Nanostructures on La-doped SrTiO3 surfaces

SrTiO(3)(100) single crystals with high donor dopant concentrations (5 at% La) were annealed at 1000 degrees C for up to 150 h in ultrahigh vacuum (UHV). By applying scanning tunneling microscopy (STM) nanostructures are observed on top of the surface with typical diameters of 20 nm and typical heights of 8 nm. To characterize their electronic structure and chemical composition, the surface was analyzed by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling spectroscopy (STS), and depth profiling Auger electron spectroscopy (AES). Investigations of the stoichiometry suggest that the secondary phases consist of LaTiO(3). We present a defect chemistry model which attempts to explain the observed effects.     

Interaction of O2, CO and CO2 with Co films

Metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS(HeI)) and x‐ray photoelectron spectroscopy (XPS) were applied to study the interaction of O₂, CO and CO₂ with Co films at room temperature. The films were produced on Si(100) surfaces under the in situ control of MIES, UPS and scanning tunnelling microscopy (STM). For O₂, dissociative adsorption takes place initially and then incorporation of oxygen starts at exposures of ～5 L. Comparison of the MIES and UPS spectra with those published for CoO shows that near‐stoichiometric CoO films can be obtained by co‐deposition of Co and O₂. The CO is adsorbed molecularly up to a maximum coverage of ～0.6 monolayer, with the C‐end pointing towards the surface. The CO₂ adsorption is dissociative, resulting in the formation of Co–CO bonds at the surface. The resulting oxygen atoms are mostly incorporated into the Co layer. For all studied molecules the interaction with Co is similar to that with Ni. Copyright © 2005 John Wiley & Sons, Ltd.     

XPS Depth Profile Analysis of a Thin Non-Conducting Titanate Superlattice

 New BaTiO₃-SrTiO₃ (BTO-STO)-superlattices which may be interesting for future electronic applications have been investigated by X-ray photoelectron spectroscopy (XPS) depth profiling. At first XPS measuring conditions were optimized for that non-conducting and thin layer systems (21 nm double layer thickness) considering the practical instrumental limitations. Second a simulation of the sputtering process for the concrete experimental conditions were done by a dynamic T-DYN code. By comparison of experimental and simulated depth profiles the maximum sample roughness could be estimated to be in the range of 2 nm.     

Energy gaps, electronic structures, and x-ray spectroscopies of finite semiconductor single-walled carbon nanotubes

We report hybrid density functional theory calculations for electronic structures of hydrogen-terminated finite single-walled carbon nanotubes (6,5) and (8,3) up to 100 nm in length. Gap states that are mainly arisen from the hydrogen-terminated edges have been found in (8,3) tubes, but their contributions to the density of states become invisible when the tube is longer than 10 nm. The electronic structures of (6,5) and (8,3) tubes are found to be converged around 20 nm. The calculated band-gap energies of 100 nm long nanotubes are in good agreement with experimental results. The valence band structures of (6,5), (8,3), as well as (5,5) tubes are also investigated by means of ultraviolet photoelectron spectra (UPS), x-ray emission spectroscopy (XES), and the resonant inelastic x-ray scattering (RIXS) spectra theoretically. The UPS, XES and RIXS spectra become converged already at 10 nm. The length-dependent oscillation behavior is found in the RIXS spectra of (5,5) tubes, indicating that the RIXS spectra may be used to determine the size and length of metallic nanotubes. Furthermore, the chiral dependence observed in the simulated RIXS spectra suggests that RIXS spectra could be a useful technique for the determination of chirality of carbon nanotubes.     

Selective binding of the cyano group in acrylonitrile adsorption on Si(100)-2 x 1

The covalent binding of acrylonitrile (CH(2)=CH-C triple bond N) and the formation of a C=C-C=N structure on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. For chemisorbed acrylonitrile, the absence of nu(C triple bond N) at 2245 cm(-1) and the appearance of nu(C=N) at 1669 cm(-1) demonstrate that the cyano group directly participates in the interaction with Si(100), which is further supported by XPS and UPS observations. Our experimental results and DFT calculations unambiguously demonstrate a [2 + 2] cycloaddition mechanism for acrylonitrile chemisorption on Si(100) through the binding of C triple bond N to Si dimers. The resulting chemisorbed monolayer with a C=C-C=N skeleton can serve as a precursor for further chemical syntheses of multilayer organic thin films in a vacuum and surface functionalization for in situ device fabrication.     

Work function and negative electron affinity of ultrathin barium fluoride films

Thin films of barium fluorides with different thicknesses were deposited on GaAs substrate by electron beam evaporation. The aim of the work was to identify the best growth conditions for the production of coatings with a low work function suitable for the anode of hybrid thermionic-photovoltaic (TIPV) devices. The chemical composition and work function φ of the films with different thicknesses were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The lowest value of φ = 2.1 eV was obtained for the film with a thickness of ~2 nm. In the valence band spectra of the films at low kinetic energy, near the cutoff, a characteristic peak of negative electron affinity was present. This effect contributed to a further reduction of the film's work function.     

Cr掺杂金红石相TiO2(110)单晶薄膜的制备、表征及光催化活性

采用脉冲激光沉积术(PLD)同质外延生长了表面原子级平整的6%(原子比)Cr 掺杂的金红石相TiO₂(110)单晶薄膜, 采用扫描隧道显微镜(STM)、扫描隧道谱(STS)、X 射线光电子能谱(XPS)和紫外光电子能谱(UPS)对其进行了表征. 结果表明: Cr 掺杂对TiO₂(110)-(1×1)表面的形貌没有明显影响, 但是提高了掺杂薄膜在负偏压的导电性; Cr与晶格O键合而呈现+3价态, 由此在TiO₂的价带顶上方~0.4 eV处引入杂质能级. 紫外-可见光吸收谱显示薄膜的光吸收能力被扩展到~650 nm, 处于可见光范围. 借助STM以单个甲醇分子的光解反应检测了薄膜的光催化活性. 仅观察到紫外光照射下甲醇分子的脱氢反应, 在可见光照射下(λ>430 nm)甲醇分子没有发生反应, 表明单独的Cr掺杂可能不足以提高TiO₂在可见光下的催化活性.     

Synthesis,structures, and electroluminescence properties of a 1D zinc(II) coordination polymer containing both dicyanamide and pyrazinamide ligands

A 1D coordination polymer, {[Zn(μ_1,5-dca)₂(PZA)₂](PZA)₂}_n (1), has been synthesized and characterized by single-crystal X-ray crystallography. The coordination modes of the dicyanamide (dca) and the pyrazinamide (PZA) were inferred by IR spectroscopy. The complex was applied to organic electroluminescent (EL) devices as the emitting materials. The electroluminescent device of ITO/NPB (40 nm)/Zn polymer: CBP (30 nm) (30 nm)/BCP (15 nm)/Alq (30 nm)/LiF (1 nm)/Al (100 nm) was fabricated. The EL device emits cyan light originating from this complex with high brightness and efficiencies. For 1, a maximum luminance of 34.9 cd/A was achieved at 9 V.     

A comparative study between inductively and capacitively coupled plasma deposited polystyrene films: chemical and morphological characterizations

Thin polystyrene films were deposited on stainless steel substrates by capacitively and inductively coupled radio frequency glow discharge plasma, in order to compare their chemical and morphological properties. The films were characterized by Fourier‐transform infrared spectroscopy (FTIR), X‐Ray photoelectron spectroscopy (XPS), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), atomic force microscopy (AFM) and scanning electron microscope (SEM). Wettability properties were also determined by contact angle measurements. Ageing effect was studied by analysing films aged for 15 min and for 1 week. Results from both capacitively and inductively plasma polymerized polystyrene (pPS) films aged for 15 min showed that the chemical structure of the bulk, chemical composition of the surface (depth < 10 nm) and wettability properties were rather similar. Only their microstructures were very different: the pPS_capa film's microstructure showed homogeneous distribution of spherical particles of about 100 nm in diameter but the pPS_ind film's microstructure seemed to be mainly influenced by the surface of the metallic substrate: orientated ‘lamellae‐like layers’ of polymers were observed on each metallic grain. Ageing for 1 week in ambient air induced low oxygen uptake in the surface of both pPS films. The pPS_ind topmost surface (depth < 3 nm) was more oxidized than that of pPS_capa but no modification of the chemical structure of the bulk or of the morphology was noticed after ageing. Copyright © 2006 John Wiley & Sons, Ltd.     

One-Step Synthesis of Silicon Oxynitride Films Using a Steady-State and High-Flux Helicon-Wave Excited Nitrogen Plasma

A steady-state and high-flux helicon-wave excited N₂ plasma was used to oxynitride Si substrates for the synthesis of silicon oxynitride (SiON) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been extensively used to characterize surface quality of the SiON films, and it is found that a large amount of nitrogen (N) can be incorporated into the films. The result of XPS depth profiles shows that the N concentration is high near the surface and the oxide/Si interface. In the UPS spectra, absence of the reappearance of surface states suggests a resistance to clustering of the oxynitride layer. The N₂ flux and Ar mixture quantity can facilitate tuning of the dissociation characteristics in N₂ discharge. By modulating the N₂ fractions, the N⁺ density reaches maximum at a N₂/(N₂ + Ar) flow-rate ratio of 0.5, resulting in incorporation of more N atoms into the SiON films. Considering the easy control of N₂ plasma, our work opens up a new avenue for achieving high-yield SiON films at low temperature.     

Antimony-doped tin(IV) oxide: Surface composition and electronic structure

Antimony-doped tin(IV) oxide Sn_1?xSb_xO₂ prepared by a high-temperature (1300 K) solid-state synthetic procedure has been studied over the composition range0 < x < 0.03by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron-energy-loss spectroscopy (HREELS). Pronounced enrichment by antimony close to the surface is evident from XPS with a heat of segregation approaching 30 kJ/mole. However, no increase in the surface free-carrier concentration is evident from the conduction-to-valence band intensity ratio in UPS or from the surface plasmon frequency in EELS. It is concluded that electrons associated with segregated Sb ions occupy a lone-pair-likesp hybrid surface state whose energy lies well below that of the conduction band.     

Investigation of physicochemical properties of CuSn-based PAN nanofibers prepared via electrospinning method

Copper-tin (CuSn) nanomaterials have been receiving substantial attention due to their excellent thermal, electrical, and optical properties. However, how such properties are affected based on heat treatment temperature and chemical composition of copper and tin is still not very well understood. In this paper, CuSn nanofibers were fabricated by electrospinning a precursor solution of polyacrylonitrile, copper, and tin. Calcination temperatures were selected using thermogravimetric/differential thermal analysis (TG/DTA) and Fourier transform infrared (FT-IR) results. Analytical techniques such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) were employed to investigate the physicochemical properties of the CuSn nanofibers. SEM images and EDS revealed the formation of pores on the nanofibers and high concentrations of tin were in the core, while copper was located on the surface. XRD results confirmed the monoclinic phase of Cu₆Sn₅ for the CuSn nanofibers because peaks for diffraction angles at 27.6°, 53.4°, and 60.0° were observed. XPS results showed that Cu―C and Sn―C bonds occur at binding energies around 932 and 484 eV, respectively. The work function of the CuSn NF heat treated at 150°C was calculated from the UPS spectra, and the value was 4.19 eV.     

Surface characterisation of laser irradiated SiC ceramics by AES and XPS

Samples of sintered silicon carbide (SSiC) were irradiated with a KrF excimer laser (λ = 248 nm) at energy densities of 10, 15 and 25 J/cm² in He atmosphere. The composition of the near surface region was investigated by Auger electron spectroscopy (AES) and photoelectron spectroscopy (XPS) after lapping, laser irradiation and tribological treatment, respectively. By laser irradiation a surface layer is formed which contains about 30% oxygen. The existence of different bonding states of Si, C and O was established by factor analysis of the AES depth profiles and by XPS. By laser irradiation SiC is decomposed and a siliconoxycarbide with the average composition SiC_3.5O_1.5 is formed. Beneath the oxidised surface layer the nominal elemental composition SiC is found but the sample represents a mixture of Si, graphite and siliconoxycarbide with a small amount of SiC only. Obviously, the decomposition zone exceeds in a depth > 300 nm.     

A [4+2]-like cycloaddition of methyl methacrylate on Si(100)-2 x 1

The attachment of methyl methacrylate (MMA) on Si(100)-2x1 was investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. The HREELS spectra of chemisorbed MMA show the disappearance of characteristic vibrations of C=O (1725 cm(-1)) and C(sp(2))-H (3110, 1400, and 962 cm(-1)) coupled with the blue shift of the C=C stretching mode by 34 cm(-1) compared to those of physisorbed molecules. These results clearly demonstrate that both C=C and C=O in MMA directly participate in the interaction with the surface to form a SiCH(2)C(CH(3))=C(OCH(3))OSi species via a [4+2]-like cycloaddition. This binding configuration was further supported by XPS, UPS, and DFT studies.     

Application of Cr Kα X‐ray photoelectron spectroscopy system to overlayer thickness determination

A laboratory hard X‐ray photoelectron spectroscopy (HXPS) system at 5.4‐keV excitation energy was used to measure the angle dependence of a silicon oxide overlayer on a Si(0 0 1) substrate with overlayer thickness ranging from 4 to 25 nm. The thickness values of the SiO₂ overlayers were determined by utilizing a focused monochromatized Cr Kα source and a high‐energy hemispherical analyzer with an angle‐resolved wide acceptance angle objective lens. The modulation of the photoemission intensity due to photoelectron diffraction, which deteriorates high‐precision thickness determination, was suppressed significantly by continuous sample rotation around the sample's normal during the measurements. The resultant thickness values very well agree with those determined by ellipsometry in the same sample set. To demonstrate merits of the large information depth measurements, profiling of a wedged SiO₂ layer buried in a gate stack model structure with Ir (8 nm) and HfO₂ (2 nm) overlayers was performed. Copyright © 2011 John Wiley & Sons, Ltd.     

Bonding structure of phenylacetylene on hydrogen-terminated Si(111) and Si(100): surface photoelectron spectroscopy analysis and ab initio calculations

Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s → π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH(2)) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.     

Lithium adsorption on TiO2: studies with electron spectroscopies (MIES and UPS)

The adsorption of lithium atoms on rutile TiO₂(110) single crystals was studied with metastable‐induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS(HeI)) between 130 K and room temperature. Some auxiliary measurements on W(110) required for data interpretation are also reported. At 130 K ionic adsorption at titania prevails up to 0.3 monolayer equivalents (MLE) as judged from the weak Li(2s) emission in MIES for these exposures. The reduction of the Ti⁴⁺ cation is manifested by the growth of an occupied bandgap state in UPS: the alkali s‐electron is transferred to a near‐surface cation, thereby reducing it to Ti³⁺ 3d. The transfer of the s‐electron is responsible for the observed work function decrease up to ～0.5 MLE coverage. From the analysis of the UPS Ti³⁺ 3d signal, as well as from the Li(2s) emission, it is concluded that the degree of ionicity of the adsorbed Li decreases from 100% at 0.3 MLE to 40% at 0.7 MLE. Above 0.5 MLE the MIES spectra are dominated by an Li(2s)‐induced peak indicating the presence of Li with an at least partially filled 2s orbital. At temperatures above 160 K this peak is almost absent. Excluding Li desorption at these temperatures, we suggest that Li moves into or below the rutile TiO₂(110) surface above 160 K. Lithium insertion into the surface and intercalation are discussed. Copyright © 2004 John Wiley & Sons, Ltd.     

Preparation and characterization of poly(MMA-M12-BPMA)/TiO2 composite particles

In this paper, poly(MMA-M12-BPMA)/TiO₂ composite particles were prepared by the copolymerization of a reactive surfactant sodium sulfopropyl-laurylmaleate (M12) and a reactive UV-stabilizer 2-hydroxy-4-(3-methacryloxy-2-hydroxylpropoxy) benzophenone (BPMA) with methyl methacrylate (MMA) in the presence of TiO₂. The structure and performance of composite particles were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, gel permeation chromatography, ultraviolet–visible absorption spectroscopy, differential scanning calorimeter, and scanning electron microscope.The measurement results indicate that the percentage of grafting and the grafting efficiency can reach 336.87% and 36.38%, respectively, and the glass transition temperature of poly(MMA-M12-BPMA)/TiO₂ composite particles is higher than that of poly(MMA-M12-BPMA); the size of the composite particles is about 130–200 nm. The poly(MMA-M12-BPMA) grafted from the surface of TiO₂ keeps the excellent characteristic of BPMA that possesses high absorbance of ultraviolet light, which is very important for improving UV-resistant performance of the polymethyl methacrylate. These research results are very useful for preparing polymethyl methacrylate with resistance to UV light.     

Dynamic mechanical behavior and optical limiting property of multifunctional fullerenol/polymer composite

The dynamic mechanical behavior of materials based on multifunctional fullerenol and poly(styrene-co-4-vinylpyridine)/poly(styrene-co-butadiene) matrix was studied. The interaction between the hydroxyl group of fullerenol and the pyridine group as evidenced by X-ray photoelectron spectroscopy results in a significant increase of the storage modulus. The optical limiting property of fullerenol was studied at 532 nm with nanosecond laser pulses. The optical limiting performance of fullerenol, which is poorer than its parent C₆₀, is slightly improved upon the incorporation of poly(styrene-co-4-vinylpyridine).     

Surface investigation of intermetallic PdGa(111)

The intermetallic PdGa is a highly selective and potent catalyst in the semihydrogenation of acetylene, which is attributed to the surface stability and isolated Pd atom ensembles. In this context PdGa single crystals of form B with (111) orientation were investigated by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), X-ray photoelectron diffraction (XPD), and low-energy electron diffraction (LEED) to study the electronic and geometric properties of this surface. UPS and thermal desorption spectroscopy (TDS) were used to probe the chemisorption behavior of CO. The PdGa(111) surface exhibits a (1 × 1) LEED and a pronounced XPD pattern indicating an unreconstructed bulk-truncated surface. Low-temperature STM reveals a smooth surface with a (1 × 1) unit cell. No segregation occurs, and no impurities are detected by XPS. The electronic structure and the CO adsorption properties reveal PdGa(111) to be a bulk-truncated intermetallic compound with Pd-Ga partial covalent bonding.