Effect of edge-hydrogen passivation and nanometer size on the electronic properties of phagraphene ribbons
Graphical abstract
Introduction
Carbon is one of the most attractive and versatile elements in the periodic table and forms a large number of allotropes due to its variant electron orbital characteristics. From zero-dimensional fullerene (C60) [1] to 1D nanotube [2], quasi-one dimensional graphene nanoribbon [3], 2D graphene [4], and 3D graphite, diamond as well as metallic carbon phase [5], [6], all of them are a member of the carbon family and the emergence of these materials greatly promoted the development of the material science, physics, chemistry and other related disciplines.
In particular, 2D carbon allotrope graphene has been extensively studied, due to its remarkable properties, such as the ultrahigh carrier mobility [7], superior thermal conductivity [8] and quantum Hall effect [9], [10], which have triggered considerable interest in exploring novel 2D carbon-based nanomaterials. Therefore, a series of two-dimensional carbon materials have been investigated, some of them exist only in theory, may be not stable, such as graphyne [11], [12], [13], [14] and penta-graphene [15], [16], some have been synthesized as well, such as graphane [17], [18].
Recently, a new polymorph structure of carbon, named phagraphene [19], was predicted by calculation. Distinguished with graphene hexagonal honeycomb structure, phagraphene is two-dimensional structure composed of 5–6–7 carbon rings. This planar carbon allotrope is metastable compared with graphene, but it is more favorable than other carbon allotropes proposed in previous works due to its sp2-hybridization and dense atomic packing structure [20]. As we all know, the structure of the material has important influence on its performance. Because of the structural characteristics of phagraphene, it not only shares much of same nature with graphene, such as zero-gap electronic structure, but also has some differences, like distorted Dirac cones [19], that allows it to be considered an advanced material for flexible electronic devices, transistors, solar batteries, display units and many other things. Unfortunately, a band gap will be needed, if we apply it in the field of nanoelectronics. Similar to graphene [21], an approach is to reduce the dimensionality of phagraphene from 2D to 1D by cutting phagraphene into narrow ribbons, phagraphene nanoribbons (PHAGNRs).
PHAGNRs have been preliminary studied used theoretical calculation. The calculation indicated that PHAGNRs with mixture of armchair and zigzag shaped edges are semiconducting, and that PHAGNRs with pure zigzag shaped edges are metallic. Although preliminary calculation has been completed, while a careful consideration of edge effects in nanometer sized ribbons are required to determine their bandgaps because, unlike the situation in phagraphene, the bonding characteristics between atoms change abruptly at the edges. We will explore the relation between the bandgap and the geometries of PHAGNRs. In this paper, we carried out first-principles calculations to investigate the structure and electronic properties of PHAGNRs with bare edges and hydrogen passivated edges. We also obtained the size effect on nanometer scale by examining the electronic structure of PHAGNRs of different widths.
Section snippets
Calculation method and model
The calculations in this paper were performed within the framework of the density function theory (DFT) using the projector-augmented wave (PAW) method [22], as implemented in the Vienna ab-initio Simulation Package (VASP) computer code [23], [24]. The electron exchange–correlation energy was treated by using the Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [25], which yielded the correct ground-state structure of the combined systems. The cutoff
Structures of phagraphene nanoribbons
We first optimized the typical ZPHAGNRs and MPHAGNRs with bare and hydrogen-terminated edges, named bare 6-MPHAGNR, 6-H-MPHAGNR, bare 4-ZPHAGNR and 4-H-ZPAHGNR, respectively. Fig. 2 displays the structures after geometry optimization and we defined the L1, L2, L3, L4 and d1, d2, d3, d4 representative as unit cell width and length of bare 6-MPHAGNR, 6-H-MPHAGNR, bare 4-ZPHAGNR, 4-H-ZPAHGNR, respectively.
We compared the changes of the structural parameters for bare and hydrogen-terminated edges
Conclusions
In this study, the structures and electronic properties have been investigated for both pure zigzag and mixture phagraphene nanoribbons terminated edges with H atoms or bare edges by using the first-principles projector augmented wave (PAW) potential within the density function theory (DFT) framework under the generalized gradient approximation (GGA). Structure parameters were compared between passivated and bare edge phagraphene nanoribbons (PHAGNRs) after geometry optimization. Our
Acknowledgement
This research was supported by the National Natural Science Foundation of China (NNSFC) (Grant Nos. 51474176 and 51475378).
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