"Spicy world of NanoScience"
Vertical nanocrystal whiskers or nanowiskers are one-dimensional crystals that look like rods or threads several nanometers in diameter. The rods are often cultivated using the elements of the third and fifth groups of the periodic table. The most common nanocrystals of the third and firth groups are gallium arsenide and indium phosphide. These semiconducting materials present competition to silicon in the electronics field as diodes and transistors built using them are faster in operation. In addition, due to their ability to emit and absorb radiation with 1.55 micrometer wavelength such nanocrystals can be used to transmit information via fiber-optic lines.
Up until recently, it was common knowledge that a cubic structure is formed with all nanowhiskers of the 3-5 groups (except for nitrogen-containing) during crystallisation. Several experiments showed, however, that the structure can be hexagonal as well, with a hexagon as a base. This finding draw increased attention to nanowhiskers. On the one hand, such structure is unstable and can have a negative effect on the quality of the materials being produced, but on the other hand the hexagonal crystals of the 3-5 group are not very well studied yet and therefore it is possible to anticipate the discovery of new promising features. In any case, the primary target of the research must be the synthesis condition responsible for the formation of cubic or hexagonal nanowhisker crystal lattice.
Researchers at the St. Petersburg Academic University, and the Ioffe Technical Physical Institute have been exploring the subject for a long time. An article by Vladimir Dubrovsky et al. has been published recently in the Journal of Technical Physics, describing a model of nanowhisker growth and structure that they had developed. The study was supported by the Russian Foundation for Basic Research, the Presidium of Russian Academy of Sciences, and the Ministry of Education and Science of the Russian Federation.
The researchers studied the most common mechanism of nanowhisker growth — vapour-liquid-crystal. This method cultivation requires the vapours of the substance in question are precipitated on a substrate in the form of a drop that later solidifies forming a homogeneous structure. The growth surfaces are often activated using metallic catalyst drops which is presented by gold. In that case, a nanocrystal grows under the drop and after solidification it looks like a rod with a golden cap on its upper end. The size of the crystal depends on the amount of the substance precipitating on the substrate it grows on.
The model developed by St. Petersburg-based physicists takes into account not only the consecutive transition of the atoms from gaseous form into liquid and then into solid forms, but also other processes such as immediate precipitation of the gas on the surface of the crystal, and the diffusion of adsorbed atoms (adatoms) from the substrate surface into the growing crystal. The model allows to determine the probability of formation of cubic and hexagonal crystals depending on the precipitation conditions, substrate properties, and gaseous medium saturation. According to the discovery of the same research group made in 2009, the most important factor influencing the crystal structure if its transversal dimension. Nanowhiskers up to 50–70 nanometers in diameter grow in the cubic phase, whereas crystals of larger sizes grow in the hexagonal phase. The researchers note that the model can be used to forecast the crystal growth with various precipitation methods applied.