Clustering in nanostructured carbon: Evidence of electron delocalization

The electronic properties of disordered carbon-based materials can be discussed in terms of the clustering of the sp² carbon phase and delocalization of the electron wave function. In smooth amorphous carbon thin films this results in a mixed phase material of conductive sp² clusters embedded in an electrically insulating sp³ matrix. The delocalization of the electron wave function associated with the sp² clusters is shown to play an important role in understanding many of the electronic and optical properties of the films. It is demonstrated that the extent of the electron delocalization and clustering can be estimated using magnetic resonance methods. Evidence for delocalization in a range of carbon-based materials such as diamond-like carbon thin films produced by chemical vapour deposition, nanostructured carbon produced by pulsed laser ablation and ultrananocrystalline diamond is presented.     

Magnetic field induced delocalization in multi-wall carbon nanotube-polystyrene composite at high fields

In well dispersed multi-wall carbon nanotube-polystyrene composite of 15 wt%, with room temperature conductivity of ∼5 S/cm and resistivity ratio [R_2 K/R_200 K] of ∼1.4, the temperature dependence of conductivity follows a power-law behavior. The conductivity increases with magnetic field for a wide range of temperature (2–200 K), and power-law fits to conductivity data show that localization length (ξ) increases with magnetic field, resulting in a large negative magnetoresistance (MR). At 50 T, the negative MR at 8 K is ∼13% and it shows a maximum at 90 K (∼25%). This unusually large negative MR indicates that the field is delocalizing the charge carriers even at higher temperatures, apart from the smaller weak localization contribution at T < 20 K. This field-induced delocalization mechanism of MR can provide insight into the intra and inter tube transport.     

Using resonance Raman spectroscopy to examine vibrational barriers to electron transfer and electronic delocalization

A time-dependent approach to the interpretation of resonance Raman scattering intensities has been used to obtain quantitative vibrational mode displacement information from scattering intensities associated with charge-transfer excitation. The displacements and associated frequencies are the key parameters needed to understand Franck-Condon effects in electron-transfer kinetics, and to delineate in a mode-specific way the composition of vibrational reorganization energies. Application of the approach to a number of types of electron-transfer reactions is described, including symmetrical and unsymmetrical intervalence electron transfers in inorganic and organic redox systems, metal-to-ligand charge-transfer reactions, and interfacial electron-transfer reactions. Also described is how the approach can be used to elucidate mechanisms for valence delocalization in strongly interacting redox systems.     

Buckling and postbuckling analysis of single-walled carbon nanotubes in thermal environments via molecular dynamics simulation

Buckling and postbuckling analysis of single-walled carbon nanotubes (SWCNTs) with (n, n)- and (n, 0)-helicity, when acted upon by the destabilizing loads of axial compression, torsion and external pressure, is presented by using molecular dynamics simulation. Based on the interatomic interactions given by Brenner and Lennard-Jones potentials, the molecular dynamics method is used to determine the postbuckling equilibrium paths as well as the variation of strain energy. Temperature changes and van der Waals interaction forces between the opposite walls of SWCNTs are both taken into account. Comprehensive numerical results for armchair (12, 12)- and zigzag (21, 0)-tubes are presented. The results reveal that the effect of van der Waals interactions on the postbuckling behavior of SWCNTs under axial compression can be negligible, while the additional van der Waals forces will affect the postbuckling equilibrium paths of SWCNTs under torsion and external pressure when the deformation of the tube is sufficiently large. The results also show that the temperature change has a significant effect on the postbuckling response of SWCNTs under axial compression, but it has a small effect in the loading case of torsion. In contrast, it only has a less effect on the postbuckling response of SWCNTs under external pressure.     

Control of crystal morphology via molecular imprinting

Imprinted polymers were prepared from divinylbenzene and 6‐methacrylamidohexanoic acid as the functional monomer, using calcite crystals of three different morphologies as templates. After template removal voids in the polymer remained which reflected the size and shape of the template crystals. The use of these imprinted polymers in aqueous supersaturated calcium carbonate solution resulted in the formation of some crystal objects of unusual morphology. In the case of spheroidal imprints, the nucleated crystals grow via the {104} faces in many directions creating clusters whose surface consists of many small rhombohedral crystallites. However, in the case of rhombohedral imprints, no conflict between crystal growth and the constraints of nucleation at the polymer surface arises and the new crystals very closely resemble the templates. © 2001 Society of Chemical Industry     

Terminology for the structural evaluation of coke via scanning electron microscopy

Cokes used as fillers for graphite electrodes are heterogeneous materials exhibiting at least three predominate structures as determined by scanning electron microscope techniques. A detailed set of definitions has been prepared to illustrate the terminology used in describing the morphology of cokes. Each definition is accompanied by a photomicrograph and the structures are ranked in order of decreasing usefulness to the electrode manufacturer. Several commercially available cokes are then characterized according to this scheme.     

Amplifying the molecular sieving capability of polyimide membranes via coupling of diamine networking and molecular architecture

The integration of molecular design with diamine modification generates synergistic effects for enhancing and fine tuning the molecular sieving potential of polyimide membranes. Polymer free volume and rigidity represent crucial conformational parameters that influence the effectiveness of diamine modification for elevating the H₂/CO₂ permselectivity of polyimide membranes. Experimental and molecular dynamics simulation results suggest that polyimides with higher intrinsic free volume and rigidity are ideal for diamine treatment, yielding greater increment in H₂/CO₂ selectivity. A series of copoly(4,4′-diphenyleneoxide/1,5-naphthalene-2,2′-bis(3,4-dicarboxylphenyl) hexafluoropropane diimide) (6FDA-ODA/NDA) membranes are modified with 1,3-diaminopropane (PDA). Polyimides with higher free volume intensify the methanol swelling effect which facilitates the transport and subsequent reaction of the diamine molecules. Gel content analyses of the PDA-modified films prove that the penetration depth of the diamine molecules and the extent of crosslinking are higher for polyimides with greater NDA content. 6FDA-NDA has the highest free volume and rigidity, thus exhibiting impressive improvement in ideal H₂/CO₂ selectivity from 1.8 to 120 after PDA modification. Conversely, 6FDA-ODA which is deficient in terms of free volume and rigidity, demonstrates a much lower increment in H₂/CO₂ selectivity from 2.5 to 8.2. The inherent heterogeneity of the PDA-modified polyimide films results in thickness-dependent gas permeability and selectivity. The potential of merging macromolecular tailoring with diamine networking to enhance the H₂/CO₂ separation performance of polyimide membranes is evident.     

Paramagnetic cp/dithiolene complexes as molecular hinges: interplay of metal/ligand electronic delocalization and solid-state magnetic behavior

Paramagnetic, flexible organometallic dithiolene complexes associating cyclopentadienyl (Cp) and dithiolate (dt) ligands, such as CpM(dt), Cp2M(dt), or CpM(dt)2, are investigated in the solid state through their structural and magnetic properties. The degree of delocalization of the spin density between the metal and the dithiolene fragments in a given complex, its varying molecular geometry and frontier orbitals, and the structures adopted in the solid state are intimately correlated and adapt mutually to each other. A variety of magnetic structures follows, from noninteracting spins to dyads, spin chains, spin ladders, or antiferromagnetic ground state, the detailed properties of which are highly sensitive to "minor" molecular modifications.     

Synthesis of AuPd alloyed nanoparticles via room-temperature electron reduction with argon glow discharge as electron source

Argon glow discharge has been employed as a cheap, environmentally friendly, and convenient electron source for simultaneous reduction of HAuCl₄ and PdCl₂ on the anodic aluminum oxide (AAO) substrate. The thermal imaging confirms that the synthesis is operated at room temperature. The reduction is conducted with a short time (30 min) under the pressure of approximately 100 Pa. This room-temperature electron reduction operates in a dry way and requires neither hydrogen nor extra heating nor chemical reducing agent. The analyses using X-ray photoelectron spectroscopy (XPS) confirm all the metallic ions have been reduced. The characterization with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) shows that AuPd alloyed nanoparticles are formed. There also exist some highly dispersed Au and Pd monometallic particles that cannot be detected by XRD and transmission electron microscopy (TEM) because of their small particle sizes. The observed AuPd alloyed nanoparticles are spherical with an average size of 14 nm. No core-shell structure can be observed. The room-temperature electron reduction can be operated in a larger scale. It is an easy way for the synthesis of AuPd alloyed nanoparticles.     

Tailoring molecular structure via nanoparticles for solvent-free processing of ultra-high molecular weight polyethylene composites

Composites of conventional ultra-high molecular weight polyethylene (UHMWPE) and nanoparticles, such as carbon nanotubes or ceramics, require special processing techniques due to the high melt viscosity of the polymer matrix. Recently, we have shown that polymerization with a single-site catalytic system in suitable reaction conditions produces “disentangled” UHMWPE that can be processed in the solid state. In this study, nanoparticles have been used as carriers for the single-site catalytic system in the polymerization of UHMWPE. The high-surface area of the nanoparticles, coupled with controlled reaction conditions, favors the growth of polyethylene chains with a reduced number of entanglements. This novel synthetic route offers several advantages: 1) the catalytic system is more stable and less fouling occurs during the polymerization reaction; 2) nanoparticles are directly embedded in an otherwise intractable polymer matrix; 3) the low amount of entanglements in the UHMWPE matrix allows the resulting composites to be processed in the solid state well below the equilibrium melting temperature in a broad temperature window, to give high strength/high modulus tapes.     

Gelatin/tannin complex nanospheres via molecular assembly

Nanospheres were produced by molecular assembly between tannin and gelatin because of the synergistic interaction of the hydrophobic effect and hydrogen bonding. The factors that influenced the production of nanospheres, such as sample concentration, mass ratio between tannin and gelatin, reaction temperature, pH, and reaction time, were studied. Moreover, the nanospheres were analyzed and characterized by a particle size analyzer, UV–vis spectrophotometer, and TEM. It was concluded that the critical point was important for the assembled nanospheres. The tannin/protein mass ratio should be lower than the critical point. The concentration of tannin should be confined to a relatively low level. The proper range of the reaction temperatures was usually between 10°C and 50°C. It was steady for nanospheres assembled when the pH value of the gelatin solution was within ±1 IEP of the gelatin. After the reaction had gone on for more than 48 h, the assembled nanospheres became stable. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3125–3130, 2006     

LiX沸石分子筛的铈离子改性研究

采用阳离子交换法对Li X沸石分子筛进行Ce~(3+)改性,制得不同Ce~(3+)交换量的Ce Li X沸石分子筛。通过XRD、SEM、BET、XRF对Ce Li X沸石分子筛进行表征,通过气体吸附仪测得了Ce Li X沸石分子筛在25℃下对氮气、氧气的吸附等温线。结果表明,Ce~(3+)通过离子交换法可以替换Li X沸石分子筛中的非骨架Li+,Ce~(3+)的引入不会改变其原本的晶体结构,且能显著提高Li X沸石分子筛对O2/N2的吸附选择性。     

LiX沸石分子筛的铈离子改性研究

采用阳离子交换法对Li X沸石分子筛进行Ce⁽³⁺⁾改性,制得不同Ce(3+)改性,制得不同Ce⁽³⁺⁾交换量的Ce Li X沸石分子筛。通过XRD、SEM、BET、XRF对Ce Li X沸石分子筛进行表征,通过气体吸附仪测得了Ce Li X沸石分子筛在25℃下对氮气、氧气的吸附等温线。结果表明,Ce(3+)交换量的Ce Li X沸石分子筛。通过XRD、SEM、BET、XRF对Ce Li X沸石分子筛进行表征,通过气体吸附仪测得了Ce Li X沸石分子筛在25℃下对氮气、氧气的吸附等温线。结果表明,Ce⁽³⁺⁾通过离子交换法可以替换Li X沸石分子筛中的非骨架Li+,Ce(3+)通过离子交换法可以替换Li X沸石分子筛中的非骨架Li+,Ce⁽³⁺⁾的引入不会改变其原本的晶体结构,且能显著提高Li X沸石分子筛对O2/N2的吸附选择性。     

Inelastic relaxation in silica via reactive molecular dynamics

Silica glass exhibits rate-dependent and irreversible processes during deformation and failure, resulting in inelastic effects. To explore this phenomena, molecular dynamics simulations of structural relaxation surrounding a crack tip in silica glass were performed at four different temperatures (100, 300, 600, 900 K) using a reactive force field. Per-atom stresses were found to relax during the simulation, with the highest stress relaxation occurring at 900 K. Stress relaxation was radially dependent relative to the crack tip, with stress dissipation occurring primarily within a 25–30 Å inelastic region. Within 10 Å of the crack tip, the defect concentration decreased from 0.18 to 0.09 #/nm² during inelastic relaxation at 900 K. Conversely, the defect concentration 20 Å from the crack tip increased from 0.105 to 0.118 #/nm² at 300 K, and from 0.113 to 0.126 #/nm² at 600 K, which formed a defect-enriched region ahead of the crack tip. The difference in defect concentrations suggests the possibility of a stress mediated defect migration mechanism, where defects move away from the crack tip during inelastic relaxation. Additionally, defect speciation indicated that undercoordinated silica defects, such as non-bridging oxygen, were removed through the formation of higher coordination defects during relaxation. Overall, stress relaxation causes changes in the defect concentration profile near the crack tip, which has the potential to alter the properties of silica glass in the inelastic region during relaxation.     

Mechanical characterization of nanoindented graphene via molecular dynamics simulations

The mechanical behavior of graphene under various indentation depths, velocities, and temperatures is studied using molecular dynamics analysis. The results show that the load, elastic and plastic energies, and relaxation force increased with increasing indentation depth and velocity. Nanoindentation induced pile ups and corrugations of the graphene. Resistance to deformation decreased at higher temperature. Strong adhesion caused topological defects and vacancies during the unloading process.