Raman scattering at pure graphene zigzag edges

Graphene: nanoscale processing and recent applications

One of the most interesting features of graphene is the rich physics set up by the various nanostructures it may adopt. The planar structure of graphene makes this material ideal for patterning at the nanoscale. The breathtakingly fast evolution of research on graphene growth and preparation methods has made possible the preparation of samples with arbitrary sizes. Available sample production techniques, combined with the right patterning tools, can be used to tailor the graphene sheet into functional nanostructures, even whole electronic circuits. This paper is a review of the existing graphene patterning techniques and potential applications of related lithographic methods.

Raman spectroscopic determination of the length, strength, compressibility, Debye temperature, elasticity, and force constant of the C-C bond in graphene

From the perspective of bond relaxation and bond vibration, we have formulated the Raman phonon relaxation of graphene, under the stimuli of the number-of-layers, the uni-axial strain, the pressure, and the temperature, in terms of the response of the length and strength of the representative bond of the entire specimen to the applied stimuli. Theoretical unification of the measurements clarifies that: (i) the opposite trends of the Raman shifts, which are due to the number-of-layers reduction, of the G-peak shift and arises from the vibration of a pair of atoms, while the D- and the 2D-peak shifts involve the z-neighbor of a specific atom; (ii) the tensile strain-induced phonon softening and phonon-band splitting arise from the asymmetric response of the C(3v) bond geometry to the C(2v) uni-axial bond elongation; (iii) the thermal softening of the phonons originates from bond expansion and weakening; and (iv) the pressure stiffening of the phonons results from bond compression and work hardening. Reproduction of the measurements has led to quantitative information about the referential frequencies from which the Raman frequencies shift as well as the length, energy, force constant, Debye temperature, compressibility and elastic modulus of the C-C bond in graphene, which is of instrumental importance in the understanding of the unusual behavior of graphene.

Growth mechanism of hexagonal-shape graphene flakes with zigzag edges

The properties of a graphene nanostructure are strongly influenced by the arrangement of the atoms on its edge. Growing graphene nanostructures with specified edge types in practical, scalable ways has proven challenging, with limited success to date. Here we report a method for producing graphene flakes with hexagonal shape over large areas, by a brief chemical vapor deposition growth at atmospheric pressure on polished Cu catalyst foil, with limited carbon feedstock. Raman spectra show evidence that the edges of the hexagonal crystallites are predominantly oriented along the zigzag direction. Density functional theory calculations demonstrate that the edge selectivity derives from favorable kinetics of sequential incorporation of carbon atoms to the vacancies in nonzigzag portions of the edges, driving the edges to pure zigzag geometry. This work represents an important step toward realization of graphene electronics with controlled edge geometries, which might find use in digital logic applications or zigzag-edge-based spintronic devices.

Ferromagnetism in hydrogenated graphene nanopore arrays

Theoretically, the so-called zigzag edge of graphenes provides localized electrons due to the presence of flat energy bands near the Fermi level. Spin interaction makes the localized spins strongly polarized, yielding ferromagnetism. However, in most experimental studies, ferromagnetism has been observed in uncontrollable and complicated carbon-based systems. Here, we fabricate graphenes with honeycomblike arrays of hexagonal nanopores, which have a large ensemble of hydrogen-terminated and low-defect pore edges that are prepared by a nonlithographic method (nanoporous alumina templates). We observe large-magnitude ferromagnetism derived from electron spins localizing at the zigzag nanopore edges. This promises to be a realization of graphene magnets and novel spintronic devices.

Optical control of edge chirality in graphene

We performed optical annealing experiments at the edges of nanopatterned graphene to study the resultant edge reconstruction. The lithographic patterning direction was orthogonal to a zigzag edge. μ-Raman spectroscopy shows an increase in the polarization contrast of the G band as a function of annealing time. Furthermore, transport measurements reveal a 50% increase of the GNR energy gap after optical exposure, consistent with an increased percentage of armchair segments. These results suggest that edge chirality of graphene devices can be optically purified post electron beam lithography, thereby enabling the realization of chiral graphene nanoribbons and heterostructures.

Raman signature of graphene superlattices

When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An atomic force microscopy system is used to produce and identify specific rotationally stacked bilayer graphenes that demonstrate the validity of our model.

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.

Quantifying defects in graphene via Raman spectroscopy at different excitation energies

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

Functionalizing metal nanostructured film with graphene oxide for ultrasensitive detection of aromatic molecules by surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) as a powerful analytical tool has gained extensive attention. Despite of many efforts in the design of SERS substrates, it remains a grand challenge for creating a general substrate that can detect diverse target analytes. Herein, we report our attempt to address this issue by constructing a novel metal-graphene oxide nanostructured film as SERS substrate. Taking advantages of the high affinity of graphene oxide (GO) toward aromatic molecules and the SERS property of nanostructured metal, this structure exhibits great potential for diverse aromatic molecules sensing, which is demonstrated by using crystal violet (CV) with positive charge, amaranth with negative charge, and neutral phosphorus triphenyl (PPh(3)) as model molecules.

Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene

We show that graphene chemical vapor deposition growth on copper foil using methane as a carbon source is strongly affected by hydrogen, which appears to serve a dual role: an activator of the surface bound carbon that is necessary for monolayer growth and an etching reagent that controls the size and morphology of the graphene domains. The resulting growth rate for a fixed methane partial pressure has a maximum at hydrogen partial pressures 200-400 times that of methane. The morphology and size of the graphene domains, as well as the number of layers, change with hydrogen pressure from irregularly shaped incomplete bilayers to well-defined perfect single layer hexagons. Raman spectra suggest the zigzag termination in the hexagons as more stable than the armchair edges.

All-optical high-resolution nanopatterning and 3D suspending of graphene

We introduce a laser-based technique capable of both imaging and patterning graphene with high spatial resolution. Both tasks are performed in situ using the same confocal microscope. Imaging graphene is based on the recombination of a laser-created electron-hole plasma yielding to a broadband up- and down-converted fluorescence. Patterning is due to burning graphene by local heating causing oxidation and conversion into CO(2). By shaping the laser beam profile using 1D phase-shifting plates and 2D vortex plates we can produce graphene dots below 100 nm in diameter and graphene nanoribbons down to 20 nm in width. Additionally, we demonstrate that this technique can also be applied to freely suspended graphene resulting in freely suspended graphene nanoribbons. We further present a way of freely hanging graphene vertically and imaging it in 3D. Taking advantage of having vertically hanging graphene for the first time, we measure the out-of-plane anisotropy of the upconversion fluorescence.

A precise method for visualizing dispersive features in image plots

In order to improve the advantages and the reliability of the second derivative method in tracking the position of extrema from experimental curves, we develop a novel analysis method based on the mathematical concept of curvature. We derive the formulas for the curvature in one and two dimensions and demonstrate their applicability to simulated and experimental angle-resolved photoemission spectroscopy data. As compared to the second derivative, our new method improves the localization of the extrema and reduces the peak broadness for a better visualization on intensity image plots.

Atomic resolution imaging of the edges of catalytically etched suspended few-layer graphene

Nanostructured graphene and graphene nanoribbons have been fabricated by catalytic hydrogenation, and the edge smoothness has been examined via direct imaging with atomic resolution. When abstaining from solvents during sample preparation, the prepared nanoribbons possess clean edges ready for inspection via transmission electron microscopy (TEM). Edges with subnanometer smoothness could be observed. A method has been developed to make catalytic hydrogenation experiments compatible with TEM, which enables monitoring of the nanoparticles prior to and after hydrogenation. In this way, etching of free-standing few-layer graphene could be demonstrated. Our results enable evaluation of the degree of edge control that can be achieved by means of catalytic hydrogenation.

Localized States and resultant band bending in graphene antidot superlattices

We fabricated dye sensitized graphene antidot superlattices with the purpose of elucidating the role of the localized edge state density. The fluorescence from deposited dye molecules was found to strongly quench as a function of increasing antidot filling fraction, whereas it was enhanced in unpatterned but electrically backgated samples. This contrasting behavior is strongly indicative of a built-in lateral electric field that accounts for fluorescence quenching as well as p-type doping. These findings are of great interest for light-harvesting applications that require field separation of electron-hole pairs.

Low temperature raman study of the electron coherence length near graphene edges

We developed a novel optical defocusing method for studying spatial coherence of photoexcited electrons and holes near edges of graphene. Our method is applied to measure the localization l(D) of the disorder-induced Raman D band (∼1350 cm(-1)) with a resolution of a few nanometers. Raman scattering experiments performed in a helium bath cryostat reveal that as temperature is decreased from 300 to 1.55 K, the length l(D) increases. We found that the localization of the D band varies as 1/T(1/2), giving strong evidence that l(D) scales with the coherence length of photoexcited electrons near graphene edges.