Effect of Si on the electronic structure of sputter-deposited C films: an electron spectroscopy study

The evolution of the electronic structure of C films is followed through the independent measurement of their s and p partial `density of states' (DOS). The interest in such an approach is that the mutual relationship between the two partial DOS is a powerful indicator of the diamond-like or graphite-like character of a given system. It is shown here by this approach that a graphite-like structure is brought about in C films by thermal treatments, while a diamond-like structure is induced by Si addition during the deposition process.     

New insights into the density of states of graphene oxide using capacitive photocurrent spectroscopy

Capacitive photocurrent spectroscopy is used to probe the electronic states of graphene-oxide, and reduced graphene-oxide. Three peaks are observed whose intensities scale with the oxygen coverage. The energy of these peaks correlate with the luminescence spectra reported for graphene-oxide. Using a fitting procedure, the density of states for graphene oxide is extracted from the data. It consists of the π/π¹ states along with a distribution of mid-gap states centered at three different energies near the Dirac point. X-ray photoelectron spectroscopy measurements are used to identify the oxygen functional groups corresponding to the observed state distribution.     

The application of electron spectroscopy to the study of adhesion

The relevance of electron spectroscopic surface measurements for adhesion studies is discussed, with particular emphasis on surfaces prior to the formation of an adhesive joint and in examination of the surfaces of failed adhesive joints. Auger Electron Spectroscopy (aes) and X-ray Photoelectron Spectroscopy (xps) are reviewed in terms of their basic phenomena and their analytical advantages and disadvantages. Surface chemical contamination is known to be a major cause of adhesive failure. Sources of such contamination are reviewed and particular examples discussed.     

Excited states of polysilanes: High-resolution spectroscopy and molecular modeling

High-resolution spectroscopy at low temperatures (1.5–77 K) is used to study the electronic properties of the low-temperature phase of poly-di-n-hexyl-silane in solution. The absorption spectra exhibit an unexpected sensitivity to sample preparation. Experiments probing the energy transfer dynamics are reported and a model for the electronic excitation and the disorder reigning in the polymer is presented.This paper is from the Second International Topical Workshop, Advances in Silicon-Based Science.     

A model of density of states in amorphous Si, C and SiC from time-of-flight measurement

The temperature dependence of the electron drift mobility in glow-discharged undoped hydrogenated amorphous silicon, carbon and silicon carbide films with stoichiometric compositional (a-Si_0.5C_0.5:H) has been measured by the time-of-flight method. All films displayed the same behaviour of the transient current and dispersion parameters, which can be explained by assuming a gaussian distribution of tail states near the conduction band. The results obtained results corroborated the common nature and degree of disorder of the conduction band tail in all four-coordinated amorphous semiconductors.     

N2 and CO molecules as probes of zeolite acidity: an infrared spectroscopy and density functional investigation

The interaction of N₂ with Brønsted acid centers of H-ZSM-5 zeolite has been investigated employing Fourier transform infrared spectroscopy and cluster model calculations based on a gradient corrected density functional method. A comparison is made with CO, which is widely used as a probe for surface acidity. It is shown that the computational approach is capable of almost quantitatively reproducing a number of sensitive parameters of the H-bonded dinitrogen and carbonyl complexes, like adsorption energy, adsorption-induced changes of the vibrational frequencies and of their intensities. According to a constraint space orbital variation analysis, the carbonyl and dinitrogen complexes mainly differ by the somewhat stronger donation ability of CO as compared to N₂. It is concluded that dinitrogen may serve as a convenient probe for the acidity of zeolites.     

A contribution of radial electron density distribution measurements to the study of PTFE-Carbon

The sp-hybridized bonds in elemental carbon expected as primary product of low temperature electrochemical reduction of poly(tetrafluoroethylene) were proved by determination of C-C bond length from radial electron density distribution measurements. A value of (0.139 ± 0.002) nm was found for the C-C bonding distance for the carbon phase in the “as received” reduction products. This value, lower than 0.142 nm for graphitic sp₂ bonds, can be explained only by the presence of a remainder (about 5–20%) of sp-hybridized bonds with a length of about 0.120 nm, whereas the majority of very reactive sp carbon chains was transformed to six-membered rings (sp₂ bonds). On the contrary, a higher bonding distance of (0.145 ± 0.001)nm was found for carbons isolated from the reduction products, an evidence for the formation of a portion of tetrahedral sp₃ bonds in edge-to-edge cross-linkages. Interlayer distances of 0.342 nm, constituted at 950°C, were unusually low in comparison with polymeric carbons prepared by pyrolysis.     

Escherichia coli aspartate carbamoyltransferase: the probing of crystal structure analysis via site-specific mutagenesis

Crystal structures are known for aspartate carbamoyltransferase(ATCase) in the T and R states, with and without the allostericactivator adenosine triphosphate (ATP) or inhibitor cytidinetriphosphate (CTP). Visual inspection of X-ray crystal structuresdoes not provide all of the information necessary for the determinationof structure-function relationships in protein molecules. Thisproblem is compounded because the crystalline states of themolecule may introduce effects due to crystal packing, restrictedflexibility and less than optimum enzymatic conditions. Therefore,alternative techniques are required to test mechanisms conjecturedfrom three-dimensional crystal structures of proteins. The techniqueof site-specific mutagenesis allows the researcher to test structure- function models based on threedimensional structures and toobtain further insight into characteristics of the enzyme. Site-specificmutagenesis has been used to probe residues believed to be criticalin the structure and function of ATCase. Selection of residuesto be mutated has depended extensively on three-dimensionalcrystal structures of the enzyme. To date, 48 site-specificmutations at 37 different amino acid sites have been published.Although a total of 118 mutants at 58 different sites has beencommunicated to our laboratory, only published mutants willbe considered in this review. In this paper, we compile forthe first time, review, and analyze the site-specific mutantsof ATCase. Site-specific mutagenesis of proteins has becomea powerful technique in modern-day molecular biology, especiallyin studying a molecule as large as aspartate carbamoyltransferase.In this review, the role of site-specific mutagenesis of ATCaseis discussed and improvements in the analysis are suggested.     

Copolymers of 2-hydroxyethyl methacrylate and methyl methacrylate: an electron beam resist study

The electron beam development characteristics of copolymers of HEMA and MMA are reported. A copolymer with 4% of HEMA was found to have comparable sensitivity and enhanced contrast compared with conventional PMMA and this material would appear to be useful in lithographic applications.     

Localisation and density of states in amorphous carbon-based alloys

The aim of this work was to thoroughly discuss all local effects affecting the nature and energy distribution of π and π* states. In particular, the role of overlap between atomic orbitals, disorder and localisation will be treated. It will be shown that, while the overlap of atomic p-orbitals leads to states asymmetric with respect to the non-bonding level, the disorder-induced fluctuations of the clusters non-bonding level lead to the broadening of Gaussian π and π* density-of-states (DOS) bands. The role of charge transfer and its effect on the correlation energy will be discussed as well. On such basis, the optical and paramagnetic properties of (t)a-C(:H) will be successfully modelled.     

Thermosetting mechanism study of silicon-containing polyarylacetylene via in situ FTIR and solid-state NMR spectroscopy

Silicon-containing polyarylacetylene (abbreviated as PSA) resins, which contain Si-(R₁, R₂) (R₁ and R₂ represent methyl or phenyl groups) and —CC—, exhibits high thermal stability upon curing up to 250 °C. The structure and thermosetting mechanism of PSA were characterized using FTIR, in situ FTIR, ¹³C and ²⁹Si CP-MAS spectroscopy, and thermogravimetric analysis. From the experimental results we can conclude that: (1) biphenyl and naphthalene rings are formed via a Diels–Alder reaction between the Ph—CC and CC groups at 210 °C, (2) the terminal alkyne mainly transforms into ethylenic bonds at 170 and 210 °C, and (3) an oxidation reaction occurs to give the oxide structure (Si—O—Si) and carbon dioxide at 250 °C. A new curing procedure has been proposed to maximize the T_d5 up to 635.2 °C on that basis. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47301.     

Characterization of surface phenomena: probing early stage degradation of low‐density polyethylene films

Photo‐oxidative degradation of polyethylene triggers significant deterioration in the polymer properties. Much interest is aimed at characterizing and possibly predicting photo‐oxidative damages, at early stages, prior to the occurrence of profound changes in mechanical properties. Herein, we study the degradation of low‐density polyethylene (LDPE) films, focusing on surface deterioration processes. Thin films of various molecular weights are exposed to accelerated weathering while their chemical, mechanical and morphological characteristics are monitored throughout by apparent contact angle (CA) measurements, Fourier‐transform infrared spectroscopy (FTIR), tensile test, and electron microscopy. A significant decrease in the films' CA during degradation is observed. CA is highly sensitive to two simultaneous phenomena with opposing effect: nanoscale surface roughening and composition changes. We found the latter (specifically the evolution of polar groups) to be the dominant parameter affecting the CA behavior. Consequently, simple CA measurements coincide well with conventional FTIR analysis, and are more sensitive to changes occurring at early stages of degradation. The deterioration in the mechanical properties is also characterized and is found to present poor sensitivity and high variability at these early aging stages. Thus, simple CA measurements could potentially be used as a qualitative indicator for evaluating the aging impact on LDPE. POLYM. ENG. SCI., 59:E129–E137, 2019. © 2018 Society of Plastics Engineers     

Interaction between single gold atom and the graphene edge: a study via aberration-corrected transmission electron microscopy

Interaction between single noble metal atoms and graphene edges has been investigated via aberration-corrected and monochromated transmission electron microscopy. A collective motion of the Au atom and the nearby carbon atoms is observed in transition between energy-favorable configurations. Most trapping and detrapping processes are assisted by the dangling carbon atoms, which are more susceptible to knock-on displacements by electron irradiation. Thermal energy is lower than the activation barriers in transition among different energy-favorable configurations, which suggests electron-beam irradiation can be an efficient way of engineering the graphene edge with metal atoms.     

Noble metal nanocrystals: plasmon electron transfer photochemistry and single-molecule Raman spectroscopy

The excited electronic states of noble metal Au and Ag nanocrystals are very different than those of molecules. Ag and Au nanocrystal optical transitions (plasmons) in the visible can be so intense that they significantly modify the local electromagnetic field. Also, coherent elastic Rayleigh light scattering is stronger than normal electronic absorption of photons for larger nanocrystals. These two facts make Au and Ag nanocrystals ideal nanoantennas, in that they focus incident light into the local neighborhood of subwavelength size. Surface-enhanced Raman scattering (SERS), in which the Raman scattering rate of nearby molecules increases by many orders of magnitude, is a consequence of this nanoantenna effect. Metallic nanocrystals also have no band gap; this makes them extraordinarily polarizable. Their electronic transitions sense the local environment. An extreme case is the interaction of two 30 nm Ag nanocrystals separated by a 1 nm gap. Their mutual polarization completely transforms the nature of the metallic excited electronic state. Single particles have an excited state uniformly distributed throughout the interior, while the nanocrystal dimer has its excited state localized on the metal surface in the junction. This creates an electromagnetic "hot spot" in the junction, enabling the observation of single-molecule SERS. The fact that surface molecules are typically chemisorbed and exchange electrons with the metal has interesting chemical consequences. First, the enhanced Raman intensities are controlled by quantum mechanical coupling of the molecular lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) with the optically excited electrons in the metal. Second, charge-transfer photochemistry can result from metal plasmon excitation. In crystalline Ag nanocrystals the photochemistry quantum yield can be high because the nanocrystal surface dominates plasmon nonradiative relaxation. Colloidal Ag nanocrystals stabilized by sodium citrate build up a photovoltage under visible excitation, caused by irreversible "hot hole" photo-oxidation of adsorbed citrate anion. This creates a driving force for photochemical transformation of round 8 nm Ag seeds into 70 nm single-crystal disk prisms under room lights, in a novel type of light-driven Ostwald ripening.     

Semi-quantitative study on the fabrication of densely packed and vertically aligned single-walled carbon nanotubes

This paper presents, for the first time, a semi-quantitative study on the production of densely packed and vertically aligned (DPVA) single-walled carbon nanotubes (SWNTs) from ultra-thin catalytic films. An up-to-date highest volume density (60-70 kg m⁻³) and the corresponding high surface density on the order of 10¹⁶ m⁻² of DPVA-SWNTs have been achieved by point-arc microwave plasma chemical vapor deposition. The precise thickness control of the sandwich-like catalytic nanostructure of 0.5 nm Al₂O₃/0.5 nm Fe/>5 nm Al₂O₃, developed by the authors, and a short-time (5 min) heat pretreatment of substrates at a temperature as low as 600 °C play the very key role in the process of fabricating DPVA-SWNTs.     

Stabilisation of lithiated graphite in an electrolyte based on ionic liquids: an electrochemical and scanning electron microscopy study

1-Ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (EMI-TFSI) is shown to reversibly permit lithium intercalation into standard TIMREX^® SFG44 graphite when vinylene carbonate (VC) is used in small amounts as additive. The best performance was obtained when 5% of VC was added to a 1 M solution of LiPF₆ in EMI-TFSI. Intercalation of lithium in the SFG44 graphite host was demonstrated over 100 cycles without noticeable capacity fading. The reversible charge capacity was around 350 mA h g⁻¹ and an only small irreversible capacity loss per cycle could be observed. Li₄Ti₅O₁₂ was used as counter electrode material. Scanning electron microscopy indicates the reduction of the electrolyte without graphite exfoliation in the neat electrolyte and the formation of a passivation film in the case of a VC-containing electrolyte. Other additives that were tested comprise ethylene sulphite and acrylonitrile which show also a positive effect, but a smaller one than vinylene carbonate. LiCoO₂ positive electrodes were cycled in a 1 M solution of LiPF₆ in EMI-TFSI with good charge capacity retention over more than 300 cycles, when Li₄Ti₅O₁₂ was used as counter electrode. The formation of a passivation film is proven on the LiCoO₂-electrodes, when the electrolyte contained VC, but not in the neat ionic liquid. Finally, the stable cycling of a full cell configuration is proven in this electrolyte system. An ammonium-containing ionic liquid (methyltrioctylammonium-bis(trifluoromethylsulfonyl)-imide, MTO-TFSI) is shown to permit the cycling of both, graphite and lithium cobalt oxide when VC is used as additive in small amounts, but at slightly elevated temperatures.     

Pillaring of smectites using an aluminium oligomer: A study of pillar density and thermal stability

Two smectite samples having different layer charges were pillared using hydroxy aluminium oligomers at a OH/Al ratio of 2.5 and at pH 4.3 to 4.6. Pillaring was carried out at different conditions such as ageing, temperature and base addition time of the pillaring solution, and also in the presence of nonionic surfactant polyoxyethylene sorbitanmonooleate (Tween-80). The primary objective of preparing at different conditions was to introduce varied quantities of aluminium oligomer between the layers and to study its effect on the properties of the pillared products. A simple method has been followed to estimate the amount of interlayer aluminium. A quantity called pillar density number (PDN) based on the ratio of interlayer Al adsorbed to CEC of the parent clay has been effectively used to evaluate the nature of the resulting pillared product. PDN, for a given clay, was found to correlate well with the sharpness of the d(0001) peaks for the air dried samples. The calculated number of pillars, varied from 3.00 × 10¹⁸ to 5.32 × 10¹⁸ per meq charge. The present study shows that a higher value of PDN is indicative of better thermal stability. Pillar density number may be conveniently used as a measure of the thermal stability of pillared samples.     

A star-function based density functional study of the adsorption of Lennard-Jones fluid near its supercritical states

The goal of this paper is to show that the wetting behavior of simple fluids on a repulsive solid surface - especially the “drying” phenomenon - is closely related to the proximity of the supercritical state of the bulk fluids to their vapor-liquid coexistence region. We propose here a new DFT (the star-function based density functional theory, s-DFT) that is based on the functional Taylor expansion of the intrinsic free energy F[ρ] and the singlet direct correlation to arrive at closed-form expressions for both quantities without truncations or approximations. The two formulas are mutually consistent because of the introduction of a star function Sw that has been shown to be the functional primitive of the bridge function Bw, i.e. (L.L. Lee, J. Chem. Phys. 97 (1992) 8606 [34]). The new formulation is applied to the Lennard-Jones molecules adsorbed on a planar hard wall (LJ/HW). We carried out new Monte Carlo simulations for this system. Since the s-DFT uses a bridge function Bw, we demonstrate (i) the existence of a set of data (inverted from the MC information) that can perform as the bridge function and reproduce accurately the density profiles ; (ii) this set of data can be “fitted” by a function-form with acronym ZSEP; finally (iii) ZSEP expresses the bridge function Bw in terms of a new renormalized basis function γ_H, i.e. Bw(γH). The existence of a bridge function dispels some of the misconceptions that the bridge-function based formulations did not describe the “drying” behavior. We also show that for the high density case ZSEP equation can qualify as a “closure relation”, but seems to deteriorate for the two low-density supercritical states that are close to the bulk saturated liquid phase boundaries.     

A study of intramolecular electron exchange in copper-radical complexes involved in catalysis using ESR spectroscopy

ESR simulation and experimental results have been presented to show that transition metal-radical complexes can engage in intramolecular electron exchange and that the exchange gives rise to ESR asymmetric line broadening effect. Depending on the relative concentration of the redox-exchange pair, metal or radical-like spectrum can be obtained. Simulation results show that a metal-radical complex can be masked by its redox counterpart upto a relative concentration of 1:2 at a modest exchange rate of 3 x 10⁸/s. Asymmetric line broadening was predicted to occur upon such metal-radical complexation. Experimentally, a pronounced ESR asymmetric line broadening was observed for Cu(II) complexes of various redox-active ligands. Cu(II) complex with redox-inert ammonia, however, showed no such evidence. Ligand displacement experiments established the reversibility of metalradical complexation and the associated ESR line broadening.     

Investigating the structure of biomass-derived non-graphitizing mesoporous carbons by electron energy loss spectroscopy in the transmission electron microscope and X-ray photoelectron spectroscopy

We have investigated the microstructure and bonding of two biomass-based porous carbon chromatographic stationary phase materials (alginic acid-derived Starbon® and calcium alginate-derived mesoporous carbon spheres (AMCS)) and a commercial porous graphitic carbon (PGC), using high resolution transmission electron microscopy, electron energy loss spectroscopy (EELS), N₂ porosimetry and X-ray photoelectron spectroscopy (XPS). The planar carbon sp²-content of all three material types is similar to that of traditional non-graphitizing carbon although, both biomass-based carbon types contain a greater percentage of fullerene character (i.e. curved graphene sheets) than a non-graphitizing carbon pyrolyzed at the same temperature. This is thought to arise during the pyrolytic breakdown of hexauronic acid residues into C5 intermediates. Energy dispersive X-ray and XPS analysis reveals a homogeneous distribution of calcium in the AMCS and a calcium catalysis mechanism is discussed. That both Starbon® and AMCS, with high-fullerene character, show chromatographic properties similar to those of a commercial PGC material with extended graphitic stacks, suggests that, for separations at the molecular level, curved fullerene-like and planar graphitic sheets are equivalent in PGC chromatography. In addition, variation in the number of graphitic layers suggests that stack depth has minimal effect on the retention mechanism in PGC chromatography.