The primary method for isolating graphene, micromechanical cleavage of graphite, is di? Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly a? Here, we show that epitaxy on Ru produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the?
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The primary method for isolating graphene, micromechanical cleavage of graphite, is difficult to scale up for applications. Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly affect the electronic properties of epitaxial graphene layers.
Here, we show that epitaxy on Ru produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the first graphene layer indeed interacts strongly with the metal substrate, the second layer is almost completely detached, shows weak electronic coupling to the metal, and hence retains the inherent electronic structure of graphene.
Our findings demonstrate a route towards rational graphene synthesis on transition-metal templates for applications in electronics, sensing or catalysis. A very high carrier mobility4 and long-range ballistic transport at room temperature5 , quantum confinement in nanoscale ribbons6 and single-molecule gas detection sensitivity7 qualify graphene as a promising material for large-scale applications in microelectronics and sensing.
To d realize this potential, reliable methods for fabricating large-area single-crystalline graphene domains are required. Graphene synthesis by epitaxy on transition metals has been considered recently9— Observations of the structural coherence of b graphene across steps suggest that the sizes of graphene domains e will not be limited by the substrate step spacing in this case Yet, it remains uncertain if the surface diffusion of carbon adatoms can be of sufficiently long range to achieve sparse graphene nucleation and hence epitaxial graphene domains of macroscopic size.
Here, we combine real-time observations of graphene growth Figure 1 Morphology of epitaxial graphene on Ru At high low-energy electron diffraction electron energy: The ability to nucleate and grow further graphene layers in a controlled way enables determination of the effects of a progressive weakening of substrate interactions on few-layer graphene epitaxy on Ru —and possibly on other the electronic properties of the epitaxial graphene.
Whereas the first transition-metal substrates—integrated by selective growth on graphene layer couples strongly to the Ru substrate, the second layer transition-metal template pads or combined with methods for is essentially decoupled and largely recovers the electronic structure transfer to other substrates can provide high-quality material for of free-standing graphene.
Our findings suggest that single- and applications in electronics and sensing. Numbers indicate elapsed time in seconds after the nucleation of the graphene island. Substrate steps, visible as faint dark lines, are aligned from lower left to upper right.
On Ru , single-layer Intensity a. A Energy eV marked lowering of the work function compared with that of both b clean Ru and bulk graphite indicates strong substrate bonding and C significant charge transfer from the metal to the graphene overlayer. Intensity a. The interaction of the growing islands with atomic substrate steps is an important factor in enabling monocrystalline graphene domains with size exceeding the average step spacing by several orders of magnitude.
In contrast to previous work that showed 0 10 20 30 40 dense nucleation at Ru step edges11 , epitaxial graphene sheets Energy eV on Ru nucleate very sparsely and rapidly expand by C incorporation into graphene edge sites under our growth c conditions.
This picture breaks down Figure 3 Identification of the layer spacing of one- and two-layer when the graphene edge meets a substrate step. Plan-view substrate steps. At than the substrate step spacing 0.
The result is an array of lens-shaped islands of macroscopic of magnitude. If sufficient C segregates from the Ru bulk or is deposited Ultrahigh-vacuum UHV CKLL scanning Auger microscopy shows from a suitable hydrocarbon precursor, such as ethylene18 , the that these islands are indeed C-rich, and surrounded by Ru metal graphene islands grow to a size corresponding to the spacing of the with negligible CKLL signal. A smaller separation of second- layer.
Already with the second layer, however, the interlayer spacing layer nuclei suggests a C adatom mobility that is lower than on comes close to that of bulk graphite 3. Island edges aligned affected little by the adjacent metal substrate.
Raman At this point, the surface consists of two different phases. Areas spectra on cleaved monolayer and few-layer graphene on SiO2 , with two-layer graphene coexist with regions covered by a single which served as reference samples, show two primary features: a graphene layer. Charge transfer from the substrate to due to phonons in the highest optical branch near the K point at subsequent graphene layers should diminish progressively, with the the Brillouin zone boundary21 Fig.
The 2D band results from interlayer coupling asymptotically approaching the van der Waals a double-resonance process, which links the phonon wave vectors interaction of bulk graphite. Assessing this transition is of central to the electronic band structure, that is, its line shape can serve as a importance for evaluating epitaxy on transition metals as a fingerprint of the electronic structure of massless Dirac fermions of scalable synthesis route of one- or few-layer material with the monolayer or few-layer graphene Going beyond the Figure 4 summarizes micro-Raman experiments at nm existing surface-science studies on epitaxial graphene on Ru excitation on single- and two-layer epitaxial graphene on Ru Samples with a to address this key issue.
With the addition of the second LEEM19 , a technique capable of structural fingerprinting in graphene layer, peaks appear at frequencies close to those of the submicrometre surface areas. Spectroscopic stacks of images of G and 2D bands Fig.
In Raman maps, the centre position and given surface domain. When combined with dynamical multiple width of these bands remain constant over large areas within scattering calculations of the low-energy electron reflectivity20 , two-layer epitaxial graphene domains several square micrometres this information can be used to determine the spacings of our in size Fig.
We have applied this method to bare continuous area within one of the sampled islands. Ru , as well as single- and two-layer epitaxial graphene on The double-resonance process that gives rise to the 2D band Ru Measured and simulated I V curves are in excellent has been used to distinguish monolayer and two-layer graphene On graphene, a best For cleaved graphene, the 2D band is defined largely by the fit between experimental and theoretical I V curves is obtained dispersion and splitting of electronic bands at the Brillouin zone for a unique set of layer spacings.
We determine a separation of boundary: a single peak is observed for monolayer graphene, 1. Figure 5 Measurement of interlayer electrical transport. Dark and light blue curves correspond to mechanical loading and unloading, respectively. Inset: Exponential scaling of interlayer resistance with calculated layer spacing. The observation of a single narrow peak suggests that our At identical probe spacing, the measured intralayer and two-layer samples closely match the electronic structure of cleaved interlayer resistances differ significantly.
The interlayer layer and Ru metal. It determines consistent with results obtained on cleaved graphene, the carrier the anisotropy between the in-plane and c -axis conductance of density of which is increased by a gate-induced electric field23—25 , bulk graphite26 , affects electronic transport in multiwall carbon suggesting that the Fermi level in the epitaxial two-layer graphene nanotubes27,28 and nanotube bundles29 and governs charge transfer is shifted away from the Dirac point.
The across the graphene sheets Fig. Early the coupled vibrational and electronic signatures in Raman experiments on graphite subjected to high hydrostatic pressures spectroscopy indicate that the second-layer epitaxial graphene indeed showed a lowering of the c -axis resistance at high pressure A similar type of measurement can be realized on individual This decoupling should also be reflected in the interlayer micrometre-sized graphene domains using our nanomanipulated electronic transport.
To evaluate carrier transport through epitaxial electrical probes Fig. With probes 3 and 4 placed on G1 , graphene stacks, we have carried out room-temperature four- probes 1 and 2—in contact with G2 —are moved along the sample probe transport measurements in UHV, using an instrument normal to deform G2.
The relative stiffness of the tungsten probe tip that enables the controlled positioning of probes on the sample and the graphene layer generates a large mechanical advantage, n, surface under a field-emission scanning electron microscope in the range — , that is, a sub-angstrom deformation of G2 can Fig.
Selected-area low-energy electron diffraction was over a much larger range of loading. Local intensity—voltage I V characteristics spacing between G2 and G1. For low bias voltages few millivolts , were obtained from real-space images of uniform Ru metal, one-layer and all measured current—voltage characteristics, I1,4 V2,3 , are linear. We thus achieved a faithful representation of respectively, and me denotes the electron effective mass.
Assuming the out-of-plane layer separations at reasonable computational efficiency. A comparison with graphene on Raman maps were acquired by measuring complete spectra on a 0.
Figure 4b,c was obtained by lorentzian fits to shows surprisingly similar substrate interactions in both cases: a the G and 2D Raman bands, and plotting the spatial distribution of the Raman first graphene layer is spaced closely 1. This Electrical transport measurements were carried out in UHV in a commercial layer, which will have distinct electronic and chemical properties system Omicron Nanotechnology that enables positioning of four that are yet to be explored, may be seen as a buffer layer supporting independent probe tips with nanometre accuracy on the sample while the second graphene sheet that is largely decoupled structurally and observing the process by field-emission scanning electron microscopy SEM.
Their tips were placed above selected epitaxial in the level of structural control achievable. The large first- resistance between G1 and G2. The different stiffnesses of the probe wire layer graphene domains could be isolated if etch processes are found 10 mm long, 0. It can be predicted that the weakly bound second deformations of G2. The resulting reduction of the separation between G1 and graphene layer be transferred to another substrate, for example, G2 Fig.
Using the interlayer resistances at 3. All four-probe current—voltage graphene sheets conformally on semiconductor Ge ref.
Finally, our demonstration of an atomic-layer programmable semiconductor test system Keithley, model SCS. Geim, A. The rise of graphene. Nature Mater. Novoselov, K. Two-dimensional gas of massless Dirac fermions in graphene. Bostwick, A. Quasiparticle dynamics in graphene. Nature Phys. Electric field effect in atomically thin carbon films. Science , — Berger, C.
Electronic confinement and coherence in patterned epitaxial graphene. Chen, Z. Graphene nano-ribbon electronics. Physica E 40, — Schedin, F. Detection of individual gas molecules adsorbed on graphene. Epitaxial graphene. Solid State Commun. Two-dimensional Ir cluster lattice on a Time-lapse LEEM movies were obtained during growth of the first and second graphene moire on Ir Coraux, J.
Structural coherency of graphene on Ir Nano Bachtold, A. Aharonov—Bohm oscillations in carbon nanotubes.
Epitaxial graphene on ruthenium
Abstract Graphene has been used to explore the fascinating electronic properties of ideal two-dimensional carbon, and shows great promise for quantum device architectures. The primary method for isolating graphene, micromechanical cleavage of graphite, is difficult to scale up for applications. Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly affect the electronic properties of epitaxial graphene layers. Here, we show that epitaxy on Ru produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the first graphene layer indeed interacts strongly with the metal substrate, the second layer is almost completely detached, shows weak electronic coupling to the metal, and hence retains the inherent electronic structure of graphene. Our findings demonstrate a route towards rational graphene synthesis on transition-metal templates for applications in electronics, sensing or catalysis. Graphene, a two-dimensional honeycomb lattice of sp2-bonded carbon atoms1, has shown a wealth of exceptional properties such as anomalous behaviour in the integer quantum Hall effect2 and in quasiparticle coupling3, which are signatures of charge carriers behaving as massless Dirac fermions.