Electronic structure, carrier mobility and device properties for mixed-edge phagraphene nanoribbon by hetero-atom doping
Graphical abstract
Introduction
Graphene, one of the most extensively studied two-dimensional (2D) materials in recent years, consists of carbon atoms by hexagonal symmetric lattice, resulting in its band structure like Dirac cones with linear dispersion near the Fermi level [1]. And graphene presents many novel properties, such as quantum Hall effect [2], [3], ultrahigh thermal conductivity and super-flexibility [4]. In particular, its carrier mobility can reach 105 cm2 V−1 s−1 orders of magnitude at room temperature [5]. This make it possible becoming a promising alternative for developing the next-generation high-performance functional devices. Meanwhile, the successful fabrication of graphene also encourage researchers to explore other 2D materials. So far, beyond graphene, numerous other 2D materials have also been proposed or fabricated, such as silicene [6], h-BN [7], [8], [9] and borophene [10], MoS2 [11], and phosphorene [12]. Especially for carbon-based 2D atomic crystals, such as graphane [13], graphdiyne [14], [15], and penta-graphene [16], [17], all of which demonstrate novel and exceptional electronic features.
Recently, a new carbon allotrope with a planer structure composed of 5-6-7 carbon rings, phagraphene, was predicted to be stable [18]. This 2D material made out of sp2-hybrid carbon atoms possesses a high atomic packing density, allowing that its physical properties could be comparable with graphene and its geometrical structure is energetically more favorable than other carbon allotropes proposed previously [18], [19]. Intrinsic phagraphene is a semimetal with the band structure like distorted Dirac cones near the Fermi level [19]. By using a molecular dynamics simulations, Pereira et al. [20]reported that the thermal conductivity of phagraphene is anisotropic, and the predicted electronic properties suggest that phagraphene could be a better candidate than graphene in future carbon-based thermoelectric devices. Unfortunately, the lack of band gap in phagraphene will be an obstacle for realistic applications in future electronic devices. Similar to graphene, the very simple and feasible strategy to solve this issue is cutting phagraphene sheet into quasi-one dimensional narrowed nanoribbons, phagraphene nanoribbons (PHAGNRs). The electronic properties of PHAGNRs would be ruled by atomic geometries along edges. For example, Liu et al. [21]reported that hydrogen-terminated PHAGNRs with mixture of armchair and zigzag shaped edges are semiconducting, and hydrogen-terminated pure zigzag PHAGNRs behave as a metal.
In previous works, in order to tune functional properties of graphene or derived graphene nanoribbons adequately for realizing applications in more fields, one of the usually used routines is doping them with foreign atoms [22], [23], [24], [25], [26], [27], [28], [29], [30] or to constitute the hetero-structure [31], [32], [33], [34]. Among these doping schemes, B or/and N atom doping is more preferred. This is because the chemical properties of N and B atoms are similar to C atoms, and thus forming a quite strong covalent B (N)-C bond, similar to the CC bond. The geometric deformation for graphene induced by the N or/and B doping is also very small. They modulate electronic properties of graphene only by introducing extra carries and changing energy band structures, therefore, the N or B atom or BN molecule doping has become typical substitutional doping in graphene. So far, B- or/and N-doped graphene have been synthesized in many experiments [28], [29], [30], [35], [36], including a large area h-BNC film being fabricated, which presents different physical properties from h-BN film and graphene sheet [36]. Particularly, with the continuous enhancement of the experimental technology, for example, single carbon atom can be knocked off by focused electron beam of 1 Å diameter [37], and an atomic force microscope (AFM)has been applied to achieve various single-atom manipulations [38], the atom-doping trends to be more ordered, and even to be feasible to realize site-selective substitutional doping with atomic precision [39]. However, there are no reports about doping for phagraphene to modulate electronic properties until now, including B or/and N doping.
In this present work, based on the first-principles method, we study the geometrical stability, electronic structure, carrier mobility, and device property for a mixed-edge phagraphene ribbon with B-, N-, and BN- doping, respectively. Calculations show that the energetic and thermal stability for these ribbons is very high, and they hold diverse electronic structures upon the dopant types and doping sites. For this, the charge transfer moving edge bands and new formed hybridized bands due to doping play a crucial role. In particular, doping can regulate the carrier mobility of ribbons, and the difference of two orders of magnitude for hole mobility can be generated by BN-doping. In addition, constructed devices based on these ribbons exhibit there is a prominent negative differential resistance characteristics occurring in a BN-doped ribbon device.
Section snippets
Structure Models and Theoretical Method
The schematic diagram for the atomic structure of 2D phagraphene is demonstrated in Fig. 1(a), and when tailoring it along x direction, we can obtain one kind of typical nanoribbon, MPHAGNR, whose each edge is a mixture structure of alternating armchair and zigzag segments. A MPHAGNR contains two classes of carbon chains across its width direction, both pure zigzag-type carbon chains and mixture-type carbon chains consisting of alternating armchair and zigzag segments. Thus, the width of a
Structure stability
After full relaxations, the geometrical deformations occurring in the area near a dopant are extremely small, and all atoms, including carbon atoms and a dopant, are all stay in the same plane. To assess structural stability, we firstly calculate the binding energy of all doped ribbons to demonstrate their energetic stability, and the binding energy is defined as:where EDNR is the total energy of one unit cell for doped ribbons. EC, EH, EB, and EN
Conclusion
Using first-principles calculations based on the density functional theory, we study the structural stability, electronic structure, carrier mobility, and device property of the mixed-edge phagraphene nanoribbon (MPHAGNR) with B-, N-, and BN-doping, respectively. The calculated binding energy suggests that these structures are energetically stable. Born-Oppenheimer molecular dynamics (BOMD) simulations demonstrate that the thermal stability of all systems is also higher. In particular, our
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61371065 and 51604042), the Hunan Provincial Natural Science Foundation of China (Grant Nos. 12JJ3004, 2015JJ3002, 2015JJ2009, 2015JJ2013), and Project supported by the scientific research project of the Education Department of Hunan Province (Grant No. 16C0029).
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