The Experts below are selected from a list of 174 Experts worldwide ranked by ideXlab platform
Gianaurelio Cuniberti - One of the best experts on this subject based on the ideXlab platform.
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vibrational modes and low temperature thermal properties of graphene and carbon nanotubes minimal Force Constant model
Physical Review B, 2008Co-Authors: Janina Zimmermann, P Pavone, Gianaurelio CunibertiAbstract:We present a phenomenological Force-Constant model developed for the description of lattice dynamics of $s{p}^{2}$ hybridized carbon networks. Within this model approach, we introduce a set of parameters to calculate the phonon dispersion of graphene by fitting the ab initio dispersion. Vibrational modes of carbon nanotubes are obtained by folding the two-dimensional (2D) dispersion of graphene and applying special corrections for the low-frequency modes. Particular attention is paid to the exact dispersion law of the acoustic modes, which determine the low-frequency thermal properties and reveal quantum size effects in carbon nanotubes. On the basis of the resulting phonon spectra, we calculate the specific heat and the thermal conductance for several achiral nanotubes of different diameters. Through the temperature dependence of the specific heat, we demonstrate that phonon spectra of carbon nanotubes show one-dimensional behavior and that the phonon sub-bands are quantized at low temperatures. Consequently, we prove the quantization of the phonon thermal conductance by means of an analysis based on the Landauer theory of heat transport.
Janina Zimmermann - One of the best experts on this subject based on the ideXlab platform.
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vibrational modes and low temperature thermal properties of graphene and carbon nanotubes minimal Force Constant model
Physical Review B, 2008Co-Authors: Janina Zimmermann, P Pavone, Gianaurelio CunibertiAbstract:We present a phenomenological Force-Constant model developed for the description of lattice dynamics of $s{p}^{2}$ hybridized carbon networks. Within this model approach, we introduce a set of parameters to calculate the phonon dispersion of graphene by fitting the ab initio dispersion. Vibrational modes of carbon nanotubes are obtained by folding the two-dimensional (2D) dispersion of graphene and applying special corrections for the low-frequency modes. Particular attention is paid to the exact dispersion law of the acoustic modes, which determine the low-frequency thermal properties and reveal quantum size effects in carbon nanotubes. On the basis of the resulting phonon spectra, we calculate the specific heat and the thermal conductance for several achiral nanotubes of different diameters. Through the temperature dependence of the specific heat, we demonstrate that phonon spectra of carbon nanotubes show one-dimensional behavior and that the phonon sub-bands are quantized at low temperatures. Consequently, we prove the quantization of the phonon thermal conductance by means of an analysis based on the Landauer theory of heat transport.
Stefano De Gironcoli - One of the best experts on this subject based on the ideXlab platform.
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phonons in si ge systems an ab initio interatomic Force Constant approach
Physical Review B, 1992Co-Authors: Stefano De GironcoliAbstract:The vibrational properties of Si-Ge systems are studied theoretically with ab initio techniques. Full dispersion relations for pure silicon and germanium crystals under several (homogeneous and epitaxial) strain conditions are obtained from interatomic Force Constants (IFC's). High-symmetry vibrations for a few short-period ordered superlattices (SL's) are studied from first principles as well
Peter Politzer - One of the best experts on this subject based on the ideXlab platform.
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analysis of diatomic bond dissociation and formation in terms of the reaction Force and the position dependent reaction Force Constant
Journal of Molecular Modeling, 2009Co-Authors: Peter Politzer, Jane S Murray, Alejandro Torolabbe, Timothy ClarkAbstract:Bond dissociation and formation in diatomic molecules are analyzed in terms of the reaction Force F(R) and the reaction Force Constant κ(R). These were determined for a group of 13 molecules from their extended-Rydberg potential energy functions V(R), which are of near-experimental quality. From F(R) and κ(R) comes a two-stage description of dissociation/formation. In dissociation, the first stage involves stretching of the bond, which is opposed by an increasingly negative retarding Force F(R). This reaches a minimum and then begins to weaken in the second stage, which is the transition from stretched molecule to free atoms. Bond formation begins with the reverse transition, driven by a positive F(R) which reaches a maximum for the stretched molecule and then becomes a decreasing restoring Force. In the stages in which the system is a stretched molecule, κ(R) is positive with its maximum at the equilibrium bond length; it is zero at the minimum or maximum of F(R), and negative throughout the transition stages, going through a minimum. κ(R) <0 has been found to characterize the transition portion of a reaction. This description of dissociation/formation is reinForced by computed B3LYP and Hartree-Fock Force Constants at different atom separations for the singlet molecules. Hartree-Fock wave function stability assessments suggest that, for the single-bonded singlet molecules, the onset of electron unpairing in dissociation comes in the neighborhood of the F(R) minimum.
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Reaction Force Constant and projected Force Constants of vibrational modes along the path of an intramolecular proton transfer reaction
Chemical Physics Letters, 2008Co-Authors: Pablo Jaque, Peter Politzer, Alejandro Toro-labbé, Paul GeerlingsAbstract:Abstract We have explored the relationships between the reaction Force F ( ξ ), the reaction Force Constant κ ( ξ ) and the projected Force Constants of the intramolecular proton transfer HO−N S → O N−SH along the intrinsic reaction coordinate ξ . The structural changes and energetics associated with the reaction are analyzed in terms of the three regions defined by F ( ξ ): reactant, transition and product. The significance of the similarity between κ ( ξ ) and the variation of the Force Constant associated to the reaction coordinate mode, k ξ ( ξ ), is discussed in detail.
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the position dependent reaction Force Constant in bond dissociation formation
Collection of Czechoslovak Chemical Communications, 2008Co-Authors: Peter Politzer, Jane S MurrayAbstract:The concept of a position-dependent reaction Force Constant K(R) can be used to distinguish the two phases of bond dissociation or formation: stretched bond, K(R) > 0, and interacting but separate fragments, K(R) < 0. The transition between these phases is at K(R) = 0, which coincides with the minimum (for dissociation) or maximum (for formation) of the reaction Force. As was shown earlier, all of these occur (for diatomic molecules) at the separation R at which the system's energy relative to equilibrium is about 27% of its dissociation energy.
Paul Geerlings - One of the best experts on this subject based on the ideXlab platform.
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Reaction Force Constant and projected Force Constants of vibrational modes along the path of an intramolecular proton transfer reaction
Chemical Physics Letters, 2008Co-Authors: Pablo Jaque, Peter Politzer, Alejandro Toro-labbé, Paul GeerlingsAbstract:Abstract We have explored the relationships between the reaction Force F ( ξ ), the reaction Force Constant κ ( ξ ) and the projected Force Constants of the intramolecular proton transfer HO−N S → O N−SH along the intrinsic reaction coordinate ξ . The structural changes and energetics associated with the reaction are analyzed in terms of the three regions defined by F ( ξ ): reactant, transition and product. The significance of the similarity between κ ( ξ ) and the variation of the Force Constant associated to the reaction coordinate mode, k ξ ( ξ ), is discussed in detail.