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Electronic and magnetic properties of silicon adsorption on graphene
Solid State Communications 151 (2011) 1128–1130
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Electronic and magnetic properties of silicon adsorption on graphene C.H. Hu, Y. Zheng, Y. Zhang, S.Q. Wu ∗ , Y.H. Wen, Z.Z. Zhu ∗ Department of Physics and Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China
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info
Article history: Received 12 January 2011 Received in revised form 21 April 2011 Accepted 15 May 2011 by V. Fal’ko Available online 23 June 2011 Keywords: A. Surfaces and interfaces C. Point defects D. Electronic states
abstract Graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate and guest adatoms. In this work, the structural stabilities, and electronic and magnetic properties of graphene with low-coverage adsorption of Si atoms and dimers are studied using a firstprinciples method. Our results show that graphene with Si adatoms is metallic and magnetic with a tiny structural change in the graphene, while graphene with Si addimers is semi-metallic and nonmagnetic with a visible deformation of the graphene. The spin-polarized density of states is calculated in order to identify the electronic origin of the magnetic and nonmagnetic states. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions. © 2011 Elsevier Ltd. All rights reserved.
Graphene, a one-atom-thick carbon sheet with carbon atoms arranged in a two-dimensional (2D) honeycomb configuration, exhibits a number of intriguing properties. Graphene has been widely studied and is expected to have applications in many fields, and especially in nanoelectronic devices [1–4]. However, being an all-surface material, graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate (e.g., an Si and/or SiO2 substrate) [5,6], dielectric media [7–9] and adatoms [10–15]. Already, great attention has been paid to studying the effect of adsorbed guest atoms or molecules on graphene [10–15]. It has been reported that for all the metal adatoms from groups I–III [15] of the periodic table, adsorbing on the hollow site (above the center of the hexagon) is favored, whereas the noble metal adatoms (Au, Ag and Cu) [10–12] tend to adsorb on the carbon top sites. Even at low coverage, remarkable changes in the electronic properties of graphene may still be induced. In this work, we focus on studies of the adsorption of Si atoms and dimers on graphene in an effort to understand their structural stabilities, and electronic and magnetic properties on the basis of first-principles calculations. Calculations relating to the adsorption of Si atoms and dimers on graphene were performed using a first-principles method based on density-functional theory with the generalized gradient approximation (GGA) in the Perdew and Wang 91 (PW91) exchange–correlation functional [16,17], as implemented in the Vienna Ab initio Simulation Package (VASP) [18,19]. In our calculations, the system was modeled by an Si adatom or addimer
∗
Corresponding authors. Tel.: +86 592 2182248; fax: +86 592 2189426. E-mail addresses: [email protected] (Z.Z. Zhu), [email protected] (S.Q. Wu).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.05.027
on a 6 × 6 graphene supercell, plus a 20 Å vacuum layer in the Z -direction, so the inter-layer interactions will be negligible. We anticipated that the inter-adatom or inter-addimer interactions could also be neglected, so a low-coverage adsorption is simulated. Periodic boundary conditions were employed. A plane-wave basis set with a kinetic energy cutoff of 450 eV was used. The Brillouin zone integrals were performed using a Monkhorst–Pack [20] sampling scheme with a 5 × 5 × 1 Γ -centered grid, which had been optimized such that the total energy change would be less than ∼1 meV when the k-point mesh was increased. All the atomic positions were fully relaxed in each system until the forces on all atoms were smaller than 0.01 eV/Å. The calculated lattice parameter for pristine graphene is 2.467 Å, which is in excellent agreement with the experimental value (2.46 Å) [21]. In the case of the adsorption of a single Si atom on graphene, adsorption sites of three types, i.e., H sites (hollow sites above the centers of hexagons), T sites (top sites directly above carbon atoms) and B sites (bridge sites above the midpoints of the C–C bonds), are considered, and these are shown in Fig. 1(a). In the case of Si dimer adsorption on graphene, seven initial adsorption sites are considered, i.e., (1) the CON1 site, an Si dimer parallel to the graphene sheet with its two atoms sitting above the two neighboring T sites of the same sublattice; (2) the CON2 site, an Si dimer parallel to the graphene sheet with its two Si atoms sitting above the opposite B sites; (3) the CON3 site, an Si dimer parallel to the graphene sheet with its two Si atoms sitting above two next neighboring B sites; (4) the CON4 site, an Si dimer perpendicular to the graphene sheet with its two Si atoms sitting above one B site; (5) the CON5 site, an Si dimer parallel to the graphene sheet with its two Si atoms sitting above two neighboring H sites; (6) the CON6 site, an Si dimer perpendicular to the graphene sheet with its two Si atoms sitting above the T site; and (7) the CON7 site, an Si dimer
C.H. Hu et al. / Solid State Communications 151 (2011) 1128–1130
b
a
e
c
f
d
g
h
Fig. 1. (Color online) The supercell model and the adsorption sites for (a) the Si adatom and ((b)–(h)) Si addimers on graphene.
a
c
b
d
e
Fig. 2. (Color online) ((a), (b)) Top and side views of the most stable configurations of the adsorption of the Si atom and dimer on graphene, respectively. ((c), (d)) The corresponding charge density difference plots of the plane perpendicular to the graphene sheet and passing through Si atoms for the Si adatom and addimer ¯ ) plane of on graphene, respectively. (e) The spin density (ρ↑ − ρ↓ ) on the (1010 the graphene with the Si adatom. In panels (c) and (d), the green solid line is for charge accumulation, and the blue dashed line is for charge depletion. In panel (e), the orange solid line is for the majority spin density and the blue dashed line is for the minority spin density.
perpendicular to the graphene sheet with its two Si atoms sitting above the H site, as shown in Fig. 1(b)–(h). Table 1 shows the adsorption energy, the nearest Si–C and Si–Si bond lengths, the angles (denoted as θ SiC−G ) between the nearest neighbor Si–C bonds and the graphene sheet (treated as a plane), and the induced magnetic moments of graphene with the adsorption of Si atoms and Si dimers. The adsorption energy is defined as Ead = ESi–G –nESi –EG , where ESi–G represents the total energy of the silicon–graphene system, ESi the energy of an isolated Si atom, and EG the total energy of the isolated graphene, per 6 × 6 supercell. Of all the adsorption sites for Si adatoms and addimers considered, the site with the lowest adsorption energy is referred to as the most stable one. In the case of Si atom adsorption on graphene, the B site is found to be the most stable adsorption site. The corresponding adsorption energy is about 0.07 and 0.40 eV lower than those of the T and H sites, respectively (see Table 1). This is similar to the case for palladium adatoms, but different from the cases for metal adatoms of the groups I–III where the favored site is the H site [15]. On the other hand, for Si dimer adsorption on graphene, the CON1 site is found to be the most stable site, as indicated in Table 1. In such a case, for an Si addimer, adsorbing on top of two carbon atoms in the same sublattice is favored, as shown in Fig. 2(b). Each Si atom in an addimer is now threefold coordinated (with two carbon atoms and one Si atom). The length of the bond between Si and its nearest neighbor carbon is a bit larger than that in the Si adatom–graphene system (see Table 1). Meanwhile, due to the interaction between the addimer and graphene, the Si–Si bond length in the Si addimer is slightly elongated compared to that in the isolated Si dimer (∼2.18 Å). Moreover, the adsorption sites CON4, CON6 and CON7, where the Si dimer is perpendicular to the graphene plane, are all not favored. Generally, the adsorption energy of the Si addimer is much larger than that of the Si adatom, as a result of remarkable additional
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Table 1 The adsorption energies (Ead ), the nearest Si–C bond lengths (dSi–C ), the Si–Si bond lengths in Si addimers (dSi–Si ), the Si–C bond–graphene sheet angles (θSiC–G ) and the magnetic moments (M) of the Si adatom and addimer on graphene. Adsorption site
Ead (eV)
dC–Si (Å)
dSi–Si (Å)
θSiC–G (◦)
M (µB )
H B T CON1 CON2 CON3 CON4 CON5 CON6 CON7
−0.07 −0.47 −0.41 −3.62 −3.43 −3.60 −3.48 −3.58 −3.36 −3.10
2.48 2.22 2.27 2.25 2.63 2.34 3.90 4.14 3.37 4.10
– – – 2.26 2.32 2.27 2.17 2.28 2.16 2.15
51.20 70.93 90.00 87.59 79.49 74.59 79.48 69.97 90.00 69.74
1.86 1.74 1.83 0.00 0.13 0.00 1.91 −1.99 1.89 1.90
interactions between the Si addimer and graphene, and atoms in the Si addimer. In Fig. 2(a)–(d), the most stable configurations for the adsorption of Si atoms and dimers on graphene together with the corresponding charge density difference plots for the plane perpendicular to the graphene plane and passing through the Si atoms are presented. The charge density difference is defined as 1ρ(⃗ r) = ∑ ρSi–G (⃗r ) − µ ρatom (⃗r − R⃗µ ), where ρSi–G (⃗r ) represents the charge
⃗µ ) the density of the silicon–graphene system, and ρatom (⃗ r − R atomic charge density. As can be seen from Fig. 2(a) and (c), for the adsorption of Si atoms on graphene, Si binds covalently and somewhat ionically to its two nearest neighbor C atoms and this induces a tiny deformation in the graphene. This kind of deformation is characterized by the slight protrusion of the Si nearest neighboring C atoms and small displacements of all of the other neighboring carbon atoms. For Si dimer adsorption on graphene, in addition to the covalent Si–C bond, a strong Si–Si covalent bond in the Si addimer can also be seen from Fig. 2(d). Moreover, in contrast to the Si adatom–graphene system case, a structurally visible deformation in the graphene sheet is found in the Si addimer–graphene system, as can be seen in Fig. 2(b). The spin-polarized total and partial densities of states (TDOS and PDOS) for the most stable configurations of the adsorptions of Si atoms and dimers on graphene are presented in Fig. 3. Fig. 3(a) presents the spin-polarized TDOS together with the PDOS of the Si atom and its two nearest neighbor C atoms in the case of Si atom adsorption on graphene. It shows that graphene that has been adsorbed by low-coverage Si atoms is metallic with a substantial TDOS at the Fermi level mainly contributed by the Si–px py and the C–pz states. This is consistent with the findings of a recent study which showed that the electronic behavior of graphene could be transformed from semi-metallic to metallic even with just a low concentration of Cu adatoms [12]. In more detail, the DOS below −10 eV comes totally from the C–C (sp2 –sp2 ) σ bonds. The C–Si (pz –s) σ -bond, which is responsible for the C–Si covalent interaction, is found to be located around −8.5 eV. Due to the existence of Si dangling bonds, most of the Si p states are unoccupied. For low-coverage adsorption of Si dimers, the spinpolarized PDOS of two Si atoms and one of their nearest neighbor C atoms is presented in Fig. 3(b). Graphene with Si dimers adsorbed is found to be semi-metallic. Like for the Si adatom–graphene system, the states below −10 eV come again from the C–C (sp2 –sp2 ) σ bonds. However, in the energy range from about −10 to −3 eV, the DOS are found to be chiefly contributed by the C sp3 -like states and the Si s state. Namely, the Si–Si (s–s) σ -bond locates around −8.5 eV while the C–Si (sp3 –s) σ -bond is around −8.5 to −3 eV. In the vicinity of the Fermi level, the DOS are predominantly contributed by the Si–Si (px , py –px , py ) π -bonds. It can be further seen from Fig. 3 and Table 1 that graphene with Si atoms adsorbed is magnetic with an induced magnetic moment of about 1.74 µB , and the spin-polarized charge is
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C.H. Hu et al. / Solid State Communications 151 (2011) 1128–1130
In summary, we have studied the structural stabilities, and electronic and magnetic behaviors for graphene with low-coverage Si atom and dimer adsorption, using first-principles calculations. Our results show that for Si atoms, adsorbing on the C–C bridge site is favored, while the Si dimers tend to adsorb on top of two carbon atoms in the same sublattice (CON1 sites). Furthermore, it is also found that graphene with Si atoms adsorbed is metallic and magnetic, with the spin-polarized charge mainly arising from the Si p electrons and partially from the C p electrons. The distribution of the magnetization in the Si adatom–graphene system is closely associated with the distributed defect states induced by the Si adatoms. However, graphene with Si dimers adsorbed is shown to be semi-metallic and nonmagnetic, resulting from there being no residual polarized charge of the Si atoms in the dimers. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions. Acknowledgments This work was supported by the National 973 Program of China (Grant No. 2007CB209702), the National Natural Science Foundation of China under grant No. 11004165, and the Natural Science Foundation of Fujian Province of China (Grant No. 2008J04018). Fig. 3. (Color online) The spin-polarized total and partial densities of states for the systems of (a) Si adsorption on graphene and (b) Si dimer adsorption on graphene. The energy is relative to the Fermi energy, which is indicated by the vertical dashed lines.
contributed mainly by the Si p electrons and a small part by the C p electrons. The Sipx , py and the Cpz majority spin states are partially occupied while the minority spin states are mostly unoccupied (see Fig. 3(a)), resulting in the magnetic state in this ¯ ) system. Fig. 2(e) illustrates the spin density (ρ↑ −ρ↓ ) of the (1010 plane of the Si adatom–graphene system, showing that the spin density is chiefly localized on the Si adatom and its neighboring carbon atoms in the same sublattice. Such a picture is similar to that derived in a previous study of the defect (H chemisorption and vacancy defects) induced magnetism in graphene reported by Yazyev et al. [22]. This kind of magnetic pattern can be explained by the fact that the defect state is distributed over the sites of a sublattice complementary to the one in which the defect was created. In this work, the distributed defect state is the consequence of the electron redistribution and the structural deformation created by the Si adatom on the graphene sheet. In contrast, the adsorption of Si dimers on graphene is nonmagnetic. This is intimately related to the totally bonded electrons of the Si addimer–graphene system, leading to there being no residual spinpolarized charge, as can be seen from Fig. 3(b). Considering the fact that the signature of magnetic adatoms can be experimentally probed using scanning tunneling spectroscopy (STS) [23], such distinctive magnetic behaviors of the adsorption of Si atoms and dimers on graphene would lead to potential applications in the field of data storage.
References [1] F. Chen, J.L. Xia, D.K. Ferry, N.J. Tao, Nano Lett. 9 (2009) 2571. [2] H.E. Romero, N. Shen, P. Joshi, H.R. Gutierrez, S.A. Tadigadapa, J.O. Sofo, P.C. Eklund, ACS Nano. 2 (2008) 2037. [3] M.C. Lemme, T.J. Echtermeyer, M. Baus, B.N. Szafranek, J. Bolten, M. Schmidt, T. Wahlbrink, H. Kurz, Solid State Electron. 52 (2008) 514. [4] L.F. Mao, Nanotechnology 20 (2009) 275203. [5] S. Adam, E.H. Hwang, V.M. Galitski, S. Das Sarma, Proc. Natl. Acad. Sci. U.S.A. 104 (2007) 18392. [6] K. Nomura, A.H. MacDonald, Phys. Rev. Lett. 98 (2007) 076602. [7] P.K. Ang, W. Chen, A.T.S. Wee, K.P. Loh, J. Am. Chem. Soc. 130 (2008) 14392. [8] F. Chen, J.L. Xia, N.J. Tao, Nano Lett. 9 (2009) 1621. [9] C. Jang, S. Adam, J.H. Chen, D. Williams, S. Das Sarma, M.S. Fuhrer, Phys. Rev. Lett. 101 (2008) 146805. [10] J.P. Jalkanen, M. Halonen, D. Fernandez-Torre, K. Laasonen, L. Halonen, J. Phys. Chem. A 111 (2007) 12317. [11] A. Ishii, M. Yamamoto, H. Asano, K. Fujiwara, J. Phys: Conf. Ser. 100 (2008) 052087. [12] M. Wu, E.Z. Liu, M.Y. Ge, J.Z. Jiang, Appl. Phys. Lett. 94 (2009) 102505. [13] T.O. Wehling, K.S. Novoselov, S.V. Morozov, E.E. Vdovin, M.I. Katsnelson, A.K. Geim, A.I. Lichtenstein, Nano Lett. 8 (2008) 173. [14] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Nat. Mater. 6 (2007) 652. [15] K.T. Chan, J.B. Neaton, M.L. Cohen, Phys. Rev. B 77 (2008) 235430. [16] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [17] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. [18] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15. [19] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. [20] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [21] Y. Baskin, L. Meyer, Phys. Rev. 100 (1955) 544. [22] O.V. Yazyev, L. Helm, Phys. Rev. B 75 (2007) 125408. [23] B. Uchoa, L. Yang, S.-W. Tsai, N.M.R. Peres, A.H. Castro Neto, Phys. Rev. Lett. 103 (2009) 206804.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Electronic and magnetic properties of silicon adsorption on graphene C.H. Hu, Y. Zheng, Y. Zhang, S.Q. Wu ∗ , Y.H. Wen, Z.Z. Zhu ∗ Department of Physics and Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China
article
info
Article history: Received 12 January 2011 Received in revised form 21 April 2011 Accepted 15 May 2011 by V. Fal’ko Available online 23 June 2011 Keywords: A. Surfaces and interfaces C. Point defects D. Electronic states
abstract Graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate and guest adatoms. In this work, the structural stabilities, and electronic and magnetic properties of graphene with low-coverage adsorption of Si atoms and dimers are studied using a firstprinciples method. Our results show that graphene with Si adatoms is metallic and magnetic with a tiny structural change in the graphene, while graphene with Si addimers is semi-metallic and nonmagnetic with a visible deformation of the graphene. The spin-polarized density of states is calculated in order to identify the electronic origin of the magnetic and nonmagnetic states. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions. © 2011 Elsevier Ltd. All rights reserved.
Graphene, a one-atom-thick carbon sheet with carbon atoms arranged in a two-dimensional (2D) honeycomb configuration, exhibits a number of intriguing properties. Graphene has been widely studied and is expected to have applications in many fields, and especially in nanoelectronic devices [1–4]. However, being an all-surface material, graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate (e.g., an Si and/or SiO2 substrate) [5,6], dielectric media [7–9] and adatoms [10–15]. Already, great attention has been paid to studying the effect of adsorbed guest atoms or molecules on graphene [10–15]. It has been reported that for all the metal adatoms from groups I–III [15] of the periodic table, adsorbing on the hollow site (above the center of the hexagon) is favored, whereas the noble metal adatoms (Au, Ag and Cu) [10–12] tend to adsorb on the carbon top sites. Even at low coverage, remarkable changes in the electronic properties of graphene may still be induced. In this work, we focus on studies of the adsorption of Si atoms and dimers on graphene in an effort to understand their structural stabilities, and electronic and magnetic properties on the basis of first-principles calculations. Calculations relating to the adsorption of Si atoms and dimers on graphene were performed using a first-principles method based on density-functional theory with the generalized gradient approximation (GGA) in the Perdew and Wang 91 (PW91) exchange–correlation functional [16,17], as implemented in the Vienna Ab initio Simulation Package (VASP) [18,19]. In our calculations, the system was modeled by an Si adatom or addimer
∗
Corresponding authors. Tel.: +86 592 2182248; fax: +86 592 2189426. E-mail addresses: [email protected] (Z.Z. Zhu), [email protected] (S.Q. Wu).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.05.027
on a 6 × 6 graphene supercell, plus a 20 Å vacuum layer in the Z -direction, so the inter-layer interactions will be negligible. We anticipated that the inter-adatom or inter-addimer interactions could also be neglected, so a low-coverage adsorption is simulated. Periodic boundary conditions were employed. A plane-wave basis set with a kinetic energy cutoff of 450 eV was used. The Brillouin zone integrals were performed using a Monkhorst–Pack [20] sampling scheme with a 5 × 5 × 1 Γ -centered grid, which had been optimized such that the total energy change would be less than ∼1 meV when the k-point mesh was increased. All the atomic positions were fully relaxed in each system until the forces on all atoms were smaller than 0.01 eV/Å. The calculated lattice parameter for pristine graphene is 2.467 Å, which is in excellent agreement with the experimental value (2.46 Å) [21]. In the case of the adsorption of a single Si atom on graphene, adsorption sites of three types, i.e., H sites (hollow sites above the centers of hexagons), T sites (top sites directly above carbon atoms) and B sites (bridge sites above the midpoints of the C–C bonds), are considered, and these are shown in Fig. 1(a). In the case of Si dimer adsorption on graphene, seven initial adsorption sites are considered, i.e., (1) the CON1 site, an Si dimer parallel to the graphene sheet with its two atoms sitting above the two neighboring T sites of the same sublattice; (2) the CON2 site, an Si dimer parallel to the graphene sheet with its two Si atoms sitting above the opposite B sites; (3) the CON3 site, an Si dimer parallel to the graphene sheet with its two Si atoms sitting above two next neighboring B sites; (4) the CON4 site, an Si dimer perpendicular to the graphene sheet with its two Si atoms sitting above one B site; (5) the CON5 site, an Si dimer parallel to the graphene sheet with its two Si atoms sitting above two neighboring H sites; (6) the CON6 site, an Si dimer perpendicular to the graphene sheet with its two Si atoms sitting above the T site; and (7) the CON7 site, an Si dimer
C.H. Hu et al. / Solid State Communications 151 (2011) 1128–1130
b
a
e
c
f
d
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Fig. 1. (Color online) The supercell model and the adsorption sites for (a) the Si adatom and ((b)–(h)) Si addimers on graphene.
a
c
b
d
e
Fig. 2. (Color online) ((a), (b)) Top and side views of the most stable configurations of the adsorption of the Si atom and dimer on graphene, respectively. ((c), (d)) The corresponding charge density difference plots of the plane perpendicular to the graphene sheet and passing through Si atoms for the Si adatom and addimer ¯ ) plane of on graphene, respectively. (e) The spin density (ρ↑ − ρ↓ ) on the (1010 the graphene with the Si adatom. In panels (c) and (d), the green solid line is for charge accumulation, and the blue dashed line is for charge depletion. In panel (e), the orange solid line is for the majority spin density and the blue dashed line is for the minority spin density.
perpendicular to the graphene sheet with its two Si atoms sitting above the H site, as shown in Fig. 1(b)–(h). Table 1 shows the adsorption energy, the nearest Si–C and Si–Si bond lengths, the angles (denoted as θ SiC−G ) between the nearest neighbor Si–C bonds and the graphene sheet (treated as a plane), and the induced magnetic moments of graphene with the adsorption of Si atoms and Si dimers. The adsorption energy is defined as Ead = ESi–G –nESi –EG , where ESi–G represents the total energy of the silicon–graphene system, ESi the energy of an isolated Si atom, and EG the total energy of the isolated graphene, per 6 × 6 supercell. Of all the adsorption sites for Si adatoms and addimers considered, the site with the lowest adsorption energy is referred to as the most stable one. In the case of Si atom adsorption on graphene, the B site is found to be the most stable adsorption site. The corresponding adsorption energy is about 0.07 and 0.40 eV lower than those of the T and H sites, respectively (see Table 1). This is similar to the case for palladium adatoms, but different from the cases for metal adatoms of the groups I–III where the favored site is the H site [15]. On the other hand, for Si dimer adsorption on graphene, the CON1 site is found to be the most stable site, as indicated in Table 1. In such a case, for an Si addimer, adsorbing on top of two carbon atoms in the same sublattice is favored, as shown in Fig. 2(b). Each Si atom in an addimer is now threefold coordinated (with two carbon atoms and one Si atom). The length of the bond between Si and its nearest neighbor carbon is a bit larger than that in the Si adatom–graphene system (see Table 1). Meanwhile, due to the interaction between the addimer and graphene, the Si–Si bond length in the Si addimer is slightly elongated compared to that in the isolated Si dimer (∼2.18 Å). Moreover, the adsorption sites CON4, CON6 and CON7, where the Si dimer is perpendicular to the graphene plane, are all not favored. Generally, the adsorption energy of the Si addimer is much larger than that of the Si adatom, as a result of remarkable additional
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Table 1 The adsorption energies (Ead ), the nearest Si–C bond lengths (dSi–C ), the Si–Si bond lengths in Si addimers (dSi–Si ), the Si–C bond–graphene sheet angles (θSiC–G ) and the magnetic moments (M) of the Si adatom and addimer on graphene. Adsorption site
Ead (eV)
dC–Si (Å)
dSi–Si (Å)
θSiC–G (◦)
M (µB )
H B T CON1 CON2 CON3 CON4 CON5 CON6 CON7
−0.07 −0.47 −0.41 −3.62 −3.43 −3.60 −3.48 −3.58 −3.36 −3.10
2.48 2.22 2.27 2.25 2.63 2.34 3.90 4.14 3.37 4.10
– – – 2.26 2.32 2.27 2.17 2.28 2.16 2.15
51.20 70.93 90.00 87.59 79.49 74.59 79.48 69.97 90.00 69.74
1.86 1.74 1.83 0.00 0.13 0.00 1.91 −1.99 1.89 1.90
interactions between the Si addimer and graphene, and atoms in the Si addimer. In Fig. 2(a)–(d), the most stable configurations for the adsorption of Si atoms and dimers on graphene together with the corresponding charge density difference plots for the plane perpendicular to the graphene plane and passing through the Si atoms are presented. The charge density difference is defined as 1ρ(⃗ r) = ∑ ρSi–G (⃗r ) − µ ρatom (⃗r − R⃗µ ), where ρSi–G (⃗r ) represents the charge
⃗µ ) the density of the silicon–graphene system, and ρatom (⃗ r − R atomic charge density. As can be seen from Fig. 2(a) and (c), for the adsorption of Si atoms on graphene, Si binds covalently and somewhat ionically to its two nearest neighbor C atoms and this induces a tiny deformation in the graphene. This kind of deformation is characterized by the slight protrusion of the Si nearest neighboring C atoms and small displacements of all of the other neighboring carbon atoms. For Si dimer adsorption on graphene, in addition to the covalent Si–C bond, a strong Si–Si covalent bond in the Si addimer can also be seen from Fig. 2(d). Moreover, in contrast to the Si adatom–graphene system case, a structurally visible deformation in the graphene sheet is found in the Si addimer–graphene system, as can be seen in Fig. 2(b). The spin-polarized total and partial densities of states (TDOS and PDOS) for the most stable configurations of the adsorptions of Si atoms and dimers on graphene are presented in Fig. 3. Fig. 3(a) presents the spin-polarized TDOS together with the PDOS of the Si atom and its two nearest neighbor C atoms in the case of Si atom adsorption on graphene. It shows that graphene that has been adsorbed by low-coverage Si atoms is metallic with a substantial TDOS at the Fermi level mainly contributed by the Si–px py and the C–pz states. This is consistent with the findings of a recent study which showed that the electronic behavior of graphene could be transformed from semi-metallic to metallic even with just a low concentration of Cu adatoms [12]. In more detail, the DOS below −10 eV comes totally from the C–C (sp2 –sp2 ) σ bonds. The C–Si (pz –s) σ -bond, which is responsible for the C–Si covalent interaction, is found to be located around −8.5 eV. Due to the existence of Si dangling bonds, most of the Si p states are unoccupied. For low-coverage adsorption of Si dimers, the spinpolarized PDOS of two Si atoms and one of their nearest neighbor C atoms is presented in Fig. 3(b). Graphene with Si dimers adsorbed is found to be semi-metallic. Like for the Si adatom–graphene system, the states below −10 eV come again from the C–C (sp2 –sp2 ) σ bonds. However, in the energy range from about −10 to −3 eV, the DOS are found to be chiefly contributed by the C sp3 -like states and the Si s state. Namely, the Si–Si (s–s) σ -bond locates around −8.5 eV while the C–Si (sp3 –s) σ -bond is around −8.5 to −3 eV. In the vicinity of the Fermi level, the DOS are predominantly contributed by the Si–Si (px , py –px , py ) π -bonds. It can be further seen from Fig. 3 and Table 1 that graphene with Si atoms adsorbed is magnetic with an induced magnetic moment of about 1.74 µB , and the spin-polarized charge is
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In summary, we have studied the structural stabilities, and electronic and magnetic behaviors for graphene with low-coverage Si atom and dimer adsorption, using first-principles calculations. Our results show that for Si atoms, adsorbing on the C–C bridge site is favored, while the Si dimers tend to adsorb on top of two carbon atoms in the same sublattice (CON1 sites). Furthermore, it is also found that graphene with Si atoms adsorbed is metallic and magnetic, with the spin-polarized charge mainly arising from the Si p electrons and partially from the C p electrons. The distribution of the magnetization in the Si adatom–graphene system is closely associated with the distributed defect states induced by the Si adatoms. However, graphene with Si dimers adsorbed is shown to be semi-metallic and nonmagnetic, resulting from there being no residual polarized charge of the Si atoms in the dimers. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions. Acknowledgments This work was supported by the National 973 Program of China (Grant No. 2007CB209702), the National Natural Science Foundation of China under grant No. 11004165, and the Natural Science Foundation of Fujian Province of China (Grant No. 2008J04018). Fig. 3. (Color online) The spin-polarized total and partial densities of states for the systems of (a) Si adsorption on graphene and (b) Si dimer adsorption on graphene. The energy is relative to the Fermi energy, which is indicated by the vertical dashed lines.
contributed mainly by the Si p electrons and a small part by the C p electrons. The Sipx , py and the Cpz majority spin states are partially occupied while the minority spin states are mostly unoccupied (see Fig. 3(a)), resulting in the magnetic state in this ¯ ) system. Fig. 2(e) illustrates the spin density (ρ↑ −ρ↓ ) of the (1010 plane of the Si adatom–graphene system, showing that the spin density is chiefly localized on the Si adatom and its neighboring carbon atoms in the same sublattice. Such a picture is similar to that derived in a previous study of the defect (H chemisorption and vacancy defects) induced magnetism in graphene reported by Yazyev et al. [22]. This kind of magnetic pattern can be explained by the fact that the defect state is distributed over the sites of a sublattice complementary to the one in which the defect was created. In this work, the distributed defect state is the consequence of the electron redistribution and the structural deformation created by the Si adatom on the graphene sheet. In contrast, the adsorption of Si dimers on graphene is nonmagnetic. This is intimately related to the totally bonded electrons of the Si addimer–graphene system, leading to there being no residual spinpolarized charge, as can be seen from Fig. 3(b). Considering the fact that the signature of magnetic adatoms can be experimentally probed using scanning tunneling spectroscopy (STS) [23], such distinctive magnetic behaviors of the adsorption of Si atoms and dimers on graphene would lead to potential applications in the field of data storage.
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