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Lattice vibrations of {gamma}- and b{eta}-coronene from Raman and theory. (arXiv:1910.04101v1 [cond-mat.mtrl-sci])

Publications on 2D Materials - Thu, 10/10/2019 - 10:29am

We combine polarization-resolved low frequency Raman microscopy and dispersion-corrected density-functional calculations (DFT-D3) to study polymorph-dependent lattice vibrations in coronene, a model molecular system for nanographenes and disc-like organic semiconductors that exhibits two crystalline structures with distinct electronic and optical properties. Changes in low energy Raman-active lattice phonons are followed across the {gamma}- to b{eta}-phase transition at 150 K. Raman frequencies calculated using DFT-D3 agree to within 4 cm-1, and on the basis of polarisation dependence of peak positions and intensities we achieve a clear mode assignment. Studies of the associated atomic motions show how the pure librational and rotational modes of {gamma}-coronene change into mixed roto-librations in the b{eta}-phase, explaining the remarkable differences in Raman spectra of the two phases.

Published in: "arXiv Material Science".

The post Lattice vibrations of {gamma}- and b{eta}-coronene from Raman and theory. (arXiv:1910.04101v1 [cond-mat.mtrl-sci]) appeared first on 2D Research.

From Highly Purified Boron Nitride to Boron Nitride‐Based Heterostructures: An Inorganic Precursor‐Based Strategy

Publications on 2D Materials - Thu, 10/10/2019 - 8:32am

A simple yet highly efficient approach is developed for the fabrication of high‐purity, crystalline, nanoporous hexagonal boron nitrides (h‐BN) from inorganic starting materials. h‐BN/carbon heterostructures exhibiting improved supercapacitive behavior are also created. Abstract The current approaches used to fabricate hexagonal boron nitrides (h‐BN) from boron trioxide and urea always results in contamination of the h‐BN product with carbon/oxygen. Thus, discovering a facile way of mass producing high‐purity h‐BN remains a challenge. A simple yet highly efficient thermal treatment approach to large‐scale fabrication of nanoporous h‐BN with high yield, high purity, and high crystallinity is described using NaNH2 and NaBH4 as the oxygen‐ and carbon‐free precursors. The unique properties of inorganic metal salts, i.e., high melting point and strong electrostatic interaction with carbon substrates, render this strategy suitable for the production of homogeneous h‐BN/mesoporous carbon and h‐BN/carbon nanotube heterostructures of high crystallinity, high h‐BN dispersity, and with a strong interfacial effect. These unique features make them promising candidates for supercapacitor applications, resulting a significantly enhanced specific capacitance. This study provides new insight into the fabrication of high‐purity h‐BN and h‐BN‐based heterostructures thus expanding their application in the field of energy storage and transformation.

Published in: "Advanced Functional Materials".

The post From Highly Purified Boron Nitride to Boron Nitride‐Based Heterostructures: An Inorganic Precursor‐Based Strategy appeared first on 2D Research.

Ultrasensitive and Highly Stretchable Multifunctional Strain Sensors with Timbre‐Recognition Ability Based on Vertical Graphene

Publications on 2D Materials - Thu, 10/10/2019 - 8:32am

An ultrasensitive and highly stretchable multifunctional strain sensor with timbre‐recognition ability based on high‐crack‐density vertical graphene is fabricated using an ultrasonic peeling method. The strain sensor has a gauge factor of 22 000 at a strain of 100%, and can distinguish frequencies of sounds higher than 2500 Hz. Abstract Stretchable/wearable strain sensors are attracting growing interest due to their broad applications in physical and physiological measurements. However, the development of a multifunctional highly stretchable sensor satisfying the requirements of ultrahigh sensitivity (able to distinguish sound frequency) remains a challenge. An ultrasensitive and highly stretchable multifunctional strain sensor with timbre‐recognition ability based on high‐crack‐density vertical graphene (VGr) is fabricated using an ultrasonic peeling (UP) method. It can distinguish frequencies of sounds higher than 2500 Hz. Detailed microscopic examinations reveal that their ultrahigh sensitivity stems from the formation of high‐density nanocracks in the graphitic base layer, which is bridged by the top branched VGr nanowalls. These nanocracks cut the VGr film into a large number of nanopieces, which increase the natural frequency of the sensors, enabling the sensors to distinguish the sound frequency. Demonstrations are presented to highlight the sensors’ potential as wearable devices for human physiological signal and timbre detections. This is the first multifunctional highly stretchable strain sensor with timbre‐recognition ability.

Published in: "Advanced Functional Materials".

The post Ultrasensitive and Highly Stretchable Multifunctional Strain Sensors with Timbre‐Recognition Ability Based on Vertical Graphene appeared first on 2D Research.

One‐Step Scalable Fabrication of Graphene‐Integrated Micro‐Supercapacitors with Remarkable Flexibility and Exceptional Performance Uniformity

Publications on 2D Materials - Thu, 10/10/2019 - 8:32am

A one‐step simplified and scalable fabrication method is demonstrated for the facile construction of highly integrated all‐solid‐state planar graphene‐based micro‐supercapacitors, free of metal current collectors, interconnects, and extra substrates. The resulting micro‐supercapacitors exhibit shape diversity, remarkable mechanical flexibility, customized output voltage and current, exceptional performance uniformity, and outstanding high‐temperature stability. Abstract The rapid development of miniature electronics has accelerated the demand for simplified and scalable production of micro‐supercapacitors (MSCs); however, the preparation of active materials, patterning microelectrodes, and subsequent modular integration of the reported MSCs are normally separated and are involved in multiple complex steps. Herein, a one‐step, cost‐effective strategy for fast and scalable fabrication of patterned laser‐induced graphene (LIG) for all‐solid‐state planar integrated MSCs (LIG‐MSCs) with various form factors of designable shape, exceptional flexibility, performance uniformity, superior modularization, and high‐temperature stability is demonstrated. Notably, using the conductive and porous LIG patterns composed of randomly stacked graphene nanosheets simultaneously acting as both microelectrodes and interconnects, the resulting LIG‐MSCs represent typical electrical double capacitive behavior, having an impressive areal capacitance of 0.62 mF cm−2 and long‐term stability without capacitance degeneration after 10 000 cycles. Furthermore, LIG‐MSCs display exceptional mechanical flexibility and adjustable voltage and capacitance output through arbitrary arrangement of cells connected in series and in parallel, indicative of exceptional performance customization. Moreover, all‐solid‐state LIG‐MSCs working at ionogel electrolyte exhibit highly stable performance even at high temperature of 100 °C, with 90% capacitance retention over 3000 cycles, suggestive of outstanding reliability. Therefore, the LIG‐MSCs offer tremendous opportunities for miniature power source‐integrated microelectronics.

Published in: "Advanced Functional Materials".

The post One‐Step Scalable Fabrication of Graphene‐Integrated Micro‐Supercapacitors with Remarkable Flexibility and Exceptional Performance Uniformity appeared first on 2D Research.

Bioinspired Graphene Oxide Membranes with Dual Transport Mechanisms for Precise Molecular Separation

Publications on 2D Materials - Thu, 10/10/2019 - 8:32am

A bioinspired membrane with dual transport mechanisms of molecular sieving and carrier‐facilitated transport is designed by the fixation of a silver ion carrier and the impregnation of ionic liquid within 2D nanochannels of graphene oxide for precise molecular separation. Abstract The implementation of membrane technology to replace or combine with energy‐intensive cryogenic distillation for precise separation of ethylene/ethane mixture proves an extremely important yet highly challenging task. Inspired by the hierarchical structure and facilitated gas transport of biological membranes, a highly selective ethylene/ethane separation membrane is explored through the fixation of a silver ion carrier and the impregnation of ionic liquid within 2D nanochannels of graphene oxide laminate, where plenty of ethylene‐permeating in‐plane nano‐wrinkles and ethylene‐facilitated plane‐to‐plane nanochannels are constructed. By virtue of synergistic effects of molecular sieving and carrier‐facilitated transport, an unprecedented combination of high ethylene permeance (72.5 GPU) and superhigh ethylene/ethane selectivity (215) is achieved, out‐performing currently reported advanced membranes. Moreover, molecular dynamics simulations verify a favorable membrane nanostructure for fast and selective transport of ethylene molecules. This bioinspired approach with dual transport mechanisms may open novel avenues to the design of high‐performance membranes for precise molecular separation.

Published in: "Advanced Functional Materials".

The post Bioinspired Graphene Oxide Membranes with Dual Transport Mechanisms for Precise Molecular Separation appeared first on 2D Research.

Phase Transition and Superconductivity Enhancement in Se‐Substituted MoTe2 Thin Films

Publications on 2D Materials - Thu, 10/10/2019 - 2:40am

Consecutively tailoring few‐layer transition metal dichalcogenides from the 2H to Td phase may realize the long‐sought topological superconductivity by incorporating the quantum spin Hall effect and superconductivity. This study demonstrates that the transitions from Td to 1T’ to 2H phase can be realized in Se‐substituted MoTe2 thin films. More importantly, the observed superconductivity enhancement can be interpreted as two‐band superconductivity. Abstract Consecutively tailoring few‐layer transition metal dichalcogenides MX2 from 2H to T d phase may realize the long‐sought topological superconductivity in a single material system by incorporating superconductivity and the quantum spin Hall effect together. Here, this study demonstrates that a consecutive structural phase transition from T d to 1T′ to 2H polytype can be realized by increasing the Se concentration in Se‐substituted MoTe2 thin films. More importantly, the Se‐substitution is found to dramatically enhance the superconductivity of the MoTe2 thin film, which is interpreted as the introduction of two‐band superconductivity. The chemical‐constituent‐induced phase transition offers a new strategy to study the s+− superconductivity and the possible topological superconductivity, as well as to develop phase‐sensitive devices based on MX2 materials.

Published in: "Advanced Materials".

The post Phase Transition and Superconductivity Enhancement in Se‐Substituted MoTe2 Thin Films appeared first on 2D Research.

Tunable electronic structure in gallium chalcogenide van der Waals compounds

Publications on 2D Materials - Thu, 10/10/2019 - 12:32am

Author(s): Brian Shevitski, Søren Ulstrup, Roland J. Koch, Hui Cai, Sefaattin Tongay, Luca Moreschini, Chris Jozwiak, Aaron Bostwick, Alex Zettl, Eli Rotenberg, and Shaul AloniTransition-metal monochalcogenides comprise a class of two-dimensional materials with electronic band gaps that are highly sensitive to material thickness and chemical composition. Here, we explore the tunability of the electronic excitation spectrum in GaSe by using angle-resolved photoemission spe…[Phys. Rev. B 100, 165112] Published Wed Oct 09, 2019

Published in: "Physical Review B".

The post Tunable electronic structure in gallium chalcogenide van der Waals compounds appeared first on 2D Research.

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