Search

Fast charging rate and large energy storage are becoming key elements for the development of next-generation batteries, targeting high-performance electric vehicles. Developing electrodes with high volumetric and gravimetric capacity that could be operated at a high rate is the most challenging part of this process. Using silicon as the anode material, which exhibits the highest theoretical capacity as a lithium-ion battery anode, we report a binder-free electrode that interconnects carbon-sheathed porous silicon nanowires into a coral-like network and shows fast charging performance coupled to high energy and power densities when integrated into a full cell with a high areal capacity loading. The combination of interconnected nanowires, porous structure, and a highly conformal carbon coating in a single system strongly promotes the reaction kinetics of the electrode. This leads to fast-charging capability while maintaining the integrity of the electrode without structural collapse and, thus, stable cycling performance without using binder and conductive additives. Specifically, this anode shows high specific capacities (over 1200 mAh g-1) at an ultrahigh charging rate of 7 C over 500 charge-discharge cycles. When coupled with a commercial LiCoO2 or LiFePO4 cathode in a full cell, it delivers a volumetric energy density of 1621 Wh L-1 with a LiCoO2 cathode and a power density of 7762 W L-1 with a LiFePO4 cathode.

In this work, we study the properties of re-assembled graphene nanoplatelets as an effective anticorrosion coating for aluminum (Al) substrate in 0.5 M Sodium Chloride (NaCl) at room temperature (30°C). Scanning and Transmission Electron Microscopy (TEM) as well as Raman spectroscopy reveal the high quality multilayer graphene nanoplatelets. The modifications of the corrosion resistance characteristic were investigated by Open Circuit Potential (OCP), followed by electrochemical tests such as potentiodynamic polarization (Tafel curves) and Electrochemical Impedance Spectroscopy (EIS). The electrochemical results show that the graphene nanoplatelets provide effective resistance against the corrosive medium during the two weeks of immersion in the saline medium. Scanning Electron Microscopy (SEM), Raman spectroscopy and Energy Dispersive X-ray (EDX) studies carried out after immersion in the corrosive medium confirm that a graphene-coated aluminum surface is well protected compared to an uncoated substrate.

In this work, we present a practical approach for controlling single walled carbon nanotubes (SWCNTs) diameter distribution through thin film Ru catalyst, coupled with hydrogen pre-treatment. Uniform and stable Ru nanoclusters were obtained after dewetting the Ru thin films under atomic hydrogen pre-treatment. SWCNTs were synthetized by double hot filament chemical vapor deposition (d-HFCVD) on SiO2/Si substrates at different temperatures. We found that the temperature is an important synthesis parameter that in fluences the diameter distribution of the final SWCNTs. Statistical analysis of the Raman radial breathing modes evidences the growth of highly enriched semi-conducting SWCNTs (about 90%) with narrow diameter distribution that correlates directly with the catalyst particle size distribution. Electrical measurement results on as-grown SWCNTs show good thin-film transistor characteristics.

Kinking of semiconductor nanowires grown by the vapour–solid–liquid (VSL) mechanism has long been observed and studied, particularly for Si. A large variety of turning angles for kinked Si nanowires (KSiNWs) has been reported in the literature, but most authors have studied the kinking mechanism rather than the structure and corresponding geometrical features of the kinks. Here, we have investigated the relationship between the turning angles and the structure (down to atomic level) of KSiNWs grown by VSL from indium nanoparticles. By using transmission electron microscopy, we have characterized the transition regions between different segments of KSiNWs of various crystallographic orientations. We have found that most turning angles can be viewed as rich combinations of different types of {111} coherent twins that coexist within the transition regions between different segments of KSiNWs.

We have studied the electrochemical polishing of copper foils at elevated temperature, in H3PO4 electrolytes of various concentrations. Atomic force microscopy, surface reflectance measurements as well as optical microscopy and scanning electron microscopy (including electron backscattering diffraction) have been used throughout this study to characterize the surface of the electropolished foils. We have found that copper foils electropolished at 65 °C in 2.17 M H3PO4, exhibited a lower surface roughness and a higher percent specular reflection, comparing with values obtained after classical electropolishing in concentrated H3PO4 at room temperature or comparing with values obtained after chemical-mechanical polishing. This work could open up new prospects for the preparation of copper foils before the growth of high quality graphene layers.

Silicon exhibits the largest known capacity for Li insertion in anodes of Li-ion batteries. However, because of large volume expansion/phase changes upon alloying, Si becomes powder-like after a few charge-discharge cycles. Various approaches have been explored in the past to circumvent this problem, including the use of nanomaterials, particularly Si nanowires. However, even though nanowires resist cracking very well, anodes based on Si nanowires still see their original capacity fade away upon cycling, because of wire detachment from the substrate, due to the stress generated at their roots upon alloying with Li. Here, we present a silicon nanowire growth strategy yielding highly interconnected specimens, which prevents them from being individually detached from the substrate. We report a ∼ 100% charge retention after 40 cycles at C/2 rate, without charging voltage limitation. We also show that our anodes can be cycled at 8C rates without damage and we grow nanowires with a density of 1.2 mg/cm 2, yielding anodes delivering a 4.2 mAh/cm 2 charge density. Finally, we point out that a better understanding of the interactions of silicon with electrolytes is needed if the field is to progress in the future. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

International audience; A Hot Wire assisted Chemical Vapor Deposition (HWCVD) process has been developed for producing highdensity arrays of parallel, straight and organized silicon nanowires (SiNWs) inside vertical Porous Anodic Alumina (PAA) templates, exploring temperatures ranging from 430 °C to 600 °C, and pressures varying between 2.5 and 7.5 mbar. In order to prevent parasitic amorphous silicon (a-Si) deposit and to promote the crystalline SiNWs growth, we used a tungsten hot wire to partially crack H2 into atomic hydrogen, which acts like a selective etchant regarding a-Si. Here we describe the optimization route we followed to limit the deposit of a-Si onto the surface of the porous membrane and on the walls of the pores, which led to the possibility to grow SiNWs inside the PAA membranes. Such an approach has high potentialities for device realization, like PIN junctions, FETs or electrodes for Li-ion batteries.

International audience; The electrochemical and structural responses of silicon nanowires deposited by chemical vapor deposition are investigated. Transmission electron microscopy and X-ray diffraction experiments show the electrochemical lithiation of SiNWs is not a quantitative process in good agreement with cycling experiments performed as a function of cycling limits. The SiNWs are not deeply lithiated as revealed by TEM micrographs. Our results suggest the existence of two well-defined lithiation steps, first at ∼0.2 V into amorphous silicon and then into crystalline silicon at ∼0.1 V. Cycling SiNWs above 100 mV avoid the lithiation of c-Si which preserves the silicon 3-D architecture and results in good cycling performances. A stable capacity value of ∼500 mAh g−1 is achieved over at least 50 cycles.

Facet apparition during selective epitaxial growth of silicon and silicon–germanium alloys is reported in terms of morphology and kinetics. Epitaxial growth was performed on (0 0 1) Si wafers by chemical vapour deposition using the H2/HCl/SiH2Cl2 chemistry for silicon and GeH4 addition for silicon–germanium alloy. The (0 0 1) Si and SiGe growth rate was found to be limited by chlorine desorption at low temperature. The creation and development of (3 1 1) facets has been clearly explained by the epitaxial growth kinetics considerations. The impact of the deposition conditions, of the pattern structure and also of the dielectric nature on faceting are discussed here and analysed, thanks to cross section scanning electron microscopy (XSEM) and cross section transmission electron microscopy (XTEM) observations.

A fabrication method for silicon beams and membranes defined in lateral and vertical dimensions, as well as superposed silicon membranes, all realized in bulk silicon using only one lithographic step is proposed. This proposal is based on observations made on structures obtained by High Temperature Annealing (HTA) in hydrogen atmosphere process. The combination of design configuration and materials technology (hard mask) with the process shows the possibility of new 3D devices and cavities beyond previously reported capabilities of with this technique. The specific design and hard mask engineering presented can lead to structures used in a bulk silicon platform for 3D devices with optical and electronic functions for the fabrication of bulk silicon waveguides and transistors on stressed membranes with enhanced mobility.

The effects of an atomic hydrogen ( H at ) pretreatment of the catalyst layer on the low temperature growth of single-walled carbon nanotubes (SWCNTs) have been investigated using a modified catalytic chemical vapor deposition system. Well-defined and isolated individual Fe nanoparticles as a catalyst are successfully formed on the defects with high trapping energy which are created on the Al 2 O 3 surface by H at pretreatment, yielding highly dense SWCNTs. The pretreatment mechanism of H at , compared to H 2 , is also discussed. It was also found that the quality of SWCNTs can be enhanced when H at is flowed with CH 4 during nanotubes growth at low temperature. In this case, the undesired carbon products and defects on catalyst seeds and nanotube walls can be selectively removed by H at . Therefore it is essential to use H at in the pretreatment stage for increasing catalytic activity and to keep the size of nanoparticles in the nm range. H at can also be employed in growth stage for enhancing SWCNTs quality and density at low temperature.

International audience; The low temperature catalytic growth of multiwall carbon nanotubes (MWCNTs) rests on the continuous nucleation and growth of graphene layers at the surface of crystalline catalystparticles. Here, we study the atomic mechanisms at work in this phenomenon, by observing the growth of such layers in situ in the transmission electron microscope, in the case of iron-based catalysts. Graphene layers, parallel to the catalyst surface, appear by a mechanism of step flow, where the atomic layers of catalyst are "replaced" by graphene planes. Quite remarkably, catalyst facets systematically develop while this mechanism is at work. We discuss the origin of faceting in terms of equilibrium particle shape and graphene layer nucleation. Step bunching due to impeded step migration, in certain growth conditions, yields characteristic catalyst nail-head shapes. Mastering themechanisms of faceting and step bunching could open up the way to tailoring the structure of low temperature-grown MWCNTs, e.g. with highly parallel carbon walls and, ultimately, with controlled structure and chirality.

We have studied the influence of the surface roughness of copper foils on the sheet resistance of graphene sheets grown by chemical vapor deposition. The surface roughness of the copper foils was reproducibly controlled by electropolishing. We have found that the graphene sheet resistance monotonically decreases as the surface roughness of the copper foils decreases. We show that a pre-annealing treatment combined with an optimized electropolishing process of the Cu foils and a fast CVD growth prevents the evolution of the Cu surface roughness during graphene synthesis. This combination of fabrication conditions produces small grain polycrystalline graphene films with a sheet resistance of 210 Ω □(-1) and carrier mobility values as high as 5450 cm(2) V(-1) s(-1) after transfer onto SiO2/Si.

In this work, the excimer-laser-induced crystallization of a-Si films on SiO2 was investigated using a long-pulse-duration (200 ns) XeCl source. The microstructural analysis of the laser-irradiated area, for incident energy densities comprised between the thresholds corresponding to the surface and full melting, respectively, of the Si layer, was performed by scanning electron microscopy and Raman spectroscopy. A super-lateral-growth regime was evidenced quite comparable to that which occurs when classical excimer laser pulses of short duration (≈20 ns) are used. A numerical simulation of the surface melt dynamics was also performed and compared to the experimental observations.

Comparisons of polycrystalline silicon materials are made using both density of states measurements made on thin films transistors and transmission and scanning electron microscopy micrographs. We show that in solid phase crystallised materials, in-grain defects control electrical properties whereas in laser crystallised materials, it is grain boundaries.

International audience; Y- and comb-type carbon nanotrees formed from branched carbon nanofibres grown by plasma-enhanced chemical vapour deposition were studied by transmission electron microscopy. Different growth mechanisms are proposed for the two types of nanotrees based on the observed and reconstituted dynamic transformations of the catalyst particles during synthesis. However, the splitting of the larger catalyst particles is required for both kinds of nanotrees, whatever the involved growth mechanism. The carbon nanotrees are well crystallized and connections of the branches are continuous, which may be interesting for future applications in nanoelectronic devices and also composite materials.

Since chemical vapor deposition of carbon-containing precursors onto transition metals tends to develop as the preferred growth process for the mass production of graphene films, the deep understanding of its mechanism becomes mandatory. In the case of nickel, which represents an economically viable catalytic substrate, the solubility of carbon is significant enough so that the growth mechanism proceeds in at least two steps: the dissolution of carbon in the metal followed by the precipitation of graphene at the surface. In this work, we use ion implantation to dissolve calibrated amounts of carbon in nickel thin films and grow graphene films by annealing. Observations of those graphene films using transmission electron microscopy , directly on the growth substrate as well as transfered on TEM grids, allowed us to precisely study the mechanisms that lead to their formation.

We compare the field emission characteristics of dense (109 nanofibers/cm2), sparse (107 nanofibers/cm2), and patterned arrays (106 nanofibers/cm2) of vertically aligned carbon nanofibers on silicon substrates. The carbon nanofibers were prepared using plasma-enhanced chemical vapor deposition of acetylene and ammonia gases in the presence of a nickel catalyst. We demonstrate how the density of carbon nanofibers can be varied by reducing the deposition yield through nickel interaction with a diffusion layer or by direct lithographic patterning of the nickel catalyst to precisely position each nanofiber. The patterned array of individual vertically aligned nanofibers had the most desirable field emission characteristics, highest apparent field enhancement factor, and emission site density.

International audience; Iron is a widely used catalyst for the growth of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) by catalytic chemical vapor deposition. However, both Fe and Fe−C compounds (generally, Fe3C) have been found to catalyze the growth of CNTs/CNFs, and a comparison study of their respective catalytic activities is still missing. Furthermore, the control of the crystal structure of iron-based catalysts, that is α-Fe or Fe3C, is still a challenge, which not only obscures our understanding of the growth mechanisms of CNTs/CNFs, but also complicates subsequent procedures, such as the removal of catalysts for better industrial applications. Here, we show a partial control of the phase of iron catalysts (α-Fe or Fe3C), obtained by varying the growth temperatures during the synthesis of carbon-based nanofibers/nanotubes in a plasma-enhanced chemical vapor deposition reactor. We also show that the structure of CNFs originating from Fe3C is bamboo-type, while that of CNFs originating from Fe is not. Moreover, we directly compare the growth rates of carbon-based nanofibers/nanotubes during the same experiments and find that CNFs/ CNTs grown by α-Fe nanoparticles are longer than CNFs grown from Fe3C nanoparticles. The influence of the type of catalyst on the growth of CNFs is analyzed and the corresponding possible growth mechanisms, based on the different phases of the catalysts, are discussed.

We report that highly crystalline graphene can be obtained from well-controlled surface morphology of the copper substrate. Flat copper surface was prepared by using a chemical mechanical polishing method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. We also found that grain boundaries and intentionally formed scratched area play an important role for nucleation seeds. Although the best monolayer graphene was grown from polished Cu with a low sheet resistance of 260 Ω/sq, a small portion of multilayers were also formed near the impurity particles or locally protruded parts.

This paper describes a novel photocathode which is an array of vertically aligned multi-walled carbon nanotubes (MWCNTs), each MWCNT being associated with one p-i-n photodiode. Unlike conventional photocathodes, the functions of photon-electron conversion and subsequent electron emission are physically separated. Photon-electron conversion is achieved with p-i-n photodiodes and the electron emission occurs from the MWCNTs. The current modulation is highly efficient as it uses an optically controlled reconfiguration of the electric field at the MWCNT locations. Such devices are compatible with high frequency and very large bandwidth operation and could lead to their application in compact, light and efficient microwave amplifiers for satellite telecommunication. To demonstrate this new photocathode concept, we have fabricated the first carbon nanotube based photocathode using silicon p-i-n photodiodes and MWCNT bunches. Using a green laser, this photocathode delivers 0.5 mA with an internal quantum efficiency of 10% and an I(ON)/I(OFF) ratio of 30.

10 pages; International audience; Carbon nanofibres (CNFs) obtained by plasma enhanced chemical vapour deposition are made of cone-shaped graphene layers, the opening angle of which has a significant influence on their properties: the smaller the angle, the closer the properties to those of carbon nanotubes. That angle is determined by the shape of the metal nanoparticle used to catalyse the growth. We show in this paper that the shape of Ni nanoparticle catalysts, and in turn the CNF properties, can be tuned during plasma-enhanced chemical vapour deposition, by the choice of the etchant gas. We show in particular that a water-containing etchant (H2O or H2O+H2) increases the growth rate by an order of magnitude at 600°C compared to an ammonia-containing etchant (NH3 or NH3+H2), and leaves more elongated Ni particles with a cone angle three times smaller. We conclude that the cone angle and the growth rate are directly related, and propose a mechanism to explain that large difference between the two etchants.

International audience; Lateral porous anodic alumina (PAA) templates were used to organize carbon nanotubes (CNTs) grown by a hot-filament assisted chemical vapor deposition (HFCVD) process. For the CNT growth, we used a modified "home-made" HFCVD system with two independently powered filaments which are fitted respectively on the methane (CH4) gas line, which serves as a carbon precursor and on the hydrogen (H2) gas line, which acts as an etching agent for the parasitic amorphous carbon. Various activation powers of the hot filaments were used to directly or indirectly decompose the gas mixtures at relatively low substrate temperatures. A parametric study of the HFCVD process has been carried out for optimizing the confined CNTs growth inside the lateral PAA templates.

International audience; The synthesis of few-layered graphene is performed by ion implantation of carbon species in thin nickel films, followed by high temperature annealing and quenching. Although ion implantation enables a precise control of the carbon content and of the uniformity of the in-plane carbon concentration in the Ni films before annealing, we observe thickness non-uniformities in the synthesized graphene layers after high temperature annealing. These non-uniformities are probably induced by the heterogeneous distribution/topography of the graphene nucleation sites on the Ni surface. Taken altogether, our results indicate that the number of graphene layers on top of Ni films is controlled by the nucleation process on the Ni surface rather than by the carbon content in the Ni film.

International audience; Growth on transition metal substrates is becoming a method of choice to prepare large-area graphene foils. In the case of nickel, where carbon has a significant solubility, such a growth process includes at least two elementary steps: (1) carbon dissolution into the metal, and (2) graphene precipitation at the surface. Here, we dissolve calibrated amounts of carbon in nickel films, using carbon ion implantation, and annealing at 725 °C or 900 °C. We then use transmission electron microscopy to analyse the precipitation process in detail: the latter appears to imply carbon diffusion over large distances and at least two distinct microscopic mechanisms.

In order to utilize the unique properties of carbon nanotubes in microelectronic devices, it is necessary to develop a technology which enables high yield, uniform, and preferential growth of perfectly aligned nanotubes. We demonstrate such a technology by using plasma-enhanced chemical-vapor deposition (PECVD) of carbon nanotubes. By patterning the nickel catalyst, we have deposited uniform arrays of nanotubes and single free-standing aligned nanotubes at precise locations. In the PECVD process, however, detrimental amorphous carbon (a-C) is also deposited over regions of the substrate surface where the catalyst is absent. Here, we show, using depth-resolved Auger electron spectroscopy, that by employing a suitable deposition (acetylene, C2H2) to etching (ammonia, NH3) gas ratio, it is possible to obtain nanotube growth without the presence of a-C on the substrate surface.

In this work, we will present a novel graphene synthesis technique, based on the ion implantation of carbon ions into polycrystalline transition metal thin films. Ion implantation is now a routine process developed by the semiconductor industry over the past three decades. It is used to dope the source and drain contact regions as well as to adjust the channel characteristics of CMOS transistors. One of the advantages of ion implantation is that it allows one to precisely control the carbon dose introduced in a particular substrate.

We successfully synthesized organized Carbon nanotubes (CNTs) and Silicon Nanowires (SiNWs) arrays using LPAA. This approach can yield very dense assemblies of nano-objects with a planar-type organization compatible with existing tools inherited from advanced microelectronic processes and adapted to electronic devices as field effect transistors, interconnects, sensors, etc. CNTs/SiNWs were grown using Hot-filament Chemical Vapor Deposition (HFCVD) within lateral-type porous anodic alumina. We demonstrate that the pulsed electrodeposition of metal nanoparticles to be further used as catalysts inside the membranes requires specific thinning and pore widening process to remove the alumina barrier layer located at the bottom of the pores. The growth of CNTs was found to strongly depend on the electrodeposition conditions as well as on the CVD parameters. In addition, we found that introducing atomic hydrogen (generated using a hot-wire) as etching agent was essential to prevent parasitic carbon/silicon deposition on the surface of PAA or on the wall of pores and to improve CNTs/NWs growth. Such organized CNTs/SiNWs arrays are very promising as advanced microelectronic devices and their potentiality for photosensing applications were investigated.

Graphene has been given great attention to overcome current physical limits in electronic devices and its synthesis routes are developing rapidly. However, graphene film manufacturing is still hindered by either low throughput or low material quality. Here, we present a low temperature PE-CVD assisted graphene growth process on nickel thin films deposited on silicon oxide. Furthermore, our process leads to the formation of two separated graphene films, one at the nickel surface and the other at the Ni/SiO2 interface. A mixture of methane and hydrogen was employed as carbon precursor and activated by DC plasma. We found that the number of graphene layers on top of nickel can be controlled by carbon exposure time, from 1 to around 10 layers. Further annealing process of samples allowed us to achieve improved graphene films by the dissolution and segregation-crystallization process.

In this paper we demonstrate the efficiency of porous anodic alumina (PAA) to confine the growth of silicon nanowires (SiNWs). High-density arrays of parallel, straight and organized SiNWs have been realized, by Hot Wire Chemical Vapor Deposition (HW-CVD) growth process inside PAA templates with electrodeposited copper as catalyst. The PAA was made by the anodization of an aluminium layer, followed by the catalysts electrodeposition at the bottom of the pores. Subsequently, SiNWs were grown in a modified HW-CVD reactor with SiH 4 as the precursor gas. The morphology and the structure of the wires have been investigated by SEM and TEM, and their collective electrical behavior has been characterized with a 2-probes device.

The aim of this paper is to review our recent results on the growth and optimization of carbon nanotubes (CNTs) and CNT/Zinc Oxide nanostructures and present and discuss their suitability for various applications such as cold cathode electron sources for use in x-ray sources and lighting.

Highly crystalline few-graphene layers were synthesized on poly-nickel, Ni(111) and Ni-deposited substrates by optimizing the mixing ratio of C 2 H 2 /H 2 and C 2 H 4 /H 2 and growth time. The hydrogen effect was investigated to minimize defects and maintain uniformity of the synthesized few-layer graphenes. Using the optimized ratio of hydrogen and acetylene mixture, few graphene layers with large sizes of up to 4 inches in diameter were also synthesized on Ni evaporated Si substrate with different thicknesses and were transferred successfully onto PET film. We also found that the wrinkles, different from inherent ripples, were formed in the graphene layer independent of the location of the grain boundary of poly-Ni substrate and growth conditions. This was attributed to the formation of a step terrace followed by the terrace bunching to result in higher wrinkles due to the thermal mismatch existing between Ni substrate and graphene layers during thermal quenching. A sheet resistance of 233 Ω/sq was obtained at a transmittance of 65%.

Carbon nanotubes and semiconductor nanowires have been thoroughly studied for the future replacement of silicon-based complementary metal oxide semiconductor (CMOS) devices and circuits. However, the organisation of these nanomaterials in dense transistor arrays, where each device is capable of delivering drive currents comparable with those of their silicon counterparts is still a big challenge. Here, we present a novel approach to the organisation of carbon nanotubes and semiconductor nanowires, based on the use of porous lateral alumina templates obtained by the controlled anodic oxidation of aluminium thin films. We discuss the growth of nanomaterials inside the pores of such templates and show the feasibility of our approach. Our first results point to further work on controlling the synthesis of catalyst nanoparticles at the bottom of the pores, these particles being necessary to nucleate and sustain the growth of carbon nanotubes or semiconductor nanowires. To cite this article: D. Pribat et al., C. R. Physique 10 (2009).

The CNTs were synthesized with various growth durations from 10min to 120min by dc-plsam enhanced chemical vapor deposition (dc-PECVD). The mixture of C2H2/NH3 and H2 gases was used for CNT growth. As the growth time is increase, the CNTs show the tendency gathering into the center of catalyst pattern due to the van der waals interaction between the CNTs and bonding to each other. In the 80 min growth, the CNTs show that the ~ 1μm thick diamter and up to 35μm length. We investiged these CNTs by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (TEM-EDS) and Raman. The bonded CNTs was covered by silicon comes from the silicon substrate and nitrogen was incoporated by hiding of catalyst metals. We obtained ratio of N/C and Si/C is ranged 0.25 ~ 0.55, 0.74 ~ 1.19 respectively.

We present some early results on the controlled growth of carbon nanotubes inside lateral porous alumina templates. Such lateral templates provide an easy way of organizing nano-objects in the plane of their supporting substrate, with potential densities of more than 100/µm, thus paving the way for the realization of dense circuits. Here we discuss the growth conditions inside the lateral pores of the templates, with the aim of avoiding the parasitic deposition of amorphous carbon. Our organization method should also apply to other nanostructures such as semiconductor nanowires.

International audience; The increasingly demand on secondary batteries with higher specific energy densities requires the replace- ment of the actual electrode materials. With a very high theoretical capacity (4200 mAh g−1 ) at low voltage, silicon is presented as a very interesting potential candidate as negative electrode for lithium-ion micro- batteries. For the first time, the electrochemical lithium alloying/de-alloying process is proven to occur, respectively, at 0.15 V/0.45 V vs. Li+ /Li with Si nanowires (SiNWs, 200-300 nm in diameter) synthesized by chemical vapour deposition. This new three-dimensional architecture material is well suited to accom- modate the expected large volume expansion due to the reversible formation of Li-Si alloys. At present, stable capacity over ten to twenty cycles is demonstrated. The storage capacity is shown to increase with the growth temperature by a factor 3 as the temperature varies from 525 to 575 ◦ C. These results, showing an attractive working potential and large storage capacities, open up a new promising field of research.

International audience; Aluminium wires are electrochemically sculptured into bi-directional templates for the templated growth and contacting of nanowires as three terminal devices. The use of this nanostructured template is demonstrated by a ZnO nanowire surrounding-gate field-effect transistor. This bottom-up approach to a 3D nanowire transistor is unique in that it can be almost entirely fabricated in a beaker using aqueous, room temperature electrochemistry. The fabrication procedures and preliminary device characteristics of this new approach to nanowire transistors are shown

Modifications of noise performances induced by hot-carrier degradation in polycrystalline silicon thin-film transistors, made by excimer laser crystallization, are presented. In particular, the normalized drain current spectral density of these devices shows an evident 1/f behavior, and as the device characteristics are degraded by prolonged bias stressing, the noise performances worsen. Hot-carrier degradation results in the formation of both interface states, that have been evaluated through the analysis of the sheet conductance, as well as of oxide traps near the insulator/semiconductor interface, as evidenced by the 1/f noise measurements. A strong correlation between interface state and oxide trap densities has been found, suggesting a common origin for the generation mechanism of the two types of defects.

A systematic study of the noise performances of polycrystalline silicon (polysilicon) thin-film transistors (TFTs) made by excimer laser crystallization is presented. The drain current spectral density of these devices shows an evident 1/f behavior and the origin of the noise was attributed to carrier number fluctuations. The flat-band voltage spectral density was found to be strongly correlated with the field-effect mobility, suggesting that the microscopic mechanism causing the carrier number fluctuations involves the localized states present at the grain boundaries. The noise level in the devices with the best electrical characteristics is comparable with that observed in c-Si metal–oxide–semiconductor field effect transistors, a major improvement if compared to polysilicon TFTs made by solid-phase crystallization.

Carbon nano-tubes are alternatives to conventional metal/silicon tips for field emission sources. They exhibit extraordinary field emission properties because of their high electrical conductivity, their high aspect ratio "whisker-like" shape for optimum geometrical field enhancement, and remarkable thermal stability. This paper will review the PECVD growth process, and the micro-fabrication techniques needed to produce well defined carbon nano-tube based micro-electron sources for use in a variety of applications.

Conformal growth is a confined epitaxial lateral overgrowth technique previously used to achieve low dislocation density GaAs films on Si. Conformal growth was used here to obtain high quality InP films on silicon, starting from GaAs seeds. Detailed characterization of the InP films (chemical etching of dislocations, photoluminescence at 300 K, cross‐section transmission electron microscopy) proved the high defect filtering power of this technique. Dislocations initially present in the GaAs seeds and dislocations generated at the InP/GaAs interface were blocked and did not propagate through the growing films. Dislocations densities below 5×105 cm−2 have been obtained in submicrometer‐thick conformal InP films on Si.

A systematic study of the noise performances of polycrystalline silicon thin film transistors is presented. The drain current spectral density of these devices shows an evident 1/ f behavior and scales, when operating in the linear regime, with the square of the mean value of the drain current. The origin of the noise can be ascribed to carrier number fluctuations related to the dynamic trapping and detrapping of the oxide traps.

The strain of conformal LnP films on Si substrate grown by HVPE from GaAs seeds has been investigated by photoluminescence and X-ray diffraction measurements. A weak tensile strain (E = 1.3x10-3 at 300 K) is observed in the GaAs seeds grown by MOCVD. It is induced by the difference of the thermal dilatation of GaAs and Si lattices between growth and room temperatures. In contrast, InP lateral films are found quasi relaxed at 300 K. They present a low density of dislocations because of the defect blocking mechanism inherent to the crystal refining technique. Conformal growth is a promising technique to achieve monolithic III-V devices integration on Si chips. >

In order to utilise the full potential of carbon nanotubes/nanofibers, it is necessary to be able to synthesize well aligned nanotubes/nanofibres at desired locations on a substrate. This paper examines the preferential growth of aligned carbon nanofibres by PECVD using lithographically patterned catalysts. In the PECVD deposition process, amorphous carbon is deposited together with the nanotubes due to the plasma decomposition of the carbon feed gas, in this case, acetylene. The challenge is to uniformly nucleate nanotubes and reduce the unwanted amorphous carbon on both the patterned and unpatterned areas. An etching gas (ammonia) is thus also incorporated into the PECVD process and by appropriately balancing the acetylene to ammonia ratio, conditions are obtained where no unwanted amorphous carbon is deposited. In this paper, we demonstrate high yield, uniform, ‘clean’ and preferential growth of vertically aligned nanotubes using PECVD.

Conformal epitaxy is an epitaxial growth technique capable of yielding low dislocation density III-V films on Silicon. In this technique, the growth of the III-V material occurs parallel to the silicon substrate, from the edge of a previously deposited III-V seed, the vertical growth being stopped by an overhanging capping layer. As an example, conformal GaAs layers on Silicon, presenting dislocation densities below 105cm−2, have been obtained using selective vapor phase epitaxy. These layers have then been used as high quality GaAs on Si substrates for subsequent vertical MBE regrowth of active structures. In this paper, we report on the integration of surface-emitting microcavity LEDs with their silicon drivers using this conformal growth technique. The global technology concept and the design of the active structures are first presented. The compatibility of the conformal growth technique with CMOS technology is then checked: the impact of the integration process on the performances of the drivers is for example quantified. Characterisations of the high crystalline quality of the conformal layers and of the LEDs structures grown on it are then shown. The electro-optical characteristics of the LEDs on Si are finally compared to those of reference LEDs on GaAs substrates in order to prove the efficiency of the integration procedure.

After more than 20 years of steady progress, lithium-ion batteries still exhibit modest energy capacities that seem to have reached their asymptotic values with the present combination of graphite at the anode and insertion oxide or phosphate materials at the cathode. New applications, particularly for all-electric vehicles are pushing the development of electrode materials with higher Li storage capabilities, for both electrodes. Silicon, which exhibits the highest known Li-alloying capacity is one of the most promising anode materials. However, Li alloying with Si is accompanied by a large volume change which induces cracking and rapid pulverization of Si-based anodes. Significant improvements in the anode's lifetime as well as charge–discharge rates have been obtained over the past few years by employing Si nanostructures, particularly nanowires. In this paper, we present the main synthesis methods for Si nanowires as well as the alloying properties of Li with Si and review how the use of Si-based nanowires has evolved, thanks to sophisticated material/structure combinations, including core–shell nanowires, composites as well as hollowed nanotube-like approaches.

Amongst other requirements, a good anode for Li-ion battery applications must exhibit dimensional stability upon Li insertion, as well as chemical inertness with respect to the electrolyte. This latter characteristic is usually provided by the so-called solid electrolyte interphase (SEI), a passivating film that is formed at the end of the first lithiation step, originating from the partial reduction of the electrolyte and Li salt. However, silicon, which exhibits the highest known capacity for Li alloying, possesses none of the above attributes when used as an anode material. Actually, the large volume expansion of Si upon alloying with Li induces a mechanical instability of the SEI film, which therefore fails to provide its protective role. In this paper, we have studied the effect of thin alumina deposits on top of Si-based nanowires. A thin alumina deposit will act as a substitute for the SEI, preventing electrolyte decomposition. We observe that even if alumina films crack during lithiation–delithiation steps of the Si-based nanowires, they still provide some kind of protection, prolonging the lifetime of the anode. Using Al2O3-coated Si-based nanowires, we have been able to obtain a lifetime of 1280 cycles when the capacity of the anode was limited to 1200 mA h g−1. We also show that uncoated Si nanowires degrade more rapidly when cycled under identical conditions.

Electronic transport properties of silicon nanowires grown by chemical vapor deposition, embedded in an insulating alumina nanoporous membrane are studied. Transport measurements were performed from 300to4.2K, which revealed a scaling law of the conductance as a function of the temperature and the dc bias voltage, which the authors interpreted as a Coulomb blockade manifestation. Magnetoconductive measurements at low temperature revealed a positive magnetoconductance which can be well fitted by quasi-one-dimensional (quasi-1D) weak localization theory. These results seem to indicate that electron-electron interactions and quasi-1D effect predominate on the electronic transport properties of these systems.