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International audience; Ordered porous anodic alumina (PAA) templates are of great interest as they facilitate the future development of nanodevices. The present study focuses on the impact of substrates with different crystallographic orientations on the template's pore structure. Characteristics such as pore diameter, interpore distance, pore regularity, porosity, and circularity are calculated as a function of the anodization potential for three different Al crystal orientations. The presented experiments reveal that the different crystallographic orientations mainly impact the pore ordering, while other structural parameters, such as the pore diameter and interpore distance, are not significantly affected.

International audience; The synthesis of carbon nanotubes (CNTs) within porous anodic alumina (PAA) templates requires better understanding regarding their dynamic growth evolution, and how this is influenced by the geometrical features of the PAA. The growth of nanostructures, such as CNTs, becomes more complex when an exponential voltage decrease process is applied to thin the dielectric layer of the PAA matrix, because this process leads to the formation of a branched structure at the bottom of the pores. Here, we present direct evidence of the impact of the branched structure at the bottom of the PAA at different time-stages of the CNT synthesis. These studies reveal that when numerous branches are created, competition between the growing nanotubes is established during their formation. Additionally, large pores lead to flattened catalyst electrodeposition, promoting tube entanglement since more than one tube can grow in each pore. Interestingly, two different electrical behaviors are measured when considering PAA/CNT devices: linear response in the case of CNTs connecting two parallel electrodes, and nonlinear when junctions between the tubes are being promoted. These results highlight the relevance of having an in-depth understanding of the CVD growth evolution of carbon nanostructures within PAA templates, and simultaneously serve as a guiding procedure towards the fabrication of devices based on parallel organized CNT arrays.

The effect of atomic hydrogen assisted pre-treatment on the growth of vertically aligned carbon nanotubes using hot-filament chemical vapor deposition was investigated. Iron nanoparticle catalysts were formed on an aluminum oxide support layer by spraying of iron chloride salt solutions as catalyst precursor. It is found that pre-treatment time and process temperature tune the density as well as the shape and the structure of the grown carbon nanotubes. An optimum pre-treatment time can be found for the growth of long and well aligned carbon nanotubes, densely packed to each other. To provide insight on this behavior, the iron catalytic nanoparticles formed after the atomic hydrogen assisted pre-treatment were analyzed by atomic force microscopy. The relations between the size and the density of the as-formed catalyst and the as-grown carbon nanotube's structure and density are discussed.

High performance optoelectronic devices based on cup-stacked carbon nanotubes are realized. Based on a Schottky barrier model, rectifying behaviors and high photoresponse are observed by using growth catalysts as nanoscale electronic contacts. Similar performances are also obtained and furthermore tuned by using nanotube’s defective surface as effective decoration sites transforming nanotubes resistors into Schottky diodes.

International audience; Since their discovery in 1991 carbon nanotubes (CNTs) become one of the most emblematic type of nanostructures due to their unique electrical, mechanical, thermal and optical properties. Despite these promising and astonishing properties, the critical subject of their manipulation remains a primordial challenge. Very expensive and time consuming techniques, like e-beam lithography, have been developed in order to precisely control their growth, or to obtain a nano-object from the etching of a bulk material. Such techniques are suitable as proofs of concept; however the object-by-object manipulation approach is unrealistic because they are not compatible with a massive integration. On the other hand the miniaturization of the fabricated devices is reaching a bottleneck since the scaling-down for enhancing their performance is approaching many physical limitations. The efforts concentrated to achieve device fabrication by continuous miniaturization are nowadays struggling with performance decline in the electrical conductivity transports values and a significantly loss of reliability. Self-organized templates such as porous anodic alumina (PAA) templates provide several advantages for controlling the nanostructures growth.[1,2] Its well-ordered structure and the confinement imposed by the nanopores the PAA template offer a promising approach for cost-effective, stable and efficient fabrication of carbon nanotubes based devices.[3] Here we present a complete 2D analysis based on advanced electron microscopy techniques devoted to the full characterization of both the PAA structure and the as-grown CNTs using a dHF-CVD (double-Hot Filament assisted CVD) synthesis method. More exactly, we combine the FIB (Focused Ion Beam) preparation technique with advanced TEM characterization techniques such as STEM-EDX and EELS spectroscopy for the assessment of an accurate correlation between the synthesis parameters and the morphological, structural and chemical characteristics of both the PAA structure and the as-grown CNTs. In a first step, the TEM analysis of different PAA cross-sections prepared using the FIB technique, allowed us accessing precise characteristics such as the pore length (800nm) and their diameter (30nm) as well as the inter-pore distance (30nm) (see figure1). A more detailed analysis on the bottom part of the PAA structure helped us evidencing the presence of a branched nanopores structure product of an exponential voltage decrease process, applied in order to thin the oxide barrier layer at the bottom of the pores. For the CNTs, we first examined the impact of the catalyst pretreatment step performed prior to the CNTs growth step. Secondly, by varying the hot-filaments power applied during the growth, we investigated the impact of the additional gas phase activation conditions over the synthesized carbon nanostructures. The results revealed that the pretreatment conditions determine the catalyst distribution at the bottom pores of the PAA membranes, with a strong impact on the CNTs growth within the PAA templates. Another important finding concerns the amount of defects incorporated into the grown CNTs walls which could be related to the hot-filament power applied during the synthesis.

International audience; Laser pulses can effectively induce local structural changes and modify the physical properties of carbon allotropes. So far, only graphitization has been demonstrated using low laser energies (≤1 J/cm2). The novelty of this paper is a result of laser-induced amorphization of a highly anisotropic carbon allotrope by using high energy (1.5–15.4 J/cm2) 5 ns, 532 nm Nd-YAG laser pulses. Moreover, cycling phase change, between an amorphous and a crystalline phase, is also obtained by adjusting the pulse energy. However, cycling ability is restricted to a few cycles as a consequence of laser-induced surface damages caused by both high temperatures during and high thermal gradients during and after laser exposure. The occurrence of graphitization or amorphization depends on the amount of solid crystalline seeds during solidification from the melt, which is controlled by the post-pulse temperature of the carbon surface. This study uncovers new applications of carbon allotropes, such as optically-controlled reversible phase-change memories.

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.

In spite of numerous efforts for developing the applications of graphene, it remains difficult to put the remarkable physical properties of this material into devices. This is mainly due to the fact that large-area (industrial) graphene includes in its structure and on its surfaces a significant density of defects that make as many traps and scattering centres for charge carriers. The idea of the present work, contrary to diminishing the defect density, is to use the defects and the very large surface to volume ratio of that 2D material, to transform it into high sensitivity sensors. When defects are useful, low-temperature growth becomes the method that best satisfies both physical and financial demands. Here, we further decrease preparation costs by performing growth not only at low temperature directly on the final insulating substrate (glass), but also by printing the device contacts by ink-jet printing. Graphene layers actually develop at the interface between a metallic catalytic film and the insulating substrate during plasma-enhanced chemical vapour deposition (PE-CVD).1,2. Resistivity of the graphene foils was measured by the four-point methods using ink-jet printed electrods, and a resistivity as low as 820 ohms/sq were obtained. Moreover, the sensitivity of such graphene foils to water vapour was evaluated, with the prospect to use them in humidity sensors for civil engineering. In this presentation, we explain how graphite may precipitate at the interface in addition to the surface.2,3 Then we show examples of graphene obtained at temperatures in between 450 and 550°C, on glass (Fig.), fused silica, alumina and SiO2//Si. Transmission electron microscopy indicates that the structure is nanocrystalline. We finally show the humidity response of the fabricated device. Results seem to indicate that high-defect density, thin deposits are more sensitive to water vapour than thicker ones.

Highly densely packed, self-organized silicon nanowires with very narrow diameter distribution were synthesized within porous anodic alumina templates with electrodeposited catalytic metal nanoparticles. For successful catalytic metal nanoparticle deposition, electrochemical-, and chemical barrier layer thinning process was investigated following anodization process. Controlled pulsed electrodeposition process was carried out for a volume calibration of desired catalytic metal nanoparticle deposition inside nanopore arrays using different metal-ion containing electrolyte. Not only single metal nanoparticles, but also multi metal nanoparticles layers were filled inside PAA to enhance metal filling aspect, and to control the volume of nanoparticles more precisely. Using multilayered metal nanoparticles resulted on different SiNW’s growth behavior depending on the types of underlying metal nanoparticles.SiNWs were successfully synthesized using hot-filament assisted chemical vapor deposition system. Although silicon precursor gas can generally be dissociated at relatively low temperatures, the use of a hot filament activation help decreasing process temperature, and also, highly activated atomic hydrogen generation via the tungsten hot filament placed at gas inlet helps preventing parasitic amorphous silicon deposition on either the alumina membrane surface or the pore wall which hinders appropriate growth of SiNWs in PAA by nanopores clogging. Such densely packed, self-organized SiNWs are of high interest in many application fields like nanoelectronics, optoelectronics, and energy storage/conversion devices etc.

A novel vertical nanoporous structure is reported as a starting point for the fabrication of a fully-surround gate field effect transistor (FET) based on well-ordered nanostructures array. The proposed porous stacking is perfectly suited both for the collective organization of high density (up to 1011.cm-2) arrays of nanostructures like nanowires (NWs) or nanotubes (NTs), as with calibrated diameters (during growth), as well as for easing the Source, Gate, and Drain electrodes connections for individual or groups of nanostructures. Moreover the unique fully-surround gate architecture enables a quasi-ideal coupling between the gate and the channel, theoretically leading to improved devices performance and reduced global power consumption.In this paper we describe the main steps for this versatile and lithography-free technique to fabricate a multi-layer porous template down to the nanometer scale, as well as the first nanostructures (carbon NTs) growth attempts inside such functional template. We highlight the fact that the proposed porous structure may acts as a passive template for the one-dimensional nanomaterials growth as well as an active element in the future device.The proposed approach is in line with bottom-up fabrication approach to provide smaller devices, and is fully-compatible with classical processes used in the silicon industry.

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; 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.

International audience; Porous alumina templates have been fabricated by applying an exponential voltage decrease at the end of the anodization process. The time constant η of the exponential voltage function has been used to control the average thickness and the thickness distribution of the barrier layer at the bottom of the pores of the alumina structure. Depending on the η value, the thickness distribution of the barrier layer can be made very uniform or highly scattered, which allows us to subsequently fine tune the electrodeposition yield of nickel nanoparticles/nanowires at low voltage. As an illustration, the pore filling percentage with Ni has been varied, in a totallyreproducible manner, between ∼3 and 100%. Combined with the ability to vary the porediameter and repetition step over ∼2 orders of magnitude (by varying the anodization voltageand electrolyte type), the control of the pore filling percentage with metal particles/nanowires could bring novel approaches for the organization of nano-objects.

International audience; Post-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq-1. Finally the samples equipped with printed contacts appear to be efficient humidity sensors.