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Heteroepitaxial growth of GeSi alloys on Si (001) under deposition conditions that partially limit surface mobility leads to an unusual form of strain-induced surface morphological evolution. We discuss a kinetic growth regime wherein pits form in a thick metastable wetting layer and, with additional deposition, evolve to a quantum dot molecule---a symmetric assembly of four quantum dots bound by the central pit. We discuss the size selection and scaling of quantum dot molecules. We then examine the key mechanism---preferred pit formation---in detail, using ex situ atomic force microscopy, in situ scanning tunneling microscopy, and kinetic Monte Carlo simulations. A picture emerges wherein localized pits appear to arise from a damped instability. When pits are annealed, they extend into an array of highly anisotropic surface grooves via a one-dimensional growth instability. Subsequent deposition on this grooved film results in a fascinating structure where compact quantum dots and molecules, as well as highly ramified quantum wires, are all simultaneously self-assembled.

Using transmission electron microscopy, we identify the temperature-dependent interaction pathway of carbon-supported Au nanoparticles. At low temperature (room temp. to 400 degrees C), Au nanoparticles predominantly interact by coalescence initiated by an atomic Au bridge. At high temperature (400-800 degrees C), the particles assemble into Au/C core-shell nanostructures. C-shells around individual nanoparticles passivate their surface and prevent coalescence. Ultimately coalescence does occur via rupture of the passivating shells, which invariably follows the assembly of C sheets enveloping and compressing multiple closely spaced Au/C nanoparticles.

We assess scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) for thickness measurements on few-layer hexagonal boron nitride (h-BN), the layered dielectric of choice for integration with graphene and other two-dimensional materials. Observations on h-BN islands with large, atomically flat terraces show that the secondary electron intensity in SEM reflects monolayer height changes in films up to least 10 atomic layers thickness. From a quantitative analysis of AES data, the energy-dependent electron escape depth in h-BN films is deduced. The results show that AES is suitable for absolute thickness measurements of few-layer h-BN of 1 to 6 layers.

The growth of two-dimensional (2D) materials is a topic of very high scientific and technological interest. While chemical vapour deposition on catalytic metals has become a well developed approach for the growth of graphene and hexagonal boron nitride (BN), very few alternative approaches for synthesis on non-reactive supports have been explored so far. Here we report the growth of BN on gold, using magnetron sputtering of B in N2/Ar atmosphere, a scalable method using only non-toxic reagents. Scanning tunnelling microscopy at low coverage shows primarily triangular monolayer BN islands exhibiting two 'magic' orientations on the Au(111) surface. Such rotational alignment of BN on Au(111) is surprising, given the expected weak binding and the high lattice mismatch (~14%) between BN and Au. Our observations are consistent with a strong coupling between the edges of BN flakes and the substrate, which leads to the selection of BN orientations that maximize the orbital overlap between edge atoms and Au surface atoms. Diverse flake morphologies resembling the shape of butterflies, six-apex stars and diamonds, implying alternating B- and N- terminated edges, are observed as well. Our results provide insight into the growth mechanisms of 2D materials on weakly interacting and chemically inert substrates, and provide the basis for integrating other 2D materials with atomically precise graphene nanostructures synthesized from molecular precursors on Au.

Large and homogeneous layers of graphene are obtained by annealing silicon carbide in a dense noble gas atmosphere that controls the way in which silicon sublimates. Epitaxial graphene thus gets back on track towards future electronic applications.

We introduce a method for isolating graphene grown on epitaxial Ru(0001)/α-Al2O3. The strong graphene/Ru(0001) coupling is weakened by electrochemically driven intercalation of hydrogen underpotentially deposited in aqueous KOH solution, which allows the penetration of water molecules at the graphene/Ru(0001) interface. Following these electrochemically driven processes, the graphene can be isolated by electrochemical hydrogen evolution and transferred to arbitrary supports. Raman and transport measurements demonstrate the high quality of the transferred graphene. Our results show that intercalation, typically carried out in vacuum, can be extended to solution environments for graphene processing under ambient conditions.

Synthesis of bilayer graphene by chemical vapor deposition is of importance for graphene-based field effect devices. Here, we demonstrate that bilayer graphene preferentially grows by carbon-segregation under graphene sheets that are rotated relative to a Ni(111) substrate. Rotated graphene monolayer films can be synthesized at growth temperatures above 650 °C on a Ni(111) thin-film. The segregated second graphene layer is in registry with the Ni(111) substrate and this suppresses further C-segregation, effectively self-limiting graphene formation to two layers.

We investigate the room-temperature oxidation of In nanoparticles at the tips of Si nanowires. This geometry allows a direct comparison of oxidation at the gas-indium interface and the interface to the Si nanowire. While the In2O3 at the free surface is polycrystalline with small limiting thickness, the oxidation at the nanoscale interface to Si gives rise to single crystalline In2O3 with a tenfold-enhanced thickness. Our results demonstrate interfacial modifications of oxidation at the nanoscale, which need to be considered in scaling electronic devices, and which can become a route for forming high-quality semiconductor-oxide interfaces in nanostructured materials, such as nanowires.

In contrast to the commonly employed high temperature chemical vapor deposition growth that leads to multilayer graphene formation by carbon segregation from the bulk, we demonstrate that below 600 °C graphene can be grown in a self-limiting monolayer growth process. Optimum growth is achieved at ∼550 °C. Above this temperature, carbon diffusion into the bulk is limiting the surface growth rate, while at temperatures below ∼500 °C a competing surface carbide phase impedes graphene formation.

Single crystalline Ru(0001) thin films epitaxially grown on sapphire (0001) substrates were used as sacrificial metal templates for the synthesis of high-quality graphene with uniform monolayer thickness and full surface coverage. Removal of the metal template by etching transferred monolayer graphene with good crystal quality onto the insulating sapphire support. Our findings demonstrate epitaxial Ru(0001) films on sapphire (0001) as a substrate for the scalable synthesis of high-quality graphene for applications.

In the literature, there are controversies on the interpretation of the appearance in InN Raman spectra of a strong scattering peak in the energy region of the unscreened longitudinal optical (LO) phonons, although a shift caused by the phonon-plasmon interaction is expected for the high conductance observed in this material. Most measurements on light scattering are performed on ensembles of InN nanowires (NWs). However, it is important to investigate the behavior of individual nanowires and here we report on micro-Raman measurements on single nanowires. When changing the polarization direction of the incident light from parallel to perpendicular to the wire, the expected reduction of the Raman scattering was observed for transversal optical (TO) and E(2) phonon scattering modes, while a strong symmetry-forbidden LO mode was observed independently on the laser polarization direction. Single Mg- and Si-doped crystalline InN nanowires were also investigated. Magnesium doping results in a sharpening of the Raman peaks, while silicon doping leads to an asymmetric broadening of the LO peak. The results can be explained based on the influence of the high electron concentration with a strong contribution of the surface accumulation layer and the associated internal electric field.

Monolayer graphene has been grown on polycrystalline Ru thin films on SiO2/Si substrates. The Ru films have columnar structure with strongly aligned grains exposing flat (0001) surface facets. Adjacent grains show small relative tilts of their [0001] axes and variations in in-plane orientation. Graphene layers grown on this template cover the entire surface and have uniform monolayer thickness. Analysis of the graphene/Ru moire structure shows that monocrystalline graphene domains are coherent across a large number of substrate grains. Hence, the size of monolayer graphene domains is not limited by grain boundaries in the metal template.

The atomic structure of epitaxial single and bilayer graphene on Ru(0001) was studied by scanning tunneling microscopy (STM). High-resolution imaging of the surface of single layer graphene shows a moire with pronounced buckling and broken A/B carbon sublayer symmetry due to a strong interaction with the metal substrate. The top sheet of bilayer graphene is largely unperturbed by residual interactions with the substrate. Screened from the metal substrate, it shows the hallmarks of freestanding monolayer graphene: a honeycomb structure with equivalent carbon sublattices imaged in STM and a linear dispersion of π-bands near the Dirac point.

We report on a one-step process based on thermal evaporation at moderate temperatures that yields single-crystalline Ge nanowires (NWs) encapsulated in SiO2 shells. The dielectric shell forms around the Ge NW core during the NW growth process itself, an advantage in the assembly of NW devices such as surround-gate NW field-effect transistors (FETs). The formation of the core-shell structures proceeds via an unconventional vapor-liquid-solid process involving root growth of SiGe NWs and selective Si oxidation by background oxygen in the reactor. Electrical measurements of the p-type Ge–SiO2 FET devices show efficient gate control and hole mobilities of 20 cm2/V s.

Using scanning tunneling microscopy and density functional theory, we characterize the population of low-coverage Ti atoms on Al(111) as a model surface system for transition metal doped alanate hydrogen storage compounds, such as NaAlH4. When deposited at room temperature, Ti is kinetically trapped in first-layer substitutional sites, avoids nearest-neighbor locations, and preferentially forms next-nearest-neighbor pairs, similar to a structure that has been predicted to dissociate H2 with no energy barrier. The results on this well-defined system suggest the presence of a large population of Ti-pair complexes that may catalyze the dissociative chemisorption of hydrogen in Ti-doped alanate storage materials.

In situ annealing experiments on individual group III-nitride nanowires (NWs) in a transmission electron microscope demonstrate the encapsulation of GaN wires in crystalline carbon shells in the presence of C at moderate temperatures. The complete encapsulation of GaN wires in carbon shells can be achieved when small indium metal clusters are introduced on the wire surface. No encapsulation is observed on pure GaN wires under the same conditions. The observations suggest a general processing route for the formation of semiconductor core/C-shell NW structures based on surface decoration with small metal clusters. (c) 2007 American Institute of Physics.

Annealing of ultrathin silicon-on-insulator drives the formation of pinholes in the Si template, which in turn triggers a dewetting of the monocrystalline Si slab that is strongly affected by its crystallographic structure. An initial phase of well-defined square openings in the Si is followed by a hierarchical sequence of more complex branched shapes of the dewetting front. Annealing temperatures of about 800°C, near the onset of significant thermally activated mass transport, drive a slow and controlled evolution that allows us to identify the mechanism underlying the dewetting and explain the spontaneous formation of well-defined Si patterns at submicron dimensions.

In Ge heteroepitaxy on vicinal Si(001), miscut by 4.8° toward [100], pyramid-shaped faceted quantum dot islands (“huts”) form continuously from individual (105) facets on a wetting layer of coexisting (105) and (001) segments. Via this barrierless kinetic route the first three-dimensional islands rapidly form wherever there are substantial local gradients along ⟨100⟩ in-plane directions.

We present the results of a renormalization-group calculation of the specific heat of confined4 He near Tλ for three different geometries: parallel plates, rectangular ☐, and cube in three dimensions. Our results for the plate geometry are compared with existing data and with recent 4−e expansion results at e=1. Predictions are made for measurements close to Tλ under microgravity conditions.

The cyclization of 2-(2-chloro(bromo)phenoxy)anilines and 3-(phenoxy)pyridines to 10-methylphenoxazines and 10-methyl[1,4]benzoxazino[3,2-b]pyridines, respectively, by means of dimethyl methylphosphonate is reported. The cyclization reaction proceeded with expulsion of methyl ether. Demethylation of some 10-methylphenoxazines was achieved with pyridine hydrobromide. Nitration was carried out with sodium nitrite, and by reduction of the nitro groups the corresponding amines were prepared.

A partir d'oxo-4 azetidinecarboxylate-2, synthese des derives trimethylsilyl-1 ou -3; le derive trimethylsilyl-3 est transforme en acetoxy-4 trimethylsilyl-3 azetidinone-2 dans laquelle on substitue ensuite le groupe acetoxy par RO−, RS− ou RSO 2 −

8-Quinolinols which are substituted in the aromatic nucleus by nitro-, chloro-, or by sulfonic acid groups underwent a neat thermal fragmentation upon heating in 75% nitric acid as reaction medium yielding 2,3-dicarboxypyridinium nitrate. The scope and the mechanism of these reactions are discussed.

Meerwein reactions of 1,5-anthraquinone bis(diazonium hydrogen sulfate) with sufficiently activated olefins, such as acrylonitrile, acrylic esters, methacrylonitrile and its esters, styrene and α-methylstyrene yielded derivatives of the new heterocyclic system anthra[9,1-bc:10,5-b'c']-2,3,7,8-tetrahydrodipyran. Isolation of derivatives was realized and yields enhanced by using dimethyl methylphosphonate as reaction medium. Compounds 5–18 were isolated as mixtures of diastereomers, some of which were separated by crystallization. Proof of structure and stereochemistry was obtained for some of the compounds by X-ray crystallographic analysis. The 1H- and 13C-nmr data provided further support. Surprisingly large chemical shift differences between some of the proton signals of isomers were observed although the two asymmetric centers are far apart through bonds and through space. The mechanism of formation is explained on the basis of redox modulation of a charge transfer complex in which the aliphatic radical attacks the oxygen atom of the anthraquinone. It is proposed to classify this new type of reaction as an intramolecular Meerwein reaction.

The high specificity of the reduction system zinc and ammonium salts in dimethyl methylphosphonate as solvent is demonstrated in several reductive dechlorination reactions of polychloropyridines. The reduction of pentachloropyridine with zinc/ammonium chloride system in dimethyl methylphosphonate yielded solely 2,3,5,6-tetrachloropyridine. Similarly, the reduction of 2,3,4,5-tetrachloropyridine with zinc and the tetramethylammonium salt of methyl methylphosphonate furnished exclusively 2,3,5-trichloropyridine. A synthetic procedure for the preparation of the new ammonium salts of methyl methylphosphonate is given.

Arylation of α-methyleneglutaronitrile (MGN) occurred when arylamines, aminopyridines and 3-aminothiophene were treated with alkylnitrites and copper(I) chloride in dimethyl methylphosphonate (DMMP) solutions. Results from this study exemplified the synthetic advantage of DMMP as a solvent in Meerwein reactions with an olefin of low reactivity, yielding 1-aryl-2 chloro-2,4-dicyanobutanes, and its 3-pyridyl- and thiophen-3-yl analogues. Partial hydrolysis, followed by subsequent ring closure of the substituted 2,4-dicyanobutanes to 2,6-piperidinediones (glutaric acid imides) was effected with a solution of sulfuric acid in acetic acid. The aromatization of the substituted 2,6-piperidinediones with phosphorus oxychloride in the presence of hexamethylphosphoric acid triamide (HMPT) yielded the 2,6-dichloropyridine moiety substituted at the carbon atom C-3.

Nucleophilic attack of anthra[1,9-cd]isoxazol-6-ones, and of anthra[1,9-cd:5,10-c'd']isoxazole by various sulfoxides, triphenyl phosphine, and aliphatic or aromatic phosphites proceeded with cleavage of the nitrogenoxygen bond of the isoxazole ring. This method provided ready access to substituted anthraquinones bearing sulfoximido-, triphenylphosphazo-, and dialkyl(aryl)phosphoramidic groups attached to the 1- and the 1,5-positions of the anthraquinone system. The structures of the newly synthesized anthraquinone derivatives were supported by analytical and spectral data. l-S,S-dimethyl-N-(5-benzamidoanthraquinon-1-yl)-sulfoximide yielded in 90% sulfuric acid the l-amino-5-sulfoximido anthraquinone without hydrolyzing the sulfoximido group.

Variable temperature transmission electron microscopy of individual 5 nm Au nanoparticles shows a striking increase in the particle size on raising the temperature from room temperature to 500 °C in the presence of carbon from amorphous carbon support. Using the assembly of ordered graphene shells on the surface of individual nanoparticles at elevated temperatures—and the high pressures induced by such shells—as an experimental tool to study the origins of this swelling, we find that the volume increase is associated with the uptake of carbon to concentrations exceeding the bulk solubility by more than four orders of magnitude. The formation of stable metal–carbon nanostructures that have no bulk equivalent may have important implications on the functional properties of metal nanoparticles.

Arylation of α-methyleneglutaronitrile (MGN) occurred when arylamines, aminopyridines and 3-aminothiophene were treated with alkylnitrites and copper(I) chloride in dimethyl methylphosphonate (DMMP) solutions. Results from this study exemplified the synthetic advantage of DMMP as a solvent in Meerwein reactions with an olefin of low reactivity, yielding 1-aryl-2 chloro-2,4-dicyanobutanes, and its 3-pyridyl- and thiophen-3-yl analogues. Partial hydrolysis, followed by subsequent ring closure of the substituted 2,4-dicyanobutanes to 2,6-piperidinediones (glutaric acid imides) was effected with a solution of sulfuric acid in acetic acid. The aromatization of the substituted 2,6-piperidinediones with phosphorus oxychloride in the presence of hexamethylphosphoric acid triamide (HMPT) yielded the 2,6-dichloropyridine moiety substituted at the carbon atom C-3.

The methylating properties of dimethyl methylphosphonate have been investigated. Aromatic mono- and polycarboxylic acids were converted into their methyl esters by heating with dimethyl methylphosphonate. Similarly phenols or amines gave their respective O- or N-methyl derivatives. The sodium salts of sulfinic acids also reacted yielding sulfones in high yield.

The Meerwein reaction of 1-anthraquinone diazonium hydrogensulfate with 2-methyleneglutaronitrile in methanol yielded predominantly 3-hydroxy-4-(1-anthraquinone)-1,3-butanedicarbonitrile (5), while the corresponding 2-methyleneglutaric acid diethyl ester furnished a derivative of 1-(anthraquinone)butanolide 10. Catalytic dehydrocyanation of 5 was effected with Dowex-50 to furnish 3-oxo-4-(anthraquinone)butanecarbo-nitrile (6). Both nitriles 5,6 gave in a double cyclization reaction a benzanthrone lactone 12 from which a crystalline derivative of the enol form was isolated, the structure of which was elucidated by 1H- and 13C-nmr spectra. The methylene group of the lactone ring underwent coupling reactions with aromatic diazonium salts to form hydrazone derivatives.

We demonstrate the controlled manipulation of the 2D-electronic transport in the surface state of Bi(111) through the deposition of small amounts of Bi to generate adatoms and 2D islands as additional scatterers. The corresponding increase in resistance is recorded in situ and in real time. Model calculations based on mean-field nucleation theory reveal a constant scattering efficiency of adatoms and of small 2D Bi islands, independent of their size. This finding is supported by a detailed scanning tunneling microscopy and spectroscopy study at 5 K which shows a highly anisotropic scattering pattern surrounding each surface protrusion.

The oxidation of 2,6-dichloro-3-benzylpyridines and bis(2,6-dichloro-3-pyridyl)methanes, respectively, was accomplished using chromic trioxide in a highly acidic medium. The synthesis of several phenyl-3-(2,6-dichloropyridyl)ketones and of bis(2,6-dichloro-3-pyridyl)ketone are reported.