The growth of one-dimensional (1D) nanocrystals represents an important research topic in crystal engineering for nanotechnology. The growth of 1D fullerene (C60) nanocrystals (or nanowires) has proven to be of considerable scientific and technological interest because of the properties associated with the low-dimensionality, quantum confinement effect, and potential electronic, magnetic and photonic applications.
Figure 1. Fullerene (C60) nanowire polymer could be more attractive than carbon nanotubes in many cases especially for bio-applications
In a recent study [Geng, et al., JACS 130, 2527 (2008); Geng, Solov'yov, Zhou, Solov'yov, Johnson, (2009)], it was demonstrated that exceptionally long fullerene nanowires, with a length-to-width aspect ratio as large as ~3000 (see Fig. 1), can be grown from 1,2,4-trimethylbenzene (TMB) solution of C60. These nanowires, denoted as C60•TMB, have been observed to possess a highly unusual shape that retains unchanged even after removing the solvent at elevated temperatures. This excellent property has offered a new approach to the formation of a fullerene-based carbon 1D nanostructure, but importantly, without the involvement of any metal species as the growth catalyst. Consequently, the normally employed post-growth purification process for the removal of metal is no longer necessary, in a marked contrast to the chemical vapor deposition (CVD) technique for growing carbon nanotubes.
Optimization of the unit cell
By fixing the fullerene molecules to the experimentally observed positions (see Fig. 1) and introducing TMB molecules into the unit cell, we performed structural optimization using conjugate gradient method implemented in the MBN Explorer program (see our MBN Explorer website). The structures of the stable, low-energy isomeric configurations of the unit cell are shown in Fig. 2. The energies calculated for these isomers are indicated in the figure. A specific structure of the unit cell isomer depends on the relative orientation of C60 and TMB molecules and also on the location of the TMB molecules inside the cell. We found that the orientation of the C60 molecules may affect the energetic of the unit cell but has a minor impact on the relative adhesion energy along the three principal growth directions. However, the total energy of the unit cell and the relative adhesion energy largely depend on the location and orientation of TMB molecules inside the cell; for example, in isomer 6, E = -1.528 eV, but in isomer 1, E = -1.899 eV. This relation can be clearly seen in Figure 2.
Figure 2. Optimized isomeric states in the C60•TMB nanowire unit cell as derived from the calculations. The number below each image shows the energy of the structure (in eV). The coordinate frames used in the present work are also indicated.
We considered [Geng, Solov'yov, Zhou, Solov'yov, Johnson, (2009)] how the different isomeric structure of the unit cell would affect the crystal shape in the C60•TMB nanowires. Figure 3 shows the saturated adhesion energy, corresponding to the adhesion energy of a unit cell to an infinitely thick crystal having 3x3 and 4x4 unit cells in the cross section. Comparing the values suggests that the growth along the c-axis would be most difficult, consistent with the experimental observation.
Figure 3. Adhesion energy of a unit cell to an infinitely thick crystal along the a- (squares), b- (circles), and c-nanowire growth directions (triangles) calculated for different isomeric states of the crystal's unit cell. (a) and (b) correspond to the adhesion of a nanolayer consisting of 3x3 and 4x4, cells respectively.
However, depending on the type of isomer, the lowest adhesion energy could be along either the a or b direction. The adhesion energy difference between a and b, can be sufficiently large, as for example, -0.30 eV for isomer 1 and +0.15 eV for isomer 6. Such a large energy difference strongly suggests that the crystals with a given isomeric structure would preferably grow along one direction that leads to the formation of a wirelike 1D shape. For isomer 1, this direction is axis a; but for other isomers, it is axis b.
Close examination of the two growth possibilities along either the direction a or b also indicates that the degree or extent of their growth is different. When certain crystals grow along a (such as for isomer 1), their simultaneous growth along b would be significantly suppressed because of the much higher adhesion energy in the b direction (~0.30 eV higher). In this case, the crystals favorably grow almost only along a, which could result in the development of a thin nanowire. However, if a crystal grows along b (such as for isomer 3), its simultaneous growth along a may still be able to reasonably develop because of the relatively smaller adhesion energy difference between the a and b (0.07...0.15 eV, depending on the structure of the isomer). This is the case where a crystal is more likely to grow into a thick nanowire with wider wings. Because there are more isomeric structures favoring the growth along the b-axis, as illustrated in Figure 3, it is expected that in comparison with the thin nanowires, more crystals would grow into thick ones possessing wider wings, and this has been confirmed by our extensive electron microscopic observations [Geng, Solov'yov, Zhou, Solov'yov, Johnson, J. Phys. Chem. C 113, 6390 (2009)].
Effect of electron polarization
The effect of electron polarization has been originally suggested to aid the formation process of the C60-based nanowires [Solov'yov, Geng, Solov'yov, Johnson, Chem. Phys. Lett 472, 166 (2009)]. However, the performed theoretical estimates show that at room temperature the effect of electron polarization is negligibly small and, therefore, cannot become the driving force for nanowire growth along one preferential direction. The adhesion energy of a single fullerene due to the polarization forces in the system was estimated as 9.0365x105 (eV). It is almost negligible since the energy of thermal vibrations in the system at 300 K is approximately 0.026 eV.
Figure 4. Scanning electron microscopic (SEM) images of the fullerene (C60) crystals (either in the form of wires or particles) grown by using the solvents of 1,2,3-trimethylbenzene (a), 1,2,4-trimethylbenzene (b), 1,3,5-trimethylbenzene (c), benzene (d), and toluene (e). The insets of (a), (b) and (c) are the corresponding optical microscopic images, with the scale bars being 2 µm, 10 µm and 200 µm, respectively. The insets of (c) and (d) are SEM images of higher magnifications to show the individual crystalline particles, with the corresponding scale bars being 5 µm and 10 µm, respectively.
The theoretical analysis may be compared with the experimental result. As shown in Figure 4, toluene, 1,2,3-TMB and 1,2,4-TMB are all polar molecules, but benzene and 1,3,5-TMB are non-polar owing to their symmetrical structures. This conclusion follows from the ab initio B3LYP/cc-pVTZ calculation. The crystallization in benzene did not result in nanowires, as expected, but only flake-like small particles (Figure 4d). However, crystallizations of C60 from 1,2,3-TMB and toluene did not yield the nanowire-like crystals as well, even though they are polar and thus able to cause the fullerene to polarize (Figure 4a and e). This is in a clear contrast to the 1,2,4-TMB case where massive C60 nanowires were formed (Figure 4b). Interestingly, we also observed that 1,3,5-TMB did assist the massive growth of nanowires which morphologies resemble those yielded in the 1,2,4-TMB solution (Figure 4c), although this molecule is non-polar and not able to cause any electron polarization to C60. These experimental results are clearly in agreement with the above theoretical analysis.
The solubility of the nanowires was found to change with time. Unlike raw C60 powder, well known to be highly soluble in aromatic solvents, the as-made nanowires were only partially soluble in these solvents, and this solubility decreased further with time [Geng, Solov'yov, Reid, Skelton, Wheatley, Solov'yov, Johnson, Phys. Rev. B 81, 214114 2010]. After ageing for a period of ~10 months, the nanowires became totally insoluble in common organic solvents including benzene, toluene, 1,2,4-TMB, and carbon tetrachloride. This initial observation led us to speculate that a polymerization reaction might have occurred within the system, and that this had led to interesting new physical and chemical properties, including excellent thermal stability and solvent resistant behavior.
Figure 5. A schematic shows the polymerization reaction pathway. The label X in the dimer indicates a residue derived from the TMB molecule during the reaction. Hydrogen atoms are omitted in this diagram for clarity.
Detailed investigations of the polymerization reaction mechanism and the nature of the associated bonding mode have been undertaken theoretically. A possible polymerization scenario has been considered, in which two C60 molecules and a TMB molecule form two methylene bridges (see Fig. 5). The product predicted from the reaction between two C60 molecules and one TMB molecule is a molecular complex, C60XC60, where X denotes a residue derived from the TMB molecule. There are two possibilities regarding the reaction pathway:
where TMB-2H represents the TMB molecule with two hydrogen atoms removed and ΔE1 and ΔE2 are the corresponding reaction enthalpies.
In reaction (1), two C-H bonds in the TMB molecule (likely at the 1- and 4-methyl positions) are broken and two C-C60 bonds are formed. As a consequence, an H2 molecule is released. Reaction (2) occurs in a similar way, but instead of forming a H2 molecule, the released H-atoms bond to either fullerene. The structures of the computed (by the ab initio B3LYP/6-21G method) reaction products, C60-TMB-2H-C60 and HC60-TMB-2H-C60H, are shown in Figure 6a and 6b, respectively, along with their associated distances between the two fullerenes.
Figure 6. Computed structures of the reaction products C60-TMB-2H-C60 (a) and HC60-TMB-2HC60H (b), corresponding to reactions (1) and (2). The distances between the fullerenes are indicated and they are the values calculated using the B3LYP/6-21G method. The inset shows the sp3 hybridization of the fullerene carbon atoms resulting from the formation of the covalent bonds with TMB-2H.
To quantify ΔE1 and ΔE2, we first optimized the structures of all the reaction partners and calculated their energies using the methods described in [Geng, Solov'yov, Reid, Skelton, Wheatley, Solov'yov, Johnson, Phys. Rev. B 81, 214114 2010]. The calculated enthalpies are as follows: ΔE1=59.8 kcal/mol and ΔE2=-11.7 kcal/mol.
The fact that reaction (1) is endothermic can be understood in terms of the changes in electron configuration for carbon atoms in C60 following the reaction. Every carbon atom in a fullerene has three covalent bonds with its neighbors, two of which are single, and one of which is double. The carbon atoms in a fullerene therefore exhibit sp2 hybridization, but the framework is slightly distorted because of the surface curvature. To attach a TMB molecule to a fullerene, it is necessary to break a double bond in the carbon shell, leading to the formation of two unsaturated carbon atoms. In reaction (1) the TMB-2H complex caps one of these carbon atoms in each fullerene, rendering the carbon sp3 hybridized but leaving the neighboring carbon unsaturated – an energetically unfavorable scenario. In contrast, reaction (2) is exothermic because the hydrogen atoms released from the TMB molecule cap the neighboring unsaturated carbon atoms in each of the C60 fragments. Thus, both carbon atoms that have undergone reaction in either fullerene have become sp3 hybridized (as illustrated by the inset of Figure 6b).
In order to understand how the 1,2,4-TMB molecules bond to fullerenes in the nanowires, we have studied geometrical constraints of the unit cell of the crystal lattice. Figure 7a shows the geometry of such a unit cell and its dimensions, based on the experimental data [Geng, et al., JACS 130, 2527 (2008)]. For simplicity, here we do not show the embedded TMB molecules. As discussed before, a TMB molecule may link two fullerenes together, and if this mode is repeated periodically, the crystalline nanowire would turn into a nanopolymer. However, this is only possible if a TMB fits into the space between two fullerenes without significantly disturbing the unit cell structure.
Figure 7. Possible polymerization scenarios of the C60TMB 1D nanocrystals. In (a) we show the geometry of the unit cell (the TMB molecules embedded in the unit cell are not shown). The distances between the centers of the fullerenes are indicated. The geometry of a C60-TMB-C60 molecule is shown in (b). The distance between the two fullerenes in this case is 14.4 Å, very close to the dimensions of the unit cell. Plots (c) and (d) demonstrate the cross section of a nanowire, viewed along the c-axis, and illustrate two possible linking schemes between the fullerenes. The bars between the fullerenes indicate the TMB molecules. The polymerization can occur along either the a- (plot c) or the b-axis (plot d) of a nanowire.
In Figure 7b we show a C60-TMB-C60 complex, where two fullerenes are covalently linked by a 1,2,4-TMB molecule. In this case the distance between the two fullerenes is 14.4 Å, very close to one of the inter-fullerene spacings measured for the unit cell (14.5 Å, Figure 7a). The linkage is thus highly likely to occur in the unit cell and the C60-TMB-C60 complex is likely to be a possible building block for the polymerization. Figures 7c and 7d illustrate two possible linking schemes between the fullerenes, where the bars between fullerenes represent TMB molecules. Based on the preferential crystal growth directions studied previously, this polymerization may occur along either the a- (see Figure 7c) or the b-axis (see Figure 7d), with the linkages corresponding to the two schemes as shown in the figures.