Fig. 2. Control of the lateral size and thickness of the BP films during growth.

a Schematic view of the experimental setup for the BP growth. Specially, the RP was evaporated to P₄ and diffused from low temperature (LT) side toward high temperature (HT) side. And, a massive of Au/Si substrates were vertically stacked and placed in the evacuated silica tube. These routes were employed for reducing the P₄ source involved in the transformation into BP. b–e BP films with lateral size ranging from several micronmeter to sub-millimeter under a proper control of the temperature gradient between the source at LT and substrates at HT. f–i The corresponding AFM images indicating the thicknesses of the BP films, ranging from several nanometers to hundred of nanometers.