Search

International audience; Having clear and deep information on the surface/interface of deposited molecules is of crucial importance for the development of efficient optoelectronic devices. This paper reports on a joint experimental/theoretical approach based on Raman spectroscopy in order to provide information on the orientation of deposited push-pull chromophores onto a gold surface. In addition, several parameters can strongly control or impede the orientation of such molecules on the surface such as: the molecular structure, the surface itself, the method of deposition and the solvents used. From this approach, additional information has been highlighted such as perpendicularly depositing the molecule on the surface, the bithiophene compounds displaying more solvent effects compared to terthiophene molecules and so on. According to the results, the joint SERS/DFT study proves to be an effective tool for probing the arrangement of push-pull chromophores and selecting right experimental conditions to tune the surface properties.

International audience; The photoluminescence of two series of A- and B-site Eu3+ substituted (SrxBa1-x)(2)CaWyMo1-yO6 double perovskite phosphor materials, (SrxBa1-x)(1.96)Eu0.02K0.02CaWyMo1-yO6 and (SrxBa1-x)(2)Ca0.96Eu0.02Li0.02WyMo1-yO6 (x and y=0, 0.25, 0.50, 0.75, and 1), were studied systematically as a function of stoichiometry and crystal structure. The Eu3+ lattice sites controlled by co-doping with either K or Li were confirmed by Raman spectroscopy. The variation in integrated emission intensity and emission colour over the experimental matrix was examined using statistical tools, and the observed trends were rationalized based on the physical and electronic structure of the phosphors. Phosphors with Eu on B-site with maximum Sr content had remarkably higher emission intensities than all other materials, but the emission was more orange than red due to domination of the D-5(0) - F-7(1) (595 nm) transition of Eu3+. The relative intensities of the D-5(0) - F-7(2) (615 nm) and D-3(0) - F-7(1) transitions of Eu3+, and thus the red-shift of the emission, decreased linearly with increasing Sr content in the A-site Eu-substituted phosphors, and reached a maximum for Sr1.96Eu0.02K0.02CaW0.25Mo0.75O6. A maximum external quantum efficiency of 17% was obtained for the phosphor Sr2Ca0.7Eu0.15Li0.15W0.5Mo0.5O6 with Eu on B-site. (C) 2016 Elsevier Inc. All rights reserved.

The solid solution Sr{sub 1−x}Pb{sub x}WO{sub 4} based on luminescent tungstates SrWO{sub 4} and PbWO{sub 4} has been synthesized by solid-state reaction for all compositions 0≤x≤1. Using Rietveld method, the structural data of all polycrystalline samples have been refined and crystal cell parameters exhibited a linear behavior as a function of x. All substituted structures are of scheelite type. Scanning electron microscopy showed that a high level of crystallization characterized the samples, with modifications in sizes and shapes depending on composition x. Infrared and Raman spectroscopy have been performed to characterize the evolution of vibrational modes with substitution rate. Finally, a systematic study of luminescence under X-ray excitation has been performed: in the composition range x=0.2 to 0.4, intensities of emission exhibited increased values. The luminescence profiles have been interpreted in terms of four Gaussian components, two of them depending on substitution rate. - Graphical abstract: Photoluminescence under X-ray excitation of Sr{sub 1−x}Pb{sub x}WO{sub 4} solid solution: (left) decomposition of emission large band into four components for composition x=0.3; (right) variation of total emission intensity with composition x. - Highlights: • Structural study of solid solution Sr{sub 1−x}Pb{sub x} WO{sub 4} using Rietveld analyses. • Variation of Debye–Waller factor withmore » composition x, disorder parameter. • Existence of four components of luminescence under X-ray excitation. • Effect of substitution of Sr by Pb on two components, due to Pb–O–W interactions. • Enhancement of luminescence intensity in a specific composition range.« less

A series of polycrystalline barium-lead tungstate Ba1-xPbxWO 4 with 0 ≤ x ≤ 1 was synthesized using a classical solid-state method with thermal treatment at 1000 °C. These materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Raman (FT-Raman) spectroscopy. X-ray diffraction profile analyses were performed using Rietveld method. These materials crystallized in the scheelite tetragonal structure and behaved as quasi ideal solid solution. Raman spectroscopy confirmed the formation of the solid solution. Structural distortions were evidenced in X-ray diffraction profiles and in vibration Raman spectra. The scanning electron microscopy experiments showed large and rounded irregular grains. Luminescence experiments were performed under X-ray excitation. The luminescence emission profiles have been interpreted in terms of four Gaussian components, with a major contribution of blue emission. The integrated intensity of luminescence reached a maximum value in the composition range x = 0.3–0.6, in relation with distortions of crystal lattice.

International audience; The bismuth lutetium tungstate series Bi(2−x)Lu(x)WO6 with 0.1 ≤ x ≤ 1 were synthesized by solid state reaction of oxide precursors at 1000 °C for 3 h. The as-prepared polycrystalline compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) analyses. Biphasic samples were obtained in the composition range of 0.1 ≤ x ≤ 0.3. Solid solutions were obtained in the composition range of 0.4 ≤ x ≤ 1, and their monoclinic crystal structure was refined using the Rietveld method. SEM micrographs showed that solid solutions presented homogeneous morphologies. Attributions of Raman vibrational modes were proposed. A shift in the vibrational wavenumber depending on the lutetium composition was observed. The specific broadening of the spectral bands was interpreted in terms of long range Bi/Lu disorder and local WO6 octahedron distortions in the structure. The PL experiments were performed under UV-laser light irradiation. Each PL band was decomposed into three Gaussian components with energies close to 1.25, 1.80 and 2.1 eV. Their integrated intensities increased with the value of x. The presence of the near infrared band at 1.25 eV is discussed.

Nanocrystals of 20 mol % Gd3+-doped CeO2 dispersible in basic aqueous solutions were grown via hydrothermal treatment of anionic Ce(IV) and Gd(III) carbonate complexes at 125–150 °C for 6–24 h with N(CH3)4+ as a capping agent. The nanocrystals were characterized in detail using dynamic light scattering (DLS), ζ-potential measurements, X-ray diffraction (XRD), specific surface area measurements based on the Brunauer–Emmett–Teller theory (SSABET), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy, and Raman spectroscopy. DLS analysis revealed that the highly transparent product solution consisted of nanocrystals approximately 10–20 nm of hydrodynamic diameter with a very narrow size distribution, while the ζ-potential analysis results strongly suggested that the N(CH3)4+ capped negatively charged sites on the nanocrystals’ surface and provided sufficient repulsive steric effect to prevent agglomeration. Moreover, the crystallite size (dXRD) estimated from the XRD patterns and the ...

Hydrated nanostructured cerium dioxide CeO2, xH2O (hydrated nanoceria) has been synthesized in room conditions via a precipitation route. This hydrated nanoceria phase has been subjected to thermal decomposition in the temperature range from 25 °C to 800 °C. At least three decomposition steps have been observed in thermal and thermogravimetric analyses. Three different samples of cubic nanoceria respectively obtained at room temperature (RT-nanoceria), 80 °C (80-nanoceria) and 600 °C (600-nanoceria) have been studied by X-Ray diffraction, Raman spectroscopy and scanning electron microscopy analyses. The analyses of X-ray diffraction profiles and Raman vibrational bands have clearly shown that dehydration is accompanied by increasing crystallite size, lattice parameter contraction. The cubic structure of hydrated RT-nanoceria might be associated with a complex chemical formula unit involving Ce4+, Ce3+ mixed valences, oxygen vacancies, lattice and surface water and OH− proton species.

International audience; We compared the surface enhanced spectroscopy signal arising from dye molecules (methylene blue, crystal violet and octadecylrhodamine B) deposited on gold nanostructures fabricated onto two kinds of substrates: silicon wafer or glass. For any molecule and excitation wavelength, the observed signal was always higher on glass compared to silicon. This experimental result is confirmed by Finite-Difference Time Domain calculations: the enhancement in the electric field is much higher with gold nanoparticles on glass than on silicon, even if a thin oxide layer is present. These results clearly demonstrate that the nature of the substrate, independently of metallic nanoparticles, is important for the preparation of highly efficient SERS nanostructures.

A series of ceramics samples belonging to theCeO2-Bi2O3phase system have been prepared via a coprecipitation route. The crystallized phases were obtained by heating the solid precursors at600∘Cfor 6 hours, then quenching the samples. X-ray diffraction analyses show that forx<0.20a solid solutionCe1−xBixO2−x/2with fluorine structure is formed. For x ranging between 0.25 and 0.7, a tetragonalβ′phase coexisting with the FCC solid solution is observed. For x ranging between 0.8 and 0.9, a new tetragonalβphase appears. Theβ′phase is postulated to be a superstructure of theβphase. Finally, close tox=1, the classical monoclinicα Bi2O3structure is observed. Raman spectroscopy confirms the existence of the phase changes as x varies between 0 and 1.

In this paper, strontium lead molybdate Sr1-xPbxMoO4 polycrystalline samples with 0

International audience; Two series of A-site and B-site Eu3+ doped (SrxBa1-x)(2)CaWyMo1-yO6 double perovskite phosphor materials were prepared via a modified Pechini sol-gel route; (SrxBa1-x)(1.96)Eu(0.02)K(0.02)CaW(y)Mo(1-)yO(6) and (SrxBa1-x)(2)Ca0.96Eu0.02Li0.02WyMo1-yO6 (x and y=0, 0.25, 0.50, 0.75, 1). The Sr/Ba ratio was the main determinant for the crystal symmetry of the series, while variation in the W/Mo ratio did influence the crystal symmetry significantly. The crystal structure evolved with Sr/Ba ratio from cubic Frn (3) over barm for x=0, via tetragonal 14/m for x=0.25, to monoclinic P2(1)/n for x >= 0.5, as verified by Rietveld refinement of X-ray diffractograms as well as by Raman spectroscopy. The reported boundaries for the compositionally induced phase transitions are in very good agreement with reported optical properties. (C) 2015 Elsevier Inc. All rights reserved.

We compare the Raman scattering properties of hybrid nanostructures consisting of Ag nanoparticles (NPs) in disordered and aligned arrangements on single-walled carbon nanotubes (SWCNTs) as a result of chemical and photoreduction methods. In the latter case, the unique structure of the very small Ag NP (from 4 to 7 nm) chains generated an extremely large mode at 969 cm(-1) that was assigned to the sulphate-silver interaction at the NP surface. Another strong mode was present at 1201 cm(-1) and was assigned to an IR-active mode of sodium dodecyl sulphate (SDS); this mode was observed because the symmetry changes altered the selection rules. We demonstrate that both the UV photoreduction of silver and the presence of SWCNTs are necessary to produce this very strong Raman scattering. The Raman modes of the SWCNTs are also significantly modified by the presence of Ag NP chains along the nanotubes.

Tip-Enhanced Raman Spectroscopy (TERS) is a very promising analytical technique for high sensitivity, high spatial resolution analysis of the physical and chemical properties of nanoscale materials including nanocrystals, biomolecules, carbon-based nanostructures and nanometer-size devices. Polarized TERS, in addition, offers novel opportunities for high contrast spectroscopy and imaging of semiconductor crystals and crystalline nanostructures. This chapter reviews the current state-of-the-art in polarization-sensitive TERS focusing the attention on the experimental implementations of the technique, on the light scattering properties of the metallic probes, on the Raman signal enhancement mechanisms and, finally, on the applications of this technique.

Copper oxalate has been precipitated in water and acetone. Raman spectroscopy is performed to validate the chemical composition of the obtained powders. Aqueous precipitation has led to micrometer-scaled agglomerates, whereas acetone precipitation has led to nanorods showing a two-dimensional (2D) self-organization when deposited on a Si substrate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show an anisotropic structure of the agglomerates with two kinds of faces. Nanoplatelets with self-assembly abilities are also detected in the copper oxalate aqueous suspension. These observations reveal a supracrystalline self-assembly process leading to micrometer-scaled three-dimensional (3D) superstructures. AFM measurements on nanoplatelets and nanorods are correlated to X-ray diffraction results. A kinetic study carried out by SEM highlights four main steps in the 3D self-assembly process. Each step corresponds to a specific assembly speed ratio between the two directions that define the...

1 - Article; Tip-enhanced Raman spectroscopy has proven to be a promising technique for stress/strain mapping of silicon-based semiconductor devices on the nanometer scale. Field enhancement factors of up to 104 have been reported and a spatial resolution down to 20 nm has been claimed through exploiting far-field suppression techniques based on an appropriate choice of the excitation/detection polarization states. In this paper, we show that depolarization of light due to scattering from the tip plays a key role in the selective enhancement of the one-phonon optical mode peak at 520 cm−1 with respect to the two-phonon ones. The spatial confinement of the selective enhancement has been studied by means of approach curves, and its dependence on the excitation wavelength and power further explored. Conclusions on the physical nature of the enhancement (depolarization- or plasmonic-based) are presented. Copyright © 2009 John Wiley & Sons, Ltd.

Copper oxalate 1 �m-sized nanocrystalline assemblies with several shapes (cushions, lenses, drilled cushions, and square rods) have been obtained by aqueous precipitation without additive, with glycerol, with PEG and with HPMC. Respective influences of these additives on the nanocrystals self-assembly are suggested from the obtained particles morphologies to provide a better understanding of this assembly process. Raman spectroscopy is used to highlight fluorescence occurring on the copper oxalate samples synthesized with additives. This additive induced fluorescence is suggested to result from specific interactions between the nanocrystals and the adsorbed additives.

Copper-7,7',8,8'-tetracyanoquinodimethane (CuTCNQ) is an interesting material for memory applications since it reversibly switches from high to low resistance states under external voltage. In this paper, CuTCNQ material was prepared from the gas/solid reaction between metallic copper and hot gaseous TCNQ at low pressure to form CuTCNQ nanowires. Nanowires were first grown on Au stripes and aluminum top electrode deposition enabled formation of crossbar memory structures. Electrical testing including standard current-voltage and data retention measurements were performed on the Au/CuTCNQ/AI stacks. Besides, in order to apprehend the resistive switching mechanisms, Raman spectroscopy was carried out on packaged structures enabling measurements under electrical field. Finally, in the perspective of high-density memory devices, CuTCNQ nanowires were grown in via arrays and characterized by electron transmission microscopy.

International audience; The autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces allow creating a wide range of surface nanostructures, opening an alternative 'bottom-up' route to the traditional 'top-down' fabrication methods of the microelectronics industry now approaching their fundamental limits. This review summarises some recent efforts of our team to grow molecular arrays on metal, insulating or semiconductor surfaces. In a fundamental approach, two-dimensional surface arrays of nanometre size have been obtained under ultrahigh vacuum by evaporation of molecules, functionalised to favour the intermolecular links rather than molecule-substrate ones. Intermolecular links such as hydrogen bonds, covalent or coordination bonding were profitably used to create various molecular networks. Alternatively, we also investigated molecular self-assembly from the solution whose architectures are mainly fixed by the molecule-substrate adsorption forces. Molecular assemblies were characterised using Scanning Near-Field Microscopies (Scanning Tunnelling Microscopy, non-contact-Atomic Force Microscopy), whereas electronic and vibrational properties were investigated by surface spectroscopy such as Ultra-Violet and X-ray Photoelectron Spectroscopy, infrared or Surface-Enhanced Raman Spectroscopy.