Characterizing the maximum number of layers in chemically exfoliated graphene.

An efficient route to synthesize macroscopic amounts of graphene is highly desired and bulk characterization of such samples, in terms of the number of layers, is equally important. We present a Raman spectroscopy-based method to determine the typical upper limit of the number of graphene layers in chemically exfoliated graphene. We utilize a controlled vapour-phase potassium intercalation technique and identify a lightly doped stage, where the Raman modes of undoped and doped few-layer graphene flakes coexist. The spectra can be unambiguously distinguished from alkali doped graphite, and modeling with the typical upper limit of the layers yields an upper limit of flake thickness of five layers with a significant single-layer graphene content. Complementary statistical AFM measurements on individual few-layer graphene flakes find a consistent distribution of the layer numbers.

Synthesis and first X-ray structure of a hexa-peri-hexabenzocoronene-fullerene-dyad: a model for an inter-carbon-allotrope hybrid.

The synthesis of a new hexa-peri-hexabenzocoronene (HBC)-fullerene dyad was accomplished involving a covalent linkage between both the planar and the spherical conjugated π-system of the two chromophores. We also present the first X-ray single crystal structure of a HBC-fullerene conjugate. A very short HBC-fullerene distance of 3.2 Å is observed. For the synthesis of this molecule, a new versatile applicable template, namely, a mono-functionalized hexa-peri-hexabenzocoronene was attached to a fullerene. Absorption and fluorescence spectroscopy, as well as quantum yield measurements of , indicated close electronic communication between the two subunits, which is promising for possible applications in molecular electronics.

Screening of the chemical reactivity of three different graphite sources using the formation of reductively alkylated graphene as a model reaction.

Reductive alkylation of three graphite starting materials G(flake), G(powder), and G(spherical) reveals pronounced differences in the obtained covalently functionalized graphene with respect to the degree of functionalization, exfoliation efficiency and product homogeneity, as demonstrated by statistical Raman microscopy (SRM), TGA/MS, IR-spectroscopy and solubility behavior.

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.

HBC-porphyrin--close contact chromophores.

A photo/redoxactive hexa-peri-hexabenzocoronene-porphyrin conjugate with a direct connection between the two chromophores was synthesised using a formylated hexaphenylbenzene precursor.

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.

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 graphene's 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.

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.

Effect of Polymer Molecular Weight and Solution Parameters on Selective Dispersion of Single-Walled Carbon Nanotubes.

The selective dispersion of single-walled carbon nanotube species (n,m) with conjugated polymers such as poly(9,9-dioctylfluorene) (PFO) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) in organic solvents depends not only on the type of solvent but also on the molecular weight of the polymer. We find an increasing amount of nanotubes and altered selectivities for dispersions with higher molecular weight polymers. Including the effects of different aromatic solvents, we propose that solution viscosity is one of the factors influencing the apparent selectivity by changing the reaggregation rate of the single-walled carbon nanotubes (SWNT). The type of solvent, polymer molecular weight, concentration, and viscosity should thus be taken into account when screening for new polymers for selective SWNT dispersion.

Chemical methods for the generation of graphenes and graphene nanoribbons.

Synthetic carbon allotrope chemistry is currently among the most rapidly growing topics in materials chemistry. The youngest and at the same time probably the most promising representative of new carbon allotropes is graphene. In this article we outline our recent contributions to chemical graphene formation and functionalization.

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.

Exfoliation of hexa-peri-hexabenzocoronene in water.

Unsubstituted hexa-peri-hexabenzocoronene (HBC)-a small and well defined model system for graphite/graphene-was exfoliated and dissolved in water and the resulting individualized nano graphene sheets were characterized spectroscopically for the first time.