J. Dian

Determination of sensoric parameters of porous silicon in sensing of organic vapors

Photoluminescence (PL) properties of as-prepared and surface derivatized porous silicon (PS) in the presence of organic compounds in gas phase were studied. Surface derivatization, aimed at increasing stability of porous silicon properties, was performed by Lewis acid mediated hydrosilylation with methyl 10-undecenoate. We have systematically measured changes in photoluminescence intensity for a set of alcohols from C 1 to C 6 . From the variation of the photoluminescence quenching response as a function of alcohol concentration, we determine the sensitivities and detection limits of our porous silicon sensors and these correlate with physical and chemical properties of studied species. For methyl 10-undecenoate derivatized PS samples, we have observed a remarkable enhancement of the selectivity for C 4 -C 6 alcohols as compared with C 1 -C 3 alcohols.

Photoluminescence properties of sol–gel derived SiO2 layers doped with porous silicon

Abstract We proposed and studied a new light emitting material based on silicon nanocrystals (Si-nc). The new material was fabricated using a low cost way by incorporating Si-nc into a sol–gel derived SiO2 matrix. The Si-nc were prepared (i) by electrochemical etching of monocrystalline Si wafers and (ii) pulverizing the obtained porous silicon film. The porous silicon powder was then dispersed in the SiO2 sol. After solidification, we obtained transparent and self-supporting SiO2 layers of about 1 mm thickness containing Si nanocrystals. The sol–gel layers exhibit bright red photoluminescence (PL) under UV lamp excitation at room temperature. This novel method circumvents the usual Si+-implantation step. We present first basic experimental studies of the PL properties of these sol–gel derived SiO2 layers and compare them with that of as-prepared porous silicon. We mention also observation of a non-linear behavior in the PL spectra. We emphasize potential advantages of this technology compared to the standard Si+-implanted SiO2 layers.

Red electroluminescence in Si+-implanted sol–gel-derived SiO2 films

We report on a continuously emitting electroluminescent device fabricated by Si+-ion implantation and subsequent annealing of a SiO2 layer on a silicon substrate. The SiO2 layer with a thickness of 250 nm was prepared by the sol–gel technique. Four different Si+-ion energies and implantation doses were applied in order to obtain a flat Si+-ion profile across the SiO2 film thickness with an atomic Si excess of 5%. Electroluminescence (EL) occurs above a low-voltage threshold (∼5 V, 1 A/cm2) at one bias polarity only even if the device in fact does not exhibit rectifying properties. EL microscopy reveals that EL at 295 K is emitted from a small number of bright spots with diffraction-limited size. EL spectra of individual bright spots were measured using an imaging spectrometer. The wide EL emission band (situated in the red region ∼750 nm) obtained with spatial averaging over the semitransparent indium–tin–oxide contact represents the envelope of these individual contributions. We suggest that the EL is du...

Concept of effective and non-effective inclusion complex formation in isotachophoresis

Abstract On the basis of experimental experiences, a simple mathematical model of 1:1 cyclodextrin-solute complex formation is proposed. A possible role of two limiting cyclodextrin-solute interactions, introduced as effective and non-effective complex formation, on the quality of isotachophoretic resolution has been studied and is discussed. Additional parameters, e.g., the solute molecular weight, its effective charge and the molecular weight of the cyclodextrin used, are involved in the proposed model. Their influence on the isotachophoretic separation was estimated by comparing the computer simulated mobility ratio curves of cyclodextrin-complexed solutes obtained at various values of the corresponding parameters.

Visible photoluminescence and electroluminescence in wide-bandgap hydrogenated amorphous silicon

Abstract The work describes basic photoluminescence (PL) and electroluminescence (EL) properties of wide-bandgap (2.0 eV or greater) hydrogenated amorphous silicon (a-Si: H). Thin films of wide-bandgap (high-hydrogen-content) a-Si: H were prepared by microwave electron cyclotron resonance plasma-enhanced chemical vapour deposition from SiH4 under high dilution with He. The films exhibit spectrally broad (full width at half-maximum, 0.4 eV or greater) visible PL at room temperature, peaked at about 1.5eV. On the basis of measurements of the PL temperature dependence, PL dynamics, infrared absorption spectra, picosecond pump-and-probe experiments and hydrogen thermal desorption spectroscopy the dominant microscopic mechanism of visible PL has been revealed to be the radiative de-excitation of oligosilane (-(SiH2)2-) units or of a specific defect in their close vicinity. EL has been investigated in sandwich p+‒i‒n+ and p+‒p‒n‒n+ structures with (Cr‒Ni)/indium tin oxide contacts. The as-grown structures exhibit good rectifying properties, low conductivity and no EL. After being subjected to a ‘forming’ procedure (application of a sufficiently high voltage), the current flowing through the structures increases abruptly by several orders of magnitude, up to about 10 mA, and the structures begin to emit weak EL at room temperature. The EL occurs in reverse bias only and its external quantum efficiency is about 10−5%. It is concluded that the forming procedure leads to partial crystallization of a-Si: H between the contacts and impact lattice ionization can participate in the light emission.

Hole burning study of excited state structure and energy transfer dynamics of bacteriochlorophyll c in chlorosomes of green sulphur photosynthetic bacteria

Results of low temperature fluorescence and spectral hole burning experiments with whole cells and isolated chlorosomes of the green sulfur bacterium Chlorobium limicola containing BChl c are reported. At least two spectral forms of BChl c (short-wavelength and long-wavelength absorbing BChl c) were identified in the second derivative fluorescence spectra. The widths of persistent holes burned in the fluorescence spectrum of BChl c are determined by excited state lifetimes due to fast energy transfer. Different excited state lifetimes for both BChl c forms were observed. A site distribution function of the lowest excited state of chlorosomal BChl c was revealed. The excited state lifetimes are strongly influenced by redox conditions of the solution. At anaerobic conditions the lifetime of 5.3 ps corresponds to the rate of energy transfer between BChl c clusters. This time shortens to 2.6 ps at aerobic conditions. The shortening may be caused by introducing a quencher. Spectral bands observed in the fluorescence of isolated chlorosomes were attributed to monomeric and lower state aggregates of BChl c. These forms are not functionally connected with the chlorosome.

Optical sensing system for ATP using porphyrin–alkaloid conjugates

Tetrabrucin-porphyrin as a sensor for ATP was designed and tested; selectivity for ATP was proved in the presence of ADP and AMP.

Optical properties of Si+-ion implanted sol–gel derived SiO2 films

Abstract Optical properties of commonly investigated Si + -implanted SiO 2 layers (thin SiO 2 films prepared by thermal growth on c-Si substrate) are compared with those of non-conventional SiO 2 layers fabricated by sol–gel process. The sol–gel films, deposited on different substrates, were implanted and annealed in a similar way as the thermal SiO 2 films on c-Si. Striking differences between these two types of samples were found. The conventional Si + -implanted SiO 2 /c-Si layers contain Si nanocrystals and their photoluminescence (PL) properties were found very similar to porous silicon (PL in the red/NIR spectral regions, PL decay time of the order of 10 μs, PL temperature dependence well described by the exciton singlet–triplet splitting model). On the other hand, the Si + -implanted sol–gel SiO 2 films exhibit room temperature PL spectra peaked in the blue–green region, PL decay is considerably faster (of the order of 1 ns) and also PL temperature dependence differs substantially from the samples of the first type. Possible influence of different substrates (silica, c-Si) is also investigated and it is shown that the observed PL is an inherent property of the implanted sol–gel SiO 2 layers. The slow red PL, in agreement with other authors, is ascribed to radiative recombination of excitons in Si nanocrystals, while the fast blue PL characteristic of ion-implanted sol–gel derived SiO 2 films is obviously of defect origin.

HOLE-BURNING SPECTROSCOPY OF ACTIVE AND INACTIVATEE) PHOTOSYSTEM II PARTICLES

This paper reports transient and persistent hole‐burning of photosynthetically active as well as chemically reduced and heat inactivated photosystem II particles isolated from cyanobacteria. Transient spectra of active and non‐active particles are significantly different. For both, the possible origin of the bottle‐neck state is discussed. Persistent holes were ascribed to the antenna complex of photosystem II. From their width the energy transfer rate was estimated to be 4.8 ps at 4.2 K.

VISIBLE PHOTOLUMINESCENCE IN HYDROGENATED AMORPHOUS SILICON GROWN IN MICROWAVE PLASMA FROM SIH4 STRONGLY DILUTED WITH HE

Room temperature visible photoluminescence (PL) of wide-band gap hydrogenated amorphous silicon (a-Si:H) thin films prepared in SiH4 microwave plasma strongly diluted with He is reported. Films were characterized by means of optical and infrared absorption, hydrogen thermal desorption, and Raman scattering. The band gap of a-Si:H films varies within the interval 2.0–2.2 eV, corresponding PL maxima are located at 1.4–1.6 eV. The highest PL intensity was observed in samples with a position of H–Si–H symmetric stretching vibration of the –(SiH2)n– units near the frequency of 2100 cm−1. The strong evidence for two distinct types of PL processes is presented: one being linked with oligosilanes and the second one attributed to electron-hole recombination in tail states.