Phase-Determining Factors in the Colloidal Synthesis of Metastable Transition-Metal Chalcogenide Nanocrystals

Book excerpt: In nanochemistry, it is a common goal to be able to synthesize materials with a specific morphology, composition, size, and crystal structure. This type of synthetic control is desirable as even small changes in one of these four factors can lead to drastically different material properties. An example of this is the formation of non-equilibrium crystal phases which are metastable in bulk systems and often exhibit characteristics unique from the thermodynamically stable phase. Consequently, these metastable phases are favored for certain applications due to their properties. However, attempts to synthesize a phase pure sample of a non-equilibrium crystal phase are often fraught with difficulties as phase determining factors are not well understood for nanoscale syntheses. As a result, nanoscale synthetic research is frequently performed in an exploratory, semi-serendipitous method. It is therefore desirable to expand our understanding of nanoscale reactions so as to facilitate the directed synthesis of specific nanoscale targets. In the research reported herein, attempts were made to elucidate the phase determining factors in two systems: polyol synthesis of manganese(II) sulfide, and cation exchange with cobalt and manganese. First, a manganese(II) sulfide, MnS, polyol synthesis was studied to elucidate which reaction variables have the greatest impact in crystal phase determination. Initial research showed that reaction temperature, aging time, and manganese source had the greatest influence on the crystal phase of the MnS product. Further investigation of this reaction implied that a complex relationship exists between the decomposition rate of the sulfur source, the concentration of the tetramethylammonium hydroxide pentahydrate, and the presence of an in situ intermediate. Although this complex relationship was not deconvoluted through this research, these results nevertheless help to shed light on the phase determining factors in the polyol synthesis of manganese(II) sulfide. In addition, cation exchange was investigated as a method for the synthesis of metastable phases. Although the use of ion exchange to access metastable phases had been reported previously, much of this research was limited to copper, silver, cadmium, lead and zinc chalcogenides.1-3 In this work, cation exchange with cobalt and manganese from a roxbyite-type Cu2-xS precursor was explored. Ultimately, metastable wurtzite-type cobalt- and manganese sulfides were synthesized through this exchange process. This result is of great interest for several reasons. First of all, cation exchange was performed with two transition metal cations which had previously only served as dopants in exchange reactions. This work also demonstrates a colloidal method for the synthesis of wurtzite-type CoS which had previously only been synthesized through chemical bath deposition methods.4 Finally, formation of the metastable wurtzite-type phases demonstrates the retention of both anion and cation sublattice features -- a result which has not been reported in literature to date.