The sensing properties of carbon nanotubes (CNTs) decorated with gold nanoparticles have been investigated by means of combined theoretical and experimental approaches. On one hand, first-principles and nonequilibrium Green's functions techniques give access to the microscopic features of the sensing mechanisms in individual nanotubes, such as electronic charge transfers and quantum conductances. On the other hand, drop coating deposition of carbon nanotubes decorated with gold nanoparticles onto sensor substrates and their characterization in the detection of pollutants such as NO(2), CO, and C(6)H(6) provide insight into the sensing ability of nanotube mats. Using the present combined approaches, the improvement in the detection of some specific gases (NO(2) and CO) using Au-functionalized nanotubes is explained. However, for other gases such as C(6)H(6), the Au nanoparticles do not seem to play a crucial role in the sensing process when compared with pristine CNTs functionalized with oxygen plasma. Indeed, these different situations can be explained by identifying the relationship between the change of resistance (macroscopic feature) and the shift of the Fermi level (microscopic feature) after gas adsorption. The understanding of the sensing ability at the atomic level opens the way to design new gas sensors and to tune their selectivity by predicting the nature of the metal that is the most appropriate to detect specific molecular species.

No abstract

We report the first quasiparticle calculations of the newly observed wurtzite polymorph of InAs and GaAs. The calculations are performed in the GW approximation using plane waves and pseudopotentials. For comparison we also report the study of the zinc-blende phase within the same approximations. In the InAs compound the In 4d electrons play a very important role: whether they are frozen in the core or not, leads either to a correct or a wrong band ordering (negative gap) within the Local Density Appproximation (LDA). We have calculated the GW band structure in both cases. In the first approach, we have estimated the correction to the pd repulsion calculated within the LDA and included this effect in the calculation of the GW corrections to the LDA spectrum. In the second case, we circumvent the negative gap problem by first using the screened exchange approximation and then calculating the GW corrections starting from the so obtained eigenvalues and eigenfunctions. This approach leads to a more realistic band-structure and was also used for GaAs. For both InAs and GaAs in the wurtzite phase we predict an increase of the quasiparticle gap with respect to the zinc-blende polytype.

First-principles and non-equilibrium Green's function approaches are used to predict spin-polarized electronic transport in monatomic carbon chains covalently connected to graphene nanoribbons, as recently synthetized experimentally (Jin, C.; et al. Phys. Rev. Lett. 2009, 102, 205501-205504). Quantum electron conductances exhibit narrow resonant states resulting from the simultaneous presence of open conductance channels in the contact region and on the chain atoms. Odd-numbered chains, which acquire metallic or semiconducting character depending on the nature of the edge at the graphene contact, always display a net spin polarization. The combination of electrical and magnetic properties of chains and contacts results in nanodevices with intriguing spintronic properties such as the coexistence of magnetic and semiconducting behaviors.