Interaction between Atomic Lanthanide Impurities and Ultrashort

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Interaction between Atomic Lanthanide Impurities and Ultrashort Carbon Nanotubes of the Zigzag Type Jianhua Wu Department of Physics, Atmospheric Sciences, and Geoscience, Jackson State University, Jackson, Mississippi 39217, United States

Frank Hagelberg* Department of Physics and Astronomy, East Tennessee State University, Johnson City, Tennessee 37614, United States ABSTRACT: Composites of two atomic lanthanide impurities (2Ln, where Ln = La, Ce, Sm, Gd) and a finite single-walled carbon nanotube (SWCNT) are investigated by use of a plane-wave density functional theory method. A symmetrically truncated SWCNT of the zigzag type is chosen as host species. Endohedral and exohedral configurations of the 2Ln(n,0), system with n = 9, 10 are explored. A molecular dimer solution for Ln = Sm, Gd, involving an Ln2 subunit at the tube center, is found to be substantially less stable than alternative geometries with separated Ln atoms, each localized at a tube edge. Addition of the Ln impurities strongly affects the magnetism of the pure SWCNT host. For the most stable isomers identified, the magnetic structure of 2Ln(n,0) is entirely determined by the magnetic moments of the Ln(4f) shell. Analyzing 2Ln insertion into the host along the SWCNT axis yields the prediction of vanishing energy barrier for the special case of 2Gd(10,0).

I. INTRODUCTION Single-walled carbon nanotubes (SWCNTs) with encapsulated lanthanide atoms are of interest for a diversity of disciplines within materials science and nanotechnology, and various applications have been envisaged for these species. In particular, closed-shell carbon nanostructures containing a lanthanide impurity, both fullerenes and axially bounded nanotubes, are currently discussed as candidates for novel contrast agents in magnetic resonance imaging (MRI).1,2 These could provide a safer and more efficient alternative to the prevailing chelate technology, as the carbon cage is of sufficient stability to prevent the potentially toxic metal species from intruding into the surrounding tissue. Recent laboratory tests have established ultrashort SWCNTs of not more than 100 nm length as highly promising for future use as MRI contrast agents.2,3 Sitharaman et al.2 reported internal loading of SWCNTs 20-100 nm in length with aqueous GdCl3 molecules. By use of nuclear magnetic resonance dispersion (NMRD), the resulting gadonanotubes were investigated with respect to their r1 proton relaxivities. Extremely high effects were found, exceeding the relaxivities of conventional MRI contrast agents, that is, small-molecule Gd3þ chelates (e.g., ref 4), by factors on the order of 40. Apart from their efficiency in the enhancement of MRI signals, various other clinical advantages have been identified for these composites. More specifically, they r 2011 American Chemical Society

may be functionalized for biocompatibility by attaching external groups to their surface,5 are largely bioinert,6 and are also pHsensitive, as they respond, according to recent observation,3 more strongly to low-pH tissue and thus select cancerous over healthy tissue. Further interesting applications may be expected to emerge in other areas than biomedicine. Thus, exceptionally high thirdorder susceptibilities were observed in laser experiments with the metallofullerene Gd2@C80,7 which poses the question of nonlinear optical effects in analogous systems based on SWCNTs. In view of the actual and potential realizations of nanotubes with internal Gd atoms, a theoretical understanding of the architecture of these units, as well as their physical properties, appears desirable. Computational modeling can provide guidance to the efficient fabrication of these systems and also to their applications in varying contexts. While the subject of lanthanide impurities in fullerenes has been treated extensively by computational theory (for a survey, see ref 8), corresponding studies involving SWCNTs seem to be restricted to peapod structures where the lanthanide component is enclosed in fullerenes embedded in SWCNTs.9 Received: December 15, 2010 Revised: January 19, 2011 Published: March 02, 2011 4571

dx.doi.org/10.1021/jp111927r | J. Phys. Chem. C 2011, 115, 4571–4577

The Journal of Physical Chemistry C This paper focuses on the geometric as well as electronic and magnetic properties of SWCNTs interacting with two Ln atoms where the lanthanide species Ln is exemplified by the four elements La, Ce, Sm, and Gd, and finite nanotubes of the (n,0) type with n = 9, 10 are employed. Particular emphasis is placed on the question of lanthanide insertion into the SWCNT. While the experimental configurations described in refs 2 and 3 allowed for Gd transition into the SWCNT through sidewall defects, we will discuss here lanthanide implantation into an intact finite SWCNT. Various models for finite zigzag SWCNTs, differing from each other with respect to their modes of tube edge termination, have been investigated.10 Thus, the tube ends may be partially saturated by addition of H atoms or fullerene hemispheres, or they may be left open, where admission is made for geometric relaxation. We will consider the latter alternative as it results in the formation of carbon polygon rings at both tube ends, which defines a suitable entrance window for the lanthanide guest atoms. It has recently been noted10-13 that finite SWCNTs of the zigzag type are intrinsically magnetic for a dimensional reason. Magnetism emerges here as a consequence of reducing the periodic system to finite length. Unpaired electrons that localize at the edges of the finite structure impose magnetic boundary conditions on the system as a whole and so exert a spin-polarizing effect on the delocalized π-electrons in the interior of the SWCNT. Understanding the parameters that govern finite SWCNT magnetism is a project of both systematic and practical interest and counts among the objectives of the work presented here. With reference to molecular electronics, the resistance encountered by spin-polarized electrons traversing an SWCNT used as transmission element depends on the magnetic state of the SWCNT.14 A traditional way to tailor the magnetism of a closed-shell carbon nanostructure is by encapsulating transitionmetal atom impurities.15 Determining the magnetic structure of a finite zigzag SWCNT with endohedral lanthanide atoms requires assessing the interaction between the magnetic moment imported by the metal atoms with that inherent in the pure SWCNT. Both sources of magnetism may combine to yield a rich variety of magnetic phenomena, to be exploited in spintronics applications. Accordingly, computational treatment of the symmetric implantation of two lanthanide atoms into a finite SWCNT of the zigzag type is faced with three main tasks: modeling the insertion process of the two impurities, determining the equilibrium state adopted by the nanotube-metal composite, and deriving the corresponding magnetic structure.

II. COMPUTATIONAL DETAILS The calculations were performed by density functional theory (DFT) employing a plane-wave basis set as implemented by the Vienna ab initio simulation package (VASP).16,17 More specifically, the generalized Kohn-Sham equations18 were solved by utilizing a residual minimization scheme, namely, direct inversion in the iterative subspace (RMM-DIIS) method.19,20 Geometry optimization was carried out by conjugate-gradient minimization. A kinetic energy cutoff of 400 eV was applied, and a total energy difference between subsequent optimization steps below 1 meV was enforced as convergence criterion. Only the Γ point of the Brillouin zone was included. Admission was made for Gaussian broadening of the one-electron energy levels, where the Gaussian distribution width was set at 10-4 eV. The total energy of the system was defined as the limit of vanishing width.

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The projector-augmented wave (PAW) method21 was used to describe the interaction between valence electrons and core ions. All DFT calculations involved the generalized gradient approximation (GGA) for the exchange-correlation functional as prescribed by Perdew, Burke, and Ernzerhof.22 In order to represent impurities with localized f electrons (namely, Ce, Sm, and Gd), spin-polarized DFT with on-site Coulomb interaction (GGAþU) has been applied. This procedure makes it possible to include the 4f electrons in the valence electron subsystem. The corresponding valence electron configurations for Ce, Sm, and Gd are 4f15s25p65d1s2, 4f65s25p66s2, and 4f75s25p65d16s2, respectively. The GGAþU scheme was realized by adopting the simplified rotationally invariant approach by Dudarev et al.23 The parameters for the on-site interaction (U) and exchange interaction (J) are U = 6.2, 5.4, and 6.7 eV and J = 0.5, 0.6, and 0.7 eV for Ce, Sm, and Gd, respectively. The studied SWCNTs comprise 10 transpolyene chains, corresponding to a length of 2.1 nm. Periodic boundary conditions were imposed on a cubic cell with of dimension 20 20 40 Å3 for all the calculations. From inspection of the converged equilibrium geometries, the nearest-neighbor distance between atoms in adjacent supercells exceeds 12 Å, making the interaction between supercells negligible. Two Ln atoms were initially located along the center axis of the nanotube at the top and bottom positions. Local minima were determined for Ln atoms positioned both outside and inside the nanotube. The energy barrier encountered by the Ln atoms upon entering the SWCNT was evaluated by variation of the Ln atom positions along the center axis of the SWCNT, moving the Ln atoms symmetrically from the outside to the inside of the SWCNT in stepwise progression. At each step, the axial positions of both the Ln atoms and the SWCNT top and bottom layers remained fixed while all other nuclear degrees of freedom were treated as variables.

III. RESULTS In the following, we comment on the geometric and energetic properties of truncated (n,0) SWCNTs (n = 9, 10) tubes in combination with two endohedral Ln (Ln = La, Ce, Sm, Gd) atoms, including the insertion mechanism of the metal atoms into the SWCNT host (section IIIa), where particular attention will be paid to the Gd containing systems. In sections IIIb and IIIc, we report and discuss observations related to the charge transfer and the magnetic order, respectively, characteristic for these units. IIIa. Geometric Structure of 2Ln(n,0) (Ln = La, Ce, Sm, Gd; n = 9, 10) and Insertion Mechanism. We studied the symmetric

insertion of two Gd atoms into an ultrashort SWCNT of the (n,0) type, where n = 9, 10, along the principal axis of the tube. In both cases, the SWCNTs are truncated by cleaving two transpolyene rings, where allowance is made for subsequent relaxation of the tube structure. The ground-state energy difference between this structure and a comparable, less stable species that is generated by an alternative truncation mode, namely, by separating adjacent transpolyene rings, is on the order of 20-25 eV for the (10,0) system with 3-10 transpolyene rings.10 As a consequence of SWCNT host truncation and relaxation, the Gd atoms have to pass through 9- and 10-membered polygon rings upon entering the interior of the (9,0) and (10,0) structures, respectively. In both cases, the total energy of the combined system was recorded as a function of the tube axis coordinate. The symmetric insertion process of the two metal atoms into the tube interior is characterized by an exterior and an interior 4572

dx.doi.org/10.1021/jp111927r |J. Phys. Chem. C 2011, 115, 4571–4577

The Journal of Physical Chemistry C

Figure 1. Total energy of (a) 2Gd(9,0) and (b) 2Gd(10,0) versus the axial SWCNT coordinate. The C atom positions are indicated by vertical dashed lines. The labels out, in, and dimer refer to external, internal, and molecular equilibrium states of 2Gd along the SWCNT axis, as discussed in the text (see section IIIa).

equilibrium state. These result from the interaction between the entrance polygon with an impurity located outside or inside the tube. This is substantiated for the (9,0) host by Figure 1a, where the total cluster energy is shown as a function of the tube axis coordinate. Acknowledging the symmetry of the system with respect to the vertical reflection plane of the tube, we show only the left half of the curve. As illustrated by Figure 2a,b, the two minima that determine the insertion of 2Gd into the (9,0) host are associated with bond formation between each Gd atom and the respective tube-terminating nonagon. The external minimum exhibits a somewhat lower vertical distance from the nonagon ring than the internal minimum, a feature that carries over to the case of 2Gd(10,0) and is also observed for the three other Ln impurities considered in this work, with slight deviation for 2Sm(9,0). Both configurations, the exterior as well as the interior equilibrium, release the strain associated with SWCNT truncation and reflected by the presence of unsaturated bonds in the pure SWCNT host system.10,11 The two minima are separated by a transition state associated with the passage through the tube opening, where the energy barrier toward insertion of 2Gd into the SWCNT is found to be 3.88 eV (see Table 1). We point out that the global minimum of the system 2Gd(9,0) differs from the interior equilibrium state shown in Figure 2b. The most stable state identified in this work involves a deviation

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Figure 2. Representations of (a) external equilibrium state of 2Gd along the (10,0) SWCNT axis, (b) internal equilibrium state of 2Gd along the (10,0) SWCNT axis, (c) most stable configuration of 2Gd in the (10,0) SWCNT, and (d) molecular equilibrium state of 2Gd along the SWCNT axis.

of the impurities from the tube axis, as displayed in Figure 2c. Interaction with the entrance nonagon, or equivalently, the ring of nine adjacent pentagons, is here replaced by bonding to a pentagon-hexagon junction. This feature allows for comparison with recent studies on di- and trigadolinium1,24 systems enclosed in fullerene cages. In fact, novel metallofullerenes of composition Sm2@C10425 have been characterized as nanocapsules, resembling small SWCNTs with suitably modified end-caps.25 In these units, the Ln atom has been shown to bond to a three-hexagon motif. With reference to Gd, a preference for bonding with a single cage hexagon has been observed for endohedral impurities of the type 2Gd1 as well as Gd3N.24 As the distance from the SWCNT edges increases, shallow minima are still adopted wherever the Gd atoms occupy a place near the midpoint between consecutive transpolyene rings, although at substantially higher energy than in the vicinity of the tube ends. Close to the tube midpoint, a pronounced minimum is encountered as the two Gd atoms bond with each other to form a molecular Gd2 substructure, shown in Figure 2d. The equilibrium bond length of this Gd2 molecule is determined to be 2.90 Å. This value is close to the bond distance obtained for gas-phase Gd2 when calculated with the same computational approach as that used to analyze 2Gd(n,0); namely, 2.92 Å. The 4573



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Table 1. Characteristic Parameters for Geometric, Energetic, and Magnetic Properties of 2Ln(n,0) [n = 9, 10; Ln = La, Ce, Sm, Gd]a 2Gd SWCNT

2Sm SWCNT

2Ce SWCNT

2La SWCNT

(9,0)

(10,0)

(9,0)

(10,0)

(9,0)

(10,0)

(9,0)

(10,0)

dLn-C(out), Å

2.498

2.473

2.712

2.742

2.622

2.601

2.658

2.640

dLn-C(in), Å dz(out), Å

2.547 1.251

2.642 0.149

2.686 1.684

2.764 1.344

2.645 1.500

2.735 0.906

2.670 1.563

2.753 1.022

dz(in), Å

1.353

1.072

1.642

1.384

1.523

1.271

1.570

1.320

EFM - EAFM, eV

0.000

0.000

0.003

0.032

0.001

0.001

0.000

0.000

ΔEin-out, eV

3.168

1.262

1.524

1.491

3.221

1.920

3.136

2.089

Ebarrier, eV

3.88

0

8.43

2.15

mFM, μB

14

14

12

12

2

2

0

0

dLn-C, distance between Ln and the closest C atom at the top (bottom) layer. dz, vertical distance between Ln and the top (bottom) layer. EFM - EAFM, energy difference between the states of FM and AFM order. ΔEin-out, energy difference between the external and internal minima of 2Ln (n,0). Ebarrier, energy barrier toward inserting 2Ln into the SWCNT. mFM, magnetic moment in the FM state. a

Figure 3. Radial electron density distributions for 2Sm@(10,0). Upper panel: distributions for SWCNT (blue) and 2Sm (red) subsystems alone, along with that of the entire composite (black). Lower panel: Difference between the distributions of the combined system and the two subsystems {F(r;Sm(10,0)) - [F(r;Sm) þ F(r;(10,0))], see section IIIb}.

latter result is in satisfactory agreement with the quantum chemical calculations of Cao and Dolg,26 who used several types of coupled cluster procedures to investigate Gd2 and arrived at bond distances varying from 2.87 to 2.91 Å. Upon going from the (9,0) to the (10,0) host, a qualitative change of the insertion process is observed. Once more, the most stable state along the tube axis corresponds to the interior minimum. Where an external minimum is expected to occur (labeled out in Table 1), however, a point of inflection is found instead, which suggest that insertion of the two impurities is a barrierless process in this case. At the out position, the vertical distance between the location of Gd and tube entrance decagon is found to be much lower than the corresponding distance for 2Gd(9,0): 0.149 versus 1.251 Å (see Table 1). This difference is a reflection of the different SWCNT diameters, as the distances between Gd and any C atom of the polygon ring at the SWCNT edges are nearly identical in both cases. The plausible location of the external minimum of 2Gd(10,0) is therefore extremely close

to the place of the transition state at the center of the decagon, and both stationary points merge into a slightly slanted plateau structure. This model implies that the two Gd impurities move spontaneously into the (10,0) SWCNT. For the parallel case of 2Sm(n,0), n = 9, 10, the trend encountered in the gadonanotube complex is found as well. From Table 1, the insertion barrier of (9,0) is seen to exceed that of (10,0) by a dramatic margin. For the latter system, however, the barrier does not vanish, contrary to the case of 2Gd(10,0). In the following, we will add some observations on the charge transfer and magnetic properties of 2Gd(n,0), n = 9, 10. IIIb. Electron Transfer. To assess the charge transfer characteristics of 2Ln(n,0) (n = 9, 10), we explored the SWCNT electron density rearrangement due to the presence of the metal impurities at various positions along the SWCNT axis for Ln = Sm and Gd. Figures 3 and 4 display the electronic charge distribution for 2Sm(10,0) and 2Gd(10,0) as a function of the SWCNT radius r, multiplied by a factor 2πrdz, with dz as the length of the SWCNT axis. Both figures refer to the most stable position along the SWCNT axis, as presented in Figure 2b. Specifically, the radial density (r;2Ln(10,0)), as found by integration of F(x) over the angular as well as the axial variable, is shown in Figures 3a and 4a along with the two distributions F(r;2Ln) and F(r;(10,0)), while Figures 3b and 4b show the difference F(r;2Ln(10,0)) - [F(r;2Ln) þ F(r;(10,0)]. Clearly, the electron density distribution of the SWCNT with the two endohedral Ln impurities is determined by two maxima, corresponding to the 2Ln subsystem and the (10,0) host. The density difference distributions shown in Figures 3b and 4b consist of a sequence of alternating maxima and minima, which reflect electron gain or loss, respectively. For both systems, the dominant maximum is found between the regimes of the metal atoms and the SWCNT enclosure. This structure is thus attributed to electron donation from both subsystems into an intermediate zone where covalent interaction takes place. From a comparison between Figures 3b and 4b, this effect is more pronounced for 2Sm(10,0) than for 2Gd(10,0), documenting stronger coupling between the Ln impurities and the SWCNT host in the latter case than in the former. The relation between the two systems is plausible in view of the more complex valence electron system of Gd as compared to Sm. IIIc. Magnetic Order. The finite SWCNT host structures (9,0) and (10,0) are both found to be magnetic, with asymptotic 4574



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Figure 4. Radial electron density distributions for 2Gd@(10,0). Upper panel: distributions for SWCNT (blue) and 2Gd (red) subsystems alone, along with that of the entire composite (black). Lower panel: Difference between the distributions of the combined system and the two subsystems {F(r;2Gd(10,0)) - [F(r;2Gd) þ F(r;(10,0))], see section IIIb}.

Table 2. Parameters for Gd2@(10,0), Gd2@(9,0), and Sm2@(10,0)a Gd2@(10,0)

a

Gd2@(9,0)

Sm2@(10,0)

dLn-Ln, Å

2.873

2.900

3.900

mFM, μB ΔE, eV

22 5.370

22 7.100

12 5.379

dLn-Ln, interatomic distance of Ln2. ΔE = Edimer - Eground.

magnetic moments of 6μB in their ferromagnetic states of lowest energy. We note that the most stable coordination is antiferromagnetic in both cases, with local moments of 3μB at the opposite ends of the SWCNT. The magnetism of the system 2Gd (n,0), with n = 9, 10 at its most stable minimum along the insertion path (see Figure 2b), however, is seen to be governed entirely by the 4f magnetic moments of Gd, as the total ferromagnetic moment amounts to 14μB. The contributions of unpaired C atom spins at the SWCNT edges are therefore completely quenched by the presence of the lanthanide impurities. The same observations are made for the analogous states in 2Ln(n,0) with Ln = La, Ce, Sm and n = 9, 10. The total ferromagnetic moments turned out to be 0 (2μB, 12μB) for Ln = La (Ce, Sm). The 2Ln(n,0) (n = 9, 10) systems discussed here proved to be essentially indifferent with respect to the distinction between ferromagnetic and antiferromagnetic coordination, as documented in Table 1 by the respective energy differences. With the exception of 2Sm(10,0), these were found to be nearvanishing. For Gd2@(n,0), the interaction with local magnetic moments of the SWCNT differs substantially from that observed for 2Gd(n,0), n = 9, 10. For both SWCNT hosts, our computations for the ferromagnetic alternative yield mFM = 22 μB (see Table 2), implying the persistence of SWCNT edge magnetism in these systems. To analyze the magnetic structure of (n,0) with an internal Gd2 dimer in further detail, we computed the linear spin density for Gd2@(9,0) and Gd2@(10,0), as obtained by integrating the spin density over the angular SWCNT coordinate and

Figure 5. (a) Spin density for pure (10,0) SWCNT versus the axial SWCNT coordinate. (b) Spin density of Gd2@(9,0) as a function of the tube length coordinate. Upper panel: spin density of Gd2@(9,0) (red) and (9,0) (black). Lower panel: difference between the spin density of the combined system and the contribution of (9,0) to the overall spin density. (c) Spin density of Gd2@(10,0) as a function of the tube length coordinate. Upper panel: spin density of Gd2@(10,0) (red) and (10,0) (black). Lower panel: difference between the spin density of the combined system and the contribution of (10,0) to the overall spin density.

shown in Figure 5b,c. By comparison with the magnetic structure of the pure SWCNT substrate (Figure 5a), the overall magnetism of the species is understood as an effect resulting from both the Gd2 impurity and the SWCNT edges. For Gd2, a high magnetic moment of 18μB has been predicted by quantum chemical computation26 and detected by electron 4575



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documented by Figure 6, which shows the dominant C(2s,2p) and Gd(5d,4f) lines, subdivided into spin up and down contributions, in the vicinity of the Fermi energy for (a) Gd2@(9,0) and (b) Gd2@(10,0). Two chief observations are made that help rationalize the deviation between the magnetic moments of Gd2 in the two different SWCNTs: (1) the intensity ratio between the Gd2(5d, spin up) lines at about -4.7 and -3.7 eV is reversed as one goes from the (9,0) to the (10,0) host, such that the more prominent line appears at the higher of the two energies in the (10,0) system. (2) In the latter case, the Gd2(5d, spin up) of higher energy nearly coincides with a marked C(2p, spin down) line. This finding suggests pairing between Gd2(5d, spin up) and C(2p, spin down) states for Gd2@(10,0) while the corresponding process is suppressed in Gd2@(9,0) as a consequence of energy mismatch.

Figure 6. Partial density of states for (a) Gd2@(9,0) and (b) Gd2@ (10,0) in the energy interval [-5.0, -3.0 eV]. Upper panels: s and p lines of C, subdivided into spin up and down components. Lower panels: d and f lines of Gd, subdivided into spin up and down components. Vertical parallels indicate the integration limits for evaluation of the Gd2 magnetic moment m.

spin resonance experiment.27 In order to distinguish between magnetic features due to Gd2 and due to the SWCNT host, we integrated the spin density distributions for Gd2@(n,0), n = 9, 10, within the limits indicated in Figure 5b,c, which include the central double peak substructure. For Gd2@(9,0), this procedure yields a partial magnetic moment of 17.9μB and thus a value close to the magnetic moment of the pure Gd2 species. This result demonstrates that encapsulation into (9,0) largely preserves the magnetism of molecular Gd2. The difference between the magnetic moments of the isolated and SWCNT-embedded molecules may be ascribed to a small amount of hybridization between the least strongly bound Gd2 valence electrons and the π electron system of the SWCNT. This is suggested by the Gd2 ground-state configuration (4f7)(4f7)σg2σu1σg1πu2, where the πu2 orbitals are predominantly of 5d character, with 6p admixtures, as determined by natural population analysis.26 For Gd2@(10,0), in contrast, the same integration procedure yields a magnetic moment of 17μB for Gd2, which is lower by 1μB than that of the gas-phase dimer. The characteristic difference between the magnetic structure of the two gadonanotubes is substantiated by partial density of states analysis. This is

IV. CONCLUSION Systems of the form 2Ln(n,0), where Ln = La, Ce, Sm, Gd and n = 9, 10, were studied with respect to the geometric, energetic, magnetic, and charge-transfer features, where finite truncated SWCNTs were chosen as host structures. Emphasis was placed on understanding the insertion mechanism of the Ln impurities into the SWCNT, and a strong dependence on both the nature of the Ln atom and the chiral index n of the host was found. For Sm and Gd as Ln species, insertion into the (9,0) SWCNT requires overcoming a markedly higher energy barrier than into the (10,0) host, corresponding to the smaller tube radius in the former than in the latter case. From our calculations, the insertion barrier vanishes in the special case of 2Gd(10,0), and spontaneous encapsulation of the lanthanide impurity is predicted for this combination. Ln dimer formation within the SWCNT was investigated for three configurations: Gd2@(n,0) (n = 9 and 10) as well as Sm2@ (10,0). In all cases, the Ln2@(n,0) phase is found to be substantially less stable than the 2Ln(n,0) alternative, involving localization of the metal impurities at the SWCNT edges. For all situations considered, the isomers containing Ln2 were seen to be separated from the most stable minima along the insertion coordinate by an energy margin of 5-7 eV. This ordering of stabilities is of direct consequence for the magnetic structure of the composites, implying that solutions with spatially wellseparated magnetic moments are strongly preferred over those with magnetic moments induced by Ln2 and thus localized at the tube center. The spin distributions of the units that contain Ln2 dimers resemble those of the pure truncated SWCNTs, with edge moments of strongly enhanced magnitude, where different schemes of hybridization between the two subsystems, Ln2 and (n,0), may be realized as n is varied. For all species considered, Ln edge localization quenches the magnetic moments of the host, leaving the Ln 4f shell as the source of magnetism. Due to minimal energy differences between parallel and antiparallel orientation of the Gd(4f) moments in 2Gd(n,0), both configurations should appear with equal probability. If suitably prepared magnetic environments at the tube ends are assumed, the 2Gd(10,0) system is conceivable as a component of a spin valve transmission element, involving parallel or antiparallel magnetic moments localized at the SWCNT edges. Such an arrangement would capitalize on the strong magnetic effects associated with Gd 4f shell. Recent studies demonstrated that the magnetic structure of finite truncated SWCNTs of the zigzag type is largely resistant with respect to irregularities in the 4576


The Journal of Physical Chemistry C geometric order of the tube, such as topological defects.28 These results suggest that the same holds for composites described here, as is to be established by future research. Furthermore, in terms of the fabrication and practical application of SWCNTs enclosing Ln atoms, as proposed here, their biocompatibility should be assessed. Both 3d transition metals and lanthanides are known to catalyze the growth of SWCNTs.29 In particular, an extensive body of data exists on the former group of impurities, which has been shown to assist the synthesis of SWCNTs with great efficiency.30 As the toxicity of carbon nanotubes can be ascribed in large part to residual metal impurities within the tube, and transition metals encapsulated in carbon nanotubes have been found to be redox-active at already small traces in the order of some parts per million,31 it will be interesting to study the influence of internal lanthanide species on the electrochemical properties of the ultrashort SWCNTs considered in this paper.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was performed in collaboration with Jackson State University and supported by the DoD through the US Army/ Engineer Research and Development Center (ERDC). Vicksburg, MS Contract #W912HZ-10-C-0107.

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