Ralf Graupner

Raman spectra of epitaxial graphene on SiC(0001)

We present Raman spectra of epitaxial graphene layers grown on 63×63 reconstructed silicon carbide surfaces during annealing at elevated temperature. In contrast to exfoliated graphene a significant phonon hardening is observed. We ascribe that phonon hardening to a minor part to the known electron transfer from the substrate to the epitaxial layer, and mainly to mechanical strain that builds up when the sample is cooled down after annealing. Due to the larger thermal expansion coefficient of silicon carbide compared to the in-plane expansion coefficient of graphite this strain is compressive at room temperature.

Doping of single-walled carbon nanotube bundles by Brønsted acids

Using X-ray induced photoelectron spectroscopy, the influence of Bronsted acids, namely sulfuric, nitric, and hydrochloric acid on the electronic properties of single-walled carbon nanotubes (SWCNTs) was investigated. Doping effects were monitored by changes in binding energy of the C 1s core level of the nanotubes. For all three acids, an acceptor type doping of the SWCNTs was observed by a shift of the C 1s core level towards lower binding energies. The inferred change of the Fermi-level position was 0.5 eV in the case of H2SO4, 0.2 eV in the case of HNO3, and 0.1 eV in the case of HCl. For HNO3 and HCl the doping was found to be unstable. The S 2p, N 1s, and Cl 2p core level spectra of the corresponding acid showed spectral features which can be attributed to the respective oxidation state of these anions in the acid, indicating that doping was induced by intercalation.

Chemical functionalization of single walled carbon nanotubes

Abstract Chemical modification has been performed on purified single walled carbon nanotubes. XPS spectrum shows that the peak corresponding to C (1s) centered at 284.38 eV in pure nanotubes (graphitic C) is 0.4 eV downshifted in chlorinated sample. Subsequent coupling reactions were carried out with diamine molecules to form intertube connections. Tripropylentetramine and phenylendiamine have been chosen as a molecular linker. End-to-side and end-to-end nanotube interconnections are formed and then observed by atomic force microscopy (AFM). Statistical analysis made from AFM images shows around 30% junctions in functionalized and less than 2% in pristine material. Remarkable features can be observed in the Raman spectra at different functionalization steps. Simple conductance measurements on bucky papers prepared from prestine nanotubes and from nanotubes modified at various steps have been made and are discussed.

Carbon Nanotube Sidewall Functionalization with Carbonyl Compounds—Modified Birch Conditions vs the Organometallic Reduction Approach

Covalent addition reactions turned out to be one of the most important functionalization techniques for a structural alteration of single walled carbon nanotube (SWCNT) scaffolds. During the last years, several reaction sequences based on an electrophilic interception of intermediately generated SWCNT(n-) carbanions, obtained via Birch reduction or by a nucleophilic addition of organometallic species, have been developed. Nevertheless, the scope and the variety of potential electrophiles is limited due to the harsh reaction conditions requested for a covalent attachment of the functional entities onto the SWCNT framework. Herein, we present a significant modification of the reductive alkylation/arylation sequence, the so-called Billups reaction, which extends the portfolio of electrophiles for covalent sidewall functionalization to carbonyl compounds--ketones, esters, and even carboxylic acid chlorides. Moreover, these carbonyl-based electrophiles can also be used as secondary functionalization reagents for anionic SWCNT intermediates, derived from a primary nucleophilic addition step. This directly leads to the generation of mixed functional SWCNT architectures, equipped with hydroxyl or carbonyl anchor groups, suitable for ongoing derivatization reactions. A correlated absorption and emission spectroscopic study elucidates the influence of the covalent sidewall functionalization degree onto the excitonic transition features of carbon nanotubes. The characterization of the different SWCNT adducts has been carried out by means of Raman, UV-vis/nIR, and fluorescence spectroscopy as well as by thermogravimetric analysis combined with mass spectrometry and X-ray photoelectron spectroscopy analysis.

Reductive retrofunctionalization of single-walled carbon nanotubes.

The chemical functionalization of fullerenes, carbon nanotubes, and graphene is a prerequisite for the use of these synthetic carbon allotropes in high-performance applications. For this reason, addition reactions to the conjugated p system of fullerenes and single-walled carbon nanotubes (SWCNTs) have been intensively investigated. 2] The intrinsic chemical properties of these carbon allotropes can be determined by cage functionalization, but the new derivatives also offer new perspectives such as: 1) increased solubility, processability, and functionality; and 2) properties combined with those of other compound classes. Addition reactions to the sphybridized carbon framework are always accompanied by the generation of sp-hybridized atoms in the cages and, as a consequence, by changes of properties, in particular the electronic structure. This might be considered a drawback because the electronic properties of the parent sp allotropes are in many regards outstanding and unprecedented. However, once the attached addends have completed their assignment in a given process chain, they may be removed such that the structure and function of the SWCNTs are recovered. Cleavage of covalently bound addends, however, often requires high temperatures, conditions that might not be compatible with many applications. In fullerene chemistry mild retrofunctionalization methods have already been discovered such as retro-Diels–Alder reactions and retrocyclopropanations induced by cage reduction with at least two electrons (retro-Bingel reaction, Scheme 1). In the case of the retro-Prato reaction the removal of the pyrrolidine ring from the sp-hybridized carbon sphere is supported by Lewis acid catalysis or microwave irradiation. We have recently developed a very versatile method for the functionalization of the sidewalls of SWCNTs, namely, the nucleophilic addition of metal alkylides and amides followed by reoxidation of the negatively charged intermediates RnSWCNT n to give the neutral derivatives RnSWCNT (Scheme 2). Like the Billups alkylation of carbon nano-

Surface vibrations on clean, deuterated, and hydrogenated single crystal diamond(100) surfaces studied by high-resolution electron energy loss spectroscopy

We present a detailed study of the clean, hydrogenated, and deuterated (2×1)-reconstructed (100) surface of diamond by angular dependent high-resolution electron energy loss spectroscopy (HREELS). We were able to identify two Rayleigh modes on all surfaces. The measured dispersion widths for the two modes are 40 meV and 43 meV on the clean diamond surface. On the hydrogenated and the deuterated surface the same dispersion widths are measured, namely 43 meV and 60 meV, indicating that no isotope shift is observed for the Rayleigh modes between these two surfaces. Bending modes could be assigned to structures at 119 meV and 183 meV for the hydrogen covered and to those at 87 meV and 137 meV on the deuterated surface. Moreover, on the clean surface modes at 180 meV and 93 meV are ascribed to the dimer stretch or dimer twist and the dimer bounce vibration, respectively.

Photoelectron spectroscopy of clean and hydrogen-exposed diamond (111) surfaces

Abstract The modifications of the (111) surfaces of a type-IIb natural diamond and a CVD diamond film upon annealing and exposure to atomic hydrogen have been examined by ultraviolet-excited (UPS) and X-ray excited (XPS) photoelectron spectroscopy. Besides the two surface configurations commonly described in the literature, i.e. the clean reconstructed surface and the one associated with a monohydride passivation, a third configuration which evolves upon extreme hydrogen exposure has been observed.

Electronic properties of single crystalline diamond surfaces

Abstract Key parameters of the surface electronic structure of diamond (111), such as surface core level shifts, surface state dispersion, band bending, and electron affinity, are presented and discussed. The first actual value for the negative electron affinity of diamond (111):H 1×1 is measured as χ =−1.27 eV. A model is given that describes the variation in χ with hydrogen coverage quantitatively in terms of C–H bond dipoles.

Initial stages in the growth of polycrystalline diamond on silicon

Abstract Polycrystalline diamond films prepared in a hot filament chemical vapour deposition reactor were investigated with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) in order to identify chemically and structurally distinguishable phases during nucleation and early stages of diamond growth. This was achieved by investigating a series of films grown under identical conditions for 5 min to 4 h. In addition, the interface between a solid diamond film and its silicon substrate was studied after the film had been removed from the substrate. Carbon deposition commences initially with the simultaneous growth of diamond crystallites along scratches and a layer of microcrystalline graphite covering the remainder of the substrate. Small amounts of SiC could also be identified during the first 100 min of deposition. Once the individual diamond crystallites have grown together to form a continuous layer, the graphitic phase in the spectra is replaced by an amorphous carbon phase which we attribute to the grain boundaries between the crystals. Inspection of the film backside revealed that the amorphous carbon had merely overgrown the microcrystalline graphite which was still present as the major component. Only after prolonged growth times (24 h) did the Raman and XPS spectra exhibit the characteristic diamond features free from any other contributions. On the basis of these observations a model for the initial stages of diamond growth on Si is developed.

The influence of surface treatment on the electronic structure of CVD diamond films

Abstract The electronic structure of CVD diamond surfaces has been studied by core level and valence band photoelectron spectroscopy. Upon annealing at 870 °C, a band of surface states appears within the top 6 eV of the valence bands which is accompanied by a decrease in spectral intensity around 9 eV and a band bending of 0.4 eV. Similar changes could be induced by Ar ion bombardment. Exposure to atomic hydrogen largely restored the original spectra. These observations are compared with corresponding data on single-crystalline diamond surfaces. Similarities and differences are discussed in terms of structural models of the CVD diamond surface.