Single-layer Graphene Films Research Articles

Enhanced Thermopower of Graphene Films with Oxygen Plasma Treatment

In this work, we show that the maximum thermopower of few layers graphene (FLG) films could be greatly enhanced up to ∼700 μV/K after oxygen plasma treatment. The electrical conductivities of these plasma treated FLG films remain high, for example, ∼10(4) S/m, which results in power factors as high as ∼4.5 × 10(-3) W K(-2) m(-1). In comparison, the pristine FLG films show a maximum thermopower of ∼80 μV/K with an electrical conductivity of ∼5 × 10(4) S/m. The proposed mechanism is due to generation of local disordered carbon that opens the band gap. Measured thermopowers of single-layer graphene (SLG) films and reduced graphene oxide (rGO) films were in the range of -40 to 50 and -10 to 20 μV/K, respectively. However, such oxygen plasma treatment is not suitable for SLG and rGO films. The SLG films were easily destroyed during the treatment while the electrical conductivity of rGO films is too low.

In-plane lattice thermal conductivities of multilayer graphene films

The in-plane lattice thermal conductivities of a single layer and multilayer graphene films are investigated using nonequilibrium molecular dynamics simulations. It is found the thermal conductivity of a single layer graphene is higher than that of multilayer graphene. Increasing the bonding strength between neighboring layers will reduce the in-plane thermal conductivity for multilayer graphene films. The constraints from the neighboring layer play the role of impeding phonon transport along the in-plane direction in multilayer graphene films. This observation implies the thermal conductivity of a single layer graphene will be reduced in practical applications once it is bonded on a substrate.

Synthesis of large area, homogeneous, single layer graphene films by annealing amorphous carbon on Co and Ni

The synthesis of large area, homogenous, single layer graphene on cobalt (Co) and nickel (Ni) is reported. The process involves vacuum annealing of sputtered amorphous carbon (a-C) deposited on Co/sapphire or Ni/sapphire substrates. The improved crystallinity of the metal film, assisted by the sapphire substrate, proves to be the key to the quality of as-grown graphene film. The crystallinity of the Co and Ni metal films was improved by sputtering the metal at elevated temperature as was verified by X-ray diffraction (XRD). After sputtering of a-C and annealing, large area, single layer graphene that occupies almost the entire area of the substrate was produced. With this method, 100 mm2-area single layer graphene can be synthesized and is limited only by the substrate and vacuum chamber size. The homogeneity of the graphene film is not dependent on the cooling rate, in contrast to syntheses using polycrystalline metal films and conventional chemical vapor deposition (CVD) growth. Our facile method of producing single layer graphene on Co and Ni metal films should lead to large scale graphene-based applications.

Layer-by-layer synthesis of large-area graphene films by thermal cracker enhanced gas source molecular beam epitaxy

A thermal cracker enhanced gas source molecular beam epitaxy system was used to synthesize large-area graphene. Hydrocarbon gas molecules were broken by thermal cracker at very high temperature of 1200 °C and then impinged on a nickel substrate. High-quality, large-area graphene films were achieved at 800 °C, and this was confirmed by both Raman spectroscopy and transmission electron microscopy. A rapid cooling rate was not required for few-layer graphene growth in this method, and a high-percentage of single layer and bilayer graphene films was grown by controlling the growth time. The results suggest that in this method, carbon atoms migrate on the nickel surface and bond with each other to form graphene. Few-layer graphene is formed by subsequent growth of carbon layers on top of existing graphene layers. This is completely different from graphene formation through carbon dissolving in nickel and then precipitating from the nickel during rapid substrate cooling in the chemical vapor deposition method.

Interfacial capacitance of single layer graphene

The interfacial capacitance of large area, single layer graphene was directly measured with electrolyte accessing both sides of the graphene sheet. PMMA and photoresist patterns were used as supports to suspend the CVD grown graphene in electrolyte during electrochemical testing. Both one and two sides of single layer graphene films were measured and compared. The results show that the area normalized charge that can be stored simultaneously on both sides is significantly lower than could be stored on just one side of single layer graphene, consistent with charge storage having a quantum capacitance component. These measurements are also consistent with the specific capacitance of graphene materials as previously measured in supercapacitor cells and provide a basis for the further understanding and development of graphene based materials for electrical energy storage.

Terahertz imaging and spectroscopy of large-area single-layer graphene

We demonstrate terahertz (THz) imaging and spectroscopy of a 15 × 15-mm2 single-layer graphene film on Si using broadband THz pulses. The THz images clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing us to measure sheet conductivity with sub-mm resolution without fabricating electrodes. The THz carrier dynamics are dominated by intraband transitions and the THz-induced electron motion is characterized by a flat spectral response. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet conductivity varies across the sample from σ(s) = 1.7 × 10(-3) to 2.4 × 10(-3) Ω(-1) (sheet resistance, ρ(s) = 420 - 590 Ω/sq).

Observation of the integer quantum Hall effect in high quality, uniform wafer-scale epitaxial graphene films

We report the observation of the integer quantum Hall states at Landau level fillings of ν=2, 6, and 10 in a Hall bar device made of a single-layer epitaxial graphene film on the silicon-face of silicon-carbide prepared via argon-assisted graphitization. The two-dimensional electron gas exhibits a low-temperature (at 4 K) carrier mobility of ∼14 000 cm2/V s at the electron density of 6.1×1011 cm−2. Furthermore, the sheet resistance obtained from four-probe measurements across the whole area (12×6 mm2) of another specimen grown under similar condition displays roughly uniform values (∼1600 Ω/square), suggesting that the macroscopic steps and accompanying multilayer graphene domains play a minor role in the low-temperature electronic transport.

Growth of large-area graphene films from metal-carbon melts

We have demonstrated a new method for the large-area graphene growth, which can lead to a scalable low-cost high-throughput production technology. The method is based on growing single layer or few-layer graphene films from a molten phase. The process involves dissolving carbon inside a molten metal at a specified temperature and then allowing the dissolved carbon to nucleate and grow on top of the melt at a lower temperature. The examined metals for the metal-carbon melt included copper and nickel. For the latter, the high-quality single layer graphene was grown successfully. The resulting graphene layers were subjected to detailed microscopic and Raman spectroscopic characterization. The deconvolution of the Raman 2D band was used to accurately determine the number of atomic planes in the resulting graphene layers and access their quality. The results indicate that our technology can provide bulk graphite films, few-layer graphene as well as high-quality single layer graphene on metals. Our approach can also be used for producing graphene-metal thermal interface materials for thermal management applications.

Large low-frequency resistance noise in chemical vapor deposited graphene

We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO2 substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as 0.1–0.5. We also find the variation in the noise magnitude with the gate voltage (or carrier density) and temperature to be surprisingly weak, which is also unlike the behavior of noise in other forms of graphene, in particular those from exfoliation.

Structural and frictional properties of graphene films on SiC(0001) studied by atomic force microscopy

Structural and frictional properties of single-layer and bilayer graphene films on a SiC(0001) substrate are studied by means of atomic force microscopy with atomic resolution. Friction on single-layer graphene is found to be a factor of two larger than on bilayer films for a variety of experimental situations. The friction contrast is found not to originate in differences in structural properties, in lateral contact stiffness, or in contact potential. The transition from atomic stick-slip friction to a regime of ultralow friction is found to occur at normal loads of 40 nN when the tip-sample interaction potential approaches 0.1--0.2 eV.

Aromatic Molecules Doping in Single-Layer Graphene Probed by Raman Spectroscopy and Electrostatic Force Microscopy

Electrostatic force microscopy (EFM) and Raman spectroscopy are used to study the effects of aromatic molecules on the doping level of single layer graphene (SLG). The EFM results demonstrate that the aromatic molecules with electron-donating group (n-doping) decrease the surface potential of the SLG films, while the molecules with electron-withdrawing groups (p-doping) increase their surface potential. Meanwhile, Raman spectra indicate that the aromatic molecules with electron-withdrawing groups make the SLG p-dope and upshifts in both Raman G and 2D frequencies. While the aromatic molecules with electron-donating groups n-dope the SLG and up-shift the 2D-but down-shift the G-band frequencies.

Reduction Kinetics of Graphene Oxide Determined by Electrical Transport Measurements and Temperature Programmed Desorption

The thermal stability and reduction kinetics of graphene oxide were studied by measuring the electrical resistivity of single-layer graphene films at various stages of reduction in high vacuum and by performing temperature programmed desorption (TPD) measurements of multilayer films in ultrahigh vacuum. The graphene oxide was exfoliated from the graphite oxide source material by slow-stirring in aqueous solution, which produces single-layer platelets with an average lateral size of ∼10 μm. From the TPD measurements, it was determined that the primary desorption products of the graphene oxide films for temperatures up to 300 °C are H2O, CO2, and CO, with only trace amounts of O2 being detected. Resistivity measurements on individual single-layer graphene oxide platelets resulted in an activation energy of 37 ± 1 kcal/mol. The TPD measurements of multilayer films of graphene oxide platelets give an activation energy of 32 ± 4 kcal/mol.

Photoelectrical Response in Single‐Layer Graphene Transistors

The illumination of single-layer graphene (SLG) transistors with visible light causes a negative shift in their transfer curves, attributable to the desorption of oxygen. However, their hysteresis is not affected by illumination, which suggests that charge traps are not affected by the visible-light exposure. When SLG transistors are covered with a layer of photoactive polymer, the photodesorption-induced current change in the transistors becomes less significant than the effects caused by the surrounding photoactive polymer. These observations demonstrate that the photoelectrical response of SLG transistors is dominated by extrinsic mechanisms rather than by the direct photocurrent process. The results suggest a new strategy for achieving light detection. The large cross section of SLG films for receiving photons and the capability of tailoring photoelectrical properties on them is potentially useful for optoelectronic applications.

Growth mechanism for epitaxial graphene on vicinal6H-SiC(0001)surfaces: A scanning tunneling microscopy study

The inability to grow large well-ordered ultra high vacuum (UHV) graphene with a specific number of layers on SiC(0001) is well known. The growth involves several competing processes (Si desorption, carbon diffusion, island nucleation, etc.) and because of the high temperatures, it has not been possible to identify the growth mechanism. Using scanning tunneling microscopy and a vicinal 6H-SiC(0001) sample, we determine that the Si desorption from steps is the main controlling process. Adjacent steps retract with different speeds and the released carbon produces large areas of bilayer graphene with characteristic ``fingers'' emanating from steps. If faster heating rates are used, the different Si desorption rates are avoided and single-layer graphene films extending over many microns are produced.

Doping Single‐Layer Graphene with Aromatic Molecules

Recently discovered single-layer graphene (SLG) has attracted great attention not only because this perfect 2-dimensional carbon crystalline structure enables unprecedented explorations of fundamental physics but also because of its exciting potentials in the post-silicon nanoeletronics 1-6 . As the electrical properties of SLG films are very sensitive to the local perturbations such as from surface charges 7-9 and adsorbed gas molecules 6 , it is plausible that the electronic structures, hence the performance, of SLG may be tailored by molecular doping on its surface. Herein, we demonstrated that the electronic structures of SLG can be differentially modulated by doping from various aromatic molecules. We also show that a simple spectroscopic method based on the Raman 2D and G band frequency sampling can be used to distinguish the n- and p-doped SLG. Raman spectroscopy is a powerful tool to rapidly and nondestructively examine intrinsic physical properties of various carbon nanostructures, including flat and one-atom thick carbon crystalline layer (graphene monolayer), stacked graphenes (graphite), and roll-up graphene monolayer (single-walled carbon nanotube–SWNT). The characteristic G (~1580-1590 cm -1 ) and 2D (~2690-2710 cm -1 ) Raman bands are able to reveal the number of stacked graphene layer 10-12 and the changes in charge carrier concentration (or Fermi energy shift) induced by static electrical field 13-14 .

Field Emission of Single‐Layer Graphene Films Prepared by Electrophoretic Deposition

Homogeneous single-layer graphene films are fabricated using an electrophoretic deposition technique, and their field-emission properties are investigated. The graphene films show high density, uniform thickness, numerous edges normal to the film surface, and good interface contact and adhesion with the substrate, and consequently show excellent field-emission properties.

Effective doping of single-layer graphene from underlyingSiO2substrates

When a single-layer graphene (SLG) is on ${\text{SiO}}_{2}$ substrates, the charge exchange at their interface results in a dipole, which direction strongly depends on the contact potential difference between the SLG and the substrates. Due to the longer experimental charge screening length of SLG than its thickness, the charge redistribution imposes effective $p$ or $n$ doping to SLG films. The substrate-dependent doping of SLG films is further confirmed by Raman and electrical measurements. Also, the unique electronic structures of SLG films make them sensitive to the doping rather than effective gating from the ${\text{SiO}}_{2}$ substrates.

Scanning Tunneling Microscope Studies of Ultrathin Graphitic (Graphene) Films on an Insulating Substrate under Ambient Conditions

In the present study, a scanning tunneling microscope (STM), modified to include active lateral position feedback control, is employed to image single and few layer graphene films placed on a nonconductive substrate under ambient conditions. The return path for tunneling electrons was provided by gold electrodes produced by either electron beam lithography or shadow evaporation techniques. STM images of graphene films with a thickness of two or more layers display topographs that are similar to those obtained from a bulk graphite crystal. For single layer graphene sheets, the ability to obtain atomically resolved images was found to be extremely sensitive to sample preparation methods. Graphene microdevices produced by electron beam lithography with edges covered by gold electrodes show hexagonal patterns similar to those obtained from ultrahigh vacuum STM images reported earlier. Ambient STM measurements of graphene microdevices made by shadow mask evaporation, whose edges were exposed to air, exhibited ...

Morphology of graphene thin film growth on SiC(0001)

Epitaxial films of graphene on SiC(0001) are interesting from a basic physics as well as an applications-oriented point of view. Here, we study the emerging morphology of in vacuo prepared graphene films using low-energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES). We obtain an identification of single-layer and bilayer graphene films by comparing the characteristic features in electron reflectivity spectra in LEEM to the π-band structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to accurately determine the local extent of graphene layers as well as the layer thickness.