Electron and spin transport in molecular systems

This research focuses at a quantum mechanical investigation of fundamental mechanisms of electron and spin transport through organic molecules. The major objective is to develop theoretical methods and computational tools aimed at predicting electrical characteristics of molecular devices including their current-voltage characteristics based on underlying atomic and electronic structure. Specific research goals include:

1. Development of a first-principles theory of electron transport in single-molecule nanodevices that will allow us to model transport properties of large, complicated molecular circuitry based on a first-principles description of atomic and electronic structure and Green’s function theory of sub-barrier scattering.
2. Application of the theoretical and computational tools to perform transport calculations including current-voltage (I-V) characteristics of saturated -bonded and unsaturated -bonded molecules, di-block oligomer diode and switching molecules, and other systems with interesting and unusual properties.
3. Investigation of novel molecular electronic elements that exhibit non-linear current-voltage characteristics such as molecular resonant tunneling diodes, field effect transistors, Coulomb blockade switches and molecular hysteresis elements.

First-principles theory of sub-barrier scattering and fundamental electron transport mechanisms in organic molecules.

The Green’s function theory of sub-barrier scattering has been developed to include both resonant and ordinary tunneling mechanisms of electron and spin transport through organic molecules. The amplitude of the tunneling transition is expressed via the Green’s function of the tunneling electron at negative energies. Calculation of the electron Green’s functions in the presence of bridging molecules was a major task that was accomplished by the application of a novel variational-asymptotic method for calculating electron sub-barrier scattering operators. This approach allowed us to incorporate many body effects, including the exchange interaction within a one-electron framework. The Green’s function of the tunneling electron was expressed via a total scattering operator which was determined based on a knowledge of scattering operators of individual fragments (or scattering centers) of the molecule within Multiple Scattering Theory (MST), see Fig. 1. This approach naturally dissected a very complex and computationally difficult problem into sub-tasks that can be readily accomplished and the results can be stored for subsequent use in future calculations. The developed approach gives the Green’s function both for tunneling and resonance cases, and therefore permits us to calculate tunneling and resonant currents simultaneously.

Rectification mechanism in di-block oligomer molecular diodes.

We investigated a mechanism of rectification in di-block oligomer diode molecules that have recently been synthesized and showed a pronounced asymmetry in the measured I-V spectrum. The diode was studied using the Green’s function sub-barrier scattering code developed within this project.

Success was achieved in explaining the mechanism of rectification: the pronounced asymmetry in the measured I-V spectrum is due to the resonant nature of electron transport in the molecular system and unique localization properties of bound-state wave functions of these resonant states of the tunneling electron interacting with an asymmetric molecule in an electric field. This diode molecule, consisting of thiophene and thiazole structural units, possesses a chemical asymmetry. Therefore, the left and right parts of the molecule interact differently with electrons that tunnel through the molecule and this interaction is increased or decreased as the local electric potential is increased or decreased.

Assume that the right half of the molecule interacts with passing electrons more strongly than the left half of the molecule. If the right half is attached to the positively charged electrode (anode), then the interaction is strongly increased, which results in a strong increase in the conductance of the molecule. When the polarity of applied voltage is changed, the anode will be connected to the left part of the molecule. Consequently, the interaction with the passing electron will only weakly increase and, correspondingly, the molecular conductance will increase slower than in the positive polarity case, see Fig. 2. This strong asymmetry of the conductance as a function of the polarity of applied voltage constitutes the rectification of the current by the molecule.

This work was highlighted at the main NSF web page in March 2006 and in review journal “Materials Today”.

Selected publications:

1. M.A. Kozhushner, V.S. Posvyanskii, and I.I. Oleynik, “Bound states of tunneling electrons in molecular chains”, Phys. Rev. B 74, 165103 (2006).
2. I. I. Oleynik, M.A. Kozhushner, V.S. Posvyanskii, and L. Yu, “Rectification mechanism in di-block oligomer molecular diodes”, Physical Review Letters, 96, 096803 (2006).
3. M.A. Kozhushner, V.S. Posvyanskii, and I. I. Oleynik, “Tunneling and resonant conductance in one-dimensional molecular structures”, Chemical Physics, 319, 368 (2005).