Tuning the Anisotropic Thermal Transport in {110}-Silicon Membranes with Surface Resonances.

Understanding the thermal transport in nanostructures has important applications in fields such as thermoelectric energy conversion, novel computing and heat dissipation. Using non-homogeneous equilibrium molecular dynamic simulations, we studied the thermal transport in pristine and resonant Si membranes bounded with {110} facets. The break of symmetry by surfaces led to the anisotropic thermal transport with the thermal conductivity along the [110]-direction to be 1.78 times larger than that along the [100]-direction in the pristine structure. In the pristine membranes, the mean free path of phonons along both the [100]- and [110]-directions could reach up to ∼100 µm. Such modes with ultra-long MFP could be effectively hindered by surface resonant pillars. As a result, the thermal conductivity was significantly reduced in resonant structures, with 87.0% and 80.8% reductions along the [110]- and [100]-directions, respectively. The thermal transport anisotropy was also reduced, with the ratio κ110/κ100 decreasing to 1.23. For both the pristine and resonant membranes, the thermal transport was mainly conducted by the in-plane modes. The current work could provide further insights in understanding the thermal transport in thin membranes and resonant structures.

Adsorption of azide-functionalized thiol linkers on zinc oxide surfaces.

A comprehensive understanding of the interactions between organic molecules and a metal oxide surface is essential for an efficient surface modification and the formation of organic-inorganic hybrids with technological applications ranging from heterogeneous catalysis and biomedical templates up to functional nanoporous matrices. In this work, first-principles calculations supported by experiments are used to provide the microstructural characteristics of (101̄0) surfaces of zinc oxide single crystals modified by azide terminated hydrocarbons, which graft on the oxide through a thiol group. On the computational side, we evaluate the specific interactions between the surface and the molecules with the chemical formula N3(CH2) n SH, with n = 1, 3, 6, 9. We demonstrate that the molecules chemisorb on the bridge site of ZnO(101̄0). Upon adsorption, the N3(CH2) n SH molecules break the neutral (Zn δ+-O δ-) dimers on ZnO(101̄0) resulting in a structural distortion of the ZnO(101̄0) substrate. The energy decomposition analysis revealed that such structure distortion favors the adsorption of the molecules on the surface leading to a strong correlation between the surface distortion energy and the interaction energy of the molecule. An azide-terminated thiol with three methylene groups in the hydrocarbon chain N3(CH2)3SH was synthesized, and the assembly of this linker on ZnO surfaces was confirmed through atomic force microscopy. The bonding to the inorganic surface was examined via X-ray photoelectron spectroscopy (XPS). Clear signatures of the organic components on the oxide substrates were observed underlying the successful realization of thiol-grafting on the metal oxide. Temperature-dependent and angle-resolved XPS were applied to examine the thermal stability and to determine the thickness of the grafted SAMs, respectively. We discuss the high potential of our hybrid materials in providing further functionalities towards heterocatalysis and medical applications.

The influence of a solvent on the electronic transport across diamondoid-functionalized biosensing electrodes.

Electrodes embedded in nanopores have the potential to detect the identity of biomolecules, such as DNA. This identification is typically being done through electronic current measurements across the electrodes in a solvent. In this work, using quantum-mechanical calculations, we qualitatively present the influence of this solvent on the current signals. For this, we model electrodes functionalized with a small diamond-like molecule known as diamondoid and place a DNA nucleotide within the electrode gap. The influence of an aqueous solvent is taken explicitly into account through Quantum-Mechanics/Molecular Mechanics (QM/MM) simulations. From these, we could clearly reveal that at the (111) surface of the Au electrode, water molecules form an adlayer-like structure through hydrogen bond networks. From the temporal evolution of the hydrogen bond between a nucleotide and the functionalizing diamondoid, we could extract information on the conductance across the device. In order to evaluate the influence of the solvent, we compare these results with ground-state electronic structure calculations in combination with the non-equilibrium Green's function (NEGF) approach. These allow access to the electronic transport across the electrodes and show a difference in the detection signals with and without the aqueous solution. We analyze the results with respect to the density of states in the device. In the end, we demonstrate that the presence of water does not hinder the detection of a mutation over a healthy DNA nucleotide. We discuss these results in view of sequencing DNA with nanopores.

A multi-dimensional quasi-zeolite with 12 × 10 × 7-ring channels demonstrates high thermal stability and good gas adsorption selectivity.

A novel quasi-zeolite PKU-15, with a rare 3-dimensional structure containing interconnected large (12-ring), medium (10-ring) and small (7-ring) multi-pore channels, was hydrothermally synthesised and characterised. A unique tri-bridging O2- anion is found to be encapsulated in the cage-like (Ge,Si)12O31 building unit and energetically stabilises the PKU-15 framework. The removal of this oxygen atom would convert PKU-15 into a hypothetical zeolite PKU-15H. Thus, PKU-15 can be considered as a unique 'quasi-zeolite', which bridges porous germanates and zeolites. Owing to the absence of terminal Ge-OH groups in its structure, PKU-15 shows a remarkably high thermal stability of up to 600 °C. PKU-15 is also the first microporous germanate that exhibits permanent porosity, with a BET area of 428 m2 g-1 and a good adsorption affinity toward CO2.