Dissertationen zum Thema „Two-dimensional nanomaterials“

2-Dimensional (2D) materials are characterised by atomic thickness and significantly larger edge-lengths, producing particles which are highly confined in 1 direction. Reducing a material to one or few atomic layers gives rise to structural and electronic properties that deviate significantly from those of the bulk crystal. For this reason 2D nanosheets have been investigated for potential application in sensing, catalysis, capacitance, photovoltaics and for flexible circuits (among others).Despite rapid progress in understanding the synthesis and properties of 2D nanosheets in recent years, there remain significant problems surrounding the development of scalable production methods, understanding and tuning fundamental properties, and controlling the size and monodispersity of semiconductor crystals. In addition, new materials with novel properties are constantly sought in order to meet specific requirements. Although the tools developed over the last 12 years can often be applied to the fabrication of these materials, understanding their behaviour and limitations is ongoing. The following thesis discusses the routes to the fabrication of 2-dimensional materials and explores the production of MoS2, black phosphorus and tin(II) sulfide nanosheets. The aim of each piece of work is determined by the level of development of the field; MoS2 nanosheets have been known for several years and therefore the work presented was motivated by a desire to impart size control for specific applications. The study of phosphorene and 2D tin(II) sulfide is in its infancy; as such the focus remains on scalable nanosheet exfoliation and developing an understanding of their properties. The following studies on phosphorene report the exfoliation of nanosheets in organic and aqueous surfactant solutions and an investigation of the stability and breakdown products of the resulting colloidal suspensions. The stabilisation of phosphorene in aqueous media paves the way for its use in biological systems. Band-gap tuning in IV-VI analogues of phosphorene is demonstrated by size-selection of exfoliated SnS nanosheets. Although the physical characteristics of nanosheets and their incorporation into devices receive some attention, this thesis will focus mainly on the synthetic aspects of 2D materials research.

Cette étude de la fonctionnalisation de graphène se base principalement sur la monocouche de graphène épitaxiée sur SiC. Les propriétés électroniques, structurales et les compositions chimiques du graphène fonctionnalisé sont étudiées. L'incorporation d'azote dans le graphène réalisée par les procédures à base de plasma montre un décalage de niveaux inoccupés du graphène vers EF , obtenue par les analyses spectroscopie de photoémission inverse en résolution angulaire. Ce dopage-n est attribué à la présence de graphitique-N. De plus, la configuration des espèces de N substitués dans le graphène peut être contrôlée efficacement par l'énergie, les espèces d'azote incidentes, et l'épaisseur du graphène de départ. L'hydrogénation de la couche tampon de graphène (BLG) à température variante sature les liaisons pendantes de Si de l'interface différemment, soit par la formation de nouvelles liaisons C-Si à température ambiente, soit par les hydrogènes intercalés. Le BLG devient fortement-isolant dans le premier cas, et devient une monocouche de graphène quasi-autoportante (QFSG) dans le second, permettant un nouveau concept de fabrication des dispositifs à base de graphène sur SiC. La réaction/couplage entre des molécules pi-conjugué et les graphène vierge ou fonctionnalisé est aussi étudiée. Les états inoccupés des molécules à base de perylene sont légèrement modiffiées sur le graphène dopé N à cause d'un renforcement de transfert de charge. Des réactions chimiques entre les molécules perylenes et le graphène sont observées aprés l'exposition aux électrons de basse énergie. En résumé, cette étude permettra une meilleure maîtrise des propriétés des matériaux 2D comme le graphène
In order to promote 2D materials like graphene to their numerous applications, new methodsaltering their electronic and chemical properties have to be mastered. In this thesis, theprocesses of chemical doping and hydrogenation of monolayer graphene grown on SiC are investigated. Nitrogen atoms are successfully substituted in the graphene lattice using plasma-basedmethods. The bonding configurations of the incorporated N can be controlled via the nature and energy of exposing species and the thickness of the pristine graphene. An n-type doping, revealed by angle-resolved inverse photoemission spectroscopy (ARIPES), is found in most N-doped graphene and is assigned to the presence of graphitic-N. Hydrogenations of the buffer layer of graphene (BLG) on SiC at ambient or high temperatures saturate the remaining Si dangling bonds at BLG/SiC interface in two different ways, either by inducing additional C-Si bonds or by H intercalation. This results in 2D materials with distinct characters, an insulating, graphane-like H-BLG or a quasi-free-standing graphene, which may be used as a new concept for the engineering of graphene-based devices. The interactions between pi-conjugated molecules and the functionalized graphene are also investigated. The unoccupied states of molecules are altered by the presence of incorporated N, but the degradation of molecules due to low-energy electron exposure seems not enhanced by the doping nitrogen under the studied conditions. Nevertheless, the functionalization of graphene is demonstrated and its electronic and chemical properties are carefully studied, which should help to faster further applications employing functionalized graphene

Developing scalable nanomaterial production methods is necessary for realising nanomaterial commercialisation. In principle, production via Liquid Phase Exfoliation satisfies this need. However, techniques reliant on energy input damage the material via mechanical stress, yielding suspensions of multi-layer stacks, stable only for days, and necessitating centrifugation for manipulation. An alternative, emerging technique relies upon the charging of material to allow spontaneous dissolution of pristine 2d nanomaterials. Here this is explored for pnictogen chalcogenide layered materials, focusing on unanswered questions relating to the practicality of the method. In this thesis, ion intercalated Bi2Te3 and Sb2Te3 were dissolved within the aprotic solvents: N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF). Successful exfoliation of undamaged, hexagonal 2d nanomaterials was confirmed. A range of complementary experimental techniques were used including TEM, AFM, and SAXS. From the analysis of thousands of nanosheets it was found that gradual diffusion of nanosheets, as a result of their spontaneous exfoliation, lead to fractionation of nanosheets of differing lateral width throughout the liquid volume without need for centrifugation. Nanosheet lateral dimension was also controlled by stoichiometry of the intercalant metal, with an optimum intercalant stoichiometry of 0.1 < x < 1.5 for Kx. Bi2Te3 for production of pristine nanomaterial. The chemical stability of the solution was investigated in relation to exposure to air, water, and heating, with a focus on tellurium impurities. Using SEM and TEM it was shown that tellurium impurities resulted from the presence of alkali metal polytellurides, which could be minimised by optimising the Kx. Bi2Te3 stoichiometry. However, the existence of nanosheets in a 16 month old solution demonstrates stability of these liquids when handled under inert conditions. Together these results demonstrate that this scalable method allows material manipulation and tailoring of nanosheet dimensions, whilst also giving weight to the argument that the liquids can be described as true thermodynamic solutions.

The ability to convert photons into mechanical motion is of significant importance for many energy conversion and reconfigurable technologies. Establishing an optical-mechanical interface has been attempted since 1881; nevertheless, only few materials exist that can convert photons of different wavelengths into mechanical motion that is large enough for practical import. Recently, various nanomaterials including nanoparticles, nanowires, carbon nanotubes, and graphene have been used as photo-thermal agents in different polymer systems and triggered using near infrared (NIR) light for photo-thermal actuation. In general, most photomechanical actuators based on sp bonded carbon namely nanotube and graphene are triggered mainly using near infra-red light and they do not exhibit wavelength selectivity. Layered transition metal dichalcogenides (TMDs) provide intriguing opportunities to develop low cost, light and wavelength tunable stimuli responsive systems that are not possible with their conventional macroscopic counterparts. Compared to graphene, which is just a layer of carbon atoms and has no bandgap, TMDs are stacks of triple layers with transition metal layer between two chalcogen layers and they also possess an intrinsic bandgap. While the atoms within the layers are chemically bonded using covalent bonds, the triple layers can be mechanically/chemically exfoliated due to weak van der Waals bonding between the layers. Due to the large optical absorption in these materials, they are already being exploited for photocatalytic, photoluminescence, photo-transistors, and solar cell applications. The large breaking strength together with large band gap and strong light- matter interaction in these materials have resulted in plethora of investigation on electronic, optical and magnetic properties of such layered ultra-thin semiconductors. This dissertation will go in depth in the synthesis, characterization, development, and application of two- dimensional (2D) nanomaterials, with an emphasis on TMDs and molybdenum disulfide (MoS2), when used as photo-thermal agents in photoactuation technologies. It will present a new class of photo-thermal actuators based on TMDs and hyperelastic elastomers with large opto-mechanical energy conversion, and investigate the layer-dependent optoelectronics and light-matter interaction in these nanomaterials and nanocomposites. Different attributes of semiconductive nanoparticles will be studied through different applications, and the possibility of globally/locally engineering the bandgap of such nanomaterials, along with its consequent effect on optomechanical properties of photo thermal actuators will be investigated. Using liquid phase exfoliation in deionized water, inks based on 2D- materials will be developed, and inkjet printing of 2D materials will be utilized as an efficient method for fast fabrication of functional devices based on nanomaterials, such as paper-graphene-based photo actuators. The scalability, simplicity, biocompatibility, and fast fabrication characteristics of the inkjet printing of 2D materials along with its applicability to a variety of substrates such as plastics and papers can potentially be implemented to fabricate high-performance devices with countless applications in soft robotics, wearable technologies, flexible electronics and optoelectronics, bio- sensing, photovoltaics, artificial skins/muscles, transparent displays and photo-detectors.

This thesis focuses on the design of metal oxide based two‐dimensional nanomaterials for various sustainable applications. The as-prepared 2D TiO2-based nanomaterials and their hybrid compounds have been characterized and applied in different sustainable environmental and energy applications and showed superior properties. It is believed that the research and investigations on 2D nanomaterials based sustainable applications is of great significance for the further development of a green, sustainable, and environmentally friendly society.

Semiconducting nanocrystals in open-access microcavities are promising systems in which enhanced light-matter interactions lead to quantum effects such as the modulation of the spontaneous emission process and exciton-polariton formation. In this thesis I present improvements of the open cavity platform which serves to confine the electromagnetic field with mode volumes down to the λ³ regime and demonstrate results in both the weak and strong coupling regimes of cavity quantum electrodynamics with a range of different low-dimensional materials. I report cavity fabrication details allowing a peak finesse of 5 × 10⁴ and advanced photonic structures such as coupled cavities in the open cavity geometry. By incorporating two-dimensional materials and nanoplatelets in the cavity I demonstrate the strong coupling regime of light-matter interaction with the formation of exciton-polaritons, quasi-particles composed of both photon and exciton, at room temperature. In the perturbative weak coupling regime I show pronounced modulation of the single-photon emission from CdSe/ZnS quantum dots and the two-dimensional material WSe₂ and demonstrate Purcell enhancement of the spontaneous emission rate by factors of 2 at room temperature and 8 at low temperature. The findings presented in this thesis pave the way to establish open microcavities as a platform for a wide range of applications in nanophotonics and quantum information technologies.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.
Includes bibliographical references.
Biological systems inherently posses the ability to synthesize and assemble nanomaterials with remarkable precision, as evident in biomineralization. These unique abilities of nature continue to inspire us to develop new approaches of nanobiotechnology to integrate advanced materials into medicine and electronics. Particularly, peptides are believed to play an important role in biotemplating and biological self-assembly. In order to understand the interface between inorganic materials and peptides and realize biological self-assembly, this work adopted M13 virus as a model system. The genetic engineering of M13 viruses enables us to grow various nanomaterials and achieve virus monolayer assembly on charged polyelectrolyte multilayers. The fundamental understanding and new discoveries obtained by this work can mature into an engineering discipline demonstrating that biological approaches may represent a new paradigm to provide novel technological advantages. The use of a biological template for a nanostructured battery electrode ramps up the device's performance and scales down its overall size. This work presents a new way of exploiting biological entities for the bottom-up assembly of battery devices by utilizing biological self-assembly and biotemplating. Viruses are genetically engineered such that they function as a toolkit for constructing the battery.
by Ki Tae Nam.
Ph.D.

This thesis deals with one-dimensional (1D) and two-dimensional nanomaterials (2D) in terms of their utilization for new types of gas sensors. Thesis focuses on study of sensing elements for gas sensors based on semiconductor metal oxide materials (MOX) and their manufacturing technology. The objective of the thesis is the design and implementation of a sensing elements formed by selected nanomaterials based on the structure of interdigital electrodes. The result of the practical part of the thesis is the characterization and comparison of materials in terms of their detection parameters in the presence of selected test gases. The first part of thesis hierarchically defines chemoresistive gas sensor, characterizes and explains its operation principle. Second part studies 1D and 2D nanomaterials of sensing elements for MOX chemoresistive gas sensors, contains a research of their properties and describes their methods of manufacturing and implementation. The last part deals with the implementation of the sensitive layer of the sensor with selected nanomaterials, characterizes and compares their detection properties.

This research aims to explore the optimization strategies of two-dimensional (2D) nanostructures for high-performance rechargeable batteries. Three effective strategies, including 2D-based phase engineering, component engineering and van der Waals (vdW) heterostructures, were proposed for improving electrochemical properties of 2D nanomaterials. These effective strategies will offer good references for researchers to develop practical next-generation rechargeable batteries using the emerging 2D nanomaterials.

The production of large area, high quality two-dimensional (2D) materials using chemical vapour deposition (CVD) has been an important and difficult topic in contemporary materials science research, after the discovery of the diverse and extraordinary properties exhibited by these materials. This thesis mainly focuses on the CVD synthesis of two 2D materials; bilayer graphene and monolayer tungsten disulphide (WS2). Various factors influencing the growth of each material were studied in order to understand how they affect the quality, uniformity, and size of the 2D films produced. Following this, these materials were combined to fabricate 2D vertical heterostructures, which were then spectroscopically examined and characterised. By conducting ambient pressure CVD growth with a flat support, it was found that high uniform bilayer graphene could be grown on the centimetre scale. The flat support provides for the consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights not only into the upscaling of CVD methods for growing large area, high quality graphene and but also in how to transfer the product onto flexible substrates for potential applications as a transparent conducting electrode. Monolayer WS2 is of interest for use in optoelectronic devices due to its direct bandgap and high photoluminescence (PL) intensity. This thesis shows how the controlled addition of hydrogen into the CVD growth of WS2 can lead to separately distributed domains or centimetre scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates or low pressure vacuum systems. This CVD reaction is simple and efficient, ideal for mass-production of large area monolayer WS2. Subsequent studies showed that hexagonal domains of monolayer WS2 can have discrete segmentation in their PL emission intensity, forming symmetric patterns with alternating bright and dark regions. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. These results provide important insights into the spatially varying properties of these CVD-grown TMDs materials, which may be important for their effective implementation in fast photo sensors and optical switches. Finally, by introducing a novel non-aqueous transfer method, it was possible to create vertical stacks of mixed 2D layers containing a strained monolayer of WS2, boron nitride, and graphene. Stronger interactions between WS2 on graphene was found when swapping water for IPA, likely resulting from reduced contamination between the layers associated with aqueous impurities. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, a result which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with large area coverage and controllable thickness on the atomic level.

The popularity of graphene owing to its unique properties has triggered huge interest in other two-dimensional (2D) nanomaterials. Among them, silicene shows considerable promise for electronic devices due to the expected compatibility with silicon electronics. However, the high-end potential application of silicene in electronic devices is limited owing to the lack of an energy band gap. Hence, the principal objective of this research is to tune the electronic and magnetic properties of silicene related nanomaterials through first-principles models. I first explored the impact of edge functionalization and doping on the stabilities, electronic, and magnetic properties of silicene nanoribbons (SiNRs) and revealed that the modified structures indicate remarkable spin gapless semiconductor and half-metal behaviors. In order to open and tune a band gap in silicene, SiNRs were perforated with periodic nanoholes. It was found that the band gap varies based on the nanoribbon’s width, nanohole’s repeat periodicity, and nanohole’s position due to the quantum confinement effect. To continue to take advantage of quantum confinement, I also studied the electronic and magnetic properties of hydrogenated silicene nanoflakes (SiNFs). It was discovered that half-hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flake’s size. Next, I studied the adsorption behavior of various gas molecules on SiNRs. Based on my results, the SiNR could serve as a highly sensitive gas sensor for CO and NH3 detection and a disposable gas sensor for NO, NO2, and SO2. I also considered adsorption behavior of toxic gas molecules on boron carbide (BC3) and found that unlike graphene, BC3 has good sensitivity to the gas molecules due to the presence of active B atoms. My findings divulged the promising potential of BC3 as a highly sensitive molecular sensor for NO and NH3 detection and a catalyst for NO2 dissociation. Finally, I scrutinized the interactions of CO2 with lithium-functionalized germanene. It was discovered that although a single CO2 molecule was weakly physisorbed on pristine germanene, a significant improvement on its adsorption energy was found by utilizing Li-functionalized germanene as the adsorbent. My results suggest that Li-functionalized germanene shows promise for CO2 capture.

Nanomaterials have attracted great interest due to the unique physical properties and great potential in the applications of nanoscale devices. Two dimensional atomic crystals, which are atomic thickness, especially graphene, have triggered the gold rush recently due to the fascinating high mobility at room temperature for future electronics. The crystal structure of nanomaterials will have great influence on their physical properties. Thus, this thesis is focused on developing the methods to control the crystal structure of nanomaterials, namely quantum dots as semiconductor, boron nitride (BN) as insulator, graphene as semimetal, with low cost for their applications in photonics, structural support and electronics. In this thesis, firstly, Mn doped ZnSe quantum dots have been synthesized using colloidal synthesis. The shape control of Mn doped ZnSe quantum dots has been achieved from branched to spherical by switching the injection temperature from kinetics to thermodynamics region. Injection rates have been found to have effect on controlling the crystal phase from zinc blende to wurtzite. The structural-property relationship has been investigated. It is found that the spherical wurtzite Mn doped ZnSe quantum dots have the highest quantum yield comparing with other shape or crystal phase of the dots. Then, the Mn doped ZnSe quantum dots were deposited onto the BN sheets, which were micron-sized and fabricated by chemical exfoliation, for high resolution imaging. It is the first demonstration of utilizing ultrathin carbon free 2D atomic crystal as support for high resolution imaging. Phase contrast images reveal moiré interference patterns between nanocrystals and BN substrate that are used to determine the relative orientation of the nanocrystals with respect to the BN sheets and interference lattice planes using a newly developed equation method. Double diffraction is observed and has been analyzed using a vector method. As only a few microns sized 2D atomic crystal, like BN, can be fabricated by the chemical exfoliation. Chemical vapour deposition (CVD) is as used as an alternative to fabricate large area graphene. The mechanism and growth dynamics of graphene domains have been investigated using Cu catalyzed atmospheric pressure CVD. Rectangular few layer graphene domains were synthesized for the first time. It only grows on the Cu grains with (111) orientation due to the interplay between atomic structure of Cu lattice and graphene domains. Hexagonal graphene domains can form on nearly all non-(111) Cu surfaces. The few layer hexagonal single crystal graphene domains were aligned in their crystallographic orientation over millimetre scale. In order to improve the alignment and reduce the layer of graphene domains, a novel method is invented to perform the CVD reaction above the melting point of copper (1090 ºC) and using molybdenum or tungsten to prevent the balling of the copper from dewetting. By controlling the amount of hydrogen during the growth, individual single crystal domains of monolayer over 200 µm are produced determined by electron diffraction mapping. Raman mapping shows the monolayer nature of graphene grown by this method. This graphene exhibits a linear dispersion relationship and no sign of doping. The large scale alignment of monolayer hexagonal graphene domains with epitaxial relationship on Cu is the key to get wafer-sized single crystal monolayer graphene films. This paves the way for industry scale production of 2D single crystal graphene.

Graphene has unrivalled properties and is heralded as a revolutionary material for the 21^st century. Chemical vapour deposition (CVD) on metals is a promising method to produce large-area graphene. Controlling the properties of CVD graphene is vital for its integration in a wide-range of future applications. Many factors can influence the CVD growth of graphene and its properties, therefore further investigations will be beneficial to fully understand and control this technique. In this thesis I expand the knowledge about the growth of pure and heteroatom-doped graphene by low pressure chemical vapour deposition (LPCVD) and atmospheric pressure chemical vapour deposition (APCVD) on commercially available Cu and Pt foils. Using a range of characterisation techniques, I investigate the influence of the substrate’s properties and the synthesis conditions on the growth of graphene, in pursuit of improved, controlled or optimised production, which can promote high quality, large-area, single-layer graphene, or other as desired. By characterising the topography, surface roughness, crystallographic orientations, and chemical composition of six Cu foils, I find that their properties vary greatly and this influences the growth of CVD graphene. I elucidate that the commonly used 99.8 % Alfa Aesar Cu foil has a surface coating composed of calcium, chromium, and phosphorus, which detrimentally influences graphene growth. Cleaning Cu foils with CH₃COOH is shown to reduce the concentration of surface contaminants, consequently reducing the nucleation density and increasing the growth rate of CVD graphene. I also demonstrate that the shape, orientation, edge-geometry and thickness of CVD graphene domains can be controlled by the Cu crystallographic orientations. Single layer LPCVD graphene domains align with zigzag edges parallel to a single <101> direction on Cu{111} and Cu{101}, while bilayer domains align to two directions on Cu{001}. Hexagonal APCVD domains also preferentially align with edges parallel to the <101> direction(s). This discovery resolves a key challenge of controlling the orientation of individual graphene domains and opens a new avenue for tailored production of large-area CVD graphene with improved properties. By controlling the synthesis conditions of APCVD graphene on Pt foils I optimise production of ~0.5 mm single layer graphene domains with reduced nucleation density and increased growth rate of ~100 μm/min by synthesis at 1150°C, a higher temperature than previously reported. The absence of large, hexagonal, single-crystal domains on pristine Pt foil, and observation of a reaction between quartz and Pt that promotes hexagonal domains, suggests that a silicon or platinum silicide surface layer may be advantageous for improved growth of graphene. Finally, I demonstrate that the dopant concentration of nitrogen-doped graphene is increased at lower synthesis temperatures and higher NH₃ concentration, up to 1.3 %, but with an associated decrease in the growth rate. Direct visualisation, elemental confirmation, and electronic characterisation of individual nitrogen atoms is shown for the first time using aberration corrected scanning transmission electron microscopy and electron energy loss spectroscopy. Boron-doped graphene is also synthesised. The implications of these findings, and many additional minor contributions, are wide-ranging and of considerable importance for the future understanding of CVD growth of graphene on metals, and more generally for the advancement of scientific knowledge for manufacturing large-area graphene. Collectively, these discoveries represent a significant body of work that can improve the efficiency of production and assist with controlling the properties of large-area CVD graphene.

In this thesis, the principal focus is the rational design and fabrication of two-dimensional (2D) nanoarchitectures, e.g., low-cost metal oxide nanosheets and earth-abundant transition metal layered double hydroxides (LDHs) for enhanced electrocatalysis. The related hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance not only demonstrated the advances of 2D nanomaterials, such as unique physical and mechanical properties, unprecedented electronic features, and ultrahigh surface areas but also indicated the possible mechanisms behind boosted activity and stability, e.g., phase engineering function and oxygen vacancies influence.

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The two-dimensional (2D) materials display many interesting properties, which are not found in bulk structure because the original electronic structure is substantially altered from its three-dimensional (3D) characteristics. Black and blue phosphorene are 2D materials which are attracting many fields interest because of their electronic and magnetic properties, making them possible materials for spintronics devices application. These nanomaterials display some characteristics that allow their use on sensors, thus, this work aims to evaluate the changes in black and blue phosphorenes’ electronic and magnetic properties, before and after the chemical groups functionalizations. We utilized the amide, amine, carboxyl and hydroxyl chemical groups because they are into many living organisms, directing the results for possible systems of molecules adsorption with chemical and/or biological interest application. First-principles calculations based on Density Functional Theory with the Local Density Approximation (LDA) were performed using the SIESTA code. Blue phosphorene systems show a higher structure disturbance level when functionalized, so as atoms displacement, when compared with black phosphorene respective systems. Its binding energy also presents higher values when compared to black phosphorene systems. Both configurations 1-2, show the more stable systems for carboxyl groups functionalized on black and blue phosphorenes, presenting a 2.34 and 2.72 eV binding energy, respectively. The configuration 1-2 take on this post because reestablish the systems' symmetry. The symmetry reestablishment effect occurs in every kind of chemical group. These results imply in a promising black and blue phosphorenes application in systems of molecules adsorption with chemical and/or biological interest.
Diversas estruturas bidimensionais (2D) vêm apresentando propriedades interessantes, pois as mesmas são substancialmente alteradas quando comparadas às suas formas tridimensionais (3D). As estruturas de fosforeno negro e azul são materiais 2D que atraíram o interesse de muitas áreas pelo fato de suas propriedades eletrônicas e magnéticas indicarem o seu possível uso em dispositivos spintrônicos. Esses nanomateriais possuem características que permitem sua utilização em sensores, desta forma, este trabalho visa avaliar as mudanças nas propriedades eletrônicas e estruturais dos fosforenos causadas pelas funcionalizações dos grupos químicos amida, amina, carboxila e hidroxila, os quais compõem grande parte das moléculas biológicas presentes em organismos vivos para possível aplicação em sistemas de adsorção de moléculas de interesse biológico. Para desenvolver este trabalho fez-se uso de simulação computacional com cálculos de primeiros princípios, utilizando a Teoria do Funcional da Densidade (DFT), e Aproximação Local da Densidade (LDA) implementada no código computacional SIESTA. Este estudo demonstra que a estrutura de fosforeno azul apresenta maiores perturbações estruturais, como a modificação da posição inicial dos átomos de fósforo, devido às funcionalizações dos grupos químicos, se comparado aos sistemas funcionalizados com o fosforeno negro. Outro fator que chama a atenção são os valores de energia de ligação, onde todos os sistemas de fosforeno azul apresentam módulos maiores neste parâmetro para as respectivas configurações de funcionalização de grupos químicos. Os sistemas mais estáveis de fosforeno negro e azul funcionalizados com dois grupos carboxílicos apresentaram 2,34 e 2,72 eV, respectivamente, para a energia de ligação. Estes sistemas apresentam maior estabilidade devido ao fato de que restabelecem a simetria do sistema, em comparação com as outras configurações. Efeitos semelhantes ocorrem para todos os grupos químicos funcionalizados. Estes resultados indicam uma promissora aplicação das estruturas de fosforeno negro e azul como sistemas de adsorção de moléculas de interesse químico e/ou biológico.

Two dimensional (2D) materials have unique properties that make them exciting candidates for various optical and electronic applications. Materials such as graphene and transition metal dichalcogenides (TMDCs) have been intensively studied recently with researchers racing to show advances in 2D device performance while developing a better understanding of the material properties. Despite recent advances,there are still significant roadblocks facing the use of 2D materials for real-world applications. The ability to make reliable, low-resistance electrical contact to TMDCs such as molybdeum disulfide (MoS22) has been a challenge that many researchers have sought to overcome with novel solutions. The work laid out in this dissertation uses novel techniques for addressing these issues through the use of improved device fabrication and with a clean, and potentially scalable doping method to tune 2D material properties.A high-performance field-effect transistor (FET) was fabricated using a new device platform that combined graphene leads with dielectric encapsulation leading to the highest reported value for electron mobility in MoS2. Device fabrication techniques were also investigated and a new, commercially available lithography tool (NanoFrazor) was used to pattern contacts directly onto monolayer MoS2. Through a series of control experiments with conventional lithography, a clear improvement in contact resistance was observed with the use of the NanoFrazor. Plasma-doping, a dry and clean process, was investigated as an alternative to traditional wet-chemistry doping techniques. In addition to developing doping parameters with a chlorine plasma treatment of graphene, a series of experiments on doped graphene were conducted to study its effect on optical properties. Whereas previous studies used electrostatic gating to modify graphene’s optical properties, this work with plasma-doped graphene showed the ability to tune absorbence and plasmon wavelength without the need for an applied bias opening the door to the potential for low-power applications. This work is a just small contribution to the larger body of research in this field but hopefully represents a meaningful step towards a greater understanding of 2D materials and the realization of functional applications.

The development of efficient and clean energy conversion technologies is a key issue for the sustainability of energy technologies. Hydrogen is one of the best fuels for clean energy systems as its combustion product is only water. Therefore, the development of cost-effective energy conversion technologies for hydrogen generation is significant. Electrocatalytic water splitting using renewable energy is one of the best ways to obtain high-purity hydrogen and the process emits no carbon dioxide. Electrocatalytic reactions occur on the surface of electrode materials. Consequently, understanding and tuning the surface properties of electrode materials is a key aspect in the design and preparation of efficient electrocatalysts. Compared to other nanomaterials, such as nanowires or nanoparticles, the most important two features of two-dimensional (2D) nanomaterials for electrocatalysis are their tunable and uniformly exposed lattice plane and unique electronic state. This Thesis aims to synthesize and optimize novel 2D nanomaterials for the study of hydrogen-related electrocatalytic reactions. Our target reactions include the hydrogen evolution reaction (HER) and the nitrogen reduction reaction (NRR), which have great potential in hydrogen-related clean energy conversion systems. The first two chapters provide a systematic review of the development of 2D nanomaterials for electrocatalysis. The unique advances of 2D electrocatalysts are discussed based on different compositions and functions followed by specific design principles. Following this, various 2D electrocatalysts for a series of electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle are discussed from their fundamental conception to their functional application. A significant emphasis is placed on various engineering strategies for 2D nanomaterials and their influence on intrinsic material performance, such as electronic properties and adsorption energetics. The first part of this Thesis focuses on the understanding of alkaline HER mechanism using 2D electrocatalyst as the platform. So far, the mechanistic understandings of alkaline HER are inapplicable to highly active nanostructured catalysts in practice. This is because most of nanostructured catalysts have complicated active sites, which cannot be identified carefully using theoretical calculations. Compared to other nanomaterials, 2D nanomaterials have uniformly exposed lattice plane which is considered as the ideal platform for the investigation of electrocatalytic reactions. Consequently, various 2D electrocatalysts with tunable active sites were synthesized and studied via advanced experimental measurements and theoretical calculations. First, a hybrid material of 2D C3N4@MoN was prepared using an interface engineering strategy. The intimate interaction of both inert C3N4 and MoN surfaces induced a highly active interface with tunable dual-active sites for alkaline HER. The enhanced activity originates from the synergy between the optimized hydrogen adsorption energy on the g-C3N4 sites and enhanced hydroxyl adsorption energy on the MoN sites. Second, atomically thin nitrogen-rich nanosheets, Mo5N6, were synthesized using a Ni-induced growth method. The 2D Mo5N6 nanosheets exhibit high HER activity and stability in natural seawater, which were superior to other TMNs and even the Pt benchmark. The superior performance of the nitrogen-rich Mo5N6 nanosheets originates from their Pt-like electronic structure and the high valence state of Mo atoms. Thirdly, a multi-faceted heteroatom-doping method (nitrogen, sulfur, and phosphorus) was applied to tune the electronic structure and HER activity of non-noble metals (Ni and Co) 2D layer directly and continuously without changing their chemical composition. The dopant-induced charge redistribution in the Ni metal 2D layer significantly influences its catalytic performance for the HER in alkaline media. The principle that bridges the dopant effect with the resultant HER activity is visualized with a volcano relationship. The second part of the thesis focuses on the exploration and synthesis of new 2D layered transition metal nitrides (TMNs) for hydrogen-related energy conversion. Firstly, the 2D layered W2N3 nanosheets with nitrogen vacancies was successfully obtained for the NRR. In this work, a new 2D layered W2N3 nanosheet was syntheiszed and the nitrogen vacancies demonstrate activity for electrochemical NRR A series of ex-situ synchrotron based characterizations show that the nitrogen vacancies in the 2D W2N3 nanosheets are stable due to the high valence state of the tungsten atoms and 2D confinement effect. Density function theory calculations suggest that the nitrogen vacancies provide an electron-deficient environment which facilitate nitrogen adsorption and lower the thermodynamic limiting potential of the NRR. Secondly, alkali molten salts were employed as the catalyst to explore and synthesize a new family of 2D layered TMNs under atmospheric pressure. The resultant 2D layered TMNs show superior performance for the HER with small overpotentials of 129 mV and 122 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4 and 1 M KOH, respectively. This level of performance surpasses most of the 2D layered electrocatalysts reported in the literature. They also exhibit excellent oxidation resistance and film-forming properties for practical applications. At last, the challenges and perspectives of 2D nanomaterials for electrocatalysis were discussed. The novel 2D nanomaterials demonstrate great potential for energy-relevant electrocatalytic processes such as HER and NRR. By rationally modifying the surface property and electronic structure at atomic level, the 2D nanomaterials can be extended to more research areas. Moreover, insightfully unveiling the reaction mechanisms of electrocatalysis can lay a solid foundation for designing more efficient 2D electrocatalysts.
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 2020

The utilisation of renewable solar energy shows great potential in tackling the problem of increasing carbon emission due to the combustion of traditional fossil fuels. To achieve carbon neutrality, more efforts need to be made on the exploration of efficient conversion of solar energy into chemical fuels/feedstocks. Particularly, chemical fuels with high energy density are ideal for storage and transportation. For the efficient transformation of solar energy into chemical fuels, high-performance photocatalysts are required to facilitate this process. Therefore, this thesis aims to find out a universal strategy for designing and fabricating novel nanomaterials as efficient photocatalysts for photocatalytic reactions. Besides, photocatalysts for emerging reactions fabricated via advanced delicate techniques and demonstrated for widespread applications are also reviewed and discussed comprehensively. Thanks to the ultrathin layered structure and exposed uncoordinated atoms, the exposed edges of 2D nanomaterials have shown great potential in acting as reactive sites for various photocatalytic reactions. In this thesis, two-dimensional Co-MOF, Ni-MOF and FePS3 (discussed in chapters 3-5) have been introduced to cooperate with the main photocatalysts for hydrogen evolution and it turns out that they have significantly improved the initial catalytic performances of the counterparts without them. These twodimensional nanomaterials play key roles in improving the photocatalytic activity of the main photocatalyst by providing sufficient reactive sites and facilitating charge separation/transfer. Also, the recent research progress of solar-driven simultaneous production of hydrogen and value-added chemicals (chapter 2), and the single-atom-based photocatalysts for emerging reactions (chapter 6) have been reviewed and discussed in this thesis. The combination of searching for high-performance photocatalysts and adapting for emerging reactions will promote the transformation and utilisation of solar energy. Probing into the origin of the structure-performance relationship and finding out a universal strategy for designing and screening outstanding high-performance photocatalysts for various reactions is of great importance for research on solar energy transformation. Meanwhile, finding approaches to improving solar-driven reaction efficiency will be of great benefit to the development of the solar energy industry as well.
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 2022

碩士
國立成功大學
光電科學與工程學系
104
Researchers have examined recent developments in research on optical features of Raman spectra and Photoluminescence (PL) spectra among several two-dimensional materials and nanomaterials. Such research could be applied to realize surface plasmon transport in AAO film. Others have focused on strain-induced electronic structure change, polarization-dependent studies on Molybdenum disulfide (MoS2). Laser annealing on AAO film and electric properties of MoS2 transistor with gate voltage were relatively unexplored. Thus, the paper studied the evolution of PL spectra as a function of laser annealing power on AAO film and the Raman profiles with different top gate voltage on MoS2 transistor, especially for direct-current (DC) and alternating-current (AC) applied gate voltage. The findings of PL spectra and Raman spectra were categorized and quantified in order to ascertain the optical mechanisms of the samples under distinct external field effects. The results revealed that the annealed AAO film with less anion impurity content due to the pyrolysis, resulting in weaker attraction of Al^(3+) to form Al2O3, as a function of annealing power. The broadening of linewidth for A1g mode was a result of strengthening of electron-phonon coupling with increased gate voltage (Vg). Peak intensity increased slightly with AC gate frequency, from 0 MHz to 5 MHz, and decreased to 20 MHz.

Breath based Diagnostics (BbD) can enable a paradigm shift in the Point-of-Care Diagnostic (PoCD) devices. Exhaled human breath has been demonstrated to contain over 2000 volatile organic and inorganic compounds, some of which report marked change in concentration under diseases conditions. A sensitive, selective, cost effective and portable gas sensing system could thus non-invasively diagnose multiple diseases from a single breath sample. However, there is a need to develop highly sensitive gas sensors with very low limit of detection (LLoD) down to ppb to ppt and high selectivity to meet this requirement. This thesis focuses on developing such gas sensors based on novel 2D nanomaterials and their hybrids while using a simple, scalable synthesis route. This is in contrast to the conventional choice of sensing materials (Metal Oxides, polymers, CNT’s etc.) and expensive fabrication methods. Here, we explored layered materials namely Transition Metal Dichalcogenides (TMDC) and Layered Transition metal oxides (TMO) and their hybrids for the detection of Ammonia (NH3), Hydrogen Sulphide (H2S) and Nitrogen Dioxide (NO2), three important constituents of exhaled breath. The synthesis of these layered materials was carried out at room temperature via the liquid phase exfoliation (LPE) technique using low boiling point solvents. This technique is attractive because it is simple, scalable and does not require sophisticated instrumentation. The key findings from this work can be summarized as follows. Layered Transition metal oxide (TMO) namely 2D MoO3 based devices demonstrated reasonable response to NH3 at room temperature but only down to 300 ppb which was not sufficient for our intended application. Further, we observed that the layered TMD’s WS2, WSe2 and its hybrid with Fe3O4 demonstrate remarkable ammonia sensing. WS2 demonstrated high sensitivity towards NH3 (detection down to 50 ppb) with fair selectivity but at an elevated operating temperature of 250oC. On the other hand, WSe2/fe3O4 hybrid-based devices demonstrated enhanced sensitivity and selectivity towards ammonia, that too at room temperature, with a 50 ppb LLoD. Another notable observation was the similar response of pristine WSe2 nanosheets towards NO2 as NH3. Hence, we enhanced the NO2 sensing performance of WSe2 based sensors by functionalizing their surface with noble metals such as Au and Pt using a simple wet chemical route. Interestingly, we obtained highly sensitive (down to 100 ppb) and selective response towards NO2 at room temperature. More importantly, the complete recovery to the original baseline without any external energy source was remarkable since it is known to be challenging. While exploring other inorganic TMO’s, we observed that 2D V2O5 based devices detect H2S non-selectively at 350oC and down to only 500 ppb. Further improvement in H2S sensing is helped by TMD’s again as we modified the surface of WS2 in such a manner that it suppressed NH3 sensing, by using low temperature microwave irradiation assisted synthesis technique. Thus, it demonstrated highly selective, sensitive, and prompt H2S detection, though at an elevated temperature of 250oC. Later, we observed that a novel material of this same class (1T-TiS2) could provide similar attributes at room temperature. This material was not investigated before for gas sensing; hence we conducted a theoretical study and presented a plausible mechanism based on vdW interaction, substantiating physisorption between adsorbate and adsorbent. Thus, this thesis investigates novel materials, hybrids, and methods for scalable production of ultrasensitive, selective, stable, and low-cost sensors for NH3, H2S and NO2, which can potentially find applications for field-usable breath-based diagnostics in the future
MHRD, DEITY, DST Nanomission through NNeTRA

Graphene, two dimensional lattice of covalent bonds of carbon atoms, has been studied as a prospective new material for the next generation. Pristine graphene, mechanically exfoliated graphene from graphite, has gained much attention due to its outstanding properties: conductivity, permeability, transparency, and mechanical stability. While pristine graphene has shown great promise as an innovative new material, the limitations from the randomness of sizes and domains are challenging for uniform mass production. In this dissertation, we present graphene produced by chemical vapor deposition (CVD) synthesis for producing designated sizes and domains. In order to prospect the utilization, the mechanical stability of CVD graphene should be determined. We first present mechanical properties of CVD graphene. Introducing transfer method, we present how to minimize damages on graphene during the fabrication. For the measurement of mechanical properties of CVD graphene, we introduce nanoindentation test with AFM and nanoindenter. Experimental results are demonstrated by the results of FEA analysis on the basis of nonlinear elastic behaviors. Through the experiment and simulation, we verify the ultra-high mechanical strength of CVD graphene. We also present defect-engineered graphene for the utilization. To determine the change of the status of defects on pristine graphene, we employed plasma etching to induce defects gradually. Through the observation of change of defects from sp3 type to sp2 type on pristine graphene, we understand how the phase changes depending on defects. Using nanoindentation, the mechanical strength of defective graphene is determined and we discuss its utilization based on the mechanical stability. We next exploit grains and grain boundaries of polycrystalline graphene. Transmission electron microscope (TEM) is used for precise observation of suspended membrane with grains and grain boundaries. Applying the same nanoindentation test, we compare the values of grain boundaries to pristine lattice in order to determine how grains and grain boundaries affect the ultra-high mechanical properties of graphene as defects. We finally present angular dependence of the mechanical properties of grains and grain boundaries. Although previous research reported the angular dependence of graphene regarding its mechanical strength, it was questionable that tilt angles among grains could not affect mechanical strength based on our previous experimental data. Therefore, here we reveal that how tilt angles among grains affect the mechanical properties. Furthermore, we investigate the crack propagation at rupture of graphene in both nanoindentation and e-beam exposure. Hence, we conclude the dissertation by a discussion of directions for future work, proposing well-stitched condition of graphene, and HR TEM for the verification of real structure of grain boundaries to apply into simulation. Therefore, this thesis is an arrangement of the outstanding mechanical properties of graphene from pristine graphene to CVD graphene in both small grain and large grain type, and from macroscopic region of interests over suspended membrane to microscopic observation such as the mechanical behaviors of grains and grain boundaries.

abstract: Liquid-phase exfoliation (LPE) is a straightforward and scalable method of producing two-dimensional nanomaterials. The LPE process has typical been applied to layered van der Waals (vdW) solids, such as graphite and transition metal dichalcogenides, which have layers held together by weak van der Waals interactions. However, recent research has shown that solids with stronger bonds and non-layered structures can be converted to solution-stabilized nanosheets via LPE, some of which have shown to have interesting optical, magnetic, and photocatalytic properties. In this work, two classes of non-vdW solids – hexagonal metal diborides and boron carbide – are investigated for their morphological features, their chemical and crystallographic compositions, and their solvent preference for exfoliation. Spectroscopic and microscopic techniques are used to verify the composition and crystal structure of metal diboride nanosheets. Their application as mechanical fillers is demonstrated by incorporation into polymer nanocomposite films of polyvinyl alcohol and by successful integration into liquid photocurable 3D printing resins. Application of Hansen solubility theory to two metal diboride compositions enables extrapolation of their affinities for certain solvents and is also used to find solvent blends suitable for the nanosheets. Boron carbide nanosheets are examined for their size and thickness and their exfoliation planes are computationally analyzed and experimentally investigated using high-resolution transmission electron microscopy. The resulting analyses indicate that the exfoliation of boron carbide leads to multiple observed exfoliation planes upon LPE processing. Overall, these studies provide insight into the production and applications of LPE-produced nanosheets derived from non-vdW solids and suggest their potential application as mechanical fillers in polymer nanocomposites.
Dissertation/Thesis
Doctoral Dissertation Chemistry 2020

Copper (Cu) has been used as the main conductor in interconnects due to its low resistivity. However, because of its high diffusivity, diffusion barriers/liners (tantalum nitride/tantalum; TaN/Ta) must be incorporated to surround Cu wires. Otherwise, Cu ions/atoms will drift/diffuse through the inter-metal dielectric (IMD) that separates two distinct interconnects, resulting in circuit shorting and chip failures. The scaling limit of conventional Cu diffusion barriers/liners has become the bottleneck for interconnect technology, which in turn limits the IC performance. The interconnect half-pitch size will reach ~20 nm in the coming sub-5 nm technology nodes. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be > 4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultra-scaled interconnect will be compromised by the thick bilayer. Therefore, two dimensional (2D) layered materials have been explored as diffusion barrier alternatives owing to their atomically thin body thicknesses. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued.

The objective of the thesis is to develop a 2D barrier/liner that overcomes the issues mentioned. Therefore, we first visit various 2D layered materials to understand their fundamental capability as barrier candidates through theoretical calculations. Among the candidates, hexagonal-boron-nitride (h-BN) and molybdenum disulfide (MoS₂) are selected for experimental studies. In addition to studying their fundamental properties to know their potential, we have also developed techniques that can realize low-temperature-grown 2D layered materials. Metal-organic chemical vapor deposition (MOCVD) is adopted for the synthesis of BEOL-compatible MoS₂. The electrical test results demonstrate the promises of integrating 2D layered materials to the state-of-the-art interconnect technology. Furthermore, by considering not only diffusion barrier properties but also liner properties, we develop another 2D layered material, tantalum sulfide (TaS_x), using plasma-enhanced chemical vapor deposition (PECVD). The TaS_x is promising in both barrier and liner aspects and is BEOL-compatible. Therefore, we believed that the conventional TaN/Ta bilayer stack can be replaced with an ultra-thin TaS_x layer to maximize the Cu volume for ultra-scaled interconnects and improve the performance. Furthermore, Since via resistance has become the bottleneck for overall interconnect performance, we study the vertical conduction of TaS_x. Both the intrinsic and extrinsic properties of this material are investigated and engineering approaches to improve the vertical conduction are also tested. Finally, we explore the possibilities of benefiting from 2D materials in other applications and propose directions for future studies.

碩士
中原大學
化學工程研究所
107
In recent years, there are many studies focus on two-dimensional materials due to their excellent properties, such as high stability, high conductivity and good electronic properties. Since the interactions between the layers of the bulk material are determined by van der Waals forces, they can be exfoliated easily. Moreover, few-layer two-dimensional materials always have better properties compared to bulk. In our study, a green, facile, low-cost and scalable liquid exfoliation method using jet cavitation was employed to rapidly exfoliate few-layered two-dimensional nanomaterials. We choose the hexagonal boron nitride(h-BN) which has high thermal conductivity, and tungsten diselenide(WSe2), type of transition metal dichalcogenides (TMDs) as our precursors. We successfully prepared few-layered h-BN(FL-hBN) and few-layered WSe2(FL-WSe2) by using jet cavitation. We then applied FL-hBN and FL-WSe2 filler for silicone and as anode materials for lithium-ion battery, respectively. The structure of hexagonal boron nitride is similar to that of graphite. h-BN has many advantages such as exceptional electrical insulation, excellent thermal conductivity and chemical stability, ultralow dielectric constant, and a negative thermal expansion coefficient. We prepared few-layered FL-hBN and analyzed its thickness and the atomic force microscopy (AFM) revealed the average thickness of FL-hBN is about 4.1 nm. Moreover, the specific surface area of FL-hBN is higher than that of h-BN. The h-BN and the as-synthesized FL-hBN were applied as fillers in silicone composite. At a loading of 30 wt.% FL-hBN and h-BN, the thermal conductivities of silicone composites were enhanced by 230% and 189%, respectively. In addition, the composites containing FL-hBN also possessed excellent thermal stability. WSe2 is a graghene-like material. Monolayer WSe2 is a semiconductor material with direct bandgap of ~1.2 eV, it has valuable application in optoelectronic devices. According to atomic force microscopy (AFM), thickness of FL-WSe2 is 9.9 nm. The specific surface area of FL-WSe2 is also higher than that of WSe2. WSe2 and FL-WSe2 were applied as anode for lithium ion battery. The results show that the reversible capacity of FL-WSe2 is 288.6 mAh/g at a current density of 10C, when cycled back to 0.1C, the electrode was able to regain an average capacity of 508.3 mAh/g. The reversible capacity of FL-WSe2 is higher than WSe2 at all current density. We also obtained a reversible capacity of 435.3 mAh/g of FL-WSe2 after 80 cycles. These results show the FL-WSe2 battery possess high reversible capacity performance. In this study, few-layered two-dimensional nano-materials were successfully prepared. They have excellent performance for heat dissipation and lithium-ion battery applications.

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.

Preface Ever since the discovery of graphene in 2004, there has been considerable interest in two dimensional (2D) nanomaterials due to their distinctive properties and the prospect of potential applications. A 2D-nanomaterial may be obtained from the bulk layered material by procedures that can overcome the van der Waals attractive force that hold adjacent layers together. Historically this was first achieved by micro-mechanical cleavage by the deceptively simple procedure of peeling atomically thin single layers from the bulk material using scotch tape. The procedure, unfortunately, is not scalable and consequently alternate procedures, both top-down as well as bottom-up, have been extensively explored. One of the simplest methods to obtain defect-free 2D nanosheets is the sonication assisted liquid phase exfoliation of the bulk layered material in a suitable solvent. The role of the solvent is crucial to the liquid phase exfoliation process, as the formation of stable dispersions require that the exfoliated sheets, produced on sonication, be prevented from re-aggregating. A wide range of solvents, solvent mixtures and surfactant solutions have been investigated and solvent systems that favour formation of stable dispersions identified. Much of the current understanding of the role of the solvent is based on phenomenological models, matching surface energies of the solvent and the layered material so as to minimize the surface tension between the two. What has remained elusive, however, is a molecular perspective of the nature of interactions between the solvent and the exfoliated nanosheet. This the focus of the present study. This thesis reports results of investigations on dispersions of graphene in aqueous and non-aqueous media as well as dispersions of boron nitride in water using solution and solid-state Nuclear Magnetic Resonance (NMR) spectroscopy aided by Molecular Dynamics (MD) simulations for interpreting the experimental observations. The thesis is organized as five chapters with Chapter 1 providing a brief overview of 2D nanomaterials with focus on graphene and boron nitride (BN); their properties and applications. The chapter discusses the methods for obtaining graphene and BN nanosheets with emphasis on the sonication assisted liquid phase exfoliation approach. The chapter also provides a brief review of the phenomenological models that have been advanced to understand the stability of dispersions of 2D nanomaterials in different solvents. The stability of the nanosheet dispersions require that solvent or ligand molecules be in close association with the nanosheets with properties and mobilities quite different from those of the bulk solvent molecules. The challenge for in-situ measurements is to be able to probe the bound or associated solvent/ ligand molecules in the presence of a large excess of the bulk. NMR methods from the solution chemists toolbox are known to provide methodologies that can distinguish bound ligand molecules from those in the bulk and are, therefore, ideal techniques for investigating nanosheet dispersions. In particular transfer Nuclear Overhauser Effect Spectroscopy (tr-NOESY) as well as Rotating-frame Overhauser Effect Spectroscopy (ROESY) are well suited for systems where bound and free solvent/ ligand molecules are in continuous exchange. This chapter also provides a brief overview of the NMR experiments used in studying nanosystem-ligand interactions. The results from NMR measurements provide a spectroscopic signature of solvent nanosheet interactions in the dispersions and in conjunction with Molecular Dynamics (MD) simulations provide a molecular level understanding of the stability of the dispersions and the role of the solvent. The MD simulation methodology used in this study are discussed in Chapter 2 along with the computational tools employed in the thesis. Graphene is perhaps the most studied 2D nanomaterial and its distinctive properties has paved the way for the commercial use of graphene-based materials in a variety of applications. Sonication of bulk graphite in an organic solvent or aqueous surfactant solutions has been considered a simple and scalable route for the production of defect free graphene nanosheets. In aqueous solutions the interaction of surfactant chains with the graphene sheets is crucial to the stability of the dispersion. In Chapter 3, 1H two-dimensional Nuclear Overhauser Effect spectroscopy (NOESY) and classical MD simulations have been used to probe these interactions in graphene dispersions stabilized by the cationic surfactant cetyltrimethylammonium bromide (CTAB). It is shown from the presence of intense negative transfer-NOESY cross peaks that the surfactant chains are quasi-bound to the graphene sheets and undergo rapid exchange with free surfactant ligands present in the dispersion. A surprising feature of the NOESY is the presence of cross-peaks between groups that are separated by more than 5_A along the chain even between protons of the `head' group of the CTA surfactant chain and protons of the `tail' methyl group. This observation of apparent very short separation of protons of distal groups of the surfactant chain corroborated reects the arrangement adopted by the surfactant chains in the quasi-bound state in the dispersion. Classical MD simulations of the dispersion provides a simple interpretation of these observations. The simulations show that surfactant CTA chains lie at on the graphene sheets adopting a random arrangement with the head of one chain in close proximity to the tail of another chain. This arrangement can give rise to cross peaks in the NOESY between groups that are apparently far separated along the chain. One of the most efficient organic solvents for the sonication assisted liquid phase exfoliation of graphite is N-methyl-2-pyrrolidone (NMP). Much of the success of phenomenological models based on surface energies has been correctly predicting that NMP would be good solvent because its surface energy and that of graphite are comparable. A molecular level understanding of the interaction of NMP and graphene sheets in the dispersion is, however, not available. In Chapter 4, it is shown that NMR methods can provide a spectroscopic signature for these interactions. The 2D ROESY NMR shows significant differences in the spectra of graphene dispersions in NMP and the pure solvent. MD simulations of a graphene sheet immersed in NMP solvent molecules show that these differences arise because of induced layering of solvent molecules in the vicinity of the sheet. The arrangement facilitates lowering of the rotational correlation time of the NMP molecules near the surface of the graphene sheet that are easily captured in the experimental two-dimensional ROESY NMR and which manifests as enhanced cross-peak intensities as compared to the bulk solvent. Among the graphene analogues boron nitride nanosheets has been considered the closest because of the similarities in structures of hexagonal BN and graphite as well as the positions of the respective elements in the periodic table. Their aqueous dispersibilities are, however, very different. While graphene does not exfoliate or form stable dispersions in water the hydrophobic BN forms stable dispersions on sonication in water, without the need for surfactants or stabilizers. In Chapter 5, it is shown from zeta potential measurements that the sheets are positively charged and the stability of the dispersions are electrostatic in origin. The observations indicate that BN reacts with water on sonication. Ab initio (Car-Parinello) MD simulations and reactive force-field (ReaxFF) MD simulations were performed to understand the reactivity, and the origin of the stability of the aqueous dispersions of BN. The simulations showed that water molecules dissociate at the edges of the BN sheets leading to the to the formation of NH bonds with the release of OH into the bulk. The simulations explain why the dispersions are basic and the exfoliated BN nanosheets in the dispersion positively charged. 1H and 11B solid-state NMR spectroscopy were used to identify the chemical species as predicted by the MD simulations. The combination of MD simulations and NMR measurements are able to provide a comprehensive understanding of the origin of the aqueous dispersibility of the hydrophobic BN nanosheets. The results are summarized in Chapter 6.

Phosphorene is a new member of the family of two-dimensional (2D) materials. Compared with other 2D materials, phosphorene exhibits a layer-dependent direct bandgap in the range of mid-infrared to near-infrared wavelengths, which can bridge the gap between relatively large bandgap transition metal dichalcogenide semiconductors and gapless graphene. The predicable direct bandgap in few-layer phosphorene will facilitate the delivery of high-performance optoelectronic devices. Besides, the high surface-volume ratio in few-layer phosphorene enables strong light-matter interactions, making phosphorene very promising for applications in different optical components. In this work, the inelastic and elastic light-matter interactions in few-layer phosphorene were investigated. For the study of inelastic light-matter interactions in phosphorene, photoluminescence (PL) and Raman effects were investigated to explore photon-electron and photon-phonon energy transfers respectively. Strong and highly layer-dependent PL was observed in few-layer phosphorene (2-5 layers). The results confirmed the theoretical prediction (by other researchers) that few-layer phosphorene had a direct and layer-sensitive bandgap. The work also demonstrated by Raman scattering that few-layer phosphorene was more sensitive to temperature modulation than graphene and MoS2, which could be due to the superior mechanical flexibility of phosphorene originating from its unique puckered crystal structure. The anisotropic Raman response in few-layer phosphorene enabled the use of a pure optics method to quickly determine crystalline orientation without a tunnelling electron microscope or a scanning tunnelling microscope. The results obtained provided much-needed experimental information about the band structures and exciton nature in few-layer phosphorene, paving the way for various optoelectronic and electronic applications. Further study in terms of inelastic light-matter interaction was undertaken regarding PL emission from excitons and trions in few-layer phosphorene. Electrostatic modulation was applied to tune the density of trions. Thereby, a trion binding energy of 162 meV was found in few-layer phosphorene at room temperature. Such a large binding energy had previously only been observed in truly 1D materials such as carbon nanotubes, whose optoelectronic applications had been severely hindered by their intrinsically small optical cross-sections. Phosphorene offers an elegant way to overcome this hurdle by enabling quasi-1D excitonic and trionic behaviours in a large 2D area, allowing optoelectronic integration. Moreover, the quasi-1D nature of excitonic and trionic dynamics in phosphorene was validated experimentally and theoretically. The implications of the extraordinarily large trion binding energy in a higher-than-one-dimensional material are far-reaching. In addition to inelastic light-matter interactions, elastic light-matter interactions in few-layer phosphorene were also studied by optical path length (OPL) and micro-lens. The giant OPL achieved in few-layer phosphorene was more than 20 times the physical thickness achieved in this few-layer phosphorene. Based on the layer-dependent OPL information, the number of layers could be more quickly and accurately identified compared with the conventional method for identifying number of layers. Black-phosphorus-based micro-lens optical properties were also studied to obtain information about elastic light-matter interactions in black phosphorus and its oxides.

Surfaces and interfaces are of fundamental importance from the nucleation to growth of crystals formed under different conditions such as vapor phase, liquid phase including biomineralisation conditions. Recently there is lot of interest in controlling the shape of nanoparticles during the synthesis due to their excellent shape dependent properties. Understanding the role of surfaces and interfaces is vital for such shapecontrolled synthesis of nanomaterials. On the surface, coordination number, structure, density and composition are different from that of bulk and hence the properties are completely different in the surfaces and interfaces of any crystalline material. Especially when the length scale become nanoscale, the surface and interface play a dominant important role and leads to several new and interesting phenomena. In this dissertation, the role of surfaces and interfaces on the synthesis and the properties of inorganic functional nanostructures have been studied. The work primarily relies on basic chemistry to synthesize nanostructures that brings the importance of surfaces/interfaces into the picture. Though several basic characterization techniques have been used, electron microscopy has been the emphasis and has been used extensively through the work to probe and explore the materials for characterizing the structures over a variety of length scales. The entire thesis based on the results and findings obtained from the present investigation are organized as follows: Chapter1 gives a general introduction to the surfaces and interfaces to create a background for the investigation. This emphasizes the role of surfaces and interfaces in several aspects starting from nucleation, growth to the properties of inorganic crystals. It gives some exposure in to the different type of surface phenomenon which is common in nanoscale materials. Chapter 2 deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter 3 deals with predicting the morphology of 2D nanostructures by combining the crystal growth theory into chemical thermodynamics. Morphology diagrams have been developed for Au, Ag, Pt and Pd to predict conditions under which two-dimensional nanostructures form as a result of a chemical reaction. In addition, it provides the general understanding of shape control in 2D nanostructures with atomistic mechanism. The validity of the morphology diagram has been tested for various noble metals by carrying out critical experiments. As a result, 2D nanostructures of metals projecting the lowest energy facet resulted in a complete novel way in the absence of any capping/reducing agents. Chapter 4 deals with predicting the formation of 2D nanostructures of inorganic crystals formed as a result of precipitation reaction. Morphology diagram has been developed for the case of hydroxyapatite, an inorganic part of the human bone. This answers some of the long standing question related to the shape of the HA crystals formed in the bone by biomineralisation. The generality of the method has been tested to few other inorganic crystals such as CaCO3, ZnO and CuO formed through precipitation reaction. The key finding of the above two chapter is that the low driving force of the chemical reactions results in two dimensional nanostructures. On contrary, high chemical driving force combined with the optimum zeta potential results in porous aggregate of nanoparticles. Chapter 5 discusses the formation of porous clusters of metals and ceramics at specific conditions. The mechanism behind the formation of monodisperse aggregates are investigated based on the interaction energies of nanoparticles in aqueous medium. This chapter reveals the role of surface charge and the surface energy in controlling the stability of nanoparticles in aqueous medium. In addition, it provides the simple methodology to produce well controlled porous clusters by exploiting the competition between surface charge and surface energy during the aggregation. The application of the porous clusters of Pt has been tested for methanol oxidation which is essential for fuel cell applications. Chapter 6 deals with the development of porous biphasic scaffolds through the morphology transition of nanorods. Rod shape is not stable when subjected to high temperature due to instability and spherodisation takes place to minimize the surface energy. Here in this chapter, by exploiting spherodisation along with the phase transition, highly interconnected porous structure of hydroxyapatite and tricalcium phosphate is achieved. Combined with the morphology transition, by adding naphthalene as a template, the possibility of achieving hierarchical porous structure also presented. The mechanical strength of the biphasic porous scaffold has been tested by microindentation. Mechanical properties of apatite are generally poor and there are lots of efforts to improve the mechanical properties apatite by the composite approach. Chapter 7 deals with the HA-Alumina and HA-TCP composites. In spite of much attention given to the mechanical properties of the composites, the interfacial phenomenon that takes place between the components of the nanocomposite has not been studied in detail. In the present study, interfacial reactions in hydroxyapatite-alumina nanocomposites have been investigated and new reaction mechanism also proposed. The degradation of densification process has been observed for the HATCP composites due to the creation of porous interface between HA crystals and TCP matrix. Mechanical properties of these two composites have been studied using microindentation. The mechanical properties of HA and TCP single crystals are important for developing the biphasic composites with reliable mechanical properties. Chapter8deals with the mechanical behavior of hydroxyapatite and tricalcium phosphate single crystals. The mechanical properties of HA and TCP have been studied by performing nanoand microindentation on specific crystallographic facets. In case of hydroxyapatite, the anisotropy in mechanical properties has been explored by performing indentation on its prism and basal planes. Nanoscale plasticity is observed in both HA and TCP crystals which arise due to the easy movement of surface atoms with lesser coordination compared to the bulk. Nanoindentation has been performed in the calciumdeficient HA platelets provides important clues about the role of calcium deficiency on the mechanical behavior of bone and has implications for the properties of osteoporotic bones.

Surfaces and interfaces are of fundamental importance from the nucleation to growth of crystals formed under different conditions such as vapor phase, liquid phase including biomineralisation conditions. Recently there is lot of interest in controlling the shape of nanoparticles during the synthesis due to their excellent shape dependent properties. Understanding the role of surfaces and interfaces is vital for such shapecontrolled synthesis of nanomaterials. On the surface, coordination number, structure, density and composition are different from that of bulk and hence the properties are completely different in the surfaces and interfaces of any crystalline material. Especially when the length scale become nanoscale, the surface and interface play a dominant important role and leads to several new and interesting phenomena. In this dissertation, the role of surfaces and interfaces on the synthesis and the properties of inorganic functional nanostructures have been studied. The work primarily relies on basic chemistry to synthesize nanostructures that brings the importance of surfaces/interfaces into the picture. Though several basic characterization techniques have been used, electron microscopy has been the emphasis and has been used extensively through the work to probe and explore the materials for characterizing the structures over a variety of length scales. The entire thesis based on the results and findings obtained from the present investigation are organized as follows: Chapter1 gives a general introduction to the surfaces and interfaces to create a background for the investigation. This emphasizes the role of surfaces and interfaces in several aspects starting from nucleation, growth to the properties of inorganic crystals. It gives some exposure in to the different type of surface phenomenon which is common in nanoscale materials. Chapter 2 deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter 3 deals with predicting the morphology of 2D nanostructures by combining the crystal growth theory into chemical thermodynamics. Morphology diagrams have been developed for Au, Ag, Pt and Pd to predict conditions under which two-dimensional nanostructures form as a result of a chemical reaction. In addition, it provides the general understanding of shape control in 2D nanostructures with atomistic mechanism. The validity of the morphology diagram has been tested for various noble metals by carrying out critical experiments. As a result, 2D nanostructures of metals projecting the lowest energy facet resulted in a complete novel way in the absence of any capping/reducing agents. Chapter 4 deals with predicting the formation of 2D nanostructures of inorganic crystals formed as a result of precipitation reaction. Morphology diagram has been developed for the case of hydroxyapatite, an inorganic part of the human bone. This answers some of the long standing question related to the shape of the HA crystals formed in the bone by biomineralisation. The generality of the method has been tested to few other inorganic crystals such as CaCO3, ZnO and CuO formed through precipitation reaction. The key finding of the above two chapter is that the low driving force of the chemical reactions results in two dimensional nanostructures. On contrary, high chemical driving force combined with the optimum zeta potential results in porous aggregate of nanoparticles. Chapter 5 discusses the formation of porous clusters of metals and ceramics at specific conditions. The mechanism behind the formation of monodisperse aggregates are investigated based on the interaction energies of nanoparticles in aqueous medium. This chapter reveals the role of surface charge and the surface energy in controlling the stability of nanoparticles in aqueous medium. In addition, it provides the simple methodology to produce well controlled porous clusters by exploiting the competition between surface charge and surface energy during the aggregation. The application of the porous clusters of Pt has been tested for methanol oxidation which is essential for fuel cell applications. Chapter 6 deals with the development of porous biphasic scaffolds through the morphology transition of nanorods. Rod shape is not stable when subjected to high temperature due to instability and spherodisation takes place to minimize the surface energy. Here in this chapter, by exploiting spherodisation along with the phase transition, highly interconnected porous structure of hydroxyapatite and tricalcium phosphate is achieved. Combined with the morphology transition, by adding naphthalene as a template, the possibility of achieving hierarchical porous structure also presented. The mechanical strength of the biphasic porous scaffold has been tested by microindentation. Mechanical properties of apatite are generally poor and there are lots of efforts to improve the mechanical properties apatite by the composite approach. Chapter 7 deals with the HA-Alumina and HA-TCP composites. In spite of much attention given to the mechanical properties of the composites, the interfacial phenomenon that takes place between the components of the nanocomposite has not been studied in detail. In the present study, interfacial reactions in hydroxyapatite-alumina nanocomposites have been investigated and new reaction mechanism also proposed. The degradation of densification process has been observed for the HATCP composites due to the creation of porous interface between HA crystals and TCP matrix. Mechanical properties of these two composites have been studied using microindentation. The mechanical properties of HA and TCP single crystals are important for developing the biphasic composites with reliable mechanical properties. Chapter8deals with the mechanical behavior of hydroxyapatite and tricalcium phosphate single crystals. The mechanical properties of HA and TCP have been studied by performing nanoand microindentation on specific crystallographic facets. In case of hydroxyapatite, the anisotropy in mechanical properties has been explored by performing indentation on its prism and basal planes. Nanoscale plasticity is observed in both HA and TCP crystals which arise due to the easy movement of surface atoms with lesser coordination compared to the bulk. Nanoindentation has been performed in the calciumdeficient HA platelets provides important clues about the role of calcium deficiency on the mechanical behavior of bone and has implications for the properties of osteoporotic bones.