Alexander Satka

Influence of surface oxidation on plasmon resonance in monolayer of gold and silver nanoparticles

Surface plasmon resonance of gold and silver nanoparticle (NP) layers is investigated by the experiment as well as simulations. Although the good agreement was found for gold NP film, a significant mismatch in the resonance energy for silver NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in silver NPs. As an alternative to the NP size-dependent Drude model, the analysis based on effective medium approximation for refractive index of Ag-Ag2O material system is carried out and compared with the core-shell model. Both Mies model and numerical simulation results illustrate shift of the surface plasmon resonance due to silver NP surface oxidation.Surface plasmon resonance of gold and silver nanoparticle (NP) layers is investigated by the experiment as well as simulations. Although the good agreement was found for gold NP film, a significant mismatch in the resonance energy for silver NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in silver NPs. As an alternative to the NP size-dependent Drude model, the analysis based on effective medium approximation for refractive index of Ag-Ag2O material system is carried out and compared with the core-shell model. Both Mies model and numerical simulation results illustrate shift of the surface plasmon resonance due to silver NP surface oxidation.

Towards strain gauges based on a self-assembled nanoparticle monolayer--SAXS study.

An in situ small-angle x-ray scattering study of the nanoparticle displacement in a self-assembled monolayer as a function of a supporting membrane strain is presented. The average nanoparticle spacing is 6.7 nm in the unstrained state and increases in the applied force direction, following linearly the membrane strain which reaches the maximum value of 11%. The experimental results suggest a continuous mutual shift of the nanoparticles and their gradual separation with the growing stress rather than nanoparticle islands formation. No measurable shift of the nanoparticles was observed in the direction perpendicular to the applied stress.

Facet recovery and light emission from GaN/InGaN/GaN core-shell structures grown by metal organic vapour phase epitaxy on etched GaN nanorod arrays

The use of etched nanorods from a planar template as a growth scaffold for a highly regular GaN/InGaN/GaN core-shell structure is demonstrated. The recovery of m-plane non-polar facets from etched high-aspect-ratio GaN nanorods is studied with and without the introduction of a hydrogen silsesquioxane passivation layer at the bottom of the etched nanorod arrays. This layer successfully prevented c-plane growth between the nanorods, resulting in vertical nanorod sidewalls (∼89.8°) and a more regular height distribution than re-growth on unpassivated nanorods. The height variation on passivated nanorods is solely determined by the uniformity of nanorod diameter, which degrades with increased growth duration. Facet-dependent indium incorporation of GaN/InGaN/GaN core-shell layers regrown onto the etched nanorods is observed by high-resolution cathodoluminescence imaging. Sharp features corresponding to diffracted wave-guide modes in angle-resolved photoluminescence measurements are evidence of the uniformity of the full core-shell structure grown on ordered etched nanorods.

Light emission from InGaN quantum wells grown on the facets of closely spaced GaN nano-pyramids formed by nano-imprinting

InxGa1-xN/GaN quantum wells have been grown on the {1011} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.

Modified Langmuir-Blodgett deposition of nanoparticles - measurement of 2D to 3D ordered arrays

Modified Langmuir-Blodgett deposition of nanoparticles - measurement of 2D to 3D ordered arrays The ordered nanoparticle monolayers and multilayers over macroscopic areas were prepared by the modified Langmuir-Blodgett method. Using this approach, the nanoparticle monolayer is formed on the water surface by compression and subsequently it is transferred onto the substrate by a controlled removal of the water subphase. The ordering and homogeneity of the prepared mono- and multilayers was studied by scanning electron microscope (SEM), grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR) techniques. From the results it follows that an ordered nanoparticle monolayer was formed over a large area. For the multilayer, the layering and lateral ordering of each layer was confirmed by XRR and SEM performed after the deposition of each nanoparticle layer.

Kinetics of nanoparticle reassembly mediated by UV-photolysis of surfactant.

Real-time reassembly of an ordered nanoparticle monolayer due to UV-photolysis of the surfactant shell of nanoparticles was observed. The technique of grazing-incidence small-angle X-ray scattering provided the possibility to track in situ the nanoparticle pair correlation function of the sample processed in a UV-ozone reactor. The analysis revealed a total shift of approximately 1 nm of the nanoparticle nearest-neighbor distance. The temporal evolution of the interparticle distance proved to be the first-order process governed by the UV-photolysis and described by a single-exponential decay function. The nanoparticles tend to agglomerate into a labyrinth-like structure with a typical length scale of some 30 nm.

Characterization of mechanochemically synthesized lead selenide

Mechanochemical synthesis of lead selenide PbSe nanocrystals was performed by high-energy milling of lead and selenium powders in a planetary ball mill. The structure properties of synthesized lead selenide were characterized by XRD analysis that confirmed crystalline nature of PbSe nanocrystals. Calculated average size of PbSe crystallites was 37 nm. The methods of particle size distribution analysis, specific surface area measurement, SEM and TEM were used for the characterization of surface and morphology of PbSe nanocrystals. SEM analysis revealed agglomerates of PbSe particles. However, HRTEM analysis confirmed perfect stoichiometric PbSe cubes with NaCl structure as well. UV-VIS-NIR spectrophotometry was used to confirm the blue shift of the small particles occurring in the powder product obtained by the mechanochemical synthesis.

Real-time tracking of superparamagnetic nanoparticle self-assembly.

The spontaneous self-assembly process of superparamagnetic nanoparticles in a fast-drying colloidal drop is observed in real time. The grazing-incidence small-angle X-ray scattering (GISAXS) technique is employed for an in situ tracking of the reciprocal space, with a 3 ms delay time between subsequent frames delivered by a new generation of X-ray cameras. A focused synchrotron beam and sophisticated sample oscillations make it possible to relate the dynamic reciprocal to direct space features and to localize the self-assembly. In particular, no nanoparticle ordering is found inside the evaporating drop and near-surface region down to a drop thickness of 90 microm. Scanning through the shrinking drop-contact line indicates the start of self-assembly near the drop three-phase interface, in accord with theoretical predictions. The results obtained have direct implications for establishing the self-assembly process as a routine technological step in the preparation of new nanostructures.

Advanced Methodology for Fast 3-D TCAD Device/Circuit Electrothermal Simulation and Analysis of Power HEMTs

This paper introduces an advanced methodology for fast 3-D Technology Computer Aided Design (TCAD) electrothermal simulation for the analysis of power devices. The proposed methodology is based on coupling finite element method (FEM) thermal and circuit electrical simulation in a mixed-mode setup. A power InAlN/GaN high-electron mobility transistor (HEMT) is used to perform validation of the designed electrothermal simulation. A new equivalent temperature-dependent nonlinear analytical large signal circuit model of HEMT is proposed. The model is implemented to Synopsys TCAD Sentaurus using compact model interface. The designed electrothermal simulation methodology is developed to shorten the simulation time for complex 3-D devices. This approach combines the speed and accuracy, and couples temperature nonuniformity to the active device electrothermal behavior. The simulation results are compared with the measured data and results of 2-D FEM simulations. The features and limitations of the methods are analyzed and presented.

PbS nanostructures synthesized via surfactant assisted mechanochemical route

PbS nanocrystals using surfactant assisted mechanochemical route has been successfully prepared. The methods of XRD, SEM, surface area and particle size measurements were used for nanocrystals characterization. The XRD patterns confirmed the presence of galena PbS (JCPDS 5–592) whatever treatment conditions were applied. The strong observable peaks indicate the highly crystalline nature in formation of PbS nanostructures where preferential crystal growth in the (200) direction after chelating agent (EDTANa2•2H2O) addition has been observed. The mean volume weighted crystallite size 4.9 nm and 35 nm has been calculated from XRD data using Williamson-Hall method for PbS synthesized without and/or with chelating agent, respectively corresponding with surface weighted crystallites sizes of 2.9 and 18.8 nm. The sample prepared without surfactant yields the smaller crystallites and the higher microstrain compared with surfactant assisted synthesis. The obtained results illustrate a possibility to manipulate crystal morphology by combining effect of milling and surfactant application.