Author
Sunmin Ryu
Other affiliations: Columbia University, Kyung Hee University, Korea Research Institute of Standards and Science ...read more
Bio: Sunmin Ryu is an academic researcher from Pohang University of Science and Technology. The author has contributed to research in topics: Graphene & KEKB. The author has an hindex of 54, co-authored 241 publications receiving 19267 citations. Previous affiliations of Sunmin Ryu include Columbia University & Kyung Hee University.
Papers published on a yearly basis
Papers
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TL;DR: This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime by characterized by Raman spectroscopy.
Abstract: Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.
3,969 citations
TL;DR: The fabrication of top-gate phototransistors based on a few-layered MoS(2) nanosheet with a transparent gate electrode exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection.
Abstract: We report on the fabrication of top-gate phototransistors based on a few-layered MoS2 nanosheet with a transparent gate electrode. Our devices with triple MoS2 layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the number of MoS2 layers. The photoelectric probing on working transistors with the nanosheets demonstrates that single-layer MoS2 has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS2 reduce to 1.65 and 1.35 eV, respectively.
1,247 citations
TL;DR: In this paper, a variation of the work function for single and bilayer graphene devices measured by scanning Kelvin probe microscopy (SKPM) is reported, by using the electric field effect, which can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point.
Abstract: We report variation of the work function for single and bilayer graphene devices measured by scanning Kelvin probe microscopy (SKPM). By use of the electric field effect, the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Upon biasing the device, the surface potential map obtained by SKPM provides a reliable way to measure the contact resistance of individual electrodes contacting graphene.
1,205 citations
TL;DR: Variation of the work function for single and bilayer graphene devices measured by scanning Kelvin probe microscopy (SKPM) is reported, by use of the electric field effect.
Abstract: We report variation of the work function for single and bi-layer graphene devices measured by scanning Kelvin probe microscopy (SKPM). Using the electric field effect, the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Upon biasing the device, the surface potential map obtained by SKPM provides a reliable way to measure the contact resistance of individual electrodes contacting graphene.
1,130 citations
TL;DR: It is found that O 2 etching kinetics vary strongly with the number of graphene layers in the sample, and three-layer-thick samples show etching similar to bulk natural graphite.
Abstract: Patterned graphene shows substantial potential for applications in future molecular-scale integrated electronics. Environmental effects are a critical issue in a single-layer material where every atom is on the surface. Especially intriguing is the variety of rich chemical interactions shown by molecular oxygen with aromatic molecules. We find that O2 etching kinetics vary strongly with the number of graphene layers in the sample. Three-layer-thick samples show etching similar to bulk natural graphite. Single-layer graphene reacts faster and shows random etch pits in contrast to natural graphite where nucleation occurs at point defects. In addition, basal plane oxygen species strongly hole dope graphene, with a Fermi level shift of ∼0.5 eV. These oxygen species desorb partially in an Ar gas flow, or under irradiation by far UV light, and readsorb again in an O2 atmosphere at room temperature. This strongly doped graphene is very different from “graphene oxide” made by mineral acid attack.
884 citations
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TL;DR: In this paper, the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations, are discussed.
Abstract: This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.
20,824 citations
28 Jul 2005
TL;DR: PfPMP1）与感染红细胞、树突状组胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作�ly.
Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。
18,940 citations
TL;DR: Graphene is established as the strongest material ever measured, and atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.
Abstract: We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m(-1)) and -690 Nm(-1), respectively. The breaking strength is 42 N m(-1) and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of sigma(int) = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.
18,008 citations
TL;DR: This work reviews the historical development of Transition metal dichalcogenides, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
Abstract: Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.
13,348 citations
TL;DR: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy and the effect of quantum confinement on the material's electronic structure is traced.
Abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material
12,822 citations