62 research outputs found
Bioactivity of TiN-coated titanium implants
No full textTitanium nitride has excellent corrosion and wear resistance properties, and has been used as a hard coating material on titanium hip prostheses. Analysis of explants reveals that calcium phosphate phases grow spontaneously and stick strongly on TiN-coated hip prosthesis heads, indicating a degree of bioactivity of the implant surface which is absent in standard uncoated titanium implants. We investigate the mechanism of TiN oxidation using spectroscopic and first principles molecular dynamics techniques. We find that the deposition of Ca2+ ions which is the first step of calcium phosphate nucleation is favoured by TiOxNy oxynitride surface phases. This is due to the presence of mixed-valence states of the surface Ti atoms which leads to localisation of negative charge on surface oxygens, promoting the adsorption of Ca2+ ions. These results indicate that nitridation and controlled oxidation of titanium implant surfaces can promote the in vivo formation of bone-like material
Optical phonons of graphene and nanotubes
No full textWe review the optical phonon dispersions of graphene. In particular, we focus on the presence of two Kohn anomalies in the highest optical phonon branch at the Gamma and K points of the Brillouin zone. We then show how graphene can be used as a model for the calculation of phonons in carbon nanotubes. Finally, we present the beyond Born-Oppenheimer corrections to their phonon dispersions. These are experimentally revealed in the Raman spectra of doped samples
Phonon Linewidths and Electron Phonon Coupling in Nanotubes
No full text5 pages, 4 figures (correction in label of fig 3)International audienceWe prove that Electron-phonon coupling (EPC) is the major source of broadening for the Raman G and G- peaks in graphite and metallic nanotubes. This allows us to directly measure the optical-phonon EPCs from the G and G- linewidths. The experimental EPCs compare extremely well with those from density functional theory. We show that the EPC explains the difference in the Raman spectra of metallic and semiconducting nanotubes and their dependence on tube diameter. We dismiss the common assignment of the G- peak in metallic nanotubes to a Fano resonance between phonons and plasmons. We assign the G+ and G- peaks to TO (tangential) and LO (axial) modes
Electron Transport and Hot Phonons in Carbon Nanotubes
No full text4 pages, 1 figureWe demonstrate the key role of phonon occupation in limiting the high-field ballistic transport in metallic carbon nanotubes. In particular, we provide a simple analytic formula for the electron transport scattering length, that we validate by accurate first principles calculations on (6,6) and (11,11) nanotubes. The comparison of our results with the scattering lengths fitted from experimental I-V curves indicates the presence of a non-equilibrium optical phonon heating induced by electron transport. We predict an effective temperature for optical phonons of thousands Kelvin
Electron-electron interactions and doping dependence of the two-phonon Raman intensity in Graphene
No full textErratum: Electron transport and hot phonons in carbon nanotubes (Phys. Rev. Lett. (2005) 95 (236802))
No full textOptical phonons in nanotubes: electron-phonon coupling, Kohn anomalies, Peierls distortions and dynamic effects
No full textDFT modelling of ceramic materials and interfaces
No full textS.271-292The introduction of computer simulations has promoted significant design optimisation and cost reduction in many fields of engineering. Nowadays, atomistic modelling of materials is becoming time and cost effective not only for pure research, but also for cutting-edge engineering applications. In this paper we present an overview of atomistic materials modelling, with particular emphasis on Quantum Mechanical (QM) simulations based on the Density Functional Theory (DFT). As examples of applications to ceramics, we report about computational investigations of the oxidation of metal surfaces, of the chemical reactivity of biomaterials, of the reinforcement mechanisms in nano-composites, and of graphitisation of SiC.35Nr.3-
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