ARTICLE pubs.acs.org/JPCC
Structural and Electronic Properties of Hybrid FluorographeneGraphene Nanoribbons: Insight from First-Principles Calculations Shaobin Tang* and Shiyong Zhang Key Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Gannan Normal University, Ganzhou 341000, China
bS Supporting Information ABSTRACT: The first-principles calculations have been used to investigate the structural and electronic properties of fluorographene nanoribbons and hybrid fluorographenegraphene nanoribbons. Our results indicate that both zigzag and armchair fluorographene nanoribbons are always semiconductors with width-dependent band gaps. The zigzag hybrid nanoribbons with F-terminated graphene edge are half-semiconductors with the two spin channels having a different band gap, qualitatively different from the graphene nanoribbons and hybrid graphanegraphene nanoribbons. The electronic properties of partially fluorinated zigzag graphene nanoribbons were well tuned by edge chemical modification and by control of the number of fluorinated zigzag carbon chains. With NH2 or CH3 groups passivating the graphene side edge, the half-metallicity in zigzag hybrid nanoribbons was realized. Interestingly, the partially fluorinated zigzag graphene nanoribbons with NH2 and CH3 groups may produce half-metallic f half-semiconducting behavior transition when the fluorographene widths gradually increase due to the chemical potential difference change. Therefore, fluorination of graphene nanoribbons will provide a novel way to tune the properties with potential applications to nanotechnology.
1. INTRODUCTION Graphene, a single layer of graphite, has attracted a large amount of attention, both experimentally and theoretically, due to its unique structural and electronic properties and potential applications in nanoscale electronics.13 Pristine graphene, however, is a zero-band gap semiconductor, giving rise to poor onoff ratios in transistor performance.4 Several ways, such as patterning into nanoribbons,69 have been suggested to open a band gap in graphene by breaking sublattice symmetry.510 The graphene nanoribbons (GNRs) with zigzag and armchair shaped edge are semiconductors with width-dependent energy gaps.6,8,11,12 Furthermore, the half-metallicity in zigzag graphene nanoribbons (ZGNRs) can be realized by applying an external electric field7 or by edge chemical modification.13 Our recent calculations show that the carbon chain-doped zigzag boron nitride nanoribbons (ZBNNRs) may produce half-semiconducting f half-metallic f metallic behavior transitions without external electric field.14 Chemical modification provides a promising way for tuning the mechanical and electronic properties of graphene-based nanomaterials. The graphene sheet with adsorptions of hydroxyl and epoxy groups, called graphene oxides (GO), has emerged as a new class of carbon-based nanoscale materials because of its potential applications,1519 such as opening the bandgap of graphene18 and improving the sensor response to nitrogen oxides.19 However, GO is inherently amorphous due to randomly decorated oxygen-containing groups.20 Very recently, the r 2011 American Chemical Society
hydrogenated derivative of graphene with new two-dimensional (2D) crystal, namely as graphane, were obtained by exposing graphene to hydrogen plasma discharge.21 Owing to the hybridization change from sp2 to sp3, the graphane transforms the highly conductive zero-band gap semiconductor into an insulator.21,22 Based on the density functional theory, the partially hydrogenated graphene on one side has been shown to possess magnetic properties.2325 Similar to the GO and graphane, fluorine chemistry may effectively modify the chemical and structural properties of carbon-based materials. A new fluorinated graphene derivative, referred to as fluorographene or graphene fluoride, is now attracting considerable interest because of promising properties revealed for graphane.2630 Graphene fluoride shows strongly insulating behavior with resistance exceeding 1012 Ω,29 qualitatively different from the known graphene-based nonstoichiometric derivatives. The structural and electronic properties of partially and fully fluorinated graphene with CnF for n e 4 have been extensively investigated, both experimentally and theoretically.2632 However, the complete structure of fluorinated graphene remains elusive due to the Raman spectrum limited in specifying CnF structures,29 as well as different preparation conditions.2730 Received: May 25, 2011 Revised: July 23, 2011 Published: July 25, 2011 16644
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Figure 1. Optimized geometric structures of (a) graphene and (b) fluorinated graphene in the chair conformation. Schematic representation of fully fluorinated zigzag (c) and (d) armchair graphene nanoribbons. The WZ and WA (n) denote the widths of the zigzag and armchair nanoribbons, respectively. The F and C atoms were denoted by blue and gray balls, respectively.
Since the properties of GNR are totally different from those of graphene, it is attractive to explore the fluorinated graphene nanoribbons. Recent theoretical studies have shown that the graphane nanoribbons,33 hybrid graphanegraphene nanoribbons,34 and fully hydrogenated boron nitride nanoribbons35 (BNNRs) present completely different electronic properties compared with corresponding GNRs and BNNRs. Because of the difference in electronegativity between C and F atoms, a hybrid system consisting of graphene and fluorographene nanoribbons may be expected to achieve the desired electronic properties, qualitatively different from that of GNRs and graphane nanoribbons. In the present work, extensive firstprinciples density functional calculations have been performed to explore the structural and electronic properties of the fluorinated graphene nanoribbons.
2. COMPUTATIONAL DETAILS All calculations in this work have been performed by using the plane-wave technique implemented in the Vienna ab initio simulation package.36 The projector augmented wave method (PAW)37 was used to describe the electronion interaction. A plane wave basis set with a cutoff energy of 400 eV was used for the electron wave function. The local-spin density approximation (LSDA)38 was employed to describe the exchange-correction potential in calculation. The 1D and 2D periodic boundary conditions were applied along the growth directions of nanoribbons and sheet, respectively. Vacuum spacings of 12 Å for layers and 10 Å for edges between periodic cells were considered in calculation, keeping the negligible interaction between sheet or ribbons in adjacent cells. Monkhost pack meshs of k-points 1 1 13 and 7 7 1 were used for sampling the 1D and 2D Brillouin zones during the geometry optimization, respectively. All atomic positions were optimized by the conjugate gradient method with the converging tolerance of 0.02 eV/Å for the forces on all atoms. The convergence criteria of energy for self-consistent field computations is 105 eV. The convergence of our calculated parameters for the geometry optimization was checked by calculating the CC bond length of carbon materials. Our calculated results of 1.42 Å for
Figure 2. Optimized geometric structures of partially fluorinated zigzag and armchair graphene nanoribbons. Top view and side view of zigzag (a) and armchair (b) nanoribbons with F-terminated edge. Top view of zigzag nanoribbons with (c) NH2 and (d) CH3 groups. Note that the arrows in (b) represent the distorted orientation of bonds at edge. The widths of graphene with and without fluorination were denoted by n and m in (a) and (b), respectively.
graphene, 1.53 Å for graphane, and 1.56 Å for fluorographene are consistent with experimental values.21,27,29 The electronic structure calculations were preformed by using 22 k-points along the periodic orientation. A 4 4 unit cell consisting of 32 carbon atoms was used to simulate the hexagonal graphene sheet and graphene fluoride as shown in Figure 1, panels a and b. The fully fluorinated graphene nanoribbons cutting from the fluorographene in the chairlike configuration were denoted as F-GNRs, whose both edges were saturated by fluorin atoms. Following the previous convention,3941 the fluorinated zigzag and armchair graphene nanoribbons (F-ZGNRs and F-AGNRs) were labeled by the number of parallel zigzag chains and by the number of parallel columns across the ribbon width, respectively, i.e., n-F-ZGNR and n-F-AGNR as shown in Figure 1, panels c and d. Accordingly, the partially fluorinated zigzag (armchair) nanoribbons, which include fluorographene side with width n and pristine graphene side with width m, were denoted as n-F-m-ZGNR (n-F-m-AGNR) as shown in Figure 2, panels a and b. Similarly, the zigzag graphene nanoribbons with partially and fully hydrogenation were denoted as n-H-m-ZGNR and n-H-ZGNR, respectively. In order to investigate the effect of edge modification on the electronic properties of zigzag hybrid ribbons, the edges of graphene side in n-F-m-ZGNR were saturated by F atoms, NH2 and CH3 groups [Figure 2, panels a, c, and d].
3. RESULTS AND DISCUSSION 3.1. Structural and Electronic Properties of Fluorographene Nanoribbons. Although the fluorinated graphene with
fluorine atoms on one and both sides of graphene (Figure 1a) may generate different configurations,28,29,31,32 the fully fluorinated 16645
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Figure 3. Band gaps of fully fluorinated zigzag and armchair graphene nanoribbons (n-F-ZGNR and n-F-AGNR) as a function of the ribbon width n.
Figure 4. Band structures of (a) 12-F-ZGNR and (b) 20-F-AGNR. The partial charge densities (top view and side view) of (c) valence band maximum (Γ1) and (d) conduction band minimum (Γ2) in (a). The isosurface is 0.0015 e/Å3. The Fermi level is set to zero.
graphene in chair conformation (CF) has relatively high stability. As Figure 1b shows, fluorographene with F atoms alternating on both sides of sheet maintains a hexagonal network structure after full geometry optimization. The optimized structure shows that the length of sp3 CC bond is 1.56 Å consistent with 1.5532 and 1.579 Å31 by previous first-principles calculations, larger than 1.42 Å for sp2 bond in graphene sheet. The predicted CF bond distance of 1.35 Å is larger than CH bond with 1.09 Å in graphane due to the difference in electronegativity between C and F atoms. The calculated band structures show that fluorographene is semiconductor with wide band gap of 3.0 eV, in agreement with 3 eV29 by experiment observation and 2.96 eV by LDA.32 Similar to GNRs, the fluorinated graphene nanoribbons cutting from fluorographene also have two main classes, zigzag and armchair. The geometrical structures of fluorinated zigzag and armchair nanoribbons were first explored. The optimized structures of 12-F-ZGNRs and 20-F-AGNRs were shown in Figure 1, panels c and d. In both nanoribbons, all of C atoms have the same sp3 hybridization as the fluorographene sheet. For zigzag nanoribbons, the length of CC bonds in zigzag chains is 1.542 Å, slightly smaller than 1.553 Å of CC bonds connecting two neighboring chains. Such difference between distance of CC bonds along dimer line and connecting two neighboring dimer lines was also found in fluorinated armchair nanoribbons. However, the
sites have less effect on the CF bond lengths (1.366 Å) except for the edge CF bonds (1.347 Å). As shown in Figure 3, the fully fluorinated graphene nanoribbons with zigzag and armchair edge are semiconductors with direct band gaps, which decrease as the ribbon width increases and converge to the value of fluorographene (CF). The band gaps of n-F-ZGNRs (n-F-AGNRs) are changed from 3.33 eV for n = 4 to 2.95 eV for n = 12 (from 3.89 eV for n = 6 to 3.06 eV for n = 21). As an example, the band structures of 12-F-ZGNR and 20-F-AGNR were shown in Figure 4, panels a and b, respectively. Compared with zigzag nanoribbons, the armchair structures with same width have the larger band gaps as shown in Figure 3. Such width-dependent energy gaps have been found in graphane nanoribbons.33,34 In order to determine the ground state of fully fluorinated zigzag and armchair nanoribbons, both spin-polarized and spin-unpolarized computations were performed. Our results show that no energy difference was found. This differs distinctly from the fully hydrogenated ZBNNRs with the ferromagnetic (FM) ground state35 and ZGNRs with antiferromagnetic (AFM) ground state.6 The energy gaps of fully fluorinated graphene nanoribbons are smaller than that of graphane nanoribbons with same width due to the difference in electronegativity between F and H atoms. For example, the band gaps of 12-F-ZGNR and 20-F-AGNR are 2.96 and 3.07 eV, respectively, but the values are changed to 3.79 and 3.65 eV for the corresponding hydrogenated nanoribbons. To 16646
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Figure 5. Formation energies of the fully fluorinated zigzag and armchair graphene nanoribbons as a function of the ribbon width.
have insight into the nature of valence band maximum (VBM) and the conduction band minimum (CBM) determining the band gap, we calculate the partial charge densities of 12-F-ZGNR as shown in Figure 4, panels c and d. The VBM located at Γ1 mainly consists of the bonding state between the hybridized orbitals of carbon atoms, whose charge densities decay to the central domain from both edges. The highest occupied state is distinctly different from that of zigzag graphane nanoribbons, where the VBM is arised from the inner carbon atoms.33 As Figure 4d shows, however, the CBM is antibonding state between py orbitals of inner C and F atoms perpendicular to the ribbon. In order to evaluate the thermodynamic stability of fluorination of nanoribbons depending on the widths, the formation energy used in analysis of the relative stability of endohedral silicon nanowires42 and armchair GNRs11 was calculated in the following way Εf ¼ ΕGF χC μC χF μF
ð1Þ
where EGF is the cohesive energy per atom of fully fluorinated graphene nanoribbons, χi is the molar fraction of atom i (i = C or F) satisfying the relation ∑iχi = 1, and μC is the cohesive energy per atom of a single graphene sheet. The binding energy per atom of F2 molecule is chosen as μF. Note that the negative formation energy Ef indicates the fluorinated GNRs with higher stability than its constituents. As shown in Figure 5, the calculated formation energies of both nanoribbons increase monotonically with the increasing ribbon width, suggesting that the narrow ribbons are more likely accessible than the wider one. Such width-dependent stabilities have been found in graphane nanoribbons33 but contrasting with the fully hydrogenated boron nitride nanoribbons.35 3.2. Structural and Electronic Properties of Hybrid Nanoribbons. The structural and electronic properties of partially fluorinated graphene nanoribbons, namely as hybrid fluorographenegraphene nanoribbons, were discussed. Similar to partially hydrogenated graphene nanoribbons,34,43 the graphene side of both zigzag and armchair structures remains essentially flat as shown in Figure 2, panels a and b, but a tilt has been introduced relative to the fluorographene side (see side view). In addition, two interface carbon atoms connecting the fluorographene side in n-F-m-AGNRs move out of the plane in opposite directions as shown in Figure 2b. More importantly, owing to the larger distance, two edge-decorated F atoms at graphene side for
armchair hybrid nanoribbons lead to the local out-of-plane structural distortion of these edge CF bonds (see side view in Figure 2b), compared to planar edge structure for AGNRs with H-passivated edge. The spin-polarized and spin-unpolarized calculations were performed to determine the electronic structures of zigzag hybrid fluorographenegraphene nanoribbons with F-terminated edge. Both antiferromagnetic (AFM) and ferromagnetic (FM) states of n-F-m-ZGNR are energetically more favorable than the spin-unpolarized states. The energy differences between the antiferromagnetic and nonmagnetic states for n + m = 12 with n = 3, 4, 5, 6, and 7 are always 20 meV/cell. Such trend is different from that of ZGNRs6 and partially hydrogenated ZGNRs,34,43 where the energy difference increases with increasing the width. According to our calculations, the AFM and FM states in n-F-m-ZGNR are nearly degenerate with energy difference of 4 meV/cell for n + m = 12. The band structures show that the ferromagnetic state of n-Fm-ZGNRs is metallic with total magnetic moments of 0.48 μB/ cell independent of the width of graphene part, consistent with the results for ZGNRs.6,34,43 However, the electronic properties of n-F-m-ZGNRs in AFM state are qualitatively different from that of partially hydrogenated graphene nanoribbons. The zigzag hybrid fluorographenegraphene nanoribbons are spin-polarized halfsemiconductors41,44 with different band gaps for both spin channels, compared to semiconductor for partially hydrogenated ZGNRs.34,43 The calculated band structures show that the partially hydrogenated ZGNRs in their AFM states are semiconductors with less spin splitting, in agreement with previous results.34,43 The spinup and spin-down states near the Fermi level in 5-H-7-ZGNR are almost degenerate, leading to the energy gaps of 0.32 eV for spin up channel and 0.36 eV for spin down channel [Figure 6a]. In contrast, the AFM state of 5-F-7-ZGNR is half-semiconductor with distinct band gaps of 0.29 eV for majority spin and 0.07 eV for minority spin because the spin degeneracy near the Fermi level was completely broken as shown in Figure 6b. The electronic properties of halfsemiconductor for partially fluorinated zigzag nanoribbons may be attributed to the chemical potential difference between both sides.13 The widths of fluorographene side in n-F-m-ZGNRs have less effect on the halfsemiconducting behavior. As shown in Figure 6c, the band gaps of n-F-m-ZGNRs with n + m = 12 (n = 3, 4, 5, 6, 7 and 8) for spin up channels are 0.24, 0.23, 0.29, 0.32, 0.36, and 0.34 eV, respectively, whereas the values for spin down channels are 0.06. 0.05, 0.07, 0.08, 0.07, and 0.06 eV due to the spin splitting. Although the ferromagnetic ground state could be achieved in partially hydrogenated graphene nanoribbons with interface zigzag carbon chain half-covered by hydrogen by Feng et al.,34 such hydrogenation of chains is energetically less favorable compared to the carbon chains with hydrogen on both sides.43 The spin densities of 5-F-7-ZGNR with AFM state show that the spin are mainly located at the edge carbon and interface carbon atoms with antiferromagnetic coupling [Figure 6b], consistent with the partially hydrogenated ZGNRs [Figure 6a]. For comparison, we performed calculations for the same n-Fm-ZGNRs with n + m = 12 and 5-H-7-ZGNRs by using the generalized gradient approximation (GGA) with the Perdew BurkeErnzerhof (PBE)45 functional. Our results reveal that the electronic properties of halfsemiconductor in zigzag hybrid fluorographenegraphene nanoribbons with F passivating the edge is unaffected by the PBE functional. The band 16647
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Figure 6. Band structures and spin densities of (a) 5-H-7-ZGNR with H-terminated edge and (b) 5-F-7-ZGNR with F-terminated edge in AFM state. The isosurface is 0.005 e/Å3 with blue for spin up and yellow for spin down. (c) The band gaps of 5-H-7-ZGNR and n-F-m-ZGNR with n + m = 12 for spinup and spin-down channels as a function of the number of fluorinated zigzag chains n. (d) width-dependent band gaps of n-F-m-AGNR with n + m = 20.
structures and corresponding spin densities of nanoribbons in their AFM state by PBE were shown in Supporting Information, Figure S1. The band gaps of n-F-m-ZGNRs with n = 5, 6, and 8 for spin up channels are 0.6, 0.7, and 0.8 eV, respectively, but the corresponding values are 0.16, 0.18, and 0.2 eV for spin down channels, quite different from the partially hydrogenated ZGNRs. Comparing the calculated results from both functionals, it can be seen that the predicted band gaps by PBE are larger than that of LSDA. In contrast, the armchair hybrid fluorographenegraphene nanoribbons n-F-m-AGNRs are nonmagnetic semiconductors with increasing band gaps for small m due to quantum confinement.6 The calculated band gaps for n-F-m-AGNRs with n + m = 20 were shown in Figure 6d. The variation in band gaps was separated into three different families with a hierarchy of gap size given by Δ3p > Δ3p+1 > Δ3p+2 (except for n = 3 and 6), where the p is integer with p = 20 n. Such width-dependent band gaps are similar to the patterned armchair graphene nanoroads,43 but different from the hierarchy reported for the AGNRs6 because of the sp2sp3 hybridization of interface carbon atoms connecting the fluorographene side. 3.3. Tuning Electronic Properties of Hybrid Nanoribbons by Edge Chemical Modification. As discussed above, the partially fluorinated zigzag graphene nanoribbons with
F-terminated edge are halfsemiconductor regardless of the number of fluorinated chains, in which the spin down channels have small band gaps. Therefore, one is expected to realize the half metal by tuning the chemical potential difference between both sides. Some donor groups such as NH2 species [see Figure 2c], which were used to passivate the graphene side edge, may be one such candidate. The band structure calculations for n-F-m-ZGNRs with n + m = 12 show that the hybrid nanoribbons with NH2 groups in AFM state are halfmetal as the n are 1, 2, 3, 4, 5, 6 and 7, corresponding to the semiconductor spin-up channels with energy gaps of 0.16, 0.18, 0.21, 0.22, 0.32, and 0.28 eV and metallic spin-down channels. As an example, the band structures of 5-F-7-ZGNR with NH2 group in AFM state were shown in Figure 7a. The spin-up channel is semiconductor with energy gap of 0.32 eV, but the band across the Fermi level results in the metal for spin down. We also perform the calculations for the zigzag graphene nanoribbon 12-ZGNRs with F at one edge and NH2 at other edge. However, the results show that the AFM state is half-semiconductor, suggesting that the half metal in n-F-m-ZGNRs does not arise from the pristine nanoribbons with same edge passivation. Interestingly, when the numbers of fluorinated zigzag chains n were changed to 7 and 8, the zigzag hybrid graphene nanoribbons 16648
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Figure 7. Band structures and spin densities of 5-F-7-ZGNR with (a) NH2 and (c) CH3 groups and 8-F-4-ZGNR with (b) NH2 and (d) CH3 groups in their AFM states. The isosurface is 0.005 e/Å3 with blue for spin up and yellow for spin down. (e) The spin density of state of 5-F-7-ZGNR with CH3 group (left panel) and the partial density of state of interface C atoms connecting fluorinated chain (middle panel) and of edge C atoms (right panel).
realize half-metallic f half-semiconducting behavior transitions. The band structures of 8-F-4-ZGNR in Figure 7b show that the ribbon with NH2 group is half-semiconductor with energy gaps of 0.27 eV for spin up and 30 meV for spin down. The chemical potential difference change between the graphene and fluorographene sides may be responsible for the electronic behavior transition due to increasing the F concentration. Such half-metallic f half-semiconducting behavior transitions were also realized in partially fluorinated ZGNRs with CH3 group [see Figure 2d]. As shown in Figure 7c, the spin-up channel for
the band structures of 5-F-7-ZGNR with CH3 group is a semiconductor with a energy gap of 0.28 eV, whereas the spindown channel includes a band across the Fermi level, leading to the half metal. The total spin density of states (DOS) for the hybrid nanoribbons with CH3 group also show that the AFM state is half-metallicity, where the majority spin is semiconductor and the minority spin electrons are metallic [left panel in Figure 7e]. Similarly, when the number of fluorinated zigzag chains n is 8, the 8-F-4-ZGNR is half-semiconductor with distinct band gaps of 0.37 eV for spin up and 53 meV for spin down [see Figure 7d]. 16649
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Figure 8. Top view and side view of partial charge densities of (a) the valence band maximum (VBM) of spin-up channel and (c) the states near the Fermi level of spin-down channel for the nanoribbons with NH2 group in the AFM state. The (b) and (d) for the nanoribbons with CH3 group are similar to (a) and (c), respectively. The isosurface is 0.02 e/Å3 and the corresponding band structures were shown in Figure 7, panels a and c.
The geometric structures of hybrid fluorographenegraphene nanoribbons with NH2 and CH3 groups were discussed. The optimized structures of supercell consisting of two unit cells for edge-decorated nanoribbons were shown in the Supporting Information, Figure S2. Similar to the case of one unit cell (Figure 2c), no structural distortion of NH2 groups was observed in 5-F-7-ZGNR when a supercell including two unit cells was used. This is due to the formation of hydrogen bond NH 3 3 3 N between two nearest NH2 groups with length of 1.74 Å, which mitigates the steric interaction. However, the two edge CH3 functional groups in supercell of 5-F-7-ZGNR induce the local out-of-plane structural distortions at each edge site because of strong steric repulsive interactions. Such structural distortions have also been investigated by the previous theoretical study for zigzag graphene nanoribbons with various edge modified chemical groups.46 The novel spin-polarized magnetic and electronic features of zigzag GNRs and BNNRs are generally dominated by the localized electronic states at the edge.6,7,47,48 According to the partial density of state (PDOS) as shown in Figure 7d, the electronic states near the Fermi level are mainly localized at the edge carbon atoms of graphene side and interface carbon atoms connecting fluorinated chains. To have insight into the localized states and the magnetic properties of hybrid ZGNRs with NH2 and CH3 groups, the partial charge densities of VBM for spin up and the states near the Fermi level for spin down and the spin densities were depicted in Figures 8 and 7ad, respectively. The magnetic moments in AFM state are mainly contributed by edge carbon atoms and interface carbon atoms with antiparallel spin orientation [Figure 7ad]. The electronic states in VBM for spin up as shown in Figure 8, panels a and b, mainly consist of the py orbitals of interface carbon atoms perpendicular to the sheet, whereas the partial charge densities near Fermi level for spin down are predominantly located at the edge carbon atoms of graphene side [Figure 8, panels c and d] and also partially at doping groups. Such localized electronic states are similar to that of ZGNRs6,13 and patterned graphene nanoroads.43
4. CONCLUSION In summary, the first-principles DFT calculations have been employed to investigate the electronic structures of hybrid fluorographenegraphene nanoribbons. Both fully fluorinated zigzag and armchair graphene nanoribbons are semiconductors with width-dependent energy gaps. The zigzag hybrid nanoribbons
with F-terminated edge in AFM state are half-semiconductors, in which the spin degeneracy near the Fermi level was completely broken. The hybrid armchair nanoribbons are nonmagnetic semiconductors with a hierarchy of gaps size given by Δ3p > Δ3p+1 > Δ3p+2. Interestingly, the electronic properties of partially fluorinated zigzag graphene nanoribbons were well tuned by edge chemical modification and by control of the width of fluorinated graphene. The zigzag hybrid graphene nanoribbons realize half-metallic f half-semiconducting behavior transition when the fluorographene widths gradually increase due to the chemical potential difference change. The tunable magnetic and electronic properties arising from the fluorination are promising for the realization of the graphene materials-based spintronics applications.
’ ASSOCIATED CONTENT
bS
Supporting Information. Band structures and spin densities of zigzag hybrid nanoribbons by using PBE functional and optimized structures of supercell including two unit cells for zigzag hybrid nanoribbon with edge-decorated groups. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +86-797-8393536. E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge simulating discussions with Z. Cao and thank the computational resources and assistance provided by the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University). ’ REFERENCES (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009, 81, 109. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (4) Schwierz, F. Nat. Nanotechnol. 2010, 5, 487. 16650
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