Mechanisms of precipitation and shape evolution of mercury telluride nanocrystallites in mercury telluride-lead telluride induced by controlled precipitation technique

The first comprehensive study on the mechanisms of precipitation and shape evolution of mercury telluride (HgTe) nanocrystallites precipitated in lead telluride (PbTe) matrix by controlled precipitation technique is presented.; The present dissertation presents the results of the controlled precipitation of HgTe nanocrystals in a PbTe semiconductor matrix and demonstrates its effectiveness in producing well-organized and crystallographically aligned semiconductor nanocrystals. Following the same procedure used in metallic alloys, the semiconductor alloys are treated at 600°C for 48 hours, quenched and aged up to 500 hours at 300°C and 450°C to induce homogeneous nucleation and growth of HgTe precipitates. Examination of the resulting precipitates using transmission electron microscopy (TEM) and high resolution TEM (HRTEM) reveals that the perfectly coherent HgTe precipitates form as thin ellipsoidal discs along the {lcub}100{rcub} habit planes of PbTe matrix due to disordering induced lattice mismatch. It is also found that the precipitate undergoes a gradual thickening and a faceting as a result of strain relaxation under isothermal aging up to 500 hours. Also, the HgTe platelets experience a gradual ordering induced by the coherency strain. Furthermore these nanocrystallites exist as a metastable precipitate without any noticeable coarsening up to 500 hours. The evidence for the order-disorder transition of HgTe nanocrystallites, the major cause of this unique morphology change, is characterized by both HRTEM and Raman spectroscopy study. Characterization of Raman frequency shift as well as atomistic HRTEM images of HgTe confirm its importance in the shape evolution in this system. These results, combined with the extreme dimension of the precipitates (4 nm in length and sub-nanometer in thickness) and the simplicity of the formation process, leads to the conclusion that controlled precipitation is an effective method for preparing desirable quantum-dot nanostructures.