Hexagonal boron nitride encapsulation is the method of choice for protecting graphene from environmental doping and impurity scattering. It was previously demonstrated that metal-organic vapor phase epitaxy (MOVPE) grows epitaxially ordered, uniform BN layers on epigraphene (graphene grown on SiC). Due to graphene's nonwetting properties, h-BN growth starts preferentially from the graphene ledges. We use this fact here to selectively promote the growth of high-quality flat h-BN on epigraphene by patterning epigraphene microstructures prior to BN growth. Thin h-BN films (down to 6 nm) grown by MOVPE show a smooth and pleated surface morphology on epigraphene, whereas crumpled BN is observed on the SiC. Cross-sectional high-resolution transmission electron microscopy images and fluorescence imaging confirm the higher BN quality grown on the epigraphene. Transport measurements reveal p-doping, as expected from hydrogen intercalation of epigraphene and regions of high and low mobility. This method can be used to produce structurally uniform high-quality h-BN/epigraphene micro/nanoscale heterostructures.

Analytical chemistry on archaeological material is an essential part of modern archaeological investigations and from year to year, instrumental improvement has made it possible to generate data at a high spatial and temporal frequency. In particular, Raman spectral imaging can be successfully applied in archaeological research by its simplicity of implementation to study past human societies through the analysis of their material remains. This technique makes it possible to simultaneously obtain spatial and spectral information by preserving sample integrity. However, because of the inherent complexity of the samples in Archaeology (e.g. seniority, fragility, lack or full absence of any information about its composition), chemical interpretation can be difficult at first glance. Indeed, specific problems of spectral selectivity related to unexpected chemical compounds could appear due to their state of conservation. Furthermore, detecting minor compounds becomes challenging as major components impose their contributions in the acquired spectra. Therefore, a relevant chemometric approach has been introduced in this context to characterize distinct spectral sources in a Raman imaging dataset of an archaeological specimen - a mosaic fragment. The fragment was unearthed during the Ruscino archaeological dig on the outskirts of Perpignan, France. It dates back to the oppidum period. The aim is to extract selective spectral information from pixel clustering analysis in order to enhance the initial optimisation step within the Multivariate Curve Resolution and Alternating Least-Squares (MCR-ALS) algorithm, a well-known signal unmixing technique. The underlying principle of the MCR-ALS is that the acquired spectra can be expressed as linear combinations of pure spectra of all individual components present in the chemical system under study. Sometimes it can be difficult to obtain the desired results through the algorithm, particularly if initial estimates of spectral or concentration profiles are inaccurate due to complex signals, noise or lack of selectivity, resulting in rank deficiency (i.e. a poor estimation of the total number of pure signals). For this reason, an innovative threshold-based clustering algorithm, combined with multiple Orthogonal Projection Approaches (OPA), has been developed to improve matrix rank investigation and thus the initialisation step of the MCR-ALS approach before optimisation. The effective analysis of Raman imaging data for an archaeological mosaic played a crucial role in uncovering significant chemical information about a particular biogenic material. This insight sheds light on the origins of mortar manufacture during the oppidum period.

Passivating graphene with a layer of hexagonal boron nitride (hBN) is known to protect it from environment effects that degrade its mobility. However, growth of high-quality BN on graphene is challenging because of the lack of surface dangling bonds. Here, we report the growth of thin BN films (down to 10 nm) on monolayer epigraphene grown on silicon carbide single-crystal substrates using metal-organic vapor phase epitaxy (MOVPE). The BN film has continuous coverage on the epigraphene surface, with smooth morphology. Particles consisting of layered BN are also observed on the surface with a higher density at the step edges. High-Resolution Scanning Transmission Electron Microscopy (HRSTEM) reveals high structural quality BN layers, with a clean and abrupt interface with graphene. The BN/epigraphene/SiC heterostructure is stable up to high temperature (1550 degrees C), and annealing improves its crystallinity. These results show that MOVPE growth technique has a potential for large-scale production of BN fully coated graphene and high-temperature applications.

The THz and sub-THz polarized Raman response was measured in tetragonal and cubic phases of single domain BaTiO3 crystal. A large peak was detected at very low wavenumber, within a scattering geometry in which all phonon lines are Raman inactive. It lies below 700 GHz in the whole temperature range of the tetragonal phase, and is clearly distinct from the soft phonon band. This peak has relaxational behavior with a slowing down on approaching the phase transition from above and from below.

BaTiO3 (BTO) is considered as a textbook material for the description of structural phase transitions (SPT) and the appearance of ferroelectricity. [...]

We show the interest in recording both Stokes and anti-Stokes Raman spectra in two peculiar applications. In one case, the comparison between these two parts of the spectrum, measured at one temperature only, allows to distinguish between first- and second-order phonon bands. In the second example, the difference between Stokes and anti-Stokes spectra provides separation between vibrational and emission lines. We describe the different stages needed in the measurements and data treatment.

We report and analyze Raman spectra recorded on crystals of LiH2PO4 (LDP) and KLi(H2PO4)(2) (KLDP). We provide a new and complete assignment of Raman lines in both crystals from their behavior as function of temperature. Among internal vibrations, we discern the modes within PO4 tetrahedra, the modes related to Li-O bonds, and those due to the O-H vibrations. In the wavenumber range between 300 and 600 cm(-1), the lines associated with O-P-O or Li-O show different temperature dependencies. The bands of PO4 are red-shifted with increasing temperature according to thermal dilatation, whereas the positions related to Li-O are temperature independent.

Raman measurements were carried out on Ho3+ doped Lithium Niobate crystals. When the excitation wavelength of 532nm is used, in addition of expected Raman modes, forbidden bands are detected, while when exciting with 785nm, "classical" Raman spectrum was recorded with expected modes according to Raman selection rules. Additional lines are attributed to emission lines of Ho doped crystals. We detect, within a very good resolution, in the same Stokes spectrum, the transitions between the electronic states, and the vibrational states as well. We report on the analysis of these data as function of Ho-content, for different polarizations and wavelengths, of the incident laser beam.

We investigate Raman scattering measurements on polycrystalline LiH2PO4 (LDP) between 30 and 240 degrees C, in order to detect phase transformations. The thermal behavior of main lines in different wavenumber ranges is discussed and analyzed. Raman spectra show two anomalies at 176 degrees C and 210 degrees C, reflecting changes in chemical bonds, and are attributed to progressive polymerization, in agreement with earlier electrical measurements.

Les defauts jouent un role important dans les proprietes des solides. Que leur effet soit benefique (cas des dopants) ou non, leur etude est un enjeu essentiel pour la maitrise et l’optimisation des materiaux. La spectroscopie Raman est une technique d’analyse de milieux permettant d’acceder a la structure par la caracterisation des vibrations des molecules. L’objectif de cet article est de decrire les effets causes par les defauts sur le spectre Raman: les modifications des raies propres de la matrice hote, par l'apparition de raies specifiques, ou par l’activation de raies normalement interdites par les regles de selection.

The paper reports on a methodology for the determination, by means of Raman spectrometry, of the concentration of inorganic salts dissolved in aqueous solutions. This procedure is then applied to nitrate solutions with a varying content from 0 to 100mM (mmol/l). For this we exploit the concentration dependence of the Raman peak lying at 1047cm−1 specific to the NO3− anion. Different signal processing and normalization methods are used and compared to deduce the most reliable and robust calibration. Then cross-validations are done using the “leave-one-out” method to validate the prediction models. Several figures of merit such as sensitivity, signal to noise ratio, limits of detection and quantification are calculated to estimate the efficiency of our methodology for nitrate solutions. Various sources of uncertainties are considered and evaluated according to the ISO GUM and the standard uncertainty of the concentration is then calculated.

The soil and water preservation requires new sensitive techniques. Raman probe was recently shown to be an interesting alternative to electrical sensors to detect several contaminants diluted in water. Here we report on the procedure to reliably determine the substance concentration within a fairly good accuracy and a high speed.

The control of water quality requires rapid and efficient monitoring techniques. A Raman sensor is shown to present interest for the analysis of chemical substances dissolved in water. The identification of chemical pollutants and the determination of their content as well are reliably obtained. The main characteristics and abilities of this multi-component Raman sensor are reported.

Raman sensors are more and more used for various applications. In particular they were recently used for in situ and real time monitoring of inorganic substances in aqueous solution in the sea, in tanks or in vicinity of industrial sites. Among several advantages they provide the identification of many species with a fairly good component resolution and the accurate determination of their concentration. Raman probes are based upon different data treatment of the Raman spectra. Two main methods are emphasized here. In the first method suitable for diluted salts with polyatomic anions, the Raman spectrum exhibits a large and well resolved peak used as the direct signature of a peculiar anion to be detected. Salts with monoatomic anions are studied via their indirect influence on the large OH stretching band of the water. The advantages and limitations of each method are discussed and illustrated with some examples derived from our own experience. The process giving the quantification in the analysis of the species is described, with a particular attention devoted to the data treatment within the calibration procedure. Analysis with lineshape treatment and chemometric methods are both reported and discussed. A comparison with different approaches reported in the literature is provided. Finally, the main devices used for in situ monitoring are briefly described.

We propose to detect by Raman spectrometry the presence of several anions in mixtures of salt solutions and to measure simultaneously their respective concentrations. In this study we show how chemometric methods based on statistical analysis like Principal Components Analysis (PCA) and Partial Least Square Regression (PLSR) can be applied for the spectra treatment. Unlike current water probes often based on conductivity, our new Raman sensor is able to identify simultaneously and in one single measurement several anions such as NO3-, SO42-, HPO42- or Cl- sought for the water pollution monitoring. Moreover our sensor gives rise to reliable, accurate and fast measurement (a few seconds) and can therefore be very useful for in situ water quality control applications.

Raman spectroscopy was used to analyze mixtures of urea and water in order to identify the influence of the urea concentration on the solution's freezing point. Our approach consisted in the analysis of urea aqueous solutions and the determination of their phase transitions at low temperatures. Hence, Raman spectra of these solutions were acquired in a −30 to 10 °C temperature range. This enabled us to build the experimental phase diagram of the urea–water binary system.

Raman spectra of potassium, sodium, and ammonium sulfates (K2SO4, Na2SO4, and (NH4)2SO4) are reported and analyzed. These sulfates have been investigated under two states: solid (anhydrous and hydrated) salts and aqueous solutions. The effects of monovalent ions (K+, Na+, and NH4+) and hydration on the position of Raman lines assigned to internal vibrations of sulfate anion SO42− are discussed. In solid salts, the line position of each Raman peak is shown to decrease with increasing radius of the cation. The main ν1 mode of sulfate molecule is particularly affected. It is emphasized that this sensitivity in solid sulfates vanishes in aqueous solutions. As a consequence, this mode can be probed by Raman spectroscopy as the main signature of SO42− to determine its concentration within a single calibration. Copyright © 2013 John Wiley & Sons, Ltd.

We propose to detect by Raman spectrometry the presence of chlorid anion in mixtures of salted solutions. Here is shown how Cl-influence the OH stretching band of the water spectrum and how its concentration is determined using chemometrics methods applied on Raman spectra recorded on mixtures in few seconds.