arXiv subjects

Sunmin Ryu

Publications and source records attributed to Sunmin Ryu.

At least 37 records · Page 2Linked to original sources

arXiv2020

Interface-Confined Doubly Anisotropic Oxidation of Two-Dimensional MoS2

Despite their importance, chemical reactions confined in a low dimensional space are elusive and experimentally intractable. In this work, we report doubly anisotropic, in-plane and out-of-plane, oxidation reactions of two-dimensional crystals, by resolving interface-confined thermal oxidation of a single and multilayer MoS2 supported on silica substrates from their conventional surface reaction. Using optical second-harmonic generation spectroscopy of artificially stacked multilayers, we directly proved that crystallographically oriented triangular oxides (TOs) were formed in the bottommost layer while triangular etch pits (TEs) were generated in the topmost layer and that both structures were terminated with zigzag edges. The formation of the Mo oxide layer at the interface demonstrates that O2 diffuses efficiently through the van der Waals (vdW) gap but not MoO3, which would otherwise sublime. The fact that TOs are several times larger than TEs indicates that oxidation is greatly enhanced when MoS2 is in direct contact with silica substrates, which suggests a catalytic effect. This study indicates that the vdW-bonded interfaces are essentially open to mass transport and can serve as a model system for investigating chemistry in low dimensional spaces.

cond-mat.mtrl-sci↗

arXiv2020

On-Stack Two-Dimensional Conversion of MoS2 into MoO3

Chemical transformation of existing two-dimensional (2D) materials can be crucial in further expanding the 2D crystal palette required to realize various functional heterostructures. In this work, we demonstrate a 2D 'on-stack' chemical conversion of single-layer crystalline MoS2 into MoO3 with a precise layer control that enables truly 2D MoO3 and MoO3/MoS2 heterostructures. To minimize perturbation of the 2D morphology, a nonthermal oxidation using O2 plasma was employed. The early stage of the reaction was characterized by a defect-induced Raman peak, drastic quenching of photoluminescence (PL) signals and sub-nm protrusions in atomic force microscopy images. As the reaction proceeded from the uppermost layer to the buried layers, PL and optical second harmonic generation signals showed characteristic modulations revealing a layer-by-layer conversion. The plasma-generated 2D oxides, confirmed as MoO3 by x-ray photoelectron spectroscopy, were found to be amorphous but extremely flat with a surface roughness of 0.18 nm, comparable to that of 1L MoS2. The rate of oxidation quantified by Raman spectroscopy decreased very rapidly for buried sulfide layers due to protection by the surface 2D oxides, exhibiting a pseudo-self-limiting behavior. As exemplified in this work, various on-stack chemical transformations can be applied to other 2D materials in forming otherwise unobtainable materials and complex heterostructures, thus expanding the palette of 2D material building blocks.

cond-mat.mtrl-sci↗

arXiv2020

Reversible Sulfuric Acid Doping of Graphene Probed by in-situ Multi-Wavelength Raman Spectroscopy

Since lattice strain and charge density affect various material properties of graphene, a reliable and efficient method is required for quantification of the two variables. While Raman spectroscopy is sensitive and non-destructive, its validity towards precise quantification of chemical charge doping has not been tested. In this work, we quantified in-situ the fractional frequency change of 2D and G peaks in response of charge density induced by sulfuric acid solution as well as native lattice strain. Based on the experimental data and theoretical corroboration, we presented an optical method that simultaneously determines strain and chemically-induced charge density for three popular excitation wavelengths of 457, 514 and 633 nm. In order to expedite intercalation of dopant species through the graphene-SiO2 substrates, dense arrays of nanopores were precisely generated in graphene by thermal oxidation. The nano-perforated graphene membrane system was robust for multiple cycles of doping and undoping processes, and will be useful in studying various types of chemical interactions with graphene.

cond-mat.mes-hall↗

arXiv2020

Direction-Controlled Chemical Doping for Reversible G-Phonon Mixing in ABC Trilayer Graphene

Not only the apparent atomic arrangement but the charge distribution also defines the crystalline symmetry that dictates the electronic and vibrational structures. In this work, we report reversible and direction-controlled chemical doping that modifies the inversion symmetry of AB-bilayer and ABCtrilayer graphene. For the top-down and bottom-up hole injection into graphene sheets, we employed molecular adsorption of electronegative I2 and annealing-induced interfacial hole doping, respectively. The chemical breakdown of the inversion symmetry led to the mixing of the G phonons, Raman active Eg and Raman-inactive Eu modes, which was manifested as the two split G peaks, G- and G+. The broken inversion symmetry could be recovered by removing the hole dopants by simple rinsing or interfacial molecular replacement. Alternatively, the symmetry could be regained by double-side charge injection, which eliminated G- and formed an additional peak, Go, originating from the barely doped interior layer. Chemical modification of crystalline symmetry as demonstrated in the current study can be applied to other low dimensional crystals in tuning their various material properties.

cond-mat.mes-hall↗

arXiv2020

Crossover between Photochemical and Photothermal Oxidations of Atomically Thin Magnetic Semiconductor CrPS4

Many two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have tunable optical bandgaps in the visible or near IR-range standing as a promising candidate for optoelectronic devices. Despite this potential, however, their photoreactions are not well understood or controversial in the mechanistic details. In this work, we report a unique thickness-dependent photoreaction sensitivity and a switchover between two competing reaction mechanisms in atomically thin chromium thiophosphate (CrPS4), a two-dimensional antiferromagnetic semiconductor. CrPS4 showed a threshold power density 2 orders of magnitude smaller than that for MoS2 obeying a photothermal reaction route. In addition, reaction cross section quantified with Raman spectroscopy revealed distinctive power dependences in the low and high power regimes. On the basis of optical in situ thermometric measurements and control experiments against O2, water, and photon energy, we proposed a photochemical oxidation mechanism involving singlet O2 in the low power regime with a photothermal route for the other. We also demonstrated a highly effective encapsulation with Al2O3 as a protection against the destructive photoinduced and ambient oxidations.

cond-mat.mtrl-sci↗

arXiv2020

Structural and optical properties of single- and few-layer magnetic semiconductor CrPS4

Atomically thin binary 2-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure and thickness. By adding another element in those materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS4), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffractions. Transmission electron microscopy showed that preferentially-cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy for the first time and exhibited significant dependence on thickness of crystals exfoliated down to single layer. Optical absorption determined by reflectance contrast was dominated by d-d type transitions localized at Cr3+ ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness-dependence and hinted a high photochemical activity for single layer CrPS4. The current structural and optical investigation will provide a firm basis for future study and application of this novel magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2020

Redox-Governed Charge Doping Dictated by Interfacial Diffusion in Two-Dimensional Materials

Controlling extra charge carriers is pivotal in manipulating electronic, optical, and magnetic properties of various two-dimensional (2D) materials. Nonetheless, the ubiquitous hole doping of 2D materials in the air and acids has been controversial in its mechanistic details. Here we show their common origin is an electrochemical reaction driven by redox couples of oxygen and water molecules. Using real-time photoluminescence imaging of WS2 and Raman spectroscopy of graphene, we capture molecular diffusion through the 2D nanoscopic space between 2D materials and hydrophilic substrates, and show that the latter accommodate water molecules also serving as a hydrating solvent. We also demonstrate that HCl-induced doping is governed by dissolved O2 and pH in accordance with the Nernst equation. The nanoscopic electrochemistry anatomized in this work sets an ambient limit to material properties, which is universal to not only 2D but also other forms of materials.

cond-mat.mes-hall↗

arXiv2020

Varying Electronic Coupling at Graphene-Copper Interfaces Probed with Raman Spectroscopy

As the synthesis of graphene on copper became one of the primary preparation methods for both fundamental research and industrial application, Raman spectra of graphene/Cu systems need to be quantitatively understood regarding how their interactions affect the electronic structure of graphene. Using multi-wavelength Raman spectroscopy, we investigated three types of graphene bound on Cu: graphene grown on Cu foils and Cu film/SiO2, and Cu-evaporated exfoliated graphene. 2D peak frequencies of the first two samples were ~17 cm-1 higher than expected for 1.96 eV excitation even when the effect of strain was considered. More notably, the upshift in 2D decreased with increasing excitation energy. Based on control experiments using Cu-evaporated graphene, we revealed that the spectral anomaly was induced by environment-dependent nonlinear dispersion in the electronic bands of graphene and determined the degree of the electronic modification. We also showed that the large upshifts of G and 2D peaks originating from differential thermal expansion of Cu could be significantly reduced by backing Cu films with dielectric substrates of insignificant thermal expansion. The quantitative analysis of electronic coupling between graphene and Cu presented in this study will be highly useful in characterizing as-grown graphene and possibly in other forms.

cond-mat.mtrl-sci↗

arXiv2018

Variation of photoluminescence spectral line shape of monolayer WS2

The origin of the variation of photoluminescence (PL) spectra of monolayer tungsten disulfide (WS2) is investigated systematically. Dependence of the PL spectrum on the excitation power show that the relatively sharp component corresponds to excitons whereas the broader component at slightly lower energy corresponds to negatively charged trions. PL imaging and second harmonic generation measurements show that the trion signals are suppressed more than the exciton signals near the edges, thereby relatively enhancing the excitonic feature in the PL spectrum and that such relative enhancement of the exciton signals is more pronounced near approximately armchair edges. This effect is interpreted in terms of depletion of free electrons near the edges caused by structural defects and adsorption of electron acceptors such as oxygen atoms.

cond-mat.mes-hall↗

arXiv2014

Extremely high mobility over 5000 cm2/Vs obtained from MoS2 nanosheet transistor with NiOx Schottky gate

Molybdenum disulfide (MoS2) nanosheet, one of two dimensional (2D) semiconductors, has recently been regarded as a promising material to break through the limit of present semiconductors including graphene. However, its potential in carrier mobility has still been depreciated since the field-effect mobilities have only been measured from metal-insulator-semiconductor field effect transistors (MISFETs), where the transport behavior of conducting carriers located at the insulator/MoS2 interface is unavoidably interfered by the interface traps and gate voltage. Here, we for the first time report MoS2-based metal semiconductor field-effect transistors (MESFETs) with NiOx Schottky electrode, where the maximum mobilities or carrier transport behavior of the Schottky devices may hardly be interfered by on-state gate field. Our MESFETs with single-, double-, and triple-layered MoS2 respectively demonstrate high mobilities of 6000, 3500, and 2800 cm2/Vs at a certain low threshold voltage of -1 ~ -2 V. The thickness-dependent mobility difference in MESFETs was theoretically explained with electron scattering reduction mechanisms.

cond-mat.mes-hall↗

arXiv2014

Two-Dimensional Water Diffusion at a Graphene-Silica Interface

Because of the dominant role of the surface of molecules and their individuality, molecules behave dis-tinctively in a confined space, which has far-reaching implications in many physical, chemical and bio-logical systems. Here, we demonstrate that graphene forms a unique atom-thick interstitial space that enables the study of molecular diffusion in 2-dimensions with underlying silica substrates. Raman spec-troscopy visualized intercalation of water from the edge to the center underneath graphene in real time, which was dictated by the hydrophilicity of the substrates. In addition, graphene undergoes reversible deformation to conform to intercalating water clusters or islands. Atomic force microscopy confirmed that the interfacial water layer is only ca. 3.5 angstroms thick, corresponding to one bilayer unit of normal ice. This study also demonstrates that oxygen species responsible for the ubiquitous hole dop-ing are located below graphene. In addition to serving as a transparent confining wall, graphene and possibly other 2-dimensional materials can be used as an optical indicator sensitive to interfacial mass transport and charge transfer.

cond-mat.mes-hall↗

arXiv2013

Optical Probing of Electronic Interaction between Graphene and Hexagonal Boron Nitride

Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilized to detect a few % decrease in the Fermi velocity (vF) of graphene caused by the vdW interaction with underlying hexagonal boron nitride (hBN). Our study also establishes Raman spectroscopic analysis which enables separation of the effects by the vdW interaction from those by mechanical strain or extra charge carriers. The analysis reveals that spectral features of graphene on hBN are mainly affected by change in vF and mechanical strain, but not by charge doping unlike graphene supported on SiO2 substrates. Graphene on hBN was also found to be less susceptible to thermally induced hole doping.

cond-mat.mes-hall↗

arXiv2013

Origin of New Broad Raman D and G Peaks in Annealed Graphene

Since graphene, a single sheet of graphite, has all of its carbon atoms on the surface, its property is very sensitive to materials contacting the surface. Herein, we report novel Raman peaks ob-served in annealed graphene and elucidate their chemical origins by Raman spectroscopy and atomic force microscopy (AFM). Graphene annealed in oxygen-free atmosphere revealed very broad additional Raman peaks overlapping the D, G and 2D peaks of graphene itself. Based on the topographic confirmation by AFM, the new Raman peaks were attributed to amorphous carbon formed on the surface of graphene by carbonization of environmental hydrocarbons. While the carbonaceous layers were formed for a wide range of annealing temperature and time, they could be effectively removed by prolonged annealing in vacuum. This study underlines that spectral features of graphene and presumably other 2-dimensional ma-terials are highly vulnerable to interference by foreign materials of molecular thickness.

cond-mat.mtrl-sci↗

arXiv2012

Optical Separation of Mechanical Strain from Charge Doping in Graphene

Graphene, due to its superior stretchability, exhibits rich structural deformation behaviors and its strain-engineering has proven useful in modifying its electronic and magnetic properties. Despite the strain-sensitivity of the Raman G and 2D modes, the optical characterization of the native strain in graphene on silica substrates has been hampered by excess charges interfering with both modes. Here we show that the effects of strain and charges can be optically separated from each other by correlation analysis of the two modes, enabling simple quantification of both. Graphene with in-plane strain randomly occurring between -0.2% and 0.4% undergoes modest compression (-0.3%) and significant hole doping upon thermal treatments. This study suggests that substrate-mediated mechanical strain is a ubiquitous phenomenon in two-dimensional materials. The proposed analysis will be of great use in characterizing graphene-based materials and devices.

cond-mat.mtrl-sci↗

arXiv2012

Water-Gated Charge Doping of Graphene Induced by Mica Substrates

We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm-thick bilayers were found to be present in regions of the interface of graphene/mica hetero-stacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene, with hole densities of $(9 \pm 2) \times 1012 cm${-2}$. The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.

cond-mat.mtrl-sci↗

arXiv2012

Raman Spectroscopy of Lithographically Patterned Graphene Nanoribbons

Nanometer-scale graphene objects are attracting much research interest because of newly emerging properties originating from quantum confinement effects. We present Raman spectroscopy studies of graphene nanoribbons (GNRs) which are known to have nonzero electronic bandgap. GNRs of width ranging from 15 nm to 100 nm have been prepared by e-beam lithographic patterning of mechanically exfoliated graphene followed by oxygen plasma etching. Raman spectra of narrow GNRs can be characterized by upshifted G band and prominent disorder-related D band originating from scattering at ribbon edges. The D-to-G band intensity ratio generally increases with decreasing ribbon width. However, its decrease for width < 25 nm, partly attributed to amorphization at the edges, provides a valuable experimental estimate on D mode relaxation length of <5 nm. The upshift in the G band of the narrowest GNRs can be attributed to confinement effect or chemical doping by functional groups on the GNR edges. Notably, GNRs are much more susceptible to photothermal effects resulting in reversible hole doping caused by atmospheric oxygen than bulk graphene sheets. Finally we show that the 2D band is still a reliable marker in determining the number of layers of GNRs despite its significant broadening for very narrow GNRs.

cond-mat.mtrl-sci↗

arXiv2011

Raman Spectroscopy Study of Annealing-Induced Effects on Graphene Prepared by Micromechanical Exfoliation

In this note, we report a Raman spectroscopy study of annealing-induced effects on graphene samples prepared by the microexfoliation method. It was shown that randomly located adhesive residues often contaminate nearby graphene sheets during thermal annealing. The contamination on graphene can be as thin as ~1 nm, but gives several new Raman bands of unusually strong intensity. We also find that their intensity is strongly dependent on the excitation wavelength implying that graphene-induced Raman enhancement may be operative. The current study also suggests that graphene can be selectively sensitive towards certain molecular species in binding, which can be exploited for interesting application.

cond-mat.mtrl-sci↗

arXiv2010

Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene on a SiO2 Substrate

Using micro-Raman spectroscopy and scanning tunneling microscopy, we study the relationship between structural distortion and electrical hole doping of graphene on a silicon dioxide substrate. The observed upshift of the Raman G band represents charge doping and not compressive strain. Two independent factors control the doping: (1) the degree of graphene coupling to the substrate, and (2) exposure to oxygen and moisture. Thermal annealing induces a pronounced structural distortion due to close coupling to SiO2 and activates the ability of diatomic oxygen to accept charge from graphene. Gas flow experiments show that dry oxygen reversibly dopes graphene; doping becomes stronger and more irreversible in the presence of moisture and over long periods of time. We propose that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface.

cond-mat.mtrl-sci↗