Simulation of dimensionality effects in thermal transport

The discovery of nanostructures and the development of growth and fabrication techniques of one- and two-dimensional materials provide the possibility to probe experimentally heat transport in low-dimensional systems. Nevertheless measuring the thermal conductivity of these systems is extremely challenging and subject to large uncertainties, thus hindering the chance for a direct comparison between experiments and statistical physics models. Atomistic simulations of realistic nanostructures provide the ideal bridge between abstract models and experiments. After briefly introducing the state of the art of heat transport measurement in nanostructures, and numerical techniques to simulate realistic systems at atomistic level, we review the contribution of lattice dynamics and molecular dynamics simulation to understanding nanoscale thermal transport in systems with reduced dimensionality. We focus on the effect of dimensionality in determining the phononic properties of carbon and semiconducting nanostructures, specifically considering the cases of carbon nanotubes, graphene and of silicon nanowires and ultra-thin membranes, underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture Notes in Physics volume "Thermal transport in low dimensions: from statistical physics to nanoscale heat transfer" (S. Lepri ed.

NH3\u2013NO Coadsorption System on Pt(111). II. Intermolecular Interaction

Coadsorption of ammonia and nitric oxide on the (111) surface of platinum causes the mutual stabilization of the two adsorbed species, arranged in an ordered 2 7 2 mixed layer. Furthermore, their interaction leads also to stable, isolated triangular units, which we observe on the surface after annealing to 345 K. Having provided in the preceding article (10.1021/jp406068y) a detailed structural description of the NH3\u2013NO mixed layer, we focus here on the stabilizing intermolecular interactions. By combining scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we identify the isolated triangular units as formed by one NH3 and three NO molecules, and we characterize them in terms of structure, energetics, and charge rearrangement. Eventually, we investigate the nature of the chemical bond between the coadsorbed NH3 and NO both in the mixed layer and in the isolated triangular units, pointing out the essential role of the surface mediation in inducing attractive dipole\u2013dipole interactions and the presence of hydrogen bonds

NH3\u2013NO Coadsorption System on Pt(111). I. Structure of the Mixed Layer

In the selective catalytic reduction (SCR) process, nitrogen oxides are selectively transformed to N2 by reductants such as ammonia. The specificity of this reaction on platinum-based catalysts was tentatively attributed to the formation of NH3\u2013NO coadsorption complexes, as indicated by several surface science techniques. Here we combine scanning tunneling microscopy (STM) and density functional theory (DFT) calculations to characterize the NH3\u2013NO complex at the atomic scale on the (111) surface of platinum, investigating the intermolecular interactions that tune the selectivity. In this first article, we analyze the structures that arise upon coadsorption of NH3 and NO in terms of adsorption sites, geometry, energetics, and charge rearrangement. An ordered 2 7 2 adlayer forms, where the two molecules are arranged in a configuration that maximizes mutual interactions. In this structure, NH3 adsorbs on-top and NO on fcc-hollow sites, leading to a cohesional stabilization of the extended layer, calculated to be 0.29 eV/unit cell. The calculated vibrational energies of the coadsorption structure agree with the experimental values found in the literature

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors

Trapping of Charged Gold Adatoms by Dimethyl Sulfoxide on a Gold Surface

10siWe report the formation of dimethyl sulfoxide (DMSO) molecular complexes on Au(111) enabled by native gold adatoms unusually linking the molecules via a bonding of ionic nature, yielding a mutual stabilization between molecules and adatom(s). DMSO is a widely used polar, aprotic solvent whose interaction with metal surfaces is not fully understood. By combining X-ray photoelectron spectroscopy, low temperature scanning tunneling microscopy, and density functional theory (DFT) calculations, we show that DMSO molecules form complexes made by up to four molecules arranged with adjacent oxygen terminations. DFT calculations reveal that most of the observed structures are accurately reproduced if, and only if, the negatively charged oxygen terminations are linked by one or two positively charged Au adatoms. A similar behavior was previously observed only in nonstoichiometric organic salt layers, fabricated using linkage alkali atoms and strongly electronegative molecules. These findings suggest a motif for anchoring organic adlayers of polar molecules on metal substrates and also provide nanoscale insight into the interaction of DMSO with gold.partially_openembargoed_20160616Feng, Zhijing; Velari, Simone; Cossaro, Albano; Castellarin-Cudia, Carla; Verdini, Alberto; Vesselli, Erik; Dri, Carlo; Peressi, Maria; De Vita, Alessandro; Comelli, GiovanniFeng, Zhijing; Velari, Simone; Cossaro, Albano; Castellarin Cudia, Carla; Verdini, Alberto; Vesselli, Erik; Dri, Carlo; Peressi, Maria; DE VITA, Alessandro; Comelli, Giovann