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This work investigates the energy-level alignment of hybrid inorganic/organic systems (HIOS) comprising ZnO as the major inorganic semiconductor. In addition to offering essential insights, the thesis demonstrates HIOS energy-level alignment tuning within an unprecedented energy range. (Sub)monolayers of organic molecular donors and acceptors are introduced as an interlayer to modify HIOS interface-energy levels. By studying numerous HIOS with varying properties, the author derives generally valid systematic insights into the fundamental processes at work. In addition to molecular pinning levels, he identifies adsorption-induced band bending and gap-state density of states as playing a crucial role in the interlayer-modified energy-level alignment, thus laying the foundation for rationally controlling HIOS interface electronic properties. The thesis also presents quantitative descriptions of many aspects of the processes, opening the door for innovative HIOS interfaces and for future applications of ZnO in electronic devices.
In recent years, ever more electronic devices have started to exploit the advantages of organic semiconductors. The work reported in this thesis focuses on analyzing theoretically the energy level alignment of different metal/organic interfaces, necessary to tailor devices with good performance. Traditional methods based on density functional theory (DFT), are not appropriate for analyzing them because they underestimate the organic energy gap and fail to correctly describe the van der Waals forces. Since the size of these systems prohibits the use of more accurate methods, corrections to those DFT drawbacks are desirable. In this work a combination of a standard DFT calculation with the inclusion of the charging energy (U) of the molecule, calculated from first principles, is presented. Regarding the dispersion forces, incorrect long range interaction is substituted by a van der Waals potential. With these corrections, the C60, benzene, pentacene, TTF and TCNQ/Au(111) interfaces are analyzed, both for single molecules and for a monolayer. The results validate the induced density of interface states model.
The first advanced textbook to provide a useful introduction in a brief, coherent and comprehensive way, with a focus on the fundamentals. After having read this book, students will be prepared to understand any of the many multi-authored books available in this field that discuss a particular aspect in more detail, and should also benefit from any of the textbooks in photochemistry or spectroscopy that concentrate on a particular mechanism. Based on a successful and well-proven lecture course given by one of the authors for many years, the book is clearly structured into four sections: electronic structure of organic semiconductors, charged and excited states in organic semiconductors, electronic and optical properties of organic semiconductors, and fundamentals of organic semiconductor devices.
Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Seven Volume Set summarizes current, fundamental knowledge of interfacial chemistry, bringing readers the latest developments in the field. As the chemical and physical properties and processes at solid and liquid interfaces are the scientific basis of so many technologies which enhance our lives and create new opportunities, its important to highlight how these technologies enable the design and optimization of functional materials for heterogeneous and electro-catalysts in food production, pollution control, energy conversion and storage, medical applications requiring biocompatibility, drug delivery, and more. This book provides an interdisciplinary view that lies at the intersection of these fields. Presents fundamental knowledge of interfacial chemistry, surface science and electrochemistry and provides cutting-edge research from academics and practitioners across various fields and global regions
Modern devices such as organic light emitting diodes use organic/oxide and organic/metal interfaces for crucial processes such as charge injection and charge transfer. Understanding fundamental physical processes occurring at these interfaces is essential to improving device performance. The ultimate goal of studying such interfaces is to form a predictive model of interfacial interactions, which has not yet been established. To this end, this thesis focuses on obtaining a better understanding of fundamental physical interactions governing molecular self-assembly and electronic energy level alignment at organic/metal and organic/oxide interfaces. This is accomplished by investigating both the molecular adsorption geometry using scanning tunneling microscopy, as well as the electronic structure at the interface using direct and inverse photoemission spectroscopy, and analyzing the results in the context of first principles electronic structure calculations. First, we study the adsorption geometry of zinc tetraphenylporphyrin (ZnTPP) molecules on three noble metal surfaces: Au(111), Ag(111), and Ag(100). These surfaces were chosen to systematically compare the molecular self-assembly and adsorption behavior on two metals of the same surface symmetry and two surface symmetries of one metal. From this investigation, we improve the understanding of self-assembly at organic/metal interfaces and the relative strengths of competing intermolecular and molecule-substrate interactions that influence molecular adsorption geometry. We then investigate the electronic structure of the ZnTPP/Au(111), Ag(111), and Ag(100) interfaces as examples of weakly-interacting systems. We compare these cases to ZnTPP on TiO2(110), a wide-bandgap oxide semiconductor, and explain the intermolecular and molecule-substrate interactions that determine the electronic energy level alignment at the interface. Finally we study tetracyanoquinodimethane (TCNQ), a strong electron acceptor, on TiO2(110), which exhibits chemical hybridization accompanied by molecular distortion, as well as extreme charge transfer resulting in the development of a space charge layer in the oxide. Thus, we present a broad experimental and theoretical perspective on the study of organic/metal and organic/oxide interfaces, elucidating fundamental physical interactions that govern molecular organization and energy level alignment.
Reviewing recent progress in the fundamental understanding of the molecule-metal interface, this useful addition to the literature focuses on experimental studies and introduces the latest analytical techniques as applied to this interface. The first part covers basic theory and initial principle studies, while the second part introduces readers to photoemission, STM, and synchrotron techniques to examine the atomic structure of the interfaces. The third part presents photoelectron spectroscopy, high-resolution UV photoelectron spectroscopy and electron spin resonance to study the electronic structure of the molecule-metal interface. In the closing chapter the editors discuss future perspectives. Written as a senior graduate or senior undergraduate textbook for students in physics, chemistry, materials science or engineering, the book's interdisciplinary approach makes it equally relevant for researchers working in the field of organic and molecular electronics.
This book provides the first complete and up-to-date summary of the state of the art in HAXPES and motivates readers to harness its powerful capabilities in their own research. The chapters are written by experts. They include historical work, modern instrumentation, theory and applications. This book spans from physics to chemistry and materials science and engineering. In consideration of the rapid development of the technique, several chapters include highlights illustrating future opportunities as well.
Revised and expanded second edition of the standard work on new techniques for studying solid surfaces.
The alignment between the energy levels of the constituent materials of metal-oxide-semiconductor field effect transistors (MOSFET's) and dye sensitized solar cell (DSSC's) is a key property that is critical to the functions of these devices. We have measured the energy level alignment (band offsets) for metal/oxide/semiconductor (MOS) systems with high-k gate oxides and metal gates, and for organic dye/oxide systems. The combination of UV photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS) in the same vacuum system was used to measure both the occupied and unoccupied density of states (DOS), respectively, of these materials systems. Additional soft X-ray photoemission spectroscopy (SXPS) measurements were made of both the valence bands and core levels of the high-k systems. The combination of the UPS, IPS and SXPS measurements were used to determine the band offsets between the high-k oxides and the Si substrates of thin film oxide/Si samples. To find the metal-oxide band offsets, thin metal layers were sequentially deposited on the oxide surfaces, followed by spectroscopic measurements. These measurements, combined with the measurements from the clean oxide surfaces, were used to find the metal-oxide band offsets. Metal-oxide band offset values were also calculated by the Interface Gap State (IGS) model. We compared the experimental metal-oxide conduction band offset (CBO) values with those calculated using the IGS model, and found that they tended to agree well for Ru/oxide and Ti/oxide systems, but not as well for Al/oxide systems. Through core level spectroscopy, we correlated observations of the composition of the metallic layers with the trends in agreement between the experimental and IGS CBO values, which led to the conclusion that the IGS model gives accurate values for the CBO for systems with chemically abrupt interfaces. Core level spectroscopy of the MOS systems also showed that Al and Ti overlayers reduced the interfacial SiO2 layers of HfO2/SiO2/Si and Hf0:7Si0:3O2/SiO2/Si systems, while leaving the composition of high-k layers essentially unchanged. We also measured the energy level alignment for 3 organic dye/oxide systems, N3 dye on rutile TiO2(110), N3 dye on anatase TiO2 nanoparticle, and N3 dye on epitaxial ZnO(1120) film substrates. For the N3/TiO2(110) system, we found the the N3 highest occupied molecular orbital (HOMO) was 0.9 eV above the TiO2 valence band maximum (VBM) and the N3 lowest unoccupied molecular orbital (LUMO) was 0.5 eV above the TiO2 conduction band minimum (CBM). The energy level alignment for the N3/TiO2 nanoparticle system was similar to that for N3/TiO2(110). The alignment between the N3 HOMO and oxide VBM for the N3/ZnO systems was found to be similar to those of the N3/TiO2 systems, whereas the alignment between the N3 LUMO and oxide CBM alignment was found to differ markedly between the N3/ZnO and the respective N3/TiO2 systems. The difference in the LUMO-CBM alignments is attributed to the different interactions between the N3 LUMO and the ZnO and TiO2 conduction bands. In addition, we measure the energy level alignment for a prototype dye molecule, isonicotinic acid (INA), on TiO2(110) and ZnO substrates. These measurements showed that the LUMO of INA is similar to that of N3, and that the HOMO of INA is much different than that of N3, in keeping with expectations based on the compositions and theoretical electronic structures of these molecules.