Mariana Sendova

LASER-INDUCED SUB-HALF-MICROMETER PERIODIC STRUCTURE ON POLYMER SURFACES

Periodic structures of sub‐half‐micrometer size are produced on the surface of different polymers [poly(ethylene terephthalate)], polyimide‐type Kapton (DuPont), and polyimide‐type XU 293 (Ciba‐Geigy)] by Nd:YAG laser irradiation at 266 nm wavelength. The obtained relief is observed by scanning electron microscopy characterized by x‐ray photoelectron spectroscopy, and an atomic force microscopy. A phenomenological explanation for the ripple formation in terms of generating waveguide modes on the polymer surface is proposed.

Sub-Half-Micron Periodic Structures on Polymer Surfaces with Polarized Laser Irradiation

A quarter-micron space-and-width linear periodic structures and an array of dot images are obtained with the Nd:YAG 4th harmonic laser irradiation on Kapton polyimide films, poly(ethylene terephthalate) films, spin-coated polyimide films and others. Our solid state Nd:YAG laser source is different from the excimer laser irradiation, in that it provides polarized beams without the need for a polarizer. Atomic force microscope (AFM) and scanning electron microscope (SEM) studies have been carried out. X-ray photoelectron spectroscopy (XPS) studies of laser-exposed areas indicate that no significant chemical reactions took place on exposed areas.

Real-Time Monitoring of Plasmonic Evolution in Thick Ag:SiO2 Films: Nanocomposite Optical Tuning

An in situ optical microspectroscopy study of the surface plasmon resonance (SPR) evolution of Ag nanoparticles (NPs) embedded in thick SiO(2) films deposited on soda-lime glass has been conducted during thermal processing in air. The temperature and time dependences of the SPR were analyzed in the context of Mie extinction and crystal growth theories and were discussed along with consideration of oxidation processes and film/substrate physicochemical interactions. At relatively high temperatures, Ag NPs were indicated to grow first through a diffusion-based process and subsequently via Ostwald ripening. At lower temperatures, an initial decrease in Ag particle size was indicated due to oxidation, followed by NP diffusion-based growth. The growth and oxidation stages appeared temperature and time dependent, allowing for the tuning of material properties. The product of Ag NP oxidation was revealed by photoluminescence spectroscopy performed ex situ as single Ag(+) ions. The oxidative effect of the air atmosphere on Ag NPs was shown to be ultimately circumvented by the thick nanocomposite film. The phenomenon was explained on the basis of the displacement of the Ag/Ag(+) redox equilibrium toward Ag NP stability after ion migration toward the substrate being self-constrained. In addition, the current spectroscopic approach has been proposed for estimating the activation energy for silver diffusion in the SiO(2) matrix.

Spectroscopic and magnetochemical studies on the active site copper complex in galactose oxidase

Abstract Galactose oxidase is a radical copper oxidase, an enzyme making use of a covalently modified tyrosine residue as a free radical redox cofactor in alcohol oxidation catalysis. We report here a combination of spectroscopic and magnetochemical studies developing insight into the interactions between the active site Cu(II) and two distinct tyrosine ligands in the biological complex. One of the tyrosine ligands (Y495) is coordinated to the Cu(II) metal center as a phenolate in the resting enzyme and serves as a general base to abstract a proton from the coordinated substrate, thus activating it for oxidation. The structure of the resting enzyme is temperature-dependent as a consequence of an internal proton equilibrium associated with this tyrosine that mimics this catalytic proton transfer step. The other tyrosine ligand (Y272) is covalently crosslinked to a cysteine residue forming a tyrosine–cysteine dimer free radical redox site that is required for hydrogen atom abstraction from the activated substrate alkoxide. The presence of the free radical in the oxidized active enzyme results in formation of an EPR-silent Cu(II) complex shown by multifield magnetic saturation experiments to be a diamagnetic singlet arising from antiferromagnetic exchange coupling between the metal and radical spins. A paramagnetic contribution observed at higher temperature may be associated with thermal population of the triplet state, thus permitting an estimate of the magnitude of the isotropic exchange coupling ( J >200 cm −1 , JS 1 · S 2 ) in this complex. Structural correlations and the possible mechanistic significance of metal–radical coupling in the active enzyme are discussed.

Enhanced 1.53 μm emission of Er3+ ions in phosphate glass via energy transfer from Cu+ ions

Optimizing the efficiency of Er3+ emission in the near-infrared telecommunication window in glass matrices is currently a subject of great interest in photonics research. In this work, Cu+ ions are shown to be successfully stabilized at a high concentration in Er-containing phosphate glass by a single-step melt-quench method, and demonstrated to transfer energy to Er3+ thereby enhancing the near-infrared emission about 15 times. The spectroscopic data indicate an energy conversion process where Cu+ ions first absorb photons broadly around 360 nm and subsequently transfer energy from the Stokes-shifted emitting states to resonant Er3+ absorption transitions in the visible. Consequently, the Er3+ electronic excited states decay and the 4I3/2 metastable state is populated, leading to the enhanced emission at 1.53 μm. Monovalent copper ions are thus recognized as sensitizers of Er3+ ions, suggesting the potential of Cu+ co-doping for applications in the telecommunications, solar cells, and solid-state lasing ...

In situ optical microspectroscopy approach for the study of metal transport in dielectrics via temperature- and time-dependent plasmonics: Ag nanoparticles in SiO2 films.

This study proposes in situ optical microspectroscopy as a means for the investigation of particle growth and metal transport in nanocomposite systems based on the temperature- and time-dependent optical response of the material. The technique has been successfully employed for the real-time monitoring of the growth of Ag nanoparticles (NPs) in SiO(2) films deposited on soda-lime glass during thermal processing in nitrogen atmosphere. By fitting the surface plasmon resonance (SPR) profiles with spectra calculated by Mie theory in the quasi-static regime, the time variation in effective Ag particle size was determined and subsequently analyzed in the context of crystal growth theory. The Ag NPs were indicated to grow first through a diffusion-based process and subsequently via Ostwald ripening. The experimental determination of the activation energies associated with each one of the particle growth mechanisms was carried out based on the time evolution of the SPR of Ag NPs. Arrhenius-type analyses of a set of time-dependent isotherms allowed for estimating the activation energies at 2.3 eV for the diffusion-based growth and 2.8 eV for the ripening stage.

Micro-Raman scattering of selenium-filled double-walled carbon nanotubes: Temperature study

Selenium-filled double-walled carbon nanotubes (Se@DWNT) have been studied by high resolution transmission electron microscopy (HRTEM) and micro-Raman spectroscopy in the temperature interval from 80to600K employing 785nm excitation wavelength. The temperature dependences of the dominant bands (G-band and G′-band) are analyzed in terms of the model developed by Klemens [Phys. Rev. 148, 845 (1966)], Hart et al. [Phys. Rev. B 1, 638 (1970)], Cowley [J. Phys. (France) 26, 659 (1965)] and extended by Balkanski et al. [Phys. Rev. B 26, 1928 (1983)] for anharmonic decay of optical phonons. The findings were compared to analogous study for empty double-walled carbon nanotubes (DWNTs). The DWNT interatomic force constant modification as a result of the presence of the Se atoms inside the tubes is revealed through larger anharmonicity constants describing the temperature dependences of the G′-band and the inner tube tangential modes (G-band).

Energy gap and optical dielectric constant of Pb1−xCdxSe films

Abstract The transmission spectra of laser-deposited Pb 1− x Cd x Se films ( x = 0, 0.02, 0.05, 0.08 and 0.12) have been measured over the energy range 0.1–0.65 eV at two different temperatures 90 and 300 K. From these spectra the dispersion of the refractive index and the absorption coefficient-energy dependence have been obtained. Thus the dependences of the gap and the optical dielectric constant on the Cd content x have been investigated. The experimental results have been discussed in the framework of the recently developed theoretical model by Volkov and Pankratov et al 1,2 .

Near-IR Photoluminescence of Pr/Cu/Sn Tridoped Phosphate Glass: Nonplasmonic Material System Versus Plasmonic Nanocomposite

An optical spectroscopy study of Pr2O3, CuO, and SnO tridoped barium phosphate glass prepared by the melt-quenching technique has been carried out, emphasizing near-infrared (IR) emission properties. The material is studied in its nonplasmonic state (as synthesized) and plasmonic form (heat-treated), aiming to elucidate the effects of Cu nanoparticles. The data indicate that Cu+ ions and Sn centers are stabilized in the melt-quenched glass. Broad ultraviolet excitations of both species can lead to near-IR emission of Pr3+ ions via energy transfer. The plasmonic nanocomposite is produced upon heat treatment as Sn2+ reduces Cu+ to Cu0 atoms, ultimately precipitating as Cu nanoparticles sustaining the surface plasmon resonance. Consequently, depletion of primarily Cu+ modified the ultraviolet excitation properties for the sensitized near-IR Pr3+ emission. Further, suppression of the Pr3+ emission from near-IR emitting states 1D2 and 1G4 was observed in the Cu nanocomposite in accord with a “plasmonic diluent” role of the nanoparticles.

Temperature dependence of Raman scattering in filled double-walled carbon nanotubes

Four types of filled double-walled carbon nanotubes (DWNTs) (Se@DWNT; Te@DWNT; HgTe@DWNT; and PbI2@DWNT) have been studied by high-resolution transmission electron microscopy and micro-Raman spectroscopy in the temperature interval from 80 to 700 K employing 785 nm excitation wavelength. The temperature dependence of the dominant bands (D-band, G-band, and the (2D)-band) are analyzed in terms of the model developed by Klemens, Hart, Agraval, Lax, and Cowley and extended by Balkanski for anharmonic decay of optical phonons. The quasiharmonic frequencies and the anharmonicity constants were obtained from the temperature dependences of the analyzed Raman bands. The findings were compared to analogous study for empty DWNTs. The strength of the van der Waals interaction between the guest material and the carbon nanotube (CNT) estimated through the quasiharmonic frequencies was found to decrease in the following order: Se@DWNT; Te@DWNT; PbI2@DWNT, and HgTe@DWNT. In agreement with this, the anharmonicity due to ...