Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport properties calculations, that this assumption can not be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energetically more stable than their thiolate counterparts. Due to the fact that density functional theory (DFT) within the local density approximation (LDA) underestimates the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital by several electron-volts, and that it does not capture the renormalization of the energy levels due to the image charge effect, the conductance of the Au-benzene-1,4-dithiol-Au junctions is overestimated. After taking into account corrections due to image charge effects by means of constrained-DFT calculations and electrostatic classical models, we apply a scissor operator to correct the DFT energy levels positions, and calculate the transport properties of the thiol and thiolate molecular junctions as a function of the electrodes separation.Comment: 14 pages, 13 figures, to appear in Nanoscal

NINGUÉM SABE TUDO, NINGUÉM SABE NADA CURIOSANDO E ANDARILHANDO COM ANA MARIA ARAÚJO FREIRE - NITA FREIRE –

Esta é uma entrevista-diálogo com Ana Maria Araújo Freire, Nita Freire, mas com um enfoque um pouco distinto das demais entrevistas que ela costuma conceder, pois vamos olhar/ad-mirar a Nita como intelectual e escritora, com intensa produção sobre o pensamento de Paulo Freire, de quem é sucessora legal. Nita vem cumprindo com esmero a tarefa a que se propôs: manter o legado de Paulo Freire vivo, atual e necessário, pois tem se dedicado a organizar e publicar tanto obras revisitadas como livros inéditos. Mesmo sabendo, e respeitando, que as fronteiras entre Nita e Paulo são quase invisíveis e/ou inexistentes, buscamos ao longo de mais de duas horas garantir o foco do diálogo, priorizando neste registro os temas que colocam a história de vida de Nita no centro da roda

Electronic Structure Calculations in a 2D SixGe1-x Alloy Under an Applied Electric Field.

The recent advances and promises in nanoscience and nanotechnology have been focused on hexagonal materials, mainly on carbon-based nanostructures. Recently, new candidates have been raised, where the greatest efforts are devoted to a new hexagonal and buckled material made of silicon, named Silicene. This new material presents an energy gap due to spin-orbit interaction of approximately 1.5 meV, where the measurement of quantum spin Hall effect(QSHE) can be made experimentally. Some investigations also show that the QSHE in 2D low-buckled hexagonal structures of\ud germanium is present. Since the similarities, and at the same time the differences,\ud between Si and Ge, over the years, have motivated a lot of investigations in these\ud materials. In this work we performed systematic investigations on the electronic\ud structure and band topology in both ordered and disordered SixGe1-x alloys monolayer\ud with 2D honeycomb geometry by first-principles calculations. We show that\ud an applied electric field can tune the gap size for both alloys. However, as a function\ud of electric field, the disordered alloy presents a W-shaped behavior, similarly to the\ud pure Si or Ge, whereas for the ordered alloy a V-shaped behavior is observed.CAPESCNPqFAPES

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time,1,2 and attempts to address it require a clear un derstanding of how ecological communities respond to environmental change across time and space.3,4 While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes,5–7 vast areas of the tropics remain understudied.8–11 In the American tropics, Amazonia stands out as the world’s most diverse rainforest and the primary source of Neotropical biodiversity,12 but it remains among the least known forests in America and is often underrepre sented in biodiversity databases.13–15 To worsen this situation, human-induced modifications16,17 may elim inate pieces of the Amazon’s biodiversity puzzle before we can use them to understand how ecological com munities are responding. To increase generalization and applicability of biodiversity knowledge,18,19 it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple or ganism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region’s vulnerability to environmental change. 15%–18% of the most ne glected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lostinfo:eu-repo/semantics/publishedVersio

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time,1,2 and attempts to address it require a clear understanding of how ecological communities respond to environmental change across time and space.3,4 While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes,5,6,7 vast areas of the tropics remain understudied.8,9,10,11 In the American tropics, Amazonia stands out as the world's most diverse rainforest and the primary source of Neotropical biodiversity,12 but it remains among the least known forests in America and is often underrepresented in biodiversity databases.13,14,15 To worsen this situation, human-induced modifications16,17 may eliminate pieces of the Amazon's biodiversity puzzle before we can use them to understand how ecological communities are responding. To increase generalization and applicability of biodiversity knowledge,18,19 it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple organism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region's vulnerability to environmental change. 15%–18% of the most neglected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lost

Theoretical study of the phase transition between diamond and \'beta\'-\'SN\' structures for the \'SI IND alloy. 1-X\' \'GE IND. X\'

O estudo do comportamento dos mais variados materiais sob pressão, tem sido objeto de intensa pesquisa nas últimas décadas. O \'SI\' e o \'GE\', por serem úteis na construção de dispositivos eletrônicos, estão entre os mais estudados. É conhecido da literatura que o \'SI\' e o \'GE\', sob pressão, apresentam diagramas de fases semelhantes, embora para as fases métalicas, as pressões de transição de fase do \'SI\' e do \'GE\' são bem diferentes. Essas semelhanças, e ao mesmo tempo diferenças entre o \'SI\' e o \'GE\', motivam o estudo da liga \'SI IND. 1-X\' \'GE IND. X\' sob pressão. A liga \'SI\"GE\' também tem despertado interesse principalmente devido a sua aplicação em dispositivos óticos e eletrônicos. O objetivo deste trabalho é mostrar como a pressão de transição de fase se comporta em função de X, onde x é a concentração de Ge na liga. Para determinar as pressões de transição, perfizemos cálculos ab initio de energia total com o método de pseudopotenciais baseados na Teoria do Funcional da Densidade (DFT), com a aproximação da densidade local (LDA) utilizando o funcional de Ceperley-Alder para a energia de troca-correlação. Neste trabalho utilizamos uma célula unitária de 16 átomos e o método \"Special Quasirandom Structures\" (SQS) desenvolvido por Wei et al. [?] para simular a distribuição dos átomos na liga. Tais cálculos foram feitos para diferentes volumes das células unitárias das duas fases e através da construção de Gibbs (ou método da tangente comum) obtivemos as pressões de transição. Em particular, este procedimento foi aplicado para o estudo da transição de fase entre as estruturas Diamante e \'beta\'-Sn para X = 0,0625, 0,50, 0,9375, 1,0Theoretical study of the phase transition between diamond and \'beta\'-\'SN\' structures for the \'SI IND alloy. 1-X\' \'GE IND. X\