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Functional Nanostructured Materials (including low-D carbon)

Analysis of Defect Recovery in Reduced Graphene Oxide and Its Application as a Heater for Self-Healing Polymers Hyun Gu Kim, Il-Kwon Oh, Seungmin Lee, Sera Jeon, Hyunyong Choi, Kwanpyo Kim, Joo Ho Yang, Jae Woo Chung, Jaekwang Lee, Woo-Hee Kim, and Han-Bo-Ram Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19955 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Analysis of Defect Recovery in Reduced Graphene Oxide and Its Application as a Heater for Self-Healing Polymers Hyun Gu Kim1, Il-Kwon Oh2, Seungmin Lee2, Sera Jeon6, Hyunyong Choi2, Kwanpyo Kim3, Joo Ho Yang4, Jae Woo Chung4, Jaekwang Lee6, Woo-Hee Kim5*, and Han-Bo-Ram Lee1,* 1Department 2School

of Materials Science and Engineering, Incheon National University, Incheon, 22012, Korea

of Electrical and Electronic Engineering, Yonsei University, Seoul, 03722, Korea

3Department

of Physics, Yonsei University, Seoul 03722, Korea

4Department

of Organic Materials and Fiber Engineering, Soongsil University, Seoul 06978, Korea

5Department

of Materials Science and Chemical Engineering, Hanyang University, Ansan 15588, Korea

6Department

of Physics, Pusan National University, Busan 46241, Korea

Abstract Reduced graphene oxide (RGO) obtained from graphene oxide has received much attention because of its simple and cost-effective manufacturing process. Previous studies have demonstrated the scalable production of RGO with relatively high quality; however, irreducible defects on RGO deteriorate the unique intrinsic physical properties of graphene, such as high-mobility electrical charge transport, limiting its potential applicability. Using the enhanced chemical reactivity of such defects, atomic layer deposition (ALD) can be a useful method to selectively passivate the defect sites. Herein, we analyzed the selective formation of Pt by ALD on the defect sites of RGO and investigated the effect of Pt formation on the electrical properties of RGO by using ultrafast terahertz (THz) laser spectroscopy. Time-resolved THz measurements directly corroborated that the degree of the defect-recovering property of ALD Pt-treated RGO appeared as Auger-type subpicosecond relaxation, which is otherwise absent in pristine RGO. In addition, the conductivity improvement of Pt-recovered RGO was theoretically explained by density functional theory calculations. The ALD Pt-passivated RGO yielded a superior platform for the fabrication of a highly conductive and transparent graphene heater. By using the ALD Pt/RGO heater embedded underneath scratched self-healing polymer materials, we also demonstrated the effective recovery property of self-healing polymers with high-performance heating capability. Our work is expected to result in significant advances toward practical applications for RGObased flexible and transparent electronics.

Keywords: Reduced graphene oxide, Atomic layer deposition, Pt, Defect recovery, RGO heater, Self-healing polymer Corresponding author e-mail: [email protected], [email protected]

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Introduction Graphene, an atomically thin hexagonal lattice of sp2-bonded C atoms1,2 exhibits many excellent properties, such as high transparency, high flexibility, excellent mechanical strength, and high electrical conductivity.1,2 By virtue of these excellent properties, graphene has been widely employed in various applications, such as flexible displays, energy devices, and transparent electrodes.2,3 Until now, many methods to synthesize graphene have been reported, which include physical exfoliation, epitaxy, chemical vapor deposition (CVD), and reduction of graphene oxide (GO). Compared to others methods, the reduction of GO has been considered as a simpler and more cost-effective method for high-volume manufacturing.4-6 Large quantities of GO flakes are easily obtainable from the chemical exfoliation process of graphite, and post-thermal and chemical reduction processes readily transform GO flakes to graphene.7,8 Reduced graphene oxide (RGO) can be simply coated on a planar substrate by spincoating without using expensive vacuum tools.9 However, it has not been possible to obtain high-quality graphene that is comparable to CVD-grown graphene through the RGO methods because the conventional thermal and chemical reduction processes cannot fully recover GO. To overcome the limitation of incomplete reduction of RGO, various post-synthesis processes have been previously explored to improve the properties of RGO, especially for its application to transparent electrodes.10-12 However, these processes for improving the conductivity are often accompanied by undesired degradation of the properties, such as the reduction of optical transparency.11,12 Recently, we demonstrated that many defect sites of CVD-grown graphene, such as the grain boundaries of polycrystalline graphene, cracks, wrinkles, and internal/external step edges, can be selectively passivated by atomic layer deposition (ALD) or the galvanic displacement reaction of metals.13,14 Metals in the defective sites of graphene can form additional conducting paths in hybrid samples, resulting in improved electrical conductivity without significant degradation of the sample transparency. The idea of selective metal formation can also be applied to RGO and related graphene structures for the improvement of electrical properties in various applications such as transparent graphene heaters; however, the effects of metals on electrical property improvement have not been clearly described.13-19 More importantly, owing to the chemical bonds formed with graphene defects


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and charge transfer and the accompanying structural reconstruction/passivation effect, metals can also induce significant modifications to the electrical properties and charge dynamics of graphene. However, in graphene-metal hybrid systems, various property measurements are often masked by the metallic component, and it is quite challenging to clearly examine the changes to the properties of graphene. In this study, we investigated the formation of Pt metal on the RGO surface by ALD and the effect of metal formation on the electrical properties of RGO. CVD-grown graphene was also used for comparison. The growth characteristics of the Pt obtained by ALD, which pertain to the defect sites and surface species on RGO and CVD-grown graphene, were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The degree of defect recovery for the Pt-deposited RGO was further characterized by ultrafast terahertz (THz) spectroscopy. Time-resolved THz spectroscopy directly characterize the recombination dynamics of the photoexcited carriers, and is used to show that pristine RGO is dominated by the conventional defect-trapped recombination, whereas Pt/RGO shows an Auger-type relaxation similar to graphene.20-29 In addition, density functional theory (DFT) was applied to the band structure calculation for the cases of the Pt-deposited RGO and the defective RGO. The results from the THz spectroscopy and the DFT calculation were consistent in that we confirmed that the conductivity significantly improved via the defect recovery process when we applied the ALD Pt to RGO. The efficiency of the improvement of the conductivity of the defectrecovered RGO was evaluated by fabricating a Pt-deposited RGO heater system. As a result, compared to the untreated RGO, the Pt-recovered RGO heater showed much improved electrical properties without the degradation of optical transparency. Finally, the defect-healed RGO was embedded beneath scratched self-healing polymer materials, and its thermal heating capability was successfully demonstrated by recovering the scratched self-healing polymer to its pristine condition.

Results and Discussion Figure 1 shows the growth behavior of the ALD Pt with increasing number of ALD cycles on the three surfaces, SiO2, RGO, and CVD-grown graphene, the surface coverages of which were determined from SEM observations. Negligible growth of the Pt was observed on the three surfaces



during the very early stage of growth below 20 cycles, as shown in Figure 1a. Above 20 cycles, however, the surface coverage of the ALD Pt on the SiO2 surface almost linearly increased with increasing number of ALD cycles. Similarly, for the Pt ALD on RGO, a gradual increase in the surface coverage was observed after 20 cycles. In contrast to the SiO2 and RGO surfaces, however, a significantly retarded growth of the Pt was noticed on the CVD-grown graphene. More specifically, there was negligible growth up to 120 cycles, and then only a small amount of Pt nucleation was observed after 150 cycles (see Figure S1 of Supporting Information for magnified SEM images and coverage analysis images). Consistent with the SEM results, similar changes in Pt quantities were observed in the XPS data (see Figure S2 of Supporting Information). These results indicate that both the RGO and CVDgrown graphene surfaces are chemically more stable against Pt growth by ALD than the SiO2 surface, but the Pt nucleation behavior can also critically depend on the prepared graphene surfaces, which in turn depend on the synthesis methods. In our previous reports, we revealed that reactions leading to ALD of Pt can take place only on chemically active defect sites, such as grain boundaries, and that no chemically active site on the ideal graphene surface is composed of sp2 hybridized bonds.30 Compared to the few-layered CVD graphene, RGO seems to be composed of multilayered structures owing to the spincoating of the RGO solution;31-33 however, Pt was randomly formed during the ALD on the surface, and not along the edge, of each RGO flake (see Figure S3 of Supporting Information). In Figure 1b and 1c, the C 1 s highresolution (HR) XPS patterns of the RGO and CVD-grown graphene are deconvoluted, with the deconvoluted peaks assigned to the C=C (284.6 eV), C-C (285.4 eV), C–O (286.4 eV), C-O-C (287.5 eV), C=O (288.6 eV), and O=C-O (289.4 eV) moieties.34,35 Herein, the C-O peak intensity of the RGO relative to those of C=C and C-C appears to be much higher than that of the CVD-grown graphene (see Figure S4 of Supporting Information for the peak areas). In addition, the C-O-C and O=C-O bonds were detected only in the RGO. This corresponds well with previous literature that showed incomplete reduction of GO and increased number of defects during the reduction process.36,37 Similarly, the defect density observed in the RGO was higher than that in the CVD-grown graphene, as seen from the Raman results (see Figure S5, Table S1 of Supporting Information). As a result, hydrophilic functional groups


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remain on the RGO surface. Further, based on the DFT calculations, C atoms having O-containing functional groups exist in the defective region of RGO, allowing the reaction of the Pt precursors with such defects.16 Therefore, it is inferred that during the ALD of Pt, the MeCpPtMe3 precursors can react with C-O bonds such as C-O, C-O-C, C=O, and O=C-O,38 leading to more favorable Pt growth compared to the CVD graphene surface. Figure 1d and 1e show the changes in the relative peak ratios of the C-O bonds, obtained from the deconvoluted C 1 s signal as a function of the number of ALD cycles (the full HR spectra are shown in Figure S4.). In Figure 1d, the C-O, O-C-O, C=O, and O=C-O bonds of the RGO gradually decreased with increasing number of ALD cycles, which also indicated that the preferential growth of Pt on the surface functional groups is associated with the C-O bonds.38 However, those bonds in the CVD-grown graphene underwent very little change until after 150 cycles of ALD of Pt, as seen in Figure 1e; this correspond well with the growth data for the ALD of Pt, shown in Figure 1a. This suggests that the amount of O-bonded C surface groups in the CVD graphene is much less compared to that in the RGO, thereby providing more chemical stability against the ALD process. Similarly, different sensitivities to humidity can be seen in Figure 1f. In contrast to CVD-grown graphene with/without ALD Pt and RGO without ALD Pt, the Pt-deposited RGO exhibited very little change in the electrical resistance with increasing humidity.39,40 The defect site could be easily oxidized by the moisture present in the air, leading to the degradation of the electrical conductivity. These results indicate that most defect sites on RGO are saturated by Pt as a result of the reaction with the Pt precursor and counter reactant during the ALD process. To further explore the degree of defect recovery from the Pt-passivated RGO, we measured the transient THz conductivity responses of the RGO samples by using optical-pump and THz-probe (OPTP) measurements. The schematic diagram is shown in Figure 2a (see Supporting Information for a more detailed description). Because of the small photon energy of the THz field (1 THz = 4.1 meV), the OPTP measurements are a powerful method to directly characterize the low-energy free-carrier transitions in a nonequilibrium state. In RGO, the defect-mediated carrier relaxation channel is distributed in the THz frequency domain.41,42 Therefore, by using the OPTP method, we can not only



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probe the defect density in the RGO sample but also verify the degree of the defects in Pt-passivated RGO (Pt/RGO). All the measurements were performed at room temperature with a pump fluence F = 100 μJ/cm2. The 50 fs, 1.55 eV pump generates nonequilibrium photocarriers, which experienced various relaxation mechanisms, such as thermalized carrier dynamics, carrier-carrier scattering and optical phonon emission, within a few hundreds of a femtosecond.43,44 Here, the photoinduced carriers in the RGO decay via the following two channels: the Auger-recombination channel and defect trapping, as schematically shown in Figure 2b. Prior studies on graphene have shown that Auger-scattering recombination is the dominant channel.45-47 For RGO, however, a considerable amount of defects generated by the O-bonded C functional groups break the Dirac-like linear dispersion property of graphene. In this case, Auger scattering is less likely to contribute, compared to the defect trapping process.36,41,48 As such, the degree of defect recovery can be identified by examining the pump-induced THz transients. Figure 2c shows the pump-induced THz-field changes for RGO and Pt/RGO as functions of the pump-probe time delay, Δt. Opposite signs for the unpumped THz-field Eo(t) and the pump-induced THz-field change ΔEo(t) indicate an increase in pump-induced absorption for both the samples. The peak THz for Pt/RGO is much larger than that for RGO, which readily represents defect recovery. The decay process of the photoinduced carriers can be understood by the coupled rate equations dN (t ) N (t ) [ N (t )]3 N (t )[T0  T (t )]     A(t ) dt  bulk 1 2 dT (t ) N (t )[T0  T (t )] ,  dt 2

(1)

(2)

where N(t) is the photoinduced carrier density and T(t) is the density of the occupied trap states. Detailed information on construction of the rate equation model can be found in Supporting Information.49,50 Herein, the first decay constant (𝜏1) corresponds to the reciprocal Auger-scattering rate (𝛾Auger), and the second decay (𝜏2) is associated with the defect-trapped relaxation rate (𝛾D).50 The rate-equation model was used to fit the experimentally measured data (Figure 2d). The extracted 𝜏1 and 𝜏2 values for RGO are 100 ps and 16 ps, respectively, while the values for Pt/RGO changed to 𝜏1 ≈ 0.25 ps and 𝜏2 ≈ 18 ps.


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For the untreated RGO, the main contributor to the decay is defect-trapped relaxation. However, for the Pt/RGO, we clearly see that Auger-scattering recombination is by far more efficient than defecttrapping. This observation corroborates that Pt treatment effectively reduces the defect density and, thereby, confirms the recovery of defects by ALD of Pt. For quantitative analysis, the values of the extracted initial trap state density, T0, for RGO and Pt/RGO were found to be 9 × 1012 cm2 and 6 × 1012 cm2, respectively. To study the effect of Pt deposition on defective graphene at the atomic scale, we now analyze the first-principles DFT calculations. As illustrated in Figure 3a, a 6 × 3 × 1 rectangular supercell consisting of 72 C atoms is used to model pristine graphene. Divacant graphene, shown in Figure 3b, is considered as the model for defective graphene, since a divacancy is one of the most abundant and most important intrinsic defects that significantly alters the electrical properties of graphene.51 For the Ptdecorated graphene, two C atoms are replaced by one Pt atom, and the defective structure is fully relaxed until the forces on all the atoms are lower than 0.002 eV/Å. As shown in Figure 3c, we find that the substituted Pt atom prefers to be placed at the center of the divacant site and participates equally in the bonding with the neighboring four C atoms. From the band structure and density of states (DOS) calculations for the rectangular pristine graphene (see Figure 3d), we can see that there exists a wellknown linear dispersion near the gamma point where the electrons behave as massless particles. However, for the divacant graphene, this linear dispersion disappears, and a parabolic band appears, as indicated in Figure 3e, suggesting that the divacancy can reduce the electrical conductivity of graphene. Interestingly, after substituting the divacant site with a Pt atom, the linear dispersion reappears near the gamma point, as shown in Figure 3f, which strongly supports the theory that the selective deposition of Pt on defect sites can definitely increase the electrical conductivity of defective graphene; this is in agreement with our experimental observation. In addition, we considered five types of defects (monovacancy, divacancy, tetravacancy, Stone-Wales defects and O-containing surface groups) in graphene as well, and found out from band structure calculations that Pt doping can significantly increase the electrical conductivity of defective graphene (See Figure S6-S10 of Supporting Information).



In Figure 3g, a proposed defect recovery process on RGO by the ALD of Pt is depicted. RGO has many intrinsic defects, such as chemical species and physical vacancies, owing to its chemical synthesis process. The defects are indirectly observed through the formation of many O-related species, as revealed in the XPS analysis results of Figure 1, because the species are mainly formed from the exposure of the defect sites of graphene to air and moisture.18,52 As reported in other papers on ALD of Pt, the surface O species initiate the nucleation of Pt during the ALD, and therefore, Pt nucleates on the defect sites, which are O-rich. For this, there are two defect recovery processes involved in the formation of Pt: one is direct occupation of the defect sites by Pt, and the other is Pt formation over the defect sites, with coexistence of O species. In a previous report, it was confirmed that Pt is directly bonded to C through the formation of Pt-C.38 However, the Auger-scattering process is mainly observed in the THz analysis in the case of the direct occupation of Pt; therefore, the model used in the DFT calculation is valid. Therefore, we can conclude that the selectively formed Pt can recover the defect sites electrically and improve the conductivity of defective graphene by modulating the electronic band structure at the atomic scale. To assess the efficacy of ALD of Pt on the electrical properties of RGO, we fabricated a simple graphene heater with Pt/RGO, which is shown in Figure 4. The photographs of the transparent RGO and Pt/RGO on a glass substrate are shown in Figure 4a. Following 100 cycles of ALD of Pt, the optical transparency appeared to remain unchanged. More specifically, it was found from the transmittance spectra in the visible wavelength ranges that the transmittance of RGO decreased by approximately 2% after the Pt deposition (RGO: 87.9%, Pt/RGO: 85.2%), as noticed in Figure 4b. However, the Pt/RGO heater showed a significantly improved conductivity versus voltage, compared to the untreated RGO heater, as shown in Figure 4c. The temperature of the RGO heater without the Pt ALD treatment showed negligible change as a function of the operating voltage (Figure 4d). In contrast, the temperature of the Pt/RGO heater revealed fast switching behavior that depended on the operating voltage, which ranged from 5 V to 20 V, as shown in Figure 4e, and table 1 summarizes the performance indicators such as heating rate, cooling rate, and saturation temperature, together with the results of previous reports. Under the current experimental condition, the best heating performance of the Pt/RGO heater was


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realized at the heating temperature of 97 °C with the sheet resistance of 6.74 kΩ/sq, which is comparable to the graphene heater synthesized by other methods; 1) chemical doping process (i.e. layer-by-layer doping methods with AuCl3 and HNO3),53 2) electrohydrodynamic (EHD) jet printing (i.e. printing of Ag lines on graphene with the EHD nozzle),54 and 3) annealing (i.e. annealing of spin-coated GO films at 800-1000 °C).55 However, it is worth noting that the current study shows not only the conductivity enhancement of graphenes but has significant implications for recovering graphene defects using ALD, and thereby improving conductivity and stability. However, the RGO heater only showed minimal changes at the temperature of 34.2 °C and displayed the high sheet resistance of 135.28 kΩ/sq. The overall values of the current, resistance, and power consumption extracted from the experimental data, are summarized in Table 2. In the preceding study, the considerable improvements in the heater performance and the electrical conductivity of the Pt/RGO sample might be ascribed to the change in the bandgap structure through the defect recovery process as a result of Pt ALD. In addition to the significantly enhanced conductivity, the efficacy of metal nanoparticles has also been reported elsewhere.56-59 According to the DFT calculation, the bonds between the metal and graphene change their Fermi levels to improve the catalytic activity and stability.56-59 Having established the capability of the Pt/RGO structure to be used as a good conductive heater, the feasibility of using the Pt/RGO heater for healing self-healing polymers was further evaluated. Figure 5a shows a schematic illustration that represents the healing performance of a scratched self-healing polymer using the Pt/RGO heater. The photographs of the scratched self-healing polymer before and after healing are shown in the top and bottom images, respectively, of Figure 5b. For this study, the healing time of the scratched polymer was approximately 35 min. The transmittance values of the self-healing polymer were 4.15-7.45% in the visible wavelength ranges before healing. Upon healing with the Pt/RGO heater, the transmittance values of the self-healing polymer returned to 52.80-79.29%, which are close to those of the pristine polymer (Figure 5c). This indicates that the healing properties of the self-healing polymer were successfully demonstrated through the use of the conductive Pt/RGO heater. The results thus show that this Pt/RGO scheme provides a potential method for realizing highly conductive and transparent graphene by virtue of selective defect passivation of



RGO enabled by the Pt ALD process. We foresee that this method will be useful for the fabrication of graphene with simplified processing and high-volume manufacturability that is potentially applicable to flexible and transparent electronics.

Conclusion We demonstrated the effective recovery of surface defects on RGO via selective Pt growth through ALD. Through detailed surface investigations with Raman and XPS analyses, we showed that Pt predominantly passivates the defective sites on RGO. In addition to the selective growth characteristics of ALD of Pt on RGO, the degree of defect recovery for the Pt-passivated RGO was further elucidated by THz laser spectroscopy; thus, we confirmed that the main carrier recombination channel switched from defect-trapped relaxation to Auger-scattering recombination. Since the ALD Pt allows for selective passivation of the defective regions on RGO, Pt-healed RGO heater revealed highperformance improvement in the electrical properties without the degradation of the optical transparency. With the success of the highly conductive and transparent Pt/RGO heater, we demonstrated effective healing of scratched self-healing polymers. The method presented here is expected to offer a new route to practical RGO-based flexible and transparent electronics.

Experimental section Synthesis of GO and CVD-grown graphene. A modified Hummer’s method was used for GO synthesis.60 Five grams of graphite (SP-1 Graphite, Bay Carbon) and 3.8 g of NaNO3 were placed in an Erlenmeyer flask and cooled in an ice bath. One hundred sixty nine milliliters of H2SO4 was added to the Erlenmeyer flask and stirred to ensure uniform mixing. Twenty two and a half grams of KMnO4 was gradually added to the solution over 1 h while stirring. After another 2 h of stirring, the ice bath was removed, and the mixture was stirred for 5 days. At that point, a brown viscous slurry was obtained. Five hundred milliliters of H2SO4 (5 wt%) was slowly added to the mixture and stirred for an hour, followed by 2 h of additional stirring. The mixture was refined by using 500 mL of an aqueous solution of H2SO4 (3 wt%) and H2O2 (0.5 wt%). After precipitating for 2 days, the supernatant solution was


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removed. After adding 2 L of deionized water and precipitating again, the supernatant solution was removed. This process was repeated until the pH became neutral (pH 6-7). Finally, the GO was filtered by using a cotton fiber mesh.61 The filtered solution was frozen and dried at 50 °C, and then, GO powder (6.3 g) was obtained.61 CVD-grown graphene was grown on a Cu foil through the typical CVD method by using CH4 gas.62,63 The graphene grown on Cu foil was transferred to a SiO2/Si wafer by using an additional layer of poly(methyl methacrylate) (PMMA).64,65 The graphene/Cu-foil samples were coated with 2% PMMA (Sigma Aldrich, average molar weight of approximately 996000, dissolved in anisole) by using a spin coater at 4000 rpm for 60 s. The PMMA-coated graphene was then floated on 0.1 M ammonium persulfate solution to etch the Cu foil. After the Cu foil was etched, the graphene with the PMMA support was placed in deionized water and incubated for 30 min while floating to rinse the remaining etchant residue. The graphene samples with the PMMA support were then manually transferred to the desired SiO2/Si substrate and then baked on a hot plate at 60 °C for 1 h. The PMMA supporting layer was finally removed by immersing the sample in acetone.

Preparation of Pt/graphene. A 1.5 mg/mL solution of GO was sonicated in deionized water to obtain a uniform dispersion. Before spincoating the GO, the substrate wafer was cut to 1 cm × 1 cm and cleaned by piranha treatment (H2SO4:H2O2 = 3:1) to improve the adhesion of the uniform GO laminates to the substrate. The dispersed GO solution was dropped on the substrate, and the substrate was rotated in the spincoater at 1000 rpm for 30 s. Before the ALD process, the GO-coated substrate was annealed at 300 °C for 20 min to reduce GO into RGO in the ALD chamber, the minimum pressure of which was 70 mTorr. Similarly, the CVD-grown graphene was also annealed at the same condition prior to the ALD to ensure that the preparation process was consistent with that of the RGO sample. Pt ALD was performed on three surfaces, bare SiO2, RGO, and CVD-grown graphene, in a commercial ALD system (SN-100, SNTEK) by using trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe3) as the precursor (Sigma-Aldrich) and O2 as the counter reactant. The ALD system is a traveling wave-type chamber that is capable of loading wafers up to 4 inches in diameter. N2 was used



as both the carrier and purging gas. The substrate temperature was maintained at 300 °C, and the bubbler was heated at 50 °C to obtain an appropriate vapor pressure. The vaporized MeCpPtMe3 molecules were transported into the reaction chamber by the N2 carrier gas flowing at the rate of 30 sccm for 2 s. The unreacted Pt precursors and byproducts were purged by the N2 gas for 15 s and exposed to the O2 counter reactant for 4 s. This was followed by N2 purging for 30 s to eliminate the ligands of the chemisorbed Pt precursors and evacuate the byproducts. Additional information on the Pt ALD process can be found in our previous paper.32 The number of ALD cycles was systematically varied from 20150.

Preparation of self-healing polymer. 2-(6-Isocyanato-hexylamino)-6-methyl-4[1H]pyrimidinone (UPy-NCO) was synthesized by using the procedure published in the literature.66 Then, 1.5-fold excess of UPy-NCO was reacted with hydroxyl-terminated linear and nonlinear polymers, which were prepared by ring-opening polymerization of -caprolactone with diethylene glycol and 1,1,1-tris(hydroxymethyl)propane in chloroform along with a catalyst for 16 h at 60 °C. After the reaction, the unfunctionalized UPy-NCO was removed by postreaction with silica and filtered. The filtrate was precipitated, filtered, and vacuum dried, resulting in linear and nonlinear supramolecular polymers (yield: 90.4%) 1H-NMR (CDCl3):  = 13.12 (s, 1H, CH3-C-NH), 11.86 (s, 1H, CH2-NH(C=O)-NH), 10.14 (s, 1H, CH2-NH-(C=O)-NH), 5.84 (s, H, CH=C-CH3), 4.90 (s, H, NH-(C=O)-O), 3.24 (m, 2H, CH2-NH-(C=O)-NH), 3.14 (m, 2H, CH2-NH-(C=O)-O), 2.23 (m, 3, CH3 at UPy), 4.22 (t, 2H, O-CH2-CH2-O-(C=O)-CH2; linear supramolecular polymer), 3.68 (t, 2H, CH2-O-CH2-CH2-O(C=O); linear supramolecular polymer), 0.90 (t, 3H, CH3-CH2-C; nonlinear supramolecular polymer). The self-healing polymer was prepared by mixing both the supramolecular polymers in the ratio of 7:3 by solvent casting.

Characterization. Field emission scanning electron microscopy (JSM-7001F, JEOL) and XPS (PHI-5000 Versa Probe II, PHYSICAL ELECTRONICS) were used for analyzing the morphology


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and chemical composition, respectively, of the ALD Pt on various substrates. A computer graphic software (ImageJ) was used to calculate the surface coverage of the Pt from the SEM images. Atomic force microscopy (Multimode 8, Bruker) was performed for the CVD-grown graphene and the Si substrate prepared by spincoating the GO solutions at 1000 rpm. The resistance values of the CVDgrown graphene and RGO were measured by a four-point probe system (4-Point Probe System, CMT). A Raman spectrometer (Raman, JOBIN YVON LabRAM Hr800, Ar laser 633 nm, Horiba) was used to analyze the physical properties of the RGO and CVD-grown graphene, such as the disorder, defects, and doping. To measure the photoinduced conductivity dynamics, we employed the OPTP spectrometer, where all pulses were delivered from a 250 kHz Ti:sapphire–regenerative amplifier laser system (Coherenet RegA 9050). For time-resolved measurements, 1.55-eV (800 nm) pulses of duration 50 fs were used to excite the samples. The THz pulses were generated and detected by optical rectification and electro-optic sampling, respectively, with a pair of ZnTe crystals. THz pulses can cover the photon energy range 3-11 meV (1 THz = 4.1 meV), which is low enough to characterize the defect response in the frequency domain (see Supporting Information for more details on the experimental setup). All the calculations were carried out by using DFT with the plane wave-based Vienna ab initio simulation package.67 We used the projector-augmented wave method of Blöchl68 in the implementation of Kresse and Joubert.69 The generalized gradient approximation was employed for the exchangecorrelation functional. We used an energy cut-off of 550 eV for the plane wave and Γ-centered 24 × 24 × 4 k-point meshes for the electronic bandstructure and DOS calculations. The calculations converged in energy to 10−6 eV/cell, and the structures were allowed to relax until the forces were less than 2 × 10−3 eV/Å. For calculations pertaining to pristine graphene, we considered a rectangular 6 × 3 × 1 supercell consisting of 72 C atoms. For divacant graphene, two C atoms were removed from the rectangular 6 × 3 × 1 supercell. For the Pt-doped graphene, two C atoms were replaced by a Pt atom, and the supercell composed of 70 C atoms and 1 Pt atom. A vacuum layer of thickness 15 Å was inserted perpendicularly to the rectangular supercell in order to avoid spurious interlayer interactions. To investigate the stability of defect-healed graphene, the resistance change under different humidity conditions was measured in a fume hood. The humidity was controlled to 30%, 40%, 70%,



and 80% by using a humidifier. The RGO and CVD-grown graphene treated by ALD of Pt were exposed to the controlled humidity conditions for an hour, and the resistance was measured by the four-point probe. To investigate graphene-based heaters, RGO was spincoated on the glass substrate (sample size: 1 cm × 1 cm), which was followed by 100 cycles of Pt ALD. Two electrodes were secured by using a Cu tape on both edges of the Pt/RGO samples. Power was supplied from a source meter (Keithley 2400) to the heater through the electrodes. A direct current (DC) power source was connected to the Ptdeposited RGO through the Cu tape electrodes, and the power was controlled under various fixedvoltage conditions ranging from 5 V to 20 V. The time-dependent temperature profiles were observed by using an infrared thermal imaging camera (FLIR t630). The synthesis of the self-healing polymer was verified by a Bruker Avance NMR spectrometer operating at 400 MHz. The self-healing polymer was placed onto the defect-healed graphene heater. A scratch on the self-healing polymer was intentionally made by using sandpaper. The output voltage was fixed at 10 V to increase the temperature of the heater to 50-57 °C. Before and after the healing process involving thermal heating, the transmittance of the self-healing polymer was measured by a UV–visible NIR spectrophotometer (JASCO V-670) to estimate the healing performance.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional information about the experimental setup of THz, and FE-SEM, XPS, AFM, Raman of graphene, and five types of defects in graphene and Pt-doped graphene obtained from DFT calculations

Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2016R1D1A1B03935611) and the MOTIE (Ministry of Trade, Industry & Energy (10080643) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device. It was also supported by the National Research Foundation of Korea (NRF) through the government of Korea


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(MSIP) (grant nos. NRF-2018R1A2A1A05079060). This research is supported by ‘‘Rediscovery of the Past R&D Result” through the Ministry of Trade, Industry and Energy (MOTIE) and the Korea Institute for Advancement of Technology (KIAT) (Grant No.: P0004074).



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Figure 1. Growth characteristics of ALD of Pt on the defect sites of RGO and CVD-grown graphene. (a) Pt surface coverages on the reference SiO2, RGO, and CVD-grown graphene obtained from the SEM images as functions of the number of ALD cycles. C 1s high-resolution XPS patterns of (b) RGO and (c) CVD graphene without the ALD of Pt. (d-e) Normalized amounts of O bonded with C, obtained from the C 1s high-resolution XPS scan (see Figure S4). (f) Resistance versus humidity of RGO and CVD-grown graphene with or without the ALD of Pt.


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Figure 2. Ultrafast optical-pump THz-probe (OPTP) measurement. (a) Schematic illustration of the OPTP measurement. (b) Schematic illustration of the decay process of the photoinduced carriers in the RGO samples. (c) The transient THz field peak for RGO (red line) and Pt/RGO (blue line). Inset: photoinduced ΔE(t) of RGO and Pt/RGO (red and blue lines, respectively, scaled up by a factor of 50), and the reference E0(t) (black line) at Δt = 0.5 (d) Normalized dynamics of the transient THz field signals (dots) and the line fitted with a coupled rate equation. The RGO (red line) and Pt/RGO (blue line) show different decay dynamics.



Figure 3. Stable atomic structures of pristine, divacant, and Pt-doped graphenes obtained from DFT calculations (upper panel) and the corresponding electronic properties (lower panel). (a)-(c) Top views of the optimized pristine, divacant, and Pt-doped graphenes. (d)-(f) Each set of left and right plots shows the electronic band structure and the DOS for the pristine, divacant, and Pt-doped graphenes. The Fermi level is set to zero. (g) A scheme of the defect recovery process on RGO realized by ALD of Pt.


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Figure 4. Graphene heater experiments involving RGO and Pt/RGO samples. (a) Photographs of the transparent Pt/RGO heater (left image) and the RGO heater (right image). (b) Optical transmittance spectra of glass, RGO, and Pt/RGO obtained in the visible wavelength region. (c) Conductivity of the RGO and Pt/RGO heaters. Time-temperature profiles with respect to different applied voltages for the (d) RGO heater and (e) Pt/RGO heater.



Figure 5. Healing experiments of scratched self-healing polymers with Pt/RGO heater. (a) Schematic diagram that shows the healing performance of a scratched self-healing polymer using the Pt/RGO heater. (b) Photographs of the self-healing polymer (top image: the scratched self-healing polymer before healing; bottom image: the self-healing polymer after healing by heating for 35 min). (c) Optical transmittance spectra of the self-healing polymers obtained in the visible wavelength region


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Table 1. Heating rates, cooling rates, and saturation temperatures of Pt/RGO heater, and previous reports Heater

Voltage (V)

Heating rate (℃/s)

Cooling rate (℃/s)

Saturation temperature (℃)

5

0.039

-0.028

39.6

10

0.124

-0.332

50.9

Pt/RGO

Synthesis method

Atomic layer deposition 15

0.270

-0.563

71.0

20

0.458

-1.003

97.0

HNO3, AuCl3doped graphene (Ref. 53)

12

-

-

65.0 (HNO3), 100.0 (AuCl3)

Chemical doping process

Ag-grid/graphene (Ref. 54)

4

-

-

145.0 (Ag 150 m)

Electrohydrodynamic jet printing

60

0.7 (RGO-800), 7 (RGO-900), 9 (RGO-1000)

-

42 (RGO-800), 150 (RGO-900), 206 (RGO-1000)

Annealing

Annealed RGO (Ref. 55)



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Table 2. Overall electrical properties such as the current, resistance, and power consumption of RGO and Pt/RGO heaters.

Heaters

Voltage

Current

Resistance

Resistance

Power consumption

(V)

(A)

(kΩ)

(kΩ/sq.)

(mW)

5

0.00389

1.29

5.83

19.45

10

0.00735

1.36

6.17

73.5

15

0.01093

1.37

6.22

163.95

20

0.01345

1.49

6.74

269

5

0.00019

26.32

119.28

0.95

10

0.00036

27.78

125.90

3.6

15

0.00053

28.30

128.26

7.95

20

0.00067

29.85

135.28

13.4

Pt/RGO

RGO


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Analysis of Defect Recovery in Reduced Graphene ... - ACS Publications

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