Abstract
Graphyne, a two-dimensional carbon allotrope like graphene but containing doubly and triply bonded carbon atoms, has been proven to possess amazing electronic properties as graphene. Although the electronic, optical, and mechanical properties of graphyne and graphyne nanoribbons (NRs) have been previously studied, their electron transport behaviors have not been understood. Here we report a comprehensive study of the intrinsic electronic and transport properties of four distinct polymorphs of graphyne (α, β, γ, and 6,6,12-graphynes) and their nanoribbons (GyNRs) using density functional theory coupled with the non-equilibrium Green's function (NEGF) method. Among the four graphyne sheets, 6,6,12-graphyne displays notable directional anisotropy in the transport properties. Among the GyNRs, those with armchair edges are nonmagnetic semiconductors whereas those with zigzag edges can be either antiferromagnetic or nonmagnetic semiconductors. Among the armchair GyNRs, the α-GyNRs and 6,6,12-GyNRs exhibit distinctive negative differential resistance (NDR) behavior. On the other hand, the zigzag α-GyNRs and zigzag 6,6,12-GyNRs exhibit symmetry-dependent transport properties, that is, asymmetric zigzag GyNRs behave as conductors with nearly linear current-voltage dependence, whereas symmetric GyNRs produce very weak currents due to the presence of a conductance gap around the Fermi level under finite bias voltages. Such symmetry-dependent behavior stems from different coupling between π* and π subbands. Unlike α- and 6,6,12-GyNRs, both zigzag β-GyNRs and zigzag γ-GyNRs exhibit NDR behavior regardless of the symmetry.
Highlights
Carbon, the element providing the basis for most organic and biological molecules, has a broad range of unique chemical and physical properties largely due to possible formation of three distinct covalent bonds, namely, sp, sp[2], sp3-hybridized bonds.[1]
Similar to graphene nanoribbons (GNRs), the a-graphyne nanoribbons (GyNRs) exhibit two typical edge structures, namely, armchair and zigzag, for which the corresponding nanoribbons are denoted as a-armchair-edged GyNRs (AGyNRs) and aZGyNRs, respectively
To analyze symmetry-dependent transport behaviors of the a-zigzag-edged GyNRs (ZGyNRs), we present some details of the transmission spectrum and band structures of le and right electrodes for 5- and 6-a-ZGyNRs
Summary
The discovery of graphene-based two-dimensional (2D) carbon materials has sparked considerable research interest in nding. On graphyne nanoribbons (GyNRs), previous theoretical studies have predicted electronic and magnetic properties of a- and g-GyNRs.[37,38] It is found that all the armchair-edged a-GyNRs (a-AGyNRs) are nonmagnetic semiconductors with their band gaps dependent on the ribbon widths. The band gap of zigzag g-GyNRs (g-ZGyNRs) shows a unique “step effect” as the width increases. These remarkable properties of graphynes afford their potential applications in optoelectronic devices, sensors, or energy storage. We show that the graphynes and their NRs exhibit many unique electronic, magnetic and transport properties very different from those of GNRs and boron-nitride (BN) NRs. We nd that all armchair-edged GyNRs (AGyNRs) are nonmagnetic semiconductors, while all the zigzag-edged GyNRs (ZGyNRs) are magnetic semiconductors with the AFM ground state except that the g-ZGyNRs can be either nonmagnetic or magnetic semiconductors. A-AGyNRs, armchair-edged 6,6,12-GyNRs (6,6,12-AGyNRs), zigzag-edged b-GyNRs (b-ZGyNRs), and g-ZGyNRs all exhibit distinct negative differential resistance (NDR) effects, whereas the a-ZGyNRs and 6,6,12-ZGyNRs exhibit symmetry-dependent transport properties
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