Optical properties of sol-gel fabricated Mn/SiO2 nanocomposites: Observation of surface plasmon resonance in Mn nanoparticles

Manganese nanoparticles were grown in silica glass and silica film on silicon substrate by annealing of the sol-gel prepared porous silicate matrices doped with manganese nitrate. Annealing of doped porous silicate matrices was performed at various conditions that allowed to obtain the nanocomposite glasses with various content of metallic Mn. TEM of Mn/SiO₂ glass indicates the bimodal size distribution of Mn nanoparticles with mean sizes of 10.5 nm and 21 nm. The absorption and photoluminescence spectra of Mn/SiO₂ glasses were measured. In the absorption spectra at 300 nm (4.13 eV) we observed the band attributed to the surface plasmon resonance in Mn nanoparticles. The spectra proved the creation of Mn²⁺ and Mn³⁺ ions in silica glass as well. The absorption spectra of Mn/SiO₂ glasses annealed in air prove the creation of manganese oxide Mn₂O₃. The measured reflection spectra of Mn/SiO₂ film manifest at 240-310 nm the peculiarity attributed to surface plasmons in Mn nanoparticles.     

Third-order nonlinear optical response of Au-core CdS-shell composite nanoparticles embedded in BaTiO<Subscript>3</Subscript> thin films

Au-core CdS-shell composite nanoparticles were synthesized by a direct self-assembly process and integrated into BaTiO₃ thin films. Characterization by transmission electron microscopy showed that the average diameter of these composite nanoparticles was about 8 nm. Using the femtosecond time-resolved optical Kerr effect method, we investigated the third-order nonlinear optical response of the Au@CdS nanoparticles embedded in the BaTiO₃ thin films at a wavelength of 800 nm. An ultrafast nonlinear response and a large effective third-order nonlinear susceptibility of χ⁽³⁾=7.7×10^-11 esu were observed. We attributed the enhancement of the third-order optical nonlinearity to a localized electric field effect originating from the core-shell structure under off-surface-plasmon resonance conditions. Received: 13 May 2002 / Revised version: 23 October 2002 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +86-21/6510-4949, E-mail: sxqian@fudan.ac.cn     

Structural, optical, and electronic properties of colloidal CuO nanoparticles formed by using a colloid-thermal synthesis process

Colloidal cupric oxide (CuO) nanoparticles were formed by using a colloid-thermal synthesis process. X-ray diffraction patterns, transmission electron microscopy (TEM) images, high-resolution TEM images, and X-ray energy dispersive spectrometry profiles showed that the colloidal CuO nanoparticles were formed. The optical band-gap energy of CuO nanoparticles at 300 K, as determined from the absorbance spectrum, was 3.63 eV. A photoluminescence spectrum at 300 K showed that a dominant emission peak appeared at the blue region. X-ray photoelectron spectroscopy profiles showed that the O 1s and the Cu 2p peaks corresponding to the CuO nanoparticles were observed.     

Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in Ag_xAu_1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag. Received 17 October 2002 / Received in final form 26 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Fabrication of a nanostructured gold-polymer composite material

A facile synthesis route is described for the preparation of a poly-(o-aminophenol)-gold nanoparticle composite material by polymerization of o-aminophenol (AP) monomer using HAuCl₄ as the oxidant. The synthesis was carried out in a methanol medium so that it could serve a dual solvent role, a solvent for both the AP and the water solution of HAuCl₄. It was found that oxidative polymerization of AP leads to the formation of poly-AP with a diameter of 50±10nm, while the reduction of AuCl₄ ^- results in the formation of gold nanoparticles (∼ 2nm). The gold nanoparticles were uniformly dispersed and highly stabilized throughout the macromolecular chain that formed a uniform metal-polymer composite material. The resultant composite material was characterized by means of different techniques, such as UV-vis, IR and Raman spectroscopy, which offered the information about the chemical structure of polymer, whereas electron microscopy images provided information regarding the morphology of the composite material and the distribution of the metal particles in the composite material.     

Using gold colloid nanoparticles to modulate the surface enhanced fluorescence of Rhodamine B

Enhanced fluorescence from Rhodamine B (RB) mixed with gold colloids has been observed under ultraviolet irradiation. Spectroscopic studies show that with the increasing gold colloids content, the fluorescence of RB at about 590 nm increases firstly and then decreases with slight red shift. These features observed in the experiment can be explained by the local electric field enhancement via surface plasmon resonance (SPR) of gold nanoparticles. Fluorescence enhancement is obtained when the emission frequency of RB lies within the bandwidth of local field enhancement from gold nanoparticles. Theoretical calculation results show that the local field band red shifts obviously with increase the thickness of dye shell which capped on gold particle, whereas the fluorescence band of RB is fixed around 590 nm. Therefore, the red shift and non-monotonic change of fluorescence intensity from RB is attributed to the dye shell dependent red shift of local field band of gold particles.     

Amplification properties of Tb (III) green emission in polymeric waveguide doped with Tb-Al nanocluster

This paper describes optical amplification properties in a polymeric waveguide doped with Tb-Al nanoclusters. The Tb-Al nanocluster is a promising fluorescent material for polymeric waveguides, which can be uniformly dispersed in polymer matrices while restraining the concentration quenching of Tb³⁺. Under the continuous optical pumping by 488 nm laser light, optical amplification for the green emission of Tb³⁺ was achieved. The optical gain coefficients were estimated to be as high as 0.25 and 0.56 mm⁻¹ at the Tb-Al nanocluster concentrations of 4.5 and 5.0 wt%, respectively. Taking into account our previous works for the polymeric waveguide doped with Eu-Al nanocluster, the rare-earth-metal nanocluster is believed to be a promising candidate for various photonic applications such as multicolor polymer lasers and waveguide-type optical amplifiers.     

Anomalous dispersion in nanocomposite films at the surface plasmon resonance

Nanocomposite thin films formed by Cu nanocrystals (NCs) embedded in an amorphous aluminium oxide (Al₂O₃) host have been prepared by alternate pulsed laser deposition. Spectroscopic ellipsometry is used to determine the effective refractive index (n=n+ik). The extinction coefficient is non-negligible and shows a broad absorption band related to the surface plasmon resonance. In the neighbourhood of this wavelength, the real part of the refractive index undergoes an anomalous dispersion, leading to a significant increase of the n value of the composite compared to that of the host. When the Cu content is low enough, about 2 at. %, the use of an effective medium approach combined with a regression method allows us to determine the metal content and film thickness from the ellipsometric measurements. For larger concentrations this approach is no longer valid. Received: 31 July 2001 / Revised version: 21 September 2001 / Published online: 15 October 2001     

Influence of the incorporation of metals on the optical properties of MCM-41

We studied the optical properties of mesoporous aluminosilica incorporated with metal nanoparticles, which can be used to efficiently enhance catalytic activity for CO oxidation. Analysis of the PL spectra indicated that the incorporation of Al greatly enhanced the generation of oxygen related defects. When Au nanoparticles were deposited onto Al-MCM-41, both the PL and PLE spectra suggested that the metal nanoparticles were strongly attracted by the electrostatic force induced by the charged oxygen defects. Electron spin resonance measurement provided further evidence, registering a spike in the intensity following incorporation of the metal particles, as the strong interaction between the F+ center and the metal particles induced electron transfer to the trapped O₂ molecule. These discoveries shed further light on the perplexing mechanism by which the Si/Al ratio and metal particles can give rise to dramatic enhancement of the catalytic activity for CO oxidation.     

Self-assembled and fluorescence enhancement of semiconductor nanoparticles induced by surfactant adsorption

When semiconductor colloidal CdS nanoparticles and nonylphenol are mixed together in dimethyl sulfoxide at room temperature, a self-assembling process is induced. In the course, the size tunable properties of CdS nanoparticles are amplified. A blue shift in the emission spectrum and a strong photoluminescence enhancement are observed without significant change in the absorption features of the colloidal nanoparticles. These results are attributed to the adsorption of nonylphenol onto the nanoparticles surface and to the association process of the surfactant molecules. The surfactant adsorption process provides a nanoparticle surface passivation and induces an associative phase that enlarges the photoluminescence stability. This strategy opens the possibility to improve simultaneously physicochemical and photoluminescence properties of nanocrystals in solution as well as to control their deposition on two-dimensional surfaces.     

Spectral dependence of the optical absorption temperature coefficient of CdS1−xSex nanocrystallites embedded in a silicate glass

The absorption temperature coefficient of CdS_1−xSe_x nanocrystallites embedded in a silicate glass has been studied in the temperature range above room temperature at different technological regimes and sizes of nanocrystals. To understand the optical properties of silicate glasses with semiconductor nanocrystallites, especially that at the initial stage of their formation, it is necessary to include the structural changes occurring in the nanocrystals during the heat treatment.     

Optical properties of graded colloidal nanocrystallines with tunable structure

We propose a class of graded colloidal crystalline materials which consist of polydisperse metallodielectric nanoshells stacked in layers. We take the Lekner-Lishchuk summation method to treat the graded systems which are not tractable by conventional approach such as Ewald-Kornfeld methods. It is demonstrated that this kind of graded materials exhibit a series of sharp peaks, which merge in a broadened resonant absorption band in the optical region, in contrast to colloidal crystal containing monodisperse nanoshells or nanoparticles. Effects of various gradient profiles of the ratio of the inner/outer radii in the nanoshells and lattice geometries on the optical properties are discussed. These materials are not hard to fabricate by contemporary nanofabrication techniques and they shall be useful in the engineering of optical nanomaterials.     

Spatial dependence of the local field enhancement in dielectric shell coated silver nanospheres

The dependence of the local field factor around dielectric shell coated silver nanospheres was investigated by theoretical calculation as a function of the spatial distance. The local field factors in the dielectric shell are sensitive to the distance from particle center and shell thickness. When the shell dielectric constant is greater than that of surrounding medium, the maximum of local field factor at inner surface of the shell red shift and increases nonlinearly with increasing the shell thickness. On the contrary, when shell dielectric constant is less than that of surrounding medium, increasing the shell thickness leads the maximum of local field factor at inner surface blue shifts and decreases nonlinearly. However, with increasing the shell thickness, the maximum of local field factor at exterior surface of the shell always decrease nonlinearly. Furthermore, with increasing shell thickness, all these variations get gentle approach to a constant value when the shell thickness is two times of the core radius. When the core and shell diameter have fixed values, the local field factors in dielectric shell decrease with increasing the distance from particle center, but the peak position is not sensitive to the distance.     

Microgeometry and effective permittivity of discontinuous metal films

This work deals with the investigation of dielectric and optical properties of discontinuous metal films over a wide range of filling factors each side of the percolation threshold. A phenomenological two-parameter representation is used for the effective permittivity &εtilde; of such systems. In order to test the proposed approach, we have taken for gold films on a glass substrate the transmittance spectra in the visible and near-IR regions. It is shown that the representation used for the effective permittivity enables us to adequately describe the experimental transmittance spectra. The parameters entering the above representation are determined and the relationship between them and film microgeometry is discussed. Received: 6 November 2000 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Synthesis and luminescence properties of nanocrystalline LiF:Mg,Cu,P phosphor

Nanocrystalline LiF:Mg,Cu,P phosphor material of different shapes and sizes (microcrystalline cubic shape, nanorod shape and nanocrystalline cubical shaped) have been prepared by the chemical co-precipitation method. Thermoluminescence (TL) and other dosimetric characteristics of the phosphor are studied and presented here. The formation of the materials was confirmed by the X-ray diffraction (XRD). Its shapes and sizes were also observed using scanning electron microscope (SEM). The TL glow curve of the microcrystalline powder shows a prominent single peak at 408 K along with another peak of lesser intensity at around 638 K. On the contrary, the nanocrystalline rod shaped particles show a peak of low intensity at 401 K and a prominent peak around 700 K while the nanocrystalline particles in cubical shapes again show two peaks, one at around 407 K and the other at around 617 K, of which the lower temperature (407 K) peak is more prominent. The glow curve structure changes at very high doses (100 kRad) and some new peaks appear at around 525 and 637 K also the first peak appearing at around 401 K becomes prominent. The observed changes in TL due to the change in the shape and sizes of the nanophosphor have been reported. The PL has also been studied and various excitation and emission peaks observed due to the presence of various impurities are explained. The observed results have been explained in the light of asymmetrical crystal field effects due to asymmetrical shapes of the nanocrystalline phosphor. The comparison of these properties with the microcrystalline material prepared by the same co-precipitation method is also done.     

Combinatorial synthesis of Li-doped NiO thin films and their transparent conducting properties

Transparent and conductive oxide (TCO) thin films of Li-doped NiO were prepared by combinatorial pulsed laser deposition technique. Composition spread approach was applied to optimize the Li concentration for achieving high conductivity and transparency in the film. Conductivity of the composition spread film was found to increase almost linearly from nearly insulating to as high as 1.41 Ω⁻¹ cm⁻¹ with increasing Li content along a 9 mm long film. Optical transparency was found to decrease in an equal proportion across the 40 nm thick film with a slight shift in the absorption edge.     

Optical properties of silylene-biphenylene copolymer films

We have measured absorption, photoluminescence, and photoluminescence excitation spectra, and the photoluminescence time response for films of silylene-biphenylene copolymer, ((C₆H₄)₂(Si(CH₃)₂)_m)_n with m=1,2,4, and 6. The excitation spectra clearly reveal that the lowest absorption band in each copolymer consists of two bands, i.e., a band at 4.7 eV and a band of which energy depends on m. Since the latter band is absent in the copolymer with m=1, the former band is attributed to the lowest π−π* transition in biphenylene subunits. The latter band is attributed to the lowest σ−σ* transition in the silylene subunits, considering its dependence on m. In contrast to the result for solution, the peak energy of photoluminescence band is independent of m. The band has a Stokes shift of more than 1 eV and a large band width of 0.5 eV. The time responses of photoluminescence intensity consist of more than two decay components and the intensity decays more slowly at smaller energy. The large Stokes shift is explained as due to excimer formation between biphenylene subunits. In order to explain the energy dependence of time responses, energy migration is discussed.     

Synthesis and fine patterning of organic-inorganic composite SiO2-Al2O3 thick films

Organic-inorganic composite SiO₂-Al₂O₃ films have been prepared by sol-gel using methacryloxypropyl trimethoxysilane and aluminum sec-butoxide as the precursors. By introduction of organic groups into the inorganic backbone, the smooth and crack-free films could be readily achieved by a one-step dip-coating process, with the thickness up to 4.6 μm after being post-baked at 200 °C for 2 h. The films presented in an amorphous phase with an acceptable chemical homogeneity. Owing to the formation of chelate rings, the gel films showed a strong photosensitivity to ultraviolet light at 325 nm. The uniform fine patterns of SiO₂-Al₂O₃ thick films could be well defined by ultraviolet light imprinting simply using a mask. These performances of SiO₂-Al₂O₃ films indicate the potential for integrated optical systems.     

Characterization of nanocrystalline gold/DLC composite films synthesized by plasma CVD technique

Composite films containing gold nanoparticles embedded in diamond-like carbon (Au-DLC) matrix were deposited on glass and Si (1 0 0) substrates by using capacitatively coupled plasma (CCP) chemical vapour deposition technique (CVD). Particle size and metal volume fraction were tailored by varying the relative amount of argon in the methane + argon gas mixture in the plasma. Optical constants of the films were evaluated. Bonding environment in these films were obtained from Raman and Fourier transformed infrared spectra (FTIR) studies. Blue-shift of the surface plasmon resonance peak in the optical absorbance spectra of the films could be associated with the reduction of the particle size while red shift was observed with the increase in volume fraction of metal particles in the DLC films. Absorption spectra recorded in the reflection mode indicated dichromatism in these films.     

Synthesis and optical properties of tetragonal KTa0.6Nb0.4O3 nanoparticles

Tetragonal phase KTa_0.6Nb_0.4O₃ (KTN) nanoparticles have been prepared by hydrothermal method. The obtained particles were characterized by X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and UV-vis absorption spectrum techniques. A systematic change in crystal structure from cubic to tetragonal is observed with the increase of reaction temperature and KOH concentration. Room temperature UV-vis absorption spectrums of KTN particles show that the band gap changes from 3.24 to 3.34 eV with grain size diminished, which reveals the existence of blue-shift phenomenon of absorption bands.