Laetitia Philippe

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

Ordered networks of ZnO-nanowire hierarchical urchin-like structures for improved dye-sensitized solar cells.

Quasi-1D ZnO nanowires (NWs) ordered as patterned 3D hollow hierarchical urchin-like structures have been prepared on transparent conducting substrates by electrodeposition. The ZnO NWs have been grown on self-assembled ordered polystyrene microspheres with electrical charge densities ranging from 5 to 30 C cm(-2) and organized arrays of mono and multi-urchin layers have been built. These layers have been sensitized by the highly absorbing D149 indoline organic dye. The optical characterizations and dye titrations have shown a significant increase in the light scattering and absorption as well as dye loading for the organized structures compared to randomly vertically aligned ZnO NWs grown under the same conditions. The dye-sensitized solar cells (DSSC) prepared using the sensitized layers have been characterized by current-voltage (J-V) measurements, IPCE and by electrochemical impedance spectroscopy. We show that the best performances are obtained for the 3D urchin monolayer structures. The conversion efficiency is increased by up to 4 times compared to their counterparts made of randomly dispersed vertical ZnO NWs. Impedance spectroscopy results show a very fast charge transfer in the ZnO NWs and urchin monolayers and that the electron lifetime is in the 4-14 ms range.

Simple synthetic route for SERS-active gold nanoparticles substrate with controlled shape and organization.

We report a simple synthetic route based on electroless deposition (galvanic displacement) and natural lithography to simultaneously control the shape and organization of Au nanoparticles (NPs). We show for the first time the formation of organized extended domains of Au nanoflowers and nanocrowns with single crystalline tips. The dimension and morphology of the desired nanostructures (NSs) can be tuned easily by controlling the deposition conditions at room temperature using saccharin as an organic additive. The exact role of saccharin on the crystal growth process of Au NPs is also discussed. A systematic surface enhancement Raman spectroscopy (SERS) study of large, ordered areas of organized gold nanoflowers using p-mercaptoaniline (pMA) as the probe molecule shows massive and reproducible enhancements of the Raman signal. By comparing the relative enhancement of the different vibrational modes as a function of the morphology, the specific charge-transfer (chemical effect) SERS mechanism can be distinguished from the general electromagnetic field enhancement (physical effect). A wide range of applications can be envisaged for these SERS substrates.

Compression of freestanding gold nanostructures: from stochastic yield to predictable flow

Characterizing the mechanical response of isolated nanostructures is vitally important to fields such as microelectromechanical systems (MEMS) where the behaviour of nanoscale contacts can in large part determine system reliability and lifetime. To address this challenge directly, single crystal gold nanodots are compressed inside a high resolution scanning electron microscope (SEM) using a nanoindenter equipped with a flat punch tip. These structures load elastically, and then yield in a stochastic manner, at loads ranging from 16 to 110 microN, which is up to five times higher than the load necessary for flow after yield. Yielding is immediately followed by displacement bursts equivalent to 1-50% of the initial height, depending on the yield point. During the largest displacement bursts, strain energy within the structure is released while new surface area is created in the form of localized slip bands, which are evident in both the SEM movies and still-images. A first order estimate of the apparent energy release rate, in terms of fracture mechanics concepts, for bursts representing 5-50% of the structures initial height is on the order of 10-100 J m(-2), which is approximately two orders of magnitude lower than bulk values. Once this initial strain burst during yielding has occurred, the structures flow in a ductile way. The implications of this behaviour, which is analogous to a brittle to ductile transition, are discussed with respect to mechanical reliability at the micro- and nanoscales.

Extended domains of organized nanorings of silver grains as surface-enhanced Raman scattering sensors for molecular detection

The possibility to synthesize large areas of silver grains organized in nanorings using a simple technique based on nanosphere lithography and electroless plating as a metal deposition method is described for the first time. In addition, we present a systematic SERS study of the obtained long-range ordered silver nanodots and nanorings. The possibility to precisely control the size, the interdistance and the morphology of these nanostructures allows us to systematically investigate the influence of these parameters on SERS. We show that the best possible SERS substrates should not only present optimal sizes, interdistances and shapes, but also a grain-like structure composed of sub-100 nm grains in order to maximize the number of hot-spots. In addition, we show that grains arranged in nanorings present higher enhancement factors (E(F) = 5.5 x 10(5)) as compared to similar arrays made of nanodots. A wide range of applications, including real-time monitoring of catalytic surface reactions, environmental and security monitoring as well as clinical and pharmaceutical screening, can be envisaged for these SERS substrates.

Elevated temperature, strain rate jump microcompression of nanocrystalline nickel

Nanocrystalline and ultrafine-grained materials show enhanced strain rate sensitivity (SRS) in comparison to their coarse grained counterparts. Majority of SRS measurements on nanocrystalline thin films reported in literature have focused on nanoindentation-based approaches. In this paper, micropillar strain rate jump tests were demonstrated on an electrodeposited nanocrystalline nickel film from 25 to 100 °C. SRS exponent, m, and activation volume, V, values were determined as a function of temperature. The measured values were found to be in good agreement with previously reported literature on bulk and nanoindentation measurements. Apparent activation energy for deformation was found to be about 100 kJ/mol, which is close to that for grain boundary diffusion in nickel. Grain boundary sliding was observed in the deformed pillars from scanning electron microscopy images.

In situ SEM indentation experiments: Instruments, methodology, and applications

The purpose of this article is to present the design and capabilities of two in situ scanning electron microscope (SEM) indentation instruments covering a large load range from μN to N. The capabilities and advantages of in situ SEM indentation are illustrated by two applications: indentation of a thin film and a nanowire. All the experiments were performed on electrodeposited cobalt, whose outstanding magnetic properties make it a candidate material for MEMS and NEMS devices. Microsc. Res. Tech., 2009.

Ordered hexagonal array of Au nanodots on Si substrate based on colloidal crystal templating

We report two types of site-selective metal deposition methods based on colloidal crystal templating. We discuss in particular the controllability of the morphology and crystallinity of Au nanodots depending of the choice of method.

Electrodeposition of gold thin films with controlled morphologies and their applications in electrocatalysis and SERS

Here, an easy and effective electrochemical route towards the synthesis of gold thin films with well-controlled roughness, morphology and crystallographic orientation is reported. To control these different factors, the applied potential during deposition played a major role. A tentative nucleation and growth mechanism is demonstrated by means of electrochemical characterizations and a formation mechanism is proposed. Interestingly, the differences in geometry and orientation of the different gold deposits have shown a clear correlation with the electrocatalytical activity in the case of oxygen sensing. In addition, not only the electrocatalytical activity but also the surface-enhanced Raman scattering of the gold deposits have been found to depend both on the roughness and on the size of the surface nanostructures, allowing a fine tuning by controlling these two parameters during deposition.

Reducing the adhesion between surfaces using surface structuring with PS latex particle.

The adhesion between a micro-object and a microgripper end-effector is an important problem in micromanipulation. Canceling or reducing this force is a great challenge. This force is directly linked to the surface chemical structure of the object and the gripper. We propose to reduce the adhesion force by using a self-assembled monolayer structuring on one surface. The surface was structured by polystyrene latex particles (PS particles) with radii from 100 to 1500 nm. The adhesion force measurements obtained by AFM were compared to a multisphere van der Waals force model. The model suggests the existence of an optimal value of the sphere radius which minimizes the adhesion. In that case, the pull-off force is reduced to 20 nN by the PS particles layer with a radius of 45 nm. A wide range of applications in the field of telecommunications, bioengineering, and more generally speaking, MEMS can be envisaged for these substrates.