Nanowire-based active matrix backplanes for the control of large area X-ray imagers

International audienceIn digital X-ray sensors, pixel complexity is limited by the instabilities of amorphous silicon transistors or by the lack of homogeneity of their polycrystalline silicon counterparts. Here, we present a novel approach to the fabrication of thin-film transistors, based on the use of silicon nanowires grown in porous alumina templates. Transistors made from silicon nanowires are essentially studied for the post-MOS era and they exhibit excellent transport characteristics. The technology we propose is simple, as it only employs chemical vapour deposition processes for transistor fabrication. Also, as far as active matrix fabrication is concerned, a small number of masks is needed and we present a 5 mask process for the fabrication of an X-ray panel with pixel amplification

On the role of activation mode in the plasma- and hot filaments-enhanced catalytic chemical vapour deposition of vertically aligned carbon nanotubes

International audienceCatalytic chemical vapor deposition (CCVD) with different activation modes (thermal; hot filaments-enhanced; direct current plasma-enhanced and both hot filament and direct current plasma-enhanced) are achieved in order to grow vertically aligned carbon nanotubes (VA CNTs). By widely varying the power of the different activation sources of the gas (plasma, hot filaments, substrate heating) while keeping identical the substrate temperature (973 K) and the catalyst preparation, the results point out the important role of ions in the nucleation of carbon nanotubes (CNTs), as well as the etching behaviour of highly activated radicals such as H˙ in the selective growth of vertically aligned films of CNTs. Moreover, it is demonstrated that, within the deposition conditions (temperature, pressure, flow rate) used in this study, oriented carbon nanotubes can be grown only when both ions, mainly generated by the gas discharge plasma, and highly reactive radicals, mainly formed by the hot filaments, are produced in the gas phase. We propose that highly energetic ions are needed to nucleate the carbon nanotubes by increasing the carbon concentration gradient whereas the highly reactive radicals allow the selective growth of vertically aligned CNTs by preventing carbon deposition on the whole surface through chemical etching of edge carbons in graphene sheets

A new vertical nanoporous functional structure process fabrication to control one dimensional nanostructure growth

International audienceA 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

Porous Alumina Template based Versatile and Controllable Direct Synthesis of Silicon nanowires

International audienceHighly 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

High-quality Single-walled carbon nanotubes synthesis by hot filament CVD on Ru nanoparticule catalyst

International audienceWe investigated the single-walled carbon nanotubes (SWCNTs) growth on Ru nanoparticle catalyst via hot filament assisted chemical vapor deposition (HFCVD) with two independent W filaments for the carbon precursor (methane) and the hydrogen dissociation respectively. The Ru nanoparticles were obtained following a two-step strategy. At first the growth substrate is functionalized by silanisation, then a self assembly of a ruthenium porphyrin complex monolayer on pyridine-functionalized metal oxide substrates. We have studied the impact of the filaments power and we optimized the SWCNTs growth temperature. The as grown SWCNTs were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. It was found that the quality, density and the diameter of SWCNTs depends on the filament and growth temperature. Results of this study can be used to improve the understanding of the growth of SWCNTs by HFCVD

Laterally organized carbon nanotube arrays based on hot-filament chemical vapor deposition

International audienceLateral 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

Growth mechanisms of carbon nanotrees with branched carbon nanofibers synthesized by plasma-enhanced chemical vapour deposition

International audienceY- 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

Graphene-based resistive humidity sensor for in-situ monitoring of drying shrinkage and intrinsic permeability in concrete

Nanosensors dedicated to the structural health monitoring of concrete structures have been only marginally studied. They would however be particularly well-suited to monitor durability-related processes, as these phenomena involve transport of gas and liquids through micro and nano-porosity. In this paper we discuss the relevance and feasibility of embedding rela-tive humidity nanosensors within concrete. It appears that the localized, continuous knowledge of relative humidity within a concrete structure could provide a useful insight into drying shrinkage; it could also contribute to improved intrinsic permeability measurements, leading to improved assessment of structural durability. For the task, we propose a low-cost, downscalable resistive device made of a 10 nm graphene sheet grown directly on glass and atop which are ink-jet printed silver electrodes. The device resistance increases significantly with relative humidity (RH), especially above 40% RH. Relative amplitude of variations are only of about 3% for the two tested devices, but absolute variations (80 Ohms/sq and 480 Ohms/sq) appear measurable by a low-cost and robust signal conditioning electronics. Thus, the idea of using our graphene-based resistive device for embedded humidity monitoring in concrete ap-pears quite promising

All-printed infrared sensor based on multiwalled carbon nanotubes

International audienceThis contribution deals with all-printed infrared sensors fabricated using multiwalled carbon nanotubes deposited on a flexible polyimide substrate. A high responsivity of up to 1.2 kV/W is achieved at room temperature in ambient air. We evidence a strong dependence of the device transduction mechanism on the surrounding atmosphere, which can be attributed to bolometric effect interference with water molecule desorption upon irradiation