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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.
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.
Femtosecond, angle resolved two photon photoemission spectroscopy is used as a probe of the metal-molecule interface. In four related investigations, monolayer or submonolayer coverages of a molecule are adsorbed onto an Ag(111) surface under ultrahigh vacuum conditions. Studies use a an optically excited electronic probe to investigate to probe the energetic landscape experienced and the response of a molecular adsorbate to an excess charge. Energy levels, electronic population, and band structure reveal molecular structure and mechanisms of dynamic response to an injected charge. Room temperature ionic liquids (RTIL), consisting of charge separated anions and cations, represent a new and poorly understood class of electrochemical solvents. The mechanism of electron solvation has been proposed to be vastly different at the electrode interface than has been observed in bulk studies. Optical excitation into the RTIL conduction band results in a localized excess charge, as measured by band dispersions. Electron solvation is measured directly as the photoemitted kinetic energy. Solvation by 200-540 meV is found to occur rapidly on the timescale of 350 fs, supporting previous predictions of fast charge solvation at the interface. Further, a previously proposed phase transition is observed between a low temperature ordered phase and high temperature disordered phase. This phase transition, which occurs at 250 K, is quantified by both the change in workfunction and a change in the energy of solvation between the two phases. The image state is often used as a sensitive probe of molecular films, however, the electronic potential experienced by the image state electron is poorly understood. Theoretical predictions of image state energy in molecular films with a positive electron affinity are often wrong by 1-3 eV, and descriptions of the band structure typically remain limited to a measurement of effective mass. An image state is investigated within a film of metallated phthalocyanines. Phthalocyanines crystallize laying flat on the substrate, with a nearly square unit cell of 14 - 15 Å on a side. Angle resolved measurements into the second Brillouin zone reveal folding of the image state due to interactions with the screened potential energy surface within the molecule. The bandgap, measured to be 150 meV, can be used to estimate the corrugation of the energy landscape. Further, a Kronig Penney model is quantitatively compared to the first several backfolded image bands. Comparision between theory and experiment reveals the effects of the fourfold symmetry of lattice on the image state bandstructure. The morphology and crystal structure of thin film molecular semiconductors is well known to directly influence the band structure and electronic properties of the material. Further, several morphologies and crystal structures can result from epitaxial growth of a single molecule on a metal surface. Although phase transitions in 2D-atomic systems have been well studied for over 80 years, molecular systems are less well understood due to their larger size and the complex interplay of relatively weak forces that govern the crystalline packing. Three phases of metallated phthalocyanines are studied as a function of substrate temperature and submonolayer coverage. Three phases, the 2D-gas phase, the low temperature commensurate phase, and the high temperature incommensurate phase, are each studied using TPPE and low energy electron diffraction. The image state is found to be an excellent probe of the local crystal structure and local workfunction. Preliminary studies focus on the temperature and coverage dependent energies and intensities of the image state peak in domains of each phase. Temperature dependent studies reveal a pseudo-isosbestic point in the transition from the low temperature commensurate to the high temperature incommensurate phases. Coverage dependence reveals a redshift of the image state with increasing molecular density and decreasing workfunction, and low temperature studies reveal long time scale kinetics of reorganization. Preliminary experimental results are interpreted with the aid of kinetic monte carlo simulations. Further studies will aim to obtain quantitative thermodynamics of these submonolayer coverages. Two photon photoemission spectrscopy has proven to be a powerful tool for understanding basic surface physics of atomic and molecular systems. In particular, the image state has proven to be a powerful probe of the first 1-2 ML coverage of a molecular system. In order to improve the applicability of TPPE to answer device relevant questions, the technique will need to resolve, identify, and characterize molecularly derived bands and excitonic states. The basic selection rules for metal-molecule charge transfer excitations and exciton formation at a surface are understood, but it is not agreed upon whether hot-electrons or interfacial electronic coupling allow for excitations that break normal selection rules. Further, classical dipole quenching is expected to play a strong role in the exciton lifetimes in molecules adsorbed on metal or semiconductor surfaces, though no TPPE study up to this point has been able to observe the expected distance dependent lifteimes. In this final chapter, selection rules and classical dipole quenching are briefly discussed, and general trends are derived for molecular systems adsorbed on highly oriented metal surfaces. Our work up to this point suggests that selection rules are followed rigourously in molecular adsorbates on Ag(111).
The study of electronic structure of materials is at a momentous stage, with new computational methods and advances in basic theory. Many properties of materials can be determined from the fundamental equations, and electronic structure theory is now an integral part of research in physics, chemistry, materials science and other fields. This book provides a unified exposition of the theory and methods, with emphasis on understanding each essential component. New in the second edition are recent advances in density functional theory, an introduction to Berry phases and topological insulators explained in terms of elementary band theory, and many new examples of applications. Graduate students and research scientists will find careful explanations with references to original papers, pertinent reviews, and accessible books. Each chapter includes a short list of the most relevant works and exercises that reveal salient points and challenge the reader.
Ab initio quantum chemistry has emerged as an important tool in chemical research and is appliced to a wide variety of problems in chemistry and molecular physics. Recent developments of computational methods have enabled previously intractable chemical problems to be solved using rigorous quantum-mechanical methods. This is the first comprehensive, up-to-date and technical work to cover all the important aspects of modern molecular electronic-structure theory. Topics covered in the book include: * Second quantization with spin adaptation * Gaussian basis sets and molecular-integral evaluation * Hartree-Fock theory * Configuration-interaction and multi-configurational self-consistent theory * Coupled-cluster theory for ground and excited states * Perturbation theory for single- and multi-configurational states * Linear-scaling techniques and the fast multipole method * Explicity correlated wave functions * Basis-set convergence and extrapolation * Calibration and benchmarking of computational methods, with applications to moelcular equilibrium structure, atomization energies and reaction enthalpies. Molecular Electronic-Structure Theory makes extensive use of numerical examples, designed to illustrate the strengths and weaknesses of each method treated. In addition, statements about the usefulness and deficiencies of the various methods are supported by actual examples, not just model calculations. Problems and exercises are provided at the end of each chapter, complete with hints and solutions. This book is a must for researchers in the field of quantum chemistry as well as for nonspecialists who wish to acquire a thorough understanding of ab initio molecular electronic-structure theory and its applications to problems in chemistry and physics. It is also highly recommended for the teaching of graduates and advanced undergraduates.