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1.
Langmuir ; 38(29): 8757-8765, 2022 Jul 26.
Artigo em Inglês | MEDLINE | ID: mdl-35834350

RESUMO

Graphene oxide (GO) paper is an attractive material because of high stiffness and strength, light weight, and multiple functionalities. While these properties are now widely exploited in nanoinclusions or flat sheets, three-dimensional (3D) structures from GO paper are not widely studied because of a lack of suitable processing methods. In this study, we report a layered assembly method to make stiff and strong 3D GO structures with the aid of a sodium tetraborate (borax) solution. By comparing mechanical properties of assembled GO paper using water or borax solution, we found that the borax-assembled layers had the highest stiffness. To demonstrate the versatility of our assembly protocol, we then fabricated a variety of 3D structures including I-beams, cylindrical tubes, and bridge-like structures from GO paper. These GO structures were stiff and light weight, and the stiffness to mass ratio was around 2-4 times higher than other polymer samples including cellulose, fluorinated ethylene propylene, and poly(vinyl alcohol). The versatile processing method to make stiff and strong GO structures will enable new engineering applications where nonplanar GO structures are required.

2.
J Phys Condens Matter ; 34(5)2021 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-34783682

RESUMO

The mineral franckeite is a naturally occurring van der Waals superlattice which has recently attracted attention for future applications in optoelectronics, biosensors and beyond. Furthermore, its stacking of incommensurately modulated 2D layers, the pseudo tetragonal Q-layer and the pseudo hexagonal H-layer, is an experimentally accessible prototype for the development of synthetic van der Waals materials and of advanced characterization methods to reveal new insights in their structure and chemistry at the atomic scale that is crucial for deep understanding of its properties. While some experimental studies have been undertaken in the past, much is still unknown on the correlation between local atomic structure and chemical composition within the layers. Here we present an investigation of the atomic structure of franckeite using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT). With atomic-number image contrast in HAADF STEM direct information about both the geometric structure and its chemistry is provided. By imaging samples under different zone axes within the van der Waals plane, we propose refinements to the structure of the Q-layer and H-layer, including several chemical ordering effects that are expected to impact electronic structure calculations. Additionally, we observe and characterize stacking faults which are possible sources of differences between experimentally determined properties and calculations. Furthermore, we demonstrate advantages and discuss current limitations and perspectives of combining TEM and APT for the atomic scale characterization of incommensurately modulated von der Waals materials.

3.
Nat Nanotechnol ; 16(8): 853-854, 2021 08.
Artigo em Inglês | MEDLINE | ID: mdl-34099900
4.
Nat Commun ; 12(1): 2542, 2021 May 05.
Artigo em Inglês | MEDLINE | ID: mdl-33953174

RESUMO

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

5.
Nanotechnology ; 32(4): 045502, 2021 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-33049728

RESUMO

The discovery of the field effect in graphene initiated the development of graphene field effect transistor (FET) sensors, wherein high mobility surface conduction is readily modulated by surface adsorption. For all graphene transistor sensors, low-frequency 1/f noise determines sensor resolution, and the absolute measure of 1/f noise is thus a crucial performance metric for sensor applications. Here we report a simple method for reducing 1/f noise by scaling the active area of graphene FET sensors. We measured 1/f noise in graphene FETs with size 5 µm × 5 µm to 5.12 mm × 5.12 mm, observing more than five orders of magnitude reduction in 1/f noise. We report the lowest normalized graphene 1/f noise parameter observed to date, 5 × 10-13, and we demonstrate a sulfate ion sensor with a record resolution of 1.2 × 10-3 log molar concentration units. Our work highlights the importance of area scaling in graphene FET sensor design, wherein increased channel area improves sensor resolution.

6.
Nat Commun ; 11(1): 3226, 2020 06 26.
Artigo em Inglês | MEDLINE | ID: mdl-32591504

RESUMO

Real-time, high resolution, simultaneous measurement of multiple ionic species is challenging with existing chromatographic, spectrophotometric and potentiometric techniques. Potentiometric ion sensors exhibit limitations in both resolution and selectivity. Herein, we develop wafer scale graphene transistor technology for overcoming these limitations. Large area graphene is an ideal material for high resolution ion sensitive field effect transistors (ISFETs), while simultaneously enabling facile fabrication as compared to conventional semiconductors. We develop the ISFETs into an array and apply Nikolskii-Eisenman analysis to account for cross-sensitivity and thereby achieve high selectivity. We experimentally demonstrate real-time, simultaneous concentration measurement of K+, Na+, [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and Cl- with a resolution of [Formula: see text] concentration units. The array achieves an accuracy of  ±0.05 log concentration. Finally, we demonstrate real-time ion concentration measurement in an aquarium with lemnoideae lemna over three weeks, where mineral uptake by aquatic organisms can be observed during their growth.


Assuntos
Grafite/química , Eletrodos Seletivos de Íons , Transistores Eletrônicos , Ânions , Cátions , Eletrólitos/química , Concentração de Íons de Hidrogênio , Imagem Óptica , Temperatura , Fatores de Tempo
7.
Nat Nanotechnol ; 13(5): 357, 2018 05.
Artigo em Inglês | MEDLINE | ID: mdl-29728670

Assuntos
Semicondutores
8.
Nano Lett ; 17(6): 3738-3743, 2017 06 14.
Artigo em Inglês | MEDLINE | ID: mdl-28471682

RESUMO

Aluminum-rich AlGaN is the ideal material system for emerging solid-state deep-ultraviolet (DUV) light sources. Devices operating in the near-UV spectral range have been realized; to date, however, the achievement of high-efficiency light-emitting diodes (LEDs) operating in the UV-C band (200-280 nm specifically) has been hindered by the extremely inefficient p-type conduction in AlGaN and the lack of DUV-transparent conductive electrodes. Here, we show that these critical challenges can be addressed by Mg dopant-free Al(Ga)N/h-BN nanowire heterostructures. By exploiting the acceptor-like boron vacancy formation, we have demonstrated that h-BN can function as a highly conductive, DUV-transparent electrode; the hole concentration is ∼1020 cm-3 at room temperature, which is 10 orders of magnitude higher than that previously measured for Mg-doped AlN epilayers. We have further demonstrated the first Al(Ga)N/h-BN LED, which exhibits strong emission at ∼210 nm. This work also reports the first achievement of Mg-free III-nitride LEDs that can exhibit high electrical efficiency (80% at 20 A/cm2).

9.
Langmuir ; 31(20): 5545-9, 2015 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-25942331

RESUMO

Graphene hydrogels/aerogels are emerging three-dimensional graphene macroscopic assemblies of potential use in many applications including energy storage, pollutant adsorption, and gas sensing. In this Letter, we identify, characterize and control the formation of the exterior shell structure of graphene hydrogels prepared via hydrothermal reduction of graphene oxide. Unlike the porous bulk of the hydrogel, the shell is a compact, highly ordered layer with a higher electrical conductivity. Shell formation is dependent upon the surface anchoring of graphene oxide at the liquid-air and liquid-container interfaces. By purposefully weakening surface anchoring of graphene oxide using mild thermal or chemical prereduction method prior to hydrothermal reduction, we have succeeded in completely suppressing shell formation in the graphene hydrogel. The resulting graphene hydrogel shows a lower volume reduction with a porous bulk structure immediately accessible from the surface, in contrast to graphene hydrogels prepared under conventional conditions.

10.
Nano Lett ; 15(4): 2263-8, 2015 Apr 08.
Artigo em Inglês | MEDLINE | ID: mdl-25730309

RESUMO

Hexagonal boron nitride (hBN) is a wide-gap material that has attracted significant attention as an ideal dielectric substrate for 2D crystal heterostructures. We report here the first observation of in-plane charge transport in large-area monolayer hBN, grown by chemical vapor deposition. The quadratic scaling of current with voltage at high bias corresponds to a space-charge limited conduction mechanism, with a room-temperature mobility reaching up to 0.01 cm(2)/(V s) at electric fields up to 100 kV/cm in the absence of dielectric breakdown. The observation of in-plane charge transport highlights the semiconducting nature of monolayer hBN, and identifies hBN as a wide-gap 2D crystal capable of supporting charge transport at high field. Future exploration of charge transport in hBN is motivated by the fundamental study of UV optoelectronics and the massive Dirac fermion spectrum of hBN.

11.
Nanoscale ; 6(12): 6710-9, 2014 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-24821216

RESUMO

We report the hydrothermal synthesis of graphene (GNP)-TiO2 nanoparticle (NP) hybrids using COOH and NH2 functionalized GNP as a shape controller. Anatase was the only TiO2 crystalline phase nucleated on the functionalized GNP, whereas traces of rutile were detected on unfunctionalized GNP. X-Ray Photoelectron spectroscopy (XPS) showed C-Ti bonds on all hybrids, thus confirming heterogeneous nucleation. GNP functionalization induced the nucleation of TiO2 NPs with specific shapes and crystalline facets exposed. COOH functionalization directed the synthesis of anatase truncated bipyramids, bonded to graphene sheets via the {101} facets, while NH2 functionalization induced the formation of belted truncated bipyramids, bonded to graphene via the {100} facets. Belted truncated bipyramids formed on unfunctionalized GNP too, however the NPs were more irregular and rounded. These effects were ascribed to pH variations in the proximity of the functionalized GNP sheets, due to the high density of COOH or NH2 groups. Because of the different reactivity of anatase {100} and {101} crystalline facets, we hypothesize that the hybrid materials will behave differently as photocatalysts, and that the COOH-GNP-TiO2 hybrids will be better photocatalysts for water splitting and H2 production.

12.
Opt Express ; 21(21): 25356-63, 2013 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-24150377

RESUMO

Terahertz plasmons and magnetoplasmons propagating along electrically and chemically doped graphene p-n junctions are investigated. It is shown that such junctions support non-reciprocal magnetoplasmonic modes which get concentrated at the middle of the junction in one direction and split away from the middle of the junction in the other direction under the application of an external static magnetic field. This phenomenon follows from the combined effects of circular birefringence and carrier density non-uniformity. It can be exploited for the realization of plasmonic isolators.

13.
Langmuir ; 28(36): 13042-50, 2012 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-22866918

RESUMO

Composites of tin nanoparticles (Sn NP) and graphene are candidate materials for high capacity and mechanically stable negative electrodes in rechargeable Li ion batteries. A uniform dispersion of Sn NP with controlled size is necessary to obtain high electrochemical performance. We show that the nucleation of Sn particles on highly ordered pyrolitic graphite (HOPG) from solution can be controlled by functionalizing the HOPG surface by aryl groups prior to Sn deposition. On the contrary, we observe heterogeneous deposition of micrometer sized Sn islands on HOPG subjected to oxidation prior to deposition in the same conditions. We demonstrate that functional groups act as nucleation sites for Sn NP nucleation, and that homogeneous nucleation of small particles can be achieved by combining surface functionalization with diazonium chemistry and appropriate stabilizers in solution.

14.
Phys Rev Lett ; 106(17): 176801, 2011 Apr 29.
Artigo em Inglês | MEDLINE | ID: mdl-21635055

RESUMO

The error rate in complementary transistor circuits is suppressed exponentially in electron number, arising from an intrinsic physical implementation of fault-tolerant error correction. Contrariwise, explicit assembly of gates into the most efficient known fault-tolerant architecture is characterized by a subexponential suppression of error rate with electron number, and incurs significant overhead in wiring and complexity. We conclude that it is more efficient to prevent logical errors with physical fault tolerance than to correct logical errors with fault-tolerant architecture.

15.
Nano Lett ; 11(1): 132-7, 2011 Jan 12.
Artigo em Inglês | MEDLINE | ID: mdl-21141990

RESUMO

Graphene field effect transistors (FETs) are extremely sensitive to gas exposure. Charge transfer doping of graphene FETs by atmospheric gas is ubiquitous but not yet understood. We have used graphene FETs to probe minute changes in electrochemical potential during high-purity gas exposure experiments. Our study shows quantitatively that electrochemistry involving adsorbed water, graphene, and the substrate is responsible for doping. We not only identify the water/oxygen redox couple as the underlying mechanism but also capture the kinetics of this reaction. The graphene FET is highlighted here as an extremely sensitive potentiometer for probing electrochemical reactions at interfaces, arising from the unique density of states of graphene. This work establishes a fundamental basis on which new electrochemical nanoprobes and gas sensors can be developed with graphene.


Assuntos
Grafite/química , Oxigênio/química , Água/química , Eletroquímica , Transporte de Elétrons , Oxirredução , Transistores Eletrônicos
16.
Opt Lett ; 35(20): 3336-8, 2010 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-20967058

RESUMO

An ellipsometer with 3µm×5µm spot size constructed with a single focusing and imaging element is used to measure the layer number of exfoliated graphene on glass and expitaxial graphene on SiC. Ellipsometric sensitivity to graphene layer number increases with decreasing layer number and decreasing substrate refractive index. Single-atomic-layer sensitivity has been achieved. High spatial resolution imaging and ellipsometry is useful for rapid characterization of epitaxially grown graphene films.

17.
Opt Express ; 16(3): 1820-35, 2008 Feb 04.
Artigo em Inglês | MEDLINE | ID: mdl-18542261

RESUMO

We applied a multiple-multipole method to calculate the field enhancement of discrete metal nanosphere assemblies due to plasma resonance, thus performing the first full electromagnetic simulation of a variety of nanoparticle assemblies for efficient field focusing, including the self-similar geometric series of spheres first proposed by Li, Stockman and Bergman. Our study captures electromagnetic resonance effects important for optimizing nanoparticle assemblies to achieve maximum electric field focusing. We predict optical frequency electric fields can be enhanced in gold nanoparticle assemblies in aqueous solution by the order of ~450, within a factor of 2 of that achievable in silver nanostructures. We find that both absorption and far-field scattering resonances of nanoparticle assemblies must be carefully interpreted when inferring near-field focusing properties.


Assuntos
Ouro/química , Modelos Teóricos , Nanosferas/química , Nanosferas/ultraestrutura , Óptica e Fotônica , Ressonância de Plasmônio de Superfície/métodos , Simulação por Computador , Tamanho da Partícula
18.
Opt Express ; 15(8): 4474-84, 2007 Apr 16.
Artigo em Inglês | MEDLINE | ID: mdl-19532694

RESUMO

The continued scaling of integrated circuits will require advances in intra-chip interconnect technology to minimize delay, density of energy dissipation and cross-talk. We present the first quantitative comparison between the performance of metal wire interconnects, operated in the traditional manner by electric charge and discharge, versus the performance of metal wires operated as surface plasmon waveguides. Surface plasmon wire waveguides have the potential to reduce signal delay, but the high confinement required for low cross-talk amongst high density plasmon wire interconnects significantly increases energy dissipation per transmitted bit, above and beyond that required for electric charge/discharge interconnects at the same density.

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