Silicon and germanium nanowires (Si and GeNWs) are promising materials for electronic applications due to their high aspect ratio and the ability to incorporate dopant molecules in-situ during growth. Unfortunately, the frequently employed gold catalyst used in vapor-liquid-solid (VLS) growth of Si and GeNWs exhibits a high solubility for dopants such as boron and phosphorous<SUP>1</SUP> - a property that leads to diffuse interfaces when attempting to grow <I>p-n</I> junctions. We have explored the abruptness of dopant homojunctions in VLS grown nanowires using atom probe tomography to correlate fundamental thermodynamic parameters with junction abruptness. Growth from alloy nanoparticles has also been explored as a potential means to rationally control dopant solubility, and hence junction abruptness, during growth. We previously demonstrated that local electrode atom probe (LEAP) tomography can be used to precisely map the three-dimensional positions and chemical identities of the atoms in a nanowire sample with sub-nm spatial resolution<SUP>1</SUP>. Si and GeNWs were grown using a hot-walled CVD reactor, with silane and germane as the semiconductor precursor sources, and p- and n-type growth was performed using diborane and phosphine, respectively. During growth, the dopant gas flows were removed, and growth was continued in order to fully deplete the catalyst of the retained dopant species. By measuring the decay in dopant concentration along the length of the nanowire using LEAP tomography, we are able to determine the dopant solubilities in Au during growth, as well as the segregation coefficients. Core-shell Au-Cu<SUB>2</SUB>O nanoparticle catalysts have also been synthesized for use as alloy seeds for NW growth. Upon annealing at 450ºC, these particles form an Au-Cu alloy, with the alloy composition determined by the thickness of the Cu<SUB>2</SUB>O shell. GeNW growth with these particles was performed at 320ºC, which is well below the Au-Ge and the Cu-Ge eutectic temperatures, suggesting growth proceeds via the vapor-solid-solid (VSS) mechanism. We anticipate that dopant solubility in a solid catalyst will be markedly lower than that of a liquid catalyst, which may enable the growth of atomically abrupt <I>p-n</I> homojunction NWs.