Two dimensional (2D) materials, such as graphene, phosphorene and transition metal chalcogenides, have received a great attention in recent years due to their unique physical and chemical properties. A majority of 2D materials is intrinsically non-magnetic, therefore, their applications in spintronics are limited. The design and synthesis of new 2D materials with intrinsic magnetism and high spin-polarization remains a challenge. Computational discovery of new 2D materials with desired magnetic and electronic properties is the subject of this thesis. Using density functional theory with PBE, PBE+U and HSE06 functionals, we have systematically investigated the structure, electronic, magnetic and topological properties of novel 2D materials. Investigated materials include MXenes and layered transition-metal trihalides, both with great potential applications in spintronic devices. Four different classes of materials showing unique magnetic properties were investigated and reported in this thesis. (1) Asymmetrically functionalized MXenes were studied. The coexistence of the fully compensated antiferromagnetic order (zero magnetization) and completely spin-polarized semiconductivity was found for the first time. Moreover, the spin carrier orientation and induced transition from bipolar antiferromagnetic...

<div>Over the last decade, Dirac matter has become one of the most prominent fields of research in contemporary material science due to the incredibly rich physics of the Dirac equation. Notable examples are the Dirac cones in graphene, Weyl points in TaAs, and gapless edge states in Bi₂Te₃. These unique phases of matter are intimately related to the topological structure of Dirac fermions. However, it remains an open question if the topological structure of Maxwell's equations predicts yet new phases of matter. This thesis will conclusively answer this question.</div><div><br></div><div>Topological electrodynamics is concerned with the geometry of electromagnetic waves in condensed matter. At the microscopic level, photons couple to the dipole-carrying excitations of a material, such as plasmons and excitons, which hybridize to form new normal modes of the system. The interaction between these bosonic oscillators is the origin of temporal and spatial dispersion in optical response functions like the conductivity tensor. Our main achievement is motivating a global interpretation of these response functions, over all frequencies and wavevectors. This theory led us to the conclusion that there are topological invariants associated with the conductivity tensor itself. In this thesis, we show exactly how to calculate these electromagnetic invariants, in both continuum and lattice theories, to identify unique Maxwellian phases of matter. Magnetohydrodynamic electron fluids in strongly-correlated 2D materials like graphene are the first candidates of this new class of topological phase. The fundamental physical mechanism that gives rise to a topological electromagnetic classification is Hall viscosity which adds a nonlocal component to the Hall conductivity. To study the topological electrodynamics, we propose viscous Maxwell-Chern-Simons theory -- a Lagrangian framework that naturally generates the equations of motion, nonlocal Hall response and the boundary conditions. We demonstrate that nonlocal Hall conductivity is the spin-1 photonic equivalent of dispersive mass and induces precession of bulk photonic skyrmions. Nontrivial photonic skyrmions are associated with Dirac monopoles in the bulk momentum space and a singular Berry gauge. A singular gauge occurs when the photonic mass changes sign. Remarkably, the boundary of this medium supports gapless chiral edge states that are spin-1 helically-quantized and satisfy open boundary conditions.</div>

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This dissertation is devoted to the development of novel devices for optoelectronic and photovoltaic applications using the promise of inkjet printing with two-dimensional (2D) materials. A systematic approach toward the characterization of the liquid exfoliated 2D inks comprising of graphene, molybdenum disulfide (MoS2), tungsten diselenide (WSe2), and 2D perovskites is discussed at depth. In the first study, the biocompatibility of 2D materials -- graphene and MoS2 -- that were drop cast onto flexible PET and polyimide substrates using mouse embryonic fibroblast (STO) and human esophageal fibroblast (HEF) cell lines, was explored. The polyimide samples for both STO and HEF showed high biocompatibility with a cell survival rate of up to ~ 98% and a confluence rate of 70-98%. An inkjet printed, biocompatible, heterostructure photodetector was constructed using inks of photo-active MoS2 and electrically conducting graphene, which facilitated charge collection of the photocarriers. The importance of such devices stems from their potential utility in age-related-macular degeneration (AMD), which is a condition where the photosensitive retinal tissue degrades with aging, eventually compromising vision. The biocompatible inkjet printed 2D heterojunction devices were photoresponsive to broadband incoming radiation in the visible regime, and the photocurrent scaled proportionally with the incident light intensity, exhibiting a photoresponsivity R ~ 0.30 A/W. Strain-dependent measurements were also conducted with bending, that showed Iph ~ 1.16 µA with strain levels for curvature up to ~ 0.262 cm-1, indicating the feasibility of such devices for large format arrays printed on flexible substrates. Alongside the optoelectronic measurements, temperature-dependent (~ 80 K to 573 K) frequency shifts of the Raman-active E12g and A1g modes of multilayer MoS2 exhibited a red-shift with increasing temperature, where the temperature coefficients for the E12g and A1g modes were determined to be ~ - 0.016 cm-1/K and ~ - 0.014 cm-1/K, respectively. The phonon lifetime τ was determined to be in the picosecond range for the E12g and A1g modes, respectively, for the liquid exfoliated multilayer MoS2. Secondly, an all inkjet printed WSe2-graphene hetero-structure photodetector on flexible polyimide substrates is also studied, where the device performance was found to be superior compared to the MoS2-graphene photodetector. The printed photodetector was photo responsive to broadband incoming radiation in the visible regime, where the photo responsivity R ~ 0.7 A/W and conductivity σ ~ 2.3 × 10-1 S/m were achieved at room temperature. Thirdly, the synthesis of solution-processed 2D layered organo-halide (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 2, 3, and 4) perovskites is presented here, where inkjet printing was used to fabricate heterostructure flexible photodetector devices on polyimide substrates. The ON/OFF ratio was determined to be high, ~ 2.3 × 103 while the photoresponse time on the rising and falling edges was measured to be rise ~ 24 ms and fall ~ 65 ms, respectively. The strain-dependent measurements, conducted here for the first time for inkjet printed perovskite photodetectors, revealed the Ip decreased by only ~ 27% with bending (radius of curvature of ~ 0.262 cm-1). This work demonstrates the tremendous potential of the inkjet printed, composition tunable, organo-halide 2D perovskite heterostructures for high-performance photodetectors, where the techniques are readily translatable toward flexible solar cell platforms as well. Fourthly, metal contacts and carrier transport in 2D (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 4) perovskites is a critical topic, where the use of silver (Ag) and graphene (Gr) inks as metallic contacts to 2D perovskites was investigated. The all inkjet printed Gr-perovskite and Ag-perovskite photodetectors were found to be photo-responsive to broadband incoming radiation where measurements were conducted from λ ~ 400 nm to 2300 nm. The photoresponsivity R and detectivity D were compared between the Gr-perovskite and Ag-perovskite photodetectors, which revealed the higher performance for the Ag-perovskite photodetector. The superior performance of the Ag-perovskite photodetector was also justified with the Schottky barrier analysis using the thermionic emission model through temperature-dependent transport measurements. Finally, this dissertation ends with the description of the first steps for using solution-processed, inkjet printed perovskites for solar cells. The preliminary investigations include the discussion of the chemical formulations for the carrier separation layers, dispersion route, and the variation of solar cell figures of merit with processing.

[pt] Neste trabalho foi estudado o atrito em escala nanométrica em materiais atomicamente finos, como o grafeno e os dicalcogenetos de metais de transição (TMD) como o dissulfeto de molibdênio (MoS2) e o dissulfeto de tungstênio (WS2). Para tanto, foi utilizado um microscópio de força atômica (AFM), de modo que uma ponta de nitreto de silício suportada por uma haste (cantiléver) é deslizada sob a superfície do material em análise, e o atrito é quantificado de acordo com a deformação lateral da haste. Diferentes parâmetros foram alterados durante a varredura para verificar suas influências, tais como a força normal aplicada durante a varredura e a velocidade relativa em que o sistema ponta-amostra desliza. Parâmetros relativos às superfícies, como número de camadas, rugosidade e adesão também foram investigados. Com a variação da velocidade de deslizamneto, verificamos uma dependência linear com o logaritmo da velocidade, até um ponto de saturação. Esta dependência é amplificada de acordo com o número de camadas do grafeno, de modo que numa monocamada essa inclinação é mais acentuada do que nas demais camadas. Usando o modelo de Prandtl-Tomlinson termicamente ativo, conseguiu-se determinar o potencial de interação entre a ponta do AFM e a superfície analisada, as forças críticas em que a saturação do atrito ocorre e a frequência estipulada com que os eventos de superação da barreira de pontecial acontecem. Com a variação da força normal aplicada, os resultados mostram que grafeno e MoS2 seguem o modelo Johnson-Kendall-Roberts (JKR) de mecânica de contato, enquanto o WS2 segue o modelo Derjaguin-Muller-Toporov (DMT). Para explicar tal diferença, uma hipótese associada ao efeito piezoelétrico é estipulada. Ademais, foi observado que a contaminação das superfícies de grafeno por adsorção de hidrocarbonetos pela exposição ao ar aumenta o atrito medido, e altera sua relação à carga aplicada. Os estágios iniciais da contaminação foram observados, e notou-se que esta se propaga da monocamada para as demais camadas da folha de grafeno, com diferentes taxas de área contaminada por tempo. / [en] In this work, the friction mechanism at the nanoscale of atomically thin materials such as graphene, transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), and muscovite mica was studied with the use of an atomic force microscope (AFM). The AFM scans these materials surfaces with a silicon nitride tip which is attached at the end of a cantilever. The tips slides through the surface and friction is measured by the torsional deflection of the cantilever. Parameters such as applied normal load and sliding speed were varied in order to verify their influences. Surfaces properties such as number of layers, roughness and tip-sample adhesion were also analyzed. The sliding speed experiment shows a linear dependence with the logarithm of the scanning velocity, until friction reaches a saturation point, where it remains the same even at higher velocities. Such dependence is amplified with the number of graphene layers, as a monolayer presents a steeper curve than few layers graphene. The data was fitted using the thermally active Prandtl-Tomlinson model and the tip-sample interaction potential was estimated, as well as the critical forces at which friction saturation occurs and the hop frequency at which a potential barrier is surpassed. In the applied normal load experiment, results shows that both graphene and MoS2 follow the Johnson-Kendall-Roberts (JKR) model, while WS2 and mica follows the Derjaguin-Muller-Toporov (DMT) model. In order to explain the different behavior in both TMDs samples, a hypothesis associated with the piezoelectric effect is proposed. Furthermore, the influence of airborne contamination in the friction of graphene was studied. Results shows that the contact mechanics is altered due to adsorbed hydrocarbon molecules on the graphene flakes. Initial stages of contamination shows that it propagates from the monolayer to subsequent layers, with a different contaminated area over time rate.

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<p dir="ltr">Since the discovery of graphene in 2004, numerous other materials with intriguing electronic, optical, and magnetic properties have been found to be layered and exfoliatable down to atomic thickness. Owing to their weak interlayer coupling, mediated only by van der Waals forces, this new class of 2-dimensional materials, also known as van der Waals (vdW) materials, allows layer-by-layer stacking, overcoming some of the limitations of growth techniques. In particular, the growing inventory of vdW materials has expanded to include magnetic materials, further broadening the possibilities of novel devices based on stacked heterostructures. These magnetic heterostructures can find applications in spintronics and memory devices and may be combined with other vdW materials with optical properties for applications in optoelectronics. In this thesis, we assembled heterostructures via mechanical transfer or growth to modify the magnetism in these vdW materials. We used various optical and electrical techniques to probe the modified magnetism or its effects on the novel heterostructure. Thus, we observed the emergence of the magnetic proximity effect on the topological insulator BiSbTeSe₂ after dry transferring a thin flake of Cr₂Ge₂Te₆ on top, taking steps towards the observation of novel topological phases, such as the quantum Hall insulator. Additionally, we demonstrated an increased Curie temperature and magnetic anisotropy, effectively enhancing the magnetism, in thin flakes of Cr₂Ge₂Te₆ and Cr₂Si₂Te₆ after sputtering NiO or MgO. Finally, noting that the effect of modified magnetism in Cr2Ge2Te6 after sputtering NiO or MgO is induced due to wrinkle formation and strain, we further reproduce similar wrinkle formation on other 2D materials such as hBN, graphite, and 2D antiferromagnets (XPS₃, (X= Mn, Fe, Ni), CrSBr, RuCl₃). We used polarized Raman spectroscopy to characterize the induced biaxial strain in hBN and showed that such wrinkle formation can lead to moderately (up to 1.4% strain) spatially inhomogeneous and anisotropic strain profiles. These efforts demonstrate the versatility of tailoring the properties of these vdW materials.</p>

Lasers are increasingly used for a wide range of different applications for both civil and military purposes. Due to the distinct properties of laser light, use of lasers often comes with a risk of damage to the human eye and other optical sensors. Therefore, an effective laser protection is needed. 2D-materials is a relatively new class of materials, which have shown to possess many unique properties compared to its bulk counterparts. Some 2D-materials exhibit nonlinear optical (NLO) properties, and specifically optical power limiting (OPL) effects, and have therefore been researched for laser protection applications. In this work, two different 2D-materials, MXene Ti3C2 and graphene oxide (GO), have been combined with a hybrid organic-inorganic polymer, a so called melting gel (MG), to synthesise nanocomposites possessing OPL effects for laser protection applications. Different methods of incorporating the 2D-materials in the polymer matrix as well as the effect on optical properties of different concentrations of 2D-materials were investigated. The prepared nanocomposites were characterised using optical microscopy, spectroscopy and OPL measurements in order to investigate and quantify their linear and nonlinear optical properties. The MG was optically clear, mechanically stable and easy to synthesise, which makes it a suitable candidate as a matrix for a laser protection nanocomposite. Additionally, it was possible to dope the MG with the two different 2D-materials to create nanocomposites showing desirable optical properties in the visible spectrum. However, many samples showed signs of clustered 2D-particles indicating that the dispersion could be improved. Finally, OPL measurements, performed at 532 nm, showed that the MG itself exhibited OPL effects, both 2D-materials showed a stronger OPL effect than the non-doped MG and that GO-doped samples gave a better protection than the MXene samples.