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Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature (T = 5 K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), i.e. MoS2, MoSe2, WS2, and WSe2, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, i.e. in-plane E ' and out-of-plane A(1)', but the appearance of 1st, 2nd, and higher-order phonon modes is recognized. The intensity profiles of the A(1)' modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe2 ML, the A(1)' mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, i.e. bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature (T = 5 K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), i.e. MoS2, MoSe2, WS2, and WSe2, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, i.e. in-plane E ' and out-of-plane A(1)', but the appearance of 1st, 2nd, and higher-order phonon modes is recognized. The intensity profiles of the A(1)' modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe2 ML, the A(1)' mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, i.e. bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

Les hétérostructures de van der Waals en rotation relative font l'objet de nombreux travaux de recherche depuis l'observation de supraconductivité dans les bicouches de graphène moirées près des angles dits "magiques". Dans cette thèse, la supraconductivité sera étudiée dans deux types différents de bicouches pivotées. Le premier système étudié est une bicouche pivotée de WSe${}_2$, un dichalcogénure de métal de transition. Cette bicouche possède des bandes d'énergie peu dispersives pour un continuum d'angle de rotation. Nous montrons que, dans un modèle effectif décrivant ce système, une phase supraconductrice de type $d\pm id$ existe de part et d'autre du demi-remplissage. Ce résultat est en accord avec les évidences expérimentales. Nous montrons aussi que la supraconductivité dans ce système peut être contrôlée par l'angle de rotation et par l'application d'un champ électrique perpendiculaire, permettant un contrôle {\it in situ} de la phase. Le deuxième système étudié est la bicouche de cuprates en rotation relative proche de $45^\circ$. Il a été prédit par une approche de Ginzburg-Landau que l'angle de rotation permet de modifier la phase supraconductrice déjà présente dans les monocouches de cuprates vers une phase supraconductrice topologique qui brise la symétrie d'inversion du temps. Nous représentons chaque couche de ce système par le modèle de Hubbard à une bande et les sauts intercouches sont décrites par une forme exponentielle simple. Ce modèle permet de montrer que la phase supraconductrice qui brise la symétrie d'inversion du temps apparait seulement dans une région étroite en dopage. Cette phase est donc plus fragile que ce qui était initialement envisagé. La dernière partie de cette thèse aborde une version simplifiée du modèle de Hubbard sur réseau triangulaire avec sauts complexes utilisé pour décrire les bicouches de WSe${}_2$. Les résultats présentés dans cette partie permettent une meilleure compréhension des résultats obtenus pour le WSe${}_2

Recent research on two-dimensional (2D) transition metal dichalcogenides (TMDCs) has led to remarkable discoveries of fundamental phenomena and to device applications with technological potential. Large-scale TMDCs grown by chemical vapor deposition (CVD) are now available at continuously improving quality, but native defects and natural degradation in these materials still present significant challenges. Spectral hysteresis in gate-biased photoluminescence (PL) measurements of WSe2 further revealed long-term trapping issues of charge carriers in intrinsic defect states. To address these issues, we apply here a two-step treatment with organic molecules, demonstrating the ``healing'' of native defects in CVD-grown WSe2 and WS2 by substituting atomic sulfur into chalcogen vacancies. We uncover that the adsorption of thiols provides only partial defect passivation, even for high adsorption quality, and that thiol adsorption is fundamentally limited in eliminating charge traps. However, as soon as the molecular backbone is trimmed and atomic sulfur is released to the crystal, both bonds of the sulfur are recruited to passivate the divalent defect and the semiconductor quality improves drastically. Time-dependent X-ray photoelectron spectroscopy (XPS) is applied here together with other methods for the characterization of defects, their healing, leading energies and occupation. First-principles calculations support a unified picture of the electronic passivation of sulfur-healed WSe2 and WS2. This work provides a simple and efficient method for improving the quality of 2D semiconductors and has the potential to impact device performance even after natural degradation.

Bilayer graphene (BLG) was recently shown to host a band-inverted phase with unconventional topology emerging from the Ising-type spin-orbit interaction (SOI) induced by the proximity of transition metal dichalcogenides with large intrinsic SOI. Here, we report the stabilization of this band-inverted phase in BLG symmetrically encapsulated in tungsten diselenide (WSe2) via hydrostatic pressure. Our observations from low temperature transport measurements are consistent with a single particle model with induced Ising SOI of opposite sign on the two graphene layers. To confirm the strengthening of the inverted phase, we present thermal activation measurements and show that the SOI-induced band gap increases by more than 100% due to the applied pressure. Finally, the investigation of Landau level spectra reveals the dependence of the level-crossings on the applied magnetic field, which further confirms the enhancement of SOI with pressure., BUS/TNO STAFF, QRD/Goswami Lab

Non-destructive and reversible modulations of polarity and carrier concentration in transistors are essential for complementary devices. The fabricated multi-gated WSe2 devices obtain dynamic electrostatic field induced electrically configurable functions and demonstrate as diode with high rectification ratio of 4.1 × 105, as well as n- and p-type inverter with voltage gain of 19.9 and 12.1, respectively. Benefiting from the continuous band alignment induced modulation of channel underneath the dual gates, the devices exhibit high-performance photodetection in wide spectral range. The devices yield high photo-responsivity (5.16 A W−1) and large Ilight/Idark ratio (1 × 105). Besides, the local gate fields accelerate the separation of photo-induced carriers, leading to fast response without persistent current. This strategy takes the advantage of the simplified design and continues to deliver integrated circuits with high density. The superior electrical and photodetection characteristics exhibit great potency in the domain of future optoelectronics.

[EN] Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wideranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes a benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

[EN] Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wideranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes a benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

[EN] Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wideranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes a benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

Během experimentu této práce byly připraveny polykrystalické vzorky SnSe2 dopovaného MoSe2 a WSe2 v nominálních složeních (SnSe2)1-x(MeSe2)x, kde Me = Mo, W v koncentračním rozsahu x = 0 - 0,2. Syntéza vzorků probíhala v odporové peci. Vzniklé ingoty byly rozdrceny. Pomocí PXRD analýzy byla ověřena fázová čistota vzorků., During the experiment of this work, polycrystalline SnSe2 samples doped with MoSe2 and WSe2 were prepared in the nominal compositions (SnSe2)1-x(MeSe2)x, where Me = Mo, W in the concentration range x = 0 - 0.2. The synthesis of the samples was carried out in a resistance furnace. The resulting ingots were crushed. The phase purity of the samples was verified by PXRD analysis., Fakulta chemicko-technologická, Diplomantka seznámila komisi se svojí diplomovou prací. Dále reagovala na připomínky oponenta. Zodpověděla otázky členů komise: Uvažovala jste vznik WSe3 a kontrolovala jste vznik WSe2 pomocí rentgenu? Jsou známy oxidační stavy Mo a W v připravených materiálech? Diskutujte kompoziční závislost měrné elektrické vodivosti. Jsou známé fázové diagramy (metalurgické) od sloučenin v připravených materiálech? Upřesněte šířku zakázaného pásu u studovaných vzorků? Diskutujte mřížkové parametry studovaných materiálů., Dokončená práce s úspěšnou obhajobou

2D materials attracted considerable interest in the research community following the first report in 2004 on the preparation of graphene by simple micromechanical exfoliation of highly oriented pyrolytic graphite. For example, single-layer Molybdenum Di selenide (MoSe2) and Tungsten Di selenide (WSe2) transistors with on/off ratios of 108 and ultra-low standby power dissipation were demonstrated. Graphene electrodes have been used to develop fully transparent resistive memories to suppress unwanted surface effects that occur in oxide memory devices. Photodetectors based on MoSe2 with few layers have shown excellent photodetection properties. Phototransistors based on WSe2 monolayers exhibit photosensitivity up to 2200 AW, demonstrating the emerging applications of 2D materials for highefficiency optoelectronic devices. In particular, the 2D monolayers of semiconducting transition metal dichalcogenides (TMDs) exhibit direct bandgaps and have intriguing optical properties suitable for optoelectronic applications in light-emitting diodes and photovoltaics. To realize highly efficient optoelectronic devices based on TMD monolayers, it is also essential to develop a strategy for tuning the bandgaps of TMD monolayers. A significant drawback of graphene is that it does not have a bandgap and is therefore not considered the best material for light-emitting devices. This drawback of graphene has severely limited its application in the electronics industry, where semiconductor materials are widely used. In contrast, single-layer TMDs such as WSe2 and WSe2 are direct bandgap semiconductors and show good light emission properties. In this work, density functional theory is used to perform systematic studies of layered MoSe2 and WSe2. Theoretically, we transition metals Rh and Ru to perform planned studies of layered MoSe2 and WSe2 via substitutional doping and vacanc

Semiconducting transition metal dichalcogenides (TMDs) such as MoS2, WSe2 or WS2 have attracted a lot of interest in recent years because of their outstanding optoelectronic properties including tunable bandgap or strong light absorption in the visible range, even at sub-micrometer thickness. Furthermore, doping these materials with transition metal atoms is an approach which can additionally modify a TMDs’ optoelectronic properties and therefore could enable these TMDs to take part in new applications. Indeed, one method to exfoliate TMD materials is the electrochemical intercalation of TMDs with THA+ molecules. Recently this method was altered to make it more scalable, using annealed pellets made from commercially available powder. This method is readily adaptable such that bulk TMDs are doped in the annealing phase before being exfoliated into doped 2D TMD nanoflakes. In this work we present an extensive study regarding p-type doping of MoS2 with Niobium (Nb) for different atomic concentrations. We confirm the incorporation of the dopant agent to the TMD crystal lattice and observe consequential changes in the optoelectronic properties of the resulting nanoflakes. Besides, from 0.75% Nb doping the intrinsic n-type MoS2 displays ambipolar due to the p-type behavior added by Nb atoms. Consequently, NbMoS2 shows promise as a building block of ultra-thin ambipolar field effect transistors (FETs). We follow up with the incorporation of n-type dopant agent Rhenium (Re) as preliminary story. Here, Re is also confirmed to enter at MoS2 crystal structure. However, as it stands 0.5% and 1% Re doping to MoS2 are not beneficial for PEC performance of this material. Finally, we show that this method can also be used for other TMDs such as p-type WSe2 and n-type WS2, showcasing the universality of this method. Here, dopant agent incorporation to the TMDs crystal lattices is also confirmed and therefore optoelectronic properties of WSe2 and WS2 are also demonstrated to change b, Outgoing

In the 1H monolayer transition metal dichalcogenide, the inversion symmetry is broken, while the reflection symmetry is maintained. On the other hand, in the bilayer, the inversion symmetry is restored, but the reflection symmetry is broken. As a consequence of these contrasting symmetries, here we show that bilayer WS2 exhibits a quantum confined Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrast to a quadratic one for a monolayer. The interplay between the unique layer degree of freedom in the bilayer and the field driven partial interconversion between intralayer and interlayer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS2 a promising candidate for an atomically thin, tunable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultrafast nonradiative relaxation channel. By tweaking the biasing configuration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. The findings are prospective toward highly tunable, atomically thin, compact, and light, on chip, reconfigurable components for next generation optoelectronics.

In the 1H monolayer transition metal dichalcogenide, the inversion symmetry is broken, while the reflection symmetry is maintained. On the other hand, in the bilayer, the inversion symmetry is restored, but the reflection symmetry is broken. As a consequence of these contrasting symmetries, here we show that bilayer WS2 exhibits a quantum confined Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrast to a quadratic one for a monolayer. The interplay between the unique layer degree of freedom in the bilayer and the field driven partial interconversion between intralayer and interlayer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS2 a promising candidate for an atomically thin, tunable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultrafast nonradiative relaxation channel. By tweaking the biasing configuration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. The findings are prospective toward highly tunable, atomically thin, compact, and light, on chip, reconfigurable components for next generation optoelectronics.

Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct experimental determination of the exciton binding energy (Eb,exc) of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combining angle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap (Eg), and by reflectance measurements the optical excitonic band gap (Eexc). The difference of these two energies is Eb,exc. The values of Eg and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, respectively. The significant Eb,exc reduction is primarily due to a reduction of Eg resulting from enhanced screening by the metal, while Eexc is barely decreased for the metal support. Energy level diagrams determined at the K-point of the 2D TMDCs Brillouin zone show that MoS2 has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron–hole pair, i.e. the exciton. A proper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitate superior optoelectronic device function., Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659

Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct experimental determination of the exciton binding energy (Eb,exc) of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combining angle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap (Eg), and by reflectance measurements the optical excitonic band gap (Eexc). The difference of these two energies is Eb,exc. The values of Eg and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, respectively. The significant Eb,exc reduction is primarily due to a reduction of Eg resulting from enhanced screening by the metal, while Eexc is barely decreased for the metal support. Energy level diagrams determined at the K-point of the 2D TMDCs Brillouin zone show that MoS2 has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron–hole pair, i.e. the exciton. A proper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitate superior optoelectronic device function., Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659

The acoustoelectric effect in single-layer molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) is studied in a hybrid setup. Such effects, which rely on the transfer of momentum from surface acoustic waves (SAWs), are generated on the surface of lithium niobate (LiNbO3) to the carriers in MoS2 and WSe2, resulting in an attenuation and velocity shift of the wave and giving rise to an acoustoelectric current. This dissertation examines the feasibility of integrating high-quality, single-layer MoS2 and WSe2 onto LiNbO3 to ultimately fabricate and characterize a hybrid chip that combines the functionality of a field-effect transistor (FET) and SAW device. MoS2 and WSe2 were synthesized by chemical vapor deposition (CVD) directly onto a chemically-reduced LiNbO3 substrate. LiNbO3 is a ferroelectric material that offers a unique blend of piezoelectric and birefringent properties, yet it lacks both optical activity and semiconductor transport. The prototypical device exhibits electrical characteristics that are competitive with MoS2 and WSe2 devices on silicon. These results demonstrate both a sound-driven battery and an acoustic photodetector, and ultimately open directions to non-invasive investigation of electrical properties of single-layer films. The experiments reveal close agreement between transport measurements utilizing conventional contacts and SAW spectroscopy. This approach will set forth the possibility of contact-free transport characterization of two-dimensional (2D) transition metal dichalcogenides (TMD) films, avoiding such concerns as the role of charge transfer at contacts as an artifact of such measurements.

Graphene stands out for its high mobility and weak spin-orbit coupling (SOC) offering efficient transport of both electron charges and spins. However, the negligible SOC prevents novel quantum states from emerging, such as the quantum anomalous Hall state. On the other hand, owing to their strong SOC, transition metal dichalcogenides (TMDs) provide an ideal platform to increase the SOC in graphene by proximity effect. This dissertation will focus on the analysis of the induced SOC in graphene, the underlying mechanisms, and the factors that could affect its strength.The first part briefly introduces graphene and SOC. We present possible ways to increase the SOC in graphene and how to quantify its strength. The second part shows the procedures on how to build a van der Waals heterostructure like graphene/WSe2/h-BN, followed by introductions to Raman and photoluminescence (PL).In the third part we demonstrate enhanced SOC in monolayer and bilayer graphene on WS2 by magneto-conductance measurements. We will show clear weak antilocalization (WAL) in WS2-covered graphene over a wide range of carrier densities. We isolate and quantify the spin-relaxation rate caused by Rashba SOC and show its strength is tunable via transverse electric fields.Then we investigate the SOC in graphene coupled to monolayer TMD films. We show that the spin relaxation rate varies linearly with the momentum scattering time and is independent of the carrier type. Our analysis yields a Rashba SOC of ~1.5 meV in graphene/WSe2 and ~0.9 meV in graphene/MoS2. The nearly electron-hole symmetric nature of the Rashba SOC calls deeper understanding for the underlying mechanisms.Finally, we study the interactions between TMDs and graphene in graphene/WSe2/h-BN and WSe2/ graphene/h-BN. We find that strong PL quenching exists in the former stack while the PL is only weakly quenched in the latter stack. We attribute this difference to the increased interlayer distance between WSe2 and graphene caused by the h

The acoustoelectric effect in single-layer molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) is studied in a hybrid setup. Such effects, which rely on the transfer of momentum from surface acoustic waves (SAWs), are generated on the surface of lithium niobate (LiNbO3) to the carriers in MoS2 and WSe2, resulting in an attenuation and velocity shift of the wave and giving rise to an acoustoelectric current. This dissertation examines the feasibility of integrating high-quality, single-layer MoS2 and WSe2 onto LiNbO3 to ultimately fabricate and characterize a hybrid chip that combines the functionality of a field-effect transistor (FET) and SAW device. MoS2 and WSe2 were synthesized by chemical vapor deposition (CVD) directly onto a chemically-reduced LiNbO3 substrate. LiNbO3 is a ferroelectric material that offers a unique blend of piezoelectric and birefringent properties, yet it lacks both optical activity and semiconductor transport. The prototypical device exhibits electrical characteristics that are competitive with MoS2 and WSe2 devices on silicon. These results demonstrate both a sound-driven battery and an acoustic photodetector, and ultimately open directions to non-invasive investigation of electrical properties of single-layer films. The experiments reveal close agreement between transport measurements utilizing conventional contacts and SAW spectroscopy. This approach will set forth the possibility of contact-free transport characterization of two-dimensional (2D) transition metal dichalcogenides (TMD) films, avoiding such concerns as the role of charge transfer at contacts as an artifact of such measurements.

Graphene stands out for its high mobility and weak spin-orbit coupling (SOC) offering efficient transport of both electron charges and spins. However, the negligible SOC prevents novel quantum states from emerging, such as the quantum anomalous Hall state. On the other hand, owing to their strong SOC, transition metal dichalcogenides (TMDs) provide an ideal platform to increase the SOC in graphene by proximity effect. This dissertation will focus on the analysis of the induced SOC in graphene, the underlying mechanisms, and the factors that could affect its strength.The first part briefly introduces graphene and SOC. We present possible ways to increase the SOC in graphene and how to quantify its strength. The second part shows the procedures on how to build a van der Waals heterostructure like graphene/WSe2/h-BN, followed by introductions to Raman and photoluminescence (PL).In the third part we demonstrate enhanced SOC in monolayer and bilayer graphene on WS2 by magneto-conductance measurements. We will show clear weak antilocalization (WAL) in WS2-covered graphene over a wide range of carrier densities. We isolate and quantify the spin-relaxation rate caused by Rashba SOC and show its strength is tunable via transverse electric fields.Then we investigate the SOC in graphene coupled to monolayer TMD films. We show that the spin relaxation rate varies linearly with the momentum scattering time and is independent of the carrier type. Our analysis yields a Rashba SOC of ~1.5 meV in graphene/WSe2 and ~0.9 meV in graphene/MoS2. The nearly electron-hole symmetric nature of the Rashba SOC calls deeper understanding for the underlying mechanisms.Finally, we study the interactions between TMDs and graphene in graphene/WSe2/h-BN and WSe2/ graphene/h-BN. We find that strong PL quenching exists in the former stack while the PL is only weakly quenched in the latter stack. We attribute this difference to the increased interlayer distance between WSe2 and graphene caused by the h

Few-layer thick MoSe2 and WSe2 possess non-trivial spin textures with sizable spin splitting due to the inversion symmetry breaking embedded in the crystal structure and strong spin-orbit coupling. We report a spin-resolved photoemission study of MoSe2 and WSe2 thin film samples epitaxially grown on a bilayer graphene substrate. We only found spin polarization in the single- and trilayer samples-not in the bilayer sample-mostly along the out-of-plane direction of the sample surface. The measured spin polarization is found to be strongly dependent on the light polarization as well as the measurement geometry, which reveals intricate coupling between the spin and orbital degrees of freedom in this class of material.

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

As transistor dimension kept scaling down, many challenges arises such as worse electrostatic control and higher variability. In order to address these issues, thin down body thickness is widely accepted and device structures such as FinFET and SOI are employed. Although FinFET has been adopted as main device structure by major foundries like Intel and TSMC in 20nm node and beyond, its analog performances like gm and fT are still lagging behind the bulk and SOI and prevent it from applying to SOC applications. In order to maintain the scaling trend, new materials and/or novel device design is needed. Therefore, channel engineering by using laterally composed with different electron affinities along the channel is proposed to show improved analog performance.Besides thinning down body thickness, there are other methods to realize thin body and one of them is applying channel engineering by employing deeply retrograde doping profile (DRCP) in bulk device. The doping profile is designed such that it behaves like a thin body device while maintaining bulk device structure. The advantage of DRCP device is that it is relatively cheap and less complicated in terms of manufacturing than those device structures with physically thinner silicon body like FinFET and SOI. It is then instructive to understand whether DRCP device can deliver comparable device performance as FinFET and SOI in 20nm regime. The physics of DRCP device is investigated by TCAD simulation tools and compared with halo device (conventional bulk device) to show the origin of the superior performance. The device performances of bulk device, DRCP, FinFET and SOI are compared to show the capability of DRCP device.The other approach to make thin body is to utilize 2D materials like transition metal di-chalcogenides (TMDs) as channel material of transistor owning to its ultra-thin body property. 2D materials such as MoS2 and WSe2 were extensively exploited recently for FET fabrication due to the good short channe

Integrated circuits consists of building blocks called transistors. A transistor is a switch that enables logic operations to perform computing. Since the invention of the first integrated circuit, transistors have been scaled down in their dimensions to increase the density of transistors per unit area to enable more functionality. Transistor scaling is continued by introducing novel device structures and materials at each technology node. Due to the challenges such as short channel effects and the power consumption issues, novel materials are investigated as a candidate for next generation transistors. In this thesis, 2-dimensinal layered semiconductors, namely transition metal dichalcogenides (TMDCs) are studied to assess their electronic material properties as a candidate channel material for next generation electronic devices.Chapter one, introduces the challenges in the state of the art MOSFET devices. Then the motivation for the use of TMDCs in MOSFETs is explained. In chapter two, doping of the TMDCs is studied to be able to probe the intrinsic electronic properties of the devices fabricated using these materials. Contact resistance can be decreased by doping and TMDC MOSFETs with near-ideal performance metrics are demonstrated. In chapter three the CMOS integration of the devices using TMDCs are examined. Logic operations are conducted by fabricating WSe2 n-FETs and p-FETs on the same flake. Then vertical 3-dimensional integration of n-FETs and p-FETs are demonstrated using the same gate. These transistors are connected as a CMOS inverter and logic operations are performed. Chapter four presents the band structure engineering study using TMDCs. Mono-multilayer MoS2 junctions are found to have a type-I heterojunction. Optoelectronic properties of this junction are investigated and the junction is shown to have a photoresponse that dominates the photoresponse coming from the contacts. In chapter five, the tunneling devices using TMDCs are studied. Dual-gated

As transistor dimension kept scaling down, many challenges arises such as worse electrostatic control and higher variability. In order to address these issues, thin down body thickness is widely accepted and device structures such as FinFET and SOI are employed. Although FinFET has been adopted as main device structure by major foundries like Intel and TSMC in 20nm node and beyond, its analog performances like gm and fT are still lagging behind the bulk and SOI and prevent it from applying to SOC applications. In order to maintain the scaling trend, new materials and/or novel device design is needed. Therefore, channel engineering by using laterally composed with different electron affinities along the channel is proposed to show improved analog performance.Besides thinning down body thickness, there are other methods to realize thin body and one of them is applying channel engineering by employing deeply retrograde doping profile (DRCP) in bulk device. The doping profile is designed such that it behaves like a thin body device while maintaining bulk device structure. The advantage of DRCP device is that it is relatively cheap and less complicated in terms of manufacturing than those device structures with physically thinner silicon body like FinFET and SOI. It is then instructive to understand whether DRCP device can deliver comparable device performance as FinFET and SOI in 20nm regime. The physics of DRCP device is investigated by TCAD simulation tools and compared with halo device (conventional bulk device) to show the origin of the superior performance. The device performances of bulk device, DRCP, FinFET and SOI are compared to show the capability of DRCP device.The other approach to make thin body is to utilize 2D materials like transition metal di-chalcogenides (TMDs) as channel material of transistor owning to its ultra-thin body property. 2D materials such as MoS2 and WSe2 were extensively exploited recently for FET fabrication due to the good short channe

Integrated circuits consists of building blocks called transistors. A transistor is a switch that enables logic operations to perform computing. Since the invention of the first integrated circuit, transistors have been scaled down in their dimensions to increase the density of transistors per unit area to enable more functionality. Transistor scaling is continued by introducing novel device structures and materials at each technology node. Due to the challenges such as short channel effects and the power consumption issues, novel materials are investigated as a candidate for next generation transistors. In this thesis, 2-dimensinal layered semiconductors, namely transition metal dichalcogenides (TMDCs) are studied to assess their electronic material properties as a candidate channel material for next generation electronic devices.Chapter one, introduces the challenges in the state of the art MOSFET devices. Then the motivation for the use of TMDCs in MOSFETs is explained. In chapter two, doping of the TMDCs is studied to be able to probe the intrinsic electronic properties of the devices fabricated using these materials. Contact resistance can be decreased by doping and TMDC MOSFETs with near-ideal performance metrics are demonstrated. In chapter three the CMOS integration of the devices using TMDCs are examined. Logic operations are conducted by fabricating WSe2 n-FETs and p-FETs on the same flake. Then vertical 3-dimensional integration of n-FETs and p-FETs are demonstrated using the same gate. These transistors are connected as a CMOS inverter and logic operations are performed. Chapter four presents the band structure engineering study using TMDCs. Mono-multilayer MoS2 junctions are found to have a type-I heterojunction. Optoelectronic properties of this junction are investigated and the junction is shown to have a photoresponse that dominates the photoresponse coming from the contacts. In chapter five, the tunneling devices using TMDCs are studied. Dual-gated

Few-layer thick MoSe2 and WSe2 possess non-trivial spin textures with sizable spin splitting due to the inversion symmetry breaking embedded in the crystal structure and strong spin-orbit coupling. We report a spin-resolved photoemission study of MoSe2 and WSe2 thin film samples epitaxially grown on a bilayer graphene substrate. We only found spin polarization in the single- and trilayer samples-not in the bilayer sample-mostly along the out-of-plane direction of the sample surface. The measured spin polarization is found to be strongly dependent on the light polarization as well as the measurement geometry, which reveals intricate coupling between the spin and orbital degrees of freedom in this class of material.

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

The p-n diodes represent the most fundamental device building blocks for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the challenges in selectively doping them into p- or n-type semiconductors. Here, we demonstrate that an atomically thin and sharp heterojunction p-n diode can be created by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurements of the vertically staked WSe2/MoS2 heterojunctions reveal excellent current rectification behavior with an ideality factor of 1.2. Photocurrent mapping shows rapid photoresponse over the entire overlapping region with a highest external quantum efficiency up to 12%. Electroluminescence studies show prominent band edge excitonic emission and strikingly enhanced hot-electron luminescence. A systematic investigation shows distinct layer-number dependent emission characteristics and reveals important insight about the origin of hot-electron luminescence and the nature of electron-orbital interaction in TMDs. We believe that these atomically thin heterojunction p-n diodes represent an interesting system for probing the fundamental electro-optical properties in TMDs and can open up a new pathway to novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, as well as spin- and valley-polarized light emitting diodes, on-chip lasers.

The p-n diodes represent the most fundamental device building blocks for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the challenges in selectively doping them into p- or n-type semiconductors. Here, we demonstrate that an atomically thin and sharp heterojunction p-n diode can be created by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurements of the vertically staked WSe2/MoS2 heterojunctions reveal excellent current rectification behavior with an ideality factor of 1.2. Photocurrent mapping shows rapid photoresponse over the entire overlapping region with a highest external quantum efficiency up to 12%. Electroluminescence studies show prominent band edge excitonic emission and strikingly enhanced hot-electron luminescence. A systematic investigation shows distinct layer-number dependent emission characteristics and reveals important insight about the origin of hot-electron luminescence and the nature of electron-orbital interaction in TMDs. We believe that these atomically thin heterojunction p-n diodes represent an interesting system for probing the fundamental electro-optical properties in TMDs and can open up a new pathway to novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, as well as spin- and valley-polarized light emitting diodes, on-chip lasers.

Tato diplomová práce se zabývá in-situ elektronovou mikroskopií a jejím využitím při pozorování chemických reakcí na povrchu nanomateriálů. Jsou zde představeny možnosti, které tato technika nabízí, a je popsáno stávajícího řešení in-situ rastrovacího elektronového mikroskopu použitého k experimentům. V práci jsou představeny Janusovy dvoudimenzionální materiály na bázi dichalkogenidů přechodných kovů se zaměřením na jejich přípravu. Následuje podrobný popis přípravy tenkých vrstev materiálu WSe2 pomocí mechanické exfoliace a experimentů provedených pomocí in-situ mikroskopu. Tyto experimenty, které zahrnovaly zahřívání materiálu WSe2 za přítomnosti sulfanu (H2S) a za současného pozorování elektronovým mikroskopem, měly za cíl výměnu chalkogenidů v tomto materiálu. Následovala analýza morfologie, složení a optických vlastností pomocí Ramanovy spektroskopie a fotoelektronové spektroskopie. Na závěr byl představen experiment, jehož výsledkem byla úspěšná výměna síry za selen v WSe2., This diploma thesis focuses on in-situ electron microscopy and its application to monitoring the chemical reactions on the surfaces of nanomaterials. The possibilities offered by this technique are presented, and the existing solution of an in-situ scanning electron microscope used for experiments is described. The thesis introduces Janus two-dimensional materials based on transition metal dichalcogenides with a focus on their preparation. A detailed description follows regarding the preparation of thin layers of WSe2 material using mechanical exfoliation and experiments conducted using an in-situ microscope. These experiments involved annealing of WSe2 material in the presence of hydrogen sulfide (H2S) while simultaneously observing it with an electron microscope, aimed to facilitate the chalcogenide exchange in this material. The morphology, composition, and optical properties of the material were subsequently analyzed using Raman spectroscopy and photoelectron spectroscopy. Finally, an experiment resulting in the successful exchange of selenium in WSe2 for sulphur is presented.

Tato práce se zabývá využitím korelativní sondové a elektronové mikroskopie. Měření bylo provedeno pomocí atomárního silového mikroskopu LiteScope navrženého speciálně ke kombinaci s elektronovými mikroskopy. Výhody korelativního AFM/SEM zobrazení jsou demonstrovány na vybraných vzorcích z oblasti nanotechnologií a materiálových věd. Možné aplikační využití korelativního AFM/SEM zobrazení bylo navrženo a následně realizováno zejména v případě nízkodimenzionálních struktur a tenkých vrstev. Dále se tato práce věnuje možnosti kombinace korelativní AFM/SEM mikroskopie s dalšími integrovanými metodami elektronového mikroskopu jako technikou fokusovaného iontového svazku a rentgenovou spektroskopií., This thesis is dealing with application of Correlative Probe and Electron Microscopy. All measurements were carried out by atomic force microscope LiteScope which is designed especially to be combined with electron microscopes. Advantages of Correlative AFM/SEM Microscopy are demonstrated on selected samples from field of nanotechnology and material science. Application of the correlative imaging was proposed and then realized particularly in case of low-dimensional structures and thin films. Further, this thesis deals with the possibility of combining Correlative AFM/SEM Microscopy with other integrated techniques of an electron microscope such as Focused Ion Beam and Energy Dispersive X-rays Spectroscopy.

Práce se zabývá studiem tenkých vrstev dichalkogenidů přechodných kovů, především sulfidem molybdeničitým. Vytvořeny byly nanostruktury v dvojdimenzionálních krystalech MoS2 a WSe2, jenž byly následně analyzovány. Sledovány byly především jejich fotoluminiscenční vlastnosti. Součástí práce je rešerše dichalkogenidů přechodných kovů a popis způsobu přípravy vrstev upravenou metodou mikromechanické exfoliace., 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.

Tato diplomová práce se zabývá in-situ elektronovou mikroskopií a jejím využitím při pozorování chemických reakcí na povrchu nanomateriálů. Jsou zde představeny možnosti, které tato technika nabízí, a je popsáno stávajícího řešení in-situ rastrovacího elektronového mikroskopu použitého k experimentům. V práci jsou představeny Janusovy dvoudimenzionální materiály na bázi dichalkogenidů přechodných kovů se zaměřením na jejich přípravu. Následuje podrobný popis přípravy tenkých vrstev materiálu WSe2 pomocí mechanické exfoliace a experimentů provedených pomocí in-situ mikroskopu. Tyto experimenty, které zahrnovaly zahřívání materiálu WSe2 za přítomnosti sulfanu (H2S) a za současného pozorování elektronovým mikroskopem, měly za cíl výměnu chalkogenidů v tomto materiálu. Následovala analýza morfologie, složení a optických vlastností pomocí Ramanovy spektroskopie a fotoelektronové spektroskopie. Na závěr byl představen experiment, jehož výsledkem byla úspěšná výměna síry za selen v WSe2., This diploma thesis focuses on in-situ electron microscopy and its application to monitoring the chemical reactions on the surfaces of nanomaterials. The possibilities offered by this technique are presented, and the existing solution of an in-situ scanning electron microscope used for experiments is described. The thesis introduces Janus two-dimensional materials based on transition metal dichalcogenides with a focus on their preparation. A detailed description follows regarding the preparation of thin layers of WSe2 material using mechanical exfoliation and experiments conducted using an in-situ microscope. These experiments involved annealing of WSe2 material in the presence of hydrogen sulfide (H2S) while simultaneously observing it with an electron microscope, aimed to facilitate the chalcogenide exchange in this material. The morphology, composition, and optical properties of the material were subsequently analyzed using Raman spectroscopy and photoelectron spectroscopy. Finally, an experiment resulting in the successful exchange of selenium in WSe2 for sulphur is presented.

Práce se zabývá studiem tenkých vrstev dichalkogenidů přechodných kovů, především sulfidem molybdeničitým. Vytvořeny byly nanostruktury v dvojdimenzionálních krystalech MoS2 a WSe2, jenž byly následně analyzovány. Sledovány byly především jejich fotoluminiscenční vlastnosti. Součástí práce je rešerše dichalkogenidů přechodných kovů a popis způsobu přípravy vrstev upravenou metodou mikromechanické exfoliace., 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.

Tato práce se zabývá využitím korelativní sondové a elektronové mikroskopie. Měření bylo provedeno pomocí atomárního silového mikroskopu LiteScope navrženého speciálně ke kombinaci s elektronovými mikroskopy. Výhody korelativního AFM/SEM zobrazení jsou demonstrovány na vybraných vzorcích z oblasti nanotechnologií a materiálových věd. Možné aplikační využití korelativního AFM/SEM zobrazení bylo navrženo a následně realizováno zejména v případě nízkodimenzionálních struktur a tenkých vrstev. Dále se tato práce věnuje možnosti kombinace korelativní AFM/SEM mikroskopie s dalšími integrovanými metodami elektronového mikroskopu jako technikou fokusovaného iontového svazku a rentgenovou spektroskopií., This thesis is dealing with application of Correlative Probe and Electron Microscopy. All measurements were carried out by atomic force microscope LiteScope which is designed especially to be combined with electron microscopes. Advantages of Correlative AFM/SEM Microscopy are demonstrated on selected samples from field of nanotechnology and material science. Application of the correlative imaging was proposed and then realized particularly in case of low-dimensional structures and thin films. Further, this thesis deals with the possibility of combining Correlative AFM/SEM Microscopy with other integrated techniques of an electron microscope such as Focused Ion Beam and Energy Dispersive X-rays Spectroscopy.

Tato diplomová práce se zabývá in-situ elektronovou mikroskopií a jejím využitím při pozorování chemických reakcí na povrchu nanomateriálů. Jsou zde představeny možnosti, které tato technika nabízí, a je popsáno stávajícího řešení in-situ rastrovacího elektronového mikroskopu použitého k experimentům. V práci jsou představeny Janusovy dvoudimenzionální materiály na bázi dichalkogenidů přechodných kovů se zaměřením na jejich přípravu. Následuje podrobný popis přípravy tenkých vrstev materiálu WSe2 pomocí mechanické exfoliace a experimentů provedených pomocí in-situ mikroskopu. Tyto experimenty, které zahrnovaly zahřívání materiálu WSe2 za přítomnosti sulfanu (H2S) a za současného pozorování elektronovým mikroskopem, měly za cíl výměnu chalkogenidů v tomto materiálu. Následovala analýza morfologie, složení a optických vlastností pomocí Ramanovy spektroskopie a fotoelektronové spektroskopie. Na závěr byl představen experiment, jehož výsledkem byla úspěšná výměna síry za selen v WSe2., This diploma thesis focuses on in-situ electron microscopy and its application to monitoring the chemical reactions on the surfaces of nanomaterials. The possibilities offered by this technique are presented, and the existing solution of an in-situ scanning electron microscope used for experiments is described. The thesis introduces Janus two-dimensional materials based on transition metal dichalcogenides with a focus on their preparation. A detailed description follows regarding the preparation of thin layers of WSe2 material using mechanical exfoliation and experiments conducted using an in-situ microscope. These experiments involved annealing of WSe2 material in the presence of hydrogen sulfide (H2S) while simultaneously observing it with an electron microscope, aimed to facilitate the chalcogenide exchange in this material. The morphology, composition, and optical properties of the material were subsequently analyzed using Raman spectroscopy and photoelectron spectroscopy. Finally, an experiment resulting in the successful exchange of selenium in WSe2 for sulphur is presented.

Tato diplomová práce se zabývá in-situ elektronovou mikroskopií a jejím využitím při pozorování chemických reakcí na povrchu nanomateriálů. Jsou zde představeny možnosti, které tato technika nabízí, a je popsáno stávajícího řešení in-situ rastrovacího elektronového mikroskopu použitého k experimentům. V práci jsou představeny Janusovy dvoudimenzionální materiály na bázi dichalkogenidů přechodných kovů se zaměřením na jejich přípravu. Následuje podrobný popis přípravy tenkých vrstev materiálu WSe2 pomocí mechanické exfoliace a experimentů provedených pomocí in-situ mikroskopu. Tyto experimenty, které zahrnovaly zahřívání materiálu WSe2 za přítomnosti sulfanu (H2S) a za současného pozorování elektronovým mikroskopem, měly za cíl výměnu chalkogenidů v tomto materiálu. Následovala analýza morfologie, složení a optických vlastností pomocí Ramanovy spektroskopie a fotoelektronové spektroskopie. Na závěr byl představen experiment, jehož výsledkem byla úspěšná výměna síry za selen v WSe2., This diploma thesis focuses on in-situ electron microscopy and its application to monitoring the chemical reactions on the surfaces of nanomaterials. The possibilities offered by this technique are presented, and the existing solution of an in-situ scanning electron microscope used for experiments is described. The thesis introduces Janus two-dimensional materials based on transition metal dichalcogenides with a focus on their preparation. A detailed description follows regarding the preparation of thin layers of WSe2 material using mechanical exfoliation and experiments conducted using an in-situ microscope. These experiments involved annealing of WSe2 material in the presence of hydrogen sulfide (H2S) while simultaneously observing it with an electron microscope, aimed to facilitate the chalcogenide exchange in this material. The morphology, composition, and optical properties of the material were subsequently analyzed using Raman spectroscopy and photoelectron spectroscopy. Finally, an experiment resulting in the successful exchange of selenium in WSe2 for sulphur is presented.