Israel Schechter

Adsorbate effects on photoluminescence and electrical conductivity of porous silicon

Porous silicon (p type) has been exposed to several chemical vapors at various partial pressures. The quenching of the photoluminescence by the adsorbates has been quantified and correlation to the electrical conductivity of the porous silicon sample has been studied. Some gases, e.g., water and benzene, have a small effect on the photoluminescence and on the conductivity, while others, e.g., methanol, reduce the photoluminescence by a factor of 2 and increase the conductivity by four orders of magnitude. This is accompanied with a qualitative change in the current‐voltage characteristics. These changes have been found to be reversible and the temporal behavior of the system has been investigated.

Chemistry for the 21st Century

Here, numerous winners of the Wolf prize from all chemical disciplines provide an overview of the new ideas and approaches that will shape this dynamic science over the forthcoming decades and so will have a decisive influence on our living conditions.[...]

Study of matrix effects in laser plasma spectroscopy by shock wave propagation

Abstract The origin of analytical matrix effects in laser plasma spectroscopy was investigated. We focused on matrix effects in sand/soil mixtures and attempted to explain the increase in the spectral response of trace elements (at constant concentration) with sand percentage. First, it was found that the energy coupled in the plasma, and which can be calculated from the propagation of the laser induced shock wave, indeed characterizes the matrix. A simple experimental setup for such measurements was suggested. Our results indicate that previous explanations of the matrix effects in this system may not be correct. We suggested that the main matrix effects were attributed to the depth of the laser-induced crater, which was correlated to a portion of the laser energy that penetrates into sand particulates and does not cause direct ablation. This explanation holds when no other effects are present (e.g. grain size distribution). The hypothesis was validated by experimental data.

Aerosol analysis by cavity-ring-down laser spectroscopy

The cavity-ring-down technique was applied for aerosol detection. The experimental set-up was based on a pulsed dye laser pumped with the third harmonic of an Nd:YAG laser. Validation of the method was performed using calibrated aerosol flows, all under ambient conditions. The method was exemplified with non-absorbing aerosols, such as NaCl and CuCl 2·2H2O, of various sizes and concentrations. The results were used for the evaluation of the corresponding aerosol extinction coefficients as a function of size, shape and index of refraction. The thus obtained aerosol extinction efficiencies were compared to theoretical models. Good agreement with theory was observed for NaCl aerosols, while the results for CuCl2·2H2O particulates required averaging over particle size and over the orientation dependent index of refraction. The actual sensitivity currently achieved was as low as an extinction coefficient of 8 ×10 −8 cm −1 , which means detection capability of about six water micro-particulates per cm 3 . The ultimate theoretical performance of this method for aerosol detection was estimated as an extinction coefficient of 1.4 × 10 −12 cm −1 , corresponding to about 100 micro-particulates per m 3 . These figures indicate that this method has the

Spectroscopic imaging of laser-induced plasma

Spectroscopic imaging provides 2D images with full spectral resolution at each pixel. Thus, chemical imaging of an object, as well as other useful information, can be obtained. An imaging spectroscopy method in the visible range is presented and applied to laser plasma. This is a powerful research tool with numerous possible applications. This study is focused on spectroscopic imaging of laser-produced plasmas, and such spectral images (full spectrum at each pixel) are presented for the first time. Detailed information on optical and geometrical effects are obtained, and an insight to the optimization of the laser plasma spectroscopy method is achieved. The size and the spatial shape of the plasma, which can be used for matrix effect compensation, are measured. Similarity maps and classification maps of laser-induced plasma are obtained for the first time. These maps are used for allocation of chemical components in the plasma. The signal to noise ratio maps of the spectra obtained from laser-induced plasmas are provided. These surfaces possess a clear maximum, indicating that there is a preferred site in the plasma, where the emitted light provides the best signal to noise ratio. The performance of the current method is limited by the lack of temporal resolution, although it can be extended by a proper temporal gating.

Cellular Responses Evoked by Different Surface Characteristics of Intraosseous Titanium Implants

The properties of biomaterials, including their surface microstructural topography and their surface chemistry or surface energy/wettability, affect cellular responses such as cell adhesion, proliferation, and migration. The nanotopography of moderately rough implant surfaces enhances the production of biological mediators in the peri-implant microenvironment with consequent recruitment of differentiating osteogenic cells to the implant surface and stimulates osteogenic maturation. Implant surfaces with moderately rough topography and with high surface energy promote osteogenesis, increase the ratio of bone-to-implant contact, and increase the bonding strength of the bone to the implant at the interface. Certain features of implant surface chemistry are also important in enhancing peri-implant bone wound healing. It is the purpose of this paper to review some of the more important features of titanium implant surfaces which have an impact on osseointegration.

On-line remote prediction of gasoline properties by combined optical methods

Abstract On-line prediction of 10 gasoline properties, such as research and motor octane numbers, vapor pressure, API gravity, aromatic contents, etc., are carried out on-line by a remotely operated detector coupled to the main control unit by communication optical fibers. This information is of considerable importance since it is needed for process monitoring and for the preparation of final petrochemical products of well defined properties. The currently available spectroscopic methods for predicting gasoline properties, which are based on near infrared (NIR) (and recently on Fourier transform infrared (FTIR) spectrometry) coupled with chemometric algorithms, provide limited performance (e.g. 0.4 standard error of prediction (SEP), for octane number). We propose an improvement of the performance by applying combined optical methods. In this study we combine on-line information from a short-wave NIR photodiode array spectrometer (700–1000 nm) with laser induced fluorescence (LIF) spectra obtained with a PC-plugged in linear CCD spectrometer. UV excitation is performed by harmonics of a compact and low-cost Nd:YAG laser, and the system is remotely operated through optical fibers. A comparison between the octane number predictions by NIR spectrometry and by LIF from third and fourth harmonics, is provided. It is shown that the addition of the fluorescence information improves octane number prediction (

Fast aerosol analysis by Fourier transform imaging fluorescence microscopy

Fourier transform imaging spectroscopy was combined with fluorescence microscopy and a cooled CCD detector for fast analysis of aerosols contaminated with polycyclic aromatic hydrocarbons (PAHs). Aerosols were collected on glass fiber filters and inspected, for the first time, by this imaging technique, which provides a full fluorescence spectrum at each pixel. Mapping of PAH contamination was carried out and used for identification and quantification of the compounds. Quantification limits (based on 95% confidence intervals of calibration plots) in the 10 ng cm(-)(2) range on filter are reported, which corresponds to 20 ng m(-)(3) in air, integrated in 1 min. The absolute detection limit (on filter) is estimated as low as 0.25 pg, corresponding to an air concentration of 0.5 pg m(-)(3), integrated in 1 min. The method is examined for analysis of monocomponent contamination and for simple mixtures. After a proper automation, this method has the potential to provide in situ and on-line results regarding particulate airborne PAH contaminations.

A renewable liquid droplet method for on-line pollution analysis by multi-photon ionization

A multi-photon ionization based fast conductance (MPI-FC) technique was applied to detect combustion byproduct aerosols. These PAH-polluted aerosols were on-line sampled by means of renewable water microdroplets. The environmental cases considered here have involved such common air contaminants as motor car exhaust gas and cigarette smoke. The possibility of obtaining useful calibration curves has been addressed. Two droplet contamination regimes were clearly observed. These have been argued to be associated with either a volume uniform (i.e., a bulk type) or a surface-favored contamination. The latter regime is possible whenever the increasing droplet contamination extends beyond the solubility saturation of the PAH compounds. Detection limits as low as 1 pg were obtained for pyrene-contaminated renewable microdroplets.

ON-LINE SCREENING OF AIRBORNE PAH CONTAMINATION BY SIMULTANEOUS MULTIPHOTON IONIZATION AND LASER INDUCED FLUORESCENCE

In this preliminary study, the feasibility of on-line monitoring of sub-micron polycyclic aromatic hydrocarbon (PAH) aerosols by a combination of laser-induced-fluorescence (LIF) and laser multiphoton ionization (LMPI) techniques was investigated. For this purpose, an aerosol contaminated nitrogen gas was slowly bubbled through a quartz measurement chamber filled with hexane. Continuous PAH monitoring was maintained over two hours of the resultant concentration process. Simultaneous LIF and LMPI excitation was achieved by a pulsed dye laser, operated at 283 nm. For this wavelength, pyrene and naphthalene aerosols were used as target PAH materials. The resultant fluorescence light was collected by an optical fiber, while the LMPI signals were acquired by a pair of stainless steel electrodes immersed in the solution, allowing an automated readout of the photoionization current. The results indicate that PAH sub-micron aerosols, at an air concentration of 1 mg m−3, can be monitored in this way.