Surface-Mediated Charge Transfer of Photogenerated Carriers in Diamond.

Solvated electrons are highly reductive chemical species whose chemical properties remain largely unknown. Diamond materials are proposed as a promising emitter of solvated electrons and visible light excitation would enable solar-driven CO2 or N2 reductions reactions in aqueous medium. But sub-bandgap excitation remains challenging. In this work, the role of surface states on diamond materials for charge separation and emission in both gaseous and aqueous environments from deep UV to visible light excitation is elucidated. Four different X-ray and UV-vis spectroscopy methods are applied to diamond materials with different surface termination, doping and crystallinity. Surface states are found to dominate sub-bandgap charge transfer. However, the surface charge separation is drastically reduced for boron-doped diamond due to a very high density of bulk defects. In a gaseous atmosphere, the oxidized diamond surface maintains a negative electron affinity, allowing charge emission, due to remaining hydrogenated and hydroxylated groups. In an aqueous electrolyte, a photocurrent for illumination down to 3.5 eV is observed for boron-doped nanostructured diamond, independent of the surface termination. This study opens new perspectives on photo-induced interfacial charge transfer processes from metal-free semiconductors such as diamonds.

Nanointerfaces: Concepts and Strategies for Optical and X-ray Spectroscopic Characterization.

Interfaces at the nanoscale, also called nanointerfaces, play a fundamental role in physics and chemistry. Probing the chemical and electronic environment at nanointerfaces is essential in order to elucidate chemical processes relevant for applications in a variety of fields. Many spectroscopic techniques have been applied for this purpose, although some approaches are more appropriate than others depending on the type of the nanointerface and the physical properties of the different phases. In this Perspective, we introduce the major concepts to be considered when characterizing nanointerfaces. In particular, the interplay between the characteristic length of the nanointerfaces, and the probing and information depths of different spectroscopy techniques is discussed. Differences between nano- and bulk interfaces are explained and illustrated with chosen examples from optical and X-ray spectroscopies, focusing on solid-liquid nanointerfaces. We hope that this Perspective will help to prepare spectroscopic characterization of nanointerfaces and stimulate interest in the development of new spectroscopic techniques adapted to the nanointerfaces.

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters.

Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

Doping nanoparticles using pulsed laser ablation in a liquid containing the doping agent.

While doping of semiconductors or oxides is crucial for numerous technological applications, its control remains difficult especially when the material is reduced down to the nanometric scale. In this paper, we show that pulsed laser ablation of an undoped solid target in an aqueous solution containing activator ions offers a new way to synthesise doped-nanoparticles. The doping efficiency is evaluated for laser ablation of an undoped Gd2O3 target in aqueous solutions of EuCl3 with molar concentration from 10-5 mol L-1 to 10-3 mol L-1. Thanks to luminescence experiments, we show that the europium ions penetrate the core of the synthesised monoclinic Gd2O3 nanoparticles. We also show that the concentration of the activators in the nanoparticles is proportional to the initial concentration in europium ions in the aqueous solution, and a doping of about 1% ([Eu]/[Gd] atomic ratio) is reached. On the one hand, this work could open new ways for the synthesis of doped nanomaterials. On the other hand, it also raises the question of undesired penetration of impurities in laser-generated nanoparticles in liquids.

Is UV-Induced Electron-Driven Proton Transfer Active in a Chemically Modified A·T DNA Base Pair?

Transient electronic and vibrational absorption spectroscopies have been used to investigate whether UV-induced electron-driven proton transfer (EDPT) mechanisms are active in a chemically modified adenine-thymine (A·T) DNA base pair. To enhance the fraction of biologically relevant Watson-Crick (WC) hydrogen-bonding motifs and eliminate undesired Hoogsteen structures, a chemically modified derivative of A was synthesized, 8-(tert-butyl)-9-ethyladenine (8tBA). Equimolar solutions of 8tBA and silyl-protected T nucleosides in chloroform yield a mixture of WC pairs, reverse WC pairs, and residual monomers. Unlike previous transient absorption studies of WC guanine-cytosine (G·C) pairs, no clear spectroscopic or kinetic evidence was identified for the participation of EDPT in the excited-state relaxation dynamics of 8tBA·T pairs, although ultrafast (sub-100 fs) EDPT cannot be discounted. Monomer-like dynamics are proposed to dominate in 8tBA·T.