C1s Core Level Research Articles

Effects of X-ray radiation on the surface chemical composition of plasma deposited thin fluorocarbon films

Different thin fluorocarbon (FC) films were deposited on Si(111) using plasma polymerisation and then exposed to X-ray radiation. Changes in the chemical composition of the deposited fluorocarbon films as a function of irradiation time were investigated in situ using X-ray photoelectron spectroscopy. The evaluation of the C1s and F1s core level induced emission as a function of exposure to X-ray radiation (Mg Kα, hν = 1253.6 eV) reveals changes in the surface chemical composition of the FC polymer structure. The presented results indicate a high defluorination under X-ray irradiation. Additionally, binding energy shifts of the F1s and C1s peaks during the exposure associated with surface charging effects were observed. With ongoing exposure the surface charging decreases continuously and the FC surfaces become more conductive due to changes in the polymer structure. Different models have been used to describe the decomposition kinetics and surface composition.

A photoelectron spectroscopy study of ion-irradiation induced defects in single-wall carbon nanotubes

Valence band and core level photoemission spectroscopies were used to study the changes brought about by irradiation of a single-wall carbon nanotube (SWCNT) film by 3 keV Ar + ions at room temperature. At low ion doses (low defect density) an increase in spectral intensity near the Fermi level ( E F) is observed, associated with formation of localized defect-related states. These states are acceptor-like as evidenced by a shift to lower binding energy for both valence band features and the C1s core level. For large ion doses (high defect density) the spectral intensity near E F decreases, valence band features associated with delocalized π bonding disappear, and a core level component associated with sp 3 bound carbon appears. This behaviour is attributed to amorphisation of the SWCNT films and occurs at ion doses consistent with those theoretically predicted.

Effect of vacuum annealing on field emission from heavily phosphorus doped homoepitaxial (111) diamond

Vacuum anneal treatments effects on field emission properties of phosphorus doped diamond are discussed. The C1s core level of diamond is characterized by X-ray photoelectron spectroscopy (XPS). With increasing annealing temperature, firstly oxygen desorption takes place from surface, which induce surface reconstruction followed by graphitization of surface. Field emission properties, therefore, strongly depend on vacuum anneal temperature. The threshold voltage of diamond annealed at 900 °C is the lowest. Fowler–Nordheim plots indicate that diamond annealed at 900 °C has the lowest barrier height, which is in good agreement with electron affinities as measured on carbon reconstructed surface. Further increased annealing temperature induces surface graphitization, which causes a threshold voltage increase of field emission.

Tetrahedral and Trigonal Carbon Atom Hybridization in Thin Amorphous Carbon Films Synthesized by Radio-Frequency Sputtering

Ultrathin amorphous carbon (a-C) films were synthesized on Si(100) substrates by low-pressure, radio-frequency sputtering under different plasma discharge conditions. X-ray photoelectron spectroscopy (XPS) revealed different contents of C, Ar, Si, O, and N in the a-C films. The percentages of tetrahedral (sp3) and trigonal (sp2) carbon hybridizations in the films were determined from an analysis of the C1s core level spectra obtained by XPS narrow scanning. It is shown that the concentrations of surface chemisorbed O and N atoms from the ambient depend on the real surface area of the film (roughness effect). The binding energy shifts of the sp2 and sp3 hybridizations (observed after the decomposition of the C1s spectra) due to the Ar+ ion bombardment are interpreted in terms of the residual compressive stress in the deposited a-C films. The mechanisms of sp3 carbon hybridization are discussed in the context of XPS results. For negligible Ar+ ion bombardment (zero substrate bias), sp3 carbon hybridization ...

Time-Resolved Synchrotron XPS Monitoring of Irradiation-Induced Nitrobenzene Reduction for Chemical Lithography

The irradiation-induced reduction of electrochemically grafted nitrobenzene films on Si(111) was monitored by high-resolution photoelectron spectroscopy. The experiments were performed using synchrotron soft X-ray irradiation at the BESSY II synchrotron facility. The evolution of different chemical species was monitored as a function of time. Careful fitting of the Si2p, C1s, N1s, and O1s core level spectra allowed us to follow this process in detail and to determine the constants of growth and decay of the specific components. The chemical changes were caused by the X-ray irradiation-induced secondary electron current through the aryl layer. A minor fraction (approximately 25%) of the initial nitro groups was split off and desorbed. The bulk of the NO2 groups was reduced to species in an amino-like chemical environment. A desorption of carbon fragments was not observed, and benzene ring specific shakeup satellites indicated that the aromatic ring structure remained intact. Irradiation-induced line-shape changes suggest a polymerization via -NH- bridges, which were formed after the irradiation-induced N-O bond splitting. A significant part of the released oxygen appeared to contribute to an oxidation of the silicon substrate at the Si(111)/benzene interface. The irradiation-induced aryl layer modification can be exploited for chemical lithography (i.e., a lateral structuring of functionalized silicon surfaces).

The electronic properties of potassium doped copper-phthalocyanine studied by electron energy-loss spectroscopy

The authors have studied the electronic structure of potassium doped copper-phthalocyanine using electron energy-loss spectroscopy. The evolution of the loss function indicates the formation of distinct KxCuPc phases. Taking into account the C1s and K2p core level excitations and recent results by Giovanelli et al. [J. Chem. Phys. 126, 044709 (2007)], they conclude that these are K2CuPc and K4CuPc. They discuss the changes in the electronic excitations upon doping on the basis of the molecular electronic levels and the presence of electronic correlations.

Growth and field electron emission properties of nanostructured white carbon films

White carbon films were synthesized by microwave plasma chemical vapor deposition. Irregular flakiness microstructure consisting of nanocrystalline grains of the as-grown film was observed by scanning electron microscope. The films have a metallic gray to white color, with reflectivity of 12.56%–21.58% in the visible region. The diffraction line at 2θ=21.6° (d=0.4107nm), shown in x-ray diffraction spectrum of the film, corresponds to the (110) facet of β modifications of white carbon. The peak position at 283.46eV in the x-ray photoelectron spectrum represents binding energy of C1s core level of sp1 hybridization of carbon. Field electron emission characteristics of the film were tested. A turn-on field of 3.5V∕μm was obtained.

Quantitative determination of oxidative defects on single walled carbon nanotubes

AbstractA large variety of functional groups (i.e. carboxyl, carbonyl, aldehyde, alcohol) – representing the different oxidation states of carbon – can be produced by oxidative treatment of single walled carbon nanotubes (SWCNTs), depending on duration and temperature of the process. For subsequent functionalization steps it is essential to know the quantitative amount of the functional groups being converted in a chemical reaction. X‐Ray Photoelectron Spectroscopy (XPS) proved to be a useful tool for such determination. Here, the asymmetry of the C1s core level in SWCNTs and the unknown peak positions of the functional groups prevented a detailed analysis of the produced oxidized carbon species. In this work we present an effective method to overcome this problem using XPS correlated with ab‐initio calculations of the Mulliken charges in functionalized SWCNTs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High resolution photoemission spectroscopy: Evidence for strong chemical interaction between Mg and 3,4,9,10-perylene-tetracarboxylic dianhydride

The interface formation between Mg and 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was investigated by high resolution soft x-ray photoemission spectroscopy. The interface chemistry was obtained after fitting the core level spectra as a function of Mg thickness. At the initial stage of deposition, a strong chemical interaction between Mg and the single bonded oxygen atoms of PTCDA is observed leading to the formation of MgO and a modified organic molecule. Based on the experimental evidence, the molecular structure of the modified molecule is proposed. Moreover, the changes observed in the measured C1s core level spectra are supported by density functional theory calculations.

Bonding of metal-free phthalocyanine to TiO 2(1 1 0) single crystal

The metal-free phthalocyanine interface formation on rutile TiO 2(1 1 0) is investigated using scanning tunneling microscopy and photoelectron spectroscopy. The molecules are adsorbed flat on the surface, centered on the substrate oxygen rows. High-resolution core-level C1s spectroscopy indicates a strong difference between the second layer and the first monolayer bonding to the surface. The C1s core-level from the second layer has a bulk-like line shape whereas the first layer shows a strongly modified line profile. Upon thermal treatment, changes in the N1s core level line profile points to dehydrogenation of the center of the molecule.

Influence of plasma parameters on the chemical composition of steady-state fluorocarbon films deposited on carbon-doped low-k dielectric layers during etching

This study investigates the fluorocarbon-based plasma etching (FBPE) of low dielectric constant (ULK) carbon-doped oxide (CDO) films, which have a dielectric constant (k) value of 2.4. The effects of different ion density and ion energy power settings on the chemical composition of the fluorocarbon layer deposited during the etch process were investigated. X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical composition of the post-etched low-k CDO films while spectroscopic ellipsometry (SE) was used to determine the overall film thickness. XPS spectra of the C1s core levels reveal that the relative concentration of CFx species in the fluorocarbon films reduced as ion density source power and ion energy power levels were increased, and this can be correlated with a higher etch rate and thinner fluorocarbon layer. Plasma conditions which led to the deposition of a thick fluorocarbon film significantly inhibited the etch rate. This work demonstrates that the chemical composition and the thickness of the fluorocarbon film can be controlled by the plasma power parameters, and this has implications for the etching of ULK CDO layers.

Probabilistic analysis of tetrahedral carbon hybridization in amorphous carbon films

An energetic particle collision analysis of the effect of Ar+ ion bombardment on tetrahedral carbon hybridization (sp3) in amorphous carbon (a-C) thin films is presented for nonmagnetron radio-frequency sputtering. The analysis is based on sequential surface events involving the collision of Ar+ ions with carbon atoms on the surface of the growing film and subsequent collision cascades between excited carbon atoms and other surface carbon atoms, which promote the formation of sp3 carbon bonding. The model is validated by transmission electron and electron energy loss spectroscopy results that confirm the existence of a two-layer film structure consisting of an ultrathin interface layer and a continuous a-C film. Analytical results for the sp3 carbon content are shown to be in good agreement with similar experimental results obtained from the analysis of the C1s core level spectra of the sputtered a-C films.

Raman study of multiwalled carbon nanotubes functionalized with oxygen groups

We present changes in the first and second order Raman spectra of multiwalled carbon nanotubes (MWNTs) functionalized with oxygenated groups. The oxygen groups were introduced onto the nanotube surface through two strong acid purification routes: (1) reflux in concentrated (70%) HNO3 acid for 4h at 80°C and (2) ultrasonification in 3 HNO3 (70%):1H2SO4 (98%) for 8.5h. Raman spectroscopy, using two laser excitation wavelengths (514.5 and 632.8nm), x-ray photoelectron spectroscopy, and thermal gravimetric analysis were employed to study the evolution of the products. All the techniques revealed a higher degree of functionalization for scheme 2 compared to scheme 1. Charge transfer phenomena were manifested by a shift of the C1s core level towards higher binding energies. We found that the intensity of both the D and G energy Raman modes if normalized to the second order mode D* mode follows similar trends upon acid treatments. We interpret this result together with the observed dispersion of G mode as an indication that the G mode in carbon nanotubes is defect induced in a double resonant process. Both acid schemes cause an upshift of D and G Raman modes, due to intercalation of acid molecules, exerting pressure on the sp2 structure and an electron transfer from the π states in MWNTs to the oxygen atoms.

Electronic properties of the Sm∕4H-SiC surface alloy

The formation of a samarium on silicon carbide (Sm∕SiC) alloy after deposition of 2–3 monolayers of Sm in ultrahigh vacuum on clean reconstructed carbon(000-1)- and silicon(0001)-terminated SiC surfaces is studied by x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and low-energy electron diffraction (LEED). The measured work function together with core-level spectroscopy is used to differentiate the formation of samarium silicide carbide (Sm–Si–C) surface alloys on both polar faces of 4H-SiC. Both naturally n-type-doped bulk Si-face and low-doped epilayer Si-face SiC were studied. A (1×1) LEED pattern is obtained on the C-face Sm–Si–C alloy and on the Si-face epilayer Sm–Si–C alloy. Flatband voltages are estimated as a function of annealing, from the shift in the C1s and Si2p bulk core-level positions. The valency of Sm is estimated during the formation of the Sm–Si–C surface alloy. The valence of the Sm overlayer after deposition at room temperature is estimated to be approximately 2.75, while upon annealing, the Sm∕SiC surface alloys on the bulk crystal become predominately 3+ valent. In the case of the Sm∕SiC surface alloy on the Si-face epilayer, the (1×1) surface alloy is of mixed valency (2.84). In all the cases studied, the Sm 3d peak undergoes a major shift of 1.8eV accompanied with a change in the Sm 3d multiplet spectrum.

Selective effect of ion/surface interaction in low frequency PACVD of SIC:H films: Part B. Microstructural study

A capacitively coupled low frequency PACVD device ( ν = 50 kHz, DC bias: | V dc| = 0 to 400 V) is used to deposit hard amorphous silicon carbide films (a-SiC:H), from the decomposition of precursor tetramethylsilane (TMS) diluted in argon. Such a deposition process induces an ion bombardment of the growing films: energetic ions can sputter atoms from the near surface of the coatings, leading to strong alterations in the microstructure of the deposited material. In order to draw the particular influence of ion bombardment on the coatings, partly compensated by growth phenomena, some deposited films are post-treated in pure argon plasma, then submitted to a crossed analysis by RBS, FTIR and XPS techniques. These results are also compared to previous gas phase considerations [A. Soum-Glaude, L. Thomas, E. Tomasella, J.M. Badie, R. Berjoan, Surf. and Coat. Technol. 200 (1–4), (2005) 855]: Optical Emission Spectroscopy (OES) measurements near films surface, and SRIM© 2003 sputtering simulation. The confrontation of gas phase study, simulation and material analysis highlights general trends. First, silicon can be selectively sputtered in regard to carbon in the DC bias domain [150–250 V], as shown by OES observations on Si + and H emitting species, Si / C ratio given by RBS, Si–C and Si–H absorption bands on FTIR spectra, and Si sputtering yield calculation with SRIM 2003. More precisely, in this DC bias domain, XPS measurements on the C1s core level peak, SRIM sputtering yields, and the observation of C–H and Si–C absorption bands with FTIR technique, show that sp 3 carbon environments (C–C + C–H) can be favored in spite of the Si–C and C–Csp 2 ones. Beyond the [150–250 V] domain, this phenomenon is no more noticeable, meaning sputtering is no more selective at higher DC bias. This selectivity can be used to improve hardness of the coatings that can reach 30 GPa.

The electronic properties of carbon nanotubes studied by high resolution photoemission spectroscopy

High resolution photoemission spectroscopy was used to follow the modification of the electronic properties of single wall carbon nanotubes determined by the interaction with adsorbed electron donor and acceptor species. Valence band, C1s and adsorbate core level spectra were monitored to reveal the adsorption and to follow the Fermi level shift induced by the charge exchange between C lattice and dopants. In the presence of nitrogen dioxide, a considerable charge withdrawal from C atoms takes place, which is revealed by the binding energy shift of the C1s peak. On the contrary, adsorbed H 2O does not appear to interact with nanotubes. For Li doping, the occurrence of an efficient electron injection into the tube structure is demonstrated by the evidence of Li ionisation, by the enhancement of the density of states near the Fermi level and by the partial conduction band filling, which originates a rigid shift of all C spectral structures.

Interface reactivity of Pr and SiO 2 at 4H-SiC(0 0 0 1)

We use photoelectron spectroscopy to characterize the reactivity of Pr evaporated on SiO 2 covered 4H-SiC substrates. We analyze the Si2p, C1s, and O1s core levels and find that Pr reacts at RT with the SiO 2 layer to form Pr-silicate. In addition also Pr-silizide is formed as well as Pr-carbides. While the latter are formed as reaction intermediates after Pr evaporation, the Pr-silicate layer is formed upon annealing to 300 °C and is stable up to 800 °C.

Characterization of Si Nanocrystals Embedded in SiO<sub>2</sub> with X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) has been used to characterize the Si nanocrystals (nc-Si) incorporated in SiO2 by ion implantation and subsequent thermal annealing. XPS results suggest that the as-implanted films contain a composition of a few suboxide SiOx (x<2). The dependence of XPS spectra on annealing temperature show that these suboxides decompose into SiO2 and Si nanocrystals. Due to the implanted Si+ depth profile, thermal annealing will lead to the formation of nc-Si with different sizes corresponding to the depth. Si0 peak shifts arising from the size effect of nc-Si can be clearly observed. Photo-induced charge effect from the nc-Si has been studied in this work. The potential wells induced by nc-Si in the SiO2 band gap substantially enhance the charging effect, which causes a significant shift in Si0 and C1s core levels.

Characterization of trypsin immobilized on the functionable alkylthiolate self-assembled monolayers: A preliminary application for trypsin digestion chip on protein identification using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Self-assembled monolayers (SAMs) on coinage metal provide versatile modeling systems for studies of interfacial electron transfer, biological interactions, molecular recognition and other interfacial phenomena. Recently the bonding of enzyme to SAMs of alkanethiols onto Au electrode surfaces was exploited to produce a bio-sensing system. In this work, the attachment of trypsin to a SAMs surface of 11-mercaptoundecanoic acid was achieved using water soluble N-ethyl-N '-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide as coupling agent. The thickness of SAMs was determined by optical ellipsometer; contact angles of the modified Au surfaces were measured in air using a goniometer. The Second Harmony Generation data displays the last few percents of the alkylthiol molecules adsorbed and produced the complete monolayer by inducing the transition from a high number of gauche defects to an all-trans conformation. Using X-ray Photoelectron Spectroscopy (XPS) and Fourier-Transformed Infrared Reflection-Absorption and Attenuated Total Reflection Spectroscopes (FTIR-RAS and ATR), we examined the chemical structures of samples with different treatments. By matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), we demonstrated the digestion of bovine serum albumin (BSA) on the trypsin-immobilized SAMs surface. Experimental results have revealed that the XPS C1s core levels at 286.3 and 286.5 eV (Amine bond), 288.1 eV (Amide bond) and 289.3 eV (Carboxylic acid) illustrate the immobilization of trypsin. These data were also in good agreement with FTIR-ATR spectra for the peaks valued at 1659.4 cm(-1) (Amide I) and 1546.6 cm(-1) (Amide II). Using MALDI-TOF MS observations, analytical results have demonstrated the BSA digestion of the immobilized trypsin on the functionalized SAMs surface. For such surfaces, BSA was digested on the trypsin-immobilized SAMs surface, which shows the enzyme digestion ability of the immobilized trypsin. The terminal groups of the SAMs structure can be further functionalized with biomolecules or antibodies to develop surface-base diagnostics, biosensors, or biomaterials.

Hydrogen-induced metallization of a preoxidized 3C-SiC(100)3×2 surface

We investigate atomic hydrogen interaction with a preoxidized Si-rich 3C-SiC(100)3×2 surface by synchrotron radiation-based valence band, and Si2p and C1s core level photoemission spectroscopies. Atomic hydrogen exposure results in (i) Fermi level built-up in the valence band, (ii) band bending, and (iii) the three Si2p surface components shifting to lower binding energies. These features indicate H-induced surface metallization. This finding opens perspectives in the metallization at the subnanometric scale of passivated semiconductor surfaces.