- Email: [email protected]
AlN nanoribbons
Journal of Alloys and Compounds 586 (2014) 176–179
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A first-principles study on the electronic and magnetic properties of armchair SiC/AlN nanoribbons Xiu-Juan Du ⇑, Zheng Chen, Jing Zhang, Zhao-Rong Ning, Xiao-Li Fan State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR China
a r t i c l e
i n f o
Article history: Received 17 April 2013 Received in revised form 28 September 2013 Accepted 30 September 2013 Available online 12 October 2013 Keywords: AlN/SiC nanoribbons Electronic properties Magnetic properties First-principle study
a b s t r a c t Under the generalized gradient approximation (GGA), the electronic and magnetic properties of armchair SiC/AlN nanoribbons have been investigated by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. The unpassivated SiC but not AlN edge can cause magnetic moments of the nanoribbons. The net up-spin charge mainly accumulates at C1, C2 and Si1 sites for ferromagnetic nanoribbons. The increasing concentration of Si–C chains has little effect on the magnetic properties of the nanoribbon, whereas the number of the dangling bonds caused by Si and C atoms strongly affects the magnetic of the system. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Graphene is a unique monolayer membrane of carbon atoms with excellent electronic properties [1–5]. A graphene nanoribbons (GNRs) can be realized by cutting mechanically exfoliated graphene or patterning epitaxially grown graphene structures [6–9]. In particular, armchair graphene nanoribbons (AGNRs) have an direct band gap and the band gap depends on the width of the ribbon [10,11]. Recently, a theoretical study predicted that wurtzite semiconductors transform into a two dimensional (2D) graphitic-like structures when they are in the form of an ultrathin [0 0 0 1] films [12]. Aluminium nitride (AlN) and silicon carbide (SiC), which have broad band gap, can be applied to the high-power, high-temperature and high-frequency electronics devices because of their unique physical and electronic properties [13,14]. The pristine AlN and SiC nanoribbons have been widely investigated due to their high thermal stability and potential technological applications [15–20]. However, up to now, there are no reports about the electronic and magnetic properties of armchair SiC/AlN nanoribbons (ASiC/AlNNRs). The investigation on ASiC/AlNNRs will be helpful to design new nano-electronic devices. In this work, the electronic and magnetic properties of ASiC/ AlNNRs have been investigated using the first-principles projec-
⇑ Corresponding author. Tel.: +86 29 88460502. E-mail address: [email protected] (X.-J. Du).
tor-augmented wave (PAW) potential within the framework of the density function theory (DFT). The paper is organized as follows. In Section 2, the calculation method and structural models of ASiC/AlNNRs are given in detail. In Section 3, the electronic and magnetic properties of the ASiC/AlNNRs are analyzed. Finally, the conclusions of the work are presented in Section 4.
2. Calculation method and model The calculations in this paper are performed within the framework of the density function theory (DFT) using the projector-augmented wave (PAW) potentials [21] and a plane-wave basis set, as implemented in the Vienna Ab-initio Simulation Package (VASP) computer code [22–25]. Normally, the PAW potentials are more accurate than the ultrasoft pseudopotentials and require higher cutoff energies. A uniform cutoff energy of 450 eV is used for all the calculations. The electron exchange and correlation is treated by using the Perdew-Burke-Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [26]. We choose a conjugate-gradient algorithm to relax the ions into their ground states, and the energies and the forces on each ion are converged to less than 104 eV/atom and 0.02 eV/Å, respectively. The Brillouin zone (BZ) integration is performed within a gamma centered Monkhorst-Pack scheme using 1 1 11 k points. To avoid the numerical instability due to level crossing and quasi-degeneracy near the Fermi level, we use Gaussian smearing with a width of 0.2 eV. In present work, we mainly investigate the armchair (SiC)4/(AlN)8 nanoribbons (A(SiC)4/(AlN)8NRs) with different terminations, where 4 and 8 are the number of parallel armchair Si–C and Al–N chains, respectively. The width Na of A(SiC)4/(AlN)8NR is defined by the number of the total Si–C and Al–N chains across the ribbon width. Fig. 1 schematically shows the optimized geometry of A(SiC)4/(AlN)8NRs with different terminations. The red-dash-line rectangle represents the optimized periodic unit cell of the A(SiC)4/(AlN)8NR. The ribbon is assumed along the z direction and as infinite length to avoid end effects. The large enough vacuum layers (16 Å) are added to the x and y lateral direction to make sure that the interactions between a nanoribbon and its periodic images are negligible.
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.203
X.-J. Du et al. / Journal of Alloys and Compounds 586 (2014) 176–179
3. Results and discussions In pristine semiconducting systems, magnetic moments can be induced by dangling bonds. In present work, the effect of four terminations shown in Fig. 1 on A(SiC)4/(AlN)8NRs is considered. Fig. 2 shows the spin-polarized band structures of A(SiC)4/(AlN)8NRs with different terminations. Firstly, the comparison among the band structures of the four systems indicates that the unpassivated SiC but not AlN edge can cause the asymmetric majority spin and minority spin bands and thus magnetic moments of these systems. Secondly, these two systems without magnetic moments (Fig. 2(a) and (c)) both have direct band gaps and Fig. 2(c) presents a smaller band gap than Fig. 2(a) because of the dangling bonds of the AlN
177
edge (the direct band gap of 1.392 eV and 0.970 eV for Fig. 2(a) and (c) respectively). However, these two systems with magnetic moments (Fig. 2(b) and (d)) both present indirect band gaps. Compared with the band gap of Fig. 2(a), the indirect band gaps of Fig. 2(b) and (d) are very narrow (the indirect band gap of 0.260 eV and 0.243 eV for Fig. 2(b) and (d) respectively). In addition, the more bands for minority spin appear in the vicinity of the Fermi level, when compared with that of the majority spin of Fig. 2(b) or (d). This reflects that spin polarization in the proximity of the Fermi level is very obvious for these two systems. Fig. 3 shows the spin-polarized density of states (DOS) for A(SiC)4/(AlN)8NRs with different terminations. Firstly, it can be seen that the majority DOS and the minority DOS shown in
Na=12
z x (a) A(SiC)4 /(AlN)8 NR with two F-termination edges
(c) A(SiC)4 /(AlN)8 NR with F-termination SiC edge
(b) A(SiC)4 /(AlN)8 NR with F-termination AlN edge
(d) A(SiC)4 /(AlN)8 NR without F terminations
Fig. 1. Optimized geometry of A(SiC)4/(AlN)8NRs with different edge terminations. The number Na stands for the width of the ribbons. A black dash line demonstrates a Si–C or Al–N chain. The red-dash-line rectangle contains an optimized periodic unit cell. The orange, gray, pink, blue and cyan balls represent the Si, C, Al, N and F atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(a) A(SiC)4 /(AlN)8 NR with F-termination edges
(c) A(SiC)4 /(AlN)8 NR with F-termination SiC edge
(b) A(SiC)4 /(AlN)8 NR with F-termination AlN edge
(d) A(SiC)4 /(AlN)8 NR without F-terminations
Fig. 2. Spin-polarized band structures of A(SiC)4/(AlN)8NRs with different terminations. The Fermi level is set to 0 and indicated by the red dash line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
178
X.-J. Du et al. / Journal of Alloys and Compounds 586 (2014) 176–179
Fig. 3(a) and (c) are almost symmetry, indicating these two systems have a zero spin polarization. Furthermore, the DOS of A(SiC)4/(AlN)8NRs with F-termination AlN edge or without Fterminations (Fig. 3(b) and (d)) presents complete spin polarization (100%) near the Fermi level. Therefore, these two systems reflected by Fig. 3(b) and (d) can be applied to design the spin electric devices due to their charge transport totally dominated by minority spin electron at the Fermi level. The spin polarization P is defined by the following formula:
P ¼ ½N# ðEF Þ N" ðEF Þ=½N# ðEF Þ þ N" ðEF Þ where N"(EF) and N;(EF) are respectively the DOS of the majority spin and minority spin at the Fermi level. To gain deep insight into the electronic and magnetic properties of the ferromagnetic A(SiC)4/(AlN)8NRs (Fig. 1(b) and (d)), their corresponding difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) are shown in Fig. 4. A significant fact can be seen that the net up-spin charge mainly accumulates at C1, C2 and Si1 sites for these two ferromagnetic nanoribbons, indicating that the magnetic moment largely contributed by the C1, C2 and Si1 atoms. The calculated magnetic moments for C1, C2 and Si1 atoms are respectively 0.5, 0.108 and 0.441 lB for the A(SiC)4/(AlN)8NRs with Ftermination AlN edge as well as 0.499, 0.109 and 0.439 lB for the A(SiC)4/(AlN)8NR without F-termination. Two groups of data are
(a) A(SiC) 4 /(AlN) 8 NR with two F-termination edges
(c) A(SiC)4 /(AlN) 8 NR with F-termination SiC edge
basically same, which can be attributed to the contribution of the unpassivated SiC but not AlN edge to the magnetic moment. This is in good agreement with the results of the band structures. In order to investigate further the effect of the concentration of Si–C chains and the number of the dangling bonds of Si and C atoms on electronic and magnetic properties, Fig. 5 presents the difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) for (a) A(SiC)8/(AlN)4NR with F-terminations AlN edge and (b) A(SiC)4/(AlN)8NR with two dangling-bond SiC edges. The distributions of the difference charge density shown in Fig. 4(a) is similar to that shown in Fig. 5(a), reflecting the increasing concentration of Si–C chains has little effect on the magnetic properties of the nanoribbon. Comparison between Fig. 4(b) and 5(b) indicates the number of the dangling bonds caused by Si and C atoms strongly affects the magnetic of the system. In present work, we investigate the electronic and magnetic properties of an idealized stress-free flat ASiC/AlNNRs with different edge terminations. However, in a very recent study on GNRs with pristine and chemically decorated edges, Akatyeva and Dumitricâ found that these GNRs are actually twisted due to their axial strain [27]. Moreover, Zhang and Dumitricâ found that the twist can modulate bandgap of the GNRs via the effective tensional strain [28]. Considering that the ASiC/AlNNR belongs to a graphitic-like structure, we speculate that a real ASiC/AlNNR should
(b) A(SiC)4 /(AlN) 8 NR with F-termination AlN edge
(d) A(SiC) 4 /(AlN) 8 NR without F-terminations
Fig. 3. The spin-polarized density of states for A(SiC)4/(AlN)8NR with different terminations. The Fermi level is set to 0 and indicated by the red dash line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
C1
C1
Si1
Si1
C2
(a) A(SiC)4 /(AlN)8 NR with F-termination AlN edge
C2
(b) A(SiC)4 /(AlN)8 NR without F-terminations
Fig. 4. The difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) for A(SiC)4/(AlN)8NRs: (a) with F-termination AlN edge and (b) without F-terminations. In all contour plots, the blue, green and yellow represent the value of the charge density in increasing order. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
179
X.-J. Du et al. / Journal of Alloys and Compounds 586 (2014) 176–179
C1
C1
Si1 C2
Si1
Si1
C1
C2
(a) A(SiC)8 /(AlN)4 NR with F-terminations AlN edge
C2
(b) A(SiC) 4 /(AlN)8 NR with two dangling-bond SiC edges
Fig. 5. The difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) for (a) A(SiC)8/(AlN)4NR with F-terminations AlN edge and (b) A(SiC)4/(AlN)8NR with two dangling-bond SiC edges. In all contour plots, the blue, green and yellow represent the value of the charge density in increasing order. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
be also twisted and its electronic properties would be also modulated by the effective strain mechanism. 4. Conclusions In summary, under GGA, both electronic and magnetic properties of armchair SiC/AlN nanoribbons have been investigated by using the first-principles PAW potential within DFT framework. Following conclusions are drawn: a) The comparison among the band structures of the four systems indicates that the unpassivated SiC but not AlN edge can cause magnetic moments of the system. The systems without magnetic moments have direct band gaps, whereas the systems with magnetic moments present indirect band gaps. The more bands for minority spin appeared in the vicinity of the Fermi level indicates spin polarization in the proximity of the Fermi level is very obvious for Fig. 2(b) and (d). b) Fig. 3(a) and (c) are almost symmetry, indicating these two systems have a zero spin polarization. Fig. 3(b) or (d) presents complete spin polarization (100%) near the Fermi level. c) The net up-spin charge mainly accumulates at C1, C2 and Si1 sites for these two ferromagnetic nanoribbons, indicating that the magnetic moment largely contributed by the C1, C2 and Si1 atoms. d) The increasing concentration of Si-C chains has little effect on the magnetic properties of the nanoribbon. Comparison between Figs. 4(b) and 5(b) indicates the number of the dangling bonds caused by Si and C atoms strongly affects the magnetic of the system.
Acknowledgements The work was supported by the National Natural Science Foundation of China (Grant Nos: 51075335, 10902086, 51174168), the
NPU Foundation for Fundamental Research (Grant No: JC201005) and the Doctorate Foundation of Northwestern Polytechnical University (Grant No: CX201309). We would like to thank the Science computational grid (ScGrid) of Supercomputing Center of the Chinese Academy of Sciences (SCCAS) for computational facilities. References [1] A.H.C. Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81 (2009) 109. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, UK, 1998. [3] H. Raza, Graphene Nanoelectronics: Metrology, Synthesis, Properties and Applications, Springer, Berlin-Heidelberg-New York, 2011. [4] P.R. Wallace, Phys. Rev. 71 (1947) 622. [5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [6] Y.B. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201. [7] M.Y. Han, B. Özyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett. 98 (2007) 206805. [8] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Nature 438 (2005) 197. [10] M. Ezawa, Phys. Rev. B 73 (2006) 045432. [11] Y.W. Son, M.L. Cohen, S.G. Louie, Phys. Rev. Lett. 97 (2006) 216803. [12] L.F. Colin, C. Frederik, L.A. Neil, H.H. John, Phys. Rev. Lett. 96 (2006) 066102. [13] L.M. Sheppard, Am. Ceram. Soc. Bull. 31 (1990) 1801. [14] G.L. Harris, in: G.L. Harris (Ed.), Properties of Silicon Carbide, Institution of Electrical Engineers, London, 1995. [15] A.J. Du, Z.H. Zhu, Y. Chen, G.Q. Lu, Sean C. Smith, Chem. Phys. Lett. 469 (2009) 183. [16] J.M. Zhang, F.L. Zheng, Y. Zhang, V. Ji, J. Mater. Sci. 45 (2010) 3259. [17] Z.H. Zhang, W.L. Guo, Phys. Rev. B 77 (2008) 075403. [18] M. Wu, X. Wu, Y. Pei, X.C. Zeng, Nano Res. 4 (2011) 233. [19] Q. Wu, Z. Hu, X.Z. Wang, Y. Chen, J. Phys. Chem. B 107 (2003) 9726. [20] F.L. Zheng, J.M. Zhang, Y. Zhang, V. Ji, Physica B 405 (2010) 3775. [21] G. Kresse, D. Joubert, Phys. Rev. B59 (1999) 1758. [22] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558. [23] G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251. [24] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15. [25] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. [26] J. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [27] E. Akatyeva, T. Dumitricâ, J. Chem. Phys. 137 (2012) 234702. [28] D.B. Zhang, T. Dumitricâ, Small 7 (2011) 1023.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A first-principles study on the electronic and magnetic properties of armchair SiC/AlN nanoribbons Xiu-Juan Du ⇑, Zheng Chen, Jing Zhang, Zhao-Rong Ning, Xiao-Li Fan State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR China
a r t i c l e
i n f o
Article history: Received 17 April 2013 Received in revised form 28 September 2013 Accepted 30 September 2013 Available online 12 October 2013 Keywords: AlN/SiC nanoribbons Electronic properties Magnetic properties First-principle study
a b s t r a c t Under the generalized gradient approximation (GGA), the electronic and magnetic properties of armchair SiC/AlN nanoribbons have been investigated by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. The unpassivated SiC but not AlN edge can cause magnetic moments of the nanoribbons. The net up-spin charge mainly accumulates at C1, C2 and Si1 sites for ferromagnetic nanoribbons. The increasing concentration of Si–C chains has little effect on the magnetic properties of the nanoribbon, whereas the number of the dangling bonds caused by Si and C atoms strongly affects the magnetic of the system. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Graphene is a unique monolayer membrane of carbon atoms with excellent electronic properties [1–5]. A graphene nanoribbons (GNRs) can be realized by cutting mechanically exfoliated graphene or patterning epitaxially grown graphene structures [6–9]. In particular, armchair graphene nanoribbons (AGNRs) have an direct band gap and the band gap depends on the width of the ribbon [10,11]. Recently, a theoretical study predicted that wurtzite semiconductors transform into a two dimensional (2D) graphitic-like structures when they are in the form of an ultrathin [0 0 0 1] films [12]. Aluminium nitride (AlN) and silicon carbide (SiC), which have broad band gap, can be applied to the high-power, high-temperature and high-frequency electronics devices because of their unique physical and electronic properties [13,14]. The pristine AlN and SiC nanoribbons have been widely investigated due to their high thermal stability and potential technological applications [15–20]. However, up to now, there are no reports about the electronic and magnetic properties of armchair SiC/AlN nanoribbons (ASiC/AlNNRs). The investigation on ASiC/AlNNRs will be helpful to design new nano-electronic devices. In this work, the electronic and magnetic properties of ASiC/ AlNNRs have been investigated using the first-principles projec-
⇑ Corresponding author. Tel.: +86 29 88460502. E-mail address: [email protected] (X.-J. Du).
tor-augmented wave (PAW) potential within the framework of the density function theory (DFT). The paper is organized as follows. In Section 2, the calculation method and structural models of ASiC/AlNNRs are given in detail. In Section 3, the electronic and magnetic properties of the ASiC/AlNNRs are analyzed. Finally, the conclusions of the work are presented in Section 4.
2. Calculation method and model The calculations in this paper are performed within the framework of the density function theory (DFT) using the projector-augmented wave (PAW) potentials [21] and a plane-wave basis set, as implemented in the Vienna Ab-initio Simulation Package (VASP) computer code [22–25]. Normally, the PAW potentials are more accurate than the ultrasoft pseudopotentials and require higher cutoff energies. A uniform cutoff energy of 450 eV is used for all the calculations. The electron exchange and correlation is treated by using the Perdew-Burke-Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [26]. We choose a conjugate-gradient algorithm to relax the ions into their ground states, and the energies and the forces on each ion are converged to less than 104 eV/atom and 0.02 eV/Å, respectively. The Brillouin zone (BZ) integration is performed within a gamma centered Monkhorst-Pack scheme using 1 1 11 k points. To avoid the numerical instability due to level crossing and quasi-degeneracy near the Fermi level, we use Gaussian smearing with a width of 0.2 eV. In present work, we mainly investigate the armchair (SiC)4/(AlN)8 nanoribbons (A(SiC)4/(AlN)8NRs) with different terminations, where 4 and 8 are the number of parallel armchair Si–C and Al–N chains, respectively. The width Na of A(SiC)4/(AlN)8NR is defined by the number of the total Si–C and Al–N chains across the ribbon width. Fig. 1 schematically shows the optimized geometry of A(SiC)4/(AlN)8NRs with different terminations. The red-dash-line rectangle represents the optimized periodic unit cell of the A(SiC)4/(AlN)8NR. The ribbon is assumed along the z direction and as infinite length to avoid end effects. The large enough vacuum layers (16 Å) are added to the x and y lateral direction to make sure that the interactions between a nanoribbon and its periodic images are negligible.
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.203
X.-J. Du et al. / Journal of Alloys and Compounds 586 (2014) 176–179
3. Results and discussions In pristine semiconducting systems, magnetic moments can be induced by dangling bonds. In present work, the effect of four terminations shown in Fig. 1 on A(SiC)4/(AlN)8NRs is considered. Fig. 2 shows the spin-polarized band structures of A(SiC)4/(AlN)8NRs with different terminations. Firstly, the comparison among the band structures of the four systems indicates that the unpassivated SiC but not AlN edge can cause the asymmetric majority spin and minority spin bands and thus magnetic moments of these systems. Secondly, these two systems without magnetic moments (Fig. 2(a) and (c)) both have direct band gaps and Fig. 2(c) presents a smaller band gap than Fig. 2(a) because of the dangling bonds of the AlN
177
edge (the direct band gap of 1.392 eV and 0.970 eV for Fig. 2(a) and (c) respectively). However, these two systems with magnetic moments (Fig. 2(b) and (d)) both present indirect band gaps. Compared with the band gap of Fig. 2(a), the indirect band gaps of Fig. 2(b) and (d) are very narrow (the indirect band gap of 0.260 eV and 0.243 eV for Fig. 2(b) and (d) respectively). In addition, the more bands for minority spin appear in the vicinity of the Fermi level, when compared with that of the majority spin of Fig. 2(b) or (d). This reflects that spin polarization in the proximity of the Fermi level is very obvious for these two systems. Fig. 3 shows the spin-polarized density of states (DOS) for A(SiC)4/(AlN)8NRs with different terminations. Firstly, it can be seen that the majority DOS and the minority DOS shown in
Na=12
z x (a) A(SiC)4 /(AlN)8 NR with two F-termination edges
(c) A(SiC)4 /(AlN)8 NR with F-termination SiC edge
(b) A(SiC)4 /(AlN)8 NR with F-termination AlN edge
(d) A(SiC)4 /(AlN)8 NR without F terminations
Fig. 1. Optimized geometry of A(SiC)4/(AlN)8NRs with different edge terminations. The number Na stands for the width of the ribbons. A black dash line demonstrates a Si–C or Al–N chain. The red-dash-line rectangle contains an optimized periodic unit cell. The orange, gray, pink, blue and cyan balls represent the Si, C, Al, N and F atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(a) A(SiC)4 /(AlN)8 NR with F-termination edges
(c) A(SiC)4 /(AlN)8 NR with F-termination SiC edge
(b) A(SiC)4 /(AlN)8 NR with F-termination AlN edge
(d) A(SiC)4 /(AlN)8 NR without F-terminations
Fig. 2. Spin-polarized band structures of A(SiC)4/(AlN)8NRs with different terminations. The Fermi level is set to 0 and indicated by the red dash line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
178
X.-J. Du et al. / Journal of Alloys and Compounds 586 (2014) 176–179
Fig. 3(a) and (c) are almost symmetry, indicating these two systems have a zero spin polarization. Furthermore, the DOS of A(SiC)4/(AlN)8NRs with F-termination AlN edge or without Fterminations (Fig. 3(b) and (d)) presents complete spin polarization (100%) near the Fermi level. Therefore, these two systems reflected by Fig. 3(b) and (d) can be applied to design the spin electric devices due to their charge transport totally dominated by minority spin electron at the Fermi level. The spin polarization P is defined by the following formula:
P ¼ ½N# ðEF Þ N" ðEF Þ=½N# ðEF Þ þ N" ðEF Þ where N"(EF) and N;(EF) are respectively the DOS of the majority spin and minority spin at the Fermi level. To gain deep insight into the electronic and magnetic properties of the ferromagnetic A(SiC)4/(AlN)8NRs (Fig. 1(b) and (d)), their corresponding difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) are shown in Fig. 4. A significant fact can be seen that the net up-spin charge mainly accumulates at C1, C2 and Si1 sites for these two ferromagnetic nanoribbons, indicating that the magnetic moment largely contributed by the C1, C2 and Si1 atoms. The calculated magnetic moments for C1, C2 and Si1 atoms are respectively 0.5, 0.108 and 0.441 lB for the A(SiC)4/(AlN)8NRs with Ftermination AlN edge as well as 0.499, 0.109 and 0.439 lB for the A(SiC)4/(AlN)8NR without F-termination. Two groups of data are
(a) A(SiC) 4 /(AlN) 8 NR with two F-termination edges
(c) A(SiC)4 /(AlN) 8 NR with F-termination SiC edge
basically same, which can be attributed to the contribution of the unpassivated SiC but not AlN edge to the magnetic moment. This is in good agreement with the results of the band structures. In order to investigate further the effect of the concentration of Si–C chains and the number of the dangling bonds of Si and C atoms on electronic and magnetic properties, Fig. 5 presents the difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) for (a) A(SiC)8/(AlN)4NR with F-terminations AlN edge and (b) A(SiC)4/(AlN)8NR with two dangling-bond SiC edges. The distributions of the difference charge density shown in Fig. 4(a) is similar to that shown in Fig. 5(a), reflecting the increasing concentration of Si–C chains has little effect on the magnetic properties of the nanoribbon. Comparison between Fig. 4(b) and 5(b) indicates the number of the dangling bonds caused by Si and C atoms strongly affects the magnetic of the system. In present work, we investigate the electronic and magnetic properties of an idealized stress-free flat ASiC/AlNNRs with different edge terminations. However, in a very recent study on GNRs with pristine and chemically decorated edges, Akatyeva and Dumitricâ found that these GNRs are actually twisted due to their axial strain [27]. Moreover, Zhang and Dumitricâ found that the twist can modulate bandgap of the GNRs via the effective tensional strain [28]. Considering that the ASiC/AlNNR belongs to a graphitic-like structure, we speculate that a real ASiC/AlNNR should
(b) A(SiC)4 /(AlN) 8 NR with F-termination AlN edge
(d) A(SiC) 4 /(AlN) 8 NR without F-terminations
Fig. 3. The spin-polarized density of states for A(SiC)4/(AlN)8NR with different terminations. The Fermi level is set to 0 and indicated by the red dash line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
C1
C1
Si1
Si1
C2
(a) A(SiC)4 /(AlN)8 NR with F-termination AlN edge
C2
(b) A(SiC)4 /(AlN)8 NR without F-terminations
Fig. 4. The difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) for A(SiC)4/(AlN)8NRs: (a) with F-termination AlN edge and (b) without F-terminations. In all contour plots, the blue, green and yellow represent the value of the charge density in increasing order. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
179
X.-J. Du et al. / Journal of Alloys and Compounds 586 (2014) 176–179
C1
C1
Si1 C2
Si1
Si1
C1
C2
(a) A(SiC)8 /(AlN)4 NR with F-terminations AlN edge
C2
(b) A(SiC) 4 /(AlN)8 NR with two dangling-bond SiC edges
Fig. 5. The difference charge density isosurfaces of 0.002 e/Å3 (qup qdown) for (a) A(SiC)8/(AlN)4NR with F-terminations AlN edge and (b) A(SiC)4/(AlN)8NR with two dangling-bond SiC edges. In all contour plots, the blue, green and yellow represent the value of the charge density in increasing order. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
be also twisted and its electronic properties would be also modulated by the effective strain mechanism. 4. Conclusions In summary, under GGA, both electronic and magnetic properties of armchair SiC/AlN nanoribbons have been investigated by using the first-principles PAW potential within DFT framework. Following conclusions are drawn: a) The comparison among the band structures of the four systems indicates that the unpassivated SiC but not AlN edge can cause magnetic moments of the system. The systems without magnetic moments have direct band gaps, whereas the systems with magnetic moments present indirect band gaps. The more bands for minority spin appeared in the vicinity of the Fermi level indicates spin polarization in the proximity of the Fermi level is very obvious for Fig. 2(b) and (d). b) Fig. 3(a) and (c) are almost symmetry, indicating these two systems have a zero spin polarization. Fig. 3(b) or (d) presents complete spin polarization (100%) near the Fermi level. c) The net up-spin charge mainly accumulates at C1, C2 and Si1 sites for these two ferromagnetic nanoribbons, indicating that the magnetic moment largely contributed by the C1, C2 and Si1 atoms. d) The increasing concentration of Si-C chains has little effect on the magnetic properties of the nanoribbon. Comparison between Figs. 4(b) and 5(b) indicates the number of the dangling bonds caused by Si and C atoms strongly affects the magnetic of the system.
Acknowledgements The work was supported by the National Natural Science Foundation of China (Grant Nos: 51075335, 10902086, 51174168), the
NPU Foundation for Fundamental Research (Grant No: JC201005) and the Doctorate Foundation of Northwestern Polytechnical University (Grant No: CX201309). We would like to thank the Science computational grid (ScGrid) of Supercomputing Center of the Chinese Academy of Sciences (SCCAS) for computational facilities. References [1] A.H.C. Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81 (2009) 109. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, UK, 1998. [3] H. Raza, Graphene Nanoelectronics: Metrology, Synthesis, Properties and Applications, Springer, Berlin-Heidelberg-New York, 2011. [4] P.R. Wallace, Phys. Rev. 71 (1947) 622. [5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [6] Y.B. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201. [7] M.Y. Han, B. Özyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett. 98 (2007) 206805. [8] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Nature 438 (2005) 197. [10] M. Ezawa, Phys. Rev. B 73 (2006) 045432. [11] Y.W. Son, M.L. Cohen, S.G. Louie, Phys. Rev. Lett. 97 (2006) 216803. [12] L.F. Colin, C. Frederik, L.A. Neil, H.H. John, Phys. Rev. Lett. 96 (2006) 066102. [13] L.M. Sheppard, Am. Ceram. Soc. Bull. 31 (1990) 1801. [14] G.L. Harris, in: G.L. Harris (Ed.), Properties of Silicon Carbide, Institution of Electrical Engineers, London, 1995. [15] A.J. Du, Z.H. Zhu, Y. Chen, G.Q. Lu, Sean C. Smith, Chem. Phys. Lett. 469 (2009) 183. [16] J.M. Zhang, F.L. Zheng, Y. Zhang, V. Ji, J. Mater. Sci. 45 (2010) 3259. [17] Z.H. Zhang, W.L. Guo, Phys. Rev. B 77 (2008) 075403. [18] M. Wu, X. Wu, Y. Pei, X.C. Zeng, Nano Res. 4 (2011) 233. [19] Q. Wu, Z. Hu, X.Z. Wang, Y. Chen, J. Phys. Chem. B 107 (2003) 9726. [20] F.L. Zheng, J.M. Zhang, Y. Zhang, V. Ji, Physica B 405 (2010) 3775. [21] G. Kresse, D. Joubert, Phys. Rev. B59 (1999) 1758. [22] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558. [23] G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251. [24] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15. [25] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. [26] J. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [27] E. Akatyeva, T. Dumitricâ, J. Chem. Phys. 137 (2012) 234702. [28] D.B. Zhang, T. Dumitricâ, Small 7 (2011) 1023.