Studies on the mechanical stretchability of transparent conductive film based on graphene-metal nanowire structures
© Lee et al.; licensee Springer. 2015
Received: 29 November 2014
Accepted: 10 January 2015
Published: 31 January 2015
Transparent electrodes with superior flexibility and stretchability as well as good electrical and optical properties are required for applications in wearable electronics with comfort designs and high performances. Here, we present hybrid nanostructures as stretchable and transparent electrodes based on graphene and networks of metal nanowires, and investigate their optical, electrical, and mechanical properties. High electrical and optical characteristics, superb bendability (folded in half), excellent stretchability (10,000 times in stretching cycles with 100% in tensile strain toward a uniaxial direction and 30% in tensile strain toward a multi-axial direction), strong robustness against electrical breakdown and thermal oxidation were obtained through comprehensive study. We believe that these results suggest a substantial promise application in future electronics.
Transparent conductive films have been widely used in electronics such as touch screen panels, electrochromic smart windows, and displays. In addition, stretchable electronics have attracted tremendous recent attention due to their potential advantages including outstanding suitability on a nonplanar surface and excellent portability. Therefore, encoding stretchability into transparent electrodes has been actively explored as the key to emerge flexible optoelectronic devices. Indium tin oxide (ITO) is the most commonly used material in transparent electrodes. However, it has several critical disadvantages, such as (i) the increasing costs for indium, (ii) need for a complex vacuum process, and (iii) its mechanical brittleness under external stress. These weak points of ITO limit its use for flexible electronics. Thus, there is a clear and urgent need for new transparent conductive materials with superb mechanical properties. Various alternative candidates for ITO have been intensively pursued, including carbon nanotubes [1-6], conductive polymers , graphene [7-14], nanowires [2,15-22], and metal mesh structures [13,21,23-25]. A powerful candidate for a transparent electrode material comes from the random networks of metal nanowires (mNWs) [17,20,22,26,27]. First, mNWs are normally formed as networks by using various solution coating processes [28-30], which can facilitate low-cost simple fabrication of transparent electrodes. In the case of mNWs, charge transport occurs longitudinally along the NWs, and open spaces between NWs provide optical transparency. Typically, sheet resistance (R s ) of the randomly distributed mNW networks (lower than approximately 80 Ω sq−1) [17,20,26] with high transmittance (higher than approximately 90%) are lower than the R s of undoped, chemical vapor deposition (CVD)-grown graphene (higher than approximately 1 kΩ sq−1) [7,9-11] and comparable with that of ITO (lower than approximately 80 Ω sq−1 with transmittance of approximately 90% at 550 nm) . In addition to their outstanding electrical and optical properties, the mesh-type geometries of mNWs also present superb mechanical robustness under external strains, such as bending strain (approximately 1%) [20-26] or stretching strain (approximately 50%) [22,27]. Several negatives of the mNW networks include (i) high NW-NW junction resistances, (ii) high contact resistance between the mNW networks and active materials, (iii) low breakdown voltages, (iv) poor adhesion of mNWs to flexible and plastic substrates, and (v) oxidation in harsh environments. These factors have limited their integration as transparent electrodes in commercial optoelectronic devices [17,18,20-22,27].
Another promising alternative to ITO comes in the form of graphene that can be deformed up to a strain of approximately 4% with negligible cracking  as well as absorbing only approximately 2.3% of visible light . In addition, the theoretical minimum R s of pristine graphene has been estimated to be as low as approximately 30 Ω sq−1 , but the experimental R s of undoped, synthesized graphene by using various methods [7,8,10,11,32] is substantially higher than the R s of ITO. Although chemical doping processes [12,33] can further reduce R s of the CVD-grown graphene by increasing carrier density, the instability of the interactions between graphene and dopant typically limits the lifetime of this doping effect and leads to a time-variant increase of R s .
In this paper, we report graphene-mNW hybrid nanostructures as high-performance, stretchable, and transparent electrodes. Especially, we focused on the study of the mechanical flexibility of the hybrid film, so various flexible tests such as bending, folding, and uniaxial and multi-axial directional stretching tests were performed. These fabricated hybrid structures present superb mechanical flexibility (folding with bending radius of approximately 3.7 μm) and stretchability (10,000 times in a fatigue test with maximum stretching strain of 100% and stretching in tensile strain of 30% toward multi-axial direction).
Silver nanowires (AgNWs) with average length of 30 μm ± 7 μm and average diameter of 20 nm ± 5 nm dispersed in ethanol (3 mg mL−1) were purchased from Nanopyxis Co., Ltd (Cheonan, South Korea). The solution was stored at 5 °C, and then spin-coated on substrates. The spin coating condition of AgNWs was fixed at 500 rpm for 30 s, except for the density study of AgNWs, which was done according to the spin rates. After spinning the AgNW suspension on substrates (with or without graphene as a top layer), the samples were annealed at 150 °C for 90 s to evaporate the solvent completely.
The synthesis and transfer of graphene were carried out under the following conditions. A Cu foil (Alfa Aesar, item no. 13382; Ward Hill, MA, USA) was loaded onto the center of a quartz CVD reactor under low vacuum (100 mTorr). Subsequently, the reactor was heated up to 1,000 °C with introduction of the flow of Ar (200 sccm) and H2 (500 sccm). The CVD growth was carried out under CH4 (12 sccm) and H2 (500 sccm) flows for 5 min, and then the reactor was rapidly cooled to room temperature under Ar (500 sccm) flow. To transfer the synthesized graphene, we used a 200-nm-thick poly(methyl methacrylate) (Micro Chem Corp. 950 PMMA; Newton, MA, USA) film as a supporting layer which was spun on the graphene. The Cu foil was dissolved in a diluted etching solution of FeCl3:HCl:H2O (1:1:20 vol.%) with the PMMA/graphene layer floating on the surface of the solution. Subsequently, the sample was rinsed by floating it on the surface of deionized water (DI water) and then the sample was transferred onto the desired substrates. In addition, the PMMA supporting layer was removed with acetone.
We have employed two approaches to assemble the hybrid structures (i) AgNWs on graphene or (ii) graphene on AgNWs: (i) After the CVD synthesis of the graphene layer, the graphene was transferred onto a target substrate. After spin casting of the AgNW dispersions onto the graphene surface, this sample was annealed at 150 °C for 90 s. (ii) AgNW network films were coated on substrate, as described above. Then the synthesized graphene layer was transferred onto the films by using PMMA. Subsequently, the supporting layer was removed with acetone.
The graphene-AgNW hybrid electrodes can be photolithographically patterned by using an etch-back process. First, the hybrid films were dry-etched using oxygen plasma (50 W, 60 sccm, 160 s) to remove carbon-based materials. Because the residues of the AgNWs were oxidized and disconnected by oxygen plasma exposure, the exposed areas became electrically nonconductive. Subsequently, these AgNW residues can be completely eliminated by using an etching solution of H3PO4:C2H4O2:C6H4NO5SNa:H2O (55:1:4:40 vol.%).
The preparation of a 2-μm-thick polyimide substrate for the bending test was carried out by the following procedures. After casting the 200-nm-thick PMMA sacrificial layer on a bare Si wafer, a polyimide precursor (poly(pyromellitic dianhydride-co-4,4′oxydianiline); Aldrich, St. Louis, MO, USA) was subsequently spin-coated at 3,000 rpm for 30 s. The thermal curing was processed at 250 °C for 6 h. After fabricating graphene-AgNW hybrid electrodes on the prepared polyimide surface, this 2-μm-thick polyimide film together with the hybrid electrodes could be delaminated from the bare Si wafer by eliminating the PMMA layer using acetone.
For the electrical, optical, and mechanical characterization of the prepared samples, a four-point probe method was used to measure the R s using a probe station with a Keithley 4200-SCS semiconductor parametric analyzer (Keithley Instruments Inc, Solon, OH, USA). In addition, the two-point probe method was also used for the R measurements and electrical breakdown test using a Keithley 2425 source meter. R s can be acquired by calculating it from the measured R by considering the aspect ratio of patterns (R s = R × (Pattern width)/(Pattern length)). Furthermore, the change in R of hybrid films during uniaxial and multi-axial stretching was measured after fixing the sample on home-built stretching stages.
The optical film transmittance was assessed by ultraviolet–visible near-infrared (UV–vis-NIR) spectroscopy (Cary 5000 UV–vis-NIR, Agilent Technologies Inc, Santa Clara, CA, USA). The transmittance of the substrates was excluded in all our data. The optical images were taken using an optical microscope (BX 53, Olympus, Tokyo, Japan). The morphologies of the fabricated films were characterized using field emission scanning electron microcopy (FE-SEM) (S-4800, Hitachi, Seoul, South Korea) and atomic force microscope (AFM) (D3100, Veeco, Plainview, NY, USA).
Electrical and optical properties of hybrid films
Bendability of the hybrid structures
Stretchability of the hybrid structures
The effect of pattern size on electrical properties
In addition, the robustness of conductive materials that can withstand electrical load becomes an important feature for next-generation electronics, as devices are highly integrated and miniaturized. This is because the electric field is inversely proportional to the length of pattern. To assess and compare the robustness of nanostructured films against electrical loads, patterned samples of graphene, AgNWs, and their hybrid structures as channels with a width of 10 and 20 μm were prepared on a metal electrode deposited onto a Si wafer (with 300-nm-thick SiO2). Figure 6f shows an optical microscope image for one of the fabricated devices with the graphene-AgNW hybrid structures as a channel (blue area in Figure 6f) and Cr/Au (2 nm/300 nm) as S/D electrodes (yellow area in Figure 6f). Here, the width and length of hybrid channel are 20 and 50 μm, respectively. To compare the condition of channel materials before and after electrical breakdown, we presented the optical images of prepared devices with the hybrid channel with lengths that varied from 5 to 50 μm after the breakdown test in Figure 6g. In contrast to Figure 6f, all the channels with different lengths in Figure 6g were disconnected at the left end of the channels. In other words, the electrical breakdown occurred near one of the contact points between the hybrid channel and metal electrodes. The breakdown phenomena of hybrid structures are opposite to those of graphene where the breakdown point is nearly at the center of the channel due to Joule heating and are similar to those of AgNW. To inspect the electrical breakdown of the three nanomaterials, the I-V characteristics of graphene, AgNWs, and the hybrid structures with various channel lengths (L) were measured in an ambient condition. As one of the results, the top graph of Figure 6h describes breakdown behaviors of the three materials, when the length and width of channels were at 50 and 10 μm, respectively. In order to clearly distinguish the results between the three kinds of materials, magnified y-axis graphs are presented together at the bottom of Figure 6h. The slopes in the I-V curves indicate electrical conductance, and the narrow pattern size (channel length = 10 μm) caused the relatively high resistance of AgNW networks, compared to the resistance of the hybrid structures, because only small portions of the conductive pathways along the NWs remained inside the patterns. For all the three cases, currents increase almost linearly with the applied voltage before their breakdown. Based on these results, we compared breakdown bias (V breakdown) according to various L of these three different materials. In the case of AgNW networks that can be degraded by electromigration , electrical breakdown occurs at a relatively low voltage (V breakdown < approximately 4 V) for a channel length of approximately ≥30 μm (average length of AgNWs) due to the large aspect ratio of NWs and NW-NW contact resistances. On the other hand, the V breakdown of graphene is higher than that of AgNW networks. This polycrystalline graphene can be damaged by Joule heating and oxidation at the defect sites , and V breakdown of graphene also decreases when the channel length falls below the threshold related to the grain sizes. In the case of graphene-AgNW hybrid nanostructures, the graphene underneath AgNWs can create a path to dissipate heat and electrical stress, which can make AgNWs sustainable against breakdown at relatively high electric bias. Therefore, the hybrid structures exhibit the highest values of V breakdown among the three materials (graphene, AgNW networks, and the hybrid). Also, the hybrid electrode can flow the highest current against its breakdown due to its low resistance. The hybridization of the graphene and AgNWs give the hybrid structures strong endurance against electrical load, so this improved robustness is another advantage of the hybrid electrode.
The results presented here demonstrate the superior characteristics of graphene-AgNW hybrid nanostructures as flexible, stretchable, and transparent electrodes. From the fundamental study, we confirmed that the hybridization between AgNWs and CVD synthesized graphene could significantly enhance electrical properties such as low R s with negligible degradation of optical transparency and has strong robustness against electrical breakdown. In the reliability test, the hybrid structures preserve their outstanding properties against thermal oxidation due to a graphene passivation layer covering AgNW networks. Furthermore, our hybrid electrode presents superb mechanical flexibility and stretchability such as complete folding in half (27% in bending strain), 10,000 times in stretching cycles with 100% tensile strain toward the uniaxial direction, and 30% stretching in tensile strain toward the multi-axial direction. We believe that the hybridization with two-dimensional and one-dimensional nanomaterials presents a promising strategy toward stretchable, wearable electronics and implantable biosensor devices.
This work was supported by the Ministry of Science, ICT & Future Planning and the Ministry of Trade, Industry and Energy of Korea through Basic Science Research Program of National Research Foundation (2013R1A2A2A01068542), IT R&D program (10041416), Materials Original Technology Program (10041222), Technology Innovation Program (Grant 10044410), Convergence Technology Development Program for Bionic Arm (NRF-2014M3C1B2048198), and Pioneer Research Center Program (NRF-2014M3C1A3001208).
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