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Seminar Speakers: NC State MSE Grad Students Mirrielees, Spencer and Yin

September 2, 2022 @ 11:00 am 12:00 pm EDT

“Celebration of Graduate Excellence”

Kelsey Mirrielees
Kelsey Mirrielees

“Predicting Oxide Reconstructions at AlN and GaN (0001) Surfaces”

AlN and GaN (0001) surfaces are prone to oxidation. There are many possible variations of reconstructions that could form, making them challenging to study. Typically, constraints are used to predict stable oxide reconstructions prior to simulation—oxide stoichiometry and electron counting rules. However, these constraints merely narrow the space of possibilities. In this work, three groups collaborated to investigate reconstructions at AlN and GaN (0001) surfaces. Scanning transmission electron microscopy directly observed the structure of the reconstruction. Density functional theory (DFT) compared surface energies of the observed reconstruction with various compositions to reconstructions previously considered. Hybrid functionals were used to calculate surface states for the lowest energy reconstructions, which when compared to observations of Fermi level pinning, demonstrate good agreement.


Michael Spencer
Michael Spencer

“Electrochemical Proton Insertion Modulates the Hydrogen Evolution Reaction on Tungsten Oxides”

As an energy-dense fuel, hydrogen is critical for many industrial processes. The market for hydrogen expects to reach USD 225B by 2030. Electrolytic production of hydrogen from water using renewable energy sources is a clean alternative to the industry-standard method of using methane. To scale up production, cheaper and more abundant electrocatalysts are required to reduce cost. Mechanistic insight into the electrocatalytic activity of a material speeds up electrocatalyst development. The electrochemical interface is a dynamic environment during an energy conversion reaction. Our goal is to understand the behavior of a transition metal oxide (tungsten oxides) during electrocatalysis of the hydrogen evolution reaction (HER). Using materials chemistry and characterization, we investigate the influence of proton insertion on the HER activity of tungsten oxides. We show proton insertion preceding the HER leads to low overpotential for the HER in tungsten oxides, showing that the active material is the hydrogenated form of tungsten oxide.


Shichen (Mason) Yin
Shichen (Mason) Yin

“PbS QD for Short-wave Infrared Photodetectors”

Colloidal lead sulfide (PbS) quantum dots (QD) are solution-processed semiconductor nanocrystals, offering strong light absorption in the infrared wavelength range. Their ease of thin-film processing, facile integration with complementary metal–oxide–semiconductors, and superiorities to other infrared-absorbing semiconductors, have made them of interest for next-generation infrared optoelectronic devices fabrication. Solution-phase ligand exchange has been demonstrated as the most promising method to prepare QD inks for device fabrication. However, due to the unwanted QD fusions, this method is problematic in preparing PbS QDs inks for shortwave infrared range sensing. In this talk, we will first review PbS QDs from the perspective of optoelectronic properties, crystal structure, and surface chemistry, followed by presenting the challenges in preparing semiconductor ink from large-size PbS QDs via phase transfer ligand exchange. To address these challenges, we develop a new strategy to provide sufficient passivation. In the end, an efficient shortwave infrared photodiode with near-unity internal quantum efficiency is demonstrated, showing the potential of PbS QD ink prepared by our strategy in shortwave infrared optoelectronic applications.

911 Partners Way
Raleigh, NC 27695-7907 United States