The cathode is the most crucial component of the polymer electrolyte membrane fuel cell (PEMFC) because it is the most limiting in terms of performance, durability, and cost. Regarding the performance, the main losses are due to the cathode because of the negative coupling between a sluggish oxygen reduction reaction (ORR) and H+ and O2 transport loss issues. Therefore, many efforts have been conducted on the one hand to increase the kinetic of the ORR by developing a catalyst with improved activity and stability. On the other hand, attempts have been made to reduce mass transport losses in the cathode by tuning its nanostructure. This paper describes a multistep process to nanostructure the electrocatalyst in the view of simultaneously benefiting from enhanced activity toward the ORR and reduced O2 transport limitations. Thus, original carbon-free electrode's architectures made of organized and well-ordered and oriented PtNi nanowires (PtNiNWs) and nanotubes (PtNiNTs) directly embedded onto a Nafion membrane were developed. Here, the nanotubes were templated from Ni nanowires grown on an anodic aluminum oxide (AAO) template. The fabrication process was optimized to improve the quality of the nanotubes and their integration into the membrane: the process includes thermal treatment in a H2/Ar environment and an acid leaching step as key steps to obtain the desired structure. After the electrodes were integrated in a complete membrane-electrode assembly (MEA), we tested the performance and durability of these nanostructures under real operating conditions. We compared the results to a Pt/C conventional electrode at low Pt loading (similar to 35 mu gPt/cm2) exhibiting a roughness factor close to that of PtNiNWs and PtNiNTs electrodes. Results have shown a great improvement in the mass activity and stability of PtNiNTs electrodes in an accelerated stress test. Also, they have shown a significant sensitivity toward relative humidity variation.

Pnictogen and chalcogenide compounds have been seen as high-potential materials for efficient thermoelectric conversion over the past few decades. It is also known that with nanostructuration, the physical properties of these pnictogen-chalcogenide compounds can be further enhanced towards a more efficient heat conversion. Here, we report the reduced thermal conductivity of a large ensemble of Bi2Te3 alloy nanowires (70 nm in diameter) with selenium for n-type and antimony for p-type (Bi2Te3-ySey and Bi2-xSbxTe3 respectively). The nanowire growth was carried out through electrodeposition in nanoporous aluminium oxide templates with high aspect ratios leading to a forest (109 per centimetre square) of nearly identical nanowires. The temperature dependence of thermal conductivity for the nanowire ensembles was acquired through a highly sensitive 3ω measurement technique. The change in the thermal conductivity of nanowires is largely affected by the roughness in addition to the size effect due to enhanced boundary scattering. The major factor that influences the thermal conductivity was found to be the ratio of the rms roughness to the correlation length of the nanowire. With a high Seebeck coefficient and electrical conductivity at room temperature, the overall thermoelectric figure of merit ZT allows the consideration of such forests of nanowires as efficient potential building blocks of future TE devices.

The 3ω method is a dynamic measurement technique developed for determining the thermal conductivity of thin films or semi-infinite bulk materials. A simplified model is often applied to deduce the thermal conductivity from the slope of the real part of the ac temperature amplitude as a function of the logarithm of frequency, which in-turn brings a limitation on the kind of samples under observation. In this work, we have measured the thermal conductivity of a forest of nanowires embedded in nanoporous alumina membranes using the 3ω method. An analytical solution of 2D heat conduction is then used to model the multilayer system, considering the anisotropic thermal properties of the different layers, substrate thermal conductivity, and their thicknesses. Data treatment is performed by fitting the experimental results with the 2D model on two different sets of nanowires (silicon and BiSbTe) embedded in the matrix of nanoporous alumina templates, having thermal conductivities that differ by at least one order of magnitude. These experimental results show that this method extends the applicability of the 3ω technique to more complex systems having anisotropic thermal properties.

We report the successful use of in situ grazing incidence small-angle X-ray scattering to follow the anodization of aluminum. A dedicated electrochemical cell was designed and developed for this purpose with low X-ray absorption, with the possibility to access all azimuthal angles (360°) and to remotely control the temperature of the electrolyte. Three well-known fabrication techniques of nanoporous alumina, i.e., single, double, and pretextured, were investigated. The differences in the evolution of the scattering images are described and explained. From these measurements, we could determine at which moment the pores start growing even for very short anodization times. Furthermore, we could follow the thickness of the alumina layer as a function of the anodization time by monitoring the period of the Kiessig fringes. This work is aimed at helping to understand the different steps taking place during the anodization of aluminum at the very early stages of nanoporous alumina formation.

This paper describes the elaboration via an electrochemistry route of a PEM carbon‐free cathode made of a periodic array of vertically aligned platinum copper nanotubes. Nanotubes synthesis is assisted with a porous alumina mold made on a silicon substrate to perform growth of copper nanowires. After optimization of an original confined galvanic displacement with H 2 PtCl 6 salt, PtCu nanotubes are embedded into porous alumina template and directly integrated onto a Nafion ® membrane. Therefore, our process does not need any step of purification or catalyst dispersion. The performances in fuel cell of these new electrodes are compared to conventional structures made of Tanaka Pt/C with the same electrocatalytic surface area. Significant improvement in the specific activity at 0.9 V and reduction in mass transport limitations are observed compared with standard Tanaka Pt/C catalyst in real operating conditions.

Selective oxidation of the silicon element of silicon germanium (SiGe) alloys during thermal oxidation is a very important and technologically relevant mechanism used to fabricate a variety of microelectronic devices. We show a two-step epitaxy/condensation process at low temperature to produce atomically flat Ge rich layer fully strained and free of defects. We demonstrate that the condensation based process enables the total inhibition of the classical ATG morphological instability, together with the hindering of dislocations for critical thickness much greater than those commonly obtained by direct deposition. Those behaviors could be explained by the injection of self-interstitials in the Ge-rich layers during condensation. An integrative approach involving vapor–liquid–solid (VLS) growth followed by selective oxidation steps to the construction of core–shell nanowires and higher-level ordered systems with scalable configurations is used. We contrast this strategy that uses reaction-diffusion-segregation mechanisms to produce coherently strained structures with highly configurable geometry and abrupt interfaces with growth-based processes which lead to low strained systems with non uniform composition, three-dimensional morphology, and broad core–shell interface. We specially focus on SiGe small core–shell nanowires and demonstrate that they can have up to 70% Ge-rich shell and 2% homogeneous strain with core diameter as small as 14 nm. Key elements of the building process associated with this approach are identified with regard to existing theoretical models. Moreover, starting from results of ab initio calculations, we discuss the electronic structure of these novel nanostructures as well as their wide potential for advanced device applications. Similar core-shell structures can be formed around small nanocrystals for photonic applications.

Selective oxidation of the silicon element of silicon germanium (SiGe) alloys during thermal oxidation is a very important and technologically relevant mechanism used to fabricate a variety of microelectronic devices. We develop here a simple integrative approach involving vapor-liquid-solid (VLS) growth followed by selective oxidation steps to the construction of core-shell nanowires and higher-level ordered systems with scalable configurations. We examine the selective oxidation/condensation process under nonequilibrium conditions that gives rise to spontaneous formation of core-shell structures by germanium condensation. We contrast this strategy that uses reaction-diffusion-segregation mechanisms to produce coherently strained structures with highly configurable geometry and abrupt interfaces with growth-based processes which lead to low strained systems with nonuniform composition, three-dimensional morphology, and broad core-shell interface. We specially focus on SiGe core-shell nanowires and demonstrate that they can have up to 70% Ge-rich shell and 2% homogeneous strain with core diameter as small as 14 nm. Key elements of the building process associated with this approach are identified with regard to existing theoretical models. Moreover, starting from results of ab initio calculations, we discuss the electronic structure of these novel nanostructures as well as their wide potential for advanced device applications.

In this work, we present completely industry adapted processes for high-chi PS-PDMS block copolymers. DSA was performed on trenches fabricated within standard photolithography stacks and pattern transfer was made by using etching processes similar to those used for gate etching in industry. We propose the alignment of two different PS-PDMS (45.5kg/mol, 16kg/mol) solely by thermal annealing. By adding plasticizer molecules in the high molecular weight BCP (45.5k), we have not only avoided solvent vapor annealing but also reduced significantly the processing time. The properties of the guiding lines and the quality of the final BCP hard mask (CD uniformity, LWR, LER) were investigated.

A new approach to obtaining spherical nanodomains using polystyrene-block-polydimethylsiloxane (PS-b-PDMS) is proposed. To reduce drastically the process time, we blended a copolymer with cylindrical morphology with a PS homopolymer. Adding PS homopolymer into a low-molar-mass cylindrical morphology PS-b-PDMS system drives it toward a spherical morphology. Besides, by controlling the as-spun state, spherical PDMS nanodomains could be kept and thermally arranged. This PS-homopolymer addition allows not only an efficient, purely thermal arrangement process of spheres but also the ability to work directly on nontreated silicon substrates. Indeed, as shown by STEM measurements, no PS brush surface treatment was necessary in our study to avoid a PDMS wetting layer at the interface with the Si substrate. Our approach was compared to a sphere-forming diblock copolymer, which needs a longer thermal annealing. Furthermore, GISAXS measurements provided complete information on PDMS sphere features. Excellent long-range order spherical microdomains were therefore produced on flat surfaces and inside graphoepitaxy trenches with a period of 21 nm, as were in-plane spheres with a diameter of 8 nm with a 15 min thermal annealing. Finally, direct plasma-etching transfer into the silicon substrate was demonstrated, and 20 nm high silicon nanopillars were obtained, which are very promising results for various nanopatterning applications.

In this work, perfectly organized triangular arrays of vertical nanopores are formed in an alumina matrix by combining a pre-patterning technique with the natural ability of alumina to form a triangular unit cell. More precisely, we imprinted a triangular array of indents on a thin layer of aluminum deposited on silicon substrates using nano-imprint lithography. During the anodization process, we forced the growth of pores in and in-between the indents obtaining a larger number of pores in the final alumina array than the initial number of indents patterned on the aluminum. Adapting the anodization conditions, a density multiplication by three was successfully achieved with a very good surface organization. The experimental details of the process are described in this paper. We studied in details the inner organization of the pores and we identified differences in their propagation between oxalic and orthophosphoric acid. The former showed a good surface propagation until 1500 nm in depth. On the contrary, the latter showed a perturbation in the organization at 450 nm: at this depth, the induced pores stopped whereas the indented ones rearranged into two or three. A longer shift in the initiation of the induced pores seemed to causes this poor propagation. A systematic study was performed to investigate the effect of the anodization conditions on the pores' propagation. We demonstrate that the optimization of the orthophosphoric acid concentration and the applied voltage towards harder anodization conditions, i.e. to higher values, allows a better control of this self-assembling process and deeper order propagation until more than 1000 nm. (C) 2015 Elsevier B.V. All rights reserved.

Due to its wide band-gap, Al 2 O 3 is known to have a moderate leakage current and a good dielectric strength [1]. Moreover, this dielectric has a fair permittivity and so constitutes interesting candidate as dielectric for Metal-Insulator-Metal (MIM) capacitor. Atomic Layer Deposition (ALD) allows obtaining a dense and thin Al 2 O 3 amorphous layer. ALD limits problems of interlayer diffusion because Al 2 O 3 is deposited underneath 400°C [2] which is essential when MIM are co-integrated with temperature sensitive structures. The aim of our investigation is to attempt to tie aluminum oxide properties dielectric with reliability from the help of capacitors of the entire wafer. In this way, conduction mechanism analysis and capacitance measurements were statistically led on the wafer. We particularly focus our study on the quantification of defects and their influence on the leakage current in planar capacitor. Firstly, to estimate the fixed oxide charges densities in the bulk of Al 2 O 3 and to analyze conduction mechanism, Metal-Oxide-Semiconductor (MOS) (Al/Al 2 O 3 /HR-Si) is developed. Then, a MIM stack (Al/TiN/Al 2 O 3 /TiN/HR-Si) is developed in order to evaluate the leakage current and the electrical reliability of thin films Al 2 O 3 based MIM capacitors. Different performances are observed according to the area on the wafer. That could be explained by the quality of the Al 2 O 3 layer and the interfaces between TiN and the oxide.

Semiconductor nanowires attracted a great deal of attention during the last decades. Despite a large number of studies dedicated to their fabrication and physical characterization, the electronic properties have still not been up to expectations. At present, most of the studies switched to core-shell nanowires that are expected to have more revolutionary properties. Theoretical demonstration of a direct band gap metamaterial for specific coreshell Si-Ge NWs has boosted the number of studies. The composition profile, morphology and core-shell aspect ratio could fully modify the band structure of the NWs and should be determined accurately. The determination of all these parameters remains a major challenge using conventional nanocharacterization techniques. In this study we perform the growth of core-shell Ge-Si nanowires by CVD and we observe them by STEM. EDX analysis showed the core-shell Ge-Si structure is very well defined. The intensity profile of Ge and Si exhibit a slight dissymmetry which is stronger in the core. Two models of the section of the nanowires are proposed and compared to the experimental data and the presence of facets on the core is shown to be a possible cause for this dissymmetry.

A single silicon nanowire core-shell structure has been elaborated. Technological stages of the process are presented. The device results in a P-i-N radial junction: the core is a P-type silicon nanowire encapsulated in an intrinsic thin silicon layer and an N-type doped silicon layer. Scanning electron microscopy observations, as well as the electrical I(V) characterisation on single nanowires, are presented.

The core-shell geometry is a strong tool for inducing and controlling strains in nano-objects in order to tune their optoelectronic properties. We synthesized and characterized core-shell nanostructures by depositing a non-epitaxial silicon nitride shell around germanium nanowires. Scanning electron microscopy as well as energy dispersive X-ray spectroscopy confirms the structural integrity of the heterostructures, and grazing incidence X-ray diffraction measurements reveal the presence of a radial tensile strain in the Ge nanowires. A control of this strain is then demonstrated up to 0.3% by adjusting the SiNx shell thickness versus Ge nanowire diameter. ((c) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The use of nanoporous anodic alumina as a template for silicon nanowire growth enables the production of ultra dense nanowire arrays with density up to 8.109cm(-2). The integration of such nanowire arrays in micro-supercapacitor electrodes significantly improves the electrode capacitance per planar surface area and nanowires length unit compared to electrodes made from silicon nanowires grown from gold colloids (from 5.2 mu F.cm(-2).mu m to 36.7 mu F.cm(-2).mu m). Micro-supercapacitors with such electrodes and ionic liquid as electrolyte have a very promising cycling stability featuring less than 1% of losses after 300000 cycles. (C) 2013 Elsevier Ltd. All rights reserved.

The production and characterization of ultradense, planarized, and organized silicon nanowire arrays with good crystalline and optical properties are reported. First, alumina templates are used to grow silicon nanowires whose height, diameter, and density are easily controlled by adjusting the structural parameters of the template. Then, post-processing using standard microelectronic techniques enables the production of high-density silicon nanowire matrices featuring a remarkably flat overall surface. Different geometries are then possible for various applications. Structural analysis using synchrotron X-ray diffraction reveals the good crystallinity of the nanowires and their long-range periodicity resulting from their high-density organization. Transmission electron microscopy also shows that the nanowires can grow on nonpreferential substrate, enabling the use of this technique with universal substrates. The good geometry control of the array also results in a strong optical absorption which is interesting for their use in nanowire-based optical sensors or similar devices.

Presented is a structural investigation of silicon nanowires, which is conducted with grazing incidence small angle X-ray scattering. The morphology of the wires is analysed following experimental measurements. Three diameters (50, 100 and 200 nm) are investigated in relation to the aspect ratio of the wires (length 25 mu m). Periodic fringes because of the weak distribution of the diameter are observed on the experimental images. The asymptotic behaviour of the scattering signal along the q(y) direction is also analysed and presented.

Nanoporous alumina films (NPAF) have been elaborated by anodization of an aluminum film on silicon wafer. Ex situ structural characterization of the films has been achieved with grazing incidence small angle X-ray scattering under ultra high vacuum atmosphere and using a synchrotron source. The comparison of the experimental patterns with suitable modeling confirms the cylindrical geometry of the pores well as the good local hexagonal order. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this work, nanoimprint lithography combined with standard anodization etching is used to make perfectly organised triangular arrays of vertical cylindrical alumina nanopores onto standard <100>−oriented silicon wafers. Both the pore diameter and the period of alumina porous array are well controlled and can be tuned: the periods vary from 80 to 460 nm, and the diameters vary from 15 nm to any required diameter. These porous thin layers are then successfully used as templates for the guided epitaxial growth of organised mono-crystalline silicon nanowire arrays in a chemical vapour deposition chamber. We report the densities of silicon nanowires up to 9 × 109 cm−2 organised in highly regular arrays with excellent diameter distribution. All process steps are demonstrated on surfaces up to 2 × 2 cm2. Specific emphasis was made to select techniques compatible with microelectronic fabrication standards, adaptable to large surface samples and with a reasonable cost. Achievements made in the quality of the porous alumina array, therefore on the silicon nanowire array, widen the number of potential applications for this technology, such as optical detectors or biological sensors.