The bands of graphite are extremely sensitive to topological defects which modify the electronic structure. In this paper we found non-dispersive flat bands no farther than 10 meV of the Fermi energy in slightly twisted bilayer graphene as a signature of a transition from a parabolic dispersion of bilayer graphene to the characteristic linear dispersion of graphene. This transition occurs for relative rotation angles of layers around 1.5 o and is related to a process of layer decoupling. We have performed ab-initio calculations to develop a tight binding model with an interaction Hamiltonian between layers that includes the π orbitals of all atoms and takes into account interactions up to third nearest-neighbors within a layer.Graphene is a sheet of carbon atoms arranged on a honeycomb lattice recently obtained by micro-mechanical cleavage of graphite 1 . This two-dimensional lattice has a singular linear band dispersion 2 which makes the charge carriers behave as massless Dirac fermions with a speed of v F ≈ 10 6 m/s, travelling large distances without interaction. These properties change drastically in the presence of a second layer. A Bernal or AB stacked bilayer graphene (BLG) is a stack of two carbon sheets in such a way that an atom of one of the layers is in the center of the other layer's hexagon. Like single layer graphene, BLG is also a semimetal but its dispersion relation is quadratic and its charge carriers have a non-zero effective mass.Few layers of graphene grown epitaxially on different surfaces show a variety of defects including rotations of the top layer, for instance in graphene layers prepared by chemical vapor deposition (CVD) 3 . Moiré patterns are very often observed in Scanning Tunneling Microscopy (STM) measurements when a relative rotation between top layers is present 4,5 . Graphite is also extremely sensitive to topological defects which can modify its electronic structure. Extended defects like lattice dislocations lead to the presence of localized states at the Fermi energy 6,7 as is the case of graphene ribbons with zigzag edges 8 . These localized states can also be found as a result of local defects such as in graphene antidot lattices 9 . They were predicted in superstructures with honeycomb symmetry by N. Shima et al. 10 , who suggest the occurrence of ferromagnetism when electron correlation is turned on.In addition one of the mechanisms proposed to explain high-T c superconductivity is associated with the presence of extended Van Hove Singularities (VHS) near the Fermi energy 11-13 . This kind of VHS arising from a nearly dispersionless band has been observed in high-T c cuprates by angle-resolved photoemission 14,15 . The superconductivity in graphene when the fermi level is close to a VHS has been also explored theoretically 16 . Recently Guohong Li et al.17 reported the observation of two symmetric low-energy VHSs in the density of states, measured by scanning tunneling spectroscopy, in twisted graphene layers. They showed that the position of these singularities can b...

Abstract:The aim of this study was to prepare a novel targeting drug delivery system for 2-Methoxyestradiol (2ME) in order to improve the clinical application of this antitumor drug. It is based in nanoparticles (NPs) of titanium dioxide (TiO 2 ) coated with polyethylene glycol (PEG) and loaded with 2ME. A complete IR and Raman characterization have been made to confirm the formation of TiO 2 -PEG-2ME composite. Vibrational modes have been assigned for TiO 2 , PEG, and 2ME and functionalized TiO 2 -PEG and TiO 2 -PEG-2ME. The observed variation in peak position of FTIR and Raman of each for these composites has been elucidated in terms of intermolecular interactions between PEG-2ME and TiO 2 , obtaining step-by-step the modification processes that were attributed to the conjugation of PEG and 2ME to TiO 2 NPs. Modifying TiO 2 NPs with PEG loaded with the 2ME drug revealed that the titanium dioxide nanocarrier possesses an effective adsorption capability, and we discuss their potential application as a system of drug delivery.

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