Formation mechanism of peapod-derived double-walled carbon nanotubes

Abstract

Atomistic simulations and a dislocation-based analysis reveal the mechanism of carbon peapod fusion into double-walled nanotubes. They explain the trend of diameter increase for the emerging inner wall, driven by the reduction in its strain energy and the interwall van der Waals energy. Surprisingly, this is also accompanied by the systematic bias in the nanotube chirality, changing from zigzag toward armchair. This prediction agrees well with our experimental data and is further supported by the analysis of earlier observations. [5][6][7][8][9] where the encapsulated fullerenes coalescence into an inner tube at high temperature. The peapod route is the preferred method for preparing relatively pure DWNTs with welldefined structures. 1 There has been considerable effort to understand the mechanism of fullerene coalescence and the transformations of sp 2 carbon networks. 8,[10][11][12][13][14] It is well established that that rotation of a C-C bond in a sp 2 carbon network, known as the Stone-Wales ͑SW͒ transformation, is the key step of such a transformation. 10,12,13 The calculated barrier of such a bond rotation is as high as 5-9 eV ͑Refs. 11 and 15͒ which explains the requirement of high temperature for the formation of peapod-derived DWNTs. 9 Because of the high barrier, it is impossible to simulate a defect-free DWNT structure by conventional molecular-dynamics simulation due to the limited simulation time ͑time scale from picosecond to nanosecond͒. 8,14 Although a full route from two fullerenes to a short SWNT has been demonstrated in previous studies, the final SWNT formation was predetermined and thus information about the inner-tube chiral angle cannot be determined correctly by these methods. [10][11][12][13] In this Rapid Communication, we study the formation of peapod-derived DWNTs. An atomic simulation successfully reproduces the transformation from peapods into a defectfree DWNT through the SW mechanism. It is found that most of the simulated inner tubes have large chiral angles ͑e.g., Ͼ 20°͒ and detailed theoretical analysis has shown that the preference for large chiral angles is dominated by the driving force of the SW transformation during tube fattening. Through careful analyzing experimental data, we have confirmed that the abundance of large chiral-angle tubes is in agreement with most experimental observations.It is well established that the kinetic Monte Carlo ͑KMC͒ method can simulate a long-time process by neglecting the thermal vibrations of atoms and considering the process of overcoming barriers directly. In mimicking the KMC method, we propose a similar method to study the coalescence of fullerenes but by considering instead the energy change in the barrier between two states. In detail, the sp 2 carbon network is described by the most used second generation Tersoff-Brenner potential in which the van der Waals interactions have been properly incorporated 15 and the energy of the relaxed initial structure is denoted as E i . A C-C bond is then selected randomly and rotated by 90°. Th...

Multiple intra-tube junctions in the inner tube of peapod-derived double walled carbon nanotubes: theoretical study and experimental evidence

“…38 The agreement 75 between these theoretical predictions and experimental measurements validated the EDKMC method. 38 Beyond, previous study has shown that the space constrain of outer tube plays a crucial role in the inner tube formation. The final inner tube always tends to fit the outer tube with a wall-wall distance range of 0.34-0.38 nm, which is exact the same as the experimentally observed wall-wall distance of DWNTs.…”

“…More details of the EDKMC 35 method and its validity are shown in previous publication. 38 In previous studies, we have shown that there's no tube chiral angle correlation between the inner wall tube and outer wall tube and thus a (18, 0) SWNT was chosen as the only outer wall tube in all trajectories. The Monte Carlo temperature of all trajectories was 40 set as 2000 K, which is close to the experimental temperature of deriving peapods into DWNTs.…”

“…The 70 EDKMC method has been successfully applied to simulate the coalescence of two fullerenes in a SWNT recently. 38 The study shows that there is no chiral angle correlation between the inner and outer tubes and the initial inner tube always tends to have a large chiral angle which is near 30 degree. 38 The agreement 75 between these theoretical predictions and experimental measurements validated the EDKMC method.…”

“…38 Here we consider 5 the inner tube elongation from a short C 120 SWNT. In the elongation process, if a C 120 is surrounded by C 60 s, the next step must be the fusion of the C 120 with a nearby C 60 , C 120 + C 60  C 180 , and then, C 180 + C 60  C 240 … Beyond the continuous growth of an inner tube by adsorbing C 60 s, the meeting of two 10 tubes and their fusion is another important process towards inner tube elongation, e.g., C 120 + C 120 , C 120 +C 180 or C 180 +C 180 .…”

“…[38][39][40][41] In a EDKMC trajectory, the energy change of a C-C bond rotation is used as the driving force. The 70 EDKMC method has been successfully applied to simulate the coalescence of two fullerenes in a SWNT recently.…”