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Large area TMD-based van der Waals heterostructures featuring enhanced photoconversion in the flat optics regime
No full textOne of the most urgent technological needs of this century concerns the research of innovative nanomaterials for applications in optoelectronics, nanophotonics and photovoltaics for renewable energy conversion. In fact, downscaling of the silicon-based devices (e.g. field-effect transistors) has come to an end due to intrinsic physical limitations such as size, inadequate carrier mobility, short channel effects, atomic-scale interactions, heat generation, and energy consumption at atomic scale thickness, requiring innovative materials to overcome such difficulties.
Graphene discovery in 2004 by Novoselov and Geim arose a tremendous interest for the two-dimensional (2D) materials, in which the reduced dimensionality brings new properties with respect to their bulk counterparts. Despite the extraordinary properties exhibited by graphene, its gapless nature strongly limits the fabrication of graphene-based optoelectronic devices. For this reason, the interest of researchers shifted towards the class of 2D semiconductors. Among these materials, Transition Metal Dichalcogenides (TMDs) represent the most important family due to suitable bandgap energy values which match the Schockley-Queisser efficiency criterion for solar photoconversion, and to their extraordinary optical absorption coefficient.
Additionally, 2D materials can be vertically stacked to form the so-called van der Waals heterostructures that are endowed by pristine interface thanks to the relatively weak van der Waals interactions that keep the stack together. This offers the opportunity to fabricate heterostructures of arbitrary 2D materials, independently by their crystal structure, with no limitations on the engineering of the optoelectronic and photonic properties of the new stacked metamaterial. In particular, the possibility to realize van der Waals p-n junctions by coupling 2D-TMDs layers is very intriguing for photoconversion and photovoltaic applications.
So far, TMDs employment has been mostly limited to the fabrication of prototypical devices such as field-effect transistors and photodetectors, most of which were realized via mechanical exfoliation from single crystal of flakes with thickness ranging from one to few atomic layers. Despite their intriguing properties, these materials do not represent an industrially relevant alternative to traditional semiconductors, being the exfoliation a randomic process with very low yields and limited areas in the micrometer range. Consequently, one of the key frontiers of TMDs research is the large area growth of homogeneous ultra-thin layers with controlled thickness over macroscopic areas. To meet this requirement, several large area techniques have been studied to synthesize TMDs layers extending over cm2 areas. However, in contrast with the exfoliation process, these techniques result in polycrystalline layers where the high concentration of grain boundaries degrades the macroscopic conduction.
The second crucial issue to be solved for ultra-thin TMD-based devices to be used in photoconductive and photodetection applications is the maximization of the optical absorption. Despite the excellent optical absorption coefficient, an ultra-thin TMD film indeed cannot absorb efficiently the incoming light due to the ultimately reduced thickness, which means a nanometric optical path. It is thus evident that new strategies for the optical absorption amplification in ultra-thin 2D semiconductors need to be developed to allow proper performances in TMD-based photodetection and photovoltaics applications. Because of the atomic thickness of the 2D layer, traditional solutions developed for the light harvesting amplification in conventional silicon-based photovoltaic devices, based on pyramidal microstructuring or on the addition of antireflective coatings, cannot be transferred to these materials.
Recently, the research group where I carried out my activity worked on large area ultra-thin MoS2 films conformally grown on self-organized rippled substrate fabricated by ion beam sputtering. Their results clearly showed a modification of the TMD optical and electronic properties grown on the nanostructured substrate with respect to a flat one. This observation was explained with the stress induced by the substrate morphology in the MoS2 layer in correspondence to the high curvature regions given by the crests and valleys of the ripples, meaning that by control of the substrate morphology it is possible to engineer the material intrinsic properties.
Additionally, the nanostructures anisotropy adds a polarization-dependent optical response, offering a way to engineer the optical absorption of the 2D material in view of photoconversion applications. However, self-organized nanostructures suffer from a relevant size dispersion and long range disorder, whereas more interesting optical effects are expected for periodic nanogratings in which the subwavelength TMD layers reshaping provide them the functionality of flat optic diffractive elements.
Starting from here, I devoted the most of my research activity to face the two main challenges described above: developing a growth process for large area ultra-thin TMD films, and studying an efficient light harvesting strategy to maximize the optical absorption in few-layer semiconductor films. An overview of the state-of-the-art regarding 2D materials and nanophotonics approaches for light harvesting in ultra-thin films is given in Chapter 1, while the growth synthesis techniques are postponed in Chapter 2.
At the beginning of my PhD, MoS2 films were grown by external collaborators of my group, thus limiting the activities. In the first phase of my research activity, I developed a novel large area growth process based on the physical deposition of solid precursor films and sulfurization, as I will describe in Chapter 2. This novel technique enabled not only the in-house growth of ultra-thin MoS2 films for the first time, but also allowed to extend the process to ultra-thin WS2 layers.
Having now the capability to control the deposition of two different TMDs layers, I moved to the growth of large area van der Waals heterostructures. Due to the type-II heterojunction formed by the band structure coupling of MoS2 and WS2, such heterostructures are expected to have a high potential in photoconversion applications. In Chapter 3 I will show the nanofabrication of planar MoS2/WS2 heterostacks, and their application in photocatalytic experiments and in a prototype of photonic device, featuring first evidence of photovoltage and photocurrent under illumination. This latter application also required me to develop large area transparent electrodes, so that I will dedicate a part of the chapter to indium tin oxide thin films and large area graphene.
In the second phase of my research activity, I focused on light harvesting in ultra-thin TMDs layers. Thanks to the conformality achieved by the physical deposition process, I explored a nanophotonic approach based on the optical anomalies arising from periodic modulation of the TMD layer at the subwavelength scale, obtained by conformal growth of the TMD layers onto nanostructured substrates. To this end, periodic nanogratings have been used as a template for the growth of ultra-thin MoS2 layers. Differently from the self-organized nanostructures mentioned before, the periodicity induces diffractive effects that are exploited to steer the light parallel to the active 2D material enhancing the optical absorption, as demonstrated in Chapter 4 both directly by absorption measurements and indirectly by a photo-to-chemical energy conversion experiment where we detected enhanced photocatalytic performances.
In the final part of my project, I started preliminary studies on the elastic scattering properties of subwavelength periodical lattices based on nanostructured tilted TMD layers. By defocused ion beam sputtering, I was able to reshape the morphology of the initial nanograting templates to further engineer the TMD optical response. In particular, by off-normal incidence sputtering it is possible to tailor a specific slope of the tilted nanofacets, on top of which I deposited laterally disconnected MoS2 nanostripes by grazing angle physical deposition. By developing a custom-made scatterometer, optical characterization of ultra-thin MoS2 nanostripes and thicker MoS2 films was performed, giving interesting preliminary results on the directional light scattering properties of these reshaped layers, as reported in Chapter 5. Finally, I adopted a similar deposition approach for the nanofabrication of large area heterostructures nanoarrays based on few-layer WS2 nanostripes coated by a conformal MoS2 layer, demonstrating further engineering of the optical response of few-layer TMD films with impact in photoconversion
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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