Organic host–guest molecular assemblies

The utility of materials possessing molecular-scale porosity has been amply demonstrated by the use of inorganic zeolite frameworks as functional catalysts, separating media, and novel electronic materials. This has prompted an increase in the development of porous organic host materials capable of including guest molecules within their pores, thereby mimicking the behavior of inorganic zeolites. Such organic host–guest complexes may prove to be superior to their inorganic counterparts in some applications because they are formed spontaneously by self-assembly from molecular building blocks that can be customized through organic synthesis, yielding host structures in which the pores have precisely defined size, shape and composition. However, in order to design and synthesize host–guest complexes possessing unique and useful properties it is essential to control the assembly of molecules into preordained, desirable solid-state architectures. Understanding how molecules arrange in the solid state and using this information rationally to ‘engineer’ solid-state properties is essential to numerous technologies, including electronics, optoelectronics, sensors, information storage, and chemical separations.     

Dielectric relaxation and electrical conduction at low field of mineral-filled epoxy

Dielectric properties and conduction of the epoxy and its composites were measured over the temperature range — 20 to 70°C and the frequency range 10^–4-10^–1 Hz. Dielectric properties were obtained by performing Fouriertransforms on the charging and discharging curves. The resulting isothermal frequency spectra of dielectric constants and dielectric loss factors were analysed using the Cole-Cole law to obtain the activation energy for each material. The activation energies were also obtained for isothermal d.c. current. Current density-electric field-temperature characteristics are obtained for field levels up to 60 kV cm^–1, with step excitation of the applied field and currents recorded after a delay time of 10 min. Current density and electric field were computed and plotted for constant temperature. The linear (ohmic) curves were obtained for fields up to about 60 kV cm^–1 for temperatures up to about 20 °C. The non-linearity at the higher fields and temperatures did not imply the occurrence of non-ionic conduction. It has been demonstrated that both electric conduction and relaxation behaviour were ionic and could be fit by the Nakajima model for the unfilled epoxy and the Taylor model for the composites.     

Shallow defect states in hydrogenated amorphous silicon oxide

The theoretical cluster-Bethe-lattice method is used in this study to investigate the shallow defect states in hydrogenated amorphous silicon oxide. The electronic density of states (DOS) for the SiO₂ Bethe lattice of various Si–O–Si angles, non-bridging oxygen Si–O^√, peroxyl radical Si–O–O^√, threefold coordinated O₃ and Si–H bonds are calculated. The variation of the Si–O–Si bond angle causes the bandgap fluctuation and induces tail states near the conduction band minimum. The Si–O^√ and Si–O–O^√ bonds introduce shallow defect states in the energy gap near the top of the valence band. The Si–H bond induces a defect state, in the energy gap near the conduction band minimum, in a-SiO_x with high oxygen concentration, but not low oxygen concentration. The O₃ bond itself does not induce defect state in the energy gap. The O₃⁺D⁻ complex, formed by the O₃ and threefold coordinated silicon, induces shallow state in the energy gap near the conduction band minimum. This defect state can explain the energy shift of photoluminescence of a-SiO_x:H under annealing.     

Electronic structure of the filled tetrahedral compound LiCdP and zinc–blende InP: Application of the interstitial insertion rule

We report on first principles studies of the electronic properties of the filled tetrahedral compound LiCdP and zinc–blende InP, using the full potential linearized augmented plane wave method within the local density approximation. The total energy calculations show that the α phase (Li⁺ near the anion) to be more stable than the β one (Li⁺ near the cation) for the LiCdP. The conduction band valleys follow the Γ–L–X ordering of increasing energy for β-LiCdP and InP, and the Γ–X–L one for α–LiCdP. The conduction band modifications are discussed and found to obey the interstitial insertion rule except for the Γ state of β-LiCdP. The valence charge density analysis shows that the Cd–P bond is covalent whereas the Li–P and the Li–Cd ones in α and β phases, respectively, are ionic.     

Soluble axially substituted phthalocyanines: Synthesis and nonlinear optical response

This review lays special stress on describing the synthesis of soluble axially substituted or bridged indium, gallium and titanium phthalocyanine complexes and their electronic absorption characteristics, photophysical and nonlinear optical properties. The enhanced solubility of the axially substituted or bridged phthalocyanine monomers and dimers, compared to the chloro analogues, shows that the usual tendency of phthalocyanines to form aggregates can be effectively suppressed by axial substitution. Axial substitution in phthalocyanine complexes has provoked relevant changes on the electronic structure of the molecule by altering the π-electronic distribution due to the dipole moment of the central metal-axial ligand bond. The nanosecond nonlinear absorption and the optical limiting of indium, gallium and titanium phthalocyanines seem to be dominated by a strong triplet state absorption in the optical region comprised between the Q- and B-bands in their UV/Vis absorption spectra. Graphical Abstract A series of highly soluble axially substituted and bridged phthalocyanine and naphthalocyanine complexes have been synthesized. Axial substitution in phthalocyanine complexes has provoked relevant changes on the electronic structure of the molecule by altering the π-electronic distribution due to the dipole moment of the central metal-axial ligand bond.     

Electrical conductivity of (ZrO2)0.85(CeO2)0.12 (Y2O3)0.03

A number of zirconia-based materials show promise as electrode materials in magnetohydrodynamic (MHD) generators. As a part of an exploratory programme to find suitable materials for graded electrode applications in MHD generators, partially stabilized and fully stabilized sintered ceramic materials are prepared and characterized. The oxygen ion transference number t _ion(O^2–) and electrical conductivity of this material are measured up to 1670 K in the oxygen partial pressure range 1 to 10^–6 atm. The activation energies for conduction are determined. The electrical properties of this material are characterized by mixed conduction, ionic and electronic. The observed conductivity data are explained in terms of the defect equilibrium reactions between tetravelent Ce⁴⁺ and trivalent Ce³⁺ ions.     

Study on electrical characteristics of fluorinated polyimide film

The electrical characteristics (I–V and I–t) of fluorinated polyimide dielectric films were measured as function of time, electric field and sample temperature and the results were discussed in view of existing conduction and charge-transport mechanisms. Experiments and analysis to elucidate conduction mechanisms at low and high electric fields were carried out. The results are presented and discussed. It is concluded that the bulk-limited Pool–Frenkel conduction mechanism is likely to dominate for the steady state current at high electric field above 1 MV cm-1, and in the range of low electric field below 1 MV cm-1, the Ohmic conduction is the main conduction mechanism. © 1998 Kluwer Academic Publishers     

Noncovalent attachment of CdSe quantum dots to single wall carbon nanotubes

Noncovalent attachment of CdSe quantum dots (QDs) to single wall carbon nanotubes (SWNTs) through an intermediary 1-pyrenebutyric acid N-hydroxy-succinimide ester (PBASE) molecule has been performed. The ligand exchange process from trioctylphosphine oxide (TOPO)-capped CdSe to the 4-aminothiophenol (ATP) ligand is supported by solvent solubility, NMR spectroscopy, and IR spectroscopy, with an estimated molecular efficiency > 50:1. Noncovalent coupling of the PBASE molecule causes a redshift in the SWNT interband electronic transitions, consistent with a π–π interaction that promotes electron delocalization. TEM analysis after chemical coupling of the CdSe–ATP QDs to the PBASE–SWNTs shows an abundant coverage of QDs along the SWNT bundles. Raman spectra (1.96 eV excitation) of PBASE–SWNTs and the noncovalent product demonstrate that each of the major Raman modes (RBM, D-, G-, or G′-bands) is unaltered by the noncovalent interaction with PBASE or attachment of CdSe QDs, indicating that the structural integrity of the SWNTs is maintained. However, upshifts in the Raman modes are observed, the largest being for the G′-band, indicating charge transfer between the SWNTs and attached CdSe QDs.     

Engineering Chemically Active Defects in Monolayer MoS2 Transistors via Ion‐Beam Irradiation and Their Healing via Vapor Deposition of Alkanethiols

Irradiation of 2D sheets of transition metal dichalcogenides with ion beams has emerged as an effective approach to engineer chemically active defects in 2D materials. In this context, argon‐ion bombardment has been utilized to introduce sulfur vacancies in monolayer molybdenum disulfide (MoS₂). However, a detailed understanding of the effects of generated defects on the functional properties of 2D MoS₂ is still lacking. In this work, the correlation between critical electronic device parameters and the density of sulfur vacancies is systematically investigated through the fabrication and characterization of back‐gated monolayer MoS₂ field‐effect transistors (FETs) exposed to a variable fluence of low‐energy argon ions. The electrical properties of pristine and ion‐irradiated FETs can be largely improved/recovered by exposing the devices to vapors of short linear thiolated molecules. Such a solvent‐free chemical treatment—carried out strictly under inert atmosphere—rules out secondary healing effects induced by oxygen or oxygen‐containing molecules. The results provide a guideline to design monolayer MoS₂ optoelectronic devices with a controlled density of sulfur vacancies, which can be further exploited to introduce ad hoc molecular functionalities by means of thiol chemistry approaches.     

Optimization of chalcogenide glass in the As–Se–S system for automotive applications

Infrared transmitting glasses in the As–Se–S system have been optimized by taking in account specifications for automotive application. The As₂Se₃ glass has been chosen as the starting composition and sulfur is introduced in order to improve some optical and mechanical properties. As expected, the Sulfur introduction decreases the refractive index as well as its change as function of temperature. The relationship between the refractive index and the glass composition has been established. The thermal coefficient of refractive index for sulfur containing glasses can be lower than 4 × 10⁻⁵ K⁻¹, which is considerably lower than that of germanium, the most used infrared transmitting materials. Influence of sulfur on other properties has also been studied.     

Controlling Electronic States and Transport Properties at the Level of Single Molecules

Since molecular electronics has been rapidly growing as a promising alternative to conventional electronics towards the ultimate miniaturization of electronic devices through the bottom‐up strategy, it has become a long‐term desire to understand and control the transport properties at the level of single molecules. In this Research News article it is shown that one may modify the electronic states of single molecules and thus control their transport properties through designing and fabrication of functional molecules or manipulating molecules with scanning tunneling microscopy. The rectifying effect of single molecules can be realized by designing a donor–barrier–acceptor architecture of Pyridine–σ–C₆₀ molecules to achieve the Aviram–Ratner rectifier and by modifying electronic states through azafullerene C₅₉N molecules. The effect of the negative differential resistances can be realized by appropriately matching the molecular orbital symmetries between a cobalt phthalocyanine (CoPc) molecule and a Ni electrode. The electronic states and transport properties of single molecules, such as CoPc and melamine molecules, can be altered through manipulation or modifying molecular structures, leading to functionalized molecular devices.     

The thermodynamic similarity of nitrogen,oxygen, and air

The thermodynamic similarity of nitrogen, oxygen, and air is established. The data for nitrogen are used to calculate the thermodynamic properties of oxygen at pressures of (1–1500)·10⁵ N/m² and temperatures of 170–1000 deg K. Tables of specific volume, enthalpy, entropy, and heat capacity of oxygen are given.     

Three-Dimensional Hard Dumbbell Solid Free Energy Calculation Via the Fluctuating Cell Model

Determination of the solid–liquid phase transition point of a molecular substance requires calculation of the free energy in both phases. Progress has been made on this problem by modeling molecules as fused hard spheres and adding attraction and electric multipole moments perturbatively. The solid free energy of hard heteronuclear dumbbells of bond length L ^*, used to model diatomic molecules, can in principle be calculated exactly via the Frenkel–Ladd method, but this is computationally intensive. Use of Lennard Jones–Devonshire fixed cells to calculate free energy is much simpler computationally but is an approximation. The fluctuating cell model is investigated as an alternative intermediate method which is still computationally simpler than the Frenkel–Ladd method. As was found earlier in two dimensions, for small L ^* the simple cell model is in better agreement with Frenkel–Ladd than the fluctuating cell model, but for larger L ^* the fluctuating cell model is in better agreement. The probability distributions of free volumes are also analyzed and show different functional behavior for near-zero bond length and appreciable bond length.     

Optical Properties of Solid Pentacene

We have prepared pentacene films on Si, Al₂O₃, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al₂O₃ and glass substrates is 1.85 eV.     

NO adsorption effects on various functional molecular nanowires

A systematic theoretical study of the electronic structure of metal tape–porphyrin (MTP, M = Ni, Cu, Zn) has been carried out using density functional theory method. The calculations provide a clear elucidation of ground states for the MTPs as well as the adsorption of NO molecule on MTPs. We found that MTPs have small energy gap, which decreases from NiTP to ZnTP. The adsorption of NO on these systems has a minor effect on the geometric structures of MTPs which is attributed to the 3d orbitals of Ni, Cu, and Zn lying lower in energy compared with the Fermi level. This leads to the adsorption of NO on top of the metal atom being a physical adsorption. However, despite the small binding of NO on MTPs, large changes in the electronic properties as well as the magnetic properties of metal tape–porphyrin systems are noted. There is a transition from an insulator to a metal in NiTP and CuTP and an opening of the energy gap in ZnTP. This is due to the small amount of charge transfer from NO molecule to tape–porphyrin, leading to a shift of the Fermi level to a higher energy. Moreover, a change from non-spin polarization to spin polarization is observed in NiTP and ZnTP. The change in magnetization is derived from the unpaired electron in π^* orbital of NO molecule.     

The shapes and sizes of vapor bubbles during boiling on downward-facing flat surfaces

The effect of the properties of the liquid (water, ethanol, nitrogen, and oxygen) on the shapes and sizes of bubbles during boiling on downward-facing plates has been investigated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 51, No. 5, pp. 709–715, November, 1986.     

Effect of acidic anode treatment on carbon fibers for increasing fiber-matrix adhesion and its relationship to interlaminar shear strength of composites

An anodic surface treatment of PAN-based carbon fibers has been studied in increasing the surface functional groups on fibers, resulting in increasing composite mechanical behavior. With a viewpoint of acid-base interaction chemistry, the 10 wt% phosphoric acid solution is selected for the electrolyte. As an experimental result, very low electric current densities, such as 30–300 A·m^–2, are need for the changing of morphological and mechanical properties. According to FT-IR and XPS measurements, it reveals that the oxygen functional groups on fibers are largely dominated in ILSS of the composites, whereas the nitrogen functional groups are not affected in this system. Also, it is found that a moderate 70 A·m^–2 treatment on fibers seems promising to assess the improving of the physical and mechanical properties.     

High Temperature Transport Properties of Dilute Nitrogen Atoms

Calculations of the transport coefficients viscosity and thermal conductivity and the diffusion collision cross section of nitrogen atoms have been carried out as a function of temperature. The dilute gas transport properties of nitrogen atoms depend only on the interactions between two nitrogen atoms along various electronic potential energy curves. The results presented here include contributions from 16 potential energy curves, four of which dissociate to two ground-state nitrogen atoms with the others also dissociating to two nitrogen atoms, at least one of which is in an excited electronic state. Thirteen of the potential energy curves are represented by the Hulburt–Hirschfeleder potential which is the best general purpose atom–atom potential. This potential depends only on the experimental spectroscopic constants and not on any adjustable parameters. Where spectroscopic constants are unavailable, fits of the Hulburt–Hirschfelder potential to ab initio quantum mechanical results are used for two states and a fit of the Morse potential is used for the other state. The results presented here should be especially useful under conditions where nitrogen atoms are at high temperatures, such as during Space Shuttle re-entry.     

Electrical properties of r.f. magnetron sputtered BaxSr1−xTiO3 films on multi-layered bottom electrodes for high-density memory application

Ba_xSr_1–xTiO₃ (BST) thin films have been deposited by r.f. magnetron sputtering on silicon and platinum-coated silicon substrates with different buffer and barrier layers. Electrical properties of BST films have been evaluated using both metal–insulator–semiconductor (MIS) and metal–insulator–metal (MIM) structures. MIS capacitor C–V and G–V characteristics have been utilized to determine the fixed charge density, interface trap density and the trap distribution in the silicon bandgap. BST films deposited on Si/SiO₂/SiN/Pt and Si/SiO₂/Ti/TiN/Pt multilayer bottom electrodes have been used for the fabrication of MIM capacitors. The role of bottom electrode, processing temperature and Ba to Sr ratio on the electrical properties of Ba_xSr_1–xTiO₃ films have been investigated. Current–voltage behavior has indicated an ohmic nature at lower voltages and Poole–Frenkel conduction at higher voltages. Deposited films have shown an excellent time-dependent dielectric breakdown under constant-current stressing.