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The thermoelectric transport properties of a metal–ceramic interface based on Al and γ-Al2O3 are explored by employing the non-equilibrium Green's function formalism (NEGF) coupled with density functional theory (DFT). However, to acquire the phonon thermal conductance, the parameterized ReaxFF potential is utilized for computing the intrinsic force constants of propagating phonons across the interface. Several interfacial electronic properties such as the charge transfer, the potential barrier, and the atomic orbital overlap are critically analyzed based on the DFT derived results of the electrostatic difference potential, the electron density difference, and the spin-polarized density of states in the fully relaxed structure of the interface. Within the NEGF framework, both the electron and phonon transmission coefficients are estimated for the variations of bias voltage and temperature gradient across the interface. The strong orbital overlap and the scattering of electrons and phonons at the nanometer-size interface suppress the lattice thermal conductivity significantly compared to the electron transport, which in turn enhances the thermoelectric performance of the Al/Al2O3 composite, in contrast to the bulk material of Al. Moreover, a steep rise of power factor induced by the increased transmission of charge carriers with temperature improves the energy conversion efficiency of the material. The present findings could pave the way for developing thermoelectric materials based on metal–ceramic composites. [ABSTRACT FROM AUTHOR]

Binding capabilities of hydrogen molecules on Li doped metal carbide nanotubes (MCNT, M = Si, Ge, Sn) have been compared from ab initio-based calculations using quantum mechanical /molecular mechanics methodology. Geometry optimizations are carried out at the ONIOM2 (B3LYP/6-31 + G(d)//LANL2DZ:UFF) level of theory with coronene ring as a model in the high layer. Lithium binding energies on each of the pure armchair (5,5) and zigzag (6,0) MCNTs are calculated. Adsorptions of one and two H2 molecules on the Li doped nanotubes are successively studied. In both cases the complexes are found to be stable and the average adsorption energy falls in between −18 and −38 kcal/mol. Stabilities of Li@MCNT, H2/Li@MCNT, and (H2)2/Li@MCNT systems are confirmed from the HOMO-LUMO gap and three chemical reactivity descriptors such as chemical potential, hardness and electrophilicity index. The charge polarization mechanism plays a pivotal role in the H2 adsorption on the surface of Li@MCNT. The dispersion corrected M06-2X functional is also included in the calculation to investigate the changes in the H2 binding properties. The interaction of H2 with the lithium doped MCNT has also been analyzed from the total electron density maps and density of states projected to different atoms. [ABSTRACT FROM AUTHOR]

Adsorptions of small toxic molecules such as CO, N2, HCN, SO2, H2CO, and NH3 on a single-walled (6,0) SnC nanotube (SnCNT) are investigated using Quantum Mechanics/Molecular Mechanics (QM/MM) methodology. The calculations are carried out at the B3LYP/6-311++G(d,p)//LANL2DZ:UFF level of theory. The high layer of the model consists of a pyrene-type ring on the nanotube surface as the adsorption site, where one gas molecule is allowed to interact. Conversely, for the adsorption of the two molecules, a larger site like a coronene ring is used for the high layer. Adsorption energy, Gibbs free energy change, Mulliken charge transfer, and total electron-density maps are computed in each case. The adsorption strength of the gas molecule on the SnCNT surface is also analyzed from the density of states projected to different atoms (PDOS) of the nanotube-adsorbate complexes. The adsorptions of CO and N2 on the (6,0) SnCNT surface require to cross potential barriers, and the corresponding transition structures are identified by ONIOM-IRC calculations. For the remaining four molecules, the processes of adsorption are predicted to be barrier-less. The calculations for the adsorption of H2CO on (5,0) and (7,0) SnCNT surfaces are extended to study the effect of the size of the nanotube. Results for the adsorption of a single molecule on (6,0) SnCNT using B3LYP functional are compared with those obtained from a dispersion corrected functional such as M06-2X. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Highlights: [•] The absorptions of O2 on pure and binary clusters of germanium and tin are studied. [•] PSPW-DFT calculations are carried out to optimize the structure of the complexes. [•] Adsorption energy, HOMO–LUMO gap, ΔG, and ΔH due to adsorption are calculated. [•] NBO charges show the amount of charge transfer between the cluster and oxygen. [ABSTRACT FROM AUTHOR]

Highlights: [•] Adsorptions of small molecules on (6,0)GeCNT are studied using ONIOM calculations. [•] The interactions of N2 and CO are found to be endothermic with a large barrier. [•] Adsorption energy, charge transfer, and DOS are used to determine the binding. [•] SO2, NH3, and H2CO molecules bind strongly towards GeCNT. [ABSTRACT FROM AUTHOR]

Electronic structures of neutral lead–silicon clusters of up to six atoms are determined by global optimization of the potential energy hyper surface using Metropolis Monte Carlo and genetic algorithm (GA) methods followed by a bond strength propensity model, and by local optimization using density functional theory at the B3LYP/aug-cc-pVTZ-pp level. For each cluster, geometries of those stable isomers whose relative energies are within 1.0 eV are reported. Average binding energy per atom, band gap, ionization potential (IP), electron affinity (EA), and vertical detachment energy of the most stable isomer of each type of cluster are computed to study their dependences on the size of the cluster. Polarizability, chemical potential, hardness and electrophilicity index of the most stable isomers are estimated to study their relative stabilities and chemical reactivities. Relative strengths of metal–metal interactions are determined from a study of all one-step fragmentation processes. IR intensities and Raman activities for all the vibrational modes of the most stable isomers are calculated. [ABSTRACT FROM AUTHOR]

Abstract: Electronic structures of tin–germanium binary clusters up to five atoms obtained by searching global optimization of the energy surface using Metropolis Monte Carlo (MMC) and genetic algorithm (GA) followed by bond strength propensity model (BSPM) and by local optimization using density functional theory (DFT) are reported. Geometries and properties of the most stable cation, neutral, and anion clusters are calculated at the B3LYP/aug-cc-pVTZ-pp level of theory. The average binding energy per atom is calculated to study its dependence on the size of the cluster. The HOMO–LUMO energy gap, ionization potential (IP), electron affinity (EA), and vertical detachment energy (VDE) are also computed for the most stable isomer of each cluster. The reactivity descriptors such as chemical potential, chemical hardness, and electrophilicity index of the lowest-energy isomers of these clusters are estimated to study their relative stabilities. The one-step fragmentation patterns of the clusters are studied to determine the relative strength of the Ge–Ge, Sn–Ge, and Sn–Sn interactions by computing the fragmentation energies. A four membered ring formed by the Sn2Ge2-b cluster shows π-aromaticity with a NICS(1) value of −13.2ppm. [Copyright &y& Elsevier]