Nonequilibrium quantum chemical molecular dynamics simulations of C₆₀ to SiC heterofullerene conversion

Abstract

Nonequilibrium high-temperature quantum chemical molecular dynamics simulations based on the self-consistent-charge density-functional tight-binding (DFTB) method for the conversion of C₆₀ to SiC fullerene by way of periodic Si atom supply are presented. Random supply of Si atoms on the surface of a perfect I_h-C₆₀ buckminsterfullerene without simultaneous carbon atom removal merely leads to formation of an exohedrally adsorbed Si cluster during the entire length of our simulations via an Ostwald ripening process, whereas supply of Si atoms in combination with simultaneous carbon atom removal affords the formation of SiC fullerene structures up to a lower limit of 2:1 for the C:Si ratio. Our simulations demonstrate the importance of vacancy defects for atomic substitution-based approaches for heterofullerene cages, and hint at inherent difficulties of such approaches for the actual synthesis of hypothetical, idealized sp²-hybridized SiC nanostructures with a 1:1 ratio featuring fully alternating atomic structures and no Si–Si and C–C bonds.

Introduction

Silicon carbide (SiC) is a wide-bandgap (2.0 eV ⩽ E_gap ⩽ 7.0 eV) semiconductor with many potential applications [1], [2], [3]. It exhibits desirable physical, mechanical and chemical properties such as low density, high strength, high thermal conductivity, stability at high temperature, low thermal expansion, high refractive index, and chemical inertness. With reduction in size, SiC nanomaterials offer novel physical, mechanical and chemical properties, due to effects of quantum confinement [4], [5]. Therefore, the synthesis of SiC nanostructures has received great attention, from SiC nanotubes (SiCNTs) [5], [6], [7], nanowires (SiCNWs) [5], [7], [8], nanocubic cages [9], nanocones [10], and nanobelts [11] to nanoribbons [12]. At the turn of the century, the synthesis and structural characterization of SiC heterofullerenes was studied under the perspective that Si-covered fullerenes possess increased chemical reactivity, opening a pathway towards the polymerization of fullerenes via SiC bonds [13]. Later, a templating effect of C₆₀ on Si cluster growth had been observed by Reinke and Oelhafen [14], who produced complex Si_n–C₆₀ aggregates by using electron beam evaporation of Si and deposited it onto a corrugated C₆₀ layer.

Remarkably, true heterofullerene cages had been reported even earlier. In 1996, Shinohara and co-workers [15] and later Mélinon and co-workers [16], [17], [18] employed the laser vaporization technique on Si-doped carbon targets to produce true heterofullerenes Si_mC_2n (m < 4). Such heterocage molecules can also be synthesized in the vapor phase using Si-doped graphite rods in the Krätschmer–Huffman process [19]. On the basis of mass spectroscopy, photofragmentation, and gas phase ion mobility experiments, clear evidence was presented that Si_mC_2n (m < 4) clusters possess a fullerene cage wherein Si substitutes C atoms [16], [17], [18], [19]. However, direct experimental evidence for the existence of Si_mC_2n (m ⩾ 4) heterofullerenes is elusive, and the maximum number of Si atoms that can be substituted in the carbon network remains unclear [5]. Moreover, to the best of our knowledge, no experimental report of SiC fullerenes or single-walled silicon-carbide nanotubes (SWSiCNTs) with 1:1 stoichiometry has been reported, due to the lack of successful synthetic routes towards fully sp²-hybridized SiC nanostructures. On the other hand, multiwalled SiCNTs with a C:Si ratio of 1:1 featuring an sp³-hybridized silicon carbide configuration were already successfully synthesized [7].

During the last decade, several theoretical studies based on semiempirical and ab initio methodologies have been used to investigate the structure, stability, thermal and electronic properties, and dynamics of SiC heterofullerenes [20], [21], [22], [23], [24], [25], [26], [27], [28]. The stability of Si_mC_60−m heterofullerenes has been studied based on first-principles approaches [20], [21]. Using density functional theory (DFT) geometry optimizations and limited molecular dynamics (MD) simulations, it was found that the critical number of Si atoms (m) for transition from thermally stable to unstable Si_mC_60−m heterofullerenes is around 20 [20], at least when maximum segregation of C and Si atoms is assumed in the heterofullerenes. Contradicting this theoretical prediction, a rough experimental estimate placed the stability threshold value at around m = 12 heteroatoms [17], [18]. Depending on the level of C–Si segregation, the SiC fullerenes have also been reported to exhibit many different shapes [23], [24], [25], [26], [27], [28]. Very recently, Yu et al. [29] predicted on the basis of semiempirical calculations that a SiC buckydiamond cluster with the specific composition of Si₆₈C₇₉ corresponds is magic; this sp³-hybridized cluster was encapsulated within a C₁₁₂ fullerene-like shell. Despite these efforts, the assessment of the structural stability of SiC heterofullerenes is far from complete. Even more elusive remains the formation mechanism of SiC heterofullerenes.

Therefore, in this work, we investigated the conversion of C₆₀ to SiC heterofullerenes by performing direct, nonequilibrium MD simulations with periodic Si atom addition, based on an accurate quantum chemical potential. Since under experimental conditions, carbon vacancy defects are important promoters of fullerene chemical reactivity [30], we also considered the possibility of carbon removal occurring simultaneously to the Si addition, thereby effectively simulating an atomic substitution-based approach towards SiC fullerenes. The technique of “removal” of atoms during nonequilibrium MD has been used before on several occasions [31], [32] as it allows to include important, but high-barrier dissociation reactions that occur on timescales out of reach of our computational capabilities. The simulations we performed demonstrate how Si atoms can indeed substitute carbon in a fullerene cage during annealing of carbon vacancies, and thus highlight the importance of such structural defects in the carbon network for the development of SiC bonds with sp²-hybridized silicon atoms.