A. Kassiba

Vibrational density of states in silicon carbide nanoparticles: experiments and numerical simulations

The vibrational properties of silicon carbide nanoparticles (np-SiC) were investigated as function of the nanocrystal size (5-25 nm) and the features of their outermost surfaces. Raman experiments and numerical methods were conjugated to characterize the signatures from the active SiC normal modes and the vibrational density of states (VDOS). The Raman spectra of the nanopowders were marked by VDOS signals which correlate with the SiC amorphous fractions favoured by the high specific surfaces of the nanoparticles and their surface reconstruction. Quantitative interpretation of the experimental VDOS features, IR absorption and Raman scattering properties in nanosized SiC were carried out by means of numerical methods developed on SiC clusters with suitable structures and sizes.

Local electrooptic effect of the SiC large-sized nanocrystallites incorporated in polymer matrices

Linear electro-optical effect (LEO) is experimentally demonstrated in large-sized (10 and 30 nm) silicon carbide nanocrystallites (nc-SiC) incorporated in poly(N-vinylcarbazole) (PVK) blended with coumarine (MK). By using the angle-dependent polarimetric method at λprobe=694 nm, in both static and photoinduced regimes, we have shown that embedding the nc-SiC within polymer host matrices gives rise to an enhancement of the LEO effective parameters up to 1.325 pm/V. The absence of these phenomena in the separate components—i.e., pure wurtzite-like SiC crystals as well as the organic matrices—suggests that the observed local electro-optical response is intimately driven by the interfacial hyperpolarizabilities appearing between the nc-SiC and the surrounding organic matrix.

Structural and optical characterization of ball-milled copper-doped bismuth vanadium oxide (BiVO4)

Copper-doped BiVO4 nanoparticles were synthesized by a mechano-chemical method under optimized conditions to obtain a monoclinic scheelite structure. The crystal structure and its evolution with doping were investigated by X-ray powder diffraction, micro-Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). Spherical shape particles with sizes ranging between 40 and 160 nm, which possess a monoclinic scheelite structure, were obtained. From the structural data analysis, it can be observed that the particle size decreases and distortions occur as the copper content increases in doped BiVO4. Chemical bonding and valence states of the Bi-4f, V-2p, O-1s and Cu-2p ions were investigated by XPS which revealed the location of Cu ions in the host lattice of BiVO4 in agreement with EPR investigations. UV-visible absorption experiments showed a broad band in the visible range with a small shift of the energy band-gap from 2.41 eV for undoped BiVO4 to 2.34 eV for 10 at.% Cu–BiVO4. Additional absorption band shoulders and widening of the optical absorption spectrum in the visible range with a well crystalline monoclinic scheelite structure pave the way for efficient visible light-driven photocatalytic activity. Photocatalytic measurements reprinted in supplementary data.

Multi-approach Electron Paramagnetic Resonance Investigations of UV-Photoinduced Ti3+ in Titanium Oxide-Based Gels

EPR investigations of the photoreduction of Ti(4+) into Ti(3+) under UV irradiation were carried out on three titanium-based materials for which the initial concentration of Ti(4+) was defined in the ternary phase diagram (TiOCl(2), H(2)O, DMF). The kinetics of this photoreduction was monitored at 200 K and related to the initial concentration of Ti(4+) in the solution. This study was complemented by a multi-approach EPR method (pulsed electron paramagnetic resonance (EPR), pulsed electron nuclear double resonance, and hyperfine sublevel correlation spectroscopy (HYSCORE)) with the aim of probing the proton environment of the Ti(3+) ions. Indeed, many species such as H(2)O, OH(-), HCOO(-) are located in the immediate vicinity of Ti(3+). Although we found that a distribution of g tensors was involved, for simplicity, two types of g tensor were used to describe the main features of the EPR signal related to the paramagnetic ions. Additionally, we have evidenced that two kinds of protons are identified next to Ti(3+) species, with specific distances determined from the hyperfine coupling parameters obtained by the HYSCORE method.

Solid-State NMR Correlation Experiments and Distance Measurements in Paramagnetic Metalorganics Exemplified by Cu-Cyclam

We show how to record and analyze solid-state NMR spectra of organic paramagnetic complexes with moderate hyperfine interactions using the Cu-cyclam complex as an example. Assignment of the (13)C signals was performed with the help of density functional theory (DFT) calculations. An initial assignment of the (1)H signals was done by means of (1)H-(13)C correlation spectra. The possibility of recording a dipolar HSQC spectrum with the advantage of direct (1)H acquisition is discussed. Owing to the paramagnetic shifting the resolution of such paramagnetic (1)H spectra is generally better than for diamagnetic solid samples, and we exploit this advantage by recording (1)H-(1)H correlation spectra with a simple and short pulse sequence. This experiment, along with a Karplus relation, allowed for the completion of the (1)H signal assignment. On the basis of these data, we measured the distances of the carbon atoms to the copper center in Cu-cyclam by means of (13)CR2 relaxation experiments combined with the electronic relaxation determined by EPR.

Stoichiometry and interface effects on the electronic and optical properties of SiC nanoparticles

The physical properties of nanosized SiC particles are investigated as a function of the C/Si ratios. The synthesis by the laser pyrolysis process monitors the particle stoichiometry by the initial fluxes of the reactants (SiH4 and C2H2) and leads to Si- or C-rich batches. Spectroscopic methods such as EPR and Raman are used to probe the paramagnetic active electronic centres as well as the role played by the external surface composition on the effective vibrational and optical properties of the nanoparticles. Numerical methods are undertaken in order to point out the role played by the carbon or silicon excess on the interface properties. In particular, simulations by quantum chemistry codes within the configuration interaction and the parametric method (PM3) evidence the relevant features of the Raman spectra induced by a suitable distribution of C or Si excess within the SiC nanoparticles.

Dielectric and EPR investigations of stoichiometry and interface effects in silicon carbide nanoparticles

The dielectric properties of non-stoichiometric silicon carbide nanoparticles (np-SiC), with a silicon-rich composition in the atomic ratio C/Si = 0.85, are investigated in a wide frequency and temperature range. Two samples, namely S1400 and S1700, obtained from the same synthesis batch but submitted to annealing at, respectively, 1400 and 1700 °C, are representative of this class of material. In particular, quasi-insulating and semiconducting states are evidenced in these samples, and they point out the influence of the annealing treatment. The dielectric spectra of S1400 are marked by a relaxation process and by a thermally activated conductivity, whereas only a large dc-conductivity is found in the S1700 sample. The temperature dependence of both relaxation times and dc-conductivity are determined and interpreted taking into account the transport process in these media. EPR investigations are carried out to correlate the electrical and dielectric behaviour with the surface states of the nanoparticles which exhibit heterogeneous composition and surface active electronic centres. The experimental conductivity and dielectric functions in these silicon-rich materials are discussed and compared with those from carbon-rich np-SiC samples.

Nickel titanate (NiTiO3) thin films: RF-sputtering synthesis and investigation of related features for photocatalysis

Polycrystalline nickel titanate thin films were deposited on silicon substrates by the radio-frequency (rf) sputtering method using NiTiO3 targets, which in turn were sintered from powders obtained by solid state reaction. The deposition parameters such as rf-powers, partial pressures of argon and oxygen, as well as substrate temperatures were optimized to ensure the homogeneous and stoichiometric composition of NiTiO3 thin films. Post-synthesis annealing at suitable temperatures and time durations was realized to stabilize the ilmenite structure with improved crystalline features deduced from structural and vibrational investigations by using XRD and Raman spectroscopy. The surface roughness and thin film microstructure were characterized by AFM which points out the organisation of the film surface. Representative films with ilmenite structure and defined surface microstructure and optical absorbance were tested as visible light driven heterogeneous photocatalysts for the degradation of organic dyes.

Some fundamental and applicative properties of (polymer/nano-SiC) hybrid nanocomposites

Hybrid nanocomposites which combine polymer as host matrix and nanocrystals as active elements are promising functional materials for electronics, optics or photonics. In these systems, the physical response is governed by the nanocrystal features (size, surface and defect states), the polymer properties and the polymer-nanocrystal interface. This work reviews some selective nanostructured architectures based on active elements such as silicon carbide (SiC) nanocrystals and polymer host matrices. Beyond an overview of some key properties of the nanocrystals, a main part will be devoted to the electro-optical (EO) properties of SiC based hybrid systems where SiC nanocrystals are embedded in polymer matrices of different chemical nature such as poly-(methylmethacrylate) (PMMA), poly-vinylcarbazole (PVK) or polycarbonate. Using this approach, the organic-inorganic interface effects are emphasised with regard to the dielectric or hole transporting behaviour of PMMA and PVK respectively. These effects are illustrated through different EO responses associated with hybrid composites based on PMMA or PVK.

Electronic, optical and vibrational features of BiVO4 nanostructures investigated by first-principles calculations

Numerical models based on DFT and semi-empirical methods were developed on bulk and nano-sized BiVO4 semiconducting oxide. Two different approaches were implemented to calculate the electronic properties of the bulk BiVO4 crystal. One approach used the X-ray specified atomic positions with the defined lattice parameters involved in the monoclinic crystalline structure of BiVO4, for which geometry optimization was performed before the electronic properties calculations. The second approach considered the atomic positions of the scheelite structure, which was maintained frozen without any lattice rearrangement. The obtained data were considered as a guideline to choose appropriate methodology to calculate the physical properties of (BiVO4)n nanoparticles. A semi-empirical method with the PM6 parameterization was applied to calculate the electronic and vibrational properties of (BiVO4)n nanostructures versus their sizes. The quantum confinement effect was shown for the clusters through a blue-shift of the optical absorption bands compared to an infinite system. It was demonstrated that the optical and vibrational properties of the nanoparticles are determined by the internal atoms keeping their positions as in the crystalline structure and also that they feel surface reconstruction effects according to the environmental interactions. The above mentioned behaviors were analyzed (using UV-vis absorption, IR and Raman features), theoretically predicted and then compared with the experimental results. The performed analyses underlines the evolution of the electronic density of states and the optical absorption peculiarities between a bulk infinite system and nano-sized objects dedicated to realize visible-light-driven photocatalysts.