Molecular Junctions for Nanoelectronics: Electron Transport through Zinc-Porphyrin

Molecular electronics is an exciting new field where molecules are introduced as circuit elements to create new electronic devices. This is an attractive option due to the scaling down of the device size (~ 2 nm) and also the new functionalities molecules can bring into existing microelectronics. For our study, we have used the highly conjugated aromatic system of porphyrin molecules which gives a measurable conductance at room temperature and is known to behave similar to a molecular switch. Our objective in this thesis is to: (1) explore an alternative fabrication method to create nanojunctions for molecular devices, (2) study molecular switching and (3) understand electron transport mechanisms responsible for conductance in Zn-Porphyrin molecules. To realize such devices, we developed a progressive electromigration technique to fabricate nanogaps of size ~ 2 nm from a gold nanowire and characterized the gap for molecular junction measurements. Porphyrin molecules ligated to a Zn atom (ZnP) was positioned in this gap. We investigated three different porphyrin molecules of various lengths and rigidness (ZnP-F, ZnP-A1 and ZnP-A2) to compare and contrast how the side groups connected to the ZnP affects the transport behavior. Current-voltage (I-V) and differential conductance (dI/dV and d2I/dV2) of the ZnP molecular junction was measured at 300 K and 4.2 K. All three molecules showed asymmetric I-V and dI/dV characteristics in contrast to an empty nanogap that displayed symmetric I-V and dI/dV. ZnP-F showed gate dependent I-V measurement at room temperature. The current through the ZnP-F molecular junction was suppressed with increasing gate voltage indicating a highest occupied molecular orbital (HOMO) mediated tunneling. For ZnP-A1, we observed two "switch-like" behaviors at 4.2 K, one in the positive bias and the other in the negative bias. The "switch-like" behavior in the negative bias disappeared after two successive I-V measurements and only the one in the positive bias remained. Similar behavior was not observed at 300 K. ZnP-A2 showed an obvious switching at 300 K which was absent at 4.2 K. We believe the switching behavior is temperature dependent and affected by the side groups to ZnP. We also hypothesize that the switching is electric field driven and is caused by conformational change resulting from twisting/rotation of bonds in the ZnP molecules. To gain insight to the mechanism of switching behavior and understand how the electrode metal-molecule coupling affects the transport behavior, we completed inelastic electron tunneling spectroscopy (IETS). IETS is a powerful technique that probes the vibrational modes of the molecule at temperatures <10 K in molecular junctions. From the spectra, we were able to identify vibrational mode excited by the metal-molecule interface (Au-S bond) for each molecule. This unambiguously proved that we were successful in creating molecular junctions and all the measured transport characteristics are due to ZnP in the junction. We also identified vibrational modes intrinsic to the porphyrin molecule by comparing the peaks in the IET spectra to infra-red (IR) and Raman spectra. Additionally, by correlating peaks in the IET spectra to switching, we were able to recognize that switching is a result of inelastic electron tunneling and believe that the energy lost by the electron is used to switch the molecule to a different conformation state. We expect that our results will enable fundamental understanding of metal-molecule-metal junction behavior and impact the design of molecular based electronics that can use porphyrins as switches.