Lattice Thermal Transport in Monolayer Group 13 Monochalcogenides MX (M = Ga, In; X = S, Se, Te): Interplay of Atomic Mass, Harmonicity, and Lone-Pair-Induced Anharmonicity.

We perform a systematic study of the lattice dynamics and the lattice thermal conductivity, κ, of monolayer group 13 monochalcogenides MX (M = Ga, In; X = S, Se, Te) by combining an iterative solution for linearized phonon Boltzmann transport equation and density functional theory. Among the competing factors influencing κ, harmonic parameters along with the atomic masses dominate over anharmonicity. An increase in atomic mass leads to a decrease in phonon frequencies and phonon group velocities and consequently in κ. At T = 300 K, the calculated κ values are 54.9, 48.1, 44.3, 25.0, 22.3, and 17.3 W m-1 K-1 for GaS, InS, GaSe, InSe, GaTe, and InTe monolayers, respectively. Further analysis of anharmonic scattering rates and average scattering matrix elements evidences that the anharmonicity characterized by the third-order IFCs in GaS and InS are the largest among all monolayer group 13 monochalcogenides despite the largest κ values. This is attributed to a strong interaction between nonbonding lone-pair s electrons around the S atom and adjacent bonding electrons. In addition, the κ of these monolayers further reduces to 50% for sample sizes 300-400 nm. Our findings provide fundamental insights into thermal transport in monolayer group 13 monochalcogenides and should stimulate further experimental exploration of thermal transport in these materials for possible theromoelectric and thermal management applications.

Phonon transport and thermoelectric properties of semiconducting Bi2Te2X (X = S, Se, Te) monolayers.

Confinement or dimensionality reduction is a novel strategy to reduce the lattice thermal conductivity and, consequently, to improve the thermoelectric conversion performance. Bismuth and tellurium based low-dimensional materials have great potential in this regard. The phonon transport and thermoelectric properties of Bi2Te2X (X = S, Se, Te) monolayers are systematically investigated by employing density functional theory and the Boltzmann transport equation. The calculated lattice thermal conductivity of these 2D systems ranges from ∼1.3 W m-1 K-1 (Bi2Te2Se) to ∼1.5 W m-1 K-1 (Bi2Te3) for a 10 µm system size at room temperature and considering spin-orbit coupling in harmonic force constants. This remarkably low lattice thermal conductivity is attributed to small group velocities and enhanced anharmonic phonon scattering rates. A detailed analysis is presented in terms of mode-level phonon group velocities, anharmonic scattering rates and phonon mean free paths. Our results reveal that the thermal transport in these 2D systems is dominated by in-plane transverse acoustic modes. Additionally, the thermal conductivity can be further reduced by decreasing the sample size due to phonon-boundary scattering. The thermoelectric properties including the Seebeck coefficient, power factor and electrical conductivity are calculated using the semi-classical Boltzmann transport equation within the rigid band approximation. The low thermal conductivities coupled with their high carrier mobilities lead to good thermoelectric power factors. With optimal carrier doping, a figure of merit ∼0.6 can be achieved at room temperature, which increases to ∼0.8 at 700 K, thus making them promising candidates for thermoelectric applications.

Synthesis, characterization, DNA binding and in vitro antimicrobial studies of a novel tetra-substituted N-isopropyl-N-(4-ferrocenylphenyl)-N'-(2,6-diethylphenyl)-N″-benzoylguanidine: crystallographic structure and quantum chemical computations.

A novel tetra-substituted guanidine, N-isopropyl-N-(4-ferrocenylphenyl)-N'-(2,6-diethylphenyl)-N″-benzoylguanidine (1), [(CH3)2CH)(C5H5FeC5H4C6H4)NC(NHCOC6H5)(NHC6H3(CH2CH3)2] has been synthesized and characterized by elemental analysis, FT-IR, multinuclear ((1)H, (13)C) NMR spectroscopy, single crystal X-rays diffraction analysis and density functional theory based quantum chemical calculations. The torsion angles indicating that the guanidine moiety and carbonyl group are almost co-planar, due to the pseudo hexagonal ring formed by intramolecular N-H⋯O hydrogen bonds. The DNA interaction studies performed by cyclic voltammetry and UV-visible spectroscopy are in close agreement with the binding constants (K) 1.4×10(4) and 1.2×10(4) respectively. The shift in peak potential, current and absorption maxima of the studied ferrocenyl guanidine in the presence of DNA discovered that CV coupled with UV-vis spectroscopy could provide an opportunity to elaborate DNA interaction mechanism, a prerequisite for the design of new drug like agents and understanding the molecular basis of their action. The synthesized compound (1) has also been screened for their antibacterial and antifungal.

A quadratically convergent VBSCF method.

A quadratically convergent valence bond self-consistent field method is described where the simultaneous optimisation of orbitals and the coefficients of the configurations (VB structures) is based on a Newton-Raphson scheme. The applicability of the method is demonstrated in actual calculations. The convergence and efficiency are compared with the Super-CI method. A necessary condition to achieve convergence in the Newton-Raphson method is that the Hessian is positive definite. When this is not the case, a combination of the Super-CI and Newton-Raphson methods is shown to be an optimal choice instead of shifting the eigenvalues of the Hessian to make it positive definite. In the combined method, the first few iterations are performed with the Super-CI method and then the Newton-Raphson scheme is switched on based on an internal indicator. This approach is found computationally a more economical choice than using either the Newton-Raphson or Super-CI method alone to perform a full optimisation of the nonorthogonal orbitals.

Zinc coordination to the bapbpy ligand in homogeneous solutions and at liposomes: zinc detection via fluorescence enhancement.

In this work, the complexation of the bapbpy ligand to zinc dichloride is described (bapbpy = 6,6'-bis(2-aminopyridyl)-2,2'-bipyridine). The water-soluble, colorless complex [Zn(bapbpy)Cl]Cl·2H2O (compound 2·H2O) was synthesized; its X-ray crystal structure shows a mononuclear, pentacoordinated geometry with one chloride ligand in apical position. Upon excitation of its lowest-energy absorption band (375 nm) compound 2 shows intense emission (Φ = 0.50) at 418 nm in aqueous solution, and an excited state lifetime of 5 ns at room temperature. Photophysical measurements, DFT, and TD-DFT calculations prove that emission arises from vibronically coupled Ligand-to-Ligand Charge Transfer singlet excited states, characterized by electron density flowing from the lone pairs of the non-coordinated NH bridges to the π* orbitals of the pyridine rings. Monofunctionalization of the ligand with one long alkyl chain was realized to afford ligand 3, which can be inserted into dimyristoylphosphatidylglycerol (DMPG) or dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles. For negatively charged DMPG membranes the addition of a zinc salt to the vesicles leads to an enhancement of the fluorescence due to zinc coordination to the membrane-embedded tetrapyridyl ligand. No changes were observed for the zwitterionic DMPC lipids, where binding of the Zn ions does not take place. A modest binding constant was found (5 × 10(6) M(−1)) for the coordination of zinc cations to bapbpy-functionalized DMPG membranes, which allows for the detection of micromolar zinc concentrations in aqueous solution. The influence of chloride concentration and other transition metal ions on the zinc binding was evaluated, and the potential of liposome-supported metal chelators such as ligand 3 for zinc detection in biological media is discussed.

Resonance and aromaticity: an ab initio valence bond approach.

Resonance energy is one of the criteria to measure aromaticity. The effect of the use of different orbital models is investigated in the calculated resonance energies of cyclic conjugated hydrocarbons within the framework of the ab initio Valence Bond Self-Consistent Field (VBSCF) method. The VB wave function for each system was constructed using a linear combination of the VB structures (spin functions), which closely resemble the Kekulé valence structures, and two types of orbitals, that is, strictly atomic (local) and delocalized atomic (delocal) p-orbitals, were used to describe the π-system. It is found that the Pauling-Wheland's resonance energy with nonorthogonal structures decreases, while the same with orthogonalized structures and the total mean resonance energy (the sum of the weighted off-diagonal contributions in the Hamiltonian matrix of orthogonalized structures) increase when delocal orbitals are used as compared to local p-orbitals. Analysis of the interactions between the different structures of a system shows that the resonance in the 6π electrons conjugated circuits have the largest contributions to the resonance energy. The VBSCF calculations also show that the extra stability of phenanthrene, a kinked benzenoid, as compared to its linear counterpart, anthracene, is a consequence of the resonance in the π-system rather than the H-H interaction in the bay region as suggested previously. Finally, the empirical parameters for the resonance interactions between different 4n+2 or 4n π electrons conjugated circuits, used in Randic's conjugated circuits theory or Herdon's semi-emprical VB approach, are quantified. These parameters have to be scaled by the structure coefficients (weights) of the contributing structures.

Generation of Kekulé valence structures and the corresponding valence bond wave function.

A new scheme, called "list of nonredundant bonds", is presented to record the number of bonds and their positions for the atoms involved in Kekulé valence structures of (poly)cyclic conjugated systems. Based on this scheme, a recursive algorithm for generating Kekulé valence structures has been developed and implemented. The method is general and applicable for all kinds of (poly)cyclic conjugated systems including fullerenes. The application of the algorithm in generating Valence Bond (VB) wave functions, in terms of Kekulé valence structures, is discussed and illustrated in actual VB calculations. Two types of VBSCF calculations, one involving Kekulé valence structures only and the second one involving all covalent VB structures, were performed for benzene, pentalene, benzocyclobutadiene, and naphthalene. Both strictly local and delocalised p-orbitals were used in these calculations. Our results show that, when the orbitals are restricted to their own atoms, other VB structures (Dewar structures) also have a significant contribution in the VB wave function. When removing this restriction, the other VB structures (Dewar and also the ionic structures) are accommodated in the Kekulé valence structures, automatically. Therefore, at VBSCF delocal level, the ground states of these systems can be described almost quantitatively by considering Kekulé valence structures only at a considerable saving of time.