Electronic and Structural Properties of C60 and Sc3N@C80

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Article 60

Electronic and Structural Properties of C and ScN@C Supported on Graphene Nanoflakes 3

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Jose Ulises Reveles, Nakul N Karle, Tunna Baruah, and Rajendra R. Zope J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07405 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

Electronic and Structural Properties of C60 and Sc3N@C80 Supported on Graphene Nanoflakes Jose U. Reveles,a,b,* Nakul N. Karle,a Tunna Baruah,a and Rajendra R. Zopea,** a b

Department of Physics, University of Texas El Paso, El Paso, TX 79958, USA

Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA

Abstract A theoretical study on the geometric and electronic structure of C60 and Sc3N@C80 absorbed on pristine graphene nanoflakes (GNF) is presented. C60 is found to adsorb in two nearly degenerate configurations: with a pentagon facing the GNF which is the most stable one, and with a hexagon facing the GNF in a face-to-face perfect alignment, rarely common in π−π interactions, 0.06 eV higher in energy. The calculated binding energy of 0.76 eV, which includes dispersion effects, is in good agreement with previous theoretical and experimental reports. On the other hand, Sc3N@C80 adsorption on the GNF resulted in a higher binding energy of 1.00 eV for nearly degenerate isomers that have a pentagon and a hexagon facing the SLG. This larger binding energy is explained in terms of a higher dispersion interaction between the larger metallofullerene and the GNF and due to the fact that charge separation in Sc3N@C80, which results in a positively charged Sc3N inside a negatively charged C80, favors binding with the GNF. Further, the Sc3N moiety is found to rotate inside the supported C80 fullerene which in combination with the orientation of the fullerene on the SLG leads to a series of isomers with binding energies ranging from 0.76 to 1.00 eV. Our results show that it could be possible to adsorb metallofulleres on graphene nanoflakes with an energy large enough to prevent diffusion and therefore opening the possibility to potential applications. *Email: [email protected]. Tel. (804) 828-5443 **Email: [email protected], Tel. (915) 747-8742

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1. Introduction Isolation of graphene in 20041 and the subsequent experiments to further characterize it 2, 3

fueled a great area of research aimed to find applications for this novel material. Graphene, a

single sheet of crystalline carbon just one atom thick, being the thinnest, strongest, and stiffest material known has proved to have great potential in electronics and photonics,4 and as support for metal particles that exhibit catalytic properties,5-7 and high hydrogen storage capacity.8-10 On the other hand, fullerenes that contain metal particles in their interior, known as endohedral metallofullerenes (EMF) have also attracted increasing attention over the last few years due to their potential applications in materials science and medicine.11-15 Particular attention has been devoted to Sc3N@Ih-C80, being the third most abundant fullerene after C60 and C70 that can be prepared in an arc reactor, and the prototype of nitride containing EMFs.16 The electronic structure of nitride EMFs can be understood using an ionic model that considers a charge transfer of up to six electrons from the endohedral species to the carbon cage, leading to a (M3N)6+@(C2n)6- species. Notably, this charge transfer provides unique properties to EMFs that enhance their reactivity, i.e. addition reactions and adsorption energy on surfaces, as compared to their hollow counterparts. Importantly, practical applications of EMFs require them to either being functionalized to make them water soluble or to deposit them on adequate surfaces. For example, water-soluble derivatives of Gd3N@Ih-C80 have been shown to be effective relaxation agents for magnetic resonance imaging (MRI),17 and the first described functionalization of a nitride EMF was the Diels–Alder cycloaddition on the icosahedral (Ih) isomer of Sc3N@C80 reported by Iezzi et al. in 2002.18 Moreover, recent studies of exohedral derivatization of EMFs have also shown that partial saturation of the fullerene π-system substantially changes the way the endohedral cluster interacts with the carbon cage,19 and that redistribution of the frontier


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orbitals as well as a change of the spatial distribution of the metal-cage bonding sites can modify the internal dynamics of the endohedral cluster. In this paper we investigate the interaction of Sc3N@C80 with a GNF and compare it with the interaction of a hollow C60 with the GNF. We aim to understand the nature of the chemical interaction and the role the endohedral Sc3N moiety plays on the fullerene-nanoflake binding. The charge transfer from the endohedral Sc3N species to the C80 carbon cage in the supported Sc3N@C80 fullerene was calculated to be (Sc3N)+3.6@(C80)-3.6 leading to an electron-richer carbon cage, a factor that enhances the intermolecular dispersion interaction between the fullerene and the graphene nanoflake and thus increases the binding energy. Further, we found that the Sc3N moiety rotates inside the supported C80 fullerene, as it does in free C80, and this rotation, in combination with the fullerene orientation on the nanoflake, generates a series of isomers with binding energies ranging from 0.76 to 1.00 eV. Remarkably, the possibility of using nanoflakes as a support of fullerenes with a binding energy large enough to prevent diffusion opens up possibilities for potential applications. Our paper is organized as follows. Section 2 describes our computational and theoretical method, results and discussion are presented in section 3, and summary and conclusions are given in the last sections. 2. Computational Method All electron theoretical calculations were carried out within the density functional formalism (DFT)20,21 and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE96) to describe the exchange and correlation effects.22 The electronic orbitals and eigenstates were determined by using a linear combination of Gaussian type atomic orbital molecular orbital (LCGTO) approach within the auxiliary function DFT formalism (ADFT) in the deMon2k23-24 and with the formalism (DFT) in the NRLMOL software.25-27 Electron



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repulsion integrals were accurately and efficiently calculated using a variational mesh and the self-consistency cycle was carried out until the energies converged. All-electron DZVP28 and QZVP like quality29 basis sets for the C, H, N and Sc atoms were used in deMon2k and NRLMOL respectively. The geometry optimization employed a quasi-Newton method in internal delocalized coordinates30-31 to fully pre-optimize the structures in deMon2k without any symmetry constraints, followed by an optimization with the larger basis set in NRLMOL, in which symmetry unrestricted geometry optimizations employed a LBFGS method in Cartesian coordinates with a maximum force threshold of 0.001 a.u. All reported geometries and binding energies are based on NRLMOL calculations. The large basis sets of QZVP quality in NRLMOL have shown to have negligible basis set superposition error (BSSE) in the calculated binding energies, and therefore we have not included BSSE in our results. The dispersion interaction was included in deMon2k with the empirical dispersion energy term (DFT-D),32 and in NRLMOL using the DFT-D3 parameters.33 A natural bond orbital (NBO) analysis34 was performed to analyze the charge transfer between the Sc3N, fullerenes, and GNF. 3. Results and Discussion 3.1 Graphene nanoflakes, C60 and Sc3NC80 optimized geometries and HOMO-LUMO gaps. After testing various possible sizes the graphene nanoflake was simulated with a cluster model composed of 96 C and 24 H atoms (C96H24) forming a symmetric hexagonal nanoplatelet as shown in Figure 1. This cluster size was determined to be large enough to serve as a support for the fullerenes that were deposited at its center. In this model, hydrogen atoms were added to the dangling bonds of the carbon edge atoms to passivate them. The optimized GNF was found to preserve the symmetry of extended graphene with bonds in excellent agreement with the experimental C-C bond length of 1.42 Å.35 The optimized C-C bonds can be grouped into three


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sets. The carbon atoms at the center of the cluster forming six hexagons, highlighted in Figure 1a, presenting an average C-C bond length of 1.423 Å. Then, the first neighbor hexagons of this center cluster with carbon atoms having C-C bond lengths ranging from 1.421 to 1.428 Å, and finally, the second neighbor hexagons with C-C bonds of 1.44 Å and alternating bonds of 1.417 Å and 1.395 Å for the edge carbon atoms. The calculated gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap) was 1.35 eV, thus showing a semiconducting behavior in agreement with previous theoretical studies that report HOMOLUMO gaps ranging from 0.73-2.89 eV depending on the cluster’s size.36 The optimized geometry in a single closed shell configuration for C60 having Ih symmetry is shown in Figure 1b. It presented the two characteristic C-C bond lengths. The 6:6 ring bonds, between adjacent hexagons, of 1.399 Å, and the longer 6:5 bonds, between adjacent hexagons and pentagons of 1.453 Å, in excellent agreement with the experimental values of 1.391 Å and 1.455Å respectively.37 Further, our calculated HOMO–LUMO gap for C60 of 1.28 eV agreed well with the previous works.38-40 The optimized geometry for Sc3N@C80 is shown in Figure 1c. It presents C-C bond lengths ranging from 1.435 to 1.455 Å, and the endohedral moiety Sc-C and N-Sc bond lengths of 2.262 Å and 2.035 Å respectively. The calculated HOMO-LUMO gap for Sc3N@C80 was 1.42 eV thus exhibiting a noticeable chemical stability similar to C60. Note that species with a large HOMO-LUMO gap are known to be quite unreactive, having a low ionization energy and a high electron affinity, as they neither want to give nor to gain electrons. The special properties of these class of fullerenes is based on the transfer of up to six electrons from the endohedral Sc3N species to the cage,41 which leads to stabilization of large fullerenes which usually are highly reactive.42-44 According to experimental and theoretical studies, the endohedral Sc3N cluster is



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highly mobile. First evidence for its rotation was found in the experimental

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13

C NMR pattern

which reveals Ih symmetry, that is, the same point group as for the empty C80.45 Given that the D3h symmetry of the Sc3N cluster would lower the Ih symmetry of the Sc3N@C80 complex, the experimental results are only consistent with a nearly free rotation of Sc3N which averages out the lower symmetries. A similar rotation, and thus generation of a series of close-energy isomers is found for the supported Sc3N@C80 as discussed below. Cartesian coordinates of all optimized geometries are given in the supporting information. 3.2. C60 adsorption on the graphene nanoflake It was found that both C60 and Sc3N@C80 adsorbed and directly interacted with a fragment of the GNF composed of seven hexagonal carbon rings (highlighted in Figure 1a), and that the second and third carbon neighbors were less affected by the fullerenes. The optimized side and top view for the ground state and lowest energy isomer of C60/GNF clusters are presented in Figure 2. Also included in this figure is the calculated interaction potential for the lowest energy isomer between C60 and GNF as a function of the distance between these two species. The potential shows an attractive interaction which extends up to a distance of 5.5 Angstroms. Out of the two nearly energy degenerate structures found (within a 0.065 eV energy range), the most stable one, in which a pentagon of C60 faces the GNF presented a tilted configuration with C(fullerene)-C(GNF) bond lengths ranging from 3.21 to 3.62 Å. These distances are within the range of distances reported using the optB86B functional in the work of S. Laref et al. which range from 2.81 to 3.35 for different absorption geometries,46 and are similar to the experimental estimation of 2.9 Å.47 It should be noted that different cluster models and the C60 orientation have an important effect in the calculated geometries.


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We found that the GNF reacted to the C60 adsorption by distorting outward from planarity in the opposite side of the fullerene, presenting a maximum out-of-plane distance of 0.507 Å (Fig. 2), which resulted in an elongation of the GNF C-C bond lengths by less than one picometer (0.010 Å) attesting for the known structural robustness of nanoflakes. On the other hand, for the second energy isomer C60 adsorbed a hexagon symmetrically facing the GNF in a π-π stacking interaction at a distance of 3.561 Å, which is within the distance range for a faceto-face perfect alignment of 3.3 to 3.8 Å as suggested by Janiak48 based on the van der Waals radius (VdW) of the carbon atoms. Note that the reported face-to-face alignment of the rings here is extremely rare as the usual π interaction is an offset or slipped stacking, that is, the rings are parallel and displaced. The binding energy of C60 to the GNF was calculated according to equation 1: BE = E(M) + E(GNF) - E(M-GNF) with M= C60 and Sc3N@C80

(1)

Where E(M), E(GNF), E(C60-GNF) are the total energies, including dispersion effects, for the M, GNF, M-GNF species. Note that according to this equation a positive BE implies a favorable binding and that the larger the BE, the more stable is the complex formed. The calculated BE is 0.76 eV and 0.70 eV for the most stable and lowest energy isomers respectively. This BE is in good agreement with the theoretical binding energy (0.85 eV) reported by Berland et al.49 using the vDW-DF1 van der Waals density functional, and with the experimental binging energy (0.85 eV) by Ulbricht, et al.47 Note that dispersion effects need to be taken into account in the theoretical modeling of the fullerene-graphene systems given that the chemical interaction of these species, dipole induced – dipole induced, are the London dispersion intermolecular forces as discussed below.



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It should also be noted that non-polar aromatic systems, similar to the system studied here, have been identified as having pi stacking (also called π–π stacking) that are attractive, non-covalent interactions which for benzene range between 0.08 to 0.12 eV. Although this is a small energy for a pair of benzene rings, this interaction accounts for the binding of larger systems with delocalized pi electrons as shown in the experimental work of a buckycatcher by Sygula et al. in which a C60 binds to a convex fullerene molecule.50 3.3. Sc3N@C80 adsorption on GNF The optimized side and top view for the ground state and four lowest energy structures of the Sc3N@C80-GNF systems (isomers 1 to 5) are presented in Figure 3. Also shown in this figure is the calculated interaction potential between Sc3N@C80 and GNF for the lowest energy isomer as a function of the distance between them, which as in the case of the C60/GNF system shows and attractive interaction up to a distance of 5.5 Angstroms. The isomer reported in Figure 3 lie within a 0.234 eV energy range above the ground state structure, and can be grouped into two sets. The first set is composed of isomer 1 (ground state structure) and isomer 2, lie within a very close energy range of less than 0.06 eV and present a C80 encapsulated Sc3N moiety nearly perpendicular to the graphene nanoflake. The second set is composed of isomers 3 to 5 in which Sc3N presents a parallel or slightly tilted structure with respect to the nanoflake and are between 0.15 eV and 0.23 eV less stable than the ground states geometry. Note that both experimental and theoretical studies of gas phase Sc3N@C80 have shown that the endohedral Sc3N cluster is highly mobile, and we found that a similar behavior is presented in the GNF supported cluster, although having found the two sets of isomers above mentioned make us think that the support tend to favor an axis of rotation of Sc3N in a particular orientation, in this case, perpendicular to the nanoflake plane. Furthermore, our results agree with a recent report by Osuna et al.51 which


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study Diels-Alder additions to Sc3N@C80 fullerenes, and found that isomers in which two Sc atoms are pointing to the two sides of the functionalized bond (in our case towards the graphene nanoflake) are the most stable ones. All isomers in our investigation are found to present a binding of the Sc3N@C80 cage slightly tilted, with a 6-6 bond oriented towards the GNF and presenting similar fullerenenanoflake bond lengths ranging from 3.02 to 3.36 Å as shown in Figure 3. This distance is shorter than the one found for the C60-GNF system (ranging from 3.21 to 3.60 Å) and attest for the existence of a stronger interaction of Sc3N@C80 with the nanoflake as compared with C60. Further, an out the plane distortion of the nanoflake layer was found ranging from 0.48 to 0.72 Å in the direction opposite to the side at which Sc3N@C80 adsorbs, this distortion gave place to a very small stretching of the C-C bond lengths of the nanoflake layer of less than 1 pm. The binding energy of Sc3N@C80 to the GNF was calculated according to equation 1 and gives results from 1.0 to 0.76 eV for the five Sc3N@C80–GNF isomers reported, thus being consistently larger that the binding energy of C60-graphene of 0.76 – 0.70 eV, as summarized in Table 1. As in the case of the structural analysis, this larger binding energy of Sc3N@C80 confirms the existence of a stronger chemical London dispersion interaction with the nanoflake and we discuss in detail the binding mechanism in the following section. Cartesian coordinates of all optimized geometries are given in the supporting information. 3.4 Binding Mechanism and Chemical Stability To analyze the binding mechanism of C60 and Sc3N@C80 to GNF we plotted the oneelectron energy levels and molecular orbital isosurfaces for the frontier orbitals as shown in Figure 4. Bucky ball C60 is a closed-shell species with sp2 carbon atoms forming three strong σ bonds presenting delocalized electronic π−bands that surround the cage. It has a five-fold



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degenerated (HOMO) and a tri-fold degenerated (LUMO) 1.28 eV higher in energy. This large gap is a signal of chemical stability and functionalization of the C60 cage has been characterized by additional reactions that transfer charge to the LUMO in bonding interaction, i.e. OH addition that further enables the cage to be water soluble by inducing local polarization.52 On the other side, GNF’s frontier orbitals presented a HOMO-LUMO gap of 1.35 eV between the two-fold degenerated HOMO and LUMO. These orbitals can be visualized by its characteristic π orbitals as a pair of symmetric lobes oriented along the z-axis and centered on the nucleus. Each atom has one of these π-bonds, which are then hybridized together to form what are referred to as the π-bands and π∗-bands which are responsible for most of the peculiar electronic properties of graphene. Interaction between C60 and GNF’s π-bands leads to a London dispersion bonding interaction of the order of 0.76 eV as described above. An analysis of the one-electron energy levels of the C60-GNF species allowed us to identify the energy-levels with both C60 and GNF character. The two-fold degenerated HOMO of the complex is given by GNF, and the three-fold degenerated LUMO by C60. Interestingly, the LUMO of C60 is destabilized by 0.3 eV, i.e. moves to a higher energy as compared to the free C60, thus enhancing its reactivity and making it a better electron-acceptor. The HOMO-LUMO gap of C60-GNF is reduced to 0.69 eV because of the effect of the C60 adsorption. In a similar way to C60, C80 presents sp2 carbon atoms and delocalized π-bands. However, in contrast to C60, C80 is not stable by itself, and it is only after the charge transfer from the endohedral Sc3N species to the carbon cage that it becomes a stable Sc3N@C80 in a closed-shell with a significant HOMO-LUMO gap of 1.42 eV. As in the case of C60-GNF, interaction between Sc3N@C80 and GNF’s π-bands leads to a London dispersion bonding interaction ranging from 1.00 to 0.74 eV for the studied isomers


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(Table 1). An analysis of the one-electron energy levels shows a HOMO and LUMO with GNF and Sc3N@C80 character respectively. The LUMO of Sc3N@C80 is destabilized by 0.28 eV, after its absorption on to the nanoflake, thus enhancing its reactivity and making it a better electronacceptor. The HOMO-LUMO gap of Sc3N@C80-GNF is reduced from the free Sc3N@C80 (1.34 eV) and ranges from 1.01 to 1.07 eV for the different isomers. It is worth to note that this gap is larger than the one found for C60-GNF of 0.66 eV. The formation of a chemically more stable Sc3N@C80 species is attributed to a stronger London dispersion intermolecular interaction between the large fullerene and graphene, as proved by the large binding energy. In order to investigate the effect of the fullerenes’ orientation on the graphene and that of the internal degree of freedom of Sc3N we plot the density of states (DOS) for all the calculated isomer in Figures 5 and 6. It can be noted that DOS present for the two isomers of C60-GNF (Figure 5) are very similar and that the found out-of-plane distortion of graphene in the ground state (0.5 Å) does not have a noticeable effect in the electronic structure, thus confirming the robustness of graphene. Similarly, the five investigated isomers for Sc3N@C80-GNF presented similar DOS, independently of the fullerene-nanoflake distance and position of the internal Sc3N moiety. 3.5 Charge Distribution and Induced Dipole Moment Charge separation within the Sc3N@C80 was another factor to be taken into account to investigate the fullerenes-GNF chemical interaction. As mentioned above charge transfer between Sc3N to C80 stabilizes the species producing a positive core within a negative carbon cage. This electron-richer cage can then induce a large dispersion force with the nanoflake support. An analysis of the electronic charges show that Sc3N acquires a positive charge of 3.59 electrons, C80 a negative charge of -3.63 electrons, and the GNF becomes slightly positive. This



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charge distribution generates a dipole moment between 1.15 and 1.65 Debye for the studied isomers as shown in Figure 7. This induced electrostatic moments may contribute to dipole – dipole induced electrostatic interactions between graphene and Sc3N@C80 and thus being an factor that enhances the binding energy to around 1.0 eV, compared with 0.76 eV for the C60/GNF species in which only dispersion forces are present, as this system has a very small charge transfer and therefore a smaller dipole moment between 1.15 and 1.43 Debye. (See Table 2). Conclusions An electronic and structural investigation of the interaction of a bare fullerene (C60) and a larger metallofullerene (Sc3N@C80) with a pristine graphene nanoflake is presented. Our results indicate a bonding interaction resulting from London dispersion forces. C60 attaches with an energy of 0.76 eV and Sc3N@C80 with a larger energy of the order of 1.0 eV to GNF. We explore different isomers and investigate how orientation of the cage and internal rotation of Sc3N affects the binding and determine that GNF are robust enough to tolerate out-of-plane distortions from 0.30 to 0.76 Å without losing its characteristic structural and electronic properties. Two interesting facts for the C60-GNF system were found. First, a low energy isomer presented a hexagonal carbon ring facing graphene in a face-to-face perfect alignment rarely in π−π interactions, and thus opening the possibility to use this system to investigate π−π interactions. Second, adsorption of C60 destabilizes its LUMO and reduces the HOMO-LUMO gap from 1.28 eV in the free C60 to only 0.69 eV on the supported cluster, thus enhancing its reactivity by making it a better electron-acceptor. Finally, the unveiled binding energies shows it could be possible to adsorb metallofulleres on graphene nanoflakes without diffusion, therefore


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opening the possibility to potential applications. We hope that our results motivate future experimental studies that confirm predictions. Associated Content Supporting Information. Optimized Cartesian geometries for all the studied systems are given in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Authors *Email: [email protected]. Tel. (804) 828-5443 **Email: [email protected], Tel. (915) 747-8742

Notes The authors declare no competing ﬁnancial interest. Acknowledgements Authors thank the Texas Advanced Computing Center (TACC) from the National Science Foundation (NSF; Grant No. TG-DMR090071). This research was supported in part by the DOE Basic Energy Science under Award DE-SC0006818 (R.R.Z.) and by Virginia Commonwealth University (J.U.R.). TOC



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19. Elliott, B.; Pykhova, A. D.; Rivera, J.; Cardona, C. M.; Dunsch, L.; Popov, A. A.; Echegoyen, L.“Spin density and cluster dynamics in Sc3N@C80 upon [5,6] exohedral functionalization: An ESR and DFT study. J. Phys. Chem. C 2013, 117, 2344−2348 20. Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. B 1964, 136, B864. 21. Kohn, W.; Sham, L.J. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140, 1133-&. 22. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple Phys. Rev. Lett. 1996, 77, 3865-3868. 23. Koster, A. M.; Calaminici, P.; Casida, M. E.; Flores-Moreno, R.; Geudtner, G.; Goursot, A.; Heine, T.; Ipatov, A.; Janetzko, F.; del Campo, J. M.; et al. deMon2k, version 2.3.6; The deMon Developers: Cinvestav, Mexico, 2010. http://www.deMon-software.com. 24. Geudtner, G.; Calaminici, P.; Carmona-Espindola, J.; M. del Campo, J.; DominguezSoria, V. D.; Flores Moreno, R.; Gamboa, G.U.; Goursot, A.; Koster, A. M.; Reveles, J. U.; et al. deMon2k. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2011, 2, 548−555. 25. Pederson, M.R.; Jackson, K.A. Variational mesh for quantum-mechanical simulations. Phys. Rev. B 1990, 41, 7453-7461. 26. Pederson, M.R.; Jackson, K.A. Pseudoenergies for simulations of metallic systems. Phys. Rev. B 1991, 43, 7312-7315. 27. Jackson, K. A.; Pederson, M. R. Accurate forces in a local-orbital approach to the localdensity approximation. Phys. Rev. B 1990, 42, 3276-3281. 28. Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Optimization of Gaussian-type basis-sets for local spin-density calculations. 1. Boron through Neon, optimization technique and validation. Can. J. Chem. 1992, 70, 560−571. 29. Porezag, D.; Pederson, M. R. Optimization of Gaussian basis sets for density-functional calculations. Phys. Rev. A 1999, 60, 2840-2847. 30. Reveles, J. U.; Koster, A.M. Geometry optimization in density functional methods. J. Comput. Chem. 2004, 25, 1109−1116. 31. Reveles, J. U.; Khanna, S. N.; Köster, A. M. Equivalent delocalized internal coordinates. J. Mol. Struct.: THEOCHEM 2006, 762, 171−178.


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32. Goursot, A.; Mineva, T.; Kevorkyants, R.; Talbi, D. Interaction between n-alkane chains: Applicability of the empirically corrected density functional theory for van der Waals complexes. J. Chem. Theor. Comput. 2007, 3, 755-763 33. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. 34. Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.G, Theoretical Chemistry Institute; University of Wisconsin: Madison, WI, 2004. http://www.chem.wisc.edu/~nbo5 (accessed October 10, 2016) 35. Cooper, D. R.; D’Anjou, B.; Ghattamaneni, N.; Harack, B.; Hilke, M.; Horth, A.; Majlis, N.; Massicotte, M.; V. L.; Whiteway, E.; Yu, V. Experimental review of graphene. Cond. Matt. Phys. 2011, 2012, 1–56. 36. Silva, A. M.; Pires, M. S.; Freire, V. N. Graphene nanoflakes: Thermal stability, infrared signatures, and potential applications in the field of spintronics and optical nanodevices. J. Phys. Chem. C 2010, 114, 17472–17485. 37. David, W. I. F.; Ibberson, R. M.; Matthewman, J. C.; Prassides, K.; Dennis, T. J. S.; Hare, P. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Crystal-structure and bonding of ordered C60. Nature 1991, 353, 147-149. 38. Green Jr., W. H.; Gorun, S. M.; Fitzgerald, G.; Fowler, P. W.; Ceulemans, A.; Titeca, B. C. Electronic structures and geometries of C60 anions via density functional calculations. J. Phys. Chem. 1996, 100, 14892-14898. 39. Diener, M. D.; Alford, J. M. Isolation and properties of small-bandgap fullerenes. Nature 1998, 393, 668-671. 40. Dunlap, B. I.; Brenner, D. W.; Mintmire, J. W.; Mowrey, R. C.; White, C. T. Local density functional electronic-structures of 3 stable isosahedral fullerenes. J. Phys. Chem. 1991, 95, 8737-8741. 41. Kobayashi, K.; Sano, Y.; Nagase, S. Theoretical study of endohedral metallofullerenes: Sc3-nLanN@C80 (n=0-3). J. Comput. Chem. 2001, 22, 1353-1358. 42. Aihara, J. Many reactive fullerenes tend to form stable metallofullerenes. J. Phys. Chem. A 2002, 106, 11371-11374.



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Table 1. Relative energy for the different isomers, dipole moment, HOMO-LUMO gap and binding energy for the GNF, C60, Sc3N@C80, C60/GNF and Sc3N@C80/GNF clusters.

Cluster

Isomers

Relative

Dipole Moment

HOMO-LUMO

Binding

Energy (eV)

(Debye)

gap (eV)

Energy (eV)

GNF

0.92

1.35

C60

0.00

1.28

Sc3N@C80

0.59

1.42

C60/GNF

1

0.00

1.15

0.69

0.76

2

0.07

1.43

0.66

0.70 0.85a) 0.85b)

Sc3N@C80/GNF

1

0.00

1.65

1.04

1.00

2

0.056

1.58

1.07

0.94

3

0.151

1.17

1.01

0.85

4

0.218

1.15

1.02

0.78

5

0.234

1.36

1.06

0.76

a)

Theoretical binding energy of C60 on graphene using the vdW-DF1 van der Waals density functional.49 b)

Experimental binding energy of C60 on graphene.47



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Table 2. Natural Bonding Orbital (NBO) atomic charges for the different isomers of C60/GNF and Sc3N@C80/GNF clusters. Atomic Charges Cluster C60/GNF

Sc3N@C80/GNF

Isomer

GNF

C60

Sc3N

C80

Sc3N@C80

1

0.029

-0.029

2

0.018

-0.018

1

0.029

3.591

-3.620

-0.029

2

0.025

3.584

-3.610

-0.025

3

0.032

3.598

-3.630

-0.032

4

0.035

3.598

-3.634

-0.035

5

0.040

3.582

-3.690

-0.040


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Figure Captions Figure 1. Optimized ground state geometries for a) graphene nanoflake (GNF), b) C60 and c) Sc3N@C80. Carbon, hydrogen, scandium, and nitrogen atoms are represented by dark grey, white, blue, and light gray circles respectively. Bond lengths are given in Angstroms. The center atoms in the GNF are highlighted in purple color. Figure 2. Side and top views for the optimized ground state and lowest energy isomer C60-GNF clusters. Bond lengths are given in Angstroms and the relative energies (Erel) in units of eV. The interacting potential profile for the lowest energy isomer (Isomer 1) is also given. Figure 3. Side and top views for the optimized ground state and four lowest energy Sc3N@C80GNF isomers. Carbon, hydrogen, scandium, and nitrogen atoms are represented by dark grey, white, green, and blue circles respectively. Bond lengths are given in Angstroms and the relative energies (Erel) in units of eV. The interacting potential profile for the lowest energy isomer (Isomer 1) is also given. Figure 4. a) One electron energy levels for C60, GNF and C60-GNF clusters, and b) Sc3N@C80, GNF, and Sc3N@C80-GNF clusters. Degeneracy and isosurfaces (isoval = 0.01 au) is given for selected frontier orbitals. Solid black lines and dotted red lines represent, respectively, occupied and unoccupied states. Total electronic density isosurfaces (isoval = 0.05 au) are also given. Figure 5. Density of states for C60, GNF, and C60-GNF isomers. Figure 6. Density of states for Sc3N@C80, GNF, and Sc3N@C80-GNF isomers. Figure 7. Electric dipole moment vector marked with a red arrow for the a) C60-GNF, and b) Sc3N@C80-GNF isomers. Natural population charges are marked next to the structures.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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Figure 6.



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Figure 7.


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Table of Contents Graphics 95x66mm (300 x 300 DPI)



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Figure 1. Optimized ground state geometries for a) graphene nanoflake (GNF), b) C60 and c) Sc3N@C80. Carbon, hydrogen, scandium, and nitrogen atoms are represented by dark grey, white, blue, and light gray circles respectively. Bond lengths are given in Angstroms. The center atoms in the GNF are highlighted in purple color. 224x95mm (300 x 300 DPI)


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Figure 2. Side and top views for the optimized ground state and lowest energy isomer C60-GNF clusters. Bond lengths are given in Angstroms and the relative energies (Erel) in units of eV. The interacting potential profile for the lowest energy isomer (Isomer 1) is also given. 223x109mm (300 x 300 DPI)



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Figure 3. Side and top views for the optimized ground state and four lowest energy Sc3N@C80-GNF isomers. Carbon, hydrogen, scandium, and nitrogen atoms are represented by dark grey, white, green, and blue circles respectively. Bond lengths are given in Angstroms and the relative energies (Erel) in units of eV. The interacting potential profile for the lowest energy isomer (Isomer 1) is also given. 215x227mm (300 x 300 DPI)


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Figure 4. a) One electron energy levels for C60, GNF and C60-GNF clusters, and b) Sc3N@C80, GNF, and Sc3N@C80-GNF clusters. Degeneracy and isosurfaces (isoval = 0.01 au) is given for selected frontier orbitals. Solid black lines and dotted red lines represent, respectively, occupied and unoccupied states. Total electronic density isosurfaces (isoval = 0.05 au) are also given. 237x276mm (300 x 300 DPI)



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Figure 5. Density of states for C60, GNF, and C60-GNF isomers. 154x119mm (300 x 300 DPI)


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Figure 6. Density of states for Sc3N@C80, GNF, and Sc3N@C80-GNF isomers. 157x202mm (300 x 300 DPI)



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Figure 7. Electric dipole moment vector marked with a red arrow for the a) C60-GNF, and b) Sc3N@C80GNF isomers. Natural population charges are marked next to the structures. 207x164mm (300 x 300 DPI)


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Electronic and Structural Properties of C60 and Sc3N@C80

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