Enhancement of the photoluminescence of porous silicon by sputter deposition of semitransparent metal films

Optoelectronic devices based on porous silicon (PS) undergo substantial degradation in luminescence with aging due to atmospheric oxidation. The passivation of PS has been reported with a transparent conducting material or a semi-transparent metal. In this paper, we report enhancement of the photoluminescence (PL) of PS by the passivation of PS with ultrathin metal films such as silver (Ag), aluminum (Al), and gold (Au). It has been found that Ag and Au are respectively most and least effective in enhancing the PL of PS among those three different metals. The highest PL enhancing effect of Ag is mostly attributed to the high electrical conductivity of Ag, whereas the lowest PL enhancing effect of Au is due to the lowest optical transmittance of Au. The details of the PL enhancing effect of metal passivation are discussed with the aid of FTIR analysis results.     

SERS,FT-IR and photoluminescence studies on single-walled carbon nanotubes/conducting polymers composites

Surface-enhanced Raman scattering (SERS), Fourier transform infrared (FT-IR) and photoluminescence (PL) spectroscopies were used to investigate composites based on single-walled carbon nanotubes (SWNTs) and different conducting polymers like polyaniline (PANI), poly-paraphenylene vinylene (PPV) and poly 3-hexylthiophene (3-PHT). In the case of SWNTs/PANI, different composites are obtained by adding dispersed SWNTs powder to the polymer solutions and by chemical polymerization of aniline in presence of SWNTs. The difference originates in the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs in fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FT-IR spectra of the composites PANI/SWNTs and PANI/C₆₀, chemically synthesized. Electrochemical polymerization of aniline in an HCl solution on a SWNT film leads to a covalent functionalization of SWNTs with PANI. In this case, Raman scattering data suggest an additional nanotubes roping with PANI as a binding agent. A post treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. In the case of PPV and 3-PHT, the effect of the concentration of SWNTs on the photoluminescence properties will be described and compared, as probes of interaction.     

Electrolyte composition dependence of the morphological and nanostructural features of porous silicon prepared by electrochemical anodic etching

Porous silicon layers were formed by electrochemical anodic etching of p-type Si wafers. The electrostatic condition at the interface between the Si wafers and electrolytes was affected sensitively by the addition of isopropyl alcohol (IPA) in the etchant. As the IPA ratio was varied in the range of 0 to 75%, the ideality factor in the ln I-V relationships and the viscosity of the electrolytes changed from 27.2 to 16.0, and from 1.0 to 3.3 cp, respectively. The etched surface exhibited three different morphologies, such as ‘turtle-back’-, ‘column’-, and ‘mountain’-like structures depending on the electrolyte composition. The etched layers contained Si nanocrystallites, ~5.5 to ~2.6 nm in size, which exhibited photoluminescence features in the wavelength range, 733 to 624 nm. The variation of the band gap was determined by size of the nanocrystallites, whereas the nanostructural and morphological features were dependent on the IPA ratios of the etchants.     

Reactive infiltration of porous graphite by NiSi alloys

Infiltration of reactive NiSi alloys into porous graphite preforms is studied following in situ the variation with time of the volume of a droplet placed on the porous substrate. The shape and depth of the infiltration zone as well as the degree of transformation of carbon to silicon carbide are determined by post-mortem characterizations by scanning electron microscopy and image analysis. Experiments are performed by varying the temperature, the mole fraction of Si in the alloy and the infiltration time. The results are used to determine the process governing reactive infiltration.     

NMRD of polyaniline-based conducting polymers

Nuclear magnetic resonance dispersion (NMRD) is a technique for measuring NMR relaxation times, which is commonly used to study dynamics in condensed matter. In this paper, we present NMRD profiles, for polyanilines doped with trifluoromethanesulfonic acid (TFSA), recorded as a function of temperature and dopant concentration. This work demonstrates that NMRD can be used to characterise the critical processes that determine the conductivity of the materials. The NMRD profile of the bulk polymer is sensitively dependent on the concentration of the dopant, while the response of the dopant is insensitive to concentration. Complete analysis of the NMRD profiles of these ICPs will give important information, including the rates of spin diffusion and of charge transport between dopant-rich domains.     

Thermodynamics of chemical and electrochemical stability of aluminum, silicon, and tin bronzes

Phase diagrams copper-element-oxygen and potential-pH diagrams are plotted for alloys of the Cu-Al, Cu-Si, and Cu-Sn systems at 25°C. Thermodynamic peculiarities of the corrosion and electrochemical behavior of phase components of aluminum, silicon, and tin bronzes are analyzed. The surface activity of alloying elements is assessed. Original Russian Text ? A.G. Tyurin, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 3, pp. 312–320.     

Mechanochemical preparation of conducting polymers and oligomers

Three types of conducting polymers – polypyrrole (PPy), polythiophene (PTh), and polyparaphenylene (PPP), – are prepared by environmentally friendly and efficient solvent-free mechanochemical method. The experimental data obtained in the work argue for a high molecular weight for PPy, whereas PTh and PPP are represented by different oligomers. Measurements of conductivity confirm formation of the doped state of PPy, weakly doped state of PTh and non-doped state of PPP. Interrelation between the doping state and degree of polymerization is discussed.     

Interpenetrating networks of two conducting polymers

Interpenetrating networks (IPNs) of two conjugated polymers are prepared by a combination of a chemical oxidation step and a vapour phase polymerisation step on non-conducting surfaces. In this work ferric tosylate was used as the oxidant as it gives very smooth and homogeneous coatings, and because its reaction products can be removed efficiently after the formation of the composite. Several combinations of polymers are demonstrated, and the versatility of the proposed method allows extensions to a wide range of conjugated polymers. The IPNs show optical and electrochemical characteristics, which are sums of the characteristics from the participating conducting polymers.     

Nonlinear optical susceptibilities of conducting polymers

Using the Keldysh non-equilibrium Green's function methods we compute analytic expressions over the full frequency range for the nonlinear optical response functions for a non-interacting model of a conducting polymer. The model has no adjustable parameters. Our results for 3rd harmonic generation are in excellent agreement with experiments on polyacetylene. This constitutes strong evidence that polyacetylene is well described by a model of weakly interacting quasi-particles. Further nonlinear optical effects, which should be experimentally observable, are predicted.     

渗流法制备高孔隙率多孔铝

采用负压渗流铸造法,通过控制铝熔体浇铸量,制备了高孔隙率(70%～90%)、高通孔度(25%～50%)的多孔铝;研究了渗流驱动压力、铝熔体浇铸温度及模具预热温度对渗流过程的影响;测试了多孔铝的渗透系数,并分析了孔结构对渗透系数的影响.结果表明:通过正交实验选择渗流参数,可以降低多孔铝在渗流方向上的孔隙率梯度;多孔铝的渗透系数与其孔结构密切相关:对于相同孔径的多孔铝,渗透系数随孔隙率增大而提高;在孔隙率相同的条件下,随着孔径增大,表面积减小,摩擦阻力减小,渗透系数相应增大.     

Dielectric properties of thiophene based conducting polymers

Dielectric properties of some conducting polymers were investigated and their electrical properties are compared with each other. Temperature dependencies of their electrical properties are investigated throughout the work. Electrical behaviors of the synthesized systems constitute a good infrastructure for both understanding the intrinsic mechanisms and optimizing new synthesizes. It is found that, it can be possible to attain desired electrical properties by small synthesis modulations in thiophene based polymer materials.     

X-ray absorption spectroscopy of iron-doped conducting polymers

X-ray absorption spectroscopy (XAS) measurements were performed at the Fe K edge to determine the iron local structure in chemically prepared polyaniline (PANI) and polypyrrole (PPy) samples prepared with FeCl₃ as an oxidant. The samples were conditioned at different pHs by an acid–base treatment. In both as synthesized-doped polymers, the observed dispersed Fe atoms were predominantly coordinated to chlorine as Fe(III)Cl_x species, where x=6 for PANI and x=4 for PPy samples suggesting that the polymer–counteranion bonding is weaker than the iron–chelate one. For the PANI samples submitted to basic treatment, three different sets of Fe distances were found: five Fe–O at 2.00 Å, three Fe–Fe at approximately 2.80 Å and five Fe–Fe at 3.00 Å and the near-edge spectra showed the presence of octahedrally coordinated Fe⁺³. These results strongly suggest the presence of small oxide/hydroxide aggregates. Similar data were obtained for PPy treated with NH₄OH.     

液相渗硅法和原位反应法制备硅化石墨及性能研究

分别采用液相渗硅法和原位反应法制备了硅化石墨,讨论了石墨基体密度和渗硅方法对硅化石墨力学性能和摩擦性能的影响.结果 表明:2种方法都能明显提升石墨的抗弯强度,液相渗硅法制得的硅化石墨表面有一层浓度梯度分布的SiC层,石墨基体的耐磨性能得到明显改善.综合力学性能和摩擦性能考虑,选择低密度的石墨通过石墨化处理并采用液相渗硅...     

Structural properties of some conducting polymers and carbon nanotubes investigated by SERS spectroscopy

In this paper, we present Surface Enhanced Raman Scattering (SERS) experiments performed successfully on some conducting polymers and carbon nanotubes deposited in thin films on rough metallic surfaces. The enhancement mechanism in SERS has a twofold origin: electromagnetic and chemical. The electromagnetic enhancement is, however, the dominant mechanism and consists in the excitation of localized and delocalized surface plasmons (SP). The second enhancing mechanism for SERS, not yet convincingly supported by experimental data, is of chemical origin. This mechanism is due to the increase of the polarizability of the molecules on the metal surface under the action of the incident radiation, leading to the formation of new chemical bonds between the molecules and the metal surface. Unfortunately, the experimental data available to argue the presence of a chemical process are scarce and scattered. In this paper, we try to present some details regarding this aspect. In order to do so, we studied poly 3-hexylthiophene (3-PHT) and polyaniline (PAN) as a function of the type of the rough metallic support (Ag, Au or Cu), the oxidization state and thickness of the polymer layer. We studied also carbon single walled nanotubes. Our experiments reveal the existence of a chemical surface effect. The results obtained for 3-PHT show that SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds. The SERS measurements reported here reveal an increase of the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. We observe for the first time by SERS spectroscopy that doping of 3-PHT with FeCl₃ leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. The type of the rough metallic support can modify SERS spectra and such an effect is clearly shown for the polyaniline case. No such dependence on metallic support type is observed on the SERS spectra of carbon nanotubes.     

Electrical and optical properties of molecularly doped conducting polymers

Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C₆₀, C₇₀ etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C₆₀-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.     

Decay mechanisms of photoexcitations in conducting polymers probed by picosecond photoinduced absorption

The dynamics of picosecond photoinduced absorption (PA) in a host of conducting polymers with varying degrees of ground-state degeneracy are compared for low-density excitation ( 10¹⁸ carriers/cm³). The results are consistent with dispersive recombination associated with the 1-D diffusion of the photoexcited species on chain segments distributed in length in the polymers polythiophene, trans-polyacetylene, and in polydiacetylene at high pressure. We find that, within the 5 ps time resolution of the experiment, the transient electronic response is approximately independent of probe wavelength in the range 1.2 to 2.1 eV for all polymers except polydiethynylsilane. In that material recombination occurs within 1.4 ps after photoexcitation at all energies except those between 1.8 and 2.0 eV, coincident with a narrow band in the PA spectrum that is linked to thermal modulation of the absorption edge.     

Oxidation mechanisms of C-SiC-TaC-C composites prepared by chemical vapor infiltration

To improve the oxidation-resistance properties, SiC and TaC species were introduced in C/C composites by chemical vapor infiltration （CVI） methods. The oxidation-resistance properties of C-SiC-TaC-C composites were studied by X-Ray diffractometry （XRD）, JEOL-6360LV scanning electronic microscopy （SEM） and AdventurerTM electronic balance with precision of 0.1 mg. The results show that, 1） the oxidation rate of the composites increases continuously with time at all experimental temperatures; 2） The oxidation rate increases with temperature within 700-1 100℃, slowly in 700-800 ℃, acutely in 800-1 100℃; it reaches a maximum value at 1 100℃, then decreases within 1 100-1 400 ℃; 3） The relationship curve of oxidation rate with temperature can be divided into three regions. The oxidation rate is controlled by reactivity in region I, the mixed effects of reactivity and gas diffusion in region II, gas phase diffusion in region III; 4） The composites exhibit a higher oxidation onset temperature in low temperature region and a lower oxidation rate at high temperature due to the oxidation of TaC to （Ta, O） and the formation of the dense SiO2-Ta2O5 oxide layer respectively. With the addition of SiC/TaC species, the oxidation-resistant properties of C/C composites can be improved effectively.