LiF Particles for Chemical Seeding of Diamond Films: Nucleation and Growth in Microwave Plasma CVD

Polycrystalline diamond films and coatings are widely used in optical components, thermal management, and cutting tools. Chemical vapor deposition (CVD) is widely used for their synthesis, while the initial nucleation stage largely determines the continuity and morphology of the diamond layer. Conventional nucleation is achieved by surface seeding with nanodiamond particles. An alternative approach is chemical nucleation, extensively studied for carbon-containing precursors and polymers, whereas nucleation on non-carbon particles remains comparatively unexplored. Here, lithium fluoride (LiF) particles were used as a model non-carbon system to study diamond nucleation and subsequent growth in CH4–H2 microwave plasma CVD. Surface morphology and Raman spectroscopy showed that LiF promotes localized formation of well-faceted diamond crystallites without conventional nanodiamond seeding. Increasing the substrate temperature to 900°C produced a nearly linear increase in nucleation density and enabled the formation of nearly continuous polycrystalline films, whereas further heating to 950°C promoted island-like growth despite increasing growth rate. Methane concentration mainly affected the growth regime: at CH4 contents of 10% and higher, secondary nucleation became pronounced, suppressing crystallite enlargement and promoting the transition from microcrystalline to nanocrystalline diamond. Raman analysis showed that substrate temperature had little effect on the sp3/sp2 ratio, although the diamond Raman line broadened at higher temperatures, whereas elevated methane concentrations increased the contribution of non-diamond carbon phases. These findings suggest that non-carbon chemical nucleation can provide a new perspective on diamond formation during CVD, including the emergence of diamond nuclei under conditions where diamond remains thermodynamically metastable.