High Cohesive Energy Research Articles

Ultrarobust Thin-Film Devices from Self-Assembled Metal-Terpyridine Oligomers.

Ultrathin molecular layers of Fe(II) -terpyridine oligomers allow the fabrication of large-area crossbar junctions by conventional electrode vapor deposition. The junctions are electrically stable for over 2.5 years and operate over a wide range of temperatures (150-360 K) and voltages (±3 V) due to the high cohesive energy and packing density of the oligomer layer. Electrical measurements reveal ideal Richardson-Shottky emission in surprising agreement with electrochemical, optical, and photoemission data.

Structural evolution of TaN-alloyed Cr–Al–Y–N coatings

Abstract Substitutions of metallic elements in Al-containing ternary transition metal nitrides are a promising method for improving the alloy thermal stability and oxidation resistance. In this work, combined experimental and ab initio analysis of thermal stability and structural evolution of CrAlYN alloyed to tantalum nitride is presented. As-deposited reactively-sputtered Cr 1 − x − y − z Al x Y y Ta z N coatings (z = 0 ÷ 0.21) exhibit single phase cubic sodium chloride (B1) structure identified as fcc-CrAlY(Ta)N solid solution. The presence of Ta in the solid solution shifts the decomposition process to higher temperatures (> 1000 °C for Ta content of z = 0.21) compared to CrAlYN (~ 900 °C), thus enhancing the alloy thermal stability. Improved thermal stability of tantalum-containing solid solutions may be attributed to their higher cohesive energies, as revealed from ab initio calculations. X-ray diffraction and transmission electron microscopy investigation of the N 2 -depleted structure after exposure at high temperature (1200 °C) in Ar + H 2 atmosphere revealed the presence of wurtzite AlN (w-AlN), cubic, hexagonal and tetragonal Cr- and Ta-containing binary or ternary nitrides (h-Cr 2 N, h-TaN, h-Ta 2 N, t-Cr 0.8 Ta 1.2 N) and metallic phases (bcc-Cr(AlTa), fcc-Cr 2 Ta). First-principle calculations show negative values of mixing free energies for fcc-Cr 1 − x Ta x N over the whole composition range at 1600 K indicating its enhanced thermodynamic stability compared to cubic Al 1 − x Ta x N and Al 1 − x Cr x N.

Structure and cohesive energy of dipolar helices.

This paper deals with the investigation of cohesive energy in dipolar helices made up of hard spheres. Such tubular helical structures are ubiquitous objects in biological systems. We observe a complex dependence of cohesive energy on surface packing fraction and dipole moment distribution. As far as single helices are concerned, the lowest cohesive energy is achieved at the highest surface packing fraction. Besides, a striking non-monotonic behavior is reported for the cohesive energy as a function of the surface packing fraction. For multiple helices, we discover a new phase, exhibiting markedly higher cohesive energy. This phase is referred to as ZZ tube consisting of stacked crown rings (reminiscent of a pile of zig-zag rings), resulting in a local triangular arrangement with densely packed filaments parallel to the tube axis.

Coulomb frustration of the multiphoton ionization of metallic clusters under intense EUV FEL evidenced by ion spectrometry

Free electron laser light sources delivering high intensity pulses of short wavelength radiation are opening novel possibilities for the investigation of matter at the nanoscale and for the discovery and understanding of new physical processes occurring at the exotic transient states they make accessible. Strong ionization of atomic constituents of a nano-sized sample is a representative example of such processes and the understanding of ionization dynamics is crucial for a realistic description of the experiments. We report here on multiple ionization experiments on free clusters of titanium, a high cohesive energy metal. The time of flight ion spectra reveal a saturation of the cluster ionization at ∼1016 photons per pulse per cm2. Our results also show a clear lack of any explosion process, opposite to what is observed for a rare-gas cluster under similar conditions. A simple and generalized multi-step ionization model including Coulomb frustration of the photoemission process effectively reproduces with a good agreement the main features of the experimental observation and points to an interpretation of the data involving a substantial energy deposition into the cluster through electronic system heating upon scattering events within photoemission.

Interfacial effects on dielectric properties of polymethylmethacrylate‐titania microcomposites and nanocomposites

It is discussed that dielectric properties of polymethylmethacrylate (PMMA) composites are influenced by the particle size of titania as a result of changes in the structure and dynamics of polymer chains in the interphase regions around the particles. It is hypothesized that a loosely packed interphase layer, resulted from high stiffness and high cohesion energy of PMMA chains, provides enough free volume for segmental chain movements and dipole orientation along the applied electric field and causes an increase in dielectric properties of nanocomposites at low filler contents. It is shown that the attraction of PMMA toward micro‐titania and nano‐titania is similar by measuring the surface energy of fillers. Dynamics of polymer chains around fillers is investigated by dynamic‐mechanical‐thermal analysis and broadband‐dielectric spectroscopy, and presence of a loosely packed layer is confirmed by measuring thermal and mechanical glass transition temperature of composites. It is concluded that the anomalous enhancement in dielectric properties of 2 vol% nanocomposite, compared to the corresponding microcomposite, can be attributed to the higher volume fraction of loosely packed interphase layers in the nanocomposite. POLYM. COMPOS., 38:1158–1166, 2017. © 2015 Society of Plastics Engineers

Growing high-quality ternary CdMnTe epilayers by molecular beam epitaxy on Si substrates and its mechanism

Cd(Mn,Zn)Te-based ternary compound semiconductors with wide band-gaps are important in the detection of radiation and photovoltaic applications. This study characterizes Cd1-xMnxTe epilayers on Si substrates with various Mn compositions grown by molecular beam epitaxy. The surface smoothness, crystallinity and optical quality all are significantly improved with increasing Mn content. The Cd0.61Mn0.39Te epilayer with a thickness of only about 500 nm yields a full width at half maximum of the X-ray rocking curve of 165 arcsec. Photoluminescence spectra at 10 K show that the intensity of defect-related emissions is much lower than that of binary CdTe epilayers, reaching zero from the samples with high Mn content, while the integral intensity of the exciton-related emissions is increased by more than two orders of magnitude. Raman scattering spectra reveal that the intensity of the Te–Te related defect vibration modes falls significantly as the Mn content increase, even disappearing altogether in the samples with high Mn content. This work proposes that incorporating Mn atoms during epitaxial growth can promote the decomposition of Te2 sources, owing to the high sticking coefficient of Mn and the high cohesive energy of the Mn–Te bond, and then reduce the number of Te–Te related stacking fault defects, yielding high-quality CdMnTe epilayers. Our results herein demonstrate that the CdMnTe ternary epilayers are much more promising in terms of material quality than the CdZnTe ternary epilayers.

ChemInform Abstract: Synthesis of Various Heterocyclic Compounds via Multi‐Component Reactions in Water

AbstractReview: 279 refs.

Conformational Preadjustment in Aqueous Claisen Rearrangement Revealed by SITS-QM/MM MD Simulations

An efficient sampling method was implemented in QM/MM hybrid molecular simulations to study aliphatic Claisen rearrangement in aqueous solutions. On the basis of the computational results, the necessary conformational adjustment to trap the reactant into a favorable compact conformation specifically in water was observed. The conformational equilibrium was shown to be important to the elucidation of the "water-acceleration" effect of Claisen rearrangement. Thus, a two-step process of aqueous Claisen rearrangement was proposed. It was similar to the pseudodiaxial-pseudodiequatorial conformational equilibrium observed in the enzymatic reaction of chorismate acid but with explicit inclusion of the solvent coordinates to explain the solvation effects. Polarization was found to occur during the reactant conformational transition. A solvent with high cohesive energy density (CED) like water was suggested to accommodate compact conformers better, thus facilitating the following reaction by concentrating the real "active" reactant. The substituent effects also manifested, leading to varied conformational distributions of different substituted allyl vinyl ethers (AVEs). The application of the enhanced sampling method allowed a systematic analysis of thermodynamic information without loss of solvent coordinates. These data showed the conformational transition of AVEs was an entropy-driving process which was sensitive to the substituent, and enthalpy played an important role in the solvation effect on the conformational equilibrium.

Stabilizing graphene-based organometallic sandwich structures through defect engineering

In this theoretical work, we propose a chemical route to creating stable benzene-transition metal-graphene sandwich structures. The binding energy of the transition metal to graphene is enhanced through adsorption at appropriate defects, immobilizing the metal onto the graphene web. Capping the metal with a benzene ring further stabilizes the structure. The stability and the magnetic properties of these composite structures vary for different defects such as vacancies and nitrogen substitutionals in graphene. The proposed complexes have high cohesive energies and are either metallic or are small-band-gap semiconductors. Several of the proposed structures also have large spin polarization energies that make them suitable for use as nanomagnets in ambient conditions. This work also sheds light on the experimental results in the field where the sandwich structures may have been successfully created. We show that defect engineering is a viable option for creating designer, graphene-based structures that may play an important role in fields as diverse as spintronics, nanoelectronics, hydrogen storage, and catalysis.

A first-principles study of the III-IV-V semiconductor nanosheets.

Alloying the III-V and IV-IV sheets leads to III-IV-V nano-composites, such as the BC2N sheet, having a lower band gap than their parent III-V counterparts while having higher cohesive energies. Unlike the well known BC2N sheet, the formation energy of the III-IV-V sheets with high Z atomic constituents is much low suggesting in favour of their experimental realisation. From first-principles hybrid density functional calculations, we report a family of group III-IV-V nano-sheets that have their electronic band gap lying between 0.13-1.0 eV, which is ideal for device applications. In particular, we compare the electronic, vibrational, mechanical and thermal properties of a set of III-IV-V sheets with their III-V and IV-IV counterparts. The cohesive energies of these III-IV-V sheets are found to be the intermediate of their parent III-V and IV-IV counterparts. The puckered geometry and the presence of heterogeneous bonds may result in low thermal conductivity (due to scattering of low energy phonons at the junction of the IIIV and IVIV chains) of these sheets.

Synthesis of various heterocyclic compounds via multi-component reactions in water

Due to special effects of water such as abundant, environmentally benign, hydrogen bonding in the transition state and high cohesive energy density as well as negative activation volume; the use of water as a green solvent is emphasized in green chemical processes. Heterocyclic compounds play an important role in the treatment of many diseases and also are effective in biological processes. Multi-component reactions are powerful synthetic methods in construction of new organic molecules because in these methods product is formed as one-pot and reduced number of reaction separation steps. For these reasons, the synthesis of heterocyclic systems using a simple multi-component approach in water is investigated greatly in the recent years. In this review, multi-component reactions that have been used for the synthesis of heterocyclic compounds in aqueous media during the last two decades are described.

Wetting Reversal at Gelation Transition Freezes Thermodynamically Unstable States

The contact angle of a drop of gelling solution on a flat, solid surface was monitored as the hot solution was allowed to cool. When a solvent with a high cohesive energy and a wettable solid surface was used, a wetting solution turned into a dewetting solid at the gelation transition. The density profiles in gel as probed by confocal Raman microscopy reveal that the adsorption of both gelator and solvent shifts at the transition and the solvent is severely depleted from the interfacial region. Thus, the wetting reversal is accompanied by the interfacial desolvation. As a result of the adsorption shift during the gelation process in progress, a locally concentrated region of the gelator is frozen in space far away from the surface. This is a thermodynamically unstable state but can be realized reproducibly. The profile analysis also shows that the effect of the surface extends out to a few hundred micrometers, 2 orders of magnitude larger than the bulk correlation length.

Optimization of Surface Layers for Suppression of Agglomeration in Ag Films

We previously reported that Al, Ti, and Nb surface layers can effectively suppress agglomeration in Ag films, which is dominated by the surface diffusion of Ag atoms. In this study, Ag films with Pd, Ni, and W surface layers were prepared by RF magnetron sputtering, and the properties of these films were compared with previous observations of the effects of Ti and Nb surface layers. The most important properties of the appropriate surface layers for the suppression of agglomeration in Ag films are a high cohesive energy or a high Gibbs free energy of formation of the oxide, and a low solid solubility in Ag.

ELECTRONIC AND ELASTIC PROPERTIES OF ZINC-BLENDE MgSe

In the present study, ground state and elastic properties of semiconductor MgSe in zinc-blende phase are investigated using density functional theory (DFT). Exchange-correlation potentials are approximated with the generalized gradient approximation (GGA). From the calculated bulk modulus, we determine the refractive index, plasmon energy, cohesive energy and micro-hardness of the MgSe semiconductor binary alloy. The density of state (DOS), projected density of state (PDOS), phonon dispersion frequencies, charged density, electronic band structure and dielectric functions are also reported. From the band structure, a direct band gap of 2.50 eV was observed in close agreement with other reported calculations, but lower than the experimental value of 3.60 eV. Along the high symmetries directions, we found a striking resemblance between MgSe and a III–V semiconductor, while the high cohesive energy in MgSe suggests filled bonding orbitals which might result in decrease in atomic volume with corresponding increased compression of the s-orbitals under any transition series.

Silicene beyond mono-layers—different stacking configurations and their properties

We carry out a computational study on the geometric and electronic properties of multi-layers of silicene in different stacking configurations using state-of-the-art ab initio density functional theory based calculations. In this work we investigate the evolution of these properties with increasing number of layers (n) ranging from 1 to 10. Although a mono-layer of silicene possesses properties similar to those of graphene, our results show that the geometric and electronic properties of multi-layers of silicene are strikingly different from those of multi-layers of graphene. We observe that strong inter-layer covalent bonding exists between the layers in multi-layers of silicene as opposed to weak van der Waals bonding which exists between the graphene layers. The inter-layer bonding strongly influences the geometric and electronic structures of these multi-layers. Like bi-layers of graphene, silicene with two different stacking configurations AA and AB exhibits linear and parabolic dispersions around the Fermi level, respectively. However, unlike graphene, for bi-layers of silicene, these dispersion curves are shifted in the band diagram; this is due to the strong inter-layer bonding present in the latter. For n > 3, we study the geometric and electronic properties of multi-layers with four different stacking configurations, namely AAAA, AABB, ABAB and ABC. Our results on cohesive energy show that all the multi-layers considered are energetically stable. Furthermore, we find that the three stacking configurations (AAAA, AABB and ABC) containing tetrahedral coordination have much higher cohesive energy than the Bernal (ABAB) stacking configuration. This is in contrast to the case of multi-layers of graphene, where ABAB is reported to be the lowest energy configuration. We also observe that bands near the Fermi level in lower energy stacking configurations AAAA, AABB and ABC correspond to the surface atoms and these surface states are responsible for the semi-metallic character of these multi-layers.

Tailoring structural and electronic properties of RuO2 nanotubes: a many-body approach and electronic transport

The electrical conduction properties of ruthenium oxide nanocables are of high interest. These cables can be built as thin shells of RuO2 surrounding an inner solid nanowire of a dielectric insulating silica material. With this motivation we have investigated the structural, electronic and transport properties of RuO2 nanotubes using the density functional formalism, and applying many-body corrections to the electronic band structure. The structures obtained for the thinnest nanotubes are of the rutile type. The structures of nanotubes with larger diameters deviate from the rutile structure and have in common the formation of dimerized Ru-Ru rows along the axial direction. The cohesive energy shows an oscillating behavior as a function of the tube diameter. With the exception of the thinnest nanotubes, there is a correlation such that the electronic band structures of tubes with high cohesive energies show small gaps at the Fermi energy, whereas the less stable nanotubes exhibit metallic behavior, with bands crossing the Fermi surface. The electronic conductance of nanotubes of finite length connected to gold electrodes has been calculated using a Green-function formalism, and correlations have been established between the electronic band structure and the conductance at zero bias.

Research on starch‐g‐polyvinyl acetate and epoxy resin‐modified corn starch adhesive

AbstractA new corn starch adhesive modified by starch‐g‐polyvinyl acetate (starch‐g‐PVAc) and epoxy resin is described in this study. Starch‐g‐PVAc is used as high cohesive energy component to improve the dry shear strength of the starch adhesive. Although the epoxy resin, which can easily crosslink with the oxidized starch, is used as water‐resistant component to improve the wet shear strength. Because there is no chemical reaction happening between polyvinyl acetate and epoxy resin, both the dry shear strength and the wet shear strength of the corn starch adhesive are notably increased. Considering all the related factors, the optimum of the modification is achieved when the dosage of starch‐g‐PVAc and epoxy resin is 70% of the oxidized starch latex with m(Ep): m(starch‐g‐PVAc) = 1:2. That is, the epoxy resin is 23% in mass fraction and starch‐g‐PVAc 47% in mass fraction. The dry shear strength is 4.50 MPa, and the wet shear strength is 2.51 MPa. The modified corn starch has a broad prospect in the application of plywood industry. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers

Periodic Trends in Adsorption and Activation Energies for Heterometallic Diffusion on (100) Transition Metal Surfaces

A first-principles analysis of trends in metal-on-metal hopping diffusion for 64 admetal/substrate systems is presented. Focusing on the (100) facets of various transition metal substrates, we demonstrate that the calculated hopping diffusion barriers may be interpreted in terms of the cohesive energies of the admetals and substrates, as well as the lattice constants of the substrates. We further show that general linear relationships exist between the diffusion barriers and the corresponding adsorption energies on each transition metal substrate. The slopes in these Brønsted–Evans–Polanyi relationships are related to the degree of resemblance between the initial states and the transition states for hopping diffusion, and the slopes are found to depend sensitively on the nature of the transition metal substrate. Substrates with higher cohesive energies and smaller lattice constants generally exhibit smaller slopes and, therefore, a closer correspondence between the transition states and the initial states. These relationships, in addition to providing fundamental insights into trends in diffusion across different transition metal surfaces, give a powerful and convenient means of predicting diffusional kinetics from purely thermodynamic quantities. The results may ultimately provide a useful input to kinetic Monte Carlo (kMC)-type simulations, enabling efficient and accurate studies of heteroepitaxial metal-on-metal growth.

Assessment of intertube interactions in different functionalized multiwalled carbon nanotubes incorporated in a phenoxy resin

AbstractIn this work, about 1 wt% of different functionalized carbon nanotubes (CNTs), namely CNTCOOH (CNT with carboxylic groups), CNTNH2 (CNT with amine groups) and CNTOH (CNT with hydroxyl groups), as well as nonfunctionalized CNTs were incorporated into a phenoxy resin via a melt mixing process. The extent of intertubes and polymer–tubes interactions and their influence on state of CNTs dispersion were assessed through determination of electrical, rheological, and morphological characteristics. CNTNH2 showed the lowest intertubes interactions followed by CNTOH and CNTCOOH. Nanocomposite made from CNTCOOH showed the poorest state of CNTs dispersion and the biggest CNTs agglomerates and it remained nonconductive. The acid‐functionalized CNTs were not able to form strong polymer–tube interactions because of their high cohesive energy and therefore in the melt rheological investigations they exhibited the lowest storage modulus and complex viscosity as well as the highest loss factor among all the studied CNTs. A good balance between intertubes and polymer–tube interactions is necessary through proper selection of CNTs functional groups for achieving a good state of CNTs dispersion and consequently obtaining enhanced electrical and viscoelastic properties. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Molecular and nanoscale factors governing pressure‐sensitive adhesion strength of viscoelastic polymers

AbstractPressure‐sensitive adhesives (PSAs) are finding increasing applications in various areas of industry and medicine. PSAs are a special class of viscoelastic polymers that form strong adhesive joints with substrates of varying chemical nature under application of light external bonding pressures (1–10 Pa) over short periods of time (1–5 s). To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, and high cohesive strength and elasticity, which are necessary for resistance to debonding stresses and for dissipation of mechanical energy at the stage of adhesive bond failure under detaching force. For rational design of novel PSAs, molecular insight into mechanisms of their adhesive behavior is necessary. As shown in this review, strength of PSA adhesive joints is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms. At the molecular level, strong adhesion is the result of a narrow balance between two generally conflicting properties: high cohesive strength and large free volume. These conflicting properties are difficult to combine in a single polymer material. Individually, high cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. Evident correlations are observed between the adhesive bond strengths of different PSAs, and their relaxation behaviors are described by longer relaxation times. Innovative PSAs with tailored properties can be produced by physical mixing of nonadhesive long‐ and short‐chain linear parent polymers, with groups at the two ends of the short chains complementary to the functional groups in the recurring units of the long chains. Although chemical composition and molecular structure of such innovative adhesives are unrelated to those of conventional PSAs, their mechanical properties and adhesive behaviors obey the same general laws, such as the Dahlquist's criterion of tack. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012