Tricarbonate Research Articles

A study of very high resolution visible spectra of Titan: Line characterisation in visible CH[formula omitted] bands and the search for C[formula omitted

The atmosphere of Titan is a unique natural laboratory for the study of atmospheric evolution and photochemistry akin to that of the primitive Earth, with a wide array of complex molecules discovered through infrared and sub-mm spectroscopy. Here, we explore high resolution visible spectra of Titan (obtained with VLT-UVES) and retrieve an empirical high resolution list of methane absorption features at high resolution, (R=100.000) between 5250 Å and 6180 Å, for which no linelists are yet available. Furthermore, we search for the predicted, but previously undetected carbon trimer, C3, on the atmosphere of Titan, at its 4051 Å band. Our results are consistent with the presence of C3 at the upper atmosphere of Titan, with a column density of 1013 cm−2. This study of Titan’s atmosphere with high-resolution visible spectroscopy presents a unique opportunity to observe a planetary atmosphere where CH4 is the main visible molecular absorber, from which CH4 optical proprieties can be studied. It also showcases the use of a close planetary target to test new methods for chemical retrieval of minor atmospheric compounds, in preparation for upcoming studies of cold terrestrial exoplanet atmospheres.

Empirical rovibronic energy levels of C3

ABSTRACT The carbon trimer, C 3 also known as propadienediylidene, is a quasi-linear molecule with an unusual electronic structure and a very flat bending potential in its ground electronic state. C 3 is an important species in astrophysics and carbon plasmas. Observed transition wavenumbers within and between the X ~ 1 Σ g + and A ~ 1 Π u states of C 3 are extracted from 21 papers and then subjected to a Measured Active Rotational-Vibrational Energy Levels (MARVEL) analysis: a corrected list of 4940 transitions are inverted to yield 1887 empirical energy levels. Uncertainties for these levels are determined using a newly implemented bootstrap method. These levels will provide input for developing a full spectroscopic model for C 3 which can used to generate a line list for the X ~ 1 Σ g + and A ~ 1 Π u states.

A triatomic carbon and derived pentacarbides with superstrong mechanical properties

SummaryDiamond has the largest hardness of any natural material with an experimental Vickers hardness value of 90–150 GPa. Here, we reported the stable triatomic carbon allotrope with giant hardness closing that of diamond and a family of pentacarbides with superstrong mechanical properties from the state-of-the-art theoretical calculations. The triatomic carbon allotrope can be transformed into a two-dimensional carbon monolayer at a high temperature. We predicted that the triatomic carbon allotrope holds a hardness of 113.3 GPa, showing the potential capability of cracking diamond. Substitution with Al, Fe, Ir, Os, B, N, Si, W, and O element resulted in strong pentacarbides with Young’s modulus of 400–800 GPa. SiC5, BC5, IrC5, and WC5 are superhard materials with Vickers hardness over 40 GPa, of which BC5 was successfully synthesized in previous experimental reports. Our results demonstrated the potential of the present strong triatomic carbon and pentacarbides as future high-performance materials.

Ab initio and group theoretical study of properties of a carbon trimer defect in hexagonal boron nitride

Hexagonal boron nitride (h-BN) is a promising platform for quantum information processing due to its potential to host optically active defects with attractive optical and spin properties. Recent studies suggest that carbon trimers might be the defect responsible for single-photon emission in the visible spectral range in h-BN. In this theoretical study, we combine group theory together with density-functional theory (DFT) calculations to predict the properties of the neutral ${\mathrm{C}}_{2}{\mathrm{C}}_{\mathrm{N}}$ carbon trimer defect. We find the multi-electron states of this defect along with possible radiative and nonradiative transitions assisted by the spin-orbit and the spin-spin interactions. We also investigate the Hamiltonian for external magnetic-field and ground-state hyperfine interactions. Lastly, we use the results of our investigation in a Lindblad master-equation model to predict an optically detected magnetic resonance signal and the ${g}^{2}(\ensuremath{\tau})$ correlation function. Our findings can have important outcomes in quantum information applications such as quantum repeaters used in quantum networks and quantum sensing.

Carbon trimer as a 2 eV single-photon emitter candidate in hexagonal boron nitride: A first-principles study

Identifying the atomic origin of the well-recognized 2 eV single-photon emitters in hexagonal boron nitride (hBN) has remained a long debate and important for controllable single-photon emission generation. Here, the authors investigate the static and dynamic properties of carbon trimers from first-principles with many-body interactions. Based on the calculated results, they propose C${}_{2}$C${}_{N}$ to be a promising candidate, which has Zero-Phonon Line, Huang-Rhys factor, photoluminescence lifetime and lineshape all in agreement with the current experimental observations. The near-infrared emission from C${}_{2}$C${}_{N}$ is suggested to be another identifier for experimental verification of the nature of single-photon emitters in hBN.

MXene Phase with C3 Structure Unit: A Family of 2D Electrides

AbstractA new structural phase is discovered for M2CO2 MXenes with M = Sc, Y, La, Lu, Tm, and Ho. The hexagonal carbon layer sandwiched between M atoms, typical for MXenes, is transformed into C3 trimers with anionic electrons localized in quasi zero‐dimensional lattice spaces in‐between the C3 units, so the systems can be described as [M6 C3 O6]+II : 2e− electrides. The systems are readily ionized into [M6 C3 O6]+II with very low ionization energy via an anti‐doping mechanism. It is shown that this new structure of Sc2CO2 can bind multiple lithium atoms, with low migration barriers. The findings indicate that these M2CO2 MXenes with unusual carbon trimers are a new family of 2D electride insulators with the potential for charge storage applications, thermal field emission, and as anode material in lithium batteries.

Influence of graphite crystalline orientation on the carbon dissolution reaction in liquid iron: A ReaxFF molecular dynamics simulation study

Understanding the interaction of iron and carbon is of vital importance to guide the fundamental research in the fields of metallurgy and materials science. Reaxff molecular dynamics simulation method was used to study the carburizing reaction process between graphite and molten iron. It was found that the rise of temperature can promote the carburization reaction, and the graphite prism model needs to break through a potential energy barrier when the temperature rises to 2000 K, and then carbon atoms can diffuse into the molten iron more easily. Above 2000 K, the carbon saturation of the molten iron of the graphite prism model is significantly higher than that of the graphite basal model, which proves that different carbon materials and contact ways will have different effects on the carburizing results. And it also found that high temperature can promote the mass transfer. In addition, it is found that the carburizing reaction in the graphite basal model mainly occurred in the defect parts of the graphite sheet layer. There are no defects in the graphite prism model, but the carbon atoms can be stripped from the graphite structure in the form of single atoms, diatomic carbon chains and triatomic carbon chains.

First-Principles Identification of Single Photon Emitters Based on Carbon Clusters in Hexagonal Boron Nitride

A recent study associates carbon with single photon emitters (SPEs) in hexagonal boron nitride (h-BN). This observation, together with the high mobility of carbon in h-BN, suggests the existence of SPEs based on carbon clusters. Here, by means of density functional theory calculations, we studied clusters of substitutional carbon atoms up to tetramers in h-BN. Two different conformations of neutral carbon trimers have zero-point line energies and shifts of the phonon sideband compatible with typical photoluminescence spectra. Moreover, some conformations of two small C clusters next to each other result in photoluminescence spectra similar to those found in the experiments. We also showed that vacancies are unable to reproduce the typical features of the phonon sideband observed in most measurements because of the large spectral weight of low-energy breathing modes, ubiquitous in such defects.

Structure of graphene and a surface carbide grown on the (0001) surface of rhenium

Transition metal surfaces catalyse a broad range of thermally-activated reactions involving carbon-containing-species -- from atomic carbon to small hydrocarbons or organic molecules, and polymers. These reactions yield well-separated phases, for instance graphene and the metal surface, or, on the contrary, alloyed phases, such as metal carbides. Here, we investigate carbon phases on a rhenium (0001) surface, where the former kind of phase can transform into the latter. We find that this transformation occurs with increasing annealing time, which is hence not suitable to increase the quality of graphene. Our scanning tunneling spectroscopy and reflection high-energy electron diffraction analysis reveal that repeated short annealing cycles are best suited to increase the lateral extension of the structurally coherent graphene domains. Using the same techniques and with the support of density functional theory calculations, we next unveil, in real space, the symmetry of the many variants (two six-fold families) of a rhenium surface carbide observed with diffraction since the 1970s, and finally propose models of the atomic details. One of these models, which nicely matches the microscopy observations, consists of parallel rows of eight aligned carbon trimers with a so-called $(7\times\sqrt{\mathrm{19}})$ unit cell with respect to Re(0001).

Quantum Scattering Calculations for Rotational Excitations of C3 by Hydrogen Atom: Potential Energy Surfaces and Rate Coefficients

Ab initio calculations are performed to determine new potential energy surfaces for the ground state and low-lying excited states of C3 induced by collision with atomic hydrogen. The calculations are performed using the multireference configuration interaction method including Davidson's correction using aug-cc-pVQZ (augmented correlation consistent polarized valence quadruple zeta) basis sets. Nonadiabatic effects leading to avoided crossings are observed between ground and excited states. The computed points of the ground-state surface are fitted to an analytical form suitable for time-independent quantum scattering calculations of the state-to-state collisional cross sections. The close-coupling calculations are performed up to 1000 cm-1. Resonances are observed at very low energies. Among all the rotational transitions, Δj = 2 is found to be predominant for excitation. After Boltzmann thermal averaging collisional cross sections, rate coefficients for rotational levels j = 0, 2, ..., 8 are obtained and discussed covering the temperature up to 100 K. The magnitude of the state-to-state excitation rate obtained is maximum for j = 0 → 2 transition and decrease for other higher excitations. The results computed in this work will be crucially required to accurately model the abundance of carbon trimer and its hydrocarbon form in space.

In silico carbon molecular beam epitaxial growth of graphene on the h-BN substrate: carbon source effect on van der Waals epitaxy.

Against the presumption that hexagonal boron-nitride (h-BN) should provide an ideal substrate for van der Waals (vdW) epitaxy to grow high quality graphene films, carbon molecular beam epitaxy (CMBE) techniques using solid carbon sublimation have reported relatively poor quality of the graphene. In this article, the CMBE growth of graphene on the h-BN substrate is numerically studied in order to identify the effect of the carbon source on the quality of the graphene film. The carbon molecular beam generated by the sublimation of solid carbon source materials such as graphite and glassy carbon is mostly composed of atomic carbon, carbon dimers and carbon trimers. Therefore, the graphene film growth becomes a complex process involving various deposition characteristics of a multitude of carbon entities. Based on the study of surface adsorption and film growth characteristics of these three major carbon entities comprising graphite vapour, we report that carbon trimers convey strong traits of vdW epitaxy prone to high quality graphene growth, while atomic carbon deposition is a surface-reaction limited process accompanied by strong chemisorption. The vdW epitaxial behaviour of carbon trimers is found to be substantial enough to nucleate and develop into graphene like planar films within a nanosecond of high flux growth simulation, while reactive atomic carbons tend to impair the structural integrity of the crystalline h-BN substrate upon deposition to form an amorphous interface between the substrate and the growing carbon film. The content of reactive atomic carbons in the molecular beam is suspected to be the primary cause of low quality graphene reported in the literature. A possible optimization of the molecular beam composition towards the synthesis of better quality graphene films is suggested.

Defect stability in thorium monocarbide: An ab initio study**Project supported by the International S&T Cooperation Program of China (Grant No. 2014DFG60230), the National Natural Science Foundation of China (Grant No. 91326105), the National Basic Research Program of China (Grant No. 2010CB934504), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA02040104).

The elastic properties and point defects of thorium monocarbide (ThC) have been studied by means of density functional theory based on the projector-augmented-wave method. The calculated electronic and elastic properties of ThC are in good agreement with experimental data and previous theoretical results. Five types of point defects have been considered in our study, including the vacancy defect, interstitial defect, antisite defect, schottky defect, and composition-conserving defect. Among these defects, the carbon vacancy defect has the lowest formation energy of 0.29 eV. The second most stable defect (0.49 eV) is one of composition-conserving defects in which one carbon is removed to another carbon site forming a C2 dimer. In addition, we also discuss several kinds of carbon interstitial defects, and predict that the carbon trimer configuration may be a transition state for a carbon dimer diffusion in ThC.

Structural rearrangement in gas phase niobium carbide clusters, NbnC3 and NbnC6

Niobium carbide clusters were formed in the gas phase by mixing vapors of niobium and carbon. When a carbon trimer, C3, was richly formed in the vapor, clusters with compositions NbnC3k+ (n=2–10, k=0–4) were abundantly observed after photoionization at 215nm, enabling the presumption that a C3 unit could be a building block of the observed clusters. Contrary to this presumption, photoionization efficiency spectra in 215–286nm suggest that the clusters should be assigned to atomically mixed niobium carbide. Structural rearrangement of the clusters into the atomically mixed form is consistent with the results of DFT calculations.

Structural and electronic properties of the transition layer at the SiO2/4H-SiC interface

Using first-principles methods, we generate an amorphous SiO2/4H-SiC interface with a transition layer. Based this interface model, we investigate the structural and electronic properties of the interfacial transition layer. The calculated Si 2p core-level shifts for this interface are comparable to the experimental data, indicating that various SiCxOy species should be present in this interface transition layer. The analysis of the electronic structures reveals that the tetrahedral SiCxOy structures cannot introduce any of the defect states at the interface. Interestingly, our transition layer also includes a C-C=C trimer and SiO5 configurations, which lead to the generation of interface states. The accurate positions of Kohn-Sham energy levels associated with these defects are further calculated within the hybrid functional scheme. The Kohn-Sham energy levels of the carbon trimer and SiO5 configurations are located near the conduction and valence band of bulk 4H-SiC, respectively. The result indicates that the carbon trimer occurred in the transition layer may be a possible origin of near interface traps. These findings provide novel insight into the structural and electronic properties of the realistic SiO2/SiC interface.

Excitation and abundance of C3in star forming cores

We present spectrally resolved observations of triatomic carbon (C3) in several ro-vibrational transitions between the vibrational ground state and the low-energy nu2 bending mode at frequencies between 1654-1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N, using Herschel's HIFI instrument. We detect C3 in absorption arising from the warm envelope surrounding the hot core, as indicated by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C3 absorption due to diffuse foreground clouds. From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T_rot) of ~50--70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on the derived T_rot, we get column densities N(C3) ~7-9x10^{14} cm^{-2} and abundance x(C3)~10^{-8} with respect to H2. For W31C, using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x(C3)=10^{-8}, T_kin=30-50 K, N(C3)=1.5 10^{15} cm^{-2} fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis.

Surface interactions of C3 radicals during the deposition of fluorocarbon and hydrocarbon films

The gas-phase density and surface interactions of the carbon trimer C3 have been examined in fluorocarbon and hydrocarbon plasmas. The Πu1-Σg+1 fluorescence excitation spectra and relative gas-phase densities of C3 radicals have been collected using laser-induced fluorescence (LIF) spectroscopy. The relative C3 density increases significantly with CH2F2 in the feed, indicating that C3 is primarily produced via decomposing CH2F2 and chemical reactions in the gas phase. In addition, the surface reactivity R of C3 has been measured during fluorocarbon and hydrocarbon film depositions using C3F8∕CH2F2 and CH4∕CH2F2 13.56 MHz rf plasmas. The C3 radicals were characterized using our LIF-based imaging of radicals interacting with surfaces technique. R values for C3 range from 0.10 to 0.38, depending on plasma conditions, but show no clear dependence on the gas mixture or the plasma conditions used. X-ray photoelectron spectroscopy measurements of the films deposited in these systems provide additional evidence that suggests that C3 carbon clusters may be contributing to the formation of more cross-linked films.

Spectroscopic signatures of nonrigidity: Algebraic analyses of infrared and Raman transitions in nonrigid species

An algebraic scheme is proposed to explore the influence of nonrigidity upon spectroscopic (infrared/Raman) transitions. Application of this approach to large amplitude bending motion relies upon a Hamiltonian that embodies both rigidly-linear and rigidly-bent behavior, with the attendant U(3) dynamical algebra uniformly describing all pertinent operators. The bending mode supported by the quasilinear carbon trimer (C 3) is investigated, yielding algebraic predictions for vibrational energy level patterns and infrared transition matrix elements that are in reasonable accord with their ab initio counterparts. A coherent state formalism is employed to extract dependence of potential energy and dipole moment functions on C 3 bending coordinate.

Molecular Dynamics Simulation of Hydrocarbon Reflection and Dissociation Coefficients from Fusion-Relevant Carbon Surfaces

Reflection coefficients for carbon and hydrocarbon atoms/molecules on carbon-based surfaces are critically needed for plasma-surface interaction analysis in fusion devices, as carbon will continue to be used in next step devices like ITER. These have been calculated at different energies and angles with a molecular dynamics code using the Brenner hydrocarbon potential. Hydrogen saturated graphite was prepared by bombarding a graphite lattice with hydrogen, until a saturation at ~0.42 H:C. Carbon at 45° has a reflection coefficient (R) of 0.64 ± 0.01 at thermal energy, decreasing to 0.19 ± 0.01 at 10 eV. Carbon dimers (Rthermal = 0.51, R>1 eV ~ 0.10) tend to stick more readily than carbon trimers (Rthermal = 0.63, R10 eV = 0.16). Hydrocarbons reflect as molecules at thermal energies and break up at higher energies. The total reflection via these fragments decreases with energy, the number of unpaired electrons, and changing hybridization from sp3 to sp2 to sp. The results compare reasonably well with binary collision modeling for higher energies and experimental sticking data at thermal energies. A second surface, representing a “soft” redeposited carbon layer formed by the deposition of hydrocarbons onto a graphite surface, is also analyzed. In general, reflection is lower from the “soft” surface by 0.1–0.2. This reflection data can and has been incorporated in erosion/redeposition codes to allow improved modeling of chemically eroded carbon transport in fusion devices.

Interstellar C$_\mathsf{3}$ toward HD 210121

We report the detection of the 405 nm band of interstellar C3 in absorption toward HD 210121. The abundance of triatomic carbon is approximately 1/17 of that of diatomic carbon in the same diuse molecular cloud. Rotational levels of C3 up toJ = 14 are seen in this cloud. The rotational excitation of C3 in the interstellar medium may reflect a competition between inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. The abundance of C3 is compared with chemical models. Attention is called to molecular properties that need to be better determined.

Carbon-induced reconstructions on Si(111) investigated by RHEED and molecular dynamics

The adsorption of carbon trimers on Si(1 1 1) 1×1 is investigated by means of empirical molecular dynamics based on a Tersoff potential. A priori unknown carbon terminated Si(1 1 1) surfaces are modeled. The energetics of different adsorption sites, specifically the S5, T4 and H3 sites are investigated. The obtained structural models are used to simulate their response to a RHEED experiment. This enables us to further elucidate on the feasibility of the models by comparing theoretical data to in situ RHEED observations during the MBE experiments. Implications of the models to growth of 3C SiC by carbonization are discussed.