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.

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.

Die Art der zwischen den 2D-Materialien und den festen Substraten eingeschlossenen Wasserschichten ist umstritten, sowie auch ihr Einfluss auf die Eigenschaften der 2D-Materialien. In-situ-Rasterkraftmikroskopie (SFM) wurde eingesetzt, um den Benetzungsprozess von Wasser an der Grenzfläche zwischen trockenem graphen- und molybdändisulfid (MoS2)- und Glimmer zu visualisieren. In-situ Raman- und Photolumineszenzmessungen (PL) wurden durchgeführt, um zu untersuchen, wie sich die Ladungsdotierung von Graphen und die Dehnung von Graphen und MoS2 bei der Benetzung verändern. SFM-Ergebnisse zeigen, dass Wassermoleküle, die die trockene Grenzfläche benetzen, bei hoher relativer Luftfeuchtigkeit eine homogene monomolekulare Schicht ausbilden. Aus Raman-Messungen kann man schließen, dass die Wasserschicht vorhandenen Ladungstransfer an der trockenen Grenzfläche blockiert, während eine Schicht aus Ethanolmolekülen dafür nicht ausreicht. Der Austausch von Ethanol gegen Wasser und umgekehrt ermöglicht eine reversible Umschaltung des Ladungstransfers an der Grenzfläche. Dehnungsänderungen von 2D-Materialien auf Glimmer mit eingeschlossenen Flüssigkeitsschichten wird in dieser Arbeit durch Dehnung eines Glimmersubstrats mit darauf exfoliertem 2D-Material untersucht. Die dadurch induzierte Dehnung in Graphen und MoS2 wird durch die Analyse der Veränderungen in den Raman- bzw. PL-Spektren ermittelt. Dabei kann eine Dehnungsrelaxation in Graphen beobachtet werden, die sich von einer „Stick-Slip-Bewegung“ bei trockener Grenzfläche zu viskosem Relaxationsverhalten verändert, wenn Wasser in die Grenzfläche interkaliert. Im Gegensatz dazu findet man in MoS2 unabhängig von der Hydratation keine viskose Relaxation. / The nature of the water layers confined between 2D materials and solid substrates is disputed, also their influences on properties of 2D materials are in debate. I employ In-situ scanning force microscopy (SFM) to visualize wetting of water at the dry graphene-/molybdenum disulfide (MoS2) - mica interface. In-situ Raman and photoluminescence (PL) measurements probe charge-doping and strain change of graphene and MoS2 upon wetting. SFM results show that water molecules wetting the dry interface form a monomolecular layer at high relative humidity (RH). Raman results imply that the water layer blocks charge transfer from mica to graphene, while an ethanol monolayer allows for it. Strain changes of both 2D materials on mica with confined liquid layers are investigated by stretching a mica substrate with the 2D material exfoliated on it. The strain induced in graphene and MoS2 is inferred by analyzing changes in Raman and PL spectra, respectively. Strain relaxation in graphene changes from stick-slip for dry interface to viscous when intercalated by water. In contrast, there is no viscous relaxation in MoS2 regardless of hydration.

La construction d’un NSOM, dans ce manuscrit de thèse, est décrite en détail. Lacombinaison du système NSOM construit avec un interféromètre est proposée afin d’accéderà des mesures de phase, à la fois de ultra-haute sensibilité mais également de très granderésolution spatiale. Le nom de l’instrument développé est un interferomètre optique àbalayage en champ proche (NSOI, pour l’acronyme en anglais). Le principe est basé surl’utilisation d’un diapason accordable en cristal de quartz, sur lequel se trouve une pointe,afin de sonder le matériau étudié. La mesure de la force de cisaillement de la pointe sondeau voisinage de la surface permet d’assurer la régulation et la stabilité de la distance depositionnement de la pointe par rapport à la surface considérée. Le dispositif est construit encombinant différents éléments électroniques pilotés par un logiciel développé en langageLab-VIEW. Le bruit de la mesure en NSOI est supprimé par un calcul simple basé sur lathéorie de l’optique ondulatoire et des interférences associées. Le système permet deréaliser des mesures optiques en champ proche ainsi que la détermination en hauterésolution de la phase du champ optique. L’échantillon SNG01 (l’un des réseaux utilisés pourcaractériser notre microscope à balayage en champ proche), ainsi que des disques optiques(CD, DVD and disques blu-ray) ont été utilisés pour tester la faisabilité et les performancesde notre système.Dans ce manuscrit de thèse, le graphène et les monocouches de MoS2 sont étudiés. Nous montrons qu’une épaisseur à l’échelle atomique peut être résolue par notresystème NSOI, avec l’utilisation de l’algorithme de suppression du bruit de mesure. Lesjoints de grain du graphène sont observés à grande échelle, via la technique d’imagerie parcollection en champ proche et par la réalisation de cartographies de phase. En particulier,les tensions internes à une couche de graphène sont observées, uniquement dans le casd’une imagerie de phase. / In this thesis, near-field scanning optical interferometry (NSOI), which combinesNSOM with interferometer, is proposed for the phase measurement. The shear-forcedetection scheme is applied for distance regulation. The hardware of the systemis constructed by combining various electronic devices, and the operating softwareis coded by LabVIEW. Unwanted background signal is removed by simple calculationbased on interference theory. By using this, the near-field optical measurementand the ultra-sensitive phase investigation of nano-materials are performed. 2D materialssuch as graphene and monolayer MoS2 are investigated. It is shown thatatomic-scale thickness can be resolved by the NSOI. Especially, the grain boundariesof graphene and the seed of MoS2 can be found by phase detection. In addition,direct laser writing (DLW) on silver-containing glass is observed by using NSOM,and NSOI. For the first time, the writing threshold is correlatively observed in thefluorescence imaging and the near-field phase image.