Effect of Peierls transition in armchair carbon nanotube on dynamical behaviour of encapsulated fullerene
 Nikolai A Poklonski^{1}Email author,
 Sergey A Vyrko^{1},
 Eugene F Kislyakov^{1},
 Nguyen Ngoc Hieu^{2},
 Oleg N Bubel'^{1},
 Andrei M Popov^{3}Email author,
 Yurii E Lozovik^{3, 6},
 Andrey A Knizhnik^{4, 5},
 Irina V Lebedeva^{4, 5, 6} and
 Nguyen Ai Viet^{7}
DOI: 10.1186/1556276X6216
© Poklonski et al; licensee Springer. 2011
Received: 31 July 2010
Accepted: 14 March 2011
Published: 14 March 2011
Abstract
The changes of dynamical behaviour of a single fullerene molecule inside an armchair carbon nanotube caused by the structural Peierls transition in the nanotube are considered. The structures of the smallest C_{20} and Fe@C_{20} fullerenes are computed using the spinpolarized density functional theory. Significant changes of the barriers for motion along the nanotube axis and rotation of these fullerenes inside the (8,8) nanotube are found at the Peierls transition. It is shown that the coefficients of translational and rotational diffusions of these fullerenes inside the nanotube change by several orders of magnitude. The possibility of inverse orientational melting, i.e. with a decrease of temperature, for the systems under consideration is predicted.
Introduction
The structure and elastic properties of carbon nanotubes are studied in connection with the perspectives of their applications in nanoelectronic and nanoelectromechanical devices and composite materials, and are also of fundamental interest, particularly for physics of phase transitions. For example, superconductivity [1], commensurateincommensurate phase transition in doublewalled nanotubes [2], spontaneous symmetry breaking with formation of corrugations along nanotube axis [3] and structural Peierls transition in armchair nanotubes [4–9] have been considered. In the present Letter, we consider a fundamentally new phenomenon related to phase transitions in nanosystems. In other words, we consider the possibility of inverse orientational melting for molecules encapsulated inside nanotubes caused by structural Peierls transition in the nanotubes.
Note that density functional theory (DFT) calculations for the (5,5) nanotube of a finite length [10, 11] also gave a 60 atom periodicity of physical properties on the length of nanotube segment which is consistent with Kekule structure for infinite armchair nanotubes. Moreover, Xray crystallographic analysis of chemically synthesized short (5,5) nanotubes [12] shows the Kekule bond length alternation pattern, which was in good agreement with DFT and PM3 calculations also performed in [12]. By the example of the infinite (5,5) nanotube, it was demonstrated that the structural Peierls transition connected with spontaneous symmetry breaking takes place not only with an increase of temperature, but also can be controlled by uniaxial deformation of armchair nanotubes [9].
A dynamical behaviour of molecules encapsulated inside nanotubes can correspond to the following regimes: oscillations about a fixed position and/or a fixed orientation of the molecule (regime A), hindered motion along the nanotube axis and/or rotation of the molecule (regime B) and free motion and/or rotation of the molecule (regime C). In the present Letter, we show the possibility of changes of the dynamical behaviour of molecules encapsulated inside armchair nanotubes as a result of the Peierls transition in the nanotube structure. In other words, these changes can include switching between the regimes A and B, switching between the regimes B and C, and the changes in diffusion coefficients corresponding to the hindered motion and/or rotation of the molecule (regime B). The considered changes are possible in the case where the regime B takes place for at least one phase of the nanotube, i.e. the temperature T _{P} of the Peierls transition should correspond to the temperature range of the regime B (the hindered motion and/or rotation of the molecule). In other words, the temperature T _{P} should be of the same order of magnitude (or a few orders of magnitude less) as energy barriers ΔE for motion and/or rotation of the molecule inside the nanotube at this phase. Note that inverse melting of motion and/or rotation of the molecule is possible, if the Peierls transition from the high to lowtemperature phase of the nanotube occurs with switching from the regime B to the regime C or switching from the regime A to the regime B.
Estimations showed that the Peierls transition temperature is T _{P} ≃ 115 K [4, 5, 8]. According to calculations [13, 14], the barriers of the value close to this temperature range were obtained for rotation of the fullerene C_{60} inside the C_{60}@C_{240} nanoparticle. It was also found that the changes of bond lengths of the fullerene C_{240}, the outer shell of these nanoparticles, within 0.06 Å lead to an increase of the barriers for rotation by more than an order of magnitude [13, 14]. The changes of the nanotube bond lengths caused by the Peierls distortions are of the same order of magnitude (about 0.03 Å for the (5,5) nanotube [9]). Note also that the size of an encapsulated molecule, and therefore, the nanotube radius cannot be too large, since the magnitude of the Peierls distortions decreases with an increase of the armchair nanotube radius [6].
Thus, taking into account the above considerations, we have chosen the smallest fullerene C_{20} and the magnetic endofullerene Fe@C_{20} to investigate changes in the dynamical behaviour of molecules inside nanotubes at the Peierls transition. It has been shown that the (8,8) nanotube is the smallest armchair carbon nanotube which can encapsulate the fullerene C_{20}[15]. A carbon nanotube with the fullerene C_{20} inside was also used as a model system to simulate a drug delivery via the nanotube [16].
This Letter is organized as follows: "Fullerene and nanotube structures" section presents the DFT calculations of the structure of the C_{20} and Fe@C_{20} fullerenes and the PM3 calculations of the structure of the (8,8) nanotube. "Fullerenenanotube interaction" section presents the semiempirical calculations of the barriers for motion and rotation of the fullerenes inside the nanotube. The section that succeeds the latter is devoted to the dynamical behaviour of molecules inside the nanotubes. Our conclusions are summarized in the final section.
Fullerene and nanotube structures
Structures of the C_{20} and Fe@C_{20} fullerenes have been calculated using the spinpolarized density functional theory implemented in NWChem 4.5 code [17] with the BeckeLeeYangParr exchangecorrelation functional (B3LYP) [18, 19]. Eighteen inner electrons of the iron atom are emulated with the help of the effective core potential  CRENBS ECP [20] (only 8 valence sd electrons are taken into account explicitly). The 631G* basis set is used for describing electrons of the carbon atoms.
The ground state of the fullerene C_{20} is found to be a singlet state and has D _{2h }symmetry. The calculated energy of the triplet state of the fullerene C_{20} is found to be 64 meV greater than the energy of the ground state. The ground state of the endofullerene Fe@C_{20} is found to be a septet state and has C _{2h }symmetry.
The semiempirical method of molecular orbitals modified for onedimensional periodic structures [21] with PM3 parameterization [22] of the Hamiltonian has been used to calculate the structure of the (8,8) nanotube. The method was used previously for calculating the Kekule structure of the (5,5) nanotube ground state and for studying structural transitions controlled by uniaxial deformation of this nanotube [9]. The adequacy of the PM3 parameterization of the Hamiltonian has been demonstrated [23] by the calculation of bond lengths of the C_{60} fullerene with I _{ h }symmetry: the calculated values of the bond lengths agree with the measured ones [24] at the level of experimental accuracy of 10^{3} Å. The calculated Kekule structure of the (8,8) nanotube ground state is shown in Figure 1. The difference between the lengths of short and long bonds of this Kekule structure of the (8,8) nanotube is close to such a difference of the (5,5) nanotube [9]. The Peierls distortions include also radial distortions of the armchair carbon nanotube with periodicity of half of the translational period of the nanotube (for details see [9]). In the case of the (8,8) nanotube, the longest nanotube radius is 0.547 nm, while the shortest radius is 0.544 nm.
Fullerenenanotube interaction
with the parameters ε = 2.755 meV, σ = 3.452 Å. These parameters of the LennardJones potential for the fullerenenanotube interaction are obtained as the average values of the parameters [26] for fullerenefullerene and fullerenegraphene interactions, in accordance with the procedure described in [26]. Here, the LennardJones potential is used for calculating the potential surface of the interaction energy E _{W} between the fullerene and the infinite nanotube, and we believe that this gives adequate qualitative characteristics of the potential surface shape. The cutoff distance, r = r _{c} of the LennardJones potential is taken equal to r _{c} = 15 Å. For this cutoff distance the errors of calculation of the interaction energy E _{W} between the fullerenes and the (8,8) nanotube and the barriers for relative motion and rotation of the fullerenes inside the nanotube are less than 0.1%. Both the fullerenes and the nanotube are considered to be rigid. An account of structure deformation is not essential for the shape of the potential surface both for the interwall interaction of carbon nanotubes [25, 27] and the intershell interaction of carbon nanoparticles [13, 14]. For example, the account of the structure deformation of the shells of C_{60}@C_{240} nanoparticle gives rise to changes of the barriers for relative rotation of the shells which are less than 1% [13, 14]. It should also be noted that the symmetry of interaction energy as a function of coordinates describing relative positions of interacting objects is determined unambiguously by symmetries of the isolated objects and does not change if the symmetries of the objects are broken because of their interactions.
The ground state interaction energies between the C_{20} and Fe@C_{20} fullerenes, and the (8,8) nanotube with Kekule structure are found to be 1.596 and 1.598 eV, respectively. The angles between the C _{2} symmetry axes of the C_{20} and Fe@C_{20} fullerenes and the nanotube axis at the ground states are 49.6° and 53.1°, respectively. The metastable states with the C _{2} symmetry axes of the fullerenes perpendicular to the nanotube axis are also found for both C_{20} and Fe@C_{20}. At the metastable states, the interaction energies are greater by 4.58 and 3.04 meV than the ground state energies, for C_{20} and Fe@C_{20}, respectively.
Calculated characteristics of the dynamical behaviour of the C_{20} and Fe@C_{20} fullerenes inside the (8,8) nanotube of different structure
Nanotube structure  Kekule structure  Structure of metallic phase  

Fullerene  C _{ 20 }  Fe@C _{ 20 }  C _{ 20 }  Fe@C _{ 20 } 
ΔE _{d} (meV)  1.68  1.73  0.87  0.44 
ΔE _{r} (meV)  0.33  0.05  0.87  0.44 
ν _{d} (GHz)  72.2  54.8  75.7  51.3 
ν _{ z } (THz)  0.439  0.144  0.558  0.329 
ν _{ x } (THz)  1.30  1.25  1.36  1.13 
ν _{ y } (THz)  2.15  2.82  2.08  2.14 
The frequencies of small vibrations of the fullerenes along the nanotube axis (ν _{d}), rotational vibrations about the nanotube axis (ν _{ z } ) and rotational vibrations about two mutually perpendicular lateral axes (ν _{ x } , ν _{ y } ) are also calculated and listed in Table 1. The most remarkable change of frequency as a result of the structural phase transition corresponds to rotational vibrations of the Fe@C_{20} fullerene about the nanotube axis (this agrees with the changes of the barriers).
Dynamical behaviour of molecules inside nanotube
Let us consider the possible changes of the dynamical behaviour of the C_{20} and Fe@C_{20} fullerenes inside the (8,8) nanotube caused by the structural phase transition. The Peierls instability transition temperature T _{P} was estimated for the (5,5) nanotube to correspond to temperature range T _{P} ≃ 115 K [4, 5, 8]. Both barriers ΔE _{d} and ΔE _{r} and the thermal energy k _{B} T _{P} are of the same order of magnitude at the structural Peierls phase transition (see Table 1). Therefore, dramatic changes of the diffusion and drift over these barriers can take place at the Peierls transition for the considered pairs of the encapsulated molecules and the nanotube.
where Ω_{d} and Ω_{r} are the preexponential multipliers in the Arrhenius formula for the frequency of jumps of the molecule between two neighbouring global minima of the potential surface E _{W}(φ, z), δ _{d} is the distance between neighbouring global minima for the motion of the molecule along the nanotube axis, δ _{r} is the angle between neighbouring global minima corresponding to the molecule rotation about the nanotube axis and k _{B} is the Boltzmann constant. The mobility B _{d} for the motion along the axis can be easily obtained from the diffusion coefficient D _{d} using the Einstein ratio D _{d}/B _{d} = k _{B} T. Figure 3 shows that δ _{d} = 0.123 nm and δ _{d} = 0.37 nm for the (8,8) nanotube with the structure of the metallic phase and the Kekule structure, respectively, and δ _{r} = 22.5° for the both structures of this nanotube.
The value of the preexponential multiplier Ω in the Arrhenius formula is usually considered to be related with the frequency ν of corresponding vibrations. We suppose that the ratio Ω/ν remains the same for relative motion of different carbon nanoobjects with graphenelike structure (nanotube walls and fullerenes). For reorientation of the fullerenes of the C_{60}@C_{240} nanoparticle, the frequency multiplier Ω was estimated by molecular dynamics simulations having the value of 650 ± 350 GHz [13, 14]. We expand the potential surface of the intershell interaction energy near the minimum using the same empirical potential as in [13, 14], and calculate the frequencies of small relative librations of the shells. The calculated libration frequency has the value ν ≈ 50 GHz, an order of magnitude less than that of the frequency multiplier Ω. In the estimations of this study, we use the values Ω_{d} ≈ 10ν _{d} and Ω_{r} ≈ 10ν _{ z } for the preexponential multipliers.
If a molecule is encapsulated inside a nanotube without a structural phase transition, the jump rotational diffusion takes place at low temperatures, ΔE _{r}/k _{B} T > 1, and the free rotation of the molecule occurs at high temperature, ΔE _{r}/k _{B} T < 1. Orientational melting (a loss of the orientational order with an increase of temperature) has a crossover behaviour if the structural phase transition is absent. Firstly, orientational melting was considered for twodimensional clusters with shell structure [34–37] and later for doubleshell carbon nanoparticles [13, 14], doublewalled carbon nanotubes [25, 38, 39] and carbon nanotube bundles [40]. In the case where a molecule is encapsulated inside a nanotube with a structural phase transition and the barrier ΔE _{r} for rotation of the molecule is greater for the hightemperature phase than for the lowtemperature phase, an inverse orientational melting (a loss of the orientational order with a decrease of temperature) is possible. In other words, the inverse orientational melting takes place if the Peierls transition temperature lies in the range ΔE _{rl} < k _{B} T _{P} < ΔE _{rh}, where ΔE _{rh} and ΔE _{rl} are the barriers for molecule rotation corresponding to hightemperature and lowtemperature phases of the nanotube, respectively. For the considered molecules inside the (8,8) nanotube, these temperature ranges are estimated to be 3.8 < T _{P} (K) < 10 and 0.58 < T _{P} (K) < 5.1 for the C_{20} and Fe@C_{20} fullerenes, respectively (see Table 1). As these temperature ranges are in agreement with the Peierls transition temperature estimates T _{P} ≃ 115 K [4, 5, 8], we predict that the inverse orientational melting is possible for the systems considered. The inverse orientational melting should be more prominent for the case of the Fe@C_{20} fullerene with the greater ratio of the barriers ΔE _{rh}/ΔE _{rl}.
Let us discuss the possibility of observing the changes of the dynamical behaviour of molecules inside armchair carbon nanotubes at the Peierls transition. We believe that the most promising method is highresolution transmission electron microscopy. This method was used for visualizing dynamics of processes inside nanotubes, such as reactions of fullerene dimerization with monitoring of timedependent changes in the atomic positions [41] and rotation of fullerene chains [42]. The rotational dynamics of C_{60} fullerenes inside carbon nanotube was studied also by analysing the intermediate frequency mode lattice vibrations using nearinfrared Raman spectroscopy [43]. The orientational melting in a single nanoparticle may be revealed also by IR or Raman study of the temperature dependence of width of spectral lines. A specific heat anomaly in multiwalled carbon nanotubes may be caused by the orientational orderdisorder transition [44]. In the case of encapsulated magnetic molecules (for example, the Fe@C_{20} endofullerene considered above), the study of the temperature dependence of the electron spin resonance spectra could yield information on the molecule rotational dynamics of these molecules [45].
Conclusive remarks
In this letter, we consider the changes of dynamical behaviour of fullerenes encapsulated in armchair carbon nanotubes caused by the Peierls transition in the nanotube structure by the example of the C_{20} and Fe@C_{20} fullerenes inside the (8,8) nanotube. We apply the DFT approach to calculate the structure of the C_{20} and Fe@C_{20} fullerenes. The ground state of the (8,8) nanotube is found to be the Kekule structure using the method of molecular orbitals. The LennardJones potential is used for calculating the barriers for motion of the fullerenes along the axis and rotation about the axis of the (8,8) nanotube with the Kekule structure and the structure with all equal bonds corresponding to lowtemperature and hightemperature phases, respectively. We show that the changes in the coefficients of diffusion of the fullerenes along the nanotube axis and their rotational diffusion at the Peierls transition can be as much as several orders of magnitude. The possibility of the inverse orientational melting at the Peierls transition is predicted. The analogous changes of dynamical behaviour are also possible for other large molecules inside armchair nanotubes. We believe that the predicted dynamical phenomena can be observed using highresolution transmission electron microscopy, nearinfrared Raman spectroscopy, specific heat measurements, and by study of electron spin resonance spectra for magnetic molecules.
Abbreviation
 DFT:

density functional theory.
Declarations
Acknowledgements
This work has been partially supported by the RFBR (Grants 110200604a and 100290021Bel) and BFBR (Grant Nos. F10R062, F11V001). The atomistic calculations are performed on the SKIF MSU Chebyshev supercomputer and on the MVS100K supercomputer at the Joint Supercomputer Center of the Russian Academy of Sciences.
Authors’ Affiliations
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