Armchair Graphene Nanoribbon Junctions Research Articles

Topologically protected edge and confined states in finite armchair graphene nanoribbons and their junctions

Topologically protected edge and junction states, previously predicted, have recently been observed in armchair graphene nanoribbon (AGNR) heterojunctions. Here, via tight-binding-based calculations, we explain the relation between the nature and number of the zero-mode edge states of finite-length AGNRs and their structure, topological invariants, and winding number. This allows us to rationalize the design of AGNR heterojunctions and superlattices with tailored phases. We show how the choice of widths, interface coupling geometry, and boundaries determines the emergence of topological states following patterns that depend on the structure and family of the constituent AGNRs. Furthermore, we prove that quantum-well-like states confined in one of the constituent ribbons develop in all the AGNR junctions irrespective of their trivial or topological character. The bipartite nature of the honeycomb lattice is determinant for the topological properties of the junctions: their electronic states can be topologically trivial or nontrivial depending on subtle differences at the boundaries of the ribbons.

Spin-polarized electron transmission through B-doped graphene nanoribbons with Fe functionalization: a first-principles study

In this study, we investigate the electron transport properties of a B-doped armchair graphene nanoribbon (AGNR) suspended between graphene electrodes based on first-principles calculations. Our calculations reveal that one of the electron transmission channels of a pristine AGNR junction is closed by the B-doping. We then proceed to explore the effect of the B-doping on the spin-polarized electron transport behavior of a Fe-functionalized AGNR junction. As a result, transmission channels for majority-spin electrons are closed and the spin polarization of the electron transmission is enhanced from 0.60 for the Fe-functionalized AGNR junction to 0.96 for the B- and Fe-codoped one. This observation implies that the codoped AGNR junction can be employed as a spin filter. In addition, we investigate the electronic nature of the transmission suppression caused by the B-doping. A detailed analysis of the scattering wave functions clarifies that a mode modulation of an incident wave arises in the B-doped AGNR part and the incident wave connects to an evanescent wave in the transmission-side electrode. For pristine and Fe-functionalized AGNR junctions, such a mode modulation is not observed and the incident wave connects to a propagating wave in the transmission-side electrode. Tuning of electron transport property by exploiting such a mode modulation is one of promising techniques for designing functionality of spintronics devices. We also discuss the general correspondence between the electron transmission spectrum and the density of states of a junction.

Isolated pentagons induced enhancement of conductance in ultra-narrow armchair graphene nanoribbon junctions

Recently, the narrowest armchair graphene nanoribbon (AGNR) in the family of width n = 3p + 2 has been fabricated [Kimouche et al., Nat. Commun. 6, 10177 (2015)]. Here, the effects of isolated pentagonal structures that are identified as the contacts of the formed AGNRs by monomers have been investigated. Our first principles results show that the coupling of pentagons dominates the transport properties of the junctions, i.e., a weak coupling enhances the conductance, remarkably higher than that of a perfect AGNR, whereas a strong coupling is opposite and the threshold voltage is not affected at all. Such a contrastive capacity implies the promotion of introducing isolated pentagons as a feasible technique to tune the transport properties of AGNR junctions. The underlying microscopic mechanisms are revealed for this technique.

The Electronic Transport Properties in Boron-Doped Armchair Graphene Nanoribbon Junctions

An electronic component using individual molecules is one of the ultimate goals in nanotechnology. In this work, by performing non-equilibrium Green's functions in combination with density-functions theory, the transport properties of armchair graphene nanoribbon (AGNR) devices are investigated, in which one lead is undoped AGNRs and the other is Boron (B)-doped AGNRs. The current voltage (I V) characteristic of B-doped AGNRs devices depends on their width and exhibits three distinct family (3n, 3n +1, 3n +2) behaviors. It was found that the 3n+2 system exhibits a metallic property and 3n and 3n +1 systems exhibit semi conductive property. Moreover, it shows rectifying behavior for W7 system.

Low-bias negative differential resistance effect in armchair graphene nanoribbon junctions

Graphene nanoribbons with armchair edges (AGNRs) have bandgaps that can be flexibly tuned via the ribbon width. A junction made of a narrower AGNR sandwiched between two wider AGNR leads was recently reported to possess two perfect transmission channels close to the Fermi level. Here, we report that by using a bias voltage to drive these transmission channels into the gap of the wider AGNR lead, we can obtain a negative differential resistance (NDR) effect. Owing to the intrinsic properties of the AGNR junctions, the on-set bias reaches as low as ∼0.2 V and the valley current almost vanishes. We further show that such NDR effect is robust against details of the atomic structure of the junction, substrate, and whether the junction is made by etching or by hydrogenation.

Large magnetoresistance from long-range interface coupling in armchair graphene nanoribbon junctions

In recent years, bottom-up synthesis procedures have achieved significant advancements in atomically controlled growth of several-nanometer-long graphene nanoribbons with armchair-shaped edges (AGNRs). This greatly encourages us to explore the potential of such well-defined AGNRs in electronics and spintronics. Here, we propose an AGNR based spin valve architecture that induces a large magnetoresistance up to 900%. We find that, when an AGNR is connected perpendicularly to zigzag-shaped edges, the AGNR allows for long-range extension of the otherwise localized edge state. The huge magnetoresistance is a direct consequence of the coupling of two such extended states from both ends of the AGNR, which forms a perfect transmission channel. By tuning the coupling between these two spin-polarized states with a magnetic field, the channel can be destroyed, leading to an abrupt drop in electron transmission.

First-principles study of rectifying and switching behavior for different contact positions between sulfur-terminated armchair graphene nanoribbons junctions

Transport properties of armchair graphene nanoribbon junctions with different widths are investigated on the zigzag edges terminated with sulfur atoms. The first-principles calculations based on the non-equilibrium Green's functions together with the density-functional theory show that their characteristics display obvious rectifying performance and switching behavior which are sensitive to the contact points and external activation. The analysis of the Mulliken charge distribution and projected Hamiltonian energy spectrum provides an inside view of the electronic structure of the ground state. The non-equilibrium states analysis, incorporating the density of states and projected density of states as well as the evolutions of frontier orbitals under various external biases, reveals that the intrinsic origin of the different rectifying performances is a result of the asymmetric movement of conducting states and contributing orbitals.

Negative differential resistance behavior in phosphorus-doped armchair graphene nanoribbon junctions

In this present work, we investigate the electronic transport properties of phosphorus-doped armchair graphene nanoribbon (AGNR) junctions by employing nonequilibrium Green's functions in combination with the density-function theory. Two phosphorus (P) atoms are considered to substitute the central carbon atom with the different width of AGNRs. The results indicate that the electronic transport behaviors are strongly dependent on the width of the P-doped graphene nanoribbons. The current-voltage characteristics of the doped AGNR junctions reveal an interesting negative differential resistance (NDR) and exhibit three distinct family (3 n, 3 n + 1, 3 n + 2) behaviors. These results display that P doping is a very good way to achieve NDR of the graphene nanoribbon devices.

Transport properties of armchair graphene nanoribbon junctions between graphene electrodes

The transmission properties of armchair graphene nanoribbon junctions between graphene electrodes are investigated by means of first-principles quantum transport calculations. First the dependence of the transmission function on the size of the nanoribbon has been studied. Two regimes are highlighted: for a small applied bias transport takes place via tunneling and the length of the ribbon is the key parameter that determines the junction conductance; at a higher applied bias resonant transport through the HOMO and LUMO starts to play a more determinant role, and the transport properties depend on the details of the geometry (width and length) of the carbon nanoribbon. In the case of the thinnest ribbon it has been verified that a tilted geometry of the central phenyl ring is the most stable configuration. As a consequence of this rotation the conductance decreases due to the misalignment of the π orbitals between the phenyl ring and the remaining part of the junction. All the computed transmission functions have shown a negligible dependence on different saturations and reconstructions of the edges of the graphene leads, suggesting a general validity of the reported results.