Anisotropic Thermal Transport in Spray-Coated Single-Phase Two-Dimensional Materials: Synthetic Clay Versus Graphene Oxide.

Directional control on material properties such as mechanical moduli or thermal conductivity are of paramount importance for the development of nanostructured next-generation devices. Two-dimensional materials are particularly interesting in this context owing to their inherent structural anisotropy. Here, we compare graphene oxide (GO) and synthetic clay sodium fluorohectorite (Hec) with respect to their thermal transport properties. The unique sheet structure of both allows preparation of highly ordered Bragg stacks of these pure materials. The thermal conductivity parallel to the platelets strongly exceeds that perpendicular to them. We find a significant difference in the performance between GO and synthetic clay. Our analysis of the textured structure, size of the platelets, and chemical composition shows that Hec is a superior two-dimensional component to GO. Consequently, synthetic clay is a promising material for thermal management applications in electronic devices where electrically insulating materials are prerequisites.

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement.

Controlling thermomechanical anisotropy is important for emerging heat management applications such as thermal interface and electronic packaging materials. Whereas many studies report on thermal transport in anisotropic nanocomposite materials, a fundamental understanding of the interplay between mechanical and thermal properties is missing, due to the lack of measurements of direction-dependent mechanical properties. In this work, exceptionally coherent and transparent hybrid Bragg stacks made of strictly alternating mica-type nanosheets (synthetic hectorite) and polymer layers (polyvinylpyrrolidone) were fabricated at large scale. Distinct from ordinary nanocomposites, these stacks display long-range periodicity, which is tunable down to angstrom precision. A large thermal transport anisotropy (up to 38) is consequently observed, with the high in-plane thermal conductivity (up to 5.7 W m-1 K-1 ) exhibiting an effective medium behavior. The unique hybrid material combined with advanced characterization techniques allows correlating the full elastic tensors to the direction-dependent thermal conductivities. We, therefore, provide a first analysis on how the direction-dependent Young's and shear moduli influence the flow of heat.

Simple and High Yield Synthesis of Metal-Polymer Nanocomposites: The Role of Theta-Centrifugation as an Essential Purification Step.

Nanocomposites are an important materials class, which strives to foster synergistic effects from the intimate mixture of two vastly different materials. Inorganic nanoparticles decorated with polymer ligands, for instance, aim to combine the processing flexibility of polymers with the mechanical robustness of solid state materials. The fabrication and purification of such composite nanoparticles, however, still presents a synthetic challenge. Here, we present a simple synthesis of silver polystyrene nanocomposites with a controllable interparticle distance. The interparticle distance can be well-controlled with a few nanometer precision using polystyrene ligands with various molecular weights. The nanoparticle and polymer ligand synthesis yield both materials on gram scales. Consequently, the polymer nanocomposites can also be fabricated in such large amounts. Most importantly, we introduce Θ-centrifugation as a purification method, which is capable of purifying large nanocomposite batches in a reproducible manner. We employ a range of characterization methods to prove the successful purification procedure, such as transmission electron microscopy, thermogravimetric analysis, and dynamic light scattering. Our contribution will be of high interest for many groups working on nanocomposite materials, where the sample purification has been a challenge up to now.

Electronic excited states of tetracyanonickelate(II).

We revisit the assignment of the absorption spectrum of tetracyanonickelate(II) by calculating energies of excitations with time-dependent density functional theory. Our results give strong evidence that the original assignment of the spectrum is only partially correct. We thus propose an alternative assignment consistent with our theoretical calculations and all available experimental evidence. In particular, we reassign the bands at 22 400 and 32 300 cm(-1) to the (1)A(1g) --> (3)A(2g) (b(2g) --> b(1g)) and (1)A(1g) --> (1)A(2g) (b(2g) --> b(1g)) excitations.

Electronic structures of trans-dioxometal complexes.

We have employed computational methods based on density functional theory to elucidate the effects of equatorial ligands on the electronic structures of trans-dioxometal complexes. In complexes with amine (sigma-only) equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) excitation energy increases with metal oxidation state: Mo(IV) < Tc(V) < Ru(vi) and W(IV) < Re(V) < Os(VI). Increasing transition energies are attributed to enhanced oxometal pi-donor interactions in the higher valent central metals. But in complexes with cyanide equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) energy remains roughly independent of metal oxidation state, likely owing to the compensating increased pi-donation from the pi(CN) orbitals to the metal d(xy) orbitals as the oxidation state of the metal increases.

Ligand-field excited states of metal hexacarbonyls.

Over 35 years ago, the low-lying bands in the absorption spectra of metal hexacarbonyls were assigned to ligand-field (LF) excitations. Recent time-dependent density functional theory (TDDFT) calculations on M(CO)6 (M = Cr, Mo, W) are not in accord with this interpretation. Here we extend TDDFT calculations to the isoelectronic series V(CO)6-, Cr(CO)6, and Mn(CO)6+. By analyzing the trends in the energies of the various electronic excitations, we are able to fully assign the spectra of the complexes. In particular, we demonstrate that the LF excitation 1A1g -->1T1g is observed at 4.12 eV in the Mn(CO)6+ spectrum, but all LF features in the spectra of V(CO)6- and Cr(CO)6 are obscured by intense metal-to-ligand charge-transfer absorptions. Our results suggest that use of B3LYP as the exchange-correlation functional and inclusion of solvation effects through a continuum solvation model lead to the most accurate calculated transition energies.

Test of the Binding Threshold Hypothesis for olfactory receptors: explanation of the differential binding of ketones to the mouse and human orthologs of olfactory receptor 912-93.

We tested the Binding Threshold Hypothesis (BTH) for activation of olfactory receptors (ORs): To activate an OR, the odorant must bind to the OR with binding energy above some threshold value. The olfactory receptor (OR) 912-93 is known experimentally to be activated by ketones in mouse, but is inactive to ketones in human, despite an amino acid sequence identity of approximately 66%. To investigate the origins of this difference, we used the MembStruk first-principles method to predict the tertiary structure of the mouse OR 912-93 (mOR912-93), and the HierDock first-principles method to predict the binding site for ketones to this receptor. We found that the strong binding of ketones to mOR912-93 is dominated by a hydrogen bond of the ketone carbonyl group to Ser105. All ketones predicted to have a binding energy stronger than EBindThresh = 26 kcal/mol were observed experimentally to activate this OR, while the two ketones predicted to bind more weakly do not. In addition, we predict that 2-undecanone and 2-dodecanone both bind sufficiently strongly to activate mOR912-93. A similar binding site for ketones was predicted in hOR912-93, but the binding is much weaker because the human ortholog has a Gly at the position of Ser105. We predict that mutating this Gly to Ser in human should lead to activation of hOR912-93 by these ketones. Experimental substantiations of the above predictions would provide further tests of the validity of the BTH, our predicted 3D structures, and our predicted binding sites for these ORs.

Alpha-synuclein structures from fluorescence energy-transfer kinetics: implications for the role of the protein in Parkinson's disease.

Parkinson's disease is associated with the deposition and accumulation of alpha-synuclein fibrils in the brain. A30P and A53T mutations have been linked to the early-onset familial disease state. Time-resolved tryptophan fluorescence energy-transfer measurements have been used to probe the structures of pseudo-wild-type and mutant (A30P) alpha-synucleins at physiological pH (7.4), in acidic pH (4.4) solutions, and in the presence of SDS micelles, a membrane mimic. Fluorescent donor-energy acceptor (DA) distance distributions for six different tryptophan/3-nitro-tyrosine pairs reveal the presence of compact, intermediate, and extended conformations of the protein. CD spectra indicate that the protein develops substantial helical structure in the presence of SDS micelles. DA distributions show that micelles induce compaction in the N-terminal region and expansion of the acidic C terminus. In acidic solutions, there is an increased population of collapsed structures in the C-terminal region. Energy-transfer measurements demonstrate that the average DA distances for the W4-Y19 and Y19-W39 pairs are longer in one of the two disease-related mutants (A30P).