Arrays of gold nanoparticles as a platform for molecular electronics

Abstract

The field of molecular electronics has grown rapidly in the past few years. The people working in the field are faced with great technological challenges in order to discover interesting physics and chemistry. The greatest challenge people face is how to contact individual molecules with metallic electrodes in a controlled manner. Because of how small molecules are it is difficult to address or handle a single molecule. In order to prepare junctions, comprising of a molecule contacted by two metal electrodes, people have come up with a range of device. The devices can be split into roughly two categories, those utilizing a monolayer of molecules and those utilizing a metal-molecule-metal junction capturing a single molecule. In the first mentioned devices, those utilizing a monolayer of molecules, the molecules are usually encapsulated and can not be influenced from the outside. This prevents those devices to be used to investigate how the electrical properties of molecules change as their environment changes. The devices which capture a single molecule between two electrodes suffer from instability and the fact that a molecule can bind to the electrodes in more than one way. In this thesis we use an array of gold nanoparticles as a platform to perform electrical transport measurements on molecular junctions. The diameter of the gold nanoparticles, 10 nm, is comparable with the length of the molecules used. The array can be easily contacted with large electrodes. The nanoparticles in the array are stable, mechanically and chemically. Once the nanoparticle array has been prepared the molecules of interest can be inserted into it. The distance between the nanoparticles in the array and the organic ligands covering them control how a molecule can bridge a pair of neighbouring nanoparticles. Using the nanoparticle arrays we were able to investigate the influence of the oxidation state of a redox active molecule on the electrical transport through it. We see clear evidence of the influence in our data, by changing the oxidation state of the molecule an order of magnitude change in conductance was observed. By bringing the electrodes connecting to the nanoparticle array close together we were able to make small arrays, comprising of only 20 x 20 nanoparticles. By performing electrical transport measurements on those devices at a temperature of 4 K we could detect the phonon modes of the molecules bridging the nanoparticles.