Coulomb blockade in a granular material made of gold nanowires

Uniform, 2 nm diameter gold nanowires were synthesized through the reduction of gold(III) chloride in an oleylamine matrix. They were top-contacted on a Si/SiO2 substrate with metallic electrodes to manufacture back-gated transistors. Due to thermal breakage, the gold nanowires were fragmented into a granular material and the non-linear current-bias voltage characteristics measured on the devices from 7 K up to 300 K were described by the Coulomb blockade theory in a nearly one-dimensional quantum dot array. The electronic transport was governed by sequential tunneling at an applied bias above the global Coulomb blockade threshold, whereas in the Coulomb blockade regime, inelastic cotunneling was dominant up to 70 K, at which point it crossed over to activated behavior. The current dependence on the gate voltage that showed irregular oscillations was explained by the superimposition of Coulomb oscillation patterns generated by each different dot in the one-dimensional array. The competitive effects of excitation energy and stochastic Coulomb blockade balanced the number of current peaks observed

Extended gate field-effect-transistor for sensing cortisol stress hormone

Cortisol is a hormone released in response to stress and is a major glucocorticoid produced by adrenal glands. Here, we report a wearable sensory electronic chip using label-free detection, based on a platinum/graphene aptamer extended gate field effect transistor (EG-FET) for the recognition of cortisol in biological buffers within the Debye screening length. The device shows promising experimental features for real-time monitoring of the circadian rhythm of cortisol in human sweat. We report a hysteresis-free EG-FET with a voltage sensitivity of the order of 14 mV/decade and current sensitivity up to 80% over the four decades of cortisol concentration. The detection limit is 0.2 nM over a wide range, between 1 nM and 10 mu M, of cortisol concentrations in physiological fluid, with negligible drift over time and high selectivity. The dynamic range fully covers those in human sweat. We propose a comprehensive analysis and a unified, predictive analytical mapping of current sensitivity in all regimes of operation.Wearable, real-time devices that can monitor hormones are important for personalized healthcare. Here, a platinum/graphene aptamer extended gate field effect transistor is shown to detect cortisol, the primary stress hormone, in physiological fluid

Carbon nanotube gas sensor array for multiplex analyte discrimination

The lack of selectivity toward a particular analyte has always been the primary concern regarding CNT-based gas sensors. For that reason, in here we present a gas discrimination strategy that focuses on the electrode-CNT junction. The junction is shown to play a key role in the sensing mechanism. Resistive gas sensors based on horizontal CNT arrays have been fabricated using various designs and different top-contacting metals: Pt, Pd and Au. Arrays of devices have been exposed to a series of gases to monitor their resistive response. It was found for our system that the sensor response does not significantly change as a function of the device design or the available CNT sensing area in between the anchoring electrodes. On the contrary, responses to gases are observed to be specific to each sensor electrode metal. Exposure of locally passivated devices (for which distinct areas have been covered) to NO2, H-2 and NH3 highlights different sensing mechanisms for each gas. Multiplex gas discrimination for room temperature can be achieved by strategically choosing the right metal/CNT combination in a complete sensor system. (C) 2014 Elsevier B.V. All rights reserved

High-yield, in-situ fabrication and integration of horizontal carbon nanotube arrays at the wafer scale for robust ammonia sensors

This paper reports the successful experimental demonstration of the localized growth of horizontal, dense carbon nanotube (CNT) arrays in situ and at the wafer scale. The selectivity and directionality of the CNT catalytic growth process along with the adequate design and fabrication of the catalyst support enables the direct integration of nanotubes arrays into heterogeneous devices. This novel CNT integration method is developed to manufacture conductance based gas sensors for ammonia detection and is demonstrated to produce a yield above 90% at the wafer scale. Owing to its flexibility, the integration process can be useful for a wide range of applications and complies with industrial requirements in terms of manufacturability and yield, requirements for the acceptance of CNTs as alternate materials. A state-of-the-art CNT array resistivity of 1.75 x 10(-5) m has been found from the CNT characterization. When exposed to low NH3 concentrations, the CNT sensors show good repeatability, long-term stability, and high design robustness and tackle the reproducibility challenge for CNT devices. Individual device calibration is not needed. The ammonia adsorption isotherm on the sensor is well fitted by Freundlich equation. The extrapolated detection limit is about 1 ppm. The dependence of the sensitivity with temperature indicates that ammonia sensing is likely to involve an endothermic process. Finally, relative humidity cross sensitivity has been found to have no adverse effect on the ammonia response enabling NH3 monitoring in ambient conditions. (C) 2014 Elsevier Ltd. All rights reserved