1,720,956 research outputs found
Electronic properties and quantum confinement in Bi2S3 ribbon-like nanostructures
No full textIn this work we study the morphology and electronic properties of Bi2S3 nanostructures by means of atomistic simulations. We focus on elongated nanoribbons that are the building blocks of the corresponding crystal structures, and we study saturated and unsaturated nanocrystals of finite size in comparison with one-dimensional infinite ones. By means of (time-dependent) density functional theory calculations we provide evidence that the optical gap can be tuned through quantum confinement with sizable effects for ribbons smaller than three nanometers. By a comparison with Sb2S3, we conclude that Bi2S3 nanostructures have similar tunability of the bandgap and a better tendency of passivating defects at the (010) surfaces through local reconstructions
Atomistic Investigation of the Solid–Liquid Interface between the Crystalline Zinc Oxide Surface and the Liquid Tetrahydrofuran Solvent
No full textZinc oxide is typically used as an electron acceptor in the preparation of hybrid solar cells. Such hybrids are, in many cases, synthesized from solutions. In this work, we investigate the possible persistence of solvent tetrahydrofuran molecules at the interface with zinc oxide by means of atomistic simulations based on model potentials and density functional theory calculations. We found a strong interaction between the solvent and the zinc oxide that leads to the formation of an ordered layer of tetrahydrofuran molecules bound to the zinc oxide substrate
Atomistic modeling of morphology and electronic properties of colloidal ultrathin Bi<inf>2</inf>S<inf>3</inf> nanowires
No full textUltrathin crystalline Bi2S3 nanostructures are studied by first-principles atomistic modeling and supported by experiments. Consistent with previous findings, the theoretical analysis shows that nanowires with lateral sizes as small as a few nanometers are energetically possible. Also, we were able to synthesize ultrathin nanowires by means of a low-cost, nontoxic colloidal route. Transmission electron microscopy data reveal a coherence length of the nanowires exceeding 30 nm. The simulations show that surfaces affect the electronic structure of the material inducing peculiar 1D-like electronic states on the nanowire edges that are located 300 meV above the valence band. Sulfur vacancies are instead responsible for localized states a few hundred millielectronvolts below the conduction band. The possibility of eliminating the surface-induced intragap states is theoretically investigated by passivating the surfaces of the nanowires with carboxylic and amine groups that are commonly employed in colloidal synthesis. Small molecules methylamine and acetic acid are expected to fully passivate the surfaces of the nanowires, removing the edge states and restoring a clean band gap. Present results suggest a possible route for improving optoelectronic properties of Bi2S3 nanostructures by tuning the size of the ligand molecules
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Atomistic investigation of morphology and optoelectronic properties of bismuth sulfide nonostructures
No full textNanostructured metal sulfides (MS) have attracted great interest in
recent years because of the possibility to synthesize nanoparticles from
solution and tuning their optoelectronic properties through quantum
confinement phenomena. A large number of applications have been
reported in the field of solar cells, light-emitting diodes, lithium-ion
batteries, thermoelectric devices, sensors, fuel cells and nonvolatile
memory devices. Among the large family of semiconductor sulfides,
the present Thesis is focussed on bismuthinite. The choice was motivated
by a combination of factors. Its non-toxicity, low cost synthesis
and high absorption properties make Bi2S3 a promising material for
several application, such as solid-state semiconductor-sensitized solar
cells. The intrisic anisotropy in the crystal structure of this material
facilitates the formation of elongated nanostructures, in particular
nanorods, nanoribbons, and nanowires. These structures find important
applications in many nanodevices, for example field emitters,
solar cells, and lithium-ion batteries.
On the other hand, researchers are still far from a complete understanding
of Bi2S3 properties. The colloidal synthesis of bismuthinite
nanostructures, although cheap and environmentally friendly, does
not allow a perfect control on stoichiometry and surface passivation.
The large majority of the experimental studies does not report photoluminescence
of the nanocrystals, which indicates the presence of
trap states and a low defect tolerance of the material. These facts
cause a lower efficiency of the devices based on Bi2S3 nanoparticles
with respect to anologous system based on other nanomaterials (e.g
Sb2S3 in solid-state sensitized solar cells). The absence of linear optical
data makes more difficult to investigate the electronic structure of
the material by means of spectroscopic techniques. Ab initio atomistic
simulations represent a valid alternative to get an insight on the
optoelectronic properties of bismuth sulfide. Despite some computational
work on bulk Bi2S3 is already present in literature, there are
currently no ab initio studies concerning nanostructures of this material.
Such lack of information motivates the work of the present
thesis that focuses on the investigation of morphology and electronic
properties of Bi2S3 nanocrystals.
The thesis is organized as follows. In the first chapter the main differences
and advantages of nanostructured materials over the bulk counterpart
are presented. I put the accent on metal sulfide nanocrystals,
in particular those of the Bi2S3 family (pnictogen chalcogenide) and
their application in several fields of physics, environmental science,
and engineering. A section is reserved to report the development
of elongated semiconductor nanostructures and their peculiarity with
respect to nanocrystals with lower aspect ratio.
The second chapter describes the computational and experimental
methods used in this study. The basic concepts of density functional
theory and its implementation in quantum-chemistry codes are reported.
A description of the synthesis and spectroscopic methods
used to check the validity of the theoretical predictions is also given.
For the detailed list of the basis sets, pseudopotentials, and exchangecorrelation
functionals used in each calculation I refer to the end of
Chapter 3 and 4.
Chapter 3 deals with the bulk properties of Bi2S3. Atomic and crystal
cell relaxation are performed. Also I investigated electronic properties
from the calculation of the band structure, density of states, and
efficient mass. These simulations are an important preliminary to
the study of Bi2S3 nanostructures. By comparing my results with
the previous studies present in literature it is possible to validate
the method (functionals, pseudopotentials, etc) and proceed with the
study of unexplored systems.
Chapter 4 investigates the properties of Bi2S3 nanostructures. First,
I focus on elongated nanoribbons (that are the building blocks of the
crystal structure) and study saturated and unsaturated nanocrystals
of finite size in comparison with one-dimensional infinite ones. By
means of (time-dependent) density functional theory calculations it
is demonstrated that the optical gap can be tuned through quantum
confinement with sizable effects for nanoribbons smaller than three
nanometers. A comparison with Sb2S3, shown that Bi2S3 nanostructures
have similar tunability of the band gap and a better tendency of
passivating defects at the (010) surfaces through local reconstruction.
Then, the focus shifts over ultrathin nanowires formed by the aggregation
of a small number of nanoribbons, with lateral sizes as small as
3 nm as in fact observed by transmission electron microscopy. Their
electronic properties are investigated finding that surfaces induce peculiar
1D-like electronic states on the nanowire edges that are lo- cated
300 meV above the valence band. Sulfur vacancies are also responsible
for localized states a few hundreds meV below the conduction
band. The possibility to remove the surface-induced intragap states
is further investigated by passivating the surfaces of the nanowires
with carboxylic and amine groups that are commonly employed in
colloidal synthesis. The small methylamine and acetic acid molecules
are expected to fully passivate the surfaces of the nanowires removing
the edge states and restoring a clean band gap.
Conclusions are finally reported in Chapter 5. The results of the
present Thesis provide a characterization of the energetics and optoelectronic
properties of bismuth sulfide nanostructures showing the
relevance of surface defects and suggesting a possible route for improving
optoelectronic properties of Bi2S3 nanostructures by tuning
the size of the ligand molecules
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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