274 research outputs found
Crystal engineering using functionalized adamantane
We performed a first principles investigation on the structural, electronic,
and optical properties of crystals made of chemically functionalized adamantane
molecules. Several molecular building blocks, formed by boron and nitrogen
substitutional functionalizations, were considered to build zincblende and
wurtzite crystals, and the resulting structures presented large bulk moduli and
cohesive energies, wide and direct bandgaps, and low dielectric constants
(low- materials). Those properties provide stability for such
structures up to room temperature, superior to those of typical molecular
crystals. This indicates a possible road map for crystal engineering using
functionalized diamondoids, with potential applications ranging from space
filling between conducting wires in nanodevices to nano-electro-mechanical
systems
Structural, Electronic, and Vibrational Properties of Amino-adamantane and Rimantadine Isomers
We performed a first principles total energy investigation on the structural,
electronic, and vibrational properties of adamantane molecules, functionalized
with amine and ethanamine groups. We computed the vibrational signatures of
amantadine and rimantadine isomers with the functional groups bonded to
different carbon sites. By comparing our results with recent infrared and Raman
spectroscopic data, we discuss the possible presence of different isomers in
experimental samples
Functionalized adamantane: fundamental building blocks for nanostructure self-assembly
We report first principles calculations on the electronic and structural
properties of chemically functionalized adamantane molecules, either in
isolated or crystalline forms. Boron and nitrogen functionalized molecules,
aza-, tetra-aza-, bora-, and tetra-bora-adamantane, were found to be very
stable in terms of energetics, consistent with available experimental data.
Additionally, a hypothetical molecular crystal in a zincblende structure,
involving the pair tetra-bora-adamantane and tetra-aza-adamantane, was
investigated. This molecular crystal presented a direct and large electronic
bandgap and a bulk modulus of 20 GPa. The viability of using those
functionalized molecules as fundamental building blocks for nanostructure
self-assembly is discussed
Electronic properties and hyperfine fields of nickel-related complexes in diamond
We carried out a first principles investigation on the microscopic properties
of nickel-related defect centers in diamond. Several configurations, involving
substitutional and interstitial nickel impurities, have been considered either
in isolated configurations or forming complexes with other defects, such as
vacancies and boron and nitrogen dopants. The results, in terms of spin,
symmetry, and hyperfine fields, were compared with the available experimental
data on electrically active centers in synthetic diamond. Several microscopic
models, previously proposed to explain those data, have been confirmed by this
investigation, while some models could be discarded. We also provided new
insights on the microscopic structure of several of those centers.Comment: 21 pages, 8 figure
Electrical conductivity of MgSiO3 at high temperatures and pressures: implications for the Earth's mantle
The electrical conductivity of magnesium silicate MgSiO3 has been studied,
using the framework of the first-principles density functional theory and the
Boltzmann transport theory, under the thermodynamic conditions of the Earth's
lower mantle. We find that the conductivity of pristine MgSiO3 depends strongly
on the structural phase of the material, as well as on temperature and
pressure. The conductivity of the perovskite phase increases with increasing
pressure (depth of the lower mantle) up to 90 GPa, then decreases at higher
pressures due to a change in the material's band gap transition from direct to
indirect. Finally, the structural phase transition that MgSiO3 undergoes near
the bottom of the lower mantle, from perovskite to post-perovskite, causes an
increase in the conductivity of MgSiO3, which should contribute to the increase
in the electrical conductivity of the Earth's mantle under the thermodynamic
conditions of the Earth's D" layer.Comment: 16 pages, 4 figures, 2 table
Doped 2D diamond: properties and applications
In the present paper, we investigate the structural, thermodynamic, dynamic,
elastic, and electronic properties of doped 2D diamond CX (X = B or N)
nanosheets in both AAA and ABC stacking configurations, by
first-principles calculations. Those systems are composed of 3 diamond-like
graphene sheets, with an undoped graphene layer between two 50% doped ones. Our
results, based on the analysis of ab-initio molecular dynamics simulations,
phonon dispersion spectra, and Born's criteria for mechanical stability,
revealed that all four structures are stable. Additionally, their standard
enthalpy of formation values are similar to the one of pristine 2D diamond,
recently synthesized by compressing three graphene layers. The CX (X =
B or N) systems exhibit high elastic constant values and stiffness comparable
to the diamond. The CN nanosheets present wide indirect band gaps that
could be advantageous for applications similar to the ones of the hexagonal
boron nitride (h-BN), such as a substrate for high-mobility 2D devices. On the
other hand, the CB systems are semiconductors with direct band gaps, in
the 1.6 - 2.0 eV range, and small effective masses, which are characteristics
that may be favorable to high carrier mobility and optoelectronics
applications
Alterações estruturais de Cr-beidelita tratada até 1350 °C em atmosferas de oxigênio ou nitrogênio
Beidellite clay mineral after intercalation of OH-Cr(III) species were thermally analyzed up to 1350 °C in oxygen and nitrogenatmospheres. OH-Cr-beidellite can be used as a pillared clay precursor for catalysis or as adsorbent applications. However, inthis paper beidellite enriched in chromium were analyzed at different thermal treatments up to high temperature for evaluatingstructural changes for possible future ceramic applications. The structural changes were followed by thermal analysis and X-raydiffraction. The thermal treatment of OH-Cr-beidellite in oxygen and nitrogen atmospheres developed different mineralogicalphases up to 1050 °C, but at higher temperatures, the same phases were developed in both atmosphere treatments. Eskolaitephase (Cr2O3) appeared in the sample after heating at 400 °C in oxygen atmosphere, whereas grimaldite (CrO-OH) in nitrogenatmosphere, maintaining the starting phases. At 1000 °C the raw clay minerals disappeared, as it is knew. At 1050 °C in nitrogenatmosphere, grimaldite was absent and eskolaite appeared. At 1350 °C in the samples calcined in both atmospheres, quartz,cristobalite and mullite as the main phases and in lower contents aluminum oxide and aluminum-chromium oxide [(Al,Cr)2O3]were present.O mineral de argila de beidelita após intercalação das espécies OH-Cr(III) foi termicamente analisado até 1350 °C em atmosferas de oxigénio e nitrogênio. A OH-Cr-beidelita pode ser utilizada como um precursor de argila pilarizada para catálise ou para aplicações adsorventes. No entanto, neste trabalho, a beidelita enriquecida em cromo foi analisada em diferentes tratamentos térmicos até alta temperatura para avaliação de mudanças estruturais para possíveis aplicações cerâmicas futuras. As alterações estruturais foram acompanhadas por análise térmica e difração de raios X. O tratamento térmico da OH-Cr-beidelita em atmosferas de oxigênio e nitrogênio desenvolveu diferentes fases mineralógicas até 1050 °C, mas em temperaturas mais altas, as mesmas fases foram desenvolvidas em ambas atmosferas. A fase eskolaita (Cr2 O3 ) apareceu na amostra após aquecimento a 400 °C em atmosfera de oxigênio, enquanto grimaldita (CrO-OH) em atmosfera de nitrogênio, mantendo as fases iniciais. Em 1000 °C, os minerais originais da argila desapareceram, como é conhecido. Em 1050 °C em atmosfera de nitrogênio, a grimaldita estava ausente e apareceu eskolaita. Em amostras calcinadas a 1350 °C em ambas as atmosferas, estavam presentes quartzo, cristobolita e mulita como fases principais e em menores teores óxido de alumínio e óxido de alumínio e cromo [(Al,Cr)2 O3 ].Centro de Tecnología de Recursos Minerales y Cerámic
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