Jan M. Englert

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.

Wet Chemical Functionalization of Graphene

The fullerenes, carbon nanotubes, and graphene have enriched the family of carbon allotropes over the last few decades. Synthetic carbon allotropes (SCAs) have attracted chemists, physicists, and materials scientists because of the sheer multitude of their aesthetically pleasing structures and, more so, because of their outstanding and often unprecedented properties. They consist of fully conjugated p-electron systems and are considered topologically confined objects in zero, one, or two dimensions. Among the SCAs, graphene shows the greatest potential for high-performance applications, in the field of nanoelectronics, for example. However, significant fundamental research is still required to develop graphene chemistry. Chemical functionalization of graphene will increase its dispersibility in solvents, improve its processing into new materials, and facilitate the combination of graphenes unprecedented properties with those of other compound classes. On the basis of our experience with fullerenes and carbon nanotubes, we have described a series of covalent and noncovalent approaches to generate graphene derivatives. Using water-soluble perylene surfactants, we could efficiently exfoliate graphite in water and prepare substantial amounts of single-layer-graphene (SLG) and few-layer-graphene (FLG). At the same time, this approach leads to noncovalent graphene derivatives because it establishes efficient π-π-stacking interactions between graphene and the aromatic perylene chromophors supported by hydrophobic interactions. To gain efficient access to covalently functionalized graphene we employed graphite intercalation compounds (GICs), where positively charged metal cations are located between the negatively charged graphene sheets. The balanced combination of intercalation combined with repulsion driven by Coulombic interactions facilitated efficient exfoliation and wet chemical functionalization of the electronically activated graphene sheets via trapping with reactive electrophilic addends. For example, the treatment of reduced graphite with aryl diazonium salts with the elimination of N(2) led to the formation of arylated graphene. We obtained alkylated graphene via related trapping reactions with alkyl iodides. These new developments have opened the door for combining the unprecedented properties of graphene with those of other compound classes. We expect that further studies of the principles of graphene reactivity, improved characterization methods, and better synthetic control over graphene derivatives will lead to a whole series of new materials with highly specific functionalities and enormous potential for attractive 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 ]

Scanning-Raman-Microscopy for the Statistical Analysis of Covalently Functionalized Graphene

We report on the introduction of a systematic method for the quantitative and reliable characterization of covalently functionalized graphene based on Scanning-Raman-Microscopy (SRM). This allows for recording and analyzing several thousands of Raman spectra per sample and straightforward display of various Raman properties and their correlations with each other in histograms or coded 2D-plots. In this way, information about the functionalization efficiency of a given reaction, the reproducibility of the statistical analysis, and the sample homogeneity can be easily deduced. Based on geometric considerations, we were also able to provide, for the first time, a correlation between the mean defect distance of densely packed point defects and the Raman ID/IG ratio directly obtained from the statistical analysis. This proved to be the prerequisite for determining the degree of functionalization, termed θ. As model compounds, we have studied a series of arylated graphenes (GPh) for which we have developed new synthetic procedures. Both graphite and graphene grown by chemical vapor deposition (CVD) were used as starting materials. The best route toward GPh consisted of the initial reduction of graphite with a Na/K alloy in 1,2-dimethoxyethane (DME) as it yields the highest overall homogeneity of products reflected in the widths of the Raman ID/IG histograms. The Raman results correlate nicely with parallel thermogravimetric analysis (TGA) coupled with mass spectrometry (MS) studies.

On the Way to Graphane—Pronounced Fluorescence of Polyhydrogenated Graphene

Chemistry meets graphane: a Birch-type reaction using frozen water as a gentle proton source causes the exfoliation of graphite and formation of hydrogenated graphene with electronically decoupled π-nanodomains. This highly functionalized graphene displays pronounced fluorescence.

Functionalization of graphene by electrophilic alkylation of reduced graphite

The reaction of Na/K-reduced graphite with hexyliodide represents a new, versatile and mild approach to synthesize alkylated graphene derivatives, which were characterized by a combination of Raman spectroscopy, TEM and TGA/MS analysis.

Statistical Raman Spectroscopy: A Method for the Characterization of Covalently Functionalized Single-Walled Carbon Nanotubes†

It allowsa number of intrinsic hurdles to be overcome, such asaggregation, low solubility, and difficult processability,which impede a straightforward development of SWCNTsas building blocks in high-performance materials. Chemicalfunctionalization offers the opportunity to combine theirunprecedented properties with those of other compoundclasses and it is a promising approach for separation byelectronic properties. A number of reactions have beenreported that show preferential attack of addends to eithermetallic or semiconducting tubes.

Extrinsic corrugation-assisted mechanical exfoliation of monolayer graphene.

Figure 1 . a) The optical microscopy (OM) image of ≈ 100 nm thick graphite fl akes on silicon substrate after thermal annealing at 350 ° C for 2 h. b) The OM image of the same position after the reverse exfoliation [∗] S. Pang , Dr. H. N. Tsao , Dr. Y. Hernandez , Dr. X. Feng , Prof. K. Mullen Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz (Germany) E-mail: feng@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de J. M. Englert , Prof. A. Hirsch Zentralinstitut fur Neue Materialien und Prozesstechnik Dr.-Mack Str. 81 D-90762 Furth (Germany) Since the reports of the fi rst isolation and observation of the exceptional electronic, mechanical, and chemical properties, single layer graphene has attracted intense interest from both academic and industrial communities. [ 1–3 ] While the mechanical exfoliation method led to many exciting discoveries, several promising approaches have been reported for the preparation of monolayer graphene including solution exfoliation of graphite, [ 4 , 5 ] epitaxial growth from SiC, [ 6 , 7 ] reduction of graphene oxide, [ 8–10 ] and chemical vapor deposition (CVD) on metal surfaces. [ 11–13 ] Up to now, the CVD growth method seems to be the most promising technique for the production of large-scale few layer graphene fi lms. [ 13 , 14 ] However, this surface-mediated process requires very high temperatures, and tedious additional steps involving etching and transfer, thus rendering the production of graphene-based electronic devices diffi cult. Ultra-large monolayer graphene [ 15–17 ] and patterned graphene structures [ 18–22 ] constitute two important aspects for graphene fabrication technology. A simple way to directly “print” a high-quality graphene monolayer on insulating substrates from a graphite stamp would be particularly appealing for electronic applications. Covalent immobilization [ 23 , 24 ] and electrostatic forces [ 18 , 25 ] have been devised for the modifi cation of mechanical exfoliation, with the aim to improve the yield of monolayer graphene or to achieve patterned graphene structures. Despite the success in deposition of graphene patterns over a large area, these procedures suffer from a low monolayer yield ( < 10%). [ 18 , 19 , 26 ] Here, we describe an extrinsic corrugationassisted mechanical exfoliation (ECAME) for synthesizing monolayer graphene on substrates. This strategy involves a simple thermal treatment of deposited graphite on a silicon wafer in association with a wafer processing tape peeling process. This work reveals that underlayer graphene sheets can corrugate following the rough SiO 2 surface when the thick graphite fl ake is thermally annealed. Such a surface-mediated extrinsic corrugation process thus serves as the key driving force for exfoliation leading to more than 60% high-quality monolayer graphene. This protocol can be further employed to fabricate graphene patterns on the surface, a technique that may be explored for graphene-based electronic device fabrication.

Robust graphene membranes in a silicon carbide frame.

We present a fabrication process for freely suspended membranes consisting of bi- and trilayer graphene grown on silicon carbide. The procedure, involving photoelectrochemical etching, enables the simultaneous fabrication of hundreds of arbitrarily shaped membranes with an area up to 500 μm(2) and a yield of around 90%. Micro-Raman and atomic force microscopy measurements confirm that the graphene layer withstands the electrochemical etching and show that the membranes are virtually unstrained. The process delivers membranes with a cleanliness suited for high-resolution transmission electron microscopy (HRTEM) at atomic scale. The membrane, and its frame, is very robust with respect to thermal cycling above 1000 °C as well as harsh acidic or alkaline treatment.

Robust Two-Dimensional Electronic Properties in Three-Dimensional Microstructures of Rotationally Stacked Turbostratic Graphene

Carbon nanomaterials continue to amaze scientists due to their exceptional physical properties 1 . Recently there have been theoretical predictions and first reports on graphene multilayers, where, due to the rotation of the stacked layers, outstanding electronic properties are retained while the susceptibility to degradation and mechanical stress is strongly reduced due to the multilayer nature 2 . Here we show that fully turbostratic multilayer graphitic microstructures combine the