Single-layer Graphene Films Research Articles

Photo-thermal chemical vapor deposition of graphene on copper

We demonstrate the synthesis of single-layer graphene films on copper by photo-thermal chemical vapor deposition (PTCVD) realized using a rapid thermal processing system typically used in CMOS processing. Influence of the temperature on the low-pressure (10mbar) graphene synthesis using methane precursor was characterized by analyzing the crystalline quality, thickness and electronic properties of the films. Using a growth time of only 60s, for graphene fabricated at 950°C the sheet resistance and mobility show equivalent quality compared to thermal CVD graphene. Moreover, μ-Raman mapping reveals very low defect density and high 2D to G band ratio similar to the fingerprint of exfoliated single-layer graphene. The synthesis process was found to exhibit a threshold at around 900°C at which (and below) the single-layer graphene film does not contain adlayer flakes typically observed in high temperature CVD graphene on copper. Our study shows that PTCVD can be used for the high throughput fabrication of high-quality single-layer graphene on copper and is therefore a promising method while pursuing cost-effective graphene fabrication.

Co‐Percolating Graphene‐Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes

AbstractTransparent conducting electrodes (TCEs) require high transparency and low sheet resistance for applications in photovoltaics, photodetectors, flat panel displays, touch screen devices and imagers. Indium tin oxide (ITO), or other transparent conductive oxides, have typically been used, and provide a baseline sheet resistance (RS) vs. transparency (T) relationship. However, ITO is relatively expensive (due to limited abundance of Indium), brittle, unstable, and inflexible; moreover, ITO transparency drops rapidly for wavelengths above 1000 nm. Motivated by a need for transparent conductors with comparable (or better) RS at a given T, as well as flexible structures, several alternative material systems have been investigated. Single‐layer graphene (SLG) or few‐layer graphene provide sufficiently high transparency (≈97% per layer) to be a potential replacement for ITO. However, large‐area synthesis approaches, including chemical vapor deposition (CVD), typically yield films with relatively high sheet resistance due to small grain sizes and high‐resistance grain boundaries (HGBs). In this paper, we report a hybrid structure employing a CVD SLG film and a network of silver nanowires (AgNWs): RS as low as 22 Ω/□ (stabilized to 13 Ω/□ after 4 months) have been observed at high transparency (88% at λ = 550 nm) in hybrid structures employing relatively low‐cost commercial graphene with a starting RS of 770 Ω/□. This sheet resistance is superior to typical reported values for ITO, comparable to the best reported TCEs employing graphene and/or random nanowire networks, and the film properties exhibit impressive stability under mechanical pressure, mechanical bending and over time. The design is inspired by the theory of a co‐percolating network where conduction bottlenecks of a 2D film (e.g., SLG, MoS2) are circumvented by a 1D network (e.g., AgNWs, CNTs) and vice versa. The development of these high‐performance hybrid structures provides a route towards robust, scalable and low‐cost approaches for realizing high‐performance TCE.

The study of the effects of cooling conditions on high quality graphene growth by the APCVD method

The effects of cooling conditions on graphene growth by the atmospheric pressure chemical vapor deposition (APCVD) method on platinum are investigated. It is found that the cooling conditions are the key to better control of the coverage, thickness and uniformity of the graphene films. A series of experiments were carried out with different cooling rates and gas atmospheres. Various characterization techniques (SEM, Raman, AFM and TEM) were applied to examine the graphene morphology and quality. A "three competitive reactions" mechanism is proposed to explain the cooling condition effects. Based on optimized growth conditions, a high-coverage, single-layer graphene film could be grown on Pt using the APCVD method. Back-gated field effect transistors were fabricated and the measured carrier mobility is about 1600 cm(2) V(-1) s(-1).

Three step fabrication of graphene at low temperature by remote plasma enhanced chemical vapor deposition

In this report, we systematically studied low temperature fabrication of graphene from precursors containing cyclobenzene groups by remote plasma enhanced chemical vapor deposition. A new three-step growth procedure was developed with good control of the nucleation, domain growth and domain connection stages. Based on this growth procedure, high quality continuous graphene films could be obtained using naphthalene as the graphene precursor at temperatures lower than 600 °C. A transmittance of ∼96.4% and continuous optical images confirmed the successful fabrication of uniform single-layer graphene films with desirable quality at temperatures lower than 400 °C. Carrier mobility of graphene synthesized at 400 °C reached ∼682 cm2 V−1 s−1, indicating the samples are of reasonable quality. Low temperature graphene synthesis may pave the way for low cost large scale graphene fabrication, and for production of flexible substrates, especially polymer substrates.

Revealing anisotropic strain in exfoliated graphene by polarized Raman spectroscopy

We report on a polarized Raman study on mechanically cleaved single-layer graphene films. Under a specific orientation of scattering measurement, the width and position of the G peak change with the incident polarization direction, while the integrated intensity of that is unaltered. This phenomenon is explained by a proposed mode in which the peak is contributed by a mixture of un-, compressive-, and tensile-strained G sub-modes. The compression and tension are both uniaxial and approximately perpendicular to each other. They are undesigned and located in either separated or overlapped sub-areas within the probed local region. Compared to the unstrained wavenumber of 1580 cm(-1), compression induces a blue shift while tension causes a red one. The sub-modes correlated with the light polarization through different relationships split the G peak into three sub-ones. We develop a method to quantitatively analyze the positions, widths, intensities, and polarization dependences of sub-peaks. This analysis quantitatively reveals local strain, which changes with the detected area of a graphene film. The method presented here can be extended to detect the strain distribution in the film and thus is a promising technology for graphene characterization.

Few-layer graphene membrane as an ultrafast mode-locker in erbium-doped fiber laser

Single-layer graphene film is fabricated on the copper foil by using chemical vapor deposition method, and the corresponding Raman spectrum is measured. The graphene membrane is transferred to the facet of zirconium oxide of a fiber jumper. The mode-lock Er-doped fiber laser with few-layer graphene membrane as saturable absorber is experimentally studied, which has a ring structure and uses a 10% portion of a fiber coupler as the laser output. The laser generates a pulse train at a 7.69 MHz repetition rate, with a full width at half maximum of 58.8 ps. The corresponding time-bandwidth product is 1.98, indicating that the pulses are chirped. The peak wavelength and 3 dB bandwidth of the laser are 1560.1 nm and 0.27 nm respectively. Through changing the air gap between two the fiber ends, 4 nm wavelength tuning is observed.

Hybrid Low Resistance Ultracapacitor Electrodes Based on 1-Pyrenebutyric Acid Functionalized Centimeter-Scale Graphene Sheets

Ultracapacitors are promising candidates for alternative energy storage applications since they can store and deliver energy at relatively high rates. Here, we present hybrid nanocarbon ultracapacitor electrodes with a low equivalent series resistance (ESR) of 7 ohms. 1-pyrenebutyric acid treated large-area single layer graphene (SLG) sheets covered with shortened multi-walled carbon nanotubes (MWNTs) have been utilized as highly conductive and percolated networks of hybrid carbon nanomaterial composites or thin films as ultracapacitor electrodes. Uniform centimeter scale single layer graphene sheets were produced via low pressure chemical vapor deposition using copper foil substrates and then subsequently modified by 1-pyrenebutyric acid functionalization. Chemically shortened MWNTs ranging in length of 200-500 nm, were deposited by drop casting on 1-pyrenebutyric acid functionalized SLG films. SLG/MWNT nancomposite hybrid films of different thicknesses were obtained by controlling the density of MWNT suspension. Surface morphology and nanostructure of the hybrid nanocomposites indicated relatively dense and homogeneous web-like networks. Specific capacitance values of the hybrid electrodes were substantially increased by 200% compared to those ultracapacitors fabricated using buckypaper electrodes. Average values of specific capacitance and energy density obtained were 140.64 F/g and 21.54 Wh/kg respectively. SLG/MWNT nanocomposite electrodes are very promising for future ultracapacitor devices with their low ESR value that is 95% lower than that of buckypaper based ultracapacitors.

Few-Layer Graphene Direct Deposition on Ni andCu Foil by Cold-Wall Chemical Vapor Deposition

We report an alternative synthesis process, cold-wall thermal chemical vapor deposition (CVD), is replied to directly deposit single-layer and few-layer graphene films on Ar plasma treated Ni and Cu foils using CH4 as carbon source. Through optimizing the process parameters, large scale single-layer graphene grown on Ni foil is comparable to that grown on Cu foil. The graphene films were able to be transferred to other substrates such as SiO2/Si, flexible transparent PET and verified by optical microscopy, Raman microscopy and scanning electron microscopy. The sheet resistance and transmission of the transferred graphene films on PET substrate were also discussed.

Process Optimization for Synthesis of High-Quality Graphene Films by Low-Pressure Chemical Vapor Deposition

Low-pressure chemical vapor deposition (LPCVD) is a simple and useful method for the large-area synthesis of graphene films. Here, we have investigated how to adjust and optimize process conditions for the synthesis of single-layer graphene films by LPCVD. Through our experimental procedure, uniform and high-quality graphene films could be grown on Cu foil at 1000 °C for 20 min with an H2 flow rate of 20 sccm, CH4 flow rate of 40 sccm, total pressure of 1.7 Torr, and a fast cooling rate (>10 °C/s). In a Raman spectrum measured from synthesized graphene film, we found that the full width at half-maximum (FWHM) of a symmetric 2D peak centered at 2682.5 cm-1 was 34 cm-1 and the 2D-to-G intensity ratio was 1.35.

Substrate effect on the electronic structures of CuPc/graphene interfaces

The interfacial electronic structures of copper phthalocyanine (CuPc) deposited on a single-layer graphene (SLG) film prepared on Cu and SiO2 substrates (SLG/Cu and SLG/SiO2) were investigated using ultraviolet photoelectron spectroscopy. The ionization energy of CuPc on SLG/Cu and SLG/SiO2 substrate is, respectively, 5.62 eV and 4.97 eV. The energy level alignments at the two interfaces were estimated. The results revealed that the height of the electron (hole) injection barriers are 1.20 (1.10) and 1.38 (0.92) eV at CuPc/SLG/Cu and CuPc/SLG/SiO2 interfaces, respectively.

Direct growth of graphene films on TEM nickel grids using benzene as precursor

We report a direct synthesis of single-layer, bi-layer, and multi-layer graphene films on a nickel grid surface by chemical vapor decomposition of liquid precursor benzene. The direct route, by avoiding several wet chemical steps and accompanying mechanical stresses and contamination common to all presently reported graphene transfer to microscopy grid method, enables providing pristine microstructure information for as-prepared graphene films. A liquid precursor based growth route opens a new door to tailor the electronic properties of graphene by doping heteroatoms using various organic liquids containing the dopant atoms.

Graphene induced tunability of the surface plasmon resonance

Tunability of the surface plasmon resonance wavelength is demonstrated by varying the thickness of Al2O3 spacer layer inserted between the graphene and nanoparticles. By varying the spacer layer thickness from 0.3 to 1.8 nm, the resonance wavelength is shifted from 583 to 566 nm. The shift is due to a change in the electromagnetic field coupling strength between the localized surface plasmons excited in the gold nanoparticles and a single layer graphene film. In contrast, when the graphene film is absent from the system, no noticeable shift in the resonance wavelength is observed upon varying the spacer thickness.

Step-templated CVD growth of aligned graphene nanoribbons supported by a single-layer graphene film

We present chemical vapor deposition (CVD) growth of a hybrid structure of aligned graphene nanoribbons (GNRs) supported by a single-layer graphene sheet. The step structure created on the epitaxial Co film is used to segregate arrays of aligned GNRs. Reflecting the highly ordered step structure of the Co catalyst, straight nanoribbons with high aspect ratio (>100) are formed. Analysis suggests that a large-area, single-layer graphene film also grows over the aligned GNRs, making a GNR-graphene hybrid structure. We also demonstrate the isolation of aligned GNRs by oxygen plasma treatment or partial transfer of the hybrid film. These findings on the formation of highly aligned GNRs give new insights into the formation mechanism of graphene and can be applied for more advanced graphene structure for future electronics.

Internal Friction and Shear Modulus of Graphene Films

We report internal friction and shear modulus measurements of several types of synthesized graphene films. They include reduced graphene oxide, chemical-vapor deposited (CVD) graphene films on thin nickel films and on copper foils. These films were transferred from their host substrate into a water bath, and re-deposited onto to a high-Q single crystal silicon mechanical double-paddle oscillator. A minimal thickness dependence of both internal friction and shear modulus was found for reduced graphene oxide films varying thickness from 4 to 90 nm and CVD graphene films on nickel from 6 to 8 nm. The shear modulus of these multilayered films averages 53 GPa. Their internal friction exhibits a temperature independent plateau below 10K. The values of the plateaus are similar for both the reduced graphene oxide films and CVD graphene films on nickel, and they are as high as the universal "glassy range" where the tunneling states dominated internal friction of amorphous solids lies. In contrast, CVD graphene films on copper foils are 90~95% single layer. The shear modulus of these single layer graphene films are about five times higher, averaging 280 GPa. Their low temperature internal friction is too small to measure within the uncertainty of our experiments. Our results demonstrate the dramatic difference in the elastic properties of multilayer and single layer graphene films.

Efficiency enhancement of graphene/silicon-pillar-array solar cells by HNO3 and PEDOT-PSS

A single-layer graphene film was grown on copper foil by chemical vapor deposition and transferred onto a silicon-pillar-array (SPA) substrate to make a Schottky junction solar cell. The SPA substrate was specifically designed to suppress reflectance and enhance light absorption. The energy conversion efficiency of the prepared graphene/SPA solar cells achieved a maximum of 2.90% with a junction area of 0.09 cm(2). HNO(3) was employed to dope the graphene in the solar cells, and the time dependence of HNO(3) treatment on the cell performance was studied. Poly(3,4-ethylenedioxythiophene) polystyrenesulfonic acid (PEDOT-PSS) was also introduced in graphene/SPA solar cells by spin coating on top of the graphene film, and its modification on the cell performance was characterized. The results show that both HNO(3) and the PEDOT-PSS film could enhance the energy conversion efficiency of graphene/SPA solar cells.

Large scale metal-free synthesis of graphene on sapphire and transfer-free device fabrication

Metal catalyst-free growth of large scale single layer graphene film on a sapphire substrate by a chemical vapor deposition (CVD) process at 950 °C is demonstrated. A top-gated graphene field effect transistor (FET) device is successfully fabricated without any transfer process. The detailed growth process is investigated by the atomic force microscopy (AFM) studies.

The Application of Highly Doped Single-Layer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells

A single-layer graphene film with high conductance and transparency was realized by effective chemical doping. The conductance of single-layer graphene was increased for more than 400% when it was doped with Au nanoparticles and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid). Then semitransparent organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with single-layer graphene and indium tin oxide (ITO) as the top and bottom electrodes, respectively. The performance of the devices was optimized by tuning the active layer thickness and doping the single-layer graphene electrodes. The maximum efficiency of 2.7% was observed in the devices with the area of 20 mm(2) illuminated from graphene electrode under the AM1.5 solar simulator. It is notable that all of the devices showed higher efficiency from the graphene than ITO side, which was attributed to the better transmittance of the graphene electrodes. In addition, the influence of the active area of the organic solar cell on its photovoltaic performance was studied. We found that, when the active areas increased from 6 to 50 mm(2), the power conversion efficiencies decreased from 3% to 2.3% because of the increased series resistances and the decreased edge effect of the devices.

Control of thickness uniformity and grain size in graphene films for transparent conductive electrodes

Large-scale and transferable graphene films grown on metal substrates by chemical vapor deposition (CVD) still hold great promise for future nanotechnology. To realize the promise, one of the key issues is to further improve the quality of graphene, e.g., uniform thickness, large grain size, and low defects. Here we grow graphene films on Cu foils by CVD at ambient pressure, and study the graphene nucleation and growth processes under different concentrations of carbon precursor. On the basis of the results, we develop a two-step ambient pressure CVD process to synthesize continuous single-layer graphene films with large grain size (up to hundreds of square micrometers). Scanning electron microscopy and Raman spectroscopy characterizations confirm the film thickness and uniformity. The transferred graphene films on cover glass slips show high electrical conductivity and high optical transmittance that make them suitable as transparent conductive electrodes. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model has been proposed. Our results provide important guidance toward the synthesis of high quality uniform graphene films, and could offer a great driving force for graphene based applications.

Substrate grain size and orientation of Cu and Cu–Ni foils used for the growth of graphene films

Graphene growth on Cu foils by catalytic decomposition of methane forms predominantly single-layer graphene films due to the low solubility of carbon in Cu. On the other hand, graphene growth on Cu–Ni foils can result in the controlled growth of few-layer graphene films because of the higher solubility of carbon in Ni. One of the key issues for the use of graphene grown by chemical vapor deposition for device applications is the influence of defects on the transport properties of the graphene. For instance, growth on metal foil substrates is expected to result in multidomain graphene growth because of the presence of grains within the foil that exhibit a variety of surface terminations. Therefore, the size and orientation of the grains within the metal foil should influence the defect density of the graphene. For this reason, we have studied the effect of total anneal time and temperature on the orientation and size of grains within Cu foils and Cu–Ni alloy foils with a nominal concentration of 90/10 by weight. The graphene growth procedure involves preannealing the foil in a H2 background followed by the graphene growth in a CH4/H2 atmosphere. Measurements of the substrate grain size have been performed with optical microscopy and scanning electron microscopy. These results show typical lateral dimensions ranging from a few millimeters up to approximately a centimeter for Cu foils annealed at 1030 °C for 35 min and from tens of microns up to a few hundred microns for the 90/10 Cu–Ni foils annealed at 1050 °C for times ranging from 45 to 90 min. The smaller grains within the Cu–Ni foils are attributed to the higher melting point of the Cu–Ni alloy. The crystallographic orientation within each substrate grain was studied with electron backscatter diffraction, and shows that the preferred orientation for the Cu foil is primarily toward the (100) surface plane. For the 90/10 Cu–Ni foils, the orientation of the surface of the grains is initially toward the (110) plane and shifts into an orientation midway between the (100) and (111) planes as the anneal time is increased.

Effects of methane flux on structural and transport properties of CVD-grown graphene films

Large area, single-layer graphene films were synthesized on copper foils by chemical vapor deposition. We have investigated the effects of methane flux on structural and transport properties of graphene. Raman spectra and electrical results reveal that methane flux has almost no influence on the thickness of graphene, but clearly influences the structural defects of graphene. In addition, graphene field effect transistors with a gate length of 10 μm were fabricated, exhibiting obvious field effects and p-type characteristics.