1,720,985 research outputs found
MoS₂ Enhanced T-Phase Stabilization and Tunability Through Alloying
Full text linkTwo-dimensional MoS₂ is a promising material for nanoelectronics and catalysis, but its potential is not fully exploited since proper control of its multiple phases (H, T, ZT) and electronic properties is lacking. In this theoretical study, alloying is proposed as a method to stabilize the MoS₂ T-phase. In particular, MoS₂ is alloyed with another material that is known to exist in a monolayer MX₂ T-structure, and we show that the formation energy difference among phases decreases even for low impurity concentrations in MoS₂, and a relationship between impurity concentration and alloy band gap is established. This method can be potentially applied to many two-dimensional materials to tune/enhance their electronic properties and stabilities in order to suit the desired application
Simulating Li-ion Batteries and Beyond: Developing Electrode Materials for Next-Generation Batteries
No full textIn order to curve the environmental impact of fossil fuels, many countries have turned to "cleaner” energy sources and storage devices. One hope is that next-generation batteries may completely revolutionize energy storage. For in- stance, much research has been done concerning replacing the internal-combustion engine of automobiles with electric powertrains. However, it is essential that these batteries not just be energy and power dense, but also sustainable. This means that next-generation battery materials must be acquired through (relatively) en- vironmentally friendly and humane means. Towards this goal, there are two main options: select battery materials that can be responsibly obtained, or design bat- teries to not contain those unsustainable materials. With careful research, com- putational researchers can test which materials and designs theoretically work to save manufactures time and money, and to help explain their experimental results.To guide experimentalists, theorists run quantum mechanical simulations to test the stability, electronic structures, and diffusion on such candidate materials. One of the most popular methods is DFT, an approximate method that is usually accurate and computationally efficient. For more exact answers, one can use more advanced methods such as QMC. In this work, we explain our work using DFT to study anode materials for various ion batteries. In particular, we studied Li absorption and diffusion on 2D regular and Janus TMDs, and found that the sides with the lower electronegative chalcogenides had lower diffusion energy barriers than regular TMDs. We performed similar studies of adsorption and diffusion of various ions on three types of transition-metal carbide monolayers with Sulfur terminations (referred to as a type of ”MXene”). In one study, we closely examined Na and Mg ion interaction with these structures, and found that Na diffuses more efficiently on them than Mg. In another study, we build a physics-based machine learning model to use on our adsorption energy data. For the dilute case, ionization potential of the adatoms and workfunction of the materials are the most significant features to the model. For the full-coverage case, repulsion energy of the adatoms is the most significant feature. Finally, we also studied the discharge products of a novel battery design, the Li-air battery. We performed QMC calculations on the discharge products at the cathode, and found that Fo ?ppl Li2O2 is the most stable product
Beyond DFT: Accurately Engineering the Properties of 2D Materials for Energy and Device Applications
No full textIn recent years, two-dimensional (2D) materials have emerged as an important class of nanomaterials for novel applications in optoelectronic and energy related devices. These potential applications are due to the unusual electronic, optical and magnetic properties that arise from the reduced dimensionality of 2D materials. In addition, various methods have been employed to further engineer the properties of 2D materials such as alloying, chemical functionalization and creating heterostructures. To guide experimentalists in the process of materials discovery and design, accurate computational methodologies must be used. These quantum simulations can achieve a fundamental understanding of the electronic structure of a given material by approximately solving the many-electron Schrodinger equation. Currently, density functional theory (DFT) is the most widely used method due to its relative accuracy and computational efficiency. Despite this advantage, there are significant shortcomings of DFT that can be addressed by using more accurate many-body methodologies such as Quantum Monte Carlo (QMC). In this thesis, we transition from DFT to more sophisticated methods (QMC) to study and engineer the electronic, optical and magnetic properties of 2D materials with a higher degree of accuracy for next generation devices
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Investigations of Two-Dimensional Materials for Next-Generation Electronic Devices
No full textSemiconducting transition metal dichalcogenides (TMDs) such as MoS2 exhibit a direct band gap when isolated in monolayer form. They also exhibit high carrier mobility and are therefore attractive for use as channel materials in metal-oxide semiconductor field-effect transistors (MOSFETs) and other devices. MOSFETs rely on the integration of a metal-oxide dielectric layer with the semiconducting channel material. Atomic layer deposition (ALD) is often used for the deposition of dielectrics, but the MoS2 surface is hydrophobic and lacks dangling bonds. Because of this, ALD films on MoS2 are often poor quality, containing many gaps or pinholes. This work studies the effects of MoS2 surface preparation on ALD film quality. ALD films of TiO2 and Al2O3 are deposited on both mechanically exfoliated and chemical vapor deposition (CVD) grown MoS2. It is found that mechanically exfoliated MoS2 samples exhibit a wide range of film surface coverage as a result of variations in surface defect concentration and contamination from the exfoliation process. CVD-grown MoS2 offers more reliable starting surfaces. Therefore, CVD-grown MoS2 was chosen to study surface engineering and functionalization for ALD. Reactive sulfur vacancies were formed in the MoS2 surface via argon ion sputtering, and thiol molecules were introduced to passivate the vacancies and leave the surface amenable to ALD. Films grown on thiol-treated surfaces and vacancy-containing surfaces were continuous and morphologically similar. DFT calculations showed that the thiol molecules attach primarily to sulfur vacancies. The ALD precursor molecule also binds strongly to the sulfur vacancies, thus leading to uniform film growth on vacancy-containing surfaces. The remainder of this work focuses on the discovery of novel electronic materials using DFT. The electronic and magnetic properties of Janus TMD nanoribbons were studied. Janus TMDs are created by replacing one side of chalcogen atoms (e.g. S) of a TMD monolayer with another (e.g. Se). Nanoribbons of Mo- and W-based Janus TMDs were studied in zigzag and armchair configurations. Zigzag nanoribbons are found to be magnetic metals, while armchair nanoribbons are semiconductors. The magnetic and electronic properties of these nanoribbons can be modulated by controlling the ribbon width and the saturation of edge atoms. Further studies investigate the surface functionalization of two-dimensional tellurene (Te). DFT is used to investigate the electronic structure of ?- and ?-Te sheets functionalized with hydrogen, oxygen, or fluorine. When functionalized, ?- and ?-Te become metallic, except for hydrogenated ?-Te which remains semiconducting. Fluorinated Te structures are unstable, but H and O functionalized structures may be suitable for use in nanoscale electronics
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Magnetism of transition metal nanowires
No full textCataloged from PDF version of article.Includes bibliographical references leaves 70-73.In this thesis we investigated structural, electronic and magnetic properties
of 3d (light) transition metal (TM) atomic chains and Cr nanowires using firstprinciples
pseudopotential plane wave calculations. Infinite periodic linear, dimerized
linear and planar zigzag chain structures, as well as their short segments
consisting of finite number of atoms and chromium nanowires have been considered.
For most of the infinite periodic chains, neither linear nor dimerized linear
structures are favored; to lower their energy the chains undergo a structural transformation
to form planar zigzag and dimerized zigzag geometries. Dimerization in
both infinite and finite chains are much stronger than the usual Peierls distortion
and appear to depend on the number of 3d-electrons. As a result of dimerization,
a significant energy lowering occurs which, in turn, influences the stability and
physical properties. Metallic linear chain of vanadium becomes half-metallic upon
dimerization. Infinite linear chain of scandium also becomes half-metallic upon
transformation to the zigzag structure. The end effects influence the geometry,
energetics and the magnetic ground state of the finite chains. Structure optimization
performed using noncollinear approximation indicates significant differences
from the collinear approximation. Variation of the cohesive energy of infinite and
finite-size chains with respect to the number of 3d-electrons are found to mimic
the bulk behavior pointed out by Friedel.
Furthermore, we considered Cr nanowires, which have cross section comprising
a few (4,5 - 9,12) atoms. Chromium nanowires are found to be in a local
minimum in the Born-Oppenheimer surface and are ferrimagnetic metals. The
type of coupling, as for ferromagnetic or antiferromagnetic, between neighboring
Cr atoms depends on their interatomic distances. The spin-orbit coupling of finite
chains are found to be negligibly small for finite molecules and Cr nanowires.Ataca, Ca
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Nanoscience for sustainable energy production
No full textCataloged from PDF version of article.Includes bibliographical references leaves 130-160.Hydrogen economy towards the utilization of hydrogen as a clean and sustainable
energy source has three ingredients. These are (i) hydrogen production;
(ii) hydrogen storage; and (iii) fuel cells. Optimization of fuel cells for desired applications
is a challenging engineering problem. The subject matter of my thesis
is to develop nanostructures and to reveal physical and chemical mechanisms for
the production of free hydrogen and its high capacity storage. The predictions of
this study are obtained from rst-principles density functional theory and nite
temperature molecular dynamics calculations, phonon calculations and transition
state analyses.
Recent studies have revealed that single layer transition metal oxides and
dichalcogenides (MX2; M:Transition metal, X:Chalcogen atom) may o er properties,
which can be superior to those of graphene. Synthesis of single layer free
standing MoS2 and its nanoribbons, fabrication of transistor using this nanostructure,
active edges of akes of MoS2 taking a part in hydrogen evolution reaction
(HER) boost the interest in these materials. The electronic, magnetic, mechanical,
elastic and vibrational properties of three-, two- and quasi one-dimensional
MoS2 are investigated. Dimensionality e ects such as indirect to direct band gap
transition, shift of phonon modes upon three- to two- dimensional transition, half
metallic nanoribbons are revealed. Functionalization of single layer MoS2 and its
nanoribbons are achieved by creating vacancy defects and adatom adsorption.
Moreover, out of 88 di erent combinations of MX2 compounds (transition metal
dichalcogenides) it is also predicted that more than 50 single layer, free standing
MX2 can be stable in honeycomb like structures and o er novel physical and
chemical properties relevant for hydrogen economy.
It is predicted that H2O can be split spontaneously into its constituents O and
H at speci c vacancy defects of single layer MoS2 honeycomb structure. Interacting
with the photons of visible light, H atoms adsorbed to two folded S atoms
surrounding the vacancy start to migrate and eventually form free H2 molecules,
which in turn, are released from the surface. Not only taking a part in HER, but
also it is shown that MoS2 as a catalyst can release H2 molecule from water. Also
other possible candidates among the manifold of stable MX2 compounds, which
are capable of presenting similar catalytic activities are deduced.
In an e ort to obtain a high capacity hydrogen storage medium, the functionalization
of graphene with adatoms is investigated. It is found that Li-graphene
complex can serve as a high capacity hydrogen storage medium. A gravimetric
storage capacity of 12.8 wt % is attained, whereby each Li atom donates the significant
part of its charge to graphene and eventually attracts up to four H2 through
a weak interaction. Similarly Ca adatoms can hold H2 molecule on graphene up
to 8.4 wt % through an interesting mechanism involving charge exchange among
Ca, graphene and H2.Ataca, Ca
- …
