Sheet Resistance Of Graphene Research Articles

The sheet resistance of graphene under contact and its effect on the derived specific contact resistivity

It is normally assumed that the sheet resistances under and outside the metal contact are identical when deriving specific contact resistivity of graphene from transmission line model. We considered the contact end resistance and obtained the sheet resistance under contact of 670Ω/□, which is much different from that outside the contact of 1840Ω/□. Considering the difference, the value of specific contact resistivity is determined to be 3.3×10−6Ωcm2, which is three times as large as the unmodified value. This indicates that the difference between the sheet resistances under and outside the contact affects the derived specific contact resistivity of graphene significantly.

Sheet resistance variation of graphene grown on annealed and mechanically polished Cu films

Graphene was grown on a Cu film, processed by annealing and mechanical polishing. The sheet resistance of graphene shows ∼57% decrease by mechanical polishing for raw Cu and ∼20% decrease by annealing for 50 nm SiO2 polishing.

One-step etching, doping, and adhesion-control process for graphene electrodes

Enhancing the electrical conductivity and reliability of graphene electrodes is critical for the practical realization of graphene-based electronics, since these factors influence the electrical performance of devices. To achieve such improvements in graphene electrodes requires additional processes such as doping and surface treatments, which inevitably complicate device fabrication. Here, we introduce a novel, straightforward one-step etching method, in which a catalytic copper substrate is etched in imidazole-containing ammonium persulfate solution, of simultaneously enhancing the electrical and adhesion properties of graphene grown on copper foil by chemical vapor deposition. Applying one-step etching method, the sheet resistance of monolayer graphene with 270Ω/sq is obtained, while the adhesion of graphene is improved by 20%. Moreover, the electrical conductivity of graphene remained improved after storage for 30days in ambient conditions without any passivation layers, and the graphene was almost transparent with transmittance of 97.7% at 550nm. The enhancement of the electrical and adhesion properties of graphene originated from the synergistic adsorption of imidazole and etched Cu ions, which results in p-doping of graphene.

Ultraclean transfer of CVD-grown graphene and its application to flexible organic photovoltaic cells

We demonstrate a PMMA reverse transfer method, where a PMMA/graphene bilayer was reversely transferred onto target substrates, to better control both contamination and crack formation relative to conventional approaches. Based on this novel transfer process, the graphene sheet resistance was greatly reduced by about 50% at the same transmittance, exhibiting ∼200% higher efficiency when applied in a solar cell as the anode.

Improvement of Conductivity in Graphene by Ag Nanowires under a Non-Uniform Electric Field

We demonstrated the precise positioning of silver nanowires (AgNWs) assisted by a non-uniform electric field to improve the sheet resistance of a graphene layer. The distribution of AgNWs aligned by dielectrophoretic force at a peak-to-peak voltage of 20 V was well matched to the results of the finite element method calculation. The sheet resistance of graphene obtained by the circular transmission line method was improved by the controlled integration with AgNWs. Effects of thermal annealing on the graphene-AgNW hybrid structures were also systematically characterized, demonstrating decreases in the sheet resistance by up to 20%. The results from our study suggest that the conductivity of graphene can be locally tailored by positioning AgNWs using dielectrophoresis.

Step-edge-induced resistance anisotropy in quasi-free-standing bilayer chemical vapor deposition graphene on SiC

The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps ∼10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be ∼190 Ωμm for step height hS = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications.

Fluoropolymer-assisted graphene electrode for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) were fabricated on a graphene electrode, with synthesized graphene being transferred and simultaneously doped with supporting polymers. Poly[methyl methacrylate] (PMMA) and fluoropolymer (CYTOP) layers were used as the supporting polymers. The sheet resistance of CYTOP-assisted graphene (CYTOP-G) with 4 layers of graphene is 200Ω/sq., which is lower than that of PMMA-assisted graphene (PMMA-G, 330Ω/sq.) The transmittance value of PMMA-G and CYTOP-G with 4 graphene layers is higher than 85%. CYTOP-G is shown to exhibit a higher tolerance to UV–O3 treatment and thermal annealing than PMMA-G. Work function of CYTOP-G is 4.7eV, which is higher than that of PMMA-G (4.3eV). X-ray photoemission and Raman spectroscopy data indicate that CYTOP-G has numerous C-F bonds on the surface exhibiting p-type semiconductor properties, owing to the high electronegativity of fluorine. The turn-on voltage of an OLED based on CYTOP-G with 4 graphene layers is 4.2V, which is lower than that of indium tin oxide (ITO)-based one (4.5eV). Furthermore, the luminance ratio of graphene-based OLEDs to ITO-based OLEDs was calculated to be 104% for CYTOP-G, and 97% for PMMA-G. According to the ultraviolet photoemission spectra, the hole injection barrier in CYTOP-G is lower by about 0.5eV than the hole injection barrier in PMMA-G. These results are very encouraging to the prospect of replacing ITO electrodes with graphene ones in OLED applications.

Iron (III) Chloride doping of CVD graphene

Chemical doping has been shown as an effective method of reducing the sheet resistance of graphene. We present the results of our investigations into doping large area chemical vapor deposition graphene using Iron (III) Chloride (FeCl3). It is shown that evaporating FeCl3 can increase the carrier concentration of monolayer graphene to greater than 1014 cm−2 and achieve resistances as low as 72 Ω sq−1. We also evaluate other important properties of the doped graphene such as surface cleanliness, air stability, and solvent stability. Furthermore, we compare FeCl3 to three other common dopants: Gold (III) Chloride (AuCl3), Nitric Acid (HNO3), and TFSA ((CF3SO2)2NH). We show that compared to these dopants, FeCl3 can not only achieve better sheet resistance but also has other key advantages including better solvent stability.

Contact properties to CVD-graphene on GaAs substrates for optoelectronic applications

The optimization of contacts between graphene and metals is important for many optoelectronic applications. In this work, we evaluate the contact resistance and sheet resistance of monolayer and few-layered graphene with different metallizations using the transfer length method (TLM). Graphene was obtained by the chemical vapor deposition technique (CVD-graphene) and transferred onto GaAs and Si/SiO2 substrates. To account for the quality of large-area contacts used in a number of practical applications, a millimeter-wide TLM pattern was used for transport measurements. Different metals—namely, Ag, Pt, Cr, Au, Ni, and Ti—have been tested. The minimal contact resistance Rc obtained in this work is 11.3 kΩ μm for monolayer CVD-graphene, and 6.3 kΩ μm for a few-layered graphene. Annealing allows us to decrease the contact resistance Rc and achieve 1.7 kΩm μm for few-layered graphene on GaAs substrate with Au contacts. The minimal sheet resistance Rsh of few-layered graphene transferred to GaAs and Si/SiO2 substrates are 0.28 kΩ/□ and 0.27 kΩ/□. The Rsh value of monolayer graphene on the GaAs substrate is 8 times higher (2.3 kΩ/□), but it reduces for the monolayer graphene on Si/SiO2 (1.4 kΩ/□). For distances between the contacts below 5 μm, a considerable reduction in the resistance per unit length was observed, which is explained by the changes in doping level caused by graphene suspension at small distances between contact pads.

Bilayer graphene synthesis by plasma treatment of copper foils without using a carbon-containing gas

Bilayer graphene has been synthesized by using hydrogen plasma treatment of copper foils for 30s at the temperature of 850°C together with joule-heating treatment of the foils without using a carbon-containing gas such as methane in order to suppress the nucleation density of graphene. The effect of plasma provides active species of carbon atoms on copper substrate and a selective bilayer graphene formation of AB-stacking in a very short time. Carbon to be precipitated is delivered from the copper foil and/or the environment in the reaction chamber. The domain size of synthesized graphene, the controllability of a few layers and the electrical conductivity have been significantly improved compared with plasma chemical vapor deposition (CVD) using carbon-containing gas. The sheet resistance of bilayer graphene exhibits 951Ω in average. The carrier mobility shows 1000cm2/Vs in maximum at room temperature. The sheet resistance of 130±26Ω has been attained after the doping by gold chloride solution.

In-situ monitoring of AuCl3-doping and -dedoping behaviors in graphene

In-situ variations of the surface morphology, transmittance, sheet resistance, and Dirac curve of graphene have been monitored during AuCl3-doping and annealing-induced-dedoping processes. The transmittance of graphene shows a monotonically-decreasing behavior due to doping and subequent annealing at higher temperatures (T A ), resulting from the continuing increase in the surface corrugations due to intensified coalescing of Au clusters. Electronic parameters, such as the sheet resistance, work function, Dirac point, and electron mobility, show their extreme values when annealed at 50 °C after doping, which can be attributed to further enhancement of a reaction donating electrons to Cl ions from graphene at T A = 50 °C. The annealing at higher T A induces a dedoping effect due to the Cl desorption, thereby decreasing the doping efficiency. These results suggest that Cl anions play a crucial role in the doping and the dedoping processes, especially causing variations in the electronic properties of graphene.

Graphene Doping Effect of Thin Film and Contact Mechanisms

The contact mechanism of devices is usually researched at electrode contacts. However, the contact between a dielectric and channel at the MOS structure is more important. The graphene was used as a channel material, and the thin film transistor with MOS structure was prepared to observe the contact mechanism. The graphene was obtained on Cu foil by the thermal decomposition method with <TEX>$H_2$</TEX> and <TEX>$CH_4$</TEX> mixed gases at an ambient annealing temperature of <TEX>$1000^{\circ}C$</TEX> during the deposition for 30 min, and was then transferred onto a <TEX>$SiO_2/Si$</TEX> substrate. The graphene was doped in a nitrogen acidic solution. The chemical properties of graphene were investigated to research the effect of nitric atoms doping. The sheet resistance of graphene decreased after nitrogen acidic doping, and the sheet resistance decreased with an increase in the doping times because of the increment of negative charge carriers. The nitric-atom-doped graphene showed the Ohmic contact at the curve of the drain current and drain voltage, in spite of the Schottky contact of grapnene without doping.

Effect of transition-metal chlorides on graphene properties

The doping mechanisms of transition-metal chlorides such as NiCl2, CuCl2, CoCl2, ZnCl2, TiCl3, and InCl3 in graphene were investigated. The sheet resistance of graphene decreased from 780 to 400–700 Ω sq.−1 on doping; further, its transmittance at 550 nm also decreased from 97% to 90–96% owing to reduction of metal cations. Both the G and 2D peak in Raman spectra were shifted to higher wavenumbers after metal-chloride doping, which indicates a charge transfer from graphene to metal cations. The peak position of the CC bond in synchrotron radiation photoemission spectroscopy shifted to lower binding energy, suggesting that the Fermi level of doped graphene was lowered by spontaneous electron transfer between graphene and metal cations. Secondary cut-off spectra showed that the work function of doped graphene increased from 4.32 eV to 4.88, 4.85, 4.82, 4.81, 4.8, and 4.64 eV upon doping with 20 mM of NiCl2, CuCl2, CoCl2, ZnCl2, TiCl3, and InCl3, respectively. The tendency of an increasing work function is strongly related to the work function value of the bulk metallic state. It is considered that spontaneous electron transfer and strong carbon–chlorine bond formation induce a specific energy-level engineering in doped graphene sheets, thereby increasing its work function and conductivity.

Arrays of carbon nanoscrolls as deep subwavelength magnetic metamaterials

We demonstrate theoretically that an array of carbon nanoscrolls acts as a hyperbolic magnetic metamaterial in the THz regime with genuine subwavelength operation corresponding to wavelength-to-structure ratio of about 200. Due to the low sheet resistance of graphene, the electromagnetic losses in an array of carbon nanoscrolls are almost negligible offering a very sharp magnetic resonance of extreme positive and negative values of the effective magnetic permeability. The latter property leads to superior imaging properties for arrays of carbon nanoscrolls which can operate as magnetic endoscopes in the THz where magnetic materials are scarce. Our optical modelling is supplemented with ab initio density-functional calculations of the self-winding of a single layer of graphene onto a carbon nanotube so as to form a carbon nanoscroll. The latter process is viewed as a means to realize ordered arrays of carbon nanoscrolls in the laboratory based on arrays of aligned carbon nanotubes which are nowadays routinely fabricated.

Work‐Function Engineering of Graphene Anode by Bis(trifluoromethanesulfonyl)amide Doping for Efficient Polymer Light‐Emitting Diodes

Graphene has been considered to be a potential alternative transparent and flexible electrode for replacing commercially available indium tin oxide (ITO) anode. However, the relatively high sheet resistance and low work function of graphene compared with ITO limit the application of graphene as an anode for organic or polymer light‐emitting diodes (OLEDs or PLEDs). Here, flexible PLEDs made by using bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]2NH) doped graphene anodes are demonstrated to have low sheet resistance and high work function. The graphene is easily doped with TFSA by means of a simple spin‐coating process. After TFSA doping, the sheet resistance of the TFSA‐doped five‐layer graphene, with optical transmittance of ≈88%, is as low as ≈90 Ω sq−1. The maximum current efficiency and power efficiency of the PLED fabricated on the TFSA‐doped graphene anode are 9.6 cd A−1and 10.5 lm W−1, respectively; these values are markedly higher than those of the PLED fabricated on pristine graphene anode and comparable to those of an ITO anode.

Thermal reversibility in electrical characteristics of ultraviolet/ozone-treated graphene

Changes in electrical properties of a bilayer graphene-based field-effect transistor (G-FET) after being oxidized through ultraviolet (UV)/ozone (O3) treatment are presented. A decrease in conductivity and carrier mobility was observed after oxidation. However, electrical properties recovered after annealing oxidized G-FET with H2/Ar, indicating that oxidation with UV/O3 treatment was thermally reversible. Raman spectroscopy was conducted to verify that no defects were introduced after oxidation. The existence of chemical bonds between oxygen and graphene was confirmed from the X-ray photoelectron spectroscopy. Moreover, we found that graphene's sheet resistance increased after oxidation. Nevertheless, contact resistivity at graphene-Au/TiN electrode interface remained unchanged.

Delaminated Graphene at Silicon Carbide Facets: Atomic Scale Imaging and Spectroscopy

Atomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitization of a hexagonal SiC(0001) substrate by high temperature annealing. This growth technique is known to result in a pronounced electron-doping (∼10(13) cm(-2)) of graphene, which is thought to originate from an interface carbon buffer layer strongly bound to the substrate. The scanning transmission electron microscopy analysis, carried out at an energy below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (112n) facets of SiC surface steps. In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp2-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions. A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination.

Terahertz and optical study of monolayer graphene processed by plasma oxidation

We report on our terahertz and optical study of monolayer graphene grown by chemical vapor deposition and processed by plasma oxidation. The plasma oxidation induces oxygen-related defects, and the resulting disorder increases the sheet resistance of graphene as measured via terahertz spectroscopy. The excitonic absorption peak weakens considerably and blue shifts upon plasma oxidation, resulting in higher transmittance in both the visible and ultraviolet regions. Our oxygen plasma-treated graphene also exhibits a free-carrier doping effect as confirmed by the blue shift of the Raman G band.

Graphene-P(VDF-TrFE) Multilayer Film for Flexible Applications

A flexible, transparent acoustic actuator and nanogenerator based on graphene/P(VDF-TrFE)/graphene multilayer film is demonstrated. P(VDF-TrFE) is used as an effective doping layer for graphene and contributes significantly to decreasing the sheet resistance of graphene to 188 ohm/sq. The potentiality of graphene/P(VDF-TrFE)/graphene multilayer film is realized in fabricating transparent, flexible acoustic devices and nanogenerators to represent its functionality. The acoustic actuator shows good performance and sensitivity over a broad range of frequency. The output voltage and the current density of the nanogenerator are estimated to be ∼3 V and ∼0.37 μAcm(-2), respectively, upon the application of pressure. These values are comparable to those reported earlier for ZnO- and PZT-based nanogenerators. Finally, the possibility of rollable devices based on graphene/P(VDF-TrFE)/graphene structure is also demonstrated under a dynamic mechanical loading condition.

Measurement of the gauge factor of few-layer graphene

Graphene, one of the recently discovered carbon nanostructures, has shown good piezoresistive properties. One of the most important areas of research for graphene sheets, in terms of basic science and application in strain or stress sensors, is the measurement of gauge factors. The gauge factors of various layers of graphene sheets are measured based on the equivalent stress beam. The measurement is carried out using a beam-bending method to detect the change in resistance of graphene sheets in different bending states. The gauge factor ranges from 10 to 15, depending on the number of layers in the graphene sheet. These results reveal the piezoresistance effect of single- and multi-layer graphene sheets, which will be of benefit in the fabrication of microsensors. The resistance of graphene sheets decreases as temperature increases from 20°C to 60°C, and the gauge factor is not very sensitive to changes in environmental temperature.