Monika Benkovicova

Fe3O4/γ-Fe2O3 nanoparticle multilayers deposited by the Langmuir-Blodgett technique for gas sensors application.

Fe3O4/γ-Fe2O3 nanoparticles (NPs) based thin films were used as active layers in solid state resistive chemical sensors. NPs were synthesized by high temperature solution phase reaction. Sensing NP monolayers (ML) were deposited by Langmuir-Blodgett (LB) techniques onto chemoresistive transduction platforms. The sensing ML were UV treated to remove NP insulating capping. Sensors surface was characterized by scanning electron microscopy (SEM). Systematic gas sensing tests in controlled atmosphere were carried out toward NO2, CO, and acetone at different concentrations and working temperatures of the sensing layers. The best sensing performance results were obtained for sensors with higher NPs coverage (10 ML), mainly for NO2 gas showing interesting selectivity toward nitrogen oxides. Electrical properties and conduction mechanisms are discussed.

Nonequilibrium Phases of Nanoparticle Langmuir Films

We report on an in-situ observation of the colloidal silver nanoparticle self-assembly into a close-packed monolayer at the air/water interface followed by a 2D to 3D transition. Using the fast tracking GISAXS technique, we were able to observe the immediate response to the compression of the self-assembled nanoparticle layer at the air/water interface and to identify all relevant intermediate stages including those far from the equilibrium. In particular, a new nonequilibrium phase before the monolayer collapse via the 2D to 3D transition was found that is inaccessible by the competing direct space imaging techniques such as the scanning and transmission electron microscopies due to the high water vapor pressure and surface tension.

Preparation of sterically stabilized gold nanoparticles for plasmonic applications

Plasmonic nanoparticles such as those of gold or silver have been recently investigated as a possible way to improve light absorption in thin film solar cells. Here, a simple method for the preparation of spherical plasmonic gold nanoparticles in the form of a colloidal solution is presented. The nanoparticle diameter is controlled in the range from several nm to tens of nm depending on the synthesis parameters with the size dispersion down to 14 %. The synthesis is based on thermal decomposition and reduction of the chloroauric acid in the presence of a stabilizing capping agent (surfactant) that is very slowly injected into the hot solvent. The surfactant prevents uncontrolled nanoparticle aggregation during the growth process. The nanoparticle size and shape depend on the type of the stabilizing agent. Surfactants with different lengths of the hydrocarbon chains such as Z-octa-9-decenylamine (oleylamine) with AgNO3 and polyvinylpyrrolidone with AgNO3 were used for the steric stabilization. Hydrodynamic diameter of the gold nanoparticles in the colloidal solution was determined by dynamic light scattering while the size of the nanoparticle metallic core was found by small-angle X-ray scattering. The UV-VIS-NIR spectrophotometer measurements revealed a plasmon resonance absorption in the 500–600 nm range. Self-assembled nanoparticle arrays on a silicon substrate were prepared by drop casting followed by spontaneous evaporation of the solvent and by a modified Langmuir-Blodgett deposition. The degree of perfection of the self-assembled arrays was analyzed by scanning electron microscopy and grazing-incidence small-angle X-ray scattering. Homogeneous close-packed hexagonal ordering of the nanoparticles stretching over large areas was evidenced. These results document the viability of the proposed nanoparticle synthesis for the preparation of high-quality plasmonic templates for thin film solar cells with enhanced power conversion efficiency, surface enhanced Raman scattering, and other applications.

P3HT:PCBM Based Organic Solar Cells: Structure Optimization and Improving External Quantum Efficiency by Plasmonic Nanoparticles Incorporation

We optimized morphology of the active layer of organic solar cells based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction. The optimization process includes a 20 min solvent annealing followed by a 5 min thermal annealing at 110 ∘C. After such a procedure a ∼ 3 % power conversion efficiency (PCE) was achieved. Adding of gold plasmonic nanoparticles (0.1 wt %) resulted in external quantum efficiency improvement.

Sensitivity and long-term stability of γ-Fe 2 O 3 and CoFe 2 O 4 nanoparticle gas sensors for NO 2 , CO and acetone sensing — A comparative study

Gas sensors based on γ-Fe₂O₃ and CoFe₂O₄ nanoparticles (NPs) were prepared by Langmuir-Schaeffer deposition onto Al₂O₃ and auxilliary Si substrates. NPs with the size of 6.4 nm (Fe₂O₃) and 7.6 nm (CoFe₂O₄) and polydispersity of 9 % were fabricated by chemical route. NP arrays were analyzed by GIXRD, SEM, TEM, DLS and ellipsometry. Fe₂O₃ sensors show high response of 38 for 500 ppb of strongly oxidizing NO₂. The response of CoFe₂O₄ sensors is 10 at 5 ppm of NO₂. Sensitivities are lower with reducing CO and acetone gases. After one year storage at room temperature, a 6 % decrease of response was found. The sensor sensitivities are discussed comparing with the best published results so far.

Silicon substrates for nanoparticle gas sensors with embedded electrodes and planar surface

Standard ceramic substrates with patterned surface electrodes few hundred nm thick are not appropriate for advanced nanoparticle gas sensors. Smooth silicon planar substrates with embedded metallic electrodes are introduced in our work. They give the chance to cover the small substrate surface irregularities by fragile nanoparticle arrays and provide an opportunity to integrate sensors into electronic circuits. The suppression of high power consumption of integrated sensor heater due to the high thermal conductivity of Si is resolved using substrates hanging on thin contacting wires without getting in touch with the socket. Embedded Au electrodes were created in sputtered amorphous silicon layer by lift-off technique. The ridges at Au – a:Si interface were reduced due to improved Au wettability of a:Si vertical walls. γ-Fe2O3 nanoparticle deposits on the Si patterned substrates were studied.Standard ceramic substrates with patterned surface electrodes few hundred nm thick are not appropriate for advanced nanoparticle gas sensors. Smooth silicon planar substrates with embedded metallic electrodes are introduced in our work. They give the chance to cover the small substrate surface irregularities by fragile nanoparticle arrays and provide an opportunity to integrate sensors into electronic circuits. The suppression of high power consumption of integrated sensor heater due to the high thermal conductivity of Si is resolved using substrates hanging on thin contacting wires without getting in touch with the socket. Embedded Au electrodes were created in sputtered amorphous silicon layer by lift-off technique. The ridges at Au – a:Si interface were reduced due to improved Au wettability of a:Si vertical walls. γ-Fe2O3 nanoparticle deposits on the Si patterned substrates were studied.

Control of interparticle distance of ordered iron-oxide nanoparticle assemblies by means of surfactant design

Effect of the nanoparticle effective size on interparticle distance in ordered nanoparticle films was investigated. To obtain different interparticle distance, the iron oxide nanoparticles (IONPs) with the size of 6.5 nm were functionalized by three kinds of surfactants: mixture of oleic acid/oleylamine, polybutylacrylate and polystyrene. Both hydrodynamic diameter and size distribution of nanoparticles in colloidal solution were measured by dynamic light scattering (DLS) in range from 8 to 17 nm. The ordering and homogeneity of the prepared Langmuir monolayers on solid surface was studied by scanning electron microscope (SEM) and atomic force microscopy (AFM). Method of pair correlation function was used for calculation of interparticle distance ensuing from SEM images. The distances from center to center of particles varied in the range from 6.75 to 11.75 nm. From the results it follows that we are able to change of interparticle distance on a solid substrate using of nanoparticles with different surfactant size.Effect of the nanoparticle effective size on interparticle distance in ordered nanoparticle films was investigated. To obtain different interparticle distance, the iron oxide nanoparticles (IONPs) with the size of 6.5 nm were functionalized by three kinds of surfactants: mixture of oleic acid/oleylamine, polybutylacrylate and polystyrene. Both hydrodynamic diameter and size distribution of nanoparticles in colloidal solution were measured by dynamic light scattering (DLS) in range from 8 to 17 nm. The ordering and homogeneity of the prepared Langmuir monolayers on solid surface was studied by scanning electron microscope (SEM) and atomic force microscopy (AFM). Method of pair correlation function was used for calculation of interparticle distance ensuing from SEM images. The distances from center to center of particles varied in the range from 6.75 to 11.75 nm. From the results it follows that we are able to change of interparticle distance on a solid substrate using of nanoparticles with different surfac...

Iron Oxides Nanoparticles Langmuir-Schaeffer Multilayers for Chemoresistive Gas Sensing

Gas sensors based on mixtures of iron oxides nanoparticles (NPs) modified with Pd NPs at the different NPs Pd: Fe3O4/γ-Fe2O3 proportions were prepared and characterized. Nanoparticles diameter was 6–7 nm. NPs films were deposited from the mixtures of Fe3O4/γ-Fe2O3 and Pd NPs colloidal solutions by modified Langmuir-Schaeffer technique onto alumina substrates. Samples were composed from 4 Fe3O4/γ-Fe2O3 NP monolayers (MLs). Sensors were tested toward acetone vapors and NO2 at concentration range of 5–200 and 0.5–12 ppm respectively in dry air, and at different working temperatures. Pd-doped Fe3O4/γ-Fe2O3 sensors showed p-type gas response, higher conductance and higher conduction activation energies compared to pure Fe3O4/γ-Fe2O3 sensor that gives n-type gas response. Results were tentatively explained in terms of surface interactions with oxidizing and reducing species, and Nearest Neighbor Hopping conduction model (NNH).

Nitrogen Dioxide and Acetone Sensors Based on Iron Oxide Nanoparticles

The Fe2O3 and CoFe2O4 nanoparticle-based Langmuir-Blodgett lms for sensingof nitrogen dioxide (NO2) and acetone vapours have been explored. Both the sensitivity of thechemiresistors and dynamic properties, such as the response/recovery time, have been probed independence of the number of nanoparticle monolayers and working temperatures. The responseof 23 at the NO2 concentration of 1 ppm has been monitored suggesting the pertinent sensitivityin the deep sub-ppm range, i.e. approaching the canine detection limit, and likewise implyingthe supposable detection of nitrate-based explosives.

Gas sensing properties and electrical resistance of Langmuir-Blodgett iron oxide nanoparticle arrays

Nanoparticle (NP) sensors are prepared by Langmuir-Blodgett deposition of colloid Fe₂O₃ or CoFe₂O₄ NPs onto alumina substrates. The thickness of the deposit is 1, 2, 4 or 7 NP monolayers (ML). NP arrays are analysed by GIXRD, SEM, SAXS, DLS and XANES. The average NP diameters are 6.4 nm and 7.6 nm for Fe₂O₃ and CoFe₂O₄, respectively. The response current of the sensor is measured at ≈ 400 °C in the mixtures of dry air with the monitored CO and NO₂ gases. The best relative response is 38 for 500 ppb of NO₂. The resistance of NP arrays was studied between 250 and 500 °C. With 2 ML Fe₂O₃ deposit the sheet resistance decreases with increasing temperature from 10¹¹ to 10⁹ Ω. Activation energy of the semiconductor type conductivity is 0.75 eV. High gas monitoring sensitivity of NP sensors is due to the big effective surface of Fe₂O₃ and CoFe₂O₄ NP arrays.