Martin Weis

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.

Gradual channel approximation models for organic field-effect transistors: The space-charge field effect

The gradual channel approximation is widely used for organic field-effect transistors with an assumption of linear potential profile across the channel. However, this is in contradiction with reported potential profiles. Here, we discuss linear and nonlinear potential profiles in the meaning of the space-charge field generated by injected carriers. The influence on current-voltage relation used for mobility evaluation in linear and saturated regions is proposed as well as transition between these states. In addition, the effect of the space-charge on the potential drop and field around the drain electrode in the saturation region is discussed.

Plasmonic properties of Au-Ag nanoparticles: Distinctiveness of metal arrangements by optical study

The core-shell arrangement of binary compound plasmonic nanoparticles (NPs) is usually verified by plasmonic extinction spectra, since microscopy-based methods cannot provide analysis of many NPs. Here, we discuss possible scenarios of different metal arrangements: (i) core-shell model, (ii) bimetallic model, and (iii) mixture of pure metal NPs. The possibility of distinguishing individual cases is discussed in accordance with numerical simulations and an alternative characterization is suggested.

Improved Tolerance Against UV and Alpha Irradiation of Encapsulated Organic TFTs

In this work, we analyzed the effects of alpha and ultraviolet irradiation on encapsulated and non-encapsulated thin film-transistors. Up to -35% mobility variation occurs on unencapsulated devices after UV irradiation. The same UV irradiation induced less than -3% on encapsulated devices. By investigating the charge trapping kinetics, we show that the increased robustness comes from the reduced air absorption, rather than solely attenuating UV the components. Beside the degradation mechanisms seen on conventional MOS structures, alpha irradiation also induces another form of degradation in air, on devices without encapsulation, due to the reaction of the exposed pentacene with ozone or ionized oxygen generated by alpha, through air ionization. The encapsulation partially mitigates this mechanism, but it is ineffective in reducing the ionizing radiation effects intrinsic to the MOS structure.

Orientation Ordering of Nanoparticle Ag/Co Cores Controlled by Electric and Magnetic Fields

The effect of electric and magnetic fields on the sandwich structure Pt/hydrogenated amorphous silicon (a-Si:H)/stearic acid monolayer/Langmuir-Blodgett film of Ag/Co nanoparticles encapsulated in an organic envelope is studied. This structure is used as a working electrode in an electrochemical cell filled with NaCl solution (1 mM) and equipped with an Ag/AgCl reference electrode. Reversible changes in voltammograms are observed due to treatments (negative or positive bias voltage and simultaneous laser irradiation) applied to the designed structure before measurements. As an explanation of the observed phenomena we suggest that both the Co-up and Ag-up (on the a-Si:H surface) orientation orderings of nanoparticle Ag/Co cores are repeatedly reached. The role of the photovoltaic material (a-Si:H) in the observed behavior is explained. Voltammetric measurements with an applied magnetic field support our idea about the orientation ordering of nanoparticle cores.

Charge injection and transport properties of an organic light-emitting diode

Summary The charge behavior of organic light emitting diode (OLED) is investigated by steady-state current–voltage technique and impedance spectroscopy at various temperatures to obtain activation energies of charge injection and transport processes. Good agreement of activation energies obtained by steady-state and frequency-domain was used to analyze their contributions to the charge injection and transport. We concluded that charge is injected into the OLED device mostly through the interfacial states at low voltage region, whereas the thermionic injection dominates in the high voltage region. This comparison of experimental techniques demonstrates their capabilities of identification of major bottleneck of charge injection and transport.

Effects of positive and negative constant voltage stress on organic TFTs

We subjected Organic TFTs to positive and negative constant gate voltage stresses. Stress not only induces temporary charge trapping, but also permanent transconductance degradation. The permanent degradation is accelerated if the devices are stressed under illumination. Negative stress degrades the TFT much faster than the positive stress, at the same voltage. Furthermore, hard breakdowns are observed on devices stressed under light, with VGS= -65V. The degradation mainly comes from stress-induced interface trap generation, rather than photochemical reactions as observed with UV irradiations.

Self-Assembly of Nanoparticles at Solid and Liquid Surfaces

The research field of nanoparticle synthesis and related nanoparticle applied sciences have been steadily growing in the past two decades. The chemical synthesis of nanoparticles was improved up to the point that the organic and inorganic nanoparticle colloids are produced with a low size dispersion and with a well defined nanoparticle shape in large quantities. A stunning feature of a drying nanoparticle colloidal solution is the ability to create selfassembled arrays of nanoparticles. The self-assembled nanoparticle arrays mimic the natural crystals. The size of perfectly ordered domains is limited by the size dispersion of nanoparticles. Consequently the defects in the self-assembled structure are obvious and unavoidable. Despite these defects, the self-assembled nanoparticle arrays represent a new class of nanostructures built on “bottom-up” technological approach to fabrication. The traditional way of “top-down” fabrication technology primarily based on nano-lithography is complex, including many technological steps, time consuming and expensive. The main advantage is the tight control of all parameters governing the final nanostructures. On the other hand, the emerging fabrication technologies based on the self-assembled nanoparticles are fast, less complex and more price competitive. An extensive research is now focused on a deeper understanding of processes that control the self-assembly. New routes for directed or stimulated self-assembly are studied to achieve a tighter control than readily available in the spontaneous self-assembly. In this chapter we will discuss the spontaneous nanoparticle self-assembly with emphasis on characterization of nanoparticle arrays at various stages of the self-assembly process. The main diagnostic technique used throughout this chapter will be the grazing-incidence small-angle X-ray scattering (GISAXS) that represents a reliable and simple monitor of nanoparticle arrangement. The theoretical background of GISAXS and required instrumentation are described in Section 2. The most flexible surface to study the nanoparticle self-assembly processes is the liquid surface. The Section 3. reviews the latest results of studies combing the GISAXS technique with Langmuir nanoparticle layers on the water subphase. Almost all relevant nanoparticle applications rely on self-assembled arrays on solid surfaces. The Section 4 describes in detail the possibilities of nanoparticle transfer from liquid onto solid surfaces. The post-processing of self-assembled nanoparticle arrays and their applications are reviewed in the last Section 5.

Impact of the interfacial traps on the charge accumulation in organic transistors

A silver nanoparticles (NPs) self-assembled monolayer (SAM) was introduced in the pentacene organic field-effect transistors using modified Langmuir–Schäffer (horizontal lifting) technique, and its effect was being evaluated electrically using current–voltage (I−V) and impedance spectroscopy (IS) measurements. The I−V results showed a significant negative threshold voltage shift indicating that hole trapping phenomena exist inside the devices with about two orders higher in the contact resistance and an order lower in the effective mobility when a SAM was introduced. The IS measurements with the simulation of Maxwell–Wagner equivalent circuit revealed that the incorporation of the NPs SAM had a comparable negative voltage shift as in I–V measurements with a higher trapping time based on the simulation results.

Surface plasmon resonance of gold and silver nanoparticle monolayers: effect of coupling and surface oxides

Material properties are described by some physical parameters such as temperature or pressure. Optical properties of materials are very important for applications where is light as electromagnetic wave dominant. Behavior of the light in interaction with materials depends on refractive indices. These indices are same for various sizes of materials, but in nanoscale dimensions, they depend on some phenomena. Herein, we present the study of the silver (Ag) nanoparticle (NP) monolayer film and its dielectric properties. The aim of the study is to explain phenomenon why it is necessary to use effective material properties for Ag NPs, where these properties are size-dependent. The plasmonic properties of NP have been investigated by the finite domain time difference (FDTD) simulation methods. Although the good agreement of plasmonic resonances was found for gold (Au) NP film, a significant mismatch in the resonance energy for Ag NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in Ag NPs as a surface layer. This real structure of Ag NPs can be replaced by structure with suitable effective material properties. Results depict importance of the effective material properties in Ag NP film for reason of the presence of silver oxide. The Ag NPs with surface oxide exhibits linear tendency in the deviation of the effective dielectric function, which agrees with the experimental observations.