Electron transport in iron porphyrin molecular junctions and layered vanadium disulfide ultrathin flakes

Abstract

To date, the experimental studies of the voltage-dependent-electron-transport (VDET) of iron porphyrin is limited. In this thesis, Fe(III) 5,15-di[4-(s-acetylthio)phenyl] 10,20-diphenyl porphine acetate (FeP) is used for VDET measurements. The MJs were formed with self-assembled FeP molecules on a gold (Au) film and a conductive atomic force microscope (AFM) tip. The surface characterization by AFM of the self-assembled FeP indicates the successful deposition of FeP on the Au film with a finite roughness. From the Raman spectra, vibrational modes of FeP are consistent with mode assignments for Fe(TPP)2O reported in literature. The electrical current versus bias voltage (I-V) measurements of FeP MJs show current fluctuations and both symmetric and asymmetric behaviors. The variation in the microgeometry of the MJs and the roughness of the self-assembled FeP likely contribute to the current fluctuations. To understand the VDET property of FeP, a quantitative analysis is performed using the averaged I-V curves with low current fluctuations.

The study of electron transport in nanoscale materials is one of the keys to utilize them successfully in future electronic applications. This thesis focuses on two nanoscale materials: iron porphyrin molecular junctions (MJ) and layered ultrathin vanadium disulfide (VS2).

In the second transport study, layered VS2 was grown on SiO2/silicon substrate by atmospheric pressure chemical vapor deposition. The resistivity of ~2 μm estimated from the room temperature measurement of an as grown ~13 nm thick VS2 agrees with values reported in the literature. In addition, the temperature dependent resistance from ~15 and ~25 nm thick VS2 were measured using microfabricated four-point probe. Both thicknesses show increasing resistance (R) as the temperature (T) is lowered to 4.2 K. This behavior of ultrathin VS2 resembles a semimetal in contrast to a ~205 nm thick VS2 where the R decreases as the T decreases, which represents metallic behavior. Recent spin-polarized density functional theory (DFT) calculation shows that the band edges (bottom of the conduction band, top of the valence band, or both) of monolayer (ML) VS2 are very close or overlap their Fermi levels. This means ML VS2 is a semimetal or an ultra-narrow-band semiconductor. This supports our observation of ultrathin VS2, which is likely the first semimetal experimentally observed among layered transition metal dichalcogenides. For the magnetoresistance (MR) study, no significant MR signal was observed at room temperature and 4.2 K due to the VS2 flake is too thick.

The transport theory based on a rectangular barrier with constant barrier height proposed by Simmons is used to interpret the VDET from the averaged I-V curve of FeP. All averaged I-V curves are well described by this theory in the low voltage region. The Beebe model is introduced to identify the transition between conduction mechanisms. This model predicts a conduction mechanism transition from direct tunneling (DT) at low-voltage (LV) to Fowler-Nordheim tunneling at high-voltage (HV). This implies that DT is the main conduction mechanism in the FeP at LV. The measured barrier heights 0 ranged from ~0.3-0.6 eV and the range of the attenuation coefficient was ~0.6-0.8 Å-1. This value is nearly three to four times larger than the attenuation coefficient of FeP in electromigration break junction measured at 4.2 K in vacuum from a previous study. However, Simmons theory cannot explain the I-V behavior well at HV. To overcome this, a bias voltage that is linearly dependent on an effective barrier height  = cV + i, where c is the effective barrier height constant and φ_i is the barrier height at zero bias, is introduced to the Simmons theory. With this modification to the Simmons model, the Simmons theory can describe the averaged I-V curves much better in the entire voltage range including the HV range. The extracted barrier height i ranges from ~0.2-0.4 eV. This modification implies that the bias voltage influences the molecular orbital of FeP by increasing the energy difference between the Fermi level of the electrode and the highest occupied molecular orbital (HOMO) level of the FeP. This conclusion qualitatively agrees with the molecular projected self-consistent Hamiltonian (MPSH) calculations of iron porphyrin molecules between Au electrodes presented by Wang, et al. (2009).