We start with the demonstration of the high quality single layer graphene synthesis on commercially available ultra-smooth Cu foils. In order to compare the surface quality, we used relatively rough Alfa Aesar Cu foil which is commonly used for CVD of graphene (Fig. 1(a–c)) and commercially available ultra-smooth surface Cu foils (Mitsui mining and smelting co., LTD, B1-SBS) (Fig. 1(d–f)) in the same CVD chamber. Deep trenches on the surface of rough copper foils are clearly seen in both optical microscope (Fig. 1(a)) and scanning electron microscope images. (Fig. 1(b,c)). On the other hand, commercially available smooth copper foils show uniform surface properties (Fig. 1(d–f)). We investigated the surface topography of the ultra-smooth Cu foils via scanning electron microscopy (SEM) and atomic force microscopy (AFM). The root mean square (RMS) surface roughness of the ultra-smooth Cu is around 100 nm before annealing which is two times lower than the value reported for the Cu foils commonly used in graphene growth30.

Figure 1 Effects of Cu surface morphology on graphene growth: (a) The optical microscope image of rough Cu foil (Alfa Aesar Cu foil) commonly used in graphene growth. Deep scratches on the rough Cu foil due to the rolling process are clearly visible (b) Scanning electron microscopy (SEM) image of the rough Cu surface. (c) The magnified SEM image of graphene flakes on rough Cu foil surface. The growth was terminated after 10 seconds to obtain dispersed graphene flakes. (d) Optical microscope image of commercially available ultra-smooth Cu foil (Mitsui mining and smelting co. LTD., B1-SBS) and (e) SEM image of ultra-smooth Cu surface. (f) Ultra-smooth Cu surfaces with graphene flakes. The density and shape of the graphene flakes are different on the rough and smooth foils. The sizes of individual grains along the Cu surface are clearly visible. Full size image

By sending the methane gas into the chamber with different intervals, we obtain both the graphene flakes (Fig. 2(a,b)) and full coverage graphene film (Fig. 2(c)) on the ultra-smooth Cu foils. Figure 2(a) shows the SEM image of Cu surface partially covered by graphene flakes. The average size of the flakes are ~1100 μm2 in the form of a snow-flake (Fig. 2(b)). Also we observed that the flake size and shape are directly related to the ratio of flow rate and partial pressure of hydrogen to methane gases, as reported in previous work31.

Figure 2 Graphene flake formation at early stages of growth: (a,b) SEM images of the surface of the ultra-smooth Cu partially covered by graphene flakes. To achieve ~1100 μm2 flakes, methane gas was flushed into chamber for 5 seconds then left for cooling to room temperature. Smoothness of the surface and size of the grain boundaries of the Cu are clearly visible. (c) AFM image showing the surface topography of full graphene layer on the ultra-smooth Cu foil. Buckling on the graphene surface is clearly visible. Full size image

As a next step, the partial pressure and flow of the gases were set to have full coverage single layer graphene on both the ultra-smooth Cu foils and rough copper foils. We followed the similar growth conditions as in ref. 22. According to our observations, the formation of continuous graphene layer is initiated with single crystal graphene flakes. Increasing the growth time, under the same methane flow rate and partial pressure, results in the formation of continuous graphene layer. Moreover, the synthesized graphene flakes and continuous graphene layers includes ripples (Fig. 2(c)) because of the physical instability of perfectly flat graphene layer32,33,34,35.

To investigate the effect of the surface roughness on the quality of the graphene layer, both the ultra-smooth Cu and the rough Cu foils were placed in the same growth chamber so as to expose them to the same growth conditions. Once the graphene layers were formed on the both the smooth and the rough Cu surfaces, we transferred them to 100-nm-thick SiO 2 coated Si wafers for structural characterization and transistor applications. (Supplementary Fig. S1)

From now on, we will denote the graphene synthesized on commercially available ultra-smooth Cu foils as “smooth-Cu-graphene” and the graphene synthesized on standard rough copper foils as “rough-Cu-graphene”. To present a structural comparison, we performed Raman mapping for both the smooth-Cu-graphene and rough-Cu-graphene. Figure 3(a) shows the optical microscope image of rough-Cu-graphene transferred on SiO 2 /Si. Rough-Cu-graphene includes longitudinal cracks along the graphene layer. These cracks are the graphene-free areas and resulted from the deep trenches of the rough Cu surface. Graphene synthesized on these trenches are not compatible to transfer techniques since they are formed in different height with respect to overall surface and when transferred to solid substrates these areas remain without graphene. Figure 3(b) shows the Raman mapping of the rough-Cu-graphene. Longitudinal cracks on the surface appears as low intensity black areas. Figure 3(c) shows the comparison of Raman spectrum taken from the different parts of rough-Cu-graphene including both the cracks and the full coverage graphene. Due to lacking of graphene in the cracked areas, Raman fingerprints of graphene such as G band and 2D band can hardly be detected in these areas and the Raman intensity is relatively low when compared to full coverage graphene areas. Figure 3(d) shows the optical microscope image of the smooth-Cu-graphene. After transferring to dielectric surfaces we didn’t observe any cracks on the smooth-Cu-graphene. We performed the Raman mapping in same conditions for smooth-Cu-graphene (Fig. 3(e)) and we obtain a stable Raman signal throughout the whole scanning area (25 μm x 25 μm) due to high surface coverage of the smooth-Cu-graphene. Figure 3(f) shows the Raman signal taken from the smooth-Cu-graphene surface. The Raman fingerprints of the single layer graphene, namely the peaks of G band and 2D band are clearly seen in the spectrum. We observed the G peak at 1587 nm with the full width at half maximum (FWHM) of ~23. The FWHM of 2D peak at 2658 cm−1 is ~40 and G/2D ratio is ~1.4.

Figure 3 Raman investigation of graphene synthesized on rough and smooth copper foils: (a) The optical microscope image of rough-Cu-graphene on SiO2/Si. Formation of the cracks on the graphene layer is a due to the deep trenches of rough-surface Cu foil. (b) Raman mapping of rough-Cu-graphene transferred on SiO2/Si. Cracked and graphene free areas appear as low intensity dark areas in Raman mapping image. (c) Raman spectrum of rough-Cu-graphene taken from fully graphene coated and graphene-free cracked areas. The G and 2D bands are hardly detectable in the cracked areas and the Raman intensity is very low when compared to the Raman signal taken from full coverage graphene area. (d) Optical microscope image of smooth-Cu-graphene transferred on SiO2/Si. In contrast to rough-Cu-graphene there is no graphene-free areas throughout the 1 cm2 area. (e) Raman mapping of smooth-Cu-graphene. Due to high surface coverage of the graphene layer we didn’t observe any low intensity graphene-free areas in Raman mapping. (f) Raman spectrum of smooth-Cu-graphene. We obtain a uniform Raman signal from the smooth-Cu-graphene throughout the 25 μm × 25 μm scan area. Full size image

To compare the electronic properties, we fabricated the graphene field effect transistors based on smooth and rough-Cu-graphene. Schematic drawing of the fabricated back-gated transistors are given in Fig. 4(a). Optical microscope images of the devices using smooth and rough-Cu-graphene are given in Fig. 4(b,c) respectively. The graphene-free spots can also cause variations in charge carrier transport and optical properties. This means the sensors in an array (e.g. active matrix display) or electronic devices in a circuit over large area graphene are likely to have non-uniform response. The transfer characteristics of the fabricated graphene transistors using rough and smooth-Cu-graphene are given in Fig. 4(d) and (e) respectively. We recorded the drain current as a function of gate voltage for six different devices at constant drain voltage. The variation of on-current (Fig. 4(d)) is significantly larger for the FETs using rough-Cu-graphene due to the non-uniform graphene formed on rough Cu foils. The calculated field effect mobility of the devices (Fig. 4(f)) scales with the channel length (See Supplementary Fig. S2 for device statistics). As it seems from the scattered plot, for longer channel lengths where the graphene uniformity starts to play critical role on the transport through the channel, field effect mobility values of the devices using smooth-Cu-graphene are enhanced compared to rough-Cu-graphene. This electrical transport measurements over large graphene areas simply shows the effects of usage of smooth Cu foils in CVD synthesis.

Figure 4 Transport properties of fabricated graphene based transistors: (a) Schematic drawing of the fabricated graphene field effect transistor. The transistors use 100 nm thick SiO 2 as a dielectric and highly doped Si as a back gate electrode. The source (S) and drain (D) electrodes are 50 nm thick thermally evaporated gold. (b,c) Optical microscope images of the fabricated transistors based smooth-Cu-graphene and rough-Cu-graphene respectively. The deep scratches on the Cu foils results in visible cracks in transferred graphene. (d,e) Transfer curves of the six different transistors with a channel length (L c ) of 64 μm, based on the smooth-Cu-graphene and rough-Cu-graphene respectively. Drain voltage (V d ) was kept constant at 1 V during the measurement. The transistors based on rough-Cu-graphene, show significantly larger variation in the on-current values. (f) The channel length (L c ) scaling of the field effect mobility of the transistors. The scattered plot shows the averaged value of the field effect mobility of 10 identical transistors based on the smooth-Cu-graphene and rough-Cu-graphene. Full size image

After the electrical measurements, to examine the optical properties of smooth-Cu-graphene we performed a large area optical scan. Figure 5(a) shows the schematic illustration for the experimental setup. We used 635 nm diode laser as a light source and with the help of motorized stage we recorded the transmittance on 5 cm x 5 cm graphene area. Figure 5(b) shows the large area graphene (~400 cm2) on flexible, 125 μm thick PVC substrate. After the CVD synthesis on ultra-smooth Cu foils, we transferred the graphene to the flexible PVC substrates by using hot lamination method. During the Cu etching, we protected the Cu foils at the edges of the sample to use them as contact electrodes for resistivity measurements and application of bias voltage. The measured total resistance of graphene is superposition of contact resistance and the sheet resistance terms ( ). By using transfer length method (TLM) we extracted the sheet resistance of smooth-Cu-graphene as ~2.68 kΩ and contact resistance value as ~0.4 kΩ (Supplementary Fig. S3). Figure 5(c) shows the transmittance measurement of the smooth-Cu-graphene on glass substrates by using conventional spectrometer. Theoretically, free-standing single layer graphene absorbs 2.3% of the incident light. We observed the transmittance of smooth-Cu-graphene on glass substrate is changing from 96% to 97.5% going from UV to near-infrared wavelengths. This is due to the substrate effect during transmittance measurement throughout the broad wavelength range. To check the optical uniformity in large area graphene holding PVC substrate we used the experimental setup given in Fig. 5(a) and mapped the transmittance behaviour in x and y directions. Figure 5(d) shows the transmittance mapping of graphene-free PVC substrate at 635 nm. Due to the scattering on the PVC surface, we observe a small deviation (0.5%) from a fully transparent state. Then we scanned the transmittance of smooth-Cu-graphene transferred on PVC. Figure 5(e) shows the transmittance mapping of smooth-Cu-graphene on PVC. We recorded the absorption of graphene which is stable throughout the scan area and around 1.6%. The difference between the theoretical value (2.3%) for free standing graphene and our result is likely due to the effect of PVC substrate to the measurement. Since the graphene layer is lying on the PVC surface, we also observed the same small deviation resulting from the scattering on the PVC surface in the large area optical scan for graphene. Figure 5(f) shows the histogram for the transmittance mapping of graphene-free PVC and smooth-Cu-graphene on PVC. The transmittance difference due to small optical absorption of graphene and the deviations regarding the PVC surface can be easily seen.

Figure 5 Optical characterization for the flexible graphene electrodes. (a) The schematic illustration of the experimental setup. Graphene holding PVC substrates are placed on a motorized stage for the large area optical scan in x and y directions. We used 635 nm diode laser as a light source and recorded the signal changes in the photodetector. (b) Photograph of the graphene transferred on PVC substrates by using hot lamination method. During the Cu etching process, we protected the Cu lines at the edges of the sample as an electrical contact pads. (c) Transmittance spectra of single layer graphene on PVC substrate. As a reference measurement we used the laminated PVC substrate without graphene. The spot size in the measurement is 2 mm. (d) Large area optical scan for a PVC substrate without graphene. We first scanned the reference substrate in x and y directions for 25 cm2 area and mapped the normalized transmittance at 635 nm. Due to scattering on the plain PVC surface there are small deviations in transmittance up to ± 0.5%. (e) Large area optical scan for a graphene holding PVC substrate. After we transferred smooth-Cu-graphene on PVC, we performed the same large area optical scan and recorded the normalized transmittance and compared with the reference substrate. The transmittance of the single layer graphene is around 1.6% and the total deviation on the 25 cm2 graphene area is recorded as ± 0.5% likely due to the scattering from the PVC surface. (f) Histogram plot for the large area optical scan of PVC with and without graphene. While the average transmittance of the plain PVC is around 1, the same value for PVC with smooth-cu-graphene is 0.984 due to the small optical absorption single layer graphene defined by the fundamental constants. Full size image

Previously we showed that the supercapacitor device structure can provide an efficient electrostatic doping of graphene which cannot be achieved by graphene based transistors using solid insulating dielectric layers29. To test and compare the effects of rough and smooth-Cu-graphene in large area electro-optic devices, we fabricated 20 cm × 20 cm graphene supercapacitors by using large area flexible graphene electrodes. Figure 6(a) shows the photograph of large area graphene based flexible supercapacitor. We formed the supercapacitor structure by putting the top and bottom graphene holding flexible PVC electrodes together and filling the gap with ionic liquid electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate). The inset shows the previously reported rigid device having ~2.5 cm2 graphene area. Before forming the supercapacitor structure, ionic liquid electrolyte was soaked into optical cleaning tissue to prevent the electrical shortage between top and bottom graphene electrodes which is due to the sagging of the large area flexible substrate. Figure 6(b,c) show the transmittance modulation of smooth and rough-Cu-graphene based supercapacitors which are operating as voltage controlled optical modulators. To avoid the scattering and absorption effects of PVC we used the unbiased device as reference, then recorded the transmittance changings with the applied bias voltage. Increasing the bias voltage causes the ions inside the electrolyte to form a double layer in the graphene electrolyte interface and the created electric field effectively dopes the graphene due to the extremely small separation between the graphene and the ion accumulation layer. Electrostatic doping of charge carriers in graphene shifts the Fermi level and this causes the inter-band transitions to be blocked by Pauli blocking. For Pauli blocking to take place, one should shift the Fermi level (E F ) above the half of the incoming photon energy. This blocks the inter-band transitions and makes the graphene more transparent by blocking optical absorption mechanism of the material. By blocking the inter-band transitions, we achieved the light modulation over a broad range of wavelengths with the large area graphene supercapacitors. We achieve more efficient transmittance modulation in the case of smooth-Cu-graphene based supercapacitors due to the lower sheet resistance and high coverage of the smooth-Cu-graphene film. Figure 6(d) shows the transmittance versus voltage plot for both rough and smooth-Cu-graphene based supercapacitors at 700 nm. Transmittance modulation at single wavelength gives clear picture about the efficiency of using smooth-Cu-graphene in large area optoelectronic devices. Maximum modulation obtained from rough-Cu- graphene based supercapacitor is 1.16% at 4.4 V while the smooth-Cu-graphene based supercapacitor provides 2.35%. The transmittance spectra shown in Fig. 6(b,c) is a spectral distribution of the change of the transmission and it has a step-like behavior with transition centre at 2E F . Therefore we were able to extract the Fermi energy values corresponding to each curve. Figure 6(e) shows the Fermi energy level shift of the rough and smooth-Cu-graphene with respect to applied bias voltage. According to our observation, maximum Fermi level shift obtained by using rough-Cu-graphene is 0.89 eV while for the smooth-Cu-graphene it is 1.13 eV at 4.4 V. The small Fermi level difference between the rough-Cu-graphene and smooth-Cu-graphene (max. 6 meV) at lower voltages is due to the unintentional doping of graphene during the Cu etching and transfer processes.

Figure 6 Electro-optic response of large area flexible graphene supercapacitors using rough and smooth-Cu-graphene. (a) Photograph of ~400 cm2 flexible graphene supercapacitor. Inset shows the previously reported rigid supercapacitor having ~2.5 cm2 graphene area. (b,c) Transmittance modulation of smooth-Cu-graphene and rough-Cu-graphene based supercapacitors respectively. Increasing the bias voltage shifts the Fermi level of graphene and blocks the optical absorption mechanism of the material. Smooth-Cu-graphene provides more profound light modulation especially in the higher voltage. (d) Transmittance versus voltage plot at 700 nm for supercapacitors using both rough and smooth-Cu-graphene. Supercapacitor using rough-Cu-graphene shows less efficient modulation when compared to the one using smooth-Cu-graphene (e) Extracted Fermi energy levels with respect to applied bias voltages. Due to lower sheet resistance and uniformity of smooth-Cu-graphene, it gives more efficient Fermi level shift with the applied bias voltage. (f) Resistance and capacitance modulation with respect to applied bias voltage. Electrostatic doping of graphene changes the resistance and quantum capacitance of top and bottom graphene electrodes by increasing charge carrier concentration. Full size image

Here the limiting factor for the bias voltage is the electrochemical window of the electrolyte used in the supercapacitor structure. Since our devices use mutual gating between top and bottom graphene electrodes, they are limited with the two times of the electrochemical window of the electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate) which is around 4.5 V. In mutual gating; the Dirac points of the top and bottom graphene shifts with respect to each other leading to two different dips in the capacitance modulation. Figure 6(f) shows the capacitance and resistance modulation of the graphene supercapacitor with respect to the applied bias voltage. As the voltage is increased to both polarizations, we observed a dramatic capacitance and resistance change due to the electrostatic doping of the top and bottom graphene layers. Detailed explanation of the physics behind this observations can be found in our previous work29.

As a result, we obtained a more efficient transmittance modulation up to two times (at 700 nm) by using smooth-Cu-graphene in supercapacitors operating as optical modulators. We also observed 27% more efficient Fermi level shift compared to rough-Cu-foils (at 4.4 V). By using electrolytes with broader electrochemical windows, the enhancement in transmittance and Fermi energy shift by using smooth-Cu-graphene can be further demonstrated in optoelectronic devices.