Evidence for direct tunneling and studies of the electron attenuation coefficient in porphyrin molecular junctions

Abstract

The goal of this thesis is to verify that the electron conduction mechanism through metal-porphyrin-metal junctions (MJs) is direct tunneling, and compute the electron attenuation coefficient. Porphyrins are of interest for their applications in molecular electronics due to their use as electrical switches, rectifiers, dye-sensitized solar cells, and organic field effect transistors. To understand the behavior of the molecules as an electronic component, it is necessary to study both the electron transport and electron-phonon coupling in molecular junctions. Towards this goal, this thesis investigates: (1) temperature dependent electron transport measurements to determine the electron conduction mechanism through porphyrin MJs and the electron attenuation coefficient, β, which describes how the molecules transport charge over distances as molecular wires; and (2) inelastic electron tunneling spectroscopy (IETS) on these MJs to confirm the presence of these molecules in the junctions by identifying electron-phonon coupling modes that are compared to Fourier transform infrared (FTIR) spectra. The IETS spectra determines which modes play a role in electron transport.

IETS of the three porphyrin analogs were performed at 4.3 K to reveal the vibrational modes of the molecular junction. The position of these peaks were associated with a particular vibrational mode of the molecule. For these measurements, peaks observed in the 0-40 mV range were due to the molecule electrode coupling. Vibrations due to the phenyl groups, vibrations of the carbon-carbon bonds in the porphyrin, and deformation of the porphine ring were present in the measurements. These peaks were compared to Fourier transform infrared spectroscopy (FTIR) measurements of porphyrin self-assembled monolayers (SAM). Peaks were observed at 797 1/cm in both FBP and Zn-P, which indicate the presence of β-hydrogen wagging on the porphyrins. Additionally, the peaks at 966 1/cm for FBP and 999 1/cm for ZnP were vibrations caused by the breathing mode of the pyrrole. The significance of observing at least some of the possible vibrational modes of the molecules with IETS confirms that a molecular junction is created. Modes that describe molecule-electrode coupling were inconsistent between samples, suggesting that the molecules' position in the gap will effect IETS measurements.

To determine the electron conduction mechanism through the molecular junction, I/V measurements were collected from 4.3 K - 300 K. Measurements revealed no significant change in the I/V characteristics and the electron conduction mechanism was determined to be direct tunneling. From these studies the electron attenuation coefficient β was determined. The average β for FBP is 0.231 1/Å, for Zn-P is 0.188 1/Å, and for Fe-P is 0.177 1/Å. The low electron attenuation coefficient is comparable to porphyrin oligomers (0.04 1/Å) that are considered for use as molecular wires. The barrier height of the porphyrin molecular junctions was experimentally determined from a combination of ultraviolet/visible (UV/Vis), fluorescence (FL) spectroscopy, and cyclic voltammetry (CV). From UV/Vis and FL spectroscopy, the energy level between the molecule's highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) or the HOMO-LUMO gap was 1.92 eV and 2.06 eV for FBP and Zn-P, respectively. The barrier heights were found to be 1.1 eV and 1.5 eV for FBP and Zn-P, respectively.

Three types of porphyrin molecules are studied: 5,15-di[4-(S-acetylthio) phenyl] 10, 20-diphenylporphine (FBP), Zn(II) 5,15-di [4-(S-acetylthio)phenyl] 10,20- di phenyl porphine (Zn-P), and Fe(III) 5,15-di[4-(S-acetylthio) phenyl] 10,20-diphenyl porphine acetate (Fe-P). For these experiments, MJs are formed using a zig-zag electromigration technique. Prior to electromigration, molecules are drop dried on intact 50 nm x 30 nm gold wires, and the voltage across the wire is increased until a 3-5 nm junction is formed (i.e. when a current density of about 10^12 A/m^2 is achieved). During this process, the molecules bridge the gap to form the molecular junction. Current/voltage (I/V), dI/dV, and IETS (d^2I/dV^2) are collected simultaneously over a range of temperatures.