Shape control in nanocrystal growth

Shape control in nanocrystal growth is the control of the shape of nanocrystals (crystalline nanoparticles) formed in their synthesis by means of varying reaction conditions. This is a concept studied in nanosciences, which is a part of both chemistry and condensed matter physics. There are two processes involved in the growth of these nanocrystals. Firstly, volume Gibbs free energy of the system containing the nanocrystal in solution decreases as the nanocrystal size increases. Secondly, each crystal has a surface Gibbs free energy that can be minimized by adopting the shape that is energetically most favorable. Surface energies of crystal planes are related to their Miller indices, which is why these can help predict the equilibrium shape of a certain nanocrystal.

Because of these two different processes, there are two competing regimes in which nanocrystal growth can take place: the kinetic regime, where the crystal growth is controlled by minimization of the volume free energy, and the thermodynamic regime, where growth is controlled by minimization of the surface free energy. High concentration, low temperatures and short aging times favor the kinetic regime, whereas low concentration, high temperatures and long aging times favor the thermodynamic regime.

The different regimes lead to different shapes of the nanocrystals: the kinetic regime can give anisotropic shapes which are often connected to the kinetic Wulff construction, whereas the thermodynamic regime gives equilibrium, isotropic shapes, which can be determined using the Wulff construction.

The shape of the nanocrystal determines many properties of the nanocrystal, such as the band gap and polarization of emitted light.