This thesis focuses on the preparation of nanostructures for electron emission sources, with an emphasis on field emission. The theoretical part summarizes the types of electron sources, the mechanisms of electron emission into vacuum, and practical aspects of Fowler–Nordheim analysis, including the interpretation of the effective field-enhancement factor and possible sources of nonlinearity. The experimental part develops a low-temperature growth of GaN nanocrystals based on Ga deposition from an effusion cell followed by nitridation using hyperthermal nitrogen ions with an energy of 50 eV. The method is applied to two substrates: (1) the preparation of GaN on tungsten tips of real emission cathodes, where the roles of surface properties, Ga diffusion, and the influence of the electric field on the reproducibility of growth on the very tip are discussed; and (2) the growth of GaN nanocrystals on graphene-coated copper substrates. In the second system, the nanocrystal density and morphology, including the formation of locally hollow, edge-rich structures, are controlled by varying the deposition temperature (100–200 °C) and the ion current density during nitridation (1–1.9 µA/cm^2). The morphology is correlated with the emission characteristics. The best sample exhibits a turn-on field of 3.4 V/µm and an effective field-enhancement factor of β = 1470. The thesis establishes a relationship between the morphology of low-temperature nitride nanostructures and their emission characteristics and formulates recommendations for their successful growth on both types of substrates.