Carbon nanotube forests as top electrode in electroacoustic resonators

We grow carbon nanotube forests on piezoelectric AlN films and fabricate and characterize nanotube-based solidly mounted bulk acoustic wave resonators employing the forests as the top electrode material. The devices show values for quality factor at anti-resonance of ∼430, and at resonance of ∼100. The effective coupling coefficient is of ∼6%, and the resonant frequencies are up to ∼800 MHz above those observed with metallic top electrodes. AlN promotes a strong catalyst-support interaction, which reduces Fe catalyst mobility, and thus enforces the growth of forests by the base growth mechanism.This work was partially supported by the European Commission through the project GRAFOL and the COST action IC1208 and by the Ministerio de Economía y Competitividad del Gobierno de España through project MAT2013-45957.This is the accepted manuscript. The final version is available at http://scitation.aip.org/content/aip/journal/apl/107/13/10.1063/1.4932197

Several New Active Galactic Nuclei Among X-ray Sources Detected by INTEGRAL and SWIFT Observatories

We present the results of the optical identifications of a set of X-ray sources from the all-sky surveys of INTEGRAL and SWIFT observatories. Optical data were obtained with Russian-Turkish 1.5-m Telescope (RTT150). Nine X-ray sources were identified as active galactic nuclei (AGNs). Two of them are hosted by nearby, nearly exactly edge-on, spiral galaxies MCG -01-05-047 and NGC 973. One source, IGR J16562-3301, is most probably BL Lac object (blazar). Other AGNs are observed as stellar-like nuclei of spiral galaxies, with broad emission lines in their spectra. For the majority of our hard X-ray selected AGNs, their hard X-ray luminosities are well-correlated with the luminosities in [OIII],5007 optical emission line. However, the luminosities of some AGNs deviate from this correlation. The fraction of these objects can be as high as 20%. In particular, the flux in [OIII] line turns to be lower in two nearby edge-on spiral galaxies, which can be explained by the extinction in their galactic disks.Comment: 9 pages, 3 figures, accepted for publication in Astronomy Letters, the original text in Russian can be found at http://hea.iki.rssi.ru/~rodion/poptid.pd

Surface Plasmon Frequency Spectrum in a System of Two Spherical Dielectric Coated Metallic Nanoparticles

Semi-analytical method of surface plasmon frequency calculation for the system of two almost touching coupled dielectric coated metallic nanospheres is presented. The method allows transforming the problem to numerical solution of two simple algebraic equations for arbitrary values of parameters - particle radius, distance between sphere centers, dielectric permittivity of the matrix, dielectrics and metals. It is especially easy to get the obvious graphical solution. The surface plasmon frequencies of longitudinal and transversal oscillations of the system are calculated and good agreement with the experimental results is achieved

Comparison of carbon nanotube forest growth using AlSi, TiSiN, and TiN as conductive catalyst supports

We evaluate carbon nanotube growth by employing AlSi, TiSiN and TiN as conductive catalyst supports. Using a wide range of chemical vapour deposition conditions, we find that only AlSi and TiSiN yield homogeneously-sized nanoparticles, which are stable throughout both catalyst preparation and nanotube synthesis processes. This favours the growth of forests with area densities of the order of 1012 nanotubes cm-2. TiN, in contrast, yield lower density forests in a very narrow window process. The forests and the three screened catalyst supports show ohmic conductivity. TiSiN, however, is the only conductor that leads to the very robust growth process. This suggests TiSiN is useful for applications requiring forest growth on conductors and thus warrants further assessment for reducing nanotube diameter and improving area density of the forests

Growth Kinetics and Growth Mechanism of Ultrahigh Mass Density Carbon Nanotube Forests on Conductive Ti/Cu Supports

We evaluate the growth kinetics and growth mechanism of ultrahigh mass density carbon nanotube forests. They are synthesized by chemical vapor deposition at 450 °C using a conductive Ti/Cu support and Co-Mo catalyst system. We find that Mo stabilizes Co particles preventing lift off during the initial growth stage, thus promoting the growth of ultrahigh mass density nanotube forests by the base growth mechanism. The morphology of the forest gradually changes with growth time, mostly because of a structural change of the catalyst particles. After 100 min growth, toward the bottom of the forest, the area density decreases from ∼3-6 × 1011cm-2to ∼5 × 1010cm-2and the mass density decreases from 1.6 to 0.38 g cm-3. We also observe part of catalyst particles detached and embedded within nanotubes. The progressive detachment of catalyst particles results in the depletion of the catalyst metals on the substrate surfaces. This is one of the crucial reasons for growth termination and may apply to other catalyst systems where the same features are observed. Using the packed forest morphology, we demonstrate patterned forest growth with a pitch of ∼300 nm and a line width of ∼150 nm. This is one of the smallest patterning of the carbon nanotube forests to date

Hollow MXene Spheres and 3D Macroporous MXene Frameworks for Na-Ion Storage

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2D transition metal carbides and nitrides, named MXenes, are attracting increasing attentions and showing competitive performance in energy storage devices including electrochemical capacitors, lithium- and sodium-ion batteries, and lithium–sulfur batteries. However, similar to other 2D materials, MXene nanosheets are inclined to stack together, limiting the device performance. In order to fully utilize MXenes' electrochemical energy storage capability, here, processing of 2D MXene flakes into hollow spheres and 3D architectures via a template method is reported. The MXene hollow spheres are stable and can be easily dispersed in solvents such as water and ethanol, demonstrating their potential applications in environmental and biomedical fields as well. The 3D macroporous MXene films are free-standing, flexible, and highly conductive due to good contacts between spheres and metallic conductivity of MXenes. When used as anodes for sodium-ion storage, these 3D MXene films exhibit much improved performances compared to multilayer MXenes and MXene/carbon nanotube hybrid architectures in terms of capacity, rate capability, and cycling stability. This work demonstrates the importance of MXene electrode architecture on the electrochemical performance and can guide future work on designing high-performance MXene-based materials for energy storage, catalysis, environmental, and biomedical applications

Development of asymmetric supercapacitors with titanium carbide-reduced graphene oxide couples as electrodes

Two-dimensional (2D) nanomaterials have attracted significant interest for supercapacitor applications due to their high surface to volume ratio. Layered 2D materials have the ability to intercalate ions and thus can provide intercalation pseudocapacitance. Properties such as achieving fast ion diffusion kinetics and maximizing the exposure of the electrolyte to the surface of the active material are critical for optimizing the performance of active materials for electrochemical capacitors (i.e. Supercapacitors). In this study, two 2D materials, titanium carbide (TiCT) and reduced graphene oxide (rGO), were used as electrode materials for asymmetric supercapacitors, with the resulting devices achieving high capacitance values and excellent capacitance retention in both aqueous and organic electrolytes. This work demonstrates that TiCT is a promising electrode material for flexible and high-performance energy storage devices.This work was financially supported by the Graphene Flagship (Grant agreement no: 604391. Call: FP7-ICT-2013-FET-F). AMNS was supported by CIC energiGUNE, the Basque Government Scholarship for pre-doctoral formation (PRE_2015_2_0096) and the Egonlabur Traveling Grant (EP_2016_1_0030). KLVA was funded by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.Peer Reviewe

MoS<inf>2</inf>-on-MXene Heterostructures as Highly Reversible Anode Materials for Lithium-Ion Batteries

© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Two-dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition-metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS2-on-MXene heterostructures through in situ sulfidation of Mo2TiC2Tx MXene. The computational results show that MoS2-on-MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li2S adsorption during the intercalation and conversion reactions. These characteristics render the as-prepared MoS2-on-MXene heterostructures stable Li-ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications

Evaluation of bimetallic catalysts for the growth of carbon nanotube forests

We systematically study the growth of carbon nanotube forests by chemical vapor deposition using evaporated monometallic or bimetallic Ni, Co, or Fe films supported on alumina. Our results show two regimes of catalytic activity. When the total thickness of catalyst is larger than nominally 1nm, bimetallic catalysts tend to outperform the equivalent layers of a single metal, yielding taller forests of multi-walled carbon nanotubes (CNTs). In contrast, for layers thinner than ~1nm, bimetallic catalysts are notably less active than individually. However, the amount of small diameter and single-walled CNTs is significantly increased. This possible transition at ~1nm might be related to different catalyst composition after annealing, depending whether or not the films overlap during evaporation and alloy during catalyst formation. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Carbon nanotube growth on conductors: Influence of the support structure and catalyst thickness

We investigate the formation and stability of Fe nanoparticles on TiN and poly-crystalline PtSi films, and their ability to grow carbon nanotubes forests. Using different-microstructure films, coated with or without their native oxides, we show that, upon purely-thermal catalyst pretreatment, PtSi favours the formation of homogenously sized nanoparticles and forest growth, partly due to its low surface energy. TiN, in contrast, leads to much less controllable processes and only when coated with its native oxide, or with thick catalyst films, yields large diameter nanotube forests. The microstructure of the material can dramatically limit catalyst diffusion into the bulk of the support during nanotube growth. These results allow us to establish the general behaviour expected for nanotube growth on any conductive materials. © 2014 Elsevier Ltd. All rights reserved