Ultrafast Resonant Magnetic Responses of Non-magnetic 2D Semiconductors to Low-Frequency Optical Fields

Mahmut Sait Okyay,Bumseop Kim,Noejung Park 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2

All-optical helicity-dependent switching of magnetism has attracted broad attention from the perspective of non-magnetic fast control of spins. We examine ultrafast spin dynamics of two-dimensional non-magnetic semiconductors, particularly focusing on alteration of material’s time-reversal and valley symmetries driven by a circularly polarized light. Monolayer MoS₂ was selected as an exemplary system and the real-time time-dependent density functional theory (rt-TDDFT) calculations were carried out, together with a model Hamiltonian analysis. As a distinction from some of the previous reports, this study was circumscribed to the non-excitonic regime, i.e., an interval of frequency values below the band gap. 〈그림 본문참조〉 Our real-time ab initio calculations show that a circularly polarized light can induce a net magnetization in a non-magnetic two-dimensional material. We demonstrate that the magnetic responses are particularly amplified when the applied electric fields are in resonance with the spin-flipping band transition between valence and conduction bands through the second harmonic of the light. We forecast that a tunable spin dynamics can be achieved from these non-magnetic semiconductors when the light of the resonant frequency is combined with a proper setting of a few parameters such as field strength, pulse duration and the magnitude of spin-orbit coupling.

All-optical fast control of band topology by exploiting the correlation between Berry curvature and magnetic anisotropy

Bumseop Kim,Mahmut Sait Okyay,Noejung Park 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1

The effect of light pulse on magnetic materials have attracted broad interest in the perspective of laser-induced demagnetization and ultrafast controls of spin [1-3]. Here we focus on the fact that, beyond such passive roles, appropriately selected light conditions can result in more active effect in terms of magnetization density and magnetization direction. We selected a few examples of two-dimensional ferromagnetic insulators, and performed real-time time-dependent density functional theory by implementing the external light field in terms of vector potential. We find that asymmetric single-cycle light pulses can rotate the anisotropic local magnetization direction smoothly as shown in Fig 1. We discuss the effect of this optical reorientation of local moment, in the context of Mermin-Wagner’s theorem, on the long-range order of two-dimensional magnet. Our first-principles computations of real materials expect that this optical control of magnetization direction can adjust the Berry curvature distribution over the Brillouin zone, which eventually can switch the topological nature of quantum anomalous Hall state of ferromagnetic insulators.

A resonated breaking of time-reversal symmetry: a first-principles TDDFT study for the case of 2-dimensional semiconductors

Adem Halil Kulahlioglu,Mahmut Sait Okyay,Bumseop Kim,Min Choi,Dongbin Shin,Noejung Park 한국자기학회 2019 한국자기학회 학술연구발표회 논문개요집 Vol.29 No.1

Bifunctional sulfur-doped cobalt phosphide electrocatalyst outperforms all-noble-metal electrocatalysts in alkaline electrolyzer for overall water splitting

Anjum, Mohsin Ali Raza,Okyay, Mahmut Sait,Kim, Minkyung,Lee, Min Hee,Park, Noejung,Lee, Jae Sung Elsevier 2018 Nano energy Vol.53 No.-

<P><B>Abstract</B></P> <P>Sulfur-doped CoP (S:CoP) nanoparticles are synthesized as a noble metal-free electrocatalyst <I>via</I> a novel and eco-friendly thiourea-phosphate-assisted solvothermal route. When used as a bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions from water splitting in an alkaline solution, the electrode exhibits excellent activity and stability outperforming noble mental-based Pt/C, IrO<SUB>2</SUB>, and reported non-noble metal-based electrocatalysts. Density functional theory calculations indicate that the excellent performance is attributable to the improved charge-transfer characteristics of the S:CoP nanoparticles owing to their modified electronic structure. It also increases the number of exposed active sites especially on the conductive substrates. A bifunctional S:CoP catalyst-based alkaline electrolyzer for overall water splitting exhibits a stable current density of 100 mA/cm<SUP>2</SUP> at an overvoltage of 0.55 V during a long-term operation; this performance is superior to that obtained from all-noble metal electrolyzer with a Pt/C cathode and an IrO<SUB>2</SUB> anode.</P> <P><B>Highlights</B></P> <P> <UL> <LI> S:CoP is synthesized by eco-friendly thiourea-phosphate-assisted solvothermal route. </LI> <LI> S:CoP/NF becomes an active and stable bifunctional water splitting electrocatalyst. </LI> <LI> S-doping improves charge transfer and increases density of active sites. </LI> <LI> An alkaline electrolyzer with bifunctional S:CoP outperforms all-noble-metal electrolyzer. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>The electronic properties of CoP are modulated by doping S into its structure through an environment-friendly thiourea-phosphate route, and produced S:CoP electrocatalysts efficiently catalyze both HER and OER in alkaline media.</P> <P>[DISPLAY OMISSION]</P>

Macroporous Inverse Opal-like Mo_<i>x</i>C with Incorporated Mo Vacancies for Significantly Enhanced Hydrogen Evolution

Li, Feng,Zhao, Xianglong,Mahmood, Javeed,Okyay, Mahmut Sait,Jung, Sun-Min,Ahmad, Ishfaq,Kim, Seok-Jin,Han, Gao-Feng,Park, Noejung,Baek, Jong-Beom American Chemical Society 2017 ACS NANO Vol.11 No.7

<P>The hydrogen evolution reaction (HER) is one of the most important pathways for producing pure and clean hydrogen. Although platinum (Pt) is the most efficient HER electrocatalyst, its practical application is significantly hindered by high cost and scarcity. In this work, an MoxS with incorporated Mo vacancies and macroporous inverse opal-like (IOL) structure (MoxC-IOL) was synthesized and studied as a low-cost efficient HER electrocatalyst. The macroporous IOL structure was controllably fabricated using a facile-hard template strategy. As a result of the combined benefits of the Mo vacancies and structural advantages, including appropriate hydrogen binding energy, large exposed surface, robust IOL structure and fast mass/charge transport, the synthesized MoxC-IOL exhibited significantly enhanced HER electrocatalytic performance with good stability, with performance comparable or superior to Pt wire in both acidic and alkaline solutions.</P>

An efficient and pH-universal ruthenium-based catalyst for the hydrogen evolution reaction

Mahmood, Javeed,Li, Feng,Jung, Sun-Min,Okyay, Mahmut Sait,Ahmad, Ishfaq,Kim, Seok-Jin,Park, Noejung,Jeong, Hu Young,Baek, Jong-Beom Nature Publishing Group, a division of Macmillan P 2017 Nature nanotechnology Vol.12 No.5

<P>The hydrogen evolution reaction (HER) is a crucial step in electro-chemical water splitting and demands an efficient, durable and cheap catalyst if it is to succeed in real applications(1-3). For an energy-efficient HER, a catalyst must be able to trigger proton reduction with minimal overpotential(4) and have fast kinetics(5-9). The most efficient catalysts in acidic media are platinum-based, as the strength of the Pt-H bond(10) is associated with the fastest reaction rate for the HER11,12. The use of platinum, however, raises issues linked to cost and stability in non-acidic media. Recently, non-precious-metal-based catalysts have been reported, but these are susceptible to acid corrosion and are typically much inferior to Pt-based catalysts, exhibiting higher overpotentials and lower stability(13-15). As a cheaper alternative to platinum, ruthenium possesses a similar bond strength with hydrogen (similar to 65 kcal mol(-1))(16), but has never been studied as a viable alternative for a HER catalyst. Here, we report a Ru-based catalyst for the HER that can operate both in acidic and alkaline media. Our catalyst is made of Ru nanoparticles dispersed within a nitrogenated holey two-dimensional carbon structure (Ru@C2N). The Ru@C2N electrocatalyst exhibits high turnover frequencies at 25 mV (0.67 H-2 s(-1) in 0.5 M H2SO4 solution; 0.75 H-2 s(-1) in 1.0 M KOH solution) and small overpotentials at 10 mA cm(-2) (13.5 mV in 0.5 M H2SO4 solution; 17.0 mV in 1.0 M KOH solution) as well as superior stability in both acidic and alkaline media. These performances are comparable to, or even better than, the Pt/C catalyst for the HER.</P>

Coordination Polymers for High-Capacity Li-Ion Batteries: Metal-Dependent Solid-State Reversibility

Lee, Hyun Ho,Lee, Jae Bin,Park, Yuwon,Park, Kern Ho,Okyay, Mahmut Sait,Shin, Dong-Seon,Kim, Sunghwan,Park, Jongnam,Park, Noejung,An, Byeong-Kwan,Jung, Yoon Seok,Lee, Hyun-Wook,Lee, Kyu Tae,Hong, Sung American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.26

<P>Electrode materials exploiting multielectron-transfer processes are essential components for large-scale energy storage systems. Organic-based electrode materials undergoing distinct molecular redox transformations can intrinsically circumvent the structural instability issue of conventional inorganic-based host materials associated with lattice volume expansion and pulverization. Yet, the fundamental mechanistic understanding of metal-organic coordination polymers toward the reversible electrochemical processes is still lacking. Herein, we demonstrate that metal-dependent spatial proximity and binding affinity play a critical role in the reversible redox processes, as verified by combined <SUP>13</SUP>C solid-state NMR, X-ray absorption spectroscopy, and transmission electron microscopy. During the electrochemical lithiation, in situ generated metallic nanoparticles dispersed in the organic matrix generate electrically conductive paths, synergistically aiding subsequent multielectron transfer to π-conjugated ligands. Comprehensive screening on 3d-metal-organic coordination polymers leads to a high-capacity electrode material, cobalt-2,5-thiophenedicarboxylate, which delivers a stable specific capacity of ∼1100 mA h g<SUP>-1</SUP> after 100 cycles.</P> [FIG OMISSION]</BR>