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In the present thesis, research work to tackle challenges such as large-scale integration, selectivity and low efficiency around different types of nanocarbon based sensors is performed. The findings of these studies are given in the form of peer-reviewed publications and conclusions with future recommendations proposed as a summary. The work focuses on three key sensors types, gas sensors, biosensors and photodetectors. The first key aspect is dielectrophoretic (DEP) deposition of nitrogen doped single-walled carbon nanotubes (N-SWCNTs) and it is used as a route to large-scale assembly of increased reactivity, and thus selectivity, gas sensors. Furthermore, suspended SWCNTs and few layer graphene (FLG) devices are fabricated through a novel process which results in increased surface area transducers and low resistance SWCNTs based devices. Moreover, biosensors face similar challenges to gas sensors with the addition that their selectivity needs to be engineered through the formation of a biomimetic interface due to the nature of the analytes they are destined to investigate. Non-covalent functionalization of graphene using self-assembled phospholipid membranes delivered in a controlled and precise manner by dip-pen nanolithography (DPN) was demonstrated together with a high-speed fabrication process of bioassays onto patterned CVD graphene using a parallel tips system. Lastly, for the case of photodetectors, a SWCNT – nanoplasmonic system is proposed as a solution to the major issue of low quantum efficiency in low dimensionality materials. First, the performance of various geometries and arrangements of Au nanoparticles is explored by transferring a micromechanically exfoliated graphene flake onto them and studying the Raman enhancement that arises due to uncoupled and coupled near-fields. An increase of graphene Raman signal of 103 was observed for the areas suspended between two closely spaced dimers as a result of strong near field coupling when the polarisation of the incident light is parallel to the nanostructures axis. A large-scale integration of SWCNTs positioned in between the dimers using DEP is performed as a demonstration of the scalability of the system.

Monolayers and heterostructures of two-dimensional (2D) electronic materials with spin-orbit interactions offer the promise of observing many novel physical effects. While theoretical predictions of 2D layered materials based on density functional theory (DFT) are many, the DFT approach is limited to small simulation sizes (several nanometers), and thus inhomogeneous strain and boundary effects that are often observed experimentally cannot be simulated within a reasonable time. The aim of this thesis is (i) to study effects of strain on 2D materials beyond graphene using first-principles and tight-binding methods and (ii) to investigate the effects of cuts--"kirigami"-- on 2D materials using molecular dynamics and machine learning approach. The first half of this thesis focuses on the effects of strain on manipulating spin and valley degrees of freedom for two classes of 2D materials--monochalcogenide and lead chalcogenide monolayers--using DFT. A tight-binding (TB) approach is developed to describe the electronic changes in lead chalcogenide monolayers due to strains that often persist in real devices. The strain-dependent TB model allows one to establish a relationship between the Rashba field and the out-of-plane strain or electric polarization from a microscopic view, a connection that is not well understood in the ferroelectric Rashba materials. This framework connecting strain fields and electronic changes is important to overcome the size and computational limitations associated with DFT. The second part of the thesis focuses on defect engineering and design of 2D materials via the "kirigami" technique of introducing different patterns of cuts. A machine learning (ML) approach is presented to provide physical insights and an effective model to describe the physical system. We demonstrate that a machine learning model based on a convolutional neural network is able to find the optimal design from a training data set that is much smaller than the design space.

This thesis describes a detailed study of advanced fibre optic sensors and their applications for label-free biochemical detection. The major contributions presented in this thesis are summarised below. A self-assembly based in-situ layer-by-layer (i-LbL) or multilayer deposition technique has been developed to deposit the 2D material nanosheets on cylindrical fibre devices. This deposition technique is based on the chemical bonding associated with the physical adsorption, securing high-quality 2D materials coating on specific fibre cylindrical surface with strong adhesion as well as a prospective thickness control. Then a " Photonic-nano-bio configuration", which is bioprobes immobilised 2D-(nano)material deposited fibre grating, was built. 2D material overlay provides a remarkable analytical platform for bio-affinity binding interface due to its exceptional optical and biochemical properties. EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) and NHS (NHydroxysuccinimide) were used to immobilise bioprobes. This kind of configuration is considered to have many advantages such as: enhanced RI sensitivity, enrich immobilisation sites, improved binding efficiency, selective detection. Followed by this configuration, several label-free biosensors were developed. For example, graphene oxide coated dual-peak long period grating (GO-dLPG) based immunosensor has been implemented for ultrasensitive detection of antibody/antigen interaction. The GO-LPG based biosensor has been developed for label-free haemoglobin detection. Apart from biosensors, the black phosphorus (BP) integrated tilted fibre grating (TFG) has been proposed, for the first time, as BP-fibre optic chemical sensor for heavy metal (Pb2+ ions) detection, demonstrating ultrahigh sensitivity, lower limit of detection and wider concentration range. Ultrafast laser micromachining technology has been employed to fabricate long period grating (LPG) and microstructures on optical fibre. The ultrafast laser micromachined polymer optical fibre Bragg grating (POFBG) has been developed for humidity sensing, showing the significant improvement with the reduced response time.

Graphene and other two-dimensional materials can be stacked together to form vander Waals heterostructures: synthetic crystals composed of different atomically thin layers with a bespoke electronic band structure. Structural characterisation of vander Waals heterostructures is difficult using conventional methods as the properties are almost entirely defined by the nature of the buried interfaces between dissimilar crystals. These methods also fall short of resolving the atomic structure of buried defects in van der Waals materials such as graphite. This work demonstrates the refinement and successful application of ion beam specimen preparation to produce cross sectional slices through these unique crystals so that they can be characterised by high resolution scanning transmission electron microscopy (STEM). Cross sectional specimen were prepared using in situ lift-out in a focused ion beam (FIB) dual-beam instrument. The fine polishing steps were optimised to prevent damage to the core of the specimen. High resolution STEM imaging of twin defects in graphene, hexagonal boron ni-tride and MoSe2 revealed that the boundaries are not atomically sharp but extended across many atoms. Advanced processing and analysis of these images uncovered fundamental mechanics which govern their geometry. This technique was further applied to complex transition metal dichalcogenide heterostructures to quantitatively determine the properties of buried interfaces between atomically thin crystals.

This thesis explores the use of 2D materials (graphene and hBN) for spintronics. Interest on these materials in spintronics arose from theoretical predictions of high spin filtering in out-of-plane transport through graphene and hBN sandwiched by ferromagnets. Similarly, 5-layer graphene was forecast to be a perfect spin filter. In the case of in-plane spin transport, graphene was expected to be an excellent material due to its low spin-orbit coupling and low number of defects. Although there already exist experimental works that attempted to explore the aforementioned predictions, they have failed so far to comply with the expected results. Earlier experimental works in graphene and hBN out-of-plane spin transport achieved low spin filtering on the order of a few percent; while spin relaxation parameters in graphene for in-plane spin transport remained one or two orders of magnitude below the predicted values. In the case of vertical devices, the failure to meet the theoretical expectations was attributed to the oxidation of the ferromagnets and the lack of an epitaxial interface between the later and the graphene or hBN. Similarly, the exact mechanisms that lead to high spin relaxation for in-plane spin transport in graphene are not completely understood, in part due to the low-quality of the explored devices. In this thesis we analyze new architectures and procedures that allowed us to fabricate ultraclean and oxidation-free interfaces between ferromagnets and graphene or hBN. In these devices we encountered negative and reversible magnetoresistance, that could not be explained with the previous theoretical models. We propose a new model based on a thorough characterization of the devices and well-known properties of graphene that were not taken into account in the previous model. We also employed a novel type of contact to graphene (1D-contacts) and applied it for the first time to achieve spin-injection in graphene. The main advantage of this type of contact is the full encapsulation of graphene with hBN, which leads to high quality graphene spintronic devices.

A large family of materials, collectively known as "van der Waals materials," have attracted enormous research attention over the past decade following the realization that they could be isolated into individual crystalline monolayers, with charge carriers behaving effectively two-dimensionally. More recently, an even larger class of composite materials has been realized, made possible by combining the isolated atomic layers of different materials into "van der Waals heterostructures," which can exhibit electronic and optical behaviors not observed in the parent materials alone. This thesis describes efforts to characterize the atomic-scale structural and electronic properties of these van der Waals materials and heterostructures through scanning tunneling microscopy measurements. The majority of this work addresses the properties of monolayer and few-layer graphene, whose charge carriers are described by massless and massive chiral Dirac Hamiltonians, respectively. In heterostructures with hexagonal boron nitride, an insulating isomorph of graphene, we observe electronic interference patterns between the two materials which depend on their relative rotation. As a result, replica Dirac cones are formed in the valence and conduction bands of graphene, with their energy tuned by the rotation. Further, we are able to dynamically drag the graphene lattice in these heterostructures, owing to an interaction between the scanning probe tip and the domain walls formed by the electronic interference pattern. Similar dragging is observed in domain walls of trilayer graphene, whose electronic properties are found to depend on the stacking configuration of the three layers. Scanning tunneling spectroscopy provides a direct method for visualizing the scattering pathways of electrons in these materials. By analyzing the scattering, we can directly infer properties of the band structures and local environments of these heterostructures. In bilayer graphene, we map the electrically field-tunable band gap and extract electronic hopping parameters. In WSe₂, a semiconducting transition metal dichalcogenide, we observe spin and layer polarizations of the charge carriers, representing a coupling of the spin, valley and layer degrees of freedom.

Defective structures in two-dimensional (2D) materials have been proved to have significant influences on the materials' properties. Understanding structural defects in 2D materials at atomic scale is therefore required. With the use of advanced imaging techniques, one of the main approaches applied in this project was aberration-corrected transmission electron microscopy (AC-TEM), the structures are able to be resolved with single-atom sensitivity with the reduction of both spherical aberration and the influence of chromatic aberration. This laid the foundation for the first two experiments, which involve the bond length measurement of each C-C bond within three types of divacancies and Si-C bonds at graphene edges. The former explains the tendency of bond rotations within the divacancies from the perspective of strain inside the defective areas and surrounding lattice; the latter revels the interactions between isolated Si atoms and zigzag/armchair graphene edges. The use of in-situ heating holder in the AC-TEM makes the direct visualization of structures and their dynamics at elevated temperatures possible. The Si-graphene edge interactions, as well as the following two experiments are all designed to study the high-temperature performances for different systems. Gold nanoclusters are introduced to monolayer graphene by thermal evaporation to study the interaction between gold and graphene at elevated temperature. Due to the strong interaction between gold and graphene, gold crystals are able to adapt to planar configurations with two different crystalline forms, and an epitaxial relationship was found for planar gold crystals and graphene. Atomically flat and long line defects and zigzag edges in monolayer molybdenum disulfide (MoS₂) are successfully created by in-situ thermal annealing. The relationship between S vacancy mobility and defect forms are revealed based on the experiment. High-temperature atomic configurations of line defects and edge terminations are resolved in the first time. Their electronic properties are also explored with the support of density functional theory calculations.

abstract: The continuous random network (CRN) model of network glasses is widely accepted as a model for materials such as vitreous silica and amorphous silicon. Although it has been more than eighty years since the proposal of the CRN, there has not been conclusive experimental evidence of the structure of glasses and amorphous materials. This has now changed with the advent of two-dimensional amorphous materials. Now, not only the distribution of rings but the actual atomic ring structure can be imaged in real space, allowing for greater charicterization of these types of networks. This dissertation reports the first work done on the modelling of amorphous graphene and vitreous silica bilayers. Models of amorphous graphene have been created using a Monte Carlo bond-switching method and MD method. Vitreous silica bilayers have been constructed using models of amorphous graphene and the ring statistics of silica bilayers has been studied. / Dissertation/Thesis / Doctoral Dissertation Physics 2014

abstract: There has been a surge in two-dimensional (2D) materials field since the discovery of graphene in 2004. Recently, a new class of layered atomically thin materials that exhibit in-plane structural anisotropy, such as black phosphorous, transition metal trichalcogenides and rhenium dichalcogenides (ReS2), have attracted great attention. The reduced symmetry in these novel 2D materials gives rise to highly anisotropic physical properties that enable unique applications in next-gen electronics and optoelectronics. For example, higher carrier mobility along one preferential crystal direction for anisotropic field effect transistors and anisotropic photon absorption for polarization-sensitive photodetectors. This dissertation endeavors to address two key challenges towards practical application of anisotropic materials. One is the scalable production of high quality 2D anisotropic thin films, and the other is the controllability over anisotropy present in synthesized crystals. The investigation is focused primarily on rhenium disulfide because of its chemical similarity to conventional 2D transition metal dichalcogenides and yet anisotropic nature. Carefully designed vapor phase deposition has been demonstrated effective for batch synthesis of high quality ReS2 monolayer. Heteroepitaxial growth proves to be a feasible route for controlling anisotropic directions. Scanning/transmission electron microscopy and angle-resolved Raman spectroscopy have been extensively applied to reveal the structure-property relationship in synthesized 2D anisotropic layers and their heterostructures. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018