DSpace at VNU: Electronic and magnetic properties of C-60-Fe-n-graphene intercalating nanostructures (n=1-6) predicted from first-principles calculations

Nga, Hajime Hiraoa,∗ a Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371,

j o ur na l h o me p a g e :w w w e l s e v i e r c o m / l o c a t e / c p l e t t

Hung M Lea,b,∗, Wilson K.H Nga, Hajime Hiraoa,∗

a Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,

Singapore 637371, Singapore

b Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam

Article history:

Received 10 September 2014

In final form 21 October 2014

Available online 30 October 2014

GrapheneandC60canestablishcoordinationbondswithtransitionmetalatoms/clusters.Using first-principlesmodelingmethods,weexploretheC60–Fen–grapheneintercalatingnanostructures(n=1–6), whichmayhavepotentialapplicationsin,e.g.,spintronics.Twelveoptimizedconfigurationsarefoundto possessgoodenergeticstability(withbindingenergiesof4.22–20.54eV).Elevenstructureshavedifferent magnitudesofmagnetism(2.00–12.75␮B/cell),whereasoneisnon-magnetic.Themagnetismishighly correlatedwiththebondingorientationsbetweenFeatomsandC60.Sevennanostructurespossessgood halfmetallicity(withthespinpolarizationeffects>0.8),whilethenon-magneticstructureisfoundtobe insulating

©2014ElsevierB.V.Allrightsreserved

1 Introduction

Since the successful experimental synthesis of graphene, a

material that features a two-dimensional carbon-made

struc-ture,many advanced technologieshave been inventedthrough

itsutilization.Atthenanometerscale,graphenehasremarkable

mechanical stability because of the fully sp2-bonding

arrange-ment of carbon atoms [1] Moreover, graphene is regarded as

a zero-gap semiconductor that exhibits superconductivity and

potentiallyoffersusefulapplicationsinelectronicdevices[2]

Reac-tioncatalysisisanothernoticeableapplicationbecausegraphene

canbeemployedasahostingmaterialtocarrycatalyzingmetal

atoms/clusters/complexes[3–5].Forthatreason,thechemistryof

metal-bondinginteractionsofgrapheneisanimportantaspectthat

hasbeenintensivelyinvestigatedoverthepastfewyears [6–8]

Themetal–graphenecontactisformeduponhybridizationofdand

porbitals,similartothatintheintercalatingstructuresofmetal

andbenzene[9].Thesp2bondsareresponsiblefortheformation

ofagraphenemonolayer;however,the2pzorbitals,whicharenot

involvedinthesp2bonds,tendtointeractwiththevacantdshells

ofmetalatomsandformcoordinationbonds.Sofar,therehave

beenalargenumberofexperimentalstudiesofmetal–graphene

nanostructures,suchasthoseinvolvingNi[10],Au,Fe,Cr[11–13],

∗ Corresponding authors.

E-mail addresses: hung.m.le@hotmail.com (H.M Le), hirao@ntu.edu.sg

(H Hirao).

orAg[14].Byemployingelectrolysis,Zhangetal.[15]investigated theintercalatingcompoundsofironchlorideongraphite.In addi-tion,therehavebeeneffortstoattachgrapheneonmetalsurfaces

toderiveinterestingelectronicproperties[16] Theoretical and computational investigations of graphene– metalinteractionshavebeenconductedusingdensityfunctional theory (DFT) calculations [17,18] Significant achievements in grapheneresearchhavebeenattainedduringthepastfewyears, which have enhanced attentionto theelectronicand magnetic propertiesof graphene.Importantly,Nakataand Ishiihave pro-videdtheoreticalevidencethat3dtransitionmetalsbindstrongly

tographene[19] Moreover,theattachmentof varioustypesof ligandsongrapheneviaatransition-metalatombridgehasbeen investigatedinseveralpreviousstudies[6,9,20,21].Recently,we have exploredC60–M–Gnanostructures, inwhich buckminster-fullerene[22,23](C60)wassteadiedonagraphenesurfaceviaone bridgingtransition-metalatom[4,24].Interestingly,whenCr,Mn,

Fe,orNiisusedasabridgingmetalatom,C60–Mdoesnotstand uprightongraphene; instead,we observegeometrydistortions that correlatewithspinpolarization inthe3dorbitals and dis-persioninteractionsbetweengrapheneandC60.Wehypothesize thatsuchageometrydistortingfeaturemaybeeffectivelyexploited

todesignnewnanostructures,inwhichmultipletransition-metal atomsarearrangedinacrown-likemanner.Thisstrategymayallow onetoconstructmorestablegraphene–metal–C60nanostructures thatmightfindapplicationsinspintronicsorcatalysis[25].Inthis Letter,wepresentaDFTinvestigationofbridgingC60andgraphene usingseveralFeatoms(uptosixatoms)

http://dx.doi.org/10.1016/j.cplett.2014.10.051

0009-2614/© 2014 Elsevier B.V All rights reserved.

Figure 1.C 60 steadied on graphene using six bridging Fe atoms Twelve

configu-rations (pre-optimized) based on Fe allocations are suggested: (a) C 60 –Fe–G, (b)

C 60 –Fe 5 –G, (c) C 60 –Fe 6 –G, three C 60 –Fe 2 –G configurations, namely (d) (1,2), (e)

(1,3), and (f) (1,4), three C 60 –Fe 3 –G configurations, namely (g) (1,2,3), (h) (1,2,4),

and (i) (1,3,5), and three C 60 –Fe 4 –G configurations, namely (j) (1,2,3,4), (k) (1,2,3,5),

and (l) (1,2,4,5) For convenience, the given nomenclatures are used to address the

structures throughout the Letter.

2 Computational details

DFT calculations are executed using the Quantum

Espresso (QE) package [26] Specifically, we employ the

Perdew–Burke–Ernzerhof (PBE) exchange-correlation

func-tional[27,28]withtheultrasoftpseudopotentials[29,30](USPP)

Thekineticenergycutofffor plane-waveexpansionissetto45

Rydberg.Theempiricalcorrectionsforlong-rangedispersionsare

alsoincluded [31,32] To approximate thecontinuity of energy

bands, we employ the Gaussian smearing technique with a

smallsmearingwidth (0.002Rydberg).Structural optimizations

are performed with an energy-convergence criterion of 10−6

Rydberg/cell.Initially,fullstructuraloptimizations areexecuted

at the -point by relaxing theatomic positions and unit cells

simultaneously.Then,thefinalrelaxedstructuresaredetermined

byfurtherrelaxingtheatomicpositionswitha k-pointmeshof

(6×6×1)

Thetheoreticalmodelshave two-dimensionalcharacteristics

and consist of three major buildingunits: a periodic graphene

monolayer (54 C atoms), bridging atoms (i.e.Fen), and C60 In

thosetwo-dimensionalslabs,thelengthofc-axisissetto40Bohr

(21.17 ˚A)toallowvacuumtreatmentsinthezdirection

perpen-diculartothegraphenesheet.IntheC60–Fe–Gstructure(Figure

S2,SupplementaryMaterial(SM)),C60–Fedoesnotstandupright

onthegraphenesheet,andFeinteractswithonlytwoCatomsin

C60.InFigure1,weshowacomplexstructurewithamaximum

loadofsixFeatoms,whichfullyinteractwithahoneycombring

ofC60.Also,allotherpossiblestructuresofC60–Fen–G(n<6)are

constructed.Forillustrationpurposes,thetopandsideviewsofall

optimizedC60–Fen–GstructuresarepresentedintheSM

Afterageometryoptimization,thebindingenergyofacomplex

nanostructurewithnFeatomscanbecalculatedusingthefollowing

equation:

Ebinding=EC 60+nEFe+EG−Estructure, (1)

whereEC60,EFe,andEGarethetotalenergiesofC60,anisolatedFe

atom,andtheperiodicgraphenelayer,respectively,whileEstructure

denotesthetotalenergyofthecomplexobtainedfromDFT

calcula-tions.Forfaircomparisonsamongtheinvestigatedcases,wedefine

averagestabilizationenergyforaC60–Fen–Gnanostructureas

ES= Ebinding

Table 1

Binding energies, average Fe stabilization energies, M T and M A for the investigated nanostructures.

Fe distribution E binding E S M T M A

(eV) (eV) (␮ B /cell) (␮ B /cell)

C 60 –Fe 2 –G (1,2) 7.62 3.81 4.11 6.33

C 60 –Fe 3 –G (1,2,3) 10.68 3.56 6.13 8.58

(1,2,4) 10.38 3.46 6.11 8.51 (1,3,5) 9.74 3.25 6.00 8.02

C 60 –Fe 4 –G (1,2,3,4) 13.88 3.47 8.50 11.31

(1,2,3,5) 13.98 3.49 8.14 10.55 (1,2,4,5) 14.05 3.51 8.09 11.30

C 60 –Fe 5 –G 17.73 3.55 10.36 14.12

C 60 –Fe 6 –G 20.54 3.42 12.75 16.60

3 Results and discussion

As shown in Table 1, with a full load of six Fe atoms, the

C60–Fe6–Gnanostructure(FigureS3,SM)ishighlystablewitha bindingenergyof20.54eV.Onaverage, thestabilizationenergy arisingfromeachFeatominthiscaseis3.42eV,whichindicates goodstabilizationofthenanostructure,althoughthisstabilization energyislowerthanthatofC60–Fe–G(4.22eV).TheDOSdataallow

ustoestimatethetotalmagnetization(MT)andabsolute magneti-zation(MA)asreportedinTable1

MT and MA of C60–Fe6–G are calculated as 12.75 and 16.60␮B/cell, respectively Such ferromagnetism is mainly pro-duced by the strong spin polarization of the Fe atoms in the nanostructure(withthemajorcontributionofdorbital polariza-tion).Recallthatinapreviouswork[24],withtheuseofoneFe atom,C60–Fe–Gwasreportedtoexhibitatotalmagneticmoment

of2.00␮B/cell(Figure2a),whichisslightlysmallerthanonesixth

Figure 2.Spin-polarized total DOS (left panels) and PDOS of Fe atoms (right pan-els) in (a) C 60 –Fe–G, (b) C 60 –Fe 4 –G (1,2,3,4), (c) C 60 –Fe 4 –G (1,2,3,5), (d) C 60 –Fe 4 –G (1,2,4,5), (e) C –Fe –G, and (f) C –Fe –G The Fermi level is positioned at 0.

Table 2

Magnetic contributions (␮ B ) from Fe atoms (and their 3d shells) and spin

polariza-tion effects for the investigated C 60 –Fe n –G nanostructures.

Fe distribution Fe magnetic contribution P

(␮ B ) Total 3d

C 60 –Fe 3 –G (1,2,3) 7.01 6.87 0.81

C 60 –Fe 4 –G (1,2,3,4) 9.46 9.30 0.61

ofMTexhibitedbyC60–Fe6–G.Interestingly,weobservean

alter-natepatternofFeoccupationsinC60–Fe6–G:threearecloserto

C60andpossessslightlylargermagneticterms(2.50␮B),whereas

theotherthreeareclosertographeneandpossesssmallerterms

(2.25␮B)asillustratedinFigure2 Moreover,theLöwdincharge

[33]analysisindicatesthatFe2,Fe4,andFe6havesmallerpositive

chargesthantheothers(seeTableS1,SM)

The half-metallicity is an interesting feature that can be

observedinseveralnanostructures.Inthosenanostructures,while

oneelectronicspinstate(up)indicatesinsulation,theotherspin

state (down) is conductive In a typical case of perfect

half-metallicity,thespin-upDOSshouldcompletelyvanishattheFermi

level.ParticularlyintheC60–Fe6–Gcase,thespin-upDOSdoesnot

vanishatthe Fermilevel, but it isvery smallcompared tothe

spin-downDOS asshown inthetotal DOS diagram (Figure2f)

Therefore,weregardthisstructureasanimperfecthalfmetal.The

half-metallicpropertycanbeevaluatedbythespin-polarization

effectP[34,35]:

P=

↑(EF −↓(EF

↑(EF +↓(EF



where↑(EF)and↓(EF)representthespin-upandspin-downDOS

attheFermilevel,respectively.IfPisunity,thematerialcanbe

regardedasaperfecthalfmetal.Indeed,thespinpolarizationeffect

ofC60–Fe6–Gis0.89.IntheC60–Fe–Gcase,thespinpolarization

effectisdeterminedtobeunity,whichindicatesagoodhalf-metal

Forconvenience,wesummarizethemagneticcontributionsfrom

themetalatoms(and their3d orbitals)andthecalculated spin

polarizationeffectsoftheinvestigatednanostructuresinTable2

WhenfiveFeatomsareused(C60–Fe5–G),thebonding

interac-tionsbetweenthemetalatomsandgraphenechangesignificantly

Fromthetopview(FigureS4,SM),itcanbeseenthatthosefive

Featomsconstituteapentagon-likestructure.Closerinspection

showsthattherearethreetypesofFe–grapheneinteractionswith

differentdegreesofspinpolarization.TwoFeatoms(2.52␮B/atom)

interact with graphene via three Fe–C bonds, two Fe atoms

(2.33␮B/atom)interactwithfullhoneycombunitsofgraphene(but

dislocatedfromthecenterofthehoneycombrings),andoneFe

(2.24␮B/atom)islocatedabovethecenterofahoneycombring

ThedifferenceinFelocationscanalsobeobservedfromthePDOS

ofFe(Figure2e).ThebindingenergyofC60–Fe5–Gis17.73eV,while

thestabilizationenergyforoneFeatomis3.55eV,slightlyhigher

thanthatinC60–Fe6–G.C60–Fe5–Gpossessesweakhalfmetallicity

becauseofitslowspin-polarizationeffect(0.39)

AsshowninFigure1,threedifferentC60–Fe4–Gnanostructures are optimized.When fourFeatoms are locatedatthe (1,2,3,5) positions(FigureS5,SM),thestructurehasanintermediatebinding energy(13.98eV)andexhibitsanintermediatemagneticmoment (8.14␮B/cell)amongthethreepossibilities.Inthisstructure,fourFe atomsfullyinteractwithfourcorrespondinghoneycombunitsfrom graphene.Eachofthefirstthreemetalatoms(Fe1,Fe2,Fe3) inter-actswithC60viatwoFe–Clinkages,whiletheremainingFeatom (Fe4inFigure2c)fullyinteractswithafive-memberedpentagonal ringfromC60.Indeed,thisspecialFeatomhasthesmallestspin polarizationterm(1.01␮B)amongthefourandanegativecharge (−0.06),whiletheotherthreeFeatomshavepositivechargesand exhibitgreatermagneticmomentsof2.31–2.64␮B.ThepartialDOS (PDOS)profilesforFe1 andFe3 areverysimilar,andthehighest polarizationtermoriginatesfromFe2.Overall,thisnanostructure hasaspinpolarizationeffectof0.76

WhenfourFeatomsresideatthe(1,2,3,4)positions(FigureS6, SM),theresultingstructurehasabindingenergyof13.88eV (low-estofthethreecases),whileithasthestrongestferromagnetism

ofthethree(8.50␮B/cell).Inthiscase,theFeatomsareobserved

tobehaveinslightly differentmanners (seeFigure2b).EachFe atomhasapositivechargeandproducesastrongferromagnetic moment(higherthan2␮B/atom).IntheDOSplot(Figure2b),it

is observedthatthe(1,2,3,4)structure hasa low spin polariza-tioneffect (0.61).Similartothecase of(1,2,3,4),there aretwo differenttypesofFeallocationsinthe(1,2,4,5)structure(Figure S7,SM),whichhasthelargestbindingenergy(14.05eV)of the threeC60–Fe4–Gcases.Allfouratomsarefoundtoshiftslightly away fromthecenterofthehoneycomb ringsingrapheneand eachFeinteractswithtwoCatomsfromC60,whichresultsina strongferromagneticmoment(2.25–2.41␮B/atom).Consequently,

astrongmagneticmomentof8.09␮B/cellisfound(butthe small-estofthethreeC60–Fe4–Gcases).Likeinthecasesof(1,2,3,4)and (1,2,3,5),the(1,2,4,5)structure doesnotreallypossessthe half-metalcharacteristics,becausethereisstillelectrondensityinthe spin-upstateattheFermilevel(seetheDOSplotinFigure2d), andthespinpolarizationeffectforthe(1,2,4,5)structureisaslow

as0.68.AdditionalvalidationcalculationsareexecutedusingQE withtheUSPPandtheViennaAbInitioPackage[36–38](VASP4.6) withtheprojector-augmented-wavemethodfortheinspectionof ferromagnetic/anti-ferromagneticstatesinC60–Fe4–G(1,2,4,5)and threeotherstructures(FigureS1,SM).Weconcludethat neighbor-ingFeatomsfavortheferromagneticspinalignmentanddonot haveopposingmagneticmoments

There are three possibilities to distribute two Fe atoms in

C60–Fe2–G.Inthosethreecases,thetwoFeatomsareobserved

toshiftslightly awayfromthecenters ofthehoneycombunits

ingraphene;however,themajordistinctionscomefromvarious interactingschemesbetweenFeandC60.WhentwoFeatomsare placedin the(1,2)arrangement(FigureS8,SM)(resultinginan Fe–Fedistanceof2.47 ˚A),eachFeatominteractswithC60viatwo Fe–Clinkages,receivesapositivecharge,andexhibitsalarge fer-romagneticmoment(2.41␮BpereachFeatom).Overall,the(1,2) structureexhibitsatotalmagneticmomentof4.11␮B/cell.From bindingenergycalculations,itisshownthatthe(1,2)structureis themoststableofthethreeC60–Fe2–Gstructuresexaminedandthe correspondingaveragestabilizationenergyisthelargest(3.81eV)

ofallstructuresreportedin thisstudy(excludingtheC60–Fe–G case).AccordingtotheDOSdistribution(Figure3a),the(1,2) struc-turecanberegardedasaperfecthalf-metalwiththePvalueof1.00 (largestamongthreeC60–Fe2–Gcases)

The(1,3)and(1,4) structures(FiguresS9andS10intheSM, respectively)arelessstable,withthebindingenergiesbeing6.78 and6.83eV,respectively.ItisseenfromtheDOSplots(Figure3 andc)thatthe(1,3)nanostructureisanon-magnetic and insu-lating material, while (1,4) possesses half-metallicity with the

Figure 3. Spin-polarized total DOS (left panels) and PDOS of Fe atoms (right panels)

in (a) C 60 –Fe 2 –G (1,2), (b) C 60 –Fe 2 –G (1,3), (c) C 60 –Fe 2 –G (1,4), (d) C 60 –Fe 3 –G (1,2,3),

(e) C 60 –Fe 3 –G (1,2,4), and (f) C 60 –Fe 3 –G (1,3,5) The Fermi level is positioned at 0.

spin-polarizationeffectestimatedas0.87.Inthe(1,3)case, two

Featomsplay similarrolesinthebondinginteractionwithC60,

andeachofthemestablishesbondingtoC60viathreeFe–C

link-ages.Interestingly enough,suchanunusualinteractingscheme

causesthespin-upandspin-downDOStocanceleachotherout

andconsequentlyproducesanon-magneticstructure(illustrated

inFigure3b).Thebandgapofthisinsulatingcaseisverynarrow (0.09eV)accordingtoourbandenergyexamination.BothFeatoms

inthe(1,3)casehavenegativecharges.Ontheotherhand,twoFe atomsinthe(1,4)casebehavedifferentlyfromeachother.While

Fe1(asdenotedinFigure3)makesbondstoafive-memberedring fromC60andexhibitsaweakferromagneticterm(1.30␮B)with

a negativecharge (−0.04), Fe2 interacts withtwo C atoms and exhibitsastrongerferromagneticmoment(2.38␮B)witha posi-tivecharge(0.13).UnliketheothermagneticstructureswhereFe atomscontributeferromagnetictermsandCatomscontribute anti-ferromagneticterms,the(1,4)structureisthesolecasewhereFe,

C60,andgraphenejointlycontributeferromagnetism

Withtheinclusionofthreemetalatoms,eachFeisfoundto interactfullywiththehoneycombringsingraphenewhilebinding

toC60viatwoFe–Clinkages.Whenthreemetalatomsarelocatedat the(1,2,3)positions(FigureS11,SM),Fe1andFe3behavesimilarly

intheirinteractionswithgrapheneandC60asprovedbythePDOS

ofFe1andFe3(withamagneticalignmentof2.23␮B/atom).Fe2,on theotherhand,exhibitsalargermagneticcontribution(2.55␮B) thanthe others.The total ferromagneticmoment ofthe (1,2,3) structure(6.13␮B/cell)isactuallyobservedtobethelargestofall

C60–Fe3–Gcases.Inthenextstructurehavingthe(1,2,4) arrange-mentforthreeFeatoms(FigureS12,SM),theroleofeachmetal atomisdifferentfromthatoftheothers,asseeninthePDOS distri-butioninFigure3e.Thisstructureexhibitsamagneticmomentwith

anintermediatemagnitudeamongthreecases(6.11␮B/cell).Inthe lastcase,threeFeatomsareequallydistributedonthegraphene sheet(FigureS13,SM),sothattheyinteractwithC60inanalmost similarmanner(seeFigure3f).Asaresult,theyestablishthesame spinpolarizations,whichproduceanapproximatemagnetic align-mentof2.14␮B/Featom.Thetotalmagneticmomentgivenbythis structureis6.00␮B/cell.Itcanbeseenthatthecomputedtotal mag-netizationsofthree C60–Fe3–Gstructuresvaryinasmallrange

Figure 4.Charge density plots of the planes containing three Fe atoms in the (a) (1,2,3) and (b) (1,2,4) C 60 –Fe 3 –G nanostructures The Fe atoms are represented by red

semi-metallicity.AmongthethreeC60–Fe3–Gnanostructures,the(1,2,3)

structurehasthelowestspinpolarizationeffect(0.81).Atthesame

time,thisstructurealsohasthelargestbindingenergyand

mag-neticmoment(showninTable1).Incontrast,(1,2,4)and(1,3,5)

haveperfectspinpolarizationeffects(1.00),whiletheyarequite

lessstable(withbindingenergiesof10.38and9.74eV,respectively)

andexhibitsmallermagneticmoments(6.11and6.00␮B/cell)

TheinteractionsbetweenintercalatedFeatomsandgraphene

havelargebindingenergiesand thuscanberegarded asstrong

coordinationbonds.Interestingly,thisbondinginteractionalters

theelectronicstructureof graphenebydopingelectrondensity

tobuildupthehighestoccupiedbandsin the␤-spin.Notonly

doesthisbehavior resultin magnetism,but italsocauses

half-metallicityofmostnanostructures,ascanbeseeninFigureS14

(SM)whereweobservesignificantcontributionsofC60–Fengroups

atthehighestoccupiedbands.Semi-metallicity,whicharisesfrom

theincorporationofthed-bandsacrosstheFermienergylevel,has

alsobeenfoundinothercarbonmaterialsintercalatedwith

transi-tionmetalatoms[20,39].Incyclopentadienyl–Fe–carbonnanotube

(Cp–Fe–CNT)[39],themagneticmomentofFeisquenchedto0.00

or0.97␮B/cell,whereasinbenzene–Fe–graphene[20]andthe

cur-rentsystem,themagneticmomentofFeremainslarge.Thesmaller

magneticmomentinCp–Fe–CNTisduetothestronginteractions

and multiplechemical bondsbetweenmetal and CNT Another

notabledifferencebetweenpreviousstudiesandoursistheslant

orientationofC60intheone-Fecase,whiletheCpringandbenzene

remainflatandpreservethe5and6hapticities,respectively.In

fact,2hapticityinorganometalliccompoundscontaininga

buck-minsterfullereneligandiscommon[40]

ItisofparticularinteresttoinspecttheFe–Feinteractions,which

mayaffectthestabilityofthenanostructurestosomeextent.In

order toverifythe possibleinteractions betweenFeatoms, we

choosetoexaminechargedensitydistributionsintwoC60–Fe3–G

models:(1,2,3)and(1,2,4).Inthe(1,2,3)structure,therearetwo

possibleFe–Febonds(with2.37 ˚Ainlength),whileinthe(1,2,4)

structure, we suspect that there is only one Fe–Fe interaction

(2.35 ˚A)becauseoneFeisdistantfromtheothertwoFeatoms.In

thetwo-dimensionalchargedensityplotsoftheFeatoms(Figure4

weobservethatthereareactuallytwoweakFe–Feinteractions

inthe(1,2,3)case, whilethere is onlyone Fe–Feinteractionin

(1,2,4).Suchmetal–metalinteractionsexplainwhythe

stabiliza-tionenergyof(1,2,3)isthelargestandthestabilizationenergyof

(1,3,5)isthesmallest.Thecontributionofmetal–metalinteractions

instructuralstabilizationsisalsosignificantinC60–Fe2–G,because

the(1,2)structurewithaFe–Fedistanceof2.47 ˚Ahasthelargest

stabilizationenergy

4 Conclusions

In summary, the C60–Fen–G nanostructures (n≤6)

investi-gatedin this studyare highly stable The nanostructuresseem

tobestabilizedsignificantlywiththeaveragestabilization

ener-giesamountingto>3eV.Moststructuresexhibitferromagnetism

(excepttheC60–Fe2–G(1,3)casewheremagnetismvanishesand

anarrowbandgapof0.09eVisopen).Themagneticalignment

of each Featomexhibits dependencyonthe bondingsituation

betweengrapheneandC60.Forinstance,whenthemetalatomis

boundtoC60viatwoFe–Clinkages,theelectronspinin3dorbitals

ishighlypolarizedtoproduceamagnetizationabove2␮B

Other-wise,themetalatomexhibitsa weakmagneticmoment(when

it interacts fullywith a five-memberedring from C60)or even

becomesnon-magnetic(whenitinteractswiththreeCatomsfrom

afive-memberedringofC60).Fromthechargedistributionplots (Figure4 itis observedthatthere isa weakFe–Fe interaction whenthedistanceisrelativelyshort(∼2.3 ˚A),whichcontributes somewhattotheenhancedstabilizationofFeatomsinthe struc-tures.Importantly,sevenhalf-metalswithspinpolarizationeffects greaterthan0.8arefound.Withtheinterestingmagnetismand sta-bility,theC60–Fen–Gnanostructuresmayfindusefulapplications

inspintronics,catalysis,etc

Acknowledgments

TheauthorsthanktheHigh-PerformanceComputingCentreat NanyangTechnologicalUniversityandtheInstituteforMaterials ResearchatTohokuUniversity,Japan(underVNUB2014-18-03)for computerresources.ThisworkissupportedbyaNanyangAssistant ProfessorshipandanAcRFTier1grant(RG3/13)

Appendix A Supplementary data

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.cplett.2014.10.051

References

[1] R Prasher, Science 328 (2010) 185.

[2] A.K Geim, K.S Novoselov, Nat Mater 6 (2007) 183.

[3] Y.-H Lu, M Zhou, C Zhang, Y.-P Feng, J Phys Chem C 113 (2009) 20156 [4] H.M Le, H Hirao, Y Kawazoe, D Nguyen-Manh, Phys Chem Chem Phys 15 (2013) 19395.

[5] L Shang, et al., Angew Chem Int Ed 53 (2014) 250.

[6] V.Q Bui, H.M Le, Y Kawazoe, D Nguyen-Manh, J Phys Chem C 117 (2013) 3605.

[7] H.R Byon, J Suntivich, Y Shao-Horn, Chem Mater 23 (2011) 3421 [8] H Huang, H Zhang, Z Ma, Y Liu, H Ming, H Li, Z Kang, Nanoscale 4 (2012) 4964.

[9] I.S Youn, D.Y Kim, N.J Singh, S.W Park, J Youn, K.S Kim, J Chem Theory Comput 8 (2012) 99.

[10] M Weser, et al., Appl Phys Lett 96 (2010) 012504.

[11] R Muszynski, B Seger, P.V Kamat, J Phys Chem C 112 (2008) 5263 [12] W Hong, H Bai, Y Xu, Z Yao, Z Gu, G Shi, J Phys Chem C 114 (2010) 1822.

[13] R Zan, U Bangert, Q Ramasse, K.S Novoselov, Nano Lett 11 (2011) 1087 [14] Y Hajati, et al., Nanotechnology 23 (2012) 505501.

[15] H.Y Zhang, W.C Shen, Z.D Wang, F Zhang, Carbon 35 (1997) 285.

[16] J Wintterlin, M.L Bocquet, Surf Sci 603 (2009) 1841.

[17] P Hohenberg, W Kohn, Phys Rev 136 (1964) B864.

[18] W Kohn, L.J Sham, Phys Rev 140 (1965) A1133.

[19] K Nakada, A Ishii, Solid State Commun 151 (2011) 13.

[20] P Plachinda, D.R Evans, R Solanki, J Chem Phys 135 (2011) 044103 [21] S.M Avdoshenko, I.N Ioffe, G Cuniberti, L Dunsch, A.A Popov, ACS Nano 5 (2011) 9939.

[22] H.W Kroto, Nature 329 (1987) 529.

[23] H.W Kroto, J.R Heath, S.C O’Brien, R.F Curl, R.E Smalley, Nature 318 (1985) 162.

[24] H.M Le, H Hirao, Y Kawazoe, D Nguyen-Manh, J Phys Chem C 118 (2014) 21057.

[25] A Ludwig, et al., in: H Zabel, M Farle (Eds.), Magnetic Nanostructures, 246, Springer, Berlin, Heidelberg, 2013, p 235.

[26] P Giannozzi, et al., J Phys.: Condens Matter 21 (2009) 395502.

[27] J.P Perdew, K Burke, M Ernzerhof, Phys Rev Lett 77 (1996) 3865.

[28] J.P Perdew, K Burke, M Ernzerhof, Phys Rev Lett 78 (1997) 1396.

[29] A Dal Corso, Phys Rev B 64 (2001) 235118.

[30] D Vanderbilt, Phys Rev B 41 (1990) 7892.

[31] S Grimme, J Comput Chem 27 (2006) 1787.

[32] V Barone, M Casarin, D Forrer, M Pavone, M Sambi, A Vittadini, J Comput Chem 30 (2009) 94.

[33] D Sanchez-Portal, E Artacho, J.M Soler, Solid State Commun 95 (1995) 685 [34] R.J Soulen, et al., Science 282 (1998) 85.

[35] V.Q Bui, T.-T Pham, H.-V.S Nguyen, H.M Le, J Phys Chem C 117 (2013) 23364 [36] G Kresse, J Hafner, Phys Rev B 47 (1993) 558.

[37] G Kresse, J Furthmüller, Comput Mater Sci 6 (1996) 15.

[38] G Kresse, J Furthmüller, Phys Rev B 54 (1996) 11169.

[39] Z Zhang, C.H Turner, J Phys Chem C 117 (2013) 8758.

[40] A.L Balch, M.M Olmstead, Chem Rev 98 (1998) 2123.