Flexible and transparent gold network electrodes on fluorinated graphene†

Yuna Lee‡ ^a, Eunji Ji‡ ^b, Min Jung Kim ^b and Gwan-Hyoung Lee *^a
aDepartment of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea. E-mail: gwanlee@snu.ac.kr
^bDepartment of Materials Science and Engineering, Yonsei University, Seoul, Republic of Korea

First published on 9th August 2024

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Introduction

Manipulating the morphology of metal films deposited on various substrates has been widely studied by altering the interfacial characteristics between metals and substrates.^1–4 The deposition of metals on a range of 2D materials, in particular, has attracted considerable interest for its potential in applications, such as electronic devices,^5–7 catalysts,^8–10 Surface-Enhanced Raman Spectroscopy (SERS)^11,12 and sensors.^13,14 Graphene, a prominent 2D material, presents a pristine surface free of dangling bonds, attributed to its sp² bonded carbon atoms, and offers opportunities for various applications including oxidation,¹⁵ hydrogenation,¹⁶ fluorination,¹⁷ and the addition of –OH,¹⁸ –COOH,¹⁹ –RNH,²⁰etc. Such functionalization of graphene can tailor the interactions with deposited metals by introducing doping or altering polarity, enabling precise control over metal deposition to achieve either island-like structures or uniform films.²¹

In this study, we fabricate a flexible and transparent mesh-like 5 nm gold network electrode on FG by tuning the fluorination of graphene. Previous studies have explored various treatments of the substrates adjusting the interface characteristics between the metal and the substrate to control the morphology of metal. However, it is still challenging to achieve interconnected structures in ultrathin metal films (below 5 nm).^22,23,47 By fine-tuning the conditions of XeF₂ treatment, we could precisely control the connectivity between gold particles, thus effectively adjusting the coverage, electrical sheet resistance, and optical transmittance of the gold network electrode. This interlinked gold network shows potential for use as a transparent conducting electrode (TCE), exhibiting a high transmittance of 80.2% at a wavelength of 550 nm and a sheet resistance of 1.66 Ω □⁻¹ at a gold thickness of 5 nm. The empty regions within the gold network allow the electrode to accommodate significant strain, maintaining stable resistance even under bending, which renders the gold network electrode fabricated on FG highly adaptable for use in flexible electronics.

Results and discussion

Graphene has a planar crystal structure with sp² bonded carbon atoms arranged in a hexagonal lattice. When fluorine atoms are chemically absorbed on the surface of graphene, the sp² bonded planar structure partially transformed into a three-dimensional structure with sp³ C–F bonds. The C–F sp³ bonding formed on the surface of graphene through XeF₂ treatment can be quantified by using Raman spectra and X-ray Photoelectron Spectroscopy (XPS). Fig. 1a shows the Raman spectra of FG under varying XeF₂ gas pressures, keeping the treatment time constant at 50 s. The Raman spectrum of as-exfoliated graphene (indicated by a black line) shows G and 2D peaks without a D peak, indicating no defects. After XeF₂ gas exposure at low pressure (0.5 Torr), the D and D′ peaks appear, highlighting induced structural disorder by sp³ fluorine bonding, which also leads to broadening of the G peak and reduction of the 2D peak.²⁴ Higher pressures of XeF₂ gas (>1 Torr) lead to the merging and broadening of the D′ and G peaks while the 2D peak diminishes further. As the fluorination level increases, the intensities of the D and G peaks progressively reduce, leading to a decrease in the I_D/I_G ratio. We further quantified this disorder by analyzing Raman intensity ratios of I_2D/I_G, I_D/I_G, and I_D′/I_G, revealing a relationship between defect spacing and treatment conditions (Fig. S1†). A specific focus on the I_D/I_G ratio demonstrates how defect density changes with gas pressure, offering insights into the material's structural alterations upon fluorination.²⁵ We observed that I_D/I_G increases and decreases beyond the maximum value with increasing XeF₂ pressure as previously reported.²⁶Fig. 1b presents further analysis of C–F bonding through XPS, indicating a transition in C–F bonds from ionic, through semi-ionic, to covalent as the fluorination conditions change, evident in the F/C ratio alterations. At a low level of fluorination, fluorine atoms are sporadically bonded to the graphene surface, resulting in the formation of semi-ionic C–F bonds rather than complete covalent bonds. As the degree of fluorination increases, some fluorine atoms form covalent C–F bonds causing a higher proportion of covalent C–F bonds.^27–29 This transition is marked by an increase in C–F covalent bonds (∼688 eV) and a decrease in semi-ionic bonds (∼686.5 eV) with increasing XeF₂ gas pressures, indicating more sp³ bonding between carbon and fluorine on the graphene surface. The C1s peak reveals three distinct bonding types: C–C, C–CX (indicative of sp³ hybridization and potential interactions with various functional groups), and C–F as shown in Fig. S2.†^30,31 The high-resolution transmission electron microscopy (HR-TEM) images in Fig. 1c show the crystalline structure of FG treated at 2 Torr, indicating that the fluorination process transforms the graphene lattice into an sp³ configuration without the formation of pores, confirming that the FG can serve as an impermeable barrier. The selected area electron diffraction (SAED) pattern in Fig. 1d confirms that the crystallinity of graphene is maintained after fluorination, which agrees with previous results.¹⁷

To investigate the effect of fluorination on the morphology of deposited gold, we compared 1 nm gold films deposited on as-exfoliated and fluorinated graphene sheets by e-beam evaporation as shown in Fig. 2. Fig. 2a and b show scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the deposited gold on the as-exfoliated graphene, respectively. The deposited gold forms triangular nanoparticles 20–40 nm in lateral size and ∼2.8 nm in thickness, which aligns with the previous results that weakly bonded gold particles on graphene have a van der Waals crystalline relation.^32–34 The gold film deposited on the FG was uniform, as shown in the AFM image in Fig. 2c, and consists of gold nanoparticles as verified in the HR-TEM image in Fig. 2d. As shown in the inset of Fig. 2d, the gold nanoparticles have diameters of 2–5 nm and a round shape. As confirmed in Fig. S3,† 1 nm-thick gold films deposited on a SiO₂ substrate and FG exhibited different roughness values of 0.522 and 0.156 nm, respectively. These results indicate that the gold can be more uniformly deposited on the FG with stronger bonding strength. The electronegativity difference between the fluorine and carbon atoms offers a high charge polarization^35–37 and anchoring of gold particles on the fluorinated graphene surface.

The unique morphology of deposited gold on FG enables the fabrication of ultrathin gold films with high conductivity and transparency. The HR-TEM images in Fig. 3a show different morphologies and thicknesses of the deposited gold on the FG obtained from exfoliated graphene, depending on the fluorination extent and gold thickness. A detailed explanation regarding the impact of the fluorination level on gold deposition is provided in Fig. S4.† By adjusting the XeF₂ treatment conditions, we observed variations in the deposited gold with different thicknesses of 1, 3, and 7 nm. The changes in the intensity ratio (I_D/I_G) of the Raman spectrum due to different XeF₂ treatment conditions are summarized in Table S1.† For example, exposure to XeF₂ at P = 2.25 Torr for 300 s resulted in an I_D/I_G ratio of 1.60, which decreased to 1.50 after treatment for 2400 s under the same pressure. A decrease in the I_D/I_G ratios leads to more uniform deposition of smaller gold islands, particularly noticeable when depositing 7 nm gold, where enhanced necking among particles forms a two-dimensional network. Notably, gold deposition at 1 and 3 nm exhibits significant differences in the distributions of gold island areas as shown in Fig. 3b and c. A lower I_D/I_G value at a deposition of 1 nm gold correlates with an increased number of gold islands smaller than 5 nm², while a lower I_D/I_G value is linked to an increase of larger gold islands at a deposition of 3 nm gold. This indicates a more uniform distribution of small gold islands on the FG surface with a lower I_D/I_G value. It also suggests that, with an increase in gold deposition thickness, the gold network, connected between gold islands, becomes denser on the FG surface. Fig. 3d shows the coverage of deposited gold as a function of gold thickness. At a lower I_D/I_G ratio, the gold coverage is higher on the FG surface. Specifically, at 7 nm gold deposition, the decrease in I_D/I_G values from 1.60 to 1.50 results in a rise in coverage from 60.0% to 72.9%.

To assess their potential as a transparent conducting electrode (TCE), we investigated the morphology of a 5 nm gold film deposited on FG using TEM as shown in Fig. 4a. We synthesized monolayer graphene on copper foil by chemical vapor deposition (CVD), followed by fluorination through XeF₂ treatment and gold deposition. A detailed analysis of the pores in the Au/FG electrodes is provided in Fig. S5.† Even at a small thickness of 5 nm, the deposited gold (dark regions) forms a mesh-like network with gold coverage increasing alongside a lower I_D/I_G ratio as shown in Fig. 4b. The sheet resistance of the 5 nm gold network decreases as gold coverage increases. Fig. 4c shows that, as the I_D/I_G ratio decreases from 1.50 to 1.35, the sheet resistance (R_sh), measured from three samples for each, decreases from 2.09 to 1.66 Ω □⁻¹, which is two orders of magnitude lower than that of 5 nm gold deposited on as-grown graphene (128.4 Ω □⁻¹). Note that the gold films deposited at 5 and 7 nm on a SiO₂ substrate have extremely high sheet resistance beyond the measurement limit because the gold islands are isolated and un-connected as shown in Fig. S6.† Furthermore, the 5 and 7 nm gold network electrodes exhibit high transparency due to their minimal thickness of gold islands and gold-free regions. The transmittance at a wavelength of 550 nm (T₅₅₀) increases from 77.7% at I_D/I_G = 1.50 to 80.2% at I_D/I_G = 1.35 (Fig. 4d). This indicates that a higher metal coverage in highly fluorinated graphene leads to fewer gold-free regions and more uniform deposition of thinner gold islands, thereby enhancing transmittance. We calculated the figure of merit (FoM) that is widely used for assessing the optoelectrical characteristics of TCEs:³⁸

In Fig. 4e, we compared the FoMs of our Au/FG network electrode with various TCEs, such as metal nanowires (NWs),^39–42 oxide/metal/oxide,^43,44 Indium Tin Oxide (ITO),^45,46 conductive polymers,^47,48 metal nanomesh (NM)⁴⁹ and carbon nanotubes (CNTs).^50,51 Our 5 nm Au/FG network electrode showed an exceptional FoM of 973.8, with a transmittance (T₅₅₀) of 80.2% and a sheet resistance of 1.66 Ω □⁻¹. This performance markedly exceeds those of traditional TCEs.

To evaluate the flexibility of the gold network electrode, we transferred the Au/FG prepared from CVD graphene onto a flexible polyethylene terephthalate (PET) substrate (Fig. 5a) and conducted a cyclic bending test using a custom-designed bending instrument (Fig. 5b). We conducted the bending test by fixing one side of the electrode and moving the opposite side in the Δx-direction, as shown in Fig. 5b. For each R_bending, the electrode was bent at a rate of 0.1 cm s⁻¹ in the Δx-direction, held for 2 seconds, and then returned to the original position at the same speed. Even after a bending test of 100 cycles, the gold networks remained interconnected as shown in Fig. 5c. This suggests that the gold network electrode can retain its interconnectedness even when subjected to considerable strain as verified in the resistance measurements in Fig. 5d. After conducting a bending test of 10 cycles across varying bending radii (R_bending), the change in resistance (ΔR/R₀) for the gold network electrodes fabricated on the FG was measured, at different I_D/I_G ratios (1.72, 1.50, 1.42, and 1.35). Across all XeF₂ treatment conditions, the ΔR/R₀ values approached nearly zero regardless of R_bending. Fig. S7† presents the percentage change in current of the Au/FG electrode during bending (ΔI_bending = I_flat − I_bent/I_flat). The Au/FG electrode shows a slight difference in current between flat and bent states, ranging from 0.69% to 14.86%, depending on the R_bending and I_D/I_G ratio. This suggests that the Au/FG can be tailored for flexible electrodes regardless of R_bending and the I_D/I_G ratio. Fig. 5e presents the repetitive bending fatigue test results of the gold network electrodes fabricated on various FGs over 100 cycles at R_bending = 2 mm. The resistance change during the 100 bending cycles remained remarkably low (∼10⁻²) across all tested I_D/I_G ratios, confirming the high resilience of the gold network electrode to extensive strain. In contrast, CVD graphene transferred on the PET substrate showed a significant resistance change. Finally, we benchmarked the ΔR/R₀ with respect to bending strain for various flexible electrodes.^52–64 We compared the ΔR/R₀ value of our gold network electrode in relation to bending strain with other flexible electrodes, calculating the bending strain using the formula of ε_b = t/2R_bending,⁶⁵ where t is the total thickness of the sample (Fig. 5f). In our Au/FG/PET system, we consider t as ∼0.1880055 mm (the sum of 188 μm thickness of PET, 5 nm of gold, and approximately 0.5 nm of fluorinated graphene) with 1 mm of R_bending, resulting in a ε_b = 0.094. The gold network electrode outperformed other flexible electrodes, exhibiting a small resistance change of ∼10⁻² at a bending strain of 9.4%. In conjunction with the exceptional FoM value in Fig. 4e, the durability of the gold network electrode under strain indicates its suitability for use in transparent and flexible electronic applications.

Conclusion

We successfully demonstrated the fabrication of flexible and transparent gold network electrodes using FG as a deposition template. The fluorinated graphene surface allows for the controlled deposition of a mesh-like gold network rather than isolated islands. The resulting gold network on FG exhibited exceptional optical transparency and high conductivity. Furthermore, the gold network electrodes showed remarkable flexibility, enduring significant bending strains and fatigue tests with minimal resistance change. Our study suggests the critical role of the functionalization between metal and graphene in determining the morphology and characteristics of the deposited metal films. By manipulating the surface conditions of the substrates, particularly through the functionalization of graphene, we have illustrated the potential to control metal deposition outcomes, shifting from discrete islands to uniform, interconnected networks. This control over deposition morphology has significant implications for the development of transparent conducting electrodes (TCEs) and flexible electronic devices. Our findings present a straightforward method for crafting ultrathin metal network electrodes that combine transparency and flexibility, offering promising avenues for applications in bendable and transparent electronics.

Experimental section/methods

Sample preparation

We used mechanical exfoliated monolayer graphene on SiO₂(285 nm)/Si and CVD graphene as substrates of gold electrodes. For CVD graphene, monolayer graphene was synthesized on 2 cm × 5 cm of copper foil (25 μm thickness, 99.8%, Alfa Aesar) in a quartz-tube furnace by CVD. The copper foil was annealed at 1060 °C in Ar and H₂ (50/50 sccm) for 18 h under atmospheric conditions. After the annealing process, CH₄ (1 sccm) and H₂ (100 sccm) gases were flown into the furnace under a vacuum of 10⁻⁴ Torr for 30 min. After growth, the samples were naturally cooled to room temperature. To remove the graphene grown on the backside of copper foil, Gr/Cu was etched by using oxygen plasma (100 W, 100 kHz, 10 s, 20 sccm of oxygen, RIE mode). For fluorination, graphene was treated with XeF₂ (SAMCO, VPE-4F and SPTS, Xactix® e2). Finally, a gold metal film was deposited on fluorinated graphene in a high vacuum of <10⁻⁷ Torr.

Material characterization

The samples were examined by Raman spectroscopy (HORIBA, LabRAM HR Evolution) with a 532 nm laser, XPS (KRATOS, AXIS-His), TEM (JEOL, JEM-2100F and JEM-ARM200F), AFM (Park Systems, NX10 and XE-100) and SEM (Carl Zeiss AG, SUPRA 55VP).

Measurement of transmittance and electrical properties

For measurements of the transmittance and electrical properties of the Au/FG electrode, we transferred the Au/FG onto a 188 μm PET (2 cm × 2 cm) substrate. Ethyl Cellulose (EC) was spin-coated on Au/FG/Cu at 2000 rpm for 90 s. The copper foil was etched by using ammonium persulfate (25 wt%) and rinsed in DI water. EC/Au/FG was scooped on the PET substrate and EC was removed by using an ethanol bath (50 °C). We measured the transmittance and sheet resistance of the Au/FG electrode using UV-vis spectroscopy (JASCO, V-770) and a four-point probe measurement system (AIT, CMT-SR2000N).

Bending tests of Au/FG electrodes

For the bending tests, we used a home-built bending instrument (Fig. 5b) and connected the electrodes to a semiconductor parameter analyzer (Keithley, 4200A-SCS).

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Y. L., E. J., and G. H. L. designed this work. Y. L. and E. J. fabricated and analyzed the samples. M. K. synthesized the graphene. Y. L., E. J. and G. H. L. wrote the manuscript together.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by Samsung Electronics Co., Ltd (IO201210-07987-01) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2021R1A2C3014316). G.H.L. acknowledges the support from the Research Institute of Advanced Materials (RIAM), the Institute of Engineering Research (IER), the Institute of Applied Physics (IAP), and the Inter-University Semiconductor Research Center (ISRC) at Seoul National University.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03468f

‡ These authors are co-first authors in this work.

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