1. Introducing EAM
The development of advanced materials with properties tailored on the nano- and mesoscale is expected to revolutionize key application fields such as information technology, catalysis, energy and transportation. Committed to this goal, the Cluster of Excellence “Engineering of Advanced Materials – Hierarchical Structure Formation for Functional Devices” (hereafter referred to as EAM) is an interdisciplinary research collaboration located at the Friedrich-Alexander University (FAU) of Erlangen-Nuremberg. Funds totaling €40M to cover scientific personnel and equipment acquisitions for the first 5 years of EAM were awarded in 2007 by the German Research Foundation in the framework of the national “Excellence Initiative”. Additional investments totaling €38M for further research funding and buildings have been received from state and federal government as well as from industry. Today, the Cluster stands out as one of the few undertakings of its type and size worldwide to focus on the fusion of the fundamental science of matter with process engineering. This special issue of Advanced Materials is dedicated to the extraordinary breadth of expertise and profound progress in technologically significant materials research achieved so far by EAM. Along with a Review, five Progress Reports and five Research News articles, nine Communications by EAM researchers highlight some of our most notable contributions to the state of the art in a range of key areas of materials research.
1.1. Interdisciplinarity: A New Approach
The EAM network comprises roughly 200 scientists, ranging from doctoral researchers to full professors, from eight university departments: Applied Mathematics, Chemical Engineering, Chemistry, Computer Science, Electrical Engineering, Materials Science, Mechanical Engineering and Physics. Significant contributions also come from the Erlangen-based Max Planck Institute for the Science of Light, the Fraunhofer Institute for Integrated Systems and Device Technology IISB and the Development Center for X-ray Technology EZRT of the Fraunhofer Institute for Integrated Circuits IIS. Strategic partnerships with key industries complete the loop to realize EAM's vision of a process chain: from molecules to materials to applications. The coordination of such a large and multifaceted project requires novel forms of interdisciplinary cooperation. The structure of EAM (Figure1) has been designed with this in mind. In particular, research areas devoted to the application fields of electronics, photonics, catalysis and structural materials are closely supported by so-called “cross-sectional topics”. The latter constitute a strategic pooling of resources and expertise in areas of particle technology, advanced characterization techniques as well as modeling and simulation. The aim here is the development of common methods for the design, fabrication and analysis of building blocks (molecules and particles) and assemblies that are required for the hierarchical structuring of materials and devices with superior properties. These activities have led to the creation of three interdisciplinary centers representing significant added value to the university landscape with far-reaching implications for future internal and external collaborations:
Center for Functional Particle Systems
Center for Nanoanalysis and Electron Microscopy
Center for Multiscale Modeling and Simulation
1.2. Hierarchical Materials That do Something
Besides academic excellence, the success of EAM can be gauged very effectively by its juxtaposition of fundamental materials and processing advances in the context of real-world applications. To this end, a milestone of EAM is to produce a number of “demonstrators”, i.e. functional systems or devices which show the benefits afforded by hierarchical design. In addition to inspiring team-working across traditional academic boundaries, the demonstrators provide a feedback route for identifying new directions for fundamental research. Typical demonstrators currently under development include dye-sensitized solar cells comprising novel sensitizer and photoanode materials as well as catalytic microreactors based on photonic crystal fibers.
1.3. Investing in the Future
To assist with achieving its strategic goals, the development of EAM has been catalyzed by the acquisition of world-class scientific instruments. These include an FEI Titan3 80–300 transmission electron microscope (Figure2) boasting an aberration corrector to enable the high resolution imaging of nanomaterials in unprecedented clarity. The instrument also includes a full suite of state of the art spectroscopic and tomographic techniques. Substantial investment has also been made in personnel. The appointment of new faculty, with particular focus on the promotion of scientists in their early- or mid-career stages, has both brought new ideas and consolidated EAM's original vision. Support is also provided for the next generation of scientists, namely doctoral researchers, for whom a dedicated graduate school has been founded. Undergraduate students, too, are being inspired by EAM's presence, not only in the traditional disciplines, but also those enrolled in recently created degree courses including a Bachelor-Master degree in Nanotechnology and International Masters courses in Advanced Materials and Processes (MAP) and Advanced Optical Technologies (MAOT), both taught in English.
1.4. Engineering of Advanced Materials: A Special Issue
With contributions covering all of the EAM research areas, this special edition provides an excellent opportunity to introduce our interdisciplinary interests in more depth. In the field of electronic materials our aim is to develop new concepts leading to a toolbox of complementary components for the construction of low-cost and versatile devices. In particular, the production and patterned deposition of three main classes of material are being investigated: organics (i.e., molecular and polymeric materials), carbon allotropes and inorganic nanoparticles. The former are of special interest for the low-cost engineering of new hierarchically ordered multilayer assemblies for devices including solar cells and field effect transistors. In order to optimize and miniaturize such devices, the formation of self-assembled monolayers containing the active molecular components represents a very promising approach. This concept has enabled the demonstration of highly integrated devices, sensor arrays and memories. The current state of the art is detailed in the Reseach News of Halik and Hirsch: The Potential of Molecular Self-Assembled Monolayers in Organic Electronic Devices (p. 2689, DOI: 10.1002/adma.201100337).
EAM is also focusing considerable effort on hybrid electronic systems which exploit the advantages of both organic and inorganic materials. Here, functional nanoparticles (Figure3) are being integrated with organic molecules which provide, in addition to stabilization, an electrical link to nearby redox-active components such as π-conjugated polymers and carbon nanostructures. Regarding the latter, significant challenges remain in the stabilization of, and electrical connection to, individual carbon nanotubes, fullerenes or graphene sheets. These operations can be aided by appropriate chemical functionalization. This, along with an assessment of the state of the art of nanotube and graphene dispersion and non-covalent doping is addressed in the Progress Report of Backes et al.: The Potential of Perylene Bisimide Derivatives for the Solubilization of Carbon Nanotubes and Graphene (p. 2588, DOI: 10.1002/adma.201100300). The functionalization of graphene, besides enabling its integration with other components, modifies its intrinsic electronic properties. The exciting potential of such doping is illustrated by its fundamental theoretical investigation detailed in a Communication by Kozlov et al.: Bandgap Engineering of Graphene by Physisorbed Adsorbates (p. 2638, DOI: 10.1002/adma.201100171).
Due to a strong reliance on a hierarchical structure, the development of improved third-generation (polymer heterojunction and dye sensitized) photovoltaic devices is also a major focus of EAM. Besides improvements in the charge carrier separation and transport, we are developing approaches to widen the broad spectral coverage of the photoactive material. More efficient light harvesting of the solar irradiance, which spans well into the near infrared can be achieved using up-converting materials, a subject highlighted in the Research News by Wang et al.: Rare-Earth Ion Doped Up-Conversion Materials for Photovoltaic Applications (p. 2675, DOI: 10.1002/adma.201100511).
Whether it is the delamination of graphene from graphite or the deposition of a molecular multilayer, the formation of thin films for optoelectronics or indeed many other applications in nanotechnology is reliant on the careful tuning of interactions between individual building blocks. The huge potential for the microscopic tailoring of macroscopic structures is highlighted in the Communication by Fenz et al.: Switching from ultraweak to strong adhesion (p. 2622, DOI: 10.1002/adma.201004097). This work presents experiments and simulations that indicate how to create and detect ultra-weak adhesion in the context of two dimensional membranes interacting via specific ligand/receptor bonds. The authors demonstrate the surprising result that the avidin/biotin pair – famous for forming the strongest receptor/ligand bond known in nature, mediates ultra-weak adhesion under suitable circumstances.
Another research area of EAM is devoted to the study of optical and photonic materials. Here, interests are divided between the design and fabrication of composite nanoparticles, hierarchically structured coatings, novel glasses and the application of photonic crystal fibers (Figure4) to sensing, photochemistry or particle trapping. Optical metamaterials are macroscopically homogeneous structures displaying novel properties by virtue of an inherently anisotropic sub-wavelength architecture. They promise to play an important role in future applications of communications, imaging, security and sensing. Romanov et al. contribute to this field in their Review of the integration of dielectric photonic crystals with a continuous metal film: Hybrid colloidal plasmonic-photonic crystals (p. 2515, DOI: 10.1002/adma.201100460). This approach leads to novel resonance phenomena, broadly tunable through the hybrid's architecture. Besides their role in advanced optical coatings, surface plasmon polaritons are also very promising as short-range carriers of electromagnetic energy. Here, new techniques are needed for the controlled sub-wavelength patterning of metallic coatings. To this end, a versatile direct-writing method leading to gold features possessing very low levels of carbon contamination is reported by Höflich et al. in the Communication: The Direct Writing of Plasmonic Gold Nanostructures via Electron Beam Induced Deposition (p. 2657, DOI: 10.1002/adma.201004114).
A broad interdisciplinary collaboration within EAM is considering how nanostructured particles can be designed to yield enhanced optical properties for applications such as transparent sunscreen or highly efficient thermal management coatings. This work is linking optimization techniques to advances in optical simulation and is feeding ideas for novel structures back into synthetic projects with very promising results. In a Progress Report, Klupp Taylor et al. present an overview of the theory-led design of particle-based optical materials: “Painting by numbers: Nanoparticle based colorants in the post-empirical age” (p. 2554, DOI: 10.1002/adma.201100541). The same group also contribute a Communication, by Bao et al. detailing unprecedented synthetic advances in the scalable and tuneable formation of asymmetric coatings on spherical particles: One-Pot Colloidal Synthesis of Plasmonic Patchy Particles (p. 2644, DOI: 10.1002/adma.201100698). Besides the near-IR plasmon resonance of such structures, the heterogeneity of their surface is expected to contribute to their exploitation in the area of self-assembly of novel mesostructures for a broad array of nanotechnology applications.
Magneto-optical glasses are of considerable current interest, primarily for applications in fiber circuitry, optical isolation, all-optical diodes, optical switching and modulation. In comparison to conventional fiber drawing processes, the pressure-assisted melt-filling of microcapillaries or photonic crystal fibers with specialized glasses offers an attractive route to creating complex waveguide architectures from unusual materials combinations. A magneto-optical waveguide can be created, for example, by the filling of a hollow silica fiber with high refractive index and strongly diamagnetic tellurite or chalcogenide glass. Further details are presented in the Research News by Schmidt et al.: Complex Faraday Rotation in Microstructured Magneto-optical Fiber Waveguides (p. 2681, DOI: 10.1002/adma.201100364).
In the field of catalytic materials, EAM research focuses on the development of advanced catalysts for the selective coupling or cleavage of C–C-bonds, reactions of utmost significance for refining processes, chemical production and environmental catalysis. In particular we are responding to the great need to improve on today's heterogeneous catalysts, which, due to current synthetic procedures, show dispersity in the behavior of active centers and often present issues during upscaling from laboratory to technical reactors. Our research therefore spans the range from the design of catalytic surfaces on the molecular scale to the development of production and process technologies for the final hierarchical catalyst materials. Within our catalysis research portfolio we can highlight advances in materials (catalytic nanoparticles, ionic liquids, zeolites, biomimetic structures, advanced immobilization techniques) as well as mechanistic insights through the development of cutting edge spectroscopic techniques.
Articles in this special issue highlight two aspects of EAM's catalysis research–the use of ionic liquids, either as “Supported Ionic Liquid Phase (SILP)” or “Solid Catalyst with Ionic Liquid Layer (SCILL)” systems, and the design, synthesis and application of novel, highly porous and hierarchically structured inorganic catalyst materials. Regarding the former topic, Steinrück et al. consider in a Progress Report, Surface Science and Model Catalysis with Ionic Liquid-modified Materials (p. 2571, DOI: 10.1002/adma.201100211), recent studies aiming for a better mechanistic understanding of the sometimes drastic activity and selectivity effects observed for ionic liquid-modified catalytic surfaces. This work considers the application of surface science techniques to well-defined model systems and also addresses structural aspects of ionic liquid layer formation on supports. Finally a number of very relevant studies on ionic liquid/catalyst complex interactions are highlighted. Sobota et al. contribute a Communication, Ligand Effects in SCILL Model Systems: Site-Specific Interactions with Pt and Pd Nanoparticles (p. 2617, DOI: 10.1002/adma.201004064) demonstrating for the first time both metal- and metal site-specific interactions of a deposited ionic liquid film with supported noble metal catalysts. These findings suggest that the systematic exploration of specific IL-ligand effects offers high potential for the identification of SCILL systems with enhanced catalytic performance.
The exploitation of highly porous materials such as zeolites represents an important approach to produce more efficient catalytic materials (Figure5). In particular, the superior heat and mass transfer characteristics of zeolites, as well as their low pressure drop and high diffusivity of reactants and products through their meso- and macropores and can be optimized via synthetic control over the material structure. The Progress Report Zeolitic Materials with Hierarchical Porous Structures (p. 2602, DOI: 10.1002/adma.201100462) by Lopez-Orozco et al. systematically summarizes the materials and process-relevant properties and advantages of different types of zeolites. Impressive results regarding the formation of another type of mesoporous material using a partially hydrophobic organosilane precursor are shown in a Communication by Zhou et al.: Mesoporous Organosilicas with Large Cage-like Pores for High Efficiency Immobilization of Enzymes (p. 2627, DOI: 10.1002/adma.201004054). The presented materials show a novel multiple-shell vesicle-like structure and display very attractive properties for the physical adsorption of lipases from aqueous media leading to enhanced enzymatic activity.
The final application-oriented research area of EAM considers the design and fabrication of lightweight materials. Here the challenge is to develop hierarchical micro- and macrostructures that enable a material to be lighter, stronger and more affordable than conventional structural components. Such improvements are particularly relevant to mobility and energy applications since they would result in reduced fuel consumption and environmental impact at the same time as increased safety and reliability.
Auxetic materials are of particular interest due to their inherently low-density and inhomogeneous microstructure. Indeed, they can be considered as mechanical metamaterials since they possess – in contrast to almost all engineering materials – a negative Poisson's ratio. This means that auxetic components demonstrate the unusual effect of expansion under tension and dilation under compression and offer promise as shock absorber and fastener components with excellent fracture resistance. To identify novel auxetic materials, one approach is analogous to the case of nanostructured optical particles already described i.e. a shift from intuitive- to theory-based design making use of mathematical optimization techniques. The Communication by Schwerdtfeger et al., Design of Auxetic Structures via Mathematical Optimization (p. 2650, DOI: 10.1002/adma.201004090) combines this novel approach with the versatile morphological control of the selective electron beam melting (SEBM) technique. The latter additive process can build up a complex cellular metal structure layer-by-layer according to an arbitrary design such as that resulting from topology optimization (Figure6). Auxeticity is, in essence, a result of the geometric properties of an underlying spatial structure. As detailed in the Research News, Finding auxetic frameworks in periodic tessellations (p. 2669, DOI: 10.1002/adma.201100268) by Mitschke et al., inspiration for auxetic designs can be sought from the analysis of existing repositories and databases of geometric structures in general and planar tesselations in particular. The analysis of the latter has produced novel auxetic mechanisms in skeletal structures and in the corresponding linear-elastic cellular solids manufactured by SEBM. The method also generalizes to 3D, allowing the use of existing databases of spatial networks and tessellations to guide the design of inherently three-dimensional auxetic mechanisms.
Enhanced material properties often result from improvement in the control over material microstructure. This, in turn, requires concise methods to describe “shape” – both qualitatively and quantitatively. Minkowski tensors are tensorial shape indices for the quantitative characterization of orientation-dependent properties and of complex spatial structure that are particularly well suited for the development of structure-property relationships for tensor-valued physical properties. An overview of the state of the art of this method is given in the Progress Report by Schröder-Turk et al., Minkowski Tensor Anisotropy Analysis of Cellular, Granular and Porous Structures (p. 2535, DOI: 10.1002/adma.201100562), which include both experimental (electric field alignment of co-polymer films, surface orientation in open-cell solid foams, X-ray tomographic analysis of bead packs) and theoretical (defect detection in molecular dynamics simulations of crystalline copper, surface distribution in anisotropic porous media models). This work highlights the need for improved techniques for the quantitative structural analysis of real materials imaged by 3D microscopic techniques such as X-ray imaging at the micrometer-scale. Related to this, the Communication, Multi-energy X-ray imaging as a quantitative method for materials characterization (p. 2655, DOI: 10.1002/adma.201004111), by Firsching et al. presents two- and three-dimensional multi-energy X-ray analysis as a method for material characterization. The Communication by Nachtrab et al., Morphology and Linear-Elastic Moduli of Random Network Solids (p. 2633, DOI: 10.1002/adma.20104094), analyzes the effective linear-elastic moduli of disordered network solids by finite element methods. This work again emphasizes the need for sound geometric modeling, by showing a connection between the Possion-Voronoi network model and real-world biopolymer structures and by a unified analysis of network-like cellular solids and high-density porous media.
In contrast to the cellular and network solids described above, EAM is also developing structural materials with tailored nanostructure and function. One approach, accumulative roll bonding, is a top-down process which can produce high strength ultrafine-grained alloy materials by sequential cutting, stacking and compressive rolling of sheets. The tailoring of materials properties through this technique is described in the Research News, Tailoring nanostructured, graded and particle reinforced Al laminates by accumulative roll bonding (ARB) (p. 2663, DOI: 10.1002/adma.201100407) by Göken and Höppel. Materials with superior ductility, high corrosion resistance and good visual surface properties are accessible by this route. Two approaches described are the grading of the material composition across the laminate and introduction of ceramic particles of various shapes in order to improve its properties and accelerate grain refining during the fabrication process.