Influence of molecular adsorption on the dielectric properties of a single wall nanotube: a model sensor.

Recent measurements of the resonance frequency of a copper disk covered with carbon nanotube bundles have shown characteristic resonance shifts during exposure with various gas molecules. The shifts were interpreted as the change of the dielectric permittivity of the system forming the sensor due to the electric properties of the adsorbed molecules. Starting from a simplified sensor model formed by one single wall nanotube, we develop a self-consistent approach to describe the variation of the linear dielectric susceptibility of the tube at the atomic scale when molecules are adsorbed at its external surface. The sensitivity of this model sensor is tested as a function of the apolar or polar nature of the admolecules, their adsorption geometry, their concentration, and the characteristics of the tube (length, diameter,...). The comparison with data on dielectric constant changes vs adsorption, coming from measurements of the resonance frequency shifts, displays striking agreement for most of the molecular species considered.

Discernibility criterion in Fresnel projection microscopy.

The applicability of the Rayleigh separation criterion in Fresnel electron projection microscopy is studied theoretically. Quantum mechanical simulations of electron scattering by two C60 molecules, at energies of the order of 150 eV, are analysed on the basis of the discernibility criterion of two diffracting apertures. The simple separation criterion derived from Fresnel theory is found to remain pertinent, except for small source to object distances. At these distances, the critical separation distance between the two C60 molecules is such that the 'sucking-in' of the beam between the two nearby C60 molecules cannot be neglected. This leads to a discrepancy with the Rayleigh criterion.

Theoretical study of field emission by a four atoms nanotip: implications for carbon nanotubes observation.

A fully three-dimensional quantum model is developed to simulate the emission of electrons by a nanotip in applied fields ranging from 0.4 to 0.8 V/A and their diffusion by an extended molecule. It is shown that the widening of the beam, when the applied field is increased, can be attributed to an increase in the number of emitting atoms. Simulated images of a (9,0) carbon nanotube, in a Fresnel projection microscope-type setup, for various applied fields, reproduce the experimental, so-called, "sucking-in" effect. The relationship between this effect and the transmission probability of the nanotube is studied.