Doping-induced large spin-filter behavior and rectification behavior in zigzag graphene nano-ribbon junction
Introduction
In the past two decades, with quick developments of nano and single-molecule technologies [[1], [2], [3], [4], [5], [6], [7], [8]], low-dimensional systems have attracted significant attention due to their potential for excellent device applications [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [9]]. Owing to the atomic-scaled thickness, two-dimensional (2D) materials possess unique electronics, magnetics and mechanics properties [[21], [22], [23], [24], [25], [26], [27]]. Moreover, the 2D materials can also be used to fabricate functional heterojunction devices by doping, which is an extensive strategy in constructing traditional electronics devices [28,29]. The common 2D materials include graphene, arsenene, MoS2, etc [[30], [31], [32]]. Among them, graphene is an extensively studied 2D material due to its attractive properties of high carrier mobility, good toughness and excellent thermal conductivity [[33], [34], [35]]. However, pristine graphene is a kind of zero-bandgap material, which limits its practical applications. Fortunately, the bandgap can be opened when the graphene is cut into quasi one-dimension nanoribbons [36]. The bandgap is inversely proportional to the nanoribbon width, so the bandgap can be tuned by precisely controlling the width of graphene nanoribbon (GNR) [[37], [38], [39], [40]].
In general, there are two types of GNRs according to the shapes of their edges. One is zigzag graphene nanoribbon (ZGNR) and the other is armchair graphene nanoribbon (AGNR). Recently, both ZGNR and AGNR have been successfully synthesized experimentally with atomically precise width (e.g. 6-ZGNRs) via bottom-up approach [[41], [42], [43], [44]]. It is significant that ZGNRs have edge states which have been observed not only by scanning tunneling spectroscopy image but also by optical absorption resonance [44,45]. The edge states of two ZGNR edges may show spin parallel or spin anti-parallel [46,47], i.e., ZGNR may show ferromagnetic or antiferromagnetic properties [[48], [49], [50], [51]]. Moreover, the edge states can be changed by doping or cutting the edges of ZGNRs [41,52]. The magnetic properties of ZGNRs also can be controlled by applying electric fields across ZGNR plane [53] or magnetic field perpendicular to the ZGNR planes [54]. Therefore, functional devices with high spin polarization [[55], [56], [57]], high spin filter efficiency (SFE) [[58], [59], [60], [61], [62]] and even half-metallic behavior [53,54] are promised to be fabricated by doping or modifying the edges of ZGNR, or applying external gate field on ZGNR. Among them, doping with boron or nitrogen atoms is a very useful strategy to tune the edge states, band structures and further the spintronic properties of ZGNR [41].
In this paper, four kinds of nitrogen-doped and boron-doped ZGNR-based heterojunctions are modeled with ab initio calculations and non-equilibrium Green's function (NEGF). For the dopant atoms, hydrogenation and non-hydrogenation effects are considered. Based on the numerical results we find that, due to the localized spin-dependent sub-band lying on the Fermi level, non-hydrogenated systems show excellent spin filter performance, for which 100% SFEs are obtained in lower negative bias regime. When the dopant nitrogen and boron atoms are hydrogenated, the systems exhibit reduced spin filter performances, while demonstrating outstanding rectification behavior.
Section snippets
Theoretical models and computational details
The GNR systems studied here are two-probe 4-ZGNRs with nitrogen and boron-doped edges, where the number 4 denotes that the ZGNR is composed of 4 zigzag carbon chains (Fig. 1). In order to avoid the direct coupling between the edge states of source electrode and drain electrode, the 4-ZGNRs are trimmed with the two sides of the central scattering region being cut. In what follows we refer to such systems as H-shaped 4-ZGNRs. We find that the H-shaped 4-ZGNRs with one side of the electrodes
Results and discussion
Fig. 2 shows the total currents and spin-dependent currents of H1–H4 systems as functions of bias voltages. The figure shows that, all four systems manifest both rectification and spin filter behavior, but the currents, the rectification ratios (RRs) and the SFEs of these four systems show large differences. The total currents of H1 and H2 systems demonstrate small rectification, while the spin-up currents are characterized with large rectification ratios in lower bias regime with the maximum RR
Conclusions
Based on the ab initio calculations, the spin filter performances and the rectification behavior of nitrogen/boron-doped 4-ZGNR-based heterojunctions are studied by using non-equilibrium Green's function method. The numerical results show that, when the dopant boron atoms of 4-ZGNR electrode is not hydrogenated, a large band gap is presented for spin-up bands except for a localized narrow band on the Fermi level of the system, which hinders the spin-up electrons entering into this electrode
Author statement
All authors provided essential contributions to the manuscript. All authors have given approval to the final version of the manuscript. Li Z.L. and Wang C.K. contributed the ideas and discussed the manuscript in detail. Niu L.L. and Fu H.Y. performed the calculations and the data analysis. Li Z.L. and Niu L.L. wrote and revised the manuscript. Suo Y.Q., Liu R., Sun F., Wang S.S. and Zhang G.P. analyzed the data and discussed the manuscript in details with the corresponding authors.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11974217 and 11874242), the Natural Science Foundation of Shandong Province, China (Grant No. ZR2018MA037) and the Postdoctoral Science Foundation (Grant No. 2017M612321) of China. Thanks to the supporting of Taishan Scholar Project of Shandong Province (Grant No. ts201511025).
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These authors contributed equally to this work.