Synthesis and surface plasmon resonance of Au–ZnO Janus nanostructures

Metal–semiconductor Janus nanostructures with asymmetry and directionality have recently aroused significant interest, both in fundamental light–matter interactions mechanism and in technological applications. Here we report the synthesis of different Au–ZnO Janus nanostructures via a facile one-pot colloid method. The growth mechanism is revealed by a series of designed synthesis experiments. The light absorption properties are determined by both the decrease of dipole oscillations of the free electrons and the plasmon-induced hot-electron transfer. Moreover, the finite-difference time-domain(FDTD) simulation method is used to elucidate the electric field distributions of these Janus nanostructures.     

Fluorescence spectroscopic study to characterize and monitor TEOS based sol–gel process for development of optical biosensors

Development of optical biosensors is an active area of research in the field of medical technology. Sol–gel matrices made from alkoxide silicates, tetraethyl orthosilicate (TEOS) appear to be suitable glassy host matrix for the sensing system. However, the major problem in the TEOS based sol–gel matrices is stability. So it is important to study dopant–matrix interaction as a function of time. In the present study, we report fluorescence emission and excited state lifetime measurements on fluorescent probes entrapped in TEOS sol–gel for monitoring the physico-chemical processes for characterization and monitoring of local environment (pores) of dopant molecule (fluorescent probes) for construction of sensing layer for optical transducer. Different types of fluorescent probes viz., Hoechst 33258 (H258) and pyranine (PY) were used. Sol–gels containing these probes were prepared at pH=6.0 and the physical and spectroscopic parameters were monitored as a function of storage time (days). The emission intensity from entrapped H258 has shown relatively higher extent of decrease during aging. The excited state fluorescence lifetime measurements on these probes depicted single exponential decay component at 5.4 ns (PY) and 3.6 ns (H258) in fresh sol–gels. After a few days of storage the sol–gel containing H258 revealed an additional short decay component whereas no such alteration could be observed with the probe molecule PY. Further confirmation of multicomponents decay was obtained by distribution analysis of lifetime of H258 where an increase in width of mean lifetime was observed with storage whereas no such change was indicated from PY. Thus it appears that H258 is a better probe molecule for characterizing and monitoring local environment of pores in sol–gel.     

Atomic composition and stability of Langmuir–Blodgett monolayers based on siloxane dimer of quaterthiophene on the surface of polycrystalline gold

Physics of the Solid State - Atomic composition of monolayers based on siloxane dimer of quaterthiophene deposited by Langmuir–Blodgett technique on a silicon dioxide surface partially...     

Comparative PEEM and AES study of surface morphology and composition of Cu films on Si and 3C–SiC substrates

We have conducted photoemission electron microscope (PEEM) and Auger electron spectroscopy (AES) studies on the Cu(30 nm)/3C–SiC(1 0 0) and Cu(30 nm)/Si(1 0 0) samples annealed successively up to 850 °C. With PEEM, lateral diffusion of Cu atoms on the 3C–SiC substrate was observed at 400 °C while no lateral diffusion was seen for the Cu/Si(1 0 0) samples up to 850 °C. The 30 nm Cu thin film on 3C–SiC began to agglomerate at 550 °C, similar to the case for the Cu/Si(1 0 0) system. No further spread of the lateral diffusion region was found in subsequent annealing up to 850 °C for Cu/3C–SiC while separated regular-sized dot structures were found at 850 °C for Cu/Si(1 0 0). AES studies of Cu/Si(1 0 0) system showed partial interface reaction during Cu deposition onto the Si(1 0 0) substrate and oxidation of the resultant Cu₃Si to form SiO₂ on the specimen surface at room temperature in air. Surface segregation of Si and C was observed after annealing at 300 °C for Cu/Si(1 0 0) and at 850 °C for the Cu/3C–SiC system. We have successfully elucidated the observed phenomena by combining PEEM and AES considering diffusion of the constituent elements and/or reaction at interfaces.     

An experimental and kinetic modeling study of the auto-ignition of natural gas blends containing C1–C7 alkanes

Ignition delay time measurements for multi-component natural gas mixtures were carried out using a rapid compression machine at conditions relevant to gas turbine operation, at equivalence ratios of 0.5–2.0 in ‘air’ in the temperature range 650–1050 K, at pressures of 10–30 bar. Natural gas mixtures comprising C₁–C₇ n-alkanes with methane as the major component (volume fraction: 0.35–0.98) were considered. A design of experiments was employed to minimize the number of experiments needed to cover the wide range of pressures, temperatures and equivalence ratios. The new experimental data, together with available literature data, were used to develop and assess a comprehensive chemical kinetic model. Replacing 1.875% methane with 1.25% n-hexane and 0.625% n-heptane in a mixture containing C₁–C₅ components leads to a significant increase in a mixture's reactivity. The mixtures containing heavier hydrocarbons also tend to show a strong negative temperature coefficient and two-stage ignition behavior. Sensitivity analyses of the C₁–C₇ blends have been performed to highlight the key reactions controlling their ignition behavior.     

A comparative study of conventional type II and inverted core–shell nanostructures based on CdSe and ZnS

The objective of this work is to investigate structural, morphological and optical properties of conventional CdSe/ZnS core–shell and inverted ZnS/CdSe core–shell nanostructures for opto-electronic device applications. For this purpose both nanostructures were synthesized using chemical bath deposition technique in thin film form. The structural properties were studied using X-ray diffraction technique with Rietveld refinement and transmission electron microscopy (TEM). The surface morphology of synthesized thin film was illustrated in the form 2D and 3D images using atomic force microscopy (AFM). The optical properties were explained using UV–Vis absorption spectroscopy and photo luminescence (PL) spectroscopy in in situ monitoring process. A comparison of estimated particle size from XRD, high resolution AFM and TEM images was resulted in good agreement as 2.1, 2.4 and 2.1 nm respectively for conventional CdSe/ZnS core–shell and as 2.5, 2.5 and 2.2 nm respectively for inverted ZnS/CdSe core–shell nanostructures.     

An experimental and quantum mechanical study on luminescence properties of SM(β-Nbm)3·(Pd)

A new ternary samarium complex Sm(β-NBM)₃·(PD) has been synthesized by the reaction of SmCl₃·⁶H₂O with β-naphthoylbenzoylmethane (β-HNBM) and 1,10-phenanthroline-5,6-dione (PD) in stoichiometry. The new samarium complex obtained was characterized by elemental analysis, ¹H NMR, and FT-IR spectroscopy. The absorption and emission of this complex were systematically investigated. Photoluminescence studies indicated that the energy absorbed by the organic ligands was efficiently transferred to the central Sm³⁺ ions and the complex showed intensely and characteristically orange emission due to the ⁴G_5/2→⁶H_j transitions of the central Sm³⁺ ions. The energy levels (HOMO and LUMO) of the ligands and the complex were further confirmed by computer simulation and cyclic voltammetry, respectively. All the results suggested that the synthesized Sm(β-NBM)₃·(PD) would have the potential application for organic light-emitting diodes.     

Molecular dynamic simulation of core–shell structure: study of the interaction between modified surface of nano-SiO2 and PAMAA in vacuum and aqueous solution

The interaction between acrylamide acrylicacid copolymer (PAMAA) and the modified surface of nano-SiO₂ is investigated using the molecular dynamic (MD) simulation. The binding energies (E_binding) of interface, the concentration profiles of PAMAA and functional groups (carboxyl and acylamino) of corresponding model, the mean square displacements (MSD) and diffusion coefficients (D) of PAMAA in four systems with different modifiers are all calculated at 325 K in vacuum. Vinyl trimethoxy silane (VTEOS) shows best modification effect in the systems mentioned above. Furthermore, the effects of temperature on the interaction between VTEOS modified surface of nano-SiO₂ and PAMAA are studied at 300, 325, 350, 375 and 400 K in aqueous solution. Interesting results show that, water molecular layer reduces with the increase of temperature, and then improves the interaction between PAMAA and VTEOS modified surface of nano-SiO₂. The corresponding E_binding of interface, the radial distribution functions (RDF) of carbon atoms on the surface and oxygen atoms of water molecules, the concentration profiles of PAMAA on the surface of nano-SiO₂, the MSD and D of PAMAA are all studied seriously to find the reason of this counterintuitive phenomenon.     

Effects of pressure and fuel dilution on coflow laminar methane–air diffusion flames: A computational and experimental study

In this study, the influence of pressure and fuel dilution on the structure and geometry of coflow laminar methane–air diffusion flames is examined. A series of methane-fuelled, nitrogen-diluted flames has been investigated both computationally and experimentally, with pressure ranging from 1.0 to 2.7 atm and CH₄ mole fraction ranging from 0.50 to 0.65. Computationally, the MC-Smooth vorticity–velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modelled by sectional aerosol equations. The governing equations and boundary conditions were discretised on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, chemiluminescence measurements of CH* were taken to determine its relative concentration profile and the structure of the flame front. A thin-filament ratio pyrometry method using a colour digital camera was employed to determine the temperature profiles of the non-sooty, atmospheric pressure flames, while soot volume fraction was quantified, after evaluation of soot temperature, through an absolute light calibration using a thermocouple. For a broad spectrum of flames in atmospheric and elevated pressures, the computed and measured flame quantities were examined to characterise the influence of pressure and fuel dilution, and the major conclusions were as follows: (1) maximum temperature increases with increasing pressure or CH₄ concentration; (2) lift-off height decreases significantly with increasing pressure, modified flame length is roughly independent of pressure, and flame radius decreases with pressure approximately as P^?1/2; and (3) pressure and fuel stream dilution significantly affect the spatial distribution and the peak value of the soot volume fraction.     

Bistatic electromagnetic scattering from a three-dimensional perfect electric conducting object above a Gaussian rough surface based on the Kirchhoff–Helmholtz and electric field integral equation

A hybrid integral equation is developed to solve the problem of electromagnetic (EM) scattering from a three-dimensional (3D) perfect electric conducting (PEC) object above a two-dimensional (2D) PEC or dielectric Gaussian rough surface. Firstly, the Kirchhoff–Helmholtz (KH) equation is adopted to describe the wave reflection on the rough surface; only one integral operation on the rough surface is needed, and the scattering from the object can be described by solving the electric field integral equation (EFIE) on the surface of the object. Moreover, according to scattering theory, the KH equation and the EFIE are coupled together (KH-EFIE) to describe wave propagation between the object and the rough surface. Then method of moments (MoM) is adopted to solve the KH-EFIE, and the current is obtained to calculate the scattering field. Finally, compared with other methods, the accuracy of the proposed approach is validated, and its efficiency is proved to be much higher than numerical solutions. Furthermore, by calculating the statistic composite radar cross-section (RCS) of the object/surface and the difference radar cross-section (DRCS) of the object, the influence of the rough surface root mean square (rms) height, the correlation length, the medium permittivity, the shape of the object, and the altitude of the object on the scattering characteristic is investigated.     

Development of the device prototype based on the semiconductor–carbon nanotubes structure for optical radiation detection and study of its parameters

A light-receiving device prototype based on the semiconductor–carbon nanotubes (CNTs) structure consisting of 16 cellular structured sensitive elements grown on the same substrate is developed. The topology of sensitive cells represents holes through metallization and insulator layers to the semiconductor from which the CNT array grows to the top metallization layer. The device prototype parameters are determined as follows: the effective wavelength range is within 400–1100 nm, the operational speed is no longer than 30 μs, the coefficients of peak sensitivity reached at wavelengths of 640 and 950 nm are 197 and 193 μA/W, respectively.     

PBE–DFT theoretical study of organic photovoltaic materials based on thiophene with 1D and 2D periodic boundary conditions

Conjugated organic systems such as thiophene are interesting topics in the field of organic solar cells. We theoretically investigate π-conjugated polymers constituted by n units (n = 1–11) based on the thiophene (Tn) molecule. The computations of the geometries and electronic structures of these compounds are performed using the density functional theory (DFT) at the 6–31 G(d, p) level of theory and the Perdew–Burke–Eenzerhof (PBE) formulation of the generalized gradient approximation with periodic boundary conditions (PBCs) in one (1D) and two (2D) dimensions. Moreover, the electronic properties (HOCO, LUCO, E _gap, V _oc, and V _bi) are determined from 1D and 2D PBC to understand the effect of the number of rings in polythiophene. The absorption properties—excitation energies (E _ex), the maximal absorption wavelength (λ_max), oscillator strengths, and light harvesting—efficiency are studied using the time-dependent DFT method. Our studies show that changing the number of thiophene units can effectively modulate the electronic and optical properties. On the other hand, our work demonstrates the efficiency of theoretical calculation in the PBCs.     

Monte Carlo simulated annealing study of mixed Si–Ge and C–C dimer adsorption on a Si (001) 2×1 surface

Energetics and structural relaxations related to the surface complexes formed by mixed Si–Ge and C–C dimer adsorption on predefined adsorption sites on a (2×1) reconstructed Si (001) surface are investigated. Monte Carlo simulated annealing procedure is used in conjugation with Tersoff's semi-empirical potentials. The reliability check of the method is performed by comparing our results for the case of Si–Ge dimer adsorption with the results reported by using ab initio pseudo-potential calculations. The agreement is found to be good. For carbon dimer adsorption, the nucleation centers are found to be different from those for Si and Ge. It is seen that carbon has a tendency to get adsorbed at the dangling bond site, or to form a Si–C–C–Si chain like structure under specific conditions.     

CCSD(T) study on the structures and chemical bonds of AnO molecules (An = Bk–Lr)

The molecular geometries and dissociation energies of AnO (An = Bk–Lr) molecules were first obtained at thecoupled-cluster single-, double-, and perturbative triple-excitations [CCSD(T)] level of theory. Four hybrid functionals,B3LYP, M06-2X, TPSSh, and PBE0, were also employed in the calculations for the sake of comparison. In comparison ofthe CCSD(T) results, B3LYP, TPSSh, and PBE0 functionals can obtain more appropriate results than M06-2X and MP2.The analyses on molecular orbitals show that the 7s, 6d, and 5f atomic orbitals of actinide (An) atoms participate in thebonding of An–O bonds. The partial covalent nature between An and O atoms is revealed by QTAIM analyses.     

Characterization of Rh,Mo and Rh–Mo particles formed on TiO2(110) surface: A TDS,AES and RAIRS study

The structural and chemical characterization of Rh, Mo and Rh–Mo nanosized clusters formed by physical vapor deposition on TiO₂ single crystal was performed by Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS) and Reflection Absorption Infrared Spectroscopy (RAIRS), applying CO as test molecule. On a slightly reduced titania surface 2D-like growth of Rh was revealed at 300 K up to 0.23 ML coverage by AES and CO-desorption experiments. For CO-saturated Rh particles TDS showed molecular CO desorption in a broad temperature range with T_p = 400, 440, 490 and 540 K (α-states), the latter state appearing only on the smallest Rh particles. The population of γ-state (T_p = 780–820 K) originating from the recombination of C and O atoms on the support began at Θ_Rh = 0.23ML and was maximized at around 1–2 ML Rh coverage, corresponding to 30% dissociation of CO. A possible dissociation precursor on Rh particles is identified as linearly bonded CO on step sites characterized by ν(C–O) of 2017 cm^? 1. Deliberation of CO₂ could not be detected between 170 and 900 K, showing the absence of disproportionation reaction. Instead of oxidizing CO molecules, oxygen atoms stemming from the dissociation of CO attached to the reduced centers of titania, indicating the role of adsorption sites at the perimeter of Rh particles in the decomposition process. 2 ML of predeposited Mo enhanced markedly the dispersion of Rh particles as a result of strong Rh–Mo interaction, but it slightly reduced the molecular α-CO desorption possibly due to enhanced dissociation. The formation of γ-CO was suppressed considerably through elimination of adsorption centers by Mo on the TiO₂ substrate. The reactivity of Rh layers deposited on Mo-covered surface towards CO was reduced after repeated annealing to 600 K due to partial encapsulation of Rh by titania, manifesting in the suppression of the more strongly bonded α-state. Mo-deposits (up to 0.5ML) on Rh particles decreased the saturation coverage of α-CO through a site-blocking mechanism without detectable influence on the binding energy of CO to Rh, indicating Mo island formation. The carbon arising from the decomposition of CO dissolved in the Mo-containing particles formed a solid solution stable even at 900 K, suggesting a possible role of molybdenum carbide regarding the enhanced catalytic activity of Rh clusters.     

Adsorption of γ-mercaptopropyltrimethoxysilane on zinc: A study of the competition between thiol and silanol functions related to the age of the siloxane solution,its pH and the oxidation state of the surface

We describe the adsorption of γ–mercaptopropyltrimethoxysilane (γ-MPS) on zinc under various experimental conditions, including the age of the siloxane solution (t_ag), its pH (7 or 4), and the mode of preparation of the surface (RCA treatment or in situ polishing). It is shown by XPS studies that the structure of the adsorbed monolayer varies dramatically with the pH of the solution. At the natural pH of the siloxane solution (pH 7) where no hydrolysis of the SiOCH₃ group occurs, adsorption proceeds through the SH moiety and not through SiOCH₃ groups. This preferential attachment through SH is found whatever the age of the solution and the treatment of the zinc. It is confirmed by the fact that n-propyltrimethoxysilane (PSi) does not interact with the surface in the case of very old solutions (adsorption is not observed when Zn is polished in situ and only occurs with RCA zinc treatment for t_ag > 40 min). With siloxane solutions at pH 4, adsorption of γ-MPS is more complex and the structure of the adsorbed layer depends mainly on the age of the solution. With a fresh solution, hydrolysis is not very advanced and, as mentioned previously, adsorption occurs through the SH group. With older solutions and as a consequence of the progressive hydrolysis of the SiOCH₃ group to SiOH, the density of the grafted siloxane monolayer increases (6 min < t_ag < 10 min), followed by a mixed adsorption through SH and SiOH (10 min < t_ag < 40–50 min) revealed by the decrease in the normalised (Si2p/S2p)* intensity ratio. Finally, adsorption of dimers and oligomers is observed with still older siloxane solutions. In contrast to PSi whose adsorption on zinc is favoured by the RCA treatment, neither treatment of the surface changes the results significantly in the case of γ-MPS. Comparison with alkanethiols confirms the transition from monomer to dimer adsorption and IRRAS studies clearly indicate a condensation reaction between OH and SH groups.     

Dynamic phase transitions and dynamic phase diagrams of the Blume–Emery–Griffiths model in an oscillating field: the effective-field theory based on the Glauber-type stochastic dynamics

Using the effective-field theory based on the Glauber-type stochastic dynamics (DEFT), we investigate dynamic phase transitions and dynamic phase diagrams of the Blume–Emery–Griffiths model under an oscillating magnetic field. We presented the dynamic phase diagrams in (T/J, h₀/J), (D/J, T/J) and (K/J, T/J) planes, where T, h₀, D, K and z are the temperature, magnetic field amplitude, crystal–field interaction, biquadratic interaction and the coordination number. The dynamic phase diagrams exhibit several ordered phases, coexistence phase regions and special critical points, as well as re-entrant behavior depending on interaction parameters. We also compare and discuss the results with the results of the same system within the mean-field theory based on the Glauber-type stochastic dynamics and find that some of the dynamic first-order phase lines and special dynamic critical points disappeared in the DEFT calculation.