Kostas Papagelis

Novel Hybrid Materials Consisting of Regioregular Poly(3‐octylthiophene)s Covalently Attached to Single‐Wall Carbon Nanotubes

Facile routes for the synthesis of hybrid materials consisting of regioregular poly(3-octylthiophene)s covalently attached to single-wall carbon nanotubes are presented for the first time. These materials are easily processable using common organic solvents, and at the same time combine the properties of regioregular poly(3-alkylthiophene)s with those of single-wall carbon nanotubes. Moreover, studies of the properties of these materials have provided strong evidence for an electron transfer from the regioregular poly(3-octylthiophene) to the single-wall carbon nanotube.

Experimentally derived axial stress–strain relations for two-dimensional materials such as monolayer graphene

A methodology is presented here for deriving true experimental axial stress–strain curves in both tension and compression for monolayer graphene through the shift of the 2D Raman peak (Δω) that is present in all graphitic materials. The principle behind this approach is the observation that the shift of the 2D wavenumber as a function of strain for different types of PAN-based fibres is a linear function of their Young’s moduli and, hence, the corresponding value of Δω over axial stress is, in fact, a constant. By moving across the length scales we show that this value is also valid at the nanoscale as it corresponds to the in-plane breathing mode of graphene that is present in both PAN-based fibres and monolayer graphene. Hence, the Δω values can be easily converted to values of σ in the linear elastic region without the aid of modelling or the need to resort to cumbersome experimental procedures for obtaining the axial force transmitted to the material and the cross-sectional area of the two-dimensional membrane.

Single-walled carbon nanotubes decorated with a pyrene-fluorenevinylene conjugate

Single-walled carbon nanotubes are noncovalently functionalized using a pyrene-fluorenevinylene dye and the resulting nanohybrids are isolated from the free molecules. The tubes modified by means of this noncovalent approach show enhanced solubility in organic media. The structure and morphology of this hybrid material are fully characterized using absorption, infrared and Raman spectroscopies as well as atomic force and scanning electron microscopies. Steady state fluorescence measurements reveal that significant quenching of the pyrene derivative excited state takes place through an energy transfer mechanism.

N-Octyl-O-sulfate chitosan stabilises single wall carbon nanotubes in aqueous media and bestows biocompatibility

A non-covalent approach to debundle single wall carbon nanotubes using a biocompatible chitosan-derivative, namely N-octyl-O-sulfate chitosan (NOSC), was investigated. The resulting stable dispersions were characterised by Raman spectroscopy, UV-Vis spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM) and zeta-potential measurements. Both AFM and TEM studies revealed the presence of individual carbon nanotubes wrapped with the polymer (diameters up to 7 nm). Raman spectra showed radial breathing mode frequency shifts, after the addition of NOSC, due to the wrapping of the biomolecules onto the graphitic sidewalls. Molecular modelling studies were employed to investigate the mode of binding of the NOSC chains to the surface of the nanotubes. In agreement with the experiments, modelling studies predicted that the wrapped tube has a maximum thickness of approximately 7 nm. Studies on the anticoagulant properties of these complexes revealed that NOSC coated SWCNTs exhibit similar activity to the polymer alone, this property would eliminate the risk for SWCNTs to induce coagulation as a host reaction process when used in vivo.

High-pressure effects on the Raman spectrum and the force constants of the rare-earth aluminium garnets (RE3Al5O12)

The pressure dependence of the Raman-active modes of the rare-earth aluminium garnet compounds (RE3Al5O12, RE: Tb, Dy, Tm, Yb, Lu) has been measured at room temperature and discussed in terms of the rigid-ion model (RIM). The Raman modes in the high-frequency region (ω>650 cm-1) exhibit systematically larger pressure slopes (4-5.6 cm-1 GPa-1) than the rest, which have slopes ranging between -0.1 and 3.3 cm-1 GPa-1. The values of the Γi ( = ∂ln ωi/∂P) parameters show that the corresponding bonding in the RE3Al5O12 family is nearly of the same nature and order of magnitude. No pressure-induced phase transitions could be detected up to 25 GPa. Using the RIM, the pressure dependence (up to 10 GPa) of the first-neighbour stretching and bending force constants for the Al-O (in the octahedra and tetrahedra) and RE-O bonds (in the dodecahedra) of the Yb3Al5O12, Dy3Al5O12 and Tm3Al5O12 has been calculated. The calculated force constants show that the corresponding bonds in the samples investigated are of comparable strength and their pressure variation is quite similar. Furthermore, it has been found that they increase according to the sequence dodecahedra → octahedra → tetrahedra and that the compressibility follows the same trend, while the electronic configuration of the dodecahedral cation might play a role in the total compressibility.

Compressive response and buckling of graphene nanoribbons

We examine the mechanical response of single layer graphene nanoribbons (GNR) under constant compressive loads through molecular dynamics simulations. Compressive stress-strain curves are presented for GNRs of various lengths and widths. The dependence of GNR’s buckling resistance on its size, aspect ratio, and chiral angle is discussed and approximate corresponding relations are provided. A single master curve describing the dependence of the critical buckling stress of GNRs on their aspect ratio is presented. Our findings were compared to the continuum elasticity theories for wide plates and wide columns. In the large width limit, the response of the GNRs agrees with the predictions of the wide plates theory and thus, with that of wide graphenes. In the small width limit, the behavior of graphene nanoribbons deviates from that of periodic graphenes due to various edge related effects which govern the stiffness and the stability of the graphene membranes, but it qualitatively agrees with the theory of wide columns. In order to assess the effect of thermal fluctuations on the critical buckling stress a wide range of temperatures is examined. The findings of the current study could provide important insights regarding the feasibility and the evaluation of the performance of graphene-based devices.