First-principles electronic structure studies on the ground state geometry and electronic and magnetic properties of bare and hydrogen coated metallofullerene Gd3N–C80 have been carried out within a density functional formalism. The correlation effects are incorporated either through a generalized gradient corrected functional or through an on-site Coulomb interaction (LDA+U). It is shown that the bare Gd3N–C80 possess a ferromagnetic ground state with a large spin moment of 21μB that is highly stable against spin fluctuations. The simulated Raman spectrum shows that the low-energy peaks are contributed by the floppy movement of N atom. As to the effect of addition of hydrogens, it is shown that the most favorable site for the hydrogen adsorption is an on-top site where the H atom is located above a five-member carbon ring with a binding energy of 1.92eV, while the least stable site corresponds to an on-top absorption above a six-member ring. A study of the energetics upon multiple adsorption of H shows that the binding energy of the H to metallofullerene drops after 11 H atoms. This shows that it should be possible to attach multiple ligands offering the potential that the Gd3N–C80 can be functionalized with ligands or assembled in cluster assemblies

Khanna, S. N.

Qian, M. C.

VCU Scholars Compass

Virginia Commonwealth UniversityVCU Scholars CompassPhysics Publications Dept. of Physics2007An ab initio investigation on the endohedralmetallofullerene Gd 3 N – C 80M. C. QianVirginia Commonwealth UniversityS. N. KhannaVirginia Commonwealth University, snkhanna@vcu.eduFollow this and additional works at: http://scholarscompass.vcu.edu/phys_pubsPart of the Physics CommonsQian, M. C., and Khanna, S. N. An ab initio investigation on the endohedral metallofullerene Gd 3 N – C 80. Journal ofApplied Physics, 101, 09E105 (2007). Copyright © 2007 American Institute of Physics.This Article is brought to you for free and open access by the Dept. of Physics at VCU Scholars Compass. It has been accepted for inclusion in PhysicsPublications by an authorized administrator of VCU Scholars Compass. For more information, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/phys_pubs/119An ab initio investigation on the endohedral metallofullerene Gd3N–C80M. C. Qiana and S. N. KhannaDepartment of Physics, Virginia Commonwealth University, Richmond, Virginia 23284-2000Presented on 9 January 2007; received 31 October 2006; accepted 12 December 2006;published online 2 May 2007First-principles electronic structure studies on the ground state geometry and electronic andmagnetic properties of bare and hydrogen coated metallofullerene Gd3N–C80 have been carried outwithin a density functional formalism. The correlation effects are incorporated either through ageneralized gradient corrected functional or through an on-site Coulomb interaction LDA+U. It isshown that the bare Gd3N–C80 possess a ferromagnetic ground state with a large spin moment of21B that is highly stable against spin fluctuations. The simulated Raman spectrum shows that thelow-energy peaks are contributed by the floppy movement of N atom. As to the effect of additionof hydrogens, it is shown that the most favorable site for the hydrogen adsorption is an on-top sitewhere the H atom is located above a five-member carbon ring with a binding energy of 1.92 eV,while the least stable site corresponds to an on-top absorption above a six-member ring. A study ofthe energetics upon multiple adsorption of H shows that the binding energy of the H tometallofullerene drops after 11 H atoms. This shows that it should be possible to attach multipleligands offering the potential that the Gd3N–C80 can be functionalized with ligands or assembled incluster assemblies. © 2007 American Institute of Physics. DOI: 10.1063/1.2711420Endohedral metallofullerenes have attracted consider-able attention over the past 14 years as they provide uniqueopportunity of preserving selected properties of encapsulatedmetal core while the unit is exposed to solvents or otheragents. This has led to numerous applications, in particular,in biological sciences where the metal alone could lead toharmful consequences. In this regard, the trimetal nitridecontaining endohedral fullerene1 has generated the tremen-dous interest. Specifically, the metallofullerenes Gd3N–C80are paid much attention2–4 since they are potential contrastenhancing agents for magnetic resonance imaging MRI. Anisolated Gd3N molecule has a large spin magnetic moment of23B. When embedded inside the carbon cage, the Gd atomsbond through the delocalized s ,d states with the s , p states ofC atom, leaving the spin magnetic moment from f electronsalmost intact. The fullerene cage not only acts to protect theinterior metal atoms from the leakage and prevent their ac-cumulation in human organs and tissues, but can also befunctionalized with ligands for medicinal applications. Inparticular, experiments indicate that trigadolinium nitridefullerenes Gd3N–C80 can lead to more over 20 times higherproton relaxivities5 in comparison with the commercial Gd3+chelate contrast agent. Further, the substitution of Gd by Tbcan generate fluorescent particles6 for another imaging tech-nique. Although there are exciting developments on themedical applications of nanoparticles,7 a fundamental under-standing of such endohedral metallofullerenes is still lacking.In this paper, we present investigations on the electronic andmagnetic properties of endohedral metallofullereneGd3N–C80 using a first-principles method. We demonstratethat Gd atomic magnetic moments inside the carbon cage areferromagnetically coupled and that the Gd3N presents afloppy interior where N atom can easily tunnel through theGd3 plane. We suggest that the observed low-energy Ramanlines3 are related with this floppy nature of Gd3N in the cage.By examining the attachment of hydrogen atoms, we inves-tigate the chemical functionalization of the metallofullerenes.The total energy and electronic structure calculations arecarried out using VASP program,8 which is the pseudopoten-tial plane-wave method based on density functional theory9within the generalized gradient approximation GGA.10Since the correlation effects are important to properly treatGd f states, the density functional based correlated bandtheory LDA+U method11 was chosen in order to separate theoccupied 4f orbitals from the unoccupied ones. The Hubbardparameter U=8 eV and the exchange parameter J=0.8 eVwere taken for the effective coulomb interaction12 and theexchange interaction on the Gd ion, respectively. A plane-wave basis set and the projector augmented wave PAWpseudopotentials for Gd, N, and C elements with 18, 5, and 4valence electrons, respectively, were employed. The 4f elec-trons are included in the valence shell of Gd atom. The en-ergy cutoff was set to 300 eV. The convergence in energywas set as 1 meV.The cluster Gd3N–C80 was placed inside the cubic su-percell with the size of 20 Å, which is large enough to elimi-nate the interaction between the neighboring cells. The point is used to integrate over the first Brillouin zone. Thegeometry is fully optimized by the criterion that the force oneach atom is less than 1 meV/Å.Figure 1 shows the one electron energy levels for theendohedral metallofullerene Gd3N–C80. The calculationssuggest a large highest occupied molecular orbital HOMO-lowest unoccupied molecular orbital LUMO gap of 1.48and 1.54 eV for the majority and minority spin channels,respectively, which is in good agreement with the photoemis-aElectronic mail: mquian@vcu.eduJOURNAL OF APPLIED PHYSICS 101, 09E105 20070021-8979/2007/1019/09E105/3/$23.00 © 2007 American Institute of Physics101, 09E105-1 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:128.172.48.59 On: Fri, 06 Nov 2015 17:34:00sion experimental gap3 of 1.75 eV. The electronic structuresof bare C80 and Gd3N–C80 show that six electrons are trans-ferred from the Gd3N to the carbon cage to fill the emptystates near the HOMO in the bare C80. The hybridizationbetween the s ,d states of Gd atoms and the s , p states ofsix-member ring C atoms creates strong bonds and stabilizesthe whole metallofullerene. The calculated binding energy isas large as 13.63 eV. The geometry of Gd3N inside the cageis kept to be a pyramidal shape, while the Gd–Gd bondlength is slightly stretched and the N atom is closer to theGd3 plane compared to the case for the isolated molecule.Because Gd f electrons are fully spin polarized, both occu-pied and unoccupied f states are located away from theFermi level, which leads to atomic magnetic moment of 7Bfor each Gd ion. In the right panel of Fig. 1, we show theisosurface of the magnetic charge densities with the value of0.8 electrons/Å3 when the Gd3N are embedded inside theC80 cage. The ground state is marked by ferromagnetic cou-pling among three Gd ions and gives rise to the total mag-netic moment of 21B. Using the fixed magnetic momentcalculations, we found that the exciting magnetic states withspin moments of 19B and 23B are higher by 1.48 and1.41 eV, respectively.To investigate the floppy movement of N atom betweenthe two sides of Gd3 plane, we solved the one-dimensionalSchödinger equation in the double-well potential. In Fig.2a, we give the potential for N atom as a function of thedisplacement away from the Gd3 plane when the moleculeGd3N is embedded inside the fullerene C80 cage. We shouldpoint out that it is important to treat the movement of N atomby quantum mechanism, because the double well is shallowand the energy barrier is only 78 meV when N atom movesfrom one side to another side of the double well. The poten-tial is obtained from the total energy calculations by the dis-placement step of 0.02 Å. In every step, the N atom is fixedand all other atoms of the metallofullerene are fully relaxed.Then, more densely spaced points corresponding to the dis-placement step of 0.001 Å are derived by a cubic spline in-terpolation in order to numerically solve the Schödingerequation using the plane wave basis. The convergence wastested by varying the number of plane waves, and we foundthat 500 plane waves were sufficient to give converged ei-genvalues and eigenstates over the entire energy range exam-ined in this paper. The calculated energy eigenvalues areshown in Fig. 2a and the lowest-energy eigenvalue indi-cates the quantum zero-point energy. The energy spacing be-tween the adjacent energy levels is interesting. If the poten-tial for the movement of N atom is harmonic, the energyspacing would be exactly equal, which corresponds to thephonon energy using the force field approximation. Appar-ently, the movement of N atom is anharmonic and floppy.Inside the double well, there are three energy levels corre-sponding to the N atom vibrating around the local minima.For the energy levels nearby the barrier, the energy spacingsdeviate from those of the harmonic treatment. On the otherhand, the energy spacings are almost constant in the range ofhigh energy.FIG. 1. Color online One electron energy levels in majority and minorityspin channels for the Gd3N embedded in the C80 cage. The right panel givesthe isosurface of the magnetic valence charge densities with the value of0.8 electrons/Å3.FIG. 2. Color online a Calculated potential energy curve for the N atomas a function of the displacement away from the Gd3 plane. The horizontallines represent the calculated energy eigenvalue spectra and the lowest-energy space between the adjacent energy levels corresponds to the quantumzero-point energy. b Simulated Raman spectrum of Gd3N embedded insidethe C80 cage. The observed peak positions in the experiment are indicated byhashmarks.09E105-2 M. C. Qian and S. N. Khanna J. Appl. Phys. 101, 09E105 2007 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:128.172.48.59 On: Fri, 06 Nov 2015 17:34:00We now focus on the energy adsorption in the Ramanscattering measurements. In the dipole approximation, thetransition can occur among the energy eigenstates. We usethe calculated “one atom energy eigenvalues and eigenstatesto simulate the Raman spectra at room temperature shown inFig. 2b. The Gaussian smearing of 0.2 meV is taken torepresent the precision of the experimental instruments. TheRaman lines at 223, 140, 85, and 19 cm−1 are obtained andqualitatively agree with the experimental ones.3 In particular,the low-energy lines below 100 cm−1 are unique features forthe Gd3N embedded inside the C80 cage and can be assignedto the floppy movement of N atom between the two sides ofGd3 plane. We notice that the discrepancies between theoret-ical and experimental results mainly arisen from the frozenphonon approximation in our simulation.For medical application, it is necessary that the metallof-ullerene be functionalized with ligands as well as watersoluble. Theoretically we examined the chemical propertiesof the metallofullerene Gd3N–C80 simply by adding hydro-gen atoms. Figure 3 shows the four possible addicted sites ofhydrogen atom outside the carbon cage. We use the follow-ing formula to calculate the binding energy:BE = EGd3N – C80 + H − EGd3N – C80 − EH ,where EGd3N–C80+H and EGd3N–C80 are the total en-ergies of metallofullerene with and without hydrogen atom,and EH is the energy of hydrogen atom. We found that themost favorable site is on top of a five-member ring carbonatom site A with a binding energy of 1.92 eV. The on-topsite over a six-member ring carbon atom site B is the leastfavorable with a binding energy of 1.64 eV. The bridge sitesC and D give lower binding energies. At the on-top site, thebond length between C and H is about 1.17 Å. We thenexamined the number of hydrogen atoms that can be ad-dicted outside the metallofullerene. In Fig. 4, we plot theremoval energy as a function of the number of hydrogenatoms outside the metallofullerene. The larger the removalenergy, the more stable the attachment of hydrogen atoms.We found that up to 11 hydrogen atoms can be stronglybound to the carbon cage of the metallofullerene Gd3N–C80.Our results support the finding that the endohedral metallof-ullerene Gd3N–C80 are water soluble by coating with hy-droxyl groups.In summary, the electronic and magnetic properties ofthe endohedral metallofullerene Gd3N–C80 have been stud-ied via first-principles calculations. The Gd3N molecule isfound to be strongly bound to the C80 cage inside and has ahigh spin magnetic moment of 21B that is largely localizedon the f electrons of three Gd atoms. The large binding en-ergy makes it unlikely that the cage could break and releaseGd atoms. Based on the quantum mechanical treatment, wepropose that the observed experimental low-energy Ramanspectrum is associated with the floppy movement of N atombetween the two sides of the Gd3 plane. Finally, we examinethe attachment of hydrogen outside the metallofullerene andsuggest that the endohedral metallofullerene Gd3N–C80 canbe functionalized with ligands or assembled in cluster assem-blies for medical applications.This work is supported by National Institute of Healthwith Grant No. R01-CA119371-01. One of the authorsS.N.K. is grateful to VCU for a study/research leave.1S. Stevenson et al., Nature London 401, 55 1999.2S. Stevenson, J. P. Phillips, J. E. Reid, M. M. Olmstead, S. P. Rath, and A.L. Balch, Chem. Commun. Cambridge 2004, 2814.3M. Krause and L. Dunsch, Angew. Chem., Int. Ed. 44, 1557 2005.4S. Stevenson, R. R. Stephen, T. M. Amos, V. R. Cadorette, J. E. Reid, andJ. P. Phillips, J. Am. Chem. Soc. 127, 12776 2005.5P. Fatouros et al. private communication.6L. Dunsch, M. Krause, J. Noack, and P. Georgi, J. Phys. Chem. Solids 65,309 2004.7L. J. Wilson, D. W. Cagle, T. P. Thrash, S. J. Kennel, S. Mirzadeh, J. M.Alford, and G. J. Ehrhardt, Coord. Chem. Rev. 190–192, 199 1999.8G. Kresse and J. Hafner, J. Phys.: Condens. Matter 6, 8245 1994; G.Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 1996.9P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 1964; W. Kohn and L.J. Sham, ibid. 140, A1133 1965.10J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 38651996.11V. I. Anisimov, F. Aryasetiawan, and A. I. Lichtenstein, J. Phys.: Condens.Matter 9, 767 1997.12M. D. Johannes and W. E. Pickett, Phys. Rev. B 72, 195116 2005.FIG. 3. Color online The adsorption sites of hydrogen atoms outside theC80 cage as well as their binding energies in unit of eV. The gray, green,blue, and red circles refer to the C, Gd, N, and H atoms, respectively.FIG. 4. Removal energy as a function of the number of hydrogen atomswhen the hydrogen atoms are attached to the outside of the metallofullereneGd3N–C80. The circle corresponds to the most favorable attachment of hy-drogen atoms.09E105-3 M. C. Qian and S. N. Khanna J. Appl. Phys. 101, 09E105 2007 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:128.172.48.59 On: Fri, 06 Nov 2015 17:34:00

An ab initio investigation on the endohedral metallofullerene Gd 3 N – C 80

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An ab initio investigation on the endohedral metallofullerene Gd 3 N – C 80

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