214 research outputs found
Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes
Nanolaminated materials are important because of their exceptional properties
and wide range of applications. Here, we demonstrate a general approach to
synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating
from the replacement reaction between the MAX phase and the late transition
metal halides. The approach is a top-down route that enables the late
transitional element atom (Zn in the present case) to occupy the A site in the
pre-existing MAX phase structure. Using this replacement reaction between Zn
element from molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2,
Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC
were synthesized. When employing excess ZnCl2, Cl terminated MXenes (such as
Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and
Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate
that A-site element replacement in traditional MAX phases by late transition
metal halides opens the door to explore MAX phases that are not
thermodynamically stable at high temperature and would be difficult to
synthesize through the commonly employed powder metallurgy approach. In
addition, this is the first time that exclusively Cl-terminated MXenes were
obtained, and the etching effect of Lewis acid in molten salts provides a green
and viable route to prepare MXenes through an HF-free chemical approach.Comment: Title changed; experimental section and discussion revise
Інтеграція знань з астрономії та фізики щодо уявлень про приливи та відливи
(uk) В статті розкриті окремі гравітаційні, енергетичні та екологічні особливості припливів та відливів, які у підручниках для середніх загальноосвітніх навчальних закладів та вищої школи мало висвітлені. Автором якісну картину припливів та відпливів доповнено кількісною.(en) The article revealed specific gravity, energy and environmental features tides and low tides that textbooks for secondary schools and higher education was highlighted. The author of picture quality tides supplemented with quantitative
American Military Culture and Civil-Military Relations Today
We demonstrate a standard-free method to retrieve compositional information in AlxIn1-xN thin films by measuring the bulk plasmon energy (E-p), employing electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Two series of samples were grown by magnetron sputter epitaxy (MSE) and metal organic vapor phase epitaxy (MOVPE), which together cover the full compositional range 0 <= x <= 1. Complementary compositional measurements were obtained using Rutherford backscattering spectroscopy (RBS) and the lattice parameters were obtained by X-ray diffraction (XRD). It is shown that E-p follows a linear relation with respect to composition and lattice parameter between the alloying elements from AlN to InN allowing for straightforward compositional analysis. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Regulating Sustainable Finance in Capital Markets: A Perspective from Socially Embedded Decentered Regulation
We present an ab initio theoretical analysis of the temperature-dependent stability of inherently nanolaminated (Cr1−xMnx)2AlC. The results indicate energetic stability over the composition range x = 0.0 to 0.5 for temperatures ≥600 K. Corresponding thin film compounds were grown by magnetron sputtering from four elemental targets. X-ray diffraction in combination with analytical transmission electron microscopy, including electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy analysis, revealed that the films were epitaxial (0001)-oriented single-crystals with x up to 0.16.Funding Agencies|European Research Council under the European Community|258509227754|Knut and Alice Wallenberg Foundation||Swedish Research Council|||DFG-SPP 1299|</p
Theory of the Eigler-swith
We suggest a simple model to describe the reversible field-induced transfer
of a single Xe-atom in a scanning tunneling microscope, --- the Eigler-switch.
The inelasticly tunneling electrons give rise to fluctuating forces on and
damping of the Xe-atom resulting in an effective current dependent temperature.
The rate of transfer is controlled by the well-known Arrhenius law with this
effective temperature. The directionality of atom transfer is discussed, and
the importance of use of non-equlibrium-formalism for the electronic
environment is emphasized. The theory constitutes a formal derivation and
generalization of the so-called Desorption Induced by Multiple Electron
Transitions (DIMET) point of view.Comment: 13 pages (including 2 figures in separate LaTeX-files with
ps-\specials), REVTEX 3.
Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers
Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schr\uf6dinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9
7 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity
Phase formation in CrFeCoNi nitride thin films
As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material
related to the archetypical high-entropy Cantor alloy CrFeCoNiMn. For thin
films, CrFeCoNi of approximately equimolar composition tends to assume an fcc
structure when grown at room temperature by magnetron sputtering. However, the
single-phase solid solution state is typically not achieved for thin films
grown at higher temperatures. The same holds true for Cantor alloy-based
ceramics (nitrides and oxides), where phase formation is extremely sensitive to
process parameters such as the amount of reactive gas. This study combines
theoretical and experimental methods to understand the phase formation in
nitrogen-containing CrFeCoNi thin films. Density functional theory calculations
considering three competing phases (CrN, Fe-Ni and Co) show that the free
energy of mixing, delta G of (CrFeCoNi)1-xNx solid solutions has a maximum at x
= 0.20-0.25, and delta G becomes lower when x less than 0.20, greater than
0.25. Thin films of (CrFeCoNi)1-xNx (x = 0.14-0.41) grown by magnetron
sputtering show stabilization of the metallic fcc when x lesser than or equal
to 0.22 and the stabilization of the NaCl B1 structure when x is greater than
0.33, consistent with the theoretical prediction. In contrast, films with
intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show
segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer
an explanation for the requirement of kinetically limited growth conditions at
low temperature for obtaining single-phase CrFeCoNi Cantor-like
nitrogen-containing thin films and are of importance for understanding the
phase-formation mechanisms in multicomponent ceramics
Chemical-scissor-mediated structural editing of layered transition metal carbides
Intercalation of non-van der Waals (vdW) layered materials can produce new 2D
and 3D materials with unique properties, but it is difficult to achieve. Here,
we describe a structural editing protocol for 3D non-vdW layered ternary
carbides and nitrides (MAX phases) and their 2D vdW derivatives (MXenes).
Gap-opening and species-intercalating stages were mediated by chemical scissors
and guest intercalants, creating a large family of layered materials with
unconventional elements and structures in MAX phases, as well as MXenes with
versatile termination species. Removal of surface terminations by metal
scissors and stitching of carbide layers by metal atoms leads to a reverse
transformation from MXenes to MAX phases, and metal-intercalated 2D carbides.
This scissor-mediated structural editing may enable structural and chemical
tailoring of other layered ceramics
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