Phyllosilicates as earth-abundant layered materials for electronics and optoelectronics: Prospects and challenges in their ultrathin limit

Phyllosilicate minerals are an emerging class of naturally occurring layered insulators with large bandgap energy that have gained attention from the scientific community. This class of lamellar materials has been recently explored at the ultrathin two-dimensional level due to their specific mechanical, electrical, magnetic, and optoelectronic properties, which are crucial for engineering novel devices (including heterostructures). Due to these properties, phyllosilicates minerals can be considered promising low-cost nanomaterials for future applications. In this Perspective article, we will present relevant features of these materials for their use in potential 2D-based electronic and optoelectronic applications, also discussing some of the major challenges in working with them.Comment: 29 pages, 4 figure

Estudo eletroquímico de eletrodos extensos e estruturados com nanotubos de carbono

Exportado OPUSMade available in DSpace on 2019-08-13T17:41:40Z (GMT). No. of bitstreams: 1 dissertacao_versao_biblioteca.pdf: 10950349 bytes, checksum: 9652c9bbcc42f1d39f22514763811383 (MD5) Previous issue date: 3No presente trabalho, estudamos a influência de nanoestruturas em eletrodos de uma célula eletroquímica pela técnica de Voltametria Cíclica. O material utilizado como eletrodo estruturado é composto por nanotubos de carbono de parede múltipla, diretamente crescidos em uma fibra de carbono utilizando-se a técnica de CVD. A fim de estudarmos os efeitos introduzidos por partículas metálicas nas propriedades de transferência de elétrons, algumas amostras foram decoradas com platina por eletrodeposição. Foram utilizados dois tipos de eletrólitos com diferentes características eletroquímicas. A corrente elétrica observada na célula eletroquímica foi descrita usando dois modelos de circuito elétrico, formados resistores e capacitor associados de diferentes maneiras. Esta análise permitiu uma descrição fenomenológica da dependência da capacitância de dupla camada elétrica com a taxa de variação da diferença de potencial elétrico na célula eletroquímica preenchida com eletrólito contendo as espécies K+ e Cl . Os resultados permitem concluir-se que os eletrodos estruturados apresentam maiores capacitâncias especificas. Além disto, nossa análise usando as espécies 3, 4 Fe(CN)6 mostrou que nossos eletrodos estruturados aumentam as taxas dos processos de transferência de elétrons. Detalhes não triviais da dinâmica de formação da dupla camada elétrica em eletrodos extensos foram também relatados.In this work we studied the influence of nanostructures in electrodes of a electrochemical cell using the Cyclic Voltammetry technic. The material used as structured electrode is composed by multiwall carbon nanotubes directly grown on a carbon cloth. In order to study effects introduced by metallic particles on electron transfer properties, some samples were decorated with platinum. We made use of two kinds of electrolytes with different electrochemical properties. The observed electrical current is then described using two models of electrical circuits composed by capacitors and resistors associated in different configurations. This analysis allowed a phenomenological description of the dependence of the double layer capacitance with the rates of variation of the electrical potential difference, in the case of a cell filled with the electrolyte with the species K+ and Cl The results allowed one to conclude that this kind of structured electrodes show greater specifics capacitances. Besides, our analysis using 3, 4 Fe(CN)6 electrolytes showed that our structured electrodes increased the rate of electron transfer processes. Non trivial details of the dynamics of formation of the electrical double layer capacitance in extended electrodes are also reported

Theoretical Chemistry of α‑Graphyne: Functionalization, Symmetry Breaking, and Generation of Dirac-Fermion Mass

We investigate the electronic structure and lattice stability of pristine and functionalized (with either hydrogen or oxygen) α-graphyne systems. We identify lattice instabilities due to soft-phonon modes and describe two mechanisms leading to gap opening in the Dirac-Fermion electronic spectrum of these systems: symmetry breaking, connected with the lattice instabilities, and partial incorporation of an sp³-hybrid character in the covalent-bonding network of a buckled hydrogenated α-graphyne lattice that retains the symmetries of the parent pristine α-graphyne. In the case of an oxygen-functionalized α-graphyne structure, each O atom binds asymmetrically to two twofold-coordinated C atoms, breaking inversion and mirror symmetries, and leading to the opening of a sizable gap of 0.22 eV at the Dirac point. Generally, mirror symmetries are found to suffice for the occurrence of gapless Dirac cones in these α-graphyne systems, even in the absence of inversion symmetry centers. Moreover, we analyze the gapless and gapped Dirac cones of pristine and functionalized α-graphynes from the perspective of the dispersion relations for massless and massive free Dirac Fermions. We find that mirror-symmetry breaking mimics a Dirac-Fermion mass-generation mechanism in the oxygen-functionalized α-graphyne, leading to gap opening and to isotropic electronic dispersions with a rather small electron–hole asymmetry. In the hydrogen-functionalized case, we find that carriers show a remarkable anisotropy, behaving as massless Fermions along the <b>M</b>–<b>K</b> line in the Brillouin zone and as massive Fermions along the <b>Γ</b>–<b>K</b> line

Raman and Far-Infrared Synchrotron Nanospectroscopy of Layered Crystalline Talc: Vibrational Properties, Interlayer Coupling, and Symmetry Crossover

Talc is an insulating layered material that is stable at ambient conditions and has high-quality basal cleavage, which is a major advantage for its use in van der Waals heterostructures. Here, we use near-field synchrotron infrared nanospectroscopy, Raman spectroscopy, and first-principles calculations to investigate the structural and vibrational properties of talc crystals, ranging from monolayer to bulk, in the 300-750 and &lt;60 cm-1 spectral windows. We observe a symmetry crossover from mono to bilayer talc samples, attributed to the stacking of adjacent layers. The in-plane lattice parameters and frequencies of intralayer modes of talc display weak dependence on the number of layers, consistent with a weak interlayer interaction. On the other hand, the low-frequency (&lt;60 cm-1) rigid-layer (interlayer) modes of talc are suitable to identify the number of layers in ultrathin talc samples, besides revealing strong in-plane and out-of-plane anisotropy in the interlayer force constants and related elastic stiffnesses of single crystals. The shear and breathing force constants of talc are found to be 66 and 28%, respectively, lower than those of graphite, making talc an excellent lubricant that can be easily exfoliated. Our results broaden the understanding of the structural and vibrational properties of talc at the nanoscale regime and serve as a guide for future ultrathin heterostructures applications