Fouad Maroun

In situ semiconductor surface characterisation: a comparative infrared study of Si, Ge and GaAs

The surface of Si, Ge and GaAs electrodes has been investigated by infrared spectroscopy, which provides in situ information on chemical species at the electrochemical interface. Special attention was paid to adsorbed hydrogen and hydroxyl and to oxide formation. In fluoride medium and at open circuit potential (OCP) the Si surface is completely hydrogenated and is oxidised only upon imposing a large anodic current. Conversely, GaAs is oxidised at OCP, and AsH bonds appear on the surface only in the presence of a cathodic current sufficient to compete with surface oxidation by oxygen dissolved in the electrolyte. Upon potential cycling, the surface of germanium undergoes nearly reversible changes between a hydroxylated and a hydrogenated state, and may be found in either state at OCP, depending upon its history. In situ IR spectroscopy allows one to rationalise the very different behaviours of surface reactivity found for these otherwise rather similar semiconductors and to determine the electrochemical reaction mechanisms.

The structure, growth and reactivity of electrodeposited Ru/Au(111) surfaces

In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru:Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H2SO4 solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (\0.1 V), (defective) monolayer (] 0.1 V), and multilayer films (B 0.1 V). At potentials \ 0.6 V irreversible formation of Ru oxide:hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at :0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film.

Luminescence thermochromism of acrylic materials incorporating copper iodide clusters

By incorporating molecular copper iodide clusters of formula [Cu4I4L4] (L = PPh2(CH2)2CH3) into an UV-polymerizable acrylic resin, namely BEDA (Bisphenol-a-EthoxylateDiAcrylate), transparent and highly emitting photoluminescent composite materials have been synthesized. In these materials, the original luminescence properties of the copper iodide cluster and the transparency and processability of the acrylic matrix are combined. Study of the photopolymerization kinetics shows that the clusters incorporated in low concentration have limited influence on the polymerization reaction leading to a highly cross-linking polymeric matrix. These composite materials exhibit thermochromic luminescence properties with intense emissions varying with the temperature. A perfectly controlled luminescence thermochromism is observed due to a ‘protecting effect’ of the matrix preventing the non-radiative phenomenon of the cluster luminescence. The patterning of these UV-polymerizable photoluminescent films has been also realized by the UV-NIL technique to optimize the light-emitting properties of these materials. The surface patterning acts as a diffraction grating to extract the light which was previously guided inside the film. Moreover, the nanopatterning allows tuning of the emission color of the film as a function of the viewing angle. These composite materials present potential applications as photoactive systems with emission wavelength sensitive to the temperature and the surface nanostructuration.

Self-ordered electrochemical growth on single-crystal electrode surfaces

This paper is a brief review of self-ordered electrochemical growth on single-crystal electrode surfaces on which arrays of nanostructures are created by replication of patterns made by atomic steps or surface reconstruction. Whenever possible the parallel is made between electrochemical growth and molecular beam epitaxy in ultrahigh vacuum. An atomistic view of electrodeposition is given first to help with identifying the similarities of and specific differences between the two techniques of growth. Recent examples where self-organized nanostructures are prepared on metals and silicon substrates are discussed.

Admetal-induced substrate surface restructuring during metal-on-metal electrochemical deposition studied by in situ scanning tunneling microscopy

Abstract Based on time-dependent in situ scanning tunneling microscopy (STM) studies, we demonstrate that for Ni on Ag(111) and Ru on Au(111), electrochemical metal-on-metal deposition can result in pronounced substrate surface restructuring. For Ni/Ag(111), we observe that at low deposition flux and low coverage, Ni submonolayer islands at steps are partly embedded in the Ag terraces, whereas at higher deposition flux and higher coverage, substrate restructuring results in the formation of monolayer bays in the Ag terraces. We suggest that this restructuring process proceeds predominantly via step edge diffusion of Ag atoms. For Ru/Au(111), the formation of fjords and monolayer holes in the Au terraces is observed at low and high Ru coverage, respectively. The importance of the Au surface mobility for the restructuring process is demonstrated by comparing experiments in H2SO4 and HCl solutions, in which Au exhibits strongly different surface mobilities. For this system, restructuring involves Au diffusion along Au steps, Au atom detachment from the Au steps, and upward exchange diffusion. According to these observations and their comparison with similar findings for vacuum deposition, we conclude that this restructuring requires (i) a high substrate surface mobility and (ii) a stronger bonding of substrate atoms to deposit islands than to the substrate.

Epitaxial Growth of Gold on HSi(111): The Determining Role of Hydrogen Evolution

The potential dependence of gold electrodeposition on H-terminated Si(111) is studied in acidic electrolyte by means of atomic force microscopy and X-ray diffraction. The Au films (≤66 monolayers (ML)≈16 nm) are found to be (111)-oriented and in strong epitaxy with the Si(111) surface lattice, with two in-plane orientations separated by 180°. The deposit morphology is controlled by the deposition potential and can be islandlike or atomically flat. The flat morphology is accompanied by a preferential growth of 180°-rotated Au planes with respect to the Si bulk lattice which takes place at potentials where the hydrogen evolution reaction occurs. Obtaining ultraflat Au layers on Si(111) contrasts with the commonly observed islandlike morphology of electrodeposited films on semiconductors. This behavior is discussed in terms of a nucleation coupled with hydrogen evolution reaction (HER) and an enhanced Au adatom mobility induced by this reaction.

Reversibility of the GeH/GeOH transformation at a Ge electrode: an EQCM study

We present a combined cyclic voltammetry and electrochemical quartz-crystal micro-balance study of a thin-layer germanium electrode in 1 M HClO4 electrolyte. When the potential is more positive than −0.2 V versus SCE, significant mass loss occurs due to anodic dissolution. However, when the potential is cycled in a range below −0.2 V versus SCE, only periodic variations of mass are observed, corresponding to the quasi-reversible change of the surface between a hydrogenated state and a hydroxylated state. These data provide the as yet missing clearcut proof that this change of state takes place without any dissolution of the electrode.

In situ infrared investigation of metals electrodeposited for SEIRAS

Abstract We report on the use of in situ infrared spectroscopy for monitoring the electrodeposition of metals on semiconductors. The deposit is modelled as a slice of an effective medium consisting of a mixture of the deposited metal and the electrolyte, and the infrared absorption is fitted using Bruggemans and Bergmans effective medium theories. In the case of Ag deposition on Si, this analysis brings information not only on the amount of deposited metal but also on the morphology of the deposit, as confirmed by ex situ SEM observations. In situ monitoring of the morphology provides a means for realizing surface enhanced infrared absorption geometry with electrodeposited metal islets.

In situ infrared monitoring of hydrogen adsorption/desorption at a germanium electrode

Upon potential cycling in acidic electrolyte, the coverage of Ge electrodes can be changed from GeH to GeOH. In situ infrared spectroscopy reveals that hydrogen adsorbs and desorbs at Ge in mono- and dihydride configurations. Comparison of (111)- and (100)-oriented electrodes shows that the Ge surface is rough at the atomic scale and that this roughening takes place through (111) microfacetting. This facetting can account for the attenuated polarization dependence of the GeH absorption and its enhancement through local field effects. Finally, the band edge shifts which are observed upon hydrogen adsorption/desorption appear related to a change in the Ge surface charge rather than to a dipolar effect.

In situ infrared monitoring of electrochemical adsorption/desorption of hydrogen on germanium

Abstract In an acidic electrolyte, germanium exhibits voltammetric peaks which have been ascribed to a change between a hydrogenated and a hydroxylated surface. Using in situ infrared spectroscopy, we have obtained clearcut confirmation of this prediction. GeH and GeH2 species are identified at the hydrogenated surface. The bidentate Ge sites are hydrogenated and reoxidized at more positive potentials than the monodentates. The roughness of the surfaces on the atomic scale accounts for the dependence of the absorption intensities upon infrared polarization and crystal orientation. The various voltammetric peaks are correlated with the reversible adsorption/desorption of hydrogen on steps and (111) facets.