The enhancement of thermoelectric figure of merit ZT requires to either increase the power factor or reduce the phonon conductance, or even both. In graphene, the high phonon thermal conductivity is the main factor limiting the thermoelectric conversion. The common strategy to enhance ZT is therefore to introduce phonon scatterers to suppress the phonon conductance while retaining high electrical conductance and Seebeck coefficient. Although thermoelectric performance is eventually enhanced, all studies based on this strategy show a significant reduction of the electrical conductance. In this study we demonstrate that appropriate sources of disorder, including isotopes and vacancies at lowest electron density positions, can be used as phonon scatterers to reduce the phonon conductance in graphene ribbons without degrading the electrical conductance, particularly in the low-energy region which is the most important range for device operation. By means of atomistic calculations we show that the natural electronic properties of graphene ribbons can be fully preserved while their thermoelectric efficiency is strongly enhanced. For ribbons of width M = 5 dimer lines, room-temperature ZT is enhanced from less than 0.26 to more than 2.5. This study is likely to set the milestones of a new generation of nano-devices with dual electronic/thermoelectric functionalities.

The thermoelectric properties of in-plane heterostructures made of Graphene and hexagonal boron nitride (BN) have been investigated by means of atomistic simulation. The heterostructures consist in armchair graphene nanoribbons to the sides of which BN flakes are periodically attached. This arrangement generates a strong mismatch of phonon modes between the different sections of the ribbons, which leads to a very small phonon conductance, while the electron transmission is weakly affected. In combination with the large Seebeck coefficient resulting from the BN-induced bandgap opening or broadening, it is shown that large thermoelectric figure of merit ZT > 0.8 can be reached in perfect structures at relatively low Fermi energy, depending on the graphene nanoribbon width. The high value ZT = 1.48 may even be achieved by introducing appropriately vacancies in the channel, as a consequence of further degradation of the phonon conductance.

The ab initio band structure of 2D graphene sheet is well reproduced by the third nearest neighbor tight binding model proposed by Reich et al [Phys. Rev. B 66, 035412]. For ribbon structures, the existing sets of tight binding parameters can successfully explain semi-conducting behavior of all armchair ribbon structures. However, they are still failing in describing accurately the slope of the bands while this feature is directly associated to the group velocity and the effective mass of electrons. In this work, both density functional theory and tight binding calculations were performed and a new set of tight binding parameters up to the third nearest neighbors including overlap terms is introduced. The results obtained with this model offer excellent agreement with the predictions of the density functional theory in most cases of ribbon structures, even in the highenergy region. Moreover, this set can induce electron-hole asymmetry as manifested in density functional theory. Relevant outcomes are also demonstrated for armchair ribbons of various widths as well as for zigzag structures, thus opening a route for multi-scale simulations.

We investigate the effects of external electric fields on the electronic properties of bilayer armchair graphene nano-ribbons. Using atomistic simulations with Tight Binding calculations and the Non-equilibrium Green's function formalism, we demonstrate that (i) in semi-metallic structures, vertical fields impact more effectively than transverse fields in terms of opening larger bandgap, showing a contrary phenomenon compared to that demonstrated in previous studies in bilayer zigzag graphene nano-ribbons; (ii) in some semiconducting structures, if transverse fields just show usual effects as in single layer armchair graphene nano-ribbons where the bandgap is suppressed when varying the applied potential, vertical fields exhibit an anomalous phenomenon that the bandgap can be enlarged, i.e., for a structure of width of 16 dimer lines, the bandgap increases from 0.255 eV to the maximum value of 0.40 eV when a vertical bias equates 0.96 V applied. Although the combined effect of two fields does not enlarge the bandgap as found in bilayer zigzag graphene nano-ribbons, it shows that the mutual effect can be useful to reduce faster the bandgap in semiconducting bilayer armchair graphene nano-ribbons. These results are important to fully understand the effects of electric fields on bilayer graphene nano-ribbons (AB stacking) and also suggest appropriate uses of electric gates with different edge orientations..

No abstract

We theoretically investigate effect of a transverse electric field generated by side gates and a vertical electric field generated by top/back gates on energy bands and transport properties of zigzag bilayer graphene ribbons (Bernal stacking). Using atomistic Tight Binding calculations and Green's function formalism we demonstrate that bandgap is opened when either field is applied and even enlarged under simultaneous influence of the two fields. Interestingly, although vertical electric fields are widely used to control bandgap in bilayer graphene, here we show that transverse fields exhibit more positive effect in terms of modulating a larger range of bandgap and retaining good electrical conductance. Seebeck effect is also demonstrated to be enhanced strongly about 13 times for a zigzag bilayer graphene ribbons with 16 chain lines. These results may motivate new designs of devices made of bilayer graphene ribbons using electric gates.

Gaussian deformation in graphene structures exhibits an interesting effect in which flowershaped confinement states are observed in the deformed region [Carrillo-Bastos et al., Phys. Rev. B 90 041411 (2014)]. To exploit such a deformation for various applications, tunable electronic features including a bandgap opening for semi-metallic structures are expected.Besides, the effects of disorders and external excitations also need to be considered. In this work, we present a systematic study on quantum transport of graphene ribbons with Gaussian deformation. Different levels of deformation are explored to find a universal behavior of the electron transmission. Using a tight-binding model in combination with Non-Equilibrium Green's Functions formalism, we show that Gaussian deformation influences strongly the electronic properties of ribbons in which the electron transmission decreases remarkably in high energy regions even if small deformations are considered. Interestingly, it unveils that the first plateau of the transmission of semi-metallic armchair ribbons is just weakly affected in the case of small deformations. However, significant large Gaussian bumps can induce a strong drop of this plateau and a transport gap is formed. The transmission at the zero energy is found to decrease exponentially with increasing the size of the Gaussian bump. Moreover, the gap of semi-conducting ribbons is enlarged with large deformations. The opening or the widening of the transport gap in large deformed armchair structures is interpreted by a formation of a threezone behavior along the transport direction of the hopping profile. On the other hand, a transport gap is not observed in zigzag ribbons regardless of the size of Gaussian bumps. This behavior is due to the strong localization of edge states at the energy point E = 0. Furthermore, under the effect of a positive vertical electric field +Ez, it shows an enhancement of electron transport in the conduction region and a suppression in the valence one. The effect of a negative field -Ez is reverse. Additionally, it is also pointed out that the electronic behavior of a Gaussian deformed

Among the ferromagnetic binary alloys, body centered cubic (bcc) Fe-Co is the one showing the highest magnetization. It is known experimentally that ordered Fe-Co structures show a larger magnetization than the random solid solutions with the same Co content. In this work, based on density functional theory (DFT) studies, we aim at a quantitative prediction of this feature, and point out the role of the orbital magnetic moments. Then, we introduce a DFT-based analytical model correlating local magnetic moments and chemical compositions for Co concentrations ranging from 0 to 70 at.%. It is also extended to predict the global magnetization of both ordered and disordered structures at given concentration and chemical short range orders. The latter model is particularly useful for interpreting experimental data. Based on these models, we note that the local magnetic moment of a Fe atom is mainly dictated by the Co concentration in its first two neighbor shells. The detailed local arrangement of the Co atoms has a minor effect. These simple models can fully reproduce the difference in magnetization between the ordered and disordered Fe-Co alloys between 30% and 70% Co, in good agreement with experimental data. Finally, we show that a similar model can be established for another bcc binary Fe alloy, the Fe-Ni, also presenting ferromagnetic interactions between atoms.