9 research outputs found
Stress variations near surfaces in diamond-like amorphous carbon
Using Monte Carlo simulations within the empirical potential approach, we
examine the effect produced by the surface environment on the atomic level
stresses in tetrahedral amorphous carbon. Both the distribution of stresses and
the distributions of sp^2 and sp^3 atoms as a function of depth from the
surface are highly inhomogeneous. They show the same close relationship between
local stress and bonding hybridization found previously in the bulk of the
material. Compressive local stress favors the formation of sp^3 sites, while
tensile stress favors the formation of sp^2 sites.Comment: 7pages, 4 figure
Probing the sp^2 dependence of elastic moduli in ultrahard diamond films
The structural and elastic properties of diamond nanocomposites and
ultrananocrystalline diamond films (UNCD) are investigated using both empirical
potentials and tight binding schemes. We find that both materials are extremely
hard, but their superb diamondlike properties are limited by their sp^2
component. In diamond composites, the sp^2 atoms are found in the matrix and
far from the interface with the inclusion, and they are responsible for the
softening of the material. In UNCD, the sp^2 atoms are located in the grain
boundaries. They offer relaxation mechanisms which relieve the strain but, on
the other hand, impose deformations that lead to softening. The higher the sp^2
component the less rigid these materials are.Comment: 10 pages, 3 figures. to appear in Diamond and Relarted Material
Structure, stability and stress properties of amorphous and nanostructured carbon films
Structural and mechanical properties of amorphous and nanocomposite carbon
are investigated using tight-binding molecular dynamics and Monte Carlo
simulations. In the case of amorphous carbon, we show that the variation of
sp^3 fraction as a function of density is linear over the whole range of
possible densities, and that the bulk moduli follow closely the power-law
variation suggested by Thorpe. We also review earlier work pertained to the
intrinsic stress state of tetrahedral amorphous carbon. In the case of
nanocomposites, we show that the diamond inclusions are stable only in dense
amorphous tetrahedral matrices. Their hardness is considerably higher than that
of pure amorphous carbon films. Fully relaxed diamond nanocomposites possess
zero average intrinsic stress.Comment: 10 pages, 6 figure
Energetics and stability of nanostructured amorphous carbon
Monte Carlo simulations, supplemented by ab initio calculations, shed light
into the energetics and thermodynamic stability of nanostructured amorphous
carbon. The interaction of the embedded nanocrystals with the host amorphous
matrix is shown to determine in a large degree the stability and the relative
energy differences among carbon phases. Diamonds are stable structures in
matrices with sp^3 fraction over 60%. Schwarzites are stable in low-coordinated
networks. Other sp^2-bonded structures are metastable.Comment: 11 pages, 7 figure
Calorimetry of hydrogen desorption from a-Si nanoparticles
The process of hydrogen desorption from amorphous silicon (a-Si) nanoparticles grown by plasma-enhanced chemical vapor deposition (PECVD) has been analyzed by differential scanning calorimetry (DSC), mass spectrometry, and infrared spectroscopy, with the aim of quantifying the energy exchanged. Two exothermic peaks centered at 330 and 410 C have been detected with energies per H atom of about 50 meV. This value has been compared with the results of theoretical calculations and is found to agree with the dissociation energy of Si-H groups of about 3.25 eV per H atom, provided that the formation energy per dangling bond in a-Si is about 1.15 eV. It is shown that this result is valid for a-Si:H films, too
Thermal transport in free-standing silicon membranes: influence of dimensional reduction and surface nanostructures
Nanostructuring provides a viable route to improve the thermoelectric performance of materials, even of those that in bulk form have very low figure of merit. This strategy would potentially enable the fabrication of thermoelectric devices based on silicon, the cheapest, most integrable and easiest to dope Earth-abundant semiconductor. A drastic reduction of the thermal conductivity, which would lead to a proportional enhancement of the figure of merit, was observed for silicon low-dimensional nanostructures, such as nanowires and ultra-thin membranes. Here we provide a detailed analysis of the phononic properties of the latter, and we show that dimensionality reduction alone is not sufficient to hinder heat transport to a great extent. In turn, the presence of surface roughness at the nanoscale reduces the thermal conductivity of sub-10 nm membranes up to 10 times with respect to bulk