Brandi M. Cossairt

Investigating the role of amine in InP nanocrystal synthesis: destabilizing cluster intermediates by Z-type ligand displacement

The reaction of primary amines with In37P20(O2CR)51 is found to remove In(O2CR)3 subunits from In37P20(O2CR)51. This loss of Z-type ligands coincides with structural rearrangement to alleviate core strain and passivate phosphorus atoms. This result consolidates conflicting claims that primary amines both promote and retard precursor conversion rates for InP nanocrystals.

A compact dispersive refocusing Rowland circle X-ray emission spectrometer for laboratory, synchrotron, and XFEL applications

X-ray emission spectroscopy is emerging as an important complement to x-ray absorption fine structure spectroscopy, providing a characterization of the occupied electronic density of states local to the species of interest. Here, we present details of the design and performance of a compact x-ray emission spectrometer that uses a dispersive refocusing Rowland (DRR) circle geometry to achieve excellent performance for the 2-2.5 keV range, i.e., especially for the K-edge emission from sulfur and phosphorous. The DRR approach allows high energy resolution even for unfocused x-ray sources. This property enables high count rates in laboratory studies, approaching those of insertion-device beamlines at third-generation synchrotrons, despite use of only a low-powered, conventional x-ray tube. The spectrometer, whose overall scale is set by use of a 10-cm diameter Rowland circle and a new small-pixel complementary metal-oxide-semiconductor x-ray camera, is easily portable to synchrotron or x-ray free electron laser beamlines. Photometrics from measurements at the Advanced Light Source show excellent overall instrumental efficiency. In addition, the compact size of this instrument lends itself to future multiplexing to gain large factors in net collection efficiency or its implementation in controlled gas gloveboxes either in the lab or in an endstation.

A doubly deprotonated diimine dioximate metalloligand as a synthon for multimetallic complex assembly

An electrocatalytically active cobalt diimine monoxime monoximate complex was deprotonated by 1-methylimidazole affording a doubly deprotonated complex that serves as a versatile precursor for synthesis of a variety of multimetallic complexes with Co-Zn, -Cd, -Mn and -Ru coordination. These complexes were studied using a combination of spectroscopic, analytical and electrochemical techniques, revealing the electronic and structural parameters unique to this new class of compounds. The ability of these complexes to catalyze proton reduction was also investigated. These complexes are homogeneous electrocatalysts for the hydrogen evolution reaction through reduction of [NEt3H][BPh4] in CH3CN, however decompose under extended electrolysis conditions.

CdSe on a mesoporous transparent conducting oxide scaffold as a photocathode

We report here a photocathode based on a high surface area conductive metal oxide scaffold sensitized by CdSe quantum dots attached via organic linkers. Photoreduction of methylviologen demonstrates efficient photoreactions occuring at electrode surfaces and verifies that the high surface area scaffold is promising for use as a photocathode material.

Kinetically controlled assembly of cadmium chalcogenide nanorods and nanorod heterostructures

While it is well understood that controlling anisotropic nanostructure growth can be accomplished by establishing kinetic growth conditions, the practical translation of this knowledge to access nanorods with a specific aspect ratio has not been realized. In this study we empirically determine the precursor consumption rates for growing nanorods and use this data to customize the size and shape of anisotropic nanostructures. The purpose of this work is to go beyond simply creating a set of growth conditions to obtain rods, dots, rice, and tetrapods by describing how to synthesize a nanomaterial of desired dimensions and aspect ratio in a pre-meditated fashion. Measured growth rates for model systems of CdSe (3.5 monomers rod−1 s−1 at 250 °C) and CdS nanorods (36 monomers rod−1 s−1 at 340 °C) were used to design elongated nanorods with enhanced aspect ratios and synthesize dot in rod CdS/CdSe and CdSe/CdS heterostructures. These model systems enable us to establish a rubric for the synthesis of customizable nanostructures and serve as a test case for understanding heterostructure assembly in colloidal systems.

Deterministic Positioning of Colloidal Quantum Dots on Silicon Nitride Nanobeam Cavities

We experimentally demonstrated deterministic positioning of solution processed colloidal quantum dots on a silicon nitride nanobeam resonator, with potential applications in nonlinear optics, multi-functional optical devices, and on-chip, solid-state quantum simulators.

Improved HER Catalysis through Facile, Aqueous Electrochemical Activation of Nanoscale WSe2

In the search for nonprecious metal catalysts for the hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) have been proposed as promising candidates. Here, we present a facile method for significantly decreasing the overpotential required for catalyzing the HER with colloidally synthesized WSe2. Solution phase deposition of 2H WSe2 nanoflowers (NFs) onto carbon fiber electrodes results in low catalytic activity in 0.5 M H2SO4 with an overpotential at -10 mA/cm2 of greater than 600 mV. However, two postdeposition electrode processing steps significantly reduce the overpotential. First, a room-temperature treatment of the prepared electrodes with a dilute solution of the alkylating agent Meerweins salt ([Et3O][BF4]) results in a reduction in overpotential by approximately 130 mV at -10 mA/cm2. Second, we observe a decrease in overpotential of approximately 200-300 mV when the TMDC electrode is exposed to H+, Li+, Na+, or K+ ions under a reducing potential. The combined effect of ligand removal and electrochemical activation results in an improvement in overpotential by as much as 400 mV. Notably, the Li+ activated WSe2 NF deposited carbon fiber electrode requires an overpotential of only 243 mV to generate a current density of -10 mA/cm2. Measurement of changes in the material work function and charge transfer resistance ultimately provide rationale for the catalytic improvement.

II3V2 (II: Zn, Cd; V: P, As) Semiconductors: From Bulk Solids to Colloidal Nanocrystals

II3 V2 semiconductors have become increasingly popular for a variety of applications including solar light harvesting, near-IR imaging, and low energy light detection. The bulk physical and electronic structure of these materials is highlighted, followed by an in-depth survey on progress in synthesizing these semiconductors as colloidal nanocrystals. Interestingly, no universal synthetic approach has yet been developed to access all compounds within this family. A discussion on how the complex crystal structure of these materials translates to small domain sizes will highlight current challenges in the characterization of II3 V2 nanocrystals. Finally, potential avenues for further research will be proposed as a way to advance this field towards greater utilization in light harvesting applications.