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Polarization-Aligned, Spectrally Consistent Quantum Emitters in As-Exfoliated Carbon-Doped Hexagonal Boron Nitride
Authors:
Sofiya Karankova,
Yeunjeong Lee,
Seungmin Park,
Kenji Watanabe,
Takashi Taniguchi,
Jin-Dong Song,
Young Duck Kim,
Yong-Won Song,
Hyowon Moon
Abstract:
Solid-state quantum emitters constitute an essential building blocks of integrated quantum photonic circuits. Among potential emitter platforms, hexagonal boron nitride (hBN) hosts single-photon emitters in an atomically thin lattice amenable to photonic integration. However, multi-step fabrication approaches, limited defect specificity, and poor emission wavelength repeatability limit the perform…
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Solid-state quantum emitters constitute an essential building blocks of integrated quantum photonic circuits. Among potential emitter platforms, hexagonal boron nitride (hBN) hosts single-photon emitters in an atomically thin lattice amenable to photonic integration. However, multi-step fabrication approaches, limited defect specificity, and poor emission wavelength repeatability limit the performance of hBN quantum light sources relative to established solid-state architectures. Developing methods to induce emitters that are both suitable for planar photonic devices and that exhibit consistent optical properties remains a key objective. In this work, we identify quantum emitters in as-exfoliated carbon-doped hBN that exhibit both stable and repeatable emission energies together with polarization-aligned dipoles. Owing to the high lattice crystallinity, these single-photon light sources demonstrate exceptional spectral stability with a standard deviation of 7 $μ$eV. The emission energy is reproducible and confined within a narrow range of 2.2825 ${\pm}$ 0.0042 eV. Notably, consistent dipole alignment for absorption and emission polarization suggests that the intrinsic defects are of the same nature. The color centers are observed in as-exfoliated hBN without any post-treatment, significantly facilitating further interfacing with planar photonic structures. These reproducible, polarization-aligned quantum emitters in as-exfoliated hBN provide a versatile platform for scalable integration, offering a pathway toward a broad range of quantum technologies.
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Submitted 18 March, 2026;
originally announced March 2026.
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Twist-Controlled Modulation of Quantum Emitters in a Van der Waals Bilayer
Authors:
Angus Gale,
Seungjun Lee,
Seungmin Park,
Evan Williams,
Helen Zhi Jie Zeng,
James Liddle-Wesolowski,
Young Duck Kim,
Milos Toth,
Tony Low,
Igor Aharonovich
Abstract:
Stacking and twisting two dimensional materials has garnered enormous attention across the condensed matter and the nanophotonic communities. The surge of interest stems from the emergence of novel photophysical phenomena that arise due to the interlayer coupling of the individual layers. Here, we demonstrate that the twist degree of freedom can modulate a single quantum emitter at room temperatur…
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Stacking and twisting two dimensional materials has garnered enormous attention across the condensed matter and the nanophotonic communities. The surge of interest stems from the emergence of novel photophysical phenomena that arise due to the interlayer coupling of the individual layers. Here, we demonstrate that the twist degree of freedom can modulate a single quantum emitter at room temperature. We employ a van der Waals homobilayer of hexagonal boron nitride (hBN) and model the emission properties of quantum emitters as a function of the twist angle. Density functional theory results show that the embedded emitters are strongly influenced by the twist angle and the stacking of the top hBN layer. We consequently engineer these systems experimentally, and demonstrate in-situ tuning of embedded quantum emitters by mechanically twisting the top hBN layer, achieving tunability of over 30 nm (~ 100 meV). Our work demonstrates that mechanical twisting can be harnessed to modulate the embedded quantum emitters in a vdW material, marking a crucial step towards a programmable on-chip quantum circuitry.
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Submitted 5 March, 2026;
originally announced March 2026.
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Self-patterning of Liquid Field's Metal for Enhanced Performance of Two-dimensional Semiconductor
Authors:
Kwanghee Han,
Heeyeon Lee,
Minseong Kwon,
Vinod Menon,
Chaun Jang,
Young Duck Kim
Abstract:
Two-dimensional (2D) van der Waals semiconductors show promise for atomically thin flexible and transparent optoelectronic devices in future technologies.However, developing high-performance field-effect transistors (FETs) based on 2D materials is impeded by two key challenges, the high contact resistance at the 2D semiconductors-metal interface and the limited effective doping strategies. Here, w…
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Two-dimensional (2D) van der Waals semiconductors show promise for atomically thin flexible and transparent optoelectronic devices in future technologies.However, developing high-performance field-effect transistors (FETs) based on 2D materials is impeded by two key challenges, the high contact resistance at the 2D semiconductors-metal interface and the limited effective doping strategies. Here, we present a novel approach to overcome these challenges using self-propagating liquid Fields metal, a eutectic alloy with a low melting point of approximately 62 C. By modifying pre-patterned electrodes on WSe2 FETs through the deposition of Fields metal onto contact pad edges followed by vacuum annealing, we create new semimetal electrodes that seamlessly incorporate the liquid metal into 2D semiconductors. This integration preserves the original electrode architecture while transforming to semimetal compositions of Fields metal such as Bi, In, and Sn modifies the work functions to 2D semiconductors, resulting in reduced contact resistance without inducing Fermi-level pinning and charge carrier mobilities. Our method enhances the electrical performance of 2D devices and opens new avenues for designing high-resolution liquid metal circuits suitable for stretchable, flexible, and wearable 2D semiconductor applications.
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Submitted 24 May, 2025;
originally announced May 2025.
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Flux pinning in superconducting multilayer 2H-NbSe$_2$ nano-step junction
Authors:
Minseong Kwon,
Mingi Kim,
Yoonji Gong,
Heeyeon Lee,
Young Duck Kim
Abstract:
Superconductors exhibit dissipationless supercurrents even under finite bias and magnetic field conditions, provided these remain below the critical values. However, type-II superconductors in the flux flow regime display Ohmic dissipation arising from vortex dynamics under finite magnetic fields. The interplay between supercurrent and Ohmic dissipation in a type-II superconductor is dictated by v…
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Superconductors exhibit dissipationless supercurrents even under finite bias and magnetic field conditions, provided these remain below the critical values. However, type-II superconductors in the flux flow regime display Ohmic dissipation arising from vortex dynamics under finite magnetic fields. The interplay between supercurrent and Ohmic dissipation in a type-II superconductor is dictated by vortex motion and the robustness of vortex pinning forces. In this study, we present an experimental investigation of the superconducting phase transitions and vortex dynamics in the atomically thin type-II superconductor 2H-NbSe$_2$. We fabricated a high-quality multilayer 2H-NbSe$_2$ with a step junction, demonstrating supercurrent in clean limit below a critical temperature of 6.6 K and a high residual resistance ratio of 17. The upper critical field was estimated to be 4.5 T and the Ginzburg-Landau coherence length 8.6 nm. Additionally, we observed phase transitions induced by vortex viscous dynamics in the 2H-NbSe$_2$ step junction. Analysis of the pinning force density using the Dew-Hughes model indicates that the pinning force in the 2H-NbSe$_2$ device can be attributed to step junction, related to the surface-$Δκ$ type of pinning centers. Our findings pave the way for engineering pinning forces by introducing artificial pinning centers through partial atomic thickness variation in layered 2D superconductors while minimizing unwanted quality degradation in the system.
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Submitted 7 January, 2025;
originally announced January 2025.
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Highly tunable moiré superlattice potentials in twisted hexagonal boron nitrides
Authors:
Kwanghee Han,
Minhyun Cho,
Taehyung Kim,
Seung Tae Kim,
Suk Hyun Kim,
Sang Hwa Park,
Sang Mo Yang,
Kenji Watanabe,
Takashi Taniguchi,
Vinod Menon,
Young Duck Kim
Abstract:
Moiré superlattice of twisted hexagonal boron nitride (hBN) has emerged as an advanced atomically thin van der Waals interfacial ferroelectricity platform. Nanoscale periodic ferroelectric moiré domains with out-of-plane potentials in twisted hBN allow the hosting of remote Coulomb superlattice potentials to adjacent two-dimensional materials for tailoring strongly correlated properties. Therefore…
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Moiré superlattice of twisted hexagonal boron nitride (hBN) has emerged as an advanced atomically thin van der Waals interfacial ferroelectricity platform. Nanoscale periodic ferroelectric moiré domains with out-of-plane potentials in twisted hBN allow the hosting of remote Coulomb superlattice potentials to adjacent two-dimensional materials for tailoring strongly correlated properties. Therefore, the new strategies for engineering moiré length, angle, and potential strength are essential for developing programmable quantum materials and advanced twistronics applications devices. Here, we demonstrate the realization of twisted hBN-based moiré superlattice platforms and visualize the moiré domains and ferroelectric properties using Kelvin probe force microscopy. Also, we report the KPFM result of regular moiré superlattice in the large area. It offers the possibility to reproduce uniform moiré structures with precise control piezo stage stacking and heat annealing. We demonstrate the high tunability of twisted hBN moiré platforms and achieve cumulative multi-ferroelectric polarization and multi-level domains with multiple angle mismatched interfaces. Additionally, we observe the quasi-1D anisotropic moiré domains and show the highest resolution analysis of the local built-in strain between adjacent hBN layers compared to the conventional methods. Furthermore, we demonstrate in-situ manipulation of moiré superlattice potential strength using femtosecond pulse laser irradiation, which results in the optical phonon-induced atomic displacement at the hBN moiré interfaces. Our results pave the way to develop precisely programmable moiré superlattice platforms and investigate strongly correlated physics in van der Waals heterostructures.
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Submitted 29 October, 2024;
originally announced October 2024.
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Color Centers in Hexagonal Boron Nitride
Authors:
Suk Hyun Kim,
Kyeong Ho Park,
Young Gie Lee,
Seong Jun Kang,
Yongsup Park,
Young Duck Kim
Abstract:
Atomically thin two-dimensional (2D) hexagonal boron nitride (hBN) has emerged as an essential material for the encapsulation layer in van der Waals heterostructures and efficient deep ultra-violet optoelectronics. This is primarily due to its remarkable physical properties and ultrawide bandgap (close to 6 eV, and even larger in some cases) properties. Color centers in hBN refer to intrinsic vaca…
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Atomically thin two-dimensional (2D) hexagonal boron nitride (hBN) has emerged as an essential material for the encapsulation layer in van der Waals heterostructures and efficient deep ultra-violet optoelectronics. This is primarily due to its remarkable physical properties and ultrawide bandgap (close to 6 eV, and even larger in some cases) properties. Color centers in hBN refer to intrinsic vacancies and extrinsic impurities within the 2D crystal lattice, which result in distinct optical properties in the ultraviolet (UV) to near-infrared (IR) range. Furthermore, each color center in hBN exhibits a unique emission spectrum and possesses various spin properties. These characteristics open up possibilities for the development of next-generation optoelectronics and quantum information applications, including room-temperature single-photon sources and quantum sensors. Here, we provide a comprehensive overview of the atomic configuration, optical and quantum properties, and different techniques employed for the formation of color centers in hBN. A deep understanding of color centers in hBN allows for advances in the development of next-generation UV optoelectronic applications, solid-state quantum technologies, and nanophotonics by harnessing the exceptional capabilities offered by hBN color centers.
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Submitted 12 September, 2024;
originally announced September 2024.
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Moiré exciton polaron engineering via twisted hBN
Authors:
Minhyun Cho,
Biswajit Datta,
Kwanghee Han,
Saroj B. Chand,
Pratap Chandra Adak,
Sichao Yu,
Fengping Li,
Kenji Watanabe,
Takashi Taniguchi,
James Hone,
Jeil Jung,
Gabriele Grosso,
Young Duck Kim,
Vinod M. Menon
Abstract:
Twisted hexagonal boron nitride (thBN) exhibits emergent ferroelectricity due to the formation of moiré superlattices with alternating AB and BA domains. These domains possess electric dipoles, leading to a periodic electrostatic potential that can be imprinted onto other 2D materials placed in its proximity. Here we demonstrate the remote imprinting of moiré patterns from twisted hexagonal boron…
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Twisted hexagonal boron nitride (thBN) exhibits emergent ferroelectricity due to the formation of moiré superlattices with alternating AB and BA domains. These domains possess electric dipoles, leading to a periodic electrostatic potential that can be imprinted onto other 2D materials placed in its proximity. Here we demonstrate the remote imprinting of moiré patterns from twisted hexagonal boron nitride (thBN) onto monolayer MoSe2 and investigate the resulting changes in the exciton properties. We confirm the imprinting of moiré patterns on monolayer MoSe2 via proximity using Kelvin probe force microscopy (KPFM) and hyperspectral photoluminescence (PL) mapping. By developing a technique to create large ferroelectric domain sizes ranging from 1 μm to 8.7 μm, we achieve unprecedented potential modulation of 387 +- 52 meV. We observe the formation of exciton polarons due to charge redistribution caused by the antiferroelectric moiré domains and investigate the optical property changes induced by the moiré pattern in monolayer MoSe2 by varying the moiré pattern size down to 110 nm. Our findings highlight the potential of twisted hBN as a platform for controlling the optical and electronic properties of 2D materials for optoelectronic and valleytronic applications.
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Submitted 11 September, 2024;
originally announced September 2024.
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Spin-orbit-splitting-driven nonlinear Hall effect in NbIrTe4
Authors:
Ji-Eun Lee,
Aifeng Wang,
Shuzhang Chen,
Minseong Kwon,
Jinwoong Hwang,
Minhyun Cho,
Ki-Hoon Son,
Dong-Soo Han,
Jun Woo Choi,
Young Duck Kim,
Sung-Kwan Mo,
Cedomir Petrovic,
Choongyu Hwang,
Se Young Park,
Chaun Jang,
Hyejin Ryu
Abstract:
The Berry curvature dipole (BCD) serves as a one of the fundamental contributors to emergence of the nonlinear Hall effect (NLHE). Despite intense interest due to its potential for new technologies reaching beyond the quantum efficiency limit, the interplay between BCD and NLHE has been barely understood yet in the absence of a systematic study on the electronic band structure. Here, we report NLH…
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The Berry curvature dipole (BCD) serves as a one of the fundamental contributors to emergence of the nonlinear Hall effect (NLHE). Despite intense interest due to its potential for new technologies reaching beyond the quantum efficiency limit, the interplay between BCD and NLHE has been barely understood yet in the absence of a systematic study on the electronic band structure. Here, we report NLHE realized in NbIrTe4 that persists above room temperature coupled with a sign change in the Hall conductivity at 150 K. First-principles calculations combined with angle-resolved photoemission spectroscopy (ARPES) measurements show that BCD tuned by the partial occupancy of spin-orbit split bands via temperature is responsible for the temperature-dependent NLHE. Our findings highlight the correlation between BCD and the electronic band structure, providing a viable route to create and engineer the non-trivial Hall effect by tuning the geometric properties of quasiparticles in transition-metal chalcogen compounds.
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Submitted 21 August, 2024;
originally announced August 2024.
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Pixel-wise classification in graphene-detection with tree-based machine learning algorithms
Authors:
Woon Hyung Cho,
Jiseon Shin,
Young Duck Kim,
George J. Jung
Abstract:
Mechanical exfoliation of graphene and its identification by optical inspection is one of the milestones in condensed matter physics that sparked the field of 2D materials. Finding regions of interest from the entire sample space and identification of layer number is a routine task potentially amenable to automatization. We propose supervised pixel-wise classification methods showing a high perfor…
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Mechanical exfoliation of graphene and its identification by optical inspection is one of the milestones in condensed matter physics that sparked the field of 2D materials. Finding regions of interest from the entire sample space and identification of layer number is a routine task potentially amenable to automatization. We propose supervised pixel-wise classification methods showing a high performance even with a small number of training image datasets that require short computational time without GPU. We introduce four different tree-based machine learning algorithms -- decision tree, random forest, extreme gradient boost, and light gradient boosting machine. We train them with five optical microscopy images of graphene, and evaluate their performances with multiple metrics and indices. We also discuss combinatorial machine learning models between the three single classifiers and assess their performances in identification and reliability. The code developed in this paper is open to the public and will be released at github.com/gjung-group/Graphene_segmentation.
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Submitted 24 August, 2022;
originally announced September 2022.
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Triaxially strained suspended graphene for large-area pseudo-magnetic fields
Authors:
M. Luo,
H. Sun,
Z. Qi,
K. Lu,
M. Chen,
D. Kang,
Y. Kim,
D. Burt,
X. Yu,
C. Wang,
Y. D. Kim,
H. Wang,
Q. -J. Wang,
D. Nam
Abstract:
Strain-engineered graphene has garnered much attention recently owing to the possibilities of creating substantial energy gaps enabled by pseudo-magnetic fields. While theoretical works proposed the possibility of creating large-area pseudo-magnetic fields by straining monolayer graphene along three crystallographic directions, clear experimental demonstration of such promising devices remains elu…
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Strain-engineered graphene has garnered much attention recently owing to the possibilities of creating substantial energy gaps enabled by pseudo-magnetic fields. While theoretical works proposed the possibility of creating large-area pseudo-magnetic fields by straining monolayer graphene along three crystallographic directions, clear experimental demonstration of such promising devices remains elusive. Herein, we experimentally demonstrate a triaxially strained suspended graphene structure that has the potential to possess large-scale and quasi-uniform pseudo-magnetic fields. Our structure employs uniquely designed metal electrodes that function both as stressors and metal contacts for current injection. Raman characterization and tight-binding simulations suggest the possibility of achieving pseudo-magnetic fields over a micrometer-scale area. Current-voltage measurements confirm an efficient current injection into graphene, showing the potential of our devices for a new class of optoelectronic applications. We also theoretically propose a photonic crystal-based laser structure that obtains strongly localized optical fields overlapping with the spatial area under uniform pseudo-magnetic fields, thus presenting a practical route towards the realization of graphene lasers.
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Submitted 9 February, 2022; v1 submitted 8 October, 2021;
originally announced October 2021.
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Anisotropic Behavior of Excitons in Single Crystal α-SnS
Authors:
Van Long Le,
Do Duc Cuong,
Hoang Tung Nguyen,
Xuan Au Nguyen,
Bogyu Kim,
Kyujin Kim,
Wonjun Lee,
Soon Cheol Hong,
Tae Jung Kim,
Young Dong Kim
Abstract:
We investigate analytically the anisotropic dielectric properties of single crystal α-SnS near the fundamental absorption edge by considering atomic orbitals. Most striking is the excitonic feature in the armchair- (b-) axis direction, which is particularly prominent at low temperatures. To determine the origin of this anisotropy, we perform first-principles calculations using the GW0 Bethe-Salpet…
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We investigate analytically the anisotropic dielectric properties of single crystal α-SnS near the fundamental absorption edge by considering atomic orbitals. Most striking is the excitonic feature in the armchair- (b-) axis direction, which is particularly prominent at low temperatures. To determine the origin of this anisotropy, we perform first-principles calculations using the GW0 Bethe-Salpeter equation (BSE) including the electron-hole interaction. The results show that the anisotropic dielectric characteristics are a direct result of the natural anisotropy of p orbitals. In particular, this dominant excitonic feature originates from the py orbital at the saddle point in the Γ-Y region.
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Submitted 7 July, 2020;
originally announced July 2020.
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High-performance monolayer MoS2 field-effect transistor with large-scale nitrogen-doped graphene electrodes for Ohmic contact
Authors:
Dongjea Seo,
Dong Yun Lee,
Junyoung Kwon,
Jea Jung Lee,
Takashi Taniguchi,
Kenji Watanabe,
Gwan-Hyoung Lee,
Keun Soo Kim,
James Hone,
Young Duck Kim,
Heon-Jin Choi
Abstract:
A finite Schottky barrier and large contact resistance between monolayer MoS2 and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS2 FETs. However, int…
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A finite Schottky barrier and large contact resistance between monolayer MoS2 and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS2 FETs. However, intrinsic misalignment between the work function of pristine graphene and the conduction band of MoS2 results in a large threshold voltage for the FETs, because of which Ohmic contact behaviors are observed only at very high gate voltages and carrier concentrations (~1013 cm-2). Here, we present high-performance monolayer MoS2 FETs with Ohmic contact at a modest gate voltage by using a chemical-vapor-deposited (CVD) nitrogen-doped graphene with a high intrinsic electron carrier density. The CVD nitrogen-doped graphene and monolayer MoS2 hybrid FETs platform exhibited a large negative shifted threshold voltage of -54.2 V and barrier-free Ohmic contact under zero gate voltage. Transparent contact by nitrogen-doped graphene led to a 214% enhancement in the on-current and a four-fold improvement in the field-effect carrier mobility of monolayer MoS2 FETs compared with those of a pristine graphene electrode platform. The transport measurements, as well as Raman and X-ray photoelectron spectroscopy analyses before and after thermal annealing, reveal that the atomic C-N bonding in the CVD nitrogen-doped graphene is responsible for the dominant effects of electron doping. Large-scale nitrogen-doped graphene electrodes provide a promising device platform for the development of high-performance devices and the study of unique quantum behaviors.
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Submitted 18 July, 2019;
originally announced July 2019.
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Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures
Authors:
Tae-Eon Park,
Licong Peng,
Jinghua Liang,
Ali Hallal,
Fehmi Sami Yasin,
Xichao Zhang,
Sung Jong Kim,
Kyung Mee Song,
Kwangsu Kim,
Markus Weigand,
Gisela Schuetz,
Simone Finizio,
Joerg Raabe,
Karin Garcia,
Jing Xia,
Yan Zhou,
Motohiko Ezawa,
Xiaoxi Liu,
Joonyeon Chang,
Hyun Cheol Koo,
Young Duck Kim,
Mairbek Chshiev,
Albert Fert,
Hongxin Yang,
Xiuzhen Yu
, et al. (1 additional authors not shown)
Abstract:
Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing properties and integration with other 2D family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy (PMA). Such a strong SOC could also lift the chiral degeneracy…
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Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing properties and integration with other 2D family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy (PMA). Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here, we report the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe3GeTe2 (FGT). We demonstrate the ability to drive individual skyrmion by short current pulses along a vdW heterostructure, FGT/h-BN, as highly required for any skyrmion-based spintronic device. Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states. Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application.
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Submitted 25 June, 2020; v1 submitted 2 July, 2019;
originally announced July 2019.
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Ultrafast Graphene Light Emitter
Authors:
Young Duck Kim,
Yuanda Gao,
Ren-Jye Shiue,
Lei Wang,
Ozgur Burak Aslan,
Myung-Ho Bae,
Hyungsik Kim,
Dongjea Seo,
Heon-Jin Choi,
Suk Hyun Kim,
Andrei Nemilentsau,
Tony Low,
Cheng Tan,
Dmitri K. Efetov,
Takashi Taniguchi,
Kenji Watanabe,
Kenneth L. Shepard,
Tony F. Heinz,
Dirk Englund,
James Hone
Abstract:
Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achie…
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Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth, across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge carrier dynamics in graphene, and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460 % enhancement compared to the grey-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2,000 K under ambient conditions, as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.
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Submitted 24 October, 2017;
originally announced October 2017.
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Bright visible light emission from graphene
Authors:
Young Duck Kim,
Hakseong Kim,
Yujin Cho,
Ji Hoon Ryoo,
Cheol-Hwan Park,
Pilkwang Kim,
Yong Seung Kim,
Sunwoo Lee,
Yilei Li,
Seung-Nam Park,
Yong Shim Yoo,
Duhee Yoon,
Vincent E. Dorgan,
Eric Pop,
Tony F. Heinz,
James Hone,
Seung-Hyun Chun,
Hyeonsik Cheong,
Sang Wook Lee,
Myung-Ho Bae,
Yun Daniel Park
Abstract:
Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible, and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators, and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficien…
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Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible, and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators, and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here we report the observation of bright visible-light emission from electrically biased suspended graphenes. In these devices, heat transport is greatly minimised; thus hot electrons (~ 2800 K) become spatially localised at the centre of graphene layer, resulting in a 1000-fold enhancement in the thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be utilized to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene bright visible-light emitters. These results pave the way towards the realisation of commercially viable large-scale, atomically-thin, flexible and transparent light emitters and displays with low-operation voltage, and graphene-based, on-chip ultrafast optical communications.
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Submitted 13 September, 2017;
originally announced September 2017.
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Surface buckling of phosphorene materials: determination, origin and influence on electronic structure
Authors:
Zhongwei Dai,
Wencan Jin,
Jie-Xiang Yu,
Maxwell Grady,
Jerzy T. Sadowski,
Young Duck Kim,
James Hone,
Jiadong Zang,
Richard M. Osgood, Jr.,
Karsten Pohl
Abstract:
The surface structure of phosphorene crystals materials is determined using surface sensitive dynamical micro-spot low energy electron diffraction (μLEED) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (\textit{i}) crystalline cleaved black phosphorus (BP) at 300 K and (\textit{ii}) exfoliated few-layer phosphorene (FLP) of about 10 nm thicknes, w…
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The surface structure of phosphorene crystals materials is determined using surface sensitive dynamical micro-spot low energy electron diffraction (μLEED) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (\textit{i}) crystalline cleaved black phosphorus (BP) at 300 K and (\textit{ii}) exfoliated few-layer phosphorene (FLP) of about 10 nm thicknes, which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 Å and 0.30 Å, respectively, is measured, which is one order of magnitude larger than previously reported. Using first principle calculations, the presence of surface vacancies is attributed not only to the surface buckling in BP and FLP, but also the previously reported intrinsic hole doping of phosphorene materials.
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Submitted 22 April, 2017;
originally announced April 2017.
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Interfacial Charge Transfer Circumventing Momentum Mismatch at 2D van der Waals Heterojunctions
Authors:
Haiming Zhu,
Jue Wang,
Zizhou Gong,
Young Duck Kim,
Martin Gustafsson,
James Hone,
Xiaoyang Zhu
Abstract:
Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K-valleys in each layer. Since these high-momentum valleys are usually not aligned across th…
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Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K-valleys in each layer. Since these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technical of femtosecond pump-probe spectroscopy. Upon photo-excitation specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe2 to MoS2, independent of the angular alignment and, thus, momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiatively recombination, with rates varying by up to three-orders of magnitude from sample to sample, but with no correlation with inter-layer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively.
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Submitted 21 February, 2017;
originally announced February 2017.
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High Electric Field Carrier Transport and Power Dissipation in Multilayer Black Phosphorus Field Effect Transistor with Dielectric Engineering
Authors:
Faisal Ahmed,
Young Duck Kim,
Min Sup Choi,
Xiaochi Liu,
Deshun Qu,
Zheng Yang,
Jiayang Hu,
Irving P. Herman,
James Hone,
Won Jong Yoo
Abstract:
This study addresses high electric field transport in multilayer black phosphorus (BP) field effect transistors (FETs) with self-heating and thermal spreading by dielectric engineering. Interestingly, we found that multilayer BP device on a SiO2 substrate exhibited a maximum current density of 3.3 x 10E10 A/m2 at an electric field of 5.58 MV/m, several times higher than multilayer MoS2. Our breakd…
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This study addresses high electric field transport in multilayer black phosphorus (BP) field effect transistors (FETs) with self-heating and thermal spreading by dielectric engineering. Interestingly, we found that multilayer BP device on a SiO2 substrate exhibited a maximum current density of 3.3 x 10E10 A/m2 at an electric field of 5.58 MV/m, several times higher than multilayer MoS2. Our breakdown thermometry analysis revealed that self-heating was impeded along BP-dielectric interface, resulting in a thermal plateau inside the channel and eventual Joule breakdown. Using a size-dependent electro-thermal transport model, we extracted an interfacial thermal conductance of 1-10 MW/m2 K for the BP-dielectric interfaces. By using hBN as a dielectric material for BP instead of thermally resistive SiO2 (about 1.4 W/m K), we observed a 3 fold increase in breakdown power density and a relatively higher electric field endurance together with efficient and homogenous thermal spreading because hBN had superior structural and thermal compatibility with BP. We further confirmed our results based on micro-Raman spectroscopy and atomic force microscopy, and observed that BP devices on hBN exhibited centrally localized hotspots with a breakdown temperature of 600K, while the BP device on SiO2 exhibited a hotspot in the vicinity of the electrode at 520K.
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Submitted 31 October, 2016;
originally announced October 2016.
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Influence of the Substrate Material on the Optical Properties of Tungsten Diselenide Monolayers
Authors:
Sina Lippert,
Lorenz Maximilian Schneider,
Dylan Renaud,
Kyung Nam Kang,
Obafunso Ajayi,
Marc-Uwe Halbich,
Oday M. Abdulmunem,
Xing Lin,
Jan Kuhnert,
Khaleel Hassoon,
Saeideh Edalati-Boostan,
Young Duck Kim,
Wolfram Heimbrodt,
Eui-Hyeok Yang,
James Hone,
Arash Rahimi-Iman
Abstract:
Monolayers of transition-metal dichalcogenides such as WSe2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe2. To accomplish this study, pump-density-dependent…
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Monolayers of transition-metal dichalcogenides such as WSe2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe2. To accomplish this study, pump-density-dependent micro-photoluminescence measurements are performed with time-integrating and time-resolving acquisition techniques. Spectral information and power-dependent mode intensities are compared at 290K and 10K for exfoliated WSe2 on SiO2/Si, sapphire (Al2O3), hBN/Si3N4/Si, and MgF2, indicating substrate-dependent appearance and strength of exciton, trion, and biexciton modes. Additionally, one CVD-grown WSe2 monolayer on sapphire is included in this study for direct comparison with its exfoliated counterpart. Time-resolved micro-photoluminescence shows how radiative decay times strongly differ for different substrate materials. Our data indicates exciton-exciton annihilation as a shortening mechanism at room temperature, and subtle trends in the decay rates in correlation to the dielectric environment at cryogenic temperatures. On the measureable time scales, trends are also related to the extent of the respective 2D-excitonic modes' appearance. This result highlights the importance of further detailed characterization of exciton features in 2D materials, particularly with respect to the choice of substrate.
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Submitted 30 September, 2016;
originally announced October 2016.
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Optical Characterization of PtSi/Si by Spectroscopic Ellipsometry
Authors:
Van Long Le,
Tae Jung Kim,
Han Gyeol Park,
Hwa Seob Kim,
Chang Hyun Yoo,
Hyoung Uk Kim,
Young Dong Kim,
Junsoo Kim,
Solyee Im,
Won Chul Choi,
Seung Eon Moon,
Eun Soo Nam and
Abstract:
We report optical characterization of PtSi films for thermoelectric device applications by nondestructive spectroscopic ellipsometry (SE). Pt monolayer and Pt-Si multilayer which consists of 3 pairs of Pt and Si layers were deposited on p-doped-silicon substrates by sputtering method and then rapid annealing process was done to form PtSi films through intermixing of Pt and Si atoms at the interfac…
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We report optical characterization of PtSi films for thermoelectric device applications by nondestructive spectroscopic ellipsometry (SE). Pt monolayer and Pt-Si multilayer which consists of 3 pairs of Pt and Si layers were deposited on p-doped-silicon substrates by sputtering method and then rapid annealing process was done to form PtSi films through intermixing of Pt and Si atoms at the interface. Pseudodielectric function data <ε> = <ε1> + i<ε2> of the PtSi/Si samples were obtained from 1.12 to 6.52 eV by using spectroscopic ellipsometry. Employing Tauc-Lorentz and Drude models, the dielectric function (ε) of PtSi films were determined. We found that the composition ratio of Pt:Si is nearly 1:1 for PtSi monolayer and we observed transitions between occupied and unoccupied states in Pt 5d states. We also observed formation of PtSi layers in Pt-Si multilayer sample. The SE results were confirmed by the transmission electron microscopy and energy dispersive X-ray spectroscopy.
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Submitted 12 June, 2016;
originally announced June 2016.
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Nature of the Quantum Metal in a Two-Dimensional Crystalline Superconductor
Authors:
A. W. Tsen,
B. Hunt,
Y. D. Kim,
Z. J. Yuan,
S. Jia,
R. J. Cava,
J. Hone,
P. Kim,
C. R. Dean,
A. N. Pasupathy
Abstract:
Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate metallic state o…
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Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate metallic state occupying a substantial region of the phase diagram whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe$_2$ in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the intermediate metallic state. We find a new power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field.
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Submitted 5 October, 2015; v1 submitted 30 July, 2015;
originally announced July 2015.
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Structure and Control of Charge Density Waves in Two-Dimensional 1T-TaS2
Authors:
A. W. Tsen,
R. Hovden,
D. Z. Wang,
Y. D. Kim,
J. Okamoto,
K. A. Spoth,
Y. Liu,
W. J. Lu,
Y. P. Sun,
J. Hone,
L. F. Kourkoutis,
P. Kim,
A. N. Pasupathy
Abstract:
The layered transition metal dichalcogenides host a rich collection of charge density wave (CDW) phases in which both the conduction electrons and the atomic structure display translational symmetry breaking. Manipulating these complex states by purely electronic methods has been a long-sought scientific and technological goal. Here, we show how this can be achieved in 1T-TaS2 in the two-dimension…
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The layered transition metal dichalcogenides host a rich collection of charge density wave (CDW) phases in which both the conduction electrons and the atomic structure display translational symmetry breaking. Manipulating these complex states by purely electronic methods has been a long-sought scientific and technological goal. Here, we show how this can be achieved in 1T-TaS2 in the two-dimensional (2D) limit. We first demonstrate that the intrinsic properties of atomically-thin flakes are preserved by encapsulation with hexagonal boron nitride in inert atmosphere. We use this facile assembly method together with TEM and transport measurements to probe the nature of the 2D state and show that its conductance is dominated by discommensurations. The discommensuration structure can be precisely tuned in few-layer samples by an in-plane electric current, allowing continuous electrical control over the discommensuration-melting transition in 2D.
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Submitted 14 May, 2015;
originally announced May 2015.
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Multi-terminal electrical transport measurements of molybdenum disulphide using van der Waals heterostructure device platform
Authors:
Xu Cui,
Gwan-Hyoung Lee,
Young Duck Kim,
Ghidewon Arefe,
Pinshane Y. Huang,
Chul-Ho Lee,
Daniel A. Chenet,
Xian Zhang,
Lei Wang,
Fan Ye,
Filippo Pizzocchero,
Bjarke S. Jessen,
Kenji Watanabe,
Takashi Taniguchi,
David A. Muller,
Tony Low,
Philip Kim,
James Hone
Abstract:
Atomically thin two-dimensional (2D) semiconductors such as molybdenum disulphide (MoS2) hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena such as coupled spin-valley physics and the valley Hall effect. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, particularly at l…
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Atomically thin two-dimensional (2D) semiconductors such as molybdenum disulphide (MoS2) hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena such as coupled spin-valley physics and the valley Hall effect. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, particularly at low temperature (T), which has hampered efforts to observe its intrinsic quantum transport behaviors. Potential sources of disorder and scattering include both defects such as sulfur vacancies in the MoS2 itself, and extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering and approach the intrinsic limit, we developed a van der Waals (vdW) heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride (hBN), and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Multi-terminal magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm2/Vs for 6-layer MoS2 at low T. Comparison to theory shows a decrease of 1-2 orders of magnitude in the density of charged impurities, indicating that performance at low T in previous studies was limited by extrinsic factors rather than defects in the MoS2. We also observed Shubnikov-de Haas (SdH) oscillations for the first time in high-mobility monolayer and few-layer MoS2. This novel device platform therefore opens up a new way toward measurements of intrinsic properties and the study of quantum transport phenomena in 2D semiconducting materials.
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Submitted 22 December, 2014; v1 submitted 18 December, 2014;
originally announced December 2014.
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Valley Splitting and Polarization by the Zeeman Effect in Monolayer MoSe2
Authors:
Yilei Li,
Jonathan Ludwig,
Tony Low,
Alexey Chernikov,
Xu Cui,
Ghidewon Arefe,
Young Duck Kim,
Arend M. van der Zande,
Albert Rigosi,
Heather M. Hill,
Suk Hyun Kim,
James Hone,
Zhiqiang Li,
Dmitry Smirnov,
Tony F. Heinz
Abstract:
We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of $\mp$ 0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley de…
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We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of $\mp$ 0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to $\mp$ 0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.
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Submitted 30 September, 2014;
originally announced September 2014.
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Methane as an effective hydrogen source for single-layer graphene synthesis on Cu foil by plasma enhanced chemical vapor deposition
Authors:
Yong Seung Kim,
Jae Hong Lee,
Young Duck Kim,
Sahng-Kyoon Jerng,
Kisu Joo,
Eunho Kim,
Jongwan Jung,
Euijoon Yoon,
Yun Daniel Park,
Sunae Seo,
Seung-Hyun Chun
Abstract:
A single-layer graphene is synthesized on Cu foil in the absence of H2 flow by plasma enhanced chemical vapor deposition (PECVD). In lieu of an explicit H2 flow, hydrogen species are produced during methane decomposition process into their active species (CHx<4), assisted by the plasma. Notably, the early stage of growth depends strongly on the plasma power. The resulting grain size (the nucleatio…
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A single-layer graphene is synthesized on Cu foil in the absence of H2 flow by plasma enhanced chemical vapor deposition (PECVD). In lieu of an explicit H2 flow, hydrogen species are produced during methane decomposition process into their active species (CHx<4), assisted by the plasma. Notably, the early stage of growth depends strongly on the plasma power. The resulting grain size (the nucleation density) has a maximum (minimum) at 50 W and saturates when the plasma power is higher than 120 W because hydrogen partial pressures are effectively tuned by a simple control of the plasma power. Raman spectroscopy and transport measurements show that decomposed methane alone can provide sufficient amount of hydrogen species for high-quality graphene synthesis by PECVD.
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Submitted 26 June, 2013; v1 submitted 5 March, 2012;
originally announced March 2012.
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Interference effect on Raman spectrum of graphene on SiO_2/Si
Authors:
Duhee Yoon,
Hyerim Moon,
Young-Woo Son,
Jin Sik Choi,
Bae Ho Park,
Young Hun Cha,
Young Dong Kim,
Hyeonsik Cheong
Abstract:
The intensity ratio between two major Raman bands in graphene is one of the most important information for physics of graphene and has been believed to represent various intrinsic properties of graphene without critical assessment of extrinsic effects. We report a micro Raman spectroscopy study on the Raman intensity ratio of the 2D band to the G Raman band of graphene varying the thickness of d…
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The intensity ratio between two major Raman bands in graphene is one of the most important information for physics of graphene and has been believed to represent various intrinsic properties of graphene without critical assessment of extrinsic effects. We report a micro Raman spectroscopy study on the Raman intensity ratio of the 2D band to the G Raman band of graphene varying the thickness of dielectric layers (SiO_2) underneath it. The ratio is shown to change by almost 370% when the thickness is varied by 60%. The large variation in the ratio is well explained by theoretical calculations considering multiple Raman scattering events at the interfaces. Our analysis shows that the interference effect is critical in extracting the intrinsic 2D to G intensity ratio and therefore must be taken into account in extracting various physical properties of graphene from Raman measurements.
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Submitted 29 August, 2009;
originally announced August 2009.
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X-ray absorption spectroscopy study of diluted magnetic semiconductors: Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y = Se, Te)
Authors:
Kwanghyun Cho,
Hoon Koh,
S. -J. Oh,
Hyeong-Do Kim,
Moonsup Han,
J. -H. Park,
C. T. Chen,
Y. D. Kim,
J. -S. Kim,
B. T. Jonker
Abstract:
We have investigated 3d electronic states of doped transition metals in II-VI diluted magnetic semiconductors, Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y = Se, Te), using the transition-metal L2,3-edge X-ray absorption spectroscopy (XAS) measurements. In order to explain the XAS spectra, we employed a tetragonal cluster model calculation, which includes not only the full ionic multiplet structu…
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We have investigated 3d electronic states of doped transition metals in II-VI diluted magnetic semiconductors, Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y = Se, Te), using the transition-metal L2,3-edge X-ray absorption spectroscopy (XAS) measurements. In order to explain the XAS spectra, we employed a tetragonal cluster model calculation, which includes not only the full ionic multiplet structure but also configuration interaction (CI). The results show that CI is essential to describe the experimental spectra adequately, indicating the strong hybridization between the transition metal 3d and the ligand p orbitals. In the study of Zn1-xMnxY (Y = Se, Te), we also found considerable spectral change in the Mn L2,3-edge XAS spectra for different ligands, confirming the importance of the hybridization effects in these materials.
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Submitted 18 September, 2000;
originally announced September 2000.