Fabian M. Koehler

Selective Chemical Modification of Graphene Surfaces: Distinction Between Single- and Bilayer Graphene

Graphene modifications with oxygen or hydrogen are well known in contrast to carbon attachment to the graphene lattice. The chemical modification of graphene sheets with aromatic diazonium ions (carbon attachment) is analyzed by confocal Raman spectroscopy. The temporal and spatial evolution of surface-adsorbed species allows accurate tracking of the chemical reaction and identification of intermediates. The controlled transformation of sp(2) to sp(3) carbon proceeds in two separate steps. The presented derivatization is faster for single-layer graphene and allows controlled transformation of adsorbed diazonium reagents into covalently bound surface derivatives with enhanced reactivity at the edge of single-layer graphene. On bilayer graphene the derivatization proceeds to an adsorbed intermediate, which reacts slower to a covalently attached species on the carbon surface.

Permanent Pattern‐Resolved Adjustment of the Surface Potential of Graphene‐Like Carbon through Chemical Functionalization

The exceptional electronic and optical properties of graphene have caught the attention of physicists and materials scientists since the first effective preparation of this two-dimensional form of carbon by Novoselov et al. in 2004. Much effort is currently being invested in the large-scale production of graphene surfaces 3] and in the investigation of its peculiar quantum effects. Graphene is viewed as a potential alternative to silicon as a material for the construction of nanoscale electronic circuits. The use of graphene in this way would require control of its electronic band structure and the withdrawal or injection of electron density to adjust or tilt the Fermi level in a graphene sheet. Such pattern-resolved control of the energy level is the two-dimensional equivalent of n or p doping in classical semiconductors. In contrast to silicon, graphene has a continuous band structure with zero band gap. Thus, single adsorbed molecules modify the band structure and affect the electronic properties of graphene significantly, 10] which makes graphene difficult to handle. Device fabrication requires reliable and permanent control over the different electronic states and the Fermi energy of an air-stable material. The adsorption of organic molecules can result in p-type doping through a sandwichlike p-stacking arrangement on graphene. The injection of electrons is possible through n-type doping with potassium; however, such materials are highly sensitive to air and water. In the search for a robust and highly precise doping method, we investigated well-established protocols from organic radical chemistry to attach an air-stable dopant covalently and thus permanently alter the electronic structure of graphene sheets. The relative surface charge levels were measured by Kelvin force microscopy (KFM). The application of the linear free-enthalpy relationship for substituted aromatic compounds enabled the direct prediction of the charge-withdrawing or charge-injecting effect of graphene modification. We therefore concluded that this approach should enable direct control of the surface potential, Y, of modified graphene. Furthermore, the Hammett concept enabled a precise correlation between the observed change in the surface potential, DY, and the structure of the covalently bonded reagents. This concept was confirmed experimentally by using strongly electron withdrawing (p-nitrophenyl, s = 0.78) and electron donating substituents (p-methoxyphenyl, s = 0.23). For our experiments, we used the top graphene layer of highly ordered pyrolytic graphite (HOPG) as a model material. From a physical point of view, this model is not representative for detailed investigations on band structure or electronic effects. However, from a chemical point of view, the reactivity of the graphene stacks of HOPG is comparable to that of single-walled carbon nanotubes, which can be considered as rolls of graphene. For additional experimental validation, we also carried out the graphene modifications described herein on carbon-coated nanoparticles (two or three layers of graphene on copper). Detailed structural evidence was then provided by diffuse reflectance FTIR to characterize the products and confirm the direct covalent attachment of the modifying groups to the top graphene layer. This functionalization approach extends p and n doping based on adsorbed molecules or ions to make it a systematic and robust method with which molecular electronics elements can be attached perpendicular to the graphene plane in a third dimension. The experimental approach to covalent graphene modification is shown in Figure 1. The model material (top layer of a monocrystalline graphene stack) was first patterned by lithography, so that a plain (unfunctionalized) graphene surface would be preserved below the photoresist. The unmasked areas were functionalized by exposure to highly diluted diazonium reagents (see the Supporting Information). After removal of the photoresist, the graphene surface was investigated by scanning electron microscopy (SEM) and Kelvin force microscopy (KFM) in tapping mode to image the relative surface-potential levels of modified and native areas of the graphene surface. The chemical derivatization depends [*] MSc Chem. Eng. F. M. Koehler, MSc Mat. Sci. N. A. Luechinger, Dipl.-Chem.-Ing. E. K. Athanassiou, Dr. R. N. Grass, Prof. Dr. W. J. Stark Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences, ETH Zurich Wolfgang-Pauli-Strasse 10, 8093 Zurich (Switzerland) Fax: (+ 41)44-633-1083 E-mail: wendelin.stark@chem.ethz.ch Homepage: http://www.fml.ethz.ch

Gold adsorption on the carbon surface of C/Co nanoparticles allows magnetic extraction from extremely diluted aqueous solutions

The elusive chemistry of gold has made refining from ores a difficult task and often involves handling of large volumes of water at low pH values with associated high environmental burden. As a result, the broader use of gold in environmental catalysis, organic synthesis and in electronics is still limited in spite of its most attractive chemistry. Present gold extraction suffers from metal loss in the form of gold adsorbed on active carbon particles that are washed out of the extraction process. Here, we investigate the use of magnetic carbon in the form of carbon-coated metal nanomagnets for ionic gold recovery. In contrast to acid-labile iron oxide nanoparticles, the carbon/cobalt nanomagnets resisted dissolution in acidic refining/recycling waters. Repetitive extraction runs demonstrated the possibility to recycle the magnetic reagent. A series of dilution studies showed a high affinity of the ionic gold to the carbon surfaces of the nanomagnets which enabled gold extraction down to the part per billion level (microgram per litre). Detailed investigations on the morphology of the Au-loaded nanomagnets after use suggest a mechanism based on the selective reduction of ionic gold on the C/Co surface and transfer of cobalt through the carbon shell. The resulting irreversible deposition of metallic gold correlated with the release of oxidized (ionic) cobalt into the aqueous phase.

Efficient Magnetic Recycling of Covalently Attached Enzymes on Carbon-Coated Metallic Nanomagnets

In the pursuit of robust and reusable biocatalysts for industrial synthetic chemistry, nanobiotechnology is currently taking a significant part. Recently, enzymes have been immobilized on different nanoscaffold supports. Carbon coated metallic nanoparticles were found to be a practically useful support for enzyme immobilization due to their large surface area, high magnetic saturation, and manipulatable surface chemistry. In this study carbon coated cobalt nanoparticles were chemically functionalized (diazonium chemistry), activated for bioconjugation (N,N-disuccinimidyl carbonate), and subsequently used in enzyme immobilization. Three enzymes, β-glucosidase, α-chymotrypsin, and lipase B were successfully covalently immobilized on the magnetic nonsupport. The enzyme-particle conjugates formed retained their activity and stability after immobilization and were efficiently recycled from milliliter to liter scales in short recycle times.

Magnetic switching of optical reflectivity in nanomagnet/micromirror suspensions: colloid displays as a potential alternative to liquid crystal displays.

Two-particle colloids containing nanomagnets and microscale mirrors can be prepared from iron oxide nanoparticles, microscale metal flakes and high-density liquids stabilizing the mirror suspension against sedimentation by matching the constituents density. The free Brownian rotation of the micromirrors can be magnetically controlled through an anisotropic change in impulse transport arising from impacts of the magnetic nanoparticles onto the anisotropic flakes. The resulting rapid mirror orientation allows large changes in light transmission and switchable optical reflectivity. The preparation of a passive display was conceptually demonstrated through colloid confinement in a planar cavity over an array of individually addressable solenoids and resulted in 4 x 4 digit displays with a reaction time of less than 100 ms.

Electrical resistivity of assembled transparent inorganic oxide nanoparticle thin layers: influence of silica, insulating impurities, and surfactant layer thickness.

The electrical properties of transparent, conductive layers prepared from nanoparticle dispersions of doped oxides are highly sensitive to impurities. Production of cost-effective thin conducting films for consumer electronics often employs wet processing such as spin and/or dip coating of surfactant-stabilized nanoparticle dispersions. This inherently results in entrainment of organic and inorganic impurities into the conducting layer leading to largely varying electrical conductivity. Therefore, this study provides a systematic investigation on the effect of insulating surfactants, small organic molecules and silica in terms of pressure dependent electrical resistivity as a result of different core/shell structures (layer thickness). Application of high temperature flame synthesis gives access to antimony-doped tin oxide (ATO) nanoparticles with high purity. This well-defined starting material was then subjected to representative film preparation processes using organic additives. In addition ATO nanoparticles were prepared with a homogeneous inorganic silica layer (silica layer thickness from 0.7 to 2 nm). Testing both organic and inorganic shell materials for the electronic transport through the nanoparticle composite allowed a systematic study on the influence of surface adsorbates (e.g., organic, insulating materials on the conducting nanoparticles surface) in comparison to well-known insulators such as silica. Insulating impurities or shells revealed a dominant influence of a tunneling effect on the overall layer resistance. Mechanical relaxation phenomena were found for 2 nm insulating shells for both large polymer surfactants and (inorganic) SiO(2) shells.

Chemical modification of graphene characterized by Raman and transport experiments

A chemical approach to modify the electronic transport of graphene is investigated by detailed transport and Raman spectroscopy measurements on Hall bar shaped samples. The functionalization of graphene with nitrobenzene diazonium ions results in a strong p-doping of the graphene samples and only slightly lower mobilities. Comparing Raman and transport data taken after each functionalization step allowed the conclusion that two preferential reactions take place on the graphene surface. In the beginning a few nitrobenzene molecules are directly attached to the graphene atoms creating defects. Afterwards these act as seeds for a polymer like growth not directly connected to the graphene atoms. The effects of solvents were excluded by thorough control measurements.

Incorporation of Penicillin-Producing Fungi into Living Materials to Provide Chemically Active and Antibiotic-Releasing Surfaces

Living materials: artificial biological niches are loaded with the penicillin-producing mold Penicillium chrysogenum. This living material consumes food through a nanoporous top layer and releases the antibiotic on-site. No reloading of the active compound is needed. Gram-positive bacteria were efficiently killed if nearby, whereas Gram-negative bacteria (control experiment, not sensitive to penicillin) were not affected.

Towards electron transport measurements in chemically modified graphene: effect of a solvent

The chemical functionalization of graphene modifies the local electron density of carbon atoms and hence electron transport. Measuring these changes allows for a closer understanding of the chemical interaction and the influence of functionalization on the graphene lattice. However, not only chemistry, in this case diazonium chemistry, has an effect on electron transport. The latter is also influenced by defects and dopants resulting from different processing steps. Here, we show that the solvents used in the chemical reaction process change the transport properties. In more detail, the investigated combination of isopropanol and heating treatment reduces the doping concentration and significantly increases the mobility of graphene. Furthermore, isopropanol treatment alone increases the concentration of dopants and introduces an asymmetry between electron and hole transport, which might be difficult to distinguish from the effect of functionalization. The results shown in this work demand a closer look at the influence of solvents used for chemical modification in order to understand their influence.

Incorporating microorganisms into polymer layers provides bioinspired functional living materials

Artificial two-dimensional biological habitats were prepared from porous polymer layers and inoculated with the fungus Penicillium roqueforti to provide a living material. Such composites of classical industrial ingredients and living microorganisms can provide a novel form of functional or smart materials with capability for evolutionary adaptation. This allows realization of most complex responses to environmental stimuli. As a conceptual design, we prepared a material surface with self-cleaning capability when subjected to standardized food spill. Fungal growth and reproduction were observed in between two specifically adapted polymer layers. Gas exchange for breathing and transport of nutrient through a nano-porous top layer allowed selective intake of food whilst limiting the microorganism to dwell exclusively in between a confined, well-enclosed area of the material. We demonstrated a design of such living materials and showed both active (eating) and waiting (dormant, hibernation) states with additional recovery for reinitiation of a new active state by observing the metabolic activity over two full nutrition cycles of the living material (active, hibernation, reactivation). This novel class of living materials can be expected to provide nonclassical solutions in consumer goods such as packaging, indoor surfaces, and in biotechnology.