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.

Terahertz (THz) technology stands to solve a number of problems in everyday life, from next generation wireless communication to spectroscopic identification and imaging. However it is technically challenging to make a high power, compact source for terahertz radiation. The Quantum Cascade Laser (QCL), which produces gain at THz frequencies by exploiting inter-sub-band transitions in quantum wells, offers one solution to this problem. However controlling and detecting the emission from such sources remains a major challenge. This thesis investigates the theory and measurement of emission frequencies from aperiodic lattice THz QCLs. Crucially, realising both frequency control and detection provides a complete system for coherent THz characterisation of devices at precise, user defined frequencies. The author starts by studying the emission frequencies and threshold of discretely tuned aperiodic lattice lasers. This is achieved using a numerical transfer matrix method (TMM), which allows the calculation of the aperiodic lattice threshold spectrum for the first time. Calculations reveal that the low threshold modes of aperiodic lattice lasers form at peaks in the electromagnetic density of modes. This shows that lasing in aperiodic lattices arises from slow light propagation induced by multiple photonic band gaps, leading to both band edge and defect laser modes. Frequency selective lasing is maintained even under the influence of external facet feedback, albeit at the cost of precise knowledge of the mode frequency. Importantly this framework allows the understanding of essentially any aperiodic lattice laser system. Most significantly, the TMM is exploited in order to understand how graphene can be used to control a THz laser. Graphene interacts strongly with THz waves, and can be easily integrated with semiconductor structures such as lasers and waveguides. Here, numerical calculations reveal that graphene can be introduced into the waveguide of a THz QCL, generating electrically tunable THz surface plasmons. Such surface plasmons couple into an aperiodic lattice to change the scattering strength of each individual grating element. The TMM reveals that this change in scattering strength controls the modal selectivity of an aperiodic lattice THz QCL. This hypothesis successfully explains both earlier experiments and those performed by the author. Crucially, this model was central to a publication in the journal Science. Finally, this thesis demonstrates a novel coherent detection system for the characterisation of THz QCL emission. The technique exploits non-linear up-conversion of THz waves to a telecoms frequency side-band, a process shown to be sensitive to THz waveguide dispersion. By mixing the up-converted THz wave with a near infra-red local oscillator laser, coherent detection of QCL emission using all fibre coupled components is demonstrated for the first time. This measurement allows for the characterisation of laser emission with high frequency and temporal resolution. Specifically sub-microsecond pulses of THz emission and transients can be detected. When taken as a whole, the work of this thesis constitutes a major step towards realising cost effective THz characterisation and spectroscopy using QCLs.

Microelectronic fabrication of Si typically involves high-temperature or high-energy processes. For instance, wafer fabrication, transistor fabrication, and silicidation are all above 500°C. Contrary to that tradition, we believe low-energy processes constitute a better alternative to enable the industrial application of single-molecule devices based on 2D materials. The present work addresses the postsynthesis processing of graphene at unconventional low temperature, low energy, and low pressure in the poly methyl-methacrylate- (PMMA-) assisted transfer of graphene to oxide wafer, in the electron-beam lithography with PMMA, and in the plasma patterning of graphene with a PMMA ribbon mask. During the exposure to the oxygen plasma, unprotected areas of graphene are converted to graphene oxide. The exposure time required to produce the ribbon patterns on graphene is 2 minutes. We produce graphene ribbon patterns with ∼50 nm width and integrate them into solid state and liquid gated transistor devices. / )e submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract DE-AC02-06CH11357. )e U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the government. Funding text #2 )e Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. )e authors also acknowledge financial support from Argonne National Laboratory’s Laboratory-Directed Research and Development Strategic Initiative. / Revisión por pares

Graphene is a single atomic sheet of graphite that exhibits a diverse range of unique properties. The electrons in intrinsic graphene behave like relativistic Dirac fermions; graphene has a record high Young's modulus but extremely low bending rigidity; and suspended graphene exhibits very high thermal conductivity. These properties are made more intriguing because with a thickness of only a single atomic layer, graphene is both especially affected by its environment and readily manipulated. In this dissertation the interaction between graphene and its environment as well as the exciting new physics realized by manipulating graphene's lattice are investigated. Lattice manipulations in the form of strain cause alterations in graphene's electrical dispersion mathematically analogous to the vector potential associated with a magnetic field. We complete the standard description of the strain-induced vector potential by explicitly including the lattice deformations and find new, leading order terms. Additionally, a strain engineered device with large, localized, plasmonically enhanced pseudomagnetic fields is proposed to couple light to pseudomagnetic fields. Accurate strain engineering requires a complete understanding of the interactions between a two dimensional material and its environment, particularly the adhesion and friction between graphene and its supporting substrate. We measure the load dependent sliding friction between mono-, bi-, and trilayer graphene and the commonly used silicon dioxide substrate by analyzing Raman spectra of circular, graphene sealed microchambers under variable external pressure. We find that the sliding friction for trilayer graphene behaves normally, scaling with the applied load, whereas the friction for monolayer and bilayer graphene is anomalous, scaling with the inverse of the strain in the graphene. Both strain and graphene's environment are expected to affect the quadratically dispersed out of plane acoustic phonon. Although this phonon is believed to provide the majority of graphene's very high thermal conductivity, its contributions have never been isolated. By measuring strain and pressure dependent thermal conductivity, we gain insight into the mechanism of graphene's thermal transport.

>Magister Scientiae - MSc / Electrochemical platforms were developed based on pencil graphite electrodes (PGEs) modified with electrochemically deposited graphene (EG) sheets and Nafion-graphene (NG) nanocomposites in conjunction with an in situ plated bismuth-film (EG-PG-BiE and NG-PG-BiE). The EG- and NG-PG-BiEs were used as sensing platforms for determining Zn2+, Cd2+ and Pb2+ by square wave anodic stripping voltammetry (SWASV). EG sheets were deposited onto pencil graphite electrodes by cyclic voltammetric reduction from a graphene oxide (GO) solution, while a dip coating method was used to prepare the NG-PG-BiE. The GO and graphene, with flake thicknesses of 1.78 (2 sheets) and 2.10 nm (5 sheets) respectively, was characterized using FT-IR, HR-SEM, HR-TEM, AFM, XRD and Raman spectroscopy. Parameters influencing the electroanalytical response of the EG-PG-BiE and NG-PG-BiE such as, bismuth ion concentration, deposition potential, deposition time and rotation speed were investigated and optimized. The EG-PG-BiE gave well-defined, reproducible peaks with detection limits of 0.19 μg L-1, 0.09 μg L-1 and 0.12 μg L-1 for Zn2+, Cd2+ and Pb2+ respectively, at a deposition time of 120 seconds. The NG-PG-BiE showed similar detection limits of 0.167 μg L-1, 0.098 μg L-1 and 0.125 μg L-1 for Zn2+, Cd2+ and Pb2+ respectively. For real sample analysis, the enhanced voltammetric sensor proved to be suitable for the detection and quantitation of heavy metals below the US EPA prescribed drinking water standards of 5 mg L-1, 5 μg L-1 and 15 μg L-1 for Zn2+, Cd2+ and Pb2+ respectively.

Molecular dynamics (MD) simulations have been applied to study the interactions between different carbohydrates and graphene. In cellulose-graphene complexes, the behaviour of hydrophobic and hydrophilic faces of cellulose chains on a single layer of graphene in aqueous solvent have been investigated. The hydrophobic cellulose face forms a stable complex with graphene and the interface remains solvent-excluded over the course of the simulation. Cellulose chains contacting graphene preserved their intra- and inter-chain hydrogen bonds and maintaining a tg orientation of its hydroxymethyl groups that is similar to that found for the sugar in a vacuum environment. The solvent-exposed cellulose chains of the complex showed more flexibility. By contrast, over the course of the 300 ns MD simulation, the hydrophilic face of cellulose exhibits progressive rearrangement as it seeks to present its hydrophobic face, with disrupted intra- and inter-chain hydrogen bonding; sequential residue twisting to form CH-pie interactions with graphene; and permeation then expulsion of interstitial water. This transition is also accompanied by a more favourable cellulose-graphene adhesion energy as predicted at the PM6-DH2 level of theory. The stability of the cellulose-graphene hydrophobic interface in water reflects the amphiphilicity of cellulose and provides insight into favoured interactions within graphene-cellulose nanocomposites. Furthermore, water is observed to permeate cellulose during rearrangement of the hydrophilic face which may have application in addressing cellulose recalcitrance. In addition, the interaction of six different types of monosaccharide (β/alpha-D-Glc, β/alpha-D-Gal and β/alpha-D-Man) on the surface of graphene has been studied, using PM6-DH2 and PMF calculations in both gas phase and explicit water. The parameters studied included anomer, epimer, saccharide face, hydroxymethyl orientation and solvation. Binding of graphene to monosaccharide is more preferred in vacuum than in water; solvation of the complexes leads to reduction in the number of pie-interactions formed with graphene. In almost all studied complexes, β-anomers bind stronger to graphene compared to alpha-anomers in gas phase and water. Each monosaccharide has two unique faces parallel to the plane of the pyranose ring and these surfaces determine the interaction formed with graphene and water. Binding of graphene with different faces significantly influences the value of the computed interaction and binding free energy. We also find that the interactions between graphene and saccharide are mainly controlled by the number of CH-pie and OH-pie interactions formed between saccharides and graphene. The interaction energy and binding energy values suggest that the a-face of β-D-Glc is the most preferred to bind on graphene in vacuum while the b-face of β-D-Glc is preferred in the aqueous phase.

Magister Scientiae - MSc (Chemistry) / Electrochemical platforms were developed based on pencil graphite electrodes (PGEs) modified electrochemically with reduced graphene oxide metal nanoparticles (ERGO-metalNPs) composite and used for the high-sensitivity determination of Bisphenol A (BPA) in water samples. Synergistic effects of both reduced Graphene Oxide sheets and metal nanoparticles on the performance of the pencil graphite electrode (PGE) were demonstrated in the oxidation of BPA by differential pulse voltammetry (DPV). A solution of graphene oxide (GO) 1 mg mL-1 and 15 ppm of metal stock solutions (1,000 mg L-1, atomic absorption standard solution) (Antimony or Gold) was prepared and after sonication deposited onto pencil graphite electrodes by cyclic voltammetry reduction. Different characterization techniques such as FT-IR, HR-SEM, XRD and Raman spectroscopy were used to characterize the GO and ERGO-metalNPs. Parameters that influence the electroanalytical response of the ERGO-SbNPs and ERGO-AuNPs such as, pH, deposition time, deposition potential, purging time were investigated and optimized. Well-defined, reproducible peaks with detection limits of 0.0125 ?M and 0.062 ?M were obtained for BPA using ERGO-SbNPs and ERGO-AuNPs respectively. The rGO-metalNPs-PGE was used for the quantification of BPA in tap water sample and proved to be suitable for the detection of BPA below USEPA prescribed drinking water standards of 0.087 ?M. / 2021-12-31

Magister Scientiae - MSc (Chemistry) / Electrochemical platforms were developed based on pencil graphite electrodes (PGEs) modified electrochemically with reduced graphene oxide metal nanoparticles (ERGO–metalNPs) composite and used for the high-sensitivity determination of Bisphenol A (BPA) in water samples. Synergistic effects of both reduced Graphene Oxide sheets and metal nanoparticles on the performance of the pencil graphite electrode (PGE) were demonstrated in the oxidation of BPA by differential pulse voltammetry (DPV). A solution of graphene oxide (GO) 1 mg mL-1 and 15 ppm of metal stock solutions (1,000 mg L-1, atomic absorption standard solution) (Antimony or Gold) was prepared and after sonication deposited onto pencil graphite electrodes by cyclic voltammetry reduction. Different characterization techniques such as FT-IR, HR-SEM, XRD and Raman spectroscopy were used to characterize the GO and ERGO–metalNPs. Parameters that influence the electroanalytical response of the ERGO–SbNPs and ERGO–AuNPs such as, pH, deposition time, deposition potential, purging time were investigated and optimized. Well-defined, reproducible peaks with detection limits of 0.0125 μM and 0.062 μM were obtained for BPA using ERGO–SbNPs and ERGO–AuNPs respectively. The rGO-metalNPs–PGE was used for the quantification of BPA in tap water sample and proved to be suitable for the detection of BPA below USEPA prescribed drinking water standards of 0.087 μM.

Orientador: Ivana Cesarino / Resumo: O glifosato é o agrotóxico mais usado no mundo e atualmente é detectado em amostras por métodos cromatográficos. A fim de contribuir com as limitações desta técnica, este trabalho apresenta uma nova alternativa para a análise deste pesticida através de uma técnica eletroanalítica. Trata-se de uma molécula não eletroativa e que, portanto, não é detectada voltametricamente com sua estrutura básica, seja na forma molecular ou iônica. No entanto, é possível detectá-la de forma indireta devido à sua propriedade de formar complexos com íons cúpricos. Um sensor foi desenvolvido para a detecção de glifosato, usando a técnica de voltametria de pulso diferencial (DPV) e um eletrodo de carbono vítreo (GC) modificado com um compósito de óxido de grafeno reduzido (rGO) e nanopartículas de cobre (CuNPs), sintetizado por método químico. O comportamento eletroquímico dos eletrodos GC/rGO-CuNPs foi caracterizado por voltametria cíclica (CV) em solução tampão fosfato (PBS) pH 7,4no intervalo de -0,3 a +0,2 V vs. Ag/AgCl/KCl (3,0 mol L-1), com velocidade de varredura de 50mV s-1. Foram observados processos de oxidação em 30 mV e redução em -180 mV, que comprovam que rGO foi modificado com as CuNPs.Por DPV, a diminuição na corrente de pico anódica do cobre em presença de glifosato, atribuída formação do complexo Cu(II)-glifosato, foi usada para quantificar o analito em amostras. Assim, o eletrodo desenvolvido foi avaliado e otimizado na detecção glifosato por DPV, e os melhores resultados obtido... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Glyphosate is the most widely used pesticide in the world and is currently detected in samples by chromatographic methods. In order to contribute to the limitations of this technique, this work presents a new alternative for the analysis of this pesticide through an electroanalytical technique. It is a non-electroactive molecule and therefore is not detected voltammetrically with its basic structure, either in molecular or ionic form. However, it is possible to detect it indirectly because of its property of forming complexes with cupric ions. A sensor was developed for the detection of glyphosate using the differential pulse voltammetry (DPV) technique and a modified graphene oxide (GC) electrode with reduced graphene oxide (rGO) and copper nanoparticles (CuNPs), synthesized by chemical method. The electrochemical behavior of the GC / rGO-CuNPs electrodes was characterized by cyclic voltammetry (CV) in phosphate buffered saline (PBS) pH 7.4 in the range of -0.3 to +0.2 V vs. Ag / AgCl / KCl (3.0 mol L-1), with a scanning speed of 50mV s-1. Oxidation processes were observed at 30 mV and reduction at -180 mV, which proved that rGO was modified with CuNPs. By DPV, the decrease in the anodic peak current of copper in the presence of glyphosate, attributed the formation of Cu (II) -glyphosate, was used to quantify the analyte in samples. Thus, the electrode developed was evaluated and optimized for glyphosate detection by DPV, and the best results were obtained in the following c... (Complete abstract click electronic access below) / Mestre

ARAÚJO, F. R. V. Transporte Eletrônico em Phased Arrays de Nanofitas de Grafeno. 2017. 70 f. Dissertação (Mestrado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017. / Submitted by Pós-Graduação em Física (posgrad@fisica.ufc.br) on 2017-08-23T17:06:25Z No. of bitstreams: 1 2017_dis_frvaraujo.pdf: 3006955 bytes, checksum: 96cafe22b25ecd5c8d4c555c7a4a5c83 (MD5) / Approved for entry into archive by Giordana Silva (giordana.nascimento@gmail.com) on 2017-08-23T17:44:16Z (GMT) No. of bitstreams: 1 2017_dis_frvaraujo.pdf: 3006955 bytes, checksum: 96cafe22b25ecd5c8d4c555c7a4a5c83 (MD5) / Made available in DSpace on 2017-08-23T17:44:16Z (GMT). No. of bitstreams: 1 2017_dis_frvaraujo.pdf: 3006955 bytes, checksum: 96cafe22b25ecd5c8d4c555c7a4a5c83 (MD5) Previous issue date: 2017 / Graphene, a layer of carbon atoms arranged in a honeycomb crystal lattice, has remarkable physical properties. After its experimental obtaining in 2004 by A. K. Geim and K. S. Novoselov, several researches were carried out aiming to understand such physical properties and several possibilities of applications were proposed. At the low energy limit, there is a linearity relationship between energy and momentum for the electric charge carriers in this material and, therefore, they behave as relativistic particles of zero mass, described by the Dirac equation. One of the implications is that the electron-associated eigenfunctions that cross a potential barrier may not undergo damping under certain circumstances, a phenomenon known as Klein's paradox. Even without damping, these eigenfunctions acquire a phase factors that may depend only on the height and width values of the potential barrier. In this study, we investigate the properties transport in two electronic devices that use this phenomenon and that may be associated to phased arrays (electronic systems that have several emitters of waves, mechanically or electromagnetic, properly organized). We studied the electronic transport mechanisms in these physical systems and performed numerical simulations of electrical conductance as a function of energy and electrical conductance as a function of the electric potential and it was observed that the direction of propagation of the electrons can be controlled by varying the values of height and width of potential barriers. / O grafeno, uma camada de átomos de carbono arranjados em uma rede cristalina honeycomb (favo de mel), possui propriedades físicas notáveis. Após sua obtenção experimental em 2004 por A. K. Geim e K. S. Novoselov, várias pesquisas foram realizadas objetivando compreender tais propriedades físicas e diversas possibilidades de aplicações foram propostas. No limite de baixas energias, existe uma relação de linearidade entre a energia e o momento para os portadores de carga elétrica nesse material e, com isso, os mesmos comportam-se como partículas relativísticas de massa nula, descritas pela equação de Dirac. Uma das implicações disso é que as autofunções associadas aos elétrons que atravessam uma barreira de potencial podem não sofrer amortecimento em dadas circunstâncias, fenômeno esse conhecido como paradoxo de Klein. Mesmo sem sofrer amortecimento, essas autofunções adquirem fatores de fase que podem depender apenas dos valores de altura e largura da barreira de potencial. Nesse trabalho investigamos as propriedades de transporte em dois dispositivos eletrônicos que utilizam-se desse fenômeno e que podem ser associados a phased arrays (sistemas eletrônicos que possuem vários emissores de ondas, mecânicas ou eletromagnéticas, devidamente organizados). Estudamos os mecanismos de transporte eletrônico nesses sistemas físicos e realizamos simulações numéricas da condutância elétrica em função da energia e da condutância elétrica em função do potencial elétrico e observamos que a direção de propagação dos elétrons pode ser controlada através da variação dos valores de altura e largura das barreiras de potencial.