Thermal conductivity of graphene nanoribbons with defects and nitrogen doping

Thermal conductivity of defective graphene nanoribbons doped with nitrogen for different distributions around the defect edge at nanoscale is investigated using the reverse non-equilibrium molecular dynamics (RNEMD) method, which explores ways to improve thermal management. In addition, thermal conductivity of graphene nanoribbons with both defects and nearby nitrogen doping is investigated in comparison to that of nanoribbons with defects alone. The simulation results are analyzed from three perspectives: phonon match, concentration of N doping, and distribution of N doping. This approach reveals that a coupling effect is the cause of the observed results. Nitrogen doped graphene nanoribbons (both perfect and defective variants) perform better with thermal management than do graphene nanoribbons with defects alone, which is of considerable interest. Based on these investigations, a guide for graphene-interconnected circuits design is implied.     

Efficient Exciton Quenching by Hole Polarons in the Conjugated Polymer MEH-PPV

Recently published near field scanning optical measurements (NSOM) on the conjugated polymer MEH-PPV exhibited a strong dependence of the photoluminescence intensity on the applied electric fields at the NSOM tip. The observed effect is apparently due to exciton quenching by hole polarons. In the present paper, a model “single carrier” electro-modulated-photoluminescence device is used to further explore the exciton quenching effect of hole polarons in MEH-PPV. Hole polarons, created by charge injection from an ITO electrode, are observed to dramatically quench the photoluminescence intensity of MEH-PPV. The Stern-Volmer quenching efficiency of a hole polaron in conjugated polymer thin films was measured to be 390 nm³. This value, and other data presented herein, are consistent with the published NSOM photoluminescence modulation measurements and offer further evidence that hole polarons are efficient photoluminescence quenchers in MEH-PPV.     

Site-dependent doping effects on quantum transport in zigzag graphene nanoribbons

We investigate the site-dependent effects of a substitutional nitrogen or boron atom on quantum transport in zigzag graphene nanoribbons from first principles and tight-binding model calculations. The former show three characteristics in transmission spectra: drops around the Fermi level, dips at the bottom of the second conduction (nitrogen) or valence (boron) band, and sharp peaks at the Fermi level. Comparing with the latter, the origins of the transmission features are revealed. The drops are attributed to the impurity onsite potential or its Coulomb interaction, depending on its location. The dips come from the interaction between the impurity and its neighbor atoms. The peaks are associated with the resonant long-rang components of the Coulomb interaction.     

A polaron in molecular chains with dispersion

The polaron or soliton mechanism is one of the possible mechanisms of charge transfer in biopolymers. The states with the lowest energy, i.e., stationary polarons, and their dependence on the dispersion in a classical chain have been investigated using numerical experiments in terms of the one-dimensional discrete Holstein model. It has been shown that an increase in the dispersion leads to an increase in the total energy of the system and in the polaron radius. The mobility of charge carriers in molecular chains has been calculated for different dispersions. It has been demonstrated that, in the case of polynucleotide chains, the inclusion of the dispersion can substantially increase the hole mobility.     

Thermal control in graphene nanoribbons: thermal valve, thermal switch and thermal amplifier

We study the thermal transport in graphene nanoribbons by using nonequilibrium molecular dynamics simulations. It is reported that the three-terminal graphene nanoribbons can perform some functions of thermal devices such as thermal valve, thermal switch and thermal amplifier. Electronic devices have transformed almost all aspects of our lives. It has not escaped our attention that the graphene nanoribbons we have presented here may have similar surprising applications in devices that allow the flow of heat to be controlled in a short future.     

Ultrafast Dynamics of Localized and Delocalized Polaron Transitions in P3HT/PCBM Blend Materials: The Effects of PCBM Concentration

Nowadays, organic solar cells have the interest of engineers for manufacturing flexible and low cost devices. The considerable progress of this nanotechnology area presents the possibility of investigating new effects from a fundamental science point of view. In this letter we highlight the influence of the concentration of fullerene molecules on the ultrafast transport properties of charged electrons and polarons in P3HT/PCBM blended materials which are crucial for the development of organic solar cells. Especially, we report on the femtosecond dynamics of localized (P₂ at 1.45 eV) and delocalized (DP₂ at 1.76 eV) polaron states of P3HT matrix with the addition of fullerene molecules as well as the free-electron relaxation dynamics of PCBM-related states. Our study shows that as PCBM concentration increases, the amplified exciton dissociation at bulk heterojunctions leads to increased polaron lifetimes. However, the increase in PCBM concentration can be directly related to the localization of polarons, creating thus two competing trends within the material. Our methodology shows that the effect of changes in structure and/or composition can be monitored at the fundamental level toward optimization of device efficiency.     

Evaluation of Polaron Transport in Solids from First-principles

Polarons are formed in polar or ionic solids, either molecular or crystalline, due to local distortions of the lattice induced by charge carriers. Polaron hopping is the primary mechanism of charge transport in these materials, such as functional ceramic compounds, with applications in photovoltaics, thermoelectrics, two-dimensional electron gas transistors, magnetic sensors, spin valve devices, and memories. Understanding the fundamental physics of polaron hopping is, therefore, of prime technological importance. This article provides a brief physical background of polarons and their hopping mechanism, focusing on first-principles calculations of polaron properties. Herein, we review recent selected studies applying the density functional theory (DFT), and describe the merits and challenges in applying DFT for such calculations, highlighting the need to address both electronic and vibrational aspects. The vibrational component of the polaron is evaluated based on structural and total energy calculations, whereas the electronic component is derived from both total energy and electron density calculations. To address the most compelling challenge of calculating polaron properties using DFT, which is the issue of electron localization, we propose to employ calculations of selected vibrational properties, such as the sound velocity, shear modulus, and Grüneisen parameter, to represent the polaron hopping energy; all of which originate from the stiffness of inter-atomic bonds. Such methodology is expected to be more straightforward than the existing ones, however demands standardization.     

Electromechanical properties of armchair graphene nanoribbons under local torsion

While graphene nanoribbons are prone to twist intrinsically, the effect of local twist on the electromechanical properties remains unexplored. By using the density functional theory in combination with the nonequilibrium Green’s function method, we investigate the responses of structural evolution and electrical transport of armchair graphene nanoribbons to local torsion. We show that local twist can alter their transport properties significantly. The current at a given bias can switch on/off or change many times with twist angle, which is related with twist-induced changes in electronic structures of graphene nanoribbons. Our results can provide a valuable guideline for design and implementation of graphene nanoribbons in nanoelectromechanical systems and devices.     

Edge effect on electronic transport properties of graphene nanoribbons and presence of perfectly conducting channel

Numerical calculations have been performed to elucidate unconventional electronic transport properties in disordered nanographene ribbons with zigzag edges (zigzag ribbons). The energy band structure of zigzag ribbons has two valleys that are well separated in momentum space, related to the two Dirac points of the graphene spectrum. The partial flat bands due to edge states make the imbalance between left- and right-going modes in each valley, i.e. appearance of a single chiral mode. This feature gives rise to a perfectly conducting channel in the disordered system, i.e. the average of conductance 〈g〉 converges exponentially to 1 conductance quantum per spin with increasing system length, provided impurity scattering does not connect the two valleys, as is the case for long-range impurity potentials. Ribbons with short-range impurity potentials, however, through inter-valley scattering, display ordinary localization behavior. Symmetry considerations lead to the classification of disordered zigzag ribbons into the unitary class for long-range impurities, and the orthogonal class for short-range impurities. The electronic states of graphene nanoribbons with general edge structures are also discussed, and it is demonstrated that chiral channels due to the edge states are realized even in more general edge structures except for armchair edges.     

Thermal conduction and rectification in few-layer graphene Y junctions

By using molecular dynamics simulations, we have studied heat flux in graphene Y junctions with lengths of 16.7 nm. It is found that the heat flux runs preferentially from the branches to the stem, which demonstrates an obvious thermal rectification effect in these asymmetric graphene ribbons. More interesting, compared to single-layer graphene Y junctions, a larger rectification ratio can be achieved in double-layer structures, due to the presence of layer-layer interactions. Combined with the availability of high quality few-layer graphene materials, our results shed light on heat conduction in graphene nanoribbons and may open up few-layer graphene applications in thermal management of nano electronics.     

First-principles prediction of charge mobility in carbon and organic nanomaterials

We summarize our recent progresses in developing first-principles methods for predicting the intrinsic charge mobility in carbon and organic nanomaterials, within the framework of Boltzmann transport theory and relaxation time approximation. The electron-phonon couplings are described by Bardeen and Shockley's deformation potential theory, namely delocalized electrons scattered by longitudinal acoustic phonons as modeled by uniform lattice dilation. We have applied such methodology to calculating the charge carrier mobilities of graphene and graphdiyne, both sheets and nanoribbons, as well as closely packed organic crystals. The intrinsic charge carrier mobilities for graphene sheet and naphthalene are calculated to be 3 × 10(5) and ～60 cm(2) V(-1) s(-1) respectively at room temperature, in reasonable agreement with previous studies. We also present some new theoretical results for the recently discovered organic electronic materials, diacene-fused thienothiophenes, for which the charge carrier mobilities are predicted to be around 100 cm(2) V(-1) s(-1).     

Conductance of Graphene Nanoribbon Junctions and the Tight Binding Model

Planar carbon-based electronic devices, including metal/semiconductor junctions, transistors and interconnects, can now be formed from patterned sheets of graphene. Most simulations of charge transport within graphene-based electronic devices assume an energy band structure based on a nearest-neighbour tight binding analysis. In this paper, the energy band structure and conductance of graphene nanoribbons and metal/semiconductor junctions are obtained using a third nearest-neighbour tight binding analysis in conjunction with an efficient nonequilibrium Green's function formalism. We find significant differences in both the energy band structure and conductance obtained with the two approximations.     

Synthesis and characterization of nanocomposites of thermoplastic polyurethane with both graphene and graphene nanoribbon fillers

Thermoplastic polyurethane (TPU) nanocomposites containing graphene and graphene nanoribbons were obtained by polymerizing 1,4-butanediol with two diisocyanates (namely, 1,6-hexane diisocyanate or isophorone diisocyanate), in which the nanofillers were previously dispersed. Raman spectroscopy and Transmission Electron Microscopy demonstrated the formation of few-layer graphene and graphene nanoribbons dispersed in the monomers. At variance to the methods commonly reported in literature, that used in this work consists of the direct exfoliation of graphite without any chemical manipulation. Apart from the obvious cost and ease advantages, the so-obtained graphene does not contain any carboxy or alkoxy groups formed during the exfoliation process, which, at variance, are typically present in the most commonly reported methods. This finding paves the way toward the large-scale production of graphene and its nanoribbons, which are considered even more interesting than graphene itself for many potential applications. The obtained nanocomposites show a peculiar thermal and rheological behavior due to the presence of the nanofillers and to their reinforcing or plasticizing effect exerted on the TPU matrices.     

Alignment of graphene nanoribbons by an electric field

In this paper, we develop an analytical approach to predict the field-induced alignment of cantilevered graphene nanoribbons. This approach is validated through molecular simulations using a constitutive atomic electrostatic model. Our results reveal that graphene’s field-oriented bending angle is roughly proportional to the square of field strength or to the graphene length for small deformations, while is roughly independent of graphene width. The effective bending stiffness and the longitudinal polarizability are found to be approximately proportional to the square of graphene length. Compared with carbon nanotubes, graphene nanoribbons are found to be more mechanically sensitive to an external electric field.     

Influence of chemisorption on the thermal conductivity of graphene nanoribbons

The thermal conductivity of graphene nanoribbons (GNRs) functionalized by the chemical attachment of methyl and phenyl groups at random positions is calculated using reverse nonequilibrium molecular dynamics. The GNRs exhibit a rapid drop in thermal conductivity with increasing degree of functionalization; a functional group coverage regime of as little as 1.25% of GNR atoms reduces the thermal conductivity by about 50%. The thermal conductivity of nanoribbons with zigzag edges is more sensitive in the degree of functionalization than nanoribbons with armchair edges. The simulation results indicate that the rapid drop in thermal conductivity is a consequence of the higher angular momentum of functional groups, which rotate the unsupported sp³ bonds and thus reduce the phonon mean free paths.     

Beryllium doping graphene,graphene-nanoribbons,C60-fullerene,and carbon nanotubes

Beryllium substitutional doping within graphene, graphene nanoribbons, and carbon nanotubes are investigated using first-principles density functional theory calculations. Nanoribbons with armchair and zigzag edges, semiconducting (10,0) and metallic (6,6) carbon nanotubes, and C₆₀ fullerene structures are analyzed. Binding energy, doping energy, band structure, electronic density of states (DOS), and magnetic ordering are calculated. Our results demonstrate that conversely to perfect graphene, Be-doped graphene reveals a semiconducting behavior with an indirect band gap of 0.298 eV. Formation energy analysis reveals that Be into graphene and ribbons is more energetically favorable, but the energies involved are larger than those obtained for B- and N-doped nanocarbons. For nanoribbons, two different ways of incorporating the Be atom are explored (dopant placed in the center or edge), demonstrating that armchair nanoribbons preserve the semiconducting behavior with a reduced band-gap whereas that zigzag nanoribbons exhibit a half-metallic behavior with magnetic order along the edges. Results on Be-doping zigzag (10,0) semiconducting and armchair (6,6) metallic nanotubes and C₆₀ fullerene reveal the appearance of additional electronic states around the Fermi level. We envisage that the present investigation could motivate the realization of future experiments to introduce Be into sp² graphite-like lattice using high temperature chemical vapor deposition method.     

Stacking dependent electronic structure and transport in bilayer graphene nanoribbons

The stacking-dependent electronic structure and transport properties of bilayer graphene nanoribbons suspended between gold electrodes are investigated using density functional theory coupled with non-equilibrium Green’s functional method. We find substantially enhanced electron transmission as well as tunneling currents in the AA stacking of bilayer nanoribbons compared to either single-layer or AB stacked bilayer nanoribbons. Interlayer separation between the nanoribbons appears to have a profound impact on the conducting features of the bilayer nanoribbons, which is found to be closely related to the topology and overlap between the edge-localized π orbitals.     

Step-templated CVD growth of aligned graphene nanoribbons supported by a single-layer graphene film

We present chemical vapor deposition (CVD) growth of a hybrid structure of aligned graphene nanoribbons (GNRs) supported by a single-layer graphene sheet. The step structure created on the epitaxial Co film is used to segregate arrays of aligned GNRs. Reflecting the highly ordered step structure of the Co catalyst, straight nanoribbons with high aspect ratio (>100) are formed. Analysis suggests that a large-area, single-layer graphene film also grows over the aligned GNRs, making a GNR-graphene hybrid structure. We also demonstrate the isolation of aligned GNRs by oxygen plasma treatment or partial transfer of the hybrid film. These findings on the formation of highly aligned GNRs give new insights into the formation mechanism of graphene and can be applied for more advanced graphene structure for future electronics.     

Knitted graphene-nanoribbon sheet: a mechanically robust structure

In this paper, a new nanostructure is proposed, namely, the knitted graphene-nanoribbon sheet (KGS), which consists of zigzag and/or armchair graphene nanoribbons. The knitting technology is introduced to graphene nanotechnology to produce large area graphene sheets. Compared with pristine graphene, the chirality of a knitted graphene-nanoribbon sheet is much more flexible and can be designed on demand. The mechanical properties of KGSs are investigated by molecular dynamics simulations, including the effect of vacancies. With hydrogen atoms saturating the ribbon edges, the structure (KGS + H) is found to be of significant mechanical robustness, whose fracture does not rely on the critical bonds. The fracture strain of KGS + H remains nearly unchanged as long as there remains a single defect-free graphene nanoribbon in the tensile direction. This graphene nano knitting technique is experimentally feasible, inspired by a recent demonstration by Fournier et al. [Phys. Rev. B, 2011, 84, 035435] of lifting a single molecular wire using a combined frequency-modulated atomic force and tunnelling microscope.