Advanced branching control and characterization of inorganic semiconducting nanocrystals

The ability to finely tune the size and shape of inorganic semiconducting nanocrystals is an area of great interest1-5, as the more control one has, the more applications will be possible for their use. The first two basic shapes developed in nanocrystals were the sphere6-8 and the anisotropic nanorod1, 9. The II-VI materials being used such as Cadmium Selenide (CdSe) and Cadmium Telluride (CdTe), exhibit polytypism, which allows them to form in either the hexagonally packed wurtzite or cubically packed zinc blende crystalline phase10, 11. The nanorods are wurtzite with the length of the rod growing along the c-axis1. As this grows, stacking faults may form, which are layers of zinc blende in the otherwise wurtzite crystal. Using this polytypism, though, the first generation of branched crystals were developed in the form of the CdTe tetrapod 12. This is a nanocrystal that nucleates in the zinc blende form, creating a tetrahedral core, on which four wurtzite arms are grown. This structure opens up the possibility of even more complex shapes and applications. This dissertation investigates the advancement of branching control and further understanding the materials' polytypism in the form of the stacking faults in nanorods.;Understanding the nature of the polytypism in these materials is paramount to controlling their branching. Thus the first step is understanding the formation of stacking faults, which are the most common appearance of polytypism in these materials. By performing a thorough statistically analysis of the growth of stacking faults in these rods, a better understanding is obtained on how and where the faults form, and how best to encourage branching. With this knowledge, more complex structures begin to make more sense, such as heterostructures. The semiconductor heterostructures developed here incorporate multiple materials into a single nanocrystal. Additionally, they can incorporate a second generation of branching as well to form even more complex structures. One example of such a structure is a CdSe tetrapod with branching CdTe at the end of each original rod, resulting in a nanocrystal with a total of 12 arms. In addition to this method, oriented attachment is also investigated here as a viable means of branching. Using this technique, gold is used as an intermediate method of attachment for two CdSe rods. Once the gold joint is ultimately removed a new piece of CdSe is grown between the two original rods, and its crystalline phase appears to be dictated by the angle and orientation of the joining rods. Through these methods of crystalline growth and characterization new progress is made toward the ultimate goal of complete structural control over materials such as these II-VI semiconductor nanocrystals.