Cem Sevik

Thermal conductivity of BN-C nanostructures

Chemical and structural diversity present in hexagonal boron nitride ((h-BN) and graphene hybrid nanostructures provide new avenues for tuning various properties for their technological applications. In this paper we investigate the variation of thermal conductivity (

Mechanical and thermal properties of h-MX2 (M = Cr, Mo, W; X = O, S, Se, Te) monolayers: A comparative study

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Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

) of hybrid graphene/h-BN nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are investigated using equilibrium molecular dynamics. To simulate these systems, we have parameterized a Tersoff type interaction potential to reproduce the ab initio energetics of the B-C and N-C bonds for studying the various interfaces that emerge in these hybrid nanostructures. We demonstrate that both the details of the interface, including energetic stability and shape, as well as the spacing of the interfaces in the material exert strong control on the thermal conductivity of these systems. For stripe superlattices, we find that zigzag configured interfaces produce a higher

Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals

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Efficiency and harmonic enhancement trends in GaN-based Gunn diodes: Ensemble Monte Carlo analysis

in the direction parallel to the interface than the armchair configuration, while the perpendicular conductivity is less prone to the details of the interface and is limited by the

Gain and temporal response of AlGaN solar-blind avalanche photodiodes: An ensemble Monte Carlo analysis

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High dielectric constant and wide band gap inverse silver oxide phases of the ordered ternary alloys of SiO2, GeO2, and SnO2

of h-BN. Additionally, the embedded dot structures, having mixed zigzag and armchair interfaces, affects the thermal transport properties more strongly than superlattices. Though dot radius appears to have little effect on the magnitude of reduction, we find that dot concentration (50% yielding the greatest reduction) and composition (embedded graphene dots showing larger reduction that h-BN dot) have a significant effect.

Gunn oscillations in GaN channels

Using density functional theory, we obtain the mechanical and thermal properties of MX2 monolayers (where M = Cr, Mo, W and X = O, S, Se, Te). The Γ-centered phonon frequencies (i.e., A1, A2″, E′, and E″), relative frequency values of A1, and E′ modes, and mechanical properties (i.e., elastic constants, Young modulus, and Poissons ratio) display a strong dependence on the type of metal and chalcogenide atoms. In each chalcogenide (metal) group, transition-metal dichalcogenides (TMDCs) with W (O) atom are found to be much stiffer. Consistent with their stability, the thermal expansion of lattice constants for TMDCs with O (Te) is much slower (faster). Furthermore, in a heterostructure of these materials, the difference of the thermal expansion of lattice constants between the individual components becomes quite tiny over the whole temperature range. The calculated mechanical and thermal properties show that TMDCs are promising materials for heterostructures.

Harmonic enhancement of Gunn oscillations in GaN

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO2, GeO2, Al2O3, Si3N4, and Ge3N4. Their important polymorphs are considered which are for SiO2: α-quartz, α- and β-cristobalite and stishovite, for GeO2: α-quartz, and rutile, for Al2O3: α-phase, for Si3N4 and Ge3N4: α- and β-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving ab initio pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO2. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si3N4 and Ge3N4.

Significant effect of stacking on the electronic and optical properties of few-layer black phosphorus

For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger recombination (AR) and carrier multiplication (CM) lifetimes are computed exactly in a three-dimensional real space grid using empirical pseudopotential wave functions. Our results in support of recent experimental data offer new predictions. We extract simple Auger constants valid for NCs. We show that both Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lions share from the total excess energy and can cause a CM. We predict that CM becomes especially efficient for hot electrons with an excess energy of about 1 eV above the CM threshold.