Search

This thesis presents optical methods to split the energy levels of electronic valleys in transition-metal dichalcogenides (TMDs) by means of coherent light-matter interactions. The electronic valleys found in monolayer TMDs such as MoS2, WS2, and WSe2 are among the many novel properties exhibited by semiconductors when thinned down to a few atomic layers, and have have been proposed as a new way to carry information in next generation devices (so-called valleytronics). These valleys are, however, normally locked in the same energy level, which limits their potential use for applications. The author describes experiments performed with a pump-probe technique using transient absorption spectroscopy on MoS2 and WS2. It is demonstrated that hybridizing the electronic valleys with light allows one to optically tune their energy levels in a controllable valley-selective manner. In particular, by using off-resonance circularly polarized light at small detuning, one can tune the energy level of one valley through the optical Stark effect. Also presented within are observations, at larger detuning, of a separate contribution from the so-called Bloch--Siegert effect, a delicate phenomenon that has eluded direct observation in solids. The two effects obey opposite selection rules, enabling one to separate the two effects at two different valleys.

This book serves as a concise guide to essential topics in Transition Metal Organometallic Chemistry for senior undergraduate and graduate students; it blends qualitative theoretical approach with experimental description of the facts. Its content emphasizes on the orbital description of M-L bonds; the electronic structures of the main types of organometallic complexes (ML2 to ML6); main types of organometallic reactions; organometallic compound synthesis, analytical characterization and the reactivity and lastly the applications of transition metals in homogeneous catalysis.

The structural phase transition is one of the most fundamental problems in solid state physics. Layered transition-metal dichalcogenides provide us with a most exciting area for the study of structural phase transitions that are associated with the charge density wave (CDW). A large variety of structural phase transitions, such as commensurate and incommensurate transitions, and the physical proper­ ties related to the formation of a CDW, have been an object of intense study made for many years by methods employing modem microscopic techniques. Rather recently, efforts have been devoted to the theoretical understanding of these experimental results. Thus, McMillan, for example, has developed an elegant phenomenological theory on the basis of the Landau free energy expansion. An extension of McMillan's theory has provided a successful understanding of the successive phase transitions observed in the IT- and 2H-compounds. In addition, a microscopic theory of lattice instability, lattice dynamics, and lattice distortion in the CDW state of the transition-metal dichalcogenides has been developed based on their electronic structures. As a result, the driving force of the CDW formation in the IT- and 2H-compounds has become clear. Furthermore, the effect of lattice fluctuations on the CDW transition and on the anomalous behavior of various physical properties has been made clear microscopically.

The continually growing contribution of transition metal chemistry to synthetic organic chemistry is, of course, widely recognized. Equally well­ known is the difficulty in keeping up-to-date with the multifarious reactions and procedures that seem to be spawned at an ever-increasing rate. These can certainly be summarized on the basis of reviews under the headings of the individual transition metals. More useful to the bench organic chemist, however, would be the opposite type of concordance based on the structural type of the desired synthetic product. This is the approach taken in the present monograph, which presents for each structural entity a conspectus of the transition metal-mediated processes that can be employed in its production. The resulting comparative survey should be a great help in devising the optimum synthetic approach for a particular goal. It is presented from an essentially practical viewpoint, with detailed direc­ tions interspersed in the Houben-Weyl style. The wide scope of the volume should certainly encourage synthetic organic chemists to utilize fully the range and versatility of these transition metal-mediated processes. This will certainly be a well-thumbed reference book! R. A. RAPHAEL Cambridge University v Preface In recent years an enormous amount of work has been done on the catalysis of organic reactions by various transition metal species and on the organic reactivity of organo-transition-metal compounds.

Materials with layered structures remain an extensively investigated subject in current physics and chemistry. Most of the promising technological applications however deal with intercalation compounds of layered materials. Graphite intercalation compounds have now been known for a long time. Intercalation in transition metal dichalcogenides, on the other hand, has been investigated only recently. The amount of information on intercalated layered materials has increased far beyond the original concept for this volume in the series Physics and Chemistry of Materials with Layered Structures. The large size of this volume also indicates how important this field of research will be, not only in basic science, but also in industrial and energy applications. In this volume, two classes of materials are included, generally investigated by different scientists. Graphite intercalates and intercalates of other inorganic com­ pounds actually constitute separate classes of materials. However, the similarity between the intercalation techniques and some intercalation processes does not justify this separation, and accounts for the inclusion of both classes in this volume. The first part of the volume deals with intercalation processes and intercalates of transition metal dichalcogenides. Several chapters include connected topics necessary to give a good introduction or comprehensive review of these types of materials. Organic as well as inorganic intercalation compounds are treated. The second part includes contributions concerning graphite intercalates. It should be noted that graphite intercalation compounds have already been mentioned in Volumes I and V.

Studying the interactions between heavy hydrogen isotopes and hydride forming metals or intermetallic compounds (IMC) is of importance for both fundamental and applied sciences. These systems offer, for example, the possibility of technical hydrogen isotope separation due to their considerable isotope effects. In addition, quite a lot of problems of hydrogen recovery, hydrogen purification, and tritium storage can be solved. This review deals with theoretical aspects of the interaction of heavy hydrogen isotopes with metals and IMC, and contains detailed information on phase and isotopic equilibrium and of the kinetics of isotope exchange in systems with hydride phases. Numerical data and results from theoretical and experimental studies are presented as well.

Interest in the title compounds has fallen into three phases: initial study of their solution chemistry, study of their role in the electrode reactions of alkaline batteries during and since World War II, and present concern over the mechanisms by which these elements can enter the environment through dissolution processes, sometimes with toxic results. This volume provides a complete compilation of solubility data published up to 1984, including all pertinent articles, together with critical evaluations of the data. Almost all the measurements relate to aqueous systems.