Free-standing Assemblies Research Articles

(Invited) Three-dimensional Free-standing Assemblies of Electrodeposited Nanowires for Photoelectrochemical and Catalytical Applications

The implementation of metal and semiconductor nanostructures in devices for catalysis, sensing, and energy conversion requires an excellent control on geometry, crystallinity and composition of the individual nanostructures, as well as its successful assembly into 2-D and 3-D architectures. In particular, nanowire ensembles offer important advantages compared to planar films including e.g. larger surface area and surface to volume ratio, and better crystalline quality.Ion-track nanotechnology is a powerful technique to create templates for the controlled synthesis of such 3-D nanowire architectures. Etched ion-track membranes are fabricated in two separate process steps: (i) sequential irradiation of polymer foils from one or various directions with swift heavy ions resulting in the creation of tracks; (ii) converting the ion tracks into nanochannels by selective chemical etching. Channel density and orientation, as well as diameter and geometry are adjusted by the irradiation and etching conditions, respectively. Afterwards, by electrodeposition in the nanochannels and subsequent removal of the polymer template 3-D assemblies of nanowires are synthesized. Since the nanowires adopt the exact shape of the host channel, their diameter can be adjusted between ~15 nm and a few µm, and their length between ~1 and several tens of µm.In this talk, we will discuss the electrochemical deposition of Au1-xAgx, Cu, and Cu2O in interconnected nanochannel networks, the characterization of the resulting 3-D nanowire assemblies, and the investigation of their photoelectrochemical and catalytical properties.

Scalable CVD synthesis of three-dimensional graphene from cast catalyst

Three-dimensional (3D) graphene has become an area of great interest due to its unique properties, and different synthesis routes and structures have been reported in the literature. However, the challenges of retaining high quality, scaling up and improving economic feasibility are apparent. Here we describe 3D graphene sheet (3DGS), which is a novel, electrically conductive and porous 3D graphene material that addresses some of these challenges. Due to the combination of polymer binders and a casting technique, scale up for this material is possible. The 3DGS reported in this work can be described as a seamless and freestanding assembly with low structural defects, electrical conductivity of 14.71 S/cm and reasonable mechanical integrity. This material exhibits an average density of 21.65 ± 1.49 kg/m3 and a mesoporous structure with a specific surface area of 16.5 m2/g. The Chemical Vapor Deposition (CVD) synthesis and the cast catalyst preparation method is simple and versatile in terms of tuning the material properties such as areal density and electrical conductivity. These and other properties of 3DGS enable its use in many applications, especially as binder-free, flexible electrodes and current collectors for energy storage devices and sensors.

Combined simulation and optical measurement technique for investigation of system effects on components solder fatigue

Among others, physics of failure related concepts are being developed to address the thermo-mechanical reliability challenges in automotive electronics. Limitations in particular applied for finite element (FE-) analyses are models of limited size, which rarely address the system character of failure. Also in testing a system view on fully mounted electronic control units (ECU) and loaded by environmental and active loading cannot be taken performing end-of-life tests for time limitations. Accelerated testing is done instead, however, mostly on board level.To overcome some of these limitations, a combined measuring-simulation technique is being developed, which is described in the paper. System level view on boards mounted in automotive ECUs is taken by a newly developed high-precision optical deformation measuring system. The multi-sensor measuring method combines a chromatic sensor for topography and warping analysis with an optical sensor for in-plane deformation and strain field analysis. By this combination, a high resolution can be reached for all three components of displacement vectors. Additionally, software tools allow the determination of derived quantities like strains, local curvatures and local warpage radius. The latter can be taken as input for FE-simulations. It is shown that some components, in particular QFNs, are sensitive to thermally induced cyclic warpage even if the corresponding bending deflections are in the micrometers range. Worst case in-plane stretching and cyclic warpage of a board mounted in an ECU have been measured. By corresponding simulations on QFN solder fatigue, mounted on a special test board, the critical fatigue life can be determined dependent on the interaction to the case, which differs by several hundred percent from a free-standing assembly.

Proton transport in assemblies of silica colloidal spheres

We studied the proton transport in free-standing assemblies of surface-sulfonated silica nanospheres, either randomly packed or self-assembled into a close-packed arrangement. The transport studies were performed in the absence of solvent, under either dry or high humidity conditions. We demonstrated that colloidal assemblies prepared using surface-sulfonated silica nanospheres possess proton conductivity that depends on the ordering of the material, temperature and relative humidity. Based on the comparison between the close-packed and disordered assemblies made of the same spheres we conclude that the increase in structural organization of the self-assembled colloidal materials leads to increased proton conductivity and better water retention.

Tow Methods for Deepwater Riser-Tower Transportation in West-of-Africa Environment

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17892, "Novel Tow Methods for Deepwater Riser-Towers Transportation in West-of-Africa Environment," by R. Di Silvestro, F. Casola, G. Fatica, A. Mameli, and A. Prandi, Saibos SAS, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. Riser towers are characterized by very low service-fatigue damage in deep-water fields with floating production facilities. However, transportation from the fabrication yard to site can be of concern. This phase can use a significant portion of the allowable fatigue damage. Longer tow distances use more of the allowable fatigue damage. Two tow configurations that minimize the fatigue damage and allow longer tow distances are presented. Introduction The use of riser towers in deepwater fields west of Africa has increased in recent years. In addition to low service-fatigue damage, high thermal performance and the ability to accommodate many risers and umbilicals are advantages over the more traditional riser concepts. The presence of a floating production, storage, and offloading (FPSO) vessel is not required before installation of the tower, improving the installation sequence and reducing the time to first oil production after the FPSO arrives. Transportation of the tower from the fabrication yard to the field is a critical operation that consumes much of the allowed fatigue. Management of towing is a key aspect in this phase of the oil industry driven by the growing distance between fabrication and installation sites as new fields are found in deeper waters. Two tow methods were developed for transporting the riser tower developed in line with the bundle-hybrid-offset-riser (BHOR) concept. Both surface and submerged configurations were considered to mitigate the dynamic-fatigue response of the tower to wave periods and facilitate tow operation lasting many days. BHOR Overview The BHOR is based on the steel-hybrid-riser concept and is a freestanding assembly with several risers in a bundle configuration. In Fig. 1, the assembly is top tensioned by a buoyancy tank and anchored to the seabed by means of a gravity-skirted foundation. The connection of the bundle main section (BMS) to the seabed foundation is made by means of a mechanical flexible joint similar to that used for a tension-leg-platform tendon that is able to accommodate the small angular excursions of the bundle with a limited stiffness. A similar connection is used to connect the buoyancy tank to the top part of the tower. The buoyancy tank is a standalone structure that is installed after the upending of the BMS and docking it to the foundation. The buoyancy-tank structure is derived from the typical pressure-vessel design. The schedules for fabrication and installation of the bundle and the buoyancy tank are independent.

Cd2P2Se6 nanolenses formed at a water–air interface

Self-assembly offers an attractive approach for the organization of nanocrystals (NCs) into functional superstructures. Well-ordered arrays with various configurations have been produced upon solid substrates [1– 6], but there are obvious advantages to be exploited if self-supporting films containing assemblies of functional NCs can be produced. To this end, the selfassembly behavior of NCs on liquid substrates, such as water surfaces, has been investigated. Lim et al. produced close-packed arrays of 5 nm silver NCs at a water–air interface by introducing a NC dispersion in the presence of polysterene [7]. Well-organized monolayer and bilayer regions were embedded in a continuous layer of the polymeric species. In a similar manner, Park et al. produced free-standing photonic bandgap crystals by developing 3-D assemblies of colloidal polysterene particles (∼240 nm in diameter) [8]. In the present letter we describe attempts to apply this approach to tri-n-octyl phosphine oxide (TOPO)coated CdSe NCs. The authors have had considerable success in assembling such NCs into assemblies on solid substrates, even to the point of producing wellordered monolayers with strong crystallographic texture [9]. Attempts to produce free-standing assemblies at a water–air interface, however, led to the serendipitous observation of nanoscale lens-shaped particles of the cadmium hexachalcogeno-hypodiphosphate phase Cd2P2Se6. A summary of these observations is presented here together with an outline of possible routes by which this phase could form and how the morphology might develop. The method for producing TOPO-coated CdSe NCs has been described in detail elsewhere [10]. Briefly, the precursors used are dimethyl cadmium and a solution of selenium in tri-n-octyl phosphine (TOP). The precursors were pre-mixed and subsequently introduced into a reaction flask containing a degassed mixture of TOPO and TOP under a nitrogen atmosphere at an elevated temperature (∼300 ◦C). The growth of the NCs in the reaction solution was monitored by ultraviolet visible spectrometry. The surfaces of the assynthesised CdSe NCs were passivated by a layer of TOPO molecules in the presence of the TOPO/TOP solvent mixture. These solvents were then removed by precipitation and repeated rinsing with methanol. Fractionation was performed using a solvent/non-solvent pair (chloroform/methanol) to further narrow the initial size distribution of the CdSe NCs [11]. This procedure was used to produce NCs of various core diameters, corresponding to different luminescent wavelengths. The CdSe NCs used in this work had a core diameter of 6.20 ± 0.36 nm, i.e. a variation of ±6.0%. Dilute dispersions of these fractionated CdSe nanocrystals in toluene were slowly deposited onto the surface of water contained in a 100 mm diameter Pyrex petri dish. Since toluene and water are immiscible, the dispersion formed a smooth floating circular layer on the water surface. The petri dish was then covered and the toluene allowed to evaporate for a period of 4 days. When the evaporation was completed, a fragmented thin film remained on the water surface. These fragments were carefully transferred on to a TEM copper grid and examined in a Philips EM420T operating at an accelerating voltage of 100 kV. Many regions of the thin film fragments were too thick for the electron beam to penetrate. In those regions that were electron-transparent, no assemblies of close-packed CdSe NCs were observed. Instead, large numbers of disc-like nanoparticles were present. These varied in diameter from 20 nm to 400 nm and were typically ∼100 nm in diameter. Fig. 1 is a brightfield TEM micrograph obtained from a representative area showing such particles. It was not possible to measure the thickness of the discs accurately but tilting experiments indicated that this was far less than the diameter of the discs in each case and probably <10 nm. The composition of the particles was investigated using energy dispersive spectrometry (EDS) in the TEM. The spectra exhibited characteristic peaks corresponding to Cd Lα , Se Lα , and P Kα , together with Kα system peaks for Cu and Zn, which arose from the support grid and specimen holder. Accurate quantification was not possible in the absence of a suitable standard, but standard-less analysis indicated that the particles certainly contained far more Se and P than one would

One-, two-, and three-dimensional organization of colloidal particles using nonuniform alternating current electric fields.

We demonstrate here the use of nonuniform alternating current (AC) electric fields, generated by planar electrodes, for the organization of num-sized particles into one-, two-, and three-dimensional assemblies. The electrodes, with separations that vary from 35 to 300 num, are made of gold deposited on glass substrata. Latex, silica and graphite particles have been examined inside organic or aqueous media in order to illustrate the general applicability of the technique. Theoretical predictions of the particle response under the electric fields are experimentally confirmed for all the above particle/media combinations and can thus be used as a valuable design tool. The size and shape of the final structures are mainly dependent on the electrode shape and dimensions, but are also subject to the particle type and operating conditions. Particle organization in one dimension (strings) is achieved under conditions of positive or negative dielectrophoresis in the space between two energized electrodes. Two-dimensional particle organization (ordered, planar particles assemblies) was observed under conditions of negative dielectrophoresis, when quadrupole electrodes were employed. Moreover, when negative dielectrophoresis and stronger electric fields are applied (of the order of 50 kV(rms) m(-1)), three-dimensional, pyramid-like structures with a vertical dimension 1000-fold higher than that of the corresponding (planar) electrodes can be assembled. These 3-D structures can grow as free-standing assemblies, or inside templates etched in the substratum. The dielectrophoresis (DEP)-organized particle assemblies can subsequently be rendered permanent via the in situ fixing (cross-linking) of the individual particles.