Mina Yoon

Oak Ridge National Laboratory | ORNL · Center for Nanophase Materials Sciences

In this work, we demonstrate the formation and electronic influence of lateral heterointerfaces in FeSn containing Kagome and honeycomb layers. Lateral heterostructures offer spatially resolved property control, enabling the integration of dissimilar materials and promoting phenomena not typically observed in vertical heterostructures. Using the mo...

Delafossites, typically denoted by the formula ABO2, are a class of layered materials that exhibit a wide range of electronic and optical properties. Recently, the idea of modifying these delafossites into ordered kagome or honeycomb phases via strategic doping has emerged as a potential way to tailor these properties. In this study, we use high-th...

Using an evolutionary algorithm in combination with first-principles density-functional theory calculations, we identify a two-dimensional (2D) CaP3 monolayer as a new Dirac semimetal due to inversion and nonsymmorphic spatial symmetries of the structure. This new topological material, composed of light elements, exhibits high structural stability...

In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band gap size and high absorption coefficient. However, challenges such as structural defects...

Solvent-based CO2 capture consumes significant energy in solvent recovery. To improve energy efficiency, this study investigates CO2 fixation in a solid form through solvation followed by ionic self-assembly-aided precipitation without requiring high energy for solvent regeneration. Based on hypothesis that CO2-3 ions may bind with monovalent metal...

The rapid development of 6G communications using terahertz (THz) electromagnetic waves has created a demand for highly sensitive THz nanoresonators capable of detecting these waves. Among the potential candidates, THz nanogap loop arrays show promising characteristics but require significant computational resources for accurate simulation. This req...

Chromium atoms in graphene have been proposed to exhibit magnetic moments and spin-selective conducting states depending on the local bonding geometry within the graphene structure, which could lead to interesting applications in spintronics. Despite this interest, there are few direct experimental reports of Cr dopants in graphene even though it i...

Atomic-scale fabrication is an outstanding challenge and overarching goal for the nanoscience community. The practical implementation of moving and fixing atoms to a structure is non-trivial considering that one must spatially address the positioning of single atoms, provide a stabilizing scaffold to hold structures in place, and understand the det...

Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic scale features scattered probabilistically throughout the material. In a top-down approach, diff...

We study the effect of stacking faults on the topological properties of the magnetic topological insulator MnBi$_{2}$Te$_{4}$ (MBT) using density functional theory calculations and the Hubbard $U$ being tuned with many-body diffusion Monte Carlo techniques. We show that a modest deviation from the equilibrium interlayer distance leads to a topologi...

We investigate the stability of MnPb$_{2}$Bi$_{2}$Te$_{6}$ (MPBT), which is predicted to be a magnetic topological insulator (TI), using density functional theory calculations. Our analysis includes various measures such as enthalpies of formation, Helmholtz free energies, defect formation energies, and dynamical stability. Our thermodynamic analys...

Sb thin films have attracted wide interests due to their tunable band structure, topological phases, and remarkable electronic properties. We successfully grow epitaxial Sb thin films on a closely lattice-matched GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic directional dependence of their structural, morphological, and e...

Atmospheric pressure plasma (AP) treatment, using an open-air jet of ionized CO2, N2, or air, was applied to AZ91D Mg alloy surfaces to investigate its effects on primer coating adhesion and corrosion resistance. The CO2 and air AP treatments formed an O- and C-rich surface layer (Mg-O-C) consisting of agglomerated nanoparticles and pits with a dep...

The kagome system of ScV$_6$Sn$_6$ displays a transition towards a charge density (CDW) state at $T_{\textrm{CDW}}$ = 91 K. By comparing to the isostructural non-CDW compound LuV$_6$Sn$_6$, we unravel interesting electrical transport properties in ScV$_6$Sn$_6$, above and below the charge ordering temperature. We observed that by applying a magneti...

PdCrO 2 films are synthesized on CuCrO 2 buffer layers on Al 2 O 3 substrates. This synthesis is accompanied by impurity phase segregation, which hampers the synthesis of high quality PdCrO 2 films. Potential causes ofth impurity phase segregation are studied by using a combination of experiments and ab initio calculations. X‐ray diffraction and sc...

PdCrO$_2$ films are synthesized on CuCrO$_2$ buffer layers on Al$_2$O$_3$ substrates. This synthesis is accompanied by impurity phase segregation, which hampers the synthesis of high quality PdCrO$_2$ films. The potential causes of impurity phase segregation were studied by using a combination of experiments and ab initio calculations. X-ray diffra...

Using an evolutionary algorithm in combination with first-principles density functional theory calculations, we identify two-dimensional (2D) CaP$_3$ monolayer as a new Dirac semimetal due to inversion and nonsymmorphic spatial symmetries of the structure. This new topological material, composed of light elements, exhibits high structural stability...

Topological kagome systems have been a topic of great interest in condensed matter physics due totheir unique electronic properties. The vanadium-based kagome materials are particularly intrigu-ing since they exhibit exotic phenomena such as charge density wave (CDW) and unconventionalsuperconductivity. The origin of these electronic instabilities...

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-tempera...

Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic scale features scattered probabilistically throughout the material. In a top-down approach, diff...

Carbon and oxygen-rich corrosion barrier layer formed on Mg by a simple and scalable CO2 atmospheric plasma (CO2-AP) process. The reactive CO2-AP interacts with the Mg surface and forms a unique layered structure with the top MgCO3/MgO-intermixed particulates pillars and the bottom dense layer. The surface features were simultaneously formed on the...

Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic pro...

In many complex oxides, the oxygen vacancy formation is a promising route to modify the material properties such as a superconductivity and an oxygen diffusivity. Cation substitutions and external strain have been utilized to control the concentration and diffusion of oxygen vacancies, but the mechanisms behind the controls are not fully understood...

PdSe2 has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se-Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe2 is conventionally orthorhombic, and direct synthe...

Scanning tunneling microscopy (STM) is one of the indispensable tools to characterize surface structures, but the distinction between atomic geometry and electronic effects based on the measured tunneling current is not always straightforward. In particular, for single-atomic-thick materials (graphene or boron nitride) on metallic substrates, count...

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entangleme...

Using ab initio tight-binding approaches, we investigate Floquet band engineering of the 1T’ phase of transition metal dichalcogenides (MX2, M = W, Mo and X = Te, Se, S) monolayers under the irradiation with circularly polarized light. Our first principles calculations demonstrate that light can induce important transitions in the topological phases...

2D Materials In article number 2106674, Kai Xiao and co‐workers develop a nonequilibrium chemical vapor deposition approach for selective formation of antisite defects in atomically thin 2D tungsten sulfide monolayer crystals by regulating the diffusion of tungsten into gold substrates. This work demonstrates a novel strategy for the selective form...

Defects are ubiquitous in 2D materials and can not only affect the structure and properties of the materials but can also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. In this paper,...

Ternary chalcogenides, such as parkerites and shandites, are a broad class of materials exhibiting a rich diversity of transport and magnetic behavior and an array of topological phases, including Weyl and Dirac nodes. However, they remain largely unexplored as high-quality epitaxial thin films. Here, we report the self-regulated growth of thin fil...

Using ab initio tight-binding approaches, we investigate Floquet band engineering of the 1T' phase of transition metal dichalcogenides (MX2, M = W, Mo and X = Te, Se, S) monolayers under the irradiation with circularly polarized light. Our first principles calculations demonstrate that light can induce important transitions in the topological phase...

Understanding the bottom-up synthesis of atomically thin two-dimensional (2D) crystals and heterostructures is important for the development of new processing strategies to assemble 2D heterostructures with desired functional properties. Here, we utilize in situ laser-heating within a transmission electron microscope (TEM) to understand the stages...

Tailoring the grain boundaries (GBs) and twist angles between two-dimensional (2D) crystals are two crucial synthetic challenges to deterministically enable envisioned applications such as moiré excitons, emerging magnetism, or single-photon emission. Here, we reveal how twisted 2D bilayers can be synthesized from the collision and coalescence of t...

Atomic-scale fabrication is an outstanding challenge and overarching goal for the nanoscience community. The practical implementation of moving and fixing atoms to a structure is non-trivial considering that one must spatially address the positioning of single atoms, provide a stabilizing scaffold to hold structures in place, and understand the det...

Chromium atoms in graphene have been proposed to exhibit magnetic moments and spin-selective conducting states depending on the local bonding geometry within the graphene structure, which could lead to interesting applications in spintronics. Despite this interest, there are few direct experimental reports of Cr dopants in graphene even though it i...

In Situ Laser Synthesis of 2D WSe2 Within TEM - Chenze Liu, Yu-Chuan Lin, Yiling Yu, Alexander Puretzky, Mina Yoon, Gyula Eres, Christopher Rouleau, Kai Xiao, David Geohegan, Gerd Duscher

Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers....

Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here, we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT)....

Cubic TiO 2 was predicted to be a great visible light absorption material partially due to the smallest bandgap among all the TiO 2 polymorphs. The cubic TiO 2 can be synthesized using the common TiO 2 polymorph i.e. anatase as a precursor, but it mixes with other metastable phases as byproducts including baddeleyite, TiO 2 (OI) and TiO 2 (OII). Th...

Two-dimensional (2D) crystal growth over substrate features is fundamentally guided by the Gauss-Bonnet theorem, which mandates that rigid, planar crystals cannot conform to surfaces with nonzero Gaussian curvature. Here, we reveal how topographic curvature of lithographically designed substrate features govern the strain and growth dynamics of tri...

Solar‐thermal driven desalination based on porous carbon materials has promise for fresh water production. Exploration of high‐efficiency solar desalination devices has not solved issues for practical application, namely complicated fabrication, cost‐effectiveness, and scalability. Here, direct solar‐thermal carbon distillation (DS‐CD) tubular devi...

Density functional theory (DFT) has been widely employed to study the gas adsorption properties of surface-based or nanoscale structures. However, recent indications raise questions about the trustworthiness of some literature values, especially in terms of the DFT exchange–correlation (XC) functional. Using hydrogen adsorption on metalloporphyrin-...

Atomically-thin two-dimensional (2D) materials display widely varying electronic and vibronic properties compared to their bulk counterparts. Laser interactions with 2D materials are central to their development. Here we attempt to overview recent progress and define the current challenges in the broad range of laser interactions involved in the sy...

We investigate phonon transport in dicalcium nitride (Ca2N), an electride with two-dimensional confined electron layers, using first-principles density functional theory and the phonon Boltzmann transport equation. The in-plane (κ100) and out-of-plane (κ001) lattice thermal conductivities at 300 K are found to be 11.72 W m⁻¹ K⁻¹ and 2.50 W m⁻¹ K⁻¹,...

The doping of monolayer MoSe2 by tungsten (W) can suppress the Se vacancy concentration, but how doping and resulting change in defect concentration can tune its thermal properties is not understood yet. We use first-principles density functional theory (DFT) along with the phonon Boltzmann transport equation (BTE) to study the phonon transport pro...

It is well understood that defect engineering can give rise to exotic electronic properties in transition metal dichalcogenides, but to this date, there is no detailed study to illustrate how defects can be engineered to tailor their thermal properties. Here, through combined experimental and theoretical approaches based on the first-principles den...

Non-equilibrium growth pathways for crystalline nanostructures with metastable phases are demonstrated through the gas-phase formation, attachment, and crystallization of ultrasmall amorphous nanoparticles as building blocks in pulsed laser deposition (PLD). Temporally- and spatially-resolved gated-ICCD imaging and ion probe measurements are employ...

Charge transfer between an electron donor and an electron acceptor is widely accepted as being independent of their relative configurations if the interaction between them is weak; however, the limit of this concept for an interacting system has not yet been well established. Our study of prototypical electron donor–acceptor molecules, Tetrathiaful...

Controlling the spin states of the surface and interface is a key to spintronic applications of magnetic materials. Here, we report the evolution of surface magnetism of Co nanoislands on Cu(111) upon hydrogen adsorption and desorption, with the hope to realize reversible control of spin dependent tunneling. Spin-polarized scanning tunneling micros...

The confinement effect of intercalated atoms in van der Waals heterostructures can lead to interesting interactions between the confined atoms or molecules and the overlaying two-dimensional (2D) materials. Here we report the formation of ordered Cu(100) p(2×2) oxygen superstructures by oxygen intercalation under the monolayer hexagonal boron nitri...

Defect engineering has been a critical step in controlling the transport characteristics of electronic devices, and ability to create, tune, and annihilate defects is essential to enable the range of next-generation devices. Whereas defect formation has been well demonstrated in three-dimensional semiconductors, similar exploration of the heterogen...

TVan der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive charac...

Ultrafast pump-probe spectroscopy of two-dimensional tungsten disulfide monolayers (2DWS2) grown on sapphire substrates revealed two transient absorption spectral peaks that are attributed to distinct negative trions at ∼2.02eV (T1) and ∼1.98eV (T2). The dynamics measurements indicate that trion formation by the probe is enabled by photodoped 2D WS...

The remarkable properties of black TiO2 are due to its disordered surface shell surrounding a crystalline core. However, the chemical composition and the atomic and electronic structure of the disordered shell and its relationship to the core remain poorly understood. Using advanced transmission electron microscopy (TEM) methods, we found that the...

div class="title">Phase Determination of Black TiO 2 Nanoparticles - Volume 21 Issue S3 - Meng-kun Tian, Masoud. Mahjouri-Samani, Gyula Eres, Ritesh Sachan, Matthew F. Chisholm, Kai Wang, Alexander. A. Puretzky, Christopher. M. Rouleau, Mina. Yoon, David. B. Geohegan, Gerd Duscher

Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially-structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to stamping, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here, we explore...

The formation of semiconductor heterojunctions and their high-density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional crystalline semiconductors as building blocks in next-generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate an a...

Bilayer graphene (BLG) with a tunable bandgap appears interesting as an alternative to graphene for practical applications, thus its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLGs in various stackings rel...

Characterizing and controlling the interlayer orientations and stacking orders of two-dimensional (2D) bilayer crystals and van der Waals (vdW) heterostructures is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) crystals that result from different layer stackings provide an...

Two-dimensional interfaces between crystalline materials have been shown to generate unusual interfacial electronic states in complex oxides. Recently, a one-dimensional interface has been realized in hexagonal boron nitride and graphene planar heterostructures, where a polar-on-nonpolar one-dimensional boundary is expected to possess peculiar elec...

The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces...

Compared with their bulk counterparts, atomically thin two-dimensional (2D) crystals exhibit new physical properties, and have the potential to enable next-generation electronic and optoelectronic devices. However, controlled synthesis of large uniform monolayer and multi-layer 2D crystals is still challenging. Here, we report the controlled synthe...

Laser interactions have traditionally been at the center of nanomaterials science, providing highly nonequilibrium growth conditions to enable the synthesis of novel new nanoparticles, nanotubes, and nanowires with metastable phases. Simultaneously, lasers provide unique opportunities for the remote characterization of nanomaterial size, structure,...

We propose a scheme to obtain a system-dependent fraction of exact exchange (α) within the framework of hybrid density functional theory (DFT) that is consistent with the G_{0}W_{0} approach, where G_{0} is the noninteracting Green function of the system and W_{0} the screened Coulomb interaction. We exploit the formally exact condition of exact DF...

A theoretical model is presented that explains spontaneous changes in the crystalline orientation of nanoparticles. The spontaneous changes in crystalline orientation are attributed to the crystal anisotropy of the surface energy of nanocrystalline particles. We consider an important specific case of the chemical vapor deposition growth of carbon n...

Our recent studies have demonstrated how substrates can be used to control the synthesis of nanoscale organic semicorductors. In particular, we study the growth mechanism of oriented crystalline organic nanowires consisting of M-TCNQF4 (M=Cu or Ag) from vapor-solid chemical reaction (VSCR). Our experimental and theoretical study combining time-reso...

Detailed physisorption data from experiment for the H_2 molecule on low-index Cu surfaces challenge theory. Recently, density-functional theory (DFT) has been developed to account for nonlocal correlation effects, including van der Waals (dispersion) forces. We show that the functional vdW-DF2 gives a potential-energy curve, potential-well energy l...

The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date; however, the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF(4) (where M is Cu-...

Using first-principles density functional theory we investigated the binding mechanism of hydrogen to thin Mg films and alloyed films. In ultrathin Mg films the stability of hydrides is much lower than in the corresponding bulk systems and it can be modified by metal alloying. We calculated the chemical potential of hydrogen in Mg films for differe...

We study the performance of density-functional theory (DFT) with various exchange-correlation (XC) functionals in describing electronic and structural properties of the prototypical donor/acceptor complex TTF/TCNQ. We find that the binding energetics and the amount of electron transfer between TTF and TCNQ depends strongly on the functional. In par...

We have studied the dynamics of electron transfer between the molecules of an organic donor–acceptor pair upon absorption of light. Specifically, we considered the tetrathiafulvalene (TTF)–7,7,8,8-tetracyanoquinodimethane (TCNQ) donor–acceptor pair using time-dependent density functional theory with local-density approximation. The molecular planes...

Non-equilibrium, pulsed gas delivery and pulsed heating synthesis approaches are used to explore and compare the kinetics and mechanisms of carbon nanotube and graphene growth on metal thin-films. Time-resolved, in situ optical reflectivity of growing nanotubes and graphene reveal the growth kinetics resulting from well-controlled, pulsed fluxes of...

We use first-principles density functional theory to study the binding mechanism of hydrogen to nanoscale systems. We investigate the performance of the exchange-correlation functional in describing the interaction between hydrogen and metal systems and the importance of the vibrational contribution in the formation enthalpy. In ultrathin Mg films...

Organic materials are promising candidates for a next generation of electronic devices, since they offer a variety of new intriguing electronic phenomena while being environmentally friendly, low cost, and mechanically flexible. Here we study the donor/acceptor interface of TTF and TCNQ organic molecular crystals which was found to exhibit metallic...

We have investigated the photo-induced electron dynamics in donor-acceptor pairs of organic molecules. Specifically we will discuss TTF and TCNQ molecules and study their electron dynamics under illumination by means of time-dependent density functional theory within the local-density approximation. In their stable molecular structure, we find that...

The search for technologically and economically viable storage solutions for hydrogen fuel would benefit greatly from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density-functional theory to investigate theoretically the structural and electr...

We study the equilibrium structure of large but finite aggregates of magnetic dipoles, representing a colloidal suspension of magnetite particles in a ferrofluid. With increasing system size, the structural motif evolves from chains and rings to multi-chain and multi-ring assemblies. Very large systems form single- and multi-wall coils, tubes and s...

Within first-principles density functional theory, we explore the feasibility of using metallofullerenes as efficient hydrogen storage media. In particular, we systematically investigate the interaction between hydrogen molecules and La encapsulated all-carbon fullerenes, C(n) (20 < or = n < or = 82), focusing on the role of transferred charges bet...

We propose charge compensated organic molecular crystals as a promising class of materials for hydrogen storage. Using quantum mechanical first-principles calculations based on numerical atom-centered orbitals as all-electron basis functions [1] we study the basic structural properties of molecular crystals consisting of parallel sheets of cations...

Using ab initio spin density functional theory, we investigate the energetics and kinetics of Ti clustering on both neutral and charged C(60) surfaces. We compare the formation energy of sparsely dispersed zero-dimensional (0D), compact single-layered two-dimensional (2D), and clustered three-dimensional (3D) Ti(N) configurations as a function of c...

Endohedral metallofullerenes constitute an appealing class of nanoscale building blocks for fabrication of a wide range of materials. One open question of fundamental importance is the precise nature of charge redistribution within the carbon cages (Cn) upon metal encapsulation. Using ab initio density functional theory, we systematically study the...

We explore theoretically the feasibility of functionalizing carbon nanostructures for hydrogen storage, focusing on the coating of C60 fullerenes with light alkaline-earth metals. Our first-principles density functional theory studies show that both Ca and Sr can bind strongly to the C60 surface, and highly prefer monolayer coating, thereby explain...

We propose functionalizing carbon nanostructures with light alkaline-earth metals for use as hydrogen storage media. To support this idea, we investigate the feasibility of coating C60 fullerenes with light alkaline-earth metals and analyze the hydrogen storage capacities of the resulting compounds. We find a new and unique binding mechanism respon...