Nanoscale Interface for Organic Electronics

The following sections are included:

• PREFACE

• CONTENTS

In this chapter, an introduction to nanoscale interface is described in terms of organic electronics as well as molecular electronics. In contrast to scale-down technology, so-called “top-down” technology, “bottom-up” technology is a key in this field. Using this technique, artificial molecular systems comprised of organic molecules are designed, and utilized to fruition novel functions at nanoscale. As a result, understanding of alignment of molecules, interaction between molecules and substrates and so forth are getting important in nanodevice and organic electronic devices. In this chapter we briefly summarize the feature of nanoscale organic interface from viewpoint of organic and molecular device physics.

We present a brief review on the limitation of charge injection to organic field-effect transistor due to interface metal-organic phenomenon, the contact resistance. Experimental methods for estimation of the contact resistance are summarized and their analysis is discussed. Voltage dependence of the contact resistance is carefully studied in respect to the definition of contact resistance and its time dependence during the device charging is illustrated. The physical background provides design of contact resistance.

Organic field-effect transistors (OFETs) have been studied extensively for flexible displays, organic sensors and radio frequency identification tags. However, conventional OFETs have remaining issues such as higher driving voltage, lower current operation and lower frequency response due to lower carrier mobility of organic semiconductors than their inorganic counter parts. Vertical-type organic transistors are strong candidates for improvement in the performance of the OFETs. This article describes an overview of the vertical-type organic transistors. We will focus on the progress in our recent experimental studies on organic static induction transistors (OSITs) as one type of the vertical-type organic transistors. OSITs are proposed for application in flexible sheet displays, logic devices, and organic light-emitting transistors (OLETs). In particular, this article describes our recent efforts to improve the performance of the OSITs by controlling the interface states between source electrode and organic semiconductor (pentacene). These results showed that both the high on/off ratio more than 10³ and the high-current value of larger than 40 μA were achieved. In addition, vertical-type OLET and logic devices were fabricated using the OSITs for the realization of flexible sheet displays. These results exhibit that the OSITs attract much attention for application in organic flexible displays.

Viologen derivatives, which have been widely investigated their well-behaved electrochemistry including electron transfer mediation, are attractive materials because of their chemical stability, their relatively simple behavior of redox reaction and their possible practical applications due to their electrochemical properties. The self-assembled monolayers (SAMs) were prepared surfaces by use of a viologen. The electrochemical property of the self-assembled viologen derivatives has been investigated with a quartz crystal microbalance (QCM), which has been known as a nanogram order mass detector. After then we attempted to apply this viologen derivative for biosensor design. A gold electrode modified by viologen-thiol modification is used to design a biosensor in corporate with hemoglobin (Hb). By incorporating with SAMs viologen and Hb, viologen can act as an electron transfer mediator between Hb and gold electrode. Experimental conditions influencing the biosensor performance such as pH and/or potential are optimized and assessed. This sensor offered an excellent electrochemical response for H₂O₂ concentration below μmol level with high sensitivity, selectivity and short time response.

In this chapter we present a review on the organometallic complexes which are employed for the organic light-emitting diodes (OLED) having nanoscale multilayer structures. The device fabrication and operation mechanism of OLED are followed by the ligands and metal complexes of Ir(III), Zn(II) and Sn(IV). The discussions have emphasized on their chemical structures and the electronic properties at the multilayer interfaces. The organometallic complexes are classified based on their colors and luminescent efficiencies.

White organic light-emitting diodes (WOLEDs) have proven to have great utility in solid-state light sources (SSLs) as well as backlights in liquid-crystal displays (LCDs) and full-color OLEDs, due to their light weight, low operating voltage, thinness, diffusiveness, and harmlesslight source for the restriction of certain hazardous substances directive/waste electrical and electronic equipment. In this article, advances in the understanding of WOLEDs are reviewed with dry- and wet-process and structure optimization for high efficiency WOLEDs. This article also reviews the highlights of the last few years and summarizes recent trends on WOLEDs. The technical discussions explore the question of whether WOLEDs are a viable alternative component in SSLs and backlights in LCD and full-color OLEDs.

We present a review on the electronic transport of alkanethiols molecular junctions in microscale or nanoscale via-hole structure. The statistical analysis provides criteria for defining “working” molecular electronic devices and selecting “representative” devices. Based on statistical analysis, we proposed a multi-barrier tunneling (MBT) model. The charge conduction mechanisms in molecular junctions are studied. These statistical analyses on the electronic transport properties provide an insight of metal-molecule contact effect in the conduction mechanism.

We review on the current-voltage response of redox-active ruthenium complex molecules in self-assembled monolayers. In single molecular junctions with STM, the threshold voltage to the high conductance state in the molecular junctions of RuII complexes was consistent with the electronic energy gap between the Fermi level of the gold substrate and the lowest ligand-centered redox state of the metal complex molecule. As an active redox center leading to conductance switching in the molecule, the lowest ligand-centered redox state of Ru^II complexes was suggested to trap an electron injected from the gold substrate. In molecular monolayer devices of Ru^II complexes with conducting polymer (PEDOT:PSS), the hydrophilic terminal thiol group of the alkylthiolate-tethered Ru^II terpyridine complexes can interact with the hydrophilic sulfonic acid groups of PEDOT:PSS, preventing the penetration of soft and metal top electrodes into the SAMs to result in high yields. As the number of alkyl chains or as alkyl chain length increases, the device yield of molecular monolayer memory will improve and its retention time will increase, guiding a new strategy for the development of volatile to non-volatile memory.

This study performed a study on the characteristics of self-assembled monolayers by fabricating monolayers using a self-assembly method in which viologen molecules were applied to an Au(111) substrate. The forming process of such monolayers was verified through the change in the resonance frequency of quartz crystal microbalance (QCM) and the morphology determined by using a scanning tunneling microscopy (STM). Based on the results of the measurement of the I-V characteristics of self-assembled viologen monolayers at a nanolevel using STM, it can be seen that the currents in its I-V characteristics according to the change in the length of viologen molecules were reduced from 292 nA to 40.6 nA in the region of the forward bias of +1.65 V. In addition, in the electric conduction mechanism at a high electric field region more than 1 V, it was verified that the Fowler-Nordheim tunneling dominated the mechanism.

There have been many investigations of the alignment of nematic liquid crystals by either a magnetic and/or an electric field. The basic features of these hydrodynamic processes have been characterized for the systems in their equilibrium and non-equilibrium states. To complement the experiments theoretical models, based on continuum theory, have been developed which successfully describe the static and dynamic phenomena. This technique has proved to be especially important for the investigation of liquid crystals. In this chapter, we describe some of our studies of the field-induced oscillatory behaviour of the nematic director for low molar mass nematics using deuterium NMR but with a sinusoidal electric as well as a static magnetic field to align the director. The director orientations have been observed from the time-resolved NMR experiments but we have found that they are also available, more directly, from the time-averaged NMR spectrum. The maximum and minimum angles made by the director with the magnetic field were determined, as a function of frequency, from the NMR spectrum averaged over many thousand cycles of the oscillations. These NMR experiments can give dynamic as well as static information concerning the nematic phase and could be used to provide a new route to certain material properties of the nematic.

Artificial muscles (soft actuators) based on electrochemomechanical deformation of conducting polymer films, e.g. polypyrrole and polyaniline are mentioned in their behavior of strain, stress, time response and training under tensile loads. The typical strain and stress of polypyrrole actuators are 20-30% and 22 MPa, respectively, which are larger than those of skeletal muscles. Under higher tensile loads, the artificial muscle shows creeping, which is serious problem for the practical application. The mechanism of creeping has been studied and found that the artificial muscles are strengthened by creeping, namely, the training or experience of high tensile loads. Fatigue of artificial muscle by electrochemical cycles is also found to originate from loose of conductivity due to stress.

The research is reviewed on the attenuated total reflection (ATR) method utilizing surface plasmon (SP) excitations at the interface between metal and dielectric ultrathin films, that is, the surface plasmon resonance (SPR) method. The ATR method is quite useful for evaluation of the dielectric ultrathin films and applications to sensors and plasmonic devices. Emission lights due to SP excitations are observed through the prism in the Kretschmann configuration for the ATR method, when metal films on the prism or organic films with metal films on the prism are directly irradiated from air by a laser beam, that is, reverse irradiation. The emission lights depend on the surface roughness of the films and are also related to the photoluminescence of organic molecules. The properties and applications of the SP emission lights are described. Among the SP excitation methods, one of the most well known techniques is a prism coupling technique based on the ATR configuration, while a grating coupling technique can be widely used for the sensing or plasmonic devices. The grating coupling excitation method is also described.

Some experimental results on the electrophoretic deposition of conjugated polymer are reported. The scanning electron micrographs clearly indicate that the morphology of films of a fluorine-based polymer PDOF-MEHPV by the electrophoretic deposition strongly depends on the toluene/acetonitrile ratio of the parent suspensions. It is also shown that the suspension prepared from a 0.1 g/l toluene solution of the polymer, can give flat and dense polymer films with a thickness in the order of 100 nm by using the electrophoretic deposition. The electrical current originating from the movement of colloidal particles has been measured for the suspensions containing polymer more than 0.2 g/l. However, the current due to impurities in the solvent makes it difficult to detect the electrical current corresponding to the particle movement in the suspensions containing smaller amount of the polymer. Polymer light-emitting devices having PDOF-MEHPV films with various thickness prepared by electrophoretic deposition have also been demonstrated.

Roll-to-roll printing process has been recently considered as a key technology for the realization of a penny RFID tag. To employ the current commercially avaiable R2R printing process to print RFID tags including antenna, rectifier, and digital processors on plastic or paper foils, materials and device circuits should be first well optimized to the R2R printers and substrates. In this chapter, we would like to briefly discuss about our own device circuits with our designed materials for R2R gravure printing RFID tags on plastic foils.

This chapter describes novel method for preparing polymeric thin films by way of physical vapor deposition (PVD) instead of the wet-coating techniques that have been used conventionally. The PVD has advantages in preparing highly pure nanometric thin films and their multilayered structures, and is suitable for constructing electronic and optical devices. Four methods of polymer PVD, including direct evaporation of polymers, coevaporation of bifunctional monomers, radical polymerization from single monomer, and surface-initiated deposition polymerization are described in this chapter. The type of polymers includes polyethylene, polytetrafluoroethylene, polyimide, polyurea, vinyl or acryl polymers, and polypeptide. It was found that the deposition polymerization provides organic thin films that are superior in stability and uniformity compared to the conventional vapor-deposited thin films of small molecules. In addition, the surfaceinitiated deposition polymerization have possibility of controlling the interface between inorganic substrate and polymer thin films. These characteristics can be utilized for improving the device characteristics such as the lifetime of organic light emitting diodes (OLEDs).

Currently, a nanoscle bioelectronic devices consisting of biomolecules and the interaction mechanisms of biomolecules have been developed in various industrial fields such as clinical diagnosis, electronic device, pharmaceutical screening, bioprocess, photonic device, environmental pollution detection, and etc. Biomaterials such as protein, DNA/RNA, pigment, and cells have been introduced into inorganic electronic structure in order to construct the biochip devices such as biophotodiode,bio-information storage device and bioelectroluminescence device, protein chip, DNA chip, and cell chip. Several nanobio technologies (nano-scaled immobilization technology of recombinant biomolecules, nanopatterning technology, detection technology and micro- or nano-electromechanical systems technology) have been applied in order to provide new functions to a nanobio device with advantages such as high density immobilization, orientation and control of immobilized biomolecules. In this review, we describe the nano scale fabrication of biomolecules based on electronic device using immobilizing technology (langmuir-blodgett and self-assembly) and also briefly prospect a bioelectronic device as one of the alternatives of current inorganic electronic devices.