Armchair Edge Structures Research Articles

Density Functional Theory Study of Adhesion Mechanism between Epoxy Resins Cured with 4,4'-Diaminodiphenyl Sulfone and 4,4'-Diaminodiphenylmethane and Carboxyl Functionalized Carbon Fiber.

The molecular mechanism of adhesion of two epoxy resins based on diglycidylether of bisphenol A (DGEBA) cured with 4,4'-diaminodiphenyl sulfone (DDS) and 4,4'-diaminodiphenylmethane (DDM) to the carbon fiber (CF) surface is investigated by employing density functional theory (DFT) calculations. The CF surface was modeled by the armchair-edge structure of graphite functionalized with carboxyl (COOH) groups. Two adhesion interfaces were constructed using the CF surface: one with the DGEBA-DDS molecule (CF/DGEBA-DDS interface) and the other with the DGEBA-DDM molecule (CF/DGEBA-DDM interface). The interfacial properties were analyzed by calculating the maximum adhesion stress (Smax) at the interface. The adhesion stress-displacement curve revealed that Smax is 1160.37 MPa, higher for the CF/DGEBA-DDS interface compared to the CF/DGEBA-DDM interface, which is 1060.48 MPa. The energy decomposition analysis showed a similar DFT contribution to adhesion stress for both interfaces, but the dispersion contribution is more significant at the CF/DGEBA-DDS interface. The crystal orbital Hamilton population (COHP) analysis revealed distinct interfacial interactions despite similar DFT contributions. Hydrogen bonding (H-bonding) between the functional groups at both interfaces including feeble OH-π interactions between the benzene rings of epoxy resins and COOH groups on the CF surface were observed. The orbital interaction energies calculated from integrated COHP, i.e., IpCOHP, revealed that the CF/DGEBA-DDS interface has six H-bonding interactions with large absolute IpCOHP values (>1 eV), whereas the CF/DGEBA-DDM interface has five. The interaction between the amine group of the DGEBA-DDM molecule and the CF surface has a large IpCOHP value among all interactions. The sulfone group being at the center of the DDS molecule and its strong surface interaction positioned the DGEBA-DDS molecule closer to the CF surface than the DGEBA-DDM molecule, enhancing dispersion interaction at the CF/DGEBA-DDS interface. Hence, the CF surface exhibits a stronger affinity toward the DGEBA-DDS molecule than the DGEBA-DDM molecule through dispersion interaction.

Electronic Properties of Single‐Layer and Bilayer Graphene Nanoribbons

Herein, a tight‐binding description is utilized to investigate electronic structures and density of states of single‐layer (SL) and bilayer (BL) graphene ribbons with and without the influence of external electric fields. Analyses are implemented to reveal the similarity and difference among electronic properties of three types of structures (specifically SL and BL ribbons with AA and AB stackings). Moreover, both armchair and zigzag edge orientations are considered. It is indicated in the results that variation in electronic properties of these structures in the presence of external electric fields depends on both structural form and edge orientation. The following two points are demonstrated: 1) a transverse field has a significant effect on adjusting bandgap of the zigzag configurations, whereas a vertical electric field has a distinct impact on energy bands of armchair edge structures, and 2) among considered structures, AB‐stacking BL ribbons are invariably the structures that are most strongly influenced by external fields. Interestingly, AB‐stacking BL ribbons are also capable of enlarging bandgap with both edge terminations. Herein, an insight into how the electronic structure and charge distribution of both SL and BL graphene nanoribbons can be controlled not only in armchair but also in zigzag edge terminations using electric stimulants is provided by the results.

A Curved Graphene Nanoribbon with Multi-Edge Structure and High Intrinsic Charge Carrier Mobility

Structurally well-defined graphene nanoribbons (GNRs) have emerged as highly promising materials for the next-generation nanoelectronics. The electronic properties of GNRs critically depend on their edge topologies. Here, we demonstrate the efficient synthesis of a curved GNR (cGNR) with a combined cove, zigzag, and armchair edge structure, through bottom-up synthesis. The curvature of the cGNR is elucidated by the corresponding model compounds tetrabenzo[a,cd,j,lm]perylene (1) and diphenanthrene-fused tetrabenzo[a,cd,j,lm]perylene (2), the structures of which are unambiguously confirmed by the X-ray single-crystal analysis. The resultant multi-edged cGNR exhibits a well-resolved absorption at the near-infrared (NIR) region with a maximum peak at 850 nm, corresponding to a narrow optical energy gap of ∼1.22 eV. Employing THz spectroscopy, we disclose a long scattering time of ∼60 fs, corresponding to a record intrinsic charge carrier mobility of ∼600 cm2 V-1 s-1 for photogenerated charge carriers in cGNR.

Tightly Pitched Sub-10 Nm Nanoribbons Grown Via Seeded Anisotropic Synthesis on Ge(001)

Graphene nanoribbons with sub-10 nm widths and smooth armchair edges possess exceptional electrical and thermal properties making them a material of interest for next-generation electronics. However, the difficulty of synthesizing ribbons with the qualities required for device applications has remained a challenge. Ribbons must have sub-10 nm widths, lengths greater than 100 nm, controlled orientation and placement, and a smooth, armchair edge structure. Top-down lithographic patterning of monolayer graphene into ribbons can address the issues of placement, orientation, and length. However, patterning resolution limits ribbons to greater than 10 nm widths and they suffer from poor edge structure, which degrades electrical and thermal properties. Bottom-up techniques such as the unzipping of graphite and nanotubes can result in narrow ribbons with smooth edges but do not provide control over placement, orientation, and edge chirality. Another method is polymerization, which results in a smooth, armchair edge structure and sub-10 nm widths but it lacks control over orientation and placement and the ribbons possess short lengths (~20 nm). Our group has shown the CVD growth of graphene on Ge(001) directly yields long, smooth armchair edged, semiconducting graphene nanoribbons [1-2]. The ribbons evolve from the bottom-up from CH4 due to highly anisotropic growth kinetics. The ribbons possess exceptional charge transport properties, as compared to others in literature, demonstrating an on/off conduction of 2x104 and an on-state conductance of 5 µS [3]. However, this fabrication method causes nanoribbons (with two orientations possible) to stochastically nucleate at random locations and times giving rise to length, width, and bandgap polydispersity. We have recently shown that seed-mediated growth of graphene nanoribbons on Ge(001) enables control over ribbon position and orientation, reduces length and width polydispersity, and enables the fabrication of nanoribbon arrays [4]. However, the diameter of our graphene seeds before growth was unknown, the effect of pitch (ribbon to ribbon distance) on ribbon growth was not studied, and sub-10 nm ribbon arrays were not achieved. Here, we present on the synthesis of sub-10 nm graphene nanoribbons from sub-10 nm seeds, at pitches varying from 50 to 500 nm, for the first time. We use lithography to create graphene seeds that vary from 20 to 55 nm in diameter and then etch the seeds in an Ar/H2 environment before nanoribbon synthesis to create sub-10 nm seeds. We find that the evolution of nanoribbons from the seeds proceeds similarly, regardless of pitch – indicating that growth is dictated by attachment limited processes that do not depend on inter-nanoribbon separation. These findings are used to fabricate tightly pitched arrays of registered, unidirectionally aligned nanoribbons, with sub-10 nm widths and lengths exceeding 200 nm, for the first time. These results show that seed-mediated nanoribbon growth is a viable route for creating dense arrays of nanoribbons for semiconductor electronics. [1] Jacobberger, R. M. et al. Nat. Commun. 2015, 6, 8006. [2] Kiraly, B. et al. Appl. Phys. Lett. 2016, 108, 213101. [3] Jacobberger, R. M. et al. ACS Nano 2017, 11, 8924−8929. [4] Way, A. J. et al. Nano Letters 2018, 18, 898.

Density Functional Theory Study of the Role of an Carbon–Oxygen Single Bond Group in the NO–Char Reaction

The mechanism of the reaction between char and nitric oxide (NO) on zigzag and armchair edge structures was studied based on density functional theory (DFT). Four possible pathways were presented t...

Synergy of Electrostatic and van der Waals Interactions in the Adhesion of Epoxy Resin with Carbon-Fiber and Glass Surfaces

Abstract The adhesion between epoxy resin and carbon fiber is investigated by using pair interaction energy decomposition analysis (PIEDA), by which the adhesive interaction energy and adhesive force can be partitioned into the electrostatic, exchange-repulsion, charge-transfer, and van der Waals (dispersion) contributions. The three stabilizing electrostatic, charge-transfer, and dispersion effects are correlated with the destabilizing exchange-repulsion effect. The surface structures of carbon fiber are modeled by the basal face, the armchair-edge structure, and the OH-functionalized armchair-edge structure of graphite. The surface of α-cristobalite (covered with OH groups), which can be viewed as a good model of a hydrophilic glass surface, is also studied. Adhesive properties of the model interfaces are evaluated on the basis of the binding energy of the resin with the carbon and glass surfaces and the adhesive force acting at the interfaces in terms of energy decomposition. PIEDA calculations demonstrate that only dispersion interactions can substantially work in the hydrophobic surfaces of the basal face and armchair-edge structures. This is a direct consequence of the electrostatic and charge-transfer interactions being cancelled by the exchange-repulsion interactions. On the other hand, both electrostatic and dispersion interactions are significant in the OH-functionalized surfaces of the armchair edge of graphite and α-cristobalite.

Poly(ethylene oxide) Functionalized Graphene Nanoribbons with Excellent Solution Processability.

Structurally well-defined graphene nanoribbons (GNRs) have attracted great interest as next-generation semiconductor materials. The functionalization of GNRs with polymeric side chains, which can widely broaden GNR-related studies on physiochemical properties and potential applications, has remained unexplored. Here, we demonstrate the bottom-up solution synthesis of defect-free GNRs grafted with flexible poly(ethylene oxide) (PEO) chains. The GNR backbones possess an armchair edge structure with a width of 1.0-1.7 nm and mean lengths of 15-60 nm, enabling near-infrared absorption and a low bandgap of 1.3 eV. Remarkably, the PEO grafting renders the GNRs superb dispersibility in common organic solvents, with a record concentration of ∼1 mg mL(-1) (for GNR backbone) that is much higher than that (<0.01 mg mL(-1)) of reported GNRs. Moreover, the PEO-functionalized GNRs can be readily dispersed in water, accompanying with supramolecular helical nanowire formation. Scanning probe microscopy reveals raft-like self-assembled monolayers of uniform GNRs on graphite substrates. Thin-film-based field-effect transistors (FETs) of the GNRs exhibit a high carrier mobility of ∼0.3 cm(2) V(-1) s(-1), manifesting promising application of the polymer-functionalized GNRs in electronic devices.

Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum

Transferring molecular nanostructures from one surface to another in ultrahigh vacuum (UHV) by mechanical contact might be a possible route to avoid the severe limitations of in situ molecular synthesis on technologically relevant template surfaces. Here, transfer printing in UHV of molecular structures between metal surfaces is investigated by a combination of scanning tunneling microscopy and scanning electron microscopy/energy dispersive x-ray spectroscopy. The authors present the complete procedure of the printing and characterization process. Microstructured Au-coated MoS₂ samples exhibiting a periodic pillar structure are used as stamp surfaces with Au(111) single crystals as target surface. Polymers of 1,3,5-tris(4-bromophenyl)benzene molecules and graphene nanoribbons with an armchair edge structure are grown on the pillars of the stamp surface. After bringing the two surfaces in mechanical contact, the transferred material is found on the target while decapping occurs on the stamp surface. Polymer structures are probably buried under the transferred stamp material, and in rare cases, evidence for molecular structures is found in their vicinity.

Polarized micro Raman scattering spectroscopy for curved edges of epitaxial graphene

This letter performed polarized microscopic laser Raman scattering spectroscopy on the curved edges of transferred epitaxial graphene on SiO2/Si. The intensity ratio between the parallel and perpendicular polarized D band is evolved, providing a spectroscopy-based technique to probe the atomic-scale edge structures in graphene. A detailed analysis procedure for non-ideal disordered curved edges of graphene is developed combining the atomic-scale zigzag and armchair edge structures along with some point defects. These results could provide valuable information of the realistic edges of graphene at the atomic-scale that can strongly influence the performance of graphene-based nanodevices.

Anisotropic Etching of Graphite Flakes with Water Vapor to Produce Armchair‐Edged Graphene

A one-step anisotropic etching method is developed to specifically obtain armchair-edged graphene directly from graphite flakes on various substrates. The armchair edge structure of the produced graphene is verified by the atomic resolution images obtained from the fluid mode peakforce tapping AFM and the relatively high intensity of D band in the Raman spectra.

Role of Edge Oxygen Atoms on the Adhesive Interaction between Carbon Fiber and Epoxy Resin

A mechanism of the adhesion between carbon fiber and epoxy resin is studied by using density functional theory (DFT) calculations. Surface structures of carbon fiber were modeled by the armchair-edge structure of graphite functionalized with OH and COOH groups. DFT calculations were performed to construct two realistic models of adhesion interface consisting of the functionalized carbon surface and a fragment of epoxy resin. Adhesive properties of the model interfaces were evaluated based on the binding energy (Eb) between the carbon surface and the resin as well as the maximum adhesive force (Fmax) acting at the interface. Calculated values of Eb are 13.8 kcal/mol for the OH-functionalized surface and 19.1 kcal/mol for the COOH-functionalized surface. The binding energy per hydrogen bond is calculated to be 6.9 kcal/mol (OH model; two H-bonds) and 6.3 kcal/mol (COOH model; three H-bonds), both of which are virtually similar and reasonable for the bond energy of a typical OH···O hydrogen bond. Analysis of adhesive force–displacement curves derived from energy–displacement plots revealed that Fmax is 0.52 nN for the OH model and 0.70 nN for the COOH model. Calculated adhesive properties are in good agreement with those previously reported for the interface between an aluminum oxide surface and an epoxy resin [J. Phys. Chem. C 2011, 115, 11701], strongly suggesting that hydrogen bonds between the oxygen-containing functional groups play a crucial role in the adhesive interaction in the carbon fiber/epoxy resin system.

Revealing the Dual Role of Hydrogen for Growth Inhibition and Defect Healing in Polycyclic Aromatic Hydrocarbon Formation: QM/MD Simulations

Quantum mechanical molecular dynamics simulations are employed to reveal the influence of hydrogen on polycyclic aromatic hydrocarbon (PAH) formation in oxygen-lean combustion. While higher hydrogen concentration leads to the inhibition of PAH growth, it simultaneously facilitates pentagon and heptagon defect healing, leading to thermodynamically more stable PAH fragments with more hexagons. We therefore propose the existence of an optimal H/C ratio that facilitates the growth of all-hexagon-containing PAH species. Analysis of the PAH edge reconstruction in our simulations shows a near-equal ratio of armchair and zigzag edge structures. As armchair edge structures are thermodynamically considerably more stable than zigzag edge structures, the present simulations show that both kinetic and thermodynamic factors are needed to understand PAH/graphene edge reconstruction.

Direct and diffuse reflection of electron waves at armchair edges of epitaxial graphene

Scanning tunneling microscopy (STM) is adopted to characterize the commonly formed armchair edge structure of epitaxial graphene prepared by thermal decomposition of 6H-SiC. At the smooth armchair edges, patterns are usually observed as a result of quantum interference (QI) of the incident and directly reflected Bloch waves of electrons at the Fermi level and, the distinct morphologies, such as, dumbbell- and horseshoes-shaped ones can be ascribed to the phase shift of the reflected Bloch wave. However, atomically-resolved graphene lattice instead of QI patterns is imaged near the ridged armchair edges. On the analogy of light interference, when the electron waves encounter such a “rough” edge, diffuse reflection occurs and coherent conditions between the incident and reflected waves cannot be satisfied and so no QI pattern is observed. This provides a unified model describing the electronic states on graphene edges and facilitates optimal design of graphene-based electronics.

Chemically Derived, Ultrasmooth Graphene Nanoribbon Semiconductors

We developed a chemical route to produce graphene nanoribbons (GNR) with width below 10 nanometers, as well as single ribbons with varying widths along their lengths or containing lattice-defined graphene junctions for potential molecular electronics. The GNRs were solution-phase-derived, stably suspended in solvents with noncovalent polymer functionalization, and exhibited ultrasmooth edges with possibly well-defined zigzag or armchair-edge structures. Electrical transport experiments showed that, unlike single-walled carbon nanotubes, all of the sub-10-nanometer GNRs produced were semiconductors and afforded graphene field effect transistors with on-off ratios of about 10(7) at room temperature.

Metallic edges in zinc oxide nanoribbons

First principles calculations of zinc oxide nanoribbons are investigated for zigzag and armchair terminated edges. We present the effects of the atomic relaxation on the shape of the nanoribbons. The electronic properties of the nanoribbons as a function of width and thickness are studied systematically. We found that the stability of armchair edge structures is greater than the zigzag edge configurations. Our results indicate that single layered armchair ribbons are semiconductors, whereas the zigzag counterparts are metallic. However, the electronic behavior changes considerably when the thickness of the ribbon is increased.