High purity chemically vapor deposited (CVD) diamond single crystals are now widely available. However, the reduction of dislocations in this material still remains an important challenge that will ...strongly condition its adoption in areas such as optics, electronics, and spintronics, where these defects have a disastrous effect on the properties. In this work we report on a methodology that allows a complete identification of the type, density, and distribution of dislocations in a high quality CVD single crystal. A good agreement between all characterization techniques was established. When the surface is adequately prepared, a simple plasma etching allows evidencing 2 main dislocation types: 45° mixed and edge, with the latter one being dominant with a density of around 4.5 × 104 cm–2. This investigation paves the way to the development of a quick and simple process to analyze dislocations toward getting a better understanding of their impact on the material properties and eventually elaborating strategies to eliminate them.
MXenes are two-dimensional (2D) materials with a great potential for sensor applications due to their high aspect ratio and fully functionalized surface that can be tuned for specific gas adsorption. ...Here, we demonstrate that the Nb2CTz-based sensor exhibits high performance towards alcohol vapors at temperatures up to 300–350 °C, with the best sensitivity towards ethanol. We attribute the observed remarkable chemiresistive effect of this material to the formation of quasi-2D Nb2O5 sheets as the result of the oxidation of Nb-based MXenes. These findings are supported by synchrotron X-ray photoelectron spectroscopy studies together with X-ray diffraction and electron microscopy observations. For analyte selectivity, we employ a multisensor approach where the gas recognition is achieved by linear discriminant analysis of the vector response of the on-chip sensor array. The reported protocol demonstrates that MXene layers are efficient precursors for the derivation of 2D oxide architectures, which are suitable for developing gas sensors and sensor arrays.
Al4C3 single crystals are synthesized by the reaction between SiC and aluminum and are structurally verified and analyzed by Raman and X‐ray diffraction spectra. The optical absorption measurements ...of the Al4C3 crystals indicated that the bandgap is about 2.3 eV, which surprisingly is much higher than the bandgap value from the literature. In order to elucidate the discrepancy between the new experimental results and the literature, an advanced ab initio based investigation on the structural, electronic and vibrational properties of the Al4C3 is performed. Remarkably, the scGW approach arrived at an indirect band gap Γ→L equal to 2.12 eV while the HSE06 calculated band gap was found even closer to the experimental value equal to 2.27 eV. In addition, the hole effective mass is found to be considerably high due to the flat nature of the valence band at the Γ, indicating low hole mobility. Moreover, phonon calculations indicated that the vibrations of the Al atoms contribute mostly on low frequency acoustic branches, while vibrations from C atoms have a stronger contribution to optical phonons.
Structural analysis by Raman and X‐ray diffraction spectra verifies the synthesis of Al4C3 single crystals. The optical absorption measurements of the Al4C3 crystals indicate that the bandgap is about 2.3 eV. Remarkably, the GW approach yields an indirect band gap equal to 2.12 eV while the HSE06 calculated band gap is even closer to the experimental value equal to 2.27 eV.
To assist the development of high quality single crystalline SiC ingot using the top seeded solution growth process, we have implemented a numerical model with the aim of giving quantitative outcomes ...in addition to qualitative information. The major role of the convection patterns on the carbon flux is demonstrated. We also evidence that the carbon solubility in liquid silicon is the actual limiting parameter of the SiC solution growth process. A good agreement between computed and experimental growth rates is obtained as a function of temperature, making simulation an adapted predictive tool for the further development of the process.
Herein, we critically assess magnetotransport in the MAX phases and their 2D derivatives, MXenes. For some MAX phases, a simple, 2D hexagonal metal model describes weak-field magnetotransport of ...their nearly free electrons reasonably well. For others, experimental and/or theoretical Fermi surfaces need to be mapped-a crucial task required for true understanding. Even less is known about MXenes. The density of apparent mobile carriers in Ti
3
C
2
T
x
-assuming a single-band model-is ≈1 × 10
14
cm
−2
(10
28
cm
−3
), which justifies it being sometimes described as a 2D metal. Much work is needed before a clearer picture emerges.
IMPACT STATEMENT
Magnetotransport in the MAX phases and their 2D derivatives MXene are critically reviewed for the first time. For some, a 2D hexagonal metal can explain magnetotransport; in others not.
Here, we report on synthesis and characterization of single crystals of the hexagonal Ga-containing κ-carbide phase Mo3(2+y)Ga3(1-x)Fe1+3(1-y+x)C4 with x = 0.15, y = 0.35 and mixed occupation of two ...6 h sites. We refined its crystal structure as hexagonal with P63/mmc symmetry. We combined polarized Raman spectroscopy, X-ray absorption near edge structure, X-ray magnetic circular dichroism and electrical resistivity measurements with density functional theory in order to give a first consistent description of the structural, electronic and magnetic properties of this new compound. The phase exhibits metallic conductivity, has a high thermal stability in inert atmosphere and surprisingly has a non-magnetic ground state. Our study provides further impetus for the rediscovery of the κ-phase family, which, due to its chemical diversity, should have many physical properties to be easily tailored.
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In the present work, the electronic and vibrational properties of both pristine V2C and fully terminated V2CT2 (where T=F, O, OH) two-dimensional monolayers are investigated using density functional ...theory. First, the atomic structures of V2C-based MXene phases are optimized, and their respective dynamical stabilities are discussed. Second, electronic band structures are computed indicating that V2C is metallic as well as all the corresponding functionalized systems. Third, the vibrational properties (phonon frequencies and spectra) of V2C-based MXenes are computed with the density functional perturbation theory and reported. Both Raman-active (Eg, A1g) and infrared-active (Eu, A2u) vibrational modes are predicted ab initio with the aim to correlate the experimental Raman peaks with the calculated vibrational modes and to assign them to specific atomic motions. The effect of the terminal groups on the vibrational properties is emphasized, along with the effect on the presence and position of the corresponding Raman peaks. Our results provide insights for the identification and characterization of V2C-based samples using Raman spectroscopy.
Elementary deformation mechanisms have been investigated in a single-crystalline Cr2AlC MAX phase. Plastic deformation has been induced at room temperature by spherical nanoindentation, with a ...crystal orientation chosen to minimize basal slip and to promote local crystal reorientation. The combination of Atomic Force Microscopy (AFM) surface observation, Transmission Electron Microscopy (TEM) characterization and Automated Crystal Orientation Mapping (ACOM) in a same region has provided a full 3D characterization of the deformation microstructure. Localized and isolated deformation twins have been identified below the indents. The whole microstructure consists in interactions between dislocations and {112¯4} twin variants. The TEM analysis also revealed activation of a 13〈112¯0〉{1¯103} slip system never reported before in a MAX phase.
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