Kevlar-functionalized graphene nanoribbon for polymer reinforcement

Multiwalled carbon nanotubes (MWCNTs) have been widely used as reinforcement fillers in past decades. However, the reinforcement effect has been greatly hindered by the limited available interface area (AIA) with polymer matrices for polymer composites. Successively, the method of oxidative unzipping MWCNTs into graphene nanoribbons (GNRs) was demonstrated to be the effective way for addressing the inherent drawback of MWCNTs. However, the GNRs are easy to agglomerate in polymer matrix even at relatively low loading amount. In this paper, we found that the functionalization of GNRs with Kevlar^® can significantly improve the dispersion state of GNRs in polymer matrix. Consequently, Kevlar^®-functionalized graphene nanoribbons (KGNRs) were successfully prepared through non-covalent functionalization of π–π stacking interaction between the aromatic area of Kevlar^® and the graphitic surface of GNRs. As-prepared KGNRs were characterized by FT-IR, TGA, XRD and TEM measurements. Poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) were selected as model polymers to investigate the reinforcement effect of KGNRs. The KGNRs could be well dispersed in PVC and PMMA matrices at relatively high loading level. Meantime, the ultimate tensile strengths and Young's modulus of KGNRs/PVC and KGNRs/PMMA composite films were significantly improved. Based on the observations above, KGNRs hold great promise in many potential applications in the future.     

Graphene nanoribbons (GNRs) by unzipping MWCNTs for the improvement of PMMA microcellular foams

This work presents the cellular microstructures and properties of PMMA/graphene nanoribbons (GNRs) microcellular foams. GNRs were obtained by oxidative unzipping multiwalled carbon nanotubes and solvent thermal reduction in dimethylformamide (DMF), then they were mixed with PMMA to fabricate PMMA/GNRs nanocomposites by solution blending. Subsequently, supercritical carbon dioxide (scCO₂) as a friendly foaming agent was applied to fabricate PMMA/GNRs microcellular foam by a batch foaming in a special mold. The morphology of cell structure was analyzed by scanning electron microscopy and image software, showing that the addition of a smaller content of GNRs caused a fine cellular structure with a higher cell density (～3 × 10¹¹ cells/cm³) and smaller cell sizes (～1 μm) due to their remarkable heterogeneous nucleation effect. The mechanical testing of PMMA/GNRs microcellular foams demonstrated that the obtained GNRs also could be used as a reinforcing filler to increase the mechanical properties of PMMA foams. An improvement in the compressive strength of ～80% (about 39% increase standardized by specific compressive strength) was achieved by 1.5 wt % GNRs addition, and the thermal stability of PMMA/GNRs foams was enhanced too. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45182.     

The production of multi-layer graphene nanoribbons from thermally reduced unzipped multi-walled carbon nanotubes

An easy and scalable approach is reported for the production of multi-layer graphene nanoribbons (GNRs) from thermally treated unzipped multi-walled carbon nanotubes (MWCNTs) by controlled oxidation and intercalation. The prepared GNRs are characterized using transmission and scanning electron microscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. AFM studies show that the thickness of the unzipped MWCNTs lies in the range of 100–124 nm, which correspond to ∼150–185 GNRs, whereas the width is in the ranges of 500–700 nm. This could be due to the displacement of loose individual graphene layers in the solvent during sonication process. The irregular edges of the multi-layer GNR appeared due to the presence of functional groups attached at the edges, is confirmed by XPS. The XPS studies reveals that the amount of defects present on the nanoribbons after heat treatment at 1000 °C is almost same as that of as synthesized MWCNTs. However, on heat treatment at 2500 °C, defects are reduced and the quality of ribbon is improved. Also, Raman spectroscopy has confirmed that on heat treatment at 2500 °C the quality of GNRs is improved and I(D)/I(G) ratio decreases from 0.92 to 0.44.     

In situ transmission electron microscopy of electrochemical lithiation,delithiation and deformation of individual graphene nanoribbons

We report an in situ transmission electron microscopy study of the electrochemical behavior of few-layer graphene nanoribbons (GNRs) synthesized by longitudinal splitting the multi-walled carbon nanotubes (MWCNTs). Upon lithiation, the GNRs were covered by a nanocrystalline lithium oxide layer attached to the surfaces and edges of the GNRs, most of which were removed upon delithiation, indicating that the lithiation/delithiation processes occurred predominantly at the surfaces of GNRs. The lithiated GNRs were mechanically robust during the tension and compression tests, in sharp contrast to the easy and brittle fracture of the lithiated MWCNTs. This difference is attributed to the unconfined stacking of planar carbon layers in GNRs leading to a weak coupling between the intralayer and interlayer deformations, as opposed to the cylindrically confined carbon nanotubes where the interlayer lithium produces large tensile hoop stresses within the circumferentially-closed carbon layers, causing the ease of brittle fracture. These results suggest substantial promise of graphene for building durable batteries.     

Conductance recovery and spin polarization in boron and nitrogen co-doped graphene nanoribbons

We present an ab initio study of the structural, electronic, and quantum transport properties of B–N-complex edge-doped graphene nanoribbons (GNRs). We find that the B–N edge codoping is energetically a very favorable process and furthermore can achieve novel doping effects that are absent for the single B or N doping. The compensation effect between B and N is predicted to generally recover the excellent electronic transport properties of pristine GNRs. For the zigzag GNRs, however, the spatially localized B–N defect states selectively destroy the doped-side spin-polarized GNR edge currents at the valence and conduction band edges. We show that the energetically and spatially spin-polarized currents survive even in the fully ferromagnetic metallic state and heterojunction configurations. This suggests a simple yet efficient scheme to achieve effectively smooth GNR edges and graphene-based spintronic devices.     

Spectral phonon thermal properties in graphene nanoribbons

This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.     

The effect of the functionalization of carbon nanofibers on their electronic conductivity

The effects of the functionalization of carbon nanofibers (CNFs) on their electronic conductivity, in addition to their physico-chemical properties have been studied. Oxygen surface groups have been created on the surface of three CNFs with different properties, following three oxidation treatments with diverse severity. The oxygen content increased from two to six times the original content, depending on the CNF texture, from 1.5–2.6 wt.% up to 15.1 wt.%. Whereas some important properties are not significantly modified after functionalization (texture, crystalline structure, etc.), other properties like the electronic conductivity are affected depending on the extent of the process. The electronic conductivity of CNFs decreases from 200–350 S m⁻¹ up to 20–100 S m⁻¹ (the precise value depends on carbon crystallinity and compaction degree) when surface oxygen content increases from 1.5 wt.% to 5 wt.%. A further oxidation degree leads to a 90% decrease in conductivity, and in the end can even destroy the original fibrous structure. As a first approach, oxidizing at room temperature with rather strong acid solutions is a better strategy to create functional groups and maintain the electronic conductivity than increasing the process temperature with less severe oxidizing agents.     

Graphene nanoribbon ceramic composites

By unzipping multi-walled carbon nanotubes (MWCNTs) it is possible to obtain graphene nanoribbons (GNRs) that could then be used as fillers in ceramic composites. Here we report the fabrication of silicon nitride (Si₃N₄) ceramics with different contents of GNRs by spark plasma sintering. The GNR fillers confer electrical conductivity to the Si₃N₄ composites, following a semiconducting-like behavior at relatively low volume filler concentrations (0.04). In addition, a toughening effect, produced by GNRs bridging the cracks was observed. GNRs appear to be an efficient alternative to graphene-based composites, useful in the fabrication of novel multifunctional ceramic composites.     

Thermal conductivity of MWCNT/epoxy composites: The effects of length,alignment and functionalization

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ Tⁿ (n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.     

Correlation between atomistic morphology and electron transport properties in defect-free and defected graphene nanoribbons: An interpretation through Clar sextet theory

Structural, electronic and transport properties of defect-free, defected and functionalized armchair and zig-zag graphene nanoribbons (GNRs) are investigated with density functional theory and non-equilibrium Green’s function calculations and rationalized in terms of Clar’s theory of the aromatic sextet. Calculations suggest a tight relationship between the transport properties of nanoribbons and the underlying bond patterning as described by valence bond and Clar sextet theory. Namely, armchair GNRs exhibit a strong dependence of the transport properties on the ribbon width, as a consequence of different valence bond representations. The occurrence of localized defects involving electron pairs does not significantly alter this behavior. Conversely, transport properties of zigzag GNRs are less affected by morphological details, such as width and occurrence of defects, as expected from the application of Clar’s theory. However, controlled edge functionalization and morphology modifications in zigzag GNRs can potentially lead to localization of aromatic sextets and, consequently, to strong changes in the transport properties. Our work indicates Clar sextet theory as a powerful and accurate tool to rationalize and predict the electronic and transport properties of complex carbon nanostructures based on GNRs. These principles can be extended to the design of novel systems with potential applications in nanoelectronics.     

Evolution of graphene nanoribbons under low-voltage electron irradiation

Though the all-semiconducting nature of ultrathin graphene nanoribbons (GNRs) has been demonstrated in field-effect transistors operated at room temperature with ～10(5) on-off current ratios, the borderline for the potential of GNRs is still untouched. There remains a great challenge in fabricating even thinner GNRs with precise width, known edge configurations and specified crystallographic orientations. Unparalleled to other methods, low-voltage electron irradiation leads to a continuous reduction in width to a sub-nanometer range until the occurrence of structural instability. The underlying mechanisms have been investigated by the molecular dynamics method herein, combined with in situ aberration-corrected transmission electron microscopy and density functional theory calculations. The structural evolution reveals that the zigzag edges are dynamically more stable than the chiral ones. Preferential bond breaking induces atomic rings and dangling bonds as the initial defects. The defects grow, combine and reconstruct to complex edge structures. Dynamic recovery is enhanced by thermal activation, especially in cooperation with electron irradiation. Roughness develops under irradiation and reaches a plateau less than 1 nm for all edge configurations after longtime exposure. These features render low-voltage electron irradiation an attractive technique in the fabrication of ultrathin GNRs for exploring the ultimate electronic properties.     

Effects of quantum statistics of phonons on the thermal conductivity of silicon and germanium nanoribbons

We present molecular dynamics simulation of phonon thermal conductivity of semiconductor nanoribbons with an account for phonon quantum statistics. In our semiquantum molecular dynamics simulation, dynamics of the system is described with the use of classical Newtonian equations of motion where the effect of phonon quantum statistics is introduced through random Langevin-like forces with a specific power spectral density (color noise). The color noise describes interaction of the molecular system with the thermostat. The thermal transport of silicon and germanium nanoribbons with atomically smooth (perfect) and rough (porous) edges are studied. We show that the existence of rough (porous) edges and the quantum statistics of phonon change drastically the low-temperature thermal conductivity of the nanoribbon in comparison with that of the perfect nanoribbon with atomically smooth edges and classical phonon dynamics and statistics. The rough-edge phonon scattering and weak anharmonicity of the considered lattice produce a weakly pronounced maximum of thermal conductivity of the nanoribbon at low temperature.     

Ab initio density functional studies of the restructuring of graphene nanoribbons to form tailored single walled carbon nanotubes

A method based on density functional theory calculations is proposed for the preparation of chiral controlled single walled carbon nanotubes (SWCNTs) by tailoring the edges of bi-layered graphene nanoribbons (GNRs). We find that armchair edged bi-layered GNRs are highly stable and need to be compressed to overcome the energy barrier to form zigzag SWCNTs, while the zigzag edged bi-layered GNRs are intrinsically highly unstable and immediately form armchair SWCNTs. We have investigated the rehybridization of orbitals of carbon atoms in the process of nanotube formation. Nanotube formation is found to be assisted by the edge ripples along with the intrinsic edge reactivity of different types of bi-layered GNRs. Utilizing these results we show that it may be possible to produce high specificity chiral controlled SWCNTs and pattern them for nanoscale device applications.     

Bromohydration and bromination of the pendant vinylbenzene groups of commercial poly(divinylbenzene-co-ethylvinylbenzene) resins with subsequent modifications of the (1,2-dibromoethyl)benzene groups: thiol-containing styrenic resins

Amberlite XAD-4, the commercial form of poly(divinylbenzene-co-ethylvinylbenzene), was modified by bromohydration and bromination of its pendant vinylbenzene groups. The vinylbenzene groups were completely converted to (1,2-dibromoethyl)benzene groups, with a degree of functionalization of 35%. Bromohydration of XAD-4 created a resin with (1-hydroxy-2-bromoethyl)benzene groups, with a degree of functionalization of 24%. Further modifications to the (1,2-dibromoethyl)benzene groups of the brominated resin were done using thiourea, which led to a resin with a mixture of functional groups, including isothiouronium bromide; and using N,N-dimethylthioformamide with subsequent methanolysis, which led to a resin with thiol groups.     

Structure-mediated thermal transport of monolayer graphene allotropes nanoribbons

We report the study of the thermal transport management of monolayer graphene allotrope nanoribbons (size ∼20 × 4 nm²) by the modulation of their structures via molecular dynamics simulations. The thermal conductivity of graphyne (GY)-like geometries is observed to decrease monotonously with increasing number of acetylenic linkages between adjacent hexagons. Strikingly, by incorporating those GY or GY-like structures, the thermal performance of graphene can be effectively engineered. The resulting hetero-junctions possess a sharp local temperature jump at the interface, and show a much lower effective thermal conductivity due to the enhanced phonon–phonon scattering. More importantly, by controlling the percentage, type and distribution pattern of the GY or GY-like structures, the hetero-junctions are found to exhibit tunable thermal transport properties (including the effective thermal conductivity, interfacial thermal resistance and rectification). This study provides a heuristic guideline to manipulate the thermal properties of 2D carbon networks, ideal for application in thermoelectric devices with strongly suppressed thermal conductivity.     

Electronic thermal conductivity and thermopower of armchair graphene nanoribbons

A theoretical investigation of the diffusion contribution to thermopower, S_d, and the electronic thermal conductivity, κ_e, of semiconducting armchair graphene nanoribbons (GNRs) is made for T ⩽ 300 K. Considering the electrons to be scattered by edge roughness, impurities and deformation-potential coupled acoustic phonons and optical phonons, expressions for S_d and κ_e are obtained. Numerical calculations of S_d and κ_e, as functions of temperature and linear carrier density, bring out the relative importance of the contributing scattering mechanisms. A GNR of width 5 nm, supporting an electron density 2 × 10⁸ m⁻¹, is found to exhibit room temperature values of S_d and κ_e as 42 μV/K and 26.5 W/mK, respectively. A decrease in armchair GNR width, is found to enhance S_d and reduce κ_e. The effect of varying the electron density is to increase their magnitude when Fermi energy moves into the second subband. An analysis of thermopower and thermal conductivity data in clean armchair GNR samples will enable better understanding of the electron–phonon interaction.     

Synthesis of graphene nanoribbons with various widths and its application to thin-film transistor

Novel precursor polymers containing phenylene, naphthalene and anthracene units were synthesized for fabrication of graphene nanoribbons (GNRs) by the Suzuki coupling reaction between dibrominated monomers and diboronic ester monomers. The precursor polymers were converted into GNRs by intramolecular cyclodehydrogenation using FeCl₃ as a catalyst. The degree of cyclodehydrogenation was determined by analysis of nuclear magnetic resonance spectra. All GNR films show ambipolar charge transport behavior in thin-film transistor (TFT). The GNR film prepared from anthracene-based polymer exhibits the highest TFT performance due to its longer conjugation length and larger width of nanoribbon than GNRs prepared from phenylene and naphthalene-based polymers.     

A microexplosion method for the synthesis of graphene nanoribbons

Graphene nanoribbons (GNR) have been fabricated by a microexplosion method without severe oxidation – filling multi-walled carbon nanotubes (MWCNT) with potassium and then reacting with water vigorously. Transmission electron microscopy and scanning transmission electron microscopy have verified the synthesis mechanism: when MWCNTs are effectively filled with potassium, the microexplosion generated by reaction between water and potassium can split the MWCNTs to form GNRs. Most of the obtained GNRs have smooth edges and the maximum wall thickness of MWCNTs that can be split by this method is around 10 nm.     

Edge-decorated graphene nanoribbons by scandium as hydrogen storage media

On the basis of density functional theory calculations, we show that edge-decorated graphene nanoribbons (GNRs) by scandium can bind multiple hydrogen molecules in a quasi-molecular fashion. The average adsorption energy of H(2) on Sc ranges from 0.17 to 0.23 eV, ideally suited to hydrogen storage. For the narrowest GNR with either armchair or zigzag edges, the predicted weight percentage of H(2) is >9 wt%, exceeding the gravimetric target value set by the Department of Energy (DOE). The bonding energy between Sc and the GNR is significantly greater than the cohesive energy of bulk Sc so that clustering of Sc will not occur once Sc is bonded with carbon atoms at the edge of GNRs. Moreover, the adsorption energy of H(2) can be modestly tuned (either enhanced or reduced) by applying an external electric field.     

A theoretical analysis of the thermal conductivity of hydrogenated graphene

We investigate the thermal conductivity of hydrogenated graphene using non-equilibrium molecular dynamics simulations. It is found that the thermal conductivity greatly depends on the hydrogen distribution and coverage. For random hydrogenation, the thermal conductivity decreases rapidly with increasing coverage up to about 30%. Beyond this limit, however, the thermal conductivity is almost insensitive to the coverage. For patterned hydrogenation with stripes parallel to the heat flux, the thermal conductivity decreases gradually with increasing coverage from 0% to 100%. In contrast, when the stripe direction is perpendicular to the heat flux, a small (5%) coverage causes a sharp (60%) drop of thermal conductivity. The deterioration of thermal conductivity is due to the sp²-to-sp³ bonding transition upon hydrogenation, which softens the G-band phonon modes. Percolation theory can be used to explain the variation of thermal conductivity at different hydrogenation distributions and coverages. The applicability of the rule of mixtures in predicting the thermal conductivity is also discussed. The work suggests that hydrogenation is a possible route to tune graphene thermal conductivity and manage heat dissipation in graphene-based nanoelectronic devices.