Comparative X-ray absorption investigation of fluorinated single-walled carbon nanotubes

The C 1s and F 1s X-ray absorption spectra of pristine and fluorinated single-walled carbon nanotubes with different fluorine contents and nanodiamond as a reference compound have been measured with the aim of characterizing single-walled carbon nanotubes and their products formed upon treatment of the nanotubes with molecular fluorine at a temperature of 190°C. The spectra obtained have been analyzed by thoroughly comparing with the previously measured spectra of highly oriented pyrolytic graphite and fluorinated multiwalled carbon nanotubes and the spectrum of nanodiamond. It has been established that the fluorination of single-walled and multiwalled carbon nanotubes leads to similar results and is characterized by the attachment of fluorine atoms to carbon atoms on the lateral surface of the nanotube with the formation of the σ(C-F) bonds due to the covalent mixing of F 2p and C 2p _z π valence electron states.     

Electron spectroscopy of graphite,graphite oxide and amorphous carbon

Core-level and valence-band photoelectron spectra and Auger spectra for the basal plane of highly ordered graphite, the edge plane of highly ordered graphite, the basal plane of graphite oxide, and for the basal plane of disordered graphite are compared in an effort to determine spectral features that may be used to identify these chemical species in carbon-rich specimens. The photoelectron spectra were recorded using 1253.6 eV X-ray excitation. The Auger spectra were obtained using both X-ray (1253.6 eV) and electron (3 keV) excitation. The differential X-ray excited C-KVV spectra were the most useful in distinguishing the different types of carbon. In particular, the plasmon structure and the energy separation between the two major excursions were very sensitive to changes in the surface chemistry and structure. Changes in peak position and peak width observed in the valence band and C(1s) photoelectron spectra were also very helpful in distinguishing the pristine, structurally damaged, and the oxidized graphite surfaces. Much of the structural and chemical information apparent in the X-ray excited C-KVV spectra was lost upon electron beam exposure. Differences in the C-KVV Auger lineshapes for X-ray and electron excitation were attributed to both structural and chemical changes induced by electron beam exposure.     

Characterization of fluorinated multiwalled carbon nanotubes by x-ray absorption spectroscopy

The C 1s and F 1s x-ray absorption spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents and reference compounds (highly oriented pyrolytic graphite crystals and “white” graphite fluoride) were measured using the equipment of the Russian-German beamline at the BESSY II storage ring with a high energy resolution. The spectra obtained were analyzed with the aim of characterizing multiwalled carbon nanotubes and their products formed upon treatment of the nanotubes with fluorine at a temperature of 420°C. It was established that, within the probing depth (～15 nm) of carbon nanotubes, the process of fluorination occurs uniformly and does not depend on the fluorine concentration. The interaction of fluorine atoms with multiwalled carbon nanotubes in this case proceeds through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton and is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp ²) to tetrahedral (sp ³) hybridization.     

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.     

Auger spectra of CO,C2H4, C2N2 and C6H6 adsorbed on Pt(111)

Electron excited carbon KVV Auger spectra of CO, C₂H₄, C₂N₂ and C₆H₆ adsorbed on Pt(111) are compared. By estimating the effective Coulomb interaction between the final-state holes it is possible to associate some features with transitions observed in free molecule spectra, but others must involve at least one electron with energy within the conduction band of the metal. Such “cross-transitions” are associated with strong 2π* character of filled states in the presence of a core hole in molecules such as CO.     

Tunneling-current-induced light emission from individual carbon nanotubes

Tunneling electrons from a scanning tunneling microscope were used to excite light emission from individual multiwalled carbon nanotubes adsorbed on a highly ordered pyrolytic graphite surface. In the integral photon-intensity map, spatially uniform emission in the visible region was observed along the identical multiwalled carbon nanotubes. The emission spectra for the individual nanotubes showed unique profiles which differed with each nanotube, and were classified into two types. Our results indicate that the light emission is due to not the localized electronic states at the tube ends or defects but radiative transitions of electrons between the one-dimensional van Hove singularities, indicating that the two types of spectra are attributed to metallic and semiconducting nanotubes.     

Electron energy loss spectrum of graphane from first-principles calculations

In this study, the energy loss near edge structure (ELNES) of carbon atoms in chair and tricycle conformers of hydrogenated graphene, namely ‘graphane’, has been calculated in the density functional theory using FP-LAPW method, and then, it has been compared with that of graphite and graphene. Using ELNES from chair conformer, the carbon K-edge was found to have a few main features including electron transition from 1s orbital of carbon atom to π*, σ*, and a hybridization of these two states. The first feature in tricycle conformer, however, has contributions of both π* and σ* states. The comparison of ELNES and the unoccupied density of states in each structure also justifies this. The energy difference between π* and σ* features of graphane conformers was decreased relative to it in graphite and graphene. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in graphite and graphene, it may be essential as well for the ELNES spectra of graphane conformers.     

Electronic structure of pyrolytic graphite by auger electron spectroscopy

We have used Auger electron spectroscopy to study the effect of defects on the surface electronic states of graphite. We modeled the effects of outer layer inclusions composed of carbon and cesium atoms. The experimental Auger spectra are compared with theoretical calculations of the density of states in the valence band of graphite. An increase in defect density deforms the energy bands. Chelyabinsk State Teachers University. Translated from Izvestiya Vysshikh Uchebnykh, Zavedenii, Fizika, No. 10, pp 84–88, October, 1997.     

Spectroscopic study of plasmons in ion-irradiated single-walled carbon nanotubes

The electronic structure of single-walled carbon nanotubes was experimentally investigated using x-ray photoelectron spectroscopy, reflection electron energy-loss spectroscopy, and Auger electron spectroscopy. A shake-up satellite structure observed near the C 1s core-level lines in the x-ray photoelectron spectra at high binding energies in the range 284–330 eV due to excitation of π and π + σ plasmons was studied. The effect of irradiation by 1-keV argon ions on the shape of the spectra was analyzed. The shape of the C 1s satellite spectra was found to be sensitive to Ar⁺ irradiation in the electron energy loss range 10–40 eV corresponding to excitation of π + σ plasmons. Auger spectroscopy revealed the presence of argon on the surface of ion-irradiated samples. The argon content increased to ～4 at. % with increasing irradiation dose. An analysis of the results obtained and their comparison with the data available in the literature led to a qualitative conclusion that the bond angles of the carbon atoms making up the walls of single-walled carbon nanotubes are distorted at sites exposed to Ar⁺ irradiation.     

Quantized bimolecular auger recombination of excitons in single-walled carbon nanotubes

Auger-like exciton-exciton annihilation in isolated single-walled carbon nanotubes (SWNTs) has been studied by femtosecond transient absorption spectroscopy. We observe a quantization of the Auger recombination process and extract dynamics for 2 and 3 electron-hole pair excited states. We further demonstrate that Auger recombination in SWNTs is a two-particle process involving strongly bound excitons and not a three-particle Auger process involving unbound electrons and holes. We thus provide explicit experimental evidence for one-dimensional discrete excitons in SWNTs.     

碳纳米管拉曼光谱的温度效应

碳纳米管拉曼光谱的温度效应黄福敏１余国滔２谭平恒１张树霖１（北京大学物理系１和生命科学院２北京１００８７１）施祖进周锡煌顾镇南（北京大学化学与分子工程学院北京１００８７１）ＴｅｍｐｅｒａｔｕｒｅＥｆｆｅｃｔｏｆｔｈｅＲａｍａｎＳｐｅｃｔｒａｏｆＣａｒ...     

Formation of intercalate-like systems of graphite-ytterbium monolayers on the Ni(111) surface

The ytterbium intercalation under a graphite monolayer formed on the Ni(111) surface has been studied by Auger electron and angle-resolved photoelectron spectroscopy. The features of the electronic structure of the intercalate-like thin-film compound formed in this process are analyzed. It is shown that the energy shift of the π and σ states in the valence band toward higher binding energies (by ～2 and ～1 eV, respectively) can be described in terms of hybridization of the carbon π states in the graphite monolayer with the d states of the underlying metal.     

N23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N₂₃-excitation and the low energy N₂₃VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N₂₃VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.     

The calculation of energy gaps in small single-walled carbon nanotubes within a symmetry-adapted tight-binding model

This paper studies in detail the electronic properties of the semimetallic single-walled carbon nanotubes by applying the symmetry-adapted tight-binding model.It is found that the hybridization of π-σ states caused by the curvature produces an energy gap at the vicinity of the Fermi level.Such effects are obvious for the small zigzag and chiral single-walled carbon nanotubes.The energy gaps decrease as the diameters and the chiral angles of the tubes increase,while the top of the valence band and the bottom of the conduction band of armchair tubes cross at the Fermi level.The numeral results agree well with the experimental results.     

Obtaining density of states information from self-deconvolution of Auger band-type spectra

A systematic revision of the techniques to isolate Auger peaks in experimental spectra obtained with LEED/AES analyzers has been carried out: Background subtraction, deconvolution with the elastic peak and inversion of self-convolution. New techniques have been devised and applied to the Auger spectra of graphite, aluminum, magnesium, magnesium oxide and silicon. Self-deconvolution of these spectra leads to a transition density function that has been compared with the density of states of the valence band near the surface, as given by other competitive techniques (XPS, UPS, XES) and by theory. This comparison shows that in those meterials, Auger electron spectroscopy (AES) is a sensitive probe of the electron environment of surface atoms.     

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.     

The bonding state of carbon segregated to α-iron surfaces and on iron carbide surfaces studied by electron spectroscopy

Segregated carbon on the Fe(100) surface has been studied by means of X-rayand ultraviolet photoelectron (XPS, UPS), Auger electron (AES) and electron energy loss spectroscopy (ELS). For comparison, the surfaces of polycrystalline graphite and of iron carbides stabilized by chromium or manganese additions have been investigated. On the iron surface, carbon exists as a chemisorbed state or graphitic multilayer. The two states exhibit different energy positions in XPS, and are different in energy positions and lineshapes in AES and ELS. During the transition of graphitic carbon to chemisorbed carbon on Fe(100) a novel coverage-dependent Auger feature is reported. The spectra of graphitic carbon on the iron surface always coincide with those of solid graphite. The carbon Auger transitions of chemisorbed carbon and of iron carbides exhibit very similar lineshapes, but the energy positions of both states differ in AES as well as XPS.     

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.     

Characteristic energy gain and loss,double ionization,and ionization loss events in Be and BeO secondary electron spectra

Two satellite peaks have been observed on the high energy side of the Be KVV Auger peak. The lower energy satellite is attributed to coupling of energy from bulk plasmon de-excitations with Auger electrons, and the higher energy event to Auger electrons ejected from Be atoms with doubly ionized K levels. Following oxidation, the ionization loss spectra of BeO were observed to have structure which is interpreted as being related to the density of unfilled electron states above the BeO valence band. In addition, the characteristic loss and the low energy (“true secondary”) spectra of Be and BeO were determined. Peaks in these spectra are discussed in terms of characteristic energies related to excited electron states in the solids.     

Insulating properties of ultrathin KF layers on Cu(1 0 0): Resonant Auger spectroscopy

Solid-state effects in the creation and decay of K 2p core excitations in thin KF films on Cu(1 0 0) surface have been studied in resonant Auger spectra, excited using synchrotron radiation. The spectra of films of various thickness starting from a single monolayer were measured.The photoabsorption spectra reveal crystal field splitting already at film thickness of about 1 monolayer. The Auger decay spectra of the K 2p⁻¹3d core excitations in films of thickness up to 2 monolayers exhibit a band characteristic of the decay of core ionised states, showing that the excited electron delocalises into substrate before the core hole decays. In thicker films the coexistence of the decay of excited states in the bulk of the KF crystalline film and of ionised states at the KF-metal interface is observed, indicating that the charge transfer probability from the upper layers of the film into the metallic substrate is strongly reduced.