Few-layer Graphene Nanoribbons Research Articles

Emerging Trends in Thermal Properties of Graphene

We go over the thermal characteristics of graphene, few-layer graphene, and graphene nanoribbons, and we go over some real-world uses for graphene in energy storage and thermal control. The first section of the paper discusses the most recent developments in the graphene thermal area with an emphasis on the experimental and theoretical data on heat conduction in graphene and graphene nanoribbons that have recently been published. In the summary, the effects of the sample's size, shape, quality, strain distribution, isotope composition, and point-defect concentration are discussed. The thermal characteristics of materials used in energy storage that have been increased by graphene are described in the second section of the review. It has been established that the usage of liquid-phase-exfoliated graphene as a filler in phase change materials has promise for the thermal control of high-power density battery parks. The experimental and modelling findings have been disclosed.

Structural effect of Low-dimensional carbon nanostructures on Long-term stability of dye sensitized solar cells

A major challenge in upscaling and commercializing dye-sensitized solar cells (DSSC) is to simultaneously rely on a robust photoconversion efficiency (PCE) along with long-term device stability. Two-dimensional (2D) nanocarbon materials are known to improve the PCE and stability of devices made of hybrid nanocomposite photoanodes. However, there is inadequate knowledge on how the morphology of 2D nanocarbon materials plays an important role in improving photovoltaic (PV) performance and stability. Here we report the comparative effect on the PV performance and the long-term stability of DSSCs made of optimal loadings of few-layer graphene (FLG) flakes and graphene nanoribbons (GNR) into the mesoporous TiO2 active layer. DSSCs were fabricated by using standard nanocrystalline TiO2, GNR-TiO2 and FLG-TiO2 hybrid mesoporous films as anodes and subjected to continuous visible light irradiation to monitor their long-term stability. Results show that the optimized incorporation of GNR (0.005 wt%) and FLG (0.010 wt%) in the mesoporous TiO2 active layer enhances the PCE by 34% and 21% compared to pristine TiO2, respectively. Moreover, the operational stability of hybrid devices shows a 51.6% (for GNR-TiO2) and a 10% (for FLG-TiO2) sustained higher PCE than the device based on bare TiO2, after 273 h of continuous one sunlight soaking. The electrochemical impedance spectroscopy (EIS) and transient photovoltage decay were studied to investigate how GNR strikingly reduces degradation of device performance as compared to its counterpart FLG-TiO2 and TiO2 based cells. This significant enhancement can be attributed to the structural modifications induced by GNR within the TiO2 photoanode, which improves the electron lifetime and reduces non-radiative carrier recombination.

Electrical and thermal conductivities of few-layer armchair graphene nanoribbons

The tight-binding Hamiltonian model and the Green’s function formalism have been employed to calculate the temperature dependent electrical and electronic thermal conductivities of metal and few-layer armchair graphene nanoribbon semiconductors and the results were compared with the mono-layer system. It was observed that due to the overlapping of the nonhybridized p z orbital perpendicular to the sheets, increasing the layers of the systems causes the conductivities of the layers to decrease. Also, these quantities are calculated for three different values of interlayer hopping of the nonhybridized p z orbitals. The results show that in low temperatures, the electrical and thermal conductivities of the system increase when the interlayer hopping term is increased. However, by increasing the temperature, the curves representing electrical conductivities converge to the same value while thermal conductivity decreases.

Vacuum or flowing argon: What is the best synthesis atmosphere for nanodiamond-derived carbon onions for supercapacitor electrodes?

We present a comprehensive study on the influence of the synthesis atmosphere on the structure and properties of nanodiamond-derived carbon onions. Carbon onions were synthesized at 1300 and 1700°C in high vacuum or argon flow, using rapid dynamic heating and cooling. High vacuum annealing yielded carbon onions with nearly perfect spherical shape. An increase in surface area was caused by a decrease in particle density when transitioning from sp3 to sp2 hybridization and negligible amounts of disordered carbon were produced. In contrast, carbon onions from annealing nanodiamonds in flowing argon are highly interconnected by few-layer graphene nanoribbons. The presence of the latter improves the electrical conductivity, which is reflected by an enhanced power handling ability of supercapacitor electrodes operated in an organic electrolyte (1M tetraethylammonium tetrafluoroborate in acetonitrile). Carbon onions synthesized in argon flow at 1700°C show a specific capacitance of 20F/g at 20A/g current density and 2.7V cell voltage which is an improvement of more than 40% compared to vacuum annealing. The same effect was measured for a synthesis temperature of 1300°C, with a 140% higher capacitance at 20A/g for argon flow compared to vacuum annealing.

Low-frequency phonons of few-layer graphene within a tight-binding model

Few-layer graphene is a layered carbon material with covalent bonding in the layers and weak van der Waals interactions between the layers. The interlayer energy is more than two orders of magnitude smaller than the intralayer one, which hinders the description of the static and dynamic properties within parameter-free electron band structure models. We overcome this difficulty by introducing two sets of matrix elements - one set for the covalent bonds in the graphene layers and another one for the van der Waals interactions between adjacent graphene layers in a tight-binding model of the band structure. Both sets of matrix elements are derived from an ab-initio study on carbon dimers. The matrix elements are then applied in the calculation of the phonon dispersion of graphite and few-layer graphene with AB and ABC layer stacking. The results for AB stacking agree well with the available experimental data, which justifies the application of the matrix elements to other layered carbon structures with van der Waals interactions such as few-layer graphene nanoribbons, multiwall carbon nanotubes, and carbon onions.

Graphene Thermal Properties: Applications in Thermal Management and Energy Storage

We review the thermal properties of graphene, few-layer graphene and graphene nanoribbons, and discuss practical applications of graphene in thermal management and energy storage. The first part of the review describes the state-of-the-art in the graphene thermal field focusing on recently reported experimental and theoretical data for heat conduction in graphene and graphene nanoribbons. The effects of the sample size, shape, quality, strain distribution, isotope composition, and point-defect concentration are included in the summary. The second part of the review outlines thermal properties of graphene-enhanced phase change materials used in energy storage. It is shown that the use of liquid-phase-exfoliated graphene as filler material in phase change materials is promising for thermal management of high-power-density battery parks. The reported experimental and modeling results indicate that graphene has the potential to outperform metal nanoparticles, carbon nanotubes, and other carbon allotropes as filler in thermal management materials.

Electronic properties of impurity-infected few-layer graphene nanoribbons

Spurred by achievements in devising different multilayered graphene-based nano-systems, based on the random tight-binding Hamiltonian model and within the coherent potential approximation, the influence of varying the number of layers and the effect of doping by the boron and nitrogen impurities on the density of states of a mono- and few-layer armchair- and zigzag-edge graphene nanoribbons are theoretically investigated. When the nanoribbons are pristine, with increasing the number of layers the band gap of the armchair nanoribbons is decreased, yet the zigzag ribbons remain metallic and depending on the number of the layers few peaks are appeared around the zero-energy level. Moreover, in the presence of impurities, the band gap of the armchair nanoribbons is decreased for each number of layers. The Van-Hove singularities are steadily broadened and the density of states move to a higher (lower) value of the energy as a result of doping with boron (nitrogen) atoms. This study could provide with us to explore and devise new optoelectronic devices based on the impurity-infected graphene nanoribbons with tunable widths and edges.

Correlating atomic structure and transport in suspended graphene nanoribbons.

Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components; this fact motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (<10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. Few-layer GNRs very frequently formed bonded-bilayers and were remarkably robust, sustaining currents in excess of 1.5 μA per carbon bond across a 5 atom-wide ribbon. We found that the intrinsic conductance of a sub-10 nm bonded bilayer GNR scaled with width as GBL(w) ≈ 3/4(e2/h)w, where w is the width in nanometers, while a monolayer GNR was roughly five times less conductive. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry. Finally, we report on simulations of quantum transport in GNRs that are in qualitative agreement with the observations.

Versatile and scalable synthesis of graphene nanoribbons

The inability to readily upscale nanofabrication of carbon nanomaterials often restricts their application, despite outstanding performances reported in both the research laboratory and prototype stages. Here we report the direct chemical synthesis of graphene nanoribbons by a bottom-up approach based on the common laboratory reagents sodium and propanol; these are solvothermally reacted to give an intermediate precursor that is then rapidly pyrolized yielding single- and few-layer graphene nanoribbons. Our results show that confinement of the lateral dimensions of graphene can be achieved simply by varying the alcohol feedstock. The ability to produce bulk quantities of graphene nanoribbons by a low cost and scalable approach is anticipated to enable a wider range of affordable real-world graphene applications.

Controlling Thermal Conductivity of Few-Layer Graphene Nanoribbons by Using the Transversal Pressure

We study the thermal transport of few-layer graphene nanoribbons in the presence of the transversal pressure by using molecular dynamics simulations. It is reported that the pressure can improve the thermal conductivity of few-layer graphene nanoribbons. This improvement can reach 37.5% in the low temperature region. The pressure dependence of thermal conductivity is also investigated for different length, width and thickness of few-layer graphene. Our results provide an alternative option to tuning thermal conductivity of few-layer graphene nanoribbons. Furthermore, it maybe indicate a so-called pressure-thermal effect in nanomaterials.

Nanomechanics of Twisted Mono- and Few-Layer Graphene Nanoribbons.

Enabled by the technique of objective molecular dynamics, we reveal the unusual mechanics exhibited by nanoscale twisted graphene nanoribbons containing up to seven layers. Unlike in a linear-elastic plate, we find that the deformation practically does not contain contributions associated with in-plane shearing but largely with inhomogeneous stretching and compression of the constituent layers. The whole twisted structure undergoes shortening when no axial force is applied, while the constituent layers store various strain energies, depending on their location. We capture this behavior with a simple model and show that the deviations from the plate model are increasing with the number of layers and width of the ribbon. Our results are especially relevant for the experimental efforts of measuring graphene's shear modulus.

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.

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.

Photoinduced Optical Transparency in Dye-Sensitized Solar Cells Containing Graphene Nanoribbons

We describe the use of few-layer graphene nanoribbons, either attached to counter electrodes or dispersed into electrolyte, to induce optical transparency of an iodide/triiodide redox couple in a dye-sensitized solar cell (DSSC). We then evaluate the effect of reversible bleaching of the electrolyte on the DSSC performance. This bleaching effect is related to an energy transfer from photoexcited quantum-dot-like regions to the triiodide (I3–) radical ions in the electrolyte, saturating their absorption in the visible optical range. DSSC power conversion efficiency using few-layer graphene nanoribbons at the counter electrode (5.8%) did not deteriorate when the electrolyte became optically transparent. The increased transparency of the electrolyte resulted in a decreased photocurrent density (from 17.6 to 14.2 mA/cm2), an unchanged open circuit voltage of 750 mV, and a slightly increased fill factor (from 0.45 to 0.55). When the few-layer graphene nanoribbons were introduced into the electrolyte directly by ultrasonication, a semitransparent DSSC was found to have increased its power conversion efficiency in an optically inverted setup from 5.75% to 7.01%, arising from an increase in photocurrent from 9.9 to 12.1 mA/cm2. This significant photocurrent increase demonstrates that the effect of electrolyte bleaching can be used for further improving power conversion efficiency for inverted and tandem DSSCs, in which light has to pass through the electrolyte to generate photocurrent on one or more photocells.

Electrical transport properties of graphene nanoribbons produced from sonicating graphitein solution

A simple one-stage solution-based method was developed to produce graphenenanoribbons by sonicating graphite powder in organic solutions with polymersurfactant. The graphene nanoribbons were deposited on a silicon substrate, andcharacterized by Raman spectroscopy and atomic force microscopy. Single-layerand few-layer graphene nanoribbons with a width ranging from sub-10 nmto tens of nanometers and lengths ranging from hundreds of nanometers to1 µm were routinely observed. The electrical transport properties of individual graphenenanoribbons were measured in both the back-gate and polymer–electrolyte top-gateconfigurations. The mobility of the graphene nanoribbons was found to be over an order ofmagnitude higher when measured in the latter than in the former configuration (withoutthe polymer–electrolyte), which can be attributed to the screening of the chargedimpurities by the counter ions in the polymer–electrolyte. This finding suggests that thecharge transport in these solution produced graphene nanoribbons is largely limited bycharge impurity scattering.

Multilayer graphene nanoribbons exhibit larger capacitance than their few-layer and single-layer graphene counterparts

This article demonstrates that it is not always beneficial to exfoliate graphitic structures to single-layer graphene to achieve maximum electrochemical performance. Using electrochemical impedance spectroscopy, we show that multilayer graphene nanoribbons with cross sections of 100 × 100 nm provide larger capacitance (15.6 F/g) than do few-layer graphene nanoribbons (14.9 F/g) and far larger capacitance than single-layer graphene nanoribbons (10.9 F/g) with the same cross section.

Edge phonon state of mono- and few-layer graphene nanoribbons observed by surface and interference co-enhanced Raman spectroscopy

[Ren, Wencai; Gao, Libo; Wu, Zhongshuai; Liu, Bilu; Zhao, Jinping; Chen, Zongping; Cheng, Hui-Ming] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. [Saito, Riichiro; Zheng, Fawei; Furukawa, Masaru] Tohoku Univ, Dept Phys, Sendai, Miyagi 9808578, Japan.;Ren, WC (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;wcren@imr.ac.cn cheng@imr.ac.cn

Electronic structure of epitaxial graphene nanoribbons on SiC(0001)

We present electronic structure calculations of few-layer epitaxial graphene nanoribbons on SiC(0001). Trough an atomistic description of the graphene layers and the substrate within the extended Hückel theory and real/momentum space projections we argue that the role of the heterostructure’s interface becomes crucial for the conducting capacity of the studied systems. The key issue arising from this interaction is a Fermi level pinning effect introduced by dangling interface bonds. Such phenomenon is independent from the width of the considered nanostructures, compromising the importance of confinement in these systems.

Electric-field induced modification of electronic properties of few-layer graphene nanoribbons

In the presence of electric fields, the low-energy electronic properties of AB-stacked few-layer graphene nanoribbons are studied by using the tight-binding model. They are strongly dependent on the geometric structures (the interlayer interactions, the ribbon edges, the ribbon width Ny, and the ribbon number Nz) and the field strength. The interlayer interactions significantly affect density of states (DOS), energy gap (Eg), band structure, and free carriers. DOS exhibits many special structures including plateau, discontinuities, and divergent peaks. The effective electric field modifies the energy dispersions, alters the subband spacing, changes the subband curvature, produces the new edge state, switches the band gap, and causes the metal-semiconductor (or semiconductor-metal) transitions. In gapless zigzag ribbons, electric fields not only lifts the degeneracy of partial flatbands at EF but also induces an energy gap. Eg is dependent on the ribbon width, ribbon edges, and the field strength. The semiconductor-metal transitions occur in both armchair ribbons and zigzag ribbons in the increase in electric fields. Due to electric fields, the above-mentioned effects are completely reflected in the features of DOS, such as the generation of special structures, the shift of peak position, the change in peak height, and the alternation of band gap. The predicted electronic properties could be examined by the experimental measurements on absorption spectra and transport properties.