Physica E 43 (2011) 889–892

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

yfzhang

journal homepage: www.elsevier.com/locate/physe

Electronic and magnetic properties of metal atom adsorption on SWNT

Liang Wang n, YaFei Zhang

National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nanometer

Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China

a r t i c l e i n f o

Article history:

Received 19 June 2010

Received in revised form

30 September 2010

Accepted 5 November 2010Available online 15 December 2010

77/$ - see front matter & 2010 Elsevier B.V. A

016/j.physe.2010.11.007

esponding author. Tel./fax: +86 21 3420 566

ail addresses: shliangw@hotmail.com (L. Wan

@sjtu.edu.cn (Y.F. Zhang).

a b s t r a c t

The electronic structures and magnetic properties of metal (Cu, Ti) atom adsorption on the SWNT (metal/

SWNT) were investigated. A model of electron transfer in metal/SWNT system without and under external

electric field was proposed that may explain some experimental data. In addition, an explanation for the

difference in the contact resistance between Cu/SWNT and Ti/SWNT was proposed. It was found that the

Ti/SWNT system has a magnetic moment without electric field, but no magnetic moment when a certain

value of electric field is applied, which provides a theoretical foundation for fabricating storage devices by

such metal/SWNT structure. The results should be helpful to understand in depth the metal/SWNT and

should also cover all the metal/SWNT structures.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Single-wall carbon nanotubes (SWNTs) are quasi-one-dimen-sional systems, and their thermal, electrical, and mechanicalproperties are unique [1]. The SWNTs are the promising candidatesof materials for numerous breakthrough applications, such asmultifunctional high-strength fiber, coatings, and conducting films[2]. Especially, in recent reports the synthesis of macroscopic SWNTfibers and sheets is developed, and tens of meters of continuousfiber can be easily prepared in a matter of minutes [2]. Thistechnique is an important foundation for bottom-up assembly ofnanotubes and nanorods into functional material. In general, themetallic nanowires are considered important in nanodevices asconducting connects. In addition, there are two possible clues forSWNTs being used as conducting connects. One is coating of metalson the SWNTs. For example, coating of various metals on sus-pended SWNTs is reported [2,3]. Another is to prepare the metal/SWNT composite materials by incorporation techniques [4]. How-ever, the nature of the productions is the same by differentprocesses, which is the metal/SWNT interaction. To understandthe nature of metal/SWNT interaction is a crucial point for theirapplication in nanodevices. Recently, some works on metal/SWNTinteraction have been reported [3,5–7]. However, systematicexperimental and theoretical work with deep understanding hasbeen lacking.

It is clear that the study of adsorption of metal atoms on thesurfaces of SWNTs is helpful for understanding the metal/SWNTinteractions in depth. In addition, it is essential to achieve

ll rights reserved.

5.

g),

low-resistance ohmic contact nanotubes, to produce nanowireswith controllable size, and to fabricate functional nanodevices [6].It is likely that CNT/Cu composite from experimental data [5] issuitable to be used as conducting interconnects of ultralarge scaleintegrated circuits instead of copper, as the CNTs/Cu composite hasbetter electrical properties than those of conventional pure Cu. Butthe relevant mechanism is still not clear. The experimental resultsof metal coating on suspended nanotubes show that Ti coating onthe suspended tubes is continuous or quasi-continuous; never-theless, certain metal coatings are isolated discrete particles on thenanotubes, mostly due to the binding energy of metal [3]. Accord-ing to the theoretical results on the binding energy [6], Cu coatingon the suspended nanotube should not form the isolated discreteparticles continuously. Results of some works also demonstrate thatthe contact resistance of Ti/SWNT is lower than that of Cu/SWNT [8],and even obtained the high-transconductance nanotubes field-effecttransistors with Ti contacts [9]. Some works have been done onmetal atoms adsorption on the SWNT [6], but more relevant studiesare still needed.

2. Computational details

The binding geometry and binding energy, and resulting electro-nic structure of Cu and Ti atoms adsorbed on a (5, 5) armchair SWNThave been calculated by the pseudo-potential plane wave method[10], with the generalized gradient approximation [11]. Spin-unpolarized and spin-polarized relaxation calculations have beenperformed for bare SWNT and single-atom adsorbed SWNT in aperiodically occurring tetragonal supercell, respectively. We havebuilt SWNT with 100 atoms. Ultrasoft pseudo-potentials and planewaves up to an energy cutoff of 300 eV are used. The Brillouin zone of

Fig. 1. Charge distributions of HOMO and LUMO without electric field (left) and

with electric field along z axis (right).

1200its

)Cu/SWNT

L. Wang, Y.F. Zhang / Physica E 43 (2011) 889–892890

the 1�1�10 supercell is sampled by the Monkhorst–Pack scheme[12]. For the adsorption of individual atoms three possible sites wereconsidered. They are labeled as site I above the hexagon, site II abovethe middle of the axial C–C bonds, and site III above the C atoms.Besides single-atom adsorption on the SWNT, some moleculeadsorption on different sites of the SWNT has also been investigated[13]. The binding energies are determined by the expression

Eb ¼ ETðSWNTÞ þETðMetalÞ�ETðMetalþSWNTÞ

in terms of the total energies of the fully optimized bare nanotube(ET(SWNT)), free metal atom (ET(Metal)), and the metal atom adsorbedon the SWNT (ET(Metal+SWNT)). From the calculated results withoutspin-polarized for the system of single-atom adsorbed SWNT, the Cuadatom seems to prefer site II with Eb of about 0.69 eV, and Ti adatomprefers site I with Eb of about 1.62 eV, which are close to the availabledata in the literature [3,6].

The difference in charge densities in the Mulliken scheme wascalculated to analyze the charge transfer between metal adatomsand SWNT. An external electric field was also applied to the metal/SWNT model so that the electronic structures of metal/SWNT canbe analyzed under the external field and without the external field.In this work, an electric field of 0.5 eV/A was applied along the axisof SWNT of Cu/SWNT and Ti/SWNT model. In addition, it wassupposed that SWNT was in a uniform electric field [14]. The lowestunoccupied molecular orbital (LUMO) and the highest occupiedmolecular orbital (HOMO) were also calculated.

-100

400

800

DO

S (A

rb. U

n

E (eV)

S d

p

total

-100

400

800

DO

S (A

rb. U

Uni

ts)

E (eV)

Cu/SWNT

total

p

s d

-8 -6 -4 -2 0 2

-8 -6 -4 -2 0 2

Fig. 2. Total and partial densities of states of Cu/SWNT system: (a) without electric

field and (b) under the electric field. Fermi level is set to 0 eV.

3. Results and discussion

The Mulliken population analysis of Cu/SWNT shows that the Cuatom carries positive charge, and each of the two nearest carbonatoms each carries a little negative charge. Furthermore, thedifference in charge density also demonstrates that charges aretransferred from the Cu adatom to the SWNT. It can be found thatsuch transferred charges increase several times with electric fieldthan without electric field. The Ti/SWNT system is similar. Forexample, in the Ti/SWNT system the Mulliken charge that a Ti atomcarries is 0.719, and the six nearest neighbor C atoms carry is�0.198, �0.145, 0.149, 0.152, �0.193, and �0.140. The atom whichcarries negative charge can aggregate more electrons. If 0.5 eV/Aelectrical field is applied, the Mulliken charge each atom carriesobviously changes to 2.857 for Ti atom and �0.371, �0.325, 0.512,0.549, �0.373, and �0.319 for the six nearest neighbor C atoms.Transferred charges from Ti atom increased about 4 times andcharges were redistributed on the body of SWNT after the electricalfield is applied. The lowest unoccupied molecular orbital (LUMO)and the highest occupied molecular orbital (HOMO) were alsocalculated and are shown in Fig. 1. The corresponding results alsoshow the transfer of electronic charge from metal to SWNT bycomparing the case without electric field to the case under electricfield. It is clear from Fig. 1 that the charge density on the Cu atom ismostly bare with the electric field. Experiments demonstrated thatCNTs/Cu composite thin films have better electrical properties thanthose of conventional pure Cu thin films; however, the correspond-ing mechanism is not clear [5]. Such charge transfer of the metaladsorpbed on the SWNTs may explain the experimental results. Dueto the incorporation of SWNTs into metal matrix in the Cu/SWNTcomposite, SWNTs are enwrapped with a large number of Cu atoms.Refering to the pattern of metal adsorption on the SWNT, underthe electric field the SWNTs act like the liquid channels. The Cufilm provides the free electron ‘‘sea’’. The free electrons prefer toaggregate on the SWNT and flow along the axis, which improve theconductivity of Cu/SWNT composite compared with that of pure Cufilm. Such transfer and aggregation of the charges between CNTs andCu may improve the conductivity. Based on this model, the long

high-conductivity SWNT coating of metal [2,3] may become true forpower transfers in the future.

Fig. 2 shows the total and partial densities of states (DOS) of Cuadsorption of the SWNT (Cu/SWNT) system. It can be found that theCu/SWNT features metallic without electric field as shown inFig. 2(a). In addition, in the vicinity of Fermi level, the relativelylocalized s states of Cu atom can be found and the relativelylocalized d electronic states of Cu atom at about 2.71 eV lower thanthe Fermi level. For Ti/SWNT system the s electronic states of Tiatom are also relatively localized, but the d states of Ti atom aremore extended than those of the Cu atom of Cu/SWNT. The

20-2-40

50

100

150

200

250

DO

S 9

Arb

. Uni

ts)

E (eV)

20-2-4E (eV)

Ti

sp

d total

0

5

10

15

20

DO

S (A

rb. U

nits

)

Ti

total

ds

p

Fig. 3. Total and partial densities of states of Ti atom in the Ti/SWNT system:

(a) without electric field and (b) under electric field. Fermi level is set to 0 eV.

-800-600-400-200

0200400600800

-10

-600-400-200

0200400600

DO

S (A

rb. U

nits

)

E (eV)

spin-up

spin-down

a

Ti/SWNT

spin-up

spin-down

a

-12

-600-400-200

0200400600

-800-600-400-200

0200400600800

DO

S (A

rb. U

nits

)

spin-up

spin-down

b

Cu/SWNT

spin-up

spin-down

b

E (eV)

-8 -6 -4 -2 0 2

-10 -8 -6 -4 -2 0 2

Fig. 4. (a) Spin DOS of Ti/SWNT system without electric field is shown in the upper

figure and under electric field is shown in the subjacent one and (b) spin DOS of

Cu/SWNT system without electric field is shown in the upper figure and under

electric field is shown in the subjacent one. Fermi level is set to 0 eV.

L. Wang, Y.F. Zhang / Physica E 43 (2011) 889–892 891

corresponding results are shown in Fig. 3(a). The localized electro-nic states are unfavorable for binding in the metal/SWNT. It is easyto understand why the binding energy of Cu adsorption on theSWNT is lower than that of Ti adatom. As mentioned above, themetal–C bond can be seen as a ‘‘bridge’’ for the transfer of freeelectrons from the ‘‘electron sea’’ of metal/SWNT composite to theSWNT and then the SWNT acts as a channel for electronic flow. It isalso helpful to understand the nature of metal/SWNT interactions.The relatively localized electronic states in the vicinity of Fermilevel of the metal/SWNT system handicap the electron transfer,which decreases its conductivity resulting in high contact resis-tance. The contact resistance of the Ti/SWNT is much lower thanthat of Cu/SWNT due to its more extended electronic states in thevicinity of Fermi level. When an electric field was applied, the DOSin the vicinity of Fermi level changed remarkably. Firstly, Fig. 2(b)shows that in the vicinity of Fermi level total DOS of Cu/SWNTsystem has larger relative value. Especially, the numbers of thefilled occupied states and the empty unoccupied states increaseobviously at the same time. The total DOS strides over the Fermilevel and extends into the conduction bands. Secondly, the s and ppartial DOS changed obviously. For Cu atom, the relative localized sstates basically vanished and d states became more extended. TheCu–C bond is generally seen as ionic [8]. The electrons of s states ofCu atom may transfer to the C atom that it is binding with. Theresults of the DOS seem to demonstrate that the Cu/SWNT systemeasily has high conductivity. The Ti/SWNT basically has similarresults. However, the DOS of Ti shows that the electrons of d statesare predominant in the Ti/SWNT and the spin-polarization shouldnot be ignored.

Spin DOS of the Ti/SWNT system and Cu/SWNT system areshown in Fig. 4(a) and Fig. 4(b), respectively. Without electric fieldfor the Ti/SWNT system it can be found that the total spin DOS inthe vicinity of Fermi level mainly consists of spin-down DOS, whileat the higher energy side of conduction bands the spin-up DOS ispredominant as shown in Fig. 4(a). The net magnetic moment of theTi/SWNT has a value of about 2.20mB [6]. When electric field was

applied, as shown in Fig. 4(a), the spin-up and spin-down DOSall change. Especially, the features of spin-up and spin-downDOS became more symmetrical. The net magnetic moment of theTi/SWNT decreased close to 0mB. This implies the possibility offabricating the storage devices using such a Ti/SWNT structure.Without external electric field, the system has a magnetic momentthat can correspond to one storage unit (1 or 0). For a certain valueof electric field the system shows no magnetic moment, which cancorrespond to another storage unit (0 or 1). For Cu/SWNT system,in Fig. 4(b), it is shown that the features of spin DOS changeddrastically, but the net magnetic moment remains constant. Theelectrical field has basically no influence on magnetic properties ofCu/SWNT system, whose value is about 0.48mB.

4. Conclusions

In conclusion, the electronic structures and magnetic properties ofmetal/SWNT (metal¼Cu and Ti) were investigated. Models of onemetal atom adsorption on the SWNT were constructed. The resultsshow that the charges transferred from metal atom to C atom. Modelsof electron transfer in the metal/SWNT system without and underexternal electric field was proposed. By the analysis of the DOS of themetal/SWNT without and under electric field, the mechanism ofimproving conductivity from the experimental data [5] may beexplained. In addition, the reason for difference in contact resistance

L. Wang, Y.F. Zhang / Physica E 43 (2011) 889–892892

between Cu/SWNT and Ti/SWNT was provided. When spin polarizationwas considered for the Ti/SWNT, it was found that the Ti/SWNT systemhas a magnetic moment, but there is no magnetic moment when acertain value of electric field is applied. Such interesting results canprovide a theoretical foundation for fabricating storage devices by suchmetal/SWNT. The results should be helpful to understand in depth themetal/SWNT and should also be applicable to all the metal/SWNTstructures.

Acknowledgments

The authors would like to thank Prof. X.S. Chen and Prof. W. Lu atNational Laboratory for Infrared Physics, SITP, CAS, for their helpfuldiscussions and computational support. This work is supported byNational Natural Science Foundation of China no. 50730008, ShanghaiScience and Technology Committee Grant nos. 09JC1407400 and1052nm02000.

References

[1] R.H. Baughman, Science 290 (2000) 1310.[2] V.A. Davis, A.N.G. Parra-Vasquez, M.J. Green, P.K. Rai, N. Behabtu, V. Prieto,

R.D. Booker, J. Schmidt, E. Kesselman, W. Zhou, H. Han, W.W. Adams,R.H. Hauge, J.E. Fiischer, Y. Cohen, Y. Talmon, R.E. Smalley, M. Pasquali, Nat.Nanotechnol. 302 (2009) 1.

[3] Y. Zhang, N.W. Franklin, R.J. Chen, H.J. Dai, Chem. Phys. Lett. 331 (2000) 35.[4] A. Bezryadin, C.N. Lau, M. Tinkham, Nature 404 (2000) 971.[5] P. Liu, D. Xu, Z.J. Li, B. Zhao, E.S. Kong, Y.F. Zhang, Microelectron. Eng. 85 (2008)

1984.[6] E. Durgun, S. Dag, V.M.K. Bagci, O. Gulseren, T. Yildirim, S. Ciraci, Phys. Rev. B 67

(2003) 201401 R.[7] A. Kanda, Y. Ootuka, K. Tsukagoshi, Y. Aoyagi, Appl. Phys. Lett. 79 (2003) 1354.[8] A.N. Andriotis, M. Menon, G.E. Froudakis, Appl. Phys. Lett. 76 (2000) 3890.[9] C.W. Zhou, J. Kong, H.J. Dai, Appl. Phys. Lett. 76 (2000) 1597.

[10] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh,C. Fiolhois, Phys. Rev. B 46 (1992) 6671.

[11] D. Vanderbilt, Phys. Rev. B 41 (1990) 7892.[12] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188.[13] V. Gayathri, R. Geetha, Adsorption 13 (2007) 53.[14] L. Qiao, W.T. Zheng, H. Xu, L. Zahng, Q. Jiang, J. Chem. Phys. 126 (2007) 164702.