Thermal transport in nanocrystalline graphene investigated by approach-to-equilibrium molecular dynamics simulations

Approach-to-equilibrium molecular dynamics simulations have been used to study thermal transport in nanocrystalline graphene sheets. Nanostructured graphene has been created using an iterative process for grain growth from initial seeds with random crystallographic orientations. The resulting cells have been characterized by the grain size distribution based on the radius of gyration, by the number of atoms in each grain and by the number of atoms in the grain boundary. Introduction of nanograins with a radius of gyration of 1 nm has led to a significant reduction in the thermal conductivity to 3% of the value in single crystalline graphene. Analysis of the vibrational density of states has revealed a general reduction of the vibrational intensities and broadening of the peaks when nanograins are introduced which can be attributed to phonon scattering in the boundary layer. The thermal conductivity has been evaluated as a function of the grain size with increasing size up to 14 nm and it has been shown to follow an inverse rational function. The grain size dependent thermal conductivity could be approximated well by a function where transport is described by a connection in series of conducting elements and resistances (at boundaries).Comment: 9 pages, 9 figure

Thermal boundary resistance at Si/Ge interfaces determined by approach-to-equilibrium molecular dynamics simulations

The thermal boundary resistance of Si/Ge interfaces as been determined using approach-to-equilibrium molecular dynamics simulations. Assuming a reciprocal linear dependence of the thermal boundary resistance, a length-independent bulk thermal boundary resistance could be extracted from the calculation resulting in a value of 3.76x10$^{-9}$ m$^2$ K/W for a sharp Si/Ge interface and thermal transport from Si to Ge. Introducing an interface with finite thickness of 0.5 nm consisting of a SiGe alloy, the bulk thermal resistance slightly decreases compared to the sharp Si/Ge interface. Further growth of the boundary leads to an increase in the bulk thermal boundary resistance. When the heat flow is inverted (Ge to Si), the thermal boundary resistance is found to be higher. From the differences in the thermal boundary resistance for different heat flow direction, the rectification factor of the Si/Ge has been determined and is found to significantly decrease when the sharp interface is moderated by introduction of a SiGe alloy in the boundary layer.Comment: 7 pages, 6 figure

Effect of asymmetric concentration profile on thermal conductivity in Ge/SiGe superlattices

The effect of the chemical composition in Si/Ge-based superlattices on their thermal conductivity has been investigated using molecular dynamics simulations. Simulation cells of Ge/SiGe superlattices have been generated with different concentration profiles such that the Si concentration follows a step-like, a tooth-saw, a Gaussian, and a gamma-type function in direction of the heat flux. The step-like and tooth-saw profiles mimic ideally sharp interfaces, whereas Gaussian and gamma-type profiles are smooth functions imitating atomic diffusion at the interface as obtained experimentally. Symmetry effects have been investigated comparing the symmetric profiles of the step-like and the Gaussian function to the asymmetric profiles of the tooth-saw and the gamma-type function. At longer sample length and similar degree of interdiffusion, the thermal conductivity is found to be lower in asymmetric profiles. Furthermore, it is found that with smooth concentration profiles where atomic diffusion at the interface takes place the thermal conductivity is higher compared to systems with atomically sharp concentration profiles

chiral modification of platinum ab initio study of the effect of hydrogen coadsorption on stability and geometry of adsorbed cinchona alkaloids

Hydrogen coadsorption affects the orientation and stability of cinchona alkaloids used for the chiral modification of platinum and thus can influence their enantiodifferentiating ability

First Principles Analysis of H2O Adsorption on the (110) Surfaces of SnO2, TiO2 and Their Solid Solutions

Both associative and dissociative H2O adsorption on SnO2(110), TiO2(110), and Ti-enriched Sn1-xTixO2(110) surfaces have been investigated at low (1/12 monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO2(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H2O configurations. Increasing the coverage from 1/12 to 1 ML stabilized the (associatively or dissociatively) adsorbed H2O on SnO2(110) because of the formation of intermolecular H bonds. In contrast, on TiO2(110), the adsorption of isolated H2O groups (1/12 ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H2O adsorption on Ti-enriched Sn1-xTixO2(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti-6c and bridging O atoms that had to be broken upon H2O adsorption. As a result, the H2O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for 1/12 ML. This behavior was even more accentuated at high H2O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn1-xTixO2 gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H2O is facilitated at a high fractional coverage

Grain size-dependent thermal conductivity of polycrystalline twisted bilayer graphene

Abstract We report the room temperature thermal conductivity of polycrystalline twisted bilayer graphene (tBLG) as a function of grain size measured by employing a noncontact optical technique based on micro-Raman spectroscopy. Polycrystalline tBLG sheets of different grain sizes were synthesized on copper by hot filament chemical vapor deposition. The thermal conductivity values are 1305 ± 122 , 971 ± 73 , and 657 ± 42 W m − 1 K − 1 for polycrystalline tBLG with average grain sizes of 54, 21, and 8 nm, respectively. Based on these thermal conductivity values, we also estimated the grain boundary conductance, 14.43 ± 1.21 × 10 10 W m − 2 K − 1 , and the thermal conductivity for single crystal tBLG, 1510 ± 103 W m − 1 K − 1 . Our results show that the relative degradation of thermal conductivity due to grain boundaries is smaller in bilayer than in monolayer graphene. Molecular dynamics simulations indicate that interlayer interactions play an important role in the heat conductivity of polycrystalline bilayer graphene. The quantitative study of the grain size dependent thermal conductivity of polycrystalline bilayer graphene is valuable in technological applications as well as for fundamental scientific understanding

Effect of structural features on the thermal conductivity of SiGe-based materials

Approach-to-equilibrium molecular dynamics have been utilized to investigate the thermal transport in SiGe-based materials focusing on the effect of structural changes while the chemical composition has been kept constant. At a Si:Ge ratio of 1:1, the thermal conductivity has been evaluated for crystalline SiGe with a periodic (ZnS-like) and random (alloy) distribution of Si and Ge atoms, for SiGe nanocomposite and for amorphous SiGe. Thermal conductivity has been found to be lowest in amorphous SiGe (0.9 W/mK). In the regime studied here, a non-linear behavior of 1/κ to 1/Lz has been observed for amorphous SiGe, while a linear trend is found for all crystalline materials (ZnS-like, alloy and nanocomposite). This has been attributed to a wide spread in the mean free path of phonons dominating the thermal transport in the amorphous system

Coadsorption and Interaction of Quinolines and Hydrogen on Platinum Group Metals and Gold: A First-Principles Analysis

The coadsorption of quinoline and hydrogen, a crucial step in the hydrogenation of quinoline, has been examined by first-principles DFT simulations using the (111) face of the catalytically essential face-centered cubic metals rhodium, palladium, platinum, and gold employed for this reaction. Special attention was given to the energetics of coadsorbed hydrogen and quinoline, the configuration of quinoline, and changes in these properties with hydrogen surface coverage and hydrogen transfer from the metal surface to quinoline. In the absence of hydrogen, quinoline is found to be chemisorbed nearly flat-lying on the surface of platinum group metals via the π-bonding system of the aromatic moiety. In contrast, the favored adsorption mode of quinoline on Au is tilted to the metal surface, and σ-bonding via the N-lone pair prevails. These binding modes are in line with the calculated Löwdin charges and projected density of states of the quinoline–metal systems. The adsorption strengths of quinoline on the pristine (111) surfaces of the metals follow the order Rh > Pd > Pt ≫ Au. The stability of the coadsorbed configurations has been analyzed in terms of the hydrogen-binding energy, showing a preference of H to bind to the N atom of quinoline on Au and Pt, while on Pd and Rh, adsorption of H atoms to the metal surface is favored. On Au, hydrogen adsorption is energetically disfavored compared to the platinum group (PG) metals. Insertion of a bulky substituent at the C4 atom of quinoline has only little influence on its energetics and adsorption configuration, as exemplified by the adsorption of cinchonidine for imparting chirality to PG metal surfaces in catalytic enantioselective hydrogenations. Possible impacts of these adsorption behaviors on the hydrogenation of quinoline and the use of the quinoline derivative cinchonidine as a chiral modifier in enantioselective hydrogenation are discussed

Structural, Vibrational, and Thermal Properties of Nanocrystalline Graphene in Atomistic Simulations

Two different methods have been applied to create differently structured nanocrystalline graphene samples used in molecular dynamics simulations. In the first method, graphene sheets are generated by grain growth from individual nucleation seeds. The second method applies Voronoi tessellation to define single crystalline domains in the simulation cells. The differently generated nanocrystalline graphene sheets show significant variations in the grain size distribution and the shape of the crystalline domains. Furthermore, out-of-plane corrugation is found to be more pronounced in samples generated by the Voronoi method, in particular for small grain sizes (≤14 nm). Marginal differences are observed in the distribution of polygonal rings in the grain boundaries which might result from the geometrical shape of the grain boundaries. Thermal conductivity has been determined using the approach-to-equilibrium molecular dynamics formalism. A lower thermal conductivity is observed in Voronoi samples for grain sizes between 5 and 14 nm which is attributed to the stronger out-of-plane corrugatio

Thermal conduction and rectification phenomena in nanoporous silicon membranes

Non-equilibrium molecular dynamics simulations have been applied to study thermal transport properties, such as thermal conductivity and rectification, in nanoporous Si membranes. Cylindrical pores have been generated in crystalline Si membranes with different configurations, including step-like, ordered and random pore distributions. The effect of interface and overall porosity on thermal transport properties has been investigated as well as the impact of the porosity profile on the direction of the heat current. The lowest thermal conductivity and highest thermal rectification for equal porosity have been found for a step-like pore distribution. Increasing interface porosity resulted in an increase of thermal rectification, which has been found to be systematically higher for random pore distribution with respect to an ordered one. Furthermore, a maximum in rectification of 5.5% has been found for a specific overall porosity (phi(tot) = 0.02) in samples with constant interface porosity and ordered pore distribution. This has been attributed to an increased effect of asymmetric interface boundary resistance resulting from increased fluctuations of the latter with altering temperature. The average value of the interface boundary resistance has been found to decrease with increasing porosity for samples with ordered pore distribution leading to a decrease in thermal rectification