Resumen de Rectification and magnetism in single molecule junctions
The electronics industry has been consistently in demand to develop miniaturized devices by scaling down the size of the electronic components. Gordon E. Moore observed this trend in size reduction and proposed his famous Moore's Law . The present top-bottom approaches used in the production line are reaching the theoretical limit. One way to overcome this limit is to use functionalized molecules that can perform as electronic components. Aviram and Ratner first proposed this concept in 1974. They suggested that a molecule could act as a rectifier, a primary electronic component that allows the flow of current only in one direction. Ever since researchers have been trying to develop molecule based devices that could replace various electronic components. Molecules are small and must be considered as quantum mechanical systems, which opens the possibility of devices based on quantum properties such as quantum computers, quantum interference devices, etc. To develop such devices, it is necessary to understand the properties of the molecules at the individual level. Measuring electronic transport across the molecule is one of the methods to probe these quantum properties. This is a challenging task that requires contacting the molecule with external electrodes. A scanning tunnelling microscopy (STM) is an ideal tool for studying electronics and magnetic properties of single molecules deposited on surfaces. The main motivation of this thesis is to investigate rectification and magnetism in single molecule STM junctions at both room and cryogenic temperatures.
One of the prerequisites for single molecule experiments using STM is to have a "good" sample, i.e., well separated individual molecules spread on a flat conducting substrate. The first part of the thesis deals with a technique for depositing the molecules on a suitable substrate. We have successfully deposited K12(DyP5W30O110).nH2O molecules on an Au(111) substrate. This was not an easy task due to the size of the molecule. In the second part we investigate the rectification observed in the molecule. The rectification ratio attained for this molecule is quite high, more than ten times larger than previously reported results in molecular junctions. We could also explain unambitiously that the rectification in this molecular system is arising from the asymmetric coupling of the electrodes and molecule. The third part of the thesis deals with the magnetism in the same molecule. This molecule is single-ion molecular magnet that exhibit magnetic properties at low temperature in bulk samples. We addressed the following question: Does the molecule preserve its magnetic behaviour when deposited on a metallic substrate? Inelastic spin flip tunnelling spectroscopy probing the magnetism in the individual molecules showed that the molecules did preserve their magnetic properties on the surface. The last part of the thesis, focuses on fabricating a superconducting Graphene substrate that could eventually be used in single-molecule experiments. We show that Graphene on lead becomes superconducting by proximity effect.
