The role of chemical contacts in molecular conductance

Density-functional calculations show that biaryl dithiolates compared with analogues having dicarbene and diisocyanide alligator clips have a lower conductance arising from a lower density of electronic states at the metal electrode Fermi energy. The differences in state density originate from shifts in the energies of molecular orbitals that extend across the molecule to the electrodes. Nonlocal effects consistent with the compounds' electronegativities account for the shifts in orbital energies, beyond the electrostatic potential variations from charge transfer at the metal-molecule interface.     

Thermal contact conductance of pressed contacts at low temperatures

The influence of variations of interface temperature in the range 50-300 K on the thermal contact conductance between aluminium and stainless steel joints was determined. Predictions were done by modeling the deformation at the interface for different values of surface finish and contact pressure over the range of interface temperatures. Both elastic and plastic deformation was considered. Experiments were carried out in a closed loop cryostat and the results were shown to compare well with the predictions. A reduction of the interface temperature resulted in a smaller value of thermal contact conductance. Interfacial pressure variation had much lower influence at the smaller value of temperatures. The role of surface roughness at the contact was also seen to be less significant at lower interface temperatures and the zone of hysteresis was smaller. A correlation was developed for estimating thermal contact conductance at joints over this temperature range. An explicit dependence of contact conductance on temperature was not seen to be necessary as long as the changes in the hardness and thermal conductivity of the material with temperature are incorporated in the correlation.     

Brightening of the lowest exciton in carbon nanotubes via chemical functionalization

Using time-dependent density functional theory, we found that chemical functionalization at low concentrations of single-walled carbon nanotubes (SWNTs) locally alters the π-conjugated network of the nanotube surface and leads to a spatial confinement of the electronically excited wave functions. Depending on the adsorbant positions, the chemisorption significantly modifies the optical selection rules. Our modeling suggests that photoluminescent efficiency of semiconducting SWNT materials can be controlled by selective chemical functionalization.     

Negative differential conductance of quasi-one-dimensional contacts

The negative differential conductance (NDC) with an S-shaped static current-voltage characteristic is investigated in quasi-one-dimensional microcontacts. It is shown that NDC can be caused by the space charge of the carriers in a channel with narrowings. Pis’ma Zh. Tekh. Fiz. 24, 40–44 (June 12, 1998)     

Remarkable enhancement in failure stress and strain of penta-graphene via chemical functionalization

Penta-graphene (PG),a newly proposed two-dimensional material composed entirely of carbon pentagons,is believed to possess much lower failure stress and strain than those of graphene.An open question is whether and how these properties can be enhanced.Herein using molecular dynamics simulations,we examine the deformation and failure processes of PG functionalized with different functional groups.We reveal that complete chemical functionalization leads to remarkable increases in the failure stress and strain of PG by up to 86.6％ and 82.4％,respectively.The underlying reason for this enhancement is that the buckled pentagonal rings in pristine and partially functionalized PGs can easily transform into planar polygon rings under stretching;in contrast,complete functionalization of PG strongly stabilizes its structure and prevents such transformation,thereby significantly increasing the failure stress and strain.Our findings suggest a possible route to enhance the mechanical properties of PG for potential applications in nanocomposites and nanodevices.     

Manipulating the chemical affinity and kinetics of 3D silica particle network via the phase-separation technique

We developed a simple and versatile technique for a particle’s self-organizing-network based on a non-solvent induced micro-phase separation (NIPS). When a good solvent vaporizes from a particle dispersion in a ternary solution including the polymer, good solvent and non-solvent, the suspension is separated into the polymer network and non-solvent phase. If the affinity between the particles and polymer is sufficient enough, the particles are entrapped in the polymer network and particle network can be achieved. To expand this technique to particles with various physical properties, the surface of the particles was identified using the Hansen dispersibility parameter (HDP). From a comparison of the HDP of the unmodified and modified silica, an NH₂ group is suitable for entrapment of the silica by cellulose acetate as the polymer. However, with an increase in number of the silica particles, entrapment of the silica in the polymer was prevented. Control of the phase separation rate by the lowering temperature leaded to entrapment of silica particles in the polymer network. The proposed technique is effective not only for spherical oxide particles, but also for non-oxides, various shapes and structures. Depending on particle characteristics, functional films and bulk materials for thermal insulation, light diffusion, and electro conductivity can be obtained.     

Manipulating Kondo temperature via single molecule switching

Two conformations of isolated single TBrPP-Co molecules on a Cu(111) surface are switched by applying +2.2 V voltage pulses from a scanning tunneling microscope tip at 4.6 K. The TBrPP-Co has a spin-active cobalt atom caged at its center, and the interaction between the spin of this cobalt atom and free electrons from the Cu(111) substrate can cause a Kondo resonance. Tunneling spectroscopy data reveal that switching from the saddle to a planar molecular conformation enhances spin-electron coupling, which increases the associated Kondo temperature from 130 to 170 K. This result demonstrates that the Kondo temperature can be manipulated just by changing molecular conformation without altering chemical composition of the molecule.     

Apparatus to measure thermal conductance

An apparatus for measuring the thermal conductance of insulating mechanical supports under compression is described. It requires no installation of thermometers on the sample. Results on sample supports are presented by plots of the heat conducted between two temperatures as a function of the force compressing the support. The importance of contact resistance is discussed.     

Manipulating feature sizes in Si-based grating structures by thermal oxidation

We report a method for manipulating feature sizes in Si-based grating structures by thermal oxidation, which allows the realization of fin width/period ratios not directly accessible by laser interference lithography. Taking advantage of the expansion in volume associated with the thermal oxidation of Si, grating structures with very high fin width/period ratios of the order of 0.96 were obtained, whereas subsequent chemical etching of the oxide yields grating structures with fin width/period ratios as small as ～0.06.     

Rapid thermal annealing of Ti Schottky contacts ton-GaAs

Ti Schottky contacts were formed onn-GaAs surfaces and were subjected to rapid thermal annealing (RTA) at various temperatures. Low temperature RTA (<500 °C) results in a reduction in the diode leakage currents and increase in the barrier voltage. High temperature RTA (>500 °C) results in progressive degradation of the diode parameters. The improvement in diode parameters is expected to be due to better adhesion of Ti Schottky contact resulting in a more intimate contact of the Schottky metal to the GaAs substrate.     

Liquid Plasmonics: Manipulating Surface Plasmon Polaritons via Phase Transitions

This paper reports the manipulation of surface plasmon polaritons (SPPs) in a liquid plasmonic metal by changing its physical phase. Dynamic properties were controlled by solid-to-liquid phase transitions in 1D Ga gratings that were fabricated using a simple molding process. Solid and liquid phases were found to exhibit different plasmonic properties, where light coupled to SPPs more efficiently in the liquid phase. We exploited the supercooling characteristics of Ga to access plasmonic properties associated with the liquid phase over a wider temperature range (up to 30 °C below the melting point of bulk Ga). Ab initio density functional theory-molecular dynamic calculations showed that the broadening of the solid-state electronic band structure was responsible for the superior plasmonic properties of the liquid metal.     

Interfacial thermal conductance of in situ aluminum-matrix nanocomposites

Journal of Materials Science - Thermal performance of Al nanocomposites is of significance for broad applications. In this study, we successfully fabricated Al–TiC, Al–ZrB2, and...     

Phase formation at rapid thermal annealing of Al/Ti/Ni ohmic contacts on 4H-SiC

Ohmic contacts to the top p-type layers of 4H-SiC p⁺–n–n⁺ epitaxial structures having an acceptor concentration lower than 1×10¹⁹ cm⁻³ were fabricated by the rapid thermal anneal of multilayer Al/Ti/Pt/Ni metal composition. The rapid thermal anneal of multilayer A1/Ti/Pt/Ni metal composition led to the formation of duplex cermet composition containing Ni₂Si and TiC phases. The decomposition of the SiC under the contact was found to be down to a depth of about 100 nm. The contacts exhibited a contact resistivity R_c of 9×10⁻⁵ Ω cm⁻² at 21°C, decreasing to 3.1×10⁻⁵ Ω cm⁻² at 186°C. It was found that thermionic emission through the barrier having a height of 0.097 eV is the predominant current transport mechanism in the fabricated contacts.     

Selective surface functionalization of silicon nanowires via nanoscale joule heating

In this letter, we report a novel approach to selectively functionalize the surface of silicon nanowires located on silicon-based substrates. This method is based upon highly localized nanoscale Joule heating along silicon nanowires under an applied electrical bias. Numerical simulation shows that a high-temperature (>800 K) with a large thermal gradient can be achieved by applying an appropriate electrical bias across silicon nanowires. This localized heating effect can be utilized to selectively ablate a protective polymer layer from a region of the chosen silicon nanowire. The exposed surface, with proper postprocessing, becomes available for surface functionalization with chemical linker molecules, such as 3-mercaptopropyltrimethoxysilanes, while the surrounding area is still protected by the chemically inert polymer layer. This approach is successfully demonstrated on silicon nanowire arrays fabricated on SOI wafers and visualized by selective attachment of gold nanoparticles.     

Formation of new phases at nickel-phosphorus contacts

The formation of new phases at a contact of pure nickel and phosphorus was studied using optical microscopy, X-ray diffraction and electron microprobe analysis. In the temperature interval between 500 and 800°C (except 600°C) two phases were formed: Ni₂P and Ni₅P₄. In the given temperature interval, the stability of these phases does not depend on the temperature but on the phosphorus content (the content of nickel is constant). Both phases are formed from the very beginning of the process and show a deviation from stoichiometry. No regularity in the deviation from stoichiometry could be found.