Miles A. White

Many technologically relevant semiconductors contain toxic, heavily regulated (Cd, Pb, As) or relatively scarce (Li, In) elements, and often require high manufacturing costs. We report a facile, general, low-temperature, and size tunable (4–28 nm) solution phase synthesis of ternary APnE2 semiconductors based on Earth-abundant and biocompatible elements (A = Na, Pn = Bi, E = S or Se). The observed experimental band gaps (1.20–1.45 eV) fall within the ideal range for solar cells. Computational investigation of the lowest energy superstructures that result from ‘coloring’, caused by mixed cation sites present in their rock salt lattice agree with other better-known members of this family of materials. Our synthesis unlocks a new class of low cost and environmentally friendly ternary semiconductors that show properties of interest for applications in energy conversion.

Filled tetrahedral semiconductors are a rich family of compounds with tunable electronic structure, making them ideal for applications in thermoelectrics, photovoltaics, and battery anodes. Furthermore, these materials crystallize in a plethora of related structures that are very close in energy, giving rise to polytypism through the manipulation of synthetic parameters. This Minireview highlights recent advances in the solution‐phase synthesis and nanostructuring of these materials. These methods enable the synthesis of metastable phases and polytypes that were previously unobtainable. Additionally, samples synthesized in solution phase have enhanced thermoelectric performance due to their decreased grain size.

In this invited paper, we highlight some of our most recent work on the synthesis of low dimensional nanomaterials. Current graduate students and members of our group present four specific case systems: Nowotny–Juza phases, nickel phosphides, germanium-based core/shells, and organolead mixed-halide perovskites. Each system is accompanied by commentary from the student involved, which explains the motivation behind their work, as well as a protocol detailing the key experimental considerations involved in their synthesis. We trust these and similar efforts will help further advance our understanding of the broader field of synthetic nanomaterials chemistry, while, at the same time, highlighting how important this area is to the development of new materials for technologically relevant applications.

The first example of polytypism in the I–II–V semiconductors has been demonstrated with the synthesis of cubic LiZnSb by a low-temperature solution-phase method. This phase exhibits a unique coloring pattern that is novel for this class of compounds. The choice of site configuration has a considerable impact on the band structure of these materials, which in turn affects the transport properties. While the hexagonal polytype has been suggested as a promising n-type and extremely poor p-type thermoelectric material, the cubic analogue is calculated to have high efficiencies for both the n- and p-type derivatives (1.64 and 1.43, respectively, at 600 K). Furthermore, the cubic phase is found to be the energetically favored polytype. This surprising result provides a rationale for the lack of success in synthesizing the hexagonal polytype in either stoichiometric or n-type compositions.

Nanocrystalline LiZnP was synthesized using a flexible low temperature solution phase method that is generally applicable to other Nowotny–Juza phases.

Low-dimensional cuprous nitride (Cu₃N) was synthesized by nitridation (ammonolysis) of cuprous oxide (Cu₂O) nanocrystals using either ammonia (NH₃) or urea (H₂NCONH₂) as the nitrogen source. The resulting nanocrystalline Cu₃N spontaneously decomposes to nanocrystalline CuO in the presence of both water and oxygen from air at room temperature. Ammonia was produced in 60% chemical yield during Cu₃N decomposition, as measured using the colorimetric indophenol method. Because Cu₃N decomposition requires H₂O and produces substoichiometric amounts of NH₃, we conclude that this reaction proceeds through a complex stoichiometry that involves the concomitant release of both N₂ and NH₃. This is a thermodynamically unfavorable outcome, strongly indicating that H₂O (and thus NH₃ production) facilitate the kinetics of the reaction by lowering the energy barrier for Cu₃N decomposition. The three different Cu₂O, Cu₃N, and CuO nanocrystalline phases were characterized by a combination of optical absorption, powder X-ray diffraction, transmission electron microscopy, and electronic density of states obtained from electronic structure calculations on the bulk solids. The relative ease of interconversion between these interesting and inexpensive materials bears possible implications for catalytic and optoelectronic applications.