Direct Growth of Substrate-Adhered Graphene on Flexible Polymer

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Direct Growth of Substrate-Adhered Graphene on Flexible Polymer Substrates for Soft Electronics Eunho Lee, Seung Goo Lee, and Kilwon Cho Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00948 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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Chemistry of Materials

Direct Growth of Substrate-Adhered Graphene on Flexible Polymer Substrates for Soft Electronics Eunho Lee1, Seung Goo Lee2, and Kilwon Cho1*

1Department

of Chemical Engineering and Center for Advanced Soft Electronics, Pohang University of Science and Technology, Pohang 37673, Korea

2Department

of Chemistry, University of Ulsan, Ulsan 44610, Korea

*E-mail: [email protected]

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Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract This article describes a novel method of growing graphene directly on a flexible substrate at low temperatures using plasma-enhanced chemical vapor deposition (PECVD) with a solid aromatic hydrocarbon source, 1,2,3,4-tetraphenylnaphthalene (TPN), which acts as the feedstock for graphene growth. The TPN is embedded with copper ions that are reduced under the growth conditions to copper nanoparticles that catalyze the graphene growth and then evaporate to leave pristine graphene. Strong covalent bonds between the TPN film and the flexible substrate, prepared by depositing an aluminum oxide (Al2O3) layer on a colorless polyimide (PI) layer, is generated by exposing the TPN film to UV/ozone. The TPN/substrate interfacial adhesive bonds impede sublimation of the TPN from the flexible substrate at the growth temperature, and TPN can convert directly to graphene. The synthesized substrateadhered graphene shows excellent bending stability, with small electrical resistance changes (the resistance R during bending over initial resistance R0 was R/R0 < 1.2 for compressive strain, and R/R0 < 1.4 for tensile strain at   4.68 %). The graphene is appropriate for use in flexible and transparent electrodes for electronic device applications. The proposed method for directly synthesizing substrate-adhered graphene on a flexible substrate is expected to have wide applications in flexible and wearable electronics.


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1. Introduction Graphene synthesis via chemical vapor deposition (CVD) can yield a high-quality, largearea product on a transition metal catalyst.1-5 It must then be transferred using an organic supporting layer, such as poly(methyl methacrylate) (PMMA) onto the target substrate.6-9 This transfer generates numerous defects, such as cracks, wrinkles, and polymer residues, which critically degrade the electrical and mechanical properties of the graphene.10, 11 In addition, CVD processes require high growth temperatures TG > 1000°C because adsorption and dehydrogenation of hydrocarbon sources onto metal catalyst surfaces rarely occur at low TG.12, 13

The high-temperature processes cause many problems, such as contamination from the

evaporated metal catalyst, the requirement for hazardous processes, and high costs.14, 15 To achieve process compatibility with well-developed Si-based technologies and graphene commercialization, it is important to lower the growth temperature TG.16-19 A flexible polymer substrate with limited thermal stability can be easily deformed at high TG, representing a serious impediment to the direct growth of graphene on flexible polymer substrates. To avoid this problem, approaches for synthesizing graphene on flexible substrates have been suggested, including a low-temperature (TG = 300°C) method that uses plasmaenhanced CVD to grow graphene–graphite carbon (G-GC) composite films on deposited Cu film/PI substrates.20 Graphene grown by acetylene (C2H2) was directly transferred to the flexible substrate by wet etching the deposited Cu film. The graphitic films could be formed directly on a flexible substrate; however, the metal catalyst deposition and wet-etching processes were strictly limited in the large-area mass production methods used for industrial applications. Another approach used a thermal CVD process at high temperatures to synthesize graphene–dielectric bi-layer films on a deposited Ni film/polydimethylsiloxane (PDMS)/Si substrate.21 Graphene was directly formed from the methyl functional group of the PDMS substrate using this method. This process still required high growth temperatures to decompose



the carbon source (TG = 1,000°C) and used metal catalyst deposition and wet-etching methods. The methods listed above entail decomposition/wet-etching processes that restrict their application to the fabrication of wearable/flexible electronic devices.22 In addition, the adhesion between graphene and substrates is very important in terms of the application of electronic devices such as electrodes. However, since the adhesion of the conventionally transferred graphene is poor, it is especially limited in flexible electronics. Therefore, a strategic approach for the direct growth of substrate-adhered graphene on flexible substrates at low temperatures has been developed. Here, we propose a new method for synthesizing substrate-adhered graphene directly on a flexible substrate using a solid carbon source with embedded Cu2+ ions at low TG. Deposition of a thin inorganic film on a flexible substrate and subsequent UV/ozone exposure of the carbon source film promoted strong covalent bonding between the carbon source and the flexible substrate.23 The UV/ozone treatment promoted adhesion between the carbon source and the substrate and impeded carbon sublimation from the substrate during the CVD process. The efficient conversion of a surface-adhered carbon source film to graphene was obtained by infusing Cu2+ ions as a catalyst into the carbon source film. During the growth stage, the embedded Cu2+ ions in the carbon source were reduced to Cu nanoparticles (NPs) under the growth conditions, which catalyzed the conversion of the carbon source to graphene prior to evaporation. As a result, substrate-adhered graphene could be synthesized directly on the flexible substrate. Graphene synthesized using our proposed method showed better bending stability than the conventionally transferred graphene and was suitable for use in transparent and flexible conducting electrodes in various flexible electronic device applications.

2. Experimental Section Substrate-adhered graphene synthesis using CVD. 6 nm thick Al2O3 layer was firstly


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deposited on the colorless polyimide film (Kolon Industry) by atomic layer deposition at 150 °C. Then, a 1,2,3,4-tetraphenylnapthalene (TPN) (Sigma-Aldrich) dissolved in chloroform (20 mg/ml) was spin-coated on the prepared flexible substrate (Al2O3/PI) by an angular velocity of 2,000 rpm for 30 sec. In order to form the interfacial adhesion bonding, UV/Ozone was exposed to the TPN-coated film with time 10-20 min. Then the TPN-coated substrate was immersed in 0.1 M solution of Cu(NO3)2 to infuse the TPN with Cu2+ ions. The residual salts were removed by rinsing in deionized DI water, then the Cu2+ ion infused TPN-coated substrate was loaded into a quartz tube that was subsequently evacuated. The prepared substrate was heated to 500 °C to synthesize graphene under optimal growth conditions (power 10 W; pressure 2.3×10-2 Torr; H2 flow rate 50 sccm; growth time 30 min). When the growth of graphene was finished, the chamber was rapidly cooled to room temperature and it was removed from the plasma-enhanced CVD chamber.

Synthesis of the conventionally transferred graphene. Cu foil was firstly heated to 1000 °C under H2 flowing at 10 sccm for reduction of copper oxide on the surface. Subsequently, 45 sccm CH4 gas flowed for 20 min, then the chamber was rapidly cooled to room temperature. The CVD-grown graphene on Cu foil was spin-coated by PMMA which acts as a supporting layer. This bilayer of graphene/PMMA was floated in an aqueous solution of 0.1 M ammonium persulfate for Cu foil etching. After this sample was transferred to the target substrates, the PMMA layer was removed by organic solvents such as acetone for leaving the only graphene on the substrates.

Flexible organic field-effect transistors (OFETs) fabrication and electrical characteristics measurements. For the fabrication of graphene-based flexible organic field-effect transistors, patterned graphene was firstly grown on Al2O3/PI substrate by selective UV/Ozone exposure



using shadow mask. Then, 20 nm-thick Al2O3 layer as gate dielectric was deposited by ALD on patterned graphene gate at 150 °C. Repeatedly, patterned graphene was grown on Al2O3 gate dielectric layer for source and drain electrode formation. An organic semiconductor, PDBT-co-TT, which is composed of 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) and thieno[3,2b]thiophene moieties was dissolved in chloroform (7 mg/ml) and spin-coated on the prepared substrate. Finally, the electrical characteristics of the fabricated devices were characterized at room temperature in a dark environment using a Keithley 2636A instrument under vacuum (103

Torr).

3. Results and discussion A colorless polyimide (PI) substrate was rinsed with ethanol, acetone, and isopropyl alcohol (IPA) to remove surface organic contaminants, then an Al2O3 layer (5 nm thick) was deposited onto the PI substrate using atomic layer deposition (ALD) (Figure 1a). The Al2O3 layer induced covalent bonding between the carbon source and the flexible substrate after subsequent UV/ozone exposure. The Al2O3-deposited polymer substrates have been widely used as passivation layers in various electronic device applications rather than using bare polymer substrates. Therefore, an Al2O3-deposited PI substrate was selected as the graphene growth template. The effects of the deposited Al2O3 are discussed below. A 1,2,3,4-tetraphenylnaphthalene (TPN) solution (50 mg/mL in chloroform) was spincoated onto the Al2O3/PI substrate. The deposited TPN film was exposed to UV/ozone under ambient conditions for 10–20 min. The UV/ozone exposure induced crosslinking within the TPN film and covalent bonding between the TPN film and the Al2O3/PI substrate. The TPNcoated substrate was then immersed in 0.1 M Cu(NO3)2 to infuse the TPN with Cu2+ ions. 24 The residual salts were removed by rinsing in deionized DI water, then the Cu2+ ion-infused TPN-coated Al2O3/PI substrate was loaded into a chamber that was subsequently evacuated.


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The prepared substrate was heated to TG = 500°C to synthesize graphene under optimal growth conditions (power 10 W; pressure 2.30 × 10–2 Torr; H2 flow rate 50 sccm; growth time 30 min) (see Figure S1, Supporting Information). The prepared Al2O3/PI substrate did not lose weight under the growth conditions and did not change its morphology (see Figure S2, Supporting Information). Once the growth of graphene was completed, the chamber was cooled to room temperature and the substrate was removed from the PECVD chamber.

Characterization of synthesized graphene on substrates The presence of synthesized graphene on the Al2O3/PI (flexible) and SiO2/Si (rigid) substrates under optimized conditions was confirmed by optical microscopy (Figure 1b) and single Raman spectrum (Figure 1c). The single Raman spectrum (𝜆𝑙𝑎𝑠𝑒𝑟 = 532 nm) of graphene on an Al2O3/PI and SiO2/Si substrate showed the unique D-peak (1,354 cm–1), G-peak (1,587 cm–1), and 2D-peak (2,641 cm–1) characteristic of graphene. The fluorescence background of the Al2O3/PI substrate weakened the intensity of the characteristic peaks of graphene. On the other hand, the measured single Raman spectrum on the SiO2/Si substrate clearly showed characteristic peaks of graphene. The ratio, ID/IG, of the intensity of the D-peak to the intensity of the G-peak, was strongly related to the defect density in the graphene. The synthesized graphene displayed a ID/IG = 2.36, indicating that it might have been damaged by the plasma.2527

The ratio I2D/IG of the intensity of the 2D-peak I2D to the IG in the single Raman spectrum

was < 0.58, indicating that few-layer graphene was synthesized.28 The measured full width at half maximum (FWHM) of the 2D peak was 66 cm–1 indicated that the number of graphene layers was approximately ≈ 4.29 The uniformity of the synthesized graphene layers was also analyzed by measuring the mapping of electrical sheet resistances and the FWHM of the Raman 2D-peak (see Figure S3, Supporting Information). Previous studies have reported the synthesis of multilayer graphene on dielectric substrates when solid hydrocarbon sources were



used as a precursor instead of gaseous hydrocarbon sources. Unlike the surface-mediated growth mechanisms involving the use of gaseous hydrocarbon sources on a transition metal catalyst surface, carbon atoms in the solid phase can rearrange and form multi-graphitic films on dielectric substrates.30, 31 Several lines of evidence indicated that the graphene layers were four layers thick. Highresolution transmission electron microscopy (HRTEM) of a cross-section (Figure 1d) and a depth profile (red dashed line) confirmed that the graphene synthesized on the Al2O3/PI substrate layers was four layers thick. The UV-vis transmittance spectrum was consistent with our TEM results, and the measured transmittance difference between the Al2O3/PI and graphene/Al2O3/PI substrates was 9.32% at λ = 532 nm. The transmittance of graphene was reduced by 2.3% per layer,32 indicating that the synthesized graphene was four layers thick.

Substrate-adhered graphene growth mechanism Efficient covalent bonding (Al-O-C) that enabled surface adherence to TPN was achieved by depositing a thermally stable Al2O3 layer (6 nm thick) onto the PI substrate. This bonding did not occur on a bare PI substrate (Figure 2a).33, 34 UV/ozone exposure of the Al2O3/PI substrate generated interfacial adhesion bonding (IAB) between the TPN film and the substrate and helped prevent sublimation of the TPN from the substrate at the growth temperature. The IAB was confirmed in the single Raman spectroscopy results (see Figure S4, Supporting Information). The Raman spectrum of the substrate without an Al2O3 layer showed no peaks, indicating that no graphene had formed. On the other hand, the Al2O3/PI substrate displayed a single Raman spectrum with a D-peak, G-peak, and 2D-peak. This result confirmed that graphene was successfully grown on this substrate, thereby increasing surface adhesion via IAB. Al2O3 deposition on the PI substrate reduced thermal shrinkage of the substrate and successfully maintained the initial structure at the growth temperature (see Figure S5,


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Supporting Information). The influence of UV/ozone exposure on the interface chemistry was investigated using the X-ray photoelectron spectroscopy (XPS) depth profile of C1s at the interface between TPN and the Al2O3/PI substrate. The deconvoluted peaks were analyzed, as shown in Figure 2b. The TPN-coated substrate not exposed to UV/ozone displayed only a C-C/C-H (285.3 eV) and a C=C (284.5 eV) peak corresponding to TPN molecules at the interface. UV/ozone exposure resulted in the appearance of various functional groups (C=O (286.2 eV), C-O (286.2 eV), AlO-C (284.1 eV), and Al-C (283.4 eV)) in the C1s XPS, presumably as a result of ozonolysis.35, 36

Most of functional groups (e.g., carbonyl, carboxyl, and hydroxyl) were generated at the

edge or basal plane of graphene. Interestingly, covalent Al-O-C and Al-C species formed at the interface between the TPN film and Al2O3/PI substrate. Dangling hydroxyl bonds on the Al2O3 layer provided active sites for the generation of IAB. These sites tightly bridged the TPN film and the Al2O3 substrate to prevent easy sublimation of TPN. The generation of IAB was confirmed by the depth profiles measured using the O1s (Figure 2c) and Al2p XPS (see Figure S6, Supporting Information). The Al2p XPS showed a clear Al-O-C peak (533.8 eV) after the TPN film was exposed to UV/ozone.37, 38 These results suggested that the IABs induced the direct growth of graphene on the Al2O3 substrate, without extensive sublimation of the carbon source. To determine how the Cu2+ ions affected graphene growth, we obtained atomic force microscopy (AFM) morphology and scanning electron microscopy (SEM) images before, during, and after graphene growth (Figure 3a). Before growth, these images showed that the Cu(NO3)2 salts were adsorbed onto the surface of the TPN film. The XPS depth-profile results corresponding to the Cu2p peaks were homogeneous throughout the film thickness, suggesting that the Cu(NO3)2 salts were embedded in the TPN film (see Figure S7, Supporting Information). During growth (tg=10 min), these salts were completely converted into metallic



Cu nanoparticles via thermal and hydrogen reduction at TG. Finally, the Cu NPs evaporated after the growth of graphene had reached completion. The Cu was poorly soluble in graphene, and the synthesized graphene was not contaminated by Cu.39 The Cu2p XPS results confirmed these result (Figure 3b). Prior to thermal treatment of the TPN film, two broad peaks corresponding to Cu2+ satellites (958.7 eV and 940.0 eV) and two sharp peaks corresponding to the metallic Cu (952.4 eV and 933.1 eV) were observed in the XPS of Cu2p.40 The Cu2+ satellite peaks were attributed to Cu(NO3)2, and they disappeared after thermal treatment. We inferred that the Cu2+ ions were reduced to metallic Cu NPs under the growth conditions, which helped the TPN film convert to graphene because the Cu NPs catalyzed graphene growth from the solid carbon source. The N1s XPS data were consistent with this inference. Two peaks corresponding to the NO3– ions vanished after thermal treatment. This result provided strong evidence for the reduction of Cu(NO3)2 salts (Figure 3c). The mechanism by which Cu2+ ions assisted graphene growth from a solid carbon source, such as the TPN film, is not yet understood, but it is thought that the reduced Cu particles assist the conversion of the solid carbon source to graphene. Our observations suggested the following mechanism underlying the growth of graphene in our method. First, the TPN film was strongly surface-adsorbed to the target substrate by UV/ozone exposure. The infused Cu2+ ions in the TPN film were reduced to the Cu NPs under the growth conditions, then the TPN film was converted into graphene on the substrate with the assistance of the Cu NPs. The limited solubility of Cu in graphene expelled the NPs, which then evaporated to leave multilayer graphene on the substrate (Figure 3d).

Bending stability and electrical characteristics of the DiGr-based flexible devices The graphene synthesized on the flexible substrate using our method was expected to adhere strongly to the substrate due to the IAB induced by UV/ozone exposure. The IAB was


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expected to improve the bending stability. Conventionally transferred graphene (TrGr) usually forms unstable contacts with the substrate, and weak interface between the TrGr and substrate promoted crack propagation that critically degraded the bending stability.41, 42 By contrast, directly grown graphene (DiGr) was strongly bound to the flexible substrate by IAB, which was expected to prevent crack propagation at the interface. The bending stability of DiGr was investigated by fabricating rod-shaped graphene with Au pads on the flexible substrate. The prepared sample was loaded into a homemade flexibility tester that controlled the bending radius of the substrate for device bending tests (see Figure S8, Supporting Information). Initially, the loaded samples were flat, then the gap distance was decreased by pushing each panel toward the bending radius of the flexible substrate, 1.4 mm. Bending the substrate upward (outer bending) applied a tensile strain. Conversely, bending the substrate downward (inner bending) applied a compressive force to the graphene. The applied tensile strain  of the substrate could be calculated using the following simple equation43: 𝜀=

𝑡 , 2𝑟𝑏

where t is the total thickness [mm] and rb is the bending radius [mm], respectively.  was estimated at various bending radii. The bending stabilities of DiGr and TrGr were compared by measuring the electrical resistance changes ( = R/R0, where R is the resistance measured in the bent state and R0 is the resistance measured in the flat state) at 1.0 mm ≤ rb ≤ 9.5 mm. A tensile strain can generate cracks that disrupt electrically -conductive pathways, increasing R. As rb decreased from 9.5 mm to 1.0 mm, the electrical resistance changes in TrGr rapidly increased to 2.2 under outer bending (Figure 4a). During the return to a flat state (recovery),  slowly decreased to only 1.2 because the TrGr was irreversibly degraded by ; however, the change in the electrical



resistance of DiGr was relatively small, 1.4 at rb = 1.0 mm. In addition, DiGr was more stable than TrGr during repeated outer bending cycles (Figure 4b). In TrGr, the electrical resistance change increased to 2.4, but the electrical resistance changes in DiGr remained constant, 1.4. An inner bending test of DiGr and TrGr on the flexible substrate was conducted. In this test, a compressive force was applied to the substrate to induce a corrugated topography in the graphene. The electrical resistance changes (R/R0) displayed by DiGr and TrGr were expected to be small. Under the applied tensile strain, the generated cracks usually broke the electrical current pathway, which increased the electrical resistance; however, the electrical resistance changes could be small under a compressive force if the electrical current pathways in the corrugated graphene were maintained. As expected, the measured R/R0 of DiGr was less than that of TrGr: 1.2 and 1.9, respectively (Figure 4c). The presence of IAB lent to DiGr a better bending stability compared to TrGr during repeated inner bending cycle tests (Figure 4d). In addition, in order to exclude the layer effect, we experimented with transferred graphene with the same number of layers using layer-by-layer transfer method (see Figure S9, Supporting Information). The electrical resistance changes were reduced by the "covered up" effect rather than the single layer graphene, but still, the obtained values were larger than those value of DiGr due to the weak interaction between the substrate and the transferred graphene. These flexion results clearly indicated that the IAB increased graphene’s bending stability, the most important factor for flexible/wearable electronic device applications. Then, we obtained SEM images by varying the number of bending cycles to confirm the crack generation and crack densities in graphene (see Figure S10, Supporting Information). In the case of DiGr, the line density of crack was increased from 0 µm-1 to 0.0512 µm-1 as the number of repeated bends increased (Figure 4e). For more bending, the density of the cracks was saturated. On the other hand, in the case of TrGr, it was found that the line density of the cracks was largely increased to 0.1875 µm-1 compared with DiGr (Figure 4f). For


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conventionally transferred TrGr, only a relatively weak van der Waals force acts on the substrate, so cracks can easily occur from weak points such as defects or wrinkles in graphene or Al2O3 layer itself under the repeated bending (see Figure S11, Supporting Information). However, since the graphene obtained through our method is strongly adsorbed by interfacial adhesion bonding on the substrate, the crack generation is largely reduced. From these results, we confirmed that graphene strongly adsorbed to the substrate has high bending stability. Flexible/transparent electronics applications require electrodes that are compatible with organic semiconductors to achieve efficient charge injection and a high bending stability.44 Graphene displays excellent compatibility and flexibility, and is an ideal candidate for flexible/transparent electrodes. We fabricated the DiGr-based flexible/transparent organic field-effect transistors (OFETs) (Figure 5a). First, the graphene (DiGr) gate electrode was grown directly on an Al2O3/PI substrate using our method. An Al2O3 (20 nm thick) gate dielectric layer was then deposited using atomic layer deposition, and another DiGr layer was directly grown on the gate dielectric as a source/drain electrode. Finally, we spin-coated an organic semiconductor (OSC), PDBT-co-TT, composed of 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) and thieno[3,2-b]thiophene moieties on the prepared substrate (bottom contact). PDBTco-TT had a high hole mobility.45 The devices fabricated with all-carbon electrodes was flexible and transparent. The output characteristics of the fabricated OFET device were typical of p-type transistors, with a well-resolved linear current regime (Ohmic region) at low drain voltages. These characteristics indicated that hole injection from the DiGr synthesized using our method to the PDBT-co-TT semiconductor was efficient (Figure 5b). The transfer curves obtained from this device provided the field-effect hole mobility µ for the PDBT-co-TT device using the following equation:



𝐼𝐷 =

𝑊 𝐶𝜇(𝑉𝐺 ― 𝑉𝑇)2 2𝐿

where L is channel length [µm], ID [µA] is the drain current, W [µm] is the channel width, C [F/cm2] is the normalized capacitance of the dielectric layer, VG [V] is the gate voltage, and VT [V] is the threshold voltage (Figure 5c). In the saturation regime (VDS = –6 V) of the measured transfer curve, we obtained  = 8.6 × 10–2 cm2 V–1 s–1, comparable to the values obtained from devices prepared with the same bottom contact structure but fabricated using directly grown graphene on a rigid substrate.20 We also evaluated the feasibility of deploying DiGr and TrGr in flexible electrode applications by using a homemade flexibility tester. We bent the fabricated devices repeatedly 103 times at rb = 10 mm, and we measured the field-effect hole mobility µ of the PDBT-co-TT device during bending. µ for the DiGr-based OFETs was maintained over the first 100 bending cycles, then decreased to a value 0.9 times the original value after 1000 cycles. By contrast, µ for the TrGr-based OFETs decreased rapidly as soon the test started (Figure 5d). As discussed previously, the IAB generated by UV/ozone treatment strengthened adhesion and prevented crack formation at the interface between the graphene and the substrate. These results indicated that the graphene grown using our method might be appropriate for use in flexible electrodes by providing a good bending stability. In addition, we fabricated flexible temperature sensors which can be applied to a human body or flexible surface using the same method (Figure 5e). In order to measure the change of electrical resistance (R/R0) according to the applied temperature, the temperature was changed from 40 oC to 80 oC overtimes (Figure 5f). Temperature coefficient of resistance (TCR) is a characteristic parameter for a temperature sensor and it is expressed by TCR = 1/R0 × (dR/dT). The TCR was purely extracted from the slope of temperature versus electrical resistance of graphene and the calculated TCR values for our graphene-based sensors is -5.08 × 10-3 oC-1


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(Figure 5g). The higher the value of TCR indicates the more sensitive device. When compared with commercially available temperature sensors, e.g. a Pt temperature sensor (3.92 × 10-3 oC1),

our devices showed higher TCR values. It indicated that the temperature sensor based on

our graphene could be more sensitive than commercially available temperature sensor.

4. Conclusion In conclusion, we developed a simple method of growing substrate-adhered graphene directly on a flexible substrate onto which a thin Al2O3 layer had been deposited. The method used a solid carbon source adhered to the substrate surface using UV/ozone exposure. This method permitted the efficient synthesis of graphene on a flexible substrate at low temperatures without the use of post-transfer processes, which critically degrade the properties of graphene and inhibit various graphene-based applications. UV/ozone exposure generated interfacial adhesion bonds that guided the direct growth of graphene and improved the bending stability. Cu2+ ions embedded in the TPN carbon source were reduced, and the resulting Cu nanoparticles catalyzed the conversion of the carbon source to graphene, then evaporated to leave uncontaminated graphene. We fabricated flexible/transparent OFETs and temperature sensors based on graphene electrodes synthesized using our method. The fabricated OFETs and temperature sensors exhibited excellent electrical characteristics and high bending stabilities. Our proposed method provides a facile approach to the synthesis of substrate-adhered graphene on flexible substrates at low temperatures for use in flexible electronic device applications.



Acknowledgements This work was supported by a grant (Code No.2012M3A6A5055728) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science and ICT. The authors thank D. Kim for help with graphic imaging. Supporting Information description The Supporting Information is available free of charge on the ACS Publication website or from the author. -

CVD experimental condition, thermal characteristics and surface morphology of the substrates, Thermal shrinkage profile of the substrates, Depth-profile XPS analysis, OM image of bending stability test, Line density of cracks.


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Figure captions Figure 1. Graphene synthesis and characterization. (a) Schematic diagram of synthesis process of graphene on Al2O3/PI substrate using plasma-enhanced CVD. (b) Optical images of synthesized graphene on PI (left) and on SiO2/Si substrate (right). (c) Single Raman spectrum of synthesized graphene on PI substrate (left) and SiO2/Si substrate (right). (d) Cross-sectional TEM image of graphene on Al2O3/PI substrate (left) and depth-profile of dashed red line (right). (e) Average UV-vis transmittance data of bare Al2O3/PI substrate (red) and with synthesized graphene (yellow). Figure 2. X-ray photoelectron spectroscopy analysis for interface chemistry. (a) Schematic of graphene synthesis on Al2O3 deposited flexible PI substrate with comparison to the synthesis on bare PI substrate. (b) C1s XPS analysis and (c) O1s XPS analysis of peak at the interface between TPN and the Al2O3/PI substrate before UV/Ozone exposure (top) and after UV/Ozone exposure (bottom). Figure 3. Substrate-adhered Graphene Growth Mechanism. (a) AFM and SEM morphology images of the TPN film infused by Cu(NO3)2 on the Al2O3/PI substrate (left). Cu2+ ions are reduced to Cu particles during growth stage at tg=10min (middle) and the synthesized graphene morphology after plasma-enhanced CVD growth (right). (b) Analysis of Cu2p and (c) N1s XPS peaks on the TPN-coated Al2O3/PI substrate before (red) and after (yellow) thermal treatment. (d) Schematic of possible mechanism of substrate-adhered graphene growth on the Al2O3/PI substrate by plasma-enhanced CVD. Figure 4. Bending stability tests for graphene on flexible Al2O3/PI substrate. (a) Resistance changes of DiGr and TrGr with varying bending radius by outer bending test, and (b) repeated cycle test: resistance change ratio of DiGr and TrGr on Al2O3/PI substrate at bending radius 1R; (c) inner bending test, and (d) repeated cycle test on Al2O3/PI substrate. The measured initial electrical resistances of DiGr and TrGr are 2.51 and 2.37 kohm, respectively. (e) Scanning electron microscopy (SEM) images of the DiGr (top) and TrGr (bottom) with different bending cycles. Figure 5. Electronic device application for graphene-based flexible OFETs and temperature sensors. (a) Schematic diagram of fabrication process flexible/transparent OFETs using DiGr as gate electrode and S/D electrodes. PDBT-co-TT was spin-coated on a DiGr/Al2O3 layer (𝐶𝐴𝑙2𝑂3 = 0.247 μF·cm-2). (b) Output (VG = 0 V to -6 V) and (c) transfer characteristics of the OFETs, channel length L = 200 µm and width W = 400 µm. (d) Carrier mobility changes of the fabricated OFETs using DiGr electrodes (red) and TrGr electrodes under repeated bending cycles to bending radius 10 R. (Inset) Optical microscopy image of the fabricated flexible/transparent OFET devices on the flexible Al2O3/PI substrate. (e) Optical images of the fabricated wearable DiGr-based temperature sensor. (f) Electrical resistance characteristic changes with time, and (g) temperatures.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Direct Growth of Substrate-Adhered Graphene on Flexible Polymer

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