Carbon Doping in Boron Suboxide: Structure, Energetics, and Elastic Properties

Promising initial results on bulk Mg(B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> prepared with carbon doped boron are presented. Carbon doping is achieved by reaction of ethylene gas on boron powder using a stainless steel tube furnace, a technique suitable for industrial scale processing. The nominal amount of doping was controlled by varying the reaction time with a fixed volume of ethylene gas, and the actual carbon uptake was determined by weight change after the reaction. The amount of carbon substitution x in the Mg(B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was found using the angular shift in the (100) X-ray reflection. Carbon substitution by the full nominal content in the C doped precursor boron was obtained for doping up to 7.2 at%, as shown by a o-axis compression consistent with that of carbon doped single crystals. The critical current density of the 4 at% C doped sample for temperatures at 20-30 K and fields up to 4 T, relevant to high temperature applications, was significantly higher than those in the published literature. The J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of a 10 wt% nano-SiC doped sample, used as a comparative benchmark, was found to be lower than the C doped sample at field below 2 T, but to reduce slower at higher fields. Structure analysis of the SiC doped sample revealed a coexistence of two C substitution levels of 2.25% at and 5.25% at.

We have fabricated the multiwalled carbon nanotube (MWCNT) and nano-C doped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bulk and wire to investigate the effect of carbon substitution in the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Carbon substitution into boron site was confirmed by the shrinkage of the lattice parameter a with the increasing of the doping level. It was found that the lattice distortion and optimum doping level are different in the CNT and nano-C samples. This can be explained by carbon substitution, which is a more direct substitution process with the nano-C precursor. With CNT doping, carbon substitution only occurred after break out of carbon atoms from CNT. Both dopants showed significant enhancement in both magnetic critical current density, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (H), and transport critical current at high sintering temperature like 900degC . Transport I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of magnitude 14 A and 30 A was observed at 4.2 K and 12 T for CNT and nano-C doping, respectively.

We report on the electrical activation of neutral carbon-oxygen complexes in Si by wet-chemical etching at room temperature. Two deep levels, E65 and E75, are observed by deep level transient spectroscopy in n-type Czochralski Si. The activation enthalpies of E65 and E75 are obtained as EC-0.11 eV (E65) and EC-0.13 eV (E75). The electric field dependence of their emission rates relates both levels to single acceptor states. From the analysis of the depth profiles, we conclude that the levels belong to two different defects, which contain only one hydrogen atom. A configuration is proposed, where the CH1BC defect, with hydrogen in the bond-centered position between neighboring C and Si atoms, is disturbed by interstitial oxygen in the second nearest neighbor position to substitutional carbon. The significant reduction of the CH1BC concentration in samples with high oxygen concentrations limits the use of this defect for the determination of low concentrations of substitutional carbon in Si samples.

Biomass-derived carbon doping to enhance the current carrying capacity and flux pinning of an isotopic Mg11B2 superconductor

The incorporation of germanium and carbon by RP-CVD epitaxy in silicon brings lots of interest in silicon devices thanks to the variety of properties that can be addressed: band gap or lattice parameter engineering, dopant diffusion reduction, chemical properties, optical properties...For all the applications requiring SiGeC materials, carbon atoms must be incorporated in fully substitutional site (Cs) despite its low bulk solubility into the Si and SiGe lattice (3x1017 at.cm-3 at the Si melting point). Beyond a total carbon concentration (depending on process parameter), carbon atoms are also incorporated into interstitial sites (Ci). These Ci atoms usually form extended defects such as clusters or SiC precipitates which are harmful for devices performances.Solubility limits of carbon can be extended into the metastable domain by optimizing the epitaxy process. Indeed, it is well established that low temperature and high growth rate are favorable to obtain high Cs atoms incorporation [1].This work compares silane (SiH4) and disilane (Si2H6) precursors coupled with germane (GeH4) and methylsilane (SiH3CH3 or MS) to process low temperature thin film of SiGeC with the highest amount of Cs without Ci.First, the SiGe growth kinetics using silane and disilane have been studied. The temperature range investigated varied from 500 °C to 600 °C at low pressure. Using similar atomic fluxes of Si, and with all the other parameters constant, a higher growth rate and lower germanium content has been observed with disilane compared to silane. For instance, there is a factor of 1.85 at 550°C. The germanium concentration was varying from 30.4 to 22.3 % and from 19.4 to 15.7 % for silane and disilane, respectively. To have a better understanding about this kinetics difference, an impoverishment rate or theoretical yield, of the gas phase, in reactive species has been calculated. The hypothesis is that the impoverishment rate is linked to the reactivity of a molecule, and thus related to the sticking probability of a molecule onto a same surface [2].The impoverishment rate is defined as the ratio between the number of moles of Si (or Ge) deposited per minute (nDeposited) and the number of moles of silicon precursor (or germanium) injected per minute(nInjected) in the epitaxy reactor: αPrecursor = nDeposited/nInjected. The impoverishment rate of the disilane molecule increases much more rapidly than the silane molecule as the temperature increases (Fig.1a). Disilane is more consumed than silane which is relevant of the higher reactivity of disilane compared to silane.Moreover, the impoverishment of germane increases more rapidly with disilane than with silane (Fig.1b). The GeH4 contribution to epitaxy is more significant using disilane than silane. The impoverishment rate of precursors according to the germane flow will be presented later.Then, silane and disilane precursors have been compared with respect to the C incorporation at 550 °C. Ge concentration was fixed to 20 % with both silicon precursors. This leads to a growth rate of SiGe equal to 2.9 and 14.1 nm.min-1 using silane and disilane, respectively. XRD has been used to gain access to the “apparent” Ge concentration in Si1-x-yGexCy layers (Fig.2(a), (b)). Carbon atoms, much smaller than silicon and germanium, compensate the compressive strain induced by germanium in the SiGe layers. It then yields smaller Ge concentration in XRD than the real Ge concentration. The carbon in substitutional position can therefore be extracted using this coefficient: 1 % of carbon compensate 12 % of Ge [3].Using disilane, the substitutional carbon concentration linearly increases with the MS flow over the whole studied range (Fig.3). However, using silane, a deviation from the linearity is observed (above 0.7 %). In the linearity part of the curves, all the carbon incorporated are in substitutional site (as the total amount of carbon atoms increasing linearly with the MS flow [4]). When a non-linearity occurs, C atoms started to be incorporated in interstitial position also. SIMS measurement will be performed to confirm that. At 550 °C, the growth rate of SiGe is more important with disilane precursor due its higher reactivity than silane. Its leads to a better substitutional carbon incorporation. Indeed, using disilane, up to 1.15 % of fully substitutional carbon can be reached while using silane, only 0.7 % is reached.[1] V. Loup et al., J. Vac. Sci. Technol. B 21(1), Jan/Feb 2003.[2] D.J. Robbins et al., Journal of Applied Physics 69, 3729 (1991).[3] D. De Salvador et al., Phys. Rev. B, vol. 61, pp. 13005, 2000.[4] V. Loup et al., J. Vac. Sci. Technol. B 20(3), May/Jun 2002. Figure 1

We perform first principles calculations based on density functional theory withthe generalized gradient approximation (GGA) on the electronic and magneticproperties of carbon dopant in graphitic boron nitride (graphitic-BN) by the supercellmethod. It is found that carbon substitution for either boron or nitrogen atomsin graphitic-BN favours spin polarization over non-spin-polarization by 0.1 eV.The calculated electronic band structures show a spin-polarized, dispersionlessband near the Fermi energy. The spin polarization is found to originate fromthe carbon dopant and can be attributed to the carbon 2p electron. The lowerenergies and the resistivity to curvature effect of the spin polarization suggest apossibility of ferromagnetic ordering of the carbon dopants in many realistic BN-basednanostructures which possess a hexagonal network such as nanotubes. Energiesrequired for carbon substitution turn out to be within experimental access. Therandom substitution and the low doping energy indicate that graphitic-BN-basednanostructures can be viewed as candidates for metal-free magnet applications.

The ferromagnetic (FM) ordering in rutile TiO2 has been theoretically studied by substituting different [Formula: see text]-block elements (B, Al, C, Si, N, P and As) doped at oxygen site (B[Formula: see text], Al[Formula: see text], C[Formula: see text], Si[Formula: see text], N[Formula: see text], P[Formula: see text] and As[Formula: see text]) as well as at titanium site. Ab initio calculations in the frame work of density functional theory indicates that the [Formula: see text]-block elements (B, C, Al, Si, N, P and As) when substituting the oxygen site give significant amount of magnetic moment, but induce zero magnetic moment in case of substitution at Ti site. Spin–spin interaction for (Ti[Formula: see text]O[Formula: see text]X)2 with X = B, Al, C, Si, N, P and As system has also been studied with two different doping distances. Among all the possibilities, carbon substitution at oxygen site (C[Formula: see text]) results the most stable FM ordering in rutile TiO2.

We performed ab initio calculation on the pristine and carbon-doped (5,5) and (9,0) BN nanotubes. It was found that carbon substitution for either a single boron or a single nitrogen atom in the BN nanotubes can induce spontaneous magnetization. Calculations based on density functional theory with the local spin density approximation on the electronic band structure revealed a spin polarized, dispersionless band near the Fermi energy. The magnetization can be attributed to the carbon 2p electron. Compared to other theoretical models of light-element or metal-free magnetic materials, the carbon-doped BN nanotubes are more experimentally accessible and can be potentially useful.

It is known that the presence of even a small amount of impurity in interstitial positions can, depending on temperature, have a drastic influence on the one-dimensional (1-D) motion of self-interstitial atom (SIA) loops, and thus, on the accumulation of radiation damage in materials. In this study, atomic-scale computer simulations based on a recently developed optimization technique have been performed to evaluate the binding energies of SIA loops with interstitial carbon, a vacancy-carbon (V-C) complex, and a vacancy as a function of loop size in α-iron. While weak and strong attractive interactions are found when an interstitial carbon atom and a vacancy, respectively, are located on the perimeter of an SIA loop, the interactions for both quickly weaken approaching the loop center. In contrast, for a wide range of loop sizes, significantly higher binding energies are obtained between an SIA loop and a V-C complex located within the habit plane of the loop. A cluster dynamics model was developed by taking into account the trapping effects of V-C complexes on 1-D migrating SIA loops, and preliminary calculations were performed to demonstrate the validity of the assumed trapping mechanism through a comparison of the microstructural evolution with experimental data in neutron-irradiated α-iron.

Photo-catalytic degradation of Rhodamine B on C-, S-, N-, and Fe-doped TiO2 under visible-light irradiation

In order to study a magnetic principle of carbon based materials, multiple spin state of zigzag edge modified graphene molecules are analyzed by the first principle density functional theory to select suitable modification element. Radical carbon modified C64H17 shows that the highest spin state is most stable, which arises from two up-spin's tetrahedral molecular orbital configuration at zigzag edge. In contrast, oxygen modified C59O5H17 show the lowest spin state to be most stable due to four spins cancellation at oxygen site. Boron modified C59B5H22 have no {\pi}-molecular orbit at boron site to bring stable molecular spin state to be the lowest one. Whereas, C59N5H2 have two {\pi}-electrons, where spins cancel each other to give the stable lowest spin state. Silicon modified C59Si5H27 and Phosphorus modified C59P5H22 show curved molecular geometry due to a large atom insertion at zigzag site, which also bring complex spin distribution. Radical carbon and dihydrogenated carbon modification are promising candidates for designing carbon-based magnetic materials.

A multiscale approach including density functional theory (DFT) and atomistic kinetic Monte Carlo (AKMC) simulations is applied to investigate the diffusion of interstitial oxygen atoms in bcc Fe under the influence of substitutional foreign atoms (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo, and W). The substitutional atoms can be assumed to be immobile since their diffusion coefficient is much smaller than that of oxygen. First, jumps of oxygen in pure bcc Fe between first-, second-, and third-neighbor octahedral interstitial sites are investigated. It is found that the first-neighbor jump is most relevant, with the tetrahedral site as the saddle point. The second-neighbor jump consists of two consecutive first-neighbor jumps. The barrier for a direct third-neighbor jump is too high to be significant for the diffusion process. In the presence of substitutional atoms the most important migration paths are first-neighbor jumps between modified octahedral sites with modified tetrahedral sites as saddle points. Calculations show that Si, P, Ni, Mo, and W cause some modifications of the migration barriers of oxygen and their interaction with O is mainly repulsive. Al, Cr, and Mn have a significant influence on the barriers and they exhibit strong attractive interactions with O. The most important modification of the barriers is found for S, Ti, and Y where deep attractive states exist. Based on the migration energies obtained by DFT, AKMC simulations on a rigid lattice are employed to determine the diffusion coefficient of oxygen in a dilute iron alloy containing the different substitutional atoms. It is found that Si, P, Ni, Mo, and W have almost no influence on the diffusivity of O. The presence of Al, Cr, Mn, S, Ti, and Y causes a significant reduction of the mobility of oxygen. In these cases the temperature dependence of the oxygen diffusion coefficient shows considerable deviations from an Arrhenius law. These phenomena are discussed in detail by considering the occupation time for the different states. The present findings on the strong dependence on the kind of substitutional atoms change the picture of oxygen diffusion in dilute iron alloys substantially.

The elastic properties, electronic structure, and optical properties of X ([Formula: see text], Al, Ga) atomic substitution for arsenene are calculated based on density functional theory. The results show that, at the same concentration, B-substituted arsenene shows the best structural stability, followed by Ga-substituted arsenene, with Al-substituted arsenene being the least. Arsenene and its substitution systems possess excellent mechanical stability. The substitution of X atoms leads to a reduction in the bulk modulus, shear modulus, and Young’s modulus of arsenene systems, while simultaneously increasing the Poisson’s ratio. For the substitution systems, the four parameters all gradually increase as the substitution concentration increases. The substitution of X ([Formula: see text], Al, Ga) atoms into arsenene can facilitate the transformation from an indirect to direct bandgap (except for Al and Ga substitution at a concentration of 3.13%) with a narrower bandgap for increasing substitution concentration. The B–As bond shows the strongest covalency, followed by the Ga–As bond, while the Al–As bond exhibits the strongest ionicity. Arsenene substituted with X atoms shows anisotropy of optical properties in both the xy-plane and along the z-axis direction. The static dielectric constant increases with higher substitution concentration. The imaginary part of the dielectric function shows the extent of excited electron transitions between bands. After X atom substitution, arsenene exhibits a redshift in its absorption edge, which is wider with the higher substitution concentrations. The peak reflectance and valley transmittance values of arsenene and its substitution systems are located in the ultraviolet region in both xy-plane and z-axis.

Carbon is a typical impurity in thin films fabricated by metal-organic chemical-vapor deposition (MOCVD). The role of carbon in undoped and nitrogen-doped ZnO thin films was studied experimentally and theoretically to understand the possible compensation effects. ZnO thin films are fabricated by low-pressure MOCVD using diethylzinc, nitric oxide (for nitrogen-doped films), or oxygen precursors (for undoped films). Compared with sputtering-fabricated ZnO film, the carbon concentration in the MOCVD-fabricated ZnO film is very high. Furthermore, the MOCVD-fabricated ZnO:N film has an even higher carbon concentration than the undoped ZnO. Considering the signal observed previously by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, it is possible that the incorporated carbon has formed complexes with doped nitrogen. The first-principles calculations predict that the formation energy for carbon interstitial (Ci) is relatively high. However, due to the large binding energy between Ci and NO (nitrogen substitute on oxygen site), the formation energy of the carbon-nitrogen (CN) defect complex is lower than that of the Ci. As a result, with nitrogen doping, the carbon-impurity concentration would be high. In the insulator or p-type ZnO films, the CN defect complexes have 1+ charge state. Therefore, the existence of carbon in the ZnO:N film could be another possible passivation factor to the nitrogen acceptor, in addition to hydrogen.

The way in which martensitic transformation in iron alloys in high pulsed magnetic field can proceed via two different routes is explained. The first route corresponds to the Clausius–Clapeyron equation and can be considered “typical structural phase transition”. The second route is governed by the redistribution of the exchange-interaction forces between electrons in the fcc-lattice phase under the action of external factors which increase the interatomic distance (magnetostriction of paraprocess). Such a transformation route can be regarded as “the first-order magnetic phase transition”. In magnetic field, in some alloys the “structural phase transition” changes to “the first-order magnetic phase transition”. The hypothesis that the martensitic transformation in some steels and iron alloys can be regarded as “the first-order magnetic phase transition” even in the absence of a magnetic field (expansion of the lattice by spontaneous magnetostriction, carbon doping, etc.) is expressed.