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Kuei-Hsien Chen

Other affiliations: Massachusetts Institute of Technology, KAIST, CERN  ...read more
Bio: Kuei-Hsien Chen is an academic researcher from Academia Sinica. The author has contributed to research in topics: Carbon nanotube & Thin film. The author has an hindex of 75, co-authored 652 publications receiving 24809 citations. Previous affiliations of Kuei-Hsien Chen include Massachusetts Institute of Technology & KAIST.


Papers
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TL;DR: A simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region is reported.
Abstract: Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military.

1,012 citations

Journal ArticleDOI
Khachatryan, Albert M. Sirunyan, Armen Tumasyan, Wolfgang Adam  +2118 moreInstitutions (3)
TL;DR: In this article, the performance and strategies used in electron reconstruction and selection at CERN LHC are presented based on data corresponding to an integrated luminosity of 19.7 inverse femtobarns, collected in proton-proton collisions at sqrt(s) = 8 TeV.
Abstract: The performance and strategies used in electron reconstruction and selection at CMS are presented based on data corresponding to an integrated luminosity of 19.7 inverse femtobarns, collected in proton-proton collisions at sqrt(s) = 8 TeV at the CERN LHC. The paper focuses on prompt isolated electrons with transverse momenta ranging from about 5 to a few 100 GeV. A detailed description is given of the algorithms used to cluster energy in the electromagnetic calorimeter and to reconstruct electron trajectories in the tracker. The electron momentum is estimated by combining the energy measurement in the calorimeter with the momentum measurement in the tracker. Benchmark selection criteria are presented, and their performances assessed using Z, Upsilon, and J/psi decays into electron-positron pairs. The spectra of the observables relevant to electron reconstruction and selection as well as their global efficiencies are well reproduced by Monte Carlo simulations. The momentum scale is calibrated with an uncertainty smaller than 0.3%. The momentum resolution for electrons produced in Z boson decays ranges from 1.7 to 4.5%, depending on electron pseudorapidity and energy loss through bremsstrahlung in the detector material.

633 citations

Journal ArticleDOI
TL;DR: The systematic evolution of the electronic structure and comprehensive analysis of steady-state and transient PL along with photoluminescence excitation (PLE) spectroscopy measurements indicate that two different types of electronically excited states are responsible for the observed emission characteristics.
Abstract: Graphene oxide (GO) is a graphene sheet modified with oxygen functional groups in the form of epoxy and hydroxy groups on the basal plane and various other types at the edges. It exhibits interesting steady-state photoluminescence (PL) properties. For example, low-energy fluorescence in red to near infrared (NIR) wavelengths (from 600– 1100 nm) has been detected for suspensions and solid thin films of as-synthesized GO. 3] In addition, broad luminescence from 400 to 800 nm from oxygen plasma-treated, mechanically exfoliated, single-layer graphene sheet has been reported. Blue fluorescence with a relatively narrow bandwidth when excited with UV irradiation has also been detected from chemically reduced GO (rGO) and graphene quantum dots. 6] Recently, chemically modified GO or rGO with n-butylamine or Mn has also demonstrated PL emission at a range of energies. 10] A detailed explanation of the origin of such variable energy PL in GO has yet to be elucidated. This is partly because the sample preparation and reduction methods varied, making it difficult to compare the results. Herein, we have prepared GO suspensions that exhibit virtually all of the PL features observed by different groups, through careful and gradual reduction of the GO. The systematic evolution of the electronic structure and comprehensive analysis of steady-state and transient PL along with photoluminescence excitation (PLE) spectroscopy measurements indicate that two different types of electronically excited states are responsible for the observed emission characteristics. GO was synthesized using the modified Hummers method, the details of which have been reported. GO usually contains a large fraction of sp hybridized carbon atoms bound to oxygen functional groups, which makes it an insulator. Reduction can be achieved chemically (e.g. hydrazine exposure) or by thermal annealing in inert environments. Photothermal reduction of GO can be achieved by exposing GO samples to a Xenon flash in ambient conditions. In this study, we prepared aqueous GO solutions and subjected them to steady-state Xe lamp irradiation (500 W) with different exposure times of up to three hours. In contrast to reduction by an instantaneous flash, this method provides a controllable, gradual transformation from GO to rGO, allowing exploration of the PL evolution and emission mechanisms from as-synthesized GO to rGO. The deoxygenation of GO after reduction was confirmed by X-ray photoelectron spectroscopy (XPS), as shown in Figure 1. The C 1s signals of the original GO can be deconvoluted into signals for the C=C bond in aromatic rings (284.6 eV), C O bond (286.1 eV), C=O bond (287.5 eV), and C(=O) OH bond (289.2 eV), in agreement with previous assignments. Increased sp carbon bonding with increased reduction time can be clearly measured, which

589 citations

Journal ArticleDOI
TL;DR: In this article, a review of state-of-the-art anti-reflecting (AR) techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field are presented.
Abstract: Optical reflection, or in other words the loss of reflection, from a surface becomes increasingly crucial in determining the extent of the light-matter interaction. The simplest example of using an anti-reflecting (AR) surface is possibly the solar cell that incorporates an AR coating to harvest sunlight more effectively. Researchers have now found ways to mimic biological structures, such as moth eyes or cicada wings, which have been used for the AR purpose by nature herself. These nanoscopic biomimetic structures lend valuable clues in fabricating and designing gradient refractive index materials that are efficient AR structures. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. In addition to the challenge of broader bands, the performance of AR structures is also limited by factors such as omnidirectional properties and polarization of incident light. This review presents selected state-of-the-art AR techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field.

565 citations

Journal ArticleDOI
S. Chatrchyan, Khachatryan1, Albert M. Sirunyan, Armen Tumasyan  +2384 moreInstitutions (207)
26 May 2014
TL;DR: In this paper, a description of the software algorithms developed for the CMS tracker both for reconstructing charged-particle trajectories in proton-proton interactions and for using the resulting tracks to estimate the positions of the LHC luminous region and individual primary-interaction vertices is provided.
Abstract: A description is provided of the software algorithms developed for the CMS tracker both for reconstructing charged-particle trajectories in proton-proton interactions and for using the resulting tracks to estimate the positions of the LHC luminous region and individual primary-interaction vertices. Despite the very hostile environment at the LHC, the performance obtained with these algorithms is found to be excellent. For tt events under typical 2011 pileup conditions, the average track-reconstruction efficiency for promptly-produced charged particles with transverse momenta of p_T > 0.9GeV is 94% for pseudorapidities of |η| < 0.9 and 85% for 0.9 < |η| < 2.5. The inefficiency is caused mainly by hadrons that undergo nuclear interactions in the tracker material. For isolated muons, the corresponding efficiencies are essentially 100%. For isolated muons of p_T = 100GeV emitted at |η| < 1.4, the resolutions are approximately 2.8% in p_T, and respectively, 10μm and 30μm in the transverse and longitudinal impact parameters. The position resolution achieved for reconstructed primary vertices that correspond to interesting pp collisions is 10–12μm in each of the three spatial dimensions. The tracking and vertexing software is fast and flexible, and easily adaptable to other functions, such as fast tracking for the trigger, or dedicated tracking for electrons that takes into account bremsstrahlung.

559 citations


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TL;DR: The thermal properties of carbon materials are reviewed, focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder, with special attention given to the unusual size dependence of heat conduction in two-dimensional crystals.
Abstract: Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.

5,189 citations

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TL;DR: It is anticipated that this review can stimulate a new research doorway to facilitate the next generation of g-C3N4-based photocatalysts with ameliorated performances by harnessing the outstanding structural, electronic, and optical properties for the development of a sustainable future without environmental detriment.
Abstract: As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

5,054 citations

Journal ArticleDOI
TL;DR: In this paper, the authors review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder.
Abstract: Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range – of over five orders of magnitude – from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.

3,609 citations

Journal ArticleDOI
TL;DR: This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core-shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts and metal-free catalysts.
Abstract: The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

2,964 citations

Journal ArticleDOI
TL;DR: The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature, and challenges in producing high-performing electrolytes are analyzed.
Abstract: Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

2,480 citations