Metal-halide semiconductors are an attractive class of materials for advanced photonic and optoelectronic applications including photovoltaics, photodetection, and photon sources. Much of the focus in this research area has been on solution processing of these materials, but solvent-free methods would be desirable for many applications. Recently, we reported the development of methods to synthesize compositionally complex metal-halide semiconductors and process them into high-quality thin films via single-source flash sublimation for conformal, large-area coatings. Here, we extend this approach to explore the rich compositional space of metal-halide semiconductors, including hybrid perovskites and allinorganic perovskite and elpasolite materials. These results demonstrate a promising approach for advanced materials discovery and a pathway toward realizing high-throughput vapor processing of metal-halide semiconductor coatings for next-generation photonic and optoelectronic applications.
A simple, versatile, and scalable procedure for synthesizing high-quality colloidal ZnO quantum dots from homogeneous polar aprotic solutions has been developed. TEM and x-ray diffraction data show that the nanocrystals synthesized by this procedure are highly crystalline, pseudo-spherical, and have a narrow size distribution. This procedure allows large quantities of homogeneous ZnO quantum dots to be prepared with relative ease, and is also conducive to the introduction of dopants.
Ligand field electronic absorption spectroscopy has been applied as a direct probe of Co<sup>2+</sup> dopant ions in II-VI based diluted magnetic semiconductor quantum dots. Synthesis of Co<sup>2+</sup>-doped CdS (Co<sup>2+</sup>:CdS) quantum dots by simple coprecipitation in inverted micelle solutions has been found to yield predominantly surface bound dopant ions, which are unstable with respect to solvation in a coordinating solvent (pyridine). The solvation kinetics are biphasic, involving two transient intermediates. In contrast, Co<sup>2+</sup> ions are doped much more isotropically in ZnS QDs, and this difference is attributed to the similar ionic radii of Co<sup>2+</sup> and Zn<sup>2+</sup> ions (0.74 Å), as opposed to Cd<sup>2+</sup> ions (0.97 Å). We have developed an isocrystalline core/shell synthetic methodology that enables us to synthesize high quality internally doped Co<sup>2+</sup>:CdS quantum dots. The effect of Co<sup>2+</sup> binding on the surface energies of CdS and ZnS quantum dots is discussed and related to the growth mechanism of diluted magnetic semiconductor quantum dots.
We report the use of electronic absorption and magnetic circular dichroism (MCD) spectroscopies to probe the magneto-optical properties of Co<sup>2+</sup> dopant ions in diluted magnetic semiconductor quantum dots. Emphasis is placed on observation and analysis of the ligand field transitions of the Co<sup>2+</sup> ions. Because the ligand field transitions may be observed in an energy region where the semiconductor host is transparent, ligand field absorption and MCD spectroscopies serve as excellent site-specific spectroscopic methods for studying the dopant ions within DMS nanocrystals. Cobalt-doped CdS nanocrystals (Co<sup>2+</sup>CdS) prepared in solution by the isocrystalline core/shell method are shown by high-resolution TEM to be of high crystallinity. The ligand field spectroscopy demonstrates substitutional doping of Co<sup>2+</sup> at Cd<sup>2+</sup> sites. The MCD spectra show a 10<sup>3</sup> enhancement in sensitivity for the Co<sup>2+</sup> ligand field transitions relative to the CdS bandgap transitions. Saturation magnetization experiments yield optically detected ground state magnetization data for these materials, and show that both the ligand field and bandgap MCD intensities follow S = 3/2 Brillouin saturation behavior associated with the isolated Co<sup>2+</sup> ions. The <sup>4</sup>A<sub>2</sub>--><sup>4</sup>T<sub>1</sub>(P) ligand field bandshape and the sign of the bandgap MCD feature are analyzed in terms of electronic structural parameters for this material.