Study of collective radial breathing-like modes in double-walled carbon nanotubes: Combination of continuous two-dimensional membrane theory and Raman spectroscopy

Radial breathing modes (RBMs) are widely used for the atomic structure characterization and index assignment of single-walled carbon nanotubes (SWNTs) from resonant Raman spectroscopy. However, for double-walled carbon nanotubes (DWNTs), the use of conventional ¿RBM(d) formulas is complicated due to the van der Waals interaction between the layers, which strongly affects the frequencies of radial modes and leads to new collective vibrations. This paper presents an alternative way to theoretically study the collective radial breathing-like modes (RBLMs) of DWNTs and to account for interlayer interaction, namely the continuous two-dimensional membrane theory. We obtain an analytical ¿RBLM(do, di) relation, being the equivalent of the conventional ¿RBM(d) expressions, established for SWNTs. We compare our theoretical predictions with Raman data, measured on individual index-identified suspended DWNTs, and find a good agreement between experiment and theory. Moreover, we show that the interlayer coupling in individual DWNTs strongly depends on the interlayer distance, which is manifested in the frequency shifts of the RBLMs with respect to the RBMs of the individual inner and outer tubes. In terms of characterization, this means that the combination of Raman spectroscopy data and predictions of continuous membrane theory may give additional criteria for the index identification of DWNTs, namely the interlayer distance

Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates

Atomically inspired k · p approach and valley Zeeman effect in transition metal dichalcogenide monolayers

International audienceWe developed a six-band k · p model that describes the electronic states of monolayer transition metal dichalcogenides (TMDCs) in K-valleys. The set of parameters for the k · p model is uniquely determined by decomposing tight-binding (TB) models in the vicinity of K ±-points. First, we used TB models existing in literature to derive systematic parametrizations for different materials, including MoS2, WS2, MoSe2 and WSe2. Then, by using the derived six-band k · p Hamiltonian we calculated effective masses, Landau levels, and the effective exciton g-factor g X 0 in different TMDCs. We showed that TB parameterizations existing in literature result in small absolute values of g X 0 , which are far from the experimentally measured g X 0 ≈ −4. To further investigate this issue we derived two additional sets of k · p parameters by developing our own TB parameterizations based on simultaneous fitting of ab-initio calculated, within the density functional (DFT) and GW approaches, energy dispersion and the value of g X 0. We showed that the change in TB parameters, which only slightly affects the dispersion of higher conduction and deep valence bands, may result in a significant increase of |g X 0 |, yielding close-to-experiment values of g X 0. Such a high parameter sensitivity of g X 0 opens a way to further improvement of DFT and TB models

Polyiodide structures in thin single-walled carbon nanotubes: A large-scale density-functional study

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Phonon contribution to electrical resistance of acceptor-doped single-wall carbon nanotubes assembled into transparent films

The electrical resistance of pristine and acceptor-doped single-wall carbon nanotubes assembled into transparent films was measured in the temperature range of 5 to 300 K. The doping was accomplished by filling the nanotubes with iodine or CuCl from the gas phase. After doping the films resistance appeared to drop down by one order of magnitude, to change the nonmonotonic temperature behavior, and to reduce the crossover temperature. The experimental data have been perfectly fitted in frames of the known heterogeneous model with two contributions: from the nanotube bundles (with quasi-one-dimensional conductivity) and from the interbundle electron tunneling. The doping was observed to decrease the magnitudes of both contributions. In this paper we have revealed the main reason of changes in the nanotube part. It is considered to be connected with the involvement of low-energy phonons, which start to participate in the intravalley scattering due to the shift of the Fermi level after doping. The values of the Fermi level shift into the valence band are estimated to be equal to -0.6 eV in the case of iodine doping and -0.9 eV in the case of CuCl doping. These values are in qualitative agreement with the optical absorption data.Peer reviewe

Optical properties and charge transfer effects in single-walled carbon nanotubes filled with functionalized adamantane molecules

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Intense Raman D band without disorder in flattened carbon nanotubes

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