Browsing by Author "Huang, Guan-Min"

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Atomic-Scale Fabrication of In-Plane Heterojunctions of Few-Layer MoS2 via In Situ Scanning Transmission Electron Microscopy
(1970-01-01)
Tai, Kuo-Lun
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Huang, Chun-Wei
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Cai, Ren-Fong
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Huang, Guan-Min
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Tseng, Yi-Tang
Layered MoS2 is a prospective candidate for use in energy harvesting, valleytronics, and nanoelectronics. Its properties strongly related to its stacking configuration and the number of layers. Due to its atomically thin nature, understanding the atomic-level and structural modifications of 2D transition metal dichalcogenides is still underdeveloped, particularly the spatial control and selective precision. Therefore, the development of nanofabrication techniques is essential. Here, an atomic-scale approach used to sculpt 2D few-layer MoS2 into lateral heterojunctions via in situ scanning/transmission electron microscopy (STEM/TEM) is developed. The dynamic evolution is tracked using ultrafast and high-resolution filming equipment. The assembly behaviors inherent to few-layer 2D-materials are observed during the process and included the following: scrolling, folding, etching, and restructuring. Atomic resolution STEM is employed to identify the layer variation and stacking sequence for this new 2D-architecture. Subsequent energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy analyses are performed to corroborate the elemental distribution. This sculpting technique that is established allows for the formation of sub-10 nm features, produces diverse nanostructures, and preserves the crystallinity of the material. The lateral heterointerfaces created in this study also pave the way for the design of quantum-relevant geometries, flexible optoelectronics, and energy storage devices.
Dynamic observation of reversible lithium storage phenomena in hybrid supercapacitor devices
(2017-11-01)
Huang, Guan-Min
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Tsai, Tsung-Chun
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Huang, Chun-Wei
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Kumar, Nagesh
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Tseng, Tseung-Yuen
Carbonaceous composites have attracted much attention as electrode materials for hybrid supercapacitors. Additionally, transition metal oxides, such as Co3O4, have high specific capacitance in the charging/discharging process. Here, we investigated the lithium storage mechanism of Co3O4/CNTs material via in situ transmission electron microscopy (TEM). Additionally, we analyzed the structure and composition of the anode material by high-resolution TEM, electron diffraction, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Using our unique in situ experimental setup that employs colloidal electrolyte, we elucidate two different mechanisms during operation, including the electrochemical reaction (battery-type) and ions intercalation (supercapacitor-type) of the electrode material. The cube-like Co3O4 nanoparticles were converted to Co nanograins dispersed in the Li2O matrix after the first charging cycle. Subsequent cycles presented a reversible reaction between Co/Li2O and CoO/Li2O. Furthermore, the porous structure of the CNTs and conservation of the Li2O matrix allow for the excellent ability to accommodate tremendous volume expansion, which enhances the life of hybrid supercapacitors. Our observations not only provide direct evidence of the electrochemical behavior but also improve the structure to promote enhanced performance for the application of hybrid supercapacitors.
Electron Beam Irradiation-Induced Deoxidation and Atomic Flattening on the Copper Surface
(2019-10-30)
Lin, Ting-Yi
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Lee, Shih-Kuang
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Huang, Guan-Min
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Huang, Chun-Wei
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Tai, Kuo-Lun
Electron beam (e-beam) has been developed for nanomaterial observation and moreover to induce structural evolutions in atomic scale. In this work, we demonstrate the deoxidation of cuprous oxide (Cu2O) and the formation of an atomically flat surface on a Cu nanowire by e-beam irradiation. To develop e-beam irradiation applications, the relation between e-beam radiation and the atomic surface is significant. Through the density functional theory simulation of atomic sputtering, an obvious disparity in the sputtering threshold has been found under different structural conditions, which leads to different structural evolutions. Both surface deoxidation and atomic surface flattening reactions have been identified as self-limiting and irreversible processes via in situ transmission electron microscope observation. Under e-beam irradiation, the dynamic mechanism of atomic surface flattening is driven by the convergence of total surface energy and confirmed by climbing-image nudged elastic band (ci-NEB) calculations. With precise control, e-beam irradiation reveals enormous potentials in atomic surface engineering.
Facile synthesis of mesoporous NiFe2O4/CNTs nanocomposite cathode material for high performance asymmetric pseudocapacitors
(2018-03-01)
Kumar, Nagesh
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Kumar, Amit
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Huang, Guan-Min
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Wu, Wen-Wei
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Tseng, Tseung Yuen
Morphology and synergistic effect of constituents are the two very important factors that greatly influence the physical, chemical and electrochemical properties of a composite material. In the present work, we report the enhanced electrochemical performance of mesoporous NiFe2O4 and multiwall carbon nanotubes (MWCNTs) nanocomposites synthesized via hexamethylene tetramine (HMT) assisted one-pot hydrothermal approach. The synthesized cubic phase spinel NiFe2O4 nanomaterial possesses high specific surface area (148 m(2)g(-1)) with narrow mesopore size distribution. The effect of MWCNTs addition on the electrochemical performance of nanocomposite has been probed thoroughly in a normal three electrode configuration using 2 M KOH electrolyte at room temperature. Experimental results show that the addition of mere 5 mg MWCNTs into fixed NiFe2O4 precursors amount enhances the specific capacitance up to 1291 Fg(-1) at 1 Ag-1, which is the highest reported value for NiFe2O4 nanocomposites so far. NiFe2O4/CNT nanocomposite exhibits small relaxation time constant (1.5 ms), good rate capability and capacitance retention of 81% over 500 charge-discharge cycles. This excellent performance can be assigned to high surface area, mesoporous structure of NiFe2O4 and conducting network formed by MWCNTs in the composite. Further, to evaluate the device performance of the composite, an asymmet-ric pseudocapacitor has been designed using NiFe2 04/CNT nanocomposite as a positive and N-doped graphene as a negative electrode material, respectively. Our designed asymmetric pseudocapacitor gives maximum energy density of 23 W h kg(-1) at power density of 872W kg(-1). These promising results assert the potential of synthesized nanocomposite in the development of efficient practical high-capacitive energy storage devices. (C) 2017 Published by Elsevier B.V.
In Situ Investigation of Defect-Free Copper Nanowire Growth
(2018-02-01)
Lin, Ting-Yi
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Chen, Yong-Long
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Chang, Chia-Fu
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Huang, Guan-Min
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Huang, Chun-Wei
The fabrication and placement of high purity nanometals, such as one-dimensional copper (Cu) nanowires, for interconnection in integrated devices have been among the most important technological developments in recent years. Structural stability and oxidation prevention have been the key issues, and the defect control in Cu nanowire growth has been found to be important. Here, we report the synthesis of defect-free single-crystalline Cu nanowires by controlling the surface-assisted heterogeneous nucleation of Cu atomic layering on the graphite-like loop of an amorphous carbon (a-C) lacey film surface. Without a metal-catalyst or induced defects, the high quality Cu nanowires formed with high aspect ratio and high growth rate of 578 nm/s. The dynamic study of the growth of heterogeneous nanowires was conducted in situ with a high-resolution transmission electron microscope. The study illuminates the new mechanism by heterogeneous nucleation control and laying the groundwork for better understanding of heterosurface-assisted nucleation of defect-free Cu nanowire on a-C lacey film.
In situ TEM investigation of electron beam-induced ultrafast chemical lithiation for charging
(2020-01-14)
Huang, Guan-Min
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Huang, Chun-Wei
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Kumar, Nagesh
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Huang, Chih-Yang
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Tseng, Tseung-Yuen
The enhancement of operational life and charging speed have been considered to be two of the major factors that will influence the development of energy storage devices in the future. Here, we utilized an electron beam (e-beam) to retrieve Li metal from LiF, the degradation product of LiPF6, and trigger the further lithiation of NiFe2O4/carbon nanotubes (CNTs) to relieve the degradation of the electrolyte and achieve ultrafast lithiation. Accordingly, in situ transmission electron microscopy (in situ TEM) was used to investigate the comprehensive mechanism of the whole process. The e-beam acting on the degradation product, i.e., LiF clusters, led to the generation of Li flakes, which served as the source for the subsequent lithiation. Then, with these Li flakes, the chemical lithiation of the NiFe2O4/CNTs was triggered, resulting in phase transformation to Ni and Fe nanograins. Compared to "electrochemical" lithiation, ultrahigh reaction speed and the ability to charge without a directly applied potential in the "chemical" lithiation are expected to extend lithiation to more diverse applications. As a result of this investigation, we have provided a new strategy for designing novel energy storage devices for the energy-harvesting field.
In Situ TEM Investigation of the Electrochemical Behavior in CNTs/MnO2-Based Energy Storage Devices
(2017-09-19)
Tsai, Tsung-Chun
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Huang, Guan-Min
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Huang, Chun-Wei
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Chen, Jui-Yuan
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Yang, Chih-Chieh
Transition metal oxides have attracted much interest owing to their ability to provide high power density in lithium batteries; therefore, it is important to understand the electrochemical behavior and mechanism of lithiation-delithiation processes. In this study, we successfully and directly observed the structural evolution of CNTs/MnO2 during the lithiation process using transmission electron microscopy (TEM). CNTs/MnO2 were selected due to their high surface area and capacitance effect, and the lithiation mechanism of the CNT wall expansion was systematically analyzed. Interestingly, the wall spacings of CNTs/MnO2 and CNTs were obviously expanded by 10.92% and 2.59%, respectively. The MnO2 layer caused structural defects on the CNTs surface that could allow penetration of Li+ and Mn4+ through the tube wall and hence improve the ionic transportation speed. This study provided direct evidence for understanding the role of CNTs/MnO2 in the lithiation process used in lithium ion batteries and also offers potential benefits for applications and development of supercapacitors.
Observing phase transformation in CVD-grown MoS2 via atomic resolution TEM
(2018-09-14)
Tai, Kuo-Lun
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Huang, Guan-Min
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Huang, Chun-Wei
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Tsai, Tsung-Chun
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Lee, Shih-Kuang
We utilized in situ transmission electron microscopy to observe phase transformation in CVD-grown MoS2. Significantly, the reaction was performed under electron irradiation through appropriate control of the electron dose and exposure time. Moreover, we proposed a new route between the 2H and 1T phases that involved the higher energy states TS1/TS2.
Revealing conducting filament evolution in low power and high reliability Fe3O4/Ta2O5 bilayer RRAM
(2018-11-01)
Chang, Chia-Fu
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Chen, Jui-Yuan
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Huang, Guan-Min
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Lin, Ting-Yi
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Tai, Kuo-Lun
In this work, we used the polycrystalline-Fe3O4 to improve the reliability of the Ag/Ta2O5/Pt resistive random access memory (RRAM). In both the Ag/Ta2O5/Fe3O4/Pt and Ag/Fe3O4/Ta2O5/Pt structures, the switching properties for these bilayer RRAMs were measured in atmosphere and vacuum environments. The results demonstrated that the humidity would affect the Ag filament formation in different environments, and the Ta2O5 and Fe3O4 interface in a different sequence would change the performance of the device, particularly the Forming voltage. Furthermore, the switching voltage and reliability of these bilayer RRAMs was better than single-layer RRAM device, which significantly increased endurance, especially in the Ag/Fe3O4/Ta2O5/Pt device. We also observed the conducting filament shape and evolution during Forming via in/ex-situ transmission electron microscopy (TEM) in the Ag/Fe3O4/Ta2O5/Pt system. In low humidity, the conducting filament was composed of many weak filaments in a low-resistance state (LRS), where the grain boundaries in the Fe3O4 layer limited filament size. The results of energy dispersive spectrometry (EDS) analysis demonstrated that the filament was composed of Ag metal. This study provided detailed switching knowledge of the bilayer RRAM for improving the reliability and power consumption of the device and new design viewpoints of the RRAM structure in future applications.