DSpace at VNU: Naphthalene adsorptions on graphene using Cr Cr-2 Fe Fe-2 linkages: Stability and spin perspectives from first-principles calculations

Chemical Physics Letters 614 (2014) 238–242 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett Naphthalene adsorptions on graphene using Cr/Cr2 /Fe/Fe2 linkages: Stability and spin perspectives from first-principles calculations Viet Q Bui, Hung M Le ∗ Department of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam a r t i c l e i n f o Article history: Received August 2014 In final form 18 September 2014 Available online 28 September 2014 a b s t r a c t We present a first-principles study of naphthalene adsorption on graphene via coordination bonds with Cr/Cr2 /Fe/Fe2 The obtained structures possess great binding stability, and the geometry alignment of C10 H8 is distorted Especially, the use of Cr/Fe dimer further enhances the binding stability of C10 H8 on graphene From binding energy analysis, the adsorption of C10 H8 on metal–graphene is observed to be more favorable than the adsorption of metal–C10 H8 on graphene When empirical dispersion corrections are introduced, the binding energy is improved by 0.78–1.40 eV Interestingly, various degrees of magnetism are observed with respect to the metal identity, atom/dimer utilization, and bonding interactions © 2014 Elsevier B.V All rights reserved Graphene has been a hot trend in material research for years [1,2] because of its excellent electronic properties and great thermostability as proven through a variety of experimental studies [3–5] Especially, the spintronic–electronic aspects with the aim to control the electronic properties tend to get more particular interests [6,7] During the past few years, studies of the electronic structures and magnetic properties of those two-dimensional structures have gained remarkable achievements Among them, the studies of metal atom adsorptions on graphene [8–16] using density functional theory (DFT) calculations [17,18] have shown different perspectives in the insights of electronic structures Besides, various aspects such as system configurations, binding energies (stability), atomic diffusion barriers, magnetization, and work function are also of concern Those studies can be regarded as a premise for developing applications in advanced semiconductors, nanomagnetic devices, as well as gas sensors The adsorptions of transition metal dimers on graphene were shown to have relatively low binding energies (from 0.16 to 0.27 eV), which might result in high mobility of the adatoms and dimers on the surface [12] Therefore, it is of importance to seal the adsorbed metal atoms with functional groups (ligands) By adopting first-principles calculations, Avdoshenko et al [19] explored the electronic properties of graphene–metal–benzene complex, and the ligand was capable of controlling the properties ∗ Corresponding author E-mail addresses: hung.m.le@hotmail.com, lmhung@hcmus.edu.vn (H.M Le) http://dx.doi.org/10.1016/j.cplett.2014.09.047 0009-2614/© 2014 Elsevier B.V All rights reserved of absorbed metal atoms, thereby imposed bosonic/fermionic characters The theoretical interactions between two graphene layers and Cr were also investigated in the graphene–Cr–graphene intercalating nanostructures [20] From the analysis of binding energy and electronic structures, it was shown that high Cr-occupation rate gave an unstable structure, while the loosened occupations (lower concentration) of Cr resulted in more stable structures In another study reported by Le et al [21], C60 was utilized as a ligand to decorate the graphene surface via coordination bonds with Cr (G–Cr–C60 ); meanwhile, interesting magnetic properties exhibited by G–Cr–C60 itself and its combination with a metal cluster (Ni4 /Pd4 /Pt4 ) were found Acknowledging the significance of graphene–metal–ligand structures in spintronic and electronic applications, in the present study, we demonstrate a theoretical investigation of threecomponent graphene-based nanostructures, in which naphthalene (C10 H8 , abbreviated as Np) is attached to graphene via coordination bonds with a Cr/Fe atom or Cr2 /Fe2 dimer (for convenience, the structures are denoted as G–M–Np or G–M2 –Np) Structural stability and magnetic moments of those structures will be deliberately discussed in the light of electronic structure data given by DFT calculations First-principles calculations based on DFT are performed within local spin density approximation in order to evaluate the degree of spin polarization in the bonding schemes between Cr/Fe and graphene/naphthalene All DFT calculations are executed using Quantum Espresso, an open-source computational package [22] The Perdew–Burke–Ernzerhof (PBE) exchange-correlation V.Q Bui, H.M Le / Chemical Physics Letters 614 (2014) 238–242 functional [23,24] is employed with the ultrasoft pseudopotentials for all atoms involved [25,26] The k-points are generated using the Monkhorst–Pack method with a chosen mesh of (6 × × 1), which is sufficient to ensure the reliability of total energy calculations The kinetic energy cut-off is selected as 45 Rydberg (612 eV) for planewave expansions The two-dimensional lattices (atomic positions and unit cell parameters) are simultaneously optimized using the Broyden–Fletcher–Goldfarb–Shanno algorithm [27] with an energy convergence criterion of 10−5 eV/cell and a gradient convergence ˚ criterion of 10−3 eV/A/atom The two-dimensional unit cells are constructed by employing large c lattice parameters (30 Bohr or ˚ The optimized structures and electronic structure data 15.88 A) herein are reported without considering the empirical dispersion correction terms It was shown in an earlier study that dispersion interactions contributed a significant role in the binding between graphene and aromatic molecules [28] Therefore, we also perform additional optimizations with the D2 empirical dispersions corrections [29,30] and update the binding energies A (4 × 4) supercell of graphene containing 32 carbon atoms is employed to host the M–Np and M2 –Np complexes The chosen graphene cell is wide enough to avoid possible interactions between the attached complexes due to periodic boundary conditions In the first part, the periodic graphene sheet is decorated with the C10 H8 ligand using only one transition metal atom (either Cr or Fe) as the bridging atom In the G–Cr–Np structure (Figure 1a), the Cr atom clings to the hollow site of two honeycomb units in graphene and C10 H8 This binding behavior is similar to that observed in two previous studies [19,21] In the case of G–Fe–Np, two different configurations are found As shown in Figure 1b and c, the Fe atom can settle either on the hollow site or on top of a C atom in graphene, while it only binds to the hollow site of Np In the second part, we attempt to use a transition-metal dimer to fix both aromatic rings of naphthalene instead of only one like in the previous cases In the case of Cr dimer, only one stable configuration can be found, in which two Cr atoms occupy the hollow sites in two adjacent hexagonal honeycomb units in both graphene and naphthalene, as shown in Figure 1d In the G–Fe2 –Np case, there are two stable configurations given by structural optimizations; one structure (Figure 1f) is similar to the Cr dimer case, while the other is much distinctive because one Fe atom accommodates at the hollow site and another locates on top of a carbon atom (Figure 1e) Interestingly enough, the different arrangements of Fe atoms in G–Fe–Np and G–Fe2 –Np result in different magnetic behaviors as will be discussed in a later part of this Letter In total, we report six configurations in this study: G–Cr–Np, G–Fe–Np(1) , G–Fe–Np(2) , G–Cr2 –Np, G–Fe2 –Np(1) , and G–Fe2 –Np(2) (see Figure 1) In the first structure (G–Cr–Np), it can be seen clearly that Cr connects to six C atoms in both naphthalene and graphene; however, the plane containing naphthalene is not parallel to graphene Indeed, the ligand is highly distorted as we observe vari˚ while the Cr–C(graphene) ous Cr–C(Np) bond lengths (2.14–2.23 A), ˚ In terms of bonding, the interbonds are almost identical (2.20 A) acting behavior of naphthalene as a ligand is distinguished from that of benzene as seen in a previous study, where benzene was symmetrically aligned [19] As a ligand, C60 even behaves more differently because the geometry of C60 allows itself to rotate and achieve most stable states by assembling high geometry distortions [31] The structural stability of a structure can be evaluated by calculating two different binding energies: (a) (1) (b) (2) Ebinding = Egraphene + EM –Np − Estructure Ebinding = Egraphene–M + ENp − Estructure 239 Table Binding energies and magnetizations of the investigated nanostructures (the values given by PBE calculations with D2 dispersion corrections are shown in parentheses) Binding energy (eV) (a) G–Cr–Np G–Fe–Np(1) G–Fe–Np(2) G–Cr2 –Np G–Fe2 –Np(1) G–Fe2 –Np(2) MT ( B /cell) MA ( B /cell) 0.11 (0.11) 1.89 (1.06) 2.00 (2.00) 2.47 (2.44) 1.03 (0.94) 1.34 (1.32) 0.20 (0.19) 2.39 (1.52) 3.27 (3.23) 3.36 (3.24) 2.36 (2.01) 1.88 (1.77) (b) Ebinding Ebinding 1.78 (2.90) 1.27 (2.24) 1.62 (2.08) 2.71 (4.04) 1.62 (2.99) 1.42 (2.82) 1.80 (2.82) 1.85 (2.69) 2.25 (3.03) 3.34 (4.48) 2.10 (3.42) 2.43 (3.51) where Egraphene , Egraphene–M , ENp , and ENp–M represent the total energies of pure graphene, graphene decorated with metal atom/dimer, C10 H8 , and metal–C10 H8 , respectively, while Estructure denotes the total energy of the optimized complex The positive binding energy value is indicative of a stable structure The two binding energy quantities above critically demonstrate the relative stability with respect to experimental synthesizing methods In experiments, the synthesis of G–M–Np/G–M2 –Np can be achieved by either attaching the C10 H8 –metal complex on a pure graphene surface (a) (expressed by Ebinding ) or attaching C10 H8 on a metal–graphene (b) surface (expressed by Ebinding ) The favorability of a synthesizing method in experimental reality can be evaluated by making direct comparisons of the two binding energies (a) By applying Eqs (1) and (2) for the G–Cr–Np case, Ebinding (b) and Ebinding are 1.78 and 1.80 eV, respectively The binding energy results indicate that the resulted structure is highly stable; also, it is demonstrated that the direct adsorption of C10 H8 on metal–graphene surface is somewhat more favorable In other words, the bond between Cr adatom and C10 H8 seems to be quite stronger than the bond between Cr and the graphene For con(a) (b) venience, we summarize Ebinding and Ebinding of the investigated nanostructures in Table The updated binding energies with empirical dispersion corrections are raised by 0.78–1.40 eV, as listed in Table (shown in parentheses) It can be seen in Table (a) that solely for G–Cr–Np, Ebinding (2.90 eV) is quite higher than the (b) corresponding Ebinding (2.82 eV) when van der Waals corrections are introduced Besides studying structural stability, we also evaluate spin polarization, which have much influence on the magnetic property By interpreting density of states (DOS) from electronic structure data, the total (MT ) and absolute (MA ) magnetizations (also listed in Table 1) are derived and reported for each structure The estimation of total magnetization in G–Cr–Np shows that the structure exhibits a weak magnetic moment of 0.11 B /cell, while the absolute magnetic moment is 0.20 B /cell In fact, the absolute magnetic moment indicates a significant anti-ferromagnetic amount within the structure For the electronic structure analysis, we only examine data from the PBE calculations without dispersion corrections By examining the total DOS of G–Cr–Np (Figure 2a), we observe that the weak spin polarization mainly occurs prior to the Fermi level (highest-occupied bands) While Cr appears to contribute a positive magnetic moment (0.15 B ), C atoms (from both graphene and naphthalene) tend to contribute a resisting amount, which causes anti-ferromagnetic effects within the structure Interestingly, the spin polarization of Cr 3d orbitals contributes the ferromagnetism 240 V.Q Bui, H.M Le / Chemical Physics Letters 614 (2014) 238–242 Figure Side and top views of six optimized nanostructures using PBE calculations without dispersion corrections: (a) G–Cr–Np, (b) G–Fe–Np(1) , (c) G–Fe–Np(2) , (d) G–Cr2 –Np, (e) G–Fe2 –Np(1) , and (f) G–Fe2 –Np(2) Teal is used for graphene Figure Total DOS of (a) G–Cr–Np, (c) G–Fe–Np(1) , (e) G–Fe–Np(2) and PDOS of 3d orbitals of (b) G–Cr–Np, (d) G–Fe–Np(1) , (f) G–Fe–Np(2) The Fermi level is positioned at (as much as 98%) Subsequently, we calculate the degree of spin polarization in each 3d subshell by analyzing the partial DOS (PDOS) of 3d orbitals Compared to the other 3d orbitals, the 3dz2 subshell is most polarized as shown in Figure 2b The spin polarization terms of metal 3d orbitals in all investigated structures are summarized in Table The most stable form of G–Fe–Np(1) (shown in Figure 1b) is 0.11 eV lower in energy compared to G–Fe–Np(2) (Figure 1c) The Table Spin polarization terms ( B) of G, Np, and 3d orbitals of each metal atom G Np 3dz2 3dzx 3dzy 3dx2 −y2 3dxy −0.03 0.17 0.05 −0.04 −0.01 0.02 −0.20 −0.19 G–Fe2 –Np(1) 0.01 −0.14 G–Fe2 –Np(2) 0.07 −0.01 0.12 0.05 0.28 0.42 0.42 −0.01 0.09 0.02 0.02 0.00 0.64 0.57 0.10 0.10 −0.10 0.43 0.33 0.34 0.00 0.55 0.56 0.18 0.18 −0.11 0.60 0.18 0.18 0.01 0.20 0.33 0.42 0.42 −0.03 0.16 0.05 0.05 0.01 0.25 0.31 0.23 0.23 −0.05 0.13 0.07 0.07 G–Cr–Np G–Fe–Np(1) G–Fe–Np(2) G–Cr2 –Np geometric configuration of G–Fe–Np(1) is somewhat similar to that of G–Cr–Np, i.e Fe is bound to two honeycomb units in naphthalene and graphene Binding energy calculations (Table 1) suggest that the Fe–naphthalene interaction is stronger than the interaction between Fe and graphene in both G–Fe–Np(1) and G–Fe–Np(2) The metastable configuration, G–Fe–Np(2) , has an odd bonding behavior, in which Fe only establishes a bond to one C atom from graphene, while it interacts with six C atoms from naphthalene with different bond distances Lowdin charge analysis [32] shows that the charge of Fe (+0.26) in G–Fe–Np(1) is actually less positive than that (+0.31) in the metastable state, G–Fe–Np(2) Indeed, this is explainable because the Fe atom in G–Fe–Np(2) locates on top of C in graphene, which allows the 2pz orbital of that C atom to receive more partial charge from the metal In terms of magnetic alignments, G–Fe–Np(1) and G–Fe–Np(2) exhibit total magnetizations of 1.89 B /cell and 2.00 B /cell, respectively Interestingly, the absolute magnetization of G–Fe–Np(2) (3.27 B /cell) is 37% higher than the absolute magnetization of G–Fe–Np(1) (2.39 B /cell), which indicates higher anti-ferromagnetic effect in G–Fe–Np(2) (see Figure 2b and c) An interesting phenomenon can be observed as we look at the binding energies of G–Fe–Np(1) and G–Fe–Np(2) Even though (a) (b) G–Fe–Np(1) is the most stable configuration, its Ebinding and Ebinding are lower than those of G–Fe–Np(2) Therefore, the adsorption of naphthalene on graphene–Fe (with Fe sitting on top of a C atom) would stabilize the valence electrons of Fe better; as a result, the binding energy for this process becomes higher When graphene is directly decorated with the Fe–naphthalene complex, the binding energy results suggest that Fe favor to bind on top of C Overall, the binding energy results suggest the favorability of C10 H8 adsorption on graphene–Fe in actual experimental synthesis In general, the use of metal dimer enhances the bonding strength between C10 H8 and graphene When a Cr dimer is utilized to bridge C10 H8 and graphene, only one stable configuration can be observed Two Cr atoms are likely to establish bonds with the honeycomb units in both naphthalene and graphene Moreover, there is an ˚ interaction between the two Cr atoms with a distance of 2.65 A In this case, two aromatic rings in C10 H8 are held tightly by the two metal atoms Thus, the resulted binding energies are larger than the previously reported cases Again, we conceive that covering graphene–Cr2 with C10 H8 might be more favorable, because (b) (a) Ebinding is larger (3.34 eV) than Ebinding While the use of a single bridging Cr atom results in a small magnetic moment, we observe that the use of Cr dimer raises the total magnetic moment to V.Q Bui, H.M Le / Chemical Physics Letters 614 (2014) 238–242 Figure Total DOS of (a) G–Cr2 –Np, (c) G–Fe2 –Np(1) , (e) G–Fe2 –Np(2) and PDOS of 3d orbitals of (b) G–Cr2 –Np, (d) G–Fe2 –Np(1) , (f) G–Fe2 –Np(2) The Fermi level is positioned at The PDOS of the second metal atom are given in dashed lines 2.47 B /cell (MA = 3.36 B /cell) From Lowdin charge analysis, it is found that the spin of one Cr atom not oppose the other; in fact, both contribute similar positive amounts of magnetization (1.34 B ), while graphene and naphthalene give negative (antiferromagnetic) moments (Figure 3a and Table 2) Among the 3d orbitals, 3dz2 and 3dx2 −y2 are the most polarized subshells with spin polarization terms of 0.42 B as shown in Figure 3b and Table The other 3d subshells give less significant magnetic contributions ranging from 0.10 B to 0.23 B In the last case, we investigate the possibility of attaching C10 H8 on graphene using two Fe atoms We observe two distinctive equilibrium structures (shown in Figure 1e and f) as introduced previously The former structure, which has one Fe atom locating on top of C in graphene, is more stable by 0.20 eV in total energy This result is somewhat contradicting to the previous observations in the G–Fe–Np cases Recall that when bridging of C10 H8 and graphene using one Fe atom, the structure with Fe located on the hollow sites of two honeycomb units is more energetically stable (a) (b) The binding energies (Ebinding and Ebinding ) of G–Fe2 –Np(1) are 1.62 eV and 2.10 eV, respectively, and those of G–Fe2 –Np(2) are 1.42 and 2.43 eV, respectively At this point, the calculated binding energies allow us to arrive at the first conclusion regarding experimental synthesis, i.e attaching the C10 H8 group on a metal–graphene sur(b) face should be more favorable due to the fact that Ebinding is higher (a) than the corresponding Ebinding in most cases Both structures exhibit smaller magnetic moments (1.03 B /cell from G–Fe2 –Np(1) and 1.34 B /cell from G–Fe2 –Np(2) ) compared to the single Fe case (G–Fe–Np) Especially, G–Fe2 –Np(1) is the sole case in which we observe a small anti-ferromagnetism from one Fe atom (see Table 2) For G–Fe2 –Np(2) , in terms of bonding and magnetic alignments, the roles of two Fe atoms are almost similar as illustrated by the PDOS distributions (Figure 3f) It is of particular interest to examine how G–Fe–Np(1) could be transformed to G–Fe–Np(2) and vice versa This can be achieved by performing a nudged-elastic-band (NEB) scan [33] By optimizing 10 intermediate structures, we have found activation energies of 0.84 and 0.73 eV for the forward and backward directions, respectively At the transition state, the structure is found to exhibit a total magnetic moment of 2.11 B , slightly higher than those found in the two equilibrium structures Moreover, a local minimum is also found near the transition state as shown in Figure 4a The 241 Figure Relative energy profiles of (a) G–Fe–Np(1) ↔ G–Fe–Np(2) and G–Fe2 –Np(1) ↔ G–Fe2 –Np(2) transformations given by NEB scans using PBE calculations without D2 corrections The reaction barriers of both transformations are found to be above 0.8 eV same procedure is executed to find 10 intermediate structures of the G–Fe2 –Np(1) ↔ G–Fe2 –Np(2) transformation; however, due to energy convergence difficulty, a rough result is reported herein, which indicates a barrier height of 0.88 and 0.67 eV for the forward and backward directions, respectively (Figure 4b) The magnetic moment found at the transition state is 0.71 B , which is lower than those of the two equilibrium structures From the NEB results, we can make a second conclusion, i.e the mobility of Fe atoms on the graphene surface is more prohibited due to strong interactions between Fe and naphthalene In summary, we have shown in this study that naphthalene can be attached to the graphene surface via coordination bonds with Cr/Fe atom or dimer with great stability In a previous study, the binding of transition metal dimers on graphene was found to be weak and have high surface mobility [12] With the use of naphthalene as a binding ligand on metal, the metal–graphene binding is shown to be more stabilized As a reference for experimental synthesis, we adopt different binding energy evaluation schemes, and the adsorption of C10 H8 molecule on a metal-attached graphene surface is more thermodynamically favorable in most cases; in other words, the metal–naphthalene bond is stronger than the interaction between metal and graphene The inclusion of dispersion corrections for long-range interactions shows that the attachment of Np on graphene–metal is further enhanced by 0.78–1.40 eV When Fe is utilized as bridging atoms, we observe different configurations of G–Fe–Np/G–Fe2 –Np, which possess different structure stability and magnetic moments Overall, good structural stability and interesting magnetism of the investigated nanostructures may elicit potential spintronic and electronic applications Supplementary material ˚ of the optiThe unit cell parameters and atomic positions (in A) mized structures (with and without D2 dispersion corrections) are given in one supplementary file Acknowledgement We are grateful to the computing support from the University of Science, Vietnam National University This work is funded by Vietnam National University under grant B2014-18-03 242 V.Q Bui, H.M Le / Chemical Physics Letters 614 (2014) 238–242 Appendix A Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.cplett.2014.09.047 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] K.S Novoselov, et al., Science 306 (2004) 666 K.S Novoselov, et al., Nature 438 (2005) 197 R Prasher, Science 328 (2010) 185 C Berger, et al., J Phys Chem B 108 (2004) 19912 H.C Schniepp, et al., J Phys Chem B 110 (2006) 8535 E Rudberg, P Salek, Y Luo, Nano Lett (2007) 2211 Y.W Son, M.L Cohen, S.G Louie, Nature 444 (2006) 347 M Wu, E.-Z Liu, M.Y Ge, J.Z Jiang, Appl Phys Lett 94 (2009) 102505 C Cao, M Wu, J Jiang, H.-P Cheng, Phys Rev B 81 (2010) 205424 V Zólyomi, A Rusznyák, J Kürti, C.J Lambert, J Phys Chem C 114 (2010) 18548 S Malola, H Häkkinen, P Koskinen, Appl Phys Lett 94 (2009) 043106 H Johll, H.C Kang, E.S Tok, Phys Rev B 79 (2009) 245416 X Liu, C.Z Wang, Y.X Yao, W.C Lu, M Hupalo, M.C Tringides, K.M Ho, Phys Rev B 83 (2011) 235411 [14] H Sevinc¸li, M Topsakal, E Durgun, S Ciraci, Phys Rev B 77 (2008) 195434 [15] G Cantele, Y.S Lee, D Ninno, N Marzari, Nano Lett (2009) 3425 [16] A Krasheninnikov, P Lehtinen, A Foster, P Pyykkö, R Nieminen, Phys Rev Lett 102 (2009) 126807 [17] P Hohenberg, Phys Rev 136 (1964) B864 [18] W Kohn, L.J Sham, Phys Rev 140 (1965) A1133 [19] S.M Avdoshenko, I.N Ioffe, G Cuniberti, L Dunsch, A.A Popov, ACS Nano (2011) 9939 [20] V.Q Bui, H.M Le, Y Kawazoe, D Nguyen-Manh, J Phys Chem C 117 (2013) 3605 [21] H.M Le, H Hirao, Y Kawazoe, D Nguyen-Manh, Phys Chem Chem Phys 15 (2013) 19395 [22] P Giannozzi, et al., J Phys Condens Matter 21 (2009) 395502 [23] J.P Perdew, K Burke, M Ernzerhof, Phys Rev Lett 77 (1996) 3865 [24] J.P Perdew, K Burke, M Ernzerhof, Phys Rev Lett 78 (1997) 1396 [25] D Vanderbilt, Phys Rev B 41 (1990) 7892 [26] A Dal Corso, Phys Rev B 64 (2001) 235118 [27] D.F Shanno, J Optim Theory Appl 46 (1985) 87 [28] E.G Gordeev, M.V Polynski, V.P Ananikov, Phys Chem Chem Phys 15 (2013) 18815 [29] S Grimme, J Comput Chem 27 (2006) 1787 [30] V Barone, M Casarin, D Forrer, M Pavone, M Sambi, A Vittadini, J Comput Chem 30 (2009) 934 [31] H.M Le, H Hirao, Y Kawazoe, D Nguyen-Manh, J Phys Chem C 118 (2014) 21057 [32] D Sanchez-Portal, E Artacho, J.M Soler, Solid State Commun 95 (1995) 685 [33] D Sheppard, R Terrell, G Henkelman, J Chem Phys 128 (2008) 134106  ... calculations (Table 1) suggest that the Fe naphthalene interaction is stronger than the interaction between Fe and graphene in both G Fe Np(1) and G Fe Np(2) The metastable configuration, G Fe Np(2)... 0.07 0.07 G Cr Np G Fe Np(1) G Fe Np(2) G Cr2 –Np geometric configuration of G Fe Np(1) is somewhat similar to that of G Cr Np, i.e Fe is bound to two honeycomb units in naphthalene and graphene. .. result is somewhat contradicting to the previous observations in the G Fe Np cases Recall that when bridging of C10 H8 and graphene using one Fe atom, the structure with Fe located on the hollow