Asymmetric Graphene Nanoribbons Research Articles

Tunable Fano resonance in mid-infrared region based on asymmetric graphene nanoribbon arrays

Tunable Fano resonance in mid-infrared region based on asymmetric graphene nanoribbon arrays

Electrical rectification in asymmetric graphene nanoribbons with pores

Electrical rectification, which is a key phenomenon for active current flow control, is expected to occur in asymmetric nanostructures, in particular in trapezoidal graphene nanoribbons (GNRs). We have investigated the influence of geometry of such GNRs on transport of charge carriers in order to find the optimum configuration for a high rectification effect. We found that the rectification factor depends on the trapezoidal shape (rectangular versus isosceles), on the asymmetry of the structure (the base ratio), as well as on the shape of pores introduced in the GNRs. Through simulations of the wavefunction modulus for different GNR geometries we have established that the rectification factor depends on the dissimilarity of localization probabilities of charge carriers at the drain regions for the opposite charge flow directions, dissimilarity determined by interference effects between different propagating modes in the GNRs. • We study the trapezoidal graphene nanoribbon shape effect on electric rectification. • The rectification depends on the asymmetry and shape of the structure and pores. • It depends on charge localization probability difference at drain for opposite flows. • This difference is determined by interference effects between propagating modes.

Recovery of the scattering symmetry looking at the conductance in asymmetric graphene nano-ribbons systems using pseudo-spin filters

In this work we study some applications for pseudo-spin filters. The filters are potential barriers with hyperboloid sub-band contributions that are locally applied over graphene nano-ribbons. These filters modulate the pseudo-spin and the quirality of the wave-function allowing the recovery of the conductance loss due to imperfections, bends, or constrictions (asymmetries) found in the system. The recovery of the conductance is fulfilled by a direct manipulation of the pseudo-spin polarization at both sides of the device by localizing the filters at the system’s entrance and exit points. This procedure allows the recovery of the wave-function symmetry at these points with the consequent recovery of the conductance, even when it is zero, regardless of the different internal regions that affect the transmission, i.e. the filters are used as patches for damaged regions. Our results can be extrapolated for spatially asymmetrical potentials generated by electrical (or magnetic) impurities.

A local resonance mechanism for thermal rectification in pristine/branched graphene nanoribbon junctions

Using non-equilibrium molecular dynamics simulations, we investigate thermal rectification (TR) in pristine/branched graphene nanoribbon (GNR) junctions. The results indicate that the TR ratio of such junctions can reach 470% under small temperature bias, which has distinct superiority over asymmetric GNR and many other junctions. Moreover, the TR ratio decreases rapidly as the applied temperature bias increases. It seems to be against common sense that the TR ratio generally increases with temperature bias. Phonon spectra analyses reveal that the observed phenomena stem from the local resonance of longitudinal phonons in branched GNR region under negative temperature bias. Furthermore, the influence of ambient temperature, system length, branch number, and defect density is studied to obtain the optimum conditions for TR. This work extends local resonance mechanism to GNR for thermal signal manipulation.

Thermal Rectification Characteristics of Graphene Nanoribbons of Asymmetric Geometries

ABSTRACTThe rectification of heat in graphene nanoribbons (GNRs) of asymmetric geometries is investigated by means of nonequilibrium molecular dynamics (NEMD). Two kinds of geometries of GNRs are addressed; a trapezoidal or T-shaped step is inserted halfway through a GNR in its longitudinal direction. The thermal conductivities (TCs) of the GNRs in the two longitudinal directions, forward and backward, are calculated making their width and temperature change. It is revealed that the thermal rectification ratio (TRR) of T-shaped GNRs are larger than those of trapezoidal GNRs and that the characteristics of heat transport in such asymmetric GNRs can be understood by considering the local phonon density of states (DOSs).

Giant rectification in graphene nanoflake molecular devices with asymmetric graphene nanoribbon electrodes

By applying density functional theory based nonequilibrium Green's function method, we theoretically investigate the electron transport properties of a zigzag-edged trigonal graphene nanoflake (ZTGNF) sandwiched between two asymmetric zigzag graphene nanoribbon (zGNR) and armchair graphene nanoribbon (aGNR) electrodes with carbon atomic chains (CACs) as the anchoring groups. Significant rectifying effects have been observed for these molecular devices in low bias voltage regions. Interestingly, the rectifying performance of molecular devices can be optimized by changing the width of the aGNR electrode and the number of anchoring CACs. Especially, the molecular device displays giant rectification ratios up to the order of 104 when two CACs are used as the anchoring group between the ZTGNF and the right aGNR electrode. Further analysis indicates that the asymmetric shift of the perturbed molecular energy levels and the spatial parity of the electron wavefunctions in the electrodes around the Fermi level play key roles in determining the rectification performance. And the spatial distributions of tunneling electron wavefunctions under negative bias voltages can be modified to be very localized by changing the number of anchoring CACs, which is found to be the origin of the giant rectification ratios.

Thermal AND Gate Using a Monolayer Graphene Nanoribbon.

The first ever implementation of a thermal AND gate, which performs logic calculations with phonons, is presented using two identical thermal diodes composed of asymmetric graphene nanoribbons (GNRs). Employing molecular dynamics simulations, the characteristics of this AND gate are investigated and compared with those for an electrical AND gate. The thermal gate mechanism originates through thermal rectification due to asymmetric phonon boundary scattering in the two diodes, which is only effective at the nanoscale and at the temperatures much below the room temperature. Due to the high phonon velocity in graphene, the gate has a fast switching time of ≈100 ps.

Phonon Lateral Confinement Enables Thermal Rectification in Asymmetric Single-Material Nanostructures

We show that thermal rectification (TR) in asymmetric graphene nanoribbons (GNRs) is originated from phonon confinement in the lateral dimension, which is a fundamentally new mechanism different from that in macroscopic heterojunctions. Our molecular dynamics simulations reveal that, though TR is significant in nanosized asymmetric GNRs, it diminishes at larger width. By solving the heat diffusion equation, we prove that TR is indeed absent in both the total heat transfer rate and local heat flux for bulk-size asymmetric single materials, regardless of the device geometry or the anisotropy of the thermal conductivity. For a deeper understanding of why lateral confinement is needed, we have performed phonon spectra analysis and shown that phonon lateral confinement can enable three possible mechanisms for TR: phonon spectra overlap, inseparable dependence of thermal conductivity on temperature and space, and phonon edge localization, which are essentially related to each other in a complicated manner. Under such guidance, we demonstrate that other asymmetric nanostructures, such as asymmetric nanowires, thin films, and quantum dots, of a single material are potentially high-performance thermal rectifiers.

Stable ferromagnetism and doping-induced half-metallicity in asymmetric graphene nanoribbons

We propose a class of graphene nanoribbons showing strong intrinsic ferromagnetic behavior due to their asymmetry. Such ribbons are based on a zigzag-edged backbone surmounted by a periodic, triangular notched region of variable size. The electronic properties as a function of the topology are investigated. Interestingly, substitutional doping by boron or nitrogen induces half-metallicity. The most effective doping sites can be inferred from the band structure. Given the present rapid development of bottom-up strategies for the synthesis of atomically precise carbon nanostructures the proposed class of nanoribbons emerges as a real candidate for spintronic applications at ambient temperature.

Thermal rectification in thickness-asymmetric graphene nanoribbons

Thermal rectification in thickness asymmetric graphene nanoribbons connecting single-layer with multi-layer graphene is investigated by using classical nonequilibrium molecular dynamics. It is reported that the graphene nanoribbons with thickness-asymmetry have a good thermal rectification. The thermal rectification factor depends on temperature as well as the thickness-ratio of the two-segment. Our results provide a direct evidence that the thermal rectifier can be achieved in a nanostructure crossing two- and three-dimension.

Thermal Conductivity and Thermal Rectification in Graphene Nanoribbons: A Molecular Dynamics Study

We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in size (up to approximately 4 nm wide and approximately 10 nm long). For symmetric nanoribbons, the calculated thermal conductivity (e.g., approximately 2000 W/m-K at 400 K for a 1.5 nm x 5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have appreciably larger thermal conductivity than nanoribbons with armchair edges. For asymmetric nanoribbons, we have found significant thermal rectification. Among various triangularly shaped GNRs we investigated, the GNR with armchair bottom edge and a vertex angle of 30 degrees gives the maximal thermal rectification. We also studied the effect of defects and found that vacancies and edge roughness in the nanoribbons can significantly decrease the thermal conductivity. However, substantial thermal rectification is observed even in the presence of edge roughness.