The adsorption of CO2 and CO on Ca and CaO films studied with MIES, UPS and XPS

Metastable Induced Electron Spectroscopy (MIES), Ultraviolet Photoelectron Spectroscopy (UPS), and X-ray Photoelectron Spectroscopy (XPS) are employed to study the adsorption of CO₂ and CO on Ca and CaO films. Ca films are prepared by evaporation of Ca onto clean Si(1 0 0) substrates. CaO films are produced by Ca evaporation in an oxygen atmosphere at a substrate temperature of 670 K. CO₂ interaction with the Ca films is initiated by dissociation of the impinging molecules leading to the formation of Ca-O bonds. These Ca-O bonds are subsequently consumed in the formation of a closed CaCO₃ layer on top of the surface. CO interaction with the Ca surfaces also leads to the dissociation of the molecule and the formation of Ca-O bonds. We find evidence for the subsequent formation of complexes on top of the surface. On CaO surfaces, both CO₂ and CO lead to the formation of a closed CaCO₃ top layer, though displaying very different reaction rates.     

Domain structure and optical property of epitaxial indium oxide film deposited on MgO(100) substrate

Indium oxide (In₂O₃) film has been deposited on MgO(100) substrate at 600 °C by metalorganic chemical vapor deposition (MOCVD). The crystal structure and epitaxial relationship of the sample were examined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The results showed a clear epitaxial relationship of In₂O₃(111)||MgO(100) with In₂O₃[011?]||MgO<072> or In₂O₃[01$\overline{1}$]||MgO<011>. A multiple domain structure was found inside the In₂O₃ film and a theoretical model clarifying the geometrical relationships between each domain and the substrate has been proposed. Scanning electron microscopy (SEM) micrograph showed that the main surface features were triangle-shaped grains on the film. The absolute average transmittance of the obtained film in the visible range was over 93%.     

The Fate of a Cluster Colliding onto a Substrate Dissipation of Translational Kinetic Energy

The fate of clusters emitted onto a substrate falls into several categories including repulsion, soft landing, migration, and explosion, depending on the interaction between the cluster and the substrate. This dynamic behavior of the clusters controls thin-film formation processes from clusters such as cluster ion beam deposition and chemical vapor deposition. Here we describe the collision processes of both Al and Au clusters with a kinetic energy of 0.56 eV/atom onto an amorphous SiO₂ substrate studied by molecular dynamics simulation, focusing on the dissipation of translational kinetic energy during the collision process. The simulation elucidated that the activation of thermal vibrational energy of the substrate promoted the sticking of the colliding clusters on the substrate. This result suggests that the dissipation of the translational kinetic energy of the colliding cluster is one of the factors that determine the structure formed on a substrate from vapor phase.     

Pulsed laser synthesis of titanium silicides using a Q-Switched Nd: Glass laser

Titanium films 120 nm thick deposited on single-crystalline silicon (c-Si) as well as poly-Si/SiO₂/c-Si substrates were subjected to Nd: glass laser irradiation. Laser fluences of 1,1.5, and 2 J/cm² were used at the pulse duration of 30 ns. From RBS analysis it follows that on c-Si substrate titanium suicide is formed using one pulse of 1.5 J/cm² energy density. On the substrate with surface overlayers lower fluence (1 J/cm²) was sufficient. Under these conditions the sheet resistance of the samples decreased from the initial value 5 / to 2–3 /. The smaller threshold density of energy for suicide formation in Ti/polySi/SiO₂/c-Si structure is shown to be a consequence of the SiO₂ underlayer, which is a poorer heat conductor than silicon. The experimental results of the suicide synthesis are in semi-quantitative accordance with the numerical computations of the temperature vs time evolution and depth temperature distribution in our samples.     

Theoretical study of CO and Pb adsorption on the Ni(1 1 1) and Ni3Al(1 1 1) surfaces

Adsorption of CO molecules and Pb atoms on the Ni(1 1 1) and Ni₃Al(1 1 1) substrates is studied theoretically within an ab initio density-functional-theory approach. Stable adsorption sites and the corresponding adsorption energies are first determined for stoichiometric surfaces. The three-fold hollow sites (fcc for Pb and hcp for CO) are found most favourable on both substrates. Next, the effect of surface alloying by a substitution of selected topmost substrate atoms by Pb or Ni atoms on the adsorption characteristics is investigated. When the surface Al atoms of the Ni₃Al(1 1 1) substrate are replaced by Ni atoms, the Pb and CO adsorption energies approach those for a pure Ni(1 1 1) substrate. The Pb alloying has a more substantial effect. On the Ni₃Al(1 1 1) substrate, it reduces considerably adsorption energy of CO. On the Ni(1 1 1) substrate, CO binding strengthens slightly upon the formation of the Ni(1 1 1)p(2×2)-Pb surface alloy, whereas it weakens drastically when the Ni(1 1 1)-Pb surface alloy is formed.     

Structural analysis of In/Ag, In/Cu and In/Pd thin films on tungsten by ellipsometric, XRD and AES methods

Compositional, microstructural and optical properties of In/Cu, In/Ag and In/Pd thin films evaporated on W substrate in a vacuum were investigated by means of Auger electron spectroscopy, X-ray diffractometry, scanning electron microscopy, and spectroscopic ellipsometry methods. Thicknesses of deposited pure metal layers were adjusted to atomic concentration ratios In:Ag = 1:2, In:Cu = 2:1 and In:Pd = 3:1. Interdiffusion of metals and creation of intermetallic compounds AgIn₂, Ag₂In and CuIn₂were detected at room temperature. Phase transformation and changes in the surface morphology due to annealing of samples at 393 K for 60 min were revealed. Combined investigations indicated a layered structure of In/Ag films. A tendency of island-like structure formation was stronger for In/Cu and In/Pd films. The complex dielectric functions of In/Ag, In/Cu and In/Pd composite layers were determined from spectroellipsometric data.     

Photocatalytic reduction of oxygen molecules at the (1 0 0) TiO2 anatase surface

The adsorption, photoreduction and chemical activity of oxygen molecules on the (1 0 0) anatase surface have been investigated here together with the effects that surface oxygen vacancies (V_O) can have on these O₂-related processes. We use an original approach by treating molecules on the TiO₂ surface like surface defects in the same framework successfully used for defects in semiconductors. The achieved results: (i) give the first theoretical evidence of an acceptor behaviour of an adsorbed O₂ molecule, which is at the origin of its photoreduction; (ii) show that the V_O donor character is strongly affected by the interaction with O₂; and (iii) suggest that the release of radicals as well as the formation of O₂-related radicals may be favoured by photogenerated electrons in presence of surface V_O’s.     

Electronic structure of gold nanoclusters on oxidized Ni(755) surface

The electronic energy structure of gold nanoclusters grown on oxidized single-crystal stepped surface Ni(755) is studied. It is shown that oxidation of the stepped Ni(755) surface results in the formation of a well-ordered continuous structure O(2 × 2) similar to that grown on a flat Ni(111) single-crystal surface. Evaporation of gold on such a surface leads to the formation of gold nanoclusters of a size determined by the size of the terraces on the Ni(755) surface. A comparison of the photoelectron spectra of the Au 4f _{5/2, 7/2} core levels in clusters grown on clean and oxidized Ni(755) surfaces reveals that the spectra obtained for a gold cluster system on an oxidized Ni(755) surface contain not only the spectral components characteristic of metallic gold but also additional components of Au^+δ. It is assumed that additional components for gold clusters on the oxidized Ni(755) surface originate from partial oxidation of gold atoms with the participation of defects inherent in the stepped relief of the nickel substrate.     

Effect of growth parameters on crystallinity and properties of ZnO films grown by plasma assisted MOCVD

Thin films of ZnO have been grown by plasma assisted metal–organic chemical vapour deposition (PA-MOCVD) using a 13.56 MHz O₂ plasma and the (HTTA=2-thenoyltrifluoroacetone, TMED=N,N,N′,N′-tetramethylethylendiamine) precursor. The effects of growth parameters such as the plasma activation, the substrate, the surface temperature, and the ratio of fluxes of precursors on the structure, morphology, and optical and electrical properties of ZnO thin films have been studied. Under a very low plasma power of 20 W, c-axis oriented hexagonal ZnO thin films are grown on hexagonal sapphire (0001), cubic Si(001) and amorphous quartz substrates. The substrate temperature mainly controls grain size.     

Inclined ZnO thin films produced by pulsed-laser deposition

ZnO thin films with a uniformly inclined structure are grown by pulsed-laser deposition on SrTiO₃. The c-axis of ZnO films is inclined by an angle =20±0.5° and =42±0.5° against the surface normal of 25° miscut (100) SrTiO₃ and (110) SrTiO₃ single crystal substrates, respectively. The inclined structure is due to epitaxial growth of hexagonal ZnO on cubic SrTiO₃ as evidenced by X-ray diffraction and high-resolution transmission electron microscopy investigations. The range of deposition parameters (substrate temperature, oxygen background pressure) to achieve epitaxial growth is determined. The inclined films are smooth with an rms surface roughness of 1.5 nm for layer thicknesses up to 700 nm. PACS 61.10.-i; 68.37.-d; 81.15.Fg     

P-Polarized nonlinear surface polaritons in layered structures

We found an exact solution of Maxwell's equations, which describes the propagation ofp-polarized nonlinear surface polaritons and ofp-polarized nonlinear guided wave polaritons in two cases:i) in a film of a surface active material placed on a substrate described by a diagonal dielectric tensor whose elements depend on the amplitude of the electric field according to _{1 1}=_{2 2}= + (|E ₁|² + |E ₂|²), _{3 3}=, andii) in a film described by the same dielectric tensor (optically uniaxial nonlinear crystal) placed on a substrate with dielectric constant ₃ (optically linear medium). The power carried in the surface waves has also been exactly calculated.     

Influence of aliphatic spacer group on adsorption mechanisms of phosphonate derivatives of l-phenylalanine: Surface-enhanced Raman, Raman, and infrared studies

The nature of phosphonopeptides containing N-terminal l-phenylalanine (l-Phe), namely l-Phe-dl-NH-CH(CH(CH₃)₂)-PO₃H₂ (A), l-Phe-l-NH-CH(CH₃)-PO₃H₂ (B), and l-Phe-dl-NH-CH(CH₂CH₂COOH)-PO₃H₂ (C) (Fig. 1 presents molecular structure of these molecules), adsorbed on electrochemically roughened and colloidal silver surfaces has been explored by surface-enhanced Raman spectroscopy (SERS). To reveal adsorption mechanism of these species on the basis of their SERS spectra at first Fourier-transform Raman (FT-RS) and absorption infrared (FT-IR) spectra of non-adsorbed molecules were measured. Examination of enhancement, frequency shifts, and changes in relative intensities of SERS bands due to adsorption and surface roughens variation reveals that the tilted compounds adsorb on the electrochemically roughened silver substrate in similar way, while they behave differently on the colloidal silver surface. A stronger enhancement of in-plane ring vibrations of the l-Phe ring, i.e., ν₃ and ν_18b (B₂), over these of the A₂ symmetry in all SERS spectra on the electrochemically roughened silver substrate suggests that the ring interacts with this surface adopting slightly deflect orientation from the perpendicular one. Also, enhancement of PO and -CH₂-/-CH₃ fragments vibrations points out that they are involved in adsorption process on this substrate. This conclusion was drawn on the basis of the enhancement of 1274-1279 and 1138-1152 (ν(PO)), 1393-1400 (δ(CH) + ρ_b(CNH₂) + ν(C-C_O) + δ(CH₃)), ∼1455 (δ(CCH₃/CCH₂) + ρ_b(CH₃/CH₂), and 1505-1512 cm⁻¹ (δ(CH₂) + Phe(ν_19a)) bands. Although a relative intensity ratio of these bands in the presented SERS spectra is different. On the other hand, on the colloidal silver nanoparticles, the aromatic ring of all molecules is lying flat or takes almost parallel orientation to this surface. Besides, A interacts also via P-terminal group (568, 765, 827, 1040, and 1150 cm⁻¹), whereas B mainly through NH₂-C-(CO)-CNH-(712 and 1255 cm⁻¹). In the case of C, it adsorbs on the silver colloidal surface mainly through the aromatic ring of l-Phe, while other fragments of the molecule are in close proximity to this surface as comes off the weak enhancement of bands due to the aliphatic vibrations.     

Stranski-Krastanov like oxide growth on Ag(1 1 1) at atmospheric oxygen pressures

The oxidation behavior of Ag(1 1 1) was studied by means of in situ surface X-ray diffraction at atmospheric oxygen pressure. Exposure to 1 bar oxygen at 773 K reveals a competing growth of three different oxygen-induced structures on Ag(1 1 1), namely the well-known p(4 × 4) reconstruction, a surface oxide in a p(7 × 7) coincidence structure and the bulk oxide Ag₂O in orientation. The latter two exhibit the same honeycomb on hexagon arrangement of the Ag sublattice with respect to the Ag(1 1 1) surface. An inverted stacking of Ag planes in the bulk oxide islands is observed as compared to the Ag(1 1 1) substrate, which sheds new light on the Ag₂O formation process. Finally, we present a structural model of the p(7 × 7) reconstruction, based on a three-layer O-Ag-O slab of Ag₂O(1 1 1).     

Electromagnetic waves in thin-film superconducting lines

Conclusions The distinctive features discussed about the propagation of electromagnetic waves in a structure containing a thin-film superconducting line are associated with the fact that the screening properties of the superconducting layer are determined by the distribution of superconducting currents in it. In the quasistatic case under discussion this distribution depends on the nature of the nonuniformity of the magnetic field along the normal to the layer. At specified angles of incidence of a wave the layer is equivalent to an ideal conductor or a diamagnetic; in the intermediate region the screening effect is far smaller. Due to this a thin-film superconducting line possesses some directionality of the leakage of the nonuniform waves when ₁ > ₂. The wave which is emergent at a specified angle which depends on the parameters of the layered structure (₁, ₂,l, _L) possesses the maximum amplitude.When ₁ < ₂, the structure in question does not have a sharply pronounced directionality of the leakage, and at angles of incidence on a thin layer corresponding to total internal reflection it supports the propagation of waves along a line with as small a film thickness as desired. The unscreening effect of a superconducting layer is exhibited here in the fact that the propagation of waves excited in the line occurs at specified angles of incidence onto the layer mainly along its outer surface.Institute of Applied Physics, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Vol. 25, No. 5, pp. 527–535, May, 1982.     

An investigation into the role of macromolecules of different polarity as passivating agent on the physical,chemical and structural properties of fluorescent carbon nanodots

In this study, comparative evaluation of fluorescent carbon nanodots (C-Dots) prepared using carob molasses was reported by screening various biocompatible macromolecules as passivating agent (PA). Incorporation of PAs with different molecular weight, polarity, and chemical structure was examined, and compared with the polyethylene glycol (PEG, Mn = 10 kN) passivated and pristine C-Dots. Not only the fluorescence properties but also many other features including size, crystal structure, colloidal conductivity, resistance to photobleaching, quantum yield, and UV-modulated surface interaction of them with the reactive oxygen species (ROS) as well as ROS production were investigated. Photoluminescence (PL) capacity of C-Dots was found to be associated with the number of surface alkyl groups and polymeric hydrogen bounding present on the C-Dot surface (increased number is associated with decreased PL) while surface conductivity of C-Dots in water was proportional to the PL intensity. More importantly, C-Dots with relatively poorer fluorescent were investigated in various organic solvents (hexane, methanol, acetone, ethanol, dimethylformamide (DMF), and DMSO). As happens with the fluorescent dyes, their PL intensities were significantly enhanced (even for pristine C-Dots) depending on the solvent characteristics. All of the C-Dots synthesized were further evaluated by means of UV-induced generation of ROS and inhibition of ROS by using H₂O₂ as model. In contrary to other carbonaceous nanomaterials, they did not show any ROS generation, on the contrary, they showed ROS scavenging activity that can be modulated by UV-irradiation (λ _exc = 365 nm). PEG and alginate passivated C-Dots inhibited H₂O₂ activity at LC₅₀ values below 10 mg/mL.

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Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Characteristics of surface acoustic waves in (11$$\bar 2$$ 0)ZnO film/ R-sapphire substrate structures

(11\(\bar 2\)0)ZnO film/R-sapphire substrate structure is promising for high frequency acoustic wave devices. The propagation characteristics of SAWs, including the Rayleigh waves along [0001] direction and Love waves along [1ī00] direction, are investigated by using 3 dimensional finite element method (3D-FEM). The phase velocity (v _p), electromechanical coupling coefficient (k ²), temperature coefficient of frequency (TCF) and reflection coefficient (r) of Rayleigh wave and Love wave devices are theoretically analyzed. Furthermore, the influences of ZnO films with different crystal orientation on SAW properties are also investigated. The results show that the 1st Rayleigh wave has an exceedingly large k ² of 4.95% in (90°, 90°, 0°) (11\(\bar 2\)0)ZnO film/R-sapphire substrate associated with a phase velocity of 5300 m/s; and the 0th Love wave in (0°, 90°, 0°) (11\(\bar 2\)0)ZnO film/R-sapphire substrate has a maximum k ² of 3.86% associated with a phase velocity of 3400 m/s. And (11\(\bar 2\)0)ZnO film/R-sapphire substrate structures can be used to design temperature-compensated and wide-band SAW devices. All of the results indicate that the performances of SAW devices can be optimized by suitably selecting ZnO films with different thickness and crystal orientations deposited on R-sapphire substrates.     

FTIR, XRD, SEM and solid state NMR investigations of carbonate-containing hydroxyapatite nano-particles synthesized by hydroxide-gel technique

Nano-crystalline hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂ has been synthesized by a precipitate conversion technique using hydroxide gel at lower temperatures, e.g. 80 °C. HA crystallizes in hexagonal structure (space group: P6₃/m) having lattice parameters: and and around 17 nm in crystallite-size for the 80 °C-heated sample. SEM micrographs show hexagonal crystallites of average particle dimensions approximately 50×20 nm for 80 °C heated sample. The structure analysis by XRD, FTIR, ¹H and ³¹P MAS NMR show the existence of structural disorder at the particle surface that either does not form hydrogen bonding due to lack of adequate bonding parameters or forms a very weak dipolar bonding. The structural disorder has been explained as a result of chemical interactions between the phosphate groups either with the surface adsorbed water or the hydroxyl groups at the surface of the nano-particles.