Raman-scattering study of the phonon dispersion in twisted bi-layer graphene

Bi-layer graphene with a twist angle \theta\ between the layers generates a superlattice structure known as Moir\'{e} pattern. This superlattice provides a \theta-dependent q wavevector that activates phonons in the interior of the Brillouin zone. Here we show that this superlattice-induced Raman scattering can be used to probe the phonon dispersion in twisted bi-layer graphene (tBLG). The effect reported here is different from the broadly studied double-resonance in graphene-related materials in many aspects, and despite the absence of stacking order in tBLG, layer breathing vibrations (namely ZO' phonons) are observed.Comment: 18 pages, 4 figures, research articl

Phase separation methodology for physicochemical studies of soils.

A detailed study of a soil in every country is of a paramount importance, because it determines an entire economic strategy. The mineralogical properties of soils have been studied in the world for more than 100 years by various characterization techniques, X-ray diffraction being the most prominent. The main difficulty in most of employed techniques is the dominance of the majority phases in the response or the measured signal from the sample that becloud minority phases preventing their identification. The application of methods of phases'separation would provide the possibility to discern minority phases in soils. This work presents a phase separation method that employs a combination of two phenomena based on principles of fluid dynamics: flotation and sedimentation. Different characterization methods were used to analyse the produced soil samples. The methodology employed for separation of phases allowed the complete separation of clay phase from heavier mineral phases. This result makes it possible to discern minority mineral phases of soils that are difficult to detect. A more accurate determination of the mineralogical composition of a soil becomes feasible

Interface Structure of a V2O3 Layer Grown on Cu3Au (001) by Cs Corrected Transmission Electron Microscopy

This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008Peer Reviewe

Silicon-incorporated diamond-like coatings for Si3N4 mechanical seals

Amorphous silicon carbide (a-SiC) and silicon-incorporated diamond-like carbon films (DLC-Si) were evaluated as protective and friction reduction coatings onto Si3N4 rings. Unlubricated tribological tests were performed with a pin-on-disk apparatus against stainless steel pins with loads ranging from 3 N to 55 N and sliding velocities from 0.2 m/s to 1.0 m/s under ambient air and 50-60% relative humidity. At the lowest loads, a-SiC coatings present a considerable improvement with respect to the behavior of uncoated disks since the friction coefficient is reduced to about 0.2 and the system is able to run stably for thousands of meters. At higher loads, however, a-SiC coatings fail. DLC-Si coated rings, on the other hand, presented for loads up to 10 N a steady state friction coefficient below 0.1 and very low wear rates. The lowest steady-state mean friction coefficient value of only 0.055 was obtained with a sliding velocity of 0.5 m/s. For higher loads in the range of 20 N the friction coefficient drops to values around 0.1 but no steady state is reached. For the highest loads of over 50 N a catastrophic behavior is observed. Typically, wear rates below 5 x 10-6 mm3/N.m and 2 x 10-7 mm3/N.m were obtained for the ceramic rings and pins, respectively, with a load of 10 N and a sliding velocity of 0.5 m/s. Analysis of the steel pin contact surface by SEM-EDS and Auger spectroscopy revealed the formation of an adherent tribo-layer mainly composed by Si, C and O. The unique structure of DLC-Si films is thought to be responsible for the formation of the tribo-layer

Quantifying defects in graphene via Raman spectroscopy at different excitation energies.

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity

Characterization of ultra-hard silicon carbide coatings deposited by RF magnetron sputtering

Silicon carbide films were deposited onto crystalline silicon substrates from a sintered SiC target using a RF magnetron sputtering system. The influence of substrate temperature (150-500 degreesC) and polarization (0-100 V), Ar pressure (0.05-4 Pa) and RF power (50-400 W) on the mechanical properties (hardness and stress) of the resulting films was studied. Films with hardness values larger than 40 GPa could be obtained, provided that Si and C sputtered atoms can reach the surface of the growing film with sufficient high energy and low deposition rates in order to guarantee a high surface mobility. At high deposition rates the surface mobility is limited, but the increase in substrate temperature can contribute to stress relief. Upon thermal annealing at high temperatures, completely stress-free films could be produced without affecting the material hardness. This effect is accompanied by an increased structural and chemical order. Substrate bias was found not to be beneficial to the film properties, since it leads to substantial argon incorporation into the material. (C) 2000 Elsevier Science B.V. All rights reserved