Date of Award
Doctor of Philosophy
Filled tetrahedral semiconductors comprised of elements from group I, II, and V of the periodic table are of interest to the thermoelectric, photovoltaic, and battery fields due to their tunable electronic structures. However, until recently, the synthesis of these materials in a facile and scalable manner had remained elusive. In this thesis, we demonstrate the solution phase synthesis of three members of this class of compounds (LiZnP, LiCdP, and LiZnSb). Furthermore, we explore the presence of polytypism in these compounds and computationally investigate which additional phases would be expected to display polytypism. Finally, we closely investigate the phase space of Li-Zn-Sb made in solution to determine the relevant factors for phase and polytype selectivity.
We begin by demonstrating the first solution phase synthesis of a I-II-V semiconductor by utilizing lithium hydride, diethylzinc, and tri-n-octylphosphine as precursors to synthesize LiZnP. We generalized this synthesis to be successful with multiple Li (lithiumdiisopropylamide, phenyllithium, n-butyllithium, and lithium hydride), Zn (zinc stearate, zinc chloride, and diethylzinc), and P (triphenylphosphine and tri-n-octylposphine) precursors as well as substituting Cd for Zn by utilizing dimethylcadmium. Additionally, we were able to determine the mechanism of formation of these nanocrystals which agreed with prior literature reports for binary phosphides.
Following which, we further extended this synthetic method to yield LiZnSb in solution. Interestingly, despite all prior literature reports showing LiZnSb crystallizing in the hexagonal LiGaGe-type, LiZnSb prepared in solution was found to crystalline in the cubic MgAgAs-type. This report was the first example of polytypism within the ternary filled tetrahedral semiconductors. Given the promising thermoelectric properties of hexagonal LiZnSb, we calculated transport properties and found cubic LiZnSb to be comparable to its hexagonal polytype but with the advantage of having high figure of merit in both p and n-type variants.
With the surprising observation of polytypism within LiZnSb, we sought to better map out the phase space of Li-Zn-Sb to see if we could selectively target both hexagonal and cubic LiZnSb. We utilized a high throughput synthetic robot to screen the effects of precursor concentration, injection order, nucleation and growth temperatures, and reaction time on reaction products. Surprisingly, we found another previously unreported ternary phase which adopts a variant of the hexagonal LiGaGe-type. Additionally, we were able to obtain 6 unique crystalline products dependent on the reaction parameters used.
The results of this work will open the door for increased application of I-II-V semiconductors. By synthesizing these phases in solution, their utility in thermoelectric devices is enhanced through a reduction in grain size and subsequent suppression of thermal conductivity. Furthermore, synthesizing these compounds by low temperature solution phase techniques significantly decreases the barrier for large-scale implementation. From a more fundamental perspective, the discovery of polytypism within this family of compounds offers a rich frontier to explore from a crystallographic perspective.
Miles Arthur Burris White
White, Miles Arthur Burris, "Solution phase synthesis and computational investigation of pnictide based Nowotny-Juza phases" (2018). Graduate Theses and Dissertations. 17356.