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Coherent phonons in carbon nanotubes and graphene
Kim, J.H.,Nugraha, A.R.T.,Booshehri, L.G.,Haroz, E.H.,Sato, K.,Sanders, G.D.,Yee, K.J.,Lim, Y.S.,Stanton, C.J.,Saito, R.,Kono, J. Elsevier Science Publishers [etc.] 2013 Chemical physics Vol.413 No.-
We review recent studies of coherent phonons (CPs) corresponding to the radial breathing mode (RBM) and G-mode in single-wall carbon nanotubes (SWCNTs) and graphene. Because of the bandgap-diameter relationship, RBM-CPs cause bandgap oscillations in SWCNTs, modulating interband transitions at terahertz frequencies. Interband resonances enhance CP signals, allowing for chirality determination. Using pulse shaping, one can selectively excite specific-chirality SWCNTs within an ensemble. G-mode CPs exhibit temperature-dependent dephasing via interaction with RBM phonons. Our microscopic theory derives a driven oscillator equation with a density-dependent driving term, which correctly predicts CP trends within and between (2n+m) families. We also find that the diameter can initially increase or decrease. Finally, we theoretically study the radial breathing like mode in graphene nanoribbons. For excitation near the absorption edge, the driving term is much larger for zigzag nanoribbons. We also explain how the armchair nanoribbon width changes in response to laser excitation.
Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials
Coleman, J. N.,Lotya, M.,O'Neill, A.,Bergin, S. D.,King, P. J.,Khan, U.,Young, K.,Gaucher, A.,De, S.,Smith, R. J.,Shvets, I. V.,Arora, S. K.,Stanton, G.,Kim, H.-Y.,Lee, K.,Kim, G. T.,Duesberg, G. S.,H American Association for the Advancement of Scienc 2011 Science Vol.331 No.6017
<P>If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS(2), WS(2), MoSe(2), MoTe(2), TaSe(2), NbSe(2), NiTe(2), BN, and Bi(2)Te(3) can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS(2) and MoS(2) effectively reinforce polymers, whereas WS(2)/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.</P>