Graphene

/Tag: Graphene

Valley filtering effect of phonons in graphene with a grain boundary. (arXiv:1902.07372v1 [cond-mat.mes-hall])

2019-02-21T04:30:34+00:00February 21st, 2019|Categories: Publications|Tags: , |

Due to their possibility to encode information and realize low-energy-consumption quantum devices, control and manipulation of the valley degree of freedom have been widely studied in electronic systems. In contrast, the phononic counterpart–valley phononics–has been largely unexplored, despite the importance in both fundamental science and practical applications. In this work, we demonstrate that the control of “valleys” is also applicable for phonons in graphene by using a grain boundary. In particular, perfect valley filtering effect is observed at certain energy windows for flexural modes and found to be closely related to the anisotropy of phonon valley pockets. Moreover, valley filtering may be further improved using Fano-like resonance. Our findings reveal the possibility of valley phononics, paving the road towards purposeful phonon engineering and future valley phononics.

Published : "arXiv Mesoscale and Nanoscale Physics".

Marginally Self-Averaging One-Dimensional Localization in Bilayer Graphene. (arXiv:1902.07428v1 [cond-mat.mes-hall])

2019-02-21T04:30:28+00:00February 21st, 2019|Categories: Publications|Tags: |

The combination of field tunable bandgap, topological edge states, and valleys in the band structure, makes insulating bilayer graphene a unique localized system, where the scaling laws of dimensionless conductance g remain largely unexplored. Here we show that the relative fluctuations in ln g with the varying chemical potential, in strongly insulating bilayer graphene (BLG) decay nearly logarithmically for channel length up to L/${xi}$ ${approx}$ 20, where ${xi}$ is the localization length. This ‘marginal’ self averaging, and the corresponding dependence of <ln g> on L, suggest that transport in strongly gapped BLG occurs along strictly one-dimensional channels, where ${xi}$ ${approx}$ 0.5${pm}$0.1 ${mu}$m was found to be much longer than that expected from the bulk bandgap. Our experiment reveals a nontrivial localization mechanism in gapped BLG, governed by transport along robust edge modes.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electrochemical Mechanism Investigation of Cu2MoS4 Hollow Nanospheres for Fast and Stable Sodium Ion Storage

2019-02-21T02:33:04+00:00February 21st, 2019|Categories: Publications|Tags: , |

Bimetallic Cu2MoS4 (CMS) hollow nanospheres (CMS1) are obtained by modified solvothermal reactions. CMS1 delivers notably improved rate performance than CMS in ether‐electrolyte, in which an intercalation‐dominant electrochemical reaction mechanism is revealed. With further incorporation of reduced graphene oxide (rGO), the CMS1–rGO composites deliver excellent cycling stability, allowing fast and stable Na+ storage in full cell by coupling with an optimized Na3V2(PO4)3 cathode. Abstract Sodium ion batteries (SIBs) are promising alternatives to lithium ion batteries with advantages of cost effectiveness. Metal sulfides as emerging SIB anodes have relatively high electronic conductivity and high theoretical capacity, however, large volume change during electrochemical testing often leads to unsatisfactory electrochemical performance. Herein bimetallic sulfide Cu2MoS4 (CMS) with layered crystal structures are prepared with glucose addition (CMS1), resulting in the formation of hollow nanospheres that endow large interlayer spacing, benefitting the rate performance and cycling stability. The electrochemical mechanisms of CMS1 are investigated using ex situ X‐ray photoelectron spectroscopy and in situ X‐ray absorption spectroscopy, revealing the conversion‐based mechanism in carbonate electrolyte and intercalation‐based mechanism in ether‐electrolyte, thus allowing fast and reversible Na+ storage. With further introduction of reduced graphene oxide (rGO), CMS1–rGO composites are obtained, maintaining the hollow structure of CMS1. CMS1–rGO delivers excellent rate performance (258 mAh g−1 at 50 mA g−1 and 131.9 mAh g−1 at 5000 mA g−1) and notably enhanced cycling stability (95.6% after 2000 cycles). A full cell SIB is assembled by coupling CMS1–rGO with Na3V2(PO4)3‐based cathode, delivering excellent cycling stability (75.5% after 500 cycles). The excellent rate performance

Published in: "Advanced Functional Materials".

Bioinspired Superhydrophobic Papillae with Tunable Adhesive Force and Ultralarge Liquid Capacity for Microdroplet Manipulation

2019-02-21T02:32:44+00:00February 21st, 2019|Categories: Publications|Tags: , |

Inspired by the multiscale papillae on the rose petal, a template‐free 3D‐shrinking method is developed. The obtained bioinspired papillae array exhibits strong superhydrophobicity (CA > 170°), ultralarge liquid capacity (25 µL), and tunable adhesive force (39.2–129.4 µN). Its application for programmable transfer of microdroplets and for multistep microreaction platforms is demonstrated. Abstract Template‐free, highly efficient, and large‐area construction of complex multiscale architectures is still a great challenge for microfabrications. Inspired by the hierarchical micropapillae on the superhydrophobic surface of natural rose petals, here, a facile 3D shrinking method is reported to build a graphene oxide (GO) papillae array. Circular GO speckles with a gradient of thickness are deposited on an inflated latex balloon through the water‐evaporation‐driven assembly of GO nanosheets, which then shrink into hierarchical papillae under compressive stresses upon deflation. The fluoroalkylsilane modified GO papillae array exhibits a combined performance of strong superhydrophobicity (CA > 170°), tunable adhesive force (39.2–129.4 µN), and ultralarge liquid capacity (25 µL). The wetting states (Wenzel, Cassie‐I, and Cassie‐Baxter), the adhesive forces, and the liquid capacities all can be tuned by varying the buckling topography (microwrinkle or microfold), the papillae number (3, 4, 6, or 7), and the array arrangement (triangle, square, or hexagon). For one single papillae, the highest adhesive force and the highest liquid capacity incresed to a record breaking value of 26.5 µN and 4.2 µL, respectively, which are promising for programmable manipulations of microdroplets and relevant for multistep microreactions.

Published in: "Advanced Functional Materials".

Scalable Production of Graphene Inks via Wet‐Jet Milling Exfoliation for Screen‐Printed Micro‐Supercapacitors

2019-02-21T02:32:35+00:00February 21st, 2019|Categories: Publications|Tags: , |

Single‐/few‐layer graphene flakes, produced via scalable wet‐jet milling exfoliation of graphite, are used as the active material for micro‐supercapacitors (MSCs). The formulation of aqueous‐alcohol graphene inks, as well as the addition of carbon nanotubes, allows interdigitated structure to be screen‐printed on plastic substrates for metal‐free, flexible, solid‐state, and washable MSCs with high areal performance. Abstract The miniaturization of energy storage units is pivotal for the development of next‐generation portable electronic devices. Micro‐supercapacitors (MSCs) hold great potential to work as on‐chip micro‐power sources and energy storage units complementing batteries and energy harvester systems. Scalable production of supercapacitor materials with cost‐effective and high‐throughput processing methods is crucial for the widespread application of MSCs. Here, wet‐jet milling exfoliation of graphite is reported to scale up the production of graphene as a supercapacitor material. The formulation of aqueous/alcohol‐based graphene inks allows metal‐free, flexible MSCs to be screen‐printed. These MSCs exhibit areal capacitance (C areal) values up to 1.324 mF cm−2 (5.296 mF cm−2 for a single electrode), corresponding to an outstanding volumetric capacitance (C vol) of 0.490 F cm−3 (1.961 F cm−3 for a single electrode). The screen‐printed MSCs can operate up to a power density above 20 mW cm−2 at an energy density of 0.064 µWh cm−2. The devices exhibit excellent cycling stability over charge–discharge cycling (10 000 cycles), bending cycling (100 cycles at a bending radius of 1 cm) and folding (up to angles of 180°). Moreover, ethylene vinyl acetate‐encapsulated MSCs retain their electrochemical properties after a home‐laundry cycle, providing waterproof and

Published in: "Advanced Functional Materials".

Chemical Vapor Deposition Growth of Boron-Carbon-Nitrogen layers from Methylamine Borane Thermolysis Products. (arXiv:1902.07525v1 [cond-mat.mtrl-sci])

2019-02-21T02:29:26+00:00February 21st, 2019|Categories: Publications|Tags: |

This work investigates the growth of B-C-N layers by chemical vapor deposition using methylamine borane (MeAB) as single-source precursor. MeAB has been synthesized and characterized, paying particular attention to the analysis of its thermolysis products, which are the gaseous precursors for B-C-N growth. Samples have been grown on Cu foils and transferred onto different substrates for their morphological, structural, chemical, electronic and optical characterizations. The results of these characterizations indicate a segregation of h-BN and Graphene-like (Gr) domains. However, there is an important presence of B and N interactions with C at the Gr borders, and of C interacting at the h-BN-edges, respectively, in the obtained nano-layers. In particular, there is significant presence of C-N bonds, at Gr/h-BN borders and in the form of N doping of Gr domains. The overall B:C:N contents in the layers is close to 1:3:1.5. A careful analysis of the optical bandgap determination of the obtained B-C-N layers is presented, discussed and compared with previous seminal works with samples of similar composition.

Published in: "arXiv Material Science".

What retards the response of graphene based gaseous sensor. (arXiv:1902.07543v1 [cond-mat.mes-hall])

2019-02-21T02:29:23+00:00February 21st, 2019|Categories: Publications|Tags: |

Graphene based sensor to gas molecules should be ultrasensitive and ultrafast because of the single-atomic thickness of graphene, while the response is not fast. Usually, the measured response time for many molecules, such as CO, NH3, SO2, CO2 and NO2 and so on, is on the scale of minutes or longer. In the present work, we found via emph{ab initio} calculations there exists a potential barrier larger than 0.7 eV that hinders the gas molecule to land directly at the defective sites of graphene and retards the response. An efficient approach to the problem is suggested as modifying the graphene sheet with other molecules to reduce the potential barrier and was demonstrated by a graphene sheet modified by Fe2O3 molecules that shows fast response to H2S molecule, and the calculated response time is close to the measured one, 500 $mu$s.

Published in: "arXiv Material Science".

Exploring Approaches for the Synthesis of Few‐Layered Graphdiyne

2019-02-21T00:38:30+00:00February 20th, 2019|Categories: Publications|Tags: , |

The state‐of‐art research of graphdiyne (GDY) and focus on exploring approaches for few‐layered GDY synthesis are critically summarized. The obstacles and challenges of GDY synthesis are also analyzed in detail. The advantages and limitations of different methods are analyzed comprehensively. These synthetic methods provide considerable inspiration to approaching the synthesis of single/few‐layered GDY film. Abstract Graphdiyne (GDY) is an emerging carbon allotrope in the graphyne (GY) family, demonstrating extensive potential applications in the fields of electronic devices, catalysis, electrochemical energy storage, and nonlinear optics. Synthesis of few‐layered GDY is especially important for both electronic applications and structural characterization. This work critically summarizes the state‐of‐art of GDY and focuses on exploring approaches for few‐layered GDY synthesis. The obstacles and challenges of GDY synthesis are also analyzed in detail. Recently developed synthetic methods are discussed such as i) the copper substrate‐based method, ii) the chemical vapor deposition (CVD) method, iii) the interfacial construction method, and iv) the graphene‐templated method. Throughout the discussion, the superiorities and limitations of different methods are analyzed comprehensively. These synthetic methods have provided considerable inspiration approaching synthesis of few‐layered or single‐layered GDY film. The work concludes with a perspective on promising research directions and remaining barriers for layer‐controlled and morphology‐controlled synthesis of GDY with higher crystalline quality.

Published in: "Advanced Materials".

Ultrathin Cobalt Oxide Layers as Electrocatalysts for High‐Performance Flexible Zn–Air Batteries

2019-02-21T00:37:54+00:00February 20th, 2019|Categories: Publications|Tags: , |

Ultrathin cobalt oxide layers on the surface of a metallic cobalt/N‐doped graphene substrate synergistically enhance the electrical conductivity and catalytic activity, leading to high performance in oxygen reactions under alkaline conditoins. A flexible Zn‐air battery built with this electrocatalyst has an ultrahigh output power capability and a record‐high specific power of 300 W gcat−1, which is essential for portable devices. Abstract Synergistic improvements in the electrical conductivity and catalytic activity for the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) are of paramount importance for rechargeable metal–air batteries. In this study, one‐nanometer‐scale ultrathin cobalt oxide (CoOx) layers are fabricated on a conducting substrate (i.e., a metallic Co/N‐doped graphene substrate) to achieve superior bifunctional activity in both the ORR and OER and ultrahigh output power for flexible Zn–air batteries. Specifically, at the atomic scale, the ultrathin CoOx layers effectively accelerate electron conduction and provide abundant active sites. X‐ray absorption spectroscopy reveals that the metallic Co/N‐doped graphene substrate contributes to electron transfer toward the ultrathin CoOx layer, which is beneficial for the electrocatalytic process. The as‐obtained electrocatalyst exhibits ultrahigh electrochemical activity with a positive half‐wave potential of 0.896 V for ORR and a low overpotential of 370 mV at 10 mA cm−2 for OER. The flexible Zn–air battery built with this catalyst exhibits an ultrahigh specific power of 300 W gcat−1, which is essential for portable devices. This work provides a new design pathway for electrocatalysts for high‐performance rechargeable metal–air battery systems.

Published in: "Advanced Materials".

Interface‐Driven Partial Dislocation Formation in 2D Heterostructures

2019-02-21T00:37:52+00:00February 20th, 2019|Categories: Publications|Tags: , , |

Herein, extended 1D AB–AC stacking boundaries in WS2 are fabricated by using morphological defects on graphene substrates, such as graphene wrinkles. When the perfect basal dislocation direction is more perpendicular to the wrinkles, high‐density stacking boundaries are induced, due to the anisotropic friction of wrinkles. Abstract Van der Waals (vdW) epitaxy allows the fabrication of various heterostructures with dramatically released lattice matching conditions. This study demonstrates interface‐driven stacking boundaries in WS2 using epitaxially grown tungsten disulfide (WS2) on wrinkled graphene. Graphene wrinkles function as highly reactive nucleation sites on WS2 epilayers; however, they impede lateral growth and induce additional stress in the epilayer due to anisotropic friction. Moreover, partial dislocation‐driven in‐plane strain facilitates out‐of‐plane buckling with a height of 1 nm to release in‐plane strain. Remarkably, in‐plane strain relaxation at partial dislocations restores the bandgap to that of monolayer WS2 due to reduced interlayer interaction. These findings clarify significant substrate morphology effects even in vdW epitaxy and are potentially useful for various applications involving modifying optical and electronic properties by manipulating extended 1D defects via substrate morphology control.

Published in: "Advanced Materials".

Recent Advances in 2D Inorganic Nanomaterials for SERS Sensing

2019-02-21T00:37:49+00:00February 20th, 2019|Categories: Publications|Tags: , |

Layered 2D inorganic nanomaterials are emerging as high‐performance materials for surface‐enhanced Raman scattering (SERS)‐based chemical sensors, due to their tunable physicochemical and enhanced charge‐transfer properties. Recent advances in the application of various layered 2D nanomaterials for SERS chemical sensing are reviewed, and the perspectives for the future development of SERS chemical sensors using 2D nanomaterials are discussed. Abstract Surface‐enhanced Raman spectroscopy is a powerful and sensitive analytical tool that has found application in chemical and biomolecule analysis and environmental monitoring. Since its discovery in the early 1970s, a variety of materials ranging from noble metals to nanostructured materials have been employed as surface enhanced Raman scattering (SERS) substrates. In recent years, 2D inorganic materials have found wide use in the development of SERS‐based chemical sensors owing to their unique thickness dependent physico‐chemical properties with enhanced chemical‐based charge‐transfer processes. Here, recent advances in the application of various 2D inorganic nanomaterials, including graphene, boron nitride, semiconducting metal oxides, and transition metal chalcogenides, in chemical detection via SERS are presented. The background of the SERS concept, including its basic theory and sensing mechanism, along with the salient features of different nanomaterials used as substrates in SERS, extending from monometallic nanoparticles to nanometal oxides, is comprehensively discussed. The importance of 2D inorganic nanomaterials in SERS enhancement, along with their application toward chemical detection, is explained in detail with suitable examples and illustrations. In conclusion, some guidelines are presented for the development of this promising field in the future.

Published in: "Advanced Materials".

Graphene Oxide: Realizing Ultralow Concentration Gelation of Graphene Oxide with Artificial Interfaces (Adv. Mater. 8/2019)

2019-02-21T00:37:46+00:00February 20th, 2019|Categories: Publications|Tags: , |

A cocktail preparation is employed by Wei Lv, Jia Li, Quan‐Hong Yang, and co‐workers in article number 1805075 to demonstrate the ultralow‐concentration gelation of graphene oxide (GO) in a water–isopropyl‐alcohol mixture, where the two different colored liquids represent water and isopropyl alcohol respectively. Microphase separation takes place, due to the different intercalation energies of the GO in the two liquids, generating artificial liquid–liquid interfaces that facilitate interactions between the sheets and the formation of a three‐dimensional graphene network.

Published in: "Advanced Materials".

Adhesive Hemostatic Conducting Injectable Composite Hydrogels with Sustained Drug Release and Photothermal Antibacterial Activity to Promote Full‐Thickness Skin Regeneration During Wound Healing

2019-02-20T22:34:45+00:00February 20th, 2019|Categories: Publications|Tags: , |

A series of hydrogel dressings with multifunctions including adhesive hemostatic antioxidative conductive photothermal antibacterial property based on hyaluronic acid‐graft‐dopamine and reduced graphene oxide (rGO) with a H2O2/HPR (horseradish peroxidase) system is prepared and the high promotion repair effect for full‐thickness skin wound regeneration confirms their great potential for clinical application. Abstract Developing injectable nanocomposite conductive hydrogel dressings with multifunctions including adhesiveness, antibacterial, and radical scavenging ability and good mechanical property to enhance full‐thickness skin wound regeneration is highly desirable in clinical application. Herein, a series of adhesive hemostatic antioxidant conductive photothermal antibacterial hydrogels based on hyaluronic acid‐graft‐dopamine and reduced graphene oxide (rGO) using a H2O2/HPR (horseradish peroxidase) system are prepared for wound dressing. These hydrogels exhibit high swelling, degradability, tunable rheological property, and similar or superior mechanical properties to human skin. The polydopamine endowed antioxidant activity, tissue adhesiveness and hemostatic ability, self‐healing ability, conductivity, and NIR irradiation enhanced in vivo antibacterial behavior of the hydrogels are investigated. Moreover, drug release and zone of inhibition tests confirm sustained drug release capacity of the hydrogels. Furthermore, the hydrogel dressings significantly enhance vascularization by upregulating growth factor expression of CD31 and improve the granulation tissue thickness and collagen deposition, all of which promote wound closure and contribute to a better therapeutic effect than the commercial Tegaderm films group in a mouse full‐thickness wounds model. In summary, these adhesive hemostatic antioxidative conductive hydrogels with sustained drug release property to promote complete skin regeneration are an excellent wound dressing for full‐thickness skin repair.

Published in: "Small".

Trifunctional Electrocatalysis on Dual‐Doped Graphene Nanorings–Integrated Boxes for Efficient Water Splitting and Zn–Air Batteries

2019-02-20T22:33:34+00:00February 20th, 2019|Categories: Publications|Tags: , |

An easy yet robust route, for the first time, is developed to craft N, O‐codoped graphene‐integrated boxes by employing hybrids containing polymers and Prussian blue analogue cubes as precursors. Benefiting from the hierarchically porous nanostructures and highly active N, O‐codoped graphene, the resulting electrocatalysts display excellent performance on the overall water splitting and Zn–air battery. Abstract Despite the exciting achievements made in synthesis of monofunctional electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), or hydrogen evolution reaction (HER), it is challenging to develop trifunctional electrocatalysts for both ORR/OER/HER. Herein, N, O‐codoped graphene nanorings‐integrated boxes (denoted NOGB) are crafted via high‐temperature pyrolysis and following acid etching of hybrid precursors containing polymers and Prussian blue analogue cubes. The electrochemical results signified that the resulting NOGB‐800 (800 refers to pyrolysis temperature) is highly active for trifunctional electrocatalysis of ORR/OER/HER. This can be reasonably attributed to the advanced nanostructures (i.e., the hierarchically porous nanostructures on the hollow nanorings) and unique chemical compositions (i.e., N, O‐codoped graphene). More attractively, the rechargeable Zn–air battery based on NOGB‐800 displays maximum power density of 111.9 mW cm−2 with small charge–discharge potential of 0.72 V and excellent stability of 30 h, comparable with the Pt/C+Ir/C counterpart. The NOGB‐800 could also be utilized as bifunctional electrocatalysts for overall water splitting to yield current density of 10 mA cm−2 at a voltage of 1.65 V, surpassing most reported electrocatalysts. Therefore, the NOGB‐800 is a promising candidate instead of precious metal–based electrocatalysts for the efficient Zn–air battery and water splitting.

Published in: "Advanced Energy Materials".

Bioinspired Superhydrophobic Papillae with Tunable Adhesive Force and Ultralarge Liquid Capacity for Microdroplet Manipulation

2019-02-20T22:32:54+00:00February 20th, 2019|Categories: Publications|Tags: , |

Inspired by the multiscale papillae on the rose petal, a template‐free 3D‐shrinking method is developed. The obtained bioinspired papillae array exhibits strong superhydrophobicity (CA > 170°), ultralarge liquid capacity (25 µL), and tunable adhesive force (39.2–129.4 µN). Its application for programmable transfer of microdroplets and for multistep microreaction platforms is demonstrated. Abstract Template‐free, highly efficient, and large‐area construction of complex multiscale architectures is still a great challenge for microfabrications. Inspired by the hierarchical micropapillae on the superhydrophobic surface of natural rose petals, here, a facile 3D shrinking method is reported to build a graphene oxide (GO) papillae array. Circular GO speckles with a gradient of thickness are deposited on an inflated latex balloon through the water‐evaporation‐driven assembly of GO nanosheets, which then shrink into hierarchical papillae under compressive stresses upon deflation. The fluoroalkylsilane modified GO papillae array exhibits a combined performance of strong superhydrophobicity (CA > 170°), tunable adhesive force (39.2–129.4 µN), and ultralarge liquid capacity (25 µL). The wetting states (Wenzel, Cassie‐I, and Cassie‐Baxter), the adhesive forces, and the liquid capacities all can be tuned by varying the buckling topography (microwrinkle or microfold), the papillae number (3, 4, 6, or 7), and the array arrangement (triangle, square, or hexagon). For one single papillae, the highest adhesive force and the highest liquid capacity incresed to a record breaking value of 26.5 µN and 4.2 µL, respectively, which are promising for programmable manipulations of microdroplets and relevant for multistep microreactions.

Published in: "Advanced Functional Materials".

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