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![Fig. S5. Armchair edges. The figure shows the edge of a ribbon in a natural step along the [[ ̅ ] direction. The atomic resolution allows to determine the armchair orientation of the edge.](profile/Maya-Narayanan-Nair/publication/269183540/figure/fig6/AS:557991810535425@1510047022966/Fig-S5-Armchair-edges-The-figure-shows-the-edge-of-a-ribbon-in-a-natural-step-along.png)
Fig. S5. Armchair edges. The figure shows the edge of a ribbon in a natural step along the [[ ̅ ] direction. The atomic resolution allows to determine the armchair orientation of the edge.
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Graphene nanoribbons grown on sidewall facets of SiC have demonstrated exceptional quantized ballistic transport up to 15µm at room temperature. Angular-Resolved Photoemission spectroscopy (ARPES) has shown that the ribbons have the band structure of charge neutral graphene, while bent regions of the ribbon develop a bandgap. We present scanning tu...
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Citations
... 24,25 Thereby, the width and edge topology of the GNRs is tunable and dened by the SiC-mesas and allows the growth of both gapped armchair and ballistic zigzag GNRs due to connement and hybridization of the edges, respectively. 8,26,27 Armchair graphene nanoribbons (AGNRs) provide a large exibility and are either metallic or semiconducting depending on the number N of sp 2 hybridized C-atoms across the ribbon. In a recent photoemission experiment the formation of 1D conned AGNRs was demonstrated. ...
... 8 The average ribbon width was around 2 nm and all features of the measured 2D band structure of an ensemble of around 500 AGNRs were consistent with 1D connement eects, assuming AGNRs with mainly three dierent widths N =16, 18,20. Compared to the on-surface synthesized AGNRs, the epitaxial ribbons on SiC supports are dierent as seen by scanning tunneling microscopy: 25,26 the instability of the SiC sidewall running along the [1120]-direction and which are on average inclined by approximately 27 • comes along with step bunching resulting in the formation of small stripes of SiC(0001) terraces of 1 nm in width. As a consequence, the p z states of the selectively grown graphene get saturated from the dangling bonds of the Si-terminated SiC(0001) terraces forming stripes of buer layers (BL) with sp 3 -like hybridized C-atoms with suspended AGNRs in between. ...
... N=18 AGNR) in agreement with previous studies. 26 This one-dimensional connement gives rise to robust quantization and formation of subbands, as seen by recent ARPES measurements. 8 Despite the outstanding homogeneity, which we achieved, the facets reveal partly irregularities and the local variation of the widths manifests with the observation of metallic and semiconducting bands. ...
... Made of carbon with sp 2 hybridization, GNRs can be obtained as graphene cuttings or unzipped carbon nanotubes [4,5]. GNRs can also be produced using bottom-up approaches, which lead to outstanding edge control [6][7][8]. Indeed, the edge of a GNR significantly influences its physical properties, especially its electronic structure, which can be metallic or semiconducting [9][10][11]. The edge geometry is defined by the chiral angle of the ribbon, which can take any value between 0 • and 30 • . ...
Zigzag graphene nanoribbons (GNRs), demonstrating edge magnetism, are fascinating materials for electronic and spintronic applications. In this paper, we investigate with explicit consideration of electron-phonon scattering the Seebeck effect and thermal conductivity at various carrier densities in zigzag GNRs, which undergo antiferromagnetic, ferromagnetic, and paramagnetic transitions. Seebeck coefficients are spin dependent in ferromagnetic zigzag GNRs and can have opposite signs for the majority and the minority spin carriers, which enables the separation of spin carriers spatially under a thermal gradient. The electronic thermal conductivity is small compared to the lattice thermal conductivity, except for GNRs in the ferromagnetic state. A 100% increase in thermal conductivity is expected at antiferromagnetic to ferromagnetic transition. These results demonstrate that zigzag GNRs are also materials of high interest for spin caloritronics.
... G raphene nanoribbons (GNRs) epitaxially grown on a SiC substrate have attractive properties, such as band-gap opening, [1][2][3] ballistic transport, 4,5) and quantum interference, 6) and they can be grown on the wafer scale, which gives them the advantage of scalability 7,8) for GNR applications. 9,10) One of the distinguishing features of GNR on SiC is a boundary structure in which the edge of the GNR connects to a buffer layer, a non-conductive carbon monolayer with a graphene-like honeycomb lattice partially bonded to the Si atoms of the SiC substrate. ...
An array of embedded graphene ribbons, whose edges connect to a buffer layer, can be grown on 4HSiC(0001). The Raman D peak intensity of the armchair edge of the ribbon shows the same polarization dependence as that of the non-connected armchair edge of graphene. Considering the Raman scattering process of the D peak, this polarization dependence indicates that electrons and holes in the embedded graphene by incident photons are scattered back at the boundary of the embedded graphene ribbon and buffer layer. These results indicate polarized Raman scattering spectroscopy is useful for investigating the edge structure of embedded graphene.
... In addition, high-quality crystalline SiC has been used as substrate for growing epitaxial graphene (epi-Gr), potentially considered for the next generation of field effect transistors (FETs) by taking advantage of its exceptional transport properties. Among the graphene family, epi-Gr has been highly regarded to establish post-CMOS electronics, not only because using epi-Gr can avoid contamination and damage due to extra transfer process, which is unavoidable for exfoliated and CVD graphene, but also for its unique tunability through substrate orientation [5][6][7]. Integrating epi-Gr devices in electronic circuits requires establishing reliable connections with low resistance. ...
One of the reasons why graphene attracts so much attention is its large ballistic transport mean free path, which could lead to novel electronic devices with very low power dissipation, breaking one of the key barriers that currently limit further electronics miniaturization. In particular, epi-graphene with scalable growth and high compatibility with modern semiconductor fabrication procedures, make it the most promising candidate for graphene based integrated circuits. However, in order to preserve that power advantage, it is essential to create interconnections with low contact resistance. This has been a long-standing challenge as metal contacts directly deposited on graphene tend to form weak interfaces, and any impurities and processing residues on graphene can deteriorate its electrical properties. In this work, side contacts with low resistance are fabricated to connect to the edge of epitaxial graphene grown on the non-polar face of silicon carbide. The procedure starts by depositing aluminum oxide on graphene, which serves both as protective coating and dielectric layer, before device fabrication. To assure obtaining a high quality Al2O3 layer using Atomic Layer Deposition (ALD), graphene is treated by hydrogen through plasma enhanced chemical vapor deposition forming reversible hydrogen functionalization. This is followed by ALD to grow a 15 nm-thick oxide, which covers the epitaxially grown graphene on SiC. Finally, the edge contact is built to connect to the single layer graphene, reaching remarkably low contact resistance width of about 340 Ω µm.
... This is expected to facilitate orbital mapping since the spectral features are not limited by the energy resolution of the electron source but by physical phenomena linked to, e.g., the excited state lifetime broadening, core-hole screening, or other multielectronic interactions. The π Ã and σ Ã fine structures are in good agreement with existing work on free-standing graphene layers [39,40], with a sharp excitonic feature visible around 294.5 eV. Although the edges overall look comparable for the in-and out-of-C plane probe positions, the π Ã intensity increases noticeably between the epitaxial graphene layers at out-of-C-plane positions. ...
The spatial distributions of antibonding π Ã and σ Ã states in epitaxial graphene multilayers are mapped using electron energy-loss spectroscopy in a scanning transmission electron microscope. Inelastic channeling simulations validate the interpretation of the spatially resolved signals in terms of electronic orbitals, and demonstrate the crucial effect of the material thickness on the experimental capability to resolve the distribution of unoccupied states. This work illustrates the current potential of core-level electron energy-loss spectroscopy towards the direct visualization of electronic orbitals in a wide range of materials, of huge interest to better understand chemical bonding among many other properties at interfaces and defects in solids.
... For example, graphene grown on transition metals nucleates at step edges, 33,34 and graphene grown epitaxially on highly stepped silicon carbide can exhibit pinning at step edges. 35 Two-dimensional films grown on substrates can alternatively exhibit carpet growth, flowing across the step edge. Therein, the atomic scale steps in the substrate have little to no impact on the atomic scale structure of the two-dimensional film; this is the case for carpet growth of graphene. ...
Silica films represent a unique two-dimensional film system, exhibiting both crystalline and vitreous forms. While much scientific work has focused on the atomic-scale features of this film system, mesoscale structures can play an important role for understanding confined space reactions and other applications of silica films. Here, we report on mesoscale structures in silica films grown under ultrahigh vacuum and examined with scanning tunneling microscopy (STM). Silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. Both holes in the film structure and atomic-scale substrate steps are observed to influence these coexisting phases. In particular, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions where the majority film structure is crystalline. At high coverages mixed zigzag and bilayer phases are observed at step edges, while at lower coverages silica phases with lower silicon densities are observed more prevalently near step edges. The STM images reveal that silica films exhibit rich structural diversity at the mesoscale.
... Several studies have focused on graphene grown on vicinal silicon carbide substrates, although the strongly interacting graphene buffer layer beneath the monolayer graphene is the responsible there of the modification of graphene's electronic structure [13,[27][28][29][30]. It is known that some growth procedures of epitaxial graphene grown on Pt(997), a (2x2) multidomain superstructure of graphene with weakly affected π bands and superperiodicity replicas [31].In other substrates, graphene band structure can be affected by the vicinal superperiodicity. ...
The atomic structure and electronic properties of monolayer graphene on a curved multi-nano-vicinal Pt (111) substrate are investigated with a low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Despite graphene grows continuously over the terraces and step bunching areas, the spatial periodicity also varies by varying the vicinal angle of the substrate. This superperiodicity has been evidenced by splitting of first order Pt spots in LEED and from STM. The photoemission spectroscopy unravels that the linearly dispersing π band of graphene is affected by the step periodicity as evidenced by the opening of bandgaps. The gaps opening occurs at the intersection of the main graphene band with Umklapp bands due to the superperiodicity of the one-dimensional nanostructured substrate. The energy and momentum locations of the minigaps change with the superperiodicity, which is related to the spatial periodicity and vicinal angle of the substrate. Our results show a simple way to tune the electronic properties of epitaxial graphene by tuning the substrate vicinality.
... 26,27 Electron microscopy revealed that the sidewall ribbons terminated in the SiC, thereby stabilizing and passivating the edges. [28][29][30] Sidewall ribbons exhibit dissipationless (ballistic) transport over tens of microns, even at room temperature. Surprisingly, the conductance was found to be 1 G0=1 e 2 /h rather than the predicted 2 G0 7,8,[13][14][15] where e is the charge and h is Planck's constant. ...
... In the process C-C and Si-C bonds are formed, 61 which fuse the ribbon edges to the SiC, thereby producing stable neutral edges that terminate in the SiC, as occurs in the self-assembled ribbons that are annealed 8 at ≈1500°C. [26][27][28][29]55 The alumina coating, that is used as the top gate dielectric, greatly reduces the mobility of the graphene bulk, with mfp's <10 nm ( Supplementary Fig. S7). However it does not affect the EGES with mfp's > 20 µm, leading to the observed vivid contrast between EGES transport and bulk transport. ...
The graphene edge state is essential for graphene electronics and fundamental in graphene theory, however it is not observed in deposited graphene. Here we report the discovery of the epigraphene edge state (EGES) in conventionally patterned epigraphene using plasma-based lithography that stabilizes and passivates the edges probably by fusing the graphene edges to the non-polar silicon carbide substrate, as expected. Transport involves a single, essentially dissipationless conductance channel at zero energy up to room temperature. The Fermi level is pinned at zero energy. The EGES does not generate a Hall voltage and the usual quantum Hall effect is observed only after subtraction of the EGES current. EGES transport is highly protected and apparently mediated by an unconventional zero-energy fermion that is half electron and half hole. Interconnected networks involving only the EGES can be patterned, opening the door to a new graphene nanoelectronics paradigm that is relevant for quantum computing.
... Duo to the sp 2 hybridization of the carbon atoms in graphene, many excellent properties of electricity and magnetism have been found in the atomically thin carbon film [1][2][3][4][5]. Moreover, modifying the graphene by cutting it into narrow ribbons [6][7][8][9][10], adsorbing exotic atoms [11][12][13][14][15] and through displacement doping [16][17][18] can result in various interesting proper- * Author to whom any correspondence should be addressed. 4 Request for materials should be addressed to Yan Han. ...
Our theoretical calculation and analysis show that the configuration of transition-metal (TM) atoms on iridium-doped graphene depends on the number of the d-state valence electrons of the TM atoms. TM atoms with three or less d-state valence electrons prefer to form a horizontal configuration and destroy the original C3v symmetry of the substrate. If there are more than three (but not five) d-state valence electrons in a TM atom, the TM atom selects the site just on the top of the iridium atoms and thus forms a vertical configuration, and the C3v symmetry of the iridium-doped graphene remains. For TM atoms with five d-state valence electrons and a closed s shell, the TM atoms and the iridium-doped graphene prefer to form an inclined configuration. The configuration regularity of the iridium-doped graphene-adsorbing TM atoms is attributed to the unique spin and orbital angular momentum of the electron in the iridium-doped graphene and the unique selection rule of the charge transfer under spin polarization.
... 205 Furthermore, the specific SiC polytype (i.e., 4H-SiC versus 6H-SiC) profoundly influences the structural and electrical properties of the resulting sidewall GNRs due to intricate differences in GNR−substrate interactions, termination, and facet morphology. 206,207 For instance, trenches that yield armchair GNRs on 4H-SiC (i.e., parallel to ⟨1120⟩) predominantly evolve into a single (1107) facet upon annealing, bordered by (1105) minifacets at the top and bottom terraces, 208 whereas trenches that yield zigzag GNRs on 6H-SiC (i.e., parallel to ⟨1100⟩) evolve into an array of 2−3 nm wide (11222) minifacets yielding decoupled GNRs (termed miniribbons) with different widths and densities. Similarly, whereas zigzag GNRs on 6H-SiC exhibit ballistic transport, zigzag GNRs on 4H-SiC are diffusive conductors (i.e., GNR conductance inversely correlates with length). ...
... For example, in the case of armchair GNRs synthesized on 4H-SiC, an appreciable band gap (>0.5 eV) is observed in regions of the GNR draped over (1105) minifacets due to the binding of the edge of the GNR to the buffer layer graphene, whereas the central region of the GNR draped over the central (1107) facet exhibits roomtemperature ballistic transport with no significant band gap. 208,209 Consequently, it might be possible to realize dense FET arrays wherein the semimetallic portion of the GNRs is utilized as source−drain contacts to the semiconducting regions of the GNRs. 72,210 Charge transport through the semiconducting regions, however, has not yet been measured. ...
... Although GNRs greater than 20 nm wide could be used as interconnects, 208 their BEOL integration would be challenging due to the high thermal budget of synthesis. 211 To utilize GNRs grown on SiC nanofacets as a semiconducting FET channel material, it will be necessary to either (i) reduce the overall GNR width to less than 10 nm to open an appreciable band gap in the semimetallic GNR region or (ii) learn how to exclusively synthesize miniribbons isolated from the semimetallic regions. ...
Graphene nanoribbons (GNRs) have recently emerged as promising candidates for channel materials in future nanoelectronic devices due to their exceptional electronic, thermal, and mechanical properties and chemical inertness. However, the adoption of GNRs in commercial technologies is currently hampered by materials science and integration challenges pertaining to synthesis and devices. In this Review, we present an overview of the current status of challenges, recent breakthroughs toward overcoming these challenges, and possible future directions for the field of GNR electronics. We motivate the need for exploration of scalable synthetic techniques that yield atomically precise, placed, registered, and oriented GNRs on CMOS-compatible substrates and stimulate ideas for contact and dielectric engineering to realize experimental performance close to theoretically predicted metrics. We also briefly discuss unconventional device architectures that could be experimentally investigated to harness the maximum potential of GNRs in future spintronic and quantum information technologies.
