Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms

Roy, Shrawan,Choi, Wooseon,Jeon, Sera,Kim, Do-Hwan,Kim, Hyun,Yun, Seok Joon,Lee, Yongjun,Lee, Jaekwang,Kim, Young-Min,Kim, Jeongyong American Chemical Society 2018 NANO LETTERS Vol.18 No.7

<P>Chemical treatment using bis(trifluoromethane) sulfonimide (TFSI) was shown to be particularly effective for increasing the photoluminescence (PL) of monolayer (1L) MoS<SUB>2</SUB>, suggesting a convenient method for overcoming the intrinsically low quantum yield of this material. However, the underlying atomic mechanism of the PL enhancement has remained elusive. Here, we report the microscopic origin of the defect healing observed in TFSI-treated 1L-MoS<SUB>2</SUB> through a correlative combination of optical characterization and atomic-scale scanning transmission electron microscopy, which showed that most of the sulfur vacancies were directly repaired by the extrinsic sulfur atoms produced from the dissociation of TFSI, concurrently resulting in a significant PL enhancement. Density functional theory calculations confirmed that the reactive sulfur dioxide molecules that dissociated from TFSI can be reduced to sulfur and oxygen gas at the vacancy site to form strongly bound S-Mo. Our results reveal how defect-mediated nonradiative recombination can be effectively eliminated by a simple chemical treatment method, thereby advancing the practical applications of monolayer semiconductors.</P> [FIG OMISSION]</BR>

Silver nanoflowers for single-particle SERS with 10 pM sensitivity

Roy, Shrawan,Muhammed Ajmal, C,Baik, Seunghyun,Kim, Jeongyong IOP Pub 2017 Nanotechnology Vol.28 No.46

<P>Surface-enhanced Raman scattering (SERS) has received considerable attention as a noninvasive optical sensing technique with ultrahigh sensitivity. While numerous types of metallic particles have been actively investigated as SERS substrates, the development of new SERS agents with high sensitivity and their reliable characterization are still required. Here we report the preparation and characterization of flower-shaped silver (Ag) nanoparticles that exhibit high-sensitivity single-particle SERS performance. Ag nanoflowers (NFs) with bud sizes in the range 220–620 nm were synthesized by the wet synthesis method. The densely packed nanoscale petals with thicknesses in the range 9–22 nm exhibit a large number of hot spots that significantly enhance their plasmonic activity. A single Ag NF particle (530–620 nm) can detect as little as 10<SUP>−11</SUP> M 4-mercaptobenzoic acid, and thus provides a sensitivity three orders of SERS magnitude greater than that of a spherical Ag nanoparticle. The analytical enhancement factors for single Ag NF particles were found to be as high as 8.0?×?10<SUP>9</SUP>, providing unprecedented high SERS detectivity at the single particle level. Here we present an unambiguous and systematic assessment of the SERS performances of the Ag NFs and demonstrate that they provide highly sensitive sensing platforms by single SERS particle.</P>

Modulation of optoelectric properties of monolayer transition metal dichalcogenides placed on a metal pattern

Roy Shrawan,Doan Manh-Ha,Kim Jeongyong,Kang Seon Kyeong,Ahn Gwang Hwi,Lee Hyun Seok,Yun Seok Joon 한국물리학회 2021 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.78 No.8

Atomically thin monolayer transition-metal dochalcogenides (1L-TMDs) are optically active direct band gap semiconducting materials with interesting properties; they are appropriate platform to study and investigate the modulated optoelectronic properties due to locally induced charge transfer phenomenon using various approaches. Herein, 1L-TMDs ( MoS2, WS2 and WSe2) grown using chemical vapor deposition (CVD) were transferred above 10-nm-thick patterned platinum (Pt) stripes deposited on SiO2/ Si substrate to fabricate a local vertical heterostructure of 1L-TMDs with Pt. The optical characterization showed that the PL intensities of n (p)-type 1L-TMDs, namely MoS2 and WS2 ( WSe2), deposited above Pt were reduced with peak positions blue (red)-shifted by 40 (16) meV compared to the samples on SiO2/ Si substrates. This was attributed to the transfer of electrons from the 1L-TMDs to the Pt due to a charge transfer process at the interface. At the same time, an enhanced photocurrent, in comparison to 1L-MoS2 alone was observed under a negative gate voltage of − 40 V from the homojunctions of 1L-MoS2 and 1L-MoS2/Pt formed within the same grain structure due to a Pt-induced local p-doping effect. The charge modulation of the opto-electrical properties of 1L-TMDs due to charge transfer caused using patterned metal provides a simple lateral homojunction for enhanced photovoltaic applications.

Observation of Charge Transfer in Heterostructures Composed of MoSe₂ Quantum Dots and a Monolayer of MoS₂ or WSe₂

Roy, Shrawan,Neupane, Guru P.,Dhakal, Krishna P.,Lee, Jubok,Yun, Seok Joon,Han, Gang Hee,Kim, Jeongyong American Chemical Society 2017 The Journal of Physical Chemistry Part C Vol.121 No.3

<P>Monolayer transition metal dichalcogenides (TMDs) are atomically thin semiconductor films that are ideal platforms for the study and engineering of quantum heterostructures for optoelectronic applications. We present a simple method for the fabrication of TMD heterostructures containing MoSe2 quantum dots (QDS) and a MoS2 or WSe2, monolayer. The strong modification of photoluminescence and Raman spectra that includes the quenching of MoSe2 QDs and the varied spectral weights of trions for the MoS2 and WSe2 monolayers were observed, suggesting the charge transfer Occurring in these TMD heterostructures. Such optically active heterostructures, which can be conveniently fabricated by dispersing TMD QDs onto TMD monolayers, are likely to have various nanophotonic applications because of their versatile and controllable properties.</P>

Heterogeneous modulation of exciton emission in triangular WS₂ monolayers by chemical treatment

Dhakal, Krishna P.,Roy, Shrawan,Yun, Seok Joon,Ghimire, Ganesh,Seo, Changwon,Kim, Jeongyong The Royal Society of Chemistry 2017 Journal of materials chemistry. C, Materials for o Vol.5 No.27

<▼1><P>Spatially heterogeneous effects of bis(trifluoromethane)sulfonimide (TFSI) and benzyl viologen (BV) treatment on the optical properties of triangular monolayer tungsten disulfides are investigated by nanoscale spectral imaging.</P></▼1><▼2><P>Chemical treatments were recently shown to be very effective in enhancing the exciton emission of monolayer transition metal dichalcogenides (1L-TMDs) by suppressing the exciton quenching caused by structural defects. However, the effects of these chemical treatments varied greatly depending on the synthesis method and the type of 1L-TMD; therefore, the exact origin of the emission enhancement is still elusive. Here we report the spatially heterogeneous effects of bis(trifluoromethane)sulfonimide (TFSI) and benzyl viologen (BV) treatment on the optical properties of triangular 1L-WS2 grown by chemical vapor deposition (CVD). Nanoscale photoluminescence (PL) and Raman spectral maps showed that TFSI had a minimal effect on the inner region of the triangular WS2 grain, whereas the PL of the edge region was enhanced up to 25 times; further, BV reduced the PL, also more strikingly in the edge region. Systematic variation of the spectral weights among neutral excitons, trions, and bi-excitons indicated that p-doping and n-doping with TFSI and BV, respectively, occurred in both the inner and edge regions; however, the PL enhancement was attributed mainly to the reduction of structural defects caused by TFSI treatment. Our observation of the spatially heterogeneous effects of chemical treatment suggests that the inner and edge regions of CVD-grown 1L-WS2 are populated with different types of structural defects and helps in clarifying the mechanism by which chemical treatment enhances the optical properties of 1L-TMDs.</P></▼2>

Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides

Dhakal, Krishna P.,Roy, Shrawan,Jang, Houk,Chen, Xiang,Yun, Won Seok,Kim, Hyunmin,Lee, JaeDong,Kim, Jeongyong,Ahn, Jong-Hyun American Chemical Society 2017 Chemistry of materials Vol.29 No.12

<P>The photocarrier relaxation between direct and indirect band gaps along the high symmetry K-Gamma line in the Brillion zone reveals interesting electronic properties of the transition metal dichalcogenides (TMDs) multilayer films. In this study, we reported on the local strain engineering and tuning of an electronic band structure of TMDs multilayer films along the K-Gamma line by artificially creating one-dimensional wrinkle structures. Significant photoluminescence (PL) intensity enhancement in conjunction with continuously tuned optical energy gaps was recorded at the high strain regions. A direct optical band gap along K-K points and an indirect optical gap along Gamma-K points measured from the PL spectra of multilayer samples monotonically decreased as the strain increased, while the indirect band gap along Lambda-Gamma was unaffected owing to the same level of local strain in the range of 0%-2%. The experimental results of band gap tuning were in agreement with the density functional theory calculation results. Local strain modified the band structure in which K-conduction band valley (CBV) was aligned below the Lambda-CBV, and this explained the observed local PL enhancement that made the material indirect via the K-Gamma transition. The study also reported experimental evidence for the funneling of photogenerated excitons toward regions of a higher strain at the top of the wrinkle geometry.</P>

Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films

Kim, Min Su,Roy, Shrawan,Lee, Jubok,Kim, Byung Gu,Kim, Hyun,Park, Ji-Hoon,Yun, Seok Joon,Han, Gang Hee,Leem, Jae-Young,Kim, Jeongyong American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.42

<P>Monolayer transition-metal dichalcogenides (1L-TMDs) are atomically thin direct band gap semiconductors, from which the emission of light is determined by optical transitions of exciton complexes such as neutral excitons and trions. While the quantum yields of 1L-TMDs are quite low, the ability to control the populations of exciton complexes in 1L-TMDs through various doping processes is an interesting advantage, and provides ample possibilities for engineering the optical properties of these semiconductor monolayers. Here we demonstrate a simple method of controlling the populations of excitons and trions to enhance the light emission of 1L-TMDs by having them form heterostructures with ZnO thin films (TFs). 1Ls of MoS2 or MoSe2 showed up to 17-fold increases in photoluminescence (PL) when they were placed on similar to 50 nm thick ZnO TFs. This enhancement of the PL was due to charge exchanges occurring through the 1L-TMD/ZnO interface. The PL enhancements and changes in the PL spectra of the 1L-TMDs were greater when the 1L-TMD/ZnO heterostructures were subjected to 355 nm wavelength laser excitation than when they were excited with a 514 nm wavelength laser, which we attributed to the onset of energy transfer by photoexcited excitons and/or the additional p-doping by photoexcited holes in ZnO. The p-doping phenomenon and the enhanced light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized in spatially resolved PL and Raman spectral maps. Our approach using the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of options for engineering the optical properties of 1L-TMDs may be made available by carrying out simple and intuitive manipulations of exciton complexes, and these endeavors may yield practical applications for 1L-TMDs in nanophotonic devices.</P>

Measurement of lateral and axial resolution of confocal Raman microscope using dispersed carbon nanotubes and suspended graphene

Youngbum Kim,이은지,Shrawan Roy,Anir S. Sharbirin,Lars-Gunnar Ranz,Thomas Dieing,Jeongyong Kim 한국물리학회 2020 Current Applied Physics Vol.20 No.1

A confocal Raman microscope (CRM) facilitates visualization of the spatial distribution of molecular bonds or phonon modes at the submicron level and has been extensively used in the characterization of nanomaterials and devices. The lateral and axial resolution is a key specification that defines the performance of CRM, however, the interpretation of spatial resolution in the literature is often ambiguous, making it often difficult to directly compare Raman images obtained under different conditions. In this report, a convenient and reliable measurement protocol using dispersed carbon nanotubes and suspended graphene as test specimens is proposed to facilitate the determination of the lateral and axial resolutions of a CRM. Spatial resolution values comparable to the results based on Rayleigh criterion calculations were obtained using Raman mapping images of test specimens. This was achieved without the need for complex deconvolution processes or the consideration of an asymmetric dielectric environment.

Near-field exciton imaging of chemically treated MoS₂ monolayers

Kim, Youngbum,Lee, Yongjun,Kim, Hyun,Roy, Shrawan,Kim, Jeongyong The Royal Society of Chemistry 2018 Nanoscale Vol.10 No.18

<P>The exciton-dominated light emission of two-dimensional (2D) semiconductors is determined largely by the doping state and the formation of defects. Extensive studies have shown that chemical treatment critically modifies the doping state and defect state of chemical vapor deposition (CVD)-grown or exfoliated monolayer MoS2 (1L-MoS2), suggesting a promising possibility for engineering the optoelectronic properties of 2D semiconductors. However, chemical treatment inevitably modifies both the doping state and defect states, and their independent roles in the exciton emission of 1L-MoS2 have been difficult to study, significantly limiting the practical and reliable uses of chemical treatment to improve the optical properties of 1L-TMDs. Herein, we used near-field imaging and spectroscopy to investigate the effects of chemical treatment on the exciton emission of 1L-MoS2. CVD-grown 1L-MoS2 was treated with bis(trifluoromethane)-sulfonimide (TFSI) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), and nanoscale maps of neutral exciton and trion emission before and after chemical treatment were obtained with 80 nm spatial resolution. A comparison of the local spatial and spectral compositions of neutral excitons and trions suggested that the p-doping effect of TFSI was especially strong around local defects, whereas electron depletion by TCNQ was spatially uniform. The specific reaction of TFSI to defect locations observed in our study provides the clue for the reason that TFSI is notably effective at improving the light emission of 1L-MoS2.</P>

Composition-Tunable Synthesis of Large-Scale Mo_1-<i>x</i>W_<i>x</i>S₂ Alloys with Enhanced Photoluminescence

Park, Juhong,Kim, Min Su,Park, Bumsu,Oh, Sang Ho,Roy, Shrawan,Kim, Jeongyong,Choi, Wonbong American Chemical Society 2018 ACS NANO Vol.12 No.6

<P>Alloying two-dimensional transition metal dichalcogenides (2D TMDs) is a promising avenue for band gap engineering. In addition, developing a scalable synthesis process is essential for the practical application of these alloys with tunable band gaps in optoelectronic devices. Here, we report the synthesis of optically uniform and scalable single-layer Mo<SUB>1-<I>x</I></SUB>W<SUB><I>x</I></SUB>S<SUB>2</SUB> alloys by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The amount of W content (<I>x</I>) in the Mo<SUB>1-<I>x</I></SUB>W<SUB><I>x</I></SUB>S<SUB>2</SUB> alloy is systemically controlled by the co-sputtering technique. The post-laser process allows layer-by-layer thinning of the Mo<SUB>1-<I>x</I></SUB>W<SUB><I>x</I></SUB>S<SUB>2</SUB> alloys down to a single-layer; such a layer exhibits tunable properties with the optical band gap ranging from 1.871 to 1.971 eV with variation in the W content, <I>x</I> = 0 to 1. Moreover, the predominant exciton complexes, trions, are transitioned to neutral excitons with increasing W concentration; this is attributed to the decrease in excessive charge carriers with an increase in the W content of the alloy. Photoluminescence (PL) and Raman mapping analyses suggest that the laser-thinning of the Mo<SUB>1-<I>x</I></SUB>W<SUB><I>x</I></SUB>S<SUB>2</SUB> alloys is a self-limiting process caused by heat dissipation to the substrate, resulting in spatially uniform single-layer Mo<SUB>1-<I>x</I></SUB>W<SUB><I>x</I></SUB>S<SUB>2</SUB> alloy films. Our findings present a promising path for the fabrication of large-scale single-layer 2D TMD alloys and the design of versatile optoelectronic devices.</P> [FIG OMISSION]</BR>