We have studied the binding energies and electronic structures of metal (Ti,
Al, Au) chains adsorbed on single-wall carbon nanotubes (SWNT) using first
principles methods. Our calculations have shown that titanium is much more
favored energetically over gold and aluminum to form a continuous chain on a
variety of SWNTs. The interaction between titanium and carbon nanotube
significantly modifies the electronic structures around Fermi energy for both
zigzag and armchair tubes. The delocalized 3d electrons from the titanium chain
generate additional states in the band gap regions of the semiconducting tubes,
transforming them into metals.Comment: 4 pages, 3 figure

A. Bachtold

A.N. Andriotis

Chih-Kai Yang

G. Kresse

J.J. Zhao

Jian Ping Lu

Jijun Zhao

K. Kong

M.B. Nardelli

P. Delaney

Y. Zhang

English

arXiv

Crossref

Binding energies and electronic structures of adsorbed titanium chains on carbon nanotubes

We have studied the binding energies and electronic structures of metal (Ti, Al, Au) chains adsorbed on single-wall carbon nanotubes (SWNT) using first principles methods. Our calculations have shown that titanium is much more favored energetically over gold and aluminum to form a continuous chain on a variety of SWNTs. The interaction between titanium and carbon nanotube significantly modifies the electronic structures around Fermi energy for both zigzag and armchair tubes. The delocalized 3d electrons from the titanium chain generate additional states in the band gap regions of the semiconducting tubes, transforming them into metals. Comment: 4 pages, 3 figure

Yang, Chih Kai

Zhao, Jijun

Lu, Jian Ping

Carolina Digital Repository

arXiv:cond-mat/0202150v1  [cond-mat.mtrl-sci]  8 Feb 2002Binding energies and electronic structures of adsorbed titanium chains on carbonnanotubesChih-Kai Yang,1 Jijun Zhao,2 and Jian Ping Lu21 Center for General Education, Chang Gung University, Kueishan, Taiwan 333, Republic of China2 Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USAWe have studied the binding energies and electronic struc-tures of metal (Ti, Al, Au) chains adsorbed on single-wall car-bon nanotubes (SWNT) using first principles methods. Ourcalculations have shown that titanium is much more favoredenergetically over gold and aluminum to form a continuouschain on a variety of SWNTs. The interaction between ti-tanium and carbon nanotube significantly modifies the elec-tronic structures around Fermi energy for both zigzag andarmchair tubes. The delocalized 3d electrons from the tita-nium chain generate additional states in the band gap regionsof the semiconducting tubes, transforming them into metals.61.48.+c, 71.15.Nc, 73.20.Hb, 71.15.HxRecent experiments1,2 have offered definitive evidencethat titanium atoms deposited on a single-wall carbonnanotube are capable of forming continuous wires. Com-pared with other metals such as gold, palladium, iron,aluminum, and lead, Ti atoms have stronger interactionswith carbon nanotube, higher nucleation, and a lowerdiffusion rate. Nanotubes coated with a Ti buffer layeralso have a better chance of forming continuous wiresmade of other metals. Thus, they function as templatesfor growing metal nanowires, which may be suitable forvarious applications in nanoscale materials and devices.A nanotube with adsorbed materials may also signifi-cantly changes its physical properties, providing usefulmeans for manipulating electronic transport for nanoelec-tronic devices. Moreover, understanding the interactionbetween titanium and nanotube will be helpful in reduc-ing the contact resistance between nanotubes and metalleads, which is a critical problem in the nanoelectronicapplications3–5.In this article we present ab initio results on severalSWNTs with adsorbed metal chains. We demonstratetheoretically that Ti is favored energetically over Au andAl, two popular materials for nanowires, for the for-mation of a continuous chain on the carbon nanotubes.Detailed analysis shows significant modification of elec-tronic states of the carbon nanotube after incorporatinga Ti chain. In particular, semiconducting zigzag SWNTstransform into metals upon adsorption of Ti chain.Electronic structure calculations and geometry opti-mization were performed using the Vienna ab initio sim-ulation package VASP,6–8 a plane-wave pseudopotentialprogram. Ultrasoft pseudopotentials were used with auniform energy cutoff of 210 eV. There were 31 uniformlydistributed k points along the nanotube symmetry axisΓX for both the pure SWNT and the SWNT depositedwith a metal chain. Ion positions were relaxed into theirinstantaneous ground states using a conjugate-gradientalgorithm. During each ionic step, a maximum of 120self-consistent cycles were executed and the Hellmann-Feynman forces were calculated. We included two zigzagSWNTs, (10,0) and (14,0), and two armchair ones, (6,6)and (8,8) in our calculations. Different initial configura-tions of the adsorbed metal chains have been used in ourcalculation to obtain the lowest-energy structures.We have optimized the equilibrium structures and cal-culated total energies for each pure SWNT, free-standingmetal chain, and metal-deposited SWNT. The bindingenergy of the metal chain on SWNT is defined as the dif-ference between the total energy of the metal-adsorbedSWNT and the sum of total energies of the individualSWNT and free-standing metal chain. For both Al andAu, we find interaction between metal atoms and thecarbon nanotube is weak. The binding energy obtainedfor aluminum chain on armchair (6,6) SWNT is 0.52 eVper metal atom and that for gold on (6,6) SWNT is only0.25 eV/atom. Similar results are found in the case ofzigzag (10,0) SWNT for Al and Au. The weak inter-actions between Al and carbon nanotube are consistentwith a recent ab initio calculation of the nanotube onaluminum surface5. The low binding energies (≤ 0.5 eV)suggest that the interaction between Al, Au metal andcarbon nanotube is most likely physisorption. Thus, alow barrier against the diffusion of Al, Au adatoms onSWNTs is expected. Between these two metals, Al hasa higher binding energy than Au, confirming part of thesuggested order of binding energy in Ref. 2. With onlyone valence electron in the 3p shell Al is more likely tohave stronger interaction with the nanotube than Au,which has nearly filled outer sd shells.Table I. Binding energy per metal atom Eb and equilib-rium metal-nanotube distance d for adsorbed Ti chainson SWNTs. The tube radiuses are also presented.Tube (10,0) (6,6) (8,8) (14,0)Radius (Å) 3.91 4.07 5.42 5.48d (Å) 2.23 2.09 2.11 2.24Eb (eV) 1.94 2.04 1.62 1.971By contrast, the binding energy between Ti and thenanotube is much greater than the cases of Al and Au, asshown in Table I. It is known that 3d orbitals in titaniumare rather delocalized and function as valence orbitals9.Thus, the unfilled 3d shell of Ti may lead to certain hy-bridization between 3d orbitals of Ti and 2p orbitals ofC, resulting in a stronger interaction between them. In-deed, the calculated binding energies of the adsorbed Tichains range from 1.62 to 2.04 eV per titanium atomfor the SWNTs studied. As shown in Table I, the bind-ing energy is not sensitive to either tube size or chiral-ity. Such higher adsorption energy favors nucleation ofTi atoms and is capable of forming continuous wires, asshown by the recent experiments.1,2 According to an em-pirical estimate10 the diffusion activation energy Ediff isabout a quarter of the binding energy and the diffusionrate is proportional to exp(−Ediff/kBT ), where kB isthe Boltzmann constant and T is the absolute temper-ature. If we follow this scheme and calculate diffusionrates for Ti, Al, and Au, we will see that Ti has a muchsmaller probability than Al and Au to diffuse. A 1.5eV difference in binding energy per atom, for instance,makes Ti harder to diffuse by an order of 10−7 at roomtemperature. Thus, Ti atoms are much more favored forforming the stable structures coated on the surface ofcarbon nanotubes than Al or Au atoms1,2.FIG. 1. Geometric structures of adsorbed Ti chains on(14,0) (top) and (8,8) (bottom) tubes. Along the tube axis,each unit cell contains two Ti atoms for (14,0) tube and oneTi atom for (8,8) tube. Several unit cells are shown for pur-pose of visualization. Distances between the two neighboringTi atoms in the Ti chain are 2.6 Å for (14,0) and 2.7 Å for(8,8) tubes.The fully relaxed atomic structures of Ti chains ad-sorbed on carbon nanotubes are illustrated in Figure 1for (8,8) and (14,0) SWNT respectively. Along the tubeaxis, per unit cell there are two Ti atoms for the zigzagtube and one for the armchair tube. The deformation ofcarbon nanotube due to titanium adsorption is relativelysmall. The deviation from circular tube as measured bythe aspect ratio (rmax/rmin)11 is less than 2% for allSWNTs studied (see also Figure 2(c) and Figure 3(c)).Distances between the two neighboring Ti atoms in theadsorbed metal chain are about 2.6 Å for zigzag tubesand 2.7 Å for armchair tubes, which are smaller than thebulk interatomic distance 2.89 Å and comparable to thebond length in titanium clusters9.-4-3-2-10123kk kX XΓ ΓEFEnergy (eV)020020 EFDensity of State (arb. unit)-4 -2 0 2 4020Energy (eV)FIG. 2. (a) Band structure for a free standing Ti chain(left), a pure (14,0) nanotube (middle), and (14,0) SWNTwith an adsorbed Ti chain (right); (b) Corresponding Densityof states from top to bottom. A Fermi broadening of 0.03 eV isused. (c) Isosurface for the electron density of the four closestbands around the Fermi level for the Ti-adsorbed (14,0) tube.It clearly shows that the conduction electrons near the Fermilevel are delocalized and distributed over both Ti chain andcarbon nanotube.The electronic band structure for Ti-adsorbed (14,0)2nanotube is presented in the right part of Figure 2(a).For comparison the middle part of the diagram showsthe band structure for the pure (14,0) SWNT, and theleft part shows the band structure of the free-standingTi chain. The pure (14,0) SWNT is a semiconductorwith small gap. The adsorbed Ti chain, however, cre-ates several new states in the band gap region, effectivelytransforms the semiconducting SWNT into a metal. Thismetallic behavior is also seen in the bottom of Figure2(b), which displays the density of states (DOS) for theTi-adsorbed SWNT, as opposed to the middle of the fig-ure showing a band gap for the pure (14,0) and the up-per one showing the DOS for the titanium chain. Thesenewly generated bands in the gap region are apparentlyrelated to the delocalized states from the adsorbed Tichain. Moreover, as shown in Figure 2(a), the bandstructure of Ti-adsorbed (14,0) nanotube is not a sim-ple superposition of those of individual nanotube and ti-tanium chain. The significant modification on the elec-tronic states pertaining to titanium and carbon nanotubeimplies certain charge transfer between them.To further explore the interaction between titaniumand carbon nanotube and understand the delocalizedelectronic states around the gap region, Figure 2(c) showsthe isosurface for the electron density of four bands thatclosest to the Fermi level. We find that the electrons fromthese bands are delocalized and distributed on both ti-tanium and carbon atoms. From the band structuresshown in Figure 2(a), we know that the bands aroundFermi level are originally from titanium chain. There-fore, it is not surprising to find certain portion of electrondensity at the titanium sites. Our detailed analysis onthe partial electron local density of states find that theDOS at Fermi level are composed of 3d electrons from ti-tanium and 2p electrons from carbon. The metallizationof semiconducting nanotubes caused by charge transferfrom metal atoms to SWNT has also been found in thealkali-metal intercalated SWNTs12. The delocalized na-ture of the conduction electrons and the charge transferimply a possible good contact between titanium and car-bon nanotube. This finding might be helpful in reduc-ing the contact resistance between nanotube and metalleads3,4. We have also carried out detailed analysis on(10,0) nanotube with adsorbed titanium chain. Similarresults are obtained. Thus, we believe these behaviorsare general for all semiconducting zigzag nanotubes.Shown in the middle part of Figure 3(a) is the bandstructure of the pure (8,8) nanotube. At the Fermi level,there is a well-known band crossing at about two thirdsof the distance between Γ and X , making it a metallicnanotube12. Again, the electronic structures of carbonnanotube have been significantly modified by the tita-nium chain. The band structure of the Ti-adsorbed (8,8)nanotube is not simply the superimposition of those of in-dividual nanotube and titanium chain (see Figure 3(a)).It is interesting to note that the adsorbed Ti chain in-duces a small gap of about 0.14 eV. As shown in theright part of Figure 3(a), an almost dispersionless bandlies just above the Fermi level, while another equally flatband lies lower in energy, both implying large effectivemass for the electron. Such opening of a small gap hasbeen reported for the Cu-adsorbed SWNT13. Due tothe presence of the adsorbed chain, the mirror symme-try of the armchair nanotube is broken and degeneracyof the π and π∗ bands is removed14. After a 0.03 eVFermi broadening, a low but finite value of DOS at theFermi level is found in the bottom of Figure 3(b) for theTi-adsorbed nanotube with a small pseudogap. Similareffect has been found in Ti-adsorbed (6,6) tube, wherethe titanium-induced pseudogap is smaller (∼ 0.1 eV).-4-3-2-10123k k kΓ ΓX XEFEnergy (eV)020020 EF-4 -2 0 2 4020Density of State (arb. unit)Energy (eV)FIG. 3. The same plot of those of Figure 2 but for (8,8)tube.Analysis of charge density for the bands around the3Fermi level shows that they are mostly located on thecarbon nanotube (see Figure 3(c)). Moreover, the circu-lar symmetry of electron density distribution is disturbedby Ti chain, which leads to reduced electron density onthe carbon sites near the Ti atoms. It is worthy to notethat there is much less electron density on titanium inthe case of armchair (8,8) tube than that in the zigzag(14,0) tube. Such difference might be understood by theexistence of conduction bands at the Fermi level in thepure armchair nanotube, which leads to more electrondensity distribution on carbon atoms. Nevertheless, inboth cases there are significant charge transfers from theTi delocalized 3d electrons to carbon nanotube.In summary, we have performed first-principles calcu-lations on metal-adsorbed single-wall carbon nanotubes.We conclude that Ti is much more likely than Al and Auto form continuous chain on the surface of SWNTs dueto much higher binding energies. Al-adsorbed SWNTs inturn have higher binding energies than the Au-adsorbedones. The delocalized 3d electrons from the titaniumchain partially transfer to carbon nanotube and generateadditional states around the Fermi level. The band struc-tures analysis shows that pure zigzag nanotubes trans-form from semiconductors into metals with the adsorp-tion of Ti atoms, while armchair nanotubes may openup a pseudogap due to the symmetry breaking. A noveltype of functional nanostructures can be anticipated withtitanium adsorbed carbon nanotubes. The modificationof electronic properties by the adsorbed Ti atoms alsoprovides a useful means for a new generation of nano-electronic devices.This work was supported by the National ScienceCouncil of Taiwan, the Republic of China, under con-tract number NSC 90-2112-M-182-005 and NASA AmesResearch Center. The authors gratefully acknowledgecomputational support from the North Carolina Super-computer Center and the National Center for High-Performance Computing of ROC.1 Y. Zhang and H. Dai, App. Phys. Lett. 77, 3015 (2000).2 Y. Zhang, N.W. Franklin, R.J. Chen, and H. Dai, Chem.Phys. Lett. 331, 35 (2000).3 A. Bachtold, M. Henny, C. Terrier, C. Strunk, C. Schonen-berger, J. P. Salvetat, J. M. Bonard, L. Forro, Appl. Phys.Lett. 73, 274(1998).4 For a summary, see: A. N. Andriotis, M. Menon, G. E.Froudakis, Appl. Phys. Lett. 76, 3890(2000).5 M. B. Nardelli, J. L. Fattebert, J. Bernholc, Phys. Rev. B64, 245423(2001).6 G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993);Phys. Rev. B 49, 14251 (1994).7 G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11196(1996).8 G. Kresse and J. Furthmüller, Comput. Mater. Sci. 6, 15(1996).9 J.J. Zhao, Q. Qiu, B.L. Wang, J.L. Wang, and G.H. Wang,Solid State Commun. 118, 157 (2001).10 K.L. Chopra, Thin Film Phenomena, (McGraw-Hill, NewYork, 1969).11 J.J. Zhao, A. Buldum, J. Han, and J.P. Lu, Phys. Rev.Lett. 85, 1706 (2000).12 Carbon Nanotube: Preparation and Properties, edited by T.Ebbesen (CRC Press, Boca Raton, Florida, 1997); R. Saito,G. Dresselhaus, and M.S. Dresselhaus, Physical Proper-ties of Carbon Nanotubes (Imperial College Press, London,1998).13 K. Kong, S. Han, and J. Ihm, Phys. Rev. B 60, 6074 (1999).14 P. Delaney, H.J. Choi, J. Ihm, S.G. Louie, and M.L. Cohen,Nature 391, 466 (1998); Phys. Rev. B 60, 7899 (1999).4

Binding energies and electronic structures of adsorbed titanium chains
  on carbon nanotubes

Binding energies and electronic structures of adsorbed titanium chains on carbon nanotubes

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