Nanoparticle Self-assembly on Prefabricated Nano Strucutres

Doktorsavhandling, 2018

The demand for more powerful computers have been increasing ever since the first semiconducting devices were invented in the second half of the 20th century. The demand for an increase in computing power have escalated since the start of modern computer technology, however conventional techniques are reaching the limits regarding the downscaling of semiconducting based logic circuits. One way of circumventing these challenges is to utilize molecules in integrated circuits instead of conventional semiconducting designs. Molecules can be synthesised in molar amount and can act as rectifiers, transistors, switches and conducting wires. They are also a factor ten smaller than the node size of commercial available transistors. However a method for integrating single molecule into an electronic grid in order to construct molecular based logic circuits is not known today. Scientists have at this point been able to contact and measure on molecules, using techniques such as the break-junction method, however it has been difficult to contact single molecules in parallel, which is needed in order to compete with conventional semiconducting industry.

This thesis focus on contacting single molecules by isolating them between nanoparticles (or dimers), that can be guided onto prefabricated structures in an approach that utilizes both top-down and bottom up concepts. The deposition efficiency was tested on a variety of materials, including metals and functionalized surfaces. This was done as a pre-study in order to determine the optimum conditions for particle deposition. A model based on a combination of DLVO-theory (Derjaguin, Landau, Verwey and Overbeek) and RSA (random sequential adsorption) was developed in order to explain the deposition process and the interactions between particles and a substrate. Spatial descriptive statistics were used to see if the pattern of the particles from simulated and real depositions deviated from CSR (complete spatial randomness) and to compare the inter-particle distances. Potential measurements were compared to the nanoparticle densities. The experiment showed that materials such as nickel and aluminium attract the negatively charged particles used in this thesis. As a next step, particles were deposited on arrays of nanosized objects of different shape and size in order to optimize deposition parameters and electrode design. Finally, electrical measurements of BDT (benzene-1,4-dithiol) and HDT (1,6-hexanedithiol) linked dimers were performed as a proof of principal, indicating that conductance through BDT is higher compared to HDT. More experiments is needed in order to confirm this. However, the deposition is still inefficient, only 5 % of the nanogaps are filled with a dimer. This number needs to be increased in order for molecular electronics to be able to compete with upcoming techniques such as extreme UV-lithography.