Surface‐Dominated Sodium Storage Towards High Capacity and Ultrastable Anode Material for Sodium‐Ion Batteries

2018-11-19T14:32:30+00:00November 19th, 2018|Categories: Publications|Tags: |

Surface‐dominated Na storage is demonstrated on the oxygen‐functionalized graphene nanosheets with fast surface‐capacitive storage and robust structural stability. The enhanced Na storage is realized by tuning the overall oxygen content and specific C = O groups on the graphene matrix for adequate surface‐active sites, accompanying with the high electrical conductivity, high specific surface area, and optimized Na+ transport paths. Abstract The development of sodium‐ion batteries is hindered by the poor Na+ transport kinetics and structural instability of electrode materials during Na+ intercalation/deintercalation. In this work, surface‐dominated Na storage is demonstrated on the oxygen‐functionalized graphene nanosheets (FGS) with fast surface redox reaction and robust structural stability. The FGS samples with tunable oxygen contents and species are fabricated via a two‐step thermal exfoliation method from graphite oxides. The surface‐induced oxygen functional groups can serve as the surface‐redox sites for the FGS electrode, attaining a high specific capacity of 603 mAh g−1 at a current density of 0.05 A g−1, excellent rate capability (214 mAh g−1 at 10 A g−1), and ultrastable cycling stability (capacity retention close to 100% after 10 000 cycles at 5 A g−1). Even at a slow scan rate of 0.1 mV s−1 for cyclic voltammetry, about 67.7% capacity is contributed from the surface adsorption/desorption and surface‐redox reaction, suggesting surface‐dominated Na storage for the FGS‐700 (FGS sample obtained at 700 °C) electrode. The present work demonstrates that the surface oxygen functionalization is an effective strategy to develop high‐performance graphene‐based anodes due to the surface‐dominated Na storage with improved

Published in: "Advanced Functional Materials".

Toward Air Stability of Thin GaSe Devices: Avoiding Environmental and Laser‐Induced Degradation by Encapsulation

2018-11-19T14:32:29+00:00November 19th, 2018|Categories: Publications|Tags: |

The degradation of thin GaSe in air is a complex process that involves photooxidation and generates various byproducts such as elemental selenium and Ga2O3. This degradation has profound influences on the optoelectronic properties of devices based on this layered material. Abstract Gallium selenide (GaSe) is a novel 2D material, which belongs to the layered III–VIA semiconductors family and attracted interest recently as it displays single‐photon emitters at room temperature and strong optical nonlinearity. Nonetheless, few‐layer GaSe is not stable under ambient conditions and it tends to degrade over time. Here atomic force microscopy, Raman spectroscopy, and optoelectronic measurements are combined in photodetectors based on thin GaSe to study its long‐term stability. It is found that the GaSe flakes exposed to air tend to decompose forming first amorphous selenium and Ga2Se3 and subsequently Ga2O3. While the first stage is accompanied by an increase in photocurrent, in the second stage, a decrease in photocurrent is observed, which leads to the final failure of GaSe photodetectors. Additionally, it is found that the encapsulation of the GaSe photodetectors with hexagonal boron nitride (h‐BN) can protect the GaSe from degradation and can help to achieve long‐term stability of the devices.

Published in: "Advanced Functional Materials".

Interlayer Expansion of 2D MoS2 Nanosheets for Highly Improved Photothermal Therapy of Tumor in vitro and in vivo

2018-11-19T12:35:51+00:00November 19th, 2018|Categories: Publications|Tags: |

Chem. Commun., 2018, Accepted ManuscriptDOI: 10.1039/C8CC08279K, CommunicationChanghui Fu, Longfei Tan, Xiangling Ren, Qiong Wu, Haibo Shao, Jun Ren, Yuxia Zhao, Xianwei MengHerein, the interlayer-expanded MoS2 (E-MoS2) nanosheets with a value of 0.94 nm are demonstrated to show a much higher photothermal conversion efficiency (~62%) than normal MoS2 nanosheets (45%). More importantly,…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Chemical Communications".

Clinical Applications of Carbon Nanomaterials in Diagnostics and Therapy

2018-11-19T10:37:20+00:00November 19th, 2018|Categories: Publications|Tags: |

Biomedical application of carbon nanomaterials as well as the potential clinical implementation of carbon nanomaterials includes enhanced drug delivery, improved sensitivity of clinical diagnostic tools and imaging drugs, as well as more effective regenerative medical platforms. Abstract Nanomaterials have the potential to improve how patients are clinically treated and diagnosed. While there are a number of nanomaterials that can be used toward improved drug delivery and imaging, how these nanomaterials confer an advantage over other nanomaterials, as well as current clinical approaches is often application or disease specific. How the unique properties of carbon nanomaterials, such as nanodiamonds, carbon nanotubes, carbon nanofibers, graphene, and graphene oxides, make them promising nanomaterials for a wide range of clinical applications are discussed herein, including treating chemoresistant cancer, enhancing magnetic resonance imaging, and improving tissue regeneration and stem cell banking, among others. Additionally, the strategies for further improving drug delivery and imaging by carbon nanomaterials are reviewed, such as inducing endothelial leakiness as well as applying artificial intelligence toward designing optimal nanoparticle‐based drug combination delivery. While the clinical application of carbon nanomaterials is still an emerging field of research, there is substantial preclinical evidence of the translational potential of carbon nanomaterials. Early clinically trial studies are highlighted, further supporting the use of carbon nanomaterials in clinical applications for both drug delivery and imaging.

Published in: "Advanced Materials".

2D Phosphorene: 2D Phosphorene: Epitaxial Growth and Interface Engineering for Electronic Devices (Adv. Mater. 47/2018)

2018-11-19T10:37:16+00:00November 19th, 2018|Categories: Publications|Tags: |

In article number 1802207, Wei Chen and co‐workers highlight their recent progress in the interface engineering of 2D phosphorene for applications in both epitaxial growth and electronic devices. Their detailed investigations reveal the critical role of substrates for epitaxial growth of 2D phosphorene, demonstrate a highly efficient surface transfer doping method, and provide a comprehensive understanding of the oxidation mechanism of black phosphorus in air.

Published in: "Advanced Materials".

Molybdenum Disulfid: Differentiating Polymorphs in Molybdenum Disulfide via Electron Microscopy (Adv. Mater. 47/2018)

2018-11-19T10:37:13+00:00November 19th, 2018|Categories: Publications|Tags: |

The presence of rich polymorphs and stacking polytypes in molybdenum disulfide (MoS2) endows it with a diverse range of properties. This has stimulated a lot of interest in the unique properties associated with each polymorph. In article number 1802397, Kian Ping Loh and co‐workers discuss the use of electron microscopy for identifying the atomic structures of several important polymorphs in MoS2 and establishing the correlation between structure and properties.

Published in: "Advanced Materials".

The Atomic Circus: Small Electron Beams Spotlight Advanced Materials Down to the Atomic Scale

2018-11-19T10:36:49+00:00November 19th, 2018|Categories: Publications|

Direct visualization of structural defects, especially for atomic‐scale defects, and thorough understanding of their nature and dynamics, are crucial to unravel structure–property relationships. Such insights can therefore guide further property optimization through appropriate defect engineering. Two representative examples are coexisting nanophases with polarization rotation at the atomic‐scale of piezoelectrics, and atomic‐scale interstitials for simultaneously optimizing thermal and electrical transport properties in classic PbTe thermoelectric materials. Abstract Defects in crystalline materials have a tremendous impact on their functional behavior. Controlling and tuning of these imperfections can lead to marked improvements in their physical, electrical, magnetic, and optical properties. Thanks to the development of aberration‐corrected (scanning) transmission electron microscopy (STEM/TEM), direct visualization of defects at multiple length scales has now become possible, including those critically important defects at the atomic scale. Thorough understanding of the nature and dynamics of these defects is the key to unraveling the fundamental origins of structure–property relationships. Such insight can therefore allow the creation of new materials with desired properties through appropriate defect engineering. Herein, several examples of new insights obtained from representative functional materials are shown, including piezoelectrics/ferroelectrics, oxide interfaces, thermoelectrics, electrocatalysts, and 2D materials.

Published in: "Advanced Materials".

Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides (MXenes)

2018-11-19T10:36:14+00:00November 19th, 2018|Categories: Publications|Tags: |

The electronic and optical properties, as well as related applications of 2D transition metal carbides, carbonitrides, and nitrides (MXenes) are reviewed. This very large and rapidly growing family of 2D materials has demonstrated attractive electrical, optical, electrochemical, and mechanical properties, which lead to numerous plasmonic, optoelectronic, and other applications. Abstract 2D transition metal carbides, carbonitrides, and nitrides, known as MXenes, are a rapidly growing family of 2D materials with close to 30 members experimentally synthesized, and dozens more studied theoretically. They exhibit outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. They have shown promise in many applications, such as energy storage, electromagnetic interference shielding, transparent electrodes, sensors, catalysis, photothermal therapy, etc. The high electronic conductivity and wide range of optical absorption properties of MXenes are the key to their success in the aforementioned applications. However, relatively little is currently known about their fundamental electronic and optical properties, limiting their use to their full potential. Here, MXenes’ electronic and optical properties from both theoretical and experimental perspectives, as well as applications related to those properties, are discussed, providing a guide for researchers who are exploring those properties of MXenes.

Published in: "Advanced Materials".

Pseudo-Euler equations from nonlinear optics: plasmon-assisted photodetection beyond hydrodynamics. (arXiv:1811.06540v1 [cond-mat.mes-hall])

2018-11-19T04:30:18+00:00November 19th, 2018|Categories: Publications|Tags: |

A great deal of theoretical and experimental efforts have been devoted in the last decades to the study of long-wavelength photodetection mechanisms in field-effect transistors hosting two-dimensional (2D) electron systems. A particularly interesting subclass of these mechanisms is intrinsic and based on the conversion of the incoming electromagnetic radiation into plasmons, which resonantly enhance the photoresponse, and subsequent rectification via hydrodynamic nonlinearities. In this Article we show that such conversion and subsequent rectification occur well beyond the frequency regime in which hydrodynamic theory applies. We consider the nonlinear optical response of generic 2D electron systems and derive pseudo-Euler equations of motion for suitable collective variables. These are solved in one- and two-dimensional geometries for the case of graphene and the results are compared with those of hydrodynamic theory. Significant qualitative differences are found, which are amenable to experimental studies. Our theory expands the knowledge of the fundamental physics behind long-wavelength photodetection.

Published : "arXiv Mesoscale and Nanoscale Physics".

Towards the growth of single-crystal boron nitride monolayer on Cu. (arXiv:1811.06688v1 [cond-mat.mtrl-sci])

2018-11-19T02:29:26+00:00November 19th, 2018|Categories: Publications|Tags: |

Atom-layered hexagonal boron nitride (hBN), with excellent stability, flat surface and large bandgap, has been reported to be the best 2D insulator to open up the great possibilities for exciting potential applications in electronics, optoelectronics and photovoltaics. The ability to grow high-quality large single crystals of hBN is at the heart for those applications, but the size of single-crystal 2D BN is less than a millimetre till now. Here, we report the first epitaxial growth of a 10*10 cm2 single-crystal hBN monolayer on a low symmetry Cu(110) “vicinal surface”. The growth kinetics, unidirectional alignment and seamless stitching of hBN domains are unambiguously illustrated using centimetre- to the atomic-scale characterization techniques. The findings in this work are expected to significantly boost the massive applications of 2D materials-based devices, and also pave the way for the epitaxial growth of broad non-centrosymmetric 2D materials.

Published in: "arXiv Material Science".

Impact of grain boundary characteristics on thermal transport in polycrystalline graphene: Analytical results. (arXiv:1811.06695v1 [cond-mat.mtrl-sci])

2018-11-19T02:29:24+00:00November 19th, 2018|Categories: Publications|Tags: |

The effect of grain boundary (GB) structure, size and shape on thermal conductivity of polycrystalline graphene is studied in the framework of the deformation potential approach. Precise analytical expressions for the phonon mean free path (MFP) are obtained within the Born approximation. We found exactly two types of behavior in the long-wavelength limit: MFP varies as $k^{-1}$ for open GBs of any shape while it behaves as $k^{-3}$ for closed configurations (loops). In the short-wavelength limit MFP tends to a constant value for any configuration. Oscillatory behavior is observed for all GBs which indicates that they serve as diffraction grating for phonons. This property is also inherent in GBs with irregularities caused by partial disclination dipoles. The thermal conductivity is calculated in the framework of Callaway’s approach with all main sources of phonon scattering taken into account. Reduction of the heat conductivity with decreasing grain size is obtained in a wide temperature range. Most interesting is that we found a marked decrease in the thermal conductivity of polycrystalline graphene containing GBs with changes in their misorientation angles.

Published in: "arXiv Material Science".

Distinctive signatures of the spin- and momentum-forbidden dark exciton states in the photo-luminescences of strained WSe$_2$ monolayers under thermalization. (arXiv:1811.06728v1 [cond-mat.mes-hall])

2018-11-19T02:29:22+00:00November 19th, 2018|Categories: Publications|Tags: |

With the both spin and valley degrees of freedom, the low-lying excitonic spectra of photo-excited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intra-valley bright exciton states as well as various intra- and inter-valley dark ones. The latter states can be classified as those of the spin- and momentum-forbidden dark excitons according to the violated optical selection rules. Because of the optical invisibility, the two types of the dark states are in general hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photo-luminescence spectra of strained tungsten diselenide monolayers (WSe$_2$-MLs) where the intra-valley spin-forbidden dark exciton lies in the lowest exciton states and other momentum-forbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe$_2$-ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin- and momentum-forbidden exciton states of strained WSe$_2$-MLs in the temperature-dependent photo-luminescences and present the guiding principle to infer the relative energetic locations of the two types of DX’s.

Published in: "arXiv Material Science".

Planar Hall effect in the Dirac semimetal PdTe2. (arXiv:1811.06767v1 [cond-mat.mtrl-sci])

2018-11-19T02:29:20+00:00November 19th, 2018|Categories: Publications|

We report the synthesis and magneto-transport measurements on the single crystal of Dirac semimetal PdTe$_2$. The de Haas-van Alphen oscillations with multiple frequencies have been clearly observed, from which the small effective masses and nontrivial Berry phase are extracted, implying the possible existence of the Dirac fermions in PdTe$_2$. The planar Hall effect and anisotropic longitudinal resistivity originating from the chiral anomaly and nontrivial Berry phase are observed, providing strong evidence for the nontrivial properties in PdTe$_2$. With the increase of temperature up to 150 K, planar Hall effect still remains. The possible origin of mismatch between experimental results and theoretical predictions is also discussed.

Published in: "arXiv Material Science".

Interaction of Fluorescent Gold Nanoclusters with Transition Metal Dichalcogenides Nanosheets: A Spectroscopic Study. (arXiv:1811.06910v1 [cond-mat.mtrl-sci])

2018-11-19T02:29:17+00:00November 19th, 2018|Categories: Publications|Tags: , |

In this paper, the interaction of few tens of atoms containing gold nanoclusters with two dimensional nanosheets of transition metal dichalcogenides nanosheets has been explored. The gold nanoclusters have been synthesized using chemical reduction method in presence of protein molecules as stabilizing agent. The transition metal dichalcogenides nanosheets of molybdenum disulfide (MoS$_2$) has been chemically exfoliated. Different microscopic and optical spectroscopic tools have been used for characterizing the physical properties of the gold nanoclusters and the two dimensional nanosheets of MoS$_2$. The gold nanoclusters exhibit fluorescence emission at 690 nm. However, the interaction with transition metal dichalcogenides diminishes drastically the fluorescence intensity of the nanoclusters. The spectroscopic methods used for understanding the interaction in the system reveals the absence of energy transfer and dynamic nature of the fluorescence quenching.

Published in: "arXiv Material Science".

Promoted Glycerol Oxidation Reaction in an Interface‐Confined Hierarchically Structured Catalyst

2018-11-18T00:34:15+00:00November 17th, 2018|Categories: Publications|Tags: , |

The confinement of Pt nanosheets is realized in a vertically erected graphene array with hierarchically porous architecture to address the mass‐diffusion limitation in interface‐confined catalysis. Such a confined 3D catalyst exhibits a much stronger oxidation and CC bond cleaving ability for the glycerol oxidation reaction, leading to a superior mass activity and selectivity toward C1 products than commercial Pt/C catalysts. Abstract Confined catalysis in a 2D system is of particular interest owing to the facet control of the catalysts and the anisotropic kinetics of reactants, which suppress side reactions and improve selectivity. Here, a 2D‐confined system consisting of intercalated Pt nanosheets within few‐layered graphene is demonstrated. The strong metal–substrate interaction between the Pt nanosheets and the graphene leads to the quasi‐2D growth of Pt with a unique (100)/(111)/(100) faceted structure, thus providing excellent catalytic activity and selectivity toward one‐carbon (C1) products for the glycerol oxidation reaction. A hierarchically porous graphene architecture, grown on carbon cloth, is used to fabricate the confined catalyst bed in order to enhance the mass‐diffusion limitation in interface‐confined reactions. Owing to its unique 3D porous structure, this graphene‐confined Pt catalyst exhibits an extraordinary mass activity of 2910 mA mgPt−1 together with a formate selectivity of 79% at 60 °C. This paves the way toward rational designs of heterogeneous catalysts for energy‐related applications.

Published in: "Advanced Materials".

Electronic and Optical Properties of 2D Materials Constructed from Light Atoms

2018-11-18T00:34:12+00:00November 17th, 2018|Categories: Publications|Tags: |

2D materials constructed from boron, carbon, nitrogen, and oxygen atoms show a rich structural diversity. This makes possible engineering of their electronic and optical properties through a refined structural control. Abstract Boron, carbon, nitrogen, and oxygen atoms can form various building blocks for further construction of structurally well‐defined 2D materials (2DMs). Both in theory and experiment, it has been documented that the electronic structures and optical properties of 2DMs are well tunable through a rational design of the material structure. Here, the recent progress on 2DMs that are composed of B, C, N, and O elements is introduced, including borophene, graphene, h‐BN, g‐C3N4, organic 2D polymers (2DPs), etc. Attention is put on the band structure/bandgap engineering for these materials through a variety of methodologies, such as chemical modifications, layer number and atomic structure control, change of conjugation degree, etc. The optical properties, such as photoluminescence, thermoluminescence, single photon emission, as well as the associated applications in bioimaging and sensing, are discussed in detail and highlighted.

Published in: "Advanced Materials".

An Ambipolar Superconducting Field‐Effect Transistor Operating above Liquid Helium Temperature

2018-11-18T00:34:08+00:00November 17th, 2018|Categories: Publications|Tags: |

An ambipolar superconducting field‐effect transistor is developed using a strongly correlated molecular system laminated on a SiO2/Si substrate. The low‐temperature electronic state is fine tuned in the vicinity of the superconductor‐to‐Mott‐insulator transition, utilizing the negative pressure effect from the substrate, which allows a small dose of hole or electron injection by the SiO2 dielectric to control the superconductivity above 4.2 K. Abstract Superconducting (SC) devices are attracting renewed attention as the demands for quantum‐information processing, meteorology, and sensing become advanced. The SC field‐effect transistor (FET) is one of the elements that can control the SC state, but its variety is still limited. Superconductors at the strong‐coupling limit tend to require a higher carrier density when the critical temperature (T C) becomes higher. Therefore, field‐effect control of superconductivity by a solid gate dielectric has been limited only to low temperatures. However, recent efforts have resulted in achieving n‐type and p‐type SC FETs based on organic superconductors whose T C exceed liquid He temperature (4.2 K). Here, a novel “ambipolar” SC FET operating at normally OFF mode with T C of around 6 K is reported. Although this is the second example of an SC FET with such an operation mode, the operation temperature exceeds that of the first example, or magic‐angle twisted‐bilayer graphene that operates at around 1 K. Because the superconductivity in this SC FET is of unconventional type, the performance of the present device will contribute not only to fabricating SC circuits, but also to elucidating phase transitions

Published in: "Advanced Materials".

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2018-11-18T00:34:06+00:00November 17th, 2018|Categories: Publications|Tags: , |

Contact resistance between the channel and electrodes in MoS2 devices is significantly reduced using a low work function metal (Ag) and graphene as an interfacial layer between MoS2 and Ag because the Schottky barrier height is lowered at the contacts. Using graphene/Ag contacts instead of Ti/Au improves the field‐effect mobility, on/off current ratio, and photoresponsivity of the devices. Abstract 2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS2 devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS2 film and a Ag electrode as an interfacial layer. The MoS2 field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm2 V−1 s−1, an on/off current ratio of 4 × 108, and a photoresponsivity of 2160 A W−1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS2 and graphene is a key step toward the practical application of atomically thin TMDC‐based devices

Published in: "Advanced Materials".

Ultrasensitive 2D Bi2O2Se Phototransistors on Silicon Substrates

2018-11-18T00:34:04+00:00November 17th, 2018|Categories: Publications|

Ultrasensitive Bi2O2Se phototransistors are demonstrated by utilizing a unique method to transfer CVD‐grown Bi2O2Se from mica onto silicon substrates. Compared with devices on mica, photodetectors on silicon maintain high photoresponsivity, high photoconductive gain, fast response rate, and at the same time, the dark current is much lowered, which yields ultrahigh on/off ratio and specific detectivity. Abstract 2D materials are considered as intriguing building blocks for next‐generation optoelectronic devices. However, their photoresponse performance still needs to be improved for practical applications. Here, ultrasensitive 2D phototransistors are reported employing chemical vapor deposition (CVD)‐grown 2D Bi2O2Se transferred onto silicon substrates with a noncorrosive transfer method. The as‐transferred Bi2O2Se preserves high quality in contrast to the serious quality degradation in hydrofluoric‐acid‐assisted transfer. The phototransistors show a responsivity of 3.5 × 104 A W−1, a photoconductive gain of more than 104, and a time response in the order of sub‐millisecond. With back gating of the silicon substrate, the dark current can be reduced to several pA. This yields an ultrahigh sensitivity with a specific detectivity of 9.0 × 1013 Jones, which is one of the highest values among 2D material photodetectors and two orders of magnitude higher than that of other CVD‐grown 2D materials. The high performance of the phototransistor shown here together with the developed unique transfer technique are promising for the development of novel 2D‐material‐based optoelectronic applications as well as integrating with state‐of‐the‐art silicon photonic and electronic technologies.

Published in: "Advanced Materials".

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