Sergei Alekseev

Chemically modified porous silicon for laser desorption/ionization mass spectrometry of ionic dyes.

Desorption/ionization on silicon (DIOS) mass spectra of model ionic dyes methylene blue (MB+Cl-) and methyl orange (Na+MO-) were studied using p+ type-derived porous silicon (PS) free layers. As-prepared PS (PS-H), the PS thermally oxidized at 300 degrees C (PS-OX), PS with chemically grafted cation-exchanging alkylsulfonic acid (PS-SO(3)H) and anion-exchanging propyl-octadecyldimethylammonium chloride (PS-ODMA+Cl-) groups was tested as ionization platforms. Two mechanisms of the methylene blue desorption/ionization were found: (1) the formation of [MB + H]+* ion due to the reduction/protonation of MB+, which is predominant for PS-H and PS-OX platforms and (2) direct thermal desorption of the MB+ cation, prevailing for PS-SO3H. The fragmentation of the cation is significantly suppressed in the latter case. The samples of PS-SO3H and PS-ODMA+ Cl- efficiently adsorb the dyes of the opposite charge from their solutions via the ion-exchange. Consequent DIOS MS studies allow to detect only low fragmented ions (MB+ and MO-, respectively), demonstrating the potential of the ion-exchange adsorption combined with DIOS MS for the analysis of ionic organic compounds in solutions.

Photoluminescence of silicon nanoparticles chemically modified by alkyl groups and dispersed in low-polar liquids

A detailed comparative analysis of photoluminescence behavior of silicon nanoparticles in air and dispersed in low-polar liquids is reported. Efficient dispersion and excellent stability of the chemically modified nanoparticles in low-polar liquids are achieved. Influence of the chemical functionalization and of the low-polar liquids on steady-state and time-resolved photoluminescence of the silicon nanoparticles is investigated. Role of low-polar liquids on recombination mechanisms taking place in the nanoparticles is discussed in terms of Förster resonant energy transfer processes. Effect of exciting laser power on photoluminescence spectra of the silicon nanoparticles both in air and in low-polar liquids is investigated and the electronic mechanisms involved into the observed phenomena are discussed.

Effect of Silanol Groups on the Acidic and Catalytic Properties of Alkylsulphoacidic Silicas and SiO2/Nafion Nanocomposites

Sets of silicas covalently modified with alkylsulphoacidic groups (SiO2-SO3H) and nanocomposites of silica with superacidic Nafion® polymer (SiO2/Nafion) were synthesised. End-capping the silanol groups with trimethylsilyl groups was used to obtain surface hydrophobisation. The materials obtained were characterised by nitrogen and water adsorption measurements, MAS NMR spectroscopy and dielectric relaxation spectroscopy (DRS). The acidity of the materials was tested by 31P MAS NMR spectroscopic examination of samples with adsorbed triethylphosphine oxide (TEPO). It was demonstrated that end-capping excluded part of the surface silanol groups from interaction with acidic groups, thereby causing the sulphonic groups to exhibit an increased acidity. With SiO2-SO3H, end-capping also increased their catalytic activity in the synthesis of ethyl t-butyl ether (ETBE). However, end-capping had a negligible effect on the catalytic properties of SiO2/Nafion, probably because the catalytic reaction occurred inside the Nafion polymer nanoparticles.

Synthesis and structure of grafted layer of silicas modified with alkanesulfonic acid

A convenient method for preparation of silicas modified with alkanesulfonic acid through the reaction of NaHSO3 with the C2H3 groups grafted on the SiO2 surface was proposed. The influence of the carrier and the structure of the grafted layer on the modification process was studied by DRIFT, MAS NMR, and temperature-programmed desorption mass spectrometry (TPD-MS). It was found that at most 40% of the C2H3 groups transform to the acid sites, which mostly have the structure of 2-Si-ethanesulfonic acid. The mechanism of the thermal decomposition of the grafted groups, which are stable up to 240 °C, was proposed. The monomeric structure of the grafted layer leads to partial olygomerization of the C2H3 groups through the N=N bonds.

SiC as a Biocompatible Marker for Cell Labeling

This chapter focuses on the application of SiC quantum dots (QDs) for fluorescent cell labeling. Two different top-down processes are investigated to produce SiC QDs from bulk or polycrystalline SiC substrates: electrochemical etching and laser ablation. In both cases, a broad size distribution of crystalline SiC nanoparticles is obtained (ranging from few tens of nanometers down to 2nm in diameter). In electrochemical etching, the original polytype is conserved, whereas in laser ablation, some stacking faults are found in the naoparticles. The mechanism of photoluminescence has been investigated as a function of QD spatial and chemical configuration, i.e., interconnected QDs in a freestanding nanoporous layer, dry and wetted nanopowder obtained from the nanoporous layer, dispersion of QDs in various solvents (with different dielectric constants) and with various surfactants, and centrigugated solutions of QDs to keep the smallest ones. From all these experiments, both surface-related photoluminescence, probably due to C═O double bonds, and photoluminescence, due to discrete levels of quantum-confined carriers, have been evidenced.

Luminescence behavior of silicon and carbon nanoparticles dispersed in low-polar liquids

A comparative photoluminescence analysis of as-prepared and chemically modified (by alkyl chains -C18H37) silicon and carbon nanoparticles dispersed in low-polar liquids is reported. Influence of the low-polar liquid nature and ambient temperature on photoluminescence of the nanoparticles has been investigated from the point of view of their possible application as thermal nanoprobes.

FEATURES OF FUNCTIONALIZATION OF MCM-41 SILICA MESOPOROUS MOLECULAR SIEVES DURING THEIR TEMPLATE SYNTHESIS

We have shown that functionalization of MCM-41 mesoporous molecular sieves during their template synthesis using the effect of solubilization of organic hydrophobic co-condensing compounds makes it possible to obtain highly ordered mesoporous molecular sieves, with concentrations of vinyl or allyl groups covalently bonded to the surface of the silica matrix of about 3.5 mmol/g, which after sulfonation have acidic properties and exhibit catalytic activity in the process of ethyl tert-butyl ether synthesis.

Impact of Water Adsorption on Nonlinear Optical Properties of Functionalized Porous Silicon

The porous silicon (PS) surface modification diagnostics due to functionalization and water adsorption/desorption processes were provided by the self-action effects of picosecond range pulsed laser radiation at 1064 nm. It was shown that the PS surface functionalization—oxide removal, alkylation, and oxidation—resulted in a refractive nonlinear optical (NLO) response sign turn to self-focusing (Δn>0) versus the self-defocusing (Δn<0) observed in the aged PS. The sensitivity of the proposed technique was revealed to water adsorption/desorption from the chemically oxidized PS interface. For the dried PS, the self-defocusing effect with corresponding NLO cubic susceptibility Re(χ(3))∼−4.7·10−9 esu was observed versus the self-focusing one (∼5·10−8 esu) for the PS positioned in saturated water vapor at room temperature. The obtained results demonstrate high sensitivity and wide versatility of the proposed readout technique based on pulsed laser radiation self-action at 1064 nm to the PS surface modification monitoring/diagnostics applications.

Mesoporous silicon carbide via nanocasting of Ludox® xerogel

Porous SiC with uniformly sized 12 nm and 22 nm spherical mesopores was synthesized from nanocomposites of polycarbosilane (PCS) preceramic polymer and xerogels of Ludox® SiO2 nanoparticles as templates. The influence of PCS type (Mw 800 and 2000 Da), PCS : SiO2 ratio, pyrolysis temperature 1200–1400 °C, and addition of Ni complex to the preceramic composite was studied with respect to the SiC porous morphology, crystalline structure and chemical properties. We found that the pore walls of Ni-free por-SiC are composed of relatively large (20 nm) crystallites embedded inside a poorly crystalline SiC/SiC1+x phase. Increasing the pyrolysis temperature resulted in an increase of the large crystallites fraction, as well as of the stability with regard to air oxidation; however, some degradation of the porous morphology was noted too. The presence of Ni (1.5% wt relatively to PCS) noticeably improved the crystallinity of por-SiC prepared at 1200–1300 °C, with no degradation of the porous morphology occurring. On the other hand, higher Ni loadings and temperatures led to the transformation of the porous morphology into aggregates of irregularly packed large crystallites.

Size and Surface Chemistry Tuning of Silicon Carbide Nanoparticles

Chemical transformations on the surface of commercially available 3C-SiC nanoparticles were studied by means of FTIR, XPS, and temperature-programmed desorption mass spectrometry methods. Thermal oxidation of SiC NPs resulted in the formation of a hydroxylated SiO2 surface layer with C3Si-H and CHx groups over the SiO2/SiC interface. Controllable oxidation followed by oxide dissolution in HF or KOH solution allowed the SiC NPs size tuning from 17 to 9 nm. Oxide-free SiC surfaces, terminated by hydroxyls and C3Si-H groups, can be efficiently functionalized by alkenes under thermal or photochemical initiation. Treatment of SiC NPs by HF/HNO3 mixture produces a carbon-enriched surface layer with carboxylic acid groups susceptible to amide chemistry functionalization. The hydroxylated, carboxylated, and aminated SiC NPs form stable aqueous sols.