Presentation on theme: "1 Graphene-based transparent conductive thin films prepared using surface wave plasma CVD[12-4] A low-temperature process is particularly essential for."— Presentation transcript:
1 Graphene-based transparent conductive thin films prepared using surface wave plasma CVD[12-4] A low-temperature process is particularly essential for the Si-based device industry and would pave the way for the direct synthesis of graphene-based films onto glass and plastic substrates. Two methods for synthesizing graphene-based films have been previously proposed; the thermal chemical vapor deposition CVD method on metal surfaces and the chemical reduction method of graphite oxides. the former method is restricted to a high deposition temperature limit of around 1000 °C and the latter method requires time consuming procedures and complicated liquid waste treatment. The large-area surface wave plasma SWP CVD apparatus with a plasma area of 40 x 60 cm2 by adopting an array configuration of microwave slot antennas for synthesizing carbon nanomaterials was proposed.
2 The original large-area SWP operated at gas pressures less than 5 Pa was performed. Such a low-pressure process allows to keep a substrate temperature lower due to low neutral gas temperatures and also uniform CVD growth over large areas due to the enhanced diffusion of plasma. SWP is able to provide high- density plasmas and radicals and has relatively low electron temperature below 2 eV and also low plasma space potential below 10 V in bulk region even at the low gas pressures. As a result SWP can reduce ion bombardment on the substrate surface. Fig. 3. Corona and dielectric barrier discharge (DBD) arrangements. (a) Corona with streamers; (b) dielectric barriers on electrodes; (c) DBD arrangement for treatment of large-area planar substrates; (d) DBD arrangement for two-sided treatments; and (e) multiple grid-type DBD arrangement (200 μm×200 μm windows). The arrows in (c), (d), and (e) indicate the direction of substrate motion.
3 The polycrystalline Cu and Al foils were treated to clean their surfaces by using Ar/H2 plasmas at 5 Pa for 20 min before the CVD process. After the treatment, we turned off the microwave generators, evacuated the discharge chamber to base pressure of 10−3 Pa, introduced a reaction gas mixture into the discharge chamber. We then performed CVD using these foils as substrates at 3 or 5 Pa with microwave powers of 3 to 4.5 kW for each microwave generator for various growth times ranging from 30 to 180 s. The formation of the graphene-based films using CH4 gas mixed with Ar and/or H2 was examined. Although graphene can be grown using CH4 alone in our plasma CVD process, the addition of Ar improves the stability and uniformity of the plasma at low gas pressure. The addition of H2 is expected to improve the quality of the films because hydrogen is known to selectively etch amorphous carbon defects. The composition CH4 /Ar=30/10 SCCM and CH4 /Ar/H2=30/20/10 SCCM standard cubic centimeters per minute in this experiment. The reaction gas mixtures enable the synthesis of graphene-based films on Cu and Al foils at substrate temperatures lower than 400 °C.
4 FIG. 1. A Raman spectrum of a typical graphene-based film deposited on Cu foil CVD conditions: 5 Pa, CH4 /Ar/H2=30/20/10 SCCM, b Raman spectrum of a graphene-based film deposited on Al foil CVD conditions: 3 Pa, CH4 /Ar/H2 =30/20/10 SCCM, c Substrate temperature profile during the Ar/H2 plasma treatment and the plasma CVD for the synthesis of the film on Al foil shown in Fig. 1b. The high-intensity D peak and low-intensity D shoulder in the Raman spectrum shown in Fig. 1a are attributed to the edges and the boundaries of the flakes, which suggests that the film contains submicrometer sized flakes of graphene- based materials. Figure 1a suggests that Raman peaks are assigned to the peaks of few-layer graphene in the deposited film.
5 (a) Optical transmittance of a graphene-based film transferred on a glass plate. The inset shows a picture of the film with 81% transparency. (b) Plot of sheet resistance vs optical transmittance average between 400 and 800 nm of the films obtained by SWP-CVD with various CVD conditions. FIG. 3. Color online Synthesis of large-area graphene- based films with an area of 23 x 20 cm using SWP-CVD CVD conditions: 3 Pa, CH4 /Ar/H2=30/20/10 SCCM, 4.5 kW per a MW generator. (a) Cu film after plasma CVD. (b) Transferred film on an acrylic plate deposited for 180 s. (c) Transferred film on an acrylic plate deposited for 90 s. (d) Spatial distribution of the sheet resistance of the graphene-based film transferred on an acrylic plate shown in Fig. 3b.
6 12.2 Large area flexible transparent conductive thin films produced through Langmuir-Blodgett Assembly A LB film contains one or more monolayers of an organic material, deposited from the surface of liquid onto a solid by immersing the solid substrate into (or from) the liquid. A monolayer is adsobbed homogeneously with each immersion or emersion step. GO sheets are strongly hydrophilic and can produce stable and homogeneous colloidal suspensions in aqueous and various polar organic solvents due to the electrostatic repulsion between the negatively charged GO sheets. Before the LB deposition, the as-prepared GO dispersion was screened to separate the dispersions containing the smallest and the largest groups of GO sheets, namely, the small graphene oxide (S-GO, several m 2 ) and ultra large graphene oxide (UL-GO, 1~10000 m 2 ) dispersions.
7 The work involves pre-exfoliation of natural graphite flakes to produce gram quantities of ultral arge graphene oxide (UL-GO) sheets, up to ∼ 50~200 μm in lateral size with a yield exceeding 50% by weight. The LB assembly technique is then used to transfer the GO monolayers onto substrates and produce highly conducting transparent LB thin films. After thermal reduction and chemical doping treatments, the transparent conductor made from the UL-GO sheets shows a sheet resistance of 459 Ω/sq at a transmittance of 90% along with a remarkable σ DC /σ OP ratio of 7.29. where Z 0 =377 ohm is the impedance of free space. A high ratio represents a high transmittance and a low sheet resistance, and thus high opto-electrical properties of transparent conductors,
8 Parameters affect LB films. Surface pressure The typical surface pressure-area isotherm shown in Figure 3a presents the change in the slope corresponding to the phase transition of GO sheets from gas to condensed liquid and to solid state. There existed an initial gas phase where the surface pressure remained essentially constant (stage a). The pressure began to increase as the compression continued (stage b) and the GO sheets were about to touch one another, tiling( 貼磚 ) over the entire surface. The increase in surface pressure was likely due to the electrostatic repulsion between the GO sheets. A further increase in surface pressure followed (stage c) when the monolayer was compressed beyond the close-packed stage. This occurred because the GO sheets started to fold at the touching points along their edges instead of overlapping on top of another. At a higher pressure (stage d), partial overlapping and wrinkling happened, leading to a nearly complete monolayer of interlocked GO.
9 The LB transfer of flat GO sheets is a self-assembly process whose quality depends largely on the evaporation of water molecules present between UL-GO sheets and substrate. The relatively small size of S-GO allowed water to evaporate easily, and thus wrinkle-free S-GO films were obtained (Figure 4). However, the water droplets are often trapped between the UL-GO sheets, and the capillary force induced by water evaporation causes wrinkling of the sheets. In particular, if a high pulling speed is applied, there would be too short a time for the UL-GO sheets to relax into a flat state and transfer onto the substrate. In this case, water may not be fully evaporated, causing wrinkling to occur during the transfer process due to the capillary force and gravity. Pulling rate and evaporation rate Figure 4. SEM images of S-GO sheets collected on a silicon substrate at different stages of surface pressure (see Figure 3a). The packaging density increases from (a) isolated S-GO sheets to (b) close-packed S-GO sheets, (e) over packed S-GO sheets with folded edges, and (f) over packed S- GO sheets with overlapping.
10 Figure 5. SEM images of UL-GO sheets collected on a silicon wafer at different stages of surface pressure at a pulling speed of 0.1 mm/min. The packaging density increases from (a) isolated UL-GO sheets to (b) close-packed UL-GO sheets, (c) overlapped UL-GO sheets with some wrinkles, and (d) overlapped UL-GO sheets with extensive wrinkles.
11 Surface morphology When the second GO layer was deposited on top of the first layer, these two layers were likely to experience both electrostatic repulsion and van der Waals attraction. Since the GO sheets are brought together on top of another, their van der Waals potential can be scaled with (1/d 2 ). The residual π-conjugated domains can also contribute to the attraction between GO sheets. While these attractive forces dominate and lead to successful layer-by-layer deposition of GO sheets, the GO sheets also experience electrostatic repulsion from both their neighbors and those already deposited, causing wrinkling to occur. Figure 6. AFM images of GO films consisting of two layers (a,b) and eight layers (c,d) of monolayer GO sheets taken before (a,c) and after thermal treatment (b,d). It is also worth noting that the surface roughness was consistently reduced after the thermal treatment due to the removal of oxygenated functional groups and graphitization of the films at an elevated temperature.
12 Comparison of optical and electrical properties between ULGO films with different number of layers obtained at different stages of treatment is presented in Figure 8. A thicker film resulted in a higher degree of absorption of light and thus a lower transparency at all treatment stages studied. The transparency was significantly deteriorated after the thermal treatment, whereas part of the lost transparency was restored after the chemical treatments (Figure 8a,b).The films darkened after the thermal treatment due to the reduction of GO and the adsorption of impurity particles on the other side of quartz substrates, which was an artifact. The removal of these impurities by the subsequent acid treatment contributed to the improvement of transparency. Optical Transmittance and Electrical Conductivity. Figure 8. Comparison of optical and electrical properties between UL-GO films of different number of layers taken at different stages of treatment: (a) transmittance measured at 550 nm wavelength; (b) transmittance spectra of C-rUL-GO films as a function of wavelength; and (c) sheet resistance at different stages.
13 (f) Figure 9. (a,c) Raman spectra for natural graphite, UL-GO, rUL-GO, and C-rUL-GO; and (b,d) the corresponding D/G and 2D/(D+G) intensity ratios. Schematic illustrations of (e) molecular structure of SOCl2 molecules absorbed onto graphene surface; and (f) chemical structure of graphene sheet after chemical doping treatments.
14 Besides the G- and D-bands, there are two other Raman bands, called 2D and D+G at 2600-3000 cm -1 (Figure 9c), which are often ignored due to their weak intensities compared to D- and G-bands. The 2D-band is Raman-active for crystalline graphitic materials and is sensitive to the π-band in the graphitic electronic structure, while the combination mode of D+G is induced by disorder.43 The intensity ratio I 2D /I D+G shown in Figure 9d indicates that the I 2D /I D+G ratio was more sensitive to the change in electronic conjugation from UL-GO to rUL-GO than the I D /I G ID/IG ratio (Figure 9b), as a reflection of the recovery of graphitic electronic conjugation. The reduction of the I 2D /I D+G ratio corresponding to the modification from rUL-GO to C-rULGO indicates that the newly doped functional groups, such as -Cl, -SOCl, and -COOH, introduced disorder again.
15 12.2 Large area flexible transparent conductive thin films produced through the roll- to-roll method [12-6, 12-7] There are three essential steps in the roll-to-roll transfer (Fig. 1a): (i)adhesion of polymer supports to the graphene on the copper foil; (ii)etching of the copper layers; (iii)release of the graphene layers and transfer onto a target substrate. In the adhesion step, the graphene film, grown on a copper foil, is attached to a thin polymer film coated with an adhesive layer by passing between two rollers. In the subsequent step, the copper layers are removed by electrochemical reaction with aqueous 0.1 M ammonium persulphate solution (NH 4 ) 2 S 2 O 8. Finally, the graphene films are transferred from the polymer support onto a target substrate by removing the adhesive force holding the graphene films.
16 In the first step of synthesis, the roll of copper foil is inserted into a tubular quartz tube and then heated to 1,000C with flowing 8 sccm. H 2 at 90 mtorr. After reaching 1,000C, the sample is annealed for 30 min without changing the flow rate or pressure. The copper foils are heat-treated to increase the grain size from a few micrometers to 100 m, as we have found that the copper foils with larger grain size yield higher-quality graphene films. The gas mixture of CH 4 and H 2 is then flowed at 460 mtorr with rates of 24 and 8 sccm. for 30 min, respectively. Finally, the sample is rapidly cooled to R.T. (~10 C/s) with flowing H 2 under a pressure of 90 mtorr.
17 After growth, the graphene film grown on copper foil is attached to a thermal release tape by applying soft pressure (0.2 MPa) between two rollers. After etching the copper foil in a plastic bath filled with copper etchant, the transferred graphene film on the tape is rinsed with deionized water to remove residual etchant, and is then ready to be transferred to any kind of flat or curved surface on demand. The graphene film on the thermal release tape is inserted between the rollers together with a target substrate and exposed to mild heat (~90–120C), achieving a transfer rate of ~150–200 mm min -1 and resulting in the transfer of the graphene films from the tape to the target substrate (Fig. 2b). By repeating these steps on the same substrate, multilayered graphene films can be prepared that exhibit enhanced electrical and optical properties, As the graphene layers are transferred one after another, the intensities of the G- and 2D- band peaks increase together, but their ratios do not change significantly. This is because the hexagonal lattices of the upper and lower layers are randomly oriented, unlike in graphite, so the original properties of each monolayer remain unchanged, even after stacking into multilayers; this is clearly different from the case of multilayer graphene exfoliated from graphite crystals.
18 The randomly stacked layers behave independently without significant change in the electronic band structures, and the overall conductivity of the graphene films appears to be proportional to the number of stacked layers. The optical transmittance is usually reduced by 2.2–2.3% for an additional transfer, implying that the average thickness is approximately a monolayer The unique electronic band structure of graphene allows modulation of the charge carrier concentrations in dependence on an electric field induced by gate bias or chemical doping, resulting in enhancement of sheet resistance. We tried various types of chemical doping methods, and found that nitric acid (HNO 3 ) is very effective for p-doping of graphene films.
19 Figure 3c shows Raman spectra of the graphene films before and after doping with 63 wt% HNO3 for 5 min. The large peak shift ( =18 cm -1 ) indicates that the graphene film is strongly p-doped. The shifted G peak is often split near the randomly stacked bilayer islands. as shown in Fig. 3c. The lower graphene layer, which is screened by top layers, experiences a reduced doping effect, leading to G- band splitting. In X-ray photoelectron spectra (XPS), the C1s peaks corresponding to sp2 and sp3 hybridized states are shifted to lower energy, similar to the case for p-doped carbon nanotubes25. However, multilayer stacking results in blueshifted C1s peaks. We suppose that weak chemical bonding such as the p–p stacking interaction causes descreening of nucleus charges, leading to an overall increase in core electron binding energies. We also find that the work functions of graphene films as estimated by UV photoelectron spectroscopy (UPS) are blue-shifted by 130 meV with increasing doping time. (Fig. 3d, inset). The multiple stacking also changes the work functions which could be very important in controlling the efficiency of photovoltaic or light- emitting devices based on graphene transparent electrodes.
20 Usually, the sheet resistance of graphene film with 97.4% transmittance is as low as 125 Ω / □ (Fig. 4a) when it is transferred by a soluble polymer support such as polymethyl methacrylate (PMMA). In the process of roll-to-roll dry transfer, the first layer sometimes shows approximately two to three times larger sheet resistance than that of the PMMA- assisted wet-transfer method. As the number of layers increases, the resistance drops faster compared to the wet-transfer method (Fig. 4a).
21 Preparation of a 100-m-long graphene transparent conductive film [12-7] Owing to the small emissivity (e 0.04) and heat conduction of thin copper foil, a large area copper foil can be selectively heated to 1000 C in the vacuum chamber equipped with a conventional roll-to-roll system. A cold rolled copper foil (230mm wide, >100m long, 36 m thick, >99.9% pure) was enclosed in the vacuum chamber, in which the pressure was maintained at 1000 Pa under a continuous flow of CH4 (450 sccm) and H2 (50 sccm). The pressure in the chamber at a higher value than used in a typical low-pressure CVD synthesis was used to suppress the sublimation of copper at high temperature.
22 A high methane partial pressure of 900 Pa was used to achieve a high growth rate of graphene at a relatively low growth temperature. The copper foil was wound on the roll-to-roll system for 100 m at a velocity of 0.1 m/min. Owing to the precisely controlled handling of the copper foil and the small heat load to the roll- to-roll system. The first 52 m of graphene film was grown at temperature Tg~950C by applying a constant direct current J=82 A/mm 2 to the suspended copper foil, and the remaining 48 m of graphene film was grown at J=83 A/mm 2 (Tg~980C). Since Joule heating of a copper foil may result in a nonuniform temperature distribution, the spatial uniformity of the temperature within the copper foil was investigated by growing graphene films that only partially covered the copper foil surface, and then measured the coverage distribution [Fig. 2(a)]. After baking the copper foil at 180 C for 5 min, the graphene-covered regions can be observed with an optical microscope because of oxidation of the bare copper surface. Near the center of the foil, the graphene coverages were89% and 98% for J=82 and 83 A/mm 2, respectively.
23 Raman spectroscopy [Fig. 2(d)] revealed that the graphene film consisted of predominantly single-layer graphene. A scanning electron microscopy (SEM) image analysis [Fig. 2(b)] showed that each graphene grain is nearly hexagonal, and in many cases, a thicker multi-layer grain existed at the center. We observed more than 800 grains and found that multi-layer grains with a diameter larger than 100 nm covered ~5% of the film. The lower coverage near the edge, as shown in Fig. 2(c), suggests that there is a non-negligible temperature drop due to a larger heat loss at the edges. However, uniform graphene coverage was observed near the center of the copper foil, indicating that the temperature variation across the majority of the surface area of the copper foil is negligibly small. These results demonstrate that selective Joule heating is a reasonable heating technique for graphene CVD synthesis.