The fabrication of integrated multimaterial structures with nanoscale accuracy is a challenging pursuit essential for novel advances in electronics and photonics. The atomic force microscope precisely controls the synthesis of these nanostructures by triggering chemical reactions at determined substrate locations. Using a biased atomic force microscope tip, we have recently demonstrated the localized synthesis of germanium and silicon from organometallic liquid precursors. This localized synthesis yields the direct-write of germanium and silicon nanostructures onto the substrate. Here, we elucidate the reactions occurring during this synthesis in the tip–sample proximity. Reactions for both precursors are triggered by field-emitted electrons that fragment the precursor molecules into the desired products. In this reaction pathway, the synthesis of germanium is more favourable than the synthesis of silicon from precursors containing the same ligands. We explain these differences by comparing experimental data of reaction onset and kinetics to continuum/VASP simulations. This data is supplemented with electron impact mass spectrometry of the precursors and compared with the simulation results.