Deliverables Synthesis of functional conjugated molecules with thiol end groups We have developed the synthesis of new long-chain organic molecules with lengths of between 3 and 5nm that can span nanogaps. These molecules have been designed to make them soluble in most organic solvents so that they are easy to deposit onto the surfaces of supports and into nanogaps. The electronic properties of the long-chain molecules have been altered by altering the organic functional groups along the chains, in one of two ways: - By introducing donor-acceptor groups on either side of the molecules which introduces a polarity perpendicular to the chain direction, or - By using steric control to introduce a twist into the molecules along the chain, and by altering the angle of the twist altering the electron flow along the chains. In the latter stages of the project, transition-metal-containing units have been introduced into the chains at regular intervals. The introduction of the metals means that it is easier to grow longer chains, and that there is potentially greater control over the electronic properties of the chains because the redox properties of the metals can be tailored to enhance or decrease the electron flow. Modelling of energy levels and electrical properties of conjugated molecules connected to realistic gold substrates/ and molecules and nanocrystals with anchors connected to gold electrodes Modelling of energy levels and electrical properties of conjugated molecules connected to realistic gold substrates, using a new programme called TranSIESTA: We have developed a new modelling programme (TranSIESTA) to calculate transport through conjugated molecules attached to realistic substrates. Arrays or individual molecules can be handled. This has produced important insights into the causes of the switching in molecules consisting of chains of one or more benzene rings. Modelling of energy levels and electrical properties of conjugated molecules and nanocrystals with anchors connected to gold electrodes: We have identified from electronic structure calculations molecular features required to promote an NDR signal in molecular junctions; according to these calculations, resonant tunnelling processes can be generated in conjugated wires by introducing a twist or a saturated spacer along the molecular backbone. This is further supported by sophisticated I-V simulations, thus pointing to the fact that simple molecular modelling can prove very useful to design wires with specific functionalities and electrical characteristics for use in molecular junctions. The calculations have also rationalized a large number of recent experimental data, thus providing a unified picture of the NDR behaviour in molecular wires. Results SANEME on nanocrystals Modelling of tunnelling-based static random access memory (TSRAM) architecture with conjugated molecules and nanocrystals: Computer simulations have been used to determine the performance required for molecular electronic devices to be useful in computer circuits. One of the possible applications of molecular electronics in combination with silicon technology is for the refresh of DRAM cells. The simulations show that no molecules with resonant electrical properties published so far in the literature have suitable properties for this particular application. Thus the aim should be to find molecules which show NDR at lower applied voltages and lower current levels than the molecules published so far. Fabrication of gold nanogaps using shadow evaporation, and self-assembly and current-voltage measurements of nanocrystals or short conjugated molecules within them: The edge of one layer of gold shadows the region next to it from gold atoms evaporated at an angle, forming a gap of a controllable size, usually between 2 and 6 nm. CdSe nanocrystals with dithiol linker molecules, or short conjugated molecules with thiol end groups, were self-assembled in this gap. The I-V characteristics of some of these samples showed a staircase structure (at 4K), indicative of the quantised energy levels or of Coulomb blockade of single-electron tunnelling. Physical and electrical characterisation of conjugated molecules and nanocrystals on gold and silicon surfaces using a low-temperature UHV scanning tunnelling spectroscopy: Selected molecules and nanocrystals have been investigated by scanning tunnelling spectroscopy with regard to their electrical properties and their suitability for an application in nanoelectronic devices. I-V characteristics collected at different molecules or nanocrystals have been measured and interpreted. The I-V characteristics on semiconductor substrates are found to be strongly dependent on whether the substrate is doped heavily p- or n-type. Development of selectively-etched nanogaps (size about 5nm) on silicon wafers, and incorporation and current-voltage measurements of nanocrystals within them; use of such gaps as a template for metal: The SiO(2) oxide in a conventional MOSFET structure is etched away, leaving a gap between highly doped silicon electrodes. The size of the gap (~5nm) can be tailored to the length of the molecule or diameter of the nanocrystal. We have observed reproducible negative differential resistance (NDR) when CdSe nanocrystals are incorporated into such gaps. We have developed a technique of forming a silicide on the nanogap surfaces, and have shown that molecules with thiol end groups will self-assemble on such surfaces. We have also used the selectively etched gap as a template for the formation of a gap between metal electrodes. These two techniques offer the prospect of easy attachment of molecules to a silicon device. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
