The University of Tennessee, Knoxville

Joint Institute for Advanced Materials


JIAM Seminar Series

2018

The seminars are held every Friday in the seminar room of the Joint Institute for Advanced Materials (Room 147).

Date Speaker Title

February 16

Yan Jiaqiang

February 23

An-Ping Li

Magnetism and spin-transport in 2D quantum materials revealed with spin-polarized scanning probe techniques

March 2

R. Morris Bullock

Design of Molecular Electrocatalysts for the Production and Oxidation of Hydrogen

March 23

Mina Yoon

First-principles materials by design for thermodynamically stable low-dimensional electrides

February 23

An-Ping Li

Magnetism and spin-transport in 2D quantum materials revealed with spin-polarized scanning probe techniques

Many of the defining discoveries in the quantum materials were made by studying the magnetism and spin dynamics ranging from topological textures such as magnetic skyrmions, topological insulators and quantum spin Hall, to extreme quantum states of matter such as quantum criticality and quantum spin liquids. A key advantage of scanning probe microscopy techniques, such as spin-polarized scanning tunneling microscopy (SP-STM) and SP 4-probe STM, is that they can probe both static distributions and transport of spin textures in real-space and in atomic fidelity and illuminate complex interactions of quantum states with their lattice, dopant, electronic, and magnetic environments.

In this talk, I will present two examples to demonstrate the recent development of spin-polarized scan probes in the Center for Nanophase Materials Sciences of Oak Ridge National Laboratory and the applications in understanding the magnetic behaviors of quantum materials. The first is the search of magnetic ground states of Fe3GeTe2. The magnetic 2D layered material is known as a ferromagnet. Our recent study using the SP-STM shows that its magnetic domain structures resemble many of the phenomena of magnetic skyrmions. This observation raises interesting questions on the nature of these magnetic domains and the potential emergence of magnetic skyrmions in a “conventional” magnet system. The second example is direct detection of a spin current carried by topological surface states in Bi2Te2Se. The measured spin current is directly locked to electron momentum and comes from the 2D charge current. In this manner, we achieve a quantitative measurement of spin current generation efficiency in topological states and demonstrate a spin voltmeter approach to understanding the spin physics of quantum materials.



March 2

R. Morris Bullock

Design of Molecular Electrocatalysts for the Production and Oxidation of Hydrogen

Solar and wind are carbon-neutral, sustainable energy sources, but they are intermittent, so energy storage is required. Catalysts that efficiently interconvert between electrical energy and chemical bonds (fuels) are needed for sustainable, secure energy. Electrocatalysts based on inexpensive, earth-abundant metals (“Cheap Metals for Noble Tasks”) are needed since low-temperature fuel cells generally use platinum, an expensive, precious metal. We developed nickel(II) complexes for the electrocatalytic production of H2 by reduction of protons. Pendant amines in the ligand function as proton relays, facilitating intramolecular and intermolecular proton mobility. Turnover frequencies up to 107 s-1 have been observed. Iron complexes with pendant amines in diphosphine ligands have been developed for the opposite reaction, oxidation of H2 (1 atm). The proposed mechanism involves binding of H2, heterolytic cleavage of H2, and removal of two protons and two electrons. Our results document the rational design of catalysts based on abundant, inexpensive metals as alternatives to precious metals.



March 23

Mina Yoon (R&D staff at ORNL, Joint Prof. at UTK)

First-principles materials by design for thermodynamically stable low-dimensional electrides

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. A realization of electrides containing anionic electrons has been a great challenge because of their thermodynamic stability. For example, experimentally, only a couple of layered nitrides and carbides have been identified as 2D electrides. We developed a materials by design scheme and applied it to the computational exploration of new low-dimensional electrides. Our approach here offers an important alternative that overcomes the current limitation on discovery of new 2D inorganic electrides. By combining the global structure optimization method and first-principles calculations, we identified new thermodynamically stable electrides that are experimentally accessible. Most remarkably, we, for the first time, reveal an effective design rule for 2D electrides [1]. We then discover another new class of electrides based on 1D building blocks by coupling materials database searches and first-principles-calculations-based analysis. This new class of electrides, composed of 1D nanorod building blocks, has crystal structures that mimic β-TiCl3 with the position of anions and cations exchanged. Unlike the weakly coupled nanorods of β-TiCl3, Cs3O and Ba3N retain 1D anionic electrons along the hollow inter-rod sites; additionally, strong inter-rod interaction in C3O and Ba3N induces band inversion in a 2D superatomic triangular lattice, resulting in Dirac nodal lines [2]. Our work [1,2] represents an important scientific advancement over previous knowledge of realizing electrides in terms of both materials and design principles, and should interest the communities of catalytic chemistry, surface physics, and structural chemistry, as well as the related engineering disciplines.

Reference
1. First-Principles Prediction of Themodynamically Stable Two-Dimensional Electrides, W. Ming, M. Yoon, M.-H. Du, F. Liu, K. Lee, and S. W. Kim, J. Am. Chem. Soc. 138, 15336 (2016).
2. New electrides based on one-dimensional building blocks, Changwon Park, Sung Wng Kim, Mina Yoon (2017, submitted to Phys. Rev. Lett.).



 

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