102 research outputs found
A Brief Review on Syntheses, Structures and Applications of Nanoscrolls
Nanoscrolls are papyrus-like nanostructures which present unique properties
due to their open ended morphology. These properties can be exploited in a
plethora of technological applications, leading to the design of novel and
interesting devices. During the past decade, significant advances in the
synthesis and characterization of these structures have been made, but many
challenges still remain. In this mini review we provide an overview on their
history, experimental synthesis methods, basic properties and application
perspectives
One-dimensional Silicon and Germanium Nanostructures With No Carbon Analogues
In this work we report new silicon and germanium tubular nanostructures with
no corresponding stable carbon analogues. The electronic and mechanical
properties of these new tubes were investigated through ab initio methods. Our
results show that the structures have lower energy than their corresponding
nanoribbon structures and are stable up to high temperatures (500 and 1000 K,
for silicon and germanium tubes, respectively). Both tubes are semiconducting
with small indirect band gaps, which can be significantly altered by both
compressive and tensile strains. Large bandgap variations of almost 50% were
observed for strain rates as small as 3%, suggesting possible applications in
sensor devices. They also present high Young's modulus values (0.25 and 0.15
TPa, respectively). TEM images were simulated to help the identification of
these new structures
Mechanical Properties and Fracture Dynamics of Silicene Membranes
As graphene became one of the most important materials today, there is a
renewed interest on others similar structures. One example is silicene, the
silicon analogue of graphene. It share some the remarkable graphene properties,
such as the Dirac cone, but presents some distinct ones, such as a pronounced
structural buckling. We have investigated, through density functional based
tight-binding (DFTB), as well as reactive molecular dynamics (using ReaxFF),
the mechanical properties of suspended single-layer silicene. We calculated the
elastic constants, analyzed the fracture patterns and edge reconstructions. We
also addressed the stress distributions, unbuckling mechanisms and the fracture
dependence on the temperature. We analysed the differences due to distinct edge
morphologies, namely zigzag and armchair
Structure and Dynamics of Boron Nitride Nanoscrolls
Carbon nanoscrolls (CNSs) are structures formed by rolling up graphene layers
into a papyruslike shape. CNNs have been experimentally produced by different
groups. Boron nitride nanoscrolls (BNNSs) are similar structures using boron
nitride instead of graphene layers. In this work we report molecular mechanics
and molecular dynamics results for the structural and dynamical aspects of BNNS
formation. Similarly to CNS, BNNS formation is dominated by two major energy
contributions, the increase in the elastic energy and the energetic gain due to
van der Waals interactions of the overlapping surface of the rolled layers. The
armchair scrolls are the most stable configuration while zigzag scrolls are
metastable structures which can be thermally converted to armchair. Chiral
scrolls are unstable and tend to evolve to zigzag or armchair configurations
depending on their initial geometries. The possible experimental routes to
produce BNNSs are also addressed
Dynamical Aspects Of The Unzipping Of Multiwalled Boron Nitride Nanotubes.
Boron nitride nanoribbons (BNNRs) exhibit very interesting magnetic properties, which could be very useful in the development of spintronic based devices. One possible route to obtain BNNRs is through the unzipping of boron nitride nanotubes (BNNTs), which have been already experimentally realized. In this work, different aspects of the unzipping process of BNNTs were investigated through fully atomistic molecular dynamics simulations using a classical reactive force field (ReaxFF). We investigated multiwalled BNNTs of different diameters and chiralities. Our results show that chirality plays a very important role in the unzipping process, as well as the interlayer coupling. These combined aspects significantly change the fracturing patterns and several other features of the unzipping processes in comparison to the ones observed for carbon nanotubes. Also, similar to carbon nanotubes, defective BNNTs can create regions of very high curvature which can act as a path to the unzipping process.1519147-5
Curved Graphene Nanoribbons: Structure and Dynamics of Carbon Nanobelts
Carbon nanoribbons (CNRs) are graphene (planar) structures with large aspect
ratio. Carbon nanobelts (CNBs) are small graphene nanoribbons rolled up into
spiral-like structures, i. e., carbon nanoscrolls (CNSs) with large aspect
ratio. In this work we investigated the energetics and dynamical aspects of
CNBs formed from rolling up CNRs. We have carried out molecular dynamics
simulations using reactive empirical bond-order potentials. Our results show
that similarly to CNSs, CNBs formation is dominated by two major energy
contribution, the increase in the elastic energy due to the bending of the
initial planar configuration (decreasing structural stability) and the
energetic gain due to van der Waals interactions of the overlapping surface of
the rolled layers (increasing structural stability). Beyond a critical diameter
value these scrolled structures can be even more stable (in terms of energy)
than their equivalent planar configurations. In contrast to CNSs that require
energy assisted processes (sonication, chemical reactions, etc.) to be formed,
CNBs can be spontaneously formed from low temperature driven processes. Long
CNBs (length of 30.0 nm) tend to exhibit self-folded racket-like
conformations with formation dynamics very similar to the one observed for long
carbon nanotubes. Shorter CNBs will be more likely to form perfect scrolled
structures. Possible synthetic routes to fabricate CNBs from graphene membranes
are also addressed
Rotação adequada.
Entre os fatores capazes de proporcionar redução na produtividade das culturas encontram-se as plantas daninhas, que podem afetar a produção agrícola e econômica devido, principalmente, às interferências negativas impostas por sua presença, como a competição por água, nutrientes, luz e efeitos alelopáticos. Atualmente, o controle de plantas daninhas se dá, predominantemente, pelo método químico, com a aplicação de herbicidas. No entanto, essa medida utilizada isoladamente não é suficiente para eliminar toda a interferência das plantas daninhas sobre as culturas, exigindo medidas integradas de controle. A execução de um programa de manejo integrado de plantas daninhas prevê o pleno atendimento a quatro etapas de planejamento: o diagnóstico do problema, a escolha do método a ser utilizado, a seleção e, por último, a avaliação do programa de controle. A prática de pousio para o controle de plantas daninhas tem uma longa história de sucesso, especialmente quando imperava o cultivo convencional. A não movimentação do solo, a cobertura vegetal permanente e a rotação de culturas, preconizadas no Sistema Plantio Direto (SPD), podem resultar em menor germinação das sementes no solo. De acordo com a espécie e a quantidade dessa cobertura, substâncias alelopáticas (inibidoras da germinação ou desenvolvimento de outras espécies) e o efeito do sombreamento determinam variações na intensidade e frequência de emergência das espécies daninhas. Neste sentido, trabalhos sugerem que culturas de entressafra podem ser supressoras das plantas daninhas, assim como sistemas alternados, como o safra ? safrinha, safra ? adubo verde e, mais recentemente, a Integração Lavoura Pecuária Floresta (iLPF), método que consiste na coexistência parcial de uma cultura graníferas e uma forrageira, onde a pastagem permanece na área por toda a entressafra. O levantamento de espécies daninhas, por amostragens da flora emergente, deve permitir a identificação e a quantificação das plantas infestantes, bem como a determinação da sua evolução. Esses conhecimentos podem ser usados para indicar a necessidade de controle, adequando diferentes manejos de solo, da cultura e as sucessões utilizadas buscando a racionalização de uso de herbicidas, com base em considerações de custo/benefício do sistema de produção agrícola. Em função disso, a Techfield, em parceria com os professores e pesquisadores, realiza estudos em diferentes sistemas de produção de grãos e rotações de culturas, com o objetivo de constatar a influência da dinâmica de ocorrência e controle de plantas daninhas destas rotações, o que pode acarretar em redução da utilização de herbicidas
CARMA1 is a novel regulator of T-ALL disease and leukemic cell migration to the CNS
No abstract available
AFLOW-CHULL: Cloud-Oriented Platform for Autonomous Phase Stability Analysis
A priori prediction of phase stability of materials is a challenging practice, requiring knowledge of all energetically-competing structures at formation conditions. Large materials repositories - housing properties of both experimental and hypothetical compounds - offer a path to prediction through the construction of informatics-based, ab-initio phase diagrams. However, limited access to relevant data and software infrastructure has rendered thermodynamic characterizations largely peripheral, despite their continued success in dictating synthesizability. Herein, a new module is presented for autonomous thermodynamic stability analysis implemented within the open-source, ab-initio framework AFLOW. Powered by the AFLUX Search-API, AFLOW-CHULL leverages data of more than 1.8 million compounds currently characterized in the AFLOW.org repository and can be employed locally from any UNIX-like computer. The module integrates a range of functionality: the identification of stable phases and equivalent structures, phase coexistence, measures for robust stability, and determination of decomposition reactions. As a proof-of-concept, thorough thermodynamic characterizations have been performed for more than 1,300 binary and ternary systems, enabling the identification of several candidate phases for synthesis based on their relative stability criterion - including 18 promising C15b-type structures and two half-Heuslers. In addition to a full report included herein, an interactive, online web application has been developed showcasing the results of the analysis, and is located at aflow.org/aflow-chull
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