arXiv subjects

Geunsik Lee

Publications and source records attributed to Geunsik Lee.

At least 19 recordsLinked to original sources

arXiv2025

BO-graphane and BO-diamane

The adsorption of boron and oxygen atoms onto mono- and multi-layer graphene leads to the formation of a buckled graphene layer (BO-graphane) and a 2D diamond-like structure (BO-diamane) sandwiched between boron monoxide layers per DFT calculations. BO-graphane has a calculated Young's modulus ($\it{E}$) of 750 GPA and BO-diamane 771 GPa, higher than the calculated $\it{E}$ of -F,-OH, and -H diamanes; this is due to the presence of B-O bonds in the functionalizing layers. Electronic band structure calculations show BO-graphane and BO-diamane are wide band gap semiconductors with an indirect band gap up to a thickness of three layers (3L). Phonon dispersion and $ab-initio$ molecular dynamics (AIMD) simulations confirm dynamic and thermal stability, maintaining structural integrity at 1000 K. The room-temperature lattice thermal conductivity of BO-graphane and BO-diamane is found to be 879 W/m.K and 1260 W/m.K, respectively, surpassing BeO (385 W/m.K), MgO (64 W/m.K), and Al$_2$O$_3$ (36 W/m.K); and F-diamane (377 W/m.K), and comparable to H-diamane (1145-1960 W/m.K), suggesting them as candidates for thermal management in applications.

cond-mat.mtrl-sci↗

arXiv2024

Surface plasmon-mediated photoluminescence boost in graphene-covered CsPbBr$_3$ quantum dots

The optical properties of graphene (Gr)-covered CsPbBr$_3$ quantum dots (QDs) were investigated using micro-photoluminescence spectroscopy, revealing a remarkable three-orders-of-magnitude enhancement in photoluminescence (PL) intensity compared to bare CsPbBr$_3$ QDs. To elucidate the underlying mechanisms, we combined experimental techniques with density functional theory (DFT) calculations. DFT simulations showed that the graphene layer generates interfacial electrostatic potential barriers when in contact with the CsPbBr$_3$ surface, impeding carrier leakage from perovskite to graphene and enhancing radiative recombination. Additionally, graphene passivates CsPbBr$_3$ surface defect states, suppressing nonradiative recombination of photo-generated carriers. Our study also revealed that graphene becomes n-doped upon contact with CsPbBr$_3$ QDs, activating its plasmon mode. This mode resonantly couples with photo-generated excitons in the perovskite. The momentum mismatch between graphene plasmons and free-space photons is resolved through plasmon scattering at Gr/CsPbBr$_3$ interface corrugations, facilitating the observed super-bright emission. These findings highlight the critical role of graphene as a top contact in dramatically enhancing CsPbBr$_3$ QDs' PL. Our work advances the understanding of graphene-perovskite interfaces and opens new avenues for designing high-efficiency optoelectronic devices. The multifaceted enhancement mechanisms uncovered provide valuable insights for future research in nanophotonics and materials science, potentially leading to breakthroughs in light-emitting technologies.

cond-mat.mtrl-sci↗

arXiv2022

Freeze-frame approach for robust single-molecule tip-enhnaced Raman spectroscopy at room temperature

A quantitative single-molecule tip-enhanced Raman spectroscopy (TERS) study at room temperature remained a challenge due to the rapid structural dynamics of molecules exposed to air. Here, we demonstrate the single-molecule level hyperspectral TERS imaging of brilliant cresyl blue (BCB) at room temperature for the first time, along with quantitative spectral analyses. Freeze-frame approach using a thin Al2O3 capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contaminations in air, enabled reliable and robust chemical imaging. For the molecules resolved spatially in the TERS image, a clear Raman peak variation up to 7.5 cm-1 is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks, we reveal the conformational heterogeneity at the single-molecule level. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy.

physics.chem-ph↗

arXiv2022

Conformational heterogeneity of molecules physisorbed on a gold surface at room temperature

A quantitative single-molecule tip-enhanced Raman spectroscopy (TERS) study at room temperature remained a challenge due to the rapid structural dynamics of molecules exposed to air. Here, we demonstrate the hyperspectral TERS imaging of single or a few brilliant cresyl blue (BCB) molecules at room temperature, along with quantitative spectral analyses. Robust chemical imaging is enabled by the freeze-frame approach using a thin Al$_{2}$O$_{3}$ capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contaminations in air. For the molecules resolved spatially in the TERS image, a clear Raman peak variation up to 7.5 cm$^{-1}$ is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks, we reveal the conformational heterogeneity at the single-molecule level. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy studies.

physics.optics↗

arXiv2022

Fe- and Co-based magnetic tunnel junctions with AlN and ZnO spacers

AlN and ZnO, two wide band-gap semiconductors extensively used in the display industry, crystallise in the wurtzite structure, which can favour the formation of epitaxial interfaces to close-packed common ferromagnets. Here we explore these semiconductors as material for insulating barriers in magnetic tunnel junctions. In particular, the {\it ab initio} quantum transport code {\it Smeagol} is used to model the $X$[111]/$Y$[0001]/$X$[111] ($X=$ Co and Fe, $Y=$ AlN and ZnO) family of junctions. Both semiconductors display a valance-band top with $p$-orbital character, while the conduction band bottom exhibits $s$-type symmetry. The smallest complex-band decay coefficient in the forbidden energy-gap along the [0001] direction is associated with the $Δ_1$ symmetry, and connects across the band gap at the $Γ$ point in 2D Brillouin zones. This feature enables spin filtering and may result in a large tunnelling magnetoresistance. In general, we find that Co-based junctions present limited spin filtering and little magnetoresistance at low bias, since both spin sub-bands cross the Fermi level with $Δ_1$ symmetry. This contrasts the situation of Fe, where only the minority $Δ_1$ band is available. However, even in the case of Fe the magnitude of the magnetoresistance at low bias remains relatively small, mostly due to conduction away from the $Γ$ point and through complex bands with symmetry different than $Δ_1$. The only exception is for the Fe/AlN/Fe junction, where we predict a magnetoresitance of around 1,000\% at low bias.

cond-mat.mes-hall↗

arXiv2021

Co/AlP/Co, Co/GaN/Co as magnetic tunnel junctions

AlP and GaN are wide band-gap semiconductors (SC) uses in opto-electronic industry as light emitting diodes. Here we investigate it as future perspective candidate for insulating barrier in magnetic tunnel junctions. We employ density functional theory for ground state electronic properties and non-equilibrium Green's function method for quantum transport and examined Co/AlP/Co and Co/GaN/Co MTJs. We find that both AlP and GaN valance band maxima are predominantly made with $p_z$-type orbitals while conduction band minima are $s$-type symmetry. We find that both AlP and GaN filter $Δ_1$ symmetry of Bloch states at $Γ$-point and transmission coefficient at any energy level in-between the band-gap of materials, is mostly driven by $Δ_1$ symmetry of Bloch states tunnel via $Γ$-point in first Brillouin zones. We find large magneto-resistance $\sim$300\% in Co/AlP/Co MTJs at zero-bias. In Co/GaN/Co MTJs we find $\sim$300\% TMR at 1.25 eV below the Fermi energy ($E_{F}-1.25$)~eV, while $\sim$10\% TMR around $E_F$ in zero-bias calculations. We notice that both majority and minority $Δ_{2'}$ symmetry of Bloch states with rather different $spd$-orbitals compositions tunnel in Co[0001]/AlP[0001] MTJs and exhibit non-zero TMR, whereas in Co[111]/GaN[0001] MTJs, the both majority and minority $Δ_1$ symmetry of Bloch states with different energy-gradient tunnels at Fermi energy level along [111] transport direction. Our work accentuate the process for systematic, efficient, accurate and versatile framework to design the semiconductors based MTJs for low power electronics.

cond-mat.mtrl-sci↗

arXiv2021

Room temperature self-assembly of cation-free guanine quartet network nucleated from Mo-induced defect on decorated Au(111) with graphene nanoribbons

Guanine-quadruplex, consisting of several stacked guanine-quartets (GQs), has emerged as an important category of novel molecular targets with applications from nanoelectronic devices to anticancer drugs. Incorporation of metal cations into GQ structure is utilized to form stable G-quadruplexes, while no other passage has been reported yet. Here we report the room temperature (RT) molecular self-assembly of extensive metal-free GQ networks on Au(111) surface. Surface defect induced by an implanted molybdenum atom within Au(111) surface is used to nucleate and stabilize the cation-free GQ network. Additionally, the decorated Au(111) surface with 7-armchair graphene nanoribbons (7-AGNRs) results in more extensive GQ networks by curing the disordered phase nucleated from Au step edges spatially and chemically. Scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations confirm GQ networks' formation and unravel the nucleation and growth mechanism. This method stimulates cation-free G-quartet network formation at RT and can lead to stabilizing new emerging molecular self-assembly.

cond-mat.mes-hall↗

arXiv2020

Tip-induced nano-engineering of strain, bandgap, and exciton dynamics in 2D semiconductors

The tunability of the bandgap, absorption and emission energies, photoluminescence (PL) quantum yield, exciton transport, and energy transfer in transition metal dichalcogenide (TMD) monolayers provides a new class of functions for a wide range of ultrathin photonic devices. Recent strain-engineering approaches have enabled us to tune some of these properties, yet dynamic control at the nanoscale with real-time and -space characterizations remains a challenge. Here, we demonstrate a dynamic nano-mechanical strain-engineering of naturally-formed wrinkles in a WSe2 monolayer, with real-time investigation of nano-spectroscopic properties using hyperspectral adaptive tip-enhanced PL (a-TEPL) spectroscopy. First, we characterize nanoscale wrinkles through hyperspectral a-TEPL nano-imaging with <15 nm spatial resolution which reveals the modified nano-excitonic properties by the induced tensile strain at the wrinkle apex, e.g., an increase in the quantum yield due to the exciton funneling, decrease in PL energy up to ~10 meV, and a symmetry change in the TEPL spectra caused by the reconfigured electronic bandstructure. We then dynamically engineer the local strain by pressing and releasing the wrinkle apex through an atomic force tip control. This nano-mechanical strain-engineering allows us to tune the exciton dynamics and emission properties at the nanoscale in a reversible fashion. In addition, we demonstrate a systematic switching and modulation platform of the wrinkle emission, which provides a new strategy for robust, tunable, and ultracompact nano-optical sources in atomically thin semiconductors.

cond-mat.mtrl-sci↗

arXiv2018

Unified low-energy effective Hamiltonian and the band topology of $p$-block square-net layer derivatives

In recent years, low-dimensional materials with tetragonal $P4/nmm$ (orthorhombic $Pnma$) space group having square-net (chain-like) substructure of $p$-block elements have been studied extensively. By using a first-principles calculation and a two-sites $\otimes$ two-orbitals tight-binding model, we construct the unified low-energy effective Hamiltonian and the $\mathbb{Z}_{2}$ topological phase diagram for such materials with different filling factors. Near the chemical potential, we show that the staggered arrangement of ions at 2c (4c) site yields the virtual hopping that have the same form with the second nearest-neighbor hopping between the square-net (chain-like) ions. We show that this hybridization and low-symmetry of the chain-like structure protects the quantum spin Hall insulator phase. Finally, the second order spin-orbit coupling on top of the atomic spin-orbit coupling is considered to clarify the origin of the non-zero Berry phase signals reported in recent quantum oscillation experiments.

cond-mat.mtrl-sci↗

arXiv2018

La-Doped BaSnO$_3$ Electron Transport Layer for Perovskite Solar Cells

Due to the photo-instability and hysteresis of TiO$_2$ electron transport layer (ETL) in perovskite solar cells (PSCs), novel electron transport materials are highly demanded. Here, we show ideal band alignment between La-doped BaSnO$_3$ (LBSO) and methyl ammonium (MA) lead iodide perovskite (MAPbI$_3$). The CH$_3$NH$_3$PbI$_3$/La$_x$Ba$_{(1-x)}$SnO$_3$ interface forms a stable all-perovskite heterostructure. The selective band alignment is manipulated with band gap renormalization by La-doping on the Ba site. LBSO shows high mobility, photo-stability, and structural stability, promising the next generation ETL materials.

cond-mat.mtrl-sci↗

arXiv2018

Rashba-Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces

Despite the imperative importance in solar-cell efficiency, the intriguing phenomena at the interface between perovskite solar-cell and adjacent carrier transfer layers are hardly uncovered. Here we show that PbI$_2$/AI-terminated lead-iodide-perovskite (APbI$_3$; A=Cs$^+$/ methylammonium(MA)) interfaced with the charge transport medium of graphene or TiO2 exhibits the sizable/robust Rashba-Dresselhaus (RD) effect using density-functional-theory and ab initio molecular dynamics (AIMD) simulations above cubic-phase temperature. At the PbI$_2$-terminated graphene/CsPbI3(001) interface, ferroelectric distortion towards graphene facilitates an inversion breaking field. At the MAI-terminated TiO$_2$/MAPbI$_3$(001) interface, the enrooted alignment of MA$^+$ towards TiO$_2$ by short-strong hydrogen-bonding and the concomitant PbI$_3$ distortion preserve the RD interactions even above 330 K. The robust RD effect at the interface even at high temperatures, unlike in bulk, changes the direct-type band to the indirect to suppress recombination of electron and hole, thereby letting these accumulated carriers overcome the potential barrier between perovskite and charge transfer materials, which promotes the solar-cell efficiency.

cond-mat.mtrl-sci↗

arXiv2017

A New Perspective on the Role of A-site Cation in Perovskite Solar Cells

As the race towards higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport towards higher power efficiency has been urgently demanded. Here, we unravel a hidden role of A-site cation of PSCs in carrier transport which has been largely neglected, i.e., tuning the Frohlich electron-phonon (e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Frohlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ion. The coordination to I alleviates electron-phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses lower electron-phonon coupling by several promising organic cations including hydroxyl-ammonium cation (NH$_3$OH$^+$) and possibly Li$^+$ solvating methylamine (Li$^+$NH$_2$CH$_3$) than methyl-ammonium cation. A new perspective on the role of A-site cation could help in improving power efficiency and accelerating the application of PSCs.

cond-mat.mtrl-sci↗

arXiv2016

Two-dimensional Excitonic Photoluminescence in Graphene on Cu surface

Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on Cu(111) surface, which shows an unexpected remarkably sharp and strong emission near 3.16 eV (full-width at half-maximum $\leq$ 3meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue-shift, showing the characteristic negative thermal coefficient of graphene. Observed PLs originate from significantly suppressed dispersion of excited electrons in graphene caused by hybridization of graphene $π$ and Cu d orbitals of the 1st and 2nd Cu layers at a shifted saddle point 0.525(M+K) of Brillouin zone. This finding provides a new pathway to engineering novel optoelectronic graphene devices, whilst maintaining the outstanding electrical properties of graphene.

cond-mat.mes-hall↗

arXiv2013

Anisotropic Dirac electronic structures of AMnBi$_2$ (A=Sr, Ca)

Low energy electronic structures in AMnBi2 (A=alkaline earths) are investigated using a first-principles calculation and a tight binding method. An anisotropic Dirac dispersion is induced by the checkerboard arrangement of A atoms above and below the Bi square net in AMnBi2. SrMnBi2 and CaMnBi2 have a different kind of Dirac dispersion due to the different stacking of nearby A layers, where each Sr (Ca) of one side appears at the overlapped (alternate) position of the same element at the other side. Using the tight binding analysis, we reveal the chirality of the anisotropic Dirac electrons as well as the sizable spin-orbit coupling effect in the Bi square net. We suggest that the Bi square net provides a platform for the interplay between anisotropic Dirac electrons and the neighboring environment such as magnetism and structural changes.

cond-mat.mtrl-sci↗

arXiv2012

Orbital selective Fermi surface shifts and mechanism of high T$_c$ superconductivity in correlated AFeAs (A=Li,Na)

Based on the dynamical mean field theory (DMFT) and angle resolved photoemission spectroscopy (ARPES), we have investigated the mechanism of high $T_c$ superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the observed ARPES measurement. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect with the DMFT method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

cond-mat.supr-con↗

arXiv2011

Density functional calculations of the electronic structure and magnetic properties of the hydrocarbon K3picene superconductor near the metal-insulator transition

We have investigated the electronic structures and magnetic properties of of K3picene, which is a first hydrocarbon superconductor with high transition temperature T_c=18K. We have shown that the metal-insulator transition (MIT) is driven in K3picene by 5% volume enhancement with a formation of local magnetic moment. Active bands for superconductivity near the Fermi level E_F are found to have hybridized character of LUMO and LUMO+1 picene molecular orbitals. Fermi surfaces of K3picene manifest neither prominent nesting feature nor marked two-dimensional behavior. By estimating the ratio of the Coulomb interaction U and the band width W of the active bands near E_F, U/W, we have demonstrated that K3picene is located in the vicinity of the Mott transition.

cond-mat.supr-con↗

arXiv2011

Self-trapping nature of Tl nanoclusters on Si(111)-7$\times$7 surface

We have investigated electronic and structural properties of thallium (Tl) nanoclusters formed on the Si(111)-7$\times$7 surface at room temperature (RT) by utilizing photoemission spectroscopy (PES) and high-resolution electron-energy-loss spectroscopy (HREELS) combined with first principles calculations. Our PES data show that the state S2 stemming from Si restatoms remains quite inert with Tl coverage $θ$ while S1 from Si adatoms gradually changes, in sharp contrast with the rapidly decaying states of Na or Li nanoclusters. No Tl-induced surface state is observed until $θ$=0.21 ML where Tl nanoclusters completely cover the faulted half unit cells (FHUCs) of the surface. These spectral behaviors of surface states and a unique loss peak L$_2$ associated with Tl in HREELS spectra indicate no strong Si-Tl bonding and are well understood in terms of gradual filling of Si dangling bonds with increasing $θ$. Our calculational results further reveal that there are several metastable atomic structures for Tl nanoclusters at RT transforming from each other faster than 10$^{10}$ flippings per second. We thus conclude that the highly mobile Tl atoms form self-trapped nanoclusters within FHUC at RT with several metastable phases. The mobile and multi-phased nature of Tl nanoclusters not only account for all the existing experimental observations including the fuzzy scanning tunneling microscope images and a dynamical model proposed by recent x-ray study but also provides an example of self-trapping of atoms in a nanometer-scale region.

cond-mat.mes-hall↗

arXiv2011

Electron correlation induced small anisotropy in iron-based superconductors

We have investigated the electron correlation effect on the electronic structures and transport properties of the iron-based superconductors using the density functional theory (DFT) and dynamical mean field theory (DMFT). By considering the Fe 3d electron correlation using the DMFT, the bandwidth near the Fermi level is substantially suppressed compared to the conventional DFT calculation. Because of the different renormalization factors of each 3d orbital, the DMFT gives considerably reduced electrical anisotropy compared to the DFT results, which explains the unusually small anisotropic resistivity and superconducting property observed in the iron-based superconductors. We suggest that the electron correlation effect should be considered to explain the anisotropic transport properties of the general d/f valence electron system.

cond-mat.str-el↗