In our work, we take advantage of the fact that silicon can be transformed from a disordered amorphous form into a crystalline form. Crystallization of amorphous silicon films is traditionally studied as an alternative method for producing large-area electronics, such as displays and solar cells, because silicon in an amorphous form can be grown easily in thin films on arbitrary substrates.

We employed a solid-state process at room temperature where the crystallization is facilitated by the presence of metal (nickel in this case) and the application of an electric field. We localized this process by applying an electric field via an atomically sharp tip – the tip commonly used in atomic force microscopes (AFM).

The challenge is gaining control over the flowing electrical currents (in the 0.1–10 nA range) so that the film is not destroyed by dielectric breakdown and the affected area is minimized. Controlling the discharge currents by a specialized circuit leads to more balanced dynamics of the crystallization process and enables miniaturization below 100 nm.

Furthermore, inherently precise positioning of AFM (facilitated by a piezocrystal element) enables the fabrication of microscopic matrices of crystallized dots. As shown in the figure, each dot is embedded in a nanoscale pit. The nanocrystals are visualized in situ by AFM in the regime where electrical current is measured at a given bias voltage as the tip is scanning the surface.

We envision that the presented technology may be useful in creating/positioning silicon nanocrystals in predefined locations with nanoscale accuracy for optoelectronic elements, creating crystalline (conductive) pathways in the amorphous matrix, or creating nanowells for microscale chemistry or data storage.

Further research is driven by the question of whether further miniaturization is possible by optimizing the layer thickness, energy transfer rates, current amplitudes, loading force on the tip, tip material, ambient conditions and other parameters.