Hélène Le Poche

Tunable MEMS capacitors using vertical carbon nanotube arrays grown on metal lines

In this work, tunable MEMS capacitors are realized using a vertically grown carbon nanotube array. The vertical CNT array forms an effective CNT membrane, which can be electrostatically actuated like the conventional metal plates used in MEMS capacitors. The CNT membrane is grown on titanium nitride metal lines, with a Al/Fe bi-layer as buffer layer and catalyst material respectively, using chemical vapor deposition process. Two different anchor configurations are investigated. A maximum capacitance of 400 fF and maximum tunability of 5.8% is extracted from the S-parameter measurements. Using the tunable MEMS vertical array capacitor a voltage controlled oscillator (VCO) is demonstrated showing promise for integrating CNTs for communications applications.

Impact of a van der Waals interface on intrinsic and extrinsic defects in an MoSe2 monolayer

In this work, we study growth and migration of atomic defects in MoSe2 on graphene using multiple advanced transmission electron microscopy techniques to explore defect behavior in vdW heterostructures. A MoSe2/graphene vdW heterostructure is prepared by a direct growth of both monolayers, thereby attaining an ideal vdW interface between the monolayers. We investigate the intrinsic defects (inversion domains and grain boundaries) in synthesized MoSe2, their evolution amid growth processing steps, and their influence on the formation and movement of extrinsic defects. Electron diffraction identifies a preferential interlayer orientation of 2° between MoSe2 and graphene, which is caused by the presence of intrinsic IBD defects. Extrinsic defects (point and line defects) are generated by in situ electron irradiation in the MoSe2 layer. Our results shed light on how to independently modify the MoSe2 atomic structure in vdW heterostructures for potential utilization in device processing.

Carbon nanotube resistors as gas sensors: Towards selective analyte detection with various metal-nanotubeinterfaces

This paper reports the fabrication of conductance-based gas sensors based on horizontal, dense carbon nanotube (CNT) arrays directly integrated between metal electrodes via a selective and directional catalytic growth process performed in-situ and at the wafer-scale. We originally propose to use a functionalization strategy focusing on the key role of the electrode-CNT junction, for increased selectivity. Devices based on CNT arrays have been fabricated using various designs and different metals: Pt, Pd and Au, to top-contact the CNT arrays so as to provide several types of metal-CNT junctions. The sensors have been exposed to low concentrations of H2 as well as NH3, toluene, ethanol and relative humidity. We demonstrate that the CNT resistors responses to gases are hardly dependent on the design, the CNT surface developed between the electrodes, but very specific to the metal-CNT interface. Therefore, we postulate that gases interact in a metal-specific way and modify the metal-CNT contact resistance of the resistors but the device sensitivity cannot simply be ascribed to the modulation of Schottky barriers as reported in literature. Since the main issue with CNT sensors is their lack of selectivity, we believe that this method provides an easy to implement strategy and shows potential for room temperature discriminative gas detection.