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1.
Nature ; 582(7813): 511-514, 2020 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-32581381

RESUMO

Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics1-3. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal-oxide-semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends4 the development of dielectrics with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivatives, organic compounds and aerogels) have κ values greater than 2 and poor thermo-mechanical properties5. Here we report three-nanometre-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-κ dielectric characteristics for high-performance electronics.

2.
Nanoscale ; 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32083259

RESUMO

Edge contacts between two-dimensional (2D) materials in the in-plane direction can achieve minimal contact area and low contact resistance, producing atomically thin devices with improved performance. Particularly, lateral heterojunctions of metallic graphene and semiconducting transition metal dichalcogenides (TMDs) exhibit small Schottky barrier heights due to graphene's low work-function. However, issues exist with the fabrication of highly transparent and flexible multi-functional devices utilizing lateral heterostructures (HSs) of graphene and TMDs via spatially controlled growth. This review demonstrates the growth and electronic applications of lateral HSs of graphene and TMDs, highlighting key technologies controlling the wafer-scale growth of continuous films for practical applications. It deepens the understanding of the spatially controlled growth of lateral HSs using chemical vapor deposition methods, and also contributes to the applications that depend on the scale-up of all-2D electronics with ultra-high electrical performance.

3.
Chin J Integr Med ; 26(2): 130-137, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31872368

RESUMO

OBJECTIVE: To determine the effects of Hydrangeae Dulcis Folium (EHDF) on physical stress, changes in the whole-body cortisol level and behaviour in zebrafish (Danio rerio). METHODS: One hundred and seventy-four fish were randomly divided into 4 [adrenocorticotropin hormone (ACTH) challenge test: 4 fish per group] or 6 groups (behavioural test: 10-12 fish per group, whole-body cortisol: 4 fish per group). Net handling stress (NHS) was used to induce physical stress. Fish were treated with vehicle or EHDF (5-20 mg/L) for 6 min before they were exposed to stress. And then, fish were sacrificed for collecting body fluid from whole-body or conducted behavioural tests, including novel tank test and open field test, and were evaluated to observe anxiety-like behaviours and locomotion. In addition, to elucidate the mode of action of the anti-stress effects of EHDF, ACTH (0.2 IU/g, i.p.) challenge test was performed. RESULTS: The increased anxiety-like behaviours in novel tank test and open field test under stress were prevented by treatment with EHDF at 5-20 mg/L (P <0.05). Moreover, compared with the unstressed group, which was not treated with NHS, the whole-body cortisol level was significantly increased by treatment with NHS (P <0.05). Compared with the NHS-treated stressed control group, pre-treatment with EHDF at concentrations of 5-20 mg/L for 6 min significantly prevented the NHS-increased whole-body cortisol level (<0.05). In addition, ACTH challenge test showed that EHDF completely blocked the effects of ACTH on cortisol secretion (P <0.05). CONCLUSION: EHDF may be a good antistress candidate and its mechanism of action may be related to its positive effects on cortisol release.

4.
Food Sci Nutr ; 7(2): 547-553, 2019 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-30847133

RESUMO

To find an economic use of red snow crab (Chionoecetes japonicus) shell waste, we focused on its high mineral content. To evaluate its usability as a coagulant for tofu making, the effects of the crab shell extracts on the textural and sensorial properties of the tofu samples were investigated. The crab shell powder (CSP) and ash (CSA) were used for their extract preparation, and 1%-5% acetic acid treatment led to an abundance of calcium in the resulting extracts. The tofu yields of all the acetic acid extracts were comparable with those of the commercial coagulants MgCl2 and glucono-δ-lactone (GDL). Furthermore, the results for the textural attributes and sensorial acceptability demonstrated that either the extract from CSP prepared with 3% acetic acid or the extracts from CSA prepared with 1% or 3% acetic acid could be used as coagulants, because all the values of the extracts were statistically equivalent to those of the MgCl2 and GDL (p < 0.05).

5.
Nat Commun ; 10(1): 987, 2019 02 25.
Artigo em Inglês | MEDLINE | ID: mdl-30804336

RESUMO

The original version of this Article contained an error in the spelling of the author Matthew Holwill, which was incorrectly given as Mathew Holwill. This has now been corrected in both the PDF and HTML versions of the Article.

6.
Nat Commun ; 10(1): 230, 2019 01 16.
Artigo em Inglês | MEDLINE | ID: mdl-30651554

RESUMO

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.

7.
ACS Nano ; 12(11): 10764-10771, 2018 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-30335961

RESUMO

Hexagonal boron nitride (h-BN) and graphene have emerged as promising materials for proton exchange membranes because of their high proton conductivity and chemical stability. However, the defects and grain boundaries generated during the growth and transfer of two-dimensional materials limit their practical applicability. Here, we report the fabrication of membrane electrode assemblies using large-area single-oriented AA'-stacked trilayer h-BN (3L-BN), which exhibits very few defects during the growth and transfer, as a proton exchange membrane for use in fuel cell systems. The fuel cell based on AA'-stacked 3L-BN showed a H2 permeation current density as low as 2.69 mA cm-2 and an open circuit voltage (OCV) as high as 0.958 V; this performance is much superior to those for cells based on Nafion (3.7 mA cm-2 and 0.942 V, respectively) and single-layer h-BN (10.08 mA cm-2 and 0.894 V, respectively). Furthermore, the fuel cell with the AA'-stacked 3L-BN membrane almost maintained its original performance (OCV, maximum power density, and H2 permeation current density) even after 100 h of an accelerated stress test at 30% RH and 90 °C, while the fuel cells with the Nafion and single-layer BN membranes exhibited severely deteriorated performances. The stability of the cell based on the AA'-stacked 3L-BN membrane was better because the membrane prevented gas crossover and suppressed the generation of reactive radicals during cell operation.

9.
ACS Nano ; 11(7): 7084-7090, 2017 07 25.
Artigo em Inglês | MEDLINE | ID: mdl-28613831

RESUMO

Transition to a commensurate state changes the local symmetry periodicity on two-dimensional van der Waals superstructures, evoking distinctive properties far beyond individual layers. We investigate the morphology of moiré superstructures of graphene on hexagonal boron nitride (hBN) with a low twist angle (≈0°) through moiré fringe analyses with dark field transmission electron microscopy. The moiré fringes exhibit local variation, suggesting that the interaction between graphene and hBN depends on the stacking configuration and that local transition to the commensurate state occurs through the reduced crystalline mismatch (that is, by lattice stretching and twisting on the graphene lattices). This moiré superstructure analysis suggests an inventive method for studying the interaction between stacked van der Waals layers and for discerning the altered electronic and optical properties of graphene on hBN superstructures with a low twist angle, even at low magnification.

10.
ACS Nano ; 11(4): 4041-4050, 2017 04 25.
Artigo em Inglês | MEDLINE | ID: mdl-28363013

RESUMO

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe2/WSe2 vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe2 and WSe2 layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe2 and WSe2.

11.
ACS Nano ; 10(12): 11156-11162, 2016 12 27.
Artigo em Inglês | MEDLINE | ID: mdl-28024355

RESUMO

We report on an insulating two-dimensional material, hexagonal boron nitride (h-BN), which can be used as an effective wrapping layer for surface-enhanced Raman spectroscopy (SERS) substrates. This material exhibits outstanding characteristics such as its crystallinity, impermeability, and thermal conductance. Improved SERS sensitivity is confirmed for Au substrates wrapped with h-BN, the mechanism of which is investigated via h-BN thickness-dependent experiments combined with theoretical simulations. The investigations reveal that a stronger electromagnetic field can be generated at the narrowed gap of the h-BN surface, which results in higher Raman sensitivity. Moreover, the h-BN-wrapped Au substrate shows extraordinary stability against photothermal and oxidative damages. We also describe its capability to detect specific chemicals that are difficult to analyze using conventional SERS substrates. We believe that this concept of using an h-BN insulating layer to protect metallic or plasmonic materials will be widely used not only in the field of SERS but also in the broader study of plasmonic and optoelectronic devices.

12.
ACS Nano ; 10(9): 8973-9, 2016 09 27.
Artigo em Inglês | MEDLINE | ID: mdl-27563804

RESUMO

Transition metal dichalcogenides (TMDs) have recently received increasing attention because of their potential applications in semiconducting and optoelectronic devices exhibiting large optical absorptions in the visible range. However, some studies have reported that the grain boundaries of TMDs can be easily degraded by the presence of oxygen in water and by UV irradiation, ozone, and heating under ambient conditions. We herein demonstrate the photodegradation of WSe2 and MoSe2 by laser exposure (532 nm) and the subsequent prevention of this photodegradation by encapsulation with hexagonal boron nitride (h-BN) layers. The photodegradation was monitored by variation in peak intensities in the Raman and photoluminescence spectra. The rapid photodegradation of WSe2 under air occurred at a laser power of ≥0.5 mW and was not observed to any extent at ≤0.1 mW. However, in the presence of a water droplet, the photodegradation of WSe2 was accelerated and took place even at 0.1 mW. We examined the encapsulation of WSe2 with h-BN and found that this prevented photodegradation. However, a single layer of h-BN was not sufficient to fully prevent this photodegradation, and so a triple layer of h-BN was employed. We also demonstrated that the photodegradation of MoSe2 was prevented by encapsulation with h-BN layers. On the basis of X-ray photoelectron spectroscopy and scanning photoemission microscopy data, we determined that this degradation was caused by the photoinduced oxidation of TMDs. These results can be used to develop a general strategy for improving the stability of 2D materials in practical applications.

13.
Nano Lett ; 16(5): 3360-6, 2016 05 11.
Artigo em Inglês | MEDLINE | ID: mdl-27120101

RESUMO

Large-scale growth of high-quality hexagonal boron nitride has been a challenge in two-dimensional-material-based electronics. Herein, we present wafer-scale and wrinkle-free epitaxial growth of multilayer hexagonal boron nitride on a sapphire substrate by using high-temperature and low-pressure chemical vapor deposition. Microscopic and spectroscopic investigations and theoretical calculations reveal that synthesized hexagonal boron nitride has a single rotational orientation with AA' stacking order. A facile method for transferring hexagonal boron nitride onto other target substrates was developed, which provides the opportunity for using hexagonal boron nitride as a substrate in practical electronic circuits. A graphene field effect transistor fabricated on our hexagonal boron nitride sheets shows clear quantum oscillation and highly improved carrier mobility because the ultraflatness of the hexagonal boron nitride surface can reduce the substrate-induced degradation of the carrier mobility of two-dimensional materials.

14.
Nano Lett ; 15(7): 4769-75, 2015 Jul 08.
Artigo em Inglês | MEDLINE | ID: mdl-26083832

RESUMO

Heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications in 2D materials. Although several methods have been developed to produce this material through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we report herein a new chemical route in which the Pt substrate plays a catalytic role. We propose that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enables the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate.

15.
Nanoscale ; 7(24): 10600-5, 2015 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-25960354

RESUMO

The production of holes by electron beam irradiation in hexagonal boron nitride (hBN), which has a lattice similar to that of graphene, is monitored over time using atomic resolution transmission electron microscopy. The holes appear to be initiated by the formation of a vacancy of boron and grow in a manner that retains an overall triangular shape. The hole growth process involves the formation of single chains of B and N atoms and is accompanied by the ejection of atoms and bundles of atoms along the hole edges, as well as atom migration. These observations are compared to density functional theory calculations and molecular dynamics simulations.

16.
Nano Lett ; 13(4): 1834-9, 2013 Apr 10.
Artigo em Inglês | MEDLINE | ID: mdl-23527543

RESUMO

Hexagonal boron nitride (h-BN) is gaining significant attention as a two-dimensional dielectric material, along with graphene and other such materials. Herein, we demonstrate the growth of highly crystalline, single-layer h-BN on Pt foil through a low-pressure chemical vapor deposition method that allowed h-BN to be grown over a wide area (8 × 25 mm(2)). An electrochemical bubbling-based method was used to transfer the grown h-BN layer from the Pt foil onto an arbitrary substrate. This allowed the Pt foil, which was not consumed during the process, to be recycled repeatedly. The UV-visible absorption spectrum of the single-layer h-BN suggested an optical band gap of 6.06 eV, while a high-resolution transmission electron microscopy image of the same showed the presence of distinct hexagonal arrays of B and N atoms, which were indicative of the highly crystalline nature and single-atom thickness of the h-BN layer. This method of growing single-layer h-BN over large areas was also compatible with use of a sapphire substrate.


Assuntos
Compostos de Boro/química , Grafite/química , Nanopartículas/química , Microscopia Eletrônica de Transmissão , Nanoestruturas/química , Tamanho da Partícula , Platina/química , Propriedades de Superfície
17.
ACS Nano ; 6(10): 9207-13, 2012 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-22980316

RESUMO

Graphene has been functionalized with spiropyran (SP), a well-known photochromic molecule. It has been realized with pyrene-modified SP, which has been adsorbed on graphene by π-π interaction between pyrene and graphene. The field-effect transistor (FET) with SP-functionalized graphene exhibited n-doping effect and interesting optoelectronic behaviors. The Dirac point of graphene in the FET could be controlled by light modulation because spiropyran can be reversibly switched between two different conformations, a neutral form (colorless SP) and a charge-separated form (purple colored merocyanine, MC), on UV and visible light irradiation. The MC form is produced during UV light irradiation, inducing the shift of the Dirac point of graphene toward negative gate voltage. The reverse process back to the neutral SP form occurred under visible light irradiation or in darkness, inducing a shift of the Dirac point toward positive gate voltage. The change of the Dirac point by UV and visible light was reproducibly repeated. SP molecules also improved the conductance change in the FET device. Furthermore, dynamics on conversion from MC to SP on graphene was different from that in solution and solid samples with SP-grafted polymer or that on gold nanoparticles.


Assuntos
Benzopiranos/química , Grafite/química , Indóis/química , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nitrocompostos/química , Refratometria/instrumentação , Transistores Eletrônicos , Desenho de Equipamento , Análise de Falha de Equipamento , Luz , Teste de Materiais , Tamanho da Partícula , Espalhamento de Radiação
18.
J Oleo Sci ; 61(6): 311-9, 2012.
Artigo em Inglês | MEDLINE | ID: mdl-22687776

RESUMO

Detailed fatty acid compositions of five species of the brown algae Sargassum (S. fulvellum, S. horneri, S. boreale, S. thunbergii, and S. yezoense) were determined using silver ion solid phase extraction, gas chromatography (GC), and GC-mass spectrometry (GC-MS) techniques. In addition to a high number of typical saturated and unsaturated fatty acids, the GC-MS spectra of the 4,4-dimethyloxazoline derivatives of fatty acids revealed the occurrence of small amounts of unusual non-methylene-interrupted (NMI) fatty acids with Δ5 unsaturation, namely, 5,9-eicosadienoic (5,9-20:2), 5,11,14-eicosatrienoic (5,11,14-20:3), and 5,11,14,17-eicosatetraenoic (5,11,14,17-20:4) acids. Of these three NMI acids, the 5,9-20:2 acid was found to be the most abundant (0.4%-2.3% of the total fatty acids) and was detected for the first time in algae.


Assuntos
Ácidos Graxos Insaturados/análise , Ácidos Graxos/análise , Sargassum/metabolismo , Algoritmos , Química Farmacêutica/métodos , Ácidos Graxos Insaturados/química , Cromatografia Gasosa-Espectrometria de Massas/métodos , Lipídeos/química , Modelos Químicos , Oxazóis/química , Feófitas/metabolismo , Especificidade da Espécie , Fatores de Tempo
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