Fabrication of highly conducting and transparent graphene films

Graphene-based transparent conducting films were prepared using the following method. A chemically-reduced graphene dispersion was synthesized and graphene films were prepared from it by transfer printing, followed by thermal treatment. The resulting graphene films possessed an excellent electrical conductivity with a high transparency. A sheet resistance lower than ∼2 KΩ/sq and a transparency well over 80% were achieved at a typical wavelength of 550 nm. These properties are considered quite sufficient for many applications, such as transparent conductor films for touch panels.     

Fabrication of graphene films with high transparent conducting characteristics

We present a study on the transparent conducting characteristics of graphene-based films prepared by means of rapid chemical vapor deposition. The graphene films were grown on quartz slides with a CH₄/Ar mixed gas under a constant flow at 950°C and then annealed at 1,000°C. It was found that the graphene films present excellent electrical conductivity with high transparency. The conductivity is up to 1,240 S/cm, the sheet resistance is lower than 1 kΩ/sq, and the transparency is well over 85% in the visible wavelength range of 400 to 800 nm, showing that the graphene films have very low resistivity and superior transparency and completely satisfy the need for transparent conductors. These properties can be used in many applications, such as transparent conductor films for touch panels.

PACS

61.48.+c, 78.67.Pt, 68.37.Hk, 68.65.Ac     

Thermally stable and transparent F-doped SnO2 (FTO) /Ag/FTO films for transparent thin film heaters used in automobiles

We investigated the characteristics of F-doped SnO₂ (FTO)/Ag/FTO films prepared using thermal evaporation at room temperature for the application of the as-formed films in transparent thin film heaters (TFHs) of automobiles. To optimize the electrical and optical properties of the FTO/Ag/FTO multi-layer, the figure of merit (FoM) values of the FTO/Ag/FTO multi-layers were compared as a function of the thickness of the Ag and FTO layers. The sheet resistance and optical transmittance of the FTO/Ag/FTO multi-layer were primarily affected by the Ag inter-layer and bottom/top FTO thicknesses, respectively. At optimized Ag (10 nm) and FTO (40 nm) thicknesses, we fabricated a FTO/Ag/FTO electrode with a sheet resistance of 8.00 Ohm/square, an optical transmittance of 83.04 % at a visible wavelength (400–800 nm) and a FoM value of 19.49 Ohm^-1. The TFHs comprising the optimal FTO/Ag/FTO electrode exhibited a saturated temperature of 117 °C at a low operating direct current of 6 V, owing to the low sheet resistance. In addition, the FTO/Ag/FTO-based TFHs exhibited thermally stable performances owing to the stability of the bottom and top FTO electrodes. The performance of the FTO/Ag/FTO-based TFHs demonstrated that the thermally evaporated FTO/Ag/FTO multi-layer is a promising, stable, and transparent electrode material for application in the front window TFHs used in automobiles.     

Conductive and transparent multilayer films for low-temperature TiO2/Ag/SiO2 electrodes by E-beam evaporation with IAD

Conductive and transparent multilayer thin films consisting of three alternating layers (TiO₂/Ag/SiO₂, TAS) have been fabricated for applications as transparent conducting oxides. Metal oxide and metal layers were prepared by electron-beam evaporation with ion-assisted deposition, and the optical and electrical properties of the resulting films as well as their energy bounding characteristics and microstructures were carefully investigated. The optical properties of the obtained TAS material were compared with those of well-known transparent metal oxide glasses such as ZnO/Ag/ZnO, TiO₂/Ag/TiO₂, ZnO/Cu/ZnO, and ZnO/Al/ZnO. The weathering resistance of the TAS film was improved by using a protective SiO₂ film as the uppermost layer. The transmittance spectra and sheet resistance of the material were carefully measured and analyzed as a function of the layer thickness. By properly adjusting the thickness of the metal and dielectric films, a low sheet resistance of 6.5 ohm/sq and a high average transmittance of over 89% in the 400 to 700 nm wavelength regions were achieved. We found that the Ag layer played a significant role in determining the optical and electrical properties of this film.     

Controlled growth of carbon nanotube-graphene hybrid materials for flexible and transparent conductors and electron field emitters

We report a versatile synthetic process based on rapid heating and cooling chemical vapor deposition for the growth of carbon nanotube (CNT)-graphene hybrid materials where the thickness of graphene and density of CNTs are properly controlled. Graphene films are demonstrated as an efficient barrier layer for preventing poisoning of iron nanoparticles, which catalyze the growth of CNTs on copper substrates. Based on this method, the opto-electronic and field emission properties of graphene integrated with CNTs can be remarkably tailored. A graphene film exhibits a sheet resistance of 2.15 kΩ sq(-1) with a transmittance of 85.6% (at 550 nm), while a CNT-graphene hybrid film shows an improved sheet resistance of 420 Ω sq(-1) with an optical transmittance of 72.9%. Moreover, CNT-graphene films are demonstrated as effective electron field emitters with low turn-on and threshold electric fields of 2.9 and 3.3 V μm(-1), respectively. The development of CNT-graphene films with a wide range of tunable properties presented in this study shows promising applications in flexible opto-electronic, energy, and sensor devices.     

Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids

We report a simple but highly-effective hydrohalic acid reducing method to reduce graphene oxide (GO) films into highly conductive graphene films without destroying their integrity and flexibility at low temperature based on the nucleophilic substitution reaction. GO films reduced for 1 h at 100 °C in 55% hydroiodic (HI) acid have an electrical conductivity as high as 298 S/cm and a C/O ratio above 12, both of which are much higher than films reduced by other chemical methods. The reduction maintains good integrity and flexibility, and even improves the strength and ductility, of the original GO films. Based on this reducing method, a flexible graphene-based transparent conductive film with a sheet resistance of 1.6 kΩ/sq and 85% transparency was obtained, further verifying the advantage of HI acid reduction.     

Large-size graphene microsheets as a protective layer for transparent conductive silver nanowire film heaters

A new strategy is reported for the fabrication of silver nanowire (AgNW) film heaters using reduced small/or large-size graphene oxide (rSGO or rLGO) sheets as an over-coating protective layer. The results show that ultrathin rLGO microsheets provide the best combination of protective effect and electrical properties on AgNW networks and thus could enable the design of high-performance transparent film heaters. As a consequence, good optical transparency and electrical conductivity, good oxidation resistance and thermal stability, and good heating performances are achieved with as-made rLGO/AgNW film heaters. Specifically, the rLGO/AgNW hybrid film annealed at 700 °C shows a low sheet resistance of 27 Ω sq⁻¹ and a good optical transparency of 80%. Furthermore, it exhibits good heating characteristics and defrosting performance at low voltages. The results presented here may pave the way for a new promising application of rLGO/AgNW hybrid film in transparent film heaters and other electrical devices.     

壳聚糖/结冷胶双层膜制备工艺优化及表征

以壳聚糖（CS）和结冷胶（GG）为原料，采用溶液流延法并结合层层自组装技术制备了CS/GG双层膜，通过单因素和响应面试验优化制备工艺条件，并对双层膜的微观形貌、化学结构、力学性能、光学性能、阻水性能和抗氧化性能进行评价。结果表明，CS质量分数1.7 %、GG质量分数1.5 %、CS与GG成膜液体积比4/6、甘油质量分数25 %条件下双层膜的拉伸强度为38.83 MPa、透湿量为750.76 g/（m²·d）；与单层膜相比，CS/GG双层膜表面光滑、截面致密，红外光谱分析结果表明双层膜具有良好的相容性；双层膜可以有效改善纯CS膜、纯GG膜的力学性能和阻水性能。     

Tri-layer graphene films produced by mechanochemical exfoliation of graphite

We report a method for the production of transparent thin films composed in majority (59%) by tri-layer graphene, in which both the synthesis and processing are combined in one single step. Starting from graphite, two mechanical exfoliation processes are performed, followed by chemical/thermal exfoliation in a water/oil two phase system, which yields few layer graphene flakes spontaneously self-assembled at the water/oil interface as a thin film that is easily transferred to an arbitrary substrate.     

The production of flexible and transparent conductive films of carbon nanotube/graphene networks coordinated by divalent metal (Cu, Ca or Mg) ions

Divalent metal ions (M²⁺, M = Cu, Ca or Mg) are used to coordinate oxidized carbon nanotubes (CNTs) and chemically modified graphene (CMG) by the oxygen-containing functional groups. Flexible and transparent conductive films of M²⁺-coordinated CNT/CMG networks are produced by spin coating. The lowest sheet resistance is 680 Ω/sq at 86% transmittance. Chemical doping with SOCl₂ further decreases the value to 370 Ω/sq with only 1% loss in transmittance. The M²⁺-coordinated CNT/CMG films can withstand 200 bending cycles without losing much in sheet resistance. This method provides a new route to transparent electrode materials with high flexibility and strength.     

Improved electrical conductivity of a non-covalently dispersed graphene–carbon nanotube film by chemical p-type doping

Using a high-pressure air spray we developed a method to deposit electrically-conducting thin films consisting of non-covalently dispersed graphene and carbon nanotubes. The graphene–carbon nanotube film was immersed in a nitric acid and followed by exposure to fuming nitric acid. The acid treatment induced an increased concentration of atomic nitrogen on the graphene basal plane and carbon nanotube sidewall. This result indicates chemical p-type doping of the graphene oxide–carbon nanotube film. After the two acid treatments, the spray coated graphene oxide–carbon nanotube films on a glass substrate exhibit a low sheet resistance of 171 Ω/sq, and a high transmittance of 84% at a wavelength of 550 nm.     

High performance of carbon nanotubes/silver nanowires-PET hybrid flexible transparent conductive films via facile pressing-transfer technique

To obtain low sheet resistance, high optical transmittance, small open spaces in conductive networks, and enhanced adhesion of flexible transparent conductive films, a carbon nanotube (CNT)/silver nanowire (AgNW)-PET hybrid film was fabricated by mechanical pressing-transfer process at room temperature. The morphology and structure were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM), the optical transmittance and sheet resistance were tested by ultraviolet-visible spectroscopy (UV-vis) spectrophotometer and four-point probe technique, and the adhesion was also measured by 3M sticky tape. The results indicate that in this hybrid nanostructure, AgNWs form the main conductive networks and CNTs as assistant conductive networks are filled in the open spaces of AgNWs networks. The sheet resistance of the hybrid films can reach approximately 20.9 to 53.9 Ω/□ with the optical transmittance of approximately 84% to 91%. The second mechanical pressing step can greatly reduce the surface roughness of the hybrid film and enhance the adhesion force between CNTs, AgNWs, and PET substrate. This process is hopeful for large-scale production of high-end flexible transparent conductive films.     

Polymer photovoltaic devices with transparent graphene electrodes produced by spin-casting

Large-area, smooth, transparent and conductive graphene films were produced by a spin-coating method using graphene solutions. Bulk heterojunction polymer organic photovoltaic devices using these pure graphene films as a transparent anode were fabricated and studied. A direct pure graphene film electrode ensured that the device fabrication cost remained low and the processing was simple. The photovoltaic device displayed a power-conversion efficiency of 0.13%.     

The production of graphene nanosheets decorated with silver nanoparticles for use in transparent, conductive films

Aggregation and restacking of graphene nanosheets (GNS) can be efficiently inhibited by decorating the silver nanoparticles on the surface of GNS to form GNS/silver (GNS-Ag) composites, which can construct high transparent and electrically conductive thin films. Silver nanoparticles act as a useful nanospacer and conductor, which not only increase the interlayer distance but also improve the electrical conductivity between layers. A two-step reduction process using sodium borohydride and ethylene glycol was also demonstrated reducing graphene oxide to GNS efficiently. The GNS-Ag composite films showed a maximum sheet resistance of 93 Ω□⁻¹, while maintaining up to 78% light transmittance, which was two order of magnitude lower than that of GNS (8.2 × 10³ Ω□⁻¹, 81%), and the value of DC conductivity to optical conductivity ratio was 13.5 instead of 0.25.     

Direct laser fabrication of large-area and patterned graphene at room temperature

Direct fabrication of graphene on solid carbon coated nickel surface was realized by laser irradiation at room temperature. High-quality graphene was obtained rapidly, e.g. 28.8 cm²/min. Arbitrary patterns designed by computer aided design (CAD) software were fabricated directly on Ni substrates without additional mask or setup. Raman mapping results showed that monolayer/bilayer graphene regions accounted for 64% of the film area. Graphene films exhibited excellent resistance to corrosion. The extremely low corrosion current density and high free corrosion potential in 3.5% NaCl aqueous (aq) solutions showed that the as-produced graphene had a superior anti-corrosion performance. The penetration and precipitation mechanism of carbon into Ni substrate during the fabrication process were also discussed. This approach may reach the scale large enough for practical applications.     

Transparent and high infrared reflection film having sandwich structure of SiO2/Al:ZnO/SiO2

New transparent and high infrared reflection films having the sandwich structure of SiO₂/Al:ZnO(AZO)/SiO₂ were deposited on the soda-lime silicate glass at room temperature by radio frequency (R.F.) magnetron sputtering. The optical and electrical properties of SiO₂ (110 nm)/AZO (860 nm)/SiO₂ (110 nm) sandwich films were compared with those of single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. The results show that these sandwich films exhibit high transmittance of over 85% in the visible light range (380–760 nm), and low reflection rate of below 4.5% in the wavelength range of 350–525 nm, which is not shown in the conventional single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. Further these sandwich films display a low sheet resistance of 20 Ω/sq by sheet resistance formula and high infrared reflection rate of above 80% in the wavelength range of 15–25 μm. In addition, the infrared reflection property of these sandwich films is determined mainly by the AZO film. The outer SiO₂ film can diminish the interference coloring and increase transparency; the inner SiO₂ film improves the adhesion of the coating to the glass substrate and prevents Ca²⁺ and Na⁺ in the glass substrate from entering the AZO film.     

还原温度对氧化石墨烯的湿度-甲醛交叉敏感性能的影响

以氧化石墨烯溶胶为前体,通过旋涂工艺制备薄膜型气敏元件,在低温80～180℃下进行热处理,获得系列不同还原程度的还原氧化石墨烯气敏元件,采用XRD、AFM、FT-IR、XPS对样品的层结构、薄膜厚度及含氧官能团变化属性进行表征,将气敏薄膜元件在相对湿度为11.3%～93.6%的范围内进行预湿处理,并测定元件对甲醛气氛的敏感性能。结果表明：随热还原处理温度的升高,氧化石墨烯的结构逐渐向类石墨结构转变,含氧官能团逐渐脱失,缺陷增多,薄膜的方块电阻呈数量级地减小,从41 MΩ减小至928 Ω;经不同湿度预处理的气敏元件置于甲醛气氛中产生了水分子与甲醛分子的竞争吸附,从而导致电阻的明显变化;在10^?4甲醛气氛下,未还原或热还原温度较低的气敏元件适用于低、高湿环境下甲醛气氛的气敏测试,最大灵敏度为69.1%,而还原温度适中的元件则适用于中湿环境的甲醛测试,最大灵敏度为80.3%。     

Synthesis of large area carbon nanosheets for energy storage applications

We report the large area growth of highly conductive carbon nanosheets (CNS) composed of few layer graphene on 200 mm diameter Si substrates using conventional radio frequency plasma-enhanced chemical vapour deposition. Raman spectroscopy is used to characterise the evolution of the CNS nucleation and growth with time in conjunction with TEM revealing the nano-sized graphene-like nature of these films and the intimate contact to the substrate. An individual sheet can have edges as thin as 3 graphene layers. The influence of the growth support layer is also discussed as film growth is compared on titanium nitride (TiN) and directly on Si. Electrochemical cyclic voltammogram (CV) measurements reveal these layers to form an excellent electrical contact to the underlying substrate with excellent stability towards oxidation whilst having a large electrochemical surface area. The resistance of a 150 nm film was measured to be as low as 20 μohm cm. The high percentage of narrow few layer graphene edge sites exposed allows for faster electrochemical reaction rates compared to carbon nanotubes (CNTs) and other electrode materials (glassy carbon and Pt).     

Electrical conductivity of poly(3,4-ethylenedioxythiophene):p-toluene sulfonate films hybridized with reduced graphene oxide

Reduced graphene oxide-poly(3,4-ethylenedioxythiophene):p-toluene sulfonate (rGO-PEDOT:PTS) hybrid electrode films were synthesized directly on a substrate by interfacial polymerization between an oxidizing solid layer and liquid droplets of 3,4-ethylenedioxythiophene (EDOT) produced by electrospraying. The EDOT reduced the graphene oxide by donating electrons during its transformation into PEDOT:PTS, and hybrid films consisting of rGO distributed in a matrix of PEDOT:PTS were obtained. These rGO-PEDOT:PTS hybrid films showed excellent electrical conductivities as high as 1,500 S/cm and a sheet resistance of 70 Ω sq^-1. The conductivity values are up to 50% greater than those of films containing conductive PEDOT:PTS alone. These results confirm that highly conductive rGO-PEDOT:PTS hybrid films can potentially be used as organic transparent electrodes.     

Synthesis of high-quality graphene films on nickel foils by rapid thermal chemical vapor deposition

We report the synthesis of high-quality graphene films on Ni foils using a cold-wall reactor by rapid thermal chemical vapor deposition (CVD). The graphene films were produced by shortening the growth time to 10 s, suggesting that a direct growth mechanism may play a larger role rather than a precipitation mechanism. A lower H₂ flow rate is favorable for the growth of high-quality graphene films. The graphene film prepared without the presence of H₂ has a sheet resistance as low as ～367 ohm/sq coupled with 97.3% optical transmittance at 550 nm wavelength, which is much better than for those grown by hot-wall CVD systems. These data suggest that the structural and electrical characteristics of these graphene films are comparable to those prepared by CVD on Cu.