Xuedong Bai

Technical Institute of Physics and Chemistry, Peping, Beijing, China

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Publications (78)549.4 Total impact

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    ABSTRACT: Nanostructured silicon anodes, which possess extremely high energy density and accommodate large strain without pulverization, have been developed rapidly for high-power lithium ion batteries. Here, using in situ transmission electron microscopy, the lithiation behavior of silicon nanowires with diameters smaller than 60 nm was investigated. The study demonstrated a direct dependence of the self-limiting lithiation on the pristine diameter. A "punch-through" lithiation process at the core of nanowires with pristine diameters slightly larger than the self-limiting threshold is suggested to occur with the consequent formation of a stage structure. Our work demonstrates the crucial role of mechanical stress and local defects in determining the migration and geometry of the reaction front at the mesoscopic scale. This intriguing finding holds critical significance for the application of silicon nanostructures in high-power lithium ion batteries.
    ACS Nano 07/2014; · 12.03 Impact Factor
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    ABSTRACT: The possibility to induce magnetism in light-element materials that contain only s and p electrons is of fundamental and practical importance. Here, weak high-temperature ferromagnetism is observed in carbon-doped boron nitride (B-C-N) nanosheets. The bulk-quantities of B-C-N nanosheets that are free of metallic impurities are prepared through a multi-step process. These B-C-N samples exhibit ferromagnetic hysteresis stable at room temperature and above, with saturation magnetization and coercivity comparable to the previously reported results of defective graphite samples. The ferromagnetic response disappears upon the removal of carbon dopants from the BN lattice, indicating that the observed magnetism originates from substitutional carbon-doping rather than from extrinsic magnetic impurities. On the basis of first-principle calculations it is shown that not only substitutional carbon doping in a honeycomb BN lattice favors spontaneous spin polarization and local moment formation, but also that the spin moments can exhibit long-range magnetic ordering.
    Advanced Functional Materials 07/2014; · 9.77 Impact Factor
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    ABSTRACT: Carbon nanotubes have many material properties that make them attractive for applications. In the context of nanoelectronics, interest has focused on single-walled carbon nanotubes (SWNTs) because slight changes in tube diameter and wrapping angle, defined by the chirality indices (n, m), will shift their electrical conductivity from one characteristic of a metallic state to one characteristic of a semiconducting state, and will also change the bandgap. However, this structure-function relationship can be fully exploited only with structurally pure SWNTs. Solution-based separation methods yield tubes within a narrow structure range, but the ultimate goal of producing just one type of SWNT by controlling its structure during growth has proved to be a considerable challenge over the last two decades. Such efforts aim to optimize the composition or shape of the catalyst particles that are used in the chemical vapour deposition synthesis process to decompose the carbon feedstock and influence SWNT nucleation and growth. This approach resulted in the highest reported proportion, 55 per cent, of single-chirality SWNTs in an as-grown sample. Here we show that SWNTs of a single chirality, (12, 6), can be produced directly with an abundance higher than 92 per cent when using tungsten-based bimetallic alloy nanocrystals as catalysts. These, unlike other catalysts used so far, have such high melting points that they maintain their crystalline structure during the chemical vapour deposition process. This feature seems crucial because experiment and simulation both suggest that the highly selective growth of (12, 6) SWNTs is the result of a good structural match between the carbon atom arrangement around the nanotube circumference and the arrangement of the catalytically active atoms in one of the planes of the nanocrystal catalyst. We anticipate that using high-melting-point alloy nanocrystals with optimized structures as catalysts paves the way for total chirality control in SWNT growth and will thus promote the development of SWNT applications.
    Nature 06/2014; 510(7506):522-4. · 38.60 Impact Factor
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    ABSTRACT: Optical absorption is the most fundamental optical property characterizing light-matter interactions in materials and can be most readily compared with theoretical predictions. However, determination of optical absorption cross-section of individual nanostructures is experimentally challenging due to the small extinction signal using conventional transmission measurements. Recently, dramatic increase of optical contrast from individual carbon nanotubes has been successfully achieved with a polarization-based homodyne microscope, where the scattered light wave from the nanostructure interferes with the optimized reference signal (the reflected/transmitted light). Here we demonstrate high-sensitivity absorption spectroscopy for individual single-walled carbon nanotubes by combining the polarization-based homodyne technique with broadband supercontinuum excitation in transmission configuration. To our knowledge, this is the first time that high-throughput and quantitative determination of nanotube absorption cross-section over broad spectral range at the single-tube level was performed for more than 50 individual chirality-defined single-walled nanotubes. Our data reveal chirality-dependent behaviors of exciton resonances in carbon nanotubes, where the exciton oscillator strength exhibits a universal scaling law with the nanotube diameter and the transition order. The exciton linewidth (characterizing the exciton lifetime) varies strongly in different nanotubes, and on average it increases linearly with the transition energy. In addition, we establish an empirical formula by extrapolating our data to predict the absorption cross-section spectrum for any given nanotube. The quantitative information of absorption cross-section in a broad spectral range and all nanotube species not only provides new insight into the unique photophysics in one-dimensional carbon nanotubes, but also enables absolute determination of optical quantum efficiencies in important photoluminescence and photovoltaic processes.
    Proceedings of the National Academy of Sciences 05/2014; · 9.74 Impact Factor
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    ABSTRACT: We report a scalable growth of monolayer MoS2 films on SiO2 substrates by chemical vapor deposition. As-grown polycrystalline MoS2 films are continuous over the entire substrate surface with a tunable domain size from ~20nm up to ~1μm. An obvious blue-shift (up to 80meV) of photoluminescence peaks was observed from a series samples with different domain sizes. Back-gated field effect transistors based on polycrystalline MoS2 film with a typical domain size of ~600nm shows field mobility of ~7cm2/Vs and on/off ratio of ~106, comparable to those achieved from exfoliated MoS2. Our work provides a route towards scaled-up synthesis of high-quality monolayer MoS2 for electronic and optoelectronic devices.
    ACS Nano 05/2014; · 12.03 Impact Factor
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    Lifen Wang, Zhi Xu, Wenlong Wang, Xuedong Bai
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    ABSTRACT: Layered molybdenum disulfide (MoS2) has been studied for decades for its diversity of structure and properties, where the structural dynamic evolution during lithium intercalation is an important but still indistinct, controversial topic. Here the electrochemical dynamic process of MoS2 nanosheets upon lithium intercalation has been systematically investigated by in situ high-resolution transmission electron microscopy. The results indicate that the lithiated MoS2 undergoes a trigonal prismatic (2H) - octahedral (1T) phase transition with lithium ion occupying the interlayer S-S tetrahedron site in the 1T-LiMoS2. A pseudo-periodic structural modulation composed by polytype superlattices is also revealed as a consequence of the electron-lattice interaction. Furthermore, the shear mechanism of 2H-1T phase transition has been confirmed by probing the dynamic phase boundary movement. The in situ real-time characterization at atomic scale provides a fundamental understanding of the lithium ion storage mechanism in MoS2 with a great leap forward, which should be also of help for other transition metal dichalcogenides.
    Journal of the American Chemical Society 04/2014; · 10.68 Impact Factor
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    ABSTRACT: Defect engineering in graphene is important for tailoring graphene's properties thus applicable in various applications such as porous membranes and ultra-capacitors. In this paper, we report a general route towards defect- and pore- engineering in graphene through remote plasma treatments. Oxygen plasma irradiation was employed to create homogenous defects in graphene with controllable density from a few to ≈10(3) (μm(-2) ). The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well-defined pore size of a few nm or above. The achieved smallest nanopores are ≈2 nm in size, showing the potential for ultra-small graphene nanopores fabrication.
    Small 03/2014; · 7.82 Impact Factor
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    ABSTRACT: The fatigue behavior of ZnO nanowires (NWs) and microwires was systematically investigated with in situ transmission electron microscopy electromechanical resonance method. The elastic modulus and mechanical quality factors of ZnO wires were obtained. No damage or failure was found in the intact ZnO wires after resonance for about 108–109 cycles, while the damaged ZnO NW under electron beam (e-beam) irradiation fractured after resonance for seconds. The research results will provide a useful guide for designing, fabricating, and optimizing electromechanical nanodevices based on ZnO nanomaterials, as well as future applications.
    01/2014; 14(2).
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    ABSTRACT: The fatigue behavior of ZnO nanowires (NWs) and microwires (MWs) was systematically investigated with in situ transmission electron microscopy (TEM) electromechanical resonance method. The elastic modulus and mechanical quality factors of ZnO wires were obtained. No damage or failure was found in the intact ZnO wires after resonance for about 108~109 cycles, while the damaged ZnO NW under electron beam (e-beam) irradiation fractured after resonance for seconds. The research results will provide a useful guide for designing, fabricating and optimizing electromechanical nanodevices based on ZnO nanomaterials, as well as future applications.
    Nano Letters 01/2014; · 13.03 Impact Factor
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    ABSTRACT: High-energy lithium battery materials based on conversion/alloying reactions have tremendous potential applications in new generation energy storage devices. However, these applications are limited by inherent large volume variations and sluggish kinetics. Here we report a self-adaptive strain-relaxed electrode through crumpling of graphene to serve as high-stretchy protective shells on metal framework, to overcome these limitations. The graphene sheets are self-assembled and deeply crumpled into pinecone-like structure through a contraction-strain-driven crumpling method. The as-prepared electrode exhibits high specific capacity (2,165 mAh g(-1)), fast charge-discharge rate (20 A g(-1)) with no capacity fading in 1,000 cycles. This kind of crumpled graphene has self-adaptive behaviour of spontaneous unfolding-folding synchronized with cyclic expansion-contraction volumetric variation of core materials, which can release strain and maintain good electric contact simultaneously. It is expected that such findings will facilitate the applications of crumpled graphene and the self-adaptive materials.
    Nature Communications 01/2014; 5:4565. · 10.02 Impact Factor
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    ABSTRACT: Getting a grip on the switching mechanism in nanoionic resistive memories: the bipolar electrochemical mechanism for mass transfer of Ag in nanoscale SiO2 is disclosed. The in-situ atomic-level experiments provide detailed evidence of the mass-transfer process under external electric fields. The mass transfer of Ag directly leads to conductive filament formation and disruption, which is responsible for the switching mechanism in nanoionic resistive memories.
    Advanced Materials 01/2014; · 14.83 Impact Factor
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    ABSTRACT: Resistive switching random access memories (RRAM) have been considered to be promising for future information technology with applications for non-volatile memory, logic circuits and neuromorphic computing. Key performances of those resistive devices are approaching the realistic levels for production. In this paper, we review the progress of valence change type memories, including relevant work reported by our group. Both electrode engineering and in- situ transmission electron microscopy (TEM) high-resolution observation have been implemented to reveal the influence of migration of oxygen anions/vacancies on the resistive switching effect. The understanding of resistive memory mechanism is significantly important for device applications.
    Science China: Physics, Mechanics and Astronomy 12/2013; 56(12):2361-2369. · 1.17 Impact Factor
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    ABSTRACT: Determination of optical absorption cross-section is always among the central importance of understanding a material. However its realization on individual nanostructures, such as carbon nanotubes, is experimentally challenging due to the small extinction signal using conventional transmission measurements. Here we develop a technique based on polarization manipulation to enhance the sensitivity of single-nanotube absorption spectroscopy by two-orders of magnitude. We systematically determine absorption cross-section over broad spectral range at single-tube level for more than 50 chirality-defined single-walled nanotubes. Our data reveals chirality-dependent one-dimensional photo-physics through the behaviours of exciton oscillator strength and lifetime. We also establish an empirical formula to predict absorption spectrum of any nanotube, which provides the foundation to determine quantum efficiencies in important photoluminescence and photovoltaic processes.
    11/2013;
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    ABSTRACT: Tin oxide (SnO2) is one of the most promising electron transporters to further enhance the performance of quantum dots sensitized solar cells (QDSCs). Unfortunately, the performance of SnO2-based QDSCs is still poor. It was observed that surface modification toward a SnO2 photoelectrode such as a TiCl4 treatment is crucial to dramatically increase the performance of the devices. However, the mechanism of the TiCl4 treatment remains poorly understood. Here, systematic studies on the photoelectrochemical properties of SnO2-based QDSCs were performed in order to clarify the mechanism by which the TiCl4 treatment improves the performance of solar cells. Impendence spectroscopy results reveal that the photogenerated electrons transport in the porous SnO2 network rather than the TiO2 coating. Furthermore, a physical model considering the existence of monoenergetic surface states at the SnO2 surface was used to simulate the behavior of chemical capacitance at various forward biases. The accordance between the decrease of the surface states and the recombination reduction clearly indicates that the surface states act as the recombination centers to influence the device performance, which can be well described by Marcus-Gerischer theory. These combined findings provide new understanding of the recombination mechanism of SnO2-based sensitized solar cells and guidelines for further improving the performance of this system.
    The Journal of Physical Chemistry C. 05/2013; 117(21):10965–10973.
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    ABSTRACT: Improvement of energy density is an urgent task for developing advanced supercapacitors. In this paper, aqueous supercapacitors with high voltage of 1.8 V and energy density of 29.1 W h kg(-1) were fabricated based on carbon nanofibers (CNFs) and Na2SO4 electrolyte. The CNFs with radially grown graphene sheets (GSs) and small average diameter down to 11 nm were prepared by electrospinning and carbonization in NH3. The radially grown GSs contain between 1 and a few atomic layers with their edges exposed on the surface. The CNFs are doped with nitrogen and oxygen with different concentrations depending on the carbonizing temperature. The supercapacitors exhibit excellent cycling performance with the capacity retention over 93.7% after 5000 charging-discharging cycles. The unique structure, possessing radially grown GSs, small diameter, and heteroatom doping of the CNFs, and application of neutral electrolyte account for the high voltage and energy density of the present supercapacitors. The present supercapacitors are of high promise for practical application due to the high energy density and the advantages of neutral electrolyte including low cost, safety, low corrosivity, and convenient assembly in air.
    Nanoscale 04/2013; · 6.23 Impact Factor
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    ABSTRACT: A self-limiting growth process based on the interface-controlled reaction of molten boron oxide (B2 O3 ) with ammonia (NH3 ) is demonstrated for the facile and lost-cost synthesis of ultrathin (20-30 nm) crystalline hexagonal boron nitride (h-BN) films over large areas. The as-grown h-BN films are of high quality, being densely continuous, uniform and smooth, and highly transparent over a broad wavelength range.
    Small 03/2013; · 7.82 Impact Factor
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    ABSTRACT: Van der Waals-coupled materials, ranging from multilayers of graphene and MoS(2) to superlattices of nanoparticles, exhibit rich emerging behaviour owing to quantum coupling between individual nanoscale constituents. Double-walled carbon nanotubes provide a model system for studying such quantum coupling mediated by van der Waals interactions, because each constituent single-walled nanotube can have distinctly different physical structures and electronic properties. Here we systematically investigate quantum-coupled radial-breathing mode oscillations in chirality-defined double-walled nanotubes by combining simultaneous structural, electronic and vibrational characterizations on the same individual nanotubes. We show that these radial-breathing oscillations are collective modes characterized by concerted inner- and outer-wall motions, and determine quantitatively the tube-dependent van der Waals potential governing their vibration frequencies. We also observe strong quantum interference between Raman scattering from the inner- and outer-wall excitation pathways, the relative phase of which reveals chirality-dependent excited-state potential energy surface displacement in different nanotubes.
    Nature Communications 01/2013; 4:1375. · 10.02 Impact Factor
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    ABSTRACT: Honeycomb porous MnO2 nanofibers (HMONFs) have been prepared by solution reaction between KMnO4 and electrospun carbon nanofibers (CNFs). The HMONFs are entirely composed of radially grown MnO2 nanosheets with thickness about 4–7 nm, which interconnect each other, forming the honeycomb pores. Formation of this unique structure occurs only at very low KMnO4 concentrations and sufficiently long reaction time. The constituting MnO2 nanosheets in the HMONFs are well separated with the sheet edges oriented on the surface, leading to excellent supercapacitive performance. Symmetric aqueous supercapacitors are assembled using the HMONFs and 1M Na2SO4 electrolyte, which exhibits a working voltage as high as 2.2 V and high energy density of 41.1 Wh/kg at the power density of 3.3 kW/kg. The supercapacitor capacity can be retained about 76% of its initial value after 3500 cycles, which is acceptable due to its high energy density. These results indicate that the HMONFs are of high promise in developing advanced supercapacitors with high working voltage and energy density for practical applications.
    Nano Energy. 01/2013;
  • Science China Technological Sciences 01/2013; 56(11):2630-2635. · 1.19 Impact Factor
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    ABSTRACT: Ultra-thin solid-state nanopore with good wetting property is strongly desired to achieve high spatial resolution for DNA sequencing applications. Atomic thick hexagonal boron nitride (h-BN) layer provides a promising two-dimensional material for fabricating solid-state nanopores. Due to its good oxidation resistance, the hydrophilicity of h-BN nanopore device can be significantly improved by UV-Ozone treatment. The contact angle of a KCl-TE droplet on h-BN layer can be reduced from 57° to 26° after the treatment. Abundant DNA translocation events have been observed in such devices, and strong DNA-nanopore interaction has been revealed in pores smaller than 10 nm in diameter. The 1/f noise level is closely related to the area of suspended h-BN layer, and it is significantly reduced in smaller supporting window. The demonstrated performance in h-BN nanopore paves the way towards base discrimination in a single DNA molecule.
    Scientific Reports 01/2013; 3:3287. · 5.08 Impact Factor

Publication Stats

832 Citations
549.40 Total Impact Points

Institutions

  • 2009–2014
    • Technical Institute of Physics and Chemistry
      Peping, Beijing, China
  • 2008–2014
    • Chinese Academy of Sciences
      • Institute of Physics
      Peping, Beijing, China
  • 2013
    • University Town of Shenzhen
      Shen-ch’üan-shih, Zhejiang Sheng, China
  • 2012–2013
    • University of California, Berkeley
      • Department of Physics
      Berkeley, CA, United States
    • Peking University
      • International Center for Quantum Materials
      Beijing, Beijing Shi, China
  • 2006–2013
    • Northeast Institute of Geography and Agroecology
      • • Institute of Physics
      • • State Key Laboratory for Surface Physics
      Beijing, Beijing Shi, China
  • 2009–2011
    • Harbin Institute of Technology
      • School of Materials Science and Engineering
      Harbin, Heilongjiang Sheng, China
  • 2007
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan