Hydrogen addition for centimeter-sized monolayer tungsten disulfide continuous films by ambient pressure chemical vapor deposition

Monolayer tungsten disulfide (WS 2 ) offers great prospects for use in optoelectronic devices due to its direct bandgap and high photoluminescence intensity. Here, we show how the controlled addition of hydrogen into the chemical vapor deposition growth of WS 2 can lead to the formation of centimeter scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates, or low pressure vacuum systems. Modifications of the reaction conditions, including growth time and hydrogen to argon ratio, allow for control over the domain size, film coverage, and film uniformity of the prepared WS 2 film. The combined control of hydrogen and a double-furnace system enables an increases in the growth rate of WS 2 , which results in fully merged films with cm 2 coverage and reduced multilayer content. Field effect transistors are fabricated to demonstrate that WS 2 has high quality for electronic applications. Our ambient pressure chemical vapor deposition reaction is simple and efficient, ideal for mass-production of large area monolayer WS 2

Estudo da dinâmica da magnetização em filmes finos de dissulfeto de tungstênio

Dissertação apresentada à Universidade Federal da Integração Latino-Americana, como parte das exigências do Programa de Pós-Graduação em Física Aplicada, área de concentração em Física da Matéria Condensada, para a obtenção do título de Mestre. Orientador: Prof. Dr. Rafael Otoniel Ribeiro Rodrigues da Cunha e Co-Orientador Prof. Dr. Yunier Garcia BasabeNeste trabalho investiga-se as propriedades de conversão de corrente de spin em corrente de carga do semicondutor WS2. Filmes finos bidimensionais compostos por bicamadas de Pd(10 nm) / NiFe(10 nm) e tricamadas de WS2 / Pd(10 nm) / NiFe(10 nm) foram investigados por meio do efeito Hall de spin inverso (ISHE) usando efeito de bombeamento de spin (spin pumping) (SPE) e efeito Seebeck de spin (SSE), nos quais uma corrente de spin gerada na camada ferromagnética é injetada na(s) camada(s) adjacente(s). O filme de WS2 foi crescido pela técnica de spin coating, enquanto que as camadas de Pd e NiFe foram fabricadas por magnetron sputtering. Caracterizações estruturais foram realizadas pelas técnicas de espectroscopia Raman, difração de raios-X para amostras na forma de pó (PXRD) e difração de raios-X em baixo ângulo (LIXRD). Medindo a corrente ISHE de cada amostra e fazendo um comparativo, consegue-se obter um parâmetro-chave para a spintrônica: o ângulo Hall de spin () para a tricamada WS2/Pd/NiFe, com o valor de = 0,015. Com esta investigação, mostra-se que o filme de WS2 tem contribuição significativa para corrente de spin, demonstrando ser um material promissor para aplicação em spintrônica de semicondutores.In this work we investigate the spin current to charge current conversion properties of the semiconductor WS 2 . Two-dimensional thin films composed of bilayer Pd (10nm) / NiFe (10nm) and trilayer of WS 2 / Pd (10nm) / NiFe (10nm) were investigated by means of the inverse spin Hall effect (ISHE) using spin pumping effect (SPE) and spin Seebeck effect (SSE) experiments in which a spin current generated in the ferromagnetic layer is injected into the adjacent layer. The WS 2 film was grown by the spin coating technique, while the Pd and NiFe layers were grown by magnetron sputtering. Structural characterizations have been done by Raman spectroscopy, X-ray powder diffraction (PXRD) and low-angle X-ray diffusion (LIXRD). By measuring the ISHE current of each sample and making a comparative, we were able to obtain a key spintronic parameter: the spin Hall angle θ SH for a WS 2 /Pd/NiFe, with the value of θ SH = 0,015. With this research, we show that the WS 2 film has a significant contribution to the spin current, proving to be a promising material for the spintronic application of semiconductors.Programa de Demanda Social da Universidade Federal da Integração Latino-Americana (DS-UNILA

Tribological properties of PVD coatings with lubricating films

Abstract. This work reports on the tribological performance of three different commercial hard PVD coatings (TiN, TiCN and nACo) with lubricating extra films of Al 2 O 3 , Ni-WS 2 and diamondlike carbon (DLC). WC-Co hardmetal has been used as substrate material. Wear tests, employing two counter bodies of Al 2 O 3 and hardmetal WC-Co, were performed for the PVD coatings with and without the extra films. The results showed that the presence of DLC extra film reduces the coefficient of friction of the PVD hard coatings TiN and nACo. Furthermore, the wear of TiN coatings was reduced in the presence of an extra Ni-WS 2 lubricant film

They Soon Forgot Their Words - Documentary

This documentary film, They Soon Forgot Their Words, is about the language issues that threaten the Kaqchikel Mayan language spoken in Guatemala. It is one of 21 Mayan languages spoken in the country and is one of the most widely used with more than 500,000 speakers. Unfortunately, however, scholars have noticed that Kaqchikel is in a state of “shift” a process where one language combines with another in speech. As is the case with many Mayan languages in Guatemala, Kaqchikel is often combined with Spanish in speech. The colloquial name used for the combining of these two languages is “kaqchinol.” Shift is considered the first step in language loss and 90 percent of the world’s languages are expected to disappear within 100 years (Nettle & Romaine, 2000). This is the focus of the film. To provide an in-depth examination of one language threatened by a global issue: language attrition. There are two sides to this story which have shaped this documentary, one of language use in rural Guatemala and one of language use in urban Guatemala. Because of this, I have focused this film on individuals from opposing communities to provide a broad scope of the current language use/disuse, circumstances and issues that affect Kaqchikel and its continued vitality as a spoken language. I also rely on history and expert commentary to provide historical and social context. The goal of this film is to shed light on the global issue of language attrition, an issue that effects most of the worlds languages, by providing an in-depth look at the unique issues threatening one language

Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film

Electrons on liquid helium can form different phases depending on density, and temperature. Also the electron-ripplon coupling strength influences the phase diagram, through the formation of so-called "ripplonic polarons", that change how electrons are localized, and that shifts the transition between the Wigner solid and the liquid phase. We use an all-coupling, finite-temperature variational method to study the formation of a ripplopolaron Wigner solid on a liquid helium film for different regimes of the electron-ripplon coupling strength. In addition to the three known phases of the ripplopolaron system (electron Wigner solid, polaron Wigner solid, and electron fluid), we define and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the transitions between these four phases and calculate the corresponding phase diagrams. This reveals a reentrant melting of the electron solid as a function of temperature. The calculated regions of existence of the Wigner solid are in agreement with recent experimental data.Comment: 12 pages, 6 figures. arXiv admin note: text overlap with arXiv:1012.4576, arXiv:0709.4140 by other author

Proximity effect model of ultra-narrow NbN strips

We show that narrow superconducting strips in superconducting (S) and normal (N) states are universally described by the model presenting them as lateral NSN proximity systems in which the superconducting central band is sandwiched between damaged edge-bands with suppressed superconductivity.The width of the superconducting band was experimentally determined from the value of magnetic field at which the band transits from the Meissner state to the static vortex state. Systematic experimental study of 4.9 nm thick NbN strips with widths in the interval from 50 nm to 20 ${\mu}$m, which are all smaller than the Pearl's length, demonstrates gradual evolution of the temperature dependence of the critical current with the change of the strip width

Lessons from Oxypnictide Thin Films

First experiments on the growth of oxypnictide F-doped LaFeAsO thin films indicated an incomplete normal-to-superconducting transition and offered a work programme challenging to overcome possible difficulties in their fabrication. In this regard the possibility of an all in-situ epitaxial growth appeared to be a matter of time and growth parameters. The following review clarifies that F-doped oxypnictide thin films are extremely difficult to grow by in-situ PLD due to the formation of very stable impurity phases such as oxyfluorides (LaOF) and oxides (La2O3) and the loss of stoichiometry possibly due to incongruent evaporation of the target or re-evaporation of volatile elements at the substrate surface. However, the review also demonstrates that the employed two-step fabrication process for oxypnictide thin films has been successfully applied in the preparation of clean polycrystalline as well as of epitaxial thin films. Fundamental investigations on the upper critical field, its temperature dependence and its anisotropy contributed to an understanding of multiband superconductivity in oxypnictides.Comment: accepted, pre-print versio

The in-plane electrodynamics of the superconductivity in Bi2Sr2CaCu2O8+d: energy scales and spectral weight distribution

The in-plane infrared and visible (3 meV-3 eV) reflectivity of Bi2Sr2CaCu2O8+d (Bi-2212) thin films is measured between 300 K and 10 K for different doping levels with unprecedented accuracy. The optical conductivity is derived through an accurate fitting procedure. We study the transfer of spectral weight from finite energy into the superfluid as the system becomes superconducting. In the over-doped regime, the superfluid develops at the expense of states lying below 60 meV, a conventional energy of the order of a few times the superconducting gap. In the underdoped regime, spectral weight is removed from up to 2 eV, far beyond any conventional scale. The intraband spectral weight change between the normal and superconducting state, if analyzed in terms of a change of kinetic energy is ~1 meV. Compared to the condensation energy, this figure addresses the issue of a kinetic energy driven mechanism.Comment: 13 pages with 9 figures include

Up-dating the Cholodny method using PET films to sample microbial communities in soil

The aim of this work was to investigate the use of PET (polyethylene terephtalate) films as a modern development of Cholodny’s glass slides, to enable microscopy and molecular-based analysis of soil communities where spatial detail at the scale of microbial habitats is essential to understand microbial associations and interactions in this complex environment. Methods. Classical microbiological methods; attachment assay; surface tension measurements; molecular techniques: DNA extraction, PCR; confocal laser scanning microscopy (CLSM); micro- focus X-ray computed tomography (μCT). Results. We first show, using the model soil and rhizosphere bacteria Pseudomonas fluorescens SBW25 and P. putida KT2440, that bacteria are able to attach and detach from PET films, and that pre-conditioning with a filtered soil suspension improved the levels of attachment. Bacteria attached to the films were viable and could develop substantial biofilms. PET films buried in soil were rapidly colonised by microorganisms which could be investigated by CLSM and recovered onto agar plates. Secondly, we demonstrate that μCT can be used to non-destructively visualise soil aggregate contact points and pore spaces across the surface of PET films buried in soil. Conclusions. PET films are a successful development of Cholodny’s glass slides and can be used to sample soil communities in which bacterial adherence, growth, biofilm and community development can be investigated. The use of these films with μCT imaging in soil will enable a better understanding of soil micro-habitats and the spatially-explicit nature of microbial interactions in this complex environment