Time‐resolved ultrafast carrier dynamics in as‐grown nanocrystalline silicon films: the effect of film thickness and grain boundaries

In this letter, we have studied transient photoinduced absorption in as‐grown nanocrystalline silicon films with thickness varied from 5 to 30 nm. Effects of quantum confinement (QC) in z ‐direction and grain boundary distortions alter the carrier dynamics of these films considerably. Based on the determination of critical points in the first Brillouin zone of the band structure of materials, we have time‐resolved the relaxation times of surface‐related states and indirect valleys. When decreasing the film thickness down to the QC limit (∼10 nm) new ultrafast relaxation mechanisms start to play a dominant role in carrier dynamics due to the topological disordering of these ultrathin films. These relaxation mechanisms seem to be related with the traping/de‐traping of the excited carriers prior to recombination. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of photoluminescence intensity from Si nanodots using Al2O3 surface passivation layer grown by atomic layer deposition

Temperature-dependent photoluminescence (PL) from Si nanodots with Al₂O₃ surface passivation layers was studied. The Si nanodots were grown by low pressure chemical vapor deposition and the Al₂O₃ thin films were prepared by atomic layer deposition (ALD), respectively. The BOE (Buffer-Oxide-Etch) treatment resulted in the damaged surface of Si nanodots and thus caused dramatic reduction in the PL intensity. Significant enhancement of the PL intensity from Si nanodots after the deposition of Al₂O₃ thin films was observed over a wide temperature range, indicating the remarkable surface passivation effect to suppress the non-radiative recombination at the surface of Si nanodots. The results demonstrated that the Al₂O₃ surface passivation layers grown by ALD are effectually applicable to nanostructured silicon devices.     

Deposition of Ag nanostructures on TiO2 thin films by RF magnetron sputtering

Ag nanostructures on TiO₂ films were deposited by RF magnetron sputtering under variable deposition parameters, such as DC potential, RF-power and total pressure. The concentration, shape, and distribution of the deposited nanostructures and continuous Ag films on thin films of TiO₂ can be tailored by careful variation of the deposition parameters. Controllable clusterlike, islandlike and film Ag structures on TiO₂ film were obtained, respectively. DC potential was found as an appropriate parameter to tailor the change of Ag nanostructure and the overall Ag amount. The compositions, nanostructures and morphologies of nanocomposite films appreciably influence the optical response.     

Optical spectroscopy studies on FeTelxSex and AxFe2uSe2 （A = K, Rb, Cs）： A brief overview

In this short overview, we summarize the optical spectroscopy FeTezxSex and AxFe2-ySe2. We elaborate that optical spectroscopy band structure evolution across the AFM phase transition temperature, studies on iron selenide superconducting systems measurements yield fruitful information about the the electronic correlation effect, the superconduct- ing pairing energy gap, the condensed carrier density or penetration depth, the inhomogeneity and the nanoscale phase separation between superconductivity and antiferromagnetism in those systems.     

Optical characterization and manipulation of alkali metal nanoparticles in porous silica

Rubidium and cesium metal nanoparticles were grown in nanoporous silica samples placed in alkali vapor cells. Their size and shape were investigated by measuring the sample optical transmittance. Spectral changes due to photodesorption processes activated by weak light were also analyzed. Alkali atoms photoejected from the silica walls diffuse through and out of the nanopores, modifying both the nanoparticle distribution in the silica matrix and the atomic vapor pressure in the cell volume. The number of rubidium and cesium atoms burst out of the samples was measured as a function of photon energy and fluence. The optical absorption measurements together with the analysis of the photodesorption yield give a complete picture of the processes triggered by light inside the nanopores. We show that atomic photodesorption, upon proper choice of light frequency and intensity, induces either growth or evaporation of nanosized alkali metal clusters. Cluster size and shape are determined by the host-guest interaction.     

Optical and nonlinear optical properties of copper nanocomposite glasses annealed near the glass softening temperature

Copper nanocomposite glasses have been prepared by the ion-exchange method, and annealed at different temperatures up to and above the glass softening temperature. The absorption spectra, fluorescence spectra, and nonlinear optical transmission of the samples at 532 nm for nanosecond laser pulses, have been investigated. The optical and nonlinear optical properties of the glasses are found to be distinctly different below and above the glass softening temperature. For instance, thermal annealing up to the glass softening temperature makes the samples behave like saturable absorbers, while annealing at higher temperatures makes them behave like optical limiters. Such flexibility in controlling the optical nonlinearity in these materials makes them potential candidates for photonic applications.     

A new blue-emitting phosphor, SrZnO2:Pb, synthesized by the adipic acid templated sol-gel route

A new blue-emitting phosphor, Sr_1−xPb_xZnO₂, was prepared by a novel adipic acid templated sol-gel route. Photoluminescence and crystalline properties were investigated as functions of calcination temperatures and the Pb²⁺ doping levels. It was found that under UV excitation with a wavelength of 283 or 317 nm, the phosphors gave emission from 374 to 615 nm with a peak centered at 451 nm. This broad-band was composed of UV and the visible range was attributed to an impurity-trapped exciton-type emission. The maximum emission intensity of the Sr_1−xPb_xZnO₂ phosphors occurred at a Pb concentration of x=0.01. The decay time was observed to be ∼33 ms for the compound doped with 1 mol% Pb prepared at 1000 °C. Diffuse reflectance spectra revealed the characteristic absorption peaks and the bandgap energy of SrZnO₂ was found to be 3.4 eV. SEM analysis indicated that phosphor particles have an irregularly rounded morphology and the average particle size was found to be approximately 1 μm.     

Shell thickness dependence of exciton trapping in colloidal core/shell nanorods

We quantitatively investigated, by time-resolved photoluminescence (PL) spectroscopy, the shell thickness dependence of exciton trapping and its effects on the PL quantum yield (QY) in colloidal CdSe/CdS/ZnS core/shell quantum rods. The defects passivation, due to a thin shell (0.6 monolayer), leads to a 2 times reduction of the trapping from both emitting and high-energy excited states, thus explaining the observed 4.3 times increase of the PL QY. Moreover, the QY decrease in the thick shell (1.3 monolayers) sample is fully explained in terms of increased trapping from the emitting states, which is ascribed to new defects caused by the strain relaxation at the core-shell interface.     

Porous silicon as efficient surface enhanced Raman scattering (SERS) substrate

Silver nanocrystallites are obtained through immersion of porous silicon samples in AgNO₃ solutions and a successive thermal annealing. The efficiency of nanostructures as surface enhanced Raman scattering (SERS) substrates is checked on cyanine-based dyes and horseradish peroxidase, evidencing detectable concentrations as low as 10⁻⁷ to 10⁻⁸ M. The substrate efficiency is strictly related to the Ag particle morphology, which could yield to either local surface plasmons (LSP) coupled to individual particles or to inter-particle short-range interaction.     

Colloidal semiconductor quantum dots: Potential tools for new diagnostic methods

We present and discuss the application of colloidal semiconductor quantum dots for diagnostic purposes, with special emphasis for cancer. We prepared and applied core-shell cadmium sulfide-cadmium hydroxide (CdS/Cd(OH)₂) semiconductor quantum dots in aqueous medium. Tissue and cells labeling was evaluated by laser scanning confocal microscopy as well as by conventional fluorescence microscopy. The procedure presented in this work, shown to be a promising tool for fast, low-cost and precise cancer diagnostic protocols.