Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2

Infrared spectroscopy of wafer-scale graphene

We report spectroscopy results from the mid- to far-infrared on wafer-scale graphene, grown either epitaxially on silicon carbide or by chemical vapor deposition. The free carrier absorption (Drude peak) is simultaneously obtained with the universal optical conductivity (due to interband transitions) and the wavelength at which Pauli blocking occurs due to band filling. From these, the graphene layer number, doping level, sheet resistivity, carrier mobility, and scattering rate can be inferred. The mid-IR absorption of epitaxial two-layer graphene shows a less pronounced peak at 0.37 ± 0.02 eV compared to that in exfoliated bilayer graphene. In heavily chemically doped single-layer graphene, a record high transmission reduction due to free carriers approaching 40% at 250 μm (40 cm(-1)) is measured in this atomically thin material, supporting the great potential of graphene in far-infrared and terahertz optoelectronics.

Quantum Hall effect in twisted bilayer graphene

We address the quantum Hall behavior in twisted bilayer graphene transferred from the C face of SiC. The measured Hall conductivity exhibits the same plateau values as for a commensurate Bernal bilayer. This implies that the eightfold degeneracy of the zero energy mode is topologically protected despite rotational disorder as recently predicted. In addition, an anomaly appears. The densities at which these plateaus occur show a magnetic field dependent offset. It suggests the existence of a pool of localized states at low energy, which do not count towards the degeneracy of the lowest band Landau levels. These states originate from an inhomogeneous spatial variation of the interlayer coupling.

Observation of Raman g-peak split for graphene nanoribbons with hydrogen-terminated zigzag edges

Raman scattering of individual hydrogen-terminated zigzag-edged graphene nanoribbons (Z-GNRs) was studied with focus on the G-peak. In addition to the bulk graphene G-peak appearing at ∼1594 cm(-1) (G(+)), an edge-related G-peak at ∼1583 cm(-1) (G(-)) was observed for Z-GNRs. This additional Raman vibrational mode originates from the zigzag edges where localized metallic edge states are present. The relative intensity ratio G(-)/G(+) displays a strong dependence on the ribbon width (W). It increases gradually with decreasing W, and the G(+) finally vanishes at W = 5(±3) nm. Polarized Raman scattering was also employed to confirm the four-fold symmetry of the split TO modes, and the results are in good agreement with previous theoretical predictions. Our work offers the first direct experimental evidence to confirm the validity of predicted Raman scattering of GNRs.

Giant two-photon absorption in bilayer graphene

We present a quantum perturbation theory on two-photon absorption (2PA) in monolayer and bilayer graphene which is Bernal-stacked. The theory shows that 2PA is significantly greater in bilayer graphene than monolayer graphene in the visible and infrared spectrum (up to 3 μm) with a resonant 2PA coefficient of up to ∼0.2 cm/W located at half of the bandgap energy, γ(1) = 0.4 eV. In the visible and terahertz region, 2PA exhibits a light frequency dependence of ω(-3) in bilayer graphene, while it is proportional to ω(-4) for monolayer graphene at all photon energies. Within the same order of magnitude, the 2PA theory is in agreement with our Z-scan measurements on high-quality epitaxial bilayer graphene deposited on SiC substrate at light wavelength of 780 and 1100 nm.

Direct growth of graphene/hexagonal boron nitride stacked layers

Graphene (G) and atomic layers of hexagonal boron nitride (h-BN) are complementary two-dimensional materials, structurally very similar but with vastly different electronic properties. Recent studies indicate that h-BN atomic layers would be excellent dielectric layers to complement graphene electronics. Graphene on h-BN has been realized via peeling of layers from bulk material to create G/h-BN stacks. Considering that both these layers can be independently grown via chemical vapor deposition (CVD) of their precursors on metal substrates, it is feasible that these can be sequentially grown on substrates to create the G/h-BN stacked layers useful for applications. Here we demonstrate the direct CVD growth of h-BN on highly oriented pyrolytic graphite and on mechanically exfoliated graphene, as well as the large area growth of G/h-BN stacks, consisting of few layers of graphene and h-BN, via a two-step CVD process. The G/h-BN film is uniform and continuous and could be transferred onto different substrates for further characterization and device fabrication.

Nonperturbative chemical modification of graphene for protein micropatterning

Graphene's extraordinary physical properties and its planar geometry make it an ideal candidate for a wide array of applications, many of which require controlled chemical modification and the spatial organization of molecules on its surface. In particular, the ability to functionalize and micropattern graphene with proteins is relevant to bioscience applications such as biomolecular sensors, single-cell sensors, and tissue engineering. We report a general strategy for the noncovalent chemical modification of epitaxial graphene for protein immobilization and micropatterning. We show that bifunctional molecule pyrenebutanoic acid-succinimidyl ester (PYR-NHS), composed of the hydrophobic pyrene and the reactive succinimide ester group, binds to graphene noncovalently but irreversibly. We investigate whether the chemical treatment perturbs the electronic band structure of graphene using X-ray photoemission (XPS) and Raman spectroscopy. Our results show that the sp(2) hybridization remains intact and that the π band maintains its characteristic Lorentzian shape in the Raman spectra. The modified graphene surfaces, which bind specifically to amines in proteins, are micropatterned with arrays of fluorescently labeled proteins that are relevant to glucose sensors (glucose oxidase) and cell sensor and tissue engineering applications (laminin).

Laser-synthesized epitaxial graphene

Owing to its unique electronic properties, graphene has recently attracted wide attention in both the condensed matter physics and microelectronic device communities. Despite intense interest in this material, an industrially scalable graphene synthesis process remains elusive. Here, we demonstrate a high-throughput, low-temperature, spatially controlled and scalable epitaxial graphene (EG) synthesis technique based on laser-induced surface decomposition of the Si-rich face of a SiC single-crystal. We confirm the formation of EG on SiC as a result of excimer laser irradiation by using reflection high-energy electron diffraction (RHEED), Raman spectroscopy, synchrotron-based X-ray diffraction, transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). Laser fluence controls the thickness of the graphene film down to a single monolayer. Laser-synthesized graphene does not display some of the structural characteristics observed in EG grown by conventional thermal decomposition on SiC (0001), such as Bernal stacking and surface reconstruction of the underlying SiC surface.

Layer-by-layer transfer of multiple, large area sheets of graphene grown in multilayer stacks on a single SiC wafer

Here we report a technique for transferring graphene layers, one by one, from a multilayer deposit formed by epitaxial growth on the Si-terminated face of a 6H-SiC substrate. The procedure uses a bilayer film of palladium/polyimide deposited onto the graphene coated SiC, which is then mechanically peeled away and placed on a target substrate. Orthogonal etching of the palladium and polyimide leaves isolated sheets of graphene with sizes of square centimeters. Repeating these steps transfers additional sheets from the same SiC substrate. Raman spectroscopy, scanning tunneling spectroscopy, low-energy electron diffraction and X-ray photoelectron spectroscopy, together with scanning tunneling, atomic force, optical, and scanning electron microscopy reveal key properties of the materials. The sheet resistances determined from measurements of four point probe devices were found to be ∼2 kΩ/square, close to expectation. Graphene crossbar structures fabricated in stacked configurations demonstrate the versatility of the procedures.

Epitaxial growth of graphitic carbon on C-face SiC and Sapphire by chemical vapor deposition (CVD)

Epitaxial, graphitic carbon thin films were directly grown on C-face/ (0001̄) SiC and (0001) sapphire by chemical vapor deposition (CVD), using propane as a carbon source and without any catalytic metal on the substrate surface. Raman spectroscopy shows the signature of multilayer graphene/graphite growth on both the SiC and sapphire. Raman 2D-peaks have Lorentzian lineshapes with FWHM of ~60 cm(-1) and the ratio of the D-peak to G-peak intensity (I(D)/I(G)) linearly decreases (down to 0.06) as growth temperature is increased. The epitaxial relationship between film and substrates were determined by x-ray diffraction. On both substrates, graphitic layers are oriented parallel to the substrate, but exhibit significant rotational disorder about the surface normal, and predominantly rhombohedral stacking. Film thicknesses were determined to be a function of growth time, growth temperature, and propane flow rate.

Epitaxial graphene growth by carbon molecular beam epitaxy (CMBE)

A novel growth method (carbon molecular beam epitaxy (CMBE)) has been developed to produce high-quality and large-area epitaxial graphene. This method demonstrates significantly improved controllability of the graphene growth. CMBE with C(60) produces AB stacked graphene, while growth with the graphite filament results in non-Bernal stacked graphene layers with a Dirac-like electronic structure, which is similar to graphene grown by thermal decomposition on SiC (000-1).

Patterned growth of graphene over epitaxial catalyst

Rectangle- and triangle-shaped microscale graphene films are grown on epitaxial Co films deposited on single-crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few-layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO(2)/Si substrate while maintaining the original shape and field-effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.

Auger electron spectroscopy: a rational method for determining thickness of graphene films

We report the determination of the thickness of graphene layers by Auger electron spectroscopy (AES). We measure AES spectra of graphenes with different numbers of layers. The AES spectroscopy shows distinct spectrum shape, intensity, and energy characteristics with an increasing number of graphene layers. We also calculate electron inelastic mean free paths for graphene layers directly from these measurements. The method allows unambiguous and high-throughput determination of thickness up to six graphene layers and detection of defect and dopant in graphene films on almost any substrate. The availability of this reliable method will permit direct probing of graphene growth mechanisms and exploration of novel properties of graphenes with different thicknesses on diverse substrates.

Direct chemical vapor deposition of graphene on dielectric surfaces

Direct deposition of graphene on various dielectric substrates is demonstrated using a single-step chemical vapor deposition process. Single-layer graphene is formed through surface catalytic decomposition of hydrocarbon precursors on thin copper films predeposited on dielectric substrates. The copper films dewet and evaporate during or immediately after graphene growth, resulting in graphene deposition directly on the bare dielectric substrates. Scanning Raman mapping and spectroscopy, scanning electron microscopy, and atomic force microscopy confirm the presence of continuous graphene layers on tens of micrometer square metal-free areas. The revealed growth mechanism opens new opportunities for deposition of higher quality graphene films on dielectric materials.

Graphene on a hydrophobic substrate: doping reduction and hysteresis suppression under ambient conditions

The intrinsic doping level of graphene prepared by mechanical exfoliation and standard lithography procedures on thermally oxidized silicon varies significantly and seems to depend strongly on processing details and the substrate morphology. Moreover, transport properties of such graphene devices suffer from hysteretic behavior under ambient conditions. The hysteresis presumably originates from dipolar adsorbates on the substrate or graphene surface. Here, we demonstrate that it is possible to reliably obtain low intrinsic doping levels and to strongly suppress hysteretic behavior even in ambient air by depositing graphene on top of a thin, hydrophobic self-assembled layer of hexamethyldisilazane (HMDS). The HMDS serves as a reproducible template that prevents the adsorption of dipolar substances. It may also screen the influence of substrate deficiencies.

Hydrogen Intercalation below Epitaxial Graphene on SiC(0001)

In this report we review how intrinsic drawbacks of epitaxial graphene on SiC(0001) such as n-doping and strong electronic influence of the substrate can be overcome. Besides surface transfer doping from a strong electron acceptor and transfer of epitaxial graphene from SiC(0001) to SiO2 the most promising route is to generate quasi-free standing epitaxial graphene by means of hydrogen intercalation. The hydrogen moves between the (6p3×6p3)R30◦ reconstructed initial carbon (so-called buffer) layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free standing graphene monolayer, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 °C. This technique offers significant advances in epitaxial graphene based nanoelectronics.

Technique for the dry transfer of epitaxial graphene onto arbitrary substrates

To make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO(2), GaN and Al(2)O(3) substrates using a thermal release tape. Subsequent Hall effect measurements illustrated that minimal degradation in the carrier mobility was induced following the transfer process in lithographically patterned devices. Correspondingly, a large drop in the carrier concentration was observed following the transfer process, supporting the notion that a gradient in the carrier density is present in C-face EG, with lower values being observed in layers further removed from the SiC interface. X-ray photoemission spectra collected from EG films attached to the transfer tape revealed the presence of atomic Si within the EG layers, which may indicate the identity of the unknown intrinsic dopant in EG. Finally, this transfer process is shown to enable EG films amenable for use in device fabrication on arbitrary substrates and films that are deemed most beneficial to carrier transport, as flexible electronic devices or optically transparent contacts.

Marker-free on-the-fly fabrication of graphene devices based on fluorescence quenching

Graphene has been dominating the electronic research community recently, with a brisk surge in proposals for its use in novel devices. The aspirations of 2D-carbon-based electronics largely rely on the availability of a mass-production technique to obtain wafer-scale graphene circuits. In this paper, we take a first step towards fulfilling this aspiration by demonstrating a rapid prototyping route for graphene-based devices. The method is based on our observation that graphene quenches the fluorescence from dyes. Utilizing this property, we use a confocal microscope to identify graphene flakes and perform the required lithography steps, bypassing the need for markers and other infrastructure such as atomic force microscopy or e-beam lithography. The versatility of this technique enables it to harbour ambitions of an automated process for large scale in situ assembly of graphene-based circuits.

Synthesis of graphene on silicon dioxide by a solid carbon source

We report on a method for the fabrication of graphene on a silicon dioxide substrate by solid-state dissolution of an overlying stack of a silicon carbide and a nickel thin film. The carbon dissolves in the nickel by rapid thermal annealing. Upon cooling, the carbon segregates to the nickel surface forming a graphene layer over the entire nickel surface. By wet etching of the nickel layer, the graphene layer was allowed to settle on the original substrate. Scanning tunneling microscopy (STM) as well as Raman spectroscopy has been performed for characterization of the layers. Further insight into the morphology of the layers has been gained by Raman mapping indicating micrometer-size graphene grains. Devices for electrical measurement have been manufactured exhibiting a modulation of the transfer current by backgate electric fields. The presented approach allows for mass fabrication of polycrystalline graphene without transfer steps while using only CMOS compatible process steps.

Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation

Quasi-free-standing epitaxial graphene is obtained on SiC(0001) by hydrogen intercalation. The hydrogen moves between the (6 square root(3) x 6 square root(3))R30 degrees reconstructed initial carbon layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free-standing graphene monolayer with its typical linear pi bands. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 degrees C.

Transfer printing of graphene using gold film

We report a facile transfer printing process for easily exfoliating prepatterned graphene from HOPG surfaces by using gold film as the transfer stamp. The transferred printed patterns consist of single- and few-layer graphene with macroscopic continuity. Raman spectra show that some defects appear along the edges of the graphene patterns, which were induced by the oxygen plasma-etching treatment of the HOPG surface. This transfer-printing technique paves a new and simple way to get large-scale graphene patterns on to any substrates.

Epitaxial graphene on SiC(0001) and [Formula: see text]: from surface reconstructions to carbon electronics

Graphene with its unconventional two-dimensional electron gas properties promises a pathway towards nanoscaled carbon electronics. Large scale graphene layers for a possible application can be grown epitaxially on SiC by Si sublimation. Here we report on the initial growth of graphene on SiC basal plane surfaces and its relation to surface reconstructions. The surfaces were investigated by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and x-ray photoelectron spectroscopy (XPS). On SiC(0001) the interface is characterized by the so-called [Formula: see text] reconstruction. The homogeneity of this phase is influenced by the preparation procedure. Yet, it appears to be crucial for the quality of further graphene growth. We discuss the role of three structures with periodicities [Formula: see text], (6 × 6) and (5 × 5) present in this phase. The graphitization process can be observed by distinct features in the STM images with atomic resolution. The number of graphene layers grown can be controlled by the conical band structure of the π-bands around the [Formula: see text] point of the graphene Brillouin zone as measured by laboratory-based ARUPS using UV light from He II excitation. In addition we show that the spot intensity spectra in LEED can also be used as fingerprints for the exact determination of the number of layers for the first three graphene layers. LEED data correlated to the ARUPS results allow for an easy and practical method for the thickness analysis of epitaxial graphene on SiC(0001) that can be applied continuously during the preparation procedure, thus paving the way for a large variety of experiments to tune the electronic structure of graphene for future applications in carbon electronics. On [Formula: see text] graphene grows without the presence of an interface layer. The initial graphene layer develops in coexistence with intrinsic surface reconstructions of the [Formula: see text] surface. In high resolution STM measurements we show atomically resolved graphene layers on top of the (3 × 3) reconstruction with a Moiré type modulation by a large superlattice periodicity that indicates a weak coupling between the graphene layer and the substrate.