Article Cite This: Chem. Mater. 2019, 31, 4451−4459
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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*,† †
Department of Chemical Engineering, Center for Advanced Soft Electronics, Pohang University of Science and Technology, Pohang 37673, Korea ‡ Department of Chemistry, University of Ulsan, Ulsan 44610, Korea
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S Supporting Information *
ABSTRACT: This article describes a novel method of growing graphene directly on a flexible substrate at low temperatures using plasma-enhanced chemical vapor deposition with a solid aromatic hydrocarbon source, 1,2,3,4tetraphenylnaphthalene (TPN), which acts as the feedstock for graphene growth. 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 layer, are generated by exposing the TPN film to ultraviolet/ozone. The TPN/substrate interfacial adhesive bonds impede the sublimation of TPN from the flexible substrate at the growth temperature, and TPN can convert directly to graphene. The synthesized substrate-adhered 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 ε = 4.68%). 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. carbon (G−GC) composite films on deposited Cu film/ polyimide (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 bilayer films on a deposited Ni film/poly(dimethylsiloxane) (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 = 1000 °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
1. INTRODUCTION Graphene synthesis via chemical vapor deposition (CVD) can yield a high-quality, large-area 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 the adsorption and dehydrogenation of hydrocarbon sources onto metal catalyst surfaces rarely occur at low TG.12,13 The hightemperature 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 plasma-enhanced CVD to grow graphene−graphite © 2019 American Chemical Society
Received: March 7, 2019 Revised: May 29, 2019 Published: May 30, 2019 4451
DOI: 10.1021/acs.chemmater.9b00948 Chem. Mater. 2019, 31, 4451−4459
Article
Chemistry of Materials
Figure 1. Graphene synthesis and characterization. (a) Schematic diagram of the synthesis process of graphene on the Al2O3/PI substrate using plasma-enhanced CVD. (b) Optical images of the synthesized graphene on the PI (left) and on the SiO2/Si substrates (right). (c) Single Raman spectrum of the synthesized graphene on the PI substrate (left) and the SiO2/Si substrate (right). (d) Cross-sectional transmission electron microscopy (TEM) image of graphene on the Al2O3/PI substrate (left) and the depth-profile of the dashed red line (right). (e) Average UV−vis transmittance data of the bare Al2O3/PI substrate (red) and with synthesized graphene (yellow).
2. EXPERIMENTAL SECTION
transferred graphene (TrGr) 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 substrateadhered 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 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. The 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.
2.1. Substrate-Adhered Graphene Synthesis Using CVD. Six nanometer thick Al2O3 layer was first deposited on the colorless polyimide film (Kolon Industry) by atomic layer deposition (ALD) at 150 °C. Then, a 1,2,3,4-tetraphenylnaphthalene (TPN) (SigmaAldrich) dissolved in chloroform (20 mg mL−1) was spin-coated on the prepared flexible substrate (Al2O3/PI) by an angular velocity of 2000 rpm for 30 s. To form an interfacial adhesion bonding (IAB), the TPN-coated film was exposed to UV/ozone for 10−20 min. Then, the TPN-coated substrate was immersed in 0.1 M solution of Cu(NO3)2 to infuse TPN with Cu2+ ions. The residual salts were removed by rinsing in deionized (DI) water, then the Cu2+-ioninfused 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 the graphene was removed from the plasma-enhanced CVD chamber. 2.2. Synthesis of the Conventionally Transferred Graphene. Cu foil was first heated to 1000 °C under H2 flow at 10 sccm for the reduction of copper oxide on the surface. Subsequently, 45 sccm CH4 gas flowed for 20 min and then the chamber was rapidly cooled to room temperature. The CVD-grown graphene on the 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 4452
DOI: 10.1021/acs.chemmater.9b00948 Chem. Mater. 2019, 31, 4451−4459
Article
Chemistry of Materials
Figure 2. X-ray photoelectron spectroscopy (XPS) analysis for interface chemistry. (a) Schematic of the graphene synthesis on the Al2O3-deposited flexible PI substrate compared to the synthesis on bare PI substrate. (b) C 1s XPS analysis and (c) O 1s 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). organic solvents such as acetone, leaving only graphene on the substrates. 2.3. 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 first grown on Al2O3/PI substrate by selective UV/ozone exposure using a shadow mask. Then, a 20 nm thick Al2O3 layer as a gate dielectric was deposited by atomic layer deposition (ALD) on a patterned graphene gate at 150 °C. Repeatedly, the patterned graphene was grown on the Al2O3 gate dielectric layer for source and drain electrode formation. An organic semiconductor (OSC), PDBT-co-TT, which is composed of 1,4diketopyrrolo[3,4-c]pyrrole (DPP) and thieno[3,2-b]thiophene moieties, was dissolved in chloroform (7 mg mL−1) 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 (10−3 Torr).
mer substrates have been widely used as passivation layers in various electronic device applications rather than 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 solution of 1,2,3,4-tetraphenylnaphthalene (TPN) (50 mg mL−1 in chloroform) was spin-coated 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 cross-linking within the TPN film and covalent bonding between the TPN film and the Al2O3/PI substrate. The TPN-coated substrate was then immersed in 0.1 M Cu(NO3)2 to infuse TPN with Cu2+ ions.24 The residual salts were removed by rinsing in deionized DI water, and then the Cu2+-ion-infused TPN-coated Al2O3/PI substrate was loaded into a chamber that was subsequently evacuated. 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 complete, the chamber was cooled to room
3. RESULTS AND DISCUSSION A colorless polyimide (PI) substrate was rinsed with ethanol, acetone, and isopropyl alcohol to remove surface organic contaminants, and 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 poly4453
DOI: 10.1021/acs.chemmater.9b00948 Chem. Mater. 2019, 31, 4451−4459
Article
Chemistry of Materials
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, Supporting Information). The influence of UV/ozone exposure on the interface chemistry was investigated using the X-ray photoelectron spectroscopy (XPS) depth profile of C 1s 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 CC (284.5 eV) peak corresponding to the TPN molecules at the interface. The UV/ozone exposure resulted in the appearance of various functional groups (CO (286.2 eV), C−O (286.2 eV), Al−O−C (284.1 eV), and Al−C (283.4 eV)) in the C 1s XPS, presumably as a result of ozonolysis.35,36 Most of the 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 were formed at the interface between the TPN film and the 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 an easy sublimation of TPN. The generation of IAB was confirmed by the depth profiles measured using the O 1s (Figure 2c) and Al 2p XPS (see Figure S6, Supporting Information). The Al 2p 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 the 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 Cu 2p 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. Cu was poorly soluble in graphene, and the synthesized graphene was not contaminated by Cu.39 The Cu 2p XPS results confirmed these result (Figure 3b). Prior to thermal treatment of the TPN film, two broad peaks corresponding to Cu2+ satellites (958.7 and 940.0 eV) and two sharp peaks corresponding to metallic Cu (952.4 and 933.1 eV) were observed in the XPS of Cu 2p.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 N 1s XPS data were consistent with this inference. Two peaks corresponding to the NO3− ions vanished after thermal treatment. This result provided a 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.
temperature and the substrate was removed from the plasmaenhanced chemical vapor deposition chamber. 3.1. 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 the Al2O3/PI and SiO2/Si substrate showed a unique D-peak (1354 cm−1), G-peak (1587 cm−1), and 2D-peak (2641 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 graphene. The synthesized graphene displayed a ID/IG = 2.36, indicating that it might have been damaged by plasma.25−27 The ratio (I2D/IG) of the intensities of the 2D-peak (I2D) and the G-peak (IG) in the single Raman spectrum was
