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.

Graphite and graphene electrodes are used to study two electrochemical processes: the decoration of these electrodes with Au metallic nanoparticles through the use of electrodeposition, and electrowetting, the potential-dependent change in hydrophobicity of a surface. Electrodeposition provides a useful route to electrode functionalisation, in particular to combine the enhanced properties of metallic nanoparticles with the advantageous features of carbon materials. A combination of cyclic voltammetry, chronoamperometry, and both ex situ and in situ atomic force microscopy are used to deduce the mechanism of Au electrodeposition on graphite and graphene. Notably, the mechanism of Au nanoparticle formation cannot be deduced from simple voltammetry alone, and the spontaneous formation of Au within the timescale of the electrodeposition experiment is confirmed. Electrowetting is a uniquely responsive method to manipulate the wetting properties of an electrode. However, a dielectric coating is commonly required to protect the surface from electrolysis, which in turn further increases the potentials needed to perform electrowetting. In contrast to this, here it is shown that bare graphite and graphene electrodes support electrowetting without the disadvantages of a dielectric coating, allowing an unprecedented combination of performance and efficiency. Furthermore, the ideal behaviour this system demonstrates is implemented as a platform to study electrowetting itself. The influence of electrolyte composition, surface defects and electrode-blocking dielectric-like films are investigated to determine the factors that impede electrowetting, a key step to understanding the phenomenon that is normally hindered by the use of the dielectric.

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.

Projects were undertaken investigating the functionalisation of polyaromatic cores (chrysene, pyrene and perylene) for use in organic electronics and aqueous graphene stabilisation. In each case an iridium-catalysed aromatic C-H borylation formed a key synthetic step, allowing access to unique substitution patterns. The development of strategies for the orthogonal and asymmetric functionalisation of polyaromatic hydrocarbons was explored. In a key synthetic step 4,10-dichlorochrysene was regioselectively borylated in high yields at the 2,8-positions though C-H activation chemistry. The subsequent application of sequential palladium-catalysed Suzuki and Kumada coupling reactions to this intermediate enabled the synthesis of a series of chrysene derivatives with a unique orthogonal "A2B2" 2,8- and 4,10-substitution pattern. In addition the application of a trifluoromethylation at the borylated 2,8-positions enabled the synthesis of a donor-acceptor chrysene derivative. The effect of these substitution patterns on the photophysical and electrochemical properties of these derivatives was investigated and their potential use as organic semiconducting materials evaluated. In particular the synthesised chrysene derivatives displayed broadened UV-vis absorption spectra, redshifted fluorescence spectra, increased HOMO levels and decreased band gaps. In an extension of these aromatic substitution methodologies, pyrene and perylene aromatic cores were functionalised to perform as stabilisers for aqueous graphene dispersions, investigating asymmetric motifs that may maximise performance. A series of amphiphilic pyrene- and perylene-based alkylsulfonic acid salts were synthesised via their intermediate hydroxyalkyl derivatives. In addition the application of the previously explored aromatic C-H borylation allowed access to 7- and 5,8,11- asymmetrically substituted pyrene and perylene derivatives. Through collaboration, initial steps have been undertaken to compare and evaluate these novel stabilisers for their ability exfoliate graphite to graphene in aqueous solution.

The spintronic materials graphene and FeRh are of great scientific and technological interest due to their unique properties. Graphene's remarkable electronic transport and low spin interaction suggest it could be a near-perfect spin-transport material, while the equiatomic alloy FeRh undergoes a first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition when heated through a critical temperature ~370 K. Combining these materials could lead to a single multifunctional spin injection, transport and detection device in which a range of stimuli - heat, magnetic field, strain etc. - could be used to manipulate the device state. However, realisation of such a multifunctional device is extremely challenging. This thesis describes the progress made in developing a novel method of spin injection into graphene, and details a study of the metamagnetic phase transition in FeRh nanowires suitable for use as spin injection and detection electrodes. The measured values of spin lifetime and spin diffusion length in graphene are an order of magnitude lower than those predicted theoretically. In this project, a novel 1D contact geometry was investigated to determine whether the dwelling of spins underneath tunnel barrier contacts was the cause of the discrepancy. Although these devices exhibited very high charge carrier mobility - indicating successful device fabrication, defect-free graphene flakes and low levels of contamination - no spin signals were observed. Through a thorough investigation of this unexpected result it was determined that the quality of the graphene/- ferromagnetic interface was limiting the polarisation of injected spin current. The use of FeRh as a novel spin injection and detection material was investigated through magnetic force microscopy imaging of the AF and FM phases during heating and cooling sweeps. The results from FeRh full-films showed a strong dependence on surface morphology, as certain surface types were observed to favour the FM phase. These behaviours were confirmed in patterned nanowire devices, which indicated that the dependence on surface topology dominated over spatial confinement effects. In order to perform these studies a magneto-transport measurement system capable of performing measurements over a wide temperature range 2 K - 500 K in a rotatable magnetic field of up to 750 mT was developed. The noise base of the completed system was measured at just 10% above the theoretical minimum level.

石墨烯，由單層碳原子緊密排列成蜂巢狀的晶體結構，擁有卓越的電學性能和光學性能。這些優異的性能包括：極高的電子遷移率，超寬的吸收光譜，栅電壓調控光躍遷性質，飽和吸收性質。石墨烯的獨特光學性質已經被用來製造多種高性能的光電器件，包括光電探測器， 飽和吸收器， 光調製器和四波混頻的介質。同時，石墨烯的製造工藝可以與現代半導體標準製造工藝相兼容， 加上它穩定的化學性質， 石墨烯非常有潛力在未來半導體工業發展中發揮重要作用。 / 單層石墨烯可以透過97.7% 垂直入射的光，吸收2.3% 左右，並且吸收的光頻非常寬。雖然對於單層碳原子結構，這個吸收率很大，但是在一些器件中，我們需要更高的光吸收率。爲了突破單層石墨烯光吸收的極限，一種可行的辦法是延長石墨烯與光相互作用的長度。將單層石墨烯轉移到長的硅波導上，可以延長石墨烯與光作用的長度。除了線性光吸收特性，石墨烯的飽和吸收特性也有廣泛的應用。很多脉衝激光發生器是利用被動鎖模的原理，即激光器中的飽和吸收器將連續的光波轉變成頻率固定的脉衝。石墨烯已經被證明是恢復速度快、調製範圍大的飽和吸收器。 但是，石墨烯和硅波導結合作為飽和吸收器的特性和它們的應用還從來沒有被研究過。 / 在論文中，我們研究了石墨烯轉移到硅波導后的線形光學性質和飽和吸收特性。首先我們討論了石墨烯轉移的方法，然後我們通過實驗調查了石墨烯在貴波導上的線形光學性質和飽和吸收特性。爲了研究石墨烯/硅波導在被動鎖模光纖激光器中的作用，我們利用石墨烯/硅波導作為飽和吸收器製造了一個光纖激光器。之後，通過在激光腔體中加入可调滤波器， 我們用成功的演示了波長可調製被動鎖模光纖激光器。 / 多脉衝激光光源在光纖通信，測量學和光學器件性能鑒定中有重要作用。所以研究緊湊、穩定並且價格實惠的多脉衝鎖模激光器非常有意義。爲了實現多脉衝鎖模光纖激光器，我們將硅基濾波器和石墨烯/硅波導集成在一起. 在論文中，我們設計并優化了能被應用於多脉衝鎖模激光器的寬帶寬濾波器凹凸光栅。 / Graphene, a single 2D sheet of carbon atoms arranged in a honeycomb lattice, has superior electrical and optical properties including extremely high charge-carrier mobility, broadband optical absorption, gate-variable optical transitions and saturable absorptions. Its unique optical properties have led to a range of promising optoelectronic devices, such as photo detectors, saturable absorbers, optical modulators and nonlinear media for four-wave mixing. Graphene’s complementary metal-oxide-semiconductor (CMOS) integration processes at wafer scale and its electrochemical stability make it a promising candidate for post CMOS electronics. / Monolayer grapheme transmits 97.7% of the normal incident light and absorbs 2.3%, independent of wavelength. In order to overcome the challenge of limited absorption of a monolayer and better exploit graphene optical properties, a long interaction length is needed. By integrating graphene directly on top of silicon waveguides, longer light/graphene interactions can be achieved. In addition to the linear optical properties of the graphene, its saturable absorption also finds useful applications. Many ultra-short pulse lasers are based on passive mode-locking, where a saturable absorber turns continuous wave output of the laser into a train of optical pulses. And graphene has been shown to make an excellent saturable absorber. However, the saturable absorption behavior of graphene-silicon wavguides and their applications have never been studied. / In this thesis, the linear and saturable absorption of monolayer graphene films transferred onto silicon waveguide are investigated. The transfer process of monolayer graphene to silicon waveguides is studied and linear and saturable absorption measurements are carried out. To investigate applications of graphene-silicon waveguides, a passive mode-lock fiber lasers in which graphene-silicon waveguides act as saturable absorbers to mode-lock pulses is constructed. By adding a tunable filter in cavity, a tunable mode-locked fiber laser based on graphene-silicon waveguide is demonstrated. / Multi-wavelength pulse sources are important for applications including optical fiber communication, instrumentation, and photonic component characterization. The availability of compact, reliable and cost effective multi-wavelength mode-locked lasers is of great importance. We also hope to build multiwavelength mode-locked fiber lasers by integrating CWDM silicon filters with graphene-silicon waveguides. The design process and optimization of a silicon filter called echelle grating is demonstrated. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Shi, Zerui. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstracts also in Chinese. / Chapter Chapter 1: --- Introduction --- p.1 / Chapter 1.1 --- Photonic Integrated Circuit (PIC) --- p.1 / Chapter 1.2 --- Silicon on Insulator (SOI) Photonics --- p.3 / Chapter 1.2.1 --- SOI Waveguides --- p.5 / Chapter 1.2.2 --- SOI Waveguide Based Wavelength Division Multiplexing --- p.6 / Chapter 1.3 --- Graphene Photonics --- p.9 / Chapter 1.3.1 --- Band Structure of Monolayer Graphene --- p.9 / Chapter 1.3.2 --- Optical Absorption of Graphene --- p.13 / Chapter 1.3.3 --- Graphene Based Passive Mode-Locked Fiber Lasers --- p.18 / Chapter 1.4 --- Motivation --- p.22 / Chapter 1.5 --- Thesis Outline --- p.23 / Chapter 1.6 --- Reference --- p.24 / Chapter Chapter 2: --- Optical Properties of Graphene-Silicon Waveguides --- p.27 / Chapter 2.1 --- Fabrication and Transfer Process of Graphene-Silicon Waveguides --- p.28 / Chapter 2.1.1 --- Fabrication of SOI Waveguides --- p.28 / Chapter 2.1.2 --- Transfer Process of Monolayer Graphene onto SOI Waveguides --- p.29 / Chapter 2.2 --- Linear Optical Absorption of Graphene-Silicon Waveguides --- p.35 / Chapter 2.2.1 --- Introduction to Photoexcitation in Monolayer Graphene --- p.35 / Chapter 2.2.2 --- Experimental Results of Linear Absorption --- p.37 / Chapter 2.3 --- In-Plane Saturable Absorption of Graphene-Silicon Waveguides --- p.38 / Chapter 2.3.1 --- Experimental Result of In-Plane Saturable Absorption of Graphene-Silicon Waveguides --- p.39 / Chapter 2.3.2 --- Mechanism of Saturable Absorption of Graphene-Silicon Waveguides --- p.42 / Chapter 2.4 --- Summary --- p.44 / Chapter 2.5 --- Reference --- p.45 / Chapter Chapter 3: --- Hybrid Graphene-Silicon Waveguides Based Mode-Locked Fiber Lasers --- p.47 / Chapter 3.1 --- Background of Graphene Based Mode-Locked Fiber Lasers --- p.47 / Chapter 3.1.1 --- Passive Mode-Locked Lasers Fundamentals --- p.47 / Chapter 3.1.2 --- Review of Recent Works of Graphene Based Mode-Locked Fiber Lasers . --- p.52 / Chapter 3.2 --- Hybrid Graphene-Silicon Waveguides Based Mode-Locked Fiber Lasers --- p.55 / Chapter 3.2.1 --- Experimental Set Up and Results --- p.55 / Chapter 3.2.2 --- Discussion --- p.59 / Chapter 3.3 --- Tunable Graphene-Silicon Waveguides Based Mode-Locked Fiber Lasers --- p.62 / Chapter 3.4 --- Summary --- p.66 / Chapter 3.5 --- Reference --- p.66 / Chapter Chapter 4: --- Conclusion and Future Work --- p.68 / Chapter 4.1 --- Conclusion --- p.68 / Chapter 4.2 --- Future Work --- p.69 / Chapter 4.2.1 --- Design and Simulation Results of Echelle Grating --- p.69 / Chapter 4.2.2 --- Optimization and Experiment Results of Echelle Gratings --- p.77 / Chapter 4.2.3 --- Integration of Echelle Grating with SOI Waveguides --- p.82 / Chapter 4.3 --- Summary --- p.84 / Chapter 4.4 --- Reference --- p.85 / Chapter APPENDIX A: --- PUBLICATION LIST --- p.86 / Chapter APPENDIX B: --- LIST OF FIGURES --- p.87 / Chapter APPENDIX C: --- LIST OF TABLES --- p.91 / Chapter APPENDIX D: --- atlab Code of Simulation of Echelle Grating --- p.92

Orientador: Maria Isabel Felisberti / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-09-27T12:42:23Z (GMT). No. of bitstreams: 1 Nogueira_HeltonPereira_M.pdf: 2762504 bytes, checksum: 2a6c093233b2870f10da6a8408841655 (MD5) Previous issue date: 2012 / Resumo: As cargas de carbono, tais como grafite, nanotubos de carbono, fulerenos e mais recentemente o grafite tem atraído a atenção de inúmeros pesquisadores, que buscam formas de aproveitar suas excepcionais propriedades elétricas no desenvolvimento de compósitos com potenciais aplicações nas áreas de sensores eletroquímicos, células solares, barreira eletromagnética entre outras. Neste trabalho teve-se como objetivo o desenvolvimento de compósitos baseados em poli(estireno-co-butadieno-co-estireno) ¿ SBS e as cargas de carbono, grafite e grafite. Para tanto, o grafite foi obtido pela oxidação do grafite empregando-se a metodologia de Hummers, seguida de redução por hidrazina. Os compósitos de grafite e "grafite¿ foram preparados por mistura mecânica e por casting de soluções em diferentes solventes na faixa de composição de 0,5 a 10 % em massa de carga. O processamento mecânico resultou em compósitos que apresentaram comportamento mecânico dependente do tipo e quantidade de carga. Porém, esses compósitos apresentaram-se como isolantes elétricos devido a não formação de uma rede de percolação. A matriz de SBS nos compósitos obtidos por casting apresentou morfologia dependente do solvente empregado, o que se deve aos parâmetros de solubilidade dos solventes e dos blocos de poliestireno e polibutadieno do SBS, e a dispersão da carga pela matriz foi mais uniforme do que no caso de compósitos preparados por mistura mecânica, promovendo uma melhora nas propriedades mecânicas e conferindo propriedades de condução elétrica / Abstract: Carbon fillers such as graphite, carbon nanotubes, fullerenes and currently graphene have attracted the attention of many researchers, due to their exceptional and useful electrical properties for the development of composites with potential applications in electrochemical sensors, solar cells, electromagnetic barrier, and so on. The objective of the herewith work is the development of composites made up of poly(styrene-co-butadiene-co-styrene) ¿ SBS and carbon fillers (graphite or graphene). The graphene was obtained by oxidation of the graphite by the Hummers¿ method, followed by hydrazine¿s reduction. The graphite and graphene - based composites were prepared by melt compounding and by solvent casting using different organic solvents in a composition range of 0,5 to 10% in weight of filler. The results showed that the melt compounding produced composites with mechanical properties dependent on the type and amount of filler. Also, these composites behave as electrical insulators due to the absence of percolation thresholds. The morphology of SBS matrix in the composites obtained by solvent casting is determined by the organic solvent, more specifically by the interaction paramenter of solvent and of the polybutadiene and polystyrene blocks of the SBS. The filler dispersion throughout the matrix in the composites obtained by solvent-casting was more uniform than the dispersion level in the composites obtained by melt compounding. Therefore, the first materials showed higher mechanical performance and electrical conducting / Mestrado / Físico-Química / Mestre em Química

Um dispositivo eletroquímico foi desenvolvido para a determinação simultânea de sulfametoxazol (SMX) e trimetoprim (TMP), utilizando voltametria de pulso diferencial e um eletrodo de carbono vítreo (GC) modificado com o compósito óxido de grafeno reduzido (rGO) e nanopartículas de prata (AgNPs), sintetizados por dois métodos: químico e o eletroquímico. A morfologia e o comportamento eletroquímico dos eletrodos GC modificados com os compósitos rGO/AgNPs (método químico) e rGO-AgNPs (método eletroquímico) foram caracterizados por microscopia eletrônica de varredura acoplada a um canhão de elétrons (FEG-SEM) e voltametria cíclica (CV). Essas técnicas demostraram que, nos dois métodos utilizados, o óxido de grafeno (GO) foi modificado com as AgNPs, e que o compósito sintetizado pelo método eletroquímico apresentou uma melhor dispersão das nanopartículas, resultando em um aumento da área superficial quando comparado ao compósito rGO/AgNPs. Assim, o eletrodo GC/rGO-AgNPs foi avaliado e otimizado na determinação simultânea de SMX e TMP e atingiu limites de detecção de 0,6 µmol L-1 para o SMX e 0,4 µmol L-1 para o TMP. O dispositivo eletroquímico proposto, GC/rGO-AgNPs, foi aplicado com sucesso na determinação simultânea de SMX e TMP em águas residuárias. / An electrochemical device was developed for the simultaneous determination of sulfamethoxazole (SMX) and trimethoprim (TMP) using differential pulse voltammetry, and a glassy carbon electrode (GC) modified with the reduced graphene oxide composite (rGO) and silver nanoparticles (AgNPs), synthesized by two methods: chemical and electrochemical. The morphology and the electrochemical behavior of the GC electrode modified with composite rGO/AgNPs (chemical method) and rGO-AgNPs (electrochemical method) were characterized by scanning electron microscopy coupled to an electron gun (FEG-SEM) and cyclic voltammetry (CV). These techniques demonstrated that, in both methods, the graphene oxide (GO) was modified with AgNPs, and the composite synthesized by the electrochemical method showed a better dispersion of the nanoparticles, resulting in an increased surface area when compared to the composite rGO/ AgNPs. Thus, the GC/rGO-AgNPs electrode was evaluated and optimized for simultaneous determination of SMX and TMP and achieved detection limits of 0,6 µmol L-1 to SMX and 0,4 µmol L-1 for the TMP. The proposed electrochemical device, GC/rGO-AgNPS, was successfully applied for the simultaneous determination of SMX and TMP in wastewater.

Esta tese descreve a esfoliação e modificação do grafeno oxidado (GO) na obtenção de grafeno em sua forma reduzida (RGO) para aplicações biomédicas, que envolve sensoriamento e biossensoriamento, além de aplicação em fototerapia. Nas aplicações em sensores, inicialmente o RGO juntamente com o surfactante aniônico Dihexadecilfosfato (DHP), foi utilizado na fabricação de filmes por drop casting em eletrodo de carbono vítreo (CGE), na detecção do hormônio Estradiol. O eletrodo modificado (RGO-DHP/CGE) foi caracterizado por voltametria cíclica e impedância de espectroscopia eletroquímica. Os resultados mostraram uma corrente de pico de oxidação irreversível em 0,6 V. Sob as condições experimentais ideais, usando a voltametria linear, o limite de detecção para este hormônio foi de 7,7 × 10-8 mol L-1. Foram fabricados também dispositivos de efeito de campo (FET) de RGO via porta líquida em eletrodos interdigitados, para a detecção de Cistatina-C, um marcador de doença renal crônica. Os filmes foram fabricados utilizando a técnica de automontagem de interação eletrostática, nos quais, como polieletrólito de carga positiva foi utilizado o RGO modificado via ligação covalente de APTES, e como polieletrólito de carga negativa, o RGO dopado com nitrogênio, através da redução via micro-ondas. Estes dispositivos apresentaram uma sensibilidade de (1,94 ± 0,29) ΔIDS(%)ngmL-1. O LD foi de 0,39 ngmL-1 e a região linear entre 5 ngmL-1 100 ngmL-1, quando utilizados em urina sintética. Avaliamos também o uso de RGO em sistemas de fototerapia, utilizando GO reduzido com NH4OH na presença de L-Glutamina (RGO-Glu), onde observamos um aumento de temperatura localizado quando o material é irradiado por um laser (808 nm). Este sistema apresentou uma boa estabilidade e baixa agregação em dispersão aquosa e em meio de cultura, devido à formação de uma corona proteica. O RGO-Glu mostrou-se mais eficiente para o aquecimento que o RGO sem a modificação, na absorção do laser em 808 nm, com valores de eficiência de conversão de energia de 63% e 50% respectivamente. Estudos utilizando célula HeLa mostram que a internalização do RGO-Glu foi mais eficiente do que o RGO sem a modificação. Estes estudos mostram a versatilidade do grafeno quimicamente esfoliado em aplicações biomédicas quando convenientemente modificado, que pode ser utilizado em diagnóstico e em terapia. / This thesis describes the exfoliation and modification of graphene oxide (GO) to obtain reduced graphene oxide (RGO), for biomedical applications, namely: (bio)sensing for diagnostics and as active material in phototherapy. For (bio)sensing applications, RGO was used in combination with the anionic surfactant Dihexadecylphosphate (DHP) in the fabrication of drop-cast thin films onto carbon glass electrode (CGE), to be used in the detection of the hormone Estradiol. The modified electrode (RGO-DHP/CGE) was characterized by cyclic voltammetry and electrochemical spectroscopy impedance (EIS). The results showed an irreversible oxidation peak current at 0.6 V. Under ideal experimental conditions, and using linear voltammetry, the detection limit obtained for this sensor was 7.7 × 10-8 mol L-1. In the second part of the study, RGO was used in the fabrication of field effect transistors (FETs) via liquid gate, and the devices were applied in the detection of Cystatin-C, a biomarker for chronic renal disease. The films were made using the electrostatic layer-by-layer technique, in which APTES-modified RGO was used as positive polyelectrolyte, whereas nitrogen-doped RGO was used as the negative species. These devices exhibited a sensitivity of (1,94 ± 0,29) ΔIDS(%)ngmL-1, whereas LD was 0,39 ng.mL-1 and the linear region of detection was between 5 ng.mL-1 100 ngmL-1 when used in synthetic urine. The studies on the use of RGO in phototherapy were carried out using NH4OH -reduced GO in the presence of L-Glutamine (RGO-Glu) for subsequent cell internalization and irradiation under an 808 nm lase line to promote hiperthermia. This system showed good stability and low aggregation in aqueous dispersions and culture medium, due to the formation of a protein corona. RGO-Glu was more efficient than the RGO without the modification in the absorption of the laser at 808 nm, resulting in an efficiency of heat generation (energy conversion efficiency) of 63% and 50% respectively. Cytotoxicity studies using HeLa cell lines revealed that the internalization of RGO-Glu was more efficient than RGO without modification. These studies show the versatility of chemically exfoliated graphene oxides for biomedical applications, including diagnosis and therapy.

Two-dimensional (2D) materials have attracted extensive research interest in recent years. Among them, graphene and the semiconducting transition metal dichalcogenides (TMDs) are considered as promising candidates for future device applications due to their unique atomic thickness and outstanding properties. The study on graphene and TMDs has demonstrated great potential to further push the scaling of devices into the sub-10 nanometer regime and enable endless opportunities of novel device architectures for the next generation. In this thesis, crucial challenges facing 2D materials are investigated from material synthesis to electronic applications. A comprehensive review of the direct synthesis of graphene on arbitrary substrates with an emphasis on the metal-catalyst-free synthesis is given, followed by a detailed study of the contact engineering in TMDs with a focus on the strategies to lower the contact resistance. Effective approaches have been demonstrated to solve these issues. These include: (1) metal-catalyst-free synthesis of graphene on various insulating substrates; (2) Fermi level pinning observed in TMDs and integration of graphene contact to lower the contact resistance; and (3) application of metal-insulator-semiconductor (MIS) contact in TMD field-effect transistors (FETs). First, a direct low-temperature synthesis of graphene on insulators without any metal catalysts has been realized. The effects of carbon sources, NH3/H2 concentrations, and insulating substrates on the material synthesis have been systematically investigated. Graphene transistors based on the as-grown material have been fabricated to study the electronic properties, which can further confirm the nitrogen-doped graphene has been synthesized from the electrical characterizations. Then electronic devices focusing on the semiconducting TMDs has been studied. The Fermi level pinning has been observed and studied in WS2 FETs with four metal materials. A novel method of using graphene as an insertion layer between the metal and TMDs has been proven to effectively reduce the contact resistance. Owing to the benefit of tuning the graphene work function via the electric field, the contact resistance can further be reduced. Finally, the effectiveness of MIS contacts in WS2 FETs has been demonstrated. A thickness dependence research has been conducted to find the optimal thickness of the inserted insulator. Moreover, the possible physical mechanism of how this MIS contact reduces the contact resistance in 2D materials has been discussed.