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Wireworms, the larvae of the click beetle are a major polyphagous soil pest, damaging crops such as corn, wheat, or potatoes. The withdrawal of registration of many chemical insecticides left farmers with a limited number of solutions against wireworms and soil pests. Entomopathogenic fungi (EPF) can be useful against wireworm; yet they are slow acting and species dependent. To override these limitations, stress and kill strategies were tested, combining spinosad and different Metarhizium strain against three different wireworm species from the genera Agriotes. Combining spinosad and M. brunneum could increase the mortality of the EPF strains; yet, the susceptibility of the Agriotes species differs for each combination (Chapter I). The aim of the PhD was to develop new integrated pest management strategies against wireworms to avoid important corn yield losses. Developing pest management strategies using EPF seemed unreliable; thus, fungal volatile organic compounds (VOCs) were tested for their fumigant and attractant/repellent activity (Chapter II and III). In Chapter II, fumigation using 1-octen-3-ol and 3-octanone was tested against wireworm (Agriotes lineatus), corn rootworm (Diabrotica virgifera virgifera) and garden chafer (Phyllopertha horticola). The VOCs were highly efficacious in a closed environment (Chapter II); yet, in open containers, no mortality was observed (Chapter III). In this experiment, the VOCs had attractant/repellent properties depending on the species. In silico study, confirmed that corn rootworm could detect and recognize the VOCs with their odorant binding proteins. As odorant binding proteins are well conserved between species, it is expected that wireworm and garden chafer can also detect and recognize the VOCs. In chapter IV, the VOCs were formulated as slow-release granules. These granules facilitate application of the VOCs in the field and increase the remanence of the VOCs in the soil. In fact, to avoid economic damage, wireworms need to be repelled from maize seedlings during the three first weeks of growth. Four doses of 1-octen-3-ol cyclodextrin granules were tested to reduce wireworm damage on maize. The lowest dose (2.26kg/ha) was attractive and increased damage; but doses of 4.52kg/ha or higher, significantly reduced damage for 18 days. The granule formulation needs to be optimized to increase the duration of the protection, but early indications suggest that VOCs granules appear to be a possible candidate to achieve wireworm control.

The discovery of magnetism in two dimensional materials, first found in CrI₃ and Cr₂Ge₂Te₆, has opened up many possibilities to explore, concerning novel phenomena and potential applications in spintronics. Prior to these discoveries, it was thought that magnetism in two dimensions was impossible due to the conditions of the Mermin-Wagner theorem. In this work, the ferromagnet CrI₃ is examined in the monolayer to understand how its magnetism may be utilised, potentially in racetrack memory. Ab initio methods provides a way to study CrI₃'s nature by calculating individual exchange values and using them to find an appropriate spin model. In the initial literature, CrI₃ was identified as an Ising magnet based on experimental data. By using a Monte Carlo atomistic simulation approach, the Curie temperature (T_C) of the material can be calculated. Thus, the suitability of the Ising model can be assessed. The atomistic spin model shows that the Ising model overestimates the T_C, and we show that by using a modified Heisenberg model, a value of T_C much closer to experiment can be calculated. Using this model and our calculated exchange parameters, the spin dynamics in monolayer CrI₃ can be examined with the Landau-Lifshitz-Gilbert equation. Spins in monolayer CrI₃ appear to fluctuate in zero-field cooling simulations, seemingly as a result of the system's high anisotropy. Zero-field cooling simulations are also performed on bulk CrI₃ revealing a multi-step transition in the material. To consider CrI₃'s potential in domain wall motion based technology, simulations of a moving domain wall are carried out on a short section of nanowire to assess domain wall motion under applied field and with the use of a spin-transfer torque, applied current. High velocities of 1020 m/s are reached under applied current. At this point the material undergoes a Walker breakdown collapse of the domain wall, so the limits of CrI₃ domain wall motion capabilities are shown.

Magnetism in recently discovered van-der-Waals (vdW) materials such as CrI3 has opened new avenues in the study of fundamental spin interactions in two dimensions (2D). Such developments allow us to assess whether magnetism in 2D is a smooth continuation from low-dimensional magnetic materials, or if new physics and magnetic behaviour arise due to the reduced dimension. In this thesis, we present an investigation of 2D magnetism through the use of a multiscale approach involving atomistic scale models with Hubbard-U corrected Density Functional Theory (DFT+U) and mesoscale models with Monte-Carlo (MC) simulations. Firstly, we performed a high throughput screening over 150 compounds, identifying 59 potential two-dimensional magnets. To computed their critical temperature Tc, we parametrised the generalised Heisenberg model (XXZ) using DFT+U, and then we ran MC simulations. Most of the materials have a critical temperature below 100 K, but two of them, i.e. VPS3 and VSe2, may potentially show magnetism at room temperature. Some of these materials also stabilise vortex-antivortex spin texture at low temperatures. These vortex structures are similar to those from the XY model. We find that the XXZ model can not reproduce the magnitude of the experimental Tc of CrI3 if only first-nearest neighbour interactions are included. We thus extended the XXZ model to include more neighbour interactions as well as higher-order exchange terms and Dzyaloshinskii-Moriya interactions. This extended model gives a better estimation of the critical temperature for several materials, including the chromium trihalides family (CrX3, X=F, Cl, Br, I). Modeling the magnon spectra using this model within the linear spin-wave theory gives good agreement with the experimental results. In particular, we show that for CrI3 the quantum effects are essential when it comes to describing the critical exponent in a Curie-Bloch law, and the shape of magnetisation versus temperature curve. We present a study of the magnetic domain in CrI3 monolayer and show that the domains are metastable. The boundary between the magnetic domains called the domain walls (DW) is in the majority of Néel type (97 %), with the rest being either Bloch type or hybrids. Finally, we show the nucleation of topological spin texture (TST) in CrCl3. We identify these TSTs as merons and anti-merons. The TSTs are stabilised through the dipole-dipole interaction. They are dynamic and can annihilate through non-trivial processes.

The limitations of the down-scaling of the silicon metal-oxide-semiconductor (MOS) field-effect transistor (FET) and the demand for energy-efficient appliances motivate the study of alternative devices. The device under consideration is the biaxial tensile-strained Germanium on InGaAs alloy tunnel FET (TFET). The type of device depends on the band alignments at the heterojunction. For example, it determines if it can work as a TFET or a MOSFET. In turn, the band alignments depend on the atomistic details of the interface structure. First-principles calculations were performed to obtain the band alignments of the simpler Ge/GaAs system. The difference of the Ge and GaAs band gaps, calculated at Many-Body Perturbation Theory (MBPT) level, is added to the potential line-up of a supercell structure containing the Ge/GaAs interface, calculated using Density Functional Theory (DFT). It was studied how the band alignment is affected by the disorder, the stoichiometry composition and the diffusion of atoms at the interface. Calculations on the relative interface formation energy were performed to determine the relative stability of the various models for the interface. Results show how the heterojunction type changes when going from As-rich to Ga-rich conditions and with the diffusion of atoms over a few layers. These results explain the interval range of the experimental measurements and highlight the challenge of obtaining a Ge/GaAs TFET as this implies the control of atomic diffusion at the interface. Results also show that As-rich interface conditions are more favourable than Ga-rich conditions and that the contribution of disorder at the interface to the potential line-up is negligible. Further, preliminary studies were performed for band gaps of strained Ge and InGaAs alloys. Besides, an alternative approach to the expensive MBPT was developed based on enforcing Koopman's theorem to DFT to obtain accurate band gaps at a low computational cost.

Lead telluride is a reference high-performance thermoelectric which possesses a remarkable efficiency at intermediate temperatures in its pristine form, largely due to its low lattice thermal conductivity. Several studies have been proposed to analyse this thermal conductivity, but their range of application is usually limited: first-principles simulations do not allow for the study of intrinsic defects in large structures and the available classical potentials tend to overestimate the lattice thermal conductivity of the bulk material. Here, we present an optimised Coulomb-Buckingham potential that provides an improved description of the material when compared to other force fields in the literature, especially for the bulk lattice thermal conductivity, and that allows for the study of intrinsic defects. Using this potential, we studied how intrinsic defects influence the thermal conductivity, paying special attention to vacancies, interstitials, voids and grain boundaries. Our results show that the presence of both vacancies and grain boundaries can separately reduce the value of the thermal conductivity to ~ 0.5 W/mK, regardless of the temperature, i.e. a factor of 4 at room temperature. A similar decrease is observed in the presence of interstitials and voids, but in this case concentrations are more limited. However, small grains are not stable at high and intermediate temperatures, so in the second part of the thesis we study if grain boundaries survive or not against thermal coarsening under operational conditions. We attacked this problem by means of a multiscale dynamical modelling approach applied to porous, polycrystalline PbTe through a phase-field model informed by molecular dynamics simulations. The main hypothesis considered was that voids act as Zener pinning particles to stop grain growth and stabilise grain boundaries. We analyzed the stability of voids and determined metastable void sizes at different temperatures and observed that the presence of voids can stabilise grains with radii of the order of hundreds of nm. A simple serial model allowed us to calculate the thermal conductivity of these porous polycrystalline materials and showed that it can be reduced by a 35 %, with lower reductions at higher temperatures.

Nanostructured materials have recently attracted much interest due to their unique properties which make them appealing for the next generation of efficient and novel optical/electrical devices. In particular, two-dimensional (atomically thin) materials such as transition metal dichalcogenides (TMDs) have been extensively researched and shown to exhibit, e.g., strong optical absorption when compared to their bulk counterparts, with potential applications in photovoltaics for solar energy. The optical absorption in such materials may be further improved by decoration with metal nanoparticles (MNPs) due to a plasmonic enhancement effect. While such an effect has been demonstrated experimentally, it is hard to show theoretically from an ab initio point of view due to the computational demand of such calculations: the state-of- the-art approaches commonly used to determine the optical spectra of, e.g, the semiconducting TMD (i.e. density functional theory (DFT) and many-body perturbation theory) may not be applied to the full TMD-MNP structure as the MNP is often on a much larger scale than the semiconductor rendering the ab initio methods infeasible. In this thesis, we develop a dynamical, hybrid approach (the projected equations of motion (PEOM) method) where a composite material is split into subsystems which are then coupled electromagnetically. The primary system (e.g., the TMD) may be treated using any time- dependent theory, while the secondary system (e.g., the MNP) is modelled entirely through its frequency-dependent polarisability which is fitted to obtain parameters which enter into the PEOMs. As a proof of concept, the PEOM method is first applied to a semiconducting quantum dot- MNP system and the obtained energy absorption rates and population dynamics are compared with results from existing analytical approaches. Good agreement between the two methods is shown, while the PEOM proves to be an improvement when investigating ultrafast excitations in such systems. The method is then further tested by comparing absorption spectra in bilayers of hexagonal boron nitride and MoS2 obtained via the PEOM approach with those obtained by time-dependent DFT and the Bethe-Salpeter equation. Again, the two approaches are in good, qualitative agreement. The thesis is left open as a starting point towards investigating the TMD-MNP structures, as well as stating the limitations of the PEOM method and suggesting possible improvements.