Combining equilibrium and non-equilibrium molecular dynamics simulations with accurate carbon potentials, we determine the thermal conductivity λ of carbon nanotubes and its dependence on temperature. Our results suggest an unusually high value λ≈6, 600 W/m·K for an isolated (10, 10) nanotube at room temperature, comparable to the thermal conductivity of a hypothetical isolated graphene monolayer or diamond. Our results suggest that these high values of λ are associated with the large phonon mean free paths in these systems; substantially lower values are predicted and observed for the basal plane of bulk graphite. 61.48.+c, 66.70.+f, 63.22.+m, 68.70.+w With the continually decreasing size of electronic and micromechanical devices, there is an increasing interest in materials that conduct heat efficiently, thus preventing structural damage. The stiff sp 3 bonds, resulting in a high speed of sound, make monocrystalline diamond one of the best thermal conductors [1]. An unusually high thermal conductance should also be expected in carbon nanotubes [2,3], which are held together by even stronger sp 2 bonds. These systems, consisting of seamless and atomically perfect graphitic cylinders few nanometers in diameter, are self-supporting. The rigidity of these systems , combined with virtual absence of atomic defects or coupling to soft phonon modes of the embedding medium, should make isolated nanotubes very good candidates for efficient thermal conductors. This conjecture has been confirmed by experimental data that are consistent with a very high thermal conductivity for nanotubes [4]. In the following, we will present results of molecular dynamics simulations using the Tersoff potential [5], augmented by Van der Waals interactions in graphite, for the temperature dependence of the thermal conductivity of nanotubes and other carbon allotropes. We will show that isolated nanotubes are at least as good heat conductors as high-purity diamond. Our comparison with graphitic carbon shows that inter-layer coupling reduces thermal conductivity of graphite within the basal plane by one order of magnitude with respect to the nanotube value which lies close to that for a hypothetical isolated graphene monolayer. The thermal conductivity λ of a solid along a particular direction, taken here as the z axis, is related to the heat flowing down a long rod with a temperature gradient dT /dz by 1 A dQ dt = −λ dT dz , (1) where dQ is the energy transmitted across the area A in the time interval dt. In solids where the phonon contribution to the heat conductance dominates, λ is proportional to Cvl, the product of the heat capacity per unit volume C, the speed of sound v, and the phonon mean free path l. The latter quantity is limited by scattering from sample boundaries (related to grain sizes), point defects, and by umklapp processes. In the experiment, the strong dependence of the thermal conductivity λ on l translates into an unusual sensitivity to isotopic and other atomic defects. This is best illustrated by the reported thermal c...

Chemical modification by SOCl2 of an entangled network of purified single-wall carbon nanotubes, also known as 'bucky paper', is reported to profoundly change the electrical and mechanical properties of this system. Four-probe measurements indicate a conductivity increase by up to a factor of 5 at room temperature and an even more pronounced increase at lower temperatures. This chemical modification also improves the mechanical properties of SWNT networks. Whereas the pristine sample shows an overall semiconducting character, the modified material behaves as a metal. The effect of SOCl2 is studied in terms of chemical doping of the nanotube network. We identified the microscopic origin of these changes using SEM, XPS, NEXAFS, EDX, and Raman spectroscopy measurements and ab initio calculations. We interpret the SOCl2-induced conductivity increase by p-type doping of the pristine material. This conclusion is reached by electronic structure calculations, which indicate a Fermi level shift into the valence band, and is consistent with the temperature dependence of the thermopower.

We investigate spin conductance in zigzag graphene nanoribbons and propose a spin injection mechanism based only on graphitic nanostructures. We find that nanoribbons with atomically straight, symmetric edges show zero spin conductance but nonzero spin Hall conductance. Only nanoribbons with asymmetrically shaped edges give rise to a finite spin conductance and can be used for spin injection into graphene. Furthermore, nanoribbons with rough edges exhibit mesoscopic spin conductance fluctuations with a universal value of rmsG s 0:4e=4 . DOI: 10.1103/PhysRevLett.100.177207 PACS numbers: 85.75.ÿd, 72.25.ÿb, 73.22.ÿf, 73.63.ÿb After their experimental discovery in 2004 [1], monolayers of graphite have attracted much experimental and theoretical attention owing to their unusual band structure [2]. Graphene has also been suggested as a good candidate for spin-based quantum computing and spintronics [3], as it is expected to have long spin decoherence or relaxation times [4]. This prospect led to the recent interest in generating and manipulating net spin distributions in graphene. Recently, spin injection from ferromagnetic metal contacts into graphene has been achieved [5][6][7][8].Transport properties of graphene nanoribbons (GNRs) are expected to depend strongly on whether they have an armchair or zigzag edge [9]. In GNRs with zigzag edges, transport is dominated by edge states which have been observed in scanning tunneling microscopy [10]. Moreover, owing to their high degeneracy, these states are expected to be spin-polarized [11], making zigzag GNRs attractive for spintronics [12]. In addition, edge states are expected to occur also in nanoribbons with other edge orientations [13]. Recently, the first transport experiments have been performed in narrow ribbons of graphene [14], albeit with not well defined edges. Recent theoretical work focused on charge transport through rough GNRs [15], but spin transport properties have not been explored yet.In the present work, we focus on spin transport in GNRs with rough zigzag edges. Ideal zigzag GNRs are not efficient spin injectors due to the symmetry between the edges with opposite magnetization. In order to obtain net spin injection, this symmetry must be broken. Existing proposals to achieve this require very large transverse electric fields [12]. We sidestep this difficulty by showing that edge imperfections (such as vacancies), which usually cannot be avoided experimentally, break the symmetry between the edges and lead to a finite spin conductance of the GNR. Thus, rough zigzag GNRs can be used as spin injectors or detectors in graphene spintronics.We start with a description of the electronic ground state properties of the zigzag GNR, which captures the essential physics relevant to spin transport, given by the single band tight- Here t ij t if i and j are nearest neighbors, t ij t 0 if i and j are next nearest neighbors [16], and are the Pauli matrices corresponding to the spin degree of freedom. The local magnetization m i can be obtained from the self-con...

We apply the ab initio spin density functional theory to study magnetism in all-carbon nanostructures. We find that particular systems, which are related to schwarzite and contain no undercoordinated carbon atoms, carry a net magnetic moment in the ground state. We postulate that, in this and other nonalternant aromatic systems with negative Gaussian curvature, unpaired spins can be introduced by sterically protected carbon radicals.

No abstract

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of ≤21 mJ/cm 2 , lattice vibrations are observed to thermalize on a time scale of ≈8 ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp 3 -like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photo-excitation.

We combined resonant photoabsorption and vibration spectroscopy with scanning tunneling microscopy ͑STM͒ to unambiguously identify the presence of Stone-Wales ͑SW͒ defects in carbon and boron nitride nanotubes. Based on extensive time-dependent ab initio density functional calculations, we propose to resonantly photoexcite SW defects in the infrared and ultraviolet regime as a means of their identification. Onset of nonradiative decay to a local defect vibration with a frequency of 1962 cm Ϫ1 serves as a fingerprint of such defects in carbon nanotubes. The bias dependence of the STM images shows distinct features associated with the presence of SW defects.

We combine scanning tunneling microscopy (STM) measurements with ab initio calculations to study the self-assembly of long chain alkanes and related alcohol and carboxylic acid molecules on graphite. For each system, we identify the optimum adsorption geometry and explain the energetic origin of the domain formation observed in the STM images. Our results for the hierarchy of adsorbate-adsorbate and adsorbate-substrate interactions provide a quantitative basis to understand the ordering of long chain alkanes in self-assembled monolayers and ways to modify it using alcohol and acid functional groups.