Auger-electron-ion coincidence study of photon-stimulated ion desorption for condensed acetonitrile

Auger-electron-photoion coincidence (AEPICO) studies of photon-stimulated ion desorption (PSID) for condensed acetonitrile induced by carbon core excitation have been performed to elucidate the desorption mechanism related to the Auger process. We have detected only the H⁺ ion in AEPICO spectra. The total ion yield spectrum divided by the total electron yield shows that the desorption efficiency is largely increased at the resonant excitation to C---H*. We have also measured the Auger electron spectrum and the AEPICO yield spectrum at the C---H* excitation. The AEPICO yield spectrum shows enhancement at 245–250 eV electron energy. This seems to be related to the spectator resonant Auger stimulated ion desorption. That is, H⁺ desorption is enhanced due to a two-hole-one-electron state at which the electron is in an anti-bonding C---H* orbital and the two holes can be in a 1π bonding orbital localized on the ---CH₃ group. We have also measured similar spectra for other resonant excitation (π*_CN, σ*_CC, σ*_CN). The results are discussed in connection with the bonding/anti-bonding character and localized character of the excited state.     

Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption

Adsorption behavior of atomic deuterium on a hexagonal boron nitride (h-BN) thin film is studied by photon-stimulated ion desorption (PSID) of D⁺ and near edge X-ray absorption fine structure (NEXAFS) at the B and N K-edges. After the adsorption of atomic deuterium, D⁺ desorption yield η(hν) shows clear enhancement at the B K-edge and almost no enhancement at the N K-edge. NEXAFS spectra show a large change in the B K-edge and a small change in the N K-edge after the adsorption. We propose selective adsorption of atomic deuterium on the h-BN thin film based on the experimental results, and mention the effectiveness of applying the PSID method with X-ray to study hydrogen storage materials.     

Defect electron states in carbon nanotubes and graphite from the NEXAFS spectroscopy data

Experimental results of studying the electronic structure of single-walled and multi-walled carbon nanotubes as well as graphite by X-ray absorption spectroscopy (or NEXAFS spectroscopy) are presented. The C1s absorption spectra are measured with high energy resolution using the equipment of the Russian-German beamline of the BESSY electron storage ring. Features found in absorption spectra of carbon nanotubes and graphite for the first time are interpreted in the case of carbon nanotubes as the contribution of electron states appearing due to the imperfection of their structure both under the nonequilibrium synthesis conditions and during the subsequent producing manipulations.     

Functional multi-walled carbon nanotube/polysiloxane composite films as supports of PtNi alloy nanoparticles for methanol electro-oxidation

We demonstrate the use of molecular monolayers to enhance the nucleation of electrocatalytically active PtNi alloy nanoparticles onto the multi-walled carbon nanotubes (MWCNTs). After the siloxane was polymerized on the nanotube surfaces, the carbon nanotubes were embedded within the polysiloxane shell with a hydrophilic amino group situated outside. Subsequent deposition of PtNi nanoparticles led to high density of 3-10 nm diameter PtNi alloy nanoparticles uniformly deposited along the length of the carbon nanotubes. The presence of MWCNTs and PtNi in the composite films was confirmed by transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersion X-ray spectra analysis (EDS). The electrocatalytic activity of the PtNi-modified MWCNT/polysiloxane (PtNi/Si-MWCNT) composite electrode for electro-oxidation of methanol was investigated by cyclic voltammetry (CV), and excellent electrocatalytic activity can be observed.     

X-ray induced electron stimulated desorption versus photon stimulated desorption: NH3 on Ni(110)

The X-ray induced desorption of H⁺ ions from NH₃ layers adsorbed at T = 90 K on Ni(110) has been compared to the corresponding total electron yield (TY) in the photon energy range 390 to 900 eV. The H⁺ yield exhibits a jump at the N K-edge and the Ni L-edge which inversely varies with the NH₃ layer thickness. The H⁺ Ni L-edge jump is closely correlated to the TY jump. Both vanish for the saturated NH₃ multilayer, indicating that the observed Ni L-edge jump in the H⁺ yield is exclusively due to X-ray induced electron stimulated desorption (XESD). At the N K-edge, the near edge absorption fine structure of the H⁺ yield and TY of the saturated NH₃ multilayer are distinctly different. This is interpreted as the H ⁺ yield being the superposition of direct photon stimulated ion desorption (PSID) and XESD. Based on the observed variation of the H⁺ yield near edge fine structure with varying NH₃ layer thickness, a deconvolution of the PSID and XESD contributions is used to derive the relative contribution of PSID versus XESD to be 40% versus 60%, respectively. The relevance of this result for future PSID-SEXAFS studies is discussed. For monolayer NH₃ on Ni(110) the polarization dependence of the N K-edge fine structure in the N(KVV) Auger yield indicates that the symmetry axis of NH₃, is perpendicular to the surface.     

Effect of pulsed ion irradiation on the electronic structure of multi-walled carbon nanotubes

The effect of pulsed ion irradiation and vacuum annealing on the ratio of sp ²- and sp ³-hybridized orbitals of carbon atoms in the layers of oriented multi-walled carbon nanotubes has been studied by analyzing the photoemission spectra of the C1s core level and the valence band of carbon, which were obtained using the equipment of the BESSY II Russian-German beamline of synchrotron radiation and a Riber analytical system. It has been shown that the ion irradiation leads to a significant decrease in the fraction of atoms with the sp ³ hybridization of electrons. On the contrary, the annealing reduces the fraction of the sp ³-component in the spectra of carbon. Typical features of the valence band of multi-walled carbon nanotubes in the annealed and irradiated states have been established.     

Positive and negative ionic desorption from condensed formic acid photoexcited around the O 1s-edge: Relevance to cometary and planetary surfaces

Photon stimulated ion desorption (PSID) studies have been performed in condensed formic acid using oxygen 1s-edge synchrotron radiation from the Brazilian synchrotron light source (LNLS), operating in a single-bunch mode. Ion formation was discussed in terms of the Auger stimulated ion desorption (ASID) and X-ray induced electron stimulated desorption (XESD) mechanisms. It is found that O 1s(C-OH) → π*(CO) and O 1s(CO) → 3s/σ*(HCO) transitions favored the production of C⁺, CH⁺, O⁺, O⁻ and H⁻ ions. The hydroxyl anion has not been observed while the hydroxyl cation showed low intensity or was absent. Some anion formation routes from dissociative reactions are suggested taking into account the positive ion yields.     

Improving the surface properties of multi-walled carbon nanotubes after irradiation with gamma rays

The effects of gamma-irradiation on the modification of the surface and structure of multi-walled carbon nanotubes were studied. Gamma-irradiation affected the graphitization properties of functional groups, and decreased the diameter of multi-walled carbon nanotubes. The irradiated multi-walled carbon nanotubes with the absorbed dose of 100 kGy exhibited a larger specific surface area and microporous volume as compared with the other samples. The Raman spectroscopy and X-ray photoelectron spectroscopy showed that the interaction between the gamma-irradiation and the multi-walled carbon nanotubes with the absorbed dose of 150 kGy destroyed the nanostructure of carbons, leading to the formation of diamond-like structures and carbon oxides. In addition, gamma-irradiation with the absorbed dose of 100 kGy improved multi-walled carbon nanotubes graphitization and surface properties while at higher absorbed dose (150 kGy), it induced damaged structures (sp³ bonds and oxygen compositions).     

Geometrically nonlinear free vibration of shear deformable piezoelectric carbon nanotube/fiber/polymer multiscale laminated composite plates

The nonlinear free vibration of carbon nanotubes/fiber/polymer composite (CNTFPC) multi-scale plates with surface-bonded piezoelectric actuators is studied in this paper. The governing equations of the piezoelectric nanotubes/fiber/polymer multiscale laminated composite plates are derived based on first-order shear deformation plate theory (FSDT) and von Kármán geometrical nonlinearity. Halpin–Tsai equations and fiber micromechanics are used in hierarchy to predict the bulk material properties of the multiscale composite. The carbon nanotubes are assumed to be uniformly distributed and randomly oriented through the epoxy resin matrix. A perturbation scheme of multiple time scales is employed to determine the nonlinear vibration response and the nonlinear natural frequencies of the plates with immovable simply supported boundary conditions. The effects of the applied constant voltage, plate geometry, volume fraction of fibers and weight percentage of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) on the linear and nonlinear natural frequencies of the piezoelectric nanotubes/fiber/polymer multiscale composite plate are investigated through a detailed parametric study.     

可溶性高聚物-多壁碳纳米管复合物的吸收光谱和光限幅性能研究

合成了三种可溶性高聚物嫁接的多壁碳纳米管复合物,包括聚乙烯基咔唑-多壁碳纳米管复合物(MWNTs-PVK),聚苯乙烯-多壁碳纳米管复合物(MWNTs-PSt)和聚甲基丙烯酸甲酯-多壁碳纳米管复合物(MWNTs-PMMA)。合成样品的TEM图显示出高聚物成功地包裹在碳纳米管外。样品的氯仿溶液UV-Vis 吸收谱显示,复合物的紫外区与纯的多壁碳纳米管不同,有一些复杂的特征吸收峰,反映了高聚物与碳纳米管共价连接的作用。利用Z扫描技术通过527 nm的纳秒激光脉冲激发研究了这三种复合物的非线性光学性质和光限幅性能。这三种复合物的氯仿溶液的光限幅性能很相似,并且优于纯多壁碳纳米管氯仿悬浮液和C60甲苯溶液。为解释观察到的结果,研究了样品的非线性折射,非线性吸收和非线性散射机制。结果显示,非线性吸收是引起样品光限幅的主要机制。     

Photon stimulated ion desorption of H+ ions from GaAs (110)/H2O

The photon stimulated ion desorption yield of H⁺ ions from a H₂O dosed GaAs (110) surface has been measured in the range 18eV ? hυ ? 30eV. There is a direct correspondence between the PSID H⁺ yield, reflectance, and the secondary electron yield spectrum of GaAs (110). The data provides evidence that the initial stages of PSID involve core level (Ga(3d), O(2s)) → conduction band excitation followed by Auger decay.     

Origin of the low-frequency band in Raman spectra of multi-walled carbon nanotubes synthesized by the CVD method

The origin of the low-frequency band (250–300 cm^?1) in the Raman spectra of multi-walled carbon nanotubes (MWCNTs) produced by the CVD method has been studied. The studies performed by Raman spectroscopy, transmission electron microscopy, Auger spectroscopy, and X-ray photoelectron spectroscopy after chemical and thermal treatments allow the assumption that this band belongs to radial vibrations of carbon atoms in internal walls of MWCNTs.     

Ionic desorption in valence- and core-excited poly(vinyl chloride)

Photon stimulated ion desorption (PSID) studies have been performed in poly(vinyl chloride) (PVC) using synchrotron radiation, encompassing the valence and core electron (Cl 2p and C 1s) energy ranges. Data acquisition was performed at the Brazilian synchrotron light source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer (TOF-MS). A pulsed high voltage applied to the sample was used as a trigger for the TOF-MS experiments. Ionic desorption from PVC shows strong selectivity in the formation of chlorine ions around the Cl 2p-edge while very similar fragmentation patterns are observed for the other energies studied.     

Deposition of silver nanoparticles on carbon nanotube by chemical reduction method: Evaluation of surface,thermal and optical properties

Silver was stabilized on multi-walled carbon nanotubes (MWCNTs) by chemical-reduction technique using N,N-dimethylformamide (DMF) as a reducing agent. The influence of silver on the performance of carbon nanotubes (CNTs) was investigated by employing Fourier-transform infrared spectra (FTIR), Raman spectroscopy (RAS), thermal gravimetric analysis (TGA), zeta potential measurement, scanning electron microscope (SEM), electron dispersive X-ray spectrometer (EDX), transmission electron microscopy (TEM), and reflectance spectroscopy (RS). FTIR as well as RS methods evidenced the synthesis procedure using chemical reduction method was successful. Performing TGA of the samples under oxygen atmosphere demonstrated that the silver nanoparticles (Ag NPs) generated on MWCNTs surface can decrease the thermal stability of the particles by the catalytic oxidation of CNTs. In contrary, the thermal stability of the MWCNTs has improved under nitrogen atmosphere. EDX results showed the presence of Ag, Au and Co on the surface of deposited sample. The synthesised silver multi-walled carbon nanotubes (Ag–MWCNTs) were found to have higher UV reflection activity compared with untreated particles. The Ag–CNTs can be used in producing anti-UV composites.     

Raman and SERS studies of carbon nanotube systems

In this paper, we report on Raman studies carried out on different carbon nanotube systems, namely single-walled and multi-walled carbon nanotubes and polymer/nanotube composites. We focus on different types of interactions which can take place in these materials. In single-walled nanotubes, the introduction of van der Waals interactions between tubes when arranged in bundles leads to an upshift of the radial breathing mode (RBM) ranging from 11 to 16 cm⁻¹ depending on the size of the bundle. In multi-walled carbon nanotubes, similar interactions between concentric tubes permit to interpret the low frequency Raman modes. In composites, PMMA/nanotubes, an upshift of the RBM is also observed, explained by the dynamical strain applied by the polymer on the bundles, in response to the breathing vibration. In addition, surface enhanced Raman scattering experiments have demonstrated the occurrence of interfacial reactions between the nanotubes and the metallic support. This is put in evidence by the degradation of tubes, especially metallic ones, and reconstruction of C₆₀-like molecules are in some cases observed.     

Micromechanical properties of poly(butylene terephthalate) nanocomposites with single- and multi-walled carbon nanotubes

Polymer nanocomposites with carbon nanotubes (CNT) are becoming important structural materials because of their superior mechanical properties and easy processability. The objective of the work is to investigate the influence of small amounts of single walled carbon nanotubes (SWCNT), as well as multi-walled carbon nanotubes (MWCNT), on the microhardness of a thermoplastic polymer such as poly(butylene terephthalate) (PBT). The nanocomposites were obtained by introducing the CNT into the reaction mixture during the synthesis of PBT. The polymers without carbon nanotubes (reference material) and with carbon nanotubes were synthesized using an in-situ polycondensation reaction process. Weight percentages ranging from 0.01 to 0.2 wt% of the single walled and from 0.01 to 0.35 wt% of the multi-walled nanotubes were dispersed in 1,4-butanediol (BD) by ultrasonication and by ultra high speed stirring. The nanocomposites were extruded followed by injection molding. The samples were characterized by electron microscopy and microindentation hardness techniques. The variations of the micromechanical properties (indentation hardness) of the nanocomposites with nanotube content and with temperature are discussed in the light of the stress transfer between the polymer matrix and nanotubes, the degree of dispersion, the nature of the tubes and other structural parameters.     

Electron stimulated desorption of H3O from 316L stainless steel

Surface ions generated by electron stimulated desorption from mass spectrometer ion source grids are frequently observed, but often misidentified. For example, in the case of mass 19, the source is often assumed to be surface fluorine, but since the metal oxide on grid surfaces has been shown to form water and hydroxides, a more compelling case can be made for the formation of hydronium. Further, fluorine is strongly electronegative, so it is rarely generated as a positive ion. A commonly used metal for ion source grids is 316L stainless steel. Thermal vacuum processing by bakeout or radiation heating from the filament typically alters the surface composition to predominantly Cr₂O₃. X-ray photoelectron spectral shoulders on the O 1s and Cr 2p_3/2 peaks can be attributed to adsorbed water and hydroxides, the intensity of which can be substantially increased by hydrogen dosing. On the other hand, the sub-peak intensities are substantially reduced by heating and/or by electron bombardment. Electron bombardment diode measurements show an initial work function increase corresponding to predominant hydrogen desorption (H₂) and a subsequent work function decrease corresponding to predominant oxygen desorption (CO). The fraction of hydroxide concentration on the surface was determined from X-ray photoelectron spectroscopy and from the deconvolution of temperature desorption spectra. Electron stimulated desorption yields from the surface show unambiguous H₃O⁺ peaks that can be significantly increased by hydrogen dosing. Time of flight secondary ion mass spectrometry sputter yields show small signals of H₃O⁺, as well as its constituents (H⁺, O⁺ and OH⁺) and a small amount of fluorine as F⁻, but no F⁺ or F⁺ complexes (HF⁺, etc.). An electron stimulated desorption cross-section of σ⁺ ∼ 1.4 × 10⁻²⁰ cm² was determined for H₃O⁺ from 316L stainless steel for hydrogen residing in surface chromium hydroxide.     

Correlation between carbon nanotube microstructure and their catalytic efficiency towards the p-coumaric acid degradation

Pristine and annealed multi-walled C nanotubes are preliminarily tested as catalysts towards the degradation of p-coumaric acid with better performances than the widely used activated carbon. In order to clarify the origin of their different catalytic efficiency, the samples are analysed by means of high-resolution transmission electron microscopy, X-ray photoemission, reflection electron energy loss and micro-Raman spectroscopy. Chemical composition, mass density, bonding configurations and threefold- to fourfold-coordinated carbon bonding ratios are determined. The comparative discussion of the results obtained leads to attribute the higher catalytic activity of the pristine nanotubes to their larger sp²/sp³ carbon bonding fraction and greater amount of oxygen containing functional groups.     

Polyelectrolyte Nanocomposite Membranes with Imidazole-Functionalized Multi-Walled Carbon Nanotubes for Use in Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® and imidazole modified multi-walled carbon nanotubes (MWCNT-Im), for direct methanol fuel cell (DMFC) applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of multi-walled carbon nanotubes (MWCNT), and the negatively charged sulfonic acid groups of Nafion®, new electrostatic interactions can be formed in the interface of the Nafion® and MWCNT-Im which result in both lower methanol permeability and also higher proton conductivity. The physical characteristics of these manufactured nanocomposite membranes were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/MWCNT-Im membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter in comparison to neat Nafion® or Nafion® containing –OH functionalized multi-walled carbon nanotubes (MWCNT-OH) membranes. The obtained results indicated that the Nafion®/MWCNT-Im membranes could be utilized as efficient polyelectrolyte membranes for direct methanol fuel cell applications.     

The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.