Ferromagnetism in zigzag GaN nanoribbons with tunable half-metallic gap

doi:10.1016/j.commatsci.2016.02.012

Computational Materials Science

Volume 117, May 2016, Pages 300-305

https://doi.org/10.1016/j.commatsci.2016.02.012 Get rights and content

Highlights

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The ZGaNNRs with hydrogenated nitrogen edge are wide band-gap semiconductors.
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The ZGaNNRs with unpassivated nitrogen edge are half metals with ferromagnetism.
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The half-metallicity results from the interaction between N-2p and Ga-4p orbitals.
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The band gap of only gallium edge hydrogenated ZGaNNRs can be tuned in a wide range.

Abstract

Using first-principles calculations, we investigate the electronic and magnetic properties of pristine and hydrogen-terminated zigzag GaN nanoribbons (ZGaNNRs). When the nitrogen edge of the ZGaNNRs is passivated, regardless of the gallium edge, the ZGaNNRs are wide band-gap semiconductors. However, when the nitrogen edge is unpassivated, the ZGaNNRs have 100% spin polarization around the Fermi level and become half metals. It is the strong interaction between the N-2p orbitals and Ga-4p orbitals that leads to the half-metallic ferromagnetism. What’s more, with the ribbons width increases, the half-metallic gap of only gallium edge hydrogenated ZGaNNRs decreases monotonously in a wide range. The tunability of half-metallic gap for ZGaNNRs can be applied to electronic and spintronic devices with wide or specific energy gaps.

Graphical abstract

Introduction

Since the discovery of graphene in 2004 [1], two-demensional nanosheets and the corresponding nanoribbons have attracted considerable research attention due to their high potential applications in spintronics. [2], [3], [4], [5], [6], [7], [8] In addition to graphene nanoribbons (GNRs), other graphene analogues, such as BN [9], [10], [11], [12], Si [13], [14], SiC [15], MoS₂ [16], g-C₃N₄ [17], [18], GaN [19], and AlN [20], [21] nanoribbons are also the research hotspots in material science and condensed-matter physics for their unique quantum confinement and physical properties. Due to the extensive studies on these materials, half-metallicity gradually becomes a research focus of spintronics.

Half-metallicity appears when one of the electron spins shows insulating behavior while the other shows metallic behavior. [22], [23] There are many ways to induce half-metallicity in these graphene-based or graphene-like materials, such as external electric field [24], impurity doping [5], [25], [26], defects [27], and edge modification [28], [29], [30], etc. The edge modification is by far the simplest and most effective method to realize half-metallicity in these ribbons, and edge hydrogenation, edge fluorination, edge chlorination and terminal groups are its several major means. In the last decade, edge hydrogenation method has been applied to many one-dimensional materials [31], [32], [30]. For example, Kan et al. [3] investigated the edge-modified zigzag graphene nanoribbons (ZGNRs) with different terminal groups, such as H, NH₂, NO₂, and CH₃, and half-metallicity was observed when ZGNRs were terminated by NO₂ groups at one edge and by CH₃ groups on another. Zheng et al. [23] claimed that the zigzag boron nitride nanoribbons (ZBNNRs) could be half metallic if their B edge was passivated with hydrogen, while the N edge was unpassivated. Du et al. [20] found that H-terminated zigzag AlN nanoribbons had a non-direct band gap and were nonmagnetic, while the nanoribbons with the N edge unpassivated displayed strong spin-polarization close to the Fermi level.

Among the group III nitrides, GaN is a wide and direct band-gap semiconductor with a value of 3.4 eV, which is a promising material for its outstanding performance in optics, electronics, as well as photoelectronics. [33] Low-dimensional GaN nanostructures, such as GaN nanoribbons (GaNNRs), is widely considered to have great potential applications, too. In this paper, we systematically research the effect of edge-hydrogenation on the electronic and magnetic properties of ZGaNNRs. The calculations demonstrate that when the Ga edge, but not the N edge, of the ZGaNNRs is passivated by H, the ribbons show typical half-metallic ferromagnetism. Moreover, as the ribbons width increases, the half-metallic gap of the ZGaNNRs decreases monotonously in a wide range, and eventually very close to zero.

Section snippets

Method and models

Our first-principles calculations are based on spin-polarized density functional theory in conjunction with the projector-augmented-wave (PAW) potential [34], [35] as implemented by the Vienna ab initio simulation package (VASP) [36], [37]. The electron exchange–correlation functional is treated using the generalized gradient approximation (GGA) in form of Perdew-Burke-Ernzerhof formula (PBE). The cutoff energy for the plane-wave basis set is 500 eV and an energy criterion of 10⁻⁶ eV is selected

Results and discussion

Firstly, we investigate the magnetic couplings between the magnetic moments localized at the edges and plot the spin density distributions of ground states for these systems, as shown in Fig. 1. Here, FM denotes ferromagnetically ordered spins at the edges with the paralled spin directions, and AFM stands for ferromagnetically ordered spins at each edge but with the antiparalled spin directions at the edges. $Δ E$ denotes the energy difference between AFM state and FM state, $Δ E = Δ E_{AFM} - Δ E_{FM}$ . One can

Conclusions

In summary, using first-principle density-functional calculations, we have systematically investigated the electronic and magnetic properties of ZGaNNRs with different edge treatments. Both the electronic and magnetic properties of the ZGNNRs depend critically on how the edges are passivated. When both of the edges of 8-ZGaNNRs are terminated with hydrogen, the system is a wide gap semiconductor with nonmagnetic ground state. When only the gallium edge is passivated, the system is a half-metal

Acknowledgements

We acknowledge the supports from Natural Science Foundation of China Grant Nos. 61176116 and 11347015, Hunan province postdoctoral daily funding No. 201567480511, and Guangxi Natural Science Foundation No. 2013GXNSFBA019002.

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¹: Principal corresponding author.

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Ferromagnetism in zigzag GaN nanoribbons with tunable half-metallic gap

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Method and models

Results and discussion

Conclusions

Acknowledgements

Comput. Mater. Sci.

Appl. Surface Sci.

Chem. Phys. Lett.

Comput. Mater. Sci.

Comput. Mater. Sci.

Physica E: Low-dimensional Syst. Nanostruct.

Science

Nature

J. Am. Chem. Soc.

Rev. Modern Phys.

Phys. Rev. Lett.

J. Phys. Chem. C

Appl. Phys. Lett.

EPL (Europhys. Lett.)

Nano Lett.

Nano Lett.

RSC Adv.

Phys. Rev. Lett.

J. Chem. Phys.