Session L33: Graphene: Synthesis and Nanoribbons

Band alignment in atomically precise graphene nanoribbon junctions

Building atomically precise graphene nanoribbon (GNR) heterojunctions down to molecular level opens a new realm to functional graphene-based devices. By employing a surface-assisted self-assembly process, we have synthesized heterojunctions of armchair GNRs (aGNR) with widths of seven, fourteen and twenty-one carbon atoms, denoted 7, 14 and 21-aGNR respectively. A combined study with scanning tunneling microscopy (STM) and density functional theory (DFT) allows the visualization of electronic band structures and energy level alignments at the heterojunctions with varying widths. A wide bandgap (\textasciitilde 2.6 eV) has been identified on semiconducting 7-aGNR, while the 14-aGNR appears nearly metallic and the 21-aGNR possesses a narrow bandgap. The spatially modulations of the energy bands are strongly confined at the heterojunctions within a width of about 2 nm. Clear band bending of about 0.4 eV and 0.1 eV are observed at the 7--14 and 14--21 aGNR heterojunctions, respectively. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.   

Single-Step Seeded-Growth of Graphene Nanoribbons (GNRs) via Plasma-Enhanced Chemical Vapor Deposition (PECVD)

One of the main challenges in the fabrication of GNRs is achieving large-scale low-cost production with high quality. Current techniques, including lithography and unzipped carbon nanotubes, are not suitable for mass production. We have recently developed a single-step PECVD growth process of high-quality graphene sheets without any active heating. By adding some substituted aromatic as seeding molecules, we are able to rapidly grow GNRs vertically on various transition-metal substrates. The morphology and electrical properties of the GNRs are dependent on the growth parameters such as the growth time, gas flow and species of the seeding molecules. On the other hand, all GNRs exhibit strong infrared and optical absorption. From studies of the Raman spectra, scanning electron microscopic images, and x-ray/ultraviolet photoelectron spectra of these GNRs as functions of the growth parameters, we propose a model for the growth mechanism. Our findings suggest that our approach opens up a pathway to large-scale, inexpensive production of GNRs for applications to supercapacitors and solar cells.   

Theory of pore-edge interactions and resulting patterns in graphene nanoribbons

We report a systematic analysis of pore-edge interactions in graphene nanoribbons (GNRs) and their outcomes based on first-principles calculations and atomic-scale simulations according to reliable interatomic potentials. Energetic computations reveal strongly attractive interactions when the edge of a nanopore (vacancy cluster) in the GNR and a GNR edge are in close proximity to each other. We derive functional forms for these interaction potentials and characterize the effects of pore size and GNR edge type on these interactions. We conduct molecular-dynamics (MD) simulations of nanopore dynamics at high temperature for nanopores in the vicinity of GNR edges under the thermodynamic driving force for nanopore migration provided by the attractive pore-edge interactions. We find that pore migration toward the GNR edge is mediated by a sequence of carbon ring reconstructions, which cause pore coalescence with the GNR edge. Subsequent morphological evolution of the GNR edge leads to edge pattern formation, controlled by the pore size and the GNR edge type. We construct the underlying optimal kinetic pathways through nudged-elastic-band calculations and characterize the electronic structure of the resulting GNRs with patterned edges.   

Graphene nanoribbons and nanosheets incorporated in Ag, Al alloys and Cu bulk and thin films

Incorporation of carbon nanostructures in materials has gained high interest because of the possibility of improving the properties of the material due to the properties of the carbon nanostructures. In many instances, however, the material obtained is a composite in which the metal and the carbon do not share any bonds. We are investigating metals with high carbon content, called covetics, where the carbon is introduced by the application of a DC current \textgreater 100 A to a mixture of the liquid metal and particles of activated carbon. The carbon in covetics forms ribbon like structures of graphene and nano sheets which are bonded to the metal and are, therefore, very stable. We are investigating the structure and properties of Ag, Al and Cu covetics in bulk and thin films as well as the form and structure of the carbon in the metal. 3D epitaxy of graphene nanoribbons with the lattice is observed in both Ag and Al covetics. The carbon has primarily sp2 bonding as obtained by Raman scattering, XPS and EELS. Films of copper covetics grown by PLD preserve the carbon structure and present higher transmittance to light and higher resistance to oxidation than pure copper films making them great candidates for transparent electrodes.   

Ab initio study of the mechanism of bottom-up synthesis of graphene nanoribbons

Graphene nanoribbons (GNRs) can be fabricated with atomic precision by using molecular precursors deposited on a metal substrate, and potentially form the basis for future molecular-scale electronics. The precursor molecules are first annealed to form a polymer, and further annealing at a higher temperature leads to the formation of a GNR. We systematically study the reaction pathways of this cyclodehydrogenation process, using density functional theory and the nudged elastic band method. We find that the Au substrate reduces the reaction barriers for key steps in the cyclodehydrogenation process: cyclization, hydrogen migration and desorption. Furthermore, our calculations explain recent experiments showing that an STM-tip can induce local polymer-to-GNR transition, which can be used to fabricate atomically precise heterojunctions: at a negative bias, the STM tip injects excess holes into the polymer HOMO state, lowering the energy barrier in agreement with Woodward-Hoffmann rules. At a positive bias, when excess electrons are injected into the LUMO state, the energy barrier is not significantly lowered and the transition is not observed.   

Designing the Gauge Potential for Dirac Fermions in Nanoscale Strain-Engineered Graphene

Non-trivial strain in graphene is known to induce pseudomagnetic fields ($B_{s})$ that can significantly affect the properties of Dirac fermions. We have employed nearly strain-free PECVD-grown graphene$^{1}$ to induce controllable strain and gauge potentials by placing graphene on substrates with either lithographically prepared nanostructures or synthesized nanocrystals.$^{2}$ Here we report the use of Pd tetrahedron nanocrystals (55nm laterally and 45nm in height) for strain-engineering of graphene. The nanocrystals were spin-coated on a Si substrates and then covered by a monolayer of h-BN followed by a monolayer of graphene. Comparison between the Raman 2D band of strained-graphene and that of as-grown graphene confirmed an increase of average strain in the former. Molecular dynamics simulations revealed alternating signs of $B_{s}$ with three-fold symmetry, and the maximum magnitude of $B_{s}$ was up to \textasciitilde 2000 T. These results will be compared with scanning tunneling spectroscopic studies for spatially varying $B_{s}$ and alternating presence and absence of the zero mode at two inequivalent sites of graphene due to local time-reversal symmetry breaking.$^{3}$ \newline [1] D. A. Boyd \textit{et al. Nat. Comm.} \textbf{6}, 6620 (2015). \newline [2] N.-C. Yeh \textit{et al. Acta Mech. Sin.} \textbf{32}, 497 (2016). \newline [3] N.-C. Yeh \textit{et al. Surf. Sci.} \textbf{605}, 1649 (2011).   

Near-field microscopy of transferred graphene-graphene heterostructures: Interplay between materials properties and preparation procedure

Micro-Raman and scattering-type Scanning Near-field Optical Microscopy (s-SNOM) are well established graphene characterization methods that can go beyond imaging mode: thorough analysis of the signals obtained can provide direct mapping of materials properties of graphene. While pristine graphene has been already studied intensively, heterostructures, including those combining “infinite” 2D-layers with confined 1D- or 0D-objects (wires or dots), received less attention so far. Starting with the high quality CVD graphene (on Cu) we mastered transfer techniques that produce monolayers with a series of islands of a second layer, making natural bilayer heterostructures of different type. Both properties determined by the symmetry of graphene-graphene heterostructures (such as size quantization, edge symmetry, rotational commensurability) and by the transfer procedure and the substrate (like doping level, strain, warping and wrinkling) can reflect on the optical response. Mid-infrared s-SNOM interferometric microscopy has revealed sensitivity of the signal to lattice matching. This was correlated to the large scale micro-Raman mapping followed by statistical principal component analysis to obtain relation between intrinsic materials properties and those from the preparation methods.   

Theory of near-field response of graphene-graphene plasmonic heterostructures: Symmetry breaking in rotationally disordered bilayer graphene

High field confinement – one of the important goals in nanophotonics devices - has been already demonstrated in graphene plasmonics. Confined (vs. propagating) plasmons, that give even stronger field localization, were observed in graphene with metallic resonators/antennas and in nanostructured graphene. However hybridization of plasmons with the resonances of the substrate which happens frequently in ordinary devices impeded studies of intrinsic wavefunctions of plasmons so far. With the help of the mid-infrared scattering-type Scanning Near-field Optical Microscopy direct mapping of angular distribution of plasmon wave functions is demonstrated here in graphene-graphene heterostructure made of a nano-disk covered by a monolayer graphene. Using excitation frequencies well above SiO2 substrate resonances a clean quantized plasmonic signal was obtained. This talk will further present the model of plasmon hybridization which allows to explain experimental angular patterns of confined plasmonic modes. In particular, mixing of angular momentum is induced by the rotational disorder in bilayer disk lattices. Moire pattern of the rotationally displaced graphene lattice is shown to produce confined modes of the mixed symmetry, not expected in the ordinary 2DEG plasmonics.   

Low temperature synthesis of graphene on arbitrary substrates and its transport properties

Here we report the direct synthesis of uniform and vertically oriented graphene films on multiple substrates including glass, Si/SiO2, and copper foil by radio-frequency plasma enhanced chemical vapor deposition (PECVD) using methane as the carbon precursor at relatively low temperatures. Raman spectra of all the samples show characteristic Raman peaks of graphene. The temperature dependence of electrical transport properties such as 4-probe resistance, thermo electrical power and hall mobility were measured for graphene grown on glass substrates at varying temperature from 500 $^{\circ}$ C to 700 $^{\circ}$ C. The morphological and surface characteristics were also studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This work demonstrates the potential of low temperature and transfer-free graphene growth for future graphene-based electronic applications.   

Investigating the two-dimensional conductivity of CVD graphene using surface acoustic waves

Surface acoustic waves (SAW) propagating on a piezoelectric substrate can be a sensitive probe of the dynamical conductivity of a nearby two-dimensional electron system (2DES). Enhanced absorption of acoustic energy can occur when the wavelength, or frequency, of the SAW become comparable to some other length, or time, scale within the 2DES. We implement SAW measurements of the frequency and wavevector dependent conductivity of graphene grown via chemical vapor deposition (CVD). We measure the conductivity at low temperatures and high magnetic fields utilizing a flip-chip SAW device, with access to multiple frequencies by employing higher SAW harmonics. Finally, we report on progress to extend these measurements to higher mobility graphene devices.   

Cu Single Crystal Substrates for Growth of CVD Graphene

To provide a systematic study of the CVD graphene growth process, a study of the growth of graphene on single crystal Cu substrates, with terminations along the (100), (110), and (111) planes, was performed. Synthesis was performed in an ultra-high vacuum (UHV) chamber using a modified setup to allow growth at pressures as high as 1 Torr. Ethylene was used as the precursor gas. To control Cu sublimation at the elevated growth temperatures, an Ar overpressure was used. This arrangement allowed for the preparation of clean Cu surfaces by sputtering and annealing the Cu crystals in UHV, followed by graphene growth at low pressure, and in-situ analysis with low energy electron diffraction. It was found that surface termination plays a strong role in the rotational alignment of the nucleating graphene grains and the decomposition rate of the ethylene. It was observed that single-domain epitaxy is possible on Cu(111) when the ethylene pressure is 5 mTorr or less. However, growth on both Cu(100) and Cu(110) result in a minimum of two domains. In addition, ex-situ EELS is currently being performed on well-ordered epitaxial graphene films grown on Cu(111) and Cu(100) to determine the effect of the graphene-Cu interaction on the electronic properties of the graphene.