Covalent functionalization of silicon nitride surfaces by semicarbazide group

The paper reports on the covalent attachment of semicarbazide functional groups onto hydrogen-terminated silicon-rich silicon nitride (Si_xN₄; 4 < x < 5) surface under photochemical conditions. UV-irradiation (254 nm) of the surface coated with a thin film of tert-butyl 2-[(allylamino)carbonyl]hydrazine-carboxylate yields an organic monolayer covalently attached to the surface via Si-C bonds and terminated with t-Boc-protected semicarbazide moieties. Removal of the protecting group produces a reactive semicarbazide-terminated monolayer. The photo-induced reaction was investigated by means of X-ray photoelectron spectroscopy (XPS) and contact angle measurements.     

Investigation of monolayer dispersion of benzoic acid supported on the surface of H-titanate nanotubes

Benzoic acid (BA) can disperse spontaneously onto the surface of H-titanate nanotubes (HTNTs) in a sub-monolayer state by heating mechanical mixtures method. The structure of BA-HTNTs system has been characterized by X-ray diffraction (XRD), thermogravimetric (TG), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) in detail. The results show that the H-bond association structure among BA molecules collapses and the carboxyl groups react with the surface hydroxyl group of HTNTs to form a salt-like structure on the surface after dispersion. The monolayer dispersion capacity determined by XRD is ca. 0.305 g BA g⁻¹ HTNTs, which is lower than the utmost monolayer dispersion capacity 0.550 g BA g⁻¹ HTNTs calculated according to a model that the benzene ring in BA molecules is perpendicular to the surface of HTNTs. At the same time, the dispersion capacity is also measurement by the fit of C 1s XPS peak at various BA loadings at first time.     

Evaluation of copper ion of antibacterial effect on Pseudomonas aeruginosa, Salmonella typhimurium and Helicobacter pylori and optical, mechanical properties

Antibacterial effect on Pseudomonas aeruginosa, Salmonella typhimurium and Helicobacter pylori of copper ion was researched. Also, additional effects of copper ion coating on optical and mechanical properties were researched as well. Copper ion was coated on glass substrate as a thin film to prevent bacteria from growing. Cupric nitrate was used as precursors for copper ion. The copper ion contained sol was deposited by spin coating process on glass substrate. Then, the deposited substrates were heat treated at the temperature range between 200 °C and 250 °C. The thickness of deposited copper layer on the surface was 63 nm. The antibacterial effect of copper ion coated glass on P. aeruginosa, S. typhimurium and H. pylori demonstrated excellent effect compared with parent glass. Copper ion contained layer on glass showed a similar value of transmittance compared with value of parent glass. The 3-point bending strength and Vickers hardness were 209.2 MPa, 540.9 kg/mm² which were about 1.5% and 1.3% higher than the value of parent glass. From these findings, it is clear that copper ion coating on glass substrate showed outstanding effect not only in antibacterial activity but also in optical and mechanical properties as well.     

Thermal decomposition mechanisms of tungsten nitride CVD precursors on Cu(1 1 1)

Chemisorption and thermal decomposition of metallorganic chemical vapor deposition precursors, (t-BuN)₂W(NHBu-t)₂, bis(tert-butylimido)bis(tert-butylamido)tungsten (BTBTT) and (t-BuN)₂W(NEt₂)₂, bis(tert-butylimido)bis(diethylamido)tungsten (BTBDT), on Cu(1 1 1) have been investigated by means of thermal desorption spectroscopy (TDS) and synchrotron-based X-ray photoelectron spectroscopy (SR-XPS) under ultrahigh vacuum conditions. The precursors remain intact upon chemisorption on Cu(1 1 1) at 100 K, and at 300 K both precursors decompose readily via the characteristic hydride abstraction/elimination pathways to produce two stable surface intermediates for each precursor. For BTBTT, one species is W(=NBu-t)₃ and the other is proposed to be a bridged amido complex, [(t-BuN)₂W(μ-NBu-t)]₂. In comparison, a W-imine complex and a W-N-C metallacycle are two intermediates produced from BTBDT. Annealing toward 800 K further decomposes the intermediates and the detectable desorption species are completely derived from the ligands. The desorption products from BTBTT include t-butylamine generated from α-H abstraction, isobutylene from γ-H elimination, acetonitrile from β-methyl elimination, and molecular hydrogen. In addition to these desorption species, BTBDT produces hydrogen cyanide and imine (EtN = CHMe) via β-H elimination, not possible with BTBTT due to the absence of β-H in the ligands. Eventually, tungsten nitrides incorporating oxygen atoms and a small amount of graphitic carbons are formed and the stoichiometry is approximated as WN_1.5O_0.1. Oxygen incorporation, driven by a large oxide formation enthalpy, is sensitively dependent on the moisture exposure in UHV environment.     

Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr-O valence electron matching to the number of dangling bonds per surface Si atom. The Zr-O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C-O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.     

Optical absorption of 1-phenyl-3-methyl-1H-pyrazolo[3,4-b]quinoline derivatives: Molecular dynamics modeling

Paper deals with quantum chemical modeling of the optical absorption spectra of 6-fluoro, 7-trifluoromethyl, 6-cyano, 6-carboethoxy and 6-tert-butyl derivatives of 1-phenyl-3-methyl-1H-pyrazolo[3,4-b]quinoline. The calculations are performed by means of the semiempirical quantum chemical methods (AM1 or PM3) applied to the equilibrium molecular conformation in vacuo (T = 0 K) or molecular dynamic (MD) trajectories as obtained within fragmental or total MD simulations at T = 300 K. The results of these calculations are compared with the measured spectra of optical absorption. The quantum chemical analysis show that the dynamics of the methyl, trifluoromethyl, carboethoxy and tert-butyl groups practically does not influence the absorption spectra whereas the strongest their modifications are found to be related with dynamics of the aromatic group. The semiempirical method AM1 in combination with MD simulations gives for all dyes the best agreement between the calculated and measured spectral positions of absorption bands. In most cases the quantum chemical calculations describe properly the trends of their changes depending on the lateral substituent.     

BaO/W(1 0 0) thermionic emitters and the effects of Sc, Y, La, and the density functional used in computations

Density functional theory is used to predict workfunctions, φ. For relaxed clean W(1 0 0), the local density approximation (LDA) agrees with experiment better than the newer generalized gradient approximation, probably due to the surface electron self-energy. The large Ba metallic radius indicates it covers W(1 0 0) at about 0.5 monolayer (ML). However, Ba²⁺, O²⁻, and metallic W all have similar radii. Thus 1 ML of BaO (one BaO unit for each two W atoms) produces minimum strain, indicating commensurate interfaces. BaO (1 ML) and Ba (1/2 ML) have the sameφ to within 0.02 V, so at these coverages reduction or oxidation is not important. Due to greater chemical activity of ScO vs. highly ionic BaO, when mixing the latter with this suboxide of scandia, the overlayer always has BaO as the top layer and ScO as the second layer. The BaO/ScO bilayer has a rocksalt structure, suggesting high stability. In the series BaO/ScO/, BaO/YO/, and BaO/LaO/W(1 0 0), the latter has a remarkably low φ of 1.3 V (LDA), but 2 ML of rocksalt BaO also has φ at 1.3 V. We suggest BaO (1 ML) does not exist and that it is worthwhile to attempt the direct synthesis and study of BaO (2 ML) and BaO/LaO.     

Synthesis of magnetic and lightweight hollow microspheres/polyaniline/Fe3O4 composite in one-step method

After hollow microspheres (HM) were surface modified, a layer of electromagnetic polyaniline/Fe₃O₄ composite (PAN/Fe₃O₄) was successfully grafted onto the surface of the self-assembled monolayer coated HM, resulting in HM/PAN/Fe₃O₄ composites. In this approach, γ-aminopropyltriethoxy silane was adopted to form a well-coating monolayer with amino groups for the graft polymerization of aniline, which played an important role in fabricating the core-shell structure. FeCl₃ was used as the oxidant not only for aniline to form PAN, but also for FeCl₂ to prepare the magnets. The structure, morphologies, and magnetic properties of the as-prepared samples were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and vibrating sample magnetometer. The results indicated that the HM/PAN/Fe₃O₄ composites possess low density (ρ < 1.0 g/cm³), controllable morphology, and good magnetic properties at room temperature (saturation magnetization Ms = 8.32 emu g⁻¹ and coercive force Hc ≈ 0).     

Studies of the phosphorescence of polycrystalline hafnia

Persistent phosphorescence induced by ultraviolet light in polycrystalline HfO₂ and enhancement of the phosphorescence by sintering are investigated. The phosphorescence afterglow emission is in the 1.8-3.2 eV spectral range, with a peak at 2.53 eV. The afterglow intensity is significantly increased by sintering in either inert atmosphere or air. The afterglow light sum measured at room temperature for samples sintered at 1500 °C is more than an order of magnitude higher than that before sintering. In the temperature range −50 to 200 °C, three thermoluminescence (TL) peaks are observed near −10, 30, and 100 °C. The relative contribution of the low-temperature TL peak to the total TL intensity decreases after sintering, and this effect is more pronounced upon sintering in inert atmosphere. Conversely, the contribution of the TL peak near 100 °C increases after sintering. The enhancement of the afterglow by sintering is associated with the observed increase in the intensity of TL peaks at and above room temperature and attributed to an increase in the number of deep charge traps. The room-temperature afterglow time decay has a form consistent with the second-order mechanism, ∝(t₀+t)⁻ⁿ, and the best-fit values of both fitting parameters t₀ and n tend to increase with the sintering temperature.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Pattern guided structure formation in polymer films of asymmetric blends

Two off-critical blends of poly(2-vinylpyridine) and polystyrene, 2:3 and 3:2 (w:w) PVP:PS, were spin-cast (with varied domain scale R) onto periodically (λ = 4 μm) patterned substrate. The pattern consisted of two alternating symmetric stripes: Au attracting PVP and neutral self-assembled monolayer. The resulting droplet-type morphologies were recorded with Scanning Force Microscopy and examined with integral geometry approach. PVP-rich islands of the 2:3 PVP:PS films form, for a wide R/λ range, strongly anisotropic morphologies. They show up, for R/λ ∼ 0.5, a weak λ/2-substructure of smaller PVP droplets in addition to the domains periodic with λ. The 3:2 blend exhibits morphologies with dominant λ-structure of PVP ribbons, which encircle PS droplets. For R/λ ∼ 0.5, smaller PS domains are also present but no λ/2-substructure is formed. The |χ_E|-values of droplet surface density are reduced, as compared to homogeneous substrate, for the 3:2 blend (with |χ_E| → 0 for R ∼ λ). This effect is absent for the 2:3 mixture.     

Reaction of tert-butyl isocyanate and tert-butyl isothiocyanate at the Ge(100) − 2 × 1 Surface

The adsorption of tert-butyl isothiocyanate and tert-butyl isocyanate at the Ge(100) ? 2 × 1 surface was probed using multiple internal reflection Fourier transform infrared (FTIR) spectroscopy X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling. Results indicate that there are multiple surface products for each molecule. FTIR studies of tert-butyl isothiocyanate reveal adsorption through an S-dative bonded state, while XPS studies further suggest a reactive desorption product which leaves excess sulfur atoms at the surface. Studies of tert-butyl isocyanate indicate that the molecule dissociatively adsorbs at the surface, resulting in tert-butyl and germyl isocyanate groups, as the major pathway, in addition to forming several minor products, including a [2 + 2] cycloaddition product across the C=N bond. DFT was used to simulate vibrational spectra and map the reaction pathways, and confirms that the assigned products are energetically favorable.     

NMR of Xe on CO/Ir(1 1 1) and on multilayer Xe/Ir(1 1 1)

Nuclear magnetic resonance (NMR) is performed on monolayer (ML) amounts of adsorbed ¹²⁹Xe on a single crystal substrate. The inherently low sensitivity of NMR is overcome by using highly nuclear spin polarized ¹²⁹Xe that has been produced by optical pumping. A polarization of 0.8 is regularly achieved which is 10⁵ times the thermal (Boltzmann) polarization. The experiments are performed with a constant flux of xenon atoms impinging on the surface, typically 4 ML/s. The chemical shift (σ) of ¹²⁹Xe is highly sensitive to the Xe local environment. We measured profoundly different shifts for the Xe bulk, for the surface of the Xe bulk, and for Xe on CO/Ir(1 1 1). The growth of the bulk is seen in a phase transition like change of σ as a function of temperature at constant Xe flux. At temperatures where no bulk forms at a flux of 4 ML/s, the xenon exchange rate was measured by a spin inversion/recovery method. The exchange time of Xe is found to be 0.24 s at 63.4 K and 64.4 K and somewhat longer at 61.2 K. An analysis is given involving the desorption out of the second layer and fast mixing of first and second layer atoms at these temperatures.     

Nanostructuring of Fe films by oblique incidence deposition on a FeSi2 template onto Si(1 1 1): Growth, morphology, structure and faceting

The growth of thin Fe films deposited at oblique incidence on an iron silicide template onto Si(1 1 1) single crystal has been investigated as a function of Fe thickness (0 < t_Fe ? 180 monolayers (MLs)) and incidence angle (0 ? θ ? 80°). The growth mode is determined in situ by means of scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Stripes oriented perpendicularly to the incident atomic flux are formed for θ ? 30°. Self-correlation functions are used to extract characteristic lengths from STM images. The correlation lengths in the direction of the incident flux (ξ_x) and perpendicular to the atomic flux (ξ_y) grow with different powers versus time (ξ_x∝t^σ and ξ_y∝t^ρ, with σ = 0.34 ± 0.03 and ρ = 0.67 ± 0.03) following the exact solution of the (1 + 1) dimensional Kardar-Parisi-Zhang (KPZ) equation. The root mean square roughness follows also a scaling law for t_Fe < 120 ML leading to a growth exponent β = 0.73 ± 0.02. Shadowing and steering effects are discussed on the basis of our STM data.     

Thermal stability and switching field of hard/soft-stacked perpendicular media

The time dependence of remanence coercivity and thermal stability were investigated for hard/soft-stacked media consisting of a magnetically hard granular layer underneath a very thin soft layer with a large saturation magnetization, M_s. The values of remanence coercivity at measurement times t′=10³ and 10⁻⁵ s (pulse field) were measured, and defined as H_r and H_r^P. The remanence coercivity on the recording time scale, H_r (1 ns), and the energy barrier, ΔE/kT, were evaluated by fitting H_r and H_r^P to Sharrock's equation taking into account the power law variation of the energy barrier, n. The value of H_r (1 ns) for a (Co–Pt)–SiO₂ (9 nm)/Co–SiO₂ (2 nm) stacked medium with an interfacial coupling control layer was about 9 kOe, which was less than half of that of a (Co–Pt)–SiO₂ (9 nm) conventional medium (=21.3 kOe). The value of ΔE/kT for the stacked medium was about 111 (n=0.7), and was not significantly different from the conventional medium. Moreover, no significant difference in the rate of decrease of H_r with increasing temperature was observed between media with and without interlayers. These results indicate that the use of a thin soft layer with high M_s was effective at significantly reducing H_r with no notable change in thermal stability.     

Causal particle detectors and topology

We investigate particle detector responses in some topologically non-trivial spacetimes. We extend a recently proposed regularization of the massless scalar field Wightman function in four-dimensional Minkowski space to arbitrary dimension, to the massive scalar field, to quotients of Minkowski space under discrete isometry groups and to the massless Dirac field. We investigate in detail the transition rate of inertial and uniformly accelerated detectors on the quotient spaces under groups generated by (t, x, y, z) ? (t, x, y, z + 2a), (t, x, y, z) ? (t, −x, y, z), (t, x, y, z) ? (t, −x, −y, z), (t, x, y, z) ? (t, −x, −y, z + a) and some higher dimensional generalizations. For motions in at constant y and z on the latter three spaces the response is time dependent. We also discuss the response of static detectors on the RP³ geon and inertial detectors on RP³ de Sitter space via their associated global embedding Minkowski spaces (GEMS). The response on RP³ de Sitter space, found both directly and in its GEMS, provides support for the validity of applying the GEMS procedure to detector responses and to quotient spaces such as RP³ de Sitter space and the RP³ geon where the embedding spaces are Minkowski spaces with suitable identifications.     

Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films

H₂ interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes ΔΦ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10⁻³ Pa, increasing the work function. Under higher H₂ pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.     

Vortex structure of Gross-Pitaevskii model

The vortex line of the Gross-Pitaevskii model is studied. The kinetic helicity of the vortex is discussed, and vortex structure is classified by the Hopf index, linking number in geometry. A mechanism of generation and annihilation of vortex lines is given by the method of phase singularity theory. The dynamic behavior of the vortex at the critical points is discussed in detail, and three kinds of length approximation relations at the neighborhood of a critical point are given: l ∝ (t − t^∗)^1/2, l ∝ t − t^∗, l = const.     

Growth and thermal stability of ultra-thin Ag and Au layers on Mo(1 1 1) surface

The influence of temperature on the growth process of ultra-thin Ag and Au layers on the Mo(1 1 1) surface was investigated. At 300 K growth of the Stranski-Krastanov type was found for Ag; for Au growth of the monolayer plus simultaneous multilayers type was found, where a base layer is one physical layer. The first three geometrical adsorbed layers for Ag are thermally stable. For annealed Au layers triangle features with base side length from 15 to 35 Å were formed for θ < 6 monolayer (ML), and for θ > 6 ML part of the Au formed a flat adlayer with Au atoms grouped in equilateral triangles with side length 7 Å. The presence of Au layers does not cause faceting, layers are not smooth which could be caused by the fact that Au does not wets the substrate. For Ag thick layers reversible wetting/non-wetting transition was observed at 600 K. Ag layers on Mo(1 1 1) surface did not lead to faceting.     

Evaluating the surface density and heterogeneity of a dithiobis (succinimidylpropionate) self-assembled monolayer on gold and its coupling with DNA embedded within a matrix

The homogeneity of a self-assembled monolayer (SAM) on a surface is an important parameter which affects the ability of a SAM to fulfill its intended function. As an example, SAMs formed from octanethiols can form an impermeable surface, while SAMs based on a bifunctional coupling reagent can form a surface with uniform reactivity. Exposure of gold nanoparticles or gold surfaces to solutions of dithiobis (succinimidylpropionate) (DSP) gives rise to a surface which can react with DNA. Atomic force microscopy, UV-vis and gel electrophoresis experiments indicate that a self-assembled monolayer of DSP on gold nanoparticles can attenuate aggregation, inhibit the “lying down” of covalently-bound single-stranded (ss) DNA and promote more efficient hybridization. The determination of the point of aggregation after reacting DSP with colloidal gold yields 2.86 × 10⁻¹⁰ mol/cm² or 42% of the value determined from molecular modeling. Cyclic voltammetry experiments validate that DSP on a gold quartz crystal (6.3 × 10⁻¹⁰ mol/cm²) forms a fairly uniform SAM that is within 94% of maximum coverage when compared with results obtained from molecular modeling (6.67 × 10⁻¹⁰ mol/cm²). Surface plasmon resonance experiments indicate that the reaction of a DSP coated gold surface with (ss) DNA yields 2.4 × 10⁻¹² mol/cm² or reaction with about 1% of the available surface area. Subsequent reactions of the DSP surface with the filler, n-boc-1,4-phenylene diamine (n-boc), yield a total surface coverage of 1.8 × 10⁻¹¹ mol/cm². The surrounded (ss) DNA yields a surface with 97% hybridization efficiency toward the complement.