No presente estudo, amostras policristalinas ricas em Ta e com estequiometrias Ta1-xZrx; x < 0.15; foram preparadas através da mistura apropriada dos elementos metálicos, os quais foram fundidos em forno a arco elétrico sobre uma placa de cobre refrigerada a água e sob atmosfera de argônio de alta pureza. Os padrões de difração de raios-X das ligas, como fundidas (as cast) e tratadas termicamente a 850 °C por 24 h, revelaram a ocorrência de uma estrutura cristalina cúbica de corpo centrada bcc, tipo W, e parâmetros de rede que aumentam suavemente com o aumento do teor de Zr nas ligas. Medidas de susceptibilidade magnética dc, conduzidas nas condições de resfriamento da amostra em campo zero (ZFC) e do resfriamento com o campo magnético aplicado (FC), indicaram que supercondutividade volumétrica é observada abaixo de ~ 5.8, 6.9, 7.0 K em amostras com x = 0.05, 0.08, e 0.10, respectivamente. Essas temperaturas críticas supercondutoras são bastante superiores àquela observada no Ta elementar ~ 4.45 K. Medidas de resistividade elétrica na presença de campos magnéticos aplicados de até 9 T confirmaram a temperatura crítica supercondutora das amostras estudadas. O campo crítico superior Hc2 e o comprimento de coerência E foram estimados a partir dos dados de magnetorresistência. Os valores estimados de Hc2 foram de ~ 0.46, 1.78, 3.85 e 3.97 T, resultando em valores de E ~ 26.0, 13.6, 9.2 e 9.1 nm para as ligas as cast com x = 0.00, 0.05, 0.08 e 0.10, respectivamente. A partir dos dados experimentais do calor específico Cp das ligas, magnitudes estimadas do salto em Cp nas vizinhanças das transições supercondutoras indicaram valores maiores que o previsto pela teoria BCS. Utilizando as equações analíticas derivadas da teoria do acoplamento forte da supercondutividade foi então proposto que o aumento da temperatura de transição supercondutora nas ligas devido a substituição parcial do Ta por Zr está intimamente relacionado ao aumento do acoplamento elétron-fônon, visto que a densidade de estados eletrônicos no nível de Fermi foi estimada ser essencialmente constante através da série Ta1-xZrx com x < 0.10. / In the present study, polycrystalline samples of Ta-rich binary alloys with stoichiometry Ta1-xZrx; x < 0.15; were prepared by mixing appropriate amounts of the metallic elements which were arc-melted on a water-cooled hearth under high-purity argon atmosphere. The X-ray diffraction patterns of the as cast alloys and heat treated ones at 850 °C by 24 h revealed the occurrence of the body-centered cubic crystal structure bcc, type W, and lattice parameters that increase slightly with increasing Zr content. Magnetic susceptibility measurements dc, performed in zero-field cooling ZFC and field cooling FC processes, indicated that bulk superconductivity is observed below ~ 5.8, 6.9, and 7.0 K, in samples with x = 0.05, 0.08, and 0.10, respectively. These superconducting critical temperatures are higher than that of ~ 4.45 K found in elemental Ta. Electrical resistivity measurements under applied magnetic fields to 9 T corroborated the superconducting critical temperatures for the samples studied. The thermodynamic upper critical field Hc2 and the coherence length E were estimated from the magnetoresistance data. The estimated values of Hc2 were 0.46, 1.78, 3.85, and 3.97 T, leading to E 26.0, 13.6, 9.2, and 9.1 nm for the as cast alloys with x = 0.00, 0.05, 0.08, and 0.10, respectively. In addition to this, from the results of heat capacity Cp data, jumps in the vicinity of the superconducting transition were estimated and found to be larger than the one expected from the BCS theory. By using analytic equations derived from the strong coupling theory of superconductivity we argued that the enhancement of Tc in alloying Ta with Zr is due to the increase of the electron-phonon coupling, provided that the density of states in the Fermi level was found to be essentially constant in the series Ta1-xZrx; x < 0.10.

Processos fotofísicos em polímeros conjugados estão intimamente relacionados com a conformação dos segmentos moleculares. Filmes finos de polímeros conjugados apresentam uma anisotropia intrínseca resultante da conformação dos segmentos moleculares tornando esses materiais atrativos em estudos fotofísicos pela emissão polarizada apresentada quando excitado via radiação eletromagnética ou sob aplicação de uma diferença de potencial elétrico. Neste trabalho procuramos correlacionar o espectro de emissão fotoluminescente de filmes finos de um derivado do polifluoreno, nominalmente poli(9,9-dioctilfluorenil-2-7-diil) terminado com dimetilfenil, com sua anisotropia molecular. Filmes finos mecanicamente estirados sofrem um processo de reordenamento molecular induzindo a emissão de luz polarizada predominantemente na direção de estiramento. O estiramento ocasiona ainda um aumento no comprimento de conjugação efetivo dos segmentos moleculares influenciando no acoplamento elétron-fônon. Através da técnica de elipsometria, foi possível determinar os estados de polarização da luz (através dos parâmetros de Stokes) e medidas de fotoluminescência estacionária dependente da temperatura nos possibilitou aferirmos sobre o acoplamento elétron-fônon a partir do Princípio de Franck-Condon. Medidas de fotoluminescência de excitação (PLE) determinou que o espectro da PL consiste da sobreposição espectral de duas espécies emissoras: a espécie isolada e a espécie agregada. Para baixas temperaturas a PL apresenta picos de emissão bem definidos como resultado da dinâmica molecular do PFO correspondendo ao favorecimento de emissão da espécie isolada. Para temperaturas acima da temperatura de transição \'beta\' (~270 K), a emissão da espécie agregada é favorecida, ocorrendo uma possível transferência de energia da espécie isolada para a agregada. O estiramento induz um aumento do comprimento de conjugação, refletido na diminuição do fator de Huang-Rhys, \'S IND. ISO\'POT. LO\'|140 K = 0,40 para amostra não estirada e \'S IND. ISO\'POT.2LO\'| 140 K = 0,19 para a amostra com a maior taxa de estiramento, tornando o espectro mais resolvido. Amostras não estiradas sob excitação paralela ao estiramento apresentaram polarização total de emissão P = 3,4% linearmente paralela ao estiramento e anisotropia de fluorescência de r = 0,025 e amostras com estiramento L = 2Lo apresentaram P = 46,1% de emissão polarizada ao longo da direção de estiramento e uma anisotropia de fluorescência de r = 0,27. A emissão polarizada mostrou ser independente da temperatura. A anisotropia de fluorescência mostrou ser fortemente dependente do estiramento e da anisotropia para temperaturas acima de 340 K, temperatura característica de um inicio de transição de fase do PFO. / Photophysics processes in conjugated polymer are closely related with the molecular segments conformation. Conjugated polymers thin films has shown an intrinsic anisotropy due to the molecular segments conformation making this materials attractive in photophysics studies by its polarized emission when stimulated by light or biased. In this work, we correlated the photoluminescence spectra of a derivative PFO polymer thin films, namely poly(9,9-dioctylfluorenyl-2,7-diyl) end capped with dimethylphenyl, with the molecular anisotropy. Mechanically stretched thin films undergo a molecular rearrangement process of inducing emission of light predominantly polarized in the direction of stretch. The stretching also causes an increase in the effective conjugation length of the molecular segments influencing the electron-phonon coupling. By ellipsometry technique, it was possible to determine the polarization states of light (by the Stokes parameters) and temperature dependent stationary photoluminescence measurements enabled us to get the electron-phonon coupling from the Franck-Condon principle. Measurements of photoluminescence excitation (PLE) have determined that the PL spectrum consists of spectral overlap of the two emitting species: the isolated and aggregated species. At low temperatures the PL emission peaks has presented well-defined as a result of PFO molecular dynamics favoring the emission of the isolated species. For temperatures above the transition beta temperature (270 K), the emission of aggregated species is favored, causing a possible energy transfer isolated to aggregate species. The stretching induces an increase in the conjugation length, reflected in the decreasing Huang-Rhys factor \'S IND. ISO\' POT. LO\'|140 K = 0,40 to non-stretched samples and \'S IND. ISO\' POT. 2Lo\'| 140 K = 0,19 for the sample with the highest draw ratio, making the spectrum more resolved. Unstretched samples under polarized excitation parallel to the stretching showed total polarized emission P = 3,4% linearly parallel to the stretching and fluorescence anisotropy of r = 0,025 and the L = 2Lo samples showed P = 46,1% of polarized emission along the direction of stretching and fluorescence anisotropy r = 0,27. The polarized emission was found to be independent of temperature. The fluorescence anisotropy was found to be strongly dependent of stretching rates and for temperatures above 340 K, a characteristic onset temperature of phase transition of the PFO.

Electronics industry has been developing at a tremendous rate for last five decades and currently is one of the biggest industries in the world. The key to the rapid growth of electronics industry is innovation that made possible the constant scaling of transistors with reduced cost and improved performance. Scaling transistors were simpler at the beginning, but currently as the gate length of transistors has reached few nanometers, different short channel effects have emerged and power density of transistors has also increased drastically, which made further scaling much more challenging. To study electro-thermal transport in these reduced dimensionality devices, continuum models are no longer sufficient. In this work, the electrical and thermal transport properties have been modeled by solving Boltzmann Transport Equation (BTE) for electrons and phonons, respectively, using the Monte Carlo (MC) technique. To solve BTE for the phonons, a coupled Molecular Mechanics-Monte Carlo approach is employed where phonon band-structure is obtained using the atomistic modified Valence Force Field (VFF) model and is coupled with a Monte Carlo Phonon Transport kernel which solves the BTE for phonons. The phonon-phonon scattering is modeled in relaxation time approximation (RTA) using Holland’s formalism. Diffusive boundary scattering for phonons has been modeled using the Beckmann-Kirchhoff (B-K) surface roughness scattering model taking into account the effects of phonon wavelength, incident angles and degree of surface roughness. The effect of rough surface on longitudinal acoustic (LA) and transverse acoustic (TA) phonon branches has been studied with the help of the B-K model and it has been found that, at elevated temperatures, there is less backscattering to the LA branch due to rough surface. Effort has been made then to couple the developed phonon Monte Carlo transport simulator with an electron Monte Carlo transport simulator to study the origin and effects of self-heating in a nanoscale field-effect transistor (FET). In contrast to the widely used continuum model, where Fourier heat diffusion equation is usually solved to describe the thermal transport, the simulator developed in this dissertation treats both the electrons and the phonons at the particle level. Acoustic and intervalley g and f type electron-phonon scattering mechanisms are considered and the resulting local temperature modification has been used to bridge the electron and phonon transport paths. Phonon transport at the oxide-silicon interface has been modeled using the Diffuse Mismatch (DM) model, whereas, the phonons in the oxide have been described using the Debye model and temperature and frequency dependent relaxation time. The simulator is then benchmarked and used to study the electron-phonon transport processes in a FinFET device with a gate length of 18 nm, channel width of 4 nm, and a fin height of 8 nm. Preliminary results show that there can be a current degradation of as high as ~9.56% due to self-heating effect. Also, temperature in the entire channel region could rise due to self-heating. The maximum temperature rise in the channel region is found to be ~30K.

In absence of external fields, vibrational modes of periodic systems are usually considered as linearly polarized and, as such, they do not carry angular momentum. Our work proves that non-adiabatic effects due to the electron-phonon coupling are time-reversal symmetry breaking interactions for the vibrational field in systems with non-collinear magnetism and large spin-orbit coupling. Since in these systems the deformation potential matrix elements are necessarily complex, a nonzero synthetic gauge field (Berry curvature) arises in the dynamic equations of the ionic motion. As a result, phonon modes are elliptically polarized in the non-adiabatic framework and intrinsic vibrational angular momenta occur even for non-degenerate modes and without external probes. These results are validated by performing fully relativistic ab-initio calculations on two insulating platinum clusters and a metallic manganese compound, with non-collinear magnetism. In both cases, non-adiabatic vibrational modes carry sizeable angular momenta comparable to the orbital electronic ones in itinerant ferromagnets.

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Les phonons mous et le couplage électron-phonon sont considérés comme responsables d’un grand nombre de transition de phase. Pour en comprendre complétement les mécanismes, il est nécessaire d’étudier à la fois les modifications structurales, les changements de configuration électronique et les dynamiques de réseau cristallin. De toute évidence, cela représente une charge de travail expérimental et théorique considérable, voire même hors d’atteinte.Néanmoins, il est parfois possible d’introduire certaines simplifications et d’ainsi rendre une telle étude réalisable. C’est le cas pour les deux systèmes au cœur de ce travail de thèse, pour lesquels la transition de phase peut être diviser en deux instabilités : l’une est structurale, intrinsèque aux éléments constitutifs du système et l’autre, superposée, provient de la configuration électronique. L’interaction entre ces instabilités est illustrée à travers l’exemple de deux systèmes à priori hétérogènes, la famille des bronzes monophosphate de tungstène d’une part et l’antimoine d’autre part, qui révèleront finalement posséder des similarités inattendues. La combinaison des techniques de diffusion diffuse et de diffusion inélastique de rayons X permet des observations qualitatives et une meilleure compréhension de la situation pour les deux systèmes.Les bronzes monophosphate de tungstène font partie de la famille des oxydes quasi-2D, (PO2)4(WO3)2m, qui ont la particularité d’être sujet à des instabilités de type onde de densité de charges (ODC). Ces bronzes sont constitués d’une structure de perovskite vide composée par des couches octaédriques (WO3)2m. L’épaisseur de chacune de ces couches est définie par la valeur de m, qui mène ainsi à différents types de phase d’ODC. Le cas du terme m=2 a aussi été étudié car le fait que les chaînes zig-zag y soient isolées conduit à une instabilité quasi-1D. La présence d’une phase d’ODC a été découverte à TC=270K avec q=0.25b*. Cette phase est engendrée par le mouvement à corps rigide, plus exactement, par les basculements corrélés des octaèdres. Pour les autres termes (m=6,7 et 8), l’instabilité structurale a une origine différente et est liée à l’agencement en couches de WO3, plus particulièrement aux déplacements corrélés des chaînes W-O-W-O. Ces derniers sont la cause d’une forte diffusion diffuse sur des plans spécifiques, résultant de la présence de phonons ’relativement’ mous localisés dans la même région. Ensuite, l’emboitement de la surface de Fermi quasi-2D est à l’origine de l’ancrage du vecteur de modulation sur une valeur spécifique de transfert de moment, définit par l’interaction de deux instabilités, structurale et électronique. De façon remarquable, l’amplitude des déplacements des atomes de tungstène dans le terme m=8 est beaucoup plus élevée que dans le m=6.L’antimoine à température ambiante possède une structure rhomboédrique, dérivant d’une légère distorsion de la structure cubique primitive (CP) par transition de Peierls. Sous pression, la distorsion se réduit sans toutefois disparaître complétement, puisque l’antimoine se transforme dans un premier temps en une série de structures complexes, pour finalement adopter celle possédant la plus grande symétrie, la structure cubique centrée (CC). De la même façon que pour les bronzes, les caractéristiques de la diffusion diffuse ainsi que, dans une certaine mesure, les particularités de la dynamique du réseau rhomboédrique, s’expliquent à travers de l’instabilité du réseau cubique primitif. Cette dernière est liée aux déplacements corrélés dans les chaînes avec direction pseudo-cubique <100>. En outre, les détails de la transition de phase peuvent être explicités par l’association de l’analyse des vecteurs critiques de la transformation CC-CP avec les résultats expérimentaux obtenus sur la dépendance en pression de l’énergie des phonons. / A large number of phase transitions can be interpreted as being driven by phonon softening and/or electron-phonon coupling. Thus, a full mechanistic description requires the understanding of structural transformation, changes in electronic structure and lattice dynamics. All together this represents an enormous, for many cases unrealisable, experimental and theoretical effort.However, with the introduction of appropriate assumptions the problem may be simplified. Here we concentrate on two systems, where the interpretation of the phase transition may be split into an intrinsic instability of the building blocks combined with a superimposed electronic instability. We illustrate the interplay between the framework and electron-phonon-related instabilities using the seemingly heterogeneous examples of phosphate tungsten bronzes and elementary antimony. Based on the combined results from diffuse and inelastic X-ray scattering, we propose for the two systems a picture that explains the experimental observations. The similarities found between these two systems are deemed to be rather surprising.Monophosphate tungsten bronzes are a family of quasi-2D-oxides, (PO2)4(WO3)2m, that exhibits charge density wave (CDW) instability. They contain empty perovskite WO3 slabs with varying thickness between different members, characterised by the $m$ value. This thickness defines the sequence of charge density wave phases that appear on cooling. The degenerate case of $m$=2, presenting a quasi-1D instability, was explored since the WO3-octahedra zig-zag chain is isolated. A CDW phase (TC=270K and q=0.25b*) is found to be linked to a rigid-body motion, precisely, to a correlation in the tilting of the octahedra. For the others studied members, as m=6,7 and 8, we found another kind of structural instability. In this case the origin comes from the WO$_3$ slabs framework, realised as correlated displacements of tungsten atoms along the octahedral 4-fold axis direction (W-O-W-O direction). This leads to a strong x-ray diffuse scattering localised in specific planes, linked to relatively soft phonons modes. Specific Fermi surface nesting, close to the 2D case, gives rise to a freezing of the modulations at the specific momentum transfer, defined by the interplay of two instabilities, the structural and electronic one. Remarkably, the displacements of W for m=8 are much superior than in m=6.Elemental antimony at ambient condition has an A7 rhombohedral structure, obtained by small distortion from primitive cubic (PC) lattice through a Peierls transition. Under pressure, the distortion is reduced, but remains finite, as antimony transforms through a series of highly complex structures, before adopting as last the highest-symmetry body-centred cubic (BCC) phase. The main diffuse scattering features and to some extent the peculiarities in the lattice dynamics of the A7 phase – as above - can be explained by the instability of the primitive cubic network with respect to correlated displacements along the chains with <100> pseudo-cubic directions. Analysis of critical vectors for the BCC-PC transformation together with experimentally obtained phonon-energies pressure dependence provides further insights into the details of the phase transformation.

<div>Metallic components and metal-dielectric interfaces appear widely in modern electronics and the thermal management is an important issue. A very important feature that has been overlooked in the conventional Fourier's equations analyses is the nonequilibrium thermal transport induced by selective electron-phonon (e-p) coupling and phonon-phonon (p-p) coupling. It signicantly affects many processes such as laser heating and ignoring this phenomenon can lead to wrong or misleading predictions. On the other hand, as devices shrink into nano-scale, heat generation and dissipation at the interfaces between different components start to dominate the thermal process and present a challenge for thermal mitigations. Many unresolved issues also arise from interfaces, such as the unexpected high interfacial thermal conductance (ITC) at metal-diamond interfaces. Both of these require a deep understanding of the physics at interfaces.</div><div><br></div><div><div>Therefore in this work, I present multi-scale simulations in metals/dielectrics and interfaces based on two-temperature model (TTM) and establish the new multitemperature model (MTM). The methods are combined with Fourier's Law, molecular dynamics (MD), Boltzmann transport equations (BTE) and implemented to predict the thermal transport in several materials and interfaces where e-p coupling and p-p coupling are important. First-principles studies based on density functional theory (DFT) are also presented as predictive approaches to acquire the properties, as well as investigating the new physical phenomenon of non-local e-p coupling in metals. This research seeks to provide general, sophisticated but also simple simulation approaches which can help people accurately predict the thermal transport process. It also seeks to explore new physics which cannot be captured and predicted by conventional analyses based on Fourier's Law and can advance our understanding as well as providing new insights in the current thermal analysis paradigm.</div></div><div><br></div><div><div>The rst part of this thesis focuses on the non-equilibrium thermal transport in metals and across metal-dielectric interfaces based on TTM. First of all, nonequilibrium thermal transport in metal matrix composites (MMC) is investigated. Metal particle is usually added to polymer matrix for enhanced thermal performance. Here we apply TTM calculations and manifest a \critical particle size" above which the thermal conductivity of the composite material can be enhanced. MD simulations are performed to predict the thermal properties. TTM-Fourier and TTM-BTE calculations are conducted as comparisons. The widely used Au-SAM (self-assemblymonolayers) material pair is chosen to demonstrate our models. For a 1-D SAMAu-SAM sandwich system, the two calculation approaches present almost identical results, and the critical particle size is 10.7 nm. A general interpretation of thermal transport in sandwiched metal thin lms between two dielectric materials is also presented. It is found that when the lm thickness is on the order of several nanometers, due to strong e-p non-equilibrium the thermal transport is dominated by phonons</div><div>and electrons hardly contribute.</div></div><div><br></div><div><div>Then the e-p non-equilibrium thermal transport across metal-dielectric interfaces is investigated using TTM-MD. One possible explanation to the unexpected ITC at metal-diamond interfaces is the cross-interface e-p coupling mechanism, which is based on the hypothesis that electrons can couple to phonons within a certain distance rather than just those at the same location. Therefore we extend TTM-MD by modifying its governing equation to a non-local integral form. Two models are proposed to describe the coupling distance: the \joint-phonon-modes" model and the \phonon-wavelength" model. A case study of thermal transport across Cu-Si interfaces is presented, and both models predict similar coupling distances of 0.5 nm in Cu and 1.4 nm in Si near the interfaces. The cross-interface e-p coupling can increase the ITC by 20% based on our models. Based on the results, we construct a new mixed series-parallel thermal circuit. It is shown that such a thermal circuit is essential for understanding metal-nonmetal interfacial transport, while calculating a single resistance without solving temperature proles as done in most previous studies is generally incomplete.</div></div><div><br></div><div><div>Inspired by the previous work, we investigate further into the physics of nonlocal e-p coupling. First-principles calculations based on DFT is used due to their predictive feature without assumptions or adjustable parameters. By calculating the e-p coupling in metal lms of different sizes, we nd that e-p coupling has size effect which can only be explained by a non-local coupling picture. Results show that in Al, electrons and phonons can couple to each other in a range of up to 2 lattice-constants, or 0.8 nm. The coupling strength between electrons and phonons in adjacent atomic layers still has 75% of that in the same layer. Comparative studies are also performed on Cu and Ag. Results show that their non-local e-p coupling is not as signicant as in Al, with coupling distances of 0.37 nm for Cu and 0.49 nm for Ag. Similar results in Cu and Ag also indicate that materials with similar electronic structures have similar non-local e-p coupling properties.</div></div><div><br></div><div><div>In TTM, it is assumed that phonons are in thermal equilibrium and have a common temperature. In the second part of this thesis we go beyond TTM to investigate the non-equilibrium between phonons as well. TTM is extended to a general MTM with e-p coupling strength for each phonon branch. An averaged scattering lattice reservoir is dened to represent p-p scattering. The thermal transport process in single-layer graphene under constant and pulse laser irradiation is investigated. Results show that the phonon branches are in strong non-equilibrium. A comparison with TTM reveals that MTM can increase the thermal conductivity prediction by 50% and the hot electron relaxation time by 60 times. We also perform MTM simulations on Si-Ge interfaces to investigate the effect of non-equilibrium thermal transport on ITC. Results show that thermal non-equilibrium between phonons will introduce additional resistance at the interfaces, which is similar with e-p non-equilibrium's impact on ITC at metal-dielectric interfaces.</div></div>

Comprendre le transport électronique dans les cristaux bidimensionnels est un enjeu conceptuel majeur pour la nanoélectornique de demain. Dans cette thèse, on dévelloppe des méthodes ab initio pour étudier l'interaction électron-phonon, l'écrantage et le transport dans le graphène. Pour surpasser les limites des méthodes ab initio en ondes planes, à l'origine destinées aux matériaux périodiques en trois dimensions, on tronque l'interaction coulombienne dans la troisième dimension, isolant ainsi le système bidimensionnel de ses images périodiques. Ceci est réalisé au sein de la théorie de la fonctionnelle de la densité en perturbation, afin de calculer la réponse de la densité de charge et le spectre des phonons dans un cadre bidimensionnel. On utilise ces méthodes pour obtenir un modèle quantitatif du couplage électron-phonon dans le graphène pour une configuration de type transistor à effet de champ. Le couplage aux phonons acoustiques est dominé par le champ de jauge non-écranté, que nous calculons en incluant l'effet des interactions électron-électron au niveau GW. Nos simulations des propriétés d'écrantage statiques du graphene valident les modèles analytiques et montrent que le potentiel de déformation est fortement écranté, de sorte que sa contribution à la diffusion des électrons par les phonons acoustiques est négligeable. On montre également que le couplage avec les phonons hors-plan est faible mais fini. On obtient la contribution de la diffusion par les phonons à la résistivité en résolvant l'équation de Boltzmann pour le transport. En dessous de la température ambiante, nos résultats confirment le rôle des phonons acoustiques et une augmentation de 15% du paramètre de jauge it ab initio permet un excellent accord avec l'expérience. Au dessus de la température ambiante, on dénote l'importance des phonons optiques intrinsèques. / Understanding the transport properties of two-dimensional crystals doped by field effect is a conceptual milestone for tomorrow's nanoelectronics. In this thesis we develop first-principles methods to investigate electron-phonon interactions, screening and phonon-limited transport in graphene. To overcome the limitations of existing plane-wave ab initio packages, originally devised for three-dimensional periodic solids, we truncate the Coulomb interaction in the third direction and isolate the 2D system from its periodic images. This is implemented in density-functional perturbation theory to calculate charge density responses and phonon spectra in a two-dimensional framework. We use those methods to develop a quantitative model of electron-phonon coupling for graphene in the field effect transistor configuration. We find that the coupling of electrons to acoustic phonons is dominated by the unscreened gauge field, which we compute with full inclusion of electron-electron interactions at the GW level. Our simulations of the static screening properties of graphene validate analytical models and reveal that the deformation potential is strongly screened, such that its contribution to acoustic phonon scattering is negligible. We find a small but finite linear coupling with out-of-plane phonons. By solving the Boltzmann transport equation we obtain the phonon-limited resistivity. Below room temperature, our results confirm the role of acoustic phonons and a 15% increase of the ab initio gauge field parameter leads to an excellent quantitative agreement with experiment. Above room-temperature, we point to the importance of the coupling with intrinsic optical phonons.