Zigzag Orientation Research Articles

Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties

We investigated the atomic structures, Raman spectroscopic and electrical transport properties of individual graphene nanoribbons (GNRs, widths ~10-30 nm) derived from sonochemical unzipping of multiwalled carbon nanotubes (MWNTs). Aberration-corrected transmission electron microscopy (TEM) revealed a high percentage of two-layer (2 L) GNRs and some single-layer ribbons. The layer-layer stacking angles ranged from 0° to 30° including average chiral angles near 30° (armchair orientation) or 0° (zigzag orientation). A large fraction of GNRs with bent and smooth edges was observed, while the rest showed flat and less smooth edges (roughness ≤1 nm). Polarized Raman spectroscopy probed individual GNRs to reveal D/G ratios and ratios of D band intensities at parallel and perpendicular laser excitation polarization (D(∥)/D(⊥)). The observed spectroscopic trends were used to infer the average chiral angles and edge smoothness of GNRs. Electrical transport and Raman measurements were carried out for individual ribbons to correlate spectroscopic and electrical properties of GNRs.

Graphene Edge from Armchair to Zigzag: The Origins of Nanotube Chirality?

The energy of an arbitrary graphene edge, from armchair (A) to zigzag (Z) orientation, is derived in analytical form. It contains a "chemical phase shift" determined by the chemical conditions at the edge. Direct atomistic computations support the universal nature of the relationship, definitive for graphene formation, and shapes of the voids or ribbons. It has further profound implications for nanotube chirality selection and possibly control by chemical means, at the nucleation stage.

Direct determination of the crystallographic orientation of graphene edges by atomic resolution imaging

In this letter, we show how high-resolution scanning tunneling microscopy (STM) imaging can be used to reveal that certain edges of micromechanically exfoliated single layer graphene crystals on silicon oxide follow either zigzag or armchair orientation. Using the cleavage technique, graphene flakes are obtained that very often show terminating edges seemingly following the crystallographic directions of the underlying honeycomb lattice. Performing atomic resolution STM-imaging on such flakes, we were able to directly prove this assumption. Raman imaging carried out on the same flakes further validated our findings.

First-principles study of graphene edge properties and flake shapes

We use density functional theory to determine the equilibrium shape of graphene flakes, through the calculation of the edge orientation dependence of the edge energy and edge stress of graphene nanoribbons. The edge energy is a nearly linear function of edge orientation angle; increasing from the armchair orientation to the zigzag orientation. Reconstruction of the zigzag edge lowers its energy to less than that of the armchair edge. The edge stress for all edge orientations is compressive, however, reconstruction of the zigzag edge reduces this edge stress to near zero. Hydrogen adsorption is favorable for all edge orientations; dramatically lowering all edge energies and all edge stresses. It also removes the reconstruction of the zigzag edge. Using the new edge energy data, we determine the equilibrium shape of a graphene sheet (with unreconstructed edges) to be hexagonal with straight armchair edges in the presence and absence of hydrogen. However, zigzag edge reconstruction produces graphene flakes with a six-fold symmetry, but with rounded edges. This shape is dominated by near zigzag edges. The compressive edge stresses will lead to edge buckling (out-of-the-plane of the graphene sheet) for all edge orientations, in the absence of hydrogen. Exposing the graphene flake to hydrogen dramatically decreases the buckling amplitude.

Evaluation of the effect of column orientation in type-I high-speed counter-current chromatography

The performance of type-I high-speed counter-current chromatography was evaluated by changing the column inclination against the rotating centrifugal force field. The separations were performed with two different solvent systems composed of 1-butanol-acetic acid-water (4.75:0.25:5, v/v) (BAW) and hexane-ethyl acetate-methanol-0.1 M HCl (1:1:1:1, v/v) (HEMW) using dipeptides and DNP-amino acid as test samples, respectively. A set of short coiled columns connected in series is mounted around the holder hub in two different ways: in the parallel orientation, all column units are arranged in parallel to each other and mounted on the holder at various angles against the horizontal plane. In the zigzag configuration, the neighboring units of the same column are mounted symmetrically forming various angles apart. In the parallel configuration, for both the BAW and HEMW systems, Sf (the retention of stationary phase) first increased as the column angle decreased from 90 degrees to 60 degrees and then decreased, as the column angle further decreased from 60 degrees to 0 degrees, while Rs (peak resolution) continually declined over the entire column angle range from 90 degrees to 0 degrees. But, for both solvent systems, with the zigzag configuration, retention of stationary phase and resolution both decreased as the column angle decreased from 90 degrees to 0 degrees. In general, Sf and Rs for separation of dipeptides in the BAW system, from 90 degrees to 15 degrees, is better for the parallel orientation than for the zigzag configuration. However, at 0 degrees, Sf and Rs are better for the zigzag orientation. In the DNP-amino acid separation with the HEMW system, retention of the stationary phase and Rs for the parallel orientation is better than that for the zigzag orientation from 90 degrees to 30 degrees, whereas from 30 degrees to 0 degrees the results are opposite. Over all results of our studies revealed that the formally used column orientation at 90 degrees inclination yields the highest peak resolution in both solvent systems.

Reciprocal Translocations in Grass Pea (Lathyrus sativus L.): Pattern of Transmission, Detection of Multiple Interchanges and their Independence

Grass pea (Lathyrus sativus L.) is a diploid crop having 2n = 14 chromosomes. Four different plant types, heterozygous for reciprocal translocation (RT), RT-1, RT-2, RT-3, and RT-4 showing pollen semisterility, were isolated in induced mutant population of three varieties of grass pea. Transmission rate of these 4 translocations was studied in advanced selfed and intercrossed progenies, whereas multiple chromosomal interchanges were detected by the pattern of chromosomal ring formation at meiosis I in the F(1) progeny of RTxRT. RT transmitted at an average of 48.89% in selfed progeny and along with normal plants, RT and translocation homozygotes produced some trisomic plants in the segregation progeny. RT-3 showing maximum frequency (68.20%) of zig-zag or alternate chromosome orientation exhibited highest transmission (55.20%). Presence of a ring of 6 or 2 rings of 4 chromosomes in F(1) double heterozygotes obtained from RTxRT suggested involvement of one common chromosome in translocations of RT-1, RT-2, and RT-4 but a different one in common between RT-2 and RT-3. No common chromosome, however, was shared by RT-3 and RT-4. Thus, 5 out of 7 chromosomes are involved in the present RTs, and one of the translocated chromosomes in RT-1 line was always associated with nucleolar organizing region.

Structural transformations in graphene studied with high spatial and temporal resolution

Graphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects, and has also been used as a support for samples in high-resolution transmission electron microscopy and as a transparent electrode in photovoltaic devices. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beam-induced sputtering. Aberration-corrected, low-voltage, high-resolution transmission electron microscopy with sub-ångström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process. We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations.

Electronics and Magnetism of Patterned Graphene Nanoroads

Individual ribbons of graphene show orientation-dependent electronic properties of great interest, yet to ensure their perfect geometry and integrity or to assemble free ribbons into a device remains a daunting task. Here we explore, using density functional theory, an alternative possibility of "nanoroads" of pristine graphene being carved in the electrically insulating matrix of fully hydrogenated carbon sheet (graphane). Such one-dimensional entities show individual characteristics and, depending upon zigzag (and their magnetic state) or armchair orientation, can be metallic or semiconducting. Furthermore, the wide enough zigzag roads become magnetic with energetically similar ferro- and antiferromagnetic states. Designing magnetic, metallic, and semiconducting elements within the same mechanically intact sheet of graphene presents a new opportunity for applications.

Raman Spectroscopy of Graphene Edges

Graphene edges are of particular interest since their orientation determines the electronic properties. Here we present a detailed Raman investigation of graphene flakes with edges oriented at different crystallographic directions. We also develop a real space theory for Raman scattering to analyze the general case of disordered edges. The position, width, and intensity of G and D peaks are studied as a function of the incident light polarization. The D-band is strongest for polarization parallel to the edge and minimum for perpendicular. Raman mapping shows that the D peak is localized in proximity of the edge. For ideal edges, the D peak is zero for zigzag orientation and large for armchair, allowing in principle the use of Raman spectroscopy as a sensitive tool for edge orientation. However, for real samples, the D to G ratio does not always show a significant dependence on edge orientation. Thus, even though edges can appear macroscopically smooth and oriented at well-defined angles, they are not necessarily microscopically ordered.

Charge transport in disordered graphene-based low dimensional materials

Two-dimensional graphene, carbon nanotubes, and graphene nanoribbons represent a novel class of low dimensional materials that could serve as building blocks for future carbon-based nanoelectronics. Although these systems share a similar underlying electronic structure, whose exact details depend on confinement effects, crucial differences emerge when disorder comes into play. In this review, we consider the transport properties of these materials, with particular emphasis on the case of graphene nanoribbons. After summarizing the electronic and transport properties of defect-free systems, we focus on the effects of a model disorder potential (Anderson-type), and illustrate how transport properties are sensitive to the underlying symmetry. We provide analytical expressions for the elastic mean free path of carbon nanotubes and graphene nanoribbons, and discuss the onset of weak and strong localization regimes, which are genuinely dependent on the transport dimensionality. We also consider the effects of edge disorder and roughness for graphene nanoribbons in relation to their armchair or zigzag orientation.

A dimer-to-dimer metal–metal linear aggregate from a nitrate-bridgedcis-(2,2′:6′,2′′-terpyridyl-κ3N)palladium(II) cofacial dimer

The asymmetric unit of the title compound, μ-nitrato-bis­[(2,2′:6′,2′′-terpyridyl-κ3N)palladium(II)] hexa­kis(hexa­fluorido­phosphate) acetonitrile solvate, [Pd2(C15H11N3)2NO3]PF6·C2H3N, contains two cationic dimers, [Pd2(C15H11N3)2NO3]3+, six (PF6)− anions and one CH3CN solvent mol­ecule. Each cationic dimer is built upon (μ-1,3-NO3) bridging coordination to two cis-(2,2′:6′,2′′-terpyridine)PdII units in a cofacial arrangement. The Pd atom is four-coordinated by a tridentate chelating 2,2′:6′,2′′-terpyridine ligand and one bridging nitrate ion, to form a distorted square-planar geometry. The two dimers aggregate alternately along a linear chain via Pd⋯Pd inter­actions [intra­dimer, 3.09 (1) Å; inter­dimer, 3.33 (1) Å]. The dimer-to-dimer aggregates arrange in a zigzag orientation along the b axis through π–π inter­actions [centroid–centroid distances 3.521 (6)–3.894 (6)Å] between terpyridine ligands and weak inter­molecular hydrogen bonds involving hexa­fluorido­phosphate anions (C—H⋯F) and the solvent acetonitrile mol­ecule (C—H⋯N).

A proton-doped calix[4]arene-based conducting polymer.

Segmented conducting polymers based upon a calix[4]arene scaffold are reported. The cone conformation creates a zigzag orientation of the polymer segments. Their acid-dependent conductivities are similar to the strong pH conductivity dependence of polyaniline which is said to be acid dopable. On the other hand, they have a segmented structure that imposes greater localization of the carriers. The conductivity of such a system can be considered to result from rapid self-exchange between discrete units. Hence, electron exchange between radical cations and p-diquinone salts produces the high conductivity of these polymers.

Influence of tool path strategy on the cycle time of high-speed milling

This work at first look discusses the influence of the tool path strategy on the cycle time of high-speed milling operations. Experiments and predictions were focused on pocketing operations with a zig-zag tool path, quantifying the significant discrepancy between the programmed feed rate and the actual average feed rate. A mechanistic approach for cycle time evaluation is proposed. The mechanistic model construction is based on the experimental measurement of the machine tool acceleration and specific geometric assumptions regarding tool motion. For high feed rates, the proposed approach is capable of capturing the influence of the zig-zag tool path orientation on the machining cycle time.

STM investigation of the growth structure of Cu-phthalocyanine films with submolecular resolution

In this study Cu-phthalocyanine (CuPc) films have been investigated by scanning tunneling microscopy to get quantitative insight into the morphology of the films from the micrometer range down to nanometer resolution to see the arrangement of the molecules. The molecules on the surface have been found to be arranged in a slipped stacking order rather than in the expected zig-zag orientation normally found for CuPc crystals. The cystal structure could be identified to be an α-polymorph. The STM image of the individual molecules on the surface is in good agreement with calculations of the charge density of the LUMO for an isolated molecule. A model for the growth of the crystals was derived from this study.

Solidification-induced and shear-induced band texture in thermotropic liquid crystalline polymer films

Abstract It has been observed that the band texture of thermotropic liquid crystalline polymers (LCPs) is formed at the shearing temperature during the relaxation of shear. The band spacing increases with increasing shearing temperature between T m and T i, but is independent of shearing rate within the range of our experiments. The angle of zigzag orientation of the molecular chain fibrils with respect to the direction of shear is independent of the shearing temperature. The relaxation rate of the shear-induced band texture depends on the width of the bands formed. The wider the band spacing is, the slower the relaxation. When specimens with shear-induced bands of different spacings are annealed at a certain temperature for 10 min, they are all converted to a new band texture of the same spacing of around 1 μm. These bands with a small spacing generated during cooling are termed the solidification-induced band texture. When the specimen with shear-induced bands is annealed for a short time, so that the b...

Trends in the electrosorption of terminal glycols at the mercury/water interface

The adsorption behaviour of terminal glycols from ethanediol (ED) to 1,7-heptanediol (HPD) at the mercury/water interface was determined from capacitive charge measurements carried out by a computerized chronocoulometric technique. The low value of the maximum surface coverage of 1,4-butanediol (BD) with respect to 1,3-propanediol (PPD) and 1,5-pentanediol (PTD), as well as the high values of the differential capacity at maximum coverage of both ED and BD with respect to PPD and PTD, strongly suggest that the even members of the series are adsorbed on the electrode surface in a flatter orientation than the corresponding odd members. Further evidence in favour of this interpretation is provided by the comparatively higher values of ▪ and by the less attractive values of the Frumkin interaction factor exhibited by ED and BD with respect to PPD and PTD; here, Δ G o ads is the standard Gibbs energy of adsorption and σ m is the charge density of maximum adsorption. This behaviour is explained by noting that one molecule of ED or BD in a flat zig-zag orientation may accommodate exactly one or two water molecules bridging the two terminal hydroxyl groups, respectively. 1,6-Hexanediol shares some features of ED and BD, although to a minor extent.

Epitaxial polymerization of acetylene

AbstractEpitaxial polymerization of acetylene was studied using systematic series of homologous isomorphic compounds as substrates for the purpose of establishing techniques for preparing well oriented polyacetylenes. From electron microscopy, it was confirmed that polyacetylenes grow on the substrate in the form of fibrils whose orientation is strongly dependent on the lattice matching between polyacetylene and substrate crystals: e.g. with naphthalene and anthracene as substrates the fibrils assume zigzag orientation in three specific directions on the crystal but with biphenyl and terphenyl the zigzag fibrils assume predominantly two specific directions. The mechanism of epitaxial growth during polymerization on the various substrates is discussed on the basis of the features of the epitaxial fibrils.

The rotational angle dependence of 5JmH,SH in benzenethiol

It is suggested that the five-bond spin–spin coupling constant between the sulfhydryl proton and the ring proton in the meta position is given by [Formula: see text]. Here θ is the angle by which the S—H bond twists out of the benzene plane and angular brackets indicate expectation or average values, determined by the twofold barrier to internal rotation about the C—S bond. [Formula: see text] is the π electron contribution and has a maximum at θ = 90° and [Formula: see text] is the σ electron contribution with a maximum at θ = 180° (zig-zag orientation). For eight para substituted derivatives of benzenethiol, the 5J numbers can be reproduced by this equation, which also agrees with the observed couplings in 2-hydroxybenzenethiol. In this compound θ lies near 90°. It appears that this approach will allow an extension of the J method to the evaluation of two-term potential functions in, for example, ortho substituted benzene derivatives.