Capillary Imbibition of PbI2 Melt by Inorganic and Carbon Nanotubes

Jul 9, 2009 - We present the results of molecular dynamics simulations for the capillary imbibition of a drop of an ionic PbI2 melt into inorganic BN,...
0 downloads 0 Views 4MB Size
Download PDF
13664

J. Phys. Chem. C 2009, 113, 13664–13669

Capillary Imbibition of PbI2 Melt by Inorganic and Carbon Nanotubes Andrey N. Enyashin,*,†,‡ Ronen Kreizman,§ and Gotthard Seifert†,| Physical Chemistry, Technical UniVersity Dresden, 01062 Dresden, Germany; Institute of Solid State Chemistry UB RAS, 620990 Ekaterinburg, Russia; and Department of Materials and Interfaces, Weizmann Institute of Sciences, 76100 RehoVot, Israel ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: June 2, 2009

We present the results of molecular dynamics simulations for the capillary imbibition of a drop of an ionic PbI2 melt into inorganic BN, MoS2, and carbon nanotubes. Radial atomic distribution functions are used to characterize the structure of the liquid inside of nanotubes and the kinetics of the penetration. We find that in all cases the PbI2 melt remains liquid but a shell-like structure of the melt along the nanotube axis becomes visible. The filling of MoS2 nanotubes obeys the Lucas-Washburn equation within each shell, whereas the filling of carbon and BN nanotubes follows a modified expression. Both the chemical nature and roughness of the nanotube walls play an important role in the capillary kinetics and the habitus of the embedded liquid. 1. Introduction Carbon nanotubes (CNTs) have attracted much attention in the research due to their specific electronic, thermal, and mechanical properties, giving a great expectation of their successful applications in the creation of materials for microand nanoelectronics as well as reinforced composites,1 though one of the first suggestions in the application of CNTs was based on their genuine cavity structure, making them real nanocapillars and providing their application as a gas storage media and nanocontainers with spontaneous filling of a liquid.2 This idea was proved by Ajayan et al., investigating the wettability and filling of CNTs using the capillary effect with molten lead oxidized in air or with low-melting Bi2O3 and V2O5 oxides.3-5 Acidic solutions of salts, such as nitrates or chlorides, can also enter the cavity of nanotubes.6,7 Furthermore, calcination enables the formation of nanoparticles or nanowires of oxides on the inner surface of CNTs, which can further be reduced to the corresponding metals.8 Such a route could become a basis for the preparation of low-dimensional metallic nanostructures with intriguing electronic and magnetic characteristics. Carbon nanotubes are usually capped; therefore, a successful imbibition of a liquid into CNTs is possible only after oxidation and opening of their caps. In the above-mentioned examples it was achieved by the oxidative ability of the filling medium itself. Otherwise, the insertion of a liquid could only be performed on nanotubes, oxidized before by nitric acid or in an oxidative atmosphere (air, CO2). Several studies have shown that the surface tension of the liquid is important, whether it can enter into an open CNT or not. At values of the surface tension less than 0.1-0.2 N/m, the wetting of the inner concave surface of the nanotube is sufficient to initiate capillary action.8 This condition fails for many pure metals; however, it is fulfilled for many metal oxides and other compounds, including anhydrous metal halides. A big effort was achieved in the fabrication of metal halides as nanowires (KI,9 AgCl1-xIx,10 CuI11) and * To whom correspondence should be addressed. E-mail: Enyashin@ ihim.uran.ru. † Technical University Dresden. ‡ Institute of Solid State Chemistry UB RAS. § Department of Materials and Interfaces. | E-mail: [email protected].

nanostripes (PbI212) by means of capillary insertion of molten salts and the following slow cooling in narrow single-walled CNTs with diameters less than 2 nm. Both extensive microscopic and theoretical investigations of solid halides in such CNTs show that, in many cases, these compounds appear with an atomic coordination which is not typical for their bulk structures (for example, BaI2,13 ErCl314) or even with a partially different stoichiometry (like LaI215). This change in coordination or stoichiometry can change the electronic, luminescent, and magnetic properties. A detailed review about these nanostructures is given by Sloan et al.16 In contrast to the wettability and capillary characteristics of carbon nanotubes, these properties are almost not studied for inorganic nanotubes. BN nanotubes (BNNTs) of hexagonal BN, which is isoelectronic and isostructural to graphite, have been filled using capillary effects by potassium halides.17 There are known other examples of filled BNNTs. However, they are usually fabricated in situ, like BNNTs with encapsulated nanowires of MgO2.18 The well established filling techniques for carbon nanotubes do not work efficiently for BN nanotubes, whereas wetting properties such as contact angles with various liquids and the surface tension of BNNTs are comparable or only slightly different from those for CNTs.19 In our opinion, the failure of a direct capillary entering into free BNNTs can be explained in many cases considering the high chemical endurance of BN, which hampers the opening of the nanotubular caps. Recently, progress in the study of the capillary properties of inorganic nanotubes was outlined by Kreizman et al. using the filling of tungsten disulfide WS2 nanotubes by molten lead iodide.20 However, the process of capillary insertion into this kind of nanotubes has some peculiarities in comparison with CNTs. Due to the larger thickness of a chalcogenide monolayer than a graphene layer, the corresponding nanotubes have larger strain energies.21 As a consequence, the diameters of such inorganic nanotubes are typically an order of magnitude larger than the diameters of CNTs. The high strain energy of chalcogenide hollow nanostructures leads also to the formation of mainly open nanotubes, because corresponding fullerenic structures, which could form the caps of chalcogenide nanotubes, are less stable than carbon fullerenes.22 Thus, chalcogenide

10.1021/jp903649w CCC: $40.75  2009 American Chemical Society Published on Web 07/09/2009

Capillary Imbibition of PbI2 Melt

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13665

TABLE 1: Parameters of Pair Interactions A (in eV · Å12), B (in eV · Å6), and Atomic Charges q (in e) Used in Eq 1 for Molecular Dynamics Simulations of Molten PbI2 Imbibition by Carbon and Inorganic BN and MoS2 Nanotubes 10-3A 0.00

C B+0.66 N-0.66 Mo+1.44 S-0.72 Pb+1.26 I-0.63

B

Pb+1.26

I-0.63

Pb+1.26

I-0.63

185.93 287.37 284.01 701.08 153.81 3.21 678.25

438.31 626.23 672.18 583.96 1060.49 678.25 646.12

77.17 107.25 118.50 639.87 457.36 176.85 763.74

111.38 148.83 171.38 548.93 285.61 763.74 700.75

nanotubes are genuine nanocapillars, and the imbibition of a liquid would be one-step process enabled without any preliminary oxidation. The final product after capillary imbibition and cooling of molten PbI2 within WS2 nanotubes may differ from those of bulk nanocrystallites. Even the formation of multiwall nanotubes of layered PbI2 coaxial to the WS2 nanotubes can be observed.20 This opens new horizons in the fabrication of hollow halide nanostructures, which until now have been prepared on the basis of NiCl2, FeCl2, CdCl2, CdI2, and NiBr2 compounds by laser, electron beam, or thermal evaporation and condensation.23-27 While “classical” methods give a mess of halide nanoparticles with different sizes and morphologies (nanotubes, nanooctahedra, fullerene-like particles), which are unstable against humid atmosphere, capillary filling of chalcogenide nanotubes can offer in the future a production of protected and well crystallized halide nanostructures with more or less a uniform size, depending only on the size of the chalcogenide nanotubes. Evidently, the diameters of chalcogenide nanotubes are large enough to provide a nanocoating of the inner surface by a halide and not only the formation of halide nanocrystal or nanowire in the cavity of the nanotube. However, the possible growth mechanism of halide nanotubes within nanotubes of chalcogenides remains unclear. There are two obvious possibilities, either Via imbibition of a continuous drop of melt and its uniform distribution over the inner tubular surface before crystallization or by diffusion of ions along the inner surface from a drop located nearby the end of a nanotube. In order to get a better understanding of the structure formation inside of nanotubes by capillar filling with liquids, we will compare in this paper the capillarity properties of inorganic BN and MoS2 nanotubes with those of carbon nanotubes. As an example, we have investigated the imbibition of PbI2 into carbon, BN, and WS2 nanotubes. The insertion of PbI2 was also experimentally studied on carbon12 and inorganic nanotubes.20

Figure 1. Snapshots for imbibition of molten a PbI2 drop by (25,25) carbon, (25,25) BN, and (21,21) MoS2 nanotubes (from left to right) at different times. For clearness, the front walls of nanotubes are removed.

Figure 2. Mean radial (black) and axial (gray) atomic velocities of a PbI2 melt entering into (25,25) carbon (1), (25,25) BN (2), and (21,21) MoS2 nanotubes (3). Accelerating growth of axial velocity in the case of C and BN nanotubes shows the absence of an essential friction force after the full imbibition of melt.

2. Calculations As the models for the study of imbibition of molten PbI2 into carbon and inorganic nanotubes, we have chosen single-walled (25,25) carbon, (25,25) BN nanotubes consisting of 50 unit cells, and (21,21) MoS2 nanotubes consisting of 40 unit cells with open ends. The methodologies for the construction of the carbon, BN, and MoS2 nanotubes (MoS2 NTs) were reported earlier.1,21 All these tubes have similar geometrical parameters: the lengths and their inner radii are nearly the same: 123.0 Å for CNT and BNNT and 126.4 Å for MoS2 NT, and 16.95 Å and 16.71 Å, respectively. The melt of PbI2 was modeled as a drop composed of 840 atoms (280 stoichiometric units of PbI2). In our theoretical description of capillary filling, we will not consider

a steady-state capillary flow but will pay attention to the filling of nanotubes by a finite amount of matter, as is realized in the experiments.12,20 For the calculations we have employed an effective pair potential, which is based on the Born-Mayer model. This approach is widely used for the simulations of the stability and structure of many ionic compounds in bulk structures and nanostructures and considers the information about the formation and stability of KI, KBr, and AgI nanowires and clusters in narrow CNTs.28-30 The long-range Coulomb interaction between two atoms (i and j) with charges qi and qj at a distance rij is described by the pair potential

13666

J. Phys. Chem. C, Vol. 113, No. 31, 2009

Vij )

Aij Bij e2 qiqj + 12 - 6 4πε0 rij rij rij

Enyashin et al.

(1)

where Aij and Bij are the parameters specific to each type of pair interaction modeling short-range forces (repulsion of nuclei, repulsion of electron shells, van der Waals forces). In our calculations, we used the parameters, which were fitted and successfully applied for bulk PbI2, carbon, and BN nanotubes.31-33 Missing parameters for pair interactions between Pb and I ions with C, B, N, Mo, and S atoms were chosen by applying the Lorentz-Berthelot mixing rules. To estimate the atomic charges q for PbI2, BN, and MoS2, we applied the simple Pauling scheme using Pauling electronegativities.34 All parameters used in eq 1 are listed in Table 1. 3. Results and Discussion The penetration of a PbI2 drop was investigated using molecular dynamics simulations under the conditions of an NVT ensemble with the temperature T ) 1000 K. During the MD simulation the atomic coordinates of the nanotubes were fixed; that is, the nanotubes were assumed to be rigid. All the molecular dynamics simulations were performed for up to 1 ps for carbon and BN nanotubes, whereas for the case of the MoS2 nanotube it was extended up to 2 ps. Already a first view at the insertion of molten PbI2 into carbon, BN, and MoS2 nanotubes shows two different types of this process (see Figure 1 and the Supporting Information). CNT

and BNNT behave nearly identically. At the initial step, a formation of a convex meniscus is obtained due to the weak wetting of graphene-like surfaces by an ionic melt. At the initial stage, the penetration of the melt into the nanotubes is quite slow in the case of the CNT and the BNNT in comparison to the MoS2 nanotube. Pb and I ions, adsorbed on the outer surface of a tube wall, were not observed at all for the CNT and the BNNT, which is evidence for stronger interatomic forces within the PbI2 drop, than with C, B, and N atoms of the tubes. As the second step in the melt imbibition starts, the radius of the outer segment becomes close to the radius of the already penetrated part. At this point, a fast acceleration of the drop can be observed, which is not bothered by friction-like forces due to the high smoothness of a graphene-like surface (Figure 2). Such slippage causes a fast motion of the molten PbI2 drop until it reaches the opposite end of the tube. Afterward the drop slows down and returns to the middle of tube. The only difference between capillary filling of carbon and BN nanotubes consists in the velocity of imbibition. In the case of the BN nanotube the melt enters into the cavity faster and already after ∼0.4 ps, whereas for the carbon nanotube it takes about 0.65 ps. The capillary imbibition of the PbI2 melt into the MoS2 nanotube has a different character. First, in contrast to graphenelike nanotubes, the penetration of the PbI2 melt is accompanied by the partial adsorption of Pb and I ions on the outer surface of the tube. Second, it starts with a rapid adsorption of Pb and I ions at the edge of the inner nanotube wall, and a concave meniscus of a monatomic thickness at its edge occurs almost

Figure 3. Distribution functions gij(r) for interatomic distances in molten PbI2 as a free drop and the drops within different nanotubes after 1 ps.

Capillary Imbibition of PbI2 Melt

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13667

Figure 4. Average number of Pb and I atoms 〈N〉 found a certain distance from a tube axis in the drop of molten PbI2 penetrated into (21,21) MoS2 (1), (25,25) BN (2), and (25,25) carbon nanotubes (3) after t ) 1 ps. The views on cross sections along the tubes’ axes demonstrate the shell-like character of the liquid PbI2 within these nanotubes.

immediately. Since the average interatomic distance between the sulfur atoms in the MoS2 NTs is considerably larger than that in a CNT or a BNNT, the inner sulfuric surface in a MoS2 NT has a more pronounced roughness than the inner surface of CNTs and BNNTs. This more pronounced roughness obviously hinders an acceleration of the penetration into the nanotube. Therefore, the penetration of the PbI2 drop into the nanotubes takes (with ∼1 ps) considerably longer than in the case of the CNT and the BNNT. Afterward, the growth of a meniscus along the wall continues by diffusion of atoms from the inner part of the melt, and second, a concave meniscus appears at the rear part of the injected drop. The kinetics of the penetration process can be visualized by means of the radial and axial (relative to a tubes axis) velocities of the atoms within the PbI2 drop (Figure 2). The change in axial velocities during time illustrates well the basic steps in PbI2 entering, as was described above: more or less constant invasion of the melt into MoS2 NT with a velocity of ∼20 m/s and the presence of an induction time in the case of CNT and BNNT, after which the axial velocity increases up to ∼100 m/s. Moreover, a comparison of axial velocities for the CNT and the BNNT shows that the interaction between the ionic melt and the polar BN surface is stronger than that in the case of a pure covalent carbon surface. This can be seen from the rapid increase in the velocity for the melt entering BNNT, which happens at an earlier time, and a very rapid deceleration after the insertion of the drop. The penetration of liquid lead iodide into a nanotube raises the question whether the phase state of PbI2 is kept after its

insertion. The analysis of the radial distribution functions gij(r) for Pb-I, Pb-Pb, and I-I within a nanotube and in the free drop shows that the PbI2 melt still remains a liquid and is not affected by the character of the nanotube wall (Figure 3). There is no essential difference in the shape and intensity profiles between gij(r) for the free PbI2 drop and inside the nanotubes. The peaks in these profiles have very similar positions. The first peak in gPbI(r) function is located at 2.3 Å, which corresponds also to the value of the Pb-I bond length in the bulk compound PbI2.35 A view on the cross sections of the final atomic structure for PbI2 melts along the tubes’ axes shows a possible shell-like structure of the Pb and I ions in the nanotubes (Figure 4). This observation is supported by the radial distribution function of the Pb and I atoms 〈N〉. A very clear alternation of shells can be seen, especially for the BNNT. The dependence of 〈N〉 as a function of time shows that a quite pronounced shell-like structure of the liquid PbI2 appears already at the beginning of the imbibition and becomes more evident during the penetration process (Figure 5). However, even after a long time, the most pronounced shells appear close to the nanotube walls, whereas the liquid in its middle part remains disordered. We have considered the filling of nanotubes by a drop of finite size. It could be interesting to investigate the validity of macroscopic phenomenological equations known for a steadystate capillary flow in the case of nanotube filling. The Lucas and Washburn theory for a capillary flow36 was derived for an incompressible Newtonian fluid in a circular tube. It gives the dependence of the height h of the adsorbed liquid column

13668

J. Phys. Chem. C, Vol. 113, No. 31, 2009

Enyashin et al. TABLE 2: Mean Radii for Pb and I Shells within Molten PbI2 Injecting Different Nanotubes and Parameters a and b for Eq 2 nanotube (25,25) C (25,25) BN (21,21) MoS2

rPb, Å

a

b

r I, Å

a

b

14.4 11.2 6.5 14.4 11.2 6.5 14.2 10.8 6.8

0.692 2.520 2.494 4.410 5.184 7.334 10.143 2.490 2.252

0.017 0.142 0.196 0.070 0.109 0.214 0.000 0.000 0.000

13.2 9.0 5.1 13.2 9.0 5.1 13.2 9.2 5.0

3.411 1.900 2.529 8.530 5.439 6.109 8.862 2.307 1.642

0.160 0.052 0.162 0.113 0.091 0.179 0.000 0.000 0.000

The same eq 2 can be used also for the case of the MoS2 nanotube, although the parameter b is found to be equal to 0, which means that the penentration of a molten PbI2 drop in this case may be well described using classical Lucas-Washburn theory even at small t. This behavior is obtained for all the shells and at t < 1.0 ps (before the full drop is penetrated). After the second meniscus appears, the shells closest to the nanotube wall will continue to grow using the inner part of the melt as a source with the same a, b parameters, whereas for inner shells the equation will not be valid anymore. 4. Conclusions

Figure 5. Evolution of the average number of Pb and I atoms 〈N〉 found at a certain distance from a tube axis 〈N〉 during time in the drops of molten PbI2 penetrating into different nanotubes.

(penetration length) at time t as h ∼ t. It was found that this dependence may fail at early times, when the liquid is accelerated by the capillary forces, and inertial effects can lead to h ∼ t2 or h ∼ t dependencies for the meniscus rise.37,38 Our simulations show that the filling of nanotubes by a drop of liquid PbI2 can be described by the formation of separate Pb and I shells within this drop. In the case of carbon and BN nanotubes, this formation does not follow the Lucas-Washburn equation (〈N〉 ∼ t), but it can also not be described by a 〈N〉 ∼ t2 dependence for an initial flow behavior. However, as a good description for the dependence of 〈N〉 on t, the following conciliating function can be used:

〈N〉 )

at2 b + t3/2

(2)

which at small t behaves like t2 and at high t approaches t dependence. This expression reflects the existence of a conduction time and initially slow penetration of the ionic melt into CNT or BNNT. For both carbon and BN nanotubes, three Pb and three I shells can be found with their own a and b parameters, which are valid until t < 0.5-0.6 ps, i.e. before the penetration process is complete and the drop is fully inserted (Table 2).

In summary, the way how inorganic nanotubes imbibe a fluid, particularly a molten salt, is of broad interest. We have investigated the imbibition of molten PbI2 salt in carbon and inorganic BN and MoS2 nanotubes by molecular dynamics simulations at 1000 K. Our results show that the main difference in the capillary filling process among all three kinds of nanotubes with the same inner radius will be caused by the structure of the walls. Graphene-like nanotubes of carbon and BN, having weak interaction with the melt, need a larger penetration time than a nanotube of MoS2 having stronger van der Waals’ interaction, due to the S atoms. Though smooth surfaces of carbon and BN nanotubes provide a fast and friction-free saltdrift into these nanotubes, the sulfur surface of MoS2 does not support an acceleration of the molten PbI2 drop. All nanotubes have a complex dynamics of the filling, which releases in a shell-like structure of embedded liquid. The filling of MoS2 NTs obeys a Lucas-Washburn equation within every shell, whereas the filling of CNT and BNNT needs a modified expression, as described above. Both factors, the strength of interaction and friction with the surface of a nanotube, also play an important role for the final morphology and the location of a “sucked” drop. In the case of a CNT with such a large radius as considered here, one may expect that a drop of an ionic melt can move in a tube far from the entrance point. In the case of a BNNT, a drop of a salt will be located inside of the tube but close to the edge. A localization more deeply in the tube could be achieved through thermal diffusion during a long time of heating in a furnace. After cooling, in both cases a fragment of bulk compound (nanowire or nanocrystal) will be formed because the drops of ionic melt within CNTs and BNNTs were observed with two convex menisci. A different situation may be obtained in the case of a MoS2 NT due to the occurrence of concave menisci. Good wettability during a long time can lead to the total spread of melt over the inner surface of the nanotube, which can later crystallize into cylindrical nanostructures. Thus, not numerous experimental data17,20 about capillary filling of inorganic BN and MoS2 nanotubes are supported by our molecular-dynamics

Capillary Imbibition of PbI2 Melt simulations. Certainly, the present study is only a first step into the understanding of filling and crystallization processes of salts in inorganic nanotubes. Many more experimental and theoretical studies are needed. Acknowledgment. The visualization of atomic structure and MD trajectories were performed using VMD software.39 The authors are grateful to the support from the European Commission (ERC Grant INTIF 226639). We thank Prof. Reshef Tenne from the Weizmann Institute of Science (Israel) for helpful discussions. Supporting Information Available: Dynamics movies for the capillary imbibition of a drop of ionic PbI2 melt into carbon (25,25), BN (25,25), and MoS2 (21,21) nanotubes.This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jorio, A., Dreselhaus, G., Dresselhaus, M. S., Eds. Carbon Nanotubes, Topics in Applied Physics; Springer-Verlag: Berlin Heidelberg, 2008; Vol. 111. (2) Pederson, M. R.; Broughton, J. Q. Phys. ReV. Lett. 1992, 69, 2689– 2692. (3) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333–334. (4) Ajayan, P. M.; Ichihashi, T.; Iijima, S. Chem. Phys. Lett. 1993, 202, 384–388. (5) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564–567. (6) Tsang, S.C.; Chen, Y. K.; Harris, P. J. P.; Green, M. L. H. Nature 1994, 372, 159–162. (7) Sloan, J.; Chen, Y. K.; Zweifka-Sibley, M.; Green, M. L. H. Chem. Commun. 1998, 347. (8) Dujardin, E.; Ebbesen, T. W.; Hiura, H.; Tanigaki, K. Science 1994, 265, 1850–1852. (9) Meyer, R. R.; Sloan, J.; Dunin-Borkowski, R. E.; Kirkland, A. I.; Novotny, M. C.; Bailey, S. R.; Hutchison, J. L.; Green, M. L. H. Science 2000, 289, 1324–1326. (10) Sloan, J.; Terrones, M.; Nufer, S.; Friedrichs, S.; Bailey, S. R.; Woo, H.-G.; Ru¨hle, M.; Hutchison, J. L.; Green, M. L. H. J. Am. Chem. Soc. 2002, 124, 2116–2117. (11) Kiselev, N. A.; Zakalyukin, R. M.; Zhigalina, O. M.; Grobert, M.; Kumskov, A. S.; Grigoriev, Y. V.; Chernysheva, M. V.; Eliseev, A. A.; Krestinin, A. V.; Tretyakov, Y. D.; Freitag, B.; Hutchison, J. L. J. Microscopy 2008, 232, 335–342. (12) Flahaut, E.; Sloan, J.; Friedrichs, S.; Kirkland, A. I.; Coleman, K. S.; Williams, V. C.; Hanson, N.; Hutchison, J. L.; Green, M. Chem. Mater. 2006, 18, 2059–2069. (13) Sloan, J.; Grosvenor, S. J.; Friedrichs, S.; Kirkland, A. I.; Hutchison, J. L.; Green, M. L. H. Angew. Chem., Int. Ed. 2002, 41, 1156–1159.

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13669 (14) Kitaura, R.; Ogawa, D.; Kobayashi, K.; Saito, T.; Ohahima, S.; Nakamura, T.; Yoshikwa, H.; Awaga, K.; Shinohara, H. Nano Res. 2008, 1, 152–157. (15) Friedrichs, S.; Falke, U.; Green, M. L. H. Chem. Phys. Chem. 2005, 6, 300–305. (16) Sloan, J.; Chen, Y. K.; Kirkland, A. I.; Hutchison, J. L.; Green, M. L. H. Chem. Commun. 2002, 1319–1322. (17) Han, W.-Q.; Chang, C. W.; Zettl, A. Nano Lett. 2004, 4, 1355– 1357. (18) Golberg, D.; Bando, Y.; Mitome, M.; Fushimi, K.; Tang, C. Acta Mater. 2004, 52, 3295–3303. (19) Golberg, D.; Bando, Y.; Tang, C.; Zhi, C. AdV. Mater. 2007, 19, 2413–2432. (20) Kreizman, R.; Hong, S. Y.; Sloan, J.; Popovitz-Biro, R.; AlbuYaron, A.; Tobias, G.; Ballesteros, B.; Davis, B. G.; Green, M. L. H.; Tenne, R. Angew. Chem., Int. Ed. 2009, 48, 1230–1233. (21) Seifert, G.; Terrones, H.; Terrones, M.; Jungnickel, G.; Frauenheim, T. Phys. ReV. Lett. 2000, 85, 146–149. (22) Enyashin, A. N.; Gemming, S.; Bar-Sadan, M.; Popovitz-Biro, R.; Hong, S. Y.; Prior, Y.; Tenne, R.; Seifert, G. Angew. Chem., Int. Ed. 2007, 46, 623–627. (23) Rosenfeld Hacohen, Y.; Grunbaum, E.; Tenne, R.; Sloan, J.; Hutchison, J. L. Nature 1998, 395, 337–338. (24) Popovitz-Biro, R.; Twersky, A.; Rosenfeld Hacohen, Y.; Tenne, R. Isr. J. Chem. 2001, 41, 7–14. (25) Rosenfeld Hacohen, Y.; Popovitz-Biro, R.; Prior, Y.; Gemming, S.; Seifert, G.; Tenne, R. Phys. Chem. Chem. Phys. 2003, 5, 1644–1651. (26) Popovitz-Biro, R.; Sallacan, N.; Tenne, R. J. Mater. Chem. 2003, 13, 1631–1634. (27) Bar-Sadan, M.; Popovitz-Biro, R.; Prior, Y.; Tenne, R. Mater. Res. Bull. 2006, 41, 2137–2146. (28) Bishop, C. L.; Wilson, M. Mol. Phys. 2008, 106, 1665–1674. (29) Baldoni, M.; Leoni, S.; Sgamellotti, A.; Seifert, G.; Mercuri, F. Small 2007, 3, 1730–1734. (30) Yang, C.; Zhu, X.; Lu, X.; Feng, X. THEOCHEM 2009, 896, 6– 11. (31) Winkler, B.; Dove, M. T.; Salje, E. K. H.; Leslie, M.; Palosz, B. J. Phys.: Condens. Matter 1991, 3, 539–550. (32) Lee, J. H. J. Korean Chem. Soc. 2006, 49, 172–176. (33) Brunier, T. M.; Drew, M. G. B.; Mitchell, P. C. H. J. Chem. Soc., Faraday Trans. 1992, 88, 3225–3232. (34) Pauling, L. The nature of the chemical bond; Cornell University Press: 1960. (35) Konings, R. J. M.; Cordfunke, E. H. P.; van der Laan, R. R. J. Alloys Compd. 1995, 230, 85–88. (36) Washburn, E. W. Phys. ReV. 1921, 17, 273–283. (37) Zhmud, B. V.; Tiberg, F.; Hallstensson, K. J. Colloid Interface Sci. 2000, 228, 263. (38) Supple, S.; Quirke, N. Phys. ReV. Lett. 2003, 90, 214501(1-4). (39) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33–38.

JP903649W