Ismail Bilgin - Academia.edu (original) (raw)

Papers by Ismail Bilgin

Research paper thumbnail of Direct and Scalable Chemical Vapor Deposition of Ultrathin Low-Noise MoS2 Membranes on Apertures

arXiv (Cornell University), May 11, 2015

We show that atomically thin molybdenum disulfide (MoS 2) crystals can grow without any underlyin... more We show that atomically thin molybdenum disulfide (MoS 2) crystals can grow without any underlying substrates into free-standing atomically-thin layers, maintaining their planar 2D form. Using this property, we present a new mechanism for 2D crystal synthesis, i.e. reagent-limited nucleation near an aperture edge followed by reactions that allow crystal growth into the free-space of the aperture. Such an approach enables us, for the first time, the direct and selective growth of freestanding membranes of atomically thin MoS 2 layers across micrometer-scale prefabricated solid-state apertures in SiN x membranes. Under optimal conditions, MoS 2 grows preferentially across apertures, resulting in sealed membranes that are one to a few atomic layers thick. Since our method involves "free-space growth" and is devoid of either substrates or "transfer", it is conceivably the most contamination-free method for obtaining 2D crystals reported so far. The membrane quality was investigated using atomic-resolution transmission electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, and lownoise ion-current recordings through nanopores fabricated in such membranes.

Research paper thumbnail of Energy dispersive X-ray spectroscopy of atomically thin semiconductors

We report the implementation of energy dispersive X-ray spectroscopy for layered semiconductors i... more We report the implementation of energy dispersive X-ray spectroscopy for layered semiconductors in the form of atomically thin transition metal dichalcogenides. The technique is based on a scanning electron microscope equipped with a silicon drift detector for energy dispersive X-ray analysis. By optimizing operational parameters in numerical simulations and experiments, we achieve layer-resolving sensitivity for few-layer crystals down to the monolayer limit and demonstrate elemental composition profiling in vertical and lateral heterobilayers of transition metal dichalcogenides. The technique can be straight-forwardly applied to other layered two-dimensional materials and van der Waals heterostructures, thus expanding the experimental toolbox for quantitative characterization of layer number, atomic composition, or alloy gradients for atomically thin materials and devices.

Research paper thumbnail of Stacking-dependent exciton multiplicity in WSe2 bilayers

Twisted layers of atomically thin two-dimensional materials realize a broad range of novel quantu... more Twisted layers of atomically thin two-dimensional materials realize a broad range of novel quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report experimental and theoretical studies of WSe2 homobilayers obtained in two stable configurations of 2H (60° twist) and 3R (0° twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence spectroscopy at cryogenic temperatures, we uncover marked differences in the optical characteristics of 2H and 3R bilayer WSe2 which we explain on the basis of beyond-DFT theoretical calculations. Our results highlight the role of layer stacking for the spectral multiplicity of momentum-direct intralayer exciton transitions in absorption, and relate the multiplicity of phonon sidebands in the photoluminescence to momentum-indirect excitons wi...

Research paper thumbnail of O 2 -induced in-situ manipulation of exciton recombination pathways in 2D heterostructures: Submicron, intensity-programmable pixels with rapid write-read-erase capability, as well as 2D O 2 -sensor applications

Bulletin of the American Physical Society, 2019

Research paper thumbnail of 2D Heterocrystals: Vertical stacking of rotationally aligned electronically, structurally and chemically dissimilar 2D materials

Bulletin of the American Physical Society, 2017

BANSIL, SWASTIK KAR, Northeastern University-We present a new species of 2D materials called 'Het... more BANSIL, SWASTIK KAR, Northeastern University-We present a new species of 2D materials called 'Heterocrystals' (HCs), which are layered stacks of chemically, structurally, and electronically dissimilar 2D materials that grow with perfect rotational alignment and long-range order, despite substantial lattice mismatch. We have successfully grown a family of 2D HCs using chemical vapor deposition. Our investigations reveal a novel lattice matching such that n unit lengths of one lattice approximately matches m lengths of the other lattice, e.g. 3x3 unit cells of Bi2Se3 match nearly perfectly with 4x4 unit cells of MoS2, forming the larger HC unit cell. The HC exhibits a variety of electronic and optical properties different from its parent 2D crystals. Such large changes in properties are in sharp contrast with those seen from randomly stacked 2D materials, or those grown with high crystallographically commensurate structures. We will present a range of novel, tunable and reconfigurable optical and electronic properties of these new systems, including transition from direct to indirect band gap, tunable and reversible photoluminescences, electronic transport, and Raman spectroscopic results. Our experimental results are in good agreement with theoretical results.

Research paper thumbnail of Resolving surface potential variation in Ge/MoS2 heterostructures with Kelvin probe force microscopy

AIP Advances, 2021

This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Enhanced van der Waals epit... more This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Enhanced van der Waals epitaxy of germanium by out-of-plane dipole moment induced from transferred graphene on TiN/AlN multilayers

Research paper thumbnail of Direct observation and in-situ control of interfaces between heterostructures of chemically and structurally distinct 2D materials

Bulletin of the American Physical Society, 2018

Research paper thumbnail of Ab-initio study of heterostructures of vertically stacked and rotationally aligned incommensurate 2D-films

Bulletin of the American Physical Society, 2017

3 quintuple-layer (QL) deposited on an MoS 2 trilayer (TL) can stack aligned rotationally with lo... more 3 quintuple-layer (QL) deposited on an MoS 2 trilayer (TL) can stack aligned rotationally with long-range crystallographic order, despite the incommensurability of their lattices to form a new type of well-defined heterocrystal. Surprisingly, interaction between the Bi 2 Se 3 and MoS 2 layers leads to electronic properties of the heterocrystal that are quite distinct from those of the parent films. We discuss our experimental findings in terms of first-principles computations of electronic and spin-structures, as well as charge densities for heterostructures of Bi 2 Se 3 stacked layer-by-layer on MoSe 2 and WS 2 films.

Research paper thumbnail of Second- and third-order optical susceptibilities across excitons states in 2D monolayer transition metal dichalcogenides

2D Materials, 2021

Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In th... more Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In this work we have used second-harmonic generation and the four-wave mixing spectroscopy in resonance with the excitons in MoS2, MoSe2, and WS2 monolayers to characterize the nonlinear optical properties of these materials. We show that trions and excitons are responsible for enhancing the nonlinear optical response and determine the exciton and trion energies by comparing with the photoluminescence spectra. Moreover, we extract the second- and third-order optical sheet susceptibility (χ (2) and χ (3)) across exciton energies and compare with values found in the literature. We also demonstrate the ability to generate different nonlinear effects in a wide spectral range in the visible region for monolayer MoS2, opening the possibility of using two-dimensional materials for nonlinear optoelectronic and photonic applications.

Research paper thumbnail of Nonlinear Dark-Field Imaging of One-Dimensional Defects in Monolayer Dichalcogenides

Research paper thumbnail of Polariton hyperspectral imaging of two-dimensional semiconductor crystals

Scientific Reports, 2019

Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong bin... more Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong binding energies and sizable light-matter interactions. Coupled to optical cavities, monolayer TMDs routinely reach the regime of strong light-matter coupling, where excitons and photons admix coherently to form polaritons up to room temperature. Here, we explore the two-dimensional nature of TMD polaritons with scanning-cavity hyperspectral imaging. We record a spatial map of polariton properties of extended WS2 monolayers coupled to a tunable micro cavity in the strong coupling regime, and correlate it with maps of exciton extinction and fluorescence taken from the same flake with the cavity. We find a high level of homogeneity, and show that polariton splitting variations are correlated with intrinsic exciton properties such as oscillator strength and linewidth. Moreover, we observe a deviation from thermal equilibrium in the resonant polariton population, which we ascribe to non-Markovi...

Research paper thumbnail of Widely tunable Bi 2 Se 3 /transition metal dichalcogenide 2D heterostructures for write-read-erase-reuse applications

Research paper thumbnail of Metallic and highly conducting two-dimensional atomic arrays of sulfur enabled by molybdenum disulfide nanotemplate

npj Computational Materials, 2017

Element sulfur in nature is an insulating solid. While it has been tested that one-dimensional su... more Element sulfur in nature is an insulating solid. While it has been tested that one-dimensional sulfur chain is metallic and conducting, the investigation on two-dimensional sulfur remains elusive. We report that molybdenum disulfide layers are able to serve as the nanotemplate to facilitate the formation of two-dimensional sulfur. Density functional theory calculations suggest that confined inbetween layers of molybdenum disulfide, sulfur atoms are able to form two-dimensional triangular arrays that are highly metallic. As a result, these arrays contribute to the high conductivity and metallic phase of the hybrid structures of molybdenum disulfide layers and two-dimensional sulfur arrays. The experimentally measured conductivity of such hybrid structures reaches up to 223 S/ m. Multiple experimental results, including X-ray photoelectron spectroscopy (XPS), transition electron microscope (TEM), selected area electron diffraction (SAED), agree with the computational insights. Due to the excellent conductivity, the current density is linearly proportional to the scan rate until 30,000 mV s −1 without the attendance of conductive additives. Using such hybrid structures as electrode, the two-electrode supercapacitor cells yield a power density of 10 6 Wh kg −1 and energy density~47.5 Wh kg −1 in ionic liquid electrolytes. Our findings offer new insights into using two-dimensional materials and their Van der Waals heterostructures as nanotemplates to pattern foreign atoms for unprecedented material properties.

Research paper thumbnail of Resonant Raman and Exciton Coupling in High-Quality Single Crystals of Atomically Thin Molybdenum Diselenide Grown by Vapor-Phase Chalcogenization

ACS nano, Jan 23, 2018

We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, h... more We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, high-quality single-crystal atomically thin molybdenum diselenide samples. Measurements were performed in samples with four different incident laser excitation energies ranging from 1.95 eV ⩽ E ⩽ 2.71 eV, revealing rich spectral information in samples ranging from N = 1-4 layers and a thick, bulk sample. In addition to previously observed (and identified) peaks, we specifically investigate the origin of a peak near ω ≈ 250 cm. Our density functional theory and Bethe-Salpeter calculations suggest that this peak arises from a double-resonant Raman process involving the ZA acoustic phonon perpendicular to the layer. This mode appears prominently in freshly prepared samples and disappears in aged samples, thereby offering a method for ascertaining the high optoelectronic quality of freshly prepared 2D-MoSe crystals. We further present an in-depth investigation of the energy-dependent variatio...

Research paper thumbnail of Layer dependence of the electronic band alignment of few-layer MoS2 on SiO2 measured using photoemission electron microscopy

Physical Review B, 2017

Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconduct... more Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultra-thin, flexible, and efficient. 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here we

Research paper thumbnail of Tunable and laser-reconfigurable 2D heterocrystals obtained by epitaxial stacking of crystallographically incommensurate Bi2Se3 and MoS2 atomic layers

Science advances, 2017

Vertical stacking is widely viewed as a promising approach for designing advanced functionalities... more Vertical stacking is widely viewed as a promising approach for designing advanced functionalities using two-dimensional (2D) materials. Combining crystallographically commensurate materials in these 2D stacks has been shown to result in rich new electronic structure, magnetotransport, and optical properties. In this context, vertical stacks of crystallographically incommensurate 2D materials with well-defined crystallographic order are a counterintuitive concept and, hence, fundamentally intriguing. We show that crystallographically dissimilar and incommensurate atomically thin MoS2 and Bi2Se3 layers can form rotationally aligned stacks with long-range crystallographic order. Our first-principles theoretical modeling predicts heterocrystal electronic band structures, which are quite distinct from those of the parent crystals, characterized with an indirect bandgap. Experiments reveal striking optical changes when Bi2Se3 is stacked layer by layer on monolayer MoS2, including 100% pho...

Research paper thumbnail of Transparent Electrophysiology Microelectrodes and Interconnects from Metal Nanomesh

ACS Nano, 2017

Mapping biocurrents at both microsecond and single-cell resolution requires the combination of op... more Mapping biocurrents at both microsecond and single-cell resolution requires the combination of optical imaging with innovative electrophysiological sensing techniques. Here, we present transparent electrophysiology electrodes and interconnects made of gold (Au) nanomesh on flexible substrates to achieve such measurements. Compared to previously demonstrated indium tin oxide (ITO) and graphene electrodes, the ones from Au nanomesh possess superior properties including low electrical impedance, high transparency, good cell viability, and superb flexibility. Specifically, we demonstrated a 15 nm thick Au nanomesh electrode with 8.14 Ω•cm 2 normalized impedance, >65% average transmittance over a 300−1100 nm window, and stability up to 300 bending cycles. Systematic sheet resistance measurements, electrochemical impedance studies, optical characterization, mechanical bending tests, and cell studies highlight the capabilities of the Au nanomesh as a transparent electrophysiology electrode and interconnect material. Together, these results demonstrate applicability of using nanomesh under biological conditions and broad applications in biology and medicine.

Research paper thumbnail of Charge transfer in crystalline germanium/monolayer MoS2heterostructures prepared by chemical vapor deposition

Nanoscale, 2016

Heterostructuring provides novel opportunities for exploring emergent phenomena and applications ... more Heterostructuring provides novel opportunities for exploring emergent phenomena and applications by developing designed properties beyond those of homogeneous materials. Advances in nanoscience enable the preparation of heterostructures formed incommensurate materials. Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, are of particular interest due to their distinct physical characteristics. Recently, 2D/2D heterostructures have opened up new research areas. However, other heterostructures such as 2D/three-dimensional (3D) materials have not been thoroughly studied yet although the growth of 3D materials on 2D materials creating 2D/3D heterostructures with exceptional carrier transport properties has been reported. Here we report a novel heterostructure composed of Ge and monolayer MoS2, prepared by chemical vapor deposition. A single crystalline Ge (110) thin film was grown on monolayer MoS2. The electrical characteristics of Ge and MoS2 in the Ge/MoS2 heterostructure were remarkably different from those of isolated Ge and MoS2. The field-effect conductivity type of the monolayer MoS2 is converted from n-type to p-type by growth of the Ge thin film on top of it. Undoped Ge on MoS2 is highly conducting. The observations can be explained by charge transfer in the heterostructure as opposed to chemical doping via the incorporation of impurities, based on our first-principles calculations.

Research paper thumbnail of Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality

ACS nano, Jan 13, 2015

The ability to synthesize high-quality samples over large areas and at low cost is one of the big... more The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few-layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm(2)/(V s) in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Because of their high crystalline quality, Raman spectroscopy on these samples reveal a range...

Research paper thumbnail of Direct and Scalable Deposition of Atomically Thin Low-Noise MoS2 Membranes on Apertures

ACS nano, Jan 30, 2015

Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a pla... more Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a planar freestanding crystal. When the substrate edge is in the form of an aperture, reagent-limited nucleation followed by edge growth facilitate direct and selective growth of freestanding MoS2 membranes. We have found conditions under which MoS2 grows preferentially across micrometer-scale prefabricated solid-state apertures in silicon nitride membranes, resulting in sealed membranes that are one to a few atomic layers thick. We have investigated the structure and purity of our membranes by a combination of atomic-resolution transmission electron microscopy, elemental analysis, Raman spectroscopy, photoluminescence spectroscopy, and low-noise ion-current recordings through nanopores fabricated in such membranes. Finally, we demonstrate the utility of fabricated ultrathin nanopores in such membranes for single-stranded DNA translocation detection.

Research paper thumbnail of Direct and Scalable Chemical Vapor Deposition of Ultrathin Low-Noise MoS2 Membranes on Apertures

arXiv (Cornell University), May 11, 2015

We show that atomically thin molybdenum disulfide (MoS 2) crystals can grow without any underlyin... more We show that atomically thin molybdenum disulfide (MoS 2) crystals can grow without any underlying substrates into free-standing atomically-thin layers, maintaining their planar 2D form. Using this property, we present a new mechanism for 2D crystal synthesis, i.e. reagent-limited nucleation near an aperture edge followed by reactions that allow crystal growth into the free-space of the aperture. Such an approach enables us, for the first time, the direct and selective growth of freestanding membranes of atomically thin MoS 2 layers across micrometer-scale prefabricated solid-state apertures in SiN x membranes. Under optimal conditions, MoS 2 grows preferentially across apertures, resulting in sealed membranes that are one to a few atomic layers thick. Since our method involves "free-space growth" and is devoid of either substrates or "transfer", it is conceivably the most contamination-free method for obtaining 2D crystals reported so far. The membrane quality was investigated using atomic-resolution transmission electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, and lownoise ion-current recordings through nanopores fabricated in such membranes.

Research paper thumbnail of Energy dispersive X-ray spectroscopy of atomically thin semiconductors

We report the implementation of energy dispersive X-ray spectroscopy for layered semiconductors i... more We report the implementation of energy dispersive X-ray spectroscopy for layered semiconductors in the form of atomically thin transition metal dichalcogenides. The technique is based on a scanning electron microscope equipped with a silicon drift detector for energy dispersive X-ray analysis. By optimizing operational parameters in numerical simulations and experiments, we achieve layer-resolving sensitivity for few-layer crystals down to the monolayer limit and demonstrate elemental composition profiling in vertical and lateral heterobilayers of transition metal dichalcogenides. The technique can be straight-forwardly applied to other layered two-dimensional materials and van der Waals heterostructures, thus expanding the experimental toolbox for quantitative characterization of layer number, atomic composition, or alloy gradients for atomically thin materials and devices.

Research paper thumbnail of Stacking-dependent exciton multiplicity in WSe2 bilayers

Twisted layers of atomically thin two-dimensional materials realize a broad range of novel quantu... more Twisted layers of atomically thin two-dimensional materials realize a broad range of novel quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report experimental and theoretical studies of WSe2 homobilayers obtained in two stable configurations of 2H (60° twist) and 3R (0° twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence spectroscopy at cryogenic temperatures, we uncover marked differences in the optical characteristics of 2H and 3R bilayer WSe2 which we explain on the basis of beyond-DFT theoretical calculations. Our results highlight the role of layer stacking for the spectral multiplicity of momentum-direct intralayer exciton transitions in absorption, and relate the multiplicity of phonon sidebands in the photoluminescence to momentum-indirect excitons wi...

Research paper thumbnail of O 2 -induced in-situ manipulation of exciton recombination pathways in 2D heterostructures: Submicron, intensity-programmable pixels with rapid write-read-erase capability, as well as 2D O 2 -sensor applications

Bulletin of the American Physical Society, 2019

Research paper thumbnail of 2D Heterocrystals: Vertical stacking of rotationally aligned electronically, structurally and chemically dissimilar 2D materials

Bulletin of the American Physical Society, 2017

BANSIL, SWASTIK KAR, Northeastern University-We present a new species of 2D materials called 'Het... more BANSIL, SWASTIK KAR, Northeastern University-We present a new species of 2D materials called 'Heterocrystals' (HCs), which are layered stacks of chemically, structurally, and electronically dissimilar 2D materials that grow with perfect rotational alignment and long-range order, despite substantial lattice mismatch. We have successfully grown a family of 2D HCs using chemical vapor deposition. Our investigations reveal a novel lattice matching such that n unit lengths of one lattice approximately matches m lengths of the other lattice, e.g. 3x3 unit cells of Bi2Se3 match nearly perfectly with 4x4 unit cells of MoS2, forming the larger HC unit cell. The HC exhibits a variety of electronic and optical properties different from its parent 2D crystals. Such large changes in properties are in sharp contrast with those seen from randomly stacked 2D materials, or those grown with high crystallographically commensurate structures. We will present a range of novel, tunable and reconfigurable optical and electronic properties of these new systems, including transition from direct to indirect band gap, tunable and reversible photoluminescences, electronic transport, and Raman spectroscopic results. Our experimental results are in good agreement with theoretical results.

Research paper thumbnail of Resolving surface potential variation in Ge/MoS2 heterostructures with Kelvin probe force microscopy

AIP Advances, 2021

This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Enhanced van der Waals epit... more This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Enhanced van der Waals epitaxy of germanium by out-of-plane dipole moment induced from transferred graphene on TiN/AlN multilayers

Research paper thumbnail of Direct observation and in-situ control of interfaces between heterostructures of chemically and structurally distinct 2D materials

Bulletin of the American Physical Society, 2018

Research paper thumbnail of Ab-initio study of heterostructures of vertically stacked and rotationally aligned incommensurate 2D-films

Bulletin of the American Physical Society, 2017

3 quintuple-layer (QL) deposited on an MoS 2 trilayer (TL) can stack aligned rotationally with lo... more 3 quintuple-layer (QL) deposited on an MoS 2 trilayer (TL) can stack aligned rotationally with long-range crystallographic order, despite the incommensurability of their lattices to form a new type of well-defined heterocrystal. Surprisingly, interaction between the Bi 2 Se 3 and MoS 2 layers leads to electronic properties of the heterocrystal that are quite distinct from those of the parent films. We discuss our experimental findings in terms of first-principles computations of electronic and spin-structures, as well as charge densities for heterostructures of Bi 2 Se 3 stacked layer-by-layer on MoSe 2 and WS 2 films.

Research paper thumbnail of Second- and third-order optical susceptibilities across excitons states in 2D monolayer transition metal dichalcogenides

2D Materials, 2021

Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In th... more Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In this work we have used second-harmonic generation and the four-wave mixing spectroscopy in resonance with the excitons in MoS2, MoSe2, and WS2 monolayers to characterize the nonlinear optical properties of these materials. We show that trions and excitons are responsible for enhancing the nonlinear optical response and determine the exciton and trion energies by comparing with the photoluminescence spectra. Moreover, we extract the second- and third-order optical sheet susceptibility (χ (2) and χ (3)) across exciton energies and compare with values found in the literature. We also demonstrate the ability to generate different nonlinear effects in a wide spectral range in the visible region for monolayer MoS2, opening the possibility of using two-dimensional materials for nonlinear optoelectronic and photonic applications.

Research paper thumbnail of Nonlinear Dark-Field Imaging of One-Dimensional Defects in Monolayer Dichalcogenides

Research paper thumbnail of Polariton hyperspectral imaging of two-dimensional semiconductor crystals

Scientific Reports, 2019

Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong bin... more Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong binding energies and sizable light-matter interactions. Coupled to optical cavities, monolayer TMDs routinely reach the regime of strong light-matter coupling, where excitons and photons admix coherently to form polaritons up to room temperature. Here, we explore the two-dimensional nature of TMD polaritons with scanning-cavity hyperspectral imaging. We record a spatial map of polariton properties of extended WS2 monolayers coupled to a tunable micro cavity in the strong coupling regime, and correlate it with maps of exciton extinction and fluorescence taken from the same flake with the cavity. We find a high level of homogeneity, and show that polariton splitting variations are correlated with intrinsic exciton properties such as oscillator strength and linewidth. Moreover, we observe a deviation from thermal equilibrium in the resonant polariton population, which we ascribe to non-Markovi...

Research paper thumbnail of Widely tunable Bi 2 Se 3 /transition metal dichalcogenide 2D heterostructures for write-read-erase-reuse applications

Research paper thumbnail of Metallic and highly conducting two-dimensional atomic arrays of sulfur enabled by molybdenum disulfide nanotemplate

npj Computational Materials, 2017

Element sulfur in nature is an insulating solid. While it has been tested that one-dimensional su... more Element sulfur in nature is an insulating solid. While it has been tested that one-dimensional sulfur chain is metallic and conducting, the investigation on two-dimensional sulfur remains elusive. We report that molybdenum disulfide layers are able to serve as the nanotemplate to facilitate the formation of two-dimensional sulfur. Density functional theory calculations suggest that confined inbetween layers of molybdenum disulfide, sulfur atoms are able to form two-dimensional triangular arrays that are highly metallic. As a result, these arrays contribute to the high conductivity and metallic phase of the hybrid structures of molybdenum disulfide layers and two-dimensional sulfur arrays. The experimentally measured conductivity of such hybrid structures reaches up to 223 S/ m. Multiple experimental results, including X-ray photoelectron spectroscopy (XPS), transition electron microscope (TEM), selected area electron diffraction (SAED), agree with the computational insights. Due to the excellent conductivity, the current density is linearly proportional to the scan rate until 30,000 mV s −1 without the attendance of conductive additives. Using such hybrid structures as electrode, the two-electrode supercapacitor cells yield a power density of 10 6 Wh kg −1 and energy density~47.5 Wh kg −1 in ionic liquid electrolytes. Our findings offer new insights into using two-dimensional materials and their Van der Waals heterostructures as nanotemplates to pattern foreign atoms for unprecedented material properties.

Research paper thumbnail of Resonant Raman and Exciton Coupling in High-Quality Single Crystals of Atomically Thin Molybdenum Diselenide Grown by Vapor-Phase Chalcogenization

ACS nano, Jan 23, 2018

We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, h... more We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, high-quality single-crystal atomically thin molybdenum diselenide samples. Measurements were performed in samples with four different incident laser excitation energies ranging from 1.95 eV ⩽ E ⩽ 2.71 eV, revealing rich spectral information in samples ranging from N = 1-4 layers and a thick, bulk sample. In addition to previously observed (and identified) peaks, we specifically investigate the origin of a peak near ω ≈ 250 cm. Our density functional theory and Bethe-Salpeter calculations suggest that this peak arises from a double-resonant Raman process involving the ZA acoustic phonon perpendicular to the layer. This mode appears prominently in freshly prepared samples and disappears in aged samples, thereby offering a method for ascertaining the high optoelectronic quality of freshly prepared 2D-MoSe crystals. We further present an in-depth investigation of the energy-dependent variatio...

Research paper thumbnail of Layer dependence of the electronic band alignment of few-layer MoS2 on SiO2 measured using photoemission electron microscopy

Physical Review B, 2017

Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconduct... more Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultra-thin, flexible, and efficient. 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here we

Research paper thumbnail of Tunable and laser-reconfigurable 2D heterocrystals obtained by epitaxial stacking of crystallographically incommensurate Bi2Se3 and MoS2 atomic layers

Science advances, 2017

Vertical stacking is widely viewed as a promising approach for designing advanced functionalities... more Vertical stacking is widely viewed as a promising approach for designing advanced functionalities using two-dimensional (2D) materials. Combining crystallographically commensurate materials in these 2D stacks has been shown to result in rich new electronic structure, magnetotransport, and optical properties. In this context, vertical stacks of crystallographically incommensurate 2D materials with well-defined crystallographic order are a counterintuitive concept and, hence, fundamentally intriguing. We show that crystallographically dissimilar and incommensurate atomically thin MoS2 and Bi2Se3 layers can form rotationally aligned stacks with long-range crystallographic order. Our first-principles theoretical modeling predicts heterocrystal electronic band structures, which are quite distinct from those of the parent crystals, characterized with an indirect bandgap. Experiments reveal striking optical changes when Bi2Se3 is stacked layer by layer on monolayer MoS2, including 100% pho...

Research paper thumbnail of Transparent Electrophysiology Microelectrodes and Interconnects from Metal Nanomesh

ACS Nano, 2017

Mapping biocurrents at both microsecond and single-cell resolution requires the combination of op... more Mapping biocurrents at both microsecond and single-cell resolution requires the combination of optical imaging with innovative electrophysiological sensing techniques. Here, we present transparent electrophysiology electrodes and interconnects made of gold (Au) nanomesh on flexible substrates to achieve such measurements. Compared to previously demonstrated indium tin oxide (ITO) and graphene electrodes, the ones from Au nanomesh possess superior properties including low electrical impedance, high transparency, good cell viability, and superb flexibility. Specifically, we demonstrated a 15 nm thick Au nanomesh electrode with 8.14 Ω•cm 2 normalized impedance, >65% average transmittance over a 300−1100 nm window, and stability up to 300 bending cycles. Systematic sheet resistance measurements, electrochemical impedance studies, optical characterization, mechanical bending tests, and cell studies highlight the capabilities of the Au nanomesh as a transparent electrophysiology electrode and interconnect material. Together, these results demonstrate applicability of using nanomesh under biological conditions and broad applications in biology and medicine.

Research paper thumbnail of Charge transfer in crystalline germanium/monolayer MoS2heterostructures prepared by chemical vapor deposition

Nanoscale, 2016

Heterostructuring provides novel opportunities for exploring emergent phenomena and applications ... more Heterostructuring provides novel opportunities for exploring emergent phenomena and applications by developing designed properties beyond those of homogeneous materials. Advances in nanoscience enable the preparation of heterostructures formed incommensurate materials. Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, are of particular interest due to their distinct physical characteristics. Recently, 2D/2D heterostructures have opened up new research areas. However, other heterostructures such as 2D/three-dimensional (3D) materials have not been thoroughly studied yet although the growth of 3D materials on 2D materials creating 2D/3D heterostructures with exceptional carrier transport properties has been reported. Here we report a novel heterostructure composed of Ge and monolayer MoS2, prepared by chemical vapor deposition. A single crystalline Ge (110) thin film was grown on monolayer MoS2. The electrical characteristics of Ge and MoS2 in the Ge/MoS2 heterostructure were remarkably different from those of isolated Ge and MoS2. The field-effect conductivity type of the monolayer MoS2 is converted from n-type to p-type by growth of the Ge thin film on top of it. Undoped Ge on MoS2 is highly conducting. The observations can be explained by charge transfer in the heterostructure as opposed to chemical doping via the incorporation of impurities, based on our first-principles calculations.

Research paper thumbnail of Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality

ACS nano, Jan 13, 2015

The ability to synthesize high-quality samples over large areas and at low cost is one of the big... more The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few-layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm(2)/(V s) in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Because of their high crystalline quality, Raman spectroscopy on these samples reveal a range...

Research paper thumbnail of Direct and Scalable Deposition of Atomically Thin Low-Noise MoS2 Membranes on Apertures

ACS nano, Jan 30, 2015

Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a pla... more Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a planar freestanding crystal. When the substrate edge is in the form of an aperture, reagent-limited nucleation followed by edge growth facilitate direct and selective growth of freestanding MoS2 membranes. We have found conditions under which MoS2 grows preferentially across micrometer-scale prefabricated solid-state apertures in silicon nitride membranes, resulting in sealed membranes that are one to a few atomic layers thick. We have investigated the structure and purity of our membranes by a combination of atomic-resolution transmission electron microscopy, elemental analysis, Raman spectroscopy, photoluminescence spectroscopy, and low-noise ion-current recordings through nanopores fabricated in such membranes. Finally, we demonstrate the utility of fabricated ultrathin nanopores in such membranes for single-stranded DNA translocation detection.