Phonons and Thermal Transport in Si/SiO2 Multishell Nanotubes: Atomistic Study

Thermal transport in the Si/SiO2 multishell nanotubes is investigated theoretically. The phonon energy spectra are obtained using the atomistic lattice dynamics approach. Thermal conductivity is calculated using the Boltzmann transport equation within the relaxation time approximation. Redistribution of the vibrational spectra in multishell nanotubes leads to a decrease of the phonon group velocity and the thermal conductivity as compared to homogeneous Si nanowires. Phonon scattering on the Si/SiO2 interfaces is another key factor of strong reduction of the thermal conductivity in these structures (down to 0.2 Wm−1K−1 at room temperature). We demonstrate that phonon thermal transport in Si/SiO2 nanotubes can be efficiently suppressed by a proper choice of nanotube geometrical parameters: lateral cross section, thickness and number of shells. We argue that such nanotubes have prospective applications in modern electronics, in cases when low heat conduction is required.

Extent partitions and context extensions

We prove that the extent partitions of a formal context $$\mathbb{K}: = (G,M,I)$$ can be constructed from the box extents of it, which form a complete atomistic lattice. $$\mathbb{K}$$ is called a one-object extension of the subcontext (H, M, J) if it is obtained by adding a new element with attributes in M to the set H. We investigate the interplay between the box extents of (H, M, J) and those of its one-object extension $$\mathbb{K}$$, and describe those extent partitions of (H, M, J) which can be extended to $$\mathbb{K}$$.

Atomistic modeling of Ru nanocluster formation on graphene/Ru(0001): Thermodynamically versus kinetically directed-assembly

ABSTRACTAtomistic lattice-gas models for thermodynamically and kinetically directed assembly are applied to Ru nanocluster formation on a monolayer of graphene supported on Ru(0001) at 309 K. Nanocluster density, mean size, height distribution, and spatial ordering are analyzed by kinetic Monte Carlo simulations. Both models can reproduce the experimental data, but additional density functional theory analysis favors the former.

Comparison of Atomistic and Continuum Methods for Calculating Ballistic Phonon Transmission in Nanoscale Waveguides

We compare two methods for the calculation of mode dependent ballistic phonon transmission in nanoscale waveguides. The first method is based on continuum acoustic waveguide theory and uses an eigenmode expansion to solve for phonon transmission coefficients. The second method is an atomistic, lattice dynamics (LD)-molecular dynamics (MD) hybrid that uses LD computed mode shapes to excite guided phonon wavepackets in a nonequilibrium MD simulation and calculates phonon transmission from the final distribution of system energy. The two methods are compared for a planar waveguide with a t-stub irregularity, a geometry which has been proposed for the tuning of phonon transmission and nanostructure thermal conductance. Our comparison highlights advantages and disadvantages of the two methods and illustrates regimes when atomistic effects are prominent and continuum approaches are not appropriate.

Morphological Evolution during Growth and Erosion on Vicinal Si(100) Surfaces: From Electronic Structure Analyses to Atomistic and Coarse-Grained Modeling

ABSTRACTStepped Si(100) surfaces exhibit alternating stiff SA and meandering SB steps, and thus constitute a so-called AB-vicinal surface. Both growth by Molecular Beam Epitaxy (MBE) or Chemical Vapor Deposition (CVD), and erosion by ion sputtering or chemical etching, induce step pairing, although different factors contribute. In addition, more complex pattern formation often occurs during step train motion. We synthesize recent developments in modeling of these processes ranging from ab-initio electronic structure approaches for key surface energetics, to atomistic lattice-gas modeling, to coarse-grained sharp-interface (front-tracking) and smeared-interface (phase-field) step dynamics approaches. We briefly describe development of new formalisms related to coarse-grained approaches, as well as selected results for step pairing.

Competitive Etching and Oxidation of Vicinal Si(100) Surfaces

ABSTRACTExposure of a vicinal Si(100) surface to oxygen at around 550 C produces etching-mediated step recession. In addition, some oxide islands are formed which locally pin receding steps. We develop an atomistic lattice-gas model for this process which accounts for the interplay between oxygen surface chemistry (adsorption, diffusion, oxide formation, and etching via SiO desorption) and the silicon surface and step dynamics (anisotropic diffusion and aggregation of di-vacancies formed by etching, and ad-dimer attachment-detachment dynamics at steps incorporating anisotropic energetics). Kinetic Monte Carlo simulation of this model produces step morphologies retaining some qualitative but not quantitative features of their equilibrium structure (alternating rough SB steps and smooth SA steps), except for pinning which produces protruding “fingers”. These features are seen in Scanning Tunneling Microscopy studies.