Thermal transport in multilayer silicon carbide nanoribbons: reverse non-equilibrium molecular dynamics

The heat conduction performance of materials has a crucial role in deciding their functional efficiency. For this purpose, the present study explores the structural and thermal properties of multilayer silicon carbide nanoribbons (SiCNRs). At first, we realize that the smallest values of cohesive energy correspond to the system with the largest interlayer distance due to vdW forces. The effects of stacking layers, their number, edge chirality, ribbon width, temperature (T) as well as coupling strength between the layers on the thermal conductivity, are all examined and discussed, using reverse nonequilibrium molecular dynamics. This results in an anisotropic trend of κ in terms of some parameters due to phonon scattering. By analyzing the various phonon properties, including phonon density of states, phonon dispersion relations as well as phonon mean free path, we gain critical insights into the mechanism of heat conduction in the systems. System size results reveal that thermal conductivities follow an increasing behavior with length and a decreasing trend with width as well as temperature, which is attributed to the phonon-phonon scattering rate. Furthermore, the thermal conductivities drift from the normal 1/T law and show an anomalous decreasing behavior above room temperature. Overall, these results offer a deep understating towards the thermal conductivity of n-SiCNRs and could promote their potential applications in thermoelectric and nanoelectronic devices.

Graphene-based SiC Van der Waals heterostructures: nonequilibrium molecular dynamics simulation study

The structural properties and thermal conductivity of graphene-based SiC heterostructures are investigated using the reverse nonequilibrium molecular dynamics. The C/SiC/C heterostructure has the greatest value of cohesive energy due to the effect of vdW interactions between layers. The surfaces of heterostructures begin to ripple as a direct consequence of the plane fluctuations observed around T = 400 K. The thermal conductivity at room temperature is determined. The length and the armchair and zigzag orientations increase the magnitude of κ which decreases with increasing temperature. This change is attributed to the phonon Umklapp scattering and phonon cross-plane couplings. The impact of point vacancy, bi-vacancy and edge vacancy in a concentration range up to 2% is also discussed. The localization of low-frequency phonons around the vacancy induces a decaying characteristic of thermal conductivity. The effect depends on the type of vacancy and is more pronounced in heterostructures with point vacancy. The present results make pristine and defective heterostructures promising materials for various thermoelectric applications with tunable functionalities.

Thermal strain engineering of mechanical properties in Si-based hybrid sheets via molecular dynamics simulations

The mechanical properties of pristine and defective Si-based hybrid sheets are studied using molecular dynamics calculations for a temperature ranging from 100 to 800 K, in conjunction with a variable strain rate. When increasing temperature, the melting phase of the hybrids occurs from the solid to the liquid phase, while the increase in the strain rate enhances their elastic parameters. The absence of plastic stage reveals that the fracture pattern is brittle in these 2D materials. Under the uniaxial loading, the systems stretch, resulting in the failure of the crystalline skeletons that lose their rigidity with anisotropic behavior observed only for SiC. In defective hybrids, the point defects reduce the values of fracture strength and strain without affecting the brittle behavior of the sheets. The results impart that coupling high temperature to SiC material offers new possibilities for MEMS devices, whereas SiGe is a promising candidate for microelectronic devices.

Engineering silicon-carbide quantum dots for third generation photovoltaic cells

Interested in the recent development of the building up of photovoltaic devices using graphene-like quantum dots as a novel electron acceptor; we study in this work the optoelectronic properties of edge-functionalized SiC quantum dots using the first principles density functional. For an accurate quantitative estimation of key parameters, a many-body perturbation theory within GW approximation is also performmed. We examine the ability to tailor the electronic gap and optical absorption of the new class of QDs through hydroxylation and carboxylation of seam atoms, in order to improve their photovoltaic efficiency. The HOMO-LUMO energy gap was significantly altered in terms of the type, the concentration and the position of functional groups. The spatial charge separation and charge transfer characterizing our systems seem very prominent to use as dye-sensitized solar cells. Furthermore, the optical band gap of all our compounds is in the NIR-visible energy window, and exhibits a magnitude smaller than that calculated in the pristine case, which enhances the photovoltaic efficiency. Likewise, absorption curves, exciton binding energy and singlet-triplet energy splitting have been broadly modified by functionalization confirming the great luminescent yield of SiCQDs. Depending on the size, SiC quantum dots absorb light from the visible to the near-infrared region of the solar spectrum, making them suitable for third generation solar cells.

A signature index for third order topological insulators

In this work, we develop an index signature characterising the third order topological phases in 3D systems. This index is an alternating sum of monomial signatures of Higgs triplet values at 3D corners. We extend our method toN-dimensional systems with open boundaries, and demonstrate that the topological invariant can be efficiently generalised to any space dimension including the second order topological insulators. Known results on lower dimensional systems are recovered and an interpretation in the Higgs space parameters is given.

New highly efficient 2D SiC UV-absorbing material with plasmonic light trapping

The present paper is a systematic analysis of the thermoelectric and optical properties of the SiC monolayer. Based on the density functional theory (DFT) combined with the Boltzmann transport theory, the thermal conductivity, the electrical conductivity and the figures of merit are all determined and discussed for the SiC hybrid. At room temperature, it is found that SiC shows interesting values with respect to its counterparts graphene and silicene. To improve the absorption of the SiC sheet, a strategy is proposed using finite-difference time-domain (FDTD) combing with PSO-based approach. The absorbance of the UV-photodector with SiC monolayer and the SiC-based photodector with Au plasmonic grating are studied. Among our findings, the Au plasmonic grating enhances the absorbance of SiC to reach a maximum absorbance of 99.6% at the resonance wavelength of ([Formula: see text] nm), which significantly improves the performance of UV-sensors. Therefore, by combining optical DFT analysis with FDTD simulation supported by global PSO optimization, we have been able to develop a new SiC monolayer high performance UV photodetector suitable for advanced optoelectronic applications.

Graphene and silicene quantum dots for nanomedical diagnostics

In the present work, the prominent effects of edge functionalization, size variation and base material on the structural, electronic and optical properties of diamond shaped graphene and silicene quantum dots are investigated. Three functional groups, namely (–CH₃, –OH and –COOH) are investigated using the first principles calculations based on the density functional, time-dependent density functional and many-body perturbation theories. Both the HOMO–LUMO energy gap, the optical absorption and the photoluminescence are clearly modulated upon functionalization compared to the H-passivated counterparts. Besides the functional group, the geometric distortion induced in some QDs also influences their optical features ranging from near ultra-violet to near infra-red. All these results indicate that edge-functionalizations provide a favorable key factor for adjusting the optoelectronic properties of quantum dots for a wide variety of nanomedical applications, including in vitro and in vivo bioimaging in medical diagnostics and therapy.

In the present work, the prominent effects of edge functionalization, size variation and base material on the structural, electronic and optical properties of diamond shaped graphene and silicene quantum dots are investigated.

Size engineering optoelectronic features of C, Si and CSi hybrid diamond-shaped quantum dots

Based on the density functional theory and many-body ab initio calculations, we investigate the optoelectronic properties of diamond-shaped quantum dots based graphene, silicene and graphene-silicene hybrid. The HOMO-LUMO (H-L) energy gap, the exciton binding energy, the singlet-triplet energy splitting and the electron-hole overlap are all determined and discussed. Smaller nanostructures show high chemical stability and strong quantum confinement resulting in a significant increase in H-L gap and exciton binding energy. On the other hand, the larger configurations are reactive which implies characteristics favorable to possible electronic transport and conductivity. In addition, the typically strong splitting between singlet and triplet excitonic states and the big electron-hole overlap make these QDs emergent systems for nanomedicine applications.

Electron-phonon dynamics in 2D carbon based-hybrids XC (X = Si, Ge, Sn)

The effect of the presence of electron-phonon (e-ph) coupling in the SiC, GeC and SnC hybrids is studied in the framework of the ab initio perturbation theory. The electronic bang gap thermal dependence reveals a normal monotonic decrease in the SiC and GeC semiconductors, whereas SnC exhibits an anomalous behavior. The electron line widths were evaluated and the contributions of acoustic and optical phonon modes to the imaginary part of the self-energy were determined. It has been found that the e-ph scattering rates are globally controlled by the out-of-plane acoustic transverse mode ZA in SiC while both ZA and ZO are overriding in GeC. In SnC, the out-of-plane transverse optical mode ZO is the most dominant. The relaxation lifetime of the photo-excited electrons shows that the thermalization of the hot carrier occurs at 90 fs, 100 fs and 120 fs in SiC, GeC and SnC, respectively. The present study properly describes the subpicosecond time scale after sunlight illumination using an approach that requires no empirical data. The results make the investigated structures suitable for providing low cost and high-performance optical communication and monitoring applications using 2D materials.

Tunable optical and excitonic properties of phosphorene via oxidation

The optical properties and excitonic wave function of phosphorene oxides (PO) are studied using the first principle many-body Green function and the Bethe-Salpeter equation formalism. In this work, the optical properties are determined using ab initio calculations of the dielectric function. At the long wavelength limit q [Formula: see text] of EM wave (i.e. [Formula: see text]), the dielectric function, the absorption spectrum, the lectivity, the electron energy loss spectra (EELS) and the wave function are calculated. The results show an excitonic binding energy of 818 meV with a bright exciton located in the armchair direction in pristine phosphorene. For PO, the arrangement of the oxygen atoms significantly influences the optical properties. In particular, the absorption spectrum is extended along the solar spectrum, with a high absorption coefficient observed in the dangling structures. The maximum lectivity values are observed for the high energies of the light spectrum. Moreover, the first EELS peak is located in the visible region in all the structures except for one configuration that exhibits the same behavior as pure phosphorene. Finally, the exciton effect reveals that all PO conformers have a dark exciton state, which is suitable for long-lived applications.

Tuning Optoelectronic Properties of the Graphene-Based Quantum Dots C16- xSi xH10 Family

The electronic and optical properties of graphene-based quantum dots (QDs) are investigated using DFT and many-body perturbation theory. Formation energy, hardeness and electrophilicity show that all structures, from pyrene to silicene QD passing through 15 CSi QD configurations, are energetically and chemically stable. It is also found that they are reactive which implies their favorable character for the possible electronic transport and conductivity. The electronic and optical properties are very sensitive to the number and position of the substituted silicon atoms as well as the directions of the light polarization. Moreover, quantum confinement effects make the exciton binding energy of CSi quantum dots larger than those of their higher dimensional allotropes such as silicene, graphene, and SiC sheet and nanotube. It is also higher those of other shapes of quantum dots like hexagonal graphene QDs and can be tailored from the ultraviolet region to the visible one. The values of the singlet-triplet splitting determined for the X- and Y-light polarized indicate that all configurations have a high fluorescence quantum yield compared to the yield of typical semiconductors, which makes them very promising for various applications such as the light-emitting diode material and nanomedicine.

Halogenation of SiC for band-gap engineering and excitonic functionalization

The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their realization in laboratory is thus possible. Large band-gaps of the order of 4 eV are found in the so-called GW approximation, with the occurrence of bright excitons, optically active in the four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV depending on the decoration choice and in one case, a dark exciton is foreseen to exist in the fully chlorinated SiC. The wide variety of opto-electronic properties suggest halogenated SiC as interesting materials with potential not only for solar cell applications, anti-reflection coatings or high-reflective systems but also for a possible realization of excitonic Bose-Einstein condensation.

Half-oxidized phosphorene: band gap and elastic properties modulation

Based on a first principles approach, we study structural, electronic and elastic properties, as well as stabilities of all possible half-oxidized phosphorene conformers. Stability analysis reveals that oxygen chemisorption is an exothermic process in the six configurations despite the formation of interstitial oxygen bridges in three of them. Electronic structure calculations show that oxidation induces a band gap modulation ranging between 0.54 and 1.57 eV in the generalized gradient approximation corrected to 1.19 and 2.88 eV using GW. The mechanical response of the conformers is sensitively dependent on direction and indicates that the new derivatives are incompressible materials and one configuration has an auxetic behavior. The present results provide a basis for tailoring the electronic and elastic properties of phosphorene via half oxidation.

DFT investigations of the hydrogenation effect on silicene/graphene hybrids

We report here a study on the effect of hydrogenation on a new one-atom thick material made of silicon and carbon atoms (silicene/graphene (SG) hybrid) within density functional theory. The structural, electronic and magnetic properties are investigated for non-, semi- and fully hydrogenated SG hybrids in a chair configuration and are compared with their parent materials. Calculations reveal that pure SG is a non-zero band gap semi-conductor with stable planar honeycomb structure. So mixing C and Si in an alternating manner gives another way to generate a finite band gap in one-atom thick materials. Fully hydrogenation makes the gap larger; however half chemical modification with H reduces the gap in favor of ferromagnetism order. The findings of this work open a wide spectrum of possibilities for designing SG-based nanodevices with controlled and tuned properties.