Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

International audienceElectrical transport in ultrathin Metal-insulator-semiconductor (MIS) tunnel junctions is analyzed using the temperature dependence of current density and admittance characteristics, as illustrated by Hg//C12H25 - n Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to Si(111). The voltage partition is obtained from J(V, T) characteristics, over eight decades in current. In the low forward bias regime (0.2-0.4 V) governed by thermionic emission, the observed linear T-dependence of the effective barrier height, qΦEFF(T) = qΦB+(kT)β0dT, provides the tunnel barrier attenuation, exp(-β0dT), with β0= 0.93 Å−1 and the thermionic emission barrier height, ΦB = 0.53 eV. In the high-forward-bias regime (0.5-2.0 V), the bias dependence of the tunnel barrier transparency, approximated by a modified Simmons model for a rectangular tunnel barrier, provides the tunnel barrier height, ΦT = 0.5 eV; the fitted prefactor value, G0 = 10−10 Ω−1, is four decades smaller than the theoretical Simmons prefactor for MIM structures. The density distribution of defects localized at the C12H25 - n Si interface is deduced from admittance data (low-high frequency method) and from a simulation of the response time τR(V) using Gomila's model for a non equilibrium tunnel junction. The low density of electrically active defects near mid-gap (DS < 2 × 1011 eV−1.cm−2) indicates a good passivation of dangling bonds at the dodecyl - n Si (111) interface

Fonctionnalisation et structuration par microscopie à force atomique (AFM) de surfaces de silicium hydrogéné

Organic monolayers covalently bound to semiconductors allow modulation of their electronic characteristics. In this context, silicon substrates bring high interest and have already been used in microelectronics manufacturing. One's of nanotechnology's challenges is the building of organized assemblies of nano-objects in order to locate and orientate precisely working components for future applications. Among lithography techniques, Scanning Probe Microscopy is a powerful tool to engineer and design nanostructures on surfaces. Here, we present use of Atomic Force Microscopy as a tool for surface patterning and more precisely, the Local Anodic Oxidation with a conducting AFM-tip of silicon surfaces which have been previously passivated by a well-ordered organic monolayer. The generated silicon oxide nanopatterns are then removed by dipping the surface in diluted fluorohydric acid. So, the produced surface contains potentially reactive areas of hydrogenated silicon nanopatterns surrounded by an insulating organic matrix. Secondly, we focuse on the proof-of-concept results of the selective deposition of gold particles in these active patterns. Due to the difference between redox potentials of silicon and gold salt, gold reduced spontaneously on silicon surface. Direct application could be the development of nanoelectrode architectures with well-controlled surface area and geometrical pattern. It could also be possible to use silicon chemistry to directly functionalize the active patterns. The last part of this work deals with the immobilization of electroactive units (ferrocene) and nano-objects (metal clusters and carbon nanotubes) on homogeneous and structured silicon surfaces.Le greffage covalent de monocouches organiques à la surface du silicium permet de contrôler ses propriétés électroniques. Un des challenges des nanotechnologies est de construire des assemblages ordonnés de nano-objets pour positionner et orienter précisément les éléments actifs des futurs dispositifs. Parmi les techniques de lithographie à l'échelle nanométrique, l'utilisation de microscopes à sonde locale est une méthode très versatile pour l'élaboration de nanostructures superficielles. Ces travaux présentent l'utilisation du microscope à force atomique comme un outil de structuration de surface et plus précisément, l'oxydation anodique locale avec une pointe AFM conductrice de surfaces de silicium précédemment passivé par une monocouche organique dense et organisée. L'oxyde de silicium généré est ensuite dissout par trempage dans de l'acide fluorhydrique dilué. Les surfaces obtenues possèdent alors des sites de silicium hydrogéné potentiellement réactifs entourés d'une matrice organique isolante. Ensuite, nous présentons les résultats de dépôt sélectif de particules d'or dans ces structures réactives. De part la différence de potentiel d'oxydoréduction, le sel d'or se réduit spontanément à la surface du silicium. Il serait aussi possible d'utiliser la chimie du silicium pour fonctionnaliser directement les structures de silicium hydrogéné. La dernière partie de cette thèse rapporte l'étude d'immobilisation d'unités électroactives (ferrocènes) et de nano-objets (clusters métalliques et nanotubes de carbone) sur des surfaces de silicium homogènes et structurées

Automated sub-100 nm local anodic oxidation (LAO)-directed nanopatterning of organic monolayer-modified silicon surfaces

International audienceThe fabrication of silicon oxide nanopatterns using the local anodic oxidation (LAO) by a conducting AFM tip of different organic monolayer (decyl, dodecyl, hexadecyl and undecanoic acid)-modified Si(111) surfaces is reported. It is demonstrated that the threshold bias voltage of oxide formation increases with the monolayer thickness and decreases when acid end groups are used instead of methyl. The automatization of the LAO process has allowed the perfect replication of vectorized complex drawings (e.g. compass card and longship) on silicon using the undecanoic acid monolayer as the model molecular resist. Under optimized experimental conditions (namely, bias voltage of 8 V applied to the surface and writing rate of 1 μm s−1), highly reproducible and uniform 2-3 nm thick oxide patterns have been electrogenerated with a minimum lateral resolution of 80 nm. Interestingly, the silicon oxide nanopatterns have been selectively dissolved in diluted hydrogen fluoride solution to generate potentially reactive hydrogenated areas. These hydrogenated sites have then been used for the electroless deposition of gold nanoparticles by galvanic displacement. This approach was found to be convenient and fast to metallize the patterns initially produced by LAO. The average diameter of deposited gold particles decreased from 20 to 2 nm with decreasing the gold salt concentration from 100 to 1 μM

Correction de dérive pour l'interférométrie de Mach-Zehnder

International audience-Les interféromètres de Mach-Zehnder (IMZ) peuvent être utilisés pour mesurer les changements d'indice optique d'une surface. Cela permet de construire par exemple des capteurs chimiques, qui mesurent des changements de la composition chimique d'une surface à travers des changements l'indice optique. Pour ce genre d'applications, il est important que l'IMZ soit correctement calibré. Or, des dérives instrumentales peuvent causer des pertes de calibration, et par là-même biaiser l'interprétation des données. Dans cet article nous proposons une méthode simple de correction en ligne de certains types de dérive instrumentale affectant les IMZ. Notre analyse fait émerger des liens intéressants avec le suivi des courants triphasés, ainsi qu'avec des techniques de factorisation de matrices. Nous appliquons nos résultats à un capteur chimique de type nez artificiel développé par la société Aryballe

Real-time gas recognition and gas unmixing in robot applications

International audienceRobot olfaction takes inspiration from animals for locating a gas source in the environment, such as a gas leak to fix or an explosive to neutralize. In these cases, gas sources emit Volatile Organic Compounds (VOCs) which can be measured with an electronic nose. This instrument can detect a broad variety of VOCs, so the same device can then be used for many different applications. In a realistic environment, several VOCs of interest can be present at the same time and mix. This creates difficulties for gas recognition, and in the literature, the problem is often ignored. In this article, we deal both with gas recognition of a large number of VOCs and gas unmixing. For that, we use a recently developed optoelectronic nose which uses peptides as sensing materials and Surface Plasmon Resonance imaging as transduction method. We present two different setups. The first setup studies the recognition of 24 gas sources of 12 VOCs disseminated in the environment. The second setup studies various realistic scenarios in which mixtures occur, due to gas sources being spatially close. We propose a real-time dictionary-based algorithm for dealing both with gas recognition and gas unmixing. We succeed in obtaining a score of 73% for the gas recognition task, meaning that 73% of the 24 gas sources have been well identified over several runs. For the unmixing issue, we correctly identify the VOCs composing the mixtures. However, we also show that this performance is strongly related to the VOCs used in the dictionary

Correction of 2π phase jumps for silicon photonic sensors based on Mach Zehnder Interferometers with application in gas and biosensing

International audienceSilicon photonic sensors based on Mach Zehnder Interferometers (MZIs) have applications spanning from biological and olfactory sensors to temperature and ultrasound sensors. Although a coherent detection scheme can solve the issues of sensitivity fading and ambiguity in phase direction, the measured phase remains 2π periodic. This implies that the acquisition frequency should ensure a phase shift lower than π between each measurement point to prevent 2π phase jumps. Here, we describe and experimentally characterize two methods based on reference MZIs with lower sensitivities to alleviate this drawback. These solutions improve the measurement robustness and allow the lowering of the acquisition frequency. The first method is based on the phase derivative sign comparison. When a discrepancy is detected, the reference MZI is used to choose whether 2π should be added or removed from the nominal MZI. It can correct 2π phase jumps regardless of the sensitivity ratio, so that a single reference MZI can be used to correct multiple nominal MZIs. This first method relaxes the acquisition frequency requirement by a factor of almost two. However, it cannot correct phase jumps of 4π, 6π or higher between two measurement points. The second method is based on the comparison between the measured phase from the nominal MZI and the phase expected from the reference MZI. It can correct multiple 2π phase jumps but requires at least one reference MZI per biofunctionalization. It will also constrain the corrected phase to lie in a limited interval of [−π, +π] around the expected value, and might fail to correct phase shifts above a few tens of radians depending on the disparity of the nominal sensors responses. Nonetheless, for phase shift lower than typically 20 radians, this method allows the lowering of the acquisition frequency almost arbitrarily

Correction of 2<i>π</i> Phase Jumps for Silicon Photonic Sensors Based on Mach Zehnder Interferometers with Application in Gas and Biosensing

Silicon photonic sensors based on Mach Zehnder Interferometers (MZIs) have applications spanning from biological and olfactory sensors to temperature and ultrasound sensors. Although a coherent detection scheme can solve the issues of sensitivity fading and ambiguity in phase direction, the measured phase remains 2π periodic. This implies that the acquisition frequency should ensure a phase shift lower than π between each measurement point to prevent 2π phase jumps. Here, we describe and experimentally characterize two methods based on reference MZIs with lower sensitivities to alleviate this drawback. These solutions improve the measurement robustness and allow the lowering of the acquisition frequency. The first method is based on the phase derivative sign comparison. When a discrepancy is detected, the reference MZI is used to choose whether 2π should be added or removed from the nominal MZI. It can correct 2π phase jumps regardless of the sensitivity ratio, so that a single reference MZI can be used to correct multiple nominal MZIs. This first method relaxes the acquisition frequency requirement by a factor of almost two. However, it cannot correct phase jumps of 4π, 6π or higher between two measurement points. The second method is based on the comparison between the measured phase from the nominal MZI and the phase expected from the reference MZI. It can correct multiple 2π phase jumps but requires at least one reference MZI per biofunctionalization. It will also constrain the corrected phase to lie in a limited interval of [−π, +π] around the expected value, and might fail to correct phase shifts above a few tens of radians depending on the disparity of the nominal sensors responses. Nonetheless, for phase shift lower than typically 20 radians, this method allows the lowering of the acquisition frequency almost arbitrarily

A calibrant-free drift compensation method for gas sensor arrays

International audienceGas sensors lack repeatability over time. They are affected by drift, the result of changes at the sensor level and in the environment. A solution is to design software methods that compensate for the drift. Existing methods are often based on calibration samples acquired at the start of each new measurement session. However, finding a good reference compound is a difficult task and generating calibration samples is time-consuming. We propose a model-based correction method which does not require any calibration sample over time, operating 'blindly'. In this study, we focus on the drift affecting electronic noses. To this end, we built a real data set acquired over 9 months in real-life conditions. By using the proposed method, we show that the drift is partly compensated, thus increasing the reliability of the electronic nose. Besides, we also show that the algorithm can easily adapt if the target compounds are not all sampled during every session

Real-time gas recognition and gas unmixing in a robot application

Robot olfaction takes inspiration from animals for locating an object in the environment. This object can be a gas leak to fix or an explosive to neutralise. In these cases, the objects will emit Volatile Organic Compounds (VOCs) which can be measured with an electronic nose. This instrument has the great advantage of being able to detect a broad variety of VOCs, so the same device can then be used for a lot of different applications. In a realistic environment, several VOCs of interest can be present at the same time and mix. This creates difficulties for gas recognition, and in the literature, the problem is often ignored. In this article, we deal both with gas recognition of a large number of VOCs and gas unmixing. For that, we use a recently developed optoelectronic nose which uses peptides as sensing materials and Surface Plasmon Resonance imaging as transduction method. We present two different setups. The first setup studies the recognition of 24 gas sources of 12 VOCs disseminated in the environment. The second setup studies various realistic scenarios in which mixtures occur, due to gas sources being spatially close. We propose a real-time dictionary-based algorithm for dealing both with gas recognition and gas unmixing. We succeed in obtaining a score of 73% for the gas recognition task. For the unmixing issue, we reach at least 72% but we also show that this performance is strongly related to the VOCs used in the dictionary