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

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.

<p>Beyond graphene, two-dimensional (2D) atomic layered materials have drawn considerable attention as promising semiconductors for future ultrathin layered nano-electronic device applications, transparent/flexible devices and chemical sensors. But, they exhibit high levels of low-frequency due to interfacial scattering (small thickness) and interlayer coupling (large thickness). The sources and mechanisms of low frequency noise should be comprehensive and controlled to fulfill practical applications of two-dimensional transistors. This work seeks to understand the fundamental noise mechanisms of 2D transistors to find ways to reduce the noise level. It also verifies how noise can provide a spectroscopy for analysis of device quality.</p> <p>Most noise analysis tend to apply classical MOSFET models to the noise and electrical transport of 2D transistors, which put together all possible independent noise sources in 2D transistors, ignoring the contact effects. So this could lead to wrong estimation of the noise analysis in 2D transistors. This work demonstrates how the noise components can come from the channel and contact/access regions, all independently adding to the total noise. Each noise source can contribute and may dominate the total noise behavior under the specific gate voltage bias. Herein, the measured noise amplitude in our MoS₂ and MoSe₂ FETs shows a direct crossover from channel- to contact-dominated noise as the gate voltage is increased. The results can be interpreted in terms of a Hooge relationship associated with the channel noise, a transition region, and a saturated high-gate voltage regime whose characteristics are determined by a voltage-independent conductance and noise source associated with the metallurgical contact and the interlayer resistance. The approach for separating channel contributions from those contact/access region allows clear evaluation of the channel noise mechanism and also can be used to explain the qualitative differences in the transition regions between contact- and channel-dominated regimes for various devices.</p>

Les monofeuillets de dichalcogénures de métaux de transition (TMDC) tels que MoS2 ou WSe2 sont des semiconducteurs bidimensionnels à gap direct, dont les allées K et K' sont inéquivalentes dans la première zone de Brillouin : la levée de dégénérescence induite par le couplage spin-orbite entre les bandes de spin up et dawn est inversée entre les vallées K et K'. Des contacts métalliques magnétiques devraient permettre une injection de spin efficace depuis une électrode magnétique vers un TMDC. Les indices de vallée (Kou K') et de spin (up ou dawn) étant fortement couplés, cela permettrait de sélectionner électriquement l'une ou l'autre des vallées et de réaliser des dispositifs à base de TMDC pour la spintronique (exploitant le spin des électrons) ou pour la valléetronique (exploitant l'indice de vallée des électrons). Dans cette thèse, nous explorons les propriétés physiques des interfaces Co(OOOl)/MoS2 et Ni(lll)/WSe2 par des méthodes de calcul ab-initia basées sur la théorie de la fonctionnelle de la densité. Nous démontrons la nature covalente des liaisons à l'interface entre les monofeuillets de TMDC et les surfaces magnétiques Co(OOOl) et Ni(lll). Nous décrivons la structure atomique de ces interfaces, ainsi que la modification des moments magnétiques induite par des transferts de charge électrique entre atomes. Les liaisons covalentes aux interfaces confèrent aux monofeuillets de MoS2 et de WSe2 un caractère métallique. Nos calculs donnent finalement accès à la polarisation en spin au niveau de Fermi du TMDC connecté à ces électrodes magnétiques, ainsi qu'à la hauteur de la barrière Schottky (différence entre le niveau de Fermi dans la phase métallique du TMDC situé sous le contact magnétique et le bas de la bande de conduction du TMDC pur dans le canal). / Transition metal dichalcogenide (TMDC) single layers like MoS2 or WSe2 are direct band gap two-dimensional semiconductors, with non-equivalent K and K' valleys in the first Brillouin zone. The degeneracy liftingbetween spin-up and spin-down energy bands induced by spin-orbit coupling is inverted between the K and K' valleys . Magnetic metallic contacts should allow spin-injection from a magnetic electrode to a TMDC single layer. The valley (K or K') and spin (up or down) indexes being strongly coupled, this should also allow to electrically select one of the valleys in TMDC-based spintronic or valleytronic deviees. In this Thesis, we have studied the physical properties of the Co(OOOl)/MoS2 and Ni(lll)/WSe2 interfaces with first-principles methods based on the density functional theory. We demonstrated that the TMDC single layers are covalently bound to the Co(OOOl) and Ni(lll) surfaces. We describe the atomic structure of these interfaces and the modification of the magnetic moments induced by charge transfer between interface atomes. The MoS2 and WSe2 single layers become metallic when they are covalently bound to the magnetic metals. We also calculated the spin-polarization at the Fermi level of the TMDC layers connected to th Co and Ni electrodes and the Schottky barrier height (difference between the Fermi level in the metallic phase of the TMDC below the magnetic contact and the bottom of the conduction band in a pure TMDC channel).

Two-dimensional transition metal dichalcogenides (TMDCs) aroused significant interest in the last years as semiconductor materials for application in the field of electronics, due to their tunable bandgap, good carrier mobility, and strong light absorption. Among TMDCs, two-dimensional molybdenum disulfide (2D-MoS2) has been the most investigated for electronic and optoelectronic applications, like transistors and photodetectors. 2D-MoS2 can particularly benefit from the excellent light matter interaction properties in the UV-VIS spectrum combined with good charge carrier transport properties. The literature reports photodetectors based on 2D-MoS2 fabricated with different techniques, including exfoliation, chemical vapor deposition (CVD) and wet chemical synthesis. However, it is still challenging to scale the proposed devices to the industrial level, due to the lack of a versatile fabrication process that ensures both reproducibility and scalability. A possible solution to this could rise from wet chemical synthesis. In the first part of this work, I discuss the development and optimization of a fabrication method for MoS2 thin films based on a sol-gel process which allows for scalable productions. This route allowed the fabrication of large area (~cm2) MoS2 thin films of 200 nm thickness on technological relevant substrates (i.e., glass, gold, silicon). The films displayed good uniformity, although the crystallinity was affected by residual impurities. The films produced with this technique were employed for the fabrication of photodetectors, displaying responsivity of few mA/W in the NUV-VIS-NIR spectrum. However, the performance of the device was affected by a still limited quality of the MoS2 films obtained with the current method that require further optimization. Further studies will overcome the current limitations and solutions to be investigated in future works are proposed. The second part of this work focuses on expanding the detection capability of 2D-MoS2 (currently limited to the UV-VIS-NIR spectrum), by exploring for the first time X-rays sensing, taking advantage of the X-ray cross section of MoS2 associated with the high atomic number Z of Mo. A detector based on an exfoliated MoS2 monolayer (1L-MoS2) was fabricated and characterized for the purpose. The detector showed direct detection of ~10^2 keV X-rays down to dose rates of 0.08 mGy/s, with X-ray sensitivity is in the range 10^8-10^9 µC ⋅Gy-1·cm-3, outperforming most of the reported organic and inorganic materials. A strategy to improve the device response was also studied by adding a scintillator film, which resulted in a three-fold increase of the signal. These results suggest to consider 2D-MoS2 for in-vivo dosimetry applications.

<p dir="ltr">In response to the ongoing global energy crisis, significant efforts have been made to enhance the efficiency of energy conversion devices that utilize renewable resources. This has spurred the development of multi-component semiconductors, which combine the strengths of both organic and inorganic materials to offset their individual limitations. This dissertation investigates advanced band engineering strategies to manipulate photophysical phenomena in these hybrid systems for specific applications. By focusing on two-dimensional perovskites and heterostructures formed from transition metal dichalcogenides and organic molecules, we explore how the inorganic components can sensitize their organic counterparts, examining the charge and energy transfer processes occurring at their interfaces and giving rise to unique excited states with diverse optical properties which hold significant potential for energy harvesting technologies.</p><p dir="ltr"><b>CHAPTER 1</b> furnishes readers with extensive insights into the semiconducting materials discussed in this dissertation, along with essential knowledge of fundamental concepts (such as excitons, charge and energy transfer, singlet fission, Marcus Theory, etc.) crucial for a deeper understanding of subsequent chapters. It also highlights the unresolved questions addressed later in this dissertation.</p><p dir="ltr"><b>CHAPTER 2 </b>provides a comprehensive overview of the spectroscopic and other characterization techniques used to study these materials.</p><p dir="ltr"><b>CHAPTER 3 </b>illustrates how the relative band alignment and coupling between the organic and inorganic layers of 2D perovskites impact the rates and dynamics of the transfer of triplet excitons across the hybrid interface. It also demonstrates how one can utilize extremely fast triplet transfer times to induce rapid photon upconversion in the perovskite, facilitated by doping of the organic layer. Triplet energy transfer driven photon upconversion is a promising method for enhancing the efficiencies of solar cells; therefore, this chapter makes a stride towards contributing newer insights for optimizing solar energy conversion.</p><p dir="ltr"><b>CHAPTER 4</b> focuses on how tuning the dimensionalities of 2D perovskites can modify their energy landscapes and further impact the interfacial photophysics, leading to the creation of long lived and mobile ‘interfacial’ excitons with enhanced electro-optic properties, promising for potential applications in solar cells and quantum computing.</p><p dir="ltr">In <b>Chapter 5</b>, the focus shifts to organic molecules that can undergo singlet fission, which are sensitized by highly absorbing monolayer transition metal dichalcogenides (TMDCs). This chapter explores how to induce intense emission from triplet pairs of the organic molecules — the critical yet elusive intermediate species in singlet fission, by engineering direct energy transfer into them from the TMDCs. Singlet fission-based technologies hold the potential to significantly enhance solar cell efficiencies, driving extensive research into optimizing the behavior of multi-excitonic triplet pairs. These triplet pairs offer the exciting possibility of multiphoton emission and/or the donation of multiple electrons (or excitons) in a single step. Our work aims to advance the understanding of these prospects and contribute to their practical application.</p><p dir="ltr"><b>CHAPTER 6 </b>summarizes the key findings from the previous chapters and explores potential future research directions. This dissertation, as a whole, contributes to and paves the way for further investigation into optimizing band engineering-based functionalities in 2D organic-inorganic semiconductors. These efforts aim to advance photophysical applications focused on improving energy conversion for a cleaner, more sustainable future.</p>

<p dir="ltr">Photovoltaics are leading devices in green energy production. Understanding the fundamental physics behind energy transport in candidate materials for future photovoltaic and optoelectronic devices is necessary to both realize material limitations and improve efficiency. Excitons, which are bound electron-hole pairs, are central to determining how energy propagates throughout semiconductors. Exciton transport is greatly influenced by material dimensionality. In highly ordered quantum dot (QD) systems, electronic coupling between individual QDs can lead to coherent exciton transport, whereas in two-dimensional heterostructures, excitons can form at the interface of a heterojunction, creating charge-transfer excitons.</p><p dir="ltr">This dissertation is dedicated to summarizing the studies of exciton transport and behavior in two systems: perovskite QD superlattices and transition metal dichalcogenide (TMDC)/polyacene heterostructures. Chapter 1 provides readers with details on these materials in addition to information on the fundamental concepts (i.e., excitons, phonons, energy transfer) needed to best appreciate further chapters. Chapter 2 summarizes the spectroscopic techniques (photoluminescence and transient absorption spectroscopy and microscopy) used to examine exciton behavior. Next, the effects of disorder and dephasing pathways on the ability of perovskite QDs to coherently couple is investigated through the lens of superradiance in Chapter 3. After this, the temperature-dependent exciton transport within perovskite QD superlattices is imaged with high spatial and temporal resolutions in Chapter 4. The experimental transport data on these superlattices provides evidence for environment-assisted quantum transport, which, until this study, had yet to be realized in solid-state systems. In Chapter 5, attention is switched to verifying the existence and deepening the understanding of the behavior of several spatially separated interlayer excitons in a tungsten disulfide/tetracene heterostructure. Finally, Chapter 6 summarizes the preliminary results obtained through transient absorption spectroscopy on other TMDC/polyacene heterostructures where separation of the triplet pair state is attempted. </p><p dir="ltr">It is this author’s hope that this dissertation will not only summarize their graduate work but will also serve as inspiration for others to continue learning and contribute to the advancement of the energy research field.</p>

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.

Zweidimensionale (2D) Übergangsmetalldichalcogenide (TMDCs) gehören zu den attraktivsten neuen Materialien für optoelektronische Bauelemente der nächsten Generation. Um die überlegene Funktionalität der mit TMDCs verbundenen Bauelemente zu realisieren, ist ein umfassendes Verständnis ihrer elektronischen Struktur, einschließlich, aber nicht beschränkt auf die Auswirkungen von Defekten auf die elektronischen Eigenschaften und die Ausrichtung der Energieniveaus (ELA) an den TMDCs-Grenzflächen, unerlässlich, aber derzeit nicht ausreichend. Um einen tieferen Einblick in die elektronischen Eigenschaften von TMDCs und den damit verbundenen Grenzflächen in Kombination mit molekularen Halbleitern (MSCs) zu erhalten, untersuchen wir i) die fundamentale Bandstruktur von Monolagen (ML) TMDCs und die durch Schwefelfehlstellen (SVs) induzierte Renormierung der Bandstruktur, um eine solide Grundlage für ein besseres Verständnis der elektronischen Eigenschaften von polykristallinen dünnen Filmen zu schaffen, und ii) die optoelektronischen Eigenschaften ausgewählter MSC/ML-TMDCs-Grenzflächen. Darüber hinaus wird iii) der Einfluss des Substrats auf die elektronischen Eigenschaften einer MSC/ML-TMDC-Grenzfläche untersucht, um das Bauelementedesign zu steuern. Die Charakterisierung erfolgt hauptsächlich durch winkelaufgelöste Photoelektronenspektroskopie (ARPES), ergänzt durch Photolumineszenz (PL), Raman-Spektroskopie, UV-Vis-Absorption, Rastertransmissionselektronenmikroskopie (TEM) und Rasterkraftmikroskopie (AFM). Unsere Ergebnisse tragen zu einem besseren Verständnis der Auswirkungen von Defekten auf ML-TMDC und verwandte Grenzflächen mit MSCs bei, wobei auch die Auswirkungen der Substrate berücksichtigt werden, und sollten dazu beitragen, unser Verständnis des elektronischen Verhaltens in TMDC-verwandten Geräten zu verbessern. / Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are amongst the most attractive emerging materials for next-generation optoelectronic devices. To realize the superior functionality of the TMDCs related devices, a comprehensive understanding of their electronic structure, including but not limited to the impact of defects on the electronic properties and energy level alignment (ELA) at TMDCs interfaces, is essential but presently not sufficient. In an attempt to get a deep insight into the electronic properties of TMDCs and the related interfaces combined with molecular semiconductors (MSCs), we investigate i) the fundamental band structure of monolayer (ML) TMDCs and band structure renormalization induced by sulfur vacancies (SVs), in order to provide a solid foundation for a better understanding the electronic properties of polycrystalline thin films and ii) the optoelectronic properties of selected MSC/ML-TMDC interface. In addition, iii) the impact of the substrate on the electronic properties of the MSC/ML-TMDC interface is investigated for guiding device design. The characterization is mainly performed by using angle-resolved photoelectron spectroscopy (ARPES), with complementary techniques including photoluminescence (PL), Raman spectroscopies, UV-vis absorption, scanning transmission electron microscopy (TEM), and atomic force microscopy (AFM) measurements. Our findings contribute to achieving a better understanding of the impact of defects on ML-TMDC and related interfaces with MSCs considering the substrates’ effect and should help refine our understanding of the electronic behavior in TMDC-related devices.