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Alice (and Bob) in Flatland
No full text2D quantum materials have opened infinite doors, hosting intriguing phenomena and featuring incredible engineering potential. Whether these qualities can boost the use of 2D crystals for quantum applications remains an open field with yet unexplored paths
Dome-shaped two-dimensional crystals: a playground for the study of the crystal mechanical and optoelectronic properties
No full textGraphene -a single layer of carbon atoms tightly packed into a 2D honeycomb lattice- was first isolated in 2004 by A. K. Geim and K. S. Novoselov. The ground-breaking discovery that graphene can be isolated, previously thought to be impossible, opened the doors of Flatland to the condensed matter physics community. Since then, the family of 2D systems has grown rapidly, as many other crystals have been found to be characterised by a layered structure akin to graphite, with different layers bound together by weak van der Waals (vdW) forces. Among them, graphene features a semi-metallic nature and is characterised by exceptionally high carrier mobilities; hexagonal boron nitride (hBN) is an extremely good insulator and dielectric with a large bandgap; and the family of transition metal dichalcogenides (TMDs, such as MoS2 WS2, MoSe2, WSe2, MoTe2, WTe2, NbSe2, etc.) is richly varied, as it comprises superconducting materials with charge density waves and Weyl semimetal properties, as well as several semiconducting materials, with bandgaps ranging from the visible to the near infrared spectral region. In the single layer limit, semiconducting TMDs are characterised by extremely efficient light emission, which makes them ideal candidates for the realisation of innovative, flexible optoelectronic devices.
Aside from the possibility of exploring the effects of lower dimensionality on the properties of atomically thin crystals, the existence of these crystals in stable form opens new avenues to materials engineering. Indeed, the inherent all-surface nature of these systems entails a higher sensitivity to external perturbations, which can in turn be exploited to modify the material properties. Among all possible external perturbations, the incredible mechanical flexibility and robustness of 2D crystals have offered the possibility to subject them to high mechanical deformations, engendering strains larger than 10 %. Such strains are able to induce major modifications in the electronic, optical, magnetic, transport and chemical properties of 2D materials, leading to the observation of a plethora of intriguing phenomena---ripe with new physics and novel opportunities.
In the past decade, great attention has been thus devoted to the development of methods to mechanically deform 2D crystals and on the study of the effects of strain on these materials.
This thesis will be focused on the development of an original strategy to induce strain fields in 2D crystals, and on the study of the effect of strain on the peculiar properties of the material. The thesis will be articulated as follows:
The Prologue will briefly introduce the reader to the 2D world, especially highlighting the peculiar properties of 2D TMDs and hBN, thanks to which a flourishing interest in these materials has arisen. The final part of the Prologue will instead provide the reader with an overview of the field of strain engineering of 2D crystals.
Chapter 1 will present the innovative method to induce strain in TMDs and hBN pioneered by the candidate and her group. It will be discussed how, by irradiating bulk flakes of these materials with low-energy hydrogen ions, it is possible to induce on the flake surface the formation of domes with thickness of one-to-few layers and filled with pressurised hydrogen. The basic properties of these structures will be discussed. This Chapter will also discuss the effects of hydrogen-ion irradiation of other crystals, where the formation of domes was not achieved but other interesting phenomenologies were observed.
Chapter 2 will present a characterisation of the vibrational properties of the domes, highlighting their link with the strain distribution. This chapter will focus in particular on Raman studies of TMD domes and on Raman and infrared (IR) characterisations of hBN domes. The observed huge shifts and splittings of the vibrational modes will be correlated with the strain magnitude and character, which will be estimated by numerical calculations.
Chapter 3 will focus on the possibility to engineer the domes. Lithography-based approaches will be used to achieve control over their size and position, and eventually over the strain magnitude.
Chapter 4 will investigate from a fundamental point of view the morphology and mechanics of the system. In particular, an analytical method to describe the system will be presented. The model, coupled to morphological and mechanical experimental measurements, allows one to obtain precious information on the elastic properties of the membrane and on the adhesion energy between the monolayer and the bulk crystal.
Chapter 5 will discuss how strain affects the optoelectronic properties of TMDs. In particular, this chapter will present steady-state and time-resolved photoluminescence (PL) studies aimed at highlighting the effect of strain on the free excitons. Such measurements highlight intriguing behaviours, such as strain-induced direct-to-indirect exciton crossovers, that deeply affect the emitted light intensity and decay time.
Chapter 6 will present a characterisation of direct and indirect excitons when subjected to high magnetic fields. Indeed, magnetic fields induce a Zeeman effect in TMD MLs, which is promising for their utilisation for valleytronics. The effect of strain on the Zeeman effect has however not been investigated so far. The results presented in this chapter shed light on this, and highlight an unexpected behaviour, that unveils hybridisation phenomena between nearly resonant direct and indirect excitons.
Chapter 7 will demonstrate the possibility to exploit strained 2D materials for quantum applications. In particular, this chapter will discuss the observation of single photon emitters at cryogenic temperatures in hBN-capped TMD domes.
Chapter 8 will investigate a novel perspective: that of exploiting selective strain engineering in van der Waals heterostructures. Specifically, we will here focus on heterostructures made of a TMD dome and of an InSe unstrained layer, showing how strain is able to modify the electronic properties of the heterostructure
One (photon), two(-dimensional crystals), a lot (of potential): a quick snapshot of a rapidly evolving field
Full text linkWe present a concise overview of the state of affairs in the development of single-photon sources based on two-dimensional (2D) crystals, focusing in particular on transition-metal dichalcogenides and hexagonal boron nitride. We briefly discuss the current level of advancement (i) in our understanding of the microscopic origin of the quantum emitters (QEs) identified in these two material systems, and (ii) in the characterisation of the optical properties of these emitters; then, we survey the main methods developed to enable the dynamic control of the QEs' emission energy. Finally, we summarise the main results stemming from the coupling of QEs embedded in 2D materials with photonic and plasmonic structures
Fine-tuning of the excitonic response in monolayer WS2 domes via coupled pressure and strain variation
No full textWe present a spectroscopic investigation of the vibrational and optoelectronic properties of WS2 domes in the 0-0.65 GPa range. The pressure evolution of the system morphology, deduced by the combined analysis of Raman and photoluminescence spectra, revealed a significant variation in the dome's aspect ratio. The modification of the dome shape caused major changes in the mechanical properties of the system resulting in a sizable increase of the out-of-plane compressive strain while keeping the in-plane tensile strain unchanged. The variation of the strain gradients drives a nonlinear behavior in both the exciton energy and radiative recombination intensity, interpreted as the consequence of a hybridization mechanism between the electronic states of two distinct minima in the conduction band. Our results indicate that pressure and strain can be efficiently combined in low dimensional systems with unconventional morphology to obtain modulations of the electronic band structure not achievable in planar crystals
Strain engineering of the transition metal dichalcogenide chalcogen-alloy WSSe
Full text linkAlloying has been a powerful and practical strategy to widen the palette of physical properties available to semiconductor materials. Thanks to recent advances in the synthesis of van der Waals semiconductors, this strategy can be extended to monolayers (MLs) of transition metal dichalcogenides (TMDs). Due to their extraordinary flexibility and robustness, strain is another powerful means to engineer the electronic properties of two-dimensional (2D) TMDs. In this article, we combine these two approaches in an exemplary metal dichalcogenide chalcogen-alloy, WSSe. Highly strained WSSe MLs are obtained through the formation of micro-domes filled with high-pressure hydrogen. Such structures are achieved by hydrogen-ion irradiation of the bulk material, a technique successfully employed in TMDs and h-BN. Atomic force microscopy studies of the WSSe ML domes show that the dome morphology can be reproduced in terms of the average of the elastic parameters and adhesion energy of the end compounds WSe2 and WS 2. Micro-photoluminescence measurements of the WSSe domes demonstrate that the exceedingly high strains ( epsilon similar to 4 %) achieved in the domes trigger a direct-to-indirect exciton transition, similarly to WSe2 and WS2. Our findings heighten the prospects of 2D alloys as strain- and composition-engineerable materials for flexible optoelectronics
Brightly Luminescent and Moisture Tolerant Phenyl Viologen Lead Iodide Perovskites for Light Emission Applications
Full text linkLead halide perovskites are outstanding materials for optoelectronics, but they typically feature low stability against external agents. To overcome this drawback, LHPs based on quaternary ammonium cations, such as phenyl viologen lead iodide (PhVPI), were found to be promising candidates, being water-resistant and thermally stable. In this Letter, the optoelectronic properties of the PhVPI are investigated by a combined experimental-theoretical approach. Although the as-prepared material is photoluminescence-inactive, a short thermal (5 min @ 290 °C) or laser annealing turns PhVPI into a highly luminescent material, in the 600-1000 nm range. The PhVPI PL emission was characterized at different annealing conditions, and the structural evolution following thermal treatments was investigated by means of X-ray diffraction, Raman, and NMR spectroscopies. Besides this, the electronic structure and emission properties were investigated by density functional theory simulations. The intense optical emission and high stability make PhVPI an intriguing material for applications related to light-emitting 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
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
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
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