Dissertations / Theses on the topic 'TMDC materials'

This thesis explores structural and electronic properties of layered materials at the nanometre scale. Room temperature and low temperature ultrahigh vacuum scanning probe microscopy (scanning tunneling microscopy, scanning tunneling spectroscopy, atomic force microscopy) is used as the primary characterization method. The main findings in this thesis are: (a) observations of the atomic lattice and imaging local lattice defects of semiconducting ReS2 by scanning tunneling microscopy, (b) measurement of the electronic band gap of ReS2 by scanning tunneling spectroscopy, and (c) scanning tunneling microscopy study of 1T-TaS2 lattice and chemically functionalizing its defects with magnetic molecules.

Le stockage de l'énergie solaire en énergie chimique est une approche hautement souhaitable pour résoudre le défi énergétique. Les cellules photo-électrochimiques combinent la collecte de l'énergie solaire et la décomposition de l'eau. Les nanofeuillets semiconducteur 2D de di-chalcogenures de métaux de transition (TMDC) sont considérés comme des matériaux attrayants pour l'élaboration de photocatalyseur efficace pour la conversion de l'énergie solaire en hydrogène. Malgré les propriétés optoélectroniques uniques des TMDC, la passivation de défauts de surface présents en concentration élevée est un défi important pour le développement de cette classe de matériaux. Dans ce contexte, le présent travail a concerné l'élaboration d'un photocatalyseur 2D TMDC pour la photo-décomposition de l'eau. Le développement de photocatalyseurs de haute performance a été examiné suivant deux axes principaux. Un premier axe de recherche consiste à passiver les défauts de surface des nanofeuillets 2D p-WSe2 à l'aide de complexes Mo-S pour diminuer la recombinaison des porteurs de charge photo-générés et améliorer l'activité photocatalytique. Nous avons démontré que des couches ultra minces de complexes thio et oxo-thio-Mo moléculaires représentent une classe idéale de catalyseurs, bien adaptée pour fonctionnaliser les matériaux 2D car ils sont stables dans des environnements aqueux, bon marché, respectueux de l'environnement. Des densités de courant de -2 mA cm-2 à -0.2 V par rapport à l'électrode d'hydrogène (NHE) ont été obtenues pour la nouvelle photocathode p-WSe2/ MoxSy. En plus d'offrir une activité électro-catalytique élevée, les films complexes Mo se sont révélés capables de guérir les défauts de surface. Les contributions respectives aux effets catalytiques et cicatrisants observées expérimentalement pour les divers complexes moléculaires de Mo impliquaient la forte adsorption sur les défauts ponctuels du substrat 2D WSe2 de complexes de Mo tels que (MoS4)2-, (MoOS3)2- et (Mo2S6O2)2-. Il a été démontré que ces couches de co-catalyseur Mo-S formés à un pH bien défini présentent un comportement n-semi-conducteur et l'ingénierie des bandes formées avec p-WSe2 s'est révélée appropriée pour assurer la séparation des charges et la migration efficace des électrons photo-induits pour la RDH, représentant un exemple de couche de passivation multi-composant avec de multiples propriétés. Un deuxième axe de travail concerne l'optimisation de la nanostructure du film de WSe2 comme objectif l'obtention d'une surface spécifique élevée et des parois de pores composées de quelques monofeuillets. Des films de WSe2 nanostructurés de haute surface et à bonne collecte de porteurs de charge ont été obtenus par co-assemblage des nanofeuillets de WSe2 et des nanofeuillets d'oxyde de graphène réduit (rGO) avec un rapport de nanofeuillets rGO/WSe2 optimal.[...]
Collecting and storing solar energy in chemical energy is a highly desirable approach to solve energy challenge. The great potential of a photoelectrochemical cell technology combines the harvesting of solar energy with the water splitting into a single device. 2D semiconducting nanosheets of Transition Metal Di-Chalcogenides (TMDC) are seen as an attractive material to design an efficient photocatalyst for the conversion of solar energy into hydrogen. Despite the unique optoelectronic properties of the TMDCs, the passivation of surface defects in high concentration is a remaining challenge for the development of this class of materials. In this context, the present work has aimed the elaboration of thin 2D TMDC photocatalyst for solar water splitting. The development of high performance photocatalysts was evaluated following two main axis. A first strategy consists in the surface defects passivation of 2D p-WSe2 nanosheets using Mo-S complexes to decrease the photogenerated charge carrier recombination and improve photocatalytic activity. We demonstrated these Mo thio and oxo-thio- molecular complexes films as an ideal class of catalysts, well-suited to functionalize 2D materials since they are stable in aqueous environments, cheap and environmentally benign. Current densities of -2 mA cm-2 at -0.2 V vs NHE electrode were obtained for the new p-WSe2/MoxSy photocathode. Besides developing high electro-catalytic activity, the Mo complexes films were shown to display ability to heal surface defects. The respective contributions in catalytic and healing effects observed experimentally for the various molecular Mo complexes involved strong adsorption on point defects of the 2D WSe2 substrate of Mo complexes such as (MoS4)2-, (MoOS3)2-and (Mo2S6O2)2-. The Mo complexes films spontaneously formed at well-defined pH were demonstrated to present n-semi-conducting behaviour and band engineering formed with p-WSe2 showed to be suitable for ensuring charge separation and efficient migration of the photo-induced electrons for the Hydrogen Evolution Reaction, thus representing an example of multicomponent passivation layer exhibiting multiple properties. A second strategy focus in the nanostructure optimization of WSe2 with high specific surface area and pore walls composed of few layers. Nanostructured WSe2 films of high surface area and good charge carrier collection were obtained by co-assembling WSe2 nanosheets and reduced graphene oxide (rGO) nanosheets with an optimal rGO/WSe2 nanosheet ratio. After deposition of co-catalyst thin layer, the new layered nanojunctions of rGO-WSe2/MoxSy exhibited photocurrents up to -5 mA cm-2 at -0.2V vs NHE. Incident-photon-to-current efficiency conversion of 10% were achieved for WSe2 nanoflakes of 70 nm thickness in presence of rGO and MoxSy co-catalyst.[...]

Les membranes nanoporeuses solides [en anglais, SSN (Solid State Nanopore)] sont devenues des dispositifs polyvalents pour l'analyse des biomolécules. L'une des applications les plus prometteuses des SSN est le séquençage de l'ADN et des protéines avec un coût réduit et une vitesse d’exécution plus rapide que les méthodes actuelles de séquençage. Le séquençage par SSN est basé sur la mesure des variations de courant ionique observées quand unebiomolécule, dans un milieu électrolytique, est forcée de traverser de manière séquentielle un nanopore sous l’action d’une différence de potentiel électrique appliquée. Lorsque la biomolécule passe au travers du nanopore, elle occupe de manière transitoire le volume du nanopore et bloque ainsi le passage des ions du milieu électrolytique. Le blocage du courant est dépendant de la nature et de l’encombrement stérique des groupements chimiques des monomères constituant la biomolécule. Donc, la détection ultra-rapide des variations de courant ionique lors du passage de celle-ci au travers du nanopore, peut fournir des informations sur sa séquence. La résolution avec laquelle la séquence peut être déterminée dépend de la taille des nanopores et de l'épaisseur de la membrane. Les matériaux à deux dimensions tels que le graphène et les matériaux dichalcogénures de métaux de transition (MoS2 , WS2, …) sont des candidats très prometteurs pour ledéveloppement des applications de séquençage par SSN. A partir de simulations de dynamique moléculaire (DM) tous atomes, nous avons étudié la faisabilité d'utiliser des SSN de type MoS2 pour le séquençage desprotéines. En premier lieu, nous avons étudié la conductance d’une membrane nanoporeuse de MoS2 de 1 à 5 couches d’épaisseur possédant un seul nanopore de diamètre compris entre 1.0 et 5.0 nm et plongée dans un électrolyte de KCl. Nous avons démontré que le modèle de conductance macroscopique des membranes nanoporeuses cessait d’être valable pour les plus petits nanopores (diamètre < 5 nm). En analysant les simulations de DM des membranes de MoS2, nous avons développé un modèle modifié qui permet d’interpréter les mesures de courant ionique quel que soit le diamètre du nanopore. En second lieu, nous avons simulé le passage de la lysine et du di-lysine, ainsi que d'une protéine modèle, au travers de nanopores de membranes de MoS2, plongées dans un électrolyte de KCl, et soumises à une différence de potentiel électrique. A partir de nos résultats, nous avons proposé que l'utilisation d'acidesaminés chargés positivement ou négativement fixés de manière covalente à une protéine pourrait s’avérer une technique efficace pour favoriser l'entrée des protéines à travers des nanopores dans des expériences de translocation. De plus, nous avons établi la relation entre la trajectoire de la protéine au travers du nanopore et les fluctuations de courant ionique simulées.En troisième lieu, nous avons examiné la conductance ionique de membranes de MoS2 dont les pores ont un diamètre inférieur au nanomètre (sub-nm). Nous avons effectué des simulations de DM de ces systèmes en utilisant le potentiel réactif ReaxFF. Ce potentiel nous a permis de caractériser les variations de la structure atomique de ces très petits pores dans le vide et de simuler la conductance ionique de ce type de membranes. En utilisant le potentiel ReaxFF, des calculs préliminaires de la réactivité des nanopores de membranes de MoS2 à des molécules d’éthanol, utilisées dans le protocole expérimental de la préparation des membranes de MoS2, ont été réalisés
Solid-state nanopores (SSN) have emerged as versatile devices for biomolecule analysis. One of the most promising applications of SSN is DNA and protein sequencing, at a low cost and faster than the current standard methods. SSN sequencing is based on the measurement of ionic current variations when a biomolecule embedded in electrolyte is driven through a nanopore under an applied electric potential. As a biomolecule translocates through the nanopore, it occupies the pore volume and blocks the passage of ions. Hence, ultrafast monitoring of ionic flow during the passage of a biomolecule yields information about its structure and chemical properties. The size of the sensing region in SSN is determined by the size and thickness of the pore membrane. Therefore, two-dimensional (2D) transition metal dichalcogenides such as molybdenum disulfide (MoS2) arise as great candidates for SSN applications as an alternative to graphene. In the present work, we investigated the feasibility of using MoS2 nanopores for protein sequencing from all-atom molecular dynamics (MD) simulations. First, we studied the ionic conductance of MoS2 nanoporous membranes by characterizing the KCl electrolyte conductivity through MoS2 nanopores with diameters ranging from 1.0 to 5.0 nm and membranes from single to five-layers. Using MD simulations, we showed the failure of the usual macroscopic model of conductance for the nanoporous membranes with the smallest diameters and developed a modified model which proves usefulness to interpret experimental data. Second, we investigated the threading and translocation of individual lysine residues and a model protein with poly-lysine tags through MoS2 nanopores under the application of an electric potential. A proof-of principle technique based on the use of positively or negatively charged amino acids for protein translocation was proposed to promote the entrance of proteins through SSN in experiments. By analyzing the current-voltage curves simulated, we established the relationship between the translocation sequence events through the nanopores observed at the atomic scale in MD simulations, and the computed current fluctuations. Finally, experimental evidence of ionic conductance measurements in sub-nanometer (sub-nm) pores made of atomic defects has been recently reported. To give a better insight of the ionic transport through atomic scale pores, we performed MD simulations of sub-nm defect MoS2 pores using the reactive potential ReaxFF. Here, we characterized the variations of the atomic structure of the pores in vacuum and then we investigated the ionic conductance performance of one of the MoS2 defect pore membranes. ReaxFF potential was also useful to investigate the possible reactivity of MoS2 defect pore membranes with ethanol molecules. In addition, these simulations might provide a better understanding of the experimental setup of DNA sequencing, in which ethanol plays an unknown role in the sample preparation of the SSN

Two-dimensional (2D) materials show novel electronic, optical and chemical properties and have great potential in devices such as field-effect transistors (FET), photodetectors and gas sensors. This thesis focuses on scanning tunneling microscopy and spectroscopy (STM/STS) investigation of interfaces and defects 2D transition metal dichalcogenides (TMDCs). The first part of the thesis focuses on the synthesis of 2D TiSe2 with chemical vapor transport (CVT). By properly choosing the growth condition, Sub-10 nm TiSe2 flakes were successfully obtained. A 2 × 2 charge density wave (CDW) was clearly observed on these ultrathin flakes by scanning tunneling microscopy (STM). Accurate CDW phase transition temperature was measured by transport measurements. This work opens up a new approach to synthesize TMDCs. The second part of the thesis focuses on monolayer vacancy islands growing on TiSe2 surface under electrical stressing. We have observed nonlinear area evolution and growth from triangular to hexagonal driven by STM subjected electrical stressing. Our simulations of monolayer island evolution using phase-field modeling and first-principles calculations are in good agreement with our experimental observations. The results could be potentially important for device reliability in systems containing ultrathin TMDCs and related 2D materials subject to electrical stressing. The third part of the thesis focuses on point defects in 2D PtSe2. We observed five types of distinct defects from STM topography images and measured the local density of states (LDOS) of those defects from scanning tunneling spectroscopy (STS). We identified the types and characteristics of these defects with the first-principles calculations. Our findings would provide critical insight into tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering or varying growth condition in few-layer 1T-PtSe2 and other related 2D materials.
Doctor of Philosophy
Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted more and more attentions. When the thickness of a layered material thinned to one or few atoms, it shows interesting properties different from its bulk phase. Due to the reduced dimensionality, interfaces and defects in 2D materials will significantly affect the electronic property and chemical activity. However, such nanometer scale features are several orders of magnitude smaller than the wavelength of visible light, which is the limit of resolution for optical microscope. Scanning tunneling microscope (STM) is widely used in study of 2D materials not only because it can provide the topography and local electronic information at atomic scale, but also because of the possibility of directly fabricate atomic scale structure on the surface. The first part of the thesis focuses on the synthesis of 2D TiSe2 with chemical vapor transport (CVT). TiSe2 belongs to the transition metal dichalcogenides (TMDCs) family, showing a sandwiched layered structure. When the temperature goes down to 200K, a 2 × 2 superlattice called charge density wave (CDW) will show up, which is clearly observed in our STM images. The second part of the thesis focuses on monolayer vacancy islands growing on TiSe2 surface controlled by electrical stressing. During continuous STM scanning, we have observed nonlinear area growth of the vacancy islands. The shape of those islands transfers from triangular to hexagonal. We successfully simulated such growth using phase-field modeling and first-principles calculations. The results could be potentially important for device reliability in systems containing ultrathin TMDCs and related 2D materials subject to electrical stressing. The third part of the thesis focuses on defects in 2D PtSe2. We observed five types of distinct defects in our STM topography images. By comparing them with DFT-calculated simulation images, we identified the types and characteristics of these defects. Our findings would provide critical insight into tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering in few-layer 1T-PtSe2 and other related 2D materials.

Graphene and layered two-dimensional (2D) materials have set a new paradigm in modern solid-state physics and technology. In particular their exceptional optical and electronic properties have shown great promise for novel applications in light detection. However, several challenges remain to fully exploit such properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors (PDs), the efficient extraction of photoexcited charges and ultimately the environmental stability of such atomically-thin materials. In order to overcome the aforementioned limits, novel approaches to tune the properties of graphene and semiconducting \ce{HfS2} are explored in this work, using chemical functionalisation and laser-irradiation. Intercalation of graphene with \ce{FeCl3} is shown to lead to a highly tunable material, with unprecedented stability in ambient conditions. This material is used to define photo-active junctions with an unprecedented LDR via laser-irradiation. Intercalation with \ce{FeCl3} is also used to demonstrate the first all-graphene position-sensitive photodetector (PSD) promising for novel sensing applications. Finally, laser-irradiation is employed, to perform controlled oxidation of ultra-thin \ce{HfS2}, which leads to induced strain in the material and a consequent spatially-varying bandgap. Such structure is used to demonstrate, for the first time, efficient extraction of photogenerated carriers trough the so-called ``charge-funnel'' effect, paving the way to the development of ultra-thin straintronic devices.

La miniaturisation des MOSFET a permis une forte diminution des transistors et des puces, ainsi qu’une augmentation exponentielle des capacités de calcul. Cette miniaturisation ne peut néanmoins continuer ainsi: de nos jours, un microprocesseur peut contenir des dizaines de milliards de transistors et la chaleur dégagée par ces composants peut fortement détériorer ses performances. De plus, du fait de leur principe même de fonctionnement, la tension d’alimentation des MOSFET ne peut être réduite sans en impacter les performances. De nouvelles architectures telles que le TFET -basé sur l’effet tunnel bande-à-bande et pouvant fonctionner à des tensions d’alimentation très basses- ainsi que de nouveaux matériaux pourraient donc apporter une alternative au MOSFET silicium. Les monocouches de dichalcogènures de métaux de transitions (TMDs) -des semiconducteurs à bande interdite directe d’environ 1 à 2 eV- possèdent un fort potentiel pour l’électronique et la photonique. De plus, dans le cas de contraintes appropriées, ils peuvent conduire un alignement de bandes présentant un broken-gap; cette configuration permet de surpasser les limites habituelles du TFETs, à savoir de faibles courants dus à l’effet tunnel sur lequel ces dispositifs reposent. Dans ce travail de thèse, des hétérojonctions planaires de TMD sont modélisées via une approche atomistique de liaisons fortes, et une configuration broken-gap est observée dans deux d’entre elles (MoTe2/MoS2 et WTe2/MoS2). Leur potentiel dans le cadre de transistors à effet tunnel (TFETs) est évalué au moyen de simulations de transport quantique basées sur un modèle TB atomistique ainsi que la théorie des fonctions de Green hors-équilibre. Des TFETs type-p et type-n basés sur ces hétérojonctions sont simulés et présentent des courants ON élevés (ION > 103 µA/µm) ainsi que des pentes sous-seuil extrêmement raides (SS < 5 mV/dec) à des tensions d’alimentation très faibles (VDD = 0.3 V). Plusieurs architectures novatrices basées sur ces TFETs et découlant de la nature 2D des matériaux utilisés sont également présentées, et permettent d’atteindre des performances encore plus élevées
Nowadays, microprocessors can contain tens of billions of transistors and as a result, heat dissipation and its impact on device performance has increasingly become a hindrance to further scaling. Due to their working mechanism, the power supply of MOSFETs cannot be reduced without deteriorating overall performance, and Si-MOSFETs scaling therefore seems to be reaching its end. New architectures such as the TFET, which can perform at low supply voltages thanks to its reliance on band-to-band tunneling, and new materials could solve this issue. Transition metal dichalcogenide monolayers (TMDs) are 2D semiconductors with direct band gaps ranging from 1 to 2 eV, and therefore hold potential in electronics and photonics. Moreover, when under appropriate strains, their band alignment can result in broken-gap configurations which can circumvent the traditionally low currents observed in TFETs due to the tunneling mechanism they rely upon. In this work, in-plane TMD heterojunctions are investigated using an atomistic tight-binding approach, two of which lead to a broken-gap configuration (MoTe2/MoS2 and WTe2/MoS2). The potential of these heterojunctions for use in tunnel field-effect transistors (TFETs) is evaluated via quantum transport computations based on an atomistic tight-binding model and the non-equilibrium Green’s function theory. Both p-type and n-type TFETs based on these in-plane TMD heterojunctions are shownto yield high ON currents (ION > 103 µA/µm) and extremely low subthreshold swings (SS < 5 mV/dec) at low supply voltages (VDD = 0.3 V). Innovative device architectures allowed by the 2D nature of these materials are also proposed, and shown to enhance performance even further

The isolation of single layers of van der Waals materials has shown that their properties can be significantly different compared to their bulk counterparts. These observations, illustrates the importance of interface interactions for determining the materials properties even in weakly interacting materials and raise the question if materials properties of single layer van der Waals materials can be controlled by appropriate hetero-interfaces. To study interface effects in monolayer systems, surface science techniques, such as photoemission spectroscopy and scanning probe microscopy/spectroscopy, are ideally suited. However, before these characterization methods can be employed, approaches for the synthesis of hetero-van der Waals systems must be developed, preferably in-situ with the characterization methods, i.e. in ultra-high vacuum. Therefore, in this thesis, we explored novel approaches for creating van der Waals heterostructures and characterized fundamental structural and electronic properties of such systems. Specifically, we developed an approach to decouple graphene from a Ir(111) growth substrate by intercalation growth of a 2D-FeO layer, and we investigate van der Waals epitaxy of MoSe2 on graphite and other transition metal dichalcogenide substrates. For the Ir(111)/2D-FeO/graphene heterostructure system, we first demonstrated the growth of 2D-FeO on Ir(111). The FeO monolayer on Ir(111) exhibits a long range moiré structure indicating the locally varying change of the coordination of the Fe atoms with respect to the substrate Ir atoms. This variation also gives rise to modulations in the Fe2+-O2- separation, and thus in the monolayer dipole. We demonstrated that this structure can be intercalated underneath of graphene grown on Ir(111) by chemical vapor deposition. The modulation of the dipole in the 2D-FeO moiré structure consequently gives rise to a modulated charge doping in the graphene. This effect has been studied by C-1s core level broadening. In general, this study demonstrates that modulated substrates can be used to periodically modify 2D materials. Growth of transition metal dichalcogenides (TMDCs) by molecular beam epitaxy (MBE) is a very versatile approach for growing TMDC heterostructures. However, there may be unforeseen challenges in the synthesis of some of these materials. Here we show that in MBE growth of MoSe2, the formation of twin grain boundaries is very abundant. While this is detrimental in our efforts for characterizing interface properties of TMDC heterostructures, however the twin grain boundaries have exciting properties. Since the twin grain boundaries are aligned in an epitaxial film we were able to characterize their properties by angle resolved photoemission spectroscopy (ARPES), which may be the first time a material’s line defects could be studied by this method. We demonstrate that the line defects are metallic and exhibit a parabolic dispersing band. Because of the 1D nature of the metallic lines, embedded in a semiconducting matrix, the electronic structure follows a Tomonaga Luttinger formalism and our studies showed strong evidence of the predicted so-called spin charge separation in such 1D electron systems. Moreover, a metal-to-insulator Peierls transition has been observed in this system by scanning tunneling microscopy as well as in transport measurements. Finally, we have shown that the defect network that forms at the surface also lends itself for decoration with metal clusters. Although unexpected, the formation of grain boundary networks in MoSe2 marks the discovery of a new material with exciting quantum properties.

Two-dimensional (2D) materials have attracted extensive research interest in recent years. Among them, graphene and the semiconducting transition metal dichalcogenides (TMDs) are considered as promising candidates for future device applications due to their unique atomic thickness and outstanding properties. The study on graphene and TMDs has demonstrated great potential to further push the scaling of devices into the sub-10 nanometer regime and enable endless opportunities of novel device architectures for the next generation. In this thesis, crucial challenges facing 2D materials are investigated from material synthesis to electronic applications. A comprehensive review of the direct synthesis of graphene on arbitrary substrates with an emphasis on the metal-catalyst-free synthesis is given, followed by a detailed study of the contact engineering in TMDs with a focus on the strategies to lower the contact resistance. Effective approaches have been demonstrated to solve these issues. These include: (1) metal-catalyst-free synthesis of graphene on various insulating substrates; (2) Fermi level pinning observed in TMDs and integration of graphene contact to lower the contact resistance; and (3) application of metal-insulator-semiconductor (MIS) contact in TMD field-effect transistors (FETs). First, a direct low-temperature synthesis of graphene on insulators without any metal catalysts has been realized. The effects of carbon sources, NH3/H2 concentrations, and insulating substrates on the material synthesis have been systematically investigated. Graphene transistors based on the as-grown material have been fabricated to study the electronic properties, which can further confirm the nitrogen-doped graphene has been synthesized from the electrical characterizations. Then electronic devices focusing on the semiconducting TMDs has been studied. The Fermi level pinning has been observed and studied in WS2 FETs with four metal materials. A novel method of using graphene as an insertion layer between the metal and TMDs has been proven to effectively reduce the contact resistance. Owing to the benefit of tuning the graphene work function via the electric field, the contact resistance can further be reduced. Finally, the effectiveness of MIS contacts in WS2 FETs has been demonstrated. A thickness dependence research has been conducted to find the optimal thickness of the inserted insulator. Moreover, the possible physical mechanism of how this MIS contact reduces the contact resistance in 2D materials has been discussed.

The project has two main objectives. First, the demonstration of the existence of single photon emitters in WSe2 monolayers. We recognized localized defect-bound excitons as responsible for this. Second, the demonstration of the evanescent coupling of a localized defect-bound exciton emission to a tapered optical nano fiber. To achieve these goals, we produced a detailed study ofWSe2, starting from the fabrication process in the clean room, up to the characterization ofthe emission spectrum and the proof of the existence of single photon emitters. Then, before fabricating the nanofibers and demonstrating the coupling result, we tested the feasibility ofthe evanescent coupling with COMSOL and MATLAB simulations. In particular, we performed a detailed optical characterization of two samples of WSe2 monolayers produced via exfoliation in clean room. We used an all dry deterministic transfer to encapsulate the samples in two layers of hexagonal boron nitride (hBN). We performed micro-photoluminescence, lifetime and degree of second-order temporal coherence measurements. We focused our attention in localized defect-bound excitons due to the high intensity PL signal and sharp linewidth. Moreover, in this work we have demonstrated the evanescent coupling of a single localized defect-bound exciton emitter with a tapered optical nano fiber we produced. For the fabrication of the nanofiber, through COMSOL and MATLAB simulations we found the right size to have a sufficiently intense evanescent field to allow coupling to the emitter. We managed, through several repetitions, to produce autonomously the nanofiber of the desired size. Finally we were able to demonstrate the feasibility of the evanescent coupling of the emission to the fiber from a chosen localized defect-bound exciton. Thus, our results provide evidence of the possibility to integrate quantum emitters in 2D materials with photonic structures.

This thesis describes the fabrication and characterization of two-dimensional transition dichalcogenides (2D TMDs) nanolayers for various applications in electronic and opto-electronic devices applications. In Chapter 1, crystal and optical structure of TMDs materials are introduced. Many TMDs materials reveal three structure polytypes (1T, 2H, and 3R). The important electronic properties are determined by the crystal structure of TMDs; thus, the information of crystal structure is explained. In addition, the detailed information of photon vibration and optical band gap structure from single-layer to bulk TMDs materials are introduced in this chapter. In Chapter 2, detailed information of physical properties and synthesis techniques for molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum ditelluride (MoTe2) nanolayers are explained. The three representative crystal structures are trigonal prismatic (hexagonal, H), octahedral (tetragonal, T), and distorted structure (Tʹ). At room temperature, the stable structure of MoS2 and WS2 is semiconducting 2H phase, and MoTe2 can reveal both 2H (semiconducting phase) and 1Tʹ (semi-metallic phase) phases determined by the existence of strains. In addition, the pros and cons of the synthesis techniques for nanolayers are discussed. In Chapter 3, the topic of synthesized large-scale MoS2, WS2, and MoTe2 films is considered. For MoS2 and WS2 films, the layer thickness is modulated from single-layer to multi-layers. The few-layer MoTe2 film is synthesized with two different phases (2H or 1Tʹ). The all TMDs films are fabricated using two-step chemical vapor deposition (CVD) method. The analyses of atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy confirm that the synthesis of high crystalline MoS2, WS2, and MoTe2 films are successful. The electronic properties of both MoS2 and WS2 exhibit a p-type conduction with relatively high field effect mobility and current on/off ratio. In Chapter 4, vertically-stacked few-layer MoS2/WS2 heterostructures on SiO2/Si and flexible polyethylene terephthalate (PET) substrates is presented. Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) show the structural integrity of two distinct 2D TMD layers with atomically sharp van der Waals (vdW) heterointerfaces. Electrical transport measurements of the MoS2/WS2 heterostructure reveal diode-like behavior with current on/off ratio of ~ 104. In Chapter 5, optically uniform and scalable single-layer Mo1-xWxS2 alloys are synthesized by a two-step CVD method followed by a laser thinning. Post laser treatment is presented for etching of few-layer Mo1-xWxS2 alloys down to single-layer alloys. The optical band gap is controlled from 1.871 to 1.971 eV with the variation in the tungsten (W) content, x = 0 to 1. PL and Raman mapping analyses confirm that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to SiO2/Si substrate, resulting in fabrication of spatially uniform single-layer Mo1-xWxS2 alloy films.

Etude rpe de (ch::(3))::(4) nmn::(1-x) cd::(x) cl::(5) entre 20 et 300 k, pour x = 0,02; 0,08 et 0,2; mesure des temps de relaxation spin-reseau. Mise en evidence de variations importantes en fonction de la composition. Interpretation des resultats au moyen du formalisme de la fonction memoire et d'un modele de la fonction de correlation de spins des chaines finies

Il ditellurio di tungsteno (WTe2) appartiene alla classe dei dicalcogenuri di metalli di transizione (TMDs), che rappresentano attualmente i materiali più promettenti, insieme al grafene, nel campo di ricerca dei cristalli bidimensionali (2D). Grazie ad una caratteristica struttura stratificata, con differenti piani atomici legati da forze di van der Waals, mediante esfoliazione è possibile isolare strati di spessore quasi-atomico di TMDs, detti “monostrati”, con proprietà spesso molto diverse dal materiale bulk originario. Il WTe2 nella sua forma a monostrato, è stato recentemente oggetto di interesse scientifico, in quanto teoricamente predetto essere un isolante topologico (TI) bidimensionale. Un TI è un materiale che internamente si comporta come un isolante elettrico ma che sulla superficie manifesta stati conduttivi. Lo scopo di questa tesi è studiare le proprietà si trasporto di monostrati di WTe2 in micro-dispositivi realizzati con opportune tecniche di nanofabbricazione. L'ossidazione della superficie esterna del WTe2, dovuta ad una non-perfetta stabilità in aria, influenza significativamente il trasporto elettronico in cristalli costituiti da pochi strati atomici ed è causa di una transizione metallo-isolante. Una possibile soluzione per evitare la degradazione del materiale consiste nell' “incapsulamento” di un monostrato di WTe2 fra materiali 2D chimicamente inerti, come il nitruro di boro esagonale. A tale proposito, si è sviluppata una tecnica di “trasferimento” che permette di sollevare e allineare con precisione micrometrica strati di spessore atomico di differenti materiali, assemblando eterostrutture di van der Waals. Campioni selezionati sono studiati mediante misure di magneto-transporto a bassa temperatura (fino a 0.250 K). I dati analizzati evidenziano l'esistenza di un gap di energia in monostrati di WTe2 e la presenza di una corrente localizzata ai bordi del sistema, coerentemente con l'ipotesi di un isolante topologico 2D.

This work is focused on the study of luminescence of atomic thin layers of transition metal chalkogenides (eg. MoS2). In the experimental part, the work deals with the preparation of atomic thin layers of semiconducting chalcogenides and the subsequent manufacturing of plasmonic interference structures around these layers. The illumination of the interference structure will create a standing plasmonic wave that will excite the photoluminescence of the semiconductor. Photoluminescence was studied both by far-field spectroscopy and near-field optical microscopy.

In this work, we explored one material from the broad family of 2D semiconductors, namely WSe2 to serve as an enabler for advanced, low-power, high-performance nanoelectronics and optoelectronic devices. A 2D WSe2 based field-effect-transistor (FET) was designed and fabricated using electron-beam lithography, that revealed an ultra-high mobility of ~ 625 cm2/V-s, with tunable charge transport behavior in the WSe2 channel, making it a promising candidate for high speed Si-based complimentary-metal-oxide-semiconductor (CMOS) technology. Furthermore, optoelectronic properties in 2D WSe2 based photodetectors and 2D WSe2/2D MoS2 based p-n junction diodes were also analyzed, where the photoresponsivity R and external quantum efficiency were exceptional. The monolayer WSe2 based photodetector, fabricated with Al metal contacts, showed a high R ~502 AW-1 under white light illumination. The EQE was also found to vary from 2.74×101 % - 4.02×103 % within the 400 nm -1100 nm spectral range of the tunable laser source. The interfacial metal-2D WSe2 junction characteristics, which promotes the use of such devices for end-use optoelectronics and quantum scale systems, were also studied and the interfacial stated density Dit in Al/2D WSe2 junction was computed to be the lowest reported to date ~ 3.45×1012 cm-2 eV-1. We also examined the large exciton binding energy present in WSe2 through temperature-dependent Raman and photoluminescence spectroscopy, where localized exciton states perpetuated at 78 K that are gaining increasing attention for single photon emitters for quantum information processing. The exciton and phonon dynamics in 2D WSe2 were further analyzed to unveil other multi-body states besides localized excitons, such as trions whose population densities also evolved with temperature. The phonon lifetime, which is another interesting aspect of phonon dynamics, is calculated in 2D layered WSe2 using Raman spectroscopy for the first time and the influence of external stimuli such as temperature and laser power on the phonon behavior was also studied. Furthermore, we investigated the thermal properties in 2D WSe2 in a suspended architecture platform, and the thermal conductivity in suspended WSe2 was found to be ~ 1940 W/mK which was enhanced by ~ 4X when compared with substrate supported regions. We also studied the use of halide-assisted low-pressure chemical vapor deposition (CVD) with NaCl to help to reduce the growth temperature to ∼750 °C, which is lower than the typical temperatures needed with conventional CVD for realizing 1L WSe2. The synthesis of monolayer WSe2 with high crystalline and optical quality using a halide assisted CVD method was successfully demonstrated where the role of substrate was deemed to play an important role to control the optical quality of the as-grown 2D WSe2. For example, the crystalline, optical and optoelectronics quality in CVD-grown monolayer WSe2 found to improve when sapphire was used as the substrate. Our work provides fundamental insights into the electronic, optoelectronic and quantum properties of WSe2 to pave the way for high-performance electronic, optoelectronic, and quantum-optoelectronic devices using scalable synthesis routes.

Since the discovery of graphene, substantial effort has been put toward the synthesis and production of 2D materials. Developing scalable methods for the production of high-quality exfoliated nanosheets has proved a significant challenge. To date, the most promising scalable method for achieving these materials is through the liquid-based exfoliation (LBE) of nanosheetsin solvents. Thin films of nanosheets in dispersion can be modified with additives to produce 2D inks for printed electronics using inkjet printing. This is the most promising method for the deposition of such materials onto any substrate on an industrial production level. Although well-developed metallic and organic printed electronic inks exist on the market, there is still a need to improve or develop new inks based on semiconductor materials such as transition metal dichalcogenides (TMDs) that are stable, have good jetting conditions and deliver good printing quality.The inertness and mechanical properties of layered materials such as molybdenum disulfide (MoS2) make them ideally suited for printed electronics and solution processing. In addition,the high electron mobility of the layered semiconductors, make them a candidate to become a high-performance semiconductor material in printed electronics. Together, these features make MoS2 a simple and robust material with good semiconducting properties that is also suitable for solution coating and printing. It is also environmentally safe.The method described in this thesis could be easily employed to exfoliate many types of 2D materials in liquids. It consists of two exfoliation steps, one based on mechanical exfoliation of the bulk powder utilizing sand paper, and the other inthe liquid dispersion, using probe sonication to liquid-exfoliate the nanosheets. The dispersions, which were prepared in surfactant solution, were decanted, and the supernatant was collected and used for printing tests performed with a Dimatix inkjetprinter. The printing test shows that it is possible to use the MoS2 dispersion as a printed electronics inkjet ink and that optimization for specific printer and substrate combinations should be performed. There should also be advances in ink development, which would improve the drop formation and break-off at the inkjet printing nozzles, the ink jetting and, consequently, the printing quality.
Sedan upptäckten av grafen har mycket arbete lagts på framställning och produktion av 2D-material. En viktig uppgift har varit att ta fram skalbara metoder för produktion av högkvalitativa  nanosheets via exfoliering. Den mest lovande skalbarametoden hittills har varit vätskebaserad exfoliering av nanosheets i lösningsmedel. Tunna filmer av nanosheets i dispersion kan anpassas med hjälp av tillsatser och användas för tillverkning av halvledare strukturer med inkjet-skrivare, vilket är den mest lovande metoden för på en industriell produktions nivå beläggaden typen av material på substrat. Även om det finns välutvecklade metalliska och organiskabläck för tryckt elektronik, så finns det fortfarande ett behov av att förbättra eller utveckla nya bläck baserade på halvledarmaterial som t.ex. TMD, som är stabila, har goda bestryknings  egenskaper och ger bra tryckkvalitet. Den inerta naturen tillsammans med de mekaniska egenskaperna som finns hosskiktade material, som t.ex. molybdendisulfid (MoS2), gör demlämpliga för flexibel elektronik och bearbetning i lösning. Dessutom gör den höga elektronmobiliteten i dessa 2D-halvledaredem till en stark kandidat som halvledarmaterial inom trycktelektronik. Det betyder att MoS2 är ett enkelt och robust material med goda halvledaregenskaper som är lämpligt för bestrykning från lösning och tryck, och är miljömässigt säker.Den metod som beskrivs här kan med fördel användas föratt exfoliera alla typer av 2D-material i lösning. Exfolieringensker i två steg; först mekanisk exfoliering av torr bulk med sandpapper, därefter används ultraljudsbehandling i lösning för att exfoliera nanosheets. De dispersioner som framställts i lösning med surfaktanter dekanterades och det övre skiktetanvändes i trycktester med en Dimatix inkjet-skrivare.Tryckprovet visar att det är möjligt att använda MoS2 -dispersion som ett inkjet-bläck och att optimering för särskildaskrivar- och substratkombinationer borde göras, såsom förbättringav bläcksammansättningen med avseende på droppbildning och break-off vid skrivarmunstycket, vilket i sin tur skulleförbättra tryckkvaliteten.
KM2
Paper Solar Cells

The thesis deals with study of thin layers of transition metal dichalcogenides, especially of molybdenum disulfide. Nanostructures were fabricated on two-dimensional crystals of MoS2 and WSe2. Within followed analysis attention was paid to the photoluminescence properties. In the thesis transition metal dichalcogenides are reviewed and description of the modified process of preparation by micromechanical exfoliation is given.

Deeper understanding and technological progress in materials physics demand exploration of soft and hard matter at their relevant length scales. This research focuses on the nanometer length scale investigation of structural changes required for membrane fusion in virus nanoparticles and nano-spectroscopic investigation of layered material surfaces implementing scattering type scanning near-field optical microscopy (s-SNOM). Spectroscopy and imaging experiments were deployed to investigate the chemical and structural modifications of the viral protein and lipid bilayer under various environmental pH variations. It has been shown that breakage of viral membrane could occur even without the presence of a targeting membrane, if the environment pH is lowered. This is in contrary to the current viral fusion model, which requires virus binding to a host cell membrane for forming the fusion pore to release the viral genome. The fusion inhibitor compound 136 can effectively prevent the membrane breakage induced by low pH. The chemical surface stability and degradation of black phosphorus (BP) under ambient conditions have been studied using s-SNOM. We found that the degraded area and volume on the surface of black phosphorus increase with time slowly at the start of degradation and enlarge rapidly (roughly exponentially) afterward and reach saturation growth following S-shaped growth curve (sigmoid growth curve). The theoretical model presented suggests that the degraded sites in the adjacent surrounding causes the experimentally observed exponential growth of degraded area at the initial stage. By studying the BP surfaces coated by Al2O3, boron nitride (BN) and hybrid BN/Al2O3 layers through the period up to 6 months, it has been concluded that ~5 nm thin hybrid layer of BN/Al2O3 helps the surface passivation of BP flakes of thickness ~30 nm. This is supported by the electrical characterization results of BP field effect transistor coated with a BN/Al2O3 layer. We have performed infrared nano-spectroscopy on muscovite mica exfoliated on silicon and silicon dioxide substrates. We show that the near-field profile in s-SNOM can penetrate down to several hundreds of nanometers and enable spectroscopy of buried structures. We found spectral broadening of mica as its thickness increases revealing clearly the effect of size on the absorption response.

abstract: Layered chalcogenides are a diverse class of crystalline materials that consist of covalently bound building blocks held together by van der Waals forces, including the transition metal dichalcogenides (TMDCs) and the pnictogen chalcogenides (PCs) among all. These materials, in particular, MoS2 which is the most widely studied TMDC material, have attracted significant attention in recent years due to their unique physical, electronic, optical, and chemical properties that depend on the number of layers. Due to their high aspect ratios and extreme thinness, 2D materials are sensitive to modifications via chemistry on their surfaces. For instance, covalent functionalization can be used to robustly modify the electronic properties of 2D materials, and can also be used to attach other materials or structures. Metal adsorption on the surfaces of 2D materials can also tune their electronic structures, and can be used as a strategy for removing metal contaminants from water. Thus, there are many opportunities for studying the fundamental surface interactions of 2D materials and in particular the TMDCs and PCs. The work reported in this dissertation represents detailed fundamental studies of the covalent functionalization and metal adsorption behavior of layered chalcogenides, which are two significant aspects of the surface interactions of 2D materials. First, we demonstrate that both the Freundlich and Temkin isotherm models, and the pseudo-second-order reaction kinetics model are good descriptors of the reaction due to the energetically inhomogeneous surface MoS2 and the indirect adsorbate-adsorbate interactions from previously attached nitrophenyl (NP) groups. Second, the covalent functionalization using aryl diazonium salts is extended to nanosheets of other representative TMDC materials MoSe2, WS2, and WSe2, and of the representative PC materials Bi2S3 and Sb2S3, demonstrated using atomic force microscopy (AFM) imaging and Fourier transform infrared spectroscopy (FTIR). Finally, using AFM and X-ray photoelectron spectroscopy (XPS), it is shown that Pb, Cd Zn and Co form nanoclusters on the MoS2 surface without affecting the structure of the MoS2 itself. The metals can also be thermally desorbed from MoS2, thus suggesting a potential application as a reusable water purification technology.
Dissertation/Thesis
Doctoral Dissertation Materials Science and Engineering 2019

Nanomaterials are playing an increasing role in novel technologies, and it is important to develop optical methods to characterize them in situ.  To that end, backside absorbing layer microscopy (BALM) has emerged as a powerful tool, being capable to resolve sub-nanometer height profiles, with video-rate acquisition speeds and a suitable geometry to couple live experiments.  In the internship, several techniques involving BALM were developed, and applied to study optical and electrical properties of the transition metal dichalcogenide (TMD) monolayer MoS2, a type of 2-dimensional (2D) crystalline semiconductor.  A simulations toolkit was created in MATLAB to model BALM, a workflow to reliably extract linear intensities from the CMOS detector was realized, and 2D MoS2 was synthesized by chemical vapor deposition followed by transfer to appropriate substrates.  BALM data of the 2D MoS2 was acquired and combined with simulations, giving a preliminary result for its complex refractive index at 5 optical wavelengths.  In addition, the first steps towards coupling BALM with a gate biased 2D MoS2 field-effect transistor were explored.  To complement BALM measurements, the grown samples were also characterized by conventional optical microscopy, scanning electron microscopy, atomic force microscopy, photoluminescence spectroscopy, and Raman spectroscopy.  This work provides new additions to an existing platform of BALM techniques, enabling novel BALM experiments with nanomaterial systems.  In particular, it introduces a new alternative for local extraction of optical parameters and for probing of electrical charging effects, both of which are vital in the research and development of nano-optoelectronics.

Nano-materials are interesting material category with a single unit size between 1 and 1000 nanometers and possess unique mechanical, electrical, optical, and other physical properties that make them stand out from ordinary materials. With increasing demand for reduced size of electronic devices and integrated micro/nano-electro-mechanical systems (MEMS / NEMS), there is a high driving force in scientific research and technological advancement in nanotechnology. My research is about two popular novel nanomaterials: carbon nanotubes (1-dimensional material) and thin-layer transition metal dichalcogenides (2-dimensional materials). My first research direction is about the characterization of electrical properties of carbon nanotubes and using them as bio-sensors. Carbon nanotubes (CNTs), in general, are a material of great interest for many applications since their first discovery in 1991 [1], due to their unique structure, extraordinary electrical and mechanical properties, and unusual chemical properties. High-throughput fabrication of carbon nanotube field effect transistors (CNTFETs) with uniform properties has been a challenge since they were first fabricated in 1998. We invent a novel fabrication method to produce a 1×1 cm2 chip with over 700 CNTFETs fabricated around one single carbon nanotube. This large number of devices allows us to study the stability and uniformity of CNTFET properties. We grow flow-aligned CNTs on a SiO2/Si substrate by chemical vapor deposition and locate a single long CNT (as long as 1 cm) by scanning electron microscopy. Two photolithography steps are then used, first to pattern contacts and bonding pads, and next to define a mask to ‘burn’ away additional nanotubes by oxygen plasma etch. A fabrication yield of ~72% is achieved. The authors present statistics of the transport properties of these devices, which indicates that all the CNTFETs share the same threshold voltage, and similar on-state conductance. These devices are then used to measure DNA conductance by connecting DNA molecule of varying lengths to lithographically cut CNTFETs. While one single carbon nanotube is considered 1-dimensional material because it only has one side with “non-nano” length, the thin-layer transition metal dichalcogenides (TMDCs) are called the 2-dimensional materials since they have two sides of normal lengths and the other side of atomic size. Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides have attracted extensive interests in recent years, motivating investigation into multiple properties. We use a refined version of the optothermal Raman technique [2][3] to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bi-layer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ~40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different positions in radial direction. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are (80±17) W/mK and (55±18) W/mK, respectively. For 2L MoS2 and MoSe2, we obtain values of (73±25) W/mK and (39±13) W/mK. Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m2K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

abstract: Two-dimensional quantum materials have garnered increasing interest in a wide variety of applications due to their promising optical and electronic properties. These quantum materials are highly anticipated to make transformative quantum sensors and biosensors. Biosensors are currently considered among one of the most promising solutions to a wide variety of biomedical and environmental problems including highly sensitive and selective detection of difficult pathogens, toxins, and biomolecules. However, scientists face enormous challenges in achieving these goals with current technologies. Quantum biosensors can have detection with extraordinary sensitivity and selectivity through manipulation of their quantum states, offering extraordinary properties that cannot be attained with traditional materials. These quantum materials are anticipated to make significant impact in the detection, diagnosis, and treatment of many diseases. Despite the exciting promise of these cutting-edge technologies, it is largely unknown what the inherent toxicity and biocompatibility of two-dimensional (2D) materials are. Studies are greatly needed to lay the foundation for understanding the interactions between quantum materials and biosystems. This work introduces a new method to continuously monitor the cell proliferation and toxicity behavior of 2D materials. The cell viability and toxicity measurements coupled with Live/Dead fluorescence imaging suggest the biocompatibility of crystalline MoS2 and MoSSe monolayers and the significantly-reduced cellular growth of defected MoTe2 thin films and exfoliated MoS2 nanosheets. Results show the exciting potential of incorporating kinetic cell viability data of 2D materials with other assay tools to further fundamental understanding of 2D material biocompatibility.
Dissertation/Thesis
Masters Thesis Materials Science and Engineering 2019

碩士
臺北市立教育大學
數學資訊教育學系數學資訊教育教學碩士學位
97
This research tried to investigate the design guidelines of multimedia unit instructional materials. By way of context analysis and interviews, we explored some notes for the teaching implementation and material design of multimedia unit materials. Besides, we used tailored research tool to examine the outstanding cases of the group of elementary schools from Taipie Multimeida Resource Center. After analyzing the collected data, we obtained the following conclusions: 1. Among the 60 outstanging cases of the group of elementary schools from multimedia unit materials selection activities held by Taipei Multimedia Resource Center during the years from 2001 to 2008, there were 39 cases selected in the years 2002 and 2003. The distribution of these outstanding cases was: the first three learning areas were Science and Technology, Social Studies, and Arts and Humanities; the first three amounts were Life Education, Visual Arts, and Dialects. These outstanding cases were best suited for grade 5 and secondly for grade 6. 2. To design more applicable multimedia unit materials, eight aspects should be considered, namely, demands analysis, subjects analysis, instruction goals, curriculum content, media design, instructional design, instructional evaluation, and reveal of creativity. Material designers should use the guidelines for each aspects to inspect and modify the mutimedia unit material during the instructional planning, instructional design and instructional application stages. 3. After applying the 38 design guidelines to inspect the outstanding cases of the group of elementary schools from Taipei Multimedia Resource Center, we found that 24 design guidelines conformed to 90% above, accounts for 63.2%; 10 design guidelines conformed the range between 60% and 90%, accounts for 26.3%; 4 design guildlines did not tally with 90% above, accounts for 10.5%. According to the result of the study, we proposed some suggestions to the educational administrative organization, elementary schools, teachers, and for future research respectively.

Discovery of grapheme has paved way for experimental realization of many physical phenomena such as massless Dirac fermions, quantum hall effect and zero-field conductivity. Search for other two dimensional (2D) materials led to the discovery of boron nitride, transition metal dichalcogenides(TMDs),transition metal oxides(MO2)and silicene. All of these materials exhibit different electronic and transport properties and are very promising for nanodevices such as nano-electromechanical-systems(NEMS), field effect transistors(FETs),sensors, hydrogen storage, nano photonics and many more. For practical utility of these materials in electronic and photonic applications, varying the band gap is very essential. Tuning of band gap has been achieved by doping, functionalization, lateral confinement, formation of hybrid structures and application of electric field. However, most of these techniques have limitations in practical applications. While, there is a lack of effective method of doping or functionalization in a controlled fashion, growth of specific sized nanostructures (e.g., nanoribbons and quantum dots),freestanding or embedded is yet to be achieved experimentally. The requirement of high electric field as well as the need for an extra electrode is another disadvantage in electric field induced tuning of band gap in low dimensional materials. Development of simpler yet effective methods is thus necessary to achieve this goal experimentally for potential application of these materials in various nano-devices. In this thesis, novel methods for tuning band gap of few 2D materials, based on strain and stacking, have been proposed theoretically using first principles based density functional theory(DFT) calculations. Electronic properties of few layered nanomaterials are studied subjected to mechanical and chemical strain of various kinds along with the effect of stacking pattern. These methods offer promising ways for controlled tuning of band gap in low dimensional materials. Detailed methodology of these proposed methods and their effect on electronic, structural or vibrational properties have also been studied. The thesis has been organized as follows: Chapter1 provides a general introduction to the low dimensional materials: their importance and potential application. An overview of the systems studied here is also given along with the traditional methods followed in the literature to tune their electronic properties. The motivation of the current research work has also been highlighted in this chapter. Chapter 2 describes the theoretical methodology adopted in this work. It gives brief understanding of first principles based Density Functional Theory(DFT) and various exchange and correlation energy functionals used here to obtain electronic, structural, vibrational and magnetic properties of the concerned materials. Chapter 3 deals with finding the origin of a novel experimental phenomenon, where electromechanical oscillations were observed on an array of buckled multiwalled carbon nanotubes (MWCNTs)subjected to axial compression. The effect of structural changes in CNTs in terms of buckling on electronic properties was studied. Contribution from intra-as well as inter-wall interactions was investigated separately by using single-and double-walled CNTs. Chapter 4 presents a method to manipulate electronic and transport properties of graphene bilayer by sliding one of the layers. Sliding caused breaking of symmetry in the graphene bilayer, which resulted in change in dispersion in the low energy bands. A transition from linear dispersion in AA stacking to parabolic dispersion in AB stacking is discussed in details. This shows a possibility to use these slid bilayers to tailor graphene based devices. Chapter 5 develops a method to tune band gap of bilayers of semiconducting transition metal dichalcogenides(TMDs) by the application of normal compressive strain. A reversible semiconductor to metal(S-M) transition was reported in this chapter for bilayers of TMDs. Chapter 6 shows the evolution of S-M transition from few layers to the bulk MoS2 under various in-plane and out of plane strains. S-M transition as a function of layer number has been studied for different strain types. A comparison between the in-plan and normal strain on modifying electronic properties is also presented. Chapter 7 discusses the electronic phase transition of bulk MoS2 under hydrostatic pressure. A hydrostatic pressure includes a combined effect of both in-plane and normal strain on the structure. The origin of metallic transition under pressure has been studied here in terms of electronic structure, density of states and charge analysis. Chapter 8 studies the chemical strain present in boron nitride nanoribbons and its effect on structural, electronic and magnetic properties of these ribbons. Properties of two achiral (armchair and zig-zag) edges have been analyzed in terms of edge energy and edge stress to predict stability of the edges. Chapter9 summarizes and concludes the work presented in this thesis.

Recent advances in atomically thin two-dimensional materials have led to various promising technologies such as nanoelectronics, sensing, energy storage, and optoelectronics applications. Graphene with sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice has attracted tremendous interest with excellent electrical, optical, mechanical, and chemical properties. In this work, graphene’s mechanical properties, chemical properties, and piezoelectric properties are explored as graphene is implemented in the automotive electrical distribution system. Graphene is useful in friction reduction, corrosion protection, and piezoelectric energy harvesting cell improvement. Besides graphene, transition metal dichalcogenides (TMDs), which are the metal atoms sandwiched between two chalcogen atoms, have also attracted much attention. Unlike graphene, many TMDs are semiconductors in nature and possess enormous potential to be used as a potential channel material in ultra-scaled field-effect transistors (FETs). In this work, chemical doping strategies are explored for the tunnel FETs applications using different metal phthalocyanines and polyethyleneimines as dopants. TMDs FETs can also be used as a selective NO₂ gas sensor with a polydimethylsiloxane filter and a highly sensitive photo-interfacial gated photodetector application.

Magnetic random­access memory (MRAM) devices have been widely studied since the 1960s. During this time, the size of spintronic devices has continued to decrease. Conse quently, there is now an urgent need for new low­dimensional magnetic materials to mimic the traditional structures of spintronics at the nanoscale. We also require new effective mechanisms to conduct the main functions of memory devices, which are: reading, writ ing, and storing data. To date, most research efforts have focused on MRAM devices based on magnetic tun nel junction (MTJ), such as a conventional field­driven MRAM and spin­transfer torque (STT)­MRAM devices. Consequently, many efforts are currently focusing on new alterna tives using different techniques, such as spin­orbit torque (SOT) and magnetic skyrmions (a skyrmion is the smallest potential disruption to a uniform magnet required to obtain more effective memory devices). The most promising memory devices are SOT­MRAMs and skyrmion­based memories. This study investigates the magnetic properties of 1T­phase vanadium dichalcogenide (VXY) Janus monolayers, where X, Y= S, Se, or Te (i.e., monolayers that exhibit inversion symme try breaking due to the presence of different chalcogen elements). This study is developed along four directions: (I) the nature of the magnetism and the SOT effect of Janus mono layers; (II) the Dzyaloshinskii Moriya interaction (DMI); (III) investigation of stability en hancement by adopting practical procedures for industry; and (IV) study of the effect of a hexagonal boron nitride (h­BN) monolayer as an insulator on the magnetism of the VXY monolayer. This study provides a clear perspective for the next generation of memory de vices, such as SOT­MRAMs based on transition metal dichalcogenide monolayers.

Dissertação de Mestrado em Engenharia Mecânica, apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra.
Transition metal dichalcogenides (TMD) coatings are solid low-friction self-lubricant materials enabling easy sliding by formation of a tribo lm at the interface in the contact region. W-S coating, which belongs to the TMDs family, was doped with N in order to combine low friction of the coating with good hardness due to addition of a third element. In this work, W-S and W-S-N coatings with di erent composition have been deposited by magnetron sputtering, characterised and evaluated with respect to the structure, mechanical properties and tribological performance. The composition was varied by changing the ow of N2 into the deposition chamber, leading to nitrogen contents ranging from 0 to 33 at.%. All coatings were deposited with Cr interlayer. Films W-S and W-S-N deposited with the lowest content of N had a crystalline structure, while coatings with the higher N content were amorphous. The coating were tribologically tested against nitrile-butadiene rubbe (NBR) balls at room temperature and 200 C using a pin-on-disc tribometer. Energy dispersive spectroscopy, optical microscopy observations and 2D pro lometry were performed after the tribological tests. The results showed that the coatings had better tribological performance at elevated temperature. The wear and friction behaviour of the coatings was interpreted as a function of several factors including: mechanical strength, structure and formation of the tribo lm. It was concluded that WS- N coatings supposedly could be material for applications requiring contact with rubber at temperatures greater than room temperature.
Os revestimentos de Dicalcogenetos de Metais de Transi c~ao (DMT) s~ao materiais s olidos, auto-lubri cantes com baixo-atrito que permitem um f acil deslizamento atrav es da forma c~ao de uma camada transfer^encia na interface da zona de contacto. Os revestimentos W-S, que pertencem a familia dos DMTs, foram dopados com N de forma a combinar o baixo atrito dos revestimentos com elevada dureza, atrav es da adi c~ao de um terceiro elemento. Neste trabalho, os revestimentos W-S e W-S-N com diferentes composi c~oes foram depositados, caraterizados e avaliados no que diz respeito o sua estrutura, propriedades mec^anicas e comportamento tribol ogico. A composi c~ao foi variada atrav es da mudan oa de uxo de N2 no interior c^amara de deposi c~ao, levando a uma varia c~ao at omica em azoto nos revestimentos de 0 a 33 %. Todos os revestimentos foram depositados com uma intercamada de Cr. Os lmes W-S-N depositados com o menor teor e sem N apresentam alguma cristalinidade, enquanto os revestimentos com maior percentagem de N s s~a amorfos. Os revestimentos foram tribologicamente testados por pino-disco com um contracorpo esf erico de borracha NBR o temperatura ambiente e a 200 C. Ap os os ensaios tribol ogicos, as amostras foram caracterizadas por EDS, microscopia otica e per lometria 2D. Os resultados demostraram que os revestimentos apresentam melhor performance a elevadas temperaturas. A taxa de desgaste e o coe ciente de atrito foram interpretados como fun c~ao de v arios fatores, incluindo: dureza, estrutura e forma c~ao da camada transfer^encia. Deste trabalho, podemos concluir que os revestimentos W-S-N podem ser usados em aplica c~oes que requeiram o contacto com borracha a temperaturas acima da temperatura ambiente havendo uma redu c~ao do atrito em rela c~ao ao deslizamento de metal borracha para as mesmas condi c~oes.