192
S. Taioli
In the 3D case of study, we assessed the frequency-dependent dielectric response
and energy-loss functions of diamond and graphite in two ways: a full ab initio
approach, in which we carry out time-dependent density functional simulations in
linear response for different momentum transfers, and a semiclassical model, based
on the Drude-Lorentz extension to finite momenta of the optical dielectric function.
We conclude that ab initio calculated dielectric functions lead to better agreement
with measured energy-loss spectra compared to the widely used Drude-Lorentz
model. This discrepancy is particularly evident for insulators and semiconductors
beyond the optical limit (q = 0), where single-particle excitations become relevant.
Furthermore, we show that the behaviour of the energy-loss function obtained at
different accuracy levels has a dramatic effect on other physical observables, such
as the inelastic mean free path and the stopping power in the low energy (<100 eV)
regime and thus on the accuracy of MC simulations of REEL spectra. Thus, the
major point of this work is to show that an accurate treatment of the electronelectron correlations beyond the random-phase approximation of the homogeneous
Fermi gas is necessary to increase the overall accuracy of the simulations to be
compared with REEL experiments. We also discuss a theoretical approach for taking
into account the anisotropic structure of HOPG in the Monte Carlo simulations of
charge transport. The anisotropic description of the dielectric response is achieved
by linearly combining the contributions to the inverse inelastic mean free path
and energy losses along the two main orthogonal directions identifying the layered
crystalline structure of graphite. Monte Carlo simulated spectra, obtained with our
anisotropic approach, are compared with acquired experimental data of reflection
electron energy loss and secondary electron spectra, showing a good agreement.
These findings validate the idea of the importance of considering properly weighted
interplanar and intra-planar interactions in the simulation of electron transport in
layered materials.
Furthermore, we discussed the mechanical behaviour of single and multilayer
graphene armours subjected to hypervelocity impacts of fullerene molecules, due to
their outstanding mechanical stability. The investigation of the interlayer synergy
between adjacent layers at the nanoscale determines that an optimal number
of layers, between 5 and 10, emerges that maximizes also the specific energy
dissipation under impact. These results suggest that multilayer 2D material-based
armours should be structured and optimized at the nanolevel, not relying on the
mere high-specific mechanical properties of the constituent materials.
Finally, we studied the mechanical properties of nanotruss networks and of
random foams, as a further example of 3D materials based on carbon. In this respect,
we conclude that nanotruss networks and random foams may represent exceptional
candidates for porous, flexible and high-strength materials. Basically they inherit
the marvellous mechanical properties of the parent material, graphene, adding a
third bearing stress dimension. In applications to gas adsorption and sieving, other
convenient 3D architectures are represented by the PGFs. Our results show that
the density of pillars dramatically affects the adsorption rather than the pillar type
as, under saturation conditions, the increase of pillar density results in a sensible
decrease of the amount of gas adsorbed.
S. Taioli
In the 3D case of study, we assessed the frequency-dependent dielectric response
and energy-loss functions of diamond and graphite in two ways: a full ab initio
approach, in which we carry out time-dependent density functional simulations in
linear response for different momentum transfers, and a semiclassical model, based
on the Drude-Lorentz extension to finite momenta of the optical dielectric function.
We conclude that ab initio calculated dielectric functions lead to better agreement
with measured energy-loss spectra compared to the widely used Drude-Lorentz
model. This discrepancy is particularly evident for insulators and semiconductors
beyond the optical limit (q = 0), where single-particle excitations become relevant.
Furthermore, we show that the behaviour of the energy-loss function obtained at
different accuracy levels has a dramatic effect on other physical observables, such
as the inelastic mean free path and the stopping power in the low energy (<100 eV)
regime and thus on the accuracy of MC simulations of REEL spectra. Thus, the
major point of this work is to show that an accurate treatment of the electronelectron correlations beyond the random-phase approximation of the homogeneous
Fermi gas is necessary to increase the overall accuracy of the simulations to be
compared with REEL experiments. We also discuss a theoretical approach for taking
into account the anisotropic structure of HOPG in the Monte Carlo simulations of
charge transport. The anisotropic description of the dielectric response is achieved
by linearly combining the contributions to the inverse inelastic mean free path
and energy losses along the two main orthogonal directions identifying the layered
crystalline structure of graphite. Monte Carlo simulated spectra, obtained with our
anisotropic approach, are compared with acquired experimental data of reflection
electron energy loss and secondary electron spectra, showing a good agreement.
These findings validate the idea of the importance of considering properly weighted
interplanar and intra-planar interactions in the simulation of electron transport in
layered materials.
Furthermore, we discussed the mechanical behaviour of single and multilayer
graphene armours subjected to hypervelocity impacts of fullerene molecules, due to
their outstanding mechanical stability. The investigation of the interlayer synergy
between adjacent layers at the nanoscale determines that an optimal number
of layers, between 5 and 10, emerges that maximizes also the specific energy
dissipation under impact. These results suggest that multilayer 2D material-based
armours should be structured and optimized at the nanolevel, not relying on the
mere high-specific mechanical properties of the constituent materials.
Finally, we studied the mechanical properties of nanotruss networks and of
random foams, as a further example of 3D materials based on carbon. In this respect,
we conclude that nanotruss networks and random foams may represent exceptional
candidates for porous, flexible and high-strength materials. Basically they inherit
the marvellous mechanical properties of the parent material, graphene, adding a
third bearing stress dimension. In applications to gas adsorption and sieving, other
convenient 3D architectures are represented by the PGFs. Our results show that
the density of pillars dramatically affects the adsorption rather than the pillar type
as, under saturation conditions, the increase of pillar density results in a sensible
decrease of the amount of gas adsorbed.