Methods of fabricating polycrystalline diamond include subjecting a particle mixture to high pressure and high temperature (HPHT) conditions to form inter-granular diamond-to-diamond bonds. Before being subjected to HPHT conditions, the particle mixture includes a plurality of non-diamond nanoparticles, diamond nanoparticles, and diamond grit. The non-diamond nanoparticles includes carbon-free cores and at least one functional group attached to the cores. Cutting elements for use in an earth-boring tool include a polycrystalline diamond material formed by such processes. Earth-boring tools include such cutting elements.