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There has been a long-standing theoretical interest in the physical properties of solid state systems in less than three dimensions. Some well known many body problems that have defied solution in three dimensions can be solved exactly in one or two dimensions; well known examples are Bethe's theory of quantum spin chains and Onsager's solution of the two-dimensional Ising model. Further, because of an increased phase space for thermal and quantum fluctuations, the physics of such low dimensional systems can be fundamentally different from that governing three-dimensional bulk matter. For instance, whereas three-dimensional metals can be theoretically described as "Fermi liquids" (essentially superpositions of single-electron quantum states), their one-dimensional analogues are believed to behave like "Luttinger liquids" with quantum numbers fundamentally different from those of individual electrons.
For a long time, these concepts remained entirely in the realm of theory and were believed to be of no practical consequence. Only recently has it become possible to prepare and test real low dimensional quantum systems. First, solid state chemists were able to synthesize bulk organic and inorganic materials with electronic interactions largely confined to atomic chains or layers. The capability to prepare high-purity single crystals of these novel materials has enabled solid state physicists to accurately probe their thermodynamic and transport properties as well as their excitation spectra. Discoveries such as the "Haldane gap" phenomenon in even-integer spin chains, and high temperature superconductivity in layered copper oxides, have inspired an explosion of recent theoretical activity in quantum many body physics. Chemical synthesis techniques, experimental capabilities and theoretical methods are continuing to evolve at a very rapid rate.
In a second, complementary approach, modern lithographic and atomic manipulation techniques have allowed researchers to build individual micro- or nanoscale solid state systems subject to boundary conditions on a length scale where quantum coherence prevails. Quantum size effects then restrict electronic motion to two, one, or even zero dimensions. In addition, atomic self-organization processes have been developed for the controlled fabrication of a plethora of nanostructures and nanostructure arrays. The physical and chemical properties of such atomic-scale structures differ largely from the behavior of bulk matter, and fundamental discoveries such as the integer and fractional quantum Hall effects have revolutionized condensed matter physics in recent years. New experimental tools such as scanning probe techniques with sub-Angstrom resolution are only beginning to be explored, and the potential for further discoveries remains extremely high.
In zero-dimensional solid state systems, quantum size effects arising from the wave character of the electron restrict electronic motion in all three dimensions. The chemical and physical properties ofclusters or "quantum dots", aggregates of atoms with diameters of 1 to 10 nm, are in between those of atoms on the one hand and those of bulk materials on the other. Entirely new effects can also appear on this length scale; for instance, it is possible to build magnetic clusters out of elements that are nonmagnetic in bulk. The evolution of their physical properties is related partially to the variation of the surface-to-volume ratio, and partially to quantum size effects.
Individual clusters are generated and studied by a variety of experimental techniques and in a variety of environments, from the gas phase to a physisorbed state on a well prepared surface. When absorbed on a surface, their electronic properties are studied by modern scanning probe and optical techniques. Metal and semiconductor clusters already find many applications, for instance as miniature lasers and as fluorescence markers in molecular and cellular biology, which require that they be isolated from their environment in order to prevent radiationless recombination of electron-hole pairs. To this end, researchers at the MPI-FKF and at the University of Stuttgart are pursuing a strategy in which the clusters are coated by an organic passivation layer, a typical example of an interface-controlled material where there will be a maximum of synergy effects within the IMPRS-AM.
Self-organization is extensively used to produce arrays of quantum dots. This approach is based on a detailed understanding of the microscopic pathways of diffusion, nucleation and aggregation. These processes are governed by activation barriers, the delicate balance of which determines the growing adlayer structure and morphology. The hierarchy in migration barriers can be translated into geometric order in the resulting aggregates, whose shapes and length scales can be controlled experimentally in a well defined way. This novel nanostructuring technique allows in particular the fabrication of a very high density of identical nanostructures, which can then be characterized by integrative experimental techniques. Quantum mechanical interactions between dots also enable unique computing and optoelectronic applications. Finally, it has become possible to synthesize bulk compounds that can be regarded as arrays of identical, largely separated voids in which clusters of extraneous materials can be adsorbed. Zeolites, with their wide variety of crystalline structures, offer the unique opportunity to study the competition between adsorbate-adsorbate and adsorbate-interface interaction in three-dimensional networks of confined small clusters (e.g. in faujasites). In the IMPRS-AM, the structures and interactions of paramagnetic clusters and ions of transition metals in zeolites will be investigated by electron spin resonance.
Self-organization and atomic manipulation techniques are also used to produce one-dimensional arrays of clusters and large molecules ("nanowires"). For instance, through molecular nanostructuring at surfaces it has recently proven possible to build one-dimensional supramolecular architectures out of large, functional organic molecules. Other examples are polymers based on fullerenes, large carbon-based "supermolecules" whose oxidation state can easily be reduced from -1 to -6. Such fullerides can exhibit interesting physical properties. In particular, superconductivity has been detected in various compounds containing C603-. Another class of fascinating fullerene-derivatives are endohedral fullerenes, which offer additional properties, e.g. the endohedral modification leads to molecules where valence electrons from the enclosed atom can be transferred to the cage. Such "supermolecules" are highly promising as building blocks for oligomeric or polymeric structures extending in one dimension.
One big advantage of synthetic nanowires is their inherent ("natural") growth in nanometer dimensions. Carbon nanotubes are another example of such synthetic nanostructures, which may be regarded as individual macromolecules of extraordinary length. Both single- and multiwalled nanotubes are studied extensively with respect to their structural, mechanical and electrical properties. It has turned out that these remarkable wires, with diameters in the molecular scale, can behave like ballistic conductors showing quantized conductance. Some of the most convincing evidence of the above-mentioned Luttinger liquid behavior has indeed been found in carbon nanotubes. Moreover, they have been used as electrically active components in room temperature field-effect transistors or chemical sensors. Carbon nanotubes, like other synthetic nanowires or nanoparticles, are therefore promising functional building blocks of future molecular electronic devices. For instance, the availability of nanowires of regular physical and electronic structure, and with low defect densities, could lead to their application as electrical interconnections between nanoparticles. Molecular electronics may be able to overcome the limitations of silicon technology, especially because it is possible to build devices that are organized cheaply in parallel by chemical self-assembly. However, the connection of molecular switching elements to the molecular wires that will be required for high-density integration remains a substantial challenge. Another challenge particular to carbon nanotubes is that conventional bulk chemical synthesis results in molecular species with a variety of chiralities whose physical properties differ widely. Research within the IRMPS-AM will therefore focus on improved synthesis routes as well as alternative nanowire systems such as vanadium pentoxide (V2O5) nanofibers that are not plagued by such nonuniformity problems.
A complementary approach to one-dimensional physics is based on the chemical synthesis of bulk compounds containing chains of functional chemical units. Examples of particular interest to the IMPRS-AM are the organic "Bechgaard" salts whose backbone is a stack of planar organic molecules, and inorganic oxoniobates, oxomolybdates, rare earth carbide halides, and copper and silver oxides with quasi-one-dimensional electronic structure. Silver(I)-oxide, for instance, can be reacted in the solid state with virtually any other binary oxide as long as its thermal decomposition is suppressed e.g. by maintaining elevated oxygen pressures during synthesis. In this way many new ternary silver-oxides have been prepared. As an unconventional common structural feature, the silver ions aggregate to form cluster-like assemblies, most pronounced in the silver-rich representatives. These silver partial structures are mostly low-dimensional, corresponding to chains, ladders or two-dimensional nets.
The very high density of functional chains realized in these compounds enables elaborate experiments that are difficult or impossible to perform on freestanding one-dimensional units. They therefore serve as powerful model systems for quantum many body calculations. Neutron scattering, for instance, can provide maps of the magnetic excitation spectra of insulating spin chain compounds and is thus a uniquely powerful technique to test quantum theories of one-dimensional magnetic systems. These predict, for instance, that the elementary excitations in antiferromagnetic spin chains (spinons) are fermionic and thus behave in a fundamentally different manner than bosonic spin waves in three-dimensional antiferromagnets.
Bulk compounds with quasi-one-dimensional electronic structure are also host to unusual phenomena not predicted, and still incompletely understood, by current many body theories. These include especially superconductivity and quantized Hall conductance, both realized in the Bechgaard salts. Within the IMPRS-AM, the correlation effects that are crucial for these phenomena will be studied by a multidisciplinary team consisting of theorists as well as experimentalists probing the spin dynamics with neutron scattering, the charge dynamics with optical techniques. The effective dimensionality of both organic and inorganic quasi-one dimensional compounds can be tuned by external pressure and chemical substitution, so that the effects of interchain interactions can be studied all the way from the weak limit to the quasi-two-dimensional limit.
Similar considerations apply to the chemical buildup and physical properties of compounds with two-dimensional electronic structure. Examples include organic compounds based on the electron donor BEDT-TTF (bis-ethylene-dithiolo-terathiafulvalene), which often show quasi-two-dimensional electronic properties due to the fact that the donor molecules are arranged in parallel sheets, and inorganic layered rare earth halide carbides, silver oxides or copper oxides. Again, the physical properties of two-dimensional magnetic systems are very incompletely understood, and unusual phenomena such as "colossal magnetoresistance" have been discovered at an extraordinarily rapid rate in recent years. Through collaborations between solid state chemists, experimental physicists and theoretical physicists, research in the framework of the IMPRS-AM is expected to be at the forefront of this exciting field of research.
A particularly interesting phenomenon found in quasi-two-dimensional materials is superconductivity which sometimes appears at remarkably high temperatures, as in the case of the layered copper oxides. A common element of the superconducting states realized in these quasi-two-dimensional compounds are electronic correlations. The larger energy scale of the (purely repulsive) electronic interactions as compared to that of the (attractive) electron-electron coupling mediated by the electron-phonon interaction is undoubtedly responsible for the high temperature scale of the superconducting state. The mechanism of superconductivity in these strongly correlated systems is one of the most important problems in current solid state physics. This question will be addressed within the IMPRS-AM through elaborate experiments on the spin and charge dynamics of these materials, as well as parallel theoretical investigations using state-of-the-art band structure calculations and quantum many body techniques. The same arsenal of experimental and theoretical techniques will be brought to bear on competing two-dimensional ground states exhibiting static charge and/or spin order.
In addition to quasi-two-dimensional bulk compounds containing a large number of densely spaced atomic layers, research within the IMPRS-AM will focus on isolated two-dimensional solid state structures at interfaces and thin films. We expect that particularly important synergy will be realized by the joint research and educational activities proposed here. Some of the most important discoveries in solid state physics in recent years, the integer and fractional quantum Hall effects, were in fact made at an interface between two dissimilar semiconductors. Since these initial discoveries, there has been a huge amount of experimental and theoretical work on two-dimensional electron systems in magnetic fields, much of which was stimulated by research at the MPI-FKF. Recent advances include the discovery of "composite fermions", unusual elementary excitations fundamentally different from their analogues in three-dimensional electron systems, and the direct visualization of one-dimensional "edge channels" in these two-dimensional systems. These advances were to a large extent driven by progress in the preparation of clean interfaces through advanced techniques such as molecular beam epitaxy. Very recently, remarkable progress has been made in this regard on two-dimensional organic layers, where integer and fractional quantum Hall effects could be observed up to unprecedentedly high temperatures. Organic films and interfaces will be an important focus of interdisciplinary research at the IMPRS-AM.
Finally, the surface of bulk crystals constitutes another prototypical two-dimensional solid state system of interest to both experimental and theoretical physicists in the IMPRS-AM. Surface segregation is an omnipresent deviation of the surface composition from the bulk value of a binary system. This phenomenon has a strong influence on the ordering behaviour at the surface and in the subsurface regime. Particular attention will be focused on single crystal surfaces of Cu-, Fe- and Ni-based binary alloys and on the formation of short-range and long-range ordered structures in the subsurface regime. In-situ temperature-dependent x-ray scattering experiments will be performed at modern synchrotron radiation facilities and will be complemented by theoretical studies.
Geometrical constraints have large effects on small-scale materials and structures. Basic metallurgical mechanisms are explored in bulk materials and transferred to continuous and patterned thin film components, with typical dimensions of 1 µm and less. The overall aim is to investigate how basic materials concepts and measurement techniques can be transferred from the macroscopic to the micro-regime, where the high density of interfaces affects the materials behaviour. The studies are predominantly fundamental in nature, but are oriented along the lines of materials problems in mechanical design and in microtechnology.
The nature of size effects on stress build-up and its relaxation by deformation processes is studied in thin-film systems on a length scale of 10-1000 nm. Because of the growing importance of interfaces with shrinking dimensions, the mechanisms of interaction between lattice defects and interfaces need to be better understood; for this purpose extensive studies with TEM in situ techniques are planned as well as atomistic modeling. Mechanical testing using X-ray diffraction will be moved to a synchrotron facility to allow faster stress measurements and, consequently, testing at higher strain rates and higher temperatures. A long-term objective of these studies is to produce size-dependent deformation mechanism maps for pure and multiphase materials. A comprehensive view of mechanical metallurgy in small dimensions would not only be a desirable intellectual achievement, but would also impact issues of reliability of small-scale systems. Also the limitations in electrical reliability due to electromigration in miniaturized nanowires will be at the focus of our research.
Surface and interface energies as well as stresses play an important role in the behaviour of small scale materials, such as thin films. Thus unusual (as compared to bulk materials) phenomena can be observed: favourable interface and surface energies can stabilize amorphous phases or, generally, phases that would be unstable as bulk material. Thereby exciting new electric, magnetic and mechanical properties become accessible for practical applications. The thermodynamics for such systems need to be developed, because at the moment only, at best, semi-empirical approaches are available.
Reactions at surface and interfaces and within the (thin film) systems are decisive for the resulting properties of these small scale structures. One of the great challenges is to find the relation between the thermodynamics (driving force) and the kineticsof the solid-solid, solid-liquid or solid-gas reactions/phase transformations involved. The hardest problem in the modelling of these (interface/interphase/surface) reactions is the quantitative prediction of nucleation. Experimental and theoretical (computer simulations) work in this virginal area is very necessary.
Electronic and material transport in thin films of metals and semiconductors is crucially affected by the real structure of the material at hand. In this project the effect of the real structure in thin (epitaxial) systems on the transport properties are considered. The focus is on the interfacial roughness, misfit dislocations and interfacial strain onto the electrical conductivity and on the role of (planar) stress on diffusion in thin film systems, The structural characterization of the films is done by combining novel x-ray scattering techniques with scanning tunneling microscopy as well as (HR)TEM.
The scope of the research on small-scale materials will be widened to include biologicalsystems. Many biological processes are controlled by mechanical phenomena operating on the molecular level, at the scale of individual cells, and in complete organisms. It will be studied how small-scale test methods, such as nanoindentation, AFM and micro-tensile testing, can be applied to biological matter to gain insight into such diverse questions such as the microtribology of biological surfaces and the deformation mechanisms of the cytoskeleton.
The term "interface" includes grain boundaries, phase boundaries, grain boundary phases and grain boundary segregations. Interfaces can affect the properties in numerous ways. The interface can be viewed as a discontinuity. The structure, composition and chemical bonds at the interface differ from the bulk. Therefore, interfaces govern diffusion, gliding, viscous flow, microcracking, cleavage and cavity formation, to mention just a few items. Changes of elastic and thermoelastic constants at the interface create stresses under mechanical or thermal load.
The current strategy of materials science is directed towards a controlled morphology of interface structures. The concept of interface design has to be adapted to the type of material under consideration, as, for example among the diverse classes of ceramic materials, segregated glasses, precursor-derived ceramics, glass ceramics, solid-state- or liquid-phase-sintered ceramics, fiber-reinforced composites, particle-reinforced materials or multi-layer structures.
For example the fracture toughness strongly depends on interface structure and morphology. Crack deflection can occur at weak interfaces. Friction at the interface controls the complementary effects of pull-out and crack bridging. The mutual interaction of these effects is responsible for closure stresses along the crack profile and the related increase of the fracture toughness. Weak interfaces can be due to a disordered atomic structure or, for instance in a ceramic, a glassy phase at the grain boundaries. Strong interfaces can occur between fibers and matrix, in reinforced ceramics, by the presence of carbon or boron nitride coatings with a two-dimensional structure. Weak interfaces occur between the originally separated domains in polymers produced from solution by wet spinning processing (after coagulation and solvent removal).
Interfacial defect chemistry in ionic materials determines the carrier distribution at interfaces and the related transport phenomena. Of particular interest are phenomena in systems in which the spacing of interfaces is comparable to or smaller than the Debye-length (nano-ionics). Then additional size effects occur. Defect models will be developed that predict interfacial effect. Understanding should be obtained of how the local effects influence the overall materials response. Finally strategies will emerge for purposefully structuring matter, having a certain property in mind. This work is relevant for electrochemical energy conversion, photo electrochemistry, sensors, actuators in particular and electroceramics in general.
Not only the solid-solid interfaces are of interest, solid-gas (oxidation) and solid-liquid interfaces are important as well. In the case of solid-liquid interfaces the structure of the solid influences the structure of the adjacent fluid. Research in this area could lead to a better understanding of the phase behaviour of fluids in porous materials. The performance of fluid materials, such as in liquid crystal devices, can be optimized by tuning of the geometrical and chemical properties of their confining interfaces. Crystalline-amorphous interfaces can occur in polymeric systems as well: when polymers are melt processed, crystallization often occurs in oriented systems. The particular course of crystallization strongly affects the interface between the crystalline and the amorphous material, in particular with regard to the portion of backfolding chains. The mechanical properties depend decisively on these crystalline-amorphous interfaces.
Since in interface-controlled materials the main defects affecting the properties are the interfaces it is important to investigate their structure, composition and bonding. This is possible with transmission electron microscopy methods (TEM). The structure of interfaces can best be determined by quantitative high-resolution transmission electron microscopy (QHRTEM). The QHRTEM method was developed at MPI-MF and was applied to different homophase boundaries (grain boundaries in Cu, a-Al2O3, SrTiO3, and heterophase boundaries (metal/ceramic interfaces. Nb/Al2O3, Cu/Al2O3, Pt/SrTiO3, ....). It is also important to understand the composition of grain boundaries on the atomic scale. This is possible by analytical advanced microscopy where the chemical composition can be determined on a sub-nanometer scale. It is expected that in the foreseeable future column by column analysis will be possible. The techniques will include electron energy loss spectroscopy (EELS) as well as energy dispersive X-ray spectroscopy (EDS). Both techniques are applied to metal/ceramic systems.
Last but not least, information on the bonding across interfaces can also be obtained by advanced TEM, i.e. by investigating the near-edge fine structures of the EELS spectra. These signals give information on the distribution of the projected unoccupied density of states. The experimental spectra have to be compared to calculated spectra yielding some information on the electron density distribution on the atomic scale which reflects the bonding and changes in the bonding across interfaces. Of great interest will be, of course, the change of bonding due to the introduction of segregated impurities.
All these techniques are well-established at the MPI-MF. Advanced instruments for high-resolution TEM and analytical electron microscopy are available. By 2002 a sub-eVolt sub-Angstroem electron microscope (SESAM) will be available which will allow to perform all experiments on the same area simultaneously.
To determine the macroscopic and microscopic structure and property parameters of advanced materials, dedicated methods and techniques for measurements at all length scales, from say 0.1 nm to 1 m, are required. A variety of diffraction, microscopic, spectroscopic, calorimetric, dilatometric, electric, magnetic, etc, and also computer simulation techniques are used within the IMPRS-AM research program. Often, within a research project, it is necessary to develop a technique further in order to perform the measurements needed. Therefore, within the IMPRS-AM research program also method and technique development occurs at the forefront of science.