Scanning tunneling microscopy has been employed to create nanostructures at both polarities of the applied bias voltage, using the hexafluoroacetylacetonate Cu (I) vinyltrimethylsilane precursor molecule on the Si(111) surface at 300 K. Two distinctive mechanisms controlling the formation of nanostructures have been delineated. The first mechanism causes the dissociation of the molecule by an electron attachment process. The second process involves excitation of the molecule by the applied electric field; here field induced surface diffusion acts to supply molecules to the nanostructure, growth region under the tip. Both mechanisms have a threshold in the range of 2.5-4.5 V, and while the electron current induced process operates only at positive sample bias, the field induced mechanism works at both bias polarities. Model calculations of the field assisted growth mechanism, which involves both the radial diffusion of the precursor species from outer surface regions and their activation underneath the tip, show a very reasonable quantitative agreement with the experimental data.