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A Fazzio

Publications and source records attributed to A Fazzio.

At least 19 recordsLinked to original sources
PubMed2006

Structure and energetics of molecular point defects in ice Ih.

We present a first-principles study of the molecular vacancy and three distinct molecular interstitial structures in ice Ih. The results indicate that, due to its bonding to the surrounding hydrogen-bond network, the bond-center (Bc) configuration is the favored molecular interstitial in ice Ih. A comparison between the vacancy and the Bc interstitial suggests that the former is the predominant molecular point defect for T approximately < 200K although a crossover scenario in which the latter becomes favored below the melting point is conceivable.

Journal Article↗
PubMed2006

C59Si on the monohydride Si(100):H-(2 x 1) surface.

We propose the use of the Si atom in the experimentally observed C59Si molecule as a possible way to controllably anchor fullerene molecules on a Si surface, due to the formation of a strong bond to one of the Si surface atoms. All our results are based on ab initio total energy density functional theory, and we obtain that the binding energy is on the order of 2.1 eV, approximately 1.4 eV more stable than a C60 bonded in a similar situation. A possible route to obtain such adsorption via a (C59Si)2 dimer is examined, and we find the whole process to be exothermic by approximately 0.2 eV.

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PubMed2006

Orientational defects in ice Ih: an interpretation of electrical conductivity measurements.

We present a first-principles study of the structure and energetics of Bjerrum defects in ice Ih and compare the results to experimental electrical conductivity data. While the DFT result for the activation energy is in good agreement with experiment, we find that its two components have quite different values. Aside from providing new insight into the fundamental parameters of the microscopic electrical theory of ice, our results suggest the activity of traps in doped ice in the temperature regime typically assumed to be controlled by the free migration of L defects.

Journal Article↗
PubMed2006

Oxygen clamps in gold nanowires.

We investigate how the insertion of an oxygen atom in an atomically thin gold nanowire can affect its rupture. We find, using ab initio total energy density functional theory calculations, that O atoms when inserted in gold nanowires form not only stable but also very strong bonds, in such a way that they can extract atoms from a stable tip, serving in this way as a clamp that could be used to pull a string of gold atoms.

Journal Article↗
PubMed2005

Vacancy formation process in carbon nanotubes: first-principles approach.

The electronic and structural properties of a single-walled carbon nanotube (SWNT) under mechanical deformation are studied using first-principles calculations based on the density functional theory. A force is applied over one particular C-atom with enough strength to break the chemical bonds between the atom and its nearest neighbors, leading to a final configuration represented by one tube with a vacancy and an isolated C-atom inside the tube. Our investigation demonstrates that there is a tendency that the first bond to break is the one most parallel possible to the tube axis and, after, the remaining two other bonds are broken. The analysis of the electronic charge densities, just before and after the bonds breaking, helps to elucidate how the vacancy is formed on an atom-by-atom basis. In particular, for tubes with a diameter around 11 angstroms, it is shown that the chemical bonds start to break only when the externally applied force is of the order of 14 nN and it is independent of the chirality. The formation energies for the vacancies created using this process are almost independent of the chirality, otherwise the bonds broken and the reconstruction are dependent.

Journal Article↗
PubMed2005

Bundling up carbon nanotubes through Wigner defects.

We show, using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right beside a vacancy, which are present in irradiated graphite, can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizable amount, clearly indicating its importance for the mechanical properties of nanotube bundles.

Carbon↗
PubMed2003

First-principles calculations of carbon nanotubes adsorbed on Si(001).

We report first-principles calculations on the adsorption of a metallic (6,6) single-walled carbon nanotube (SWCN) on the Si(001) surface. We find stable geometries for the nanotube between two consecutive dimer rows where C-Si chemical bonds are formed. The binding energy in the most stable geometry is found to be 0.2 eV/A. Concerning the electronic properties, the most stable structure shows an increase in the density of states near the Fermi level due to the formation of C-Si bonds enhancing the metallic character of the nanotube by the contact with the surface. These properties may lead one to consider metallic SWCNs adsorbed on Si substrates for interconnections and contacts on future nanoscale devices. Finally, the nature of the nanotube-surface interaction for nanotubes of larger diameters is also discussed.

Journal Article↗
PubMed2003

Effect of impurities in the large Au-Au distances in gold nanowires.

Experimentally obtained atomically thin gold nanowires have presented exceedingly large Au-Au interatomic distances before they break. Since no theoretical calculations of pure gold nanowires have been able to produce such large distances, we have investigated, through ab initio calculations, how impurities could affect them. We have studied the effect of H, B, C, N, O, and S impurities on the nanowire electronic and structural properties, in particular how they affect the maximum Au-Au bond length. We find that the most likely candidates to explain the distances in the range of 3.6 A and 4.8 A are H and S impurity atoms, respectively.

Journal Article↗
PubMed2003

Oxidation at the Si/SiO2 interface: influence of the spin degree of freedom.

We show, using first-principles spin-polarized total-energy calculations, that depending on the spin configuration of the system, the reaction of an O2 molecule with a Si-Si bond in a suboxidized region might result either in a peroxy linkage defect (for a singlet spin state) or in a perfect Si-O-Si bond plus an interstitial O atom (for a triplet spin state). Even though the singlet has a lower energy than the triplet configuration, we find a rather small probability for triplet to singlet conversion. Therefore, as the O2 in an SiO2 interstitial site has a triplet configuration, this reaction spin dependence may have a strong influence on the high quality of the Si/SiO2 interface.

Journal Article↗
PubMed2001

How do gold nanowires break?

Suspended gold nanowires have recently been made in an ultrahigh vacuum and were imaged by electron microscopy. Using realistic molecular dynamics simulation, we study the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. We show how defects induce the formation of constrictions that eventually will form the one-atom chains. We find that these chains, before breaking, are five atoms long, which is in excellent agreement with experimental results. After the nanowire's rupture, we analyze the structure of the Au tip, which we believe will be universally present due to its highly symmetric nature.

Journal Article↗
PubMed2001

O2 diffusion in SiO2: triplet versus singlet.

We address the diffusion of the oxygen molecule in SiO2, using first-principles spin-polarized total-energy calculations. We find that the potential energy surfaces for the singlet and triplet states are very different in certain regions, and that the O2 molecule preserves its spin-triplet ground state not only at its most stable interstitial position inside the solid but also throughout its diffusion pathway. Therefore, the singlet state is not a good approximation to describe the behavior of O2 inside SiO2, and spin-polarization effects are fundamental to understand the properties of this system.

Journal Article↗
PubMed2001

Two-atom structures of Ge on Si(100): dimers versus adatom pairs.

We present an ab initio study of the properties of structures composed of two and four Ge atoms adsorbed on the troughs of the Si(100) surface, and we conclude that these structures are all composed of dimers, with a chemical bonding between the adatoms. We compare our calculated local density of states with scanning tunneling microscope (STM) images, and we show that these Ge dimers adsorbed on the troughs between the substrate dimer rows can be identified with the adatom pairs observed experimentally. We also show that the local buckling of the substrate dimers can give rise to similar structures with very different STM images.

Journal Article↗
PubMed1999

Signal transduction in rat vascular smooth muscle cells: control of osmotically induced aldose reductase expression by cell kinases and phosphatases.

We have studied the osmotically induced gene expression (measured as chloramphenicol acetyl transferase (CAT) reporter gene expression) in rat smooth muscle cell primary cultures (rSMC), under the control of osmotic response elements (ORE). It was found that osmotically induced gene expression is sensitive to signal transduction inhibitors and activators. In particular, protein kinase C inhibition by calphostin C prevented gene expression by osmotic response. On the other hand, receptor tyrosine kinase inhibition has been shown to produce an enhancement of gene expression. This suggests that tyrosine kinase receptor activation exerts an inhibitory action on ORE induced gene expression. Gene expression was also induced by treating cells with PD098059, a specific inhibitor of mitogen-activated protein kinase kinase. Moreover, the same inhibitors and activators have been shown to affect the hyperosmosis induced expression of aldose reductase gene.

Aldehyde Reductase↗
PubMed1998

RRR-alpha-tocopherol regulation of gene transcription in response to the cell oxidant status.

RRR-alpha-Tocopherol, but not RRR-beta-tocopherol, negative regulates proliferation of vascular smooth muscle cells at physiological concentrations. At the same concentrations RRR-alpha-tocopherol inhibits protein kinase C activity, whereas RRR-beta-tocopherol is ineffective. Furthermore, RRR-beta-tocopherol prevents the inhibition of cell growth and of protein kinase C activity caused by RRR-alpha-tocopherol. The negative regulation by RRR-alpha-tocopherol of protein kinase C activity appears to be the cause of smooth muscle cell growth inhibition. RRR-alpha-Tocopherol does not act by binding to protein kinase C directly but presumably by preventing protein kinase C activation. A second RRR-alpha-tocopherol effect has been found at the level of AP 1, the latter becoming activated by RRR-alpha-tocopherol under condition of protein kinase C inhibition or down regulation. AP-1 inhibition by RRR-alpha-tocopherol is seen, however, under condition of protein kinase C stimulation. Compositional changes of AP-1 have been found to be at the basis of the RRR-alpha-tocopherol effects. RRR-beta-tocopherol, provided with similar antioxidant properties, not only it does not affect AP 1 but it prevents the effects of RRR-alpha-tocopherol. Moreover, it has been observed that RRR-alpha-tocopherol is able to affect TRE regulated gene transcription. It is concluded that RRR-alpha-tocopherol acts specifically in vascular smooth muscle cells, by controlling a signal transduction pathway leading to cell proliferation by a non-antioxidant mechanism.

Animals↗
PubMed1997

The effect of alpha-tocopherol and beta-tocopherol on proliferation, protein kinase C activity and gene expression in different cell lines.

alpha-Tocopherol, but not beta-tocopherol, negatively regulates proliferation of A7r5 vascular smooth muscle cells at physiological concentration. The HeLa cell line was not affected whereas the Chinese hamster ovary cell line (CHO) was slightly inhibited by both alpha-tocopherol and beta-tocopherol. In A7r5 cells alpha-tocopherol inhibited protein kinase C activity, and this correlated with inhibition of proliferation. beta-Tocopherol did not inhibit either protein kinase C or proliferation. In HeLa cells no inhibition by alpha-tocopherol or beta-tocopherol of protein kinase C activity and cell proliferation was observed. In Chinese hamster ovary cells both tocopherols inhibited protein kinase C activity but not proliferation. Thus in the latter cells proliferation was not protein kinase C-dependent. In A7r5 cells alpha-tocopherol but not beta-tocopherol activated AP-1-mediated gene expression. In HeLa cells no change in gene expression was observed in agreement with the finding that also protein kinase C was not affected. In CHO cells gene-expression was activated by both alpha-tocopherol and beta-tocopherol. In this case also a positive correlation was found with similar inhibition of protein kinase C activity. In these cells, however, the changes at the level of protein kinase C activity and gene expression did not result in proliferation changes. The effect of alpha-tocopherol and beta-tocopherol on protein kinase C activity and gene expression suggest a cause-to-effect relationship. Inhibition of proliferation, however, correlates in the case of A7r5 and HeLa cells but not in the case of CHO suggesting a different proliferation pathway for these cells.

Animals↗