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1.
Sci Adv ; 6(12): eaay8163, 2020 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-32219163

RESUMO

The most effective approach to practical exploitation of the layered solids that often have unique valuable properties-such as graphene, clays, and other compounds-is by dispersion into colloidal suspensions of monolayers, called liquid exfoliation. This fundamentally expected behavior can be used to deposit monolayers on supports or to reassemble into hierarchical materials to produce, by design, catalysts, nanodevices, films, drug delivery systems, and other products. Zeolites have been known as extraordinary catalysts and sorbents with three-dimensional structures but emerged as an unexpected new class of layered solids contributing previously unknown valuable features: catalytically active layers with pores inside or across. The self-evident question of layered zeolite exfoliation has remained unresolved for three decades. Here, we report the first direct exfoliation of zeolites into suspension of monolayers as proof of the concept, which enables diverse applications including membranes and hierarchical catalysts with improved access.

2.
Nano Lett ; 19(8): 4974-4980, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31265300

RESUMO

Boron nitride nanotubes (BNNTs) are promising for mechanical applications owing to the high modulus, high strength, and inert chemical nature. However, up to now, precise evaluation of their elastic properties and their relation to defects have not been experimentally established. Herein, the intrinsic elastic modulus of BNNTs and its dependence on intrinsic and deliberately irradiation-induced extrinsic defects have been studied via an electric-field-induced high-order resonance technique inside a high-resolution transmission electron microscope (HRTEM). Resonances up to fourth order for normal modes and third order for parametric modes have been initiated in the cantilevered tubes, and the recorded frequencies are well consistent with the theoretical calculations with a discrepancy of ∼1%. The elastic moduli of the BNNTs measured from high-order resonance is about 906.2 GPa on average, with a standard deviation of 9.3%, which is found to be closely related to the intrinsic defect as cavities in the nanotube walls. Furthermore, electron irradiation in HRTEM has been used to study the effects of defects to elastic moduli and to evaluate the radiation resistance of the BNNTs. Along with an increase in the irradiation dose, the outer diameter has linearly reduced due to the knock-on effects. A defective shell with nearly constant thickness has been formed on the outer surface, and as a result, the elastic modulus decreases gradually to ∼662.9 GPa, which is still 3 times that of steel. Excellent intrinsic elastic properties and decent radiation-resistance prove that BNNTs could be a material of choice for applications in extreme environments, such as those existing in space.

3.
Nat Mater ; 18(1): 62-68, 2019 01.
Artigo em Inglês | MEDLINE | ID: mdl-30455446

RESUMO

Inorganic chalcogenides are traditional high-performance thermoelectric materials. However, they suffer from intrinsic brittleness and it is very difficult to obtain materials with both high thermoelectric ability and good flexibility. Here, we report a flexible thermoelectric material comprising highly ordered Bi2Te3 nanocrystals anchored on a single-walled carbon nanotube (SWCNT) network, where a crystallographic relationship exists between the Bi2Te3 <[Formula: see text]> orientation and SWCNT bundle axis. This material has a power factor of ~1,600 µW m-1 K-2 at room temperature, decreasing to 1,100 µW m-1 K-2 at 473 K. With a low in-plane lattice thermal conductivity of 0.26 ± 0.03 W m-1 K-1, a maximum thermoelectric figure of merit (ZT) of 0.89 at room temperature is achieved, originating from a strong phonon scattering effect. The origin of the excellent flexibility and thermoelectric performance of the Bi2Te3-SWCNT material is attributed, by experimental and computational evidence, to its crystal orientation, interface and nanopore structure. Our results provide insight into the design and fabrication of high-performance flexible thermoelectric materials.

4.
Ultramicroscopy ; 194: 108-116, 2018 11.
Artigo em Inglês | MEDLINE | ID: mdl-30107290

RESUMO

Physical properties of carbon nanotubes (CNTs) are closely related to the atomic structure, i.e. the chirality. It is highly desirable to develop a technique to modify their chirality and control the resultant transport properties. Herein, we present an in situ transmission electron microscopy (TEM) probing method to monitor the chirality transition and transport properties of individual few-walled CNTs. The changes of tube structure including the chirality are stimulated by programmed bias pulses and associated Joule heating. The chirality change of each shell is analyzed by nanobeam electron diffraction. Supported by molecular dynamics simulations, a preferred chirality transition path is identified, consistent with the Stone-Wales defect formation and dislocation sliding mechanism. The electronic transport properties are measured along with the structural changes, via fabricating transistors using the individual nanotubes as the suspended channels. Metal-to-semiconductor transitions are observed along with the chirality changes as confirmed by both the electron diffraction and electrical measurements. Apart from providing an alternative route to control the chirality of CNTs, the present work demonstrates the rare possibility of obtaining the dynamic structure-properties relationships at the atomic and molecular levels.


Assuntos
Nanotubos de Carbono/química , Elétrons , Microscopia Eletrônica de Transmissão/métodos , Simulação de Dinâmica Molecular , Semicondutores
5.
Sci Adv ; 4(5): eaap9264, 2018 05.
Artigo em Inglês | MEDLINE | ID: mdl-29736413

RESUMO

Single-wall carbon nanotubes (SWCNTs) are ideal for fabricating transparent conductive films because of their small diameter, good optical and electrical properties, and excellent flexibility. However, a high intertube Schottky junction resistance, together with the existence of aggregated bundles of SWCNTs, leads to a degraded optoelectronic performance of the films. We report a network of isolated SWCNTs prepared by an injection floating catalyst chemical vapor deposition method, in which crossed SWCNTs are welded together by graphitic carbon. Pristine SWCNT films show a record low sheet resistance of 41 ohm □-1 at 90% transmittance for 550-nm light. After HNO3 treatment, the sheet resistance further decreases to 25 ohm □-1. Organic light-emitting diodes using this SWCNT film as anodes demonstrate a low turn-on voltage of 2.5 V, a high current efficiency of 75 cd A-1, and excellent flexibility. Investigation of isolated SWCNT-based field-effect transistors shows that the carbon-welded joints convert the Schottky contacts between metallic and semiconducting SWCNTs into near-ohmic ones, which significantly improves the conductivity of the transparent SWCNT network. Our work provides a new avenue of assembling individual SWCNTs into macroscopic thin films, which demonstrate great potential for use as transparent electrodes in various flexible electronics.

6.
Nat Mater ; 17(6): 535-542, 2018 06.
Artigo em Inglês | MEDLINE | ID: mdl-29686277

RESUMO

Chemical vapour deposition of two-dimensional materials typically involves the conversion of vapour precursors to solid products in a vapour-solid-solid mode. Here, we report the vapour-liquid-solid growth of monolayer MoS2, yielding highly crystalline ribbons with a width of few tens to thousands of nanometres. This vapour-liquid-solid growth is triggered by the reaction between MoO3 and NaCl, which results in the formation of molten Na-Mo-O droplets. These droplets mediate the growth of MoS2 ribbons in the 'crawling mode' when saturated with sulfur. The locally well-defined orientations of the ribbons reveal the regular horizontal motion of the droplets during growth. Using atomic-resolution scanning transmission electron microscopy and second harmonic generation microscopy, we show that the ribbons are grown homoepitaxially on monolayer MoS2 with predominantly 2H- or 3R-type stacking. Our findings highlight the prospects for the controlled growth of atomically thin nanostructure arrays for nanoelectronic devices and the development of unique mixed-dimensional structures.

7.
Nat Commun ; 9(1): 402, 2018 01 26.
Artigo em Inglês | MEDLINE | ID: mdl-29374156

RESUMO

Tin and its compounds hold promise for the development of high-capacity anode materials that could replace graphitic carbon used in current lithium-ion batteries. However, the introduced porosity in current electrode designs to buffer the volume changes of active materials during cycling does not afford high volumetric performance. Here, we show a strategy leveraging a sulfur sacrificial agent for controlled utility of void space in a tin oxide/graphene composite anode. In a typical synthesis using the capillary drying of graphene hydrogels, sulfur is employed with hard tin oxide nanoparticles inside the contraction hydrogels. The resultant graphene-caged tin oxide delivers an ultrahigh volumetric capacity of 2123 mAh cm-3 together with good cycling stability. Our results suggest not only a conversion-type composite anode that allows for good electrochemical characteristics, but also a general synthetic means to engineering the packing density of graphene nanosheets for high energy storage capabilities in small volumes.

8.
Adv Mater ; 29(28)2017 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-28523720

RESUMO

Engineering of the optical, electronic, and magnetic properties of hexagonal boron nitride (h-BN) nanomaterials via oxygen doping and functionalization has been envisaged in theory. However, it is still unclear as to what extent these properties can be altered using such methodology because of the lack of significant experimental progress and systematic theoretical investigations. Therefore, here, comprehensive theoretical predictions verified by solid experimental confirmations are provided, which unambiguously answer this long-standing question. Narrowing of the optical bandgap in h-BN nanosheets (from ≈5.5 eV down to 2.1 eV) and the appearance of paramagnetism and photoluminescence (of both Stokes and anti-Stokes types) in them after oxygen doping and functionalization are discussed. These results are highly valuable for further advances in semiconducting nanoscale electronics, optoelectronics, and spintronics.

9.
Nano Lett ; 15(8): 4922-7, 2015 Aug 12.
Artigo em Inglês | MEDLINE | ID: mdl-26114583

RESUMO

The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology, and metallurgy of different materials confined in such nanospace. Here, we investigate the nonequilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Because of nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (∼10(13) K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt, and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-to-medium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nanowire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.

10.
Nanotechnology ; 26(15): 154001, 2015 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-25797523

RESUMO

A high-precision technique was utilized to construct and characterize axial nanowire heterojunctions inside a high-resolution transmission electron microscope (HRTEM). By an in-tandem technique using an ultra-sharp tungsten probe as the nanomanipulator and an optical fiber as the optical waveguide the nanoscale CdS/p-Si axial nanowire junctions were fabricated, and in situ photocurrents from them were successfully measured. Compared to a single constituting nanowire, the CdS/p-Si axial nanowire junctions possess a photocurrent saturation effect, which protects them from damage under high voltages. Furthermore, a set of experiments reveals the clear relationship between the saturation photocurrent values and the incident light intensities. The applied technique is expected to be valuable for bottom-up nanodevice fabrications, and the regarded photocurrent saturation feature may solve the Joule heating-induced failure problem in nanowire optoelectronic devices caused by a fluctuating bias.

11.
Nano Lett ; 15(1): 689-94, 2015 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-25555238

RESUMO

We report the comparative in situ fracture toughness testing on single-edge V/U-notched multilayer graphenes and boronitrenes in a high-resolution transmission electron microscope (HRTEM). The nanostructures of notch tips and fracture edges of the tested specimens are unambiguously resolved using HRTEM. By analyzing the notch tip stresses using finite element method, the fracture toughness of multilayer graphenes and boronitrenes is determined to be 12.0 ± 3.9 and 5.5 ± 0.7 MPa√m, respectively, taking into account the notch tip blunting effects.

12.
Nanoscale ; 6(14): 8084-90, 2014 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-24915978

RESUMO

CdS/ZnO branched heterostructures have been successfully synthesized by combining thermal vapour deposition and a hydrothermal method. Drastic optoelectronic performance enhancement of such heterostructures was revealed, compared to plain CdS nanobelts, as documented by comparative in situ optoelectronic studies on corresponding individual nanostructures using an originally designed laser-compatible transmission electron microscopy (TEM) technique. Furthermore, flexible thin-film based photodetectors based on standard CdS nanobelts and newly prepared CdS/ZnO heterostructures were fabricated on PET substrates, and comparative photocurrent and photo-responsivity measurements thoroughly verified the in situ TEM results. The CdS/ZnO branched heterostructures were found to have better performance than standard CdS nanobelts for optoelectronic applications with respect to the photocurrent to dark current ratio and responsivity.

13.
Adv Mater ; 26(27): 4659-64, 2014 Jul 16.
Artigo em Inglês | MEDLINE | ID: mdl-24861825

RESUMO

Novel cathodes based on Sb-doped tin oxide (STO)-supported Ru particles enable Li-O2 batteries to be operated below 4.0 V, which is of crucial importance for the realization of rechargeable Li-O2 batteries, and to deliver a high specific capacity of 750 mA h g(-1) even after 50 discharge-charge cycles at 0.1 mA cm(-2) .


Assuntos
Antimônio/química , Fontes de Energia Elétrica , Lítio/química , Nanopartículas Metálicas/química , Oxigênio/química , Rutênio/química , Compostos de Estanho/química , Eletrodos
14.
Nat Commun ; 5: 3631, 2014 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-24698887

RESUMO

The discovery of two-dimensional materials became possible due to the mechanical cleavage technique. Despite its simplicity, the as-cleaved materials demonstrated surprising macro-continuity, high crystalline quality and extraordinary mechanical and electrical properties that triggered global research interest. Here such cleavage processes and associated mechanical behaviours are investigated by a direct in situ transmission electron microscopy probing technique, using atomically thin molybdenum disulphide layers as a model material. Our technique demonstrates layer number selective cleavage, from a monolayer to double layer and up to 23 atomic layers. In situ observations combined with molecular dynamics simulations reveal unique layer-dependent bending behaviours, from spontaneous rippling (<5 atomic layers) to homogeneous curving (~ 10 layers) and finally to kinking (20 or more layers), depending on the competition of strain energy and interfacial energy.

15.
Nano Lett ; 14(3): 1164-71, 2014 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-24479759

RESUMO

Distinct from pure graphene, N-doped graphene (GN) has been found to possess high rate capability and capacity for lithium storage. However, there has still been a lack of direct experimental evidence and fundamental understanding of the storage mechanisms at the atomic scale, which may shed a new light on the reasons of the ultrafast lithium storage property and high capacity for GN. Here we report on the atomistic insights of the GN energy storage as revealed by in situ transmission electron microscopy (TEM). The lithiation process on edges and basal planes is directly visualized, the pyrrolic N "hole" defect and the perturbed solid-electrolyte-interface configurations are observed, and charge transfer states for three N-existing forms are also investigated. In situ high-resolution TEM experiments together with theoretical calculations provide a solid evidence that enlarged edge {0002} spacings and surface hole defects result in improved surface capacitive effects and thus high rate capability and the high capacity are owing to short-distance orderings at the edges during discharging and numerous surface defects; the phenomena cannot be understood previously by standard electron or X-ray diffraction analyses.

16.
Small ; 10(4): 685-93, 2014 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-24030778

RESUMO

Boron nanowires (BNWs) are considered as an ideal optoelectronic nanomaterial, but controlling them in identical growth mode and large-area patterns is technically challenging. Here, large-scale BNW patterns with a uniform base-up growth mode are successfully fabricated by choosing Ni film as the catalyst. Moreover, they exhibit low turn-on field (4.3 V/µm) and excellent field emission uniformity (88%).

17.
ACS Nano ; 8(1): 292-301, 2014 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-24354297

RESUMO

We report a simple, versatile in situ transmission electron microscopy (TEM) approach for investigating the nucleation and growth mechanism of carbon nanotubes (CNTs), by which the composition, phase transition, and physical state of various catalysts can be clearly resolved. In our approach, catalyst nanoparticles (NPs) are placed in a multiwall CNT "tubular furnace" with two open ends, and a high temperature is obtained by Joule heating in the specimen chamber of a TEM. The carbon is supplied by electron irradiation-induced injection of carbon atoms. Comparative studies on the catalytic behavior of traditional iron oxide and recently discovered gold catalysts were performed. It was found that the growth of CNTs from iron oxide involves the reduction of Fe2O3 to Fe3C, nucleation and growth of CNTs from partially liquefied Fe3C, and finally the formation of elemental Fe when the growth stops. In contrast, while changes in shape, size, and orientation were also observed for the fluctuating Au NPs, no chemical reactions or phase transitions occurred during the nucleation of CNTs. These two distinct nucleation and growth processes and mechanisms would be valuable for the structure-controlled growth of CNTs by catalyst design and engineering.

18.
Sci Rep ; 3: 2975, 2013 Oct 17.
Artigo em Inglês | MEDLINE | ID: mdl-24131954

RESUMO

A small volumetric capacitance resulting from a low packing density is one of the major limitations for novel nanocarbons finding real applications in commercial electrochemical energy storage devices. Here we report a carbon with a density of 1.58 g cm(-3), 70% of the density of graphite, constructed of compactly interlinked graphene nanosheets, which is produced by an evaporation-induced drying of a graphene hydrogel. Such a carbon balances two seemingly incompatible characteristics: a porous microstructure and a high density, and therefore has a volumetric capacitance for electrochemical capacitors (ECs) up to 376 F cm(-3), which is the highest value so far reported for carbon materials in an aqueous electrolyte. More promising, the carbon is conductive and moldable, and thus could be used directly as a well-shaped electrode sheet for the assembly of a supercapacitor device free of any additives, resulting in device-level high energy density ECs.

19.
ACS Nano ; 7(11): 10112-20, 2013 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-24093621

RESUMO

Boron nanowires (BNWs) may have potential applications as reinforcing materials because B fibers are widely known for their excellent mechanical performance. However until now, there have been only few reports on the mechanical properties of individual BNW, and in situ transmission electron microscopy (TEM) investigations shining a light on their fracture mechanism have not been performed. In this paper, we applied in situ high-resolution TEM (HRTEM) technique to study the mechanical properties of individual BNWs using three loading schemes. The mean fracture strength and the maximum strain of individual BNWs were measured to be 10.4 GPa and 4.1%, respectively, during the tensile tests. And the averaged Young's modulus was calculated to be 308.2 GPa under tensile and compression tests. Bending experiments for the first time performed on individual BNWs revealed that their maximum bending strain could reach 9.9% and their ultimate bending stress arrived at 36.2 GPa. These figures are much higher than those of Si and ZnO nanowires known for their high bending strength. Moreover, the BNWs exhibited very high specific fracture strength (3.9 (GPa·cm(3))/g) and specific elastic modulus (130.6 (GPa·cm(3))/g), which are several dozens of times larger compared to many nanostructures known for their superb mechanical behaviors. At last, the effect of surface oxide layer on the Young's modulus, fracture strength and maximum bending strength of individual BNWs was elucidated to extract their intrinsic mechanical parameters using calculated corrections. All experimental results suggest that the present BNW are a bright promise as lightweight reinforcing fillers.

20.
Nano Lett ; 13(10): 4702-7, 2013 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-24063602

RESUMO

Ru nanoparticles deposited on a conductive support indium tin oxide (Ru/ITO) were applied as a carbon-free cathode in a nonaqueous Li-O2 battery. The Li-O2 battery with Ru/ITO showed much lower charging overpotentials and better cycling performance at 0.15 mA/cm(2) than those with Super P (SP) and SP loaded with Ru nanoparticles (Ru/SP) as the cathodes. The carbon-free cathode Ru/ITO can effectively reduce formation of Li2CO3 or other Li carbonates in a discharging process, which cannot be completely decomposed upon charging, in comparison with the carbon based cathode. The improved performance of Ru/ITO can be attributed to the superior catalytic activity of Ru nanoparticles toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the absence of carbon that has been reported to react with Li2O2 to form Li2CO3.

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