XPS Intensity Research Articles

XPS depth profiling of nano-layers by a novel trial-and-error evaluation procedure

In spite of its superior chemical sensitivity, XPS depth profiling is rarely used because of the alteration introduced by the sputter removal process and the resulting inhomogeneous in-depth concentration distribution. Moreover, the application of XPS becomes increasingly challenging in the case of the analysis of thin layers, if the thickness is in the range of 2–3 inelastic mean free paths (IMFP) of the photoelectrons. In this paper we will show that even in these unfavorable cases the XPS depth profiling is applicable. Herein the XPS depth profiling of a model system tungsten-carbide-rich nano-layer of high hardness and corrosion resistance is presented. We will show that the problems arising because of the relatively high IMFP can be corrected by introducing a layer model for the calculation of the observed XPS intensities, while the alteration, e.g. ion mixing, compound formation and similar artefact, introduced by the sputter removal process can be handled by TRIDYN simulation. The method presented here overcomes the limitation of XPS depth profiling.

Wafer-scale development, characterization, and high temperature stabilization of epitaxial Cr2O3 films grown on Ru(0001).

This work outlines conditions suitable for the heteroepitaxial growth of Cr2O3(0001) films (1.5-20nm thick) on a Ru(0001)-terminated substrate. Optimized growth is achieved by sputter deposition of Cr within a 4mTorr Ar/O2 20% ambient at Ru temperatures ranging from 450 to 600 °C. The Cr2O3 film adopts a 30° rotated honeycomb configuration with respect to the underlying Ru(0001) substrate and exhibits a hexagonal lattice parameter consistent with that for bulk Cr2O3(0001). Heating to 700 °C within the same environment during film preparation leads to Ru oxidation. Exposure to temperatures at or above 400 °C in a vacuum, Ar, or Ar/H2 3% leads to chromia film degradation characterized by increased Ru 3d XPS intensity coupled with concomitant Cr 2p and O 1s peak attenuations when compared to data collected from unannealed films. An ill-defined but hexagonally well-ordered RuxCryOz surface structure is noted after heating the film in this manner. Heating within a wet Ar/H2 3% environment preserves the Cr2O3(0001)/Ru(0001) heterolayer structure to temperatures of at least 950 °C. Heating an Ru-Cr2O3-Ru heterostacked film to 950 °C within this environment is shown by cross-sectional scanning/transmission electron microscopy (S/TEM) to provide clear evidence of retained epitaxial bicrystalline oxide interlayer structure, interlayer immiscibility, and epitaxial registry between the top and bottom Ru layers. Subtle effects marked by O enrichment and O 1s and Cr 2p shifts to increased binding energies are noted by XPS in the near-Ru regions of Cr2O3(0001)/Ru(0001) and Ru(0001)/Cr2O3(0001)/Ru(0001) films after annealing to different temperatures in different sets of environmental conditions.

Transformation of Light Alkanes into High-Value Aromatics

This research work is focused on the transformation of light alkane (propane) into high-value aromatics using gallo-alumino-silicate catalysts. Two sets of gallo-alumino-silicates were synthesized for this study. In the first set, the ratio of Ga/(Al+Ga) was modified, while the Si/(Al+Ga) ratio was held constant. In the subsequent set, the Si/(Al+Ga) ratio was adjusted, while maintaining a consistent Ga/(Al+Ga) ratio. This approach aimed to directly assess the impact of each ratio on catalyst performance. The comprehensive characterization of all catalysts was conducted using various instrumental techniques, i.e., BET surface area, XRD, NH3-TPD, 27Al, 71Ga and 29Si MAS NMR, and XPS. A gradual reduction in the percentage of crystallinity and rise in meso-surface area was noticed with a rise in Ga/(Al+Ga) ratio. The total acidity (NH3-TPD) demonstrated a decline as the Si/(Al+Ga) ratio increased, attributed to an overall decline in Al3+ or Ga3+ species. The XPS intensity of the Ga 2p3/2 peak rose in correlation with an elevated ratio of Ga/(Al+Ga), suggesting the formation of extra-framework Ga species. The propane conversion, aromatic yield, and aromatization/cracking ratio exhibited an increase with an increasing Ga/(Al+Ga) ratio, reaching an optimum value of 0.46 before declining. Conversely, an appreciable drop in the conversion of propane and yield of aromatics was detected with the rise in Si/(Al+Ga) ratio, attributing to the decline in acidity. The catalyst having a Ga/(Al+Ga) ration of 0.46 exhibited the highest propane conversion and aromatic yield of 83.0% and 55.0% respectively.

XPS modeling of GaN/GaAs nanostructure grown by the droplet epitaxy technique

The contribution of XPS spectroscopy and its associated modeling to the study of metallic and semiconductor nanostructure design is highlighted in this work. We first give a brief description of the growth kinetics of pure and binary materials on hydrophilic substrates showing the real shape of the nanodroplets to be a spherical cap. This morphology is implemented in the XPS signal modeling of pure and binary nanodroplets. Then, the theoretical calculations will be compared to the XPS intensities in situ carried out on the Ga/GaAs(111)A and GaN/GaAs(111)A nanostructures. This comparison allows the shape of Ga and GaN nanodroplets to be determined, giving their radius, height and coverage. Finally, XPS results are compared to AFM and SEM studies to confirm the XPS modeling. Moreover, the influence of using inelastic mean free paths or effective attenuation lengths in the XPS signal modeling is discussed.

Origin of the complex main and satellite features in Fe 2p XPS of Fe2O3.

Although the origin and assignment of the complex XPS features of the cations in ionic compounds has been the subject of extensive theoretical work, agreement with experimental observations remains insufficient for unambiguous interpretation. This paper presents a rigorous ab initio treatment of the main and satellite features in the Fe 2p XPS of Fe2O3. This has been possible using a unique methodology for the selection of orbitals that are used to form the ionic wavefunctions. This orbital selection makes it possible to treat both the angular momentum coupling of the open shell core and valence electrons as well the shake excitations from the closed shell orbitals associated with the O ligands into the valence open shell orbitals associated with the Fe 3d shell. This allows the character of the ionic states in terms of the occupations of the open shell core and valence orbitals and of the contributions of 2p1/2 and 2p3/2 ionization to the XPS intensities to be determined. Our analysis gives strong evidence that many body effects are essential for a correct description of the ionic states and, in general the states cannot be described by a single configuration over the open shell orbitals. An important consequence is that the Fe 2p XPS intensity in most of the features arises from small contributions from the ionization to many, tens to hundreds, of often unresolved ionic states. While the usual understanding of the lower binding energy main and satellite features as being dominantly from 2p3/2 ionization is confirmed, this is not the case for the higher binding energy features where 2p1/2 and 2p3/2 ionization and shake excitations in the valence space mix strongly. Furthermore, we have been able to show that a very large fraction, 88%, of the total Fe 2p XPS intensity is contained in a relatively small binding energy range of ∼35 eV. This is relevant if one wants to extract the stoichiometry of Fe2O3 from Fe 2p/O 1s intensity ratios. Similar considerations about the importance of many-body effects are likely to be relevant for other ionic compounds as well.

Shake loss intensities in x-ray photoelectron spectroscopy: Theory, experiment, and atomic composition accuracy for MgO and related compounds

The relative intensities of XPS core levels, scaled by their photoionization cross sections, are regularly used to determine sample atomic composition. Cross sections, however, give the intensity to all possible final states for the core ionizations, not just to the main peak. This includes all intrinsic satellite structure (shake states and, for open shell systems, the different ionic multiplets). In practice, for solids, this is usually experimentally impossible to determine accurately because such a satellite structure sits on the inelastically scattered electron background and cannot be easily separated. Therefore, usually, only the intensity of the main peak is used. This limits the ultimate possible accuracy of XPS composition determination. The purpose of the present paper is to examine the contributions that a theoretical analysis of losses of intensity can make to improve quantitation. For an MgO single crystal, we show that the correct stoichiometry of 1:1 can be recovered using the theoretical analysis of the experimental MgO peak ratio intensities. For materials with a sufficient bandgap for the XPS main peaks to be separated from the scattered background, the intensity of main peaks can often be accurately determined. Thus, if one uses theory to calculate that fraction of the total intensity lost from a main peak into all its satellite structure, the intensity of just main peaks could then be used to more accurately determine relative atom % composition. This work tests this approach using a single crystal MgO (50% Mg, 50% O) standard. Ab initio electronic structure theory of representative MgO clusters is used to determine Hartree–Fock wave functions for the ground state and final ionized states corresponding to the main Mg 2p and O1s XPS peaks of the oxide. The sudden approximation, SA, is used to determine the fractional losses from these main peaks to shake satellites, which is found to be greater for O1s than Mg2p. This results in predicted “apparent composition” for stoichiometric MgO of 55.2% Mg, 44.8% O instead of the true 50% Mg, 50% O. Equivalent theory for CaO results in a predicted apparent Ca value of 53.4%. Experimentally, using Mg2s or 2p intensity ratio to O1s, we find values between 52.2% and 56.0% Mg using two crystals and four different instrument electron pass energies. The average value of the measurements is 54.5% Mg when corrected for the presence of an adventitious carbon overlayer and slight surface hydroxide. Though this agreement with theory may be somewhat fortuitous, given the potential experimental errors, which are fully discussed, it is similar to that in our earlier study on LiF. We also present preliminary experimental data on Mg(OH)2 and MgSO4, which show a similar trend of apparently higher than 50% Mg, but we have no theory values. We are not yet able to experimentally test for validation of the difference between apparent composition for MgO (55.2% Mg) and CaO (53.4% Ca), owing to significant carbonate formation at the surface of the single crystal CaO. An important conclusion is that the theoretical determination of shake losses, obtained with ab initio wavefunctions and the SA, is likely to be a useful way to calibrate the accuracy and reliability of compositions obtained from XPS intensities and merits further study.

Adsorption of Carbon Dioxide on Mono-Layer Thick Oxidized Samarium Films on Ni(100).

Studies of adsorption of CO on nanoscopic surfaces are relevant for technological applications in heterogeneous catalysis as well as for sorption of this important greenhouse gas. Presently, adsorption of carbon dioxide on pure and oxidized thin samarium layers near mono-layer thickness on Ni(100) has been investigated by photoelectron spectroscopy and temperature programmed desorption. It is observed that very little CO adsorb on the metallic sample for exposures in the vacuum regime at room temperature. For the oxidized sample, a large enhancement in CO adsorption is observed in the desorption measurements. Indications of carbonate formation on the surface were found by C 1s and O 1s XPS. After annealing of the oxidized samples to 900 K very little CO was found to adsorb. Differences in desorption spectra before and after annealing of the oxidized samples are correlated with changes in XPS intensities, and with changes in sample work function which determines the energy difference between molecular orbitals and substrate Fermi level, and thus the probability of charge transfer between adsorbed molecule and substrate.

Thermal oxidation of Ru(0001) to RuO2(110) studied with ambient pressure x-ray photoelectron spectroscopy

The thermal oxidation of Ru(0001) has been extensively studied in the surface science community to determine the oxidation pathway towards ruthenium dioxide (RuO2(110)), improving the knowledge of Ru(0001) surface chemistry. Using time-lapsed ambient-pressure x-ray photoelectron spectroscopy (APXPS), we investigate the thermal oxidation of single-crystalline Ru(0001) films toward rutile RuO2(110) in situ. APXPS spectra were continuously collected while the Ru(0001) films were exposed to a fixed O2 partial pressure of 10−2 mbar and the sample temperature was increased stepwise from room temperature to 400 °C. We initially observe the removal of adventitious carbon and subsequent formation of a chemisorbed oxygen overlayer at 250 °C. Further annealing to 300 °C leads to an increase in thickness of the oxide layer and a shift in the Ru–O component of the Ru 3d spectra, indicating the presence of a metastable O–Ru–O trilayer structure. A rapid formation of the RuO2 rutile phase with an approximate thickness of at least 2.6 nm is formed about four minutes after stabilizing the temperature at 350 °C and subsequent annealing to 400 °C, signaled by a distinct binding energy shift in both the Ru 3d and O 1s spectra, as well as quantitative analysis of XPS intensities. This observed autocatalytic oxidation process agrees well with previous theoretical models and experimental studies, and the data provide the unambiguous spectral identification of one proposed metastable precursor required for full oxidation to rutile RuO2(110). Further ex situ characterization of the grown oxide with x-ray photoelectron diffraction confirms the presence of three rotated domains of rutile RuO2(110) and reveals their orientation relative to the substrate lattice.

Orbitals Permit the Interpretation of Core-Level Spectroscopies in Terms of Chemistry

We present evidence that the use of orbital based analyses provides an important fundamental basis for interpreting and understanding the significance of features in core-level spectroscopies, in particular, X-ray photoelectron spectroscopy, XPS. A recent paper by Truhlar and colleagues cautions about the limitations of orbital based interpretations of photoelectron spectroscopy for the accurate modeling of the electronic structure of the ions. We provide an alternate view that does use orbitals to derive detailed information from XPS. We discuss the appropriate meaning of Koopmans’ theorem as a way to separate the chemically interesting initial state contributions to XPS binding energies, BEs, from the less interesting properties of highly excited ionic states, rather than as a way to estimate values of BEs and BE shifts. The use of orbitals and orbital occupations to take into account the spin and orbital angular momentum coupling that leads to the observed XPS multiplets is shown to be important to understand the number and energy spread of XPS features. The use of a standard approximation to determine the XPS intensities, essential for the theoretical prediction of XPS spectra, is shown to depend on the concept of ionization of electrons from specific orbitals. Finally, the use of orbital properties to identify and include the main many electron correlation of the ionic states in XPS is described. The importance of these considerations is shown by describing examples where they are used in the theoretical determination and the theoretical analysis of the XPS of representative systems. The orbital description of XPS is shown schematically below for the Ne atom. An orbital description can be applied to and is appropriate to understand the photoemission of complex systems.

Evaluation of procedures for overlayer thickness determination from XPS intensities

The attenuation length approach for determination of an overlayer thickness is analyzed in the case of overlayer/substrate systems in which both materials distinctly differ; in particular, they exhibit considerably different electron transport properties. Typical measurement conditions for overlayer-thickness determination were simulated for numerous systems using an advanced Monte Carlo strategy. Three experimental procedures were considered: (i) measurement of only one signal intensity from an overlayer; (ii) only one signal intensity from a substrate; and (iii) measurement of two signal intensities from an overlayer and a substrate. It turned out that differences in electron transport properties of the overlayer and substrate materials (elastic scattering cross sections, the inelastic mean free path) can generally be ignored. One can safely assume that the overlayer and substrate materials are identical, and the effective attenuation length (EAL) values needed in calculations should correspond to the overlayer material only. Furthermore, one can use for that purpose the EALs estimated for the overlayer material from available predictive formulas. However, care must be taken to estimate correctly the needed albedo values. In that case, the overlayer thicknesses assumed in Monte Carlo simulations are reproduced from calculated signal intensities within 2 – 3%.

Minimization of SiO2/4H-SiC (0001) Interface State Density by Low-Temperature Post-Oxidation-Annealing in Wet Ambient after Nitrogen Passivation

4H-SiC has attractive properties for power devices of high voltage applications, however, the performance of MOSFETs is often severely limited by the poor SiO2/SiC interface quality. One of the common techniques to passivate the interface defects is the nitrogen introduction by NO annealing [1] while introduction of wide variety of impurity elements have also been studied [2]. Recently we have demonstrated the benefit of post-oxidation annealing in wet ambient at low-temperature as an alternative way to improve the SiO2/4H-SiC (0001) interface quality, which effectively works with only < 1nm additional growth of oxide at the interface [3]. In this study we investigated the validity of the combined passivation process of the NO annealing and the low-temperature H2O-annealing after the dry oxidation of 4H-SiC, to take the advantages of both the nitrogen passivation and the good quality interface formation by H2O-oxidation [4]. After HF cleaning of 4º off-axis 4H-SiC (0001) wafers covered with 5 μm-thick n-type epitaxial layers (ND ~ 1×1016 cm-3), ~30 nm-thick SiO2 layers were thermally-grown in dry oxygen at 1300ºC. As the post-oxidation annealing techniques, the annealing in NO:N2 =1:2 ambient at 1150ºC for 2hrs (NO-POA) and the additional annealing in H2O:O2=9:1 ambient at 800ºC (LT-H2O-POA) for various duration time, were sequentially conducted. Finally Au electrode was deposited to form a MOS capacitor. For the NO-POA process of SiC after thermal growth of SiO2, it has been clarified that nitrogen atoms are introduced mostly in the first few monolayers of SiC, and remains even after removal of SiO2 by chemical etching completely [5]. We evaluated the effects of our NO-POA and LT-H2O-POA on the amount of nitrogen introduced at the interface, from the relative change of core level XPS intensity ratio of N1s to the substrate component of Si2p, after the complete removal of SiO2 layer. It was found that the long-time LT-H2O-POA results in a significant reduction of nitrogen density at the interface after the growth of several MLs of SiO2, due to the consumption of nitrogen-introduced SiC surface layers for the oxidation by H2O. However, it would be noteworthy that most of the nitrogen still remains at the interface after a short-time LT-H2O-POA to cause the growth of limited amount of SiO2 around 1 ML. Next we evaluated the interface state density (Dit) of SiO2/SiC MOS capacitors fabricated with different duration time of LT-H2O-POA after NO-POA, by using conductance method. We found Dit was significantly reduced by short-time LT-H2O-POA, and minimized when only 1 ML of SiO2 was grown during the H2O-POA. The Dit value at the energy level at 0.2 eV below the conduction band edge of SiC was reduced below 5×1010 cm-2eV-1, which is less than half of the value observed just after the NO-POA (without H2O-POA). This would be the minimum Dit ever reported for 4H-SiC (0001) MOS capacitor at this energy level. On the other hand, LT-H2O-POA with excess duration time resulted in an increase of Dit. After the long-time LT-H2O-POA to cause ~1 nm SiO2 growth, the Dit value became even larger than the one just after the NO-POA. This deterioration would be caused by the partial lost of passivating nitrogen at the interface by the growth of new oxide layers, as indicated by the XPS analysis. One of the methods to evaluate the density of such slow traps in SiC MOS capacitors is the characterization of the hysteresis of CV curve observed for a voltage sweep just after the ejection of trapped charges by UV irradiation [6]. We observed that the slow trap density also significantly decreased by short-time LT-H2O-POA when the additional growth of SiO2 was limited to ~1 ML, but gradually increased by a long-time one, which is a quite similar behavior as Dit change by H2O-POA. These results show that the roles of nitrogen introduction and oxidation by H2O on SiO2/SiC interface defect annihilation are different from each other, and the interface with minimized Dit and less near-interface oxide traps is achievable by the sequential combination of those two POA processes, when the amount of additional oxidation by H2O is limited to ~ 1ML of SiO2.

A New Mechanism For XPS Line Broadening: The 2p-XPS of Ti(IV)

There is almost a factor of 2 increase in the full width at half maximum (FWHM) of the main Ti 2p1/2 XPS peak compared to the 2p3/2 in the closed-shell Ti(IV) compounds TiO2 and strontium titanate, STO. Although the spectra of anatase and rutile forms of TiO2 differ slightly from STO, the 2p1/2 broadening over 2p3/2 is very similar. For STO, we show that ascribing this differential broadening to a short 2p1/2 lifetime is unphysical. For STO, rigorous and fully relativistic electronic structure calculations have been carried out for both the initial state and the 2p core-hole states; these calculations include many-body effects as well as the one-body effects of spin–orbit and ligand field splittings. The agreement of the theoretical and measured XPS data for the main 2p1/2 and 2p3/2 peaks indicate that the necessary one- and many-body effects have been included. They show that the broadening is due to the presence of XPS intensity distributed over many unresolved final states for a 2p1/2 core-hole, wherea...

Stability of Organic Cations in Solution-Processed CH3NH3PbI3 Perovskites: Formation of Modified Surface Layers

We report on a combined X-ray and UV photoemission spectroscopy study (XPS and UPS) of organic–inorganic perovskites prepared from a solution of lead chloride (PbCl2) and methylammonium iodide (CH3NH3I). The XPS intensities are consistent with a pure iodide perovskite (CH3NH3PbI3), with no detectable chloride left. However, we found that the elimination of chloride results in residual methylamine molecules (CH3NH2) trapped within the perovskite crystal lattice. Furthermore, we show that vacuum annealing or sputtering induces the formation of a thin PbI2 layer at the crystal surface which acts as a surface barrier blocking electron transfer from the underlying perovskite film.

Understanding the dispersion of Ag on high surface area TiO2 supports using XPS intensity ratios

Silver–titania (Ag/TiO2) adsorbents, in the range of 4wt% Ag, display high selectivity toward sulfur heterocyclic compounds from complex fuel streams containing other aromatics. An experimental investigation of Ag on TiO2 has been undertaken to understand the state of dispersion and growth of Ag. XPS is one of the more promising characterization tools for the state of dispersion. Silver loading from 1wt% to 20wt% on 150m2/g titania was investigated. Ag/Ti intensity ratios increased linearly with Ag content up to 4wt% and increased less significantly thereafter from 8wt% to 20wt% indicating nucleation and growth of Ag crystallites. Inelastic mean free path (IMFP) calculations were used to estimate Ag crystallite size based on the attenuation of the Ag signal, realizing in this regime there is insufficient Ag to attenuate background titania. At 4, 8, 12, and 20wt% the estimated average crystallite sizes were 0.35, 0.71, 0.84, and 1.11nm respectively. Ag loadings up to 4wt% were present in the form of Ag+1 adatoms presumably occupying TiO2 surface defects. Saturation of surface TiO2 defects is in good agreement with quantitative sulfur heterocycle adsorption.

Understanding the light soaking effect of ZnMgO buffer in CIGS solar cells.

This study investigated the mechanism underlying the light soaking effect of a ZnMgO buffer in Cu(In,Ga)Se2 (CIGS) solar cells, where the cell efficiency increased with an increase of light soaking time. The ZnMgO buffer layer was deposited by an atomic layer deposition method. With light soaking, the cell efficiency of ZnMgO/CIGS cells increased mainly by the increase of the fill factor and partly by the increase of the open-circuit voltage. With light soaking, the electron carrier concentration of the ZnMgO layer increased and the XPS intensity of the hydroxyl bond in the ZnMgO layer decreased. Based on the above results and the comparison of other buffers in literature, we assumed that the hydrogen atoms broken away from the hydroxyl bond by photon irradiation occupied the interstitial sites of the ZnMgO layer as a donor atom and also passivated the defects at the ZnMgO/CIGS interface. The increase of the fill factor and open circuit voltage was explained based on H doping in the ZnMgO layer and H passivation at the ZnO/CIGS interface.

Interpretation of nanoparticle X-ray photoelectron intensities

X-ray photoelectron (XPS) intensities have been simulated for spherical core-shell nanoparticles (NPs) in different geometrical arrangements in order to investigate the validity of commonly made assumptions for the interpretation of XPS NP intensities. The single-sphere approximation is valid for a powder sample when all spatial coordinates of the NP positions are uncorrelated. Correlations along either the depth coordinate or the lateral coordinates lead to features in the angular distribution that provide information on these correlations. The XPS intensity is proportional to the surface-to-volume ratio of nanoparticles but only for NP sizes exceeding the inelastic mean free path of the photoelectrons.

XPS study of the surface properties and Ni particle size determination of Ni‐supported catalysts

The surface properties of Ni/MgAl2O4 catalysts doped with Ce or Pr were analyzed by XPS after treatment in an inert and reductive atmosphere at 400 °C. The Ce‐promoted solids presented the Ce3+/Ce4+ couple on the surface even after treatment in a reductive atmosphere, H2(5%)/Ar. The promotion effect of Ce on these solids could be associated with their participation on the carbon deposition‐removal mechanism. Pr‐doped catalysts showed a very high concentration of Pr3+ under a reductive atmosphere and the redox behavior associated with the carbon removal could be partially inhibited or become slower. The size of the Ni0 particles after both an inert and a reductive atmosphere was estimated by XPS intensity ratio using the model proposed by Davis. The results obtained from the Davis model showed that an important increase occurred in Ni particle size after treatment in H2(5%)/Ar for the Pr‐promoted solids. The metal sintering under reductive atmosphere could be the reason for the higher loss of activity of the Pr‐doped solids under reforming conditions. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Growth and electronic structure studies of semiconducting thin films of fluorine-monosubstituted fused-thiophene derivative

Formation of high quality interface between metal and organic semiconducting film is important in realizing high-performance organic electronics devices. We have investigated the room-temperature growth of organic semiconducting thin films of monofluorine-substituted 2-phenylbenzo[d,d′]thieno[3,2-b;4,5-b′]-dithiophene (m-FP-BTDT) on Au(111), and Au(111) modified by benzenethiolate (Au-BT) by means of synchrotron-based electron spectroscopies. XPS intensity analysis shows that on Au(111), m-FP-BTDT film grows according to Stranski–Krastanov (SK) mode in which the first monolayer (ML) is completed before the 3D growth sets in. In comparison, the initial growth of m-FP-BTDT on Au-BT proceeds via a pseudo layer-by-layer growth mechanism in which the growth of a new layer starts after the underneath layers are almost finished. Orientations of m-FP-BTDT molecules for the thick films on two substrates remain different, despite the large thickness. NEXAFS data show that m-FP-BTDT molecules on Au-BT adopt a more erected configuration, resulting in a better cofacial π-stacking. Work function for the thick m-FP-BTDT film on Au-BT is determined with UPS as 4.62eV and the hole injection barrier as 0.95eV.

Calculation of layer thickness on nanotube surfaces from XPS intensity data

Surface modification of nanomaterials is the subject of numerous recent investigations. Quantitative characterisation of such modifications is of great theoretical and practical interest. As a first step, our attention is focussed on carbon nanotubes because of their unique morphology and physicochemical properties, having many potential advantages in nanotechnology, electronics, optics and other fields of material science. Surface modification or coating of the nanotubes is essential in several applications, using them as drug carriers, reinforcing component in polymeric matrices or hi‐tech structural ceramics to create reactive surface groups or protection for efficient processing. Applying the simple planar approach to cylindrical‐shaped samples, as known, leading to overestimated layer thickness values because the effective thickness of the layers (seen from the direction of analyser) varies from point to point along the convex surface of the tubes of essentially cylindrical geometry. In case of nanotubes, and any type of nano‐objects, however, a further significant difference is that they have no ‘bulk‐like’ core material because they are usually thin enough to be transparent by the photoelectrons evolving in the XPS measurements. In a step forward for quantitative characterisation, in this article, a special model was developed to calculate the thickness of the modified layer on nanotubes, taking into account that several rows of the tubes are randomly piled on each other. The applicability of the model was tested for quantitative characterisation on multiwall carbon nanotubes (MWCNT), which were treated in d.c.‐biased RF N2 plasma for covalent attachment of nitrogen to their outer shells. This model will be implemented, and thus the calculations would be conveniently performed in the latest versions (from 7.0) of the XPS MultiQuant program. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Interfacial nature of Ag nanoparticles supported on TiO2 photocatalysts

Silver nanoparticles supported on anatase TiO2 nanoparticles have been prepared by deposition–precipitation, and characterized by X-ray photoelectron spectroscopy, including scanning electron microscopy, X-ray diffraction crystallography, Raman, and UV–visible absorption spectroscopy. The Ag 3d peak and the X-ray diffraction patterns show characteristics of purely metallic Ag, with no indication of Ag oxide species. Depth-profiling X-ray photoelectron spectroscopy with Ar+ ion beam sputtering show a significant change in Ti 2p, and an asymmetric broadening of Ag 3d to a higher binding energy side. A decrease in major Ti 2p 3/2 at 459.2 eV and a significant increase in lower binding energy peak are due to change in oxidation state of Ti from +4 to +3/+2. A broadening of Ag 3d peak with sputtering time is tentatively assigned to a final state quantum size effect. Upon annealing the deposition–precipitation sample, no significant change in Ag 3d peak is observed, while Ti 2p and O 1s XPS intensities are reduced, plausibly due to change in analyzed surface area for TiO2. The photocatalytic activity for the photodegradation of methyl orange is dramatically reduced upon high Ag-loading, compared to bare TiO2. The X-ray photoelectron spectroscopy of Ag on TiO2 prepared by an electrochemical deposition reveals that Ag is also metallic, with no evidence of an oxide form. Upon annealing the electrodeposited sample, the Ag 3d peak shifts by +0.3 eV, while the Ti 2p and O 1s show no critical change in intensity and peak position.