Formation of bismuth nanocrystals on sapphire substrates

Bismuth nanocrystals and microcrystals were condensed on the sapphire (1-102) substrates by thermal evaporation in an Ar atmosphere. The shape, size and number of Bi nanocrystals were determined by intelligent image analysis. Most of Bi nanocrystals condensed after deposition for 10 s on the sapphire (1-102) substrates had hexagonal and rhombohedral shapes, which indicates the influence of a low temperature gradient at the nanocrystal-substrate interface and physicochemical interaction with the sapphire substrate, which has a similar structure. The increase of deposition time from 10 s to 20 s increased the sizes of most condensed Bi nanocrystals from 20 – 40 nm to 30 – 90 nm and promoted the formation of the microcrystals up to 160 nm. Two-layer Bi crystallites were observed at deposition time more than 15 s. Bi nanodisks and microdisks with a vertical orientation of flat surface toward the sapphire substrate surface condensed at deposition times of 15 s and 20 s. This orientation of disk-shaped crystallites is the result of anisotropy of Bi thermal conductivity and high thermal conductivity of sapphire, which affect the condensation process with the increase of the temperature gradient at the crystallite-substrate interface due to the increase of heat flux with Bi atom evaporation. X-ray images of the samples confirmed the shapes of Bi crystallites with orientation similar to the structure of Bi and the sapphire substrate and revealed Bi2O3 oxide on their surface, which was formed due to interaction with oxygen on the substrate surface and in the atmosphere during their storage.