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.     

Tuning the curing behavior of fluoroelastomer (FKM) by incorporation of nitrogen doped graphene nanoribbons (CNx-GNRs)

Graphene nanoribbons (GNRs), obtained by different methods from carbon nanotubes (CNTs) or graphene, are attractive materials for polymer nanocomposites due to their considerably high interfacial area, as compared to CNTs. Consequently, a better adhesion with a polymer matrix is anticipated for GNRs. Also, surface modification of these nanofillers, such as nitrogen doping, is known to be an efficient method to improve their properties. In this work, fluoroelastomers (FKM) were used as the polymer matrix to host GNRs. Undoped and nitrogen doped GNRs were synthesized from the parent multiwall carbon nanotubes (MWCNTs). MWCNT/FKM and GNR/FKM nanocomposites were prepared via a solution mixing/melt mixing protocol.     

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.     

Single step synthesis of graphene nanoribbons by catalyst particle size dependent cutting of multiwalled carbon nanotubes

Graphene nanoribbons are emerging as an interesting material for the study of low dimensional physics and for the applications in future electronics due to its finite energy band gap. However, its applicability for large scale nanoelectronics may not be effectively realized unless graphene nanoribbons could be produced using a simple, viable, cost-effective and scalable technique. Here, we report the one step facile synthesis of few layered graphene nanoribbons (GNRs) by catalytically unzipping multi-walled carbon nanotubes (MWCNTs) based on the solubility of carbon atoms in transition metals. The process is free from aggressive oxidants (such as KMnO(4), KClO(4), H(2)SO(4), HNO(3), etc.) and utilizes the in situ grown nickel nanoparticles for nanotube unzipping. This is an additional advantage over previously used techniques to synthesize GNRs. To observe the effect of catalyst particle size and reaction temperature on cutting length of the nanotubes, a simulation study has been done based on solubility of carbon atoms in metal nanoparticles.     

Structural and electronic properties of carbon nanowires made of linear carbon chains enclosed inside zigzag carbon nanotubes

This article presents ab initio self-consistent-field crystal orbital calculations on the structural and electronic properties for recently-discovered carbon nanowires (CNWs) made of linear carbon chains inserted inside zigzag carbon nanotubes using density functional theory. The studies focus on the change of geometric structures and electronic properties upon the encapsulation. It is found that the carbon nanotubes can stabilize the encapsulated carbon chain which prefers a dimerized structure in the tube with larger diameters. The interaction between the tube and the chain becomes more obvious when the tube size decreases, leading to the change of structures and the energy bands upon encapsulation. All the CNWs we calculated are metals with zero band gap. The encapsulation of the carbon chain may modulate the electronic properties for the CNWs depending on the tube size and the filling density of carbon atoms. Therefore, it is expected that CNWs’s electronic properties can be controlled artificially by filling carbon chains with various densities of atoms into the nanotubes.     

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.     

Separation of atoms with carbon nanotubes

Molecular dynamics are used to study the possibility of separating different species of atom using carbon nanotubes in torsion. The effect of the rate of torsional loading on a carbon nanotube on the separation of helium and carbon atoms encapsulated in the tube is examined and the release of the van der Waals energy for the atom separation is evaluated. The complete separation of atoms using carbon nanotubes would make the tubes candidates for transporting and partitioning atoms and molecules as a strong competitor for conventional membrane filters.     

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.     

Modeling of graphene nanoribbon devices

Recent advances in graphene nanoribbon (GNR) electronic devices provide a concrete context for developing simulation methods, comparing theories to experiments, and using simulations to explore device physics. We present a review on modeling of graphene nanoribbon devices, with an emphasis on electronic and magnetoresistive devices. Device modeling is reviewed in a synergistic perspective with GNR material properties, device characteristics, and circuit requirements. Similarity with and difference to carbon nanotube devices are discussed. Device modeling and simulation results are compared to experimental data, which underlines the importance of theory-experiment collaborations in this field. Importance of the GNR edges, which have a negative impact on the carrier mobility due to edge roughness but offer new possibilities of spintronic devices and edge doping, is emphasized. Advanced device modeling of GNRs needs to have the capability to describe GNR device physics, including three-dimensional electrostatics, quantum and atomistic scale effects, elastic and inelastic scattering processes, electron-electron interaction, edge chemistry, magnetic field modulation, and spintronic and thermoelectric device phenomena.     

Thermal buckling of initially compressed single-walled carbon nanotubes by molecular dynamics simulation

Thermal buckling of initially compressed single-walled carbon nanotubes subjected to a uniform temperature rise is presented by using molecular dynamics simulations. Comprehensive numerical calculations are carried out for armchair and zigzag carbon nanotubes with various geometric dimensions. The results show that thermal buckling can occur beyond a critical value of temperature when the tube is initially compressed to a point prior to buckling. The critical buckling temperature increases as the compressive load ratio parameter decreases, and varies dramatically with nanotube helicity, radius and length. Owing to strong thermal oscillations of carbon atoms, a zigzag carbon nanotube with relatively small radius can buckle at a surprisingly lower temperature than the expected one.     

Why boron nitride nanotubes are preferable to carbon nanotubes for hydrogen storage?: An ab initio theoretical study

Using ab initio calculations we investigated the hydrogen storage in single-walled boron nitride nanotubes. We present the nature of hydrogen interaction in selected sites of a (5,5) and (9,9) BN nanotube. Our results show that BN nanotubes are preferable to carbon nanotubes for hydrogen storage applications due to their heteropolar binding nature of their atoms. In addition, by increasing the nanotube's diameter – decreasing its curvature, more efficient binding energies of hydrogen can be achieved.     

Torsional buckling of carbon nanopeapods

Torsional buckling of carbon nanopeapods (carbon nanotubes filled with fullerenes) is studied using a continuum-based multi-layered shell model. The model takes into account non-bonded van der Waals interactions between nested fullerenes and the innermost layer of host nanotube. For nanopeapods with linearly arranged nested fullerenes, equivalent pressure distribution is proposed to model these interactions. Deriving explicit equations governing the torsional stability, it is concluded that the critical torsional load of a carbon nanopeapod is less than that of a carbon nanotube under otherwise identical geometric and mechanical conditions. Performing numerical calculations, it is also shown that increasing the number of layers of the host carbon nanotube decreases the weakening effect of encapsulated fullerenes on torsional stability of the nanopeapod.     

Metal-functionalized single-walled graphitic carbon nitride nanotubes: a first-principles study on magnetic property

The magnetic properties of metal-functionalized graphitic carbon nitride nanotubes were investigated based on first-principles calculations. The graphitic carbon nitride nanotube can be either ferromagnetic or antiferromagnetic by functionalizing with different metal atoms. The W- and Ti-functionalized nanotubes are ferromagnetic, which are attributed to carrier-mediated interactions because of the coupling between the spin-polarized d and p electrons and the formation of the impurity bands close to the band edges. However, Cr-, Mn-, Co-, and Ni-functionalized nanotubes are antiferromagnetic because of the anti-alignment of the magnetic moments between neighboring metal atoms. The functionalized nanotubes may be used in spintronics and hydrogen storage.     

Carbon nanotube bottles for incorporation,release and enhanced cytotoxic effect of cisplatin

Carbon nanotubes (CNTs) have emerged as promising drug delivery systems particularly for cancer therapy, due to their abilities to overcome some of the challenges faced by cancer treatment, namely non-specificity, poor permeability into tumour tissues, and poor stability of anticancer drugs. Encapsulation of anticancer agents inside CNTs provides protection from external deactivating agents. However, the open ends of the CNTs leave the encapsulated drugs exposed to the environment and eventually their uncontrolled release before reaching the desired target. In this study, we report the successful encapsulation of cisplatin, a FDA-approved chemotherapeutic drug, into multi-walled carbon nanotubes and the capping at the ends with functionalised gold nanoparticles to achieve a “carbon nanotube bottle” structure. In this proof-of-concept study, these caps did not prevent the encapsulation of drug in the inner space of CNTs; on the contrary, we achieved higher drug loading inside the nanotubes in comparison with data reported in literature. In addition, we demonstrated that encapsulated cisplatin could be delivered in living cells under physiological conditions to exert its pharmacological action.     

碳纳米管—一种新型纳米反应器

具有一维中空管结构的碳纳米管,可以用来填充各种物质。由于碳纳米管的限域作用,填充物在碳纳米管中具有新的结构和性质,其化学反应、相变等行为具有不同于常温常压状态下的特点。该文介绍了碳纳米管管内的化学反应的影响因素,反应类型和准高压效应,并总结出碳纳米管可作为一种新型纳米反应器合成不同材料,最后对这一领域的研究方向和应用进行了展望。     

Effect of crystalline filling on the mechanical response of carbon nanotubes

The electrical and mechanical properties of the same hybrid carbon nanotube before and after removal of the core Ga-doped ZnS semiconductor filling have been analysed inside a transmission electron microscope (TEM) using a conductive atomic force microscope - TEM system. It is found that the encapsulated material can substantially change the mechanical response of the turbostratic carbon tube container. Furthermore, because the extent of filling is operator-controlled, this provides a simple way to change on-demand the stiffness of hybrid carbon nanotubes.     

Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.     

Neutralized fluorine radical detection using single-walled carbon nanotube network

It is known that single-walled carbon nanotubes (SWCNTs) can be functionalized by fluorine gas. Here, we report neutralized fluorine radical detection using a matted sheet of SWCNTs, prepared by alternating current dielectrophoresis. Upon exposure to neutralized radicals containing fluorine atoms in a plasma, as model analytes, the conductance of the SWCNT matt showed fast modulation. The transduction mechanism was investigated by electrical transport measurements, X-ray photoelectron spectroscopy and Raman spectroscopy. Metallic nanotubes were shown to react covalently to the near exclusion of semiconducting species. The selectivity was promoted by the curvature-induced strain of the nanotubes. The results open new opportunities for the detection of fluorine radicals at specific locations inside the reaction zone using a simple, miniaturized carbon nanotube network.     

Effect of pentagon–heptagon defect on thermal transport properties in graphene nanoribbons

The thermal transport properties of graphene nanoribbons (GNRs) with pentagon–heptagon defect (PHD) are studied by using first principles calculations in combination with non-equilibrium Green’s function approach. The results show that the PHD effect on thermal conduction in armchair-oriented GNR is stronger than that in zigzag-oriented GNR. The out-of-plane acoustic mode is almost reflected by the PHD at a particular frequency. When the temperature is larger than 400 K, the thermal conduction ratio is only related to the PHD’s orientation, and insensitive to the width of PHD. These results will be helpful for designing high-performance thermal insulation or thermoelectric device.     

Electronic coupling in fullerene-doped semiconducting carbon nanotubes probed by Raman spectroscopy and electronic transport

We investigate the electronic properties of individual fullerene peapods by combining micro-Raman spectroscopy and (magneto)-transport measurements on the same devices. We bring evidence that the encapsulated C₆₀ molecules strongly modify the electronic band structure of semiconducting nanotubes in the vicinity of the charge neutrality point, including a rigid shift and a partial filling of the energy gap. Using a selective UV excitation of the fullerenes, we demonstrate that the electronic coupling between the contained C₆₀ molecules and the containing carbon nanotube is strongly modified by the partial coalescence of the C₆₀ and their distribution inside the tube. Our experimental results are supported by both numerical simulations of the Density of States and the measured conductance of carbon nanotubes with coalesced fullerenes inside.