Sexiphenyl on Cu(100): nc-AFM tip functionalization and identification

The contrast obtained in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images is determined by the tip termination and symmetry. Functionalizing the tip with a single metal atom, CO molecule or organic species has been shown to provide high spatial resolution and insights into tip-surface interactions. A topic where this concept is utilized is the adsorption of organic molecules at surfaces. With this work we aim to contribute to the growing database of organic molecules that allow assignment by intra-molecular imaging. We investigated the organic molecule para-sexiphenyl (C36H26, 6P) on Cu(100) using low-temperature STM and non-contact AFM with intra-molecular resolution. In the sub-monolayer regime we find a planar and flat adsorption with the 6P molecules rotated 10{\deg} off the directions. In this configuration, four of the six phenyl rings occupy almost equivalent sites on the surface. The 6P molecules are further investigated with CO- functionalized tips, in comparison to a single-atom metal and 6P-terminated tip. We also show that the procedure of using adsorbed CO to characterize tips introduced by Hofmann et al. Phys. Rev. B 112 (2014) 066101 is useful when the tip is terminated with an organic molecule

Nucleation and growth of thin films of rod--like conjugated molecules

Thin films formed from small molecules rapidly gain importance in different technological fields. To explain their growth, methods developed for zero--dimensional atoms as the film forming particles are applied. However, in organic thin film growth the dimensionality of the building blocks comes into play. Using the special case of the model molecule para--Sexiphenyl, we will emphasize the challenges that arise from the anisotropic and one--dimensional nature of building blocks. Differences or common features with other rodlike molecules will be discussed. The typical morphologies encountered for this group of molecules and the relevant growth modes will be investigated. Special attention is given to the transition between flat lying and upright orientation of the building blocks during nucleation. We will further discuss methods to control the molecular orientation and describe the involved diffusion processes qualitatively and quantitatively.Comment: as submitted to JPCM (revised version) fixed figures and a few lines of tex

Luminescent polymer blends in hybrid organic-inorganic polymer light-emitting devices (HyPLEDs)

Electronic MaterialsOrganic/polymer based light-emitting diodes(OLEDs/PLEDs) have enormous technological significance for low-cost solution processing such as spin coating, dip coating and ink-jet printing. A recent promising approach toward air-stable devices without a low work function metal is to use inorganic metal oxides as charge injection and transport layer. In particular, solution processable n-type ZnO layer as electron injection/transport layer was mainly used in hybrid organic-inorganic polymer light-emitting diodes (HyPLEDs) and organic solar cells (OSCs). Based on HyPLEDs, PLED performance was optimized by blending luminescent polymers with various functional molecules or same fluorescent polymers. Blending approaches are generally considered to offer simpler device fabrication, higher device efficiency and performance. Here, we report a luminescent polymer blend system by mixing fluorescent polymer with ionic salt for hybrid organic-inorganic polymer light-emitting electrochemical cells (HyPLECs), and fluorescent red, green, and blue (RGB) polymers for white PLEDs (WPLEDs). Further development was achieved by attaching cholesteric liquid crystal (CLC) reflector in front of the surface of WPLEDs to obtain white circularly polarized (CP) electroluminescence (EL). This thesis is organized as follow. An introduction of semiconducting polymers, characteristics of OLEDs and hybrid PLEDs with diverse transition metal oxide (TMO) layer are mainly described in the chapter 1. In particular, hybrid organic-inorganic polymer light emitting diode (HyPLEDs) are new type of top-emission device in which device can combine with n-type thin-film transistor (TFTs) for application to active matrix structure. The ZnO layer as a cathode material in the inverted configuration takes a role of the electron-injecting layer as they combine properties such as transparency, low resistance, and air-stability. In the chapter 2, we demonstrate enhanced device performance by using a blend of emissive polymer (“Super Yellow”) and mobile ionic liquid molecules (ILMs) in hybrid organic-inorganic polymeric light-emitting electrochemical cells (HyPLECs) with high air stability. The mobile anions and cations redistributed near each electrode/active layer interface make ohmic contacts, thereby enhancing current density and electroluminescence efficiency at relatively low operating voltage. Moreover, a luminescent blend of blue-emitting polymer (“M-blue”), orange-emitting dye (DCM), and ILMs was investigated to achieve white emission in HyPLECs. By using ILMs, we can observe the characteristics of LECs with low operating voltage and air stability of HyPLECs by introducing ZnO layer. Finally, we investigate RGB ternary blend in single active layer for white emission. WPLEDs using polymer blends showed low turn-on voltage, high brightness, efficiency, and color stability. Furthermore, we observed CP-EL by combination of WPLED and cholesteric liquid crystal (CLC) reflector.ope

Organic Light Emitting Devices

This book describes the state-of-the-art advancement in the field of organic electroluminescence contributed by many researchers with internationally established expertise in the field. It includes original contributions on the synthesis of suitable organic materials, fabrication of organic light emitting devices (OLEDs) and organic white light emitting devices (WOLEDs), characterization of these devices and some designs for optimal performance. All chapters are self-sufficient in presenting their contents. The cost effective chemical technology offers many exciting possibilities for OLEDs and organic solar cells (OSCs) to be futuristic solutions for lighting and power generation. A common flexible substrate can be used to fabricate OLEDs on one side facing a room and OSCs on the other side facing the sun. The device thus fabricated can generate power in the day time and light a room/house at night. The book covers developments on OLEDs, WOLEDs and briefly on OSCs as well

The growth of organic small molecule and inorganic halide perovskite crystalline thin films

Organic semiconductors have shown exceptional opportunities for manipulating energy in a range of structures in light-emitting diodes, lasers, transistors, transparent photovoltaics, etc. with the presence of excitons at room temperature that distinguishes them from traditional semiconductors. The control over the crystalline order, orientation, layer-coupling as well as defect formation are the key to the fabrication and optimization for improving the performance of organic electronics. In the first part of this thesis, we focus on understanding organic crystalline growth. Organic homoepitaxy growth mode is mapped as a function of vapor phase growth conditions on high quality organic crystalline substrates. Organic-organic hetero-quasiepitaxy is then studied to explore the design rules for ordered alternating organic growth similar to inorganic quantum well structure. A unique organic edge driven case is demonstrated providing new routes to controlling molecular orientation and multilayer ordering. These results could enable entirely new opportunities for enhancing unique excitonic tunability and could also be used as a platform to study organic exciton confinement and strong coupling.The second part of the thesis is focused on inorganic halide perovskite growth. Hybrid halide perovskites have attracted tremendous attention as an exceptional new class of semiconductors for solar harvesting, light emission, lasing, quantum dots, thin-film electronics, etc. However, the toxicity of lead devices and lead manufacturing combined with the instability of organic components have been two key barriers to widespread applications. In this work, we demonstrate the first single-domain epitaxial growth of halide perovskites. This in situ growth study is enabled by the study of homoepitaxy and mixed-homoepitaxy of metal halide crystals that demonstrates the capability of performing reflection high-energy electron diffraction (RHEED) on insulating surfaces. We then focus on tin-based inorganic halide perovskites, CsSnX3 (X = Cl, Br, and I), on lattice-matched metal halide crystals via reactive vapor growth route that leads to single-domain epitaxial films with excellent crystalline order lacking in solution processing. Exploiting this highly controllable epitaxial growth we demonstrate the first halide perovskite quantum wells that creates photoluminescent tunability with different well width. These demonstrations could spark the exploration of a full range of epitaxial halide perovskites and lead to novel applications for metal-halide-perovskite based single-crystal epitaxial optoelectronics.Thesis (Ph. D.)--Michigan State University. Materials Science and Engineering, 2019Includes bibliographical reference