Reactions of N-heterocyclic carbenes (NHCs) with one-electron oxidants: possible formation of a carbene cation radical

One-electron oxidation of N-heterocyclic carbenes (NHCs) has been carried out using oxidising agents such as tetracyanoethylene (TCNE) and ferrocenium [Cp(2)Fe](+); the formation of carbene radical cations is postulated.     

Study of chemical bonds in a-B/C:H films by electron probe microanalysis

This paper describes the study of chemical bonds in amorphous hydrogen rich boron/carbon (a-B/C:H) films by electron probe microanalysis. The films were deposited on Si single crystals by plasma chemical vapour deposition with a precursor-carborane (C₂B₁₀H₁₂) in a laboratory setup. A film thickness and B/C ratio up to a value of 8000 Å and 4, respectively, have been obtained. The analysis of boron and carbon X-ray emission spectra has shown that the nearest order in the films is characterized by the coexistence of C-C, B-C and B-B bonds for B/C 1 and of B-B and B-C bonds for B/C 4. After two years exposure in air the oxygen content in the films increases from 2–5 to 15–20 at.%.     

Microtorus: a high-finesse microcavity with whispering-gallery modes

We demonstrate a 165-mu;m oblate spheroidal microcavity with a free spectral range of 383.7 GHz (3.06 nm), a resonance bandwidth of 23 MHz (quality factor Q approximately 10(7)) at 1550 nm, and finesse F>/=10(4) . The highly oblate spheroidal dielectric microcavity combines a very high Q factor, typical of microspheres, with a vastly reduced number of excited whispering-gallery modes (by 2 orders of magnitude). The very large free spectral range in this novel microcavity-a few hundred gigahertz instead of a few gigahertz as in typical microspheres-is desirable for applications in spectral analysis, narrow-linewidth optical and rf oscillators, and cavity QED.     

Octave spanning tunable frequency comb from a microresonator

We report the generation of an octave-spanning optical frequency comb in a continuous wave laser pumped microresonator. The generated comb spectrum covers the wavelength range from 990 to 2170 nm without relying on additional external broadening. Continuous tunability of the generated frequency comb over more than an entire free spectral range is demonstrated. Moreover, the linewidth of individual optical comb components and its relation to the pump laser phase noise is studied. The ability to derive octave-spanning spectra from microresonator comb generators represents a key step towards f-2f self-referencing of microresonator-based optical frequency combs.     

Ultrasonic studies of attenuation and dispersion in NH4Br near the order-disorder transition

Velocity and attenuation of sound wave propagation along the [100] direction in NH₄Br were measured near its transition temperature T_c = 234.5 K at frequencies of 30–150 MHz. The experimental results for the high-frequency attenuation and the dispersive velocity shifts are found to be in good agreement with the model of a single critical relaxation time.     

Depth distribution of SiD and SiD2 complexes in silicon bombarded by 10 keV D+ 2 ion

Abstract

A secondary ion mass spectrometry (SIMS) and a neutral molecule mass spectrometry (NMMS) study of deuterium trapping in a single crystalline silicon as a result of ion irradiation at fluences 10^16--10¹⁸ cm-² are presented. An attempt has been made to observe the formation and evolution of defect profiles containing one or two deuterium atoms (SiD- and SiD₂-complexes). The proposed model of radiation-induced sequential reactions describes satisfactorily the accumulation of SiD- and SiD₂-complexes and the reemission of D₂-molecules.     

Pressure-induced suppression of ferromagnetic phase in LaCoO3 nanoparticles

Magnetic and structural properties of nanocrystalline LaCoO₃ with particle size ranging from 25 to 38 nm, prepared by the citrate method, were investigated. All nanoparticles exhibit ferromagnetism below T_C ≈ 85 K. It was found that the unit-cell volume increases monotonically with decreasing particle size and ferromagnetic (FM) moment increases simultaneously with lattice expansion, whereas T_C remains nearly unchanged. It appears that both magnetic and structural properties of LaCoO₃ nanoparticles are size-dependent due to the surface effect. On the other hand, an applied pressure suppresses strongly the FM phase leading to its disappearance at ～11 kbar. Remarkably, the T_C does not change visibly under pressure. Our data reveal that the ferromagnetism in LaCoO₃ nanoparticles, likely related to the intermediate-spin (IS) Co³⁺ state, is simply controlled by the unit-cell volume. Within this scenario, the FM coupled IS states appear/disappear with expanding/compressing the lattice and/or Co–O bonds.     

Synthesis of Nitrogen‐Containing Rubicene and Tetrabenzopentacene Derivatives

Carbon‐based materials, such as acenes, fullerenes, and graphene nanoribbons, are viewed as the potential successors to silicon in the next generation of electronics. Although a number of methodologies provide access to these materials’ all‐carbon variants, relatively fewer strategies readily furnish their nitrogen‐doped analogues. Herein, we report the rational design, preparation, and characterization of nitrogen‐containing rubicenes and tetrabenzopentacenes, which can be viewed either as acene derivatives or as molecular fragments of fullerenes and graphene nanoribbons. The reported findings may prove valuable for the development of electron transporting organic semiconductors and for the eventual construction of larger carbonaceous systems.