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We report total-energy electronic structure calculations that provide energetics of encapsulation of C60 in the carbon nanotube and electronic structures of the resulting carbon peapods. We find that the encapsulating process is exothermic for the (10,10) nanotube, whereas the processes are endothermic for the (8,8) and (9,9) nanotubes, indicative that the minimum radius of the nanotube for the encapsulation is 6.4 A. We also find that the C(60)@(10,10) is a metal with multicarriers each of which distributes either along the nanotube or on the C60 chain. This unusual feature is due to the nearly free electron state that is inherent to hierarchical solids with sufficient space inside.

Okada, Saito, and Oshiyama Reply: As we mentioned [1], there are only three polymerized C 60 phases which have been synthesized under pressure and whose atomic structure has been identified to date; one-dimensional orthorhombic, two-dimensional tetragonal, and twodimensional rhombohedral phases [2]. Although there have been a lot of experimental studies to explore new phases of carbon using pressure synthesis from solid C 60 [3][4][5][6] as Brazhkin and Lyapin commented [7], in most cases it is not clear what type of atomic structure the synthesized material has: Sometimes amorphous, sometimes crystal, sometimes a mixture of diamond and graphite, and sometimes totally unidentified at all.Some materials obtained via pressure synthesis from solid C 60 were reported to be superhard or even "ultrahard" (harder than diamond) and were inferred to be three-dimensional (3D) C 60 polymers from their broad x-ray diffraction profiles [8,9]. For these "superhard 3D C 60 polymers," atomic-scale network topologies had not been reported so far. Only quite recently, candidates for the atomic coordinates have been proposed [10] for the first time.In our Letter [1] we tried to provide a firm theoretical framework to consider synthesis and properties of 3D C 60 polymers. Starting from the tetragonal phase of 2D C 60 polymer, we found an ordered 3D C 60 polymer which had not been identified before and exhibited fascinating properties as described in the Letter. From the point of synthesis, for instance, the radial distribution function which experimentalists sometimes rely on to determine the structure is found to not be a simple reflection of the microscopic structures. Also the system is expected to be a candidate for a new elemental superconductor consisting entirely of carbon. At the same time, we found that our system was not a superhard or ultrahard material. Its bulk modulus is found to be 1 order of magnitude smaller than diamond [1].The present theoretical treatment (density-functional pseudopotential procedure) is expected to have enough accuracy to discuss relative hardness of various carbon based materials [11,12]. Hence there is no doubt that the system we found does not correspond directly to so-called superhard 3D C 60 polymers. On the other hand, generally it is useful to compare theoretical and experimental results carefully to innovate new materials. It is especially important in the field of nanostructure materials consisting of carbon and/or other covalent-bond elements. Their physical properties are known to depend strongly on the network topology of covalent bonds as has been clearly demonstrated in the case of carbon nanotubes [13,14].Sometimes the target new materials, with novel properties to be synthesized, can be given from the theoretical study. In this respect, a comparison between theory and experiment done by Brazhkin and Lyapin [7] is worth further consideration. The possibility of the presence of various different phases in pressure-polymerized 3D C 60 is an interesting issue to be studied theoreti...

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We have investigated by first principles the electronic, vibrational, and structural properties of bct C4, a new form of crystalline sp{3} carbon recently found in molecular dynamics simulations of carbon nanotubes under pressure. This phase is transparent, dynamically stable at zero pressure, and more stable than graphite beyond 18.6 GPa. Coexistence of bct C4 with M carbon can explain better the x-ray diffraction pattern of a transparent and hard phase of carbon produced by the cold compression of graphite. Its structure appears to be intermediate between that of graphite and hexagonal diamond. These facts suggest that bct C4 is an accessible form of sp{3} carbon along the graphite-to-hexagonal diamond transformation path.

Ever since the discovery of carbon nanotubes (CNTs), many groups have endeavored to understand the fundamental properties of the CNTs. The redox properties (i.e. electronic densities, the Fermi levels, redox potentials) of single-walled carbon nanotubes (SWNTs) are related to the structures of SWNTs that have a specified diameter and chirality angle uniquely related to a pair of integers (n,m); the so-called chiral indices. [1,2] Many attempts have been made to determine the electronic properties of SWNTs using scanning tunneling spectroscopy, [3] redox titrimetry, [4] photoluminescence (PL) measurements, [5][6][7] and spectroelectrochemistry; [8][9][10][11][12][13] however, the success in the determination of the redox properties as already reported has been low. Recently, Paolucci et al. [12] employed Vis-near-IR absorption spectroelectrochemistry to estimate the redox potentials of the SWNTs dissolved in an ultradry dimethylsulfoxide (DMSO) solution; however, it is not easy to determine the redox potentials of isolated (n,m)SWNTs using this method because SWNTs with several different chiral indices have band gaps in the near-IR region that overlap one another. We now describe a simple method for the determination of the redox potentials of many (in this study, fifteen) individual (n,m)SWNTs using near-IR PL spectroelectrochemistry in an aqueous medium.Strategic approaches toward the solubilization of CNTs are essential for many applications of CNTs [14] and numerous dispersants including carboxymethylcellulose sodium salt (CMC, Figure S1a in the Supporting Information) [15] have been used to individually dissolve SWNTs. In this study, we fabricated a non-fluorescent transparent indium tin oxide (ITO) electrode modified with a cast film of CMC/poly-(diallyldimethylammonium chloride) (PDDA; Figure S1b in the Supporting Information) that contained isolated SWNTs (for details, see Experimental Section in the Supporting Information).We have discovered that we can determine the redox potentials of isolated SWNTs having their own chirality indices by in situ near-IR PL spectroelectrochemistry at the fabricated modified ITO electrode. This modified film retains the isolated SWNTs and the spectroelectrochemical results are analyzed with the Nernst equation.Externally applied potentials were changed in the range of ) and oxidized form (SWNT n+ ) when the external potential was applied to the electrode in arbitrary steps from 0.0 V to À1.0 V and from 0.0 V to + 1.1 V, respectively. After each potential step, the applied potential was returned to 0.0 V and it was confirmed that no significant spectral change in the SWNT had occurred, namely, the SWNTs in the film are stable during these electrochemically driven redox processes. This behavior of the SWNTs is consistent with those in Visnear-IR absorption [8b] and Raman [10b] spectroelectrochemical studies.We carried out in situ near-IR absorption spectroelectrochemistry using the modified electrode. The near-IR absorption spectra of the individually solubilized SWNTs wer...

We investigate nitrogen-doping effects in a graphene sheet using a first-principles electronic-structure study in the framework of density-functional theory. As possible configurations of nitrogen impurities in graphene, substitutional nitrogen and pyridine-type defects around a monovacancy and around a divacancy are considered, and their energetics and electronic structures are discussed. The formation-energy calculations suggest that substitutional doping of the nitrogen atom into a graphene sheet is energetically the most favorable among the possible nitrogen-doping configurations. Furthermore, by comparison of the total energy of the pyridine-type defects with that of the substitutional nitrogen defect in graphene, it is revealed that formation of the pyridine-type defects becomes energetically favorable compared with formation of the substitutional nitrogen defect in the presence of a vacancy. From the results of electronic-band-structure calculations, it is found that the nitrogenimpurity states appear around the Fermi level as either acceptorlike or donorlike states, depending on the atomic geometries of the nitrogen impurities in graphene. We also calculate the scanning tunneling microscopy (STM) images associated with impurity-induced electronic states for future experimental identification of nitrogen impurities. The simulated STM images of the three N-doping configurations considered here are found to be strongly dependent on the local density of states around the nitrogen impurity, and therefore the doping configurations should be distinguishable from one another. The similarities and differences of the electronic structures and STM corrugations between N-doped and undoped graphenes are also discussed.