A New Kind of Edge-Modified Spin Semiconductor in Graphene

Feb 7, 2014 - Despite their rich electronic and magnetic properties, the free-standing or suspending zigzag edge graphene nanoribbons with n chains ...
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A New Kind of Edge-Modified Spin Semiconductor in Graphene Nanoribbons Ping Lou* Department of Physics, Anhui University, Hefei 230039, Anhui, China S Supporting Information *

ABSTRACT: Despite their rich electronic and magnetic properties, the freestanding or suspending zigzag edge graphene nanoribbons with n chains (nZGRNs) can be twisted quite easily and buckle, which makes it difficult for nanoelectronics as well as spintronics applications. Using first principles density functional theory (DFT) calculations as well as classical molecular dynamic (MD) simulations, we propose a way to overcome this problem by modifying one edge of n-ZGRNs with (m,m)single-walled carbon nanotubes ((m,m)SWCNTs) into functionalized n-ZGRNs, namely nZGNR-(m,m)SWCNTs. DFT calculations indicate that the 8ZGNR and (6,6)SWCNT are predicted to form a 8ZGNR-(6,6)SWCNT without any obvious activation barrier. Moreover, the formed 8ZGNR-(6,6)SWCNT is more energetically favorable by about 1.86 eV. Hence, the nZGNR-(m,m)SWCNT should be found in experiment under mild conditions. MD simulations indicate that the nZGNR-(m,m)SWCNT possesses significantly improved mechanical and thermal stability as compared to a n-ZGNR, such that even at 1000 K the 6ZGNR-(6,6)SWCNT can remain straight. Excitingly, we find that one edge modification with −(m,m)SWCNT transforms the n-ZGNR into a ferromagnetic spin semiconductor. By simulation field-effect transistor (FET) doping, we demonstrate that in a nZGNR-(m,m)SWNT FET completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. These findings should open a viable route for efficient spin-resolved band engineering in graphene-based devices with the current technology of the semiconductor industry. Finally, the origins of its unique electronic and magnetic properties as well as of its mechanical and thermal stabilities are discussed by using the band structures, partial charge densities of the bands at the Γ and X points, Mulliken charge analysis, as well as atomic configurations.

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INTRODUCTION It has been noted that graphene nanoribbons (GNRs) can be either magnetic or nonmagnetic semiconductors, depending on their edge structure. Both edges hydrogenation armchair edge GNR with n dimer lines (n-AGNR) is a nonmagnetic semiconductor, while both edges hydrogenation zigzag edge GNR with n chains (n-ZGNR) is an antiferromagnetic (AFM) semiconductor.1 The magnetism in n-ZGNR arises from the peculiar localized states along the zigzag edges.2−4 It provides a new mode to perform potential applications in spintronics.5−7 Son et al.5 have demonstrated that n-ZGNRs can be made to carry a spin current in the presence of a sufficiently large electric field. Kim et al.6 have reported a large magnetoresistance in the spin-valve devices based on n-ZGNRs. Martins et al.7 have put forward a class of spin filters based on σ- and πdefects at n-ZGNR edges. In general, if the edges are not straight, the edge magnetic coupling can be different;8,9 therefore, Yu et al.9 propose a unified geometric rule for designing graphene-based magnetic nanostructures. Moreover, the localized edge state has unique chemical reactivity,10,11 and thus by functionalization of the edges with various atoms and by functional groups, one can obtain various electronic and magnetic properties.12−16 For example, Kan et al.13 have © 2014 American Chemical Society

revealed that the half-metallic behavior can be observed in nZGNRs with edge decoration by electron donor/acceptor pair (CH3/NO2). Wu et al.14 have reported that not only can halfmetallicity be achieved in the modified n-ZGNRs with electron donor/acceptor pairs OH/SO2 and OH/NO2, but also metallicity can be produced by using the OH/CN pair to decorate both edges of n-ZGNRs. In addition, Wu et al.14 have revealed that using isolated SO2 functional group at one edge can lead to underlying half-metallicity in n-ZGNRs, almost independent of the type of functional group decorating the other edge, such as the sampled F, H, or OH atom/group, or even with a bare edge.14 Modification of one or both edges of n-ZGNR with functional groups −O, −F, −OH, −NH2, −CH, −BH, and −Cu was also investigated.12,15,16 On the other hand, SWCNTs, which can be considered to be rolled from GNRs,17 can be either semiconducting or metallic, depending on their chiral indices, (m,l), which are equivalently identified by the tube diameter. Namely, (m,m)single-walled carbon nanotubes ((m,m)SWCNTs) are metals (Figure 8b Received: November 25, 2013 Revised: February 6, 2014 Published: February 7, 2014 4475

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optimization with 10−4 Hartree/bohr convergence tolerance of force on each atom. The accurate band structure calculations are based on PBE only by using 121 k-points along the Z axis.41 A series of nZGNR-(m,m)SWCNTs with different ribbon widths n and tube diameters (m,m) was built; all the geometries were first optimized before calculating their band structures. The simulation field-effect transistor (FET) holes and electrons doping were performed by a shift in the Fermi level, and a uniform background charge is introduced to balance the charge neutrality of the system, which is called the Fermilevel shift (FLS) method.42 The transport properties are calculated using the nonequilibrium Green’s function formalism43 as implemented in the version 3.6 of OPENMX code.44 The nudged elastic band (NEB) method incorporated in the OpenMX computer code was used for transition state search.45 Twelve images were inserted between the initial and final states. Images were optimized until the energy and force on each atom were less than 10−8 Hartree and 10−4 Hartree/bohr. The MD simulations was used to demonstrate the mechanical and thermal stabilities of 6ZGNR-(6,6)SWCNT and 6-ZGNR. The general utility lattice program (GULP) code46,47 was employed for the MD simulations. The bondorder Brenner empirical potential48 was employed to describe the C−C and C−H interactions. The MD simulations were performed in the constant-volume and constant-temperature (NVT) ensemble at 300 and 1000 K, with a time step of 0.5 fs, for 106 MD steps.

below), while (m,l)SWCNTs with m ≠ l are semiconductors.18 Now, they have been proposed for a wide range of scientific and technology applications, such as gas adsorption and purification,18 chemical sensors,19−23 hydrogen storage,24,25 scanning probe microscopy tips,26 cathode field emitters,27 and electronic devices.28,29 Moreover, SWCNTs are renowned for their extremely high flexural rigidity.30 We note that due to intrinsic nonzero edge stress31 and buckling against compression,32 a free-standing or suspending n-ZGRN can be twisted quite easily and buckle,33 which makes it difficult for nanoelectronics as well as spintronics applications. From (m,m)SWCNT tube-shape stability structure, one can expect a method to overcome this problem by modifying one edge of n-ZGRNs with (m,m)SWCNTs into functionalized n-ZGRNs, namely, nZGNR-(m,m)SWCNT. Does nZGNR-(m,m)SWCNT really possess significantly improved mechanical and thermal stabilities? Moreover, so far the edge modifications of n-ZGNRs are done by various atoms or by functional groups.12−16 If the one edge modification of nZGNR is achieved by a (m,m)SWCNT, the other question is whether the electronic and magnetic properties of nZGNR(m,m)SWCNT are novel. If these answers are yes, then a key question is whether to form a nZGNR-(m,m)SWCNT via the interaction between n-ZGNR and (m,m)SWCNT. Motivated by these issues, we systematically investigated the electronic and magnetic properties, and the mechanical and thermal stabilities of nZGNR-(m,m)SWCNT, as well as the possible route for synthesizing nZGNR-(m,m)SWCNT, by using the spin-polarized first principles density functional theory (DFT) calculations and the classical molecular dynamic (MD) simulations. We find that the nZGNR-(m,m)SWCNT possesses not only high flexural rigidity but also novel and tunable electronic and magnetic properties. More excitingly, nZGNR(m,m)SWCNT is a ferromagnetic (FM) spin semiconductor.34 By simulation field-effect transistor (FET) doping, we show that in an nZGNR-(m,m)SWNT FET, the manipulation of spin-polarized currents can be achieved just simply by altering the sign of gate voltages. We reveal that nZGNR-(m,m)SWCNT entails much greater flexural rigidity than a n-ZGNR is due to the sp3-carbon bond “Y”-shaped beam as well as tubeshape stability structure commonly used for building construction. Its unique electronic and magnetic properties originate from the hydrogenation zigzag edge state of nZGNRas well as the Klein and Zigzag π-edge states of −(m,m)SWCNT. In addition, our results suggest a route for synthesizing nZGNR-(m,m)SWCNT.

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RESULTS AND DISCUSSION We first suggest a possible synthetic route. Intuitively, one can fabricate such a pristine 8ZGNR-(6,6)SWCNT structure by fusing a pristine 8ZGNR edge vertically to the (6,6)SWCNT sidewall (see Supporting Information, movie). Using the climbing-image nudged elastic band method,45 we calculated the corresponding minimum energy path (MEP). As shown in Figure 1a, the 8ZGNR and (6,6)SWCNT are predicted to form a 8ZGNR-(6,6)SWCNT without any obvious activation barrier. Moreover, the formed 8ZGNR-(6,6)SWCNT is more energetically favorable by about 1.86 eV. Therefore, the nZGNR(m,m)SWCNT should be feasible to be synthesized in experiment under mild conditions. We hope that the study here can stimulate experimentalists to synthesize nZGNR(m,m)SWCNT. It should be pointed out that the possible synthetic route presented in Figure 1a is, of course, idealistic and does not take into account other factors effect, such as two pristine ZGNRs may form a SWCNT17 or a larger piece of graphene, but it could serve as rough guidelines to the experimental conditions required to achieve nZGNR-(m,m)SWCNT, such as in a scenario where ZGNR/SWCNT ratio is small, so the chance of ZGNR meeting another ZGNR is small. As for the atomic structure of 8ZGNR-(6,6)SWCNT, we find that each C-0 atom (see Figure 1b) is sp3-hybridized and forms four C−C bonds with the nearest neighbors: one of them connects to the C-3 atom with a bond length of 1.53 Å, two of them connect to two adjacent C-1 atoms with the bond length of 1.48 Å, and the last one connects to the adjacent C-2 atom with the bond length of 1.56 Å. The other carbons remain sp2hybridized, whose bond length is about 1.43 Å. It should be mentioned that the minimal nZGNR-(m,m)SWCNT unit cell contains one C-0, one C-2, and one C-3, as well as one C-1 atom. Because Figure 1b consists of four unit cells, it contains four C-0 atoms. Moreover, C-0 atom originally belongs to

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COMPUTATIONAL METHODS AND MODELS All the electronic structure calculations were performed by the OpenMX computer code35 implementing the generalized gradient approximation with PBE exchange correlation functional.36 Norm-conserving Kleinman−Bylander pseudopotentials37 were employed, and the wave functions were expanded by a linear combination of multiple pseudo atomic orbitals (LCPAO)38,39 with a kinetic energy cutoff of 300 Ry. The basis functions used were C5.5-s2p2d1 and H4.5-s1p1. The first symbol designates the chemical name, followed by the cutoff radius (in Bohr radius) in the confinement scheme, and the last set of symbols defines the primitive orbitals applied. The supercell is large enough to ensure that the vacuum space is at least 20 Å to separate the interaction between periodic images. Monkhost pack mesh of K-points (1 × 1 × 61)40 is used for sampling the one-dimensional Brillium zone during geometry 4476

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Figure 1. (color online) (a) Calculated minimal energy path that synthesizes pristine 8ZGNR-(6,6)SWCNT from pristine 8ZGNR and (6,6)SWCNT. Insets show the corresponding geometric structures of initial (I), middle (M), and final (F) states along the energy path. (b) Geometric structures and spin densities of 8ZGNR-(6,6)SWCNT, where the special C sites are marked by edge-C, C-0, C-1, C-2, and C3, respectively. The red and blue surfaces represent the spin-up (↑) and spin-down (↓). (c) Typical snapshots of 6ZGNR-(6,6)SWCNT and 6-ZGNR at 298 K. (d) Typical snapshots of 6ZGNR(6,6)SWCNT and 6-ZGNR 1000 K.

Figure 2. (Color online) Geometric structures and corresponding band structures, as well as spin densities. (a) 8ZGNR-(4,4)SWCNT . (b) 8ZGNR-(9,9)SWCNT. The red solid and blue dash dotted lines denote the spin-up (↑) and spin-down (↓) bands, respectively. The common VB and CB are highlighted with opened circles. The Fermi level is set to zero. The red and blue surfaces represent the spin-up (↑) and spin-down (↓).

(m,m)SWCNT (see Figure 7a), while C-3 originally belongs to pristine n-ZGNR. Through the bonding of C-0 and C-3 atoms, the pristine n-ZGNR and (m,m)SWCNT bonded with each other. It is well-known that the sp3 carbon atoms of diamond form a very rigid three-dimensional network and are capable of resisting strong bending even under high temperature. Hence, from the sp3-hybridized Y-shape and remaining tube-shape stability structure, one can expect that the nZGNR-(m,m)SWCNT should possess significantly improved mechanical and thermal stabilities compared to nZGNR. In order to verify this expectation, the constant-volume and constant-temperature MD simulations were performed at 298 and 1000 K, respectively. As one would expect, even at 1000 K the 6ZGNR-(6,6)SWCNT can remain straight except for the occurrence of gentle ripples at the edge (Figure 1d). However, at 298 K the 6-ZGNR is already curled (Figure 1c). As for electronic and magnetic properties of nZGNR(m,m)SWCNT, Figure 2 presents the optimized geometric structures and corresponding band structures, as well as spin densities, for 8ZGNR-(4,4)SWNT and 8ZGNR-(9,9)SWNT. We note that both spin channels are gapped, but they have different band gaps. Most notably, the common VB (highlighted with opened circles) belongs to the spin-up band, while the common CB (highlighted with opened circles) belongs to the spin-down band. Hence there exists a common band gap (labeled as Δ0, see also Figure 8c). In addition, there are the relatively large local magnetic moments on the edge-C, C-1, and C-2 atoms, and their orientations are parallel (see also Figure 1b), leading to a net magnetic moment of 1 μB per unit cell. On the other hand, when Δ0 equals zero, it becomes a special type of spin-gapless semiconductor.49 Thus, 8ZGNR(4,4)SWNT and 8ZGNR-(9,9)SWNT are called FM spin semiconductors.34 Moreover, the observed phenomenon of FM spin semiconductor is essentially independent of the tube diameter (m,m) of 8ZGNR-(m,m)SWNTs.

Figure 3 presents the density of states versus ribbon width n for nZGNR-(6,6)SWCNT in the ground state. Clearly, for all nZGNR-(6,6)SWCNTs, a gap (Δ0) exists between VB and CB edges. Hence, the observed phenomenon of FM spin semiconductor is essentially independent of the ribbon width n of nZGNR-(6,6)SWNTs. We conclude that the nZGNR(m,m)SWNTs are FM spin semiconductors. More excitingly, for the band structures of nZGNR(m,m)SWCNTs (see Figure 2 and Figure 8c), one can expect that if the Fermi level can be shifted up and down by altering the sign of gate voltages (the well-known field-effect transistor (FET) doping technique50), 100% spin-polarized currents with reversible spin polarization can be achieved. In order verify this expectation, we calculated the spin transport polarization η for a 8ZGNR-(6,6)SWCNT FET under zero bias, which is defined as η = (Tup − Tdown)/(Tup + Tdown).41,51 When gate voltage (Vg) is zero, from Figure 4b, one can find that near Fermi level, both transmittances of spin-up channel (Tup) and spin-up channel (Tdown) are zero, which indicates η = 0; i.e., there is no spin transport polarization. However, when (Vg > 0), from Figure 4a, one can find that at Fermi level the transmittance of spin-up channel (Tup) is 2G0 (G0 = e2/h), while the transmittance of spin-down channel (Tdown) is zero, and then η = 1; i.e., the 8ZGNR-(6,6)SWCNT FET has 100% spin-up polarization transport around the Fermi level. In contrast, when (Vg < 0), from Figure 4c, one can find that at Fermi level the transmittance of spin-up channel (Tup) is zero, while the transmittance of spin-down channel (Tdown) is 2G0, and then η = −1; i.e., the 8ZGNR-(6,6)SWCNT FET has 100% spin-down polarization transport around the Fermi level. This is to say that in an nZGNR-(m,m)SWCNT FET, just simply by altering the sign of gate voltages, one can achieve the manipulation of spinpolarized currents. It is because of this special property that nZGNR-(m,m)SWNT is viewed as one of ideal materials for constructing spintronic nanodevices. It should be pointed out that here we only analyzed the case of zero bias. For general 4477

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Figure 3. (Color online) Density of states for nZGNR-(6,6)SWCNTs with different ribbon widths n: (a) n = 1, (b) n = 2, (c) n = 3, (d) n = 4, (e) n = 5, (f) n = 6, (g) n = 7, and (h) n = 8. The other marks are same as in Figure 2

Figure 5. (color online) Mulliken population analysis of the charge on the edge-C, C-1, and C-2 atoms versus ribbon width n for nZGNR(6,6)SWCNT in the ground state.

Figure 4. (color online) Transmission functions for the 8ZGNR(6,6)SWCNT FET by altering the sign of gate voltages under zero bias. (a) Vg > 0, (b) Vg = 0, and (c) Vg < 0, respectively, where G0 = e2/ h; the red solid and blue dash-dotted lines denote the spin-up (↑) and spin-down (↓) channels, respectively. The Fermi level is set to zero.

with increasing n and eventually reaches maximum value (0.31 μB and is equal to that of 8ZGNR). In contrast to the edge-C atom, for the C-2 atom, Q↑C‑2 − Q↓C‑2 decreases with increasing n and eventually reaches minimum value (0.09 μB). It is noted that Q↑edge‑C − Q↓edge‑C > 0, as well as Q↑C‑2 − Q↓C‑2. As a result, the local magnetic moment at the edge-C and C-2 atoms are spinup (see Figure 1c). Remarkably, the local magnetic moment of C-1 atom is 0.42 μB, independent of n. Thus, the local magnetic moments in 8ZGNR-(6,6)SWCNT are enhanced in comparison to 8-ZGNR. Note that the local magnetic moment on C-3 atom is only 0.01 μB, independent of n (not shown in Figure 5). What is the origination of the unique electronic and magnetic properties of nZGNR-(m,m)SWCNT? In order to trace the origins of them, we calculate the spin-unpolarized band structure of the 6ZGNR-(6,6)SWCNT, which is shown in Figure 6a. Obviously, there is a half-filled flat band (Half-B) located in the whole Brillouin zone from Γ to X, highlighted with opened circles. It is this Half-B that leads to the magnetism in 6ZGNR-(6,6)SWCNT, according to Stoner magnetic theory.54 The existence of such dispersionless Half-B is originated from the special edge shape of the 6ZGNR-

case, one should follow the spirit of graphene nanoribbon FET.52 In order to gain further insight into the above results, as well as analyze the local magnetic moment, we calculated Mulliken charge and spin population on the edge-C, C-1, and C-2 atoms versus ribbon width n for nZGNR-(6,6)SWCNT in the ground state, which is shown in Figure 5. One can find that the sum of the charges in spin-up (Q↑edge‑C) and spin-down channels (Q↓edge‑C) of the edge-C atom (Q↑edge‑C + Q↓edge‑C), as well as the C-1 atom (Q↑C−‑1 + Q↓C‑1) and the C-2 atom (Q↑C‑2 + Q↓C‑2), is essentially independent of n. The Q↑C‑1 and Q↓C‑1 are also independent of n. As a result, the difference of the charges in spin-up and spin-down channels of the C-1 atom (Q↑C‑1 − Q↓C‑1) remains unchanged. It is noted that Q↑C−1 is larger than Q↓C‑1, which leads to the local magnetic moment at the edge-C atom being spin-up (Q↑C‑1 − Q↓C‑1 > 0) (see Figure 8c). However, the difference of the charges in spin-up and spindown channels of the edge-C atom (Q↑edge‑C − Q↓edge‑C) increases 4478

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Figure 7. (color online) (a) Hydrogen adsorption pattern in an (m,m)SWCNT with one H atom per unit cell, where C-0 atom is hydrogen-adsorbed site. The vertical dash line corresponds to the HLines, and the arrows indicate the spin orientation at the π-edges. The Klein and zigzag π-edges at each side of the H-Line are labeled “Klein” (right) and “Zigzag” (left), respectively. (b) Spin-unpolarized band structures of the (m,m)SWCNT-H. A half-filled band across the Fermi level is highlighted with open circles. (c) Spin-polarized band structures of the H-(m,m)SWCNT in the magnetically ordered state. (d) and (e) are geometric structures as well as spin densities for the spin-unpolarized and spin-polarized states of H-(m,m)SWCNT, respectively. The other marks are same as in Figure 2

Figure 6. (color online) Band structures and partial charge densities of the bands at the Γ and X points. The isosurface is 0.08 e/Å3. The Fermi level is set to zero. (a) Spin-unpolarized band structures of the 6ZGNR-(6,6)SWCNT in the paramagnetic state, as well as partial charge densities of the bands at the Γ and X points. A half-filled band across the Fermi level is highlighted with open circles. (b) and (c) are the spin-polarized band structures of the 6ZGNR-(6,6)SWCNT in the magnetically ordered state, as well as partial charge densities of the bands at the Γ and X points.

to ferromagnetism. This is due to that a single H-Line bisects the π-bond networks, creating Klein and Zigzag π-edges at each side of the H-Line.56 According to the spin alternation rule that neighboring sites should have opposite spins, the ferromagnetic order between the edges at opposite sides of the single H-Line occurs (see Figure 7a). Thus, it is interesting to calculate the band structure and local magnetic moment of the H(m,m)SWCNT and compare the results of H-(m,m)SWCNT to that of nZGNR-(m,m)SWCNT. As shown in Figure 7b, for the spin-unpolarized band structure of the H-(m,m)SWCNT, there is a Half-B located in the whole Brillouin zone from Γ to X (highlighted with open circles). Due to the Stoner effect,54 the Half-B will drive spontaneous spin polarization, resulting in an exchange splitting of 0.652 eV for the common band gap. Indeed, Figure 7c shows that the Half-B splits into two bands near the Fermi level: the lower one is spin-up, while the upper band is spin-down. The local magnetic moment per unit cell is 1 μB. Thus, just like the 6ZGNR-(6,6)SWCNT, the H(m,m)SWCNT is a FM spin semiconductor with a common band gap of 0.652 eV. On the other hand, the local magnetic moments of C-1, C-2, and H atoms are 0.43, 0.37, and 0.05 μB, respectively. Notably, the local magnetic moment of C-1 atom is the same as that of 6ZGNR-(6,6)SWCNT. The local magnetic moment of C-2 atom is three times that of 6ZGNR(6,6)SWCNT and is nearly equal to that of 1ZGNR(6,6)SWCNT. Moreover, C-0 is an H-adsorbed site and is also the adsorbed site of edge atom (C-3 atom, see Figure 8c) of 6-ZGNR. Hence, we can conclude that the unique electronic and magnetic properties of H-(m,m)SWCNT originate from the Klein and zigzag π-edge states of −(m,m)SWCNT. In order to further trace the origins of unique electronic and magnetic properties of nZGNR-(m,m)SWCNT, we present the optimized geometric structures and corresponding band

(6,6)SWCNT. Note that both edges of 6-ZGNR are passivated with H atoms via sp2 hybridization,1 while in 6ZGNR(6,6)SWCNT, one edge is passivated with H atoms, but another edge now is connected to −(6,6)SWCNT’s C atoms via sp3 hybridization (C-0 as shown in Figure 1c and Figure 7a and d). From the partial charge densities of the bands at the Γ and X points, one can find that the dispersionless Half-B at the X point (X − 0) is originated from the hydrogenation zigzag edge of the 6ZGNR-(6,6)SWCNT, which exists in the hydrogenation 6-ZGNR. However, the dispersionless Half-B at the Γ point (Γ − 0) is originated mainly from −(6,6)SWCNT, which is absent in the hydrogenation 6ZGNR. On the other hand, that the Half-B is almost dispersionless in the whole fist Brillouin zone indicates that the corresponding electron states are heavily localized. According to Hund’s rule, partial filling of the localized band drives spontaneous spin polarization. Indeed, Figure 6b, c shows that the original Half-B splits into two bands near the Fermi level: the lower one is spin-up, which is labeled “VB” in Figure 6b. The upper band is spin-down, which is labeled “CB” in Figure 6c. Consequently, the 6ZGNR-(6,6)SWCNT is a FM spin semiconductor with a common band gap of 0.105 eV (an exchange splitting of 0.105 eV of Half-B). It has been known that one hydrogen per unit cell adsorbed on (m,m)SWCNT (H-(m,m)SWCNT),55 where the H atoms all adsorbed on the same sublattice forming an infinite straight line of H atoms (H-Line) along the tube axis (z-axis), may lead 4479

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Figure 8. (Color online) Geometric structures and corresponding band structures, as well as spin densities. (a) One-side hydrogenated 8-ZGNR. The dangling bond bands (Dangling-B) are indicated by opened circles. (b) (6,6)SWCNT. (c) 8ZGNR-(6,6)SWCNT, where Δ1 (Δ2) denotes the spin-up (spin-down) band gap; the common VB and CB are marked by “o”. The other marks are same as in Figure 2

because the (6,6)SWCNT passivation removes the dangling bond states of the one-sided hydrogenated 8-ZGNR, as well corresponding edge state. As a result, the local magnetic moment on C-3 atom (original bare edge-C atom) is only 0.01 μB, independent of n (not shown in Figure 5). On the other hand, the SWCNT absorbing the hydrogenated 8-ZGNR produces the ferromagnetic order. As a result, there are relatively large local magnetic moments on the edge-C, C-1, and C-2 atoms, and their orientations are parallel, leading to a net magnetic moment of 1 μB per unit cell. It is the SWCNT unique absorbate that can modify the magnetic features of the one-side hydrogenated ZGNR. The unique electronic and magnetic properties of nZGNR-(m,m)SWCNT originate from the hydrogenation zigzag edge state of ZGNRas well as the Klein and zigzag π-edge states of −(m,m)SWCNT. It should be mentioned that here we focus on using periodic boundary conditions to investigate the electronic and magnetic properties of the nZGNR-(m,m)SWNTs. For general case, the nZGNR-(m,m)SWNTs might display finite-size effects, just like in zigzag-edged triangular graphene molecules,58 acenes,59 and short zigzag nanotubes,60 which is an interesting open question. Thus, we hope that the research here can stimulate more studies on the nZGNR-(m,m)SWNTs.

structures as well as spin densities for one-side hydrogenated 8ZGNR, (6,6)SWCNT, and 8ZGNR-(6,6)SWCNT, respectively, in Figure 8a−c. Clearly, the one-side hydrogenated 8ZGNR has a FA (ferromagnetic spin ordering at each edge and antiparallel spin orientation between both the edges) semiconducting ground state with spin-up and spin-down channels having same band gaps almost (Figure 8a), which is agreement with previous theoretical results.57 Moreover, the edge-C atom with H has −0.287 μB, while the bare edge-C atom has 1.289 μB, leading to a net magnetic moment of 1 μB per unit cell. It is noted that the magnetic properties of one-side hydrogenated 8ZGNR are quite different from those of 8-ZGNR (both twoside hydrogenated 8-ZGNR). This is because of the contribution of the dangling bond states in the one-sided hydrogenated 8-ZGNR. Interestingly, the dangling bond state is very similar to the edge state in that it formed both in the valence bands and in the conduction bands (indicated by opened circles, see Figure 8a). Since the dangling bond state and the edge state with the same spin are localized at the same side, we have 1.289 μB magnetic moment on each bare edge-C atom, which is enhanced compared to that on each Hpassivated edge-C atom (−0.287 μB in our calculations). Thus, H passivation removes the dangling bond states and greatly reduces the magnetic moment. As for the (6,6)SWNT band structure (Figure 8b), the valence band (VB) and conduction band (CB) intersect each other with the Fermi level. Moreover, the local magnetic moment on each C atoms is zero. Thus, the (6,6)SWCNT is a normal metal.18 However, when the (6,6)SWNT-bonding on the edge of one-side-hydrogenated 8-ZGNR, both its spin channels not only are opened band gaps but also have different values (Figure 8c), and magnetic properties change. This is

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CONCLUSION In summary, based on DFT calculations and MD simulations, we have predicted a graphene-based nanostructure, namely nZGNR-(m,m)SWCNT. We find that the direct synthesis (in vacuum) of nZGNR-(m,m)SWCNT via one edge of n-ZGNR adsorbing a (m,m)SWCNT is achieved without any obvious activation barrier. The nZGNR-(m,m)SWCNT possesses not only novel electronic and magnetic properties but also high 4480

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flexural rigidity. Remarkably, the nZGNR-(m,m)SWCNTs are FM spin semiconductors semiconductors, independent of tube diameter (m,m) and ribbon width n. The nZGNR-(m,m)SWCNT FET can provide completely spin-polarized currents with tunable spin polarization just simply by applying a Vg. Moreover, compared to nZGNR, the nZGNR-(m,m)SWCNT possesses enhanced local magnetic moment. These unique electronic and magnetic properties originate from the hydrogenation zigzag edge state of nZGNR− as well as the Klein and zigzag π-edge states of −(m,m)SWCNT in nZGNR-(m,m)SWCNT. Its fascinating and tunable electronic and magnetic properties, as well as the much improved mechanical and thermal stabilities, make it a promising candidate for application in nanoelectronics and spintronics devices. These findings not only open a viable route for efficient spin-resolved band engineering in graphene-based devices but also have practical importance in pushing n-ZGRN into electronics and spintronics applications compatible with the current technology of the semiconductor industry. It should be pointed out that both n-ZGNR and (m,m)SWCNT contain only by sp2-carbon bonds. However, nZGNR(m,m)SWCNT is composed of mixed sp2- and sp3-hybridized carbon bonds, due to the sp3-hybridized connection of nZGNR/(m,m)SWCNT. It is due to the sp3-carbon bond Yshaped beam as well as tube-shape stability structure commonly used for building construction that the nZGNR-(m,m)SWCNT entails much greater flexural rigidity than a n-ZGNR. This design ideas can be applied to the graphene-like systems.

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ASSOCIATED CONTENT

S Supporting Information *

A supplementary movie is included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 11174003) and the 211 Project of Anhui University.

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REFERENCES

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