Metal doping effects in NIR Raman spectra of YBCO epitaxial films

Metal doping (Co, Zn) of YBCO epitaxial thin films leads to large changes of the scattering efficiencies of various lattice modes in near-infrared (NIR) -Raman spectra (1064 nm) if it is accompanied by a variation of the charge carrier concentration. The results resemble earlier observations in oxygen depleted YBCO thin films. In contrast, no effect is found when metal doping does not change the carrier concentration even if it leads to a massive T_c suppression.     

Aqueous-phase synthesis of ultra-stable small CdSe nanoparticles

Very stable and small CdSe nanoparticles (NPs) were synthesized from the aqueous solutions containing L-cysteine (Cys) at room temperature. The Cys-capped CdSe NPs showed a very sharp excitonic peak at 420 nm. Its very small full width at half maximum (18 nm) indicates very high quality of the CdSe NPs. Their absorption features experienced little change over a month, implying an excellent stability of the CdSe NPs. The synthesis conditions were very critical to the optical property and stability of the CdSe NPs: only those prepared at specific conditions (n(Se-precursor)/n(Cd-precursor) = 0.25-0.5, n(Cys)/n(Cd-precursor) = approximately 9, pH = approximately 12) showed very sharp absorption peaks and maintained an excellent stability against time. Under these conditions, the peaks always appear at nearly the same wavelength, indicating that these NPs are selectively stable and grow at a particular size and structure.     

Enhanced electroluminescence in emissive polymer/CdSe double-layer films

We demonstrate that the phase aggregation process can be generally applied to the fabrication of polymer/CdSe double-layer light-emitting diodes (LEDs). The current and luminance characteristics of devices with aggregated CdSe nanoparticles are much better than those of devices with well-dispersed CdSe nanoparticles. It has been found that during the spinning process, aggregated nanoparticles are separated from the polymer and become attached to the polymer film. Aggregated nanoparticles between the emitting layer and the Al cathode can act as a buffer layer, enabling polymer/CdSe-based LEDs to exhibit better luminescence efficiency than that of neat polymer-based LED.     

非理想配比二氧化钒薄膜喇曼光谱研究

采用射频(RF)磁控溅射在各种条件下制备的VO2薄膜的喇曼光谱曲线.研究发现,和理想配比VO2薄膜的主要喇曼峰相比较,无论是富钒的VO2薄膜还是富氧的VO2薄膜它们主要的喇曼峰都向高频方向移动.我们提供了实验证据并且讨论了非理想配比导致VO2薄膜喇曼光谱变化的原因.     

Electrophoretic deposition of CdSe nanocrystal films onto dielectric polymer thin films

The electrophoretic deposition of cadmium selenide (CdSe) nanocrystal films from a stable hexane suspension onto a dielectric polymer film is reported. The electrodes were prepared by spin casting a ~ 30 nm thick smooth, defect-free, polystyrene film on silicon substrates. Scanning electron microscopy and atomic force microscopy showed that the CdSe films deposited atop polystyrene possessed morphology comparable to CdSe films deposited on the bare silicon electrodes. Factors affecting deposition, such as nanocrystal charging in suspension and wetting of electrode surfaces, are reviewed. This approach to preparing nanocrystal films onto homogeneous dielectric layers will facilitate the fabrication of novel polymer-nanocrystal composites.     

Size effects in double-layer metallic films

An analysis is made of the electrical conductivity of double-layer, continouos, metallic films using the free-electron Boltzmann equation with elastic and isotropic bulk scattering. The electron-surface scattering is assumed to be specular. The analysis shows that a size effect can be expected when the thickness of the double-layer is of the same order of magnitude as the mean free path of the conduction electrons in the better conductor. The theoretical results are compared with the results obtained using the Fuchs-Sondheimer formalism and assuming diffuse electron-boundary scattering. The theoretical results are also related to published experimental results with superimposed metallic films.     

Reproducible surface-enhanced Raman scattering spectra of bacteria on aggregated silver nanoparticles

Surface-enhanced Raman scattering (SERS) is proven to be a powerful technique for rapid identification and discrimination of microorganisms. However, due to the heterogeneous nature of the samples, the acquisition of reproducible spectra hinders the further development of the technique. In this study, we demonstrate the influence of the experimental conditions on SERS spectra. Then, we report a simple sample preparation method coupled with a light microscope attached to a Raman spectrometer to find a proper spot on the sample to acquire reproducible SERS spectra. This method utilizes the excited surface plasmons of the aggregated silver nanoparticles to visualize the spots on the sample. The samples are prepared using the concentrated silver colloidal solutions. The collection time for one spectrum is 10 s and each spectrum is a very good representative of the other spectra acquired from the same sample. The nature of the surface charge of the silver nanoparticles influences the spectral features by determining the strength of the interactions between nanoparticles and bacteria and the aggregation properties of the nanoparticles. Although increasing the colloid concentration in the sample resulted in reproducible spectra from arbitrary points on the sample, a great variation from sample to sample prepared with the different colloidal solution concentrations is observed.     

Size effects in nanostructured ferroelectric films

The dependence of the effective dielectric permittivity of a nanostructured ferroelectric film on the grain size and the thickness of a dead (nonferroelectric) surface layer in inhomogeneous grains has been theoretically studied.     

Interphase evolution in polymer films by confocal Raman microspectroscopy

Liquid-glassy polymer diffusion is an important topic in polymer physics, with several mechanistic aspects that still remain unclear. Here we describe the use of confocal Raman microspectroscopy (CRM) to study directly several features of interphase evolution in a system of this type. The interphase studied was generated by contact between liquid polystyrene (PS) and glassy polyphenylene oxide (PPO). Interphase evolution on thin films made from these polymers was followed by depth profiling in combination with immersion optics. We also applied regularized deconvolution to improve the spatial resolution of the measurements. With the help of these techniques, we examined interphase PPO concentration profiles and kinetics of interphase evolution in the range 120-180 degrees C, well below the glass transition temperature of the PPO-based films (185 degrees C). Overall, the experiment captures the most important features needed to discern the mechanistic factors that control this process. In this sense, confocal Raman microspectroscopy emerges as one of the best experimental techniques for the study of diffusion kinetics in this type of system.     

Annealing effects on the photoresponse properties of CdSe nanocrystal thin films

The photoresponse properties of the as-prepared and annealed close-packed CdSe nanocrystal (NC) films were investigated under laser illumination by Kelvin probe force microscopy. The annealing process improved the photoresponse speed of the CdSe NC films. The work function of the annealed CdSe NC films changed more rapidly than that of the non-annealed film in air at room temperature. Combined with X-ray photoelectron spectroscopy measurements and thermogravimetric analysis, the observed phenomena can be interpreted that annealing process removed the organic capping agents completely and eliminated oxide on the CdSe surface, which formed the tunnel barrier between NCs in the CdSe NC films. Consequently, it improved the separation rate of photoelectric charges and thus provided high speed photoresponse.     

Raman microprobe analysis of elastic strain and fracture in electrophoretically deposited CdSe nanocrystal films

The mechanical stability of nanocrystal films is critical for applications, yet largely unexplored. Raman microprobe analysis used here to probe the nanocrystal cores of thick, fractured electrophoretically deposited films of 3.2 nm diameter CdSe nanocrystals measures approximately 2.5% in-plane tensile strain in cores of unfractured films. The crack dimensions determine the overall in-plane film strain, approximately 11.7%, and the film biaxial modulus, approximately 13.8 GPa, from which the biaxial modulus of the trioctylphosphine oxide ligand matrix is inferred, approximately 5.1 GPa.     

Size effects of polycrystalline tin films

The electrical properties of polycrystalline tin films evaporated onto a cooled quartz substrate were examined in situ and in vacuum. The relationship between internal size effect and annealing temperature was considered. It was found that the parameter * of grain-boundary scattering decreased as the annealing temperature was increased.     

Influence of metal components in Hopeite films on IR and laser Raman spectra

Hopeite films on galvannealed steel were formed in various zinc phosphate solutions. The metal component composition of various Hopeite films was determined by atomic absorption. By measuring the infrared (IR) spectra and the laser Raman spectra for these films, the influence of metal component composition on these spectra could be investigated. In IR spectra, peaks corresponding to ₃ and ₄, the basic vibration mode of PO ₄ ^3– , were seen, but the peak patterns of ₄ were different for Hopeite containing manganese and that not containing it. In the Raman spectra, four clear peaks for various films were found in the region 1300 to 800 cm^–1 . Although these spectra are due to PO ₄ ^3– , ₁ and ₃ are found in this region. Essentially, ₃ degenerates triply, but in the crystalline state of Hopeite films, the symmetry of the regular PO ₄ ^3– tetrahedron will be distorted by the interaction. At that moment, the degenerations of ₃ are untied and split into three peaks. Thus it was thought that spectra with four peaks of ₁ were generated. However, the main peak corresponds to ₁ and other three peaks correspond to ₃. The values of the Raman band for various Hopeite films were compared. The band shifted to the lower wavenumber region according to the increase of manganese or nickel in these films. The structure of Hopeite films modified by the action of manganese or nickel has been analysed to be Zn_3–x Me _x (PO₄)₂ · 4H₂O (Me = Mn or Ni), but this structure is different from that of normal Hopeite films (Zn₃(PO₄)₂ · 4H₂O).     

Raman spectra of intrinsic and doped hydrogenated nanocrystalline silicon films

Raman scattering characteristics of intrinsic and doped hydrogenated nanocrystalline silicon films which prepared by a plasma-enhanced chemical vapor deposition system are investigated. Results indicate that Raman spectra depend intensively on microstructure and impurity in the films. Taking into account phonon confinement effect and tensile strain effect in Si nanocrystals, peak redshift of measured transverse optical modes in Raman spectra of intrinsic films can be well interpreted. With respect to Raman scattering from doped samples, besides phonon confinement effect, the peak of experimental transverse optical mode further downshifts with heightening doping level, which can be primarily assigned to impurity effect from doping. In addition, the increase in relative integral intensity ratio of transverse acoustic branch to transverse optical mode and that of longitudinal acoustic branch to transverse optical mode with decreasing mean dimension of nanocrystals and heightening doping ratio, respectively, can be ascribed to disorder. Furthermore, at the same doping level, incorporation of boron can induce higher disorder than incorporation of phosphorus in nc-Si:H films.     

Influence of SOP modes on Raman spectra of ZnO(Fe) nanoparticles

Nanocrystaline samples of ZnO(Fe) were synthesized by traditional wet chemical method followed by calcinations. Samples were characterized by X-ray diffraction to determine composition of the samples (ZnO, ZnFe₂O₄ and Fe₂O₃) and the mean crystalline size (from 8 to 51 nm). In this paper we report the experimental spectra of Raman scattering (from 200 to 1600 cm⁻¹) with surface optical phonons (SOP) in range of 500–550 cm⁻¹. The phonon of registered phase’s exhibit effects connected to phase concentration, while the SOP phonon mode exhibit significant confinement effect.     

Effect of silver nanoparticles on photo-induced reorientation of azo groups in polymer films

A series of polymer films containing azo groups and silver nanoparticles were prepared. Photo-induced reorientation of the film was conducted under irradiation of polarized light with wavelength at 365 nm, 442 nm and 532 nm, respectively. The influence of the concentration of dopant silver on the reorientation of the azo groups was studied. An enhancement of about 50% for the reorientation rate and about 70% for the reorientation amplitude was achieved. From a comparison of the enhancement obtained by irradiating with three different light sources, it was realized that the mechanism for enhancement of reorientation of azo groups is due to plasmon resonance of silver nanoparticles doped in the polymer films.     

Broadband coherent anti-Stokes Raman spectroscopy characterization of polymer thin films

Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of polymer thin film materials. A simple modification to a 1 kHz broad bandwidth sum frequency generation (SFG) spectrometer permits acquisition of CARS spectra for polymer thin films less than 100 nm thick, a dimension relevant to organic electronic device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance-enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained under these conditions consistently demonstrate enhanced signal-to-noise ratio compared to the spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The dielectic properties of the substrate have a dramatic effect on the detected signal intensity. For ultrathin films, the strongest signals are obtained from fused silica surfaces. Similar to surface-enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS, no surface roughening is necessary.     

Cytotoxicity of colloidal CdSe and CdSe/ZnS nanoparticles

Cytotoxicity of CdSe and CdSe/ZnS nanoparticles has been investigated for different surface modifications such as coating with mercaptopropionic acid, silanization, and polymer coating. For all cases, quantitative values for the onset of cytotoxic effects in serum-free culture media are given. These values are correlated with microscope images in which the uptake of the particles by the cells has been investigated. Our data suggest that in addition to the release of toxic Cd(2+) ions from the particles also their surface chemistry, in particular their stability toward aggregation, plays an important role for cytotoxic effects. Additional patch clamp experiments investigate effects of the particles on currents through ion channels.     

Size and shape effects on the strength of platinum nanoparticles

Journal of Materials Science - Several previous studies demonstrated that defect-free faceted nanocrystals of face-centered cubic metals (such as Au, Ni, and Pd) exhibit extraordinarily high...     

CdSe band-splitting on thermal annealed films

Semiconducting polycrystalline CdSe thin films were prepared on glass substrates by chemical bath at 65 °C. As-deposited films grew in the metastable cubic sphalerite (S) crystalline structure with good stoichiometry. Upon thermal annealing (TA) in Ar+Se₂ atmosphere at different temperatures in the range 200–500 °C, the gradual phase transformation from cubic modification to hexagonal wurtzite (W) stable phase could be observed. From optical absorption measurements the fundamental energy band gap (E_g) and the second electronic transition (E_g+ΔE_g) were calculated for as-deposited and thermal annealed films. For TA350 °C, S-phase dominates the crystalline structure and only the spin orbit (ΔE_so) contribution to ΔE_g is present. Above 350 °C, the W-phase dominates and the energy splitting (ΔE_cf), owed to crystal field contribution and originated by the loss of lattice symmetry, should be added to ΔE_so in order to complete ΔE_g in the W-phase. The values ΔE_so=0.389±0.011 eV and ΔE_cf=0.048±0.018 eV were found from our analysis, and T_c=350 °C was here defined as the critical point of the phase transformation.