Transistors à base de fils moléculaires nanométriques isolés et fonctionnalisés : synthèse et applications aux capteurs chimiques – TRANSFILSEN

The project involved physics and chemistry teams. A new nanometers sized molécules serie has been synthesized by chemists for their relevance in molecular electronics using atom economy efficient synthetical methods. Studies of these objects will improve fundamental knowledge about electronic properties of organic molecules. ¨Physicits teams evidenced unprecedented selfassembling properties in order to achieve micrometer sized semiconductors. Studies enabled to improve technologies to insert molecules in electric devices by fabricating nanometric gold connectors. New pathways to connect molecules have been realized. Selfassembled gold nanobulbs have been made by epitaxial growing to create suitable interelectrodes distances with molecular nanowires lengths; lithography techniques on these structures to introduce metallic contacts and real time control by atomic force
microscopy of an electromigration made nanogap are additional promising results to fabricate in a very short future nanotransistors based on these molecules. A new method for alcohols sensing, based on macroscopic transistors characteristics evolution, has been worked out.

Chemists prepared about 10 nm long molecular wires. When the molecular size increased, synthetic difficulties due to the solubility and to slower reactions made the ambitious objective more difficult to reach.
Owing to a larger synthetic tools panel, an homogeneous panel pf molecular wires ranging from 1,2 nm to 11,8 nm long were elaborated. With suitable purification methods, new molecules, synthesized in
allowable quantities regarding their complexity were produced. They are also relevant for other applications. Nano-devices fabrications, the above molecule being the involved active material induced a novel conception regarding macroscopic transistors design: the interelectrodes gap should be compatible with nanowires lengths, of about 10 nm wide and the nanogaps shape ought to be obtained in a controlled manner. These two aspects were actual challenges the solutions of which were investigated according to several ways. Gold nano-blugs epitaxial growing over a
planar and isolating surface, compatible withfollowing deposits of electrical contacts on their surface led to selg assembled gold nanogaps on the surface; the inter-gold blugs distance is temperature controlled and is suitable with the lengths of the above molecular nanowires. Other ways based on electronic lithography or on gold electrodeposition under atomic force microscopy enabled to create suitable shaped and sized nanogaps. Moreover, the fundamental mechanism of electromigration created nanogap was elucidated. Independently, self-assembling properties of these molecular wires were evidenced and revealed how the above molecular wires were relevant for investigated charge transport properties. In order to mimic these unprecedented wires self-assembling properties recorded on grapheme, monocrystals were elaborated and are under
conductivity measurements. For the moment, electrical characterizations of these molécules were realized in macroscopic transistors and alcohols sensors were worked out.

This project perspectives and benefits lie in different fields: From a fundamental knowledge point of view, the prepared molecules constitute an homogeneous serie and are studies objects which enable charge transport investigations related to their length. Such study should
inform about the conduction mode in relation to the organic semi-conductor length. This work perspective has yet presently been
initiated on these wires monocrystals the conductivity properties of which being studied owing to near field microscopies. To differently investigate transport properties and molecules size relationships, another evaluation method is under progress in relation with a swedish team: gold nanoparticles are functionalized by molecular wires and the whole lot is inserted in a shape controlled larger electrodes gap. From a technological and applicative point of view, significative breakthroughs to fabricate in a controlled manner nanometric sized transistors will be of great importance
to develop nanometric sized, sensors which are smaller and more sensitive than theirlarger analogs. Applicative fields are very wide, ranging from environment with gaseous pollutants detection for example to safety
with explosives detection (this is an evidenced result of the project) and to health by adapting the sensitive material to bioanalytes sensing. Costs will be lowered and a better prevention will be envisaged. In health field, the molecular wires will be valorized owing to their emitting and biphotonic absorbing optical properties for medical bioimaging or for drug delivery in
cancer therapy.

The consortium scientific production is made of articles in international journals with lecture committee (4 articles published now
and at least five other papers will be submitted) and of conferences or numerous oral or poster communications in national or international meetings dedicated to chemistry or physics. This scientific production is single partner or involves two or three partners. Other studies that were not considered at the beginning of the project have been done: these wires biphotonic absorption or their use as nitroaromatic sensors have been yet published or are going to be published.

The project aims at synthesising, characterising and implementing into electronic devices rigid semiconducting nanowires which are about 10 nm long and whose molecular structure attenuates intermolecular interactions. These electrically insulated wires will be the functional building blocks to fabricate, in a controlled and reproducible manner, nanometric molecular transistors. The electric isolation due to negligible Pi-stacking interactions will allow us to use these devices for intramolecular charge transport studies and for applications in the field of chemical sensors. The molecular wire synthesis will be done from various pi-conjugated sequences. Several strategies will be considered in order to decrease the intermolecular coupling and thereby enhance the 1D intramolecular nature of charge transport: introduction of sterically hindered or bulky groups, nanowires sheathed by cyclodextrins, chiral conjugated oligomers whose secondary structure prevents Pi-stacking. Also, nanowire functionalisation with complementary molecular recognition patterns will be used to induce their self-assembling into a network of parallel wires. The molecular conformation, the intra-molecular electronic delocalisation, and the nanowire length and self-assembling behaviour will be studied by spectroscopic scanning tunneling microscopy. Several metallic electrode nanogap fabrication methods will be implemented to obtain suitable electrode shapes: electron beam lithography, electromigration.and electrodeposition. Metal/molecular wires/metal elemental components will be prepared. The approximately 10nm long semiconducting channel will be formed by molecular wires bonded to the metal electrodes by specific chemical moieties. The device electrical characterisation will yield new information about 1D intramolecular charge transport. A gate electrode will be implimented in order to form a field-effect transistor structure. At last, the adsorption of several chemical species onto the molecular channel and the resulting effect on the device.