We report on the effect of germanium (Ge) coatings on the thermal transport properties of silicon (Si) nanowires using nonequilibrium molecular dynamics simulations. Our results show that a simple deposition of a Ge shell of only 1 to 2 unit cells in thickness on a single crystalline Si nanowire can lead to a dramatic 75% decrease in thermal conductivity at room temperature compared to an uncoated Si nanowire. By analyzing the vibrational density states of phonons and the participation ratio of each specific mode, we demonstrate that the reduction in the thermal conductivity of Si/Ge core-shell nanowire stems from the depression and localization of long-wavelength phonon modes at the Si/Ge interface and of high frequency nonpropagating diffusive modes.

In this section we present the details on simulations we performed. Initially N + 2 graphene layers of size L × L are placed along z axis with d = 3.35Å spacing between each other. Followed is the water block of size L × L × W and N + 2 more graphene layers of the same size. The resulting system is mirrored with respect to a XY plane and shifted along Z such that distance d between graphene layers of the original system and its symmetric image is maintained. The schematic of the setup is shown in Fig. 1 of the Main Text.Each simulation consists of three stages: equilibration to isothermal-isobaric (N, P, T ) ensemble; applying temperature gradient at canonical ensemble (N, V, T ); collecting the statistics at canonical ensemble while the temperature gradient is maintained. To achieve the temperature gradient, a high temperature Nosé-Hoover heat bath is applied to four central graphene layers and a low temperature Nosé-Hoover heat bath is applied to four outermost graphene layers (two leftmost and two rightmost). Periodic boundary conditions are applied in all directions. Equilibration always takes 400 ps. Applying temperature gradient takes from 1 ns to 3 ns depending on the number of used layers, and collecting the statistics takes from 2 ns to 3 ns.Water was modeled with flexible simple point charge (SPC) model [1]. We also perform a simulation using rigid SPC water model [2]. The result are almost identical to the corresponding simulation with flexible water model: see Table S1.Typical pressure evolution versus time is shown in the Fig. S3. High pressure oscillations are observed due to the high stiffness of both water and graphene. However, the average pressure remains constant. DATA EXTRACTIONKapitza resistance R K was calculated as R K = ∆T /J, where ∆T is the temperature jump at the solid-liquid interface, and J is the heat flux through the interface. The heat flux is defined as the conducted energy from the high temperature heat bath to the low temperature heat sink per unit time across unit area. Due to the symmetry of our simulation setup (Fig. 1), the heat flux can be computed as half of the slope of the energy change with respective to time in the heat bath: J = 0.5 d dt ∆E(t), where ∆E is induced energy in heat bath (Fig. S2). In our calculations, we average the local temperature in two symmetric copies. As shown in Fig. S1, ∆T is computed from the solid and liquid temperature at the interface. Solid temperature at the interface is determined by a linear fitting of the temperatures in different graphene layers. Water is divided into bins of 0.1Å in thickness along z, thus liquid temperature at the interface is determined by fitting the temperatures of water in different bins on a straight line. The error in Kapitza resistance is given by errors of two linear fits.Important to note that with the described procedure, we calculate two Kapitza resistances -one at a high temperature interface, and the other at a low temperature interface (Fig. S1). In all our simulation, the low temperature Kapitza resis...

Silicon nanoparticles are synthesized from a mixture of argon/silane in a continuous flow atmospheric-pressure microdischarge reactor. Particles nucleate and grow to a few nanometers (1−3 nm) in diameter before their growth is abruptly terminated in the short residence time microreactor. Narrow size distributions are obtained as inferred from size classification and imaging. As-grown Si nanoparticles collected in solution exhibit room-temperature photoluminescence that peaks at 420 nm with a quantum efficiency of 30%; the emission is stable for months in ambient air.

We present a two-dimensional Monte Carlo simulation of profile evolution during the overetching step of polysilicon-on-insulator structures, which considers explicitly ͑a͒ electric field effects during the charging transient, ͑b͒ etching reactions of energetic ions impinging on the poly-Si, and ͑c͒ forward inelastic scattering effects. Realistic energy and angular distributions for ions and electrons are used in trajectory calculations through local electric fields near and in the microstructure. Transient charging of exposed insulator surfaces is found to profoundly affect local sidewall etching ͑notching͒. Ion scattering contributions are small but important in matching experimental notch profiles. The model is validated by capturing quantitatively the notch characteristics and also the effects of the line connectivity and open area width on the notch depth, which have been observed experimentally by Nozawa et al. ͓Jpn. J. Appl. Phys. 34, 2107 ͑1995͔͒. Elucidation of the mechanisms responsible for the effect facilitates the prediction of ways to minimize or eliminate notching.

Absolute photonic band gaps in two-dimensional square and honeycomb lattices of circular crosssection rods can be increased by reducing the structure symmetry. The addition of a smaller diameter rod into the center of each lattice unit cell lifts band degeneracies to create significantly larger band gaps. Symmetry breaking is most effective at filling fractions near those which produce absolute band gaps for the original lattice. Rod diameter ratios in the range 0.1-0.2 yield the greatest improvement in absolute gap size. Crystal symmetry reduction opens up new ways for engineering photonic gaps.[S0031-9007 (96) The last few years have witnessed an ongoing search for periodic dielectric structures which give rise to a photonic band gap (PBG)-a region of the frequency spectrum where propagating modes are forbidden. These "photonic crystals" could alter radiation-matter interactions and thus improve the efficiency of optical devices by controlling spontaneous emission [1]. Applications of these crystals in semiconductor lasers and solar cells [1], and high-quality resonant cavities and filters [2] have been proposed. Although three-dimensional (3D) PBG crystals suggest the most interesting ideas for novel applications, two-dimensional (2D) structures could also find several important uses, as a result of their strong angular reflectivity properties over a wide frequency band. For example, 2D PBG crystals with absolute band gaps provide a large stop band for use as a feedback mirror in laser diodes [3]. Photonic gaps at visible to near-infrared (IR) wavelengths could have the widest impact in applications. As the band gap frequency is directly related to the size of the scattering elements comprising the lattice, a near-IR band gap requires features with dimensions in the submicron size regime. Fabricating 3D periodic structures in this regime poses an overwhelming challenge, despite progress in microfabrication technology. Perhaps for this reason attention has been drawn towards 2D lattice structures. The successful fabrication of 2D crystals with near-IR band gaps has been recently reported [4,5].

International audienceThe transfer functions and penetrations of five differential mobility analyzers (DMAs) for sub-2 nm particle classification were evaluated in this study. These DMAs include the TSI nanoDMA, the Caltech radial DMA (RDMA) and nanoRDMA, the Grimm nanoDMA, and the Karlsruhe-Vienna DMA. Measurements were done using tetra-alkyl ammonium ion standards with mobility diameters of 1.16, 1.47, and 1.70 nm. These monomobile ions were generated by electrospray followed by high resolution mobility classification. Measurements were focused at an aerosol-to-sheath flow ratio of 0.1. A data inversion routine was developed to obtain the true transfer function for each test DMA, and these measured transfer functions were compared with theory. DMA penetration efficiencies were also measured. An approximate model for diffusional deposition, based on the modified Gormley and Kennedy equation using an effective length, is given for each test DMA. These results quantitatively characterize the performance of the test DMAs in classifying sub-2 nm particles and can be readily used for DMA data inversion

We demonstrate reversible wetting and filling of open single-wall carbon nanotubes with mercury by means of electrocapillary pressure originating from the application of a potential across an individual nanotube in contact with a mercury drop. Wetting improves the conductance in both metallic and semiconducting nanotube probes by decreasing contact resistance and forming a mercury nanowire inside the nanotube. Molecular dynamics simulations corroborate the electrocapillarity-driven filling process and provide estimates for the imbibition speed and electrocapillary pressure.

Nanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. Using nonequilibrium molecular dynamics, we investigate how the thermal conductivity can be altered in core-shell nanocomposites of Si and Ge. By calculating the phonon vibrational density of states and performing normal mode analysis, we show that the thermal conductivity decreases when phonon-transport becomes diffusion-dominated and unveil a competition between modes from the various regions of the nanocomposite (core, interface, and shell). The effects of nanowire length, cross section, and temperature on thermal conductivity are explicitly considered. Surprisingly, the thermal conductivity variation with nanowire length is much weaker than in pure nanowires. Also, the thermal conductivity is almost independent of temperature in the wide region between 50 and 600 K, a direct result of confinement of the core by the shell. These results suggest that core-shell nanowires are promising structures for thermoelectrics.