Abstracts Chemistry

Magnetotransport Studies of Metal-Organic Molecule-Metal Structures: Electronic Transport Through Molecular Films of Alkanedithiols and Valence Tautomers

by William Charles Rice

Considerable recent research interest has focused on developing molecule-based device structures for spintronics applications. In addition to having long lifetimes of the electron spin which make them ideal for spintronics, molecules offer the possibility for synthetically customizing their properties to suit device requirements. Integrating molecules into architectures which can exploit these controllable properties, understanding intrinsic transport and magnetotransport within the molecules, and manipulating interfaces are the current challenges in the field. In the first part of this project, the technique of nanotransfer printing (nTP) with polymer stamps was used to print Au thin film electrodes onto self-assembled monolayers (SAMs) of alkanedithiols, carbon chains with sulfur-based thiol end groups. A systematic study was conducted to determine the dependence of transport properties on the length of the alkane chains and the area of the printed Au electrodes. Current-voltage (I-V) curves analyzed with the Simmons model determined the height of tunneling barrier associated with the molecule (alkane) to be 3.5 � 0.2 eV with an upper bound of 2.4 eV for the counterpart at the interface (thiol). The former is consistent with the theoretical value of ~3.5-5.0 eV. The results demonstrated that nTP is a promising technique for producing scalable and permanent metal-molecule-metal junctions for molecule-based electronics. In the second part of this research, cobalt-based valence tautomers (VTs), which have two distinct stable magnetic forms that can be switched as a function of temperature and light, were studied for their magnetotransport behaviors. The molecules were spin-coated to form the spacer layer of a spin valve which showed a magnetoresistance (MR) effect controlled by the tautomeric switching behavior. The effective switching on or off of the MR, corresponding to the anti-parallel magnetization state of the spin valve, was observed to undergo a pronounced shift as a function of the magnetic field strength, correlating to the reported tautomeric temperature transition between 150K and 180K. Additionally, asymmetries in the MR as a function of both magnetic field sweep direction and voltage bias direction demonstrate that the VT has a bias direction-dependent spin-polarization, a novel result that makes it suitable for use in a multi-state memory component.