Claude Lévy-Clément

Determination of carrier density of ZnO nanowires by electrochemical techniques

The carrier density of ZnO nanowires has been determined by means of electrochemical impedance spectroscopy. A model taking into account the geometry of ZnO nanowires has been developed and the differences with the standard flat model, as curved Mott-Schottky plots, are discussed. The as-grown electrodeposited samples present a high donor density of 6.2×1019cm−3, dramatically reduced by two orders of magnitude after an annealing in air at 450°C during 1h. The results show that the surface of the ZnO nanowires is active; therefore this system appears as a useful structure to support a functionalized nanostructured devices.

Thin film semiconductor deposition on free-standing ZnO columns

We report the deposition of a-Si:H on thin films of free-standing single crystalline ZnO columns. The ZnO columns have a height of several μm and a diameter between 100 and 200 nm. The ZnO films are prepared in electrodeposition and have considerable potential for use in photoelectric thin film devices. Morphology, electronic parameters, and basic optical behavior, such as reflectance and light trapping efficiency, are reported. Amorphous silicon is deposited on the columns as a continuous smooth film with conformal coverage. Some possibilities of using these films in devices are discussed.

Hollow Urchin‐like ZnO thin Films by Electrochemical Deposition

Since the first report on ultraviolet lasing from ZnO nanowires (NWs), remarkable effort has been dedicated to the development of novel synthesis routes for 1D ZnO nanostructures. Ordered arrays of 1D ZnO NWs have a promising future as applications in electronic and optoelectronic devices, because they are expected to improve the performance of various nanodevices such as short-wavelength lasers, nanostructured solar cells, electroluminescent, and field-emission devices. What is now a relevant area of focus in nanoscience involves the preparation of higher-order assemblies, arrays, and superlattices of these 1D nanostructures. Recently, many efforts have focused on the integration of 1D nanoscale building blocks into 3D architectures. Hollow urchin-like ZnO NWs that combine properties of 3D and 1D materials may emerge as a more interesting alternative than simple arrays of NWs due to the higher specific surface and porosity, especially for application in dye and semiconductor-sensitized solar cells. To date, there are only two strategies to synthesize hollow urchin-like ZnO NWs. The first one is a wet-chemical route that uses a modified Kirkendall process, by which zinc powders that are spherical in shape are transformed into hollow urchin-like ZnO NWs dispersed in solution. The second strategy is based on the calcination of metallic Zn microsphere powders at relatively high temperature (500–750 8C). With these two approaches, ZnO nanostructures are often randomly distributed (in size and organization), which may limit their practical applications as building blocks in nanodevices. Nevertheless, it is essential for the fabrication of nanodevices to assemble NW-structured hollow spheres with a uniform size in ordered arrays, since such an organisation combines the merits of patterned arrays and nanometer-sized materials. Until now, a suitable technique is still missing for the fabrication of ordered arrays of hollow urchin-like ZnO NWs with tunable sizes. In this paper, we report on a novel approach to fabricate well-ordered hollow urchin-like single-crystal ZnO NWs with controlled NW and core dimensions. The method combines the formation of a polystyrene (PS) microsphere colloidal mono/ multilayer and the electrodeposition of ZnONWs, followed by the elimination of the PS microspheres, which play the role of a template. It is shown that the light scattering properties of such an ordered architecture exceed those of ZnO NW arrays. Applications as 3D building blocks in the field of nanostructured solar cells are discussed. Mono/multilayers of PS spheres covering conductive substrates have been used as templates to electrodeposit inverse opal structures. In such cases the nucleation of ZnO took place at the interstitial sites (on a conductive substrate) between the PS spheres leading to different morphologies depending on the employed method. Our strategy of electrodeposition differs from those previously described by the mode of nucleation and growth. In our case, the deposition of ZnO takes place, from the nucleation step, on the PS spheres and the conductive substrate, simultaneously. As a result, the spheres are homogenously covered by a thin film composed of single-crystal ZnO NWs connected together at their base. This approach provides a simple and versatile way to synthesize well-ordered mono/multilayers of ZnO hollow microspheres with the ability to control the sphere size in addition to the ZnO NW dimensions and morphology. A monolayer of commercially available carboxylate-modified PS spheres ( 4.3mm) is deposited directly on a transparent conductive oxide (TCO) substrate by using a self-assembly technique on a water surface. We have used the method of Zhou et al. with some modifications. A detailed description of our process is given in the experimental section. Figure 1a shows a tilted, low-magnification scanning electron microscopy (SEM) image of the self-assembled monolayer on TCO substrate. A well-organized monolayer of PS microspheres can be observed in addition to occasional point defects in some regions due to the presence of larger spheres in the commercial solution (circled region in Fig. 1a). This organization is observed throughout the entire TCO surface ( 1.5 cm). As a proof of that, the lower inset in Figure 1a shows an optical image of TCO/PS where the sample colour is perfectly homogeneous, reflecting the presence of only one PS domain (monolayer) on the substrate. The detailed organization of the spheres was investigated by a closer examination using high-magnification SEM (Fig. 1b and its inset), which shows a relatively large area of the self-assembled monolayer and a perfectly ordered array. The TCO/PS sample has then been immersed for 30min in 2 M ZnCl2 aqueous solution at room temperature and used as a working electrode in an electrochemical cell for the deposition of ZnO NWs. The electrolyte was an aqueous solution saturated by molecular O2, containing 5 10 4 M ZnCl2 (zinc precursor) and 0.1 M KCl

Electrochemical study of diamond thin films in neutral and basic solutions of nitrate

Abstract The electrochemical behavior of boron-doped diamond films has been investigated in a number of neutral and alkaline solutions with and without nitrate ions. Two kinds of diamond electrode were studied: self-supported film (100 μm) (sample A) and diamond film (10 μm) supported on a Si substrate (sample B). It was found that water oxidation and reduction appear at much larger polarizations for diamond electrodes, as compared with platinum and platinized platinum electrodes. In particular, the higher (cathodic) overpotential for hydrogen reduction permits efficient nitrate reduction to ammonia. The underlying Si substrate is shown to take part in the electrochemistry of the diamond electrodes. In the case of the Si-supported electrode (sample B) the reaction with the Si substrate was imminent. For the free-standing diamond electrode (sample A) various impurities in the grain boundaries and at the back of the electrode, including the back metallic contact, intervened with the electrochemistry of the diamond electrode, but to a much lesser extent than with sample B. Meticulous cleaning and careful working practices permitted this interference to be excluded altogether in sample A.

Electrochemical deposition of MoS2 thin films by reduction of tetrathiomolybdate

Abstract Molybdenum sulphide was cathodically electrodeposited from aqueous solutions of sodium tetrathiomolybdate. The as-deposited films were X-ray amorphous with a composition, measured by microprobe analysis, close to MoS 2 . Annealing these films in Ar resulted in highly-textured films of MoS 2 with the van der Waals planes parallel to the substrate. A small expansion in the c spacing of the annealed films was explained by the presence of oxygen in the crystals. A direct bandgap of 1.78 eV was found for the annealed films.

ZnO nanowire arrays: Optical scattering and sensitization to solar light

Arrays of ZnO nanowires with different lengths (0.5–2 μm) and diameters (100–330 nm) were electrodeposited to study the influence of the nanowire dimensions on light scattering. The nanowire length and diameter were found to be major parameters in modifying the intensity and the wavelength of the scattered radiation, respectively. A significant scattering for the whole visible wavelength range was attained in arrays of ZnO nanowires of ∼1.5 μm in length and ∼330 nm in diameter. ZnO nanowire arrays were sensitized to solar light with a conformally deposited thin CdSe layer. A clear correlation between light scattering before coating and absorption in the resulting ZnO/CdSe core-shell nanostructures was found. The enhancement in the scattering for wavelengths where CdSe exhibits a relatively low absorption coefficient resulted in an effective absorption, in the 400–725 nm range of the AM1.5 solar spectrum, as high as 88% with a CdSe shell thickness of ∼20 nm only.

Electrodeposition of zinc selenide

Abstract The synthesis of polycrystalline thin films of cubic ZnSe by electrochemical plating on conducting substrates is described. The influence of deposition parameters such as electrolyte composition, deposition potential and temperature on the crystallanity and on the chemical composition of the films is discussed. Of the various plating techniques (potentiostatic, potentiodynamic pulse and galvanostatic), potentiostatic plating offers the best control of film composition. An excess of Se with respect to perfect stoichiometry cannot be avoided by adjusting electrolyte composition and deposition potential, but can be decreased by vacuum annealing.

Morphology of Porous n‐Type Silicon Obtained by Photoelectrochemical Etching I . Correlations with Material and Etching Parameters

We describe here the experimental conditions under which the photoelectrochemical etching (PEC-etching) of n-type silicon in HF induces the formation of porous silicon. Two types of porous silicon are formed underneath an etch crater. a layer of nanoporous material with pores in the nanometer range on top of a macroporous layer with pores in the micron range. The form of the macropores changes with the crystallographic orientation. We report on the evolution of these different features with the quantity of charge passed during the PEC and other etching parameters

Chemical bath deposition of cubic copper (I) selenide and its room temperature transformation to the orthorhombic phase

The chemical bath deposition of cubic copper (I) selenide (Cu2-xSe), thin films has been achieved on an inert Pt substrate from a selenosulfite-containing bath at 75 °C. The electrochemical polarisation of this film at −0.78 V vs. SCE leads to the transformation of the compound into the orthorhombic phase. The lattice parameter of the face-centered cubic copper (I) selenide increases from 5.742 A to 5.761 A due to the decrease of the concentration of Cu vacancies upon electrochemical polarisation. The transformation of the cubic to an orthorhombic phase starts to occur when copper vacancies reach a critical value of x < 0.15. This transformation seems to be induced by a Cu3Se2 impurity in the cubic Cu2-xSe phase.

Efficient reduction of nitrite and nitrate to ammonia using thin-film B-doped diamond electrodes

Abstract Boron-doped diamond films were used as electrodes for the reduction of nitrate and nitrite ions to ammonia. Prior to the electrochemical investigation, some in-depth analysis of the diamond films, was undertaken using various techniques. It was found that the plasma-assisted chemical vapor deposition process is superior to the chemical deposition process in controlling the (boron) doping process. The analysis also indicates that some of the boron is occluded in the film along the channels (grain boundaries) between the diamond crystallites. The faradaic efficiency (FE) of reduction of the nitrite (nitrate) to ammonia was found to be greater than unity and could be partially accounted for by the dissolution of the Si substrate itself. Further experiments with free-standing diamond films showed a rather smaller FE; nevertheless it was still greater than unity. A submonolayer of bound nitrogen was found on the diamond surface after the electrolysis. The present results are attributed to a catalytic mechanism for the multi-electron process described here.