Shot-noise spectroscopy of energy resolved ballistic currents

We investigate the shot noise of nonequilibrium carriers injected into a ballistic conductor and interacting via long-range Coulomb forces. Coulomb interactions are shown to act as an energy analyzer of the profile of injected electrons by means of the fluctuations of the potential barrier at the emitter contact. We show that the details in the energy profile can be extracted from shot-noise measurements in the Coulomb interaction regime, but cannot be obtained from time-averaged quantities or shot-noise measurements in the absence of interactions.

Size-dependent surface plasmon resonance in silver silica nanocomposites

Silver silica nanocomposites were obtained by the sol–gel technique using tetraethyl orthosilicate (TEOS) and silver nitrate (AgNO3) as precursors. The silver nitrate concentration was varied for obtaining composites with different nanoparticle sizes. The structural and microstructural properties were determined by x-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopic (XPS) studies were done for determining the chemical states of silver in the silica matrix. For the lowest AgNO3 concentration, monodispersed and spherical Ag crystallites, with an average diameter of 5 nm, were obtained. Grain growth and an increase in size distribution was observed for higher concentrations. The occurrence of surface plasmon resonance (SPR) bands and their evolution in the size range 5–10 nm is studied. For decreasing nanoparticle size, a redshift and broadening of the plasmon-related absorption peak was observed. The observed redshift and broadening of the SPR band was explained using modified Mie scattering theory

A Low-Energy Ion Scattering (LEIS) Study of the Influence of the Vanadium Concentration on the Activity of Vanadium-Niobium Oxide Catalysts for the Oxidative Dehydrogenation of Propane

A series of vanadium-niobium oxide catalysts in which the vanadia content varies between 0.3 and 18mol%was prepared by coprecipitation. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and by catalytic testing in the oxidative dehydrogenation reaction of propane. The results of the surface analysis by XPS and LEIS are compared. It is concluded that the active site on the catalyst surface contains 2.0 ± 0.3 vanadium atoms on average. This can be understood byassuming the existenceof two or three different sites:isolated vanadium atoms, pairs of vanadium atoms, or ensembles of three vanadium atoms. At higher vanadium concentration more vanadium clusters with a higher activity are at the surface.LEIS revealed that as the vanadium concentration in the catalyst increases, vanadium replaces niobium at the surface. At vanadium concentrations above 8 mol%, new phases such as P-(Nb, V)20S which are less active because vanadium is present in isolated sites are formed, while the vanadium surface concentration shows a slight decrease

Evidence for the existence of multiple equilibrium states in cobalt phthalocyanine tetramer: a study by dielectric spectroscopy

In any investigation, information about the molecules under consideration is very essential for tailoring their properties. Evaluation of dispersion parameters, namely optical dielectric constant, static dielectric constant, relaxation time and spreading factor, assumes significance in this context. Dielectric spectroscopy is a useful tool for estimating these parameters. Not only does it reveal details about these constants but it also gives insight into the mechanism of conduction. In this paper the evaluation of dispersion parameters of cobalt phthalocyanine tetramer in the temperature range 300–393K is attempted using Cole–Cole plots. The temperature variation of the spreading factor indicates the existence of multiple equilibrium positions in the case of cobalt phthalocyanine tetramer. To the best of our knowledge, the evaluation of dispersion parameters for cobalt phthalocyanine tetramer is reported for the first time

Unusual surface and edge morphologies, sp2 to sp3 hybridized transformation and electronic damage after Ar+ ion irradiation of few-layer graphene surfaces

Roughness and defects induced on few-layer graphene (FLG) irradiated by Ar+ ions at different energies were investigated using X-ray photoemission spectroscopy (XPS) and atomic force microscopy techniques. The results provide direct experimental evidence of ripple formation, sp2 to sp3 hybridized carbon transformation, electronic damage, Ar+ implantation, unusual defects and edge reconstructions in FLG, which depend on the irradiation energy. In addition, shadowing effects similar to those found in oblique-angle growth of thin films were seen. Reliable quantification of the transition from the sp2-bonding to sp3-hybridized state as a result of Ar+ ion irradiation is achieved from the deconvolution of the XPS C (1s) peak. Although the ion irradiation effect is demonstrated through the shape of the derivative of the Auger transition C KVV spectra, we show that the D parameter values obtained from these spectra which are normally used in the literature fail to account for the sp2 to sp3 hybridization transition. In contrast to what is known, it is revealed that using ion irradiation at large FLG sample tilt angles can lead to edge reconstructions. Furthermore, FLG irradiation by low energy of 0.25 keV can be a plausible way of peeling graphene layers without the need of Joule heating reported previously

Structural, topographical and magnetic evolution of RF-sputtered Fe-Ni alloy based thin films with thermal annealing

Metglas 2826 MB having a nominal composition of Fe40Ni38Mo4B18 is an excellent soft magnetic material and finds application in sensors and memory heads. However, the thin-film forms of Fe40Ni38Mo4B18 are seldom studied, although they are important in micro-electro-mechanical systems/nano-electromechanical systems devices. The stoichiometry of the film plays a vital role in determining the structural and magnetic properties of Fe40Ni38Mo4B18 thin films: retaining the composition in thin films is a challenge. Thin films of 52 nm thickness were fabricated by RF sputtering technique on silicon substrate from a target of nominal composition of Fe40Ni38Mo4B18. The films were annealed at temperatures of 400 °C and 600 °C. The micro-structural studies of films using glancing x-ray diffractometer (GXRD) and transmission electron microscope (TEM) revealed that pristine films are crystalline with (FeNiMo)23B6 phase. Atomic force microscope (AFM) images were subjected to power spectral density analysis to understand the probable surface evolution mechanism during sputtering and annealing. X-ray photoelectron spectroscopy (XPS) was employed to determine the film composition. The sluggish growth of crystallites with annealing is attributed to the presence of molybdenum in the thin film. The observed changes in magnetic properties were correlated with annealing induced structural, compositional and morphological changes

Microwave exfoliated reduced graphene oxide epoxy nanocomposites for high performance applications

Graphene has captured the attention of scientific community due to recently emerging high performance applications. Hence, studying its reinforcing effects on epoxy resin is a significant step. In this study, microwave exfoliated reduced graphene oxide (MERGO) was prepared from natural graphite for subsequent fabrication of epoxy nanocomposites using triethylenetetramine (TETA) as a curing agent via insitu polymerization. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), C13 NMR spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultravioletevisible (UVevis) spectroscopy were employed to confirm the simultaneous reduction and exfoliation of graphene oxide. The reinforcing effect of MERGO on epoxy resin was explored by investigating its static mechanical properties and dynamic mechanical analysis (DMA) at MERGO loadings of 0 to 0.5 phr. The micro-structure of epoxy/MERGO nanocomposites was investigated using scanning electron microscope (SEM), transmission electron microscope (TEM) and XRD techniques. The present work reports an enhancement of 32%, 103% and 85% in tensile, impact and flexural strength respectively of epoxy by the addition of even 0.25 phr MERGO. At this loading elastic and flexural moduli also increased by 10% and 65%, respectively. Single-edge-notch three-point-Bending (SEN-TPB) fracture toughness (KIC) measurements were carried out where a 63% increase was observed by the introduction of 0.25 phr MERGO. The interfacial interactions brought about by graphene also benefited the dynamic mechanical properties to a large extent in the form of a significant enhancement in storage modulus and slightly improved glass transition temperature. Considerable improvements were also detected in dielectric properties. The epoxy nanocomposite also attained an ac conductivity of 10 5 S/m and a remarkable increase in dielectric constant. The simple and cost effective way of graphene synthesis for the fabrication of epoxy/MERGO nanocomposites may be extended to the preparation of other MERGO based polymer nanocomposites. This remarkable class of materials has thrown open enormous opportunities for developing conductive adhesives and in microelectronics

Synthesis, Characterization and Applications of Modified TiO2 Systems

This thesis is divided in to 9 chapters and deals with the modification of TiO2 for various applications include photocatalysis, thermal reaction, photovoltaics and non-linear optics. Chapter 1 involves a brief introduction of the topic of study. An introduction to the applications of modified titania systems in various fields are discussed concisely. Scope and objectives of the present work are also discussed in this chapter. Chapter 2 explains the strategy adopted for the synthesis of metal, nonmetal co-doped TiO2 systems. Hydrothermal technique was employed for the preparation of the co-doped TiO2 system, where Ti[OCH(CH3)2]4, urea and metal nitrates were used as the sources for TiO2, N and metals respectively. In all the co-doped systems, urea to Ti[OCH(CH3)2]4 was taken in a 1:1 molar ratio and varied the concentration of metals. Five different co-doped catalytic systems and for each catalysts, three versions were prepared by varying the concentration of metals. A brief explanation of physico-chemical techniques used for the characterization of the material was also presented in this chapter. This includes X-ray Diffraction (XRD), Raman Spectroscopy, FTIR analysis, Thermo Gravimetric Analysis, Energy Dispersive X-ray Analysis (EDX), Scanning Electron Microscopy(SEM), UV-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS), Transmission Electron Microscopy (TEM), BET Surface Area Measurements and X-ray Photoelectron Spectroscopy (XPS). Chapter 3 contains the results and discussion of characterization techniques used for analyzing the prepared systems. Characterization is an inevitable part of materials research. Determination of physico-chemical properties of the prepared materials using suitable characterization techniques is very crucial to find its exact field of application. It is clear from the XRD pattern that photocatalytically active anatase phase dominates in the calcined samples with peaks at 2θ values around 25.4°, 38°, 48.1°, 55.2° and 62.7° corresponding to (101), (004), (200), (211) and (204) crystal planes (JCPDS 21-1272) respectively. But in the case of Pr-N-Ti sample, a new peak was observed at 2θ = 30.8° corresponding to the (121) plane of the polymorph brookite. There are no visible peaks corresponding to dopants, which may be due to their low concentration or it is an indication of the better dispersion of impurities in the TiO2. Crystallite size of the sample was calculated from Scherrer equation byusing full width at half maximum (FWHM) of the (101) peak of the anatase phase. Crystallite size of all the co-doped TiO2 was found to be lower than that of bare TiO2 which indicates that the doping of metal ions having higher ionic radius into the lattice of TiO2 causes some lattice distortion which suppress the growth of TiO2 nanoparticles. The structural identity of the prepared system obtained from XRD pattern is further confirmed by Raman spectra measurements. Anatase has six Raman active modes. Band gap of the co-doped system was calculated using Kubelka-Munk equation and that was found to be lower than pure TiO2. Stability of the prepared systems was understood from thermo gravimetric analysis. FT-IR was performed to understand the functional groups as well as to study the surface changes occurred during modification. EDX was used to determine the impurities present in the system. The EDX spectra of all the co-doped samples show signals directly related to the dopants. Spectra of all the co-doped systems contain O and Ti as the main components with low concentrations of doped elements. Morphologies of the prepared systems were obtained from SEM and TEM analysis. Average particle size of the systems was drawn from histogram data. Electronic structures of the samples were identified perfectly from XPS measurements. Chapter 4 describes the photocatalytic degradation of herbicides Atrazine and Metolachlor using metal, non-metal co-doped titania systems. The percentage of degradation was analyzed by HPLC technique. Parameters such as effect of different catalysts, effect of time, effect of catalysts amount and reusability studies were discussed. Chapter 5 deals with the photo-oxidation of some anthracene derivatives by co-doped catalytic systems. These anthracene derivatives come underthe category of polycyclic aromatic hydrocarbons (PAH). Due to the presence of stable benzene rings, most of the PAH show strong inhibition towards biological degradation and the common methods employed for their removal. According to environmental protection agency, most of the PAH are highly toxic in nature. TiO2 photochemistry has been extensively investigated as a method for the catalytic conversion of such organic compounds, highlighting the potential of thereof in the green chemistry. There are actually two methods for the removal of pollutants from the ecosystem. Complete mineralization is the one way to remove pollutants. Conversion of toxic compounds to another compound having toxicity less than the initial starting compound is the second way. Here in this chapter, we are concentrating on the second aspect. The catalysts used were Gd(1wt%)-N-Ti, Pd(1wt%)-N-Ti and Ag(1wt%)-N-Ti. Here we were very successfully converted all the PAH to anthraquinone, a compound having diverse applications in industrial as well as medical fields. Substitution of 10th position of desired PAH by phenyl ring reduces the feasibility of photo reaction and produced 9-hydroxy 9-phenyl anthrone (9H9PA) as an intermediate species. The products were separated and purified by column chromatography using 70:30 hexane/DCM mixtures as the mobile phase and the resultant products were characterized thoroughly by 1H NMR, IR spectroscopy and GCMS analysis. Chapter 6 elucidates the heterogeneous Suzuki coupling reaction by Cu/Pd bimetallic supported on TiO2. Sol-Gel followed by impregnation method was adopted for the synthesis of Cu/Pd-TiO2. The prepared system was characterized by XRD, TG-DTG, SEM, EDX, BET Surface area and XPS. The product was separated and purified by column chromatography using hexane as the mobile phase. Maximum isolated yield of biphenyl of around72% was obtained in DMF using Cu(2wt%)-Pd(4wt%)-Ti as the catalyst. In this reaction, effective solvent, base and catalyst were found to be DMF, K2CO3 and Cu(2wt%)-Pd(4wt%)-Ti respectively. Chapter 7 gives an idea about the photovoltaic (PV) applications of TiO2 based thin films. Due to energy crisis, the whole world is looking for a new sustainable energy source. Harnessing solar energy is one of the most promising ways to tackle this issue. The present dominant photovoltaic (PV) technologies are based on inorganic materials. But the high material, low power conversion efficiency and manufacturing cost limits its popularization. A lot of research has been conducted towards the development of low-cost PV technologies, of which organic photovoltaic (OPV) devices are one of the promising. Here two TiO2 thin films having different thickness were prepared by spin coating technique. The prepared films were characterized by XRD, AFM and conductivity measurements. The thickness of the films was measured by Stylus Profiler. This chapter mainly concentrated on the fabrication of an inverted hetero junction solar cell using conducting polymer MEH-PPV as photo active layer. Here TiO2 was used as the electron transport layer. Thin films of MEH-PPV were also prepared using spin coating technique. Two fullerene derivatives such as PCBM and ICBA were introduced into the device in order to improve the power conversion efficiency. Effective charge transfer between the conducting polymer and ICBA were understood from fluorescence quenching studies. The fabricated Inverted hetero junction exhibited maximum power conversion efficiency of 0.22% with ICBA as the acceptor molecule. Chapter 8 narrates the third order order nonlinear optical properties of bare and noble metal modified TiO2 thin films. Thin films were fabricatedby spray pyrolysis technique. Sol-Gel derived Ti[OCH(CH3)2]4 in CH3CH2OH/CH3COOH was used as the precursor for TiO2. The precursors used for Au, Ag and Pd were the aqueous solutions of HAuCl4, AgNO3 and Pd(NO3)2 respectively. The prepared films were characterized by XRD, SEM and EDX. The nonlinear optical properties of the prepared materials were investigated by Z-Scan technique comprising of Nd-YAG laser (532 nm,7 ns and10 Hz). The non-linear coefficients were obtained by fitting the experimental Z-Scan plot with the theoretical plots. Nonlinear absorption is a phenomenon defined as a nonlinear change (increase or decrease) in absorption with increasing of intensity. This can be mainly divided into two types: saturable absorption (SA) and reverse saturable absorption (RSA). Depending on the pump intensity and on the absorption cross- section at the excitation wavelength, most molecules show non- linear absorption. With increasing intensity, if the excited states show saturation owing to their long lifetimes, the transmission will show SA characteristics. Here absorption decreases with increase of intensity. If, however, the excited state has strong absorption compared with that of the ground state, the transmission will show RSA characteristics. Here in our work most of the materials show SA behavior and some materials exhibited RSA behavior. Both these properties purely depend on the nature of the materials and alignment of energy states within them. Both these SA and RSA have got immense applications in electronic devices. The important results obtained from various studies are presented in chapter 9.

Femtosecond imaging-mode laser-induced breakdown spectroscopy

Many nonlinear optical microscopy techniques based on the high-intensity nonlinear phenomena were developed recent years. A new technique based on the minimal-invasive in-situ analysis of the specific bound elements in biological samples is described in the present work. The imaging-mode Laser-Induced Breakdown Spectroscopy (LIBS) is proposed as a combination of LIBS, femtosecond laser material processing and microscopy. The Calcium distribution in the peripheral cell wall of the sunflower seedling (Helianthus Annuus L.) stem is studied as a first application of the imaging-mode LIBS. At first, several nonlinear optical microscopy techniques are overviewed. The spatial resolution of the imaging-mode LIBS microscope is discussed basing on the Point-Spread Function (PSF) concept. The primary processes of the Laser-Induced Breakdown (LIB) are overviewed. We consider ionization, breakdown, plasma formation and ablation processes. Water with defined Calcium salt concentration is used as a model of the biological object in the preliminary experiments. The transient LIB spectra are measured and analysed for both nanosecond and femtosecond laser excitation. The experiment on the local Calcium concentration measurements in the peripheral cell wall of the sunflower seedling stem employing nanosecond LIBS shows, that nanosecond laser is not a suitable excitation source for the biological applications. In case of the nanosecond laser the ablation craters have random shape and depth over 20 µm. The analysis of the femtosecond laser ablation craters shows the reproducible circle form. At 3.5 µJ laser pulse energy the diameter of the crater is 4 µm and depth 140 nm for single laser pulse, which results in 1 femtoliter analytical volume. The experimental result of the 2 dimensional and surface sectioning of the bound Calcium concentrations is presented in the work.

Photoschaltbare Azobenzol-modifizierte selbstorganisierte Monoschichten auf Gold(111)-Oberflächen

Für die Entwicklung photoschaltbarer selbstorganisierter Monoschichten (SAMs) auf Gold(111)-Oberflächen wurden neue Azobenzol-terminierte Asparagussäure - und Liponsäurederivate synthetisiert. Um den Einfluss lateraler Wasserstoffbrückenbindungen auf Qualität und Orientierungsordnung der Schichten zu untersuchen, wurden Monolagen, die durch amid- und esterverknüpfte Verbindungen gebildet wurden, miteinander verglichen. Die Filmbildung aus der Lösung wurde in situ durch optische Frequenzverdopplung (SHG) untersucht und die Photoreaktivität mittels Kontaktwinkelmessungen, Oberflächen-Plasmonenresonanz (SPR) und Ellipsometrie verfolgt. SAMs auf Gold wurden außerdem mit Hilfe von Röntgenphotoelektronenspektroskopie (XPS), Nahkanten-Reflexions-Röntgenabsorptionsspektroskopie (NEXAFS) und Infrarot-Reflexionsabsorptionsspektroskopie (IRRAS) charakterisiert, um die Filmqualität, die Bindung ans Substrat und Orientierungsordnung im Film zu ermitteln. Da die Chemisorption auf polykristallinem Gold formal der Koordinationschemie von 1,2-Dithiolan-Derivaten gegenüber nullwertigen Edelmetall-Zentralatomen entspricht, wurden etliche Pt-Komplexe durch oxidative Addition an [Pt(PPh3)4] dargestellt. Im Zusammenhang mit der Darstellung der Asparagussäure wurde die Kristallstruktur von [pipH]2[WS4] und der neuen Verbindungen [pipH]3[WS4](HS) und [pipH]4[WS4][WOS3] (pip = Piperidin) bestimmt. Wasserstoffbrückenbindungen zwischen den Piperidinium-Kationen und den Thiowolframat-Anionen spielen eine dominante strukturelle Rolle.

Absolute Kr 4s-electron photoionization cross sections between 30 and 90 eV measured by photon-induced fluorescence spectroscopy (PIFS)

Absolute Kr 4s-electron photoionization cross sections as a function of the exciting-photon energy between 30 and 90 eV were measured by photon-induced fluorescence spectroscopy (PIFS). The measurements were compared with available experimental data and theoretical calculations.

Dipodal Ferrocene Derivatives for Nanostructured Redox-Functionalised Surfaces

Self-assembled monolayers (SAMs) on solid surfaces are of great current interest in science and nanotechnology. This thesis describes the preparation of several symmetrically 1,1’-substituted ferrocene derivatives that contain anchoring groups suitable for chemisorption on gold and may give rise to SAMs with electrochemically switchable properties. The binding groups are isocyano (-NC), isothiocyanato (-NCS), phosphanyl (-PPh2), thioether (-SR) and thienyl. In the context of SAM fabrication, isothiocyanates and phosphanes are adsorbate systems which, surprisingly, have remained essentially unexplored. SAMs on gold have been fabricated with the adsorbates from solution and investigated primarily by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The results of these analytical investigations are presented and discussed in matters of the film quality and possible binding modes. The quality of self-assembled monolayers fabricated from 1,1’-diisocyanoferrocene and 1,1’-diisothiocyanatoferrocene turned out to be superior to that of films based on the other adsorbate species investigated. Films of those absorbates as well as of dppf afforded well-defined SAMs of good quality. All other films of this study based on sulfur containing anchoring groups exhibit chemical inhomogeneity and low orientational order of the film constituents and therefore failed to give rise to well-defined SAMs. Surface coordination chemistry is naturally related to molecular coordination chemistry. Since all SAMs described in this thesis were prepared on gold (111) surfaces, the ferrocene-based ligands of this study have been investigated in their ability for complexation towards gold(I). The sulfur-based ferrocene ligands [fc(SR)2] failed to give stable gold(I) complexes. In contrast, 1,1’-diisocyanoferrocene (1) proved to be an excellent ligand for the complexation of gold(I). Several complexes were prepared and characterised utilising a series of gold(I) acetylides. These complexes show interesting structural motifs in the solid state, since intramolecular aurophilic interactions lead to a parallel orientation of the isocyano moieties, combined with an antiparallel alignment of neighbouring units. The reaction of 1 with the gold(I) acetylide [Au(C≡C–Fc)]n turned out to be very unusual, since the two chemically equivalent isocyano groups undergo a different reaction. One group shows an ordinary coordination and the other one undergoes an extraordinary 1,1-insertion into the Au-C bond. As a sideline of the research of this thesis several ferrocene derivatives have been tested for their suitability for potential surface reactions. Copper(I) mediated 1,3-dipolar cycloadditions of azidoferrocene derivatives with terminal alkynes appeared very promising in this context, but failed to a certain extent in terms of ‘click’ chemistry, since the formation of the triazoles depended on the strict exclusion of oxygen and moisture and yields were only moderate. Staudinger reactions between dppf and azidoferrocene derivatives were also tested. The nucleophilic additions of secondary amines to 1,1’-diisothiocyanatoferrocene led to the respective thiourea derivatives in quantitative yields.

Auger spectroscopy of foil-excited beryllium ions

We have measured prompt and delayed emission spectra of electrons from foilexcited Be, B^+, and Be^2+ ions at 300 keV. On the basis of recently calculated eigenvalues we identified two lines in the prompt Be^+ spectrum as transitions from 2s^22p and 2s2p^2. The delayed Be spectrum indicates that transitions from highly excited quintet states occur. We propose radiationless deexcitation with one excited spectator electron not involved in the transition.