Erdmann Spiecker

Covalent bulk functionalization of graphene

Graphene, a truly two-dimensional and fully π-conjugated honeycomb carbon network, is currently evolving into the most promising successor to silicon in micro- and nanoelectronic applications. However, its wider application is impeded by the difficulties in opening a bandgap in its gapless band-structure, as well as the lack of processability in the resultant intrinscially insoluble material. Covalent chemical modification of the π-electron system is capable of addressing both of these issues through the introduction of variable chemical decoration. Although there has been significant research activity in the field of functionalized graphene, most work to date has focused on the use of graphene oxide. In this Article, we report on the first wet chemical bulk functionalization route beginning with pristine graphite that does not require initial oxidative damage of the graphene basal planes. Through effective reductive activation, covalent functionalization of the charged graphene is achieved by organic diazonium salts. Functionalization was observed spectroscopically, and successfully prevents reaggregation while providing solubility in common organic media.

Assemblies of Mesoporous FAU-Type Zeolite Nanosheets†

Hierarchical porous materials are of great scientific as well as technological interest because the presence of porosity on different scales has the potential to affect the transport characteristics of molecules within the pore system. The targeted design of the pore hierarchy in such materials will result in improved performance in transport-based applications, such as adsorption, catalysis, and separation. 2] Hierarchical materials containing zeolites, combine characteristics of systems with pore sizes on at least two different length scales. Compared to the allover microporous channels in conventional zeolite crystals, such hierarchical pore systems, with wide transport pores intersecting the micropore network like motorways linking a narrow road system, are already proven to reduce diffusion limitations for molecules within zeolitic catalysts. While for high-silica zeolites, such as ZSM-5, several methods for incorporating additional transport pores were developed during the past few years, 4, 11] such a substantial collection of approaches is not available for low-silica zeolites. Herein, we present the synthesis and characterization results of Faujasite (FAU)type zeolite X (Si/Al< 1.5) grown as house-of-cards-like nanosheet assemblies with intracrystalline mesoporosity. The resulting pore system covers all three pore size levels (micro-meso-macro) in a hierarchical interconnection. The reduction of diffusion limitations in microporous zeolites can be achieved by the creation of intracrystalline transport pores (in the meso or macro pore range) or by the reduction of the zeolite crystal dimensions itself. Zeolite crystals with additional meso (or in few cases macro) pores can be produced by different methods, for example, desilication by an alkaline post-treatment 15] or by the use of hard 17] and soft 18–20] templates during zeolite synthesis. However, all the methods are usually limited to a certain group of zeolites or just to a special zeolite type. For example, for zeolites with a Si/Al molar ratio below about 15, desilication is not an appropriate method for the introduction of additional transport pores. Thus, it is very challenging to introduce transport pores into low-silica (high aluminum content) zeolites. To date, only the low-silica zeolites LTA and SOD could be synthesized with intracrystalline mesoporosity by soft-templating with organosilane surfactants such as 3(trimethoxysilyl)propyl hexadecyl dimethyl ammonium chloride (TPHAC). 20] There are a few reports on the synthesis of mesoporous FAU-type zeolite Y, either by using carbon aerogel as a hard template, by steaming, or by a combined acid–base post-treatment. With the acid–base technique a threefold hierarchical pore system, combining the zeolitic micropores and two ranges of mesopores, was obtained with a resulting Si/Al molar ratio of around 20. But owing to the underlying extraction mechanism, this combined acid–base post-treatment is (as well as the single alkaline post-treatment) not suitable for introducing mesopores into low-silica zeolites with Si/Al molar ratios close to 1. Furthermore, all the examples reported for hierarchical zeolites involve either a micro/meso 16, 17,19–22] or a micro/ macro pore size combination, but to our knowledge there is no zeolitic material which combines pores of all three (micromeso-macro) size levels hierarchically interconnected within one particle. In fact, for Faujasite-type zeolite X, a highly hydrophilic large-pore zeolite with a pore diameter of about 0.74 nm and with a low Si/Al molar ratio of 1.0–1.5, a successful method for the creation of additional transport porosity has not been reported up to now. This limitation is a drawback, especially from the viewpoint of using renewable feedstocks for the production of so-called new platform chemicals. Such processes usually involve the transformation of larger molecules, such as fatty acids or sugars. In this area especially basic catalysts such as zeolite X are of high importance, for example for transesterification reactions. 25] Thus it seems to be an essential task to design the properties of such catalysts according to the future demands. Consequently, our major aim was to find a facile synthesis pathway for the implementation of additional transport porosity in Faujasite-type zeolite X. For the synthesis of the mesoporous zeolite X nanosheets the organosilane template TPHAC was used. The as-synthesized material was designated NaX-T, and after template removal by calcination as NaX-T-cal. For comparison purpose a conventional microporous zeolite X (NaX-R, NaX-R-cal) was synthesized using the same synthesis conditions and composition but without TPHAC. As can be seen from the powder X-ray diffraction (XRD) patterns in Figure 1, the reflections of both synthesis products could be attributed to the Faujasite structure and were indexed accordingly. Competing crystalline phases such as zeolite LTA and zeolite P (GIS structure) were not observed. Furthermore, the absence of a halo between 2q = 25–308 in the XRD patterns as well as the absence of sponge-like material in the SEM pictures in Figure 2 indicate the absence of amorphous material. Elemental analysis gave a Si/Al molar ratio of 1.2 [*] A. Inayat, Prof. W. Schwieger Chair of Chemical Reaction Engineering, Department of Chemical and Bioengineering, University of Erlangen-Nuremberg Egerlandstrasse 3, 91052 Erlangen (Germany) E-mail: wilhelm.schwieger@crt.cbi.uni-erlangen.de

Carbon nanodots: toward a comprehensive understanding of their photoluminescence.

We report the characterization of carbon nanodots (CNDs) synthesized under mild and controlled conditions, that is, in a microwave reactor. The CNDs thus synthesized exhibit homogeneous and narrowly dispersed optical properties. They are thus well suited as a testbed for studies of the photophysics of carbon-based nanoscopic emitters. In addition to steady-state investigations, time-correlated single-photon counting, fluorescence up-conversion, and transient pump probe absorption spectroscopy were used to elucidate the excited-state dynamics. Moreover, quenching the CND-based emission with electron donors or acceptors helped shed light on the nature of individual states. Density functional theory and semiempirical configuration-interaction calculations on model systems helped understand the fundamental structure-property relationships for this novel type of material.

Anodic Formation of Thick Anatase TiO2 Mesosponge Layers for High-Efficiency Photocatalysis

We report a process for the fabrication of an anatase TiO(2) mesosponge (TMS) layer by an optimized Ti anodization process in a hot glycerol electrolyte followed by a suitable etching process. Such layers can easily be grown to >10 microm thickness and have regular channels and structural features in the 5-20 nm range. The layers show high photocatalytic activity and are mechanically very robust. The layers therefore open new pathways to the wide field of TiO(2)(anatase) applications.

Non‐Covalent Chemistry of Graphene: Electronic Communication with Dendronized Perylene Bisimides

Graphene is the youngest representative of synthetic carbon allotropes. Since its discovery in 2004, [ 1 ] a series of outstanding physical properties has been revealed. As a consequence, this single-layer graphite is considered to be one of the most promising materials for high-performance applications, [ 2 ] for example in the fi eld of molecular electronics. Although chemistry on graphene and highly dispersed graphite offers unprecedented opportunities, wet chemical functionalization of intact graphene [ 3 ] remains almost completely unexplored. Wet chemistry of graphene is highly attractive because: a) its unique properties can be combined with those of other compound classes, b) solubility and processability can be increased, c) fi ne tuning of the electronic characteristics (doping) can be achieved, d) synthetic routes to novel macromolecular architectures, for instance, graphanes as 2D-polymers, can be provided, and e) the inherent principles of graphene reactivity can be revealed. Herein, we report for the fi rst time on the electronic communication between graphene with the perylene bisimide (PBI) 1 [ 4 ] ( Figure 1 ) when both are deposited on a surface or dispersed in homogeneous solution. This interaction is provided by the non-covalent binding of their conjugated π -systems. Recently we have shown that amphiphilic PBIs, which are related to 1 but contain deprotected carboxylic acid groups, are very effi cient for the exfoliation of single-walled carbon nanotubes (SWNTs) [ 4 , 5 ] and graphite [ 6 ] in water. Moreover, we have demonstrated that the π – π -stacking interaction of electrondefi cient PBIs with SWNTs in water is accompanied by a p -doping of the tubes. [ 7 ] So far, evidence for graphene-dye interactions has been obtained for the solid state, exclusively, for instance, after gas-phase deposition of the dye in ultrahigh vacuum on epitaxial graphene, [ 8 ] after soaking mechanically exfoliated Kish graphite in a dye solution, [ 9 ] on gold or silver colloids, [ 10 ] and on H-passivated substrates. [ 11 ]

Overcoming the Interface Losses in Planar Heterojunction Perovskite-Based Solar Cells.

UNLABELLED A scalable, hysteresis-free and planar architecture perovskite solar cell is presented, employing a flame spray synthesized low-temperature processed NiO (LT-NiO) as hole-transporting layer yielding efficiencies close to 18%. Importantly, it is found that LT-NiO boosts the limits of open-circuit voltages toward an impressive non-radiative voltage loss of 0.226 V only, whereas PEDOT PSS suffers from significant large non-radiative recombination losses.

Low-Voltage p- and n-Type Organic Self-Assembled Monolayer Field Effect Transistors

We report on p- and n-type organic self-assembled monolayer field effect transistors. On the base of quaterthiophene and fullerene units, multifunctional molecules were synthesized, which have the ability to self-assemble and provide multifunctional monolayers. The self-assembly approach, based on phosphonic acids, is very robust and allows the fabrication of functional devices even on larger areas. The p- and n-type transistor devices with only one molecular active layer were demonstrated for transistor channel lengths up to 10 μm. The monolayer composition is proven by electrical experiments and by high-resolution transmission electron microscopy, electron energy loss spectroscopy, XPS, and AFM experiments. Because of the molecular design and the contribution of isolating alkyl chains to the hybrid dielectric, our devices operate at low supply voltages (-4 V to +4 V), which is a key requirement for practical use and simplifies the integration in standard applications. The monolayer devices operate in ambient air and show hole and electron mobilities of 10(-5) cm(2)/(V s) and 10(-4) cm(2)/(V s) respectively. In particular the n-type operation of self-assembled monolayer transistors has not been reported before. Hereby, structure-property relations of the SAMs have been studied. Furthermore an approach to protect the sensitive C(60) from immediate degradation within the molecular design is provided.

Formation of a Non-Thickness-Limited Titanium Dioxide Mesosponge and its Use in Dye-Sensitized Solar Cells

In 1998, Melody et al. introduced an electrochemical anodization process that was reported to lead to so-called nonthickness-limited (NTL) oxide growth for some refractory metals (in particular Ta). By anodization of Ta at 150–180 8C in glycerol/K2HPO4 solutions, oxide layers several tens of micrometers thick could be grown without observing a drop in the growth rate with time. In further work, this process was used with Ti, but was not successful. Herein, we show that by anodizing Ti under adequate conditions, a non-thicknesslimited oxide can indeed be grown. Moreover, by a subsequent selective etching treatment of these layers, a connected, ordered, and mesoporous TiO2 network can be obtained and is suitable for application in high-efficiency dye-sensitized solar cells. Over the past 30 years, TiO2 has attracted wide interest from both the scientific and the technological communities because of its high number of potential applications. In particular, its unique electronic properties make the material attractive, for example for photocatalysis, as well as for solar energy conversion. For this type of application, a large specific surface area is required, and therefore efficient photovoltaic or catalytic electrodes are commonly based on sintered or compacted anatase nanoparticles. In Gr tzel-type solar cells, particle sizes are typically around 10 nm, which are assembled to approximately 10 mm thick porous layers by doctor-blading or spin-coating approaches. The layers then are sensitized with a suitable dye and mounted into various solar cell configurations. 10, 11,16] Another versatile technique for the production of defined oxide layers is anodization of a suitable metal substrate. However, in the case of Ti, anodic layers of TiO2 are formed under most conditions with a compact morphology that is typically limited to a thickness of some 100 nm. To date, only the anodic growth of TiO2 nanotube layers seemed promising for the production of nanostructured geometries that have thicknesses in the 10 mm range and are interesting for solar cell applications. Herein, we introduce an anodization and selective etching approach to form a robust and ordered mesoporous TiO2 network (TiO2 mesosponge) that is tens of micrometers thick and is formed directly on a Ti substrate. Figure 1 shows a SEM cross section of a 15–18 mm thick layer of TiO2 grown by anodization of a Ti sheet in 10 wt% K2HPO4 in glycerol at (180 1) 8C. It was crucial to carefully optimize the experimental conditions in order to achieve the growth of such layers. In particular, the temperature, preanodization time, and anodization current have to be accurately controlled (details are given in the Supporting Information). Under the optimized conditions and after extended anodization, such layers can be grown to thicknesses greater than 50 mm. If the conditions are not sufficiently maintained, only compact or nanoporous oxide layers of some 100 nm thickness could be observed. Thick layers, as shown in Figure 1, have a comparably tight oxide morphology with some nanoscopic channels that are apparent in SEM (scanning electron microscopy; Figure 1b) and TEM (Figure 1c,d) images. The SEM image in Figure 1b shows that the channels typically have a preferred orientation perpendicular to the surface. This orientation is further confirmed by the TEM image (Figure 1c) and the HRTEM image (Figure 1d), which were taken in plan-view geometry and show most of the channels in an edge-on orientation. The width of these pores is in the range 5–10 nm. A main drawback in terms of applications is that the spacing between the nanoscopic channels is wide (ca. 20–50 nm), as apparent from the SEM inset in Figure 1b. However, if this structure is adequately chemically etched, a highly regular and defined sponge structure as shown in the SEM image of Figure 1 e, f is obtained. This structure was etched for 1 h in 30 wt % H2O2 under ultrasonication. A very regular mesoporous morphology is obtained over the entire sample thickness (Figure 1e), with typical TiO2 sizes in the range 5–10 nm and pores of approximately 10 nm (Figure 1 f). In comparison with the “asformed” layer, a much more open structure is present. Figure 2a shows the XRD pattern of the layers of Figure 1. The results reveal that the “as-formed” porous layers before and after etching are amorphous but contain some anatase and rutile crystallites. The diffractogram shown as inset in Figure 1d confirms the mostly amorphous nature but furthermore indicates that some metallic Ti may still be present in the structure. To make use of TiO2 in applications based on photoexcitation, a crystalline structure is desired to eliminate defects associated with the amorphous material. 21] To crystallize the mesoporous TiO2 layers, we [*] D. Kim, K. Lee, Dr. P. Roy, Dr. P. Schmuki Department of Materials Science and Engineering, WW4-LKO University of Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen (Germany) Fax: (+ 49)9131-852-7582 E-mail: schmuki@ww.uni-erlangen.de Homepage: http://www.lko.uni-erlangen.de/

Dislocations in bilayer graphene

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB  AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

Black TiO2 nanotubes formed by high energy proton implantation show noble-metal-co-catalyst free photocatalytic H2-evolution

We apply high-energy proton ion-implantation to modify TiO2 nanotubes selectively at their tops. In the proton-implanted region, we observe the creation of intrinsic cocatalytic centers for photocatalytic H2-evolution. We find proton implantation to induce specific defects and a characteristic modification of the electronic properties not only in nanotubes but also on anatase single crystal (001) surfaces. Nevertheless, for TiO2 nanotubes a strong synergetic effect between implanted region (catalyst) and implant-free tube segment (absorber) can be obtained.