Research Program
Research program
Carbon with its unique binding properties is the element in the periodic table that provides the basis for life on earth. It is the key for many technological applications that range from drugs to synthetic materials that are indispensible in our modern civilization. The reason for this outstanding role is the ability of carbon to bind to itself and to almost any other element in an almost limitless variety of ways. The resulting structural diversity of organic molecules and compounds is accompanied by a broad range of chemical and physical properties. The tools of modern synthetic chemistry allow tailor designing these properties almost at will via the controlled combination of structure- and function-defining building blocks in new target systems. Elemental carbon exists in two natural allotropes, diamond and graphite that consist of extended networks of sp3- and sp2-C-atoms, respectively. These two forms exhibit a number of unique physical properties such as hardness, thermal conductivity, lubrication behavior and electrical conductivity, which have made them attractive for many industrial applications.
Conceptually, many other ways to construct carbon allotropes are possible upon altering the periodic binding motif in networks consisting of sp3-, sp2- and sp-hybridized C-atoms.[1] As a consequence of the remarkable physical properties expected of these so-far elusive carbon allotropes, it has been appealing for quite some time to develop concepts for their preparation in macroscopic quantities. However, diamond and graphite represented the only known allotropes of carbon through centuries of scientific research. This situation changed in 1985,[2] with the advent of the fullerenes (Figure 1), whose production in macroscopic quantities succeeded in 1990.[3] The discovery of the fullerenes was awarded with the Nobel Prize for Chemistry in 1996 and marked the beginning of an “era of synthetic carbon allotropes”.[4] The next representatives of this growing family are the carbon nanotubes (1991)[5] and graphene (2004)[6] (Figure 1). The isolation and characterization of graphene was recognized by the award of the Nobel Prize in Physics in 2010. Keeping in mind the numerous possible carbon modifications and the general interest in investigating this challenge, doubtless, further revelations are waiting.
Figure 1: The world of synthetic carbon allotropes. In this family the fullerenes represent the most intensely investigated class. Many well defined fullerene derivatives with outstanding properties have been synthesized and first representatives such as organic solar cells have already entered the market.The materials properties of carbon nanotubes and especially of graphene are considered to be even more promising. However, it is still difficult to control their chemistry and device fabrication. Also the bulk production of graphene and of carbon nanotubes in uniform monodisperse samples remains a challenge. Graphene chemistry is still in its early infancy. Thinking of the future, there are a huge number of elusive carbon modifications, whose predicted properties are unprecedented.
The fullerenes, with C60 as most prominent and abundant representative, are molecular carbon allotroppes. They consist of a spherical or spheroid network of sp2-C-atoms. [7] Apart from hexagonal rings, which also represent the smallest repetition unit of planar graphite sheets, exactly 12 pentagons must be incorporated in a fullerene structure to provide the curvature required for the spherical closure of the carbon network. The 12 pentagons of C60 are surrounded by 20 hexagons, as a consequence of the isolated pentagon rule (IPR). The resulting lh-symmetrical truncated icosahedron has the shape of a soccer ball, consisting of 60 structurally equivalent C-atoms. C60 is the smallest stable fullerene. Many representatives of higher fullerenes such as D5h-C70, chiral D2-C76 and D6h-C84 have been isolated and structurally characterized. [7] A second family of fullerenes, formed by the condensation of evaporated carbon, are endohedral fullerenes incorporating guest atoms or molecules in the interior of the carbon framework. [7] Examples are La@C82 and Sc3N@C80.
Soon after the discovery of the fullerenes, the formation of the quasi 1-dimensional carbon nanotubes from evaporated graphite was reported (Figure 1).[5] Depending on the preparation conditions, either single (SWNTs) or multi-walled carbon nanotubes (MWNTs) are obtained. Carbon nanotubes can be considered as seamless rolled-up graphite sheets.[8] The corresponding tubular networks of bent sp2-C-atoms are characterized by a pronounced 1-dimensionality because of their very high aspect ratios. Typical diameters of SWNTs are in the range of 1-2 nm, whereas the lengths can easily reach values in the millimeter regime and beyond. Current production methods for carbon nanotubes lead to the formation of mixtures of tubes with many different helicities, which are characterized by the so called m,n-values of the roll-up vectors.
The youngest representative of synthetic carbon allotropes is 2-dimensional graphene, which is a single sheet of graphite. Graphene, mother of all expanded aromatic carbon modifications, had for a very long time been considered a material accessible exclusively to theoretical studies. The first groundbreaking physical experiments regarding the 2-dimensional allotrope graphene succeeded in 2004,[6] based on a simple mechanical exfoliation of graphite using ScotchÒ tape. Within these three families of synthetic carbon allotropes, graphene is the structurally most uniform macromolecular form in which only the sheet extensions and nature of edges can differ.
During the investigation of these sp2-allotropes attention has also been paid towards the development of other new synthetic carbon allotropes.[1, 9] The extension of the construction principle by allowing sp- and sp3-C-atoms also to be incorporated into carbon networks offers almost limitless structural possibilities and opportunities. The simplest example for such synthetic carbon allotropes is 1-dimensional carbyne, which consists of infinite chains of sp-C-atoms. Whereas the parent allotrope itself is still elusive, a number of model compounds, e.g. endcapped polyynes, have been synthesized and characterized. The combination of sp-, sp2- and sp3-C-atoms leads to many possible 2- and 3-dimensional carbon allotropes, of which only graphyne and graphdiyne have received any serious synthetic attention.[9c] Significant molecular segments of graphyne and graphdiyne have been synthesized, and their study suggests attractive optoelectronic and self-assembly properties. Allotropes with a skeleton of sp- and sp3-hybridized carbon are the least studied class of hybrid allotropes, and the only notable example thus far reported is expanded cubane.[9c] Motivating the study of these materials is the prediction of impressive mechanical properties such as hardness for allotropes like yne-diamond,[10] which is composed of the hypothetical building block adamantyne. Although a variety of low molecular partial structures of these networks have been synthesized successfully, preparative access to these designer allotropes is still elusive.
What makes synthetic carbon allotropes so attractive for chemists, physicists and materials scientists is not only the sheer multitude of aesthetically pleasing structures but, even more so, their outstanding and in many cases unprecedented properties. Common to fullerenes, carbon nanotubes, and graphene is the presence of fully conjugated p-electrons confined in either 0‑, quasi-1- or 2-dimensions. This leads to pronounced redox-activity and high electron mobilities accompanied by stability under ambient conditions. Indeed, metallic carbon nanotubes and graphene are the first representatives of stable organic metals, where no further activation by doping or charge transfer is required to establish high charge-carrier mobilities. No other compound class surpasses fullerenes as electron acceptors in photovoltaic devices.[11] At the same time they exhibit remarkable biological properties such as anti-HIV-activity[12] and the first example of metal-free superoxide dismutation, which is a major mechanism for efficient neuro-protection.[13] Depending on their helicity, carbon nanotubes are either semiconducting or metallic. Both properties are very appealing for applications in the field of molecular electronics or for the refining of materials such as antistatic paints and shieldings. Their position as the record-holding mechanically strongest material makes them very attractive for reinforcing polymers in composite materials.
The specific properties of the youngest representative graphene are considered to rank even higher. A distinct ambipolar field effect that indicates both excellent electron and hole conductivity has been measured in various analogously produced devices. These experiments indicate that the conductivity approaches the quantum limit (e2/h per carrier type) in undoped graphene when approaching zero gate voltage.[14] The results are accompanied with a sudden reversal in the Hall coefficient RH. All other known materials inevitably traverse a metal/insulator transition at this point at low temperatures, which is not observed for graphene down to 4K.[15] An almost temperature-independent charge-carrier mobility up to µ = 230,000 cm2/Vs in suspended state and carrier concentrations of n = 2 x 1011 translate into ballistic carrier transportation in the micrometer range.[16] No other known material has yet been able to exhibit metallic conductivity at scales comparable to those of single- or few-layer graphene. Another property that is so far unique to graphene is the ambient temperature quantum Hall effect (QHE).[16c] Graphene’s intrinsic feature of being built from a single atomic layer naturally enables detection of single molecules adsorbed onto it, since even slight energetic shifts of the Fermi-energy away from the Dirac-point change conductivity dramatically.[17] Usually the bulk nature of commonly used detectors lowers sensitivity and increases the S/N-ratio, resulting in detection thresholds that are not even close to those exhibited by graphene.
At a time when silicon-based technology is approaching its fundamental limits, any new candidate materials for future electronic devices is welcome. Owing to their spectacular electronic, thermal and mechanical properties, synthetic carbon allotropes are expected to offer an exceptional choice. Their importance is now abundantly clear in terms of fundamental physics. However, fully exploiting the proposed applications such as nanoelectronics, sensors, nanocomposites, batteries and supercapacitors is currently still hampered by a number of hurdles (Figure 2).
The scientific aim of this SFB is to solve these problems making use of the well-recognized expertise in Erlangen in the field of synthetic carbon allotropes by increasing the interactions between the disciplines within a formal environment. The joint effort and the interdisciplinary interaction between chemists, physicists, materials scientists, chemical engineers and theoreticians will form the basis of our approach to this challenge. Our common twelve years goal is to develop and characterize a growing family of carbon-based high-performance materials with tailored and tunable properties. Examples for such targeted long-term achievements are: a) the large scale production of single-layer graphene and carbon nanotubes with uniform helicity, b) theory-guided design, synthesis and characterization of new, so far elusive synthetic carbon allotropes and heterographenes, c) theoretical prediction, experimental exploration and comprehensive understanding of the covalent and non-covalent chemistry of carbon nanotubes, graphene, and of new, yet to be discovered forms of carbon, d) comprehensive understanding of charge-transport, photoinduced and optical processes in nanoscale carbon systems, e) enabling carbon allotrope based solar cells as a sustainable energy resource for society, f) development of high-performance devices such as semitransparent electrodes, low cost sensors, all-carbon integrated circuits, graphene THz emitters and transistors and NIR emitters, g) development of a self-sustainable hydrogen (H2) generation system based on SWNT- and graphene-carriers and coupled to a solar cell as energy source, h) preparation of graphane and peralkylated graphane as the first family of 2-dimensional polymers and i) exploration of various families of inter-carbon-allotrope materials.
Figure 2: Current major challenges on the way of developing synthetic carbon allotropes as components for future high-performance materials and devices.
The various projects proposed for SFB 953 Synthetic Carbon Allotropes are structured in three research areas and two central projects (Figure 3). Research Area A (Synthesis and Functionalization) provides the materials basis of the SFB. Both chemical functionalization of known synthetic carbon allotropes and development of new carbon modifications lie at the forefront.
Figure 3: Structure and workflow of the proposed SFB.
The next level within the process chain is the systematic investigation of physical and materials properties and the development of concepts for device fabrication. This is guaranteed by the close interaction with Research Area B (Electronic, Optical and Structural Properties), where systems synthesized in Research Area A will be studied in great detail. This highly integrated and interdisciplinary approach of the SFB also necessitates a close connection with Research Area C (Theory). Both classical and quantum mechanical calculations provide the basis for an in-depth understanding of reaction mechanisms, stability, electronic and optical properties, structural and mechanical properties of synthetic carbon allotropes and their derivatives. Moreover, theory will provide some of the most valuable design principles for the exploration of hitherto unknown forms of carbon. Finally, developing and strengthening fundamental and applied carbon-allotrope research requires strong support from highly sophisticated analysis and structural characterization, which is provided by the two Z Projects on tandem mass spectrometry and high-resolution electron microscopy. Making use of the latest developments in analysis of carbon materials and advanced instrumentation, like dedicated mass spectrometry and aberration-corrected TEM, the goal of the two Z Projects is to contribute to an atomic scale understanding of structure and structure-property relationships of carbon allotropes and related devices.
In order to illustrate the highly integrated and collaborative nature of the proposed SFB, the process chain of non-covalent functionalization of synthetic carbon allotropes (SCA) with rylenes is described briefly as an example: Stimulated by theoretical calculations in C2 that predict electronic modification and doping of SCAs, A1 will synthesize new rylenes and use them to functionalize, disperse, separate and sort SCAs. The next steps in the process chain are photophysical studies such as time resolved transient absorption spectroscopy by B2 on SCA-rylene complexes in solution in order to determine possible photoinduced electron or energy-transfer events. B2 will also investigate p- and/or n-doping of SCAs by rylenes by spectroelectrochemistry. The solvent-free adsorption of rylenes and other organic p-systems onto graphene will also be studied in great detail in B7, for example, by XPS measurements and simulated using classical molecular dynamics in C1. The nature of the deposits, single layer graphene (SLG) or few layer graphene (FLG) will be investigated in A1 by optical microscopy and Raman microscopy. In addition high-resolution TEM and STM will be carried out in Z2 and B4, respectively. The experimental data will be compared with calculations from C3, where theoretical predictions of thermodynamic characteristics (shapes, energetics) of single- and few layer graphene flakes will be carried out on conformational ensembles provided by C1. Based on these fundamental studies, the use of such non-covalently functionalized SCAs as components for electronic devices field effect transistors will then be studied in a close cooperation between B3 and C4.
The proposed workflow in the process chain depicted in Figure 3 will benefit greatly from the close scientific interaction that characterizes the Erlangen team. The existing collaboration between the PIs of the SFB is documented inter alia by more the 100 bi-, tri- and tetralateral publications in the field of synthetic carbon allotropes.
Research Area A (Synthesis and Functionalization):
The five projects that comprise Research Area A are unified by the general theme of creating carbon allotropes with unprecedented structures and properties. In many cases, the advancement of new and innovative synthetic methods for covalent bond formation is a central theme, and these routes will lead to functionalized carbon nanotubes/fullerenes/graphenes, nitrogen doped graphene-like sheets, porous carbon networks, linear carbyne wires, and segments of the hybrid allotrope graphyne. In other cases, supramolecular chemistry is the main focus, and the reliability of thermodynamically driven non-covalent interactions will provide for the separation and sorting of carbon nanotubes, new techniques for the exfoliation of graphene and nanoscrolls, formation of graphyrin-fullerene architectures, and creating 2D organometallic p-conjugated sheets and networks.
The five PIs involved in synthesis represent a fine balance between targeting fundamental goals and perspectives for applications. From a fundamental perspective, new methodology is proposed that will significantly expand the range of known allotrope structures and derivatives. At the same time, mechanistic and structure-property studies also feature prominently in these projects with the aim of creating a firm foundation for the discovery of new molecular structures. It is particularly noteworthy that many of the molecules, networks, and allotropes proposed in the projects of Research Area A represent ambitious synthetic targets that will greatly expand the borders of carbon-allotrope chemistry. Achieving these synthetic challenges will be aided greatly by collaborations with theorists in Research Area C and analysis in the two scientific Z Projects, who offer guidance on the prospective stability of new targets, mechanistic interpretation and geometrical and structural considerations for the systems under study.
On the application side of the coin, each of the five synthetic projects has well-defined targets in mind that will serve as components in devices or as substrates for analysis with collaborators in Research Area B. Carbon allotropes offer a wealth of opportunities as new materials, and through the appropriate functionalization they will be engineered selectively for applications in carbon-allotrope-based solar cells, light-emitting devices, field-effect transistors, memory switches, thin-film electrodes, and other optoelectronic devices or catalytic and sensoric applications. Applying the molecules from Research Area A projects to the study of electronic, optical and structural properties in Research Area B will depend strongly on the close ties between experimentalists and theorists in Research Area C, as molecular design based on predicted electronic structure will nicely guide the synthetic targets, while at the same time theory will help to explain experimental observables in the physical study of these new allotropes.
In the broadest sense, targets from Research Area A can be separated into four categories, 1) Fullerenes, 2) Carbon nanotubes, 3) Graphene and carbon nanoscrolls, and 4) Novel carbon allotropes (including N-doped graphene-like structures, graphyrins, polyynes/carbyne, and graphynes). The distribution of projects is shown in Table 1, and specific topics are summarized below.
Table 1: Involvement of individual projects in the various synthetic targets.
Project A1 “Unifying Concepts for the Chemistry of Synthetic Carbon Allotropes” by PI Hirsch will develop an integrated approach for the chemistry of the three synthetic carbon allotropes (SCAs): fullerenes, carbon nanotubes, and graphene. Both covalent (synthesis of SCA hydrides and carboxylate derivatives) and non-covalent (dispersion, exfoliation, sorting of SCAs with rylenes) chemistry will be refined and optimized. Ultimately, the ability to modify the properties of SCAs through both post-functionalization of carboxylated SCAs and electronic communication with rylene dyes will provide new high-performance materials for incorporating into optoelectronic devices.
Project A2 “Graphyrins – Graphene-Porphyrin Hybrids” by PI Jux will use the porphyrin skeleton as a platform to synthesize graphene-like carbon networks selectively doped with nitrogen atoms (called graphyrins). Smaller graphyrins would possess bowl-shaped structures and offer a class of substrates that will self-assemble with fullerenes. Larger graphyrins containing more six-membered rings (or seven-membered rings) will become planarized, providing large networks to approximate the characteristics of N-doped graphene. Both planar and non-planar graphyrins will be studied with a focus on electron-transfer properties and interactions with various surfaces, while the graphyrin properties are controlled via the degree of planarity, substitution, metal inclusion at the porphyrin core, and axial coordination.
Project A3 “Scalable Shear Force-Induced Exfoliation and Functionalization of Graphene Flakes and Nanoscrolls” by PI Peukert outlines the realization of a room temperature process for the scalable production of graphene sheets and nanoscrolls in the liquid phase. A major goal of this work is to develop a detailed understanding of the formation mechanism(s) at the microscopic and nanoscopic levels, together with subsequent optimization of the technologies necessary for separation and processing. Once formed, the graphene flakes and scrolls will be decorated with nanoparticles to modulate electronic, optical, and catalytic properties. The ability of the allotropes to shuttle and store electrons will be exploited as a crucial parameter in controlling activity, as numerous aspects are optimized for nanoparticle deposition and activity.
Project A4 “The Next Generation of Carbon Allotropes: On the Way to Carbyne and Graphyne” by PI Tykwinski focuses on the synthesis of the new carbon allotropes carbyne and graphyne, which are derived from sp-hybridized carbon building blocks (i.e., acetylenes). Monodisperse and polymeric polyynes will serve as models for the allotrope carbyne, and these molecules also form the basis of molecular wires for single molecule conductivity measurements. Polyynes will be exploited as precursors to new carbon allotropes via surface thermolysis or irradiation with ionized gases. The synthesis of graphyne segments will provide new electronic and magnetic materials, while understanding the relationship between planar and curved surfaces of graphyne is also a primary goal, analogous to the difference between graphene and carbon nanotubes.
Project A5 “Graphene-like Systems Containing Heteroatoms” by PI Kivala will construct defined molecular networks as models for N-doped graphene. The modular syntheses will derive from defined molecular building blocks (e.g., triphenylamine scaffolds/heterotriangulenes) that provide the opportunity to vary size, shape, and geometry (planar and non-planar) of the targeted p-systems. Synthetic manipulation will be used to introduce pendant functional groups that allow self-assembly via metal-ligand coordination in order to provide supramolecular nanostructures with different sizes and morphologies. The formation of nanoporous organic frameworks will then be explored through surface-assisted olefin metathesis.
In conclusion, the methods, molecules, and materials proposed in Research Area A form the fundamental basis for the discovery of new devices and applications based on carbon allotropes. The synthesis and study of these molecules, however, can only be accomplished efficiently under guidance from theorists in Research Area C projects, i.e., these cooperations will continually guide the selection and structure of the synthetic targets. On the other hand, the goals and discoveries described in Research Area A also rely on the synergistic relationship with the characterization studies of Research Area B and the Z Projects. In nearly all cases, state of the art analysis of electronic, optical and structural properties will be an integral aspect of molecular design in Research Area A.
Research Area B (Electronic, Optical and Structural Properties):
Research Area B of the SFB proposal targets understanding the fundamental optical, electronic, structural, and chemical properties of carbon allotropes as well as the design of novel functions and applications within the class of carbon allotropes. In total, nine PIs are involved in investigating the structure-property relationships of processed and functionalized derivatives of synthetic carbon allotropes with respect to their use in future high-tech applications.
Carbon allotropes are a fascinating material class with outstanding potential for both ground-breaking science and novel optoelectronic applications. Specifically, the various dimensionalities of carbon-allotrope molecules open new interdisciplinary research challenges for physicists, chemists, and materials scientists. Semiconducting or metallic carbon allotropes are available as 0D point structures (fullerenes), 1D rods (nanotubes), 2D sheets (graphenes), while 3D bulk structures are generated via thin films from fullerenes, nanotubes, etc. or blends thereof. Research Area B of the SFB is closely related and connected to the other SFB blocks. Figure 3 gives a schematic overview on the workflow and the role of Research Area B within the SFB.
Novel materials synthesized in the chemistry projects in Research Area A will be transferred to Research Area B for fundamental investigations. Experimental investigations will be accompanied by theoretical calculations and simulations (Research Area C) and further supported by extended analysis (Z Projects). Based on these findings, optoelectronic devices will be designed, processed and investigated. As a result of these cooperative efforts and activities, selected projects of Research Area B will make available novel devices based on fully characterized novel carbon allotropes.
The individual nine projects of Research Area B will work predominantly on the investigation and characterization of fullerenes, nanotubes, graphenes and innovative carbon allotropes with novel properties. A special focus across all projects will be placed on exploring organic, doped and undoped SCAs. Table 2 gives an overview of the nine subprojects and the material systems which will be mainly investigated.
Table 2: Involvement of individual projects in the various materials systems.
Project B1 “Optoelectronic Properties of Semiconducting Carbon Allotrope Heterojunction Devices” by PI Brabec will explore the design, formation and characterization of 3D bulk hetero-junctions from various carbon allotropes. Special emphasis will be put on understanding and controlling the nanoscaled microstructures of the heterojunctions with respect to their device properties in carbon-allotrope-based solar cells, photodetectors and light-emitting diodes.
Project B3 “Electrical Properties and Assembly of Carbon Allotropes” by PI Halik will focus on the controlled assembly and integration of graphene and single-walled carbon nanotubes (SWCNTs) by a self-assembled-monolayer (SAM) approach. Low dimensional composites and molecular interfaces will be designed for electronic and optoelectronic devices.
Project B4 “High Resolution Scanning Probe Microscopy/Spectroscopy of Functionalized Carbon Allotropes” by PI Maier will study the local atomic and electronic structure of functionalized graphene, heterographene and networks of graphene-like molecules. The adsorption of molecules on graphene and intentionally inserted impurities into graphene will be investigated by high resolution scanning probe microscopy and spectroscopy. The adsorption mechanism of small molecules on graphene is of special interest for understanding the mechanism of gas sensing devices.
Project B5 “Production Processes and Physical Properties of Carbon Allotropes” by PIs Müller/Koval will produce novel carbon allotropes with high conductivity and mobility by low-energy ion irradiation of carbon-based polymers and/or molecules and study their resistive memory-switching properties. A second approach will study the magnetic properties of graphene via molecular organic probes.
Project B6 “Electronic Structure and Many-Body Effects in Doped Graphene” by PI Seyller will investigate the impact and the nature of substitutional doping of graphene. The global electronic structure of substitutionally doped graphene as well as the impurity induced changes in the carrier dynamics and device-relevant localization phenomena will be explored by angle resolved UV photoelectron spectroscopy (ARPES) and transport measurements.
Project B7 “Tuning the Surface Chemical Properties of Graphene” by PIs Steinrück/Papp aims to explore the interplay of supported graphene layers. Different routes such as the insertion of heteroatoms (N, B), functionalization with rylenes and porphyrins, hydrogenation to graphane, and intercalation of metals will be investigated by mainly X-ray photoelectron spectroscopy. In addition, also the in situ formation of macromolecular structures on metal substrates will be studied.
Project B8 “Graphene and Organic Molecules: Transport Experiments” by PI Weber aims to explore the interplay of graphene and organic molecules by means of single molecule transport measurements via graphene nanogaps, which will be fabricated out of graphene nanowires by electromigration and mechanically controlled break junctions (MCBJ). These experiments are expected to allow the characterization of single molecule junctions with unprecedented quality.
Project B9 “Photo- and Electroluminescence of Carbon Allotropes” by PI Zaumseil will investigate and tailor the optoelectronic properties of carbon nanotubes, functionalized graphene and new carbon-rich molecules with respect to their photoluminescence and electroluminescence properties. Furthermore, the device characteristics of nanotube and graphene-based (light-emitting) field-effect transistors will be explored.
These fundamental electronic, optical and structural investigations will play a major role within this part of the SFB. The fields of investigation to be treated by the individual partners are summarized in Table 3.
Table 3: Physical properties investigated by the individual PIs.
A second, equally important, aspect of Research Area B are device-related investigations and developments. The concept of doping carbon allotropes and forming heterojunctions of various dimensionalities are most important here. Advanced device concepts and investigations will be implemented in various thin-film electronic applications such as carbon-allotrope-based solar cells, light-emitting diodes, transistors, switches, sensors or conductors. The device-oriented activities of the single projects are summarized in Table 4.
Table 4: Carbon allotrope based device orientated work performed by the individual PIs.
Concluding, Research Area B is well connected with Research Areas A, C and Z and is designed to give essential input to the chemistry partners (for the design of novel materials). Further, Research Area B will deliver experimental data relevant for and required by the theory partners. Research Area B of this SFB will go significantly beyond the state of the art in the characterization and investigation of novel carbon allotropes and in designing and realization of novel devices.
Research Area C (Theory):
Research Area C of the proposed SFB is devoted to the theory and modeling of synthetic carbon allotropes. The contribution to the proposed SFB is both substantial and important with a total of four theoretical projects; C1 (Clark/Meyer), C2 (Görling), C3 (Pankratov/Shallcross) and C4 (Thoss). This concentration reflects both the emphasis on modeling and simulation within the University of Erlangen-Nürnberg and the importance attributed to theory by the experimental groups within the SFB. Synthetic carbon allotropes are novel in so many ways that almost all experimental investigations must be accompanied by theoretical work in order to provide a complete understanding of the results. The traditional acceptance of theory by Erlangen experimental groups and the many established collaborations underline the benefits to be expected from the strong theoretical component. The four projects in Research Area C cover a wide range of theoretical techniques. The respective carbon allotrope spezies under investigation by the different sub-projects are summarized in Table 5.
Table 5: Involvement of individual projects in the various materials systems.
Project C1 “Large Scale Simulations on Carbon Allotropes” by PIs Clark / Meyer concentrates on what are traditionally considered the “modeling” aspects of the chemistry of carbon allotropes; large-scale calculations and molecular dynamics. To this end, fast approximate molecular orbital (MO) and density functional theory-based tight binding (TB) techniques will be developed for and applied to questions that arise in the experimental projects. Both classical and quantum mechanical molecular-dynamics simulations will be used to sample conformations of flexible systems and to determine reaction paths using the metadynamics technique.
Project C2 “Formation, Structure, Energetics, and Electronic Properties of Carbynes, Fullerenes, and Graphenes by First-Principles Calculations” by PI Görling will use existing density functional theory (DFT) techniques and develop new, more accurate ones to investigate the structures and energies of new carbon allotropes and to calculate spectroscopic and imaging data (infrared, UV/VIS, X-ray photoelectron spectroscopy, angle resolved ultraviolet photoelectron spectroscopy, and scanning tunneling microscopy) for direct comparison with experimental results. It is important that the level of theory used will be reliable enough that both real predictions and interpretations of experimental data can be provided to the experimental partners.
Project C3 “Theory of Few Layer Graphenes: Flakes, Edges and Doping” by PIs Pankratov / Shallcross will concentrate on fundamental studies of the electronic properties of few-layer graphene (FLG) flakes, the properties of which are largely unknown. Questions of interest, which will be investigated both analytically and using DFT, include finite-size effects, multilayer edge electronic structure and the energetics of the interlayer degrees of freedom. Once these fundamental questions have been answered, the doping of both FLG flakes and pristine graphene sheets by N- and B-substitution and magnetic ad-atoms will be investigated. This project is tightly integrated with a number of experimental projects dealing with production, characterization and doping of the FLG flakes and graphene-based carbon allotropes.
Project C4 “Interaction of Organic Molecules with Synthetic Carbon Allotropes: Theory and Simulation of Charge-Transport Processes and Photophysical Properties” by PI Thoss will concentrate on the study of electron transport in and photophysical properties of aggregates of synthetic carbon allotropes with organic molecules. In particular, structures that use graphene, polyynes or fullerenes in nanocontacts, as electrodes or active materials will be investigated. Such aggregate structures play an important role in many experimental projects, so that the interaction between project C4 and the experimental groups is particularly important. DFT techniques and quantum transport approaches will be developed and used to obtain a detailed understanding of the interaction of organic molecules with synthetic carbon allotropes.
The four groups in Research Area C provide the broad expertise needed to handle the diverse questions posed in the experimental projects of Research Areas A and B. Based on this expertise, the theory and simulations will assist and guide the experimental work. Employing state-of-the-art methodology, the theory projects will provide information that often cannot be obtained easily from experiment, including electronic and geometric structure, energetics, and stability of synthetic carbon allotropes, and analyses of reaction mechanisms. In addition, the theory projects aim to initiate new research directions on the basis of theoretical predictions. This includes, for example, the theoretical study of not yet synthesized forms of carbon allotropes, such as various forms of graphynes and finite segments thereof, and investigations of covalently bound molecule-graphene nanocontacts. Moreover, theory can propose promising new strategies for the formation of carbon allotropes and their modifications. Examples are hydrogenation of synthetic carbon allotropes (preliminary work in C1), template-assisted fullerene formation from short-chain hydrocarbons using supported platinum nanoparticles, graphene band-gap engineering by physisorbed molecules (preliminary work in C2), or self-assembling of magnetic structures on graphene flakes (preliminary work in C3). These examples help demonstrate the proposed active role of theory in developing and leading scientific activities within the proposed SFB.
The successful and effective functioning of Research Area C is not possible without cooperation and coordination between the theory groups, which is also a characteristic of the proposed SFB. The individual strengths of the groups complement each other and allow a very high level of interaction and validation of the results. The planned interactions include direct data exchange between the theory projects as part of the overall process chain of the SFB. Project C1, for instance, will provide the ensembles of structures of flexible systems needed by projects C3 and C4. Projects C2 and C3 will provide information on the electronic structure of graphene and molecule-graphene adsorbate systems as input for electron transport calculations in project C4. Quite generally, data obtained in methods developed in projects C2 and C3 will be transferred to very large systems (as required for some of the experimental projects) by appropriately parameterized approximate techniques implemented in massively parallel software in project C1. Moreover, within C3, the self-consistent tight binding code developed in C1 will be extended to include magnetic field and will be used for large-scale simulation of doped systems. Direct data exchange and the use of complementary techniques will allow validation of the different approaches as the basis for a further advancement of the methodology. For example, projects C2 and C3 overlap by using alternative DFT-based techniques to calculate spectral data for model systems. Similarly for larger systems, results obtained with semiempirical configuration-interaction methods (C1) and with time-dependent DFT (C4) will provide independent calculations of the photophysical properties of aggregates of synthetic carbon allotropes and organic molecules.
This is not to say that Research Area C represents and end in itself. A further characteristic of the proposed SFB is that experimental groups will often not only work together with one theoretical partner, but rather that the theory groups will combine to answer the questions posed by experimental groups in the best possible way by concentrating on their strengths and providing or using data from other theoretical projects in a problem-oriented strategy. Within Research Area A, fundamental questions of structure, stability and reactivity will be answered by the theoretical groups of Research Area C. High level DFT (C2) and wavefunction-based ab initio calculations (C1) will be used to generate definitive data on new carbon allotropes (A2, A4, A5) and for reaction energies for modification and functionalization of fullerenes, nanotubes and graphene (A1, A2, A3, A4). Fundamental data on the effect of doping on graphene (C3) will be used to help guide research in A5. The importance of the theoretical projects for Research Area B is typified by, for instance, the treatment of conformationally flexible aggregates to be investigated in projects B2, B3 and B7, whose electronic and spectroscopic properties will be calculated by the specialists in C2 and C4 using the results of molecular-dynamics simulations from C1. Similarly, the large-scale capabilities of C1 must be combined with the fundamental questions to be answered in C3 to provide the necessary theoretical support for B6 and B7.
Research Area C thus plays a key role in the proposed SFB in two ways; strong interaction with the experimental groups of Research Areas A and B will advance the general goals of the SFB, but more specifically, the theory of synthetic carbon allotropes will benefit from the interaction of a strong and varied theoretical component.
We also note that the research groups involved in Research Area C are members of the newly formed Central Institute for Scientific Computing (CISC) at the Friedrich-Alexander University of Erlangen-Nürnberg, so that contact to and support from other disciplines such as Computer Science (high-performance computing) and Applied Mathematics ensure an ideal scientific environment for Research Area C.
Because Research Area C involves both development of theoretical techniques and extensive calculations and simulations on experimental systems specific to the SFB, a significant SFB-specific computational load will result. Some of this load can be absorbed by existing local and central computers, but a dedicated cluster for sole use of the SFB will be necessary and forms part of this proposal.
Research Area Z (Central Scientific Projects):
Research Area Z comprises two central scientific projects (Z1 Drewello, Z2 Spiecker) which provide expertise and development in advanced spectroscopic, microscopic and nanoanalytical techniques for structural and chemical analysis of carbon allotrope materials. Advanced mass spectrometry, high-resolution transmission electron microscopy and nanoanalysis are of key importance for evaluating synthesis routes, analyzing functionalization schemes and characterizing device structures which are developed in Research Areas A and B, respectively. Moreover, a detailed characterization is a prerequisite for relating experiments to calculations carried out in Research Area C. In the following the two projects of Research Area Z and their relationship with the other research areas are briefly described:
Project Z1 “ Gas-phase Ion Chemistry of New and Modified Carbon Allotropes” by PI Drewello is concerned with method development for the investigation of pure and derivatized synthetic carbon allotropes (SCAs) by dedicated mass spectrometry. Advanced mass spectrometry based on ion trap, time-of-flight (ToF), quadrupole ToF and Fourier-Transform technologies will be employed to the study of isolated species in the gas-phase free of solvent effects. The project aims at the development of tailor-made ionization methods based on Electrospray Ionization (ESI) and Matrix-Assisted Laser Desorption/Ionization (MALDI). Based on ESI, the establishment of a Desorption ESI and a Dual Spray source are planned to enhance the sensitivity of the analysis of the SCAs produced in the SFB. Central to the project is the study of the non-covalent interactions of metal cations and small anions with selected carbon allotropes of the SFB, involving derivatized fullerenes (A1), expanded cyclazines (A5) and graphyrins (A2). Tandem mass spectrometry will be applied to evaluate bond strengths, whereby computational support will be provided by projects C1 and C2. Laser activation of new SCAs is a further research topic in Z1, which divides into three areas. Laser-induced hydrogen release studies from hydrogenated SCAs (produced in project A1) will provide essential insights towards the exploitation of covalent hydrogen storage on SCAs. Laser activation to produce new carbon architectures through rearrangement and coalescence will be studied with the polyynes from project A4 and with the expanded cyclazines from project A5. Pristine and modified graphene, derived from the different graphene producing groups (A1, A3, B6, B8), will be tested as matrix material for the MALDI application with large molecules of biological relevance and importance in material sciences.
Project Z2 “Aberration-Corrected High-Resolution and in situ Transmission Electron Microscopy of Carbon Allotropes and Related Device Structures” by PI Spiecker will employ the analytical and high-resolution capabilities of the new aberration-corrected FEI Titan3 80-300 microscope in Erlangen for detailed investigations of individual carbon allotropes, carbon allotrope composites and thin films as well as related device structures in collaboration with other projects within the SFB. Owing to aberration-corrected electron optics high-resolution transmission electron microscopy (HRTEM) can be carried out at reduced acceleration voltage (e.g. 80 kV) where knock-on damage of carbon allotropes is effectively suppressed. High-resolution TEM combined with electron-energy loss spectroscopy (EELS) will be used, for instance, to address the structure and chemistry of functionalization layers on SWCNTs, graphene and carbon nanoscrolls (cooperation with projects A1 and A3). Device structures containing synthetic carbon allotropes will be analysed with respect to their inner structure and their interfaces in order to be able to compare structure with property measurements thus contributing to a microscopic understanding of structure-property relationships (cooperation with various projects of Research Area B: B1-B3, B5, B6, B8, B9). In order to study device structures in cross-section geometry advanced Focused Ion Beam (FIB) techniques will be employed for sectioning. Another focus will be on the application of electron tomography techniques for characterization of the 3D structure of carbon allotrope bulk heterojunction (BHJ) films (cooperation with project B1). Finally, nanoelectrical and nanomechanical in situ TEM experiments are planned in cooperation with projects of Research Area A and B. For instance, an in situ TEM-AFM holder will be employed for studying the nanomechanics of exfoliation of graphene and the fundamental aspects of nanoscroll formation in collaboration with project A3.
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