiToyoUniversity.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 09 Mar, 2026 Reviews received at journal 06 Mar, 2026 Reviews received at journal 25 Feb, 2026 Reviews received at journal 23 Feb, 2026 Reviewers agreed at journal 17 Feb, 2026 Reviews received at journal 15 Feb, 2026 Reviewers agreed at journal 14 Feb, 2026 Reviewers agreed at journal 14 Feb, 2026 Reviewers agreed at journal 13 Feb, 2026 Reviewers agreed at journal 12 Feb, 2026 Reviewers agreed at journal 12 Feb, 2026 Reviewers invited by journal 12 Feb, 2026 Editor assigned by journal 04 Feb, 2026 Submission checks completed at journal 04 Feb, 2026 First submitted to journal 03 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8773050","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":592946312,"identity":"d5a50a43-4515-4e88-9c7a-2d608798e4db","order_by":0,"name":"S. Kanda","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9ElEQVRIiWNgGAWjYDAC/uaDHxIq2HjYmBkSH3yAiCXg1yJxLFniwxk+OX72hseGMxKI0cKQo8Y5s03OWLLn4DNpHkKKQUC+4QwbMw+bWeKGG8nJxrY/bBL7GRiePcCnxeBw7zFmHp40oJa0xMc5CWmJMxsY0g3wamE4l8bMI3EMqCUn2Tgn4XDihgMMaRL4HZZjxsxj8B+oJf+btAUxWhgO5JhxzkhgA3r/QJo0AzFaDG6AAvkAGyiQkw170tKMZzYT8It8PzAqE/9Bo/KHjY1sP3tP2gO8DkMHjg3MPGkk6WCwZ2BgP0aallEwCkbBKBjuAAA7ZVJEQ1U0kwAAAABJRU5ErkJggg==","orcid":"","institution":"Toyo University","correspondingAuthor":true,"prefix":"","firstName":"S.","middleName":"","lastName":"Kanda","suffix":""},{"id":592946314,"identity":"f3f33810-96d7-4fd4-91cd-82b740a17374","order_by":1,"name":"T. Yamashita","email":"","orcid":"","institution":"Toyo University","correspondingAuthor":false,"prefix":"","firstName":"T.","middleName":"","lastName":"Yamashita","suffix":""},{"id":592946315,"identity":"ff2aeb22-da1e-4924-b2e9-04521bd44129","order_by":2,"name":"S. Kurosu","email":"","orcid":"","institution":"Bio-nanoelectronics Research Centre","correspondingAuthor":false,"prefix":"","firstName":"S.","middleName":"","lastName":"Kurosu","suffix":""},{"id":592946317,"identity":"0cfed9e9-f947-432b-9547-f89a53c1eb9e","order_by":3,"name":"F. Sakamoto","email":"","orcid":"","institution":"Bio-nanoelectronics Research Centre","correspondingAuthor":false,"prefix":"","firstName":"F.","middleName":"","lastName":"Sakamoto","suffix":""},{"id":592946321,"identity":"d0b72982-c34f-4af3-bd8b-06c394ea139e","order_by":4,"name":"T. Hanajiri","email":"","orcid":"","institution":"Toyo University","correspondingAuthor":false,"prefix":"","firstName":"T.","middleName":"","lastName":"Hanajiri","suffix":""},{"id":592946322,"identity":"b1fa21f7-c38b-4d77-905b-de8e600c8aa8","order_by":5,"name":"Y. Nishina","email":"","orcid":"","institution":"Research Institute for Interdisciplinary Science","correspondingAuthor":false,"prefix":"","firstName":"Y.","middleName":"","lastName":"Nishina","suffix":""},{"id":592946325,"identity":"071082e4-aec7-4823-a28f-97ac016097bc","order_by":6,"name":"R. Negishi","email":"","orcid":"","institution":"Toyo University","correspondingAuthor":false,"prefix":"","firstName":"R.","middleName":"","lastName":"Negishi","suffix":""}],"badges":[],"createdAt":"2026-02-03 08:28:54","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8773050/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8773050/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103019472,"identity":"ff9bb765-4770-4da8-b9a9-19674e520357","added_by":"auto","created_at":"2026-02-19 17:34:38","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":796994,"visible":true,"origin":"","legend":"\u003cp\u003eEvaluation of rGO crystallinity based on Raman spectra obtained from rGO thin films reduced at various temperatures in an ethanol atmosphere (ethanol partial pressure: 1.33 Pa; total pressure: 1.33 kPa). (a) Raman spectra and the corresponding intensity ratio I(D)/I(G) at each reduction temperature. (b) Reduction temperature dependence of the intensity ratio I(D)/I(G) and the full width at half maximum (FWHM) of the D peak for the rGO thin films. The temperature range is classified into three stages according to changes in I(D)/I(G) and FWHM of the D peak.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/47a83370be869482f246e2ff.jpeg"},{"id":103049858,"identity":"6229e9f9-57e1-46bb-a91c-b9bbe2b2cf1d","added_by":"auto","created_at":"2026-02-20 07:46:53","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":260613,"visible":true,"origin":"","legend":"\u003cp\u003eReduction temperature dependence of the intensity ratio I(D)/I(D') and I(D)/I(G) for the rGO thin films.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/b941ad4f59056a630d00ccd1.jpeg"},{"id":103050036,"identity":"805e636c-d1a8-4194-9da2-f7e6894282df","added_by":"auto","created_at":"2026-02-20 07:47:48","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":631465,"visible":true,"origin":"","legend":"\u003cp\u003eAFM phase images of the rGO surfaces thermally annealed at 1340°C in an ethanol atmosphere, with varying ethanol partial pressures: (a) 0.27 Pa, (b) 0.67 Pa and (c) 2.67 Pa, respectively. (d) Ethanol partial pressure dependence of the intensity ratio I(D)/I(G) and grain size.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/d10f79ac99b17c41e38f0654.jpeg"},{"id":103050403,"identity":"37d25c9d-5d1a-4b23-a14f-347f73899118","added_by":"auto","created_at":"2026-02-20 07:49:52","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":714548,"visible":true,"origin":"","legend":"\u003cp\u003eAFM phase images of the rGO surface after thermal reduction in an ethanol atmosphere (ethanol partial pressure: 1.33 Pa) at (a) 1100 °C (Stage (I)), (b) 1250 °C (Stage (II)), and (c) 1400 °C (Stage (III)). (d–f) Corresponding height profiles extracted along the blue lines obtained from the AFM height images. The RMS surface roughness values were evaluated from height profiles shown in (d–f).\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/ddb502503daa279d5fe477e4.jpeg"},{"id":103503830,"identity":"2ec88011-f789-439e-8afe-439a6e804290","added_by":"auto","created_at":"2026-02-26 13:02:46","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":416593,"visible":true,"origin":"","legend":"\u003cp\u003eAFM height images of multilayered graphene islands with (a) a layer-by-layer structure and (b, c) a spiral structure. The multilayered graphene islands with a spiral structure result from (b) a screw dislocation and (c) a Frank–Read source. The corresponding height profiles along the blue lines are shown below each image in (d–f). The purple arrow in (b) indicates a smoothly merged boundary between adjacent island edges.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/8d56dac47866bb395d6f4f2b.jpeg"},{"id":103050478,"identity":"b66e39ad-470c-4ef1-a8f6-69d64a810338","added_by":"auto","created_at":"2026-02-20 07:50:13","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":378716,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Correlation between the grain size of rGO thin films, estimated from Raman spectra, and carrier mobility measured by the van der Pauw method. (b) Dependence of sheet resistance on the number of layers in the rGO thin films.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/a57744f23cefcd091533bf51.jpeg"},{"id":103050189,"identity":"cd447f05-91f8-4d3c-87ad-3c7a3b52c841","added_by":"auto","created_at":"2026-02-20 07:48:41","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":242378,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the crystallization mechanisms of rGO thin films at each stage of the thermal reduction in an ethanol atmosphere.\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/0129d5ad9ee57b04b96c31de.jpeg"},{"id":103019476,"identity":"9d7bf3fc-ac55-425c-957c-458cae21dcd7","added_by":"auto","created_at":"2026-02-19 17:34:38","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":360046,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the crystal growth processes of graphene islands via two distinct growth modes: (a) layer-by-layer growth and (b, c) spiral growth. Spiral growth originates from (b) a screw dislocation and (c) a Frank–Read source.\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/80cabc21476c33215fdd1b06.jpeg"},{"id":104397424,"identity":"708ea191-2f06-4154-99b6-d5a2911f8d15","added_by":"auto","created_at":"2026-03-11 11:47:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4555990,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/c14cadf8-12b1-4d86-8f67-047dbf87cf70.pdf"},{"id":103019471,"identity":"a5d1af5c-1558-4998-babd-2008fa5b5a2a","added_by":"auto","created_at":"2026-02-19 17:34:38","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":570961,"visible":true,"origin":"","legend":"","description":"","filename":"supplementaryManuscriptKandaSatoshiToyoUniversity.docx","url":"https://assets-eu.researchsquare.com/files/rs-8773050/v1/372211d284857cd962aa6ef6.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhanced Crystallinity and Electrical Properties of Reduced Graphene Oxide through Homoepitaxial Growth","fulltext":[{"header":"Introduction","content":"\u003cp\u003eGraphene oxide (GO) is a scalable two-dimensional material that can be synthesized at low cost and with high yield\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Owing to its potential applications in a wide range of electronic devices such as sensors, transparent conductive films and supercapacitors, GO has attracted significant attention in recent years for both its fundamental properties and practical utility\u003csup\u003e\u003cspan additionalcitationids=\"CR4 CR5 CR6\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. GO is typically produced by oxidizing graphite, a process that increases the interlayer spacing and weakens the van der Waals forces between the layers\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. The most common method to chemically exfoliate into single-layer GO is the modified Hummers method\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. The resulting GO can be deposited onto various device substrates to form large-area GO thin films. However, GO exhibits insulating behavior due to the presence of numerous oxygen-containing functional groups and structural defects. Therefore, a reduction process is essential to restore its electrical conductivity for use in electronic devices.\u003c/p\u003e \u003cp\u003eSeveral reduction methods have been explored for GO, including chemical reduction\u003csup\u003e\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e with agents such as hydrazine, photoreduction\u003csup\u003e\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e under light irradiation, and thermal reduction conducted in inert atmospheres\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e such as argon. Among these approaches, thermal reduction has proven particularly effective in removing oxygen-containing functional groups\u0026mdash;such as hydroxyl, carboxyl, and carbonyl moieties\u0026mdash;thereby yielding reduced graphene oxide (rGO) with improved electrical conductivity\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNevertheless, it has been reported that structural defects introduced during the oxidation process\u0026mdash;such as vacancies, 5- and 7-membered rings, and dislocations\u0026mdash;are not repaired by reduction treatments and remain embedded within the graphene lattice\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. These structural defects disrupt the intrinsic hexagonal lattice, thereby degrading the electrical transport properties of the material.\u003c/p\u003e \u003cp\u003ePlasma and high-temperature thermal reduction of GO have been extensively investigated under carbonaceous atmospheres such as methane\u003csup\u003e\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e and ethanol\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Under these conditions, carbonaceous gases decompose into reactive carbon precursors through plasma irradiation or high-temperature thermal reduction, which react with dangling bonds at vacancies and thereby promote partial repair of structural defects\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23 CR24 CR25 CR26\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Alcohol-based carbon sources such as ethanol differ from hydrocarbon gases in that they generate hydroxyl radicals during thermal decomposition. These hydroxyl radicals are known to suppress the formation of amorphous carbon in carbon aggregates by an etching effect\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Indeed, high-temperature thermal reduction using ethanol has been reported to dramatically enhance the crystallinity of rGO with increasing reduction temperature\u003csup\u003e\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, suggesting its effectiveness not only in removing oxygen-containing functional groups but also in repairing vacancies.\u003c/p\u003e \u003cp\u003eHowever, under excessive conditions, ethanol decomposition proceeds beyond the optimal range, leading not only to carbon-precursor-mediated repair of vacancies but also to the deposition of carbon aggregates on the rGO surface\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Thus, thermal reduction in an ethanol atmosphere intrinsically encompasses a paradox\u0026mdash;simultaneously enabling repair of vacancies that improves crystallinity while inducing amorphous carbon deposition as a side reaction\u0026mdash;thereby making process control critically important. Nevertheless, the influence of surface morphological evolution on the crystallization behavior of rGO during high-temperature thermal reduction in an ethanol atmosphere remains insufficiently understood.\u003c/p\u003e \u003cp\u003eIn this study, we systematically varied the reduction temperature and ethanol supply to investigate the relationship between crystallinity and surface morphology in rGO thin films. Our results reveal that the formation of highly crystalline rGO film is primarily driven by a homoepitaxial growth mechanism, which plays a central role in enhancing the film\u0026rsquo;s crystallinity.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eHigh crystallinity of rGO by thermal reduction in an ethanol atmosphere\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) shows typical Raman spectra of rGO reduced at various temperatures in an ethanol atmosphere with a total pressure of 1.33 kPa (ethanol partial pressure: 1.33 Pa). The spectra exhibit peaks in the D band (~\u0026thinsp;1350 cm⁻\u0026sup1;), G band (~\u0026thinsp;1600 cm⁻\u0026sup1;), and 2D band (~\u0026thinsp;2700 cm⁻\u0026sup1;) regions. Since the D peak originates from structural defects of graphene, the intensity ratio I(D)/I(G), calculated from the maximum intensities of the D and G peaks in the Raman spectrum, serves as an indicator of the crystallinity of graphene-based materials\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. At 980\u0026deg;C, the intensity ratio I(D)/I(G) is 0.99, while at 1400 ℃, it decreases to 0.31, indicating improved crystallinity of rGO as a result of high-temperature thermal reduction in an ethanol atmosphere.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) shows the reduction temperature on the horizontal axis and both the intensity ratio I(D)/I(G) and the full width at half maximum (FWHM) of the D peak (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e) as the vertical axis. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e is determined by fitting the D and G peaks with Lorentzian functions and extracting the full width at half maximum of the D peak (Supplementary Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e narrows as the \u003cem\u003esp\u0026sup2;\u003c/em\u003e carbon network expands, which reflects improved structural ordering\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBased on the variations in the intensity ratio I(D)/I(G) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e with respect to the reduction temperature, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b), the temperature ranges were classified into distinct stages. The temperature range where both an increase in the intensity ratio I(D)/I(G) and a narrowing of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e are observed is defined as Stage (I). The temperature range in which \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e remains constant is defined as Stage (II). The temperature range where the intensity ratio I(D)/I(G) begins to decrease sharply and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e narrows again is defined as Stage (III).\u003c/p\u003e \u003cp\u003eThe changes in the intensity ratio I(D)/I(G) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e with increasing reduction temperature within the temperature ranges corresponding to Stage (I) and Stage (II) are consistent with previously reported trends\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Stage (I) is primarily associated with desorption of oxygen-containing functional groups from GO\u003csup\u003e26\u003c/sup\u003e. This desorption reveals the underlying \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network, thereby enhancing structural order and crystallinity (Supplementary Fig. S2). Since the D peak originates from \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network in proximity to structural defects, the exposure of the \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network contributes to an increase in D peak intensity\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Consequently, the intensity ratio I(D)/I(G) increases and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e gradually narrows as the reduction temperature rises.\u003c/p\u003e \u003cp\u003eStage (II) is characterized by the repair of vacancies in rGO through the adsorption of carbon precursors onto dangling bonds\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. This repair process facilitates lateral expansion of the \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network and results in a lower defect density compared to Stage (I). In this stage, the crystallinity of rGO, as assessed by the intensity ratio I(D)/I(G), increases only marginally, and the change is sufficiently small that \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e remains nearly constant (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b)). At higher reduction temperatures corresponding to Stage (III), a pronounced decrease in the intensity ratio I(D)/I(G) and further narrowing of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Gamma\\:}}_{D}\\)\u003c/span\u003e\u003c/span\u003e are observed. The significant crystallization occurring in Stage (III) is clearly distinct from the trends observed in Stages (I) and (II), suggesting the involvement of a previously unreported crystallization mechanism (Supplementary Fig. S3).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eEstimation of Defect Structures\u003c/h3\u003e\n\u003cp\u003eThe reduction of GO at the temperatures corresponding to Stage (III) in an ethanol atmosphere was found to markedly enhance the crystallinity of rGO. To clarify the crystallization mechanism operative in Stage (III), the evolution of defect structures associated with this pronounced crystallization behavior was analyzed.\u003c/p\u003e \u003cp\u003eRaman spectra can be deconvoluted into multiple peaks (Supplementary Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Structural defects in graphene typically include \u003cem\u003esp\u003c/em\u003e\u0026sup3;-defects, vacancies, and grain boundaries. Since the D' peak is highly sensitive to the type of defect, the intensity ratio I(D)/I(D') between the D and D' peaks can be used to infer the nature of the defects\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. The intensity ratio I(D)/I(D') corresponds to structural defects of \u003cem\u003esp\u003c/em\u003e\u0026sup3;-defects (~\u0026thinsp;13), vacancies (~\u0026thinsp;7), and grain boundaries (~\u0026thinsp;3.5), respectively\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the intensity ratio I(D)/I(D') and I(D)/I(G) (same as Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b)) in the temperature range from Stage (II) to Stage (III). The intensity ratio I(D)/I(D') is observed to transition linearly from Stage (II) to Stage (III). Fitting results for each of the peaks yielded I(D)/I(D')\u0026thinsp;\u0026asymp;\u0026thinsp;4.4 at 1200 ℃ and I(D)/I(D')\u0026thinsp;\u0026asymp;\u0026thinsp;3.3 at 1400 ℃ (Supplementary Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Therefore, the structural defects of rGO are suggested to change from a coexistence of grain boundaries and minor vacancies to predominantly grain boundaries as the reduction temperature increases from Stage (II) to Stage (III). This is consistent with previously reported models of rGO crystallization, in which the repair of vacancies progresses through the adsorption of carbon precursors onto dangling bonds within rGO\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23 CR24 CR25 CR26\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. In contrast, a pronounced decrease in the intensity ratio I(D)/I(G) is observed at Stage (III). This suggests that the dramatic crystallization observed at Stage (III) involves a mechanism distinct from defect repair in rGO.\u003c/p\u003e\n\u003ch3\u003eChange in Surface Morphology of rGO Thin Films as a Function of Ethanol Partial Pressure\u003c/h3\u003e\n\u003cp\u003eAnalysis of defect structures based on Raman spectroscopy indicates that mechanisms beyond the simple repair of vacancies contribute to the pronounced crystallization observed in Stage (III). Previous studies have reported that high-temperature thermal reduction in an ethanol atmosphere leads to the formation of carbon aggregates on the surface of rGO\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. To clarify the crystallization mechanism operative in Stage (III), we investigated the influence of these carbon aggregates on the surface morphology and crystallinity of rGO.\u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a)-(c) show the atomic force microscopy (AFM) phase images of rGO surfaces thermally reduced at 1340\u0026deg;C under varying ethanol partial pressures. At a low partial pressure of 0.27 Pa (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a)), small amorphous carbon aggregates originating from carbon precursors are observed. At an intermediate partial pressure of 0.67 Pa (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)), two‑dimensional island structures emerge. At a higher partial pressure of 2.67 Pa (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c)), the islands further increase in lateral size and exhibit multilayered structures with hexagonal edges. These edge morphologies reflect crystallographic orientations associated with slower growth kinetics; among graphene edge structures, the zigzag edges exhibit the slowest growth rate\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Consequently, the graphene islands preferentially adopt hexagonal shapes. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d) summarizes the variation in the intensity ratio I(D)/I(G) and the grain size of rGO as a function of ethanol partial pressure during thermal reduction at 1340\u0026deg;C in an ethanol atmosphere. The grain size \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{L}_{a}\\)\u003c/span\u003e\u003c/span\u003e is evaluated using Eq.\u0026nbsp;(1)\u003csup\u003e36\u003c/sup\u003e.\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}{L}_{a}\\left[nm\\right]=\\frac{560}{{{E}_{l}}^{4}}{\\left(\\frac{I\\left(D\\right)}{I\\left(G\\right)}\\right)}^{-1}\\#\\left(1\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d), the crystallinity of rGO increases with increasing ethanol partial pressure. Because the surface morphology of the rGO is dramatically changed by the introduction of a carbonaceous gas, this enhancement is attributed to the formation of graphene islands on the rGO surface during thermal reduction in an ethanol atmosphere. However, when the ethanol partial pressure exceeds 1.33 Pa, the improvement in crystallinity gradually saturates, and asymptotically approaches a constant value.\u003c/p\u003e\n\u003ch3\u003eEvaluation of Surface Morphology of rGO Thin Films as a Function of Reduction Temperature\u003c/h3\u003e\n\u003cp\u003eFigures \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a)\u0026ndash;(c) show the AFM phase images observed on the rGO surface after thermal reduction at 1100\u0026deg;C (Stage (I)), 1250\u0026deg;C (Stage (II)), and 1400\u0026deg;C (Stage (III)), in an ethanol atmosphere with a partial pressure of 1.33 Pa. At Stage (I), no ethanol-derived carbon aggregates are observed on the rGO surface. In contrast, at Stage (II), minute carbon aggregates are uniformly formed across the surface. At the higher temperature of Stage (III), multilayered islands with hexagonal edges appear and are widely distributed over the rGO surface. Figures\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d)\u0026ndash;(f) show the height profiles along the blue lines in Figs.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a)\u0026ndash;(c) obtained from AFM height images. The value of RMS (Root Mean Square) surface roughness at Stage (I) is 0.174 nm, which is comparable to that of rGO reduced in an argon atmosphere. At Stage (II), the value of RMS surface roughness slightly increases to 0.249 nm, likely due to the formation of small carbon aggregates. In contrast, Stage (III) exhibits a significant increase in RMS surface roughness to 0.989 nm. This increase is attributed to the multilayered islands, featuring pyramid-like three-dimensional structures with heights of several nanometers. As highlighted by the green arrows in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c), part of the uppermost island layer appears to be bonded to the underlying layer. The detailed formation mechanism of these multilayered islands is discussed later.\u003c/p\u003e \u003cp\u003eIn the temperature range corresponding to Stage (III), where a significant enhancement in crystallinity is observed, three-dimensional structures are observed to form on the rGO surface as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c). Figure\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the AFM height images and corresponding height profiles of multilayered islands on the rGO surface thermally reduced at 1400\u0026deg;C (Stage III)). The multilayered islands can be classified into two types: layer-by-layer structures (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)) and spiral structures (Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) and 5(c)). The multilayered islands shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) are attributed to the homoepitaxial growth of graphene, proceeding via layer-by-layer growth in which each monolayer is sequentially stacked uniformly on the rGO surface, forming distinct step edges. As shown in the height profile in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d), steps with monolayer height are clearly observed. These observations indicate that the crystal growth process yields a highly crystalline structure with excellent lattice matching and no discernible structural defects such as dislocations.\u003c/p\u003e \u003cp\u003eBy contrast, the island with the spiral structure shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) is similar to the graphene spirals reported in previous studies\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. This structure is thought to result from screw dislocations formed at edges or defect sites on the rGO surface acting as nucleation sites, inducing spiral growth. The spiral structure exhibits a geometric feature where each layer is continuously connected along the out-of-plane direction. The morphology shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c) exhibits a characteristic feature in which a portion of the uppermost layer is continuously connected to the underlying layer, similar to the multilayered island indicated by the green arrow in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c). This morphology is attributed to spiral growth induced by a dislocation originating from a Frank\u0026ndash;Read source. Such a morphology is consistent with geometric features that have been experimentally and theoretically reported in other material systems\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. The dislocation structure generated from a Frank\u0026ndash;Read source consists of paired right-handed and left-handed screw dislocations\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. As these two screw dislocations grow in mutually outward rotational directions, multilayered islands, as observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c), are formed. Accordingly, this structure is classified as a spiral structure. The height profiles shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(e) and 5(f) reveal that the step heights of the spiral-structured multilayered islands exceed the interlayer spacing of 0.335 nm typically observed in AB-stacked graphene. This observation suggests the formation of turbostratic stacking, characterized by a defect-free in-plane orientation and weak interlayer interactions along the c-axis, arising from spiral growth. Furthermore, the area indicated by the purple arrow in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) shows where adjacent islands are connected, appearing to exhibit smoothly merged boundaries between island edges. These observations indicate that the lateral expansion of graphene islands occurs not only through layer-by-layer growth but also via the coalescence of independently grown islands originating from multiple nucleation sites. This process facilitates the formation of a highly crystalline thin film with minimal structural defects.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) shows the relationship between the grain size of the rGO thin film, evaluated from Raman spectra, and the Hall mobility measured using the Van der Pauw method. The grain size is controlled by varying the reduction temperature under constant gas-phase conditions. As the grain size increases, indicating improved crystallinity, the Hall mobility increases linearly and reaches a maximum of 365 cm\u0026sup2;/V\u0026middot;s at room temperature. This mobility represents one of the highest values reported so far for rGO-based materials\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) shows the variation in sheet resistance as a function of the number of layers in the rGO thin film. The number of rGO layers is controlled by varying the reduction time, while maintaining the GO thin film at 1\u0026ndash;3 layers and keeping gas-phase conditions constant. The layer number is evaluated using the intensity ratio I(G)/I(sub.) between the G band peak and the substrate-derived peak from fused quartz at ~\u0026thinsp;460 cm⁻\u0026sup1; in the Raman spectra\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. The solid red line represents the theoretical sheet resistance calculated based on a parallel resistance model, as illustrated on the right side of the graph, using the sheet resistance of rGO thin films without graphene islands as a reference. For films with five or more layers, the measured sheet resistance deviates from the theoretical curve and exhibits lower values. This reduction in resistance is attributed to the stacking of graphene islands that possess higher crystallinity than the original rGO thin film as the template.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows a schematic illustration of the crystallization mechanisms of rGO thin films across different temperature ranges. In Stage (I), the thermal energy supplied during the reduction process promotes the desorption of oxygen-containing functional groups, leading to progressive crystallization. However, due to the relatively low temperature, the driving force for graphene growth remains insufficient, and the generation of carbon precursors is negligible.\u003c/p\u003e \u003cp\u003eIn Stage (II), vacancies are repaired as carbon precursors adsorb onto the dangling bonds present in rGO. Meanwhile, it has also been reported that carbon precursors can adsorb and aggregate on the rGO surface, leading to the formation of carbon aggregates\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. These adsorbed carbon aggregates lack sufficient activation energy to undergo structural reorganization into a stable \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network, resulting instead in the formation of an amorphous-like structure containing \u003cem\u003esp\u003c/em\u003e\u0026sup3;-defects.\u003c/p\u003e \u003cp\u003eIn Stage (III), the high-temperature conditions provide sufficient activation energy for structural reorganization, allowing graphene islands composed of \u003cem\u003esp\u0026sup2;\u003c/em\u003e carbon networks to crystallize on the rGO surface, originating from carbon precursors. Two types of multilayer graphene islands are observed in this stage: layer-by-layer structures and spiral structures. Both types exhibit well-defined step features corresponding to the thickness of monolayer graphene.\u003c/p\u003e \u003cp\u003eAt the reduction temperature corresponding to Stage (III), the structural analysis using Raman spectra and AFM images indicates that the rGO thin film exhibits enhanced crystallinity, attributed to the crystal growth of graphene islands on the rGO template. This observation implies that the graphene islands formed atop the rGO template possess higher crystallinity than the template itself. Indeed, the 002 diffraction peak obtained from X-ray diffraction (XRD) measurements shows that the graphene islands exhibit a sharper peak than the rGO template, confirming their superior crystallinity (Fig. S4).\u003c/p\u003e \u003cp\u003eThis result, indicating that the graphene islands exhibit higher crystallinity than the rGO template, is in good agreement with the observed dependence of sheet resistance on the number of layers. The tendency for the upper epitaxially grown layers to possess higher crystallinity than the underlying template is considered a characteristic feature of van der Waals epitaxial growth in layered materials. This enhanced crystallinity of the grown layers is attributed to the weak π‑bonding interlayer interactions along the c-axis direction of graphene.\u003c/p\u003e \u003cp\u003eThe 2D band in the Raman spectrum of multilayer graphene is known to be highly sensitive to interlayer interactions\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. In this study, analysis of the 2D peak reveals a turbostratic stacking ratio of approximately 63.4% (Fig. S5). This ratio suggests that, during the crystal growth of graphene islands, interlayer interactions with the rGO template are weak. As a result, crystallization proceeds relatively independently of the crystallinity of the underlying template and is strongly influenced by the surrounding gas-phase conditions.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates the crystallization processes of graphene islands via two distinct crystal growth modes: layer-by-layer growth and spiral growth. These differences in behavior are attributed to variations in the crystal structure of the rGO template, which serves as a nucleation site for the carbon precursor. In the absence of structural defects such as dislocations (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a)), nucleation of graphene is followed by epitaxial growth, resulting in the formation of islands with a layer-by-layer structure. In contrast, when a screw dislocation is present in the rGO template (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b)), the carbon precursors are adsorbed at the dislocation, which serves as a preferential nucleation site, leading to the development of the spiral structure. Furthermore, when the template contains a dislocation originating from a Frank\u0026ndash;Read source (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c)), a geometric feature characteristic of the Frank\u0026ndash;Read mechanism is observed in the uppermost layer of the resulting graphene islands. These islands also exhibit continuously connected spiral structures, analogous to those formed via spiral growth, indicating that the \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network is preserved throughout the vertical growth process. Importantly, regardless of the crystal growth mode, in-plane crystallization proceeds through the homoepitaxial growth mechanism, enabling the formation of an extended \u003cem\u003esp\u003c/em\u003e\u0026sup2; carbon network with minimal defects. These findings indicate that, although the crystal growth modes are governed by the structure of the underlying template, highly crystalline graphene islands can be grown on rGO templates.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSummary\u003c/h2\u003e \u003cp\u003eIn this study, we systematically investigated the crystallization mechanisms of rGO thin films during thermal reduction in an ethanol atmosphere, with particular emphasis on the effects of reduction temperature and ethanol partial pressure on the structural, morphological, and electrical properties of rGO. Comprehensive characterization using Raman spectroscopy, AFM, XRD, and electrical transport measurements revealed a multi‑stage evolution of crystallinity and surface morphology. Notably, we identified a previously unreported crystallization mechanism dominated by the homoepitaxial growth of graphene islands on the rGO surface. These findings provide fundamental insights into the structural transformation of GO through thermal reduction in an ethanol atmosphere and suggest a viable strategy for the scalable and controllable fabrication of high-crystallinity rGO thin films with superior electronic properties.\u003c/p\u003e \u003c/div\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of GO thin films\u003c/h2\u003e \u003cp\u003eGO was synthesized following previously reported methods\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. GO thin films were fabricated using an electrostatic self-assembly approach\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,45\u003c/sup\u003e. A fused silica substrate (Shin-Etsu Chemical) was first immersed for 1 h in a solution of 3-aminopropyltrimethoxysilane (APTMS, purity\u0026thinsp;\u0026gt;\u0026thinsp;96.0%, Tokyo Chemical Industry) and ethanol mixed at a volume ratio of 1:9, thereby forming an amino-terminated self-assembled monolayer (SAM) on the substrate surface. After annealing the SAM-modified substrate at 120\u0026deg;C for 30 min on a hot plate, the substrate was immersed overnight in a 0.003 wt% aqueous GO dispersion, resulting in the formation of GO thin films composed of one to three layers.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eThermal reduction of GO thin films\u003c/h2\u003e \u003cp\u003eThe reduction of GO was carried out using a custom-built cold-wall infrared heating furnace. Unlike conventional hot-wall furnaces, the cold-wall configuration enables localized heating of only the substrate, thereby allowing reduction at elevated temperatures. This design also suppresses undesired heating of the surrounding chamber, effectively inhibiting secondary gas-phase association reactions of ethanol decomposition products and preventing the deposition of carbon aggregates on the rGO surface. Argon, an inert gas, was employed as the carrier gas, while ethanol served as the carbonaceous. The flow rates of all gases were precisely controlled using mass flow controllers. The temperature ramp rate was set to 100\u0026deg;C/min up to 1000\u0026deg;C, and 50\u0026deg;C/min up to 1400\u0026deg;C. Upon reaching the target temperature, the total chamber pressure was maintained at 1.33 kPa. During the 60-minute reduction process, the Ar flow rate was kept constant at 500 sccm, while the ethanol flow rate was varied between 0 and 3 sccm depending on the experimental conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization of rGO Thin Films\u003c/h2\u003e \u003cp\u003eTo investigate the surface morphology of the rGO thin film, atomic force microscopy (AFM; Jupiter, Oxford Instruments) was performed in AC mode (tapping mode). The crystallinity of the rGO thin film was evaluated by measuring Raman spectra using a micro-Raman spectrometer (LabRAM HR-800, HORIBA Jobin Yvon). Raman measurements were carried out at room temperature with a 100\u0026times; objective lens and a laser excitation wavelength of 514.5 nm. The chemical bonding states of the rGO films were analyzed using X-ray photoelectron spectroscopy (XPS; PHI Quante, Ulvac PHI) with a monochromatic Al Kα X-ray source (1486.6 eV). In addition, θ-2θ scans were conducted using an X-ray diffractometer (XRD; SmartLab, Rigaku) to assess the stacking structure of the rGO thin films. Measurements were performed over a 2θ range of 5\u0026deg;-60\u0026deg; with a step size of 0.02\u0026deg;, using a monochromatic Cu Kα radiation source (λ\u0026thinsp;=\u0026thinsp;0.154 nm).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eTransistor fabrication and Electrical transport measurements\u003c/h2\u003e \u003cp\u003eThe carrier mobility and sheet resistance of the rGO thin films were determined by Hall-effect measurements using the van der Pauw configuration with a Bio-Rad HL5500PC system. Device fabrication was carried out using photolithography (Mask-less Exposure System, DL-1000/NC2P, NanoSystemSolutions) followed by electron beam lithography (ELS-G125, Elionix).\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eDeclaration\u003c/p\u003e \u003cp\u003eThis work was supported by Grants-in-Aid for Scientific Research (C) (No. 22K04865) from the Japan Society for the Promotion of Science (JSPS) and partly by the Inoue Enryo Memorial Grant, Toyo University.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eS. Kanda and Dr. R. Negishi led the study and wrote the manuscript. T. Yamashita and Dr. R. Negishi performed the measurements and analysis of the electrical transport properties of the rGO samples. S. Kanda carried out the Raman spectroscopy, AFM, XPS, and XRD measurements and analysis. Dr. S. Kurosu assisted with Raman spectroscopy measurements and contributed to discussions of the results. F. Sakamoto assisted with XPS measurements and analyses and contributed to discussions of the results. S. Kanda, Dr. R. Negishi, and T. Hanajiri discussed and interpreted the results. Dr. Y. Nishina synthesized the GO samples. All authors reviewed and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by Grants-in-Aid for Scientific Research (C) (No. 22K04865) from the Japan Society for the Promotion of Science (JSPS) and partly by the Inoue Enryo Memorial Grant, Toyo University. This work was carried out using the advanced facilities of Bio-nano Electronics Research Centre. We gratefully acknowledge the technical assistance in photolithography provided by Dr. T. Yamaguchi and Dr. K. 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Electronic transport properties of individual chemically reduced graphene oxide sheets. \u003cem\u003eNano Lett.\u003c/em\u003e \u003cb\u003e7\u003c/b\u003e, 3499\u0026ndash;3503. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/nl901209z\u003c/span\u003e\u003cspan address=\"10.1021/nl901209z\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2007).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8773050/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8773050/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGraphene oxide (GO), which can be synthesized inexpensively and in large quantities, is regarded as a promising starting material for electronic device applications due to its ability to recover electrical conductivity through reduction. However, oxygen-containing functional groups and structural defects introduced during the oxidation and reduction process significantly impair the electrical performance of reduced graphene oxide (rGO), posing a major challenge for practical implementation. In this study, we demonstrate that high-temperature thermal reduction in the presence of a carbonaceous gas not only facilitates the repair of vacancies in rGO thin films but also induces the homoepitaxial growth of two-dimensional graphene islands, guided by the underlying rGO template. By precisely controlling the growth driving force of the carbonaceous gas, epitaxial graphene islands were successfully formed, resulting in a significant improvement in electrical performance, with Hall mobilities reaching up to 365 cm\u0026sup2;/V\u0026middot;s. These results suggest that the homoepitaxial growth of graphene islands plays a crucial role in enhancing both the crystallinity and electrical properties of rGO films.\u003c/p\u003e","manuscriptTitle":"Enhanced Crystallinity and Electrical Properties of Reduced Graphene Oxide through Homoepitaxial Growth","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-19 17:34:33","doi":"10.21203/rs.3.rs-8773050/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-09T07:12:44+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-06T09:21:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-25T22:07:39+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-23T12:07:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"229029918218363987562737721758114865441","date":"2026-02-18T04:29:33+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-15T08:36:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"249460190849034108788533429066607076439","date":"2026-02-15T03:00:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"157990448921753885273367348044486243489","date":"2026-02-14T14:33:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"277955963827415400185163575932418117514","date":"2026-02-13T20:18:29+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"129519595409234623759535365908652810899","date":"2026-02-12T19:57:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"259184839172022669246255388137782008841","date":"2026-02-12T16:58:59+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-12T14:29:15+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-05T01:05:46+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-05T01:04:26+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-02-03T07:48:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2daf6c84-2d4f-4e52-92fa-63b24202f820","owner":[],"postedDate":"February 19th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":63093878,"name":"Physical sciences/Energy science and technology"},{"id":63093879,"name":"Physical sciences/Materials science"},{"id":63093880,"name":"Physical sciences/Nanoscience and technology"},{"id":63093881,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-05-05T08:39:08+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-19 17:34:33","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8773050","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8773050","identity":"rs-8773050","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}