Anomalous temperature dependence of the band-gap in Black Phosphorus

Black Phosphorus (BP) has gained renewed attention due to its singular anisotropic electronic and optical properties that might be exploited for a wide range of technological applications. In this respect, the thermal properties are particularly important both to predict its room temperature operation and to determine its thermoelectric potential. From this point of view, one of the most spectacular and poorly understood phenomena is, indeed, the BP temperature-induced band-gap opening: when temperature is increased the fundamental band-gap increases instead of decreasing. This anomalous thermal dependence has also been observed, recently, in its monolayer counterpart. In this work, based on \textit{ab-initio} calculations, we present an explanation for this long known, and yet not fully explained, effect. We show that it arises from a combination of harmonic and lattice thermal expansion contributions, which are, in fact, highly interwined. We clearly narrow down the mechanisms that cause this gap opening by identifying the peculiar atomic vibrations that drive the anomaly. The final picture we give explains both the BP anomalous band-gap opening and the frequency increase with increasing volume (tension effect).Comment: Published in Nano Letter

Otenção e caracterização de organogéis de óleo de soja estruturado com ceras vegetais e gordura interesterificada obtidos sob condições controladas de cristalização

Orientador: Daniel Barrera-ArellanoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia de AlimentosResumo: Organogéis são misturas que apresentam características de gel, compostas por agentes estruturantes e uma fase imobilizada, formando uma rede termorreversível autosustentada, onde esta fase imobilizada é um composto orgânico, o que o difere de outros géis formados basicamente por compostos hidrossolúveis. Os principais agentes estruturantes já estudados para formação de organogéis são os diacilgliceróis, monoacilgliceróis, álcoois e ácidos graxos e suas misturas, ceras e ésteres de cera, fitosteróis e orizanol, lecitina e triestearato de sorbitana, entre outros. O objetivo desta pesquisa foi desenvolver e caracterizar organogéis elaborados a partir de óleo de soja, gordura vegetal e ceras vegetais (cera de cana-de-açúcar, cera de carnaúba e cera de candelila), e a partir dos resultados obtidos gerar uma base de conhecimento para estudos posteriores sobre aplicação destes organogéis em produtos alimentícios. Organogéis estruturados com até 4% de cera vegetal (candelila, carnaúba e cana-de-açúcar) e adição de gordura vegetal nos teores de 5, 15 e 25%, os organogéis foram feitos utilizando condições controladas de cristalização (taxa de resfriamento e cisalhamento), as amostras foram analisadas quanto à estabilidade, propriedades reológicas, comportamento térmico, conteúdo de gordura sólida, dureza e morfologia. Os organogéis desenvolvidos somente com óleo de soja foram desenvolvidos para as ceras de cana-de-açúcar e candelila usando apenas cristalização estática. As análises térmicas mostraram uma temperatura de cristalização para os organogéis variando entre 43,7 e 44,57°C para a cera de cana-de-açúcar, 42,95 e 42,92°C para cera de candelila e 56,23°C para a cera de carnaúba, todas em 4% (m/m). As análises reológicas com cristalização estática mostraram que os materiais apresentaram um comportamento típico de gel com início da mudança de comportamento reológico em condições de temperatura similares às observadas na análise térmica com valores de 43,1 e 42,1°C para os géis de cana-de-açúcar e candelila respectivamente, a mesma análise para o organogel de cera de carnaúba mostrou a mudança de comportamento reológio em temperatura de aproximadamente 61°C, todos os organogéis apresentaram o comporamento esperado na varredura de frequências com os valores de G' maiores que G". As mesmas medidas feitas usando cristalização com cisalhamento em 300s-1 até a temperatura de formação de gel seguido de cristalização estática. As amostras apresentaram comportamento similar à da cristalização estática, a inclusão de gordura interesterificada por sua vez aumentou os valores de G' para as amostras, porém na análise de varredura de frequência foi observado uma maior sensibilidade à variações de frequência. A dureza foi maior para os géis de candelila, seguido dos géis de cana-de-açúcar e por últimos os de carnaúba. Os tamanhos de cristais observados na microscopia foram maiores para os organogéis de carnaúba e para os géis de cera de cana-de-açúcar quando comparados com os da cera de candelila. A formação de organogéis permitiu um aumento na estabilidade oxidativa da fase lipídica contínua. De maneira geral os organogéis apresentaram baixa resistência mecânica que foi melhorada pela inclusão da gordura vegetal, permitindo que a modificação da fase contínua amplia as possibilidades de aplicação para organogéis em produtos alimentíciosAbstract: Organogels are mixtures that present gel like characteristics, composed of a structuring agent and an immobilized phase, forming a self-sustained thermoreversible network, where this immobilized phase is an organic compound, which differs them from other gels basically formed by water-soluble compounds. The main structurant agents already studied for the formation of organogels are diacylglycerols, monoacylglycerols, alcohols, fatty acids and their mixtures, waxes and esters of wax, phytosterols and oryzanol, lecithin and sorbitan tristearate, among others. The objective of this research was to develop and characterize organogels made from soybean oil, vegetable fat and vegetable waxes (sugarcane wax, carnauba wax and candelilla wax), and from the obtained results generate a base of knowledge for further studies on the application of these organogels in food products. Organogels structured with up to 4% of vegetable wax (candelilla, carnauba and sugar cane) and addition of vegetable fat at levels of 5, 15 and 25%, the organogels were made using controlled conditions of crystallization (cooling rate And shear), the samples were analyzed for stability, rheological properties, thermal behavior, solid fat content, hardness and morphology. The organogels developed only with soybean oil were developed for sugar cane and candelilla wax using only static crystallization. Thermal analyzes showed a crystallization temperature for the organogels ranging from 43.7 to 44.57°C for sugar cane wax, 42.95 and 42.92°C for Candelilla wax and 56.23°C for carnauba wax, all at 4% (w/w). The rheological analyzes with static crystallization showed that the materials presented a typical behavior of gel with beginning of the change of rheological behavior in conditions of temperature similar to those observed in the thermal analysis with values of 43.1 and 42.1°C to The sugarcane and candelilla gels respectively, the same analysis for the carnauba wax organogel showed the change in rheological behavior at a temperature of approximately 61°C, all the organogels presented the expected behavior at the frequency sweep with values of G' greater than G". The same measurements were taken using shear crystallization in 300s-1 to the gel-forming temperature followed by static crystallization.The samples exhibited behavior similar to that of static crystallization, tha inclusion os interesterified fat increased the G' and the frequency sweep showed that it made the material more frequency sensible. The hardness was higher for the candelilla gels, followed by the sugarcane gels and the latter for the carnauba gels, the inclusion of interesterified fats increased mechanical resistance. The crystal sizes observed in the microscopy were higher for the carnauba organogens and for the sugarcane wax gels, Of sugarcane when compared to candelilla wax. The formation of organogels allowed an increase in the oxidative stability of the continuous lipid phase. All organogels presented low mechanical resistance that was improved by the inclusion of vegetable fat, allowing the modification of the continuous phase, increasing the possibilities of application for organogels in food productsDoutoradoTecnologia de AlimentosDoutor em Tecnologia de Alimentos140517/2012-0CNP

The Nusselt numbers of horizontal convection

We consider the problem of horizontal convection in which non-uniform buoyancy, $b_{\rm s}(x,y)$, is imposed on the top surface of a container and all other surfaces are insulating. Horizontal convection produces a net horizontal flux of buoyancy, $\mathbf{J}$, defined by vertically and temporally averaging the interior horizontal flux of buoyancy. We show that $\overline{\mathbf{J}\cdot\mathbf{\nabla}b_{\rm s}}=-\kappa\langle|\boldsymbol{\nabla}b|^2\rangle$; overbar denotes a space-time average over the top surface, angle brackets denote a volume-time average and $\kappa$ is the molecular diffusivity of buoyancy $b$. This connection between $\mathbf{J}$ and $\kappa\langle|\boldsymbol{\nabla}b|^2\rangle$ justifies the definition of the horizontal-convective Nusselt number, $Nu$, as the ratio of $\kappa \langle|\boldsymbol{\nabla}b|^2\rangle$ to the corresponding quantity produced by molecular diffusion alone. We discuss the advantages of this definition of $Nu$ over other definitions of horizontal-convective Nusselt number currently in use. We investigate transient effects and show that $\kappa \langle|\boldsymbol{\nabla}b|^2\rangle$ equilibrates more rapidly than other global averages, such as the domain averaged kinetic energy and bottom buoyancy. We show that $\kappa\langle|\boldsymbol{\nabla} b|^2\rangle$ is essentially the volume-averaged rate of Boussinesq entropy production within the enclosure. In statistical steady state, the interior entropy production is balanced by a flux of entropy through the top surface. This leads to an equivalent "surface Nusselt number", defined as the surface average of vertical buoyancy flux through the top surface times the imposed surface buoyancy $b_{\rm s}(x,y)$. In experiments it is likely easier to evaluate the surface entropy flux, rather than the volume integral of $|\mathbf{\nabla}b|^2$ demanded by $\kappa\langle|\mathbf{\nabla}b|^2\rangle$.Comment: 16 pages, 7 figure

Rewriting Modulo SMT and Open System Analysis

This paper proposes rewriting modulo SMT, a new technique that combines the power of SMT solving, rewriting modulo theories, and model checking. Rewriting modulo SMT is ideally suited to model and analyze reachability properties of infinite-state open systems, i.e., systems that interact with a nondeterministic environment. Such systems exhibit both internal nondeterminism, which is proper to the system, and external nondeterminism, which is due to the environment. In a reflective formalism, such as rewriting logic, rewriting modulo SMT can be reduced to standard rewriting. Hence, rewriting modulo SMT naturally extends rewriting-based reachability analysis techniques, which are available for closed systems, to open systems. The proposed technique is illustrated with the formal analysis of: (i) a real-time system that is beyond the scope of timed-automata methods and (ii) automatic detection of reachability violations in a synchronous language developed to support autonomous spacecraft operations.NSF Grant CNS 13-19109 and NASA Research Cooperative Agreement No. NNL09AA00AOpe

First-principles calculation of hot carriers in black phosphorus

ABSTRACT Black Phosphorus (BP), a layered semiconductor, has atracted enormous attention due to its singular anisotropic electronic, optical and thickness-dependent direct bandgap properties. As a consequence, BP has been envisioned as a promising material for several technological applications including photonics electronics and optolectronics. Nonetheless, most of the materials that integrate these devices undergo scattering and decay processes that are governed by quantum mechanical effects. From this point of view, the correct understanding and prediction of hot carriers dynamics in prospective materials as BP is crucial for its succesfull integration in future technology. In this work, based on ab initio calculations, we study the carrier relaxation rates in BP. Thus, the electron-electron and electron-phonon scattering contributions are investigated. Our results suggest that for the near-infrared and visible light spectrum [1.5 to 3.5 eV], the carriers in BP follow an ultrafast dynamics with relaxation times of the order of few to tens of femtoseconds while for the far-infrared range the relaxation times is of the order of hundreds of femtoseconds. Our reults are consistent with previous studies of pump-probe measurements on carrier dynamics