53 research outputs found
Tunnel current in self-assembled monolayers of 3-mercaptopropyltrimethoxysilane
The current density-voltage (J-V) characteristics of self assembled
monolayers of 3-mercaptopropyltrimethoxysilane (MPTMS) chemisorbed on the
native oxide surface of p+-doped Si demonstrate the excellent tunnel dielectric
behavior of organic monolayers down to 3 carbon atoms. The J-V characteristics
of MPTMS SAMs on Si are found to be asymmetric, and the direction of
rectification has been found to depend upon the applied voltage range. At
voltages < 2.45V, the reverse bias current was found to be higher than forward
bias current; while at higher voltages this trend was reversed. This result is
in agreement with Simmons theory. The tunnel barrier heights for this short
chain (2.56 and 2.14 eV respectively at Au and Si interfaces) are in good
agreement with the ones for longer chains (>10 carbon atoms) if the chain is
chemisorbed at the electrodes. These results extend all previous experiments on
such molecular tunnel dielectrics down to 3 carbon atoms. This suggests that
these molecular monolayers, having good tunnel behavior (up to 2.5 eV) over a
large bias range, can be used as gate dielectric well below the limits of
Si-based dielectrics.Comment: Small, in pres
Electron transport through self-assembled monolayers of tripeptides
We report how the electron transport through a solid-state metal/Gly-Gly-His
tripeptide (GGH) monolayer/metal junction and the metal/GGH work function are
modified by the GGH complexation with Cu2+ ions. Conducting AFM is used to
measure the current-voltage histograms. The work function is characterized by
combining macroscopic Kelvin probe and Kelvin probe force microscopy at the
nanoscale. We observe that the Cu2+ ions complexation with the GGH monolayer is
highly dependent on the molecular surface density and results in opposite
trends. In the case of a high density monolayer the conformational changes are
hindered by the proximity of the neighboring peptides, hence forming an
insulating layer in response to copper-complexation. Whereas the slightly lower
density monolayers allow for the conformational change to a looped peptide
wrapping the Cu-ion, which results in a more conductive monolayer. Copper-ion
complexation to the high- and low-density monolayers systematically induces an
increase of the work functions. Copper-ion complexation to the low-density
monolayer induces an increase of electron transport efficiency, while the
copper-ion complexation to the high-density monolayer results in a slight
decrease of electron transport. Both of the observed trends are in agreement
with first-principle calculations. Complexed copper to low density
GGH-monolayer induces a new gap state slightly above the Au Fermi energy that
is absent in the high density monolayer.Comment: Full paper with supporting informatio
Molecule-Electrode Interface Energetics in Molecular Junction: a Transition Voltage Spectroscopy Study
We assess the performances of the transition voltage spectroscopy (TVS)
method to determine the energies of the molecular orbitals involved in the
electronic transport though molecular junctions. A large number of various
molecular junctions made with alkyl chains but with different chemical
structure of the electrode-molecule interfaces are studied. In the case of
molecular junctions with clean, unoxidized electrode-molecule interfaces, i.e.
alkylthiols and alkenes directly grafted on Au and hydrogenated Si,
respectively, we measure transition voltages in the range 0.9 - 1.4 V. We
conclude that the TVS method allows estimating the onset of the tail of the
LUMO density of states, at energy located 1.0 - 1.2 eV above the electrode
Fermi energy. For oxidized interfaces (e.g. the same monolayer measured with Hg
or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate),
lower transition voltages (0.1 - 0.6 V) are systematically measured. These
values are explained by the presence of oxide-related density of states at
energies lower than the HOMO-LUMO of the molecules. As such, the TVS method is
a useful technique to assess the quality of the molecule-electrode interfaces
in molecular junctions.Comment: Accepted for publication in J. Phys. Chem C. One pdf file including
manuscript, figures and supporting informatio
Contribution à l'électronique moléculaire : de la jonction au composant
The growing number of studies in molecular electronics since several decades is based on the fascinating perspective to use molecular "bricks" for nanoscale electronics. The work presented here take place in this perspective with the particularity to form the molecular system with self-assembled monolayers (SAMs). The summary of this research in molecular electronic over the last 10 years at the IEMN will be presented. During this presentation we will focused specifically on four aspects of these activities. First, we will discuss the problem of the molecular junction formation (metal or semiconductor/ molecule/ metal junctions). Then, we will detailed the experimental realization of various molecular junctions: coplanar electrodes (spaced from 50 µm to 16 nm), shadow mask, micro-nanopore, contact with liquid electrode (eGain and Hg drop) and Conducting AFM. Secondly, we will talk about mechanisms of electron transport through the junction. To do this, we will used the Transition Voltage Spectroscopy (or TVS) to measure the energy offset (i.e. the position of one of the molecular orbitals with respect to the electrode Fermi energy) at the electrode/ molecule interface in a molecular junction. Our approach in this part is based on the analysis by TVS of various molecular junctions formed by different techniques and different molecules. The results will be compared with those obtained by UPS and IPES to estimate the relevance of the TVS. We will see that this work highlights the importance of the interface on the interpretation of the results obtained by TVS. The third aspect will focus on the realization of a molecular component: a field effect transistor with a conductive channel based on a SAM. The fabrication of this component, named Self Assembled Monolayer Field Effect Transistor or SAMFET, will be described. We will see that the mobility obtained with this transistor is close to those obtained on organic transistors with a conductive channel thicker. The device operates at very low bias (less than 2V), which may open the way to low consumption device. The final part will center on the achievement of stimulable molecular junctions, i.e. the conductance of the molecular junction changes under the influence of external excitation. Three aspects will be detailed: first of all, we will compare the conditions of grafting on gold substrate for SAMs consisting of quaterthiophene derived molecules with one or two thiol group(s); then we will examine a SAM based on molecules able to react with Pb2+ and modify its electronic properties; and finally, we will present electro-optical switches. For this last example, the junction consists of a molecule with an azobenzene group. This group can switch optically between two isomers reversibly. These two isomers have different conductance, the average ratio of conductance was measured at about 1.5 103 with a maximum value of 7 103. This conductance ratio between the two isomers remains the highest measured for molecular junctions with derived azobenzene molecule.La croissance du nombre d'études en électronique moléculaire depuis plusieurs décennies repose sur la perpective fascinante d'utiliser des " briques " moléculaires nanométriques pour la fabrication de composants électroniques. Le travail présenté ici s'inscrit dans cette perspective avec comme particularité d'utiliser les monocouches auto-assemblées (les SAMs) pour former le système moléculaire à étudier. La synthèse de ces travaux de recherche en électronique moléculaire durant ces 10 dernières années à l'IEMN sera présentée en se focalisant plus particulièrement sur quatre aspects de ces activités. Tout d'abord, nous aborderons la problématique de la formation expérimentale de la jonction moléculaire (métal ou semi-conducteur/molécules/métal). Dans ce cadre, nous décrirons la réalisation expérimentale de nombreux types de jonctions moléculaires par: électrodes coplanaires (espacées de 50 µm à 16 nm), masque mécanique, micro-nanopore, contact avec une électrode liquide (eGaIn et Hg) et Conducting AFM. Dans un second temps, nous discuterons des mécanismes de transport électronique au sein de la jonction. Pour cela nous étudierons une technique très utilisée depuis quelques années appelée Transition Voltage Spectrocopy (ou TVS), qui permet théoriquement de remonter au niveau d'énergie de l'orbitale moléculaire impliquée dans le transport électronique au sein de la molécule. Notre approche dans cette partie repose sur l'analyse par TVS d'un grand nombre de jonctions moléculaires formées par différentes techniques, et différentes molécules déposées en SAM (en fait 3 familles de molécules). Les résultats obtenus seront comparés à ceux obtenus par UPS et IPES afin d'estimer la pertinence de la technique TVS. Nous verrons que ce travail met en lumière l'importance de l'interface sur l'interprétation des résultats obtenus par TVS. Le troisième aspect traitera de la réalisation d'un composant moléculaire : le transistor à effet de champ, dont le canal conducteur est constitué d'une SAM. La fabrication à l'aide d'électrodes coplanaires de ce type de composant, nommé Self Assembled Monolayer Field Effect Transistor ou SAMFET, sera décrite. Nous verrons que ce transistor donne des valeurs de mobilités comparables à celles obtenues sur des transistors organiques avec un canal conducteur plus épais. De plus, les tensions nécessaires au fonctionnement de ce SAMFET sont très faibles (inférieures à 2V). C'est la première démonstration de SAMFET avec des tensions de fonctionnement proches du volt. Le quatrième et dernier volet portera sur la réalisation de jonctions moléculaires stimulables, c'est-à-dire des molécules dont la conductance change sous l'effet d'une excitation extérieure. Trois aspects seront détaillés : tout d'abord, nous comparerons les conditions de greffage sur substrat d'or pour des SAMs constituées de molécules dérivées quaterthiophène avec une ou deux fonctions thiol ; puis nous étudierons une molécule déposée en SAM capable de réagir avec des cations Pb2+ et modifier ses propriétés électroniques ; et enfin, nous examinerons des jonctions excitables optiquement nommées commutateurs électro-optiques. Pour ce dernier exemple, la jonction est constituée d'une molécule avec un groupement azobenzène. Ce groupement peut basculer optiquement et réversiblement entre deux isomères. Ces deux isomères ont des conductances différentes, le rapport des conductances moyen a été mesuré à environ 1,5.103 et avec une valeur maximum de 7.103. Ce ratio de conductance entre les deux isomères demeure à ce jour le plus élevé mesuré pour des jonctions moléculaires à base de molécules dérivées azobenzène
Groupement de Recherche New Molecular Electronics NEMO Chroniques du GDR NEMO N°2
chroniques du GDR NEMO #
Concentric-Electrode Organic Electrochemical Transistors: Case Study for Selective Hydrazine Sensing
We report on hydrazine-sensing organic electrochemical transistors (OECTs) with a design consisting of concentric annular electrodes. The design engineering of these OECTs was motivated by the great potential of using OECT sensing arrays in fields such as bioelectronics. In this work, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based OECTs have been studied as aqueous sensors that are specifically sensitive to the lethal hydrazine molecule. These amperometric sensors have many relevant features for the development of hydrazine sensors, such as a sensitivity down to 10−5 M of hydrazine in water, an order of magnitude higher selectivity for hydrazine than for nine other water-soluble common analytes, the capability to entirely recover its base signal after water flushing, and a very low operation voltage. The specificity for hydrazine to be sensed by our OECTs is caused by its catalytic oxidation at the gate electrode, and enables an increase in the output current modulation of the devices. This has permitted the device-geometry study of the whole series of 80 micrometric OECT devices with sub-20-nm PEDOT:PSS layers, channel lengths down to 1 µm, and a specific device geometry of coplanar and concentric electrodes. The numerous geometries unravel new aspects of the OECT mechanisms governing the electrochemical sensing behaviours of the device—more particularly the effect of the contacts which are inherent at the micro-scale. By lowering the device cross-talk, micrometric gate-integrated radial OECTs shall contribute to the diminishing of the readout invasiveness and therefore further promote the development of OECT biosensors
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