Dissertations / Theses on the topic 'Two-dimensional molybdenum disulfide'

Graphene, an atomically thin material consisting of a hexagonal, highly packed carbon lattice, is of great interests in its magnetic properties. These interests can be categorized in several fields: graphene-based magnetic materials and their applications, large diamagnetism of graphene, and the heterostructures of graphene and other two dimensional materials. In the first aspect, magnetic moments can be in theory introduced to graphene by minimizing its size or introducing structural defects, leading to a very light magnetic material. Furthermore, weak spin-orbital interaction, and long spin relaxation length make graphene promising for spintronics. The first part of this thesis addressed our experimental investigation in defect-induced magnetism of graphene. Non-interacted spins of graphene have been observed by intentionally introducing vacancies and adatoms through ion-irradiation and fluorination, respectively. The defect concentration or the magnetic moments introduced in this thesis cannot provide enough interaction for magnetic coupling. Furthermore, the spins induced by vacancies and adatoms can be controlled through shifting the Fermi energy of graphene using molecular doping, where the adatoms were alternatively introduced by annealing in the inert environment. The paramagnetic responses in graphene induced by vacancy-type defects can only be diverted to half of its maximum, while those induced by sp3 defects can be almost completely suppressed. This difference is supposed that vacancy-type defects induced two localized states (pie and sigma). Only the latter states, which is also the only states induced by sp3 defects, involves in the suppression of magnetic moments at the maximum doping achieved in this thesis. The observation through high resolution transmission electron microscope (HR-TEM) provides more information to the hypothesis of the previous magnetic findings. Reconstructed single vacancy is the majority of defects discovered in proton-irradiated graphene. This result verifies the defect-induced magnetic findings in our results, as well as the electronic properties of defected graphene in the literatures. On the other hand, the diamagnetic susceptibility of neutral graphene is suggested to be larger than that of graphite, and vanish rapidly as a delta-like function when graphene is doped. In our result, surprisingly, the diamagnetic susceptibility varies little when the Fermi level is less than 0.3 eV, in contrast with the theory. When the Fermi energy is higher than 0.3 eV, susceptibility then reduces significantly as the trend of graphite. The little variation in susceptibility near the Dirac point is probably attributed to the spatial confinement of graphene nanoflakes, which are the composition of graphene laminates. In the end of this thesis, we discuss the magnetic properties in one of the other two dimensional materials, molybdenum disulfide (MoS2). It is a potential material for graphene-based heterostructure applications. The magnetic moments in MoS2 are shown to be induced by either edges or vacancies, which are introduced by sonication or proton-irradiation, respectively, similar to the suggestions by theories. However, no significant ferromagnetic finding has been found in all of our cases.

Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS2 films. An understanding of these effects is critical for integrating MoS2 into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS2 films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS2. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS2 for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS2 films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.

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.

A novel 2D hybrid with MoS₂ nanocrystals strongly coupled on nitrogen-enriched graphene (MoS₂/NGg-C₃N₄) is realized by mild temperature pyrolysis (550 °C) of a self-assembled precursor (MoS₃/g-C₃N₄–H⁺/GO). With rich active sites, the boosted electronic conductivity and the coupled structure, MoS₂/NGg₋C₃N₄ achieves superior lithium storage performance.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) show great potential for the future electronics, optoelectronics and energy applications. But, the studies unveiling their interactions with the host substrates are sparse and limits their practical use for real device applications. We report the facile nano-scratch method to determine the adhesion energy of the wafer scale MoS2 atomic layers attached to the SiO2/Si and sapphire substrates. The practical adhesion energy of monolayer MoS2 on the SiO2/Si substrate is 7.78 J/m2. The practical adhesion energy was found to be an increasing function of the MoS2 thickness. Unlike SiO2/Si substrates, MoS2 films grown on the sapphire possess higher bonding energy, which is attributed to the defect-free growth and less number of grain boundaries, as well as less stress and strain stored at the interface owing to the similarity of Thermal Expansion Coefficient (TEC) between MoS2 films and sapphire substrate. Furthermore, we calculated the surface free energy of 2D MoS2 by the facile contact angle measurements and Neumann model fitting. A surface free energy ~85.3 mJ/m2 in few layers thick MoS2 manifests the hydrophilic nature of 2D MoS2. The high surface energy of MoS2 helps explain the good bonding strength at MoS2/substrate interface. This simple adhesion energy and surface energy measurement methodology could further apply to other TMDs for their widespread use.

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.

abstract: Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) are attractive for use in biotechnology, optical and electronics devices due to their promising and tunable electrical, optical and chemical properties. To fulfill the variety of requirements for different applications, chemical treatment methods are developed to tune their properties. In this dissertation, plasma treatment, chemical doping and functionalization methods have been applied to tune the properties of TMDCs. First, plasma treatment of TMDCs results in doping and generation of defects, as well as the synthesis of transition metal oxides (TMOs) with rolled layers that have increased surface-to-volume ratio and are promising for electrochemical applications. Second, chemical functionalization is another powerful approach for tuning the properties of TMDCs for use in many applications. To covalently functionalize the basal planes of TMDCs, previous reports begin with harsh treatments like lithium intercalation that disrupt the structure and lead to a phase transformation from semiconducting to metallic. Instead, this work demonstrates the direct covalent functionalization of semiconducting MoS2 using aryl diazonium salts without lithium treatments. It preserves the structure and semiconducting nature of MoS2, results in covalent C-S bonds on basal planes and enables different functional groups to be tethered to the MoS2 surface via the diazonium salts. The attachment of fluorescent proteins has been used as a demonstration and it suggests future applications in biology and biosensing. The effects of the covalent functionalization on the electronic transport properties of MoS2 were then studied using field effect transistor (FET) devices.
Dissertation/Thesis
Doctoral Dissertation Mathematics 2018

The interest in two-dimensional materials and materials physics has grown dramatically over the past decade. The family of two-dimensional materials, which includes graphene, transition metal dichalcogenides, phosphorene, hexagonal boron nitride, etc., can be fabricated into atomically thin films since the intralayer bonding arises from their strong covalent character, while the interlayer interaction is mediated by weak van der Waals forces. Among them, molybdenum disulfide (MoS₂) has attracted much interest for its potential applications in opto-electronic and valleytronics devices. Previously, much of the experimental studies have concentrated on optical and transport measurements while neglecting direct experimental determination of the electronic structure of MoS₂, which is crucial to the full understanding of its distinctive properties. In particular, like other atomically thin materials, the interactions with substrate impact the surface structure and morphology of MoS₂, and as a result, its structural and physical properties can be affected. In this dissertation, the electronic structure and surface structure of MoS₂ are directly investigated using angle-resolved photoemission spectroscopy and cathode lens microscopy. Local-probe angle-resolved photoemission spectroscopy measurements of monolayer, bilayer, trilayer, and bulk MoS₂ directly demonstrate the indirect-to-direct bandgap transition due to quantum confinement as the MoS₂ thickness is decreased from multilayer to monolayer. The evolution of the interlayer coupling in this transition is also investigated using density functional theory calculations. Also, the thickness-dependent surface roughness is characterized using selected-area low energy electron diffraction (LEED) and the surface structural relaxation is investigated using LEED I-V measurements combined with dynamical LEED calculations. Finally, bandgap engineering is demonstrated via tuning of the interlayer interactions in van der Waals interfaces by twisting the relative orientation in bilayer-MoS₂ and graphene-MoS₂-heterostructure systems.

Since their discovery at the beginning of the 21st century, two-dimensional (2D) materials have emerged as one of the most exciting material groups offering unique properties which promise a plethora of potential applications in nanoelectronics, quantum computing, and surface science. The progress in the study of 2D materials has advanced rapidly stimulated by the ever-growing interest in their behavior and the fact that they are the ideal specimen for transmission electron microscopy (TEM), as their geometry allows to identify every single atom. Their morphology – 2D materials consist of “surface” only – at the same time makes them sensitive to beam damage, since high-energy electrons easily sputter atoms and introduce defects. While this is in general not desirable – as non-destructive imaging is aimed at – it allows to precisely quantify the damage in TEM and even pattern the 2D material with atomic resolution using the electron beam. Alternatively, patterning of 2D materials can be achieved using focused ion irradiation, which makes studying its effect on 2D materials relevant and essential. In this thesis, we theoretically study the effects of electron and ion irradiation on 2D materials, exemplarily on 2D MoS2 . Specifically, we address the combined effect of electronic excitations and direct momentum transfer by high-energy electrons (knock-on damage) in 2D MoS2 using advanced first-principles simulation techniques, such as Ehrenfest dynamics based on time-dependent density functional theory (DFT). Here, we stress the importance of the combined effect of ionization damage and knock-on damage as neither of these alone can account for experimentally-observed defect production below the displacement threshold – the minimum energy required for the displacement of an atom from the pristine system. A mechanism of defect production relying on the localization of the electronic excitation at the emerging vacancy site is presented. The localized excitation eventually leads to a significant drop in the displacement threshold. The combination of electronic excitation and knock-on damage may in addition to beam-induced chemical etching explain the observed sub-threshold damage in low voltage TEM experiments. Apart from non-destructive imaging, electrons may be used to modify the 2D material intentionally. In this light, we consider the electron-beam driven phase transformation in 2D MoS2 , where the semiconducting polymorph transforms into its metallic counterpart. The phase energetics and a possible transformation mechanism under electron irradiation are investigated using DFT based first-principles calculations. The detailed understanding of the interaction of the electron beam with the 2D material promises to improve the patterning resolution enabling circuit design on the nanoscale. Ion irradiation employed in focussed ion beams (FIB), e.g., the helium ion microscope (HIM) constitutes another tool widely used to pattern and even image 2D materials. Ion bombardment experiment usually carried out for the 2D material placed on a substrate are frequently rationalized using simulations for free-standing systems neglecting the effect of the substrate. Combining Monte Carlo with analytical potential molecular dynamics simulations, we demonstrate that the substrate plays a crucial role in damage production under ion irradiation and cannot be neglected. Especially for light ions such as He and Ne, which are usually used in the HIM, the effect of the substrate needs to be considered to account for the increased number of defects and their broadened spatial distribution which limits the patterning resolution for typical HIM energies.
Seit ihrer Entdeckung Anfang des 21. Jahrhunderts haben sich zwei-dimensionale (2D) Materialien zu einer der spannendsten Materialklassen im Forschungsfeld aus Materialwissenschaft, Physik und Chemie entwickelt. Ihre einzigartigen Eigenschaften versprechen eine Vielzahl potentieller Anwendungen in der Nanoelektronik, für Quantencomputer und in der Oberflächenwissenschaft. Beflügelt durch das wachsende Interesse an ihrem Verhalten und der Tatsache, dass sie die idealen Proben für die Transmissions-Elektronen-Mikroskopie (TEM) darstellen – ihre Geometrie erlaubt es, jedes einzelne Atom zu identifizieren – sind die Forschungen an 2D-Materialien rapide vorangeschritten. Ihre Morphologie – 2D-Materialien bestehen nur aus “Oberfläche” – bedingt zugleich ihre Sensitivität bezüglich Strahlschäden. Hochenergetische Elektronen lösen sehr leicht Atome aus dem 2D-Material und induzieren Defekte. Obwohl dies im Allgemeinen unerwünscht ist – Ziel ist eine nicht-destruktive Bildgebung – erlaubt es doch präzise Einblicke in die Schadensentstehung im TEM. Überdies können 2D-Materialien mit Hilfe des Elektronenstrahls mit atomarer Auflösung strukturiert werden. Alternativ kann die Strukturierung des 2D-Materials über fokussierte Ionenstrahlung erfolgen, weshalb es lohnenswert erscheint, auch deren Effekt auf 2D-Materialien zu untersuchen. In dieser Arbeit werden die Effekte von Elektronen- und Ionenstrahlung auf 2D-Materialien aus theoretischer Sicht exemplarisch an 2D-MoS2 untersucht. Besonderes Augenmerk liegt dabei auf dem kombinierten Effekt von elektronischer Anregung und dem direkten Impulsübertrag durch hochenergetische Elektronen (Kollisionsschaden) in 2D-MoS2 , der durch die Anwendung von Ab-Initio-Simulationstechniken wie der Ehrenfest-Molekulardynamik, basierend auf zeitabhängiger Dichtefunktionaltheorie (DFT), studiert wird. Dabei liegt die Betonung auf der Kombination beider Effekte, da weder Ionisierungs- noch Kollisionsschäden allein die experimentell beobachtete Defekterzeugung unterhalb der Displacement Threshold – der notwendigen Mindestenergie, um ein Atom aus dem reinen Material herauszulösen – erklären. Ein möglicher Mechanismus der Defekterzeugung, basierend auf der Lokalisierung der elektronischen Anregung an der entstehenden Vakanzstelle, wird vorgeschlagen. Die lokalisierte Anregung führt dabei schließlich zu einem signifikanten Absinken der Displacement Threshold. Die Kombination von elektronischer Anregung und Kollisionsschaden trägt neben strahlinduzierten chemischen Reaktionen zur Erklärung der beobachteten Schäden unterhalb der Displacement Threshold in Niederspannungs-TEM-Experimenten bei. Neben nicht-destruktiver Bildgebung können Elektronenstrahlen auch dafür benutzt werden, 2D-Materialien gezielt zu modifizieren. In diesem Sinne wird der elektronenstrahl-induzierte Phasenübergang in 2D-MoS2 , bei dem sich das Material von einem halbleitenden in einen metallischen Zustand transformiert, betrachtet. Die Phasenenergetik und ein möglicher Transformationsmechanismus werden unter Zuhilfenahme von DFT-basierten Ab-Initio-Simulationen untersucht. Das detaillierte Verständnis der Interaktion des Elektronenstrahls mit dem 2D-Material verspricht dabei die Strukturierungsauflösung zu verbessern und ermöglicht Schaltkreisdesign auf der Nanoskala. Fokussierte Ionenstrahlen, wie sie in Ionenstrahlinstrumenten – wie dem Helium-Ionen-Mikroskop (HIM) zum Einsatz kommen – stellen ein weiteres häufig verwendetes Werkzeug zur Modifikation sowie zur Bildgebung von 2D-Materialien dar. Ionenstrahlexperimente – üblicherweise mit dem auf einem Substrat platzierten 2D-Material durchgeführt – werden hingegen oft mit Simulationen für freistehende 2D-Materialien rationalisiert, wobei jegliche Einwirkung des Substrats vernachlässigt wird. Die Kombination von Monte-Carlo-Simulationen mit Molekulardynamik-Simulationen (auf der Basis analytischer Potentiale) in dieser Arbeit verdeutlicht, dass das Substrat eine wichtige Rolle in der Defekterzeugung spielt und nicht vernachlässigt werden kann. Besonders für leichte Ionen, wie He und Ne, wie sie typischerweise im HIM zum Einsatz kommen, sollte der Effekt des Substrats berücksichtigt werden. Dieses führt für typische Ionenenergien im HIM – im Vergleich zum freistehenden 2D-Material – zu einer ansteigenden Anzahl an Defekten und einer breiteren räumlichen Defektverteilung, welche die Strukturierungsauflösung begrenzt.

碩士
國立交通大學
光電工程研究所
105
Recently, increasing attentions are paid to the two-dimensional materials especially TMDCs due to its direct band gap light emission. It was view as a next generation semiconductor materials which would apply to the optoelectronic device. However, challenging of TMDCs was that they suffered weak quantum yield. Therefore, in this study, we demonstrated the spontaneous emmission enhancement of MoS2 on the planar hyperbolic metamaterials (P- HMMs). In the first part, we designed the planar P-HMM at the PL wavelength of MoS2 which have the better mode coupling in the vertical direction compared to multilayers HMMs. Moreover, its anisotropic property make the PL of MoS2 led to in-plane confinement and resonance. Therefore, the strong coupling between structure and two-dimensional materials and spontaneous emission enhancement was observed in both experiment and simulation. In the second part, we started to curved the planar 1-D HMM into the concentric planar HMM (CPHMM) because we expected the better resonance in the ring cavity. On one hand, because the concentric structure could support the Whisper Gallery Mode (WGM) resonance compared to one dimensional resonance, the higher spontaneous emission enhancement of MoS2 with CP-HMM was observed than enhancement with P-HMM. On the other hand, by the photoluminescence mapping analysis, the highest happened at the inner edge of CP-HMM than other position of CP-HMM. It meant the energy concentration at the inner edge which led to the stronger mode coupling to the two-dimensional materials led to the higher enhancement. The similar results was also confirmed by the simulation results by Finite Element Method. Therefore, it showed the possibility to develop the ultrasmall light source combing subwavelength cavity and atomic thick two-dimensional materials.

Novel two dimensional nanoscale materials like graphene and metal dichalcogenides (MX2) have attracted the attention of the scientific community, due to their rich physics and wide range of potential applications. It has been shown that novel graphene based transparent conductors and radiofrequency transistors are competitive with the existing technologies. Graphene's properties are influenced sensitively by adsorbates and substrates. As such not surprisingly, physical properties of graphene are found to have a large variability, which cannot be controlled at the synthesis level, reducing the utility of graphene. As a part of my doctorate dissertation, I have developed atomic hydrogen as a novel technique to count the scatterers responsible for limiting the carrier mobility of graphene field effect transistors on silicon oxide (SiO2) and identified that charged impurities to be the most dominant scatterer. This result enables systematic reduction of the detrimental variability in device performance of graphene. Such sensitivity to substrates also gives an opportunity for engineering device properties of graphene using substrate interaction and atomic scale vacancies. Stacking graphene on hexagonal boron-nitride (h-BN) gives rise to nanoscale periodic potential, which influences its electronic graphene. Using state-of-the-art atomic-resolution scanning probe microscope, I correlated the observed transport properties to the substrate induced extrinsic potentials. Finally in efforts to exploit graphene's sensitivity to discover new sensor technologies, I have explored noncovalent functionalization of graphene using peptides. Molybdenum disulfide (MoS2) exhibits thickness dependent bandgap. Transistors fabricated from single layer MoS2 have shown a high on/off ratio. It is expected that ad-atom engineering can be used to induce on demand a metal-semiconductor transition in MoS2. In this direction, I have explored controlled/reversible fluorination and hydrogenation of monolayer MoS2 to potentially derive a full range of integrated circuit technology. The in-depth characterization of the samples is carried out by Raman/photoluminescence spectroscopy and scanning tunneling microscopy.
Ph.D.
Doctorate
Physics
Sciences
Physics

碩士
國立臺灣科技大學
電子工程系
105
The theme of this thesis is focused on the preparation and application of molybdenum disulfide (MoS2) film at the atomic layer. MoS2 thin films were deposited by chemical vapor deposition (CVD) method on the quartz and sapphire substrates. After the MoS2 thin film was synthesized, the morphology and film continuity of the substrate were analyzed by optical microscopy. Raman scattering was used to analyze the atomic vibration mode of the MoS2 thin film. The atom force microscopy (AFM) showed the epitaxial thickness are atomic layer. Optical properties of near-band-edge emission of MoS2 thin film using photoluminescence (PL) measurements in the temperature range between 12 K and 300 K. Bound exciton has been observed at low temperature. On the other hand, transmittance experiment observed free A exciton and free B exciton which are caused by electron spin splitting. At the application of MoS2 thin film, we used MoS2 to fabricate field effect transistor (FET) channel and photodetector. Beside, we studied of the optoelectronic structure for the MoS2 thin film and we made into photoconductive detectors. Finally, the oxygen plasma treatment was used to induce oxygen atoms into the MoS2. The better optoelectronic characteristics due to the plasma treatment doped MoS2. The tunneling current increases exponentially with doping concentration. By applying our fabricating method, the MoS2 thin film provides a new application of semiconductor device.

碩士
國立臺灣大學
電子工程學研究所
103
In this thesis, the mechanically exfoliated 2D material MoS2 nanosheet was used to fabricate thin film transistor and their electrical properties were investigated as well. By checking their thickness by optical microscopy and atomic force microscopy, the nanosheet with an appropriate thickness can be selected. It was found that the ohmic contact on MoS2 can be achieved by low work function metal titanium. The performance of TFTs achieved the high on/off current ratio up to 8 th order of magnitude and the mobility of 16 cm2/V-sec. However, in order to identify MoS2 thickness on SiO2/Si substrate by means of better optical interference, the thickness of silicon dioxide was limited to 300nm thick. The thick oxide lead to very high gate control voltage. To reduce the operation voltage, the use of thinner high-k gate dielectric Al2O3 and HfO2 were used, resulting in much better performances than traditional SiO2. The on/off current ratio for Al2O3 gate insulator thin film transistor was 6×107 and mobility of 23 cm2/V-sec. HfO2 gate insulator could even boost the mobility to 28 cm2/V-sec and the sub-threshold swing to only 127mV/dec, combining with the gate operation voltage below ±1 V. In addition, it was also found that the oxygen and water molecules were easily absorbed at the MoS2 surface in air, which would deteriorate the stability and hysteresis of devices. Finally, the high vacuum measurement method and passivation layer were used to improve the stability.

碩士
國立交通大學
電信工程研究所
106
The semiconductor industry has undergone a huge development over the past fifty years. The technology node has finally scaled down to five nanometer which allows designers to devise more complicated and diverse functions and systems in the same silicon chip. Driven by the big data and artificial intelligence technology, the need for high-performance logic circuit is growing astronomically. However, miniaturization which was used to boost the performance of transistors is limited by Moore's law and quantum mechanics. Although industry is still trying to challenge the limitation of miniaturization, advanced alternative materials equipping with superb material property has gaining importance from both academia and industry. Among them, the applications of low-dimensional materials are the cynosure of all eyes. People's interest in two-dimensional burgeoned since graphene was exfoliated from graphite in 2004. However, the zero-gap property of graphene limits its potential in logic design. The invention of monolayer transition metal dichalcogenide transistor in 2011 and few-layer black phosphorus transistor in 2014 grab more attention. However, researching how to implement two-dimensional materials in next-generation integrated circuit is still an important topic of semiconductor industry. One of the emerging topics is how to modulate the electronic property of two-dimensional by doping engineering, especially for molybdenum disulfide because it is promising for optoelectronics. In the traditional silicon technology, the elements in group III and V, such as boron and arsenic, are frequently used as dopants for p-type and n-type semiconductor. However, distinct from the bulk size of silicon, the surface effect is significant in two-dimensional materials due to higher surface/volume ratio. Also, because molybdenum disulfide is constructed by molybdenum and sulfur elements, therefore it is an urgent task to find out the proper dopants and doping levels that lead to n-type or p-type property. In this thesis, we apply blocked Davidson iteration to solve Kohn-Sham equation. We also integrate the model with empirical van der Waals correction to describe the interaction among atoms and calculate the electronic structure. In practical, we exploit VASP first-principles technical software for the test of exchange-correlation functionals, the atomic relaxation, and the calculation of energy levels. Based on the methodology, we investigate the impact of transition metal dopants on the electronic property of monolayer molybdenum disulfide using first-principles calculation. Firstly, we verify and calibrate the parameters and models by comparing with experimental measurement. We discuss how the dopants modulate the vacuum potential, electron affinity, work function, and Fermi energy of monolayer molybdenum disulfide by analyzing charge transfer. Meanwhile, we consider the effect of doping level on the Fermi energy for each dopant and categorize the proper dopant and doping level for n-type and p-type monolayer molybdenum disulfide. The other urgent task is to explore the proper electrode materials for black phosphorus. Although black phosphorus has a high carrier mobility, but the high contact resistance keeps black phosphorus from fully wield its excellent material property. The significant contact resistance was also observed in one-dimensional materials, such as carbon nanotube, and other kinds of two-dimensional materials, such as molybdenum disulfide, however, they had already been well-studied. Black phosphorus, on the contrary, is the youngest member in the two-dimensional family, thus the corresponding research are still progressing. It worth noting that an international researcher used scandium as contact material and measured a high performance, but lacks a theoretical explanation. In this thesis, we first propose the revolutionary hybrid exchange-correlation functional to lift the computational accuracy and boost the reliability by calibrating with experiments. Based on the accuracy in atomic and electronic structures, we construct the interface between contact materials and trilayer black phosphorus to analyze the interfacial binding behavior and the impact of binding on the other layers of trilayer black phosphorus. We analyze the performance of contact material from the insight of interfacial potential, charge density, charge transfer, and density of states. Finally, we conclude that why scandium electrode leads to superior performance. In short, this thesis mainly focuses on how the transition metal dopants modulate the electronic property of monolayer molybdenum disulfide and the impact of contact materials on the electronic property of black phosphorus. The results of this thesis can be a valuable reference for the development of next-generation transistor and can provide contemporary semiconductor industry to develop and improve the innovative technology.

Infectious diseases caused by pathogenic bacteria are creating a global health problem. In the recent report of World Health Organization (WHO), it has been mentioned that around 7 lacks people are dying each year worldwide due to drug resistant microbials. After discovery of the lifesaving “wonder drug” molecule penicillin, it was extensively used for the treatment of bacterial infection diseases. However, the excessive use of antibiotics leads to the development of antimicrobial resistance in the pathogenic bacterial strains to overcome the bactericidal effect of antibiotics. The drug-resistance bacteria follow multiple pathways to show resistance towards the existing antimicrobial agents and eventually make them abortive. The prevalence of these drug resistant bacterial strains poses a serious threat to the present medical system. Therefore, there is an urgency to develop advanced antimicrobial agents which can restrict the spread of pathogenic bacteria to eradicate infectious diseases. In this context, the current advancement in the field of nanotechnology would help us to develop nanomaterial-based antimicrobial agents which could be one of the possible alternatives of conventionally used antibiotics. There are numerous reports, which established that nanomaterials such as graphene oxide, carbon nanotube, noble metal nanoparticles, metal oxides like ZnO2, MnO2 etc. have possessed antibacterial activity. In particular, the use of nanosized molybdenum disulfide (MoS2), a transition metal dichalcogenide showed a great potential to utilize for the development of potent antibacterial agents owing to its unique chemical and photophysical properties. Two-dimensional MoS2 nanosheets provide a large surface to volume ratio for the effective interaction with the bacterial cell membrane. For better biological interactions of MoS2 nanomaterials, its surface modification can be easily achieved through functionalization using thiol ligand molecule. Functionalization also enhances its aqueous dispersibility in manyfold. In this thesis work, I have utilized MoS2 nanomaterials and their nanocomposites to develop nanomaterial-based effective antimicrobial agents for the pathogenic bacterial strains using multiple strategies. To extend my work towards the development of nanomaterial-based antibacterial agents, I have explored antibacterial activity of the supramolecularly self-assembled nanosized cage molecule to eradicate drug-resistant bacteria. Apart from antibacterial activity, I have also expanded the scope of applicability of our newly developed nanomaterials in the direction of bio-analyte detection and environmental remediation such as degradation of organic pollutant and detoxification of the chemical warfare agent.

碩士
國立東華大學
材料科學與工程學系
107
In this thesis, large-area molybdenum disulfides (MoS2) are prepared by sulfurizing the pre-deposited transition metal films. We have demonstrated 2D tin (stanene) and germanium (germanene) film growths on MoS2/sapphire substrates by using the thermal evaporation. In the first section, the elemental 2D material of germanene was grown on the MoS2 surface at 400 °C. Observed from the cross-sectional high-resolution transmission electron microscopy image (HRTEM), the layer separation between germanene is 3.3 Å, which is consistent with the value extracted from the XRD curve. Next, we find out that the other 2D material stanene can also be grown on the MoS2 surface at 150°C and room temperature. The stanene layer separation is 2.9 Å. The value is consistent with the value extracted from the XRD curve. We also attempted to grow stanene on sapphire substrates at room temperature. The results show that stanene can also be formed on sapphire substrates at room temperature. Finally, we use stanene as the contact metals for MoS2. Four different contact metal electrodes of these samples were prepared. They are Au/Ti, stanene, Au/stanene, and Au/Al/stanene respectively. By using the transmission line model (TLM), the device using Au/Al/stanene as contact metal shows lower specific contact resistance (ρc) value compare with the device using Au/Ti as electrodes. The values of specific contact resistance (ρc) decreased from 6.63×103 Ω·cm2 to 4.04 Ω·cm2. Totally descending three orders of magnitude. We have demonstrated that the conductive 2D material stanene is a promising candidate as a contact metal for 2D devices. This significant achievement is so impressive, we succeed in stepping across this milestone and look forward to the 2D materials will have a great development potential in the future.