Skip navigation
Please use this identifier to cite or link to this item: http://repository.iitr.ac.in/handle/123456789/12886
Full metadata record
DC FieldValueLanguage
dc.contributor.authorVerma V.P.-
dc.contributor.authorDas S.-
dc.contributor.authorLahiri, Indranil-
dc.contributor.authorChoi W.-
dc.date.accessioned2020-10-15T12:28:12Z-
dc.date.available2020-10-15T12:28:12Z-
dc.date.issued2010-
dc.identifier.citationApplied Physics Letters (2010), 96(20): --
dc.identifier.issn36951-
dc.identifier.urihttps://doi.org/10.1063/1.3431630-
dc.identifier.urihttp://repository.iitr.ac.in/handle/123456789/12886-
dc.description.abstractWe present the fabrication and electrical characterization of large graphene structure on polyethylene terephthalate (PET) flexible substrate. Graphene film was grown on Cu foil by thermal chemical vapor deposition and transferred to PET by using hot press lamination. The graphene/PET film shows high quality, flexible transparent conductive structure with unique electrical-mechanical properties; ∼88.80% light transmittance and ∼1.1742 kω/sq sheet resistance. We demonstrate application of graphene/PET film as flexible and transparent electrode for field emission displays. Our proposed techniques can be tailored for any flexible substrate and large scale production, which could open up exciting device applications in foldable electronics. © 2010 American Institute of Physics.-
dc.language.isoen_US-
dc.relation.ispartofApplied Physics Letters-
dc.titleLarge-area graphene on polymer film for flexible and transparent anode in field emission device-
dc.typeArticle-
dc.scopusid23098911200-
dc.scopusid56452060700-
dc.scopusid7004250718-
dc.scopusid57202287300-
dc.affiliationVerma, V.P., Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, United States-
dc.affiliationDas, S., Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, United States-
dc.affiliationLahiri, I., Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, United States-
dc.affiliationChoi, W., Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, United States-
dc.description.fundingVerma Ved Prakash Das Santanu Lahiri Indranil Choi Wonbong a) Department of Mechanical and Materials Engineering, Florida International University , Miami, Florida 33174, USA a) Electronic mail: choiw@fiu.edu . 17 05 2010 96 20 203108 25 03 2010 28 04 2010 20 05 2010 2010-05-20T12:23:36 2010 American Institute of Physics 0003-6951/2010/96(20)/203108/3/ $30.00 We present the fabrication and electrical characterization of large graphene structure on polyethylene terephthalate (PET) flexible substrate. Graphene film was grown on Cu foil by thermal chemical vapor deposition and transferred to PET by using hot press lamination. The graphene/PET film shows high quality, flexible transparent conductive structure with unique electrical-mechanical properties; ∼ 88.80 % light transmittance and ∼ 1.1742   k Ω / sq sheet resistance. We demonstrate application of graphene/PET film as flexible and transparent electrode for field emission displays. Our proposed techniques can be tailored for any flexible substrate and large scale production, which could open up exciting device applications in foldable electronics. USAFOSR FA9550-09-1-0544 Graphene is a two-dimensional (2D) carbon material having unique band structure and outstanding thermal, mechanical, and electrical properties. 1–3 Some of the potential applications of graphene are for sensors, transistors, supercapacitors, solar cells, and flexible displays. 4–8 It is well known that graphene has high mechanical strength with flexibility, high transmittance, and high electron mobility. 8,9 These properties make graphene an emerging alternate for transparent conductive metal oxides electrodes, in particular, indium tin oxide (ITO) which contains indium as a costly and scarce element. In order to make a transparent conductive graphene film, most of the researchers have used liquid solution of graphene flakes (obtained by reducing graphene oxide flakes) for deposition of transparent conductive film. 7,10 Recently, Yamaguchi et al. 11 has deposited chemically derived graphene solution on flexible substrate for large area transparent flexible electrode which contains 2 to 30 layers of graphene. Application of graphene in flexible electronics will need synthesis of continuous graphene film on substrates and transfer it to polymeric substrate in large scale. Li et al. 12 has grown high quality, predominately monolayer graphene films on copper foils by chemical vapor deposition (CVD) method. Kim et al. 8 have demonstrated two different techniques (stamping and scooping) for transferring graphene from nickel substrate to other arbitrary substrates. These techniques are not effective for industrial application which will require low cost, high quality, and large area production of graphene flexible electrodes. Here we present a direct and effective method for synthesis of large graphene film on copper foils and transferring it to polyethylene terephthalate (PET) flexible substrate by hot press lamination process. This method provides an effective way to handle large area of graphene film with minimal physical damage to it. The resulting graphene polymer film is flexible and remains conductive under high tensile strains. The application of this graphene film as flexible transparent conductive anode has been demonstrated in carbon nanotube (CNT) field emission devices (FEDs). Graphene film was synthesized by CVD of hydrocarbon on copper foil. Commercially available, cold rolled Cu foil of 50 to 200   μ m thickness and large area (6 cm width and 15 cm length) was first annealed at 1000   ° C for 1 h under Ar atmosphere. After annealing, Cu foil was acid-treated for 10 min using 1 M acetic acid at 60   ° C . This acid treatment helps in removing oxide layer generated at the Cu foil surface during annealing process. Copper foils were thoroughly washed with de-ionized water and dried at the ambient conditions. Graphene films were grown on copper foils in a similar way to the previously reported CVD process. 12 In short, graphene on the Cu foil was synthesized at 1000   ° C and 1 atm pressure, using a 5 min flow of CH 4 and H 2 gases in 1:4 ratios. After graphene growth, the foil was cooled down to room temperature before being taken out from the furnace. Graphene growth on Cu foil has been reported as a surface-catalyzed process which indicates that Cu act as catalyst for CVD of graphene. A detailed discussion about growth mechanism of graphene formation on copper foil has been presented elsewhere. 12 We have used hot press lamination and chemical etching process for transferring graphene grown over the Cu foils to the transparent flexible substrates. Figure 1(a) shows flow diagram of graphene transfer technique. Cu foils with graphene were hot press rolled with a transparent flexible PET film having thickness ∼ 50   μ m . For complete removal of Cu from the graphene and laminated film, we have used concentrated FeCl 3 solution. Laminated polymer film with graphene and Cu foil underneath was floated over the FeCl 3 acid bath at room temperature. After 40 min of etching process Cu was completely dissolved into the solution leaving graphene film with the PET substrate. This transparent flexible film was then thoroughly washed with de-ionized water and dried in air at room temperature. Figure 1(b) demonstrates a flexible, transparent graphene film with diagonal length of ∼ 16   cm . This hot press lamination process provides a very adherent graphene film on the flexible substrate which can be deformed easily into various geometries [Fig. 1(c) ] without damaging the film. Characterization of graphene over Cu and flexible substrate were done by Raman spectroscopy. The Raman spectrum of graphene film on Cu foil and PET film are shown in Fig. 2(a) . A symmetric and sharp 2D-band indicates monolayer of graphene film over Cu foil. The I G / I 2 D ratio for graphene on Cu foil was 0.4, conforming monolayer graphene film. 12 A high quality of graphene film over Cu foil was in compliance with a small D peak. 13 The transfer of graphene film on PET substrate was evident from Raman spectrum, showing characteristic G and 2D bands. Optical transmission spectroscopy measurement was performed using Scinco, SD-1000 Ultraviolet-visible spectrophotometer. Figure 2(b) shows the transmittance of the graphene film with optical range of wavelengths. Percent transmission of graphene film on PET substrate is 88.80 at 550 nm wavelength. The graphene film shows higher transmittance compared to CNT (Ref. 14 ) and ITO films. 15 Sheet resistance of the graphene film measured by four-point probe method was ∼ 1.1742   k Ω / sq which is comparable to other reported graphene films. 8,16 Graphene has an advantage over ITO, as it shows superior electrical-mechanical properties. To investigate the electromechanical properties of graphene/PET film, bending and stretching tests was performed. Graphene/PET film (dimension 1 × 1   cm 2 ) was mounted on the lower jaws of a Vernier caliper which helps in bending and stretching the film in desired amount. The linear resistance of the film was measured at two edges across the film. Figure 2(c) shows the resistance of graphene/PET film with tensile strain ranging from 0% to 60%. Surprisingly, graphene/PET film remains conductive and shows only one order of magnitude change by ∼ 60 % stretching. Foldability of graphene/PET film was evaluated by measuring resistance with respect to bending radii [Fig. 2(d) ]. The graphene is only few nanometers thick and PET thickness ( ∼ 50   μ m ) alone is used in tensile strain calculation. Bending the film upto a radius of curvature of 1.52 mm (approximate tensile strain of 6.5%) changes the resistance of graphene from 2.09 to 2.68   k Ω , which was recovered completely after unbending the graphene/PET film. These stretching and bending tests shows that graphene film has superior mechanical and electrical properties compared to brittle ITO electrode, which generate microcracks inside the film under mechanical stress. 17 The excellent mechanical properties of graphene film can be attributed to atomically perfect lattice of hexagonally arranged strong carbon–carbon bonds. 18 This transparent and conductive graphene film was characterized for its possible application in FEDs, where graphene/PET film was used as a flexible anode. Green phosphor (Phosphor Tech; DPG01) material was deposited on the graphene/PET flexible substrate by dip coating process. This graphene/PET anode was assembled over bent (radius of curvature ∼ 13.65   mm ) CNT field emitters grown on copper foil [Fig. 3(a) ]. Details of synthesis and field emission properties of CNT field emitters are discussed in our past publication. 19 The interelectrode distance was kept constant at 600   μ m and the device was tested for its performance and stability under dc bias and high vacuum ( ∼ 1 × 10 − 7   Torr ) condition. Figure 3(b) shows the field emission response of the device in bent configuration. The emission current behavior of the device was analyzed using Fowler–Nordheim (F–N) equation I = ( aA β 2 E 2 / Φ ) exp ( − b Φ 3 / 2 / β E ) , where a = 1.54 × 10 − 6   A   eV   V − 2 and b = 6.83 × 10 7   eV 3 / 2   V   cm − 1 , respectively, A is the emission area, β is the field enhancement factor, E is the applied electric field in volt per centimeter, and Φ is the work function in electron volt. 20 Inset in Fig. 3(b) shows the corresponding F–N plot. Straight line nature of the F–N plot indicates that the emission current was due to field emission. Turn-on field for the device was found to be 1.75   V / μ m and field enhancement factor ( β ) , as calculated from the slope of F–N plot, was approximately 1000. Figure 3(c) presents the stability of the field emitter device for more than 3 h, at an average current level of 7.5   μ A . It can be clearly observed from the figure that the device could produce a stable emission for the stipulated time period, indicating the stability of the graphene anode for long period of operation. Inset of Fig. 3(c) shows the bent FED and corresponding emission image, captured by using a thin layer of green phosphor on the graphene anode. The image clearly shows presence of emission from multiple sites of the field emitter device. The performance of the transparent, flexible graphene shows its potential in low-current, flexible field emission applications. Although, there have been published reports about flexible (and nontransparent) FEDs, 21,22 our demonstration of graphene as the transparent, flexible anode of FEDs shows possibility for future flexible and transparent display devices. Graphene anodes could act as a possible replacement for toxic and expensive ITO coatings. In conclusion, we have synthesized large area graphene film and transferred it to PET flexible substrate by using hot press lamination technique. Graphene film on PET substrate shows high transmittance ( > 88 % ) and conductivity with mechanical stability. Also we have demonstrated application of this graphene /PET film as a flexible transparent anode in FED. The flexible FED with CNT emitter and graphene anode shows turn-on field of 1.75   V / μ m and β values of ∼ 1000 with good field emission stability. Our proposed techniques can be tailored for any flexible substrate and large scale production at low-cost, which could open up exciting applications in foldable electronics and electromechanical devices. The authors thank Dr. Chang-soo Han at Korea Institute of Machinery and Materials for characterizing optical transmittance. V.V. acknowledges Dissertation Year Fellowship from University Graduate School, Florida International University for financial support. This research was partly supported by AFOSR (Grant No. FA9550-09-1-0544). FIG. 1. (a) Process flow for graphene transfer from Cu foil to PET substrate. Hot press lamination and chemical etching processes were used in this method. (b) Large area graphene film transferred over PET substrate. (c) Flexibility of graphene/PET film. FIG. 2. (a) Raman spectra from graphene on copper foil and PET substrates. (b) Transmittance of graphene film over PET substrate. Inset shows large area transparent graphene/PET film. (c) Variation in resistance of graphene/PET film uniaxial stretched by 60%. (d) Resistance of graphene/PET film with different bending radii. Insets show schematic of stress modes applied to graphene/PET film. FIG. 3. (a) Schematic of CNT FED consisting flexible graphene anode. Multiwall CNT grown on Cu substrate was used as cathode and graphene/PET film was used as anode. Green phosphor deposited over graphene was used to show the illumination from flexible anode. (b) Emission current-voltage characteristics of FED. Inset shows corresponding FN plot. (c) Emission current stability for > 3   h . Inset shows bent FED and green illumination from graphene flexible electrode.-
dc.description.correspondingauthorVerma, V. P.; Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, United States-
Appears in Collections:Journal Publications [MT]

Files in This Item:
There are no files associated with this item.
Show simple item record


Items in Repository are protected by copyright, with all rights reserved, unless otherwise indicated.